EP0634618A1 - Self-refrigerating process for cryogenic fractionation and purification of gas and heat exchanger for carrying out the process - Google Patents

Abstract

The present invention relates to a self-refrigerating process for cryogenic fractionation and purification of gas, and a heat exchanger for carrying out this process. The gaseous fluid is treated in an exchanger forming a unitary assembly: it is partially condensed by cooling in the circuits (C5) and (C1) and the uncondensed gaseous fraction is reheated in the circuit (C2). The required cold is supplied by the condensates which, after supercooling (C3) and pressure reduction (V1) evaporate (C4). The process can be carried out in an exchanger comprising multiple channels for each circuit. This process allows the purification of a gaseous fluid having a plurality of components which can be condensed by cooling. <IMAGE>

Description

The present invention relates to a cryogenic fractionation process and gas purification.

It also relates to a heat exchanger for the execution of this process.

Certain gases include both constituents which are fairly easily liquefiable at low temperature and constituents which are more difficult to liquefy or non-liquefiable. It is therefore common to try to separate them by cooling in order to condense the elements which are more easily liquefiable and thus separate them from the constituents which are more difficult to liquefy or non-liquefiable.

Among the gases with several components which can thus be treated, there may be mentioned mixtures of different hydrocarbons or non-hydrocarbon components such as nitrogen, hydrogen, argon and / or carbon monoxide and, for example , catalytic cracking or steam cracking gases.

In order to obtain the necessary cooling, use is made in the prior art of heat exchangers, and in particular of reflux exchangers, also designated by "dephlegmators", the external refrigeration usually being supplied against the current by a refrigeration cycle or by dynamic gas expansion. This limits the use of these techniques to the temperatures at which the refrigeration cycles are available and to cases where the expansion of the effluents, for example hydrogen or methane, is possible.

One can also use a self-refrigeration technique. The technique consists in cooling the gas to be purified in a first exchanger, in separating the non-gas condensed from the first condensate formed, for example in a fractionation column, to further cool the non-condensed gas in a second exchanger to form a second condensate, to separate this second condensate from the non-condensed gas in a separator and to return the second condensate to the column as reflux.

The uncondensed gas separated from the second condensate constitutes the purified gas. The cooling agent for the two exchangers consists of the first condensate which is subjected to vaporization by expansion and passes successively through the second then the first exchanger. The purified gas can itself pass through the second and then the first exchanger.

The method and the device of the invention have the advantage of not generally requiring refrigeration using refrigerants foreign to the installation and of not requiring expansion of the constituent (s) ) the most difficult to liquefy (s) of the treated gas mixture. This last point is important because, on the one hand, liquefaction techniques most often require the application of high pressure and, on the other hand, certain separate gases obtained such as, for example, hydrogen and / or carbon monoxide are often reactants for chemical reactions which must themselves be carried out under high pressure. It would therefore be uneconomic to expand these gases during cryogenic separation and then have to recompress them.

On the other hand, the method and the device of the invention are more economical than the known method of self-refrigeration because they require only a unitary exchanger which is less expensive than the multiple devices (at least two exchangers, a fractionation column , a separator and many circuits) of the known method. They also reduce heat losses and avoid high expenses for insulating circuits and devices.

The gases to which the invention applies are mixtures of at least two, and preferably at least three, different chemical components and different boiling (or condensation) temperatures under the conditions of the process, and, for example, a mixture of hydrogen, methane and at least one C2 hydrocarbon such as ethane or ethylene, with or without higher hydrocarbons (C3 or more). Other mixtures also contain carbon monoxide and / or nitrogen.

