Daily question in the tower equipment technology exchange section – participate in the prize-winning activity～(02.17)

A decrease in column pressure leads to a series of adjustments to the process parameters, particularly changes in temperature. It is necessary to promptly adjust parameters such as system temperature and reflux rate in order to maintain the temperatures at the bottom and top of the distillation column, ensuring that the composition of the materials as well as the liquid levels remain within normal ranges. Sampling and analysis should be carried out promptly for verification. There are many reasons for the decrease in tower pressure, such as adjustments to the steam supply, manual control of pressure, excessively low condensation temperature at the top of the tower, and a reduction in the system’s feed rate. However, I believe it is due to a combination of factors, which should be considered as a whole.

Tower pressure is directly proportional to temperature; changes in pressure reflect changes in temperature. Even when the operating pressure is reduced to 0.1 MPa, the product still meets the required standards, indicating that the temperature remains within the normal range. Reducing the tower pressure in this way helps save energy.

Fluctuations in column pressure have the following effects on column operation: (1) Impact on product quality and material balance. Changing the operating pressure alters the composition of the gas-liquid equilibrium on each tray of the column. As the pressure increases, the heavier components in the gas phase decrease, thereby increasing the concentration of the lighter components in the gas phase ； The increase in the content of light components in the liquid phase also alters the weight ratio between the gas and liquid phases, resulting in an increase in the liquid phase volume and a decrease in the gas phase volume. The overall result is: the concentration of light components in the tower top distillate increases, but their quantity decreases relatively ； The concentration of light components in the reactor liquid increases, and the volume of the reactor liquid increases as well. Similarly, as the pressure decreases, the amount of distillate at the top of the tower increases, while the concentration of light components decreases ； The amount of liquid in the kettle decreases, and the concentration of light components drops. Under normal operation, a constant pressure should be maintained; however, if abnormal operations lead to an increase in the concentration of heavier components in the product at the top of the tower, it is possible to raise the operating pressure appropriately to ensure that the product quality remains satisfactory, but this will result in an increased loss of lighter components in the liquid phase. (2) Relative volatility between components: As pressure increases, the relative volatility between components decreases, resulting in a reduced separation efficiency; the opposite is true as well. (3) Changing the tower’s production capacity: An increase in pressure and an increase in the specific gravity of the components lead to an increased processing capacity of the tower. (4) Fluctuations in tower pressure will cause confusion in the corresponding relationship between temperature and composition. In our operations, we often use temperature as an indirect criterion for measuring product quality, but this is only correct when the tower pressure remains constant. When the column pressure changes, the dew point and bubble point of the mixture change, which leads to a change in the temperature distribution throughout the column; consequently, the relationship between temperature and product quality also changes. Based on the above analysis, changing the operating pressure will affect the performance of the entire tower; therefore, a constant pressure should be maintained during normal operation. Only when the normal operation of the tower is disrupted can the pressure be adjusted appropriately, within the limits permitted by the process parameters, in accordance with the analysis outlined above.

Thank you all! The impact of reduced tower pressure on distillation columns is now basically understood. I’m very curious to know the main reasons for the decrease in tower pressure?

The number of plates can be regarded as a reference value for separation efficiency. Similarly, due to the reduced relative volatility, the separation efficiency is low; or, with the same number of theoretical plates, the separation cannot achieve the desired purity, that is, the separation efficiency is low. If there is a problem inside a tower, could an increased reflux rate cause the reflux to accumulate at the top of the tower, thereby increasing the tower pressure? At that time, there would be an excess of light components at the top of the tower, which in turn leads to a decrease in the temperature at that location. This post was last edited by Chuangxin Filler on 2009-2-17 at 16:10.]

As the column pressure decreases, the load on the column should also decrease accordingly. Moreover, the energy consumption of the column will clearly reduce as well. As for the reason, could it be that the moisture content in the air is too high, entering the distillation column and causing partial freezing? Hehe:)

