Showing papers in "The Chemical Engineering Journal and The Biochemical Engineering Journal in 1995"
TL;DR: In this article, a comparison between the results of the numerical models (Fick and dusty-gas) and the effectiveness-factor concept was made between the two models, and it was concluded that pressure in porous catalyst with a plane, line, or point of symmetry did not affect the fluxes seriously, and, therefore, the pressure-driven flow can be omitted from the flux expression without significant loss of accuracy.
Abstract: In the present study, mass transport accompanied by chemical reactions in porous media is studied according to the Fick model and the dusty-gas model. For mass transport accompanied by a chemical reaction in catalyst structures showing a plane, line, or point of symmetry, the approximate analytical concept of an effectiveness factor, accounting for intraparticle diffusion, was also evaluated. For a variety of reaction schemes and kinetic rate equations, a comparison was made between the results of the numerical models (Fick and dusty-gas) and the effectiveness-factor concept.
From the results it was concluded that pressure in porous catalyst with a plane, line, or point of symmetry did not affect the fluxes seriously, and, therefore, the pressure-driven flow can be omitted from the flux expression without significant loss of accuracy. Furthermore, both for single and multiple reactions, the Fick model is satisfactorily accurate to estimate the transport rate in all cases, and the results deviate only slightly from the dusty-gas model. It should be noted that this latter model requires substantially more computational time.
For catalytic membranes, however, transport of inert components as well as large trans-membrane pressure differences may be present, which affect the transport of the reactants and products. The calculations showed that, in contrast to the above-mentioned structures, in this case the dusty-gas model has to be used to describe the transport.
170 citations
TL;DR: In this article, the optimum conditions for heavy metal removal in an activated sludge bacterium were determined. But the results were limited to the case of lead, copper, nickel, and iron.
Abstract: The biosorption of lead(II), copper(II), nickel(II), and iron(III) ions on Zoogloea ramigera, an activated sludge bacterium, was studied with respect to adsorption pH and temperature in order to determine the optimum conditions for heavy metal removal. Optimum initial pH for the biosorption of lead(II), nickel(II) and copper(II) ions by Z. ramigera was determined as 4.0–4.5 whereas higher biosorptive uptake of iron(III) ions by Z. ramigera was obtained at pH 2.0. Maximum biosorption rates of nickel(II) and copper(II) ions by Z. ramigera where obtained at 25°C, while the initial biosorption rates and the adsorptive capacity of the biomass for lead(II) and iron(III) ions increased with increasing temperatures in the range 25–45°C. The adsorption isotherms were developed for optimum conditions and it was seen that the adsorption equilibrium data fit both Freundlich and Langmuir isotherms within the metal ion concentrations studied (25–200 mg 1−1). The adsorption constants for lead(II) and iron(III) were higher than those for nickel(II) and copper(II) for both Langmuir and Freundlich models.
166 citations
TL;DR: In this article, the effects of turbulent mixing of incompressible fluids in single-phase systems on the course of chemical reactions are discussed in detail: the fluid mechanical interpretation of turbulent micromixing (effect of fluid element deformation on the acceleration of molecular diffusion, engulfing of environment, inertial disintegration of large eddies and local intermittency) is presented.
Abstract: The idea of micromixing, its definition and measures are outlined. The concepts of mixing environments and mixing earliness are presented. The paper concentrates on the effects of turbulent mixing of incompressible fluids in single-phase systems on the course of chemical reactions. The processes of turbulent micromixing are discussed in detail: the fluid mechanical interpretation of turbulent micromixing (effect of fluid element deformation on the acceleration of molecular diffusion, engulfing of environment, inertial—convective disintegration of large eddies and local intermittency) is presented. It is concluded that stretching of material elements and vortices, accompanied by molecular diffusion results in the growth of the mixing zones. The groth of the zone mixed on the molecular scale is a characteristic feature of micromixing and should be included in micromixing modelling. The characteristic time constants for the consecutive stages of mixing are presented and compared with the characteristic time for chemical reaction — the numerical criteria are outlined. The two approaches, i.e. eulerian and lagrangian, are described; it is shown that each requires different methods of description and generates specific problems (closure problem, problem of environment). The applications of the micromixing theory to the most important fields of industrial practice, such as complex reactions, precipitations and polymerizations, are outlined.
