J. Cent. South Univ. (2017) 24: 655-664

DOI: 10.1007/s11771-017-3466-0

Temperature fault-tolerant control system of CSTR with coil and jacket heat exchanger based on dual control and fault diagnosis

WANG Zai-ying(王再英), WANG Guo-xin(王国鑫)

School of Electrical and Control Engineering, Xi’an University of Science and Technology, Xi’an 710054, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: For the characteristics of the continuous stirred-tank reactor (CSTR) with coil and jacket cooling system, a CSTR temperature dual control solution based on the analysis of the CSTR exothermic reaction control characteristic was proposed for an organic material polymerization production. The control solution has passive fault-tolerant ability for the jacket cooling water cutting off fault and active fault-tolerant potential for the coil cooling water cutting off fault, and it has good control ability, high saving energy and reducing consumption performance. Fault detection and diagnosis and fault-tolerant control strategy are designed for the coil cooling fault to achieve the active fault-tolerant control function. The CSTR temperature dual control, process fault detection and diagnosis and active fault-tolerant control were full integrated into the CSTR temperature fault-tolerant control system, which achieve fault tolerance control of CSTR temperature for any severe malfunction of jacket cooling or coil cooling cutting off, and the security for CSTR exothermic reaction is improved. Finally, the effectiveness of this system was validated by semi-physical simulation experiment.

Key words: continuous stirred-tank reactor (CSTR); exothermic reaction; dual control; process fault diagnosis; fault-tolerant control

1 Introduction

Continuous stirred-tank reactor (CSTR) is an important process equipment used for various physical change and chemical reaction in chemical production. The number of CSTR accounting the reactors is over 90% in main three synthetic materials (plastic, synthetic rubber, synthetic fiber) productions [1-5]. Besides, it is also largely used in fine chemical industry, pharmacy and environmental protection. The reaction temperature is the most important process parameter in CSTR operation process, which is closely related to the production efficiency and economic benefit. Due to the important factor of CSTR temperature in production process and its wide application, the automatic control of CSTR temperature has been focused by professionals who put forward a variety of feasible control method. For example,the algorithm of single loop control solutions in temperature automatic control gets series of achievements. The dual control system of temperature was proposed for the CSTR polymerization process of two heat ex-changers with jacket and coil, which has considered the accuracy of dynamic control of temperature, steady production, saving energy and reducing consumption [6, 7].

During the exothermic reaction of CSRT, once the temperature rises, the speed of reaction will accelerate, which leads to releasing more heat, and then the reaction temperature continues to rise. Because the positive feedback, exists in internal process of exothermic reaction the exothermic reaction process is unstable in the condition of open-loop. In the practical polymerization exothermic reaction of CSTR, it is stable and safe to prevent heat accumulation in the reactor and ensure reaction process; the temperature of material inside the reactor is maintained at the setting value through heat exchange between cooling devices and reaction materials. Since the CSRT temperature system of exothermic reaction is open-loop and unstable, there are dangers of runaway reaction, fire and explosion in the system [8-11]. For the above problem, fault-tolerant control strategy based on the dual control is proposed, and the CSTR with coil and jacket cooling has operation redundant in the temperature control. The effectiveness of strategy is verified by semi-physical simulation experiments.

2 Temperature characteristic analysis of CSTR with dual-heat exchanger of exothermic reaction

2.1 Production process

In production process of synthetic material, CSTR is used in polymerization of organic materials. The basic structure, measured parameters and operation equipment configuration of the CSTR used in the continuous polymerization production are shown in Fig. 1. In the production process, material D can be produced from the polymerization reaction of materials A and B under the effect of catalyst C. The reaction may begin with certain temperature conditions and release great deal of the heat. In order to prevent the accumulation of heat within the reactor, cooling water in the jacket cooling or coil cooling is used to exchange heat with the reaction mass, so as to take away the heat of reaction and then make the temperature of materials in the CSTR maintain at a setting value. The diameter of reactor is 1000 mm; the effective height is 1376 mm; the practicable volume is 0.903 m³; and the upper pressure limit is 2.5 MPa. Measure parameters and installation location of operation equipment of the reactor are shown in Fig. 1.

