1. Introduction to Mass Transfer
CHAPTER 6
Introduction to Mass Transfer

2. Introduction Three fundamental transfer processes: Momentum transfer
Heat transfer
Mass transfer

3. Mass transfer may occur in a gas mixture, a liquid solution or solid.
Mass transfer occurs whenever there is a gradient in the concentration of a species.
The basic mechanisms are the same whether the phase is a gas, liquid, or solid.

4. Definition of Concentration
i) Number of molecules of each species present per unit volume (molecules/m3)
ii) Molar concentration of species i = Number of moles of i per unit volume (kmol/m3)
iii) Mass concentration = Mass of i per unit volume (kg/m3)

5. Diffusion phenomena
Fick’s law: linear relation between the rate of diffusion of chemical species and the concentration gradient of that species.
Thermal diffusion: Diffusion due to a temperature gradient. Usually negligible unless the temperature gradient is very large.
Pressure diffusion: Diffusion due to a pressure gradient. Usually negligible unless the pressure gradient is very large.

6. Forced diffusion: Diffusion due to external force field acting on a molecule. Forced diffusion occurs when an electrical field is imposed on an electrolyte ( for example, in charging an automobile battery)
Knudsen diffusion: Diffusion phenomena occur in porous solids.

7. resistance Whenever there is concentration difference in a medium,
nature tends to equalize
things by forcing a flow
from the high to the low
concentration region.
The molecular transport process of mass is characterized by the general equation:
Rate of transfer process = driving force
resistance
Before After

8. Example of Mass Transfer Processes
Consider a tank that is divided into two equal parts by a partition.
Initially, the left half of the tank contains nitrogen N2 gas while the right half contains O2 at the same temperature and pressure.
When the partition is removed the N2 molecules will start diffusing into the air while the O2 molecules diffuse into the N2.
If we wait long enough, we will have a homogeneous mixture of N2 and O2 in the tank.

9. Liquid in open pail of water evaporates into air because of the difference in concentration of water vapor at the water surface and the surrounding air.
A drop of blue liquid dye is added to a cup of water. The dye molecules will diffuse slowly by molecular diffusion to all parts of the water.

10. Molecular Diffusion Equation
Fick’s Law
is the molar flux of component A in the z direction in kg mol A/s.m2.
is the molecular diffusivity of the molecule A in B in m2/s
is the concentration of A in kg mol/m3.
z is the distance of diffusion in m

11. Fick’s Law of Diffusion
Molecular diffusion or molecular transport can be defined as the transfer or movement of individual molecules through a fluid by mean of the random, individual movements of the molecules.
If there are greater number of A molecules near point (1) than at (2), then since molecules diffuse randomly in both direction, more A molecules will diffuse from (1) to (2) than from (2) to (1).
The net diffusion of A is from high to low concentration regions.
(2) A B B B B B B B B B B B
(1) A
Figure 3: Schematic diagram of molecular diffusion process

12. The two modes of mass transfer:
Molecular diffusion
Convective mass transfer

13. Molecular diffusion
The diffusion of molecules when the whole bulk fluid is not moving but stationary. Diffusion of molecules is due to a concentration gradient.
The general Fick’s Law Equation for binary mixture of A and B
c = total concentration of A and B [kgmol (A + B)/m3]
xA= mole fraction of A in the mixture of A and B

14. Example
A mixture of He and N2 gas is contained in a pipe at 298 K and 1 atm total pressure which is constant throughout. At one end of the pipe at point 1 the partial pressure pA1 of He is 0.6 atm and at the other end 0.2 m pA2 = 0.2 atm. Calculate the flux of He at steady state if DAB of the He-N2 mixture is x 10-4 m2/s.

15. Solution
Since a total pressure P is constant, the c is constant, where c is as follows for a gas according to the perfect gas law:
Where n is kg mol A plus B, V is volume in m3, T is temperature in K, R is m3.Pa/kg mol.K or R is x 10-3 cm3. atm/g. mol. K, and c is kg mol A plus B/m3.
For steady state the flux J*Az in Eq.(6.1-3) is constant. Also DAB for gas is constant. Rearranging Eq. (6.1-3) and integrating.
(6.1-11)

16. Also, from the perfect gas law, pAV=nART, and
Substituting Eq. (6.1-12) into (6.1-11),
This is the final equation to use, which is in a form eqsily used for gases. Partial pressures are pA1 = 0.6 atm = 0.6 x x 105 = 6.04 x 104 Pa and pA2 = 0.2 atm = 0.2 x x 105 = x 104 Pa. Then, using SI units,
(6.1-13)

17. If pressures in atm are used with SI unit,
Other driving forces (besides concentration differences) for diffusion also occur because of temperature, pressure, electrical potential, and other gradients.

