2 Experimental Simulation of a Continuous Stirred Tank Reactor for Teaching Reactor Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch002
Design and Control MICHAEL L. BRISK Department of Chemical Engineering, University of Sydney, Sydney, Australia
A novel application of on-line simulation was developed to provide flexible, realistic, safe student experiments to illustrate multiple steady states and dynamic instabilities in the design and control of stirred chemical reactors. An exothermic reaction is simulated in a small CSTR using only steam and water. Heat generation in the reactor, produced by live steam injection, is controlled as a dynamic function of reactor temperature by an on-line analog computer which provides the reaction kinetics and reaction rate-temperature relationships. The reactor exhibits three steady states and under closed-loop temperature control demonstrates limit cycle phenomena. It has been a valuable teaching tool, and with the addition of direct digital computer control its application is extending to research into reactor control algorithms.
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he concepts of steady state and dynamic instabilities in continuous stirred tank reactors (CSTR's) are important topics in chemical engineering courses on reactor design and process control. They are also difficult concepts requiring fairly advanced analytical methods for their mathematical treatment, and the student's understanding of them is aided if he can experiment with these phenomena in the laboratory. However, the design and operation of experimental reactors which allow this is awkward. There can be difficulties with materials of construction, and there are problems in providing and handling reasonable quantities of the necessary chemicals. For example, in one of the few published experimental studies of CSTR stability, it was necessary to run quite a small reactor (0.6 ft ) as a batch reactor to conserve chemicals (I). Once a reaction system has been chosen the equipment tends to be rather inflexible, and it is not simple to vary operating parameters over much of a range. There is, too, an uncomfortable safety problem associated with student operation of a reactor at an unstable state. One solution to this is to write a mathematical model for a selected reaction system and simulate the reactor with associated control loops on a general purpose analog computer. Until recently this approach has been used by the 3
13 Hulburt; Chemical Reaction Engineering—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch002
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CHEMICAL REACTION ENGINEERING
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author to allow students to observe the characteristics of reactor behavior. Although it offers flexibility, this method just does not provide the same experience and "feel" for the system that is obtained by operation of a real reactor. This was noticeably true of undergraduates with no plant experience, who found it very difficult to visualize and believe in the actual system they were modeling if they could not test their model. In particular, the time scale of the analog simulation was much faster than a real system. This is normally exploited as an advantage in simulation, but it removed a further element of reality from the student's experimental study, particularly when dealing with controller responses. The novel system described in this paper has been developed in an attempt to overcome these various problems and provide students with realistic but safe laboratory experiments in reactor stability and reactor control, with emphasis on: (a) observation of stable and unstable steady states; (b) observation of dynamic behavior and the use of standard control hardware to control the reactor; (c) use of D D C by an on-line digital computer for startup and control of the reactor. Experimental Simulation In an early experimental study of CSTR dynamics (-2) a synthetic reactor was described in which an exothermic reaction could be carried out using only steam and water. Heat was generated in the reactor by injecting live steam at a rate proportional to the temperature, using a conventional proportional controller to manipulate the steam valve. This early attempt could represent only a locally linearized model of a zero-order reaction. In the present work this concept has been adapted by replacing the proportional controller by an on-line analog computer which is used to control the rate of steam injection to a small CSTR. The computer generates both the steady state Arrhenius reaction rate-temperature relationship and the dynamics of the reaction kinetics. The hardware reactor plus on-line computer together simulate a real reactor with an exothermic chemical reaction. The combination has been christened an "experimental simulation" of a CSTR. Equipment The complete system is shown in Figure 1. Cold water, representing a reactant, is metered into a 0.4 ft stirred, insulated vessel where it is heated by live steam. The steam valve is manipulated by an on-line analog computer which measures the temperature in the reactor by thermocouple and solves the kinetic equations for the heat generation rate. The computer used is a superseded 100 V , 10 amplifier valve machine which had been withdrawn from teaching use several years ago but was kept in working order. The thermocouple signal, compensated for cold junction variation, is preamplified by a factor of 1000 with a low-drift dc amplifier, the output of which provides the computer input signal. This input is further amplified (10X ) and filtered (0.5 sec time constant) by the first operational amplifier in the computer circuit (Figure 2) to give a working signal of 0.4 V / ° C . The computer output (scaled to 0 to 5 V dc) drives an electropneumatic converter with output in the range 3 to 15 psig. This in turn manipulates a %-inch steam valve through a pneumatic valve positioner which reduces valve hysteresis to a negligible level, simplifying the inclusion of the valve characteristic in the computer calculation. 3
Hulburt; Chemical Reaction Engineering—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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BRISK
Teaching Reactor Design and Control
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