Method Of Cooling A Gaseous Mixture And Installation Therefor

Abstract

A method of and apparatus for cooling and condensing a gaseous mixture by means of at least one frigorific cycle utilizing a cycle mixture which may include at least one constituent of said gaseous mixture, the cycle mixture being cooled and subjected to fractional condensation under high pressure; at least the first condensed fraction obtained during this fractional condensation is expanded to a low pressure lower than said high pressure; at least said expanded portion is vaporized and re-heated at said low pressure, in heat exchange relation with at least the cycle mixture in course of fractional condensation; at least the first heated portion is recompressed from said low pressure to said high pressure in at least one compression stage so as to re-constitute the cycle mixture, at least in part, under the high pressure, the first condensed fraction being obtained immediately after the last stage of compression; the said method further comprises the steps of expanding the portion of the first condensed fraction to the low pressure in at least one intermediate stage, consisting of expanding said portion from a pressure at most equal to the high pressure to a pressure intermediate between the said high and low pressures, a gaseous fraction from the said portion expanded to the intermediate pressure being separated out and re-compressed from the intermediate pressure to the high pressure in order to re-constitute a further portion of the cycle mixture under high pressure. The invention is especially applicable to the liquefaction of natural gas.

Description

The present invention has for its object a method of cooling and condensing a gaseous mixture, together with an installation enabling the said method to be carried into effect. THe invention is especially applicable to the liquefaction of natural gas.

At the International Refrigeration Congress of 1959 in Copenhagen, A.P. Kleemenko (Reports: pages 34 to 39) described a method of cooling and condensing a gaseous mixture by means of a frigorific cycle utilizing a cycle mixture which could comprise at least one constituent of the gaseous mixture treated.

In accordance with this method, at least the cycle mixture is cooled and subjected to fractional condensation under high pressure, at least the first condensed fraction obtained during the said fractional condensation is expanded to a low pressure lower than the high pressure, at least the first expanded fraction is vaporized and heated under the low pressure in heat exchange with the cycle mixture and the gaseous mixture in course of condensation, at least the first heated fraction is re-compressed from the low pressure to the high pressure so as to re-constitute, at least in part, the cycle mixture under the high pressure, the first condensed fraction being obtained immediately after the compression.

In addition, two distinct methods of operation of this cycle were described. In a first case, the frigorific cycle is of the open type and the gaseous mixture and the cycle mixture are combined and subjected together to the fractional condensation. In a second case, the frigorific cycle is of the closed type, and the cycle mixture and the gaseous mixture circulate in separate and distinct conduits, in which they are condensed independently.

Thie refrigeration cycle, known as "auto-refrigerated cascade cycle", is now well known. As compared with the Pictet cascade cycle, it necessitates only a single compressor, and it is therefore distinguished from this latter by smaller capital investments in equipment. Furthermore, certain improvements in this cycle have formed the subject of French Patent No. 1,302,989 and it two Certificates of Addition Nos.80,294 and 86,485.

By way of example, a frigorific cycle of this kind, utilizing a cycle mixture having the following composition by volume:

Methane -- 35 percent

Ethane -- 40 percent

Propane -- 5 percent

Butane -- 12 percent

Pentane -- 3 percent

Nitrogen and other light gases -- 5 percent

makes it possible to liquefy and sub-cool a natural gas having the following composition by volume:

Methane -- 88 percent

Ethane -- 5 percent

Propane -- 3 percent

Butane -- 2 percent

Nitrogen and other light gases -- 2 percent

While the previously described method, improved in accordance with the above-mentioned patents, and the corresponding installations give satisfaction, it must however be observed that the degree of irreversibility of certain operational phases of the method employed remains relatively large and correspondingly increases the total power consumed in condensing the gaseous mixture treated.

In this connection, the Applicant has found, in the case of liquefaction of natural gas, that the difference in temperature existing between the cycle mixture during the course of fractional condensation and the cycle mixture in course of heating remains large, especially in the first exchanger or hot exchanger of the installation, in which the vaporization of the first condensed fraction is effected and essentially in the central zone of this latter.

