CA2139950A1 - Liquid/supercritical carbon dioxide dry cleaning system - Google Patents

Abstract

A dry cleaning system particularly suited for employing supercritical CO2 as the cleaning fluid consisting of a sealable cleaning vessel (10) containing a rotatable drum (110) adapted for holding soiled substrate, a cleaning fluid storage vessel (12), and a gas vaporizer vessel (11) for recycling used cleaning fluid is provided. The drum (110) is magnetically coupled to a motor (120) so that it can be rotated during the cleaning process. The system is adapted for automation which permits in-creased energy efficiency as the heating and cooling effect associated with CO2 gas condensation and expansion can be channeled to heat and cool various parts of the system.

Description

WO94/01613 213~ PCT/US93/06509 OUID/SUP~CRITIC~T~ C~BON DIOXIDE
DRY ~T~ ING SYST~

Field of the Invention This invention generally relates to an energy efficient dry cleaning system that employs supercritical carbon dioxide and that provides im~ov~d cleaning with decreased re~Ppocition of contaminants, and reduces damage to polymer substrates.

Back~uul-d of the Invention CleAn; ng contaminants from metal, machinery, precision parts, and textiles (dry cleaning) using hydrocarbon and halogenated solvents has been practiced for many years. Traditional dry cleAninq mar-hinPc operate typically as follows: a soiled garment is placed into a cylindrical "hA~ket" inside a cleaning chamber which is then sealed. A non-polar hydrocarbon solvent is pumped into the chamber. The garment and solvent are mixed together by rotating the basket for the purpose of dissolving the soils and stains from the garment into the solvent, while the solvent is continuously filtered and recirculated in the chamber.
After the cleaning cycle, most of the solvent is removed; filtered, and reused.

W O 94/01613 ~ 2~399SO PC~r/US93/06509 . ~ .;

Recently the environmental, health, and cost risks associated with this practice has become obvious.
Carbon dioxide holds potential advantages among other non-polar solvents for this type of cleA~ . It avoids many of the environmental, health, hazard, and cost problems associated with more common solvents.
Liquid/superCritiCal fluid carbon dioxide has been suggested as an alternative to halocarbon solvents in removing organic and inorganic cont~m;n~nts from the surfaces of metal parts and in cleaning fabrics. For example, NASA Technical Brief MFA-29611 entitled "Cle~n;ng With Supercritical C02" (March 1979) discusses removal of oil and carbon tetrachloride residues from metal. In addition, Maffei, U.S. Patent No. 4,012,194, issued March 15, 1977, describes a dry cle~n; nq system in which chilled liquid carbon dioxide is used to extract soils adhered to garments.
Such methods cuggested for cle;~n~n~ fabrics with a dense gas such as carbon dioxide have tended to be restricted in usefulness because they have been based on stA~Ard extraction proCPcFes where "clean" dense gas is pumped into a chamber con~; n ~ ~ the substrate and "dirty" dense gas is drained. This dilution process severely restricts the cleaning efficiency, which is needed for quick processing.
Another problem with attempts to use carbon dioxide in cleA~in9 is the fact that the solvent power of dense carbon dioxide is not high compared to ordinary liquid solvents. Thus, there have been attempts to overcome this solvent limitation.
German Patent Application 3904514, published August 23, l9gO, describes a process in which supercritical fluid or fluid mixture, which includes polar clP;~nin~ promoters and surfactants, may be practiced for the cl~ nin~ or w~ching of clothing and textiles.

WO94/01613 ~13~ PCT/US93/06509 PCT/US89/04674, published June 14, l990, describes a process for removing two or more contaminants by contacting the contaminated substrate with a dense phase gas where the phase is then ~hifted between the liquid state and the supercritical state by varying the temperature. The phase shifting is said to provide removal of a variety of con~ n~ntS without the neC~scity of utilizing different solvents.
However, the problems of relatively slow processing, limited solvent power, and redeposition have seriously hindered the usef~ c-c of carbon dioxide cle~ning methods.
Another particularly serious obstacle to commercial acceptability of dense gas cleaning is the fact that when certain solid materials, such as polyester buttons on fabrics or polymer parts, are removed from a dense gas treatment they are liable to shatter or to be severely missh~r~np~. This problem of surface blistering and cracking for buttons or other solids has prevented the commercial utilization of carbon dioxide cl~ni n~ for consumer clothing and electronic parts.

