Chemical Engineering and Processing

Published by Elsevier
Print ISSN: 0255-2701
Retention of the stationary phase of aqueous-aqueous polymer phase systems is improved by a spiral column configuration which utilizes the radially acting centrifugal force along the spiral pitch to retain the heavier phase in the outer portion and the lighter phase in the inner portion of the spiral channel. For the separation of proteins which has low mass transfer rates, the system needs further modification of the separation channel to interrupt the laminar flow and enhance mixing of the two phases. Two spiral column assemblies were developed, one using a disk with spiral grooves and the other, the spiral tube support which accommodates the multiple spiral layers made from a single piece of fluorinated plastic tubing. In the spiral disk assembly, the best protein separation is achieved by the mixer-settler system which vigorously mixes two phases by vibrating glass beads placed in every other section of barricaded spiral channel, while in the spiral tube assembly the partition efficiency of proteins is enhanced by compressing the tubing to interrupt the laminar flow of the mobile phase. In both systems protein samples were well resolved by choosing the suitable elution modes.
The purpose of this work is to obtain a neural model of the Acid Brown 75, Acid Orange 52, Acid Orange 10 and Direct Red 28 dyes decoloration process. Dyes aqueous solutions were individually treated in plug-flow reactor, with ultraviolet radiation and hydrogen peroxide. The decoloration process was evaluated in function of the absorbance reading in each dye maximum absorbance wavelength. The input variables corresponding to the input neurons of the neural feedforward backpropagation model used in the work were comprised by structural parameters characteristic of each dye (number of azo bonds and sulphonate groups) and also by process operational variables (temperature, initial pH, hydrogen peroxide volume, reactor operation time and dyes concentration). The combination of structural and operational parameters provided a hybrid character in relation to the input variable nature. The correlation coefficients (approximately 0.96 for the data total, validation and test sets) showed the good model prediction capacity. The neural model obtained also provided, via Garson Partition Method, the determination of the influence of the decoloration process input variables.
Sulfur dioxide is one of the major pollutants resulting from fuel combustion. In this study non-isobaric pulse chromatography was used for the investigation of the adsorption properties of sulfur dioxide on molecular sieve 13X and activated carbon. The experimental procedure consists of introducing a pulse of sulfur dioxide on the inlet of a column packed with molecular sieve 13X and activated carbon adsorbents and measuring the concentration versus time response peak leaving the other end of the column. Adsorption equilibrium constants were calculated using the moment analysis of the chromatographic peaks. Non-isobaric pulse chromatographic technique necessitates the use of high carrier gas flow rates which avoids the tailing of the response peaks and reduces considerably the uncertainty in the measurement of the moments. The adsorption of sulfur dioxide was investigated in a temperature range of 353–453 K with the activated carbon, which is a typical temperature range of desulfurization of flue gas, and in a temperature range of 523–673 K with molecular sieve 13X. Adsorption equilibrium constants of sulfur dioxide were found to decrease considerably with increasing temperature. At 453 K, the adsorption equilibrium constant was found to be 18.78, whereas it has a value of 330.2 at 353 K with the activated carbon. At 673 K, the adsorption equilibrium constant was found to be 11.52, whereas it has a value of 167.82 at 523 K with molecular sieve 13X. Heat of adsorption of sulfur dioxide on molecular sieve 13X and the activated carbon was determined as −12.4 and −8.99 kcal mol−1, respectively.
The decolorization of C.I. Acid Red 14 (AR14) azo dye by electrocoagulation (EC) process was studied in a batch reactor. Response surface methodology (RSM) was applied to evaluate the simple and combined effects of the three main independent parameters, current density, time of electrolysis and initial pH of the dye solution on the color removal efficiency and optimising the operating conditions of the treatment process. A 23 full factorial central composite face centred (CCF) experimental design was employed. Analysis of variance (ANOVA) showed a high coefficient of determination value (R2 = 0.928) and satisfactory prediction second-order regression model was derived. Maximum color removal efficiency was predicted and experimentally validated. The optimum current density, time of electrolysis and initial pH of the dye solution were found to be 102 A m−2, 4.47 min and 7.27, respectively. Under optimal value of process parameters, high removal (>91%) was obtained for Acid Red 14. This study clearly showed that response surface methodology was one of the suitable methods to optimize the operating conditions and maximize the dye removal. Graphical response surface and contour plots were used to locate the optimum point.
Manufacturers of plate and frame heat exchangers nowadays mainly offer plates with chevron (or herringbone) corrugation patterns. The inclination angleof the crests and furrows of that sinusoidal pattern relative to the main flow direction has been shown to be the most important design parameter with respect to fluid friction and heat transfer. Two kinds of flow may exist in the gap between two plates (pressed together with the chevron pattern of the second plate turned into the opposite direction): the crossing flow of small substreams following the furrows of the first and the second plate, respectively, over the whole width of the corrugation pattern, dominating at lower inclination angles (lower pressure drop); and the wavy longitudinal flow between two vertical rows of contact points, prevailing at highangles (high pressure drop). The combined effects of the longer flow paths along the furrows, the crossing of the substreams, flow reversal at the edges of the chevron pattern, and the competition between crossing and longitudinal flow are taken into account to derive a relatively simple but physically reasonable equation for the friction factor ξ as a function of the angleand the Reynolds number Re. Heat-transfer coefficients are then obtained from a theoretical equation for developing thermal boundary layers in fully developed laminar or turbulent channel flow — the generalized Lévêque equation — predicting heat-transfer coefficients as being proportional to (ξ·Re2)1/3. It is shown, by comparison, that this prediction is in good agreement with experimental observations quoted in the literature.
The behaviour of several organophosphorus extractants for nickel removal from sulphate solutions was studied. Cyanex 302 was found to be the most suitable extracting reagent for nickel recovery from neutral and slightly acidic aqueous solutions. Nickel was successfully recovered from neutral and slightly acidic sulphate media (pH 6.2–7.0) applying batch pertraction in a rotating film contactor and using Cyanex 302 as a carrier. Higher pH of the feed solution favours the process of nickel transfer. Positive effect of the agitation of all three phases on the process of nickel pertraction was also observed. At relatively low rotation velocity pertraction process is controlled by both diffusion and kinetics, while at high velocity the process is governed mainly by nickel-extraction kinetics.
