Control structure for heat pump rectification

Abstract

The invention relates to the field of chemical process strengthening, in particular to a control method of a heat pump rectification system. The heat pump technology can obviously reduce the energy consumption in the rectification process, the heating and condensing processes of the rectification tower are changed after the heat pump is introduced, and the original control method or control structure also needs to be further designed and optimized. According to the control method for heat pump rectification, the heating capacity of the tower kettle of the rectifying tower is controlled by adopting the pressure ratio control loop before and after the heat pump and the stripping section temperature control loop, and the heat pump rectification system can be well controlled in combination with the rectifying section temperature control loop, the pressure control loop, the liquid level control loop and the flow control loop. The control structure is suitable for the heat pump rectification process in the fields of petrochemical engineering, medicine, fine chemicals and the like, can cope with large feeding flow fluctuation and feeding composition fluctuation, and has remarkable industrial application value.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of chemical process intensification, and specifically relates to a control method for heat pump rectification. BACKGROUND

[0002] According to statistics, the energy consumption of a rectification tower accounts for more than 50% of the total energy consumption of the petroleum and chemical industry. The heat of the rectification tower is all added to the reboiler at the bottom of the tower at the highest temperature, while the sensible heat and latent heat of the overhead gas cannot be effectively utilized, and the energy utilization efficiency is low. The heat pump technology recovers the heat of the low-temperature steam at the top of the tower through steam compression technology, and reuses it for heating the kettle after upgrading the grade, so that the energy can be recycled and utilized efficiently. Compared with the traditional process, the heat pump rectification can reduce the energy consumption by 30%-70% and reduce the carbon emissions by 20%-50%, and is one of the key technologies for the green upgrading of the chemical industry. After the introduction of the heat pump, the structure of the rectification tower has changed, and the original control method or structure also needs to be further designed and optimized.

[0003] A kind of open heat pump batch rectification system and its control method are disclosed in Chinese patent application for invention with publication number CN117065384A (publication date November 17, 2023): the key points of the concentration change of each component from the start to the stop of the rectification tower are taken as design points to design the full rectification process parameters, so that the heat pump compressor is matched with the rectification system, and the performance curve of the heat pump compressor under the parameters of each key point is calculated; after the heat pump compressor is started, the material state is judged by the material component concentration detector and the performance curve of the heat pump compressor under this state is matched; the working state of the heat pump compressor at this time is judged by the inlet and outlet pressure, temperature and motor power of the heat pump compressor, and the heat pump compressor is adjusted in real time to realize the matching of the heat pump compressor and the rectification system according to the performance curve.

[0004] For the rectification tower, the quality of the overhead and bottom products needs to be strictly controlled. There is a corresponding functional relationship between temperature and composition, and in the control process of the rectification tower, the product purity can be maintained by controlling the temperature of the sensitive plate to be constant. Therefore, the temperature control structure is widely used because of its simplicity and effectiveness. The advantage of the temperature controller is that the reaction is faster and the maximum deviation of the product purity is smaller. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a temperature control structure for a heat pump rectification tower. The control structure described in the present application can realize stable control of the heat pump rectification tower. Under the premise of not affecting the product quality and ensuring the safety of the system, the control structure of the present application can cope with obvious fluctuations in feed flow and feed composition. The control structure can be used for the control of existing and newly built heat pump devices, and has significant practicality and broad application prospects.

[0006] In order to achieve the above-mentioned application purposes, the present application adopts the following technical solutions:

[0007] A heat pump rectification system and its control structure, including a rectification column T1, a compressor COMPR1, a bottom heat exchanger HX1, and a top condenser HX2. The material to be separated enters T1 through a feed valve V1. Light components rise in the column, and heavy components descend in the column. A gas phase stream is obtained at the top of T1. The gas phase stream is compressed and heated by COMPR1, and then enters HX1 to exchange heat with the liquid at the bottom. A pressure relief valve V3 is provided at the outlet of the hot fluid of HX1, and the high-pressure fluid after heat exchange is relieved by V3. The cold fluid flowing through HX1 is evaporated into a gas phase, and enters the bottom of T1 through a back pressure valve V4 to provide heat for T1. A condenser HX2 is provided after V3 to condense the gas-liquid mixture after pressure relief into a liquid. A buffer tank R1 is provided after HX2. The liquid is buffered in R1, and then is divided into two parts, one part is taken out as qualified top product through a valve V5, and the other part is returned to T1 through a valve V6 to provide cold for T1. The bottom product flows to a product tank through a valve V7. The present application provides a control structure for heat pump rectification and heat pump stripping systems, which is easy to operate and has good control effect.

[0008] The heat pump rectification system includes a feed control loop FC. FC adjusts the opening of V1 according to the change of the feed flow.

