World petroleum refining capacity has reached about 4100 million
metric tons per annum (MMTPA) (Swaty, 2005), inclusive of India's
refining capacity of about 120 MMTPA (Goyal, 2006). Of late, the
petroleum refining industry is facing many new challenges to remain
competitive in the world fuels market. One of the major challenges is to
fully utilize the existing petroleum resources, while protecting our
environment. This very fact has led to the emphasis on the
bottom-of-the-barrel residue upgrading. Furthermore, the crudes are
getting heavier and demand for light, clean fuels is increasing, leaving
the refiners with no option but to expand their residue upgrading
capacity (Elliott, 1992; Bansal et al., 1994; Henderson et al., 2005).
As the price differential between light crudes and heavy crudes is
widening, a trend of processing heavier crudes is catching up fast in
the refineries. As a result of this, refiners are getting burdened with
heavy residues that are subsequently obtained by processing heavy
crudes. Heavy crudes ([less than or equal to] 20[degrees]API) yield
large amount of residual fractions such as atmospheric residue (AR,
initial boiling point, IBP > 343[degrees]C) and vacuum residue (VR,
IBP > 500[degrees]C) as shown in Figure 1 (Boduszynski, 2002). The
processes that convert these heavy ends into lighter, more value-added
products are termed as bottom-of-the-barrel conversion processes or
residue upgrading processes.
Among the various processes available, the delayed coking process
is a long-time workhorse as regards the bottom-of-the-barrel upgrading
(Schulman et al., 1993). World coking capacity has reached about 210
MMTPA (Swaty, 2005), comprising India's coking capacity of about 9
MMTPA. In the delayed coking process, the aim is an economical
conversion of residual feedstocks, especially vacuum residues (short
residues) to lighter, more value-added products and if possible, to
produce a coke material of desired quality. The inherent flexibility of
the delayed coking process to handle different feedstocks promises the
refiner a solution to the problem of decreasing residual fuel demand and
takes advantage of the attractive economics of upgrading it to more
valuable lighter products (Mekler and Brooks, 1959; Stormont, 1969;
Gambro et al., 1969; DeBiase and Elliott, 1982). Conventionally,
processes such as coking, visbreaking, vacuum distillation, solvent
deasphalting or residual cracking have effected the upgrading of
residual feedstocks. All the aforementioned processes result in less
residual fuel. However, except for coking, they still produce a liquid
bottoms product that requires tankage and cutter stock (Rose, 1971).
Therefore, a refinery with delayed coker is said to be "zero resid
refinery," which itself spells the importance of delayed coking in
the refinery set-up. This is one of the major advantages of the delayed
[FIGURE 1 OMITTED]
Another advantage is that delayed coking offers a potential means
of converting a variety of materials to valuable motor fuels, often
while eliminating a low-value or unmarketable refinery stream, or
eliminating a stream that not only is environmentally unfriendly, but
also involves a disposal cost (Christman, 1999). Yet another advantage
of delayed coking is that it not only complements other, more capital
intensive, bottom-of-the-barrel conversion technologies, but also works
very well as the primary upgrader in the refinery (Sloan et al., 1992;
Bansal et al., 1994). Delayed coking can also be used to produce needle
coke, a specialty product, if appropriate feedstocks, design techniques
and operating parameters are applied (Sarkar, 1998).
The worldwide trend of processing heavy feedstocks in the delayed
cokers for getting maximum yield of liquid products has led to the
production of fuel grade coke that contains large amounts of sulphur and
metals. Currently, about 65% of the petroleum coke produced is fuel
grade coke (Shen et al., 1998). Once considered a waste by-product, fuel
grade petroleum coke is now an important fuel for the cement industry
and competes successfully with coal in several industrial fuel
applications such as utilities and cogeneration facilities. Advancements
in circulating fluidized bed (CFB) boiler design have enabled the
exclusive use of coke as a fuel to steam power generators and
cogenerators (Elliott, 1992). Petroleum coke can also be successfully
employed for the production of synthesis gas via gasification route
(Furimsky, 1999). Since the fundamental objective of the refiners who
process heavy residues to the delayed cokers is to maximize liquid
product yield, any value added to the coking process by selling the fuel
grade coke is a bonus for the refiners (Elliott, 1992).
NECESSITY OF RESIDUE UPGRADING
Upgrading heavy residuals or bottom-of-the-barrel has always been
the goal of the refiner to achieve value addition by producing lighter,
more value-added products out of the residual feedstock (Rose, 1971).
The refinery scene is changing significantly all over the world and it
is driven by environmentally obligated modifications for gasoline and
diesel quality. As a result of this, there is an increased need for
capacity and flexibility in conversion technology and there is a
dramatic increase in refinery hydrogen demand (Schulman et al., 1993).
While past emphasis has been on increasing gasoline production, of late,
the middle distillates are in great demand. This situation of demand and
supply, according to Sloan (1994), calls for an increase in flexibility
not only for gas oil boiling range materials, but also for the
bottom-of-the-barrel upgrading. The demand for fuel oil is declining as
the user industry is switching over to other alternate sources of energy
like liquefied natural gas (LNG). The reserves of conventional (light)
crude oil are depleting and there is a gradual but sure decline in crude
oil quality. Therefore, there is a dire need to fully utilize the
limited petroleum resources (Schulman et al., 1993; Bansal et al., 1994;
Shen et al., 1998). Consequently, interest is focused on diverting the
crude's residual fraction from its traditional use as a heavy fuel
component to processes that either convert the residue into high-value
products or that provide additional feedstock for downstream conversion
units (Sarkar, 1998). The renewed interest in residue upgrading can be
attributed to the fact that, lately, ever-heavier crude oils are being
processed in the refineries (Christman, 1999), which subsequently
produce a significant amount of vacuum residue (may go up to 40 wt.%)
Indian refiners are equally concerned about upgrading the available
crude oil for refining, along with other international refiners in the
area of residue upgrading, to get more light distillates required for
transport fuels and also to provide the needs for the other concerned
industries using petroleum products such as fertilizer and
petrochemicals. The basic reason for giving extra attention to residue
upgrading is that India has less options than to import crude oil with
maximum percentage of residues (Sarkar, 1998).
QUALITATIVE DESCRIPTION OF DIFFERENT UPGRADING PROCESSES
Technologies for upgrading heavy feedstocks such as heavy oil,
bitumen and residua can be broadly divided into carbon rejection and
hydrogen addition processes. Carbon rejection redistributes hydrogen
among the various components, resulting in fractions with increased H/C
atomic ratios and fractions with lower H/C atomic ratios. On the other
hand, hydrogen addition processes involve the reaction of heavy
feedstock with an external source of hydrogen, which results in an
overall increase in H/C ratio. Within these broad ranges, all upgrading
technologies can be subdivided as follows:
1. Carbon rejection processes: visbreaking, steam cracking, fluid
catalytic cracking, and coking; 2. Separation processes: solvent
3. Hydrogen addition processes: hydrocracking, fixed bed catalytic
hydroconversion, ebullated catalytic bed hydroconversion,
hydrovisbreaking, hydropyrolysis, and donor solvent processes (Speight,
Table 1 outlines the comparison of different residue upgrading
processes. As can be seen from Table 1, the non-catalytic carbon
rejection processes score higher than other processes in simplicity and
operating costs and hence have large numbers of units in the world.
Table 2 shows the world residue processing capacity in different parts
of the world. It can be seen that a major portion of the petroleum
residue upgrading (about 63%) is met via thermal processes, viz.,
visbreaking and delayed coking.
There can be a brief classification of residue upgrading processes
under different headings, which have been commercially installed over
the years in the refineries. The classification can be as follows:
1. Separation processes: solvent deasphalting;
2. Catalytic process: residue fluidized catalytic cracking (RFCC);
3. Hydrogen-addition processes: residue hydrocracking;
4. Thermal conversion processes: visbreaking, delayed coking, fluid
coking and flexi coking.
The solvent deasphalting process involves physical separation and
there is no chemical conversion. The limitations of this process are
high energy costs and the limited uses of deasphalter tar. Current
interest in deasphalting is greatest in areas of the world where demand
for motor fuel is low. This suggests that, in the long run, solvent
deasphalting, as a stand-alone residue upgrading process, will be of
less interest worldwide (Christman, 1999).
Residue fluidized catalytic cracking (RFCC) involves a vapour phase
catalytic cracking reaction. The heavier and more contaminated
atmospheric and vacuum residues cannot vaporize and eventually end up
getting deposited on the surface of the catalyst and tend to increase
the production of coke and deactivate the catalyst. Thus, residue
fluidized catalytic cracking (RFCC) is limited in terms of its
applicability to process relatively low metal and low asphaltene feeds
(Shen et al., 1998).
Residue hydroprocessing can process a little heavier and high metal
content feedstocks (Conradson carbon residue (CCR) up to 10 wt.% and
metals up to 100-150 ppm) with the aid of new processes such as
Chevron's Onstream Catalyst Replacement (OCR) and Shell's
HYCON unit. Thus, residue hydroprocessing has a distinct advantage over
residue fluidized catalytic cracking as far as processing of heavier
feeds is concerned. However, it may be pointed out that although residue
hydroprocessing can produce high-quality products and meet the
requirement of the reformulated gasoline and diesel in terms of low
aromatic and low sulphur, hydrogen resource and high investment limit
its application (Shen et al., 1998). Thus, heavy residues containing
more than 10 wt.% CCR and 150 ppm of metals can only be processed by
using non-catalytic carbon rejection processes as illustrated in Figure
2 (Philips and Liu, 2002).
Thermal conversion processes can handle any kind of feedstock, even
extra heavy vacuum residues. Visbreaking is the least costly of the
residue upgrading options. However, its major product, fuel oil has a
dwindling market and provides low margins. The yield of gas and gasoline
together is generally limited to a maximum of about 7 wt.% (Zuba, 1998)
as the cracking reactions are arrested so that asphaltenes flocculation
does not take place and in turn a stable fuel oil is obtained. Like
deasphalting, current interest in visbreaking is in those areas where
motor fuel demand is relatively low. When the motor fuel demand will
increase in these areas and refiners will have no option but to process
heavier crudes, delayed coking will be more widely used (Christman,
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
As regards the status of the delayed coking process in the area of
residue upgrading, Christman (1999) reported that by using the carbon
rejection technologies, 17 residue upgrading projects were in the
construction phase worldwide, 14 in the engineering phase, and another 5
in the planning phase. Out of these 36 projects, two thirds of the
projects were getting along with the delayed coking process. This
indicates the growing interest in the delayed coking process as the
preferred petroleum residue upgrading route.
