Pressurized distillation of acetaldehyde tower
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What is the main purpose of applying pressure to an acetaldehyde distillation tower for distillation? (The pressure is maintained at around 1.36 bar gauge.) If the goal is to save on the costs associated with the cooling medium used at the top of the tower (since the boiling point of acetaldehyde is relatively low, around 20 degrees), yet currently 12-bar cooling water is still being used there. My colleague said that applying pressure can help reduce the size of the equipment and thus lower the investment costs. Are there any other factors as well?
Applying pressure can indeed reduce the diameter of the tower, which is advantageous for large-scale production facilities. However, it introduces disadvantages such as the need for additional control devices and the transformation of equipment into pressure vessels. The consideration of using pressure in acetaldehyde distillation is mainly due to its low boiling point; distillation is only possible when both gas and liquid phases are present. Just like in air treatment systems where high distillation pressures are used, the use of cold water in the top condenser is also aimed at taking advantage of the low boiling point of acetaldehyde to ensure complete condensation.
Reply to 2# chinazwr: Thank you for your guidance. It’s actually just a small tower; the pressure increase is only around 1.3 kilograms, so it can still be considered a low-pressure device. I think the main issue lies in the boiling point of acetaldehyde – increasing the pressure raises its boiling point, which makes cooling easier. I haven’t been able to find out by how much the boiling point of acetaldehyde changes under increased pressure. Cooling at 12 degrees seems to be done to ensure complete condensation. I’m not sure if there are any other benefits as well
Reply to 1# fujiicy: Hello. I’m not sure what industry you work in. In terms of acetaldehyde purification, one thing to consider is the issue of impurities; increased pressure inevitably leads to a rise in temperature, and at higher temperatures, it’s possible for impurities to aggregate on their own. Another point to keep in mind is that acetaldehyde itself has a tendency to aggregate, and under conditions such as high temperatures or the presence of rust-like impurities, it is even more likely to undergo self-aggregation. Our usual acetaldehyde delivery pipelines are cooled by 7-degree chilled water; otherwise, the delivery pump is prone to cavitation. Furthermore, it is mentioned that increasing pressure helps to reduce the tower diameter, thereby lowering equipment costs. I don’t think so. When pressure is increased and the number of theoretical plates remains the same, a much higher reflux ratio is required to ensure the quality of the product at the top of the tower. This leads to increased consumption of steam and cooling water, and the long-term operating costs are quite high; it seems like it’s not worth it. If the number of theoretical plates is increased in order to reduce the reflux ratio, then the number of tower plates will have to increase significantly, and the capital investment for the entire tower cannot be reduced. Therefore, I believe that pressurized distillation of acetaldehyde is not feasible.
Acetaldehyde tends to react at high temperatures; since there is no absolute certainty, pressurization is not recommended.
Distillation is an operation related to “volatility,” which is why the concept of relative volatility exists. We generally do not advocate applying pressure, as pressure will definitely lead to a decrease in relative volatility, and a decrease in relative volatility will certainly result in a taller tower.