Fig. 1 Diagram of CSTR with dual heat ex-changer

Under the condition of production at (72±1.0) °C, product D can be obtained from the polymerization reaction between reactant A and reactant B under the effect of catalyst C. The feed flow F4 of reactant A is controlled by regulating valve V4, the feed flow F5 of reactant B is controlled by regulating valve V5, and the feed flow F6 of catalyst C is controlled by regulating valve V6.

The weight percentage of main product D in the reactor is instructed by concentration meter A (it is only as a direct reference and cannot be used as the control signal); T1 is temperature sensor; L4 is liquid level sensor; export (material) flow F9 of reactor is controlled by regulating valve V9. Export material of the reactor is mixture composed of synthetic reaction product D, uncreatives A, B and catalyst C. The reactor is equipped with two sets of cooling devices. The first device is jacket cooling, and the cooling water entrance flow rate F7 is controlled by regulating valve V7. The second device is coil cooling, and the cooling water entrance flow F8 is controlled by regulating valve V8. In the reaction start-up phase, polymerization reaction is triggered when the heated material inside the reactor is added to the jacket, and the reactor stirrer motor switch S8 is on.

The stuff in the CSTR is stirred strongly, which plays very good function in dispersion and dilution, so as to make the state of stuff in CSTR meet the assumption of full flow. Thus, it can assume that all points temperatures in the CSTR are uniform, and the slurry concentration and temperature of the reactor outlet are equal to those in the reactor.

Through the thermodynamic analysis of CSTR, most of these temperature systems are in open-loop instability, such as the controlled variable of temperature inside the reactor, and coil cooling water flow and jacket cooling water flow ware used as manipulating variables.

2.2 Analysis of temperature control system for exothermic reaction CSTR

2.2.1 Temperature control system of single-loop for CSTR

1) Single-loop temperature control system using jacket cooling for CSTR

Jacket cooling is used in the scheme of CSTR exothermic reaction temperature control, and the flow of jacket cooling water is manipulating variable. Since the lag of measurement part is small, open-loop transfer function between CSTR reaction temperature Θ and jacket cooling water flow F_jc can be approximately shown as [12-16]

               (1)

When it brings into the specific device parameters, the actual open loop transfer function can be expressed as follows:

            (2)

Assume that single-loop control system uses the control of pure proportional action (strategy). Dynamic response curve of a single loop jacket cooling temperature of CSTR can be obtained as shown in Fig. 2.

2) Single-loop temperature control system using coil cooling for CSTR

Coil cooling is used in the scheme of CSTR exothermic reaction temperature control, and the flow of coil cooling water is manipulating variable. Assume that single-loop control system uses the control of pure proportional action (strategy). Since the lag of measurement part is small, open-loop transfer function between CSTR reaction temperature Θ and coil cooling water flow F_sc can be approximately shown as [12, 13]

               (3)

For the specific device parameters, the actual open loop transfer function is obtained as follows:

            (4)

Assume that single-loop control system uses the control of pure proportional action (algorithm). Response curve of a single loop coil cooling temperature of CSTR can be obtained as shown in Fig. 2.

Fig. 2 Temperature response curves of CSTR used single cooling control on coil or jacketed cooling:

