CLOSED CYCLE MIXED REFRIGERANT SYSTEMS

Abstract

The present invention provides for a process for providing cooling and controlling the refrigeration in a cooling loop used in the production of liquefied natural gas. A cooling loop in contact with a heat exchanger contains a refrigerant composition and by controlling the amount of a component in the refrigerant composition, the necessary level of cooling provided to the heat exchanger can be maintained.

Description

BACKGROUND OF THE INVENTION

The present invention provides for a method to continuously monitor and control the refrigerant composition in a closed cycle mixed refrigerant system.

Liquefied natural gas (LNG) is produced by both small scale and large scale plants. The overall heating value of the LNG can be affected by the percentage of heavier hydrocarbon components found in the natural gas and hence, heavier hydrocarbons may need be removed prior to liquefaction. Some natural gases will also require removal of the heavy ends to prevent operating problems in the liquefaction cycle

U.S. Pat. No. 6,530,240 B1 teaches a method for controlling a mixed refrigerant based, natural gas liquefier system that utilizes an exchange of system refrigerant between the system and an external storage tank whereby the use of extremely high pressures in the compressor discharge that is employed in conventional systems is circumvented. This reference is directed to the control of mixed refrigerant based natural gas liquefiers using low cost HVAC components due to the risk of exceeding the pressure and temperature requirements of the HVAC components. The pressure limitations are avoided by adjusting the pressure in the refrigerant circulation circuit to below about 175 psig by exchange of refrigerant with the refrigerant storage circuit.

MRC Cycle for LNG/Air Separation Saves Money, D. T. Linnett, Tenth Australian Chemical Engineering Conference, 1982, Sydney, 24-26 August teaches alternative cycles for liquefaction of natural gas and the advantages of mixed refrigerant cascade cycle. The MRC cycle uses a single multi-component refrigerant comprising five components, nitrogen, methane, ethylene, propane, and butane. The mixture composition is such that it condenses over a very wide temperature range. The mixture is compressed in a single compressor and then partially condensed against cooling water. The liquid is separated, sub cooled, expanded to a common low pressure, then evaporated and recycled to the compressor. The uncondensed vapor is further cooled and further partially condensed and the same procedure is repeated.

Closed cycle mixed refrigerant cycle (MRC) based liquefaction systems are commonly used for the liquefaction of natural gas. These systems have been identified to offer improved efficiency by way of lower power consumption compared to a single component nitrogen based system. An MRC system for natural gas liquefaction uses three or more components. One such mixture specified by the owners of the '240 patent for their small scale LNG system uses a five component refrigerant mixture consisting of nitrogen, methane, ethane, iso-butane and iso-pentane. In this mixture, the iso-pentane is liquid at the warm end or ambient temperature. A number of factors such as ambient temperature, discharge pressure of the high pressure refrigerant mixture in the cycle, two phase flow distribution in the heat exchanger, and refrigerant component solubility in oil in an oil-flooded compressor system, affect the overall cooling efficiency of the closed cycle refrigeration system. When the cooling efficiency of a system is affected, the power consumption for the liquefaction of natural gas will also be affected.

FIG. 1 illustrates a prior mixed refrigerant system. Natural gas is fed to the main heat exchanger 10 through line 1 and product liquefied is recovered via valve 3 through lines 2 and 4. A mixed refrigerant of fixed composition will enter the suction of a compressor as vapor only. The compressed refrigerant is fed through line 6 and is cooled in an evaporative cooler 20 and partially condensed. The two-phase high pressure mixture is then fed through line 7 into the main plate fin heat exchanger 10 and completely condensed as it passes down to the Joule-Thompson valve. The high pressure liquid refrigerant is expanded to a pressure close to the compressor suction pressure and produces cooling by flashing and phase change. This high pressure liquid refrigerant is fed through line 11 into the heat exchanger 10. This gas-liquid refrigerant mixture is completely vaporized in the low pressure passage of the main heat exchanger 10 and provides refrigeration to liquefy the high pressure refrigerant and the natural gas. The warm gaseous refrigerant leaving the exchanger is fed through line 5 to the compressor 15 thereby completing the refrigerant cycle.

