18.6. Mass and Energy Transfer in Packed Beds

For detailed predictions and understanding of sorption separations we need to do a detailed analysis of diffusion rates, mass and energy transfer, and mass and energy balances in the column. In order to make the results somewhat tractable, we will make all the usual assumptions listed in Table 18-6.

Table 18-6. Assumptions for mass and energy transfer analysis

Assumption		Comments
1.	Homogeneous packing (no channeling)	Valid if carefully packed and Dcol/dp > ~30.
2.	Negligible radial gradients	May not be valid if high heat losses.
3.	Neglect thermal and pressure diffusion	Usually OK.
4.	No chemical reactions except for sorption	Sorbents can act as catalysts. Need to check.
5.	Neglect kinetic and potential energy	Usually OK.
6.	No radiant heat transfer	Absolutely true only for isothermal. Usually lumped with convective heat transfer.
7.	No electrical or magnetic fields	Usually OK.
8.	No phase changes except sorption	Usually OK (watch for solute precipitation).
9.	Velocity constant across cross-section	Reasonable approximation if: no channeling, no radial gradients, no viscous fingering.
10.	No irreversible adsorption	Usually OK once sorbent has been used and regenerated.
11.	Rigid packing	OK except for soft packings (e.g., polymers).
12.	Constant sorption properties	OK if no slow degradation or poisoning.
13.	No breakage or dissolution of packing	Can be problem with moving beds or when there is chemical attack.
14.	Negligible conduction in column walls	OK for large diameter columns.