METHOD FOR EXTRACTING ALCOHOLS FROM AN INITIAL MIXTURE COMPRISING ALCOHOLS IN AQUEOUS PHASE

Abstract

The invention relates to a process for extracting alcohols from an initial mixture (1) comprising alcohols in aqueous phase, with: a distillative separation by an isopropanol-butanol distillation column (III) equipped at the top with a condenser (c3), intended for separating said mixture into a water-isopropanol azeotrope stream (6) at the top and into a water-butanol azeotrope stream (7) at the bottom, a distillative separation intended for separating the water-butanol azeotrope stream into water and butanol, by a hetero-azeotropic distillation system comprising at least one water-recovering water column (IV) and at least one butanol (9)-recovering butanol column (V) equipped with a reboiler (r5),
such that a heat transfer is performed from the top water-isopropanol azeotrope stream (6) entering the condenser of the isopropanol-butanol distillation column (III), to the stream entering the reboiler of the butanol column (V).

Claims

1. A process for extracting alcohols from an initial mixture (1) comprising alcohols, including at least isopropanol and butanol, and optionally ethanol and/or acetone, in aqueous phase, said process comprising: performing a distillative separation by at least one distillation column referred to as isopropanol-butanol column (III) and equipped at the top with a condenser (c3), and intended for separating said initial mixture or a mixture deriving from said initial mixture into a water-isopropanol azeotrope stream (6) at the top and into a water-butanol azeotrope stream (7) at the bottom, performing a distillative separation for separating the water-butanol azeotrope stream into water and butanol, using by a hetero-azeotropic distillation system comprising at least one water-recovery column referred to as water column (IV), and at least one butanol (9)-recovering column referred to as butanol column (V) and equipped with a reboiler (r5), and performing a heat transfer between the top water-isopropanol azeotrope stream (6) entering the condenser of the isopropanol-butanol distillation column (III) and the stream entering the reboiler of the butanol column (V).

2. The process as claimed claim 1, wherein the heat transfer is performed by a heat exchanger (c3+r5) which is common to the two columns (III, V), incorporating said condenser (c3) and said reboiler (r5).

3. The process as claimed in claim 1, wherein operation of the isopropanol-butanol distillation column (III) is regulated for maintaining a two-phase liquid/gas mixture in the column, especially by imposing a temperature of at least 120? ? C. and/or a pressure of at least 3 bar absolute.

4. The process as claimed in claim 1, wherein the pressure of the isopropanol-butanol distillation column (III) is regulated to a value of at least 3 bar absolute, especially at least 4 bar absolute, preferably of at most 10 bar absolute, especially of at most 7 bar absolute, especially between 4.5 and 6.5 bar absolute.

5. The process as claimed in claim 1, wherein the isopropanol-butanol distillation column (III) and the butanol column (IV) are operated at temperatures selected in such a way that the top water-isopropanol azeotrope stream (6) from the isopropanol-butanol distillation column (III) is at a temperature T1 at least 8? C., especially at least 10? C., greater than the temperature T2 of the stream exiting the bottom of the butanol column (V).

6. The process as claimed in claim 1, wherein the isopropanol-butanol distillation column (III) and the butanol column (V) are operated at temperatures selected in such a way that the top water-isopropanol azeotrope stream (6) from the isopropanol-butanol distillation column (III) is at a temperature T1 of at least 120? ? C., especially of at most 140? C.

7. The process as claimed in claim 1, wherein the initial mixture (1) also comprises ethanol, and in that the distillative separation which is performed by at least the isopropanol-butanol distillation column (III) is intended for separating said mixture or a mixture deriving from said initial mixture into a water-isopropanol-ethanol azeotrope stream (6) at the top and into a water-butanol azeotrope stream (7) at the bottom.

8. The process as claimed in claim 1, wherein the thermal transfer to the stream entering the butanol column (V) provides the entirety of the heat required to the stream entering the reboiler (r5) of the butanol column (V) for the operation of said column.

9. The process as claimed in claim 1, wherein the thermal transfer to the stream entering the butanol column (V) provides a portion of the heat required to the stream entering the butanol column (V) for the operation of said column, the heat complement being supplied by another heat source, especially by an external steam source.

10. The process as claimed in claim 1, wherein the thermal transfer to the stream entering the butanol column (V) provides more heat than that required to the stream entering the reboiler (r5) of the butanol column (V) for the operation of said column, and in that a cooler (c3) is added for evacuating the excess heat.

