Carbon dioxide transport through enzymatically active synthetic membranes

The facilitated transport of CO₂ through thin liquid membranes, Millipore filter membranes, and cross-linked protein membranes has been investigated using a tracer ¹⁴CO₂ technique. Both the uncatalyzed reactions and the enzymatic reactions catalyzed by carbonic anhydrase in homogeneous solution and immobilized in several membrane configurations were studied. The steady-state transport data were reduced in terms of an analytical model for simultaneous reaction and diffusion which permitted the direct determination of the diffusional and enzyme kinetic parameters. The experimental method illustrates a powerful technique for measuring rapid reaction kinetics. In addition, a general multi-layer membrane model is developed which is capable of treating membrane kinetic heterogeneities. The multi-layer model yields a useful definition for the reaction boundary layer and provides a comparison among different membrane kinetic configurations for membrane design purposes.     

Chloride-sulphate exchange on anion resins — kinetic investigation,IX. Direct,isotopic and reverse exchange

Experimental kinetics of direct, isotopic and reverse Cl^?/SO⁼₄ exchange on different anion resins are presented.At very low concentration (C = 6 × 10^?3 N) extremely high resin selectivity toward sulphates occurs (separation factors ranging from 52 to 589). Consequently chemical interaction of interdiffusing species within the resin seems to influence the rate determining step, exception made for isotopic exchange. Application of model equations from Nernst-Planck and SN₂ rate theories is discussed.     

Optimal conditions for obtaining polymeric membranes in binary systems: a numerical analysis

The mathematical model, describing solvent evaporation from a cast film, is used with the aim of formulating a general approach to the optimization of RO membrane production.The results were correlated in terms of a “productivity factor.” This productivity factor approaches unity when the casting film has a thin and compact layer with high asymmetric character on the evaporating surface.The model is tested for different values of the parameters contained in the concentration dependence law of the mutual diffusion coefficient in solvent-polymer binary system. Also different solvent rates at the evaporating surface (low, medium, high) are employed. The numerical results were expressed in terms of intrinsic properties of the binary system with the aid of the free volume theory.Testing the model indicates that increased productivities should be expected for solutions of low free-volume polymers in high cross-section solvents. However, in general, for a given polymer-solvent system, it is very important to adjust solvent evaporation rates by varying solvent concentration in the casting atmosphere.On the other hand it could be necessary to control the plasticizing action of the solvent on the polymer in order to influence the diffusion process.     

Initial flux decline of the cellulose acetate butylate membranes with time under RO performance

Initial flux decline, i.e. compaction, has been investigated for the cellulose acetate butylate membrane prepared by varying casting condition. The compaction behavior of membranes prepared in a short evaporation period was different from that prepared in a long evaporation period. Compression tests showed that the membrane thinned by 30 ～ 40% instantaneously when pressurized; afterwards it thinned gradually. This corresponded to the behavior of decline. It appears that a change in decline occurs reflecting membrane structure depending on casting condition. The results were discussed for a pore model based on Kozeny-Carman equation and on the viscoelastic deformation model.     

Uniqueness criteria for first order catalytic reactions with external transport limitations

Uniqueness and multiplicity criteria are developed for a non-isothermal first order reaction occurring in infinite slab or spherically shaped catalyst pellets, with external heat and mass transport limitations. The analysis is done for the analytically resolvable model wherein the catalyst pellet is assumed to be internally isothermal, but with internal gradient of concentration. The results are also compared with those obtained from numerical integration of the complete heat and mass balance equations, including internal temperature gradient as well. The general features of the results will carry over to pellets of any shape.     

Hold-up of fine particles in the packed dense bed of the multisolid pneumatic transport bed

The hold-up of fine particles in the packed bed of dense particles in the multisolid pneumatic transport bed was experimentally examined. In the experiments, three types of fine particles were used including FCC particles (65 μm), glass beads (105 μm) and fine sand (155 μm), while three types of dense particles were used including aluminum particles (5.50 mm and 6.96 mm) and iron oxide particles (11.08 mm). The hold-up of fine particles in the packed bed of dense particles was found to be higher by a factor as large as 6 compared with that observed in systems without dense particles. An interaction coefficient between fine particles and dense particles was defined based on the momentum balance equation of fine particles in the packed bed. This interaction coefficient, which is empirically correlated, was utilized to account for the hold-up of fine particles in the packed bed.     

