Article

A comparison of mass transfer coefficients between trickle-bed, Hollow fiber membrane and stirred tank reactors

Authors:
  • Indigo Ag, Charlestown, United States
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Abstract

Trickle-bed reactor (TBR), hollow fiber membrane reactor (HFR) and stirred tank reactor (STR) can be used in fermentation of sparingly soluble gasses such as CO and H(2) to produce biofuels and bio-based chemicals. Gas fermenting reactors must provide high mass transfer capabilities that match the kinetic requirements of the microorganisms used. The present study compared the volumetric mass transfer coefficient (K(tot)A/V(L)) of three reactor types; the TBR with 3mm and 6mm beads, five different modules of HFRs, and the STR. The analysis was performed using O(2) as the gaseous mass transfer agent. The non-porous polydimethylsiloxane (PDMS) HFR provided the highest K(tot)A/V(L) (1062h(-1)), followed by the TBR with 6mm beads (421h(-1)), and then the STR (114h(-1)). The mass transfer characteristics in each reactor were affected by agitation speed, and gas and liquid flow rates. Furthermore, issues regarding the comparison of mass transfer coefficients are discussed.

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... During the syngas biomethanation process, a crucial limiting factor is the slow gasliquid mass transfer since the microbial conversions occur mainly in the aqueous phase and the gas must pass the interface film of gas-liquid [10,25]. The continuous stirred tank reactor (CSTR) is commonly used in chemical industry. ...
... The continuous stirred tank reactor (CSTR) is commonly used in chemical industry. In the case of syngas biomethanation in a CSTR bioreactor, the mass transfer rate is raised by increasing the impeller speed to break up large bubbles into smaller ones [25]. Another common reactor with a simple structure is the bubble column reactor with gas circulation (BCR-C), which might be more suitable for syngas biomethanation [26]. ...
... In large industrial CSTR fermenters above 10 2 m 3 , the power input is limited to about 5 kW m −3 [28]. High power consumption of CSTR limits its scaling-up feasibility in the industry [25]. In a BCR, the power is supplied solely by external compressors with low power consumption and low equipment investment, but back-mixing and coalescence are considered to be the major drawbacks [28]. ...
Article
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In order to utilize a wider range of low-grade syngas, the syngas biomethanation was studied in this work with respect to the gas–liquid mass transfer and the reactor start-up strategy. Two reactors, a continuous stirred tank (CSTR) and a bubble column with gas recirculation (BCR-C), were used in the experiment by feeding an artificial syngas of 20% H2, 50% CO, and 30% CO2 into the reactors at 55 °C. The results showed that the CH4 productivity was slightly increased by reducing the gas retention time (GRT), but was significantly improved by increasing the stirring speed in the CSTR and the gas circulation rate in the BCR-C. The best syngas biomethanation performance of the CSTR with a CH4 productivity of 22.20 mmol∙Lr−1∙day−1 and a yield of 49.01% was achieved at a GRT of 0.833 h and a stirring speed of 300 rpm, while for the BCR-C, the best performance with a CH4 productivity of 61.96 mmol∙Lr−1∙day−1 and a yield of 87.57% was achieved at a GRT of 0.625 h and a gas circulation rate of 40 L∙Lr−1∙h−1. The gas–liquid mass transfer capability provided by gas circulation is far superior to mechanical stirring, leading to a much better performance of low-grade syngas biomethanation in the BCR-C. Feeding H2/CO2 during the startup stage of the reactor can effectively stimulate the growth and metabolism of microorganisms, and create a better metabolic environment for subsequent low-grade syngas biomethanation. In addition, during the thermophilic biomethanation of syngas, Methanothermobacter is the dominant genus.
... The syngas mass transfer rate (K L aÁDC) includes a small driving force because of the low aqueous solubility of the gaseous substrates CO and H 2, relatively low absolute pressures, and low partial pressures due to the presence of CO 2 and sometimes N 2. This needs to be compensated by the high efficiency of contacting with the liquid medium [6], hence a high value of the volumetric mass transfer coefficient, K L a. Thus, mass transfer limitations can be addressed by a selection of the bioreactor and operational conditions to achieve a higher concentration gradient DC, mass transfer coefficient K L , and/or interfacial area a [10,11]. ...
... Non-agitated reactor systems have also been investigated as suitable configurations for syngas fermentation, using much less energy than STRs [14]. In trickle bed reactors, the liquid film contacting the gas phase is very thin and therefore the liquid resistance to mass transfer is diminished [11]. Monolithic biofilm reactors may achieve very high mass transfer but could be prone to clogging by biofilms [15]. ...
... For stand-alone HFM reactors, K L a has been calculated by a steady-state mass transfer analysis (static method) according to Equation (6) [11,42]. ...
Article
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Syngas fermentation to biofuels and chemicals is an emerging technology in the biobased economy. Mass transfer is usually limiting the syngas fermentation rate, due to the low aqueous solubilities of the gaseous substrates. Membrane bioreactors, as efficient gas–liquid contactors, are a promising configuration for overcoming this gas-to-liquid mass transfer limitation, so that sufficient productivity can be achieved. We summarize the published performances of these reactors. Moreover, we highlight numerous parameters settings that need to be used for the enhancement of membrane bioreactor performance. To facilitate this enhancement, we relate mass transfer and other performance indicators to the type of membrane material, module, and flow configuration. Hollow fiber modules with dense or asymmetric membranes on which biofilm might form seem suitable. A model-based approach is advocated to optimize their performance.
... The attainment of higher mass transfer represented in the volumetric mass transfer coefficients, k L,CO a/V L and k L,H2 a/V L , is of primary concern in most discussion of syngas fermentation [82][83][84]. A model of syngas fermentation in the continuously-stirred tank reactor (CSTR) was developed to assess the potential for the production of acetate [85], and mass transfer has been studied in various configurations of fermenters [82,83,86,87]. Klasson et al. [83], however, notes that the rate of mass transfer will not exceed the rate of reaction of the slightly soluble substrates and that the applied mass transfer should balance the supply and consumption of CO and H 2 . ...
... Two-stage CSTR fermenters have been operated with the first CSTR configured to promote growth of the acetogenic culture with acid production and the second CSTR operated at low pH under nutrient limitation and low gas conversion to achieve high ethanol concentration [100]. Column fermenters that show promise include a bubble column with a ceramic monolith to support biofilm [84], a trickle bed with biofilm [87,101,102] and a biofilm supported on a hollow fiber membrane for gas dispersion [86,87]. Biofilms retain cells, but long-term mass transfer and fouling may limit application. ...
... Two-stage CSTR fermenters have been operated with the first CSTR configured to promote growth of the acetogenic culture with acid production and the second CSTR operated at low pH under nutrient limitation and low gas conversion to achieve high ethanol concentration [100]. Column fermenters that show promise include a bubble column with a ceramic monolith to support biofilm [84], a trickle bed with biofilm [87,101,102] and a biofilm supported on a hollow fiber membrane for gas dispersion [86,87]. Biofilms retain cells, but long-term mass transfer and fouling may limit application. ...
Article
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Biomass and other carbonaceous materials can be gasified to produce syngas with high concentrations of CO and H2. Feedstock materials include wood, dedicated energy crops, grain wastes, manufacturing or municipal wastes, natural gas, petroleum and chemical wastes, lignin, coal and tires. Syngas fermentation converts CO and H2 to alcohols and organic acids and uses concepts applicable in fermentation of gas phase substrates. The growth of chemoautotrophic microbes produces a wide range of chemicals from the enzyme platform of native organisms. In this review paper, the Wood–Ljungdahl biochemical pathway used by chemoautotrophs is described including balanced reactions, reaction sites physically located within the cell and cell mechanisms for energy conservation that govern production. Important concepts discussed include gas solubility, mass transfer, thermodynamics of enzyme-catalyzed reactions, electrochemistry and cellular electron carriers and fermentation kinetics. Potential applications of these concepts include acid and alcohol production, hydrogen generation and conversion of methane to liquids or hydrogen.
... The reactor with a submerged composite hollow fiber membrane module had the lowest k L a (0.4 h -1 ). Orgill et al. (2013) compared the k L a values of HFMR with different modules, TBR with different size of packing beads, and CSTR. The same authors found that the highest k L a (1062 h -1 ) 29 was obtained using HFMR with non-porous polydimethylsiloxane followed by TBR with large size (6 mm) beads (421 h -1 ) and CSTR (114 h -1 ). ...
... Efforts have been made to address the gas to liquid mass transfer limitation by designing and characterization of various types of reactors, diffusers, and impeller Munasinghe & Khanal, 2010b;Orgill et al., 2013;Shen et al., 2017b). Research was also focused on evaluating effect of media components such as minerals and trace metals on cell growth, products formation and key enzymes in acetyl-CoA pathway . ...
... One of the major bottlenecks for syngas fermentation is gas-liquid mass transfer because of low solubility of CO (83% that of O 2 ) and H 2 (71% that of O 2 ) on molar basis in the fermentation broth (Phillips et al., 2017a). The gas-liquid mass transfer can be a limiting factor when cell concentration is high (Orgill et al., 2013). Efforts have been made to increase gas-liquid mass transfer using different types of reactors such as CSTR, bubble column reactor (BCR), hollow fiber membrane reactor (HFMR), trickle bed reactor (TBR), and horizontal rotating packed bed biofilm reactor (h-RPB) (Datar et al., 2004;Riggs & Heindel, 2006;Shen et al., 2014b;Shen et al., 2017b). ...
Thesis
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Microbial fermentation of syngas (mainly CO, H2 and CO2) into biofuels (ethanol, butanol, etc.) and chemicals (carboxylic acids, 2,3-butanediol, etc.) is a flexible and promising route for producing liquid biofuels and chemicals. Biochar obtained from gasification or pyrolysis of the carbon-rich feedstocks contains rich mineral and metal compounds, high pH buffering capacity, and cation exchange capacity (CEC). The use of biochar in syngas fermentation could lead to enhanced production of biofuels and chemicals. In the present study, biochar from switchgrass (SGBC), forage sorghum (FSBC), red cedar (RCBC) and poultry litter (PLBC) were incorporated into fermentation media of C. ragsdalei and C. carboxidivorans. Fermentations were performed in 250 mL bottle reactors with 50 mL working volume fed with bottled syngas (40% CO, 30% H2, 30% CO2, by volume) at 37 C for 15 days. Fermentations with C. ragsdalei showed that PLBC and RCBC improved ethanol production by 59% and 16%, respectively, compared to standard YE medium. Fermentations with C. carboxidivorans showed that media with PLBC and SGBC enhanced ethanol production by 90% and 73%, respectively, and butanol production by four fold compared to standard YE medium. PLBC loading of 10 and 20 g L-1 was shown to enhance ethanol and acetic acid productions with C. ragsdalei and enhance ethanol, butanol, acetic acid, butyric acid, and hexanoic acid productions with C. carboxidivorans. A 10 g L-1 of PLBC was used in fed-batch fermentation with C. ragsdalei in 3-L CSTR. The highest amounts of acetic acid produced in YE medium without MES, PLBC media without and with MES were 0.6, 1.2, and 2.0 g L-1, respectively. The acetic acid produced in all media was completely converted to ethanol using a patented pH controller and syngas feeding method. Fermentations using PLBC media without and with MES resulted in production of 34% and 63%, respectively, more ethanol than in YE medium. Ca, Mg, Fe and Mn were mostly released from biochar during syngas fermentation with C. ragsdalei and C. carboxidivorans. The release of these elements was caused by neutralization with H+ from undissociated acetic acid during acetogenic phase in fermentation. The reduced H+ led to deceleration in pH drop, which extended acetogenic phase with accumulation of more acetic acid and reduced “acid stress” on the microorganisms used. Compared with other types of biochar materials, PLBC had the highest pH buffering capacity, total bound cations and acid neutralizing capacity, which contributed to the enhancement of alcohol and acid production with potential use in commercial syngas fermentation processes.
... Studies on bioreactor configuration aimed at addressing this mass transfer limitation have been conducted by evaluating the volumetric mass transfer coefficient (kLa) of the system as the key parameter. Such studies have been conducted in various bioreactor configurations, including the Continuous Stirred Tank Reactor (CSTR) (Riggs & Heindel, 2006), Bubble Column Reactor (BCR) (Datar et al., 2004), Gas Airlift Reactor (Munasinghe & Kanal, 2010), Trickle Bed Reactor (TBR) (Orgill et al., 2013), and Hollow Fiber Membrane Bioreactor (HFMBR) (Shen et al., 2014). High CO mass transfer in CSTR is normally achieved by increasing the agitation speed to increase the interface area (Orgill et al., 2013). ...
... Such studies have been conducted in various bioreactor configurations, including the Continuous Stirred Tank Reactor (CSTR) (Riggs & Heindel, 2006), Bubble Column Reactor (BCR) (Datar et al., 2004), Gas Airlift Reactor (Munasinghe & Kanal, 2010), Trickle Bed Reactor (TBR) (Orgill et al., 2013), and Hollow Fiber Membrane Bioreactor (HFMBR) (Shen et al., 2014). High CO mass transfer in CSTR is normally achieved by increasing the agitation speed to increase the interface area (Orgill et al., 2013). Microbubble sparging has been used as one tool to enhance the dispersion of gas in the CSTR system, although no significant difference in the kLa values compared to a conventional bubble system has been demonstrated (Munasinghe & Khanal, 2010). ...
... The approach frequently used to attain high gasliquid mass transfer is by increasing the agitation speed; however, this is economically infeasible due to the high-power consumption required for upscale fermentation. The application of hollow fiber membrane (HFM) as an external gas-liquid contactor connected to the bioreactor as the reservoir is able to significantly increase the CO mass transfer rate since it provides a large ratio of surface area to volume as the syngas flowing through the lumen of the membrane diffuses through the microporous membrane without forming bubbles (Orgill et al., 2013). The high gas-liquid mass transfer offered by this system enables the application of a low gas flow rate for higher gas conversion. ...