Summary of the Invention Accordingly, it is an object of the present invention to provide a cl~n; ~ system in which an environmentally safe non-polar solvent such as densified carbon dioxide can be used for rapid and efficient cle~ning, with decreased damage to solid components such as buttons and increased performance.
It is another object of the present invention to provide a cleaning system with reduced redeposition of contaminants, that is adaptable to the incorporation of active cleaning materials that are not necessarily soluble in the non-polar solvent.

WO94/01613 PCT/US93/06~09 ~1~99~iO

Yet another object is to provide a Cl~A~; ~g system that employs a rotatable inner drum designed to hold the substrate during cl~ n~ and a system in which the cle~ni~g fluid is recycled.
In one aspect of the present invention, a system i5 provided for cl~n;n~ contaminated substrates.
The system includes a sealable cle~n;ng vessel containing a rotatable drum adapted for holding the substrate, a cleAn;nq fluid storage vessel, and a gas vaporizer vessel for recycling used cleAn;ng fluid. The drum is magnetically coupled to an electric motor so that it can be rotated during the cl~; nq process.
The inventive system is particularly suited for automation so that the system can be regulated by a mi~ ecsQr. Moreover, automation permits increased energy efficiency as the heating and cooling effect associated with CO2 gas condensation and ~Yp~ncion can be exploited to heat and cool various parts of the system.

Brief Descri~tion of the Drawinqs Figure l is a diagrammatic flow sheet showing the system of the invention.
Figure 2 is a cross-sectional view of the cle~ni ng vessel.
Figure 3 graphically illustrates temperature and pressure conditions within a hatched area in which cleAni~q is preferably carried out for re~-lce~ button damage.

Description of the Preferred Embodiments A cleaning system that can use a ubstantially non-polar fluid such as densified carbon dioxide (CO2) as the cleaning fluid is chown schematically in Fig. l.
The system generally comprises three vessels, the cl~ning vessel lO, preferably a rotatable drum, the gas ~39~50.

vaporizer vessel ll, and the storage vessel 12, all of which are interco~cted. The clPAnin~ vessel, where soiled substrates (e.g. clothing) are received and placed into contact with the Cl~An; n~ fluid is also referred to as an autoclave. As will be described further below, much of the C02 cle~n;~g fluid is recycled in this system.
C02 is often stored and/or transported in refrigerated tanks at approximately 300 psi and -18C.
In charging the inventive system with C2, pump 2l is adapted to draw low pressure liquid C02 through line 92 that is connected to a refrigerated tank (not shown) through make-up heater 42 which raises the temperature of the C02. The heater preferably has finned coils through which ambient air flows and employs resistive electric heating. Pump 21 is a direct drive, single-piston pump. Liquid C02 is then stored in the storage vessel 12 at approximately 9lS psi and 25C. The storage vessel is preferably made of stainless steel.
As shown in Fig. l, conventional temperature gauges (each depicted as an encircled "T"), pressure gauges (each depicted as an encircled ~tpn)~ liquid C02 level meters (each depicted as an encircled "L"), and a flowmeter (depicted as an encircled "F") are employed in the system. In addition, conventional valves are used.
In operation, after placing soiled substrate into the cleaning vessel, the cleAn; ng vessel is then charged with gaseous CO2 (from the storage vessel) to an inter~e~te pressure of approximately 200-300 psi to prevent extreme thermal shock to the chamber. The gaseous C02 is transferred into the cle~ning vessel through lines 82 and 84. Thereafter, liquid C02 is pumped into the clP~n;ng vessel from the storage vessel through lines 80, 9l, 81, and 82 by pump 20 which preferably has dual pistons with either direct or hydraulic/electric drive. The pump raises the pressure WO94/01613 PCT/US93/06509 ~

