Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining
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Example E6.3
A vacuum residue of Conradson carbon (wt% CCR ¼ 15) is fed into a delayed
coker at a rate of 200,000 lb/h, of API¼ 8.5 and with a sulphur content of
3.0 wt%. Find the amount of yield (lb/h) and their sulphur content. Calculate
yield of liquid products in BPD.
Solution:
The solution of the example is summarized in Table E6.3.
Table E6.3 Results of delayed coking example
Feed rate ¼ 200,000 lb/h
Delayed
coker
CCR ¼ 15 wt%
S
f
¼ 3wt%
Feed API ¼ 8.5
Products yield wt% lb/h
Gas ¼ (7.8þ0.14 4
CCR%)
9.96 19,920
Naphtha ¼ (11. 29 þ 0.343
CCR%)
16.44 32,870
Coke ¼ (1.6 CCR%) 24.0 0 48,00 0
19920
HN
C
4
−
LN
LCO
Coke
HCGO
10918
21952
63987
35223
48,000
200,000
VR
Gas oil (100 Gas%
Napthta% Coke%)
49.61 99,210
100.00 200,000
Naphtha (assumed split wt%)
Light naphtha
LN ¼ 33.2%
33.22 10,918
Heavy naphtha
HN ¼ 66.78%
66.78 21,952
100.00 32,870
Gas oil (assumed split wt%)
Light gas oil LGO 64.50 63,987
Heavy gas oil HGO 35.50 35,223
100.00 99,210
Sulphur distribution in delayed coker
products (assumed wt%)
S in gas 30.00 1800
S in light naphtha 1.70 102
S in heavy naphtha 3.30 198
S in light gas oil 15.4 0 924
S in heavy gas oil 19.60 1176
S in coke 30.00 1800
100.00 6000
(continued)
Thermal Cracking and Coking 141
Gas composition
The gas produced is fuel gas, which can be sent to a fuel gas network
after amine washing. The gas can also be used in the alkylation unit because
of its high olefin content. A typical coker gas composition is given in
Table 6.11 (Gary and Handwerk, 2001).
6.5.7. Process Simulation of Delayed Coking
The composition of vacuum residue is quite complex and the thermal
decomposition reactions are difficult to simulate in a single model. For
this reason, the following model reaction is used to describe the thermal
cracking of vacuum residue.
2:3C
14
H
16
! C
2
H
4
þ 4:4H
2
þ C
6
H
6
þ C
10
H
8
þ C
12
H
10
þ 2:2C ð6:22Þ
Example E6.4
Vacuum residue at a rate of 8359 kg/h is fed into a delayed coker. The
composition of the feed used in this example is described in Table E6.4.1.
Applying a UNISIM process simulator was used to simulate the delayed coker
process using reaction (6.22) in the reactor block. The simulator flow diagram is
shown in Figure E6.4.
Table 6.11 Typical gas composition from delayed coker (sulphur-free basis)
Gas C
1
C
¼
2
C
2
C
¼
3
C
3
C
¼
4
iC
4
C
4
H
2
CO
2
Mole % 51.4 1.5 15.9 3.1 8.2 2.4 1.0 2.6 13.7 0.2
Table E6.3 (continued )
Feed rate ¼ 200,000 lb/h
Delayed
coker
CCR ¼ 15 w t%
S
f
¼ 3wt%
Feed API ¼ 8.5
Gravity of products
(assumed gravities) API SG BPD
Light naphtha 65 0.72 1041.5
Heavy naphtha 50 0.78 2894.3
Light gas oil 30 0.88 4994.0
Heavy gas oil 13 0.98 2468.5
142 Chapter 6
Solution:
Summary of the results is given in Tables E6.4.2 and E6.4.3
Table E6.4.1 Component presentation of each stream
Component Formula
Petroleum cut
representing
Composition (mole
fraction) in feed
n-Butyl-
naphthalene
C
14
H
16
Vacuum residue 1.0
Ethylene C
2
H
4
Gas 0
Hydrogen H
2
Gas 0
Benzene C
6
H
6
Naphtha 0
Naphthalene C
10
H
8
LGO 0
Di-phenyl C
12
H
10
HGO 0
Carbon C Coke 0
Products
Reactor
Heater
Column
HGO
LGO
Naphtha
Gas
Bottom
Recycle
R
C
RCY-1
Feed(VR)
Coke
q2
q4
q3
p1
q
Figure E6.4 Simul ation flow diagram in UNISIM
Table E6.4.2 Material balance sheet of simulation results
Feed (VR) Ga s Naphtha LGO HGO Coke
Vapour fraction 0 1 0 0 0 0
Temp. (
C) 176 37.8 37.8 214 252 343
Press (kPa) 294 172 172 181 196 206
Mass flow (kg/h) 8359 1917 175 4722 1052 493
Thermal Cracking and Coking 143
6.6. Fluid Coking
Fluid coking is a thermal cracking process consisting of a fluidized bed
reactor and a fluidized bed burner as shown in Figure 6.7. Vacuum residue is
heated to 260
C (500
F) and is fed into the scrubber which is located
above the reactor for coke fine particle recovery, and it operates at 370
C
(700
F). The heavy hydrocarbons in the feed are recycled with the fine
particles to the reactor as slurry recycle. The reactor operating temperature
is 510–566
C (950–1050
F). The heavy vacuum residue feed is injected
through nozzles to a fluidized bed of coke particles. The feed is cracked to
vapour and lighter gases which pass through the scrubber to the distillation
column.
