Regarding tower distillation, what are the advantages and disadvantages of reduced pressure, atmospheric pressure, and increased pressure?

Changes in pressure affect changes in boiling points; the use of reduced pressure, normal pressure, or increased pressure is determined entirely based on the properties of the substance and the requirements for separation. Reduced pressure and increased pressure are employed as necessary measures; in some substance systems, separation cannot be achieved even at high temperatures at the bottom of the tower, and raising the temperature further may lead to coking or a significant increase in costs. In such cases, reducing pressure can lower the boiling point and meet the separation requirements. The most common example of this is the distillation column operating under reduced pressure. Increased pressure is used when the separation accuracy cannot be achieved at normal pressure; raising the pressure helps to reduce the amount of heavier components in the gas phase, thereby meeting the separation requirements.

This post was last edited by lin Ling on 2019-3-4 at 14:30. I’m just passing by to share my own views; the person above has already considered this issue from a physical properties perspective, so I won’t repeat that here. This is also an issue that has been discussed in the fields of chemical engineering principles and separation engineering, namely the effect of pressure on the relative volatility of the components to be separated. However, after carrying out certain distillation processes, such as those for silicone monomers, property analyses show that pressure has little effect on the relative volatility of some of the less volatile components. Considering that an increase in pressure leads to an increase in the steam load, it would theoretically be appropriate to reduce the pressure, for example, to atmospheric pressure. But after checking the factory’s data, the pressure is generally above 1.5 bar. Later, I looked up the views mentioned in the literature; from a hydraulic calculation perspective, for low-pressure columns (p7 bar), reducing the pressure leads to a smaller diameter required for separation, whereas for medium-pressure columns (P=3–7 bar), pressure changes have little effect on the column diameter. The diameter of the distillation column is directly related to the cost of manufacturing the column; therefore, from this perspective, it is appropriate to increase the operating pressure inside the distillation column for silicone monomers in order to reduce the manufacturing cost of the column. It’s a perspective I had never considered before; my engineering experience is still limited, so if there are experts in this field, I would appreciate their guidance. Liu Z Y, Jobson M. The effect of operating pressure on distillation column throughput. Computers & Chemical Engineering, 1999, 23(S):S831–S834.