Process and system for liquefaction of hydrocarbon-rich gas stream utilizing three refrigeration cycles

Abstract

A process is disclosed for liquefying a hydrocarbon-rich gas stream such as natural gas by indirect heat exchange with refrigerants in a cascade of three closed loop refrigeration cycles, namely pre-cooling, liquefaction and subcooling. Each of the closed loop refrigeration cycles utilizes a different refrigerant composition. The pre-cooling cycle utilizes a first mixed refrigerant; the liquefaction cycle utilizes a pure refrigerant; and the subcooling cycle utilizes a second next refrigerant. The refrigerants closely match the enthalpy curve of the hydrocarbon-rich gas, thereby improving the energy efficiency of the liquefaction process.

Description

FIELD OF THE INVENTION

The present invention relates generally to the liquefaction of natural gas and more particularly to a gas liquefaction process having improved efficiency including three refrigeration cycles, namely pre-cooling, liquefaction and subcooling.

BACKGROUND OF THE INVENTION

Known processes for liquefying gas e.g. natural gas include a variety of process or flowsheet configurations. Such configurations include refrigeration cycles for liquefaction using between one and three pure refrigerants, one and three mixed refrigerants, or a combination thereof. The process can contain all closed loop refrigeration cycles, a cascade of refrigeration cycles or open loop refrigeration cycles. The liquefied natural gas industry continuously seeks to improve the efficiency of liquefaction.

SUMMARY OF THE INVENTION

The invention relates to a process for liquefying a hydrocarbon-rich stream such as natural gas by indirect heat exchange with refrigerants in a cascade refrigeration cycle comprising three closed loop refrigeration cycles, characterized in that the refrigerants are subcooled and subjected to expansion, and the resultant cooled gas is heated in indirect heat exchange with the hydrocarbon-rich stream within at least one heat exchanger in each of the three cycles, and each of the resultant vaporized refrigerants is compressed, wherein:

the first cycle pre-cools the hydrocarbon-rich stream utilizing a pre-cooling mixed refrigerant;

the second cycle liquefies the hydrocarbon-rich stream utilizing a liquefying pure refrigerant; and

the third cycle subcools the hydrocarbon-rich stream utilizing a subcooling mixed refrigerant.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the present invention will become better understood with regard to the following description, pending claims and accompanying drawings where:

FIG. 1 is a schematic diagram of the liquefaction process of one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Liquefied natural gas, or LNG, is natural gas that has been processed to remove impurities and heavy hydrocarbons and then condensed into a liquid at almost atmospheric pressure by cooling to approximately −260° F. (−163° C.).

Improving the efficiency of gas liquefaction involves matching as closely as possible the cooling curves, also referred to as the enthalpy curve, of the gas and the refrigerants utilized in the refrigeration cycles of the liquefaction process. The cooling curve is a plot of enthalpy versus temperature. As the natural gas is liquefied, there are three distinct sections within the cooling curve: pre-cooling, liquefaction, and subcooling. For ideal efficiency, the refrigerants for use in the liquefaction process are selected based on a close match of the enthalpy-temperature relationships between the refrigerants and the particular gas to be liquefied. The closer the cooling curve and refrigeration curves, theoretically the more thermodynamically efficient the liquefaction process. Therefore, the required power per unit of LNG produced is lowered. The liquefaction process described in the first embodiment of the invention includes three stages of refrigeration utilizing: (a) a first mixed refrigerant for pre-cooling, (b) a pure refrigerant for liquefaction, and (c) a second mixed refrigerant for subcooling. The three stages or cycles mimic the cooling curve of the natural gas to be liquefied thus reducing the duty or power required as compared with traditional liquefaction processes and thereby increasing efficiency. Each refrigerant loop can be optimized with respect to energy consumption, operation, and equipment availability.