[0009] Further, the heat pump rectification system includes a power control loop CPC of COMPR1. CPC adjusts the power of the compressor through the ratio of the pressures before and after COMPR1.

[0010] Further, the heat pump rectification system includes a rectification section sensitive plate temperature control loop TC1 of T1 and a stripping section sensitive plate temperature control loop TC2. TC1 controls the sensitive plate temperature of the rectification section of T1 through the opening of valve V5. TC2 controls the heat fluid flow entering HX1 through valve V2, and indirectly controls the sensitive plate temperature of the stripping section.

[0011] Further, the pressure of the system also needs to be controlled. The heat pump rectification system includes a top pressure control loop PC1, a pressure control loop PC2 of the fluid after V3, and a bottom pressure control loop PC3. PC1 controls the pressure of the top tank through the cold of HX2. PC2 controls the pressure of the fluid after V3 through the opening of V3. PC3 controls the pressure of the fluid after V4 through the opening of V4, and indirectly controls the bottom pressure.

[0012] Further, the heat pump rectification system includes a top liquid level control loop LC1 and a bottom liquid level control loop LC2. LC1 controls the liquid level of the top tank through the opening of V6, and LC2 controls the liquid level of the bottom through the opening of V7. BRIEF DESCRIPTION OF DRAWINGS

[0013] The present application will be described in detail below with reference to the accompanying drawings.

[0014] Figure 1 is a schematic diagram of temperature control structure of a conventional rectifying column. Among them:

[0015] TC1—rectifying section sensitive plate temperature control loop; TC2—stripping section sensitive plate temperature control loop; PC1—overhead pressure control loop; LC1—overhead liquid level control loop; LC2—bottom liquid level control loop; FC—feed flow control loop.

[0016] Figure 2 is a schematic diagram of control structure of a heat pump rectifying column. CPC—heat pump compressor power control loop; TC1—rectifying section sensitive plate temperature control loop; TC2—stripping section sensitive plate temperature control loop; PC1—overhead pressure control loop; PC2—pressure control loop of pressure relief valve; PC3—pressure control loop of back pressure valve; LC1—overhead liquid level control loop; LC2—bottom liquid level control loop; FC—feed flow control loop.

[0017] Figure 3 is a schematic diagram of control structure containing overhead preheater HX3. CPC—heat pump compressor power control loop; TC1—rectifying section sensitive plate temperature control loop; TC2—stripping section sensitive plate temperature control loop; PC1—overhead pressure control loop; TC3—HX3 outlet temperature control loop; PC2—pressure control loop of pressure relief valve; PC3—pressure control loop of back pressure valve; LC1—overhead liquid level control loop; LC2—bottom liquid level control loop; FC—feed flow control loop. DETAILED DESCRIPTION

[0018] The present application will be described in detail below with reference to the accompanying drawings.

[0019] The specific implementation can be determined according to different separation conditions and separation requirements. The present application is applicable to newly built heat pump rectifying columns and heat pump rectifying columns after modification of existing conventional rectifying devices. The present application takes the heat pump rectifying device in the separation process of a binary mixture of benzene-toluene and p-xylene-styrene as an example to illustrate the specific implementation, and will be described in detail below in combination with the accompanying drawings, but only for illustration and not for limitation of the present application.

[0020] Example 1 Heat pump rectifying device for separation of p-xylene-styrene

[0021] The boiling point of p-xylene is 138℃ at normal pressure, and the boiling point of styrene is 145.2℃ at normal pressure. The boiling points of p-xylene and styrene are relatively close at normal pressure. The energy consumption for directly separating p-xylene and styrene by conventional rectification is high. The heat pump rectifying device shown in the figure can greatly reduce the energy consumption for separating mixed xylene. Figure 2 The heat pump rectifying device shown in the figure can greatly reduce the energy consumption for separating mixed xylene.

[0022] The feed flow rate to the p-xylene-styrene separation unit is 100 kmol / h, with a mole fraction of 0.5 for p-xylene and 0.5 for styrene. The purity of p-xylene and styrene is not less than 0.99 under steady state. The total tray number of T1 is 110, with a top pressure of 1 atm and a tray pressure drop of 0.68 kPa. The calculated sensitive plates are the 55th and 83rd plates. The temperatures of the sensitive plates are 141.8 °C and 144.3 °C, respectively. The pressure ratio before and after COMPR1 is 1.8. The load of HX1 is 7622.4 kW, and the load of HX2 is -530.2 kW. The liquid level of R1 is 0.987 m, and the liquid level at the bottom of T1 is 1.234 m.

[0023] The control logic of the FC loop is that when the feed flow rate increases, the opening of V1 is decreased to control the flow rate of the p-xylene and styrene mixture into T1. When the feed flow rate decreases, the opening of V1 is increased.

