Literature Cited
Badger, G. M., “Aromatic Character and Aromaticity,” P. 39, Cambridge University Press, Cambridge, Great Britain, 1969. Burger, A., Mosettig, E., J . Amer. Chem. Soc. 57, 27312 (1935). Clar, E., “Polycyclic Hydrocarbons,” Academic Press, London and New York, 1964. Garlock, E . A., Mosettig, E., J . Amer. Chem. Soc. 6 7 , 22559 (1945). Glasstone, S., Laidler, K. J., Eyring, H., “Theory of Rate Processes,” Chap. 1, p. 9, McGraw-Hill, Kew York, 1941.
Johnson. W. S., J . Amer. Chem. Soc. 75, 1498-500 (1953). Pauling, L., “Nature of the Chemical Bond,” Chap. 3, Cornel1 University Press, Ithaca, N. Y., 1948. Smith, H.A., ~ ~ ~ ~p. H. ~ E~~~~~ ~ l ~Ed,,~ vel. i 5~, pp. 175-256, Reinhold, Yew York, 1955. Waterman, H. I., et al., Rec. Trav. Chim. 58, 83-92 (1939). RECEIVED for review October 20, 1969 ACCEPTED December 23, 1969 Symposium on Chemicals from Coal, Division of Fuel Chemistry. 158th Meeting, ACS, New York. N . Y., September 1969. Research supported by the U. S. Office of Coal Research and t h e bniversitv of-Utah under contract 14-01 -0001-271.
Hydrodesulfurization of Bituminous Coal Chars Charles A. Gray,’ Martin E. Sacks,’ and R. Tracy Eddinger Chemical Research and Development Center, FMC Corp., Princeton. N . J . 08540
Illinois No. 6 char was desulfurized by hydrotreatment at about 1600°F. The char initially contained about 2% organic sulfur a n d 1 YO inorganic sulfur (FeS) from pyrolysis of pyrite. Removal of the organic sulfur is rapid and limited primarily by an equilibrium between H2S in the gas and sulfur in the char. The reaction of the FeS with H? proceeds more slowly than would b e expected from equilibrium. The rate-limiting step appears t o b e either the inherent solid-gas kinetics, or the diffusion of H:S through the reacted shell of iron surrounding the FeS. About 10% of the sulfur content of the char was not removed even by prolonged hydrotreatment; however, it i s possible to remove over 80% in 20 minutes. Because of the high equilibrium ratio of HL to HlS, an H?S acceptor material would have to be admixed with the char bed to have a commercially viable process.
A
MAJOR by-product of many of the coal-conversion processes now under development is a highly porous semicoke or char. The value of this by-product could be enhanced in many instances, if it could be desulfurized. In this manner, a low-sulfur fuel might be produced from a high-sulfur coal. Previous work a t Consolidation Coal Co. (Batchelor et al., 1960; Zielke et al., 1954) established that high temperature hydrogenation effectively removed sulfur from some chars prepared from Pittsburgh-seam coals. For a char prepared from Arkwright coal, these authors found an equilibrium between sulfur in the char and hydrogen sulfide in the gas. Contrary to our findings, Zielke et al. (1954) found the inorganic sulfur more easily removed than the organic. Partial gasification of the char increased the rate of removal of sulfur, suggesting some diffusional inhibition of the organic sulfur.
Present address, Inorganic Chemicals Division, F M C Corp.. Carteret, N. J.. 07008. -€’resent address. Stevens Institute of Technology. Hoboken, N..J., 07030.
