Manipulation of the cooling - water flow

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. 

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. 

So-called water savers are cooling-water exit-temperature controls. They have the advantage of rninimizing cooling-water flow rate for any given heat load. Their use also minimizes subcooling—and there are instances where this is desirable—but at the expense of variable condensate temperature. This can cause variable internal reflux unless it is compensated for (see Chapter 11, Section 2). This kind of control is often implemented as shown in Figure 3.7 pressure is controlled by manipulation of makeup and vent valves. 

An alternative arrangement, used especially when the condenser is built into the head of the column, is that of Figure 3.15. Direct measurement and control of reflux are not possible since the flow is internal. Instead it must be controlled indirectly by manipulation of condenser cooling water, which, in turn, may be reset by a vapor-composition controller. This internal reflux arrangement works well if a heat-computation scheme is used for control. A scheme that we have used successfully is discussed in Chapter 11, Section 4. 

The lube oil temperature is the controlled variable because it is maintained at a desired value (the setpoint). Cooling water flow rate is the manipulated variable because it is adjusted by the temperature control valve to maintain the lube oil temperature. The temperature transmitter senses the temperature of the lube oil as it leaves the cooler and sends an air signal that is proportional to the temperature controller. Next, the temperature controller compares the actual temperature of the lube oil to the setpoint (the desired value). If a difference exists between the actual and desired temperatures, the controller will vary the control air signal to the temperature control valve. This causes it to move in the direction and by the amount needed to correct the difference. For example, if the actual temperature is greater than the setpoint value, the controller will vary the control air signal and cause the valve to move in the open direction. 

The performance of the convention control structure, in which cooling water flow is manipulated to control temperature, is shown in Figure 3.52. The disturbance is the same increase in cooling water temperature. Feed flowrate is constant. The cooling water flowrate more than doubles to control reactor temperature, but the temperature is returned to the desired value in about 2 h. The peak deviation in temperature is less than 0.6 K. Controller settings are those given in Table 3.2 for the 95% conversion case with a 330 K reactor temperature (the integral time is 50 min). 

The control structure shown in Figure 6.57 is installed on the flowsheet. The feed is flow-controlled. The outlet temperature is controlled by manipulating the coolant flowrate. Note that the OP signal is sent to both of the control valves on the coolant stream, opening and closing them simultaneously. The setup works in the simulations, but it is not what would be used in a real physical system. A pressure-driven simulation in Aspen Plus requires that valves be placed on both the inlet and outlet coolant streams. In a real system, the cooling water would be drawn from a supply header, which operates a fixed pressure. A single control valve would be used, either on the inlet or on the outlet, to manipulate the flowrate of coolant. 

The temperature in a chemical reactor can be controlled by manipulating cooling-water flow rate to the cooling coil. The flow rate of a reactant fed into the reactor also affects the temperature. The reaction is exothermic. 

A + C + D—>G, A + C + E— //, A + E- F, 3D — 2F. Four gas fresh feed streams enter the process. One of these is fed into the bottom of the stripper. The vapor stream leaving the reactor is cooled before entering a separator. Liquid from the separator is fed into a stripper, which produces a bottoms product stream. Vapor from the separator is split between a gas purge and a recycle stream back to the reactor. The vapor from the stripper is also fed into the reactor. The reactor is cooled by manipulating cooling-water flow rate to cooling coils. 

The flow rate of extraction water fed to the top of column C2 is ratioed to the feed to this column D1 by using a multiplier and a remote-set flow controller. The temperature of the extraction water is controlled by manipulating cooling water to the cooler. Base level is controlled by manipulating bottoms, and reflux drum level is controlled by manipulating distillate. The binary methanol/water mixture from the bottom of column C2 is fed to column C3. A constant reflux ratio is maintained in this column by adjusting reflux flow rate. 

The fresh C5 stream containing the reactive isoamylenes and the chemically inert other C5 components is fed into the reactor on flow control. The methanol fed to the prereactor is ratioed to the fresh feed flowrate. The exit temperamre of the reactor is controlled using a ternperamre/temperamre cascade structure. The reactor effluent temperamre controller changes the setpoint of the circulating cooling water temperamre controller, which manipulates the cooling water makeup valve (see Fig. 14.7). 

Figure 3.18. Reactor with control of temperature by manipulating the flow of cooling water.

The water flow to a chemical reactor cooling coil is controlled by the system shown in Figure 11-4. The flow is measured by a differential pressure (DP) device, the controller decides on an appropriate control strategy, and the control valve manipulates the flow of coolant. Determine the overall failure rate, the unreliability, the reliability, and the MTBF for this system. Assume a 1-yr period of operation. 

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