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.     

Utilization of polymeric materials in laser fusion target fabrication

A variety of coating techniques are described that are used for depositing uniform films of a predetermined thickness and high surface quality of a laser fustion target (LFT). The first coating method involves the thermal cleavage of di-p-xylylene. The product of this thermal treatment polymerizes readily to poly(p-xylylene). Addition of pendent groups onto the dimer di-p-xylylene extends the usefulness of this coating method. The modified dimers deposit films at significantly higher rates and with improved surface quality compared to films produced from the unmodified dimer. Spherulitic morphology is eliminated and the degree of crystallinity is decreased with the addition of pendent moieties on the phenylene ring. This reduction in the size of the morphological units significantly reduces the surface perturbations caused by these structures, giving rise to target coatings with the required smoothness. Complete elimination of all spherulites and also all lamellae (i.e., crystallinity) occurs with the glow discharge polymerization technique. The glow discharge polymerization of p-xylene is used to coat a LFT rapidly with a highly crosslinked, amorphous, transparent film having a high surface quality. The system is designed to eliminate powders from depositing on the target's surface. Finally, a technique is described for depositing inorganic and/or organic coatings using dilute aqueous solutions of colloidal particles. This method is unique in that the films are composed of layers of particles with each layer possessing the identical surface charge and chemical makeup.     

Water transport in ion exchange membranes

Water transport across ion exchange membranes has been studied experimentally. Water transference numbers were obtained from streaming potential measurements for cation and anion exchange membranes. At low salt concentration the water transference number reaches a limiting value which for the cation exchange membranes seems to be closely correlated with the cation—water friction in infinite dilute solutions.The observed water transference number for a given anion exchange membrane does not seem to be much dependent on the type of coion even at concentrations where the membrane is no longer perfectly permselective.     

Procedures for the initial design of chemical processing systems

Process synthesis as a problem in devising chemical processing structures is considered as a multi-step search procedure beginning with the consideration of production goals one at a time and ending with a feasible process flowsheet with regard to mass and energy balance. The theoretical foundation for the search procedure employed guarantees that a solution will be found in a finite number of steps if one exists within the information constraints imposed. These information constraints include the availability of raw materials and reaction chemistries provided to the computer program.A depth-first sequential search for a feasible process structure is based on the following rules: (a) use the compositionally most similar sources to generate products, (b) give preference to by-products instead of raw materials, and (c) reduce mass load on separation tasks.     

Computer-aided analysis and design of robust multivariable control systems for chemical processes

A conceptual framework to design robust process control systems is develope d and its realization through an interactive computer-aided design software is presented. The overall design methodology is based on a unified treatment of recent theoretical results in modern control and new computational techniques in symbolic logic manipulation, singular value decomposition and optimization. Several physical examples are given to demonstrate the application of the design approach and the utility of its computer software.     

Dynamic interfacial tension minima in finite systems

A physical model is presented to explain the dramatic interfacial tension minima that may sometimes occur during the equilibration of a surface-active solute between two liquid phases. The effects of bulk phase mass transfer resistances, interfacial sorption kinetics, and finite phase volumes are included. Adsorption and desorption barriers are treated both by linear and Langmuir kinetics. Dynamic interfacial tension minima are predicted whenever the solute mass transfer resistance is higher and/or a large desorption barrier exists in the phase towards which transfer is occurring. Experimental dynamic tension minima for acidic organic phases contacting an alkaline aqueous phase can be rationalized by the existence of desorption barriers. The relevance of such tension minima to interfacial turbulence, spontaneous emulsification, and enhanced oil recovery is discussed.     

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.     

Study of interfacial kinetics for liquid-liquid systems—II: Intrinsic kinetics of the copper chelation reaction with an oxime-based reagent

The interfacial chelation of copper from copper sulfate-sulfuric acid solutions by the commerical oxime extractant LIX65N in toluene is studied using the liquid jet recycle reactor. The intrinsic initial rate of copper chelation by LIX65N is found to be represented in mol. cm^?2-sec^?1 by the expression R = $\text{1.22 × 10}{}^{\mathrm{?8}}\text{{Cu}{}^{\mathrm{2+}}\text{}[LIX65N]}\text{{H}{}^{\mathrm{+}}\text{} + 0.0288[LIX65N]}$ where {Cu²⁺} and {H⁺} are the cupric and hydrogen activities. The interfacial rate expression above is determined as free from diffusional contributions by using a numerical solution to model mass transfer with interfacial reaction in the LJRR. The above expression is selected on statistical and physical grounds from a set of models systematically generated from the most probable reaction pathways. The liquid jet technique has been shown to be an effective and accurate (±6% experimental error) technique for the study of liquid-liquid interfacial reactions.     

On synthesis of optimal chemical-technological systems

The paper presents a technique for the synthesis of optimal systems based on the combination of the method of structure variables [1] and the “Branch-and-bound” [2] algorithm.Complex method [3] is used to solve the problems of optimization.     

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.     

An analysis of a nonisothermal one-component sorption in a single adsorbent particle

Heat and mass balances describing the process of a nonisothermal sorption within an adsorbent particle are formulated. Application to gas—solid adsor The governing equations represent a set of coupled strongly nonlinear parabolic equations. A finite-difference method based on explicit—implicit proc The equations are solved for a case of a strong adsorption accompanied with significant heat generation effects for molecular and Knudsen diffusion. It temperature differences may be of the order of magnitude 10–50°C.     

