# Chemical Engineering Science (v.63, #6)

Preparing recombinant single chain antibodies by Susanna S.J. Leong; Wei Ning Chen

*(1401-1414)*. A review of current processing practices in preparation of recombinant single chain antibody fragments is presented. Single chain antibody fragments which are superior to their Fab and IgG counterparts due to their higher affinity for target antigens while imposing minimal antigenicity in recipient hosts, have sparked breakthroughs in immunology and the medical field at large. The rapidly increasing market demand for pure single chain antibodies for research and therapeutic applications, necessitates viable manufacture routes that can produce large amounts of these antibodies efficiently and as cheaply as possible. Medium- to high-producing expression systems reported for recombinant single chain antibody production are reviewed, and their reported or potential success for efficient commercial-viable preparation of pure antibodies discussed. The effects of expression host system choice on product molecular constraints, ease of processing, and flowsheet design and scale-up are compared. It is concluded that there is no unique host system that can consistently yield high expression levels for a wide range of single chain antibodies; instead, product sequence and end application often dictate the optimum choice of expression host. Irrespective of host systems, adequate

*a priori*design and engineering of the molecular construct supported by good biophysical understanding of the single chain fragment molecules, is a crucial pre-requisite for improved product stability and downstream recovery, which will favourably impact final product yield and functionality.**Keywords:**Recombinant antibodies; Single chain variable fragment; Biomolecule; Protein; Biomolecular engineering; Processing;

A generalized segregation and dispersion model for liquid-fluidized beds by Bharatkumar K. Patel; W. Fred Ramirez; K.P. Galvin

*(1415-1427)*. This work builds upon our previous two studies [Ramirez, W.F., Galvin, K.P., 2005. Dynamic model of multispecies segregation and dispersion in liquid fluidized beds. A.I.Ch.E. Journal 51, 2103–2108; Galvin, K.P., Swann, R., Ramirez, W.F., 2006. Segregation and dispersion of a binary system of particles in a fluidized bed. A.I.Ch.E. Journal 52, 3401–3410] in establishing a dynamic continuum description of the segregation and dispersion of multiple particle species in a liquid-fluidized bed. A correlation for the particle dispersion coefficient, based on the kinetic theory of gases, is investigated in this present paper. Our adjustable model parameter was found to vary inversely with a particle Froude number, in a manner similar to the variation of the particle drag coefficient with the particle Reynolds number. Hence a new relationship for the dispersion coefficient, based around the kinetic theory of gases, was proposed and validated. The validation applies to a nominal particle Reynolds number in the range of 20–1000. A new solution strategy to the dynamic segregation and dispersion model, a shell balance approach, was developed to overcome the potential for a singularity above the bed where exceedingly low particle concentrations arise. This approach guarantees mass conservation of the particles. The model was validated against steady-state experimental data of binary particle systems consisting notionally of one particle size and two different particle densities. When the particle densities of the two species are very similar, it becomes necessary to treat each particle species as having a range of particle sizes, given that the small size range was equally significant to the difference in particle density. In this case the model was applied to 10 particle species. Moreover, the model is capable of capturing the dynamics of the layer inversion problem, illustrating the power of the model to describe complex fluidization behaviour.

**Keywords:**Fluidization; Segregation; Dispersion; Dynamic model; Slip velocity; Inversion;

Multi-objective optimization for two catalytic membrane reactors—Methanol synthesis and hydrogen production by Shueh-Hen Cheng; Hsi-Jen Chen; Hsuan Chang; Cheng-Kai Chang; Yi-Ming Chen

*(1428-1437)*. This paper provides the triple-objective-function optimization results for the catalytic membrane reactors, including one for methanol synthesis and one for hydrogen generation. A 1-D, non-isothermal model, which takes into account the intra-particle diffusion for the catalyst, and the elitist nondominated sorting genetic algorithm (NSGA-II) for the multi-objective optimization are adopted. Optimal solutions for methanol synthesis and hydrogen generation systems show distinctive feature. One is randomly scattered and the other is linearly spread out in the Pareto plot. Solution characteristics in terms of variable distribution are quite different for the two systems. Device size, including membrane area and membrane size, shows effects both on the optimal solutions and on the correlation relations between objective functions and variables.

**Keywords:**Genetic algorithm; Multi-objective optimization; Catalytic membrane reactor; Methanol synthesis; Carbon dioxide; Hydrogen; Methane;

Rheology of a thermoplastic paste through the mushy state transition by D.A. Barker; D.I. Wilson

*(1438-1448)*. The rheology of a highly filled thermoplastic paste employed in a novel continuous casting process was investigated across the melt-mushy zone temperature transition. The paste comprises of an ultrafine (or nano-particulate) zinc oxide powder ( d 1 , 0 = 85 nm , 50 vol%) and a continuous phase consisting of a blend of organic waxes with an onset of solidification around 54 ∘ C . Data were collected over nine decades of shear rate, using controlled strain and controlled stress devices, steady shear and oscillatory modes, rotational rheometry, and capillary rheometry using the multi-pass technique. The melt rheology could be described using a Carreau-type constitutive model, with strong evidence of a low shear rate viscosity plateau and a possible high shear rate viscosity plateau, with a transition between the two described by a highly shear-thinning power-law region. Nearer the solidification temperature the material exhibited strong pseudo-plastic behaviour, and capillary flow appears to be determined by pressure-dependent slip behaviour.

