GB2085310A

GB2085310A - Process and apparatus for recovery of solvents

Info

Publication number: GB2085310A
Application number: GB8130902A
Authority: GB
Original assignee: Lohmann GmbH and Co KG
Current assignee: Lohmann GmbH and Co KG
Priority date: 1980-10-14
Filing date: 1981-10-13
Publication date: 1982-04-28
Also published as: NL8104679A; FI74619B; DK453481A; GB2085310B; SE8106080L; AU545130B2; FR2491768B1; ES8302472A1; NO813461L; ES506224A0; FR2491768A1; AU7629881A; FI813200L; FI74619C; CH656549A5

Abstract

A process for the recovery of solvents is disclosed. A carrier gas stream 14 laden with solvent vapours in an evaporation space 12 is compressed by a compressor 24, cooled, e.g. by heat exchanger 26, and expanded by turbine 30 with the production of work to condense solvent vapours and separate the solvent. The carrier gas stream low in solvent vapours is returned into the evaporation space 18 after being reheated e.g. by the exchanger 26. The work arising on expansion is used in mechanical coupling with the compressor 24 for compressing the carrier gas stream laden with solvent vapours. The work produced on expansion is used in mechanical coupling with the compressor 24 for compressing the carrier gas stream laden with solvent vapours. <IMAGE>

Description

SPECIFICATION Process and apparatus for recovery of solvents This invention relates to a process and apparatus for recovery of solvents.

In the known processes and apparatus, which are described, for example, in Ull.mann's Encyclopedia of Technical Chemistry, Vol. 1(1951), page 338, a carrier gas stream; laden with solvent vapours in an evaporation space is cooled to condense out solvent vapours and to separate the solvent. After condensation and separation the carrier gas stream, low in solvent vapours, is returned into the evaporation space, after being reheated. The solvent vapou rs are not completely condensed out of the carrier gas stream since there remains in the carrier gas stream a certain residual amount of solvent vapour, corresponding to the vapour pressure of the solvent at the temperature of the coolant. In order to avoid losses of solvent, the carrier gas stream is conducted in a circuit.The ability of the carrier gas stream, which is low in solvent vapours, to take up solvent vapours is hence in fact somewhat reduced; however, this is unimportant for the efficiency of the process.

The process is quite generally suitable for removal of volatile solvents from non-volatile substrates.

One field of application for this process is the removal of solvent residues from chemical substances which have been produced or purified with the use of solvents. Further fields of application lie in the paint and lacquering fields, the field of chemical cleaning of textiles, the film and foil field, the rubber processing field, and the adhesives and adhesive materials field.

In prior art apparatus, generally, separate cooling device for condensing out the solvent vapours and devices for reheating the carrier gas stream low in solvent vapours are provided. Disadvantages of these prior art apparatus include the fact that considerable amounts of coolant are required and a high amount of energy is needed to reheat the carrier medium, low in solvent vapours. This reheating of the carrier gas stream is necessary so that the carrier gas stream can rapidly become re-laden in the evaporation space with a sufficient amount of solvent vapour, i.e. so that the substrate is rapidly dried.

One possible way to attempt to achieve a saving of energy would be to use the coolant, which has been heated on passage through the cooling device for reheating the carrier gas stream low in solvent vapours, i.e. to conduct the coolant countercurrently to the carrier gas stream. However, it is immediately obvious that only a small fraction of the heat taken from the carrier gas stream, previously, in the cooling device, can be returned to the carrier gas stream. Because of the relatively low temperature difference between the carrier gas and the coolant, the cooling device and the device for reheating the carrier gas stream have to be provided with large heat exchange surfaces.

The process is thus disadvantageous not only because of its high energy and coolant consumption, but also because of its high cost in apparatus.

An apparatus for recovery of solvent from a hot carrier gas stream laden with solvent vapours is known from DE-PS 27 25 252, in which the carrier gas stream is compressed, cooled, and expanded with production of work, for condensing out the solvent vapours and separating the solvent. The carrier gas stream low in solvent vapours is conducted back into the evaporation space after being reheated.

