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CHEMICAL ENGINEERING

a v i , v i D the summation of the special diffusion volume coefficients for components
b
a and b, given in Table 8.5.
The method is illustrated in Example 8.11.
Table 8.5. Special atomic diffusion volumes (Fuller et al., 1966)
Atomic and structural diffusion volume increments
C 16.5 Cl 19.5 Ł
H 1.98 S 17.0 Ł
O 5.48 Aromatic or hetrocyclic rings 20.0
N 5.69 Ł

Diffusion volumes of simple molecules

H 2 7.07 CO 18.9
D 2 6.70 CO 2 26.9
He 2.88 N 2 O 35.9
N 2 17.9 NH 3 14.9
O 2 16.6 H 2 12.7
Air 20.1 CCL 2 F 2 114.8 Ł
Ne 5.59 SF 6 69.7 Ł
Ar 16.1 Cl 2 37.7 Ł
Kr 22.8 Br 2 67.2 Ł
Xe 37.9 Ł SO 2 41.1 Ł
Ł Value based on only a few data points

Example 8.11
Ž
Estimate the diffusivity of methanol in air at atmospheric pressure and 25 C.

Solution
Diffusion volumes from Table 8.5; methanol:
Element v i No. of
C 16.50 ð 1 D 16.50
H 1.98 ð 4 D 7.92
O 5.48 ð 1 D 5.48

v i 29.90
a
Diffusion volume for air D 20.1.
1 standard atmosphere D 1.013 bar.
molecular mass CH 3 OH D 32, air D 29.
1.013 ð 10 7 ð 298 1.75 1/32 C 1/29 1/2
D v D 8.21
1.013[ 29.90 1/3 C 20.1 1/3 2
]
6
2
D 16.2 ð 10 m /s
2
Experimental value, 15.9 ð 10 6 m /s.