- Email: [email protected]
Dry-cleaning with liquid carbon dioxide
641 9.11 Dry-cleaning with liquid carbon dioxide Paolo Pallado via M. Ravel, 8
35132 Padova
Italy
9.11.1 Introduction Carbon Dioxide (CO2), in its liquid or supercritical state, is currently attracting much interest as an environmentally acceptable solvent, which is easy to handle, and available at low cost, even in high purity grade. In the late 1960s, the potential of the non-toxic and environmentally benign of supercritical CO2 (SC CO2) for natural product extraction was recognized by several authors [1-3]. So far, the practical use of CO2 has been addressed mainly as an extractive media in agricultural fields. The advantage of SC CO2 over conventional extraction with liquid solvents is that the SC fluid has a very low viscosity and a high diffusion coefficient [4]. It penetrates solid substances efficiently and is removed after depressurization. The excess amount of adsorbed gas diffuses from the material under room conditions. Moreover, SC fluid's solvent power, which is related to solvent density, can be tuned by varying the pressure and temperature. Some substances can be selectively stripped away from other compounds from a raw material, and then the same substances are easily separated from the fluid by reducing the pressure and controlling the temperature. Its application has been limited by the need for high pressures to dissolve substances having an affinity with CO2, mainly non-polar compounds, but even small amounts of polar, amphiphilic, organometallic, or compounds with molecular weights up to 500 Da. Nevertheless, the poor solubility of many interesting target substances places a severe restriction on the widespread use of CO2 as a solvent. Sometimes, unrealistically high pressures or expensive "CO2-philic" compounds (substances with a high affinity for CO2 solutions at lower pressures) are needed. The specific properties of SC CO2 make it an interesting "green" replacement for liquid organic solvents, which are often characterized by acute toxicity, ecological hazards, or drawbacks with disposal and recycling. The use of CO2 has currently moved from the socalled conventional extraction process to several other areas as diverse as the dyeing and cleaning of fibre and textiles, polymerization and polymer processing [5], purification and crystallization of pharmaceuticals [6,7], comminution of difficult-to-handle solids [8], controlled drug delivery systems [9], and as a reaction medium for chemical synthesis [ 10]. Last, but not least, a great effort has been made by several companies worldwide for the development of the dry-cleaning of garments and textiles with liquid CO2, in substitution for the organic solvents currently used, such as perchloroethylene, petroleum ether, and trichloroethane. The commercial target in this field is not limited to a few industrial plants, but to the huge number of industrial dry-cleaning companies, and to the dry-cleaning shops in commercial centers. Especially thanks to these efforts, SC CO2 can be handled safely in high-pressure equipment from the laboratory- to the industrial scale. Nevertheless, its limited power to dissolve polar or non-volatile compounds represents a major drawback in most of these applications, particularly in the garment-cleaning process, where the design of CO2-philic
642 solubilizers is of paramount importance. 9.11.2
Dry-cleaning processes
9.11.2.1 Conventional dry-cleaning In the conventional dry-cleaning process, liquid organic solvents such as perchloroethylene and other petroleum-based substances are used. The machines have a washing chamber, inside which a rotating cylindrical basket is used for the motion and agitation of articles and garments. The removal of dirt, stains, and other contaminants on the articles to be washed is assured by the solvent's ability to dissolve the contaminants, and by the "fall and splash" effect caused by the rotation of the internal basket. The articles and the fluid are tumbled together by rotating the basket or "tumbler", so that the solvent is brought into intimate contact with the surfaces of the articles. As the solvent is absorbed by the absorbent portions, it dissolves these contaminants or maintains them in suspension. Solid particles and chemicalor organic contaminants are then forced from the internal part of the cylinder to the annular zone between the cylinder and the chamber's internal walls, and are removed by continuously re-circulating the liquid solvent from-and to-the washing chamber. Dry-cleaning machines are usually provided with a centrifugal pump to force the liquid through a filter, and with a blower to move and filter the solvent vapour phase also. After the cleaning cycle, the solvent is drained from the cleaning chamber and may be reused, provided that the level of dissolved contaminants is not approaching saturation, otherwise it is regenerated by distillation or cleaned by mechanical filtration. The conventional dry-cleaning process involves significant health and environmental risks associated with the toxic nature of conventional cleaning solvents. Percholoroethylene is suspected to be carcinogenic, while petroleum-based solvents are flammable and smogproducing. The search for alternative cleaning technologies, which are safe and environmentally acceptable, is still in progress, to substitute the solvents and methods of controlling exposure to dry-cleaning chemicals.