REACTIONS AND REACTOR ENGINEERING
Delayed coking is a severe form of thermal cracking process that
falls in the temperature range of 450-470[degrees]C. The name
"delayed" comes from the fact that cracking reactions are
given sufficient (extended) time to proceed to completion in coke drums
that are specially designed to accumulate the coke and not in the heater
tubes, which otherwise would have led to the premature shutdown of the
unit. Sufficient heat is introduced in the heater tubes for complete
destructive distillation, but the reduction to coke does not occur
unless and until the residue enters the coke drum. In other words, it
can be said that the heating is done in a furnace to initiate cracking
and the actual reactions are complemented and completed in the huge and
tall coke drums. The first commercial delayed coker began operations at
the Whiting refinery of Standard Oil Co. (Indiana) in 1930 (Kasch and
Thiele, 1956). Foster Wheeler and ConocoPhillips are the major
contributors with regard to the design, engineering and construction of
delayed coker units. Foster Wheeler has designed, engineered and built
more than 60 delayed coking units ranging in capacity from 50 to 3300
tons per day. ConocoPhillips has worldwide delayed coking operations and
produces over 2 million tons per year of combined fuel-grade and
high-quality petroleum coke. According to the report of Sloan et al. in
1992, Kellogg had designed and constructed about one third of the
world's delayed coking capacity. Lummus and Flour are the other
licensors of delayed coking process having relatively lesser market
Process Description of Delayed Coking
Figure 3 shows the process flow diagram of a delayed coking unit.
The feedstock is fed directly to the bottom of the fractionator where it
is heated and lighter fractions are removed as side streams. The
fractionator bottoms, including a recycle stream of heavy product, are
then heated in a furnace whose outlet temperature varies from 480 to
515[degrees]C. The heated feedstock enters one of the pair of coking
drums where the cracking reactions continue. The cracked products leave
as overheads, and coke deposits form on the inner surface of the drum.
For continuous operation, two coke drums are used; while one is on
stream, the other is being cleaned. The temperature in the coke drum
ranges from 415 to 465[degrees]C and the pressure from 0.1MPa to 0.4
MPa. Overhead products go to the fractionator, where naphtha and heating
oil fractions are recovered. The heavy recycle material is combined with
preheated fresh feed and returned to the reactor. The coke drum is
usually on stream for about 24 h before getting filled with porous coke.
Figure 4 shows a cross-section of a coke drum, and demonstrates how the
coke is formed during the delayed coking operation. The coke is formed
at the rate of about 0.6 m per hour and progresses during the 24-hour
cycle. The material at the bottom is fully carbonized and develops a
porous structure through which gases and liquid can pass. The top layer
is not fully carbonized until it is subjected to heat for a prolonged
time. At the very top some foam occurs, but subsides during the steaming
and cooling cycle. It is important in filling the coke drum to avoid
carryover of foam or pitchy material into the vapour lines. Level
indicators are handy for detecting the position of the liquid or foam in
the drum. These are operated by transmitting a beam from a radioactive
source to an instrument mounted near the top of the drum (Nelson, 1970).
[FIGURE 4 OMITTED]
The following procedure is practised to remove the coke:
1. The coke deposit is cooled with water;
2. One of the heads of the coking drum is removed to permit the
drilling of a hole through the centre of the deposit;
3. A hydraulic cutting device, which uses multiple high-pressure
water jets, is inserted into the hole and the wet coke is removed from
the drum (Speight, 2000).
Feedstock Characterization Affecting Coking
As far as the feedstock for delayed coking is concerned, heavy
residues such as vacuum residue or occasionally atmospheric residue are
most commonly fed to the delayed cokers. However, there are many
feedstocks that have been fed to the delayed cokers over the years,
which include gilsonite (Anon., 1956), lignite pitch (Berber et al.,
1968), coal tar pitch (Gambro et al., 1969), refinery hazardous wastes,
and used plastics (Christman, 1999). For special applications in which
high-quality needle coke is desired, certain highly aromatic heavy oils
or blends of such heavy oils have also been processed (Acciarri and
[FIGURE 5 OMITTED]
Vacuum residue, which is by far the most common feedstock to
delayed coking unit (Rose, 1971; DeBiase and Elliott, 1982), is composed
of four fractions, viz., saturates, aromatics, resins and asphaltenes. A
brief description of these is given below:
These components are found to have an average carbon number in the
range of [C.sub.38-50] with relatively low heteroatom content. The
structural study shows that it consists of long alkyl chains with few or
negligible naphthenic and aromatic rings. The micro-carbon residue (MCR)
value (coking tendency) reported is almost 0 wt.%, which indicates that
these fractions are completely volatile and cannot directly yield coke.
The aromatic fraction has a slightly higher molecular weight (600
to 750) than saturates with an average carbon number in the range of
[C.sub.41-53.] These are simple structures (Jacob, 1971) relative to
resins and asphaltenes, having low heteroatom content and an MCR value
of about 3.7 wt.%.
These are viscous, tacky and volatile enough to be distilled with
hydrocarbons. Structurally, resins consist of an appreciable amount of
aromatic carbon content (40-53%) with intermediate paraffin chain length
on naphthenic structures and aromatic rings, and about two thirds of its
aromatic carbon atoms are non-bridged. The resin fraction acts as the
dispersant for the asphaltenic component in the maltene phase (Di Carlo
and Janis, 1992). Figure 5 shows the hypothetical structures of resin
from different sources of crude oil. Figure 5a shows the hypothetical
structure from American crude oil and Figure 5b shows the hypothetical
structure from a Turkish bitumen. As can be seen from Figures 5a and 5b,
the structures associated with resins from American crude oil and
Turkish bitumen are substantially different from each other in terms of
the linkages between the aromatic structures, heteroatoms, etc.
The asphaltene fraction of residues is a brown to black,
nonvolatile, amorphous substance, which exists as highly dispersed
colloid in the oil. Asphaltenes are insoluble in n-alkanes such as
n-pentane and n-heptane and soluble in benzene or toluene (IP-143 and
ASTM D-4124). Except for small amounts of hydrocarbons adsorbed at the
surface, asphaltenes are non-hydrocarbons made of nitrogen, oxygen,
sulphur, vanadium and nickel (Jacob, 1971). Asphaltenes resemble a stack
or cluster of naphthenic and aromatic molecules. Fused ring aromaticity,
small aliphatic side chains, and polar functional groups are the
structural features of these pseudo components. Figure 6 shows the
asphaltenes postulated as polymers of aromatic and naphthenic ring
systems. The heteroatoms (N, O, S) are present as carboxylic acids,
carbonyl, phenol, pyrroles, pyridine, thiol, thiophene and sulphones
while the metals (Ni and V) are mostly present as organometallic
compounds (Gawrys et al., 2002). Figure 7 shows the asphaltenes
postulated as polymers of aromatic and naphthenic ring systems
accompanied by heteroatoms. The molecular weight of asphaltenes lies in
between 3000 and 5000 (Jacob, 1971).
Figure 8 shows the average structural models for the asphaltene
fractions. Figures 8a and 8b show the stable asphaltenes, while Figures
8c and 8d show the unstable asphaltenes from different vacuum residues.
The asphaltenes and resins in an unstable feed are found to have low H/C
ratio, high aromaticity, highly condensed aromatic rings and less alkyl
and naphthenic substitution (Leon et al., 2000). Figures 9, 10 and 11
show the hypothetical asphaltenes structures from Venezuelan,
Californian and Iraqi crude oil, respectively. It can be seen that the
complexity of the asphaltene structures is more than the resin
structures. Also, the asphaltenes of different origins exhibit different
structures as in the case of resins. Thus, it can be said that the
physicochemical properties of the crude oil have a pronounced effect on
the structures associated with resins and asphaltenes.
The main characteristics of the feedstocks, which govern the
quality of distillate and quality of coke, are true boiling point (TBP)
cut point, carbon residue, sulphur, metals and paraffincity and
aromaticity of the feedstocks.
TBP cut point
For vacuum residues, a typical TBP cut point is 538[degrees]C, but
it may be lower or higher depending upon the crude. A TBP cut point of
343[degrees]C is typical for atmospheric residues. The TBP cut point
defines the concentration of CCR, sulphur, and metals in the feed and
thereby affects the product yield as well as product quality.
It is the most important characteristics in determining the
quantity of coke that will be produced from any particular feedstock.
The higher the CCR (ASTM D 189), the more the coke that will be
produced. In other words, it reveals the coke-forming propensity of the
feedstock. Since, in most cases, the objective of delayed coking is to
maximize the production of clean liquid products and minimize the
production of coke, the higher the CCR, more difficult it is to achieve
the same. In recent years as there has been a trend of processing
heavier crudes, values of CCR in excess of 20 wt.% and sometimes higher
than 30 wt.% are becoming more common (Meyers, 1997).
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Sulphur is an objectionable feed impurity, which tends to
concentrate in the coke and in the heavy liquid products. In a manner
similar to CCR, the sulphur content in the delayed coker feedstock has
gone up considerably because of the trend of processing heavy crudes.
This trend is going to continue as has been reported by DeBaise and
Elliott (1982), Bansal et al. (1994), and Christman (1999).
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Metals such as nickel and vanadium are objectionable feed
impurities that tend to be present in increasing quantities in heavier
feeds. It has been reported (DeBiase and Elliott, 1982) that essentially
all the metals in the feedstock concentrate in coke, thereby producing a
Table 3 shows the comparison of the delayed coker yields from
heavier feedstocks with those from conventional lighter feedstocks at
constant operating conditions. It can be seen that among the heavy
residues, the Maya residue yields more of naphtha (19.3 wt.%) as against
16.2 wt.% and 13.5 wt.% yield of naphtha, from Orinoco and heavy Arabian
residues, respectively. At the same time, the yield of coke (38.3 wt.%)
from the Maya residue can be found to be a little higher than the yield
of coke (37.9 wt.%) from the Orinoco residue. The yield of coke (33
wt.%) from the Arabian heavy residue can be found to be the lowest
amongst the three residues. This can be attributed to the fact that
Arabian heavy residue contains less asphaltenes as compared to the other
two residues, which is quite evident from the [degrees]API values of the
three residues shown in Table 3.
Figure 12 shows the variation in the feed properties as a function
of H/C atomic ratio and API gravity of the feed. From these figures, it
can be seen that: (1) the sulphur and nitrogen content increases with a
decrease in the H/C atomic ratio (Figures 12A and 12B), which indicate
the concentration of these hetero moieties in aromatic and unsaturated
compounds; and (2) H/C atomic ratio decreases with the decrease in API
gravity (Figure 12C). The reduction in the H/C ratio indicates an
increase in the content of unsaturated and polycondensed aromatic
compounds in the feedstock. This is again confirmed by the observed
trends for nC7 asphaltenes and CCR (Figures 12D and 12E).
Reaction Mechanism and Kinetics
Coke formation during the thermal cracking of residual feedstocks
indeed is an intriguing phenomenon. Over the past five decades many
researchers worldwide have put in their quality time and efforts to
learn the intricacies pertaining to coke formation during the cracking
of residual feedstocks.
Magaril and Aksenova (1968) have observed that the coke formation
begins only after an accumulation of considerable amount of asphaltenes.
The rate of coke formation in a given case is determined by the rate of
increase of the asphaltenes in the cracked residue. The process of
formation of a new solid phase is made up of the precipitation of
asphaltenes from the saturated solution and their subsequent
condensation. It was also observed that the time for the inception of
coke formation coincides with the time of maximum yield of asphaltenes
(Magaril et al., 1971).
