Formed zeolite particles, mass transfer

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

Some additional complexity arises from the possibility of different adsorption sites and the presence of pores, which reflect in nonideal adsorption isotherms and mass-transfer problems. The mass transport can be relatively slow in pores and interparticle spaces [13], as it is the case of P25, for which, in suspension, there are particles ranging from 0.2 to 2 p,m, formed by 30-nrn-sizcd primary particles. In such spaces, the diffusion coefficient is comparable to liquid diffusion in zeolites. 

In certain adsorbents, notably partially coked zeolites and some carbon molecular sieves, the resistance to mass transfer may be concentrated at the surface of the particle, leading to an uptake expression of the form... 

PSA units using adsorbent sheets have been operated by Questair Industries, Inc.46,47,53-55 The sheets were coated with a fine powder of zeolite particles (1-10/rm) and there were spacers between the sheets to establish flow channels in a flow direction tangential to the sheets and between adjacent pairs of sheets. The sheets could be made in various configurations (rectangular, annular stack, spiral-wound, etc.) and included a support for the adsorbent in the form of an aluminum foil, a metallic mesh, or a matrix that could be woven, nonwoven, ceramic, or wool. These structured materials did not fluidize at high gas velocities and exhibited equilibrium and mass transfer properties of the powdered adsorbent. 

The intrinsic rate of physical adsorption is very rapid so the overall sorption rate is generally controlled by the diffusional resistances associated with mass transfer to the adsorption site. Commercial zeoUte-based catalysts and adsorbents consist of small (micron-sized) zeolite crystals formed into macro-porous (milUmeter-sized) particles, generally with the aid of a clay binder. Such materials offer at least three and in some cases four distinct mass transfer resistances (see Fig. 10) [35] ... 

Up to now, we have only considered the thermodynamics of adsorption. However, in technical processes - for example, in a fixed bed - adsorption is a transient process until a particle or a zone of a fixed bed has reached the equilibrium loading. The intrinsic chemical process of adsorption can be regarded as instantaneous, and the mass transfer to and into the porous adsorbent determines how fast the equilibrium is reached. Experience teaches that the mass transfer resistance by film diffusion is mostly negligible, and so the adsorption is governed by pore diffusion (Topic 3.3.4). Pore diffusion can roughly be divided into macropore and micropore diffusion, for example, zeolites have macropores as well as micropores in the small crystallites, which form a sub-structure in a partide. The... 

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