Exothermic reactor

Generally speaking, temperature control in fixed beds is difficult because heat loads vary through the bed. Also, in exothermic reactors, the temperature in the catalyst can become locally excessive. Such hot spots can cause the onset of undesired reactions or catalyst degradation. In tubular devices such as shown in Fig. 2.6a and b, the smaller the diameter of tube, the better is the temperature control. Temperature-control problems also can be overcome by using a mixture of catalyst and inert solid to effectively dilute the catalyst. Varying this mixture allows the rate of reaction in different parts of the bed to be controlled more easily. 

Adiabatic operation. If adiabatic operation leads to an acceptable temperature rise for exothermic reactors or an acceptable fall for endothermic reactors, then this is the option normally chosen. If this is the case, then the feed stream to the reactor requires heating and the efiluent stream requires cooling. The heat integration characteristics are thus a cold stream (the reactor feed) and a hot stream (the reactor efiluent). The heat of reaction appears as elevated temperature of the efiluent stream in the case of exothermic reaction or reduced temperature in the case of endothermic reaction. 

Figure 13.3 shows a process represented simply as a heat sink and heat source divided hy the pinch. Figure 13.3a shows the process with an exothermic reactor integrated above the pinch. The minimum hot utility can be reduced by the heat released by reaction, Qreact-... 

By comparison, Fig. 13.36 shows an exothermic reactor integrated below the pinch. Although heat is being recovered, it is being recovered into part of the process which is a heat source. The hot utility requirement cannot be reduced because the process above the pinch needs at least Q//m-,n to satisfy its enthalpy imbalance. 

There is no obvious benefit from integrating an exothermic reactor below the pinch. The appropriate placement for exothermic reactors is above the pinch. ... 

The appropriate placement of reactors, as far as heat integration is concerned, is that exothermic reactors should be integrated above the pinch and endothermic reactors below the pinch. Care should be taken when reactor feeds are preheated by heat of reaction within the reactor for exothermic reactions. This can constitute cross-pinch heat transfer. The feeds should be preheated to pinch temperature by heat recovery before being fed to the reactor. 

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. 

But their instability makes it difficult to prepare them in good yields and to use them safely in reactions. Ozonides or ozonolysis products have at times expld on standing. Ozonolysis products are also thermally unstable. One must maintain the reaction at a certain temp in order to prepare and react these compds. Moreover, since the ozone addition reaction is highly exothermic, reactors must be cooled to maintain the desired temp (Ref 4)... 

Use of thermal stability tests (DTA s) to determine the heat sensitivity of a given process mixture is desirable. Recent advances in analytical methods permit good calorimetric determination of heat of reaction. Heat of reaction data are critical for exothermic reactor vent sizing. Heat impact from fire is usually small in comparison, but should not be neglected. 

Table 10-7 HAZOP Study Applied to the Exothermic Reactor of Example 10-2.

Regenass, W., "Safe Operation of Exothermic Reactors," technical paper, Ciba-Geigy Limited, Basel, Switzerland (1984). 

In the second phase searches were made on the system and subsystem level. This is needed for the comparison of process alternatives and for the design of the exothermic reactor and its heat transfer systems. Carbonylation of methanol is an exothermic reaction. Thus only the exothermal reactors were searched. The CBR search found two cases which are general recommendations on the design of exothermic reactors with heat transfer systems. They are shown in Fig. 8 and 9. 

The case in Figure 9 shows a heat recovery system of a reactor. The not recommended case on the left shows the feed to an exothermic reactor being heated by the product. In this case the temperature rise in the reactor may lead to the temperature rise in feed. The recommended case on right is safer since the connection is broken because the heat transfer is done by generating and using medium pressure steam. 

Figure 9. A recommendation for preheating the feed of an exothermic reactor.

Beveridge, H.J.R. and Jones, C.G., Shock effects on a bursting disk in a relief manifold, in The Protection of Exothermic Reactors and Pressurised Storage Vessels, I Chem E Symposium Series No. 85, pp. 207-14 (1984). 

In some situations it is very important to be able to increase the flow rate above the design conditions (for example, the cooling water to an exothermic reactor may have to be doubled or tripled to handle dynamic upsets). In other cases this is not as important (for example, the feed flow rate to a unit). Therefore it is logical to base the design of the control valve and the pump on having a process that can attain both the maximum and the minimum flow conditions. The design flow conditions are only used to get the pressure drop over the heat exchanger (or fixed resistance part of the process). 

Here, a control law for chemical reactors had been proposed. The controller was designed from compensation/estimation of the heat reaction in exothermic reactor. In particular, the paper is focused on the isoparafhn/olefin alkylation in STRATCO reactors. It should be noted that control design from heat compensation leads to controllers with same structure than nonlinear feedback. This fact can allow to exploit formal mathematical tools from nonlinear control theory. Moreover, the estimation scheme yields in a linear controller. Thus, the interpretation for heat compensation/estimation is simple in the context of process control. 

Another reason why calculations of the adiabatic reactor is important is for safety. Suppose we have a reactor operating in a stable fashion with cooling. What happens if the cooling is suddenly stopped The limit of this situation is the adiabatic reactor, and the engineer must always be aware of this mode because it is the worst-case scenario of any exothermic reactor. Note that if A Hr > 0, we must supply heat to maintain the reactor temperature, and loss of heat will cause the reactor to cool down and the rate will decrease safely. 

Figure 7.1 shows a typical chemical process in which a feed-effluent heat exchanger is coupled with an adiabatic exothermic reactor. The heat of reaction produces a reactor... 

How often the control loop is executed is determined by the sampling interval, which is how often the measurement is taken. A critical pressure on an exothermic reactor may be sampled several times per second, while the level of a buffer tank may only be sampled a few times an hour. The sampling time must be chosen with care to ensure that possible changes in the process are detected early enough for the controller to take appropriate action. However, sampling too often is undesirable as it may upset the process and because a large number of data points would have to be sampled and stored. 

It should be noted that interactions between control loops is not just limited to interacting units but will also occur within single units. A typical example is an exothermic reactor where a change in the control loop which controls the level in the reactor will have an effect on the amount of material in the reactor. This in turn will affect the heat removal requirements and, therefore, the cooling water control loop. Many strategies for reducing loop interactions, and for selecting control loops so as to minimise interactions, can be found in most standard control textbooks.1-11... 

The mechanism for heat transfer includes the following steps (1) conduction in the catalyst particle (2) convection from the particle to the gas phase (3) conduction at contact points between particles (4) convection between the gas and vessel wall (5) radiation heat transfer between the particles, the gas, and the vessel wall (6) conduction in the wall and (7) convection to the coolant. There are a number of ways, through reactor models, that these steps are correlated to provide design and analysis estimates and criteria for preventing runaway in exothermic reactors. 

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