EP0157090B1 - Method and apparatus for cleaning work pieces by means of a volatile solvent - Google Patents

Abstract

A method and system for cleaning workpieces with a liquid solvent in a treatment chamber which is connected into a drying gas circuit for the purpose of drying the workpieces. The drying gas circuit includes in series one after the other a ventilator, a condenser, a heating device and an adsorber containing activated carbon. The circuit is operated such that during a drying phase the condenser is cooled, the heating device switched on and the activated carbon regenerated by the hot circulating air whereupon, during an adsorption phase with the condenser switched on and the heating device switched off, the remaining solvent vapor is withdrawn from the circulating drying air by the activated carbon.

Description

The invention relates to a method for the cleaning of workpieces by means of a liquid solvent in a treatment chamber, in which the workpieces are dried after cleaning in a closed drying room by a gas stream, with at least a part of the drying gas in a drying gas cycle of a part of the in vapor form entrained solvent is freed by cooling in a condensation stage and returned to the drying room, and an adsorbent for adsorbing solvent vapor produced during drying and a heating device are also used. The solvents in question are those that can be used to remove greasy, oily or similar contaminants.

In a known method of the type mentioned above (DE-A-32 05 736), the treatment chamber also serves as a drying space and is therefore integrated into the drying gas circuit, which has a condensation stage designed as a heat exchanger, a blower and a heating device likewise designed as a heat exchanger for heating the contains circulated air as drying gas A return line for condensed solvent leads from the condensation stage to the treatment chamber. In this solvent spray nozzles are installed, which are part of a solvent cycle, i. H. the solvent is drawn off at the bottom of the treatment chamber and pumped back to the spray nozzles. Dirty solvent is drawn off from the solvent circuit and regenerated via a distillation device.

Since even in the case of a condensation stage in the drying gas circuit that works with deep-freezing, the treatment chamber still contains too much solvent vapor after completion of the drying, at least if the condensation stage is operated at temperatures that can be achieved on an industrial scale at economically justifiable costs (the frequently used trichlorethylene) e.g. still has a saturation content of almost 100 g / m ³ at -10 ° C, the drying gas circuit is switched off in the known method after drying the workpieces and the treatment chamber is flushed with room air until the solvent concentration in the Treatment chamber is below the maximum permissible workplace concentration; The ambient air sucked in from the environment and used to purge the treatment chamber is blown off through the roof, whereby it can be passed through a condensation stage or over activated carbon in order to remove most of the solvent vapor.

A disadvantage of the known system is not only the comparatively large structural outlay in the case of cleaning the exhaust air, but the room air sucked in from the environment for purging the treatment chamber leads to a loss of heating energy in winter, and the system can be emission-free only with great effort operate, since, as already mentioned, a condensation stage operated with reasonable effort means that the solvent vapors can only be inadequately removed from the room air used for purging the treatment chamber and an activated carbon adsorber has to be filled with fresh or regenerated activated carbon after a relatively short time. In the usual regeneration processes for activated carbon, water vapor is blown into it, which is then condensed in a condensation stage. However, there are numerous disadvantages associated with the method to be improved by the invention: high steam and thus energy consumption; Together with the water, the solvent also condenses, which on the one hand makes it difficult to reuse it and on the other hand can lead to wastewater problems; with some chlorinated hydrocarbons there is a risk of hydrolysis (e.g. with the very commonly used 1.1.1-trichloroethane); after blowing in the hot steam, the activated carbon must also be pre-dried before it can be used again in the adsorber; Finally, the room air used to rinse the treatment chamber contains air humidity, which can be adsorbed and desorbed together with the solvent vapor, but only if a water-specific adsorber such as e.g. B. a molecular sieve is used (DE-OS 31 39 369).