Article
Full-text available
Ethanol production via syngas fermentation obtained from lignocellulose gasification provides a method to completely utilize all the carbon content from lignocellulosic feedstock. A low mass transfer rate of less soluble gas CO and H2 to liquid has been considered as the major bottleneck of the overall process; thus, microporous membrane was proposed as a gas diffuser to improve gas-to-liquid mass transfer. In this study, a liquid batch of syngas fermentation employing Clostridium ljungdahlii with continuous gas supply was carried out with the configuration of a bioreactor connected to the microporous hydrophobic polypropylene hollow fiber membrane (HFM) as a gas diffuser. Liquid recirculation between the fermentation vessel and membrane module was applied to enhance the gas-liquid contact as well as cell-recycle. Fermentation performance with and without HFM was compared and evaluated by cell growth, CO utilization, ethanol yield, and productivity. A higher ethanol yield, 0.22 mol/mol, was achieved by the system of HFM-supported bioreactor with higher ethanol titter of 1.09 g/L and ethanol to acetate molar ratio of 1.43 mol/mol. The obtained result indicates HMF-supported bioreactor is the best fermentation system compared to STR without the membrane.
... The gas-liquid mass transfer as a reaction-limiting factor of synthesis gas fermentation receives special attention in the literature [9,27,66,67]. In the fermenter, there is a threephase system consisting of the gaseous substrate mixture, the liquid nutrient solution and the bacteria cells suspended in the fermentation broth. ...
... As gas-liquid mass transfer is considered as a reactionlimiting factor of gas fermentation, a high level of research activity with regard to k L a measurements in different reactor configurations can be observed in this context [66,67,88,[90][91][92] [7,9,29,95]. k L a values are specific for each system and depend on various parameters such as stirrer speed, gas volume flow, gas bubble size, liquid phase circulation rate, reactor geometry, internals or packing materials [9,36,96]. ...
... The gas-liquid mass transfer is an essential property of a reactor and can be improved by the choice of suitable parameters or supporting technologies [97]. However, the comparison of measured k L a values of different studies is not trivial [67]. As systems with high volumetric mass transfer rates often require an increased energy consumption and, thus, lose efficiency, k L a values must be linked to the required volumetric power input P/V for a more reliable comparison [16,98]. ...
Article
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One approach of Power‐to‐X is the coupling of the energy and chemical sector, using electrolysis for syngas generation and microbial gas conversion for the production of biochemicals. On the verge of commercialization, known challenges of gas fermentation technology are poor mass transfer of syngas, low cell concentration and productivity. These problems can be addressed by an intelligent reactor design. Thus, this article provides an overview on the current state of the art for reactor technology in syngas fermentation and discusses possible concepts with regard to an application at industrial scale. Developing the best reactor possible is important for every biotechnological or chemical process, as it plays a key role in achieving economic success. This study compares different reactor concepts for syngas fermentation, focusing on gas‐liquid mass transfer, power input and scale‐up experiences.
... The mass transfer characteristics of several types of bioreactors, including the continuously stirred tank reactor (CSTR), trickle bed reactor (TBR), hollow fiber membrane reactor (HFR), bubble column reactor (BCR), monolith biofilm reactor (MBR), horizontal rotating packed bed biofilm (h-RPB) reactor, and airlift reactor have been reported [11,13,[15][16][17][18][19]. These studies estimated the volumetric mass transfer coefficient via an air-water system, or sparging syngas into medium with and without fermentation. ...
... Recently, a new model predicting k L a of CO and H 2 in a HFR had also been developed [22]. However, when considered for syngas fermentation, each reactor has its own advantages and disadvantages in terms of operation and scale up [15]. The CSTR, as a conventional reactor, has been more extensively studied and applied in industrial fermentation processes than the HFR and TBR [11,15]. ...
... However, when considered for syngas fermentation, each reactor has its own advantages and disadvantages in terms of operation and scale up [15]. The CSTR, as a conventional reactor, has been more extensively studied and applied in industrial fermentation processes than the HFR and TBR [11,15]. In addition, the CSTR operation is simpler than other types of reactors and can provide good mixing capability and high mass transfer rates, but requires high power consumption, which becomes an issue for large reactors due to high power cost. ...
Article
Full-text available
Syngas (mixture of CO, H2 and CO2) fermentation suffers from mass transfer limitation due to low solubility of CO and H2 in the liquid medium. Therefore, it is critical to characterize the mass transfer in syngas fermentation reactors to guide in delivery of syngas to the microorganisms. The objective of this study is to measure and predict the overall volumetric mass transfer coefficient, kLa for O2 at various operating conditions in a 7-L sparged and non-sparged continuous stirred-tank reactor (CSTR). Measurements indicated that the kLa for O2 increased with an increase in air flow rate and agitation speed. However, kLa for O2 decreased with the increase in the headspace pressure. The highest kLa for O2 with air sparged in the CSTR was 116 h−1 at 600 sccm, 900 rpm, 101 kPa, and 3 L working volume. Backmixing of the headspace N2 in the sparged CSTR reduced the observed kLa. The mass transfer model predicted the kLa for O2 within 10% of the experimental values. The model was extended to predict the kLa for syngas components CO, CO2 and H2, which will guide in selecting operating conditions that minimize power input to the bioreactor and maximize the syngas conversion efficiency.
... Liquid flowed onto the surface of the string support connecting top and bottom of the reactor, which enhanced k L a of CH 4 up to 864.7/h (Mariyana et al., 2018). A similar mass transfer concept is implemented in trickle-bed systems (Devarapalli et al., 2016;Devarapalli et al., 2017;Kimmel et al., 1991;Orgill et al., 2013). The distance or thickness of bulk liquid between the gas phase and biofilm was kept very thin, leading to high k L a (Shen et al., 2014a). ...
... However, this volume cannot indicate the exact GLMT area which leads to lower k L a prediction due to the complex fluid flow on the bed layer. Accordingly, the model should define and include the GLMT area parameters depending on the shape of support media and bed (Orgill et al., 2013). ...
... Also, the concentration gradient of dissolved gases occurs between the liquids flowing in the inlet and outlet due to the consistent flow direction. Therefore, the logarithmic C L is used as a characteristic value (Orgill et al., 2013;Orgill et al., 2019), which makes k L a calculation convenient. But, it cannot exactly indicate the behavior of the dissolved gas gradient, which may lead to inaccurate prediction of k L a. ...
Article
Bioprocessing of synthesis gas (syngas) and waste gases from industrial streams has been developed as a potential option to produce biofuels and biochemicals. However, poor solubility and mass exchange of syngas, low cell biomass, and productivities are key bottlenecks for technology commercialization. All aforementioned hurdles are mainly the outcome of poor gas-liquid mass transfer (GLMT). Efforts have been devoted by employing different reactor configurations, improving gas delivery systems, and altering properties of the system. To date, all available options have not been critically reviewed and an optimal solution to get high GLMT is lacking to drive technology towards practicality and economic feasibility. Hence, aim of this article is to compare different options to increase GLMT in syngas fermentation. Comprehensive discussions are made on state of the art reactor configurations, gas delivery devices and supporting technologies. Finally, a reactor system for high GLMT, high cell concentration and productivity is proposed.
... Therefore, it is necessary to characterize the mass transfer of the reactor used for syngas fermentation to better understand how to overcome mass transfer limitations. Most mass transfer studies in syngas fermentation using Continuous Stirred Tank Reactor (CSTR) investigated operating parameters such as gas flow rate and agitation speed at a fixed working volume and one pressure in the headspace [8][9][10]. The objectives of this study were to experimentally investigate the overall volumetric mass transfer coefficient (kLa/VL) for the hollow fiber membrane (HFM) reactor as the new proposed configuration reactor to be used in syngas fermentation, in which an external microporous HFM module was used as the gas diffuser and gas-liquid contactor to enhance gas-liquid mass transfer. ...
... The experiments were conducted in duplicate. The kLa/VL value was estimated by Equation (1) [9,11]. (1) where Cs is the saturated DO concentration in the liquid (mol/m 3 ), CL is DO concentration in the bulk liquid (mol/m 3 ), VL is the liquid working volume (m 3 ) and t is time (h). ...
... The kLa/VL value for CO was calculated from measured kLa/VL for O2 using the penetration theory based on their diffusivities in water. The kLa/VL for gas species i can be calculated from kLa/VL for gas species j using Equation (2) [9,11]. ) ...
Conference Paper
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The low mass transfer rate of less soluble gas CO and H² to liquid has been considered as the major bottleneck of syngas fermentation. Gas mass transfer rate depends on many factors, including the reactor type, gas flow rate and agitation speed. This paper presents the evaluation of the mass transfer coefficient (kLa) for the hollow fiber membrane (HFM) reactor as the new proposed configuration reactor to be used in syngas fermentation, in which an external microporous HFM module was used as the gas diffuser and gas-liquid contactor to enhance gas-liquid mass transfer. Experiments were conducted at various agitation speed, gas flow rate and liquid recirculation rate. Similar experiments were also conducted for the stirred-tank reactor (STR) as the most common type of fermenter. The kLa value was determined with a dynamic method, in which the change in oxygen concentration in the liquid phase is measured with time. The kLa of CO or H2 were inferred from equations developed based on penetration theory using O2 kLa value. The maximum value of CO kLa (300.5 h⁻¹) was achieved in HFM supported reactor at the specific CO flow rate of 1.05 vvm and liquid recirculation rate of 120 ml/min. The result of this study would be used for designing configuration of syngas fermentation in the future.
... For many of these processes, poor gas-to-liquid mass transfer and low dissolved gas concentrations have been identified as a limiting factor [6][7][8][9]. Based upon that, a lot of research, for example in syngas fermentation, is focused on increasing the volumetric mass transfer coefficient (k L a) by developing innovative reactor configurations [6,10,11]. Understanding of the k L a values obtained is essential in the gas fermentation field. ...
... For example, during batch operation, different values of k L a may apply. Moreover, we recommend to obtain k L a values using representative fermentation conditions rather than using water, to compare reactor configurations [10,41] or to determine dissolved gas concentrations [42]. ...
Article
Full-text available
In gas fermentations (using O2, CO, H2, CH4 or CO2), gas-to-liquid mass transfer is often regarded as one of the limiting processes. However, it is widely known that components in fermentation broths (e.g., salts, biomass, proteins, antifoam, and organic products such as alcohols and acids) have tremendous impact on the volumetric mass transfer coefficient kLa. We studied the influence of ethanol on mass transfer in three fermentation broths derived from syngas fermentation. In demineralized water, we observed that the addition of ethanol, the expected product, increased kLa two-fold in the 0-5 g.L⁻¹ range, after which near-constant kLa values were obtained. In the fermentation broths, kLa was increased significantly (2-4 fold compared to water) by ethanol supplementation, and to be highly influenced by broth salinity. Our results indicate that kLa is a dynamic parameter in gas fermentation experiments and can be significantly increased due to broth components.
... Recent reviews of bioreactor aeration discuss a variety of methods [4][5][6]. There are also many variables that can influence aeration including temperature, mixing rate, membranes, biomass concentration, and media [7][8][9][10]. In this study, we use a novel sensor array to Recent reviews of bioreactor aeration discuss a variety of methods [4][5][6]. ...
... In this study, we use a novel sensor array to Recent reviews of bioreactor aeration discuss a variety of methods [4][5][6]. There are also many variables that can influence aeration including temperature, mixing rate, membranes, biomass concentration, and media [7][8][9][10]. In this study, we use a novel sensor array to compare different filter technologies and media to conventional methods to determine if the methods can be used in space to oxygenate yeast cultures. ...
Article
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Maintaining steady-state, aerobic cultures of yeast in a bioreactor depends on the configuration of the bioreactor system as well as the growth medium used. In this paper, we compare several conventional aeration methods with newer filter methods using a novel optical sensor array to monitor dissolved oxygen, pH, and biomass. With conventional methods, only a continuously stirred tank reactor configuration gave high aeration rates for cultures in yeast extract peptone dextrose (YPD) medium. For filters technologies, only a polydimethylsiloxan filter provided sufficient aeration of yeast cultures. Further, using the polydimethylsiloxan filter, the YPD medium gave inferior oxygenation rates of yeast compared to superior results with Synthetic Complete medium. It was found that the YPD medium itself, not the yeast cells, interfered with the filter giving the low oxygen transfer rates based on the volumetric transfer coefficient (KLa). The results are discussed for implications of miniaturized bioreactors in low-gravity environments.
... Munasinghe and Khanal (2010) compared eight reactor configurations for syngas fermentation and reported that the air-lift reactor combined with a 20-micron bulb diffuser resulted in the highest mass transfer coefficient (k L a) of up to 91.1 h −1 [45]. In a comparison performed by Orgill et al. (2013), the highest k l a (1062 h −1 ) was reported with a hollow fiber membrane reactor (HFMR) [46]. Although k L a is often used to describe GMLT, it is directly proportional to the flow rate of fluids or the impeller speed, thus resulting in increased power consumption. ...
... Munasinghe and Khanal (2010) compared eight reactor configurations for syngas fermentation and reported that the air-lift reactor combined with a 20-micron bulb diffuser resulted in the highest mass transfer coefficient (k L a) of up to 91.1 h −1 [45]. In a comparison performed by Orgill et al. (2013), the highest k l a (1062 h −1 ) was reported with a hollow fiber membrane reactor (HFMR) [46]. Although k L a is often used to describe GMLT, it is directly proportional to the flow rate of fluids or the impeller speed, thus resulting in increased power consumption. ...
