To enhance light olefin production, low hydrocarbon partial pressures (10–30
psia), elevated temperatures (1,000–1,150
F), and 10–25 wt% of shape-selective
(pentasil zeolites) additives are employed. Maximum olefin yields require high
conversion since it is the gasoline produced in the cracking process that ultimately
is made into propylene and butylene.
Cited yields for the PetroFCC are shown in Table 22.
While the feedstock properties and operating conditions are not known, the
feedstock had to be very paraffinic to yield such a high gasoline yield in the regular
FCC processing mode.
Another process designed for high olefin yields is the Maxofin FCC. This unit is
shown in Fig. 31 and has a second riser for propylene maximization. The process
uses the Orthoflow FCC Hardware, e.g., Atomax nozzles and close-connected
Table 22 Pilot yields
(wt%) – PetroFCC versus
FCC
PetroFCC
FCC
Propylene
22
4.7
Butylene
14
$6
Ethylene
6
$1
Gasoline
28
53.5
2nd Riser
for Naphtha
Recycle
Recycle
Naphtha
Injection
Primary
Feed
Riser
Closed
Cyclone
System
ATOMAX-2
Fresh Feed
Injection
Fig. 31 Maxofin process
(Kellogg)
304
W. Letzsch