Mass Transfer Operation

1. Air-Liquid Diffusion
Aim: To study the steady State Diffusion of Acetone in Air and Calculate the Diffusivity for the same.
Apparatus: Glass tube, measuring Cylinder, Heating Coils, Blower.
Chemicals: Acetone
Theory:
Diffusivities are most conveniently determined by Winkelmann’s Method, in which liquid is allowed
to evaporate in a vertical glass tube over the top of which steam of vapour free gas is passed, sufficiently
rapidly for the Vapour Pressure to be maintained at almost zero. If the apparatus is maintained at a steady
temperature, there will be no eddy currents in the vertical tube and mass transfer will take place from the
surface by means of molecular diffusion alone. The rate of evaporation can then be determined by the rate of
fall of liquid surface and since the concentration gradient is known the diffusivity can then be calculated.
Procedure:
1. Fill up the Water tank and start the water heater.
2. Fill the Glass tube with acetone upto a specified level by using long neck funnel.
3. Allow the temperature to rise upto 50o C and maintain it at same temperature.
4. Ensure that the outlet (bypass) valve of the blower is fully open and that the tube side valve is fully
closed.
5. Open the tube side valve and allow air to flow gently over the diffusing fluid (here Acetone)
6. Note the drop in height of liquid inside the tube after a time interval of every 5 minutes.
7. Calculate the diffusivity using Stefan’s correlation
Observations:
1. Temperature of Air =
2. Temperature of Water Bath =
3. Density of Acetone =
4. Molecular Weight of Acetone =
5. Velocity of Air =
6. I.D. of Tube containing Acetone =
7. Area of air outlet =
Observation Table:
Sr. No. Time t Level of Acetone (Z1
2- Z2
2) DAB* 10-3
Z1 Z2 2t
(ml) (mL) (m2/s)
1
2
3
4
5
6
Volume = Area x Height
Area = π Di
2 /4
Where, Di = I.D. of Tube containing Acetone
= 20mm

2. Calculations:
1. Total Pressure
Velocity of air =
Area of Discharge =
Discharge = Area x Velocity
= m3/s
Power of Motor = W
Pressure of Air on Acetone = Pressure / Discharge
= kPa
Vapour Pressure at 50o C
lnP = A – (B/(T+C))
A = 16.6513
B = 2940.46
C = -35.93
lnP =
P = kPa
Therefore, Total Pressure on interface
P = Pb-p
= kPa
2. Initial number of moles in the tube
n = ρZ1 / M
= kmol
Moles diffused after 10 minutes
n10 = ρ(Z1 – Z2) / M
= kmol
Therefore, mole fraction of Acetone in air
X10 = n10/n
= xA1
PA1 = xA1 * P
= kPa
Mole fraction of air in the gaseous mixture after 10 minutes
XB1 = 1-xA1
=
Therefore PB1 = kPa
Now PA2 = 0 kPa
PB2 = 101.325 kPa
PBM = (PB2 - PB1) / ln ( PB2 / PB1)
DAB = (Z12 - Z22) RT PBM
Z1 * P * (PA1 - PA2)
= m2/s

3. Results:
SR.NO. Time DAB
(s) (m2/s)
1
2
3
4
5
6
Conclusions:
1. As the time increases the concentration of Acetone (A) in air (B) increases.
2. As the time increases the diffusivity of acetone in air decreases due to the decrease in concentration
gradient.

4. Tray Dryer
Aim: To study the characteristics of Tray dryer and to calculate the rate of drying.
Apparatus: Tray dryer, weighing balance, blower.
Materials: Water, Chalk pieces.
Theory:
Equipments used for drying processes can be classified according to
1. Methods of operation
2. Methods of supplying the heat necessary for evaporating the moisture
3. Nature of substance to be dried
Tray Dryers are the batch type dryers and direct dryers. In this type of dryers, heat is supplied directly by
direct contact with the substance with the hit gas into which evaporation takes place.
Construction & Working:
A typical tray dryer consists of a cabinet containing removable trays on which the solid to be dried is
placed. After loading the solids onto the tray, the cabinet is closed and hot air is blown across and between
the trays to evaporate the moisture. Moist air is continuously vented out through a small duct.
Rate of Batch Reaction:
In order to set up drying schedules and to determine the size of equipment it is necessary to know the
time required to dry the substance from one level of moisture to another under specified conditions.
Rate of drying can be determined by measuring the weights of drying ample at different times. From
the data so obtained a graph of moisture v/s time is plotted and the rate can be calculated as
Rate = -M * Win / A
Where M = Slope of Curve
Win= Initial weight of feed
A = cross sectional area of Tray
Procedure:
1. Note the dimensions of the tray.
2. Weigh the dry feed.
3. Add a known quantity of water to the dry feed
4. Spread the sample uniformly on the tray and close the cabinet.
5. Switch on the heater and the blower.
6. Note the weight of sample after every 5 minutes and the corresponding temperature upto 30 minutes.
Graphs: The following graphs are to be plotted
1. Moisture Content v/s Time
2. Rate of Drying v/s Moisture content
Observations:
1. Weight of Dry feed =
2. Weight of Water =
3. Area of Tray =

