Reactors, batch temperature profile

The polymerization takes place in the presence of catalysis. The objective of the batch process is to maximize production of the product by controlling the amount and timing of added reactants along with the reactor s temperature profile. Of course, this objective is conditional on satisfying certain safety constraints. In the initial recipe the feedrates for components A and B, the temperature and the catalyst charge, which are also the decision variables in the optimization, are set. 

The tower reactors similar to the ethylene polymerization reactors are used in other free-radical vinyl polymerization processes. Figure 4a shows a schematic of the tower reactor for bulk styrene polymerization developed by Farben in the 1930s [4]. The prepolymers prepared in batch prepolymerization reactors to about 33-35% conversion are transferred to a tower reactor whose temperature profile is controlled from 100°C to 200°C by jackets and internal cooling coils. There is no agitation device in the tower reactor. The product is then discharged from the bottom of the tower by an extruder, cooled, and pelletized. 

A hst of polyol producers is shown in Table 6. Each producer has a varied line of PPO and EOPO copolymers for polyurethane use. Polyols are usually produced in a semibatch mode in stainless steel autoclaves using basic catalysis. Autoclaves in use range from one gallon (3.785 L) size in research faciUties to 20,000 gallon (75.7 m ) commercial vessels. In semibatch operation, starter and catalyst are charged to the reactor and the water formed is removed under vacuum. Sometimes an intermediate is made and stored because a 30—100 dilution of starter with PO would require an extraordinary reactor to provide adequate stirring. PO and/or EO are added continuously until the desired OH No. is reached the reaction is stopped and the catalyst is removed. A uniform addition rate and temperature profile is required to keep unsaturation the same from batch to batch. The KOH catalyst can be removed by absorbent treatment (140), extraction into water (141), neutralization and/or crystallization of the salt (142—147), and ion exchange (148—150). 

Figure 5.63. Temperature profiles for the batch reactor, Tl, and for the jacket TC, which lags behind.

Example 14.1 Consider again the chlorination reaction in Example 7.3. This was examined as a continuous process. Now assume it is carried out in batch or semibatch mode. The same reactor model will be used as in Example 7.3. The liquid feed of butanoic acid is 13.3 kmol. The butanoic acid and chlorine addition rates and the temperature profile need to be optimized simultaneously through the batch, and the batch time optimized. The reaction takes place isobarically at 10 bar. The upper and lower temperature bounds are 50°C and 150°C respectively. Assume the reactor vessel to be perfectly mixed and assume that the batch operation can be modeled as a series of mixed-flow reactors. The objective is to maximize the fractional yield of a-monochlorobutanoic acid with respect to butanoic acid. Specialized software is required to perform the calculations, in this case using simulated annealing3. 

Let us consider the batch reactor sketched in Fig. 3.9. Reactant is charged into the vessel. Steam is fed into the jacket to bring the reaction mass up to a desired temperature. Then cooling water must be added to the jaeket to remove the exothermic heat of reaction and to make the reactor temperature follow the prescribed temperature-time curve. This temperature profile is fed into the temperature controller as a setpoint signal. The setpoint varies with time. 

Determination of die process parameters that ensure a permissible temperature profile and optimal solidification path is based on the general principles of the theory of batch reactors formulated in Section 2.7. Let us illustrate this approach with the example of solidification of a urethane-based compound for use as a coating.176... 

In general, an objective function in the optimization problem can be chosen, depending on the nature of the problem. Here, two practical optimization problems related to batch operation maximization of product concentration in a fixed batch time and minimization of batch operation time given amount of desired product, are considered to determine an optimal reactor temperature profile. The first problem formulation is applied to a situation where we need to increase the amount of desired product while batch operation time is fixed. This is due to the limitation of complete production line in a sequential processing. However, in some circumstances, we need to reduce the duration of batch run to allow the operation of more runs per day. This requirement leads to the minimum time optimization problem. These problems can be described in details as follows. 

N. Aziz, M.A. Hussain, I.M. Mujtaba, Performance of different types of controllers in tracking optimal temperature profiles in batch reactors, Comp. Chem. Eng. 24 (2000) 1069-1075. 

In Figure 4.26 a temperature profile has been specified. The reactor starts at 300 K at time equal zero, and the temperature is ramped to 400 K at 10 min. Then it is ramped to 430 K at 20 min and remains at this temperature for the rest of the 120-min batch. Figure 4.27 shows that the Reactions page tab permits the installation of reactions in the normal way, as discussed in Chapter 2. The primary reaction is ethylene and benzene forming ethylbenzene. 

Numerous reactions are performed by feeding the reactants continuously to cylindrical tubes, either empty or packed with catalyst, with a length which is 10 to 1000 times larger than the diameter. The mixture of unconverted reactants and reaction products is continuously withdrawn at the reactor exit. Hence, constant concentration profiles of reactants and products, as well as a temperature profile are established between the inlet and the outlet of the tubular reactor, see Fig. 7.1. This requires, in contrast to the batch reactor, the application of the law of conservation of mass over an infinitesimal volume element, dV, of the reactor. In contrast to a batch reactor the existence of a temperature profile does not allow us to consider the mass balances for the reacting components and the energy balance separately. Such a separation can only be performed for isothermal tubular reactors. 

In practice the heat effects associated with chemical reactions result in nonisothermal conditions. In the case of a batch reactor the temperature changes as a function of time, whereas an axial temperature profile is established in a plug flow reactor. The application of the law of conservation of energy, in a similar... 

Most batch reactors today employ, at best, only partially automatic control. The operator must keep track of numerous valves, motors, and flow and temperature gages — just to control temperature, and just for one reactor. Failure to continuously coordinate these devices leads to varying temperature profiles from batch to batch, and, thus, inconsistent or unacceptable product quality. 

This work presents the on-line level control of a batch reactor. The on-line strategy is required to accommodate the reaction rate disturbances which arise due to catalyst dosing uncertainties (catalyst mass and feeding time). It is concluded that the implemented shrinking horizon on-line optimization strategy is able to calculate the optimal temperature profile without causing swelling or sub-optimal operation. Additionally, it is concluded that, for this process, a closed-loop formulation of the model predictive controller is needed where an output feedback controller ensures the level is controlled within the discretization intervals. 

In contrast to a batch reactor, the existence of a temperature profile does not allow us to consider the mass balances for the reacting components and the energy balance separately. Such a separation can only be performed for isothermal tubular reactors. 

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