FR2491768A1 - PROCESS AND INSTALLATION FOR RECOVERING SOLVENTS - Google Patents

Abstract

THE OBJECT OF THE INVENTION IS A PROCESS FOR RECOVERING SOLVENTS IN WHICH A CURRENT OF CARRIER GAS IS COOLED WHICH IS CHARGED WITH SOLVENT VAPORS IN AN EVAPORATION CHAMBER, TO CONDENSATE THE SOLVENT VAPORS AND SEPARATE THE SOLVENTS, AFTER WHAT. THE CARRIER GAS CURRENT LOW IN SOLVENT VAPORS IS TAKEN BACK TO THE EVAPORATION CHAMBER, AFTER HEATING IT AGAIN. THIS PROCESS IS CHARACTERIZED IN THAT THE CURRENT OF CARRIER GAS CHARGED WITH SOLVENT VAPORS IS COMPRESSED AND AFTER COOLING IT BY INDIRECT THERMAL EXCHANGE, IT IS EXPANDED WITH WORK SUPPLY, WHICH IS USED TO COMPRESS THE CHARGED CARRIER GAS CURRENT, AND IN USING THE LOW CARRIER GAS CURRENT IN SOLVENT VAPORS TO COOL, BY INDIRECT THERMAL EXCHANGE, THE CARRIER GAS CURRENT CHARGED WITH SOLVENT VAPORS.

Description

The invention relates to a method and an installation

for solvent recovery.

  In the known processes and systems described in, for example, "Ullmanns Enzydlopedie der technischen Chemie", Vol. 1 (1951), page 338, a carrier gas stream which has been charged with solvent vapors in a reactor chamber is cooled. evaporation, to condense the solvent vapors and separate the solvent, after which, once again, the low carrier gas stream is heated.

  solvent vapors, it is brought back to the evaporation chamber.

  The solvent vapors do not completely separate, by condensation, from the carrier gas stream; there is still a residual amount of solvent vapor in the latter which corresponds to the vapor pressure

  of solvent at the temperature of the coolant.

  To avoid solvent losses, the carrier gas stream is recycled. It is true that in this way, the solvent vapor absorption capacity of the carrier gas stream which is poor in solvent vapors is slightly reduced,

  but that is of no importance to the economics of the process.

  The process is quite suitable for

  separate volatile solvents from non-vaporizable substrates.

  One area of application is the removal of solvent residues contained in chemical substances

  that have been prepared or purified using solvents.

  Other fields of application are in the paints and varnishes sector, in the chemical cleaning of textiles sector, in the films and foils sector, in the rubber processing sector and in

  in the adhesives and adhesives sector.

  In known installations, separate coolers are generally provided for condensing the solvent vapors and devices are provided for heating the carrier gas stream low in solvent vapors, so that, on the one hand, quantities are required. - 2 - considerable amount of energy to reheat the carrier gas low in solvent vapors, This heating of the carrier gas stream is necessary, so that it it is possible, in the evaporation chamber, to rapidly charge again a sufficient quantity of solvent vapor,

  that is, so that the substrate is dried more quickly.

  One could try to obtain an energy saving by using the coolant that has heated through the cooler to heat the carrier gas stream low in solvent vapors, that is to say to do pass the cooling agent against the current of the carrier gas. However, it can easily be seen that, in this way, it is possible to return to the carrier gas stream only a small fraction of the heat previously borrowed from it in the cooler. Due to the relatively low temperature differences between the carrier gas and the coolant, the cooler and the carrier gas stream warmer must be provided with large heat exchange surfaces. Thus, the process is disadvantageous, not only because of its high energy consumption and coolant, but also because of its high expense.

fitting.

  In addition, DE 2 725 252 discloses a solvent recovery plant contained in a hot stream of carrier gas charged with solvent vapors, in which the gas is compressed to condense the solvent vapors and to separate the solvent. it is cooled and expanded with a supply of work, the carrier gas stream low in solvent vapors being brought back,

  after heating, to the evaporation chamber.

  However, the return of this carrier gas stream is carried out in a mixture with a charged carrier gas stream.

  solvent vapor removed from the evaporation chamber.

  This stream, after having been heated by indirect heat exchange with the stream of compressed carrier gas, is brought back by a pipe loop to the evaporation chamber together with the carrier gas stream low in solvent vapors. This is to ensure a better thermal regulation, but here we accept a disadvantage, namely that the carrier gas stream brought to the evaporation chamber has a relatively high content of solvent vapor. In this way, the drying effect in the evaporation chamber is decreased. Furthermore, in patent DE 2 725 252, it is indicated that the energy can be recovered.

  released during expansion in an expansion turbine.