**Keywords:**Complex fluid; Mushy zone; Paste; Phase change; Rheology; Suspension;

An approach for modeling two-step denitrification in activated sludge systems by Bing-Jie Ni; Han-Qing Yu

*(1449-1459)*. In the widely used Activated Sludge Models (ASMs) for biological wastewater treatment, denitrification is assumed as a single-step process, which is not true for some cases where a considerable amount of nitrite is build-up in system. This causes limitation to the application of ASMs. In this work, with a consideration of denitrification on both nitrite and nitrate, and an introduction of the simultaneous storage and growth concept, a two-step ASM3-based denitrification model is established. The sensitivity of the effluent chemical oxygen demand, NO 2 - – N and NO 3 - – N concentrations toward the stoichiometric and kinetic coefficients is analyzed. Model calibration is performed by comparing the measured and predicted values for model components. Thereafter, this model is evaluated with the experimental results of four different case studies on denitrification and compared with the original ASM3. The evaluation and comparison results demonstrate that the developed model is able to better and more mechanistically describe the denitrification process in the activated sludge systems.

**Keywords:**Activated sludge; Denitrification; Kinetics; Modeling; Wastewater;

Experimental studies and computer simulation of the preparation of nanoporous silicon-carbide membranes by chemical-vapor infiltration/chemical-vapor deposition techniques by Feng Chen; Rayan Mourhatch; Theodore T. Tsotsis; Muhammad Sahimi

*(1460-1470)*. Preparation of asymmetric nanoporous silicon-carbide membranes using chemical-vapor infiltration/chemical-vapor deposition (CVI/CVD) is described. We use macroporous SiC disks and tubes as supports, and tri-isopropylsilane as the precursor. Experimental data for the permeation and selectivity of the membranes are presented. We also develop two dynamic models to describe the preparation of the membranes. The models are shown to provide accurate predictions for the experimental data for the permeation characteristics of the membranes, as a function of the preparation conditions.

**Keywords:**Nanoporous membranes; Knudsen diffusion; Hindered diffusion; Chemical-vapor deposition; Chemical-vapor infiltration; Numerical simulation;

Theoretical and experimental study on hydrodynamic characteristics of fluidization in air–sand conical beds by Rachadaporn Kaewklum; Vladimir I. Kuprianov

*(1471-1479)*. This work was aimed at modeling hydrodynamic characteristics of fluidization in conical beds using quartz sand as the inert bed material and air as the fluidizing agent. The minimum fluidization velocity, u mf , and the minimum velocity of full fluidization, u mff , were determined by Peng and Fan's models modified for conical fluidized bed. Meanwhile, the pressure drop across a bed, Δ p (including Δ p max and Δ p mff corresponding to u mf and u mff , respectively), was predicted by using modified Ergun's equations for variable superficial air velocity at an air distributor, u 0 . The predicted results were validated by experimental data for some operating conditions. Effects of the sand particle size, cone angle and static bed height on the fluidization pattern and hydrodynamic characteristics are discussed. With the proposed models, the Δ p – u 0 diagram were obtained with rather high accuracy for the conical air–sand beds of 30– 45 ∘ cone angles and 20–30 cm static bed heights, when using 300– 1180 μ m sand particles. For the predicted u mf and u mff , the relative computational errors were found to be within 20% for wide ranges of operating variables, whereas Δ p max and Δ p mff could be predicted with lower (10–15%) relative errors. With higher cone angles and/or bed heights, the computational accuracy was found to deteriorate.

**Keywords:**Particle size; Static bed height; Cone angle; The Δ p – u 0 diagram;

Estimating spectral properties of the thermal instability in packed-bed reactors by Vladimir Z. Yakhnin; Michael Menzinger

*(1480-1489)*. In response to transient perturbations, the packed-bed reactor (PBR) can exhibit dynamic thermal instability in the form of resonant amplification of process disturbances. Based on linearized PBR model we derive estimates of the resonance frequency, the frequency range where amplification takes place and the maximum amplification. We also discuss the velocity of perturbation waves and how nonlinearity of reactor response to finite-size perturbations limits the amplification predicted by the linear analysis.

