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As drycleaning facilities are operated in closed cycles, solvent regeneration plays an important role. This is particularly important as the customer rates the performance of the drycleaner mainly by the smell, by the brightness of colors and by the cleanliness of the cleaned textiles. Therefore, professional solvent care is in direct correlation with cleaning quality and customer acceptance, and in the end it helps to preserve the value of the machines.

Basically, adsorption, distillation or a combination of both methods can be used for regenerating polluted cleaning liquors. In hydrocarbon solvent drycleaning, all three procedures are applied. In perc cleaning machines, solvent regeneration is usually achieved by distillation.

As an essential condition for distillation, the solvent must be a uniform substance with defined boiling point. The boiling point must be considerably lower than the thermal decomposition point and lie beyond the boiling range of the pollution to be separated. There must be no azeotrope formation between solvent and pollution to be separated.

Distillation problems of hydrocarbon solvent result from the fact that the boiling point of the solvent must be lowered as a result of high energy costs and to avoid thermal decomposition. In practice, this is achieved by vacuum distillation; in this case, however, a reduced separation efficiency often complicates the separation of problematic pollutions.

In many European countries, more rigorous regulations for reducing solvent emissions have caused drastic changes in the technology for perchloroethylene cleaning machines. Expander or cartridge filters and the use of filter powder in cleaning technology were replaced by modern fifth-generation machines.

Major characteristics of this machine technology are deposit-free centrifugal filters, automatic distillation boiler emptying, safety water separators, activated carbon adsorption systems and computers for controlling the cage interior limit values at the end of the drying stage of less than 2g perchloroethylene/m3. With new machines, the distinctly higher technical expenditure, particularly in drying, has resulted in batch time expansions of approximately 10 minutes for standard two-stage bath procedures.

The distillation of perc cleaning liquors starts with the azeotropic distillation in the presence of water. At a constant temperature of approximately 87°C, (176°F) perc and water distill at a ratio of 84% perchloroethylene to 16% water. Excessive heating rates and excessive water content in distillation are often the cause of boil-over. Only after the water content has been distilled off completely, the temperature in the boiler rises to the boiling temperature of perchloroethylene (121°C), (249.8°F).

If the distillation boiler is not cleaned regularly, high-boiling oils, greases and other residues accumulate in the boiler. As the content of high-boiling pollutants in the distillation residue increases, the distillation temperature in the still and therefore the amount of energy required increases distinctly.

Table 1 table shows that, under normal distillation conditions, the residual perc content in the distillation residue is always within a range of 30% to 40%. This is normal and should be accepted as such. An attempt to minimize solvent losses by blowing in hot steam at the end of the distillation cycle is hazardous. Particularly the substances that cause unpleasant odors are often steam-distillable.

Many high-boiling oils and greases can penetrate the distillate due to carrier steam or azeotrope formation with water. In addition, the residual perc content in the distillation residue ensures the required pumpability of the sludge. Manual emptying of distillation boilers as performed in former years is no longer permitted in many European countries.

Considering these circumstances, the steam pressure set for steam-heated perchloroethylene cleaning machines should not exceed 4.5-5 bar which corresponds with temperatures of approximately 145° to 150°C (293° to 302°F). If a sufficient stabilization is ensured, this should reliably prevent the thermal decomposition of perchloroethylene. For the reasons mentioned above, it is furthermore recommended to clean the distillation boiler every day (after approximately 10 batches).

Perchloroethylene is stabilized to prevent thermal or hydrolytic acid elimination as reliably as possible. The stabilizers are acid-binding substances based on amines and/or phenol. The stabilizer must not be separable by distillation. In case of thermal decomposition, the stabilizer binds acid and is consumed in this process.

Due to the considerably lower solvent losses, the demands on the stabilization of perchloroethylene are very high. In addition to the gas-chromatographic test for foreign solvent contents in the distillate, the pH is one of the essential parameters that can be easily controlled.

The pH determination can be performed using special indicator paper (test range 5-9 pH) during the aqueous phase of the water separator. The pH can also be determined using the aqueous extract of the shaken-out solvent by taking a clean perc sample from the pure tank into a glass container or test tube, adding an identical quantity of water, shaking the solution for approximately two minutes and measuring pH in the aqueous phase after successful phase separation.

Assessment of pH (in distillate cleaning machines)

pH <6: Distinct acid elimination

pH 6.1-6.9: Minor acid elimination

pH 7.0-7.2: Acceptable

pH 7.3-8.0: Ideal

pH 8.1-8.5: Acceptable

pH > 8.5: Over-stabilized

The pH of the solvent is crucial for corrosion in cleaning facilities and often allows conclusions with regard to the distillation conditions. Fresh perc brands available in the market as well as perc regenerates are in accordance with or even surpass the required purities.