3) Contrast of two single-loop control schemes

According to the scheme of previous single loop control, semi-physical simulation experiments are performed respectively for the CSRT temperature control process of coil and jacket cooling, and temperature dynamic response curves of exothermic reaction CSTR are shown in Fig. 2. As shown in Fig. 2, when t=500 s, the temperature set value increases from 70 °C to 72 °C, dynamic deviation of step response curve for the coiling cooling scheme is smaller than the dynamic deviation of jacket cooling scheme, and the adjustment time is shorter than that of jacket cooling scheme. When t=900 s, the reaction material flow is changed and disturbance is added into smooth reaction process. The suppression effect of coil cooling system for disturbance (dynamic deviation and adjustment time) is superior to the jacket cooling system, which indicates that the dynamic characteristics of coil cooling scheme is significantly better than the jacket cooling scheme. But under the same steady state load (production) conditions, cooling water flow of coil cooling scheme is much larger than the amount of jacket cooling water, which indicates that economic efficiency of jacket cooling scheme is better than the coil cooling scheme and the consumption of coil cooling scheme is high. This is because the volume of coil is smaller than that of the jacket, and the duration of cooling water in the coil is shorter than that of the jacket. Heat exchange insufficient and a low cooling efficiency of the cooling unit of water can lead to the coil cooling efficiency below the jacket cooling. When process is in steady-state, the cooling water consumption of cooling coil is greater than that of the jacket cooling water consumption. The controllability and dynamic characteristics of closed-loop system used coil cooling scheme are superior to those of jacket cooling scheme. Taking the economic production process into consideration, for the actual production process of polymerization reaction using jacket cooling scheme, the advantage is conducive to saving energy, and the disadvantage is the low polymerization temperature control accuracy which will affect product quality and easy to bring the stop accident for the uncontrolled temperature [9, 10, 16]. So, coil cooling system is just used as an emergency security measures, in the normal production process and not play a role.

2.2.2 Temperature dual-control system of CSTR

Considering the energy saving, control performance and the security of the production process, the control system should combine their respective advantages and overcome their respective disadvantages of the two single-loop schemes. So, when the reactor temperature has deviation, it should be eliminated quickly by the manipulated variable (coil cooling water flow) which has good dynamical performance; when the process has kept steady-state, the manipulated variable (jacket cooling water flow) with high cooling efficiency will gradually replace the manipulated variable with good dynamic performance and low cooling efficiency, and finally the control performance of system is kept the ideal level in terms of both dynamic performance and static performance. The control scheme must have two manipulated variables: jacket and coil cooling water flow, and the reaction temperature in the dynamic performance and static performance both are guaranteed by the system to adjust the two manipulated variables. Dual-control scheme may meet these requirements.

According to the dynamic characteristics analysis of coil and jacket cooling process and the analysis of semi-physical simulation curves in Fig. 2, the dual control system is designed by selecting the reaction temperature (represented by y_T) as the (direct) controlled variable and cooling water flow F8, F7 of coil and jacket cooler as the manipulated variables. Dual control is also called valve position control (V_PC). The main manipulated variable of dual control is reaction temperature y_T, and the manipulated variable is coil cooling water flow F7; The valve position controlled is the opening amount of coil cooling water valve V7; The manipulated variable is the jacket cooling water flow F8; The constituted CSTR dual-control system is shown in Fig. 3.

Its working principle is that the master controller TC of CSTR temperature dual-control system can control the opening of coil valve V7 and adjust the coil cooling water flow F7 according to the deviation e_T when the deviation(e_T=r_T-y_T) is generated by reactor temperature y_T deviated from the set value r_T . Since the coil cooler has good dynamic characteristics, y_T can return quickly to the set value r_T and the deviation of temperature is eliminated timely. This ensures good dynamic response, while the efficiency of coil cooling is low and the coil cooling water flow in the regulating process of temperature changes largely.

When the main controller T₁C adjusts the opening of coil cooling water valve V7, the opening of valve V7 deviates from the set value r_VPG, then valve position controller (V_PC) controls opening of jacket valve V8 and adjusts the jacket cooling water flow F8 and the jacket cooler will replace gradually the coil cooler. So, the opening of coil cooling water valve V7 returns to the set value r_VPG. Thus, in the stable condition, the main cooling load of polymerization CSTR is born by jacket. In the dynamic adjustment process, namely, the coil cooler control temperature is at the beginning, when the opening of coil valve V7 has changed, jacket cooler controls temperature through controlling the opening of valve V8. Because the heat exchange capacity of both the jacket and coil cooler is used fully, the dynamic characteristics of temperature control is further improved.

