Patent Publication Number: US-6216302-B1

Title: Carbon dioxide dry cleaning system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 08/979,060 filed Nov. 26, 1997, and now U.S. Pat. No. 5,904,737. 
    
    
     BACKGROUND 
     The present invention generally relates to carbon dioxide dry cleaning systems and, more particularly, to improved carbon dioxide dry cleaning systems that purify and reclaim carbon dioxide without the use of heaters and that do not use pumps to move liquid carbon dioxide. 
     The dry cleaning industry makes up one of the largest groups of chemical users that come into direct contact with the general public. Currently, the dry cleaning industry primarily uses perchloroethylene (“perc”) and petroleum-based solvents. These solvents present health and safety risks and are detrimental to the environment. More specifically, perc is a suspected carcinogen while petroleum-based solvents are flammable and produce smog. For these reasons, the dry cleaning industry is engaged in an ongoing search for alternative, safe and environmentally “green” cleaning technologies, substitute solvents and methods to control exposure to dry cleaning chemicals. 
     Liquid carbon dioxide has been identified as a solvent that is an inexpensive and an unlimited natural resource. Furthermore, liquid carbon dioxide is non-toxic, non-flammable and does not produce smog. Liquid carbon dioxide does not damage fabrics or dissolve common dyes and exhibits solvating properties typical of more traditional solvents. Its properties make it a good dry cleaning medium for fabrics and garments. As a result, several dry cleaning systems utilizing carbon dioxide as a solvent have been developed. 
     U.S. Pat. No. 4,012,194 to Maffei discloses a simple dry cleaning process wherein garments are placed in a cylinder and liquid carbon dioxide is gravity fed thereto from a refrigerated storage tank. The liquid carbon dioxide passes through the garments, removing soil, and is transferred to an evaporator. The evaporator vaporizes the carbon dioxide so that the soil is left behind. The vaporized carbon dioxide is pumped to a condenser and the liquid carbon dioxide produced thereby is returned to the refrigerated storage tank. 
     The system of Maffei, however, does not disclose a means for agitating the garments. Furthermore, because the system of Maffei does not disclose a means for pressurizing the chamber, the carbon dioxide must be very cold to remain in a liquid state. Both of these limitations inhibit the cleaning performance of the Maffei system. 
     U.S. Pat. No. 5,267,455 to Dewees et al. discloses a system wherein liquid carbon dioxide is pumped to a pressurized cleaning chamber from a pressurized storage vessel. The cleaning chamber features a basket containing the soiled garments. The interior of the basket includes projecting vanes so that a tumbling motion is induced upon the garments when the basket is rotated by an electric motor. This causes the garments to drop and splash into the solvent. This method of agitation, known as the “drop and splash” technique, is used by the majority of traditional dry cleaning systems. After agitation, a compressed gas is pumped into the chamber to replace the liquid carbon dioxide. The displaced “dirty” liquid carbon dioxide is pumped to a vaporizer which is equipped with an internal heat exchanger. This allows “clean” gaseous carbon dioxide to be recovered and routed back to the storage vessel. 
     While the system of Dewees et al. overcomes the shortcomings of Maffei, namely, the lack of an agitation means and a pressurized cleaning chamber, it relies upon a pump to move its liquid carbon dioxide and utilizes a heat exchanger in its vaporizer. Both of these components add complexity, cost and maintenance requirements to the system. In addition, the mechanically rotating basket, whether achieved by large, magnetically coupled drives or by shafts, is expensive and has high maintenance costs. 
     Many patents have disclosed improved agitation arrangements for carbon dioxide dry cleaning systems. For example, U.S. Pat. No. 5,467,492 to Chao et al. discloses a fixed perforated basket combined with a variety of agitation techniques. These include “gas bubble/boiling agitation” where the liquid carbon dioxide in the basket is boiled, “liquid agitation” where nozzles spraying carbon dioxide tumble the liquid and garments, “sonic agitation” where sonic nozzles create agitating waves and “stirring agitation” where an impeller creates the fluid agitation. The remaining portion of the system of Chao, however, does not provide for a significant improvement over Dewees et al. in that a pump is still relied upon to move the liquid carbon dioxide from the system storage container to the cleaning chamber. 
     U.S. Pat. No. 5,651,276 to Purer et al. discloses an agitation technique which removes particulate soils from fabrics by gas jets. This gas agitation process is performed separately from the solvent-immersion process. Purer et al. further disclose that carbon dioxide may be employed both as the gas and the solvent. U.S. Pat. No. 5,669,251 to Townsend et al. discloses a rotating basket for a carbon dioxide dry cleaning system powered by a hydraulic flow emitted by a number of nozzles. This eliminates the need for rotating seals and drive shafts. While these two patents address agitation techniques, they do not address the remaining portion of the dry cleaning system. 
