1
Desulphurization of Petroleum Coke: A Review
1
A number of processes ranging from purely physical such as solvent extraction
to thermal or chemical treatment using different agents including hydrocarbon
gases, alkali metal compounds, and hydrogen have been proposed for petroleum
coke desulphurization. The complexity of the sulfur-carbon bonds in coke,
together with the extremely variable nature of the coke itself, has precluded
a theoretical study with the emphasis being laid on empirical data as a
consequence. The conclusion that has emerged from this review is that a
thermal treatment using a suitable agent is essential for effective
desulphurization at temperatures less than 1100 K.
Introduction
Petroleum coke, or petcoke for short, is no longer a left-over byproduct of the “bottom-of-the barrel” refinery processes whose chief purpose
is the production of other products. Petcoke has become a valuable product in
its own right, and the demand for high-quality low-sulfur coke is increasing.
However, more coke with high sulfur content is being produced, and means
whereby such sulfur content is reduced to an acceptable level or eliminated
altogether
are
called
for,
in
particular
with
the
ever-tightening
restrictions on sulfur oxide emissions for environmental considerations.
Petcoke can be produced from virgin crude residues by precipitation
reactions of high molecular weight compounds, asphaltenes, and resins or from
highly aromatic tar or decanted oil stokes by condensation and polymerization
of aromatic compounds. Delayed petcoke is produced by a semicontinuous
process which can be carried through in one of the following ways: ultimate,
once through, or intermediate coking [1]. Delayed petcoke has a much more
uniform crystallinity than other types of petcoke since the delayed coking
process allows the time needed for the coke crystals to orient themselves
upon one another [2]. Green coke is made of petroleum pitches and residues by
any of several coking processes, chief of which are delayed coking, contact
coking, fluid coking, and flexicoking. Green delayed coke is a petcoke which
has not been calcined for the removal of moisture and excess volatile matter
[3]. The pores in the green coke are filled, probably with a hardened
residuum from the coker feed which distills off during calcination exposing
the pores and typical lamellar structure of the calcined coke. The main types
of green delayed petcoke include needle coke, honeycomb, sponge coke, and
shot coke [3-8].
The green coke has a low ash content between 0.3 and 0.5 wt % , and ita
fixed carbon content is generally between 83 and 90 wt % (3). A typical
chemical composition of green petcoke as obtained by ultimate analysis showed
that the total sulfur content was 1.29 wt % and the fixed carbon content was
91.8 wt % [9]. The volatile matter is composed of heavy hydrocarbons
deposited in the coke matrix. It is generally between 9 and 21 wt % and is
mainly a function of the coking drum temperature. An increase of the drum
temperature vaporizes off the lower-sulfur-bearing oils from the coke leaving
a more concentrated sulfur-bearing coke [7].
Sulfur in Petcoke
The sulfur content of the petcoke strongly depends on the nature of the
coking feedstock (crude oil) and its sulphur content. The sulfur content of
the feedstocks seems to increase with increasing the concentration of
1
Hassan Al-Haj-Ibrahim and Badie I.
Chemistry Research, 1992, 31, 1835-1840.
Morsi,
Industrial
and
Engineering
2
asphaltenes and Conradson carbon content. For instance, higher sulphur
contents were found in “sponge” coke (produced from high-resin asphaltene
feedstocks) than in “honeycomb” coke (produced from low-resin asphaltene
feedstocks) or “needle” coke (produced from highly-aromatic feedstocks).
The sulfur content in petcoke varies widely (from less than 0.5% in
gilsonite to more than 10%) mainly depending on the sulfur content of
feedstock [10]. Typically, sponge coke contains between 1 % and 6%. Sponge
coke containing 4% is used for fuel whereas that of less than 4% sulfur
content is used in anode manufacturing. Needle cokes for electrode
manufacturing are required to have lese than 1% sulfur content [11].
Sulfur contents in sponge cokes have been correlated to the sulfur
content and Conradson carbon residue of feedstock by Jacob (7). He found that
the sulfur content of sponge coke increases almost proportionally to the
feedstock sulfur contents and increases less strongly with increasing
Conradson carbon residue.
