Process For The Liquefaction And Subcooling Of Natural Gas

Abstract

A process for the liquefaction and subcooling of natural gas with a Claude closed refrigerating cycle comprising compressing gaseous cycle medium; cooling resultant compressed gaseous cycle medium; dividing cooled compressed gas into two streams; engine-expanding one stream; and cooling the other stream with the engine-expanded stream to such an extent that said other stream becomes partially liquefied after a subsequent throttle expansion thereof; the improvement comprising employing as the cycle medium, a mixture of nitrogen and methane.

Description

BACKGROUND OF THE INVENTION

This invention relates to a process for liquefaction and subcooling of natural gas by a Claude cycle. This cycle comprises compressing cycle gas; cooling same; dividing cooled compressed gas into two streams; engine-expanding one stream; and cooling the other stream with the engine-expanded stream to such an extent that said other stream becomes partially liquefied after a subsequent throttle expansion thereof. By the evaporation of this resultant liquid, peak cold is made available, namely that amount of refrigeration required for subcooling the natural gas. Before subcooling, the natural gas has already been liquefied under a higher pressure by heat exchange with the cycle gas, but the subcooling ensures that the natural gas remains practically entirely in the liquid phase even after expansion to the pressure of the storage tank. If it is desired to avoid operation of the Claude cycle under a negative pressure, it is essential to employ as the cycle medium a gas having a lower boiling point than methane, nitrogen being conventionally employed.

One disadvantage of the above-described process is that the liquid cycle nitrogen is evaporated isothermally, i.e., yields cold at a constant temperature, but the liquid natural gas to be subcooled can absorb this refrigeration only at a decreasing temperature. Consequently, because of this temperature gradient, the refrigeration is transferred at a temperature level lower than that necessary for cooling purposes. Thus, in the peak-cold generator, the occurrence of large heat transfer temperature differences .DELTA.T is unavoidable, increasing the thermodynamic irreversibility and energy requirements of the process.

Another disadvantage is to be seen from the fact that the enthalpy gradient in the expansion turbine, as well as the Joule-Thomson effect in the peak-cold generator are relatively small in case of nitrogen, so that a large amount of gas must bypass the turbine and enter through the throttle valve, and therefore cannot be utilized for the production of cold by engine expansion. For this reason, in addition to the fact that the isothermal Joule-Thomson effect of the nitrogen is also small at the warm end, the specific refrigeration capacity of the cycle per Nm.sup.3 of circulated gas is relatively low.

Finally, another disadvantage is that it is necessary, in order to provide makeup nitrogen due to cycle losses, either to keep pure nitrogen in readiness or to separate same continuously from the natural gas. The latter expedient involves a substantial plant investment, however, in case the natural gas is low in nitrogen.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for the liquefaction of natural gas by the use of a Claude closed refrigerating cycle wherein one or more of the following advantages are obtained: a lower energy requirement, a higher refrigerating capacity per unit quantity of the cycle gas, and easier availability and lower cost of makeup fluid required to effect leakage losses.

Upon further study of the specification and appended claims, other objects and advantages of the present invention will become apparent.

These objects are attained, according to this invention, by using a mixture of nitrogen and methane as the cycle medium.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a boiling point-composition diagram for N.sub.2 --CH.sub.4 at various pressures; and

FIGS. 2 and 3 are schematic views of preferred embodiments of the process.

DETAILED DISCUSSION

One advantage of this process is that the evaporation of the liquid methane-nitrogen mixture does not take place at a constant temperature, but rather at a sliding temperature in accordance with the boiling point diagram of nitrogenmethane at the evaporation pressure under consideration, each evaporation temperature being associated with a specific mixture (compare FIG. 1). Therefore, by a selection of the evaporation pressure and the composition of the cycle gas, the temperature range of the evaporation can be very well adapted to the temperature range of the subcooling. Thus, the temperature differences in the peak cooler are small and the energy losses caused thereby are minor. Since the methane, in mixture with the nitrogen, is vaporized at its partial pressure, i.e., a pressure lower than the ambient evaporation pressure, it is possible to obtain, with methane, a specific low temperature at a relatively high pressure level.

