Experiment 2
Solid Gas Diffusion
Aim:
To study the variation of mass transfer coefficient as a function of air
for the representation of naphthalene in a packed bed.
Diagram:
Apparatus:
1.Weighing Machine
2.Air blower
3.Rotameter
4.Naphthalene balls
5. Glass tube
Theory:
Diffusion can be explained as a phenomenon of material transfer by
transport from a region of high conc to region of lower
concentration.
Where DAB is the diffusivity of A in B, which depends upon the crystal
imperfection, diffusion mechanism , temperature ,type of crystal
structure , concentration of diffusion species.
Diffusivity of Naphthalene in air = 0.078 cm2/s.
The vapour Pressure of naphthalene is given by the equation:
1606.52
log P S = 6.845 - (187.2+T )
Ps in mmHg and T in °C
The governing equation for rate of mass transfer for simple
vaporization of naphthalene in a current of air in a packed column is
given by:
NA = Molar flux (kmol/hr*m2)
AP = Surface Area of solute particles (m2)
PT =Vapour pressure of solute (atm)
Po = Partial pressure of a solute (atm)
W = loss in weight sample (kg/hr)
Ms = molecular weight of naphthalene (128 g/mol)
Procedure:
1.Naphthalene balls were packed inside the glass column.
2.Initial weight of the sample was recorded.
3.Air was passed through the column at a known flow rate
(measured by rotameter).
4.Weight loss of naphthalene was measured after fixed time
intervals.
5.Procedure was repeated for different airflow rates.
6.Mass flux (Nₐ) and mass transfer coefficient (kg) were calculated
from observations.
Flow Initial Weight Weight Weight
Rate Weight At At At
(LPM) (gm) t=10mins(gm) t=20mins(gm) t=30mins(gm)
Observation Table:
Area of diffusion = Πd2 /4
Where diameter D = 7.8 cm
Area = 4.77*10-3 m2
Time = 10 min = 1/6 hr
Sample calculation:
For 40 LPM
Average weight lost = 0.103g
G = avg. weight lost / (area *time) = 0.129 kg/m2 hr
log Ps = 6.845 – 1606.529 / 187.245 + T
=0.2818 mmHg
Ps = 3.7084 * 10-4 atm
Kg = G / Ps * M = 0.129/3.7084 * 10-4 *128 = 2.73 kmol/atm m2 hr
Re = ρvd / µ = 696.85
JD = Kg. P. (Sc)2/3 /G. Ms= 3982.811 kg/ m2 hr
Result:
The experiment successfully demonstrated the principles of mass
transfer. The weight loss of naphthalene decreased gradually over
time, which is consistent with the sublimation of a solid into a
flowing air stream. The results are tabulated in the excel snippet
appended.
Furthermore, the Reynolds number (Re) and the mass transfer factor
(JD) both increased with higher air velocities. This confirms that a
higher flow rate leads to stronger convective effects, which in turn
enhance the rate of diffusion.
Conclusion:
1.The experiment successfully demonstrated that the rate of mass
transfer is directly proportional to the air velocity.
2.Higher air flow rates lead to a significant increase in the mass
transfer coefficient, which enhances the sublimation process.
3.This validates the principle that convection plays a crucial role in
boosting diffusion and mass transfer.
Precautions:
1.Naphthalene was handled safely, and inhalation/skin contact were
avoided.
2.An airtight setup and steady airflow were ensured.
3.The packed bed was not disturbed during the run.