  However, we do not indicate where this energy can be

used with profit.

  The object of the invention is to improve a method and an installation of the species defined above, so that with reduced equipment expenditure, it is possible, on the one hand, to take advantage advantageously of the energy obtained during the expansion of the carrier gas stream loaded with solvent vapors and compressed and that, on the other hand, the carrier gas stream brought back to the evaporation chamber

  as low as possible in solvent vapor.

  Thus, the subject of the invention is a solvent recovery process in which a stream of carrier gas is cooled, which is charged with solvent vapors in an evaporation chamber, for condensing the solvent vapors and separating the solvents, after which the stream of carrier gas low in solvent vapors is again brought back to the evaporation chamber, after having heated it again; this process is characterized in that the carrier gas stream charged with solvent vapors is compressed and, after being cooled by indirect heat exchange, is expanded with a work supply, which is used to compress - 4 - the stream of charged carrier gas, and in that one uses the carrier gas stream low in solvent vapor to cool, by indirect heat exchange, the current

  carrier gas charged with solvent vapors.

  Preferably, all of the energy obtained during expansion is transmitted to one of two or more compression stages with direct mechanical coupling to compress the carrier gas stream charged with solvent vapors. The invention also relates to an installation for carrying out this method, in which are connected, in a carrier gas circuit, an evaporation chamber where the stream of heated carrier gas is

  solvent vapor charge, compressor, coolant

  apparatus for condensing the solvent vapors by separating them from the carrier gas stream, an expansion machine, a solvent separator and a device for heating the carrier gas stream low in solvent vapors, said apparatus being characterized in that the compressor is mechanically coupled to the expansion machine and in that the cooler and the heating device constitute at least one heat exchanger traversed by the carrier gas stream which is low in vapors

of solvent.

  Preferably, the expansion machine is directly coupled mechanically to one of two or more compressors. In the plant according to the invention, the stream of carrier gas recycled into the evaporation chamber (usually a dryer) absorbs at a high concentration the evaporated solvent, which is again removed in the cooler, by cooling and condensation. In the known process, it is true that the carrier gas stream loaded with solvent vapors is compressed and relaxed after cooling with the supply of work, but the work of relaxation as work is not taken advantage of. compression in the system, while it is done according to the invention. Thus, it is only the difference between the work of compression and that of relaxation that must be provided to the compression stage or to one of the compression stages, from the outside, that is to say to using an additional external working machine, which is directly mechanically coupled to either the expansion machine, the compressor or one of the compressors. This difference covers the work necessary to separate the solvent vapor from the carrier gas stream as well as to overcome the losses (friction,

  transmission of cold to the environment). -

  As a result of the compression, the particle density in the carrier gas stream and solvent vapor mixture is increased. Thus, the efficiency of the heat exchanger is increased. As a result of the decreased gas volume, the heat exchanger can be kept small

  and other installation parts that are under pressure.

  Finally, during compression and expansion, no chemical influence, in particular oxidation, is exerted on the solvent vapors, unlike recovery processes in which adsorbents such as activated carbon are used. Such adsorbents can often act on solvent vapors with the formation of harmful decomposition products. Since the carrier gas stream is continuously stirred, these decomposition products would accumulate and react undesirably on the products to be dried or on the plant parts. A known case is the decomposition of chlorohydrocarbons on activated charcoal in the presence of steam with formation of hydrogen chloride. In the embodiment comprising two or more compression stages, these are preferably actuated.

  separately by energy supplied from outside, that is,

  say that the second compressor or the following ones are

  mechanically coupled to external working machines.

  This arrangement makes it possible to better control the compression process, in particular to better adjust the desired final pressure. In addition, the dimensions of the necessary transmission between the working machine and the compressor

may be limited.

  As an expansion machine, an expansion turbine is preferably used because it has a greater efficiency and can also be more easily coupled to the compressor or to one of the compressors than, for example, a piston machine. The extra work machine

  preferably constitutes an electric motor.

  The method according to the invention can be applied for example to the manufacture of thin adhesive material in which an adhesive is applied to strips or tapes of paper or textile. Ribbons of this kind can be used for example technical adhesive tapes or ribbons or tapes for medical purposes (eg adhesive bandages). For the application of the adhesive to the paper or textile web, the adhesive is brought to a fluid state by means of liquid solvents so that it can be applied in sufficiently thin and uniform layers. During drying, the solvent evaporates. For this purpose, the material to be dried remains in an evaporation chamber, for a time determined by the volatility and the amount of the solvent, in contact with the carrier gas.

which absorbs solvent vapors.

