Vacuum distillation is a crucial process in various industries, especially in petroleum refineries and molecular distillation.
The pressure range for vacuum distillation typically varies depending on the specific application.
For petroleum refinery processes, the pressure usually ranges from 10 to 40 mmHg.
This range allows for distillation at temperatures below 370 to 380 °C.
Reducing the pressure increases the vapor volume per volume of liquid.
This increase necessitates the use of very large diameter columns to accommodate the increased vapor space.
For molecular distillation, the pressures are even lower, typically below 0.01 torr (1.3 Pa).
This high vacuum is necessary for the process to operate in the free molecular flow regime.
In this regime, mass transport is governed by molecular dynamics rather than fluid dynamics.
4 Key Pressure Ranges in Vacuum Distillation
1. Vacuum Distillation in Petroleum Refineries (10-40 mmHg)
In petroleum refineries, vacuum distillation is used to further process the residual oil obtained from atmospheric distillation.
The pressure is reduced to 10-40 mmHg, which is about 5% of atmospheric pressure.
This reduction allows the distillation to occur at lower temperatures, specifically below 370-380 °C.
Operating at these lower temperatures prevents the thermal cracking of the heavier hydrocarbons.
Thermal cracking could lead to the formation of undesirable products.
The low pressure also increases the volume of vapor formed per volume of liquid.
This increase necessitates the use of very large diameter distillation columns.
2. Molecular Distillation ( <0.01 torr or 1.3 Pa)
Molecular distillation operates at pressures significantly lower than those used in vacuum distillation for petroleum refining.
It typically occurs below 0.01 torr (1.3 Pa), which is considered high vacuum.
At these pressures, the mean free path of molecules is comparable to the size of the equipment.
The gaseous phase no longer exerts significant pressure on the substance being evaporated.
This regime is known as the free molecular flow regime.
In this regime, the rate of evaporation is independent of pressure.
The process relies on a short path between the hot and cold surfaces.
This minimizes the heating time and reduces heat damage to heat-sensitive materials.
Molecular distillation is particularly useful for the purification of oils and other sensitive materials.
These different pressure ranges reflect the specific needs of the materials being processed and the goals of the distillation process.
Whether it be to prevent thermal degradation of heavy hydrocarbons in petroleum refining or to purify sensitive materials like oils in molecular distillation.
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