Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining
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Separation
Conversion Finishing
Products
Gas
Plant
Atm.
Pipe
Still
Crude
Oil
Alkylation
Hydrofiner
Reformer
Hydrofiner
Hydrofiner
Cat Cracker
Cat Cracker
Visbreaker
Coker
Deasphalter
Solvent
Extraction
Hydrogen
Gasoline
Kerosene
Kerosene,
Mild-Distillate
Naphtha,
Kerosene
Heating Oil
Fuel Oil
Distillation
Gasoline,
Kerosene,
Diesel Oil
Light Ends
Naphtha,
Kerosene
Coke
Gasoline,
Kerosene
Asphalt
Aromatic oil
Lube Oils,
Wax,
Greases
LPG,
Fuel Gases,
Petrochemical
feedstock
VGO
GO
LGO
DAO
C
4, C
4
=, iC
4
320°F
−
320/450°F
450/580°F
580/650°F
650°F
+
1050°F
+
880/1050°F
Soluble
insoluble
Hydrocracker
Hydrocracker
Hydrotreater
Vacuum
Pipe
Still
Figure 7.9 Role of the hydrocracker in the refinery
Hydroconversion 181
4. Hydroisomerization
ð7:31Þ
5. Polynuclear aromatics hydrocracking
H
13
C
6
+ 2H
2
H
13
C
6
C
H
2
H
2
C
H
13
C
6
+ 4H
2
+ H
2
+
C
2
H
5
C
2
H
5
+
C
3
H
7
CH
3
CH
3
CH
Isohexane Eth
y
l Benzene
Cyclohexane
ð7:32Þ
7.3.4. Hydrocracking Catalysts
Hydrocracking catalysts have a cracking function and a hydrogenation–
dehydrogenation function (Figure 7.10). The cracking function is provided
by an acidic support, whereas the hydrogenation–dehydrogenation
Table 7.11 Typical feedstocks and products
Feedstocks Products
Kerosene Naphtha
Straight-run diesel Naphtha and/or jet fuel
Atmospheric gas oil Naphtha, jet fuel, and/or diesel
Vacuum gas oil Naphtha, jet fuel, diesel, lube oil
FCC LCO Naphtha
FCC HCO Naphtha and/or distillates
Coker LCO Naphtha and/or distillates
Coker HCO Naphtha and/or distillates
Deasphalted oil Olefin plant feedstocks
182 Chapter 7
function is provided by active metals. The acidic support can be (a) amor-
phous oxides (e.g. silica–alumina), (b) a crystalline zeolite (mostly modified
Y zeolite) plus binder (e.g. alumina) or (c) a mixture of crystalline zeolite
and amorphous oxides. Cracking and isomerization reactions take place on
the acidic support (Secherzer and Gruia, 1996).
The metals providing the hydrogenation–dehydrogenation function can
be noble metals (palladium, platinum) or non-noble metal sulphides from
Group VI.A (molybdenum, tungsten) and group VIII.A (cobalt, nickel).
These metals catalyze the hydrogenation of the feedstock, making it more
reactive for cracking and heteroatom removal and reducing the coking rate.
They also initiate the cracking by forming a reactive olefin intermediate via
dehydrogenation. The ratio between the catalyst’s cracking function and
hydrogenation function can be adjusted to optimize activity and selectivity.
The relative strength of different hydrogenation components and cracking
(acid) components in hydrocracking catalysts are shown in Table 7.12.
Hydrocracking Catalyst
(dual function)
Cracking Function
(acid support)
Hydrogenation Function
(metals)
Amorphous
(SiO
2
, Al
2
O
3
,
Al
2
O
3
, Hal-
Al
2
O
3
)
Low Zeolite/
Amorphous
(Modified Y/
SiO
2
– Al
2
O
3
)
High Zeolite
+ Binder
(Modified Y+
Al
2
O
3
)
Noble Metals
(Pt, Pd)
Non-noble Metals
M
x
S
y
from group VIA
(Mo, W)
+ Group VIIIA (Co, Ni)
Figure 7.10 Classification of hydrocracking catalyst (Secherzer and Gruia,1996)
Table 7.12 Bifunctional catalyst strength for hydrogenation and
cracking
Hydrogenation
function
Co/Mo Ni/Mo Ni/W Pt(Pd)
x xx xxx xxxx
Cracking function Al
2
O
3
Al
2
O
3
–Hal SiO
3
–Al
2
O
3
Zeolite
x xx xxx xxxx
x represents order of strength
Hydroconversion 183
Catalysts with amorphous supports are still in commercial use, primarily
where maximizing the production of middle distillates or conversion to lube oil
blending stock are the objective. Amorphous hydrocracking catalysts contain
primarily amorphous silica–alumina. The hydrocracking catalysts described so
far are used primarily for hydrocracking gas oils and FCC cycle oils. For
hydroprocessing residues, amorphous hydrocrackingcatalystsaswellasspecially
designed hydrotreating catalysts and iron-containing catalysts are used.
