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Sodium hydroxide-assisted desulphurization of petroleum fluid coke
Sodium hydroxide-assisted of petroleum fluid coke Zacheria
M. George
desulphurization
and Linda G. Schneider
Alberta Research Council, 17315-87 (Received 1 July 1982)
Avenue, Edmonton, Alberta, T6G 2C2, Canada
Desulphurization of a fluid coke produced commercially from a conventional petroleum crude oil was attempted. Direct hydrodesulphurization of the coke at 700°C resulted in ~31 w% sulphur removal; however, impregnation of the fluid coke with trace amounts of sodium hydroxide and subsequent hydrodesulphurization resulted in > 80 wp/o sulphur removal primarily as Hz!% A significant part of the alkaline reagent could be recovered by hot water leaching of the desulphurized coke. The calorific value of the desulphurized coke is slightly lower than that of the starting material. The mechanism appears to be complex as the change in surface area was negligible upon impregnation and hydrodesulphurization. Economic evaluation of the desulphurization process, carried out at the Alberta Research Council, indicates that it has significant economic advantages over fluidized-bed combustion of the coke with limestone or combustion of the coke with flue gas desulphurization. (Keywords: desulphurization; fluid; coke; sodium hydroxide)
Although much work has been carried out on methods of desulphurizing coal and coal chars, few studies are reported in the literature on the desulphurization of petroleum cokes. In coal, a significant portion of the sulphur may be present as inorganic sulphides and methods such as that of Meyer1 are very efficient for removal of these sulphur compounds. In petroleum cokes however, a considerable part of the sulphur may be present as organic sulphur compounds and these compounds are not easily desulphurized. Further, petroleum cokes exhibit significant differences depending upon the origin of the petroleum, coking process employed and the amount and type of sulphur compounds present in the petroleum. El Kaddah and Ezz’ investigated thermal desulphurization of petroleum coke containing 8.3 wt% sulphur and observed 30 wt% sulphur removal in 30 min at 1600°C. Sef3 studied hydrodesulphurization of petroleum coke containing 2 wt% sulphur and reported 85 wt% sulphur removal using small particles of coke, a pressure of 659 Pa, and high space velocities. Mahmoud et al4 observed a maximum at 600°C for the hydrodesulphurization of petroleum coke containing 3 wt% sulphur, this maximum being attributed to the onset of sintering. George5 and Tollefson and Parmar have studied the hydrodesulphurization of Athabasca oil sands delayed coke and observed that the level of desulphurization was not significant under the experimental conditions and that the reaction was controlled by pore diffusion. Mason7 has reported the beneficial effects of preoxidation of coke on the however, the subsequent hydrodesulphurization; optimum conditions of the preoxidation appear to vary widely. Thakker* showed that impregnation of petroleum coke with sodium carbonate enhances the level of sulphur removal during hydrogenation. Ridley’ reported that 001~2361/82/12126%07%3.00 @ 1982 Butterworth & Co (Publishers)
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sodium hydroxide and sodium sulphide when impregnated on coke particles aided desulphurization. Lukasiewicz and Johnson” and Sabott” observed that significant desulphurization of petroleum coke can be achieved by impregnating the coke with an alkaline reagent, calcination of the impregnated coke in an inert atmosphere at elevated temperatures and subsequent hot water leaching of the coke to remove the metal sulphides. Parmar and Tollefson6 employed a fluidized-bed reactor to evaluate several methods to desulphurize delayed coke produced by Suncor Canada. George, Parmar and Tollefson” investigated the desulphurization of oil sands delayed coke involving impregnation with an alkaline reagent, and subsequent calcination, and George, Tollefson’3 have reported the Schneider and desulphurization of a high sulphur fluid coke by this method. These experiments showed that to achieve significant desulphurization, large (twice stoichiometric amount to form metal sulphides) quantities of the alkaline reagents were necessary and posed serious corrosion and environmental problems. The objective of this investigation was to develop an economically attractive process for the desulphurization of petroleum coke prior to combustion. A sample of petroleum fluid coke (sulphur content 7.3 f 0.3 wt%) was obtained from Getty Oil Company, Delaware, USA, and an attempt to desulphurize this coke was made so that the sulphur dioxide level produced during combustion may be tolerated with minimum environmental damage. Whilst investigating methods of desulphurizing this petroleum coke, it was observed that if the fluid coke were impregnated with a small amount of base, such as sodium hydroxide, and then hydrodesulphurized at M700°C significant sulphur removal could be achieved. This Paper summerizes results on this method of desulphurizing a sample of Getty fluid coke.
Desulphurization Tab/e 1 Analysisof Carbon Hydrogen Sulphur Nitrogen Ash Nickel Vanadium
Gettyfluid
of petroleum fluid coke: 2. M. George and 1. G. Schneider
coke (db)
hydrogen flow established, and the furnace switched on. The reactor was heated by a Lindberg heavy duty furnace (type 59344) equipped with a controller (_+5’C). It took = 30 min for the furnace to reach the desired temperature (Figure 2). The rate of hydrogen flow, 120 ml mine’,. was measured at the reactor outlet under ambient condltlons. The product stream was sampled and analysed every 5 min by gas chromatography and the reactor effluent was scrubbed with NaOH prior to venting.
86.9 wt % 1.8wt% 7.3 f 0.3 wl % 1.3wt% 0.1 w-t% 1
30 PPm 225 ppm
Ash
3. Leaching. After hyprodesulphurization the coke was leached with tap water (E 5 g coke/500 ml H,O) at 80°C for 12 h. The water was then decanted and the sample
Tab/e 2 Sulphur content of fluid cokes examined Source Getty Coke, Delaware, Petrofina, Montreal Imperial Oil, Sarnia
Sulphur USA
mntent
(wt %)
7.3 ?: 0.3 6.4 f 0.3 3.2 + 0.2
EXPERIMENTAL Materials
Hz
Most of the experiments were carried out using fluid coke obtained from Getty Oil Company, Delaware, USA. Analysis of this coke is summarized in Table 1. Limited experiments were performed on samples of fluid coke obtained from Petrofina, Montreal, and Imperial Oil, Sarnia, Canada. The sulphur contents of these cokes are listed in Table 2. The reagents used in this study were all as-received grade. The surface area of the coke samples was usually determined by the high speed surface area analyser (Micromeritics Model 2200) and periodically by the BET measurements.* The coke sample was activated in dry helium for 4 h at 100°C before surface area measurements. For the sodium hydroxide-assisted desulphurization of fluid coke the following steps are involved: (1) impregnation of the coke particles with a suitable alkaline reagent; (2) hydrodesulphurization; (3) leaching to remove the alkaline reagent; (4) sulphur and recover determination in the coke. 1. Impregnation of the coke with alkaline reagents. Coke granules, 40/60 US mesh, were slurried with an aqueous solution of the alkaline reagent and evaporated to dryness at x 80°C with stirring. The ratio of the weight of alkaline reagent to the weight of coke is defined as the weight ratio, W/R. Most of the experiments reported here involved impregnation with 1M NaOH. Impregnation and drying at higher temperatures or at room temperature resulted in significantly lower desulphurizations during the hydrogenation step. A few experiments were also carried out using KOH and LiOH. Impregnation in an air or inert atmosphere did not appear to influence the level of sulphur removal in subsequent hydrogenation. 2. Hydrodesulphurization. A fixed-bed flow reactor system (Figure I) was constructed from 316 stainless steel tubing except for the reactor which was made out of quartz tubing.? The reactor consisted of a 3.2 cm i.d. x 61 cm long quartz tube and had a quartz fibre plug midway to support the coke sample. The reagent-loaded coke (5.0 g) was charged into the reactor at room temperature, a *
BET surface area measured
by N, adsorption check the surface area measured by the one-point 7 Initially, a stainless steel reactor was employed; sulphided and interfered with gc. analysis.