9.11.2.2
Dry-cleaning with liquid carbon dioxide
As CO2 is an environmentally safe fluid, its efficient use as a dry-cleaning fluid avoids the risks and drawbacks associated with conventional cleaning solvents. Carbon dioxide has been identified as an inexpensive solvent, with an unlimited natural resource. It shows a high compatibility with garments, as it does not damage fabrics nor dissolve common dyes. In this way, CO2 has been recognized as a good medium to be used in the dry-cleaning industry. Also, the solvating properties of CO2 are quite non-typical of those of more traditional solvents. As the fluid's solvent power is a function of its density, CO2 must be compressed to enhance the cleaning efficiency. Keeping COz at relative high densities requires that the vessels have to be designed for higher pressures than those of the process itself, increasing machine's cost and safety level. As a first step to contain the machine cost, the process is performed with liquid CO2 to reduce the vessels' design-pressure values. The pressure ceiling can be fixed at 70 bar, but this constraint limits the maximum allowable working temperature to be lower than 18~ reducing the mass-transfer and solute dissolution inside the fluid. The viscosity of fatty substances shows a large dependence on temperature, decreasing dramatically above 50~ Heavy greasy compounds tend to segregate as a solid phase at temperatures lower then 10~
643 The pressure is the main cost-driver, since it affects all process-housing component costs, as well as operational costs. Standard petro-chemical components can be used, as the maximum design pressure value can be set close to the CO2 critical value. The physical characteristics of the fluid do not allow one to run the process at temperatures higher than 20~ using liquid CO2, as the process-control ceases to be affordable when the pressure and temperature approach the near-critical zone. The use of SC CO2, at temperatures higher than 35~ will require the pressure above 200 bar to have a comparable value in density, raising the costs for the hardware and process control. The cleaning of the garments is thus assured, coupling the affinity of CO2 towards dissolving greasy substances, with mechanical removal of fine particles and stains. Agitation represents another major contribution to costs. In a pressure vessel, mechanical agitation produced with moving parts is cost-prohibitive in terms of capital and maintenance expenditures. It has to be provided without moving parts, that is, with no electrical or mechanical devices which require a sealing system, to avoid CO2 leakage from the washing chamber. 9.11.3
The CO2 dry-cleaning process
9.11.3.1 Fundamentals Several dry-cleaning systems using CO2 as a solvent have been proposed and are currently under development. As the topic is mainly concerned with the definition of a cleaning process for industrial or commercial applications, the literature refers to several patents which differ in type and performance of the washing machines. The CO2 dry-cleaning process was originally disclosed by Maffei [11], who designed a simple system formed by a chamber, a storage tank and an evaporator. Garments are placed inside a basket contained in the chamber, then CO2 is gravity-fed thereto from the refrigerated tank, passes through the garments, and is finally transferred to the evaporator. In the evaporator, CO2 is evaporated and separated from the soil, which is recovered from the bottom of the tank. The vapour CO2 is compressed into a heat-exchanger to condense before being stored in the storage tank. Many improvements have been made since Maffei's system. First of all, the washing chamber needs to be pressurized to maintain the CO2 in the liquid state, otherwise the CO2 must be very cold to remain liquid. To enhance the washing efficiency, garments have to be agitated to remove solid stains and particles which are not soluble in CO2. The simplified scheme depicted in Fig. 9.11-1 is useful to understand the machine units and process steps. The main representative vessel is the washing chamber, where garments are placed inside a basket. Normally, other two main vessels are used to handle the fluid during the process, a storage tank and an operative or service tank. The clean CO2 is kept in the liquid state in the storage tank, which is used as an additional reservoir. The process liquid CO2 is stored in the operative tank. Two small filtering vessels, a lint-trap with a filter train, and a button-trap, complete the number of vessels normally present in the machine. The button-filter is located at the bottom of the chamber; a wire filter avoids the dragging of fibres, buttons, and other macroscopic pieces which could seriously damage the liquid pump and the valves. The lint-filter is used as the main filtering device to capture the fine solid particles removed from the garments in the washing step, when the liquid is continuously pumped from and to the chamber. In some versions, active-carbon powder filtering-cartridges are used to
644
c-] Figure 9.11-1: Schematic representation of the dry-cleaning process, displaying different steps from A to D. trap unpleasant odours dissolved in the fluid. The chamber is initially pressurized with vapour, fed from the still or the service tank (step 1), and then loaded with liquid from the service tank (step 2). A liquid piston pump assures the pumping of the liquid CO2 in the system. Normally, the pump is used to displace the liquid CO2 from the storage and operating vessels to the chamber, and vice versa, and through the chamber during the washing phase (step 3). Pressure-drops are limited to values lower than 20 bar, but very high flow-rates are needed to reduce the process time. At the end of the washing step, the liquid is drained from the chamber to the service tank (step 4). A compressor is required to recover the vapour CO2 from the chamber at the end of the washing cycle, to minimize the loss in CO2 for each cycle (step 5). The vapour is condensed through a heat-exchanger and the liquid is recovered and retumed to the storage vessel. The pressure-drop at the end of this step can be larger than 50 bar, with a temperature of the process-fluid higher than 80~ downstream the compressor. The vapour phase is sometimes moved by heating in the distillation step, using the positive pressure-drop generated by the increase in temperature (step 6). Nevertheless, the compressor can be used to help the distillation process, sucking the vapour from the still to the heat exchanger and then to the storage or service tank (step 7). The operative tank can be utilized as a full capacity still if it is provided with an intemal heat exchanger, otherwise an additional still, smaller in size, is necessary to perform the distillation step and to clean the "dirty" liquid CO2. As the percentage of lypophilic substances
645 in the garments is very low, the "dirty" liquid CO2 distillation by means of the little still is carried out after a series of washing cycles, depending on the amount of liquid loaded in the chamber. So, a distillation per dilution process is reached, as the operative tank remains partially dirty at its bottom. On the other hand, using a full capability still liquid CO2 vaporizes and can be completely cleaned from stains and soluble substances. Two main thermal fluxes characterize the system from a thermodynamic point of view. These come from the liquid-CO2 distillation and, consequently, the vapour condensation to recover the liquid in the operative or storage tank, and the condensation of vapour CO2 recovered from the chamber at the end of the washing step. Most frequently, the distillation is carried out in a vessel which is equipped with an intemal heat exchanger, dip in the liquid to be cleaned. The heat exchanger can be of an electric resistance type, or a series of coils through which hot water flows. The heat exchange is very efficient at the beginning, when the exchanger is completely immersed in the liquid, then the efficiency reduces dramatically as the exchange-surface in contact with the liquid decreases. As the overall heat coefficient of the vapour phase is very low, a plate heat-exchanger is used for the condensation. The machine is equipped with a refrigeration unit designed according to the thermal flux required for the condensation of the vapour CO2. The refrigeration unit is then used to control the temperature in the chamber during the washing step or to prevent over-temperatures in the storage vessels. The system is finally optimized if the refrigerating fluid used to cool down the refrigeration unit process-fluid is fed to the still to supply the heat required for the distillation process. A third heat flux is necessary for pre-heating the chamber before the gas recovery. To avoid the insertion of an additional plate heat-exchanger, it is preferable to use electric resistances which can supply high thermal fluxes per specific surface.
