WO1993024198A1

WO1993024198A1 - Process and device for treating contaminated high boiling solvents

Info

Publication number: WO1993024198A1
Application number: PCT/EP1993/001361
Authority: WO
Inventors: Michael Rannow
Original assignee: Transferon Wäschereimaschinen Gmbh
Priority date: 1992-06-04
Filing date: 1993-05-30
Publication date: 1993-12-09
Also published as: DE4218451A1

Abstract

A process and device are disclosed for treating contaminated high boiling solvents, such as those produced for example in chemical cleaning machines. The evaporable solvents and low boiling impurities are separated during a first separation step in a flash evaporator (4) from the high boiling impurities that are retained as a residue, and are supplied by a volumetric ventilator (13) to a condenser (6) where a second separation step is carried out, in which the low boiling impurities are separated from the pure solvent; the first are extracted as vapour and are condensed in a separate, outwardly cooled condenser (20), whereas the pure solvent is condensed in the first condenser (6) and recycled.

Description

Process for the preparation of contaminated higher-boiling solvents and device for carrying out the process

The invention relates to a process for the preparation of contaminated, higher-boiling solvents, in which all components of the contaminated solvent, with the exception of the higher-boiling contaminants, are converted into vapor form in one evaporation process and in which the solvent is subsequently condensed,

as well as a

Device for performing this method with

a) an evaporation device in which all components of the contaminated solvent, with the exception of the higher-boiling contaminants, are converted into vapor form;

b) a condenser in which the solvent is condensed from the vapor form.

So far, perchlorethylene has preferably been used as the organic solvent for cleaning textiles and leather. This solvent has a relatively low boiling point of around 121 ° C. The contaminated solvent is processed by simple bubble distillation at atmospheric pressure. Solvent and water are distilled off, condensed and separated from one another by gravity. All contaminants that have got into the solvent from the items to be cleaned remain in the distillation residue and become batches of residual sludge taken from the still.

Because of the ecological disadvantages of perchlorethylene, more and more solvents, especially hydrocarbons, are being used. For safety reasons, products with a boiling point above approximately 170 ° C and a flash point above ^: 55 ° C are used. Even such higher-boiling solvents must be completely recovered and regenerated in the closed circuit during drying of the items to be cleaned. This is currently done in the same manner as described above in which the perchlorethylene is also processed, namely by distillation in a still, which is however operated under vacuum. The known device is relatively simple; however, the cleaning effect - unlike when used for perchlorethylene - is poor. The solvent absorbs both non-volatile (higher-boiling) and volatile (low-boiling) components from the textiles. When working with perchlorethylene, the low-boiling constituents remain in the residue during distillation because their boiling points are higher than that of perchlorethylene. This situation changes when higher-boiling solvents are used: the boiling point of the low-boiling impurities is below that of the solvent boiling at 170 to 190 ° C .; these impurities therefore pass into the distillate during evaporation. They are thus enriched in the solvent. This ultimately leads to a lowering of the flash point of the solvent to below 55 ° C or even below 21 ° C, which places the solvent in safety groups A2 or even AI in the categories of flammable liquids. Then the chemical cleaning may no longer be operated under the conditions provided until now; Explosion-proof electrical installations must be installed and other similar safety precautions must be taken. A disadvantage of the known method and the known device for the treatment of higher-boiling hydrocarbons is also that the residues remaining in the still are exposed to a permanent, high temperature load, because these residues are removed only once a day or even less frequently and so that they remain in the heated still for a long time. This long dwell time at the high surface temperature of the heated still bottom leads to the decomposition and conversion of the contaminants, from which odors are formed. In the case of higher-boiling solvents, these substances are exaggerated in the bubble distillation and enriched in the solvent. This also means that the solvent can no longer be used for cleaning in the long term.

The object of the present invention is to design a method or a device of the type mentioned at the outset so that complete cleaning of the solvent is ensured.

