666
Ind. Eng. Chem. Process Des. Dev. 1981, 20, 666-670
Effect of Carbon Formation on Liquid Viscosity and Performance of Fischer-Tropsch 8ubble-Column Reactors Charles N. Satterfleid,’ George A. Huff, Jr., and Harvey 0. Stenger Department of Chemical Engineering, Massachusetts Institute of Technomy, CambrMge, Massachusetts 02 139
Viscosities of 0.5 to 6.8 wt % dispersions of three different carbon blacks in n-hexadecane were measured. Agglomerates are formed that subtantially increase the apparent viscosity at low shear rates. However, the large increases of viscosity in liquid-phase Fischer-Tropsch reaction, attributed to carbon formation by Farley and Ray (1964) and others, appear instead to have been caused primarily by accumulation of high molecular weight hydrocarbon products.
Introduction A recent economic study for the U.S. Department of Energy by Shinnar and Kuo (1978) pointed out that the lowest cost synthesis gas produced with the highest thermal efficiency from coal is that with a low hydrogen to carbon monoxide mole ratio, in the range of 0.6 to 0.7. Unfortunately, much higher ratios are required in conventional processes for conversion to fuels, such as the Fischer-Tropsch vapor-phase process. With the iron-based catalysts employed in the entrained-bed reactors at SASOL in South Africa, a ratio of about 6 is reportedly employed. Lower ratios accelerate the Boudouard reaction, 2CO C02 + C. In fixed- and entrained-bed reactors, carbon formation poisons the catalyst and can cause severe mechanical problems as carbon growth causes the catalyst to swell and break up into fine particles (Dry, 1976, 1980). In contrast, one of several potential advantages of a slurry reactor process for the Fischer-Tropsch synthesis is that it can purportedly utilize a hydrogen to carbon monoxide feed ratio as low as 0.6 without rapid catalyst fouling (Poutsma, 1980). Much of the carbon formed on iron and other metals under sufficiently reducing conditions appears as carbon filaments that originate from the metal surface, each containing a particle of metal or metal carbide at the head, which is the center for growth (Satterfield, 1980). In a Fischer-Tropsch bubble-column reactor, these fibrous particles will be constantly sheared from the catalyst surface and will be broken up by the stirring action of the gas bubbles, resulting in a fine solid suspension. In an early study, widely cited, Farley and Ray (1964) reported that the slurry in their Fischer-Tropsch reactor underwent a large increase in viscosity during reaction, which eventually led to shutdown of the reactor as the liquid approached “gellation” at reaction conditions. The increased viscosity was attributed to the accumulation of finely divided carbon. The accumulation of fine solids in a Fischer-Tropsch reactor is undesirable for several reasons, but increased liquid viscosity of itself causes gas bubbles to be larger and hence reduces the potential rate of reactant mass transfer from the gas bubbles to the surface of the catalyst. It is well known that the rheological properties of a dispersion of carbon black in a vehicle differ considerably from those of the matrix itself as aggregates of carbon particles coalesce in the liquid medium to form agglomerates (Donnet and Voet, 1976). However, it appeared to us that the carbon concentrations reported by Farley and Ray were too low to account for the reported viscosity increase and that perhaps instead this was caused pri-
-
Table I. Carbon Black Propertiesa arithmetic dibutyl mean surface phthalate particle volatile area, absorption, diameter, content, m’/g cm3/100g mpm wt%
BETN, black Monarch 1300 Elftex 12 Sterling R
’Source:
560 43 25
121 95 71
13 37 75
9.5 1.0 0.5
Cabot Technical Report S-36,October 1979.
marily by the accumulation of high molecular weight hydrocarbon products in the liquid. Little information is available on the viscosity of low concentrations of finely divided carbon in organic liquids; therefore, we measured the viscosity of three quite different carbon blacks in a paraffin liquid over a range of shear rates of interest and have shown the significance of the results to the liquidphase Fischer-Tropsch process.
