Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining
Подождите немного. Документ загружается.
6.4.7. Process Description
There are two types of visbreakers: coil visbreaking, in which thermal
cracking occurs in the coil of the furnace, and the soak visbreaker, in
which cracking occurs in a soak drum.
6.4.7.1. Coil Visbreaker
Vacuum or atmospheric residue feedstock is heated and then mildly cracked
in the visbreaker furnace. Reaction temperatures range from 850 to 900
F
(450 to 480
C), and operating pressures vary from as low as 3 bar to as high
as 10 bar. As shown in Figure 6.2A, coil furnace visbreaking is used and the
visbroken products are immediately quenched to stop the cracking reaction.
The quenching step is essential to prevent coking in the fractionation
tower. The gas oil and the visbreaker residue are most commonly used as
quenching streams (Parakash, 2003).
435°C
455°C
10 bar
Gases
Gasoline
Gas Oil
Residue
Fractionation
Furnace
Preheater
Soaking
reactor
Vacuum
Residue
Vacuum
Residue
485°C
3 bar
Gases
Gasoline
Gas Oil
Residue
Fractionation
Furnace
Preheater
A
B
Figure 6.2 (A) Coil type visbreaker. (B) Soaker type visbreaker (Leprince, 2001)
Thermal Cracking and Coking 131
After quenching, the effluent is directed to the lower section of the fraction-
ator where it is flashed. The fractionator separates the products into gas, gasoline,
gas oil and visbreaker tar (residu e). The gas oil withdrawn from the fractionator is
steam-stripped to remove volatile components and then blended with the vis-
breaker bottoms or routed for further processing, such as hydrotreating, catalytic
cracking or hydrocracking. The un-stabilized naphtha and fuel gas, recovered as
overhead products, are treated and then used as feedstock for catalytic reforming,
blended into finished products or sent to the fuel system. The visbreaker bottoms
are withdrawn from the fractionator, heat exchanged with the visbreaker
feedstock, mixed with stripped gas oil (optional) and routed to storage.
6.4.7.2. Soaker Visbreaker
The process scheme described above is usually referred to as furnace or coil
cracking. Some visbreakers employ a soaker between the visbreaker furnace
and the quenching step, similar to the conventional thermal cracking
processes. This type of operation is termed soaker cracking as shown in
Figure 6.2B (Leprince, 2001).
According to the fundamentals of thermal cracking technology, the
conversion is mainly a function of two operating parameters, temperature
and residence time. Coil cracking is described as a high temperature, short
residence time route whereas soaker cracking is a low temperature,
long residence time route. The yields achieved by both options are in
principle the same, as are also the properties of the products.
Both process configurations have their advantages and applications. Coil
cracking yields a slightly more stable visbreaker products, which are impor-
tant for some feedstocks and applications. It is generally more flexible and
allows the production of heavy cuts, boiling in the vacuum gas oil range.
Soaker cracking usually requires less capital investment, consumes less fuel
and has longer on-stream times. Table 6.6 shows the relation between
temperature and soaking time for equal conversion.
Example E6.2
It is required to decoke a visbreaker coil with an inside diameter ¼ 9 cm and
700 m long with a coke layer of 0.35 cm thickness. This is done in two steps:
(a) Air is introduced to combust the coke layer whose density is 1202 Kg/m
3
.
Table 6.6 Soaking time–temperature relationship for
equal conversion
Time (min)
C
F
1 485 905
2 470 878
4 455 850
8 440 825
132 Chapter 6
(b) Steam at 450
C and flow rate of 1000 Kg/hr is introduced to the coil to
remove debris and cleaning-up. The exit temperature is 700
C. Coke
contains 92 wt% carbon and 8 wt% sulphur.
For how long should steam be switched on, in hours?
