Douglas M. Ruthven

Counter-current and simulated counter-current adsorption separation processes

Abstract The application of counter-current and simulated counter-current process schemes to adsorption separations is reviewed with emphasis on industrial-scale operations and mathematical modelling.

A new experimental technique for measurement of intracrystalline diffusivity

A new experimental technique has been developed for measuring the intracrystalline diffusivities of strongly adsorbed species by following chromatographically the desorption curve for a small sample of zeolite, saturated at a low concentration level. Results obtained by this method for o -xylene in 50-μm and 100-μm NaX crystals are self-consistent and in good agreement with previously determined gravimetric values.

Diffusion in zeolites

Abstract Recent developments in the study of intracrystalline diffusion in zeolites by novel macroscopic methods and the results obtained by some of these methods are reviewed. For many systems there is a significant discrepancy between the macroscopic and microscopic (QENS, PFG NMR) diffusivity values. A possible explanation is suggested.

On the comparison between macroscopic and n.m.r. measurements of intracrystalline diffusion in zeolites

The extensive intracrystalline diffusivity data that have been obtained by both macroscopic and microscopic (principally n.m.r.) methods for a wide range of zeolitic systems are reviewed. Further experimental evidence for the self-consistency of the n.m.r. diffusion data has been obtained by applying both 1H and 19F resonances to study the self-diffusion of CHF2Cl in NaX zeolite and by varying the intensity of the external magnetic field. For relatively immobile systems, there is, in general, good agreement between the diffusivity values derived from both classes of experiment, but for the more mobile systems, the n.m.r. diffusivity values are substantially higher than are the values derived from sorption rate or membrane transport measurements. For several such systems (e.g., propane-silicalite, xylenes-NaX), both the microscopic and macroscopic measurements have been confirmed by different experimental methods, in different laboratories. In such cases, it seems unlikely that the discrepancy can be explained by mere shortcomings of the individual techniques (e.g., the intrusion of extracrystalline rate processes), and we are forced to conclude that both classes of experiment measure intracrystalline migration but on different time scales. Such behaviour is consistent with a slow equilibration between mobile and immobile intracrystalline states. Tentative evidence to support such a hypothesis is provided by frequency response measurements carried out over a wide range of time scales, but this is not confirmed by the evidence from the intensity of the n.m.r. pulsed-field gradient signals.

The nature of surface barriers on nanoporous solids explored by microimaging of transient guest distributions.

Nanoporous solids are attractive materials for energetically efficient and environmentally friendly catalytic and adsorption separation processes. Although the performance of such materials is largely dependent on their molecular transport properties, our fundamental understanding of these phenomena is far from complete. This is particularly true for the mechanisms that control the penetration rate through the outer surface of these materials (commonly referred to as surface barriers). Recent detailed sorption rate measurements with Zn(tbip) crystals have greatly enhanced our basic understanding of such processes. Surface resistance in this material has been shown to arise from the complete blockage of most of the pore entrances on the outer surface, while the transport resistance of the remaining open pores is negligibly small. More generally, the revealed correlation between intracrystalline diffusion and surface permeation provides a new view of the nature of transport resistances in nanoporous materials acting in addition to the diffusion resistance of the regular pore network, leading to a rational explanation of the discrepancy which is often observed between microscopic and macroscopic diffusion measurements.

Diffusion of C8 aromatic hydrocarbons in silicalite

Diffusion of p -xylene, ethylbenzene, and o -xylene in large (105 × 50 × 40 μm) crystals of silicalite has been studied by the zero-length column (ZLC) method and by gravimetric uptake rate measurements. The form of the uptake curves suggests that diffusion occurs primarily through the straight channels of the silicalite pore structure so both ZLC and uptake rate curves were interpreted on the basis of diffusion in a parallel-sided slab. The experimental data obtained by both techniques are consistent. The diffusivity of p -xylene is almost the same as that of benzene and about an order of magnitude larger than that of o -xylene, but there is very little difference in activation energy (~ 30 kJ/mol). The results, therefore, do not support the commonly accepted view that the difference in diffusivity between the ortho and para isomers results from the difference in their critical molecular diameters