Experimental Section Carbon Black and Liquid Vehicle. Table I lists the physical properties of the three different carbon blacks studied. All blacks were commercial grades, made by the furnace process, and were supplied by the Cabot Corporation. They were selected so as to reflect a wide range of parameters. Volatile content is the reduction in weight when the black is heated to 950 “C in an inert atmosphere. The liquid in a Fischer-Tropsch slurry reactor consists mainly of normal paraffins. For our studies we used 99% pure normal hexadecane (Humphrey Chemical Co.). Hexadecane is a Newtonian liquid with a freezing point of 18 “C, and thereby viscosity experiments can be conveniently performed at room temperature. Most carbon black loadings studied here were between 0.5 and 3.0 wt 70 but a few ranged up to 6.8 wt %. The overall range corresponds to ratios of carbon black volume to vehicle volume, F , from approximately 0.002 to 0.030, assuming a carbon black density of 1.86 g/cm3 and a hexadecane density of 0.773 g/cm3. Selection of Viscometer. The viscosities of the pure liquid and of the carbon dispersion were measured with an Ostwald capillary viscometer, chosen for its simplicity and availability. However, the shear rate is less with smaller capillary bore sizes or with increased viscosity of the dispersion. Whereas vehicle viscosity does not change with shear rate, carbon dispersed in hexadecane, on the other hand, is non-Newtonian as shear breaks up clumps of carbon aggregates that have coalesced in the liquid
0196-4305/81/1120-0666$01.25/00 1981 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 4, 1981 667
medium. It is important then that viscosity measurements be made at shear rates typical of those in a commercial bubble-column reactor. Viscometers with A.N.S.I./A.S.T.M. bore sizes of 150 and 200 were employed, which correspond to capillary diameters of 0.078 cm and 0.101 cm, respectively. They were selected to provide minimum elutriation times with a wide range of shear rates (50 to 400 s-l) and still maintain negligible kinetic energy corrections. Rheological Measurement. Measurements were conducted in a constant-temperature oil bath maintained at 23 “C. At this temperature, pure n-hexadecane has a viscosity of 0.0353 P. To prepare the mixtures, fluffy carbon black was added with the required quantity of hexadecane in the lower reservoir of the viscometer. This mixture was homogenized by an ultrasonic bath before the initial run. Viscosity measurements followed the procedure outlined in A.N. S.I./A.S.T.M. standard no. D-446-74 (“Standard Specifications and Operating Instructions for Glass Capillary Viscometers”). The dispersions measured in this study with the capillary rheometer did not exhibit thixotropic behavior in that viscosities were not a function of time. Using a 4.2 wt % Monarch 1300 carbon black dispersion, three runs over a 20-min period showed a random error of less then 2%. For non-Newtonian fluids, the wall shear in a capillary rheometer is given by 7,
= pap (4Q/~ro3)
(1)
The term papis the apparent viscosity and 4Qlrr: is referred to as the pseudo-shear rate. While these results from the capillary flow experiments do not give the basic shear diagram directly, they can be adjusted so as to correct for the non-Newtonian behavior of carbon black dispersions. This can be accomplished by assuming that the results are adequately described by a power law model as 7
= m(-du/dr)“
(2)
From this relationship, the shear rate and velocity profiles as a function of capillary radius for laminar and steadystate flow are then defined as
(-$)
=
(5)( 7) (k)’”
(3)
and
With these two relations, the shear rate that corresponds to the apparent viscosity can be solved from the expression
JO