On one hand, CSTR temperature dual-control system of polymerization and exothermic uses the good dynamic characteristics of coil cooling system to eliminate rapidly deviation and restore quickly the controlled parameters(variable) y_T to the set value r_T. It ensures that the control system has good dynamic response. On the other hand, using the good static characteristics of jacket cooler (high efficiency of cooling), the reaction temperature has good static properties. Dual control takes full advantage of the good dynamic characteristics of coil cooler and good static characteristics of jacket cooler, and CSRT temperature control of polymerization reaction takes full advantage of the characteristics of two cooling systems, overcoming their shortcomings and achieving satisfactory results in terms of dynamic and static performance.

Control performance of CSTR temperature dual control scheme can be analyzed theoretically. To simplify block diagram, valves V7, V8, and vice-regulation loop are respectively incorporated into the coil and jacket cooling system, then simplified system block diagram of CSTR dual control can be obtained as shown in Fig. 4. G_TC(s) and G_VPC(s) are the transfer functions of the temperature controller and the valve controller and G_SCO(s) and G_JCO(s) are transfer functions of generalized object of coil and jacket cooling system. Obviously, this is a double-input and single-output system and the transfer function is shown as Eq. (5).

Fig. 3 Dual control system of CSTR polymerization temperature

Fig. 4 Simplified system block diagram for temperature dual- control system of CSRT exothermic reaction

 (5)

Figure 5 shows temperature response curve of semi- physical simulation of CSTR polymerization dual- control system (the jacket and coil cooling process take the actual transfer functions Eqs. (2) and (4) of CSTR). It can be seen from the temperature response curve that the dual-control system has good dynamic characteristics. Response curve of temperature dynamics is even slightlybetter than the curve of coil cooling scheme in Fig. 2. It depends on the jacket cooler which is involved in the dynamic control process in the dynamic response process of temperature dual-control, and dynamic performance compared with the temperature control process only by the coil is improved to some extent.

Fig. 5 Response curve of semi-physical experiment of CSTR temperature dual-control solution:

After the end of adjusting process of dynamical temperature, the CSTR exothermic reaction temperature returns to the set value r_T, and then valve controller VPC adjusts slowly the opening of jacket cooling water valve V8, which replaces coil cooler of low efficiency, so that the coil cooling valve V7 (corresponding to the cooling water flow F7) recovers to the set value. The curves of jacket cooling water flow of F8 and coil cooling water flow of F7 reflect clearly this adjustment process.

By comparing the response and cooling water flow curves in Fig. 5 and Fig. 2, CSTR temperature dual- control scheme of polymerization reaction has better dynamic characteristics. The dynamic characteristics is similar with the scheme of coil cooling (slightly better), which has short transition process and high control accuracy. The static characteristics are similar with scheme which uses jacket cooling. So, the system has high-efficiency cooling and is conducive to saving energy and water.

3 Fault-tolerant analysis of temperature dual-control of CSTR with jacket and coil heat exchanger

When the control system and some components (such as actuators, sensors and component parts) have malfunctions, dynamic fault-tolerant control system can remain stable and still guarantee certain performance indicators. Fault-tolerant control in the dynamic process is achieved through the control system which has redundant function or information to some extent [17-19]. Fault-tolerant control can be divided into active and passive fault tolerant control depending on whether uses FDD or not.

Active fault-tolerant control based on FDD monitoring and diagnosis can isolate the current occurring fault mode and type, using correlatively prior knowledge and reconstructing the structure of the control system or adjusting the control strategies to ensure process control sound performance and safe operation. Passive fault tolerant control is trying to advance the design and realize fault-tolerant control using the constant control scheme in running process, and it does not require the support of FDD, and a passive fault tolerant control condition is quite harsh.

