Special thread on basic knowledge for the Chemistry and Engineering field – Discussions on key examination points in Chemistry and Engineering + Distillation + 05 + Summary of error-prone and easily confused concepts

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Summary of error-prone and easily confused concepts: 1. For a certain binary ideal solution with a composition of x = 0.6 (mole fraction), the boiling point is t1; the composition of the gas phase in equilibrium with it is y = 0.7, and the dew point is t2. Then, the relationship between t1 and t2 is that t1 = t2. Explanation: Since x=0.6 and y=0.7, these are of course the mole fractions of the light components. For a balanced two-phase system, the bubble point temperature of the liquid phase is equal to the dew point temperature of the gas phase. On the phase diagram, it is a horizontal line. 2. In simple distillation calculations, when the initial composition, mass flow rate, and final composition of the feed liquid are known and the remaining amount in the bottom liquid is to be determined, a differential equation can be formulated for solution. The conditions that must be satisfied during this solution process are: ① The gas and liquid phases are in equilibrium at any given moment. ②The relative volatility is considered constant. It is not an isothermal operation; in a simple distillation process, the temperature gradually increases, and the amount of vaporization is related to the heat supplied. 3. In distillation theory, the necessary and sufficient condition for the introduction of the concept of theoretical plates is that the two phases leaving the plate reach equilibrium. The concept of theoretical plates has nothing to do with actual trays. 4. In the calculations for binary continuous distillation, changes in the feed heat condition q cause changes in the lines on the x-y diagram: the operating line and the q-line. 5. The minimum reflux ratio in distillation is the reflux ratio that would require an infinite number of trays to meet the separation requirements. 6. For a certain degree of separation, the minimum number of theoretical plates required for a distillation column is: full reflux. 7. The reboiler and the degasifier can provide a separation effect equivalent to that of one theoretical plate. 8. A certain binary ideal solution is separated by distillation; the composition of the product is XD, xW. When the feed composition isxF1, the corresponding minimum reflux ratio is Rm1 ； When the feed composition is xF2, the corresponding minimum reflux ratio is Rm2. If xF1 < xF2 and the feed thermal state is the same, then Rm1 > Rm2. Explanation: I still haven’t figured out what the reason is. I hope those knowledgeable among you will use their brains to come up with an answer. 9. In a vacuum distillation column, the changes in vacuum level and corresponding dew point from top to bottom during pressure reduction are as follows: the vacuum level decreases while the dew point increases. The vacuum level at the bottom of the tower is the lowest. The component with the highest weight has the highest dew point. 10. In a normally operating two-component distillation column, changes in the feed material can result in an excessive level of heavy components at the top of the column; measures that can be considered in such cases include increasing the reflux ratio and lowering the feed temperature. However, reducing the bottom temperature of the tower has no direct effect on the product at the top of the tower; if it is possible to lower the feed inlet, considering increasing the number of trays in the distillation section would be an option. Such questions are difficult to answer; each measure may have an impact on other aspects, so a comprehensive consideration is necessary. 11. The tray spacing refers to the distance between any two adjacent trays within the tower; however, there is no concept of distance between theoretical trays.