A process dynamic control method based on a dividing wall distillation tower for separating alkanes

Abstract

The present invention provides a process dynamic control method based on a distillation tower for separating alkanes. This process is separated by a distillation tower, and the control method includes feed flow control, tower top pressure control, liquid level control, coupled flow control, composition control and temperature-composition cascade control loop. This process can not only make the product separation complete, but also under the disturbance of feed flow and feed composition of ±20%, this control method has a good control effect, and the product purity can reach the original set value.

Description

Technical Field

[0001] The invention belongs to the field of chemical distillation process and automation control, and relates to a process dynamic control method based on a partition wall distillation tower for separating alkanes. Background Art

[0002] The production of most chemicals in the world today requires reaction and separation processes, and distillation technology is one of the most widely used separation technologies in the chemical and petrochemical industries. However, despite the simplicity and wide application of the distillation process, it accounts for more than half of the plant's operating costs, and energy consumption in the chemical industry accounts for as much as 40%. With the increasing reduction of non-renewable energy sources, in order to save energy consumption, in-depth exploration of the energy-saving potential of the distillation process and improving its thermodynamic efficiency have become one of the urgent issues that need to be solved in the modern process industry. As one of the most important ways to intensify the process, the distillation tower has been widely studied by scholars at home and abroad.

[0003] The next-door distillation tower is a tower in which the pre-separation tower and the main tower are placed in the same tower shell and separated by a middle partition. The two towers are connected by the vapor-liquid phase streams at both ends of the partition to achieve complete heat coupling. The pre-separation tower can flexibly adjust the content of the intermediate components in the top and bottom streams, so that the stream composition matches the composition on the main tower feed plate, which avoids the energy loss caused by backmixing to a certain extent and further improves the thermodynamic efficiency of the separation process. Studies have shown that for the separation of different systems, the next-door distillation tower can save 10% to 60% of energy consumption and 10% to 50% of equipment investment. However, the next-door tower has a high degree of coupling of its coupled streams, which makes the interaction between variables more complicated. The control of the next-door distillation tower has always been an important problem that prevents the rapid development of industry. Therefore, many scholars at home and abroad have conducted many studies on the controllability and operability of the next-door tower to solve the practical problems of its industrial application.

[0004] Kiss and Rewagad discussed the control problem of the dividing-wall distillation column of the BTX system. They adopted several traditional control structures based on PID control loops (DB / LSV, DV / LSB, LB / DSV, LV / DSB) as the control basis. By using the liquid phase distribution ratio as an additional manipulated variable to control the concentration of the heavy component at the top of the pre-dividing tower, the energy demand was implicitly minimized (Kiss, AA and RRR Rewagad, Energy efficient control of a BTX dividing-wall column. Computers & Chemical Engineering, 2011. 35(12): p. 2896-2904.). Ling et al. controlled the process with pure component control and verified that the energy consumption can be reduced by manipulating the liquid phase distribution ratio to control the concentration of heavy components at the top of the pre-separation tower, and finally achieve the purpose of controlling the purity of the product (Ling, H. and WLLuyben, New Control Structure for Divided-Wall Columns. Industrial & Engineering Chemistry Research, 2009. 48 (13): p. 6034-6049.). The adjustment of the gas phase distribution ratio also plays an important role in saving energy. LI et al. studied a novel gas phase distribution ratio device and applied this control loop to the diverter tower, carried out numerical simulation and experimental testing, and achieved good energy-saving effect (Li, C., et al., Experimental study and CFD numerical simulation of an innovative vapor splitter in dividing wall column. AIChE Journal, 2020. 66 (8).).

[0005] The above research shows that by controlling the gas-liquid distribution ratio between coupled streams, the control effect of the process can be effectively improved, and it has a good energy-saving effect. In this design, by combining the control loop between coupled streams, the composition control of the main tower product, and the temperature-composition cascade control loop, when there is a large disturbance in the feed flow rate or feed composition, its control effect can still make the product return to the original set value in a shorter time and with a smaller overshoot, and there is no deviation in product purity. Summary of the invention

[0006] The problem to be solved by the present invention is the difficulty in controlling the distillation tower, the poor stability and the requirements for product quality. The purpose is to provide a process dynamic control method based on the distillation tower for separating alkanes. After a large disturbance (±20%) in the feed flow rate or feed composition, the product purity returns to the original set value in a short time and with a small overshoot. In addition, the control structure of the present invention is simple, easy to operate, and has great application prospects.

