Natural gas demand has dramatically increased due to the emerging growth of the world economy and industry. Presently, CO2 and H2S content in gas fields accounts for up to 90% and 15%, respectively. Apart from fulfilling the market demand, CO2 and H2S removal from natural gas is critical due to their corrosive natures, the low heating value of natural gas and the greenhouse gas effect. To date, several gas fields have remained unexplored due to limited technologies to monetize the highly sour natural gas. A variety of conventional technologies have been implemented to purify natural gas such as absorption, adsorption and membrane and cryogenic separation. The application of these technologies in natural gas upgrading are also presented. Among these commercial technologies, cryogenic technology has advanced rapidly in gas separation and proven ideally suitable for bulk CO2 removal due to its independence from absorbents or adsorbents, which require a larger footprint, weight and energy. Present work comprehensively reviews the mechanisms and potential of the advanced nonconventional cryogenic separation technologies for processing of natural gas streams with high CO2 and H2S content. Moreover, the prospects of emerging cryogenic technologies for future commercialization exploitation are highlighted.
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.
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