at 77K was only to system but the reactor was
He O2
HZS Flow meter H20
ie
Flow reactor for coke desulphurization. D, Sampling Figure 1 10-0~; V, 7-port sampling valve; m, metering valve; T, thermomuple; Tc, thermal mnductivity detector; CON, controller for the furnace; El, electronic integrator; PS, Power supply for gas chromatograph
8o08533
Time (mln) Coke temperature (0) and H.# mncentration in the f&urel reactor effluent gas; (A) as a function of time after reactor start-up. 5.0 g coke sample, 40160 US mesh, 2 h, 700°C, Hz flow = 120 ml min-I, W/t? = 0.04
FUEL, 1982, Vol 61, December
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Desulphurization Table3
NaOH
Summary
of petroleum fluid coke: Z. M. George and L. G. Schneider
of preliminary
experiments,
Getty
Desulphurization medium
loading
fluid coke, 7OO”C, 40160
Desulphurization
None
N2
2
N2
29
“2
31 90 85 None
‘42
H, wet None
Surface area before leaching (m2 g-l)
(96)
W/R = 0.04 None W/R = 0.04 W/R = 0.04 W/R = 0.04
US mesh, 2 h, 5.0 g sample.
11
Gas flow:
120 ml min-’
Surface area after leaching (m2 9-l)
(Ml
11
30.24
3 3 5
kg-‘)
29.08a
Wt NaOH(g)
W/R = Weight ratio, Wt coke(g) a Leached sample
dried at 100°C in air. Leaching was practised as an integral part of desulphurization as the NaOH used for the impregnation of the coke is an expensive component of desulphurization which could be recovered by leaching and may be used for reagent make-up for further impregnation.* Also, the presence of alkaline compounds in the coke may lead to serious problems in the boiler tubes during combustion. The rate of leaching was significant initially but declined with time. Approximately 50% of the base could be extracted in E 1 h and =7&75x in 12 h. Because of the small amount of NaOH used in these experiments to achieve >80% desulphurization and as a significant portion of the alkaline reagent may be recovered by leaching and reused, the NaOH used appeared to act as a catalyst. 4. Determination of sulphur removal. Gas chromatographic analysis and high temperature combustion (ASTM D-3177-75) were employed independently to determine desulphurization via H,S and the total sulphur removal, respectively. G.c. analysis To quantitate sulphur removal from the coke as H,S or other gaseous sulphur compounds during hydrodesulphurization, the effluent of the reactor was analysed by g.c. every 5 min. By switching a six-port sampling valve (Figure Z),a sample of the effluent gas (2.0 ml) was swept into the analytical column, 8 feet of Poropak Q followed by 2 feet of Poropak T, maintained at 15O”C,and analysed over a calibrated thermal conductivity detector. During the initial stages of hydrodesulphurization, CO, CH,, CO,, H,S and H,O were detected. Only H,S and H,O remained during the later stages. COS, CS, and SO, were not detected. Generally, the H,S concentration profile in the product followed that of H,O. Integration of the H,S area under the peak (i.e. graph of the partial pressure of H,S uersus time, Figure 2) was used to determine the extent of desulphurization by H,S. High temperature sulphur determination
This method is designed specifically for the rapid determination of sulphur in coal and coke and consists of burning a sample of coke within a tube furnace at * 1000°C in a stream of oxygen. Sulphur oxides are absorbed in a hydrogen peroxide solution, yielding sulphuric acid (equation 1) which is titrated against standard NaOH to a pH of 4.5. SO, + H,O, -+H,SO,
(1)
The method was tested against the Standard Eschka methodI and agreement with 4% was obtained. Duplicate analysis of the desulphurized and leached sample as well as a single analysis of the starting material, were made for each The percentage desulphurization was experiment. determined by comparing the percentage of sulphur in the residue with that of the starting sample. Any residual basic sodium compounds in the coke have been shown to form the corresponding metal sulphates and these do not decompose to form SO, during combustion. Unless specifically stated, high temperature combustion was used for sulphur determination.
RESULTS Preliminary
experiments
Before investigating the desulphurization of NaOHassisted fluid coke in detail, experiments were conducted to investigate the sulphur removal by volatilization (calcination to 700°C in flowing nitrogen) and by direct hydrodesulphurization. Samples impregnated with NaOH were investigated also under the same conditions and the results are summarized in Table 3. The significant effect of trace amounts of NaOH on the level of desulphurization was evident; in particular the effect on hydrodesulphurization was very pronounced in that, whereas direct hydrodesulphurization resulted in 30 wt% sulphur removal, incorporation of x 3 wt% NaOH in the coke led to 90 wt% sulphur removal under the same experimental conditions. Reversibility
of NaOH
impregnation
25 g of the coke was impregnated with NaOH to a weight ratio of 0.040 in a Teflon beaker. The reversibility of the impregnation was examined by serial hot water extraction of the base from a weighed sample of the impregnated coke. Determination of the sodium in the leachate by atomic absorption indicated that S&85% of the base could be leached out. The remainder was irreversibly adsorbed, probably within the coke matrix. A similar experiment carried out on NaOH-impregnated coke which had been hydrodesulphurized showed that only x 70 wt% of the base could be recovered. Loss of base could have resulted from chemical reaction with the coke and with the quartz reactor, which was extensively corroded. Sodium balance
* Although the composition of the leachate has not been determined, is probably a mixture of NaOH, Na,CO, with traces of Na,S.