Article
Full-text available
Increasing environmental awareness among the general public and legislators has driven this modern era to seek alternatives to fossil-derived products such as fuel and plastics. Addressing environmental issues through bio-based products driven from microbial fermentation of synthetic gas (syngas) could be a future endeavor, as this could result in both fuel and plastic in the form of bioethanol and polyhydroxyalkanoates (PHA). Abundant availability in the form of cellulosic, lignocellulosic, and other organic and inorganic wastes presents syngas catalysis as an interesting topic for commercialization. Fascination with syngas fermentation is trending, as it addresses the limitations of conventional technologies like direct biochemical conversion and Fischer–Tropsch’s method for the utilization of lignocellulosic biomass. A plethora of microbial strains is available for syngas fermentation and PHA production, which could be exploited either in an axenic form or in a mixed culture. These microbes constitute diverse biochemical pathways supported by the activity of hydrogenase and carbon monoxide dehydrogenase (CODH), thus resulting in product diversity. There are always possibilities of enzymatic regulation and/or gene tailoring to enhance the process’s effectiveness. PHA productivity drags the techno-economical perspective of syngas fermentation, and this is further influenced by syngas impurities, gas–liquid mass transfer (GLMT), substrate or product inhibition, downstream processing, etc. Product variation and valorization could improve the economical perspective and positively impact commercial sustainability. Moreover, choices of single-stage or multi-stage fermentation processes upon product specification followed by microbial selection could be perceptively optimized.
... A stirred-tank reactor (STR) is the most common reactor type for gas fermentation. Operating STRs with high impeller rotational speeds enables the break up into smaller gas bubbles, increasing gas-liquid interfacial areas and allowing for longer retention time of gas bubbles in the liquid medium, thereby increasing k L a [71]. From an economic aspect, STR is often equipped with a microbubble sparger to reduce the operational costs for high impeller speeds, which increases the overall k L a of CO by 6-fold compared to a conventional sparger [72]. ...
... Other energy-efficient bioreactor designs to exclude mechanical agitation include bubble column reactors, Hollow fiber membrane reactors (HFRs), and trickle-bed reactors [69,70]. The mass transfer efficiencies of various reactor configurations have been compared by many researchers [69,71,72], and one of the configurations with the highest k L a was a composite hollow fiber membrane module [73]. In the HFR, the gaseous substrate is introduced into the membrane lumen, diffuses through the membrane. ...
Article
In times of global warming and upcoming fossil fuel shortages, the demand for the replacement of current fossil fuel-based chemical production via the development of alternative technologies and sustainable resources has increased. As a possible solution, an approach that produces chemicals from C1 gases derived from industrial waste gas or syngas has been suggested, but inefficient costs and syngas contaminant-sensitive processes of chemical catalysts have limited C1 gas utilization. Recently, acetogenic bacteria have received much attention as potential biocatalysts capable of C1 gas valorization into value-added chemicals. A comprehensive overview of C1 gas conversion using acetogenic bacteria as biocatalysts and a wide range of value-added products converted from C1 gases is provided in this review. Additionally, several strategies for enhancing product yield and alcohol selectivity during the gas fermentation processes, converting native products into valuable longer carbon compounds through coupling gas fermentation with additional processes, and overcoming energetic limitations underlying acetogenic bacteria via strain engineering are discussed.
... Ancak gıda kaynaklarının enerji kaynağı olarak kullanılması konusundaki tartışmalar ikinci nesil etanol üretimini cazip hale getirmiştir. İkinci nesil etanol üretiminde son yılların popüler alternatifi ise sentezgaz fermentasyonudur [29]. Biyokütlenin termokimyasal-biyokimyasal yöntemlerle biyoyakıta dönüştürülme işlemi bir gıda kaynağının kullanılmasına gerek olmaması avantajının yanısıra sakkarifikasyon-fermentasyon proseslerine göre daha yüksek verimle çalışmaktadır [30]. ...
... Saf kültür veya ko-kültür çalışmalarında yüksek işletme maliyetleri, suşların dejenerasyonu ve sürekli kontaminasyon risklerinden dolayı, sentezgaz fermentasyonunda karışık kültür kullanılması oldukça önemli ve ekonomik bir alternatif olacaktır [29]. ...
... Apart from the microbial communities, scientific attention has also focus on the bioreactor configurations that can increase the mass transfer rate of the sparingly soluble syngas components [13][14][15]. Trickle bed and hollow fiber membrane reactors are the most attractive options for syngas biomethanation, as they allow for higher mass transfer coefficients compared to bubble columns, gas lift reactors and stirred tank reactors [4,16,17]. Recent research activity has been focused on dedicated studies of trickle bed reactors for biological hydrogen methanation and biogas upgrade by mixed microbial cultures [18,19] due to the fact that hydrogenotrophic communities are capable of developing stable biofilms that reinforce the methanogenic activity. Yet, bottlenecks such as the operational cost of the liquid recirculation pump and the expensive synthetic media should be addressed before commercialization. ...
... The container (13) was also connected to a compressible gas bag (12) filled with N 2 so as to avoid formation of vacuum as fresh medium was pumped out. Liquid effluent was collected in another container (16) with the use of a peristaltic pump (Cole Parmer) (15). Liquid samples were collected from a liquid sampling port (6) placed between the liquid reservoir and the liquid recirculation peristaltic pump. ...
Article
Syngas, mainly consisting of CO, H2 and CO2, can be generated from the gasification of biomass and organic waste and constitutes an important energy and carbon source. However, its biological conversion is still challenging due to the low solubility and the toxic nature of its components. In this study, enriched mixed microbial consortia were inoculated in trickle bed reactors operated in continuous mode with the supply of artificial syngas (45% H2, 25% CO2, 20 % CO and 10% N2) under mesophilic (37 oC) and thermophilic (60 oC) conditions. The results revealed a clear superiority of the thermophilic conditions exhibiting higher methane productivities, higher conversion efficiencies and lower yields of byproducts at all steady states tested compared to mesophilic temperature. The highest methane productivity achieved was 8.49 mmol∙lbed⁻¹∙h⁻¹. The microorganisms related to syngas biomethanation were investigated through metagenomic analysis of samples obtained from the inoculum, the liquid phase and the biofilm of the reactors. The continuous operation altered completely the dominant species in mesophilic conditions compared to the inoculum. Accumulation of volatile fatty acids (VFAs) under mesophilic conditions was attributed to the high relative abundance of the genus Sporomusa. A large quantity of acetogenic cell debris scavengers and potential acetogenic metabolism of the genus Thermincola could justify accumulation of VFAs under thermophilic conditions. Absence of aceticlastic methanogens in both reactors was also noticeable. The archaeal communities were enhanced in the biofilm compared to the liquid phase presenting a 6.2 fold and a 1.8 fold higher relative abundance at 37 oC and 60 oC, respectively.
... Many studies have explored using various reactor configurations to enhance gas-liquid mass transfer. This includes well-developed reactors such as continuous stirred tank reactor (CSTR), hollow fiber membrane reactor (HFMR) (Orgill et al., 2013), bubble column reactor (Rajagopalan et al., 2002), gas-lift reactor (Munasinghe and Khanal, 2014), trickle bed reactor (TBR) (Devarapalli et al., 2016;Devarapalli et al., 2017), and other reactors such as monolithic biofilm reactor (Shen et al., 2014a), horizontal rotating packed bed biofilm reactor (Y. Shen et al., 2017) and gas-to-atomized-liquid contactor (Sathish et al., 2019). ...
... Munasinghe and Khanal, 2010b showed that bulb diffuser in the reactor had the highest volumetric mass transfer coefficient (k L a) (91.1 h −1 ) with increased CO flow rate compared to a submerged composite hollow fiber membrane module (0.4 h −1 ). Orgill et al. (2013) compared the k L a values of different types of HFMRs, a TBR with different size of packing beads and a CSTR. Researchers found that the highest k L a (1062 h −1 ) was obtained using HFMR with non-porous polydimethylsiloxane followed by TBR with 6 mm beads (421 h −1 ) and CSTR (114 h −1 ). ...
Article
Syngas is produced by thermochemical conversion, e.g., pyrolysis and gasification, of biomass, animal waste, coal, municipal solid waste and other carbonaceous materials, or directly from CO-rich off-gases from industry, e.g. steel mills. Syngas components (mainly CO, H2 and CO2) are converted to alcohols and other chemicals by acetogenic bacteria through the Wood-Ljungdahl pathway or its derivatives. Syngas fermentation is affected by the acetogen(s), type of reactor, gas composition, medium components, operating parameters, gas-liquid mass transfer and fermentation strategies. These factors affect product distribution, titer, yield, productivity and process feasibility. In this article, syngas fermentation process development with focus on microorganisms, gas composition, medium design, gas-liquid mass transfer fermentation strategies, techno-economic analysis and commercialization efforts are critically reviewed. This review provides new insights in syngas fermentation, which can guide future research towards commercialization of renewable and sustainable biofuels and chemicals.
... HFMs diffuse gases through their micropores without forming bubbles yielding a large surface area for both gas-liquid transfer and, therefore, enhancing the amount of dissolved gas in fermentation broth [19]. HFMs have been employed successfully to enhance mass transfer of gases in wastewater and water treatments and ethanol and syngas fermentations [19][20][21]. ...
Article
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Succinic acid is one of the most useful intermediate chemicals that can be produced in a biorefinery approach. In this study, Actinobacillus succinogenes was immobilized to produce succinic acid using non-detoxified corn fiber hydrolysate (CFH) and a control mimicking the sugars in CFH. Tests were carried out in a hollow fiber membrane packed-bed biofilm reactor (HFM–PBR) operated in a continuous mode. Under steady-state conditions, the bioconversion process was characterized in terms of sugar consumption, succinic acid and other organic acid production. Steady states were obtained at dilution rates of 0.025, 0.05, 0.075, 0.1, 0.2, and 0.3 h−1. The optimal results were achieved at the dilution rate of 0.05 h−1 and recirculation rate of 50 ml/min with a maximum succinic acid concentration, yield and productivity of 31.1 g/L, 0.61 g/g and 1.56 g/L h, respectively, when control was used. Succinic acid concentration, yield and productivity of 23.4 g/L, 0.51 g/g and 1.17 g/L h, respectively, were obtained when CFH was used. Productivity in the HFM–PBR was between 1.3 and 1.9 times higher than productivities for succinic acid production from CFH stated in the literature. The results demonstrated that immobilized A. succinogenes has the potential for effective conversion of an inexpensive biomass feedstock to succinic acid. Furthermore, the process has the potential to serve as a means for value-added chemical biomanufacturing in an integrated corn biorefinery.
... However, the specific energy input for agitation is typically ca. 1 kW m -3 [131], which is too high for the commercialization of syngas fermentation processes at industrial scale, as it should not surpass 0.3 kW m -3 [132]. Trickle bed reactors have a thin liquid film contacting the gas phase and, therefore, a low liquid resistance to mass transfer [133]. Membrane reactors could reach a maximum k L a of 1096 h -1 at laboratory scale, which is three times higher than that of industrially used bubble columns [134]. ...
Article
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Heterogeneous catalysis and anaerobic syngas fermentation represent two different approaches for the conversion of synthesis gas into chemicals and fuels. This review provides a unique comparison of different reaction paths for the fixation of CO2, CO and H2 into elementary building blocks such as methanol, acetic acid and ethanol. Operating conditions, reactor engineering, influence of gas impurities, yields, conversion efficiencies as well as downstream product recovery are compared. It was found that mass‐specific productivity ranges in the same order of magnitude for both technologies, while space‐time yield of heterogeneous catalysis is up to three orders of magnitude higher.
... Because C1 gases, unlike other substrates such as glucose or glycerol, are gaseous substrates, the gas-to-liquid mass transfer rate is critically affected by the physical properties of gas solubility. Many studies have attempted to increase the fixing efficiency of C1 gases through various gas fermentation techniques, such as increasing the partial pressure of the gas (Phillips et al., 1993;Bredwell and Worden, 1998;Hurst and Lewis, 2010;Orgill et al., 2013;Sathish et al., 2019;Bae et al., 2022). However, this approach also has physical limitations, and thus, genetic attempts should be made to overcome the low productivity, yield, and cell density of acetogens by developing a platform acetogen strain with increased C1 gas fixation efficiency and expanding it to a commercial scale. ...
Article
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C1 gases, including carbon dioxide (CO2) and carbon monoxide (CO), are major contributors to climate crisis. Numerous studies have been conducted to fix and recycle C1 gases in order to solve this problem. Among them, the use of microorganisms as biocatalysts to convert C1 gases to value-added chemicals is a promising solution. Acetogenic bacteria (acetogens) have received attention as high-potential biocatalysts owing to their conserved Wood–Ljungdahl (WL) pathway, which fixes not only CO2 but also CO. Although some metabolites have been produced via C1 gas fermentation on an industrial scale, the conversion of C1 gases to produce various biochemicals by engineering acetogens has been limited. The energy limitation of acetogens is one of the challenges to overcome, as their metabolism operates at a thermodynamic limit, and the low solubility of gaseous substrates results in a limited supply of cellular energy. This review provides strategies for developing efficient platform strains for C1 gas conversion, focusing on engineering the WL pathway. Supplying liquid C1 substrates, which can be obtained from CO2, or electricity is introduced as a strategy to overcome the energy limitation. Future prospective approaches on engineering acetogens based on systems and synthetic biology approaches are also discussed.
... Oklahoma State University's collaborative network involved 3 universities in USA: Brigham Young University (12 collaborative publications), the University of Oklahoma (11 collaborative publications), and Iowa State University (16 collaborative publications). The collaboration between Brigham Young University and Oklahoma State University (7 collaborative publications) focused on the effect of partial pressure of carbon monoxide in syngas fermentation, effects of nitric oxide in ethanol production, and improving gas-liquid mass transfer (Ahmed and Lewis, 2007;Devarapalli et al., 2016Devarapalli et al., , 2017Hurst and Lewis, 2010;Orgill et al., 2013). Co-authored publications (7) between Oklahoma State University and the University of Oklahoma were directed to ethanol production from syngas (Liu et al., 2012(Liu et al., , 2014a(Liu et al., , 2014bPhillips et al., 2015;Sun et al., 2018aSun et al., , 2018b. ...
