Cascade temperature

Fig. 10. Block diagrams a) Single loop concentration controi and b) Cascade temperature control...

In all these cases the reflux rate is simply set at a safe value, enough to nullify the effects of any possible perturbations in operation. There rarely is any harm in obtaining greater purity than actually is necessary. The cases that are not on direct control of reflux flow rate are (g) is on cascade temperature (or composition) and flow control, (h) is on differential temperature control, and (i) is on temperature control of the HTM flow rate. 

A reliable control of the reaction course can be obtained by isothermal operation. Nevertheless, to maintain a constant reaction medium temperature, the heat exchange system must be able to remove even the maximum heat release rate of the reaction. Strictly isothermal behavior is difficult to achieve due to the thermal inertia of the reactor. However, in actual practice, the reaction temperature (Tr) can be controlled within 2°C, by using a cascade temperature controller (see Section 9.2.3). Isothermal conditions may also be achieved by using reflux cooling (see Section 9.2.3.3), provided the boiling point of the reaction mass does not change with composition. 

Therefore, the chapter is mainly focused on the design of model-based control approaches. Namely, a controller-observer control strategy is considered, where an observer is designed to estimate the heat released by the reaction, together with a cascade temperature control scheme. The performance of this control strategy are further improved by introducing an adaptive estimation of the heat transfer coefficient. Finally, the application of the proposed methods to the phenol-formaldehyde reaction studied in the previous chapters is presented. 

In the following, the model-based controller-observer adaptive scheme in [15] is presented. Namely, an observer is designed to estimate the effect of the heat released by the reaction on the reactor temperature dynamics then, this estimate is used by a cascade temperature control scheme, based on the closure of two temperature feedback loops, where the output of the reactor temperature controller becomes the setpoint of the cooling jacket temperature controller. Model-free variants of this control scheme are developed as well. The convergence of the overall controller-observer scheme, in terms of observer estimation errors and controller tracking errors, is proven via a Lyapunov-like argument. Noticeably, the scheme is developed for the general class of irreversible nonchain reactions presented in Sect. 2.5. 

Distillate composition can be controlled by a cascade temperature master on the upper part of the column, which manipulates the reflux flow L (left). Similarly, the bottoms composition can be controlled by a cascade temperature master located on the lower half of the column, throttling the reboiler heat input (right). 

Another approach for removing heat is a circulation loop through an external heat exchanger, as shown in Figure 3.24. The circulation rate is maximized for good heat transfer on the process side, while the heat transfer medium is throttled by the reactor temperature controller. If the reactor is small and well mixed, the cascade temperature control arrangement as shown may not be necessary, and the reactor temperature controller may be connected directly to the valve. 

Table 23.4 shows the three possible loop configurations. Number 3 corresponds to cascade temperature control and, as we have seen in Section 20.1 and Example 20.1, it provides fast compensation. Thus configuration 3 is selected for the reactor. 

Fig. 6.9. Direct-charged aluminum melting furnace with cascaded temperature control and regenerative burners. On the next 20-sec cycle, two air valves, two exhaust valves, and two fuel shutoff valves will reverse positions. Ma = milliamps. Se = suction exhaust. SP = setpoint. T/s = temperature sensor. Courtesy of North American Mfg. Co. 

Tendency Model-based Improvement of the Slave Loop in Cascade Temperature Control of Batch Process Units... 

The batch evolution presented in Figure 1 has a similar shape that the ones drawn from optimization [4]. The results show that temperature and free monomer concentration can be reliably tracked with a nearly linear multivariable calorimetric controller that manipulates the monomer addition and heat exchange rates, and with a control scheme that can be seen as the adequate coordination of two controllers that are well-known and accepted in industrial practice a cascade temperature controller that manipulates the heat exchange rate Qj for a given monomer feed rate w, and a ratio-type free-monomer controller that sets w proportionally to the heat generation rate Q. 

If water-header pressure fluctuations are a problem, use a cascade temperature water-flow control system. 

The preferred arrangement for a vacuum or a pressure column with a large amount of inerts is shown in Figure 3.10. Here the inerts are pulled off or blown out through a vent line in which there is a throtde valve manipulated by the subcooled-condensate temperature controller. For a vacuum column, the low-pressure source is usually a steam jet. If the downstream pressure fluctuates too much, it may be necessary to use a cascade temperature-vent flow-control arrangement. 

Cascading temperature or other variables to flow-ratio control should be avoided unless ratio turndown and flow turndown are less than 2 1. This applies whether one uses the divider (closed-loop) or multiplier (open-loop) flow-ratio scheme. Tlie problem is that the gain of the ratio loop as seen by the primary loop is zero at low flow and high at high flows. We have not foimd a convenient way to compensate for this with analog equipment without interfering with antireset windup. 

The figure below shows cascade temperature control of a polymerization reactor, which uses feed heat exchange to adjust the reactor temperature. Using the instrumentation diagram, explain how this cascade control system (both master and slave components) handles the following disturbances. (describe what happens to the reactor temperature.) Assume normal temperatures of coolant (70°F), polymerization feed (200°F), exchanger effluent (100°F), and reactor outlet (800°F). 

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