Method for dry cleaning with petroleum solvent

Abstract

 In a method for dry cleaning with a petroleum solvent, the inside of a washing bath in which at least fabrics to be cleaned are placed is depressurized to a predetermined value, and thereafter an inert gas is introduced into the washing bath until the oxygen density of the mixture containing the solvent gas takes a value below a predetermined one, namely, a peak value of the inflammable range, thereby eliminating the danger of solvent gas explosion and shortening the time for drying of the fabrics, these steps being done before the step of drying the fabrics.

Description

BACKGROUND OF THE INVENTION

a) Field of the Invention

[0001] The present invention relates to a dry cleaning with a petroleum solvent of which the flash point is relatively low, and more particularly to a method for dry cleaning which can be done with a high safety and without any danger of explosion of the evaporated solvent during drying of treated fabrics or garments.

b) Description of the Prior Art:

[0002] Generally, the dry cleaning, more properly called "chemical cleaning" is classified into two types depending upon the kind of the solvent used in treating the washing such as fabrics or garments, etc: one uses a petroleum solvent and the other uses a synthetic solvent such as tetra­chloroethylene. The petroleum solvent is a product obtained through refining of a crude oil taken out from underground. A variety of solvents is available as classified according to their different boiling points during refining of the crude oil. These two types of dry cleaning are different from each other in cleaning machine, soap and treatment with which they are used as well as in influence of them on the finish of the fabrics and garments.

[0003] The conventional dry cleaning comprises at least a step of washing the fabrics with a solvent supplied into a washing bath rotated at a low speed, a step of centrifuging the fabrics with the washing bath rotated at a high speed, a step of distilling the drain by an indirect heating with a steam heat source, a step of distilling/recovering at which distilling the waste liquid from the drum by an indirect heating with a steam heat source and recovering clean solvent through distillation or purification by a condenser and water separator and a step of drying/solvent recovering at which the centrifuged fabrics are dried while the evaporated solvent is recovered as condensed. Generally, the equipment for such dry cleaning is a composite-type one comprising a washing and centrifuging machine, dryer and distillator in combination, or an integrated-type one with a machine having functions of washing, centrifuging and drying and a distiller. In the conventional dry cleaning with a petroleum solvent, since the flash point of the solvent is as low as about 41°C, air is heated by the heater is introduced into the washing bath and also circulated through the drying route to dry the fabrics. As the temperature of the fabrics rises gradually, the ratio of the solvent gas to air is reduced. In such conventional dry cleaning, arcing of the static electricity on the fabrics or spark caused by collision between the metallic portion of a gas lighter or the like forgotten in the fabric or garment and the inner wall of the inner drum of the washing bath is liable to ignite the solvent vapor, causing an explosion. To avoid such danger of explosion, a vacuum dryer has been proposed as disclosed in, for example, the Japanese Unexamined Patent Publication No. 61-240999, the inside of the drying drum of which being depressurized to promote the evaporation of the solvent in the fabrics, thereby lowering the drying temperature of the fabrics. In such vacuum dryer, however, it is difficult to control the ratio of the solvent gas to air inside the drying drum and there is a possibility that the heater to supply a hot air for drying the fabrics gets out of order, so that if the solvent used is a one of which the flash point is low, namely, a petroleum solvent, it is not possible to completely prevent the explosion of the solvent gas.

[0004] Generally speaking, a mixture of a solvent vapor and air will not explode even with a large energy applied thereto unless the composition of the mixture is within a certain range of composition (density). This range is called the limits of inflammability or explosion limits, lower and upper. In this respect, the following is already known. The limits of inflammability do not only exist with a mixture of a kind of combustible material and air but also with a mixture of plural constituents. For example, in case a noncombustible material is added to a mixture of solvent vapor and air, the lower limit does not vary so much but the upper limit does considerably, so that the flammable range narrows gradually as the additive amount increases. When the additive amount reaches a certain value, the lower and upper limits coincide with each other with the result that no combustion will take place any more irrespectively of the composition of the mixture. In this sense, the above-­ mentioned point is called a peak value of the limits of inflammability which depends upon the constituents of a mixture and kinds of additive materials. Such addition of noncombustible material for limits of inflammability is widely effected for prevention of explosion and fire extinction in industries where combustible mixture is used, and it is very important against a danger such as explosion or firing.

