Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining
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This ratio influences the purity of hydrogen in the recycle gas. Typical
purge ratios are summarized in Table 7.8. In addition, some mechanical loss
of hydrogen in the compressor can take place.
7.2.9. Operating Conditions
The operating conditions of the hydrotreating processes include pressure,
temperature, catalyst loading, feed flow rate and hydrogen partial pressure.
The hydrogen partial pressure must be greater than the hydrocarbon partial
pressure. High pressure and high hydrogen flow rate (make-up and recycle)
would insure that. Increasing hydrogen partial pressure improves the
removal of sulphur and nitrogen compounds and reduces coke formation
(Heinrich and Kasztelan, 2001).
Higher temperatures will increase the reaction rate constant and improve
the kinetics. However, excessive temperatures will lead to thermal cracking
and coke formation. The space velocity is the reverse of reactor residence time
(y). High space velocity results in low conversion, low hydrogen consump-
tion and low coke formation. The range of operating conditions for
hydrotreating of different feed fractions is given in Table 7.9.
7.2.10. Hydrotreating Correlations
Empirical equations can be used to predict the amount of hydrogen
required for certain feeds. Regression of plant data resulted in the correla-
tions listed below (Maples, 1993).
7.2.10.1. Naphtha and Gas Oil Hydrotreating Correlations
The standard cubic foot of hydrogen per barrel of feed (SCFB) required for
complete sulphur removal is calculated as:
SCFB H
2
¼ 191S
f
30:7 ð7:21Þ
where S
f
is the sulphur wt% in feed.
Table 7.8 Purge requirement of HDS processes
Feed Purge H
2
/H
2
make-up
Naphtha HDS 0.10
Kerosene HDS 0.15
Diesel HDS 0.20
VGO HDS 0.30
Hydroconversion 171
The increase in the API gravity of a product is calculated as:
DðAPIÞ
p
¼ 0:01 ðSCFB H
2
Þþ0:036 ðAPIÞ
f
2:69 ð7:22Þ
where p and f refer to product and feed, respectively.
In some cases, if it is required to saturate aromatics and naphthenes to the
corresponding paraffin, a set of correlations for PNA analysis is required to
predict the naphtha composition. These are:
Vol% Paraffins ¼ 12:8K
2
f
259:5K
f
þ 1330: 0 ð7:23Þ
Vol% Naphthene ¼78:5K
2
f
þ 1776:6K
f
9993:7 ð7:24Þ
Vol% Aromatics ¼ 38:4K
2
f
894:3K
f
þ 5219:4 ð7:25Þ
where K
f
is the feed Watson characterization factor.
Example E7.3
It is required to hydrotreat naphtha which has 1 wt% S and API ¼ 50. Find:
a. How much hydrogen is required to remove all the sulphur in the feed by
empirical correlations.
b. How much of this H
2
is used for chemical requirements.
c. If the mean average boiling point of this naphtha is 135
F. Assuming that
the naphthene and aromatic present in the naphtha are cyclohexane and
benzene, respectively. Find the volume of hydrogen (SCFB) to convert all
cyclohexane and benzene into hexane.
Table 7.9 Process parameters for hydrotreating different feedstocks (Heinrich and
Kasztelan, 2001)
Feedstock Naphtha Kerosene Gas oil
Vacuum
gas oil Residue
Boiling range,
o
C 70–180 160–240 230–350 350–550 >550
Operating temperature,
o
C
260–300 300–340 320–350 360–380 360–380
Hydrogen pressure, bar 5–10 15–30 15–40 40–70 120–160
Hydrogen consumption,
wt%
0.05–0.1 0.1–0.2 0.3–0.5 0.4–0.7 1.5–2.0
a
LHSV, hr
1
4–10 2–4 1–3 1–2 0.15–0.3
H
2
/HC ratio,
std m
3
/m
3
36–48 36–48 36–48 36–48 12–24
a
LHSV ¼ Liquid volumetric flow rate at 15
C (ft
3
/h)/Volume of catalyst (ft
3
)
172 Chapter 7
Solution:
Assume a basis of one bbl naphtha feed
a. Hydrogen required for desulphurization:
SCFB H
2
¼ 191 1ðÞ30:7 ¼ 160:3
Hydrogenated naphtha API:
DðAPIÞ
P
¼ 0:01ð160:3Þþ0:036ð50Þ2:69 ¼ 0:7
ðAPIÞ
P
¼ 50 þ 0:7 ¼ 50:7
b. To find the chemical hydrogen required:
For the given feed of API ¼ 50, r
f
¼ 273 lb/bbl
Amount of S ¼ 273 0:01 ¼ 2:73 lb=bbl
Mole S ¼ 2:73=32 ¼ 0:0853 mol=bbl
For reaction S þ H
2
! H
2
S
H
2
required ¼ 0:0853 mol 379 ft
3
=mol ¼ 32:3ft
3
=bbl
(Note: At standard gas condition, 1 lb mol occupies 379 ft
3
.)
