What is the purpose of adding make-up water to the pre-distillation column in the methanol four-column process?
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As I understand it, water is added to dissolve ethanol in order to prevent high ethanol levels at the top of the tower. I’m not sure if it’s right
Bro, you might have misunderstood. Adding water is used for extraction; I guess what you mean is the extraction water, which is added to the reflux tank, the stock solution tank, or the feed pipeline. Its main purpose is to raise the boiling point of methanol, thereby helping to achieve complete separation of the light components.
To increase the boiling point difference between the light components and methanol, it is primarily to neutralize impurities with a boiling point similar to that of methanol, thereby forming an azeotrope. If the water content is controlled at too high a level, it has certain benefits for product quality; however, when this level is too high, the effects become less noticeable and energy consumption also increases. If the specific gravity is controlled at too low a level, impurities are not removed thoroughly and end up in the subsequent two towers, resulting in substandard products.
I looked up some more information. Another purpose is to form an azeotrope with the lighter components such as methanol and propanol; adding supplementary water at the top of the tower is intended to extract methanol from the azeotrope and increase the yield of methanol. This post was last edited by wdmemory on 2009-4-10 11:06.]
It is not to increase the boiling point of methanol, because in crude methanol, methanol forms azeotropes with certain alkanes (methanol-heptane, methanol-isooctane, methanol-nonane, methanol-decane), whose boiling points are 58.8, 58.3, 63.9, and 64.3 respectively; this makes separation difficult. By adding water, these compounds are extracted out, and their boiling points become 98.4, 109.8, 150.7, and 174 respectively, thus making separation much easier. Additionally, water can form azeotropes with certain alcohols, such as isobutanol and isopentanol; the formation of these azeotropes increases the boiling point difference between them and water, which facilitates their separation later on.
What was said upstairs is mostly correct; it should be the extracted water. Water is added to separate substances that are soluble in methanol but not soluble in water. Adding water at the same time can change the volatility of the components, which facilitates their separation and improves the quality of methanol. However, adding too much extraction water increases distillation energy consumption, and it also raises the power usage of the water pumps; therefore, the amount to be added must be determined by taking into account both product quality and energy consumption.
The purpose of adding water is to facilitate the extraction of light components, allowing them to be completely separated in the pre-tower and thus ensuring product quality.