This is a closed refrigerant system, so the total holdup/inventory is constant and is distributed between the gas and the liquid phase. This is true for all closed refrigerant cycles. The refrigerant in the gas phase is a function of the system pressure with the balance being in the liquid phase. The primary liquid holdup is at the cold end of the main heat exchanger. This suggests that the system pressure will determine the liquid holdup. In this case, the primary liquid holdup will be at the cold end upstream of the Joule-Thomson valve. The refrigerant composition everywhere in the loop is the same.

The present invention addresses the inefficiencies of these earlier system by exploiting the difference in the liquid and gas phase compositions at different locations in the loop.

SUMMARY OF THE INVENTION

The present invention provides for a method of controlling the liquid inventory in a closed loop refrigeration system in order to increase the quantity or relative percentage of the heavy component, iso-pentane, which is a liquid at ambient conditions (all other components are vapor), in the refrigerant mixture so that the warm end heat exchange in the refrigeration system heat exchanger is optimized. This will improve liquefaction efficiency.

In a first embodiment of the present invention there is disclosed a method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop containing a refrigerant composition is directed through a heat exchanger comprising controlling said refrigerant composition continuously by changing the liquid level in a warm end phase separator.

The refrigerant composition contains iso-pentane amongst other components. The liquid level in the warm end phase separator decreases as the heat transfer required of the heat exchanger increases. The liquid level is controlled on-line typically by a PLC in communication with the warm end phase separator. As the liquid level in the warm end phase separator increases, the heat transfer required of the heat exchanger, which is in fluid communication therewith will decrease.

The cooling loop and the heat exchanger are also in fluid communication and the control of the refrigerant composition is performed by changing the amount of iso-pentane present therein.

In another embodiment of the present invention there is disclosed a method for providing cooling to a process for producing liquefied natural gas comprising the steps:

• • a) contacting a stream of liquefied natural gas with a heat exchanger;
  • b) contacting the heat exchanger with a cooling loop containing a refrigerant composition;
  • c) adjusting the composition of the refrigerant by controlling the amount of a heavier condensable component in the refrigerant composition in response to performance variations in the heat exchanger; and
  • d) recovering liquefied natural gas.

In this embodiment, the heavier condensable component is iso-pentane. The performance variations in the heat exchanger are selected from the group consisting of a temperature increase in the heat exchanger and a temperature decrease in the heat exchanger. The amount of the heavier condensable component in the refrigerant composition will increase in response to an increase in temperature in the heat exchanger. Accordingly the amount of the heavier condensable component in the refrigerant composition will decrease in response to a decrease in temperature in the heat exchanger.

In another embodiment of the present invention, there is disclosed a method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop comprising two warm end separators and a cold end separator are in fluid connection with each other comprising controlling the liquid level in the warm end separators.

In this embodiment, the cooling loop contacts a heat exchanger. The refrigerant composition contains iso-pentane and the control of the liquid level is performed by changing the amount of iso-pentane present in the refrigerant composition. The liquid level in the warm end separators decreases as the heat transfer required of the heat exchanger increases.

The liquid level is controlled on-line typically by a PLC in communication with the warm end phase separator. A first warm end separator is in fluid communication with a second warm end separator. The liquid level in the warm end separators increases as the heat transfer required of the heat exchanger decreases.

In another embodiment of the present invention there is disclosed a method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop comprising a warm end separator and a cold end separator are in fluid connection with each other comprising the steps:

• • a) controlling the liquid level in the warm end separator;
  • b) removing vapor from the cold end separator;
  • c) heat exchanging the vapor with a separate liquid nitrogen storage system; and
  • d) returning the liquid from the exchange of step c) to a heat exchanger.

The controlling of the liquid level is performed on-line. The cooling loop will contact a heat exchanger and the liquid level in the warm end separator is controlled by changing the amount of iso-pentane present in the liquid. The liquid level in the warm end separators decreases as the heat transfer required of the heat exchanger increases.

In a further embodiment of the present invention, there is disclosed a method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop comprising a warm end separator and a cold end separator are in fluid communication with each other comprising controlling the liquid level in the warm end separator and controlling the flow of natural gas from a separator to a heat exchanger.