Upon substitution of eq 3 and 4 into eq 5, integration results in
+
‘ap
(3n 2(2n + 1)(3n
+ 2)(
5)
The basic shear diagram was obtained as follows. (1) A t a given carbon loading, n was obtained from the slope
of a log-log plot of 7, vs. 4Q/rro3 (manipulation of eq 1 and 2). (2) This value of n was then used to construct a plot of n vs. carbon weight percent for a particular black. (3) With a value of n at a given carbon loading, 4, , which corresponds to pap,was calculated from eq 6. dowever, this correction amounted to only 15% for the most extreme condition here, namely n = 0.5. Inspection of the plots of n vs. loading for the blacks studied reveals that n appears to decay exponentially and, for example, declines from 1.0 to 0.6 at Monarch 1300 loadings of 1.0 to 0.6 wt % , respectively. As expected, n is less than 1for a carbon black dispersion, which indicates that it behaves as a pseudoplastic fluid in which viscosity decreases with increasing shear. Furthermore, dilute dispersions of carbon black in the paraffin exhibit Newtonian character as n approaches unity at low loadings. Correlations Carbon particles in oil dispersions interact physically through elastic collisions and electrostatically by London-van der Waals forces. These latter attractive forces are of great importance as they cause the black to set up a network structure comprising carbon black aggregates held together in agglomerates. These flocs influence viscosity to a greater extent than the individual aggregates because their effective solid volume is much larger than that of the carbon black due to vehicle occluded in the void volume between the aggregates. Hence the network imparts a higher resistance to flow, and viscosity of the dispersion is increased. On the other hand, agitation breaks up the agglomerates, which results in a lower viscosity. Experimental evidence indicates that the carbon black aggregates are nonspherical, even at the highest shearing stresses obtained. This suggests that a fusion of elementary spherical particles into aggregates occurred during the formation of the black in the furnace (Donnet and Voet, 1976). A carbon black aggregate appears to be a significantly anisometric branched chain of coalesced elementary carbon particles. It is not surprising then that Einstein’s classic equation for the viscosity of a dispersion of spherical particles, eq 7, has not been successful in describing the viscosity of a tr = 1 + 2.5C (7) carbon black dispersion. Einstein’s relation is based on the hydrodynamics for spherical, incompressible, noninteracting particles in an infinitely diluted dispersion. Rutgers (1962) gives an exhaustive survey of existing relationships, proposed on theoretical grounds and used empirically, for predicting the relative viscosity of dispersed systems. According to Rutgers, a general power formula takes into account structure-forming dispersions and is valid up to moderate concentrations qr =
1 + kC
+ alk2C2+ a2k3C3+ ...
(8)
Recently, Graziano et ai. (1979) reported the relative viscosity of carbon black pastes to be proportional to C2 for carbon black loadings of 23 to 45 wt %, studied a t a given shear rate. Graziano’s results imply that a t high concentrations eq 8 can be simplified to qr = ulk2C2, i.e., that third- and higher-order terms are negligible. Based on this result, it is reasonable to assume that at the lower concentrations of this study eq 8 can be reduced to qr = 1 + kC + Ulk2C2 (9) Riseman and Ullman (1951) have proposed values of ul of 0.60 for flexible chain macromolecules and 0.73 for rigid rod molecules. Electroviscous effeds cause higher k values.
668
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 4, 1981
01
0
Figure 1. Effect of carbon loading and K on relative viscosity.
