Effect of the reboiler at the bottom of the tower on the separation efficiency at the top of the extractive distillation column
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Does the power of the reboiler at the bottom of the tower affect the separation performance at the top of the extractive distillation column, and if so, to what extent? Friends who have done this kind of work, please help out. I specifically use ethylene glycol as the extractant to separate ethanol from water. When performing calculations, I use sensitivity analysis to set the power at the bottom of the tower between 40 and 90 watts; the lower the power, the better the separation effect. At 40 watts, the ethanol concentration at the top of the tower is 99% – is this normal?
The products at the bottom of the tower are ethylene glycol and water. The lower the power, the less vapor of ethylene glycol and water there is. The purity of ethanol at the top of the tower is high. The owner should pay attention to the ethanol content in the product from the tower bottom. Both its purity and the purity of ethanol at the top of the tower are important factors.
When simulating an extractive distillation column, it is sufficient for the reboiler at the bottom of the column to have an adequate load. During operation, if too much heat is applied to the bottom of the column, the water content at the top of the column will increase； If the heating amount is insufficient, the ethanol content in the bottom of the tower will increase. The factors that have a significant impact on an extraction distillation tower are the amount of extractant used and the extraction temperature, as well as the reflux rate at the top of the tower and the output volume from the top of the tower
Thank you for your reply; I’m very grateful
The feed for distillation operations can be supplied in five ways, such as from cold liquid to superheated steam; the corresponding feed heat condition parameter q value varies from high to low. As q decreases, the number of theoretical plates required to meet the specified separation requirements increases. Therefore, from the perspective of separation alone, the higher the preheating degree of the feed, the more unfavorable it is for distillation separation; thus, the temperature of the feed should be reduced as much as possible. On the other hand, the higher the degree of feed preheating, the less heat is required from the reboiler, and thus the reboiler load is also lower. Although for cold liquids, the total energy required for preheating the feed to a certain temperature before feeding it in is roughly equal to that needed when feeding it in without preheating, since the energy level required for preheating is lower than that required for reboiling – for example, waste heat steam or other high-temperature liquids can be used to preheat the waste liquid (in many distillation processes, the high-temperature liquid discharged from the bottom of the tower can be used to preheat the feed) – it is advantageous from an energy consumption perspective to preheat the feed. However, the feed temperature cannot be too high either; on one hand, it affects the separation capacity of the tower, and on the other hand, if the amount of vapor entering the tower is too low, the difference in gas load between the distillation section and the reboiler section becomes too large, which poses significant difficulties in the design of the distillation tower (the diameter of the distillation tower depends primarily on the gas load). It is common to preheat the feed liquid to around its bubble point. In addition to taking the above factors into consideration, feeding at the bubble point has another advantage: it helps to avoid fluctuations in the feed liquid temperature caused by seasonal changes, which could affect the stability of distillation tower operation.
Reply to 3# *aoqiang-39: May I ask what factors influence the extraction temperature? Can the increase in extraction temperature be used to determine the effectiveness of the extraction?