The process of the invention is a self-refrigerated process of cryogenic fractionation and purification of a gaseous fluid supplying at least two condensable components of different condensation temperatures, respectively at least one relatively heavy component to remove and at least a relatively light component to be recovered, so as to produce a purified gas preferably comprising the relatively light component (s) and a separate gas preferably comprising the relatively heavy component (s), characterized in that one operates in a heat exchange zone forming a unitary assembly and comprising at least five distinct circuits, designated respectively by first, second, third, fourth and fifth circuits, in indirect heat exchange relationship with each the others at each level of the heat exchange zone, generally vertical, the first circuit, or reflux circuit, é both disposed essentially in an upper and relatively cooler portion of the heat exchange zone, and the fifth circuit being disposed essentially in a lower and relatively less cold portion of the heat exchange zone, process in which at least a fraction of the gaseous supply fluid is circulated generally from bottom to top in the fifth circuit, under conditions such that it can partially condense to give a first condensate and this first condensate is entrained without substantial reflux by said gaseous fluid, the resulting mixture of uncondensed gas and of the first condensate is discharged from the top of the fifth circuit, the said uncondensed gas is separated said first condensate, in a phase separation zone, the gas thus separated is circulated generally from bottom to top in the first circuit, or reflux circuit, under conditions such that part of the gas can give a second condensate and that this second condensate can flow back into said first circuit, and be collected at its base, at least part of the uncondensed gas, discharged from the top of the first circuit, generally from top to bottom in the second circuit, against the current of the fluid circulating in the first circuit then of the fluid circulating in the fifth circuit, and the resulting purified gas is discharged, circulate the first condensate and the second condensate overall from bottom to top in at least a third circuit to undergo a sub-cooling there, discharge from the top of the (at least one) third circuit the first and second resulting sub-cooled condensates , they are relaxed and circulated globally from top to bottom in at least a fourth circuit where they vaporize by taking heat from the fluids of the first, third and fifth circuits, finally discharging said vaporized condensates from the bottom of the (at least one) fourth circuit, these vaporized condensates constituting the separated gas.

Thus the invention uses a unitary heat exchanger (a unitary heat exchange zone) comprising, over at least part of its height, at least five circuits, each preferably of the multi-channel type, directed generally vertically . One of the circuits, called the reflux circuit or first circuit, is arranged essentially in an upper portion of the exchanger (the exchange zone), that is to say in a relatively cooler portion of the exchanger. . It is preferably a "non-tortuous" circuit, that is to say in which the condensed liquid can flow in a generally descending direction. Another circuit (fifth circuit), preferably of the tortuous type, not suitable for reflux of liquid, is essentially arranged in a lower portion of the exchanger (the exchange zone), that is to say in a portion relatively cooler of the exchanger.

By circuit of the tortuous type, directed generally vertically, is meant a circuit such that the fluid which is introduced therein at the base can progress generally from the bottom upwards without significant reflux of the liquid portions of this fluid, which supposes, by example, an average slope less steep than in the above-mentioned reflux circuit; in other words, all or almost all of the fluid (liquid and gaseous) will follow a generally ascending path in this circuit of tortuous type and will be collected at the top of said circuit, the point (or zone) of discharge being located in a portion intermediate of the heat exchanger, for example in the vicinity of the first third or half the height of the exchanger.

It is preferable that the aforementioned tortuous circuit is in whole, or almost in whole, at a level lower than the reflux circuit, and, even better, than the two circuits are arranged substantially one above the other in the exchanger.

The second, third and fourth circuits can be tortuous or not, preferably non-tortuous.

However, it is not essential to use a tortuous circuit and a non-tortuous circuit to obtain the above results (reflux and non-reflux respectively). One can indeed act on the section of the circuit and / or the speed of circulation of the supply fluid in this circuit. A low speed in a relatively wide channel allows reflux indeed while a high speed in a relatively narrow channel results in entrainment of the condensate, thus preventing it from backing up. A multi-channel circuit with a small section and a high circulation speed is therefore advantageous, in particular for the fifth circuit.

The five aforementioned circuits are in heat exchange relationship with each other at each level of the exchanger where they are present, which supposes that the exchanger is preferably made of a good heat-conducting material, with walls as thin as possible compatible with the resistance of the materials and having a high exchange surface. Specialists can easily make such exchangers from the previous indications.

According to the invention, the aforementioned multi-component gaseous fluid (at least two and preferably at least three condensable components) is circulated from bottom to top in the fifth circuit, located in a lower portion of the exchanger, in conditions of temperature and pressure such that it can partially condense without backflow into said circuit. The mixture of gas and liquid (first condensate) withdrawn from the top of the fifth circuit is separated into a gas phase and a liquid phase, in a separation zone. The resulting gas phase is circulated from bottom to top in the first circuit (reflux circuit) preferably located above the fifth circuit, as indicated above. In this relatively cold portion of the exchanger, part of the gas condenses and the condensate (second condensate) descends towards the aforementioned separation zone, this due to the non-tortuous nature of this first circuit or the low rate of rise of the gas.