11. The process as claimed in claim 1, further comprising the following series of operations for separating the initial mixture (1) comprising alcohols, including at least isopropanol and butanol, and optionally ethanol, and also acetone, in aqueous phase: (a) a separation of said mixture, by distillation performed in a beer column (I) and intended for removing at least a portion of the water from the aqueous phase, to give a concentrated mixture (2), (b) a separation of the concentrated mixture obtained from step (a), by distillation performed in a column referred to as acetone column (II), and intended for at least partly separating the acetone from said concentrated mixture (2), to give an acetone-depleted concentrated mixture (5), (c) the distillative separation performed by at least one column referred to as isopropanol-butanol distillation column (III) and intended for separating the acetone-depleted concentrated mixture obtained in step (b) into a water-isopropanol or water-isopropanol-ethanol azeotrope stream (6) at the top and into a water-butanol azeotrope stream (7) at the bottom, (d) the distillative separation intended for separating the water-butanol azeotrope stream obtained in step (c) into water and butanol, performed by a hetero-azeotropic distillation system comprising at least one water-recovery column referred to as water column (IV), and at least one butanol-recovering column referred to as butanol column (V).

12. The process as claimed in claim 1, wherein the initial mixture (1) comprising alcohols, including at least isopropanol and butanol, and optionally ethanol and/or acetone, in aqueous phase is a must obtained by fermentation of sugary liquors, especially derived from lignocellulosic biomass, under the action of microorganisms, especially obtained from a solvent-producing strain, preferably selected from at least one of the following microorganisms: bacteria, especially of the genus Clostridium, Escherichia coli, yeasts, especially of Saccharomyces cerevisiae type.

13. The process as claimed in claim 1, wherein the initial mixture (1) combines two or more mixtures of different compositions, each comprising one or more alcohols in aqueous phase, especially two or more different musts obtained by fermentation of sugary liquors.

14. A facility for extracting alcohols from an initial mixture (1) comprising alcohols, including at least isopropanol and butanol, and optionally ethanol and/or acetone, in aqueous phase, said facility comprising: a section for distillative separation performed by at least one distillation column referred to as isopropanol-butanol column (III) and equipped at the top with a condenser (c3), and intended for separating said initial mixture or a mixture deriving from said initial mixture into a water-isopropanol or water-isopropanol-ethanol azeotrope stream (6) at the top and into a water-butanol azeotrope stream (7) at the bottom, a section for distillative separation intended for separating the water-butanol azeotrope stream (7) into water and butanol, performed by a hetero-azeotropic distillation system comprising at least one water (8)-recovering column referred to as water column, and at least one butanol (9)-recovering column referred to as butanol column and equipped with a reboiler (r5), and a heat exchanger which is common to the isopropanol-butanol distillation column (III) and to the butanol column (V) and ensures a thermal transfer from the top water-isopropanol or water-isopropanol-ethanol azeotrope stream (6) from the isopropanol-butanol distillation column (III), entering the condenser (c3), to the stream entering the reboiler (r5) of the butanol column (V), said heat exchanger common to the two isopropanol-butanol distillation (III) and butanol (V) columns incorporating said condenser (c3) and said reboiler (r5), and optionally an additional condenser (c3) at the top of the isopropanol-butanol column (III) for evacuating the excess heat of the top water-isopropanol or water-isopropanol-ethanol azeotrope stream (6).

15. The facility as claimed in claim 14, comprising the following series of sections for separating the initial mixture comprising alcohols, including at least isopropanol and butanol, and optionally ethanol, and also acetone, in aqueous phase: (a) a section for separating said mixture, by distillation comprising at least one beer column (I) and intended for removing at least a portion of the water from the aqueous phase, to give a concentrated mixture, (b) a section for separating the concentrated mixture of step (a), by distillation comprising at least one column referred to as acetone column (II), and intended for separating the acetone from said concentrated mixture, to give an acetone-depleted concentrated mixture, (c) said section for distillative separation comprising at least one distillation column referred to as isopropanol-butanol column (III) and intended for separating the acetone-depleted concentrated mixture obtained in step (b) into a water-isopropanol or water-isopropanol-ethanol azeotrope stream at the top and into a water-butanol azeotrope stream at the bottom, (d) said section for distillative separation intended for separating the water-butanol azeotrope stream obtained in step (c) into water and butanol, comprising at least one hetero-azeotropic distillation system comprising at least one water-recovery column referred to as water column (IV), and at least one butanol-recovery column referred to as butanol column (V).