Functional redundancy is an inherent property of the system, which means that different devices or components overlap in the function and some components or some functions of devices or all functions may be replaced by other functional components. CSTR temperature dual-control scheme takes full advantage of its operating functions redundant, which not only has the excellent properties in the improved quality of control, but also has passive fault-tolerant features. This has very important significance for the improved safety of the production process.

3.1 Analysis of fault-tolerant for CSTR temperature dual-control system

In the conventional polymerization production of CSTR temperature control scheme, the reaction temperature control is implemented by manipulated variable using cooling water flow of jacket cooler, and the shortcoming of control is that accuracy is not high and prone to frequently cease caused by the uncontrolled temperature. In order to prevent the greater production accidents caused by runaway temperature, the system often uses the coil cooling as an emergency of forced cooling. When the situation of runaway temperature occurs, the forced cooling is implemented by staff with manual operation or coil cooling water valve V7 opened by safety interlock to avoid further accidents. However, this disposition may lead to the result that productivity and safety are not high.

The CSTR temperature fault-tolerant control scheme based on dual control is proposed using the operation redundancy of dual cooling CSTR and dual control of features.

3.1.1 Fault-tolerant analysis of dual-control scheme for jacket cooling fault

The temperature dual-control system is shown in Fig. 3. In order to improve the cooling efficiency, the most of cooling load of normal production process is produced by jacket and the cooling load of coil cooling is the minimal (it means that the set value r_VPS of valve V7 position is the minimal in the case of ensuring control performance and security needs). When the system is in the steady-state conditions, the opening of coil valve V7 and the cooling water flow F7 is very small. There is a big cooling load margin in the coil ex-changer. If the jacket cooling system is fault, eq. the cooling water valve V8 stuck, not opening, or pipe blockage, temperature controller G_TC(s) of dual control system undertakes all the cooling load through the control of coil valve V7 (cooling water flow F7). It can still control the reaction temperature y_T (T1) at the set value r_T, avoid the reaction of runway temperature and then maintain CSTR system security and normal production. The CSTR temperature dual-control system has a passive fault-tolerant capacity for the fault of jacket cooling system, which is important for the polymerization reaction CSTR system security. The temperature dual-control system has the fault-tolerant capacity for the jacket cooling system fault that can transfer automatically cooling load of malfunction or failure in the jacket cooling system to the coil cooling system, whose advantages are superior than the safety measure by taking forced cooling for maintaining normal production and improving system reliability. In the normal production process, the load of coil cooling (the cooling water flow F7) produced actually by coil is very small but has large margin of cooling and low probability of failure, which ensures reliability of the improved fault-tolerant. This can also be obtained from the transfer function of the dual-control system of analysis Eq. (5). When the jacket cooling channel fault is failure, it corresponds to the channel jacket disconnecting in Figs. 3 or 4, and then the transfer function Eq. (5) degenerates into Eq. (6).

It can be seen that the CSTR temperature dual- control system has good dynamic characteristics from the analysis above.

                (6)

Figure 6 shows CSTR temperature semi-physical simulation curve when the jacket cooling system t= 1000 s and the cooling water valve (V8) is shut down and broken. It can be seen from Fig. 6, the dual controlsystem of temperature reactor is still effective, which puts the reaction temperature to the set value; the production process of polymerization reaction is normal. It can be seen from the change of cooling water flow (F7, F8) that the dual-control system transfers automatically the stable cooling load produced by jacket to cooler, so it has the fault-tolerant capacity for the breaking of jacket cooling system. But the cost is more than cooling water flow to be used, which is the disadvantage of saving energy. It is only used as temporary emergency and safety measure.

Fig. 6 Semi-physical simulation response curve of CSTR dual- control for jacket cooling water cutoff:

3.1.2 Fault-tolerant analysis of dual control scheme for coil cooling fault

It can be seen from the analysis of simplified system block diagram of temperature dual-control system in Fig. 4 that the control signal of temperature controller G_TC(s) is not effective in the coil channel when the valve V7 in coil cooling system is closed or stuck, and G_SCO(s) cannot control temperature y_T. The valve position of controller G_VPC(s) in jacket cooling system controls the flow of jacket cooling water to control temperature affected by G_TC(s). Superficially, since CSTR has the capacity of redundant operation, the dual-control system performs dynamic control using jacket cooling system instead of coil, which shows that the dual-control system has fault-tolerant capacity for the fault of coil cooling system.