Coking temperature also affects the sulfur content of petcoke, though
in less degree, mainly due to the vaporization and removal of the low sulfur
containing volatile matter which result in a reduction of the total sulphur
content in the coke [7].
Most of the sulfur in petcoke exists as organic sulphur bound to the
carbon matrix of the coke [12]. Some sulphur could also exist as sulfates and
as pyritic sulfur [13] but these do not in general make up more than 0.02% of
the total sulfur in coke (3). In at least one case, however, pyritic sulfur
was reported to be as high as 0.4% (14). Free sulfur may occasionally be
present [1].
The structure of organic sulfur compounds in petcoke remains largely
unknown, and no precise analytical methods exist today to determine exactly
this structure [3, 15]. The organic sulfur compounds identified by the Bureau
of Mines in four crude oils include thiols (alkyl, cyclic, and aromatic),
sulfides (alkyl, cyclic, and alkylcycloalkyl), disulfides (alkyl, cyclic, and
aromatic), and thiophenes [16]. The thiophenes are most prevalent in the
heavy fractions of crude oils and hence in coke. Other sulfur compounds are
less prevalent in coke. Work done by Sabott [17] indicates that sulfur may
not be present in coke in the form of thiols or aromatic and aliphatic
sulfides, and sulfides do not make in fact more than 0.003% of the total
sulfur in petcoke [3].
Sulfur may exist in the coke in many forms [3}: (a) as thiophenes
attached to the aromatic carbon skeleton; (b) attached to side chains of
aromatic or naphthenic molecules; (c) between the aromatic sheets or on the
surface of clustered molecules; (d) on the coke surface or in coke pores
bound by capillary condensation, adsorption, or chemisorption.
Desulphurization of petcoke
The desulphurization of petcoke involves the desorption of the sulfur
present in the coke pores or on the coke surface, and the partition and
removal of the sulfur attached to the aromatic carbon skeleton. For the
removal of sulfur in the first category a purely thermal treatment at
temperatures less than 1100 K is generally sufficient. A more sever treatment
and/or the use of chemicals is, however, necessary for the removal of the
sulfur attached to the carbon skeleton, particularly in the case of the
thiophenic sulfur which is much more stable than the other organic sulfur
compounds and therefore much more difficult to remove. This makes it evident
why the effective desulphurization of petcoke, involving as it does the
rupture of the thiophenes, is not as simple a process as may be desired. The
thiophenes do make up most of the sulfur present in the petcoke. They, on the
other hand, are much less stable chemically than their aromatic isologs, and
it is always possible to find compounds that react more readily with the
thiophenes than with the aromatic or other compounds of the coke structure
[18], a fact which can be made use of in desulphurization processes.
3
Processes for petcoke desulphurization
Although much experimental work has been done on the desulphurization
of petcoke, there is still as yet no commercial process for desulphurization
[19]. One common feature of all, or most, published work is its exclusively
empirical nature. This is presumably because a theoretical, thermodynamic and
kinetic, study is not possible due to lack of essential data on the nature of
S-C bonds, their free energy change (ΔG), and the manner and speed of sulfur
replacement. In addition, some important factors such as the coke structure
and
porosity
undergo
significant
changes
during
the
course
of
desulphurization, a fact which makes the problem even more complicated [20].
1. Solvent extraction. Solvent extraction would offer the simplest approach
to desulphurization if it were possible to selectively dissolve the organic
sulphur compounds present in the coke. As materials of similar chemical
structure are more likely to be mutually soluble in one another, aromatic and
similar compounds might be used as solvents. Experience with coal indicates
that weak organic acids such as phenols and nitrobenzene are more effective
than other organic solvents [16]. These could be used to dissolve sulfides
and disulfides and possibly some thiophenes as well.
Extractions with coke using a large variety of solvents were made in a
Soxhlet extractor. No sulphur removal was reported when petroleum ether [21],
dioxane [21] or hydrochloric acid [17, 21] were used. Extractions made with
other solvents led to some sulphur removal, but in no case was more than 20%
sulphur removed (Table 1).