Growth curve under a continuous light regime (dark:light).  
Kinetic parameters for Aphanothece microscopica Nägeli in different light cycles
Growth curves in different light cycles (dark:light).  
Percent carbon fixation as related to the duration of the light periods.  
The objective of this study was to evaluate the effect of the photoperiod on the biomass production and carbon dioxide fixation rates using a photosynthetic culture of the cyanobacterium Aphanothece microscopica Nägeli in bubble column photobioreactors. The cultures were carried out at temperatures of 35 °C, air enriched with carbon dioxide at concentrations of 15% and photon flux density of 150 μmol m−2 s−1. The light cycles evaluated were 0:24, 2:22, 4:20, 6:18, 8:16, 10:14, 12:12, 14:10, 16:8, 18:6, 20:4, 22:2 and 24:0 (night:day), respectively. The results obtained indicated that the duration of the light periods was a determinant factor in the performance of the photobioreactors. A linear reduction in biomass production and carbon dioxide fixation with reductions in the duration of the light period was evident, with the exception of the 12:12 (night:day) cycles. Reductions of up to 99.69% in the carbon-fixation rates as compared with cultures under continuous illumination were obtained.
Distribution of valeric acid between water and Alamine 336, a mixture of tertiary aliphatic amines dissolved in various (proton-donating and -accepting, polar and nonpolar) diluents, as well as a comparison with the extraction equilibria of pure diluent alone, have been studied at 298 K. The influence of the acid structure over distribution has been interpreted through comparing the extractabilities of four acids containing different functional groups, i.e. valeric, formic, levulinic and nicotinic acids. The results were correlated using a modified linear solvation energy relation (METLER) and various versions of the mass action law, i.e. a chemodel approach and a modified Langmuir equilibrium model comprising the formation of one or two acids per multiple amines aggregated structures. The reliability of the proposed extraction models has been analyzed statistically on the basis of the overall loading factor of amine, Zt, using a log-ratio objective function.
The robustness, reliability and efficiency of modern numerical methods for obtaining solutions to flow problems have given rise to the adoption of Computational Fluid Dynamics (CFD) as a widely used analysis tool for membrane separation systems. In the past decade, many two-dimensional (2D) flow studies employing CFD have been published. Three-dimensional (3D) solutions are also slowly emerging. This paper reviews recent research utilizing 3D CFD models to simulate the flow conditions in narrow spacer-filled channels, such as those encountered in Spiral Wound Membrane (SWM) modules. Many of these studies have focused on optimizing spacer geometric parameters, while others have attempted to gain a better understanding of the mechanisms giving rise to mass transfer enhancement. Applications of 3D CFD to complex spacer geometries and multiple ionic component diffusion are also discussed.
Vertical profiles of local pressure, horizontal profiles of net vertical solid mass flux, and residence time distributions (RTD) of the solid phase are experimentally assessed in the riser of a small scale cold circulating fluidized bed of 9 m high having a square cross section of 11 cm × 11 cm. Air (density 1.2 kg/m3, dynamic viscosity 1.8 × 10−5 Pa s) and typical FCC particles (density 1400 kg/m3, mean diameter 70 μm) are used. The superficial gas velocity is kept constant at 7 m/s while the solid mass flux ranges from 46 to 133 kg/m2 s. The axial dispersion of the solid phase is found to decrease when increasing the solid mass flux. Simultaneously, 3D transient CFD simulations are performed to conclude on the usability of the eulerian–eulerian approach for the prediction of the solid phase mixing in the riser. The numerical investigation of the solid mixing is deferred until later since the near-wall region where the solid phase downflow and mixing are predominant is not well predicted in spite of well-predicted vertical profiles of pressure.
The 3D flow field generated by a Scaba 6SRGT impeller in the agitation of xanthan gum, a pseudoplastic fluid with yield stress, was simulated using the commercial CFD package. The flow was modeled as laminar and a multiple reference frame (MRF) approach was used to solve the discretized equations of motion. The velocity profiles predicted by the simulation agreed well with those measured using ultrasonic Doppler velocimetry, a non-invasive fluid flow measurement technique for opaque systems. Using computed velocity profiles across the impeller, the effect of fluid rheology on the impeller flow number was investigated. The validated CFD model provided useful information regarding the formation of cavern around the impeller in the mixing of yield stress fluids and the size of cavern predicted by the CFD model was in good agreement with that calculated using Elson's model.
High pressure–cold pasteurisation (HP–CP) (≤700 MPa, ≤ 25 °C) of microbial contaminants in liquid foods is an alternative to thermal processing. A rigorous and quantitative understanding of HP–CP microbial kinetics is a necessary first step to process optimisation. Here, four appropriate models for the rate coefficient for HP–CP inactivation kinetics, together with a first-order chemical reaction, are assessed against five independent data sets for the reduction in the number of viable cells of Escherichia coli, Yersinia enterocolitica and Listeria monocytogenes in three liquid foods—milk, broth and buffer solution. The four models include an nth-order polynomial (n-OP) restricted to a cubic, the square root, classical log-linear and the Davey–Linear Arrhenius (D–LA). The percent variance accounted for (%V) together with analyses of residuals was used as stringent criteria of model fit. The n-OP, square root, classical log-linear and D–LA models explained, respectively, an overall mean 77.9, 87.6, 84.1 and 89.1 %V. The D–LA model is selected as that which best satisfied criteria established for goodness of fit. The model has three principal terms: treatment pressure (HPCP), time (t) and t2. If carefully used, the model can reduce the number of challenge tests for validation of product safety.
The aim of this paper is to study the possibility to use the pertechnetate ion (99mTcO4−) as a radiotracer for the measurement of the residence time distribution (RTD) of the liquid phase in opaque anaerobic digesters. An anaerobic digester in a sugar wastewater treatment plant was selected to carry out this study. Laboratory tests showed that the measured redox potential of the medium was not enough for the chemical reduction of the species TcO4− to TcO(OH)2. However, sorption rate between 0.045 and 0.083% h−1 were observed, probably associated to the retention of the tracer by the biomass as a result of the metabolization and catalytic reduction due to microbiological activity. This phenomenon did not affect the estimation of the RTD. The selected tracer has been used to determine the RTD of the industrial digester in a sugar wastewater treatment plant and to model the flow behavior.
