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Hydrogen production in a Pore-Plated Pd-membrane reactor: Experimental analysis and model validation for the Water Gas Shift reaction
Abstract
A laboratory reactor equipped with a Pd-composite membrane prepared by ELP “poreplating”
method (Pd thickness of 10.2 mm) has been used for performing the water gas shift
reaction (WGSR). Reaction experiments were carried out with and without the membrane
at different operating conditions: H2O/CO ratio (1e3), temperature (350e400 �C) and GHSV
(4000e5500 h�1). In all cases, CO conversion was found to be higher when using the
membrane to separate hydrogen. The membrane maintained the integrity with complete
selectivity to H2. The membrane reactor has been modelled using a 2D mathematical
model, capable of modelling the non-ideal flow pattern formed in this type of reactors. The
model predicts the experimental CO conversion with an accuracy of ±10%. The proposed
model was used as a tool in the scale-up of a membrane reactor for the wateregas-shift
reaction (feed: 100 m3/h synthesis gas), designed to achieve high CO conversion (>99%) and
hydrogen recovery (>99.5%). The permeation of hydrogen through the membrane was
found to be ruled by mass transfer in the membrane support and palladium layer
... The choice of the high H2O/CO ratio is motivated also by practical reasons. Indeed, when the H2O/CO ratio is low, there is formation and deposition of carbon on the pellets leading to a deactivation of the catalyst, which is not taken into account in the models developed [32]. ...
... In order to validate the rigorous model developed, we carried out other simulations taking into account the operating conditions, the characteristics of the reactors and the size of the catalyst pellets as reported by Marin et al. [23] and Sanz et al. [32]. ...
... In order to validate the rigorous heterogeneous model developed, we proceeded to the comparison of the results obtained with the experimental and numerical results we found in the literature, under similar operating conditions. The first comparison was made with the experimental study conducted by Sanz et al. [32], which is carried out on a laboratory fixed-bed isothermal reactor. We took into consideration all the operating conditions and the characteristics of the catalyst and the reactor, which are detailed in reference [32]. ...
Today, hydrogen has become one of the most promising clean energy. Several processes allow obtaining hydrogen, among them there is the Water Gas Shift (WGS) reaction. On an industrial scale, WGS reaction takes place at high pressure [25–35 bar]. At high pressure, the cost of the process rises due to the energy consumed by compression, and the reduction in the lifetime of the equipment and the catalyst. At low pressures, catalyst lifetime can reach many years and the energy cost is reduced. It is for this reason that we are interested in modelling and simulation of a WGS converter operating at low pressures close to atmospheric pressure. In this work, a numerical study was conducted in order to determine the conditions allowing good rector operating at low pressure. A number of drawbacks of the process were identified. These drawbacks are essentially the non-negligible pressure drops and the strong intraparticle diffusion resistances. The prediction of the concentrations and the reaction rate within the pellet showed that the active zone of the pellet is located near the particle surface. It has also been shown that the resistances to interfacial mass and heat transfer are insignificant. The study of pressure effect showed that the pressure increase reduces the required catalyst mass to achieve equilibrium. Finally, this work revealed that the decrease in temperature and the increase in the concentrations of the reactants by increasing their fluxes, make it possible to increase the effectiveness factor of the catalyst and the conversion of carbon monoxide. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).
... The choice of the high H2O/CO ratio is motivated also by practical reasons. Indeed, when the H2O/CO ratio is low, there is formation and deposition of carbon on the pellets leading to a deactivation of the catalyst, which is not taken into account in the models developed [32]. ...
... In order to validate the rigorous model developed, we carried out other simulations taking into account the operating conditions, the characteristics of the reactors and the size of the catalyst pellets as reported by Marin et al. [23] and Sanz et al. [32]. ...
... In order to validate the rigorous heterogeneous model developed, we proceeded to the comparison of the results obtained with the experimental and numerical results we found in the literature, under similar operating conditions. The first comparison was made with the experimental study conducted by Sanz et al. [32], which is carried out on a laboratory fixed-bed isothermal reactor. We took into consideration all the operating conditions and the characteristics of the catalyst and the reactor, which are detailed in reference [32]. ...
Today, hydrogen has become one of the most promising clean energy. Several processes allow obtaining hydrogen, among them there is the Water Gas Shift (WGS) reaction. WGS reaction synthesizes hydrogen from carbon monoxide and water steam. On an industrial scale, WGS reaction takes place at high pressure [25-35 bars]. At high pressure, the cost of the process rises due to the energy consumed by compression, and the reduction in the lifetime of the equipment and the catalyst. At low pressures, catalyst lifetime can reach many years and the energy cost is reduced. It is for this reason that we are interested in modelling and simulation of a WGS converter operating at low pressures close to atmospheric pressure. In this work, a numerical study was conducted in order to determine the conditions allowing good rector operating at low pressure. A number of drawbacks of the process were identified. These drawbacks are essentially the non-negligible pressure drops and the strong intraparticle diffusion resistances. It has also been shown that the resistances to interfacial mass and heat transfer are not significant. Finally, this work revealed that the decrease in temperature and the increase in the concentrations of the reactants by increasing their fluxes, make it possible to increase the effectiveness factor of the catalyst and the conversion of carbon monoxide
... Compared to the conventional reactor, Sahoo et al. [45] discovered similar trends by using Co/Al 2 O 3 as the catalyst. Sanz et al. [46] compared a CMR with and without membrane using 2-D modeling and found that permeating H 2 resulted in the depletion of syngas in the WGSR. Gallucci et al. [47] also studied the role of the membrane for the WGSR. ...
... It followed that the WGSR shown in Eq. (7) did not undergo complete conversion at higher flow rates. Nevertheless, it was obvious that the CO was well-consumed at the membrane surface due to the permeation of H 2 through the membrane [46]. Fig. 6c shows the CO 2 mass fraction contours and it was in stark contrast with Fig. 6b. ...
Ethanol steam reforming (ESR) can be performed efficiently using catalytic membrane reactors (CMR) to enhance H2 production. To investigate the reaction of ESR and the effect of membrane on H2 production, a numerical model was developed to predict the chemical reaction phenomena. The simulations suggested that lower Reynolds numbers were conducive to ethanol conversion and H2 recovery. The H2 yield could be increased by recovering H2 from the ESR product gas using the Pd membrane, and the membrane had a better performance at low Reynolds numbers. Alternatively, total H2 production increased at higher Reynolds numbers, but H2 recovery decreased due to shorter residence time in the reactor. Increasing the S/E ratio enhanced the ESR performance to produce H2 due to the excessive steam supplied to the reaction, but the H2 recovery declined slightly and more energy would be required. Although a high inlet temperature increased the H2 concentration on the retentate side, it also caused the membrane to experience a higher risk of melting. An increase in pressure facilitated both the ethanol conversion and H2 recovery, scribing to more H2 permeating through the membrane. Overall, the obtained results in this study are beneficial to ESR operation for H2 production. (Reviewer #1, Reply #1).
... The general procedure for the membrane preparation consists of five successive steps: (i) initial cleaning, (ii) support calcination, (iii) CeO 2 intermediate layer incorporation, (iv) activation and (v) palladium deposition by ELP-PP. Details about the first two general steps in which the supports were cleaned and calcined can be found in previous works [26,42]. ...
... At this point, as it has been previously anticipated, different experiments were carried out at several temperatures (350-475°C) and trans-membrane pressure driving forces (0.5-2.0 bar) to analyze in detail the effect of each operating conditions. For all these experiments, the in-out permeation mode was considered to compare the obtained results with the previously reported for other pore-plated membranes in which a ceria intermediate layer was not incorporated [26,[39][40][41][42]45]. ...
This work presents the improvement of hydrogen permeance on electroless pore-plated Pd-composite membranes by the incorporation of ceria as intermediate barrier. This modification, in case of preparing a thick barrier, reduces both average pore size and external roughness of an oxidized Porous Stainless Steel (PSS) tube used as support. However, it also provokes a marked reduction of its permeance, turning more difficult the pass of the hydrazine through the modified support and, therefore, the palladium incorporation by electroless poreplating.
An optimization of this process leads to a Pd/CeO2/PSS composite membrane in which the initial roughness is halved, achieving a stable and selective Pd layer of around 15 μm. This composite membrane
exhibits a hydrogen permeance of 5.37 · 10−4 molm−2 s−1 Pa−0.5 at 400 °C, an ideal H2/N2 perm-selectivity ≥10,000 and an activation energy of 8.9 kJ mol−1. Moreover, the hydrogen flux increases around 400% with regard to previous results, in which no ceria was used as intermediate layer (measured range: 0.03–0.12 molm−2 s−1 versus 0.01–0.03 molm−2 s−1). This increase is derived from a high reduction, of around 30%, in the resistance to the permeation process when using electroless pore-plated membranes due to a lower penetration grade of the Pd external film into the support. In addition, it has been confirmed the successfully stability of the Pd membrane under thermal cycles and different operating conditions, including the
variation of permeate flux direction from the inner to the outer of the membrane, where the Pd-layer is placed, or vice versa
... In this context, the use of membrane reactors, which combine both chemical reaction and separation steps in a single device, appears as a very attractive alternative for efficient process intensification [17,18]. Selective permeation of hydrogen through an adequate membrane shifts the equilibrium, enhancing the chemical reaction and, thus, improving both conversion and global efficiency while a high-purity product is simultaneously obtained in the permeate side [4,17,18]. ...
... In this context, the use of membrane reactors, which combine both chemical reaction and separation steps in a single device, appears as a very attractive alternative for efficient process intensification [17,18]. Selective permeation of hydrogen through an adequate membrane shifts the equilibrium, enhancing the chemical reaction and, thus, improving both conversion and global efficiency while a high-purity product is simultaneously obtained in the permeate side [4,17,18]. Over recent years, multiple experimental and modeling works with membrane reactors can be found in the literature for diverse processes, mainly steam reforming [19], auto-thermal reforming [20], and water gas shift [21]. ...
Hydrogen, as an energy carrier, can take the main role in the transition to a new energy model based on renewable sources. However, its application in the transport sector is limited by its difficult storage and the lack of infrastructure for its distribution. On-board H2 production is proposed as a possible solution to these problems, especially in the case of considering renewable feedstocks such as bio-ethanol or bio-methane. This work addresses a first approach for analyzing the viability of these alternatives by using Pd-membrane reactors in polymer electrolyte membrane fuel cell (PEM-FC) vehicles. It has been demonstrated that the use of Pd-based membrane reactors enhances hydrogen productivity and provides enough pure hydrogen to feed the PEM-FC requirements in one single step. Both alternatives seem to be feasible, although the methane-based on-board hydrogen production offers some additional advantages. For this case, it is possible to generate 1.82 kmol h−1 of pure H2 to feed the PEM-FC while minimizing the CO2 emissions to 71 g CO2/100 km. This value would be under the future emissions limits proposed by the European Union (EU) for year 2020. In this case, the operating conditions of the on-board reformer are T = 650 °C, Pret = 10 bar and H2O/CH4 = 2.25, requiring 1 kg of catalyst load and a membrane area of 1.76 m2.