The liquefaction process 100 in the first exemplary embodiment is illustrated in FIG. 1. A natural gas stream 2 is fed to an acid gas removal unit 4 where the gas is first treated to remove carbon dioxide or other acid gases by any known means. The acid gas is removed at gas removal port 6. The natural gas stream is then pre-cooled by a first mixed refrigerant (MR1) utilizing first closed loop refrigeration cycle 8 at heat exchanger 10. The lower natural gas temperature enables better separation of water in dehydration column 12 where water is removed from water removal port 14. The pre-cool cycle 8 optimizes the temperature to slightly above the hydrate formation temperature. The natural gas stream is then further cooled at heat exchanger 16 utilizing the first closed loop mixed refrigerant MR1 to create an optimum temperature for the separation and removal of heavy hydrocarbons in fractionation column 18. The fractionation column includes a condenser which utilizes the pure refrigerant to meet the cooling requirement. As an example, the gas can be pre-cooled to a temperature of about 24° F. (−5° C.) entering the fractionation column and about −26° F. (−32° C.) exiting the fractionation column. The heavy hydrocarbons are removed by way of heavy hydrocarbon removal port 20. The second closed loop refrigerant is a pure refrigerant (PR) and is utilized for further cooling within the fractionation column.

The natural gas is then cross exchanged with pure refrigerant PR utilizing a second closed loop refrigeration cycle 22 for further temperature reduction at heat exchanger 24. The natural gas stream can be cooled by the pure refrigerant to approximately −105° F. (−76° C.), for example, prior to entering the final LNG heat exchanger 28. The natural gas stream liquefies within this low pressure pure refrigerant cycle.

The final, subcooling temperature reduction of the natural gas occurs in LNG heat exchanger 28 by heat exchange between the natural gas and a second mixed refrigerant (MR2), utilizing third closed loop refrigeration cycle 26. The second mixed refrigerant MR2 is utilized in a first pass through the LNG exchanger 28, and the LNG pressure is dropped for further temperature reduction followed by additional passes through the LNG exchanger 28. The LNG exchanger 28 can subcool the LNG to storage temperatures of approximately −257° F. (−161° C.), for example.

The natural gas pressure is reduced to a suitable storage pressure through the use of a Joule-Thomson valve 30, and sent to LNG storage 32.

The three closed loop refrigeration cycles 8, 22 and 26 are utilized in a cascade type process wherein the second loop pure refrigerant PR cools the first loop mixed refrigerant MR1. Similarly, the third loop mixed refrigerant MR2 cools the second loop refrigerant PR. Known process components of refrigeration cycles, namely compression, cooling and pressure drop, are utilized in each closed loop refrigeration cycle to liquefy the refrigerant and prepare for heat exchange with the natural gas. Air coolers can optionally be used for interstage cooling of the refrigerant within the compression stages. Cooling water can optionally be utilized if available; this would advantageously further reduce energy usage if the outlet temperature were lower than the ambient air temperature.

The liquefaction process illustrated in FIG. 1 was simulated as an example of the invention using HYSYS 2004.1 process simulation software (available from Aspen Technology Inc., Burlington, Mass.) to determine the overall duty requirement of the process of the simulated example. The acid gas removal, dehydration, and fractionation components of the liquefaction process were all simulated to ensure accurate values were used in calculating the overall duty required by the simulated process. Note, the rundown and the boil-off gas sections of the liquefaction plant were not simulated as these sections do not directly relate to the liquefaction process.

The feed gas composition and operating conditions assumed in the simulation are given in Table 1. Although the feed gas composition used in the simulation is natural gas, other hydrocarbon-rich gas compositions could be liquefied by the present exemplary process as well.

TABLE 1
Feed Operating Pressure 735/50.7
(psia/bara)
Feed Operating          105/40.7
Temperature (° F./° C.)
Feed Flow rate          563/5.4
(MMSCFD*/MMTPA**)
Feed Gas Composition    Mol Fraction
Methane                 0.835
Ethane                  0.062
Propane                 0.032
i-Butane                0.005
n-Butane                0.006
i-Pentane               0.002
n-Pentane               0.002
Nitrogen                0.001
Carbon Dioxide          0.054
Water                   0.001
Total                   1.000
*millions of standard cubic feet per day
**million metric tons per annum

Table 2 summarizes the final LNG composition and operating conditions as a result of the simulated liquefaction process as an example of the invention. The pre-cooling, cooling/liquefaction and subcooling temperatures in the simulation were those temperatures given as examples in the preceding paragraphs. Namely, the natural gas stream was assumed to be pre-cooled to approximately 24° F. (−5° C.) entering the fractionation column and approximately −26° F. (−32° C.) exiting the fractionation column; the natural gas stream was assumed to be cooled by pure refrigerant to approximately −105° F. (−76° C.) prior to entering the LNG heat exchanger 28; and the liquefied natural gas was assumed to be subcooled by the LNG exchanger 28 to approximately −257° F. (−161° C.).