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it
0.3 wt% sodium
and
leached
coke
was determined in the desulphurized indicating that x80 wt’? of the
Desulphurization
of petroleum fluid coke: 2. M. George and L. G. Schneider
reaches a constant level at E 85% at a weight ratio of 0.040. 32% desulphurization at 0 weight ratio refers to direct hydrodesulphurization. lime on stream. Data for these experiments presented in Figure 5 demonstrate that sulphur removal increases with time up to 2 h and reaches a constant level at ~85% desulphurization. Consequently, desulphurization experiments were set for 2 h.
I 500
600
I
1
I
700 Coke tempemture
800
900
(“C)
Figure3 Effect of coke temperature on desulphurization. W/R = 0.040 NaOH. Conditions as in Figure 2
Partial pressure of hydrogen. Desulphurization was investigated at different partial pressures of hydrogen at 700°C by keeping the total flow rate constant and diluting helium with hydrogen. Results shown in Figure 6 indicate a strong dependence on the partial pressure of hydrogen. 29% desulphurization at zero partial pressure of hydrogen refers to desulphurization in helium under the same experimental conditions. Eflect of Na+, K+ and Li’ on desulphurization. A few
I
I
I
I
1
I
experiments I
indicated
that
the effectiveness I
I
I
of the I
I
l
80-
0
I 004
I 008
I
0.12
(N&i-l/coke) Figure 4 Effect of NaOH/coke tions as in F&we 2
I 0.16
0.20
mtlo
ratio on desulphurization.
Condi-
1
0
I
I
I
I
2
4
6
8
Time on streum
impregnated NaOH was removed during the process. Whereas coke containing higher loadings of NaOH fused during laboratory combustion, coke containing 0.3 wt% sodium did not cause problems during laboratory combustion at 1000°C. Effect ofprocess variables in the hydrodesulphurization of NaOH impregnated coke Temperature. Results summarized in Figure 3 for a
weight ratio of 0.040 in the temperature range 55&85o”C for 2 h show a maximum desulphurization at ~700°C. The decrease in the extent of sulphur removal at > 700°C may be associated with the collapse of the coke structure or loss of NaOH by volatilization. To test this latter possibility, the NaOH impregnated coke was heated to and maintained at 700°C in a flow of helium (150 ml min-‘) for 4 h. The effluent of the reactor, collected in distilled water and analysed for sodium, indicated negligible loss of sodium suggesting that the maximum observed in the desulphurization may be due to factors volatilization of NaOH. In these than other desulphurization experiments, weight loss amounted to z 15% of the initial charge of the coke. The sulphur content of the coke was determined by the high temperature combustion method. Weight ratio. Coke samples with NaOH weight ratios of 0.010, 0.020, 0.040, 0.100 and 0.200 were hydrodesulphurized at 700°C and the results, summarized in Figure 4, show that as the weight ratio (NaOH) is increased, desulphurization increases quite rapidly and
F$ure5
(h)
Effect of time on hydrodesulphurization
I-
O-
O-
‘O-
I
O-
0
26.7
I 53.3
I 78.0
pH, (kPa) Fgure
6
Effect of partial pressure of hydrogen
FUEL, 1982, Vol 61, December
1263
1
Desulphurization
of petroleum fluid coke: 2. M. George and L. G. Schneider
hydrogen resulted in 85 wt% sulphur removal, but the original surface area was retained. These results, summarized in Table 3, suggest that there is no simple relation between the surface area and the level of desulphurization. Diffusional limitations
I 0.04
401
I 006 Weight mtlo
I 0.08
I 0.10
NaOH
Figure 7 Effect of NaOH/coke ratio on desulphurization via H2.S (A) and total desulphurization (0). Conditionsas in Figure 2
reagents in hydrodesulphurization (700°C 4 h, a hydrogen flow of 150 ml min- ‘) decreases in the order NaOH > LiOH > KOH. With a metal/sulphur molar ratio of 0.50, the respective desulphurizations were 88% (MaOH), 61% (LiOH), and 53% (KOH). Effect of NaOH loading on desulphurization via H,S.
Samples of coke were loaded with NaOH in the weight ratio range 0.02-0.07. Aliquots of these samples were hydrodesulphurized at 700°C for 2 hat 120 ml min-’ and the extent of desulphurization via H,S (g.c.) and the residual sulphur in the desulphurized coke were determined by the high temperature combustion method. Data summarized in Figure 7demonstrate that the level of desulphurization increases up to a W/R of 0.04 after which the total desulphurization as determined by the high temperature combustion method remains constant, but the level of desulphurization via H,S decreases. Up to a weight ratio of 0.04, the extent of desulphurization was primarily via H,S, as these desulphurized (not leached) samples failed to show the presence of Na,S by X-ray diffraction analysis or H,S production on heating the samples with HCl. However, with samples containing higher weight ratios of NaOH, significant quantities of H,S could be detected during leaching of the desulphurized sample which probably resulted from the hydrolysis of Na,S: Na,S + 2H,0+2NaOH
+ H,S
Generally desulphurization of coke increases with the rate of hydrogen flow and then reaches a constant level. This has been attributed to the presence of a film of products surrounding the coke granules, and the rate of desulphurization depends upon the thickness or concentration of this layer. As the hydrogen flow rate is increased, the thickness of this layer is decreased, enabling rapid diffusion of products (H,S) out of the coke granules thereby increasing the extent of desulphurization. Desulphurization experiments carried out at 550°C and 700°C at different rates of hydrogen flow, shown in Figure 8, show that, whereas little boundary layer diffusion control exists at 55o”C, a diffusional barrier is apparent at 700°C where a high level of desulphurization was attained. Experiments in which the coke particle size was varied but the NaOH weight ratio maintained at 0.04 indicate that pore diffusion is not significant (Figure 9). Although not shown in Figure 9, a similar trend was observed at 700°C with a hydrogen flow rate of 120 ml min- ‘. This is one of the advantages of NaOHimpregnation of the coke fluid prior to hydrodesulphurization. In contrast, direct hydrodesulphurization of a delayed coke produced by
I
,L
Fgure 8 at 700°C
I
I
I
I
I
*i
Effect of hydrogen (0) and 550°C (A)
flow rate on hydrodesulphuristion
(2)
X-ray diffraction of this sample confirmed the presence of Na,S, with d-spacings indicated at 0.380,0.232,0.198 and 0.164 nm. Surface area
The starting material had a surface area of 11.O+ 2 m* g -l, which did not change during heating in nitrogen or hydrogen. Impregnation of the coke with NaOH resulted in significant loss of surface area; this is not surprising as the pores within the coke granules are probably filled with adsorbed NaOH. Even after desulphurization at 700°C where 90 wt% sulphur removal was achieved (Table 3), the surface area was negligible. Hot water leaching of this sample (x4.5 g sample, 500 ml tap water, 80°C overnight) restored the surface area partially. Hydrodesulphurization of the impregnated sample in wet
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FUEL, 1982,
Vol 61, December
0.10
I
I
030
050 Average
coke
partlcle diameter .