In other words, during the first exchange of heat which permits the temperature of the cycle mixture to be reduced from the ambient temperature towards a temperature zone of the order of -30.degree.C., the difference in temperature existing in certain zones between the condensation curve of the cycle mixture and the vaporization curve of this latter corresponds to a considerable irreversibility of the said heat exchange and correspondingly increases the total power consumed by the installation.

The difference in temperature found depends especially on the ratio of the high pressure at which the fractional condensation of the cycle mixture is effected, and the low pressure under which the vaporization of the condensed fractions of the said mixture is effected. Certain imperatives, imposed furthermore, prevent any modification of the ratio of the high and low pressures of the frigorific cycle in order to reduce correspondingly the temperature difference found above.

Within the framework of an installation utilizing the so-called "auto-refrigerated cascade cycle", the present invention thus has the object of reducing this difference of temperature existing between the cycle mixture in course of condensation and the said mixture in course of vaporization, in the first exchanger or hot exchanger, in order to reduce the consumption of power necessary for the liquefaction of the gaseous mixture treated, and without having per contra any excessive increase in the exchange surface area of the said exchanger.

In order to achieve this result, a method according to the invention is characterized in that at least a portion of the first condensed fraction is expanded to the low pressure of the frigorific cycle in at least one intermediate stage consisting of expanding the said portion from a pressure at most equal to the high pressure of the frigorific cycle to a pressure intermediate between the high pressure and the low pressure, separating a gaseous fraction from the said portion expanded to the intermediate pressure and recompressing the separated gaseous fraction from the intermediate pressure to the high pressure, so as to re-constitute another portion of the cycle mixture under the high pressure.

Advantageously, when at least the re-heated portion of the first condensed fraction is re-compressed to the high pressure in at least one stage of compression carried out from a pressure at least equal to the low pressure to a mean pressure comprised between the low and high pressures, the said mean pressure is chosen as the intermediate pressure of the expansion stage. This makes it possible to combine, at the said mean pressure, the gaseous fractions separated from the said portion of the first fraction and at least the said re-heated portion, and then to re-compress them together at the high pressure, in at least one other stage of compression effected from the mean pressure to a pressure at most equal to the high pressure.

Preferably, at least the re-heated portion of the first condensed fraction is re-compressed in two stages of compression. In this case, the said portion of the first condensed fraction is expanded to the low pressure in a single intermediate stage, and the dividing pressure between the two compression stages is chosen as the intermediate expansion pressure.

As compared with the known method previously described, the invention makes it possible in particular to enrich the first condensed fraction of the cycle mixture in heavy constituents, and therefore in constituents having a high boiling point.

Correspondingly, the vaporization of the first condensed fraction is effected in a first exchanger, or hot exchanger, at a temperature which is everywhere higher than that previously obtained. At the level of the first exchange of heat, the difference of temperature between the vaporization curve and the condensation curve of the cycle mixture is thus correspondingly reduced. The thermodynamic efficiency of this first exchange is therefore improved and in consequence, the corresponding power consumption of the installation is reduced.

As compared with a conventional auto-refrigerated cascade cycle installation, the installation corresponding to the method according to the invention necessitates only a small additional investment. On the one hand, it is in fact found that the differences in temperature initially encountered in the hot exchanger being large, their relative reduction, obtained according to the invention, remains small. The result is therefore that the exchange surface necessary for the first exchanger is only very slightly increased. Furthermore, in certain cases, as the invention makes it possible to harmonize the vaporization and condensation curves of the cycle mixture, there results a better harmonization of the differences in temperature along the hot exchanger, and so the exchange surface area may remain unchanged. On the other hand, it must be observed that, as the gaseous fraction separated from the first condensed fraction at the intermediate pressure is not large, the corresponding intermediate separator remains of modest dimensions.