Thermodynamic and hydrodynamic properties of polymer — tetrahydrofuran — water systems: 2. Gel chromatography of polystyrene

Chromatogram shapes, concentration effects and calibration curves in g.p.c. were studied for dry tetrahydrofuran and its mixtures with water up to 8.9 vol. %. Polystyrene reference materials were separated with silica-based column packings. The results can be generalized fairly well and lead to the conclusion that the gel chromatographic data are considerably influenced by humidity present in the eluent. Ghost-peaks appeared in the domain of high elution volumes (V_e) using mixed eluents. Simultaneously, the solute peaks changed their widths and the slope of the plot of V_eversus injected polymer concentration changed with water content. However, most important were the shifts in the polymer elution volumes that were caused by the complex of interactions in the system gel-mixed eluent-solute and depended on the amount of water present in eluent. Thus, the dry eluent is an inevitable condition for obtaining g.p.c. results of high precision and reproducibility. The use of a guard column filled with a sorbent strongly trapping water is proposed in g.p.c. with hygroscopic eluents. Similar precautions are to be considered for any admixture in the eluent, especially if its content cannot be kept constant and if its polarity differs substantially from the polarity of the eluent.     

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.     

Sandstone water sensitivity: Existence of a critical rate of salinity decrease for particle capture

A critical rate of salinity decrease (CRSD) was found to exist in the water sensitivity of Berea sandstone. The CRSD was determined by conducting core flood experiments in which the salinity was decreased exponentially by placing mixers of various volumes in the inlet stream. Above the CRSD, drastic reductions in core permeability occur due to a log jam, or bridging effect, at the pore constriction. Below the CRSD, the released clay particles pass through the constriction without being captured and therefore very little reduction in core permeability occurs. A critical particle concentration was also identified above which bridging occurs. Based on the experimental observations, a mathematical model was developed to delineate the regions of high and low permeability reductions. The model predictions are in good agreement with experimental measurements.     

Verification of a model for estimating the void fraction in a three-component randomly packed bed

The void fraction in a three-component randomly packed bed was calculated from the authors' model, and the calculated values were compared with published experimental data for spherical and irregularly shaped particles and with results from computer simulations. Results from the model were in good agreement with simulated and published experimental data.     

Formation and aging of colloidal multicomponent systems

The influence of the cationic surfactant, n-dodecylamine nitrate, on negative silver iodide sols has been studied in a range of qualitative and quantitative changes in surfactant aqueous solutions (in the neighbourhood of the c.m.c.). Silver iodide was formed and aged from concentration to concentration under different conditions. The role of surfactant in silver iodide formation and aging are discussed.     

Dynamic characteristic of binary srv distillation systems

This paper reports a simulation study of SRV (secondary reflux and vaporization) distillation system using a perturbed linear model. The important factors which affect the dynamics are the heat transfer rate between the rectifying and the stripping sections, and the mean temperature differences between the process fluid and the cooling or heating medium at the condenser and the reboiler. The system becomes unstable if the mean temperature differences at the condenser and the reboiler are small, and the heat transfer rate between the rectifying and the stripping sections falls within a certain critical range of values. On either side of this range the system is insensitive to any kind of disturbances because of the inherent regulation occurring between them. Modal analysis shows that the external reflux rate activates the slowest mode which contains mainly the contributions of liquid concentrations on all stages.     

Development of a new system for circulating fluidized particles within a single vessel

A new system for circulating fluidized particles within a single vessel has been developed which has the same advantages as the dual-bed systems widely used for the FCC process and the Fluid Coker process. The interior of the vessel was divided into four sections by intersecting two flat vertical plates at right angles. Two sections were used for the upflowing bubbling fluidized beds and the other two sections were used for the downflowing bubble-free fluidized beds. Fluidized particles were circulated between the two upflowing beds, which were to be used as reactors, through the two downflowing beds, which were to be used as downcomers. Solid particles were fluidized by several streams of gas injected at several stages of the bed. The effects of these gas injection rates on the circulation rate of solids were investigated. The circulation rate of solids was measured using both a measuring box, which collected the overflowing particles, and the downward motion of a gauze net induced by the descending particles. The static pressure distribution within the vessel and the residence time distribution for coarse foreign solids in the system were measured. The present research demonstrated that the proposed system potentially had the same advantages as the conventional dual-bed system, and that the system can be applied to the simple gasification of biomass and solid wastes.     

Optimal control of a freeze Dryer—II: Dynamic analysis

The unsteady state model and Lagrangian equations of the freeze-drying process presented in Part I[4], have been solved for non-fat reconstituted milk and turkey, using the control strategies presented in [4]; the results were then used to evaluate the coefficients representing the necessary conditions of optimality. The results for non-fat reconstituted milk confirm the results of the quasi-steady state analysis [4], except for a time segment very close to t = 0. Simulations with turkey breast showed similar results at low pressures, but it was found that at higher pressures the assumed control policies which were based on a quasi-steady state analysis[4], were not optimalThe non-steady state results indicate that for turkey, intermediate rather than maximum, pressure control policies were optimal for cases (a) and (b) in[4]. However, in practice these optimal pressure policies might produce only minor improvements in drying time when compared with a near optimal policy. It was also found that changes in the interface constraint affected drying times only marginally, but that large heaters operating under optimal control had a pronounced effect on the drying time.