[0024] The control logic of the CPC loop is that when the pressure ratio after COMPR1 to the pressure before it decreases, the power of COMPR1 is increased. When the pressure ratio after COMPR1 to the pressure before it increases, the power of COMPR1 is decreased. This adjustment ensures that the overhead gas is compressed to a sufficient amount of heat for the heat exchange at the bottom, while also ensuring a lower power.

[0025] The control logic of the TC1 loop is that when the temperature of the sensitive plate in the rectifying section increases, V5 is closed to allow more condensed liquid to return. When the temperature of the sensitive plate in the rectifying section decreases, V5 is opened.

[0026] The control logic of the TC2 loop is that when the temperature of the sensitive plate in the stripping section increases, V2 is closed to decrease the flow rate of the hot fluid, thereby decreasing the heat exchange power of HX1. When the temperature of the sensitive plate in the stripping section decreases, V2 is opened.

[0027] The control logic of the LC1 and LC2 loops is that when the liquid level of buffer tank R1 increases, the opening of V6 is increased to increase the return flow of T1, thereby decreasing the liquid level of R1. When the liquid level of buffer tank R1 decreases, the opening of V6 is decreased. When the liquid level at the bottom of T1 increases, the opening of V7 is increased. When the liquid level at the bottom of T1 decreases, the opening of V7 is decreased.

[0028] The control logic of PC1 is that when the pressure of R1 increases, the condensation of HX2 is increased. When the pressure of R2 decreases, the condensation of HX2 is decreased.

[0029] The control logic of PC2 and PC3 is that when the pressure after the valve increases, the corresponding valve is closed. When the pressure after the valve decreases, the corresponding valve is opened.

[0030] The main external disturbance in the paraxylene-styrene separation process is the total feed flow fluctuation, and the composition fluctuation of benzene and toluene.

[0031] When the total feed flow decreases by 10%, the amount of paraxylene and styrene both decrease, the amount of overhead vapor decreases, but the heat required at the bottom also decreases. The power of the compressor controlled by the CPC loop decreases, and the insufficient or excessive heat is compensated by the opening of V2 controlled by TC2. The opening of V5 controlled by TC1 decreases. Other control loops are adjusted in real time to respond to the changes in the rectification system.

[0032] When the total feed flow increases by 10%, the amount of paraxylene and styrene both increase, the amount of overhead vapor increases, and the heat required at the bottom also increases. The power of the compressor controlled by the CPC loop increases, and the insufficient or excessive heat is compensated by the opening of V2 controlled by TC2. The opening of V5 controlled by TC1 increases. Other control loops are adjusted in real time to respond to the changes in the rectification system.

[0033] When the feed flow remains unchanged, and the paraxylene composition of the feed decreases by 10%, i.e., the styrene composition increases by 10%, the overhead vapor decreases, but the heat required at the bottom decreases. The power of the compressor controlled by the CPC loop decreases, and the insufficient or excessive heat is compensated by the opening of V2 controlled by TC2. The opening of V5 controlled by TC1 decreases. Other control loops are adjusted in real time to respond to the changes in the rectification system.

[0034] When the feed flow remains unchanged, and the paraxylene composition of the feed increases by 10%, i.e., the styrene composition decreases by 10%, the overhead vapor increases, but the heat required at the bottom increases. The power of the compressor controlled by the CPC loop increases, and the insufficient or excessive heat is compensated by the opening of V2 controlled by TC2. The opening of V5 controlled by TC1 decreases. Other control loops are adjusted in real time to respond to the changes in the rectification system.

[0035] Example 1 solves the problems of system lag and large fluctuation by matching and regulating the pressure, temperature, liquid level, and flow of each tower in the system, and can effectively improve the product rectification separation effect. The control method adopted by the present application is advanced in technology and easy to control, and can achieve the goal of energy saving optimization under the premise of ensuring the product index and normal operation parameters of each tower. Compared with the traditional control method, the purity fluctuation in the product is controlled within 0.005, which reduces the quality fluctuation of the downstream product. After calculation, the total annual cost can be saved by about 40%.

[0036] Example 2: Heat pump rectification device for benzene-toluene separation

[0037] The boiling point of benzene is 80°C under normal pressure, and the boiling point of styrene is 110.6°C under normal pressure. The boiling points of benzene and toluene are quite different under normal pressure. Direct use of heat pump compression of overhead gas requires a large compression ratio. The difference in boiling point between benzene and toluene can be usedFigure 3 The HX3 preheats the gas at the top of the tower and then compresses the preheated gas.

