Reactor control strategies

Figure 11 shows a system for controlling the water dow to a chemical reactor. The dow is measured by a differential pressure (DP) device. The controller decides on an appropriate control strategy and the control valve manipulates the dow of coolant. The procedure to determine the overall failure rate, the failure probabiUty, and the reUabiUty of the system, assuming a one-year operating period, is outlined hereia. 

Fig. 17. Examples of selective control strategy (a) reactor hot spot (b) level override (c) prioritized and (d) constraint controls, where...

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. 

An example of cascade control could be based on the simulation example DEACT and this is shown in Fig. 2.35. The problem involves a loop reactor with a deactivating catalyst, and a control strategy is needed to keep the product concentration Cp constant. This could be done by manipulating the feed rate into the system to control the product concentration at a desired level, Cjet- In this cascade control, the first controller establishes the setpoint for flow rate. The second controller uses a measurement of flow rate to establish the valve position. This control procedure would then counteract the influence of decreasing catalyst activity. 

Obviously the aim in operating the reactor cascade is to ensure operation at the most favourable conditions, and for this both the startup policy and control strategy are important. 

Polymer production technology involves a diversity of products produced from even a single monomer. Polymerizations are carried out in a variety of reactor types batch, semi-batch and continuous flow stirred tank or tubular reactors. However, very few commercial or fundamental polymer or latex properties can be measured on-line. Therefore, if one aims to develop and apply control strategies to achieve desired polymer (or latex) property trajectories under such a variety of conditions, it is important to have a valid mechanistic model capable of predicting at least the major effects of the process variables. 

Control of Substances Hazardous to Health Regulations (COSHH UK), 14 220 Control rods, nuclear reactor, 17 569 Control room, in plant layout, 10 514—515 Controls, food processing, 12 87-88 Control stations, 20 668 Control strategies, for fermentation, 11 36-40 Control systems... 

The example simulation THERMFF illustrates this method of using a dynamic process model to develop a feedforward control strategy. At the desired setpoint the process will be at steady-state. Therefore the steady-state form of the model is used to make the feedforward calculations. This example involves a continuous tank reactor with exothermic reaction and jacket cooling. It is assumed here that variations of inlet concentration and inlet temperature will disturb the reactor operation. As shown in the example description, the steady state material balance is used to calculate the required response of flowrate and the steady state energy balance is used to calculate the required variation in jacket temperature. This feedforward strategy results in perfect control of the simulated process, but limitations required on the jacket temperature lead to imperfections in the control. 

A variety of control schemes can be employed and are of great importance, since the reactor scheme shows a multiplicity of possible stable operating points. Obviously the aim in operating the reactor cascade is to ensure operation at the most favourable conditions, and for this both the startup policy and the control strategy are important. This example is taken from the paper of Mukesh and Rao (1977). 

The response of the kiln to a 20% increase in coke on the catalyst from the reactor is shown in Fig. 27 for no control and for schemes (a) and (b). The simpler scheme (b) is clearly superior to scheme (a) although steady-state considerations predicted that scheme (a) would be the better strategy. The fluctuations in the total air rate of scheme (a) that maintains a constant amount of oxygen to the kiln causes this difference and outweighs the effects of the fluctuations in oxygen amounts present in scheme (b). This comparison showed that control strategies designed... 

We have discussed setting up a single feedback controller and establishing a control strategy for one unit operation a reactor, a column, etc. The next level of complexity is to look at an entire operating plant which is made up of many unit... 

To control the CCD, the anthors chose a closed-loop control strategy with in-line NIR used to measure the monomer concentrations. The closed-loop control strategy nses the NIR in-line measurement to provide close to real-time information abont the concentration of both reactants and prodncts in the reactor. One prerequisite of this control strategy is that the sampling rate be faster than the kinetics of the reaction. 

In this section, the physical model used to describe the flow-through monolith reactor is outlined. Such a reactor is common to all the emissions control strategies discussed in this chapter, apart from soot filters. 

A polytropic reaction means the reactor is neither designed to work under isothermal conditions, nor under adiabatic conditions. The reactor control strategy comprises different periods of time, where different modes of temperature control are applied. These different temperature control strategies may include heating to... 

These different temperature and feed control strategies and their impact on reactor safety, together with general rules for assessing and improving process safety, are presented below. The choice of the reactor temperature and feed rate is also of primary importance for safety and this point will be discussed in the last section of this chapter. 

An often-used method for the limitation of the heat release rate is an interlock of the feed with the temperature of the reaction mass. This method consists of halting the feed when the temperature reaches a predefined limit. This feed control strategy keeps the reactor temperature under control even in the case of poor dynamic behavior of the reactor temperature control system, should the heat exchange coefficient be lowered (e.g. fouling crusts) or feed rate too high. 

This system is the most complex, but also the most versatile. In fact, with this type of system, all the previous modes are accessible without further modification. The temperature set point corresponds to a predefined function of time (Figure 9.12). Polytrophic conditions can be achieved (see Section 6.6). The reactor is heated up at a temperature lower than that of the reaction and is then run under adiabatic conditions, Finally, cooling is started to stabilize the temperature at the desired level. By doing so, energy is saved because it is the heat of reaction that attains the process temperature. Moreover, for batch reactions, the cooling capacity is not oversized, since the low temperature at the beginning of the reaction diminishes the heat production rate. Other control strategies are possible, such as the ramped reactor, where the temperature varies with time (see Section 7.7). 

Feed tank with centrifugal pump the centrifugal pump replaces the gravity, positioning the feed tank, even below the reactor level. Since a centrifugal pump is not volumetric, it is necessary to provide an additional control valve to limit the flow rate. The flow control strategies are the same as described above. 

On-line dynamic optimization and control strategy for improving the performance of batch reactors... 

Fig. 3. The proposed strategy for on-line update and control of reactor temperature profile.

The most common continuous emulsion polymerization systems require isothermal reaction conditions and provide for conversion control through manipulation of initiator feed rates. Typically, as shown in Figure 1, flow rates of monomer, water, and emulsifier solutions into the first reactor of the series are controlled at levels prescribed by the particular recipe being made and reaction temperature is controlled by changing the temperature of the coolant in the reactor jacket. Manipulation of the initiator feed rate to the reactor is then used to control reaction rate and, subsequently, exit conversion. An aspect of this control strategy which has not been considered in the literature is the complication presented by the apparent dead-time which exists between the point of addition of initiator and the point where conversion is measured. In many systems this dead-time is of the order of several hours, presenting a problem which conventional control systems are incapable of solving. This apparent dead-time often encountered in initiation of polymerization. 

The analytical predictor, as well as the other dead-time compensation techniques, requires a mathematical model of the process for implementation. The block diagram of the analytical predictor control strategy, applied to the problem of conversion control in an emulsion polymerization, is illustrated in Figure 2(a). In this application, the current measured values of monomer conversion and initiator feed rate are input into the mathematical model which then calculates the value of conversion T units of time in the future assuming no changes in initiator flow or reactor conditions occur during this time. 

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