This study was undertaken to demonstrate the technical feasibility of desulfurizing chars produced by multistage pyrolysis of coal, identify the important variables influencing the rate of desulfurization, and obtain kinetic data for process design. Experimental Apparatus
The apparatus is shown in Figure 1. The reactor was a 2-foot length of >4-inch Type 316 stainless steel pipe, mounted in a vertical tube furnace which was controlled on the basis of bed temperature. A 22-inch-long therrnowell of 14-inch 0.d. tubing ran up the center of the pipe. Reactor pressure was measured by a Rourdon gage. Gas flow was metered by rotameter and by a wet-test meter. The gas flow and reactor pressure were controlled by needle valves. Procedure
The char bed was supported in the reactor bv a ceramic wool plug resting on a coarse screen welded to the thermowell. A second ceramic wool plug blocked the reactor exit port, to retain any fines that might be elutriated. Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970 357
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THERMOWELL
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PRESSURE, P , psia WT. SPACE VELOCITY,Sw LB. H2/HR. PER LE. OF CHAR T I M E , t , MINUTES
60-
THERMOCOUPLE 3 I
Figure 1 . Char desulfurization system W
l
The reactor was brought up to temperature with nitrogen purge. Hydrogen was introduced into the system a t the start of the run. Adjustments were then made to bring the flow and pressure to the desired levels. After the run, the reactor was disassembled; the product char was weighed, and analyzed for residual sulfur. Sulfur was routinely determined by X-ray emission ’ sulfur. spectroscopy, with a precision of k0.05 weight % This routine method was standardized against the standard Eschka method. Repeated efforts were made to obtain a satisfactory off-gas analysis, but these were unsuccessful. Char Properties
Char from the Project COED process development unit was used in this study (Jones et al., 1966; Schmid et al., 1968). This char resulted from multistage pyrolysis of coal with a final stage temperature of 1600°F. The feed coal was from the Illinois No. 6 seam, the Crown mine. The chemical and physical properties of the char are presented in Table I. During pyrolysis, about half of the coal was volatilized. The ash content of the char is about 17%. The char is highly microporous-its porosity is about 60% and the surface area is over 100 m2per gram. There is no significant change in the properties of the char other than sulfur content as a result of desulfurization. The untreated char contains 3.4% sulfur. The ratio of inorganic to organic sulfur is about 0.5 in the +loomesh fraction studied. There is only a very small quantity of sulfur as sulfate. The inorganic sulfur is present primarily as either ferrous sulfide (FeS) or pyrrhotite (FesSs), the pyrolysis products of pyrite present in the coal. Microscopic examination showed that the original pyrite particles ranged from 3 to 25 microns in diameter. The data in Table I1 on sulfur content as a function of particle size show that sulfur is somewhat concentrated in the fines. Because of this, great care was taken in preparing the feed samples. The standard error due to sampling is *0.13 weight % sulfur in the feed char. This accounts for much of the error in replicate runs. Experimental Results
The early experiments were directed toward screening effects observed by previous investigators. Variables 358
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970
Figure 2. Effect of temperature on desulfurization
Table I. Physical and Chemical Properties of Crown Char Proximate analysis, wt. “c Moisture Volatile matter Fixed carbon
4.4 3.6 15.4 16.6
Ash Ultimate analysis, wt.“r, dry
C H N S 0 Ash Particle density, gicc Macropore volume, cc/g Mean macropore diameter, Surface area, m‘/g
p
76.8 1.4 1.2 3.1 0.1 17.4 1.08 0.32 5 123
Table II. Sulfur Distribution in Crown Char Wt. Yo S in Fraction Wt. O h on Sieve
Sieve Size, Tyler Mesh
28 48 100 200 Pan
’
Sulfur species Organic Inorganic (pyritic) Sulfate
39 34 20 6 1 68% 297 30;
2.57 2.59 3.14 3.80
...
Inlet H A , Vol.
Yo
Reaction Time, Hr
O h
S Remaining
1.6 3.2
15 0.5 11,150
PRESSURE, psia ( P I SUPERFICIAL V E L , ft./sec.(U) VOL. SPACE V E L , hr-I(SY)
t LOG
12 22 72 118 105
0
MID. 30 I .o 22,300
LOW
Table Ill. Effect of HzS on Desulfurization a t 1600' F and 115 PSlA
HIGH 60
2.o 44,600
P
Table IV. Effect of Particle Size on Desulfurization a t 1600' F, 15 PSIA Size Fraction
Time, M i n
Superficial Velocity, Ft/Sec
Residual Sulfur, Yo of Orig.
