Page 88 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

71
FUNDAMENTALS OF ENERGY BALANCES
to SI units, is shown as Figure 3.2. Edmister’s chart was based on hydrocarbons, but can
be used for other materials to give an indication of the likely error if the ideal gas specific
heat values are used without corrections.
The method is illustrated in Example 3.4.
Example 3.4
The ideal state heat capacity of ethylene is given by the equation:
2
5
2
9 3
Ž
C D 3.95 C 15.6 ð 10 T 8.3 ð 10 T C 17.6 ð 10 T J/mol K
p
Estimate the value at 10 bar and 300 K.
Solution
Ethylene: critical pressure 50.5 bar
critical temperature 283 K
Ž
2
C D 3.95 C 15.6 ð 10 2 ð 300 8.3 ð 10 5 ð 300 C 17.6 ð 10 9 ð 300 3
p
D 43.76 J/mol K
10
P r D D 0.20
50.5
300
T r D D 1.06
283
From Figure 3.2:
Ž
C p C ' 5J/mol K
p
So C p D 43.76 C 5 D³ 49 J/mol K
The error in C p if the ideal gas value were used uncorrected would be approximately 10
per cent.

3.8. ENTHALPY OF MIXTURES
For gases, the heats of mixing are usually negligible and the heat capacities and enthalpies
can be taken as additive without introducing any significant error into design calculations;
as was done in Example 3.3.

C ÐÐÐ . 3.20
C p mixture D x a C p a C x b C p b C x c C p c
where x a , x b , x c , etc., are the mol fractions of the components a, b, c.
For mixtures of liquids and for solutions, the heat of mixing (heat of solution) may be
significant, and so must be included when calculating the enthalpy of the mixture.
For binary mixtures, the specific enthalpy of the mixture at temperature t is given by:

H mixture,t D x a H a,t C x b H b,t C H m,t 3.21
where H a,t and H b,t are the specific enthalpies of the components a and b and H m,t
is the heat of mixing when 1 mol of solution is formed, at temperature t.