In addition, in an installation which carries into effect a method according to the invention, the flow-rate treated in the last compression stage is always larger as that of the first stage. This always leads therefore, according to the invention, to a better adaptation of the compression unit and this advantage is especially appreciable in the case of a single compressor of the axial type.

Other objects and advantages of the present invention will become apparent from examination of the detailed description which follows below, with reference to the accompanying drawings, in which the same reference numbers have been given to the same parts.

In the drawings:

FIG. 1 represents an installation for carrying into effect the so-called "auto-refrigerated cascade cycle";

FIGS. 2, 3 and 4 show three installations for following this same cycle, as improved according to the invention;

FIG. 5 shows heat exchange diagrams illustrating the theoretical considerations previously referred to. In these diagrams, the cooling and heating curves relating to the first exchanger or hot exchanger of an auto-refrigerated cascade frigorific installation have been drawn. To this end, the quantities of heat (Q) in kilocalories have been plotted in ordinates and the temperatures in degrees Celsius are plotted in abscissae. The curves in full lines correspond to the exchange diagram of a hot exchanger of an installation according to FIG. 1, and therefore of a conventional auto-refrigerated cascade installation. The curves in broken lines correspond to the exchange diagram of a hot exchanger in an installation improved according to the invention, as shown in FIG. 2, under conditions of delivery output from the compressor, of flow-rate of the gaseous mixture treated (natural gas) and pressures identical with those taken into consideration for FIG. 1;

FIG. 6 represents the total exchange surface S (not including the exchange surface of the final condenser arranged after the compressor), expressed in relative values (that is to say to liquefy 1 Nm.sup.3 of natural gas), necessary in the case of FIGS. 1, 2 and 4, as a function of the power P to be supplied to the cycle mixture.

A conventional installation of the auto-refrigerated cascade type, permitting the cooling and condensation of a gaseous mixture such as natural gas, comprises a frigorific unit such as that shown in FIG. 1, intended for the circulation of a cycle mixture comprising, if so desired, at least one constituent of the gaseous mixture treated. In the case of liquefaction of natural gas, the cycle mixture comprises a certain number of hydrocarbons of the gas to be liquefied (methane, ethane, propane, etc.) and, when so desired, nitrogen, depending on the cooling desired.

The refrigeration unit shown in FIG. 1 comprises a compressor 2, in which the suction and the delivery work under pressures respectively termed hereinafter as "low pressure" and "high pressure". The compressor 2 comprises a first compression stage 2' sucking-in at low pressure and delivering at a means pressure comprised between the high and low pressures, a second and last stage 2" sucking-in at the mean pressure and delivering at the high pressure.

A final condenser 3", the inlet of which communicates with the delivery of the compressor 2 is associated with this latter. It comprises circulating means for a refrigerant external to the frigorific unit, such as water. A first exchanger 10 or hot exchanger, a second exchanger 20, a third exchanger 30, a first separator 3, a second separator 13, a first expansion valve 4', a second valve 14', a third valve 15', permit the continuation of the fractional condensation of the cycle mixture utilized, commenced in the condenser 3".

The inlet of the first separator 3 communicates with the outlet of the condenser 3". Each exchanger 10 or 20 comprises a first passage means 51 communicating at one extremity with the gaseous outlet of a separator 3 or 13, and at the other extremity with the inlet of the second separator 13 (cf exchanger 10) or the third expansion valve 15' (cf exchanger 20); a second passage means 52, constituted by the interior of each exchanger 10 or 20, in heat exchange relation with the first passage means 51, communicating with the downstream side of an expansion valve 4' or 14' and with the suction side of the compressor 2 through the conduit 6 or through the conduits 16 and 26; a third passage means 53 for the gaseous mixture to be cooled and liquefied, in thermal exchange relation with the second passage means 52; a fourth passage means 54 in heat exchange relation with the second passage means 52, of which one extremity communicates with the liquid outlet of a separator 3 or 13, while the other extremity communicates with the upstream side of an expansion valve 4' or 14'.

