Manipulation of the cooling

A similar equation could be applied to the manipulation of the cooling water flow. 

An exothermic reaction involving two reactants is run in a semi-continuous reactor. The heat evolution can be controlled by varying the feed rate of one component This is done via feedback control with reactor temperature measurement used to manipulate the feed rate. The reactor is cooled by a water jacket, for which the heat transfer area varies with volume. Additional control could involve the manipulation of the cooling-water flow rate. 

Add 15 g, of chloroacetic acid to 300 ml. of aqueous ammonia solution d, o-88o) contained in a 750 ml. conical flask. (The manipulation of the concentrated ammonia should preferably be carried out in a fume-cupboard, and great care taken to avoid ammonia fumes.) Cork the flask loosely and set aside overnight at room temperature. Now concentrate the solution to about 30 ml. by distillation under reduced pressure. For this purpose, place the solution in a suitable distilling-flask with some fragments of unglazed porcelain, fit a capillary tube to the neck of the flask, and connect the flask through a water-condenser and receiver to a water-pump then heat the flask carefully on a water-bath. Make the concentrated solution up to 40 ml. by the addition of water, filter, and then add 250 ml. of methanol. Cool the solution in ice-water, stir well, and set aside for ca. I hour, when the precipitation of the glycine will be complete. 

If there is no possibility to maintain a constant temperature by manipulating the temperature of the cooling medium the reaction can be slowed down by diluting the reaction mixture and/or the catalyst. After some components of the reaction mixture have been consumed to a sufficient extent and the reaction becomes too slow, more catalyst or reactants can be added to complete the reaction with the rate of heat generation not yet exceeding that of heat removal. This is the normally used semibatch operation. 

The components of the basic feedback control loop, combining the process and the controller can be best understood using a generalised block diagram (Fig. 2.29). The information on the measured variable, temperature, taken from the system is used to manipulate the flow rate of the cooling water in order to keep the temperature at the desired constant value, or setpoint. This is illustrated by the simulation example TEMPCONT, Sec. 5.7.1. 

These three nonlinear ordinary differential equations will be used to simulate the dynamic performance of the CSTR. The openloop behavior applies when no controllers are used. In this case the flowrate of the cooling water is held constant. With closedloop behavior, a temperature controller is installed that manipulates cooling water flow to maintain reactor temperature. 

The reactor is the jacket-cooled CSTR with an irreversible, exothermic, liquid-phase reaction A —> B, which was considered in Section 3.1. In that section the flowrate of the cooling water Fj to the jacket was the manipulated variable for the reactor temperature controller (TR <— Fj control). In this section we explore the use of the flowrate of the fresh feed F() to control reactor temperature (TR <— F0 control). 

The flowrate of the cooling/heating medium is usually the manipulated variable that is changed by a reactor temperature controller, either directly or through a... 

Dynamic Heat Transfer Option Figure 3.99 shows the parameters that must be specified when this heat transfer option is selected. The manipulated variable with this option is the flowrate of the cooling water. The temperature of the inlet cooling water is... 

Closedloop Response Figure 6.23 shows the response of the cooled reactor system to ramped increases and decreases of 10 K in the setpoint of the inlet temperature controller Tin. Raising the inlet temperature produces a decrease in reactor exit temperature of --2.5 K. The production rate increases by only 3%, which indicates that inlet temperature is a poor manipulated variable for production rate changes in this system. 

In some situations the dynamics of the cooling system may be such that effective temperature control cannot be accomplished by manipulation of the coolant side. This could be the situation for fluidized beds using air coolers to cool the recirculating gases or for jacketed CSTRs with thick reactor walls. The solution to this problem is to balance the rate of heat generation with the net rate of removal by adjusting a reactant concentration or the catalyst flow. Such a scheme is shown in Fig. 4.24. 

In most cases the catalytically active metal complex moiety is attached to a polymer carrying tertiary phosphine units. Such phosphinated polymers can be prepared from well-known water soluble polymers such as poly(ethyleneimine), poly(acrylic acid) [90,91] or polyethers [92] (see also Chapter 2). The solubility of these catalysts is often pH-dependent [90,91,93] so they can be separated from the reaction mixture by proper manipulation of the pH. Some polymers, such as the polyethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, have inverse temperature dependent solubility in water and retain this property after functionalization with PPh2 and subsequent complexation with rhodium(I). The effect of temperature was demonstrated in the hydrogenation of aqueous allyl alcohol, which proceeded rapidly at 0 °C but stopped completely at 40 °C at which temperature the catalyst precipitated hydrogenation resumed by cooling the solution to 0 °C [92], Such smart catalysts may have special value in regulating the rate of strongly exothermic catalytic reactions. 

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