Voet and Suriani (1952), on the other hand, developed an empirical relationship between carbon loading and relative viscosity of a carbon black dispersion, in which the value of K depended on the nature of the black In vr = K F (10) This expression was found to hold up to a concentration of about 15 wt % . This exponential relationship may be compared to eq 9 by expanding it into the following power series tr = eKF= 1 + K F ( K T ) / 2 + ... (11)
+
The ratio of carbon black volume to vehicle volume, F, is defined as F=- C
1-c
This term may also be expanded into a series as
- - - C(1 + c + c2 + ...) l-c
(13)
For low concentrations of carbon black (below about 20 wt %), eq 13 can be approximated as
F = - l- - c - C ( l + C)
(14)
Upon substitution of eq 14 into eq 11 and simplification, the following results if third- and higher-order terms are neglected tr = 1 + KC + KC2[1 + ( K / 2 ) ] (15) Finally, if K >> 2 or, in other words, strong electrostatic forces exist between carbon aggregates, eq 15 simplifies to
+ KC + ( P C 2 / 2 )
(16) Comparison of eq 16 and 9 shows that they are identical if al = 0.50. The exponential relationship proposed by Voet and Suriani (eq 10) then becomes a simplified version of the power law model, but it has been quite successful in describing a number of carbon black dispersions (Donnet and Voet, 1976). For this reason, it was employed to analyze the results of this study. For orientation in the following discussion, Figure 1displays a plot of eq 10 for carbon loadings of up to 3.5 wt % and for representative values of K. tr= 1
I 100 200 APPARENT SHEAR
I
I
300 RATE
400 s-
Figure 2. Effect of shear rate on K for three carbon blacks. Open symbols for measurements with 0.078 cm capillary, dark symbols for those with 0.101 cm capillary. Concentrationsas follows: Monarch 1300, 0.6 to 6.8 w t %; Sterling R, 0.5 to 3.7 wt %; Elftex 12,0.5 to 2.6 wt %.
Results and Discussion The intent of this study was to determine the range of K factors that would be observed with three markedly different carbon blacks in hexadecane, under shear conditions representative of those in bubble-column reactors. The goal was to obtain a better understanding of the effect of free carbon on the apparent viscosity in a liquid-phase Fischer-Tropsch reactor. Experimental K values (calculated by eq 10) are plotted in Figure 2 as a function of apparent shear rate (corrected by eq 6) for each carbon. A greater degree of agitation decreases the apparent viscosity of the carbon black dispersions, which act as a pseudoplastic fluid, as reflected by a smaller K factor. Visual observations of the black networks under stagnant conditions with a light microscope at 100 magnification confirm the trend of carbon type in Figure 2. Agglomerate size decreased from Elftex to Sterling to Monarch. Once again, evidence indicates that smaller particle flocs impart less resistance to flow and the dispersions become less viscous. In comparing the plots of Figure 2 to physical and chemical properties of the three blacks in Table I, no direct relationship can be made between any one characteristic and its relative effect on the viscosity of the suspension in hexadecane. Experimental results by Graziano et al. (1979) support this observation as they demonstrated the viscosity of carbon black pastes to very with carbon black volume fraction, specific surface area (BET), aggregate structure, and shear rate. Supposedly, the effect of carbon volume fraction on viscosity is described by eq 10 so that K is a function only of the latter three variables. In order to compare these values obtained for amorphous blacks to structured carbon, viscosity measurements were also carried out with a 44-pm particle size graphite (Microfyne grade, manufactured by Dixon Crucible Co.) at various weight percents in hexadecane. Although graphite is somewhat more dense (2.25 g/cm3) than carbon black, a K value of only 8 to 10 was obtained and, within experimental error, was independent of shear. Finely divided solids formed by catalyst attrition and catalyst breakup would not be expected to contribute significantly to a viscosity increase at low loadings and this was borne out by experiment. A mechanically stirred autoclave was operated at 250 "C and with a H2/C0 mole ratio of 1.5 in the feed gas, utilizing a conventional am-
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 4, 1981 869 Table 11. Fischer-Tropsch Studies in a