Each expansion means associated with each separator 3 or 13, comprising an expansion valve 4' or 14', thus communicates at its upstream portion with the liquid outlet of a separator 3 or 13, through the intermediary of a fourth passage means 54 of an exchanger 10 or 20, and at its downstream portion with a second passage means 52 of an exchanger 10 or 20.

The exchanger 30 differs from the other exchangers 10 and 20 in that it is not provided with a fourth passage means 54, and in that its passage means 51, previously specified, communicates directly at one extremity with the third expansion valve 15', without the intermediary of a separator similar to the separators 3 and 13, and at the other extremity with the first passage means 51 of the second exchanger 20.

In operation, following the frigorific cycle described in FIG. 1, the cycle mixture previously described, issuing from the compressor 2 at the high pressure of 40 bars, is cooled and subjected to fractional condensation. For that purpose, it is first partly condensed by passing into the condenser 3". Then, when it reaches the first separator 3, the first condensed fraction obtained in the condenser 3" is separated from the remainder of the cycle mixture.

The first condensed fraction is evacuated from the separator 3 by the conduit 4, sub-cooled by passing into the fourth passage means 54 of the exchanger 10, expanded to the low pressure of 2.5 bars in an expansion means comprising the first expansion valve 4', led through the conduit 4" into the exchanger 10, vaporized and heated by passage into the second passage means 52 of the said exchanger, by heat-exchange in counter-flow with at least the first condensed fraction in course of sub-cooling, and finally evacuated from the exchanger 10 through the conduit 6.

The remainder of the cycle mixture is evacuated from the first separator 3 and its fractional condensation is continued by passing into the first passage means 51 of the exchanger 10, by heat exchange in counter-flow with the first condensed fraction in course of vaporization and heating in the second passage means 52.

The cycle mixture is then evacuated from the exchanger 10 through the conduit 5', and led to the second separator 13, in which a second condensed fraction is separated from the cycle mixture.

With regard to the gaseous mixture (natural gas) to be cooled and condensed, this is introduced through the conduit 1 into the third passage means 53 of the exchanger 10. It is then cooled by exchange of heat in counter-flow with the first fraction condensed and expanded to the low pressure, in course of vaporization, circulating in the second passage means 52 of the exchanger 10.

The second condensed fraction is evacuated from the separator 13 through the conduit 14, sub-cooled by passage into the fourth passage means 54 of the exchanger 20, expanded to the low pressure in an expansion means comprising the second expansion valve 14', led by the conduit 14" into the exchanger 20, vaporized and heated by passing into the second passage means 52 of the said exchanger, by heat exchange in counter-flow with at least the second condensed fraction in course of sub-cooling and finally evacuated from the exchanger 20 by the conduit 16.

The cycle mixture, remaining in the gaseous state, is evacuated from the second separator 13 by the conduit 15, and its fractional condensation is continued by passing into the first passage means 51 of the second exchanger 20, by heat exchange in counter-flow with the second fraction condensed during the course of vaporization and heating in the second passage means 52. The cycle mixture is then evacuated from the exchanger 20 towards the first passage means 51 of the third exchanger 30. With regard to the gaseous mixture (natural gas), this continues its condensation at a temperature level lower than that of the first exchanger 10, in the third passage means 53 of the second exchanger 20, by exchange of heat in counter-flow with the second fraction condensed and expanded to the low pressure, during the course of vaporization in the second passage means 52 of the exchanger 20.

The cycle mixture completes its condensation, and becomes sub-cooled by passing into the first passage means 51 of the third exchanger 30. The third condensed fraction thus obtained, sub-cooled, is expanded to the low pressure in the third expansion valve 15", is vaporized and heated in the second passage means 52 of the third exchanger 30 by exchange of heat in counter-flow with at least the remainder of the cycle mixture at the end of the fractional condensation, circulating in the first passage means 51, and is evacuated from the exchanger 30 by the conduit 26.

