Cryogenic Liquefaction

This invention relates to a process for the liquefaction of air, or of primary air components thereof, by heat exchange with a cycle medium containing nitrogen and hydrocarbons, wherein the cycle medium is, in several stages, cooled and partially condensed, and the liquid portions obtained during the partial condensation are expanded and rewarmed countercurrently to mixture to be cooled, vaporized, and recompressed. By "primary air components" is meant oxygen or nitrogen.

It is conventional in connection with the liquefaction of air or nitrogen to produce the required refrigeration by processes operating in accordance with the Linde or Claude principles. In these processes, especially in the Linde method, large temperature differences are encountered between the cycle medium to be warmed and the cycle medium to be cooled, thereby resulting in exergonic losses. Consequently, these processes have the disadvantage of a large specific energy consumption. To avoid this disadvantage, it is known (Leiden Communications, Supplement No. 76a) to utilize single-component cascaded cycles for refrigeration production. Such a process, while resulting in a low specific energy consumption, requires a separate compressor and ancillary apparatus for each cycle of the cascade, thereby requiring an extremely high capital outlay for such a plant.

An object of this invention is to provide a modified process of the type described above for air, or primary air components, and paricularly nitrogen liquefaction, and especially a process distinguished by low operating and investment costs.

Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.

To attain these objects, the process provides the steps of engine-expanding the gaseous portion present after the last partial condensation and warming resultant expanded gas in heat exchange with liquid product.

By including these steps, a process is provided wherein the cooling curves are closer fitting to the warming curves in the enthalpy-temperature diagram. Thereby, the temperature differences during heat exchange and the concomitant exergonic losses are substantially diminished, resulting in a process having a low specific energy consumption. In addition, because a mixture cycle is utilized for the production of cold -- the expenditure in apparatus is substantially lower than in case of a process operating with cascaded single-component cycles.

Another aspect of this invention contributing towards an even further reduction in the temperature differences occurring during heat exchange is, in accordance with a particularly advantageous embodiment of the invention the step of subdividing the liquid obtained during the partial condensation of the first stage into two partial streams. The first of these streams is reheated in indirect heat exchange with the fluid stream flowing toward the phase separator of the first stage, and the second partial stream is admixed to the liquid obtained during the partial condensation of the second stage and is warmed and vaporized together therewith.

The cycle medium ordinarily utilized is preferably a mixture of nitrogen, natural gas, ethylene, and propane. In this connection, according to a particularly advantageous aspect of this invention, the nitrogen content in the cycle medium predominates and preferably comprises at least 40% of nitrogen. Thereby the temperature differences are further reduced during the heat exchange between the cycle medium to be cooled and the cycle medium to be warmed, and especially in the cold portion of the heat exchangers.

If flammability of the cycle medium is a very important consideration because of plant location or insurance costs, or the like, it is advantageous to utilize in the cycle medium a mixture of non-inflammable halogenated hydrocarbons instead of unsubstituted hydrocarbons. Preferred compounds for this mixture are: CCl.sub.2 F.sub.2, CHClF.sub.2, and CClF.sub.3. Generally, a cycle medium based on a mixture of nitrogen and halogenated hydrocarbons would have the following composition on a molar basis:

For the purpose of facilitating an easy understanding of the prevention, the attached drawing is a schematic representation of the preferred embodiment of the aspect of the invention relating to the liquefaction of nitrogen.

Referring to the drawing, the nitrogen liquefaction plant comprises essentially a nitrogen compressor 1, a cycle compressor 2, heat exchangers 3-8, phase separators 9 and 10, and expansion turbine 11.

10,000 Nm.sup.3 /h. of nitrogen at ambient temperature is compressed in compressor 1 to 10 bar. After removal of the heat of compression by means of the heat exchanger 3, the nitrogen is cooled in heat exchanger 4 to its liquefaction temperature and then liquefied in heat exchanger 5. To subcool the liquid nitrogen, additional heat is withdrawn from the nitrogen by means of heat exchanger 6; this heat is transferred to the turbine discharge stream from the turbine 11. The liquid nitrogen is then expanded, if necessary, in the throttle valve 18 to the desired pressure.

A mixture cycle is provided to produce the refrigeration required for the cooling and liquefaction procedure: The cycle mixture, which has the following composition on a volumetric basis:

is compressed in compressor 2 from about 2 bar to 30 bar and cooled in heat exchanger 19 to about 77.degree. to 27.degree. C. After the cycle mixture has been further cooled in heat exchanger 7 to about 27.degree. to -108.degree. C., its heavy components (propane, ethylene), as well as a minor portion, e.g., about 5 to 10%, of the methane, are condensed in the liquid phase. The liquefied proportion is separated in separator 9 and subdivided into two partial streams. The first of the two partial streams (about 50 to 70, preferably about 60% of the total liquefied portion is expanded in a throttle valve 12 to the intake pressure of the compressor 2 and vaporized in heat exchanger 7 indirectly against the cycle mixture flowing toward the phase separator 9 and then recycled to the intake side of the compressor 2. The stream withdrawn in the gaseous phase from the separator 9 is partially liquefied, about 40 to 60%, in heat exchanger 8 and separated in phase separator 10. The resultant separated liquid, predominantly methane, is split into two streams, about 20 to 40% being expanded in throttle valve 13 and the remainder is expanded in throttle valve 14. The expanded liquid from throttle valve 13 is then vaporized in heat exchanger 5 in indirect heat exchange against condensing nitrogen; the resultant vapor is then heated in heat exchanger 4 and then passed to the intake side of compressor 2. In a somewhat similar manner, the expanded liquid from throttle valve 14 is vaporized in heat exchanger 8 in indirect heat exchange against condensing methane; the resultant vapor is heated in heat exchanger 7, and then also passed to the intake side of compressor 2.

The aforementioned second partial stream of the liquid obtained from phase separator 9 is expanded in throttle valve 15 and admixed to the liquid stream from throttle valve 14 to be vaporized in heat exchanger 8.

The stream withdrawn in the gaseous phase from the phase separator 10, predominantly nitrogen with minor amounts of methane, is engine-expanded in turbine 11. The thus-produced very cold, engine-expanded gas is warmed in heat exchanger 6 against liquid nitrogen thereby subcooling the latter. The resultant warmed gas is then intermixed with the liquid stream expanded in valve 13, both streams being then warmed together by indirect heat exchange contact with nitrogen and thus vaporized, and thereafter recycled to the intake side of the compressor 2. The mixing of the warmed gas and the liquid stream has the advantage, that the latent heat of the liquid is used. In practice, the liquid is distributed to the cross-sections of heat-exchanger 5 and thereby it is intermixed with the warmed gas.

To equalize the pressures between the stream warmed in heat exchanger 8 and the stream warmed in heat exchanger 5, a pressure compensation conduit 16 with a check valve 17 is provided. If one uses a cycle mixture, which has the composition described on page 3: N.sub.2, CHClF.sub.2 and CClF.sub.3, the same process steps can be performed. In phase separator 9 the liquified proportion consists of CHClF.sub.2 and in phase separator 10 the liquified proportion consists of CClF.sub.3.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modification of the invention to adapt it to various usages and conditions .