     Finally, the Hughes DRYWASH carbon dioxide dry cleaning machine, manufactured by Hughes Aircraft Company of Los Angeles, Calif., utilizes a pump to fill a pressurized cleaning chamber with liquid carbon dioxide. The cleaning chamber contains a fixed basket featuring four nozzles. As the basket is being filled with carbon dioxide, all four nozzles are open. Once the basket is filled, however, two of the nozzles are closed. The remaining two open nozzles are positioned so that they create an agitating vortex within the basket as liquid carbon dioxide flows through them. Soil-laden liquid carbon dioxide exits the basket and chamber and is routed to a lint trap and filter train. Furthermore, the system features a still that contains an electric heater so that soluble impurities may be removed. 
     While the Hughes DRYWASH system is effective, it also suffers the cost, maintenance and reliability disadvantages associated with a liquid pump and an electrically heated still. 
     Accordingly, it is an object of the present invention to provide a carbon dioxide dry cleaning system that utilizes both the solvent properties of carbon dioxide and high velocity liquid to remove insoluble particles. 
     It is a further object of the present invention to provide a carbon dioxide dry cleaning system that purifies and reclaims carbon dioxide without the use of an electrical heater or a heat exchanger. 
     It is still a further object of the present invention to provide a carbon dioxide dry cleaning system that moves liquid without the use of a pump. 
     It is still a further object of the present invention to provide an improved carbon dioxide handling system for use in a dry cleaning process. 
     It is still a further object of the present invention to provide an improved carbon dioxide dry cleaning system with adjustable agitation pressure so that delicate objects may be cleaned without damage. 
     It is still a further object of the present invention to provide an improved carbon dioxide dry cleaning system that may accommodate a solvent additive. 
     These and other objects of the invention will be apparent from the remaining portion of the Specification. 
     SUMMARY 
     The present invention is directed to a liquid carbon dioxide dry cleaning system that moves liquid carbon dioxide without the use of a pump and distills it without the use of an electric heater or a heat exchanger. Because liquid carbon dioxide, when used as a solvent, is at a high pressure and in a saturated state, suitable pumps are expensive and not nearly as reliable as devices used for ambient temperature liquids. 
     The preferred embodiment of the system features a pair of storage tanks containing liquid carbon dioxide. A compressor initially is connected in circuit between the head space of one of the storage tanks and a sealed cleaning chamber containing the objects being dry cleaned. The liquid side of the storage tank is connected to the cleaning chamber. As a result, the storage tank is pressurized so that liquid carbon dioxide flows from it to the cleaning chamber. 
     Next, the compressor is placed in circuit between the storage tanks so that gas may be withdrawn from the now empty storage tank and used to pressurize the other storage tank, also filled with liquid carbon dioxide. The liquid side of the empty storage tank remains connected to the cleaning chamber while the liquid side of the full storage tank is connected to cleaning nozzles within the cleaning chamber. As a result, when the full storage tank is pressurized, liquid carbon dioxide flows from it, through the nozzles and into the cleaning chamber so as to agitate the objects being cleaned. The displaced liquid carbon dioxide from the cleaning chamber flows back to the empty storage tank. 
     A still, submerged in the liquid carbon dioxide within one of the storage tanks, receives soiled liquid carbon dioxide from the cleaning chamber. Gas is withdrawn from the still by the compressor and is used to pressurize the storage tank containing the still. Alternatively, the still may be connected to the liquid side of a low pressure transfer tank. As a result, gas from the still is returned to the transfer tank where it is recondensed by the cold liquid carbon dioxide contained therein. In either case, the pressure difference created between the still and storage tank causes the soiled liquid carbon dioxide to boil due to the heat supplied by the liquid carbon dioxide surrounding the still. This removes the carbon dioxide in gaseous form leaving the contaminants in the still. Heat is also removed from the liquid carbon dioxide surrounding the still without reducing the heat in the system and without mechanical refrigeration. 
     Alternative embodiments of the present invention employ this distillation arrangement with a system that uses a cryogenic liquid pump to supply liquid carbon dioxide to the cleaning nozzles. An embodiment that places this pump within one of the liquid cryogen storage tanks is also disclosed. 
     The agitation pressure may be controlled so that delicate objects may be cleaned without damage. Solvent additives may also be injected into the liquid carbon dioxide. 
     For a more complete understanding of the nature and scope of the invention, reference may now be had to the following detailed description of embodiments thereof taken in conjunction with the appended claims and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1M are schematic diagrams illustrating the operation of a preferred embodiment of the carbon dioxide dry cleaning system of the present invention wherein three carbon dioxide tanks are used; 
     FIG. 2 is a schematic diagram of another embodiment of the carbon dioxide dry cleaning system of the present invention wherein two carbon dioxide tanks and a pump in a sump are used; 
     FIG. 3 is a schematic diagram of a third embodiment of the carbon dioxide dry cleaning system of the present invention wherein a pump is disposed within the high pressure carbon dioxide storage tank; 
     FIG. 4 is a schematic diagram of the embodiment of the carbon dioxide dry cleaning system of FIGS. 1A-1M showing the agitation pressure control system; 
     FIGS. 5 and 6 are schematic diagrams of a fourth embodiment of the carbon dioxide dry cleaning system of the present invention including a heat sink, recondensing coils in one of the storage tanks and a solvent additive dispenser. 
