By Bruce Grossman, EZ Timers Manufacturing
DISTILLATION IS A PROCESS USED TO PURIFY A LIQUID, IN OUR CASE SOLVENT. IN THE NEXT SEVERAL ARTICLES I’LL BE DESCRIBING HOW DISTILLATION WORKS, THE MACHINERY COMPONENTS ASSOCIATED WITH IT, AND POSSIBLE PROBLEMS YOU MIGHT ENCOUNTER.
Solvent dissolves some of the impurities it removes from garments like grease, oil, wax, and detergent. These impurities are called solutes. Solvent also suspends solid impurities like lint, dust, and soil. Such impurities are called particulates. Proper distillation in most cases returns the distilled solvent to its original pristine condition.
The following two steps can comprise the distillation process:
1. Evaporating the solvent – Liquid solvent is heated in the still until it boils and vaporizes, changing from a liquid state into a vapor much like steam exiting a teapot.
2. Condensing the solvent vapor – Solvent vapor is cooled on some surface (generally a water-cooled coil) much like steam condensing on a bathroom mirror, changing from solvent vapor into a liquid solvent.
EVAPORATING THE SOLVENT
Solvent contained in the still is vaporized using heat (usually supplied by steam) and in some machines, heat is combined with a vacuum. The addition of a vacuum lowers the temperature at which the solvent boils, maintaining the solvent contained in the still at a temperature below the solvent’s flashpoint (the lowest temperature at which a substance can ignite). For this article, we will use steam as the heating media. Anytime you use steam for heating, a steam trap is required to remove the condensate from the heated vessel, in our example the still steam jacket. A steam jacket is a separate chamber surrounding the still bottom which contains the steam and heats the still similar to a flame under a kettle. Additionally, there is an automated steam valve that turns the steam supply on and off. Steam heating also has another very positive attribute, temperature control. Steam temperature is a function of steam pressure so it’s a slam dunk to set the heating temperature of the still by setting the steam pressure, via a steam pressure regulator. Now let’s take a moment and list some of the most common still malfunctions and see if we can find the cause using our newfound knowledge about stills.
1. STILL BOIL-OVER – I’d be willing to bet big money everyone who reads this and has done some distillation has boiled over the still more than once. For those lucky few that can look forward to this delightful experience for the first time, a still boil-over occurs when the nasty witches brew in the still is forced up the still riser (the large pipe between the still and condenser) into the condenser. Since the still is chock full of detergent and other impurities that prevent solvent from separating from water, the performance of the water separator becomes non- existent. Solvent and water mix to form a dirty-looking milky liquid that results in contaminating the separator, the solvent piping going all the way to the distilled solvent storage tank (rinse tank) and the storage tank itself. What could have caused this problem? Let’s go down the list.
A. I’ve found the most common cause of a boil-over to be overfilling the still. Although all new machines have a float-type level sensing switch in the still, it is possible – if not set correctly – to pump enough solvent into the still to cause a boil-over. Solvent expands when heated. A safe level with cool solvent can quickly become a problematic high level when heated to boiling. There is a “Catch 22” to recovering from overfilling the still involving the behavior of the still safety float switch and its associated circuitry. In the infinite wisdom of the machine designers, once the safety switch activates it shuts off the pump to prevent further filling of the still which is good. Unfortunately, it also in some cases shuts off the steam valve supplying heat required for
distillation, thereby preventing further distillation. The only way to remove solvent from the still, thereby lowering the level, is to drain the liquid contained inside the still, a torturous and hazardous job; or of course to distill, DUUUH! (do you see the hole in the fabric of this logic?). The happy news is that the still float switch is easily fooled, so distillation can be used to recover from the problem.
B. Coming in at a close second is excessive moisture (water) in the still. Free water turns to steam at temperatures below the boiling point of the solvent. This steam in the still liquid agitates the witches’ brew inside the still, causing violent bubbling and foaming forcing raw still contents up the still riser contaminating the condenser. In addition, for perc users, perc and water mixed in the proper proportions form what is known as an azeotrope (I don’t know how this works with solvents that are lighter than water). I won’t begin to try to explain the many strange and amazing qualities of a perc/water azeotrope other than it reduces the boiling temperature of perc from 260 degrees F to around 190 degrees F. Since the temperature of the still is set by steam pressure (the still steam pressure regulator doesn’t know that there is water in the still) hence, with the reduction of 70 degrees in the boiling point you get a geyser of frothing perc and water vomiting up the still riser like “Old Faithfull.” The source/sources of this contaminating moisture on older perc machines could be a leaky or partially open steam sweep valve, excessive spray (slop) pre-spotting, wet garments in the drycleaning load, let your imagination be your guide.
C. Last but not least in this species of screw-ups is excessive distillation temperature. This is either caused by the failure of the still steam regulator, someone fiddling with the adjustment, or a regulator bypass valve being open (if installed).
Recovery from any of these situations requires patience and a light hand on the steam pressure throttle. First, the cause must be determined and rectified. Next, the water separator should be drained and the contaminated mixture either disposed of or stored in a container for a few days and allowed to separate. Then, the solvent can be skimmed off for reuse and the water properly discarded. If the distilled solvent storage tank (rinse tank) is contaminated it must be pumped out, usually to the basket for later re- distillation. After these steps are accomplished adjust the still pressure regulator to around 25 psi. This will allow the solvent to cook-off at a more controlled rate. If excessive moisture is the culprit you will see a foggy or streaky whitish film on the still’s sight glass (the round porthole looking glass in the still) until the moisture is cooked off at which time the sight glass will clear. At this point, it is usually safe to slowly, gradually raise the still steam pressure and watch the results. If overfilling was the cause of the problem, after
the solvent is cooked off down to a safe level the steam pressure can be raised slowly to normal.
I’ve often wondered why some drycleaners are uncomfortable with the distillation process. It’s the simplest process in a long list of those occurring during the drycleaning process and has almost no moving parts, just letting chemistry and physics do their thing. Please keep in mind, the problem situations addressed in this article are the exception. Reasonable attention to maintenance and procedures will prevent these problems from occurring.
Bruce Grossman is the Chief of R&D for EZtimers Manufacturing. EZTIMERS MANUFACTURES MACHINES THAT MAKE YOUR MACHINES RUN BETTER. For those needing to replace ZERO-WASTE or GALAXY misters see our SAHARA model, for those with boiler problems check out our EZ-LEVEL and EZ-DOSE machines. For further information on EZtimers products visit www.eztimers.com. Please address any questions or comments for Bruce at email@example.com or call (702) 376-6693.