PROBLEM 14.5
KNOWN: Mass diffusion coefficients of two binary mixtures at a given temperature, 298 K.
FIND: Mass diffusion coefficients at a different temperature, T = 350 K.
ASSUMPTIONS: (a) Ideal gas behavior, (b) Mixtures at 1 atm total pressure.
PROPERTIES: Table A-8, Ammonia-air binary mixture (298 K), DAB = 0.28 × 10-4 m2/s;
Hydrogen-air binary mixture (298 K), DAB = 0.41 × 10-4 m2/s.
ANALYSIS: According to treatment of Section 14.1.4, assuming ideal gas behavior,
3/2
AB
D~T
where T is in kelvin units. It follows then, that for
() ( )
3/2
42
3AB
NH Air : D 350 K 0.28 10 m /s 350 K / 298 K
−
−=×
()
42
AB
D 350 K 0.36 10 m /s
−
=× <
() ( )
3/2
42
2AB
H Air : D 350 K 0.41 10 m / s 350 / 298
−
−=×
()
42
AB
D 350 K 0.52 10 m /s
−
=× <
COMMENTS: Since the H2 molecule is smaller than the NH3 molecule, it follows that
2Air 3Air
HNH
DD
−−
>
as indeed the numerical data indicate.