GOOD GUY DISTILLERS
ROTARY EVAPORATION LOW PRESSURE, HIGH QUALITY DISTILLATES WRITTEN BY DAVID LETTENEY
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istillation requires a significant amount of thermal energy input, which can cause safety issues due to the heat source and ethanol vapor cloud being present. But what if the thermal energy required to distill could be lowered, making the process inherently safer? Welcome to rotary evaporation. The concept behind rotary evaporation is relatively simple. The lower the pressure of the system, the lower the boiling point of the distillate. In practice, it can be a bit more complicated. Although a rotary evaporator seems like a fancy piece of technical lab equipment, it is actually very similar to a traditional still, especially when compared to a glass lab still. Distillation works by adding enough thermal energy (heat) into a mixture of mostly ethanol and water to vaporize some of this mixture. Because ethanol has a lower boiling point than water, the vapor produced will have a higher ethanol content than the liquid does according to thermodynamic principles. When this vapor comes in contact with a cold surface (condenser), the vapor is condensed back to a liquid, which is the product. So in this sense, a rotary evaporator performs the exact same process. W W W . ARTISANSPIRITMAG . C O M
ROTARY EVAPORATOR DIAGRAM In a rotary evaporator, the substrate to be distilled (a mixture of ethanol, water, and most likely botanicals) is placed into the boiling flask. After the whole system is connected, a vacuum is pulled on the whole apparatus. The boiling flask is lowered into the water bath, where warm water is used to heat the flask to the boiling point of the mixture. Using the speed controller, the boiling flask is rotated, promoting faster evaporation (more on this later). The vapor rises through the system until it hits the cooled coils of the condenser, at which point the vapor is condensed to a liquid, and drips down into the receiving flask. One of the main differences between a rotary evaporator and a normal still is that a rotary evaporator is a closed system, so monitoring the ABV and sensory characteristics can only be done if the vacuum is broken.
LOWER PRESSURE, LOWER BOILING POINT The boiling point of a liquid is the temperature and pressure at which a liquid’s vapor pressure is equal to atmospheric pressure. The vapor pressure of any compound is an inherent property that is a reflection of the strength of the bonds in that liquid. More specifically, the vapor pressure is a measure of the pressure exerted above a liquid from molecules evaporating. For example, a liquid with a very high vapor pressure means that there are a lot of molecules evaporating. When the pressure being exerted by the molecules leaving (vapor pressure) is equal to or greater than the pressure being exerted by
air on that liquid (atmospheric pressure), the liquid begins to rapidly and more violently evaporate, which is known as boiling. This principle can be shown in the boiling points of liquids at altitude vs. at sea level. At sea level, water boils at 212 degrees F, but in Lake Tahoe (6300 feet of elevation) water boils at 200 degrees F. Because the atmospheric pressure is lower at elevation, less energy is required to increase the vapor pressure to above that of the atmospheric pressure, thus the liquid boils at a lower temperature. A rotary evaporator takes advantage of this by greatly reducing the pressure in the system so that water can be boiled at 100 degrees F or lower! 27