9.11.3.2 Garments agitation Many systems have been proposed for the agitation of garments in CO2 dry-cleaning systems. Deewes [ 12] proposed a rotating basket, used by the majority of the traditional drycleaning systems. 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, causing the garments to drop and splash into the liquid solvent. Chao [13] disclosed a variety of agitation techniques, that is a "gas bubble and boiling agitation" where the liquid CO2 in the basket is boiled, a "liquid agitation" where a nozzle spraying CO2 tumbles the liquid and the garments, a "sonic agitation" where sonic nozzles induce vibrating waves, and a "stirring agitation" where an impeller creates the fluid displacement. In this patent, as in most of the developed systems, the basket in the washing chamber is completely drilled to allow the movement of solid particles and contaminants towards the annular free volume, from which they are removed inside the solvent stream. A gas-jets agitation technique has been proposed by Purer [14] to remove particulate soils from fabric. The agitation occurs apart from the immersion of the garments in liquid CO2; nevertheless CO2 has been employed both as a gas and a solvent. Townsend and Purer [15] disclosed a rotating basket powered by a hydraulic flow generated by forcing the fluid through a number of nozzles. Finally, an improved nozzles agitation system has been applied in the Hughes Dry Wash TM machine [16]. Despite the several systems disclosed, garments' agitation is substantially operated in two ways, with a rotating basket or by fluid-jets. In the former, the mechanical force necessary to
646 dislodge the soiling from the substrate is provided by the fall-and-splash effect of the solventloaded garment from the top of the rotating drum into the solvent pool below. The magnitude of the force is dependent on the height of fall, which depends on the solvent level in the chamber, and the solvent density. The relatively low density of liquid CO2 discourages the application of this agitation mechanism. In the fluid-jet system, the garments are submerged in liquid CO2 within the perforated basket, set into motion, and are agitated by high-velocity fluid jets. The jets are discharged through nozzles, which are set in an appropriate configuration, and the garments agitated through a Venturi effect. As the garment is accelerated by the jet, its fibres are stretched, and the soil trapped into and between the fibres are expelled. As a summary, the power of the pump used to force the fluid through the nozzles is transformed into fluid velocity which is transferred to the garment as momentum allowing the expulsion of soiling. Four series of nozzles are fixed inside the chamber of the Hughes machine [16]. The chamber is completely filled with liquid CO2 and pressurized, then the liquid is forced through two nozzles each time inside the chamber by opening and closing a valve located upstream of each pair of nozzle series. The nozzles are positioned so that they create an agitating vortex inside the basket as liquid CO2 is forced through them. The soil-laden liquid CO2 which leaves the basket is moved from the chamber to a lint-trap and a filter-train by means of a circulating pump. Liquid CO2 has a lower density than the liquid organic solvents normally used in conventional dry-cleaning processes, as shown in Table 9.11-1. Moreover, its viscosity is one order of magnitude lower than organic solvents. As a consequence, the jet-cleaning agitation seems to offer the more feasible approach for CO2 dry-cleaning machines. Table 9.11-1 Physical properties of solvents commonly used in dry-cleaning processes solvent
Surface tension, Density, Dynes/cm g/cm 3
Viscosity, cPoise
P erchloro ethyl ene 1,1,1-Trichloroethane Freon-113 Freon- 114
32.0 25.6 19.6 12.0
1.60 1.40 1.60 1.50
0.88 1.20 0.68 0.38
Water
72.0
1.00
0.90
Liquid CO2
3.0
0.85
0.08
This assumption is further supported when one considers that the rotating tumbler needs a special sealing system to prevent CO2 escaping or leaking round the drive shaft which is connected to the tumbler through a wall of the pressure vessel. When a magnetically agitated shaft is used to avoid the constraint of rotating shaft seal, the thickness of stainless steel needed for pressure resistance dissipates the magnetic coupling, limiting the torque that can be transferred from the outer magnets to the internal shaft. These solutions have been found relatively ineffective, especially for commercial-size units, and have high maintenance costs. However, in the jet-cleaning agitation process the chamber has to be completely filled with
647 liquid to allow correct functioning of the system, and this feature reflects on the machine design, which requires larger tanks, more powerful pumping devices, and an increase in the thermal fluxes needed for the distillation and condensation steps. Operational times are also increased. Finally, the fall-and-splash effect requires a load of liquid which is sufficient just to submerge the garments.