Difference in H
2
requirement ¼ 160.3 – 32.3 ¼ 128 ft
3
This difference includes purge and dissolution hydrogen.
c. From API ¼ 50 then SG ¼ 0.78
K
f
¼
135 þ 460ðÞ
1=3
0:78
¼ 10:9
Using equations (7.23), (7.24) and (7.25) , the normalized vol% of naphtha
composition is:
Vol% Praffins hexaneðÞ¼22:07 vol%
Vol% Naphthenes cyclohexaneðÞ¼44:33 vol%
Vol% Aromatics benzeneðÞ¼33:60 vol%
Hexane does not need H
2
.
One mole of cyclohexane needs one mole H
2
:
C
6
H
12
þ H
2
! C
6
H
14
One mole of benzene needs 4 moles H
2
:
C
6
H
6
þ 4H
2
! C
6
H
14
Hydroconversion 173
Thus the following table can be constructed for the calculation of the volume of
H
2
required to convert cyclohexane and benzene to hexane (assuming 1 bbl
feed).
Volume
fraction
MW
(lb/lb
mol) SG
Density
(lb/bbl)
Weight
(lb) Moles
Mole H
2
required
Hexane 0.221 86 0.66 232.6 51.33 0.59 0
Cyclohexane 0.443 84 0.80 280.7 124.4 1.48 1.48
Benzene 0.336 78 0.88 308.7 103.7 1.33 5.32
1.0 279.43 6.80
The final amount of H
2
required can be summed up as:
H
2
required ¼ 6:80 mol 379 ft
3
=mol ¼ 2577 ft
3
=bbl
It can be seen that the amount of hydrogen for naphthenes and aromatics
saturation is much higher than that needed for other requirements such as
sulphur removal.
Example E7.4
It is required to remove all S in a feed of atmospheric gas oil (AGO) by
hydrotreating. The feed contains 2 wt% S and sulphur compounds distributed
is as follows:
Compound
RSH R
2
S (RS)
2
Thiophene (C
4
H
4
S)
wt % 45 25 20 10
The AGO has an API ¼ 30. Calculate the chemical hydrogen requirement in
SCFB.
Solution:
The corresponding hydrogenation reactions are:
RSH þ H
2
! RH þ H
2
S
R
2
S þ 2H
2
! 2RH þ H
2
S
RSðÞ
2
þ 3H
2
! 2R H þ 2H
2
S
C
4
H
4
S þ 4H
2
! C
4
H
10
þ H
2
S
174 Chapter 7
AGO of 30 API has a density of 306 lb/bbl. For one barrel of AGO, the
corresponding sulphur content (S
f
¼ 0.02) is:
306 lb=bbl 0:02 ¼ 6:12 lb S = bbl
The amount of hydrogen required can be calculated as shown in the following table:
S distribution in feed
(lb S/bbl feed)
Moles
(H
2
/mol S) Hydrogen required (ft
3
/bbl feed)
6.12 0.45 ¼ 2.75 1 (2.75/32) (379) (1) ¼ 32.6
6.12 0.25 ¼ 1.53 2 (1.53/32) (379) (2) ¼ 36.3
6.12 0.20 ¼ 1.22 1.5 (1.22/32) (379) (1.5) ¼ 21.8
6.12 0.10 ¼ 0.61 4 (0.61/32) (379) (4) ¼ 29.0
Total 119.7
The total chemical hydrogen required is about 120 SCF H
2
/bbl AGO.
