Why is the vacuum pump in a vacuum distillation tower calculated based on the volume of gas after condensation?

The design and operation of vacuum distillation towers are rather special, mainly because under vacuum conditions the boiling point of the materials decreases, allowing distillation to take place at lower temperatures and thus preventing the decomposition of heat-sensitive materials or reducing energy consumption. In the process of vacuum distillation, the vacuum pump at the top of the tower plays a crucial role. Here, I will try to explain it in simple terms: 1. **Relationship between top pressure and gas flow rate**: In a vacuum distillation tower, the top pressure needs to be maintained at a very low level in order to reduce the boiling point of the materials. What flows out of the tower are mainly the components that have been stripped (or distilled) along with the non-condensable gases. During actual operation inside the tower, the flow rate of the gas phase is indeed relatively high, but after passing through the condenser, most of the condensable vapors turn into liquid; as a result, the flow rate of the gas after condensation decreases significantly. 2. **Basis for vacuum pump calculation**: The design flow rate of the vacuum pump is not based on the total amount of gas exiting at the top of the tower, but rather on the flow rate of the non-condensable gases after condensation. This includes, for example, air, water vapor (if not completely condensed), and other gases that are difficult to condense. The reason is that, after passing through the condenser, most of the easily condensable components have returned to a liquid state, and the task of the vacuum pump is to remove the remaining non-condensable gases in order to maintain a low pressure inside the tower. 3. **Maintaining vacuum in the tower**: Maintaining the vacuum level inside the tower requires vacuum pumps to continuously remove non-condensable gases, thereby preventing their accumulation within the tower and avoiding any impact on the pressure there as well as on the distillation process. Although the flow rate of the gas exiting the tower is high, the vacuum pump is responsible only for that small portion of gas that is not prone to condensation. Therefore, the vacuum pump does not need to be very large; it only needs to be able to effectively remove these non-condensable gases. 4. **Vacuum pump design**: When designing a vacuum pump, many factors need to be taken into account, including the efficiency of the condenser, the operating pressure range, the leakage rate of the process, and safety margins. Furthermore, the volume of gas increases at low pressures, which means that a vacuum pump capable of handling a relatively large volume of gas is required, even if the actual mass flow rate is not high. If you want more detailed information, you can consult books that specifically cover vacuum distillation or vacuum technology, such as \"Vacuum Technology in Chemical Engineering\", or relevant engineering handbooks like Perry’s Chemical Engineers’ Handbook; these sources provide more in-depth discussions on the design and operation of vacuum systems. Additionally, you can also consult engineers who are experienced in operating such devices; they can provide more specific insights based on their practical experience. .

The condenser is also equivalent to a vacuum pump

This reply is really comprehensive and classic. This is true not only for distillation but also for vacuum pumps used in evaporation processes