The gaseous mixture (natural gas) completes its condensation at a temperature level lower than that of the second exchanger 20 by passing into the third passage means 53 of the exchanger 30, by exchange of heat in counter-flow with the last condensed fraction of the cycle mixture in course of vaporization. It may be sub-cooled, if so required, in the third exchanger 30. The condensed gaseous mixture, sub-cooled if so desired, is evacuated from the frigorific unit and expanded to its production pressure in the expansion valve 56.

As regards the three condensed fractions of the cycle mixture, vaporized respectively in the exchangers 10, 20 and 30, they are combined by means of the conduits 6, 16 and 26 and are sent back to the suction of the compressor 2, after passing into a safety separator 55. They are then re-compressed from the low pressure (2.5 bars) to the high pressure (40 bars) of the cycle in order to re-constitute the cycle mixture under high pressure. Their compression is carried out in a first step completed in the first stage 2' from the low pressure to a means pressure, and in a second and last step carried out in the second stage 2", from the means pressure to the high pressure.

FIG. 2 represents a frigorific unit similar to that previously described, but modified according to the invention. As has been previously explained, this modification concerns solely the expansion means associated with a separator of the portion of the frigorific unit in which the fractional condensation of the cycle mixture is effected.

In accordance with FIG. 2, the first expansion means associated with the first separator 3 comprises, in addition to the first expansion valve 4', a single intermediate stage. This latter comprises an intermediate expansion valve 104, working between the high pressure of the frigorific cycle and the mean cut-out pressure of the compressor 2, the upstream portion of which communicates through the conduit 56 with the liquid outlet from the first separator 3; an intermediate separator 103, the inlet of which communicates with the downstream side of the said intermediate expansion valve 104, the gaseous outlet of which communicates with the delivery of the first compression stage 2' of the compressor 2 through the conduit 105, while the liquid outlet communicates through the conduit 114 with the upstream side of the first expansion valve 4'.

The operation of the installation described with reference to FIG. 2 differs from that described in the installation shown in FIG. 1, only by the method of expansion to the low pressure of the first fraction condensed and collected in the separator 3. According to FIG. 2, the first condensed fraction extracted from the separator 3 through the conduit 56 is expanded to the low pressure with a single intermediate step.

This step consists of expanding the first condensed fraction coming from the conduit 56 in the intermediate valve 104, to an intermediate pressure equal to the means delivery pressure of the first compression step 2'.

After this, a gaseous fraction is separated from fraction expanded to the means pressure, in the separator 103. This fraction is evacuated through the conduit 105, reunited at the mean pressure, at the delivery of the first compression stage 2', with the heated portions of the cycle mixture, and re-compressed with these latter to the high pressure, in the second compression stage 2" effected from the mean pressure to the said high pressure in order to re-constitute another portion of the cycle mixture under high pressure.

In addition to the advantages previously indicated which assist in reducing the power expenditure in condensing the gaseous mixture treated, the embodiment shown in FIG. 2 further contributes to an improvement of the economy of the frigorific cycle, for the following reasons: on the one hand, the total flow-rate of the cycle mixture at the delivery of the compressor 2 remains practically unchanged; the material balance is practically the same, except as regards the gaseous fraction obtained at the intermediate pressure in the separator 103, and which is sent at a lower temperature into the second stage 2" of compression.

On the other hand, the compression is relieved in the first stage 2' of the compressor 2, of all the gaseous fraction obtained at the intermediate pressure. If, for example, the rate of compression is the same in both stages of the compressor 2, this gaseous fraction may represent 10 to 12 percent of the cycle mixture. In this case, the gain in power is from 5 to 6 percent.

The frigorific unit shown in FIG. 3 is differentiated from the unit shown in FIG. 2 by the fact that the intermediate expansion stage described in FIG. 2 further comprises, according to FIG. 3, an intermediate heat exchanger 200.

