Chemical Engineering Department Mass Transfer Spring Semester 2008-2009 Dr Mustafa Nasser Chemical Engineering Department

The following topics will be covered in this course: Introduction to Mass Transfer and Diffussion. Diffusion of the gas, liquid and solid. Mass Transfer Theories. Mass Transfer Coefficient Dimensional Analysis Multicomponent Diffusion Multi Directions Diffusion. Boundary Layer

Definition of Mass Transfer Is a process where a component in a mixture is migrate from the high concentration to the low concentration due to the concentration gradient. Mass transfer can take place in gas or vapour or in a liquid Mass transfer result from the random velocities of the molecules

Molecular Diffusion The transfer of the component in the fluid is defined as the movement of the individual molecules through a fluid. The movement of the molecule is straight line changing in the direction of these molecules by means of the collision of the molecules . Molecules are travel by random movement this process is called molecular diffusion.

Molecular Diffusion Turbulent Diffusion

Molecular Diffusion Random-walk process –molecules traveling only in straight line and changing in direction by bouncing off other molecule after collisions. The molecule travels in Random Path. The transfer of mass between adjacent layers of fluid in laminar flow. Slow process is often called laminar flow diffusion

Turbulent Diffusion Turbulent flow By supplying mechanical force or Fast process Turbulent flow By supplying mechanical force or heating

Diffusion Types Ordinary Diffusion – occurs due to concentration gradient of gravity/force 2. Thermal Diffusion – due to temperature driving force in addition to the concentration gradient.

Diffusion Types 3. Pressure diffusion – due to pressure gradient in addition to the concentration gradient. 4. Force diffusion - due to changing external force supplied in addition to the concentration gradient.

Example Properties: The molar masses of N2, O2, and H2O are 28.0, 32.0, and 18.0 kg/kmol, respectively

Example 7 Example

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Binary diffusion coefficients values

Example : Molecular diffusion of helium in Nitrogen 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 of He is 0.6 atm and at the of the other end 0.2 m = 0.2 atm. Calculate the flux of He at steady state if of the He-Ne mixture is 0.687 x Use SI and cgs Unit.

Solution You can easily calculate for cgs If the pressure in atm This is the final equation to use, which is in the form easily used for gases partial pressure are If the pressure in atm You can easily calculate for cgs

Example (one dimensional) Hydrogen is stored at elevated pressure in a rectangular container having steel walls 10 mm thick. The molar concentration of hydrogen in the steel at inner Surface is 1 km/m3, while the concentration of hydrogen in the steel at outer Surface is negligible. The binary diffusion coefficient for hydrogen in steel is 0.26 x 10-12 m2/s. what is the molar diffusive flux for hydrogen through the steel. (steady state one dimensional conditions)

Solution

Example Plastic membrane mdiff H2 Air L

b) Repeating the calculations for a 0.5-mm thick membrane gives Analysis (a) We can consider the total molar concentration to be constant (C = CA + CB  CB = constant), and the concentration of the hydrogen in the membrane is extremely low (CA << 1). Then the molar flow rate of hydrogen through the membrane by diffusion per unit area is determined from The mass flow rate is determined by multiplying the molar flow rate by the molar mass of hydrogen, b) Repeating the calculations for a 0.5-mm thick membrane gives The mass flow rate through the entire membrane can be determined by multiplying the mass flux value above by the membrane area