... Thus, development of new ultrathin membranes without jeopardizing mechanical resistance and presence of defects is the main objective of many researchers in this field [59,60,77]. This goal is usually achieved by incorporating a thin Pd layer on the surface of a porous material that provides the required mechanical resistance to the supported membrane [71,[78][79][80]. This complex task is subject of numerous studies since many factors must be considered, i.e., the compatibility between support and selective layer, which strongly determines the mechanical resistance of the membrane due to cracks can be formed at high temperatures because of different expansion coefficients, as it will be discussed in detail later. ...
... Based on these repairing procedures, other researchers have recently reported the separated supply of Pd source and reducing agent bath to prepare Pd-based membranes directly on rough commercial PSS supports [49,51,71,[78][79][80]. This novel procedure, denoted as Electroless Pore-Plating (ELP-PP), uses the wall of the support itself to maintain separated both Pd source and hydrazine solutions. ...
In the last years, hydrogen has been considered as a promising energy vector for the oncoming modification of the current energy sector, mainly based on fossil fuels. Hydrogen can be produced from water with no significant pollutant emissions but in the nearest future its production from different hydrocarbon raw materials by thermochemical processes seems to be more feasible. In any case, a mixture of gaseous compounds containing hydrogen is produced, so a further purification step is needed to purify the hydrogen up to required levels accordingly to the final application, i.e., PEM fuel cells. In this mean, membrane technology is one of the available separation options, providing an efficient solution at reasonable cost. Particularly, dense palladium-based membranes have been proposed as an ideal chance in hydrogen purification due to the nearly complete hydrogen selectivity (ideally 100%), high thermal stability and mechanical resistance. Moreover, these membranes can be used in a membrane reactor, offering the possibility to combine both the chemical reaction for hydrogen production and the purification step in a unique device. There are many papers in the literature regarding the preparation of Pd-based membranes, trying to improve the properties of these materials in terms of permeability, thermal and mechanical resistance, poisoning and cost-efficiency. In this review, the most relevant advances in the preparation of supported Pd-based membranes for hydrogen production in recent years are presented. The work is mainly focused in the incorporation of the hydrogen selective layer (palladium or palladium-based alloy) by the electroless plating, since it is one of the most promising alternatives for a real industrial application of these membranes. The information is organized in different sections including: (i) a general introduction; (ii) raw commercial and modified membrane supports; (iii) metal deposition insights by electroless-plating; (iv) trends in preparation of Pd-based alloys, and, finally; (v) some essential concluding remarks in addition to futures perspectives.
... The operating conditions are based on the study of [3]. As shown in [20] which is the extended study from [5], the numerical model was verified with the experimental data in [20]. Using the data listed in Table 1, the numerical model is verified again to ensure its validation. ...
... The operating conditions are based on the study of [3]. As shown in [20] which is the extended study from [5], the numerical model was verified with the experimental data in [20]. Using the data listed in Table 1, the numerical model is verified again to ensure its validation. ...
Water-Gas Shift Reaction (WGSR) has become
one of the well-known pathways for H2 production in industries.
The WGSR is kinetically favored at high temperature but
thermodynamically favored at low temperature, which require
careful consideration in the control design. This paper studies the
effect of a reactor arrangement with an inter-stage cooling
implemented in the packed bed reactor. A mathematical model is
developed based on one-dimensional heat and mass transfers
which incorporate the intra-particle effects. It is shown that the
placement of the inter-stage cooling an the outlet temperature
exiting the inter-stage cooling have strong influence on the
reaction conversion. Several control strategies are explored for
the process. It is shown that a feedback-feedforward control
strategy using Multi-scale Control (MSC) is effective to regulate
the reactor temperature profile which is critical to maintaining
the catalysts activity.
... Experimental and modeling investigates have proved the feasibility of Pd membrane based WGSMR. Laboratory scale and bench scale studies are conducted to investigate the effects of different catalyst, rules of permeation of hydrogen through the membrane, performance of membrane reactor at different temperatures, and so on [21,22,42]. Besides, dynamic modeling of Pd-alloy based WGSMR are performed under precombustion conditions [43,44]. ...
A mixed ionic and electronic conducting (MIEC) membrane provides an alternative to the palladium alloy membrane for water gas shift membrane reactor. It exhibits much better sulfur resistance performance than the palladium alloy membrane. In this paper, the thermodynamic performance of the integrated gasification combined cycle (IGCC) system with MIEC membrane reactor is predicted for the first time. The effects of reactor operation parameters on system flowsheet and performance are investigated and illustrated by sensitive analysis. When the reactor operation temperature is 900 °C and the H2O decomposition ratio is 0.5, the system net efficiency is about 38.90%, which is 2.6% points higher than that of the IGCC with Selexol. The system net efficiency increases with the decrease of operation temperature. With the net efficiency of the conventional system as the reference, the minimum H2O decomposition ratios at different operating temperatures are provided.
... Considering these potential benefits as well as the large amount of previous experience on designing dense metal membranes modules for hydrogen purification [36][37][38], the research group headed by Tosti also investigated the exploitation of OMW for hydrogen production in palladium membrane reactors [24,25,39]. Palladium-based dense membranes are a widespread alternative for hydrogen separation in independent equipment or coupled with a catalytic chemical reaction in a membrane reactor at moderate-high temperatures with an extremely elevated perm-selectivity, up to infinite for completely free-defect systems [33,[40][41][42]. The hydrogen permeation through these membranes can be divided in different steps, including transport in the gas phase, adsorption on the membrane surface, hydrogen dissociation, diffusion through the bulk metal membrane, hydrogen recombination, and desorption on the contrary membrane side. ...
Olive mill wastewater (OMW) presents high environmental impact due to the fact of its elevated organic load and toxicity, especially in Mediterranean countries. Its valorization for simultaneous pollutants degradation and green energy production is receiving great attention, mainly via steam reforming for hydrogen generation. Following previous works, the present research goes into detail about OMW valorization, particularly investigating for the first time the potential benefits of OMW–bioethanol mixtures co-reforming for ultra-pure hydrogen production in Pd-membrane reactors. In this manner, the typical large dilution of OMW and, hence, excess water can be used as a reactant for obtaining additional hydrogen from ethanol. Fresh OMW was previously conditioned by filtration and distillation processes, analyzing later the effect of pressure (1–5 bar), oxidizing conditions (N2 or air as carrier gas), gas hourly space velocity (150–1500 h−1), and alcohol concentration on the co-reforming process (5–10% v/v). In all cases, the exploitation of OMW as a source of environmentally friendly hydrogen was demonstrated, obtaining up to 30 NmL·min−1 of pure H2 at the most favorable experimental conditions. In the membrane reactor, higher pressures up to 5 bar promoted both total H2 production and pure H2 recovery due to the increase in the permeate flux despite the negative effect on reforming thermodynamics. The increase of ethanol concentration also provoked a positive effect, although not in a proportional relation. Thus, a greater effect was obtained for the increase from 5% to 7.5% v/v in comparison to the additional improvement up to 10% v/v. On the contrary, the use of oxidative conditions slightly decreased the hydrogen production rate, while the effect of gas hourly space velocity needs to be carefully analyzed due to the contrary effect on potential total H2 generation and pure H2 recovery.
... Sanz et al. [130] experimentally tested the WGSR before/after membrane installation under varied operating conditions. They discovered that the CO conversion and H 2 recovery could achieve 99.0% and 99.5%, respectively under the optimal conditions. ...
The water gas shift reaction is an important and commonly employed reaction in the industry. In the water gas shift reaction, hydrogen is produced from water or steam while carbon monoxide is converted into carbon dioxide. Over the years, on account of the progress in hydrogen energy and carbon capture and storage for developing alternative fuels and mitigating the atmospheric greenhouse effect, the water gas shift reaction has become a crucial route to simultaneously reach the requirements of hydrogen production and carbon dioxide enrichment, thereby enhancing CO2 capture. This article provides a comprehensive review of the research
progress in the water gas shift reaction, with particular attention paid to the thermodynamic and kinetic characteristics. The performance of the water gas shift reaction highly depends on the adopted catalysts whose progress in recent years is extensively reviewed. The behaviors of the water gas shift reaction in special environments are also illustrated, several cases have the ability to proceed with water gas shift reaction without any catalyst. The utilization of several separation technologies on the water gas shift reaction such as carbon capture and storage and membrane reactors for purifying hydrogen and enriching carbon dioxide will be addressed
as well. Reviewing past studies suggests that separating hydrogen and carbon dioxide in the product gas from the water gas shift reaction can not only increase efficiency but also enhance the usability for further application. The CO conversion is beyond the thermodynamic limitation after applying membrane for the water
gas shift reaction.
... The use of intermediate oxide support layers has been explored by some authors. In 2015, Alique et al. [178] demonstrated the use of a laboratory reactor equipped with a Pd membrane (10 mm) deposited by ELP on top of an intermediate support layer of FeeCr oxides, on a cylindrical porous stainless steel (PSS) support, for performing the WGS reaction. CO conversion was found to be higher when using the Pd/FeeCr oxides/PSS membrane to separate H 2 . ...
Planet Earth is facing accelerated global warming due to greenhouse gas emissions from human activities. The United Nations agreement at the Paris Climate Conference in 2015 highlighted the importance of reducing CO2 emissions from fossil fuel combustion. Hydrogen is a clean and efficient energy carrier and a hydrogen-based economy is now widely regarded as a potential solution for the future of energy security and sustainability. Although hydrogen can be produced from water electrolysis, economic reasons dictate that most of the H2 produced worldwide, currently comes from the steam reforming of natural gas and this situation is set to continue in the foreseeable future. This production process delivers a H2-rich mixture of gases from which H2 needs to be purified up to the ultra-high purity levels required by fuel cells (99.97%). This driving force pushes for the development of newer H2 purification technologies that can be highly selective and more energy efficient than the traditional energy intensive processes of pressure swing adsorption and cryogenic distillation. Membrane technology appears as an obvious energy efficient alternative for producing the ultra-pure H2 required for fuel cells. However, membrane technology for H2 purification has still not reached the maturity level required for its ubiquitous industrial application. This review article covers the major aspects of the current research in membrane separation technology for H2 purification, focusing on four major types of emerging membrane technologies (carbon molecular sieve membranes; ionic-liquid based membranes; palladium-based membranes and electrochemical hydrogen pumping membranes) and establishes a comparison between them in terms of advantages and limitations.