However, return of this carrier gas stream takes place in admixture with a carrier gas stream taken from the evaporation space and laden with solvent vapours. After being heated in indirect heat exchange with the compressed carrier gas stream, the admixture is conducted back into the evaporation space in a duct loop together with the carrier gas stream low in solvent vapours. Better heat regulation is accomplished, there is the disadvantage that the carrier gas stream, conducted back into the evaporation space, has a relatively high content of solvent vapours. Thus, the drying effect in the evaporation space is reduced in this manner. It is further disclosed in DE-PS 27 25 252 that the work liberated by the expansion in an expansion turbine can be recovered. However, details are lacking as to where this work can be usefully utilized.

The present invention seeks to improve upon a process and an apparatus of the kind described above, with a low cost of apparatus while utilizing the work produced from the expansion of the compressed carrier gas stream laden with solvent vapours, and conducting the carrier gas stream as low as possible in solvent vapours back into the evaporation space.

A process for recovery of solvents is provided. A carrier gas stream laden with solvent vapours in an evaporation space is cooled to condense the solvent vapours and separate the solvent. The carrier gas stream, low in solvent vapours, after being reheated, is conducted back into the evaporation space. In the process the carrier gas stream, laden with solvent vapours, is compressed and, after cooling by indirect heat exchange, is expanded with production of work.

The resultant work done is used for compressing the laden carrier gas stream. The carrier gas stream, low is solvent vapours, is used for cooling, in indirect heat exchange, the carrier gas stream laden with solvent vapours. The whole of the work produced on expansion is transferred by direct mechanical coupling to one of two or more compression stages for compressing the carrier gas stream laden with solvent vapours.

An apparatus for carrying out the process includes, in a carrier gas circuit; an evaporation space in which the heated carrier gas stream is laden with solvent vapours, a compressor, a cooling device for condensing the solvent vapours out of the carrier gas stream, an expansion apparatus, a solvent separator, and a device for reheating the carrier gas stream, low in solvent vapours. In the apparatus the compressor is mechanically coupled to the expansion apparatus and the cooling device and also the device for reheating constitute at least one heat exchanger through which the carrier gas stream, low in solvent vapours, flows. A further feature of the apparatus is that the expansion apparatus is directly mechanically coupled to one or two or more compressors.

In the apparatus according to the invention, the carrier gas stream, conducted in a circuit, takes up the evaporated solvent in high concentration in the evaporation space (usually a dryer), and the solvent is withdrawn from the carrier gas stream in the cooling device by cooling and condensing. While in the prior art proceses the carrier gas stream laden with solvent is indeed compressed and is expanded with production of work after being cooled, the work of expansion is not utilized in the system as compression work which is contrary to the present invention. Thus, in accordance with the present invention, only the difference between the compression and expansion work is to be supplied from an external source, to the compression stage, i.e. by means of a supplementary source of power which, together with the expansion apparatus, is mechanically coupled to the compressor.Preferably the difference between the compression and expansion work is supplied to one of two compression stages by means of the supplementary source of power which is directly mechanically coupled to the one compressor. This difference covers the work used for separation of the solvent vapours from the carrier gas stream and also overcomes thermodynamic losses such as, for example, friction, takeup of heat from the surroundings.

The density of the mixture of carrier gas stream and solvent vapours is increased by the compression. Hence the efficiency of the heat exchanger is increased. Because of the reduced gas volume, the heat exchanger and the other parts of the apparatus under pressure can be kept compact. On compression and expansion no chemical, and in particular no oxidative, effect on the solvent vapours takes place, in contrast to recovery processed in which adsorbents such as active carbon are used. Such adsorbents can often act on the solventvapours with the formation of injurious decomposition products. If the carrier gas stream is constantly circulated, these decomposition products would build up and react undesirably with the products to be dried or with the parts of the apparatus.A known case is the decomposition of chlorinated hydrocarbons on active carbon in the presence of water vapour, with formation of hydrogen chloride.

When more than one compression stage is used, the second or further compression stages are preferably driven separately by power supplied from outside, i.e. the second or further compressors are mechanically coupled to external sources of power.

