Why is the catalytic fractionation tower thinner at the top and thicker at the bottom?
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Why is the catalytic fractionation tower thinner at the top and thicker at the bottom?
Not all of them have a thicker base and thinner top; what you’re referring to is a reducing tower. The purpose of reduction is to increase the gas flow rate, mainly for the sake of even product distribution. This is just my personal opinion; please feel free to correct me if I’m wrong
The gas phase load decreases as one moves toward the top of the tower; therefore, the diameter is reduced in the upper part of the tower during design
Due to the high temperature at the bottom of the fractionation tower, the prolonged residence time of the material there facilitates cracking and condensation reactions, leading to coking at the bottom of the tower and severely affecting the long-term operation of the plant. To reduce the residence time of the material at the bottom of the tower, the diameter of the tower in the lower part of most distillation towers is smaller than that in the upper part.
Reply to 4# linda1027: I really have never seen the lower part of a distillation tower be smaller than its upper part. The coking you mentioned does exist, but the coking in a distillation tower is probably not mainly caused by chemical reactions
Reply to 5# Cracking Man: If you take the time to look at atmospheric distillation towers and vacuum distillation towers with larger processing capacities, it will change your opinion. Furthermore, distillation columns with a more concentrated fraction and relatively fewer light components are also designed to have a small top and a large bottom.
Generally, a fractional distillation tower does not require a reduction in diameter; if such a reduction is needed, it is due to imbalances in the load at the upper and lower sections. The case of coking-induced diameter reduction is different – increasing the linear velocity leads to a decline in separation efficiency, and it also does not help to reduce coking within the fractional distillation tower.
Reply to 6# qugd: I’m a bit confused. What was mentioned on the fourth floor refers to fractional distillation columns with a large top and a small bottom; such columns are commonly used for desulfurization and deodorization. But I’m not sure if pressure reduction columns also have a large top and a small bottom
Reply to 4# linda1027: The original poster was talking about a structure that is thin at the top and thick at the bottom; you called it a pressure reduction tower.
Logically, in a catalytic fractionation tower, the gas-liquid load should be greater the higher one is in the tower; however, if mid-stage reflux is present in the fractionation tower, it is possible to achieve a balance in the loads between the upper and lower parts of the tower. If there is a reduction in the diameter at the top of the tower, it is likely that the heat removal rate due to mid-section reflux is high; as a result, the gas-liquid load in the upper part of the tower decreases. It is necessary to reduce the diameter in order to increase the gas velocity in that upper section, thereby preventing the operation conditions there from exceeding the specified limits.
The distillation column here also has a reduced diameter. Our technicians told me that in the past the plant’s processing capacity was low, and reducing the diameter of the distillation column helped increase the gas flow rate, thereby improving the efficiency of distillation. However, now that the processing capacity has increased due to renovations, this reduced diameter actually hinders the efficiency of the distillation column. So whether to reduce the diameter or not should depend on the specific circumstances