156 citations
TL;DR: In this paper, the transition velocity Uc corresponding to the maximum rms amplitude of pressure fluctuations in a 102 mm diameter fluidized bed was found to be a strong function of measurement method.
Abstract: The transition velocity Uc corresponding to the maximum rms amplitude of pressure fluctuations in a 102 mm diameter fluidized bed was found to be a strong function of measurement method, while the other commonly used transition velocity Uk depends on the solids return system and is not a well defined parameter. Different results and trends are obtained for Uc depending on whether one uses absolute or differential pressure fluctuations and whether or not one normalizes by the time-mean local or differential pressure. For differential pressure fluctuation measurements, a higher Uc was obtained from the dimensionless standard deviation normalized by the local pressure drop, and the value decreased with height. For local voidage fluctuation measurements using an optical fibre probe, a maximum point also appeared in the standard deviation curve, although the curve was much flatter than corresponding pressure fluctuation curves. Uc corresponding to zero skewness of voidage fluctuations on the axis of the column gave a value close to Uc from differential pressure fluctuation measurements at the same level.
111 citations
TL;DR: In this paper, the behavior of friction coefficients in the liquid and solid phases of ion exchange is explored, and it is shown that friction coefficients between counter-ions and the charged matrix are similar to those between uncharged species.
Abstract: An ion exchange, water and several ions diffuse simultaneously, with different velocities. They are driven by activity, electrical and pressure gradients. We describe these complicated processes with the Maxwell-Stefan equation. This equation for multi-component diffusion requires one diffusivity or friction coefficient for each pair of components in the mixture. In this article, we explore the behaviour of these coefficients in the liquid and solid phases of ion exchange. Friction coefficients between ions and liquid are similar to those between uncharged species. They increase with the size of the ions, and depend only weakly on composition. Friction coefficients between positive and negative ions are much larger, especially between ions with multiple charges. They decrease with increasing concentrations. Ions of like charge usually have negative friction coefficients; their behaviour mirrors that of ions with unlike charges. The many coefficients in the solid can only be obtained by combining data on ion exchange and electrodialysis membranes. In the matrix, we can estimate friction coefficients with water from free solution values and a ‘tortuosity’ correction. Friction between counter-ions and the charged matrix is important. It is much larger than the (free solution with tortuosity) prediction. This is especially so for ions with multiple charges. This type of friction also seems to depend on the polymer morphology. At the high concentrations in the matrix, friction coefficients between different counter-ions with a like charge are usually positive. The friction can be appreciable. Our knowledge of friction coefficients within the solid is still incomplete; we end with a few remarks on this.
86 citations
TL;DR: A mathematical model for transient membrane mass transport has been developed using the Maxwell-Stefan formulation as mentioned in this paper, which is able to predict the observed maximum in the flux of methane, and if we assume that the Fick diffusivity is constant the transient permeation fluxes are predicted to reach their steady state values monotonically.
Abstract: Published experimental data on transient permeation fluxes of light hydrocarbons through a silicalite-1 membrane show curious behaviour. For a mixture of methane and n -butane, for example, the transient experiments showed that initially the permeation flux of methane is higher than for n -butane but that this methane flux reduces to a lower steady state value. The n -butane flux is initially low but under steady state conditions is much higher than that for methane. A mathematical model for transient membrane mass transport has been developed using the Maxwell—Stefan formulation. This model is able to predict the observed maximum in the flux of methane, and if we assume that the Fick diffusivity is constant the transient permeation fluxes are predicted to reach their steady state values monotonically. Thus the Fick formulation fails even at the qualitative level to explain the experimental results. It is concluded that the Maxwell-Stefan formulation is indispensable for the description of mass transport across zeolite membranes.
82 citations
TL;DR: In this article, the authors used a transputer-based 8-electrode capacitance tomography system for imaging gas bubbles in a fluidized bed in the vicinity of an air distributor plate.
Abstract: This paper describes the use of a transputer-based 8-electrode capacitance tomography system for imaging gas bubbles in a fluidized bed in the vicinity of an air distributor plate. The quantitative results show how the solid concentration distribution varies as a function of time for three different flow regimes: bubbling, slugging and the transition to turbulent. Bubble shape, length and coalescence can be observed.