This arrangement ensures a better control of the compression processes, and among other things, a better regulation of the desired final pressure.

Further, the dimensions of the gearing required between the power source and the compressor can be kept low.

An expansion turbine is preferably used as the expansion apparatus because of its high efficiency.

Additionally the expansion turbine is more easily coupled to a compressor and the drive motorforthe compressor than is, for example, a piston machine.

The supplementary source of power preferably represents an electric motor.

The process according to the invention can be used, for example, in connection with the production of flat adhesive material where an adhesive is applied to paperortextile lengths or tapes. Such tapes can be used, for example, as technical adhesive tapes or as tapes or lengths for medical purposes, such as, for example, adhesive plaster.

For application of the adhesive to the paper or textile length, the adhesive is brought into a flowable state by means of liquid solvents, so that it can be applied in sufficiently thin and uniform layers. The solvent evaporates on drying. The coated substrate remains in contact with the carrier gas, which takes up the solvent vapours in the evaporation space, for a time determined by the volatility and amount of solvent.

The examples of embodiments given below relate to apparatus for this special application. However, the invention is also applicable with success to the other fields of application mentioned heretofore.

As a rule, solvents for adhesives and for many other materials are solvents or solvent mixtures whose vapours are inflammable. According to the invention there is therefore used for recovery of such solvent vapours a carrier gas with an oxygen content lying below the ignition limits. For example, intrinsically inert gases such as nitrogen or carbon dioxide can be used for this purpose; however, it is also possible to reduce the oxygen content of air, by producing an admixture with inert gas, so that the ignition limits are not reached. In certain cases it is also possible to use combustion gases with a reduced oxygen content.

The inflammability of the solvent vapours is however not only a function of the oxygen content in the carrier gas, but also depends on the concentration and nature of the solvent vapour. Thus, for example, the danger of ignition is greater with low-boiling hydrocarbons and ethers than with halogenated hydrocarbons. The ignition properties of various solvent vapours are known, and the permissible solvent vapour concentrations can be taken from the literature or determined by simple tests.

The use of an inert or oxygen-poor carrier gas stream offers the advantage that the carrier gas stream can take up a large amount of solvent vapours without the danger of an explosion arising.

In this manner, the amount of carrier gas to be circulated can be kept low, so that the amount of energy required for cooling or reheating the carrier gas can be reduced.

The process according to the invention is not limited to the recovery of organic solvents; inorganic solvents such as ammonia and sulfur dioxide can also be used, and similarly solvents which come between inorganic and organic solvents, such as carbon disulfide or carbon tetrachloride. Since these solvents (carbon disulfide excepted) are incombustible, the maintenance of a given oxygen concentration in the carrier gas is unnecessary in these cases, i.e. in the simplest case air can be used as the carrier gas.

Variations of the process according to the inven tion, depending among other things, upon adapting the apparatus to various solvents or solvent vapours, is possible. For example, the speed of the material moved through the evaporation space to be dried can be varied. A further possibility is to vary the speed of the carrier gas stream. For this purpose, the rpm of the drive motor of the one or further compressors can be varied. Furthermore, bypass control of one or more compressors can be effected for this purpose.

A particularly simple way of controlling the temperature of the carrier gas stream laden with solvent vapours consists of bringing the stream before, between and/or after an individual compression, into indirect heat exchange with a coolant. For this purpose, an indirect cooler can be inserted between the evaporation space and the first compressor, between the first and second or respective following compressors and/or between the last compressor and the expansion apparatus. By control of the coolant flow in the cooler or coolers, the entry temperature of the carrier gas stream laden with solvent vapours into the first orfollowing compressors and/or into the expansion apparatus, or the entry temperature of the carrier gas stream low in solvent vapours into the evaporation space can be adapted in a simple manner to the requirements at any given time.

By the insertion of an additional indirect cooler between the last compressor and the expansion apparatus, entry of the carrier gas stream, low in solvent vapours, into the evaporation space at a lower and better controllable temperature can be achieved.

