US2310569A - Degreasing of metal articles - Google Patents

Description

.w: 1 BOOTH nseamsma OF'METAL ARTICLES;

Filed ma 29, 1942 Feb; 9, 1943;

Eb? ozjocuw Q h 7 GM .PM ELJO muh 02 3000 INVENTOR. WILLIAM EDWARD BOOTH BY r ATTORNEY.

atented Feb. 9, i943 DEGREASING F METAL ARTICLES William Edward Booth, Runcorn, 'Englamh as-' manorto Imperial Chemical industries ited, a corporationof Great Eri Application May at, rate, semi No. 445,040

' mamas tain any is, 1941 it his. origin -i) This invention relates to improvements in the degreasing of metal and-like non-porous articles by means of a volatile solvent, and more particularly to methods of removing residual solvent after the degreasing process from articles which have a large surface-volume ratio.

A degreasing apparatus in common use comprises a tank, in the lower part of which is provided one or more heated compartments for the volatile degreasing solvent, while in the upper part condensing coils are fixed round the walls to set an upper limit to the solvent vapour. -Metal and like articles are degreased insuch an apparatus by immersing them in the hot volatile solvent such as trichloroethylene and subsequently drying them by raising the articles out of the liquor into the space above it; the solvent then evaporates as a result of the heat in the articles treated. The method is, however, not entirely satisfactory when treating pressings, wire strip, or other articles with a large surface-volume ratio, since the heat content is insuillcient to evaporate all the adherent solvent. In the case of steel wool or wire cloth, which may retain considerable quantities of solvent, the method is impracticable and heat has to be supplied from an external source to remove the solvent. In British specification No. 525,995 it has been proposed to recover solvent from solvent-soaked articles by immersing them in a body of hot water at a temperature above the boiling point of the solvent, above which body of Water is an aspirating device in conjunction with a solvent recove y I 'plant by which vapours evolved may be collected and condensed, and the solvent in the condensate recovered. This method of drying involves moving hot articles wet with solvent from the degreasing vessel to the hot water, and during this time considerable losses of solvent will occur through escape into the atmosphere. Moreover, in view of the low-boiling mixtures formed between the solvent and the water, the evaporation of the solvent under such conditions is accompanied by the generation of a considerable volume of steam, particularly in the last stages of the solvent evaporation, and this is objectionable in an apparatus primarily intended for the recovery of solvent vapours.

According to the present invention I provide in a process for degreasing metal and like nonporous articles, particularly articles of that type with a large surface-volume ratio such as steel wool, with a bath of aliquid chlorinated aliphatic hydrocarbon solvent capable of forming with water an azeotrope containing a major proportion of the former and which has a boiling point below that of water, the steps of removing 'surface thereof a layer of water of substantial depth, maintaining the water layer at a temperature below its boiling point but above the boiling point of the said azeotrope, maintaining above the water layer a mixture of the water vapour and solvent vapour in azeotropic proportions, and withdrawing the treated article through the water layer.

In processes already in use for degreasing with a liquid solvent it frequently happens, when moist articles are treated, that a film or thin covering of water forms on the layer of solvent; in contra-distinction to this, however, the water layer in the present invention is of substantial depth comparable with that of the solvent bath, so that adequate opportunity occurs when the articles are withdrawn through this layer of water for adherent solvent to vaporise as a binary azeotrope with water. Preferably the water layer is deep enough for the articles to be completely immersed in the water as they are withdrawn, and the articles are held in the water layer until solvent vaporisation is complete, but this is not essential, providing that the rate of withdrawal of the articles is such thatthe evaporation of adherent solvent is completed during the passage of any part of them from the solventwater interface to the top of the water layer.

