Liquefaction Of Natural Gas Using Separated Pure Components As Refrigerants

Abstract

A gas liquefaction system wherein the refrigerant components are mixed with one another and, thereafter, separated into individual streams which enter the system as liquid streams. Cooling is produced by evaporating the cooled liquid component streams into a gas stream, which after all the individual cooled liquid component streams have been evaporated into it, forms the mixed refrigerant component stream returned to the compressor.

Parent Case Text

This application is a continuation-in-part of application Ser. No. 770,922, filed Oct. 28, 1968.

Description

This invention relates to improved methods for liquifying gases.

The method of the present invention is particularly adapted to liquify natural gas which is composed mostly of methane but which may contain heavier hydrocarbons such as, for example, ethane, propane, butane and the like. However, it will be apparent from the description below that the method can be used to liquify a wide range of normally gaseous fluids, merely by properly selecting the refrigerant components used in this system.

It is generally well known that natural gas when reduced from a gaseous state to a liquified state is reduced approximately one six-hundredth in volume, and that the natural gas, when in a liquified state, can be more easily transported and stored. For this reason, considerable effort has been made to improve the existing methods of liquifying natural gas.

Presently, gas liquefaction systems generally are of two types. One is based on an expansion cycle wherein gas at high pressure is allowed to expand, preferably with work, with a corresponding reduction in temperature until a wet gas is produced wherein the condensed portion can be removed from the dry gas residue. The latter can be recompressed and recycled with makeup gas through the expansion system. The compressed gas is usually passed in heat exchange relation with a refrigerant to remove heat of compression and to cool the gas under pressure, whereby a higher percentage of wet gas is produced during the subsequent expansion step or steps.

The other main type of gas liquefaction system is the cascade system which is based upon the use of heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature, at which point all of the gas is converted to a liquified state. Heat exchange for refrigeration is usually carried out while the gas is at some higher pressure above atmospheric pressure, whereby the phase conversion is at a higher and more accessible temperature level. The compressed and liquified gas is then let down to a lowered pressure for storage and transportation. During the reduction in pressure, some of the liquid will be flashed off as a gas with accompanying reduction in temperature.

The selection of a cascade cycle or an expansion cycle for liquefaction depends upon a number of factors, including the composition of the gas, the power requirements, the space available for the equipment, the cost of raw materials, the type of equipment, and the like.

The method of the present invention is, generally, a cascade system, however, substantial improvements over the classical cascade cycle, or the newer so-called incorporated cascade cycle which uses only one compressor, are provided. In this latter system, the refrigerant components such as propane, ethane and butane are maintained separated as individual components or streams, in both the refrigeration cycles of the system and in storage. Accordingly, as a result, this system requires a substantial amount of power to condense these refrigerant components, to maintain the efficiency of the systems. In the system of the present invention, the refrigerant components are mixed with one another and, after being compressed, are mixed, in one embodiment of the invention, with the natural gas in the process stream. Thereafter, the refrigerant components are separated into individual streams of essentially pure components which enter the system as liquid streams. In a second embodiment of the invention, the refrigerant components are separated into individual streams of different mixtures of the components. Cooling is produced by evaporating the cooled liquid component streams progressively into a gas stream which, after all of the individual cooled liquid components streams (pure or mixtures) have been evaporated into it, forms the mixed refrigerant component stream returned to the compressor. With this arrangement, only a single multistage compressor is required and the compression energy required is approximately 40 percent less than the compression energy required for a classical cascade cycle.

The method of the present invention can be employed both for the initial liquefaction of a natural gas at the source of supply, for storage or transportation, and to reliquify natural gas vapors given off during storage. Generally, in the cascade and the incorporated cascade systems, the boil-off and flash gas either are vented to the atmosphere, compressed in a second compressor or used as a fuel. If vented to the atmosphere, or used as a fuel, the stored gas may have excess heavy hydrocarbons and may not be satisfactory for general distribution. If compressed in a second compressor and returned to storage, equipment costs are increased as are power requirements. With the system of the present invention, the boil-off and flash gas can be returned and combined with the feed stream and are redelivered to the storage tanks so that no additional equipment is required and the concentration of the stored gas is maintained relatively constant.

Accordingly, it is an object of the present invention to provide an improved liquefaction system for gaseous materials. In particular, it is an object to provide improved liquefaction system for the liquefaction or reliquifaction of natural gas.

Another object is to provide improved liquefaction system which is simple in construction and economical and efficient in operation.

