Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining
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Mild hydrocracking
Mild distillate
Mild hydrocracking
Fuel oil
Total conversion
Hydrocracking
Mild distillate
Total conversion
Hydrocracking
Naphtha
Fuel
Oil
Middle
(gas oil)
Distillate
(jet fuel)
Naphtha
LPG
25 50
75
100
200
300
400
500
0
Distillation, wt%
Product of total
Conversion hydrocracking
Feedstock:
Vacuum gas oil
Product of mild
hydrocracking
Boiling point, ⬚C
Figure 7.17 Comparison of products obtained by mild and conventional hydrocracker
(Secherzer and Gruia,1996)
Table 7.16 Comparison of operating conditions of mild hydrocracking,
hydrotreating and conventional hydrocracking (Secherzer and Gruia, 1996).
Operating
conditions Hydrotreating
Mild
hydrocracking Hyd rocracking
Conversion (%) – 20–70 70–100
H
2
pressure (bars) 25–50 35–70 85–140
H
2
flow (L/min) 300 300–1000 1000–2000
Temperature (
C) 360–400 350–440 350–450
LHSV (h
1
)0.52.5 0.31.5 0.52.0
Catalyst
Hydrogen transfer Co/Mo Ni/Mo Ni/Mo
(Metal) Ni/Mo Co/Mo Ni/W Pd
Support Alumina Alumina Si–Al
Cracking (acid) – Si–Al Zeolite
Hydroconversion 191
Due to their high melting points, long-chain normal paraffins have the
most detrimental effect on low temperature properties of middle distillates and
lube oils. By reducing the amount or chain length of normal and minimally
branched paraffins in these fuels and lubricants, their cold flow properties are
improved. This can be accomplished by using a catalytic dewaxing process.
Such a process can also be used to improve the flow properties of gas oils.
A single-stage, once through hydrocracking process can be used for
catalytic dewaxing, with or without hydrotreating, depending on the sul-
phur and nitrogen content of the feedstock. The catalytic process is carried
out as a trickle bed reactor over a bifunctional zeolite catalyst under
hydrogen flow. A non-noble metal (e.g. nickel) supported on a medium-pore
zeolite, such as ZSM-5 can be used. The medium-pore zeolite ZSM-5
appears to be particularly suitable to obtain a high selectivity.
7.3.10. Hydrocracking Correlations
7.3.10.1. Yield Correlations
The yield correlations in conventional hydrocracking depend on feed proper-
ties, hydrogen severity and mode of operation. There are three modes of
operation: maximum gasoline mode, maximum ATK mode (jet fuel) and
maximum diesel fuel. In all these modes, gasoline yield is used to correlate
other yields. Using mild hydrocracking (low severity) will add a fourth mode of
operation, maximum low sulphur fuel oil (LSFO). The hydrogen severity
ranges between (1.5 – 4) wt% of feed. In mild hydrocracking it can be assumed
as1.5wt%andinconventionalhydrocrackingas3.0wt%.Inhighseverity(high
aromatic feeds) it can be assumed as 4.0 wt% of feed. ATK mode is frequently
used because of the high demand for aviation fuels, and it is presented here from
data compiled by (Maples, 1993). Table 7.17 lists typical hydrocracking yields.