The additional cooler usually precedes the heat exchanger through which the carrier gas stream, low in solent vapours, flows. Preferably, however, this cooler can be preceded by a heat exchanger ("hot" heat exchanger) and followed by a heat exchanger ("cold" heat exchanger). In this manner, a carrier gas stream arrives in the evaporation space at a lower temperature.

When the carrier gas stream laden with solvent vapours is cooled in the cooling device after leaving the compressor, a portion of the solvent vapours are condensed depending, among other things, on the temperature of the carrier gas stream, low in solvent vapours, which is used as a coolant. For example, the possibility exists that water will separate out, since its boiling point is higher than that of many organic solvents. Although water is not used in solvent mixtures for the usual self-adhesive adhesives, it is nevertheless introduced into the system, since it is adsorbed on the paper or textile lengths used as substrate for the adhesive. In a few cases it can even happen that water freezes out in the cold part of the heat exchanger or in the expansion apparatus, thus causing a blockage of the flow cross sections or possibly damaging the moving parts of the expansion apparatus.

In order to obviate this danger, a water-soluble solvent in the liquid state can be injected into the cooler carrier gas stream before expansion. When the solvent is dissolved in water, it yield a solution with a lower freezing point than that of water and remaining liquid.

When the cold solvent is not water-soluble, the water deposits on the surface of the cold solvent droplets and thus cannot deposit on the solid boundaries of the flow paths.

This measure is carried out, as regards apparatus, by providing between the heat exchanger and the expansion apparatus devices for injecting the liquid solvent into the carrier gas stream.

A part of the water-soluble solvent, condensed out and separated in the solvent separator, is most appropriately used for injection into the cooled carrier gas stream.

When it is not required to inject a liquid solvent, or when the danger exists that the condensed-out liquid will damage the moving parts of the expansion apparatus such as, e.g. the blades of the expansion turbine, a portion of the solvent vapours can be condensed out of the cooled carrier gas stream and separated before expansion. For this purpose, a further solvent separator can be provided between the heat exchanger and the expansion apparatus.

Afurther means of controlling the temperature of the carrier gas stream consists of expanding the carrier gas stream without production of work, subsequent to the partial expansion of the gas stream with production of work. In this instance, an expansion valve can be provided before the evaporation space. This expansion valve can be provided either at the entry or at the exit of the heat exchanger. By means of this expansion valve, icing in the pipe ducts to the expansion apparatus, or in the expansion apparatus itself, is prevented.

On passage through the expansion valve a further small cooling of the carrier gas stream takes place, in this case with production of work. The thus expanded carrier gas stream can now, if necessary after separation of the condensed-out solvent, be used as cooling gas in indirect heat exchange for the carrier gas stream expanded with production of work. For this purpose, a further heat exchanger through which the carrier gas stream lean in solvent vapours passes can be inserted between the expansion apparatus and the first solvent separator. The expansion valve is positioned immediately before before this heat exchanger.

The process and the apparatus according to the invention are illustrated in the following drawings: Figure 1 is an apparatus in accordance with the invention with only one solvent separator and a cooler; Figure 2 is an apparatus in accordance with the invention with a cooler and two solvent separators and an additional heat exchanger between the first solvent separator and the expansion apparatus; Figure 3 is an apparatus in accordance with the invention with an additional cooler between the compressor and the expansion apparatus, the additional cooler being preceded and followed by respective heat exchangers; Figure 4 is an apparatus in accordance with the invention with an additional cooler between the compressor and the expansion apparatus, the additional cooler only being followed by a heat ex changer.

Figure 5 is an apparatus in accordance with the invention in which the expansion apparatus is coupled to the second of two compressors, while the first compressor is coupled to a supplementary source of power; and Figure 6 is an apparatus in accordance with the invention in which the expansion apparatus is directly coupled to the first of two compressors, while the second compressor is coupled to a supplementary source of power.

In the embodiment of Figure 1, a paper or textile length is indicated by 10, and is provided with a solvent coating dissolved in solvent. This length 10 is moved (be means drive means which are not shown) in the direction of the arrow A through the schematically shown evaporation space 12. The evaporation space 12 is completely enclosed, so that no solvent vapours can emerge into the atmosphere.

