Page 113 - Modelling in Transport Phenomena A Conceptual Approach

4.4. FLOW NORMAL TO A SINGLE CYLINDER 93

For liquids the correlation obtained by Linton and Sherwood (1950) may be used:
I Sh = 0.281 Reg6 SC''~ 1 (4.411)

Equation (4.411) is valid for

400 5 Reo 5 25,000
sc 5 3,000

4.4.3.1 Calculation of the mass transfer rate
Once the average mass transfer coefficient is estimated by using correlations, the
rate of mass of species A transferred is calculated as

where MA is the molecular weight of species A.
Example 4.11 A cylindrical pipe of 5 cm outside diameter is covered with a thin
layer of ethanol. Air at 30 "C flows normal to the pipe with a velocity of 3 m/ s.
Determine the average mass transfer coeficient.
Solution

Physical properties
Diflusion coeficient of ethanol (A) in air (23) at 30 "C (303 K) :

( m2/ s
= (1.45 x - = 1.38 x

For air at 30 "C (303 K) : v = 16 x m2/ s
The Schmidt number is
v
sc = -
DAB
- 16 x = 1.16
-
1.38 x 10-5
Assumptions

1. Steady-state conditions prevail.
2. Isothermal system.