The second condensate thus formed can be mixed with the first condensate already present in the separation zone or be collected separately. The uncondensed gas collected at the top of the first circuit is returned to the exchanger by the aforementioned second circuit to circulate there from top to bottom against the current of the fluids circulating in the first circuit and in the fifth circuit. It comes out heated by constituting the purified gas, formed of the most volatile elements of the gaseous supply fluid.

The liquid phase of the separation zone, consisting of the first condensate alone or of the mixture of the first and second condensates, is circulated from the bottom to the top in the aforementioned third circuit where it undergoes sub-cooling. It is then relaxed, statically or dynamically, and circulated from top to bottom in the aforementioned fourth circuit of the exchanger where it vaporizes thanks to the heat removed from the fluids of the tortuous circuit, the first circuit and the third circuit. The gas stream discharged at the bottom of the fourth circuit contains the least volatile constituents of the gaseous supply fluid. It can, if desired, be partially recycled or otherwise treated.

According to a variant, it is possible not to mix the first and second condensates and have them run through the third and fourth circuits separately, which explains why the invention uses "at least a third circuit" and "at least a fourth circuit".

The abovementioned process diagram thus made it possible, without adding cold from an external source to the system, to fractionate a gaseous mixture at low temperature, without appreciable loss of pressure for the most volatile constituents of the charge.

Various modifications or variants can be made to the invention.

According to a first variant, only part of the gaseous phase collected at the head of the first circuit is sent to the second circuit; the other part is expanded and used in the heat exchanger in the downward direction, either by passing through a sixth exchange circuit or, preferably, by passing through the fourth circuit, in admixture with the expanded liquid phase of the (or ) condensate (s) introduced therein, to allow vaporization at higher pressure. In this case, the production of purified gas under high pressure is less important, but this does not present any drawback when the gas stream from the fourth circuit is recycled or the gas stream from the sixth circuit is recompressed. Preferably 90 to 98% by mole of the gaseous phase collected at the head of the first circuit is sent to the second circuit and the other part (2 to 10 mol%) is expanded and joined to said liquid phase of the fourth circuit.

According to another variant, part of the gas to be purified does not pass through the fifth circuit and is sent directly to the gas-liquid separation zone or to the first circuit. This allows the operation of the installation to be adapted to changes in the composition of the load. Preferably, in this case, a fraction of 80 to 95 mol% of the gas passes through the fifth circuit, and a fraction of 5 to 20 mol% is sent to the separation zone. It is thus possible to maximize the quantity of purified gas obtained by the second circuit.

Yet another variant consists in supplying a liquid phase of external origin to the exchanger, under conditions where this liquid phase can relax and evaporate after expansion during its passage from top to bottom in the exchanger. This liquid phase of external origin can first pass through the exchanger from bottom to top by an auxiliary circuit to undergo sub-cooling there before descending by an auxiliary circuit. This is advantageous when starting the installation to facilitate and accelerate its cooling. More simply, if its composition is compatible with that of the liquid of the third circuit, it can be mixed with the latter before the entry thereof into the third circuit or only before the entry of said liquid into the fourth circuit.

It is also advantageous to adjust the condensation rate of the gaseous supply fluid in the fifth circuit to a value of 2 to 20% by mole. The temperature and pressure conditions in the unitary heat exchange zone of the invention depend, obviously, the composition of the feedstock, and the technician will be able to choose these conditions in each particular case using his knowledge, the main thing being to operate under conditions allowing partial condensation of the fluid. food. Due to the fact that it is a cryogenic process, the operation is carried out below ambient temperature, for example between 0 ° C. and -150 ° C. depending on the gas treated and the pressure chosen. As, moreover, an expansion of the condensates is provided, one advantageously operates at a super-atmospheric pressure, for example between 5 and 100 bars. Below are values given as examples.

Thanks to a judicious choice of operating conditions, it is easy to obtain a purified gas containing less than 1 mol% of relatively heavy components and a separate gas containing at least 30 mol% of said relatively heavy components.