Article
Syngas fermentation, in which microorganisms convert H2, CO, and CO2 to acids and alcohols, is a promising alternative for carbon cycling and valorization. The intellectual landscape of the topic was characterized through a bibliometric analysis using a search query (SQ) that included all relevant documents on syngas fermentation available through the Web of Science database up to December 31st, 2021. The SQ was validated with a preliminary analysis in bibliometrix and a review of titles and abstracts of all sources. Although syngas fermentation began in the early 1980s, it grew rapidly beginning in 2008, with 92.5% of total publications and 87.3% of total citations from 2008 to 2021. The field has been steadily moving from fundamentals towards applications, suggesting that the field is maturing scientifically. The greatest number of publications and citations are from the USA, and researchers in China, Germany, and Spain also are highly active. Although collaborations have increased in the past few years, author-cluster analysis shows specialized research domains with little collaboration between groups. Based on topic trends, the main challenges to be address are related to mass-transfer limitations, and researchers are starting to explore mixed cultures, genetic engineering, microbial chain elongation, and biorefineries.
... Generally, hollow fiber filters have high oxygen transfer rates (Granata et al., 2021). PDMS filters have been shown to have higher oxygen transport rates than stirred reactors at lower shear rates (Orgill et al., 1996). The major difference in the configurations of the 60 ml and 10.5 ml bioreactors, was in the later the PDMS filter functioned as the growth chamber reducing the total volume of the bioreactor system. ...
Article
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Bioreactors in space have applications from basic science to microbial factories. Monitoring bioreactors in microgravity has challenges with respect to fluidics, aeration, sensor size, sample volume and disturbance of medium and cultures. We present a case study of the development of small bioreactors and a non-invasive method to monitor dissolved oxygen, pH, and biomass of yeast cultures. Two different bioreactor configurations were tested for system volumes of 60 ml and 10.5 ml. For both configurations, the PreSens SFR vario, an optical sensor array, collected data autonomously. Oxygen and pH in the cultures were monitored using chemically doped spots, 7 mm in diameter, that were fixed to the bottom of sampling chambers. Spots emitted a fluorescent signal for DO and pH when reacted with oxygen molecules and hydrogen ions, respectively. Biomass was sensed using light reflectance at centered at 605 nm. The, optical array had three light detectors, one for each variable, that returned signals that were pre- and post-calibrated. For heterotrophic cultures requiring oxygen and respiring carbon dioxide, a hollow fiber filter, in-line with the optical array, oxygenated cells and remove carbon dioxide. This provided oxygen levels that were sufficient to maintain aerobic respiration for steady state conditions. Time series of yeast metabolism in the two bioreactors are compared and discussed. The bioreactor configurations can be easily be modified for autotrophic cultures such that carbon dioxide is enhanced and oxygen removed, which would be required for photosynthetic algal cultures.
... They used glass beads with a void fraction of 0.38, which was lower than the void fraction provided by other packing materials and reported that low void fraction decreases the availability of free space for gas-liquid mass transfer. A previous study by the same researchers also showed that TBR provided greater mass transfer capabilities compared to a CSTR [133]. Liu et al. (2019) [134] studied the gasliquid mass transfer in a sparged and non-sparged CSTR with potential application in syngas fermentation and developed a model calculating kLa for syngas components CO, CO2 and H2, which could be used in selecting operating conditions in CSTRs. ...
Article
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Microbial C1 gas conversion technologies have developed into a potentially promising technology for converting waste gases (CO2, CO) into chemicals, fuels, and other materials. However , the mass transfer constraint of these poorly soluble substrates to microorganisms is an important challenge to maximize the efficiencies of the processes. These technologies have attracted significant scientific interest in recent years, and many reactor designs have been explored. Syngas fermentation and hydrogenotrophic methanation use molecular hydrogen as an electron donor. Furthermore, the sequestration of CO2 and the generation of valuable chemicals through the application of a biocathode in bioelectrochemical cells have been evaluated for their great potential to contribute to sustainability. Through a process termed microbial chain elongation, the product portfolio from C1 gas conversion may be expanded further by carefully driving microorganisms to perform acetogenesis, solventogenesis, and reverse β-oxidation. The purpose of this review is to provide an overview of the various kinds of bioreactors that are employed in these microbial C1 conversion processes.
... The transfer of oxygen was measured by duplicate at the four conditions (Table III-11). Typical values of kLa in water vary from 24 h -1 to 300 h -1 for 2 L bioreactors [191], [192], and from 12 h -1 to 426 h -1 in 30 L bioreactors [193], [194]. The values of kLa depend highly on the configuration of the bioreactor, the number and type of agitation impellers and aeration conditions. ...
Thesis
Scorpionism is a serious public health problem in tropical areas, especially in Africa, southern India, the Middle East, and Latin America. There are about 12 species of scorpions that are dangerous to humans and all belong to the Buthidae family. In particular, venom on scorpion Androctonus australis hector is particularly toxic to humans. Serotherapy uses antibodies or fragments of antibodies to target the neurotoxins of the scorpion venom. Nanobodies, camelid-derived antibodies, are more effective than the conventional fragments of antibodies due to their low molecular weight (15 kDa). The Laboratoire des Venins et Molécules Thérapeutiques of the Pasteur Institute of Tunis (LVMT-IPT, Tunis, Tunisia) has produced the bispecific nanobodies against the neurotoxins of the Androctonus australis hector scorpion.In this doctoral project, the production of the bispecific nanobodies NbF12-10 and CH10-12 as recombinant proteins in Escherichia coli was studied. Two clones of the NbF12-10 strain were used (NN and NO), and strain E. coli WK6 was used as reference. The four strains were characterized in rich medium (TB) and defined minimal medium (MM) in shake-flask cultures. In TB, all strains grow at an average µ=0.4h-1. The strains WK6 and CH10-12 had a µmax=0.6h-1 in MM. The clone NbF12-10 NO could not grow in MM, and the clone NbF12-10 NN had a µmax=0.2h-1 in MM. The yield YX/S was 0.4g/g in MM for all strains and the yield of acetate on glucose was between 0.06 and 0.14g/g. The periplasmic extraction of the nanobodies NbF12-10 and CH10-12 was tested in TB, an improvement of 300% in the release of periplasmic proteins was achieved, compared to conventional methods. The strain CH10-12 produced 20-fold higher nanobody than the strain NbF12-10 NN (1.58 vs 0.08mg/L). The quantification of the nanobody was made through a densitometry protocol developed during this thesis. The protocol uses a processing image program (ImageJ) to quantify the optical density profiles of electrophoresis gels. The band of 50kDa of the molecular weight marker was used as reference for 750ng of recombinant protein. This was the subject of an article published in MicrobiologyOpen (Wiley).The production of the nanobodies CH10-12 and NbF12-10 were tested in bioreactor cultures in high cell density cultures in MM (>25gcdw/L) through a specific fed-batch strategy. The effect of the temperature (28–37°C) during protein expression and the duration of the induction phase (6–38h) were studied. The lower temperatures (28°C, 30°C) produced the highest titer of nanobody CH10-12 (2.3mg/L) after 10h and 12h of induction, respectively. In high temperature cultures (33°C, 37°C) the production of the nanobody was lower (0.4, 0.2mg/L). In cultures where the induction phase was longer (>30h), the production of the nanobody CH10-12 was hampered and reached a plateau after 10h (32°C, 0.7mg/L) to 25h (29°C, 1.4mg/L). The nanobody NbF12-10 also reached a plateau after 10h of induction (0.7mg/L). The metabolic burden was lowered by decreasing the temperature of induction, allowing the production of higher titers of recombinant nanobody. The effect of the low specific growth rate (0.02h-1) and the long exposure to the inducer (IPTG, >30h) could have produced a stringent response in the strain. High maintenance metabolism was found during the induction phase, and the consumption of citrate could hint an inadequate composition of the culture medium for the induction phase. Two non-specific proteins were found in the purified nanobody samples: a low molecular weight (11kDa), degradation product of the nanobody caused by a high temperature, and a high molecular weight protein (84kDa), protein aggregate produced as a response to an inadequate medium composition and the low specific growth rate. The generation of unknown proteins in the purified samples of nanobody could be a response of the strain to the low specific growth rate and the long exposure time to the inducer.
... CSTR was commonly used for BM in recent years, where improvement on the gas liquid mass-transfer rate of H 2 was the main focus. A number of studies on optimization of agitation, reactor shape, gas diffusion systems and impeller design were performed to enhance gas-liquid mass transfer by decreasing the gas bubble size (Orgill et al. 2013;Wahid and Horn 2021). For example, stirring speeds of up to 1500 rpm were demonstrated as efficient at the laboratory scale (Seifert et al. 2014). ...
Article
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Power-to-methane technology is a promising solution to facilitate the use of excess variable renewable energy for biomethane production. In this approach, hydrogen produced via electrolysis is used to upgrade raw biogas, which can be subsequently used as fuel or stored in the gas grid. Ex-situ biomethanation is an emerging technology that could potentially replace conventional energy-intensive biogas upgrading methods and allow CO2 utilization for biomethane production. This work provides a comprehensive overview on the current status of ex-situ biomethanation with particular attention to trickle bed reactor. The review includes description of ex-situ biomethanation and summarizes previous works on this topic. The key elements related to operational conditions, efficiency, and microbiology of ex-situ biomethanation using trickle bed reactor are described here. Additionally, the review highlights the technical and economic issues that have to be addressed for future development and large-scale implementation of ex-situ biomethanation.
... Comparing to past years, constant progress and population growth enhanced the amount of residues generated. According to ABRELPE ( Residue pyrolysis result in synthesis gas, which is a building block in chemical industry and can be converted by anaerobic bacteria into very important biochemicals and biofuels, such as ethanol, butanol, butyric acid, 2,3-butanediol, acetic acid and more (LIOU et al., 2005;LATIF et al., 2014;MOHAMMADI et al., 2011;ORGILL et al., 2013;PANTALÉON et al., 2014). This hybrid thermochemical-biochemical process, however, has a major technological drawback, which is the low mass transfer between gas and liquid phases due to gas low solubility (KLASSON et al., 1991). ...
Thesis
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Synthesis gas fermentation has been proposed in literature to decrease urban solid waste environmental impact. However, the low gas-liquid mass transfer is one of the major bottlenecks of this process. Therefore, the present work aimed to evaluate the overall volumetric mass transfer (kLa) in a Stirred Tank Reactor using different compositions of liquid phases. Due to the absence of probes to determine carbon monoxide (CO) concentration in liquid phase and gas chromatography cost, a myoglobin bioassay technique was executed. kLa was estimated using a hybrid optimization method (Particle Swarm Optimization - PSO, and Sequential Quadratic Programming – SQP) and Maximum Likelihood Estimation (MLE) as objective function. Pure CO (99.5%) was fed into a reactor filled with 0.75 and 1.0 L of liquid mixture. Three agitation speeds and five specific gas flow rates were tested. Four different liquid mixtures were analysed: pure distilled water; distilled water and 20% perfluorodecalin (PFC); distilled water and 0.15% Tween® 80; and distilled water, 20% PFC and 0.15% Tween® 80. A kLa of 603.49 h-1 in distilled water, PFC and Tween® 80 at 500 rpm and 2.7 min-1. The highest kLa for pure distilled water reported so far was achieved in the present work: 399.06 h-1 at 500 rpm and 2.7 min-1. Therefore, hybrid optimization was successfully performed and kLa results were comparable to literature. PFC and Tween® 80 increased CO dispersion in the liquid phase, increasing mass transfer.
... Visser et al. (1990) identified this mechanism as a major source of oxygen entry and placed a separate sterile, nitrogen-sparged, stirred bioreactor just in front of the actual chemostat bioreactor. We recently found that small, autoclavable membranecontactor modules commonly used for gas exchange (Orgill et al. 2013;Bakonyi et al. 2015;Engler et al. 2018) are extremely efficient, affordable and practical devices for deoxygenating the medium feed of continuous-cultivation systems (Fig. 5). When a membrane-contactor module was placed near the medium entry point of bioreactors and connected to a flow of nitrogen (N5.5), S. cerevisiae chemostat cultures grown on glucose synthetic medium without the anaerobic growth factors ergosterol and Tween 80 completely washed out. ...
Article
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All known facultatively fermentative yeasts require molecular oxygen for growth. Only in a small number of yeast species, these requirements can be circumvented by supplementation of known anaerobic growth factors such as nicotinate, sterols and unsaturated fatty acids. Biosynthetic oxygen requirements of yeasts are typically small and, unless extensive precautions are taken to minimize inadvertent entry of trace amounts of oxygen, easily go unnoticed in small-scale laboratory cultivation systems. This paper discusses critical points in the design of anaerobic yeast cultivation experiments in anaerobic chambers and laboratory bioreactors. Serial transfer or continuous cultivation to dilute growth factors present in anaerobically pre-grown inocula, systematic inclusion of control strains and minimizing the impact of oxygen diffusion through tubing are identified as key elements in experimental design. Basic protocols are presented for anaerobic-chamber and bioreactor experiments.
... In syngas fermentations, for example, the main issues arise from slow microbial growth rates and poor mass transfer of gaseous substrates such as CO and H 2 , into the fermentation broth (Klasson et al., 1991). In specialized biofilm reactors such as hollow-fiber membrane modules, a membrane serves as biofilm growth support and can improve mass transfer coefficients by allowing for a diffusive supply of gaseous substrates into the biofilm (Orgill et al., 2013). However, despite these benefits, obstacles such as limited membrane surface area per volume ratio, and biofilm thickness, and density control have to be addressed for establishing this technology on an industrial scale (Edel et al., 2019). ...