9.11.3.3 Machine configurations We refer to the simplified scheme depicted in Fig. 9.11-2, where the main vessels are the washing chamber C, the storage tank ST, the operative tank OT, and the evaporator or still ET. Of course, the chamber is provided with an agitation device and with a completely drilled basket, into which garments are placed.
CV
or
[~ additive PL .~
II ~ U FB I~
HE2
Figure 9.11-2: Simplified scheme of the high pressure lines for a dry-cleaning C02 unit. Legend: washing chamber, C; compressor, CV; evaporator or still, ET; button filter, FB; linttrap, FT; heat-exchanger, HE#; operative tank, OT; additive pump, PA; liquid pump, PL; vacuum pump, PV; storage tank, ST. The clean liquid C02 is stored in the storage tank, and dirty C02 in the operative tank. The storage and the operative tanks have the same internal capacity, while the still can be smaller. In a steady-state configuration, the liquid CO2 stored in the operative tank is used several times for washing, while the storage tank is half empty and is used as a reservoir, to fill up the chamber. After the garments are loaded in the basket, and the chamber closed, the air in the chamber is extracted by a vacuum pump, PV, then CO2 vapour from the storage or the operative tank is fed thereto to avoid a sharp reduction in the temperature. To complete the chamber filling, liquid from the operative tank is fed by the liquid pump PL. At the end of the filling step, the chamber is over-pressurized to avoid vapour formation, and the washing step
648 can begin. The liquid is re-circulated continuously from and to the chamber, passes through the garments, and is filtered through the lint-trap, FT. A plate heat-exchanger, HE1, is used to keep the temperature lower than 20~ At the end of the washing step, the liquid is drained to the operative tank by the pump, and through the button filter, FB, then the gas-recovery step takes place to recover most of the vapour or gaseous CO2 from the chamber. As the gas-recovery can be assumed to be like an adiabatic process, the fluid in the chamber is pre-heated by an electric resistance heat exchanger, HE2, to contain the reduction in temperature. The CO2 vapour is compressed by the oil-free compressor CV and fed to HE1 to condense before being stored in the operative tank. Finally, the chamber is vented and the door can be opened to recover the cleaned garments. 9.11.4
Conclusions
The use of SC CO2 for the cleaning of garments, as an alternative method to the liquid solvents, is very attractive from an environmental point of view, and for the safety and health of the operators involved. Nevertheless, several issues have still to be addressed and improved, such as the ability of the solvent mixture to dissolve polar and heavy compounds, the entire washing and rinsing cycle to maximize stain removal, the machine's cost compared to the conventional machines. As a final remark, the machine costs matched with the innovative character of the technology could confine the application just for industrial laundries, limiting the possibility to extend dry-cleaning machines to an end-user approach. References
1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11.
Brunner, G. and Peter, S. (1982), Ger. Chem. Eng., 5, pp. 181-195. Stahl, E., Quirin, K.W. and Gerard, D. (1988). Dense Gases for Extraction and Refining, 1st Ed., Springer-Verlag, Berlin. Williams, D.F. (1981), Chem. Eng. Sci., 36 (11), 1769-1788. McHugh, M. and Krukonis, V. (1986). Supercritical Fluid Extraction: Principles and Practice, Butterworths, Boston, 217. Gallagher, P., Proc. 3rd Int. Symp. on Supercritical Fluids, Strasbourg, France, 1994, 253-264. Larson, K.A. and King, M.L. (1986), Biotech. Prog., 2 (2), 73-82. Matson, D.W., Fulton, J.L., Petersen, R.C. and Smith, R.D. (1984), Ind. Eng. Chem. Res., 26, 2298-2306. Gallagher, P.M., Coffey, M.P., Krukonis, V.J. and Klasutis, N. (1989), Supercritical Fluid Science and Technology, pp 334-354, ACS Symp. Ser., No. 406, Chap. 23, Ed. K.P. Johnston and J.M.L. Penninger. Sang-Do Yeo, Gio-Bin Lim, Debenedetti, P.G. and Bernstein, H. (1993), Biotech. and Bioeng., 41, pp. 341-346. Poliakoff, M., Howdle, S. M., Proc. 3rd Int. Symp. on Superc. Fluids, Strasbourg, France, 1994, 81-92. Maffei, R. L., U.S. Patent No. 4.012.194, 1976.
649 12.
13. 14. 15. 16.
Dewees, T. G., Knafele, F. M., Tayler R. G., Iliff, R. J., Carty, D. T., Latham, J. R., Lipton, T. M., U.S. Patent No. 5.267.455, 1990, The Clorox Company, Oakland, California. Chao, S. C., Stanford, T. B., Purer, E. M., Wilkerson, A. Y., U.S. Patent No. 5.467.492, 1995, Hughes Aircraft Company, Los Angeles, California. Purer, E. M., Wilkerson, A. Y., Townsend, C. W., Chao, S. C., U.S. Patent No. 5.651.276, 1997, Hughes Aircraft Company, Los Angeles, California. Townsend, C. W., Purer, E. M., U.S. Patent No. 5.669.251, 1997, Hughes Aircraft Company, Los Angeles, California. Chao, S. C., Purer, E. M., U.S. Patent No. 5.822.818, 1998, Hughes Engineering.
