FR2557468A1 - Process for raising the heat level of the heat energy present in a heat transfer fluid and use of the resulting heat energy for heating a fluid - Google Patents

Abstract

Process for raising the heat level of the heat energy present in a heat transfer fluid 27. It consists in transferring said heat energy by mixing and/or heat exchange operations in 16 to at least one fluid 6 of a cycle for concentrating a liquid product, in which cycle the highest heat level is higher than that of the heat energy of the heat transfer fluid, the said concentration cycle comprising one or more successive evaporation stages 1, 2, the vapour produced in at least one evaporation stage 2 of the liquid product to be concentrated, called evaporation vapour, being compressed 24 before being employed as heating vapour for the evaporation stage from which the said evaporation vapour originates or for one of the preceding evaporation stages 1.

Description

 The present invention relates to a method for raising the thermal level of the calories contained in a heat transfer fluid.

Frequently, in industry, and especially in the dairy industry, heat transfer fluids are produced as effluents from various treatment plants. When these fluids contain calories at a high thermal level, they can serve as heat sources for various purposes, but when these calories are at a low thermal level, these cannot be used efficiently.
The object of the present invention is to remedy this drawback and to do this it proposes a method for raising the thermal level of said calories, this method being characterized in that it consists in transferring said calories by mixing operations and / or heat exchange to at least one fluid of a concentration cycle of a liquid product consisting of a solid or liquid matter and a diluent or a vaporizable solvent, cycle whose highest thermal level is higher than that of the calories of the heat transfer fluid, said concentration cycle comprising one or more successive stages of evaporation each carried out by indirect heat exchange between a heating vapor and said liquid product, exchange during which said heating vapor condenses and gives up its latent heat of vaporization to said liquid product to thereby obtain condensates and a concentrate, the vapor produced at at least one stage of evaporation of the liquid product at ncentre, called evaporation vapor, being compressed before being used as heating vapor for the evaporation stage from which said evaporation vapor originates or for one of the preceding evaporation stages.

 It will be readily understood that the calories at low thermal level transferred to the fluid of the concentration cycle, constituted in particular either by the liquid product intended to be concentrated in the evaporation stage from which the evaporation vapor to be compressed or in one previous evaporation stages, either by the condensates produced in the evaporation stage from which the evaporation vapor to be compressed or in one of the preceding evaporation stages are found in the evaporation vapor to be compressed and are therefore brought to the highest thermal level of said concentration cycle during the compression of this evaporation vapor. The thermal level of the calories of the heat-carrying fluid is thus raised without it being necessary to do this. use of a compressor external to the concentration cycle, but advantageously taking advantage of the compressor of the concentration cycle. Admittedly, this latter compressor will have to be more widely dimensioned and consume more energy than that normally required when there is no transfer of external calories in the concentration cycle, but the operating cost of the process according to the invention remains despite everything much lower than that of the processes known to date and using an additional compressor and distinct from the compressor of the concentration cycle. The transfer of calories at low thermal level to the fluid (s) of the concentration cycle can be carried out in different ways.

 It should be noted that in the case where calories are transferred from the heat transfer fluid to the concentration cycle fluid, by heat exchange, the latter can be produced by using an intermediate fluid.

 Thus, according to a first embodiment of the invention, in which the heat transfer fluid is a liquid vaporizable at a thermal level higher than that of the evaporation vapor to be compressed, this fluid is introduced into an indirect heat exchange zone of the concentration cycle, zone where condensates are produced and where the thermal level is higher than that of the evaporation vapor to be compressed, but lower than that of the heat transfer fluid.

 As a variant, an indirect heat exchange can be carried out between a heat transfer fluid and condensates which are less hot than said fluid, within the heat exchange zone where said condensates are produced and where the thermal level is higher than that of the vapor d evaporation to compress.

