Temperature is the most commonly used indirect quality indicator. For a binary distillation column, at a constant pressure, there is a separate functional relationship between boiling point and product composition. Therefore, if the pressure is constant, the tray temperature can reflect the product composition. However, the situation is more complex for multi-component distillation columns. In oil refining and petrochemical production, many products are composed of homologues of hydrocarbons; at a constant pressure and a constant temperature, compositional errors can be ignored. In other cases, temperature can also reflect compositional changes to some extent. From the above analysis, it is clear that in temperature-based quality control schemes, pressure cannot fluctuate drastically. Except for atmospheric distillation columns, the temperature control system is always linked to the pressure control system.
When using temperature as the controlled variable, the choice of which temperature point to use should be based on the actual situation. The main options are as follows:
① Temperature control at the top (or bottom) of the column: Generally, if it is desired to maintain the quality requirements of the top product, i.e., when the main product distills from the top, the top temperature should be chosen as the controlled variable for better results. Similarly, to maintain the quality requirements of the bottom product... Therefore, the bottom temperature should be used as the controlled variable. To ensure the quality of the other product remains within a certain range, the column operation must have a certain margin. For example, if the main product distills from the top and the manipulated variable is the reflux rate, the reboiler heating capacity must have a certain margin to ensure that the bottom product's specifications remain within a certain range under any possible disturbance conditions.
Using the temperature at the top (or bottom) of the column as an indirect quality indicator seems to best reflect the product's condition, but this is not entirely true. When separating a purer product, the temperature difference between the plates near the top is very small, requiring extremely high precision and sensitivity from the temperature sensing device, which is practically difficult to meet. Furthermore, the presence of trace impurities (such as a lighter component) can cause significant changes in boiling point; fluctuations in column pressure can also cause significant changes in boiling point, and these disturbances are difficult to avoid. Therefore, except for fractionation of petroleum products, where fractions are cut according to boiling point ranges, distillation columns aiming to obtain a purer component typically do not place the monitoring point at the top or bottom of the column.
② Sensitive Plate Temperature Control: A sensitive plate is defined as the plate whose temperature changes simultaneously with the composition of each plate in the tower when the tower is disturbed (or subjected to control). When a new steady state is reached, the plate with the largest temperature change is called the sensitive plate. Because the temperature change of the sensitive plate under disturbance is large, the requirements for the temperature detection device are not as high, which also helps improve control accuracy.
The location of the sensitive plate can be determined through plate-by-plate calculations or computer simulations, comparing the temperature distribution curves of each plate under different conditions. However, since the efficiency of the tower tray is not easy to estimate accurately, it needs to be determined in practice. Typically, the approximate location of the sensitive plate is first determined based on calculations, then several detection points are set up near it, and the most suitable measurement point is selected as the sensitive plate during operation.
③ Intermediate Temperature Control: Using the temperature of the tray slightly above or below the feed tray, or the temperature of the feed tray itself, as the controlled variable, this type of control, with the temperature detection point selected in the middle, is usually called intermediate temperature control. While this control scheme has been successful on some distillation columns, when separation requirements are high or when the feed concentration ZF varies greatly, medium-temperature control cannot guarantee that the composition at the top or bottom of the column meets the requirements.
