Automatic control system for tetrafluoroethylene reaction conversion rate

Abstract

The application discloses an automatic control system for tetrafluoroethylene reaction conversion rate, which is provided with a temperature controller, a dilution ratio controller and a residence time (pressure) controller for the three influencing factors; the temperature control subsystem comprises three cascade control loops, wherein the first cascade control loop is used for detecting and feeding back the fuel flow provided to the superheating furnace; the second cascade control loop is used for detecting and feeding back the outlet temperature of the superheating furnace; and the third cascade control loop is used for detecting and feeding back the cracking reactor temperature so as to reach the set temperature; the dilution ratio control subsystem comprises steam flow and F22 flow ratio control; the residence time control subsystem comprises F22 buffer tank pressure control; under the stable conditions of the residence time and the dilution ratio, the reactor temperature is adjusted to adjust the conversion rate, and finally the tetrafluoroethylene yield and quality are maximized.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of tetrafluoroethylene reaction conversion rate control technology, especially in the automatic control system of tetrafluoroethylene conversion rate prepared by using F22 steam dilution cracking. BACKGROUND

[0002] Polytetrafluoroethylene (PTFE) is one of the best corrosion resistant materials in the world today, this material has the characteristics of acid and alkali resistance, resistance to various organic solvents, and is almost insoluble in all solvents. At the same time, polytetrafluoroethylene has the characteristics of high temperature resistance, and its friction coefficient is very low, so it can be used as a lubricating effect, and also becomes an ideal coating for easy cleaning water pipe inner layer.

[0003] Tetrafluoroethylene (TFE) is a monomer for the synthesis of polytetrafluoroethylene, and is currently mainly produced by the water vapor dilution cracking method of difluoro-chloromethane (F22): using preheated F22 as raw material, high temperature superheated steam as diluent and heat carrier, mixing and pyrolyzing in a tubular reactor to produce cracking gas containing TFE; cooling, washing with water and alkali to remove HCL; refining the cracking gas, including water removal, compression, deoxygenation, and removal of unsaturated hydrogen-containing fluorocarbon; distillation to obtain pure TFE, and recovery of unreacted F22; using absorption and analysis method to recover by-product hexafluoropropylene and most of the TFE in the tail gas. The conversion rate is related to the specific device and design, and generally maintained in the range of 65-75% to meet the production index requirements.

[0004] The main factors affecting the single-pass conversion rate of F22 are as follows:

[0005] 1. Temperature

[0006] The F22 cracking reaction is an endothermic reaction, and increasing the temperature of the superheated steam or the reactor temperature can increase the single-pass conversion rate. However, when the temperature is increased to above 840℃, the conversion rate increases less, and the selectivity decreases significantly, which not only reduces the yield, but also makes the post-treatment difficult, significantly affecting the product quality, therefore, the reaction temperature should not exceed 840℃.

[0007] 2. Dilution ratio:

[0008] This reaction is a gas phase reaction with increased molecular number, increasing the dilution ratio can increase the single-pass conversion rate of F22. However, when the dilution ratio is too high, the conversion rate increases less, and the energy consumption increases significantly.

[0009] 3. Residence time:

[0010] Increasing the reaction residence time can increase the single-pass conversion rate of F22. The inventors have shown through experiments that within the time period of 0.01-0.043 seconds, there is no significant influence of reaction temperature and dilution ratio.

[0011] Traditional cracking temperature control is only simple single-loop control, so that the steps of each control are inconsistent to form a system, and it is difficult to achieve the best conversion rate and selectivity, resulting in material consumption and energy consumption not reaching the ideal state.

[0012] Patent document CN218932005U discloses a TFE cracking automatic control system, which comprehensively considers the addition proportion of R22 and water vapor and the cracking temperature control through cascade control to improve and stabilize the conversion rate and selectivity of the raw material. Figure 1 As shown in the figure, the whole system includes a heating furnace (or a superheating furnace) and a cracking reactor connected with the heating furnace, the heating furnace is connected with an R22 conveying pipeline for conveying cracking raw material R22 to the heating furnace, a steam conveying pipeline for conveying steam to the heating furnace, and a water gas conveying pipeline for conveying heating fuel to the heating furnace; the R22 and steam passing through the heating furnace are transmitted to the cracking reactor; the temperature sensor T1 is arranged on the heating furnace, the temperature sensor T2 is arranged on the steam conveying pipeline after being heated by the heating furnace, the temperature sensor T3 is arranged on the cracking reactor, the R22 flow meter F3 and the R22 flow regulating valve FV3 are arranged on the R22 conveying pipeline, the steam flow meter F2 and the steam flow regulating valve FV2 are arranged on the steam conveying pipeline, the water gas flow meter F1 and the water gas flow regulating valve FV1 are arranged on the water gas conveying pipeline, the temperature sensor T1, the water gas flow meter F1 and the water gas flow regulating valve FV1 form cascade regulation, the temperature sensor T2, the steam flow meter F2 and the steam flow regulating valve FV2 form cascade regulation, and the temperature sensor T3, the R22 flow meter F3 and the R22 flow regulating valve FV3 form cascade regulation.

