Is it better to operate a distillation tower at high pressure or low pressure?
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As the pressure in the distillation tower increases, the amount of vapor rising decreases, and thus the reflux amount must be reduced. However, as the pressure increases, the relative volatility decreases, which results in an increased number of theoretical plates required for the distillation column. But the column actually does not have that many plates, so it is necessary to increase the reflux in order to reduce the required number of theoretical plates. This is a contradiction; therefore, when the cooling load at the top of the tower is sufficient, it is preferable to operate the distillation tower at a relatively lower pressure. I’m not sure if my understanding is correct; please give me some guidance.
This should be determined specifically based on the process operation requirements and equipment costs.
According to Dalton’s law of partial pressures, the lower the pressure, the easier separation becomes; less heat source and reflux are required, which facilitates energy savings and thus makes it economical.
Low pressure facilitates separation, and vacuum distillation is very common. In industrial production, the pressure may increase due to other factors; for example, considering the relationships between upstream and downstream processes, the pressure in a distillation system cannot be lower than that in other systems, which results in a higher operating pressure. Alternatively, continuing to reduce the pressure would result in higher refrigeration costs, so a higher operating pressure would need to be used. There are also those that use inert gases to increase pressure; for example, when the medium is somewhat hazardous and the sealing performance of industrial equipment is not sufficient, inert gases are injected to raise the pressure as a form of protection, and this does no harm to the separation process
Generally, low pressure is better. Compared to high pressure, at low pressure the relative volatility of the material increases, making separation easier. The cost of equipment manufacturing is reduced, and the operating temperature is also lower at low pressures, which prevents changes in some heat-sensitive materials. However, at low pressures, to ensure the heat exchange efficiency of the top condenser, a cooling medium of 7°C water or -12°C brine is required, which increases energy consumption.