    
    
     DESCRIPTION 
     A preferred embodiment of the carbon dioxide dry cleaning system of the present invention is shown in FIG. 1A. A cold transfer tank, indicated at 12, contains a supply of liquid carbon dioxide at a pressure between 200 and 250 psi and at a temperature of approximately −15° F. Preferably, the liquid carbon dioxide contains additives to promote better cleaning and deodorizing. Transfer tank  12  is sized to hold approximately two week&#39;s worth of liquid carbon dioxide. Transfer tank  12  may be refilled from a mobile delivery tanker in a conventional manner. 
     High pressure storage tanks  18  and  20  contain liquid carbon dioxide at a pressure of approximately 650 to 690 psi. The two storage tanks may be refilled from transfer tank  12  when they become depleted. This may be done between each garment load or one time in the morning. To perform refilling, the head space of transfer tank  12  is initially connected to the head spaces of storage tanks  18  and  20  so that their pressures are equalized. This is shown in FIG. 1A by line  28 . 
     Then, as shown in FIG. 1B, the head spaces of storage tanks  18  and  20  are connected to the suction side of a compressor  14 . The discharge side of compressor  14  is connected to the head space of transfer tank  12 . As a result, the pressure in transfer tank  12  is increased while the pressure in storage tanks  18  and  20  is decreased. This causes liquid carbon dioxide to flow at a high pressure, as indicated by thick line  30 , from the liquid side of transfer tank  12  to the liquid sides of storage tanks  18  and  20 . 
     Once storage tanks  18  and  20  are properly filled with a supply of liquid carbon dioxide, the dry cleaning process may begin. While the system of the present invention is described and discussed below in terms of dry cleaning fabrics, it is to be understood that the system may be used alternatively to perform other cleaning tasks where liquid carbon dioxide is an appropriate solvent. For example, the system could be used to degrease mechanical parts. 
     Referring to FIG. 1B, soiled garments or the like are placed in cleaning chamber  32 . The door  34  of the cleaning chamber  32  features a seal, such as a large rubber O-ring, so that the chamber may be pressurized when the door is closed. In addition, door  34  features an interlocking system so as to prevent the door from opening while chamber  32  is pressurized. Such interlocking systems are well known in the art. Once the garments are loaded, and cleaning chamber  32  sealed, the air therein is evacuated using compressor  14 , as shown by line  42  in FIG.  1 B. This is done to prevent condensation when the chamber is pressurized. 
     Next, as shown by line  44  in FIG. 1C, the head space of one of the storage tanks (tank  20  in FIG. 1C) is connected to the chamber so that the latter is pressurized with carbon dioxide gas to an intermediate pressure of about 70 psi. Once chamber  32  is pressurized to an intermediate pressure, it may be filled with high pressure liquid carbon dioxide without the formation of dry ice or the occurrence of extreme thermal shock. 
     As shown in FIG. 1D, high pressure liquid carbon dioxide is then fed through line  50  via the pressure differential between storage tank  20  and cleaning chamber  32 . This almost completely fills the chamber  32  without the use of a compressor or pump. Because chamber  32  and storage tank  20  (and storage tank  18 ) are approximately the same size, the carbon dioxide remaining in storage tank  20  may be used to finish filling chamber  32 . This is accomplished, as shown in FIG. 1E, by using compressor  14  to remove carbon dioxide gas from chamber  32  and direct it back to storage tank  20 . This forces the liquid carbon dioxide remaining in storage tank  20  into chamber  32  so as to completely fill it. 
     At this point, the liquid carbon dioxide within filled chamber  32  is at a pressure and temperature of about 650 psi and 54° F., respectively. It has been determined that liquid carbon dioxide is an effective solvent at such a temperature and that it will not harm most fabrics. The system is now ready to begin the agitation process. Agitation is necessary so that the system may remove non-soluble particles that are not removed merely by submersing the garments in the liquid carbon dioxide. 
     The configuration of the system during the initial portion of the agitation process is shown in FIG.  1 F. The suction side of compressor  14  is connected to the top of empty storage tank  20 . The discharge side of compressor  14  is connected to the head space of filled storage tank  18  so that the pressure therein is increased. 
     When the pressure differential between chamber  32  and storage tank  18  reaches at least 150 psi, that is, when the pressure in storage tank  18  is greater than 800 psi, high pressure liquid carbon dioxide is permitted to flow to chamber  32 , as indicated by line  52 . This flow is directed into chamber  32  through a first set of cleaning nozzles  53 . Such nozzles are known in the art. This causes the garments and fluid in chamber  32  to rotate past the cleaning nozzles. Displaced liquid flows out of the top of chamber  32 , through lint and button traps  54  and filter  56  and finally is returned to storage tank  20  at a low pressure, as indicated by cross-hatched line  58 . The angles of the nozzles may optionally be adjustable from outside of the cleaning chamber  32  so that the agitation may be tailored to the specific load. 