Catalysts used for mild hydrocracking have a composition similar to that of
hydrotreating catalysts. They consist of Group VI and VIII non-noble metals
supported on g-alumina. The metals used are cobalt, nickel, molybdenum,
and tungsten in sulphided form. Under mild process conditions, gas oil
hydrocracking catalysts may also be used for mild hydrocracking. Dewaxing
catalysts usually consist of a hydrogenation metal (Pt, Pd, Ni) supported on a
medium-pore zeolite (e.g. ZSM-5) combined with a binder, commonly
alumina. A zeolite catalyst Y type is shown in Figure 7.11.Thereacting
molecules pass inside the cages and are brought in close contact for reaction.
The zeolite channels (or pores) are microscopically small and in fact have
molecular size dimension such that they are often termed ‘‘molecular sieves.’’
The composition of a hydrocracking catalyst depends on the final product
requirements and mode of the operation (one or two-stage). A guideline of
catalysts composition is shown in Table 7.13. In the first stage of a two-stage
operation, a hydrocracking hydrogenation catalyst is usually used for hydro-
desulphurization, hydrodenitrogenation and aromatics removal. In the sec-
ond stage a hydrocracking catalyst is used for a hydrocracking and
hydroisomerization. In Table 7.14, the types of catalysts used for mild and
high conversion hydrocracking are shown along with FCC and hydrotreating
catalysts. Residue conversion processes (fixed, moving and ebulated beds) use
Sodalite Cages
Super cage
Si
Hexagonal
Prisms
Al
Figure 7.11 Structure of Y-type zeolite catalyst
184 Chapter 7
supported palletized catalysts of bifunctional composition. Slurry reactors
employ dispersed catalysts.
7.3.5. Thermodynamics and Kinetics of Hydrocracking
7.3.5.1. Thermodynamics
Aromatic hydrogenation, paraffin hydrocracking, naphthenes hydrocrack-
ing and aromatic hydrodealkylation reactions are all exothermic and careful
control of the fixed bed temperature is required. This is usually done by
gaseous quenches in the reactor. Catalyst partitioning of the bed must be
Table 7.13 Types of catalyst used in different hydrocracking processes (Billon and
Bigeard, 2001)
Application
type
Process
type
(# stages)
Hydrogenerating
function Acid function
Pd Ni^Mo Ni^W
Y-type
zeolite
SiO
2
/Al
2
O
3
amorphous
Max. Naphtha One xxx x x xxx
Max. Naphtha Two xxx xxx
Max. Kerosene One xxx x xxx
Max. Kerosene Two x x x x x
Max. Diesel oil One x x x x
Max. Diesel oil Two x x x
Max. Lube oils One x x x x
x represents frequency of use
Table 7.14 Catalysts used for VGO treatment
High conversion
hydrocracking
Bifunctional A Amorphous Silica–
Alumina
Zeolite
H Pd, Pt
Ni (Co), Mo (W)
Mild hydrocracking Mono-functional or
bifunctional
A Amorphous Silica–
Alumina
Alumina
H Ni (Co), Mo (W)
FCC Mono-functional A Zeoliteþ
Amorphous Silica–
Alumina
Hydrotreating Mono-functional H Ni (Co), Mo (W)
A is acidic function; H is hydrogenation function.
Hydroconversion 185
applied. Table 7.15 gives average heat of a reaction at 400
C (kJ/mol) of
different classes of reactions.
7.3.5.2. Kinetics
Hydrocracking conversion is usually defined in terms of a change of the end
point:
%Conversion ¼
ðEP
þ
Þ
feed
ðEP
þ
Þ
products
ðEP
þ
Þ
feed
!
100 ð7:33Þ
where EP
þ
is the fraction of material in the feed or product boiling above
the desired end point usually as wt% or vol%.
The detailed analysis of reaction kinetics is rather complicated because it
involves several successive reactions, as in the hydrocracking of polyaro-
matics and naphthenes shown in Figure 7.12. Different reaction rate con-
stants indicate the relative reaction rate of each reaction. Comprehensive
mathematical models have recently been proposed in literature to consider
all possible reactions, and adsorption constants are usually important factors
in these models. As an example, the hydrocracking of vacuum residue is
presented as a network of nested reactions Figure 7.13.
7.3.6. Hydrocracking Processes
The following factors can affect operation (product quality), yield (quan-
tity), and the total economics of the process:
Process configuration: one stage (once-through or recycle) or two stages
Catalyst type
Operating condition (depends on process objective)
○
Conversion level
○
Maximization of certain product
○
Product quality
○
Catalyst cycle
○
Partial hydrogen pressure
○
Liquid hourly space velocity
○
Feed/hydrogen recycle ratio
Table 7.15 Heat of reaction (Billon and Bigeard, 2001)
Reaction type Ave rage heat of reaction at 400
C (kJ/mol)
Aromatics hydrogenation 210
Paraffin hydrocracking 46 to 58
Naphthenes hydrocracking 42 to 50
Aromatics hydroalkylation 42 to 46
186 Chapter 7
Polyaromatics
Phenanthrenes
Fluorenes
Tetrahydo-
Phenanthrenes
Dinaphthene
Benzenes
Tetralines &
Indanes
Alkyl-
Benzenes
Dealkylation
Paraffin
Cracking
Paraffins
Single Ring
Naphthenes
2-Ring
Naphthenes
Multi Ring
Naphthenes
Naphthenes
Ring Opening Followed by Dealkylation
Hydogenation
k = 0.9–1.0
k = 0.1 k = 0.1
k = 0.1
k = 1.1
k = 1.2 k = 2.0
k = 1.6
k = 0.4
k = 0.2
k = 1.4 k = 1.0
Figure 7.12 Relative rates of reactions u nder hydrocracking conditions (Filimonov et al.,1972)
Hydroconversion 187
7.3.7. Process Configuration
The one-stage process shown in Figure 7.14 can be used for light feeds with
once through or recycle process. In commercial hydrocrackers, a conver-
sion of 40–80% of the feed can be achieved. However if high conversion is
required the product from the bottom of the distillation tower is recycled
back to the reactor for complete conversion. This configuration can be used
to maximize a diesel product, and it employs an amorphous catalyst.