[0005] The dry cleaning method according to the present invention is based on the fact that the density of oxygen at a peak value of the limits of inflammability is about 11% when the solvent used, of whichever kind it is, is diluted with nitrogen gas and about 14% when the solvent is diluted with carbon dioxide, no explosion taking place at a lower density.

SUMMARY OF THE INVENTION

[0006] The present invention has an object to provide a method for dry cleaning with a petroleum solvent of which the flash point is low, which can be done safely without any danger of explosion of the evaporated solvent.

[0007] The above-mentioned object can be achieved by providing a method for dry cleaning with a petroleum solvent, comprising, according to the present invention, a step of depressurizing to at least a predetermined value the inside of a washing bath having fabrics therein and then introducing an inert gas into the washing bath to maintain below a predetermined value the oxygen density of a mixture containing a solvent gas, which step being done at least before a step of drying the fabrics.

[0008] In the method for dry cleaning with a petroleum solvent according to the present invention, the washing bath inside can be depressurized to the predetermined value to lower the oxygen density in the mixture to below the predetermined value at least before the step of drying, and then the oxygen density after dilution of the inert gas can be maintained at the predetermined value, so that the danger of solvent gas explosion in the subsequent drying step can be radically eliminated and the inert gas serves as a heat medium in place of air to shorten the time for drying the fabrics.

[0009] Also, the step of depressurizing and inert gas introduction may be effected before a series of dry cleaning steps is done, namely, with the fabrics to be cleaned put into the washing bath, then the drum inside depressurized to the predetermined value, the inert gas introduced into the drum and thus the oxygen density of the mixture containing the solvent gas, inside the drum, maintained to below the predetermined value, subsequent steps of washing, centrifuging, rinsing, draining/distillation and drying/solve recovery can be successively effected.

BRIEF DESCRIPTION OF THE DRAWING

[0010] 

Fig. 1 is a schematic piping diagram of the system by which the method for dry cleaning according to the present invention is carried out.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0011] 

[0012] One embodiment of the method for dry cleaning according to the present invention will be described in detail with reference to the drawing. The drawing schematically illustrates the entire system for carrying out the method for dry cleaning according to the present invention. In the drawing, the reference numeral 10 indicates a washing bath consisting of an inner drum 12 in which the fabrics or garments to be cleaned are put and an outer drum provided with a door 13 for an opening through which the fabrics are inserted and removed. The washing of the fabrics in the washing bath 10 is done in two modes: one is a washing with stay of solvent in which the solvent is supplied in a predetermined amount from a base reservoir 16 for the solvent to the washing bath 10 by means of a pump 18 and washing is done with the solvent staying in the washing bath 10, and the other is a washing with circulation of solvent in which the washing is done with the solvent circulated. These modes of washing may be independently done as selected depending upon the parameters such as kinds, extent of staining and weight of the fabrics, etc. or they may be done in combination, for example, the washing with solvent stay is done as followed by the washing with solvent circulation. This embodiment will be explained herebelow for the washing with solvent circulation. The solvent used in the embodiment is "DRY SOLVENT (trade name) available from the Nippon Oil Co., Ltd. and its flash point is about 41°C.