A still further object is to provide an improved liquefaction system wherein the refrigerant components are delivered to a compressor as a mixed stream which, after compression is separated into individual streams which enter the system as liquid streams.

Still another object is to provide an improved liquefaction system wherein substantially less compression energy is required than in similar systems.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others thereof, which will be exemplified in the method hereinafter disclosed, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a flow sheet illustrating a first embodiment of the invention;

FIGS. 2 and 3 are partial flow sheets illustrating modified forms of the invention;

FIG. 4 is a flow sheet illustrating a second embodiment of the invention; and

FIG. 5 is a partial flow sheet illustrating a modified form of the embodiment of FIG. 4.

Referring now to the drawings, particularly FIG. 1 thereof, the natural gas stream 36 is delivered to the system, at a high pressure and at a temperature achievable by cooling water. Preferably this natural gas stream is fed to the demethanizer 21 of the stripper 20, wherein it is joined with the condensed mixed refrigerant stream 55, fully described below, from the multistage compressor 30. Alternatively, however, the natural gas stream 36, if it is a substantially pure methane stream, can be delivered directly to the inlet of the heat exchanger 10, as indicated by the stream 26. In the illustrated example, both the natural gas stream 36 and the condensed mixed refrigerant stream 55 are assumed to be at a pressure of 600 p.s.i.a., and at a temperature of 70.degree. F., however, different pressures and temperatures can be used, if desired, and the system appropriately modified accordingly.

The stripper 20 can be a conventional stripper of a type generally used for fractional distillation, using heavy hydrocarbons as an overall cycle reflux with the removal of heat in the reflux condenser of each column and the supply of heat in the reboiler of each column. In the illustrated example, the different columns of the stripper 20 are a demethanizer 21, a deethanizer 22, a depropanizer 23, a debutanizer 24 and a depentanizer 25, and each includes a reflux condenser 68 and a reboiler 69.

The demehtanizer 21 essentially strips the methane from the condensed mixed refrigerant stream 55, and at the same time conditions the natural gas stream 36 so as to deliver an essentially pure liquid methane stream 40 to the liquefaction cycle. Most of the heavy ends in the natural gas stream will absorb in the recycle heavy end stream 62, thereby reducing the refrigeration required for condensation of the essentially pure liquid methane stream 40 in the liquefaction cycle.

The methane stream 40 is progressively cooled in the liquefaction cycle, in the heat exchangers 10--17 in a fashion such that it is at approximately 600 p.s.i.a. and -250.degree. F, when it emerges from the heat exchanger 17, in a manner fully described below. It can then be delivered under pressure into a storage tank 18, however, preferably it is expanded through the throttling valve 41 to a storage pressure, usually near atmospheric and in the illustrated example assumed to be 15 p.s.i.a., and separated into a liquid stream 42 which is delivered into a storage tank 18 or the like and a gaseous methane stream 43, both of which are at approximately -260.degree. F.

The gaseous methane stream 43 is joined with the hereinafter described liquid ethane stream 58, and both of these streams are joined with the stream 50 returned, through each of the heat exchangers 10--17, to the multistage compressor 30. Ultimately, all of the individual refrigerant components are joined with the stream 50 before it is returned to the multistage compressor 30, hence the stream 50 hereinafter is referred to as the mixed refrigerant stream 50. The gaseous methane stream 43 functions as the carrier for the individual refrigerant components which are absorbed as they evaporate into the mixed refrigerant stream 50 and, furthermore, provides a gaseous stream which will result in efficient operation of the heat exchangers of the system. Accordingly, for proper operation of the system, it is essential that a gas methane stream be coupled to and joined with the mixed refrigerant stream 50. This gaseous methane stream can be provided in the above-described manner or, alternatively, it can be provided by, for example, coupling the boil-off from the storage tank 18 to the mixed refrigerant stream. It will be obvious to those skilled in the art that other gases or mixtures of gases can be substituted for the methane in the gaseous stream 43, if desired.

Liquefaction of the liquid methane stream 40 in the heat exchangers 10--17 is accomplished by four refrigeration cycles, including an ethane cycle, a propane cycle, a butane cycle and a pentane cycle. Each of these refrigeration cycles is provided an essentially pure liquid refrigeration component, from the stripper 20 and, from the description below, it will be apparent that each of these cycles is adapted to achieve a reduction in temperature of the liquid methane stream 40 in the area where each of them can be operated with the greatest efficiency.