This paper presents a new approach to determine the properties of liquid and two phase boiling and condensing of two alternative refrigerant/absorbent couples (methanol–LiBr and methanol–LiCl), which do not cause ozone depletion for absorption thermal systems (ATSs) using artificial neural networks (ANNs). The back-propagation learning algorithm with three different variants and logistic sigmoid transfer function were used in the network. In order to train the neural network, limited experimental measurements were used as training and test data. In input layer, there are temperatures in the range of 298–498 K (with 25 K increase), pressures (0.1–40 MPa) and concentrations of 2, 7, and 12% of the couples; specific volume is in output layer. After training, it is found that maximum error is less than 3%, average error is about 1% and R2 values are 99.999%. As seen from the results obtained the thermodynamic properties have been obviously predicted within acceptable errors. This paper shows that values predicted with ANN can be used to define the thermodynamic properties instead of approximate and complex analytic equations.
Post-combustion concepts based on absorption of CO2 in aqueous amine solutions are considered the most mature technologies for CO2 capture from power plants. Several steady-state models of the absorption process exist. However, a dynamic model is required in order to study the behavior of the absorption process downstream of a power plant that operates at varying load. In this paper, a dynamic model of an absorber is presented and the results of two transient operational scenarios are shown; start-up and load reduction. In addition, issues regarding the operability of the absorber column in case of load-varying upstream power-plants are discussed. It is concluded that the present dynamic absorber model can be applied to study operability in absorber columns during the course of dynamic operation. However, a dynamic model of the total system is required in order to evaluate all operational challenges, such as load variation and high degree of heat integration between the power plant and the absorber/stripper plant.
A laboratory differential simulation method is used for the design of carbonization columns at coal-tar processing in which phenols are regenerated from phenolate solution by carbon dioxide absorption. The design method is based on integration of local absorption rates of carbon dioxide along the column. The local absorption rates into industrial phenolate mixture are measured in a laboratory model contactor for various compositions of the gas and liquid phases under the conditions that ensure the absorption rates in the laboratory absorber simulate the local rates in the industrial column. This condition is the same value of physical liquid-side mass transfer coefficients in the laboratory model contactor and in the absorption column. As the model contactor a stirred cell is used in which the interfacial area is well defined. By using an auxiliary surface stirrer the values of the physical mass transfer coefficient are extended up to the values encountered in packed absorbers. In experiments, carbon dioxide was absorbed into industrial phenolate mixture containing Na-phenolate of the phenol-mixture of the following approximate composition: phenol 44%, o-cresol 9%, p-cresol 10%, m-cresol 15%, 2,4-xylenol 2%, 2,5-xylenol 1%, 2,6-xylenol 0.5%, 2,3-xylenol 0.5%, 3,4-xylenol 1%, 3,5-xylenol 2%, o-ethyl-phenol 0.5%, m-ethyl-phenol 2%, p-ethyl-phenol 1.5%, others 11%. The experiments were performed in the following ranges of temperature 50–90 °C and pressure 100–700 kPa. An explicit formula correlating the absorption rates with carbonization of phenolate solution, partial pressure of carbon dioxide and temperature was derived which facilitated an assessment of various technological variants.
Absorption of SO2 from SO2/air mixture into sodium citrate buffer solution was performed using air-lift-tube (ALT) absorbers in laboratory and pilot scale. The aim of the study was to improve the efficiency of this regenerative process, to find the proper operation conditions in the air-lift-tube absorbers, and to contribute to the application of this process in the industry. The SO2-removal efficiency was measured while the gas velocity, the inlet SO2 concentration, the liquid hold-up, the sodium citrate buffer concentration and the liquid temperature were changed according to experimental design.Furthermore, the influence of oxidation on the SO2-removal efficiency was measured using SO2/N2 mixture instead of SO2/air mixture; the effect of stoichiometric ratio was investigated and the SO2 removal in one and two stages of tube absorbers was measured. The performance of the laboratory and pilot scale ALT absorbers was compared with that of other laboratory and pilot size absorption columns and of some non-regenerative methods.
Valid alternatives to the selective catalytic reduction (SCR) process for the removal of nitrogen oxides from waste gases (denoxing) may be found in wet scrubbing processes where the low solubility of NO in aqueous media is enhanced by the application of chelate complexes.The most widely used complex for this purpose is FeIIEDTA (ironII ethylenediaminetetra-acetic acid). IronII nitrilotriacetic acid (FeIINTA) might be another candidate, with similar chemical properties as FeIIEDTA but available industrially at a considerably lower price.For the development of technical processes, a knowledge of the equilibrium data over a range of temperature is of prime importance. Therefore experimental investigations were carried out concerning the chemical equilibrium of the reaction:NO + FeIINTA ⇌ FeIINTA · NOThe equilibrium constant was determined in the temperature range 22–80 °C. All experimental data fit the equilibrium constant equation well:Kp = 3.05 x 10-8 exp(7332/T) (bar-1)The equilibrium constant for FeIINTA is approximately 25% lower than the constant for FeIIEDTA. However, considering the price difference between the two complexants, FeIINTA is an interesting alternative to FeIIEDTA.
A simple mathematical model has been developed and validated for the dynamic behaviour of a pilot plant absorber with random packing (stainless steel construction, column diameter: 0.08 m, height of packing zone: 1.3 m, random packing: 15 mm ceramic spheres). The model is based upon film theory, axial dispersion, and rate enhancement. Simplifications are made which are justified for the conditions of the experimental investigations in the pilot plant of this work. Simulation of other conditions like for example acid gas removal in industrial applications will require additional consideration of water evaporation and of the energy balance. The experimental part concentrates on measurements of the residence time distribution of several sections of the pilot installation. Combination of the different transition functions allows the determination of the dynamic behaviour of the packing zone. The model has been validated by experiments with absorption of CO2 into water and into carbonate solution at moderate conditions (5 barabs, 290–333 K). Although the mass transfer parameters have been strictly prescribed according to theory, the simulation results agree well with experimental data.
