Energy & Fuels 1987,1, 377-380
377
Coal Reactivity in Direct Hydrogenation Liquefaction Processes: Measurement and Correlation with Coal Properties S.-C.Shin, R. M. Baldwin,* and R. L. Miller Chemical Engineering and Petroleum Refining Department, Colorado School of Mines, Golden, Colorado 80401 Received September 23, 1986. Revised Manuscript Received May 11, 1987
A suite of 14 coals has been liquefied in a tubing-bomb microautoclave reactor under a standard set of reaction conditions. The coals were selected from the Exxon and Argonne sample banks to represent a broad distribution with respect to chemical, geochemical, and geographical factors. Prior to liquefaction of the entire suite, an extensive set of screening runs were performed to identify conditions appropriate for reactivity measurement. Parameters examined during this phase of the program included the liquefaction vehicle (solvent), temperature, pressure, and reaction time. Data on the rate of conversion to THF, toluene, and pentane solubles were collected for each coal in the suite, and the rate of gas make was quantified. These data were then employed in kinetic modeling, and a single unified rate expression was developed that was capable of representing coal reactivity in terms of both the rate and extent of conversion. Finally, a new parameter for gauging liquefaction reactivity was developed that combined both rate of reaction and extent of reaction concepts based on the measured liquefaction kinetics profiles. This new reactivity parameter has been used successfully in single-variablelinear correlations with basic coal chemical and geochemical features such as volatile matter content, atomic H/C and O/C ratios, and vitrinite reflectance.
Background The question of how to define and measure the reactivity of coal towards direct hydrogenation liquefaction has intrigued researchers for over 60 years. A great deal of effort has been expended to find a single factor or group of factors that would correlate fundamental physical, chemical, and geochemical coal properties with the degree of conversion to liquid products under some standard set of reaction conditions. As early as 1920, Bergiusl reported that coals with over 85% carbon (daf basis) made poor liquefaction feedstocks. Francis2 suggested that a relationship existed between coal reactivity and carbon content on the basis of observations of the rate of permanganate oxidation of coals. Many other investigators have also attempted to correlate reactivity with rank, with notable lack of success.*s Neavels has shown that a relationship does exist between coal rank and the rate of conversion to benzene solubles. The relationship between coal chemical composition and hydrogenation reactivity has been extensively investigated. Shatwell and Graham,7 Horton et al.,S and Fisher et al.g have described reactivity correlations with coal composition based on both small-scale laboratory autoclave experiments and large-scale pilot plant operations. Fisher
and co-workers first developed a mathematical model for liquefaction reactivity in terms of petrographic composition, which was shown to be a fair predictive model for the small number of coals examined. More recently, the work of Given and co-workers on an extensive suite of coals has established the absence of one single parameter or universal group of parameters capable of correlating coal properties with coal con~ersion.'O-'~ Mori et al.14 found reactivity to be related to volatile carbon content, while Durie15has related reactivity to maceral distribution and atomic H/C ratio. Gray16in a study of South African coals, has related reactivity to maceral distribution, reflectance, volatile matter, and atomic H/C ratio. Several researchers have also attempted to correlate liquefaction potential with mineral matter content. Mukherjee and Ch~wdhury'~ and Guin et al.18 found iron compounds to be acting as heterogeneous catalysts, while Graylg attributed the apparent activity of increasing mineral matter to a physical dispersant/diluent effect. A common thread running through most of the previous investigations of coal liquefaction reactivity has been the definition employed for reactivity. Almost universally, reactivity has been defined and measured by the point yield of some solvent classification (e.g. pyridine solubles,
(1)Bergius, F.J . Gasbeleucht. Venu. Beleuchtungsarten Wassewersorg. 1912,54. (2) Francis, W. Fuel 1932, 71. (3) Petrick, A. J.; Gaigher, B.; Groenewoud, P. J . Chem. Metall. Min. SOC.S. Afr. 1937, 38. (4)Wright, C. C.; Sprunk, G. C. Pa. State Coll., Min. Znd. Exp. Stn. Bull. 1939, No. 28. (5) Fisher, C. H.; Sprunk, G. C.; Eisner, A.; Clarke, L., Fein, M. L.; Storch, H. H. Fuel 1940.19. (6) Neavel, R. C. Fuel 1976,55. (7) Shatwell, H. G.; Graham, J. L. Fuel 1925, 4. (8) Horton, L.; Williams, F. A.; King, J. G. G. B. Dep. Sci. Znd. Res., Fuel Res., Tech. Pap. 1936, No. 42. (9)Fisher, C. H.;Sprunk, G. C.; Eisner, A.; ODonnell, H. J.; Clarke, L.; Storch, H.H. Technical Paper 642,U. S. Bureau of Mines: Washington, DC, 1942.