This indicates clearly that solvent extraction is not an effective
method of desulphurization. However, the selectivity of solvent extraction
would be enhanced if the coke macromolecule could be cleaved. Coke
depolymerization can be effected by different methods including mild
hydrogenation, oxidation and pre-pyrolysis. The effectiveness of these
depolymerization techniques has not been investigated. Phillips and Chao [14]
found that increasing the extraction temperature improved desulphurization
from 11% at 290 K to 20% at 430 K. This can be explained by the fact that
some coke depolymerization must have taken place as a result of the
extraction temperature increase.
2. Thermal desulphurization. By thermal desulphurization 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. This process was felt to be 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 [12]. Further, petcokes
are
normally
calcined
up
to
a
temperature
of
about
1700
K
and
desulphurization could be an added asset if it can be shown to take place to
a significant degree within this temperature range. The efficiency of
desulphurization, however, is not only dependent on the maximum temperature
to which the coke is subjected, but other factors affect it also including
rate of heating, gas atmosphere and in particular residence time at the
maximum temperature. In order to neutralize the effect of this last factor, a
constant value of 30 min residence time was assigned to it throughout and a
comparison of the effects of temperature were made on this basis. Table 2
gives a summary of the maximum desulphurization achieved under these
conditions at different maximum temperatures. An examination of this table
makes it evident that the process of thermal desulphurization can be divided
into four stages:
(a) The first is an initial phase of desulphurization (300-1100 K), with the
desorption of sulphur bound on the surface or in the pores, and the
simultaneous cracking of side chains of aromatic molecules. The sulphur bound
on the surface refers to the S-C bonds in the outer layers of clusters (pre
4
walls, surfaces) that can be physically exposed to the fluid environments.
The process of sulfur separation starts at 770-820 K, first increasing to
reach a maximum at about 1000 K and then decreasing until it stops at 1270 K
[22]. The maximum amount of sulphur removed in this stage is about 25% (Table
2), which can be taken as a rough indication of the amount of non-thiophenic
sulphur in coke.
Experimental work indicates that no reaction occurs during this stage
between metal contaminants and sulphur gases, since no variation in the
degree of desulphurization of different cokes with different amounts of
metals was observed [22].
(b) A second stage is that in which little or no desulphurization takes place
(1100-1600 K), particularly in the case of coke made from aromatic-type
feedstock. Most of the sulphur removed is derived from the decomposition of
the thermally-stable sulphur hydrocarbons bound in side chains.
Ash and Metal contaminants seem to have no effect on desulphurization
up to a temperature of 1500 K [22, 24, 25]. The inhibiting effect of ash
would be related to its amount and composition [26].
Metal-hydrocarbon compounds refer mainly to vanadium and nickel metals
that are trapped in the porphyrinic structures. These metals, certainly,
survive coking temperatures [23]. It should be also mentioned that the metals
in the feedstock end up in the coke structure. At temperatures greater than
or equal to 1500 K desulphurization is significantly depressed by the metalhydrocarbon compounds which react with the dissociated sulphur to form
refractory, thermally-stable metal sulphur compounds and sulfides [13, 16,
22, 24].
(c) Desulphurization is dramatically increased when coke is heated above 1600
K. The energy available at 1700 K is sufficiently high for the decomposition
of sulphur-hydrocarbon compounds of stabilities up to those of thiophene
structure [22]. Complete elimination of sulphur is not, however, likely even
at this high temperature [25, 27]. The removal of sulphur is connected with
the creation of an organized phase detectable by X-ray methods [20]. An
organized
phase
refers
to
extended
two-dimensional
and
very
short
tridimensional arrangements of aromatic layers that resemble graphitic
structures which are typical characteristics of needle cokes.
(d) Further increase in temperature (> 1800 K) is not certain to lead to more
desulphurization, but this depends also on the nature of the coke.
The degree of desulphurization is directly related to the total sulphur
content of the coke. Gimaev and Syunyaev [28], on the other hand, found that
for a particular type of coke the degree of desulphurization is affected only
by the temperature and is independent of the initial amount of sulphur.