According to Jacob (1971), two reaction mechanisms form coke at the
time-temperature conditions prevailing in the coke drums. In one, the
colloidal suspension of the asphaltene and resin compounds change
proportions resulting in the precipitation of the compounds to form a
highly cross-linked structure of amorphous coke. The compounds are also
subjected to a cleavage of the aliphatic groups following a first order
reaction kinetics. The carbon to hydrogen ratio increases from 8-10 in
the feed to about 20-24 in the coke with trapped residue. The second
mechanism involves the polymerization and condensation of aromatics and
grouping a large number of these compounds to such a degree that coke is
eventually formed. Hobson (1982) corroborated this fact and reported
that coke results from the extensive degradation of relatively heavy
molecules to form increasing quantities of light hydrocarbon gases (dry
gas) and polycyclic compounds having high carbon to hydrogen ratio. At
the temperatures and the pressures normally employed in thermal
cracking, the olefins formed from paraffin cracking tend to polymerize
into higher molecular weight products. As these molecules themselves
crack and repolymerize, their hydrogen content continues to decrease. In
addition they undergo condensation reactions with ring compounds. These
compounds are eventually converted into high molecular weight tar and
petroleum coke of low hydrogen-to-carbon ratio.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
According to DeBiase and Elliott (1982), three distinct phenomena
occur during the formation of coke in the coke drums of the industrial
delayed cokers, viz., partial vaporization and mild cracking of the feed
as it passes through the furnaces, cracking of the vapour as it passes
through the drum, and successive cracking and polymerization of the
heavy liquid trapped in the drum until it is converted to vapour and
coke. Savage et al. (1985; 1988) observed a coke induction period for
the thermolysis of asphaltenes from an off-shore California crude at
400[degrees]C. The induction period disappeared when the thermolysis
temperature was raised to 450[degrees]C. Because of the nature of the
chemical structure of asphaltene molecules present in heavy oils and
bitumen and their solubility characteristics, asphaltene molecules form
coke rapidly during thermal treatment (Wiehe, 1993). Coke formation
during heavy oil upgrading has been elucidated by Wiehe (1994) based on
the "pendant-core model." A further possible simplified form
of the pendant-core model and the possible set of reactions leading to
the formation of coke have been shown in Figure 13. It has been proposed
that the residue contains polymeric structures containing one building
block as a non-volatile "core," which represents more aromatic
parts of the molecules (lower H content) and which results into coke.
The second building block comprises "pendant" chains, which
represent more aliphatic parts of the molecules (higher H content) and
which gives volatile components of the polymeric structure.
When a residue is heated, the pendant chains separate to form
volatile liquid products. On further cracking, the weak links between
the cores break, giving rise to radicals that combine to form larger
units with high C/H ratio, which are true precursors for coke formation.
Taking a lead from the pendant-core model, Figure 14 has been
hypothesized, which elucidates the eventual formation of graphite-like
structure, formed by the clustering of aromatic free radicals and
distillate formation by the cracking of the aliphatic linkages that hold
the aromatic structures together. Wiehe and Liang (1996) reported a
microemulsion model for petroleum. Figure 15 shows a possible simplified
form of the microemulsion model reported by Wiehe and Liang (1996).
According to this model, asphaltenes are dispersed by the
surfactant-like property of resins that in turn are held in solution by
aromatics. The saturates act as non-solvents for asphaltenes. Stefani
(1995) (based on the commercial coker operation experience) reported
that the delayed coking mechanism is such that asphaltenes are the first
particles to appear in the hydrocarbon liquid and can act as
"seeds" for coke formation. Initially, these particles are
small and can easily entrain in the vapour stream during periods of high
coke drum velocity or in the presence of coke drum foam. This phenomenon
is especially prominent in the latter parts of the coking cycle when the
coke level is closest to the outlet nozzle and also during steam-out and
quench when coke drum velocities are much higher than normal.
A coherent summary of the published work can be restated as:
1. The residue comprises saturates, aromatics, resins and
asphaltenes. The coke formation is a consequence of a series of complex
reactions and proceeds from saturates to aromatics to resins to
asphaltenes and finally to coke;
2. The phase equilibrium of petroleum is a complex and interesting
area of research as petroleum itself is an ultra complex fluid. It is a
mixture of 105-106 different molecules without a repeating molecular
unit. The key step in the formation of coke is liquid-liquid phase
3. During thermal cracking, asphaltenes become more aromatic and at
a particular stage of conversion they undergo phase separation by
breaking of colloidal equilibrium of the whole residue. Asphaltenes
undergo condensation and polymerization reactions resulting into the
4. The model showing the interconversion along with parallel
formation of different boiling fractions has been proposed by Takatsuka
et al. (1989) and experimentally validated;
5. With the help of the structural changes at the molecular level
and using solvent-resid phase diagram, it was found that the shift from
one class to another such as maltenes (heptane soluble) to asphaltenes
(heptane insoluble, toluene soluble) to coke (toluene insoluble) occurs
via continuous change in the molecular weight and Conradson carbon
residue (CCR) within the same class (Wiehe, 1992). This has been
observed and corroborated by Yasar et al. (2001);
6. The concept and existence of certain threshold concentration of
asphaltenes as solubility limit (SL) was incorporated in the coke
formation model (Wiehe, 1993). Later, using optical microscopy, the
onset of neophase separation during thermal cracking was also
experimentally proved (Li et al., 1998);
7. Song et al. (1995) have studied the kinetics of coking of Gudao
vacuum residue in the temperature range of 400-440[degrees]C and
460-500[degrees]C. The thermal cracking reactions were found to follow
first order kinetics over the studied temperature range;
8. The changes in the chemical structure of resins and asphaltenes
occurring before and after thermal conversion of the Shengli vacuum
residue have been investigated by Wang et al. (1998). It was found that
during thermal conversion of vacuum residue, the resins bearing shorter
alkyl chains and more peri-condensed aromatic units are responsible for
asphaltenes formation, while asphaltenes bearing shorter alkyl chains
and more peri-condensed aromatic units get converted into coke;
9. The effect of solvent properties (with similar solubility
parameters such as maltene, 1-methyl naphthalene and tetralin) on
solubility limit and coking kinetics has also been explored (Rahmani et
al., 2002). It was observed that the hydrogen-donating ability of the
solvent and the hydrogen-accepting ability of the asphaltenes play a
major role in determining the ultimate yield of the coke;
10. The effect of structural properties of asphaltenes on the
coking rate and coke yield has also been explored by studying the
thermal cracking of asphaltenes obtained from different origins. It was
found that the coking rate depends on aliphatic sulphide content of the
asphaltene, while its aromaticity decides the yield of the coke (Wiehe,
11. The proposed coking kinetic models have been reported to follow
first order kinetics with the range of pseudo activation energies (22-83
kcal/mol) depending upon the feed properties and severity range.
[FIGURE 13 OMITTED]
A brief summary comprising the reaction conditions, different feeds
studied and the findings of the investigators is given in Table 4. From
the foregoing discussion, it can be found that considerable work has
been done on coking kinetics with special emphasis on the mechanism of
coke formation, inter-conversion of the solubility class components
during conversion, role of these components in coke formation, influence
of structural properties on coking rate and yields, etc. The proposed
models are based on the mechanistic pseudo components, phase separation,
pendant core, etc., which explains the coke formation during thermal
[FIGURE 14 OMITTED]
[FIGURE 15 OMITTED]
However, it may be pointed out that the information with regard to
the actual coke buildup (inception, growth and saturation of coke
formation) in the reactor (batch mode or continuous mode) has not been
reported in the literature. Qualitative information can be found
(Magaril and Aksenova, 1968) with regard to the coke formation. The
authors have reported that coke formation first takes place at the wall
of the coking drum. However, there is no quantification reported as to
how much temperature gradient exists between the wall and the centre of
the reactor at different processing severities. In view of this, it is
desirable to undertake a systematic investigation of the coking
behaviour of different feedstocks varying in physico-chemical
characteristics in terms of inception, growth and subsequent saturation
of coke formation at different processing severities.
With regard to the kinetic modelling, it may be pointed out that
most of the models proposed are based on structural changes within the
residue and very few models have been proposed that comprise the lumps
of industrial relevance and more importantly involve coke as one of the
components. After close scrutiny of the available literature, it was
found that the model, which comes close on this account, is proposed by
Del Bianco et al. (1993). The model proposed by the authors comprises
vacuum residue, distillate fraction, reaction intermediate and the coke.
A good account of the variation in the rate parameters has been given in
the proposed kinetic scheme. However, the proposed kinetic model does
not include gas fraction separately and therefore suffers from the lack
of information with regard to the rate variation for the paths, which
involves gas fraction. Gas ([C.sub.1]-[C.sub.4]) forms one of the major
products of delayed coking process with the yield up to 9.5 wt.% (Zuba,
1998) depending upon the processing severity. In view of this, it is
desirable to propose a kinetic model that includes gas fraction
separately so that useful information can be obtained with regard to the
rate variations for the paths involving gas fraction, along with rate
variations for the paths involving distillates and the unconverted
vacuum residue (VR).
With regard to the design of delayed coking unit, there have been
substantial changes over the past several years so as to operate the
delayed coking unit for maximum profitability and the changes have been
focused on the following areas:
2. Coke drum;
4. Hydraulic decoking;
5. Coke handling methods;
6. Blowdown recovery.
All the areas have been discussed one by one as follows:
According to Mekler and Brooks (1959), Meyer and Webb (1960),
Elliott (1992) and Sarkar (1998), the heater forms the heart of the
delayed coking assembly. The most important function of the coking
heater is to heat the feedstock very quickly to the required outlet
temperature and pressure without premature coke formation in the tubes,
which results in premature shutdown (Mekler and Brooks, 1959). Elliott
(1992) corroborated this fact and stated that coking of the coils of the
heater poses one of the greatest problems and negative economic impacts
on the unit through the loss of unit throughput. These negative impacts
are because of the very fact that the heater has to be taken out of
service even though only one of the coils is to be decoked. In fact, the
term "delayed" coking comes from the necessity of having the
coke form in the coke drum and not in the furnace tubes. Thus, while
designing the heaters, the foremost aim has always been to inhibit the
coke formation in the heater tubes and in turn to increase the run
length of the heater.
Factors that affect the heater run length are feedstock quality,
operating conditions and how these are maintained within a narrow range,
and in addition, the frequency and handling of upset operations (DeBiase
and Elliott, 1982). Until 1970, heater run lengths were to the tune of
12 months (Rose, 1971). With the advancement in the design of heaters,
the heater run length was increased to 18 months in 1980 (DeBaise and
Elliott, 1982). Presently, with the advent of on-line spalling
(decoking) of heater tubes, the run lengths have further increased to
about 24 months (Elliott, 1992).
Mekler and Brooks (1959) reported that each feedstock has its
critical zone of decomposition where actually the coke formation starts.
