Molecular Distillation for Booster & Blower Applications

Distillation begins with a liquid mixture containing components of varying boiling points. Depending on the chemical composition and the bond strength of the components, their densities vary, which has a direct influence on boiling point. When molecules are tightly packed together, it takes more energy, typically in the form of heat, to bring these molecules a sufficient distance from each other to break the intermolecular bonds and begin vaporization.

To achieve this in an industrial setting, distillation columns are often used. The feedstock is the liquid mixture, being preheated and fed into the column at a calculated height from the bottom, where a heating element is. When the feedstock reaches the bottom, it is heated at a constant temperature known to be the boiling point of one of the liquid components. That component evaporates and rises through the column, passing through manufactured membranes with pores of uniform, predetermined diameter that diminish as the gas goes up the column until the diameter reaches a size that no longer permits transport through it for that gaseous component. The component is led through that section of the column to an evacuated condenser, which condenses the gas back to liquid. The liquid passes through a reflux drum, which sends impurities back through the column, and the refined liquid is contained separately for further refining, use, or transportation to other industrial plants with more specialized processes.

There are many different methods of distillation, including fractional distillation, steam distillation, vacuum distillation, reactive distillation, catalytic distillation, and one of the finest methods of separation, molecular distillation. The difference between molecular distillation lies less in the result and more in the apparatus and method; namely, evaporators.

Two kinds of evaporators stand out in terms of distillation processes: thin film evaporators and short path evaporators. Depending on the kind of evaporator used to vaporize the liquid components, the system setup varies, but both require subatmospheric conditions in the system via a vacuum pump.

Short path evaporation, or short path distillation, requires an operating pressure of up to 10^-4 mbar and operates with a condenser placed in the center of the evaporator, reducing distance between the hot and cold surfaces and thus decreasing the pressure differential. The operating pressure can only get as low as required because of this decreased pressure differential, which would otherwise negatively impact the purity and quantity of the yield. In this process, feedstock is poured onto the evaporator, which is held at a specific temperature that is the boiling point of one of the components of the liquid mixture. The vapor immediately condenses onto the condensing plate and is piped away to separate storage while the rest of the liquid flows out of the bottom of the apparatus.

Distillation can be named molecular distillation if the distance between the evaporator and condenser reaches the mean free path of a vapor molecule. With short path evaporators, molecular distillation occurs. Short path molecular distillation takes seconds, and can be generally applied to organic compounds with high molecular weight. Organic compounds will not decay because the high vacuum level brings the boiling points lower, to a value that does not risk the deterioration of heat-sensitive materials.