18. Convection Mass Transfer
When a fluid flowing outside a solid surface in forced convection motion, rate of convective mass transfer is given by:
kc - mass transfer coefficient (m/s)
cL1 - bulk fluid conc.
cLi - conc of fluid near the solid surface
Kc depend on:
system geometry
Fluid properties
Flow velocity

19. Principles of Mass Transfer CHAPTER 2 Molecular Diffusion in Gases

20. CONTENTS

22. Molecular Diffusion in Gases
Equimolar Counterdiffussion in Gases

25. For a binary gas mixture of A and B, the diffusivity coefficient DAB=DBA

26. Example 6.2-1 Ammonia gas (A) is diffusing through a
uniform tube 0.10 m long containing N2
gas (B) at x 105 Pa pressure and
298 K. The diagram is similar to Fig. 6.2-
1. At point 1, pA1 = x 104 Pa and
at point 2, pA2 = x 104 Pa. The
diffusivity DAB = x 10-4 m2/s.
(a) Calculate the flux J*A at steady state
(b) Repeat for J*B

27. Solution
Equation (6.1-13) can be used, where P = x 105 Pa, z2-z1 = 0.10 m, and T = 298 K. Substituting into Eq. (6.1-13) for part (a),
Rewriting Eq. (6.1-13) for component B for part (b) and noting that pB1 = P – pA1 = x 105 – x 104 = x 104 Pa and pB2 = P – pA2 = x 105 – x 104 = x 104 Pa.
The negative for J*B means the flux goes from point 2 to point 1.

28. Diffusion of Gases A and B Plus Convection

30. For equimolar counterdiffussion, NA=-NB ,
then NA=J*A=-NB=-J*B

33. Example 6.2-2 Water in the bottom of a narrow metal tube is held at a
constant temperature of 293 K. The total pressure of air
(assumed dry) is x 105 Pa (1.0 atm) and the
temperature is 293 K (20 °C). Water evaporates and
diffuses through the air in the tube, and the diffusion
path z2-z1 is m (0.5 ft) long. The diagram is
similar to Fig a. Calculate the rate of evaporation
at steady state in lb mol/h.ft2 and kg mol/s.m2. The
diffusivity of water vapor at 293 K and 1 am pressure is
0.250 x 10-4 m2/s. Assume that the system is
isothermal. Use SI and English units.

34. Solution
The diffusivity is converted to ft2/h by using the conversion factor:
From Appendix A.2 the vapor pressure of water at 20 °C is mm, or pA1 = 17.54/760 = atm = ( x 105) = x 103 Pa, pA2 = 0 (pure air). Since the temperature is 20 °C (68 °F), T = °R = 293 K. From Appendix A.1, R = ft3.atm/lb mol.°R . To calculate the value of pBM from Eq. (6.2-21)

35. Since pB1 is close to pB2, the linear mean (pB1+pB2)/2 could be used and would be very close to pBM.
Substituting into Eq. (6.2-22) with z2-z1 = 0.5 ft (0.1524m),

37. Example 6.2-4 A sphere of naphthalene having a radius of 2.0
mm is suspended in a large volume of still air at
318 K and x 105 Pa (1 atm). The
surface temperature of the naphthalene can be
assumed to be at 318 K is 6.92 x 10-6 m2/s.
Calculate the rate of evaporation of
naphthalene from the surface.

38. Solution
The flow diagram is similar to Fig a. DAB = 6.92 x 10-6 m2/s, pA1 = (0.555/760)( x 105) = 74.0 Pa, pA2 = 0, r1 = 2/1000 m, R = 8314 m3.Pa/kg mol.K, pB1 = P-pA1 = x 105 – 74.0 = x 105 Pa, pB2 = x 105 – 0. since the values of pB1 and pB2 are close to each other,
Substituting into Eq. (6.2-32),

41. Example 6.2-5 Normal butanol (A) is diffusing through air (B)
at 1 atm abs. Using the Fuller et al. method,
estimate the diffusivity DAB for the following
temperatures with the experimental data:
(a) For 0 °C.
(b) For 25.9 °C.
(c) For 0 °C and 2.0 atm abs.

42. Solution
For part (a), P = 1.00 atm, T = = 273 K, MA (butanol) = 74.1, MB (air) = 29. From Table 6.2-2,
Substituting into Eq. (6.2-45),
This values deviates +10% from the experimental values of 7.03 x 10-6 m2/s from Table 6.2-1

43. For part (b), T = 273 + 25. 9 = 298. 9. Substituting into Eq. (6
For part (b), T = = Substituting into Eq. (6.2-45), DAB= 9.05 x 10-6 m2/s. This values deviates by +4% from the experimental value of 8.70 x 10-6 m2/s
For part (c), the total pressure P = 2.0 atm. Using the value predicted in part (a) and correcting for pressure,
DAB = 7.73 x 10-6(1.0/2.0) = 3.865x10-6 m2/s