TABLE 2
Operating Pressure    15/1
(psia/bara)
Operating Temperature −257/−161
(° F./° C.)
Flow rate             521/4.5
(MMSCFD/MMTPA)
Final LNG Composition Mol Fraction
Methane               0.901
Ethane                0.064
Propane               0.029
i-Butane              0.003
n-Butane              0.003
i-Pentane             0.000
n-Pentane             0.000
Nitrogen              0.000
Carbon Dioxide        0.000
Water                 0.000
Total                 1.000

The first mixed refrigerant (MR1) is utilized in the pre-cool cycle of the liquefaction process. MR1 is made-up of ethane, methane, and propane in a ratio that allows for optimizing thermodynamic properties, i.e., optimizing the match between the MR1 and the pre-cooling portion of the gas cooling curve. MR1 preferably contains ethane between about 10% and about 20%, propane between about 60% and about 70%, and i-butane between about 15% and about 25%. For the natural gas composition considered in the process simulation in this example, it was found that MR1 is preferably about 16% ethane, about 64% propane and about 20% i-butane.

MR1 is compressed by compressor 42 to a high pressure. For the purposes of the simulation, it was assumed that MR1 is compressed to approximately 240 psia, although this pressure can vary. MR1 is then cooled by either chilled water or air (indicated in FIG. 1 by compressor/air cooler combinations 34/36 and 40/38). A discharge temperature of approximately 100° F. (38° C.) was assumed in order to ensure refrigeration requirements were met. MR1 is then expanded over three pressure drops (the first indicated in FIG. 1 by expansion valves 44 and 46 and heat exchanger 48; the second indicated by expansion valves 54 and 56 and heat exchanger 50; and the third indicated by expansion valves 57 and heat exchanger 52). The first pressure drop is used to pre-cool the natural gas stream upstream of dehydration 12, and the second pressure drop is used to perform the first cooling of the pure refrigerant used in the liquefaction refrigeration cycle. The second pressure drop cools the natural gas feed upstream of the fractionation column 18 as well as further cools the pure refrigerant. The final pressure drop of MR1 is used to liquefy the pure refrigerant. The heat exchangers used within each of the closed loop refrigeration cycles of the liquefaction process in the simulation (designated by numerals 48, 50, 52, 92, and 70 in FIG. 1) were assumed to be core-in-kettle type heat exchangers. A plate-and-fin type heat exchanger was assumed for the direct exchange between the first mixed refrigerant and the natural gas stream at heat exchanger 16. After completion of cooling, MR1 is compressed and begins the closed-loop cycle 8 again.

The liquefaction cycle 22 of the liquefaction process is also a closed loop and utilizes a pure (i.e., single component) refrigerant (PR). Any refrigerant can be used within the process as long as the thermodynamic properties match the cooling curve of the natural gas in the liquefaction portion of the curve, and the boiling point of the refrigerant is lower than the lowest boiling point of MR1. For the purposes of the simulation performed, 100% ethylene was used as the PR. The PR is compressed to high pressure by compressor 74 and is then cooled by either chilling water or air (indicated by compressor/cooler combination 76/78). A 100° F. (38° C.) discharge temperature was assumed. The cold available from the MR1 cycle is utilized to cool and liquefy the PR in three stages, in heat exchangers 48, 50 and 52. PR is then expanded over three pressure drops using expansion valves 58 and 60 in the first pressure drop, expansion valves 62 and 64 in the second pressure drop and expansion valves 66 and 68 in the third pressure drop. The pressure of PR is reduced prior to fractionation column 18 and in order to liquefy the natural gas feed stream prior to the natural gas entering the LNG exchanger 28. Within all three pressure drops of the liquefaction cycle, MR2 is cooled and condensed. The PR is then compressed by compressor 72 and the cycle 22 repeats. A plate-and-fin type heat exchanger was assumed for the direct exchange between the pure refrigerant and the natural gas stream at heat exchanger 24.

