ARTICLE pubs.acs.org/EF
Effect of Process Parameters on Rheological Properties of Coal-Derived Liquids Qiujing Yang,* Arwen R. Kandt, Sharon Falcone Miller, and Bruce G. Miller Earth and Mineral Sciences (EMS) Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: This work was undertaken to systemically study the effect of various process parameters (solvent type, reaction temperature, hydrogen pressure, coal rank, and coal particle size) on the rheological properties of coal-derived liquids (CDLs). Among three investigated solvents (tetralin, a medium oil, and a heavy oil), only tetralin enhanced the coal conversion because of its superior hydrogen-donor capability. The viscosity of the CDLs exhibited a strong positive correlation with that of the solvent used. The increase of either reaction temperature or hydrogen pressure decreased the viscosity of the CDLs. A comparison of coal rank illustrated that a bituminous coal resulted in higher viscosity of the products than a subbituminous coal. The experiments performed with three different coal particle sizes (60, 100, and 200 mesh) showed that coal with a smaller particle size distribution generated lower viscosity liquid products. It can be deduced that the coal liquefaction process parameters have a strong effect on the chemical compositions of CDLs and, therefore, their physical properties, such as viscosity. To accurately predict CDL viscosity, preliminary work was conducted to correlate the viscosities of CDLs generated at different conditions with other CDL physical properties [e.g., boiling point and American Petroleum Institute (API) gravity] using three commonly used petroleum viscosity correlations. The results showed that the predicted viscosities were much lower than the measured ones because of the presence of stronger hydrogen bonding in CDLs than in petroleum fractions. A simple correction could improve the viscosity prediction for the CDLs generated from tetralin by reducing an absolute average deviation from 65 to 28%.
1. INTRODUCTION Coal liquefaction is a process that converts coal to liquid fuels with the formation of gases (e.g., H2S, NH3, CO, CO2, and C1C2) and residual solids. The liquid fuel obtained from coal liquefaction is a complex mixture consisting mainly of hydrocarbons and minor quantities of sulfur, nitrogen, oxygen, and inorganic constituents. It is believed to be a promising alternative to petroleum-derived fuels because of the increased demand upon petroleum, reduced worldwide oil reserves, and abundant availability of coal. The U.S. Department of Energy launched an active coal liquefaction program in the 1960s as did a host of agencies, companies, and state governments, but work was stopped in the late 1990s because of the wide availability of inexpensive crude oil at the time. With the remarkably increased oil price in recent years, there is a renewed interest in coal liquefaction, with many companies and states exploring the implementation of this technology. Because the U.S. has the world’s largest coal reserves, coal liquefaction is regarded as an attractive technology to produce liquid fuels to relieve its energy dependence upon petroleum-derived fuels. For the successful implementation of coal liquefaction technology, information on various transport properties, such as rheological characteristics, of coal-derived liquids (CDLs) is important for the design of coal liquefaction process equipment. CDLs with high viscosity are a challenge to fuel processing and use because of the difficulties in pumping, filtering, and transportation. Moreover, they can reduce flow velocities in pipelines, thus easily resulting in coking in process streams. This is especially undesirable if the products are not to be fractionated by distillation but rather filtered or centrifuged and transported for direct use as a fuel. Consequently, some studies have been r 2011 American Chemical Society
