The importance of tower pressure

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The importance of stable column pressure: The design and operation of columns are based on certain pressures; therefore, in a typical distillation column, it is essential to maintain a constant pressure. Fluctuations in column pressure have the following effects on column operation: a) They affect product quality and material balance. Changing the operating pressure alters the composition of the gas-liquid equilibrium on each tray in the column. As the pressure increases, the concentration of heavier components in the gas phase decreases, thereby increasing the concentration of 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 fraction increases, but their quantity decreases relatively ； The concentration of light components in the reactor broth increases, and the volume of the reactor broth increases as well. Similarly, as 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. b. Changing the 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. c. Changing the tower’s production capacity: An increase in pressure and an increase in the density of the components lead to an increased processing capacity of the tower. d. Fluctuations in tower pressure will cause confusion in the correlation between temperature and composition. In practice, temperature is often used as an indirect criterion for assessing product quality, but this is only valid when the tower pressure remains constant. When the tower pressure changes, the dew point and bubble point of the mixture change, which in turn alters the temperature distribution throughout the tower; as a result, 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, based on the analysis outlined above. The factors that affect changes in column pressure include the top temperature of the column, the bottom temperature of the column, feed composition, feed flow rate, reflux flow rate, refrigerant dosage, refrigerant pressure, etc. Additionally, instrument failures as well as freezing and blockage of equipment and pipelines can all cause changes in column 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 its normal level. When the tower pressure changes, it is necessary first to determine the cause of that change; rather than simply adjusting the pressure to restore it to normal, the underlying cause must be eliminated in order to prevent disruption to the normal operation of the tower. For example, when an increase in column pressure occurs due to insufficient cold dose or a malfunction in the top condenser equipment, and normal corrective measures are not taken but instead the amount of product taken from the top of the column is increased 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 top of the column. For example, when a low reactor temperature causes a drop in column pressure, if one tries to restore normal column pressure by reducing the amount of product taken from the top of the column without raising the reactor temperature, it will result in an increase in the amount of light components in the reactor liquid. When the reactor temperature rises suddenly, causing the column pressure to increase, it is important to restore the normal temperature in the reactor, rather than simply increasing the amount of cooling agent or the amount of product taken from the top of the column to lower the pressure; otherwise, flooding may occur, disrupting the normal operation of the column. When equipment issues affect the proper regulation of tower pressure, it is necessary to consider adjusting other operating parameters to maintain production; in severe cases, shutdown for maintenance is required.