Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing
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272 CHAPTER 6
By changing the catalyst formulation, operating parameters, and equipment design,
the product distributions shown in Table 6.3 are obtained. In the Deep Catalytic
Cracking Process, the yields of light olefins (C
2
–C
4
) and BTX aromatics are much
higher than in standard gas oil FCC units and can result in significantly improved
processing economics. As shown in equation (2), this mode encourages the deliberate
overcracking of gasoline.
Many international petrochemical producers are challenged with the need to obtain
secure, low cost naphtha supplies. Integrating petrochemical operations with refineries
can ameliorate this situation. To become and remain competitive it is essential to have
access to low cost feedstocks and to build vertically integrated, strategically located
world scale refinery and petrochemical complexes. Such integration adds value and
provides raw materials, both keys to gaining a competitive edge in global markets.
In the case of increased propylene demand, heavier, less expensive feedstocks, i.e.
gas oils can be used in a continuous catalytic process run at more favorable operating
conditions.
The product qualities of the naphtha from the Deep Catalytic Cracking Process are
compared to a conventional FCC unit and a steam cracker in Table 6.13. The higher
aromatics concentration in the naphtha is mostly a result of the concentration of
existing aromatics due to the cracking of the paraffins and cycloparaffins rather than
through dehydrogeneration of naphthenes or cyclization of paraffins. Xylenes are
the major aromatic compounds produced catalytically while benzene is the primary
thermal product. Diolefins are low in both of the catalytic processes since these result
from the largely absent thermal reactions.
Table 6.13. Comparison of DCC, FCC, and
SC naphthas
DCC FCC SC
Components
Paraffins 14.3 28.6 3.5
Olefins 32.4 35.3 13.3
Naphthenes 5.0 9.8 4.1
Aromatics 48.3 26.3 79.1
Aromatics breakdown
Benzene 1.9 0.6 37.1
Toluene 9.4 2.4 18.9
C
8
15.6 6.7 13.5
C
9
12.1 12.5 5.4
C
10
+ 9.3 4.1 4.2
Total 48.3 26.3 79.1
FLUID CATALYTIC CRACKING 273
The normal operating conditions of the cracking process are contrasted below:
Typical operating conditions
FCC DCC SC
Reactor Temp. (
◦
F) 950–1,020 980–1,100 1,400–1,600
Residence Time (Sec) 1–10 1–10 0.1–0.2
Pressure, ATM 1–2 1–2 1
Catalyst/oil 5–10 8–15 –
Steam (% fd) 0–5 10–30 30–80
A flow diagram of the DCC process is shown in Figure 6.28. The catalyst flow and oil
vapors and products follow the same path as the gas oil FCC unit described earlier.
Considerably more steam is added and the reactor can be run with a bed level to
increase the hydrocarbon residence time and facilitate the overcracking reactions.
Other options include recycling naphtha to the same feed riser or the installation of a
separate smaller riser to run at higher temperature.
Recracking of naphtha can optionally include the C
4
olefins. Further, the recycled
naphtha can be full range or cut to maximize the crackable molecules. The result of
the recycle is to increase propylene and the concentration of BTX in the naphtha.
The product slates before and after naphtha recycle are shown in Table 6.14 from the
Jinan DCC unit.
FRESH FEED
RISER STEAM
TO FLUE GAS
SYSTEM
TO GAS
RECOVERY
SLURRY RECYCLE
STRIPPING STEAM
BLOWER
REGENERATOR
REACTOR
Figure 6.28. DCC process.
274 CHAPTER 6
Table 6.14. DCC operation with
naphtha recycle
Naphtha recycle No Yes
Products (wt%)
Dry gas 6.8 7.5
LPG 39.2 43.9
Naphtha 27.8 20.9
LCO 16.8 18.2
Coke 8.9 9.0
Loss 0.5 0.5
Olefin yield
Propylene 17.4 21.0
Butylene 12.4 13.2
A pilot plant test showed that when DCC naphtha with a 150
◦
C end point was recycled
the aromatics concentration increased dramatically. The total aromatics went from
42 to 81 wt% of the naphtha with the BTX content being 4.4, 28.9, and 40.4 wt%,
respectively.
Recent advances in the technology have been directed to expanding the feedstock
types that can be processed. Successfully run feeds now include:
r
VGO
r
Hydrotreated VGO
r
Deasphalted Oil
r
Dewaxing wax
r
Coker gas oil
r
Atmospheric resid
Catalyst formulations have had to be altered to match the changing feedstock slate.
The DCC catalysts are designed with the following characteristics: high matrix ac-
tivity for primary cracking of the heavy hydrocarbons and higher metals tolerance; a
large quantity of a modified mesopore zeolite with a pentasil structure for cracking
the primary product (gasoline), isomerization activity for the light olefins and mini-
mization of hydrogen transfer. Ten different catalysts have been used commercially
in the seven operating DCC installations.
Representative yields from various commercial operations are tabulated in Table 6.15.
