İzmir Institute of Technology
CHEMICAL ENGINEERING DEPARTMENT
2008-2009 Spring Semester
CHE 310
CHEMICAL ENGINEERING LABORATORY I
Determination of
Mass Transfer Coefficient
DETERMINATION OF MASS TRANSFER COEFFICIENT
Objective
1. Calculate the mass transfer coefficient
2. Compare the effect of temperature on the diffusion rate and mass transfer coefficient
Theory and Principles
Mass transfer occurs when a component in a mixture migrates in the same phase or
from phase to phase because of a difference in concentration between two points e.g. liquid in
an open pail of water evaporates into still air because of the difference in concentration of
water vapor at the water surface and the surrounding air. This is a driving force from surface
to air.
There are two different modes of mass transfer, namely diffusion and convection.
Diffusion is the movement under the influence of an individual component through a mixture
and the physical mechanism is that of random atomic or molecular activity. The convection
mass transfer mode is comprised of two mechanisms as free and forced convection.
Free convection mass transfer involves thermally driven free convection flows which
enhance free evaporation or sublimation occurring at a surface. In many cases, the rate of
diffusion is slow, and more rapid mass transfer is desired. In the case of forced convection
mass transfer, the flow is induced by external means such as a fan, pump or atmospheric
winds etc.
Flow of a binary mixture of chemical species A and B over the flat plate as shown in
Figure 1 below is considered. If the concentration of the species A at the surface, C A,s, differs
from that in the free stream, CA,∞ , then a concentration boundary layer will develop and
species transfer by convection between the surface and the free stream will take place.
Referring to Figure 1, the molar flux of species A, NA" (kmole/s.m2) is defined as;
NA"=hm (CA,s- CA,∞)
Where hm(m/s) is the convective mass transfer coefficient. CA,s and
concentrations(kmol/m3).
CA,∞ the molar
Equipment
T∞, CA, ∞
NA”
As, CA,s
thermocouple
Balance
Figure 1. Schematic representation of experimental set-up
Experimental procedure
1. Place the pan filled with water on the balance and set the temperature at 60oC.
2. Start timer and record weight for every 30 seconds.
3. Continue experiment until all the water evaporates.
4. Record ambient temperature and humidity.
5. Repeat the temperature at 80oC.
Questions for consideration
1. Calculate mass transfer coefficients for both temperatures. Comment on the results.
2. Which properties affect mass transfer coefficient?
3. A streamline strut supporting a bearing housing is exposed to a hot airflow from an
engine exhaust. It is necessary to run experiments to determine the average convective
heat transfer coefficient h, from the air to the strut in order to be able to cool the strut
to the desired surface temperature Ts. It is decided to run mass transfer experiments on
an object of the same shape and to obtain the desired heat transfer results by using heat
and mass transfer analogy. The mass transfer experiments were conducted using a half
size model strut constructed from naphthalene exposed to an air stream at 27oC. Mass
transfer measurements yielded these results:
LH=60mm
V
T∞
Ts
p∞
strut
ShL
ReL
282
60,000
491
120,000
568
144,000
989
288,000
a) Using the mass transfer experimental results, determine the coefficients C and m for a
correlation of the form ShL= CReLmSc1/3
b) Determine the average convective heat transfer coefficient h for the full sized strut, LH=60
mm, when exposed to a free stream airflow with V=60m/s, T∞= 184oC, and p∞=1 atm when
Ts=70oC.
c) The surface of the strut can be expressed as As=2.2LH l where l is the length normal to the
page. For the conditions of part b, what is the change in the rate of heat transfer to the strut if
the characteristic length LH is doubled?
4. Would the mass transfer coefficient increase or decrease if air temperature was increased
from 20 to 30oC.
5. An experiment is conducted to determine the average convective mass transfer coefficient
of a small droplet using a heater controlled to operate at a constant temperature. The power
history requires to completely evaporate the droplet at a temperature of 37oC is shown in the
sketch. It was observed that, as the droplet dried, its wetted area on the heater surface
remained nearly constant at a value of 4 mm2.
Dry air
P(mw)
T∞, h
D
Droplet
20
Ts
t(min)
Heater
controlled to
operate at
constant temp.
Ts
50
100
a) Calculate the average mass transfer convection coefficient based upon the wetted area
during the evaporation process when the droplet, heater, and the dry ambient air are 37 oC.
b) How much energy will be required to evaporate the droplet if the dry ambient air
temperature is 27oC, while the droplet heater temperature remains at 37 oC?
6. Compare the mass transfer coefficients of water measured for two different cases.
Case A: Relative humidity of air is zero
Case B: Relative humidity of air is 30%
References
1. Incropera F.P, De Witt D. P, “Fundamentals of Heat and Mass Transfer”, 4th Edition, John
Wiley & Sons Inc., 1996.
2. Perry J.H, “Chemical Engineering Handbook”, 4th Edition, Kogakusha Company, Tokyo,
1963.
Acknowledgment: Author thanks Dr. Alsoy Altunkaya for her helps in preparing this
manual.