**Keywords:**Packed-bed reactor; Thermal instability; Dynamic instability; Resonance; Disturbance amplification; Operational safety;

A new generalized corresponding-states equation of state for the extension of the Lee–Kesler equation to fluids consisting of polar and larger nonpolar molecules by H.W. Xiang; U.K. Deiters

*(1490-1496)*. The Lee–Kesler equation of state for the thermodynamic properties of small nonpolar fluids is extended to all fluids consisting of polar and larger nonpolar molecules, based on the general corresponding-states theory for highly nonspherical fluids. The thermodynamic functions are represented by an analytical equation of state. The results for polar fluids are substantially better than those obtainable from other currently available methods, while the results for nonpolar fluids are equivalent to and mostly better than those obtained by the Lee–Kesler method. The input data required are the critical temperature, the critical volume, the acentric factor and the aspherical factor, which is related to the critical compression factor; the critical volume is therefore required in the present method. The method developed in this work shows good accuracy for 15 representative nonpolar, polar, hydrogen bonding and associating fluids and provides a simple method for industrial applications. Average deviations for the compressibility factor, the heat capacity and the speed of sound for six nonpolar and nine polar fluids from the new equation of state are 0.74%, 2.1% and 2.3%, which are about 8 times smaller than those obtained from the Lee–Kesler equation (about 5.6%, 17% and 29%, respectively).

**Keywords:**Corresponding states; Equation of state; Fundamental equation; Helmholtz energy; Lee–Kesler equation; Nonpolar; Nonspherical; Phase equilibria; Polar; Supercritical fluid; Theory of liquids; Thermodynamic properties;

Development of an artificial neural network correlation for prediction of hold-up of slurry transport in pipelines by S.K. Lahiri; K.C. Ghanta

*(1497-1509)*. In the literature, very few correlations have been proposed for hold-up prediction in slurry pipelines. However, these correlations fail to predict hold-up over a wide range of conditions. Based on a databank of around 220 measurements collected from the open literature, a correlation for hold-up was derived using artificial neural network (ANN) modeling. The hold-up for slurry was found to be a function of nine parameters such as solids concentration, particle dia, slurry velocity, pressure drop and solid and liquid properties. Statistical analysis showed that the proposed correlation has an average absolute relative error (AARE) of 2.5% and a standard deviation of 3.0%. A comparison with selected correlations in the literature showed that the developed ANN correlation noticeably improved prediction of hold-up over a wide range of operating conditions, physical properties and pipe diameters. This correlation also predicts properly the trend of the effect of the operating and design parameters on hold-up.

**Keywords:**Artificial neural network; Slurry hold-up; Slurry flow regime;

Structure of trickle-to-pulse flow regime transition and pulse dynamics at elevated temperature in slow-mode cyclic operation by Bora Aydin; Faı¨çal Larachi

*(1510-1522)*. The effects of temperature and pressure on the structure of the trickle-to-pulse flow regime transition in slow-mode cyclic operation in trickle-bed reactors were reported. The relationship between liquid holdup and liquid velocities at the trickle-to-pulse flow transition in cyclic operation, the shock wave behavior as a function of bed depth, as well as the pulsing flow regime properties were investigated for Newtonian (air–water) and non-Newtonian (air–0.25% carboxymethylcellulose (CMC)) liquids. At a given temperature, the breakthrough, plateau and decay times of the shock wave were found to decrease with bed depth. The pulse velocity and pulse frequency for pulsing flow regime both in cyclic operation and in natural pulsing (constant-throughput operation) were observed to increase with temperature. However, increasing the reactor pressure led to increased pulse frequency and decreased pulse velocity. Analysis of the transition liquid holdups for natural pulse flow and cyclic operation revealed that the liquid holdup decreased with temperature and pressure. The transition liquid holdups and superficial liquid pulse velocities in symmetric peak-base cyclic operation surpassed those in constant-throughput operation for given temperature, pressure and gas velocity, giving rise to wider trickle flow regime area in cyclic operation. The behavior of both Newtonian and power-law non-Newtonian fluids was similar regarding the effect of temperature, pressure and gas velocity.

**Keywords:**Trickle bed; Elevated temperature; Induced pulsing; Transition boundary;

A study on tangential force laws applicable to the discrete element method (DEM) for materials with viscoelastic or plastic behavior by H. Kruggel-Emden; S. Wirtz; V. Scherer