If the oil passing through this critical zone is in the liquid state and
at relatively low linear velocity, then, under the influence of
temperature, the slow-moving oil film on the inside surface of the hot
tubes tends to polymerize and ultimately deposits coke in the
intermediate portion of the heater. In order to mitigate this, it is
necessary to provide high turbulence motion in the portion of the coil
where the zone of critical decomposition is likely to occur. If the
velocity cannot be obtained through the vaporization of a portion of the
oil, a small controlled amount of steam, condensate, or boiler feed
water should be injected, usually no more than 227 kg per hour per pass
Heck (1972) reported that the furnace design should have following
1. High radiant-heat-transfer rate;
2. Good control of firebox heat distribution;
3. High cold velocity;
4. Good peripheral heat distribution to the tubes;
5. Short residence time.
DeBaise and Elliott (1982) reported that higher cold oil velocities
should be in the order of 1.82 m/s and the design should provide
multiple injections of steam into the heater coil to adjust coil
residence time and velocity. Heck (1972) reported that the firebox
design, which provides good heat distribution for the least cost, is the
horizontal tube, two-zone, floor-fired cabin heater. While designing the
firebox it should be noted that small burners are essential and gas
firing is preferred. As far as the trends are concerned, DeBaise and
Elliott (1982) stated that in addition to more liberal firebox
dimensions there has been a tendency to specify allowable average
radiant flux rates of the order of 28.39 kW/m2 to provide for longer run
lengths, future capacity allowances, and, in general, a more
conventional heater design. By way of comparison, traditional maximum
allowable radiant flux rates range from 31 to 38 kW/m2.
As feedstocks to the delayed coking unit are getting heavy, the
coking heaters are severely getting affected. As a result of this,
heaters, which would normally have had the run lengths of the order of
nine months or more, now require more frequent decoking. In this
context, Sloan et al. (1992) reported the considerations for improving
the heater run lengths and they are as follows:
1. Use of double fired heaters to reduce peak flux rates;
2. Injection of increased amounts of steam in the heater coils;
3. Design for higher cold oil velocities;
4. Minimize flame impingement.
In addition to the aforementioned considerations, temperature
monitoring in the radiant coils, minimizing upset conditions, and
determining the best coking temperature for a slate of feedstocks
processed in the unit are the other means by the virtue of which the
heater run lengths can be improved.
The advantages of the double-fired delayed coker heater (Figure 16)
reported by Elliott (1992) are as follows:
1. Capability of maximum shop assembly;
2. No field installation of the refractory bridgewall;
3. Burner access from grade;
4. Adaptability to multiply-cell/multiple pass configurations;
5. Adaptability to complete isolation of cells in terms of
maintenance, on-line spalling and decoking.
[FIGURE 16 OMITTED]
The on-stream time for coker heaters can be improved by using
on-line decoking (Sloan et al., 1992). With the advent of this on-line
decoking (spalling) the problem of taking the heater out of service has
been removed. L. Langseth has developed the technology and techniques
for on-line spalling (decoking) of delayed coker heater hydrocarbon
process coils without taking the entire heater out of service (Elliott,
Mekler and Brooks (1959) reported that the main function of coke
drums is to provide enough volume for the accumulation of coke produced
during the normal cycle of 24 h. In designing a coke drum, however, it
is necessary not only to provide sufficient volume for coke but also to
consider the maximum allowable superficial vapour velocity in the coke
drum to prevent carryover, minimum free board, and the foaming
characteristics of the charge stocks. Rose (1971) corroborated this fact
and reported that the maximum allowable superficial vapour velocity is
the first parameter to be considered. Until 1971, operations at 0.09 to
0.12 m/s without carryover have been reported.
As regards the foaming tendency of the feedstock, the injection of
foam depressants or anti-foaming agents in the coke drums during the
last 5 or 6 h of the coking cycle has contributed significantly to
longer runs (Rose, 1971). Heck (1972) has also reported that the
designer must weigh the foaming tendency of the feed vs. antifoam
effectiveness and the designer must also consider the advantages of
radioactive level detectors vs. the consequences of a coke carryover.
The most frequently reported difficulties are:
1. Distortion of bottom access hole neck flange;
2. Weld cracking between bottom cone and access hole neck;
3. Crack formation around cone nozzle attachments;
4. Deformation and warping of shell;
5. Cracking of internal alloy lining.
At the inception of the delayed coking process, the diameter of
coke drums was 3.04 m and the height was about 12.19 m. With the growing
capacity and coke production per unit, the diameter was increased to
6.09 m and the height was increased to over 24.38 m over the years from
1930 to 1955 (Mekler and Brooks, 1959). By 1967, coke drums having a
diameter of about 7.92 m were installed. Thus, there has been a
substantial increase in the diameter and height of the coke drums over
the years and as far as the recent figures are concerned, Christman
(1999) reported coke drums having a 8.53 m diameter and a 36.57 m
height, and was of the opinion that even larger drums are expected in
the future. Table 5 shows the update of coke drum size in terms of its
diameter and length.
As regards the material of construction for these coke drums,
Mekler and Brooks (1959) and Rose (1971) reported the use of 11-13
chrome lining with 2.8 mm liner thickness. Because of the heavy wall
thickness of coke drums, the alloy protection is always provided in the
form of applied liners or clad materials (Mekler and Brooks, 1959).
Most of the cokers at the beginning of the process were designed
for 20 to 24 h coking cycle times. In the late eighties and early
nineties the coking cycle time was reduced to 16 to 20 h (Sloan et al.,
1992). In the late nineties, it was reduced further to 14 h (Christman,
1999). Table 6 shows the time required for the different steps involved
in 12 h and 24 h decoking operations. As can be seen from the table, the
time required for the water cooling and drum heat-up for the 12 h
decoking cycle has been reduced significantly.
Another important advancement pertaining to coke drums is the
removal of the coke drum top and bottom heads. This was a very important
advancement in view of safety concerns related to the production of a
high flowability shot coke, which could create serious problems for the
operators (Sloan et al., 1992). Shot coke is produced when the delayed
coker is run under severe conditions with heavy, sour residues as
feedstock. Shot coke has a spherical appearance, lower surface area,
contains lower volatiles and has the tendency to agglomerate. The
deheading device is designed to allow remote control of the deheading
operation. It includes the release of the drum bottom head, lowering of
the head and moving it away. After the completion of the decoking
operation, the deheading device is then replaced and the bottom and top
heads are locked in the drum.
One of the major changes in the delayed coking process in the
recent past is the low-pressure operation at which the delayed coking
units are being run. Operating coke drums at low pressure is gaining
momentum in view of maximizing the liquid product (C5+) yield (Bansal et
al., 1994). Table 7 shows the effect of low pressure and low recycle on
coking yield of a typical feed having 20.5 wt.% CCR. As can be seen from
the table, the yield of C5+ liquid product is 72.6 wt.% at 0.1 MPa
pressure as opposed to 69.7 wt.% at 0.2 MPa pressure. It can also be
seen that the coke quantity is reduced at 0.1 MPa pressure and is 29.7
wt.% as opposed to the coke yield of 32.1 wt.% at 0.2 MPa. Although
operating at 0.1 MPa coke drum pressure gives an incremental yield of
C5+ liquid product and a reduction in coke yield, many factors can
significantly impact economics. Increased cost for larger equipment due
to increased vapour volume and piping should be evaluated when
considering a new design or revamp of a delayed coker for low coke drum
operating pressure (Elliott, 1992; Bansal et al., 1994).
The methods used for the design of the fractionator in the delayed
coking above the gas oil tray are the same as those practised in the
crude tower fractionator design. The only and major difference is that
special attention has to be given to the bottom section of the
fractionator below the gas oil tray (Rose, 1971; Heck, 1972). The
temperatures in this section of the tower are close to and above
incipient cracking temperatures (Rose, 1971). As a result of this, coke
tends to accumulate on the trays over a long period of time. Hence, it
is suggested that the trays should be designed to be self-washing and
for minimum change of efficiency with coke buildup. Heck (1972) reported
that designers should provide: (1) adequate residence time for the
furnace; (2) a so-called "heat shield" to lessen direct
condensation of gas-oil product from the hot coke-drum vapours to the
cool liquid surface; and (3) a place to collect coke solids. With regard
to the tower diameter, Sloan (1992) reported that it is usually set by
loads in the gas-oil condensing section of the tower above the draw pan.
Heat-transfer trays provide high liquid-load capability to minimize the
tower diameter. The design of the top section of the coker fractionator
is fairly standard (Heck, 1972). To avoid corrosion problems, it has
been suggested to drive all water overheads. As the trend is towards
low-pressure operation for maximizing distillate yield, the fractionator
sizing and specifications of fractionator internals have been affected
(Elliott, 1991). Compared to a higher-pressure operation, the increase
in vapour volume to be handled requires a cross-sectional area to
increase inversely proportional to the square root of the decrease in
absolute pressure. Thus, for 0.1 MPa pressure, a 3.96 m diameter
fractionator is required as opposed to 3.65 m diameter for 0.17 MPa
pressure operation (Elliott, 1992). Fractionator internals are
traditionally specified as valve trays. It has been investigated that
they are satisfactory on new designs for low pressure (0.1 MPa) cokers.
However, for revamp designs, a careful evaluation of all components has
led to the recommendations to replace pump-around trays with packed
beds, both for increased capacity and to reduce coke drum operating
pressure (Sloan, 1992). The load on coke drums and fractionator
increases as the capacity increases. Debottlenecking of the fractionator
and reducing the internals pressure drop can maximize throughput and
lower the coke yield. Grids or structured packing can be used as a
replacement for trays to make up for this. These internals are
especially desirable in the tower heat-transfer sections where
fractionation is not required and the loads are generally the highest
Hydraulic decoking has played a major role in the success story of
the delayed coking process (Welsh, 1950; Rose, 1971). Welsh (1950)
described hydraulic decoking as a method of disrupting, removing and
transporting petroleum coke from vertical coke drums through the medium
of high-velocity water. The system consists of a cutting head, cutting
stem and rotary joint guided by a travelling crosshead. A system of jet
water and rotary joint air hoses connects from the crosshead to
stationary points at the top of the structure. An air hoist, which
controls the raising and lowering operation of the cutter assembly, is
located on the operating floor near the top of the coke drum. The
high-pressure water required for the impact jets is supplied by a
multi-stage centrifugal jet pump, which feeds from a clear water tank.
Figure 17 outlines the various steps involved in the decoking of the
The discharge pressure of the hydraulic jet pumps employed for this
operation until 1950 was in the range of 6.89 to 10.13 MPa for small
coke drums (Welsh, 1950). With the advent of large coke drums, Kutler et
al. (1970) reported that the discharge pressure has increased to about
With regard to the advancement in hydraulic decoking, Rose (1971)
reported that special interlocked controls and bypass valves have been
devised to facilitate operation and protect personnel, piping and pump.
Two-piece drill stems have been replaced with a single long stem to
reduce cutting time. Labour and maintenance costs have been further
reduced by replacing the hydraulic piping and swivel joints with a
high-pressure drill hose. Steel requirements in the superstructures have
also been reduced.
