Question

from wetted wall column experiment :

how does the change of parameter ( air flow rates or tube diameters) affecting the mass transfer in the column ?

Answer 1

Mass transfer is a transport phenomenon of movement of species from one phase to another when brought in contact with each other.

Mass transfer rate is given by : Na = K* $\Delta C$

where K is mass transfer coefficient and $\Delta C$ is concentration difference.

From the above equation we can note that the rate of mass transfer is directly proportional to mass transfer coefficient. The Mass transfer coefficient depends upon many factors including the Flow rate of liquid, viscosity of liquid, pressure, temperature, diameter of column and molecular mass of the compounds.

To answer the question, the effect of air flow rate and diameter change on mass transfer will be,

AIR FLOW RATE : Generally, the mass transfer coefficient is directly proportional to Reynold's Number which depends upon the flow rate of fluid. the expressions is : $K = C1*Re^{a}$. where C1 is some constant and Re is Reynolds Number and value of 'a' lies between 0.55 to 0.88. From this we can safely say that if AIR FLOW RATE INCREASES, THE MASS TRANSFER RATE WILL ALSO INCREASES AND VICE VERSA. Another Explanation for this could be that the when air flow rate increases the boundary resistance reduces and hence mass transfer rate increases.

DIAMETER OF TUBE : Mass transfer coefficient is inversely related to the square of diameter. The expression is $K = C2 *D^{-2}$ . where C2 is some proportionality constant and D is diameter of tube. Form the expression it is clear that if DIAMETER OF TUBE INCREASES, MASS TRANSFER COEFFICIENT DECREASES, HENCE THE MASS TRANSFER RATE DECREASES AND VICE VERSA. Another explanation of this could be that from equation of continuity, The area of cross section increases the velocity decreases and hence the flow rate decreases, hence the mass transfer rate decreases.