Discussion on the relative volatility of multiple components in chemical distillation.
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For a mixed solution of A + B + water, where A has a boiling point of 80°C and B has a boiling point of 120°C. Assuming that the azeotrope temperature between A and water is 50°C, if an azeotrope-forming agent D is added, the azeotrope temperature between D and water is 45°C. In this case, should the relative volatility at the beginning of distillation be determined based on the system of A and water’s azeotrope mixture along with D and water’s azeotrope mixture, or based on the system of D and water’s azeotrope mixture along with A? The greater the relative volatility, the better the separation; that is, when the tower is not tall enough, less A is carried over along with the water. In the first system, there is no need to add this azeotrope; whereas in the second system, this azeotrope is effective.
Distillation involves the transfer of heat at the same temperature across a certain cross-section of the distillation tower. Mass and heat transfer of saturated liquids and saturated gases. There is condensation and evaporation, thereby achieving the purpose of separation. It is recommended to carefully understand the principles of distillation for reference.
The way you phrased this question is too complicated; I couldn’t understand it well at all!
After adding the azeotrope agent D, what actually occurs is the separation of the azeotrope of A and water from that of D and water; the greater the difference in boiling points between the two, the better the separation
Relative volatility is determined through regression for each pair of components; even when there are multiple components, the final calculation formula uses the volatility between two specific components. As for azeotropes, this occurs when the relative volatility between two components equals 1, at which point there is a corresponding composition and temperature. When multiple components form azeotropes with one another, such calculations become complicated, as it is difficult to distinguish between the actual data related to various binary and ternary azeotropes during the regression of binary interaction parameters, leading to significant errors in the calculations. In other words, the azeotrope temperature of A+water is 50 degrees, while that of D+water is 45 degrees. It is almost impossible to use D+water to separate out the D+water azeotrope free of A, in order to obtain A free of D and water. This is my personal understanding; for specific theoretical derivations, one can refer to the calculation methods used by NTRL for distillation, as well as those used in Aspen Plus for predicting azeotropes...
This is a separation of a multi-component system, and the addition of new components will affect the phase equilibrium; it is recommended to use Aspen for calculations.
Thank you, I think the same way. Mainly, I find it hard to understand why, in some patents, an azeotrope is used whose boiling point differs little from that of the original system.
That’s how I understand it too. I also became skeptical after looking at other people’s patents.
It is essentially a multi-component system: the boiling point of A is 110°C, and its azeotopic boiling point with water is 80°C. An azeotrope-forming agent D is introduced, and the azeotopic boiling point of D with water is 75°C. The difference between these two azeotrope temperatures is not significant, but the azeotrope temperature of D with water differs greatly from the boiling point of pure A. My question is whether it is not necessary to use many theoretical plates to separate D from water without including A in the separation process. By definition, relative volatility refers to one component in relation to another; here I am extending this concept to the relative volatility of mixed components with respect to each other.
The ease of this separation is determined by the relative volatility between the azeotrope of A and water and the azeotrope of D and water. You can consider the azeotrope of A and water as a virtual component, and the azeotrope of D and water as another virtual component; then compare the relative volatility of these two virtual components.