*(1523-1541)*. The discrete element method is a widely used particle orientated simulation approach for modeling granular systems. It is based on tracking each particle's movement and its interactions with the surroundings over time. The motion of a particle is given by a system of coupled ordinary differential equations which are solved numerically. Therefore, models for the forces acting between particles in contact need to be specified. In the past, detailed investigations dealing with the accuracy of tangential force–displacement models have been very limited, with sparse experimental data considered and the frequent restriction of including only fully elastic materials. In large scale discrete element simulations, on the other hand, viscoelastic or plastic material behavior is often assumed for normal contacts and combined with arbitrary tangential models. To address this situation a number of tangential force–displacement models are reviewed including linear models by Cundall and Strack [1979. A discrete numerical model for granular assemblies, Geotechnique 29, 47–65], Di Maio and Di Renzo [2004. Analytical solution for the problem of frictional-elastic collisions of spherical particles using the linear model. Chemical Engineering Science 59(16), 3461–3475], Brendel and Dippel [1998. Lasting contacts in molecular dynamics simulations. In: Herrmann, H.J., Hovi, J.-P., Luding, S. (Eds.), Physics of Dry Granular Media, Dordrecht. Kluwer Academic Publishers, pp. 313], Walton and Braun [1986. Viscosity, granular temperature and stress calculations for shearing assemblies of inelastic, frictional disks. Journal of Rheology 30, 949] and simple non-linear models by Brilliantov et al. [1996. Model for collisions in granular gases. Physical Review E 53(5), 5382–5392], Tsuji et al. [1992. Lagrangian numerical simulation of plug flow of cohesionless particles in a horizontal pipe. Powder Technology 71, 239–250] and Di Renzo and Di Maio [2005. An improved integral non-linear model for the contact of particles in distinct element simulations. Chemical Engineering Science 60(5), 1303–1312]. Whereas for fully elastic materials the parameters of the tangential force–displacement models can be derived directly from mechanical properties a scaling approach is proposed for the estimation of the parameters in the non-elastic case. The effect of different normal force–displacement models is analyzed. For all model combinations macroscopic final collision properties are derived and compared to experimental results by Foerster et al. [1994. Measurements of the collision properties of small spheres. Physics of Fluids 6(3), 1108–1115], Lorenz et al. [1997. Measurements of impact properties of small, nearly spherical particles. Experimental Mechanics 37(3), 292–298], Gorham and Kharaz [2000. The measurement of particle rebound characteristics. Powder Technology 112(3), 193–202] and Dong and Moys [2003. Measurement of impact behaviour between balls and walls in grinding mills. Minerals Engineering 16(6), 543–550; 2006. Experimental study of oblique impacts with initial spin. Powder Technology 161(1), 22–31].

**Keywords:**Granular flow; Collisions models; Discrete element method;

Nonlinear parameter estimation through particle swarm optimization by Marcio Schwaab; Evaristo Chalbaud Biscaia, Jr.; José Luiz Monteiro; José Carlos Pinto

*(1542-1552)*. Parameter estimation procedures are very important in the chemical engineering field for development of mathematical models, since design, optimization and advanced control of chemical processes depend on model parameter values obtained from experimental data. Model nonlinearity makes the estimation of parameter and the statistical analysis of parameter estimates more difficult and more challenging. In this work, it is shown that many of these difficulties can be overcome with the use of heuristic optimization methods, such as the particle swarm optimization (PSO) method. Parameter estimation problems are solved here with PSO and it is shown that the PSO method is efficient for both minimization and construction of the confidence region of parameter estimates. Moreover, it is shown that the elliptical approximation of confidence regions of nonlinear model parameters can be very poor sometimes and that more accurate likelihood confidence regions can be constructed with PSO, allowing for more reliable statistical analysis of the significance of parameter estimates.

**Keywords:**Parameter estimation; Particle swarm optimization; Confidence regions; Nonlinear models;

Eulerian simulation of heterogeneous gas–solid flows in CFB risers: EMMS-based sub-grid scale model with a revised cluster description by Junwu Wang; Wei Ge; Jinghai Li

*(1553-1571)*. Gas–solid two-phase flow in CFB risers is characterized by the clustering of solid particles producing dynamical multi-scale structures, and how to quantify such heterogeneity is a critical yet unsolved issue. Recently, incorporating the energy minimization multi-scale (EMMS) model with Eulerian approach has obtained encouraging results for simulating the hydrodynamics in CFB risers. However, owing to the cluster diameter correlation used, the present model is still limited to the simulation of Geldart A particles. In this study, a stochastic geometry approach named doubly stochastic Poisson processes is used to analyze the fluctuation characteristics of solid concentration in CFB risers, which provides a mean to define the solid concentration inside clusters. The predicted results are validated by experimental data available in literature, and a revised cluster diameter correlation is then proposed for EMMS model previously developed for cocurrent-up gas–solid flow. Following our previous studies, the EMMS model thus improved is incorporated into an Eulerian–Eulerian description of gas–solid flow as a sub-grid scale model for inter-phase drag force, with which the hydrodynamics of both Geldart A and Geldart B particles in CFB risers are simulated. It is shown that the experimentally found S-shaped axial voidage profiles and the choking phenomenon can be well predicted. The computed one-dimensional slip velocities decrease toward the top of the risers and increase with decreasing cross-sectional averaged voidages. The experimentally found dependence of the root mean square of the solid concentration on its mean value at a given position is also well predicted.

