Energy Balance on Reactive Processes

5 ENERGY BALANCES ON REACTIVE PROCESSES 9.5a General Procedures 

To perform energy balance calculations on a reactive system, proceed much as you did for nonreactive systems (a) draw and label a flowchart (b) use material balances and phase equilibrium relationships such as Raoult s law to determine as many stream component amounts or flow rates as possible (c) choose reference states for specific enthalpy (or internal energy) calculations and prepare and fill in an inlet-outlet enthalpy (or internal energy) table and (d) calculate AH (or AC/ or A/C), substitute the calculated value in the appropriate form of the energy balance equation, and complete the required calculation. 

Two methods are commonly used to choose reference states for enthalpy calculations and to calculate specific enthalpies and AW. We outline the two approaches below, using a propane combustion process to illustrate them. For simplicity, the material balance calculations for the illustrative process have been performed and the results incorporated into the flowchart. 

Heat of Reaction Method. This method is generally preferable when there is a single reaction for which A/ ° is known. 

Complete the material balance calculations on the reactor to the greatest extent possible. 

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An energy balance on a reactor tells the process engineer how much heating or cooling the reactor requires in order to operate at the desired conditions. In this chapter we show how enthalpy changes that accompany chemical reactions are determined from tabulated physical properties of the reactants and products and how calculated enthalpies of reaction are incorporated in energy balances on reactive processes. 

Material and energy balances on reactive processes (end of Chapter 9). 

Perform energy balance on reactive and nonreactive processes. 

When performing energy balances on a reactive chemical process, two procedures may be followed in the calculation of AH (or AH or AH) that differ in the choice of reference states for enthalpy or internal energy calculations. In the heat of reaction method, the references are the reactant and product species at 25 C and 1 atm in the phases (solid, liquid, or gas) for which the heat of reaction is known. In the heat of formation method, the references are the elemental species that constitute the reactant and product species [e.g., C(s), 02(g), H2(g), etc.] at 25°C and 1 atm. In both methods, reference slates for nonreactive species may be chosen for convenience, as was done for the nonreactive processes of Chapters 7 and 8. 

There are several contributions of thermodynamics to the field of reactive separations. Thermodynamics provides the basic relations, such as energy balances of equilibrium conditions, used in the process models, which again are the key to reactive separation design. Furthermore, thermodynamics provides models and experimental methods for the investigations of the properties of the reacting fluid that have to be known for any successful process design. We will focus on equilibrium thermodynamics here but discuss relations to kinetic models. 

In reactive distillation, chemical reactions are assumed to occur mainly in the liquid phase. Hence the liquid holdup on the trays, or the residence time, is an important design factor for these processes. Other column design considerations, such as number of trays or feed and product tray locations, can be of particular importance in reactive distillation columns. Moreover, because chemical reactions can be exothermic or endothermic, intercoolers or heaters may be required to maintain optimum stage temperatures. Column models of reactive distillation must include chemical reaction equilibrium or kinetic equations along with the material and energy balance equations and the phase equilibrium relations. These models and methods for solving them are discussed in Chapter 13. 

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