A “free volume” model of permeation of gas and liquid mixtures through polymeric membranes

A recent “free volume” model of gas permeation (3) has been extended to the transport of gas mixtures through nonporous polymeric membranes. The present model assumes that the rates of transport of the components of a mixture depend on the free volume of the gas-polymer system, and that the effect of these components on the free volume is additive. The latter assumption limits the model to relatively dilute systems, with total penetrant concentrations of perhaps less than 0·2 volume-fraction. The prediction of permeation fluxes and permeability coefficients requires the knowledge of specified free-volume parameters which can be determined from measurements of diffusion coefficients and viscosities of the pure penetrant-polymer systems. When the systems are sufficiently dilute to obey Henry's law, the permeability coefficients for the components of a gas mixture can be predicted using only permeability measurements with the pure components. The extended free-volume model can be applied also to the permeation of liquid mixtures. The theoretical predictions are compared with the results of several experimental studies, and the potential usefulness and limitations of the model are discussed.     

Mass transfer in liquid fluidized beds of ion exchange particles

Mass transfer rate data in a shallow liquid fluidized bed of ion exchange resin particles have been obtained in the range 2 < Re_s, < 25. At low voidage, ε < 0.55–0.60, the rate of mass transfer is reduced and it is inferred that a fluidized bed tends to maintain an ordered axial structure. This is lost at high voidage due to increased flow perturbation caused by the distributor plate. A generalised correlation is given for fluidized, fixed and distended beds in the range 2 < Re_s, < 25.     

Low energy ion scattering study of Ni-MoAl2O3 catalysts

The principles of low energy ion scattering (ISS) and its application to the study of real catalyst surfaces are discussed. As a special example, the effect of the calcination temperature of a Ni-Mo/ Al₂O₃ on the elemental distribution is studied. From the ISS “depth profiles” it can be concluded that at temperatures <770 K the molybdate phase on the Al₂O₃ support forms islands with three-dimensional extension while uncovered support surface remains even though the loading corresponds to the theoretical monolayer capacity. A spreading of the molybdate over the support surface occurs at 870 K to form a more or less closed monolayer. Simultaneously, Ni²⁺ ions are lost from the surface molybdate by penetration into subsurface layers of the Al₂O₃ matrix where they occupy octahedral and tetrahedral sites. The importance of the redistribution of promotor ions and the dispersion of the molybdate phase for the catalytic performance is emphasized.     

Production of zinc ferrite in a mechano-chemical reaction by grinding in a ball mill

Zinc ferrite was obtained by prolonged dry grinding of stoichiometric powder mixtures of ZnO + -Fe₂O₃ or ZnCO₃ + -Fe₂O₃. The kinetics of transformation was studied by X-ray diffraction, Mössbauer spectroscopy and DTA. Crystallite size reduction followed the process, and defect structures resulted. The carbonate decomposed as a necessary step in the process. In the mixture of NiO + -Fe₂O₃, nickel ferrite was not obtainable, although considerable grain size reduction was observed. This was attributed to the inverse spinel configuration of the nickel ferrite, differing from the normal spinel configuration of the zinc ferrite. In the -Fe₂O₃ and the zinc ferrite, iron ions occupy octahedral sites, while in ZnO and the ferrite, zinc ions occupy tetrahedral sites, and metal ion—oxygen distance is almost identical. These facilitate the transformation. In the nickel ferrite, iron ions occupy both sites, which calls for high strains of -Fe₂O₃ before it can undergo transformation. Further implications of the processes are discussed.     

Catalytic reactions in transport-line reactors

The case of an irreversible first order catalytic reaction is considered in a transport-line reactor, via a model which assumes plug flow of both the solid and gas phases, to investigate reactor performance as a function of major operating variables. Under certain fluid mechanical assumptions, a detailed study is made of the effect of catalyst particle size on reactor conversion.     

Selectivity improvement in the SRC process

The selectivity for hydrodesulfurization over hydrogenation has been examined in a new short residence time catalytic two-stage SRC process, which has the potential of producing a low-sulfur solid SRC product to meet the newly proposed EPA new point source emission standards (NPSES). In the first stage of the process, residence time and hydrogen consumption are minimized through the use of an inexpensive mineral catalyst (SRC residue ash) that has been treated under a combustion environment to improve its selectivity for hydrodesulfurization over hydrogenation. The second stage of the process involves hydrotreating the filtered liquid product with a commercial Co-Mo-Al catalyst, before splitting into a solid SRC and solvent recycle by distillation. Several process variables — such as type of coal, catalyst, temperature, hydrogen partial pressure, and reaction time — have been examined to provide information on hydrogen consumption, product distribution, sulfur removal, SRC yield and solvent quality. The results show that the ash of SRC residue can be used to selectively catalyze desulfurization over hydrogenation in SRC processing. Selectivity for desulfurization in two stage hydrodesulfurization of coal is improved by using high reaction temperatures, short residence times, the ash of SRC residue as a first stage catalyst, and Co-Mo-Al as a second stage catalyst. Two stage catalytic SRC processing is more selective for hydrodesulfurization than catalytic or non-catalytic single stage SRC processing.     