 Yet another variant of this first embodiment consists in extracting condensates which are less hot than the heat transfer fluid from the heat exchange zone where they are produced and where the thermal level is higher than that of the evaporation vapor to be compressed, subjecting part of it to an indirect heat exchange with the heat transfer fluid, then bringing the condensates which have been subjected to this heat exchange back into said zone, the thermal level of the latter being greater than that of the evaporation vapor to be compressed.

 In this first embodiment and its two variants, the calories at low thermal level are transferred to the liquid product in the course of evaporation to be finally found in the evaporation vapor to be compressed.

 According to a second embodiment, the method according to the invention consists in carrying out an indirect heat exchange between the heat transfer fluid and the liquid product to be concentrated less hot. It should be noted that before undergoing this heat exchange, the liquid product to be concentrated may be subjected to an indirect heat exchange with the condensates produced in at least one of the stages of evaporation. In this case, there is a direct transfer of calories at a low thermal level to the liquid product to be concentrated with additional heating of the latter by means of the condensates.

 A third embodiment of the invention consists in carrying out a first indirect heat exchange between the heat transfer fluid and condensates which are less hot than this fluid, the resulting condensates then being subjected to a second indirect heat exchange with the liquid product to be concentrated.

 Advantageously, before being subjected to the first heat exchange, the condensates are subjected to an indirect heat exchange with the liquid product to be concentrated resulting from the second heat exchange.

 According to a fourth embodiment, the method according to the invention consists in mixing the heat transfer fluid and condensates, then in carrying out an indirect heat exchange between the resulting mixture and the liquid product to be concentrated, the liquid product to be concentrated resulting from said heat exchange can, if desired, be subjected to an indirect heat exchange with the condensates before mixing with the heat transfer fluid.

 A fifth embodiment of the invention consists in that a fraction of the liquid product to be concti-filtered is subjected to an indirect heat exchange with the heat transfer fluid, another fraction of the liquid product to be concentrated is subjected to an indirect heat exchange with the condensates and in that the two fractions resulting from these exchanges are combined before being treated in the concentration cycle.

 A final embodiment of the invention consists in carrying out an indirect heat exchange between the liquid product to be concentrated and the hotter condensates, in subjecting a fraction of the cooled condensates resulting from this exchange, to an indirect heat exchange with the fluid warmer heat pump, then mix the condensates thus heated with those intended to be subjected to heat exchange with the liquid product to be concentrated.

 It is quite certain that the present invention applies regardless of the manner in which the evaporation of the liquid product to be concentrated is carried out. Thus, the different stages of evaporation can be arranged to lead to a single-effect evaporation system or to a multiple-effect evaporation system; in addition, each stage of evaporation can be of any type, such as for example of the falling flow type. Finally, the compression of the evaporation vapor can be of the mechanical compression, ejecto-compression or mixed type.

In the process which has just been described, the calories at low level
transferred to the concentration cycle can, depending on the calorie availability of the cycle, either fully participate in the concentration
tion of the liquid product to be concentrated; either participate only partially
or even not participate in the concentration at all. In the second hypo
thesis, the concentration cycle is excess in calories and it is then
it is possible to advantageously use this excess to heat a fluid.

Thus, the present invention further relates to a heating method which is
characterized in that it consists in transferring to this fluid calories taken from the concentration cycle implemented in the process described above. This direct debit and transfer will advantageously be obtained by
reading a heat exchange between said fluid to be heated and a fluid of the concentration cycle. Of course, this fluid will preferably be the one with the highest thermal level and it will be in particular compressed vapor of evaporation which can, if desired, be subjected to additional compression before being used for heating the fluid.

 It is quite common, especially in the dairy industry, to find close to one another, on the one hand an installation in which is implemented a concentration cycle of the type described above and d on the other hand a drying installation using a hot drying gas. Under these conditions, the fluid to be heated will advantageously consist of the gas intended to be used as drying gas in the drying cycle of a product to be dried used in said drying installation.

