Method for automatically controlling cooling capacity of distillation tower top

Abstract

The invention discloses a method for automatically controlling cooling capacity of a distillation tower top. The method comprises the following steps: acquiring actual operation parameters of distillation tower top backflow temperatures Te, tower top pressure P and tower top backflow liquid constitution X and the like, and confirming a tower top backflow capacity Re and a tower top backflow liquid temperature Td under reference working conditions; calculating according to acquired parameters so as to obtain backflow liquid gasification heat Hv and a tower top backflow liquid heat capacity Cp; according to distillation principles, calculating a backflow quantity Re' for ensuring separation quality of the tower top at different backflow liquid temperatures Te, and establishing mathematic models with the combination of theoretical derivation; and finally, configuring the mathematic models on a DCS (distributed control system) background, and realizing the function of calculating and controlling the backflow quantity Re' through the mathematic models. By optimizing control of cooling capacity input of a distillation tower, the backflow quantity Re' of the tower top is automatically adjusted according to variations of the backflow temperatures, and thus effects of stabilizing quality of tower top products, saving energy and reducing consumption are achieved.

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...

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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

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