Question

(a) Conduct your own research to study any mass diffusion process in chemical industry. Briefly, explain in your own words (maximum of 2 pages) the mechanism of mass diffusion that occurs in your selected mass transfer process. [Show clearly all references/resources used

Answer 1

Mass diffusion :-

Transportation of the molecule or mass between the species is called Mass diffusion, It is done on the basis of concentration gradient.

Suppose Two molecule A & B present in system, in Which molecule (A) has higher concentration & Molecule (B) has Lower concentration so as they are contacting with each other there is a transportation of mass from molecule (A) to molecule (B) in other words { From Higher concentration to lower concentration} this is called mass diffusion.

Example :- If Ammonia gas is contacted with water, in which Ammonia has higher concentration & water has less concentration, so there is transfer of mass tacking from ammonia to water.

We can also understand this by example, In a winter if we Heat the coal or other thing, so on that case heat produce by Coal (High temperature area) is transfer to the lower temperature area. it is called Heat transfer it is done on the basis of temperature gradient, while Mass diffusion is done on the basis of concentration gradient.

Types of Mass diffusion :-

(1) Molecular diffusion

(2) Eddy diffusion

in molecular diffusion, diffusion is taking place naturally, while in Eddy diffusion there is a need of external source to make a diffusion possible, so this is the only difference between them.

Time needed for mass diffusion :-

This can be achieved by finding out the time needed for reaction to reach equilibrium condition.

Equilibrium is defined as " Uniform concentration is achieved in entire solution", in other word we can say that once the equilibrium is achieved further mass diffusion can't possible in the given solution.

Example :- add salt in the water than mix it properly, once all the salt particle is absorbed in the water it make salty solution. We notice that, in the salty solution there is uniform concentration within the solution & after that further absorption of salt particle is not possible, because equilibrium is reached.

During equilibrium,

(1) Net Flux is zero {Flux is define as amount of species that cross unit area per unit time}

(2) Chemical Potensial is same in solution {Chemical potential defined as ability of uncharged atoms to perform physical work}

(3) Uniform concentration within solution.

Rate of Mass diffusion :-

A molecule is imagine to travel in a straight line at uniform velocity, before it's colides with another molecule as it colides with another molecule it velocity changes both in magnitude & direction, so average distance travel by molecule in particular direction in particular time is defined as rate of mass diffusion.

Diffusion notation :-

D_AB :- Diffusion of mass from molecule A to B.

D_BA :- Diffusion of mass from molecule B to A

Mechanism of Mass Diffusion :-

Fick's Law of Mass diffusion is mechanism by which Mass diffusion is taking place.

FIck's law give the relation between flux of diffusing substance & Concentration gradient responsible for the molecular diffusion.

It is applicable for binary system.

J_A = - CD_AB (dX_a / dX_b) = D_AB (dC_A/ dZ) {Equation you need to remember}

where J_A is flux,

D_AB is Diffusion of mass from molecule A to B.

Z = Distance

X_A = Mole fraction

C_A = Concentration

Thank you,

Hope you like it & understand it.