Process for a more efficient liquefaction of a low-boiling gaseous mixture by closely matching the refrigerant warming curve to the gaseous mixture cooling curve
United States Patent 11 1 [111 3,874,185  
 Etzbach 5] Apr. 1, 1975 PROCESS FOR A MORE EFFICIENT 3,271,965 9/1966 Maher 62/40 LIQUEFACTION OF A LOW-BOILING 3,364,685 g f GASEOUS MIXTURE BY CLOSELY L058 3,541,802 111970 s MATCHING THE REFRIGERANTWARMING 3,616,652 [197] CURVE TO THE GASEOUS MIXTURE 3,657,898 4/1972 Ness COOLING CURVE 3,677,019 7/1972 Olszewski 62/9 [75] Inventor: Volker Etzbach, Munich, Germany FOREIGN PATENTS OR APPLICATIONS 1 1 Assigneei Linde Aktiengesellschaft, 930,682 7/1963 UnitedKingdom 62/11 Wiesbaden, Germany [22] Filed: 1973 Primary E.raminerA. Louis Monacell [21] Appl. No; 393,331 Assistant Examiner-Frank Sever Attorney, Agent, or Firm-Millen, Raptes &amp; White [30] Foreign Application Priority Data Dec. 18, 1972 Germany 2261886 Apr. 18, 1973 Germany 2319803 [57] ABSTRACT Gases volatilizing from liquefied nitrogen-containing [52] U.S. Cl 62/40, 62/54, 62/38, natural gas are condensed efficiently employing a 62/ 9 closed nitrogen refrigeration cycle by first cooling the [51] hit. CI F25] 1/02 natural g with nitrogen which has been at least p [58] Fleld of Search 62/9 39 tially condensed by isenthalpic expansion and further cooling the natural gas with gaseous nitrogen which [56] References C&#39;ted has been isentropically expanded.  
 UNITED STATES PATENTS 3,180,709 4/1965 Yendall 62/9 9 Claims, 3 Drawing Figures PMENTEQ APR 1 191 5 PROCESS FOR A MORE EFFICIENT LIQUEFACTION OF A LOW-BOILING GASEOUS MIXTURE BY CLOSELY MATCHING THE REFRIGERANT WARMING CURVE TO THE GASEOUS MIXTURE COOLING CURVE BACKGROUND OF THE INVENTION This invention relates to a process for the liquefaction of low-boiling gaseous mixtures with the aid of a closed refrigeration cycle wherein a cycle gas is compressed, cooled, expanded, and reheated in indirect heat exchange with the gaseous mixture to be liquefied and the cycle medium.  
  A process is known for the recondensation of nitrogen-containing natural gas vaporized from a storage tank by external heat effect, wherein the required refrigeration is produced by compression, cooling, and isentropic expansion of a refrigerant into the liquidvapor region, in a closed cycle. This simple process has the disadvantage that the isentropically expanded cycle fraction transmits its peak cold essentially at a constant temperature, but the natural gas to be liquefied, due to the nitrogen enrichment, can absorb the peak cold only at a falling temperature. Thus, the refrigeration is available in the conventional process at a lower temperature level than is required for actual cooling purposes, so that necessarily larger temperature differences occur in the heat exchangers, thereby increasing the energy requirements of the system.  
  The invention is directed to the problem of developing a simple and energy-conserving process for the liquefaction of low-boiling gaseous mixtures.  
 SUMMARY OF THE INVENTION According to this invention, the abovedescribed problem is solved by a process wherein the peak cold required for the liquefaction of the gaseous mixture is provided by warming an isentropically expanded partial stream of the cycle gas and the residual cold is provided by vaporization of the residual stream of the cycle gas which has been at least partially condensed by isenthalpic expansion.  
