THERMAL TREATMENT OF SYRIAN SHOT COKE
Hassasn Al-Haj Ibrahim
Dept. of Chemical Engineering, Al-Baath University, Homs, Syria
Shot coke is a by-product of the delayed coking process, and can
represent a significant part of the petroleum coke produced. It is comprised
in general of two basic types, namely regular shot coke and clustered or
agglomerated coke. Shot coke is, however, of little or no commercial value,
particularly if it has a high sulphur content, as is becoming increasingly
the case in most oil refineries. Upgrading the coke and reducing its sulphur
content would add considerably to its economic value both as a fuel and/or
for making aluminium anodes. Towards this end, samples of Syrian shot coke,
produced by the delayed coking unit at the Homs Oil Refinery, were thermally
treated at high temperatures and increased residence time. A coke of higher
quality was thereby obtained, with reduced sulphur content (1%) and higher
real density (1.9 g/cm3). The observed weight loss was minimal, and the
decrease in the calorific value was less than the average expected for other
types of petroleum coke.
Keywords: Petroleum coke, Shot coke, Thermal treatment.
Shot coke is an abnormal form of delayed petroleum coke. Physically it
can be comprised of:
1. Individual spheres, which are hard, dense, vitreous, nonporous spheres
that may or may not be fractured. Normally, most shot coke spheres are
uniformly sized (1–2 mm dia or 2-6 mm), but they can range from as small as
buckshot to as large as basket balls. It is thought that smaller spheres are
made when very high feed rates are used in the coker. Larger spheres (up to
20 mm) and more mixed sizing tend to occur when a coker just begins to
produce shot coke. Very large spheres (up to the size of basketballs) are
usually made up of a fused aggregation of uniformly-sized smaller spheres
2. Agglomerated or clustered spheres, where individual spheres may form an
aggregate consisting of smaller individual spheres of 1 mm to 10 mm in size,
bonded together somewhat loosely. An aggregate may range in size from a few
millimeters to as much as 30 or more centimeters.
3. Bonded spheres or matrix shot. This appears as spheres of coke that melted
together on formation. When it fractures, it will fracture across the
spheres, rather than at the boundaries of the spheres as in agglomerated
The structure of shot coke is described by the use of Montage
microphotographs, comparing the structure to that of sponge coke. The
photographs reveal the fine-mosaic, high-density structure of shot coke .
Shot coke, while it may look like it is entirely made up of shot, is
not necessarily so. Shot coke is unique in that the small spheres have a
slick shiny exterior coating of needle or acicular type carbon, but the
inside of each sphere contains isotropic or amorphous type coke. A
microscopic study by Marsh, Calvert and Bacha showed that the shot coke
spheres consist of a fine-grained mosaic core (0.5–1.5 μm dia.) surrounded by
a thin, slick outer skin, 50 μm thick, of more well-developed mesophase coke
and of coarse-grained mosaic (5.0–10.0 μm dia.) and small domains (10–60 μm
dia.). Centrally located stress cracks are thought to be due to shrinkage of
the core structure. Scanning electron microscopy photos of shot coke spheres
which had been partially fractured show that the outer skin must be a
discrete layer since it is fractured in a different pattern than the core [1,
Other types of coke, notably sponge coke, may also have some embedded
shot coke in the coke structure.
Shot coke is usually produced from very heavy, low API, feedstocks that
have lower levels of aromaticity and higher levels of asphaltenes and
heteroatoms, i.e. nitrogen, sulphur and oxygen and other associated metals.
Presumably there exists a sufficiently high content of aliphatic carbon and
hydrogen and this could lead to significant volatile evolution within the
coking drum .
Shot coke is produced as the oil flows into the coke drum. With the
light ends flashing off, small spherical droplets of heavy tar are suspended
in the flow. These tar droplets rapidly form coke due to the exothermic heat
produced by asphaltene polymerization. Their formation is caused by the
shearing action of a highly turbulent dynamic environment which results from
high fluid velocities and vaporisation of existent and/or cracked products.