2~39950 of the liquid C02 to approximately 900 to 1500 psi.
Subcooler 30 lowers the temperature of the C02 by 2 to 3 below the boiling point to prevent pump cavitation.
The temperature of the C02 can be adjusted by heating/cooling coils 95 located inside the cle~n; ng vessel. Before or during the cle~i n~ cycle, cleaniny additives may be added into the cle~ni~ vessel by pump 23 through lines 82 and 83. Moreover, pump 23 through lines 82 and 83 can also be used to deliver a compressed gas into the cl~ni ng vessel as described below.
Practice of the invention requires contact of a substrate having a cont~ nt with the first, substantially non-polar fluid that is in a liquid or in a supercritical state. With reference to Fig. 3, when using C02 as the first fluid, its temperature can range broadly from ~lightly below about 20C to slightly above about 100C as indicated on the horizontal axis and the pressure can range from about lO00 psi to about 5000 psi as shown on the vertical axis. However, within this broad range of temperature and pressure, it has been discovered that there is a zone (represented by the hatched area of the left, or on the convex side, of the curve) where ~urface blistering to components such as buttons can be reduced, whereas practice outside of the zone tends to lead to button damage that can be quite severe. As is seen by the hatched region of Fig. 3, preferred conditions are between about 900 psi to 2000 psi at temperatures between about 20C to about 45C, with more preferred conditions being pressure from about 900 psi to about 1500 psi at temperatures between about 20C and 100C or from about 3500 psi to about 5000 psi at temperatures between about 20C and 37C. Where fabrics are being cl~ , one preferably works within a temperature range between about 20C to about 100C.
In addition, it has been found within this range that W O 94/01613 = ~ ~3~50 PC~r/US93/06509 proc~Q~s which raise the temperature prior to decompression reduce the damage to polymeric parts.
Suitable com~o~r.ds as the first fluid are either liquid or are in a supercritical state within the temperature and pressure hatched area illustrated by Fig. 3. The particularly preferred first fluid in practicing this invention is carbon dioxide due to its ready availability and environmental safety. The critical temperature of carbon dioxide is 31C and the dense (or compressed) gas phase above the critical temperature and near (or above) the critical pressure is often referred to as a "supercritical fluid." Other densified gases known for their supercritical properties, as well as carbon dioxide, may also be employed a~ the first fluid by themselves or in mixture.
These gases include methane, ethane, propane, ammonium-butane, n-pentane, n-h~Y~ne~ cycloh~xAn~, n-heptane, ethylene, propylene, methanol, ethanol, isopropanol, benzene, toluene, p-xylene, chlorotrifluoromethane, trichlorofluoromethane, perfluoropropane, chlorodifluoromethane, sulfur hexafluoride, and nitrous oxide.
Although the first fluid itself is substan-tially non-polar, it may include other components, such as a source of hy~Gyen peroxide and an organic bleach activator therefor, as is described in copen~i~g application Serial No. 754,809, filed September 4, 1991, inventors Mitchell et al., of common assignment herewith. For example, the source of hydrG~en peroxide can be selected from hydrogen peroxide or an inorganic peroxide and the organic bleach activator can be a carbonyl ester such as A~ oyloxybenzene. Further, the first fluid may include a cl~n;n~ adjunct such as another liquid (e.g., alkAn~c, alcohols, aldehydes, and the like, particularly mineral oil or petrolatum), as W O 94/01613 - ~ ~39~ PC~r/US93/06509 described in Serial No. 715,299, filed June 14, 1991, inventor Mitchell, of co~mon a~signment herewith.
In a preferred mode of practicing the present invention, fabrics are initially pretreated before being contacted with the first fluid. Pretreatment may be performed at about ambient pressure and temperature, or at elevated temperature. For example, pretreatment can include contacting a fabric to be cleaned with one or more of water, a surfactant, an organic solvent, and other active cleAn;~ materials such as enzymes.
Surprisingly, if these pretreating components are added to the bulk solution of densified carbon dioxide (rather than as a ~LeL~eatment), the stain removal process can actually be impeded.
Since water is not very soluble in carbon dioxide, it can adhere to the substrate being cleaned in a dense carbon dioxide atmosphere, and impede the cle~n; ng process. Thus, when a pretreating step includes water, then a step after the first fluid cleArlinq is preferable where the cl~:~n;ng fluid is contacted with a hyyLo-~pic fluid, such as glycerol, to eliminate water otherwise absorbed onto fabric.
Prior art cle~n; n~ with carbon dioxide has typically involved an extraction type of process where clean, dense gas is pumped into a chamber cont~in;ng the substrate while "dirty" ~n~ gas is drained. This type of continuous extraction restricts the ability to quickly process, and further when pressure in the cle:~n; n~ chamber is released, then residual soil tends to be redeposited on the substrate and the chamber walls. This problem is avoided by practice of the inventive method (although the present invention can also be adapted for use as continuous extraction process, if desired).
3S The time during which articles being cleaned are ~Yp~e~ to the first fluid will vary, depending upon W094/01613 2~ PCT/US93/06509 the nature of the substrate being cleaned, the degree of soiling, and so forth. However, when working with fabrics, a typical exposure time to the first fluid is between about 1 to 120 minutes, more preferably about 10 to 60 minutes. In addition, the articles being cleaned may be agitated or tumbled in order to increase cle~ninq efficiency. Of course, for delicate items, such as electronic compo~Pnts, agitation may not be recommended.
In accordance with the invention, the first fluid is replaced with a second fluid that is a compressed gas, such as compressed air or compressed nitrogen. By "compressed" is meant that the second fluid (gas) is in a condition at a lower density than the first fluid but at a pressure above atmospheric.
The non-polar first fluid, such as carbon dioxide, is typically and preferably replaced with a non-polar second fluid, such as nitrogen or air. Thus, the first fluid is removed from contact with the substrate and replaced with a second fluid, which is a compressed gas.
This removal and replacement preferably $s by using the second fluid to displace the first fluid, so that the second fluid is interposed between the substrate and the separate contaminant, which assists in retarding redeposition of the contaminant on the substrate. The second fluid thus can be viewed as a purge gas, and the preferred compressed nitrogen or compressed air is believed to diffuse more slowly than the densified first fluid, such as densified carbon dioxide. The slower diffusion rate is believed useful in avoiding or reducing damage to permeable polymeric materials (such as buttons) that otherwise tends to occur. However, the first fluid could be removed from contact with the substrate, such as by venting, and then the second fluid simply introduced. This alternative is a less preferred manner of practicing the invention.