By this means the losses attendant on the transference of the articles from a solvent bath to a water bath are entirely avoided, and the use of a. separate vessel is rendered unnecessary. It is a further advantage of this process that, just as is the case for pure solvent vapours, the vapours of the azeotrope above the water layer are dense enough to form a definite vapour-air interface, since they contain a major proportion of the solvent. The process can thus be carried out in an open-topped vessel fitted with peripheral condensing coils and no exterior solvent' recovery apparatus is necessary. This would not be possible if the space above the water layer was filled with steam only or with steam con- J taining a minor proportion of solvent vapour,

since no definite steam-air interface would form because of the relatively low density of the steam.

Suitable aliphatic hydrocarbons which may be used in this invention are chloroform, ethylene dichloride, dichloroethylene and trichloroethylene; advantageously trichloroethylene is used. All such solvents are denser than water, and

thus the relative positions of the water and the solvent in the apparatus are automatically maintained.

In one form of the invention, using trichloroethylene as solvent, a layer of the trichloroethylene deep enough for the articles to be treated is surmounted by a layer of water of about the same depth. Trichloroethylene normally boils at 86.7 C., but forms with water an azeotrope boiling at 73 C. In carrying out the process, therefore, the trichloroethylene layer'is maintained at a temperature below this, e. g. at 65-70 C. and the water is maintained at a temperature above 73 C. but below its boilin point, for example between 80 C. and 85 0., although higher temperatures up to. say. may be used. 'Vapours of water and trichloroethylene in azeotropic proportions are generated, suitably in a separate still, and delivered to the region immediately above the water layer, the upper level of this vapour being limited as by condensing coils around the inside walls of the vessel containing the liquids. It is found in practice that no difilculty arises in maintaining the water and solvent layers in this way, even though the water is at a temperature above the boiling point of the trichloroethylene. The articles to be degreased are plunged through the water'into the trichloroethylene; a little water may thus be carried down into thetrichloroethylene layer but will soon be vaporised since such water is above the boiling point of the azeotrope. The adherent grease will then be dissolved off by the trichloroethylene. When degreasing is complete the articles are raised into the water layer. The trichloroethylene carried into the water layer will then vaporise as an azeotrope with water and thus be flashed off and mingle with the vapours above the water layer which, of course, are of a similar composition; i. e., such that on condensation about 8 volumes of trichloroethylene are obtained for each volume of water. The presence of the azeotropic vapours above the part of the vessel above the water layer, a water to by the synonymous terms of distillation vessel water layer ensures that no uncontrollable mixture of steam with a small amount of solvent vapour is formed above the water layer even when the last traces of solvent are vaporised from the article, since when such last traces are vaporised, which in practice might contain a larger proportion of water, they mix with the much larger bulk of vapour containing the high proportion of solvent. When vaporisation of the adherent solvent is complete the articles can be removed free of solvent.

The condensate formed by the condensing means limiting the vapour level may be allowed to separate by. gravity into its two constituents, which can be returned in part to the layers of the degreaser and in part to the still for maintaining the azeotropic mixture of vapours above the water layers. Preferably, however, the accumulation of grease and other impurities in the degreasing liquids as degreasing proceeds is prevented by tapping off part of the water layer and the solvent layer to the still and returning all of the separate layers of condensate to the corresponding layer of the degreasing liquids.

According to a further feature of the invention I provide an apparatus for carrying out the process comprising a vessel adapted to contain in the lower part a quantity of the chlorinated hydrocarbon solvent and a supernatant layer of water of substantial depth, means for heating the water layer, means for regulating th temperature of the solvent layer, condensing means in the upper or concentrator) adapted to receive both solvent and water from the degreasing vessel and to deliver vapours thereof to the space above the water layer.

In such an apparatus the heating means for the water layer may take the form of steam coils around the inside of that part of the tank where the water layer will be situated when the plant is in operation, while the means for regulating the temperature of the solvent layer comprise coils in the bottom tank to which either steam or cooling water may be circulated as may be required, depending on the temperature of the water layer, the solvent chosen and the temperature at which it is desired to maintain it. The condensing means above the water lay'er suitably consists of coils around the inside of the top of the tank to which cooling water can be supplied, while in one side of the tank between the level of these coils and the steam coils for heating the water layer a wide duct joins the tank to the upper part of the vaporiser, providing both the means for delivering vapour from the vaporiser to the tank, and an overflow by which water may be transferred to the vaporiser. A pipe connecting the lower part of the tank to the vaporiser provides a path for transferring dirty solvent to the latter. To maintain a suitable depth of solvent in the tank the pipe may join the vaporiser at a higher level than it joins the tank. Immediately beneath the condensing means in the upper part of the tank is a trough to collect condensate and deliver it to a water separator from which the water layer of the condensate is returned to the water layer of the tank, while the solvent in the condensate is returned to the solvent'layer.

