1016
Ind. Eng. Chem. Process Des Dev. 1086, 25, 1016-1022
Experimental Study of the Thermal Conductivity of Coal Liquefaction Products and Some Constituent Pure Compounds
Richard A. Perklns, E. Oendy Sloan,' and Michael S. Graboskl Colorado School of Mines, Chemical Engineering and Petroleum Refining Lbpartment, Golden, Colorado 8040 1
Thermal conductivlty data are presented for two coal liquid fractions and five constituent pure components. Utah (Char Oil Energy Development) COED and (Solvent Refined Coal) SRC-I whole coal liquid fractions were studied in the temperature range from ambient to 105 OC and from ambient pressures to 2000 psia. Pure component data are presented for toluene, I-methylnaphthalene, decalin, tetralin, m-xylene, and methylnaphthalene, from ambient temperature to the fluid critical or decomposition temperature and from ambient pressure to 200 psia.
The design of synthetic fuel plants and heavy oil processing plants requires accurate thermophysical property data. The available data base for the thermal conductivity of heavier oil components is very limited. A recent survey of the literature (Crooks et al., 1980) demonstrates the limited nature of the liquid data base in terms of hydrocarbon species studied and temperature and pressure range. That report further indicates there are no accurate correlational procedures available for highly aromatic mixtures which are becoming important in fuel production. The only coal liquid thermal conductivity measurements which have been reported are due to Gray (1981,1982) for SRC-I1 process fractions. The data paucity is due to the lack of foresight on the part of process developers in anticipating the magnitude of the error associated with utilizing petroleum fraction correlations to predict properties for highly aromatic coal liquid materials. The present program is concerned with the lighter portions (or cuts) of heavy oil fractions, that is, cuts with boiling points below 260 OC. Figure 1 (Gray, 1981) demonstrates that the content of aromatics increases rapidly with the cut temperature in coal-derived process oils. Figure 2 (Gray, 1981) shows that the heteroatomic content of the oils increases as cut temperatures increase. Over this temperature range, the dominant species are saturates consisting mostly of single ring naphthenes and aromatics consisting of benzene and naphthalene structures. Additionally, the cuts contain hydroaromatics and phenolic compounds. Sulfur and nitrogen, while present, represent a minor fraction of the liquid up to 260 "C. These heteroatoms appear in higher boiling asphaltenes. Similar product suites are found in heavy petroleum oils and in shale oils. Based on representative oil analyses, a group of model compounds covering the pertinent boiling range has been developed. Table I shows the compound suite. This paper presents the results of the investigation of a number of these liquids using an absolute thermal conductivity instrument developed a t the Colorado School of Mines specifically for high-temperature and high-pressure
* Author to whom correspondence should be addressed. 0196-4305/86/ 1 125-1016$01.50/0
Table I. Model Compounds in Coal Liquids Boiling below 260 "C" normal boiling critical Doint, "C temu, "C saturates methylcyclohexane 113 271 101 2,2,4-trimethylpentane 260 dimethylcyclohexane 127 332 isopropylcyclohexane 149 367 151 n-nonane 321 decalin 193 427 235 n-tridecane 403 aromatics toluene 111 318 m-xylene 138 344 tetralin 427 207 methylnaphthalene 482 242 phenols phenol 281 371 m-cresol 203 432 1-naphthol 279 427 From Crooks et al., 1980.
measurements (Perkins et al., 1981). Data are reported for toluene, 1-methylnaphthalene, decalin, tetralin, mxylene, and methylcyclohexane over the temperature range of ambinet to the fluid critical temperature or the limit of thermal stability and to pressures of 2000 psia. Additionally, data for two coal liquid fractions are reported; these consist of Utah COED and SRC-I whole fractions. The data collected in this study may be used to test the adequacy of existing design correlations for liquid thermal conductivity prediction.