Experimental values of the liquid-side mass transfer coefficient kL for absorption of gases of low solubility in a liquid film flowing down a smooth and a rough wetted wall and down an expanded metal sheet are compared with selected mass transfer models. It is shown that the set of published kL values available is insufficient to evaluate the most suitable model with which to describe the mass transfer mechanism. One of the distinguishing features of the various mass transfer models is the value of exponent m in the dependence of kL on molecular diffusivity, kL ∼ Dm. The range of published experimental values is 0.5 < m < 0.7; nevertheless, these results are insufficient for this purpose as well. It is shown that experimental determination of exponent m in the range of high Schmidt numbers could give new information about the phenomena very near the gas—liquid interface which is of importance when discussing the mass transfer mechanism in a liquid film during absorption. In discussing different models in this work special attention is given to the recently published film-penetration model proposed to evaluate kL for absorption into a liquid film on an expanded metal sheet.
Operating conditions
The oxidation of hydrogenated 2-ethyl-anthraquinone in a continuous stirred tank reactor has been studied. This reaction is considered usually as slow or moderately fast and both diffusion and reaction affect the overall rate. The interesting mass-transfer parameters: mass-transfer coefficient kL and the correlation constants of the volumetric mass-transfer coefficient kLa, were estimated with a steady-state model consisting of a set of implicit non-linear, algebraic equations. Ten screening experiments and five sequentially designed experiments based on the D-optimality criterion were carried out. The agitation speed, superficial gas velocity and the pressure served as the control variables. The response monitored was the concentration of unreacted hydroquinone in the effluent stream. The agreement between model prediction and experimental data was good. As all the model parameters could not be identified satisfactorily as such, some parameter transformations were necessary to achieve more accurate parameters. For satisfactory identification of the parameters, their sensitivities were viewed by plotting R2 contours of the fit with pairwise parameter intervals. The reaction-diffusion model was simplified by assuming pseudo first-order kinetics for the reaction taking place in the film. The estimated values of mass-transfer quantities correlate well with results published earlier.
Accurate dynamic models for industrial reactive absorption processes have to be both rigorous enough in order to reflect the process complexity and simple enough in order to ensure feasibility of process simulations. A comparison of different model approaches revealed that traditional equilibrium stage models and efficiency approaches are inadequate in this context. Therefore, we have developed a new rigorous dynamic two-phase model based on the two-film theory, which serves as a reference description and takes into account the influence of chemical reactions and additional driving forces in electrolyte systems on mass transfer, thermodynamic non-idealities and the impact of structured packings and liquid distributors on the process hydrodynamics. For the model-based control and dynamic on-line simulation some model reductions are required to limit the calculation time for the numerical solution. According to sensitivity study results, simplifications for some physical parameters and a linearisation of the film concentration profiles have been accomplished leading to a reduction of the total number of equations by half. The model has been applied to an industrial sour gas purification process with many parallel and consecutive reversible chemical reactions within a multicomponent system of aqueous electrolytes and validated by pilot plant experiments.
A mixing length turbulence model is presented, which was used for the numerical calculation of turbulent falling film heat and mass transfer coefficients. Numerous proposals of different authors for the prediction of eddy transport coefficients are discussed. Good agreement between calculated heat and mass transport coefficients and experimental data was achieved by the combination of Hubbard, Mills and Chung's proposal for the calculation of eddy transport near the film surface with a modified form of van Driest's, the latter being for the eddy transport near the solid wall. Furthermore, the dependence of the turbulent falling film heat transfer coefficients on the Prandtl number is discussed. It is shown that, besides the Prandtl number, a further dimensionless group must be considered in order to describe the effect of liquid properties on heat transfer.ZusammenfassungZur numerischen Vorausberechnung von Wärme- und Stoffübergangskoeffizienten für turbulente Rieselfilme wird ein Mischungsweg- Turbulenzmodell benutzt. Zur Berechnung der dabei benötigten turbulenten Transportkoeffizienten werden verschiedene Vorschläge aus der Literatur diskutiert. Eine gute Übereinstimmung zwischen berechneten und gemessenen Wärme- und Stoffübergangskoeffizienten liefert die Kombination der Berechnungsvorschläge von Hubbard, Mills und Chung für den turbulenten Transport in der Nähe der Filmoberfläche mit demjenigen von van Driest für die Wandnähe, wobei letzterer modifiziert wird. Schließlich wird für die turbulente Filmströmung die Abhängigkeit der Wärmeübergangskoeffizienten von der Prandtlzahl diskutiert und gezeigt, daß auber der Prandtlzahl noch mindestens eine weitere Kenngröße für den Stoffwerteinfluß herangezogen werden muß.
Absorption of SO2, diluted with nitrogen up to partial pressures of 500 Pa, was performed at 20 °C and 1 atm with water and sulphuric acid solutions containing hydrogen peroxide in a cable contactor. The absorption rate was determined for various SO2 partial pressures and different concentrations of sulphuric acid (within the range 0–40 wt.%) and hydrogen peroxide in the scrubbing liquid. It was found that the SO2 absorption rate increases with the hydrogen peroxide concentration and decreases as the acidity of the scrubbing liquid increases. Experimental results were interpreted according to the two-film theory of absorption with chemical reaction, considering fast irreversible reaction of SO2 with H2O2. This led to the determination of an overall kinetic parameter, depending on the sulphuric acid concentration, which can be used to design absorption towers. A kinetic constant, which decreases strongly with the rise of cH2SO4, was deduced thanks to the estimation of diffusivities and Henry's coefficients of SO2 for sulphuric solutions.
For a physically correct analysis (and prediction) of the effect of fine, dispersed phase drops or particles on the mass transfer rate in multiphase systems, it was demonstrated that only 3-D instationary, heterogeneous mass transfer models should be used. Existing models are either homogeneous, stationary or single particle models. As a first step, a 1-D, instationary, heterogeneous multi-particle mass transfer model was developed. With this model the influence of several system parameters was studied and problems and pitfalls in the translation of modeling results for heterogeneous models into a prediction of absorption fluxes are discussed. It was found that only those particles located closely to the gas–liquid interface determine mass transfer. For these particles the distance of the first particle to the gas–liquid interface and the particle capacity turned out to be the most important parameters. Comparisons with a homogeneous model and experimental results are presented. Typical differences in results comparing a homogeneous model with the 1-D heterogeneous model developed in this work could be attributed to a change in the near interface geometry. Future work in this field should therefore be directed towards near interface phenomena. Three dimensional mass transfer models, of which a preliminary result is presented, are indispensable for this.