 DETAILED DISCUSSION The cooling curves of low-boiling gaseous mixtures, especially gaseous mixtures whose individual components have widely spaced-apart boiling points, such as, for example, nitrogen-containing natural gas, are relatively flat, with a small dQ/dT in the low-temperature range, i.e., in the range of the peak cold, i.e., the lowest portion of its liquefaction zone, when the proportion of the lower-boiling component is relatively small compared to the other components, whereas in the zone of higher temperatures, the dQ/dT first increases markedly dut to vaporization of the main component, and then decreases again due to gas warming. The closer the warming curve of the cycle gas can be adapted to the curve path of such gaseous mixtures, the lower is the energy requirements to achieve liquefaction. This adaptation is maximally provided by the process of the present invention by employing, for producing the peak cold i.e., the lowest temperatures of the gaseous mixture, an isentropically expanded first fraction of the cycle gas which, after the expansion, is entirely in the gaseous phase, i.e., which is not expanded into the liquid-vapor region. The warming-up curve of such a gaseous fraction has only a minor dQ/dT in the low temperature zones at which the heat exchange with the gaseous mixture takes place. so that a good adaptation of the courses of the cooling curve of the gaseous mixture and the warming curve of the isentropically expanded refrigerant, i.e., cycle gas, is achieved. In the higher temperature area of heat exchange, where the cooling curve of the gaseous mixture has a relatively high dQ/dT, the heat exchange is effected with an isenthalpically expanded residual second fraction of the cycle gas, the pressure of which can be higher than the final pressure resulting from the isentropic expansion. This residual fraction, which is largely in the liquid phase after its expansion, but prior to heat exchange, possesses a warming curve, due to the varporization dQ/dT, so that good adaptation is possible to the likewise large dQ/dT of the cooling curve of the gaseous mixture in this temperature range.  
  In total, a maximum adaptation to the cooling curve of a gaseous mixture to be liquefied is thus obtained by the combination according to this invention of cold production by both isentropic and isenthalpic expansion of the cycle medium. The temperature differences at which the heat exchange takes place are small, and consequently, the amound of energy which must be expended is likewise low.  
  Since the equipment for performing the process of this invention is constructionally particularly simple, the process is especially well suitable for the reliquefaction of evaporated nitrogen-containing natural gas in tanker ships.  
  When the amount of the gaseous mixture to be liquefied is not constant per unit time but instead is subject to chronological fluctuations, the cold production by the process can be simply and readily adapted to the resulting fluctuating cold requirements in accordance with a preferred embodiment of this invention, by subjecting the residual second fraction to a bar phase separation after its isenthalpic expansion but prior to heat exchange. In this process, during periods of lowest refrigeration requirements, a portion of the liquid fraction obtained by this phase separation is stored instead of being conveyed to the heat exchange zone and, during periods of highest refrigeration requirements, at least a portion of the thus-stored liquid fraction of the cycle gas is again fed into the cycle to the heat exchange zone.  
  As soon as the refrigeration requirements of the process drops, i.e., the amount of the gaseous mixture fed to the liquefaction stage per unit of time is reduced and a part of the liquid fraction obtained during the phase separation is then stored. By this technique, the amount of circulated cycle gas is decreased and, consequently, the cycle compressor achieves a correspondingly lower compression of the cycle gas, so that the total refrigeration output of the cycle is reduced. For storage purposes, if the phase separation takes place in a separator, the latter itself and/or a separate storage tank can be utilized.  
  As soon as the refrigeration requirements of the process increases again, i.e., the amount of gaseous mixture to be liquefied which is fed to the cycle per unit time is again increased, then at least a portion of the liquid cycle gas stored, for example, in the separator proper and/or in a separate storage tank, is again fed to the refrigeration cycle.  
  This measure provides two essential advantages. First, it is possible to make immediately available to the cycle refrigeration in the form of liquid cycle gas and, on the other hand, the feeding of the liquid cycle gas into the cycle causes an increase in the total circulated amount of cycle gas, which enables the cycle compressor to achieve higher compression of the cycle gas, so that the cycle itself now additionally produces a larger amount of refrigeration.  
  The use of this simple cold production control of the cycle of dependence on the available amounts of the gas to be liquefied, e.g., natural gas, is especially advantageous in the re-liquefaction of vaporized, nitrogencontaining natural gas while being conveyed in a tanker ship. The evaporation losses of natural gas, i.e., the amounts of natural gas available for re-liquefaction, vary constantly, especially when transporting liquid natural gas over large distances, since the liquid lowboiling storage material is exposed to constantly changing surroundings, such as, for example, a continuously varying atmospheric pressure, constantly varying ambient temperatures, as well as constant movements of the ship. In other words, the stored liquified gas is subjected to influences which contribute very importantly toward a constantly changing rate of vaporization.  
  In accordance with a further feature of the invention, the cold gaseous fraction ofthe cycle gas obtained during the phase separation is again amixed with the portion of the condensed fraction utilized for the heat exchange with the gaseous mixture to be liquefied, directly after the complete evaporation of this condensed fraction, so that the refrigeration capability of the gaseous fraction produced during the phase separation can also be made available to the system.  
  The energy liberated during the isentropic expansion of the partial fraction of the cyclegas can conventionally be utilized for the compression of the cycle gas.  