Experimental Procedure The experimental data reported in this paper were obtained with a new transient hot-wire thermal conductivity instrument which is specifically designed to study nonelectrolyte liquids a t temperatures ranging from ambient to 500 OC and pressures to 2000 psia. A discussion of the theory of the experimental technique is presented by Mohammadi et al. (1981). The thermal conductivity instrument is described in Perkins et al. (1981), with an 0 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 1017 1001
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extensive description of the current instrument in Perkins (1983). Basically, the instrument uses a 5 in. long by 0.0005 in diameter platinum wire to simultaneously serve as a line heating source and resistance thermometer. The temperature response to the wire to either a ramp or a step increase in power is determined by using a microcomputer-based data logging system. The temperature response is compared with the solution of the heat-transfer model describing the temperature in a radial field of fluid to determine the thermal conductivity. It is important to note that the method is absolute; the measurements do not rely on a calibration based on a reference fluid. It has been shown by Perkins (1983) that the data derived for clean stable fluids from this instrument are accurate to within 1-2% depending on the temperature of measurement with
somewhat greater uncertainty for fluids which may thermally degrade and that the effecta of liquid compressibility on the thermal conductivity are observable with this instrument. A typical experiment is 1 s in duration and utilizes a temperature rise a t the wire of 2-20 "C. Over this time period, 1022 temperature measurements are made and used to determine the thermal conductivity through the requisite data analysis procedure.
Pure Compound Data The thermal conductivity of six pure materials is reported over the range of ambient to the critical temperature or the limit of thermal stability and 2000 psia. The fluids studied were toluene, m-xylene, l-methyl-
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Ind. Eng.
0
Figure 3. Toluene thermal conductivity compared to those of Mani and Venart. n
Figure 4. m-Xylene thermal conductivity.
naphthalene, decalin, tetralin, and methylcyclohexane. The maximum temperature in the case of toluene, methylcyclohexane, and m-xylene is limited to the critical temperature. In the cases of tetralin, decalin, and 1methylnaphthalene, the maximum temperature obtained was limited by thermal decomposition of the liquids. Thermal decomposition is indicated by a change in the phase behavior associated with the presence of hydrogen and light hydrocarbons. When excessive amounts of reaction products accumulate, the thermal conductivity value fails significantly. Toluene has been widely studied with a variety of modem instruments using both steady-state and transient techniques. A comparison of the data collected with this instrument and that of Polk and Jugel (1967) (steady-state technique) as well as the hot wire techniques of DeCastro
et al. (1977) and Nagasaka and Nagashima (1981) near ambient temperature and pressure shows excellent agreement. Figure 3 presents the toluene data as a function of temperature over the entire experimental range compared with the extensive data of Mani and Venart (1973). The agreement is very good; the slight discrepancy a t higher temperatures is probably due to the differing method employed to pressurize the sample. Mani and Venart pressurized their samples with nitrogen gas; a mercury seal is used in the current work. Perkina (1983) shows that the effect of gas on the thermal conductivity can be substantial a t high temperatures. Figures 4-8 present the data, along with multiple linear regression lines of the form K = A + B(T) + C ( p ) + D(P), for the other five fluids. m-Xylene has been studied by Poltz and Jugel (1967), Riedel (1951), and Briggs (1957).
No. 4, 1986
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Figure 5. Methylcyclohexane thermal conductivity.
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Figure 6. 1-Methylnaphthalene thermal conductivity.
A comparison of the literature data with our m-xylene data shows good agreement at ambient temperature. Agreement with Briggs' data at elevated temperature is poor. Methylcyclohexane has been studied by Briggs (1957) and Mallan (1968) to about 100 OC. Good agreement is obtained with Briggs' ambient data. Mallan's data lie about 5% higher than that reported in this paper. Mallan employed a relative hot wire instrument which was calibrated with the toluene data presented by Ziebland (1960). Those toluene data are about 4% higher than currently accepted values. Briggs reports thermal conductivity data a t 20 "C for 1-methylnaphthalene, tetralin, and decalin. These measurements are in good agreement with our ambient results. No higher temperature measurements have been reported. The complete experimental data set is presented by
Perkins (1983). Table I1 presents the set of regression constants which describe the data. Coal Liquids
A detailed characterization for the Utah COED and SRC-I coal liquid fractions is available from Kidnay and Yesavage (1980), including GC/MS analysis. A brief summary of the analysis is presented in Table 111. Both materials are highly aromatic relative to normal petroleum liquids. The data for these liquids are presented in Figures 9 and 10. No other independent measurements for these liquids are reported in the literature. In both cases, the relatively low upper temperature limit represents the point where thermal degradation of the sample became significant.
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Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986
Figure 7. Tetrahydronaphthalene thermal conductivity.
Figure 8. Decahydronaphthalene thermal conductivity.
Table 11. Fluid Constantsa fluid toluene m-xvlene me
