Recovering natural gas liquids from LNG using vacuum distillation

1. A process for recovering liquefied C 3 compounds from a natural gas stream containing C 3 compounds which comprises:

(a) chilling the natural gas stream under conditions sufficient to liquefy the natural gas, whereby a mixture comprising liquid natural gas (LNG) and liquid C 3 compounds is formed;

(b) fractionating the mixture comprising liquid natural gas (LNG) and liquid C 3 compounds in a separation zone under sub-atmospheric pressure and under conditions predetermined to vaporize a significant portion of the natural gas present while retaining the C 3 compounds as a liquid fraction; and

(c) recovering separately from the separation zone natural gas and liquid C 3 compounds.

2. The process of claim 1 wherein the separation zone is maintained at a pressure of less than 12 psia and at a bottoms temperature within the range of from about 0° F. to about 200° F.

3. The process of claim 2 wherein the bottoms temperature in the separation zone is maintained at a temperature within the range from about 20° F. to about 150° F.

4. The process of claim 1 wherein the natural gas stream also contains C 4 compounds and a mixture comprising liquid C 3 -C 4 compounds are recovered from the separation zone.

5. The process of claim 4 including the additional steps of:

(d) introducing the mixture comprising liquid C 3 -C 4 compounds into a second separation zone under sub-atmospheric pressure and under conditions predetermined to vaporize a significant portion of the C 3 compounds present while retaining the C 4 compounds as a liquid fraction; and

(e) recovering separately from the second separation zone C 3 compounds and liquid C 4 compounds.

6. The process of claim 5 wherein the second separation zone is maintained at a pressure of less than 12 psia and at a bottoms temperature within the range of from about 20° F. to about 250° F.

7. The process of claim 6 wherein bottoms temperature in the second separation zone is maintained within the range of from about 40° F. to about 150° F.

8. The process of claim 5 wherein the natural gas stream also contains C 5 + compounds and a mixture comprising liquid C 4 and C 5 + compounds are recovered from the second separation zone.

9. The process of claim 8 including the additional steps of:

(f) introducing the mixture comprising liquid C 4 and C 5 + compounds into a third separation zone under sub-atmospheric pressure and under conditions predetermined to vaporize a significant portion of the C 4 compounds present while retaining the C 5 + compounds as a liquid fraction; and

(g) recovering separately from the third separation zone C 4 and C 5 + compounds.

10. The process of claim 9 wherein the third separation zone is maintained at a pressure of less than 12 psia and at a bottoms temperature within the range of from about −75° F. to about 75° F.

11. The process of claim 10 wherein the bottoms temperature in the third separation zone is maintained within the range of from about −50° F. to about 50° F.

12. A process for recovering liquefied C 3 + compounds from a natural gas stream containing C 3 , C 4 , and C 5 + compounds which comprises:

(a) chilling the natural gas stream under conditions sufficient to liquefy the natural gas, whereby a mixture comprising liquid natural gas (LNG) and liquid C 3 + compounds is formed;

(b) fractionating the mixture comprising liquid natural gas (LNG) and liquid C 3 + compounds in a separation zone under sub-atmospheric pressure and under conditions predetermined to vaporize a significant portion of the natural gas present while retaining the C 3 + compounds as a liquid fraction;

(c) recovering separately from the separation zone natural gas and a mixture comprising liquid C 3 + compounds;

(d) introducing the mixture comprising liquid C 3 + compounds into a second separation zone under sub-atmospheric pressure and under conditions predetermined to vaporize a significant portion of the C 3 and C 4 compounds present while retaining the C 5 + compounds as a liquid fraction;

(e) recovering separately from the second separation zone liquid C 5 + compounds and a mixture comprising C 3 -C 4 compounds.

13. The process of claim 12 wherein the second separation zone is maintained at a pressure of less than 12 psia and at a bottoms temperature within the range of from about −75° F. to about 75° F.

14. The process of claim 13 wherein the bottoms temperature in the second separation zone is maintained within the range of from about −50° F. to about 50° F.

15. The process of claim 12 including the additional steps of:

(f) introducing the mixture comprising the C 3 -C 4 compounds into a third separation zone under sub-atmospheric pressure and under conditions predetermined to vaporize a significant portion of the C 3 compounds present while retaining the C 4 compounds as a liquid fraction; and

(g) recovering separately from the third separation zone C 3 compounds and liquid C 4 compounds.

16. The process of claim 15 wherein the third separation zone is maintained at a pressure of less than 12 psia and at a bottoms temperature within the range of from about 20° F. to about 250° F.

17. The process of claim 16 wherein bottoms temperature in the third separation zone is maintained within the range of from about 40° F. to about 150° F.

18. A process for producing liquefied C 3 product and liquefied C 4 product comprising fractionating a liquid comprising C 3 compounds and C 4 compounds in a separation zone at pressures maintained at sub-atmospheric pressure and separately recovering at least liquefied C 3 product and liquefied C 4 product.

19. The process of claim 18 wherein the pressure in the separation zone is maintained below 12 psia.

20. The process of claim 18 wherein boil off gas from a C 3 product storage facility is added to the separation zone and recovered as part of the liquefied C 3 product.

21. The process of claim 18 wherein boil off gas from a C 4 product storage facility is added to the separation zone and recovered as part of the liquefied C 4 product.