The liquid level in the warm end separator is controlled by changing the amount of iso-pentane present in the liquid and can be performed on-line. The flow of natural gas from a separator to a heat exchanger is controlled by the temperature of the separator where a gas stream will enter the heat exchanger and a bottom liquid stream is directed from the separator for reentry into the separator.

The bottom liquid stream comprises butane, propane and pentane. The gas stream from the separator after exiting the heat exchanger is directed into the separator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a liquefied production process with a heat exchanger.

FIG. 2 is a schematic of a liquefied production process where the liquid level in the warm end separator is continuously controlled.

FIG. 3 is a schematic of a liquefied production process where second warm end storage is provided at the suction side of the refrigerant compressor.

FIG. 4 is a schematic of a liquefied production process where there is an additional cold end control by removing vapor from the cold end separator.

FIG. 5 is removal of the heavy hydrocarbon contaminants in the natural gas by an intermediate draw.

FIG. 6 is a graph showing LNG production for three cases with different warm end temperatures and adjusted refrigerant compositions.

FIG. 7 is a graph showing the heat exchanger temperature profile for the design case 1 in Table 1 and FIG. 6.

FIG. 8 is a graph showing the heat exchanger temperature profile with increased warm end temperature for case 2 in Table 1 and FIG. 6.

FIG. 9 is a graph showing the heat exchanger temperature profile for the case with the increased warm end temperature and adjusted refrigerant composition for case 3 in Table 1 and FIG. 6

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a mixed refrigerant system employing the methods of the present invention. In this system, both warm 30 and cold end 35 phase separators are present. There are different refrigerant compositions in each of the phase separators and at the bottom of the main heat exchanger upstream of the Joule-Thompson valve.

For purposes of FIGS. 2, 3, 4 and 5, common components compressor, evaporative cooler, main heat exchanger, natural gas feed and liquefied product recovery have been designated with the same numbers throughout.

By changing the relative liquid holdup in the separators and the heat exchanger backup, the refrigerant composition in the loop can be adjusted. The liquid level in the warm end separator is continuously controlled online in order to regulate the iso-pentane in the five-component refrigerant mixture depending on various input conditions. If a higher level of warm end heat transfer is required in the main heat exchanger, a higher amount of higher boiling iso-pentane is required in the refrigerant mixture and this is achieved by decreasing the level in the warm end separator.

The increased heavy components such as iso-pentane in the refrigerant mixture can help to counter the impact due to an ambient temperature increase, a discharge pressure decrease or due to increased solubility of iso-pentane in the compressor oil.

In FIG. 2, there is represented one aspect of the present invention. Natural gas is fed into line 1 and enters the main heat exchanger 10. The liquefied natural gas will leave the main heat exchanger through line 2 where its flow will be controlled by valve 3 and recovered via line 4.

The refrigerants will circulate through line 12 and enter the compressor 15 and travel through line 13 to an evaporative cooler. The colder and compressed gas stream of refrigerants will travel through line 14 to a warm end separator 30 where the warmed stream of refrigerants will leave the warm end separator 30 through line 29 and enter the top of the main heat exchanger 10. The now cooled stream of refrigerants will leave the main heat exchanger 10 through line 21 and enter a Joule-Thompson valve 22 and travel through line 23 to a cold end separator 25. The gaseous portion from the cold end separator 25 will leave through line 24 and reenter the main heat exchanger 10. The cold bottoms from the cold end separator 25 will leave either through valve 26 or valve 27 and line 28 where they will reenter the main heat exchanger 10.

The cold end bottoms from the warm end separator 30 will be pumped out through pump 16 and line 15 as well as be withdrawn through line 17 to valve 18. Opening of the valve 18 will allow the colder bottoms to travel through line 19 back to the main heat exchange 10.

In an alternative embodiment of the present invention, FIG. 3 shows a second warm end storage being provided at the suction side of the refrigerant compressor and is connected to the discharge side warm end separator. This allows for a greater change in the iso-pentane quantity in the refrigerant mixture entering the main heat exchanger by allowing the back and forth transfer of liquid between the suction side separator and the discharge side warm end separator.