     After approximately one minute, the carbon dioxide flow is terminated and the system is reconfigured as shown in FIG. 1G so that the agitation may be “reversed.” More specifically, the suction side of compressor  14  is connected to the top of nearly emptied storage tank  18  while the discharge side is connected to nearly filled storage tank  20 . Storage tank  20  is pressurized to over 800 psi by the flow of carbon dioxide gas. 
     Liquid carbon dioxide then flows out of tank  20  to chamber  32 , as illustrated by line  60 , where it passes through a second set of cleaning nozzles  61  that reverse the rotation of the garments. This causes the garments that have collected in the center of chamber  32  to now move to the outside where they will be subjected to the action of the cleaning nozzles. Displaced liquid flows out of the top of chamber  32  and through lint and button traps  54  and filter  56  and is returned to storage tank  18  at a low pressure, as indicated by cross-hatched line  62 . The cycles of FIGS. 1F and 1G are preferably repeated approximately five to seven times for a total period of about ten to twelve minutes. 
     As shown in FIG. 1F, the system includes a standard refrigeration circuit, indicated generally at  64 . The operation of such circuits is well known in the art. As is typical in the art, refrigeration circuit  64  features a compressor  65 , fan-assisted cooling coil  66  and heat exchanger  67 . Heat exchanger  67  permits refrigeration circuit  64  to cool the liquid carbon dioxide flowing to chamber  32  along line  52 . As a result, heat from chamber  32  may be removed as it warms up during agitation or if it has warmed up between garment loads or overnight. 
     Soluble contaminants, such as soils and dyes, gradually accumulate in the liquid carbon dioxide during the agitation process and must be periodically removed. Referring to FIG. 1H, this is accomplished by still  70 . Still  70 , which is positioned within, for example, storage tank  18 , operates during the agitation process and distills approximately 3% of the carbon dioxide in chamber  32  per load of garments. 
     Still  70 , filled during a previous cycle in the manner described below, contains liquid carbon dioxide from chamber  32 . Distillation is initiated by connecting the head space of still  70  with the liquid side of transfer tank  12 . As a result, carbon dioxide gas flows to transfer tank  12  from still  70 , as indicated by line  72 , so that the pressure in the still is reduced. Meanwhile, as storage tanks  18  and  20  cycle through the agitation process described above, the pressure and temperature in storage tank  18  will rise so that the warmer temperature of the liquid carbon surrounding still  70  causes the liquid carbon dioxide therein to boil. As the liquid carbon dioxide in still  70  vaporizes, soil and dye residue is left behind inside the still shell. The carbon dioxide vapor flows through line  72  to transfer tank  12  where it is condensed as pure carbon dioxide. 
     It is necessary to drain the accumulated soil and die residue from still  70  for every garment load. This is accomplished, as shown in FIG. 1H, by opening valve  74  for approximately two seconds. This allows the pressure within still  70  to “blast” the residue out of the bottom of still, as indicated by line  76 , where it is collected in a container for disposal. 
     After the completion of the agitation process, it is necessary to refill still  70  with liquid carbon dioxide from chamber  32 . This may be accomplished in the manner illustrated in FIG.  1 I. The suction side of compressor  14  is connected to the head spaces of storage tanks  18  and  20 , while the discharge is connected to chamber  32 . Accordingly, compressor  14  extracts gas from tanks  18  and  20  and uses it to pressurize chamber  32 . As indicated by line  80 , this causes the liquid carbon dioxide in chamber  32  to flow to still  70 , through lint and button traps  54  and filter  56  so that still  70  is filled and pressurized to approximately 650 to 690 psi. Once still  70  is filled with liquid carbon dioxide, the remaining liquid carbon dioxide from chamber  32  is routed, via line  82  to storage containers  18  and  20 . By draining chamber  32  in this manner, there is a reduced possibility of liquid entrapment or ice formation. 
     At this point, chamber  32  is at a pressure of about 650 psi and is empty of carbon dioxide liquid, except for a small amount trapped between the fibers of the garments. The remaining liquid in the garments may be removed in the manner illustrated in FIGS. 1J and 1K. As illustrated in FIG. 1J, the suction side of compressor  14  is connected to chamber  32 , while the discharge side is connected to the head spaces of storage tanks  18  and  20 . Compressor  14  is then activated so that the pressure in chamber  32  is reduced to about 420 psi. As this occurs, the pressure in storage tanks  18  and  20  is increased to about 670 psi. 