This object is achieved in a method of the type mentioned in that

in the subsequent condensation process by choosing

Temperature and / or pressure selectively the solvent is condensed while low-boiling contaminants remain in the steam bunny.

In the method according to the invention, unlike the known method, two “separating cuts” are thus carried out, in which the solvent is separated from impurities: the first “separating cut” removes the higher-boiling impurities from the solvent. For this purpose, solvents and all low-boiling impurities evaporated while the higher boiling Ver ^¬ impurities remain behind as a residue. In the second “separation cut”, the solvent is separated from the low-boiling impurities by choosing the process parameters temperature and / or pressure. In this case, the solvent is selectively condensed out, while the low-boiling impurities ^¬ initially remain still in vapor form and are recovered later in a separate, externally cooled condensation process. The solvent prepared with the method according to the invention is thus free of all impurities, be they low or high boiling. Enrichment with, in particular, low-boiling impurities, which could result in a shift in the flash point, no longer occurs.

In a preferred exemplary embodiment of the method, the contaminated solvent is evaporated in a flashing process. The relaxation process is in contrast to the bubble distillation previously used. The contaminated solvent, and in particular the residue that forms, is no longer in contact with a hot surface. The decomposition and conversion of contaminants, in which odor carriers can form, is thus reduced.

Advantageously, the vapors produced in the evaporation process are compressed during suction and the heat supplied to the vapors and the enthalpy of vaporization are used to preheat the contaminated solvent supplied to the evaporation. With careful management of the method according to the invention, a separate heating can largely be dispensed with due to the mechanical energy which is supplied to the vapors during the compression, and by the recovery of the enthalpy of vaporization. The vapors generated in the evaporation process should be conveyed volumetrically to the condensation process. This avoids a shift in the partial pressure ratio, which would result in a deterioration in the efficiency in the selective condensation.

The residue remaining in the evaporation process can be fed several times in the evaporation cycle until it consists almost entirely of higher-boiling impurities. In contrast to the known method, all lower-boiling components are therefore not removed from the contaminated solvent "in one step". Rather, this takes place in a large number of passes through the evaporation process, so that the residence time of the contaminated solvent or of the residue in the evaporation device is relatively short. This also reduces the risk of contamination decomposing during evaporation.

According to a further feature of the method according to the invention, the boiling point of the residue which forms on evaporation can be monitored; in the event of a significant shift, the residue in the circuit is subtracted. With this characteristic it has the following relation:

As a result of the constant circulation through the evaporation process, the residue which forms in the process is increasingly enriched with high-boiling impurities until it finally consists almost entirely of such impurities. This process is associated with a corresponding shift in the boiling point of the residue, which can be detected by monitoring the pressure and / or the temperature in the evaporation device. A significant shift in the boiling point is therefore taken as an indication that the residue is now almost exclusively from high-boiling Contamination exists and can be deducted.

The process according to the invention also contributes to a favorable energy balance if the residue remaining in the evaporation is used as a coolant in the condensation of the vapors formed.

The separation of the low-boiling contaminants from the solvent in the selective condensation process requires precise control of the pressure at which this process takes place. In this case, an embodiment of the method is particularly recommended, in which the solvent is separated from the low-boiling impurities at a pressure which corresponds to the partial pressure of a liquid used as a pump medium. This partial pressure is very well defined and can also be reproduced very well.

It is further recommended that the partial pressure of the liquid used as the pump medium is adjusted by regulating its temperature.

The liquid used as the pump medium is expediently the condensate of the low-boiling impurities.

The inventive device for performing the method is characterized by

c) a vacuum device which generates a defined vacuum in the condenser, such that the low-boiling impurities remain in vapor form at the prevailing temperature and only the pure solvent condenses.

The advantages of the device according to the invention have been already indicated above in the explanation of the method according to the invention.