More particularly, the ethane cycle is provided by the liquid ethane stream 58 which is delivered to and passed through each of the heat exchangers 10--17, from the deethanizer 22 of the stripper 20. In passing through these heat exchangers, the pressure of the ethane stream 58 is maintained substantially constant at 600 p.s.i.a. and it is progressively cooled in respective ones of these heat exchangers to 0.degree. F, -50.degree. F, -70.degree. E, -100.degree. F, -140.degree. F, -160.degree. F, -180.degree. F and -250.degree. F. Upon emerging from the heat exchanger 17, the cooled liquid ethane stream is expanded through the throttling valve 67 to 15 p.s.i.a. and -255.degree. F, and is joined with the gaseous methane stream 43. As indicated above, both of these streams are joined with the mixed refrigerant stream 50, prior to entering the heat exchanger 17. The heat of vaporization of the cooled liquid ethane is absorbed as it evaporates into the mixed refrigerant stream 50, as the latter is warmed from -255.degree. F to -180.degree. F in the heat exchanger 17, and provides the refrigeration in the heat exchanger 17. It may be noted that a portion of the cold liquid ethane stream 58, at the outlets of the heat exchangers 14, 15 and 16, also is coupled to and joined with the mixed refrigerant system 50. The cold liquid ethane stream 58 at the outlet of the heat exchanger 14 is at 600 p.s.i.a. and -140.degree. F, and the portion thereof which is coupled to the mixed refrigerant stream 50 is expanded through the throttling valve 70 to 15 p.s.i.a. This cold liquid ethane stream joined with the mixed refrigerant stream 50 provides the refrigeration for the heat exchanger 14, as it evaporates into the mixed refrigerant stream and warms from -140.degree. F to -100.degree. F. Similarly, the portions of the cold liquid ethane stream removed at the outlets of the heat exchangers 15 and 16 are expanded through the throttling valves 71 and 72, respectively, to 15 p.s.i.a. before being joined with the mixed refrigerant stream 50. The refrigeration for these heat exchangers 15 and 16 is provided by the evaporation of the expanded cold liquid ethane stream, as it warms from -160.degree. F to -140.degree. and from -180.degree. F to -160.degree. F in these heat exchangers 15 and 16, respectively.

The propane cycle is provided by the liquid propane stream 59 which is delivered to and passed through only the heat exchangers 10--13, from the depropanizer 23 of the stripper 20. The liquid propane stream 59 is progressively cooled in respective ones of these heat exchangers 10--13 to 0.degree. F, -50.degree. F, -70.degree. of and -100.degree. F, while the pressure thereof is maintained substantially constant at 600 p.s.i.a. or less. This cooled liquid propane stream 59 at -100.degree. F, is expanded through the throttling valve 74 to 15 p.s.i.a. and is joined with the mixed refrigerant stream 50. As in the case of the ethane stream 58, this cooled liquid propane stream 59 provides the refrigeration in the heat exchanger 13, as it evaporates into the mixed refrigerant stream 50 and warms from -100.degree. F to -70.degree. F.

The butane cycle is provided by the liquid butane stream 60 which is delivered to and passed through only the first three heat exchangers 10--12, from the debutanizer 24. This liquid butane stream is progressively cooled in respective ones of the heat exchangers 10--12 to 0.degree. F, -50.degree. F and -70.degree. F. The pressure again is maintained substantially constant at 500 p.s.i.a. or less. A portion of the butane stream 60, at the outlet of the heat exchanger 11, is joined with the mixed refrigerant stream 50, after being expanded through throttling valve 75 to 15 p.s.i.a., and the rest of the butane stream 60 is joined with the mixed refrigerant stream, after passing through the heat exchanger 12 and being expanded through the throttling valve 76 to 15 p.s.i.a. The refrigeration for the heat exchangers 11 and 12 is provided by this expanded butane stream, as it evaporates into the mixed refrigerant stream 50 and warms from -50.degree. F to 0.degree. F and from -70.degree. F to -50.degree. F in the heat exchangers 11 and 12, respectively.

The remaining refrigeration cycle is the pentane cycle and, in this case, an essentially pure liquid pentane stream 61, at 400 p.s.i.a. or less and 70.degree. F, is delivered to and passed through the heat exchanger 10, from the depentanizer 25. In passing through the heat exchanger 10, the liquid pentane stream 61 is cooled to 0.degree. F and is then expanded through the throttling valve 77 to 15 p.s.i.a. and joined with the mixed refrigerant stream 50. This cooled pentane stream is evaporated into the mixed refrigerant stream 50 and provides the refrigeration for the heat exchanger 10, as it warms from 0.degree. F to 70.degree. F.