When fault appears in the coil cooling water system (t=1000 s), the semi-physical simulation curve of the temperature dual-control system reactor is shown in Fig. 7. From temperature response curve in Fig. 7, when in the coiler cooling water cutting occurs, the reaction temperature may rise up quickly, and jacket cooling system can open the cooling valve V8 and increase the cooling water flow F8 to reset reactor temperature to the set value. It can be seen from temperature curve and jacket cooling water flow curve that the reactor temperature keeps rise up quickly, because the heat taken away by jacket cooling water is smaller than reaction release heat eventually and which will lead to the reaction process out of control. It means that the CSTR temperature dual-control system dose not have the capacity of fault-tolerant for the fault of coil cooling system.

3.2 CSTR fault-tolerant temperature control system based on dual control and FDD

It can be seen from the above analysis that the CSTR temperature uses dual control of reaction temperature for production process of polymerization exothermic reaction, which improves control performance of the fault-tolerant capacity for jacket cooling fault and enhances the production security. Semi- physical simulation illustrates that although CSRTtemperature dual-control system has fault-tolerant capacity for the fault of jacket cooling system, it dose not have that coil cooling valve is stuck or water is cut off. Due to the potential danger in the production process of exothermic reaction, it is necessary to design fault detection and diagnosis (FDD) and the corresponding active fault-tolerant control strategy for the production process of CSTR polymerization exothermic reaction.

Fig. 7 Semi-physical simulation response curve of CSTR dual- control for coil cooling water cut off:

3.2.1 CSTR temperature initiative fault-tolerant control

1) Initiative fault-tolerant control strategies of coil cooling system fault

The analysis and simulation of temperature dual- control system for exothermic reaction illustrate that the system has good fault-tolerant capacity for the fault of jacket cooling system, and dose not have fault-tolerant capacity for the breaking of coil cooling system. The main reason of this situation is that the control algorithm of temperature controller G_TC(s) shown in Fig. 3 is designed for the fast response performance of coil cooling process G_SCO(s). When the coil cooling system is fault as the same as the channel G_SCO(s) cutting off, G_TC(s) (series with G_VPC(s)) plays full role in jacket cooling channel, and the dual control system shown in Fig. 4 is converted into single-loop control system, and Eq. (5) transforms to Eq. (7).

         (7)

The control algorithm of temperature controller G_TC(s) is designed according to the channel G_SCO(s) of coil cooling, which dose not match the slow response characteristic of jacket cooling process, and this leads to response process of runway temperature.

From the above analysis, the dual control system cannot control effectively CSTR reaction temperature to maintain normal production and system security running when the coil cooling water is cut off. However, the operation function of jacket cooling system is not lost and still has the capacity of making reactor temperature reset the set value of process regulation and ensuring normal production process and safety equipment. Hence, CSTR jacket cooling system can still control polymerization reaction temperature and ensure equipment security and normal production process by the initiative fault-tolerant strategy for fault in the coil cooling system.

If the coil cooling water is in failure, temperature controller G_TC(s) in Fig. 8 resets the proportional unit to gain 1, the r_vps is set zero, while the G_VPC(s) is set to the single-loop tuning value, the jacket cooling water flow that is the manipulated variable only. Thus, the CSTR temperature dual-control system transforms to single- loop temperature control system whose manipulated variable is only jacket cooling water flow. It not only implements the initiative fault-tolerant control of the fault of coil cooling system, but also needs FDD system support for the coil cooling malfunction.