5. Observation Table:
Sr. Time Weight Temperature
No. (minutes) (gm) (OC)
Dry Bulb Wet Bulb
1
2
3
4
5
6
7
8
Calculations:
At time t , Rate = -M * Wini / A
Moisture content= kg moisture/kg dry feed
Results:
Sr. Time Slope Rate of Drying Moisture content
No. (minutes) (gm/min) (gm/m2s) (kg moisture/ kg feed)
1
2
3
4
5
6
7
8

6. Liquid-Liquid Diffusion
Aim: To study the steady state molecular diffusion of Acetic acid through water and determine the
diffusivity of the
Same.
Apparatus: Porous pot, beaker, conical flask, burette, pipette.
Chemicals: Acetic acid, NaOH, Phenolphthalein indicator.
Theory:
The term molecular diffusion is concerned with the movement of individual molecules through a
substance by virtue of their thermal energy.
In a liquid solution, if the solution is uniform everywhere in the concentration of its constituents, no
alteration occurs but as long as it is not uniform, the solution is spontaneously brought to uniform by
diffusion. The substance moves from the place of high concentration to the one of low concentration. The
rate at which the diffusing substance moves at any point in any direction must therefore depend upon the
concentration gradient at that point and in that direction. To describe the motion of one component into the
other, two molar fluxes are used. NA the flux relative to a fixed position in space and JA the flux relative to
the average molar velocity of all constituents.
The diffusivity of a constituent A in the solution B is a measure of the diffusive mobility and defined
as the ration of its flux to its concentration gradient.
JA= - DAB (dCA/dz)
This is known as Fick’s law written for z-direction. The negative sign indicates that diffusion occurs in a
direction of decreasing concentration, in agreement with the second law of thermodynamics. Because of
high molecular concentrations, these diffusivities are of very low value of magnitude of 10-9 .
For liquid-liquid diffusion, the following cases can be studied:
1. Steady state diffusion of A through non diffusing B
NA=constant , NB=0
NA= DAB ρavg (xA1 - xA2)
Z xBM Mavg
Where
xBM = ( xB2 – xB1)
ln( xB2 / xB1)
2. Steady state equimolar counter diffusion
NA=NB=constant
NA= DAB ρavg (xA1- xA2)
Z Mavg
Procedure:
1. Acetic Acid is titrated against NaOH to find out its initial concentration.
2. A porous pot filled with Acetic Acid is then immersed in a beaker containing 1.8 l of Water.
3. After every 10 minutes 10ml of solution is pipetted out from a fixed point in a beaker and titrated with
0.1N NaOH using phenolphthalein as an indicator.
4. The same procedure is repeated and 5 more readings are taken.
Precaution:
Care should be taken that the solution is pipetted from same point.
Observations:
1. I.D. of porous pot = 4.8cm
2. O.D. of porous pot = 6.04cm
3. Thickness = 0.62cm

7. 4. Volume of water in beaker = 1.8l
5. Height of acid in porous pot =
6. Initial conc. Of Acetic Acid =
7. Diameter of Beaker = 20cm
Observation Table:
Sr. Time Burette Normality Gm of Acetic Acid Gm of Acetic Acid
No. (minutes) Reading (N) diffused per litre diffused totally
(ml)
1.
2.
3.
4.
5.
6.
Calculations:
1. Surface Area of Porous Pot = πDoL + πDo
2/4 =
2. Normality
N1V1 = N2V2
Acetic Acid (AA) NaOH
N1 = N2V2/V1
= N
3. Grams of AA diffused per lit. = 60N1
= g/l
4. Grams of AA diffused totally =
5. NA = gm of AA totally diffused
Area * time * 60
= kmol/m2s
6. Volume of AA in porous pot = π Di
2 L / 4
7. Weight of AA present initially = ρ V
= kg
no = kmol
8. Weight of AA after 10 min = NA * A * MAA * 600
9. Mole fraction of AA
xA1 (at 10 min) = NA * A * MAA * 1200/no =
xA2 (at 20 min) = NA * A * MAA * 1200/no =
xB1 = ( 1 - xA1 ) =
xB2 = ( 1 – xA2) =
xBM = xB2 - xB1
ln( xB2 / xB1)
=
(ρavg)1 = xA1 * ρAA + xB1 * ρw
=
(ρavg)2 = xA2 * ρAA + xB2 * ρw
=
(ρavg) = { (ρavg)1 + (ρavg)2 } / 2
= kg/m3
(Mavg)1 = xA1 * MAA + xB1 * Mw
=

8. (Mavg)2 = xA2 * MAA + xB2 * Mw
=
(Mavg) = { (Mavg)1 + (Mavg)2 } / 2
= g/mol
Therefore DAB =
Result: