B. J. McCoy

Improving permeation flux by pulsed reverse osmosis

Abstract Pulsed and nonpulsed reverse osmosis experiments were conducted with a sucrose solution in cylindrical cellulose acetate membranes. For pulsing frequencies up to 1 Hz, permeation increases of more than 70 per cent were observed. A simple quasisteady-state theory for concentration polarization based on a film model of mass transfer describes the increase.

Transport phenomena in the rarefied gas transition regime

Abstract For rarefied gases the collision frequency of the Krook kinetic equation is expressed as the sum of wall and gas collision frequencies. Effective transport coefficients arise from the Chapman—Enskog theory, and are similar to those proposed by other methods. The transport of either energy, momentum, or mass is analysed for several geometries. Profiles of temperature, velocity, and concentration, which involve jump and slip at the wall, are derived, and accommodation effects are included. The expressions have the proper limits for the continuum and free-molecule regimes, and compare well with other theories and experimental data for transition regime. Heat conduction formulas apply to polyatomic gases. Plane, concentric cylinder, and spherical geometries are considered.

Supercritical fluid desorption from activated carbon

Abstract Experimental data and mathematical models are presented for desorption of ethyl acetate from activated carbon with supercritical carbon dioxide as solvent. The effluent concentration was monitored continuously with a flame ionization detector (FID). With a well-mixed reactor model for the bed of small thickness, desorption was studied as a function of flow rate, pressure, temperature and particle size. External mass transfer resistance was apparent at very low flow rates. From studies on the effect of temperature at different pressures, a cross-over effect was observed, similar to that for solubility in supercritical solvents. Intraparticle diffusion resistance was found to be significant for the largest particle size studied (dia. = 3.15 mm). A first-order, reversible adsorption model successfully fitted the experimental data. The energies of activation and heats of adsorption were obtained from the optimized adsorption rates and equilibrium constants.

An improved model for the prediction of multicomponent ion exchange equilibria

Abstract Nonideal behaviour for liquid and resin phases is investigated theoretically and experimentally for ion exchange equilibrium of the aqueous ternary system Mg 2+ , Ca 2+ , Na + with Cl − . A semi-theoretical model is proposed in which the Pitzer approach is applied to the solution nonidealities, and the Wilson approach, based on experimental binary data, is applied to the resin. Predictions of the model agree with experimental data for both binary and ternary systems. The predictions of the proposed model are more accurate than those made using existing models.

Parabolic profile approximation (linear driving-force model) for chemical reactions

Abstract The parabolic approximation for the concentration profile inside a particle yields a substantial simplification in computations. The linear driving-force model for combined internal diffusion and external mass transfer arises from the approximation. We have applied this approximation to consider the time dependence of two cases of isothermal, irreversible, first-order chemical reaction in a spherical particle and in a slab: (1) when the reactant is a nondiffusing (adsorbed or solid) component initially inside the particle and the products diffuse out of the particle (e.g. for a reaction extraction), and (2) when the reactant is diffusing into the particle (e.g. for a catalytic reaction). The approximation is found to be satisfactory over a wide range of parameters for case (1). For case (2), however, the accuracy of the approximation is limited to small Biot numbers and Thiele moduli.

Hermite polynomial representation of chromatography elution curves

Abstract Elution curves for aqueous solutions of NaCl and β-lactoglobulin were measured for columns packed with porous polyacrylamide gel or non-porous glass particles. The elution curves were mathematically represented by series of Hermite polynomials in terms of the moments of the chromatographic pulses. Fitting the representation to skewed output pulses provides a criterion for trimming tails on elution curves.

Chemical vapor infiltration: modelling solid matrix deposition in ceramic—ceramic composites

Abstract A model is developed to predict the effect of diffusion, the rate of deposition, and spatial distribution of a solid matrix in a woven fabric forming a ceramic—ceramic composite. To the knowledge of the authors there has been no prior treatment of chemical vapor infiltration in a system of plies consisting of an assembly of woven tows containing bundles of filaments. The model predicts the times to fill the gaps around the filaments of a tow, the space between plies, and the holes between the tows. It also predicts the porosity of the composite as a function of position and processing time. Predictions are made for an illustrative case of deposition of silicon carbide matrix within a woven carbon fabric.

VOC immobilization in soil by adsorption, absorption and encapsulation

Abstract Immobilization of volative organic compounds (VOCs) in contaminated soils by solidification/stabilization ( S S ) processes is an attractive potential remediation process. The objectives of this study were as follows: to evaluate S S procedures, including a new process with a VOC absorbent and silica coatings; to apply a straightforward and inexpensive analytical technique for characterizing VOC behavior during processing; and to propose the expanded use of waste materials such as contaminated soil, rice hulls, and shredded-tire particles. We investigated how the adsorbents, absorbents and encapsulating agents can be combined to retard volatilization and resist acidic aqueous leaching of toluene. The toluene released at each step of the S S process was determined by an accurate headspace analysis method. Results showed that combining shredded-tire particles for absorption and sodium silicate for encapsulation is a promising method for immobilization of toluene in soil.

Chemical vapor infiltration: modelling solid matrix deposition for ceramic composites reinforced with layered woven fabrics

Abstract A model is developed for chemical vapor infiltration in a sample of overlapping, woven plies. The holes through each ply, formed by the interweaving of filamentous tows, are considered to be staggered maximally for the overlapping plies. The results are compared with another bounding case, the aligned-hole model studied previously. The staggered-hole model predicts concentration profiles, deposition amounts, porosities, and changes of dimensions with time. The model also explains the effects of diffusion, rate of deposition, and spatial distribution of filaments in the woven fabric. Staggering the holes leads to somewhat less uniform deposition in the holes between tows and in the spaces between plies.

Theory of capillary chromatography : Effect of coiling and interphase mass transfer

Abstract A satisfactory mathematical description of gas-liquid partition capillary chromatography is desirable for design and optimization of separation processes, as well as for determination of physical properties. A mathematical model is presented that includes the effects of mass transport in the mobile and stationary phases, assumes equilibrium at the gas-liquid interface, and accounts for axial dispersion of solute. Interphase mass transfer is included, and is found to be a significant contribution to band-spreading. To account for the effect of coiling on axial dispersion, an empirical coefficient Θ is introduced in the expression for the effective axial dispersion coefficient, since a complete theory for this phenomenon does not exist. Attempts to correlate experimental Θ values with molecular weight of solutes, kinematic viscosity of carrier gases, film thickness, and temperature are described. A criterion is established for the improvement in resolution due to coiling.