The high temperature required for thermal desulphurization can affect
some critical coke properties [21] and its effect is much more than the
effect of residence time [29]. The apparent density increases from 1.2-1.4 to
1.6-2.0 g/cm3 on heating to 1600 K [15]. Above 1600 K density changes depend
on the initial sulphur content, decreasing with cokes of high sulphur content
and increasing with cokes of low sulphur content. Real density increases with
temperature up to 1700 K, and then decreases [3, 5, 30]. The decrease of real
density above 1700 K called “puffing” is a consequence of the porosity
developed when the sulphur gets out of the coke matrix at temperatures above
1700 K. In applications above these temperatures, such as arc electric
furnaces, this puffing effect can reach levels at which large pores are
opened, and stresses are concentrated leading to breakage of graphitic
electrodes. For this reason a dynamic puffing test is specified as a part of
the characterization of filler cokes for these applications. The use of
puffing inhibitors, such as Fe2O3, generally at 2 ppm level can reduce the
puffing effect to an acceptable level [23].
Coke porosity increases with temperature and so does its reactivity
[31]. The surface area increases to a maximum at about 1000 K and then
decreases to a minimum at about 1300 K, beyond which there is a further
increase in surface area [30]. Reis [3] found that the electric resistivity
5
of petcoke was reduced with increasing temperature up to 1600 K, but higher
temperatures increase the electric resistivity which reaches a maximum at
2300 K [3].
Most desulphurization seems to occur within the first 30 minutes of the
thermal treatment up to 1700 K. According to Akhmetov and Syunyaev [32] the
highest rate of desulphurization is within the first 5 minutes, but this does
not seem to apply in all cases as can be shown by the results of Brandt and
Kapner [19]. Increasing the residence time has no effect on the degree of
desulphurization except in the range 1600-1800 k [13, 19, 22, 25, 27].
It is not clear whether the elimination of sulphur is affected in any
way by the inert gas used during the thermal treatment [33]. Most workers
used nitrogen or argon. Carbon-nitrogen complexes are not likely to be formed
when petcoke is heated in nitrogen [12]. The use of argon may inhibit
desulphurization at lower temperatures, but more work is needed for more
conclusive results.
Thermal desulphurization under vacuum does not seem to be beneficial to
desulphurization [20]. Decreasing the mean diameter of coke particles may
improve the desulphurization efficiency [1, 32, 34], but there is no
consensus on this point [13]. The use of a fluidized bed may improve the
desulphurization efficiency [34].
3. Oxidative desulphurization. The use of an oxidizing gas during the thermal
treatment of petcoke may favor the removal of sulphur. This is related to the
resulting weight loss of the coke and its improved porosity.
Heating in CO2 inhibits desulphurization. In the experiments carried
out by Hussein et. Al. [12] little desulphurization was observed below 1500 K
(12%), but desulphurization was rapid at greater temperatures (67% at 1800
K). This was accompanied by lower coke yield.
When petcoke was heated in oxygen, no sulphur was removed below a
certain temperature (1000 k [33] or 1600 K [34]). Air can be used as an
oxidizing gas. When coal was heated to 450˚C in air, a maximum of 20% of the
organic sulphur was removed [16]. No tests with coke are reported. Petcoke
can be desulfurized by heating under steam. In the temperature range of 9001300 K a maximum desulphurization of 31% was obtained [35, 36] and similar
results were reported for coal [6]. More sulphur can be removed by raising
the temperature to 1600-1800 K. This value reported for petcoke [35, 36]
appears to be extremely high when compared with the desulphurization data
using oxygen [34]. The use of a mixture of air and steam seemed to inhibit
desulphurization [35].
Oxidizing agents might be used to oxidize thiols, sulfides, and
disulfides to sulfonic acids which can be hydrolyzed in boiling water to
eliminate H2SO4. Nitric acid was used for this purpose by Phillips and Chao
[14] and Syunyaev et. al. [37], and a maximum desulphurization of 31% was
achieved [14]. This was accompanied by an increase in oxygen and nitrogen and
a decrease in carbon content [37].