WO94/01613 ~3~950 PCT/US93/06509 ~

Most preferably, the second fluid is compressed to a value about equal to P1 at a temperature T1 as it displaces the first fluid. This pressure ~alue of about P1/T~ is about equivalent to the pressure and temperature in the chamber as the contaminant separates from the substrate. That is, the value P1 is preferably the final pressure of the first fluid as it is removed from contact with the substrate. Although the pressure is thus preferably held fairly constant, the molar volume can change significantly when the ch~her that has been filled with first fluid is purged with the compressed second fluid.
The time the substrate being cleaned will vary according to various factors when contacting with the first fluid, and so also will the time for contacting with the second fluid vary. In general, when clP~ni~
fabrics, a preferred contacting time will range from l to 120 minutes, more preferably from l0 to 60 minutes.
Again, the articles being cleaned may be agitated or tumbled while they are in contact with the second fluid to increase efficiency. Preferred values of P1/T, are about 800 to 5000 psi at 0C to 100C, more preferably about l000 to 2500 psi at 20C to 60C.
St~in~ and soiled garments can be pretreated with a formula designed to work in conjunction with COz.
This pretreatment may include a bleach and activator and/or the synergistic cleaning adjunct. The garments are then placed into the cle~;ng chamber. As an alternate method, the ~LeLLeatment may be sprayed onto the garments after they are placed in the chamber, but prior to the addition of CO2.
The chamber is filled with CO2 and ~LG~-ammed through the appropriate pressure and temperature cleaning pathway. Other cleaning adjuncts can be added during this procedure to improve cle~ninq. The CO2 in the cleaning chamber is then placed into contact with a 2139~

