Improving Design of a Cryogenic Process by Exergy and Process Economic Analyses

Abstract

Aim: This work compared four different designs of a cryogenic process (Linde methane liquefaction) using exergy and process economic analyses. Study Design: Simulation, energy, exergy and process economic analyses were used. Place and Duration of Study: Department of Chemical Engineering, Obafemi Awolowo University Ile-Ife and the duration of study was between March and May 2011. Methodology: Hyprotech System Simulator (HYSYS) software was used in simulating the process plants and Microsoft Excel was used for exergy, energy and process economic analyses. Annual venture profit was used as economic index in the selection of best design among the available alternatives. Result: Linde Methane Liquefaction for cryogenic process was investigated for improvement using exergy and process economic analyses. Four design cases were considered. The exergy efficiency for the base case (case 1) was 5.19%, 5.30% for case 2 when a recycle was added to the base case, 28.7% for case 3 when a recycle and a turbine were added to the base case and 39.5% for case 4 when a recycle was added and the valve was replaced with a turbine in the base case. Case 3 has the highest equipment and installation cost of $594649.14. The capital investment, operating and maintenance expenses were $88574.00/yr, $89687.30/yr, $94280.17/yr and $90672.23/yr for case 1, case 2, case 3 and case 4, respectively. Process economic analysis based on 1 kg/h throughput revealed case 2 with the highest annual venture profit of $4564265.09/yr as the best design. Conclusion: Since it is known that throughput has effect on venture profit, it was concluded that case 4 with the highest exergetic efficiency could be the best design at higher throughput. This will be investigated in a follow up study.