Process Integration and Optimization of CO2 Removal from Natural Gas Using Cryogenic Distillation System

Abstract

The development of CO2 separation technologies will enable the monetization of undeveloped gas fields with a high level of CO2, thus providing commercial enterprises a superior competitive edge for future international field acquisitions. The cryogenic distillation system has been identified as one of bulk CO2 separation technologies for high CO2 removal from natural gas. It is a more favourable CO2 separation technology than chemical or physical absorption due to its independence from absorbents, which require a greater footprint, weight and energy. It is targeted for bulk CO2 removal from the natural gas stream 80% down to 20%, and it must be efficient and cost-effective to ensure that the overall economics of a development are positive. In-house process simulation software was used to model a cryogenic distillation column system, while an experimentally validated thermodynamic model was used to verify the phase behaviour of the components, potential CO2 solidification and hydrate formation at the operating pressure and temperature conditions. This modelling encompassed critical operating conditions such as high operating pressure and low operating temperature. This is crucial especially at lower temperature and blowdown condition to prevent piping and equipment blockage which might lead to catastrophic equipment failure. A pilot scale cryogenic distillation unit was studied in this paper with pre-mixed feed consists of CO2 and natural gas to investigate separation performance as well as to examine the operational aspects of the technology. Efforts should be made to reduce energy consumption for such applications. In this paper, pinch analysis tool is utilized to analyse and optimized the Heat Exchanger Network (HEN) of the Cryogenic Distillation System for bulk CO2 separation. Column operating pressure, condenser and reboiler temperatures and feed conditions were varied to examine the effect on energy consumptions and for comparison with process simulation results. It was found that condenser duty decreased by 50% while reboiler duty increased by 100% when operating pressure was increased from 35 bar to 50 bar to achieve the same product specification. Substantial energy reduction for external cooling was attained through pinch technology by taking advantage of the Joule-Thomson effect when expanding high pressure liquid CO2 stream to a lower pressure. Optimal operating conditions, the effect of impurities and alternative refrigeration systems are identified as current gaps in this study. Operational issues were identified and mitigated in this study, which will further the understanding and scaling-up of commercial plants, particularly blowdown study and CO2 solid and hydrate formations and potential mitigations.