[0007] In order to achieve the above-mentioned invention object, the present invention adopts the following technical methods:

[0008] The process dynamic control method based on the separating wall distillation tower for alkane separation mainly includes the following control loops: feed flow control, tower top pressure control, liquid level control, coupled flow control, composition control and temperature-composition cascade control loop. The control method of each control loop is as follows:

[0009] (1) a feed flow control loop, including a flow controller for controlling the feed flow into the tower;

[0010] (2) a tower top pressure control loop, including using a flow controller to control the flow rate from the top of the pre-separation tower to the main tower, and using a pressure controller to control the heat load of the condenser at the top of the main tower, wherein the pressure controller controls the pressure at the top of the main tower and automatically adjusts the heat load of the condenser at the top of the main tower according to the pressure;

[0011] (3) a liquid level control circuit, including using a flow controller to control the flow rate from the bottom of the pre-separation tower to the main tower, using a flow controller to control the extraction rate from the top of the main tower, and using a flow controller to control the extraction rate from the bottom of the main tower;

[0012] (4) coupling control loops between the streams, including using a flow controller to control the liquid phase flow from the main tower to the pre-separation tower, and using a temperature controller to control the temperature inside the pre-separation tower, wherein the temperature controller automatically adjusts the gas phase flow entering the pre-separation tower according to the temperature of the pre-separation tower sensitive plate;

[0013] (5) a composition control loop, including using a flow controller to control the flow of the side product and using a composition controller to control the heat load of the main tower reboiler, wherein the composition controller controls the purity of the bottom product to respond to the heat load of the main tower reboiler;

[0014] (6) A temperature-composition cascade control loop, including controlling the reflux flow rate at the top of the main tower by a temperature controller and controlling a set temperature value by a composition controller, wherein the temperature controller automatically adjusts the reflux flow rate at the top of the tower according to the temperature of the main tower sensitive plate, and the composition controller adjusts the purity of the product at the top of the tower according to the set temperature value of the temperature controller.

[0015] In the above technical method, the feed flow control loop controls the feed flow entering the tower by manipulating the opening of the valve.

[0016] In the above technical method, the pressure control loop of the pre-separation tower and the main tower top controls the pressure of the pre-separation tower top by manipulating the flow from the top of the pre-separation tower to the main tower, and controls the pressure of the main tower top by manipulating the heat load of the main tower top condenser.

[0017] In the above technical method, the liquid level of the reflux tank at the bottom of the pre-separation tower and the top of the main tower, as well as the bottom liquid level control loop, controls the bottom liquid level of the pre-separation tower by manipulating the flow from the bottom of the pre-separation tower to the main tower, controls the top reflux tank liquid level of the main tower by manipulating the product output of the top of the main tower, and controls the bottom liquid level of the main tower by manipulating the output of the bottom of the main tower. The basic control loop of the process is controlled to ensure the quality and material conservation in the tower.

[0018] In the above technical method, the control loop between the flow streams is coupled to control the concentration of the heavy component from the top of the pre-separation tower to the main tower by manipulating the flow from the top of the pre-separation tower to the main tower, and the temperature inside the pre-separation tower is controlled by manipulating the flow from the main tower to the pre-separation tower.

[0019] In the above technical method, the control loop composed of the side product extraction of the main tower and the bottom product extraction controls the purity of the side product by manipulating the side product extraction amount, and controls the purity of the bottom product by manipulating the heat load of the main tower reboiler.

[0020] In the above technical method, the temperature of the product extracted from the top of the main tower forms a cascade control loop, which controls the temperature sensitive plate in the main tower by manipulating the reflux flow, and controls the purity of the top product.

[0021] In the above technical method, PID control signals and controllers are used in the control loop.