The depentanizer 25 can be adapted to also produce a side stream of hexanes, if desired. A net product stream of heavy ends from the bottom thereof is delivered to the recycle heavy end stream 62. These heavy ends are pumped by the pump 63 to the reboiler product cooler 64, and from the latter they flow through the pipe 65 to the top of the first column or demethanizer 21 of the stripper 20. These heavy ends join with the natural gas stream 36 and the mixed refrigerant stream 55 from the multistage compressor 30, and all of these streams are again separated in the stripper, in the above-described manner.

It can be seen from the above description of the refrigeration cycle of the liquefaction system that cooling is produced by evaporating the cooled individual liquid refrigerant streams 58, 59, 60 and 61 progressively into the mixed refrigerant stream 50 which is a gas stream and which initially is essentially free of these individual components. The result is a much more efficient refrigeration effect, which provides a substantial reduction, up to 40 percent in comparison to the classical cascade cycle, in the required compression energy.

The mixed refrigerant stream 50 which, as indicated above, is a gas stream, emerges from the heat exchanger 10 at a pressure of 15 p.s.i.a. and a temperature of 70.degree. F. The mixed refrigerant stream 50 is compressed in the multistage compressor 30 including the compressor stages 31--34 to 600 p.s.i.a., and is cooled in water cooled condensers 35 included between each of the compressor stages. The compressed mixed refrigerant stream 55 from the multistage compressor 30, is delivered to the top column or demethanizer 21 of the stripper 20, for reseparation, in the manner described above.

The composition of the liquid methane stream 42 fed to the storage tank 18 can be regulated by removing the quantities of heavy components which are condensed in heat exchangers 10, 11 and 12. These separated liquids are coupled from the methane stream 40 through pipes 45--47 to the mixed refrigerant stream 50. Throttling valves 51 and 52 are included in the pipes 45 and 46 (the stream in pipe 47 is expanded through the throttling valve 76) for throttling these heavy liquid components to approximately 15 p.s.i.a. These three streams will essentially remove all of the propane, butane, pentane and heavier hydrocarbons from the methane stream 40, although the total quantity of these heavy ends is usually quite small.

If it is desired to reduce the ethane content of the liquid methane stream 42 below the level in the methane stream 40, the heat exchanger 14 is split into two separate heat exchangers 14 and 14', as illustrated in FIG. 2. The first heat exchanger 14 cools the methane stream 40 from -100.degree. F to -115.degree. F, where an ethane rich liquid stream 27 can be separated, and the methane stream 40 is then cooled to -140.degree. F in the second heat exchanger 14'. This ethane rich liquid stream 27 also can be combined with the ethane stream 58 prior to the latter entering the heat exchanger 15, to help provide the lowest temperature cooling.

If it is desired to recover helium from the methane stream 40, a high pressure separator (not shown) is provided between the outlet of the heat exchanger 17 and the throttling valve 41. The gas separated in this heat exchanger will essentially contain all of the helium present in the natural gas stream 36, which can be a most valuable byproduct obtainable only with this type of cycle.

If the natural gas stream 36 is rich in heavy hydrocarbons, such as ethane, propane, butanes and the like, a net product stream of these valuable liquids can be withdrawn from the individual refrigerant component stream 58--61 as the inventory of each component exceeds the need of the liquefaction system.

Similarly, variations in the composition of the natural gas stream 36 can vary the required amount of each of the individual refrigerant components in the refrigeration cycle, and these variations can be made internally within the cycle by the recovery of the component from the natural gas stream.

As indicated above, the boil-off from the storage tank 18 or the like can be returned to the storage tank, after being passed through the liquefaction system. The boil-off stream 28 is joined with the gaseous methane stream 43, and is subsequently joined with the mixed refrigerant stream 50 and is passed through the liquefaction cycle, in the manner described above. Accordingly, the need for additional compressors is eliminated, and the concentration of the stored gas is maintained relatively constant.

