Page 340 - Chemical engineering design

Solution
Basis: 1000 kg DESIGN INFORMATION AND DATA 315
0.6 ð 1000
Volume of water D D 0.601 m 3
998.2
0.4 ð 1000 3
Volume of methanol D D 0.506 m
791.2
Total 1.107 m 3
1000 3
Density of mixture D D 903.3 kg/m
1.107
Experimental value D 934.5 kg/m 3
934.5 903.3
Error D D 3 per cent, which would be acceptable for most
903.3 engineering purposes

If data on the variation of density with temperature cannot be found, they can be
approximated for non-polar liquids from Smith’s equation for thermal expansion (Smith
et al., 1954).
0.04314
ˇ D 8.2
T c T 0.641
1
where ˇ D coefficient of thermal expansion, K ,
T c D critical temperature, K,
T D temperature, K.

8.6.2. Gas and vapour density (specific volume)
For general engineering purposes it is often sufficient to consider that real gases, and
vapours, behave ideally, and to use the gas law:
PV D nRT 8.3
2
where P D absolute pressure N/m (Pa),
3
V D volume m ,
n D mols of gas
T D absolute temperature, K,
1
R D universal gas constant, 8.314 J K 1 mol 1 (or kJ K 1 kmol ).
RT
Specific volume D (8.4)
P
These equations will be sufficiently accurate up to moderate pressures, in circumstances
where the value is not critical. If greater accuracy is needed, the simplest method is to
modify equation 8.3 by including the compressibility factor z:

PV D znRT 8.5
The compressibility factor can be estimated from a generalised compressibility plot, which
gives z as a function of reduced pressure and temperature (Chapter 3, Figure 3.8).