[0013] The dry cleaning system for the method for dry cleaning according to the present invention comprises a circulation line A through which the solvent is supplied to the washing bath 10 via a filter 15 from the base reservoir 16 and also the solvent is circulated through a button trap 17, thereby permitting the washing with solvent circulation, a line B through which the solvent (drain) removed from the fabrics due to the high speed rotation of the washing bath 10 and containing the stains is delivered to a distiller 20, a line C through which the solvent distilled in the distiller 20 as indirectly heated with a hot vapor from the vapor source is recovered as clean solvent into the purifying agent reservoir 26 through a condenser 22, water separator 24 and valve 25, a drying line D incorporating a dust filter 20, fan 31 and heater 32 to dry the fabrics from which water has been removed, a recovery line E through which the solvent evaporated from the fabrics and inner wall of the washing bath is condensed in a condenser 34 and recovered through a water separator 38 into the base reservoir 16, a vacuum pump 42 connected to the drying line D by means of a valve 44, and a nitrogen gas generator 46 connected to the washing bath 10 by means of a pressure regulator or reducing valve 48. The method for dry cleaning using the dry cleaning system of the above-mentioned construction will be described below.

[0014] First, the solvent is supplied to the washing bath 10 by means of the pump 18 along a line from the base reservoir 16, valve 50, check valve 52, filter 15, valve 54 to the washing bath 10. The level of solvent inside the washing bath 10 is set at a predetermined position by a high level float 58 or low level float 60 provided in the button trap 60. When the set liquid level is reached, the valve 50 provides a disconnection between the base reservoir 16 and pump 18 and connects the inlet of the pump 18 to the button trap 17. Therefore, after the set liquid level is reached, the solvent is circulated along a line from the washing bath 10, valve 56, button trap 17, valve 50, check valve 52, filter 15, valve 54 to the washing bath 10. With the solvent circulated in this way, the inner drum 12 of the washing bath 10 is rotated forward or reversely, whereby the fabrics are washed with solvent circulated.

[0015] After completion of this washing step, the valve 50 disconnects the base reservoir 16 from the pump 18 and also the inlet of the pump 18 from the button trap 17, and further the button trap 17 has a valve 62 thereof opened. The contaminated solvent is sent to the distiller 20 through a valve 62 and check valve 66, and thereafter the contaminated solvent is removed with the fabrics centrifuged in the inner drum 12 of the washing bath 10 rotated at high speed, and similarly sent to the distiller 20 where it is distilled. The evaporated solvent is condensed in the condenser 22. The clean solvent thus obtained through the distillation is recovered into the clean solvent reservoir 26 through the water separator 24, valve 23 and valve 25. The clean solvent thus recovered into the clean solvent reservoir 26 is reused in the step of rinsing as will be described. Also, the water separated in the water separator 24 is once preserved in a water reservoir 72, and discharged after completion of the distillation. In the aforementioned step of distillation/­solvent recovery, the distillation/solvent recovery line C is maintained under a pressure of about 160 Torr as evacuated by a vacuum pump 74. Namely, with valves 70, 78, 80 and 82 closed while the valves 23 and 25 are opened, the vacuum pump 74 is put into run, and thereafter a valve 76 us opened, the evacuation of the distillation/solvent recovery line C is started. A pressure controller 84 having a vacuum gauge provided in the distillation/solvent recovery line C adjusts the opening of the valve 76 according to the pressure in the line C as shown with dash line to maintain the pressure of about 160 Torr. Thus, even if the temperature of the vapor supplied as heat source to the distillator 20 is low, namely, even if the vapor pressure is low, it is sufficient for distillation of the contaminated solvent and permits to reduce the time for distillation.

[0016] With the inner drum 12 of the washing bath 10 rotated at high speed when the contaminated solvent has been nearly entirely discharged from the washing bath 10 and thus the contaminated solvent removed from the fabrics, the step of rinsing is done with the clean solvent supplied from the clean solvent reservoir 26 into the washing bath 10 with the valves 25 and 76 closed while the valves 70 and 78 are opened. The solvent used in the rinsing is returned into the base reservoir 16 through the valve 56 and a valve 64 provided in the button trap 17. Namely, when the solvent has been nearly entirely removed from the fabrics due to the high speed rotation of the inner drum 12 of the washing bath 10, the valve 62 is closed while the valve 64 is opened.