**Keywords:**Fluidization; Hydrodynamics; Multiphase flow; Powder technology; Gas–solid flows; Sub-grid scale model;

Macro-mixing in a draft-tube airlift bioreactor by Hu-Ping Luo; Muthanna H. Al-Dahhan

*(1572-1585)*. Airlift reactors are pneumatically agitated reactors that have been widely used in industries, particularly in bioprocesses. Extensive studies about the flow dynamics in airlift column reactors exist; however, most of these studies have focused on global hydrodynamic parameters using conventional techniques. The local flow characteristics, such as the macro-mixing and the turbulence intensity, are crucial for reliable design and scale-up, and they remain unclear. This work focuses on studying the macro-mixing in a draft-tube airlift bioreactor utilizing an advanced flow dynamic measurement technique, computer automated radioactive particle tracking (CARPT). True residence time distribution analyses for the overall column as well as individual regions, i.e., the riser, the downcomer, the top, and the bottom regions, are conducted for the first time based on CARPT measured particle trajectories. The effects of the superficial gas velocity and the top/bottom clearances on the macro-mixing are also discussed. The results suggest that although the flow structures in the overall draft-tube column reactor, as well as in the riser and in the downcomer, are close to plug flows, bypassing and stagnancy exist in the top and the bottom regions.

**Keywords:**Airlift bioreactors; Macro-mixing; Hydrodynamics; Multiphase flow; CARPT;

Single component and competitive adsorption of propane, carbon dioxide and butane on Vycor glass by Jiřina Řezníčková Čermáková; Ana Marković; Petr Uchytil; Andreas Seidel-Morgenstern

*(1586-1601)*. Equilibrium of gas phase adsorption on Vycor glass has been investigated. Adsorption isotherms for propane, carbon dioxide and butane as pure gases, binary mixtures and ternary mixtures were determined experimentally as a function of temperature using a volumetric method. The single-component isotherms were described with the Langmuir and Freundlich equations. Additionally, a second order isotherm based on statistical thermodynamics and an isotherm equation based on vacancy solution theory taking into account real phase behavior were used for fitting single-component equilibrium data. In order to describe the measured partial isotherms for binary mixtures, at first simple extensions of the single-component isotherm models were used, i.e., the conventional competitive Langmuir model and a multi-Freundlich equation based on the ideal adsorbed solution theory (IAS). Since these two simple isotherm models failed to represent the unusual competitive behavior observed, three model extensions using additional mixture parameters were applied, i.e., two modified multi-Langmuir equations based on: (a) statistical thermodynamics and (b) vacancy solution theory and a modified multi-Freundlich IAS model correcting spreading pressure uncertainties. These three model equations were found to be capable to describe the observed behavior better. Finally, the measured partial adsorption equilibrium data of the ternary system were correlated based on the extended equations using the determined additional binary parameters. The results obtained reveal the difficulty to predict accurately multi-component adsorption equilibria.

**Keywords:**Porous media; Gases; Adsorption; Single-component isotherms; Multi-component isotherms; Parameter estimation;

Growth kinetics and mechanism of wet granulation in a laboratory-scale high shear mixer: Effect of initial polydispersity of particle size by Alvaro Realpe; Carlos Velázquez

*(1602-1611)*. The effect of initial polydispersity of particle size (unimodal versus bimodal distribution) and binder characteristics on the growth kinetics and mechanism of wet granulation was studied. Wet granulation of pharmaceutical powders with initial bimodal particle size distribution (PSD) presented growth kinetics consisting of two stages: fast growth followed by slow growth. The fast stage is controlled by the amount of binder and high probability of coalescence due to the collisions of small and large particles. The second stage is characterized by slow agglomeration of powder mixtures with water content 13.6% v/w, and slow breakage of powder mixtures with water content of 9.9% and 11.7% v/w. The wet granulation of powders with initial unimodal PSD exhibited slow growth kinetics consisting of one stage, since similar particle sizes do not promote agglomeration. The experimental results were better described by a population balance equation using a coalescence kernel that favors growth rate by collision between small and large particles. In general, the probability of a successful collision increased with higher size difference between particles, smaller particle size, and higher binder content.

**Keywords:**Polydispersity; Particle size distribution; Wet granulation; Growth kinetics; Coalescence; Binder;

Diffusiophoresis of a colloidal sphere in nonelectrolyte gradients perpendicular to two plane walls by Huan J. Keh; Yi W. Wan

