The Chemical Engineering Journal and the Biochemical Engineering Journal最新文献

This paper is about sweet whey protein (α-lactalbumin and β-lactoglobulin) recovery by a fluidized ion exchange chromatographic process. Simplified models are proposed for both main steps of this cyclic process: a model with intraparticle diffusion for the adsorption of proteins (fixation step) and a lumped kinetic model for their desorption (elution step). The validity of these models and their physical background are discussed. They are then used for an economical optimization algorithm in order to determine operating conditions (fixation and elution steps durations, elution effluent recycling and liquid-phase velocity). Recycling the elution effluents leads to a slight improvement in the process but it was found to be less advantageous than an optimization of the liquid-phase velocity.

Studies on the axial solid mixing mechanisms in gas—solids cocurrent upflow and downflow circulating fluidized bed systems have revealed that, among the many influencing factors, flow direction has the most profound influence on the axial solids mixing. When the flow is in the direction of gravity (downflow in the downer), axial solids dispersion is very small and the flow pattern approaches plug flow; when the flow is against gravity (upflow in the riser), axial solids dispersion is significantly larger and the flow pattern deviates significantly from plug flow.Solids mixing is found to be mainly due to the dispersion of dispersed particles in the downer while two solids mixing mechanisms co-exist in the riser: the dispersion of dispersed particles and the dispersion of particle clusters. Dispersion due to dispersed particles is very small in both the riser and the downer, indicating that dispersed particles pass through the system in a near plug flow pattern. Dispersion due to particle clusters in the riser, on the other hand, is very significant, contributing to the large axial solids backmixing and the bimodal solids residence time distribution in the riser.

This paper focuses mainly on the development of a model for permeation through inert membranes, as encountered in many cases in ultrafiltration and in gas permeation through inert porous plugs. The ultrafiltration model is made up of a boundary layer transport model and a porous membrane model in series, which are connected by an equilibrium relation. The boundary layer model is developed with the Vieth approximation for turbulent diffusivity. For the internal membrane transport, a modification of the Maxwell-Stefan-Lightfoot equation is derived (the binary friction model), which in a natural way includes both interspecies (diffusive) and species-wall forces. Application for the partial separation of PEG-3400 from aqueous solution shows that membrane friction coefficients can simply be estimated from membrane resistance measurements and mixture viscosity data. The only adjustable parameter to be determined is the distribution coefficient between the free solution and the membrane pores. The differences between the Lightfoot approach and the dusty gas model (DGM) are shown to stem from errors in the drivations of the latter, thus invalidating the dusty gas approach in the normal region in which viscous friction effects become important. For gases, the binary friction model is developed to include Knudsen and viscous wall friction terms as well as intermolecular diffusion. It is shown to give excellent coverage of the He-Ar diffusion data of Evans et al. (J. Appl. Phys., 33 (1962) 2682; 34 (1963) 2020), with wall friction coefficients derived directly from Knudsen coefficients and gas viscosity data. The apparent success of the DGM in describing the same phenomena is shown to be caused by the relatively small importance of the wall friction forces at elevated pressures, and by the correct transition to Knudsen flow at low pressures. In addition, it is shown that diffusive slip phenomena in capillaries can be described well by the binary friction model.

The copper substitutional series Cu_xZn_1−xCr₂O₄ where x=0.0–1.0 was synthesized for an investigation of their properties as catalysts for higher alcohol synthesis. The catalysts were prepared by the high temperature aerosol decomposition technique. The process resulted in the formation of the series of metal oxides as uniform hollow spheres having surface areas of 21–35 m² g⁻¹. Characterization of the solid state materials by X-ray diffraction, d.c. arc plasma elemental analysis, BET surface area, EDX mapping, and scanning electron microscopy demonstrated that the particles were homogeneous solid solutions up to a copper substitution of 90% having an unusual cubic spinel structure throughout the series. The copper chromite structure was observed only at copper substitutions greater than 90%. Cesium was also added in varying concentration from 1 to 10 mol.% as a promoter, and these materials were shown by EDX mapping to have a homogeneous cesium dispersion. Comparison of these syntheses to classical methods of preparation demonstrated that the aerosol process resulted in a much higher phase purity with direct formation of the metastable, cubic spinel rather than the normal tetragonal structure.

There is growing interest in forming vanadium-containing mixed transition metal oxides, particularly for the preparation of oxidation catalysts. It is desirable in many cases to prepare the solid from homogeneous (stable or metastable) metal oxide solutions and gels in order to maintain the molecular-scale mixing that homogeneous solutions can provide. While this is always possible at extreme pH values, it is also desirable to use moderate pH so that the coating solution should be benign to common substrates.

We provide an overview of the insight that ⁵¹V NMR can provide into typical aqueous and alkoxide solution routes suggested by the literature for preparing multicomponent vanadium-containing solutions at moderate pH. Details of the preparation protocol can determine whether the solutions remain homogeneous; we examine the starting pH for aqueous solutions and the alcohol used for alkoxide solutions. Solution ⁵¹V NMR shows that the vanadium speciation differs markedly between stable and unstable solutions.