In FIG. 3, there is illustrated still another modification which can be made in the above-described basic liquefaction cycle to permit production of subcooled liquid natural gas, or slush natural gas, or even solid natural gas, if it is desired. This modification includes the addition of a heat exchanger 19 and separators. The refrigerant composition is also changed to include nitrogen, helium and/or hydrogen, as well as hydrocarbons. The condensed feed stream 40 from heat exchanger 17 emerges as a stream 80 which is a liquid and gas mixture, and this stream 80, in this case, is coupled to a separator 81. The gas stream 80, in this case, is coupled to a separator 81. The gas stream 82 from the separator 81 contains most of the nitrogen and helium from the feed stream 40, and is further cooled in heat exchanger 19. The liquid stream from the separator 81 is further cooled in the heat exchanger 19, then throttled by means of the throttling valve 84 to storage pressure and coupled into the storage tank 18. The liquid stream 85 can be either subcooled liquid, or a mixture of solid and liquid natural gas known in the industry as slush. Any dissolved gas liberated in the final throttling operation can be used to maintain pressure over the storage liquid, and excess gas 86 is passed back through the heat exchanger 19 where it then joins the mixed refrigerant stream 50.

The cold refrigerant gas stream 82 from the heat exchanger 19, is throttled by means of the throttling valve 88 to a pressure of 15 p.s.i.a., and thereafter subcooled liquid methane 89, from the storage tank 18 is added to it. The vaporization of this subcooled liquid methane 89 into the nitrogen-helium, or nitrogen-hydrogen gas stream 82 provides the refrigeration in the heat exchanger 19. The remainder of the above-described liquefaction cycle is a previously described.

The nitrogen and helium, or nitrogen and hydrogen, will recycle in the feed stream 40, and again be separated in the separator 81. The selection of the separation temperature depends on the particular mixture being used and processed, and relies for its success on a phenomenon known as reverse solubility which is exhibited by hydrogen and helium at low temperatures. As the methane feed stream 40 approaches its solid point, the amount of helium or hydrogen which can be dissolved becomes less, instead of greater as is normal behavior, and it is possible to obtain almost complete separations, especially at temperatures below -250.degree. F.

The cycle illustrated in FIG. 3 also can be modified by adapting the throttling valve 84 to throttle the stream to a selected lower pressure, say 300 p.s.i.a., and by adding a bleed line 100 and another throttling valve 101. The bleed line 100 is coupled to the outlet of the throttling valve 84, before the throttling valve 101, to permit nitrogen, hydrogen, or helium and methane gases at the lower pressure to be bled off for other uses, if desired. The throttling valve 101 then throttles the stream to storage pressure, prior to its being coupled into the storage tank 18.

Referring now to FIG. 4, there is illustrated the apparatus used to liquify natural gas in accordance with another, and preferred embodiment of the invention. The system disclosed here requires substantially the same refrigeration horsepower as the system described above, however, its simplified and reduced fractionation equipment requirement provides substantial economic savings.

In this system, the natural gas stream 125 preferably is a substantially pure methane stream and is delivered to the system at a high pressure and at a temperature achievable by cooling water, such as, for example, 600 p.s.i.a. and 70.degree. F. This natural gas stream is progressively cooled in the heat exchangers 130--135, in a fashion such that it is at approximately 600 p.s.i.a and -250.degree. F, when it emerges from the heat exchanger 135, in a manner fully described below. It can then be delivered under pressure into a storage tank 136, however, preferably it is expanded through the throttling valve 137 to a storage pressure, usually near atmospheric and again, in the illustrated example, assumed to be 15 p.s.i.a., and also separated into a liquid stream 138 which is delivered into the storage tank 136 or the like and a gaseous stream 139, both of which are at approximately -260.degree. F. This gaseous stream 139 is joined with an exhaust stream 140 which is coupled to and used as fuel to drive the turbines (not shown) for the multistage compressor 141.

The fractionation equipment in this system is substantially simplified, in comparison to the stripper 20 used in the system disclosed in FIG. 1, as will be apparent from the description below. The mixed refrigerant stream 160, described more fully below, is delivered from the multistage compressor 141 to the still 142, at a pressure of 600 p.s.i.a. At the top of the still 142, a vapor stream 161 consisting primarily of methane and ethane is delivered, at a pressure of 600 p.s.i.a. and at a temperature of approximately 20.degree. F, to the heat exchanger 131. Upon emerging from the heat exchanger 131, the stream 161 is at approximately 6.degree. F, and a stream 162 of methane and ethane, at a ratio of approximately 3:1, is stripped off and pumped by means of a pump 163 back to the top of the still 142, to cool the top thereof. The remaining portion of the stream 161 which now consists of equal proportions of methane and ethane passes through each of the heat exchangers 132--135 and, when it emerges from the heat exchanger 135, it is still at substantially the same pressure of 600 p.s.i.a., however, it is cooled to a temperature of -250.degree. F. The stream 161 then is expanded through the throttling valve 164 to 15 p.s.i.a. and -260.degree. F, and merged with the refrigerant stream 160 which is passed through the heat exchangers 135--130 to the multistage compressor 141. The stream 161 is a gaseous stream consisting of substantially equal amounts of methane and ethane, and functions, as the above-described gaseous methane stream 43, as the carrier for the individual mixed refrigerant stream 160, also described below. This gaseous stream also provides a gaseous stream which will result in efficient operation of the heat exchangers of the system.