[FIGURE 17 OMITTED]
[FIGURE 18 OMITTED]
Sloan et al. (1992) reported that in order to improve the safety,
working environment and operability/maintainability of delayed coking
units, the reliability of the system controls and control switches is of
utmost importance as it is more closely related to hydraulic decoking.
The system must shut down automatically prior to the cutting head being
removed from the coke drum. The operating personnel located on the
operating floor could be at risk if the system fails. In view of this,
the protection of the operating personnel against the forces of water
jet has been taken care of in the recent past.
The coke dropping out of the base of the coke drum is accompanied
by large volumes of drilling water. Different coke dewatering / handling
systems are available, the most common reported by Hobson (1982) are as
1. Direct railcar loading;
2. Pad or apron loading;
3. Pit loading;
4. Dewatering bins.
Kutler et al. (1970) reported that the direct railcar method
(Figure 18), though the cheapest, has limitations such as, more time is
required to decoke into cars than any of the other conventional methods
of decoking and more care is required when decoking into cars is to be
carried out to avoid spillage. This has been corroborated by Hobson
(1982) and reported that decoking thus becomes dependent on the railcar
movement. These disadvantages of the direct railcar method have given
way to methods like pad loading, pit systems, hydrobins and dragline.
Pad loading allows the coke and water to flow from the drum through
a chute directly onto a concrete pad, which is placed adjacent to the
coke drums. The water drains to the periphery of the pad into a settling
maze where coke fines settle out before the clear water is recycled to
the decoking water surge tank for re-use. The coke is removed from the
pad by a front-end loader or an overhead crane. The front-end loader
operation usually associated with pads can have a tendency to increase
the generation of coke fines that are environmentally detrimental
(DeBiase and Elliott, 1982).
The most widely accepted method for coke handling depicted is the
coke pit system (Sloan et al., 1992; Stefani, 1995) shown in Figure 19.
Pit loading is very similar to pad loading, except that the coke empties
into a rectangular concrete pit generally located below instead of a
concrete apron. The decoking water drains out through ports at one or
both ends of the pit, depending on the size of the facility. A
"heel" or coke located in front of those ports acts to filter
coke fines from the water. The remaining coke fines settle out in the
maze before the clear water is pumped into the decoking water storage
tank. An overhead crane with a clamshell bucket is required for coke
handling. The pit system inherently provides several days storage of
coke, presenting an advantage over pad loading (DeBiase and Elliott,
In dewatering bins, dewatering is met via special vessels, known as
dewatering bins or drainage silos. Slurry and gravity flow are the two
types of dewatering bin systems. In both designs, coke and cutting water
pass through a coke crusher (DeBiase and Elliott, 1982). Dewatering bins
have evolved to provide totally enclosed systems to meet exceptional
environmental requirements or to prevent coke contamination in areas
where sand storms may be a problem (Hobson, 1982). From 1982 to 1992, a
clear trend in selection and design of the coke handling and water
management systems was developed. Operators are now switching from
mainly capital investment considerations towards improved environmental
considerations, maintenance and reliability.
[FIGURE 19 OMITTED]
[FIGURE 20 OMITTED]
The recovery of wax tailings has been given considerable attention
over the years. In the past, wax tailings were allowed to pass into a
pond or the API separator. Over a period of time, these wax tailings
would plug up the sewer system. The modern design includes facilities
for recovering these wax tailings with a circulating oil stream (Kutler
et al., 1970).
The modern coke drum blowdown system (Figure 20) includes a coker
blowdown drum, blowdown condenser, blowdown settling drum, blowdown
circulating oil cooler and attendant pumps (DeBiase and Elliott, 1982).
The composition of the blowdown vent vapours is hydrogen and light
hydrocarbon vapours with an average molecular weight varying between 16
Increased awareness of environmental concerns has led to several
interesting trends related to delayed coker, which include the design of
new enclosed blowdown systems for older cokers that do not meet
environmental specifications. Environmentally, the scheduled flaring of
vent vapours may not be considered an acceptable practice from the
standpoint of total refinery emissions. Economically, these gases can be
recovered and used for the fuel value (Elliott, 1992). The blowdown
system is tightly integrated with the coker operating and safety
systems. The blowdown system must be checked for the maximum load
produced during the coking and decoking cycles with all potential
relieving scenarios. The incremental debottlenecking approach for the
blowdown system can decrease the loads by addressing the whole coker
Petroleum coke from the delayed coking process can be categorized
as sponge coke, honeycomb coke, shot coke and needle coke depending upon
its physical structure (Jacob, 1971; Dymond, 1991). Out of the
aforementioned cokes, needle coke is a premium-quality coke. Other types
of coke are produced as byproducts while processing residues, where the
main aim is to get maximum yield of the liquid product. Hence, the most
important aspect pertaining to needle coke is that, unlike other cokes,
needle coke is intentionally produced by refiners from selected
feedstocks (Dymond, 1991). Needle coke commands a high price
($550/metric ton) (Acciarri and Stockman, 1989) as opposed to fuel grade
coke ($25-30/ metric ton) (Elliott, 1992) because it is categorized as a
performance product, not a commodity (Swain, 1991). Different
investigators have defined needle coke on the basis of their
observations. The observations and in turn, the definitions of needle
coke as reported by different investigators, are summarized as follows:
Needle coke is a highly crystalline coke with much less
cross-linking (Reis, 1975). Coke with high crystallinity and low
coefficient of thermal expansion (CTE) is known as needle coke (Foulkes
et al., 1978). The term "needle coke" originates from the
needle-like appearance of the particles (Kuchhal, 1982). Needle coke is
highly structured, low metal and sulphur containing delayed coke, having
large unidirectional elliptical interconnected pores surrounded by thick
fragile walls (Stokes and Guercio, 1995; Singh, 1991). Needle coke is
highly sought-after for more conventional carbon and graphite uses as
well as in nuclear reactors and aerospace components (Stormont, 1969).
As regards the graphite uses of needle coke, calcined form of needle
coke is the major raw material in the making of graphite electrodes,
which in turn are used in electric arc steel furnaces (Acciarri and
Stockman, 1989). Figure 21 shows the industry relationships of needle
coke, graphite electrodes and electric arc furnace steel. An enormous
amount of iron and steel is produced in the world to meet the demands of
global growth. This very fact has caused an accumulation of large
amounts of iron and steel scraps to be regenerated and recycled.
Electric furnaces, principally operated for this purpose, require
graphite electrodes of superior performance to enable a reduction in
cost of steel making by stable long-term operation. Needle coke is used
as the best filler for high-performance graphite electrodes (Oyama and
Todo, 1993). Thus, by far the most important application of needle coke
is in electric arc furnaces used in steel industries.
[FIGURE 21 OMITTED]
Feedstocks for Needle Coke Production
Over the years different investigators have proposed/studied
different feeds for getting premium-quality needle coke. Different
feedstocks proposed/studied by different investigators are summarized in
the following text.
Slurry oil and decant oil from catalytic cracking and tars from
thermally cracked stocks form potential feedstocks for needle coke
production (Stormont, 1969). Apart from slurry oil, decant oil and
thermal tars, coal tar pitch can also form a very attractive feedstock
(Reis, 1975). Extracts from lube operations and residues also can be
possible feedstocks for the production of needle coke (Friday, 1975).
Apart from the conventional feedstocks like thermal tar, decant oil and
slurry oil, straight residua (only atmospheric or vacuum residues that
are low in sulphur, metals, asphaltenes and contain good percentage of
aromatics) may form part of streams to produce needle coke (Kuchhal,
Acciarri and Stockman (1989) observed that the needle coke
precursors generally consists of hydrocarbon streams of:
1. Low sulphur content catalytic cracker slurry oil;
2. Tars derived from the thermal cracking of refinery gas oils;
3. Hydrodesulphurized catalytic cracked slurry oils;
4. Steam cracker pyrolysis tar;
5. Coal tar pitches.
Mochida et al. (1990) studied the feasibility of producing needle
coke out of ethylene tar pitch. Singh (1991) categorized three types of
feedstocks with their origins that can be processed to get needle coke
and are as follows:
1. Those derived from cracking processes, thermal and catalytic
such as FCC decant oils, pyrolysis tar, thermal and visbroken tars;
2. Those derived from the raw lube extraction units;
3. Those derived from the coking of coal.
Todo et al. (1991) studied the cocarbonization of a low sulphur
vacuum residue (LSVR) with fluid catalytic cracking decant oil (FCCDO)
by catalytic hydrotreatment leading to the production of needle coke.
Kumar et al. (1996) studied the feasibility of producing needle coke
from three aromatic extracts obtained by solvent refining of lubricating
oil base stocks in a bench scale delayed coker.
Specific physico-chemical characteristics of the feedstocks are of
utmost importance in the production of needle coke. Nelson (1978)
pointed out that the lower the characterization factor (KUOP) of the
feed of the cracking plant, the larger the yield of aromatic rich cycle
stock suitable for producing needle coke. Thus, the aromaticity of the
feedstock is one of the very important criterions for the selection of
the feedstock. The chemical and physical characteristics considered in
choosing a proper feedstock for the manufacture of needle coke are
summarized as follows (Singh, 1991):
1. Feedstock should have high aromaticity with 60-85% aromatic
carbon. Aromaticity, as measured by the Bureau of Mines Correlation
Index (BMCI), should be over 100;
2. Feedstock should be of high initial boiling point, over
250[degrees]C with not more than 25-30% material boiling below
360[degrees]C due to safety considerations and should contain
appropriate carbon number range of coke precursors;
3. Feedstock should have characterization factor around 11.4 or
lower, with low API gravity;
4. Feedstock should have low sulphur content preferably below 1 wt.
% due to the concern for product quality;
5. Feedstock should have low metal, asphaltenes, CCR and wax
The aromatic content, sulphur content and CCR for three feedstocks
that are known to be suitable for needle coke production are listed in
Characteristics of Needle Coke
Needle coke shows strongly defined lines or striations and has a
metallic appearance even when ground to its primary particle size. When
subjected to X-ray diffraction, individual needles exhibit a
characteristic orientation of crystals not shown by non-needle coke
(Stormont, 1969). Needle coke has large unidirectional pores,
elliptical, largely interconnected and surrounded by thick walls. When
this coke is broken, it forms needle-like pieces, hence the name needle
coke (Reis, 1975). The needle structure is caused by the large, visible
pores having an elongated rather than spherical shape (Scott, 1967).
Needle Coke Production
High-quality needle coke is obtained from the well-known delayed
coking process. It is generally coupled with thermal cracker or
catalytic cracker, which produces the required raw material (thermal
tar/slurry oil) as a feed for the delayed coking operation. Needle coke
is produced in blocked operations when the usual feed to the delayed
coker is not available or is diverted to some other units (Stormont,
1969). Thus, special feedstocks are required for needle coke production
as against the residues that are normally fed to the delayed coker.