78 citations
TL;DR: In this paper, a new correlation was proposed to estimate the surface tension of organic liquids, based on predicting the data at the normal point and subsequent extension to other temperatures, and the average and maximum absolute deviations in the estimated values for forty liquids (115 points) were found to be 3.9% and 17% respectively.
Abstract: A new correlation is proposed to estimate the surface tension of organic liquids. The correlation is based on predicting the data at the normal point and subsequent extension to other temperatures. The inputs required are the normal boiling point, the critical temperature and the critical pressure. The average and the maximum absolute deviations in the estimated values for forty liquids (115 points) are found to be 3.9% and 17% respectively.
75 citations
TL;DR: This gas stream treatment process improves the quality and caloric value of the biogas and increases the methane content through the use of a chemo-autotrophic methanogen, uncoupled methanogenesis techniques and hollow fiber membranes.
Abstract: Off-gas from anaerobic digestion and landfills has significant potential as an alternative energy source. Current technologies to purify off-gas and increase its caloric value have been primarily limited to physicochemical methods. An alternative biological method has been proposed that increases the methane content. Through the use of a chemo-autotrophic methanogen (Methanobacterium thermoautotrophicum), uncoupled methanogenesis techniques and hollow fiber membranes, carbon dioxide is converted to methane and hydrogen sulfide is effectively removed from biological off-gases. This gas stream treatment process improves the quality and caloric value of the biogas. A continuous culture bench-scale system that utilizes hollow fiber membranes was employed to study the process. The gas-phase methane concentrations were found to increase from 60% to 96%.
75 citations
TL;DR: In this article, single-component permeation data are given for methane, ethane, propane, ethene and propene through a silicalite-1 membrane of approximately 40 μm thickness at 293 K as a function of their partial pressure.
Abstract: Single-component permeation data are given for methane, ethane, propane, ethene and propene through a silicalite-1 membrane of approximately 40 μm thickness at 293 K as a function of their partial pressure. The permeation fluxes generally decrease with increasing molecular size, while the alkenes permeate more rapidly than their corresponding alkanes at identical conditions. In 1:1 mixtures of ethane-ethene and propane-propene (1 bar total pressure) the alkanes permeate faster, yielding selectivity factors of 1.9 and 1.3 respectively. The generalized Maxwell-Stefan (GMS) equations, adapted for surface diffusion, could describe the permeation data well. The unary systems yielded diffusivity data that were fairly constant or varied at most by a factor of 2–3. These diffusivities compare well with literature values obtained with other (transient) techniques that yield transport diffusivities. The binary system permeation data could be quantitatively described by the GMS equations without exchange contributions (“single-file” diffusion) and need only the diffusivity values of the unary permeation experiments.
74 citations
TL;DR: In this paper, the authors applied the Maxwell-Stefan mass transfer description to two electrodialysis systems, one involving NaCl and the other involving HCl and HCl. Although it is not possible to determine all the parameters accurately, the values found are all within the ranges expected.
Abstract: The Maxwell-Stefan mass transfer description is applied to two electrodialysis systems. Firstly, a model system involving NaCl and HCl is investigated. For this system, all the model parameters are obtained either from separate experiments or existing correlations. Although it is not possible to determine all the parameters accurately, the values found are all within the ranges expected. Secondly, a two amino acid system is investigated. The separation of two amino acids using electrodialysis is found to be feasible. Unfortunately, it is too difficult to obtain sufficient data for a proper quantitative description of the process. In both cases, the Maxwell-Stefan description appears to provide a thorough model of the electrodialysis process.
TL;DR: In this article, the authors summarized the characteristics of electrical tomography techniques, highlights their current applications and gives an indication of their future applications in the chemical process engineering environment, and gave an overview of their potential applications.
Abstract: This paper summarizes the characteristics of electrical tomography techniques, highlights their current applications and gives an indication of their future applications in the chemical process engineering environment.
TL;DR: In this article, the dynamics of transport of ternary mixtures of non-adsorbable gases (hydrogen, helium, nitrogen, argon) through porous solids was studied experimentally in the dynamic version of the Wicke-Kallenbach diffusion cel.