I 070 (mm)
Figure9 Effect of coke particle size on hydrodesulphurization. 650°C. 2 h. H2 flow = 40 ml min-t
1
0.9
Desulphurization
Suncor appeared to be controlled by pore diffusion but not film diffusion.5s6 Scanning electron micrographs No significant difference could be detected between the fluid coke sample as-received and after direct hydrodesulphurization. However, hydrogenation of the impregnated samples showed deep cracks and these persisted during hot-water leaching. It is probably the combination of NaOH-impregnation and hightemperature hydrogenation that is responsible for these cracks and these do not appear to have contributed to the total surface area of the coke (Table 3). Effect of wet hydrogen on desulphurization
As the partial pressure of H,S closely followed that of wet hydrogen enhanced product water, and desulphurization of oil sands fluid coke,15 the effect of wet hydrogen on Getty Coke desulphurization was studied. Constant partial pressure of water in hydrogen was achieved by bubbling the hydrogen through a water saturator maintained at constant temperature. Although a systematic investigation was not undertaken, the experiments indicated that for this fluid coke, the level of desulphurization was not affected by water vapour in the hydrogen.
of petroleum fluid coke: Z. M. George and L. G. Schneider
level of sulphur removal achieved when the coke is impregnated with NaOH. It is likely that the organic sulphur compounds are distributed uniformly within the coke granule. Hydrogen can diffuse in and react with the sulphur compounds to form H,S; however for H,S to diffuse out appears to be difficult as the pores appear to be blocked. It is possible that during impregnation and drying, which may be considered as an activation process for this reaction, the (C-S) bonds are weakened and these reactive sulphur compounds may diffuse towards the surface of the granules where they react readily with hydrogen to form H,S. As H,S is now formed on the external surface of the granules, the rate may be limited by film and not by pore diffusion as observed, Figures 8 and 9. The following tentative mechanism may explain desulphurization of the coke. It is probable that sulphur compounds in the coke may be present as organic sulphides of the type R-S-R, where R could be an aromatic or aliphatic group: R-S-R + HO-Na+
R-S-Na+ + ROHgRONa+
Application of this method for desulphurization of other fluid cokes
Detailed experimentation was not attempted; however, at an NaOH weight ratio of 0.04 and hydrodesulphurization at 700°C (5.0 g coke and a hydrogen flow rate of 120 ml min-I), >80 wt% of the sulphur was removed from the Imperial Oil and Petrolina cokes. The calorific value of the desulphurized Getty coke was 29.08 kJ kg- ‘, slightly lower than the starting sample and the sulphur content of the product coke was 1.0 fO.l wt% compared to 7.3 +0.3 wt% in the starting material. The sulphur content of the product coke is within the limits allowed by the Environmental Protection Agency of the USA and would probably meet the specifications of the Canadian and Alberta Governments.
DISCUSSION The significant aspect of NaOH-assisted (catalysed) hydrodesulphurization of this fluid coke is the very high
+ ROH + RSH
ROH+H$RH+H,O R-S-H + H, ZRH + H,S
Effect of Na,CO,
As Na,CO, can be mixed mechanically with coke, thereby eliminating the impregnation step, and it is much simpler to handle than NaOH, experiments were carried out to determine the efficacy of Na,CO, compared to NaOH. The results indicated that to achieve 80% desulphurization, a weight ratio of 0.08 was required for Na,CO, compared to 0.04 with NaOH. To learn more about the mechanism of desulphurization, particularly the role of sulphones, the NaOH-impregnated coke was heated to 700°C in a stream of helium and the effluent of the reactor was continuously monitored. SO, was not detected indicating that for this coke desulphurization may not proceed through a sulphone intermediate.
ZR-S-Na+
R-0-Na+
+ H,O ZROH + NaOH
(4) (5) (6) (7) (8)
Is possible that the NaOH generated in-situ (equation 8) could aid in enhanced desulphurization. Although the scanning electron micrograph indicates cracks on the desulphurized coke granules, surface area determinations do not support the hypothesis that during hydrogenation of NaOH-impregnated coke, the pores are opened up leading to enhanced desulphurization. Further, as shown in Table 3, no simple relation exists between the desulphurization and surface area. Possible explanations for the maximum observed at x 7OO”C,Figure 3, are sintering, depletion of NaOH, and the formation of stable (C-S) compounds by the reverse reaction H,S+Cz(C-S)+H,
(9)
As surface area does not appear to be related to the level of desulphurization, and there is very little loss of the reagent by volatilization, the decrease in desulphurization level at temperatures > 700°C must be related to the formation of new stable (C-S) sulphur compounds by the reverse reaction. The process described in this Paper is covered by Canadian patent 1,090,464 granted to Z. M. George. ACKNOWLEDGEMENTS Valuable discussions with Prof E. Tollefson of the University of Calgary are gratefully acknowledged. This manuscript is ‘Alberta Research Council Contribution No. 157’. REFERENCES 1 2 3
Meyer, R. A. Hydrocarbon Process. June 1975, p. 75 El Kaddah. N. and E.z.z. S. Y. Fuel 1973. 52. 128 Sef, F. Ind.‘Eng. Chem. 1960, 52(7), 599
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Mahmoud, B. H., Ayad, S. and Eu, S. Y. Fuel 1968,47,455 George, Z. M. Ind. Eng. Chem. Prod. Rex Dev. 1975, 1’4(4), 298 Tollefson, E. L. and Parmar, B. S. Can. J. Chem. Eng. 1977,55,185 Mason, R. B. Ind. Eng. Chem. 1959, 51(9), 1029 Thakker, M. T. British Patent 1,221,524, 1973 Ridley, R. D. ‘Process Research on Desulfurization of Petroleum Coke’, 160th Nat. Meeting, Amer. Chem. Sot. Div. Fuel Chem., Chicago, 1970 Lukasiewia, S. J. and Johnson,G. C. Ind. Eng. Chem. 1960,52(g), 675
FUEL, 1982,
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13 14 15
Sabott, F. K. Colorado School Mines (Quart.) 1952,47(3) George, Z. M., Parmar, B. S. and Tollefson, E. L. ‘Desulfurization of High Sulfur Cokes from Processing Oil Sands Bitumen’, 2nd Pacific Chem. Eng. Congress, (Pachec. 77) Denver, Colorado, USA, 1977 George, Z. M., Schneider, L. G. and Tollefson, E. L. Fuel 1978,57, 497 Eschka method Annual Book of ASTM Standards 1971, D271, p. 18 George, Z. M. 1981, unpublished results
M. George
desulphurization
and Linda G. Schneider
Alberta Research Council, 17315-87 (Received 1 July 1982)
Avenue, Edmonton, Alberta, T6G 2C2, Canada
Desulphurization of a fluid coke produced commercially from a conventional petroleum crude oil was attempted. Direct hydrodesulphurization of the coke at 700°C resulted in ~31 w% sulphur removal; however, impregnation of the fluid coke with trace amounts of sodium hydroxide and subsequent hydrodesulphurization resulted in > 80 wp/o sulphur removal primarily as Hz!% A significant part of the alkaline reagent could be recovered by hot water leaching of the desulphurized coke. The calorific value of the desulphurized coke is slightly lower than that of the starting material. The mechanism appears to be complex as the change in surface area was negligible upon impregnation and hydrodesulphurization. Economic evaluation of the desulphurization process, carried out at the Alberta Research Council, indicates that it has significant economic advantages over fluidized-bed combustion of the coke with limestone or combustion of the coke with flue gas desulphurization. (Keywords: desulphurization; fluid; coke; sodium hydroxide)