Advantageous refinements of the device according to the invention are described in claims 11 to 22, the advantages of which have already been explained analogously above in the discussion of the inventive method. Supplementary, therefore, only a few of these device configurations are explicitly dealt with:

If, as stated in claim 13, a volumetric pump is used which conveys all vapors generated in the evaporation device, regardless of their type, to the condenser, it is avoided that there is a shift in the partial pressure ratio in the

Condenser transferred steam (mixture) results. Such a shift would result in poorer efficiency of the subsequent selective condensation.

According to claim 18, a liquid ring pump is particularly suitable for generating the negative pressure in the condenser, and operates continuously at its final pressure, which is determined by the partial pressure of the ring liquid. Then the vacuum in the condenser also corresponds to the partial pressure of the ring liquid.

According to claim 19, the liquid ring pump can be followed by an externally cooled condenser for the low-boiling impurities. In this condenser, which can contain a water cooling coil, for example, the low-boiling impurities are then obtained as a liquid.

According to claim 22, a connecting line can be provided between the condenser and the liquid ring pump be, via which permanent gases can be supplied from the condenser to the liquid ring pump. These permanent gases reduce the tendency to form cavitation within the liquid ring pump and thus reduce wear and operating noise.

An embodiment of the invention is explained below with reference to the drawing; the only figure schematically shows a device for processing higher-boiling hydrocarbons.

The e.g. High-boiling hydrocarbon coming from a chemical cleaning machine and contaminated is input via line 1 at a point 2 into a circuit which circulates in the device and is explained in more detail below. The addition is carried out continuously; if the contaminated solvent - as is the case with chemical cleaning machines - accumulates in batches, it is temporarily stored in a tank. Before being added to the circuit at point 2, the solvent can be heated to a temperature in the range between about 125 and 130 ° C. in a start-up heater.

The residue circulating in the circuit, which contains the contaminated solvent newly added at point 2, first flows into a heat exchanger 3, which it leaves at a temperature of about 140 ° C. in a superheated state. It then passes into a flash evaporator 4, in which there is an absolute pressure of approximately 50 millibars. By removing the heat of evaporation, the temperature of the solvent and the vapors within the expansion evaporator 4 drops to about 128 to 130 ° C.

At the bottom of the flash evaporator 4, the non-evaporated solvent that collects with higher boiling points Impurities is enriched. It is fed as a residue from the pump 5 to a condenser 6, through which it passes as a cooling liquid and from which it returns via a throttle 7 to the line point 2 already mentioned above.

The residue runs through the circuit just described several times, with each passage through the flash evaporator 4, a part of the solvent and the low-boiling impurities contained therein evaporate, while the residue, which is pump 5 from the flash evaporator 4 is withdrawn, increasingly enriched with higher-boiling impurities. After a certain time, the residue in the circulation has become so heavily enriched with higher-boiling impurities that it results in a considerable shift in the boiling point. This can be determined by changing the temperature and / or the pressure within the relaxation evaporator 4. For this purpose, the output signals of a temperature sensor 8 and a pressure sensor 25 are fed to evaluation electronics 26, in which the boiling point curve of the solvent is stored. If there is a significant deviation from the stored boiling point curve, the evaluation electronics 26 opens a solenoid valve 9. This is located in a discharge line 10, which branches off from the line 11 running between the pump 5 and the condenser 6. The residue in the circuit is now withdrawn via line 10; it consists almost exclusively of higher-boiling impurities.

Only the freshly supplied contaminated solvent coming from the chemical cleaning machine comes via line 1 to the flash evaporator 4. This still contains a high proportion of low-boiling components in the flash evaporator 4 evaporate in the "normal" range of the boiling point curve. The solenoid valve 9 is closed again by the evaluation electronics 26. The cycle of the residue from the line point 2 via the heat exchanger 3, the flash evaporator 4, the pump 5, the condenser 6 and the throttle 7 starts again.