Abstract: Dynamics of transport of ternary mixtures of non-adsorbable gases (hydrogen, helium, nitrogen, argon) through porous solids was studied experimentally in the dynamic version of the Wicke-Kallenbach diffusion cel. Four samples of α-alumina pellets prepared under different compressing pressures and calcination temperatures with pore structures resembling commercial catalysts/carriers were used. Responses to composition step changes at one face of cylindrical pellets are followed at the other pellet face. The dusty gas model (DGM) and mean pore transport model (MTPM), based on the modified Maxwell-Stefan equation and Darcy law, were used for simulation of experimental responses with transport parameters obtained from dynamic measurements with binary gas mixtures. The predicted and measured responses to step changes of gases agreed very well for both models.
TL;DR: The results of their investigations on methanol synthesis, carried out over a number of years, have been presented in the form of an interpretative review as mentioned in this paper, which is an attempt to summarize this research against the background of what has already been done in this area in the world.
Abstract: The results of our investigations on methanol synthesis, carried out over a number of years, have been presented in the form of an interpretative review. The present article is an attempt to summarize this research against the background of what has already been done in this area in the world. Special attention was focused on the role of CO2 in the process.
TL;DR: In this article, the authors present experimental results concerning dynamic liquid holdup, wetting efficiency and local mass transfer coefficients between the liquid and solid surface in a fixed-bed three-phase reactor, in which both the gas and liquid flow cocurrently downwards.
Abstract: The paper presents experimental results concerning dynamic liquid holdup, wetting efficiency and local mass transfer coefficients between the liquid and solid surface in a fixed-bed three-phase reactor, in which both the gas and liquid flow cocurrently downwards. The experiments were conducted for two basic regimes of operation of trickle-bed reactors, namely the gas continuous flow regime and the pulsing flow regime under atmospheric pressure. The measurements of dynamic liquid holdup have been performed for a wide range of gas and liquid flow rates, three different packing diameters, and for two systems of working media of different physical properties. The common correlation developed determines with good accuracy the holdup values in both regimes as well as in the transition region between the two hydrodynamic modes. In experiments concerning the wetting efficiency a dynamic tracer method has been employed, for which an original mathematical model has been formulated. The results are presented in the form of diagrams and appropriate correlating formulae. The experiments concerning local solid—liquid mass transfer coefficients were carried out for two diameters of spherical particles, with the flow rates of both phases and physicochemical properties of the liquid varied over a wide range. The experimental results are correlated and compared with the appropriate literature data. A mathematical model describing the time-varying solid—liquid mass transfer process is formulated for the pulsed flow regime. The time-averaged mass transfer coefficients calculated on the basis of the model developed are compared with the experimental values. A good agreement between these two sets of values fully confirms the assumptions of the model elaborated.
TL;DR: In this article, a generalized Reynolds number for cylindrical ducts of arbitrary cross-section was proposed, which involves only one geometrical parameter, which can be easily determined theoretically or experimentally.
Abstract: The pressure drop prediction requires the generalization of the Reynolds number for laminar flow of pseudoplastic fluids in cylindrical ducts. This generalized Reynolds number is now well established for very simple cross-sections, such as circular tubes or infinite parallel plane plates. To our knowledge, there is no generalization available for other cross-sections due to the complexity of the velocity profiles. The aim of this paper is to propose a generalized Reynolds number for cylindrical ducts of arbitrary cross-section. This new Reynolds number involves only one geometrical parameter, which can be easily determined theoretically or experimentally. This Reynolds number was successfully compared with definitions found in the literature. Moreover, this generalized Reynolds number seems to be in good agreement with experimental studies carried out in plate heat exchangers.
TL;DR: In this paper, a wall-to-tube heat and mass transfer in a Kenics-type static mixer allow a unification of extent data and show that the heat transfer enhancement is greatest in deep laminar flow and that the mixers exhibit modest improvements when compared on the basis of equal pressure drop.
Abstract: New results for wall-to-tube heat and mass transfer in a Kenics-type static mixer allow a unification of extent data. Nearly universal correlations are presented. They allow detailed comparisons with open tube designs. Heat transfer enhancement is greatest in deep laminar flow. Even there, the Kenics mixers exhibit modest improvements when compared on the basis of equal pressure drop. Existing designs are not yet optimized for heat transfer and studies at lower aspect ratios for the mixing elements are recommended.