The bottom product of the still 142 is primarily ethane and it is passed as a stream 166 through a heater 147. Upon emerging from the heater 147, a portion of the stream 166 is refluxed via the stream 167 back into the still 142. The remainder portion of the stream 166 is principally ethane and butane, and this gaseous mixture, also at a pressure of approximately 600 p.s.i.a., is coupled as stream 168 and is throttled to a pressure less than 500 p.s.i.a. before entering still 143.

The vapors leaving the top of the still 143 are coupled in a stream 169 to a cooler 150 wherein a portion of the stream is condensed and refluxed back into the still 143, as stream 170. The remaining portion of the stream is primarily ethane, and this is coupled as stream 171 to and through the heat exchangers 130--134 wherein it is cooled so that upon emerging from the heat exchanger 134 it is at approximately -140.degree. F. This cooled now liquid stream is expanded through throttling valve 189 to 15 p.s.i.a., and joined with the mixed refrigerant stream 160. The heat of vaporization of the cooled liquid stream 171 is absorbed as it evaporates into the mixed refrigerant stream 160, as the latter is warmed from -140.degree. F to -80.degree. F in the heat exchanger 134, and provides the refrigeration in the heat exchanger 134.

The product at the bottom of the still 143 consists primarily of pentane and butane, and is coupled as stream 175 to and through a heater 148. A portion of the stream 175 is recycled to the still 143, via stream 176, and the remaining portion thereof is expanded through throttling valve 178 to 70 p.s.i.a. and 155.degree. F. The liquid portion of this expanded stream is separated and coupled as stream 179 through a cooler 151 which cools it to approximately 80.degree. F. From the cooler 151, it is coupled to and through the heat exchanger 130. This liquid stream is cooled to 20.degree. F on passing through the heat exchanger 130, and then is expanded through the throttling valve 180 to 15 p.s.i.a. Upon being expanded, the liquid stream is joined with the mixed refrigerant stream 160. The heat of vaporization of the cooled liquid stream is absorbed as it evaporates into the mixed refrigerant stream 160, as the latter is warmed from 20.degree. F to 0.degree. F in the heat exchanger 130, and provides the refrigeration in the heat exchanger 130.

The remaining gaseous portion of the stream 175 is passed successively through the coolers 144--146. In the cooler 144, the stream is cooled to approximately 142.degree. F and, upon emerging therefrom, another liquid stream 181 which consists primarily of butane is separated and passed through a cooler 152 which cools it to approximately 80.degree. F. The liquid stream 181 then is coupled to and through the heat exchangers 130 and 131. Upon emerging from the heat exchanger 131, the liquid stream 181 is at approximately 6.degree. F. This cooled liquid stream 181 then is expanded through throttling valve 182, and joins with the mixed refrigerant stream 160. Again, the heat of vaporization of the cooled stream is absorbed as it evaporates into the mixed refrigerant stream 160, as the latter is warmed from 6.degree. F to 20.degree. F in the heat exchanger 131, and provides the refrigeration in the heat exchanger 131.

Still another liquid stream 184 is separated from the stream 175 after the latter has passed through the cooler 145 and cooled to approximately 120.degree. F. This liquid stream 184 also consists primarily of butane but has some pentane and a small amount of propane and hexane in it. The liquid stream 184 likewise is cooled to approximately 80.degree. F in passing through a cooler 153, before being coupled to and through the heat exchangers 130--132. In passing through these heat exchangers, the liquid stream 184 is cooled to approximately -40.degree. F. Thereafter, it is expanded through the throttling valve 185 to 15 p.s.i.a. and joined with the mixed refrigerant stream 160. In this case also, the heat of vaporization of the cooled liquid stream is absorbed as it evaporates into the mixed refrigerant stream 160, as the latter is warmed from -40.degree. F to 6.degree. F in the heat exchanger 132, and provides the refrigeration for the heat exchanger 132.

