Multicomponent Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 99

10

0.01

9

1

8

100

7

6

5

4

Ty p e I I

3

2

Ty p e I I I

1

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

y i

FIGURE 3.18 Different shapes of the BET isotherm. Note that the shapes vary with the
isotherm constant, K B .

equation can be recast into the following form:

1
Γ i ·

ψ i 1

K BET Γ

(K BET − 1 )

− ψ i =

i +

· ψ i , (3.83)

m

1 K BET

m

so that both K BET and

i can be obtained from the slope and intercept of a linear
regression of the LHS versus y i . The linear region of the plot typically lies between
ψ i values of 0.05 and 0.3 and extrapolation below or above this limit should be
approached with caution. If the specific area of the adsorbate,

Γ

σ m , is known, then the
total surface area of the adsorbent can be obtained from the equation

m
i

σ m N Γ = S a . (3.84)

Pure nitrogen, argon, or butane with specific surface areas of 16.2, 13.8, and 18.1 ×

10 − 16 cm 2 /molecule are used for this purpose.

E XAMPLE 3.19 U SE OF THE B ET E QUATION

Poe et al. (1988) reported that the vapor adsorption of an organic compound (ethyl ether)
on a typical dry soil (Weller soil fromArkansas) showed distinct multi-layer formation.
The following data were obtained:

continued