TL;DR: In this article, the authors defined a one-dimensional model of molecular distillation based on the direct simulation Monte Carlo method and calculated particle velocities within the distillation space by application of the model developed.
Abstract: We have defined a one-dimensional model of molecular distillation based on the direct simulation Monte Carlo method. Particle velocities within the distillation space are calculated by application of the model developed. Using the data obtained, we compute macroscopic variables such as particle density, collision frequency, mean free path and kinetic temperature related to a particular position in the distillation space. The resulting anisotropy of the values is explained by oriented motion of the particles between the surfaces of evaporation and condensation. The influence on process efficiency of both the temperatures of the evaporating and condensing surfaces, and the width of the gap between them, is discussed. We also examine the influence of the design of the distillation space on the process taking place within it. A comparison of the efficiency of molecular distillation from convex and concave surfaces of evaporation is presented.
TL;DR: In this article, the effect of particle size on the just suspension speed was compared with the Zweitering correlation, as determined from conductivity measurements, and the principle of using data derived from conventional probes and three-dimensional tomograms to yield mixing indices for control decision making was described.
Abstract: The need for tomographic technology to assist in the development of reliable correlations for scale-up of slurry mixing processes is described. In situ measurements of the axial and radial solid concentration profiles and vector velocities of particulates are required which indicate the effect of mixing impeller geometry, mixing speed, solid concentration and size distribution of solids on mixing performance. The current status of tomographic techniques that could be suited for the applications is assessed. Recent results obtained using the two methods of electrical resistance tomography (ERT) and positron emission tomography (PET) are described together with complementary measurements performed using an invasive single point conductivity probe. The effect of particle size on the “just suspension speed” N js , as determined from conductivity measurements, was compared with the Zweitering correlation (Th. N. Zweitering, Chem. Eng. Sci., 8 (1958) 244-253). Good agreement was obtained. The principle of using data derived from conventional probes and three-dimensional tomograms to yield mixing indices for control decision making is described.
TL;DR: In this article, a mathematical model of momentum, heat and mass transfer in the atomization zone was proposed to calculate spray drying in a pilot plant dryer, where the air supplied to the dryer tangentially to the axis was characterized by a high swirl.
Abstract: A mathematical model of momentum, heat and mass transfer in the atomization zone was proposed. Uneven distribution of particles and entrainment effects were taken into account in the model. To verify the model an extensive experimental investigation was performed on water evaporation at different initial air temperatures, feed rates, flow rates of the drying agent and different parameters of atomization: spray cone angle and initial particle size distribution. Changes in air temperature inside the stream of sprayed material, material temperature, evaporation rate, and changes in Sauter diameter and distribution of particle diameters vs. their distance from the atomizer were determined experimentally. Damping screens allowed a flat profile of air flow rate in the tunnel to be obtained. Good agreement between experimental results and theoretical data was achieved. In the paper an attempt was made to apply the model to the calculation of spray drying in a pilot plant dryer. The air supplied to the dryer tangentially to the axis was characterized by a high swirl. The model of air flow in the dryer was determined theoretically using the Computational Fluid Dynamics CFX program. Results obtained in this way were used in our own model to determine particle trajectories in the dryer and to solve heat and mass balances for the continuous and dispersed phases. In order to consider swirl air flow pattern in the dryer, changes in heat and mass balance were made for the particle—drying agent system. The model was verified experimentally on the basis of results of investigations on drying of a 20% solution of sodium chloride. Additionally, our own results were compared with those obtained from the particle source in the cell type model for the drying system being studied. It was found that our model well described the process of drying in the analyzed system. Results obtained from our model were also very similar to the results obtained from the particle source in the cell type model. An extensive literature survey on spray drying, including unsteady state phenomena, is presented in the paper. Main sources of errors occurring in modelling of the spray drying process were discussed. A critical estimation of the existing mathematical models of the process was made and some of them were described in detail.
TL;DR: In this article, the steady flow of a thin trickle of viscous fluid down an inclined surface is considered, via a thin-film approximation, and results obtained for uniform flow down the topside or underside of an inclined plane are used in a simple way to approximate flow down a non-plane surface.