The invention also relates to a heat exchanger making it possible to implement the method described above. This exchanger is characterized in that it comprises at least five distinct circuits, generally vertical, designated respectively first, second, third, fourth and fifth circuits, in indirect heat exchange relationship with each other at each level of said exchanger, said circuits forming a unitary assembly, the first circuit being of the non-tortuous type and the fifth circuit of the tortuous type, the first circuit being disposed at a level higher than that of the fifth circuit, at least one direct junction between the top of the first circuit and the top of the second circuit, at least one junction through an expansion means between the top of the third circuit and the top of the fourth circuit, at least one zone of separation of phases connected by its upper part to the base of the first circuit, by its lower part to the base of the third circuit and laterally at the top of the fifth circuit.

Preferably, the first circuit is superimposed on the fifth circuit.

Figures 1 and 2 attached illustrate the invention without limitation.

The heat exchanger E1 has five main circuits C1 to C5 corresponding respectively to the first, second, third, fourth and fifth circuits of the process. The gas to be purified is sent by lines 1 and 3 to the circuit C5 and leaves it by line 2 in the mixed gas / first condensate phase. The two phases separate in the flask B1: the gas phase is sent by line 16 to the circuit C1; it undergoes cooling there and a second condensate forms and flows back through line 17. The uncondensed gas leaves at the head and is sent by lines 5 and 7 to circuit C2. It comes out warmed at the bottom of this circuit by line 14. This gives the purified gas or the lightest fraction of the charge.

The condensates coming from circuits C5 and C1 respectively by lines 2 and 17 are mixed and sent by line 4 to circuit C3 where they undergo sub-cooling. They come out at the head by line 8, pass through the expansion valve V1 and are sent to circuit C4 by line 9. They can pass through a balloon B2, in which case the gas phase and the liquid phase are conveyed to C4 respectively by the lines 18 and 19 to point 10. The condensates vaporized leaves the circuit C4 via line 11. These are the least volatile fractions of the charge.

According to the first variant, part of the gas from the circuit C1 is taken from line 5 and sent through the expansion valve V2 and line 6 to the cylinder B2.

According to the second variant, part of the starting gas is sent to the balloon B1 through the line 15 and the valve V4.

According to the third variant, a liquid phase compatible with the condensate of line 4 is sent by line 12 to an auxiliary circuit C6 to undergo sub-cooling there before passing through line 13 and the expansion valve V3 and sent to the balloon. B2, preferably by line 9.

In FIG. 2, we find the unitary exchanger assembly E1, comprising a plurality of circuits fulfilling, by groups, the same function. Thus the circuit C1 of FIG. 1 is subdivided into C1, C1 'and C1' ', the circuit C2 is subdivided into C2, C2', C2 '', etc. Each circuit is separated from the neighboring circuit by a vertical sheet such as sheets 20, 21, 22, etc. Preferably, each circuit is of the multi-channel type. Circuits C1 and C3 are examples. In fact, there are vertical sheets such as 23 (corrugated sheet) or 24 (flat partition) dividing the circuits into a plurality of elementary channels such as 25 and 26.

We find laterally the output of channels 2, 16 and 17 relating to the fifth (2) and first (16 and 17) circuits of Figure 1, with their equivalents 2 ', 16', 17 ', 2'',16'' and 17 ''. The collectors placed at the top and at the bottom of the the exchanger E1 since they are of the conventional type. For example, one of the collectors will bring together the effluents from circuits C1, C1 'and C1''; the same for C2, C2 'and C2'', etc. The lateral conduits 2, 16, 17 (and their prime and second equivalents) are connected to separate B1 tanks or to a common elongated B1 tank.

The order of succession of the circuits described above, namely C1, C2, C3, C4, C5 is not essential and any other combination can be envisaged. For example, we can have the order C1, C4, C3, C2, C5 or C2, C4, C1, C3, C5, etc ... it being understood that preferably C1 is superimposed on C5.

The following examples 1 to 4, given without limitation, illustrate the invention.

EXAMPLE 1

An available gas is treated at -93 ° C under 35 bar abs. Its composition is given in table 1. Its flow rate is 121.788 kmol / h.