  The examples of embodiment indicated below concern

  facilities for this special application.

  However, the invention is also applicable successfully

  other areas mentioned above.

  Solvents for adhesives and many other applications generally employ solvents or solvent mixtures whose vapors are flammable. To recover such solvent vapors, a carrier gas is used according to the invention, the oxygen content of which is below the flammability limit. For this purpose, it is possible, for example, to use gases that are a priori inert, such as nitrogen or carbon dioxide; however, it is also possible to reduce the oxygen content of the air by mixing an inert gas to such an extent that the ignition limit is no longer reached. In some cases, it is also possible to use flue gases with a low

oxygen content.

  However, the flammability of the solvent vapor is not only a function of the oxygen content of the carrier gas but also depends on the concentration and nature of the solvent vapor. Thus, for example, the danger of ignition is greater in the case of hydrocarbons and low-boiling ethers than in the case of halohydrocarbons. However, the ignition properties of different solvent vapors are known and can be found in the literature or by simple experiments to determine the solvent vapor concentrations and oxygen levels that are permitted. The advantage of using an inert or oxygen-poor carrier gas stream is that the carrier gas stream can absorb a large amount of solvent vapor without any risk of explosion. In this way, the amount of carrier gas to be circulated can be kept low, so that the amount of energy required to cool or reheat can be reduced.

the carrier gas.

  The process according to the invention is not limited to the recovery of organic solvents; it is also possible to use mineral solvents such as ammonia and sulfur dioxide and also solvents which are between the mineral and organic solvents, such as carbon disulfide or carbon tetrachloride. Since these solvents (with the exception of carbon disulfide) are incombustible, it is not necessary, in these cases, to respect a given concentration of oxygen in the carrier gas, that is to say that one can, in the case

  the simplest, use air as a carrier gas.

  The control of the process according to the invention, in particular with a view to adapting the plant to different solvents or solvent mixtures, is possible in different ways. For example, the speed of the material to be dried can be varied as it is passed through the evaporation chamber. The most important control possibility is to vary the speed of the carrier gas stream. For this purpose, it is possible to vary the speed of rotation of the drive motor of the compressor or compressors. In addition, for this purpose, one can perform

  compressor control by bypass.

  A particularly simple possibility of adjusting the temperature of the carrier gas stream containing solvent vapors is to put it in indirect heat exchange with a coolant before, between and / or after the different stages of compression. For this purpose, between the evaporation chamber and the compressor or the first compressor, between the first and second compressors or the following and / or between the compressor or the last compressor and the expansion machine, it is possible to interpose an indirect cooler. . By adjusting the coolant flow in the cooler (s), the inlet temperature of the carrier gas stream charged with solvent vapors to the compressor (s) can be simply adapted to the needs of the particular case. following compressors and / or the expansion machine, as well as the inlet temperature of the poor carrier gas stream - 9 -

  in solvent vapor to the evaporation chamber.

  By the interposition of an additional indirect cooler between the compressor or the last compressor and the expansion machine, it is possible for the carrier gas stream low in solvent vapor to enter the evaporation chamber at a temperature of lower entry

and better adjustable.

  In general, the additional cooler is interposed before the heat exchanger traversed by the carrier gas stream which is poor in solvent vapors. However, before this cooler can be placed a heat exchanger (heat exchanger) and, after the cooler, a heat exchanger (exchanger "cold"). In this way, a lower temperature carrier gas stream reaches the chamber

evaporation.

  When the carrier gas stream charged with solvent vapors is cooled in the cooler, after its exit from the compressor or the last compressor, a portion of the solvent vapors can already be condensed, which depends, inter alia, on the temperature of the solvent stream. carrier gas low in solvent vapors used as coolant. For example, it is possible that it separates from water, since its boiling point is higher than that of many organic solvents. Although water is not used in solvent mixtures for conventional self-adhesives, water is still entrained in the system because it is adsorbed onto paper or textile webs it is used as a substrate for the adhesive material. It may even happen, in some cases, that the water freezes in the cold part of the heat exchanger or in the expansion machine and blocks the flow sections

  or damages the moving parts of the expansion machine.

  To cope with this danger, it is proposed to inject an aqueous solvent, in liquid form, into the stream

- 10 -

  cooled carrier gas prior to expansion. When the solvent is dissolved in the water, a solution with a freezing point lower than that of water is obtained.

remains liquid.

  If the cold solvent is not soluble in water, the water is deposited on the surface of the cold solvent droplets and, thus, it can not be deposited at the boundaries

solid flow paths.

