Method for automatic control of cooling capacity at the top of rectification column

Abstract

The invention discloses a method for automatic control of cooling capacity at the top of a rectification tower. The actual operating parameters such as the top reflux temperature Te, the top pressure P, and the composition X of the top reflux liquid of the rectification tower are collected, and the top reflux flow rate Re and the temperature Td of the top reflux liquid under the reference working conditions are determined. Then, the heat of vaporization Hv of the reflux liquid and the heat capacity Cp of the tower top reflux liquid are calculated according to the collected parameters. At the same time, according to the principle of rectification, calculate the reflux Re' which guarantees the separation quality at the top of the tower under different reflux liquid temperatures Te, and establish a mathematical model in combination with theoretical derivation. Finally, configure the mathematical model on the background of the distributed control system DCS to realize the function of calculating and controlling the return flow Re' through the mathematical model. The present invention optimizes the control of the cooling capacity input of the rectification tower, and automatically adjusts the amount of reflux liquid Re' at the top of the tower according to the change of the reflux temperature Te, so as to achieve the effects of stabilizing the product quality at the top of the tower, saving energy and reducing consumption.

Description

technical field [0001] The invention relates to a method for automatic control of cooling capacity at the top of a rectification tower. Background technique [0002] The energy consumption of the rectification tower accounts for a large proportion in chemical equipment, especially for the rectification tower with high-grade steam as the heat source, the energy consumption can even account for more than 50% of the entire process or device, so it is of great significance to optimize the energy control of the rectification tower . [0003] At present, the top of most rectification towers is adjusted by a fixed reflux flow rate, and the temperature of the reflux liquid is affected by the ambient temperature, and the range of variation ranges from 3 to 7 °C, and as much as 20 to 80 °C. According to operating experience or theoretical analysis, when the reflux flow rate remains constant and the reflux temperature changes, the energy carried by the reflux stream will change. The t...

AI Technical Summary

Problems solved by technology

According to operating experience or theoretical analysis, when the reflux flow rate remains constant and the reflux temperature changes, the energy carried by the reflux stream will change. The top of the distillation column is essentially a material and energy balance process between the ascending gas phase and the reflux liquid phase. , when the energy carried by the liquid phase changes, the gas-liquid separation effect of the top tray will also change. The high reflux temperature means that the reflux cooling capacity is small. It can be understood that the actual reflux is smaller than the design working condition and the separation effect is not good. , the product quality cannot meet the production requirements; the low reflux temperature means that the reflux cooling capacity is large, which can be understood as the actual reflux ratio is larger than the design working condition, the separation effect of light components is better, and the gas phase is cooled more, which means that the output At the same time, the thermal power of the tower will also increase to achieve high separation requirements, that is, the unit consumption of the product will increase

Therefore, it is necessary to ensure low energy consumption and meet product quality requirements at the same time. When the quality of the top product is controlled by the card edge, the change of the return liquid temperature will cause fluctuations in product quality, which may lead to unqualified product quality.

The external interference entering the top of the tower cannot be minimized by the traditional reflux setting adjustment

[0004] In addition, some rectification towers will adopt the scheme of cascade adjustment of the reflux tank liquid level to adjust the reflux flow rate. This scheme can ensure the stability of the reflux tank liquid level, but when the external temperature changes, the energy carried by the reflux stream will also change, which will affect the rectification. The gas-liquid balance of the tower; or some towers adopt the scheme of cascading the temperature of the sensitive plate to adjust the reflux flow. On the one hand, there may be a large lag in the temperature response of the sensitive plate, or the temperature gradient distribution is not obvious. From the perspective of self-control, it is controlled If the pure lag time of the controlled variable is too long or the gain of the manipulated variable to the controlled variable is too small, it is difficult to adjust through the conventional PID loop. On the other hand, after the reflux temperature changes, the temperature change in the tower must be gradually transmitted to the sensitive The return flow can only be adjusted after the sensitive plate responds to changes, and there is also an inevitable process lag. The adaptive control of the return flow is to adjust the return flow in advance before entering the tower. Similar to the idea of ​​feedforward control, the control is more timely and effective. ; or some towers adjust the reflux valve by controlling the amount of liquid under the first tray. In this control method, it is difficult to determine the amount of liquid under the tray under different working conditions, and the operator cannot change according to the working conditions. The set value of the liquid volume under the given tray is not suitable for the operator to control

Images