This exchanger 200 comprises a first passage means 57 constituted by the interior of the said exchanger, communicating with the downstream side of the intermediate expansion valve 104 and with the input of the intermediate separator 103; a second passage means 58 communicating at one extremity with the liquid outlet of the first separator 3 and at the other extremity with the upstream side of the intermediate expansion valve 104, in heat-exchange relation with the first passage means 57; a third passage means 59, in heat-exchange relation with the first passage means 57, communicating at one extremity with the liquid outlet of the intermediate separator 103, and at the other extremity with the upstream side of the first expansion valve 4', through the intermediary of the fourth passage means 54 of the first exchanger 10; a further passage means 60, in heat-exchange relation with the first passage means 57 for all flows in course of cooling.

The operation of the frigorific unit according to FIG. 3 is distinguished from that of the unit previously described, solely by the exchange of heat which takes place in the exchanger 200. In this latter, there is vaporized, at least partially, in the first passage means 57, the first condensed fraction expanded to the intermediate pressure in the valve 104, and passing into the exchanger through the conduit 204".

The necessary heat of vaporization is obtained by exchange of heat, firstly with the first condensed fraction in course of sub-cooling before its expansion 104 to the intermediate pressure, circulating in the second passage means 58 of the exchanger 200 and coming from the first separator 3; secondly, with the first condensed fraction, separated from the gaseous fraction derived from the intermediate separator 103 through the conduit 114, and circulating in the third passage means 59 of the exchanger so as to be sub-cooled before its expansion to a lower pressure, equal to the low pressure, in the first expansion valve 4'; thirdly, with another flow in course of cooling, passing into the exchanger 200 through the conduit 201, and circulating in the other passage means 60.

This other flow may be the cycle mixture derived from the gaseous outlet of the first separator 3, the gaseous mixture to be cooled and condensed (natural gas for example), or any other fluid at a temperature in the neighbourhood of ambient temperature, which it is necessary to cool.

It is found that the frigorific unit according to FIG. 3, by comparison with that of FIG. 1, makes it possible to obtain a still greater gain in power with respect to that of FIG. 2. In fact, the first condensed fraction of the cycle mixture being at least partly vaporized in the intermediate heat exchanger, on the one hand the percentage of the gaseous fraction separated in the separator 103 is considerably increased, and on the other hand the first condensed fraction is still further enriched in heavy constituents.

In additiion, all the cold generated in the heat-exchanger 200 is only half as expensive in energy, since the compression of the cycle mixture can be reduced by half. The exchange surface necessary is of course increased as the vaporization of the first condensed fraction becomes greater. This imposes a limit on the gain of power which can be obtained. It can however be of the order of 10 percent.

The frigorific unit shown in FIG. 4 differs from that sbown in FIG. 3 only by the fact that the intermediate exchanger 200 also comprises a fourth passage means 61 communicating at one extremity with the gaseous outlet of the first separator 3, and at the other extremity with the inlet of the second separator 13, through the intermediary of the first passage means 51 of the first exchanger 10, and a fifth passage means 62 permitting the cooling of the gaseous mixture treated to be started, communicating at one extremity with the third passage means 53 of the first exchanger 10.

In consequence, according to FIG. 4, the heat required for the vaporization of the first condensed fraction in the exchanger 200 is also supplied by exchange of heat in counter-flow with the cycle mixture in course of fractional condensation, coming from the first separator 3 and circulating in the fourth passage means 61, and by exchange of heat in counter-flow with the gaseous mixture (natural gas) in course of cooling, circulating in the fifth passage means 62 of the exchanger 200 and flowing towards the exchangers 10, 20 and 30.

Analysis of the exchange diagrams shown in FIG. 5 makes it possible to illustrate the theoretical considerations postulated above. In this figure, the cooling curves (arrows pointing downwards) represent the sum of the quantities of heat exchanged in the gaseous mixture 1 (natural gas) in course of cooling and condensation, in the cycle mixture 5 in course of cooling and fractional condensation, and in the first condensed fraction coming from the first separator 3 in course of sub-cooling.

As regards the heating curves (arrows pointing upwards), they represent the quantity of heat exchanged by the cycle mixture in course of heating, coming-in through the conduits 16 and 4", comprising the first condensed fraction in course of vaporization and heating at the low pressure.

