The pressure in a vacuum distillation column typically ranges from 10 to 40 mmHg (millimeters of mercury), which is about 5% of atmospheric pressure. This low pressure is crucial for maintaining lower operating temperatures, typically less than 370 to 380 °C, which helps prevent thermal degradation of the materials being processed.
Detailed Explanation:
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Low Pressure Operation: The vacuum distillation process operates at significantly reduced pressures compared to atmospheric conditions. This is achieved using vacuum pumps that can lower the pressure inside the distillation column to the specified range of 10 to 40 mmHg. This pressure reduction is essential because it allows the liquids to boil at lower temperatures, which is particularly important for heat-sensitive materials that could degrade at higher temperatures.
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Impact on Boiling Points: As the pressure decreases, the boiling point of a liquid also decreases. This principle is fundamental to vacuum distillation. By operating at these low pressures, the distillation can be conducted at temperatures that are lower than those required under atmospheric pressure, thus preserving the integrity of the materials being distilled.
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Column Design and Operation: The low pressure conditions in vacuum distillation columns necessitate large diameters to accommodate the increased volume of vapor formed per volume of liquid distilled. This design feature is evident in columns that can have diameters of 15 meters or more and heights up to about 50 meters. These dimensions are necessary to handle the large feed rates and to ensure efficient separation of components.
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Molecular Distillation: At even lower pressures, typically below 0.01 torr (1.3 Pa), molecular distillation is used. This process operates in the free molecular flow regime where the mean free path of molecules is comparable to the size of the equipment. Here, the gaseous phase exerts minimal pressure on the substance, and the rate of evaporation becomes independent of pressure. This method is particularly effective for the purification of oils and other heat-sensitive materials, as it allows for very short heating times and minimal heat damage.
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Application in Petroleum Refining: In petroleum refineries, vacuum distillation is used to further distill the residual oil from the atmospheric distillation column. The low pressure conditions prevent the oil from cracking at high temperatures, which is crucial for maintaining the quality and yield of the final products.
In summary, the pressure in a vacuum distillation column is maintained at a low level (10 to 40 mmHg) to facilitate distillation at lower temperatures, which is essential for the efficient and safe processing of heat-sensitive materials in industries such as petroleum refining and oil purification.
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