+28 -200 +28 -200 +28 -200
5 5 15 15 10 10
0.53 0.51 0.52 0.56 0.22 0.19
68 66 43 46 65 72
u
/LOG
Figure 3. Desulfurization experimental design
studied were temperature, H2S concentration, and particle size. The effect of temperature is shown in Figure 2. The data suggest that there is little effect of temperature above 1600" F. At 1350" F , however, the desulfurization accomplished in a given time is far below that a t 1600"F. These were the only data taken a t temperatures other than 1600OF. The reaction is so complex t h a t from these limited data it was not possible to obtain Arrhenius plots. The resiilts of the runs with gas mixtures containing 1.6 and 3.2 mole % H 2 S in hydrogen are presented in Table 111. The results of several long-duration runs are reported as per cent of initial sulfur remaining after treatment. Comparable runs without H 3 are presented for comparison. The presence of 1.67 H2S severely retards the desulfurization. The 3.2% HIS gas actually transfers sulfur to the char. In an experiment to determine the influence of particle size on desulfurization, the plus 28-mesh fraction was screened from a batch of char. Half of this was then crushed, so that the entire quantity passed a 200-mesh sieve. Pairs of experiments were made under similar conditions, using the plus 28- and the minus 200-mesh char (Table IV). The differences between paired runs are not significant. After the initial screening runs, a 23 factorial experiment was set up in variables of pressure, superficial velocity, and space velocity. The experimental design is shown in Figure 3 in a factor space diagram. A center point was also used. Reaction times were 2, 5 , 15, 30, 45, and 90 minutes. Unfortunately, two corner points in the design were not attainable, because the required bed would have been too small for analysis. The results are presented in Figure 4 as per cent of the initial sulfur remaining after treatment us. time. Inspection reveals that the last 10% of the sulfur is removed only a t a very low rate, if a t all, and that conditions 3 and 6 are grossly different from the other conditions. The scatter of the data for these two conditions is also greater than for other sets of runs. The variable separating these runs from the others is bed depth. Higher pressure increases the rate of desulfurization to some degree. and there is a barely significant effect of space velocity. For reasons not entirely clear, the primary effect is in the direction of decreased bed depth.
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44,600 11,150 22.300
60 30
1.0
A
0
0
20
40
60
80
IO0
TIME ,MINUTES
Figure 4. Per cent sulfur vs. time for factorial design Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No.
3, 1970 359
4
I
I
might be diffusional, or it might depend on a slow rate of reaction a t the solid-gas interface. If it is diffusional in nature, clearly the diffusion barrier is not the macropore structure of the char, since otherwise reduction of particle size should have increased the desulfurization rate markedly. For the “pyritic” residue, the carbon microstructure too should not be important, since the pyrite particles tended to be grouped in rather exposed veins. A reacted shell of iron surrounding a core of still unreacted FeS is a possible diffusional barrier in this system. Classically, these two possible rate-limiting steps would be discriminated on the basis of curve shape. The data from this study are not really sufficiently precise for this comparison to be meaningful. I t is possible, however, to calculate the order of magnitude of the diffusion coefficient required to explain the data. This can be done by using a slight modification of the usual “reacted shell model” discussed by Levenspiel (1962) and others. Assuming the particles of FeS to be spheres, the sulfur flow, N,,a t any radius lying in the reacted shell, r , is given by
I
CROWN 1600’F COED CHAR
I 0
I
I
I
2 3 WEIGHT % S IN CHAR
I
I
I 4
Figure 5. Inhibition isotherms for char desulfurization Discussion of Results
Inhibition Isotherm. In their study of Arkwright char, Zielke et al. 11954) found that an equilibrium existed between hydrogen sulfide in the gas phase and sulfur in the char. They expressed this as an “inhibition isotherni,” relating hydrogen sulfide- hydrogen ratio t o the sulfur content of the char. An equivalent isotherm for Crown char is shown in Figure 5 , plotted from the data of Table 111, and from a knowledge of the ferrous sulfide content of the char and of the FeS-H2 equilibrium. Equilibrium Model. Given the inhibition isotherm for Crown char, it is logical to ask whether the gases leaving the char bed are a t equilibrium. A simple material balance on the tied gives the equation
N,= 4ar2D
t
dC
iM dr
(4)
If pseudo-steady state is assumed, the sulfur flux may be related by material balance to the rate of advance of the reacting interface a t radius R.