Simultaneous diffusion and chemical reaction in two-phase systems

An approximation is given for the case of simultaneous steady state diffusion and chemical reaction in a liquid—liquid system under such conditions that diffusion and chemical reaction are of the same order of magnitude in the two phases. The two-film model was adopted to represent the system, and results were plotted as variations of the enhancement factor with the diverse parameters affecting it. Comparison was made with an exact numerical solution, and the results show that the proposed approximation leads to small errors for low reaction rates, and gives a first approximation for the mass transfer rate for very rapid chemical reactions.     

Mercury porosimetry in pharmaceutical technology

The literature on the use of mercury porosimetry in pharmaceutical technology is reviewed. The relation between the application and the preferable pore size distribution presentation is also discussed. In addition, pore size distributions of lactose tablets are presented and the calculated values for the porosity, pore diameter and specific surface area are compared with results from air permeability and nitrogen adsorption measurements. The agreement is acceptable taking into account the simplifications and the inaccuracy of the methods.     

Photocatalysis in a slurry reactor

A series of experiments have been conducted on the photocatalytic reduction of methylene blue with zinc oxide catalyst in a slurry reactor. It is shown that by using a procedure involving separate experiments to determine the shielding effects of the suspended solid (inert solid of similar particle size with a standard actinometric solution) it is possible to identify experimentally the region in the reaction system where imperfect solids mixing influences the rate of reaction. The reactor employed here is also shown to provide a relatively simple means for kinetic measurements in photocatalytic reactions. The rate constants obtained in this study for methylene blue reduction were weakly dependent on zinc oxide concentration in the suspension. A value of 3·67 M^?1 sec^?1 at 0·1 wt.% ZnO at 31·9°C was obtained, where M is the molarity of the solution in methylene blue. This may be compared with 1·25 × 10⁴M^?1 sec^?1 reported by Yoneyama [14] for methylene blue reduction on a rutile crystal of titanium dioxide at room temperature, and 3·33 M^?1 sec^?1 for p-nitrosodimethylaniline reduction in a 0·1 wt. % ZnO suspension, also at room temperature[13].     

Heat and mass transfer in honeycomb catalysts—II

Heat and mass gas-to-solid coefficients associated with the vaporisation of water and some hydrocarbons from the surface of a porous, monolithic structure were experimentally established. The mass transfer results are correlated using Reynolds and Schmidt number and the length-to-diameter ratio. The data for heat transfer are correlated on the Nu-Re(d/L) plot. The experimental results can be used for design of afterburner reactors utilizing monolithic structures.     

Oscillations in crystallite shape during heating in hydrogen of model iron/alumina catalysts

Transmission electron microscopy is used to bring evidence for an alternation in the shape of iron crystallites supported on planar, nonporous, alumina substrates, when the specimens were heated, at 400 °C, in hydrogen contaminated with traces of oxygen (less than 1 ppm) and/or moisture. The crystallites alternated between a torus which encloses a cavity and a shape in which an annular gap separates the torus from a core in the cavity. Electron diffraction indicated that this is accompanied by a corresponding alternation in the chemical state of the catalyst. The torus with an enclosed cavity corresponds to an oxidized state, in which iron oxides, resulting on oxidation by the contaminant oxygen, form solid solutions in alumina, with the tendency to approach the aluminates FeAl₂O₄ and/or Al₂Fe₂O₆. The torus containing a core in the cavity corresponds to a less-oxidized state, in which a part of the previously dissolved oxide diffuses out to be (at least partially) reduced by hydrogen to the metal and/or to a lower oxide (including possibly nonstoichiometric oxides). After prolonged heating, the two alternating shapes become less distinct, probably because of mechanical fatigue which eventually leads to a breakup of the crystallites. Similar behavior is observed at temperatures up to 600 °C, though with diminished sharpness of the two alternating shapes above 500 °C. An attempt is made to explain the shape alternation on the basis of surface phenomena resulting from the physical and chemical interactions between gas, crystallite, and substrate.