In a further embodiment of the present invention, the primary control of the refrigerant composition is performed at the cold end. For small changes in refrigerant compositions, this is achieved by increasing or lowering the level in the cold end separator. When the level is increased, typically the concentrations of the lighter components such as nitrogen and methane in the five component refrigerant mixture is decreased and heavy components such as iso-pentane and iso-butane are increased. When the level is decreased, the concentrations of lighter components are increased. The continuous control of the cold end separator level allows the heat exchanger performance to be maximized when issues such as two phase maldistribution or varying refrigerant compressor discharge pressures are encountered.

In FIG. 3, there is represented a further aspect of the present invention. Natural gas is fed into line 1 and enters the main heat exchanger 10. The liquefied natural gas will leave the main heat exchanger through line 2 where its flow will be controlled by valve 3 and recovered via line 4.

Refrigerant will leave the main heat exchanger 10 through line 36 and travel to a suction side separator 35. SP 35 provides additional valve to store liquid from SP 45. Normally there is no liquid in stream through line 36. The bottoms from the suction side separator will leave through line 38 and travel via pump 40 to line 41 or they can be recycled through valve 39 and line 37 back to the suction side separator 35. If these bottoms are not recycled, they are transmitted via pump 42 and valve 33 back through line 44A to the main heat exchanger 10.

The gas leaving the suction side separator 35 will leave through line 35A to compressor 15 and evaporative cooler 20. The cooled and compressed refrigerant stream will travel to the warm end separator 45 through line 45A and through pump 42 where they can be transmitted through valve 44 and line 44A to the main heat exchanger 10 or through valve 44 and line 43 back to the warm end separator 45. The gas from the top of the warm end separator 45 will leave through line 46 and reenter the main heat exchanger 10.

Once they have traveled through the main heat exchanger 10, the refrigerant mixture will leave through line 31 and their flow will be controlled by a Joule-Thompson valve 32. Flow control could also be a suction pressure control. The refrigerant mixture will flow through line 33 to the cold end separator 40 where the liquid from the bottom will travel through valve 34 and line 34A back to the main heat exchanger 10 or be recycled through line 34B to the cold end separator 50. The gas from the cold end separator 50 will leave through line 36 and reenter the bottom of the main heat exchanger 10.

In a further embodiment shown in FIG. 4, additional cold end control is achieved by removing vapor from the cold end separator and heat exchanging it with a separate liquid nitrogen storage system to liquefy a portion or all of it and returning the liquid to the cold side of the main heat exchanger. This allows for significant changes in the concentration of the lighter components such as nitrogen and methane in the five component refrigerant mixture.

In FIG. 4, there is represented another aspect of the present invention. Natural gas is fed into line 1 and enters the main heat exchanger 10. The liquefied natural gas will leave the main heat exchanger through line 2 where its flow will be controlled by valve 3 and recovered via line 4.

The refrigerant stream from the main heat exchanger 10 will leave through line 78 and connect with a suction side separator 55. The bottoms from the suction side separator 55 will leave through line 57 and be pumped around through pump 60 where they will either reenter the suction side separator through line 61, valve 58 and line 56 or be directed via line 62 to pump 63, although typically there will be no bottoms. The suction side separator 55 will also utilize line 59 to create a connection with line 57 and pump 60 to recirculate as necessary through valve 58 some bottoms withdrawn from the suction side separator 55. The refrigerant bottoms have passed through pump 63 and line 64 will travel through valve 66 and line 69 to reenter the main heat exchanger 10 at the top.

The gaseous refrigerant mixture from the top of the suction side separator will leave through line 55A through compressor 15 and line 13 and travel through the evaporative cooler 20 and line 14 into the warm end separator 65. There the refrigerant stream will leave through the bottom and line 65A and be returned through pump 63, line 64, valve 66 and line 69 to the top of the main heat exchanger 10. A portion of this stream may travel via valve 66 into line 67 and be returned to the warm end separator 65.

