Hysys simulation and optimization of an LNG plant’s back-end process

End-flash system is a mechanism applied in LNG processes such as the ConocoPhillips optimized cascade process to reject nitrogen content in the liquefied natural gas as consequence of the storage tank blanketing. In order to meet client LNG quality requirement the end-flash system reduces nitrogen content by rejecting the nitrogen rich natural gas as fuel to fuel up the heavy gas turbines. This project aim is to simulate and optimize a base case simulation fig. 9 by modifying the base case with the ultimate objective to increase produced LNG which could increase plant benefits and to reduce the fuel gas production. ConocoPhillips optimized cascade back-end process simulation is the base case for this study. With a start-up feed of 50000kg/hr which is computed and reduced to 15440kg/hr once the recycle flow joins and adjusts the feed, a production of LNG 13500kg/hr (87.44% feed) and fuel gas 1825 kg/hr (11.82% adjusted feed) yielding a specific power of 903kJ per Kg of LNG produced. Modified simulations have been performed exploring the opportunity to improve the correlation of LNG production and fuel gas efficiency. Two approaches has been tackled by modifying the number of sub-cooling stages in one direction by reducing number of cooling stages from three to a single stage and achieving an improvement of 9680kg/hr of more LNG production and fuel production reduced to 5.44%. On the other direction the number of sub-cooling stages was increased from three to four stages and this approach yield results of 13660kg/hr of more produced LNG and reduced the fuel gas production to 4.04%. This approach presents an overall improvement of 61% reduction of the required power to produce 1Kg of LNG, yielding to 349 KJ/Kg. Increasing the number of sub-cooling stages resulted to be the most efficient approach with optimal results.