2003 Volume 36 Issue 6 Pages 695-702
The removal of trace carbon dioxide from air was carried out in a two-bed pressure swing adsorption (PSA) packed with 1/16” MS13X zeolite as adsorbent in the depth of L = 0.79 m to find systematically the effect of design and operating variables on the product concentration Ca1 and the mass exchange efficiency ηa defined by ηa = (Ca0 – Ca1)/(Ca0 – Ca1Pd/Pa) for the feed concentration. The operation with a shorter cycle time resulted in a higher performance which approached a limiting value at a half cycle time of shorter than 20 min. The individual superficial velocities, Ua and Ud in the adsorption and desorption steps, and the velocity ratio Ud/Ua had a great effect on the product concentration Ca1. On the other hand, pressure in the desorption step Pd had little effect on the removal performance expressed in terms of the mass exchange efficiency ηa while higher pressure in the adsorption step Pa decreased the removal efficiency. This effect of pressure was explained by the decrease in the adsorption coefficient m and intraparticle diffusivity with an increasing pressure Pa. The experimental result agreed well with a simplified model called short cycle time approximation previously proposed by authors when it was applied to a linear isotherm of adsorption and a half cycle time shorter than 20 min. The model provides a distinctive relation between the product gas concentration Ca1 and operating and design parameters by the following equation
(1 – ηaUa/Ud)/(1 – ηa) = exp[KFOa(L/Ua – L/Ud)]
in which KFOa is the unified volumetric mass transfer coefficient based on gas phase driving force and inversely proportional to the sum of column pressure (Pa + Pd).