The top from the warm end separator will leave through line 68 and reenter the main heat exchanger 10 at the top. Both refrigerant streams that leave the warm end separator 65 through either line 68 or 69 will be recovered from the bottom of the main heat exchanger 10 through line 71 where they will be drawn through a Joule-Thompson valve 72. This stream will travel through line 73 into the cold end separator 70. The gaseous mixture from the cold end separator 70 will leave through line 74 and enter line 79 where they will enter the liquid nitrogen refrigerant buffer 75. This stream may also reenter the main heat exchanger 10 at the bottom. The cold ends of the cold end separator will travel to valve 77 and either be circulated through line 76 back to the cold end separator 70 or travel through line 78 into the bottom of the main heat exchanger 10.

The refrigerant stream that has entered the liquid nitrogen cooled refrigerant buffer 75 will be withdrawn through valve 85 and line 86 to reconnect with line 73 for reentry into the cold end separator 70.

The liquid nitrogen cooled refrigerant buffer 75 will use liquid nitrogen as the buffer and this enters through line 81 and valve 82 and will travel through line 83 and out through line 80 after it has absorbed heat from the refrigerant stream. The liquid nitrogen may also travel through valve 82 and line 84 where it can connect with valve 85 for either reentry into the liquid nitrogen cold refrigerant buffer 75 or pass through line 86 and line 73 to the cold end separator 70.

FIG. 5 demonstrates another aspect of the present invention where there is removal of pentane or hexane by regulating the temperature and the natural gas bypass flow. Natural gas is fed into line 1 and enters the main heat exchanger 10. The liquefied natural gas will leave the main heat exchanger through line 2 where its flow will be controlled by valve 3 and recovered via line 4

The natural gas feed can also travel through valve 106 and line 108 to a separator 105 where the top gaseous stream lean in the heavies will leave through line 109 and travel for entry into the main heat exchanger 10. The bottom liquid stream enriched in heavy components such as propane, butane and pentane when present in the natural gas feed will leave through valve 112 and line 113 to a boiler (not shown). They can also be recycled through line 111 to the separator 105. The natural gas feed may also be directed through valve 106 to lines 107 and 110 for entry into the separator 105.

The warm stream leaving the main heat exchanger 10 through line 96 will enter compressor 15 and through line 13 enter evaporative cooler 20. The cooled and compressed refrigerant stream will enter the warm end separator 95 through line 14. The top gaseous refrigerant stream from the warm end separator will leave via line 95A for entry into the main heat exchanger 10. The bottom liquid stream will leave via pump 99 and will travel to valve 100 where they may be recycle to the warm end separator through line 98.

The bottoms from the warm end separator may also continue through valve 100 and line 91 into the main heat exchanger 10. This stream having passed through the main heat exchanger 10 will leave via line 91A and pass through a Joule-Thompson valve 92 where it will enter the cold end separator 90 through line 93. The bottoms from the cold end separator 90 will travel through valve 94 and either be recirculated through line 94A to the cold end separator 90 or travel through line 96 for reentry into the main heat exchanger 10. The top gaseous stream from the cold end separator 90 will travel through line 97 for entry back into the main heat exchanger 10.

TABLE 1
Effect of warm end temperature and refrigerant
composition on liquefier efficiency.
	1	2	3
	Tmax	° C.	25.5	35	35
	x isopentane	mol %	12.9	12.9	15.5
	LNG Prod.	kg/hr	745	589	695
	Relative LNG	% of	100%	79%	93%
	Production	design
	Case 1 is the design case, case 2 is a simulation for a 9.5K higher warm end temperature for the same refrigerant composition, and case 3 is also for the higher warm end temperature, but with an adjusted refrigerant composition. Only the amount of isopentane was increased in order to remove a temperature pinch at the warm end of the heat exchanger. FIGS. 7 through 9 show the effect of the increased warm end temperature and the compensating effect of the composition adjustment.

While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.

Claims

1. A method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop containing a refrigerant composition is directed through a heat exchanger comprising controlling said refrigerant composition continuously by changing the liquid level in a warm end phase separator.

2. The method as claimed in claim 1 wherein said refrigerant composition contains iso-pentane.

3. The method as claimed in claim 1 wherein the liquid level in said warm end phase separator decreases as the heat transfer required of said heat exchanger increases.