4. Desulphurization in an atmosphere of sulfur-bearing gas. Exposing the coke
to H2S in the temperature range 1000-1200 K increases the sulphur content of
the coke. H2S reacts with the incandescent carbon to form more stable carbonsulfur compounds [19, 35, 38, 39]. At elevated temperatures, however,
sulphur-bearing gases decompose to form free sulphur or react to form free
sulphur and thus induce desulphurization, with the preservation of the
critical coke properties. Eighty-four percent sulphur removal was obtained by
Brandt and Kapner at 1600 k [19].
5. Treatment in hydrocarbon gases. Most laboratory work indicates that the
removal of sulphur can be induced at elevated temperatures by fluidizing the
hot petcoke with low molecular weight hydrocarbon gases. These decompose to
liberate hydrogen, which assists desulphurization, while the carbon is
deposited in the coke pores increasing thereby the coke density [19, 40].
Table 3 gives a summary of maximum desulphurization obtained when different
hydrocarbon gases were used.
6
6. Treatment with alkali metal compounds. Coke may be desulfurized by
treatment with an alhali metal compound. More than 98% sulphur can be removed
using NaOH [38, 41, 42]. Other compounds may also be used with varying
degrees of success [41, 42]. Different techniques were suggested in order to
improve the desulphurization efficiency including preactivating the coke by
fluidizing with air [43] or steam [44] or exposing the coke sample to
microwave radiation [44]. A major disadvantage of this process is the
consumption of large amounts of alkali compounds [38]. The presence of the
alkali metal compounds in the treated coke is also indicated.
7. Hydrodesulphurization (HDS). Hydrodesulphurization is a common process of
desulphurization in the oil industry, but it has not yet found application in
the treatment of petcoke. The hydrodesulphurization reaction involves a
number of steps. For the initial reaction, the diffusion of hydrogen into the
pores of the granules appears to be the rate controlling step. As the
reaction progresses, the rate-controlling step shifts to the reaction of
hydrogen with sulphur compounds [46]. At 900 k it may be inferred that the
chemical surface reaction and not the physical process is rate controlling
[35]. At higher temperatures (> 1000 K) the reaction of H 2S with coke
particles occurs more rapidly leading to the formation of new stable C-S
compounds (the reverse reaction).
In simple HDS petcoke is heated in a fixed bed under an atmosphere of
hydrogen. Experimental work indicates that the rate of sulphur removal is
greatest at about 1000 K, where about 50% of the sulphur is removed, but
higher temperatures (1600 K) are required for the removal of the more
refractory sulphur. At temperatures above 1300 K and below 1500 K sulphur
removal is not apparent [20]. The maximum desulphurization at the optimum
temperature
can
be
explained
by
the
fact
that
the
net
rate
of
desulphurization is the result of two competing processes, namely the removal
of the labile sulphur by hydrogen and the thermal fixation of the sulphur by
the reverse reaction which produces a more stable form of sulphur. The
reverse reaction becomes more rapid at temperatures in the range 1000-1200 K.
At elevated temperatures (1500-1800 K) the presence of H2S would induce
sulphur removal, a fact which helps to explain the degree of desulphurization
obtainable at 1600 K (92%).
The efficiency of HDS may be improved by increasing the hydrogen flow
rate to an optimum value [39] or by increasing the pressure. The use of
fluidized bed desulphurization leads to greater desulphurization at reduced
flow rates. This is probably due to the more efficient contact between
hydrogen and coke [21].
Cooling the coke at different intervals during HDS improved the level
of desulphurization, but this was effective only at certain critical
temperatures (1000 K) [47, 48]. Similarly the effect of heating the coke in
an inert atmosphere prior to introducing the hydrogen depended largely on the
temperature at which the hydrogen was introduced, with the optimum
temperature at about 720 K [49]. When the coke is heated in an inert
atmosphere,
cooled,
and
then
heated
in
hydrogen,
the
level
of
desulphurization is either decreased [33] or greatly enhanced [50] in
comparison with simple HDS depending on the pre-treatment temperature.
Preoxidizing the coke at moderate temperatures may render it more
amenable to HDS [51], but the advantage of this treatment is offset by the
increased weight loss. The optimum temperature for maximum desulphurization
and minimum weight loss was found to be in the range 570-620 K [48, 51-53].