hygroscopic fluid to aid in the removal of water from the fabric. The second fluid (compressed gas) is then pumped into the chamber at the same pressure and temperature as the first fluid. The second fluid displaces the first fluid in this step. Once the first fluid has been flushed, the chamber can then be decompressed and the clean garments can be removed.
In order to recycle most of the CO2 from the cleaning vessel as it is being replaced by the compressed gas, the CO2 is dr~ine~ from the cle~ning vessel into the vaporizer vessel ll which is equipped with an internal heat eY~h~nger 40. The cl~ing vessel is drained through lines 87, 89, 9l, and 88 by pump 20 thereby recovering gaseous CO2 at a pressure of approximately 200 psi. Du~ing the recovery process, the cle~n;~g vessel is simultaneously heated; unrecovered CO2 is vented to atmosphere. From the vaporizer vessel, CO2 is continuously repurified by stripping the gaseous CO2 with activated charcoal in filters 50 and thereafter condensing the clean gaseous Co2 by condenser 31 so that the recovered CO2 reenters the storage vessel for later use. Soil, water, additives, and other residues are periodically removed from the vaporizer vessel through valve 66.
Referring to Fig. 2 is a cross-sectional diagrammatic view of a cl~ni~g vessel that is particularly suited for cleaning fabric substrates (e.g., clothing) with supercritical CO2. The cleaning vessel comprises an outer chamber l00 having gaseous CO2 inlet and outlet ports l0l and 102, c~ ~essed gas (e.g.
air) inlet and outlet ports 103 and 104, and liquid CO2 inlet and outlet ports 105 and 106. Although the gaseous CO2, compressed gas, and liquid CO2, each have separate inlet and outlet ports, the cle~ing vessel may instead have one port for both inlet and outlet functions for each fluid. Inside the chamber is basket WO 94/01613 2~399~) PCr/US93/06509 ~

or drum llO that i5 ~u~vr l,~d by two s~3t8 o~ rollers 11 and llla. The h~ket hz~: perforation~; 130 ~o that g~
ana l~guid C02 can re~d~ly en~er ~nd exil; t~e ba~;k~t. ~ine~
11~ cr~ate~: a tumbling action when th~ drum i~ spun.
5. Sub~trates ~o ~e cl~aned are plac:ed into the basket; t~rough ~n or~ ~ n~ in the cham}: e~ ~hic~h i ~;ealed l:~y hinged door 113 when- the cleanins~ ve~;s~l i~ in use. situa~ed ~long tne peri~eter of outer c:~a~ber are coils 114 through ~hlch cnc~lant c~r heating f~uid can be circulated. q~e drum in ~ t 1~ advantageou~; at expo~ing greater surface area of fabric ~ubs~rate~; tc~ the den~;e f luid and may al80 contribute to some - ~ch~n~ical partitioning of ~:oil from fabric. Al~o, in case there i~; an interface or densi~y ~radient esta~ ed in th~ ber, rotation of the drum 1~ can t`c~ le" t:he fabrics c:au~ing partitioning of soi~ from f~bric~. Addi~ionally, th~ den~e gas can advant~geou~;ly be ~eparated or dri~re~ o~ ~rom the fabric ~y the rotational action ~f the d~n.
~e baslce~ is magnetically coupl~ad to a motor ~0 12~, ~hioh i:s prefex~bly electric, ~o ~hat the }~asket c:an be rotated. Other motiv~3 means f or ~ri~ing the basket ar~
pos;sible. Specific:ally, ~he inn~r basket is a~tached to a plat~orm member 121 re~iting rotatably on ball ~aring~ 122, 2~nd dri~re di~lc 123. The platform and dri~re di~;k are rotationally coupl~d b~ ~agnets 1~4 which are arranged, in suitable number, syDImetrically around the ci~ erenc:e of each. T~e drive digk is ~oupled to the m~tor by bel~ 125 ~nd Pul~y 126 or othar appropriate me~ns. Wh~n lthe ~asket i~; ~agnetic~lly ~ouple~ t~ a motor, the b~ket can ad~n~q~ ly be oealed f~om the exte~n~l environmen~ wi~h no loss of ~ealing integrity since drive shafts and other drive means which penetrate the basket are obviated. ~rhu~;, by usinq a ~n~gnetic cb~pling, dr~ve ~;h~fts and associated ;ealing gaskets and t~e like can ~e avoided. Further~ i~
t~e ba~:3cet is m~gnetic~ally SUBSrITUTE SHEET

