Single-unit gas separation process having expanded, post-separation vent stream

A process comprising:

• receiving an unprocessed natural gas stream, the unprocessed natural gas stream comprising methane, ethane, propane, and nitrogen, the unprocessed natural gas stream being received from a subterranean formation, the unprocessed natural gas stream having an energy content from about 900 British thermal units per cubic foot (Btu/ft3) to about 1,800 Btu/ft3, and the unprocessed natural gas stream comprising both a liquid phase and a gas phase such that a vapor fraction of the unprocessed natural gas stream is between zero and one;
• separating the unprocessed natural gas stream into a top effluent stream and a bottom liquefied petroleum gas-rich (LPG-rich) stream in a multi-stage separation column, the bottom LPG-rich stream comprising less than or equal to about 3 molar percent of the methane in the unprocessed natural gas stream, the bottom LPG-rich stream comprising from about 40 molar percent to about 50 molar percent of the ethane in the unprocessed natural gas stream, the bottom LPG-rich stream comprising greater than or equal to about 85 molar percent of the propane in the unprocessed natural gas stream, the bottom LPG-rich stream comprising none of the nitrogen in the unprocessed natural gas stream, the bottom LPG-rich stream having a vapor pressure from about 200 pounds per a square inch gauge (psig) to about 300 psig, and the bottom LPG-rich stream comprising only the liquid phase such that a vapor fraction of the bottom LPG-rich stream is zero;
• cooling the top effluent stream in a first heat exchanger using an expanded natural gas-rich stream to create a partially condensed stream comprising a vapor portion and a liquid portion, the expanded natural gas-rich stream comprising greater than or equal to about 97 molar percent of the methane in the unprocessed natural gas stream, the expanded natural gas-rich stream comprising from about 50 molar percent to about 60 molar percent of the ethane in the unprocessed natural gas stream, the expanded natural gas-rich stream comprising less than or equal to about 15 molar percent of the propane in the unprocessed natural gas stream, and the expanded natural gas-rich stream comprising all of the nitrogen in the unprocessed natural gas stream;
• separating the vapor portion from the liquid portion in a single stage separator that receives the partially condensed stream from the first heat exchanger;
• expanding the vapor portion from the single stage separator in an expander to produce the expanded natural gas-rich stream that is fed to the first heat exchanger, the expander reducing a pressure of the vapor portion, the expander being downstream from the multi-stage separation column, the expander comprising a Joules-Thompson (JT) expander, and the JT expander receiving the vapor portion from the single stage separator and feeding the expanded natural gas-rich stream to the first heat exchanger;
• passing the liquid portion from the single stage separator to the multi-stage separation column as reflux;
• heating the expanded natural gas-rich stream in the first heat exchanger using the top effluent stream;
• passing the expanded natural gas-rich stream from the first heat exchanger to a second heat exchanger;
• cooling the unprocessed natural gas stream in the second heat exchanger using the expanded natural gas-rich stream;
• passing the expanded natural gas-rich stream from the second heat exchanger to a compressor that compresses the expanded natural gas-rich stream to form a compressed natural gas-rich stream, the compressed natural gas-rich stream comprising only the gas phase such that a vapor fraction of the natural gas-rich stream is one;
• and
• feeding the unprocessed natural gas stream to the multi-stage separation column, the multi-stage separation column comprising three input streams, and the three input streams comprising the unprocessed natural gas stream, the reflux from the single stage separator, and a boil-up stream from a reboiler.

An apparatus comprising:

• a multi-stage separation column configured to separate a hydrocarbon feed stream into a top effluent stream and a bottom liquefied petroleum gas-rich (LPG-rich) stream, the hydrocarbon feed stream being received from a subterranean formation, the hydrocarbon feed stream having an energy content from about 900 British thermal units per cubic foot (Btu/ft3) to about 1,800 Btu/ft3, the hydrocarbon feed stream comprising methane, ethane, propane, and nitrogen, the hydrocarbon feed stream comprising both a liquid phase and a gas phase such that a vapor fraction of the hydrocarbon feed stream is between zero and one, the bottom LPG-rich stream comprising less than or equal to about 3 molar percent of the methane in the hydrocarbon feed stream, the bottom LPG-rich stream comprising from about 40 molar percent to about 50 molar percent of the ethane in the hydrocarbon feed stream, the bottom LPG-rich stream comprising greater than or equal to about 85 molar percent of the propane in the hydrocarbon feed stream, the bottom LPG-rich stream comprising none of the nitrogen in the hydrocarbon feed stream, the bottom LPG-rich stream having a vapor pressure from about 200 pounds per a square inch gauge (psig) to about 300 psig, and the bottom LPG-rich stream comprising only the liquid phase such that a vapor fraction of the hydrocarbon feed stream is zero;
• a first heat exchanger configured to cool the top effluent stream using an expanded natural gas-rich stream to create a partially condensed stream comprising a vapor portion and a liquid portion, the expanded natural gas-rich stream comprising greater than or equal to about 97 molar percent of the methane in the hydrocarbon feed stream, the expanded natural gas-rich stream comprising from about 50 molar percent to about 60 molar percent of the ethane in the hydrocarbon feed stream, the expanded natural gas-rich stream comprising less than or equal to about 15 molar percent of the propane in the hydrocarbon feed stream, and the expanded natural gas-rich stream comprising all of the nitrogen in the hydrocarbon feed stream;
• a single stage separator configured to receive the partially condensed stream from the first heat exchanger and separate the vapor portion from the liquid portion;
• an expander configured to receive the vapor portion from the single stage separator and expand the vapor portion into the expanded natural gas-rich stream that is fed to the first heat exchanger, the expander reducing a pressure of the vapor portion, the expander being downstream from the multi-stage separation column, the expander comprising a Joules-Thompson (JT) expander, and the JT expander receiving the vapor portion from the single stage separator and feeding the expanded natural gas-rich stream to the first heat exchanger;
• a second heat exchanger configured to receive the expanded natural gas-rich stream from the first heat exchanger and to cool the hydrocarbon feed stream using the expanded natural gas-rich stream;
• a pump configured to pass the liquid portion from the single stage separator to the multi-stage separation column as reflux, the multi-stage separation column being the only multi-stage separation column in the apparatus, the multi-stage separation column and the single stage separator being the only two separators in the apparatus, the multi-stage separation column comprising three input streams, and the three input streams comprising the hydrocarbon feed stream, the reflux from the single stage separator, and a boil-up stream from a reboiler;
• and
• a compressor that receives the expanded natural gas-rich stream from the second heat exchanger and that compresses the expanded natural gas-rich stream to form a compressed natural gas-rich stream, the compressor being coupled to the expansion turbine and using the energy generated from the expansion of the vapor portion to power the compressor, the compressed natural gas-rich stream comprising only the gas phase such that a vapor fraction of the compressed natural gas-rich stream is one, and the vapor portion having a substantially identical composition as the expanded natural gas-rich stream.

An apparatus comprising:

• a multi-stage separation column configured to receive an unprocessed natural gas stream and separate the unprocessed natural gas stream into a top effluent stream and a bottom liquefied petroleum gas-rich (LPG-rich) stream, the unprocessed natural gas stream being received from a subterranean formation, the unprocessed natural gas stream having an energy content from about 900 British thermal units per cubic foot (Btu/ft3) to about 1,800 Btu/ft3, the unprocessed natural gas stream comprising both a liquid phase and a gas phase such that a vapor fraction of the unprocessed natural gas stream is between zero and one, the unprocessed natural gas stream comprises methane, ethane, propane, and nitrogen, the bottom LPG-rich stream comprising less than or equal to about 3 molar percent of the methane in the unprocessed natural gas stream, the bottom LPG-rich stream comprising from about 40 molar percent to about 50 molar percent of the ethane in the unprocessed natural gas stream, the bottom LPG-rich stream comprising greater than or equal to about 85 molar percent of the propane in the unprocessed natural gas stream, the bottom LPG-rich stream comprising none of the nitrogen in the unprocessed natural gas stream, the bottom LPG-rich stream having a vapor pressure from about 200 pounds per a square inch gauge (psig) to about 300 psig, and the bottom LPG-rich stream comprising only the liquid phase such that a vapor fraction of the bottom LPG-rich stream is zero;
• a first heat exchanger configured to cool the top effluent stream using an expanded natural gas-rich stream to create a partially condensed stream comprising a vapor portion and a liquid portion, the expanded natural gas-rich stream comprising greater than or equal to about 97 molar percent of the methane in the unprocessed natural gas stream, the expanded natural gas-rich stream comprising from about 50 molar percent to about 60 molar percent of the ethane in the unprocessed natural gas stream, the expanded natural gas-rich stream comprising less than or equal to about 15 molar percent of the propane in the unprocessed natural gas stream, and the expanded natural gas-rich stream comprising all of the nitrogen in the unprocessed natural gas stream;
• a reflux separator configured to receive the partially condensed stream from the first heat exchanger and separate the vapor portion from the liquid portion;
• an expander configured to expand the vapor portion from the reflux separator to produce the expanded natural gas-rich stream that is fed to the first heat exchanger, the expander comprising an expansion turbine that generates energy based on the expansion of the vapor portion, the expander reducing a pressure of the vapor portion, the expander being downstream from the multi-stage separation column, the expander comprising a Joules-Thompson (JT) expander, and the JT expander receiving the vapor portion from the reflux separator and feeding the expanded natural gas-rich stream to the first heat exchanger;
• a second heat exchanger configured to receive the expanded natural gas-rich stream from the first heat exchanger and to cool the unprocessed natural gas stream using the expanded natural gas-rich stream;
• a compressor that receives the expanded natural gas-rich stream from the second heat exchanger and that compresses the expanded natural gas-rich stream to form a compressed natural gas-rich stream, the compressed natural gas-rich stream comprising only the gas phase such that a vapor fraction of the natural gas-rich stream is one;
• and
• a pump configured to pass the liquid portion from the reflux separator to the multi-stage separation column as reflux, the multi-stage separation column comprising three input streams, and the three input streams comprising the unprocessed natural gas stream, the reflux from the single stage separator, and a boil-up stream from a reboiler.