     Next, as shown in FIG. 1K, the head spaces of storage tanks  18  and  20  are connected to a set of blasting jets  83  in the bottom of chamber  32 . Such jets are known in the art. The approximately 250 psi pressure difference between storage tanks  18  and  20  and chamber  32  causes the latter to be repressurized with a blast of gas that passes through the jets and directly into the garments. This is illustrated by line  84  in FIG.  1 K. By repeating the procedure of FIGS. 1J and 1K, the carbon dioxide liquid within the garments is removed and the garments are “fluffed.” Testing has shown that two such “blasts” are usually sufficient to remove nearly all of the liquid carbon dioxide from the garments. 
     After the last “blast” of carbon dioxide gas, chamber  32  contains the liquid carbon dioxide removed from the garments and is at a pressure of about 650 psi. The liquid removed from the garments contains an abundance of soil and dies and thus requires distillation. To transfer this liquid to still  70 , the method illustrated in FIG. 1L is employed. First, still  70  is connected to transfer tank  12 . The pressure difference between the two causes a portion of the liquid carbon dioxide in still  70  to flow to transfer tank  12  as indicated by line  86 . This decreases the pressure within still  70  so that it is significantly below the pressure of chamber  32 . As a result, the liquid within chamber  32  is transferred to still  70  as indicated by line  88 . 
     Referring to FIG. 1M, with the dry cleaning process now complete, chamber  32  must be depressurized so that the chamber door  34  may be opened and the garments removed. Accordingly, the suction side of compressor  14  is connected to chamber  32  while the discharge side is connected to storage tanks  18  and  20 . The carbon dioxide gas within chamber  32  is then extracted and used to pressurize storage tanks  18  and  20  back up to approximately 650 to 690 psi, as indicated by lines  90  and  92 . Fine screen diffusers, which are known in the art, may be placed in the bottom of the storage tanks so that the gas returned will be more efficiently diffused into the liquid. When the pressure in chamber  32  drops to 400 psi, the discharge side of compressor  14  is preferably configured via line  93  to deliver gas solely to transfer tank  12 . This is done so that compressor  14  is not overloaded and heat is not produced. After chamber  32  is depressurized, the pressure therein is approximately 50 to 65 psi. At this pressure, chamber  32  contains less than 1% of the carbon dioxide that it contained when it was full. Accordingly, chamber  32  may be vented to the atmosphere, as indicated by line  94 , without causing significant waste. With the chamber at atmospheric pressure, chamber door  34  may be safely opened and the garments removed. 
     The various configurations described above, and illustrated in FIGS. 1A through 1M, are achieved by the manipulation of a number of valves. For example, in reference to FIG. 1A, valves  302 ,  304  and  306  control communication with the head spaces of tanks  12 ,  18  and  20 , respectively. Such valves are well known in the art. 
     Control of the system valves preferably is automated by way of a microcomputer. More specifically, the sequencing of the valves, so that the system operates as described above, is preferably controlled by a microcomputer that is responsive to signals generated by temperature, pressure and liquid level sensors positioned within tanks  12 ,  18  and  20  and cleaning chamber  32 . The microcomputer preferably includes a timer as well that allows it to configure the valves for a predetermined period of time. Such microcomputers and their operation are known to those skilled in the art. Suitable microcomputers are available, for example, from the Z-World corporation of Davis, Calif. 
     Referring to FIG. 1C, for example, as carbon dioxide gas flows into chamber  32  through valve  306 , and the other open valves along line  44 , a sensor within chamber  32  monitors the pressure therein. When this pressure sensor detects that the pressure within chamber  32  has risen to 70 psi, it sends a signal to a microprocessor which in turn closes valve  306 , and the other valves along line  44 , so that the flow of carbon dioxide gas into chamber  32  ceases. 
     As another example, as agitation is being performed in the manner illustrated in FIG. 1F, a timer tracks the time interval. When one minute has passed, the timer signals a microprocessor which then reconfigures the valves to the arrangement shown in FIG. 1G so that agitation may be reversed. Alternatively, pressure sensors positioned within storage tank  18  and cleaning chamber  32  may signal a microprocessor to reconfigure the system valves to the arrangement shown in FIG. 1G when a pressure drop across the cleaning nozzles  53  (FIG. 1F) occurs. A pressure sensor positioned in storage tank  20  may be used in combination with the pressure sensor in the cleaning chamber to accomplish a similar function. 
     The pressure sensors within the storage tanks  18  and  20  and cleaning chamber  32  may also be utilized to control the pressure across the nozzles  53  (FIG. 1F) and  61  (FIG.  1 G), that is, the agitation pressure, so that delicate fabrics or objects are not damaged during agitation. This may be accomplished using the agitation control system illustrated in FIG.  4 . The pressure sensors  320  and  322  in tanks  18  and  20 , respectively, are in communication with a control means such as microprocessor  324 . The control means may alternatively take the form of a process controller such as those made by the Allen Bradley Company or a similar device. A pressure sensor  326  in cleaning chamber  32  is also in communication with the microprocessor. A selector means such as switch  330  allows an operator to select, for example, a fabric setting that is communicated to the microprocessor. During the agitation cycle, the microprocessor adjusts the loading of the compressor  14  based upon the setting of switch  330  so that the pressure differential between the tanks  18  and  20 , when pressurized, and the chamber  32  is controlled. As a result, the pressures from the nozzles in the cleaning chamber are controlled. 
