CHLORINATED SOLVENTS IN DRY CLEANING D. H. KILLEFFER 300 Madison Ave., New York, N. Y.
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HLORINATED solvents are finally freeing the dry cleaning industry from its fear of fire and from the heavy burden of high insurance rates already materially reduced by safer petroleum distillate. So great has been the effect of the introduction and growing use of nonflammable solvents in this industry during the past few years that its complexion is being greatly changed and its usefulness materially extended. At the same time its increasing requirements have posed new problems for the chemical industry, already puzzled to meet those of other new users of chlorine in preparing effectively to care for the future. It is impossible yet to provide the answers to the questions of continuity and ultimate magnitude of these demands, but an analysis of the present state of things will a t least suggest some of the limits within which the answers must fall.
and dry cleaning and as refrigerants. There has been no corresponding growth in the demand for sodium hydroxide simultaneously producedjn the electrolysis of sodium chloride, and searches have been made for new methods of producing chlorine without this by-product. One of them has been successful enough to produce some 25 tons per day of chlorine by the reaction of nitric acid with sodium chloride, yielding a by-product of sodium nitrate. Other organizations are similarly searching for new production methods, but no word of their success has yet come to hand. The lengths to which one can afford to go in the development of such new methods necessarily depends upon whether current demands for chlorine are likely to continue to grow or to diminish in the future. The proposition is advanced that, because the trend of the demand curve is still upward, it will probably continue so. On the other hand, we are faced by the fact that much of present requirements is going into what may be termed a “capital” stock of chlorinated solvents which will be used over and over rather than into an expendable supply which, in the course of a relatively short time,
Demand for Chlorine Primarily the problems of the chemical industry are involved in determining how to prepare for the future. Demand for chlorine in the recent past has grown steadily, largely because of the increasing use of chlorine compounds (particularly chlorinated hydrocarbons) as solvents in oil refining 640
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will have to be completely replaced. The present demand from the lubricating oil refiners appears to be largely a capital demand, and stocks now being accumulated in plants will be put into circulation in the operating system where only unavoidable loss will be made up from time to time. It would seem that this make-up of solvent losses after systems are filled will probably be relatively small in view of the fact that the entire refining operation can be conducted on a large scale in closed equipment with high recovery of solvent. I n the refrigerating industry the demand is entirely a capital demand since each unit, and particularly the small-capacity machine, is charged with refrigerant and sealed against leakage. The make-up here is likely to be considerably smaller than it will ultimately be in the petroleum industry. However, the saturation of the refrigeration market with units is apparently far in the future, and this capital demand may be expected to continue a t least for a matter of years. The situation in the dry cleaning industry is different. Here chlorinated solvents are definitely taking the place of hydrocarbon solvent already used and have introduced new factors-the laundries-into this industry. To make the change economically, the dry cleaner is faced with the necessity of investing in new equipment to handle the new solvents and, further, of adding continually to his operating stock to offset the relatively large unavoidable losses inherent in his processes. To make this clearer, it will be necessary to go into some detail of the methods used in the industry and the changes effected to care for the new solvents.
Dry Cleaning Methods Traditionally, dry cleaning has been accomplished by the use of various low-boiling, and hence highly flammable, petroleum naphthas. The procedure has been to introduce the work into a barrel washer (similar to those used by laundries in washing clothes) containing the naphtha. Here the goods are tumbled for 20 minutes or more to remove soluble material and are then removed to centrifugal extractors in which the residual solvent is thrown out; the work is then removed to steam-heated drying chambers where the last traces of solvent are evaporated. The solvent from the extractor is conducted with that from the original washer to a purification system. Purification of petroleum naphthas in this industry has consisted principally in washing with caustic soda solution to saponify greases and thus remove them, and in filtering the washed solvent after settling to remove all traces of aqueous alkali, soap, and any suspended dirt or grime that has not been washed out with the soap. In addition to distillation as a method of solvent purification, filtration through adsorptive media is much used. Its application is simpler and the equipment in which it is done is easier to operate with the labor ordinarily available in dry cleaning establishments. The method consists in forcing the circulation solvent through a pressure filter on which is mainchar, silica tained a layer of adsorptive material-activated char, gel, diatomite, or other medium-capable of adsorbing the extracted grease and dirt picked up from the work. In such a system the dissolved solids in the solvent are kept at an equilibrium value which is considered by many preferable to complete degreasing of the work which would o c c u r if d i s t i l l e d solv e n t w e r e continuously
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ployed w i t h b o t h hyd r o c a r b o n and chlorinated solvents and is in high favor with d r y cleaners generally.