In another known cleaning method of the type mentioned at the beginning (SE-A-110 711), an adsorbent is used in the drying gas circuit, with which - in the direction of flow of the drying gas - further solvent vapor is to be removed from the drying gas behind the condensation stage before it is again used by the heating device is heated and returned to the drying room formed by a treatment chamber (drying phase). To regenerate the adsorbent, this known system has a bypass line through which the condensation stage, bypassing the drying chamber, an adsorber containing the adsorbent and the heating device, and a switchable blower are combined to form an auxiliary circuit for the drying gas, in which, after reversing the conveying direction of the Blower the adsorbent is regenerated. The drying gas is first heated using the heating device in order to heat the adsorbent through which the drying gas flows; As a result, this releases previously adsorbed solvent vapor, which partly in the subsequent condensation stage is condensed out (desorption phase). Since in this known method the adsorbent is not regenerated during the drying phase, it loses its adsorption capacity to a considerable extent in the course of the drying phase, so that this method either requires a high level of equipment (low-temperature condensation stage and / or large amount of adsorbent) or When the drying room is opened, the environment is contaminated with considerable amounts of solvent vapor, the drying room is not flushed with room air before the workpieces are removed.

The invention had for its object to provide a method of the type mentioned, which can be carried out with a simply constructed system that can be operated without exhaust air and, consequently, makes it possible to dispense with flushing the treatment chamber or the drying room with room air. According to the invention, this object can be achieved in that, in a drying and desorption phase, the drying gas in the drying gas circuit after cooling and condensing a part of the entrained solvent is passed over a heated adsorbent for the solvent vapor in order to remove and desorb the solvent vapor desorbed from the heated adsorbent Supply condensation stage, and that for further purification of the drying gas this is passed in an adsorption phase in the drying gas circuit in a cool state over an adsorbent. In the drying and desorption phase, not only is a large part of the solvent vapor carried along by the drying gas removed in the condensation stage, but at the same time the heated adsorbent is regenerated by the drying gas, so that in the subsequent adsorption phase the solvent is largely removed from the drying gas by cool adsorbent can be removed that the maximum workplace con. below the concentration and as a result the workpieces can be removed from the system. In the method according to the invention, the problematic regeneration of the adsorbent with water vapor can therefore be dispensed with, the construction of the apparatus is extremely simple, and any adsorbent which is effective for the solvent used and which has a desorption, ie. H. Regeneration allowed at elevated temperatures. Of course, the treatment chamber in which the workpieces are cleaned can also be used as a drying chamber in the method according to the invention. Activated carbon is particularly recommended as the adsorbent, and a separate heating device for heating the adsorbent could be provided for heating the adsorbent for the purpose of desorption.

A major advantage of the method according to the invention is that it can be carried out free of waste air and waste water.

It should also be pointed out that it is known per se to regenerate activated carbon with hot air or hot inert gas (DE-PS 16 19 850), the air being passed through the activated carbon in countercurrent and the mixture of air and solvent vapor then being catalytically burned and the resulting hot gas stream is partially passed through the activated carbon again. On the other hand, in a system for carrying out the method according to the invention, the changeover valves and devices for preparing (drying, cleaning and heating) the regeneration gas or the devices for producing the water vapor that can be used for the desorption and its separation from the desorbed are required in the known regeneration processes for adsorbents Solvents.

In a preferred embodiment of the method according to the invention, the adsorbent for the desorption phase is not heated directly by a heating device, but rather by the drying gas which is heated behind the condensation stage. This not only ensures that the adsorbent is heated uniformly, but also creates the conditions for reusing the condensation heat generated in the condensation stage by means of a heat pump for heating the drying gas.

In order to cool the adsorbent again during the adsorption phase and, if necessary, to recover solvents in the condensation stage, it is advisable to cool the drying gas in the condensation stage also during the adsorption phase.

In principle, the drying gas during the adsorption phase could flow through the drying circuit in the opposite direction to the flow direction during the drying and desorption phase, but it is more advantageous to choose the same flow direction for both phases so that the drying gas from the condensation stage to the adsorber via the heating device switched on or off flows.

In a preferred embodiment of the cleaning method according to the invention, this is carried out in cycles, each of which comprises a cleaning phase during which the workpieces are cleaned, a drying and desorption phase and an adsorption phase, and the workpieces are not closed to the room or the treatment chamber until after completion of the Adsorption phase removed.

If an externally specified work cycle does not allow enough time to completely regenerate the adsorbent in the drying phase, it is recommended to start regeneration already during the cleaning phase by using drying gas to bypass the treatment chamber in the cleaning phase to desorb the adsorbent Drying gas circuit passed over the heated adsorbent and its solvent content is reduced by subsequent cooling; this procedure therefore only requires a bypass line that can be switched on and off parallel to the treatment chamber.

