In last month’s installment we covered how heat exchanging took place during the drying cycle within your dry cleaning machine. This month we are going to cover heat exchange during the distillation process.
What Is Distillation?
In dry cleaning, there are two main types of soil or impurities removed from the garment during dry cleaning. First, there is the “particulate” matter. Your filtration system is used to trap and remove particulate soils that are dust, lint, clay, food particles and any other solid debris in the solvent stream. Filters are mighty fascinating animals unto themselves, but will be a subject for future articles. The second family of impurities are those which dissolve in the solvent. These impurities can be removed only by distillation or adsorption using a clay or charcoal based sorbent. Because impurities that are dissolved in solvent are left behind when the solvent evaporates (like salt or sugar left behind in a pan when the pan is heated and the water evaporates) the impurity is called a “non-volatile residue.” Yes my friends, the infamous NVR at last explained! During distillation the solvent is heated to boiling in the still. The liquid solvent is boiled off and later condensed for reuse leaving behind a residue which is a brew of solid debris, fats, oils, salts, sugars, dyes as well as detergents and other additives to the solvent. A nasty concoction called “still bottom or sludge.”
Let’s look at the components of the distillation system:
1. Still – The still is a container comprised of two closed chambers. The upper chamber is used to hold the solvent, a second lower chamber used to hold a heating fluid, usually steam, but in some cases electrically heated oil. If steam is the heating media then this chamber is referred to as the “steam jacket.” If the heating media is oil it is called strangely enough, the “oil jacket.” In the case of the heating fluid being steam, temperature is controlled by limiting the steam pressure using a steam pressure regulator. When the heating fluid is oil. temperature is controlled using thermostat to switch electric heating elements on and off.
2. Still riser – At the top of the still is a large pipe. This pipe, called a “riser” conducts the hot solvent vapor from the still into the still condenser. A word to the wiser, don’t be a miser, be sure that this riser is well insulated. Otherwise, it will act as condenser returning lots of liquid solvent to the still instead of almost all the solvent vapor into the still condenser (this is a common cause of inefficient distillation).
3. Still condenser- A cylindrical housing containing a spiral (serpentine) coil and possibly some baffles to direct vapor, and liquid flowing through the condenser housing. The spiral coil has water flowing through it which cools the hot solvent vapors being pushed from the still into the condenser by the pressure of the boiling solvent in the still. The cooling of the solvent vapor condenses it back into liquid solvent.
4. Water separator – Since water vapor is also condensed with the solvent vapor a means must be used to separate the two liquids. All the solvents used in dry cleaning have two qualities which are used to separate them from the condensed water vapor:
A. The solvents are immiscible with water (this means they will not mix or dissolve into each other to any great extent).
B. Solvents are either heavier (Perc) or lighter (DF2000, Green Earth, K4) than water. This happy situation allows the use of gravity to separate the two liquids. In other words, the solvent will either sink through the water to the bottom of the separator or float on top of the water. After separation the solvent generally flows into a storage tank (sometimes called “rinse tank”) for reuse and the water is decanted from the separator for disposal. This water, if left untreated, is a hazardous waste and must be disposed of properly. Fortunately, EZtimers Manufacturing sells equipment that purifies this water so it can be legally disposed of, saving the high cost of hazardous waste hauling (see our ad on page 11).
Temperature Considerations During Distillation
Let’s look at the two basic heat exchanges that take place during the “distillation process.”
1. The first is the heat exchange is the addition of heat energy which turns the liquid solvent into a vapor. Only steam heated still will be the subject of the remainder of this article. Steam heating simplifies the solvent heating process. A pressure regulator limits the steam pressure inside the steam jacket thereby setting the upper temperature limit. The pressure setting of the regulator will vary depending on the solvent used, and in rare cases, what stage of distillation is being performed. In addition to heating the solvent in dry cleaning systems using solvents that are flammable (most solvents except perc are flammable) a vacuum is applied to the distillation system. A vacuum pump reduces the vapor pressure felt by the liquid solvent allowing it to change into vapor at lower temperatures (this temperature is called the “flash point”). This is necessary to prevent a situation where the vapors can spontaneously ignite inside the distillation system and explode (a very bad thing).
2. Heat exchange number 2 is the removal of heat from the vaporized solvent in the still condenser, thereby changing the solvent back to a liquid.
I wanted to continue this installment through the troubleshooting section but time limitations prevent it for this month. For those of you visiting the California show next month have fun! Next month we will finish this heat exchange stuff, cross my heart!