Reactor conversion, controlling

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

Parameter Estimation Relational and physical models require adjustable parameters to match the predicted output (e.g., distillate composition, tower profiles, and reactor conversions) to the operating specifications (e.g., distillation material and energy balance) and the unit input, feed compositions, conditions, and flows. The physical-model adjustable parameters bear a loose tie to theory with the limitations discussed in previous sections. The relational models have no tie to theory or the internal equipment processes. The purpose of this interpretation procedure is to develop estimates for these parameters. It is these parameters hnked with the model that provide a mathematical representation of the unit that can be used in fault detection, control, and design. 

By maintaining the first-stage reactor just beyond the phase inversion point, the dispersed rubber phase is relatively rich in dissolved styrene. As polymerization subsequently proceeds in the LFR s, the dissolved styrene will react to form either a graft copolymer with the rubber or a homopolymer. The latter will remain within the rubber droplet as a separate occluded phase. Achieving the first-stage reactor conversion and temperature by recycling a portion of the hot second reactor effluent may permit simplification of the first reactor temperature control system. 

FIGURE 5. Monomer ratio In controlled (C) and uncontrolled (U) typical terpolymerisation taken to high conversion. Control was achieved by feeding both TBTM and MMA to a semi-batch reactor at 80°C. Monomer ratios measured by GPC. 

The available data from emulsion polymerization systems have been obtained almost exclusively through manual, off-line analysis of monomer conversion, emulsifier concentration, particle size, molecular weight, etc. For batch systems this results in a large expenditure of time in order to sample with sufficient frequency to accurately observe the system kinetics. In continuous systems a large number of samples are required to observe interesting system dynamics such as multiple steady states or limit cycles. In addition, feedback control of any process variable other than temperature or pressure is impossible without specialized on-line sensors. This note describes the initial stages of development of two such sensors, (one for the monitoring of reactor conversion and the other for the continuous measurement of surface tension), and their implementation as part of a computer data acquisition system for the emulsion polymerization of methyl methacrylate. 

A Simulation Study on the Use of a Dead-Time Compensation Algorithm for Closed-Loop Conversion Control of Continuous Emulsion Polymerization Reactors... 

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. 

Several control techniques have been developed to compensate for large dead-times in processes and have recently been reviewed by Gopalratnam, et al. (4). Among the most effective of these techniques and the one which appears to be most readily applicable to continuous emulsion polymerization is the analytical predictor method of dead-time compensation (DTC) originally proposed by Moore ( 5). The analytical predictor has been demonstrated by Doss and Moore (6) for a stirred tank heating system and by Meyer, et al. (7) for distillation column control in the only experimental applications presently in the literature. Implementation of the analytical predictor method to monomer conversion control in a train of continuous emulsion polymerization reactors is the subject of this paper. 

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. 

Figure 3. Schematic of a control system for the first reactor of a series using supervisory setpoint control of initiator feed rate to control reactor conversion...

Closed-loop response to process disturbances and step changes in setpoint is simulated with the model of Kiparissides extended to predict the behavior of downstream reactors. Additionally, a self-optimizing control loop is simulated for conversion control of downstream reactors when the first reactor of the train is operating under closed-loop control with dead-time compensation. 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

Turning to the slow dynamics, we are interested in stabilizing the reactor holdup, controlling the process outlet temperature, and controlling the temperature in the reactor to manage the conversion. The first two objectives can be addressed using simple linear controllers. In order to stabilize the reactor holdup, we implemented a PI controller using the reactor outlet flow rate, F, as a manipulated input ... 

We considered two scenarios that are typical for the operation of reactors with feed-effluent heat exchange. The first set of simuiations traced the response of the ciosed-ioop system to a 10% increase in the production rate, imposed at t = 1 h by increasing the feed flow rate. Subsequently, we analyzed the response of the same situation, but with the added complexity of an unmeasured 10 K increase in the feed temperature occurring at t = 1 h. In both cases, the setpoint of the reactor temperature controller Tjqset was increased by 2 K at t = 1 h in order to maintain reactor conversion at the higher production rate. 

For the liquid-phase reactor shown in Fig. 4.37, monomer feed is introduced and the effluent stream controls the level (residence time). Heat is removed via cooling water. We want to remove the water to push the equilibrium to the right and increase conversion. Due to its volatility, it would be natural to remove the water vapor from the reactor to control pressure. 

Effective solutions to the problems of the vacuum residue hydrodesulfurization unit equipped with the fixed bed reactors, such as a hot spot, pressure-drop buildup, and catalyst deactivation by coke fouling, were discussed. Improving liquid distribution can prevent hot spot occurrence. Dispersing inorganic solids throughout the reactors can control a pressure-drop increase in the first bed. For a high conversion operation, controlling the conversion in each bed can minimize the coke deactivation in the fourth bed. 

Next we will examine potential controlled and manipulated variables. There are five potential controlled variables production (Fb), hydrogen/toluene ratio (yns/yn), pressure, purge composition (ynp). and conversion (X). However, some of them might be left uncontrolled. For example, conversion control is difficult, because of dead-time associated with a PFR. Moreover, it requires on-line composition analyser, which is not available or expensive. For these reasons, it would be desirable to develop a control structure in which controlling the reactor inlet temperature would be sufficient. 

---