16. The process as claimed in claim 1, wherein operation of the isopropanol-butanol distillation column (III) is regulated for maintaining a two-phase liquid/gas mixture in the column by imposing a temperature of at least 120? C. and/or a pressure of at least 3 bar absolute.

17. The process as claimed in claim 1, wherein the pressure of the isopropanol-butanol distillation column (III) is regulated to a value of at least 3 bar absolute and at most 10 bar absolute.

18. The process as claimed in claim 1, wherein the pressure of the isopropanol-butanol distillation column (III) is regulated to a value between 4.5 and 6.5 bar absolute.

19. The process as claimed in claim 1, wherein the isopropanol-butanol distillation column (III) and the butanol column (IV) are operated at temperatures selected in such a way that the top water-isopropanol azeotrope stream (6) from the isopropanol-butanol distillation column (III) is at a temperature T1 which is at least 10? ? C. greater than the temperature T2 of the stream exiting the bottom of the butanol column (V).

20. The process as claimed in claim 1, wherein the isopropanol-butanol distillation column (III) and the butanol column (V) are operated at temperatures selected in such a way that the top water-isopropanol azeotrope stream (6) from the isopropanol-butanol distillation column (III) is at a temperature T1 of at least 120? C. and at most 140? ? C.

Description

LIST OF FIGURES

[0062] FIG. 1 represents an example of a facility for separating molecules of interest in aqueous solution, from the prior art.

[0063] FIG. 2 represents the facility of FIG. 1, modified according to a nonlimiting embodiment of the invention.

[0064] The references which are identical from one figure to the other represent the same streams, devices and heat exchanges.

[0065] These two figures are highly schematic: they are flow diagrams, which are not to scale. The facilities are represented in simplified form to make reading easier, especially in order to comprehend the devices/streams which the invention employs, without representing all of the devices, in the form of ovens, exchangers, coolers/compressors, column reboilers, column condensers, etc. which are effectively provided in an industrial facility of this type and are known to the skilled person.

[0066] In all of the figures: [0067] the numeric references denote fluid streams; the references in the form of Roman numerals denote the distillation columns, the references with letters the apparatuses; [0068] for clarity, the separating sections are represented with a single column in the form of a distillation column. However, it is clear that the separating sections may contain/do contain a plurality of columns, mounted in series and/or in parallel, and that they may contain other separating devices in order to fulfil the role of at least one of the distillation columns, for example one or more liquid/gas separators in the form of tanks, etc.

DESCRIPTION OF EMBODIMENTS

[0069] The objective of the invention, in the following, nonlimiting example, is to recover the majority products, namely isopropanol on the one hand and n-butanol on the other, from an aqueous solution which also contains two other, minority components, namely acetone, considered here to be an impurity for removal, and ethanol, which is selected here for retention with the isopropanol.

[0070] The composition of the initial must (entering the beer column), as indicated earlier on above, is as follows: [0071] total concentration of isopropanol I and butanol B: 8 to 30 g/L [0072] mass ratio I/B (majority products, isopropanol/butanol): between 0.25-0.5/0.75-0.5 [0073] total concentration of minority products, acetone a and ethanol e: 0.1 to 2 g/l.

[0074] The fermentation must from which the invention starts, by way of nonlimiting example, has the following composition and following features: [0075] concentration of isopropanol I, butanol B, ethanol e and acetone a in water: 19.6 g/l [0076] weight ratio I/B/e+a=40.8/56.1/3.1 [0077] concentration of acetone a approximately double that of ethanol e.

[0078] This aqueous solution is a fermentation must obtained from a liquor composed of C5 and/or C6 sugars and fermented under the action of microorganisms derived from a Clostridium genus strain, in a known way, for example according to batch or continuous processes set out in the preamble. For further details of the fermentation process itself, reference may be made to the scientific publication cited for the batch process.