The effect of crystallite shape and size distribution on diffusion measurements in molecular sieves

Abstract The problem of calculating the diffusivities of gases in molecular sieves from experimental sorption curves is discussed. Commercial molecular sieve pellets contain zeolite crystallites of various sizes and it is shown that both the shape and the size distribution of the crystallites have significant effects on the sorption curves. For this type of system it is not possible to calculate reliable values of diffusivities using a mean equivalent spherical radius and in order to interpret the sorption curves correctly detailed knowledge of the crystallite shape and size distribution is required. Systems containing spherical and cubic particles are considered in detail and generalized sorption curves showing the effect of particle size distribution are presented. Experimental crystallite size distribution measurements are used to interpret typical sorption data for Linde 5A molecular sieve. The solution of the diffusion equation, modified to take account of the particle size distribution, fits the experimental data well suggesting that the basic assumption of Fickian diffusion is justified. It is suggested that some of the apparent anomalies in reported diffusivity data for molecular sieves may in fact be due to the neglect of the effects of crystallite shape and size distribution.

Air separation by pressure swing adsorption on a carbon molecular sieve

Abstract A simplified dynamic model for a PSA air separation process is developed based on linearized mass transfer rate expressions and binary Langmuir equilibrium. Constant pressure is assumed during adsorption and desorption steps but the variation in flow rate through the column due to adsorption is accounted for. The model predictions, using independently measured kinetic and equilibrium data are compared with experimental results obtained in a simple two-bed air separation PSA system packed with a carbon molecular sieve adsorbent. The model is shown to provide a good representation of the experimentally observed behaviour over a wide range of conditions.

Adsorption and diffusion of nitrogen and oxygen in a carbon molecular sieve

Abstract The adsorption and diffusion of N2 and O2 in two samples of carbon molecular sieve (Bergbau-Forschung) have been studied by gravimetric and chromatographic methods. The uptake curves (for N2) show a rapid initial uptake followed by a much slower approach to equilibrium in the long time region. This behaviour can be accounted for by a dual stage (micropore/macropore) diffusion model. Diffusion of oxygen was too fast to be measured accurately by the gravimetric method although an approximate estimate may be obtained from the initial uptake rate. Over the range of the measurements (0–1 atm), the equilibrium isotherms for both O2 and N2 are almost linear and there is very little difference between them. Micropore diffusivities derived from the second moments of the chromotographic response peaks are in reasonable agreement with the values derived from the uptake curves [(Dc/rc2)O2 ≈ 3.7 × 10−3 s−1; (Dc/rc2)N2 ≈ 1.2 × 10−4 s−1 at 303 K], indicating a kinetic selectivity (DO2/DN2) of about 30. This is close to the kinetic ratio reported by Knoblauch (for a Bergbau-Forschung sieve) although our values of Dc/rc2 are considerably larger.

Sorption and diffusion of C8 aromatic hydrocarbons in faujasite type zeolites. I. Equilibrium isotherms and separation factors

Henrys Law constants and equilibrium isotherms have been determined for the four C8 aromatic hydrocarbons (o, m and p-xylene and ethylbenzene) in NaY, NaX and KY zeolite crystals at temperatures in the range 130°−200°C. There is little difference in the Henry constants for the four isomers on KY and NaY. The values for NaX are slightly higher but again there is little difference between the isomers. Limiting heats of sorption derived from the temperature dependence of the Henry constants are between 18 and 22 kcal mol−1. The form of the isotherms is well correlated by a simple statistical model and, from this analysis, it is shown that molecular interaction between adsorbed molecules becomes significant at high loadings, leading to the development of selectivity. The variation of selectivity with loading was measured by using mixtures of deuterated and non-deuterated species and the resulting values are in good agreement with the predictions of the theoretical model. There is a close parallel between the affinity sequence at high loadings in the vapour phase and in the liquid phase but this is quite different from the sequence at low loadings as given by the Henry constants.