MASS TRANSFER

MASS TRANSFER : 

MASS TRANSFER PREPARED BY: MR. JITENDRA L PATEL

MASS TRANSFER : 

MASS TRANSFER Mass transfer takes place from solid to liq, solid to vapor or gas, liq to liq, liq to vapour or gas. It’s a complex phenomenon taking place in many of unit operation like Humidification $ dehumidification, Extraction, Drying, Evaporation, Distilation. Mass transfer can occur by molecular diffusion, convection or bulk flow and turbulent flow.

Solid /fluid mass transfer : 

Solid /fluid mass transfer A lump or crystal of soluble material when immersed in solvent in which it is dissolving. The crystal is surrounded by a stationary layer of solvent with the bulk of fluid able to move. Movement of fluid can be natural convection or forced convection based on density and temp changes or by agitation. The transport of solid takes place in two stages 1. Molecular diffusion 2. Eddy diffusion

Solid /fluid mass transfer : 

Solid /fluid mass transfer The molecules move through the boundary layer by molecular diffusion with no mechanical movement or mixing. After mass passes through boundary layer, mass transfer takes place by bulk movement of the solution, known as a eddy diffusion. The rate limiting step is the molecular diffusion. Same thing occur in case of mass transfer from solid to gas. E.g. if a solid is drying in air, the vapor molecules must diffuse through air boundary layers. The driving force is partial vapor pressure gradient through air boundary layers.

Solid /fluid mass transfer : 

Solid /fluid mass transfer

Solid /fluid mass transfer : 

Solid /fluid mass transfer Where w= weight of solute diffusing W=wt of solute diffusing per unit time D= diffusion coefficient A= area C1= conc of solute at interface C2= conc of solute in bulk L= thickness p1= partial pressure of vapor at interface p2= partial pressure of vapor in atm.

Fluid / Fluid mass transfer : 

Fluid / Fluid mass transfer When mass transfer takes place between two immiscible fluids. In this case there will be boundary layers of both fluids on each side of the interface. The slope of the conc gradient depends on the diffusion coefficient of the two materials.

Application : 

Application In solution making of solid in liquid, the rate of solution can be increased by 1. agitation reduce the thickness of the boundary layers thus increase the conc gradient 2.Elevating the temp, increase the diffusion coefficient and decrease the viscosity of the liq, resulting in reduction of boundary layers. Reducing the size of solid, increase the surface area over which diffusion can occur.