Article
Harnessing the potential of biocatalytic conversion of renewable biomass into value-added products is still hampered by unfavorable process economics. This has promoted the use of biofilms as an alternative to overcome the limitations of traditional planktonic systems. In this paper, the benefits and challenges of biofilm fermentations are reviewed with a focus on the production of low-value bulk chemicals and fuels from waste biomass. Our study demonstrates that biofilm fermentations can potentially improve productivities and product yields by increasing biomass retention and allowing for continuous operation at high dilution rates. Furthermore, we show that biofilms can tolerate hazardous environments, which improve the conversion of crude biomass under substrate and product inhibitory conditions. Additionally, we present examples for the improved conversion of pure and crude substrates into bulk chemicals by mixed microbial biofilms, which can benefit from microenvironments in biofilms for synergistic multi-species reactions, and improved resistance to contaminants. Finally, we suggest the use of mathematical models as useful tools to supplement experimental insights related to the effects of physico-chemical and biological phenomena on the process. Major challenges for biofilm fermentations arise from inconsistent fermentation performance, slow reactor start-up, biofilm carrier costs and carrier clogging, insufficient biofilm monitoring and process control, challenges in reactor sterilization and scale-up, and issues in recovering dilute products. The key to a successful commercialization of the technology is likely going to be an interdisciplinary approach. Crucial research areas might include genetic engineering combined with the development of specialized biofilm reactors, biofilm carrier development, in-situ biofilm monitoring, model-based process control, mixed microbial biofilm technology, development of suitable biofilm reactor scale-up criteria, and in-situ product recovery.
... Authors used an intensive stirring speed (800 rpm) to decrease the size of gas bubbles and hence to improve the gas distribution which led to an enhanced G/L mass transfer. This technique, although effective for increasing G/L mass transfer, will be difficult to apply on an industrial scale due to the large amount of energy required for mixing [63,67,73]. The last study, noted 5, was carried out by Savvas et al. using a biofilm plug-flow reactor consisting of a single tube with the following dimensions: 13 mm in diameter and 7 m long [67]. ...
Article
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In order to take action against global warming and ensure a greater energy independence, countries around the world are expected to drastically increase the proportion of renewable energy in their energy mix. However, the intermittent production of energy explains why energy supply and demand do not match. In this context, biomethanation, coupled with anaerobic digestion, could be an interesting approach to transform the extra amount of produced electricity by converting hydrogen (produced by electrolysis) and carbon dioxide (present in biogas) into methane. In this review, we have summarized several recently published results which involve biological methanation processes performed by mixed cultures, with an emphasis on microbiological as well as process aspects. In particular, the different microorganisms involved in the process, as well as the used metabolic pathways, along with their kinetic and thermodynamic specificities, are described. Furthermore, the influence of process parameters such as the type of reactor, the type of diffuser and the choice of H2 injection (in situ or ex situ) or the different operating conditions are presented. Explanations of the different performances observed in literature are assumed, technical bottlenecks are listed, and possible solutions to overcome these issues are presented. Finally, the current commercial deployment of this technology is discussed through the example of three companies offering different biomethanation solutions. Graphic Abstract
... The continuous stirred-tank reactor (CSTR) offers extensive mixing capabilities by the steady distribution of gaseous and liquid substrates [28, 107,275]. The resulting high mass transfer rate is the reason why CSTRs are the first choice for gas fermentation investigations [4,11,106,107,170,199,217,275,298,299]. For industrial-scale gas fermentations, the energy demand for sufficient mixing is significantly increased in CSTRs. ...
Article
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The sustainable production of solvents from above ground carbon is highly desired. Several clostridia naturally produce solvents and use a variety of renewable and waste-derived substrates such as lignocellulosic biomass and gas mixtures containing H2/CO2 or CO. To enable economically viable production of solvents and biofuels such as ethanol and butanol, the high productivity of continuous bioprocesses is needed. While the frst industrial-scale gas fermentation facility operates continuously, the acetone–butanol–ethanol (ABE) fermentation is traditionally operated in batch mode. This review highlights the benefts of continuous bioprocessing for solvent production and underlines the progress made towards its establishment. Based on metabolic capabilities of solvent producing clostridia, we discuss recent advances in systems-level understanding and genome engineering. On the process side, we focus on innovative fermentation methods and integrated product recovery to overcome the limitations of the classical one-stage chemostat and give an overview of the current industrial bioproduction of solvents.
... One bottleneck is the low transformation efficiency of syngas due to the mass transfer limitation of sparingly soluble gaseous substrates especially CO and H 2 in aqueous phase [7]. Many studies tried to increase the gas mass transfer via improving the bioreactor design and operation [8][9][10][11][12][13]. In continuous stirred bioreactors, the most common approach is to accelerate the agitation speed to increase the gas-liquid interfacial area and then improve the mass transfer between gas and aqueous phases [7]. ...
Article
The massive consumption of fossil energy forces people to find new sources of energy. Syngas fermentation has become a hot research field as its high potential in renewable energy production and sustainable development. In this study, trophic anaerobic acetogen Morella thermoacetica was successfully immobilized by calcium alginate embedding method. The ability of the immobilized cells on production of acetic acid through syngas fermentation was compared in both airlift and bubble column bioreactors. The bubble column bioreactor was selected as the better type of bioreactor. The production of acetic acid reached 32.3 g·L⁻¹ in bubble column bioreactor with a space-time yield of 2.13 g·L⁻¹·d⁻¹. The immobilized acetogen could be efficiently reused without significant lag period, even if exposed to air for a short time. A semi-continuous syngas fermentation was performed using immobilized cells, with an average space-time acetic acid yield of 3.20 g·L⁻¹·d⁻¹. After 30 days of fermentation, no significant decrease of the acetic acid production rate was observed.
... The k L a coefficients found in these experiments performed in batch mode were lower than those generally reported in studies using continuous gas sparging and other configurations [42,43]. The difference between the k L a coefficients found in this and other studies indicates that the mass transfer was likely limiting during the fermentation, which in turn limited the microbial conversion rates. ...
Article
The syngas biomethanation process is a promising bioconversion route due to its high versatility, as it could be applied as a stand-alone technology, coupled to gasification plants, and integrated in anaerobic digestion or bioelectrochemical conversion systems. The biomethanation of syngas typically takes place through a rather complex network of interspecies metabolic interactions, which may vary significantly depending on the operating conditions applied and the diversity of microbial groups present. Despite there are several benefits derived from using microbial consortia, these also present challenges associated with limited process control and low product selectivity. To address the latter, the syngas biomethanation process carried out by mesophilic and thermophilic microbial consortia was modelled with the ultimate goal of studying possible catabolic route control strategies through the modulation of key operating parameters. The results showed that the thermophilic microbial consortium presented much higher apparent specific methane productivity (18.8 mmol/g VSS/d) than the mesophilic (4.6 mmol/g VSS/d) at an initial PCO of 0.2 atm, and that the difference increased with increasing initial PCO. This difference in productivity was found to derive from the catabolic routes used rather than the kinetic parameters of each microbial consortium. Additionally, the thermodynamic considerations included in the models revealed the possibility of controlling the catabolic routes used by each consortium through the modulation of the mass transfer and PCO2. Our results strongly indicate that modulating the PCO2 is a promising operational strategy for boosting the product selectivity towards CH4, the productivity of the system and the biomethane quality simultaneously.
... The thermochemical-biochemical conversion or hybrid route merges these two processes, converting biomass to a gaseous substrate using thermal energy, and the resulting gas is converted to bioproducts using bacterial cultures. The hybrid route consists of the fast pyrolysis or gasification of lignocellulosic biomass producing synthesis gas (syngas), which is later converted to biofuels such as ethanol, butanol, butyrate, and 2,3-butanediol, among others [8,14,16,[135][136][137]. In this way, all components of the lignocellulosic material are converted to synthesis gas and later fermented by Clostridium bacteria, overcoming the recalcitrant characteristic of this biomass and eliminating high costs related to the enzymatic pretreatment [7]. ...
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Clostridium sp. is a genus of anaerobic bacteria capable of metabolizing several substrates (monoglycerides, diglycerides, glycerol, carbon monoxide, cellulose, and more), into valuable products. Biofuels, such as ethanol and butanol, and several chemicals, such as acetone, 1,3-propanediol, and butyric acid, can be produced by these organisms through fermentation processes. Among the most well-known species, Clostridium carboxidivorans, C. ragsdalei, and C. ljungdahlii can be highlighted for their ability to use gaseous feedstocks (as syngas), obtained from the gasification or pyrolysis of waste material, to produce ethanol and butanol. C. beijerinckii is an important species for the production of isopropanol and butanol, with the advantage of using hydrolysate lignocellulosic material, which is produced in large amounts by first-generation ethanol industries. High yields of 1,3 propanediol by C. butyricum are reported with the use of another by-product from fuel industries, glycerol. In this context, several Clostridium wild species are good candidates to be used as biocatalysts in biochemical or hybrid processes. In this review, literature data showing the technical viability of these processes are presented, evidencing the opportunity to investigate them in a biorefinery context.
... Hydrophilic fibers require the membranes to undergo a process called wetting, with membranes absorbing water into the pores. This process ensures no gas bubbles are present in the membrane before commissioning [82]. While hollow fiber membranes provide instantaneous gas to liquid mass transfer, the flow rates of the system are limited due to the porosity and relatively small surface area of these membranes. ...
Article
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The rise in intermittent renewable electricity production presents a global requirement for energy storage. Biological hydrogen methanation (BHM) facilitates wind and solar energy through the storage of otherwise curtailed or constrained electricity in the form of the gaseous energy vector biomethane. Biological methanation in the circular economy involves the reaction of hydrogen – produced during electrolysis – with carbon dioxide in biogas to produce methane (4H2 + CO2 = CH4 + 2H2), typically increasing the methane output of the biogas system by 70%. In this paper, several BHM systems were researched and a compilation of such systems was synthesized, facilitating comparison of key parameters such as methane evolution rate (MER) and retention time. Increased retention times were suggested to be related to less efficient systems with long travel paths for gases through reactors. A significant lack of information on gas-liquid transfer co-efficient was identified.
... Membrane reactors, though may suffer from membrane fouling, are promising in providing good mass transfer while the membranes can serve as support for cell growth. Hollow fiber membrane bioreactor (HFMBR) can reach a high k L a of~1000 h −1 (Munasinghe and Khanal 2012;Orgill et al. 2013;Shen et al. 2014). C. carboxidivorans produced 6.5 g/L acetate, 23.9 g/L ethanol, and 0.45 g/L butanol in a continuous HFMBR from syngas with a k L a value of 1096.2 h −1 (Shen et al. 2014). ...
Article
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Clostridia are Gram-positive, spore-forming, obligate anaerobic bacteria that can produce solvents such as acetone, ethanol, and butanol, which can be used as biofuels or building block chemicals. Many successful attempts have been made to improve solvent yield and titer from sugars through metabolic engineering of solventogenic and acidogenic clostridia. More recently, cellulolytic and acetogenic clostridia have also attracted high interests for their ability to utilize low-cost renewable substrates such as cellulose and syngas. Process engineering such as in situ butanol recovery and consolidated bioprocessing (CBP) has been developed for improved solvent titer and productivity. This review focuses on metabolic and process engineering strategies for solvent production from sugars, lignocellulosic biomass, and syngas by various clostridia, including conventional solventogenic Clostridium acetobutylicum, engineered acidogens such as C. tyrobutyricum and C. cellulovorans, and carboxydotrophic acetogens such as C. carboxidivorans and C. ljungdahlii.
... [11][12][13][14][15] This approach, however, is likely to have considerable implications for the energy consumption on scale-up. 1 Hollow fibre membranes have been investigated for the supply of gases into fermenters, including research from Ju et al., 5 Orgill et al. 16 and Yasin et al. 17 Hollow fibre membranes have the advantage that mass transfer occurs at the membrane surface, so that gaseous components have already transferred into the liquid phase when exiting the membrane, and thus there are no losses of the gas species. This is by contrast to bubbling, where losses may occur if the gas is not completely absorbed before the bubble reaches the surface. ...
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BACKGROUND With high surface‐to‐volume ratios, hollow fibre membranes offer a potential solution to improve gas‐liquid mass transfer. This work experimentally determined the mass transfer characteristics of commercially available microporous hollow fibre membranes and compared this with the mass transfer from bubble column reactors. Both mass transfer systems are considered for biological methanisation, a process that faces a challenge to enhance the H2 gas‐liquid mass transfer for methanogenic Archaea to combine H2 and CO2 into CH4. RESULTS Polypropylene membranes showed the highest mass transfer rate of membranes tested, with a mass transfer coefficient for H2 measured as kL = 1.2 × 10⁻⁴ m s⁻¹. These results support the two‐film gas‐liquid mass transfer theory, with higher mass transfer rates measured with an increase in liquid flow velocity across the membrane. Despite the higher mass transfer rate from polypropylene membranes and with a liquid flow across the membrane, a volumetric surface area of α = 10.34 m⁻¹ would be required in a full‐scale in‐situ biological methanisation process with much larger values potentially required for high‐rate ex‐situ systems. CONCLUSIONS The large surface area of hollow fibre membranes required for H2 mass transfer and issues of fouling and replacement costs of membranes are challenges for hollow fibre membranes in large‐scale biological methanisation reactors. Provided the initial bubble size is small enough (de < 0.5mm) calculations indicate that microbubbles could offer a simpler means of transferring the required H2 into the liquid phase at a head typical of that found in commercial‐scale anaerobic digesters. This article is protected by copyright. All rights reserved.
Article
In the current study, Polyimide (P84)-based polymeric membranes were fabricated and used as spargers in the bubble column reactor (BCR) to get a high gas-liquid mass transfer (GL-MT) rate of oxygen in water. Different polymeric membranes were fabricated by incorporating polyvinyl pyrrolidone (PVP) as a porogen and a Zeolitic Imidazolate Framework (ZIF-8) to induce high porosity and hydrophobicity in the membranes. The GL-MT efficiency of membranes was evaluated by measuring the overall volumetric mass transfer coefficient (kLa) of oxygen in air. The kLa of O2 (in air) was measured by supplying the gas through a fixed membrane surface area of 11.94 cm² at a fixed gas flow rate of 3L/min under atmospheric pressure. The results revealed that adding porogen and ZIF-8 increased the porosity of the membranes compared to the pure polymeric membranes. In comparison, the ZIF-8 (3 wt%) based membrane showed the highest porosity (80%), hydrophobicity (95° contact angle) and kLa of oxygen in air (241.2 h⁻¹) with 78% saturation in only 60 s. ZIF-8 based membranes showed the potential to increase the amount of dissolved oxygen in BCR by reducing the bubble size, increasing the number of bubbles, and improving the hydrophobicity. The study showed that ZIF-8 based membrane diffusers are expected to produce high GL-MT in microbial syngas fermentation. To the best of our knowledge, this is the first study on the fabrication and application of polymeric membranes for GL-MT applications. Further research should be conducted under real fermentation conditions to assess the practicality of the system to support substrate utilization, microbial growth, and product formation.
