Mixed refrigerant system and method

1. A method for cooling a feed fluid, comprising:

separating, in a high pressure separation device at the same pressure, a high pressure mixed refrigerant stream, said stream comprising two or more C1-C5 hydrocarbons and optionally N2, to form a high pressure vapor stream and a mid-boiling refrigerant liquid stream;

cooling the high pressure vapor stream in a heat exchanger to form a mixed phase stream;

separating the mixed phase stream with a cold vapor separator, to form a cold separator vapor stream and a cold separator liquid stream;

condensing the cold separator vapor stream in the heat exchanger and flashing, to form a cold temperature refrigerant stream;

subcooling at least a portion of the mid-boiling refrigerant liquid stream in a first heat exchange passage in the heat exchanger to a subcooled state and temperature, where the first heat exchange passage has a first heat exchange passage length, to form a subcooled mid-boiling refrigerant liquid stream;

subcooling the cold separator liquid stream in a second heat exchange passage in the heat exchanger to a subcooled state and temperature, where the second heat exchange passage has a second heat exchange passage length, prior to combination with the subcooled mid-boiling refrigerant liquid stream to form a subcooled cold separator liquid stream, wherein the first heat exchange passage is separate and distinct from the second heat exchange passage and the first heat exchange passage length is greater than the second heat exchange passage length;

combining the subcooled mid-boiling refrigerant liquid stream and the subcooled cold separator liquid stream while the subcooled mid-boiling refrigerant liquid stream is at, or colder via expansion than, the temperature of the subcooled mid-boiling refrigerant liquid stream in the subcooled state and the subcooled cold separator liquid stream is at, or colder via expansion than, the temperature of the subcooled cold separator liquid stream in the subcooled state to form a middle temperature refrigerant stream;

combining, in the heat exchanger, the middle temperature refrigerant stream with one of a cold temperature rejoined refrigerant stream and the cold temperature refrigerant stream, after the subcooled mid-boiling refrigerant liquid stream and the subcooled cold separator liquid stream are combined to form the middle temperature refrigerant stream, and warming, to form a vapor refrigerant return stream comprising the hydrocarbons and optional N2; and

thermally contacting the feed fluid in the heat exchanger, to form a cooled feed fluid.

2. The method of claim 1 further comprising the step of expanding the combined subcooled cold separator liquid and subcooled mid-boiling refrigerant liquid streams to form the middle temperature refrigerant stream.

3. The method of claim 2 wherein the step of expanding the combined subcooled cold separator liquid and subcooled mid-boiling refrigerant liquid streams includes flashing the combined subcooled cold separator liquid and subcooled mid-boiling refrigerant liquid streams.

4. The method of claim 1 further comprising the steps of:

cooling a high-boiling liquid refrigerant stream in the heat exchanger to form a subcooled high-boiling liquid refrigerant;

expanding the subcooled high-boiling liquid refrigerant to form a pre-cool refrigerant stream;

warming the pre-cool refrigerant stream to form a pre-cool refrigerant return stream that is vapor phase or mixed phase.

5. The method of claim 4 wherein the step of expanding the subcooled high-boiling liquid refrigerant to form a pre-cool refrigerant stream includes flashing the subcooled high-boiling liquid refrigerant.

6. The method of claim 5 further comprising the steps of separating the pre-cool refrigerant stream into a pre-cool vapor refrigerant stream and a pre-cool liquid refrigerant stream and rejoining the pre-cool vapor refrigerant and pre-cool liquid refrigerant streams prior to warming.

7. The method of claim 4 further comprising the step of combining the pre-cool refrigerant stream with the combined middle temperature refrigerant and cold temperature refrigerant streams.

8. The method of claim 7 further comprising the steps of expanding the subcooled cold separator liquid stream and expanding the subcooled mid-boiling refrigerant liquid stream before combining the subcooled cold separator liquid stream and the subcooled mid-boiling refrigerant liquid stream.

9. The method of claim 8 wherein the step of expanding the subcooled cold separator liquid stream includes flashing the subcooled cold separator liquid stream and the step of expanding the subcooled mid-boiling refrigerant liquid stream includes flashing the subcooled mid-boiling refrigerant liquid stream.

10. The method of claim 7 further comprising the step of expanding the subcooled cold separator liquid stream.

11. The method of claim 7 further comprising the step of expanding the subcooled mid-boiling refrigerant liquid stream.

12. The method of claim 7 further comprising the step of expanding the combined subcooled cold separator liquid and subcooled mid-boiling refrigerant liquid streams to form the middle temperature refrigerant stream.