For wet denitrification processes nitrogen monoxide is the crucial component owing to its low water solubility. By addition of transition-metal complexes, able to form nitrosyls, the effective NO concentration in the liquid phase is enhanced. Kinetic (reaction orders and rate constants) as well as thermodynamic (stability constants) data for nitrosylation have been established. It has been found that Fe(II)-EDTA and Fe(II)-NTA react very fast to form stable NO complexes and are widely pH independent. The formation of Fe(III)-EDTA nitrosyl is found to be rapid, but the large deviation in the measured data prevents reliable evaluation. While the equilibrium constants of the Co(II)-trien and Co(II)-tetren nitrosyls are largely pH independent, the rate of formation is influenced markedly by pH. Each nitrosyl shows individual behavior towards sulfite. Fe(II)-EDTA and Fe(III)-EDTA exhibit the highest absorption capacities. The CO(II) polyamines convert the absorbed NO mainly to gaseous N2O rather than to liquid-phase products.
In the last years chemical process industries have shown permanently increasing interest in the development of reactive separation processes (RSP) combining reaction and separation mechanisms into a single, integrated unit. Such processes bring several important advantages among which are increase of reaction yield and selectivity, overcoming thermodynamic restrictions, e.g. azeotropes, and considerable reduction in energy, water and solvent consumption. Important examples of reactive separations are reactive distillation (RD) and reactive absorption (RA). Due to strong interactions of chemical reaction and heat and mass transfer, the process behaviour of RSP tends to be quite complex. This paper gives an overview of up-to-date reactive separation modelling and design approaches and covers both steady-state and dynamic issues. These approaches have been applied to several different RA and RD processes including the absorption of NOx, coke gas purification, methyl acetate synthesis and methyl tertiary butyl ether (MTBE) synthesis.
A review is presented on the gas–liquid mass transfer enhancement due to the presence of a second dispersed liquid phase. An attempt has been made to describe the mass transfer characteristics in a gas–liquid–liquid system. The ability of an immiscible oil phase to influence the possible pathway for gas transfer from the gas phase to the aqueous phase and to affect the gas–liquid interface and the volumetric mass transfer coefficient kLa is considered. Though the mass transfer in series looks the most logical explanation, there are many gaps and contradictions in the reported results of kLa, preventing any definite conclusion being reached. An enhancement factor (E), which quantifies the effect of the oil addition on the gas–liquid mass transfer, is defined. Experimental enhancement factors are reported and compared to the theoretical maximum attainable enhancement factor (Emax). Possible mechanisms (“bubble covering”, “shuttle effect” and “permeability effect”) involved in mass transfer enhancement are assessed in detail. The commonly used “shuttle effect” mechanism, whose model proposes a direct usable expression of the enhancement factor, underestimates the reported experimental enhancement factors by about 20%. However, to date, it is not possible to satisfactorily propose a unique theory explaining the influence of the presence of an immiscible oil on mass transfer enhancement. Moreover, the development of sophisticated models has not yet reached satisfactory levels. Recommendations have been made for future research.
Absorption efficiency of sulphur dioxide in spray dry absorption depends on the superposition of the absorption process with the drying process. A model has been established considering heat and mass transfer processes for a single droplet and the two phase flow inside the spray dryer. Model predictions are compared with experimental data showing the influence of drying conditions and stoichiometric ratio on the absorption efficiency.
The absorption of single gas bubbles in water preloaded with another gas is investigated theoretically and experimentally. The absorption process is determined by the solubility coefficients, the liquid-side mass transfer coefficients, the gas concentration in the water, the interfacial area and the densities of both phases. During an experimental run the change of the radius of a single gas bubble versus time is determined photographically. In a recent publication, the authors demonstrated that the absorption of pure gas bubbles is enhanced at the beginning of the absorption process. Applying this result to the present case of multicomponent diffusion, the experiments can be described well analytically.ZusammenfassungDie Absorption von einzelnen Gasblasen in Wasser, welches mit anderen Gasen vorbeladen ist, wird theoretisch und experimentell untersucht. Die Absorptions- und Desorptionsgeschwindigkeiten werden durch die Löslichkeitskoeffizienten, die flüssigseitigen Stoffübergangskoeffizienten, die Gaskonzentration, die Oberfläche der Gasblase und die molaren Dichten beider Phasen bestimmt. Die Ergebnisse der Berechnungen werden mit den Ergebnissen von Messungen verglichen, bei denen die Volumenänderung von einzelnen Gasblasen in mit anderen Gasen vorbeladenem Wasser fotografisch vermessen wurde. Die in einer früheren Veröffentlichung berichtete anfängliche Beschleunigung bei der Absorption einzelner Gasblasen aus reinen Gasen tritt auch bei der Absorption von Gasgemischen auf. Werden these Ergebnisse bei der Berechnung entsprechend berücksichtigt, so kann der Absorptionsverlauf von Gasgemischen näherungsweise beschrieben werden.
The paper presents a model of surface phenomena accompanying the absorption of carbon dioxide into aqueous solutions of monoethanolamine. The model is based on the assumption that the cellular convection is driven by surface tension gradients, induced in turn by infinitesimally small perturbations of concentration.The model derived makes it possible to study in detail certain characteristics of the process of chemical absorption. Thus, the probability of oscillatory modes occurring in the system is analysed, the dimensions of the convective cells are evaluated and the gas—liquid contact time necessary for the instability to appear is determined. The quantitative conclusions are compared with the relevant experimental data concerning the absorption of CO2 into monoethanolamine in both wetted-wall and packed columns.
Temperature as a function of time during freeze-drying.
Moisture content as a function of time.
Redydration kinetics of acerola.
