Process simulation and optimisation for acid gas removal system in natural gas processing

Acid gases such as hydrogen sulphide (H₂S) and carbon dioxide (CO₂) are abundant in natural gas, which affect the economics of plant operations and the environment. Chemical absorption is one of the most established technologies for acid gas removal. However, it suffers with a major drawback, i.e. high energy consumption. In this work, an integrated simulation-optimisation approach was employed to minimise energy consumption and hence operating cost in an acid gas removal (AGR) system for natural gas processing. The integrated approach made use of commercial simulation software Aspen HYSYS and optimisation software LINGO to establish a surrogate model that has the best operating conditions while satisfying sales gas requirements. Operational parameters such as alkanolamine flowrates, absorber pressure, and alkanolamine temperature were taken into account. Moreover, Pareto analysis is carried out for multi-objective optimisation in maximising profit and minimising CO₂ content. The integrated approach was demonstrated on a case study involving an AGR system in a natural gas processing plant. Results showed that with the optimal operating conditions, profit of the plant is predicted to increase by 9.15% for the same CO₂ basis (i.e. 0.77 mol%); the profit is expected to increase by 23.3% at higher CO₂ content (i.e. 1 mol%). It was observed that the maximum profit and minimum CO₂ content is achieved at amine recirculation rate of 1914.49 m³/h, pressure of 54 kg/cm², and temperature of 49.54 °C. Furthermore, sensitivity analysis illustrated that profit is proportional to the sweet gas price whereas electricity cost is the most vital parameter in reducing the overall profitability.