 The drying cycle will include in particular the preheating of a drying gas by means of the calories taken from the concentration cycle, the heating in a heater of the gas thus preheated, to the final temperature required for drying, the heat exchange direct between the resulting gas and the concentrated product resulting from the concentration cycle or any other product to be dried to obtain a dried product and a gaseous fluid comprising said drying gas and the solvent or diluent vapor coming from the product to be dried and the separation of this gaseous fluid of the dried product. According to an advantageous characteristic of the present invention, said gaseous fluid and / or the hot gases possibly created in the heater and / or a liquid to which the calories of said gaseous fluid and / or said gases have previously been transferred. hot will be used as a heat transfer fluid containing calories which it is desired to raise the thermal level in accordance with the process described above recently. This characteristic is particularly interesting since it makes it possible to carry out a concentration and drying operation with a minimum rejection of calories and, consequently, with a minimum consumption of energy.

The invention is illustrated in the following description, made with reference to the accompanying drawings in which
FIG. 1 represents an evaporation installation winning a concentration cycle for a liquid product, calories from the heat transfer fluid being transferred to the liquid product during concentration,
FIG. 2 represents an evaporation installation identical to that which is the object of FIG. 1, the calories of the heat-transfer fluid being transferred in the concentration cycle by indirect heat exchange of said fluid with the less hot condensates produced in said cycle,
FIGS. 3 to 6 illustrate other methods of transferring calories from the heat-transfer fluid in the concentration cycle of an evaporation installation, and
FIG. 7 represents a drying installation in which the drying air is preheated by calories taken from an evaporation installation with vapor compression in accordance with FIG. 4.

 The installation shown by way of example in FIG. 1 comprises, in a manner known per se, a multiple-effect, for example double-effect evaporator, in which a liquid product to be concentrated, such as milk for example, is evaporated in two stages successive. More precisely, this evaporator comprises two effects 1, 2 each consisting of a supply chamber 3, 4 provided with an inlet 5, 6 of product to be concentrated, this chamber being disposed above and communicating with a tube bundle 7, 8 vertical itself disposed above and communicating with an extraction chamber 9, 10 provided at its base with an extraction duct 11, 12 connected to the suction of an extraction pump 13, 14. The tube bundle 7, 8 is disposed in a heating body 15, 16 provided with an inlet 17, 18 for heating steam. Furthermore, the extraction chamber 9, 10 is provided at its upper part with a duct of steam 19, 20 opening into a vapor-liquid separator 21, 22, the separator 21 associated with effect 1 communicating through its upper part with the inlet 18 of steam and the separator 22 communicating through its upper part and via a conduit 23 with the suction of a mechanical compressor 24 whose delivery is connected by a conduit 25 to the of steam 17.

 The liquid product to be concentrated penetrates via the inlet 5 into the supply chamber 3 of effect 1, flows by trickling into the tubular bundle 7 where it undergoes a first evaporation (first stage of evaporation) and collects in the bottom of the extraction chamber 9 from which it is extracted by means of the conduit 11 and the pump 13 then sent via the conduit 26 connected to the inlet 6, in the supply chamber 4 of the second effect 2 The liquid product undergoes a new evaporation (second stage of evaporation) in effect 2 before being extracted therefrom by the conduit 12 and the pump 14. The vapor created by the evaporation in effect 1 is supplied via the steam duct 19, the separator 21 and the inlet 18 into the heating body 16 of effect 2 where it is subjected to an indirect heat exchange with the product flowing in the tubes of the tube bundle 8, product to which it yields its latent heat of vaporization by condensing to form condensa ts, the transfer of this latent heat causing the partial evaporation of the liquid product to be concentrated. The vapor produced by evaporation in effect 2 is sucked, via line 20, separator 22 and line 23, by compressor 24 where it is compressed to a pressure corresponding to the temperature prevailing in the heating body 15 before being brought into the latter by the conduit 25 and the inlet 17. The vapor thus compressed is subjected to an indirect heat exchange with the liquid product flowing in the tubes of the tube bundle 7, product to which it yields its latent heat of vaporization by condensing to give condensate.