The temperature and pressure conditions at the various points of the circuits are given in table 5.

Valves V2, V3 and V4 are closed.

Line 6,111,703 kmol / h of hydrogen-enriched gas containing less than 1 mol% of ethylene is collected under a pressure of 34.7 bar abs and 10.086 kmol / h of gas highly enriched in ethylene in line 12 under a pressure of 1.8 bar abs. The latter gas can be sent to a distillation column to obtain an even richer stream of ethylene. The current compositions of the installation are shown in Table 1.

EXAMPLE 2

The procedure is as in Example 1, however, partially opening the valve V2 to allow the vaporization of the fluid circulating in the circuit 4 at higher pressure.

Tables 2 and 6 give respectively the compositions of the fluids, at the inlet and at the outlet, and the operating conditions.

EXAMPLE 3

The procedure is as in Example 2 with, in addition, partial opening of the valve V4. Tables 3 and 7 give the compositions of the fluids and the operating conditions.

EXAMPLE 4

The procedure is as in Example 3 with, in addition, partial opening of the valve V3 allowing the introduction of a distillate consisting of a 50/50 mixture by volume of methane and ethylene, obtained by rectification of the purified gas from a previous operation.

Such an operating mode is used when starting the installation to facilitate its cooling.

Tables 4 and 8 give the compositions of the fluids and the operating conditions. TABLE 1 Gas to be purified (line 1) Purified gas (line 14) Separate gas (line 11) Molar composition Hydrogen % mol 62.3100 67.7567 1.9871 Carbon monoxide % mol 0.3814 0.4073 0.0941 Methane % mol 31.3551 31.0198 35.0688 Acetylene % mol 0.0369 0.0013 0.4312 Ethylene % mol 5.4370 0.8057 56.7295 Ethane % mol 0.4784 0.0092 5.6743 Propylene % mol 0.0012 0.0000 0.0149 Temperature ° C -93.00 -95.00 -100.00 Pressure abs bar 35.00 34.70 1.80 Molar flow Kmol / h 121,788 111.703 10,086 Gas to be purified (line 1) Purified gas (line 14) Separate gas (line 11) Molar composition Hydrogen % mol 62.3100 67.7566 12.8710 Carbon monoxide % mol 0.3814 0.4073 0.1460 Methane % mol 31.3551 31.0198 34.3984 Acetylene % mol 0.0369 0.0013 0.3600 Ethylene % mol 5.4370 0.8057 47.4753 Ethane % mol 0.4784 0.0092 4.7369 Propylene % mol 0.0012 0.0000 0.0124 Temperature ° C -93.00 -95.00 -98.30 Pressure abs bar 35.00 34.70 2.40 Molar flow Kmol / h 121,788 109.703 12,086 Gas to be purified (line 1) Purified gas (line 14) Separate gas (line 11) Molar composition Hydrogen % mol 62.3100 67.4592 18.7556 Carbon monoxide % mol 0.3814 0.4066 0.1682 Methane % mol 31.3551 31.3225 31.6311 Acetylene % mol 0.0369 0.0012 0.3385 Ethylene % mol 5.4370 0.8021 44.6420 Ethane % mol 0.4784 0.0085 4.4529 Propylene % mol 0.0012 0.0000 0.0116 Temperature ° c -93.00 -95.00 -99.39 Pressure abs bar 35.00 34.70 2.40 Molar flow Kmol / h 121,788 108,912 12,876 Gas to be purified (line 1) Purified gas (line 14) Separate gas (line 11) Distillate (line 12) Molar composition Hydrogen % mol 62.3100 67.4592 9.1827 0.0000 Carbon monoxide % mol 0.3814 0.4066 0.1097 0.0000 Methane % mol 31.3551 31.3225 34.4823 50.0000 Acetylene % mol 0.0369 