Referring now to the curves in full lines (FIG. 1) that is to say in the case of a conventional "auto-refrigerated cascade" unit, it is found that the cooling curve is a substantially linear function of the temperature, and that the heating curve comprises an angular point, corresponding to an abrupt and considerable change in slope in the central zone of the first exchanger 10. This results in a substantial difference in temperature, essentially in this zone of the exchanger, which affects the thermo-dynamic efficiency of the frigorific cycle.

Referring now to the curves in broken lines corresponding to FIG. 2, that is to say in the case of an improved frigorific unit according to the invention, it is found on the one hand that the heating curve approaches the cooling curve, and on the other hand that the heating curve is much flatter than in the previous case. The difference in temperature has therefore been reduced over the whole length of the first exchanger or hot exchanger, and this essentially in the central zone of this exchanger. The reversibility of the first exchange of heat is thus increased, and this contributes to reducing the power consumed in liquefying the gaseous mixture treated.

The curves of FIG. 6 bring out clearly the gain obtained according to the invention, for an equal exchange surface or an equal expenditure of power.

The curves VA1, VA2 and VA4 related respectively to the case of FIGS. 1, 2 and 4. A comparison of these curves will show:

- that, as compared with the case of FIG. 1, that of FIG. 2 results in gains of power of about 5 percent with equal exchange surface, and 6 to 10 percent in exchange surface for equal expanditure of power;

- that, the choice between the case of FIG. 2 and that of FIG. 4 must be made for each case as a function of economic criteria, the case of FIGS. 3 and 4 being essentially employed when the power is expensive.

It will of course be understood that the present invention is not in any way limited to the forms of embodiment described and shown. It is capable of receiving numerous alternative forms familiar to those skilled in the art, depending on the applications considered and without thereby departing from the spirit of the invention.

In the first place, the method of compression is in no way limited to a single compressor comprising two compression stages, but may correspond to a compression unit comprising a number of compressors each forming one stage of compression.

Secondly, each of the condensed fractions of the cycle mixture is capable, in the same manner as for the first condensed fraction, of being expanded to the low pressure of the frigorific cycle in at least one intermediate stage, so as to obtain the same advantages of the invention at the level of the various exchangers 20 and 30 of the frigorific unit.

Thirdly, the invention is applicable to an "auto-refrigerated cascade cycle", whether this is of the open or closed type.

Fourthly, the invention is not limited to a cycle in which the first refrigeration, in the condenser located immediately at the outlet of the compressor, is effected with a refrigerant such as water. Depending on the case, this initial refrigeration may be carried out with a frigorific cycle independent of the auto-refrigerated cascade cycle, utilizing for example, propane as the refrigerant fluid.

Claims

What I claim is:

1. In an auto-refrigerated cascade method of cool-ing and condensing a gaseous mixture (1) by means of a frigorific cycle utilizing a cycle mixture said frigorific cycle comprising:

a. partially condensing (3') said cycle mixture under a high pressure by heat exchange with an external refrigerant,

b. separating (3) a first condensed fraction from said partially condensed cycle mixture,

c. fractionately condensing (13) under said high pressure the remainder (5) of said cycle mixture separated from said first condensed fraction, and obtaining thereby a second condensed fraction (14),

d. expanding (4') at least a portion of said first condensed fraction and at least a portion of said second condensed fraction (14') to a low pressure lower than said high pressure,

e. vaporizing and reheating (52) at least said expanded portions under said low pressure, in heat exchange (51) with at least said remaining cycle mixture in the course of fractional condensation,

f. recompressing (2) at least said reheated portions (6) from said low pressure to said high pressure in at least one compression stage (2"), so as to reconstitute, at least in part, said cycle mixture under said high pressure, and effecting the partial condensation (3') of step a) immediately after the last stage (2") of compression;

the improvement comprising

g. serially expanding during step d) said portion of said first condensed fraction (56) in at least one intermediate stage from said high pressure to a pressure intermediate said high pressure and said low pressure,

h. separating (103) out a gaseous fraction (105) from said portion (114) expanded to said intermediate pressure,

i. recompressing (2") said separated gaseous fraction (105) from said intermediate pressure to said high pressure, in order to reconstitute a further portion of said cycle mixture under high pressure, and

j. expanding (4') said portion (114) separated during step h) from said gaseous fraction (105), from said intermediate pressure to a pressure at least equal to said low pressure.