The invention will now be described with refer- .ence to the accompanying drawing, which is a vertical section through the apparatus.

In this drawing, the apparatus comprises an open-topped degreasing vessel I, around the upper part of the walls of which are condensing coils 2 which can be supplied with water by means not shown, and a vaporiser, concentrator or distillation vessel 3 communicating with the vessel I immediately beneath coils 2 by a wide duct 4. The lower sloping wall 20 of the duct determines the upper level 2i of the liquid in vessel l and a pipe I8 with a valve l9 connects vessels I and 3 near the bottom. In the bottom of vessel I is a steam coil 5 carried on a door 6 removable for cleaning purposes. and compartment 3 has a similar steam coil I. Near the bottom of vessel I is a grid 8 to support articles placed in the solvent:

two coils 9 and III are provided at different heights up the side. Of these coils, 9 is in that part of the vessel intended to contain the degreesing solvent,e. g. trichloroethylene. and is connected by means not shown to a source of cooling water, while coil I0 is in that part of the vessel intended for the water layer and is connected by means not shown to a supply of steam. By means of cofl I Ii it is thus possible to maintain the water at the desired temperature, e. g. at .8085 C. if the solvent is trichloroethylene. and by means of coils 5 and 9 the temperature of the solvent can be maintained at an elevated temperature at which degreasing proceeds satisfactorily, but below the boiling point of the solvent, e. g. at 6570 in the case of trichloroethylene. The coils and 9 could be replaced by a single coil used to maintain the necessary temperature by supplying water or steam as desired, but the arrangement depicted has the advantage that cooling is supplied where the solvent is most likely to become unduly hot by reason of its proximity to the hotter water, and a steam coil in the bottom enables solvent to be recovered by distillation when it is too dirty for use, if the capacity of the concentrator 3 is insuflicient for that purpose; it also enables solvent to be distilled during such recovery which is below the level of the pipe I8 joining vessels I and 3. Immediately beneath coils catch condensate dripping from it via well I2 in trough I I, and terior of water separator I4 at a position corresponding to a level between the coils 9 and ID in vessel I. Vent I5 connects the top of the water separator to vessel I above trough II and thus prevents airlocks in the separator; the pipe I6 connects the upper part of the separator to vessel I immediately above coil III and thus in the region in vessel I intended to hold water, while pipe I! connects the lower part of the water separator to vessel I below coil 9 and thus in the region of vessel I intended for solvent. A pipe 26 and valve 28 may also be provided to withdraw solvent from the separator to a store tank when it is desired to empty the plant by distilling off the. solvent, and when such a pipe is provided a valve 24 may be provided in pipe I1 to close the latter when the solvent is being distilled oil".