 In accordance with the invention, the calories at low thermal level present in an available heat transfer fluid, are transferred to at least one fluid of the concentration cycle implemented in the installation described above, a cycle which essentially comprises the two stages of evaporation successive steps carried out in effects 1, 2 and the printing carried out in presser 24.

 This transfer of calories can be achieved by bringing said heat transfer fluid into a heating body where the thermal level is higher than that of the evaporation vapor to be compressed. Of course, in this case, the heat transfer fluid must be a vaporizable liquid having a thermal level higher than that of the heating body where it is introduced and the heat transfer fluid will advantageously be of the same nature as the condensates. In addition, the thermal level of said heat transfer fluid must be higher than that of the evaporation vapor to be compressed.

 FIG. 1 shows more precisely that the heat transfer fluid is brought by a conduit 27 into the heating body 16 of the effect 2 where it relaxes by producing steam which gives up its calories to the liquid product to be concentrated; these calories are found in the evaporation vapor which is supplied via the conduit 20, the separator 22 and the conduit 23 in the compressor 24 which thus raises the thermal level of said calories. The condensate / heat transfer fluid mixture is then extracted from the heating body 16 by a duct 28 and an extraction pump 29.

 It is quite certain that the invention is not limited to the concentration cycle described above. In fact, it is within the scope of the invention, any concentration cycle implemented in a single or multiple effect installation and with vapor compression.

 FIG. 2 illustrates another way of carrying out the transfer of calories at low thermal level from the heat transfer fluid (liquid or gaseous) to a fluid of the concentration cycle. This illustration was made on the evaporation installation object of Figure 1, but shown more schematically; however, it could have been done on any vapor compression evaporative installation.

 The means allowing said transfer consist of a surface heat exchanger 30 comprising a first circuit disposed between an inlet 31 of heat transfer fluid and an outlet 32 of cooler heat transfer fluid and a second circuit disposed between on the one hand an inlet 33 of condensates communicating with the discharge of a recovery pump 34, the suction of which communicates through the conduit 35 with the base of the heating body 16 and on the other hand an outlet 36 of heated condensates also leading to the base of the heating body 16 A bypass 37 is also provided between the pump 34 and the exchanger 30 to continuously discharge a quantity of condensate corresponding to that which has formed in the heating body 16. It will be understood that this type of transfer cannot be implemented with condensates less hot than the heat transfer fluid. In addition, the heated condensates coming from the exchanger 30 must be at a thermal level such that they undergo a e expansion by entering the heating body 16, the expansion steam thus produced giving up its calories to the liquid product to be concentrated. As in the case of FIG. 1, the calories of the heat transfer fluid are therefore transferred to the liquid product to be concentrated flowing in the tubular bundle of effect 2 in order to end up in the evaporation vapor to be compressed in the compressor 24 which falls their thermal level.

 As shown in Figure 3, the transfer of calories from the heat transfer fluid in the concentration cycle can also be achieved in an assembly comprising two heat exchangers per surface 38, 39. The exchanger 38 comprises a first circuit between an inlet 40 and an outlet 41 and a second circuit comprised between an inlet 42 and an outlet 43 and the exchanger 39 comprises a first circuit comprised between an inlet 44 and an outlet 45 and a second circuit comprised between an inlet 46 and an outlet 47. second circuit of the exchanger 38 is supplied by the inlet 42 with liquid product intended to be concentrated in an evaporation installation with vapor compression and more precisely in the evaporation stage from which the evaporation vapor to be compressed is derived. or in any one of the preceding stages of evaporation, liquid product which is heated in the exchanger 38 by condensates (taken from the concentration cycle of the evaporation installation at co vapor pressure) and entering said exchanger by the inlet 40. The outlet 43 is connected to the inlet 46 so that the liquid product heated in the exchanger 38 is brought into the exchanger 39 where the calories of a heat transfer fluid introduced into said exchanger 39 through the inlet 44, before being subjected to one or more stages of evaporation. Thus, if the vapor compression evaporation installation is constituted by those object of FIG. 1, the liquid product reheated in the exchangers 38 and 39 can in particular be that intended to be treated in effect 1, the outlet 47 then being connected to the arrival in liquid product which is provided with the supply chamber 3.