... The stability of these membranes has only been assessed in empty reactors and, when integrated with reaction, in packed bed membrane reactors [27,28]. In order to limit external mass transfer limitations (concentration polarization) and attain a better heat management over the whole reactor, the use of fluidized bed membrane reactors is preferred [8,29e31]. ...
Pd-based membranes prepared by pore-plating technique have been investigated for the first time under fluidization conditions. A palladium thickness around 20 μm was achieved onto an oxidized porous stainless steel support. The stability of the membranes has been assessed for more than 1300 h in gas separation mode (no catalyst) and other additional 200 h to continuous fluidization conditions. Permeances in the order of 5·10⁻⁷ mol s⁻¹ m⁻² Pa⁻¹ have been obtained for temperatures in a range between 375 and 500 °C. During fluidization, a small decrease in permeance is observed, as consequence of the increased external (bed-to-wall) mass transfer resistances. Moreover, water gas shift (WGS) reaction cases have been carried out in a fluidized bed membrane reactor. It has been confirmed that the selective H2 separation through the membranes resulted in CO conversions beyond the thermodynamic equilibrium (of conventional systems), showing the benefits of membrane reactors in chemical conversions.
... The model for the reactor tube is phenomenological, e.g. based on conservation equations, as shown in Table 5 [27,30]. Due to the exothermal reactions taking place and the heat transferred through the reactor wall, temperature and concentration gradients in the axial and radial coordinates of the reactor tube are expected. ...
The scope of this work is to explore the viability of the direct synthesis of dimethyl ether (DME) over bifunctional catalysts, such as mixtures of CuO/ZnO/Al2O3 and γ-Al2O3 at industrial scale. To accomplish this purpose, the process is simulated using a phenomenological mathematical model considering momentum, mass and energy balances, applied to both the catalyst particles and reactor bed, which is solved in 2D axisymmetric coordinates. This constitutes a step beyond most of the available studies for the modelling of the DME synthesis reaction, based on simple 1D isothermal models.
The use of this detailed model revealed the importance of intraparticle mass and heat transfer, with effectiveness factors within the range 0.5–1.1. At the reactor scale, radial phenomena were found to be relevant. A design-sensitivity analysis of mass flux, catalyst fraction, pressure, feed temperature, cooling potential and tube diameter on the reactor performance was carried out. An optimized reactor design that provides 80% CO conversion operating at inlet temperature and pressure 245 °C and 40 bar, corresponds to 0.02 m diameter, 8.50 m length and 3600 h⁻¹ gas-hourly space velocity with a yield of dimethyl ether of 0.53.
... Adrover et al. [23] analyzed the influence of operating pressure on the membrane reactor performance, and found that an increase in pressure led to a significant improvement in CO conversion and the conversion in the membrane reactor was higher than that in conventional fixed-bed reactor. Sanz et al. [24] experimentally studied the WGSR with and without membrane at different operating conditions, and found that the CO conversion and H 2 recovery at GHSV = 1550 h À1 could be greater than 99% and 99.5%, respectively, when using the membrane. Li et al. [25] developed a membrane reactor using a SrCe 0.9 Eu 0.1 O 3À d tubular membrane where 92% CO conversion and 32% H 2 separation were obtained. ...
The membrane reactor is a promising device to produce pure hydrogen and enrich CO2 from syngas. To
figure out the detailed reaction phenomena of high-temperature water gas shift reaction (WGSR) in a Pd-based membrane reactor, a computational fluid dynamics (CFD) model is developed to simulate the
chemical reaction where the feed gas temperature and steam-to-CO molar ratio (S/C ratio) are in the
ranges of 400–700 �C and 1–3, respectively. The predictions suggest that the WGSR proceeds from kinetically controlled reaction to thermodynamically governed one when the feed gas temperature increases.
The CO conversion at high temperatures can be improved up to 83% when the membrane is in the reactor
compared to that without the membrane. This is mainly attributed to the intensification of the membrane’s
permeance with increasing temperature, even though high temperatures disadvantage CO conversion.
The analysis also reveals that the breakthrough in the thermodynamic limit of CO conversion
can be achieved in the membrane reactor when the feed gas temperature is higher than 500 �C. The
CO conversion in the membrane reactor can be higher than the thermodynamic equilibrium up to 61%.
... The presence of alumina into the matrix gives resistance to the fatigue and higher hardness values than YSZ alone [14]. Although the pore-filled membrane may promise to be an interesting concept [12,15,16], a more systematic study on the characteristic of the support modification and porous structure is required to be able to produce stable pore-filled membranes with reproducible results. ...
Nanoporous ceramic supports for pore filled membranes were prepared on ceramic supports (a-Al2O3) with pore size of 100 nm by adding additional layers with different proportions of YSZ/g-Al2O3 (ranging from 50% to 90% of YSZ) by dip-coating and the effect of different parameters in the preparation method have been investigated. The diffusion mechanisms of N2, He and CO2 through the supported nanoporous layers have been studied in detail with permeation measurements at a temperature range of 50-400 C and pressure difference of 30-100 kPa. It was observed that as the amount of g-Al2O3 in the nanoporous layers increases, the adsorption of CO2 is favored at low temperatures and pressures. Finally, a pore-filled Pd/YSZ/g-Al2O3 (60 wt%YSZ-40 wt. g-Al2O3) membrane was successfully prepared and its permeation performance was tested over 900 h at 500 and 550C, showing relatively low ideal H2/N2 perm-selectivity of about 50 due to low hydrogen flux
... The results shown here agree with that from Brenner [20]. The size of the particles has a direct effect on the generalized Thiele modulus and hence, in turn it affects the internal effectiveness factor which contributes to more than 96% of the total resistance, i.e., see Sanz et al [21]. However, when the combination of these two parameters is studied simultaneously, it shows that when the pellet size is small, the shapes of the particles do not have serious effect on the CO conversion. ...
High-temperature Water Gas-Shift Reaction (WGSR) is a well-known chemical route to produce hydrogen (H2) from waste carbon monoxide (CO). The WGSR in an adiabatic packed bed tubular reactor (PBTR) is often vulnerable to catalyst degeneration due to high temperature under which it is operated, and therefore the reactor requires good control system to regulate its temperature. A lot of experimental and simulation studies have only focused on investigating the individual influences of important operating and design variables, such as, the catalyst pellet sizes and shapes, feed inlet temperature and concentration on the CO conversion. On the contrary, very little study has been reported on the combined interactive effects of these variables on the CO conversion and reactor behaviours. In this paper, a fundamental dynamic model based on mass-energy balances is developed. This model also incorporates the effects of intra-particle mass-heat transfer resistances via the generalized Thiele modulus relation. The generalized cascade multi-scale control (CMSC) scheme is applied to control the temperature of the PBTR system. The control performance of CMSC is compared with that of the single-loop feedback and conventional cascade control schemes. An extensive simulation study shows that: (1) the interactive effects of operating-design variables play a significant role in determining the transient conversion and reactor behaviours, (2) the location of the secondary temperature sensor has a large influence on the performance of a cascade control scheme, and (3) the generalized CMSC scheme outperforms the conventional cascade control scheme.
This work proposes a novel tubular structure of high-temperature proton exchange membrane fuel cell (PEMFC) integrated with a built-in packed-bed methanol steam reformer to provide hydrogen for power output. A two-dimensional axisymmetric non-isothermal model was developed in COMSOL Multiphysics 5.4 to simulate the performance of a tubular high temperature proton membrane fuel cell and a packed bed methanol reformer. The model considers the coupling multi-physical processes, including methanol reforming reaction, water gas shift reaction, methanol cracking reaction as well as the heat, mass and momentum transport processes. The sub-model of the tubular packed-bed methanol reformer is validated between 433 K and 493 K with the experimental data reported in the literature. The sub-model of the high temperature proton exchange fuel cell is validated between 393 K and 433 K with the published literature. Our results show that power output and temperature distribution of the integrated unit depend on methanol flow rates and working voltages. It was suggested that stable power generation performance of 0.14 W/cm2 and temperature drop in methanol steam reformer of ≤10 K could be achieved by controlling the methanol space-time ratio of ≥250 kg·s/mol with working voltage at 0.6 V, even in the absence of an external heat source.
Three novel medical waste-to-hydrogen/methanol pathways based on plasma gasification using Aspen Plus have been proposed: The first pathway is used to convert medical waste into hydrogen; the second pathway is applied to convert medical waste into methanol; the third pathway is used to convert medical waste into hydrogen & methanol. In this study, thermodynamic and techno-economic analyses were conducted to evaluate and compare the performance of three pathways. Furthermore, three schemes are assessed by comparative economic sensitivity concerning the chemical product price, electricity price, capital expenditure, and discount rate. The thermodynamic results show that process efficiency and process exergy efficiency of the third pathway can achieve 72.16% and 66.88%, which is higher than the first pathway (69.45% and 65.65%) and the second pathway (54.08% and 50.93%). The techno-economic results indicate that the first pathway has a shorter dynamic payback period of 3.83 years and a relatively higher net present value of 13,783.57 k$ than the second pathway (7.42 years and 5,700.32 k$) and the third pathway (5.27 years and 10,780.00 k$). This work provides novel methods and guidance for efficient and economical harmless treatment and resource utilization of medical waste.
Potent carbon-neutral energy carriers bring a vital solution for sustained industrialization and environmental protection. Hydrogen as a novel zero-emission energy carrier offers more than twice energy per unit mass compared to other fuels. Membrane reactor technology transforms gray hydrogen to blue by selective hydrogen separation and carbon dioxide capture from the product mixture. Moreover, improved reactant conversion during reversible steam reforming of methane, methanol, and ethanol; water gas-shift; and dehydrogenation of cyclic and aliphatic hydrocarbons as well as enhanced hydrogen yield are results of selective and distributed hydrogen separation from membrane reactor. In this review applicability of membrane reactor technology for above-mentioned reactions are discussed, effects of different membranes, reactor designs, and operating conditions on performance of membrane reactors are investigated. Finally impact of membrane reactor technology on reactant conversion, product yield, and overall process are explained, and future directions of this technology are outlined.
