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| Green Clean | Creating the Context Background Info |
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Creating the Context Home Research Focus Background Info Research Methods Data Submission Results of Study Data Analysis Conclusion Further Research Guided Research Research Question Background Info Research Methods Data Submission Results of Study Data Analysis Conclusion Further Research Research Values Student Research Doing Research |
“Green” versus Conventional Dry Cleaners: Unlike regular home washing machines that operate in a pure batch mode, dry cleaning machines continuously pump solvent through the dry cleaning machine during the wash cycle. A typical machine might use 1,500 gallons of perchloroethylene (perc) per hour. During a typical wash cycle, clothes will come into contact with up to 200 gallons of perc to insure adequate cleaning. In order for a dry cleaner to maintain a profit, it is necessary to reclaim and clean the used solvent for use in additional wash cycles. Even conventional dry cleaning processes use sophisticated solvent recovery systems and practices. These actions serve two purposes: Several technological innovations have been made to dry cleaning machines in order to minimize solvent loss. Older machines that used separate washers and dryers (like home washing systems) are being slowly replaced with new machines that act like a washer and dryer in one unit. These machines do not require “wet” clothes be handled by workers during transfer from the washer to the dryer, thus minimizing both worker exposure and environmental contamination to harmful solvents. In addition to incorporating the washing and drying cycles in one unit, these new units include sophisticated solvent recovery systems that clean used solvent and recycle it back to the washer. The most advanced machines now have special systems (carbon filters or condensers) designed to remove the vaporized solvent from the air during the drying cycle for recycle. Despite these advancements, dry cleaning processes still result in some environmental contamination and worker exposure to solvents. Solvents are still released to the atmosphere and not all of the solvent can be reclaimed during the cleaning process resulting in significant amounts of liquid/solid hazardous waste formation. The schematic below shows the throughput of an advanced dry cleaning machine. Some points of environmental contamination, hazardous waste formation, and worker exposure to solvent include atmospheric release of perc from machine and drying cycles, perc in the hazardous waste, residual perc on finished clothing, worker exposure while handling clothing, hazardous waste, and perc.
Like perc, dry cleaners using petroleum solvents use solvent recovery systems to minimize costs and worker/environmental exposure to the solvent. However, due to the flammable nature of the petroleum solvents, many safety and fire-prevention steps must be taken for its use. Despite the advancements made in dry cleaning machines, not all dry cleaners that are currently operating use such sophisticated equipment. Purchasing new dry cleaning machines and retrofitting old ones to minimize air emissions is expensive. There are two predominant types of dry cleaners currently operating today. Commercial dry cleaners are the most prevalent facilities in the US and include full service and retail operations. Individual households use commercial facilities to have their dry cleaning done. Industrial dry cleaners, however, are large facilities that simply perform dry cleaning as one of many services. Companies that rent uniforms, linens, and other materials have dry cleaning capabilities as part of their operation. These facilities do not perform dry cleaning services to the general public. Instead they serve restaurants, hotels, and other large-scale operations. Dry cleaning facilities are geographically located throughout the United States, although their numbers correlate with local population numbers. Dry cleaning facilities tend to be more highly concentrated in urban areas. Look at the map and table below to evaluate the number of dry cleaners in your area. The map below shows the geographic distribution of dry cleaning facilities in the United States as of 1992.
There are many practices that dry cleaners can implement to become more “green” in their operation. Some are simple changes in common practice, like providing a recycle option for hangers and plastic protection bags. Others are more technologically advanced and involve finding solvent substitutions (supercritical or liquid carbon dioxide) or developing new methods of recapturing, cleaning, and recycling the solvent. A “green” dry cleaner will inevitably operate in a manner that promotes material reuse and recycling, requires less solvent, and will shift to using environmentally friendly solvents such as supercritical or liquid carbon dioxide. For simplicity, changes that will promote “green” chemistry can be segregated into three different categories:
All three of these practices are implemented (to varying degrees) in the dry cleaning industry today. Substitute Solvents: CO2: Liquid carbon dioxide has become a new trend in dry cleaning. At high pressures, carbon dioxide (CO2) is liquid. When coupled with a special detergent, liquid CO2 can act as a solvent for dry cleaning processes. The machines required for this process are different than normal dry cleaning machines because they have to operate at high pressures. Instead of a normal drying cycle, the pressure is simply reduced and the liquid CO2 becomes gaseous CO2, evaporating from the clothes.In order to investigate the properties of supercritical carbon dioxide (liquid) that make it a possible dry cleaning solvent, it is important to consider the chemistry behind washing your clothes with soap and water. When you wash your clothing in water, the water acts as a solvent working to dissolve the dirt, grease, and stains from the clothing fibers. Since you already know that solvents work best when dissolving chemicals that are most like them, which stains would you expect a polar solvent like water to dissolve the best? Salty stains or grease stains? In order to enhance the cleaning properties of water, soap or detergent is added as a surfactant. When added to water, a surfactant will change the surface properties of the water by reducing the surface tension of the solvent. This will allow water to penetrate the clothing fiber and stain matrix more thoroughly. Use the laboratory protocol described below to investigate the effects that adding different chemicals to water have on the surface properties of water. Would you expect the changes to increase or decrease the effectiveness of water’s cleaning abilities? While the dry cleaning process using carbon dioxide is similar to the conventional process using perc, it varies somewhat. What are the key differences between the two processes? The carbon dioxide process is shown below.
Laboratory Experiment #1: Surfactants and Solvents
Develop a set of experiments to test the effect that your selected surfactants have on the results. Use the same pipet method used during the water-only experiment. Questions for consider:
When most people think of carbon dioxide, they tend to refer to carbon dioxide either as a gas or as a solid, dry ice. However, under extreme conditions, it is possible to create carbon dioxide liquid. It is as a liquid that carbon dioxide is used by the dry cleaning industry as a solvent. In order to more fully understand the phase or state changes that carbon dioxide can undergo, please consider the phase diagram below.
The diagram shows three different states that carbon dioxide can exist (solid, liquid, and/or gas) and two different temperature/pressure scenarios (the triple point and critical point). The triple point represents the temperature and pressure conditions for all three phases of carbon dioxide to exist in equilibrium, whereas the critical point represents the highest temperature at which carbon dioxide can exhibit liquid-vapor equilibrium. In order for dry cleaning facilities to use carbon dioxide as a liquid solvent, they must “wash” the clothing at extremely high pressures (approximately 70atm). Based on the phase diagram above, at typical ambient temperatures (25?C, or 298K), carbon dioxide will remain a gas until the pressure reaches 67bar (or approximately 66atm). Given the high pressures required, it is not possible to safely create large quantities of liquid carbon dioxide without special pressure-rated containers. However, given some standard laboratory supplies, it is possible to safely witness the phase transformation using very small quantities of dry ice. Laboratory Experiment #2: Create Liquid Carbon Dioxide
Procedure:
Questions to consider:
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