 FIG. 4 represents an indirect heat exchange assembly comprising three heat exchangers per surface 48, 49, 50. A liquid product to be concentrated is brought by the inlet 51 into the first circuit of the exchanger 48, then from the outlet 52 of this first circuit at the input 53 of the first circuit of the exchanger 50. Furthermore, condensates taken from a vapor compression evaporation installation, for example of the type of those object of FIG. 1, are brought by the input 54 into the second circuit of the exchanger 50, then from the output 55 of this second circuit to the input 56 of the first circuit of the exchanger 49 and finally from the output 57 of this first circuit at the input 58 of the second circuit of the exchanger 48 from which they are evacuated by the outlet 59. As for the input 60 of the second circuit of the exchanger 49, it is supplied with heat transfer fluid which, after having passed through this second circuit, in is evacuated through outlet 61. Thus, to the liquid product to be concentrated are transferred both calories from condensate and heat transfer fluid. The heated liquid product discharged from the first circuit of the exchanger 50 by the outlet 62 is then subjected to evaporation in the vapor compression evaporation installation (for example in effect 1 in the case of the installation of the figure 1).

 It should be noted that, as a variant, it is possible to eliminate the exchanger 49, to connect the outlet 55 to the inlet 58 by a pipe and to introduce the heat transfer fluid into this pipe, the resulting mixture then being subjected to heat exchange with the liquid product in the exchanger 48.

 The liquid product to be concentrated can also be divided into two fractions, one of the fractions being subjected to an indirect heat exchange with the condensates produced in a vapor compression vaporization installation, the other fraction being subjected to a heat exchange indirect with a heat transfer fluid, the two fractions then being combined again before being treated in said evaporation installation.

This is shown in FIG. 5 which represents an assembly comprising two heat exchangers per surface 63, 64. An inlet 65 of liquid product to be concentrated is extended by two conduits 66, 67, one 66 connected to the inlet 68 of the first circuit of the exchanger 63, the other 67 connected to the input 69 of the first circuit of the exchanger 64, the outputs 70 and 71 of the first circuits of the exchangers 63 and 64 both emerging in a conduit 72 bringing the liquid product in the evaporation installation. The second circuit of the exchanger 63 is in turn traversed by condensates from said installation, the second circuit of the exchanger 64 being for its part traversed by the heat transfer fluid.

 FIG. 6 illustrates the case where the heat exchange between the heat transfer fluid and the liquid product to be concentrated is carried out by means of an intermediate transfer fluid. The assembly represented by this FIG. 6 more precisely comprises two heat exchangers per surface 73, 74.

The condensates sampled on the concentration cycle are brought by the inlet 75 in the first circuit of the exchanger 73 and they are evacuated from this first circuit by the outlet 76. The second circuit of the exchanger 73 is included between an inlet 77 supplied with liquid product to be concentrated in the concentration cycle and an outlet 78 with heated liquid product intended to be treated in said concentration cycle. A fraction of the cooled condensates from the outlet 76 is removed by a pump 79 and then brought to the inlet 80 of the first circuit of the exchanger 74, the outlet 81 of this circuit being connected to the inlet 75. As for the second circuit of the exchanger 74, it is supplied with heat transfer fluid by the inlet 82, a fluid which after having given up its calories to the cooled condensates coming from the exchanger 73, is evacuated from this second circuit by the outlet 83. concentrates are thus transferred to both the calories of the condensates and of the heat transfer fluid.