0.0012 0.3332 0.0000 Ethylene % mol 5.4370 0.8021 51.4970 50.0000 Ethane % mol 0.4784 0.0085 4.3836 0.0000 Propylene % mol 0.0012 0.0000 0.0115 0.0000 Temperature ° c -93.00 -95.00 -99.19 -92.00 Pressure abs bar 35.00 34.70 2.40 17.80 Molar flow Kmol / h 121,788 108,912 13,076 2,000 Designation Stream # Temperature ° C Pressure bar abs. Kmol / h flow Gas to be purified 1 -93.00 35.00 121,788 Refrigerated gas to be purified 2 -100.00 34.90 121,788 Gas to be purified supplying E1 3 -93.00 35.00 121,788 B1 liquid 4 -101.47 34.90 10,086 Purified gas 5 -120.35 34.80 111.703 Purified gas injected into B2 6 -120.35 34.80 0,000 Purified gas reintroduced into E1 7 -120.35 34.80 111.703 Refrigerated B1 liquid 8 -120.00 34.80 10,086 B2 feeding 9 -136.57 1.90 10,086 Purified gas 10 -136.57 1.90 10,086 Purified gas heated 11 -100.00 1.80 10,086 Distillate 12 0,000 Chilled distillate 13 0,000 Purified gas heated 14 -95.00 34.70 111.703 Gas to be purified injected into B1 15 -93.00 35.00 0,000 Designation Stream # Temperature ° C Pressure bar abs. Kmol / h flow Gas to be purified 1 -93.00 35.00 121,788 Refrigerated gas to be purified 2 -100.00 34.90 121,788 Gas to be purified supplying E1 3 -93.00 35.00 121,788 B1 liquid 4 -101.47 34.90 10,086 Purified gas 5 -120.35 34.80 111.703 Purified gas injected into B2 6 -120.35 34.80 0,000 Purified gas reintroduced into E1 7 -120.35 34.80 111.703 Refrigerated B1 liquid 8 -120.00 34.80 10,086 B2 feeding 9 -136.57 2.50 12,086 Purified gas 10 -136.57 2.50 12,086 Purified gas heated 11 -100.00 2.40 12,086 Distillate 12 0,000 Chilled distillate 13 0,000 Purified gas heated 14 -95.00 34.70 109.703 Gas to be purified injected into B1 15 -93.00 35.00 0,000 Designation Stream # Temperature ° C Pressure bar abs. Kmol / h flow Gas to be purified 1 -93.00 35.00 121,788 Refrigerated gas to be purified 2 -100.00 34.90 101,788 Gas to be purified supplying E1 3 -93.00 35.00 101,788 B1 liquid 4 -96.75 34.90 9.576 Purified gas 5 -120.30 34.80 112,212 Purified gas injected into B2 6 120.30 34.80 3,300 Purified gas reintroduced into E1 7 -120.30 34.80 108,912 Refrigerated B1 liquid 8 -120.00 34.80 9.576 B2 feeding 9 -137.56 2.50 12,876 Purified gas 10 -137.56 2.50 12,876 Purified gas heated 11 -99.39 2.40 12,876 Distillate 12 0,000 Chilled distillate 13 0,000 Purified gas heated 14 -95.00 34.70 108,912 Gas to be purified injected into B1 15 -93.00 35.00 20,000 Designation Stream # Temperature ° C Pressure bar abs. Kmol / h flow Gas to be purified 1 -93.00 35.00 121,788 Refrigerated gas to be purified 2 -100.00 34.90 101,788 Gas to be purified supplying E1 3 -93.00 35.00 101,788 B1 liquid 4 -96.75 34.90 9.576 Purified gas 5 -120.30 34.80 112,212 Purified gas injected into B2 6 -120.30 34.80 1,500 Purified gas reintroduced into E1 7 -120.30 34.80 110,712 Refrigerated B1 liquid 8 -120.00 34.80 9.576 B2 feeding 9 -137.56 2.50 13,076 Purified gas 10 -136.87 2.50 13,076 Purified gas heated 11 -99.19 2.40 13,076 Distillate 12 -92.00 17.76 2,000 Chilled distillate 13 -120.00 17.66 2,000 Purified gas heated 14 -95.00 34.70 110,712 Gas to be purified injected into B1 15 -93.00 35.00 20,000

Claims (10)