Although much work has been carried out on methods of desulphurizing coal and coal chars, few studies are reported in the literature on the desulphurization of petroleum cokes. In coal, a significant portion of the sulphur may be present as inorganic sulphides and methods such as that of Meyer1 are very efficient for removal of these sulphur compounds. In petroleum cokes however, a considerable part of the sulphur may be present as organic sulphur compounds and these compounds are not easily desulphurized. Further, petroleum cokes exhibit significant differences depending upon the origin of the petroleum, coking process employed and the amount and type of sulphur compounds present in the petroleum. El Kaddah and Ezz’ investigated thermal desulphurization of petroleum coke containing 8.3 wt% sulphur and observed 30 wt% sulphur removal in 30 min at 1600°C. Sef3 studied hydrodesulphurization of petroleum coke containing 2 wt% sulphur and reported 85 wt% sulphur removal using small particles of coke, a pressure of 659 Pa, and high space velocities. Mahmoud et al4 observed a maximum at 600°C for the hydrodesulphurization of petroleum coke containing 3 wt% sulphur, this maximum being attributed to the onset of sintering. George5 and Tollefson and Parmar have studied the hydrodesulphurization of Athabasca oil sands delayed coke and observed that the level of desulphurization was not significant under the experimental conditions and that the reaction was controlled by pore diffusion. Mason7 has reported the beneficial effects of preoxidation of coke on the however, the subsequent hydrodesulphurization; optimum conditions of the preoxidation appear to vary widely. Thakker* showed that impregnation of petroleum coke with sodium carbonate enhances the level of sulphur removal during hydrogenation. Ridley’ reported that 001~2361/82/12126%07%3.00 @ 1982 Butterworth & Co (Publishers)
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FUEL, 1982,
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Vol 61, December
sodium hydroxide and sodium sulphide when impregnated on coke particles aided desulphurization. Lukasiewicz and Johnson” and Sabott” observed that significant desulphurization of petroleum coke can be achieved by impregnating the coke with an alkaline reagent, calcination of the impregnated coke in an inert atmosphere at elevated temperatures and subsequent hot water leaching of the coke to remove the metal sulphides. Parmar and Tollefson6 employed a fluidized-bed reactor to evaluate several methods to desulphurize delayed coke produced by Suncor Canada. George, Parmar and Tollefson” investigated the desulphurization of oil sands delayed coke involving impregnation with an alkaline reagent, and subsequent calcination, and George, Tollefson’3 have reported the Schneider and desulphurization of a high sulphur fluid coke by this method. These experiments showed that to achieve significant desulphurization, large (twice stoichiometric amount to form metal sulphides) quantities of the alkaline reagents were necessary and posed serious corrosion and environmental problems. The objective of this investigation was to develop an economically attractive process for the desulphurization of petroleum coke prior to combustion. A sample of petroleum fluid coke (sulphur content 7.3 f 0.3 wt%) was obtained from Getty Oil Company, Delaware, USA, and an attempt to desulphurize this coke was made so that the sulphur dioxide level produced during combustion may be tolerated with minimum environmental damage. Whilst investigating methods of desulphurizing this petroleum coke, it was observed that if the fluid coke were impregnated with a small amount of base, such as sodium hydroxide, and then hydrodesulphurized at M700°C significant sulphur removal could be achieved. This Paper summerizes results on this method of desulphurizing a sample of Getty fluid coke.
Desulphurization Tab/e 1 Analysisof Carbon Hydrogen Sulphur Nitrogen Ash Nickel Vanadium
Gettyfluid
of petroleum fluid coke: 2. M. George and 1. G. Schneider
coke (db)
hydrogen flow established, and the furnace switched on. The reactor was heated by a Lindberg heavy duty furnace (type 59344) equipped with a controller (_+5’C). It took = 30 min for the furnace to reach the desired temperature (Figure 2). The rate of hydrogen flow, 120 ml mine’,. was measured at the reactor outlet under ambient condltlons. The product stream was sampled and analysed every 5 min by gas chromatography and the reactor effluent was scrubbed with NaOH prior to venting.
86.9 wt % 1.8wt% 7.3 f 0.3 wl % 1.3wt% 0.1 w-t% 1
30 PPm 225 ppm
Ash
3. Leaching. After hyprodesulphurization the coke was leached with tap water (E 5 g coke/500 ml H,O) at 80°C for 12 h. The water was then decanted and the sample
Tab/e 2 Sulphur content of fluid cokes examined Source Getty Coke, Delaware, Petrofina, Montreal Imperial Oil, Sarnia
Sulphur USA
mntent
(wt %)
7.3 ?: 0.3 6.4 f 0.3 3.2 + 0.2
EXPERIMENTAL Materials
Hz
Most of the experiments were carried out using fluid coke obtained from Getty Oil Company, Delaware, USA. Analysis of this coke is summarized in Table 1. Limited experiments were performed on samples of fluid coke obtained from Petrofina, Montreal, and Imperial Oil, Sarnia, Canada. The sulphur contents of these cokes are listed in Table 2. The reagents used in this study were all as-received grade. The surface area of the coke samples was usually determined by the high speed surface area analyser (Micromeritics Model 2200) and periodically by the BET measurements.* The coke sample was activated in dry helium for 4 h at 100°C before surface area measurements. For the sodium hydroxide-assisted desulphurization of fluid coke the following steps are involved: (1) impregnation of the coke particles with a suitable alkaline reagent; (2) hydrodesulphurization; (3) leaching to remove the alkaline reagent; (4) sulphur and recover determination in the coke. 1. Impregnation of the coke with alkaline reagents. Coke granules, 40/60 US mesh, were slurried with an aqueous solution of the alkaline reagent and evaporated to dryness at x 80°C with stirring. The ratio of the weight of alkaline reagent to the weight of coke is defined as the weight ratio, W/R. Most of the experiments reported here involved impregnation with 1M NaOH. Impregnation and drying at higher temperatures or at room temperature resulted in significantly lower desulphurizations during the hydrogenation step. A few experiments were also carried out using KOH and LiOH. Impregnation in an air or inert atmosphere did not appear to influence the level of sulphur removal in subsequent hydrogenation. 2. Hydrodesulphurization. A fixed-bed flow reactor system (Figure I) was constructed from 316 stainless steel tubing except for the reactor which was made out of quartz tubing.? The reactor consisted of a 3.2 cm i.d. x 61 cm long quartz tube and had a quartz fibre plug midway to support the coke sample. The reagent-loaded coke (5.0 g) was charged into the reactor at room temperature, a *
BET surface area measured
by N, adsorption check the surface area measured by the one-point 7 Initially, a stainless steel reactor was employed; sulphided and interfered with gc. analysis.
