Page 93 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

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CHEMICAL ENGINEERING
In process design calculations it is usually more convenient to express the heat of
reaction in terms of the mols of product produced, for the conditions under which the
reaction is carried out, kJ/mol product.
Standard heats of reaction can be converted to other reaction temperatures by making a
heat balance over a hypothetical process, in which the reactants are brought to the standard
temperature, the reaction carried out, and the products then brought to the required reaction
temperature; as illustrated in Figure 3.4.
Ž
H r,t D H C H prod. H react. 3.22
r

Reaction at
Reactants Products
t°C temp. t t°C
∆H r, t

∆Η react. ∆H prod.

Reaction at
Reactants Products
25°C 25°C
25°C
∆H° r
Figure 3.4. H r at temperature t

where H r,t D heat of reaction at temperature t,
H react. D enthalpy change to bring reactants to standard temperature,
H prod. D enthalpy change to bring products to reaction temperature, t.

For practical reactors, where the reactants and products may well be at temperatures
different from the reaction temperature, it is best to carry out the heat balance over
Ž
the actual reactor using the standard temperature (25 C) as the datum temperature; the
standard heat of reaction can then be used without correction.
It must be emphasised that it is unnecessary to correct a heat of reaction to the reaction
temperature for use in a reactor heat-balance calculation. To do so is to carry out two heat
balances, whereas with a suitable choice of datum only one need be made. For a practical
reactor, the heat added (or removed) Q p to maintain the design reactor temperature will
be given by (from equation 3.10):

3.23
Q p D H products H reactants Q r
where H products is the total enthalpy of the product streams, including unreacted
Ž
materials and by-products, evaluated from a datum temperature of 25 C;
H reactants is the total enthalpy of the feed streams, including excess reagent and
Ž
inerts, evaluated from a datum of 25 C;