[0022] In the above technical method, the position of the sensitive plate in the temperature control loop is determined by the sensitivity criterion method. This method requires a certain manipulated variable (such as reflux) to change very slightly (such as 0.1% of the set value), examine the change in temperature of each tower plate and find the tower plate with the largest temperature change. For other manipulated variables (such as heat input of the reboiler), the above process can be repeated. The change in tower plate temperature divided by the change in manipulated variable is the steady-state gain between the tower plate temperature and each manipulated variable. The tower plate with the largest temperature change is the most "sensitive", so this tower plate is selected for control. The larger the temperature gain of a tower plate, the more effective the temperature of the tower plate can be controlled by the corresponding manipulated variable. Conversely, a smaller gain means that valve regulation is prone to saturation and the operating area will be limited.

[0023] In the above technical method, the tuning parameters of the controllers in the temperature and composition control loops are performed using the Tyreus-Luyben method.

[0024] In the above technical method, after the feed flow rate or feed composition changes, the various control loops work together to make the product purity return to the original set value in a shorter time and with a smaller overshoot, and there is no deviation from the set value. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention relates to a process flow diagram of a partition-wall distillation tower for separating alkanes. It is divided into a pre-separation tower and a main tower, PRE is a pre-separation tower, MAIN is a main tower, M1 is a main tower top condenser, M2 is a main tower top reflux tank, and M3 is a main tower bottom reboiler. Fresh stream F is fed from 36 plates of the pre-separation tower, the coupling positions of the pre-separation tower and the main tower are at the 4th and 61st plates, the coupled streams are L1, V1, L2 and V2, the main tower contains a total of 113 plates, the saturated gas at the top of the tower is completely condensed by the condenser and enters the reflux tank M2, a part of the condensate R enters from the top of the main tower, and the other part is extracted as a light component product D, the side product S is extracted from the 15th plate of the main tower, and the heavy component product is extracted from the bottom stream B.

[0026] Figure 2 This is a schematic diagram of a process dynamic control method based on a dividing wall distillation tower for separating alkanes described in Example 1.

[0027] Figure 2 Among them, FC is the feed flow controller, PC1 and PC2 are pressure controllers, LC1, LC2 and LC3 are liquid level controllers, CC, CC2 and CC3 are component controllers, TC is the temperature controller, and TC1-CC1 is the temperature-component cascade controller.

[0028] Figure 3 This is the dynamic response diagram of the purity of the three products after the feed flow rate is disturbed by ±20% as described in Example 1.

[0029] Figure 4 This is the dynamic response diagram of the purity of the three products after the feed composition is disturbed by ±20% of n-butane as described in Example 2.

[0030] Figure 5 This is the dynamic response diagram of the purity of the three products after the feed composition of isopentane is disturbed by ±20% as described in Example 3.

[0031] Figure 6 This is the dynamic response diagram of the purity of the three products after the feed composition is disturbed by ±20% of n-pentane as described in Example 4. DETAILED DESCRIPTION

[0032] The following will further illustrate the process dynamic control method based on the dividing wall distillation tower for separating alkanes described in the present invention in combination with the accompanying drawings and specific embodiments, but does not limit the scope of the present invention.

[0033] Example 1

[0034] A process dynamic control method based on a dividing wall distillation tower for separating alkanes

[0035] Figure 2 This is a schematic diagram of a process dynamic control method based on a distillation tower for separating alkanes. Taking the separation of a ternary mixture of n-butane, isopentane and n-pentane as an example, the light component n-butane is extracted from the top of the main tower, the middle component isopentane is extracted from the side line, and the heavy component n-pentane is extracted from the bottom of the main tower. The process flow diagram is shown below: Figure 1 The control loop of the present invention is as follows:

[0036] (1) A feed flow control circuit, including using a flow controller to control the feed flow into the tower, and controlling the feed flow into the tower by manipulating the opening of a valve.