[0013] The above-mentioned patent document simultaneously adopts five control measures: cascade control of semi-water gas into the heating furnace, cascade control of 0.35 MPa steam into the raw material R22, cascade control of the raw material R22 into the heating furnace, proportion control of the steam and R22 into the heating furnace, and interlocking control of the heating furnace over-temperature and material interruption (0.35 MPa steam and R22). Although this scheme can make the production more safe and stable in theory, in actual production process, it is not desirable to see frequent load adjustment, and once F22 or steam is adjusted, the corresponding steam and F22 are adjusted at the same time according to the dilution ratio control, and the dynamic process of the whole adjustment has deviation of the dilution ratio. The above-mentioned scheme increases the coupling of the reactor temperature and the dilution ratio control, which is not conducive to the system stability in a certain sense. SUMMARY

[0014] Therefore, the embodiments of the present application provide an automatic control system for the reaction conversion rate of tetrafluoroethylene, which aims to solve the problem of large fluctuation of cracking conversion rate in the prior art, can realize accurate control of the cracking conversion rate, and enhance the robustness of the automatic control loop, so as to finally maximize the yield and quality of tetrafluoroethylene.

[0015] To achieve the above object, the embodiment of the present application provides the technical scheme as follows.

[0016] An automatic control system for tetrafluoroethylene reaction conversion rate, comprising a temperature control subsystem, a dilution ratio control subsystem and a residence time control subsystem;

[0017] The temperature control subsystem comprises a three-stage cascade control loop, wherein the first-stage control loop is used for detecting and feedback controlling the fuel flow provided to the superheater; the second-stage control loop is used for detecting and feedback controlling the outlet temperature of the superheater, and the controller output of the second-stage control loop is used as the given value of the first-stage control loop; the third-stage control loop is used for detecting and feedback controlling the cracking reactor temperature to reach the set temperature, and the controller output of the third-stage control loop is used as the given value of the second-stage control loop;

[0018] The dilution ratio control subsystem comprises ratio control of the steam flow and the F22 flow;

[0019] The residence time control subsystem comprises pressure control of the F22 buffer tank;

[0020] The residence time and the dilution ratio are kept stable through the dilution ratio control subsystem and the residence time control subsystem, and the set temperature of the cracking reactor is adjusted based on the temperature control subsystem to adjust the tetrafluoroethylene conversion rate.

[0021] Optionally, the dilution ratio control subsystem adopts separate PID controllers and adjusting actuators for the F22 flow and the steam flow, wherein the F22 flow is set as the given value of the F22 flow control loop, and the measured value of the F22 flow is not only fed back to the F22 flow control loop but also multiplied by the set proportional coefficient to be used as the given value of the steam flow control loop.

[0022] Optionally, the residence time control subsystem comprises a pressure controller and a feed adjusting valve, and the F22 buffer tank pressure is closed-loop feedback controlled by setting the F22 buffer tank pressure as the given value of the pressure controller, thereby indirectly controlling the residence time.

[0023] Optionally, the set temperature of the cracking reactor is adjusted to adjust the tetrafluoroethylene conversion rate by controlling the residence time to be stable at 0.01-0.043 seconds and the dilution ratio to be stable at a molar ratio of 1:5.5.

[0024] Specifically, the residence time is kept stable at 0.01-0.043 seconds by setting the buffer tank pressure at 0.1 MPa based on the residence time control subsystem.

[0025] The present application has at least the following beneficial effects:

[0026] The three main factors affecting the conversion rate are controlled by closed-loop feedback control, and the reactor temperature is adjusted to adjust the conversion rate under the conditions of stable residence time and dilution ratio. The control of the three aspects is little coupled, the accurate control of the cracking conversion rate can be realized, and the robustness of the automatic control loop is enhanced. After the automatic use, the conversion rate can be stabilized at 69% to 71%, which meets the production index requirements, provides the most suitable conditions for the cracking reaction, and maximizes the yield and quality of tetrafluoroethylene.

[0027] The present application considers the energy balance relationship, selects the reactor temperature by heat control, and finally controls the reaction conversion rate. The dilution ratio control and the reactor temperature control are not coupled, and the control effect is more effective. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is a schematic diagram of the overall structure of a TFE cracking automatic control system of the prior art;

[0029] Figure 2 It is a schematic diagram of the principle of temperature control in an embodiment of the present application;

[0030] Figure 3 It is a schematic diagram of the principle of dilution ratio control in an embodiment of the present application;

[0031] Figure 4 It is a schematic diagram of the principle of residence time control in an embodiment of the present application. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0033] In the description of the present application, unless otherwise specified, the terms "comprising", "including", "having" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not have to be limited to only those steps or units explicitly listed, but can also include other steps or units inherent to the process, method, product or device, or steps or units added based on further optimization of the concept of the present application.