Thermal and mechanical resistances of palladium composite membranes prepared by Electroless Pore-Plating (ELP-PP) and containing SBA-15 as intermediate layer were improved by doping the silica material with Pd nuclei before its incorporation on the composite membrane. Textural properties of synthesized SBA-15 materials (both raw and doped ones) were analyzed by XRD, N2 adsorption-desorption at 77 K and TEM, while the main properties of the composite membrane were determined by SEM and gravimetric analyses. Moreover, membrane permeation tests were also carried out with pure gases, hydrogen and nitrogen, and binary mixtures of them at temperature of 400 °C and pressure driving forces in the range of 0.5–2.5 bar. The use of bare SBA-15 intermediate layer leads to the appearance of cracks on the Pd layer during permeation experiments at high temperature. In contrast, the use of Pd-doped SBA-15 particles avoids this problem, thus improving both thermal and mechanical resistances of the composite ELP-PP Pd-membrane. Following this preparation method, an estimated Pd thickness of 7.1 μm was obtained, reaching a hydrogen permeance of 3.81·10⁻⁴ mol s⁻¹ m⁻² Pa−0.5 and ensuring an ideal H2/N2 separation factor higher than 2550 at 400 °C.
A detailed computational fluid dynamics (CFD) model has been developed in this study for water gas shift of untreated biogas reformate in both fixed bed and membrane reactors to gain insights into the interaction of 3Ni5Cu/Ce0.5Zr0.33Ca0.17 catalyst with fluid transport phenomena taking place in the reactor. The developed CFD model closely agreed with data from both literature and experiments. An average absolute deviation of 8.59% and 6.32% was obtained when the CFD model was validated with fixed bed reactor and membrane reactor experiments respectively. The membrane reactor enabled equilibrium conversion at 2.95 gcat.h/mol CO weight-time with equilibrium conversion attained at lower weight-time and steam to carbon ratio of 3.4. Also, it was found that the counter-flow configuration resulted in better enhanced CO conversion, and wall heat transfer coefficient above 8.6 W/m²/K had negligible on conversion, amounting to only a 1% increase in conversion for both reactors.
In this work, a comparative study is carried out to analyze the behavior of a water gas shift membrane reactor operating with and without sweep gas. The present study includes thermal effects that play a key role in reaction and permeation rates involved in the membrane reactor. Based on a 1-D mathematical model, the effect of the most important operating variables on the membrane reactor performance is comparatively studied. Additionally, novel algebraic expressions are developed when operating without sweep gas, that only depend on inlet variables to calculate the limiting membrane reactor conversions and recovery values. An algebraic equation is developed to estimate limiting recovery when operating with sweep gas. The dependence of limiting membrane reactor conversion and recovery values operating with and without sweep gas on operating parameters is included. From the comparative analysis, some guidelines for an improved operation of the membrane reactor are proposed.
Numerical modeling studies have been performed for a methane dry reforming using a shell and tube type packed-bed reactor and a membrane reactor both with a heating tube as heat source in the center of a reactor. Mathematical models for a reformer bed, a heating tube, and an insulating jacket coupled with reaction kinetics were proposed to investigate axial and radial temperature and concentration profiles within both reactors with a reformer temperature of 923 K and heating tube temperatures from 1023 to 1223 K. 3-D visualizations of temperature, CH4 conversion, CH4 concentration, and H2 concentration profiles were possible by COMSOL Multiphysics modeling software and significant variations within both reactors were observed providing a critical guideline for an efficient reactor design. Further studies for the effect of hydrogen mass flux on a hydrogen yield enhancement revealed that a threshold hydrogen mass flux in a membrane reactor to outperform a packed-bed reactor exists with a trend of a lower threshold hydrogen mass flux for a higher heating tube temperature.
Composite palladium membranes are attracting great attention for hydrogen separation and membrane reactor applications, due to the complete hydrogen selectivity with reasonably high permeability and suitable mechanical stability. These membranes are usually prepared by depositing a thin Pd layer over ceramic or metallic supports, which give to the system the necessary mechanical resistance. The Pd incorporation is generally made by electroless plating (ELP), although there is still no fully optimized and universally accepted method, and many researches are currently devoted to the generation of thin and homogeneous metallic coatings with good adhesion and resistance to real conditions. Among the many studies, very few compares directly the properties of the two different supports (metallic or ceramic) on the overall membrane structure and performance. In the present work, the permeation behavior of several Pd-composite membranes, prepared by conventional ELP and by a novel pore-plating method (ELP-PP) has been studied on both ceramic and metallic supports. The membrane prepared over a tubular ceramic support by conventional ELP shows a permeability in the range of 2.5–3.6·10⁻⁶ mol s⁻¹ bar−0.5 m, with nearly complete ideal selectivity and Pd thickness around 14 μm. With the alternative preparation method, ELP-PP, despite the lower Pd thickness, 8 μm, and also complete selectivity, lower hydrogen fluxes were observed with a permeability ranging from 5.8 to 8.5·10⁻⁷ mol s⁻¹ bar−0.5 m. This behavior can be explained by considering that the pore-plating method leads to a Pd deposition over the external surface of the support but also inside the pores, generating an effective Pd thickness higher than that obtained with the conventional ELP. In this manner, the real behavior of the membrane is equivalent to a conventional 33 μm thick palladium layer. Finally, these results are compared replacing the ceramic support by a metallic one (PSS), which led to an increase in the minimum thickness necessary to achieve a totally dense membrane (Pd thickness, 9 μm), and, consequently, to a reduction of the observed transmembrane flux. However, in this case a lower deposition of palladium inside the pores of the support is observed thus causing a lower resistance to the hydrogen permeation with respect to ceramic supported membranes.
Ethanol steam reforming (ESR) was performed over Pd-Rh/CeO2 catalyst in a catalytic membrane reactor (CMR) as a reformer unit for production of fuel cell grade pure hydrogen. Experiments were performed at 923 K, 6–10 bar, and fuel flow rates of 50–200 μl/min using a mixture of ethanol and distilled water with steam to carbon ratio of 3. A static model for the catalytic zone was derived from the Arrhenius law to calculate the total molar production rates of ESR products, i.e. CO, CO2, CH4, H2, and H2O in the catalytic zone of the CMR (coefficient of determination R2 = 0.993). The pure hydrogen production rate at steady state conditions was modeled by means of a static model based on the Sieverts' law. Finally, a dynamic model was developed under ideal gas law assumptions to simulate the dynamics of pure hydrogen production rate in the case of the fuel flow rate or the operating pressure set point adjustment (transient state) at isothermal conditions. The simulation of fuel flow rate change dynamics was more essential compared to the pressure change one, as the system responded much faster to such an adjustment. The results of the dynamic simulation fitted very well to the experimental values at P = 7–10 bar, which proved the robustness of the simulation based on the Sieverts' law. The simulation presented in this work is similar to the hydrogen flow rate adjustments needed to set the electrical load of a fuel cell, when fed online by the pure hydrogen generating reformer studied.
A catalytic Pd76Ag19Cu5 alloy membrane reactor packed with 5% Ni/Ce0.6Zr0.4O2 catalyst was adopted in this study to investigate hydrogen production performance from the dry reforming reaction of methane and carbon dioxide. The 1:1 CH4/CO2 feed was introduced to the reactor with 60 mg of the catalyst at a flow rate of 20 ml/min at 550 °C. The effluent gas compositions were examined using an online gas chromatographer (GC). Compared to a conventional reactor without the membrane, the CH4 and CO2 conversions were significantly increased by 3.5-fold and 1.5-fold, respectively. Correspondingly, the overall H2 yield was greatly improved from about 10-35%. Additionally, the hydrogen selectivity increased from 47 to 53%. It is theorized that the in-situ partial hydrogen withdrawal by the membrane mainly caused the dry reforming reaction equilibrium to shift forward and created a hydrogen-deprived environment unfavorable for the competing reversible water-gas shift reaction to take place.
This paper reports the preparation and characterization of thin-film (4-5μm thick) Pd-Ag metallic supported membranes for high temperature applications. Various thin film membranes have been prepared by depositing a ceramic interdiffusion barrier layer prior to the simultaneous Pd-Ag electroless plating deposition. Two deposition techniques for ceramic layers (made of zirconia and alumina) have been evaluated: Atmospheric Plasma Spraying and dip coating of a powder suspension. Initially, the prepared ceramic layers have been characterized for nitrogen permeation at room temperature and surface roughness for the selection of the appropriate type of ceramic layer. The most promising membranes have been tested at 400-600°C for single gas permeation (H2 and N2), and have shown extremely high H2/N2 permselectivities (>200,000).
This paper attempts to explore the determinants of CO2 emissions using the STIRPAT model and data from 1980 to 2011 for OECD countries. The empirical results show that non-renewable energy consumption increases CO2 emissions, whereas renewable energy consumption decreases CO2 emissions. Further, the results support the existence of an environmental Kuznets curve between urbanisation and CO2 emissions, implying that at higher levels of urbanisation, the environmental impact decreases. Therefore, the overall evidence suggests that policy makers should focus on urban planning as well as clean energy development to make substantial contributions to both reducing non-renewable energy use and mitigating climate change.
Composite palladium membranes based on porous stainless steel (PSS) substrate are idea hydrogen separators and purifiers for hydrogen energy systems, and the surface modification of the PSS is of key importance. In this work, the macroporous PSS tubes were aluminized through pack cementation at 850 °C in argon, followed by an oxidation with air at 600 °C. Palladium membranes were prepared by electroless plating. Their permeation performances were tested, and the hydrogen permeation kinetics was discussed. The substrate materials and the palladium membranes were characterized by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD). An Al2O3-enriched surface layer with small pore size was created through aluminizing and oxidation treatments, which greatly improves the membrane integrity. The intermetallic diffusion between the palladium membranes and the PSS substrate material was not observed after a heat-treatment at 500 °C under hydrogen for 200 h. However, the aluminizing and oxidation treatments still need to be further optimized in order to improve the membrane permeability and selectivity, and particularly, the high diffusion resistance of the substrate materials greatly limited the hydrogen flux.
The necessity arose in the last decennia for the redesign of industrial processes with new unit operations for addressing environmental concerns which has led to the definition of new process indexes, so-called, metrics, that together with the traditional parameters supply additional information for technology selection and identify operating conditions for making a process more profitable. Two sustainability indexes, mass and energy intensities were used in a non-conventional evaluation of the up-grading stage in hydrogen production, i.e. the water gas shift, by means of membrane reactors. Defined as the ratio between the total inlet mass and total energy involved in the reactor, with respect to the hydrogen fed and produced by the reactor, they provide useful information about material exploitation and energy efficiency. The comparative study of membrane reactor performance with respect to conventional reactors was analysed as a function of the main process variables, i.e., pressure, feed molar ratio and space velocity. The membrane reactor resulted always in being more material (20-40%) and energy (20-35%) intensive than a traditional reactor and, in most of the cases, the values of its indexes exceeded the best one of a conventional reactor, corresponding at the equilibrium.
