What happens after solvent and water leave the still, and why getting them apart isn’t as simple as it sounds. 

By Bruce Grossman
Chief of R & D, EZ Timers Manufacturing

There are things in the dry cleaning industry you’ve been trained to do, but do you know the WHY behind them? Let’s start with solvent/water separation. 

What is solvent/water separation, and why is it necessary?

Garments absorb moisture from the surrounding air, and during the wash cycle, this moisture is transferred to the cleaning solvent. Later, during distillation and drying, the solvent and water vaporize together, then condense back into a liquid mixture. Before reusing the condensed solvent, you must remove the water. This process is known as solvent/water separation.

Fortunately, solvent and water are immiscible, meaning they do not mix. When two immiscible liquids are allowed to stand, the lighter liquid floats on top of the heavier one. Most modern hydrocarbon and other alternative dry-cleaning solvents are lighter than water, so the solvent floats on the water’s surface. (Perchloroethylene—PERC—is heavier than water and sinks.) Using gravity to separate either type of solvent from water is known as gravity separation.

What Affects Gravity Separation? 

1. Difference in specific gravity. The greater the difference in density between the solvent and water, the faster and more completely they separate. PERC has a specific gravity of approximately 1.6, compared with water at 1.0, so separation is relatively easy and usually requires only a single separation stage. Lighter-than-water solvents typically have specific gravities around 0.9, making them much closer to water in density. As a result, they separate more slowly and generally require multiple separation stages.
   
2. Separator volume. Larger separators provide more space for the liquids to separate, improving efficiency.
   
3. Turbulence. Separation is most effective when the liquid inside the separator is calm. Excessive turbulence mixes the solvent and water, slowing the separation process.
   
4. Separation time. The longer the solvent and water remain undisturbed, the more complete the separation.
   
5. Liquid temperature. Within practical limits, cooler liquids separate more effectively than warmer ones.

PERC vs. Lighter-than-Water Solvents

Separation systems used with PERC are relatively simple and differ little from one manufacturer to another. The condensed solvent/water mixture enters a separator, where it is allowed to remain undisturbed. Because PERC is heavier than water, it settles to the bottom of the separator while the water remains on top. The separated PERC is returned to the solvent storage system, and the water is discharged for disposal.

Separation systems used with lighter-than-water solvents are considerably more complex. Since these solvents float on water and have a density much closer to that of water, they require more elaborate separation methods to achieve acceptable efficiency. 

Different manufacturers use differing separator designs, making it impractical to describe every variation. Instead, this discussion focuses on the most common separation system used with lighter-than-water dry-cleaning solvents. 

Because of that smaller difference in specific gravity, additional separation volume is typically provided by using multiple separators. 

Most lighter-than-water dry cleaning solvents have a flash point (the lowest temperature at which they produce sufficient vapor to ignite in the presence of an ignition source). To reduce the fire hazard during distillation, the solvent is heated under a partial vacuum. The vacuum lowers its boiling point, allowing it to boil below its flash point. Maintaining this partial vacuum throughout the distillation process requires batch distillation (distilling the solvent in measured quantities rather than continuously). Batch distillation, in turn, requires the coordinated operation of several valves. 

What Happens During Batch Distillation? 

CLICK HERE for larger image.

A. Heating and Condensation (see diagram)

A vacuum pump draws a partial vacuum in the still. When the required vacuum level is reached, a vacuum switch actuates, opening the steam control valve and allowing steam to enter the still steam jacket (not shown).

Because the partial vacuum lowers the solvent’s boiling point, the solvent begins to boil at a reduced temperature. The resulting solvent and water vapors rise through the still riser pipe into the still condenser, where they contact a coolant-chilled serpentine coil (not shown). The vapors condense on the surface of this coil into liquid solvent and water, which then flow into the pre-separator.

B. Transfer from the Pre-Separator to the Primary Separator

The pre-separator contains a float switch that monitors the liquid level. When the liquid reaches a preset level, the float switch actuates and initiates the following sequence:

1. The pre-separator isolation valve closes. This valve, located beneath the condenser coil, isolates the pre-separator from the still, maintaining the partial vacuum in the still while the pre-separator is vented to atmosphere.
   
2. The pre-separator vent valve opens. Air enters the pre-separator, equalizing its pressure with the primary separator and the rest of the separation system.
   
3. The primary separator inlet valve opens. The contents of the pre-separator drain into the primary separator.
   
4. The primary separator also contains a float switch that monitors its liquid level. Typically, it takes several pre-separator discharge cycles before this float is actuated. When it is, the inter-separator valve opens, allowing the solvent/water mixture to flow into the still separator.
   
5. Within the still separator, the solvent and water separate by gravity.

• The recovered solvent returns to the distilled solvent base tank through the separated solvent drain valve.
• The separated water is discharged through the still separator water dump valve into the separated water collection tank, where it can later be drained and disposed of in accordance with applicable regulations.

C. The system resets for the next transfer cycle

As the liquid level in the primary separator drops, its float switch returns to the normal position. 

• The valves close. The primary separator inlet valve, inter-separator valve, and separator vent valve return to their closed positions. 
• With the pre-separator sealed once again, the vacuum pump restores the pre-separator to approximately the same vacuum level as the still. Equalizing the pressure before reopening the pre-separator isolation valve prevents a sudden pressure differential and allows the next transfer cycle to begin smoothly.

D. Putting It All Together 

1. The piping arrangement, valve locations, and exact valve sequencing vary among machine manufacturers. In addition, numerous internal venting operations occur that are not visible to the operator.
   
2. All valve operations and venting sequences are automatically controlled by the machine’s control system. Their purpose is to maintain the required partial vacuum in the still, while allowing portions of the distillation and separation system to be temporarily vented to atmosphere during liquid transfer operations.

The operator may never see most of this happening, but every valve, separator and pressure change has a purpose. Together, they allow the machine to recover usable solvent, separate the water it has picked up during cleaning, and carry out the distillation process efficiently. 

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The author gratefully acknowledges Andy Lien, Technical Director at GreenEarth Cleaning, for his technical and editorial assistance in preparing this article.