If negative pressure distillation is used, the pressure of the distillation tower is determined

Personally, I believe that determining the pressure of a distillation tower represents an optimal choice that takes into account factors such as overall energy consumption, separation efficiency, and the ease of meeting various requirements.

The operating pressure needs to be taken into consideration, as well as whether the substance is heat-sensitive; the material at the top of the tower must be able to be cooled with industrial water. The lower the pressure, the smaller the processing capacity, and the smaller the boiling point difference becomes. It is not easy to separate.

As far as I know, reducing pressure can improve separation efficiency. The greater the relative volatility between the separated mixtures, the easier it is to separate them. Under reduced pressure, generally speaking, the relative volatility of the components increases.

The greater the relative volatility, the easier it is to separate – this is the principle of distillation – but reducing pressure does not increase relative volatility. Relative volatility is reflected in the boiling points of the components; the difference in boiling points decreases under reduced pressure.

Hello, I see that different sources mention various things, and I’m also confused. Do you have any information such as charts or data showing the boiling points of two different materials at various pressures?

By looking up the saturated vapor pressures of the two pure substances and comparing them, a result will be obtained. Any two known ones.

That’s nonsense. I was in the same situation as you when I first started working with vacuum distillation towers; I also had such doubts. But over time, you’ll realize that certain parameters of a tower are determined during its design, such as the reflux ratio and the vacuum level. You can make slight adjustments, but it’s best to avoid major changes, otherwise operation will be difficult. One more thing: vacuum is the most important parameter of a tower, and it must remain stable; otherwise, it will be hard to operate the distillation tower properly

First, let’s clarify a misconception held by many people: they think that reducing pressure will increase the relative volatility, thereby facilitating separation. This is incorrect; for different systems, changes in concentration can cause the relative volatility of vapor pressure to increase or decrease. For the same system with different compositions, changes in pressure can increase or decrease the relative volatility. It might be a bit difficult to explain, so let’s use an example: in the methanol-water system, when the methanol concentration is high, for instance above 0.25 in terms of mass fraction, reducing pressure helps increase the relative volatility. However, if the purity of methanol is only 0.01, increasing pressure can still boost relative volatility. If you don’t believe it, you can try using simulation software to see it. Now, to answer the original poster’s question: temperature is directly related to pressure. Reducing pressure can lower the temperature in the bottom of the tower. Sometimes, pressure reduction is employed to prevent polymerization or similar phenomena; however, it isn’t always true that reducing pressure leads to better separation, and it has nothing to do with the reflux ratio either. Of course, if the material system and composition are specified, that’s another matter.

In fact, the determination of vacuum level during design should be considered comprehensively; for example, it is necessary to decide whether low-temperature water or circulating water should be used as the coolant in the tower top condenser. If the substance in question is not a heat-sensitive material, circulating water should be used whenever possible, as this can result in significant cost savings. Of course, for materials with particularly high boiling points, the vacuum level should be reduced as much as possible.

Hello, what simulation software are you referring to? Could you recommend one to me?