The issue of the distillation tower having a smaller diameter at the top and a larger diameter at the bottom
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In our design, the diameter of the distillation column above the feed section is 4.2 meters, while the diameter below the feed section is 5.8 meters. Why was it designed with a smaller diameter at the top and a larger diameter at the bottom? Please ask an expert to help answer this!
It’s designed to be smaller at the top and larger at the bottom, so it has a greater recombination weight.
Reducing the feed inlet to a smaller diameter within the tower, which is what we refer to as reducing the distillation section, serves to increase the resistance encountered by the light fractions during their upward movement, thereby raising the tower pressure. This allows the light fractions to be vaporized more quickly during the mass transfer process in the distillation section, reducing the loss of heavier components
The main goal is to maintain a certain linear velocity; since there are more gas components at the bottom, the relative linear velocity is higher, and this velocity will decrease slightly after the diameter is increased； And there is less gas in the upper part, so the relative linear speed is lower; as a result, the linear speed at the necking point increases relatively!
Increase resistance? Isn’t it generally the case that the pressure drop of the tower should be as low as possible?
It depends on the properties of the materials you provide to the design institute. In a distillation column with the same diameter, the pressure is too low, resulting in a slow upward gas flow and a lack of driving force for the evaporation process. The solution is to increase the temperature at the bottom of the column in order to raise the column pressure. However, increasing the bottom temperature increases the vaporization amount of the heavy components, leading to an increase in the light fraction and affecting the yield. Therefore, the only solution is to change the tower diameter and increase the pressure at the top of the tower (not by simply raising the pressure excessively), thereby increasing the linear velocity of the distillate stream in the distillation section so that the distillate can evaporate quickly, achieving the purpose of initial distillation, reducing the vaporization of heavier components, and improving the yield of the product.