Investigation of various feed conditions on NGL recovery plant energy and exergy performance: A case study

• Energy and exergy performance of NGL recovery plant at Sirri Island was investigated.
• 60% of the exergy efficiency is for the feed containing 86.68% mol methane.
• Highest exergy destruction belonged to cold heat exchanger.
• The exergy destruction of 13-E-102 and 13-E-103 heat exchangers was small.
• The maximum loss of exergy is 36% in turbo-expander (13-K-101).

NGL recovery from natural gas is an important process in gas refineries to achieve objectives including: the elimination of heavy hydrocarbons to reach the standard specification of sale gas when the possibility of condensation in pipelines is high and afterwards the possibility of selling it in the market. The high heating value of gas condensates causes the attention of gas companies to this precious resource. The existence of light and volatile particles in gas condensates and stabilization process requirement are the greatest challenges of using this resource on refineries. Therefore, this paper deals with energy and exergy performance of NGL recovery plant in Sirri Island taking various typical feed compositions classified as lean and rich. A commercial simulator was used to obtain thermodynamic properties of the process streams and to perform mass and energy balances. Due to changes in the composition entering to de-methanization unit, in this article, simulation and exergy analysis was investigated simultaneously for the five feeds with different percentages of composition. The equations of exergy destruction and exergy efficiency for the main system components such as heat exchangers, compressors and expander were developed and utilized for evaluating system performance. Simulation results and the exergy evaluation showed that with increasing compounds heavier than methane in de-methanizer feed, the efficiency of exchanger and compression work will be reduced. Approximately 60% of the exergy efficiency was for the feed containing 86.68% mol methane while for the feed containing 65.21% mol methane, exergy efficiency was reduced to 50%. Consequently, the highest exergy destruction and exergy efficiency were belonging to the cold heat exchanger and the re-compressor, respectively.

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