Secondly, for needle coke production, a higher drum pressure (0.6-1.5
MPa), temperature (475-530[degrees]C) and recycle ratio (15-20%) are
maintained than that in normal delayed coking operations since the
feedstock is refractory (highly aromatic) in nature for the former
(Kuchhal, 1982). The effect of operating variables on the yield and
quality of coke is shown in Table 9. An increase in drum pressure
increases the coke yield since more liquid is held in the drum for the
conversion. An increase in drum temperature decreases coke yield and
betters the quality of coke. Similar effects can be found by increasing
the coker recycle ratio. However, the operating variables have practical
limits that prohibit further change. Also, the limits on each will vary
with the type of feedstock employed (Friday, 1975).
Coking Mechanism Pertaining to Needle Coke Formation
Brooks and Taylor (1965) were the first to observe a second
semi-liquid phase during coking of coal tar pitch. The second phase,
often called the mesophase has also been observed in coking of petroleum
stocks, as well as the pitch. Its striking feature is its optical
activity, which suggested that it has a crystalline structure while
retaining some properties of liquids. Needle coke consists almost
entirely of mesophase (Friday, 1975).
Two different reaction mechanisms exist at any given
time-temperature conditions pertaining to the delayed coking process. In
one, high molecular weight compounds, such as asphaltenes and resins,
when subjected to high temperatures, tend to dealkylate straight chain
compounds and CH2 groups, leaving behind a residue of carbon with a
highly disordered, cross-linked structure or with many small graphite
crystals in a matrix of amorphous carbon and asphalt. Because of these
cross-linkages, the platelets loose mobility and graphitization becomes
difficult. Therefore, coke formed from this mechanism is not found
suitable for premium-grade electrodes. The second mechanism involves the
dehydrogenation of heavy oils with subsequent polymerization and
condensation of aromatics and then grouping a large number of these
compounds to such a degree that the coke is eventually formed. The coke
produced in this way contains lesser cross-linkages and has a more
crystalline appearance (needle type) than the resin-asphaltene type
(sponge, honey comb type). This mechanism has been corroborated by Jacob
(1971), who reported that the reaction mechanism involving
polymerization and condensation of a large number of aromatic compounds
with a low concentration of impurities leads to the formation of coke
containing fewer cross-linkages and has a more crystalline appearance.
Scott (1967) observed that coke precursors from the aromatic feedstocks
have simpler structures and contain fewer cross-linkages, so they
apparently remain plastic for a longer period of time during
carbonization. This plasticity permits crystallites to be oriented by
the upward flow of gases in the coke. The general theory of needle coke
formation is based on this partial alignment of crystallites within low
boiler vapour bubble walls, which form during the coking process. The
fracture of green coke occurs in such a way as to preserve the bubble
walls, and the coke has a splintery or needle-like appearance.
Petroleum-derived starting materials yield a wide variety of
mesophase morphologies ranging from fine texture isotropic constituents
to the fibrous or acicular structure required in the needle coke. As a
result, two types of precursor components and reaction sequences occur:
1. Rapid reacting components that precipitate fine mesophase
spherules under mild pyrolytic conditions undergo coalescence without
appreciable growth to yield fine isotropic micro-constituents that are
not readily deformed;
2. The slow reacting components that precipitate fine mesophase
spherules under mild pyrolytic conditions grow and coalesce to a coarse
and easily deformable mesophase. On continuous pyrolysis these
mesophases are extensively deformed by bubble percolation; eventually
viscosity significantly increases to allow further deformation and a
fine fibrous microstructure of needle coke is established as the
mesophase congeals to semicoke.
The conversion of mesophase is a function of molecular weight of
the mesophase material as well as the composition of the liquid from
which it is formed. The formation of large liquid crystals with
preferred orientation as intermediate is most essential for the coke
formation of high-quality petroleum coke. After sufficient growth of the
mesophase droplet population, a phase inversion occurs with the
mesophase becoming continuous. Thereafter, crystalline alignment
continues within the liquid mass. Finally, molecular weight becomes very
high and cross-linking inhibits graphite sheet orientation. Hence, it
has been reasoned that coke must be produced as a relatively quiescent
liquid. If the process is "rushed" by excessive temperature
gradients, or agitated, the final coefficient of thermal expansion (CTE)
of the graphite will be higher due to insufficient or improper alignment
before the structure is fixed by the cross-linking and the growth
processes. It is for this reason that coke produced by fluid coking
cannot be used in graphite manufacture for UHP (ultra high power)
electrodes. This is partly due to the high temperatures employed and
partly because of the "onion" type (layered) development in
such a coke (Friday, 1975).
Factors Affecting the Needle Coke Quality and Quantity
It is known that the petroleum coke quality depends on the type and
origin of the feedstock as well as on the operating parameters. By
careful selection of these, it is quite possible to improve upon the
quality for more selective types of coke. Following are the factors,
which are responsible for the coke quality:
The most important variable is the hydrocarbon constituents present
in the feed, which range from high and low resins and asphaltenes to
aromatics (Stormont, 1969; Kuchhal, 1982). The presence of a high
concentration of asphaltenes and resins leads to coke formation with a
highly disordered, cross-linked sponge type of coke (high CTE). On the
contrary, low percentage of these gives an ordered, crystalline type of
needle coke (low CTE). Aromatic hydrocarbons that are resistant to
thermal cracking or chemical decomposition appear in the cycle stock as
thermal tar, decant or slurry oil. The lower the characterization factor
(CF) of the feed to the cracking plant, the larger the yield of
aromatic-rich cycle stock suitable for producing needle coke. It is
known that recycling reduces the characterization factor of the cycle
Among the non-metallic impurities, sulphur is one of the most
important criterions for the applicability of needle coke in the making
of electrodes (Kuchhal, 1982; Acciarri and Stockman, 1989). In petroleum
coke, sulphur is present nearly quantitatively in organic bonding as
heteroatoms with the aromatic rings. Surprisingly, these C-S bonds have
extreme thermal stability and therefore, sulphur remains in the main
part with the coke even at calcination temperatures up to
1350[degrees]C. At elevated temperatures between 1400[degrees]C and
1500[degrees]C, sulphur breaks out suddenly by the rupture of
hetrocyclics and forms volatile C-S compounds. This behaviour is known
as "puffing effect" and must be avoided. It has been observed
that coke having more than 1.2 wt.% of sulphur gives a vapour formation
during electrode manufacture. According to Acciarri and Stockman (1989),
if the coke contains high sulphur, the electrode maker has to extend the
time requirements as regards the graphitization process in order to
minimize irreparable cracking and weakening of the in-process electrode.
As a result of slowing down of the graphitization process, energy is
consumed to a large extent and there is a loss of production capacity as
In general, sulphur in the coke is a function of sulphur in the
coker feed. To produce less than 1.5 wt.% of sulphur in the coke, under
normal operating conditions, a residue of 0.9-1.0 wt.% sulphur is
required. It has been observed that operating conditions also play some
role in deciding the sulphur and the final coke content. Coke produced
at high pressure and recycled stock have lower sulphur content than
expected, since they result directly not only from residues but also
effectively from cracked oils having low sulphur content. It has been
further observed that sulphur generally exists in larger amounts in the
sponge coke produced from high-resin-asphaltene stock than in honeycomb
or needle coke. In the super premium grade of electrode coke used as
graphite products, sulphur is not allowed to exceed beyond 0.5 wt.%.
The metal impurities found in the oil system are in the form of Ni,
V and Fe. These impurities get condensed in the needle coke during the
coking process. A high concentration of these metals in coke affects not
only its mechanical strength and resistivity but also how it works.
Vanadium in needle coke, if it is used as a graphite steel electrode,
increases the consumption of electrode due to oxidation. In the amalgam
process, if the graphite electrode is used as an anode in aqueous
chlorine-alkaline electrolysis, vanadium reduces the hydrogen over
voltage and chlorine can be enriched by hydrogen. Above 20 ppm of
vanadium leads to the danger of formation of explosive chlorine/
hydrogen gas mixture (Kuchhal, 1982).
Very high purity is required in nuclear applications for two
reasons, firstly the neutron absorption by several elements, e.g.
gadolinium, europium and boron, and secondly by the different activation
behaviour of the elements present under neutron irradiation in forming
Furnace outlet temperature, coke drum pressure and the recycle
ratio are the main operating variables, which not only affect the needle
coke yield but also its characteristics (Kuchhal, 1982). An increase in
the drum pressure results in a higher coke yield because more molecules,
even of gas-oil range, contribute to coke formation by remaining in the
liquid phase. This not only dilutes sulphur and metal contents of coke
but also decreases its CTE.
Higher furnace and drum temperatures produce hard coke, which
creates a cutting problem during recovery. Lower temperatures produce
more coke but of lower quality. Therefore, the temperature at the outlet
of the furnace should be such that the vaporization of the oil is about
5-20 mol.% with minimum coke formation in the furnace coils. To reduce
coke formation in the furnace coils, water vapours are introduced prior
to the critical zone of decomposition, as has been discussed by Mekler
and Brooks (1959). However, Stormont (1969) has observed that the coke
produced with the injection of steam in the process not only had a
higher CTE but also was more isotropic, i.e., of lower quality.
Green petroleum coke from the delayed coking process is essentially
a hydrocarbon. Before it can be used as a carbon aggregate it must be
converted into elemental carbon by a petrochemical process, and the
process, which is imparted for the above-mentioned purpose, is called
calcination (Nelson, 1970). It is basically a heat treatment in the
temperature range of 1300-1500[degrees]C given to the green coke.
Essentially, all hydrogen is removed from the coke and the structure of
the coke changes toward a graphite. Nelson (1970) pointed out that
though this process is conventionally termed as calcination, chemically
it is essentially a dehydrogenation reaction, which converts a
hydrocarbon into elemental carbon. A number of changes in basic
properties and structure of green petroleum coke can be brought about
via calcination on removing the chemically bound hydrogen from the
hydrocarbon in order to produce elemental carbon. Dehydrogenation and
dealkylation reactions bring about the fusion of large aromatic
structures into highly organized shapes, similar to the parallel layers
of condensed planer [C.sub.6] rings that constitute the graphite
crystallite (Singh, 1991).
Table 10 shows typical ultimate analysis of green petroleum coke
and calcined petroleum coke. As can be seen from the table, the green
petroleum coke is an electrical insulator while the calcined petroleum
coke is an electrical conductor. Most calciners are operated to obtain a
specified real density of the calcined product. Anode and specialty
cokes are usually calcined before proceeding to their end use. The
calcining processes used are the rotary kiln and the rotary hearth
processes. Table 11 shows typical specifications of calcined needle coke
as proposed by Acciarri and Stockman (1989). As can be seen, the
specifications for three grades of calcined needle coke, viz., super
premium, premium and intermediate needle coke have been given. Super
premium needle coke is used to produce electrodes for the most severe
electric arc furnace steelmaking applications, premium needle coke for
electrodes intended for less severe applications and intermediate needle
coke for even less critical operation.
It has largely been believed that the quality of the final coke is
heavily dependent on the manufacturing conditions of green coke.
Accordingly, various processes developed to obtain low coefficient of
thermal expansion (CTE) products centred on producing needle coke at the
delayed coking step itself. In view of this, developments in calcining
technology have been primarily aimed at improving efficiency and economy
of the calcining units. This opinion however, in the recent past, has
proved to be otherwise (Singh, 1991).
