Today the accepted practice is to allow the adsorbent material to coat the screens of the filter, so that the solvent continuously flows through it. The adsorbents may be any of various activated clays or activated carbon. Such adsorbents remove odors and odor forming materials from the solvent. Probably the most important function of such adsorbants now is the removal of color from the solvent.
This is from page 5 of the book titled Drycleaning Technology and Theory by Albert Martin and George Fulton. Copyright 1958 from Silver Spring, MD. This book was commissioned by the government for the Office of the Quartermaster General.
The book goes on to say that distillation is the preferred way to cleanse solvent of its impurities that have been retained by the solvent due to removing soil from garments that have been cleaned. Today some progressive cleaners in the United States have gone to solvent cleansing by the use of adsorbants alone or to greatly reduce the amount of solvent needing distillation thus saving on labor and utility costs.
Distillation of drycleaning solvent takes two forms. The first form is the one most people are familiar with which is atmospheric distillation and may be called simple distillation, and we can relate to it as distilling perchloroethylene. Dirty solvent is sent to a vessel with a closed heat source which heats the liquid and brings the temperature up to the set temperature for boiling perc by the setting of a thermostat if electric or a certain steam pressure if the heat source is steam. As the temperature is coming up it will reach the vapor pressure of the most volatile contaminants first before getting to the set temperature. For drycleaning solvents this would tend to be the more volatile of the liquid and may be alcohol, ammonia, water, volatile dry solvent, and perchloroethylene. After the perc is boiled out of the still then will come oils and greases, detergents, sizings, etc. The first batch of impurities will come over first and the second batch will come over as the temperature begins to rise in the still by the absence of the more volatile contaminants that are removing heat from the still.
As the temperature rises there may also be particulate matter that will attach itself to the heavier particles in the distillate and go over with the distillate. Some of these particles can be microscopic particles of dye. The dye particles may not be in enough quantity to discern with the naked eye in a large volume of solvent but if the boiling is violent due to either water in the still or excess heat there can be much more dye that is returned to the distilled tank and can give a light or even a darker cast to the solvent.
As mentioned in the referenced textbook above the solvent would first go through a filter which will remove the non-soluble particles of soil. Dye particles are microscopic in size and can easily go through the normal filter medium whether soil, diatomaceous earth or cartridges. It is customary to remove these particles through the use of activated carbon or some other adsorbant. Carbon has an affinity for dyes particulate and odors due to its physical structure. In the textbook cited above, there are various activated clays that can remove dyes as well as moisture, odors and non-volatile residue (NVR) and moisture from the solvent. When water comes in contact with activated carbon the carbon can become very compacted and filter pressure will rise rapidly thus greatly reducing the solvent flow due to increased filter pressure.
Vacuum distillation is the other method of distillation utilized for the clarification of lighter than water drycleaning solvents. Since hydrocarbon solvent, and other lighter than water solvents, have a lower vapor pressure, or need a higher temperature in order to get the liquid to boil and turn into a vapor to commence distillation, it is necessary to put the distillation vessel under a vacuum of 25 to 26 inches in order to get the solvent to boil without damaging it and creating odors.
In years past the goal was to have twenty gallons of solvent turnover for each 100 pounds of textiles cleaned. This was done with four gallons of solvent being reclaimed by the dryer and sixteen gallons of solvent going to the still. These were figures for hydrocarbon or perc solvent. This amount of turnover produced consistently clean solvent for cleaning. Some drycleaners began reducing the solvent distilled, or distilled from none to maybe four gallons per 100 pounds cleaned. In hydrocarbon this is almost guaranteeing that odors will develop in the solvent and create a monster of its own by contaminating all garments cleaned.
With the procedures being used today such as a low-level first solvent, off filter, being dropped and extracted to the still after about a three or four minute run, the chance of dye bleeds contaminating all of the wash solvent is greatly diminished. Usually the second bath of solvent has a detergent injection then washed, and this solvent becomes the first bath for the next load with the excess going to the next wash tank. The second bath is on filtration. If a moisture stock solution is added to the first bath there will be excellent water soluble soil removal of the load.
No information on distillation would be complete without encouraging the operator to thoroughly clean the still after each distillation by scraping until all residue is removed. The residue will contain dye particles, detergent, sizing, common sludge and some odors. Most odors are water-borne, meaning they attach themselves to the solvent through the detergent and the little amount of moisture that is also distilled then goes to the water separator. Odors are the reason every water separator on a lighter than water solvent drycleaning machine needs to be thoroughly cleaned once a week by draining, opening and scrubbing out with a half cup of sodium hypochlorite (Clorox) bleach per gallon of water. Lysol may also be used to clean the water separator. It will also help to add one pint of water per day to the water separator to help keep it flushed out between cleanings. This is especially important when a moisture stock solution is not used on each cleaning load as the solvent is so dry going to the water separator that adequate flushing does not take place.
In years past solvent clarification was almost a full time job with settling tanks, highly alkaline washes and even acid washes to help remove impurities. After the distilled solvent left the still it immediately went to a damp-rag separator which would help to gather odors, usually from grossly contaminated solvent with perspiration, perfumes, ammonia and spotting chemicals. The rags would frequently begin turning red, or black, blue or green due to dye particles that had carried over with the distilled solvent. If the rags were not removed and washed after every use of the still the bacteria would begin growing and contaminate all solvent in the system or storage tanks.
Virtually every distillation problem is the result of the still heating temperatures being too high and breaking down the solvent, vigorously boiling the liquid to where some of the contaminants will boil over with the distillate or the condensing water either too hot or too cold. Steam or water leaks in the still, condenser or solvent cooling water can also allow water to get into the boiling chamber causing an azetropic solution with the solvent and a guaranteed boil over if care is not taken to prevent it.