35132 Padova
Italy
9.11.1 Introduction Carbon Dioxide (CO2), in its liquid or supercritical state, is currently attracting much interest as an environmentally acceptable solvent, which is easy to handle, and available at low cost, even in high purity grade. In the late 1960s, the potential of the non-toxic and environmentally benign of supercritical CO2 (SC CO2) for natural product extraction was recognized by several authors [1-3]. So far, the practical use of CO2 has been addressed mainly as an extractive media in agricultural fields. The advantage of SC CO2 over conventional extraction with liquid solvents is that the SC fluid has a very low viscosity and a high diffusion coefficient [4]. It penetrates solid substances efficiently and is removed after depressurization. The excess amount of adsorbed gas diffuses from the material under room conditions. Moreover, SC fluid's solvent power, which is related to solvent density, can be tuned by varying the pressure and temperature. Some substances can be selectively stripped away from other compounds from a raw material, and then the same substances are easily separated from the fluid by reducing the pressure and controlling the temperature. Its application has been limited by the need for high pressures to dissolve substances having an affinity with CO2, mainly non-polar compounds, but even small amounts of polar, amphiphilic, organometallic, or compounds with molecular weights up to 500 Da. Nevertheless, the poor solubility of many interesting target substances places a severe restriction on the widespread use of CO2 as a solvent. Sometimes, unrealistically high pressures or expensive "CO2-philic" compounds (substances with a high affinity for CO2 solutions at lower pressures) are needed. The specific properties of SC CO2 make it an interesting "green" replacement for liquid organic solvents, which are often characterized by acute toxicity, ecological hazards, or drawbacks with disposal and recycling. The use of CO2 has currently moved from the socalled conventional extraction process to several other areas as diverse as the dyeing and cleaning of fibre and textiles, polymerization and polymer processing [5], purification and crystallization of pharmaceuticals [6,7], comminution of difficult-to-handle solids [8], controlled drug delivery systems [9], and as a reaction medium for chemical synthesis [ 10]. Last, but not least, a great effort has been made by several companies worldwide for the development of the dry-cleaning of garments and textiles with liquid CO2, in substitution for the organic solvents currently used, such as perchloroethylene, petroleum ether, and trichloroethane. The commercial target in this field is not limited to a few industrial plants, but to the huge number of industrial dry-cleaning companies, and to the dry-cleaning shops in commercial centers. Especially thanks to these efforts, SC CO2 can be handled safely in high-pressure equipment from the laboratory- to the industrial scale. Nevertheless, its limited power to dissolve polar or non-volatile compounds represents a major drawback in most of these applications, particularly in the garment-cleaning process, where the design of CO2-philic
642 solubilizers is of paramount importance. 9.11.2
Dry-cleaning processes
9.11.2.1 Conventional dry-cleaning In the conventional dry-cleaning process, liquid organic solvents such as perchloroethylene and other petroleum-based substances are used. The machines have a washing chamber, inside which a rotating cylindrical basket is used for the motion and agitation of articles and garments. The removal of dirt, stains, and other contaminants on the articles to be washed is assured by the solvent's ability to dissolve the contaminants, and by the "fall and splash" effect caused by the rotation of the internal basket. The articles and the fluid are tumbled together by rotating the basket or "tumbler", so that the solvent is brought into intimate contact with the surfaces of the articles. As the solvent is absorbed by the absorbent portions, it dissolves these contaminants or maintains them in suspension. Solid particles and chemicalor organic contaminants are then forced from the internal part of the cylinder to the annular zone between the cylinder and the chamber's internal walls, and are removed by continuously re-circulating the liquid solvent from-and to-the washing chamber. Dry-cleaning machines are usually provided with a centrifugal pump to force the liquid through a filter, and with a blower to move and filter the solvent vapour phase also. After the cleaning cycle, the solvent is drained from the cleaning chamber and may be reused, provided that the level of dissolved contaminants is not approaching saturation, otherwise it is regenerated by distillation or cleaned by mechanical filtration. The conventional dry-cleaning process involves significant health and environmental risks associated with the toxic nature of conventional cleaning solvents. Percholoroethylene is suspected to be carcinogenic, while petroleum-based solvents are flammable and smogproducing. The search for alternative cleaning technologies, which are safe and environmentally acceptable, is still in progress, to substitute the solvents and methods of controlling exposure to dry-cleaning chemicals.
