Equilibrium Theory Analysis of Pressure Equalization Steps in Pressure Swing Adsorption

A binary, linear isotherm, isothermal equilibrium theory analysis of Skarstrom-type PSA cycles with bed-to-bed (BB) and bed-to-tank-to-bed (BTB) equalization steps was carried out with a binary gas mixture of A and B, with A more strongly adsorbed than B. For tractability, it was assumed that the gas produced from the light end of a bed contained only B and thus the recovery of A in the heavy product was 100%. Analytic expressions for the periodic state PSA cycle performances based on the recovery of B in the light product Re LP,B , the purity of A in the heavy product y̅HP , and the final pressures of the BB and BTB equalization steps were derived. The effects of the relative size of the equalization tanks Ψ (varied from 0.1 to 500) and the number of equalization steps n (varied from 1 to 10) were studied. The initial pressure in the bed at the beginning of the countercurrent depressurization (CnD) step Π CnD,o was important. With increasing Ψ or n, Π CnD,o always decreased and both y̅HP and Re LP,B always increased, and when the BTB and BB configurations achieved the same Π CnD,o , their PSA cycle performances were identical. Increasing Ψ at constant n caused the BTB Π CnD,o to approach that of the BB Π CnD,o , and they became equal for only very large tanks (e.g., Ψ = 500). However, increasing nBTB at constant nBB and Ψ caused the BTB Π CnD,o to be even lower than the BB Π CnD,o for some reasonable Ψ. Therefore, instead of using larger tanks in BTB to achieve the same BB performance, it was better to increase nBTB at a reasonable Ψ to keep the tank volume smaller. For the same performance (i.e., the same Π CnD,o ), the total volume of all tanks (i.e., nΨ) decreased with increasing n, and in the limit of nBTB → ∞, nΨ approached a minimum total tank volume equal to nBB . This result indicated a lower limit exists on the minimum total volume of tanks required to achieve the same performance as in the BB configuration.