The most common reason for sour perchloroethylene is definitely overheating of solvent due to technical failures in heated systems or steam pressures exceeding 5 bar for steam-heated machines. The temperature limiters in the still should be set to a maximum of 145° to 150°C, (293° to 302°F), the usual distillation temperatures are adjusted to approximately 128° to 132°C, (262.4° to 269.6°F).

At temperatures of at least 160°C, (320°F), perc decomposes with the formation of hydrochloric acid. The breakdown products resulting from this thermal stress are more unstable and tend to eliminate acids earlier.

At a pH of less than 6, machine corrosion is likely to be encountered. Many machines contain a sacrificial anode in the pure tank to prevent corrosion. Should corrosion occur, the sacrificial anode, doing justice to its name, corrodes (i.e. dissolves) instead of the metallic tank.

This corrosion protection works only if the anode is inspected regularly on the occasion of tank maintenance and if it is polished with a wire brush. In such cases, machine corrosion occurs particularly in the presence of moisture at the upper side of the tank and in the water separator. For copper coolers, sour solvent corrodes the tin-plated protective layer.

As a result, the bare copper reacts with amine components from retexturing agents and/or antistatic components of the drycleaning detergent.

This copper corrosion can be easily identified due to the deeply blue color of the copper tetramine complex forming in the water phase of the water separator. When copper corrosion is encountered, the condenser should be replaced on the occasion of the next machine maintenance interval.

Adsorption is based on physical and physico-chemical interaction between adsorptive (pollution) and adsorbent (adsorber).

Activated carbon. Activated carbons are hydrocarbon structures composed of minute graphite crystals and amorphous carbon with porous structure.

The pore structure is the major characteristic of activated carbons. The specific surface is between 400 and 1,500 m2/g, depending on the individual type. Carbons with small pores (highly active, very large interior surface) are used as adsorbent for minor dirt degrees and for the adsorption of low-molecular compounds. Large-pored carbons are used for the adsorption of surfactants and colorants.

Diatomaceous earths. Dia-tomaceous earths consist of fine, micro-crystalline quartz. Due to the compact structure of the filter cake, pigments are filtered out of the polluted liquor purely mechanically.

There is only a slight tendency to adsorb polar odorous substances. Diatomaceous earths are not suited for colorant adsorption. Some years ago, they were used to a large extent as a filtering aid in perchloroethylene cleaning machines. Particularly as a result of the second German Federal Emission Protection Regulation of 1995, they were replaced by deposit-free centrifugal filter systems.

Zeolites. As zeolites have a distinct ion-exchange capability, they are used as builders in phosphate-free detergents. They adsorb polar substances and are therefore suited for the adsorption of fragrances in solvent liquors. Anionic colorants are not adsorbed by untreated zeolites, cationic colorants are bound reliably due to the negative surface charge.

Montmorillonite. With a content of approximately 93 percent, montmorillonite is the major constituent of naturally occurring bentonites. It is a low-charged sheet silicate and is a member of the clay mineral class. With regard to technical purposes, the major characteristic of montmorillonite is its excellent adsorptiveness which is directly correlated to the swelling capacity and the capability of exchanging intermediate sheet silicate ions. In solvents, montmorillonites easily adsorb polar, odor-producing substances. The affinity for water is very high.

Independent of the filter system used, heavily bleeding textiles may cause problems with liquor decoloration. To solve these problems, the tank is often filtered during the drying phase for machines with restricted distillation.

For non-distilling machines, extended drying periods such as tank filtration during the drying phase are crucial. If the powder quantity is too small in relation to the solvent quantity, the absorbency of the filtering aid may soon be exhausted for certain colorant classes in case of massive bleeding, even though other types of colorant can still be adsorbed

For colorants and other types of chemical dirt, absorbency, speed and maximum quantity depend on the development of the adsorption isotherm and therefore also on the chemical structure of the dirt.

To ensure operational efficiency and procedural safety, adsorbent materials are usually replaced before the maximum absorbency is reached to prevent exchanges between desired adsorption and undesired desorption of dirt particles.

Knowing these basics of hydrocarbon-solvent cleaning technology, it is obvious that distillation methods, as well as adsorption methods, have specific weaknesses.

However, due to their design, they do not overlap so that the weaknesses of one method can be compensated with the advantages of the other method. For cleaning companies, this means that distillation and adsorption are a useful and often even necessary mutual completion.