WO94/01613 ~ 9~ PCT/US93/06509 coupled, the h~ket can advantageously be easily removed from and replaced in the chamber. In this manner, the basket can be a comronent unit and, if desired, different loads of fabrics with different laundering requirements can be batched into different hA~kets and thus loaded individually into the chamber one after another for ease ~f cl~nin~. The cl~Aninq vessel is generally made from materials which are chemically compatible with the dense fluids used and sufficiently strong to withstand the pressures n~ce~CAry to carry out the process, such as stainless steel or aluminum. The clPA~ i n~ vessel as shown in Fig. 2 can be used as the autoclave lO in the system as shown in Fig. l.

It is to be understood that while the lS invention has been described above in conjunction with preferred specific embodiments, the description and examples are inte~P~ to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims.

Claims (10)

It is Claimed:

1. An apparatus for cleaning a substrate with a densified gas comprising:
a sealable cleaning vessel defining a compartment with temperature change means operatively associated therewith for adjusting the temperature within said compartment;
a rotatable drum adapted to receive the substrate, the drum being positionable inside the cleaning vessel compartment;
a storage vessel in fluid communication with the compartment;
a gas vaporizer vessel in fluid communication with the compartment, wherein the storage vessel is in fluid communication with the gas vaporizer vessel by first conduit means; and means for introducing a compressed second gas at a selected pressure into said compartment for displacing said densified gas.

2. The cleaning apparatus as defined in claim 1 wherein said storage means is in fluid communication with said compartment by second conduit means and wherein said apparatus further comprises:
means for injecting cleaning additives into said cleaning vessel.

3. The cleaning apparatus as defined in claim 1 wherein said apparatus further comprises:
cooling means disposed in said second conduit means for cooling gas from said storage vessel below its boiling point.

4. The cleaning apparatus as defined in claim 1 or 3 wherein said vaporizer vessel further comprises:

means for adjusting the gas temperature therein.

5. The cleaning apparatus as defined in claim 4 further comprising:
filter means for removing volatile contaminants from gases in said first conduit means.

6. The cleaning apparatus as defined in claim 5 wherein said apparatus further comprises:
condenser means for condensing filtered gas from said filter means.

7. The cleaning apparatus as defined in claim 4 wherein the drum is cylindrical and is supported by at least two sets of rollers and wherein said cleaning vessel further comprises motive means for rotating the drum, the motive means having a drive that is magnetically coupled to said drum.

8. The cleaning apparatus as defined in claim 7 wherein the motive means includes a motor that causes said drum to rotate.

9. The cleaning apparatus as defined in claim 8 wherein the motor is electric.

10. The cleaning apparatus as defined in claim 4 wherein the drum is removably positionable inside the cleaning vessel compartment.