tion efforts. Different designs for the booster reactor
can be evaluated quickly and with no risk or inter-
ruption to current production.
A schematic representation of reactor revamp and
modernization is shown in Fig. 1. It represents an
improvement for the current reactor to a fully mod-
ernized one after progressing through successive steps
of optimization and revamps. A general reactor model
is a powerful tool; it can guide the user through these
improvements at every step.
Prerequisites.
There are three prerequisites to using
this tool type:
Reactor performance data. Data covering a rea-
sonable range of the four major operating variables--
temperature, pressure, feed composition and feedrate--
are key prerequisites for a general model. The plant data
archive often is a valuable source. However, a plant nor-
mally runs at almost fixed conditions; hence, a large
variation in the operating conditions may not be found in
the archive data. Original lab data or pilot-scale infor-
mation on the reaction system that usually cover a wide
variety of operating conditions are more useful for this
purpose. Good data sometimes can also be obtained from
operating the plant by controlled step changes, and then
monitoring the effects from such changes.
Background information on reaction mechanism
and kinetics. For reactions of commercial interest, ade-
quate information is usually available in open litera-
ture. Background literature in the plant archive can also
provide valuable data on this issue. After a careful anal-
ysis of these literatures, the user can usually formulate
a satisfactory reaction mechanism for the system. For
complex nonstoichiometric refinery reactions, the reac-
tion mechanism is represented through lumping of com-
pounds. The compounds in turn are represented either
by pseudo- (C
x
H
y
O
z
for example) or model compounds.
In contrast, information on the kinetics of refinery
reactions is fragmentary. However, a careful analysis of
this fragmentary information should identify many
important rate parameters, particularly reaction acti-
vation energies. A general model can be used to deter-
mine the pre-exponential factors of reactions by using
the available data and known activation energies, and
improve these factors with additional data that are
available from the test facility. Appropriate parameter
estimation procedure is used to determine the rate
parameters.
In the absence of satisfactory information, the rate
equation for each reaction can be expressed as first
order with respect to each reacting component for most
reaction systems, at least for a preliminary model.
Small test facility for model validation. To achieve
total confidence in the model projections, a small test
facility to carry out a limited number of experiments
to validate the model is needed. Data from this facil-
ity would also provide missing information on reaction
mechanism and kinetics. This facility can be located in
an in-house laboratory or at an external laboratory con-
tracted to do the job under confidential agreement.
Such data sometimes can also be obtained from the
operating plant by controlled step changes in the oper-
ation and monitoring the effects of such changes. This
arrangement can be a substitute for the test facility.
Five steps to use this tool.
A typical general model
has five components, which vary between reaction and
reactor systems. They are:
·
Reaction system--stoichiometry and rate para-
meters, etc.
·
Catalyst and reactor specifications--catalyst size,
shape, porosity, bulk and particle densities, and reactor
diameter, height and internals, etc.
·
Feed conditions--pressure, temperature, flowrate
and composition, etc.
·
Thermophysical properties of reactants and prod-
ucts, etc.
·
Heat transfer system specifications--heat transfer
coil/jacket design, and coolant flow, properties and inlet
temperature, etc.
The five steps to run a typical general reactor model
for a single reactor type are:
Choose the reactor type that best represents the
existing reactor or the alternate design(s) of interest
from the menu of the model package
Input all five components of data representing the
reaction and reactor system
Input the rate expression for each reaction
Execute the model
Change one or more design and/or operating vari-
ables of the second step and execute the model. Repeat
the process until the best design and operating condi-
tion(s) are found. Alternatively, interface this package
with an optimization routine to find the optimum auto-
matically by repeated execution of the model.
HYDROCARBON PROCESSING / JULY 1999
Table 1. Preliminary reaction mechanism and rate
expressions for acrylonitrile production
Assumed reaction mechanism for ammoxidation of propylene to
acrylonitrile
C
3
H
6
+ NH
3
+ 1
1
/
2
O
2
à
C
3
H
3
N
+ 3 H
2
O
Rxn1
C
3
H
6
+ 1
1
/
2
NH
3
+ 1
1
/
2
O
2
à
1
1
/
2
CH
3
CN + 3 H
2
O
Rxn2
C
3
H
6
+ 3 NH
3
+
3 O
2
à
3 HCN
+ 6 H
2
O
Rxn3
C
3
H
6
+ 3 O
2
à
3 CO
+ 3 H
2
O
Rxn4
C
3
H
6
+ 4
1
/
2
O
2
à
3 CO
2
+ 3 H
2
O
Rxn5
C
3
H
6
+ 1
1
/
2
O
2
à
C
3
H
4
O
2
+
H
2
O
Rxn6
C
3
H
6
= Propylene C
3
H
3
N = Acrylonitrile CH
3
CN = Acetonitrile C
3
H
4
O
2
= Acrylic acid
The rate expressions used
r1* = k
1
*
×
C
PR
0.48
C
O
2
0.48
C
NH
3
0.19
r2* = k
2
*
×
C
PR
0.43
C
O
2
0.52
C
NH
3
0.47
r3* = k
3
*
×
C
PR
0.43
C
O
2
0.52
C
NH
3
0.47
r4* = k
4
*
×
C
PR
C
O
2
0.50
r5* = k
5
*
×
C
PR
C
O
2
0.50
r6* = k
6
*
×
C
PR
C
O
2
0.50
where:
r1* r6*
Rates of reaction 1 to 6
k
1
* k
6
*
Rate constant of reaction 1 to 6 (k
1
* = k
01
*
e
E1*/RT
, k
2
* = k
02
* e
E2*/RT
and so on)
k
01
* k
06
*
Pre-exponential factor of reaction 1 to 6
E1* E6*
Activation energy of reaction 1 to 6
R
Gas constant
T
Temperature
C
PR
, C
0
2
, C
NH
3
Molar concentration of propylene, oxygen
and ammonia