[0037] (2) A tower top pressure control loop, including using a flow controller to control the flow rate from the top of the pre-separation tower to the main tower, and controlling the pressure at the top of the pre-separation tower by manipulating the flow rate from the top of the pre-separation tower to the main tower; and using a pressure controller to control the heat load of the condenser at the top of the main tower. The pressure controller controls the pressure at the top of the main tower, and automatically adjusts the heat load of the condenser at the top of the main tower according to the pressure. The pressure at the top of the main tower is controlled by manipulating the heat load of the condenser at the top of the main tower.

[0038] (3) Liquid level control loop, including using a flow controller to control the flow from the bottom of the pre-separation tower to the main tower, and controlling the liquid level at the bottom of the pre-separation tower by manipulating the flow from the bottom of the pre-separation tower to the main tower; using a flow controller to control the output from the top of the main tower, and controlling the liquid level of the reflux tank at the top of the main tower by manipulating the output of the product at the top of the main tower; using a flow controller to control the output from the bottom of the main tower, and controlling the liquid level at the bottom of the main tower by manipulating the output from the bottom of the main tower. Control the basic control loop of the process.

[0039] (4) A control loop is coupled between the streams, including using a flow controller to control the liquid phase flow from the main tower to the pre-separation tower, and controlling the concentration of the heavy component from the top of the pre-separation tower to the main tower by manipulating the flow from the top of the pre-separation tower to the main tower; using a temperature controller to control the temperature inside the pre-separation tower, and the temperature controller automatically adjusts the gas phase flow entering the pre-separation tower according to the temperature of the sensitive plate of the pre-separation tower, and using a sensitivity criterion method to determine that the temperature sensitive plate of the pre-separation tower is at the 44th tower plate, and controlling the temperature of the sensitive plate of the pre-separation tower by manipulating the flow from the main tower to the pre-separation tower, thereby ensuring the concentration of the intermediate component at the bottom of the pre-separation tower.

[0040] (5) A composition control loop, including using a flow controller to control the flow of the side product, and controlling the purity of the side product by manipulating the side product extraction volume; using a composition controller to control the heat load of the main tower reboiler, and the composition controller controls the purity of the tower bottom product to respond to the heat load of the main tower reboiler, thereby ensuring the purity of the side and tower bottom products.

[0041] (6) A temperature-composition cascade control loop, including controlling the reflux flow rate of the main tower top with a temperature controller, and controlling the set temperature value with a composition controller. The temperature controller automatically adjusts the reflux flow rate of the tower top according to the temperature of the main tower sensitive plate, and the composition controller adjusts the purity of the tower top product according to the set temperature value of the temperature controller, thereby ensuring the purity of the tower top product.

[0042] Example 2

[0043] The dividing wall distillation tower with the control structure described in Example 1 was used to separate the ternary mixture of n-butane, isopentane and n-pentane. After the dividing wall distillation tower was operated stably for 1 hour, the feed flow rate changed by ±20%, from the original 300 kmol / h to 360 kmol / h or 240 kmol / h, from Figure 3 It can be seen that through the control loop, the product purity returns to the original set value in a very short time and with a small overshoot under the disturbance of flow rate ±20%. The maximum overshoot does not exceed 0.006, the longest time does not exceed 7h, and the final product purity returns to the set value, indicating that the control loop can well cope with disturbances within ±20%, and has good robustness and stability.

[0044] Example 3

[0045] The dividing wall distillation tower with the control structure described in Example 1 was used to separate the ternary mixture of n-butane, isopentane and n-pentane. After the dividing wall distillation tower was operated stably for 1 hour, the feed composition of n-butane changed by ±20%, and the molar fraction changed from the original 0.33 to 0.396 or 0.264, from Figure 4 It can be seen that through the control loop, the product purity returns to the original set value in a very short time and with a small overshoot under the disturbance of flow rate ±20%. The maximum overshoot does not exceed 0.006, the time is 4 to 7 hours, and the final product purity returns to the set value, indicating that the control loop can well cope with disturbances within ±20%, and has good robustness and stability.