 When, as a result of the transfer of calories from a heat transfer fluid in a vapor compression vaporization installation, this installation becomes excess in calories, this excess of calories can advantageously be used as a heat source to heat a fluid. illustrated in FIG. 7 which represents a drying unit. In this unit, a drying gas (air for example) is supplied to the inlet 84 of the first circuit of a surface heat exchanger 85, l input 86 of the second circuit of this exchanger being supplied with fluid withdrawn from a vapor compression vaporization installation in the concentration cycle from which calories at low thermal level of a heat transfer fluid have been transferred; it may be for example the installation object of Figure 2, the inlet 86 being in this case preferably supplied with steam taken from the conduit 25 of this installation. In the exchanger 85, said withdrawn steam gives up its latent heat of vaporization to the drying gas and condenses, the condensates being evacuated from the second circuit of the exchanger 85 by the outlet 87. As for the gas thus preheated and coming from the first circuit said exchanger, it is brought to its desired final temperature in a heater 88 before being introduced into a drying enclosure 89 supplied by the inlet 90 with product to be dried which may in particular consist of the concentrated liquid product obtained from the effect 2 of the installation in FIG. 2. The dried product is extracted at the base of the enclosure 89 by extraction means 91; the drying gas loaded with vapor of the solvent or diluent of the product to be dried is for its part discharged to a separator 92 where it is freed of the dried product which may be entrained. The resulting gas optionally mixed with the gases created in the heater 88 (hot gases from a heater in which there is combustion of a fuel and transfer of calories released by this combustion to the drying gas) is then used as heat transfer fluid of which the calories are intended to be transferred into the concentration cycle of the evaporation installation from which the calories were taken for the preheating of the drying gas. This transfer can be carried out in accordance with FIG. 2 (this is what shows FIG. 7) or again in accordance with FIGS. 3 to 6. As a variant, it is possible beforehand to transfer the calories from said heat transfer fluid to a liquid and then proceed to transfer the calories from the resulting heat transfer liquid, in the concentration cycle, for example in accordance to the techniques illustrated in FIGS. 1 to 6. It should also be noted that the vapor taken from the vapor compression evaporation installation can be subjected, before its heat exchange in the exchanger 85, to compression (in a mechanical compressor or in an ejector-compressor) so as to have an even hotter vapor for preheating the drying gas. It is also possible to carry out the operations illustrated in FIG. 7 and to further subject the drying gas coming from the exchanger 85 to an additional heat exchange also by means of steam taken from the evaporation installation (for example on the duct 25 of the installation of FIG. 2) but the thermal level of which has previously been raised by means of a mechanical steam compressor or by a steam ejector, the drying gas thus being gradually preheated in two stages successive ..

Claims (18)