The refrigerating capacity of the Claude cycle is likewise improved by the addition of methane to the nitrogen. For the methane, as a less than ideal gas, increases the Joule-Thomson effect in the peak cooler, so that the amount of cycle gas to be fed to the J-T valve can be smaller, and a larger proportion of the cycle gas can be fed to the engine expansion. Additionally, the enthalpy gradient in the turbine and thus the specific refrigerating capacity of the cycle, based on the unit quantity of circulated gas, is larger in the case of methane than in the case of nitrogen. The same holds true for the isothermal Joule-Thomson effect at the warm end. Therefore, in total, the amount of cycle gas required to produce a specific amount of cold is reduced.

Finally, the use of a methane-nitrogen mixture as the cycle gas affords the further advantage that the leakage losses of the cycle medium can be compensated for at less cost by utilizing natural gas therefor. This is so because the separation of a gas rich in nitrogen prior to the liquefaction is in most cases absolutely necessary, even in case of gases of low nitrogen content, since too great a drop in the liquefaction temperature of the natural gas must be avoided. The above-described advantage is especially noticeable in the processing of natural gases having a low nitrogen content, for otherwise rectification devices would have to be provided in this procedure wherein the nitrogen is separated from the natural gas not only in a high purity, but also in good yields. The process according to the present invention offers advantages even if the natural gas contains no nitrogen at all, since only that proportion of the leakage losses associated with the nitrogen need be made up from a source externally of the plant, whereas the lost methane can readily be taken from the natural gas.

In the determination of the specific quantitative ratio of nitrogen: methane to be employed, the following criteria must be observed: As set forth hereinabove, the addition of methane to the nitrogen results in an improvement of the specific refrigerating capacity of the cycle gas; therefore, as high a methane content as possible would be desirable, all other things being equal. However, it can be seen from FIG. 1 that, at a constant pressure, the boiling point temperature of a mixture in the region of high nitrogen concentrations (down to a concentration of about 30-40 percent of nitrogen equal to 60-70 mol percent CH.sub.4) is elevated to a relatively minor extent by the addition of a specific quantity of methane, e.g., a 10 mol percent, increment in the liquid, but that the addition of the same quantity of methane in a zone of lower nitrogen concentrations (e.g. below 30 mol percent nitrogen equal to above 70% CH.sub.4) causes a substantial elevation in the boiling temperature. These relationships have a significant effect on the suction intake pressure of the compressor as will now be explained with reference to numerical values derived from FIG. 1.

In order to maintain a boiling temperature of 110.degree. K, a pressure of 16 ata. (atmospheres absolute) is required in the case of pure nitrogen (point A), and in the case of a mixture containing 55 percent of methane, a boiling pressure of 8 ata. is necessary (point B). Thus, the addition of 55 percent of methane only effects a lowering of the boiling pressure to one-half the value. In contrast thereto, in the region of high methane concentrations, the boiling point pressure is lowered by the factor of one-half already with a substantially lower quantity of added methane. For example, if the methane concentration is increased only 10 percent from 85 percent (point C) to 95 percent (point D), the boiling point pressure at 110.degree. K is lowered from about 4 ata. to about 2 ata. Therefore, in the zone of high methane concentrations, an increase in the refrigeration capacity of the cycle by the further addition of methane can be obtained only at the cost of a considerable percentage-wise drop in the suction pressure; consequently, in this region of high methane concentration, the number of required compressor stages increases sharply.

In accordance with a preferred embodiment of the invention, therefore, the nitrogen concentration of the cycle gas is, on a molar basis, at least 20 percent, preferably at least 40 percent, conversely, the maximum preferred nitrogen concentration is 80 percent, particularly 60 percent.

To provide makeup for leakage losses according to this invention, a nitrogen separation unit is incorporated in the natural gas liquefaction process. From the head of this unit there is withdrawn a fraction having a nitrogen concentration of at least the same level as that of the cycle gas, and this fraction is then fed into the cycle as makeup gas. If this fraction contains more nitrogen than the cycle gas, then supplemental methane from the purified natural gas (freed of CO.sub.2, H.sub.2 O, and heavy hydrocarbons) can be added thereto.