9.11.2.2
Dry-cleaning with liquid carbon dioxide
As CO2 is an environmentally safe fluid, its efficient use as a dry-cleaning fluid avoids the risks and drawbacks associated with conventional cleaning solvents. Carbon dioxide has been identified as an inexpensive solvent, with an unlimited natural resource. It shows a high compatibility with garments, as it does not damage fabrics nor dissolve common dyes. In this way, CO2 has been recognized as a good medium to be used in the dry-cleaning industry. Also, the solvating properties of CO2 are quite non-typical of those of more traditional solvents. As the fluid's solvent power is a function of its density, CO2 must be compressed to enhance the cleaning efficiency. Keeping COz at relative high densities requires that the vessels have to be designed for higher pressures than those of the process itself, increasing machine's cost and safety level. As a first step to contain the machine cost, the process is performed with liquid CO2 to reduce the vessels' design-pressure values. The pressure ceiling can be fixed at 70 bar, but this constraint limits the maximum allowable working temperature to be lower than 18~ reducing the mass-transfer and solute dissolution inside the fluid. The viscosity of fatty substances shows a large dependence on temperature, decreasing dramatically above 50~ Heavy greasy compounds tend to segregate as a solid phase at temperatures lower then 10~
643 The pressure is the main cost-driver, since it affects all process-housing component costs, as well as operational costs. Standard petro-chemical components can be used, as the maximum design pressure value can be set close to the CO2 critical value. The physical characteristics of the fluid do not allow one to run the process at temperatures higher than 20~ using liquid CO2, as the process-control ceases to be affordable when the pressure and temperature approach the near-critical zone. The use of SC CO2, at temperatures higher than 35~ will require the pressure above 200 bar to have a comparable value in density, raising the costs for the hardware and process control. The cleaning of the garments is thus assured, coupling the affinity of CO2 towards dissolving greasy substances, with mechanical removal of fine particles and stains. Agitation represents another major contribution to costs. In a pressure vessel, mechanical agitation produced with moving parts is cost-prohibitive in terms of capital and maintenance expenditures. It has to be provided without moving parts, that is, with no electrical or mechanical devices which require a sealing system, to avoid CO2 leakage from the washing chamber. 9.11.3
The CO2 dry-cleaning process
9.11.3.1 Fundamentals Several dry-cleaning systems using CO2 as a solvent have been proposed and are currently under development. As the topic is mainly concerned with the definition of a cleaning process for industrial or commercial applications, the literature refers to several patents which differ in type and performance of the washing machines. The CO2 dry-cleaning process was originally disclosed by Maffei [11], who designed a simple system formed by a chamber, a storage tank and an evaporator. Garments are placed inside a basket contained in the chamber, then CO2 is gravity-fed thereto from the refrigerated tank, passes through the garments, and is finally transferred to the evaporator. In the evaporator, CO2 is evaporated and separated from the soil, which is recovered from the bottom of the tank. The vapour CO2 is compressed into a heat-exchanger to condense before being stored in the storage tank. Many improvements have been made since Maffei's system. First of all, the washing chamber needs to be pressurized to maintain the CO2 in the liquid state, otherwise the CO2 must be very cold to remain liquid. To enhance the washing efficiency, garments have to be agitated to remove solid stains and particles which are not soluble in CO2. The simplified scheme depicted in Fig. 9.11-1 is useful to understand the machine units and process steps. The main representative vessel is the washing chamber, where garments are placed inside a basket. Normally, other two main vessels are used to handle the fluid during the process, a storage tank and an operative or service tank. The clean CO2 is kept in the liquid state in the storage tank, which is used as an additional reservoir. The process liquid CO2 is stored in the operative tank. Two small filtering vessels, a lint-trap with a filter train, and a button-trap, complete the number of vessels normally present in the machine. The button-filter is located at the bottom of the chamber; a wire filter avoids the dragging of fibres, buttons, and other macroscopic pieces which could seriously damage the liquid pump and the valves. The lint-filter is used as the main filtering device to capture the fine solid particles removed from the garments in the washing step, when the liquid is continuously pumped from and to the chamber. In some versions, active-carbon powder filtering-cartridges are used to
644
c-] Figure 9.11-1: Schematic representation of the dry-cleaning process, displaying different steps from A to D. trap unpleasant odours dissolved in the fluid. The chamber is initially pressurized with vapour, fed from the still or the service tank (step 1), and then loaded with liquid from the service tank (step 2). A liquid piston pump assures the pumping of the liquid CO2 in the system. Normally, the pump is used to displace the liquid CO2 from the storage and operating vessels to the chamber, and vice versa, and through the chamber during the washing phase (step 3). Pressure-drops are limited to values lower than 20 bar, but very high flow-rates are needed to reduce the process time. At the end of the washing step, the liquid is drained from the chamber to the service tank (step 4). A compressor is required to recover the vapour CO2 from the chamber at the end of the washing cycle, to minimize the loss in CO2 for each cycle (step 5). The vapour is condensed through a heat-exchanger and the liquid is recovered and retumed to the storage vessel. The pressure-drop at the end of this step can be larger than 50 bar, with a temperature of the process-fluid higher than 80~ downstream the compressor. The vapour phase is sometimes moved by heating in the distillation step, using the positive pressure-drop generated by the increase in temperature (step 6). Nevertheless, the compressor can be used to help the distillation process, sucking the vapour from the still to the heat exchanger and then to the storage or service tank (step 7). The operative tank can be utilized as a full capacity still if it is provided with an intemal heat exchanger, otherwise an additional still, smaller in size, is necessary to perform the distillation step and to clean the "dirty" liquid CO2. As the percentage of lypophilic substances
645 in the garments is very low, the "dirty" liquid CO2 distillation by means of the little still is carried out after a series of washing cycles, depending on the amount of liquid loaded in the chamber. So, a distillation per dilution process is reached, as the operative tank remains partially dirty at its bottom. On the other hand, using a full capability still liquid CO2 vaporizes and can be completely cleaned from stains and soluble substances. Two main thermal fluxes characterize the system from a thermodynamic point of view. These come from the liquid-CO2 distillation and, consequently, the vapour condensation to recover the liquid in the operative or storage tank, and the condensation of vapour CO2 recovered from the chamber at the end of the washing step. Most frequently, the distillation is carried out in a vessel which is equipped with an intemal heat exchanger, dip in the liquid to be cleaned. The heat exchanger can be of an electric resistance type, or a series of coils through which hot water flows. The heat exchange is very efficient at the beginning, when the exchanger is completely immersed in the liquid, then the efficiency reduces dramatically as the exchange-surface in contact with the liquid decreases. As the overall heat coefficient of the vapour phase is very low, a plate heat-exchanger is used for the condensation. The machine is equipped with a refrigeration unit designed according to the thermal flux required for the condensation of the vapour CO2. The refrigeration unit is then used to control the temperature in the chamber during the washing step or to prevent over-temperatures in the storage vessels. The system is finally optimized if the refrigerating fluid used to cool down the refrigeration unit process-fluid is fed to the still to supply the heat required for the distillation process. A third heat flux is necessary for pre-heating the chamber before the gas recovery. To avoid the insertion of an additional plate heat-exchanger, it is preferable to use electric resistances which can supply high thermal fluxes per specific surface.