(10)Given, P. H.;Cronauer, D. C.; Spackman, W.; Lovell, H. L.; Davis, A.; Biswas, B. Fuel 1975,54. (11)Given, P. H.; Cronauer, D. C.; Spackman, W.; Lovell, H. L.; Davis, A.; Biswas, B. Fuel 1975,54. (12)Abdel-Baset, M. B.;Yarzab, R. F.; Given, P. H. Fuel 1978, 57. (13)Yarzab, R.F.;Given, P.H.; Spackman, W.; Davis, A. Fuel 1980, 59.
(14)Mori, K.; Taniuchi, M. Kawashima, A.; Okuma, 0.; Takashashi, T. Prepr. Pap.-Am. Chem. SOC.Diu. Fuel Chem. 1979, 24(2). (15)Durie, R. A. Prepr. Pap.-Am. Chem. Soc. Diu. Fuel Chem. 1979, 24(2). (16) Gray, D. Barrass, G.; Jezko, J.; Kershaw, J. R. Fuel 1980, 59. (17)Mukherjee, D. K.; Chowdhury, P. B. Fuel 1976,55. (18) Guin, J. A.; Tarrer, A. R.; Prather, J. W.; Johnson, D. R.; Lee, J. M. Znd. Eng. Chem. Process Des. Dew. 1978, 17, 118. (19) Gray, D. Fuel 1978,57.
0887-0624187 .. . , ,12501-0377$01.50/0 0 1987 American Chemical Societv I
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378 Energy & Fuels, Vol. 1, No. 4, 1987
Exxon no. 007-MK 034-SM 021-RH 054-MY 052-SR 04143 059-SD 020-CM 086-AV 097-A3 098-A4 001-YB 105-TO Argonne
rank HVA SBB-A SBB-C HVC MV HVC HVC LV HVB HVA MV HVA HVA
location Kansas Wyoming Wyoming Illinois Penn. New Mex. Colorado Penn. Australia Canada Canada Japan Australia
C 83.52 76.62 73.40 79.44 89.57 80.04 78.04 90.77 83.90 87.04 87.83 85.78 85.47
MV
Penn.
86.0
Shin et al. Table I. Coal Properties elem anal., w t % (dmmf) H N S(tot) S(organic) 6.09 1.46 4.00 2.36 4.84 0.92 0.23 0.10 5.15 0.88 0.92 0.93 6.63 1.32 3.94 3.08 4.96 1.92 1.05 0.82 5.89 1.52 0.63 0.49 5.18 0.94 0.16 0.10 4.79 1.31 0.77 0.37 4.66 2.36 1.04 0.42 5.27 1.95 0.71 0.72 4.97 1.53 0.53 0.57 6.15 1.70 0.28 0.23 10.50 0.53 0.73 0.73 5.3
ethyl acetate solubles, etc.) after reaction for a set time period at fEed reaction conditions. This type of reactivity definition has been shown to be useful as a relative ranking factor, and to be reasonably well correlated with coal chemical features, but does not address the important question of reactivity from a rate processes perspective. That is, two coals with identical yields at extended reaction time would be judged to be equally reactive by this definition, while one coal may have reached this level of conversion in a much shorter period of time. From a reaction engineering point of view, the rate of reaction is equally or perhaps even more important than the ultimate extent of reaction. Recently, research has been reported that expands the question of how to define and correlate coal liquefaction reactivity to include reactivity definitions based on rate processes. Furlong20 reported that the rate of reaction served as a more sensitive relative reactivity ranking tool than extent of reaction for coals that had little variation in composition. Following this general concept, Baldwin et aLZ1have presented correlations for kinetic liquefaction reactivity with coal properties and have investigated development of predictive models for liquefaction reactivity using pyrolysis/mass spectrometry to characterize the coals and factor analysis to formulate the models. This paper presents the results of an experimental study on the rate of reaction of 12 coals from the Exxon sample bank. Various reactivity definitions are proposed, and the data modeled by using a pseudoreversible kinetic expression. Preliminary single-variable correlations for reactivity as a function of coal properties have been developed, with the goal of formulating predictive models for coal liquefaction reactivity.
