Application of an adsorption non-flow exergy function to an exergy analysis of a pressure swing adsorption cycle

Abstract

Pressure swing adsorption (PSA) is a popular gas separation technology for the process industries and is commonly used for air separation, hydrogen purification, and isomer separation. In this study, we apply a second law analysis to this technology to identify sources of irreversibility in the process and, in particular, identify which steps in the PSA cycle are responsible for the major losses. Unlike previous exergy analyses, we derive and use expressions for the exergy of the adsorbed phase using adsorption thermodynamics. In this way, exergy loss (or entropy generation) within the adsorption cycle in each step is clearly identified. We illustrate the use of these exergy functions with the application of binary linear isotherm (BLI) theory to a four-step Skarstrom cycle. Major losses in the process are shown to be the exergy loss across the valve in the blowdown step, and feed compressor aftercooler losses. Feed repressurisation is shown to be more efficient than product repressurisation for the separation factor examined in this study since part of the feed gas is introduced at a low pressure. During the cycle, bed exergy loss during the feed step is significant, while there is no exergy loss in the adsorbent bed during the blowdown or purge steps. The exergy functions derived in this study can readily be applied to more complex PSA cycles and provide a basis for cycle design.