Encyclopedia of Environmental Science and Engineering, Volume I and II

1226 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS

TABLE 5

Henry’s Law constant for slightly soluble pollutants, H  10 ^4 atm/mole-fraction

T,C CO CO 2 NO H 2 S SO 2

0 3.52 0.0728 1.69 0.0268 0.0016

20 5.36 0.142 2.64 0.0483 0.0033

40 6.96 0.233 3.52 0.0745 0.0062

60 8.21 0.341 4.18 0.103 —

80 8.45 — 4.48 0.135 —

100 8.46 — 4.54 0.148 —

primary differences is that heterogeneous reactions require the

adsorption of a fluid phase component on the catalyst surface.

In addition to adsorption, diffusion rates may be significant

for both main stream to surface gas transfer and for transport

inside catalyst pores. The analysis is often simplified by lump-

ing the parameters of the system and assuming plug flow.

Thus for a packed bed of catalyst as in Figure 16 the

catalyst mass is lumped as a single substance and the gas

phase as another, the latter in plug flow.

The mass of catalyst required to attain a given conver-

sion may be calculated if the dependence of u and Pi on ci

is known.

M V S

d uc

B P

i

c i

c

io

i

 &

( )

∫

where:

u superficial velocity based on total cross-

sectional area S

KB  bulk density, mass of solids/unit volume

Pi(Ci–) production rate, moles of Al formed/(mass

solids) (time).

The dependence of the production rate on concentration is

usually determined assuming either a Langmuir-Hinshelwood

or a Langmuir Rideal mechanism, if diffusion rate constants

are large.

If we let Xi represent an adsorbed component Ai on an

active site X 0 , the Langmuir-Rideal sequence, which assumes

that a fluid phase reactant combines with a surface adsorbed

molecule, the sequence may be expressed as

A X X

A X X A

k

k

k

k

1 0 1

2 1 3 4

1

2

1

2



  

&

&

−

−

Langmuir adsorption

Precursor ssurface reaction

X A X Langmuir desorption.

k

k

3 3 0

3

3

&

−



Usually, to simplify the analysis all the equations save one

are assumed to be at equilibrium and the unsteady equation is

said to control the rate. Thus for the Langmuir-Hinshelwood

equation, the rate of product formation is if

Surface Reaction Controls

P

k K K C C k K K C C C

k C

s s

j j

j











( )

1 2 1 2 3 4 3 4 0.

2

1

4 2

(^1) ∑




The denominators indicate that each component competes
for adsorption sites on the catalyst surface. The rate constants
and equilibrium constants are denoted by k and K, respec-
tively, with subscripts denoting surface reaction and j refer-
ring to adsorption of component Aj. The concentration of
each component can be put in terms of C 3 by stoichiometry
considerations.
CATALYST PROPERTIES
A typical conversion described by Langmuir adsorption
followed by chemical reaction is that for SO 2 removal over
vanadium oxide catalyst (Mars and Maessen, 1961)
SO 2  ∂V O^5  ^2 � SO 3 ∂V^4 
95% Yield at 450C
r kp
Kp P
O Kp P
SO SO
SO SO

(^2) 
2 3
(^123)
1 2 2
/
[ ( / ) ]/
k  rate constant,
K  equilib. constant for above reaction,
pi  partial pressure of ith species.
Similarly for the oxidation of ethylene
r
kp p
K C K C
c H O
C H CO

 
2 2 3
2 4 2
2
1 2
1[ 2 2].
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