Data: specific heat of steam ¼ 2.13 kJ/(Kg
C) heat of carbon combustion ¼
32,770 kJ/Kg, heat of sulphur combustion ¼ 9300 kJ/Kg
Solution:
Volume of coke layer ¼ 3.14(0.09) (700) (0.0035) ¼ 0.6924 m
3
Weight of coke ¼ 0.6924(1202) ¼ 832.3 Kg
Heat of carbon combustion in coke ¼ 832.2(0.92) (32,770) ¼ 25 10
6
kJ
Heat of sulphur combustion in coke ¼ 832.2(0.08)(9300) ¼ 0.69 10
6
kJ
Total heat ¼ 25.6910
6
kJ
Rate of heat gained by steam ¼ m
s
C
s
(T
out
T
in
)
1000(2.13) (700–450) ¼ 0.5310
6
kJ/h
Time for switching steam ¼ 25.69 10
6
/0.53 10
6
¼ 48.5 h
6.5. Delayed Coking
Delayed coking is a type of thermal cracking in which the heat required to
complete the coking reactions is supplied by a furnace, while coking itself
takes place in drums operating continuously on a 24 h filling and 24 h
emptying cycles. The process minimizes residence time in the furnace,
while sufficient time is allowed in the drums where coking takes place
(hence the term ‘‘delayed coking.’’) Coke is rejected in the drums, thus
increasing the H/C ratio in the rest of the products. However, these products
are still unstable and unsaturated, and require further hydrogenation.
The feed to coker is usually vacuum residue which is high on asphal-
tenes, resins, aromatics, sulphur and metals. The deposited coke contains
most of the asphaltenes, sulphur, and metals present in the feed, and the
products are unsaturated gases (olefins) and highly aromatic liquids.
6.5.1. Role of Delayed Coker
The feed to the delayed coker can be any undesirable heavy stream containing
high metal content. A common feed is vacuum residue but it can also accept
fluid catalytic cracking (Chapter 8) slurry and visbreaking tar (residues).
The products from the coker are unsaturated gases (C
1
–C
4
), olefins
(C
¼
2
C
¼
4
)andiC
4
. The olefins are very desirable feedstocks to the petro-
chemical industry. Isobutane and olefins can be sent to alkaylation units
(Chapter 10), and the C
3
/C
4
gases are sent to the LPG plant. The coker is
Thermal Cracking and Coking 133
the only unit in the refinery which produces coke. The highly aromatic
naphtha does not need reforming and is sent to the gasoline pool. Light gas oil
(LCO) is hydrotreated and sent to the kerosene pool. Heavy coker gas oil is
sent to the FCC for further cracking. The role of the delayed coker is to
handle very heavy undesirable streams and to produce desirable refinery
products as shown in Figure 6.3.
The overall refining yield of light products increases as a result of coke
removal.
6.5.2. Process Description
A schematic flow diagram of the delayed coking is shown in Figure 6.4. The
process includes a furnace, two coke drums, fractionator and stripping
section. Vacuum residue enters the bottom of the flash zone in the
HDS
Kerosene
Diesel
LPG
Fuel
Gas
LSR
HSR
AR
LVG O
V
D
Un-Sat.
LPG
Naph.
HAGO
Gasoline
Reformer
HDS
Sat
LPG
C
O
K
E
R
F
C
C
VR
Naph.
Light
Gas Oil
Heavy
Gas Oil
Light
Cycle Oil
Crude
Oil
D
I
S
T
I
L
L
A
T
I
O
N
Figure 6.3 Role of delayed coker in the refinery
134 Chapter 6
Heavy cycle oil
Fuel
Gases
Coker naphtha
Steam
Boiler feed water
Coker fractionator
Coker drum
Cutting
Filling
Distillate stripper
Overhead accumulator
Heated feed
VR feed
Sour water
Light cycle oil
Coke
Steam
Figure 6.4 Delayed coker unit
Thermal Cracking and Coking 135
distillation column or just below the gas oil tray. Fractions lighter than heavy
gas oil are flashed off and the remaining oil are fed to the coking furnace.
Steam is injected in the furnace to prevent premature coking. The feed to the
coker drums is heated to just above 482
C (900
F). The liquid–vapour
mixture leaving the furnace passes to one of the coking drum. Coke is deposited
in this drum for 24 h period while the other drum is being decoked and cleaned.