One negative feature of the cycle of this invention is that the cycle gas must be precooled to a very low temperature prior to entering the expansion engine so that it can be cooled to a sufficiently low temperature during the expansion. This precooling can be conducted conventionally with a multistage refrigerating machine operating with freon, ammonia, or propane, also including, in many cases, an essential third stage of vacuum. These refrigerating machines exhibit the same disadvantage described above in connection with the Claude cycle, namely that the refrigeration is transferred at a constant temperature, to a stream having a sliding temperature. Thus, unnecessarily large temperature differences inherently must occur in the heat exchangers associated with the individual pressure stages. To avoid these disadvantages, according to a further embodiment of this invention, there is employed a mixture of methane, propane, and optionally ethane as the cycle medium for the precooling cycle.

A main advantage of this latter feature is that the refrigeration liberated during the evaporation is transferred at a decreasing temperature so that small temperature differences (.DELTA.T's) can be employed in the heat exchangers, resalting in a thermodynamically more efficient process. Since the propane and any ethane present evaporate under a partial pressure lower than the total pressure ambient during the evaporation, the desired low temperature is obtained at a higher total pressure than would be the case when evaporating pure ethane or propane. In other words, because the transfer of peak cold involves a smaller pressure drop, the number of compressor stages and thus the number of evaporators is decreased, and the cost of control elements is also reduced. Finally, it is to be noted that leakage losses amounting to about 1 - 5 parts per thousand of the quantity of cycle medium can frequently be covered, in the case of methane and ethane, merely by simple separation from the natural gas proper, i.e., they need not be stored in additional tanks. Propane is always available in any case so that the BTU value of the gas can be adjusted or desired. The proportion of each of methane, propane and optionally ethane in the total of the cycle gas is about 20-50 molar percent, respectively. If a very low precooling temperature is to be reached, e.g., 200.degree. to 215 .degree.K, there is employed a cycle gas having a composition, of in moles percent 40 and 60 methane; 25 and 60 percent propane; and 0 and 15 ethane.

The above-described advantages are of special importance when the precooling cycle is operated, according to a further preferred embodiment of the invention, in a single stage, i.e., when the evaporation of the compressed, cooled, and throttle-expanded refrigerant takes place at a uniform pressure level set by the compressor suction pressure. In this way, for example, a precooling temperature of -60.degree. C. can be attained in a single stage with a mixture consisting of approximately equal parts of methane and propane, whereas, in contrast, a three-stage plant would be required for this purpose if freon were employed as the refrigerant. The precooling temperature attainable in this manner is generally sufficient, even in case of low natural gas pressures, to result in the condensation of the heavy hydrocarbons which, otherwise, could lead to obstructions in the low-temperature sections. Thus, the plant can be rapidly brought to the required cold operating condition.

By utilizing the above-described single-stage precooling cycle, an even lower precooling temperature, e.g. 160.degree. to 200.degree. K, according to a still further preferred embodiment of this invention, as follows: a fraction of the cycle medium remains in the gaseous phase downstream of the final cooler of the compressor, and the fraction containing lower boiling components is separated from the liquid fraction containing predominantly the higher-boiling hydrocarbons. Both separated gas and liquid are then cooled by heat exchange with the liquid evaporating at the suction pressure of the compressor. The gas rich in lower-boiling components is thus totally condensed, and is then in the liquid phase expanded (pressurereduced) to the suction pressure of the compressor. The resultant liquid is then evaporated to effect said total condensation.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 2 and 3, the natural gas to be processed (0.6,780 Nm.sup.3 /hr.), after having been freed of water, carbon dioxide, and hydrogen sulfide, and having approximately the following composition: 2 percent nitrogen, 94 percent methane, 3 percent ethane, 1 percent propane and higher hydrocarbons, is fed to the plant, via conduit 1, at 298.degree. K and under 39 ata. and is then cooled in the heat exchanger 2.degree. to 216.degree. K. During this step, the C.sub.5 - and higher hydrocarbons are substantially condensed, which would otherwise cause clogging in subsequent sections of the plant. The liquid is thus separated from the gaseous phase in phase separator 3, evaporated and warmed in heat exchanger 2, and discharged from the plant via conduit 4. The portion remaining in the gaseous phase is further cooled in heat exchanger 5.degree. to about 190.degree. K; during this step, a liquid is obtained consisting of about 85 percent methane, 10 percent ethane, and 5 percent propane. This liquid after being passed in separator-collector 6, is branched into two streams, and the amount necessary to make up for leakage losses of the precooling cycle is introduced into the latter via conduit 7. The remainder is evaporated and warmed in heat exchangers 5 and 2, and then discharged from the plant via conduit 4.

