Page 94 - Chemical engineering design

FUNDAMENTALS OF ENERGY BALANCES
Q r is the total heat generated by the reactions taking place, evaluated from the
Ž
standard heats of reaction at 25 C (298 K). 77
Ž
Q r D H ð (mol of product formed) 3.24
r
Ž
where H is the standard heat of reaction per mol of the particular product.
r
Note: A negative sign is necessary in equation 3.24 as Q r is positive when heat is
evolved by the reaction, whereas the standard enthalpy change will be negative for
exothermic reactions. Q p will be negative when cooling is required (see Section 3.4).
3.10.1. Effect of pressure on heats of reaction
Equation 3.22 can be written in a more general form:
P ∂H prod. ∂H react.
Ž
H r,P,T D H C dP
r
1 ∂P T ∂P T
T

∂H prod. ∂H react.
C dT 3.25
298 ∂T P ∂T P
If the effect of pressure is likely to be significant, the change in enthalpy of the products
and reactants, from the standard conditions, can be evaluated to include both the effects
of temperature and pressure (for example, by using tabulated values of enthalpy) and the
correction made in a similar way to that for temperature only.

Example 3.7
Illustrates the manual calculation of a reactor heat balance.
Vinyl chloride (VC) is manufactured by the pyrolysis of 1,2,dichloroethane (DCE). The
reaction is endothermic. The flow-rates to produce 5000 kg/h at 55 per cent conversion
are shown in the diagram (see Example 2.13).
3
The reactor is a pipe reactor heated with fuel gas, gross calorific value 33.5 MJ/m .
Estimate the quantity of fuel gas required.

Reactor VC 80 kmol/h
2 bar DCE 65.5
DCE 145.5 kmol/h 500°C HCL 80
liquid 20°C

Q

Solution
Ž
Reaction: C 2 H 4 Cl 2 (g) ! C 2 H 3 Cl(g) C HCl(g) H D 70,224 kJ/kmol.
r
The small quantity of impurities, less than 1 per cent, that would be present in the feed
have been neglected for the purposes of this example. Also, the yield of VC has been
taken as 100 per cent. It would be in the region of 99 per cent at 55 per cent conversion.