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Each step of this operation is accompanied by ideal conditions for the generation of flammable vapors and hence the fire risk in such a plant is extraordinarily high. Unlike many other similarly hazardous operations, the dry cleaners always have on hand large quantities of materials, often of very high value and belonging to others, which are sure to suffer damage from any fire, however limited may be its extent. The precautions taken to locate such plants away from neighbors and in isolated spots, to ensure against accidental sparking of any kind, and to keep away every possible source of flame minimize the danger. Nevertheless, the insurance rates on such enterprises are necessarily so high as seriously to handicap the industry, and the isolated locations make service to customers both slow and expensive.
New Solvents The first step constructively to offset this general hazard was the adoption of a naphtha solvent of high flash point and limited boiling range in 1929. This distillate, known as Stoddard Safety Solvent, is made under specifications promulgated by the Bureau of Standards on the basis of joint researches by the bureau, the petroleum refiners, and the National Association of Cleaners and Dyers. To a great extent it has improved conditions in the industry. It was in reality an answer to the first inroads of chlorinated solvents into this field. The specifications require that the flash point of the solvent shall be a t least 100" F., that half of it distill in a standard distillation flask below 350" F., and that distillation be completed a t 410" F. The specifications further require water-whiteness, absence of sulfur compounds, and sweet odor. The use of this solvent went a long way to improve conditions in the industry. Its low price (9 cents per gallon from tank wagons) still left the operating cost of the industry low without requiring the installation of expensive recovery equipment, and its similarity in solvent ability, etc., to the older naphthas permitted its use without any alteration in equipment or general arrangement of plant. The reduction in hazard thus accomplished was substantial, and the industry felt itself in a much better position to cope with its problems. Still further safety has been introduced by the use of a petroleum fraction with a flash point of 140" F. lately initiated. The reason for this innovation was primarily the reduction in fire hazard, a possibility already demonstrated by the chlorinated hydrocarbon solvents. Stoddard Solvent was, however, only a partial answer to the problem of fire; and, in spite of its wide adoption, the new "synthetic" solvents, as they are called by dry cleaners, continue to find favor. Their practically negligible flammability permitted several important new factors to enter the dry cleaning business. The most important was the fact that the laundries, already equipped with facilities for the collection and delivery of work, could add dry cleaning to their operations without affecting their insurance costs when nonflammable solvents are used. The questions of economy in thus grafting one industry on another are still being argued pro and con, but the fact remains that the laundries have definitely entered the dry cleaning field. The second important consequence of the introduction of the new solvents was the installa-
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transportation of work to
1the centralwholesale plant.
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Redesign of Equipment From the dry cleaners’ point of view these are important considerations but they are by no means the whole story. The obvious differences between the new solvents and the old required a complete redesign of the equipment of the plant to avoid loss of more expensive solvent, to minimize its possible toxic effect on employees, and to offset corrosion which might be caused by hydrolysis of the solvent by water introduced accidentally or with the work. Still another problem was set for all concerned in the differences in solvent characteristics of the new solvents from the customary petroleum distillates. The first step in design was to devise a unit which could be operated entirely closed and which would embody washing, wringing, and final drying of the work as well as continuous cleaning and vapor recovery of the solvent. The earliest units, used in the now remote past of 1925, utilized much the same ideas as were embodied in the older types of dry cleaners with the simple difference that there were lids provided for openings, and solvent recovery was part of the operation of the machine. These proved too expensive to operate in competition with Stoddard Solvent systems because even slight losses of the more expensive synthetic solvents were substantial. The selection of a suitable material of construction from the multitudes of alloys available offered no serious problem once the necessity for it was realized. To understand the conditions encountered in this industry and the requirements to be met by a proper installation, it will be well to consider the methods traditionally used. The objective of dry cleaning is to dissolve greases and other materials which hold dirt and grime in the fabric and to wash them out mechanically. At the same time it is necessary to supply so large a volume of fresh solvent to the work that the dissolved matter is completely removed and no spots can be left by subsequent evaporation of the solvent. Various cleaning agents, among them high-fatty-acid soaps, are added to the solvent to help it perform its function, and these