  In terms of equipment, this measurement is made by the fact that between the heat exchanger and the expansion machine are provided devices for injecting the liquid solvent into the carrier gas stream. Preferably, for injection into the stream of cooled carrier gas, a portion of the condensed water-soluble solvent is used and is separated in the solvent separator. If it is not desired to adopt the injection of a liquid solvent or if there is a risk that the condensed liquid will not damage the moving parts of the expansion machine, for example the blade of the expansion turbine, a portion of the solvent vapors can be condensed and separated from the cooled carrier gas stream prior to expansion. For this purpose, it can be provided between the heat exchanger

  and the expansion machine an additional solvent separator

  mentary. Another possibility of adjusting the temperature of the inert gas stream is to relax again, without providing work, the carrier gas stream which has only partially relaxed by providing work; for this purpose, an expander can be provided before the evaporation chamber. This expander can be provided either at the inlet or at the outlet of the heat exchanger. With the aid of this regulator, it is also possible, for example, to regulate, so as to prevent icing in the

- he -

  pipes leading to the relaxation machine or in the

relaxing machine itself.

  When passing through the regulator, the carrier gas stream undergoes a slight cooling again, this time, without providing work. The stream of carrier gas thus expanded can then serve as a cooling gas of the carrier gas stream expanded with work supply, by indirect heat exchange, optionally after separation of the condensed solvent. For this purpose, between the expansion machine and the first solvent separator may be interposed an additional heat exchanger traversed by the carrier gas stream low in solvent vapors, the expander being then disposed immediately before this

heat exchanger.

  Some embodiments of the installation according to the invention are illustrated by the accompanying drawings, in which: FIG. 1 shows an installation comprising a single solvent separator and a single cooler; FIG. 2 represents an installation comprising a cooler and two solvent separators, as well as an additional heat exchanger between the first solvent separator and the expansion machine; FIG. 3 illustrates an installation comprising an additional cooler between the compressor and the expansion machine, this cooler being preceded by a heat exchanger and followed by a heat exchanger: FIG. 4 represents an installation comprising an additional cooler between the compressor and the expansion machine, this cooler being only followed by a heat exchanger; Figure 5 shows an installation in which the expansion machine is directly coupled to the second compressor, while the first compressor is coupled

- 12 -

  Figure 6 illustrates an installation in which the expansion machine is coupled directly to the first compressor, while the second compressor is coupled to an external drive motor. In the embodiment of FIG. 1, there is indicated by a strip of paper or textile provided with a coating of adhesive dissolved in a solvent. This strip moves in the direction of the arrow (thanks to not shown), through the evaporation chamber 12 shown schematically It is largely shielded, so that solvent vapors

  can not go out into the atmosphere.

  In the evaporation chamber, one introduces, against

  current of the paper web or textile0 a stream of hot carrier gas 14, poor in solvent vapors, for example a stream of nitrogenG Heating of this current

  carrier gas is carried out in the following manner.

  The stream of hot carrier gas 14 travels the evaporation chamber 12 upside-down the movement of the paper web or textile web and heats it to such an extent that the solvent contained in the solution

  of adhesive evaporates (cornîse indicated on the drawing by LM).

  The carrier gas is charged with solvent vapors and cools as a result of the heat of evaporation of the solvent. When the solvent (n) is used, the inlet temperature of the gas stream in the evaporation chamber 12 is, for example, 1400.degree. C. and the outlet temperature is about 10.degree. The outgoing carrier gas stream 16, charged with solvent vapors, then enters a cooler 18, which is traversed in indirect heat exchange by a coolant 20. The flow rate of the coolant and therefore the temperature of the carrier gas stream 16 charged with solvent vapors can be regulated by means of a valve

- 13 -

  In this example, the throttle valve 22 is adjusted in such a way that the carrier gas stream exiting the cooler 18 has a

  temperature of approximately 34WC, while the cooling agent

  The dye 20 heats from about 12 ° C to about 650 ° C. To further control the temperature of this carrier gas stream, a pressure reducer 38 may be provided. When the carrier gas stream passes through this expander, there is a new cooling without labor supply. The temperature of the system can therefore be regulated simply not only by the throttle valve 22, but also by the expander 38. With these two valves, without interposing other adjustment devices, the system can be adapted to the control systems. various solvents. The cooling of the carrier gas stream, which occurs at the expander 38, serves, in the variant of Figure 2, to cool the carrier gas stream after the expansion turbine 30, which will be discussed again

more precisely below.