The vapors generated in the flash evaporator 4 contain, as already mentioned above, the solvent itself and low-boiling impurities. These vapors first pass through a separator 12 within the flash evaporator 4, on which aerosols separate. The vapors are fed by a volumetric fan 13, e.g. a Roots blower, suctioned, which ensures the already mentioned absolute pressure of about 50 millibars within the flash evaporator 4. The volumetric fan 13 conveys all types of vapors and gases in the same way, so that there can be no shift in the partial pressure ratio in the steam (mixture) it moves.

Due to the work done by the blower 13 on the vapors, their temperature rises from the above-mentioned approx. 128 to 130 ° C., which prevail in the expansion evaporator 4 during normal operation, to approx. 150 ° C.

At this temperature, the vapors are fed to the heat exchanger 3 and, as has also been explained, heat the residue circulating in the circuit to the temperature required for admission to the expansion evaporator 4.

The thus pre-cooled vapors leave the heat exchanger 3 and are fed to the condenser 6, in which a very defined vacuum is maintained in a manner to be described. This vacuum is - left

The steam temperature is adjusted so that just the solvent condenses out within the condenser 6, while the low-boiling impurities remain in the vapor form and can be drawn off via a line 14. The recovered solvent, which is now freed of both higher and lower-boiling impurities, is conveyed out of the condenser 6 by the pump 15 and fed to the reuse via the line 16.

The vapors of the low-boiling impurities removed from the condenser 6 via the line 14 are drawn off by a liquid ring pump 17 via a throttle 18. The liquid ring pump 17 is operated so that the pressure in the line 14 and thus in the condenser 6 is exclusively determined by the liquid partial pressure of its ring liquid.

The liquid ring pump 17 supplies the vapors of low-boiling impurities to a condenser 19, which is cooled by a cooling coil 20. The condensate of the low-boiling impurities formed in the condenser 19 is continuously drawn off via a line 21.

A small part of the condensate of the low-boiling impurities is returned via a return line 22 to the liquid ring pump 17 and serves there as a ring liquid, which determines the pressure in the line 14 and in the condenser 6 generated by the liquid ring pump 17 . The exact setting of the partial pressure at which the "correct" separation cut takes place within the condenser 6 at the operating temperature prevailing there, at which the solvent is condensing while the low-boiling impurities remain in vapor form, is carried out by the choice of the temperature the condensate of the low-boiling impurities, which leaves the heat exchanger 19. This temperature is monitored by a temperature sensor 23, which is connected in a suitable manner to a control circuit (not shown in the drawing). This control circuit controls the cooling within the capacitor 19 so ^'that the capacitor 19 condensate leaving always has the same, selected Tempe¬ temperature.

In this way, an extremely exact and insensitive regulation of the pressure in the condenser 6 is possible by simple temperature stabilization of the condensate of low-boiling liquids, which is responsible for the correct separation cut within the condenser 6.

In the drawing, a connecting line 24 is also shown, via which permanent gases can be removed from the condenser 19 and fed to the liquid ring pump 17. The permanent gases counteract the formation of cavitation within the liquid ring pump 17.

An additional heater 27 serves to bring the residue forming in the flash evaporator 4 or already located there to the required temperature in the start-up phase of the device and, if necessary, to bring about small changes in the temperature of the residue during operation of the device. to "trim" this temperature.

In the start-up phase of the device, distillate can also be removed from the condenser 6 via a line 28 and fed to the blower 13, so that overheating of the blower 13 is avoided and at the same time the heat is transferred to the expansion evaporator 4. The entire device manages essentially without the supply of external heat, apart from any start-up heating that may be required. The clever choice of the various heat exchangers and the mechanical energy of the fan 13, which is converted into heat, enables continuous operation with a closed heat balance.

Claims

1. A process for the preparation of contaminated, higher-boiling solvents, in which all components of the contaminated solvent with the exception of the higher-boiling contaminants are converted into vapor form in an evaporation process and in which the solvent is subsequently condensed,

characterized in that

in the subsequent condensation process by choosing

Temperature and / or pressure selectively the solvent is condensed while low boiling contaminants remain in the vapor form.