Experimental Section A suite consisting of 13 coals from the Exxon sample bank and the Argonne Upper Freeport Premium coal sample were used for the experimental portion of this program. Partial characterization data for these 14 coals are shown in Table I. Experimental work was divided into two phases. The first phase dealt with identification of appropriate reactivity screening conditions, and utilized the Argonne Premium Coal sample. T h e second phase consisted of data collection for the 13 Exxon samples a t the standard reaction conditions identified in phase I, followed by kinetic modeling and reactivity correlation. All reactions were carried out in a tubing-bomb microautoclave reaction system. A schematic of the reactor and reaction system is shown in Figure (20) Furlong, M. W.; Baldwin, R. M.; Bain, R. L. Fuel 1982, 61. (21)Baldwin, R. M.: Durfee. S. L.: Voorhees. K. J. Fuel Process Technol. 1987, 15. (22) Shin, S.-C. M.S. Thesis No. T-3267, Colorado School of Mines,
1986.
...
maceral anal., w t % vit lip. inert. 85.8 9.0 5.2 93.0 2.2 4.8 85.8 11.4 2.8 91.0 2.2 6.8 89.3 0.7 10.0 82.2 6.0 11.8 77.4 2.2 20.4 91.1 0.1 8.8 46.4 2.8 50.7 96.3 0.2 3.3 0.4 18.4 81.1 95.5 2.2 2.3 1.9 96.8 1.3
ash 8.07 3.44 8.64 9.28 7.93 10.22 4.62 6.70 16.00 4.82 11.68 6.06 8.36
2.80 MICRO-AUTOCLAVE REACTION SYSTEM Vent Electric Motor
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F i g u r e 1. Tubing-bomb batch reactor schematic. 1. In d casea, the coal sample size was maintained at 1.0 g. Two small stainless-steel balls were added to the reactor to facilitate mixing. The Argonne coal was supplied in sealed glass ampoules a t -100 mesh, while the Exxon coals were received in sealed polyethylene containers as 16 X 100 mesh samples, and were ground manually in a C02-purged glovebox to 100% -100 mesh prior to use. All samples were stored in the glovebox until utilized for reactivity measurements. The phase I program investigated the effect of the following five operating parameters on liquefaction reactivity: vehicle (hydrogen donor or nondonor); vehicle/coal ratio; hydrogen partial pressure; temperature; time. Extensive experimentation on the effect of these variables on the reactivity measurements resulted in the following conditions being selected as the standard reaction parameters for this portion of the program: vehicle, l-methylnaphthalene; vehicle/coal ratio, 1/ 1(by weight); hydrogen partial pressure, 6.31 MPa (cold); temperature, 698 K, time, 3,5,10,20, and 40 min. Reproducibility of the data was determined by performing replicate experiments at a specified set of reaction conditions. Generally, the data for the T H F solubles were reproducible to within 1-2%, with toluene and pentane solubles exhibiting variations of from 2 % to 3% and 3% to 4%, respectively.
Discussion of Results Kinetic Modeling. The fundamental data gathered from the experiments with the Exxon coals consisted of reaction profila for conversion vs. time. Conversion of coal was further subdivided into compound classifications by fractionation of the products by successive extraction with tetrahydrofuran (total conversion), toluene (asphaltenes), and pentane (oils),while gas make was quantified with the
Direct Hydrogenation Liquefaction Processes aid of a krypton tracer. Since reactivity definitions based on both static and dynamic bases were to be investigated, the data were first reduced via kinetic modeling in order to identify a unified reaction rate expression suitable for description of the reaction profiles. As such, first- and second-order pseudoreversible expressions of the form 1st order dx/dt = K ( u - X) 2nd order
dx/dt = K ( u - x)'
where x = coal conversion (daf basis), a = pseudoequilibrium conversion, It = rate constant, and t = time (min), were investigated. This form for the rate expression was specifically chosen for kinetic data analysis as both kinetic (via "K") and quasi-equilibrium (via "a") parameters could be readily calculated from the model. Integrated and linearized forms of both models were fit to the kinetic data for total conversion, toluene solubles, and pentane solubles. An iterative procedure was utilized to correct the nominal reaction times cited above for the nonisothermal period prior to attainment of the desired reaction temperature. The adequacy of the models for each data set was then determined by a sum of squares procedure in order to determine the best overall descriptive expression. In general, the pseudo-second-order rate expression was found to be superior for all solvent solubility classifications for the 13 coals in this initial suite. Correlation with Coal Properties. Before correlation of liquefaction reactivity with coal properties could be initiated, a suitable definition for reactivity had to be found. Accordingly, the following variables were investigated for suitability as reactivity definitions: rate constant (12); pseudoequilibrium conversion (a); initial rate, dx/dt, at t = 0; point conversion at t = 5 min. The adequacy of any one definition for reactivity was then tested by forming single-variable linear expressions for reactivity and coal properties of the form y=a+bX where y = reactivity and X = coal property (e.g. H/C ratio, etc.), and testing for linearity via the statistical goodness of fit parameter 9.The first data set to be applied to these various definitions was for total conversion (THFsolubles). Very weak (r2C 0.1) correlations were found between all of the above reactivity definitions and conversion to THF solubles for the following coal properties (used as the dependent variables): H/C and O/C atomic ratios; fixed C; volatile matter; mean-maximum vitrinite reflectance; calorific value; total sulfur; organic sulfur; pyritic sulfur; vitrinite + liptinite. Reactivity data in terms of toluene and pentane solubles were tested next. Fairly strong correlations (0.65 C 1.2 C 0.85) were found between pseudoequilibrium conversion and the point conversion at 5 min and several of the coal properties such as fixed carbon, atomic H/C ratio, volatile matter content, and meanmaximum vitrinite reflectance. Other definitions such as those provided by the rate constant and initial rate were poorly correlated with all coal properties tested for all solubility data sets. Following analysis of the above results, a new reactivity parameter was developed that incorporated both static (quasi-equilibrium) and dynamic (reaction rate) properties. This new definition for reactivity was initially justified on the basis that neither reactivity definition alone would provide as good a measure of the true reactivity as some combination of the two, and it was hoped that better correlations with coal properties would result. Initial efforts focused on simple weighted combinations of the above kinetic and equilibrium parameters such as Ka and ha3.