  The cooled carrier gas stream then enters the compressor 24, where it is compressed to a coefficient of about 2.5, with a temperature rise of about 1400C. After the compressor, the carrier gas stream 16 enters the heat exchanger 26, in which it cools to about -10 ° C. in the example, in indirect heat exchange with the carrier gas stream 14 which is low in solvent vapors. . In the heat exchanger 26, a part of the solvent vapor is already condensing and the mixture designated 28, formed of a carrier gas stream partially charged with solvent vapors, liquid particles of solvent and possibly ice particles, can be driven to the machine of

  Expansion 30 designed as an expansion turbine.

  However, preferably, a preliminary separator (not shown, similar to the separator 34) is interposed

- 14 -

  to remove liquid and solid particles. In the turbine 30, as a result of the work provided, there is a new cooling of the carrier gas stream and the mixture designated 32, formed of a carrier gas stream low in solvent vapors, liquid particles of solvent and optionally of ice particles, arrives at the solvent separator 34 in which

the mixture 32 separates.

  The compressor 24 and the expansion turbine 30 are preferably located on a common shaft which is driven by a motor 36. Thus, the work obtained in the expansion turbine 30 can be used, virtually without losses, to compress the flow of carrier gas 16 charged with solvent vapors in the compressor 24. The engine 36 is the only source of energy of the system. The stream of carrier gas 14 low in solvent vapor which leaves the solvent separator 34 has in the embodiment considered, a temperature of about -400C and runs through the heat exchanger 26 in indirect heat exchange with the current carrier gas 16 charged with solvent vapors. Here, the first is heated to about 1400C, that is to say at a temperature necessary to evaporate

  the solvent in the evaporation chamber 12.

  In the solvent separator 34, as mentioned above, the separation of the mixture is carried out, giving a stream of carrier gas 14 which is poor in solvent and liquid solvent <possibly mixed with particles.

  of ice). The liquid solvent is removed by the pipe 40.

  Most of the liquid solvent is used to make the adhesive solution. For this purpose, it may be necessary to separate the water from the solvent or to adjust again to the initial value the ratio between the various constituents of the solvent. However, in general, once a stable operating state has been established, the ratio of solvent components remains constant, since the

- 15 -

  evaporation chamber 12 is sufficiently sealed so that it does not escape solvent vapors during operation. The recycling of most of the solvent recovered is indicated by dash 42. A smaller part of the solvent recovered is taken by the pipe 44 to a pump 46 and injected with this pump into the heat exchanger 26 and / or in the mixture 28, before the expansion machine 30. As already indicated above, using this fraction of solvent, it is to prevent the icing of the heat exchanger 26, the the expansion device 30 and the connecting pipe 28, in that the solvent forms a low melting point mixture with water or an ice separation on the cold solvent droplets. The pipes

  solvent are designated 48a, 48_.

  The mode of execution: n of FIG. 2 corresponds to the variant of FIG. 1 with regard to the order of succession of the various components, up to the heat exchanger 26. However, after the heat exchanger 26 is provided a second solvent separator 50, before the e turbine. expansion 30. A solvent separator serves mainly to separate the water and the least volatile components of the solvent mixture. After passing through the expansion turbine 30, the carrier gas mixture 3 low in solvent and soil vapors. In this variant, the expander 38 is disposed immediately following the separator sorrant 34. In passing through this expander, the flow of liquid is passed through the heat exchanger 52 to the solvent separator 34. The carrier gas cools without working and separates further solvent which can be removed in the solvent separator 51.

  cooled carrier gas 14 can serve as a cooling agent

  in the second heat exchanger 52. Then, the carrier gas stream low in solvent vapors

- 16 -

  passes, as a cooling agent, through the heat exchanger 26 in which it heats up, as in the first embodiment, to the temperature

  necessary in the evaporation chamber 12.

  Due to the interposition of the additional solvent separator 50, the risk of blading of the flash turbine 30 being damaged by solvent droplets or ice particles is reduced. However, it is still possible to inject a smaller portion of the separated solvent into the solvent separator 34, through the pipe 44, the pump 46 and the pipes 48a, 48b, in the heat exchanger 26 or in the gas-liquid mixture 28, to avoid

  icing of the heat exchanger or pipe 28.

  In the embodiments shown in FIGS. 3 and 4, the elements which are identical to those of the embodiments of FIGS. 1 and 2 or the like bear the same references. The most important difference lies in the fact that after the compressor 24 is connected an indirect cooler 25. With the aid of this indirect cooler, it is possible to simply lower the temperature of the carrier gas stream which is low in steam vapors. solvent at its entry into the evaporation chamber, for example between 70 and about 1000C in the case of hexane (n). If it were desired to obtain the same temperature lowering in the embodiments of FIGS. 1 and 2, the carrier gas stream in the cooler 18 should be cooled to such an extent that its temperature before the inlet

  compressor 24 could only be about 10 to 20 ° C.