The paper outlines the concept of energy carrier with a particular reference to hydrogen, in view of a more disseminated employment in the field of automotive applications. In particular hydrogen production is analyzed considering the actual state of the art and recent technologies applied in production from the primary sources (fossil fuels, renewable energies, and water electrolysis). Then the problem of hydrogen storage is considered both from technical and economical point of views. In particular, differences between physical and chemical storage are here considered with a particular glance to the most innovative technologies including carbon nanostructures. A review on the main problems in storage and transportation is then shown with a particular attention given to infrastructures costs that perhaps will address particular choices for the technologies of the next future. Automotive applications are called out, accounting the main current technologies and notes on fueling station for hydrogen fed vehicle. The discussion of hydrogen safety in automotive put in evidence the needs for sophisticated sensors, but a comparison with the safety of gasoline and fire risks, evidences that some common incertitudes on hydrogen usage should be overcome. Some other safety issues are introduced in the section of hydrogen transportation. An overview of costs related hydrogen production, storage and transportation is finally given. This aspect is of a capital importance for the future dissemination of the hydrogen energy carrier.
Two different catalysts, Rh(0.6% wt/wt)/La2O3(27% wt/wt)·SiO2 and Pt(0.6% wt/wt)/La2O3(27%)·SiO2, were tested in the WGS reaction. Their performances were first studied in a conventional fixed-bed reactor. Their activities were similar and they were both very stable. However, as Pt(0.6)/La2O3(27)·SiO2 showed a much higher selectivity to the desired reaction, the performance of a membrane reactor employing this catalyst was studied. The effects of the H2O/CO ratio, space velocity, sweep gas flow rate and size of the catalyst particle on CO conversion and H2 recovery were studied at laboratory scale under isothermal conditions. A 1-D heterogeneous model was developed in order to properly reproduce the experimental results obtaining good agreement between the simulation results and laboratory data. The experimental and theoretical results confirm the existence of significant external mass-transfer limitations in the fluid-particle interface for these very active formulations.
This study investigated the effect of gases such as CO2, N-2, H2O on hydrogen permeation through a Pd-based membrane -0.012 m(2) - in a bench-scale reactor. Different mixtures were chosen of H-2/CO2, H-2/N-2/CO2 and H-2/H2O/CO2 at temperatures of 593-723 K and a hydrogen partial pressure of 150 kPa. Operating conditions were determined to minimize H-2 loss due to the reverse water gas shift (RWGS) reaction. It was found that the feed flow rate had an important effect on hydrogen recovery (HR). Furthermore, an identification of the inhibition factors to permeability was determined. Additionally, under the selected conditions, the maximum hydrogen permeation was determined in pure H-2 and the H-2/CO2 mixtures. The best operating conditions to separate hydrogen from the mixtures were identified. Copyright
Thin palladium composite membrane suffers serious damage like cracks, peel-off, and so on from hydrogen absorption and desorption, especially below the critical temperature, 298 degrees C, in which alpha <->beta phase transition occurs. To prevent such problems and strengthen the palladium film, it was proposed that "metallic glue" was introduced as an adhesion interlayer between porous support and palladium film. In this study, platinum was employed as the glue by taking particular note of its physical compatibility not only with palladium but also with alumina. First, a 60-80 nm thick platinum layer was sputtered on the porous anodic oxide support, and then, 1-2 mu m thick palladium film sputtered. It was found that deformation by an internal stress produced in the Pd film could be completely depressed with the platinum glue during permeation tests, which were carried out in the range of 150-350 degrees C including up-and-down operation with temperature.
The synthesis of thin, defect-free and long lifetime Pd-based membranes for H2 separation is still a challenging task. A porous support is necessary in order to give mechanical stability to very thin layers. Sintered porous metal supports are very promising candidates having numerous advantages over the widely studied ceramic supports. The deposition of a non-metallic barrier between the stainless steel support and the Pd dense layer can prevent the intermetallic diffusion of elements from the support to the Pd-based layer and improve the characteristics (e.g. pore size, topography) of the support surface which actually limits the minimum Pd thickness required in order to obtain high selectivity membranes.
In the present paper, the deposition of intermetallic barrier layers made of alumina obtained via sol gel technique was investigated using commercial porous stainless steel supports, both with their original roughness and after a mechanical smoothing treatment. The quality of the deposited alumina layer was related to the surface characteristics of the supports, sol composition (aluminium and additive content) and viscosity. Pd dense layers were grown via electroless plating technique on the alumina barrier by activation with an additional Pd doped thin alumina layer.
This study focuses on the application of Pd-based membranes for CO2 capture in coal fuelled power plants. In particular, membranes are applied to Integrated Gasification Combined Cycle with two innovative feeding systems. In the first feeding system investigated, CO2 is used both as fuel carrier and back-flushing gas for the candle filters, while in the second case N2 is the fuel carrier, and CO2 the back-flushing gas. The latter is investigated because current dry feed technology vents about half of the fuel carrier, which is detrimental for the CO2 avoidance in the CO2 case. The hydrogen separation is performed in membrane modules arranged in series; consistently with the IGCC plant layout, most of the hydrogen is separated at the pressure required to fuel the gas turbine. Furthermore, about 10% of the overall hydrogen permeated is separated at ambient pressure and used to post-fire the heat recovery steam generator. This layout significantly reduces membrane surface area while keeping low efficiency penalties.
The resulting net electric efficiency is higher for both feeding systems, about 39%, compared to 36% of the reference Selexol-based capture plant. The CO2 avoidance depends on the type of feeding system adopted, and its amount of vented gas; it ranges from 60% to 98%. From the economic point of view, membrane costs are significant and shares about 20% of the overall plant cost. This leads in the more optimistic case to a CO2 avoidance cost of 35 €/tCO2, which is slightly lower than the reference case.
Palladium membrane was prepared on the inner surface of alumina tube by bio-membrane assisted electroless plating combined with osmosis method (BELP). In this preparation technique, an egg-shell film not only served as a semipermeable membrane to form osmotic system for preparing palladium membrane, but also acted as a protection layer to prevent the contamination of the palladium membrane from the osmotic solution. Moreover, the plating solution was circulated through the tube side to promote the mass transfer on the solid liquid interface between the plating surface and the solution. The detailed depositing process of the palladium membrane was studied by scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDXS). Both long term operation and temperature cycling test carried out for hydrogen and nitrogen permeation confirmed that the palladium membrane was stable. Copyright
In this work, H2 production via catalytic water gas shift reaction in a composite Pd membrane reactor prepared by the ELP “pore-plating” method has been carried out. A
completely dense membrane with a Pd thickness of about 10.2 mm over oxidized porous
stainless steel support has been prepared. Firstly, permeation measurements with pure
gases (H2 and N2) and mixtures (H2 with N2, CO or CO2) at four different temperatures
(ranging from 350 to 450 �C) and trans-membrane pressure differences up to 2.5 bar have
been carried out. The hydrogen permeance when feeding pure hydrogen is within the
range 2.68e3.96$10�4 mol m�2 s�1 Pa�0.5, while it decreases until 0.66e1.35$10�4 mol m�2-s�1 Pa�0.5 for gas mixtures. Furthermore, the membrane has been also tested in a WGS membrane reactor packed with a commercial oxide FeeCr catalyst by using a typical methane reformer outlet (dry basis: 70%H2e18%COe12%CO2) and a stoichiometric H2O/CO ratio. The performance of the reactor was evaluated in terms of CO conversion at different temperatures (ranging from 350 �C to 400 �C) and trans-membrane pressures (from 2.0 to 3.0 bar), at fixed gas hourly space velocity (GHSV) of 5000 h�1. At these conditions, the membrane maintained its integrity and the membrane reactor was able to achieve up to the 59% of CO conversion as compared with 32% of CO conversion reached with conventional packed-bed reactor at the same operating conditions
In this study, a combined test of the WGS (water–gas shift) reactor and a Pd-based composite membrane was carried out for pre-combustion CO2 capture in a coal gasifier. The two series of WGS reactions, i.e., a high-temperature shift and a low-temperature shift, were performed under a gas composition of 60% CO and 40% H2 at 2100 kPa to imitate coal gasification. The CO2 enrichment and H2 recovery tests at 673 K and 2100 kPa with the high-pressure membrane module after the WGS reaction presented the enriched CO2 concentration and H2 recovery ratios of ∼92% and ∼96%, respectively. The long-term stability test showed that the CO2 concentration decreased to 78.2%, and CO was generated and reached to 8.8% in the retentate stream after 47 h because of reverse WGS and CO2 hydrogenation reaction on 316L stainless steel module. The stability test for ∼3137 h showed that these catalytic activities could be successfully prevented using steel with higher Cr and Ni contents, such as 310S. The WGS-membrane combination test using the outlet gas from a real coal gasifier was continued for ∼100 h and showed that the WGS catalysts and membrane module made of 310S would be stable under real conditions.
As a novel approach to simultaneous water–gas shift (WGS) reaction and separation for the production of hydrogen, a bi-functional membrane was successfully prepared using a simple coating method. The catalyst materials, Pt and CeO2, were directly coated over the surface of a Pd–Au dense membrane for excluding slow hydrogen diffusion through the conventional catalyst bed. The coated catalyst layer did not significantly affect the hydrogen permeance of the bare membrane, due to the presence of metal within the catalyst compositions which acted as a bridge for the surface diffusion of hydrogen atoms. As a result of WGS reaction within the novel membrane reactor at 380 °C, the CO conversion was 2 times higher than that of no hydrogen separation. The catalyst-deposited membrane can make the “simultaneous chemical reaction and separation” more feasible and relatively simple, if the more robust catalysts can be loaded with the increased active surface area and the multi-membrane module reactor adopted the coin-shape bi-functional membranes can be designed to completely treat the CO and separate hydrogen from the CO mixture.
