Equilibrium processes, physisorption

With the assumption of an inert adsorbent, we may consider the physisorption process as a simple phase change of the adsorptive from the gaseous state to an adsorbed state on the surface A of the adsorbent. Since for a closed adsorption system at equilibrium dn = dnCT + dn = 0, we may express the condition of equilibrium in the form... 

For physisorption on/in porous solids, transport into mesopores and micropores often limits the rate of adsorption. Two-stage equilibria are frequently observed the more accessible outer surfaces equilibrate rapidly and remain in equilibrium with the ambient phase, acting as a source for slower transport of the adsorbate into the interior of the solid. Establishment of cmnplete equilibrium can be a slow process. 

Species formed from acetylene (Ay) adsorbed in zeolite Y, mordenite, beta and ZSM-5 have been studied by IR spectroscopy. The dynamics of Ay physisorption has been characterized by the frequency response method (FR). The rate of micropore diffusion governed the transport in Na-mordenite, while sorption was the rate limiting process step for all the H-zeolites. The equilibrium constants (Ka) of Ay sorption have been determined applying the Langmuir rate equation to describe the pressure dependence of the sorption time constants. The -octane hydroconversion activity of Pt/H-zeolites was found to increase linearly with the Ka of Ay sorption on the H-zeoIites. 

Since both types of adsorption are exothermic, raising die temperature generally decreases the equilibrium quantity of adsorbate. Physisorption is fast, and equilibrium is rapidly reached, even at low temperature. Chemisorption generally requires high activation energies. The rate of adsorption is low at low temperatures, but the process can be rapid at higher temperatures. 

To illustrate the combination of rate and equilibrium principles, we next consider a widely used separation method, which is inherently unsteady packed bed adsorption. We imagine a packed bed of finely granulated (porous) solid (e.g., charcoal) contacting a binary mixture, one component of which selectively adsorbs (physisorption) onto and within the solid material. The physical process of adsorption is so fast relative to other slow steps (diffusion within the solid particle), that in and near the solid particles, local equilibrium exists... 

The physisorption occnrs at low temperatnres and is more intense if the temper-atnre is near the condensation. Since the interaction energy with the surface is small and due to the inexistent activation energy of adsorption, the physisorption attains quickly the equilibrium, and thus it is reversible. However, materials having very small pores (zeolites, carbons) exhibit slow physisorption, which indicates that the diffusion in pores is the limiting step of this process. 

The physisorption takes place at low temperatures and is high when the temperature is close to the condensation temperature of the gas. Due to the low energy of interaction with the surface and the absence of activation energy, the physisorption quickly reaches equilibrium and is a reversible process. However, in the materials with very small pores (zeolites, carbons), the physisorption is slow and thus, the process is limited by the rate of gas diffusion into pores. 

Physisorption-based adsorption and separation processes are of primary interest for various applications, including H2 storage, carbon capture and sequestration, and hydrocarbon separations. Physisorption based separation of adsorbate mixtures can occur through a variety of mechanisms, including equilibrium, steric, kinetic, or some combination of these in more complex systems. 

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