  For this purpose, it would be necessary to give a very large dimension to the cooler 18. In addition, thanks to the interposition of the cooler 25, it is possible to adjust the temperature of the carrier gas stream in a wider range, more precisely by correspondingly actuating the agent valve

cooling 29.

  In the embodiment of FIG.

- 17 -

  25 is preceded by a heat exchanger 26a ("hot" heat exchanger) and followed by a heat exchanger 26b ("cold" heat exchanger). These two heat exchangers are covered

  by the carrier gas stream low in solvent vapors.

  The heat exchanger 26a is avoided by a bypass valve 27. If this valve is opened, a part of the carrier gas stream goes to the heat exchanger 26a and the inlet temperature of the carrier gas stream in the evaporation chamber lowers. In this way, too,

  it is easy to adjust the temperature.

  So that the temperature of the carrier gas stream charged with solvent vapor remains constant in the "cold" heat exchanger 26b (at about 200 ° C in the case of hexane (n)), it is necessary, when the valve 27 is open , increase the coolant mass flow rate

  through the additional cooler 25.

  In FIG. 3, before the expansion machine, a solvent separator 50 is interposed. This solvent separator is only necessary when the carrier gas stream contains a high concentration of solvent vapors and the quantity of solvent separated after the heat exchanger 26b is so great that it is to be feared that the expansion machine 30 is damaged by the

droplets of solvent.

  In the embodiment of Figure 3, after the cooler 18 may be provided a valve 23, with which one can adjust the flow rate

  a carrier gas stream loaded with solvent vapors.

  In addition, at the outlet of the solvent separator 34 is provided a discharge valve 39 for the condensed solvent. This can, as in the embodiments according to FIGS. 1 and 2, be brought back to the heat exchanger 26b or to the cooler 25, if there is in this case

place, a risk of icing.

  In FIG. 3, it can be seen further that the motor 36

- 18 - 2491768

  is connected via a transmission 54 to the common shaft between the compressor 24 and the expander turbine 30.The expansion turbine isdependenceune turbine guide vanes adjustable. Compressor 24 is preferably provided with an intake guide vane.

  The embodiment of FIG.

  3 simply after the additional cooler 25 is provided a single heat exchanger 26, that is to say that the heat exchanger 26% interposed before the cooler is omitted. The heat exchanger 26 is avoided, as well as the heat exchanger 26a of Figure 3, by a bypass valve 27, so that with the aid of the latter and the valve 29, it is possible to to regulate in a simple way the inlet temperature of the carrier gas stream low in solvent vapors in

the evaporation chamber.

  Also, in the embodiment of FIG. 4, before the expansion machine 30 can be provided a solvent separator similar to the separator 50 of FIG. 3. In addition, the solvent separated in the solvent separator 34 can be removed. passing through the discharge valve 39 and partially recycled, if there is a risk of

  icing in the heat exchanger 26.

  The embodiment of FIG. 5 corresponds essentially to that of FIG. 1. The stream of cooled carrier gas leaving the cooler 18 enters the first compressor 23, which is rigidly connected by a coupling to an electric motor 36 constituting a outdoor work machine. The rotational speed of the electric motor is adjustable, depending on the conditions required for the low temperature required for

  condensation or the flow required for circulation.

  After the compressor 23, the carrier gas stream 16 enters the second compressor 24, which is directly coupled mechanically to the expander 30 (a

24 91768

- 19 -

  modified turbocharger), which is indicated by the continuous shaft 31. The stream of compressed gas carrier 16 then enters the heat exchanger 26a ("hot" heat exchanger), in which it cools in indirect heat exchange with the heat exchange current. carrier gas low in solvent vapor 14. The heat exchanger 26a is avoided, as in the embodiment of Figure 3, by a bypass valve 27. The heat exchanger 26% is followed by an indirect cooler 25 with which the temperature of the carrier gas stream can be easily adjusted. Without the cooler 25, the carrier gas stream 16 in the cooler 18 should be cooled to such an extent that its temperature, prior to entry to the compressor 23, could be only about 10 to 200 ° C. For this purpose, it would be necessary to give a very large

  dimension to the cooler 18. In addition, thanks to

  position of the cooler 25, it is possible to regulate the temperature of the carrier gas stream in a wider range, more specifically by correspondingly actuating the

  coolant valve 29.

