Chapter 11 Mass Transfer

Forsberg Heat Transfer
Chapter 11
Mass Transfer




Terminology
Concentrations in a Gas Mixture
Diffusion Mass Transfer
A box contains two different species of gas.
Gas A is in the left half and Gas B in the right half.
They are separated by a partition.
When the partition is removed, Gas A starts to move
to the right and Gas B to the left.
This diffusion continues until the mixture is
homogeneous and there are no concentration
gradients in the mixture.




1

,Concentration of gas species i :
Can be expressed as a mass concentration or a
molar concentration.


Mass concentration = mass density  i (kg / m3 )
Molar concentration = C i (kmol / m3 )


One "kmol" is the amount of gas in kilograms that
is numerically equal to the molecular weight Mi
of the gas. For example, one kmol of oxygen (O2 )
is 32 kg.




The density of species i of the mixture is
i = Mi C i
The mass fraction of species i in a gaseous mixture
mi  i
mfi = =
m 
m = total mass of the mixture
mi = mass of species i in the mixture
The mole fraction of species i in a gaseous mixture
ni
yi =
n
n = total number of kmoles in the mixture
ni = number of kmoles of species i in the mixture




2

, We have a mixture of ideal gases.
PV = nR T
n =  ni = number of kmoles in the mixture
P = pressure of the mixture (kPa)
V = volume of the mixture (m3 )
T = absolute temperature of the mixture (K)
R = universal gas constant = 8.31446 kJ / kmol K
The ideal gas law may be written for each species i
Pi V = ni R T




The pressure P of the mixture equals the sum of the
partial pressures of the species in the mixture.
P =  Pi
Pi = partial pressure of species i


For a mixture of ideal gases: the volume fraction, mole
fraction, and the ratio of the partial pressure of a species
to the total pressure of the mixture are equal. That is,
ni Pi Vi
yi = = =
n P V




3