Vacuum distillation is a method of distillation in which pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure (usually less than atmospheric pressure) causing evaporation of the most volatile liquid(s) (those with the lowest boiling points). This distillation method works on the principle that boiling occurs when the vapor pressure of a liquid exceeds the ambient pressure.
The solvent recycling is accomplished by a peripheral heating jacket filled with oil, heated by an electrical element. This means that during the solvent distillation process, the heating source is never in contact with the solvent being distilled. The solvent vapors are then conveyed to a condenser, cooled by air or water. The distilled solvent is collected in a tank, for its reuse.
For solvent distillation systems and solvent recycling, vacuum systems can be attached to the distillation equipment to distill solvent while under a vacuum. Utilizing vacuum distillation equipment, the boiling point of the solvent is lowered, decreasing the temperature necessary for distillation of the solvent. Many “safety solvents” have a high boiling point and to reclaim a safety solvent, vacuum distillation is often required.
Vacuum distillation increases the relative volatility of the key components in many applications. The higher the relative volatility, the more separable are the two components; this connotes fewer stages in a distillation column in order to effect the same separation between the overhead and bottoms products. Lower pressures increase relative volatilities in most systems.
A second advantage of vacuum distillation is the reduced temperature requirement at lower pressures. For many systems, the products degrade or polymerize at elevated temperatures.
Vacuum distillation can improve a separation by:
- Prevention of product degradation or polymer formation because of reduced pressure leading to lower tower bottoms temperatures.
- Reduction of product degradation or polymer formation because of reduced mean residence time especially in columns using packing rather than trays.
- Increasing capacity, yield, and purity.
Another advantage of vacuum distillation is the reduced capital cost, at the expense of slightly more operating cost. Utilizing vacuum distillation can reduce the height and diameter, and thus the capital cost of a distillation column.