Heat Balance of Reactors

The heat balance is important as well for the design of reactors, for their scaleup, for the risk assessment, and especially for the assessment of the reactor stability. 

The heat balance is also at the center of the evaluation of calorimetric experiments as used for safety studies. Thus understanding the heat balance of a reactor is essential for the design of safe processes. Hereafter the different contributions to the heat balance, such as the heat release rate of the reaction, the heat exchange at the wall of the reactor, the heat dissipated by the stirrer, the heat accumulation in the reactor, the effects of the sensible heat of the feed, and the heat losses, will be discussed in detail. The different terms of the heat balance are expressed as heat release rates or thermal power. 

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Part I gives a general introduction and presents the theoretical, methodological and experimental aspects of thermal risk assessment. The first chapter gives a general introduction on the risks linked to the industrial practice of chemical reactions. The second chapter reviews the theoretical background required for a fundamental understanding of mnaway reactions and reviews the thermodynamic and kinetic aspects of chemical reactions. An important part of Chapter 2 is dedicated to the heat balance of reactors. In Chapter 3, a systematic evaluation procedure developed for the evaluation of thermal risks is presented. Since such evaluations are based on data, Chapter 4 is devoted to the most common calorimetric methods used in safety laboratories. 

In Section 11.2, general principles of reactor safety and heat balance of reactors are presented, with an emphasis on specific aspects of polymerizations. Section 11.3 is devoted to safety-related thermodynamics and reaction engineering aspects of polymerization reactions. In Section 11.4, cooling of polymerization reactors is reviewed. The chapter is concluded by a section describing safety aspects of industrial processes, together with technical risk-reducing solutions. 

Fig. 2. Heat balance of the FCCU. Heat balance around the reactor A, heat balance around the regenerator B, and the overall heat balance around the entire...

Heat balances of several lands of reactors are summarized in Tables 7-5, 7-6, 7-7 and 7-10. 

In chemical processing the most fundamental constraint is that of the thermodynamics of the system. This constraint defines both the heat balance of the process and whether or not the processes in the reactor will be equilibrium limited. These constraints will limit the range of chemical engineering solutions to the problems of designing an economically viable process that can be found. 

The heat balance of a reactor is made up of three terms,... 

Understanding the heat balance is essential when considering thermal process safety. This also applies to the industrial scale for reactors or storage units, as well as at laboratory scale for understanding the results of calorimetric experiments. In fact, the same heat balance terms will serve in both situations. For this reason, we first present the different terms of the heat balance of a reactor with a reacting system. This is followed by an often-used and simplified heat balance and finally we will study how reaction rate is affected by adiabatic conditions. 

This is an overall heat balance of a continuous reactor more detailed heat balances are introduced in Chapter 8. 

The overall heat balance of a semi-batch reactor can be written by using the three terms mentioned above. If the heat exchange does not compensate exactly, the other terms (heat production, effect of the feed, temperature) will vary as... 

Figure 7.2 The different terms of the heat balance of an isothermal semi-batch reactor (in kW) as a function of time. The maximal cooling capacity of the reactor (qama,) obtained with cold water at 5°C is also represented. The difference between both curves q and qa represents the cooling effect by the feed. Its disappearance at the end of the feed at 4 hours is visible.

The adiabatic mode the reaction is performed without any exchange at all. This means the heat of reaction will be converted into a temperature increase. The temperature course can be calculated from the heat balance of the reactor ... 

The condition for the practical implementation of such a feed control is the availability of a computer controlled feed system and of an on-line measurement of the accumulation. The later condition can be achieved either by an on-line measurement of the reactant concentration, using analytical methods or indirectly, by using a heat balance of the reactor. The amount of reactant fed to the reactor corresponds to a certain energy of reaction and can be compared to the heat removed from the reaction mass by the heat exchange system. For such a measurement, the required data are the mass flow rate of the cooling medium, its inlet temperature, and its outlet temperature. The feed profile can also be simplified into three constant feed rates, which approximate the ideal profile. This kind of semi-batch process shortens the time-cycle of the process and maintains safe conditions during the whole process time. This procedure was shown to work with different reaction schemes [16, 19, 20], as long as the fed compound B does not enter parallel reactions. 

The heat balance of a reactor is made up of three terms Heat of reaction -I- Heat transfer = Gain of sensible and latent heats by the mixture. This establishes the temperature as a function of the composition... 

The major terms in the heat balance of a pyrolysis reactor are ... 

Heat balance of the bench-scale reactor is maintained in one of two ways. If the feed material is of relatively high heating value (greater than 14.0 MJ kg l, 6000 Btu/lb), a small air cooler at the bottom of the furnace is used to maintain the vessel at the operating temperature. If the waste material is low in heating value, the melt temperature is maintained by an electrically heated furnace, or by adding auxiliary fuel to the waste,... 

Gradient-free reactor types do reveal neither concentration nor temperature gradients (difference between reactor entrance exit below 2 K) and hence can be operated isothermally even if reactions with significant heat generation or consumption have to be studied. If isothermal conditions cannot be warranted, the heat balance of the reactor system must be solved in addition to the mass balance. Unfortunately, this situation is related to several disadvantages ... 

The summation of the above enthalpies represents the total enthalpy change of the reactants, the products of reaction, and the system. The heat of reaction between water and sodium then equals this enthalpy change plus the heat loss. The latter is a function of the reactor temperature and can be calculated from the experimentally determined relationship of heat loss vs. temperature. This heat balance of the system is expressed as ... 

The tiiermal behaviour of a chemical reactor depends on the thermodynamics of the process, on the reaction rate, vtdiich has already been mentioned above, but also on the mode of exchange with the environment. This is completely described in the overall heat balance of the system. The following sections will present the main balance equations, which are required for the subsequent safety assessment, as well as definitions and interpretation of characteristic numbers used in their presentatioiL... 

The General Heat Balance of Cooled Ideal Reactors... 

The general heat balance of a chemical reactor can be put into words in analogy to the mass balance ... 

The equality in reactor behaviour of the stationary PFTR and unsteady BR, which was explained in Section 4.1.5, allows the direct formulation of the heat balance for the cooled BR. If the length of the PFTR is substituted by the characteristic reaction time for the batch process, and if the Stanton munber formulation is based on its general definition regarding the overall heat removal mechanism, the unsteady heat balance of the cooled BR is obtained. 

By inserting all terms derived into the general integral heat balance for cooled ideal reactors, including the thermal inertia O, the unsteady state heat balance of the cooled SBR is obtained in a partially dimensionless form ... 

Because reaction is confined in such a small space, the heat balance of the process is generally improved as well. In that much of the water is vaporized in the reactor itself by the heat of reaction, product leaving the granulator is much drier and energy consumption for drying can be reduced. The main advantages of the pipe-reactor process, compared with the preneutralizer process, can be summarized as follows [14] ... 

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