I
FRANC
SEF
Institute of Petroleum, Zagreb, Yugoslavia
Desulfurization of Petroleum Coke during Calcination Carbon electrodes for electrolytic processing require coke of low sulfur content. They are currently made from selected r a w materials of limited quantity. Desulfurization of petroleum coke appears to be an attractive addition to this source of electrode-grade coke L I T E R A T U R E available on the desulfurization of coal is extensive. Thiessen ( 7 ) ,for example, has reported on some of the older works. Zielke, Curran, and others ( 8 ) as well as Jacobs and Rlirkus (2) investigated the desulfurization of char in a fluidized bed. However, comparatively little has been reported on the desulfurization of petroleum coke. Sabott ( 3 ) and Schafer ( 4 ) investigated the effect of various gases and other materials on the removal of sulfur. The present author ( 5 , 6) earlier reported on the effects of some process variables on the desulfurization of coke by treating it in a gaseous atmosphere at elevated temperatures. In continued investigations attention has been focused on the effect of coke particle size, space velocity, and hydrogen pressure on the desulfurization of petroleum coke during calcination. According to the data obtained in these studies, the desulfurization of petroleum coke offers promising results to the industry.
Table I.
Sulfur, % Volatile
These Are Characteristics of the Samples Studied Analyses are based on dry samples I I1 I11 IV 1.84
1.97
2.01
2.00
7.4
7.1
7.4
6.7
6.6
com-
bustible
matter, yc
Ash, 600' C.,
%
...
1.67
...
...
...
Coke particle Through 72 on Through 36 on Through 20 on Through 10 on Through 5 on size 300 mesh 72 mesh 36 mesh 20 mesh 10 m e s h
tube filled with finely ground quartz sand: was tightly inserted on the top of the reactor to prevent any outflow of coke particles. Input of electric power to the reactor furnace and to the gas preheater was manually controlled. The
Experimental Techniques
All experiments were carried out with petroleum coke prepared in bench scale equipment from a heavy distillation residue of the Yugoslav Klos'tar crude oil. The coke was ground in a laboratory hammer mill to 100% through 5-mesh Tyler screen, then separated into fractions by sieving, and dried at 105" C. Table I shows the properties of the samples so obtained. The experimental equipment consisted of a n electrically heated reactor and a gas-flow, reactor-pressure, and temperature metering system, shown schematically at right. The reactor was a vertical alloyed steel tube 1200 mm. in length and 25 and 65 mm. inside and outside diameter, respectively, which was situated in an electric muffle furnace. A small amount of quartz sand of known particle size was placed on a metallic grid a t the reactor bottom to secure a good distribution of the passing gas and to prevent the coke from dropping out of the heating zone. The filter, a perforated steel
v
1 I90
This desulfurization unit can remove as much as 90% of the sulfur originally present in petroleum coke 1. Flowmeters 2. Gas preheater 3. Alloy steel reactor tube 4. Metallic grid 5. Thermawell
6. Entrained solids filter
7. Electric winding 8. W a t e r cooler 9. Back-pressure throttling valve 10. Pressure indicator
bed temperature was measured by means of a Chromel-Alumel thermocouple and could be controlled within &so C. The floic of hydrogen and nitrogen was measured by two orifices calibrated under the operating conditions and manualh controlled. The reactor pressure \\as maintained at constant level within 1 0 . 0 5 atm. by sporadically adjustinq the back pressure throttling valve. I n all experiments, a 20-gram coke sample was introduced on the top of the cold reactor. The reactor was then heated to 450' C. in a stream of nitrogen 12 liters per gram per hour). At this temperature. nitrogen was substituted by hydrogen, and the experiment continued at fixed operating conditions, established on the basis of preliminary runs-Le., temperature, 450 to 850' C.; contact time, 160 minutes; pressure, 1 atm.; and space velocity, 2 liters per gram per hour. The heating of the reactor was adjusted to allow a temperature increase of 50" C. in 20-minute periods. Deviations from these standard operating conditions in investigating the effects of pressure and space velocity variations on the desulfurization are indicated for the relevant results throughout the paper. At the end of each experiment the supply of hydrogen was disconnected, the coke sample cooled in a stream of nitrogen, removed from the reactor bottom together with the quartz sand, and then separated from the latter. VOL. 52, NO. 7
JULY 1960
599
'
100 90-
-
-
-
I
I;
'
I
2 a VI
01
0
1
0.4
1
0.8
I
1.2
J
!
1.6
8
8
1
2.0
'
1
2.4 2.8
3.2
Diameter of Coke Particle, Mm.