2. The method according to claim 1, characterized in that the evaporation of the contaminated solvent is carried out in a relaxation process.

3. The method according to claim 1 or 2, characterized in that the vapors formed in the evaporation process are compressed during suction and the heat supplied to the vapors and the evaporation enthalpy is used for preheating the contaminated solvent supplied to the evaporation.

4. The method according to any one of claims 1 to 3, characterized in that the vapors arising in the evaporation process are volumetrically conveyed to the condensation process.

5. The method according to any one of the preceding claims, characterized in that the residue remaining in the evaporation process is fed several times in the evaporation cycle until it consists almost entirely of higher-boiling impurities.

6. The method according to claim 5, characterized in that the boiling point of the evaporation

Residue is monitored and the residue in the circuit is subtracted in the event of a significant shift in the boiling point.

7. The method according to any one of the preceding claims, characterized in that the residue remaining in the evaporation is used as a coolant in the condensation of the vapors formed.

8. The method according to any one of the preceding claims, characterized in that the separation of the solvent from the low-boiling impurities in the condensation takes place at a pressure which corresponds to the partial pressure of a liquid used as a pump medium

9. The method according to claim 8, characterized in that the partial pressure used as the pumping means

Liquid is adjusted by regulating its temperature.

10. The method according to claim 8 or 9, characterized in that the liquid used as the pump medium

Is condensate of the low-boiling impurities.

11. Device for carrying out the method according to claim 1 with a) an evaporation device in which all components of the contaminated solvent, with the exception of the higher-boiling contaminants, are converted into vapor form;

b) a condenser in which the solvent is condensed from the vapor form,

marked by

c) a vacuum device (17, 19, 20, 21, 22, 23), which generates a defined vacuum in the condenser (6), such that the low-boiling impurities remain in vapor form at the prevailing temperature and only the pure solvent condenses.

12. The apparatus according to claim 11, characterized in that the evaporation device (4) is a flash evaporator in which there is a negative pressure.

13. The apparatus according to claim 12, characterized by a volumetric pump (13) which conveys all vapors arising in the evaporation device (4) without shifting the partial pressure ratio to the condenser (6).

14. The apparatus according to claim 13, characterized by a heat exchanger (3) in which heat transfer takes place from the vapors heated by the volumetrically operating pump (13) to the contaminated solvent supplied to the evaporation device (4).

15. The device according to one of claims 11 to 14, characterized in that the cooling liquid in the condenser (6) in the evaporation device (4) remaining Arrears.

16. The apparatus according to claim 15, characterized by a pump (5) which promotes the residue formed in the evaporation device (4) through the condenser (6) in the circuit back to the inlet of the evaporation device (4).

17. The apparatus according to claim 16, characterized by a device (8, 25, 26) which monitors the boiling point of the residue in the evaporation device (4) and, in the event of a significant change, opens a valve (9) via which the residue in the circuit is subtracted.

18. Device according to one of claims 11 to 17, characterized in that the vacuum device (17,

20, 21, 22, 23) comprises a liquid ring pump (17) which is operated in such a way that the negative pressure generated by it is determined exclusively by the partial pressure of the ring liquid.

19. The apparatus according to claim 18, characterized in that the liquid ring pump (17) is followed by an externally cooled condenser (19) for the low-boiling impurities.

20. The apparatus according to claim 19, characterized in that the ring liquid is the condensate of the low-boiling impurities formed in the condenser (19).

21. The apparatus according to claim 20, characterized by a temperature sensor (23) which measures the temperature of the ring liquid, and by a control device, which controls the cooling (20) of the condenser (19) according to the output signal of the temperature sensor (23) so that a constant temperature of the condensate is maintained.

22. Device according to one of claims 18 to 20, marked by a connecting line (24) between the condenser (19) and liquid ring pump (17), via which permanent gases from the condenser (19) can be supplied to the liquid ring pump.