A. M. Voloshchuk

Kinetics of nonisothermal adsorption by biporous adsorbents

Conclusions1.A method is proposed for studying the kinetics of nonisothermal adsorption under quasiadiabatic conditions that enables the role of mass and thermal transport in the overall adsorption process to be evaluated.2.For the adsorption of xenon on CaA zeolite the process is basically controlled by the dissipation of the heat of adsorption, while for the adsorption of n-butane by a plate of CaA zeolite 0.18 cm thick, heat effects can be ignored at low surface coverages.

Kinetics of nonisothermal adsorption by biporous adsorbents Communication 3. Kinetics of the adsorption of xenon by CaA zeolite

Conclusions1.Analysis of experimental data on the kinetics of xenon adsorption by CaA zeolite, using the equations for the moments of the kinetic and temperature curves for a model which takes account of internal diffusion and outward heat transfer for a linear adsorption isotherm, has shown that the rate of outward heat transfer plays the principal role in the adsorption kinetics.2.Over the pressure range studied the outward heat transfer coefficient is independent of the xenon pressure, while the diffusion coefficient in the transport pores decreases markedly with an increase in pressure.3.The possibility of determining the heat and mass transfer coefficients from an analysis of the temperature curves only is shown.

Kinetics of nonisothermal adsorption by biporous adsorbents Communication 1. Effect of finite rate of dissipation of heat of adsorption on kinetics of adsorption by zeolites

Conclusions1.An analysis of the kinetics of nonisothermal adsorption for linear adsorption isotherms has shown that the influence of a finite rate of adsorption-heat dissipation cannot be neglected a priori even when the changes in adsorbent temperature are small; this is particularly true when studying fast adsorption processes.2.In the adsorption of xenon by CaA zeolite at 303‡K, in spite of the very small increase in the adsorbent granule temperature from evolution of the heat of adsorption, the heat effects play the main role in the adsorption kinetics. In contrast, for the trans-2-butene/NaA zeolite system, the influence of nonisothermality can be neglected, even though the temperature of the adsorbent rises 20‡K in the course of the experiment.

Use of the method of moments for the determination of the coefficients of internal diffusion in the case of adsorbents with a bidispersed porous structure and linear adsorption isotherms

1. Theoretical formulas were obtained for the moments of the kinetic curve in the case of adsorbents with a bidispersed porous structure. 2. On the basis of the formulas obtained, a rather simple method was proposed for the determination of the internal diffusion coefficients from the experimental kinetic curves.

Kinetics of nonisothermal adsorption by biporous adsorbents. Communication 8. Analysis of the precision of methods of determining the parameters of heat and mass transfer from the statistical moments of kinetics and temperature curves

Conclusions1.In determining the parameters of heat and mass transport from kinetic and temperature curves, the most reliable results are obtained when the kinetic and temperature curves are analyzed together using the moments of the first order.2.Analysis of the temperature curve during adsorption of both xenon and n-butane on CaA zeolite affords the possibility of determining the rate-determining step of the adsorption process and of computing the temperature component Mq and the coefficient of heat loss h, but does not allow the coefficients of mass transfer to be computed. Using this method to determine the parameters of mass and heat transfer requires very high precision in the measurement of the temperature curve.3.In the adsorption of xenon onto finely crystalline CaA zeolite, the precision of the determination of the kinetic curve is sufficient for separating the heat and the diffusional components using the moments of the first and second orders, and in the adsorption of n-butane the lower precision due to the influence of the nonlinearity of the adsorption isotherms makes a complete analysis of the process from the kinetic curve impossible.

KINETICS OF NONISOTHERMAL ADSORPTION BY BIPOROUS ADSORBENTS. COMMUNICATION 5. ANALYTICAL DESCRIPTION OF THE TEMPERATURE CURVES AND DETERMINATION OF MASS- AND HEAT-TRANSFER PARAMETERS

Conclusions1.We have presented an analytical description of the temperature and kinetic curves for adsorbent granules in the form of plates, and we have proposed methods for the determination of the mass- and heat-transfer parameters based on experimental temperature curves.2.In the case of the Xe-CaA system the adsorption process is controlled by the external heat transfer.

Intracrystalline diffusion in zeolites. Communication 5. Kinetics of adsorption of n-butane by CaA zeolite

Conclusions1.The rate of adsorption of n-butane by large particles of CaA zeolite is governed by intraparticle diffusion.2.In the range of surface coverages 0.25 to 3 molecules per cavity, intraparticle diffusivities rise with increasing surface coverage.3.The activation energy for intraparticle diffusion and the preexponential factor decrease with increasing surface coverage.4.Intraparticle diffusivities obtained from nonequilibrium kinetic adsorption measurement are in satisfactory agreement with self-diffusivities obtained by pulse gradient NMR.

Intracrystalline diffusion in zeolites. Communication 4. Influence of heat effects and diffusion in transport pores on the kinetics of adsorption of n-butane by caa zeolite

Conclusions1.In the adsorption of n-butane by CaA zeolite from a one-component gas phase, the relative role of diffusion in transport pores decreases with an increase in the degree of filling, and the role of the thermal component increases.2.The contribution of the thermal component increases with a decrease in the size of the adsorbent granule, and when the thickness of the plate is 2L < 0.01 cm, the adsorption kinetics are basically determined by the rate of dissipation of the heat of adsorption with all degrees of filling studied.

Kinetics of nonisothermal adsorption by biporous adsorbents. Communication 6. Description of kinetic adsorption curves with account of the temperature dependence of the intracrystalline diffusion coefficients

Conclusions1.An analytical description has been given of the kinetic adsorption curves with account of the influence of the change in adsorbent temperature due to the evolution of heat and adsorption on the coefficient of intracrystalline diffusion of the adsorbate.2.The deviation of the experimental kinetic curves for the adsorption of trans-2-butene by finely crystalline NaA zeolite from those calculated from the equation of isothermal diffusion is due to the influence of temperature on the coefficient of intracrystalline diffusion.

Study of intracrystalline diffusion in zeolites. Communication 3. Kinetics of adsorption of trans-2-butene by NaA and NaMgA zeolites

Conclusions1.For the trans-2-butene-NaA zeolite system, the rate of adsorption is determined by the intracrystalline diffusion. The effect of transfer in the transport pores and the final rate of dissipation of the heat of adsorption can be neglected. The effective intracrystal-line diffusion coefficient at 303°K is equal to 0.8·10−16 m2/sec.2.In adsorption of trans-2-butene by Na8Mg2A zeolite with a stepwise change in the pressure of the adsorbate, the kinetic curves are satisfactorily described by an internal diffusion equation for the kinetics of isothermal adsorption. The effective intracrystalline diffusion coefficients are a function of the degree of filling of the adsorbent and increase from 2.5·10−16 to 32·10−16 m2/sec with an increase in the filling from 0.44 to 1.93 mmoles/g. The self-diffusion coefficients calculated with the Darken equation also increase slightly with an increase in the filling (2.3−3.7·10−16 m2/sec).3.The significant differences between the calculated and experimental curves in adsorption of trans-2-butene by microcrystalline NaA zeolite in the case of very convex adsorption isotherms could be caused by the effect of heat effects on the intracrystalline diffusion coefficient.