[0079] The composition of the stream entering the isopropanol-butanol column is especially as follows: [0080] water: 35 to 45 weight % [0081] the remainder consisting of IBea [0082] the mass ratio r I/B (majority products, isopropanol/butanol): 0.25-0.5/0.75-0.5 [0083] concentration of acetone a: 0.1 to 1 g/l [0084] concentration of ethanol e: 2 to 10 g/l

[0085] There are two possible uses for the isopropanol obtained: [0086] it may be converted into propylene. Indeed, the hydrated isopropanol product obtained (with ethanol as minority product) with the process and facility according to the invention is a raw material suitable for conversion into propylene, as it leads to the production of a hydrated isopropanol fraction (with a little ethanol), since the isopropanol exhibits an azeotrope with water that is impossible to entirely separate solely by distillation. [0087] it may also be used as a solvent, following removal of the water. In that case, an additional dehydration process must be considered in order to remove the water from the isopropanol/water azeotrope. Such processes include distillation with third substances (benzene, cyclohexane, etc.), pressure swing distillation (PSD), which is a term denoting distillation at two different pressures, temperature swing adsorption (TSA), which is a term denoting adsorption modulated by temperature, or pressure swing adsorption (PSA), which is a term denoting adsorption modulated by pressure, or pervaporation.

[0088] The butanol obtained (n-butanol in the present case), for its part, is produced in quasi-pure form in the process and the facility according to the invention. Quasi-pure refers to a butanol content of at least 98% by weight, especially of at least 98.5% or 99% in the liquid phase in question (or an impurities content of at most 2%, especially of at most 1.5% or 2% by weight).

[0089] It is possible, in the present text, for conciseness, to denote the mixture of isopropanol, butanol, ethanol and acetone in water by the term IBea mixture or simply by the term IBea.

EXAMPLES

Example 1 (Comparative)

[0090] This example corresponds to the implementation of the facility according to FIG. 1.

[0091] The separation facility represented in FIG. 1 will be described upstream to downstream, understanding these terms according to the overall flow direction of the fermentation must and of the extracted products through the facility.

[0092] The must 1 arrives from the fermentation section (not represented) with a concentration of IBea mixture of approximately 8 to 30 g/L and a temperature of between 34 and 37? C.

[0093] The first column I, usually called a beer column, preconcentrates the must 1 to approximately 60 weight % of IBea, emerging at the top of the column in the form of a stream 2 of concentrated IBea mixture, and removes approximately 97% to 99% of the incoming water at the bottom of the column. This water 3 at the bottom of the column I contains from 50 to 1000 ppm by weight of IBea and is called Vinasse. Conventionally, this column I operates substantially at atmospheric pressure, and may also be operated at a maximum of 3 bar absolute. It has between 10 and 20 theoretical plates. It represents approximately 70% to 90% of the steam consumption of the overall separation process (this term refers to the process implemented in a facility as represented in FIG. 1). Depending on its pressure, the column I operates in a temperature range of approximately 100 to 140? C. (apart from condenser). It is possible to place a feedstock/effluent exchanger on this column I in order to preheat the feedstock (the must 1). This feedstock, initially at 34-37? ? C., returns to the column I, after being passed via the feedstock/effluent exchanger el, at a temperature of between 70 and 85?C.

[0094] The role of the second column II, called the acetone column, is to remove the acetone, which is considered here to be an impurity. Conventionally, the column operates substantially at atmospheric pressure, and may also be operated at a maximum of 3 bar absolute. It comprises between 30 and 50 theoretical plates. It represents approximately 2% to 6% of the steam consumption of the overall separation process. The stream 2 enters the column II. Leaving the column are the acetone stream 4 at the top and the acetone-depleted stream 5 at the bottom.

[0095] The third column III, called the isopropanol-butanol column, produces at the top the isopropanol/water azeotrope, being the stream 6, which also comprises the small amount of ethanol entering as a minority compound with the must. The composition of this azeotrope is between 11 and 15 weight % of water. At the bottom of the column, a stream is obtained which has a composition close to that of the water/n-butanol azeotrope, i.e., a composition of approximately 50%/50% (weight), this being the stream 7. Here, this column III operates at atmospheric pressure, and the column has a pressure at the top of 1.5 bar absolute. It comprises between 30 and 70 theoretical plates; in this example, there are 50. Under these pressure conditions, a three-phase vapor/liquid/liquid zone is observed between plates 27 and 50, beginning the numbering of the plates at the condenser c3, with plates specific to the three-phase operating mode of the column. This column III represents approximately 7% to 15% of the steam consumption of the overall separation process.

