Unformatted text preview: Sg = 832 m2/g. At higher temperatures, the
experimental values for adsorption are even less, with just a slight increase in α. Therefore, the
measured values of adsorption are equivalent to far less than a monolayer.
(c) The heat of adsorption is related to adsorption isotherm data by a rearrangement of Eq. (1517), which is a form of the ClausiusClapeyron equation. For a constant amount adsorbed,
d ln p
− ∆Hads = RT 2
(1)
where, R = 1.987 cal/molK
dT
Take an arbitrary value of 0.00025 mols of benzene adsorbed per gram of silica gel. For each
temperature, calculate the partial pressure of benzene from the above fits of the Freundlich
equation (the Langmuir equation could also be used). Evaluate d ln p/dT numerically and apply
Eq. (1). The results are as follows.
Temperature,
Temperature, Benzene pressure,
ln p d ln p/dT Tavg, K  ∆Hads,
o
C
T, K
p, atm
cal/mol
70
343
0.00108 6.831
0.05615
353
13,900
90
363
0.00332 5.708
0.05180
373
14,300
110
383
0.00935 4.672
0.0347
393
10,650
130
403
0.01873 3.978
The average heat of adsorption =  13,000 cal/mol.
From the ChemCad simulation program, the heat of condensation for benzene varies from 7,500
cal/mol at 70oC to 6,620 cal/mol at 130oC. Thus, the heat of adsorption is almost twice as great
as the heat of condensation. Exercise 15.7
Subject: Use of adsorption equilibria data to select the best adsorbent. Given: Langmuir adsorption equilibrium constants, K and qm in Eq. (1524), for 3 zeolite
molecular sieves and activated carbon for propylene (C3) and propane (C3=) at 25oC:
Adsorbent
Sorbate
qm
K
ZMS 4A
C3 0.226
9.770
C3= 2.092
95.096
ZMS 5A
C3 1.919 100.223
C3= 2.436 147.260
ZMS 13X
C3 2.130
55.412
C3= 2.680 100.000
Activated carbon
C3 4.239
58.458
C3= 4.889
34.915
Find: (a)
(b)
(c)
(d) Most strongly adsorbed component for each adsorbent.
Adsorbent with greatest adsorption capacity.
Adsorbent with greatest selectivity.
Best adsorbent for the separation of propylene from propane. Analysis:
(a) The Langmuir equation is: q = Kqmp/(1 + Kp)
At high pressure, q approaches qm. Thus, qm is a measure of capacity. Because of the high
values of K, the value of qm is reached at about 1 bar. Thus, we can make the comparisons on the
basis of qm. Therefore, propylene is the most strongly adsorbed for all 4 adsorbents.
(b) The activated carbon has the highest values of qm and, therefore, has the highest capacity.
(c) The selectivity is measured by the ratio of qm for propylene to that of propane. With a value
of 2.092/0.226 = 9.26, zeolite ZMS 4A is by far the best.
(d) Because of its very high selectivity of ZMS 4A, where the second best is only 1.27, and the
high capacity of ZMS 4A, although only about 40% of that of activated carbon, ZMS 4A is the
best adsorbent for the separation of propylene and propane. Exercise 15.8
Subject: Fitting zeolite adsorption data to linear, Freundlich, and Langmuir isotherms. Given: NaX zeolite adsorbent. Adsorption equilibrium data at 547 K for 1,2,3,5tetramethylbenzene (TMB) as follows:
9.1
10.3
10.8
11.1...
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 Spring '11
 Levicky
 The Land

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