MASS TRANSFER Sheet 1

Faculty of Engineering Cairo University petroChemical Engineering Department

Mass Transfer Fourth Year

Sheet 1: Diffusion

1- Calculate the mass diffusivity of a mixture of CO and O2 at 25°C 1 atm. using Gilliland

correlation, VO2 =25.6, VCO =34 cm3 /gmole

2- Calculate the diffusion coefficient of Methanol in Water at 30°C knowing that:

VM = 14.9 cm3 /gmole and VW = 8 cm

3 /gmole

3- Determine the diffusivity of CO2 (1), O2 (2) and N2 (3) in a gas mixture having the next

composition (Mole Basis): 28.5% CO2, 15% O2 and 56.5% N2.

The gas mixture is at 273K and 1 atm., the binary diffusivities are given as follows:

D12 = 0.1874 cm2/sec, D13 = 0.1945 cm

2/sec, D23 = 0.1834 cm

2/sec

4- Consider the case in which liquid (A) evaporates and diffuses through stagnant gas (B) as

shown in the figure. Assuming the process is carried out isothermally at 25 °C and the

total pressure is 1 atm. Calculate the molar flux when the level of liquid (A) is 0.05 m

below the top of the container.

Assume that DAB = 1.0E-5 m2 /sec. The mole fraction of A in the gas phase at the gas-

liquid interface is 0.2 and at the top of the container is 0.001

5- A simple rectifying tower consists of a tube arranged vertically and supplied at the

bottom with a vapour mixture of Benzene and Toluene. At the top of the column a

condenser returns some of the product as a liquid reflux flowing in a thin film down the

inner wall of the tube. At one point in the column the Benzene concentration in the

vapour is 70 mol% and the adjacent liquid contains 60% by mol Benzene.

The temperature at this point is 365 K. Assuming the diffusional resistance in the vapour

phase is to be equivalent to a stagnant vapour layer of 0.2 mm thick.

Calculate the rate of exchange of Benzene between vapours and liquid if the diffusivity of

the vapour is 0.051 cm2/sec.

B

A

MASS TRANSFER Sheet 1

Constants of Antoine equation for toluene are: A=16.0137, B=3096.52 & C=-53.67

6- A sintered solid of Silica 4 mm thick is porous with a porosity of 0.4 and tortusity of 3.

The pores of the Silica are filled with water at 25°C. At one face of the solid Silica the

concentration of KCl is held at 0.1 gmole/lit and pure water flows rapidly by the other

face. Calculate the diffusion flux of KCl at steady state given that the diffusivity of KCl

in water at 25°C is 1.87E-9 m2 /sec.