at the same time facilitate the later cleaning of solvent for re-use. The normal operation with Stoddard Solvent is carried out in a rotary drum washer of a type similar to that used by laundries in washing clothes. The size of the unit has been gradually reduced in the past few years as efficiency in handling solvent has been increased from a drum 30-36 inches in diameter and 56-60 inches long, to one as small as 30 inches in diameter and 30 inches long. Into this drum the work with an appropriate amount of solvent is plabed, and the washing action is continued for from 20 minutes to an hour. Originally a single charge of solvent was used, but it has been found far more effective to run a continuous stream of recovered solvent through the drum during the washing operation. Recovery is accomplished by rapidly filtering the solvent and returning it to the operation. A final wash with clean solvent removes traces of soluble materials from the work and it is then ready to go into the centrifugal extractor where residual solvent is thrown out; final traces of solvent remain to be dried from the work in a steam-heated drying chamber. The recovery and cleaning of the solvent after use present interesting problems. Distillation of petroleum solvents has not found favor with the industry because the relatively high boiling points require vacuum equipment when low-pressure steam is used for heating and because their low first cost has not o r d i n a r i l y justified it. The p r e f e r e n c e h a s been for the use of soaps and filters. Much of the grime and dirt removed
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from the work by the solvent can be taken out by simple filtration. However, the dissolved matter does not yield to this treatment. Where soaps are used, even the dissolved matter can be removed completely enough for all practical purposes by filtration. The soaps used contain an excess of fatty acids and hence are soluble in the solvent. The addition of substantial amounts of moisture to the hydrocarbon soap solution precipitates out the soap, and with it come both grime and dissolved grease. This precipitated solution can be filtered and immediately re-used. In practice the solvent is ordinarily cleaned by being passed, after filtering, through a strong solution of caustic soda in the bottom of a cone-bottom tank which effectively removes both the soap and any grease that may accompany it. After settling, the solvent is pumped back into the system. Since the advent of the so-called synthetic solvent systems, many improvements have been made in the hydrocarbon solvent equipment. Most notable has been the addition of a pump to the unit to secure rapid elimination of solvent as it becomes dirty and its replacement by cleaned solvent. The result has been to reduce the time cycle of the washing operation and thus to increase the output of each cleaning unit. The improvement of dry cleaning soaps, particularly the development of organic amine soaps which are more readily soluble in the solvent, has also had an effect in improving dry cleaning practice, although there are many who prefer the older soaps in the belief that they clean better.
Equipment for Chlorinated Hydrocarbon Solvents The problems of using the more active chlorinated hydrocarbons as solvents are quite different. Their higher price requires that they be employed with the utmost economy in a closed system to prevent loss and to minimize any possible danger, remote though it ordinarily is, to employees from fumes. The most advanced types of synthetic-solvent dry cleaning equipment embody washer, extractor, drier, solvent recovery, filter, and still within a single closed unit operated from the outside according to an automatic time cycle controlled by a commutator. The work is put into the washer chamber, which also serves as extractor and drier; the solvent is pumped over it and through the filter or to a continuous still until the washing operation is complete, as indicated by the color of the solvent; and finally the work is wrung dry in the same vessel. Subsequently a current of warmed air is passed over the work to remove last traces of solvent and is then carried to a water-cooled condenser for solvent recovery. The same volume of air is used repeatedly for this purpose and is merely circulated through the unit; thus loss of solvent is reduced to the utmost minimum. All metal parts of the equipment are corrosion-resistant (monel metal, stainless steel, etc.). Iron, copper, and even zinc are unsatisfactory since no amount of care will prevent the introduction of enough moisture with the work to cause trouble. Distillation is the preferred method of cleaning the solvent because it is a simple operation with definite chemical compounds of low boiling point and low latent heat. It also ensures complete removal of all objectionable impurities to an extent beyond that attainable except by the most careful washing processes, and it also avoids the corrosion problem accompanying the operation of washing chlorinated solvents. Unquestionably the use of chlorinated compounds with their greater solvent power has required more care on the part of the operator, particularly in view of the variety of fabrics that must be handled. The greatest difficulty has been experienced with furs and other leather articles from which all of the fats and greases needed to keep them soft and pliable are likely to be removed by solvent. T o offset this, it has
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ZORIC SYSTEMINSTALLED BY THE AMERICAN LAUNDRY MACHINERY COMPANY
Courtesg, Tolrnan Laundry, WashinQton,D . C .
