Page 84 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

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FUNDAMENTALS OF ENERGY BALANCES
3.5. CALCULATION OF SPECIFIC ENTHALPY
Tabulated values of enthalpy are available only for the more common materials. In the
absence of published data the following expressions can be used to estimate the specific
enthalpy (enthalpy per unit mass).
For pure materials, with no phase change:
T
H T D C p dT 3.11
T d
where H T D specific enthalpy at temperature T,
C p D specific heat capacity of the material, constant pressure,
T d D the datum temperature.
If a phase transition takes place between the specified and datum temperatures, the
latent heat of the phase transition is added to the sensible-heat change calculated by
equation 3.11. The sensible-heat calculation is then split into two parts:

T p
T
H T D C p 1 dT C C p 2 dT 3.12
T d T p
where T p D phase transition temperature,
D specific heat capacity first phase, below T p ,
C p 1
D specific heat capacity second phase, above T p .
C p 2
The specific heat at constant pressure will vary with temperature and to use equations
3.11 and 3.12, values of C p must be available as a function of temperature. For solids
and gases C p is usually expressed as an empirical power series equation:
2
C p D a C bT C cT C dT 3 3.13a
or C p D a C bT C cT 1/2 3.13b
Ž
Absolute (K) or relative ( C) temperature scales may be specified when the relationship
is in the form given in equation 3.13a. For equation 3.13b absolute temperatures must
be used.

Example 3.2

Ž
Estimate the specific enthalpy of ethyl alcohol at 1 bar and 200 C, taking the datum
Ž
temperature as 0 C.
Ž
Ž
C p liquid 0 C24.65 cal/mol C
Ž
Ž
100 C37.96 cal/mol C
5 2
Ž
9 3
2
C p gas t C 14.66 C 3.758 ð 10 t 2.091 ð 10 t C 4.740 ð 10 t cal/mol
Ž
Boiling point of ethyl alcohol at 1 bar D 78.4 C.
Latent heat of vaporisation D 9.22 kcal/mol.