The invention also created a system for carrying out the method according to the invention described above, with one system starting from at least one closed treatment chamber for cleaning the workpieces with liquid solvent, a closed drying room for drying the cleaned workpieces and a drying gas circuit containing the drying room has, in which a cooler for the drying gas and in the direction of circulation of the drying gas after the cooler and in front of the drying room an adsorbent for the solvent receiving adsorber and a heating device for the drying gas. are; it is proposed according to the invention to arrange the heating device between the cooler and the adsorber for heating the adsorbent by the drying gas and to provide the cooler with a return line for condensed solvent. In such a system, only the heating device has to be switched off or on to switch from the drying and desorption phase to the absorption phase and vice versa, while no valves and other control devices are required. For heat recovery during the drying and desorption phase, a preferred embodiment of the system according to the invention has a heat pump via which the cooler and the heating device are coupled to one another.

Do you want to regenerate the adsorbent, d. H. With the desorption phase of the method according to the invention, regardless of the cycle time between loading and unloading the treatment chamber or the drying room, it is advisable to design the system according to the invention in such a way that the drying gas circuit has a plurality of regeneration circuits that can optionally be switched on in the latter with a drying gas circulation device , a cooler and a drying gas return line, which can be blocked by a valve, to complete the regeneration circuit.

Further features, advantages and details of the invention result from the appended claims and / or from the following description and the attached drawing of some preferred embodiments of the system according to the invention; Figures 1 to 3 represent three different embodiments schematically.

The system according to FIG. 1 has a treatment chamber 10 with a door 12 for loading and unloading, this door should be designed so that the treatment chamber can be closed gas-tight with it. The latter contains a holder, not shown, for workpieces to be cleaned, only one workpiece 14 being shown in FIG. 1. This is sprayed by means of spray tubes 16, which are held stationary or movable in the treatment chamber 10, with liquid solvent, which flows via an intermediate floor 18 and a valve 20 to an underlying collecting space 21, in which there is a filter 22, under which a line 24 into the Collection room 21 opens. The line 24 forms, together with a line 28 containing a pump 26 and a line 30 leading to the spray tubes 16, a solvent circuit, and by means of a distillation device 32 or the like, the solvent can be regenerated, e.g. B. freed from oils and fats. This distillation device is connected to the solvent circuit via valves 34 and 36, a line 38 and a pump 40.

A drying gas circuit, designated as a whole by 42, is also connected to the treatment chamber 10. This comprises a line 44 with valves 46 and 48 opening into the treatment chamber 10 with both ends, in which a fan 50, a condenser 52, a heating device 54 and an adsorber 56 are arranged one behind the other. In addition, a bypass line 62 provided with a valve 58 is provided, via which the drying gas circuit can be operated with the valves 46 and 48 closed, bypassing the treatment chamber 10. A return line 66 provided with a valve 64 leads from the condenser 52 to the treatment chamber 10 in order to be able to return the solvent condensed in the condenser 52 into the solvent circuit. The adsorber 56 should be filled with activated carbon.

After the workpiece 14 has been adequately cleaned, the pump 26 is switched off and, after the solvent has flowed off, the valve 20 is closed, whereupon with the valves 46 and 48 open and the valve 58 closed, the fan 50, the refrigerant circuit and the condenser 52, not shown, is shown the heater 54 are turned on. The air heated by the heater 54 is blown against the workpiece 14 and absorbs solvent vapor up to its saturation vapor pressure. Most of the solvent vapor condenses in the condenser 52, whereupon the air is heated again by the heating device 54 and the relative solvent vapor content is thus reduced. This in turn heats the activated carbon contained in the absorber 56, which is desorbed by the air flowing through it and thus regenerated. The solvent vapors released by the desorption in the absorber 56 are partially condensed in the condenser 52.