Abstract: Steady flow of a thin trickle of viscous fluid down an inclined surface is considered, via a thin-film approximation. Results obtained for uniform flow down the topside or underside of an inclined plane are used in a simple way to approximate flow down a non-plane surface.
TL;DR: In this article, the adsorption of copper and nickel ions on Rhizopus arrhizus, a filamentous fungus, was investigated in three batch stirred reactors operated in series.
Abstract: In this research, the adsorption of copper(II) and nickel(II) ions on Rhizopus arrhizus, a filamentous fungus, was investigated in three batch stirred reactors operated in series. The adsorption in multistage reactors can be considered as multistage equilibrium operation, which depends on two constraints: that of equilibrium and that of mass balance. The sorption phenomenon was expressed by the Freundlich adsorption isotherm developed for optimum adsorption conditions. At a given ratio V0X0 of volume of waste water containing heavy metal ions to quantity of fungus, this expression was used to calculate the residual metal ion concentration Ceq, r or adsorbed metal ion concentration Ceq, xr at equilibrium in the solution leaving each stage. Experimental Ceq, r and Ceq, xr values were compared with calculated values. High metal ion quantities per unit mass of dried biomass were removed by using low V0X0 ratios. When aqueous solutions containing copper(II) at a concentration of 99.9 mg l−1 and nickel(II) at a concentration of 97.9 mg l−1 were fed to the first reactor, 65.7 mg of copper per gram of dried biomass and 90.2 mg of nickel per gram of dried biomass were removed in the solution, leaving a third reactor, at a chosen V0X0 ratio of 1.00.
TL;DR: In this article, the optimum operation of a two-stage discontinuous operation is determined by using cake filtration theory and assuming that the backwash period depends on the geometry of the filter module.
Abstract: Microfiltration membranes can be used for the dead-end filtration of suspensions of fine particles, with a periodic backwash to remove the cake and restore filtrate flux. The optimum operation of such a two-stage discontinuous operation is determined by using cake filtration theory and assuming that the backwash period depends on the geometry of the filter module. This allows identification of the conditions where such a process can be used. The case where flux restoration by backwashing is imperfect due to progressive internal fouling of the membrane is also treated and allows the periodicity of chemical cleaning to be determined as a function of the degree of fouling. Finally, mention is made of the possible extension of the theory to the case of cross-flow filtration.
TL;DR: In this paper, an approach based on mechanistic principles was used to point out a relationship between the gas holdups in the riser and downcomer of internal-loop air-lift reactors without gas-liquid separators.
Abstract: An approach based on mechanistic principles was used to point out a relationship between the gas holdups in the riser and downcomer of internal-loop airlift reactors without gas-liquid separators. The theoretically based relationship was shown to apply to experimental observations in five reactors, including split-cylinder and concentric draft-tube types of reactor configurations, over a broader range of volumes (0.033–0.160 m 3 ), heights (1.8–3.45 m) and riser-to-downcomer cross-sectional area ratios (1.8–7.7) than any of the previously published empirical correlations. Unlike earlier equations, the proposed equation was consistent with the observation that, at low rates of gas input in the riser. there may be no gas in the downcomer.
TL;DR: In this paper, the effect of bacterial adaptation on bioleaching of three iron-bearing sulfide minerals, namely, pyrite, chalcopyrite and arsenopyrite (FeAsS), was studied.
Abstract: Thiobacillus ferrooxidans bacteria were used in this work to study the effect of bacterial adaptation on bioleaching of three iron-bearing sulfide minerals, namely, pyrite (FeS2), chalcopyrite (CuFeS2), and arsenopyrite (FeAsS). It was found that bacterial adaptation prior to bioleaching, enhanced the degree of sulfide leaching 2–4 fold over non-adapted bacteria. Also the bacterial activity was enhanced by 2–4.8 fold. The pH of leaching solution was decreased by the increase in both adaptation period and bioleaching time. A mechanistic model for bioleaching of sulfides with T. ferrooxidans is described. The model explains the role of bacteria in direct and indirect biochemical leaching reactions. It also reconciles such observed behaviors and phenomena as lag time, adaptation, pH increase lag time, and electrochemistry of bioleaching. The model shows how direct and indirect biochemical interactions are involved in bioleaching of ferrous sulfides.
