Temperature Control in .NET framework

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Temperature Control
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Endothermic reactors present no problem regarding temperature control, since they exhibit a marked degree of self-regulation. The exothermic reactors, which have already been int roduced, pose the real problem. Their negative self-regulation has been demonstrated. One facet of an cxothermic reaction that has not yet been discussed is its initiation. Because reaction rate increases with temperature, heat must be applied before any conversion is obtained-then heat must be removed. So the heat, transfer system must have t he capability of operating in either direction. This creates something of a problem: steam, for example, is most often used for heating, but is worthless for cooling. There are two general approaches to this problem: 1. Employ a two-way cooling system, i.e., one capable of heating too. 2. Split the duties by preheating the reactants and cooling the reactor.
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FIG 10.10. This system will allow easy startup as well as efficient cooling.
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Controlling Chemical Reactions
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FIG 10.11. Manipulation of exit temperature is effective when coolant is circulated at a rapid rate.
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A very effective t wo-way cooling system uses a boiling liquid to which heat may be externally applied for startup. An example is pictured in Fig. 10.10. Because the rate of heat transfer in the system shown in Fig. 10.10 is directly proportional to coolant temperature, that variable should be manipulated to control reactor temperature. The boiling point of the condensate is a function of its pressure, therefore the manipulated variable is the set point of the jacket back-pressure controller. Another system commonly used features a liquid coolant rapidly circulated past the heat transfer surfaces. Figure 10.11 shows the arrangement when wa,ter is used as the coolant. Coolant temperature is chosen as the manipulated variable, since it is linear with bot,h heat transfer rate and reactor temperature. A high rate of circulation allows maximum heat transfer and speed of response. If recirculation of coolant is not used, the dead time in the secondary temperat ure loop will vary with the coolant flow. Combined with the nonlinear variation of temperature with flow (Eq. 9.12), it results in a limit cycle, even with an equal-percentage valve. The cycle has a distinctive appearance, the high-temperature portion, when the flow is greatest, being of short durat,ion, the low-temperature part of the cycle being longer. Heat-removal syst,ems can be used for monitoring conversion. The record of condensate flow to the reactor in Fig. 10.10 should provide a reliable indication of heat evolution. With a circulating liquid coolant, however, flow must be multiplied by temperature difference from inlet to outlet in order to determine the rate of heat evolution. There are some pitfalls in this system: 1. When the primary controller calls for more cooling, the temperature at the jacket inlet will fall before that at the outlet. The difference between inlet and outlet is ordinarily only about 5 F, which could be less than a transient in inlet temperature. This makes the record of
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heat transfer appear very erratic, unless dynamic compensation is applied to the inlet measurement. 2. There are many sources of error, the principal one being a difference in temperature between the reactants and products. Dynamic compensation requires that, the inlet temperature measurement be delayed behind its actual value an amount equal to the delay through the jacket. It can be applied most effectively by simulating the jacket by a length of tubing whose dead time may be adjusted by changing the flow. The system in Fig. 10.11 uses this compensation. If the reactants are to be preheated, it should be done before mixing, unless a catalyst is necessary for the reackion to t,ake place. Otherwise there is no assurance that the reaction would not begin inside the preheater, where it could not be controlled. Adding heat in the preheater(s) and removing it in the reactor is hardly economical. But once a reaction is initiated, it is often possible to bypass the preheaters without adverse effect. Some reactors have a regenerative preheating system, in which heat is t ransferred from the product stream to the reactan& through an exchanger. Although this is economically advantageous, unless prehea t temperature is controlled, a positive feedback loop is formed which can destroy whatever self-regulation the reactor might have had. Temperature control of regenerative preheat can be accomplished as shown in Fig. 10.12. Whenever a liquid-phase reaction is conducted at a temperature near the boiling point of one of the reactants or products, heat of vaporization may be used for control. If one of the reactants vaporizes, it may be refluxed back to the reactor after condensing. If a product vaporizes, it may be removed as a vapor. This type of heat-removal system is highly self-regulating, but it is also pressure-sensitive. In fact, pressure control should be applied rather than temperat,ure control, since it is a more responsive measurement,. Throttling the reflux from the condenser, or the vapor leaving the reactor if there is no reflux, is an effective means for controlling pressure Reactants
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