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3. If micropores exist, adsorbate diffuses further in the micropores before getting adsorbed onto the surface of the micropores, otherwise it adsorbs on the surface of macropores. Consequently, the adsorbate encounters three different kinds of mass transfer resistances at each of the steps in this sequence, which are described as below: • External film resistance: This exists in the external liquid film surrounding the ad- sorbent pellet. It can be characterized by using the system’s Sherwood, Reynolds and Schmidt number. Typically this resistance is negligibly small in PSA systems [181, 156]. • Macropore diffusive resistance: This mass transfer resistance exists in the macrop- ores of the adsorbent particle, and usually is the rate-controlling step. It depends on the relative magnitude of the pore diameter and the mean free path of the adsorbate under the operating conditions in the pores. When the pore diameter is much greater than the mean free path, Bulk diffusion (Dm,i) dominates the transport, and is estimated by Chapman-Enskog equation [181]. When the mean free path is much larger than the pore diameter, Knudsen diffusion dominates the transport, and is characterized by T (2.1) Knudsen diffusion is usually dominant when the total pressure is quite low. In the intermediate case, effective diffusivity is obtained from Bosanquit’s equation Deff,i = Dm,iDK i = 1,...,Nc (2.2) Dm,i + DK Finally, macropore diffusivity (called Pore diffusion) is characterized by following equa- tion Dp,i = εpDeff,i i = 1,...,Nc (2.3) τ • Micropore diffusive resistance: Also known as Surface diffusion, this resistance exists in the micropores of the adsorbent pellet. For the adsorbents considered in this work, Chapter 2. Pressure Swing Adsorption 15 D = 48.5d K pMw 2.3 PSA ModelingPDF Image | Design and Operation of Pressure Swing Adsorption Processes
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