Calcination of the green coke has traditionally been carried out in
a single stage rotary kiln. Use of the Marathon rotating hearth process,
Petrol Chimie vertical kiln (PC), more as a desulphurizing calciner and
electric calcination have also been reported (Reis, 1975). Singh (1991)
also discussed the new calcining technology developed by Koa Oil Company
(Japan), which is a two-stage calcination process. In this, green coke
is calcined initially at a temperature from 600-900[degrees]C, cooled
and then recalcined at a temperature of about 1300-1400[degrees]C, the
same level as in the traditional method. It has been found that in the
temperature range of 700-800[degrees]C, the transition points are
observed on coke strength, CTE, porosity and X-ray parameters. This new
technology reduces the CTE of coke regardless of the type of green coke.
The pricing scenario with regard to needle coke has evolved over a
period of time. In 1969, calcined needle coke was sold at $70/ton and in
1972, the price was in the range of $80-$120/ton (Reis, 1975). In 1979,
record sales of both needle coke and graphite electrodes were posted.
The business climate of 1979 encouraged forecasts of major near-term
shortages and continued spiraling prices. Graphite electrode
manufactures initiated programs to increase production. As a result of
which, in 1980, the price approached to $400/ton (Matson, 1980).
However, the worldwide economic recession during 1982-1985 and excess
production capacity influenced a new pricing mechanism and hence a
slowdown was observed during that period. In 1989, the price approached
$550/metric ton, and it was predicted that increasing demand would make
it more costly (Acciarri and Stockman, 1989). Table 12 shows the pricing
evolution with regard to the needle coke.
Future Market for Needle Coke
According to Friday (1975), there are many reasons that strengthen
the belief of a very bright future for the production of needle coke,
which is obtained from the delayed coking process. The impact of the
economic disorder in petroleum prices, as well as environmental and
social awareness, augments the need for needle coke. The electric arc
furnace is a relatively efficient means of producing steel. The electric
arc furnace is desirable from an environmentalist's viewpoint since
it recycles scrap steel without excessive transportation costs and
because air pollution is relatively easy to control compared with other
means of steelmaking. So as long as steelmaking continues to grow,
electric steel production will grow at a faster rate. Since ultrahigh
power (UHP) furnaces require extremely high-quality electrodes and
therefore coke, the industry will be required to produce growing
quantities of needle coke. Acciarri and Stockman (1989) are of the
opinion that refiners possessing feedstock advantages, logistics
advantages, and sustained efforts to produce a quality needle coke, can
find opportunity in a global marketplace that appears to require a new
supply development of super-premium-quality needle coke. Swain (1991)
reported that the needle coke market will continue to be characterized
by contracted sales and infrequent spot market activity. It has also
been reported (Oyama and Todo, 1993) that the quality of needle coke
needs to be upgraded because the steelmakers, with the progress of
steelmaking techniques, are asking for high-quality graphite electrodes.
Shen et al. (1998) corroborated this fact and reported that along with
the rapid development of electric arc steelmaking furnaces, the demand
for premium needle coke is going to grow constantly in the future.
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
The overall conclusions and suggestions for future work can be
summarized as follows:
1. With regard to the bottom-of-the-barrel upgrading, the delayed
coking process is a long-time workhorse. From its very inception in
1930, it is still the most sought-after residue upgrading process. In
fact, it is the lowest investment route for residue upgrading, which
gives the highest return on investment (ROI);
2. There is a renewed interest in having a delayed coker unit in
the refinery set-up as the refiners are left with no option but to
process high sulphur, high metal content crudes, which in turn lead to
the production of high sulphur, high metal content residues. It has been
anticipated that as the crudes will continue to get heavier and the
demand for motor fuels will continue to increase, delayed coking would
be even more widely used throughout the world;
3. With regard to the reaction mechanism pertaining to coke
formation, it has largely been accepted that coke formation takes place
via some reaction intermediate and only after certain induction period
as it brings with it a phase inversion from liquid to solid. However, a
kinetic model consisting of industrially important fractions under
coking conditions has not been attempted as yet. Such a kinetic model
comprising industrially important pseudocuts, viz., gas, gasoline, LGO,
VGO and coke needs to be developed, which could be of significant use to
the practising refiners;
4. The inception of coke formation in coke drums is a critical
input that in turn defines the efficiency of delayed cokers in terms of
getting low boiling, more value-added products out of the residue. It
has been reported that the coke formation starts at the wall and
progresses toward the centre of the reactor. However, no quantification
is reported as to how much temperature difference occurs between the
hydrocarbon mixture at the wall and at the centre. Hence, batch reaction
studies in the temperature range of 430-500[degrees]C and a residence
times range of 5-120 min need to be carried out so as to investigate the
inception of coke formation at low temperature and high residence times
5. Advancements in the design features of the delayed coking
process are on so as to handle the heavier residues and there is a
growing trend for low-pressure operations in view of maximizing the
yield of liquid product and to minimize the coke formation;
6. Various new processes are being employed for the efficient use
of fuel grade coke, which is obtained by processing high sulphur and
high metal content residues. Among the various processes, gasification
of petroleum coke that gives rise to synthesis gas, which in turn can be
used in various applications, is gaining widespread acceptance;
7. The production of steam and electricity via cogeneration for
internal use and outside sales is emerging to be the solution to the
disposal of this coke. In fact, Petrox, a Chilean refinery has set up a
cogeneration plant that burns the coke and supplies steam and
electricity to the refinery (Karpenski and Alveal, 1999). The most
important part of this cogeneration plant is that it provides a
permanent disposal of high sulphur and high metal content green coke
produced by the delayed coker, thus solving any problems of unwanted or
8. Since a single feedstock meeting the specifications for the
needle coke production is not available in abundance, extensive study on
the blends of different feedstocks meeting the specifications needs to
be carried out. At the same time, new potentially feasible feedstocks
need to be explored for the production of needle coke;
9. The properties of the feedstocks for the needle coke production
should be analyzed (in terms of the hydrocarbon type present and more
precisely which type of aromatic hydrocarbons), which will ultimately
lead to the proper balance of aromaticity and molecular weight to
minimize resulting graphite CTE;
10. It has been found that the mechanism of the formation of needle
coke is different from the mechanism of the formation of regular coke.
However, the information on the kinetics of needle coke formation is
scarce in the published literature. In this regard, studies with
potentially feasible feedstocks for needle coke production should be
extensively carried out. The generated batch reaction data at different
temperatures and residence times under delayed coking conditions with a
little higher severity could be of great help for subsequent reaction
kinetics model development;
11. All this calls for the systematic investigations at the batch
and bench levels, which would involve number of experiments wherein, the
different process parameters and different feedstocks and their blends
can be studied extensively.
One of the authors, A. N. Sawarkar, is grateful to the University
Grants Commissions (UGC), Government of India, for providing the
financial support in the form of a research fellowship. The financial
support of United Phosphorous Limited (UPL), India is also gratefully
Manuscript received November 14, 2006; revised manuscript received
January 5, 2007; accepted for publication January 5, 2007.
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Conversion," ACS preprints, Petroleum Chemistry Division 43(1),
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Petroleum Macromolecules," Fluid Phase Equilib. 117, 201-210
Yasar, M., D. Trauth and M. T. Klein, "Asphaltene and Resid
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Ashish N. Sawarkar, Aniruddha B. Pandit, Shriniwas D. Samant and
Jyeshtharaj B. Joshi *
Institute of Chemical Technology, University of Mumbai, N. P. Marg,
Matunga, Mumbai, Maharashtra, 400019, India
* Author to whom correspondence may be addressed. E-mail address:
AR atmospheric residue (350[degrees]C+)
CCR Conradson carbon residue, wt.%
CTE coefficient of thermal expansion
E activation energy, kcal/mol
I reaction intermediate, wt.%
IBP initial boiling point, [degrees]C
k rate constant, min-1
[K.sub.UOP] characterization factor
LGO light gas oil (150-350[degrees]C)
MMTPA million metric tons per annum
SARA saturates, aromatics, resins and asphaltenes
SL solubility limit, wt.%
VGO vacuum gas oil (350-500[degrees]C)
VR vacuum residue (500[degrees]C+)
Table 1. Comparison of different processes for residue upgrading
Simplicity High Medium
Flexibility Low High
Cost Low Medium
Quality of products Low Medium
Resid conversion level Medium Medium
Rejection as fuel oil Medium Medium
Rejection as coke High Medium
No. of units in world Large Large
Recent trends High Medium
Environmental pollution High Medium
On stream factor Poor Medium
Problems Coke disposal Heavy residue
Extraction Hydrogen addition
Simplicity Medium Low
Flexibility Low High
Cost Medium High
Quality of products Medium High
Resid conversion level Medium High
Rejection as fuel oil Medium Medium
Rejection as coke Medium Medium
No. of units in world Average Average
Recent trends Medium Medium
Environmental pollution Nil Low
On stream factor Medium High
Problems High energy Hydrogen requirement
Table 2. World residue processing capacity, MMTPA (Shen et al., 1998)
Process U. S. A. Japan Europe world Total
a. Cracking/ 6.5 1 108.5 82.5 198.5
b. Coking 93 3 31.5 61 188.5
Deasphalting 13 1 0.5 5 19.5
Hydroprocessing 30.5 30.25 9 49.75 119.5
Resid FCC 31.5 12.5 10.5 37 91.5
Total 174.5 47.75 160 235.25 617.5
Table 3. Yield * comparison for light and heavy feeds (DeBiase and
Items Crude oil sources of residues
Brega Orinoco Alaskan North Slope
TBP cut point, [degrees]C 565+ 510+ 565+
Gravity, [degrees]API 12.3 2.6 8.9
Con. carbon, wt.% 14.6 23.3 16.1
Sulphur, wt.% 1.06 4.4 2.16
Dry gas and [C.sub.4], wt.% 7.0 16.3 11.3
[C.sub.5]-193 [degrees]C 18.6 16.2 14.6
Gravity, [degrees]API 60.7 50.0 57.6
Sulphur, wt.% 0.11 1.25 0.7
193+[degrees]C Gas oil, wt.% 52.4 29.6 47.6
Gravity, [degrees]API 35.7 18.8 25.1
Sulphur, wt.% 0.83 4.1 1.4
Coke, wt.% 22 37.9 26.5
Sulphur, wt.% 1.9 4.3 3
Ni+V, ppm 182 3700 607
Items Crude oil sources of residues
Maya Light Arabian Heavy Arabian
TBP cut point, [degrees]C 565+ 565+ 565+
Gravity, [degrees]API 2.6 7.4 4.5
Con. carbon, wt.% 25.5 15.4 24.2
Sulphur, wt.% 4.91 4.1 5.25
Dry gas and [C.sub.4], wt.% 13.2 11.1 13.2
[C.sub.5]-193 [degrees]C 19.3 16.1 13.5
Gravity, [degrees]API 54.9 58.8 55.6
Sulphur, wt.% 0.9 1.0 1.1
193+[degrees]C Gas oil, wt.% 29.2 45.8 40.4
Gravity, [degrees]API 20.9 28.1 26.5
Sulphur, wt.% 3.6 2.3 2.4
Coke, wt.% 38.3 27 33
Sulphur, wt.% 5.6 6.4 7.1
Ni+V, ppm 1854 366 676
* estimated at constant recycle ratio and pressure
Table 4. Reported work on the coking kinetics
Authors Experimental details
Magaril (1968) Feed Resins
Reactor Quartz test tube
Temp. ([degrees]C) 400
Magaril et al. Feed Asphaltenes isolated
(1971) from cracked residue
Solvent Transformer oil,
Reactor Autoclave without
Temp. ([degrees]C) 380-410
Takatsuka Feed Residual oil
et al. Reactor Flow reactor, semibatch,
Temp. ([degrees]C) 400-450
time (min) Pressure 0.013-0.45
Wiehe (1992) Feed VR and its SARA fractions
Reactor Batch tubing bomb
Temp. ([degrees]C) 400
Pressure 7 MPa ([N.sub.2] atm)
Wiehe Feed Cold Lake VR (3 g)
(1993) Reactor Quartz tube reactor
Temp. ([degrees]C) 400
Pressure Open reactor with
(MPa) continuous [N.sub.2] flow
Del Bianco et Feed Belaym VR
al. (1993) (CCR-20.8 wt.%)
Reactor Batch reactor
Temp. ([degrees]C) 410-470
Pressure 1 ([N.sub.2] atm)
Song et al. Feed Gudao VR (5 g)
(1995) Reactor Batch tubular reactor
Temp. ([degrees]C) 400-500
Wang et al. Feed Shengli VR (500+)
(1998) Reactor Autoclave with magnetic
Temp. ([degrees]C) 410
Li et al. (1998) Feed Shengli, Daqing, Gudao
Reactor FYX-OSA autoclave
Temp. ([degrees]C) 370-390
Rahmani et al. Feed Athabasca asphaltenes
(2002) (n[C.sub.7] insol. 3 g)
Reactor Batch microreactor (15 mL)
Temp. ([degrees]C) 430
Pressure 0-4 MPa
Rahmani et al. Feed Asphaltenes from AL,
(2003) AH, Maya, GudaoVR
Reactor Batch microreactor (15 mL)
Temp. ([degrees]C) 430
Pressure 9.8 M Pa
Schabron et al. Feed Boscon, Llyodminster,
(2003) Redwater BC, MaxCL2 (5 g)
Reactor Tubular reactor
Temp. ([degrees]C) 400-450
Authors Proposed kinetic model
Magaril (1968) --
Magaril et al. --
Takatsuka [ILLUSTRATION OMITTED]
Wiehe (1992) [ILLUSTRATION OMITTED]
Del Bianco et [ILLUSTRATION OMITTED]
Song et al. --
Wang et al. --
Li et al. (1998) --
Rahmani et al. [H.sup.+] [??] b[A.sup.+] + (1-b) V
(2002) [A.sup.=] [??] c[A.sup.*] + (1-c) ([H.sup.*] + V)
[H.sup.+] = fraction of reac. N[C.sub.7] sol.