The remaining vapor portion of stream 175 upon passing through the cooler 146 is cooled to approximately 80.degree. F, and is coupled to and through the heat exchangers 130--133 as stream 186. This stream 186 also consists primarily of butane but has a fair amount of propane and smaller amounts of ethane, pentane and hexane in it. Upon passing through the heat exchangers 130--133, the stream 186 is cooled to approximately -80.degree. F, and then is expanded through throttling valve 187 to 15 p.s.i.a. and joined with the mixed refrigerant stream 160. As in the case of the streams 171, 179, 181 and 184, the heat of vaporization of this cooled liquid stream is absorbed as it evaporates into the mixed refrigerant stream 160, as the latter is warmed from -80.degree. F to -40.degree. F in the heat exchanger 133, and provides the refrigeration in the heat exchanger 133.

The mixed refrigeration stream 160 upon emerging from the heat exchanger 130 is at a pressure of 15 p.s.i.a. and a temperature of approximately 70.degree. F. The mixed refrigerant stream then is compressed in the multistage compressor 141 including the compressor stages 190--193 to 600 p.s.i.a. and is condensed in water cooled condensers 194 included between each of the compressor stages. The compressed mixed refrigerant stream 160 from the multistage compressor 141 is delivered to the still 142 for reseparation, in the manner described above.

A bypass 196 is provided to strip off liquid forming in the compressor stages 190 and 191, to prevent damage to the compressor stage 192. Similarly, a bypass 197 is provided to strip off liquid forming in the mixed refrigerant stream, to prevent damage to the compressor stage 193. Stripping off the liquid in this manner and pumping it into the discharge of the succeeding stage, the horse power required to compress the mixed refrigerant stream is substantially reduced.

Referring now again to the natural gas stream 125, it can be seen that it is successively cooled from 70.degree. F to 20.degree. F, 6.degree. F, -40.degree. F, -80.degree. F, -140 F and -250.degree. F as it passes through the heat exchangers 130--135, respectively, in heat exchange relationship with the mixed refrigerant stream 160. As this natural gas stream is cooled, some of the heavier components such as hexane, pentane and butane are stripped off of it, as indicated by the streams 198--200, so that substantially pure methane is delivered into the storage tank 136. As indicated above, upon emerging from the heat exchanger 135, the stream 125 preferably is expanded through throttling valve 137, to approximately atmospheric pressure for storage. Boil-off from the storage tank 136 can be joined with the gaseous methane stream 139 which is subsequently joined with the exhaust stream 140, if desired.

The mixed refrigerant stream 160 consists of a mixture of methane, ethane, propane, butane, pentane and hexane, however, these components rather than being joined with it as individual substantially pure component streams as in the case of the system of FIG. 1, are joined with it in the form of mixed component streams 171, 179, 181, 184 and 186. By using mixed component streams rather than pure component streams, and coupling them into the mixed refrigerant stream where each stream can be operated with the greatest efficiency to achieve the desired cooling, substantial economic savings are realized in view of the simplified fractionation equipment which is required in the system.

Helium also can be recovered as a byproduct, with the system of FIG. 3, in the manner shown in FIG. 4. It can be seen that in this system, the helium can be recovered simply by adding an additional throttling valve 202 in series with the throttling valve 137 and coupling a bleedoff line 203 to the stream 125, between the two throttling valves 137 and 202, as illustrated in FIG. 5.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and certain changes may be made in carrying out the above method. Accordingly, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Now that the invention has been described, what is claimed as new and desired to be secured by Letters Patent is:

Claims

I claim:

1. The method of liquifying a gas comprising the steps of: supplying the gas in a process stream at an elevated pressure, supplying a mixed refrigerant component stream at an elevated pressure, separating the mixed refrigerant component stream to provide a plurality of individual liquid refrigerant component streams by fractional distillation using hydrocarbons as an overall cycle reflux with the removal of heat in a reflux condenser of a column and the supply of heat in a reboiler of the column, cooling each of said plurality of individual liquid refrigerant component streams, evaporating each of said plurality of individual liquid refrigerant component streams into a single stream to provide a refrigerant stream which is at approximately the same temperature as the process stream when the latter is liquified and which is in heat exchange relationship with the process stream to refrigerate the gas in the process stream to a liquified state, compressing and cooling said refrigerant stream after said liquid refrigerant component streams have been evaporated into it to provide said mixed refrigerant component stream.