Article
Clostridium carboxidivorans can use syngas to produce acids and alcohols. However, simulating gas fermentation dynamics remains challenging. This study employed data transformation and machine learning (ML) approaches to predict syngas fermentation behavior. Syngas composition and fermentative metabolite concentrations (features) were paired with the production rates (prediction targets) of acetate, ethanol, butyrate, and butanol at each time point. This transformation avoided the use of time as a feature. Data augmentation by polynomial smoothing of experimental measurements was used to create a database for supervised learning of 836 rate instances from 10 gas compositions. Seven ML algorithms were compared, including neural networks, support vector machines, random forests, elastic nets, lasso regressors, k-nearest neighbors, and Bayesian ridge regressors. These algorithms predicted production rates for training data with the Pearson correlation coefficients (R² > 0.9), but they all showed poor performance for predicting unseen test data. Among the algorithms, random forests and support vector machines produced the most accurate predictions for the test data, which could regenerate product concentration curves (R² ≈ 0.85). In contrast, neural networks had a higher risk of overfitting. Additionally, ML-based feature importance analysis highlighted the significance of CO and H2 for producing alcohols, offering the possibility of model predictive control. Together, the findings from this study can guide the application of ML algorithms to complex bioprocesses with limited data.
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Since the 1930s, fixed bed reactors (FBR) have been operated for continuous hydrogenation reactions in the petrochemical/fine chemical industries, with publications each year detailing engineering principles, scientific breakthroughs, equipment design and setup, from these industries. Only in the last two decades, FBR have started to be utilized in the pharmaceutical industry as a result of sustainability commitment and growing demand of complex and specialized drugs. Most of the engineering knowledge is transferable across industries; however, there are differentiators inherent to each industry, with the pharmaceutical industry having its own unique challenges. One of the main differentiators between industries is the reactor scale and, consequently, reactor catalyst requirements. Petrochemicals or fine chemicals operate on a large industrial scale, with up to 72 m high reactors, with an internal diameter on the order of 5 m (China Petroleum & Chemical Corporation), and require large volumes of catalysts because of the high product demand for thousands of tonnes per year, while for the pharmaceutical industry, a smaller scale reactor is required for product demand in the thousands of kilograms per year range. It is the reactor size that defines catalyst specifications, such as particle size and geometry. Many transformations have emerged from academic and medicinal chemistry groups utilizing the ThalesNano H-Cube; however, there are relatively few reported processes that have been scaled within the pharmaceutical industry. This Perspective outlines a review of continuous flow hydrogenation technologies; focusing on the trickle bed reactor (TBR) and the respective process operation and effect of process variables on reaction rate requirements for the pharmaceutical industry; it also reflects on transformation examples using TBR across the pharmaceutical industry.
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Microbial biomanufacturing relies heavily on sugar feedstocks, limiting efforts to transition toward carbon neutrality. In a recent issue of Nature Biotechnology, Liew and co-workers show that engineering autotrophic bacteria enables carbon-negative biomanufacturing of chemicals from waste gas feedstocks at an industrial pilot scale, which provides a roadmap for a circular carbon economy.
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Global energy demand has been escalating creating ever increasing pressure on climate crisis caused by fossil-based fuels. Humankind is now desperately in need of alternative and sustainable energy sources. Therefore, biofuels provide promising solution. Amongst the various biofuels, bioethanol from syngas, which is a mixture of, mostly, CO, CO2, N2, H2, and CH4 gases has been drawing increasing attention recently. Regarding this, the conversion of syngas to bioethanol, an alternative biofuel to fossil fuels, is considered a promising approach to reduce the negative effects of global warming by reducing greenhouse gas emissions. In this study, a novel immobilized cell bioreactor, where Clostridium ragsdalei was grown, was designed and used to achieve an efficient production of ethanol regarding volumetric production. For this purpose, a 300 mL immobilized reactor filled with ceramic balls as immobilization material was set and operated at 30oC throughout the study where CO gas as the main substrate was fed at rate of 6 ml/min continuously. Results showed ethanol and acetic acid concentrations reaching up to 1.4 g/L and 0.2 g/L, respectively, after 600h with a volumetric production rate of 0,0023g ethanol/L/h. We believe, the ceramic ball was used for bioethanol production for syngas for the first time.
Article
Increasing environmental concerns regarding fossil fuels and potential future supply constraints have driven the exploration of alternative fuel resources. Using syngas to produce biofuels through microbial fermentation processes provides an excellent option for the synthesis of fuels and chemicals in a clean and sustainable way. The fermentation of syngas by anaerobic acetogens via the Wood-Ljungdahl pathway has attracted considerable interest to for the production of biofuels. The major natural fermentation products of these bacteria are acetate, butyrate, ethanol, butanol, and 2,3-butanediol, which can be used directly or serve as precursors for biofuel and industrial chemical production. However, the widespread use of acetogens as production biocatalysts has been partially limited by their metabolic and energetic constraints for efficient conversion of syngas into target products. A comprehensive understanding of the cellular biology that enables syngas fermentation by these versatile microorganisms is necessary to model the electron and carbon flow in specific production routes, which can contribute substantially to design strategies for acetogen cell engineering and to optimize these technologies to an industrially attractive production level. In this review, we summarize the metabolic and energy conservation mechanisms of most known acetogens during syngas fermentation and discuss parameters that can be modulated to improve their metabolic efficiencies. Finally, the potential to utilize metabolic engineering to improve the spectrum of acetogen products is discussed. This will be helpful in developing acetogens as efficient syngas fermentation biocatalysts for biofuel production in large-scale industrial processes and therefore act as a novel microbial production platform that is both environmentally safe and sustainable.
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In a circular economy approach, heterogeneous wastes can be upgraded to energy in the form of syngas via pyrogasification, and then to methane via biomethanation. Working at high pressure is a promising approach to intensify the process and to reduce gas-liquid transfer limitations. However, raising the pressure could lead to reaching the CO inhibition threshold of the microorganisms involved in syngas-biomethanation. To investigate the impact on pressure on the process, a 10L continuous stirred tank reactor working at 4 bars and 55°C was implemented. Syngas (40% CO, 40% H2, 20% CO2) biomethanation was performed successfully and methane productivity as high as 6.8 mmolCH4/Lreactor/h with almost full conversion of CO (97%) and H2 (98%) was achieved. CO inhibition was investigated and carboxydotrophs appeared less resistant to high CO exposition than methanogens.
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Biogas produced in anaerobic digestion contains energetically useable methane (CH4) and unavoidable unwanted carbon dioxide (CO2). To increase the calorific value of this environmental-friendly renewable fuel recovered from wastewater, an upgrading process is necessary to reduce the high concentration of CO2 and increase the associated CH4 content. The pipe-line quality biomethane concentration can be achieved after biologically converting CO2 by either microorganisms or algae. Over the contemporary reviews published on the biogas upgrading, no paper has ever comprehensively covered the emerging biological methods for converting or reducing CO2. Thus, the biotechnologies for CO2 bioconversion such as H2-assisted chemoautotrophic reactor, gas fermentation, microbial electrochemical cells (MEC) and microalgae-based photosynthetic technique are comprehensively reviewed from the aspects of mechanisms, configurations, bottlenecks and efficiencies in this article. The strategies towards improving the performance of each technique regarding CO2 conversion are systematically analysed. The feasibility of each method from economic and environmental perspectives is also outlined. The outlook for biotechnologies with larger scalability and better economic or technical feasibility are then put forward to facilitate their applications for more efficient biogas upgrading.
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CO is one of the toxic components of syngas, which is the major source of air pollution. Syngas fermentation technology has the ability to convert toxic gases into valuable biofuels, such as ethanol. Fermentative ethanol production is an important method that can be used to promote environmental protection. CO can be converted into ethanol, via the Wood–Ljungdahl pathway, using Clostridium ljungdahlii. The components of the growing medium––especially the trace-element solution and yeast extract––are the main reasons for the high costs associated with this process, however, and this especially impacts scaled-up operations. In this study, cheaper substitutes for these components were used in order to determine their effect on ethanol production. The study comprised three main parts––the optimization of CO concentration, and the substitution of corn syrup and whey powder in the process. The optimum volume of CO for ethanol production was found to be 10 mL. Corn syrup can be used instead of trace-element solution, but the use of yeast extract with the corn syrup was determined to be essential. Up to 1.4 g/L ethanol production was observed with the addition of 15 mL corn syrup. Whey powder had the advantage of being usable without yeast extract, with up to 2.5 g/L ethanol being produced from a 30-g/L concentration. The main finding was that either corn syrup or whey powder can be used as substitutes for expensive basal-medium components.
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Hydrogen technology is essential to the decarbonisation of global economies because it addresses the variability and storage limitation of renewable energy. Several research literatures on hydrogen technology have focused on energy systems with minimum attention given to other fossil fuel driven sectors such as chemical and material production. For effective decarbonisation, the application of hydrogen in global economies must extend beyond the use of energy systems. Renewable hydrogen anaerobic fermentation is a suitable technology for converting the hydrogen substrate into gaseous fuel and precursors for material and green chemical production. The technology leverages on the well-established anaerobic digestion (AD) technology and can be selectively operated for a specific product. Although there are some problems associated with renewable hydrogen anaerobic fermentation, studies show different technological advancements in mitigating these challenges. This review focuses on the technological breakthroughs and limitations associated with renewable hydrogen anaerobic fermentation and provides insights on other products that could be derived from it, especially for a circular economy and the emerging market of green chemicals, sustainable agriculture, and bio-based product development.
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Specific mass transfer area (a) and overall volumetric mass transfer coefficient (kLa) are mass transfer parameters commonly used in the description and comparison of various chemical reactors’ constructions. Determination of these was performed for Spinning Fluids Reactor (SFR). Sodium sulfite oxidation in the presence of cobalt ion catalyst was used for the determination of a and kLa. The a and kLa were successful predicted. Overall, the a and kLa is one order of magnitude larger than any gas liquid contactor to date.
Chapter
In general, hybrid processing encompasses a wide combination of biological, thermal, and/or catalytic processes. This chapter focuses on the sequence of thermochemical deconstruction of biomass followed by biochemical upgrading to final products. Two prominent examples of hybrid thermochemical‐biochemical processing are fast pyrolysis of biomass into pyrolytic substrates followed by microbial fermentation and gasification of biomass into synthesis gas (syngas) followed by syngas fermentation. For the pyrolysis‐based hybrid processing, the thermochemical deconstruction of biomass can be performed in close proximity to biomass production sites to produce crude bio‐oil suitable for transportation to a central upgrading facility. In this manner, low‐density biomass is converted to high‐density feedstock, reducing transportation costs. For the gasification‐based hybrid processing, biomass is converted into syngas, a uniform substrate for fermentation that includes carbon from both the carbohydrate and lignin content of biomass. The chapter explores the unique features, technical challenges, and future perspectives of these two approaches to hybrid processing.
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Syngas fermentation for fuels and chemicals is limited by the low rate of gas-to-liquid mass transfer. In this work, a unique bulk-gas-to-atomized-liquid (BGAL) contactor was developed to enhance mass transfer. In the BGAL system, liquid is atomized into discrete droplets, which significantly increases the interface between the liquid and bulk gas. Using oxygen as a model gas, the BGAL contactor achieved an oxygen transfer rate (OTR) of 569 mg·L ⁻¹ ·min ⁻¹ and a mass transfer coefficient (K L a) of 2.28 sec ⁻¹ , which are values as much as 100-fold greater than achieved in other kinds of reactors. The BGAL contactor was then combined with a packed bed to implement syngas fermentation, with packing material supporting a biofilm upon which gas saturated liquid is dispersed. This combination avoids dispersing these gas-saturated droplets into the bulk liquid, which would significantly dilute the dissolved gas concentration. Although this combination reduced overall K L a to 0.45–1.0 sec ⁻¹ , it is still nearly 20 times higher than achieved in a stirred tank reactor. The BGAL contactor/packed bed bioreactor was also more energy efficient in transferring gas to the liquid phase, requiring 8.63–26.32 J mg ⁻¹ O 2 dissolved, which is as much as four-fold reduction in energy requirement compared to a stirred tank reactor. Fermentation of syngas to ethanol was evaluated in the BGAL contactor/packed bed bioreactor using Clostridium carboxidivorans P7. Ethanol productivity reached 746 mg·L ⁻¹ ·h ⁻¹ with an ethanol/acetic acid molar ratio of 7.6. The ethanol productivity was two-fold high than the highest level previously reported. The exceptional capability of BGAL contactor to enhance mass transfer in these experiments suggests its utility in syngas fermentation as well as other gas-liquid contacting processes.
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Mixing of a gas in a liquid is required in fermentation operations and a variety of oxygenation and hydrogenation processes. Agitation increases the mass transfer between the gas and the liquid phase. Gas-liquid reactors equipped with agitators are often operated at high power input and large gas holdup, making these units among the most difficult to design. Modern high-efficiency and concave-blade disc impellers provide the proper balance of flow, turbulence and shear for most applications. For an overview of the state of the art in gas-liquid agitation, a number of excellent reports are available. This article aims to present practical design guidelines while explaining the basics of gas-liquid mixing. It expands on design procedures presented in 1976 by Hicks and Gates and capitalizes on recent advances in flow-visualization techniques, development of new gas-dispersion impellers and improved understanding of the dispersion process itself. Methods to calculate power draw, gas holdup and mass-transfer rate are illustrated with examples. It is shown that concave-blade turbines provide better performance than the traditional flat-blade units.