Several quality parameters, such as water activity (aw), glass transition temperature (Tg), Vitamin C content, shrinkage and rehydration capacity were investigated for the freeze-drying of acerola fruits. The variation of temperature with time at different positions of the samples was measured during the freezing of samples, performed prior to the freeze-drying. Drying kinetic curves were obtained for different types of samples. The extent of shrinkage after freeze-drying was investigated and related to the glass transition temperature, Tg. The variation of water activity (aw) along the drying and sorption isotherms was obtained for different freezing techniques. It was observed that freeze-dried acerola fruits can be easily reconstituted, and important nutritional parameters are well preserved after the process. They may be considered as a good source of Vitamin C, whose content was best preserved for freeze-drying of fruits at an intermediary stage of ripening.
In this article, a systematic study of the separation of the n-hexane–ethyl acetate mixture with an entrainer by heterogeneous azeotropic batch distillation is performed. Based upon the thermodynamic behaviour of the ternary mixtures, potential entrainers partially miscible with one or two original azeotropic components are chosen. In all cases, the entrainer adds a heterogeneous binary or ternary azeotrope that is the lowest boiling point in the ternary diagram. Therefore, it leaves the column by the overhead stream which is subcooled to get two liquid phases in the decanter. The phase with the highest amount of the original component is removed as distillate product whereas the entrainer-rich phase is continuously refluxed to the column. Considering methanol, acetonitrile, water and nitromethane as heterogeneous entrainers, screening was performed based on the composition of the unstable heteroazeotropic mixture, the ratio of both liquid phases in the condensed top vapour and the purity of the distillate product determined by the liquid–liquid envelope at the decanter temperature. The process feasibility analysis is validated by using rigorous simulation with the batch process simulator ProSimBatch. Simulation results are then corroborated in a bench experimental column for the selected entrainer, showing several advantages of heterogeneous batch distillation (HBD) compared to homogeneous systems.
The ethyl acetate synthesis via reactive distillation is studied theoretically and experimentally using different catalytic packings. Experiments are carried out at laboratory scale in a 50 mm diameter column with a packing height of 3 m, and at semi-industrial scale in a 162 mm diameter column with a packing height of 12 m. The experimental set-up is similar for both cases. The commercially available packings studied are KATAPAK®-S and two different variants of MULTIPAK®. Modelling is performed with a rate-based stage model. The simulation environment ASPEN Custom Modeler™ is used for the implementation and solution of the model equations. The results of the rate-based simulations agree well with the corresponding experimental results. In addition, suitable operating conditions and the influence of the selected catalytic internal on conversion and product purity are investigated. The developed model enables the scale-up from laboratory to industrial size columns, based on the respective packing characteristics.
n-Hexyl acetate esterification was carried out in a reactive distillation pilot scale column (diameter 162 mm, overall packing height 11.6 m). Sulzer Katapak®-S-500.Y filled with Amberlyst CSP2 was used as catalytic internal. The new experiments extend an investigation of Schmitt et al. [1] that was carried out in the same column with Sulzer Katapak®-S-250.Y. The comparison of both series illustrates the influence of modifying the catalytic internals in pilot plant scale. Simulations with a stage-based model, that includes reaction kinetics, show good agreement with the experimental data. The characteristics of both catalytic internals are correctly predicted. Experience with the formation of 1-hexene as a by-product is discussed.
The sorption isotherms for three organic solvents on PVAc have been measured via the isopiestic method using a simple inexpensive device with a spring inside the measuring chamber as the force sensor. A robust steel spring was used successfully instead of the sensitive quartz spring described in the literature. The sample was a thin aluminium foil coated with a thin polymer layer to yield a short diffusion path length and sufficient equilibrium load at an acceptable tare mass. This enabled equilibrium to be attained within 1–2 h.The experimental data may be well described by the Flory—Huggins theory. The interaction parameter was fitted as a linear function of the volume fraction. Temperature was of no significant influence within the range 20–50 °C or at 60 °C, respectively. The sorption isotherms can also be predicted by the UNIFAP (UNIFAC for polymers) method without the use of experimental data. PVAc exhibits highly sorptive behaviour, e.g. 10 wt.% methanol at 80% activity. The measuring device employed may be useful for the study of adsorbents that can be coated as a film onto aluminium foil or exist as a thin film themselves (e.g. membranes).
The condensation reaction of furfural (F) on acetone (Ac) gives a high added value product, the 4-(2-furyl)-3-buten-2-one (FAc), used as aroma in alcohol free drinks, ice, candies, gelatines and other products of current life. This synthesis valorises the residues of sugar cane treatment since furfural is obtained by hydrolysis of sugar cane bagasse followed by vapor training extraction. In the face of numerous and complex reactions involved in this synthesis, it is very complicated to define the kinetic laws from exact stoichiometry. A solution allowing to cope the problem consists in identifying an appropriate stoichiometric model. It does not attempt to represent exactly all the reaction mechanisms, but proposes a mathematical support to integrate available knowledge on the transformation. The aim of this work is the determination of stoichiometric and kinetic models of the condensation reaction of furfural on acetone. Concentrations of reagents and products are determined by gas and liquid chromatography. Concentration profiles obtained at different temperatures are used to identify kinetic parameters. The model is then used for the optimization of the production of FAc. The interest of such tool is also shown for the scale up of laboratory reactor to a large scale. The anticipation of the reaction behaviour in large scale is crucial especially when the reactor presents important limitations of thermal exchange capacity.
The consecutive condensation, dehydration and hydrogenation of acetone were studied in a lab scale trickle bed reactor. Amberlyst® CH28, a commercial palladium impregnated cation exchange resin catalyst was used. Acetone conversions varying from 25 to 50% were obtained at operating temperatures from 130 to 150 °C and acetone LHSVs from 4 to 16 h−1. The methyl isobutyl ketone (MIBK) selectivities varied between 70 and 90%. The results indicated that the condensation–dehydration rate-limiting step severely decreases with increasing conversion, causing a serious decrease in catalyst productivity at higher liquid residence times. This is attributed to the rate-inhibiting effect of the water—formed in the dehydration reaction—on the catalyst. The mesityl oxide (MSO) content in the reaction mixture was below 0.1% (mass) except at temperatures above 140 °C and at high condensation–dehydration rates. It was further shown that an acetone saturated catalyst bed has an 18% smaller volume than an equivalent water saturated bed.