at 77K was only to system but the reactor was
He O2
HZS Flow meter H20
ie
Flow reactor for coke desulphurization. D, Sampling Figure 1 10-0~; V, 7-port sampling valve; m, metering valve; T, thermomuple; Tc, thermal mnductivity detector; CON, controller for the furnace; El, electronic integrator; PS, Power supply for gas chromatograph
8o08533
Time (mln) Coke temperature (0) and H.# mncentration in the f&urel reactor effluent gas; (A) as a function of time after reactor start-up. 5.0 g coke sample, 40160 US mesh, 2 h, 700°C, Hz flow = 120 ml min-I, W/t? = 0.04
FUEL, 1982, Vol 61, December
1261
Desulphurization Table3
NaOH
Summary
of petroleum fluid coke: Z. M. George and L. G. Schneider
of preliminary
experiments,
Getty
Desulphurization medium
loading
fluid coke, 7OO”C, 40160
Desulphurization
None
N2
2
N2
29
“2
31 90 85 None
‘42
H, wet None
Surface area before leaching (m2 g-l)
(96)
W/R = 0.04 None W/R = 0.04 W/R = 0.04 W/R = 0.04
US mesh, 2 h, 5.0 g sample.
11
Gas flow:
120 ml min-’
Surface area after leaching (m2 9-l)
(Ml
11
30.24
3 3 5
kg-‘)
29.08a
Wt NaOH(g)
W/R = Weight ratio, Wt coke(g) a Leached sample
dried at 100°C in air. Leaching was practised as an integral part of desulphurization as the NaOH used for the impregnation of the coke is an expensive component of desulphurization which could be recovered by leaching and may be used for reagent make-up for further impregnation.* Also, the presence of alkaline compounds in the coke may lead to serious problems in the boiler tubes during combustion. The rate of leaching was significant initially but declined with time. Approximately 50% of the base could be extracted in E 1 h and =7&75x in 12 h. Because of the small amount of NaOH used in these experiments to achieve >80% desulphurization and as a significant portion of the alkaline reagent may be recovered by leaching and reused, the NaOH used appeared to act as a catalyst. 4. Determination of sulphur removal. Gas chromatographic analysis and high temperature combustion (ASTM D-3177-75) were employed independently to determine desulphurization via H,S and the total sulphur removal, respectively. G.c. analysis To quantitate sulphur removal from the coke as H,S or other gaseous sulphur compounds during hydrodesulphurization, the effluent of the reactor was analysed by g.c. every 5 min. By switching a six-port sampling valve (Figure Z),a sample of the effluent gas (2.0 ml) was swept into the analytical column, 8 feet of Poropak Q followed by 2 feet of Poropak T, maintained at 15O”C,and analysed over a calibrated thermal conductivity detector. During the initial stages of hydrodesulphurization, CO, CH,, CO,, H,S and H,O were detected. Only H,S and H,O remained during the later stages. COS, CS, and SO, were not detected. Generally, the H,S concentration profile in the product followed that of H,O. Integration of the H,S area under the peak (i.e. graph of the partial pressure of H,S uersus time, Figure 2) was used to determine the extent of desulphurization by H,S. High temperature sulphur determination
This method is designed specifically for the rapid determination of sulphur in coal and coke and consists of burning a sample of coke within a tube furnace at * 1000°C in a stream of oxygen. Sulphur oxides are absorbed in a hydrogen peroxide solution, yielding sulphuric acid (equation 1) which is titrated against standard NaOH to a pH of 4.5. SO, + H,O, -+H,SO,
(1)
The method was tested against the Standard Eschka methodI and agreement with 4% was obtained. Duplicate analysis of the desulphurized and leached sample as well as a single analysis of the starting material, were made for each The percentage desulphurization was experiment. determined by comparing the percentage of sulphur in the residue with that of the starting sample. Any residual basic sodium compounds in the coke have been shown to form the corresponding metal sulphates and these do not decompose to form SO, during combustion. Unless specifically stated, high temperature combustion was used for sulphur determination.
RESULTS Preliminary
experiments
Before investigating the desulphurization of NaOHassisted fluid coke in detail, experiments were conducted to investigate the sulphur removal by volatilization (calcination to 700°C in flowing nitrogen) and by direct hydrodesulphurization. Samples impregnated with NaOH were investigated also under the same conditions and the results are summarized in Table 3. The significant effect of trace amounts of NaOH on the level of desulphurization was evident; in particular the effect on hydrodesulphurization was very pronounced in that, whereas direct hydrodesulphurization resulted in 30 wt% sulphur removal, incorporation of x 3 wt% NaOH in the coke led to 90 wt% sulphur removal under the same experimental conditions. Reversibility
of NaOH
impregnation
25 g of the coke was impregnated with NaOH to a weight ratio of 0.040 in a Teflon beaker. The reversibility of the impregnation was examined by serial hot water extraction of the base from a weighed sample of the impregnated coke. Determination of the sodium in the leachate by atomic absorption indicated that S&85% of the base could be leached out. The remainder was irreversibly adsorbed, probably within the coke matrix. A similar experiment carried out on NaOH-impregnated coke which had been hydrodesulphurized showed that only x 70 wt% of the base could be recovered. Loss of base could have resulted from chemical reaction with the coke and with the quartz reactor, which was extensively corroded. Sodium balance
* Although the composition of the leachate has not been determined, is probably a mixture of NaOH, Na,CO, with traces of Na,S.
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it
0.3 wt% sodium
and
leached
coke
was determined in the desulphurized indicating that x80 wt’? of the
Desulphurization
of petroleum fluid coke: 2. M. George and L. G. Schneider
reaches a constant level at E 85% at a weight ratio of 0.040. 32% desulphurization at 0 weight ratio refers to direct hydrodesulphurization. lime on stream. Data for these experiments presented in Figure 5 demonstrate that sulphur removal increases with time up to 2 h and reaches a constant level at ~85% desulphurization. Consequently, desulphurization experiments were set for 2 h.