*(1612-1625)*. The problem of the diffusiophoretic motion of a spherical particle in a fluid solution of a nonionic solute situated at an arbitrary position between two infinite parallel plane walls is studied theoretically in the quasisteady limit of negligible Peclet and Reynolds numbers. The applied solute concentration gradient is uniform and perpendicular to the plane walls. The particle–solute interaction layer at the particle surface is assumed to be thin relative to the particle radius and to the particle–wall gap widths, but the polarization effect of the diffuse solute in the thin interfacial layer caused by the strong adsorption of the solute is incorporated. The presence of the walls causes two basic effects on the particle velocity: first, the local solute concentration gradient on the particle surface is altered by the walls, thereby speeding up or slowing down the moving particle; second, the walls enhance the viscous retardation of the particle. A boundary-collocation method is used to semianalytically solve the solutal and hydrodynamic governing equations of the system. Numerical results for the diffusiophoretic velocity of the particle relative to that under identical conditions in an unbounded fluid solution are presented for various cases. The collocation results agree well with the approximate analytical solutions obtained by using a method of reflections. The net effect of the confining walls is always to reduce the particle velocity, irrespective of the surface properties of the particle or the relative particle–wall separation distances. The boundary effect on diffusiophoresis of a particle normal to two plane walls is found to be quite significant and generally stronger than that parallel to the walls.

**Keywords:**Diffusiophoresis; Electrophoresis; Colloidal phenomena; Fluid mechanics; Boundary effect; Plane walls;

Mechanisms of formation of iron precipitates from ferrous solutions at high and low pH by Mfandaidza Hove; Robert P. van Hille; Alison E. Lewis

*(1626-1635)*. The oxidation and subsequent precipitation of iron from solutions are very important for most hydrometallurgical wastewater treatment practitioners. The particle formation process mechanisms are important for shaping the dewaterability and particle size distribution of the precipitates. In this study the objective is to elucidate the mechanisms of iron particles formation from ferrous solutions at high (pH 9.0) and low pH (pH 6.0). The results obtained showed that the precipitation process is dominated by nucleation at the initial stages and aggregation dominated at the final stages. The precipitates formed at higher pH transform faster and result in the formation of more stable precipitates. Precipitates formed at pH 6.0 are larger but less stable chemically. The results may mean higher throughputs and lower costs of post precipitation stabilisation for operations at higher pH. The benefits for operation at lower pH would be a better solid–liquid separation due to the bigger sized particles formed.

**Keywords:**Iron oxidation; Precipitation mechanism; Wastewater treatment; Phase transformation;

Gas–liquid mass transfer in unbaffled dual-impeller mixers by F. Cabaret; L. Fradette; P.A. Tanguy

*(1636-1647)*. The gas–liquid mass transfer performance of different unbaffled dual-impeller mixers was investigated experimentally using low- and high-viscosity Newtonian and non-Newtonian liquids. The tested configurations were composed of two Rushton turbines located at two different height levels in the vessel in centered or off-centered positions. Various mixers were compared based on their respective mass transfer performance measured by means of two dissolved oxygen probes located at different levels in the vessel. k l a correlations and an axial homogeneity criterion were established for the different configurations and liquids used. It was concluded that the dual shaft mixer consisting of two off-centered shafts appears to be a very interesting and flexible configuration to deal with Newtonian liquids in a large range of viscosities. However, concerning the gas–liquid dispersion in non-Newtonian liquids, it is shown that off-centered shaft configurations have to be avoided.

**Keywords:**Multiphase reactors; Mixing; Dispersion; Absorption; Unbaffled; Shaft off-centering;

Propane ammoxidation with lattice oxygen of Mo–V–O-based complex metal oxide catalysts by J. Kubo; N. Watanabe; W. Ueda

*(1648-1653)*. MoVTeNbO complex metal oxide catalyst is active and selective in the ammoxidation of propane to acrylonitrile. In order to clarify the redox process of this catalyst, the bulk structure and catalytic behavior were investigated by a pulse technique, in which the reactivity of propane, ammonia and lattice oxygen of this catalyst was observed. The mixture of propane and ammonia was pulsed onto the MoVTeNbO catalyst in a helium stream at a temperature of 420 ∘ C in the range of 1–120 pulses and was selectively converted to acrylonitrile, while the lattice oxygen on the surface of the catalyst was abundant. And then the conversion of propane has decreased greatly as the pulse number increases to 12. The consumption of lattice oxygen to the whole of surface lattice oxygen was estimated to be 33%. On the other hand, ammonia was still converted after 12 pulses. After 120 pulses, the bulk structure was collapsed by the consumption of lattice oxygen. However, it revived again by exposing to the gaseous oxygen. This behavior indicates that only the lattice oxygen of the surface of the catalyst mainly takes part in propane ammoxidation.