Potential applications of ionic liquids (ILs) for the green separation process of acetylene in ethylene and for the storage of acetylene were investigated. To deal with this proposal, the solubilities of the unsaturated hydrocarbons in various ionic liquids were evaluated. The solubility of ethylene shows a solubility parameter-dependent behavior as indicated by the proportional relationship between the natural log value of Henry's law constant and the inverse molar volume of ILs. This correlation suggests the most important role of voids formation within IL to accommodate the solutes and the applicability of regular solution theory to model the solubility behavior. Whereas, in addition to the free-volume contribution of ILs, the solubility of acetylene is largely controlled by a specific solute–solvent interaction as a result from the association of the acidic hydrogen character in acetylene and the relative basicity of the anion. Those two different solubility behaviors result in a high absorption selectivity of acetylene over ethylene in the basic ILs. 1H NMR experiment clearly demonstrated the presence of a substantial interaction between the acetylene and the anion of IL. Interestingly, this solute–solvent interaction is reversible as indicated by the absorption–desorption test of acetylene in [BMIM][Me2PO4].
Activated carbon fibers (ACFs) based on two precursors, namely viscose rayon and phenolic resin, were prepared by carbonization followed by physical activation with steam or CO2. The prepared ACF samples were examined for their suitability of adsorbing common atmospheric gaseous pollutants, SO2 and benzene, toluene, and m-xylene (BTX), under non-equilibrium conditions in a fixed bed tubular reactor. The textural and surface properties of the ACF samples were correlated with the adsorption characteristic of ACF for BTX and SO2. The adsorption of BTX was found to increase with increase in either the BET area or micropore volume. In contrast, the surface groups containing electronegative oxygen atoms were found to have adverse impact on BTX adsorption. The adsorption of SO2 was found to decrease with increase in the O-contents. The FTIR spectra and the elemental analysis revealed that the extent of oxygen groups (OH, COOH, CO, etc.) was larger in the steam-activated samples than in the CO2-activated samples, regardless of the type of the precursor. The results obtained here are germane to a quantitative tailoring of activation routes for optimal performance of ACFs in pollution control.
The efficiency of waste acorn of Quercus ithaburensis (WAQI) as an adsorbent for removing Cr(VI) ions from synthetic wastewater has been studied. Batch adsorption experiments were carried out as a function of pH, adsorbent mass, Cr(VI) ion concentration, agitation rate and temperatures. Maximum metal sorption was found to occur at initial pH 2.0. The adsorption capacity of WAQI was found to be 31.48 mg g−1 for initial Cr(VI) concentration of 400 mg L−1 at 25 °C. Batch adsorption models, based on the assumption of the pseudo first-order, pseudo second-order mechanism and Elovich equation were applied to examine the kinetics of the adsorption. The results showed that kinetic data were followed fitting the pseudo second-order model than the pseudo first-order and Elovich equation.Thermodynamic parameters such as ΔH°, ΔS° and ΔG° were calculated. The adsorption process was found to be endothermic and spontaneous. The results were analyzed by the Langmuir, Freundlich, Dubinin–Radushkevich (D–R), Temkin, Frumkin, Harkins–Jura, and Smith equation using linearized correlation coefficient at different temperature. The characteristic parameters for each isotherm have been determined. Models and the isotherm constant were evaluated depending on temperature. Langmuir and Freundlich equation is found to best represent the equilibrium data for Cr(VI) ions–waste acorn systems.
The effects of liquid properties on the hydrodynamics of bubble columns were investigated experimentally through analysis of acoustic sound measurements, using coalescence and non-coalescence mediums. The hydrodynamics parameters such as gas holdup, average bubble radius, gas bubbling rate, root mean square of the sound pressure and damping ratio of the bubble pulsation were investigated at the sparger and bulk regions. The acoustic study revealed that the addition of carboxymethyl cellulose (CMC) and xanthan gum (XG) in small percentage increased the overall gas holdup and reduced the average bubble radius. Moreover, the bubbling rate for these solutions is lower than that for water at low superficial gas velocities. These observations were more apparent in the CMC case. The injection of KCl and silicon polymer substances however, resulted in reduction of the gas holdup and enlargement of the bubble size. In addition, the bubbling rate for the KCl and silicon polymer solutions is found to be superior to that for water. In addition, it is found that acoustic measurements can be used to detect sparger activity for both air–water and non-Newtonian solutions systems. At low superficial gas velocity, the sparger acts as a nozzle, hence heterogeneous bubble size distribution was observed and detected. At high superficial gas velocity, the sparger becomes fully activated and consequently homogeneous size distribution was detected.
Passive acoustic sound, measured by a hydrophone in an air–water bubble column, is used to study the hydrodynamics of the unit. The recorded measurements taken at different superficial gas velocities are processed using both spectral and chaos-based techniques in order to characterize the column flow regimes and to predict the transitions points. These processing tools were supported by digital video imaging of bubbles motion inside the column. The results of data analysis indicate the applicability of passive sound measurements to identify flow regimes in the bubble column. In this regard the analysis of sound spectra gives a useful qualitative comparison of flow regimes. Chaos-based techniques, on the other hand, are more successful in predicting the transition points between the homogenous and the churn-turbulent flow regimes in the column. The critical gas velocities of the transition are associated with a marked change in some of the calculated chaotic invariants of sound pressures. The calculated superficial gas velocities of the critical points are also found to be consistent with experimental observations. Moreover, a useful visualization of the dynamics induced in the column by the alternation of small and large bubbles is possible through the inspection of phase-space trajectories reconstructed from time-series measurements. The shape and size of the trajectories are closely linked to the size distribution of bubbles, and they change as the flow moves from one regime to an other.
A calculation procedure for two-dimensional elliptic flow is applied to predict the pressure drop and heat transfer characteristics of laminar and turbulent flow of air across tube banks. The turbulence model used involves the solution of two partial differential equations, one for the kinetic energy of the turbulence and the other for its dissipation rate. These differential equations are solved simultaneously with those for the conservation equations of mass, momentum and energy using an implicit finite volume procedure. The numerical methodology utilizes the stepped boundary technique to approximate the tube surface which is kept at constant temperature. The computations are extended to cover the case of two rows of tubes undergoing cross flow with in-line and staggered tube arrangements besides the case of a single row. Thereby, Reynolds number (Re) as well as the normal and parallel tube spacing-to-diameter ratios are varied. Effects of the flow and the geometry parameters on the friction factor and the local and global Nusselt number are presented. Moreover, velocity vector diagrams and temperature contours as well as axial flow velocity and turbulence kinetic energy profiles along the flow field upstream, over and downstream the tubes are also given. The theoretical results of the present model are compared with previously published experimental data of different authors. Satisfactory agreement is demonstrated.