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In principle, syngas (primarily consisting of CO and H2) can be produced from any hydrocarbon feedstock, including: natural gas, naphtha, residual oil, petroleum coke, coal, and biomass. The lowest cost routes for syngas production, however, are based on natural gas, the cheapest option being remote or stranded reserves. Economic considerations dictate that the current production of liquid fuels from syngas translates into the use of natural gas as the hydrocarbon source. Nevertheless, the syngas production operation in a gas-to-liquids plant amounts to greater than half of the capital cost of the plant. The choice of technology for syngas production also depends on the scale of the synthesis operation. Syngas production from solid fuels can require an even greater capital investment with the addition of feedstock handling and more complex syngas purification operations. The greatest impact on improving the economics of gasto liquids plants is through 1) decreasing capital costs associated with syngas production and 2) improving the thermal efficiency with better heat integration and utilization. Improved thermal efficiency can be obtained by combining the gas-to-liquids plant with a power generation plant to take advantage of the availability of low-pressure steam.
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The development of freshwater multispecies biofilms at solid-liquid interfaces occurs both in quiescent waters and under conditions of high shear rates. However, the influence of hydrodynamic shear rates on bacterial biofilm diversity is poorly understood. We hypothesized that different shear rates would significantly influence biofilm diversity and alter the relative proportions of coaggregating and autoaggregating community isolates. In order to study this hypothesis, freshwater biofilms were developed at five shear rates (<0.1 to 305 S(-1)) in a rotating concentric cylinder reactor fed with untreated potable water. Eubacterial diversity was assessed by denaturing gradient gel electrophoresis (DGGE) and culturing on R2A agar. Fifty morphologically distinct biofilm strains and 16 planktonic strains were isolated by culturing and identified by partial 16S rRNA gene sequencing, and their relatedness was determined by the construction of a neighbor-joining phylogenetic tree. Phylogenetic and DGGE analyses showed an inverse relationship between shear rate and bacterial diversity. An in vitro aggregation assay was used to assess the relative proportions of coaggregating and autoaggregating species from each biofilm. The highest proportion of autoaggregating bacteria was present at high shear rates (198 to 305 S(-1)). The intermediate shear rate (122 S(-1)) selected for the highest proportion of coaggregating bacteria (47%, or 17 of a possible 36 coaggregation interactions). Under static conditions (<0.1 S(-1)), 41 (33%) of a possible 125 coaggregation interactions were positive. Few coaggregation (3.3%) or autoaggregation (25%) interactions occurred between the 16 planktonic strains. In conclusion, these data show that shear rates affect biofilm diversity as well as the relative proportions of aggregating bacteria.
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In recent years, growing attention has been devoted to the conversion of biomass into fuel ethanol, considered the cleanest liquid fuel alternative to fossil fuels. Significant advances have been made towards the technology of ethanol fermentation. This review provides practical examples and gives a broad overview of the current status of ethanol fermentation including biomass resources, microorganisms, and technology. Also, the promising prospects of ethanol fermentation are especially introduced. The prospects included are fermentation technology converting xylose to ethanol, cellulase enzyme utilized in the hydrolysis of lignocellulosic materials, immobilization of the microorganism in large systems, simultaneous saccharification and fermentation, and sugar conversion into ethanol.
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We introduce a procedure for determining shear forces at the balance between attachment and detachment of bacteria under flow. This procedure can be applied to derive adhesion forces in weak-adherence systems, such as polymer brush coatings, which are currently at the center of attention for their control of bacterial adhesion and biofilm formation.
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This book provides a hybrid methodology for engineering of trickle bed reactors by integrating conventional reaction engineering models with state-of-the-art computational flow models. The content may be used in several ways and at various stages in the engineering process: it may be used as a basic resource for making appropriate reactor engineering decisions in practice; as study material for a course on reactor design, operation, or optimization of trickle bed reactors; or in solving practical reactor engineering problems. Facilitates development of high fidelity models for industrial applications Facilitates selection and application of appropriate models Guides development and application of computational models to trickle beds.
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Syngas fermentation is a promising technology for sustainable production of fuels and chemicals. Gas–liquid mass transfer of syngas, however, is regarded as a limiting step of the fermentation process. The authors designed an innovative external hollow fiber membrane (HFM) diffuser to remove this hurdle. In this study, the gas–liquid mass transfer of carbon monoxide, the major component of syngas, was optimized by implementing three operational factors, membrane surface area per working volume (A/v), water velocity (VL), and specific gas flow rate (Vg). The maximum observed CO mass transfer coefficient (KLa) of 385.0 1/h in water, which is higher than that yielded by previous CO transfer methods, was achieved at an A/v of 0.56 1/cm, a VL of 2.20 cm/s, and a Vg of 1.02 1/min. At these conditions, the gas void fraction rate, the syngas supply rate per working volume, was lower than all reported values as well. The high volumetric mass transfer coefficient at low gas supply rate of the HFM diffuser would make syngas fermentation a feasible alternative industrial process. A three-factor quadratic model and a dimensionless model with high correlation coefficients were developed from the experimental data for a process scale-up. These two models verified that the membrane surface area is the most significant design factor with respect to the KLa. Three screen analyses also indicated that the membrane surface area had the highest positive impact on the KLa. As a result, the external HFM diffuser appears to be a feasible technology that can considerably increase the yield of syngas fermentation to fuels and chemicals.
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At temperatures between 10 and 60°C the diffusion coefficients of helium, hydrogen, oxygen and nitrogen in water have been determined from the permeability of a stagnant liquid layer in the quasi-steady state (SLL method). With this method we actually measure the difference in diffusive flow between two gases through a horizontal stagnant liquid layer between gas-permeable membranes. If oxygen is one of the gases, we are able to determine the diffusion coefficient of oxygen with a maximum experimental error of 3.5% and that of the other gases within 5%. Results are given and compared with experimental values stated in the literature.
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Lignicellulosic biomass has become a likely feedstock for the fuel and chemical industries in the event of an increase in oil and gas prices. The features of lignocellulose that hinder enzyme access to the cellulose and hemicellulose include lignin content, hemicellulose content, acetyl content, cellulose crystallinity, degree of polymerization, and surface area and pore volume. Pretreatment of lignocellulose recalcitrance improves enzyme access by removing or altering lignin, removing hemicellulose, decrystallizing cellulose, and reducing the degree of polymerization in cellulose. The desirable characteristics of lignicellulose pretreatment process include preserving the cellulose and hemicellulose fractions, requiring minimal energy, be effective on multiple lignocellulose feedstocks, and minimizing capital and operating costs.
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Acid gases such as H[sub 2]S and CO[sub 2] are generally removed from natural gas, biogas, synthetic natural gas, and other process gas streams by means of absorption into aqueous alkanolamine solutions. A key parameter needed to model this diffusion with chemical reaction process in the liquid phase is the diffusion coefficient. A wetted-sphere absorption apparatus was used to measure the liquid-phase diffusion coefficients for hydrogen sulfide, carbon dioxide, and nitrous oxide over the temperature range 293--368 K. The experimental results obtained in this work are compared with values in the literature and with predictions from the Wilke-Chang equation. The data presented here extend the temperature range of reported diffusivities for these gases in water.
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In order to understand the membrane wetting in microporous hollow fiber membrane contactors, a theoretical model was developed by simulating CO2 absorption in water under two extreme operating conditions of the non-wetted and wetted modes. The experimental studies on CO2 absorption using an aqueous DEA solution as the absorbent in a polypropylene hollow fiber membrane contactor were also conducted over a three-month period of time.Simulation results show that the CO2 absorption rate in the non-wetted mode is six times higher than those of the wetted mode of operation. The deteriorated performance in the wetted mode is mainly attributed to the mass transfer resistance imposed by the liquid in the membrane pores. The reduction of overall mass transfer coefficient may reach 20% even if the membrane pores were 5% wetted.Experimentally, the CO2 absorption in a 2 M DEA solution was found to be influenced by the gas flow rate significantly and the CO2 flux was enhanced by the increase of CO2 volume fraction in the feed stream. Moreover, in the Celgard microporous hollow fiber MiniModule®, the CO2 flux reduced about 20% in the initial 4 days of operation and then there was no change in the performance. The membrane wetting was identified to be the main reason responsible for the performance drop, as the membrane morphology and the overall mass transfer coefficient presented corresponding changes. The retrieval of the CO2 flux to 90% of the original amount was achieved by increasing the gas phase pressure, which further validated this hypothesis. Clearly, the prevention of the membrane wetting is very critical in maintaining the high performance of CO2 absorption in the membrane contactor.
Article
Diffusion coefficients for dissolved neon, krypton, xenon, carbon monoxide and nitric oxide have been measured in water at 10°, 20°, 30°, 40°, 50° and 60°C by following the rate of collapse of small bubbles in gas-free water. The temperature dependence of the binary diffusion coefficients is expressed in terms of an Arrhenius-type exponential relationship. The sensitivity of aqueous diffusivities to solute molecular weight, solute molecule size and solute—solvent interactions is discussed.RésuméLes coefficients de diffusion pour le néon, le krypton, le xénon, l'oxyde de carbone et l'oxyde nitrique dissous, ont été mesurés dans de l'eau à 10°, 20°, 30°, 40°, 50° et 60°C, en suivant le taux d'écrasement de petites bulles dans de l'eau non gazeuse. La dépendance de température des coefficients de diffusion est exprimée selon les termes d'une relation exponentielle du type Arrhenius. On discute de la sensibilité des diffusivités aqueuses au poids moléculaire d'un solute, à la taille des molécules d'un soluté et aux inter-réactions soluté-colvent.ZusammenfassungDie Diffusionskoeffizienten für gelöstes Neon, Krypton, Xenon, Kohlenmonoxyd und Stickstoffoxyd wurden in Wasser bei 10°, 20°, 30°, 40°, 50° und 60°C aufgrund der Zusammenbruchsgeschwindigkeit von kleinen Bläschen in gasfreiem Wasser gemessen. Die Temperaturabhängigkeit der binären Diffusionskoeffizienten wird im Sinne einer Exponentialbeziehung nach Arrhenius zum Ausdruck gebracht. Die Empfindlichkeit des Diffusionsvermögen in wässriger Lösung gegenüber dem Molekulargewicht und der Molekulargrösse des gelösten Stoffes, sowie gegenüber der gegenseitigen Wirkung von gelöstem Stoff und Lösungsmittel, wird erörtert.
Article
Separation and Purification Technology j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s e p p u r a b s t r a c t The objective of this work was to characterize the main mass transfer resistance for CO 2 capture in the gas–liquid membrane contacting process by both physical and chemical absorption conditions. The char-acterization was performed based on the resistance-in-series model as well as the Wilson-plot method. In addition, a multistage cascade model, which is able to predict the time for the system to reach a steady-state condition, was developed to describe CO 2 absorption in the membrane contacting process. The cascade model was numerically solved by using the MATLAB program. It was found that the main mass transfer resistance of the physical absorption (using pure water as an absorbent) and the chemical absorption (using 2 M NaOH as an absorbent) was in the liquid phase and in the membrane, respectively. The membrane mass transfer resistance in the case of physical absorption presented approximately 36% of the total resistances at a liquid velocity of 2.13 m/s. For the chemical absorption condition applied, the membrane mass transfer resistance occupied around 99% of the total resistance. The results of simulation by the cascade model agreed well with the experimental results when the overall mass transfer coefficient obtained form the experiment was employed. The model can potentially be used with various operating conditions including the liquid velocity, gas concentration, and reactive absorbent used.
Article
Abstract Ethanol is a high performance fuel in internal combustion engines. It is a liquid, which is advantageous in terms of storage, delivery, and infrastructural compatability. Ethanol burns relatively cleanly, especially as the amount of gasoline with which it is blended decreases. Evaporative and toxicity-weighted air toxics emissions are consistently lower for ethanol than for gasoline. It is likely that vehicles can be configured so that exhaust emissions of priority pollutants are very low for ethanol-burning engines, although the same can probably be said for most other fuels under consideration. Recent work suggests that ethanol may be more compatible with fuel cell-powered vehicles than has generally been assumed. Research and development-driven advances have clear potential to lower the price of cellulosic ethanol to a level competitive with bulk fuels. Process areas with particular potential for large cost reductions include biological processing (with consolidated bioprocessing particularly notable in this context), pretreatment, and incorporation of an advanced power cycle for cogeneration of electricity from process residues. The cellulosic ethanol fuel cycle has a high thermodynamic efficiency (useful energy/high heating value = from 50% to over 65% on a first law basis, depending on the configuration), and a decidedly positive net energy balance (ratio of useful energy output to energy input). Cellulosic ethanol is one of the most promising technogical options available to reduce transportation sector greenhouse gas emissions. It may well be possible to develop biomass-based energy on a very large scale in the United States with acceptable and in some cases positive environmental impacts. To do so will however require responsible management and increased understanding of relevant technological and natural systems. The potential biomass resource is large, but so is demand for transportation fuels as well as other uses. The following hypotheses are offered as tentative hypotheses pertaining to biomass supply and demand in the United States: There will probably not be enough suitable land available to meet transportation demand if total vehicle miles traveled increase relative to current levels, and vehicle efficiency and animal protein utilization are unchanged. There probably is enough suitable land to meet transportation demand, even given some increase in vehicle miles traveled, given large but probably possible increases in vehicle efficiency, or large but probably possible decreases in reliance on animal protein, or a combination of less aggressive changes in both of these factors. The policy debate concerning fuel ethanol has tended to ignore cellulosic ethanol. It is suggested that an appropriate policy objective is to foster a transition to cellulosic feedstocks at a pace such that opportunities for ethanol producers and the farmers that supply them are expanded rather than contracted.