I n this work, the combined role of mass and heat dispersion effects on the performance of adiabatic large-scale membrane reactor modules is investigated. The model developed can be used in predicting membrane reactor performance under industrial process conditions. The membrane reactor configuration adopted in this study assumes that the water gas shift reaction (WGS) takes place in the feed side of the membrane reactor that is filled with catalytic particles. The membrane reactor is equipped with a highly selective ceramic membrane. To undertake a systematic analysis of the role of mass and heat dispersion on the performance of the WGS membrane reactor, various versions of a two-dimensional model have been applied. It is shown that none of the mass and heat dispersion effects can be neglected a priori and their impact on the membrane reactor operation depends on the process characteristics.
In this study, the application of Pd-based H2-selective membranes in integrated gasification combined cycle (IGCC) plants with CO2 capture is performed from both technical and economic perspective. After a preliminary assessment, the membrane separation section is selected and designed on several membrane modules in series with adiabatic high temperature shift in between. The adoption of membrane module with 19 membrane tubes was mainly driven by techno-economics assessment and plant operation issues. Two different plant lay-outs are proposed: in the first configuration, which is more conventional, all the hydrogen is separated at high pressure and sent to the gas turbine. This lay-out achieves very high net electric efficiency (about 40%), but requires large membrane surface area with penalties from economic point of view (cost of CO2 avoided about 50% higher than reference case). The membrane module cost accounts for more than 25% on the total investment. In the second configuration, which is an innovative part of this work, a proportion of the hydrogen is separated at low pressure and used to post-fire the heat recovery steam generator. This configuration preserves the very high efficiency of the previous case but reduces significantly the membrane surface area; the membrane cost share over the total investment drops to 15%. The resulting cost of CO2 avoided is 36 €/tCO236 €/tCO2, equal to the reference case, with large possibility of improvements thanks to the conservative membrane design selected.
This work comprises a study of hydrogen separation with a composite Pd-YSZ-PSS membrane from mixtures of H2, N2, CO and CO2, typical of a water gas shift reactor. The Pd layer is extended over a tubular porous stainless steel support (PSS) with an intermediate layer of yttria-stabilized-zirconia (YSZ). YSZ and Pd layers were incorporated over the PSS using Atmospheric Plasma Spraying and Electroless Plating techniques, respectively. The Pd and YSZ thickness values are 13.8 and 100 μm, respectively, and the Pd layer is fully dense. Permeation measurements with pure, binary and ternary gases at different temperatures (350–450 °C), trans-membrane pressures (0–2.5 bar) and gas composition have been carried out. Moreover, thermal stability of the membrane was also checked by repeating permeation measurements after several cycles of heating and cooling the system. Membrane hydrogen permeances were calculated using Sieverts' law, obtaining values in the range of 4·10−5–4·10−4 mol m−2 s−1 Pa−0.5. The activation energy of the permeance was also calculated using Arrhenius' equation, obtaining a value of 16.4 kJ/mol. In spite of hydrogen selectivity being 100% for all experiments, the hydrogen permeability was affected by the composition of feed gas. Thus, a significant depletion in H2 permeate flux was observed when other gases were in the mixture, especially CO, being also more or less significant depending on gas composition.
The performanceofselectivehydrogenpermeationofdifferentPd-containing(Pdthicknessintherange
11–30 μm) structuredmembraneshasbeenstudiedinthiswork.Pdwasdepositedovertubularporous
stainless steelsupportsbythenovelelectrolesspore-platingmethod.Thepermeationpropertiesofthe
membranes havebeentestedatdifferentoperatingconditions:retentatepressure(1–4 bar),temperature
(623–723k)andhydrogenmolarfractionoffeedgas(0.7–1). Acompleteselectivitytohydrogenwas
observed foralltestedconditions,ensuring100%purityinthehydrogenpermeate flux. Permeancesin
the rangeof2.3–6.4�10�4 mol/m2 s Pa0.5 wereobtained,maintainingagoodmechanicalstabilityofthe
composite system.
A mathematicalmodelconsideringthedifferentmasstransferresistancesinseriesofthecomposite
membrane, e.g.Pdactivelayer,poroussupportandgasphase film layer,hasbeenproposedtodescribe
the permeationofhydrogen.Themodelalsoconsiderstheaxialvariationsofhydrogenconcentration
that takesplaceintubularmembranes.Asetofexperimentaldatahasbeenusedto fit theempirical
parametersofthemodel,usinganothersetofexperimentsforvalidatingtheproposedmodel.
Composite catalytic-permselective (CCP) membrane designs, wherein a catalytic film is applied to the retentate surface of a permselective film, are capable of enhancing gas permeation rates and permselectivities by modifying the gas composition in contact with the permselective film surface via reaction–diffusion within the catalytic layer. Isothermal, two-dimensional models are employed to compare performance of a CCP membrane system against (i) an un-modified permselective film in a gas purification membrane (GPM) system, and (ii) an equivalent packed-bed membrane reactor (PBMR) system, for coupling water–gas-shift reaction with H2 purification from a typical heavy hydrocarbon reformate mixture (9%CO, 28%H2, 15%H2O, 3%CO2). Analysis is provided for the case of (i) an infinitely H2-permselective Pd film, for exploring the potential for alleviating surface inhibition via CO using the CCP design, and (ii) a moderately CO2-permselective polymeric film, for exploring the potential for enhancing CO/CO2 separation via CCP design as compared to PBMR designs. For the former case, the CCP design is capable of enhancing overall permeation rates in GPM and PBMR configurations via alleviation of surface inhibition. In the latter case, simulations predict up to a 40% enhancement in reaction product-reactant (CO2–CO) separation, at the cost of reduced product-product (CO2–H2) separation.
A simulation study of a membrane reactor for the water-gas shift reaction is presented. A pseudohomogenous 1D mathematical model is considered to describe the performance of the membrane reactor under steady-state operation. The membrane consists on a dense Pd layer (selective to H2) deposited on a porous ceramic support. The influence of operating pressure and thermal effects on the membrane reactor performance is analyzed and the results are compared with those corresponding to a reactor with no hydrogen permeation.
In the development of environmental friendly and highly efficient energy processes, membrane reactors hold an important role for their ability to carry out, simultaneously and in the same unit, the separation and reaction steps. Taking advantage of the synergies deriving from this coupling, they achieve comparable results to the conventional reactors at less severe conditions. A sensitivity analysis has been developed in order to define the role of some variables on the performance of a membrane reactor for maximizing the system efficiency. The behaviour of a membrane reactor has been investigated by means of a two-dimensional mathematical model applied to the water–gas shift reaction. By depending on operation feed pressure, a specific choice of both sweep gas flow rate and temperature can limit the occurring of dangerous temperature hot spots without compromising the performance of the system. The catalyst distribution coupled with an efficient heat exchange across the membrane have been investigated as possible technical solutions adequate to control hot spots along the membrane reactor.
This study presents numerical studies of hydrogen production performance via water gas shift reaction in membrane reactor. The pre-exponential factor in describing the hydrogen permeation flux is used as the main parameter to account for the membrane permeance variation. The operating pressure, temperature and H2O/CO molar ratio are chosen in the 1–20 atm, 400–600 °C and 1–3 ranges, respectively. Based on the numerical simulation results three distinct CO conversion regimes exist based on the pre-exponential factor value. For low pre-exponential factors corresponding to low membrane permeance, the CO conversion approaches to that obtained from a conventional reactor without hydrogen removal. For high pre-exponential factor, high CO conversion and H2 recovery with constant values can be obtained. For intermediate pre-exponential factor range both CO conversion and H2 recovery vary linearly with the pre-exponential factor. In the high membrane permeation case CO conversion and H2 recovery approach limiting values as the operating pressure increases. Increasing the H2O/CO molar ratio results in an increase in CO conversion but decrease in H2 recovery due to hydrogen permeation driving force reduction. As the feed rate increases in the reaction side both the CO conversion and hydrogen recovery decrease because of decreased reactant residence time. The sweep gas flow rate has a significant effect on hydrogen recovery. Low sweep gas flow rate results in low CO conversion H2 recovery while limiting CO conversion and hydrogen recovery can be reached for the high membrane permeance and high sweep gas flow rate cases.
Methane steam reforming is one of the most important pathways for producing high purity hydrogen. In this context, the use of fixed-bed catalytic reactors equipped with hydrogen perm-selective membranes is an interesting alternative for producing high purity hydrogen in one single step. In this work, this reactor is studied by means of numerical simulations using a 2D model, consisting of mass, energy and momentum balances. The fixed-bed is considered to be formed by Ru/SiO2 catalyst particles, especially tailored for steam reforming at low temperature and steam-to-carbon ratio, whereas a composite palladium membrane was considered for hydrogen permeation. The model was validated with experimental data, and the adequacy of a simplified 1D model to simulate the membrane reactor was evaluated and discussed in comparison to the 2D model. Then, the model was used to study the influence of the main operating variables (inlet temperature, pressure, space velocity, steam excess and sweep gas rate in the permeate side) on the reactor performance. Finally, the optimum operating conditions, corresponding to a maximum hydrogen permeation rate, were determined, and the behaviour of the optimized reactor is analysed in detail.
A new synthesis method to prepare Pd membranes by novelty modified electroless plating over tubular porous stainless steel supports (PSS) has been developed. This new pore plating method basically consists on feeding both plating solution and reducing agent from opposite sides of support, allowing the preparation of totally hydrogen selective membranes with a significantly lower Pd consumption than the corresponding to the conventional electroless plating procedure. In the latter, both reducing agent and plating solution are added simultaneously in one side of the PSS support. This new plating method has been applied over raw commercial PSS supports and air calcined supports in order to generate a Fe–Cr oxide intermediate layer.A completely dense Pd membrane with a thickness in the range 11–20 μm directly over tubular porous stainless steel tubes with a high roughness has been achieved. The permeation properties of the membranes have been tested at different operating conditions for pure feed gases: retentate pressure (1–4 bar) and temperature (350–450 °C). All membranes present good permeance reproducibility after several thermal cycles and a complete hydrogen ideal selectivity, since complete retention of nitrogen is maintained for all tested experiment conditions, ensuring 100% purity in the hydrogen permeate flux. The permeance of both membranes is maintained in the range of 1–3·10−4 mol m−2 s−1 Pa−0.5.