Figure 1. The amount of sulfur removed decreases with increasing particle size
The volatile matter and sulfur contents of the samDles before and after desulfurization were determined by the respective ASTM methods ( 7 ) . Results and Discussion
the individual coke fractions were obtained. The results indicate that in both series of experiments with increasing particle diameters up to 0.6 mm. the amount of sulfur removed rapidly decreases; a further increase in particle size has a comparatively small effect on the desulfurization. This may be explained on the one hand by the decrease of the external surface of the samples nith increasing particle diameters up to 0.6 mm., and on the other hand, by the rough porosity of coke which particularly shows effect with particles of larger size. Figure 2 showrs the desulfurization process as a function of pressure. The latter was raised to a maximum of 6.5 atni. absolute, while the other variables were left unchanged. The results indicate that with pressures gradually increasing in the specified limits, the amount of sulfur removed with the
Table II. Sulfur Content in Petroleum Coke Can Be Reduced with Hydrogen . to Desired Specifications
The operating conditions and the results obtained i r e shown in Table 11. are expressed in Of total sulfur removed from the coke, calculated according to the following equations : yo S in sample X 100 % S in product X soyield yo S.R. = yo S in sample Samples I to Lr were used for investigating the effect of coke particle size on
Space Velocity,
I I1 111 IV
V I 11
IV V I
the desulfurization with hydrogen. Two series of experiments were carried out for that purpose. The first series was carried out under the basic operating conditions already mentioned, while in the second series a pressure of 6.5 atm. was applied. T h e results are shown in Figure 1. The average particle diameter represents the arithmetical mean of the sieve meshes between which
a
Sulfur
Pres- Liter/G. Yield of in sure, Coke/ Coke, ProdrIct, A4tm. Hr.(l % % 1.0 2.0 89.3 1.03 1.0 2.0 89.6 1.03 1.0 2.0 91.0 1.37 1.0 2.0 92.1 1.55 1.0 2.0 93.1 1.67 1.0 2.0 93.5 1.75 6.5 2.0 85.4 0.50 6.5 2.0 89.1 0.88 6.5 2.0 91.3 1.22 6.5 2.0 91.4 1.25 2.2 2.0 88.6 0.82 0.63 2.0 86.7 3.7 1.26 0.5 91.1 1.0 1.15 1.0 90.5 1.0 88.5 0.91 1.0 4.0 0.92 7.0 89.0 1.0 84.5 0.45 3.7 4.0 83.0 0.42 3.7 7.0 81.1 0.39 3.7 18.0 0.22 7.0 78.3 6.5 0.16 20.5 76.5 6.5 1.89 2.0 92.8 1.0
Coke Sample No,
Hydrogen
was used
for all samples
except t h e last, which was carried out with nitrogen only.
__O ~
~20
-
-
J
___-__ 6.5
a
10
0 0
600
4
6
8
10
12
14
16
18
20
22
Space Velority of Hydrogen. L /G Cohe/Hr.
Pressure, Atm.
Figure 2. Increasing pressures have a beneficial effect on sulfur removal
2
Figure 3. Rising space velocities favor the desulfurizing effect of hydrogen
INDUSTRIAL AND ENGINEERING CHEMISTRY
10
900 98
96
94
92
90
88
16
Yidd d Cek.,
84
82
80
78
76
74
I
Figure 4. The amount of sulfur removed depends on the amount of gasified carbon
gaseous products increases, followed by a parallel drop of the sulfur content in the residue. Very likely, a further increase in pressure would have no significant effect on the desulfurization, if the other operating conditions remained unchanged. Increasing space velocities had an effect on the desulfurization similar to that of increasing pressures. Figure 3 illustrates the results obtained with sample I by increasing the hydrogen space velocity up to 20.5 liters per gram of coke per hour at three different pres. sures. When the most severe operating conditions were applied during these runs-viz., a hydrogen space velocity of 20.5 and a pressure of about 6 . 5 arm. absolute with a coke sample investigated in a fluid bed-93.6 wt. yc of the sulfur was removed. The results of these runs generally show that, as the amount of sulfur removed increases, there is a corresponding drop in coke yield (Figure 4). The curve on the diagram in Figure 4 was extrapolated so as to reach a cokeyield point at which no s d h r would be removed whatever with the aid of data obtained when during a run the same coke sample had been calcined in a stream of nitrogen. These results show that the amount of sulfur removed, under other equal basic conditions, depends only on the percentage of carbon gasified and not on the pressure, respectively, space velocity applied. Similar results were reported by Zielke and coworkers (8) whrn investigating the desulfurization of char with hydrogen. literature Cited (1) Am. SOC.Testing Materials, Method D 271-48, D 89448T. (2) Jacobs, J. K., Mirkus, J. D., IND. ENG.CHEM.50, 24 (1958). ( 3 ) Sabott, F. K., Quart. Cob. SchooI Mines 47, No. 3, 1-22 (1952). (4) Schafer, W. C., Zbid., 47, No. 3 (1952). ( 5 ) Sef, Franc, Nafta (Yugoslavia) 8, 283 11957). ( 6 j - I h i d : ,9,71 (1958). (7) Thiessen, Gilbert, "Chemistry of Coal Utilization," Lowry, H. H., ed., Vol. I, pp. 425-49, Wiley, New York, 1943. (8) Zielke, C. W., Curran, G. P., Gorin Everett, Goring, G. E., IND. ENG.CHEM. 46, 53 (1954).