In using this apparatus valve I9 is closed, a supply of solvent (say, trichloroethylene) is charged'into the vessel I up to a level 22 between coils 9 and I I], and then water is charged to fill the vessel until it commences to overflow and run down the sloping wall 20 which thus determines the upper surface ll of the water. A charge of water and solvent is also supplied to distillation vessel 3. Cooling. water is next supplied to coils 2 and II! and steam to coils 5 and I, so that distillation commences in vessels 3, the solvent layer is maintained at 65-70 C., and the water layer at 80-85 C. As distillation proceeds in vessel 3 the azeotrope of water and solvent will vaporize, be condensed on coils 2 and delivered via trough II, well I2 and pipe I 3 to water separator It. The space above the water surface 2! will thus be filled with steam and solvent vapour substantially in the ratio of the vapours in the azeotrope, which being composed mainly of solvent vapour is considerably denser than air. A definite vapour-air interface is then formed in the region of coils 2, the exact level them, and deliver pipe I3, to the inbeing dependent on the supply of cooling water to the coils. From the separator I 4 solvent will be returned to the solvent layer in vessel 1 through pipe I7, and water to the water layer by pipe I 6. Water will thereby be caused to overflow down surface 4 to the distillation vessel 3. Valve II is then opened, and adjusted so that solvent is supplied to vessel 3 at the same rate as it is being vaporized therefrom, and in this way the proper level 22 of solvent in vessel I between coils 9 and I0 is maintained. Once steady conditions are set up, degreasing can be commenced by plunging the articles to be treated through the water layer into the solvent layer and the articles are left there until the degreasing is completed. They are then drawn up into the water layer; adherent solvent then vaporizes as an azeotrope with water, and is recondensed ultimately on coils 2 is trough II to non-porous articles with 2 and delivered to separator I 4, and so back to the solvent layer, while the water component ofthe azeotrope is returned to the water layer. Finally, the articles are withdrawn from the apparatus free from adherent solvent.

Various modifications sible. Thus instead of controlling the level of the solvent-water interface in vessel I by valve I9 in conjunction with the rate of distillation in vessel 3, the valve I9 may be replaced by an inverted U tube of appropriate height, or'the join of pipe I8 with vessel 3 may be suitably higher than its join with vessel I, so that solvent is only transferred to vessel 3 if there is a sufiicient hydrostatic pressure at ,the Join of vessel I and pipe I8 corresponding to the, proper height of solvent in vessel I. An automatic control of the solvent level is thus achieved. Again, instead of supplying an exterior water separator, well I2 may act as a water separator by allowing the separation to take place therein and thewater layer thus formed to overflow its edge and fall back into the main vessel I, while a suitably shaped pipe is provided to return solvent to the lower part of the vessel.

It isnot essential for the water layer to extend across the whole of the solvent surface; Thus the degreasing vessel may be divided into two by a partition extending from above the water surface to somewhat below the solvent-water interface, and the water layer may be maintained on one side only of the partition, while a conveyor carries the goods to be treated down into the solvent on the other side of the partition, under the latter, and up into the water layer and thence out of the apparatus.

Our process may be applied to the treatment of a variety of articles having a large surfacevolume ratio such as metal swarf, pressings, wire cloth, wire gauze and steelwool.

I claim:

1. In a process for degreasing metal and like non-porous articles with a bath of a liquid chlorinated aliphatic hydrocarbon solvent capable of formingwith water an azeotrope containing a major proportion of the former which azeotrope has a boiling point below that of water, the steps for removing solvent from such articles which have been treated in the solvent bath which comprise maintaining on the surface of the solvent a layer of water of substantial depth, maintaining the water layer at a temperature below its boiling point but above that of the boiling point of the azeotrope, maintaining above'the water layer a mixture of water vapour and solvent vapour substantially in azeotropic proportions, and withdrawing articles treated in the solvent bath through thewater layer so as to vaporize solvent carried'up with the articles as a binary azeotrope with water. l

2.. A process according to claim 1 in which water and solvent are withdrawn from the respective layers of water and solvent substantially in the proportion in which they occur in the azeotrope, a mixture of the liquids so withdrawn is distilled, the distillate allowed to separate into a solvent layer and a water layer, and the said layers of distillate are separately returned to the solvent bath and to the water layer thereon respectively.

3. In a process for degreasing metal and like a bath of liquid chlorinated aliphatic hydrocarbon solvent capable of forming with water an azeotrope containing a major proportion of the former, which azeotrope of our invention are-pos-' point of the azeotrope, maintaining above. the

water layer a mixture of water vapour and solvent vapour substantially in azeotropie proportions, and withdrawing articles treated in the solvent bath through the water layer, so as to vaporise solvent carried up with the articles as binary azeotrope with water.