TL;DR: In this article, it was shown that the COSORB reaction can be regarded as instantaneous with respect to mass transfer, and the conditions of the present experiments were such that it can be concluded that for absorption equipment design, for example, it is impossible to eliminate the influence of mass transfer.
Abstract: Absorption rate experiments and equilibrium experiments were carried out for the COSORB reaction at 300 K. The equilibrium data at 300 K could reasonably well be described with the following relation: [...]
Determination of the kinetics and mechanism of a chemical reaction by means of absorption experiments is possible in the fast reaction regime, where the absorption rate is independent of the mass transfer. In the present study it turned out to be impossible to eliminate the influence of mass transfer, although the experiments were carried out at very low CO partial pressures. This made the determination of the kinetics and mechanism of the forward reaction of the COSORB reaction not possible. The conditions of the present experiments were such that it can be concluded that for absorption equipment design the COSORB reaction can be regarded as instantaneous with respect to mass transfer
TL;DR: In this paper, the influence of fill level on the particulate motion in a 5 1 ploughshare batch mixer was studied and the average tangential velocity increased linearly with increasing fill, whereas the r.m.s. axial velocity decreased.
Abstract: The technique of positron emission particle tracking was used to study the influence of fill level on the particulate motion in a 5 1 ploughshare batch mixer. The average tangential velocity was found to increase linearly with increasing fill, whereas the r.m.s. axial velocity decreased. Above 10% fill, the axial motion developed into two circulation cells, with particles transferring occasionally between the two. Varying the size of the tracer particle had little effect on the velocity estimates or on the residence time distributions found. The axial mixing time increased with increasing fill, and showed some dependence on tracer size.
TL;DR: In this paper, the effect of gas and liquid properties on the gas phase dispersion in bubble column reactors was investigated via a simple axial dispersion model with the inclusion of mass transfer term.
Abstract: The effect of gas and liquid properties on the gas phase dispersion has been investigated in bubble column reactors. Data were obtained in two 3.0 m tall bubble columns (of diameters 0.15 m and 0.25 m) and by varying superficial phase velocities. A novel experimental technique, a quadrupole mass spectrometer, was used to measure the tracer gas concentration. Data analysis was accomplished via a simple axial dispersion model with the inclusion of mass transfer term. Results indicate that an increase in liquid viscosity and decrease in the liquid surface tension leads to a decrease in the gas phase dispersion. Further, the gas properties have no influence on the gas phase dispersion so long as the mass transfer effects are properly accounted for in the model. A hydrodynamic model has been proposed to predict the gas phase dispersion in bubble column reactors. The model distinguishes various bubble fractions present in the column based on the differences in their rise velocities. The model assumes a bimodal distribution of the gas phase, i.e. fast-rising bubbles following a plug-flow behavior and slow-rising bubbles which are being entrained and partially backmixed in the liquid phase. The model has been validated by predicting experimental as well as literature data on gas phase dispersion under various operating conditions. The proposed model is easy to use since it requires few easily obtainable parameters for the prediction of gas phase dispersion, which is essential parameter in the design and upscaling of bubble column reactors.
TL;DR: In this article, the authors present a review of the state-of-the-art work in image reconstruction modeling of mixing to interpret simple tracer diagnostic tests and thereby predict chemical reactor behavior under semi-batch operation.
Abstract: Quantitative visualization of chemical processes and the consequent incorporation of images into process control loops was an idea that E.T. Woodburn et al. pioneered in mineral processing. This approach is based on the notion that many chemical processes possess intrinsic visual indicators which are otherwise difficult to instrument. Stirred vessels used as chemical reactors have the same potential to benefit from quantitative imaging, although, in this case, not just for control purposes but also as a powerful adjunct to model development and validation. This review outlines developments pursued at UMIST which seek to utilize image reconstruction modelling of mixing to interpret simple tracer diagnostic tests and thereby to predict chemical reactor behaviour under semi-batch operation. Progress from small two-dimensional axisymmetric networks-of-zones, through larger three-dimensional networks-of-zones to computational fluid dynamics predictions in full three dimensions are outlined. Problems requiring more advanced techniques and models to link macro-scale mixing to micro-mixing form a wide set of challenges for future research.