The mmHg for vacuum distillation typically ranges from 10 to 40 mmHg for petroleum refinery processes, which allows for distillation at temperatures below 370 to 380 °C. This reduced pressure increases the vapor volume per volume of liquid, necessitating very large diameter columns. For molecular distillation, pressures are even lower, typically below 0.01 torr (1.3 Pa), which is necessary for the process to operate in the free molecular flow regime, where mass transport is governed by molecular dynamics rather than fluid dynamics.
Explanation:
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Vacuum Distillation in Petroleum Refineries (10-40 mmHg): In petroleum refineries, vacuum distillation is employed 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 in pressure is crucial as it 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, which could lead to the formation of undesirable products. The low pressure also increases the volume of vapor formed per volume of liquid, which necessitates the use of very large diameter distillation columns to accommodate the increased vapor space.
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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, and the gaseous phase no longer exerts significant pressure on the substance being evaporated. This regime is known as the free molecular flow regime, where the rate of evaporation is independent of pressure. The process relies on a short path between the hot and cold surfaces to minimize the heating time and reduce heat damage to heat-sensitive materials. Molecular distillation is particularly useful for the purification of oils and other sensitive materials that could degrade under normal distillation conditions.
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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