7.3.10.2. Maximum ATK Correlations
The ATK mode yield correlations are given here as an example. These
correlations are developed from plant data. The following is the calculation
procedure for ATK yields:
Table 7.17 Typical hydrocracking yields (Baral and Miller, 1982)
Naphtha Jet Diesel
Yields on coker gas oil and FCC decanted oil (vol% on feed)
Butanes 17 8 5
C
5
–180
o
F32 15 9
180–380
o
F81 24 20
Jet or diesel – 74 84
Total 130 121 118
192 Chapter 7
^ Hydrocracking severity H where
Hwt% ¼
lb Hydrogen
lb of feed
100
For maximum ATK, assume H = 3 wt%
In this case V
H
¼
thousands of ft
3
hydrogen
bbl feed
And is related to hydrocracking severity as
V
H
¼ 0:6621 H SG
^ Calculate liquid volume% (LV%) of gasoline
Gasoline LV% ¼0:03734 API
2
f
þ 1:57575 API
f
þ 0:014923 K
1:36473 V
H
0:16324 V
H
=K
Where K = (T
B
)
1/3
/SG
^ Butanes (iC
4
and nC
4
)
C
4
LV% ¼ 0:020359 ðLV% GasolineÞ
2
þ 0:04888 ðLV% GasolineÞ
þ 0:108964API
f
^ Heavy naphtha (HN) (180–380
o
F)
HN LV% ¼0:10322 ðLV% GasolineÞ
2
þ2:981215 ðLV% GasolineÞ
0:07898 API
f
^ ATK
Calculated by mass difference between feed and products
^ The following equation is used to convert LV% to wt% of hydrocarbon
products
Product wt% ¼ 0:8672 Product LV% 0:9969
Example E7.7
A feed of VGO of 37,500 BPCD is hydrocracked to maximize the ATK
production. The API of the feed is 20 and the mean average boiling point T
B
= 575
o
F. Make material balance around this hydrocraker.
Solution:
From feed API of 20 the SG is 0.934
Assume H = 3 wt% then
V
H
¼ 0:6621 3 0:934 ¼ 1:855 ft
3
/bbl
K = (575+460)
1/3
/(0.934) = 10.83
Gasoline LV% ¼0:03734ð20Þ
2
þ 1:57575 20 þ 0:014923 10 :83
1:36473 1:855 0:16324 ð1:855=10:83 Þ¼14:18
Gasoline wt% ¼ 0 :8672 14:18 0:9969 ¼ 11:30
Hydroconversion 193
C
4
LV% ¼ 0:020359 ð14:18Þ
2
þ 0:04888 ð14:18Þþ0:108964 20 ¼ 6:96
C
4
wt% ¼ 0:8672 6:96 0:9969 ¼ 5:04
HN LV% ¼0:10322 ð14:18Þ
2
þ2:981215 ð14:18Þ0:07898 20 ¼ 19:94
HN wt% ¼ 0:8672 19:94 0:9969 ¼ 16:29
ATK wt% = 100 + 3 – 11.30 – 5.04 – 16.29 = 70.37
Summary of material balance:
Feed VGO 100
Hydrogen
3.00
103.00
Products
C
4
5.04
Gasoline (C
5
-180
o
F) 11.3
HN (180 – 400
o
F) 16.29
ATK (+400
o
F) 70.37
Total 103.0
7.3.11. Simulation of Hydrocracking Units
Hydrocracking of VGO is carried out with a bifunctional catalyst and thus,
two types of reactions take place the hydrogenation of sulphur and nitrogen
compounds and the hydrocracking of heavy paraffins to lower molecular
weight paraffins and olefins.
Example E7.8
A VGO feed containing 950 lb mol/h n-C
16
and30lbmolC
2
H
6
S (ethyl mer-
captane) and 20 lb mol/h C
5
H
5
N (pyridine) available at 500
Fand100psianeeds
to be hydrocracked. This feed is pumped to raise its pressure up to 900 psia before
mixing with hydrogen stream that also need to be compressed from 150 psia to 900
psia. The hydrogen is available at 400
F. The mixed stream of oil feed and
hydrogen is then heated to 600
F before entering the hydrocracking reactor.
The reactions involved are listed in Table E7.8.1:
Table E7.8.1 Reactions in the hydrocracking unit
Rxn # Reaction Reaction type Conversion (%)
1C
2
H
6
S þ H
2
! C
2
H
6
þ H
2
S Conversion 100
2C
5
H
5
N þ 5H
2
! C
5
H
12
þ NH
3
Conversion 100
3C
16
H
34
þ H
2
! C
6
H
14
þ C
10
H
22
Conversion 50
4C
16
H
34
þ H
2
! 2C
8
H
18
Conversion 50
194 Chapter 7
The reactor products are then cooled and enter a distillation column to separate
the gases and light components at the top. C
8
is withdrawn in a side stream
from the column. Furthermore, heavier hydrocarbons are coming from the
bottom of the distillation column. Perform a material and energy balance for
the hydrocracking process using UNISIM simulator. Utilize the flowsheet in
Figure E7.8.1.