  The cooler 25 is followed, as in the embodiment of FIG. 3, with a heat exchanger 26b ("cold" exchanger), in which the carrier gas stream 16 cools again in indirect heat exchange with the current carrier gas low in solvent vapor 14 (at about 00C in the example). It is possible to adjust the temperature of the carrier gas stream low in solvent vapors 14 by means of the bypass valve 27 so that the temperature of the carrier gas stream charged with solvent vapors remains constant in the cold exchanger. 26b (at about 0 ° C in the case of hexane (n>), it is necessary, when the valve 27 is opened, to increase the mass flow rate of coolant passing through

  the additional cooler 25.

- 20 -

  In the heat exchanger 26b, some of the solvent vapor is already condensing. This portion is removed in the solvent separator 50. However, this is only necessary if the carrier gas stream contains such a high concentration of solvent vapors and the amount of solvent separated after the heat exchanger 26b is so great that it is to be feared that the expander 30 will be damaged by the solvent droplets. The mixture 28 comprising a carrier gas stream partially charged with solvent vapors and possibly still liquid solvent particles flows to the expansion machine 30 designed as an expansion turbine (modified turbocharger). This, as already mentioned, is directly mechanically coupled to the compressor 24 by the shaft 31. Thus, one can take advantage of the work obtained in the expansion turbine 30, virtually without losses, to compress the current of carrier gas loaded with solvent vapors, 16, in the compressor 24, because there are no losses due to transmissions. The motor 36 which drives the first compressor 23 is the only source of external energy of the system and the supply of energy can be made flexible.

according to the needs of the system.

  In the expansion turbine 30, as a result of the work provided, there is a new cooling of the carrier gas stream and the mixture 32 of carrier gas stream low in solvent vapor and liquid solvent particles possibly still present (in addition A high proportion of the mixture (28) arrives at the solvent separator 34 where the mixture 32 separates. The carrier gas stream low in solvent vapors 14 which leaves the solvent separator 34 has, in the example under consideration, a temperature of approximately -400C and successively traverses the heat exchangers 26b and 26a in heat exchange.

- 21 -

  indirectly, with the carrier gas stream charged with solvent vapors 16. Here, the first is heated to about C, i.e. at a temperature necessary to

  evaporation of the solvent in the evaporation chamber 12.

  To further control the temperature of this carrier gas stream, a pressure reducer 38 may be provided. When the carrier gas stream passes through this expander, there is a new cooling without labor supply. Thus, the temperature of the system can be regulated simply, not only by the throttle valve 22, but also by the expander 38. With these two valves, without interposing other adjustment devices, the system can be adapted to various solvent systems. The cooling of the carrier gas stream, which occurs at the expander 38, can serve to cool the carrier gas stream after the expansion turbine 30 (not shown in the drawings) and after the expander 38 may be provided optionally a separator

of solvent.

  In the solvent separator 34, as indicated with respect to the embodiment of FIG. 1, the separation of the mixture results in a solvent-laden carrier gas stream 14 and liquid solvent which is then

  treated as in the embodiment of Figure 1.

  Optionally, a smaller portion of the recovered solvent can be passed through the pipe 44 to a pump 46 and injected with this pump into the heat exchanger 26b and / or into the mixture 28, before the expansion machine 30. As already mentioned above, with the aid of this water-soluble solvent fraction, it is a question of preventing icing of the heat exchanger 26b, the expansion machine 30 and the connecting pipe 28, by the fact that the solvent forms with water a low melting point mixture or that an ice separation occurs on the cold solvent droplets. The pipes

- 22 -

  solvent introduction are designated 48a, 48b.

  The icing of the expansion turbine 30, as well as the risk of damage caused to the blading of the expansion turbine by droplets of solvent or ice particles are also reduced by means of the separator.

solvent 50.

  In the embodiment shown in FIG. 6, the elements which are identical to those of FIGS. 1 and 1 or the like bear the same references. The most important difference lies in the fact that the expansion machine 30 is directly coupled mechanically to the first compressor (by the shaft 31), while the second compressor 24 is coupled to the electric motor 36. The advantage of this arrangement, relative to that of FIG. 1, it is possible to regulate the temperature of the gas stream charged with solvent vapors even more efficiently before entering the 'hot' heat exchanger 26a, since a deviation with respect to the set values in FIG. this place can be immediately neutralized with the help of

  engine 36 and the neutralization comes into action immediately.

  Furthermore, in this embodiment, the expansion machine 30 is designed as an adjustable guide blade expansion turbine, which creates a new control possibility and optimizes the efficiency of the expansion turbine. according to the pressure and flow conditions that momentarily prevail in the system. Finally, as a setting element is still provided,

  before the compressor 23, a valve 21.

  The invention is not limited to the illustrated embodiments, but can be varied in many ways

  without departing from the scope of the invention.