     As is known in the art, differential pressure gauges may be utilized to determine the liquid levels within the storage tanks  18  and  20 . When liquid carbon dioxide under high pressure is contained within the storage tanks, however, condensation may form in the normally gas-filled external tubes of the differential pressure gauges so as to provide erroneous readings. To prevent this problem, the external tubes of the differential pressure gauges may be equipped with heaters in communication with temperature controllers. Heating the external tubes prevents the condensation. 
     The system of FIGS. 1A through 1M offers significant advantages over other carbon dioxide dry cleaning systems. The system moves the liquid carbon dioxide without the use of pumps, instead relying upon a single compressor to pressurize the appropriate carbon dioxide storage tanks with carbon dioxide gas. The density of gaseous carbon dioxide is only about one-sixth of the density of liquid carbon dioxide at the pressures involved. As a result, much less mass is moved by the compressor in motivating the liquid carbon dioxide than if pumps moved the liquid directly. By handling less mass, the compressor suffers less wear and thus offers greater reliability and lower maintenance requirements as compared to cryogenic pumps. In addition, such compressors generally cost less than pumps. 
     The still  70  is advantageous over the distillation apparatus&#39;of other carbon dioxide dry cleaning systems in that it does not employ an electric heater or a heat exchanger. This increases its reliability while decreasing its cost and maintenance requirements. Accordingly, while the preferred embodiment of the system of the present invention is pumpless, the advantages of still  70  may be utilized in systems that feature pumps. Examples of such systems are presented in FIGS. 2 and 3. 
     In FIG. 2, a second embodiment of the carbon dioxide dry cleaning system of the present invention is shown. With the exception of the agitation and distillation processes, this system operates in a manner similar to the system of FIGS. 1A through 1M. A cold transfer tank  112  contains a supply of liquid carbon dioxide, preferably with cleansing additives, at a pressure of about 200 to 250 psi. Transfer tank  112  may be refilled from a mobile delivery tank in a conventional manner. 
     Transfer tank  112  is used to refill a storage tank  118 . This is accomplished by first equalizing the pressures in the two tanks with line  120 . Next, the suction side of a compressor  114  is connected to the storage tank  118  while the discharge side is connected to transfer tank  112 . This creates a pressure differential between the two tanks so that liquid carbon dioxide travels to storage tank  118  through line  122 . 
     A cleaning chamber  132  contains soiled garments and has a volume less than that of storage tank  118 . To commence the dry cleaning process, most of the air in chamber  132  must be evacuated to prevent the addition of water to the cleaning fluid. This is accomplished through line  142 , as shown with line  42  in FIG.  1 B. Chamber  132  is then pressurized to an intermediate pressure of approximately 70 psi by placing it in communication with the head space of transfer tank  118  so that gas travels through line  144  (as in FIG.  1 C). 
     Chamber  132  may next be filled with liquid carbon dioxide. The liquid side of storage tank  118  is connected to the bottom of chamber  132  with lines  146 ,  148  and  144 . The pressure difference between tank  118  and chamber  132  then causes the latter to be almost completely filled with liquid carbon dioxide. The fill is completed by connecting chamber  132  to the suction side of compressor  114  and connecting the discharge side to storage tank  118 . This allows gas to be extracted from chamber  132  and storage tank  118  to be pressurized. The resulting pressure difference causes liquid carbon dioxide to flow from storage tank  118  to chamber  132  through pump line  152 . This pre-cools pump  150  for the agitation process, described below. 
     At this point, chamber  132  is filled with liquid carbon dioxide at a pressure of about 650 to 690 psi and a temperature of about 54° F. (a temperature at which it is an effective solvent). Pump  150  is activated to initiate the agitation process so that insoluble soils may be removed from the garments. Liquid carbon dioxide is pumped by pump  150  through pump line  152  to a first set of cleaning nozzles  153  in chamber  132 . As explained in reference to FIGS. 1F and 1G above, these nozzles cause the garments and fluid in chamber  132  to rotate past the cleaning nozzles. Displaced liquid flows out of the top of chamber  132 , through lint and button trap  154  and filter  156  and finally is returned to the top of storage tank  118  via lines  148  and  158 . 
     After approximately one minute, valve  160  is adjusted so that the flow of liquid carbon dioxide is directed to a second set of cleaning nozzles  161 . These nozzles reverse the rotation of the liquid and garments in chamber  132 . After approximately one minute, valve  160  is reconfigured so that the first set of cleaning nozzles  153  are again utilized. Valve  160  is cycled in this manner preferably five to seven times for a total period of about ten to twelve minutes. 
