This revamped reactor performance is compared
with two other design options. One is a shallow flu-
idized bed with a booster fixed-bed reactor, which is
shown as case 3 in Fig. 9. The other is a single deep
fluidized-bed depicted as case 4 in Fig. 9. Comparing
both designs illustrates that although a comparable
yield may not be achieved by these designs, the MTY
achievable is significantly higher in a fluidized bed
(case 4) and even more in a fluidized fixed-bed combi-
nation (case 3). The higher MTY's are due to opera-
tion with both higher propylene feed concentration
(8.32%) and higher temperature level possible in these
designs. Of course, we assumed that both fluidized-
and fixed-bed catalysts will have the
same activity and selectivity.
Example 6--Bubbling fluidized-
bed reactor to produce maleic
anhydride.
Compared to a fixed-bed
reactor, a f luidized-bed reactor is
more flexible not only in control and
operation, but also in terms of avail-
able options for revamps and mod-
ernization. This example illustrates
a few such possibilities using a com-
mercial reaction system to produce
maleic anhydride (MA) by catalytic
air oxidation of n-butane. Proprietary
design and operating conditions and
absolute values of performance pro-
jections for this reaction system are
not presented due to proprietary rea-
sons. However, a general model
application to evaluate several
revamp options can be demonstrated
by comparing the results on a rela-
tive basis. The reaction mechanism
used for this demonstration is shown in Fig. 10. The
rate expressions used are those provided in an earlier
publication.
2
Fig. 11 shows three revamp options of an original
fluidized-bed reactor. In option 1, part of the feed air
is injected through one or more injection ports along
the bed. In this design, pure oxygen as a replacement
for air also is explored. In option 2, the feed concen-
tration is raised and unreacted n-butane is recycled
after separation and recovery. In option 3, a higher
feed concentration and recycle is used as in option 2
together with reduced reactor temperatures. In these
figures, T
BC
represents the original or base-case reac-
tor temperature.
Various alternatives under each design option are
again possible. Under option 1, various combinations
of: number of injection points (for air/ oxygen), height
of each injection point and the amount of air/ oxygen
injection at each point can be considered. Various com-
binations between the three revamp options 1, 2
and 3 are also possible. Model projections of X-S-Y-
MTY values for several possibilities are compared in
Table 3. The table shows the percentage changes over
the original design.
Revamp option 1--Multistage air/ O
2
injection.
Four possibilities--air injection in 2 and 4 stages and
oxygen injection in 2 and 4 stages--are examined. In
all cases, the total quantity of oxygen injected is kept
HYDROCARBON PROCESSING / SEPTEMBER 1999
Table 2. Reactor design conditions to produce
acrylonitrile.
Capacity
100 tpd of acrylonitrile
Feed composition
C
3
H
6
4.16%, NH
3
9.56%,
Moisture 1.54% Air balance
Feed pressure
1.72 psia
Feed gas velocity
50 cm/s (superficial velocity at
inlet conditions)
Catalyst particle size
1/8 in.
(Other conditions are shown in Fig. 7)
Table 1. Examples of poor design of process
reactors.
Gas maldistribution and hot spots
Tendency of temperature runaway
Poor hydrodynamics
Control and operational problems, production loss and
safety concerns due to reactor instability
Control and operational difficulties and production loss
due to circulation instability or choking in solids flow lines
Low productivity or poor product quality due to
Unacceptable temperature profiles
Inadequate heat transfer
Wrong locations of feed, discharge and recycle
lines on reactor
Improper design of the feed and discharge ports
Improper or inadequate design of feed distributors
Inadequate mixing
Excessive back-mixing
Wrong design of baffles and internals
Gas maldistribution and/or channeling
Improper bed voidage profiles or low bed density
Inadequate solids circulation rates
Inability to boost capacity and yield due to narrow design
margins of key variables, such as
Temperature
Pressure
Feedrate and feed composition
Heat transfer capacity
Circulation rates
Deterioration of any or all conditions faster than expected
from the normal aging process
Table 3. Revamp options of fluidized bed maleic anhydride reactor--
comparison of projected performance
Revamp option 1--Multistage air/O
2
injection
Percent change in
Conversion
Selectivity
Yield
MTY
4-stage air
3.22
-
0.17
3.02
-
9.23
2-stage air
3.85
-
0.43
3.43
-
6.92
4-stage O
2
11.25
-
2.44
8.57
-
14.61
2-stage O
2
13.77
-
3.40
9.94
-
13.85
Revamp option 2--Higher feed concentration with n-butane recycle
Percent change in
Conversion
Selectivity
Yield
MTY
50% higher n-butane concentration
-
9.99
2.96
-
7.33
39.23
100% higher n-butane concentration
-
25.65
6.36
-
20.90
58.46
Revamp option 3--Higher feed concentration with n-butane
recycle and lower bed temperatures
Percent change in
Conversion
Selectivity
Yield
MTY
100% higher n-butane concentration
-
26.12
12.02
-
17.27
64.61
with T
=
T
BC
-
10°C
100% higher n-butane concentration
-
28.64
17.94
-
15.83
65.38
with T
=
T
BC
-
20°C
