Deep recovery of NGL from natural gas.
Products: Sales gas and stable NGL, with a propane recovery as high as 99%. For an ethane-extraction
plant, ethane recovery can be over 95%, with a propane recovery of essentially 100%.
With high feed-gas pressure and particularly with dense-phase operation (above the cricondenbar), two expanders used in series enables the design pressure of the gas/gas exchanger to be reduced significantly. It also enables a plate-fin heat exchanger to be used, thus improving process efficiency as compared to using a shell and tube exchanger.
For very high-pressure ratios across the plant, two expanders in series avoid very high liquid flows in the expander exhaust, thus improving the performance and reliability of the turboexpander system.
Dehydrated feed gas is let down in pressure by the first expander (1), and the exhaust is passed to the first separator (2). Vapor from the first separator is cooled and condensed in the gas/gas exchanger (3) and let down in pressure by the second expander (4). The exhaust is passed to the second separator (5). Unstable liquids recovered from both separators are sent on to be stabilized. Cold vapor from the second separator is rewarmed and recompressed in the brake ends of the expanders to sales gas pressure.
Operating conditions: A very high propane recovery of 99% can be achieved by using a pre-absorber column and optimizing thermal integration with multistream heat exchangers.
Improved energy integration enables NGL recovery to be increased to very high levels with similar power consumption to conventional technology. Very high ethane recovery, of over 95%, can be achieved by using a portion of the sales gas for demethanizer refluxing and by utilizing refrigeration efficiently with good thermal integration.
Contributor: Costain Oil, Gas & Process Ltd.