Page 347 - Chemical engineering design

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CHEMICAL ENGINEERING
Solution
Viscosity D 0.0134 mNs/m 2
Ž
Specific heat capacity D 2.47 kJ/kg C
10.4

Ž
k D 0.0134 2.47 C D 0.038 W/m C (8.13)
30
Ž
Experimental value, 0.043 W/m C, error 12 per cent.
8.8.4. Mixtures
In general, the thermal conductivities of liquid mixtures, and gas mixtures, are not simple
functions of composition and the thermal conductivity of the components. Bretsznajder
(1971) discusses the methods that are available for estimating the thermal conductivities
of mixtures from a knowledge of the thermal conductivity of the components.
If the components are all non-polar a simple weighted average is usually sufficiently
accurate for design purposes.
k m D k 1 w 1 C k 2 w 2 C ÐÐÐ 8.14
where k m D thermal conductivity of mixture,
k 1 ,k 2 D thermal conductivity of components,
w 1 ,w 2 D component mass fractions.

8.9. SPECIFIC HEAT CAPACITY
The specific heats of the most common organic and inorganic materials can usually be
found in the handbooks.

8.9.1. Solids and liquids
Approximate values can be calculated for solids, and liquids, by using a modified form of
Kopp’s law, which is given by Werner (1941). The heat capacity of a compound is taken
as the sum of the heat capacities of the individual elements of which it is composed. The
values attributed to each element, for liquids and solids, at room temperature, are given
in Table 8.2; the method illustrated in Example 8.6.

Table 8.2. Heat capacities of the elements, J/mol ° C
Element Solids Liquids
C 7.5 11.7
H 9.6 18.0
B 11.3 19.7
Si 15.9 24.3
O 16.7 25.1
F 20.9 29.3
P and S 22.6 31.0
all others 26.0 33.5