Page 98 - Chemical engineering design

FUNDAMENTALS OF ENERGY BALANCES
From reaction (3)
Ž
Ž
H C 6 H 12 D 3949.2 D 6 393.12 C 6 285.58 H C 6 H 12 81
c f
Ž
H C 6 H 12 D 3949.2 4072.28 D 123.06 kJ/mol
f
Ž
Ž
Ž
H D H C 6 H 12 H C 6 H 6
r f f
Ž
H D 123.06 71.88 D 195 kJ/mol
r
Note: enthalpy of formation of H 2 is zero.
Method 2
Using equation 3.27
Ž
Ž
Ž
Ž
H D H C 6 H 6 C 3 ð H H 2 H C 6 H 12
c
c
r
c
D 3287.4 C 3 285.88 3949.2 D 196 kJ/mol
Ž
Heat of reaction H D 196 kJ/mol
r
3.13. COMPRESSION AND EXPANSION OF GASES
The work term in an energy balance is unlikely to be significant unless a gas is expanded
or compressed as part of the process. To compute the pressure work term:

2
W D P dv equation 3.5
1

a relationship between pressure and volume during the expansion is needed.
If the compression or expansion is isothermal (at constant temperature) then for unit
mass of an ideal gas:

Pv D constant 3.28
P 2 RT 1 P 2
and the work done, W D P 1 v 1 ln D ln 3.29
P 1 M P 1
where P 1 D initial pressure,
P 2 D final pressure,
v 1 D initial volume.
In industrial compressors or expanders the compression or expansion path will be
“polytropic”, approximated by the expression:

n
Pv D constant 3.30