become practice to add similar oils to the solvent before use on leather goods to ensure that the equilibrium will keep the required amount in the goods. Cellulose acetate has caused some trouble because of the solubility of its dyes in trichloroethylene above 80" F. It has been largely replaced by perchloroethylene (C&14) for this reason. Some few dry cleaners, even in this presumably enlightened day, lay their troubles with cellulose acetate, not to the solvent employed but rather to the ancient and threadbare excuse of poor American dyes, inferior to foreign dyes!
Economic Problems Several economic problems are encountered by dry cleaners in using the new solvents. A new plant must be bought and the old one for petroleum solvent scrapped. The cost of the better closed types is in the vicinity of $6000, as compared with $1800 for simpler, but wasteful, open machines. The new solvents are much more expensive than the old. Stoddard Solvent commands a price of 8 to 9 cents per gallon wholesale; the chlorinated solvents are many times more expensive, varying from about 70 cents to $1.35 per gallon on the same quantity basis. Carbon tetrachloride, selling for 60 cents per gallon, trichloroethylene at $1.18 per gallon, and perchloroethylene (tetrachloroethylene) at $1.35 per gallon, are the favorites. Several proprietary mixtures compounded to give constant boiling points are also on the market, containing carbon tetrachloride, ethylene dichloride, etc. To use them economically, not only must losses be minimized but a t the same time the amount of solvent in the system is reduced by rapid circulation and recovery. When properly used in the best units, the loss of solvent per unit of work is very small. Unfortunately all units now in operation are not of the most economical types. Solvent cost with Stoddard Solvent in the customary dry cleaning plant and equipment runs as high as 1.8 cents per unit of work done, including rewashing costs, soap costs, and loss of solvent. Similarly used, the cost of the synthetic solvents runs up to 8 or 9 cents per unit of work under the best conditions and even higher when open systems are used. Distillation cost of chlorinated solvents runs as much as 0.75 cent per gallon of solvent; thus, with unavoidable losses of solvent, solvent cost in the most economical units may run up to 1.5 to 2.0 cents per pound of work. With so great a differential between
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the best performance and the poorest, it is obvious that competition will ultimately force the use of the best machines by showing that their operation is cheapest in the long run. The dry cleaning industry is not large as a whole, and, as compared with most industries, its units are small. The 1929 census placed the number of establishments at about 5200 in the United States. This figure corresponds with the number operating under the industry's NRA code in 1934-35. Others estimate 11,000. Dry cleaning is done by a vast number of tailors (about a quarter of a million) and by many others who cannot properly be considered units of this industry. The number of chlorinated solvent units in operation by the industry can be only roughly approximated. The best guess is approximately 2600 total units sold, of which most, if not all, can be considered to be in operation. It is thus apparent that the industry's market is at the moment much less than half saturated since most establishments use several units. Whether the other half will ultimately be converted to the use of chlorinated solvents is highly problematical. The nature of this demand for solvent has been largely operating demand as distinct from capital demand. A dry cleaning unit of the latest types will function well on one or a t most two drums of solvent, and hence the capital demand can be estimated as being between 2600 50-gallon drums of solvent and double that amount spread over a period of several years. Future capital demand is unlikely to be greater than this amount. I n the meantime, most of the demand has been operating demand to make up process losses. With as many as 2600 distinct units requiring a gallon or so of solvent per day for make-up, this becomes a substantial part of the total. However, the increasing adoption of more efficient units is reducing this figure; probably greater losses that might be expected from increasing numbers of units will be offset by improved efficiency of individual units. In the meantime the builders of petroleum solvent equipment are very much awake to the situation and are developing more efficient and safer machines with which they expect to stem the spread of chlorinated solvents. The improvement in safety of operation with higher flash petroleum distillates is already marked, and many believe the use of chlorinated solvents has passed its peak. RECEIVEDMarch 7, 1936.