Rapid chemical reactions which occur within the droplets while they are
suspended in the vapour phase increase the viscosity of the developing ball
of immature coke. Before the droplets can return back to and become an
integral part of the bulk liquid phase the viscosity has increased
sufficiently to “fix” the macroscopic form and microstructure of the shot
coke. The coke balls formed then fall back into the drum as discrete little
spheres. In the main channel up through the drum, some of the spheres will
roll around and stick together forming large balls as large as 25 cm. When
these large balls are broken, they are found to be composed of many of the
smaller balls [1, 3].
Shot coke can only be produced in a highly turbulent dynamic
environment such as that found in commercial delayed cokers. It is very
difficult to produce in conventional pilot plants with low superficial
velocity without specially designed modifications. Also, it cannot be
produced in a laboratory pot coker; although cokes formed from a shot coke
prone feed stock will exhibit the same microstructure and coefficient of
thermal expansion regardless of whether they are generated in a commercial-
scale delayed or laboratory pot coker. Such cokes produced in a laboratory
pot coker will not however have the microscopic form of shot coke .
The concentration of shot coke particles can vary from only a small
zone in a coke drum to making up the entire coke mass. These particles are
generally harder than sponge coke and can be a problem on cutting and
grinding; and on opening the delayed coker they can pour uncontrolled from
Shot coke is normally found at lower levels in a coke drum but may, on
occasion, be found in the upper section of the drum. Occasionally it may also
appear on top of sponge coke .
The formation of shot coke is favoured in most instances by a shorter
delayed coking cycle, higher superficial velocity, low coke drum pressures,
low recycle ratios and high coke drum temperatures. Such conditions lead to
the increase of the asphaltene content, relative reactivity of the coker
feedstock and immediate coke precursors, as well as the degree of turbulence
in the coke drum. Fluidization in the coke drum may also cause shot coke
formation, and a coker feedstock high in oxygen content can produce shot coke
Reducing the coke drum temperature can be an effective means of
eliminating shot coke formation, but normally increases coke yield. The
additional coke is produced from the internal recycle or heavy gas oil
constituents which alone would produce a well-ordered coke; in combination
they dilute shot coke forming constituents .
Shot coke should not be confused with fluid coke which is formed in the
fluid coking process. This is a completely different process from delayed
coking, in which the coke is produced by spraying the heated residuum into a
fluidized bed of hot coke particles which are maintained at 1.4-2.7 bars and
500°C. The feed vapours are cracked while forming a liquid film on the coke
particles. The residuum is formed immediately into coke with complete
disorientation of the crystals in the hot coke particle. The particles grow
by layers until they are removed and new seed coke particles are added. Fluid
coke has smaller diameter spheres than shot coke.
Shot coke tends to be more isotropic than other types of coke and its
optical texture is of fine-grained mosaic (~ 1 μm dia.) . The carbon in
shot coke is in the form of ribbon and lenticular/granular anisotropic
domains arranged in concentric patterns. Magnification of the surface
fractures show whirled, or nonlinear structure in the isotropic shot coke
Shot coke is unreactive and of relatively low electrical conductivity.
It is characterized by its lack of permeability, low porosity, low Hardgrove
Grindability Index (which may be as low as 2.7) and high density which makes
it difficult to crush. When the Hardgrove Grindability index in a coke drops
suddenly, it is an indication of the coke becoming isotropic and of a
heightened potential for shot coke formation.
The coefficient of thermal expansion is used sometimes to determine a
quantitative value describing coke structure. The coke is calcined, ground to
a flour, mixed with coal tar pitch, extruded to orientate particles into 13
mm rods, baked to 850ºC and graphitized to 2900ºC, and then the difference in
expansion at 0ºC and 50ºC is measured for coefficient of thermal expansion
determination. For shot coke, this coefficient of thermal expansion is rather
high. It is typically greater than 20 (cm/cm/ºC ×10-7) .
