Using temperature as an indirect quality indicator presupposes that the pressure inside the distillation column must remain constant. Although distillation columns are generally equipped with pressure control systems, slight pressure fluctuations are inevitable. These are negligible for distillation columns with low purity requirements, but in precision distillation and other applications with high control requirements, even small pressure changes can affect the relationship between temperature and composition, making it difficult to meet process quality requirements. Therefore, pressure fluctuations need to be compensated for, commonly using differential temperature control and dual differential temperature control.
① Differential Temperature Control: In precision distillation, differential temperature control can improve product quality. In distillation, the temperature of any tray is a function of composition and pressure; factors affecting temperature changes can be either composition or pressure. In general column operation, whether it's an atmospheric, vacuum, or pressurized column, the pressure fluctuates within a very small range, so there is a direct relationship between temperature and composition. However, in precision distillation, high product purity is required, and the boiling points of the products at the top and bottom of the column are not significantly different. In this case, the temperature change caused by pressure changes is much larger than the temperature change caused by composition changes. Therefore, even small pressure fluctuations have a significant impact and cannot be ignored. For example, in the separation of benzene, toluene, and xylene, a change in atmospheric pressure of 6.67 kPa results in a 2°C change in the boiling point of benzene, exceeding the quality specifications. Such pressure changes are entirely possible, disrupting the correlation between temperature and composition. Therefore, using temperature as the controlled variable in precision distillation often fails to achieve ideal control results; thus, the impact of small pressure fluctuations should be compensated for or eliminated.
During column pressure fluctuations, although the temperature on each plate will change to some extent, the temperature difference between two plates changes very little. For example, when the pressure changes from 1.176 MPa to 1.190 MPa, the temperature difference between plate 52 and plate 65 remains approximately 2.8°C. This maintains the correlation between temperature difference and composition. Therefore, temperature difference can be used as the controlled variable to ensure the purity of the final product meets requirements.
When selecting temperature difference signals, the detection points should be chosen as follows. For example, when the top distillate is the main product, one detection point should be placed at the top of the column (or slightly below), where temperature changes are smaller, and the other detection point should be placed near the sensitive plate, where composition and temperature changes are larger and more sensitive. Then, the temperature difference between these two measurement points is taken as the controlled variable. As long as the influence of pressure changes on the temperature at these two points is equal (or very similar), the influence of pressure fluctuations will almost cancel each other out.
In petrochemical production, temperature difference control has been successfully applied to precision distillation systems such as benzene-toluene and ethylene-ethane. To achieve good control effects, the temperature difference setpoint must be reasonable and not too large, and the operating conditions must be stable.
② Dual Temperature Difference Control: Although temperature difference control can overcome the influence of pressure fluctuations within the column on the quality of the top or bottom products, it also has a drawback: changes in feed flow rate lead to changes in the rising vapor flow rate, causing changes in the pressure drop between the trays. When the feed flow rate increases, the pressure drop between the trays increases, which in turn increases the temperature difference. The relationship between the temperature difference and the composition will change, so temperature difference control is not suitable at this time.
