Page 326 - Modelling in Transport Phenomena A Conceptual Approach

306 CHAPTER 8. STEADY MICROSCOPIC BALANCES WITHOUT GEN.

Therefore, the diffusion coeficient is determined from the slope of the L2 versus t
plot. Alternatively, rearrangement of Eq. (6) yields

In this me, the digusion weficient is determined from the slope of the t/(L -Lo)
versus (L - Lo) plot. What is the advantage of using Eq. (7) over Eq. (6)P

Example 8.15 To decrease the evaporation loss from open storage tanks, it is
recommended to use a tapered top as shown in Figure 8.34. Calculate the rate of
ethanol loss from the storage tank under steady conditions at 25 "C.

I- 1.5md

Ethanol (A)

+

+-m
Figure 8.34 Evaporation from a tapered tank.
Solution

Physical properties
Digusion coeficient of ethanol (A) in air (B) at 25 "C (298 K) :

PTt = 58.6mmHg
Analysis

In order to determine the molar flow rate of species d jhm Eq. (8.5-1 7), it is first
necessary to express the variation of the cross-sectional area in the direction of I.
The variation of the diameter as a function of z is