**Keywords:**Ammoxidation; Propane; Acrylonitrile; Redox; MoVTeNbO; Pulse technique;

Coupling exothermic and endothermic reactions in adiabatic reactors by R.C. Ramaswamy; P.A. Ramachandran; M.P. Duduković

*(1654-1667)*. The steady state and the dynamic behavior of coupling exothermic and endothermic reactions in directly coupled adiabatic packed bed reactors (DCAR) are analyzed using one-dimensional pseudo-homogeneous plug flow model. Two different configurations of DCAR (simultaneous DCAR—SIMDCAR and sequential DCAR—SEQDCAR) are investigated. In SIMDCAR, the catalyst bed favors both exothermic and endothermic reactions and both reactions occur simultaneously. SEQDCAR has alternating layers of catalyst beds for exothermic and endothermic reactions and hence the exothermic and endothermic reactions occur in a sequential fashion. The performance of both reactors, in terms of conversion achieved and manifested hot spot behavior, is compared with that of the co-current heat exchanger type reactor. Various possible operational regimes in SIMDCAR have been classified and the conditions for the existence of hot spots or cold spots in SIMDCAR are obtained analytically for the first order reactions with equal activation energies. The reactor behavior for the reactions with non-equal activation energies is also presented. The preliminary criteria for the selection of suitable reactor type and the general bounds on the reaction parameters to obtain the desired conversion for endothermic reaction with minimal temperature rise are proposed. The dynamic behavior of these reactors is important for control applications and we have reported some of the transient behavior.

**Keywords:**SIMDCAR; SEQDCAR; Co-current reactor; Coupling of exothermic and endothermic reactions;

Process alternatives for methyl acetate conversion using reactive distillation. 1. Hydrolysis by Yu-Der Lin; Jun-Hong Chen; Jian-Kai Cheng; Hsiao-Ping Huang; Cheng-Ching Yu

*(1668-1682)*. In a polyvinyl alcohol (PVA) plant, reaction stoichiometry indicates that equal molar of methyl acetate is generated for every mole of PVA produced. This work explores an alternative to convert methyl acetate back to acetic acid (raw materials of PVA plant), methyl acetate (MeAc) hydrolysis. The design and control of methyl acetate hydrolysis using reactive distillation is studied. Because of the small chemical equilibrium constant ( ∼ 0.013 ) and unfavorable boiling point ranking (MeAc being the lightest boiler), the reactive distillation exhibits the following characteristics: (1) total reflux operation and (2) excess reactant (water) design. The proposed flowsheet consists of one reactive distillation column with a reactive reflux drum, two separation columns, and one water-rich recycle stream. A systematic design procedure is used to generate the flowsheet based on the total annual cost (TAC). Two dominate design variables are: recycle flow rate (for the degree of excess in water) and the overhead impurity level of acetic acid in the product column (to avoid tangent pinch). Finally, the operability of the hydrolysis plant is evaluated. A plantwide control structure is developed followed by process identification and controller tuning. The results show that reasonable control performance can be achieved using simple temperature control for feed flow and feed composition disturbances.

**Keywords:**Reactive distillation; Methyl acetate; Hydrolysis; Process design; Process control;

Modeling of mass transfer and thermal cracking during the coking of Athabasca residues by Ramin Radmanesh; Edward Chan; Murray R. Gray

*(1683-1691)*. The kinetics of thermal cracking of films of vacuum residue from Athabasca bitumen in the temperature range of 457 – 530 ∘ C was modelled with liquid-phase mass transfer, reaction-dependent fluid properties, and coke formation by reaction of cracked products in the liquid phase. Previous investigations on the thermal cracking of vacuum residue in thin films showed that at low film thickness ( ∼ 20 ± 2 μ m ) the coke yield was insensitive to the temperature and heating rate for thin films of bitumen. The coke yield increased with the thickness of the initial film, in the range from 20 to 80 μ m ( ± 2 μ m ) . At the same time, the viscosity of the reacting liquid increased rapidly with time, which would slow down the diffusion of products inside the film. This coupling of transport and reaction would enhance the formation of coke by increasing the rate of recombination reactions. The concept of intrinsic coke is used in a new kinetic model to account for the minimum observed coke formation in thin films. With increasing film thickness, the increasing yield of extrinsic coke is modelled through the change in fluid properties as a function of extent of reaction, which reduces the rate of diffusion in the reacting liquid phase. The model was able to properly account for the insensitivity of coke yield in thin films to reaction temperature and the dependence of coke yield on the thickness of the liquid film.

**Keywords:**Heavy oil; Bitumen; Thermal cracking; Coking; Mass transfer; Diffusion;

A droplet-based high-throughput tubular platform to extract rate constants of slow chemical reactions by W. Engl; M. Tachibana; A. Colin; P. Panizza

*(1692-1695)*. A high-throughput tubular millifluidic platform for performing slow kinetics measurements occurring in liquid phase, using minute amounts of reactant is described. The method consists of generating periodic trains of monodisperse droplets in a capillary tube and using these droplets as micro-reactors. In contrast to microfluidic lab-on chip devices, limited to fast kinetics measurements, this setup permits to extract kinetic rate constants of slow reactions at very low cost without requiring any use of soft lithography or glass etching technique for its design, nor miniaturization of analytical tools. It therefore appears very well suited for laboratories and industrial research and development centers.

