In practical operation, how is the tower pressure of a distillation column controlled?

Tower pressure is one of the main control parameters in distillation operations. Excessive fluctuations in tower pressure can disrupt the material balance and gas-liquid equilibrium throughout the tower, resulting in product quality that does not meet standards; therefore, distillation operations must be kept stable. Methods for adjusting tower pressure include: (l) When the top condenser is a fractional condenser, the tower pressure is generally adjusted based on the amount of vapor product taken from the top of the tower. Remove the addition, and the tower pressure drops ； The withdrawal decreases, and the tower pressure rises. (2) If the steam consumption in the reboiler is too high and the reflux volume is large, the tower pressure increases; in such cases, the steam supply should be reduced appropriately to regulate the pressure at the bottom of the tower. The factors that affect the variation in column pressure are: ① top temperature of the column, ② bottom temperature of the column, ③ feed composition, ④ feed rate, ⑤ reflux rate, ⑥ refrigerant amount and refrigerant pressure. Furthermore, instrument failures and freezing of equipment and pipelines can also cause changes in tower pressure. In fact, the pressure at the bottom of the tower can also be considered as the tower pressure; in a distillation tower system, the tower pressure is used to regulate the three key temperatures, so it is very important. Generally, factors such as the feed rate, reflux rate, and intermediate draw rate all affect tower pressure, though to varying degrees

Adding to what was said on the 2nd floor: specific operating conditions also need to be taken into account. For example, if the operating condition is a vacuum environment, then a vacuum pumping system is required to control the vacuum level in the tower (usually at the top of the tower)； Operation at atmospheric pressure is generally simpler; if the system permits, a vent valve can be installed on the top condenser or at the top of the tower ； If it is operated under high pressure, it can be adjusted using the method on the 2nd floor; of course, N2 may sometimes be added appropriately. For reference only

The most important factors are undoubtedly the steam volume and reflux rate of the reboiler, as well as some other influencing factors mentioned on the second floor

Reply to 3# sunld76: The pressure relief tower works by creating a vacuum at the top of the tower! But attention should be paid to the pressure drop across the entire tower! ! In vacuum distillation columns, packed columns or mixed columns are generally used to reduce the overall pressure drop in the column and ensure effective vacuuming at the bottom of the column!

Thank you all for your enthusiastic responses; I’m really grateful!

It also depends on what substance you want to separate

In fact, there is a positive correlation between tower pressure and load. For example, in a vacuum distillation column, there is a pressure drop from the bottom to the top of the column; when the load is high, both the pressure at the top and at the bottom of the column increase, as does the pressure drop. However, the reflux temperature also has a significant impact on pressure control – increasing the reflux rate and lowering the reflux temperature can greatly reduce the pressure at the top of the column. Additionally, if there are changes in the composition of the feed material, the tower pressure in the distillation column will also change noticeably.

This is quite simple. Taking a constant-pressure column as an example, the column pressure refers to the pressure of the gas phase; it is naturally related to the steam pressure at the bottom of the column. Additionally, the feed rate and the flow rate also affect the column pressure. A common scenario is that there is a large amount of steam at the bottom of the column (where pressure and temperature don’t reflect this situation), along with a high reflux rate, which leads to an increase in column pressure, as the liquid phase evaporates in large quantities due to the control of the liquid level at the bottom of the column

The selection of the pressure control point should be in line with the process conditions, and it should be located at the “sensitive point” of the tower. To be more specific: if the composition of the material at the top or bottom of the tower is the process control target, then the pressure control point is generally located at the top of the tower ； If the extraction point in the middle is under my control, then the pressure control point is generally located near the extraction point, that is, at some point in the middle of the tower.