[0034] The present application is a multi-loop comprehensive control method, which gives a controller for each of the three influencing factors: temperature controller, dilution ratio controller, and residence time (pressure) controller.

[0035] In one embodiment, the control strategy is: the residence time is stabilized between 0.01 and 0.043 seconds, the dilution ratio is stabilized at 1:5.5 (molar ratio), and the reactor temperature is adjusted to maintain the conversion rate.

[0036] 1. Temperature control

[0037] like Figure 2 As shown, the temperature control includes three cascade control loops: the main loop controls the temperature of the cracking reactor, the sub-loop controls the temperature of the superheater furnace outlet, and the innermost sub-loop controls the natural gas flow rate, ultimately achieving automatic and stable control of the reactor temperature. Among them:

[0038] First-level control circuit, natural gas flow-natural gas regulating valve;

[0039] Second-level control loop, superheater outlet temperature-natural gas flow setting;

[0040] The third level control loop, reactor temperature - superheater outlet temperature setting;

[0041] 2. Dilution ratio control

[0042] like Figure 3 As shown, the dilution ratio control adopts ratio control mode, where K = 5.5 (molar ratio), and the steam flow follows the F22 flow. When the operator changes the F22 flow setting when raising or lowering the load, the steam flow automatically follows to ensure a constant dilution ratio.

[0043] 3. Residence time control

[0044] The residence time is required to be stable between 0.01 and 0.043 seconds. Figure 4 As shown in the figure, the residence time can be indirectly controlled by the pressure of the F22 buffer tank. Experiments show that when the pressure of the buffer tank is 0.1 MPa, the residence time can meet the requirements.

[0045] Several application examples (conversion rates under different reactor surface temperatures and buffer tank pressure conditions) are given below, as shown in Table 1.

[0046] Table 1

[0047]

[0048] From the above application examples, it can be seen that the residence time is stabilized between 0.01 and 0.043 seconds, the dilution ratio is stabilized at 1:5.5 (molar ratio), and the temperature setting value of the cracking reactor is optimized, which can stabilize the conversion rate at 69% to 71%, meeting the production index requirements and achieving high yield and quality of tetrafluoroethylene.

[0049] The present invention has been described in detail and in general terms through a general description and specific embodiments. It should be noted that variations and modifications to these specific embodiments are possible without departing from the spirit of the present invention, and all such variations and modifications are within the scope of protection of this application. Therefore, the scope of protection of this patent application shall be determined by the appended claims.

Claims

1. An automatic control system for tetrafluoroethylene reaction conversion rate, characterized in that: Including temperature control subsystem, dilution ratio control subsystem and residence time control subsystem; The temperature control subsystem includes a three-stage cascade control loop, wherein the first stage control loop is used to detect and feedback control the fuel flow provided to the superheater; the second stage control loop is used to detect and feedback control the superheater outlet temperature, and the controller output of the second stage control loop serves as the set value of the first stage control loop; the third stage control loop is used to detect and feedback control the temperature of the cracking reactor to achieve the set temperature, and the controller output of the third stage control loop serves as the set value of the second stage control loop; The dilution ratio control subsystem includes controlling the ratio of steam flow rate to difluorochloromethane flow rate; The residence time control subsystem includes pressure control of the difluorochloromethane buffer tank; The residence time and the dilution ratio are kept stable by the dilution ratio control subsystem and the residence time control subsystem, and the tetrafluoroethylene conversion rate is adjusted by adjusting the set temperature of the cracking reactor based on the temperature control subsystem.

2. The automatic control system for tetrafluoroethylene reaction conversion rate according to claim 1, characterized in that: The dilution ratio control subsystem uses separate PID controllers and regulating actuators for the difluorochloromethane flow rate and steam flow rate, respectively. By setting the difluorochloromethane flow rate as the set value of the difluorochloromethane flow rate control loop, the measured difluorochloromethane flow rate is not only fed back to the difluorochloromethane flow rate control loop, but is also multiplied by a set proportional coefficient and used as the set value of the steam flow rate control loop.

3. The automatic control system for tetrafluoroethylene reaction conversion rate according to claim 1, characterized in that: The residence time control subsystem includes a pressure controller and a feed regulating valve. By setting the pressure of the difluorochloromethane buffer tank as a given value of the pressure controller, the pressure of the difluorochloromethane buffer tank is subjected to closed-loop feedback control, thereby indirectly controlling the residence time.

4. The automatic control system for tetrafluoroethylene reaction conversion rate according to claim 1, characterized in that: The tetrafluoroethylene conversion rate is adjusted by controlling the residence time to be stable at 0.01 to 0.043 seconds, stabilizing the dilution ratio to be stable at a molar ratio of 1:5.5, and adjusting the set temperature of the cracking reactor.

5. The automatic control system for tetrafluoroethylene reaction conversion rate according to claim 3, characterized in that: By setting the buffer tank pressure at 0.1 MPa, the residence time is stabilized at 0.01 to 0.043 seconds based on the residence time control subsystem.

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