Solution:
1. Enter Simulation Basis Environment in UNISIM
2. A dd all the components as shown in Table E7.8.2
3. Select Peng–Robinson as Fluid package
4. Insert Reaction-1 sto ichiometry and conversion, do the same for the conse-
quent reactions, Reaction-2, Reaction-3 and Reation-4
5. Enter simulation environment
6. Insert the first unit for the oil feed as shown in the flow chart with composi-
tions, temperature and pressure
7. Continue inserting units as shown in the chart
8. T he reactor is conversion reactor
9. The distillation column is 10 trays with partial reflux. The active specifica-
tions to run the distillation column are: C
10
recovery of 80%, C
8
recovery of
80%, C
6
recovery 99% and draw rate is 226.8 kg mol/h. The pressure drop
from top to bottom is 344.7 to 482.6 kPa, respectively
Table E7.8.2 lists summary of results.
Oil Feed
Hydrogen
feed
Cracking
reactor
Heat
exchanger
Pump
Compressor
Gasses
C
6
C
8
C
10
Figure E7.8.1 Flow chart of hydrocracking process
Hydroconversion 195
Table E7.8.2 Energy and material balance results
Stream name
Oil
feed
Hydrogen
feed
Pump
out
Compressor
out
Mixed
feed
Heated
feed
Reactor
product
Cooled
product Gases
C
6
stream
C
8
stream
C
10
stream
Temperature (
F) 500 400 505.4 1157 529.34 600 705.71 643.31 95.5 95.5 376 466.28
Pressure (psia) 100 150 900 900 900 898 873 871 50 50 63.33 70
Total molar flow
(lb mol/h)
1000 1081.3 1000 1081.3 2081.3 2081.3 2001.3 2001.3 21.38 621.86 858.11 500
n-C
16
H
34
(lb mol/h)
950 0 950 0 950 950 0 0 0 0 0 0
Hydrogen
(lb mol/h)
0 1081.3 0 1081.3 1081.3 1081.3 1.34 1.34 1.25 0 0 0
C
2
H
6
S (lb mol/h) 30 0 30 0 30 30 0 0 0 0 0 0
C
5
H
5
N
(lb mol/h)
20 0 20 0 20 20 0 0 0 0 0 0
H
2
S (lb mol/h) 0 0 0 0 0 0 30 30 7.06 22.93 0 0
NH
3
(lb mol/h) 0 0 0 0 0 0 20 20 2.87 17.13 0 0
C
2
H
6
(lb mol/h) 0 0 0 0 0 0 30 30 8.33 21.67 0 0
n-C
5
H
12
(lb mol/h)
0 0 0 0 0 0 20 20 0.21 19.78 0 0
n-C
6
H
14
(lb mol/h)
0 0 0 0 0 0 475 475 1.62 470.25 3.11 0
n-C
8
H
18
(lb mol/h)
0 0 0 0 0 0 950 950 0 69.98 760 120
196 Chapter 7
Question and Problems
7.1. Classify hydrogen-consuming refinery processes.
7.2. Find the catalyst volume needed for hydrotreating a gas oil containing
1.5 wt% sulphur to a final residual sulphur of 0.05 wt%. The reaction
rate can be expressed as:
k ¼ 3:7 10
9
exp
13; 200
T
The desulphurization reaction temperature is 350
C, and the pressure
is 2.1 MPa. The reaction order is 1.5. The feed flow rate is
150,000 kg/h and has a density of 850 kg/m
3
.
7.3. Coker naphtha is hydrotreated to remove 80% of the sulphur, 60% of
the nitrogen and all the olefins. Sulphur is present as thiophene and
nitrogen as pyrrole. The naphtha has the following properties: Specific
gravity: 0.763, sulphur: 1.7 wt%, nitrogen: 200 wppm, propylene:
0.2 wt%, butanes: 3.5 wt% and butadienes: 0.3 wt%.