Bubble-Column Reactor
HJCO feed ratio
inlet superficial gas velocity, cmls
1.0
1.3-3.5
65-175
0.7
3.5, 9.5
175,475
0.7
0.2-2.2
10-110
1.3
0.9-2.2
4 5-1 10
0.7
5.3-7.2
265-360
0.6
4.0
200
molar
investigators
Schlesinger et al. (1954) Kolbel et al. (1955) Calderbank et al. (1963) Mitra and Roy 11963) F&ey and Ray ( 1964) Sakai and Kunugi ( 1 9 7 4 )
apparent shear rate. s-l
monia synthesis fused-iron catalyst in 80-100 mesh size in a molten paraffin wax. After 325 h of operation the run was stopped, and after cooling, fine solids were scraped off the top of the solidified paraffin, exhaustively extracted with carbon disulfide, and heated under nitrogen at 400 "C to drive off volatiles. The solids contained 5.1% carbon, by weight loss, the remainder being finely divided iron catalyst, probably formed primarily by catalyst attrition. A suspension of 2.12 wt % of this solid in hexadecane had a relative viscosity of only 1.04 and this was independent of shear rate. Values of K range from about 10 to 100 for all carbons studied, over a shear range of 50 to 400 8-l. These K values correspond to a viscosity increase by a factor of about 1.1 to 3.6 at 3 wt % carbon in hexadecane. The data in Figure 2 may be compared to K factors of 4 to 11 reported by Voet and Suriani (1952) and Voet and Whitten (1962) for several different channel and furnace blacks in oil with a rotational rheometer at shear rates of 670 and 600 s-l, respectively. These values are in good agreement with our study at lower shear rates (400 s-l maximum) as K would be expected to approach an asymptotic value at higher degrees of agitation as agglomerates become broken down into aggregates. They are also similar to that for graphite (K = 8 to lo), which has little tendency to form a network structure in the paraffin. Nishikawa et al. (1977) developed an experimental method by which the local average shear rate was correlated with superficial gas velocity in a bubble-column reactor containing non-Newtonian liquids. At a superficial velocity higher than 4 cm/s, the following expression was recommended, independent of vessel diameter.
Tap = 5 0 ~ ~
(17)
At lower gas velocities, the local average shear rate is higher in the core section and lower at the column wall than that given by eq 17. However, Nakanoh and Yoshida (1980) recommend the use of eq 17 even at superficial gas velocities lower than 4 cm/s as a good approximation of the average shear rate over the entire column section. A summary of Fischer-Tropsch studies conducted in a bubble-column reactor is given in Table 11. Shear rates were estimated by eq 17, based on inlet gas velocities. Gas volume and hence superficial velocity and shear rate will decrease with conversion to an extent that depends on the products formed, so the shear rates in Table I1 are maximum values for each study. Nevertheless, the experimental shear rates used here, 50 to 400 s-l, adequately cover the range of those used in previous bubble column studies.
In the study by Farley and Ray (1964), a FischerTropsch reactor was operated with an iron catalyst at 275-280 "C, absolute total pressure of 1.13 MPa, and inlet H2/C0 mole ratio of 0.67. The carbon content increased with time, and slurry viscosity increased markedly as carbon content increased above 2.5 to 3 w t % carbon. After 392 h of operation, for example, the carbon content was 3.1% and the viscosity had increased 16-fold. They measured the viscosity of the liquid at 188 "C and shear rate of 44-48 s-l with an unspecified rheometer. The viscosity corresponded to a K value of 210, as calculated by eq 10. Although the shear rate of the viscometer employed by Farley and Ray was low compared to the shear rate that probably existed in their reactor (Table 11),it does not account for the fact that their value of 210 far exceeds those obtained for any of the carbons in this study. From a rheological viewpoint then, Farley and Ray's conclusion that such a large increase in slurry viscosity came from the presence of carbon in the paraffin wax liquid seems unlikely. Other factors, inherent to their pilot plant operation, undoubtedly contributed to this phenomenon and were not recognized. Specifically, other than periodic removal of the slurry medium to maintain a constant level, the liquid was operated batch-wise so that high molecular weight products produced by the synthesis remained in the reactor while lighter hydrocarbons were boiled off. It is likely