CONFIGURATIONS AND METHODS FOR NGL RECOVERY FOR HIGH NITROGEN CONTENT FEED GASES
A low cost and efficient design is used to convert a propane recovery process based on low nitrogen content feed gas to an ethane recovery process based on a high nitrogen feed gas while achieving over 95 mole % ethane recovery while maintaining a 99% propane recovery, and achieved without additional equipment.
1 . A method of flexibly recovering ethane from a feed gas, comprising:
feeding into a demethanizer a top reflux and a second reflux below the top reflux, wherein the demethanizer produces a demethanizer bottom product and a demethanizer overhead product;
feeding at least part of the demethanizer bottom product to a deethanizer to produce a deethanizer bottom product and a deethanizer overhead product;
feeding a portion of the demethanizer overhead product back to the demethanizer as the top reflux during ethane recovery, and feeding a portion of the deethanizer overhead product back to the demethanizer as the top reflux during ethane rejection; and
wherein the demethanizer is operated at a higher pressure than the deethanizer during ethane recovery, and wherein the demethanizer is operated at a lower pressure than the deethanizer during ethane rejection.
2 . The method of claim 1 further comprising a step of expanding the feed gas in a turbo expander to produce a partially expanded feed gas and a further step of cooling and additionally expanding a first portion of the partially expanded feed gas to produce the second reflux.
3 . The method of claim 2 further comprising a step of cooling a second portion of the partially expanded feed gas to produce a partially condensed feed stream, separating the partially condensed feed stream into a vapor stream and a liquid stream, and expanding the vapor and liquid stream prior to feeding into a demethanizer.
4 . The method of claim 3 further comprising a step of using a demethanizer side reboiler in cooling a third portion of the partially expanded feed gas to produce a cooled feed stream, and combining the cooled feed stream with the chilled or partially condensed feed stream.
5 . The method of claim 3 wherein flow of the third portion of the partially expanded feed gas to the demethanizer side reboiler is decreased relative to flow of the first and second portions of the partially expanded feed gas during ethane rejection.
6 . The method of claim 1 wherein a propane recovery of at least 99% is achieved during ethane recovery and during ethane rejection.
7 . The method of claim 1 wherein an ethane recovery of at least 95% is achieved during ethane recovery.
8 . A method of changing ethane recovery to ethane rejection operation in a NGL plant, comprising:
changing a top reflux of a demethanizer from a demethanizer overhead product to a deethanizer overhead product for ethane rejection;
reducing demethanizer pressure to a pressure that is lower than a deethanizer pressure for ethane rejection; and
wherein the demethanizer receives a second reflux below the top reflux, and wherein the second reflux is a portion of a feed gas, and wherein the portion of the feed gas is subcooled by the demethanizer overhead product.
9 . The method of claim 8 wherein the demethanizer produces a bottom product that is fed to a deethanizer that produces the deethanizer overhead product.
10 . The method of claim 8 wherein the feed gas is cooled before the step of sub-cooling by expanding the feed gas in a turbo expander.
11 . The method of claim 8 wherein the demethanizer is reboiled using heat from the feed gas.
12 . The method of claim 8 wherein one portion of the feed gas is cooled in a feed gas heat exchanger, wherein another portion of the feed gas is cooled in a demethanizer reboiler heat exchanger.
13 . The method of claim 12 wherein during ethane rejection flow of the one portion of the feed gas is increased relative to flow of the another portion of the feed gas.
14 . The method of claim 8 wherein the demethanizer pressure is between 445 psig and 475 psig or at least 475 psig, and wherein the deethanizer pressure is between 319 psig and 450 psig.
15 . A method of changing ethane recovery to ethane rejection operation in an NGL plant, comprising:
providing a demethanizer that receives a top reflux and a second reflux below the top reflux, wherein the demethanizer is fluidly coupled to a deethanizer;
cooling one portion of a feed gas in a feed gas heat exchanger using a demethanizer overhead product to so produce the second reflux, cooling another portion of the feed gas in a demethanizer side reboiler heat exchanger to so produce a demethanizer feed stream;
changing the top reflux of the demethanizer from the demethanizer overhead product to a deethanizer overhead product for ethane rejection; and
increasing flow of the one portion relative to flow of the another portion for ethane rejection.
16 . The method of claim 15 further comprising a step of reducing an operating pressure in the demethanizer to a pressure that is lower than an operating pressure in the deethanizer pressure for ethane rejection
17 . The method of claim 15 wherein the demethanizer produces a bottom product that is fed to the deethanizer.
18 . The method of claim 15 wherein an operating pressure in the demethanizer is between 445 psig and 475 psig or at least 475 psig, and wherein an operating pressure in the deethanizer is between 319 psig and 450 psig.
19 . The method of claim 15 wherein the deethanizer produces a deethanizer bottom product, and further comprising a step of feeding the deethanizer bottom product into a depropanizer.
20 . The method of claim 15 wherein the feed gas has a pressure of at least 1000 psig, and further comprising a step of expanding the feed gas in a turbo expander prior to the step of cooling the one and the another portion.