[0096] The last two columns, IV and V, are coupled: they constitute a hetero-azeotropic distillation system, the principle of which is known, applied to the water/n-butanol binary system. At the bottom of the water column IV, water (stream 8) contained in the hetero-azeotrope is obtained, and at the bottom of the butanol column V, n-butanol (stream 9) is obtained with a purity which may range from 98 to 99.5 weight %. The two columns operate substantially at atmospheric pressure, and may go up to 2 bar absolute. The two columns IV and V have between 7 and 15 theoretical plates. The water column IV represents 1% to 3% of the steam consumption of the overall separation process, and the butanol column V represents 4% to 8% of the overall separation process.

[0097] All of the columns I to V are equipped, in a manner known for distillation columns, [0098] with a condenser c1, c2, c3, c4 at the column top, the last condenser c4 being common to the columns IV and V, [0099] and with a reboiler r1, r2, r3, r4, r5 at the bottom of the column, for heating the bottom stream of the column. In a known manner, these reboilers may be selected from the following reboilers: vertical thermosiphon reboilers, kettle-type reboilers, oven reboilers, or else once-through reboilers, [0100] and optionally with a reflux tank b1, b2, b3, b4 downstream of the condensers c1 to c4.

[0101] The steam consumption of the overall separation process as described varies between 12 and 50 MJ of steam/kg of IBea. It depends on the IBea concentration of the initial feedstock 1 (that is, of the must recovered after fermentation), and also on various design choices for the facility (working conditions of column operation, column design in terms of number of plates, positioning of feeds, etc.).

[0102] In terms of energy consumption of the isopropanol column III and butanol column V: [0103] the condenser c3 of the isopropanol column III is required to evacuate 1.43 MJ/kg IBea of available heat at a temperature of the order of 80? C., [0104] the reboiler r3 of this same column III consumes 1.48 MJ/kg IBea of heat in the form of reboiling steam, the butanol column V consumes 0.9 MJ/kg IBea of heat in the form of reboiling steam (reboiler r5). This heat is required at a thermal level of approximately 120? C. at the operating pressure under consideration.

Example 2 (According to the Invention)

[0105] This example corresponds to the implementation of the facility according to FIG. 2, which modifies and improves the facility of FIG. 1. All other things being equal, the only features detailed/specified are those which differ from the facility scheme of FIG. 1.

[0106] According to this example 2, a thermal transfer is added from the isopropanol-butanol column III to the butanol column V. To accomplish this, the pressure of the column III is increased: rather than operating at atmospheric pressure, a choice is made to operate the column under pressure, between 4 and 7 bar absolute, and more specifically, here, at around 5.5 bar absolute (measured at the top of the column). This increase in pressure leads to an increase in the operating temperatures of the column III. Thus the temperature at the top of the column increases to at least 120?C, especially up to 125? ? C. to 130? ? C., whereas with the standard operation at close to atmospheric pressure, according to example 1, the same temperature is only of the order of 80? C. At the top of the column, it is necessary to evacuate the heat energy, and, with this increase in temperature, the amount of heat energy to be evacuated from the top effluent in the condenser c3 of the column III is close to, or even a little greater than, the amount of heat energy to be supplied at the reboiler r5 of the butanol column V, which operates at around 120? C. at atmospheric pressure.

[0107] By increasing the temperature at the top of the column III, the heat evacuated at the condenser c3 of this column III can be transferred to the reboiler r5 of the butanol column, which operates at approximately 120? C. at a pressure close to atmospheric, via a new thermal connection T. Consequently only one exchanger is needed, common to the two columns, making up both the condenser c3 of the column III and the reboiler r5 of the column V, which can be assembled into a single apparatus c3+r5. Accordingly, two apparatuses (condenser of the isopropanol-butanol column III and reboiler of the butanol column V) are replaced with a single heat exchanger, and it is possible to omit the steam production means associated with the reboiler r5 of the column 5.

[0108] Where the amount of heat to be evacuated at the column III is not strictly equal to the amount of heat to be supplied to the column V, there are a number of options: [0109] if the amount is greater, an additional condenser c3 (for example, a cooler of the type referred to as a trim cooler) is added at the top of the column III, which will be smaller than the initial condenser c3 at the top of the column, as it is required to evacuate less heat than the condenser c3 of example 1. Example 2 and FIG. 2 represent this scenario. The entirety of the top stream 6 from III enters the common exchanger c3+r5, incorporating the condenser c3 of the column III and the reboiler r5 of the column V. The hot effluent emerging from common this exchanger enters the additional condenser c3. At the exit from this condenser c3, the stream is divided into 2: one portion leaves again in reflux in the column III; the remaining portion, the distillate, emerges from the process, being a product. [0110] if the amount is less, it is necessary to provide a steam top-up at the reboiler r5 of the column V, and so to use a heating means (not represented in the figure) to produce steam; however, this top-up remains significantly less than the amount of steam needed without this thermal transfer between the two columns; it will therefore be possible to use a much smaller heating means (oven type), with a heating capacity much lower than in the case of example 1.