After completion of the drying and desorption phase, the entire system contains a solvent content, which is determined by the temperature in the condenser 52. Before the door 12 is opened and the workpiece 14 is removed from the treatment chamber 10, the drying air circulated by the fan 50 is freed by the regenerated adsorber 56 hendst from the solvent vapors still contained in it, the heating device 54 is switched off, but the condenser 52 is still kept in operation in order to cool the adsorber 56 and the line system; The regenerated activated carbon contained in the adsorber 56 then adsorbs the remaining solvent vapors. As soon as the solvent content of the circulating air is below the maximum permissible workplace concentration, the fan 50 is switched off and the workpiece can be removed from the treatment chamber.

Of course, the workpiece can also be dried in a separate drying room, which is connected to the treatment chamber 10 via a lock and is switched into the drying gas circuit 42.

If the regeneration of the adsorber 56 is to be started for reasons of time, as long as the workpiece 14 is still being cleaned, the valves 46 and 48 are closed and the valve 58 is opened so that the fan 50 can circulate air through the heating device 54 is heated and thus the activated carbon of the adsorber 56 is regenerated, while the solvent vapors condense in the condenser 52. After the cleaning process has ended, the regeneration of the adsorber 56 can then be continued during the drying phase.

The same reference numerals as in FIG. 1 have been used in FIG. 2 insofar as the two systems are identical, so that it is only necessary in the following to explain the deviations of the system according to FIG. 2 from the first embodiment.

The system has a drying gas circuit 42 connected to a treatment chamber 10 with two branches 42a and 42b connected in parallel, which are connected to the treatment chamber 10 via a line 44 and valves 46, 48. Each of the branches 42a, 42b comprises at its ends valves 70, 72 or 70 ', 72', between which in series in the flow direction of the drying gas a fan 50 or 50 ', a condenser 52 or 52', a heating device 54 or 54 'and an adsorber 56 or 56'. In order to expand the two drying gas circuit branches 42a, 42b into complete regeneration circuits 74a and 74b, lines 76 and 76 'are provided, each of which contains a valve 78 and 78', respectively.

Instead of the treatment chamber 10, another treatment chamber 10 'can also be switched into the drying gas circuit 42 via a line 44' and valves 46 ', 48' as long as the treatment chamber 10 is emptied and valves are loaded with new workpieces when the valves 46, 48 are closed.

The advantage of the system shown in FIG. 2 over that of FIG. 1 is not only that the adsorbers 56 and 56 'can be completely regenerated even if the cycle times for the drying phase are relatively short, e.g. B. because you work with several treatment chambers, but also in an energy saving: In a system as shown in Fig. 1, the adsorber must be heated up and cooled down at short intervals. A system corresponding to that according to FIG. 2 now makes it possible to use adsorbers 56 or 56 'with a large capacity, so that each solvent vapor is either adsorbed or regenerated over several cleaning cycles. So you can z. B. first use the branch 42a for the drying and desorption phase of several cleaning cycles, for whose adsorption phases the branch 42b is switched over and during which the adsorber 56 is regenerated by the regeneration circuit 74a. After a few cleaning cycles, drying is then carried out via the branch 42b and desorbed, adsorbed via the branch 42a, and at the same time the adsorber 56 'is regenerated via the regeneration circuit 74b.

Of course, in the system according to FIG. 2, the solvent taken back in the coolers or condensers 52 and 52 'is again fed to the collecting spaces 21, not shown, of the treatment chambers 10 and 10'.

3 contains means for heat recovery from the condensation stage for the purpose of heating the air circulating in the drying gas circuit and thus the adsorber for its regeneration.

A treatment chamber 100 is again in a drying gas circuit 102, which, starting from the treatment chamber, contains a fan 104, a condenser 106, a heating device 108, an additional electrical heating device 110 and an adsorber 112 in succession. Liquid solvent accumulating in the condenser 106 can be returned via a return line 66 into a space below the treatment chamber 100 corresponding to the collecting space 21 of the embodiment according to FIG. 1.

A refrigerant circuit 114 is also provided, which contains the condenser 106 as the evaporator and the heating device 108 as the condenser. In addition, a compressor 116 is provided in the refrigerant circuit 114 and behind it, in series for the refrigerant, an aftercooler 118, a collecting tank 120 and a throttle 122, which is located in front of the condenser 106 serving as an evaporator. The aftercooler 118 is supplied with cooling water or cooling air via a coolant line 126; In the coolant line there is a valve 128, which is controlled in a temperature-dependent manner by means of a temperature sensor 130. In addition, a temperature sensor 132 is provided in the refrigerant circuit 114 behind the condenser 106 serving as an evaporator in order to be able to control the throttle 122 as a function of the temperature. A post-evaporator, designated 134, which is designed as a heat exchanger for the refrigerant, serves to cool the liquid refrigerant further behind the collecting tank 120.