A portion of the gas withdrawn from the separator 6 overhead is now further cooled in the heat exchanger 8 to 163.degree. K and expanded into the nitrogen separation column 9 operating at 22 ata. The remaining gas is conducted through a heating coil located in the sump of the column 9 and is then expanded into the midsection of column 9. At the head of the column 9, a temperature of about 150 K is maintained; the gaseous head product consists of 50 percent of a methane and 50% of nitrogen. This head product is discharged from the plant via conduit 4, except for that amount of gas required for providing makeup due to the leakage losses of the Claude refrigerating cycle and which is fed into said cycle via conduit 10. From the sump of the column 9, 5,820 Nm.sup.3 /h. of liquid natural gas is withdrawn having a temperature of 167.degree. K and approximately the following composition: 97 percent methane, 1 percent nitrogen, and 2 percent ethane. This liquid is passed to the heat exchangers 11 and 12 and cooled therein to 111.degree. K so that, during the subsequent expansion in the valve 13 to the pressure of the storage tank (slightly above 1 ata.), only a minimum amount of liquid is evaporated (about 40 Nm.sup.3 /h).

The refrigeration required for the liquefaction is provided by a Claude cycle with precooling by a one-stage cycle based on a mixture of gases. This mixture comprises 50 percent methane and 50 percent nitrogen and is employed as the cycle medium. This gas (37,900 Nm.sup.3 /hr) is compressed, in compressor 14, to 25.5 ata., and after being cooled, is further compressed to 35.5 ata. in compressor 15. The gas enters the heat exchanger 2 at a temperature of 298.degree. K and is precooled therein and also in the heat exchanger 5.degree. to 197.degree. K. 35,600 Nm.sup.3 /h. of cycle gas is then expanded in the expansion turbine 16 to 8 ata. and, during this step, is cooled to 138.degree. K. A portion of this gas is branched off via conduit 17 and serves for cooling the head of column 9; the main quantity is fed, via conduit 18, to the cold end of the heat exchanger 11, heated therein and in heat exchangers 8, 5, and 2, to ambient temperature and thereafter recompressed in the compressor 14.

The proportion of the cycle medium not subjected to engine expansion, i.e., 2,300 Nm.sup.3 /h., is cooled in conduit 19 under its pressure of 35.5 ata. in heat exchangers 8, 11, and 12, to 111.degree. K. During the subsequent throttle expansion to 8 ata. in valve 20, the temperature drops to 109.degree. K, so that the liquid natural gas can be subcooled to 111.degree. K by heat exchange with the boiling cycle liquid, before it is expanded in valve 13. At 21, the throttle-expanded cycle medium is combined with the engine-expanded cycle medium, and is warmed and recompressed together therewith.

The cycle medium of the precooling cycle consists of 45 percent methane, 5 percent ethane, and 50 percent propane. 4,200 Nm.sup.3 /h. of this gas is compressed in compressor 22 from 10 ata. to 50 ata., cooled and simultaneously liquefied in heat exchanger 2, and then expanded to 10 ata. in valve 23. The precooling temperature attainable in this manner, i.e., the temperature at which the gaseous streams to be cooled leave the cold end of the heat exchanger 2, is 216.degree. K. The evaporated and warmed cycle medium is then recompressed in the compressor 22. The leakage losses of the cycle, as mentioned above, are, in part, compensated for by the liquid withdrawn from separator 6 via conduit 7. Since this liquid contains less propane than the cycle medium, pure propane or a gas more enriched therewith must be added. This can be done by conducting the gaseous stream returning to the compressor 22 via conduit 30 through the tank 28 filled with liquid propane, rather than via conduit 29. The dome 31 serves as an entrainment separator for the separation of droplets of liquid propane.

