Important Mass Transfer Concepts

Important Mass Transfer Concepts1) Equilibrium RelationIt is mainly got using a set of experimental data.The equilibrium relation is specific for certain system, temperature, and pressure.Mainly reaching equilibrium isnt economically favorable.It represents one of the constraints for the chemical engineer.It defines your borders!It is vital for the design of the mass transfer equipment.

22) Operating Curve (Line)It is got by performing overall balance on the equipment. (mass and heat balance)We can have one or more operating curves for one equipment.It is so called because any point on this line (curve) represents the bulk concentrations of the phases at any section of the equipment (As if the equipment operates along this line!)

32) Operating Curve (Line) (Contd)

(Ponchon-Savarit) methodused in distillation is an example of having more than one operating line.

43) Number of stagesBased on the overall efficiency, we have certain relation between ideal and actual stages.Ideal stages can be calculated graphically using the method shown in the figure.For packed column, we dont have stages. But, we have a concept known as number of transfer units.

54) Pinch PointNo separation will be affected, as there is zero driving force.Infinite number of stages is required.One of the flow rates is at its minimum value.The operating flow rates are determined based on it.

65) Rate of mass transferRate = mass transfer coefficient * Area * Driving forceDriving force can be the pressure or concentration difference.The units of mass transfer coefficient depends upon the driving force.The mass transfer coefficient is very important in determining the plate efficiency, and thus in the design of the plate column.The mass transfer coefficient is very important in determining the height of transfer unit, and thus in the design of the packed column.The mass transfer coefficient is mainly got using empirical equations.

76) Why do we calulate number of stages graphically?!In the mass transfer applications, we mainly have mass balance and equilibrium relations only, so why dont we solve them together to get the number of stages?!To answer this question, we will take a simple example on an absorption column:We will further assume that the equilibrium relation can be substituted by a straight line (Y= X)The overall mass balance equation is:

86) Why do we calculate number of stages graphically?! (Contd)Substituting the equilibrium relation in the mass balance gives:

The previous is known as a First order finite difference equation. It is solved by certain mathematical technique to get after more than 4 or 5 steps:

96) Why do we calculate number of stages graphically?! (Contd)Finally!

10Note that this difficult form is got when assuming straight line equilibrium relation, and binary system.

What if we have actual case with non-straight line equilibrium relation and multi-component system?