     The system of FIG. 2 also features a refrigeration circuit, indicated generally at  164 . This refrigeration circuit features a heat exchanger  167  that allows heat to be removed from the liquid carbon dioxide flowing through pump line  152 . 
     A still, indicated at  170 , contains liquid carbon dioxide that was transferred to it during the cleaning of a previous load of garments. As the agitation process is proceeding, the head space of still  170  is connected to the suction side of compressor  114 . The discharge side of compressor  114  is connected to the head space of storage tank  118 . As a result, the pressure within still  170  is decreased while the pressure in storage tank  118  is increased. Alternatively, still  170  may be connected to the liquid side of low pressure transfer tank  112 . As a result, gas from still  170  flows to transfer tank  112  where it is recondensed by the cold liquid therein. In either case, the pressure difference created between still  170  and storage tank  118  allows the temperature of the liquid carbon dioxide in tank  118  to cause the liquid carbon dioxide in still  170  to boil. 
     As boiling occurs, the residue of soluble contaminants, such as soils and dyes, is left behind in still  170  while the carbon dioxide vapor is routed to storage tank  118 . As a result, this distillation process cools storage tank  118  while simultaneously cleaning the carbon dioxide. In addition, the pressure within storage tank  118  is increased by the vapor from still  170 . For every garment load, valve  174  is opened for about two seconds to “blast” the accumulated soil and die residue from still  170  into a container for disposal. 
     Upon completion of the agitation process, the suction side of compressor  114  is connected to storage tank  118  while the discharge side is connected to chamber  132 . The bottom of chamber  132  is connected to still  170  by lines  176  and  178 . As a result, approximately 3% of the liquid carbon dioxide in chamber  132  is transferred to still  170  so as to pressurize it to about 650 to 690 psi for distillation during the next cleaning load. In addition, still  170  is connected to storage tank  118  by line  180 . Accordingly, once still  170  is full, the remaining liquid carbon dioxide from chamber  132  is transferred to storage tank  118  so that chamber  132  is drained. 
     The pressure within chamber  132  is next decreased to about 420 psi by connecting it to the suction side of compressor  114 . The discharge side of compressor  114  is connected to storage tank  118 . As a result, the pressure in storage tank  118  is increased to about 650 to 690 psi while the pressure in chamber  132  drops to about 420 psi. The resulting approximately 250 psi pressure differential allows gas to be blasted through blasting jets  183 , positioned in the bottom of chamber  132 , and into the garments, via lines  158 ,  148  and  144 , so that liquid within the garment fibers is removed. Preferably this cycle is repeated twice. The liquid carbon dioxide from the garments is then transferred from chamber  132  to still  170  in the manner described above in reference to FIG.  1 L. 
     With the cleaning process completed, the garments are ready to be removed from chamber  132 . Before this may be safely done, the pressure within chamber  132  must be reduced to atmospheric. This is accomplished by first connecting chamber  132  to the suction side of compressor  114  and the discharge side of compressor  114  to the liquid side of storage tank  118 . As a result, the carbon dioxide gas from chamber  132  is bubbled into the liquid carbon dioxide of storage tank  118 . When the pressure within chamber  132  drops to 400 psi, the discharge side of compressor  114  is preferably configured to deliver gas solely to transfer tank  112 . As a result, the pressure within chamber  132  is reduced to approximately 50 to 65 psi. The remaining carbon dioxide gas in chamber  132  may then be vented to the atmosphere and the chamber safely opened. 
     In FIG. 3, an embodiment of the system is shown wherein the system pump  250  is disposed within the storage tank  218 . The system of FIG. 3 operates in exactly the same manner as the system of FIG. 2, except that it offers the benefits of internal pump placement. More specifically, by placing pump  250  within storage tank  218 , the pressure differential between the interior and exterior of the pump is greatly reduced. This extends the life of seals around the pump shaft so that the seal replacement intervals are drastically reduced. 
     As an alternative to placing the pump in the storage tank, the pump may be placed in a sump, illustrated at  340  in FIG.  2 . The sump receives solvent from the storage tank  118  via line  342  so that the pump remains submersed. Such an arrangement allows the pump to be readily replaced or serviced without emptying the supply tank  118 . The sump could be mounted in a rotatable fashion so that it could be used in a vertical position and then drained and rotated to a horizontal position. This would allow a tall multistage pump that cannot be removed when vertical to be replaced or serviced. 
     The systems of FIGS. 2 and 3, like the system of FIGS. 1A through 1M, feature a number of control valves. The operation of these valves may also be automated by the use of a microcomputer, process controller or similar device. 
     FIG. 5 shows an alternative embodiment of the system of the present invention. With the exception of the features discussed below, the system of FIG. 5 operates in the same manner as the system of FIGS. 1A-1M. Accordingly, components that are common between FIG.  5  and FIGS. 1A-1M will feature the same reference numbers. 