4. In a process for degreasing metal and like non-porous articles with a bath of a liquid chlorinated aliphatic hydrocarbon solvent capable of iorming with water an azeotrope containing a major proportion of the former, which azeotrope has a boiling point below that of water, the steps which comprise maintaining on the surface of the solvent a layer of water of sufficient depth for the articles to be completely immersed therein, maintaining the said water layer at a temperature below its boiling point but above the boiling point of the azeotrope, maintaining above the water layer 'a mixture of solvent vapour and water vapour substantially in azeotropic proportions, treating the articles with solvent by immersing them in the solvent bath, raising the treated articles into the water layer, and retaining them therein until solvent carried up into the water layer by the articles is vaporised as the azeotrope with water.

5. A process according to claim 4 in which the solvent bath is maintained at an elevated temperature below the boiling point of the azeotrope.

6. In a process for degreasing metal and like non-porous articles with a bath of trichloroethylene, the steps for removing trichloroethylene from such articles treated therein which comprises maintaining on the surface of the trichloroethylene bath a layer of water of substantial depth, maintaining the water layer at a temperature below its boiling point but above that of the binary azeotrope of water with trichloroethylene, maintaining above the water layer a mixture of trichloroethylene vapour and water vapour substantially in azeotropic proportions,

and withdrawing articles treated in the trichloroethylene bath through the water layer so as to vaporise solvent carried up with the articles as the binary azeotrope with water.

7. Process according to claim 6 in which the trichloroethylene layer is maintained at an elevated temperature below that of the boiling point of the azeotrope.

8. In a process for degreasing metal and like non-porous articles with a bath of trichloroethylene, the steps which comprise maintaining on the surface of the said bath a layer of water of sufllcient depth for the articles to be completely immersed therein, maintaining the said water layer at a temperature below its boiling point but above the boiling point of the azeotrope of water with trichloroethylene, maintaining above the water layer a mixture of trichloroethylene vapour and water vapour substantially in azeotropic proportions, treating the articles with solvent by immersing them in the solvent bath, raising the treated articles into the water layer, and retaining them therein until solvent carried up into the water layer by the articles is vaporised as the azeotrope with water.

9. In a process for degreasing metal and like non-porous articles with a bath of trichloroethylene, the steps which comprise maintaining on the surface of the said bath a layer of water of sufilcient depth for the articles to be completely immersed therein, maintaining the said water layer at a temperature below its boiling point but above the boiling point of the azeotrope of water with trichloroethylene, maintaining the trichloroethylene at an elevated temperature below the boiling point of the azeotrope, maintaining above the water layer a mixture of trichloroethylene vapour and Water vapour substantially in azeotropic proportions, treating the articles with solvent by immersing them in the solvent bath, raising the treated articles into the water layer, and retaining them therein unt l solvent carried up into the water layer by the articles is vaporised as the azeotrope with water.

10. A process according to claim 9 in which the water layer is maintained at a temperature between 80 C. and 85 C. and the trichloroethylene bath is maintained at a temperature between C. and C.

11. An apparatus for degreasing metal and like non-porous articles with a liquid chlorinated aliphatic hydrocarbon solvent which forms with water an azeotrope containing a major proportion of the solvent, which azeotrope has a boilin point below that of water, said apparatus comprising a vessel adapted to contain in the lower part a quantity of the solvent and a supernatant layer of water of substantial depth, means for heating the water, means for regulating the temvent in the said vessel.

perature of the solvent layer, condensing means in the upper part of the vessel above the water layer, a water separator adapted to receive condensate from said condensing means, means. to return water from the separator to the said water layer, and means to return solvent from the separator to the said solvent layer, a vaporiser adapted to receive solvent from the said solvent layer and water from the said water layer, and means to deliver vapours from the vaporiser to the said vessel above the water layer therein but beneath the said condensingmeans.

12. An apparatus according to claim 11, havin means for regulating the delivery of solvent to the vaporiser so as to maintain the level of sol- WILLIAM E BOOTH.

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