13. The method of claim 4 further comprising the step of expanding the combined subcooled cold separator liquid and subcooled mid-boiling refrigerant liquid streams to form the middle temperature refrigerant stream.

14. The method of claim 7 wherein the step of expanding the subcooled high-boiling liquid refrigerant to form a pre-cool refrigerant stream includes flashing the subcooled high-boiling liquid refrigerant and further comprising the steps of separating the pre-cool refrigerant stream into a pre-cool vapor refrigerant stream and a pre-cool liquid refrigerant stream and rejoining the pre-cool vapor refrigerant and pre-cool liquid refrigerant streams prior to combination with the middle temperature refrigerant and cold temperature refrigerant streams.

15. The method of claim 1 further comprising the steps of expanding the subcooled cold separator liquid stream and expanding the subcooled mid-boiling refrigerant liquid stream before combining the subcooled cold separator liquid stream and the subcooled mid-boiling refrigerant liquid stream.

16. The method of claim 15 wherein the step of expanding the subcooled cold separator liquid stream includes flashing the subcooled cold separator liquid stream and the step of expanding the subcooled mid-boiling refrigerant liquid stream includes flashing the subcooled mid-boiling refrigerant liquid stream.

17. The method of claim 16 further comprising the steps of combining the flashed subcooled cold separator liquid stream and the flashed subcooled mid-boiling refrigerant liquid stream and forming a middle temperature refrigerant liquid stream and a middle temperature refrigerant vapor stream and rejoining the middle temperature refrigerant liquid and middle temperature refrigerant vapor streams prior to combination with the cold temperature refrigerant stream.

18. The method of claim 1 further comprising the step of expanding the subcooled cold separator liquid stream.

19. The method of claim 1 further comprising the step of expanding the subcooled mid-boiling refrigerant liquid stream.

20. The method of claim 1 further comprising the steps of separating the cold temperature refrigerant stream to form a cold temperature liquid refrigerant stream and a cold temperature vapor refrigerant stream and rejoining the cold temperature liquid refrigerant stream and the cold temperature vapor refrigerant stream to form the cold temperature rejoined refrigerant stream, prior to combination with the middle temperature refrigerant stream.

21. The method of claim 1 wherein the cold separator liquid stream is colder than the mid-boiling refrigerant liquid stream.

22. A method for cooling a feed fluid, comprising:

separating, in a high pressure separation device at the same pressure, a high pressure mixed refrigerant stream, said stream comprising two or more C1-C5 hydrocarbons and optionally N2, to form a high pressure vapor stream and a mid-boiling refrigerant liquid stream;

cooling the high pressure vapor stream in a heat exchanger to form a mixed phase stream;

separating the mixed phase stream with a cold vapor separator, to form a cold separator vapor stream and a cold separator liquid stream;

condensing the cold separator vapor stream in the heat exchanger and flashing, to form a cold temperature refrigerant stream;

subcooling at least a portion of the mid-boiling refrigerant liquid stream in a first heat exchange passage in the heat exchanger to a subcooled state and temperature, where the first heat exchange passage has a first heat exchange passage length, to form a subcooled mid-boiling refrigerant liquid stream;

subcooling the cold separator liquid stream in a second heat exchange passage in the heat exchanger to a subcooled state and temperature, where the second heat exchange passage has a second heat exchange passage length, prior to combination with the subcooled mid-boiling refrigerant liquid stream to form a subcooled cold separator liquid stream, wherein the first heat exchange passage is separate and distinct from the second heat exchange passage and the first heat exchange passage length is greater than the second heat exchange passage length;

directing the subcooled mid-boiling refrigerant liquid stream and the subcooled cold separator liquid stream individually and directly, before or after expansion, to a mixing junction;

combining the subcooled mid-boiling refrigerant liquid stream and the subcooled cold separator liquid stream while the subcooled mid-boiling refrigerant liquid stream is at, or colder via expansion than, the temperature of the subcooled mid-boiling refrigerant liquid stream in the subcooled state and the subcooled cold separator liquid stream is at, or colder via expansion than, the temperature of the subcooled cold separator liquid stream in the subcooled state at the mixing junction to form a middle temperature refrigerant stream;

combining the middle temperature refrigerant stream and the cold temperature refrigerant stream in the heat exchanger, after the subcooled mid-boiling refrigerant liquid stream and the subcooled cold separator liquid stream are combined to form the middle temperature refrigerant stream, and warming, to form a vapor refrigerant return stream comprising the hydrocarbons and optional N2; and

thermally contacting the feed fluid in the heat exchanger, to form a cooled feed fluid.

23. The method of claim 22 wherein the cold separator liquid stream is colder than the mid-boiling refrigerant liquid stream.