Shot coke can have low mineral impurities (metal and sulphur) but it is
usually rich in such impurities because it is produced most often from
feedstocks derived from high sulphur, high metals content crude oils .
Shot coke is in general of low or no commercial value; and although it
may be used as a fuel, it is less desirable in this usage than sponge coke
The chief drawback to its use as a fuel is its extreme hardness which
increases the cost of grinding [1, 4-7].
Shot coke cannot be used in making aluminium anodes. While low
grindability cokes are preferred since such cokes yield calcined coke which
exhibits a very high vibrated bulk density which is desirable in an aluminium
prebaked anode, good bonding with binder coal tar pitch is prevented by the
slickness of the outer layer of the shot spheres, their lack of permeability
and porosity and dissimilarity of the outer needle coke layer of the shot
sphere which has a very low coefficient of thermal expansion and the interior
core of the sphere which, being isotropic, has a very high coefficient of
thermal expansion. The higher coefficient of thermal expansion of the shot
coke is also thought to cause thermal shock cracking of the anode when it is
set in a hot cell [1, 4].
On the other hand, the special properties of shot coke, including its
isotropy, spherical shape, hardness, high coefficient of thermal expansion
and high density make it useful for certain specialty applications, e.g.,
titanium dioxide manufacture, nuclear graphite (if boron content is low), and
graphite mould stocks.
For the manufacture of special graphite for nuclear reactors, gilsonite
coke, i.e. shot coke produced from gilsonite ore, may be used. Air blowing of
a sponge coke precursor before delayed coking can also be used to generate a
shot coke forming feedstock, the ultimate objective being production of an
isotropic coke for nuclear graphite manufacture .
SYRIAN DELAYED COKE
Syrian delayed coke is coke produced by the delayed coking unit at the
Homs Oil Refinery. This unit was designed and built during the late sixties
of last century for the purpose of maximizing gasoline and distillate yields
using a feedstock of residue materials. The Petcoke produced is considered
merely as a by-product of little commercial value. This is mainly because of
its high sulphur content and the high percentage of fines produced.
There are at least four basic types of delayed petcoke with different
microstructures due to differences in operating variables and nature of
feedstock. The basic coke types are: needle coke, honeycomb coke, sponge coke
and shot coke. Of these four types, only the last two types, namely sponge
and shot cokes are produced at the Homs Oil Refinery. In order to
characterise Syrian green petroleum coke, samples of the coke were classified
and divided into five basic types, namely shot coke, clustered shot coke,
porous sponge coke, continuous sponge coke and fines. Significant differences
were observed in the properties of the five types of the Syrian petroleum
coke considered, particularly ash and VM contents .
Thermal treatment of petcoke is the most promising process for the
desulphurization of petcoke, and can be the only one possible when other
techniques prove to be difficult or inefficient as was found in at least one
case with Syrian petcoke [9, 10]. By thermal treatment is meant the process
whereby a fixed static bed of petcoke is heated under atmospheric pressure in
an inert atmosphere to a specified temperature and then kept at that
temperature for a specified period of time .
Thermal treatment of Syrian petcoke in general and sponge coke in
particular has already been made [11, 12, 13] with encouraging results, but
in view of the significant differences between the different coke types,
separate treatment of shot coke was deemed of benefit as shot coke represents
a major part of Syrian coke production.
For the present work, samples of Syrian shot coke were taken from the
coke heaps stored to the west of the Homs Oil refinery. The coke samples were
classified and divided into regular shot coke and clustered or agglomerated
shot coke. Matrix shot or bonded spheres of coke that melted together on
formation were discarded. Tables 1 and 2 give the results of the proximate
and ultimate analysis for the coke samples.