This paper presents the physico-chemical characteristics of coconut-based activated carbon of commercial grade (ACC) and the sorption behaviour and kinetics of adsorption of toxic metal ions onto ACC. The proximate and CHN analysis of the ACC showed the presence of ∼41% carbon, ∼16% hydrogen and ∼2% nitrogen in ACC. Bulk density and heating value of ACC were found to be 599.32 kg/m3 and 18.81 MJ/kg, respectively. The BET surface area was 171.05 m2/g whereas the BET average pore diameter was 31.03 Å. The meso-porous surface area was found to be 76% and the micro-porous surface area was 24% of the total pore surface area. The polar groups present on the ACC surface impart considerable cation exchange capacity to it. ACC worked as a very good adsorbent at an optimum initial pH (pH0) of 6.0 and at a dose of 20 g/dm3 for the adsorption of cadmium (Cd(II)), nickel (Ni(II)) and zinc (Zn(II)) metal ions at a concentration of 100 mg/dm3. The adsorption of metal ions onto ACC was found to be a gradual process and the quasi-equilibrium condition reached in 5 h. The adsorption followed pseudo-second-order kinetics. The effective diffusion coefficient was of the order of 10−12 m2/s.
This study was to investigate the impacts of operating conditions and liquid properties on the hydrodynamics and volumetric mass transfer coefficient in activated sludge air-lift reactors. Experiments were conducted in internal and external air-lift reactors. The activated sludge liquid displayed a non-Newtonian rheological behavior. With an increase in the superficial gas velocity, the liquid circulation velocity, gas holdup and mass transfer coefficient increased, and the gas residence time decreased. The liquid circulation velocity, gas holdup and the mass transfer coefficient decreased as the sludge loading increased. The flow regime in the activated sludge air-lift reactors had significant effect on the liquid circulation velocity and the gas holdup, but appeared to have little impact on the mass transfer coefficient. The experimental results in this study were best described by the empirical models, in which the reactor geometry, superficial gas velocity and/or power consumption unit, and solid and fluid properties were employed.
The purpose of the present work is to investigate electrothermal swing adsorption (ESA) process with an activated carbon monolith as adsorbent. Experiments are performed with toluene as VOC. Several parameters are investigated both experimentally and through a mathematical model. It is shown that the monolith, as a carbon material, behaves as a semi-conductor. Its resistivity decreases as temperature and amount adsorbed increase. This leads to different types of evolution of dissipated electrical power during desorption at constant current intensity. The concentration of the desorbed VOC as a function of time has the shape of a peak followed by a tail. This shape is interpreted as a dispersive wave in the sense of the equilibrium theory of adsorption columns. The performance of the process is strongly dependent on the operating conditions. The maximal (that is initial) concentration increases almost linearly with current intensity and purge gas flow rate, and increases also with preheating duration. The purge gas flow rate has mainly an effect of dilution. The efficiency of desorption (% VOC desorbed) is almost constant in average with different preheating times. It increases with current intensity and gas purge flow rate.
This study investigates the physical–chemical characteristics of sludge treated with controlled levels of ultrasound. The results indicate that the energy used for sonication strongly influences the physical–chemical characteristics of sludge. Based on turbidity and settling velocity measurements, 1000 kJ/kg total solids (TSs) is recommended as an optimal specific energy input for improving sludge settling. For purposes of sludge disintegration, applying higher specific energies to the sludge is more efficient than applying lower levels. Nevertheless, a single, optimal energy level for thoroughly disintegrating sludge could not be determined. Possible mechanisms of ultrasonic treatment are also discussed.
In order to enhance the efficiency of anaerobic digestion, the effects of ultrasounds, ozonation and thermal pre-treatment have been studied on waste activated sludge. The feature of this study was to carry out the comparison of the three pre-treatments in the same conditions and on the same sludge sample. Each treatment was tested in two conditions close to optimum conditions to maximise batch anaerobic sludge biodegradability. All treatments led to chemical oxygen demand and matter solubilisation and had little influence on mineral matter. In terms of solubilisation thermal pre-treatment was better than sonication or ozonation. But, in terms of batch anaerobic biodegradability, best results were obtained with ultrasounds with an energy of 6250 or 9350 kJ/kg TS and a thermal treatment at 170 or 190 °C. Moreover, treatments had effects on physico-chemical characteristics of sludge samples: apparent viscosity decreased after all treatments but the reduction was more important with thermal treatment. Median diameter of sludge flocs were reduced after sonication, increased after thermal treatment and did not change after ozonation. Finally, capillary suction time (CST) increased after ozonation, increased highly after sonication and was reduced after thermal treatment.
Absorption and desorption equilibria of three organic vapors on two kinds of activated carbon are investigated theoretically and experimentally. The application of the adsorption integral equation to microporous adsorbents is discussed. It is shown that the Dubinin theory of volume filling of micropores (TVFM) provides an improved method for describing the adsorption of organic vapors in the micropores of the carbon. From adsorption and desorption experiments, the transition from micropore filling to capillary condensation in mesopores can be observed. The corresponding adsorption potentials are related to a pore radius of about 1.6 nm. A modified Dubinin-Astakhov equation is proposed, which takes into account the end of micropore filling and the characteristic curves by means of a limiting potential AGr. This equation describes the isothermal field of all six investigated systems very well in a wide temperature range (20 - 150 °C) with temperature-invariant parameters.
Top-cited authors
Parag R Gogate
  • Institute of Chemical Technology, Mumbai
Shirish H. Sonawane
  • National Institute of Technology, Warangal
Patrizia Perego
  • Università degli Studi di Genova
Timothy James Mason
  • Coventry University
Erika Ortiz
  • ICESI University