Page 340 - Elements of Chemical Reaction Engineering Ebook

Sec. 6.:3 Algorithm for Solution to Complex Reactions 31 1

I The overall selectivity of xylene relative to toluene is
s,,-F" t cx= CX - 0.003 13
-
FT CT cMo-CM-Cx 0.0105-0.0029-0.00313
- 0.7 mol xylene produced

sxT = mole toluene produced

Ih the two preceding examples there was no volume change with reaction;
consequently, we could use concentration as our dependent variable. We now
consider a gas-phase reaction with volume change taking place in a PFR. [Jnder
these conditions, we must use the molar flow rates as our dependent variables.

Example 6-8 Calculating Concentrations as Functions
of Position for NH, Oxidation in a PFR

The: following gas-phase reactions take place simultaneously on a metal oxide-sup-
ponted catalyst:

1. 4NH,+5O2 + 4NO+6H20
2. 2NH3+ 1.50, ___) N2+3H@
3. 2NO+02 ___$ 2N02
4. 4NH,+6NO ___i) 5N2+6H20

Wnting these equations in terms of symbols yields
Reaction 1: 4A + 5B __3 4c + 6D -r,, = k,ACACi (B6-8.1)
2C -t B -
Reaction 2: 2A 1.5B __3 E + 3D -r2~ = k2ACACB (B6-8.2)

Reaction 3: 2F -rSB = k3BCicB (E16-8.3)
Reactioii 4: 4A + 6C + 5E + 6D -r4c = k4cCcCF3 (B6-8.4)

with7 k,, = 5.0 (m3/km0l)~/rnin k2,= 2.0 m3/kmol-min

k3B = 10.0 (m3/kmol)2/min k4c = 5.0 (m3/km01)2'3/min
Note: We have converted the specific reaction rates to aper unit volume basis by
multiplying the k' on a per mass of catalyst basis by the bulk density of the
packed bed.

Determine the concentrations as a function of position (i.e., volume) in a PFR.
AaLlitionaE infunnation: Feed rate = 10 dm3/min; volume of reactor = 10 dm3; and
cAO = cBO = 1.0 m0i/dm3
- C, = 2 mol/dm3
Reaction orders and rate constants were estimated from periscosity measurements for
a bulk catalyst density of 1.2 kg/m3.