7.2.10.2. Middle Distillate Hydrotreating Correlations
The amount of hydrogen required can be calculated as:
SCF H
2
=bbl ¼ 110:8 S
f
ðÞ
þ 10:2 HDS%
ðÞ
659:0 ð7:26Þ
where S
f
is the wt% sulphur in the feed and HDS% is the percent of
hydrodesulphurization required (degree of severity). The increase in prod-
uct API is calculated as:
DðAPIÞ
p
¼ 0:00297 ðSCF H
2
=bblÞ0:11205 ðAPIÞ
f
þ 5:54190
ð7:27Þ
This equation is used for feed sulphur content of 0.5–6.0 wt%.
Example E7.5
Gas oil has an API of 30, and a sulphur content of 1.5 wt% is fed into a
hydrotreater. It is required to carry out HDS at a severity of 90%. Calculate
the hydrogen required and the product API.
Solution:
Assume 1 bbl of GO, API ¼ 30 (306 lb/bbl)
Amount of S in feed ¼ 306 0.015 ¼ 4.6 lb/bbl feed
Total hydrogen required (SCFB H
2
)
¼ 110:8ð1:5Þþ10:2ð90Þ659 ¼ 425 ft
3
=bbl
Hydroconversion 175
DðAPIÞ
P
¼ 0:00297ð425Þ0:11205ð30Þþ5:5419 ¼ 3:4
Product API ¼ 30 þ 3.4 ¼ 33.4
7.2.10.3. Atmospheric Residue Desulphurization (ARDS)
In atmospheric residue desulphurization (ARDS), thermal cracking occurs
and no reliable correlations were found in literature. The hydrogen require-
ment, however, can be calculated for equation (7.26). The main products in
this process are low sulphur fuel oil (LSFO) with a yield from 70 to 75
weight percent. Naphtha and diesel are also produced. A typical ARDS
yields are presented in Table 7.10.
7.2.11. Simulation of ARDS Unit
The unit is basically designed to remove sulphur and other impurities
from atmospheric residue. It is important to consider two main reactions:
desulphurization and hydrocracking. The details of the simulation are given
in example E7.6.
Example E7.6
A heavy residue stream that contains mostly n-C
30
(990 lb mol/h) and some
amount of thiophene (10 lb mol/h) is prepared to enter an ARDS process to
crack the heavy component n-C
30
to more lighter components such as n-C
20
,
Table 7.10 Typical ARDS yields (Parakash, 2003)
Mass fraction
Feed
Atmospheric residue 1.0
Hydrogen 0.016
Total input 1.016
Products
Acid gas 0.038
Off gas 0.020
Naphtha 0.017
Diesel 0.186
Fuel oil 0.755
Total output 1.016
176 Chapter 7
n-C
10
and n-C
4
. In addition, th iophenes should be completely removed. The
feed stream is initially at 100
F and 120 psia. This feed needs to be mixed with
hydrogen stream (1250 lb mol/h) available at 150
F and 200 psia. The mixed
feed should be heated and compressed to 700
F and 1500 psia before entering
the reactor. The reactions are shown in Table E.7.6.1.
The reactor products are cooled to 200
F before entering a gas–liquid separa-
tor. 300 lb mol/h of the hydrogen coming from this separator is recycled back
with the feed. The rest is vented to the atmosphere. The liquid stream coming
out from the separator is then expanded by a valve to reduce the pressure to 250
psia. This makes it ready to enter a distillation column in order to separate the
extra hydrogen left with the hydrocarbons. A typical flowsheet of the ARDS
process is shown in Figure E7.6. 1. Perform a material and energy balance for the
ARDS process using UNISIM simulator.
Solution:
1. Enter the simulation basis environment in UNISIM.
2. Add the components as follows:
Thiophene, n-C
30
, n-C
20
, n-C
10
, n-C
4
,H
2
and H
2
S.
3. Select Peng–Robinson as the fluid package.
4. Insert Reaction-1 stoichiometry and conversion and do the same for
Reaction-2.
5. Enter simulation environment.
6. Insert the first unit for the oil feed as shown in the flow chart with
compositions, temperature and pressure as given in Table E7.6.2.
7. Continue inserting units as shown in the flowsheet.
8. The reactor is a conversion reactor.
9. The distillation column is 15 trays with reflux ratio equal to 1.0 and full
reflux. The active specification to run the distillation column is a hydrogen
recovery of 100% and an n-decane recovery of 90%.
