Questions about vacuum distillation
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The last edit to this post was made by xxm86106 on 2012-4-19 at 18:23. As the title indicates, the operation involved distillation using a pressure-reducing (packed tower) approach, with a vacuum level maintained at 12 kPa. To improve the purity of the product at the top of the tower, the following measures were taken: 1. The vacuum level was slightly reduced to 20 kPa, while the heating capacity of the bottom of the tower and the cooling flow rate at the top of the tower were increased. As a result of these actions, the pressure difference across the tower was reduced by nearly half, and the purity of the product increased by 2 percentage points； 2. Increase the reflux ratio appropriately, raise the feed temperature, and also increase the heating amount at the bottom of the tower accordingly. Ultimately, the purity increased by nearly 10 points compared to the original purity. Questions: 1. What is the theoretical basis for the idea that increasing the pressure appropriately can improve separation efficiency when the tower is operating at near-saturation load? Is the increase in product purity due to a decrease in tower pressure difference? 2. Why does reducing the vacuum level also lead to a decrease in tower pressure drop? ? ? 3. Increasing the reflux ratio appropriately can undoubtedly improve product purity, but isn’t an increase in feed temperature usually employed to enhance the separation efficiency in the lower section of the tower? Please everyone **explain it**
2. The vacuum level is reduced, the amount of gas being pumped out decreases, and as a result, the amount of volatilization also decreases; hence, the pressure difference is reduced!
1. What is the theoretical basis for the fact that increasing the pressure appropriately can improve separation efficiency when the tower is operating at near-saturation load? Generally, whether pressurization is beneficial for separation depends on how the relative volatility of the system changes with pressure. If the relative volatility increases with increasing pressure, then pressurization is beneficial ； Conversely, reducing pressure is beneficial. 2. Why does reducing the vacuum level also lead to a decrease in column pressure drop? This is generally the case: when pressure increases, the density of the vapor phase rises. With the same flow rate (molar or mass), the volumetric flow rate decreases, the flow velocity drops, and the F factor becomes smaller; as a result, the column pressure drop also decreases. 3. Increasing the reflux ratio appropriately can undoubtedly improve product purity, but isn’t an increase in feed temperature usually employed to enhance the separation efficiency in the lower section of the tower? The effect of feed temperature on the top product may be that increasing the temperature results in a feed with a thermal state that is more similar to that of a bubble-point feed. For a distillation column, it is best to apply cooling at the top of the column and heating at the bottom; feed at the bubble point represents the most favorable heat condition for separation.
From what you’ve said, I understand it as follows: 1. The vacuum level increased from 12 to 20; this likely means that the system pressure decreased, the amount of evaporation increased, and the tower pressure rose. However, the increase in the cooling flow rate at the top of the tower is the main reason for the decrease in tower pressure. It’s possible that the cooling capacity of your system was insufficient, which caused the tower pressure to remain high. 2. Increasing the reflux rate improved the product quality, as the reflux ratio specified in the original design was too low, resulting in poor product quality. An increased reflux rate means more cooling capacity at the top of the tower; to maintain production levels, it’s necessary to increase the heat supply. Otherwise, the tower may not be able to produce any output. Increasing the feed temperature is equivalent to adding more heat. In other words, by increasing the reflux rate, if the heating capacity at the bottom of the tower or the feed temperature isn’t increased as well, the tower’s production capacity will decrease.
1. What is the theoretical basis for the fact that increasing the pressure appropriately can improve separation efficiency when the tower is operating at near-saturation load? Is the increase in product purity due to a decrease in tower pressure difference? Increased pressure reduced the tower’s space velocity, resulting in a longer residence time inside the tower, thereby improving product quality ； It can also be put this way: since the pressure difference is reduced, this leads to a lower air velocity. 2. Why does reducing the vacuum level also result in a decrease in the tower pressure drop? ? ? The vacuum level was reduced, the amount of gas being pumped out decreased, and as a result the amount of volatilization also decreased; therefore, the pressure difference diminished! (Copied from Floor 2, haha) 3. Increasing the reflux ratio appropriately can improve product purity, which is beyond doubt; but isn’t increasing the feed temperature usually done to enhance the separation efficiency in the lower section of the tower? Raising the feed temperature increases the heat at the bottom of the tower; as a result, the heat load **increases**, which enhances the degree of gasification and thus improves the product quality
The topic of discussion raised by the original poster is quite good. Under normal circumstances, increasing the vacuum level can boost the gas flow in the distillation section, thereby raising the liquid level in the reflux tank; this may lead to issues with the quality of the product. On the other hand, increasing the reflux amount can improve the quality of the product. Increasing the feed temperature increases the volume of rising gas, thereby reducing the load on the bottom of the tower. I feel like it’s pretty much the same as what was said upstairs................. Hehe