Chemical reactor heat balance

A chemical plant includes tens to hundreds of process units, such as chemical reactors, heat exchangers, distillation columns, absorption towers, etc. For each unit, material and energy balances are used to relate input and output streams. Rate equations and equilibrium relations help describe the conversion of species, mass, and energy in the units. Collectively, these equations provide the equality constraints for the plant model. 

Steady state models of the automobile catalytic converter have been reported in the literature 138), but only a dynamic model can do justice to the demands of an urban car. The central importance of the transient thermal behavior of the reactor was pointed out by Vardi and Biller, who made a model of the pellet bed without chemical reactions as a onedimensional continuum 139). The gas and the solid are assumed to have different temperatures, with heat transfer between the phases. The equations of heat balance are ... 

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. 

Thermod5mamics is a fundamental engineering science that has many applications to chemical reactor design. Here we give a summary of two important topics determination of heat capacities and heats of reaction for inclusion in energy balances, and determination of free energies of reaction to calculate equihbrium compositions and to aid in the determination of reverse reaction... 

Our discussion of multiphase CFD models has thus far focused on describing the mass and momentum balances for each phase. In applications to chemical reactors, we will frequently need to include chemical species and enthalpy balances. As mentioned previously, the multifluid models do not resolve the interfaces between phases and models based on correlations will be needed to close the interphase mass- and heat-transfer terms. To keep the notation simple, we will consider only a two-phase gas-solid system with ag + as = 1. If we denote the mass fractions of Nsp chemical species in each phase by Yga and Ysa, respectively, we can write the species balance equations as... 

Two limiting modes of operating chemical reactors employ (a) a vessel so well stirred that the composition and temperature are the same throughout (b) a vessel typified by a tube without mixing in which all molecules have the same residence time, and in which gradients of composition and temperature exist. Material and heat balances on these devices utilize the conservation law,... 

Part 1 Control of Chemical Processes. Some common problems in chemical processes are presented and either classical solutions or physical interpretation of controllers are discussed. Thus, the first chapter includes modeling and local control whereas the second chapter is focussed on nonlinear control design from heat balance on chemical reactors. The three first chapters deal with regulation problems while the last one is devoted to a tracking one. 

In practice, of course, it is rare that the catalytic reactor employed for a particular process operates isothermally. More often than not, heat is generated by exothermic reactions (or absorbed by endothermic reactions) within the reactor. Consequently, it is necessary to consider what effect non-isothermal conditions have on catalytic selectivity. The influence which the simultaneous transfer of heat and mass has on the selectivity of catalytic reactions can be assessed from a mathematical model in which diffusion and chemical reactions of each component within the porous catalyst are represented by differential equations and in which heat released or absorbed by reaction is described by a heat balance equation. The boundary conditions ascribed to the problem depend on whether interparticle heat and mass transfer are considered important. To illustrate how the model is constructed, the case of two concurrent first-order reactions is considered. As pointed out in the last section, if conditions were isothermal, selectivity would not be affected by any change in diffusivity within the catalyst pellet. However, non-isothermal conditions do affect selectivity even when both competing reactions are of the same kinetic order. The conservation equations for each component are described by... 

Thus we see why it is essential to consider the energy balance very carefully in designing chemical reactors. The isothermal reactor assumption, while a good starting point for estimating reactor performance (the next two chapters), is seldom adequate for real reactors, and neglect of heat release and possible temperature increases can have very dangerous consequences. 

Any stirring within the reactor wiU generate heat, and we call this term Ws- We omit gravity and kinetic energy terms in the energy balance because these are usually very small compared to the other terms in the energy balance in a chemical reactor. 

The obvious way to heat or cool a chemical reactor is by heat exchange through a wall, either by an external jacket or with a cooling or heating coil. As we discussed previously, the coolant energy balance must be solved along with the reactor energy balance to determine temperatures and heat loads. 

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. 

Figure 6-1. Energy balance in chemical reactors, where G = mass flow-rate U = internal energy per unit mass P = pressure p = fluid density m = mass of fluid in the system Q = rate at which heat is transferred to the system and Ws = rate at which work is done on the surroundings.

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