 An additional understanding of the invention can be derived from the embodiments schematically illustrated in the drawings, in which:  
  FIG. 1 is a schematic flow sheet of an embodiment of the process of this invention;  
  FIG. 2 is a graphic representation of the cooling and warming curves, respectively, based on the embodiment of FIG. 1; and  
  FIG. 3 is a schematic illustration of another embodiment of the process of this invention.  
  In accordance with the embodiment shown in FIG. 1, natural gas to be liquefied is withdrawn via conduit 1 from a storage tank 2 by a blower l5, liquefied in heat exchange with cycle gas in heat exchangers 3 and 4, and recycled into storage tank 2. The portion of the natural gas which cannot be liquefied or can be liquefied only with great difficulty, i.e., at very low temperatures, is separated in separator 16 and either combusted in a steam boiler or exhausted into the atmosphere.  
  The refrigeration required for liquefying the natural gas is produced by a closed nitrogen cycle wherein nitrogen is compressed in compressors 5 and 6 and in a high-pressure compression blower 7 to the required cycle pressure of about 45 atmospheres absolute. Water coolers 8, 9, and 10, respectively, are connected after each compression stage.  
  In a first heat exchanger 11, the compressed nitrogen is cooled in heat exchange with colder nitrogen to about 166 K and then divided into two first and second fractions. The first fraction is expanded isentropically in the turbine 12 to about 4.5 atm. abs. and fed at the resultant temperature of about 93 K to the heat exchanger 4, wherein this fraction is warmed to about 105 K by heat exchange with the natural gas while giving up the peak cold required for the liquefaction of the natural gas. The fraction then flows at this temperature to a heat exchanger 13, where it is warmed by heat exchange with the second residual fraction, to about 146 K. The second residual fraction is partially liquefied in this step. The first fraction then flows to heat exchanger 1 1 where it provides the colder nitrogen and, after further warming in heat exchanger 11, the first fraction is fed to the inlet of the low-pressure compressor 5 and, after passing through heat exchanger 8, is introduced into the inlet of intermediate-pressure compressor 6.  
  The second residual fraction is partially liquefied in heat exchanger 13 and is isenthalpically expanded in a throttle valve 14 to about 10 atm. abs. and is then fed to heat exchanger 3 at a temperature of about 103 K, wherein it is vaporized by heat exchange with the natural gas and warmed to about 146 K. This second residual fraction then flows through heat exchanger 11 with further warming and is then introduced into the inlet of intermediate-pressure compressor 6, where it is further compressed as a mixture with the first fraction.  
  As a result of the fact that the cooling of the natural gas in heat exchanger 4 being conducted with the first fraction of cycle gas, which has not yet been expanded isentropically into the liquid-vapor region, and the cooling in heat exchanger 3 being effected with the partially liquid second residual fraction of cycle gas, a good adaptation of the warming and cooling curves of the cycle gas to the natural gas can be attained in these heat exchangers, so that the heat exchange can be accomplished with small temperature differences and thus in an energy-conserving operation.  
  FIG. 2 shows the course of the cooling and warming curves in heat exchangers 3 and 4, wherein the heat exchange zone of heat exchanger 3 is indicated by A and the zone heat exchange of heat exchanger 4 is denoted by B.  
  FIG. 3 shows another embodiment of the process of this invention which makes it possible to adapt the cold production of the cycle to a gaseous mixture to be liquefied which is available in varying amounts per unit time.  
  As shown in FIG. 3, the gas which evaporates from liquid, nitrogen-containing natural gas in storage tank 20 is withdrawn via conduit 21 and compressed to about 1.3 atm. abs. by a blower 22, at least partially liquefied in heat exchanger 24 in heat exchange with a isenthalpically and isentropically expanded cycle gas, and then fed to a separator 25, where a partial fraction, enriched in nitrogen and not liquefied during this heat exchange at an adjustable low temperature of, for example, K, is withdrawn via conduit 26, and vented or used, for example as fuel gas, while the residual liquid fraction is recycled to storage tank 20 via conduit 27.  
  The refrigeration required for the liquefaction of the volatilized natural gas is produced in a closed nitrogen cycle, wherein nitrogen is compressed in compressors 28 and 29, as well as in compression blower 30, to the cycle pressure of about 45 atm. abs. required during normal operation. In order to carry away the heats of compression, these three compressors are associated with water coolers 31, 32, and 33, respectively.  
  In a heat exchanger 34, the compressed nitrogen is further cooled in heat exchange with itself to about 167 K and then divided into first and second streams.  
  The first stream is isentropically expanded to about 4.5 atm. abs. by turbine 35 and then fed at the temperature of about 93 K resulting from the expansion to the heat exchanger 24, wherein it is warmed by heat exchange with the natural gas to about 103 K while providing the peak cold, i.e., lowest heat exchange temperature, required for the liquefaction of the gaseous natural gas.  