that the accumulation of these waxes contributed to or were the major source of the increase in viscosity. This is illustrated in a study by Schlesinger et al. (1951) in which operation with an iron catalyst at 1.13 MPa total pressure, H2/C0 inlet ratio of 1.0, and 255 "C for 400 h resulted in an increase in the slurry liquid from 15 to 70% of waxes that boil above 450 "C at atmospheric pressure. This accumulation of wax increased viscosity, measured at 120 "C, of the paraffin medium by a factor of about, 5. Since the contribution of a predominantly carbon suspension to apparent viscosity will vary considerably with the nature of the solid and the shear rate, we can only approximate its effect in the Farley and Ray study. Farley and Ray did not report their method of analysis for "free carbon", but it is highly probable that it contained some iron or iron compounds. Nevertheless the range of I C s in Figure 2 is probably reasonably representative. They show that free carbon could account for an increase in viscosity at 3.1 wt % loading of only about 1.2 to 3.5, compared to the increase by a factor of 16 reported. It therefore appears that the major source of viscosity increase was not carbon formation but accumulation of high molecular weight products. We conclude that the hydrodynamic effects of carbon formation are much less severe than previously reported. Nevertheless, it is evident that carbon formation must be avoided or minimized, to avoid attack on an iron catalyst that can cause catalyst deterioration and loss of activity and to minimize re-working of the liquid medium that would be necessitated by accumulation of any form of solids or nonvolatile products. Nomenclature al, a2 = constants to adjust for orientation effects of dispersed particles C = concentration of carbon, cm3carbon/cm3total dispersion F = volume fraction, cm3 carbon black/cm3 liquid vehicle k = constant that varies with electrostatic interactions of the dispersed carbon particles in power law model for relative viscosity K = constant in empirical logarithmic model m = constant in power law or Ostwald-de Waele model for non-Newtonian behavior
670
Ind. Eng. Chem. Process Des. Dev. 1981, 20,670-674
n = constant to account for the non-Newtonian behavior of carbon black dispersions; n = 1 for Newtonian and n < 1
for pseudoplastic fluids Q = volumetric flow rate of dispersion through capillary tube in viscometer, cm3/s r = radial coordinate in capillary tube; r, for working capillary radius, cm u = local velocity of dispersion, a function of capillary radius, cm/s; du/dr for local velocity gradient or shear rate, s-l PG = superficial gas velocity in bubble-column reactor, cm/s Greek Letters apparent shear rate, s-l T~ - viscosity of carbon black dispersion relative to viscosity of pure liquid vehicle M~~ = apparent viscosity of dispersion, P 7, = shear stress at the wall of the capillary rheometer, dyn/cm2 Literature Cited Calderbank, P. H.; Evans, F.; Farley, R.; Jepson, G.; Poll, A. "Catalysis in Practice", Proceedlngs of a Symposium; Institution of Chemical Engineers: London, 1963, p 66. Donnet, J.-B.; Voet, A. "Carbon Black", Marcel Dekker: New York, 1976; Chapters 1, 5.
Dry, M. E. Ind. Eng. Chem. Prod. Res. Dev. 1976, 75, 282. Dry, M. E. Hydrocarbon Process. Feb 1980, 92.
Farley, R.; Ray, D. J. J. Inst. Pet. 1964, 50, 27. Graziano, F. R.; Cohen, R. E.; Medalla, A. I. Rheol. Acta 1979, 18, 640. Kolbel, H.; Ackermann, P.;Engelhardt, F. "Proceedings of 4th World Petrob um Congress", Rome, 1955; Sect. IV/C, 227. Mitra, A. K.; Roy, A. N. Indian Chem. Eng. July 1963, 127. Nakanoh, M.; YoshMa, F. Ind. Eng. Chem. Process Des. D e v . 1980, 19, 190. Nlshikawa, M.; Kato, H.; Hashimoto, K. Ind. Eng. Chem. Process Des. Dev. 1977, 16, 133. Poutsma. M. L. Oak Rldge Natlonal Laboratory Report 5635, 1980. Rlseman, J.; Ullman, R. J. Chem. Phys. 1951, 19, 578. Rutgers, R. Rheol. Acta 1962, 2, 305. Sakai, T.; Kunugi, T. Sekiyu Gakkal Shl1974. 17, 663. Satterfield, C. N. "Heterogeneous Catalysis in Practlce"; McGraw-Hill: New York, 1980 pp 141-143. Schlesinger, M. D.; Croweli, J. H., Leva, M.; Storch, H. H. Ind. Eng. Chem. 1951, 43, 1474. Schlesinger, M. D.; Benson, H. E.; Murphy, E. M.; Storch, H. H. Ind. Eng. Chem. 1954, 46, 1322. Shinnar, R.; Kuo, J. C. W. U.S. Department of Energy Report FE-2766-13, 1978. Voet, A.; Suriani, L. B. Am. Ink Meker 1952, 30, 37. Voet, A.; Whltten, W. N., Jr. Rubber WorM 1962, 746, 77.
