Gas Liquefaction By A Fractionally Condensed Refrigerant

Abstract

A gas stream is liquefied by indirect heat exchange with a multi-component refrigerant in a closed cooling cycle. The refrigerant is fractionally condensed and the fractions are separately evaporated in the closed cycle to cool and liquefy the gas stream.

Description

BACKGROUND OF THE INVENTION

This invention relates to a process for the liquefaction of gases or gas mixtures by means of a cooling cycle in which is circulated a cooling medium or refrigerant composed of several components that are subjected to fractional condensation and in which the condensed fractions are separately evaporated. The process is especially suitable for the liquefaction of natural gas.

From German Auslegeschrift 1,272,943, there is known a process of this type in which cooling media obtained by fractional condensation of components from the natural gas to be liquefied are decompressed to a common medium pressure, mixed, recompressed to the pressure of the natural gas flowing into the plant and again mixed therewith. It is characteristic of that process that the several cooling media are decompressed to a common medium pressure.

From German Offenlegungsschrift 1,501,690, it is also known to carry out the circulation of the cooling medium in a closed cycle in such process.

In comparison with the so-called classical cascade process, these processes have some advantages since only one cooling cycle with one compressor is needed. However, this single cooling cycle must be divided up into several circulation branches, with a corresponding division of the heat exchangers and with complicated connections resulting therefrom. Furthermore, the utilization of energy is higher than in the case of the classical cascade process.

In order to avoid these disadvantages, it has been proposed to divide up a cooling medium composed of several components and obtained by fractional condensation, to decompress one partial stream to a medium pressure and the other partial stream to approximately atmospheric pressure, to combine again the decompressed partial streams, to mix the combined streams with the gas mixture to be liquefied and to compress the total gas. The process is especially suitable for comparatively small natural gas liquefaction plants since it does not require a separate natural gas compressor. In such case, it is necessary that the natural gas is supplied at low pressure. However, these prerequisites are not always fulfilled. Thus, for example, the natural gas is frequently supplied at a pressure which is already sufficiently high. Furthermore, the mixing of light natural gas and heavy hydrocarbons always leads to an increased use of energy because of the increased entropy which arises.

It is an object of this invention to avoid these disadvantages and to make the process of universal applicability.

SUMMARY OF THE INVENTION

A process has been found for the liquefaction of gas or gas mixtures by means of a closed cooling cycle in which a circulating multi-component refrigerant is compressed by a compressor, is separated by fractional condensation into fractions with different boiling ranges and these fractions are evaported as separate streams giving up refrigeration to the refrigerant undergoing fractional condensation and to the gas or gas mixture undergoing liquefaction, are recombined and again compressed. According to this invention, the condensate fractions are separately evaporated and reheated to about ambient temperature and are again mixed together during recompression.

The process of this invention is especially advantageous for the liquefaction of natural gas since only two partial refrigerant streams fromm a single separator suffice for the liquefaction. Since the entropy increase by the mixing of cooling medium and natural gas is avoided, the energy requirement is relatively low. When using only two partial refrigerant streams, the one with the higher boiling fraction is decompressed to a medium absolute pressure, preferably five to 10 ats. (atmospheres), and the one with the lower boiling fraction is decompressed to a low absolute pressure, preferably one to five ats. When the gas mixture to be liquefied is natural gas, the refrigerant or cooling medium is preferably a hydrocarbon mixture.

When the process of this invention is applied to natural gas and nitrogen is to be removed therefrom, the natural gas is preferably subjected, in the course of the cooling thereof, to a preseparation before being further cooled and fractionated in a nitrogen rectification column.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described on the basis of a specific embodiment and with reference to the accompanying drawing which illustrates a process flow diagram for the liquefaction of natural gas containing nitrogen in which the nitrogen is separated and discharged in the gaseous state.

DESCRIPTION OF A PREFERRED EMBODIMENT

The natural gas to be liquefied, which has the following composition:

N.sub.2 14.3% by volume CH.sub.4 82.4% by volume C.sub.z H.sub.6 2.6% by volume C.sub.3 H.sub.8 0.4% by volume C.sub.4 H.sub.10 0.3% by volume

enters the plant through pipe 1 at an absolute pressure of 20 ats. and at a temperature of 30.degree. C. In purification plant 2, it is freed from impurities, especially from water and carbon dioxide, and then passed via pipe 3 successively through heat exchangers 4, 5 and 6. It is thereby cooled to a temperature of about -100.degree. C. and the heavy hydrocarbons condense out. These are separated in separator 7 and withdrawn through pipe 8. The gaseous portion which remains, consisting preponderantly of methane and nitrogen, flows through pipe 9 into heat exchanger 10 where it is cooled to about -120.degree. C. and partially liquefied.

In order to simplify the separation of nitrogen in this stream in nitrogen rectification column 11, the liquid phase is separated from the gaseous phase in separator 12. The gaseous phase is enriched in nitrogen, whereas the liquid phase is depleted in nitrogen. The liquid and gaseous phases pass via pipes 13 and 14, respectively, into heat exchanger 15 where they are cooled to about -140.degree. C. and then used to heat the sump liquid of nitrogen column 11 by means of coils 17 and 16, respectively, whereby they are further cooled. The liquid phase is then decompressed via expansion valve 18 into the middle portion of nitrogen column 11, whereas the gaseous phase is first still further cooled in heat exchanger 19 to about -183.degree. C. and thus liquefied before it is decompressed via expansion valve 20 into the top portion of nitrogen column 11.