A carrier gas stream 14, low in solvent vapours, e.g. a stream of nitrogen, is introduced into the evaporation space countercurrent (in the direction designated by arrow B) to the paper of textile length.

The heating of this carrier gas stream takes place in the manner described below.

The hot carrier gas stream 14 flows through the evaporation space 12 countercurrentlyto the paper or textile length 10 and heats the paper or textile length 10 to the extent that the solvent contained in the adhesive solution evaporatestherefrom (illustrated in the drawing by Lav). The carrier gas stream thus becomes laden with solvent gas vapours, and is cooled due to the heat of evaporation of the solvent.

For example, when n-hexane is used as a solvent, the entry temperature of the gas stream 14 into the evaporation space 12, is for example 1 40or, and the exit temperature is about 10000. The carrier gas stream 16 leaving the evaporation space 12, laden with solvent vapours, now enters a cooler 18, through which flows a coolant 20 in indirect heat exchange with the stream 16. The flow rate of the cooling, and hence the temperature of the carrier gas stream 16 laden with solvent vapours, can be regulated by means of a throttle valve 22. In the example shown in Figure 1, the throttle valve 22 can be set such that the carrier gas stream leaving the cooler 18 has a temperature of about 34 C, while the coolant 20 is heated from about 12 to about 6500.

For further control of the temperature of this carrier gas stream, an expansion valve 38 can be provided. When the carrier gas stream passes through this valve, a further cooling takes place without production of work. The temperature of the system can thus be regulated in a simple manner, not only by means of throttle valve 22, but also by means of expansion valve 38. The system can be suited to the most diverse combinations of solvents with these two valves, without insertion offurther regulating devices. The cooling of the carrier gas stream occurring at expansion valve 38 is used in the alternative apparatus in Figure 2 for cooling the carrier gas stream after the expansion turbine, as further described below.

The cooled carrier gas stream now enters the compressor 24 in which it is compressed by a factor of about 2.5 with a temperature rise to about 14000.

After the compressor, the carrier gas stream 16 enters heat exchanger 26, in which it is cooled (in this example to about - 10 C) in indirect heat exchange with the carrier gas stream 14 low in solvent vapours. A portion of the solvent vapours already condenses out in heat exchanger 26, and the mixture, referenced 28, of carrier gas stream partially laden with solventvapours, liquid solvent particles, and possibly ice particles, can be conducted into the expansion apparatus 30, constructed as an expansion turbine. However, a preseparator (not shown; similar to separator 34) is appropriately inserted in front, in order to remove the liquid and solid particles.Due to the work produced in the expansion turbine 30, a further cooling of the carrier gas stream takes place, and the mixture 32 of carrier gas stream, low in solvent vapours, liquid solvent particles, and possibly ice particles, reaches solvent separator 34 in which the mixture 32 is separated.

Compressor 24 and expansion turbine 30 are appropriately on a common shaft 31 which is driven by a motor 36. The work recovered in the expansion turbine 30 can thus be used practically without losses for compression of the carrier gas stream 16 laden with solventvapours, in the compressor 24.

The motor 36 in the single energy source of the system. The carrier gas stream 14 low in solvent vapours and leaving the solvent separator 34 has a temperature of about -40 C in the embodiment Example shown, and flows through the heat exchanger 26 in indirect heat exchange with the carrier gas stream 16 laden with solvent vapours. The former is thereby heated to about 140 C, i.e. to a temperature required for evaporation of the solvent in the evaporation space 12.

In the solvent separator 34 there occurs, as stated above, the separation of the mixture into a carrier gas stream 14 low in solventvapours and liquid solvent (possibly mixed with solid ice particles). The liquid solvent is drawn off through duct 40. The major part of the liquid solvent is used for production of the adhesive solution. For this purpose it may be necessary to separate water from the solvent or to adjust again the ratio of the individual solvent components. In general, however, the ratio between the solvent components remains constant after a stable operating state has been attained, since the evaporation space is enclosed to the degree that no solvent vapours escape during operation. The return of the major amount of recovered solvent is shown by the dashed line 42.