[0038] The total feed flow rate is 100 kmol / h, and the molar fractions of benzene and toluene are both 0.5. The total number of plates in T1 is 48. The top pressure is 100 kPa, and the plate pressure drop is 0.68 kPa. Under steady-state conditions, the purities of both benzene and toluene reach 0.99. Calculations show that the sensitive plates are the 6th and 42nd plates, respectively. The temperatures of the sensitive plates are 85.52°C and 110.23°C, respectively. The pressure ratio before and after COMPR1 is 4.1. The loads of HX1 are 1296.8 kW, HX2 are -214.2 kW, and HX3 are 177.2 kW. The liquid level in R1 is 1.057 m, and the liquid level at the bottom of T1 is 1.35 m.

[0039] The control logic of HX3 is to use the heating power of HX3 to control the temperature of the gas after HX3. When the temperature of the gas after HX3 drops, the power of HX3 is increased. When the temperature of the gas after HX3 rises, the power of HX3 is reduced.

[0040] The main external disturbances in the benzene-toluene separation process are fluctuations in the total feed flow rate and fluctuations in the composition of benzene and toluene.

[0041] When the total feed flow rate drops by 10%, the amounts of benzene and toluene decrease, reducing the amount of overhead vapor and the heat required at the bottom of the tower. The CPC loop controls the compressor power reduction, and the insufficient or excess heat is compensated by TC2 controlling the opening of V2. TC1 controls the opening of V5, decreasing it. Other control loops adjust in real time to respond to changes in the distillation system.

[0042] When the total feed flow rate increases by 10%, the amounts of benzene and toluene both decrease and increase, the amount of overhead vapor increases, and the heat required at the bottom of the tower also increases. The CPC loop controls the compressor power to increase. TC2 controls the opening of V2 to compensate for any shortfall or excess heat. TC1 controls the opening of V5 to increase. Other control loops adjust in real time to accommodate changes in the distillation system.

[0043] When the feed flow rate remains constant but the benzene content decreases by 10%, meaning the toluene content increases by 10%, the overhead vapor volume decreases, but the heat required at the bottom of the column decreases. The CPC loop controls the compressor power reduction, and the insufficient or excess heat is compensated by TC2 controlling the opening of V2. TC1 controls the opening of V5, decreasing it. Other control loops adjust in real time to accommodate changes in the distillation system.

[0044] When the feed flow rate remains constant, the feed benzene composition increases by 10%, i.e. the toluene composition decreases by 10%, the overhead vapor increases, but the required heat at the bottom increases. The CPC loop controls the power of the compressor to increase, and the insufficient or excessive heat is compensated by the opening of V2 controlled by TC2. The opening of V5 controlled by TC1 decreases. Other control loops adjust in real time to respond to changes in the rectification system.

[0045] This embodiment 2 solves the problems of system lag and large fluctuation by matching and regulating the pressure, temperature, liquid level, flow rate and the like of each column in the system, and can effectively improve the product rectification separation effect. The control method and technology adopted by the present application are advanced and easy to control, and can achieve the energy-saving optimization target under the premise of ensuring the product index and normal operation parameters of each column. Compared with the traditional control mode, the purity fluctuation in the product is controlled within 0.005, and the quality fluctuation of the downstream product is reduced. After calculation, the total annual cost can be saved by about 37%.

Claims

1. A control method for heat pump distillation, characterized in that: By controlling the pressure ratio before and after the heat pump, the heating amount of the distillation tower kettle can be controlled to ensure the heating amount of the distillation tower kettle or the purity of the tower kettle product.

2. A heat pump distillation control method according to claim 1, characterized in that By controlling the pressure ratio before and after the heat pump and combining it with the temperature control loop of the distillation section, precise control of the heating amount of the distillation tower kettle or the purity of the kettle product can be achieved.

3. The control method of heat pump distillation according to claim 1, characterized in that By controlling the pressure ratio before and after the heat pump and combining the stripping section temperature control loop, the distillation section temperature control loop, the pressure control loop, the liquid level control loop, the flow control loop, etc., precise control of the heat pump distillation tower can be achieved.

4. The heat pump distillation control method according to claim 1, characterized in that: Adjust the opening of the feed inlet valve according to the change of feed flow; adjust the power of the compressor by the pressure ratio before and after the heat pump; control the temperature of the sensitive plate in the distillation section by the opening of the top product outlet valve; control the temperature of the sensitive plate in the distillation section by controlling the opening of the hot fluid inlet valve of the bottom heat exchanger; control the top pressure of the tower by controlling the power of the top condenser; control the pressure of the fluid at the pressure relief valve outlet by the opening of the pressure relief valve; control the bottom pressure of the tower by the opening of the back pressure valve at the bottom of the tower.

5. The control method for heat pump distillation according to any one of claims 1 to 4, characterized in that: The distillation tower is an atmospheric pressure tower, a pressure tower or a vacuum tower. Optionally, the distillation tower is a plate tower or a packed tower.

6. The control method for heat pump distillation according to any one of claims 1 to 5, characterized in that: The heat pump distillation can be an open heat pump distillation or a closed heat pump distillation.

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