10. Finally , add the recycle control unit to optimize the connections.
Table E.7.6.1 Typical reactions in ARDS
Rxn
# Reaction
Reactor
type Conversion
1C
4
H
4
S þ 4H
2
! C
4
H
10
þ H
2
S Conversion 100%
2C
30
H
62
þ H
2
! C
20
H
42
þ C
10
H
22
Conversion 70%
Hydroconversion 177
Oil Feed
Hydrogen
feed
Hydrogen
recycles
Pump
Compressor
Compressor
Heater
Cooler
Vent
Hydrogen
Separation
vessel
HDS
reactor
Distillation
Column
Valve
C
10
+
Figure E7.6.1 ARDS process flow chart
178 Chapter 7
Table E7.6.2 U NISIM results
Stream name
Hydrogen
feed
Comp
out 1
Oil
feed
Pump
out
Comp
out 2
Mixed
feed
Reactor
feed
Reactor
vapour
Reactor
liquid Product
Cooler
out
Separator
vapour
Separator
liquid Vent
Recycle
hydrogen
Dist
feed Hydrogen C
þ
10
Temperature
(
F)
150 790.2 100 101.5 205.2 126.4 700 756.6 756.5 756.5 200 200 200 200 200 209 444 930
Pressure (psia) 200 1500 120 1500 1500 1500 1495 1475 1475 1475 1470 1470 1470 1470 1470 250 225 240
Total molar flow
(lb mole/h)
1250 1250 1000 1000 300 2550 2550 217.5 2303 2520 2520 638.8 1881.2 338.8 300 1881 268 1613
Thiophene
(lb mol/h)
0 0 10 10 0 10 10 0 0 0 0 0 0 0 0 0 0 0
H
2
S (lb mol/h) 0 0 0 0 1 1 1 1.5 9.5 11 11 2 9 1 1 9 9 0
H
2
(lb mol/h) 1250 1250 0 0 299 1549 1549 176.5 639 815.5 815.5 635.8 179.7 337.2 298.6 180 180 0
n-C
30
(lb mol/h) 0 0 990 990 0 990 990 1 296 297 297 0 297 0 0 297 0 297
n-C
20
(lb mol/h) 0 0 0 0 0 0 0 7.5 685.5 693 693 0 693 0 0 693 0 693
n-C
10
(lb mol/h) 0 0 0 0 0 0 0 30 664 693.5 693.5 0.4 692.8 0.2 0.2 693 69 623
n-C
4
(lb mol/h) 0 0 0 0 0 0 0 1 9 10 10 0.6 9.7 0.4 0.2 10 10 0
Hydroconversion 179
7.3. Hydrocracking
Hydrocracking is a catalytic hydrogenation process in which high
molecular weight feedstocks are converted and hydrogenated to lower
molecular weight products. The catalyst used in hydrocracking is a bifunc-
tional one. It is composed of a metallic part, which promotes hydrogena-
tion, and an acid part, which promotes cracking. Hydrogenation removes
impurities in the feed such as sulphur, nitrogen and metals. Cracking will
break bonds, and the resulting unsaturated products are consequently
hydrogenated into stable compounds.
7.3.1. Role of Hydrocracking in the Refinery
Hydrocracking plays an important role as one of the main conversion pro-
cesses in the refinery as shown in Figure 7.9. It is mainly used to produce
middle distillates of low sulphur content such as kerosene and diesel. If mild
hydrocracking is used, a LSFO can be produced. More recently, it has been
used to remove wax by catalytic dewaxing and for aromatic removal by
hydrogen saturation. This has been applied to the lube oil plants and is
gradually replacing the old solvent dewaxing and aromatic solvent extraction.
7.3.2. Feeds and Products
VGO is the main feed for hydrocrakers, however a variety of feeds can be
used as shown in Table 7.11. The feedstock type has an important influence
on the final products.
7.3.3. Hydrocracking Chemistry
1. Alkane hydrocracking
R – CH
2
–––– CH
2
– R⬘ + H
2
R – CH
3
+ R⬘ – CH
3
ð7:28Þ
2. Hydrodealkylation
+ H
2
CH
2
– R
+ R – CH
3
ð7:29Þ
3. Ring opening
+ H
2
C
6
H
14
ð7:30Þ
180 Chapter 7