A smaller portion of the recovered solvent-is conducted via duct 44 to a pump 46 and is injected by means of this pump into the heat exchanger 26 and/or into the mixture 28 before the expansion apparatus. As already mentioned above, by means of this solvent fraction, icing of the heat exchanger 26, the expansion turbine 30 and the connecting duct 28 is prevented in that either the solvent forms with water a low-melting mixture, or deposition of ice takes place on the cold solvent droplets. The solvent inlet ducts are referenced 48a or 48b.

The embodiment shown in Figure 2 corresponds, as regards the sequence of the individual compo nents up to the heat exchanger 26, to the embodiment of Figure 1. After the heat exchanger 26, however, a second solvent separator 50 is provided before the expansion turbine 30. This solvent separator serves primarily for separation of water and the higher-boiling components of the solvent mixture.

After passage through the expansion turbine 30, the mixture 32 of carrier gas, low in solvent vapours and liquid solvent, is conducted through a further heat exchanger 52 and thence reaches solvent separator 34. In this embodiment, expansion valve 38 is arranged immediately after solvent separator 34. On passing through this expansion valve, the carrier gas stream is cooled without production of work, further solvent being separated which can be removed in solvent separator 51. The cooled carrier gas stream 14 can be used as coolant in the second heat exchanger 52. The carrier gas stream low in solvent vapours subsequently passes as coolant through heat exchanger 26, in which it is heated, as in the first embodiment, to the temperature required in the evaporation space 12.

The danger of damage to the blades of the expansion turbine 30 by ice particles or solvent droplets is reduced by insertion of the additional solvent separator 50. Nevertheless, a minor portion of the solvent separated in solvent separator 34 can however be injected, via duct 44, pump 46 and ducts 48a or 48b into the heat exchanger 26 or into the gas-liquid mixture 28, in order to prevent icing of the heat exchanger or of the duct 28.

In the embodiments shown in Figures 3 and 4, the elements identical or equivalent to the elements of the embodiments of Figures 1 and 2 are given the same reference numbers. The most important difference is that an indirect cooler 25 is placed after the compressor 24. By means of this indirect cooler, the temperature of the carrier gas stream, low in solvent vapours on entry into the evaporation space, can be reduced in a simple manner, e.g. in the case of n-hexane, to about 70 to 10000. If it is desired to attain the same drop of temperature in the embodiments of Figures 1 and 2, the carrier gas stream in cooler 18 would have to be cooled so much that its temperature before entry into compressor 24 would only be 10 to 2000. Cooler 18 would have to made very large for this purpose.Furthermore, by insertion of cooler 25 the temperature of the carrier gas stream can be regulated over a wide range, and in fact by corresponding actuation of coolant valve 29.

In the embodiment of Figure 3 a heat exchanger 26a ("hot" heat exchanger) precedes cooler 25, and a heat exchanger 26b ("cold" heat exchanger) follows it. The carrier gas stream low in solvent vapours flows through these two heat exchangers.

Heat exchanger 26a is bridged over by a bypass valve 27. If this valve is opened, a portion of the carrier gas stream passes into heat exchanger 26a, so that the entry temperature of the carrier gas stream into the evaporation space falls. Simple control of temperature is also possible in this manner.

In order that the temperature of the carrier gas stream laden with solvent vapours remains constant in the "cold" heat exchanger 26b (e.g. in the case of n-hexane at about 20 C), the mass flow coolant through the additional cooler 25 must be increased when valve 27 is open.

In Figure 3, a solvent separator is inserted before the expansion apparatus. This solvent separator is only necessary when the carrier gas stream contains a high concentration of solvent vapours and the amount of solvent separated after heat exchanger 26b is so large that damage to the expansion turbine 30 by the droplets of solvent is to be expected.

In the embodiment of Figure 3, a further valve 23 can be provided after cooler 18, and the flow speed of the carrier gas stream laden with solvent vapours can be regulated with it.

