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“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:

  1. They reduce the environmental and worker exposure to harmful solvents, and
  2. Solvent recovery practices greatly reduce the operating costs by minimizing the amount of new solvent that must be purchased.
  3. 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.

    Dry Cleaners in the US, current industry status:

    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.



    State/Location Number of Dry Cleaning Facilities Percent of Dry Cleaning Facilities Revenues Rank - number of facilities Rank - State Population Census Population (1000)
    California 2,737 11.8 $629,747 1 1 29,760
    New York 2,071 8.9 $346,412 2 2 17,990
    Texas 1,828 7.9 $448,292 3 3 16,987
    Florida 1,340 5.8 $273,109 4 4 12,938
    Illinois 1,043 4.5 $231,475 5 6 11,431
    New Jersey 964 4.1 $186,588 6 9 7,730
    Ohio 897 3.9 $208,832 7 7 10,847
    Pennsylvania 892 3.8 $196,682 8 5 11,881
    Georgia 807 3.5 $161,054 9 11 6,478
    Michigan 742 3.2 $161,270 10 8 9,295
    Virginia 694 3.0 $165,446 11 12 6,187
    North Carolina 677 2.9 $172,653 12 10 6,628
    Massachusetts 601 2.6 $136,666 13 13 6,016
    Maryland 4874,781 2.1 $107,265 14 19 4,781
    Missouri 470 2.0 $98,485 15 15 5,117
    Indiana 457 2.0 $102,078 16 14 5,544
    Washington 438 1.9 $79,471 17 18 4,867
    Tennessee 436 1.9 $110,116 418 17 4,877
    Alabama 418 1.8 $93,949 19 22 4,041
    Colorado 409 1.8 $77,212 20 26 3,294
    Louisiana 379 1.6 $80,484 21 21 4,345
    Connecticut 340 1.5 $90,111 22 27 3,28
    South Carolina 339 339 $78,297 23 25 3,487
    Kentucky 295 103 $61,293 24 23 3,685
    Minnesota 291 1.3 $72,772 25 20 4,375
    Arizona 291 1.2 $73,290 26 24 3,665
    Oklahoma 282 1.2 $70,665 27 28 3,146
    Wisconsin 270 1.2 $63,964 28 16 4,891
    Arkansas 230 1.0 $45,053 29 33 2,351
    Mississippi 226 1.0 $46,756 30 31 2,573
    Oregon 218 0.9 $40,728 31 29 2,842
    Kansas 204 0.9 $41,941 32 32 2,478
    Iowa 183 0.8 $36,487 33 30 2,777
    Utah 133 0.6 $26,191 34 35 1,723
    Nevada 129 0.5 $34,118 35 39 1,202
    New Mexico 125 0.5 $22,225 36 37 1,515
    Nebraska 103 0.4 $22,339 37 36 1,578
    West Virginia 100 0.4 $19,301 38 34 1,793
    Rhode Island 82 0.3 $17,081 39 43 1,003
    D.C. 80 0.3 $13,898 40 48 607
    New Hampshire 72 0.3 $17,519 41 40 1,109
    Idaho 70 0.3 $12,558 42 42 1,007
    Delaware 58 0.2 $13,530 43 46 666
    Montana 50 0.2 $6,576 44 44 799
    Maine 48 0.2 $9,623 45 38 1,228
    Hawaii 37 0.2 $21,141 41 46 1,108
    Vermont 37 0.2 $7,680 47 49 563
    South Dakota 36 0.2 $4,481 48 45 696
    North Dakota 36 0.2 $8,280 49 47 639
    Wyoming 35 0.1 $4,168 50 50 454
    Alaska 26 0.1 $17,679 51 51 550
    Totals 23,213 100.0 $5,067,031 248,831


    Changes that a dry cleaner can make to become more “Green”:

    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:

    • Retrofitting old dry cleaning machines to include air traps capable of recovering vaporized solvent in the air stream,
    • Process changes which customize the laundering process so that not all materials are immediately dry cleaned, and
    • Finding substitute solvents that can still effectively and gently clean “dry clean only” fabrics while still being worker and environmentally friendly.

    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
    from “Introduction to Green Chemistry,”
    published by the American Chemical Society

    Materials:
    • A few clean pennies
    • Thin-stem plastic pipets
    • Water
    • Surfactants:
      • Liquid Soap
      • Solid Soap
      • Cooking Oil
      • Sugar
      • Table Salt
      • Solid Wax or Paraffin
      • Any other materials that you have selected to investigate
    • Cotton Swabs
    • Paper Towels
    Procedure: After investigating the surface properties (appearance and feel) or a clean penny, place it on a paper towel. Hold a thin-stem pipet filled with water and slowly add water one drop at a time to the penny’s surface. Count the number of drops that can be added before the water spills off of the penny and record. Repeat this experiment until you feel like you have managed to successfully reproduce your results (at least three times total).

    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:

    1. How many pure water drops could you add to the clean penny surface?
    2. Which test surfactant had the largest effect on the number of drops of solution that could be added to the penny? How many drops could you add of this solution?v
    3. Based on your experiences, what can you say about the effects of the different surfactants used?
    4. What properties do the most effective surfactants have? What properties do the least effective surfactants have?

    Carbon Dioxide as a Dry Cleaning Solvent:

    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
    from “Introduction to Green Chemistry,” published by the American Chemical Society

    Materials:
    • Small amount of Dry Ice
    • Jumbo-sized plastic Beral pipet
    • Pliers
    • Zip-lock baggie

    Procedure:

    1. Cut the tip off of the plastic pipet and add 4-5 pea-sized pieces of dry ice to the pipet. Please use care (and gloves) when handling the dry ice. The temperature of dry ice is -70?C; this is cold enough to freeze skin upon immediate contact.
    2. Slip the pipet into the zip-lock baggie so that the open protrudes. Zip the baggie as much as possible to hold the pipet and baggie in place.
    3. Using the pliers, clamp the open end of the pipet. It will be necessary to fold the end of the pipet at least once to create a pressure-tight seal.


    4. Once the CO2 has become a liquid, release the hold on the pliers and record your observations.
    5. You can repeat this process several times until the dry ice has completely melted or sublimated.

    Questions to consider:

    1. Where would you place the phase transitions that you observed on the phase diagram?
    2. What does the dry cleaner need to keep in mind when trying to use liquid carbon dioxide to clean clothes?
    3. Can you calculate the approximate pressure inside the pipet by using the ideal gas law?
    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 1,500gallons of perchloroethylene (perc) per hour. During a typical wash cycle, clothes will come into contact with up to 200gallons 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 other wash cycles.

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