‘oo 80
A t a given total pressure. the inhibition isotherm gives I
70
Equation 1 can then be formally integrated to give the time necessary to achieve a given sulfur content in the char. This relation is
(3) The results of such an integration are compared with the data in Figure 6. Two computed curves suffice to describe six sets of data. The remaining set of data, condition 5 , is described by a similar curve lying very close to the vertical axis (not shown). While the equilibrium model seems t o fit the observed behavior a t most conditions up to 50% sulfur removal, it fails badly thereafter. Gross errors in the inhibition isotherm are unlikely, since region I1 is based on a wealth of experimental and thermochemical data. The breakdown of the model around 50% sulfur removal suggests a change of rate-limiting step. I t appears that the equilibrium model is primarilv applicable to the “organic” sulfur. The rate limitation applies primarily. but not exclusively. to the reiidue of the pyritic sulfur. Such a limitation 360
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970
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that the hydrodesulfurization process is strongly influenced by the source coal and by the conditions of pyrolysis. Among the variables of importance are the per cent of pyritic sulfur in the source coal, its distribution within the coal, and the pore structure of the char. The possible importance of diffusion within the reacted shell surrounding still unreacted pyrite residues is raised here. If this hypothesis is correct, the rate of removal of “pyritic” sulfur in a hydrogen atmosphere should vary with the inverse square of the diameter of the initial pyrite particles.
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Acknowledgment
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The authors acknowledge the assistance of other members of the COED team, and in particular, of Louis D. Friedman who performed some of the early experimental work.
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Figure 7. Comparison of real and hypothetical desulfurization curves where the term in parentheses is the concentration of sulfur in the unreacted region. Double integration over r and t gives
Nomenclature
C Co D f(s)
= = = = =
M N = N, = P = r = R = R, = s = so =
sL = S, = This equation may be solved implicitly for ( R / R , ) 3which , is the fraction of the sulfur removed. Figure 7 compares the data for condition 5 with a combined model in which 10% of the sulfur is assumed to be bound in forms that do not react significantly under process conditions. Thirty-five per cent is assumed to be present as particles of ferrous sulfide. The remainder of the sulfur is assumed to be associated with the carbonaceous residue and to be removable a t equilibrium. The fitting of the data to this model fixes the value of the quantity ( ~ D / T ’ )Assuming . c to be 0.6 and T’ to be 4, we calculate that D is about l o - ’ cm per second. This is not an unreasonable value for diffusion in the Knudson or transition region. In effect, this analysis proves only that diffusional limitation cannot be ruled out as a possible explanation of the observed rate of reaction. Summary
For a t least one coal char, an economically acceptable process cannot be constructed for simple hydrodesulfurization, although the saturation of the sulfur-carrying capacity of the gas phase during hydrodesulfurization is rapid. The transfer of sulfur from the char to an acceptor material-e.g., CaO-might be economically attractive, if the acceptor is very inexpensive or can be regenerated a t moderate cost. A comparison of this work with earlier studies suggests
t =
U = W = Yo = c
=
p
=
T
=
concentration, moles per cm3 equilibrium concentration diffusion coefficient, cm2 per second inhibition isotherm, mole fraction molecular weight-species indicated by subscript rate of flow, moles per second rate of sulfur removal, moles per second pressure, psi radial distance, cm or microns radial distance to reaction interface radius of particle sulfur content, weight fraction initial sulfur content v o h n e t r i c space velocity, ft’ per hour per ft’ of bed weight space velocity, pounds per hour per pound of char time, hour, minute, or second superficial velocity, feet per second bed weight, grams equilibrium mole fraction of sulfur in gas phase porosity density tortuosity
literature Cited
Batchelor, J. D., Gorin, E., Zielke, C. W., Ind. Eng. Chem. 52 (a), 161-8 (1960). Jones, J. F., Schmid, M. R., Sacks, M. E., Chen, Y., Gray, C. A., Eddinger, R. T.. “Char Oil Energy Development,” Office of Coal Research, Dept. of Interior, Contract 14-01-0001-235, January-October 1966. Levenspiel, O., “Chemical Reaction Engineering,” pp. 3468, Wiley, New York, 1962. Schmid, M. R., Jones, J. F., Eddinger, R . T., Chem Eng. Progr. 64 (85), 26-30 (1968). Ziekle, C. W., Curran, G. P., Gorin, E., Goring, G. E., Ind. Eng. Chem. 46, 53-6 (1954). RECEIVED for review December 3,1969 ACCEPTED June 15,1970 Symposium on Chemicals from Coal, Division of Fuel Chemistry, 158th Meeting, ACS, New York, N. Y., September 1969. W-ork performed by the FMC Corp. as part of Project COED for the Department of Interior, Office of Coal Research. under contract NO, 14-01-0001-498.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970 361