Hydrocarbon-solvent liquor. The total pollution or, in other words, the quality of the cleaning liquors, can be determined relatively reliably not only by a purely visual assessment, e.g. colorlessness or clarity, but also by the smell and by the summary parameter drying residue.

The drying residue comprises all substances that have not evaporated at test temperature. These substances are, for example, oils, greases, waxes from textiles, residues from drycleaning detergents, spotting agents, and proofing agents.

The drying residue of hydrocarbon-solvent liquors is usually determined with infrared lamps at approximately 140°C (284°F) to constant weight.

In publications of the Hohenstein Institutes, the drying residues in distilling machines were distinctly lower than in non-distilling machines.

For a drying residue of 1 to 1.5 percent, clouding due to non-vaporizable residues may be encountered on bright and light tissues.

When this limit is reached, distillation of the solvent liquors should be performed or the solvent should be replaced.

Perchloroethylene liquors. The predominant use of permanent distillation and dual-bath procedures with bath replacement usually prevents excessive residue formation in perc machines.

Possible problems in the practical application of textile cleaning machines

In some cases, undesired odors may be encountered in perchloroethylene and hydrocarbon solvent cleaning machines. Usually, these unpleasant odors are the result of the interaction between dirt, moisture, fluff accumulations, and bacteria in cleaning systems.

The main cause for this problem is usually lack of maintenance of the water separator or, for hydrocarbon-solvent cleaning machines, of the water separators.

The water phase of the water separator is highly polluted by volatile organic substances removed from the cleaned textiles, even if the liquid appears to be completely clear. In combination with conditions and temperatures almost ideal for bacterial growth, a gelatinous, often slightly greenish film of algae and bacteria slime is found particularly in the boundary layer solvent/water and at the walls of the water separator.

The metabolic products of many bacteria are, among others, short-chain, organic acids that often have a typical, unpleasant odor.

To prevent this, the water separator should be drained as often as possible, preferably twice a week.

On this occasion, the walls of the water separator should also be cleaned thoroughly to remove the films described above.

Distilling hydrocarbon-solvent machines usually have two water separators: one for drying and one for distillation.

Approximately 80 to 90 percent of the water used or of the moisture dried out of clothing is removed from the system by the drying phase water separator. This means that, for achieving a proper phase separation in the distillation separator, fresh water should be filled in after the cleaning of the water separator.

Another major cause for unpleasant odors is an insufficient phase separation in the water separator.

This problem may be caused by over-boiling; foreign solvents, e.g. butyl acetate from some paint dissolvers or defoamers, can also result in insufficient phase separation.

Butyl acetate has a boiling point of 131°C (267°F) and is very difficult to remove from perc liquors by distillation. Therefore, paint dissolvers should be blown out with compressed air during the basic cleaning at the spotting table.

An insufficient water supply can also impair the phase separation. This problem often occurs during summer months in cooling water circulations using cooling towers.

As a rule, the lower third of the cooling tower should feel cool to the touch of a hand to ensure sufficient cooling during distillation.

As mentioned at the beginning, textile cleaners themselves can largely influence the quality of textile cleaning and therefore the customer acceptance on which the personal success and therefore the image of the trade depends.

The following notes are meant to illustrate these interconnections and to come up to the performance expectations, i.e. cleanliness and hygiene.

Use the distillation capacity of your cleaning machine. If possible, use the dual-bath procedure with bath replacement (first bath for distillation).

Ensure correct distillation temperatures. For steam-heated machines, check the steam pressure at the still at regular intervals. Check for pH during the water phase of the water separator at regular intervals.

Clean the distillation boiler every day, if possible, or at least twice a week.

Check the filter pressure at regular intervals and, for perc machines, spin the filter off at 1.5 bar at the latest. For hydrocarbon-solvent cleaning machines, we recommend a filter regeneration at approximately 1.2 bar to ensure perfect regeneration of the filter disks.

If the filter disks are difficult to regenerate or if the filter pressure increases drastically after a few batches, remove the disks and clean or preferably wash them.

As the water separator is essential for the solvent quality, clean it at least twice a week.

Check the tanks (particularly the working tank) for sludge layers at regular intervals. When the dual-bath procedure with tank emptying at pump-over of the second bath (with liquor completion from pure tank up to level 1:5), keep the working tank in a clean condition over an extended period of time.

When checking the tank, the sacrificial anode usually installed in the tank should be replaced, or at least cleaned, if corrosion is found.

Use the service and the competence of your supplier. When in doubt, have your solvent examined in a laboratory.