How to determine the top and bottom pressures of a distillation column

Reply to 1# Caosheng: Just connect vacuum pressure gauges and temperature gauges to the top and bottom of the tower, isn’t that enough? I’m not sure if that’s what you mean.

Reply to 2# lengkai12: No, what I’m asking is that when using ASPEN for simulation, it’s necessary to determine the pressures at the top and bottom of the tower. These values are determined based on the temperature difference between the condensate at the tower top and the temperature at the top of the tower, as well as the temperature difference between the reboiling steam at the bottom of the tower and the temperature there. I don’t know what values to use for these two temperature differences, especially the one related to heat exchange between the steam at the bottom of the tower and that area.

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This process seems reversed. However, it can also be done this way: start from the top of the tower. If circulating water is to be used, then the temperature at the top of the tower must be no lower than 40 degrees; generally, it’s between 43 and 45 degrees or higher. Based on this, the pressure at the top of the tower can be determined. Once the pressure at the top is known, the pressure at the bottom of the tower can be roughly estimated based on the total pressure drop across the tower. From there, the temperature at the bottom of the tower can also be determined. At this point, the steam pressure and temperature required can be decided based on the temperature at the bottom of the tower. Calculate one first and then adjust the various parameters

The temperature difference between the bottom of the tower and the heating steam is generally not less than 20 degrees; it can range from 30 to 40 degrees Celsius. It should not be too high either; otherwise, a large temperature difference between the inner and outer walls of the heat exchange tubes in the reboiler will cause stress issues, and the structural design will become more complex. The steam pressure and temperature to be used need to be determined based on the existing steam system throughout the plant

It seems the original poster has never seen actual distillation in action. In my workshop, the behavior of batch distillation is as follows: when the composition of the material remains constant, the top temperature stays within a range of 0.2 degrees. The temperature of the tank is closely related to the actual composition of the feed material; as components are distilled off, the tank temperature gradually increases. Therefore, both the tank temperature and the top temperature should be determined based on the properties of your material. As for the pressure at the bottom of the tower you mentioned, a pressure gauge can be installed above the reboiler; the pressure in the heating jacket is directly related to the amount of evaporation occurring at that time. The pressure at the top of the tower should be ignored; in actual operation, it needs to be connected to the atmosphere to prevent pressure buildup.

The temperature is set based on the boiling point of your distillation medium; you didn’t explain that clearly.

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Refrigerant is relatively scarce; generally, a condensation temperature at the top of the tower of around 50 degrees is chosen, so that the condensate water can be used for condensation. The pressure drop across the tower is usually not significant. Thus, once the temperatures and pressures at the top and bottom of the tower are determined, and based on the principles of vapor-liquid equilibrium, it can be seen that lower pressures result in higher separation efficiency; therefore, a lower pressure is advantageous for distillation separation

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