Effect of vacuum distillation on relative volatility?

For a perfectly ideal system, the relative volatility is the ratio of the vapor-liquid equilibrium ratios of the two components. The vapor-liquid equilibrium ratio refers to the ratio of the molar fraction of a component in the vapor phase to its molar fraction in the liquid phase at vapor-liquid equilibrium. For a binary system that is perfectly ideal, this means that when the temperature remains constant, the corresponding volatility is fixed and remains a constant value; it is only when the temperature changes that the relative volatility changes as well. This post was last edited by whq520 on 2008-1-23 15:52.]

What was said upstairs is correct – the key components in actual production are not entirely ideal systems; this is related to the nature of the substances being distilled. However, in general terms, it is possible to treat them as ideal systems, while the details should be considered based on the actual circumstances

The concept of relative volatility is: the ratio of the volatility of the volatile substance in a solution to that of the less volatile substance. When the pressure is not high, the relative volatility changes little with temperature, so the average value can be used. The data for different solutions can be obtained through experiments. Under normal circumstances, as external pressure decreases, the vapor pressure of the solution also decreases; in other words, its volatility drops. However, whether the relative volatility also decreases depends on the specific substance. Personally, I think that the effect of pressure changes on relative volatility can be ignored in some cases. We introduced the concept of relative volatility to determine the ease of separating substances using distillation; the higher the relative volatility value is than 1, the easier the separation, while if it is lower than 1, the separation becomes more difficult.

Everyone’s explanation of relative volatility is correct, but there seems to be an issue with the explanations regarding the effect of pressure on relative volatility. I’ve read materials that explain that as pressure decreases, relative volatility increases, making separation easier; the same principle applies in reverse. This indicates that when pressure changes, the effect on volatile components is greater than that on non-volatile components, resulting in a change in the ratio of the equilibrium ratio of volatile components to that of non-volatile components.

In my opinion, as pressure decreases, the relative volatility decreases. Because reducing pressure lowers the boiling point difference between two substances with different boiling points, it makes it more difficult to separate them, and thus it is harder to increase the purity of the product. I’m not sure if my view is incorrect; please give me some advice

The gas phase during the depressurization process can be treated as an ideal gas. If the liquid phase mixture can be considered an ideal solution, changes in system pressure do not affect the relative volatility. For a non-ideal solution, it depends on whether there is positive deviation or negative deviation. 　　If it is a positive deviation, reducing pressure will increase the relative volatility ； 　　If it is a negative deviation, reducing pressure will decrease the relative volatility.

The reason why vacuum distillation is used in distillation towers is that the boiling point of the substances to be distilled is relatively high; high temperatures make it difficult to carry out the operation. Vacuum can lower their boiling points as well as their relative volatility. The term “relative” implies the presence of two or more substances, while boiling point refers to just one substance. The principle of distillation in distillation towers relies on the differences in relative volatility among substances, or more specifically, on the differences in their boiling points. As for the effect of vacuum on relative volatility, what was said on floor 7 explains it very clearly