The coke samples were thermally treated in an inert atmosphere of
nitrogen at atmospheric pressure. The treatment was carried out in an
electrical tube furnace heated by a SiC element fully covering the working
tube. The outside diameter of the working tube is 59 mm, and the heated
length is 250 mm. A PtRh-Pt thermocouple is placed in the centre of the
heating zone and is lead to the temperature control unit. The conditions used
in the treatment were such that were expected to lead to a maximum rate of
desulphurization at moderately high temperatures . Table 3 is a summary
of the treatment conditions used. A summary of the results of the thermal
treatment is shown in Table 4.
Table 5 shows the results of sulphur removal for both types of shot
coke as compared with other types of Syrian coke in general. Similar trends
are observed in both cases, with a notable exception in the temperature range
1450-1550 K where a marked drop in the degree of desulphurization of shot
coke was observed. At temperature ≥ 1500 K desulphurization is normally
inhibited by the formation of thermally-stable metal sulphides . This is
mostly the case with shot coke, which is usually rich in metal impurities as
was pointed out above.
In the temperature range 1075 – 1175 K, the expected increased degree
of desulphurization is not observed in the case of the clustered shot coke,
with only 6% sulphur removal as compared to 19% for regular shot coke and 18%
for other types of coke. Most of the sulphur removed during this stage is
derived from the decomposition of the thermally-stable sulphur compounds
bound in side chains . These results may indicate a significant
difference between clustered coke and other types of coke in the amount
and/or the nature of sulphur compounds bound in side chains, but further work
is necessary before a definite conclusion is reached.
The true density of petcoke is expected to increase continuously with
increasing treatment temperature. The rate of this increase is different,
however, at different temperature ranges. Three stages of density change were
1. An initial stage (300 – 800 K), with minimal density increase due probably
to the removal of moisture and some volatile matter in the coke. The density
increase observed was 0.05 g/cm3 in the case of regular shot coke.
2. A second stage (800 – 1200) characterized by rapid increase in density
related to the evaporation of the volatile matter adsorbed on the coke
surface or in the pores. For regular shot coke, with 9.9 VM (wt.%), the
density increased during this stage by 0.47 g/cm3.
3. A final stage (1200 – 1700), where the density increase may be related to
the rate of sulphur removal. The density increased by 0.10 g/cm3 for regular
The calorific value decreases, in general, with increasing temperature
of the thermal treatment. However, there were observed two exceptions to this
rule, where the calorific value increased rather than decreased.
A slight increase in the calorific value was observed towards the end
of the first initial stage of thermal treatment (300-1200 K). This is the
overall effect of the evaporation and removal of moisture and volatile matter
which take place during this stage, where the removal of moisture, as an
inert material, has an opposite effect on the calorific value to that of
removing the volatile matter. Whereas the removal of moisture is accompanied
by an increase of the calorific value, the removal of the volatile matter
tends to lower this value. The overall effect of the thermal treatment is
therefore a factor of both volatile matter and moisture content. Since the
volatile matter content of shot coke is lower, in general, than for other
types of coke, the effect of the removal of VM on the calorific value is
expected to be less also, with the result that the increase in the calorific
value is more pronounced for shot coke.
The calorific value was also observed to increase slightly in the
temperature range 1650–1700 K. This must be related no doubt to the decreased
sulphur content, as the heat of combustion of sulphur (9420 kJ/kg) is
considerably less than that of carbon (33820 kJ/kg). A similar result was
also obtained with other types of coke .
The observed weight loss at the conclusion of the thermal treatment
varied between 19 and 21%. This is in agreement with the generally observed
weight loss for other types of coke which is normally of the order of 20% or
so. This amount corresponds to the moisture and volatile matter content of
the coke as well as to the amount of sulphur removed.
Effective desulphurization of shot coke was achieved by means of
thermal treatment to a temperature of 1700 K and increased residence time
(180 minutes). The treated coke has a low sulphur content (1%) and a high
real density (1.9 g/cm3). The adverse effects normally associated with thermal
treatment at high temperatures were minimal.
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VM Volatile matter