Received for reuiew October 15, 1980 Revised manuscript receiued April 30, 1981 Accepted April 30,1981
Effects of Calcium Minerals on the Rapid Pyrolysis of a Bituminous Coal Howard D. Franklin, Willlam A. Peters, Frank Carlello, and Jack 8. Howard* &paltment of Chemical Engineering and Energy Laboratory, Massachusetts Institute of Technolwy, Cambridge, Massachusetts 02 139
Effects of calcium minerals on product compositions from rapid pyrolysis of a Pittsburgh Seam bituminous coal were investigated. Demineralized, lime-pretreated, and calcite-pretreated samples of pulverized coal were heated in helium at 1000 K/s to temperatures of up to 1300 K, and yields of char, liquid, and individual gas products were determined as a function of time-temperature history. The addition of calcium minerals to the coal resulted in a large reduction in tar yield and smaller reductions in hydrocarbon gas yields. Carbon monoxide yields at higher temperatures were strongly enhanced by calcium minerals. Carbonate decomposition occurred at lower temperatures for calcite-coal mixtures than it did for pure calcite. Catalysis by calcium minerals of hydrocarbon cracking reactions and base catalysis of a possible mechanism for the decomposition of phenolic groups in the coal are proposed to explain the effects of calcium minerals on the decomposition of the organic coal fraction.
Introduction Previous research a t M.I.T. on rapid coal pyrolysis has dealt with the kinetics of evolution of individual products as a function of temperature, pressure, particle size, reactive gas, and coal type (Anthony and Howard, 1976; Suuberg et al., 1978, 1979a,b, 1980). Since recent studies elsewhere have shown that certain minerals occurring in coal affect significantly other types of coal conversion reactions, the present study was undertaken to determine what effects these minerals may have on rapid coal pyrolysis. This paper presents results on the pretreatment of coal with calcite (CaCOJ and lime (CaO). These minerals have already been shown to influence fluidized-bed pyrolysis (Yeboah et al., 1980), steam gasification (Haynes et al., 1973; Forney et al., 1974; Feldmann et al., 1977), and COPgasification (Sears et al., 1980; McKee, 1980) of coal. 0196-4305181 I 1 120-0670$01.25/0
Results obtained with other mineral additives will be reported later. Experimental Section The coal used was a Pittsburgh No. 8 Seam bituminous coal (Table I) ground to -270 +325 mesh (45-53 pm diameter). The native mineral matter was removed from the sample by extraction with HF and HC1 followed by float-sink separation, resulting in a coal containing 4.3% by weight mineral matter, most of it pyrite. The demineralization procedure was shown to have no effect on the subsequent pyrolysis behavior of the coal (Franklin, 1980). A fraction of the demineralized sample was co-slurried in water with 0.1-Mm diameter calcite grains for 24 h and dried at room temperature. The resulting mineral-treated coal contained 20.2% by weight CaC03. A second fraction of demineralized coal was similarly treated with CaO, re0 1981 American Chemical Society
