Exothermic reactor temperature control loop

With batch reactors, it may be possible to add all reactants in their proper quantities initially if the reaction rate can be controlled by injection of initiator or acqustment of temperature. In semibatch operation, one key ingredient is flow-controlled into the batch at a rate that sets the production. This ingredient should not be manipiilated for temperature control of an exothermic reactor, as the loop includes two dominant lags—concentration of the reactant and heat capacity of the reaction mass—and can easily go unstable. 

The use of PID controllers should be restricted to those loops where two criteria are both satisfied the controlled variable should have a very large signal-to-noise ratio and tight dynamic control is really essential from a feedback control stability perspective. The classical example of the latter is temperature control in an irreversible exothermic chemical reactor (see Chap. 4). 

Step 3 of our plantwide control design procedure involves two activities. The first is to design the control loops for the removal of heat from exothermic chemical reactors. We dealt with this problem in Chap. 4, where we showed various methods to remove heat from exothermic reactors and how to control the temperature in such reactors. At that point we assumed that the heat was removed directly and permanently from the process (e.g., by cooling water). How-ever. it is wasteful to discard the reactor heat to plant utilities when we need to add heat in other unit operations within the process. Instead, a more efficient alternative is to heat-integrate various parts of the plant by the use of process-to-process heat exchangers. 

Step 3. The open-loop instability of the reactor acts somewhat like a constraint, since closed-loop control of reactor temperature is required. By design, the exothermic reactor heat is removed via cooling water in the reactor and product condenser. We choose to control reactor temperature with reactor cooling water flow because of its direct effect. There are no process-to-process heat exchangers and no heat integration in this process. Disturbances can then be rejected to the plant utility system via cooling water or steam. 

To ensure exothermic heat removal from the process, we are constrained by the process design to assign two control loops. We must control reactor inlet temperature with the furnace and control reactor exit temperature with the quench flow. Only by adjusting fuel to the furnace do we allow the reactor heat to be dissipated to the cooler. And only when the quench loop works do we guarantee that the furnace is operational. Because of this design we do not need a bypass line around the process-to-process heat exchanger. 

For exothermic reactions, heat transfer is usually also an important factor, for reasons of temperature control and energy costs. In this respect, slurry reactors are superior to fixed-bed reactors. In particular, the jet-loop reactor with its external heat exchanger provides excellent temperature control. 

In this exercise, we will evaluate the controllability of a CSTR with heating jacket (Fig. 12.10). The reaction y4 —> 5 is first-order, irreversible and moderate exothermic. Since the heat of reaction is not enough to achieve a temperature that gives high conversion, heat is provided by pressurised hot water (inlet temperature 383 K). Temperature measurements follow a first order dynamics with a time constant of 60 s. Valve dynamics is represented by first order elements with a time constant of 30 s. Study the controllability property of the SISO loop keeping the reactor temperature at set-point by manipulating the hot-water flow rate. Disturbances in reactor inlet temperature and reactor inlet concentration are expected. 

Step 3. The reaction is exothermal. After process/process energy saving for feed preheating, the excess energy is rejected to the cooling water. Because the only reason of the furnace is to ensure constant inlet reactor temperature the first control loop is inlet reactor temperature/fliel inflow. To prevent the thermal decomposition of the product, a second loop keeps constant outlet reactor temperature by manipulating the quench stream. 

Exit temperature control of an exothermic tubular reactor having inverse response, or any such loops with inverse response. 

It is through the early development of major control loops that significant design inprovements can be made. In the high-temperature exothermic reactor, for exanple, failure to consider the control loop might lead one to propose an integrated heat-exchanger network that is difficult or inpossible to control. 

Suppose that the temperature in an exothermic continuous stirred-tank reactor is controlled by manipulating the coolant flow rate using a control valve. A PID controller is used and is well-tuned. Which of these changes could adversely affect the stability of the closed-loop system Briefly justify your answers. 

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