[0046] Example 4

[0047] The dividing wall distillation tower with the control structure described in Example 1 was used to separate n-butane, isopentane and a ternary mixture of n-pentane. After the dividing wall distillation tower was operated stably for 1 hour, the feed composition of isopentane changed by ±20%, and the molar fraction changed from the original 0.33 to 0.396 or 0.264, from Figure 5 It can be seen that through the control loop, the product purity returns to the original set value in a very short time and with a small overshoot under the disturbance of flow rate ±20%. The maximum overshoot does not exceed 0.0035, the stabilization time is about 5h, and the final product purity returns to the set value, indicating that the control loop can well cope with disturbances within ±20%, and has good robustness and stability.

[0048] Example 5

[0049] The dividing wall distillation tower with the control structure described in Example 1 was used to separate the ternary mixture of n-butane, isopentane and n-pentane. After the dividing wall distillation tower was operated stably for 1 hour, the feed composition of n-pentane changed by ±20%, and the molar fraction changed from the original 0.34 to 0.408 or 0.272, from Figure 6 It can be seen that through the control loop, the product purity returns to the original set value in a very short time and with a small overshoot under the disturbance of flow rate ±20%. The maximum overshoot does not exceed 0.001, the stabilization time is about 4h, and the final product purity returns to the set value, indicating that the control loop can well cope with disturbances within ±20%, and has good robustness and stability.

[0050] Those skilled in the art should understand that by using the control structure described in the present invention and the control method based on the control structure, it is possible to achieve that the product purity of the ternary mixture of n-butane, isopentane and n-pentane can return to the original set value under the disturbance of feed flow rate and feed composition, and it is also applicable to the stable control of other ternary mixture systems.

[0051] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. Any modification, equivalent replacement, improvement, etc. made to the invention without departing from the purpose and scope of the technical method should be included in the protection scope of the present invention.

Claims

1. A process dynamic control method based on a dividing wall distillation tower for separating alkanes, characterized in that: It includes the following control loops: A feed flow control loop, including a flow controller for controlling the feed flow into the tower; The tower top pressure control loop includes using a flow controller to control the flow rate from the top of the pre-separation tower to the main tower, and using a pressure controller to control the heat load of the condenser at the top of the main tower. The pressure controller controls the pressure at the top of the main tower, and automatically adjusts the heat load of the condenser at the top of the main tower according to the pressure. The liquid level control loop includes using a flow controller to control the flow from the bottom of the pre-separation tower to the main tower, using a flow controller to control the extraction amount from the top of the main tower, and using a flow controller to control the extraction amount from the bottom of the main tower; The control loop between the coupled streams includes using a flow controller to control the liquid phase flow from the main tower to the pre-separation tower, and using a temperature controller to control the temperature in the pre-separation tower, wherein the temperature controller automatically adjusts the gas phase flow entering the pre-separation tower according to the temperature of the pre-separation tower sensitive plate; A composition control loop, including using a flow controller to control the flow of the side product, and using a composition controller to control the heat load of the main tower reboiler, wherein the composition controller controls the purity of the bottom product to respond to the heat load of the main tower reboiler; The temperature-composition cascade control loop includes controlling the reflux flow rate at the top of the main tower by using a temperature controller, and controlling the set temperature value by using a composition controller. The temperature controller automatically adjusts the flow rate of the top reflux liquid according to the temperature of the main tower sensitive plate, and the composition controller adjusts the product purity at the top of the tower according to the set temperature value of the temperature controller.

2. A process dynamic control method based on a dividing wall distillation tower for separating alkanes according to claim 1, characterized in that: The control loops described herein use PID control signals and controllers.

3. The process dynamic control method based on the dividing wall distillation tower for separating alkanes according to claim 1 is characterized in that: The position of the sensitive plate in the temperature control loop is determined by the sensitivity criterion method.

4. The process dynamic control method based on the dividing wall distillation tower for separating alkanes according to claim 1, characterized in that: The tuning parameters of the controllers in the temperature and composition control loops are performed using the Tyreus-Luyben method.

5. The process dynamic control method based on the dividing wall distillation tower for separating alkanes according to claim 1, characterized in that: After the feed flow rate and feed composition change, the various control loops work together to return the product purity to the original set value.

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