An introduction to modeling GSP gas processing plants in ProMax

There are generally three kinds of gas processing plants to get Natural Gas Liquid NGL from natural gas. The first kind is the J-T plant, which is suitable for high pressure and rich gas. The second kind is the Mechanically Refrigerated Unit MRU. The third kind is called a "Cryo" plants, which utilize cryogenic turbo-expander to extract NGL from very lean natural gas. One common process of cryogenic plants is Gas Subcooled Process, GSP, which was patented by Ortloff Engineer, LTD in the late 1970's. This video will explain how you can model GSP in ProMax.

The wet natural gas is first dehydrated in the mole sieve to remove water to prevent ice formation in cryogenic conditions. Part of the feed is precooled in the "Residue Gas Exch" and the other part first goes through "C1 Reboiler" and then two "Side Reboilers" and then mix with the precooled portion at a temperature of about -28 F. Together the gas is fed into a "Low Temperature Separator" LTS. The liquid is sent to the DeMethanizer via a JT valve at a temperature of about -56 F. Part of the gas is expandeded in the turbo-expander to a temperature of -97 F before sending to the DeMethanizer. The power of the turbine is used to drive a compressor to boost fuel gas pressure. The other portion of the gas is introduced to the "Subcooler" before expanding to the DeMethanizer in the Cold JT valve. This is why the process is called a Gas Subcooled Process. With this subcooler, the ethane recovery can be greatly enhanced. As you can see, the lowest temperature of -148 F is created at the outlet of the cold JT valve.

From the top of the column, the residue gas is first used to subcool the reflux and then used to precool the natural gas feed after all the cold being recovered and then it serves as a fuel gas after compression. Streams 21 and 23 are withdrawn at the middle of the tower and both of them are reboiled in the side reboiler before returning to the tower. The reason of these two withdraws is to increase the ethane recovery.

NGL is extracted at the bottom of the reboiler. As you can see, in this example, 81.3% of ethane is recovered in this GSP process. There is no way for a J-T plant or Mechanical Refrigeration Unit to achieve such a high ethane recovery.

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