The presence of steam in HDS can have variable effects depending on the
temperature and/or steam flow rate [33, 42, 45]. Impregnating the coke with
an aqueous solution of NaOH prior to HDS enhances desulphurization [54].
Miyoshi et. al. found that alkali desulphurization proceeded smoothly under
hydrogen when the coke was exposed to microwave radiation [45].
7
Conclusions
Table 4 gives a summary of the most effective desulfurization processes
(with sulphur removal greater than 84%). These are grouped into four classes
according to the maximum temperature required for effective desulfurization.
At temperatures >1700 K, effective desulfurization can be achieved by
purely thermal treatment. Steam, sulphur-bearing gases or effluent gas
recirculation can also be used, but with little advantage.
In the temperature range 1400-1600 K, effective desulfurization can be
achieved by simple HDS or on treatment with unsaturated hydrocarbon gases.
In the temperature range 1170-1200 K, effective desulfurization can be
achieved either by treatment with some alkali metal compounds (Na2CO3, K2CO3)
or by preoxidation and subsequent treatment with methane.
At temperatures lower than 1100 K, two processes are possible: (a) HDS
using high pressures, NaOH impregnation, preoxidation or fluidization; (b)
Treatment with NaOH or KOH. Given the fact that the consumption of great
amount of alkali metal compounds is required for effective desulfurization
and that preoxidation reduces the coke yield, it becomes evident that HDS in
a fluidized bed offers the most attractive desulfurization process. This
process is expected to be more effective with cokes of high sulphur content
and high ash and heavy metals content, as at temperatures less than 1100 K
the adverse effects of high ash and heavy metals content on desulfurization
is largely avoided. The process would result in an increased porosity and
surface area and improved reactivity. These properties are dependent on the
sulphur concentration. The greater the amount of sulphur removed, the greater
the structural changes expected in the coke treated. The coke density is
expected to increase as a result of the treatment, and there will be some
weight loss (<10%).
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Table 1. Maximum desulfurization of petcoke by
extraction with different solvents
Solvent
Max. Desulf, %
Ref
10
o-chlorophenol
Pyridine
Aqua regia
Phenol
Furfural
Chlorex
Naphthalene
p-cresol
Xylene
Benzene
Nitrobenzene
Ethanolamine
Toluene
Acetone
CCl4
CS2
20
19
14
14
14
13
13
9
9
7
7
6
5
2
1
1
14
17, 21, 35
17, 21
17
17, 35
17
17
35
21,35
17, 21, 33, 35, 39
35
17, 21
17, 21, 35
33
17, 21
17, 21, 33
Table 2. Maximum desulfurization as a function
of temperature [5, 12, 13, 22, 25, 27, 48, 55]
Temp, K
Max. Desulf, %
Temp, K
Max. Desulf, %
1100
26
1600
32
1300
28
1700
64
1400
30
1800
91
1500
27
1900
96
Table 3. Maximum desulfurization of petcoke on
treatment with hydrocarbon gases
Gas used
Temp. K
Max. Desulf. %
Methane
1140
33
Methane
1200
90
Butagas
1070
26
Effluent gas recirculation
1770
90
Butene-2
1370
92
Ethylene
1370
93
Propylene
1470
94
Ref.
33
19
39
57, 58
17
17
17
Table 4. Processes of effective desulfurization (>84%)
Process
Temp. K
Heating in sulphur-bearing gas
1810
Heating with effluent gas recirculation
1770
Heating in steam
1770
Thermal treatment
1670-1770
Simple HDS
1570
Treatment with unsaturated hydrocarbon gases
1370-1470
Preoxidation + treatment with methane
1200
Treatment with Na2CO3 or K2CO3
1170
HDS at high pressure
1070
Desulf. %
84
90
87
89-96
92
92-94
90
93
87
11
Treatment with NaOH
HDS of coke preoxidized at 610 K
HDS with NaOH impregnation
Treatment with KOH
HDS in a fluidized bed
870-1030
1030
1020
920
870
98
87
86
97
88