9.11.3.2 Garments agitation Many systems have been proposed for the agitation of garments in CO2 dry-cleaning systems. Deewes [ 12] proposed a rotating basket, used by the majority of the traditional drycleaning systems. 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, causing the garments to drop and splash into the liquid solvent. Chao [13] disclosed a variety of agitation techniques, that is a "gas bubble and boiling agitation" where the liquid CO2 in the basket is boiled, a "liquid agitation" where a nozzle spraying CO2 tumbles the liquid and the garments, a "sonic agitation" where sonic nozzles induce vibrating waves, and a "stirring agitation" where an impeller creates the fluid displacement. In this patent, as in most of the developed systems, the basket in the washing chamber is completely drilled to allow the movement of solid particles and contaminants towards the annular free volume, from which they are removed inside the solvent stream. A gas-jets agitation technique has been proposed by Purer [14] to remove particulate soils from fabric. The agitation occurs apart from the immersion of the garments in liquid CO2; nevertheless CO2 has been employed both as a gas and a solvent. Townsend and Purer [15] disclosed a rotating basket powered by a hydraulic flow generated by forcing the fluid through a number of nozzles. Finally, an improved nozzles agitation system has been applied in the Hughes Dry Wash TM machine [16]. Despite the several systems disclosed, garments' agitation is substantially operated in two ways, with a rotating basket or by fluid-jets. In the former, the mechanical force necessary to
646 dislodge the soiling from the substrate is provided by the fall-and-splash effect of the solventloaded garment from the top of the rotating drum into the solvent pool below. The magnitude of the force is dependent on the height of fall, which depends on the solvent level in the chamber, and the solvent density. The relatively low density of liquid CO2 discourages the application of this agitation mechanism. In the fluid-jet system, the garments are submerged in liquid CO2 within the perforated basket, set into motion, and are agitated by high-velocity fluid jets. The jets are discharged through nozzles, which are set in an appropriate configuration, and the garments agitated through a Venturi effect. As the garment is accelerated by the jet, its fibres are stretched, and the soil trapped into and between the fibres are expelled. As a summary, the power of the pump used to force the fluid through the nozzles is transformed into fluid velocity which is transferred to the garment as momentum allowing the expulsion of soiling. Four series of nozzles are fixed inside the chamber of the Hughes machine [16]. The chamber is completely filled with liquid CO2 and pressurized, then the liquid is forced through two nozzles each time inside the chamber by opening and closing a valve located upstream of each pair of nozzle series. The nozzles are positioned so that they create an agitating vortex inside the basket as liquid CO2 is forced through them. The soil-laden liquid CO2 which leaves the basket is moved from the chamber to a lint-trap and a filter-train by means of a circulating pump. Liquid CO2 has a lower density than the liquid organic solvents normally used in conventional dry-cleaning processes, as shown in Table 9.11-1. Moreover, its viscosity is one order of magnitude lower than organic solvents. As a consequence, the jet-cleaning agitation seems to offer the more feasible approach for CO2 dry-cleaning machines. Table 9.11-1 Physical properties of solvents commonly used in dry-cleaning processes solvent
Surface tension, Density, Dynes/cm g/cm 3
Viscosity, cPoise
P erchloro ethyl ene 1,1,1-Trichloroethane Freon-113 Freon- 114
32.0 25.6 19.6 12.0
1.60 1.40 1.60 1.50
0.88 1.20 0.68 0.38
Water
72.0
1.00
0.90
Liquid CO2
3.0
0.85
0.08
This assumption is further supported when one considers that the rotating tumbler needs a special sealing system to prevent CO2 escaping or leaking round the drive shaft which is connected to the tumbler through a wall of the pressure vessel. When a magnetically agitated shaft is used to avoid the constraint of rotating shaft seal, the thickness of stainless steel needed for pressure resistance dissipates the magnetic coupling, limiting the torque that can be transferred from the outer magnets to the internal shaft. These solutions have been found relatively ineffective, especially for commercial-size units, and have high maintenance costs. However, in the jet-cleaning agitation process the chamber has to be completely filled with
647 liquid to allow correct functioning of the system, and this feature reflects on the machine design, which requires larger tanks, more powerful pumping devices, and an increase in the thermal fluxes needed for the distillation and condensation steps. Operational times are also increased. Finally, the fall-and-splash effect requires a load of liquid which is sufficient just to submerge the garments.