I 500
600
I
1
I
700 Coke tempemture
800
900
(“C)
Figure3 Effect of coke temperature on desulphurization. W/R = 0.040 NaOH. Conditions as in Figure 2
Partial pressure of hydrogen. Desulphurization was investigated at different partial pressures of hydrogen at 700°C by keeping the total flow rate constant and diluting helium with hydrogen. Results shown in Figure 6 indicate a strong dependence on the partial pressure of hydrogen. 29% desulphurization at zero partial pressure of hydrogen refers to desulphurization in helium under the same experimental conditions. Eflect of Na+, K+ and Li’ on desulphurization. A few
I
I
I
I
1
I
experiments I
indicated
that
the effectiveness I
I
I
of the I
I
l
80-
0
I 004
I 008
I
0.12
(N&i-l/coke) Figure 4 Effect of NaOH/coke tions as in F&we 2
I 0.16
0.20
mtlo
ratio on desulphurization.
Condi-
1
0
I
I
I
I
2
4
6
8
Time on streum
impregnated NaOH was removed during the process. Whereas coke containing higher loadings of NaOH fused during laboratory combustion, coke containing 0.3 wt% sodium did not cause problems during laboratory combustion at 1000°C. Effect ofprocess variables in the hydrodesulphurization of NaOH impregnated coke Temperature. Results summarized in Figure 3 for a
weight ratio of 0.040 in the temperature range 55&85o”C for 2 h show a maximum desulphurization at ~700°C. The decrease in the extent of sulphur removal at > 700°C may be associated with the collapse of the coke structure or loss of NaOH by volatilization. To test this latter possibility, the NaOH impregnated coke was heated to and maintained at 700°C in a flow of helium (150 ml min-‘) for 4 h. The effluent of the reactor, collected in distilled water and analysed for sodium, indicated negligible loss of sodium suggesting that the maximum observed in the desulphurization may be due to factors volatilization of NaOH. In these than other desulphurization experiments, weight loss amounted to z 15% of the initial charge of the coke. The sulphur content of the coke was determined by the high temperature combustion method. Weight ratio. Coke samples with NaOH weight ratios of 0.010, 0.020, 0.040, 0.100 and 0.200 were hydrodesulphurized at 700°C and the results, summarized in Figure 4, show that as the weight ratio (NaOH) is increased, desulphurization increases quite rapidly and
F$ure5
(h)
Effect of time on hydrodesulphurization
I-
O-
O-
‘O-
I
O-
0
26.7
I 53.3
I 78.0
pH, (kPa) Fgure
6
Effect of partial pressure of hydrogen
FUEL, 1982, Vol 61, December
1263
1
Desulphurization
of petroleum fluid coke: 2. M. George and L. G. Schneider
hydrogen resulted in 85 wt% sulphur removal, but the original surface area was retained. These results, summarized in Table 3, suggest that there is no simple relation between the surface area and the level of desulphurization. Diffusional limitations
I 0.04
401
I 006 Weight mtlo
I 0.08
I 0.10
NaOH
Figure 7 Effect of NaOH/coke ratio on desulphurization via H2.S (A) and total desulphurization (0). Conditionsas in Figure 2
reagents in hydrodesulphurization (700°C 4 h, a hydrogen flow of 150 ml min- ‘) decreases in the order NaOH > LiOH > KOH. With a metal/sulphur molar ratio of 0.50, the respective desulphurizations were 88% (MaOH), 61% (LiOH), and 53% (KOH). Effect of NaOH loading on desulphurization via H,S.
Samples of coke were loaded with NaOH in the weight ratio range 0.02-0.07. Aliquots of these samples were hydrodesulphurized at 700°C for 2 hat 120 ml min-’ and the extent of desulphurization via H,S (g.c.) and the residual sulphur in the desulphurized coke were determined by the high temperature combustion method. Data summarized in Figure 7demonstrate that the level of desulphurization increases up to a W/R of 0.04 after which the total desulphurization as determined by the high temperature combustion method remains constant, but the level of desulphurization via H,S decreases. Up to a weight ratio of 0.04, the extent of desulphurization was primarily via H,S, as these desulphurized (not leached) samples failed to show the presence of Na,S by X-ray diffraction analysis or H,S production on heating the samples with HCl. However, with samples containing higher weight ratios of NaOH, significant quantities of H,S could be detected during leaching of the desulphurized sample which probably resulted from the hydrolysis of Na,S: Na,S + 2H,0+2NaOH
+ H,S
Generally desulphurization of coke increases with the rate of hydrogen flow and then reaches a constant level. This has been attributed to the presence of a film of products surrounding the coke granules, and the rate of desulphurization depends upon the thickness or concentration of this layer. As the hydrogen flow rate is increased, the thickness of this layer is decreased, enabling rapid diffusion of products (H,S) out of the coke granules thereby increasing the extent of desulphurization. Desulphurization experiments carried out at 550°C and 700°C at different rates of hydrogen flow, shown in Figure 8, show that, whereas little boundary layer diffusion control exists at 55o”C, a diffusional barrier is apparent at 700°C where a high level of desulphurization was attained. Experiments in which the coke particle size was varied but the NaOH weight ratio maintained at 0.04 indicate that pore diffusion is not significant (Figure 9). Although not shown in Figure 9, a similar trend was observed at 700°C with a hydrogen flow rate of 120 ml min- ‘. This is one of the advantages of NaOHimpregnation of the coke fluid prior to hydrodesulphurization. In contrast, direct hydrodesulphurization of a delayed coke produced by
I
,L
Fgure 8 at 700°C
I
I
I
I
I
*i
Effect of hydrogen (0) and 550°C (A)
flow rate on hydrodesulphuristion
(2)
X-ray diffraction of this sample confirmed the presence of Na,S, with d-spacings indicated at 0.380,0.232,0.198 and 0.164 nm. Surface area
The starting material had a surface area of 11.O+ 2 m* g -l, which did not change during heating in nitrogen or hydrogen. Impregnation of the coke with NaOH resulted in significant loss of surface area; this is not surprising as the pores within the coke granules are probably filled with adsorbed NaOH. Even after desulphurization at 700°C where 90 wt% sulphur removal was achieved (Table 3), the surface area was negligible. Hot water leaching of this sample (x4.5 g sample, 500 ml tap water, 80°C overnight) restored the surface area partially. Hydrodesulphurization of the impregnated sample in wet
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FUEL, 1982,
Vol 61, December
0.10
I
I
030
050 Average
coke
partlcle diameter .