(a) Find the chemical hydrogen requirement for one barrel of naphtha.
(b) Make a material balance around the hydrotreater and find the
hydrotreated naphtha API.
7.4. The atmospheric residue desulphurization unit (ARDS) is running
under the following conditions: Feed (AR) API: 13, sulphur content:
4 wt%, nitrogen content: 0.25 wt% and metal contents 110 wppm.
Make a material balance around this unit.
7.5. In a two-stage conventional hydrocracker, it is required to maximize
the production of ATK for the following feed: Feed API: 24, S: 0.5 wt
%, N: 850 wppm. T
B
¼ 800
F. Assume the required hydrogen is
3.5 wt% of feed. Calculate all the product yields.
7.6. Classify bifunctional catalysts strength and functions in hydrocracking.
7.7. For a one-stage hydrocracking reactor, what would be the best catalyst
composition to maximize the production of kerosene?
7.8. Hydroconversion unit is used to upgrade vacuum residue at a flow rate
of 16,000 bbl/d. Under the following specifications: Catalyst pore
volume ¼ 0.5 ml/g, carbon content of coke deposits ¼ 82% and density
¼ 1350 kg/m
3
. Vanadium is removed as V
2
S
3
(density ¼ 4700 kg/m
3
),
and nickel is removed as NiS (density – 5500 kg/m
3
). Vacuum residue
properties: API ¼ 2.2, sulphur 4.5 wt% and nitrogen ¼ 4700 wppm,
nickel ¼ 120 wppm, vanadium ¼ 310 wppm.
Removal efficiency in the process: De-sulphurization, wt% is 80,
vanadium is 75 wppm, nickel is 70 wppm. The spent catalyst contains
15% carbon.
Hydroconversion 197
What is the minimum catalyst consumption in (kg/h) when the
catalyst pores are completely filled with carbon and metal sulphide
deposits?
7.9. Make a comparison between the various hydroconversion processes
and reactor technology used.
7.10. Explain product conversion behaviour in the fixed bed desulphuriza-
tion reactor.
REFERENCES
Baral, W.J., and Miller, J.R. (1982). ‘‘Hydrocracking-a Rouse to Superior Distillate Pro-
ducts from Heavy Oil, Kellog Simposium on Heavy Upgrading. Nice, France, Sep 16.
Billon, A., and Bigeard, P.H. (2001). ‘‘Hydrocracking,’’ Chapter 10, In ‘‘Conversion
Processes Petroleum Refining,’’ Vol. 3, Leprince, P. ed., TECHNIP, France.
Filimonov, V.A., Popov, A., Khavkin, V. A., Perezghhina, I.Y., Opsipov, L.N., Rogov,
S.P., and Agafonov, A.V. (1972). ‘‘The rates of reaction of individual groups of
hydrocarbons in hydrocracking.’’ Int. Chem. Eng. 12(1), 7521.
Heinrich, G., and Kasztelan, S. (2001). ‘‘Hydrotreating,’’ Chapter 16, In ‘‘Conversion
Processes,’’ Petroleum Refining,’’ Vol. 3, Leprince, P., ed., TECHNIP, France.
Kaes, G.L. (2000). ‘‘Refinery Process Modeling.’’ The Athenes Printing Company, Athenes,
Georgia.
Maples, R.E. (1993). ‘‘Petroleum Refining Process Economics.’’ PennWell Books, Tulsa
Oklahoma.
Parakash, S. (2003). ‘‘Refining Processes Handbook.’’ Elsevier, USA.
Raseev, S. (2003). ‘‘Thermal and Catalytic Processes in Petroleum Refining.’’ Marcel
Dekker, New York.
Secherzer, J., and Gruia, A.J. (1996). ‘‘Hydrocracking Science and Technology.’’ Marcell
Dekker, New York.