If the natural gas is available at a lower pressure, then a lower precooling temperature is required. In order to attain such lower temperature, the precooling cycle shown in FIG. 3 is employed. The cycle medium consists of about 70 percent methane, 5 percent ethane, and 25 propane. The liquid formed in the secondary cooler of the compressor denoted by 24 is separated from the gaseous phase in the separator 25, cooled in heat exchanger 2, expanded in valve 23' from 35 to 8 ata., and reevaporated and warmed in heat exchanger 2. The gaseous phase enriched in the lower-boiling components of the cycle medium, is withdrawn from separator 25, conducted, via conduit 26, through the heat exchangers 2 and 5, being cooled and liquefied during this step, and then expanded in valve 27 from 35 to 8 ata. By the evaporation of the resultant pressure-reduced liquid, a precooling temperature of about 165.degree. K is attacined at the cold end of the heat exchanger 5. The cycle medium evaporated and warmed in heat exchanger 5 is recombined with the cycle medium expanded in valve 23', and the combined stream is then recycled to the suction side of compressor 22.

For the sake of clarity, B in FIG. 3 denotes the sum total of the remaining gaseous streams to be cooled, i.e., the natural gas to be liquefied and the compressed cycle medium of the Claude cycle; and C denotes the sum total of the other gaseous streams to be warmed, i.e., the fractions obtained during the natural gas liquefaction and to be discharged from the plant in the gaseous phase, and the expanded cycle medium of the Claude cycle.

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. Consequently, such changes and modifications are properly, equitably, and intended to be within the full range of equivalence of the following claims.

Claims

What is claimed is:

1. In a process for the liquefaction and subcooling of natural gas with a Claude closed refrigerating cycle comprising compressing gaseous cycle medium; dividing cooled compressed gas into two streams; engine-expanding one stream; cooling the other stream with the engine-expanded stream to such an extent that said other stream becomes partially liquefied after a subsequent throttle expansion thereof and passing resultant partially liquefied stream in indirect heat exchange relationship with liquid natural gas to subcool the liquid natural gas so that it remains substantially in the liquid phase after being expanded to the pressure of a storage tank;

the improvement comprising employing as the cycle medium, a mixture of nitrogen and methane.

2. A process according to claim 1, wherein the nitrogen molar concentration in the cycle medium is at least 20 percent.

3. A process according to claim 1, wherein the nitrogen molar concentration in the cycle medium is at least 40 percent.

4. A process according to claim 1 further comprising separating the natural gas from CO.sub.2, H.sub.2 O and heavy hydrocarbons; separating a nitrogen enriched stream from resultant purified natural gas, the nitrogen content of said enriched stream being at least as large as that of said cycle medium and feeding said nitrogen enriched stream into the Claude cycle in sufficient amounts to provide makeup cycle medium.

5. A process according to claim 1 further comprising precooling said one stream prior to engine expanding thereof with a refrigeration precooling cycle based on a cycle medium comprising a mixture of methane and propane ethane.

6. A process according to claim 5, wherein said precooling refrigeration cycle is conducted in a single stage.

7. A process according to claim 5, said precooling refrigeration cycle comprising a precooling compressor and a cooler downstream of the precooling compressor, and further comprising with drawing a mixture of gas and liquid from said cooler, said liquid containing predominantly higher-boiling hydrocarbons and said gas being rich in lower-boiling components; separating the gas and the liquid; said separated gas and liquid streams in heat exchange with liquid evaporating at the suction pressure of the compressor, totally condensing said gas rich in lower-boiling components, the resultant liquid being expanded to the suction pressure of the compressor, whereby the total condensation is effected by the evaporation of the thus-formed, expanded liquid.

8. A process according to claim 6, said precooling refrigeration cycle comprising a precooling compressor and a cooler downstream of the precooling compressor, and further comprising with drawing a mixture of gas and liquid from said cooler, said liquid containing predominantly higher-boiling hydrocarbons and said gas being rich in lower-boiling components; separating the gas and the liquid; said separated gas and liquid streams in heat exchange with liquid evaporating at the suction pressure of the compressor, totally condensing said gas rich in lower-boiling components, the resultant liquid being expanded to the suction pressure of the compressor, whereby the total condensation is effected by the evaporation of the thus-formed, expanded liquid.

9. A process as defined by claim 5 wherein said precooling cycle medium further comprises ethane.

10. A process as defined by claim 2 wherein the maximum molar concentration in the cycle medium is 80% nitrogen.

11. A process as defined by claim 2 wherein the maximum molar concentration in the medium is 60 percent nitrogen.

12. A process as defined in claim 3 wherein the maximum molar concentration in the cycle medium is 80 percent nitrogen.

13. A process as defined by claim 3 wherein the maximum molar concentration in the medium is 60 percent nitrogen.