Page 110 - Modeling of Chemical Kinetics and Reactor Design

80 Modeling of Chemical Kinetics and Reactor Design

(
2
∆H rxnT = ∆H + ∆a T −298 ) + ∆b ( T − 298 2 )
rxnT 298
2
+ ( 3 3 ∆d ( 4 4 ) (2-110)
∆c
T −298 ) + T −298
3 4
ENTHALP, a computer software, was developed to calculate the
heat of reaction at the system temperature. To run the program, the
user must know the following:

• The standard temperature (298 K) T and the system temperature TF.
o
• The reaction stoichiometric coefficients α , α , α , and α .
4
3
1
2
• Heat capacity constants, and a , a , a , and a .
1
3
4
2
• Standard heats of reaction of each component, dHf , dHf , dHf ,
C
B
A
and dHf .
D
HEAT CAPACITIES OF GASES
The heat capacity of gases is essential for some process engineering
design involving gas-phase chemical reactions. Here, the heat capacities,
o
C , for gases are required to determine the heat necessary to bring
p
the chemical compound increase to the reaction temperature. The heat
capacity of a mixture of gases may be found from the heat capacities
of the individual components contained in the mixtures.
o
The correlation for C of the ideal gas at low pressure is a third
p
degree polynomial, which is a function of temperature.
o
2
C = A + BT + CT + DT 3 (2-111)
p
o
where C = heat capacity of ideal gas at low pressure, cal/mol K
p
A, B, C, and D = constants for the chemical compounds
T = temperature, K
The Appendix at the end of this chapter gives values of heat
capacity constants.
HEATS OF FORMATION

o
Heats of formation, ∆H , for individual chemicals involved in
f
chemical reactions are important in determining the heat of reaction,
o
o
∆H , and associated heating and cooling requirements. If ∆H < 0, then
r
r