Optimization of Cryogenic Carbon Capture and LNG process by Mathematical programming

Abstract

Abstract Cryogenic processes are industrial processes of sub-ambient or low temperature, used for freezing or liquefaction of gas. The cryogenic condition is done by the refrigeration system with compressors and condensers consuming high operating costs of shaft work, and condenser duty, therefore mathematical programming; GAMS, is applied as a tool to optimize the energy efficiency of the refrigeration system. In this work, cryogenic carbon capture and LNG processes (Fazlollahi, 2015) was applied as a base case for improving energy efficiency by reducing shaft work and energy duty at compressors and condensers. The concept of energy-optimization model was from Non-Linear Programming model (NLP) (Colmenares and Seider, 1989). The base-case process was simulated by software Aspen plus V8.8. Representing economic change, scenarios of different operating costs were applied. Two energy-optimization models of simple and cascade refrigeration systems; under two scenarios of shaft-work cost dominance and condenser-duty cost dominance, were developed. Under shaft-work cost dominance, the simple-refrigeration model saved 72.68% and 56.12 % in shaft-work and condenser-duty costs, respectively, compared to the base case. Cascade-refrigeration model saved 48.53% and 64.77 % in shaft-work and condenser-duty costs, respectively. Under condenser-cost dominance, the simple refrigeration model saved 65.12 % and 57.73 % in shaft-work and condenser-duty costs, respectively. Cascade refrigeration model saved 8.12 % and 62.1 % in shaft-work and condenser-duty costs, respectively. As a result, cascade refrigeration model gave lower condenser-duty cost in both scenarios. However, the cascade refrigeration model had constraints to prevent negative temperature approach in condensers and constraints not to allow shaft work at compressors reach the lowest value. Simple refrigeration model gave a better design solution for only shaft-work reduction. However, cascade refrigeration model gave higher energy efficiency for both condenser-duty and shaft-work costs reductions.