Example 5. Tubular fixed-bed reactor to produce
acrylonitrile.
Design conditions used for this reactor
are listed in Table 2. Reaction mechanism for the
design is illustrated in Fig. 6. Based on the reaction
kinetics used, it is estimated that 48,000 tubes will be
required for the desired capacity. The reaction mech-
anism and kinetics are different from a previously pre-
sented example.
1
Figs. 7 and 8 show the temperature
profile and the X-S-Y-MTY profiles of this reactor as a
function of catalyst bed height (or tube length), along
with the other profiles of the revamped reactor designs.
Due to heat transfer limitation typical of this TCR
reactor, the projected conversion (X) is 73.6%, which
is far short of near-complete conversion expected for
commercial production. The projected selectivity and
yield are 80.3% and 59.1% respectively. The MTY is
0.231.
Revamp options for TCR reactors are limited when
compared to fluidized-bed reactors. Fig.7 shows one
revamp option (case 2) in which the tubes are packed
with the catalyst with varying concentration to mod-
ify the temperature profile. One way to accomplish this
is to physically mix the original catalyst with an inert
material of the same size, shape and density. Within
this arrangement, the various combinations of catalyst
concentration and bed segmentation are possible. The
optimum design can be determined by modeling and
then followed by validation with a test rig. The modi-
fied temperature profile of the revamped reactor along
with its X-S-Y-MTY performance curves, as predicted
by the model, are compared with the original reactor
design. Using this retrofit design, reactor conversion
can be raised to as high as 98.0% with a selectivity of
81.6% and yield of 80.0%. Due to catalyst dilution, the
MTY (based on active catalyst content only, and exclud-
ing inserts) also increases to 0.493. Thus, the catalyst
consumption is also reduced by more than one half.
Remember, the benefits from such a revamp depend
primarily on the reaction system and its kinetics.
HYDROCARBON PROCESSING / SEPTEMBER 1999
,,
,,
,
y
,,
,,
,,
,,
,,
yy
yy
,,
,,
,,
Fig. 9. Retrofit designs for fixed-bed acrylonitrile reactor.
Fig. 10. Reaction mechanism for n-butane to maleic anhydride
reaction systems.
,,,
,,,
,,,
,,,
,
y
,,
,,
,,
,,
,,
,,
yy
yy
,,
,,
,,
,,
,,
yy
,,
,,
,,
,,
yy
Fig. 11. Retrofit options for a fluidized-bed maleic anhydride reactor.