4. The method as claimed in claim 1 wherein said liquid level is controlled on-line.

5. The method as claimed in claim 1 wherein the liquid level in said warm end phase separator increases as the heat transfer required of said heat exchanger decreases.

6. The method as claimed in claim 1 wherein said heat exchanger and said warm end phase separator are in fluid communication.

7. The method as claimed in claim 1 wherein said cooling loop and said heat exchanger are in fluid communication.

8. The method as claimed in claim 2 wherein said control is performed by changing the amount of iso-pentane present in said refrigerant composition.

9. A method for providing cooling to a process for producing liquefied natural gas comprising the steps: a) contacting a stream of liquefied natural gas with a heat exchanger;b) contacting said heat exchanger with a cooling loop containing a refrigerant composition;c) adjusting the composition of said refrigerant by controlling the amount of a heavier condensable component in said refrigerant composition in response to performance variations in said heat exchanger; andd) recovering liquefied natural gas.

10. The method as claimed in claim 9 wherein said heavier condensable component is iso-pentane.

11. The method as claimed in claim 9 wherein said performance variations in said heat exchanger are selected from the group consisting of a temperature increase in said heat exchanger and a temperature decrease in said heat exchanger.

12. The method as claimed in claim 9 wherein the amount of said heavier condensable component in said refrigerant composition will increase in response to an increase in temperature in said heat exchanger.

13. The method as claimed in claim 9 wherein the amount of said heavier condensable component in said refrigerant composition will decrease in response to a decrease in temperature in said heat exchanger.

14. A method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop comprising two warm end separators and a cold end separator are in fluid connection with each other comprising controlling the liquid level in said warm end separators.

15. The method as claimed in claim 14 wherein said cooling loop contacts a heat exchanger.

16. The method as claimed in claim 14 wherein said refrigerant composition contains iso-pentane.

17. The method as claimed in claim 14 wherein the liquid level in said warm end separators decreases as the heat transfer required of said heat exchanger increases.

18. The method as claimed in claim 14 wherein said liquid level is controlled on-line.

19. The method as claimed in claim 14 wherein the liquid level in said warm end separators increases as the heat transfer required of said heat exchanger decreases.

20. The method as claimed in claim 16 wherein said control is performed by changing the amount of iso-pentane present in said refrigerant composition.

21. The method as claimed in claim 14 wherein a first warm end separator is in fluid communication with a second warm end separator.

22. A method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop comprising a warm end separator and a cold end separator are in fluid connection with each other comprising the steps: a) controlling the liquid level in said warm end separator;b) removing vapor from said cold end separator;c) heat exchanging said vapor with a separate liquid nitrogen storage system; andd) returning the liquid from said exchange of step c) to a heat exchanger.

23. The method as claimed in claim 22 wherein said controlling of said liquid level is performed on-line.

24. The method as claimed in claim 22 wherein the liquid level in said warm end separator is controlled by changing the amount of iso-pentane present in said liquid.

25. The method as claimed in claim 22 wherein said cooling loop contacts a heat exchanger.

26. The method as claimed in claim 22 wherein the liquid level in said warm end separators decreases as the heat transfer required of said heat exchanger increases.

27. A method for providing refrigeration to a natural gas liquefaction process wherein a cooling loop comprising a warm end separator and a cold end separator are in fluid communication with each other comprising controlling the liquid level in said warm end separator and controlling the flow of natural gas from a separator to a heat exchanger.

28. The method as claimed in claim 27 wherein the liquid level in said warm end separator is controlled by changing the amount of iso-pentane present in said liquid.

29. The method as claimed in claim 27 wherein said controlling of said liquid level is performed on-line.

30. The method as claimed in claim 27 wherein the flow of natural gas from a separator to a heat exchanger is controlled by the temperature of said separator where a gas stream will enter said heat exchanger and a bottom liquid stream is directed from said separator for reentry into said separator.

31. The method as claimed in claim 30 wherein said bottom liquid stream comprises butane, propane and pentane.

32. The method as claimed in claim 30 wherein said gas stream from said separator after exiting said heat exchanger is directed into said separator.