Bed diffusion control

The problem of nonisothermal uptake under conditions such that the main resistance to mass transfer is the diffusional resistance of the particle bcd/ b rather than the intraparticle resistance, is similar and the expression for the uptake curve may be derived in the same way. The response of the system to a step change in sorbate concentration is described by the following set of equations  

FIGURE 6.17. Variation of heat transfer parameter (ha/C,) with sorbale pressure for sorption of i-octane and n-heptane (X, 22-mg Linde I3X 0 20-mg, 17-pm I3X , 28 mg, 17-fira 13X on wide pan I4-mg, 39-pm 13X , 14-mg, 24-pm I3X A. 14-mg, 55-pm I3X). Note that the value of ha/C, depends on the bed configuration (sample weight) and pan diameter and is independent of crystal size or nature of the sorbate. (From ref. 21, with permission.) 

The variation of the heat transfer time coristant ha/C,) and the bed diffusional time constant D/l with pressure is shown, for several different 

FIGURE 6.18. Pressure dependence of macrodiffusional lime constant D/l for sorption in 13X zeolite crystals. Data are for temperatures of 370-430 K iWith comparable adsorbent bed configuration, /-octane 39-/im crystals, 24- m crystals, I. / -heptane 39- im crystals, A 17- m crystals, a. Benzene 17-/im crystals,. Cyclohcxani 17- im crystals, 0- Note that time constant is approximately independent of either sorbate or crystal size as may be expected for bed diffusion with sorbales of comparable molecular weight. Note jalso the transition from Knudsen diffusion (Z) independent of pressure) to molecular diffusion (Z) cc l/p) as pressure is increased. (From ref. 21, with permission.) 

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