Residue
Naphtha
Distillates
k
1
k
2
k
3
k
4
k
6
k
7
k
9
k
10
k
8
k
5
VGO
Gases
Figure 7.13 Proposed kinetic model for the hydrocracking of heavy oils
Jet Fuel/Kerosene
Recycled H
2
Makeup
Hydrogen
Fresh
Feed
R-1
One-Stage
Reactor
Single-Stage
Product
Product gas
Light Naphtha
Heavy Naphtha
Diesel
FractionatorSeparators
(High and Low
pressure)
Recycled Bottoms
One-Stage once-through One-Stage with recycle
Fuel oil
FCC feed
Ethylene feed
Lube oil base
Figure 7.14 Simplified flow diagram of one-stage hydrocracking process with and
without recycle
188 Chapter 7
The two-stage operation is shown in Figure 7.15. The effluent from the first
stage reactor is sent to a separator and fractionator. The fractionator bottoms are
sent to the second reactor. In both configurations, the hydrogen is separated in
the high pressure separator and recycled back to the reactor. The hydrocracking
catalyst in the first stage has a high hydrogenation/acidity ratio, causing sulphur
and nitrogen removal. In the second reactor, the catalyst used is of a low
hydrogenati on/ac idity rat io in which naphtha production is maximized. The
main reactions taking place in each reactor are shown in Figure 7.16.
However, if middle distillate, kerosene, diesel and jet fuel are to be
maximized, a high hydrogenation/acidity ratio is used. A hydrotreatment
reactor may be added before the first hydrocracker to help in removing
sulphur and nitrogen compounds from the feed. Since H
2
S and NH
3
are
separated before entering the second hydrocracker, this allows the selection
of special catalysts in the second reactor without the poisoning effect of sour
gases. This will allow deep hydroconversion. The two-stage configuration
offers more flexibility than the single stage scheme. It is better suited for
heavy feedstocks (Secherzer and Gruia, 1996).
7.3.8. Hydrocracking Severity
There are two levels of hydrocracking severity: mild and conventional.
In mild hydrocracking the process is run at less severe operating conditions
similar to desulphurization (hydrotreating) conditions (Secherzer and Gruia,
1996). A comparison of hydrotreating, mild hydrocracking and conventional
Recycled
Gas
Makeup
Hydrogen
Fresh
Feed
First Stage
Reactor
Light
Naphtha
Heavy
Naphtha
Kerosene
Diesel
FractionatorSeparators
Recycled Bottoms
Makeup Hydrogen
Product
Gas
Recycled
Gas
Second Stage
Reactor
Separators
First Stage
Product
Figure 7.15 Conventional two-stage hydrocracker
Hydroconversion 189
hydrocracking operating conditions are given in Table 7.16. A one-stage
reactor without recycle is used in mild hydrocracking. The major character-
istics of this process are the production of a high yield of fuel oil and a savings
of hydrogen, since the process is operated at almost half of the hydrogen
pressure used in conventional hydrocracking. A comparison of the products
in each process is shown in Figure 7.17.
Usually mild hydrocracking is used to produce LSFO (Secherzer and
Gruia, 1996) as shown in Figure 7.17 .
7.3.9. Catalytic Dewaxing
Catalytic dewaxing is a particular hydrocracking process used to improve
cold flow properties of middle distillates and lubricants by cracking normal
paraffins. Dewaxing can be achieved by isomerization, as carried out by
Chevron’s isodewaxing process. Isoparaffins have lower melting points than
normal paraffins. The properties targeted for improvement are pour point
and viscosity of middle distillates and lubricants, the cloud point of diesel
fuel, and the freeze point of jet fuel.
Main Reaction Main Reaction
HDS Hydrodesulfurization
HDN Hydrodenitrogenation
HDA Aromatic Hydrogenation
Control parameter: nitrogen specification
HDA Aromatic Hydrogenation
HDI Hydroisomerization
HDC Hydrocracking
Control parameter: yields specification
1
st
Stage
reactor
2
st
Stage
reactor
Feed
VGO
Separator
Products
Figure 7.16 Main reactions in two-stage hydrocracking
190 Chapter 7