Speight, J. G. (2000). ‘‘The Desulfurization of Heavy Oils and Residue.’’ Marcel Dekker,
New York.
UNISIM Design Suite R370, HoneyWell Process Solutions, Calgary, Alberta, Canada.
Yui, S.M., and Sanford, E.C. (1985). ‘‘Kinetics of aromatics hydrogenation and prediction of
cetane number of synthetic distillates.’’ Proceedings-Refining Department; 50
th
Mid
year Meeting (Kansas City) MO, May 1985, pp 290–297.
198 Chapter 7
CHAPTER EIGHT
Fluidised Catalytic Cracking
8.1. Introduction
The fluidised catalytic cracking (FCC) unit is the heart of the refinery
and is where heavy low-value petroleum stream such as vacuum gas oil
(VGO) is upgraded into higher value products, mainly gasoline and C
3
/C
4
olefins, which can be used in the alkylation unit for production of ultra-
clean gasoline (C
7
–C
8
alkylates). The FCC process has been in operation for
over 60 years during which, a great deal of developments has occurred.
Major developments have occurred in two areas: new catalysts and new
reactor and regenerator designs.
Recently, new processes have been developed, such as petro-FCC,
residue FCC (RFCC) and olefin production, by a variety of means, includ-
ing deep catalytic cracking (DCC). The original FCC has also been devel-
oped to improve yield, emission control and adaptability for different crude.
The FCC unit mainly depends on circulating a zeolite catalyst with the
vapour of the feed into a riser-reactor for a few seconds. The cracked
products are disengaged from the solids and taken out to a distillation
column for separation of the desired products. The catalyst is circulated
back into the regenerator where coke is burned and the catalyst regenerated.
The combustion of the coke generates the heat required to carry out the
generally endothermic reaction in the riser.
8.2. Role of FCC in the Refinery
A block diagram of a typical refinery is shown in Figure 8.1. This
figure shows how the FCC plays a central role when feedstocks, such as
hydrotreated VGO (HT VGO) and atmospheric residue desulphurised
(ARDS) oil, are used (Tominaga and Tameki, 1997). More recently,
vacuum residues are desulphurised and used as feed. In this case, the unit
is called a residue FCC (RFCC unit). The basic processing principles are the
same for both units.
Fundamentals of Petroleum Refining
#
2010 Elsevier B.V.
DOI: 10.1016/ B978-0-444-52785-1.00008-5 All rights reserved.
199
The role of the FCC is to take heavy desulphurised feedstock and crack
it into lighter, mainly high octane gasoline. In a typical refinery, a FCC is
installed mainly to produce gasoline. The FCC also produces olefins (C
5
=
and C
4
=
) and LPG. Olefins can be utilised for alkylation and MTBE pro-
duction. In fact, some FCC units can be dedicated for the production of
petrochemicals, as in the UOP petro-FCC process.
8.3. Feedstock and Products
The main feedstock used in a FCC unit is the gas oil boiling between
316
C and 566
C (600
F and 1050
F). This gas oil can be considered
mixtures of aromatic, naphthanic and paraffinic molecules. There are also
Gasoline
Diesel Gas Oil
Kerosene
Fuel Oil
Naphtha
Aromatic Distillation
Kerosene
Gas oil
Naphtha
Hydrotreating
FCC
Vacuum
Distillation
Methanol
LPG
LPG
Gas
C
4
H
2
Crude Oil
Isomerization
Gas Recovery
Naphtha Catalytic
Reformer
Gas
Residue Direct
Desulphurization
(AR/VR)
VR
AR
Thermal
Cracking
Kerosene
Hydrotreating
H
2
Gas
H
2
Gas
H
2
Gas
Gas oil
Hydrotreating
Vacuum GO
Hydrotreating
H
2
Gas
H
2
Gasoline Blending
Alkylation
Gasoline Blending
Gas
MTBE
Gas
LPG
Propylene
MTBE
i-C
4
FCC
Gasoline
i-C
4
Figure 8.1 Role of FCC in refining operation
200 Chapter 8