[0017] Immediately after completion of such rinsing step, the drying line D including the washing bath 10 is depressurized as evacuated by the vacuum pump 42 until the oxygen density of the mixture in the line D becomes about 5 vol%. Thereafter, nitrogen gas is introduced from the nitrogen gas generator 46 and the oxygen density of about 5.0 vol% is maintained after dilution with the nitrogen gas, thus the step of drying is started. This drying step will be explained in detail below.

[0018] Upon completion of the rinsing step, the valves 54 and 70 remain closed while the valve 56 is open. The portion of the washing bath 10 higher than the predetermined liquid level and the upper portion of the base reservoir 16 com­municate with each other through a valve 57 provided atop of the button trap 17, and valves 29 and 33 in the drying line D are opened, so that the space to be evacuated by the vacuum pump 42 is the insides of the washing bath 10 and drying line D, upper space of the button trap 17 and upper space of the base reservoir 16 (where the solvent used in the step of rinsing stays). The volume of such spaces is nearly 1,400 liters in this embodiment. The spaces to be evacuated are charged with solvent gas at completion of the rinsing step, and the evacuation is started by opening the valve 44. The pressure inside the washing bath 10 is monitored by the pressure controller 84, and when it has reached a value of from 160 to 260 Torr, the valve 44 is closed. The oxygen density in the drying line D is measured by an oxygen density sensor 92 provided in the upper space of the base reservoir 16. The oxygen density before evacuation is nearly 20.8 vol%, but after evacuation, it is about 5.0 vol%. In this state, the pressure regulator or reducing valve 48 is opened and a nitrogen gas of about 99% in purity is introduced from the nitrogen gas generator 46 until the pressure inside the drying line D reaches a value of from 560 to 610 Torr. Then the oxygen density in the drying line D is about 5.2 vol%. The step of drying/solvent recovery starts at this state.

[0019] First, the fan 31 is started, and the temperature of the heater 32 is set to about 80°C (50°C at the inlet). Also the water temperature of the chiller of the drying condenser 34 is set to a value of about 13 to 15 °C as measured at the inlet, and the step of drying/solvent recovery is started. The heat medium containing the nitrogen gas in the drying line D is heated by the heater 32, and circulated by the fan 31 through the drying line D and washing bath 10. The fabrics are dried with the heat medium thus circulated. In this system, the nitrogen gas takes the place of air as heat medium, and the system inside is depressurized, so that the solvent contained in the fabrics is actively evaporated, thus permitting to reduce the time for drying and also completely eliminate the danger of explosion. The evaporated solvent is condensed in the condenser 34 and recovered into the base reservoir 16 through the water separator 38. When the step of drying/solvent recovery has been completed, the pressure in the drying line D is lower than the atmospheric pressure, so that after a valve 90 provided in the drying line D is opened, the door 13 can be opened for taking out the dry fabrics.

[0020] According to this embodiment, the oxygen density of the mixture in the drying line D is monitored with the oxygen density sensor 92 in the series of steps from the washing to drying/solvent recovery. When the density reaches about 10 vol%, the alarm (not shown) is activated to give an alarm to the attendant of the dry cleaning system. Also the oxygen density sensor 92 may be connected along with the pressure controller 84 provided in the distillation/solvent recovery line C to a microcomputer (not shown) so that when the oxygen density has reached about 10 vol%, the nitrogen gas is introduced again into the drying line D to dilute the mixture until the oxygen density reaches about 5.2 vol%. The nitrogen gas is introduced from the nitrogen gas generator 46 until the pressure in the drying line D takes a value of from 560 to 610 Torr, but the nitrogen gas may be introduced until the oxygen density takes a value nearly equal to the atmospheric pressure. It is substantial that the oxygen density of the mixture at this time should be less than 11 vol%. In this case, the pressure is checked by reading the scale of the vacuum gauge (not shown) provided in the washing bath 10, and after the pressure in the drying line D is adjusted to the atmospheric pressure by opening the valve 90 for ensuring the safety, the door 13 is opened to take out the dry fabrics. Carbon dioxide may be used as inert gas. In this case, the oxygen density of the mixture diluted with the carbon dioxide may be less than about 14 vol%. The dust filter 30 has provided therein a safety valve 94 which is so arranged as to open under a pressure of 0.06 kg/cm² and prevent the pressure in the drying line D from rising abnormally. In the series of steps from washing with solvent circulation to drying/solvent recovery, the drying line D is always communicated with the washing bath 10. However, the system should preferably be arranged so that after the valve 90 is opened to communicate the drying line D with the atmosphere and then the valves 29, 33, 56 and 57 are closed to isolate the washing bath 10 from the other system components in order to prevent the solvent gas from entering the washing bath 10, the dry fabrics are to be taken out from the washing bath 10.