The final subcooling refrigeration cycle 26 is also a closed loop process. The subcooling cycle utilizes a second mixed refrigerant (MR2) containing components of the natural gas stream, namely nitrogen, methane, and propane, in a ratio that allows for optimizing the match between MR2 and the subcooling portion of the gas cooling curve. MR2 preferably contains nitrogen between about 30% and about 40%, methane between about 30% and about 40%, and ethane between about 30% and about 40%. For the natural gas composition considered in the example process simulation, it was found that MR2 is preferably about 33% nitrogen, about 33% methane and about 34% ethane.

MR2 is compressed to a high pressure by compressor 84 and is then cooled by either chilled water or the air (indicated by compressor/cooler combinations in 84/86 and 88/90). For the purpose of the example simulation, air was used with a discharge temperature of approximately 100° F. (38° C.). MR2 is then condensed to a vapor fraction of approximately 0.5 by cross exchanging heat between MR2 and PR at heat exchanger 92. The vapor and liquid streams are separated by separator 80 and introduced separately into LNG heat exchanger 28. A cold box type heat exchanger was assumed for use as LNG exchanger 28. The use of other LNG exchanger e.g. a main cryogenic heat exchanger could also be used in the liquefaction process. Joule-Tompson valves 94 can be used to further reduce the temperature of the MR2 streams (which recombine following the valves) within the LNG exchanger thereby subcooling the LNG. MR2 is then recycled to the beginning of the cycle 26.

The process of the invention advantageously distributes the duty among all three refrigerants and produces a total duty per ton of LNG produced. Table 3 contains the values of the required duties from each refrigeration cycle and total from the pre-cooling/liquefaction/subcooling (MR1/PR/MR2) liquefaction process.

TABLE 3
Duty
MR1 Loop                 89
PR Loop                  73
MR2 Loop                 66
Total Refrigeration Duty 228
Required
Refrigeration            51
Duty/MMTPA of LNG

The unique refrigerant sequence allows adjustments to the cooling curve for optimization of the process. The cooling curve requirements vary depending on natural gas composition. Utilizing the first mixed refrigerant in the first stage, it is possible to vary the percentages of components of the first mixed refrigerant to mimic this portion of the cooling curve. In the second stage, it has been found that pure ethylene refrigerant is appropriate to match the cooling curve. The liquefaction process requires lower duty and offers higher efficiency than traditional liquefaction processes. The process requires 51 MW/MMTPA (megawatts/million metric tons per annum).

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the invention.

Claims

1. A process for liquefying a hydrocarbon-rich stream by indirect heat exchange with refrigerants in a cascade refrigeration cycle comprising three closed loop refrigeration cycles, characterized in that the refrigerants are subcooled and subjected to expansion, and the resultant cooled gas is heated in indirect heat exchange with the hydrocarbon-rich stream within at least one heat exchanger in each of the three cycles, and each of the resultant vaporized refrigerants is compressed, wherein:

the first cycle pre-cools the hydrocarbon-rich stream utilizing a pre-cooling mixed refrigerant;

the second cycle liquefies the hydrocarbon-rich stream utilizing a liquefying pure refrigerant; and

the third cycle subcools the hydrocarbon-rich stream utilizing a subcooling mixed refrigerant.

2. The process of claim 1 wherein the pre-cooling mixed refrigerant comprises ethane, propane and i-butane; the pure refrigerant comprises ethylene; and the subcooling mixed refrigerant comprises nitrogen, methane and ethane.

3. The process of claim 1 wherein the pre-cooling mixed refrigerant consists essentially of between about 10% and about 20% ethane, between about 60% and about 70% propane and between about 15% and about 25% i-butane; the pure refrigerant consists essentially of about 100% ethylene; and the subcooling mixed refrigerant consists essentially of between about 30% and about 40% nitrogen, between about 30% and about 40% methane and between about 30% and about 40% ethane.

4. The process of claim 1 wherein the pre-cooling mixed refrigerant consists essentially of about 16% ethane, about 64% propane and about 20% i-butane; the pure refrigerant consists essentially of about 100% ethylene; and the subcooling mixed refrigerant consists essentially of about 33% nitrogen, about 33% methane and about 34% ethane.

5. The process of claim 1 wherein the hydrocarbon-rich stream comprises methane.

6. The process of claim 1 wherein the hydrocarbon-rich stream comprises natural gas.