I 070 (mm)
Figure9 Effect of coke particle size on hydrodesulphurization. 650°C. 2 h. H2 flow = 40 ml min-t
1
0.9
Desulphurization
Suncor appeared to be controlled by pore diffusion but not film diffusion.5s6 Scanning electron micrographs No significant difference could be detected between the fluid coke sample as-received and after direct hydrodesulphurization. However, hydrogenation of the impregnated samples showed deep cracks and these persisted during hot-water leaching. It is probably the combination of NaOH-impregnation and hightemperature hydrogenation that is responsible for these cracks and these do not appear to have contributed to the total surface area of the coke (Table 3). Effect of wet hydrogen on desulphurization
As the partial pressure of H,S closely followed that of wet hydrogen enhanced product water, and desulphurization of oil sands fluid coke,15 the effect of wet hydrogen on Getty Coke desulphurization was studied. Constant partial pressure of water in hydrogen was achieved by bubbling the hydrogen through a water saturator maintained at constant temperature. Although a systematic investigation was not undertaken, the experiments indicated that for this fluid coke, the level of desulphurization was not affected by water vapour in the hydrogen.
of petroleum fluid coke: Z. M. George and L. G. Schneider
level of sulphur removal achieved when the coke is impregnated with NaOH. It is likely that the organic sulphur compounds are distributed uniformly within the coke granule. Hydrogen can diffuse in and react with the sulphur compounds to form H,S; however for H,S to diffuse out appears to be difficult as the pores appear to be blocked. It is possible that during impregnation and drying, which may be considered as an activation process for this reaction, the (C-S) bonds are weakened and these reactive sulphur compounds may diffuse towards the surface of the granules where they react readily with hydrogen to form H,S. As H,S is now formed on the external surface of the granules, the rate may be limited by film and not by pore diffusion as observed, Figures 8 and 9. The following tentative mechanism may explain desulphurization of the coke. It is probable that sulphur compounds in the coke may be present as organic sulphides of the type R-S-R, where R could be an aromatic or aliphatic group: R-S-R + HO-Na+
R-S-Na+ + ROHgRONa+
Application of this method for desulphurization of other fluid cokes
Detailed experimentation was not attempted; however, at an NaOH weight ratio of 0.04 and hydrodesulphurization at 700°C (5.0 g coke and a hydrogen flow rate of 120 ml min-I), >80 wt% of the sulphur was removed from the Imperial Oil and Petrolina cokes. The calorific value of the desulphurized Getty coke was 29.08 kJ kg- ‘, slightly lower than the starting sample and the sulphur content of the product coke was 1.0 fO.l wt% compared to 7.3 +0.3 wt% in the starting material. The sulphur content of the product coke is within the limits allowed by the Environmental Protection Agency of the USA and would probably meet the specifications of the Canadian and Alberta Governments.
DISCUSSION The significant aspect of NaOH-assisted (catalysed) hydrodesulphurization of this fluid coke is the very high
+ ROH + RSH
ROH+H$RH+H,O R-S-H + H, ZRH + H,S
Effect of Na,CO,
As Na,CO, can be mixed mechanically with coke, thereby eliminating the impregnation step, and it is much simpler to handle than NaOH, experiments were carried out to determine the efficacy of Na,CO, compared to NaOH. The results indicated that to achieve 80% desulphurization, a weight ratio of 0.08 was required for Na,CO, compared to 0.04 with NaOH. To learn more about the mechanism of desulphurization, particularly the role of sulphones, the NaOH-impregnated coke was heated to 700°C in a stream of helium and the effluent of the reactor was continuously monitored. SO, was not detected indicating that for this coke desulphurization may not proceed through a sulphone intermediate.
ZR-S-Na+
R-0-Na+
+ H,O ZROH + NaOH
(4) (5) (6) (7) (8)
Is possible that the NaOH generated in-situ (equation 8) could aid in enhanced desulphurization. Although the scanning electron micrograph indicates cracks on the desulphurized coke granules, surface area determinations do not support the hypothesis that during hydrogenation of NaOH-impregnated coke, the pores are opened up leading to enhanced desulphurization. Further, as shown in Table 3, no simple relation exists between the desulphurization and surface area. Possible explanations for the maximum observed at x 7OO”C,Figure 3, are sintering, depletion of NaOH, and the formation of stable (C-S) compounds by the reverse reaction H,S+Cz(C-S)+H,
(9)
As surface area does not appear to be related to the level of desulphurization, and there is very little loss of the reagent by volatilization, the decrease in desulphurization level at temperatures > 700°C must be related to the formation of new stable (C-S) sulphur compounds by the reverse reaction. The process described in this Paper is covered by Canadian patent 1,090,464 granted to Z. M. George. ACKNOWLEDGEMENTS Valuable discussions with Prof E. Tollefson of the University of Calgary are gratefully acknowledged. This manuscript is ‘Alberta Research Council Contribution No. 157’. REFERENCES 1 2 3
Meyer, R. A. Hydrocarbon Process. June 1975, p. 75 El Kaddah. N. and E.z.z. S. Y. Fuel 1973. 52. 128 Sef, F. Ind.‘Eng. Chem. 1960, 52(7), 599
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Desulphurization 4 5 6 7 8 9
10
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of petroleum fluid coke: Z. M. George and L. G. Schneider
Mahmoud, B. H., Ayad, S. and Eu, S. Y. Fuel 1968,47,455 George, Z. M. Ind. Eng. Chem. Prod. Rex Dev. 1975, 1’4(4), 298 Tollefson, E. L. and Parmar, B. S. Can. J. Chem. Eng. 1977,55,185 Mason, R. B. Ind. Eng. Chem. 1959, 51(9), 1029 Thakker, M. T. British Patent 1,221,524, 1973 Ridley, R. D. ‘Process Research on Desulfurization of Petroleum Coke’, 160th Nat. Meeting, Amer. Chem. Sot. Div. Fuel Chem., Chicago, 1970 Lukasiewia, S. J. and Johnson,G. C. Ind. Eng. Chem. 1960,52(g), 675
FUEL, 1982,
Vol 61, December
11 12
13 14 15
Sabott, F. K. Colorado School Mines (Quart.) 1952,47(3) George, Z. M., Parmar, B. S. and Tollefson, E. L. ‘Desulfurization of High Sulfur Cokes from Processing Oil Sands Bitumen’, 2nd Pacific Chem. Eng. Congress, (Pachec. 77) Denver, Colorado, USA, 1977 George, Z. M., Schneider, L. G. and Tollefson, E. L. Fuel 1978,57, 497 Eschka method Annual Book of ASTM Standards 1971, D271, p. 18 George, Z. M. 1981, unpublished results