[0021] In the dry cleaning by the method according to the present invention, immediately after the fabrics are put into the washing bath 10, the inside of the washing bath 10 may be depressurized by means of the vacuum pump to a predetermined value, then the inert gas be introduced to maintain the oxygen density of the mixture containing the solvent gas below the predetermined value, and in this condition, the subsequent steps of washing, centrifuging, rising, water separation/distillation and drying/solvent recovery can be successively done. In such dry cleaning, with the oxygen density of the mixture containing the solvent gas at least in the washing bath 10 maintained below the predetermined value, namely, with the air containing the solvent gas diluted with the nitrogen gas, the fabrics are washed, centrifuged, rinsed and dried, thus the danger of solvent gas explosion in the series of treatment steps can be prevented.

[0022] As having been described in the foregoing, the present invention is based on the fact that the mixture containing the gas of a petroleum solvent of which the flash point is relatively low, used in the dry cleaning will not explode at the series of dry cleaning steps so long as the oxygen density of the mixture is lower than the peak value in the inflammable range between the limits of explosion or inflammability. For the purpose of preventing a danger of solvent gas explosion, a step of evacuating the washing bath 10 to the predetermined value by means of the vacuum pump 18 and then introducing the nitrogen gas is provided at least before the drying step. Therefore, the oxygen density of the mixture containing the solvent at least in the washing bath 10 is maintained at the predetermined value, namely, below the peak value of the inflammable range. It is thus possible to minimize the danger of explosion due to the flashing of the solvent gas.

Claims

1. A method for dry cleaning with a petroleum solvent, comprising a step of depressurizing to at least a predetermined value the inside of a washing bath having fabrics therein, then introducing an inert gas into the washing bath until the oxygen density of a mixture containing a solvent gas takes a value below a predetermined one, and then drying the fabrics.

2. A method for dry cleaning, comprising, at least, a washing step of placing the fabrics to be cleaned into the washing bath, supplying a solvent into the washing bath rotated at a low speed and thus washing the fabrics; a step of centrifuging the fabrics with the washing bath rotated at a high speed after completion of the washing step; and a drying/solvent recovery step of communicating, after completion of said centrifuging step, the washing bath with the drying line having a heater provided therein, circulating a hot air between the washing bath and drying line, and condensing the evaporated solvent for recovery,
said method further comprising, before said drying/solvent recovery step, a step of depressurizing to at least a predetermined value said washing bath and drying line and thereafter introducing an inert gas into the washing bath until the oxygen density of a mixture containing a solvent gas takes a value below a predetermined one.

3. A method for dry cleaning with a petroleum solvent, comprising, at least, a washing step of placing the fabrics to be cleaned into the washing bath, supplying a solvent into the washing bath rotated at a low speed and thus washing the fabrics; a step of centrifuging the fabrics with the washing bath rotated at a high speed after completion of the washing step; and a drying/solvent recovery step of communicating, after completion of the centrifuging step, the washing bath with the drying line having a heater provided therein, circulating a hot air between the washing bath and drying line, and condensing the evaporated solvent for recovery;
said method further comprising step of depressurizing the insides of said washing bath and drying line to a predetermined value after placing said fabrics into said washing bath and then introducing an inert gas into said washing bath and drying line until the oxygen density of the mixture containing the solvent gas takes a value below the predetermined one, this step being followed by said steps of washing, centrifuging and drying/solvent recovery.

Drawing