Energy & Fuels, Vol. 1, No. 4, 1987 379 Table 11. Correlation Equations" coal property fixed carbon (dry, w t % ) H/C ratio
conversion toluene pentane toluene pentane toluene pentane toluene pentane
volatile matter (dry, wt O r ) reflectance (mean-max, % )
1.2
correlation eq Y = -0.2904X + 25.396 Y = -0.3416X + 25.463 Y = 24.146X - 9.506 Y = 28.984X - 16.035 Y = 0.2678X - 0.2611 Y = 0.3400X - 5.5741 Y = -0.6099X + 13.774 Y = -7.8026X + 12.289
0.89 0.83 0.65 0.63 0.80 0.87 0.72 0.79
"In the above expressions, the dependent variable (Y) is reactivity defined by the AREA parameter while X is the independent variable (coal property).
"1
*
01 40
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60 FIXED CARBON
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70
80
Figure 2. Reactivity vs. fixed carbon.
* 151
I
-
1
10
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30 VOLATILE M A T E R
40
50
Figure 3. Reactivity vs. volatile matter.
Finally, a reactivity definition based on the following AREA concept was investigated: AREA =
1 " x dt 0
where to = the time of for the coal to attain equilibrium conversion (40 min in this study) and x = fractional conversion (daf basis). As above, T H F solubles were not correlated with this definition of reactivity, but both toluene and pentane solubles were highly correlated with this new parameter. The results of linear regression on the coal properties shown above for the most signficant correlations found are summarized in Table 11. Figures 2-6 present sample correlations for coal properties with
Shin et al.
380 Energy &Fuels, Vol. 1, No. 4, 1987 151
151
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pected to improve the degree to which the total variation in the data set is explained by the model, thus giving higher correlation coefficients (r2). Considering the high degree of correlation achieved with single-parameter models, however, such expressions have not as yet been investigated.
*
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Figure 6. Reactivity vs. vitrinite
151
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Figure 5. Reactivity vs. atomic hydrogen to carbon ratio.
the AREA reactivity parameter. As can be seen, very good correlations with fixed carbon, volatile matter, and mean-maximum vitrinite reflectance and the AREA reactivity parameter have been found for this limited data set. A lower degree of correlation exists for atomic H/C and O/C ratios and calorific value, and essentially no correlation exists for reactive macerals, total sulfur, and organic sulfur. It should be noted that these correlations are all single-parameter linear models. Combinations of properties and multiple linear expressions might be ex-
Conclusions Research thus far on a limited suite of coals has clearly demonstrated that the procedures and techniques being employed for measurement of relative coal liquefaction reactivities are valid. Striking differences in the rate and extent of reaction to THF, toluene, and pentane solubles have been found for coals ranging in rank from low-volatile bituminous to subbituminous-C, and for coals from a very wide geographical distribution. A new reactivity definition based on a combination of equilibrium and kinetic parameters has been shown to be superior to other reactivity definitions, and this new definition has been found to be highly correlated with certain coal compositional features. Further research is now in progress to expand the data base from which these correlations were derived and to investigate the utility of various instrumental techniques such as pyrolysis/mass spectrometry, 13C NMR (CP/ MAS), and photoacoustic FTIR as rapid reactivity screening tools. Acknowledgment. The authors acknowledge the financial support of the U S . Department of Energy under Grant No. DE-FG22-85PC80907. The Exxon coal samples were provided by Dr. Richard C. Neavel.