Paraffinic feedstocks give high propylene yields and the isobutylene concentration
in the C
4
stream is near the thermodynamic limit. Atmospheric resid makes an ex-
cellent feed as long as it is paraffinic. The results from commercial runs on Daqing
atmospheric resid are given in Table 6.16 along with the feedstock analysis. The com-
bination of an ARDS unit in front of a DCC unit would allow a refinery to process
FLUID CATALYTIC CRACKING 275
Table 6.15. DCC light olefin yields
Refinery Daqing Anqing TPI Jinan Jinan
Operating mode DCC + I DCC-I DCC-I DCC-I DCC-II
Feedstock Paraffinic Naphthenic Arabian Intermediate Base
VGO + VGO VGO + VGO +
AR DAO + DAO
Wax
Reaction temp. 545 550 565 564 530
Olefins
Ethylene 3.7 3.5 5.3 5.3 1.8
Propylene 23.0 18.6 18.5 19.2 14.4
Butylene 17.3 13.8 13.3 13.2 11.4
a wide variety of crudes and still produce high quantities of petrochemical base
stocks.
Other technologies designed to increase the propylene yield from heavy feeds have
been announced. Two such processes are the PetroFCC and Maxofin. Neither of the
processes has been commercialized to date but have been discussed in various papers.
The PetroFCC shown in Figure 6.29 has a recycle stream of spent catalyst from the
stripper to the base of the feed riser. The principle behind this design, as Table 6.17
shows, is that spent catalyst is less active but somewhat more selective than the clean
regenerated catalyst at constant conversion.
This makes recycling spent catalyst an option which increases the effective catalyst/oil
ratio and reduces the initial regenerated catalyst activity.
The PetroFCC utilizes many of the same technologies that are used in the standard
UOP FCC units. This includes both the optimix nozzles and vortex separator system.
Table 6.16. DCC yields for atmospheric resid
Daqing ATB Yields (wt%)
Density 0.9012 Dry Gas 11.7
CCR (wt%) 4.7 C
3
+ C
4
48.3
Hydrogen (wt%) 12.84 C
5
+ Naphtha 18.9
Saturates 55.5 LCO 12.1
Aromatics 28.0 Coke 8.0
Resins 15.7 Loss 1.0
Asphaltenes 0.8 Olefins
Nickel (ppm) 6 Ethylene 6.85
Propylene 24.83
Butylene 15.27
276 CHAPTER 6
Figure 6.29. PetroFCC process.
A second riser for recracking the naphtha is provided that operates at higher temper-
ature.
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 (Table 6.18).
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.
Table 6.17. Selectivity of spent catalyst
Regenerated Spent
Initial carbon 0.0 0.91
MAT conversion 64.5 55.0
Yields @ 70 wt% Conversion
C
2
Minus 2.0 2.1
Gasoline 49.3 50.0
Coke 2.9 1.6
FLUID CATALYTIC CRACKING 277
Table 6.18. Pilot yields–PetroFCC (wt%) versus FCC
PetroFCC FCC
Propylene 22 4.7
Butylene 14 ∼6
Ethylene 6 ∼1
Gasoline 28 53.5
Another process designed for high olefin yields is the Maxofin FCC. This unit is
shown in Figure 6.30 and has a second riser for propylene maximization. The process
uses the Orthoflow FCC Hardware, e.g. atomax nozzles and close-connected cyclones
and the stacked reactor-regenerator configuration. A special ZSM-5 additive is used
along with the FCC catalyst.
Pilot yields from the Maxofin process are shown in Table 6.19. The data are shown
for a paraffinic gas oil operating in three different modes.
Figure 6.30. Maxofin process.
278 CHAPTER 6
Table 6.19. Yields from Maxofin unit
Max C
3
= Intermediate Max fuels
Recycle Yes No No
ZSM-5 Yes Yes No
Riser temp.,
◦
C 538/593 538 538
Yields (wt%)
C
2
Minus 7.6 2.3 2.2
Ethylene 4.3 2.0 0.9
Propylene 18.4 14.4 6.2
Butylene 12.9 12.3 7.3
Gasoline 18.8 35.5 49.8
Coke 8.3 6.4 5.9
Conversion 86.4 87.7 85.4
As the need for light olefins and LPG increases, these processes will assume even
more attention.
Catalytic cracking is well positioned as we enter a new decade of petroleum refining.
The continued demand for gasoline, the need to reduce the residual portion of the crude
barrel and an ever increasing thirst for petrochemical base stocks will insure a premier
position for the fluid catalytic cracking unit in the refining and petrochemical industry.
Nomenclature
A = activation Energy for coke burning
A, A
O
=equilibrium and initial catalyst inventories
BP =boiling point
C, C
i
=carbon on catalyst and initial carbon on catalyst (wt%)
CO =carbon monoxide
DO =decant oil
E = activation energy for thermal cracking
e = natural exponent
K = rate constant
K
D
=deactivation constant
LCO =light cycle oil
L
m
O
2
=log mean oxygen concentration
LPG =liquefied petroleum gas
P
B
, P
T
=pressure at bottom and top of standpipe (psi)
P =pressure difference
Q = aeration requirements for standpipe (scfm)
R =gas constant
S =daily fractional catalyst addition rate (ton/ton)
REFERENCES 279
T = temperature
t = time (seconds)
ρ
mf
, ρ
t
=catalyst densities at minimum fluidization and top of standpipe (lb/ft
3
)
ρ
p
=skeletal density of catalyst (lb/ft
3
)
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