Diffusion molecular sieve pellets

T iffusion in porous pellets is often the rate-limiting process in industrial adsorption or catalytic processes. Much useful work in this field has been done by Smith and coworkers (3, 5), but for molecular sieve pellets the situation is complicated by diffusion in the zeolite crystal itself, as well as through the pores formed between the crystals. Few studies have been made of zeolite crystal diffusion, but Barrer and Brook (1) reported some results on diffusion of simple gases in various cation-substituted mordenites, and Wilson (7) gives some indirect results from the study of separation of CO2 from air using a fixed bed of type 4A zeolite pellets. In the present work, results have been obtained by studying self-diffusion of CO2 in a single pellet of type 5A zeolite under controlled conditions. The experimental results were fitted satisfactorily by a very simplified model of the pellet structure, which made it possible to deduce approximate values of the self-diffusion coefficients for both pore and crystal diffusion. 

FIGURE 6.6. Sorption of li l hydrocarbons in 5A molecular sieve pellets (Davison G52I) under conditions of macropore diffusion control. (uptake curves for CjH at 323 K (6) concentration dependence of effective diffusivities calculated from the experirhental uptake curves (see Eq. (6.16)] (c) pressure dependence of pore diffusivities calculated from effective diffusivities D, Z> /(1 + (1 - t ) dq /dc)/ ]. (From ref. 9 reprinted from Canadian Journal of Chemical Engineering.)... 

A modified Carbeny mixer was used by Ma and Lee to measure uptake rates for C4 hydrocarbons in 13X molecular sieve pellets using a helium carrier. Adsorption rates were slow and they concluded that the ratecontrolling mass transfer process was intracrystalline diffusion with a diffusiv-ity of order 10 cm s at 35°C. An independent study by Doelle and Riekert using large crystals of 13X zeolite ( 100 ftm) showed that the diffusivity of butane is, under comparable conditions, very much higher ( 10" -10 cm s ). The discrepancy appears to have arisen from the intrusion of external mass transfer resistance in the Carberry mixer. 

There are three distinct mass-transfer resistances (1) the external resistance of the fluid film surrounding the pellet, (2) the diffusional resistance of the macropores of the pellet, and (3) the diffusional resistance of the zeolite crystals. The external mass-transfer resistance may be estimated from well-established correlations (4, 5) and is generally negligible for molecular sieve adsorbers so that, under practical operating conditions, the rate of mass transfer is controlled by either macropore diffusion or zeolitic diffusion. In the present analysis we consider only systems in which one or other of these resistances is dominant. If both resistances are of comparable importance the analysis becomes more difficult. 

The self-diffusion of carbon dioxide in single pellets of commercial type 5A molecular sieve has been studied using C 02 as a tracer. Experiments were carried out at atmospheric pressure between - -25° and —2S°C. Using a simple model of the pellet structure, it was possible to deduce effective diffusivities for both pore and crystal diffusion. Ordinary gas diffusion occurs in the pores crystal diffusivities have values of the order of 10 cm /sec. 

The results in this study have demonstrated clearly that the rate of adsorption and desorption of ethane at low concentrations on 4A molecular sieves in the absence of binder is controlled by intracrystalline diffusion of the ethane. Furthermore, the diffusion process may be characterized by Pick s law and an effective diffusivity dependent only on temperature, and applicable to both adsorption and desorption. It may be expected, therefore, that such micropore diffusion also determines the rates of ethane sorption with commercial 4A pellets containing clay... 

A regular pore structure is found in crystalline zeolites or molecular sieves but when these materials are used as catalysts, tiny zeolite crystals (1-2 fj,m) are combined with a binder to make practical-size pellets (1-5 mm). Spaces between the cemented crystals are macropores of irregular shape and size, and diffusion in these macropores has to be considered as well as diffusion in the micropores of the zeolite crystals. The cylindrical capillary model is used to describe diffusion in zeolite catalyst and other catalysts and porous solids because of its simplicity and because most of the literature values for average pore size are based on this model. However, the... 

The standard adsorbent contactor is a randomly packed bed of pelletized or particulate adsorbents. The commonly used adsorbents include activated carbons, which separate mostly based on dispersive interactions zeolites that separate based on polarity and size carbon molecular sieves, which use the relative differences in intra-particle diffusion rates or silica gel and alumina, generally hydrophilic. The particle size and shape should provide a suitable compromise between pressure drop (AP) and mass transfer resistance. Note that AP is rarely a dominant economic problem, except for the largest systems. In order to minimize it, the cross-sectional area must be increased, leading to a small length-to-diameter ratio LID). In small systems, pressure drop is relatively less important than performance in terms of separation efficiency. 

Generally, however, the aim is to avoid conditions leading to film diffusion control. This means that the focus is shifted towards transport processes that occur at the intermediate level (that is, in the mesopores and macropores within the macroparticle or pellet itself) and those which occur at the smallest dimensional level (viz., in the very micropores of the molecular sieve) [45, 89]. Within the mesopores and macropores between the primary zeolite crystallites transport will be dominated by molecular and ionic intercrystalline diffusion possibly coupled to surface diffusion processes, while, in the zeolite micropores themselves, intracrystalline diffusion occurs, also possibly coupled... 

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