In the embodiment shown in Figure 6, the elements which are identical or equivalent to those of Figure 5 are given the same reference numerals. The most important difference is that the expansion apparatus 30 is directly mechanically coupled to the first compressor 24 (via shaft, 31), while the second compressor 21 is coupled to the electric motor 36.

This arrangement has the advantage over that of Figure 6 in that the pressure or temperature of the gas stream laden with solventvapours before entry into the "hot" heat exchanger 26a can be regulated even better, since a deviation from the set values at this point can be countered directly by means of motor 36 and the opposing control becomes immediately effective.

Furthermore, in this embodiment the expansion apparatus 30 is constructed as an expansion turbine with guide vane adjustment, giving a further possibility of control and enabling the efficiency of the expansion turbine to be optimized in correspondence with the pressure and flow conditions in the system at any given time. Finally, the further valve 23 is provided before compressor 24 as a control element.

Furthermore, an outlet valve 39 for the condensedout solvent is provided at the outlet of the solvent separator 34. This solvent can be conducted back into heat exchanger 26b or into cooler 25, as in the embodiments of Figures 1 and 2, when there is a danger of ice formation.

It can be further seen from Figure 3 that the motor 36 is connected via a gearing 54 to the common shaft between compressor 24 and expansion turbine 30.

The expansion turbine is preferably one with guide vane adjustment. The compressor 24 is preferably provided with an inlet guide vane.

The embodiment according to Figure 4 corresponds substantially to the embodiment according to Figure 3, only a single heat exchanger 26 is provided after the additional cooler 25, i.e. the preceding heat exchanger 26a is lacking. Heat exchanger 26 is bridged over by a bypass valve 27 in a manner corresponding to heat exchanger 26a of Figure 3, so that a simple regulation of the entry temperature of the carrier gas stream low in solvent vapours into the evaporation space is possible by means of this valve and valve 29.

In the embodiment of Figure 4, a solvent separator similar to the separator 50 of Figure 3 can also be provided before the expansion turbine 30. Furthermore, the solvent separated in the solvent separ ator 34 can be drawn off via outlet valve 39 and partially returned again when a danger of icing exists in heat exchanger 26.

In Figures 5 and 6 there is shown an apparatus embodying two compressors and the elements in Figure 5 which are identical or equivalent to those of Figures 1 to 4 are given the same reference numerals. In Figure 5, the cooled gas stream enters the first compressor 21 which is indirectly mechanically coupled via a coupling to an electric motor 36 which acts as external source of power. The rpm of the electric motor 36 can be controlled according to the low temperature required for condensation or the required volume for circulation. After compressor 21 the carrier gas stream 16 enters the second compressor 24 which is directly mechanically coupled, as shown by the through shaft labelled 31 to the expansion apparatus 30. The remainder of the apparatus shown in Figure 5 is in accordance with the apparatus shown in Figures 1 to 4. However, as previously described in Figure 5, the turbine 30 is connected mechanically directly, via shaft 31 to the compressor 24. The work produced in expansion turbine 30 can be utilized, practically without losses, for compression of carrier gas stream 16 in compressor 24 since no gearing losses arise. Motor 36, which drives the first compressor 21, is the external energy source for the system, and the supply of energy can be flexible according to the demands of the system.

Claims

1. A process for recovery of solvents, wherein a carrier gas stream laden with solvent vapours in an evaporation space is compressed, cooled, and expanded with production of work to condense out the solventvapours and to separate the solvent, after which the carrier gas stream low in solvent vapours is returned into the evaporation space after being reheated, the work arising on expansion being used in mechanical coupling for compressing the carrier gas stream laden with solventvapours.

2. A process as claimed in claim 1, wherein the work produced on expansion is transferred in a mechanical coupling, with additional work supplied from external sources to a compression stage.

3. A process as claimed in claim 1 or claim 2, wherein a gas with an oxygen content lying below the ignition limits of the solvent vapours is used as the carrier gas.

4. A process as claimed in claim 3, wherein the carrier gas is an inert gas.

5. A process as claimed in anyone of claims 1 to 4, wherein the temperature of the carrier gas stream laden with solvent vapours is adjusted by indirect heat exchange with an external coolant.

