Chapter 5 Energy Balances with reaction

Energy out = Energy in + generation – consumption – accumulation CONSERVATION OF ENERGY A general equation can be written for the conservation of energy: Energy out = Energy in + generation – consumption – accumulation This is a statement of the first law of thermodynamics. An energy balance can be written for any process step. Chemical reaction will evolve energy (exothermic) or consume energy (endothermic). For steady-state processes the accumulation of both mass and energy will be zero. Energy can exist in many forms and this, to some extent, makes an energy balance more complex than a material balance.

Suppose that you want to find the standard of formation of CO from experimental data. Can you prepare pure CO from reaction of C and O2' and measure the heat transfer. This wou1d be far too difficult. It would easier experimentally to find first the heat of reaction at standard conditions for the two reactions shown below for the flow process as shown in Figure E25.1.

The Heat (Enthalpy) of Reaction

EXAMPLE: calculation of Heat of Reaction from heat of formation Calculate the heat of reaction of 4 gmol NH3 at standard conditions for the following reaction:

Calculation of Heat of Reaction at different temperatures

Heat of Combustion

Heat of Combustion

EXAMPLE Heating Value of coal

Application of Energy Balances with Reactions Processes In this section we primarily illustrate the solution of continuous, steady-state processes for which the general energy balance reduces to two choices: With the effects of chemical reaction merged with the sensible heats With the effects of chemical reaction lumped in the heat of reaction.

Application of Energy Balances with Reactions Processes The steady-state with Q = 0 reduces to just ΔH = 0. If you use tables such as in Appendix D than heat capacity equations to calculate the "sensible heats" of the various streams entering and leaving the reactor, the calculations will involve trial and error. To find the exit temperature for which ΔH := 0, if tables are used as the source of the Δ values, the simplest procedure is to

EXAMPLE: Calculation of an Adiabatic Reaction (Flame) Temperature Calculate the theoretical flame temperature for CO gas burned at constant pressure with 100% excess air, when the reactants enter at 100°C and 1 atm. Solution The solution presentation will be compressed to save space. The system is shown in Figure E26.2. We will use data from Appendix. The process is a steady-state flow system. Ignore any equilibrium effects

3 glucose + 7.8 O2 5.35 BM + 2.22 CA + 4.50 CO2

IDEAL PROCESSES, EFFICIENCY, AND THE MECHANICAL ENERGY BALANCE