The production of high-purity hydrogen using the water–gas-shift reaction in both conventional fixed bed reactor and hydrogen perm-selective membrane reactor at low to medium scale is studied in this work by developing and comparing models with different complexity levels. A two-dimensional rigorous reactor model considering radial and axial variations of properties (including bed porosity), setting mass, energy and momentum differential balances, and nesting a rigorous model for mass transfer within the porous catalyst was considered as reference for comparison. Different simplifications of this model for taking into account mass-transfer effects within the catalyst pellet (Thiele modulus, evaluation of apparent kinetic constants, empirical correlation for effectiveness factors or just neglecting these effects) were tested, being observed that these effects are not negligible and that the first two approaches are accurate enough for taking into account mass transfer within catalyst pellets. Regarding to the reactor model, it was observed that one-dimensional models are not adequate, especially for the membrane reactor. Analogously, neglecting the momentum balances in the reactor (as made is most simulations reported in the literature) leads to important misspredictions in the behaviour of the membrane reactor performance. Finally, the influence of the main operation parameters (inlet temperature, pressure, space velocity, etc.) was studied using the detailed reactor model, concluding that space velocity and pressure are the most important parameters affecting reactor performance for membrane reactors.
Membranes and membrane reactors for pure hydrogen production are widely investigated not only because of the important application areas of hydrogen, but especially because mechanically and chemically stable membranes with high perm-selectivity towards hydrogen are available and are continuously further improved in terms of stability and hydrogen flux. Membrane reactors are (multiphase) reactors integrating catalytic reactions (generally reforming and water gas shift reactions for hydrogen production) and separation through membranes in a single unit. This combination of process steps results in a high degree of process integration/intensification, with accompanying benefits in terms of increased process or energy efficiencies and reduced reactor or catalyst volume.The aim of this review is to highlight recent advances in hydrogen selective membranes (from palladium-based to silica and proton conductors) along with the advances for the different types of membrane reactors available (from packed bed to fluidized bed, from micro-reactors to bio-membrane reactors). In addition, the application of membrane reactors for hydrogen production from different feedstock is also discussed.
The abatement of concentration polarization in a membrane tube is of the utmost importance for improving the efficiency of hydrogen separation. In order to enhance the performance of hydrogen separation, the characteristics of hydrogen permeation in a Pd-based membrane system under various operating conditions and geometric designs are studied numerically. The effects of Reynolds numbers, shell size, baffle, and pressure difference on hydrogen mass transfer across the membrane are evaluated. The predictions suggest that a larger shell deteriorates concentration polarization, stemming from a larger H2 concentration boundary layer. Baffles equipped in the shell are conducive to disturbing H2 concentration boundary layer and reducing concentration polarization at the retentate side, thereby intensifying H2 permeation. The more the number of baffles, the less the increment of improvement in H2 permeation is. The installation of one baffle is recommended for enhancing H2 separation and it is especially obvious under the environments of high pressure difference. Within the investigated ranges of Reynolds number at the permeate side and the retentate side, the feasible operating conditions are suggested in this study.
A palladium selective tubular membrane has been prepared to separate and purify hydrogen. The membrane consists of a composite material, formed by different layers: a stainless steel support (thickness of 1.9 mm), an yttria-stabilized zirconia interphase (thickness of 50 μm) prepared by Atmospheric Plasma Spraying and a palladium layer (thickness of 27.7 μm) prepared by Electroless Plating. The permeation properties of the membrane have been tested at different operating conditions: retentate pressure (1–5 bar), temperature (350–450 °C) and hydrogen molar fraction of feed gas (0.7–1). At 400 °C, a permeability of 1.1 × 10−8 mol/(s m Pa0.5) and a complete selectivity to hydrogen were obtained. The complete retention of nitrogen was maintained for all tested experiment conditions, with both single and mixtures of gases, ensuring 100% purity in the hydrogen permeate flux.A rigorous model considering all the resistances involved in the hydrogen transport has been applied for evaluating the relative importance of the different resistances, concluding that the transport through the palladium layer is the controlling one. In the same way, a model considering the axial variations of hydrogen concentration because of the cylindrical geometry of the experimental device has been applied to the fitting of the experimental data. The best fitting results have been obtained considering Sieverts’-law dependences of the permeation on the hydrogen partial pressure.
The water–gas shift (WGS) catalytic membrane reactor (CMR) incorporating a composite Pd-membrane and operating at elevated temperatures and pressures can greatly contribute to the efficiency enhancement of several methods of H2 production and green power generation. To this end, mixed gas permeation experiments and WGS CMR experiments have been conducted with a porous Inconel supported, electroless plated Pd-membrane to better understand the functioning and capabilities of those processes. Binary mixtures of H2/He, H2/CO2, and a ternary mixture of H2, CO2 and CO were separated by the composite membrane at 350, 400, and 450 °C, 14.4 bar (Ptube = 1 bar), and space velocities up to 45,000 h−1. H2 permeation inhibition caused by reversible surface binding was observed due to the presence of both CO and CO2 in the mixtures and membrane inhibition coefficients were estimated. Furthermore, WGS CMR experiments were conducted with a CO and steam feed at 14.4 bar (Ptube = 1 bar), H2O/CO ratios of 1.1–2.6, and GHSVs of up to 2900 h−1, considering the effect of the H2O/CO ratio as well as temperature on the reactor performance. Experiments were also conducted with a simulated syngas feed at 14.0 bar (Ptube = 1 bar), and 400–450 °C, assessing the effect of the space velocity on the reactor performance. A maximum CO conversion of 98.2% was achieved with a H2 recovery of 81.2% at 450 °C. An optimal operating temperature for high CO conversion was identified at approximately 450 °C, and high CO conversion and H2 recovery were achieved at 450 °C with high throughput, made possible by the 14.4 bar reaction pressure.
The goal of this work is to understand the effect of the relative values of membrane permselectivity, permeation flux and reaction rate on the performance of a water gas shift membrane reactor. This was achieved by simulating the operation of an isothermal tube–shell reactor. Its performance was evaluated based on the CO conversion and H2 recovery, as well as the permeate and retentate H2 molar fractions. The maximum enhancement of CO conversion has been observed when the Damkhöler number (Da) is almost equal to the permeation number (Pe). Improvements in CO conversion can be achieved even when membranes with relatively low permselectivity values (∼10) are used. Further increase of permselectivity primarily increased the purity of the H2 rich stream. The utilization of CO2 selective instead of H2 selective membranes could improve CO conversion only if the CO2 content of the feed is higher than that of H2. Finally, simulations using rate expressions that correspond to different detailed reaction mechanisms resulted only in slight differences in reactor performance.
The water gas shift (WGS) reaction is an important step of hydrogen production in industrial cycles for upgrading H2 rich streams by CO conversion present in syngas mixtures. WGS was studied in a Pd-alloy membrane reactor (MR) by means of a non-isothermal mathematical model using, as main parameter, Damköhler's number (Da), the ratio of characteristic times of flow rate and reaction, in a temperature range of 220–320°C.Two different reactant equimolecular feed streams were considered: one containing only CO and H2O, the other containing also H2 and CO2 of higher industrial interest. The permeation driving force was generated by feed pressure ranging 200–1500kPa which allows a good H2 recovery index (up to 95%) and a retentate stream rich (up to 80%) in CO2. No sweep gas was used; therefore, a pure H2 stream is obtained as permeate.CO conversion, H2 recovery index and its partial pressure are the main variables used for analysing the MR performance and showing its advantages with respect to a TR in the large feed pressure range.In addition, the volume index and conversion index are introduced for the first time and proposed as simple tools analysing the volume reduction or improved conversion shown by MRs; both lead to the catalyst amount and reactor size being reduced. The two new indexes proposed by membrane engineering open a window on the analysis of MRs for H2 production and CO2 separation for the process intensification strategy.This paper describes a modelling analysis of a packed-bed membrane reactor involving dense Pd–Ag commercial permselective membrane.
In this study the performances of a membrane reactor (MR) are estimated when both shell side stream (sweep gas) and lumen side stream are continuously either in parallel flow configuration (co-current mode) or in counter-flow configuration (counter-current mode). Two mathematical models have been formulated and steady-state mass-balance gave two-dimensional differential equations, which were solved by using the orthogonal collocation technique. Simulation results for both co-current mode and counter-current mode have been compared in terms of hydrogen molar fraction (in the shell side) vs. axial co-ordinate at different hydrogen permeances, temperatures, and lumen pressures. At the operative conditions considered, a very similar CO conversion value has been obtained for both modes.
Water gas shift (WGS) is a thermodynamically limited reaction which has to operate at low temperatures, reducing kinetics rate and increasing the amount of catalyst required to reach valuable CO conversions.It has been widely demonstrated that the integration of hydrogen selective membranes is a promising way to enhance WGS reactors performance: a Pd-based membrane reactor (MR) operated successfully overcoming the thermodynamic constraints of a traditional reactor thanks to the removal of hydrogen from reaction environment.In this work, the effect of hydrogen removal in membrane water gas shift reactors has been investigated by a two-dimensional, non-isothermal model in order to analyze the WGS reactor performance.
A simple two-step microkinetic model for the high-temperature water−gas shift was developed using existing experimental data. This two-step redox model uses only three adjustable parameters, and it is capable of predicting the inhibitory effect of CO2 on the kinetics of the reaction. It was used to simulate the performance of an adiabatic membrane reactor for the water−gas shift where the membrane is based on Pd. The simulations show that excess steam in the feed is desirable to control the adiabatic temperature rise; a 3:1 ratio of steam to carbon monoxide was found to be near optimum. The simulations further suggest that the rate of reaction is the limiting process in the membrane reactor, not the permeation of hydrogen through the membrane. At 90% hydrogen yield, finding a perfect membrane would only reduce the reactor size by 12%, whereas eliminating the inhibitory effect of CO2 would reduce the reactor size by 76%.
Water gas shift (WGS) reaction is critical to hydrogen purification for fuel cells. Being reversible and exothermic, the WGS reaction in the traditional fixed bed reactor is not efficient. Using a CO2-selective membrane reactor shifts the reaction towards the product side, which enhances the conversion of CO and increases the purity of the H2 product at a high pressure. The simultaneous reaction and transport process in the countercurrent WGS membrane reactor was simulated by using a one-dimensional non-isothermal model, and the effect of several system parameters including CO2/H2 selectivity, CO2 permeability, and sweep-to-feed molar flow rate ratio were investigated. The synthesis gases from both autothermal reforming and steam reforming were used as the feed gas, while heated air was used as the sweep gas. A published WGS reaction rate expression with the commercial Cu/ZnO catalyst was incorporated into the model. The modeling results show that a CO concentration of less than 10 ppm, a H2 recovery of greater than 97%, and a H2 concentration of greater than 54% (on the dry basis) are achievable from autothermal reforming syngas. If steam reforming syngas is used as the feed gas, H2 concentration can be as high as 99.64% (on the dry basis).