Fig. 8 Coil cooling system cutting off and then dual control system transforming to single-loop temperature control system cooled by jacket

2) Fault diagnosis of CSTR coil cooling cooler of polymerization exothermic reaction

The control system of double CSTR of temperature belongs to passive FTC, and the failures of fault-tolerant control of collier refrigeration and temperature sensor belong to active FTC. After the fault occurs, the system of fault-tolerant control resets the system according to the requires or readjusts the parameter of controller, which stabilizes the system and maintain some basic index. The precondition is that FDD subsystem can timely detect the malfunction and diagnose the type of the malfunction accurately; the active FTC can be achieved. In order to achieve the fault-tolerant control of temperature system, we need to design corresponding system of the process diagnosis. Online malfunction diagnosis is design of CSTR temperature control system can be performed according to Ref. [20]. Fault-tolerant control of polymerization CSTR reaction temperature can be realized by the control decision based on result of process detection and diagnosis.

3.2.2 CSTR temperature fault-tolerant control system structure based on dual control and FDD

Among three parts of the dual-control system, fault detection and diagnosis are combined to form the CSTR temperature tolerant control system for the CSTR double heat ex-changer, as shown in Fig. 9.

Fig. 9 CSTR temperature fault-tolerant control system based on dual-control and fault diagnosis

3.3 CSTR temperature fault-tolerant system semi- physical simulation experiment based on dual control and FDD

When the control system (Fig. 9) works on the semi-physical simulation platform and simulate the malfunction manually, the CSTR temperatures responsive curve can be got as follows.

1) At t=1000 s, the malfunction of CSTR jacket cooling system (jacket cooling water cut off abruptly), and the responsive curve of temperature are shown in Fig. 10.

At this moment, dual-control system has ability of passive fault-tolerant. CSTR temperature dual-control system can control and maintain the CSTR temperature at set value by increasing coil cooling water flow. Then the dual control degrades to single-loop control system with only coil flow manipulation. The fault tolerant of jacket cooling doesn’t need FDD support, and also doesn’t have to change the structure and controller parameter of dual-control system, which belongs to passive fault-tolerant control process. The system of FDD can diagnose the malfunction of V8 by the diagnosis constraint relations. Then the operator can be notified by malfunction alarm to take the relevant measure.

2) At t=1000 s, the malfunction of CSTR jacket cooling system (jacket cooling water is cut off abruptly),and the reactor temperature rise up. Process diagnosis system can diagnose the malfunction of V7 by diagnosis of the constraint relations, and at the same time switch the CSTR temperature dual-control to jacket single-loop control mode. Jacket cooling system will control reactor temperature and alarm. The responsive curve of CSTR temperature is shown in Fig. 11.

The experiment results from above show that, CSTR fault tolerant temperature control system based on dual control and process diagnosis has fault-tolerant ability for jacket cooling malfunction and coil cooling malfunction. It is different from that the fault tolerance for the jacket cooling fault is a passive FTC and the fault-tolerance for the coil cooling fault is an active FTC. When the severe malfunction happens in jacket cooling and coil cooling, the strategy of fault-tolerant control based on the scheme of VPC can ensure the safe production of exothermic reaction CSTR.

Fig. 10 Semi-physical simulation temperature response curve when jacket cooling water is cut off:

Fig. 11 Fault-tolerant control process based on FDD and temperature response curve when coil cooling water cut off:

4 Conclusion

Based on the analysis of the CSTR exothermic reaction control characteristic, the CSTR temperature dual-control strategy is proposed for an organic material polymerization production. Principle analysis and the experiment of semi-physical simulation prove the superiorities of the dynamic response and static character, and also confirm the fault-tolerance ability for the malfunction of jacket cooling water.To fully use CSTR and dual temperature control’s ability of operation redundancies, active fault tolerance control is designed based on FDD. Then, the dual control, FDD and passive FTC are combined to form the fault-tolerant control system for the CSTR temperature with dual-heat ex- changer, which achieves fault tolerance control of CSTR temperature, enhances the safety of the polymerization exothermic reaction production. Finally, semi-physical simulation experiment verifies the availability of the system designed.

