Continuous stirred tank reactors control system

In this short initial communication we wish to describe a general purpose continuous-flow stirred-tank reactor (CSTR) system which incorporates a digital computer for supervisory control purposes and which has been constructed for use with radical and other polymerization processes. The performance of the system has been tested by attempting to control the MWD of the product from free-radically initiated solution polymerizations of methyl methacrylate (MMA) using oscillatory feed-forward control strategies for the reagent feeds. This reaction has been selected for study because of the ease of experimentation which it affords and because the theoretical aspects of the control of MWD in radical polymerizations has attracted much attention in the scientific literature. 

Although continuous stirred-tank reactors (Fig. 3.12) normally operate at steady-state conditions, a derivation of the full dynamic equation for the system, is necessary to cover the instances of plant start up, shut down and the application of reactor control. 

This controller was applied to a methylmethacrylate (MMA) solution ho-mopol nnerization conducted in a continuous stirred tank reactor. The solvent and initiator are ethyl acetate and benzoyl peroxide, respectively. The polymerization system parameters for numerical simulations have been taken... 

The Unipol process employs a fluidized bed reactor (see Section 3.1.2) for the preparation of polyethylene and polypropylene. A gas-liquid fluid solid reactor, where both liquid and gas fluidize the solids, is used for Ziegler-Natta catalyzed ethylene polymerization. Hoechst, Mitsui, Montedison, Solvay et Cie, and a number of other producers use a Ziegler-type catalyst for the manufacture of LLDPE by slurry polymerization in hexane solvent (Fig. 6.11). The system consists of a series of continuous stirred tank reactors to achieve the desired residence time. 1-Butene is used a comonomer, and hydrogen is used for controlling molecular weight. The polymer beads are separated from the liquid by centrifugation followed by steam stripping. 

While the adiabatic batch reactor is important and presents many control issues in its own right, we are concerned here primarily with continuous systems. We consider in detail two distinct reactor types the continuous stirred tank reactor (CSTRj and the plug-flow reactor. They differ fundamentally in the way the reactants and the products... 

The sulfur dioxide-rich citrate solution in the bottom of the absorber is fed by level control through a steam-heated exchanger to a three-stage continuous stirred tank reactor system countercurrent to a flow of hydrogen sulfide gas. For this installation the gas source is a tank of liquid hydrogen sulfide. 

We will begin with the case of the batch reactor. In this case the vessel defines the control volume. We will move to systems with flow in and both flow in and out. The former is the case of semibatch operation while the latter will be treated as the continuous stirred tank reactor (CSTR) and the plug flow reactor (PFR). All the chemical kinetics that we will need can be introduced within the context of these four different kinds of reactors. 

To illustrate the design/control trade-off more quantitatively, let us consider a simple chemical engineering system a series of continuous stirred-tank reactors (CSTRs) with jacket cooling. This type of reactor system is widely used in industry. The reactions and the reactors are quite simple, but they provide some important into evaluating the tradc-offs between steady-state design id eontroL In this section we consider the sin pfe po ibie reai ih reaction A... 

Pfeil and coworkers presented a model for the synthetic pathway of formose, shown in Scheme 5. A similar, but more detailed, model was given by Mizuno and coworkers, who investigated the intermediates in the reaction by chromatographic fractionation of alditol acetate derivatives by g.l.c. (see Table IV). Weiss and coworkers conceptualized the formose reaction as a consecutive-parallel scheme (see Scheme 6) proceeding to the C level, and reported a series of experiments in the continuously stirred tank-reactor previously mentioned to determine the effect of various concentrations of formaldehyde and calcium hydroxide on the reaction rate. The advantage of the tank reactor is that conversions in the autocatalytic system can be controlled, and reaction rates can be measured directly. When the formaldehyde feed-rate was kept constant, and the feed rate for calcium hydroxide varied, products were obtained... 

In the following discussion, we will detail the results and operational experiences the enhanced SBCR system. Objectives of the ran were to 1) test the new slurry level control system 2) compare the performance of a precipitated Fe/K Fischer Tropsch Synthesis (FTS) catalyst in the enhanced SBCR and a continuous stirred tank reactor (CSTR) and 3) determine the effectiveness of the catalyst/wax filtration system. 

Since they are open systems that can exchange chemical species with their surrounding solvent, gels can also play the role of chemical reactors. In this framework, the design of open spatial gel reactors has allowed well controlled experimental studies of chemical patterns such as chemical waves or Turing structures (2). They are made of a thin film of gel in contact with one or two continuous stirred tank reactors that sustain controlled nonequilibrium conditions. 

Consider the control of a jacketed, continuous, stirred-tank reactor (CSTR) in which the exothermic reaction A — B is carried out. This system can be described by 10 variables, as shown in Figure 20.7 h, T, Ca, Cai, T Fi, F Fc, T and T oy diree of which are considered to be externally defined C I and Tco- Its model involves four equations, assuming constant fluid density. 

One unique but normally undesirable feature of continuous emulsion polymerization carried out in a stirred tank reactor is reactor dynamics. For example, sustained oscillations (limit cycles) in the number of latex particles per unit volume of water, monomer conversion, and concentration of free surfactant have been observed in continuous emulsion polymerization systems operated at isothermal conditions [52-55], as illustrated in Figure 7.4a. Particle nucleation phenomena and gel effect are primarily responsible for the observed reactor instabilities. Several mathematical models that quantitatively predict the reaction kinetics (including the reactor dynamics) involved in continuous emulsion polymerization can be found in references 56-58. Tauer and Muller [59] developed a kinetic model for the emulsion polymerization of vinyl chloride in a continuous stirred tank reactor. The results show that the sustained oscillations depend on the rates of particle growth and coalescence. Furthermore, multiple steady states have been experienced in continuous emulsion polymerization carried out in a stirred tank reactor, and this phenomenon is attributed to the gel effect [60,61]. All these factors inevitably result in severe problems of process control and product quality. 

Suppose that the temperature in an exothermic continuous stirred-tank reactor is controlled by manipulating the coolant flow rate using a control valve. A PID controller is used and is well-tuned. Which of these changes could adversely affect the stability of the closed-loop system Briefly justify your answers. 

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