9.11.3.3 Machine configurations We refer to the simplified scheme depicted in Fig. 9.11-2, where the main vessels are the washing chamber C, the storage tank ST, the operative tank OT, and the evaporator or still ET. Of course, the chamber is provided with an agitation device and with a completely drilled basket, into which garments are placed.
CV
or
[~ additive PL .~
II ~ U FB I~
HE2
Figure 9.11-2: Simplified scheme of the high pressure lines for a dry-cleaning C02 unit. Legend: washing chamber, C; compressor, CV; evaporator or still, ET; button filter, FB; linttrap, FT; heat-exchanger, HE#; operative tank, OT; additive pump, PA; liquid pump, PL; vacuum pump, PV; storage tank, ST. The clean liquid C02 is stored in the storage tank, and dirty C02 in the operative tank. The storage and the operative tanks have the same internal capacity, while the still can be smaller. In a steady-state configuration, the liquid CO2 stored in the operative tank is used several times for washing, while the storage tank is half empty and is used as a reservoir, to fill up the chamber. After the garments are loaded in the basket, and the chamber closed, the air in the chamber is extracted by a vacuum pump, PV, then CO2 vapour from the storage or the operative tank is fed thereto to avoid a sharp reduction in the temperature. To complete the chamber filling, liquid from the operative tank is fed by the liquid pump PL. At the end of the filling step, the chamber is over-pressurized to avoid vapour formation, and the washing step
648 can begin. The liquid is re-circulated continuously from and to the chamber, passes through the garments, and is filtered through the lint-trap, FT. A plate heat-exchanger, HE1, is used to keep the temperature lower than 20~ At the end of the washing step, the liquid is drained to the operative tank by the pump, and through the button filter, FB, then the gas-recovery step takes place to recover most of the vapour or gaseous CO2 from the chamber. As the gas-recovery can be assumed to be like an adiabatic process, the fluid in the chamber is pre-heated by an electric resistance heat exchanger, HE2, to contain the reduction in temperature. The CO2 vapour is compressed by the oil-free compressor CV and fed to HE1 to condense before being stored in the operative tank. Finally, the chamber is vented and the door can be opened to recover the cleaned garments. 9.11.4
Conclusions
The use of SC CO2 for the cleaning of garments, as an alternative method to the liquid solvents, is very attractive from an environmental point of view, and for the safety and health of the operators involved. Nevertheless, several issues have still to be addressed and improved, such as the ability of the solvent mixture to dissolve polar and heavy compounds, the entire washing and rinsing cycle to maximize stain removal, the machine's cost compared to the conventional machines. As a final remark, the machine costs matched with the innovative character of the technology could confine the application just for industrial laundries, limiting the possibility to extend dry-cleaning machines to an end-user approach. References
1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11.
Brunner, G. and Peter, S. (1982), Ger. Chem. Eng., 5, pp. 181-195. Stahl, E., Quirin, K.W. and Gerard, D. (1988). Dense Gases for Extraction and Refining, 1st Ed., Springer-Verlag, Berlin. Williams, D.F. (1981), Chem. Eng. Sci., 36 (11), 1769-1788. McHugh, M. and Krukonis, V. (1986). Supercritical Fluid Extraction: Principles and Practice, Butterworths, Boston, 217. Gallagher, P., Proc. 3rd Int. Symp. on Supercritical Fluids, Strasbourg, France, 1994, 253-264. Larson, K.A. and King, M.L. (1986), Biotech. Prog., 2 (2), 73-82. Matson, D.W., Fulton, J.L., Petersen, R.C. and Smith, R.D. (1984), Ind. Eng. Chem. Res., 26, 2298-2306. Gallagher, P.M., Coffey, M.P., Krukonis, V.J. and Klasutis, N. (1989), Supercritical Fluid Science and Technology, pp 334-354, ACS Symp. Ser., No. 406, Chap. 23, Ed. K.P. Johnston and J.M.L. Penninger. Sang-Do Yeo, Gio-Bin Lim, Debenedetti, P.G. and Bernstein, H. (1993), Biotech. and Bioeng., 41, pp. 341-346. Poliakoff, M., Howdle, S. M., Proc. 3rd Int. Symp. on Superc. Fluids, Strasbourg, France, 1994, 81-92. Maffei, R. L., U.S. Patent No. 4.012.194, 1976.
649 12.
13. 14. 15. 16.
Dewees, T. G., Knafele, F. M., Tayler R. G., Iliff, R. J., Carty, D. T., Latham, J. R., Lipton, T. M., U.S. Patent No. 5.267.455, 1990, The Clorox Company, Oakland, California. Chao, S. C., Stanford, T. B., Purer, E. M., Wilkerson, A. Y., U.S. Patent No. 5.467.492, 1995, Hughes Aircraft Company, Los Angeles, California. Purer, E. M., Wilkerson, A. Y., Townsend, C. W., Chao, S. C., U.S. Patent No. 5.651.276, 1997, Hughes Aircraft Company, Los Angeles, California. Townsend, C. W., Purer, E. M., U.S. Patent No. 5.669.251, 1997, Hughes Aircraft Company, Los Angeles, California. Chao, S. C., Purer, E. M., U.S. Patent No. 5.822.818, 1998, Hughes Engineering.