References

[1] ZHANG Lian. Chemical reactor analysis [M]. Shanghai: East China University of Science and Technology Press, 2005. (in Chinese)

[2] FROMENT G. F, BISCHOFF K. Chemical reactor analysic and design [M]. New York: Wiley & Son, 1983.

[3] ZHU Bin-chen. Chemical reaction engineering [M]. Beijing: Chemical Industry Press, 2007. (in Chinese)

[4] GUO Kai, TANG Xiao-heng, ZHOU Xu-mei. Chemical reaction engineering [M]. Beijing: Chemical Industry Press, 2008. (in Chinese)

[5] WANG Ji-cheng. Chemical process control engineering [M]. Beijing: Chemical Industry Press, 2005. (in Chinese)

[6] WANG Zai-ying. Coil and Jacket dual control [J]. Computer and Chemical Application, 2012, 29(7): 829-833. (in Chinese)

[7] WANG Zai-ying, LIU Yin. Study on the temperature dual control of CSTR with coil cooling and jacket cooling and application [C]// Intelligent Control and Automation. 2012: 3359-3363.

[8] CHENG Yuan-ping. Chemical reactor safety operation technology development status [J]. China Safety Science Journal, 2000, 10(1): 66-70.

[9] LIU Xiu-yu, JIANG Jun-cheng. Continuous stirred tank reactor for exothermic reaction runaway accident prevention [J]. China Safety Science Journal, 2006, 16(11): 140-144.

[10] LIU Xiao-chun, SUN Shun-li. Polypropylene workshop post polymerization autoclave in risk analysis and safety measures [J]. China Safety Science Journal, 2005, 15(3): 99-113.

[11] LIU Xiao-chun, SUN Shun-li. Polypropylene workshop post polymerization autoclave in risk analysis and safety measures [J]. China Safety Science Journal, 2005, 15(3): 99-103.

[12] SUN You-xian, SHAO Hui-he. Industry process control technology [M]. Beijing: Chemical Industry Press, 2006. (in Chinese)

[13] YU Jin-shou. Process control engineering [M]. Beijing: Electronic Industry Press, 2007. (in Chinese)

[14] SEBORG D E. Process dynamic and control [M]. New York: John Wiley & Son, 2004.

[15] YANG Hui-zhong, SU Si-xian. CSTR control based on active disturbance rejection control [J]. Control Engineering of China, 2011, 18(3): 369-372.

[16] HONG Ding-yi. The principle, process and technology of polypropylene [M]. Beijing: China Petrochamiccal Press, 2007. (in Chinese)

[17] YANG You-qi. Chemical process simulation and optimization [M]. Beijing: Chemical Industry Press, 2006. (in Chinese)

[18] YANG Wei. The fault-tolerant flight control system [M]. Xi’an: Northwestern Polytechnical University Press, 2000. (in Chinese)

[19] ZHOU Dong-hua, YE Yin-zhong. The modern fault diagnosis and fault-torlerant [M]. Beijing: Tsinghua University Press, 2000. (in Chinese)

[20] WANG Zai-ying, BAI Hua-ning. The fault detection of process control system based on the correlation coefficient and diagnosis method [J]. Chemical Industry Journal, 2013, 64(12): 4615-4621.

(Edited by YANG Hua)

Cite this article as: WANG Zai-ying, WANG Guo-xin. Temperature fault-tolerant control system of CSTR with coil and jacket heat exchanger based on dual control and fault diagnosis [J]. Journal of Central South University, 2017, 24(3): 655-664. DOI: 10.1007/s11771-017-3466-0.

Foundation item: Project(2013JM8024) Supported by Natural Science Basic Research Plan in Shaanxi Province of China

Received date: 2015-11-13; Accepted date: 2016-01-13

Corresponding author: WANG Zai-ying, Professor, PhD; Tel: +86-13609168081; E-mai: zying_wang@xust.edu.cn