[H.sup.*] = fraction of prod. N[C.sub.7] sol.
[A.sup.+] = fract. of reac. asphaltene
[A.sup.*] = fract. of prod. asphaltene
V = cracked distillate prod
[k.sub.A] [k.sub.H] = reac. rate const. for the
thermolysis of reac. asphaltene and
n[C.sup.7] sol., respectively
b, c = stoichiometric coefficients
Rahmani et al. --
Schabron et al. --
Magaril (1968) 1. There exists a certain saturation concentration
of asphaltenes beyond which they precipitate
out from saturate solution.
2. Coke formation is the consequence of
precipitation and accumulation of asphaltenes.
The coking rate is equal to the rate of
Magaril et al. 1. The effect of solvent properties on the kinetics
(1971) of coke formation was studied.
2. Assuming the spherical shapes of asphaltene and
solvent molecules and using their molecular
weight and density, the asphaltene
concentration in the solution was proposed at
3. Coking was found to initiate after certain
threshold concentration of asphaltene in the
solution and this value was found to be higher
for solvent with good solvating power.
4. Coke formation was found to occur after a
certain severity of cracking has reached.
Takatsuka 1. Atmospheric equivalent temperature and
et al. hydrocarbon (HC) partial pressure were used to
(1989) estimate the Arrhenius rate parameters.
2. Higher HC partial pressure decreases the
reaction rate of polycondensation reactions
while cracking rate increases with increasing
the reactor pressure.
3. The activation energy for the cracking and
polycondensation reactions were estimated to be
60 kcal/mol and 40-50 kcal/mol, respectively.
Wiehe (1992) 1. The solvent-resid phase diagram (plot of
molecular weight--Hydrogen content) was
proposed to distinguish one pseudocomponent
from another and to track the chemical changes
that result their movement from one solubility
class to another.
2. Elemental analysis and molecular weight of
thermal conversion products of resid and SARA
fraction was studied.
3. Thermal cracking of the SARA fractions resulted
to reduction in their molecular weight,
H-content (slightly in case of saturate and
aromatics) and coke formation.
Wiehe 1. Inhibition of heptane solubles for coke
(1993) formation, a maxima for the asphaltenes
formation, which matches with the coke
induction period, parallel decrease in the
asphaltenes concentration beyond certain
heptane solubles ([S.sub.L], solubility limit).
2. A kinetic model was proposed that explains the
above observations and estimated the
disappearance of asphaltenes by first order
with rate constant of 0.026 [min.sup.-1] while
[S.sub.L], was estimated to be 0.49 wt.%.
Del Bianco et 1. The coke formation was proposed to be formed
al. (1993) via a reaction intermediate (I)
2. [E.sub.1]-49.4 kcal/mol; [A.sub.l]-31.97
[min.sup.-1]; [E.sub.2]-63.9 kcal/mol;
3. Structural study shows that thermal cracking of
asphaltenes follow dealkylation reactions.
4. Condensation reactions prevail at higher
Song et al. 1. Assuming the first order kinetics, the
(1995) Arrhenius parameters were found to vary from
[E.sub.400-440] -170.7 kJ/mol, [A.sub.400-440]-
7.853[e.sup.10] to [E.sub.460-500]--180 kJ/mol,
Wang et al. 1. Structural parameters were compared between
(1998) SARA fractions obtained from the feed and after
its thermal cracking.
2. Peri-condensed aromatic resins get converted
into asphaltenes, which further condense to
give coke formation.
Li et al. (1998) 1. The variation in the onset of neophase
formation, necessary for the coke formation,
was studied using optical microscope for three
different VR varying largely in their
2. A stability function, based on the SARA
composition, was determined to represent the
thermodynamic stability of the VR.
Rahmani et al. 1. The solubility limit kinetic model was proposed
(2002) to study thermal cracking of asphaltenes in
maltene, 1-methyl naphthalene and tetralin.
2. Coke formation was found to be strongly
influenced by the chemical interactions
between the asphaltenes and the solvent medium.
3. The hydrogen donating ability of the solvent
and the hydrogen accepting ability of the
asphaltenes were found to have a pronounced
effect on the coke yield.
4. The model proposed was found to be consistent
with the data on coke formation from
asphaltenes in studied solvents with low
hydrogen donating ability.
Rahmani et al. 1. Phase separation model was proposed to study
(2003) the coking kinetics of asphaltenes having range
of structural properties.
2. Structural dependency on coking kinetics showed
that sulphide content of asphaltenes correlated
with the cracking rate while the aromaticity
decided the coke yield.
Schabron et al. 1. Coke formation involves a complex set of
(2003) reactions which fall somewhere between
zero-order and first-order kinetic mechanisms.
2. Kinetics followed by coke formation is most
likely dependent on the source of the residue.
3. The activation energies of the secondary coke
formation reactions of the studies feeds were
found to be 2.2 to 2.6 times higher than that
for the initial coke formation.
Table 5. Summary of the coke drum sizing update
Year Diameter (m) Length (m)
Before 1940 3.0 12.2
1946 5.2 20.7
1952 6.1 25.6
1967 7.3 About 30
1980 8.2 About 35
1999 8.5 36.6
Table 6. Time allotted for different steps involved in a typical
12-hour and 24-hour decoking cycles (Stefani, 1996)
Operation 24-hour cycle, h 12-hour cycle, h
Switch drums 0.5 0.5
Steam out to fractionator 0.5
Steam out to blowdown 1.0 0.0
Slow water cooling 1.0 3.0
Fast water cooling 5.0
Water draining 3.0 2.5
Remove heads 1.0 0.25
Hydraulic decoking 4.0 3.0
Replace heads 1.0 0.25
Steam purge and test 1.0 0.5
Drum heat up 6.0 2.0
Total 24.0 12.0
Table 7. Effects of low pressure and low recycle on coking yields
(20.5% CCR Feedstock) (Sloan et al., 1992)
Earlier designs Current designs
Coke drum pressure, MPa 0.2 0.1
Recycle ratio 10% 5%
Coke yield, wt.% 32.1 29.7
[C.sub.5]+ Liquid yield, vol.% 69.7 72.6
Table 8. Needle coke feedstock characteristics (Meyers, 1997)
Feedstock characteristics Feedstock
Slurry oil tar no. 1 tar no. 2
Aromatic content, LV% 61.7 89.8 66.1
Sulphur content, wt.% 0.48 0.07 0.56
Conradson carbon 5.7 9.4 8.6
Table 9. Effect of operating variables on the yield and quality of
coke (Friday, 1975)
Variable Effect on
Coke yield Coke quality (CTE)
Increase drum pressure Increase Variable
Increase drum temperature Decrease Improve
Increase coker recycle ratio Increase Improve to maximum
Thermal crack cycle Decrease Improve
Table 10. Typical ultimate analysis of green petroleum coke and
calcined petroleum coke (Nelson, 1970)
Constituents/properties Green Calcined
(carbonized (pure carbon)
Carbon, wt.% 91.8 98.40
Hydrogen, wt.% 3.82 0.14
Oxygen, wt.% 1.30 0.02
Nitrogen, wt.% 0.95 0.22
Sulphur, wt.% 1.29 1.20
Ash, wt.% 0.35 0.35
C/H ratio (by weight) 24 910
Real density, kg/[m.sup.3] 1300 2070
Electrical resistivity, Ohm-m 2.3x[10.sup.5] 9x[10.sup.-4]
Table 11. Typical Calcined Needle Coke Specifications (Acciarri and
Property Quality grade
Super premium Premium Intermediate
Coefficient of thermal <2.0 2.0 to 3.0 3.1 to 4.0
expansion (CTE), x
Sulphur content, wt.% <0.5 <0.6 <0.8
Real density, gm/cc >2.12 >2.12 >2.12
Ash content, wt.% <0.1 <0.2 <0.2
Particle size Maximum practical
distribution, wt. % +
6 Tylor mesh
Hardness (attrition Varies depending upon producer
factor) and user
Table 12. Summary of pricing evolution for needle coke
Year Cost ($/ton)