- 23 -

Claims (22)

  1. A process for the recovery of solvents in which a stream of carrier gas is compressed which has been charged with solvent vapors in an evaporation chamber, for condensing the solvent vapors and separating the solvents, then cooled and it relaxes it with a supply of work, after which the carrier gas stream which is low in solvent vapors is brought back to the evaporation chamber, after having heated it again, this process being characterized in that the work is used. obtained during the relaxation, with mechanical coupling, for compressing the carrier gas stream loaded with solvent vapors. 2. A process according to claim 1, characterized in that one transmits to a compression stage, with mechanical coupling, the work obtained during the relaxation, and additional work provided from the outside.  3.- Method according to claim 1, characterized in that one transmits to one of two or more stages of compression, with direct mechanical coupling, all the work obtained during the expansion, to compress the gas stream carrier laden with vapors of solvent.   4. A process according to claim 3, characterized in that the second or subsequent compression stage is operated separately by means of work provided. from the outside.   5. Method according to one of claims 1 to 4,   characterized in that an inert gas or a gas whose oxygen content is below the limit of ignition of the solvent vapors is used as the carrier gas.   6. A process according to one of claims 1 to 5,   characterized in that the temperature of the current is regulated - 24 -   carrier gas charged with solvent vapors, before, between, and / or after the different compression stages, by indirect heat exchange with an external coolant.   7. A process according to one of claims 1 to 6,   characterized in that a water-soluble solvent is injected into the stream of cooled carrier gas prior to expansion in liquid form.   8. A process according to claim 7, characterized in that, for the injection into the stream of cooled carrier gas, a part of the condensed and separated solvent is used.   9. A process according to one of claims 1 to 8,   characterized in that the cooled carrier gas stream is condensed and separated prior to expansion by part of the solvent vapors.   10.- Method according to one of claims 1 to 9,   characterized in that the carrier gas stream is only relaxed once again without the provision of work.   partially with provision of work.   11. A process according to claim 10, characterized in that the expanded carrier gas stream is used without the provision of work, with indirect heat exchange, as the cooling gas of the carrier gas stream.   relaxed with provision of work.   12.- Installation for the implementation of the method   according to one of claims 1 to 11, wherein are   connected in a carrier gas circuit, an evaporation chamber o the stream of heated carrier gas is charged with solvent vapors, a compressor, a cooler for condensing the solvent vapor by separating them from the carrier gas stream, a an expander, a solvent separator and a device for heating the carrier gas stream low in solvent vapors,   this facility being characterized in that the compres-   (24) is mechanically coupled to the machine of - 25 -   detent (30) and in that the cooler and the heating device constitute at least one heat exchanger (26, 26a, 26b) traversed by the carrier gas stream poor in solvent vapors.   13.- Installation according to claim 12, characterized   in that the expansion machine (30) is mechanically coupled   compressor (24) at the same time as a   additional outdoor work (36).   14.- Installation according to claim 12, characterized in that the expansion machine (30) is directly coupled mechanically to one of two or more compressors (23, 24).   15.- Installation according to claim 14, characterized in that the second compressor or the following are mechanically coupled to work machines exterior (36).   16.- Installation according to one of claims 12 to   , characterized in that the expansion machine (30) constitutes an expansion turbine and that the   additional work (36) constitutes an electric motor.   17.- Installation according to one of claims 12 to   16, characterized in that between the evaporation chamber (12) and the first compressor (23), between the first compressor and the second or each of the following and / or between the last compressor (24) and the expansion machine (30) is interposed an indirect cooler <18, 25) for adjusting the temperature of the carrier gas stream charged with solvent vapors.   18.- Installation according to claim 17, characterized in that the indirect cooler <25) disposed between the compressor (24) (or the last compressor) and the expansion machine (30) is preceded by a heat exchanger (26a)   and followed by a heat exchanger (26b).   19.- Installation according to one of claims 12 to   18, characterized in that between the heat exchanger (26) - 26 -   and the expansion device (30) are provided devices (48) for injecting a water-soluble liquid solvent in the carrier gas stream.   20.- Installation according to one of claims 12 to   19, characterized in that between the heat exchanger (26) and the expansion machine (30) is provided a separator of additional solvent (50).   21.- Installation according to one of claims 12 to   , characterized in that before the evaporation chamber (12) is provided a regulator (38).   22.- Installation according to one of claims 12 to   21, characterized in that between the expansion machine (30) and the first solvent separator (34) is interposed an additional heat exchanger (52), traversed by the carrier gas stream low in solvent vapors, and in that the expander (38) is arranged immediately before this exchanger and after the expander (38) is provided a solvent separator (51),