MASS TRANSFER

MASS TRANSFER:

Mass transfer is the transfer of mass from high concentration to low concentration. It includes all physical processes that involve molecular and convective transport of atoms and molecules within physical systems. Mass transfer includes both fluid flow and separation unit operations. Mass transfer is the net movement of mass from one location, usually meaning a stream, phase, fracation or component, to another. Mass transfer occurs in many processes, such as absorption, evaporation, adsorption and drying. Mass transport occurs when a particle or component of a mixture moves to another spot because of a variance in concentration. Common examples of mass transfer processes are the evaporation of water from a pond to the atmosphere. The driving force for mass transfer is a difference in concentration. The random motion of molecules causes a net transfer of mass from an area of high concentration to an area of low concentration. The amount of mass transfer can be quantified through the calculation and application of mass transfer coefficients. Mass transfer finds extensive application in chemical engineering problems, where material balance on components is performed.

ANALOGIES BETWEEN HEAT, MASS, AND MOMENTUM TRANSFER:

It is important to note that in molecular transport, heat, or mass transfer there are many similarities. The molecular diffusion equations of Newton for momentum, Fourier law for heat, and Fick’s law for mass are very similar. Reynolds analogy assumes that the turbulent diffusivities are all equal and that the molecular diffusivities of momentum (μ/ρ) and mass (DAB) are negligible compared to the turbulent diffusivities. The most successful and most widely used analogy is the Chilton and Colburn J-factor analogy. This analogy is based on experimental data for gases and liquids in both the laminar and turbulent regions. Although it is based on experimental data, it can be shown to satisfy the exact solution derived from laminar flow over a flat plate.

TRANSPORT MECHANISM:

Transport of chemical “species” in solid, liquid, or gas mixture. Transport driven by composition gradient, similar to temperature gradients driving heat transport. We will look at two mass transport mechanisms, both analogous to heat transport:

q  Diffusion mass transfer.

q  Convection mass transfer.

DIFFUSION MASS HEAT TRANSFER

Results from intermolecular collisions that lead to net motion of molecules in direction of decreasing concentration. Analogous to conduction heat transfer. Examples:

n  Doping of semiconductor wafers to achieve desired electrical properties.

n  Diffusion of carbon atoms during heat treating of steel.

n  Leaking of gas through container wall.

CONVECTION MASS TRANSFER

Combination of diffusion mass transfer combined with bulk fluid motion. Examples:

n  Drying of materials such as paper and food products.

n  Evaporation of water in power plant cooling tower.

n  Chemical reactions and combustion processes–determination of distribution of reactants and products.

THEORIES OF MASS TRANSFER:

n  Fluid Film theory: There is diffusion from one phase to another, the resistance to mas transfer in two phases are added to get overall resistance.

n  Boundary layer theory: When the flow is laminar near the wall the mass transfer is explained by this theory.

                         M/A = D_v^2/3

n  Penetration theory: For transient rate of diffusion into thick mass of fluid with a constant conc. at the surface is given by:

            k_m= 1.13 √D_v / t

Where t is time of penetration.

And depth of penetration is given by: 3.6 √D_v  t