Self-cooled process for cryogenic fractionation and purification of a gaseous fluid supplying at least two condensable components of different condensation temperatures, respectively at least one component relatively heavy to remove and at least one component relatively light to recover so as to produce a purified gas preferably comprising the relatively light component (s) and a separate gas preferably comprising the relatively heavy component (s), characterized in that one operates in a zone d heat exchange forming a unitary assembly and comprising at least five distinct circuits, designated respectively by first, second, third, fourth and fifth circuits, in indirect heat exchange relation with each other at each level of the exchange zone thermal, generally vertical, the first circuit, or reflux circuit, being arranged essentially in a e upper and relatively cooler portion of the heat exchange zone, and the fifth circuit being arranged essentially in a lower and relatively less cold portion of the heat exchange zone, process in which at least a fraction of the fluid is circulated gaseous supply generally from bottom to top in the fifth circuit under conditions such that it can condense in part to give a first condensate and that this first condensate is entrained without substantial reflux by said gaseous fluid, the resulting mixture is discharged uncondensed gas and the first condensate from the top of the fifth circuit, said uncondensed gas is separated from said first condensate, in a phase separation zone, the gas thus separated is circulated generally from bottom to top, in the first circuit, or reflux circuit, in conditions such that a part of the gas can give a second condensate and that this second condensate can flow back into said first circuit and be collected at its base, at least part of the uncondensed gas discharged from the top of the first circuit is circulated, globally from top to bottom in the second circuit, against the current of the fluid circulating in the first circuit then of the fluid circulating in the fifth circuit and the resulting purified gas is discharged, the first condensate and the second condensate are circulated generally from bottom up in at least a third circuit to undergo sub-cooling there, discharge from the top of the (at least one) third circuit the first and second resulting sub-cooled condensates, they are expanded and they are circulated globally from the top downwards in at least a fourth circuit where they vaporize by taking heat from the fluids of the first, third and fifth circuits, finally discharging said vaporized condensates from the bottom (at least one) fourth circuit, these vaporized condensates constituting the separated gas. Process according to claim 1, in which the operation is carried out under conditions such that the purified gas contains less than 1 mol% of the relatively heavy components and that the separated gas contains at least 30 mol% of said relatively heavy components. Method according to claim 1 or 2, in which a fraction of 90 to 98 mol% of the non-condensed gas, discharged from the top of the first circuit, is put into circulation in the second circuit, and another fraction of said gas representing from 2 to 10 mol% of said non-condensed gas is expanded and circulated, after expansion, in the heat exchange zone in the overall direction from top to bottom in admixture with the first condensate or the second condensate or both to allow a higher pressure vaporization of said condensate (s). Method according to one of Claims 1 to 3, in which a fraction of 5 to 20 mol% of the gaseous supply fluid does not pass through the fifth circuit and is sent directly to said phase separation zone. Method according to claim 4, in which the portion of gaseous supply fluid sent directly to the phase separation zone is varied in response to variations in the composition of said gaseous supply fluid, so as to maximize the quantity of purified gas , obtained by the second circuit. Method according to one of claims 1 to 5, in which a liquid phase is added, of external origin to the heat exchange zone, during the start of the installation to facilitate its cooling under conditions where this liquid phase can evaporate after expansion and pass through the heat exchange zone from top to bottom. Method according to one of claims 1 to 6, in which 2 to 20 mol% of the gaseous supply fluid is condensed in the fifth circuit. Heat exchanger allowing self-refrigerated purification, at reflux, of gas, for the implementation of the method according to one of claims 1 to 7, characterized in that it comprises at least five separate circuits, generally vertical, designated respectively by first, second, third, fourth and fifth circuits (C1, C2, C3, C4, C5), in exchange relationship indirect thermal with each other at each level of said exchanger, said circuits forming a unitary assembly, the first circuit (C1) being of non-tortuous type and the fifth circuit (C5) of tortuous type, the first circuit being arranged at a level higher than that of the fifth circuit, at least one direct junction (7) between the top of the first circuit and the top of the second circuit, at least one junction through an expansion means (V1) between the top of the third circuit and the top of the fourth circuit, at least one phase separation zone (B1) connected by its upper part to the base of the first circuit, by its lower part to the base of the third circuit and laterally at the top of the fifth circuit. Exchanger according to claim 8, in which the first circuit is superimposed on the fifth circuit. Exchanger according to claim 8 or 9, in which at least part of the circuits is of the multi-channel type.

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