Page 464 - Elements of Chemical Reaction Engineering Ebook

Sec. 8.2 The Energy Balance 435

Equation (8-26) gives the heat of reaction at any temperature T in terms
of the heat of reaction at a reference temperature (usually 298 K) and an inte-
gral involving the ACp term. Techniques for determining the heat of reaction at
pressures above atmospheric can be found in Chen2 For the reaction of hydro-
gen and nitrogen at 400°C it was shown that the heat of reaction increased by
only 6% as the pressure was raised from 1 atm to 200 atm.

€1.2.6 Constant or Mean Heat Capacities

For the case of constant or mean heat capacities, Equation (8-26) becomes

(8-27)

The circumflex denotes that the heat capacities are evaluated at some mean
temperature value between II( and T.

AtP
T, ---+ T (8-28)

In a similar fashion we can write the integral involving Oi and Cpi in Equation
(8-22) as

c,i is the mean heat capacity of species i between T,, and T:

(8-29)

Substituting the mean heat capacities into Equation (8-22), the steady-state
energy balance becomes

Energy balance in
terms of mean or
constant heat 1
capacities (8-30)
In almost all of the systems we will study, the reactants will be entering the
system at the same temperature; therefore, T,, = To.

N. H. Chen, Process Reactow Design (Needham Heights, Mass.: Allyn and Bacon,
1983), p. 26.