     As described earlier in reference to FIG. 1C, the head space of either storage tank  18  or  20  may be temporarily connected to the cleaning chamber  32 . As a result, the cleaning chamber is pressurized so that it may be filled with liquid carbon dioxide without the formation of dry ice or the occurrence of thermal shock. Alternatively, as illustrated by line  350  in FIG. 5, the head space of transfer tank  12  may be connected to the cleaning chamber  32  to accomplish the same result. In addition, as illustrated by line  352 , liquid carbon dioxide from the transfer tank may be added to the cleaning chamber. This may be done at the beginning of a cleaning cycle, that is, immediately after the processes illustrated in FIG. 1C or by line  350  in FIG. 5, to replenish the solvent lost during the previous cleaning cycle. As a result, solvent may be added to the system without the use of a pump or compressor. 
     Additives for enhancing cleaning such as surfactants, anti-static agents, detergents and deodorants may be injected into the liquid carbon dioxide via the solvent additive dispenser indicated at  360  in FIG.  5 . The dispenser contains a supply of additive with a head space thereabove. The dispenser head space may be placed in communication with the head space of either storage tank  18  or  20  via line  362 . The liquid side of the dispenser may be accessed either internally by a dip tube or externally through a port so that the additive may travel through line  364 . As a result, during agitation (FIG.  1 F), the dispenser is pressurized as tank  18  (for example) is pressurized so that additive is injected into the liquid carbon dioxide traveling from the cleaning chamber  32  to storage tank  20 . 
     As illustrated in FIG. 5, line  364  features a check valve  365  that prevents liquid carbon dioxide from reaching the additive dispenser  360 . This prevents the formation of dry ice in the additive dispenser  360  when the dispenser is depressurized for replenishment of the solvent additive. 
     As indicated at  370  in FIG. 5, a heat sink is connected to the outlet of the compressor  14 . Heat from the compressed carbon dioxide gas exiting the compressor is transferred to the heat sink during the agitation (FIGS. 1F and 1G) and chamber pressure reduction (FIG. 1J) cycles. As a result, the carbon dioxide gas is cooled before it enters storage tanks  18  and  20 . The undesired heating of the solvent in the storage tanks is therefore minimized. 
     The interior of the cleaning chamber is cooled as a result of the pressure reduction of FIG.  1 J. Carbon dioxide gas within the cleaning chamber may be circulated through the heat sink  370  and returned to the cleaning chamber, as illustrated by lines  372  and  374  in FIG.  6 . The circulated carbon dioxide gas is warmed by the heat sink so that the interior of the chamber is warmed. As a result, the removal of solvent from the cleaning chamber contents is enhanced. Heat sink  370  therefore acts as a “thermal battery” by storing the heat from previous cycles for use in warming the cleaning chamber. The compressor  14  is run at very low compression during this circulation. 
     As explained in reference to FIG. IF, a refrigeration circuit  64  may be used to cool liquid carbon dioxide as it flows to the cleaning chamber. This allows the chamber to be cooled if it has warmed up between garment loads or overnight. Alternatively, as illustrated in FIG. 5, a recondensing coil  380  may be placed within storage tank  20 . The recondensing coil communicates with the refrigeration circuit  64  via a heat exchanger  381 . This allows the liquid carbon dioxide within storage tank  20  to be cooled before it is transferred to the cleaning chamber. As a result, the cleaning chamber is cooled as it receives the cooled liquid carbon dioxide. As indicated by lines  382  and  384 , the heat sink  370  may also communicate with the refrigeration circuit  64  via heat exchanger  381 . This allows the temperature of the heat sink to be controlled. 
     Boiling the liquid carbon dioxide in the cleaning chamber increases the effectiveness of the cleaning process. In other words, objects are more thoroughly cleaned when the liquid carbon dioxide in the chamber is boiled. This may be accomplished as follows. After the initial fill of the cleaning chamber  32  with liquid carbon dioxide, illustrated in FIG. 1E, the line leading from the liquid side of the storage tank  20  is closed. The line between the cleaning chamber and the head space of the storage tank  20 , with the compressor  14  in circuit therebetween, remains open. The compressor may then be utilized to lower the pressure within the cleaning chamber to below the saturation pressure for the liquid carbon dioxide. This causes the liquid carbon dioxide within the chamber to boil vigorously. Alternatively, the compressor may be connected between the cleaning chamber and the head space of the transfer tank  12 . The cleaning chamber may also be connected directly to the head space of a storage tank having a sufficiently low pressure so that use of the compressor becomes unnecessary. 
     In some instances, items in the cleaning chamber may be sufficiently cleaned by the boiling liquid carbon dioxide so that agitation, and therefore usage of the cleaning nozzles (item  53  in FIG.  1 F and item  61  in FIG.  1 G), is unnecessary. Such an approach is particularly useful for cleaning electronic parts. 
     It is to be understood that the pressures and temperatures presented above are for example purposes only and that they are in no way intended to limit the scope of the invention. Furthermore, while the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.