The so-called tower pressure fluctuation refers to reducing the tower pressure as much as possible under feasible conditions, which is beneficial for energy savings. Specifically, a decrease in tower pressure can reduce energy consumption from two aspects. a) Reducing the operating pressure increases the relative volatility between the components, making their separation easier; this in turn reduces the heating required by the reboiler and saves energy. b) Reducing the operating pressure lowers the vapor-liquid equilibrium temperature of the entire distillation system, increasing the heat transfer temperature difference across the reboiler; as a result, the reboiler’s heating capacity increases while using the same amount of heat source. c) Impact on product quality and material balance d) Change in the tower’s production capacity ② Conditions for fluctuations in tower pressure: a. The selection of quality indicators must meet the requirements associated with fluctuations in tower pressure. b. The limit to the reduction in tower pressure is determined by the maximum cooling capacity of the condenser. c. The tower pressure should fluctuate without experiencing sudden changes. The factors that affect changes in column pressure include the following: top temperature of the column ； Reactor bottom temperature ； Feed composition ； Feed flow rate ； Backflow volume ； Cold dose ； Changes in refrigerant pressure, as well as instrument failures and freezing blockages in equipment and pipelines, can all cause variations in tower pressure. In production, when changes in the above factors cause variations in tower pressure, the control mechanism for regulating tower pressure will act automatically to restore the tower pressure to normal levels. When the tower pressure changes, it is necessary first to determine the cause of the change; rather than simply adjusting the pressure to restore it to normal, the underlying cause must be eliminated in order to avoid disrupting the normal operation of the tower. For example, when the amount of cold dose is insufficient or there are malfunctions in the equipment responsible for condensation at the tower top, which leads to an increase in tower pressure, if normal corrective measures are not taken and instead the amount of product taken from the top of the tower is increased merely to restore normal pressure, it is possible for the heavier components to reach the distillation section, resulting in substandard quality of the product obtained from the tower top. For example, when a low reactor temperature causes a drop in tower pressure, if the reactor temperature is not adjusted and normal tower pressure is restored merely by reducing the amount taken from the top of the tower, it will result in a significant increase in the light components in the reactor liquid. When the reactor temperature rises suddenly, causing an increase in tower pressure, it is important to restore the normal temperature at the bottom of the tower, rather than relying solely on increasing the amount of cooling agent and raising the output from the top of the tower to lower the pressure ； Otherwise, flooding is likely to occur, disrupting the normal operation of the tower. When equipment issues affect the proper regulation of tower pressure, it is necessary to consider adjusting other operating parameters in order to maintain production; in severe cases, shutdown for maintenance is required. Last edited by 263525689 on 2009-2-18 18:21]

The lower the operating pressure of the tower, the lower the energy consumption. However, the selection of operating pressure is related to the properties of the substance to be separated. It is necessary to consult phase equilibrium and gas-liquid equilibrium data to determine the optimal operating conditions for the tower. At the same time, if the pressure drop across the tower is too high, it indicates a problem with the internal components of the tower; generally, the pressure drop per tray should not exceed 3 KPA

The most important aspect of operating a distillation tower is controlling the tower pressure; only when this pressure is within a certain range can the operating parameters be determined. If the tower pressure drops, the temperature must be adjusted promptly, otherwise it is easy to cause overflow in the tower, resulting in substandard product quality. As the original poster mentioned, if all other conditions remain unchanged, it simply indicates that the evaporation rate and condensation rate are not balanced, which leads to a decrease in tower pressure

The so-called tower pressure fluctuation refers to reducing the tower pressure as much as possible under feasible conditions, which is beneficial for energy savings. Specifically, a decrease in tower pressure can reduce energy consumption from two aspects. a) Reducing the operating pressure increases the relative volatility between the components, making their separation easier; this in turn reduces the heating required by the reboiler and saves energy. b) Reducing the operating pressure lowers the vapor-liquid equilibrium temperature of the entire distillation system, increasing the heat transfer temperature difference across the reboiler; as a result, the reboiler’s heating capacity increases while using the same amount of heat source. c) Impact on product quality and material balance d) Change in the tower’s production capacity ② Conditions for fluctuations in tower pressure: a. The selection of quality indicators must meet the requirements associated with fluctuations in tower pressure. b. The limit to the reduction in tower pressure is determined by the maximum cooling capacity of the condenser. c. The tower pressure should fluctuate without experiencing sudden changes. The factors that affect changes in column pressure include the following: top temperature of the column ； Reactor bottom temperature ； Feed composition ； Feed flow rate ； Backflow volume ； Cold dose ； Changes in refrigerant pressure, as well as instrument failures and blockages in equipment and pipelines, can all cause variations in tower pressure. Based on what you’ve described, I suggest you conduct a maintenance check on the system to see if it is in good condition.