Dhananjai B. Shah

Intracrystalline diffusion of benzene in silicalite: effect of structural heterogeneity

The sorption kinetics of benzene in silicalite have been measured gravimetrically using large silicalite crystals of sizes 350 µm × 105 µm × 105 µm and 270 µm × 70 µm × 70 µm in the temperature range 283–343 K. Experiments were performed under conditions that ensured isothermal operation with intracrystalline diffusion control. An analytical expression for the Darkens correction factor (δ In P/δ In a) was derived based on the Hill–de-Boer equation and was used to determine the variation of corrected diffusivity with concentration. The transport diffusivity varied significantly with the adsorbed-phase concentration but the corrected diffusivity was found to be essentially independent of the concentration. However, at temperatures below the surface transition temperature, a maximum in corrected diffusivity was observed at an adsorbed-phase concentration of four molecules per unit cell (uc). This maximum is the direct result of the type IV isotherm exhibited by the silicalite–benzene system below the surface transition temperature. It is proposed that the increase in corrected diffusivity at the critical adsorbed-phase concentration of 4 molecules uc–1 arises from reorientation of benzene molecules resulting in a much more efficient packing in the pore channel system.

Synthesis of ZSM-5 films at elevated gravity

ZSM-5 zeolite films, with a thickness from 100 to 1,000 μm, have been synthesized in a colloidal silica-TPABr gel system without a support and under elevated gravity (16 G). Under high gravity, the films obtained were completely crystallized and were stronger than those produced under normal gravity. Parameters that influence the film crystallization process such as water content(H 2 O/SiO 2 ratio), alumina to silica ratio, and the reaction gel alkalinity were investigated. The optimum value of the water content was found to be approximately 33 in the synthesis of these films. The film growth became difficult with increasing aluminum content in the reaction mixture. The alkalinity of the reaction gel had to be increased at a higher aluminum content to form a continuous film. The alkalinity of the synthesis mixture was also used to control the thickness of the film. In zeolite film formation, the effect of providing seeds in the reaction mixture was also investigated. The thickness of the resulting films varied with the configuration of seeds used. In addition, thick zeolite films were produced when a vycor glass plate treated by a leaching and an unconsolidating process was used as a support.

Time-lag measurements for diffusion of aromatics through a silicalite membrane

Abstract Intracrystalline diffusivities of benzene, toluene, and the three xylene isomers in silicalite were determined by the membrane technique. A zeolite membrane was fabricated by embedding a large silicalite crystal in an epoxy resin and mounted in a permeability cell. The membrane was subjected to a known pressure gradient and the rate of transport and, hence, the intracrystalline diffusivity through the zeolite membrane was determined from the rate of pressure increase on the outflow side. The measured diffusivities of aromatics ranged from 5 × 10 −14 to 10 −13 m 2 /s. The technique offers a reasonable alternative to other macroscopic methods such as gravimetric and chromatographic methods for measuring intracrystalline diffusivities. However, the limitations of the technique are that (1) it measures integral diffusivity rather than differential diffusivity and (2) the fabrication of a zeolite membrane represents a formidable task.

Adsorption Dynamics In A Monolithic Adsorbent

Adsorption dynamics in a monolithic adsorbent have been modeled and the resulting equations are solved numerically under two limiting conditions of macropore and micropore diffusion control. The performance under each is then compared with that of a bed packed with spherical adsorbent particles under identical conditions. For micropore diffusion control, the performance of the monolithic adsorbent bed is identical to that of a packed bed. For the case of macropore diffusion control, the performance is generally poorer. The performance can be improved significantly but with a concomitant loss of pressure drop advantage.

Diffusion of aromatics through a silicalite membrane

Intracrystalline diffusivities of benzene, toluene, and the three xylene isomers in silicalite were determined from the membrane technique. A zeolite membrane was fabricated by embedding a large silicalite crystal (100×100×300μm) in an epoxy resin and was mounted in a specially designed permeability cell. The membrane was subjected to a known pressure gradient and the pressure increase on the outflow side was monitored as a function of time. The intracrystalline diffusivities were determined from the slope and time intercept of P versus t curve. The measured diffusivities of aromatics ranged from 5 × 10−14to 10−13m2/sec. The method has the potential for determining intracrystalline diffusivities but it measures integral rather than differential diffusivity. The fabrication of the zeolite membrane was also found to be quite difficult.

Effect of Structural Heterogeneity on the Diffusion of Aromatic Hydrocarbons in Large Silicalite Crystals

Abstract Sorption kinetics of benzene, toluene and p-xylene on silicalite have been investigated gravimetrically. The results show that at low temperatures where the systems exhibit Type IV isotherm, the transport and corrected diffusivities both show a maximum near the adsorbed phase concentration of 4 and 2 molecules per unit cell for benzene and p-xylene respectively. A physical model based on the concept of “structural heterogeneity” has been used to explain these atypical sorption and diffusion results. The structural heterogeneity arises from 1) the presence of two nearly identical but different pore sizes representing zigzag and straight channels and 2) transport of a tightly fitting molecules such as benzene and p-xylene through these channels. It is postulated that initial adsorption occurs preferentially in larger pores followed by rearrangement at or near the critical adsorbate concentration resulting into closer packing of adsorbed phase molecules.

Application of method of moments to highly skewed responses

The feasibility of applying the method of moments to highly skewed transient responses of a packed column subjected to pulse inputs to determine the Henry constants and micropore diffusivities is investigated. The method is applied to study sorption and diffusion of argon, methane and ethylene in 4A zeolites. The Henry constants, micropore diffusivities, limiting heats of sorption and energies of activation so determined compare well with the values derived from gravimetric data. The method of moments is found to provide good initial estimates of the Henry constants and micropore diffusivities. The chromatographic experiments, however, have to be carried out at different carrier gas velocities and temperatures and also with different adsorbates to ensure the consistency of the calculated values of the parameters.

Molecular Diffusion In A Single Zeolite Crystal

The single crystal membrane technique for measuring zeolitic diffusivities has been modified with aim to measure differential diffusivities. A Mass Selective Detector (MSD) has been integrated into the experimental system. The membrane is subjected to a concentration difference rather than a pressure difference as driving force, and the concentration of the diffusing species is monitored on the receiving side with the MSD. The transient response has been used to calculate the zeolitic diffusivity. The method has been applied to measure micropore diffusivity of CO2 and CH4 in silicalite. The method has significant potential: but, several operational difficulties have to be overcome before the method can be applied to measure differential diffusivities.