**Keywords:**Chemical reactors; Chemical analysis; Multiphase flows;

Computational fluid dynamics of a circulating fluidized bed under various fluidization conditions by Adnan Almuttahar; Fariborz Taghipour

*(1696-1709)*. A computational fluid dynamics (CFD) model was developed to simulate the hydrodynamics of gas–solid flow in a circulating fluidized bed (CFB) riser at various fluidization conditions using the Eulerian–Granular multiphase model. The model was evaluated comprehensively by comparing its predictions with experimental results reported for a CFB riser operating at various solid mass fluxes and superficial gas velocities. The model was capable of predicting the main features of the complex gas–solids flow, including the cluster formation of the solid phase along the walls, for different operating conditions. The model also predicted the coexistence of up-flow in the lower regions and downward flow in the upper regions at the wall of the riser for high gas velocity and solid mass flux, as reported in the literature. The predicted solid volume fraction and axial particle velocity were in good agreement with the experimental data within the high density fast fluidization regime. However, the model showed some discrepancy in predicting the gas–solid flow behavior in the riser operating in dense suspension up-flow and low density fast fluidization regimes.

**Keywords:**CFD; Computation; Mathematical modeling; Multiphase flow; Fluidization; Circulating fluidized bed;

Lift force on bubbles in a bubble column reactor: Experimental analysis by Amol A. Kulkarni

*(1710-1723)*. The lift force acting on bubbles in a swarm has been estimated by analyzing the instantaneous velocity–time data obtained using LDA in a cylindrical bubble column. Phase distinction was achieved through the multiresolution analysis of the velocity–time data. Several important issues related to the transverse motion of bubbles subjected to a shear field have been discussed quantitatively. The actually measured bubble sizes, the respective slip velocity values in transverse and axial directions and the local shear rates ( γ ) enabled the verification of known formulations for the lift coefficient ( C L ) for bubbles. At many locations in the column the radial flux of the gas phase by turbulent dispersion and the radial slip were estimated. The radially inward movement of bubbles from low to high axial velocity (from column wall to center, i.e., C L < 0 ) was observed at most of the measurement locations. The local lift coefficient was estimated using the transverse drag force and the values support the results from the material balance approach. The estimated C L values showed a wide variation over the column cross-section.

**Keywords:**Bubble; Bubble column; Lift force; LDA; Bubble size; Hydrodynamics;

Coiled flow inverter as an inline mixer by Monisha Mridha; K.D.P. Nigam

*(1724-1732)*. Helical coils are widely used in the process industries to improve the mixing efficiency under laminar flow conditions. It was further observed that in the regular helical coils, there exists a confined region in the tube cross-section where fluids are entrapped and can escape only by diffusion. In the present work, an attempt has been made to further enhance the mixing in the coiled tube at low Dean number using the phenomenon of flow inversion. The study is performed in coiled flow inverter (CFI) [Saxena, A.K., Nigam, K.D.P., 1984. Coiled configuration for flow inversion and its effect on residence time distribution. A.I.Ch.E. Journal 30, 363–368] which was developed using the concept of inverting the direction of fluid by 90 ∘ . It comprises coils with equidistant 90 ∘ bends. The scalar mixing of two miscible fluids has been quantified for different process conditions (Dean number, Schmidt number and number of bends) by using scalar transport technique. There was a significant increase in mixing performance of CFI as compared to regular helical coils at low Dean number. The mixing efficiency increased with the increase in Dean number and number of bends. It was also observed that the mixing performance was enhanced with increase in Schmidt number. A new correlation has been proposed for unmixedness coefficient of CFI as a function of Dean number, Schmidt number and number of bends. The proposed correlation has maximum error of ± 20 % with the numerical predictions.

**Keywords:**Coiled flow inverter; Computation; Laminar flow; Mixing; Scalar transport technique; Transport processes;

On the relative importance of the kinetic and frictional contributions to granular motion in an annular Couette flow by B.H. Ng; Y.L. Ding; M. Ghadiri

*(1733-1739)*. A quantitative assessment is made on the relative importance of the kinetic and frictional contributions to the motion of dry granular materials under shear in an annular Couette flow configuration. The assessment is based on comparing the modelling results using the kinetic–frictional model with the experiments. It is shown that the kinetic-theory-based model with equal weight of the collisional and frictional contributions, commonly used in the literature, gives a great deviation from the experimental results in the point of view of the dominant solids motion, while an increase in the weight of the frictional contribution improves the modelling towards the experimental results. An increase in the weight of the frictional contribution by 25–50% leads to the best match, suggesting the current constitutive relationship with equal weight of the kinetic and frictional contributions need to be refined in order to reflect the real dense granular flows.

**Keywords:**Granular flow; Constitutive relations; Couette flow; Kinetic theory; Kinetic–frictional model;

Erratum to “Air entrainment in dip coating under reduced air pressures” by H. Benkreira; M.I. Khan

*(1740)*.