 1. Method for raising the thermal level of the calories contained in a heat transfer fluid, characterized in that it consists in transferring said calories by mixing and / or heat exchange operations to at least one fluid of a concentration cycle of a liquid product consisting of a solid or liquid matter and a diluent or a vaporizable solvent, cycle whose highest thermal level is higher than that of the calories of the heat-transfer fluid, said concentration cycle comprising one or more stages of evaporation successive each carried out by indirect heat exchange between a condensable heating vapor and said liquid product, exchange during which said heating vapor condenses and gives up its latent heat of vaporization to said liquid product to thereby obtain condensates and a concentrate, the vapor produced at least one stage of evaporation of the liquid product to be concentrated, called evaporation vapor, being compressed before being used used as heating vapor for the evaporation stage from which said evaporation vapor originates or for one of the preceding evaporation stages, in such a way that the calories of the heat transfer fluid thus transferred are found in said evaporation vapor to be compressed and are therefore brought to a higher thermal level during the compression of this evaporation vapor.  2. Method according to claim 1, characterized in that the fluid of the concentration cycle consists either of the liquid product intended to be concentrated in the evaporation stage from which the evaporation vapor to be compressed originates or in one previous evaporation stages, either by the condensates produced in the evaporation stage from which the evaporation vapor to be compressed is derived or in one of the preceding evaporation stages.  3. Method according to claim 1 or 2, characterized in that it consists in transferring the calories from the heat transfer fluid to at least one fluid of the concentration cycle, by heat exchange by means of an intermediate fluid.  4. Method according to claim 2, wherein the heat transfer fluid is a liquid vaporizable at a thermal level higher than that of the evaporation vapor to be compressed, characterized in that it consists in introducing the heat transfer fluid in a zone of indirect heat exchange of the concentration cycle, area where condensates are produced and where the thermal level is higher than the thermal level of the evaporation vapor to be compressed, but lower than that of the heat transfer fluid.  5. Method according to claim 2, characterized in. what it consists in carrying out an indirect heat exchange between the heat transfer fluid and condensates which are less hot than said fluid, within the heat exchange zone where said condensates are produced and where the thermal level is higher than that of the vapor evaporation to be compressed.  6. Method according to claim 2, characterized in that it consists in extracting condensers less hot than the heat transfer fluid from the heat exchange zone where they are produced and where the thermal level is higher than that of the vapor of evaporation to be compressed, to subject part of it to an indirect heat exchange with the heat transfer fluid ,. then to bring back into said zone the condensates which have been subjected to this heat exchange, the thermal level of the latter being higher than that of the evaporation vapor to be compressed.  7. Method according to claim 2, characterized in that it consists in carrying out an indirect heat exchange between the heat transfer fluid and the liquid product to be concentrated less hot.  8. Method according to claim 7, characterized in that before being subjected to this heat exchange, the liquid product to be concentrated is subjected to an indirect heat exchange with the condensates produced in at least one of the stages of evaporation .  9. Method according to claim 2, characterized in that it consists in carrying out a first indirect heat exchange between the heat transfer fluid and condensates which are less hot than this fluid, the resulting condensates then being subjected to a second indirect heat exchange with the product. liquid to concentrate.  10. Method according to claim 8, characterized in that before being subjected to the first heat exchange, the condensates are subjected to an indirect heat exchange with the liquid product to be concentrated resulting from the second heat exchange.  11. Method according to claim 2, characterized in that it consists in mixing the heat transfer fluid and condensates, then in carrying out an indirect heat exchange between the resulting mixture and the liquid product to be concentrated.  12. Method according to claim 11, characterized in that the liquid product to be concentrated resulting from said heat exchange is subjected to an indirect heat exchange with the condensates before their mixing with the heat transfer fluid.  13. Method according to claim 2, characterized in that a fraction of the liquid product to be concentrated is subjected to an indirect heat exchange with the heat transfer fluid, another fraction of the liquid product to be concentrated is subjected to an indirect heat exchange with the condensates and in that the fractions resulting from these exchanges are combined before being treated in the concentration cycle.  14. Method according to claim 3, characterized in that it consists in carrying out an indirect heat exchange between the liquid product to be concentrated and the hotter condensates, in subjecting a fraction of the cooled con -densates resulting from this exchange, to an exchange indirect thermal with the warmer heat transfer fluid and then mixing the condensates thus heated with those intended to be subjected to thermal exchange with the liquid product to be concentrated.  15. A method for heating a fluid, characterized in that it consists in transferring to this fluid calories taken from the concentration cycle used in the method according to any one of the preceding claims.  16. Method according to claim 15, characterized in that it consists in carrying out a heat exchange between said fluid to be heated and any fluid of the concentration cycle.  17. The method of claim 15 or 16, characterized in that the fluid to be heated consists of the gas intended to be used as drying gas in the drying cycle of a product to be dried.  18. The method of claim 17 wherein the drying cycle comprises preheating a drying gas by means of the calories taken from the concentration cycle, heating in a gas heater thus preheated to the required final temperature for drying, the direct heat exchange between the resulting gas and the concentrated product resulting from the concentration cycle or any other product to be dried to obtain a dried product and a gaseous fluid comprising said drying gas and the solvent or diluent vapor coming from of the product to be dried and the separation of this gaseous fluid from the dried product, characterized in that said gaseous fluid, the hot gases possibly created in the heater and / or a liquid to which the calories of the gaseous fluid and / or said have previously been transferred hot gases are used as the heat transfer fluid for the process according to any one of the preceding claims.