Integrated pre-cooled mixed refrigerant system and method
A system and method for cooling and liquefying a gas in a heat exchanger that includes compressing and cooling a mixed refrigerant using first and last compression and cooling cycles so that high pressure liquid and vapor streams are formed. The high pressure liquid and vapor streams are cooled in the heat exchanger and then expanded so that a primary refrigeration stream is provided in the heat exchanger. The mixed refrigerant is cooled and equilibrated between the first and last compression and cooling cycles so that a pre-cool liquid stream is formed and subcooled in the heat exchanger. The stream is then expanded and passed through the heat exchanger as a pre-cool refrigeration stream. A stream of gas is passed through the heat exchanger in countercurrent heat exchange with the primary refrigeration stream and the pre-cool refrigeration stream so that the gas is cooled. A resulting vapor stream from the primary refrigeration stream passage and a two-phase stream from the pre-cool refrigeration stream passage exit the warm end of the exchanger and are combined and undergo a simultaneous heat and mass transfer operation prior to the first compression and cooling cycle so that a reduced temperature vapor stream is provided to the first stage compressor so as to lower power consumption by the system. Additionally, the warm end of the cooling curve is nearly closed further reducing power consumption. Heavy components of the refrigerant are also kept out of the cold end of the process, reducing the possibility of refrigerant freezing, as well as facilitating a refrigerant management scheme.
1. A method for cooling a gas in a heat exchanger having a warm end and a cold end comprising the steps of:
a) compressing and cooling a mixed refrigerant using first and last compression and cooling cycles;
b) equilibrating and separating the mixed refrigerant after the first and last compression and cooling cycles so that a high pressure liquid stream and a high pressure vapor stream are formed;
c) cooling and expanding the high pressure liquid and vapor streams so that at least a primary refrigeration stream is provided in a primary refrigeration passage in the heat exchanger;
d) equilibrating and separating the mixed refrigerant between the first and last compression and cooling cycles so that a pre-cool liquid stream is formed;
e) passing the pre-cool liquid stream through the heat exchanger in countercurrent heat exchange with the primary refrigeration stream so that the pre-cool liquid stream is cooled;
f) expanding the cooled pre-cool liquid stream so that a pre-cool refrigeration stream is formed;
g) passing the pre-cool refrigeration stream through a pre-cool refrigeration passage in the heat exchanger;
h) passing a stream of the gas through the heat exchanger in countercurrent heat exchange with the primary refrigeration stream and the pre-cool refrigeration stream so that the gas is cooled and a mixed phase stream is produced from the pre-cool refrigeration stream at the outlet of the pre-cool refrigeration passage of the heat exchanger and a vapor stream is produced from the primary refrigeration stream at the outlet of the primary refrigeration passage of the heat exchanger; and
i) pumping a return liquid stream and rejoining the pumped return liquid stream with the mixed refrigerant downstream of the first compression and cooling cycle and upstream of the last compression and cooling cycle, wherein the return liquid stream is produced by:
(1) equilibrating and separating at least the mixed phase stream prior to the first compression and cooling cycle, thereby producing at least the return liquid stream; or
(2) mixing the vapor stream and the mixed phase stream prior to the first compression and cooling cycle to form a mixed stream, and equilibrating and separating the mixed stream, thereby producing at least the return liquid stream.
2. The method of claim 1 wherein the equilibrating and separating of the mixed stream in step (i)(2) further produces
suction vapor stream that is provided to a first compression and cooling cycle compressor.
3. The method of claim 1 , wherein the equilibrating and separating of the mixed phase stream in step (i)(1) further produces
a return vapor stream, wherein the return vapor stream is equilibrated and separated with
the vapor stream from the primary refrigeration stream so that a combined stream is produced and directed to the first compression and cooling cycle.
4. The method of claim 1 wherein step c) includes passing the high pressure vapor and high pressure liquid streams through the heat exchanger in countercurrent heat exchange with the primary refrigeration stream and the pre-cool refrigeration stream so that the high pressure vapor and high pressure liquid streams are cooled.
5. The method of claim 1 wherein the gas is natural gas.
6. The method of claim 1 wherein the compression and cooling of the first and last compression and cooling cycles are accomplished by compressors and heat exchangers.
7. The method of claim 1 wherein the gas and the primary refrigeration stream pass through both the warm and cold ends of the heat exchanger.
8. The method of claim 7 wherein the pre-cool refrigeration stream passes through the warm end of the heat exchanger, but does not pass through the cold end of the heat exchanger.
9. The method of claim 1 wherein the expanding of steps c) and f) is accomplished by expansion devices.
10. The method of claim 9 wherein the expansion devices are expansion valves.
11. The method of claim 1 wherein the gas is liquefied in step h).
12. The method of claim 1 further comprising the step of pre-cooling the gas prior to passing the gas through the heat exchanger.
13. The method claim 1 further comprising the step of pre-cooling the mixed refrigerant after the first compression and cooling cycle.
14. The method of claim 1 further comprising the step of pre-cooling the mixed refrigerant after the last compression and cooling cycle.
15. The method of claim 1 further comprising the step of further cooling the cooled gas from step h) in a downstream mixed refrigerant system.
16. The method of claim 1 further comprising the step of liquefying the cooled gas from step h) in a downstream mixed refrigerant system.
17. The method of claim 1 wherein the gas is another mixed refrigerant.
18. The method of claim 1 wherein the gas is a single component refrigerant.