  From the central zone of the heat exchanger 24, the now partially warmed first stream of nitrogen is withdrawn, further warmed by heat exchange with the second stream of nitrogen in heat exchanger 36, and finally fed to the inlet of low-pressure compressor 28 where, after being cooled in heat exchanger 31, is fed to the inlet of intermediate pressure compressor 29.  
  The second stream of nitrogen, after being cooled in heat exchanger 36 by heat exchange with the first stream of nitrogen, is expanded in throttle valve 37 to about atm. abs. and thereafter subjected to a phase separation in separator 38.  
  The liquid portion obtained by the phase separation of the second stream is completely withdrawn via conduit 39, in normal operation, and fed to the upper section ofthe heat exchanger 24 at a temperature of about 103 K, wherein it is first completely vaporized by heat exchange with the natural gas and then combined with the gaseous portion of the second stream obtained in separator 38, and withdrawn therefrom via conduit 40. The resulting mixture is warmed to ambient temperature in heat exchanger 34 and finally fed to the inlet of the intermediate-pressure compressor 29, where it is further compressed as a mixture with the first stream.  
  When the amount of volatilized natural gas becomes smaller, thereby reducing the refrigerating requirements to liquify it, a portion of the liquid portion of the second stream obtained in separator 38 is stored therein by partially or completely closing valve 41, thereby decreasing the amount of nitrogen circulated in the closed cycle. As a result of the reduced amount of nitrogen in the closed cycle, the pressure produced by the cycle compressors is reduced thereby reducing the total refrigeration output of the cycle and, accordingly, is thus adapted to the reduced refrigeration requirements.  
  Conversely, when the refrigeration requirements increases, i.e., the volume of natural gas to be liquefied increases, liquid nitrogen stored in separator 38 is fed into the cycle by opening wider valve 41. In so doing, additional refrigeration in the form ofliquid nitrogen is made immediately available to the refrigeration cycle, and, as a result of the increased amount of cycle gas being circulated, a higher compression ofthe cycle gas is achieved in the cycle compressors, thereby increasing the total refrigeration capacity of the system.  
  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 modifications of the invention to adapt it to various usages and conditions.  
 What is claimed is:  
  1. In a process for the liquefaction of a low-boiling gaseous mixture employing a closed refrigeration cycle wherein a cycle gas is compressed, cooled, expanded, and rewarmed by indirect heat exchange with the gaseous mixture to be liquefied, the improvement which comprises providing the lowest temperature required for the liquefaction of the gaseous mixture by warming in a heat exchange zone an isentropically expanded wholly gaseous first fraction of the cooled cycle gas and providing the remaining cold at a higher temperature level by vaporization in a heat exchange zone of the residual second fraction of the cycle gas which has been at least partially condensed by isenthalpic expansion.  
  2. A process according to claim 1 wherein the gaseous mixture is nitrogen-containing natural gas volatilized from stored liquid natural gas, and the cycle gas is nitrogen.  
  3. A process according to claim 1, wherein the isentropic expansion of the first fraction of the cycle medium is carried out to a lower pressure level than the isenthalpic expansion of the residual second fraction and the first fraction is thereafter combined with the second fraction only after recompression to the pressure of the second fraction.  
  4. A process according to claim 1 wherein the resid ual second fraction, after its isenthalpic expansion, is subjected to a phase separation in a separation zone into a liquid fraction and a gaseous fraction and. during periods of lower refrigeration requirements, a portion of the liquid fraction obtained in the phase separation is stored, and, during periods of higher refrigeration requirements, at least a portion of the stored liquid fraction is recycled into the refrigeration cycle.  
  5. A process according to claim 4, wherein the gaseous mixture is nitrogen-containing natural gas volatil-. ized from stored liquid natural gas, and the cycle gas is nitrogen.  
 6. A process according to claim 4, wherein a portion of the liquid fraction obtained during the phase separation is stored in the separation zone during the periods of lower refrigeration requirement.  
  7. A process according to claim 4, wherein at least a portion of the liquid fraction obtained during the phase separation is stored in a storage zone separate from the separation zone.  
  8. A process according to claim 4, wherein the gaseous fraction obtained during the phase separation is thereafter admixed with the liquid fraction obtained during the phase separation after the latter has completely vaporized by heat exchange with the gas to be liquefied.  
  9. A process according to claim 8, wherein the gaseous mixture is nitrogen-containing natural gas volatilized from stored liquid natural gas, and the cycle gas is