In order not to expel the drying air through the condenser 108 during the adsorption phase heat, this can be bridged by a bypass line 142 provided with a valve 140. For this purpose, a valve 144 is also provided in the refrigerant circuit 114 upstream of the condenser 108.

Claims (12)

1. A method for cleaning workpieces by means of a liquid solvent in a treatment chamber, in which method the workpieces, after cleaning, are dried in a closed drying compartment by a stream of gas, at least a part of the drying gas, in a drying gas circuit, being freed, by cooling in a condensation stage, from a part of the solvent carried along in vapour form, and being conveyed back into the drying chamber, and further with the use of an adsorption agent for adsorbing the solvent vapour arising during drying, and with the use of a heating device, characterised in that, in a drying and desorption phase, the drying gas in the drying gas circuit is conveyed, after a part of the solvent carried along has been cooled and condensed, through a heated adsorption agent for the solvent vapour, in order to convey away and to the condensation stage solvent vapour desorbed by the heated adsorption agent, and in that, for further cleaning of the drying gas, the latter is conveyed in an adsorption phase in the drying gas circuit through a cool adsorption agent.

2. A method according to Claim 1, characterised in that the adsorption agent for the desorption phase is heated in that the drying gas is heated after the condensation stage.

3. A method according to Claim 1 or 2, characterised in that the drying gas is cooled in the condensation stage during the adsorption phase also.

4. A method according to one or more of Claims 1 to 3, characterised in that the drying gas, during the adsorption phase, passes through the drying gas circuit in the same direction as in the drying and desorption phase.

5. A method according to one or more of Claims 1 to 4, characterised in that the workpieces are not removed from the drying compartment until after the adsorption phase.

6. A method according to one or more of Claims 1 to 5, characterised in that the cleaning process is carried out in cycles, each of which includes a cleaning phase during which the workpieces are cleaned, a drying and desorption phase, and an adsorption phase.

7. A method according to one or more of Claims 1 to 6, characterised in that, for desorbing the adsorption agent during the cleaning phase, drying gas is conveyed in the drying gas circuit through the heated adsorption agent, the treatment chamber being bypassed, and its solvent content is reduced by subsequent cooling.

8. A method according to one or more of Claims 1 to 7, characterised by a return of heat from the condensation stage to that area of the drying gas circuit in which the drying gas or the adsorption agent is heated.

9. An installation for carrying out the method according to one or more of Claims 1 to 8, having at least one closed treatment chamber (10 ; 100) for cleaning the workpieces with liquid solvent, and having a closed drying compartment for drying the cleaned workpieces, and having a drying gas circuit (42 ; 102) containing the drying compartment, in which circuit are arranged a cooler (52; 106) for the drying gas and, after the cooler and before the drying compartment in the direction of circulation, an adsorber (56 ; 112) accommodating an adsorption agent for the solvent, and a heating device (54; 108, 110) for the drying gas, characterised in that, for heating the adsorption agent by the drying gas, the heating agent (54; 108, 110) is arranged between the cooler (52 ; 106) and the adsorber (56 ; 112), and in that the cooler (52 ; 106) is provided with a return line (66) for condensed solvent.

10. An installation according to Claim 9, characterised in that the drying gas circuit (42) contains a bypass line (62) which bridges the drying compartment (10) and which can be connected in.

11. An installation according to Claim 9 or 10, characterised in that the cooler (106) and the heating device (108) are coupled together via a heat pump (116, 108, 106).

12. An installation according to one or more of Claims 9 to 11, characterised in that the drying gas circuit (42) has several regeneration circuits (74a, 74b) - which can be alternatively connected into the drying gas circuit - with a drying gas circulating device (50 or 50'), a cooler (52 or 52'), a heating device (54 or 54'), an adsorber (56 or 56') and a drying gas return line (76 or 76') which can be blocked by a valve (78 or 78') for completing the regeneration circuit

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