6. A process as claimed in anyone of claims 1 to 5, wherein a water-soluble solvent in liquid form is injected into the cooled carrier gas stream before expansion.

7. A process as claimed in claim 6, wherein a portion of the condensed and separated solvent is used for injection into the cooled carrier gas stream.

8. A process as claimed in anyone of claims 1 to 7, wherein a portion of the solvent vapours is condensed and separated from the cooled carrier gas stream before expansion.

9. A process as claimed in anyone of claims 1 to 8, wherein the carrier gas stream, partially expanded with production of work, is again expanded, without production of work.

10. A process as claimed in claim 9, wherein the carrier gas stream expanded without production of work is used in indirect heat exchange as cooling gas for the carrier gas stream expanded with production of work.

11. A process as claimed in anyone of claims 1 to 9, wherein the whole of the work arising on expansion is transferred in direct mechanical coupling to one of two or more compression stages for compression of the carrier gas stream laden with solvent vapours.

12. A process as claimed in claim 11, wherein the last compression stage is separately driven by externally supplied work.

13. An apparatus for the recovery of solvent wherein there are included in a carrier gas circuit, an evaporation space in which the heated carrier gas stream is laden with solventvapours; a compressor; a cooling device for condensing the solvent vapours out of the varrier gas stream; a solvent separator; and, a device for reheating the carrier gas low in soivent vapours; the compressor being mechanically coupled to an expansion apparatus, and the cooling device and the device for reheating constitute at least one heat exchanger through which flows a carrier gas stream low in solvent vapours.

14. An apparatus as claimed in claim 13, wherein the expansion apparatus is coupled, with a supplementary source of power, mechanically to the compressor.

15. An apparatus as claimed in claim 13 and claim 14, wherein the expansion apparatus is an expansion turbine and the supplementary source of power is an electric motor.

16. An apparatus as claimed in anyone of claims 13 to 15, wherein an indirect cooler is inserted between the evaporation space and the compressor to regulate the temperature of the carrier gas stream laden with solventvapours.

17. An apparatus as claimed in anyone of claims 13 to 16, wherein at least one indirect cooler is inserted between the compressor and the expansion apparatus to regulate the temperature of the carrier gas stream laden solvent vapours.

18. An apparatus as claimed in claim 17, wherein a heat exchanger precedes, and a heat exchanger follows, the indirect cooler arranged between the compressor and the expansion apparatus.

19. An apparatus as claimed in anyone of claims 13 to 18, wherein devices for injection of a liquid water-soluble solvent into the carrier gas stream are provided between the heat exchanger and the expansion apparatus.

20. An apparatus as claimed in anyone of claims 13 to 19, wherein a further solvent separator is provided between the heat exchanger and the expansion apparatus.

21. An apparatus as claimed in anyone of claims 13 to 20, wherein an expansion valve is provided before the evaporation space.

22. An apparatus as claimed in anyone of claims 13 to 21, wherein a further heat exchanger, through which the carrier gas stream low in solvent vapours flow, is inserted between the expansion apparatus and the first solvent separator, and the expansion valve is arranged immediately before this heat exchanger, and a solvent separator is provided after the expansion valve.

23. An apparatus as claimed in anyone of claims 13 to 22, wherein the expansion apparatus is directly mechanically coupled to one of at least two compressors.

24. An apparatus as claimed in claim 23, wherein other than the first compressor is mechanically coupled to an external source of power.

25. An apparatus as claimed in anyone of claims 13 to 24, wherein between the evaporation space and the expansion apparatus at least one indirect cooler is inserted for regulation of the temperature of the carrier gas stream laden with solvent vapours.

26. A process as claimed in claim 1 and substantially as hereinbefore described.

27. An apparatus as claimed in claim 13, and substantially as hereinbefore described.

28. A solvent when recovered by a process as claimed in any one of claims 1 to 12 or claim 26.

29. A solvent when recovered using an apparatus as claimed in any one of claims 13 to 25 or claim 27.