Hot vapours from the coke drum are quenched by the liquid feed, thus
preventing any significant amount of coke formation in the fractionator and
simultaneously condensing a portion of the heavy ends which are then recycled.
Vapours from the top of the coke drum are returned to the bottom of the
fractionator. These vapours consist of steam and the products of the thermal
cracking reaction (gas, naphtha and gas oils). The vapours flow up through the
quench trays of the fractionator. Above the fresh feed entry in the fractionator,
there are usually two or three additional trays below the gas oil drawoff tray.
These trays are refluxed with partially cooled gas oil to provide fine trim control
of the gas oil end point and to minimize entrainment of any fresh feed liquid or
recycle liquid into the gas oil product. The gas oil side draw is a conventional
configuration employing a six-to-eight-tray stripper with steam introduced
under the bottom tray for vaporization of light ends to control the initial boiling
point (IBP) of the gas oil.
Steam and vaporized light ends are returned from the top of the gas oil
stripper to the fractionator, one or two trays above the draw tray. A pump-
around reflux system is provided at the draw tray to recover heat at a high-
temperature level and minimize the low-temperature level heat removed by
the overhead condenser. This low-temperature level heat cannot normally be
recovered by heat exchange and is rejected to the atmosphere through a water
cooling tower or aerial coolers. Eight to ten trays are generally used between
the gas oil draw and the naphtha draw or column top. If a naphtha side draw is
employed, additional trays are required above the naphtha draw tray.
A control valve system directs the feed to enter one of the drums, where
the reactions take place and coke is deposited on the drum walls, and the
products flow back to the distillation column. In this case, the drum is in the
‘‘filling’’ mode. At the same time, the other drum is cut off from the rest of
the system while the coke is being removed. The drum in this case is in the
‘‘cutting’’ mode.
6.5.3. Delayed Coking Variables
There are three classes of variables affecting coking. They are related to
process operating variables, feedstock characterization and engineering vari-
ables as shown in Table 6.7. Temperature is used to control the severity of
coking. In delayed coking, the temperature controls the quality of the coke
produced. High temperature will remove more volatile materials. Coke yield
decreases as temperature increases. If the furnace temperature is high this
might lead to coke formation in the furnace. A low inlet furnace temperature
136 Chapter 6
will lead to incomplete coking. Short cycle time will increase capacity but will
give lower amounts of liquid products and will shorten drum lifetime.
Increasing pressure will increase coke formation and slightly increase gas
yield. However, refinery economics require operating at minimum coke
formation. New units are built to work at 1 bar gauge (15 psig), while
existing units work at 2.4 bar gauge (35 psig). In a case of production of
needle coke, a pressure of 150 psig is required. Recycle ratio is used to
control the endpoint of the coker gas oil. It has the same effect as pressure.
Units are operating at a recycle ratio as low as 3%.
Feedstock variables are the characterization factor and the Conradson
carbon which affect yield production. Sulphur and metal content are usually
retained in the coke produced. Engineering variables also affect the process
performance. These include mode of operation, capacity, coke removal and
handling equipment.
6.5.4. Types of Coke and their Properties
Coke amount can be upto 30 wt% in delayed coking. It is produced as green
coke which requires calcination to remove the volatiles as fuel product.
Green coke can also be used as fuel. The most common types of coke are:
Sponge coke
Sponge coke is named for its sponge-like appearance. It is produced
from feeds having low to moderate asphaltene content.
Needle coke
This coke has a needle-like structure and is made from feed having no
asphaltene contents such as decant oils from FCC. It is used to make
expensive graphite electrodes for the steel industry.
Shot coke
This coke is an undesirable pr oduct and is produced when feedsto ck
asphaltene content is high and/or when the drum temperature is too high.
Discrete mini-balls of 0.1–0.2 in. (2–5 cm) in diameter are produced.
There are some methods of eliminating shot coke formation including
adding aromatic fee d, such as F CC decant oi l, decreas ing te mperatu re,
increasing pressure and the recycle ratio. Summary of coke types, proper-
ties and end uses is shown in Table 6.8.