Article
Synthesis gas is readily obtained by gasifying coal, oil, biomass, or waste organics and represents an abundant, potentially inexpensive, feedstock for bioprocessing. The primary components of synthesis gas, carbon monoxide and hydrogen, can be converted into methane, organic acids, and alcohols via anaerobic fermentations. Bioconversion of synthesis gas is an attractive alternative to catalytic processing because the biological catalysts are highly specific and often more tolerant of sulfur contaminants than inorganic catalysts. However, because the aqueous solubilities of carbon monoxide and hydrogen are low, synthesis-gas fermentations are typically limited by the rate of gas-to-liquid mass transfer. Consequently, a major engineering challenge in commercial development of synthesis-gas fermentations is to provide sufficient gas mass transfer in an energy-efficient manner. This paper reviews recent progress in the development of synthesis-gas fermentations, with emphasis on efforts to increase the efficiency of gas mass transfer. Metabolic properties of several microbes able to ferment synthesis gas are described. Results of synthesis-gas fermentations conducted in various bioreactor configurations are summarized. Recent results showing enhancement of synthesis-gas fermentations using microbubble dispersions are presented, and studies of the mass-transfer and coalescence properties of microbubbles are described.
Article
Carbon monoxide sparged in batch fermentations ofC. acetobutylicum inhibits the production of H2 by the hydrogenase and enhances the production of solvents by making available larger amounts of NAD(P)H2 to the cells. CO also inhibits biomass growth and acid formation. Its effect is most pronounced under fermentation conditions of excess carbon- and nitrogen-source supply.
Article
Clostridium carboxidivorans P7 is one of three microbial catalysts capable of fermenting synthesis gas (mainly CO, CO(2) , and H(2) ) to produce the liquid biofuels ethanol and butanol. Gasification of feedstocks to produce synthesis gas (syngas), followed by microbial conversion to solvents, greatly expands the diversity of suitable feedstocks that can be used for biofuel production beyond commonly used food and energy crops to include agricultural, industrial, and municipal waste streams. C. carboxidivorans P7 uses a variation of the classic Wood-Ljungdahl pathway, identified through genome sequence-enabled approaches but only limited direct metabolic analyses. As a result, little is known about gene expression and enzyme activities during solvent production. In this study, we measured cell growth, gene expression, enzyme activity, and product formation in autotrophic batch cultures continuously fed a synthetic syngas mixture. These cultures exhibited an initial phase of growth, followed by acidogenesis that resulted in a reduction in pH. After cessation of growth, solventogenesis occurred, pH increased and maximum concentrations of acetate (41 mM), butyrate (1.4 mM), ethanol (61 mM), and butanol (7.1 mM) were achieved. Enzyme activities were highest during the growth phase, but expression of carbon monoxide dehydrogenase (CODH), Fe-only hydrogenases and two tandem bi-functional acetaldehyde/alcohol dehydrogenases were highest during specific stages of solventogenesis. Several amino acid substitutions between the tandem acetaldehyde/alcohol dehydrogenases and the differential expression of their genes suggest that they may have different roles during solvent formation. The data presented here provide a link between the expression of key enzymes, their measured activities and solvent production by C. carboxidivorans P7. This research also identifies potential targets for metabolic engineering efforts designed to produce higher amounts of ethanol or butanol from syngas. Biotechnol. Bioeng. 2012; 109: 2720-2728. © 2012 Wiley Periodicals, Inc.
Article
In this study, the volumetric mass transfer coefficients (Ka) for CO were examined in a composite hollow fiber (CHF) membrane bioreactor. The mass transfer experiments were conducted at various inlet gas pressures (from 5 to 30psig (34.5-206.8kPa(g))) and recirculation flow rates (300, 600, 900, 1200 and 1500mL/min) through CHF module. The highest Ka value of 946.61/h was observed at a recirculation rate of 1500mL/min and at an inlet gas pressure of 30psig(206.8kPa(g)). The findings of this study confirm that the use of CHF membranes is effective and improves the efficiency CO mass transfer into the aqueous phase.
Article
In gas–liquid membrane contacting, it is important to know the gas permeance of microporous hydrophobic membranes used in such a system. Gas permeance of carbon dioxide from a CO2–N2 mixture having a low CO2 concentration into an aqueous KOH solution through flat microporous (Celgard 2400, Saint-Gobain R128-10)/nonporous poly(1-trimethysilyl-1-propyne) (PTMSP) membranes is therefore studied at zero net total pressure difference (ΔP = 0). Pure gas permeance data of CO2 through the same membranes for positive ΔP and gas–gas system are extrapolated to zero mean pressure () to find also gas permeance. Conventional theoretical estimates of the liquid film resistance for such systems are compared with the experimental results for the liquid film resistance; they were found to be considerably higher than that estimated from the theory based on a liquid film having a fast chemical reaction. The membrane resistance obtained by subtraction of the experimental liquid film resistance from the total resistance of the system appears to predict the CO2 permeance for the thicker PTMSP film measured under positive ΔP quite well. However, this method leads to higher estimates of membrane resistance for thinner PTMSP films, Celgard 2400 and other supposedly highly permeable porous substrates compared to those based on the data obtained by extrapolation to . There appears to be an upper limit of permeance which may be determined correctly in such experimental measurements based on ΔP = 0. This upper limit is considerably higher than what has been achieved by earlier investigators. Several factors potentially contributing to this discrepancy have been pointed out.
Article
Potential application of monolith reactors in a biological process was investigated experimentally. A possible problem when using monolith reactors in biological applications is clogging due to biofilm formation. An interesting phenomenon is the pattern in which biofilms develop inside the monolith channels. Rather unexpectedly at a first glance, it was repeatedly observed that biofilm formation started in the middle of a side of the square-section monolith channels, instead of colonizing first the low-shear areas in the corners. To explain this biofilm formation pattern, a two-dimensional mechanistic model based on substrate diffusion and consumption accompanied by microbial growth and detachment was developed in this study. Simulation results suggest that the unexpected biofilm patterns are generated by the balance between biofilm growth and biofilm detachment due to shear stress induced erosion. In the early stages, the biofilm growth in the corners is strongly limited by the external resistance to substrate transfer. As time passes and the biofilm grows in thickness, mechanical forces due to passing gas bubbles will lead to a more regular biofilm shape, including the channel corners.
Article
Bioconversion of syngas/waste gas components to produce ethanol appears to be a promising alternative compared to the existing chemical techniques. Recently, several laboratory-scale studies have demonstrated the use of acetogens that have the ability to convert various syngas components (CO, CO2, and H-2) to multicarbon compounds, such as acetate, butyrate, butanol, lactate, and ethanol, in which ethanol is often produced as a minor end-product. This bioconversion process has several advantages, such as its high specificity, the fact that it does not require a highly specific H-2/CO ratio, and that biocatalysts are less susceptible to metal poisoning. Furthermore, this process occurs under mild temperature and pressure and does not require any costly pre-treatment of the feed gas or costly metal catalysts, making the process superior over the conventional chemical catalytic conversion process. The main challenge faced for commercializing this technology is the poor aqueous solubility of the gaseous substrates (mainly CO and H-2). In this paper, a critical review of CO-rich gas fermentation to produce ethanol has been analyzed systematically and published results have been compared. Special emphasis has been given to understand the microbial aspects of the conversion process, by highlighting the role of different micro-organisms used, pathways, and parameters affecting the bioconversion. An analysis of the process fundamentals of various bioreactors used for the biological conversion of CO-rich gases, mainly syngas to ethanol, has been made and reported in this paper. Various challenges faced by the syngas fermentation process for commercialization and future research requirements are also discussed. (C) 2011 Society of Chemical Industry and John Wiley & Sons, Ltd
Article
Ethanol production from syngas using three moderately alkaliphilic strains of a novel genus and species Alkalibaculum bacchi CP11(T), CP13 and CP15 was investigated in 250 ml bottle fermentations containing 100ml of yeast extract medium at 37 °C and pH 8.0. Two commercial syngas mixtures (Syngas I: 20% CO, 15% CO(2), 5% H(2), 60% N(2)) and (Syngas II: 40% CO, 30% CO(2), 30% H(2)) were used. Syngas I and Syngas II represent gasified biomass and coal, respectively. The maximum ethanol concentration (1.7 g l(-1)) and yield from CO (76%) were obtained with strain CP15 and Syngas II after 360 h. CP15 produced over twofold more ethanol with Syngas I compared to strains CP11(T) and CP13. In addition, CP15 produced 18% and 71% more ethanol using Syngas II compared to strains CP11(T) and CP13, respectively. These results show that CP15 is the most promising for ethanol production because of its higher growth and ethanol production rates and yield compared to CP11(T) and CP13.
Article
Switchgrass (Panicum virgatum) was subjected to hydrothermolysis pretreatment and then used to study the effect of enzyme loading and temperature in a simultaneous saccharification and fermentation (SSF) with the thermotolerant yeast strain Kluyveromyces marxianus IMB3 at 8% solid loading. Various loadings of Accellerase 1500 between 0.1 and 1.1 mL g(-1) glucan were tested in SSF at 45 °C (activity of enzyme was 82.2 FPU mL(-1)). The optimum enzyme loading was 0.7 mL g(-1) glucan based on the six different enzyme loadings tested. SSFs were performed at 37, 41 and 45 °C with an enzyme loading of 0.7 mL g(-1) glucan. The highest ethanol concentration of 22.5 g L(-1) was obtained after 168 h with SSF at 45 °C, which was equivalent to 86% yield. Four different batch and fed-batch strategies were evaluated using a total solid loading of 12% (dry basis). About 32 g L(-1) ethanol was produced with the four strategies, which was equivalent to 82% yield.
Article
Production of ethanol from fermentation of CO has received much attention in the last few years with several companies proposing to use CO fermentation in their ethanol production processes. The genomes of two CO fermenters, Clostridium ljungdahlii and Clostridium carboxidivorans, have recently been sequenced. The genetic information obtained from this sequencing is aiding molecular biologists who are enhancing ethanol and butanol production by genetic manipulation. Several studies have optimized media for CO fermentation, which has resulted in enhanced ethanol production. Also, new reactor designs involving the use of hollow fiber membranes have reduced mass transfer barriers that have hampered previous CO fermentation efforts.
Article
Lignocellulosic biomass such as agri-residues, agri-processing by-products, and energy crops do not compete with food and feed, and is considered to be the ideal renewable feedstocks for biofuel production. Gasification of biomass produces synthesis gas (syngas), a mixture primarily consisting of CO and H(2). The produced syngas can be converted to ethanol by anaerobic microbial catalysts especially acetogenic bacteria such as various clostridia species.One of the major drawbacks associated with syngas fermentation is the mass transfer limitation of these sparingly soluble gases in the aqueous phase. One way of addressing this issue is the improvement in reactor design to achieve a higher volumetric mass transfer coefficient (k(L)a). In this study, different reactor configurations such as a column diffuser, a 20-μm bulb diffuser, gas sparger, gas sparger with mechanical mixing, air-lift reactor combined with a 20-μm bulb diffuser, air-lift reactor combined with a single gas entry point, and a submerged composite hollow fiber membrane (CHFM) module were employed to examine the k(L) a values. The k(L) a values reported in this study ranged from 0.4 to 91.08 h(-1). The highest k(L) a of 91.08 h(-1) was obtained in the air-lift reactor combined with a 20-μm bulb diffuser, whereas the reactor with the CHFM showed the lowest k(L) a of 0.4 h(-1). By considering both the k(L) a value and the statistical significance of each configuration, the air-lift reactor combined with a 20-μm bulb diffuser was found to be the ideal reactor configuration for carbon monoxide mass transfer in an aqueous phase.
Article
The conversion of biomass-derived synthesis gas (or syngas in brief) into biofuels by microbial catalysts (such as Clostridium ljungdahlii, Clostridium autoethanogenum, Acetobacterium woodii, Clostridium carboxidivorans and Peptostreptococcus productus) has gained considerable attention as a promising alternative for biofuel production in the recent past. The utilization of the whole biomass, including lignin, irrespective of biomass quality, the elimination of complex pre-treatment steps and costly enzymes, a higher specificity of biocatalysts, an independence of the H(2):CO ratio for bioconversion, bioreactor operation at ambient conditions, and no issue of noble metal poisoning are among the major advantages of this process. Poor mass transfer properties of the gaseous substrates (mainly CO and H(2)) and low ethanol yield of biocatalysts are the biggest challenges preventing the commercialization of syngas fermentation technology. This paper critically reviews the existing literature in biomass-derived syngas fermentation into biofuels, specifically, different biocatalysts, factors affecting syngas fermentation, and mass transfer. The paper also outlines the major challenges of syngas fermentation, key performance index and technology road map, and discusses the further research needs.
Article
In aerobic bioprocesses, oxygen is a key substrate; due to its low solubility in broths (aqueous solutions), a continuous supply is needed. The oxygen transfer rate (OTR) must be known, and if possible predicted to achieve an optimum design operation and scale-up of bioreactors. Many studies have been conducted to enhance the efficiency of oxygen transfer. The dissolved oxygen concentration in a suspension of aerobic microorganisms depends on the rate of oxygen transfer from the gas phase to the liquid, on the rate at which oxygen is transported into the cells (where it is consumed), and on the oxygen uptake rate (OUR) by the microorganism for growth, maintenance and production.
Article
Many oxidized pollutants, such as nitrate, perchlorate, bromate, and chlorinated solvents, can be microbially reduced to less toxic or less soluble forms. For drinking water treatment, an electron donor must be added. Hydrogen is an ideal electron donor, as it is non-toxic, inexpensive, and sparsely soluble. We tested a hydrogen-based, hollow-fiber membrane biofilm reactor (MBfR) for reduction of perchlorate, bromate, chlorate, chlorite, chromate, selenate, selenite, and dichloromethane. The influent included 5 mg/L nitrate or 8 mg/L oxygen as a primary electron accepting substrate, plus 1 mg/L of the contaminant. The mixed-culture reactor was operated at a pH of 7 and with a 25 minute hydraulic detention time. High recirculation rates provided completely mixed conditions. The objective was to screen for the reduction of each contaminant. The tests were short-term, without allowing time for the reactor to adapt to the contaminants. Nitrate and oxygen were reduced by over 99 percent for all tests. Removals for the contaminants ranged from a minimum of 29% for chlorate to over 95% for bromate. Results show that the tested contaminants can be removed as secondary substrates in an MBfR, and that the MBfR may be suitable for treating these and other oxidized contaminants in drinking water.