2. The method of claim 1, wherein the mixed refrigerant component stream is separated to provide a plurality of individual essentially pure single component liquid refrigerant component streams.

3. The method of claim 1, wherein the mixed refrigerant component stream is separated to provide a plurality of individual controlled composition liquid mixture refrigerant component streams.

4. The method of claim 1, wherein the cooling of each of said plurality of individual liquid refrigerant component streams and the refrigeration of the gas in the process stream to liquify the latter both are provided by evaporating said plurality of individual liquid refrigerant component streams into said refrigerant stream.

5. The method of claim 2, further including the steps of mixing together said gas in said process stream and said mixed refrigerant gas stream, and thereafter separating said mixture to provide said plurality of individual essentially pure single component liquid refrigerant component streams and a feed stream which is thereafter liquified.

6. The method of claim 2, further including the steps of passing said gas in said process stream through a plurality of heat exchangers to refrigerate said gas in said process stream to a liquified state, expanding the liquified gas in the process stream to a lower pressure to provide a liquified gas stream for storage or transportation and a cold gaseous stream resulting from a portion of the liquified gas in the process stream being flashed off upon expansion, said cold gaseous stream providing said single stream forming said refrigerant stream, passing each of said individual essentially pure single component liquid refrigerant component streams through predetermined ones of the same plurality of heat exchangers through which said gas in said process stream is passed to cool said individual essentially pure single component liquid refrigerant component streams, expanding each of said individual essentially pure single component liquid refrigerant component streams to a lower pressure and evaporating them into said refrigerant stream in heat exchange relationship with the process stream to provide the refrigeration for both liquifying said gas in said process stream and for cooling said individual essentially pure single component liquid refrigerant component streams.

7. The method of claim 1, further including the steps of including nitrogen and helium in the composition of the mixed refrigerant component stream, expanding the gas in the process stream at a predetermined intermediate point in the refrigeration cycle to a reduced pressure, separating nitrogen and helium from said gas in said process stream after reducing the pressure of said process stream, cooling the gas in the process stream to a lower temperature, expanding the further cooled gas in the process stream to a reduced pressure and passing it into a storage tank, cooling the nitrogen and helium separated from said gas in said process stream, expanding this further cooled nitrogen and helium to a reduced pressure, joining subcooled liquid gas from said storage tank with said expanded nitrogen and helium, evaporating the resulting mixture with said refrigeration stream in heat exchange relationship with both the nitrogen and helium separated from the gas in the process stream and the latter after said nitrogen and helium has been separated from it.

8. The method of claim 3, further including the steps of separating by fractional distillation said mixed refrigerant stream to provide a first and a second controlled composition liquid mixture stream at a high pressure, cooling said first controlled composition liquid mixture stream and thereafter expanding it to a lower pressure to provide a cold gaseous stream, said cold gaseous stream providing said single stream forming said refrigerant stream, said second controlled composition liquid mixture stream being further separated to provide a plurality of individual controlled component liquid mixture refrigerant component streams, each of said individual controlled component liquid mixture refrigerant component streams being cooled and expanded to a lower pressure and evaporated into said refrigerant stream in heat exchange relationship with the process stream to provide the refrigeration for both liquifying said gas in said process stream and for cooling said individual controlled composition liquid mixture refrigerant component streams.

9. The method of claim 1, further including the steps of passing said gas in said process stream through a plurality of heat exchangers to refrigerate said gas in said process stream to a liquified state, expanding the liquified gas in the process stream to a lower pressure to provide a component gas stream for storage or transportation, providing a controlled composition liquid mixture stream at a high pressure, expanding said controlled composition liquid mixture stream to a lower pressure to provide a cold gaseous stream, said cold gaseous stream providing said single stream forming said refrigerant stream, passing each of said individual controlled composition liquid mixture refrigerant component stream through predetermined ones of the same plurality of heat exchangers through which said gas in said process stream is passed to cool said individual controlled composition mixture liquid refrigerant component streams, expanding each of said individual controlled composition liquid mixture refrigerant component streams to a lower pressure and evaporating them into said refrigerant stream in heat exchange relationship with the process stream to provide the refrigeration for both liquifying said gas in said process stream and for cooling said individual controlled composition liquid mixture refrigerant component streams.

10. The method of claim 1 wherein the liquid portion of the compressed and cooled refrigerant stream is separated from the gaseous portion thereof between the compressor stages, and is pumped into the discharge of the following stage of the compressor causing further condensation of the interstage gas portion.