2. A method as claimed in claim 1, in which said portion (264") expanded to said intermediate pressure, resulting from step (g), is partly vaporized (57), prior to the separation step (h), by heat exchange with said portion (114) separated during step (h) from said gaseous fraction (105), in the course of subcooling (59) prior to its expansion (in valve 4') according to step (j), and with at least one other flow of fluid (201) in the course of cooling.

3. An auto-refrigerated cascade installation for cooling and condensing a gaseous mixture (1), comprising a frigorific unit for the circulation of a cycle mixture, said frigorific unit comprising:

- a compressor (2) having its suction and delivery operating respectively under a low pressure and a high pressure, comprising a final compression stage (2'), the suction and delivery of which operate respectively at a pressure intermediate said high and low pressures and at said high pressure;

- a condenser (3') for cooling and partial condensation under said high pressure of at least said cycle mixture, the input of said condenser (3') communicating with the delivery of said compressor (2), and comprising circulation means for an external refrigerant to said frigorific unit;

- a first separator (3) for the separation of a first condensed fraction (56) from said cycle mixture partially condensed in said condenser, the inlet of said separator (3) communicating with the outlet of said condenser (3');

- first expansion means (104, 103, 4'), comprising one first expansion valve (4') for the expansion to said low pressure of at least a portion of said first condensed fraction (56), and the upstream side of which is in communication with the liquid outlet of said first separator (3);

- at least one heat exchanger (10) for the fractional condensation under said high pressure of at least the remainder (5) of said cycle mixture, separated from said first condensed fraction (56), and comprising: a first passage means (51) for the cycle mixture (5) in the course of fractional condensation, communicating at one extremity with the gaseous outlet of said first separator (3), and at the other extremity with the inlet (5') of a second separator (13); a second passage means (52) for the portion of said first condensed fraction (4"), expanded to said low pressure and in course of vaporization and heating, in heat exchange relation with said first passage means (51), communicating with the downstream side of said first expansion valve (4') and with the suction of said compressor (2); and a third passage means (53) for said gaseous mixture (1) in the course of cooling and condensation, in heat exchange relation with said second passage means (52);

- said first expansion means (104, 103, 4') being in fluid series and comprising an intermediate stage comprising an intermediate expansion valve (104) for the expansion of said portion (56) of the first condensed fraction from said high pressure to a pressure intermediate said high and low pressures; and

- an intermediate separator (103) for the separation of a gaseous fraction (105) from said portion (56) expanded to said intermediate pressure, the inlet of which communicates with the downstream side of said intermediate expansion valve (104), the gaseous outlet (105) of which communicates with the suction of the last compression stage (2") of said compressor (2), so as to combine under said intermediate pressure, said vaporized and heated portion and said gaseous fraction (105) and to recompress them to said high pressure, and the liquid outlet (114) of which communicates with the upstream side of said first expansion valve (4').

4. An installation as claimed in claim 3, in which said intermediate stage comprises an intermediate heat exchanger (200) for the partial vaporization of the portion (204") of said first fraction expanded (104) to the intermediate pressure, and comprising: a first passage means (57), for said expanded portion (204") in the course of vaporization, communicating with the downstream side of said intermediate expansion valve (104) and with the inlet of said intermediate separator (103), a second passage means (59) for the portion (114) expanded to said intermediate pressure and separated from the gaseous fraction (105), in the course of subcooling, in heat exchange relation with said first passage means (57), communicating at one extremity with the liquid outlet (114) of said intermediate separator (103), and at the other extremity with the upstream side of said first expansion valve (4'); and at least a further passage means (60) for another flow (201) in the course of cooling, in heat exchange relation with said first passage means (57).