Mesoporous YSZ–γ-Al2O3 membranes were coated on α-Al2O3 (Ø2 mm) tube by dipping the α-Al2O3 support tube into mixed sol consists of nano-size YSZ and bohemite particles followed by drying and calcination at 600 °C. Addition of bohemite in YSZ sol helped a good adhesion and uniform coating of the membrane film onto α-Al2O3 support. The quality of the mesoporous YSZ–γ-Al2O3 membranes was evaluated by the gas permeability experiments. The number of defects was minimized when the γ-Al2O3 content became more than 40%. Addition of γ-Al2O3 inhibited the crystal growth of YSZ, sintering shrinkage and distortion stress. Increase of calcination temperature and time results in the increase of pore size and N2 permeance. A hydrogen perm-selective membrane was prepared by filling palladium into the nano-pores of YSZ–γ-Al2O3 layer by vacuum-assisted electroless plating. Crystal growth of palladium was observed by thermal annealing of the membrane at 600 °C for 40 h. The Pd–YSZ–γ-Al2O3 composite membrane revealed improved thermal stability allowing long-term operation at elevated temperature (>500 °C). This has been attributed to the improved fracture toughness of YSZ–γ-Al2O3 layer and matching of thermal expansion coefficient between palladium and YSZ. Although fracture of the membrane did not occur, decline of H2 flux was observed when the membrane was exposed in 600 °C. This has been attributed to the agglomeration of palladium particles by crystal growth and dense packing into the pore networks of YSZ–γ-Al2O3 by elevation of temperature.
In this work, an experimental and modeling study is described, focusing on the performance of a Pd–Ag membrane reactor recently proposed and suitable for the production of ultra-pure hydrogen. A packed-bed membrane reactor (MR) with a “finger-like” membrane configuration has been used for carrying out the water-gas shift reaction (WGS) in the region of low temperature operation using a simulated reformate feed.The experiments were performed under a broad range of operating conditions of temperature (200–300 °C) and space velocity (1200–10,800 LN kgcat−1 h−1); the effect of feed pressure (1–2 bar) was also analyzed, as well as the operating mode at the permeate side: vacuum (30 mbar) or sweep gas (1.0 bar; nitrogen at 1 LN min−1). A one-dimensional, isothermal and steady-state model is proposed, which assumes axially dispersed plug flow pattern and pressure drop in the retentate side and plug flow with constant pressure in the permeate side. An innovative composed kinetic model was also used to describe the catalytic activity of the catalyst for the WGS reaction. In general, the simulation results showed a good agreement to the experimental data, in terms of carbon monoxide conversion and hydrogen recovery (and also outlet retentate composition) using only two fitting parameters related to the decline of H2 permeability due to the presence of CO. Both simulation and experimental runs showed that the MR achieves high performances, for some operating conditions clearly above the maximum limit for conventional packed bed reactors. The performance reached is particularly relevant when hydrogen is recovered via sweep gas mode (a high sweep flow rate was employed), because a lower partial pressure could be reached than using vacuum pumping. In the first case, almost complete CO conversion and H2 recovery could be reached.
Extensive water–gas shift (WGS) reaction experiments have been conducted under different operating conditions to understand WGS process better. WGS kinetic rates were extracted from the experimental results and applied in a two-dimensional (2-D) unsteady kinetic model for performance and behaviour prediction. The model showed good agreement with the experimental data; a maximum error of 0.56% for hydrogen (wet basis) and 0.47% for carbon monoxide (wet basis) was observed for the H2O/CO ratio range of 1–7. Parametric studies on WGS reaction were also covered in this paper. By converting all the results of the parametric studies with respect to space velocity, the optimum range of WGS performance was determined. Our analysis showed that WGS process should be operated from 106.09 to 212.18/h and between 873 and 973 K.
In this experimental study the water gas shift (WGS) reaction is considered as a particular application of a catalytic membrane reactor (CMR). Experiments on the WGS reaction were carried out using a composite palladium membrane obtained by coating an ultrathin double-layer palladium film on the inner surface of the support of a commercial tubular ceramic membrane by a so-called co-condensation technique. The best operating conditions were determined at various molar ratios, temperature, PIumen, gas feed flow, and with and without nitrogen sweep gas. For a non-porous stainless steel tube and for the commercial ceramic membrane having the same geometrical dimensions, the conversion results are always lower than the equilibrium value. For the composite palladium membrane, the conversion also depends on the flow of the sweep gas utilized. For example, using a nitrogen sweep gas flow of 28.2 cm3/min, the maximum conversion value reaches 99.89%. The study of the effect of temperature on conversion of carbon monoxide in the WGS reaction shows that at higher reaction temperature, the thermodynamic equilibrium conversion of CO decreases. In contrast for the CMR considered in this work, there is a maximum conversion value around 600 K. This value is a compromise between the kinetic rate of the reaction (which increases with increasing temperature) and thermodynamic considerations for the WGS reaction. The effect of the time factor () on conversion of CO, with and without sweep gas at three different temperatures (595, 615 and 633 K) shows that at greater there are correspondingly higher values of the CO conversion for each temperature considered. For each temperature there is a slight effect of the sweep gas, and this is higher at 595 K. The good performance of the composite ceramic-palladium membrane is confirmed by a comparison with experimental results recently presented in the literature for the same reaction. Reaction tests have been carried out for a feed mixture also. In this case, however, the resulting values are always below the equilibrium ones.
The partial oxidation of butane to maleic anhydride in a membrane reactor with improved heat transfer through the wall has been studied in this work. The reactor consisted of a catalytic fixed bed with sintered metal membrane wall that allows the gradual feed of air from the external fluidized bed. The influence of the most important design and operation variables (reactor length, gas flow rate, inlet temperature, butane inlet concentration, and air gas flow rate) on butane conversion and maleic anhydride selectivity has been studied by means of computer simulations using an experimentally-validated detailed 2D model. The performance of this reactor was systematically compared to the corresponding conventional fixed bed reactor. The membrane reactor has been found to provide slightly higher selectivity than the fixed bed reactor. Moreover, in the membrane reactor, the mixing of butane and air takes place through the wall directly inside the catalytic bed. Since solid beds avoid flame propagation, the process can be operated with higher butane inlet concentrations under safety conditions. Hence, the fluidized bed membrane reactor represents an interesting alternative for industrial-scale operation.
Hydrogen production via the reversible water-gas shift reaction normally requires multiple catalytic reaction steps followed by CO2 separation in order to produce the required H2 purity. Removal of CO2 as it is formed via the noncatalytic gas—solid reaction between CO2 and CaO provides the opportunity to combine reaction and separation into a single processing vessel. Resultant process simplifications include elimination of the need for heat exchangers between catalyst beds as well as the absorption and stripping units required for CO2 removal (or the pressure swing adsorption unit). Process savings may be realized by reducing the quantity of excess steam to the reactor, and the expensive, sulfur-sensitive shift catalyst is not needed in the 500–600°C range of interest. The combined shift and carbonation reactions were studied in a laboratory-scale fixed-bed reactor containing dolomite sorbent precursor. The effect of shift-carbonation temperature and pressure, synthesis gas composition, space velocity, and composition and properties of the sorbent were studied. The rapid rates of the combined reactions permit equilibrium CO conversion and CO2 removal to be closely approached. Greater than 0.995 fractional removal of carbon oxides was achieved over a range of reaction conditions.
The water-gas shift reaction (WGS) over three commercial WGS catalysts and four oxide catalysts used for alkane activation has been studied at atmospheric pressure and in the temperature range of 160 to 600 °C. The oxide catalysts used were two ethane oxydehydrogenation catalysts, namely Mo19V5Nb1Ox and V5Nb1Ox, and two methane coupling catalysts, namely Ca3NiK0.05Oxand LiMgOx. The commercial water-gas shift catalysts used were two Fe3O4-Cr2O3 catalysts and one CuZnO/Al2O3 catalyst. All catalysts except the ethane oxidehydrogenation catalysts and LiMgOx showed high activity for the water-gas shift reaction below 400°C. It is evident that Fe, Cr, Zn, Cu and Ni oxides or metals enhance the water-gas shift reaction. The commercial CuZnO/Al2O3 catalyst was the most active WGS catalyst per gram of the catalyst at 160–250°C, whereas the Fe3O4-Cr2O3) catalysts showed high activity above 300 °C. The specific rates of Ca3NiK0.05Ox, and LiMgOx were, however, higher than the specific rates of the commercial catalysts. The apparent activation energies for the conversion of carbon monoxide to carbon dioxide were 53 kJ/mol for CuZnO/Al2O3, 68 kJ/mol for LiMgOx, 86 kJ/mol for Ca3NiK0.05Ox, 95 kJ/mol and 110 kJ/mol for the Fe3O4-Cr2O3 catalysts, 101 kJ/mol for Mo19V5Nb1Ox, and 132 kJ/mol for V5Nb1Ox. For the commercial catalysts, the power-law rate model with concentration exponents of carbon monoxide and water close to one and zero, respectively, gave the best results. For V5Nb1Ox, and Ca3NiK0.05Ox, the concentration exponents of carbon monoxide and water close to 0.5 fit the results best. For Mo19V5Nb1Ox, the reaction was first order in carbon monoxide concentration whereas for LiMgOx, it was zero order in carbon monoxide concentration and 0.5 order in water concentration. Ca3NiK0.05Ox and LiMgOx were active for the water-gas shift reaction in the temperature range of oxidative methane coupling. Thus, it is probable that the water-gas shift reaction can occur during methane coupling when these catalysts are used. The water-gas shift reaction is, however, unlikely to occur during the oxidative dehydrogenation of ethane since the conversions of carbon monoxide to carbon dioxide were very low at 350–500°C.
Mathematical model of a membrane reactor for the water-gas-shift (WGS) reaction and results of simulation studies are presented. The physicochemical phenomena and brief description of the model are given. Due to the fact that membranes for CO2 separation exhibit poor selectivity at the elevated temperature range (indispensable for the desired activity of WGS catalysts), the simulations were carried out only for H2 selective membranes, containing Pd or its alloys. Comparison of the conventional two-stage WGS reactor with the membrane reactor revealed that operating temperature range of the catalyst used in the membrane reactor unit has to be much wider than that in conventional industrial reactors with separate high- and low temperature catalyst stages and with cooling in between. Thus, the research of new catalysts should accompany development of the membrane reactor technology. Simulations discussed in the present paper were focused on processing the gas derived from the coal gasification plant. Some results of mathematical simulations are presented. They reveal that under some conditions the membrane reactor technology can be promising for the hydrogen production from the coal-derived gas.
Membrane reactor modelling - a comparative study to evaluate the role of combined mass and heat dispersion in large-scale adiabatic membrane modules
- Markatos