In nitrogen column 11 which is at an absolute pressure of four ats., the nitrogen contained in the natural gas is completely separated and withdrawn via pipe 21 from the top of column 11 at a temperature of -183.degree. C. It successively gives up its refrigeration in heat exchangers 19, 15, 10, 6, 5 and 4 and is then available at ambient temperature. It can also previously be decompressed to atmospheric pressure. Liquefied methane is withdrawn via pipe 22 from nitrogen column 11. It is then, in known manner, decompressed into a reservoir which is not illustrated in the drawing; the resulting flash and evaporation gases can also be utilized for recovering refrigeration therefrom.

The refrigeration necessary for cooling and liquefying the natural gas is produced in a closed cooling cycle in which a multicomponent refrigerant is circulated and subjected to fractional condensation. The refrigerant or cooling medium is preponderantly composed of methane, ethane, propane and butane. At about ambient temperature, one fraction of the refrigerant enters compressor 24 at an absolute pressure of two ats. and the other fraction enters compressor 24 at an absolute pressure of seven ats. The fractions thus recombined are compressed to about 25 ats. absolute and cooled in water cooler 25.degree. to about 30.degree. C. The refrigerant is then further cooled in heat exchanger 4 to about 0.degree. C., whereby it is partially liquefied. It passes via pipe 26 into separator 27 in which the liquid phase is separated from the gaseous phase. The two separated phases now provide the two fractions or partial refrigerant streams. The partial refrigerant stream formed from the liquid phase and containing only a little methane and the heavier hydrocarbons, ethane, propane and butane in about equal parts, is first passed by means of pipe 28 through heat exchanger 5 to effect further cooling of the partial stream and then is decompressed by expansion valve 29 to an absolute pressure of about 7.5 ats. This partial stream then evaporates at this pressure while giving up refrigeration in heat exchangers 5 and 4 and is returned to an intermediate stage of compressor 24.

The partial refrigerant stream obtained from the gaseous phase leaving separator 26 consists preponderantly of methane and has a modest content of ethane and propane, whereas the content of butane is small. This second partial stream in pipe 30 is first cooled in heat exchangers 5, 6, 10 and 15 to about -140.degree. C. and thus liquefied and then, according to this invention, is decompressed by expansion valve 31 to an absolute pressure of about 2.5 ats. Thereafter, this second stream evaporates, successively gives up refrigeration in heat exchangers 15, 10, 6, 5 and 4 and finally passes via pipe 23 back into compressor 24.

A special advantage of the process of this invention is that it can be carried out with only a single subdivision of the closed cooling cycle.

Claims

I claim:

1. A process for the liquefaction of gas or a mixture of gases by indirect heat exchange with a compressed multi-component refrigerant circulated in a closed cooling cycle comprising the following steps:

a. cooling, fractionally condensing and separating said compressed multi-component refrigerant into higher boiling liquid and lower boiling gaseous fractions,

b. subcooling all of said higher boiling liquid fractions and further cooling, liquefying and subcooling said lower boiling gaseous fractions,

c. expanding all of said subcooled higher boiling and lower boiling liquid fractions to different pressures, said subcooled higher boiling liquid fractions being expanded to a higher pressure, and said subcooled lower boiling liquid fractions being expanded to a lower pressure,

d. separately evaporating and heating to ambient temperature said subcooled liquid fractions expanded to different pressures by indirect heat exchange with said higher boiling and lower boiling fractions of said multi-component refrigerant and with said gas or gaseous mixture to be liquefied, without mixing said higher boiling and lower boiling fractions during evaporation and heating to ambient temperature,

e. separately recycling said evaporated fractions previously expanded to different pressures to a multi-stage compressor and mixing said evaporated fractions only during the compression.

2. A process for the liquefaction of natural gas by indirect heat exchange with a compressed multi-component refrigerant circulated in a closed cooling cycle comprising the following steps:

a. cooling, fractionally condensing and separating said compressed multi-component refrigerant into only a single higher boiling liquid and a single lower boiling gaseous fraction,

b. subcooling all of said higher boiling liquid fraction and further cooling, liquefying and subcooling said lower boiling gaseous fraction,

c. expanding all of said subcooled higher boiling and lower boiling liquid fractions to different pressures, said subcooled higher boiling liquid fraction being expanded to a higher pressure, and said subcooled lower boiling liquid fraction being expanded to a lower pressure,

d. separately evaporating and heating to ambient temperature said subcooled liquid fractions expanded to different pressures by indirect heat exchange with said higher boiling and lower boiling fractions of said multi-component refrigerant and with said natural gas to be liquefied, without mixing said higher boiling and lower boiling fractions during evaporation and heating to ambient temperature,

e. separately recycling said two evaporated fractions previously expanded to different pressures to a multi-stage compressor and mixing said two evaporated fractions only during the compression.

3. The process of claim 2 wherein the subcooled higher boiling liquid fraction is expanded to an absolute pressure of not less than about 5 atmospheres and the subcooled lower boiling liquid fraction is expanded to a lower absolute pressure of less than about 5 atmospheres.

4. The process of claim 2 wherein the subcooled boiling liquid fraction is expanded to an absolute pressure in the range of about five to 10 atmospheres and the subcooled lower boiling liquid fraction is expanded to a lower absolute pressure in the range of about one to five atmospheres.