Coke produced in the reactor is laid down on the coke bed particles,
typically in a layering manner. Steam is introduced at the bottom of the
reactor, where a scrubber is also added to scrub any heavy hydrocarbons
from the surface of the coke particles. This steam is also used to fluidize the
bed. Part of the coke flows into the burner where 15–30% is combusted by
the injection of air into the burner. The rest of the hot coke is recycled back
to the reactor to provide the required heat. The operating temperature of
the burner is in the range of 593–677
C (1100–1250
F) (Hammond et al.,
2003).
The combustion of the coke produces flue gases with low heating value
(20 Btu/SCF). A block diagram showing the flow of heat and mass in the
process is shown in Figure 6.8 (Hammond et al., 2003). An example of
material balance for fluid coking of Arab light vacuum residue containing
22 wt% CCR and the end use of products are shown in Figure 6.9.
Table E6.4.3 Stream compositions (mole fraction)
Feed
(VR ) Gas Naphtha LGO HGO Coke
C
14
H
16
1.000 0 0 0.007 0.341 0
C
2
H
4
0 0.160 0.004 0 0 0
H
2
0 0.705 0 0 0 0
C
6
H
6
0 0.135 0.996 0 0.007 0
C
10
H
8
0 0 0 0.528 0.111 0
C
12
H
10
0 0 0 0.445 0.541 0
C 0 0 0 0 0 1.000
144 Chapter 6
Hot
Coke
Steam
Feed
Vac Residue
Air
Net Coke
Burner
1100-1250°F
Scrubber
Reactor
Products
To Fractionator
Flue Gas To
Stack or Steam Boiler
Slurry
Recycle
^
^
^
^
^
^
^
^
^
^
Figure 6.7 Fluid coki ng process
Liquid
Products
Net Coke
Product
Upgrading
Particulate
Removal
Products
Fractionation
Boiler
Burner
Reactor
Resid
Hot Coke
Cold Coke
Air
Steam
Flue Gas
Gas
Products
Figure 6.8 Block diagram of fluid coking
Thermal Cracking and Coking 145
Example E6.5
A vacuum residue with Conradson carbon of (wt% CCR ¼ 15) is fed into a
fluid coker at a rate of 200,000 lb/h, API ¼ 8.5 and with a sulphur content of
3.0 wt%. Find the amounts of products and their sulphur content using the yield
guidelines given in Figure 6.9.
Solution:
The solution of the example is summarized in Table E6.5.
*Fuel Oil Equivalent Barrels
Burner
Reactor/
Scrubber/
Fractionator
• Refinery Fuel/Sales
• H
2
Manufacture
• LPG Recovery
• Alkylation Feed
• Gasoline Blending
• Refinery Fuel/Sales
• Petrochemical Feed
• Middle Distillate
• FCC/Hydrocracking Feed
• Petrochemical Feed
• FCC/Hydrocracking Feed
• Fuel Oil
• Steam Generation
• Power Generation
• Emission Control/Stack
• Power Generation
• Cement Industry Fuel
• Partial Oxidation Feed
Gross Coke
(27 wt%)
Arab light
VR
22 wt% CCR
Coker Naphtha
(C
5
-430⬚F)
Lt. Coker Gas Oil
(430-650°F)
Hvy. Coker Gas Oil
(650-975°F)
Fuel gas
Net
Coke
Reactor Gas, C
4
−
11 wt%
15-20 wt%
12-14 wt%
35-36 wt%
21 wt%
0.02 FOEB*
bbl Feed
Yield
On Feed
End Us
es
Figure 6.9 Yield and end uses of fluid coker process (Hammond et al., 2003)
Table E6.5 Summary of fluid coking summary results
Feed rate 200,000 lb/h
CCR 15 wt%
S
f
3 wt%
Products wt% lb/h
Gases 11 22,000
Naphtha 18 36,000
Light gas oil 14 28,000
Heavy gas oil 36 72,000
Coke 21 42,000
Total 100 200,000
146 Chapter 6
6.7. Flexicoking
The flexicoking process is a development of the fluid coking process
where only 2 wt% of coke is produced, thus most of the coke is used to heat
the feed. A fluidized bed is added to the process which acts as a gasifier in which
steam and air are injected to produce synthesis gas called Low Btu Gas (LBG)
as shown in Figure 6.10. The gasifier which operates at 816–982
C
(1500–1800
F) produces hot coke which remains after combustion. This
coke flows into the middle vessel, which acts as a heat exchanger to heat cold
coke coming from the reactor. It operates at 593
C(1100
F). The operation
of the reactor is the same as fluid coker. The net coke is purged in the heater.