[0111] Surprisingly, it has emerged that increasing the pressure of the column III provides another advantage: at atmospheric pressure, there is a liquid/liquid separation zone in this column, owing to the presence of the ternary isopropanol/n-butanol/water system. This type of three-phase liquid/liquid/vapor zone generally requires the installation of specific column internals, which are more expensive and more complicated to implement than conventional distillation trays. With the increased pressure according to the invention, the three-phase separation zone disappears from the column, and column the can therefore be operated with conventional internals such as valve trays or perforated trays, bulk or structured packings, thereby making it possible to reduce the facility investment further.

[0112] In terms of energy consumption of the isopropanol-butanol column III and butanol column V: [0113] the condenser c3 of this third column III is required to evacuate 1.44 MJ/kg IBea of heat, available at a temperature of 129.5? C.; this rise in temperature relative to example 1 is a result of the rise in operating pressure of the column.

[0114] The fifth, butanol column V requires a reboiling heat of 0.9 MJ/kg IBea in the form of steam, at a thermal level of 119.5? C. The thermal transfer between the condenser of the column III and the boiler of the column V has therefore become possible. Here, it will be necessary to evacuate 1.44-0.9=0.54 MJ/kg IBea at the level of the condenser c3 of the isopropanol-butanol column III (rather than the 1.43 MJ/kg IBea according to example 1): the heat to be evacuated is less than in the case of example 1 and the apparatus will be smaller. Lastly, the heat to be supplied to the reboiler r3 of the isopropanol column III is 1.71 MJ/kg rather than 1.48 MJ/kg, corresponding to a rise of 0.23 MJ/kg (owing to the rise in pressure).

[0115] Table 1 below summarizes the energy data for the columns III and V according to example 1 and example 2, and details the calculation of the resultant energy gains:

TABLE-US-00001 TABLE 1 Example 1 Example 2 Gain (comparative) (invention) G1, G2 Isopropanol/butanol column III Top pressure (bar 1.5 5.5 abs.) Condenser c3 (MJ/kg 1.43 1.44 IBea) Reboiler r3 (MJ/kg 1.48 1.71 IBea) Butanol column V Top pressure (bar 1.15 1.15 abs.) Reboiler r5 (MJ/kg 0.9 0.9 IBea) Heat for evacuation 1.43 1.44 ? 0.9 = G2 = 1.43 ? (MJ/kg IBea) 0.54 0.54 = 0.89 Heat for provision 1.48 + 0.9 = 1.71 + 0.9 ? G1 = 2.38 ? (MJ/kg IBea) 2.38 0.9 = 1.71 1.71 = 0.67

[0116] By comparing these data on the energy operation of the columns III and V, it is verified that with the invention, gains are achieved both: [0117] in terms of energy for evacuating the heat from the column III, with a gain G2 in the heat for evacuation of 0.89 MJ/kg IBea, corresponding to a decrease in cooling the energy consumption of (0.89/1.43?100=) 62%; [0118] and in terms of energy for supply to the column V, with a gain G1 on the heat for supply of 0.67 MJ/kg IBea, corresponding to a decrease in the heating energy consumption of (0.67/2.38?100=) 28%.

[0119] It is therefore evident that, overall, the thermal transfer between the two distillation columns according to the invention ultimately produces considerable energy gains relative to conventional use of these two columns, in terms both of cooling (spectacular decrease of more than half in consumption) and of heating (significant decrease of at least 20%), without entailing any significant extra cost in terms of apparatus. With the condenser c3 and the reboiler r5 having been incorporated in a single apparatus and with the heat for evacuation at the additional condenser c3 (the trim cooler) being 62% lower, this additional condenser c3 of example 2 according to the invention is/can be markedly smaller than the condenser c3 of example 1. It should be noted that the addition of this additional condenser remains optional, according to the specific scenarios encountered, as is indicated earlier on above.