Table 6.7 Delayed coking variables
Process variables Feedstock variables Engineering variables
Cycle time
Temperature
Characterization factor
Conradson carbon
Mode of operation
Pressure Sulphur content Capacity
Recycle ratio Metal content,
characterization
Equipment used for coke
removal and handling
Thermal Cracking and Coking 137
6.5.5. Coking and Decoking Operation
Pilot plant data have shown that coke formation in the drums takes the
‘‘channel branching theory’’ in which channel formation allows further
gas flow while coke is formed progressively. In commercial operat ions this
can take up to 24 h. The decoking operation involves drilling a vertical
hole in coke after cooling using a mechanical boring tool (shown in
Figure 6.5A ). Further coke removal is carried out by using a hydraulic
cutting tool (Figure 6.5B),whereajetofwateriscapableoftheremovalof
the remaining coke from the drum. However, this requires using a great
amount of water which has to be treated later on (Feintuch and Negin,
2003).
The progression of the decoking operation is shown in Figure 6.6
(Feintuch and Negin, 2003). The decoking cycle involves switching
Table 6.8 Coke characterization and uses from delayed coker
Type of
coke
Operating
condition
Feed
characterization Coke property
End use as
calcinated
coke
Sponge Reflux ratio
>35%
Operating
pressure
2–4 bar
Low metal
Low S
Tar residue
FCC heavy dist
Low to
moderate
asphaltene
M < 200
S < 2.5%
High density
>780
HGI
a
100
Anodes for
aluminium
industry
Shot Low pressure
Low reflux
ratio
Large drums
High S
High metal
Low asphaltene
High S and
metal
Low HGI<50
Low surface
area
Fuel (green)
Needle Pressure > 4
bar
Reflux ratio
¼ 60–100%
to maximize
coke yield
High
temperature
to reduce
volatile
material
High aromatic
content
Tars, FCC
decant
Low S <0.5
wt%
Low ash
<0.1 wt%
No asphaltene
Crystalline
structure
Small needles
of high
conductivity
Graphite
electrodes
a
HGI ¼ Hard grove grindability index
138 Chapter 6
AB
Figure 6.5 (A) Mechanical first boring tool. (B) Final hydraulic tool
Figure 6.6 Steps of decoking operation
Thermal Cracking and Coking 139
the drums, cooling by steam, draining in the coke, warming up the drum and
leaving spare time for contingency. Meanwhile, the other drum is under
coking reactions. A typical time cycle in delayed cooking is shown in
Table 6.9.
6.5.6. Delayed Coker Yield Prediction
Estimation of product yields can be carried out using correlations based on
the weight percent of Conradson carbon residue (wt% CCR) in the
vacuum residue (Gary and Handwerk, 2001).
GasðC
4
Þwt% ¼ 7:8 þ 0:144 ðwt% CCRÞð6:18Þ
Naphtha wt% ¼ 11:29 þ 0:343 ðwt% CCRÞð6:19Þ
Coke wt% ¼ 1:6 ðwt% CCRÞð6:20Þ
Gas oil wt% ¼ 100 Gas wt% Naphtha wt% Coke wt% ð6:21Þ
The naphtha can be split in light naphtha (LN) and heavy naphtha (HN).
The split in wt% is 33.22 and 66.78, respectively, assuming corresponding
gravities of 65 API and 50 API, also respectively.
The gas oil (GO) can be split also into light cycle gas oil (LCO) and
heavy cycle gas oil (HCO). The spilt in wt% is 64.5 and 35.5, respectively,
and the corresponding gravities are 30 API and 13 API.
Typical sulphur distribution in the products for delayed coking is pre-
sented in Table 6.10 (Gary and Handwerk, 2001).
Table 6.9 Time cycle for delayed coking
Operation Ti me (h)
Coking 24
Decoking 24
Switching 0.5
Steam cooling 6.0
Decoking, drain 7.0
Warm-up 9.0
Contingency time 1.5
Table 6.10 Delayed coker sulphur distribution based on the amount of sulphur
in feed
Products Gas LN HN LCO HCO Coke
S (wt%) 30 1.7 3.3 15.4 19.6 30
140 Chapter 6