A block diagram of the process is shown in Figure 6.11 (Hammond et al.,
2003).
In the oxidation zone of the gasifier, the following reactions take place
very rapidly:
C þ 0:5O
2
! CO ð6:23Þ
CO þ 0:5O
2
! CO
2
ð6:24Þ
In the reduction zone, the following reactions take place slowly:
C þ H
2
O ! CO þ H
2
ð6:25Þ
H
2
O þ CO ! CO
2
þ H
2
ð6:26Þ
6.7.1. Yield Correlations for Flexicoking
The yield correlations for flexicoking are based on the Conradson carbon
content of the vacuum residue (CCR, wt%), its API gravity and sulphur
content (S
f
). Data compiled by Maples (1993) are correlated to express
weight percent yields.
Gas wt% ¼ 0:171943 CCR wt% þ 5:206667 ð6:27Þ
Gasoline wt% ¼0:115234 CCR wt% þ 18:594587 ð6:28Þ
Coke wt% ¼ 1:037233 CCR wt% þ 1:875742 ð6:29Þ
Gas oil wt% ¼ 100 Gas wt% Gasoline wt% Coke wt% ð6:30Þ
Gas composition:
C
4
wt% ¼0:028627 CCR wt% þ 3:200754 ð6:31Þ
C
2
wt% ¼ 0:647791 ½Gas wt% C
4
wt%þ0:456001 ð6:32Þ
C
3
wt% ¼ Gas wt% C
4
wt% C
2
wt% ð6:33Þ
Thermal Cracking and Coking 147
Heavy Coker Gas Oil
(Cat Feed)
1100⬚F
1500⬚F
To
1800⬚F
950⬚F
To
1050⬚F
Reactor
Steam
Air
Purge
Coke
Scrubber
Fluidized Bed
Heater
Feed
HCGO
Product
Fractionator
Heater Overhead
LBG to Scrubbing
Gasifier
Gas to Light Ends
Coker Naphtha
Light Coker Gas Oil
(Distillate)
Recycled
Feed
Steam
Figure 6.10 Flexicoking process
148 Chapter 6
Sulphur distribution in products:
S wt% in Gasoline ¼ 0:193461S
f
ð6:34Þ
S wt% in Gas oil ¼ 0:91482S
f
þ 0:16921 ð6:35Þ
S wt% in Coke ¼ 1:399667S
f
þ 0:18691 ð6:36Þ
S in Gas ¼ S in Feed S in Gasoline S in Gas oil S in Coke
ð6:37Þ
Gravity of flexicoker feed and gas oil
Feed API
f
¼ 0:5 CCR wt% þ 0:932644 ð6:38Þ
Gas oil API ¼ 1:264942 API
f
þ 0:506675 CCR wt% 0:79976
ð6:39Þ
Example E6.6
Resolve example E6.1 for the case of flexicoking.
Solution:
The result of this example for flexicoking is given in Table E6.6.
LBG
Clean Up
Products
Fractionation
Heater
Reactor
Resid
Hot Coke
Cold Coke
Net Coke
Air
Steam
Sweet LBG
Liquid
Products
Gas
Products
Gasifier
Coke Fines
Hot LBG
Gasifier feed Coke
Coke
Steam
Product
Upgrading
LBG Cooling
& Fines
Removal
Figure 6.11 Blockdiagram for flexicoking
Thermal Cracking and Coking 149
Table E6.6 Flexicoking example
Feed rate ¼ 200,000 lb/h
Flexicoking
CCR ¼ 15 wt%
S
f
¼ 3wt%
Feed API ¼ 8.5
Products yield wt% lb/h
Gas wt% ¼ (0.171943 CCR% þ 5.206667) 7.79 15,572
Gasoline wt% ¼ (0.115234 CCR% þ 18.594587) 16.87 33,732
Coke wt% ¼ (1.037233 CCR% þ 1.875742) 17.43 34,868
Gas oil wt% ¼ (100 Gas% Gasoline% Coke%) 57.91 115,828
100.00 200,000
Gases
Gas
VR
Gas Oil
Gasoline
Coke
34,868
15,572
33,732
115,828
C
4
wt% ¼ (0.028627 CCR% þ 3.200754) 2.77 5543
C
2
wt% ¼ (0.647791 (Gas% C
4
%) þ 0.456001) 3.70 7409
C
3
wt% ¼ (Gas% C
4
% C
2
%) 1.31 2620
7.79 15,572
Sulphur in flexicoker products
S in Gasoline ¼ (0.193461 S
f
) 0.58 196
S in Gas oil ¼ (0.91482 S
f
þ 0.16921) 2.91 3375
S in Coke ¼ (1.399667 S
f
þ 0.18691) 4.39 1529
S Gas ¼ (S Feed S in Gasoline S in Gas oil S in Coke) 0.06 900
6000
Gravity of flexicoker products API SG
Gas oil API ¼ (1.264942 API
f
þ 0.506675 CCR% 0.79976) 17.6 0.95
Feed API ¼ (0.5 CCR% þ 0.932644) 8.43 1.00
150 Chapter 6