conducted to investigate the cause for the high viscosity of CDLs. Okuma et al.1 studied the viscosity of the liquids obtained from the two-stage brown coal liquefaction process. They found that the viscosities of the secondary hydrogenation distillates were much lower than those of the primary hydrogenation distillates at the same temperature because of the weaker molecular interaction in the former. Gould et al.2 reported that removing hydroxyl groups from Wyodak (Wyoming, subbituminous coal) coalliquefaction bottoms reduced the viscosity by about 80%. This fact indicates that the hydroxyl interactions have a profound effect on the viscosity of CDLs. Arganinski and Jones3,4 suggested that hydrogen bonding had a greater contribution to the viscosity of coal-derived pre-asphaltenes than molecular weight, and both of them showed a much larger influence than the degree of aromatic condensation. Although efforts have been devoted to understanding the rheological behavior of CDLs in recent years, the effect of various liquefaction process parameters has not been systemically explored thus far. Accurately predicting viscosity is not easily achieved and, therefore, is regarded as one of the critical problems in coal liquefaction technology. Some models for the viscosity of pure components and mixtures have been summarized by Brush,5 Monnery et al.,6 and Mehrotra et al.7 However, no model has been proven to be applicable to all fluids, especially for undefined mixtures, such as CDLs. Because CDLs are considered as an alternative to the petroleum-derived fuels, some viscosity predictions for hydrocarbons and petroleum fractions Received: January 24, 2011 Revised: March 20, 2011 Published: March 29, 2011 2119
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Energy & Fuels might be used for CDLs. Unfortunately, Hwang’s study8 indicated that the two existing correlations developed for petroleum fractions did not fit the viscosity of CDLs well. Krishnamoorthy et al.9 developed a more accurate viscosity correlation by considering the property difference between petroleum and CDLs. As a result, one must be cautious when applying viscosity correlations developed for petroleum fractions to coal-derived liquids, and the applicability of correlations should be strictly tested prior to use. In this study, the effect of various process variables (solvent type, reaction temperature, hydrogen pressure, coal rank, and coal particle size) on rheological properties of CDLs was systematically studied. In addition, applicability of three commonly used petroleum viscosity correlations to CDLs was examined to establish the relationship between the viscosity of CDLs with their two important properties: boiling point range and American Petroleum Institute (API) gravity.
ARTICLE
Table 1. Composition and Characteristics of the Coal Samples Penn State Sample Bank number
The API gravity was corrected from the observed temperature to 60 °F using API/ASTM Petroleum Measurement Tables: Table 5A.11 Fourier transform infrared (FTIR) spectra were measured by a Nicolet NEXUS 470 FTIR spectrometer at a resolution of 4 cm1 by co-adding 32 scans. Samples for FTIR measurements were prepared by diluting 2 mg of sample in 200 mg of KBr.
seam
Dietz
Illinois No. 6
state ASTM ranka
Montana SubB
Illinois hvCb
moisture (as received, wt %)
22.01
13.20
ash (dry, wt %)
4.80
13.39
volatile matter (dry, wt %)
44.34
47.14
fixed carbon (dry, wt %)
50.86
45.78
carbon hydrogen
76.52 5.32
76.26 5.30
nitrogen
1.03
1.32
total sulfur
0.46
6.38
oxygen (by difference)
16.67
10.74
ultimate analysis (wt %, daf)
2.1. Materials. Coal samples were obtained from the Penn State
API ðdegÞ ¼ ð141:5=SGÞ 131:5
DECS-24
proximate analysis
2. EXPERIMENTAL SECTION Coal Sample Bank. Two coal samples were used in the experiments: Dietz coal DECS-38 (subbituminous B coal) and Illinois No. 6 DECS24 (high-volatile C bituminous coal). The Dietz coal was pulverized to 8590 wt %, passing through 60, 100, and 200 mesh (250, 150, and 75 μm, respectively) screens. The three differently sized coal samples are referred to as 60, 100, and 200 mesh in this study. 1,2,3,4Tetrahydronaphthalene (tetralin), a medium oil, and a heavy oil were used as solvents. The medium oil and heavy oil were the boiling fractions obtained from the fluid catalytic cracking process of a refinery, obtained from Chevron and ConocoPhillips, respectively. 2.2. Characterization of Feedstocks. Proximate and ultimate analyses were conducted on the coal samples and the solvents, and the data are shown in Tables 1 and 2. The proximate analysis (moisture, volatile matter, ash, and fixed carbon) and the ultimate analysis (total carbon, hydrogen, and nitrogen) were performed with a LECO MAC 400 proximate analyzer and a LECO CHN analyzer, respectively. A LECO SC144DR was used to determine the total sulfur content. Simulated distillation (SimDis) measurements were made according to the American Society for Testing and Materials (ASTM) 2887 method using a HP 5890 gas chromatographflame ionization detector (GCFID) fitted with a MXT-500 simulated distillation column (10 m length, 0.53 mm inner diameter, and 2.65 μm pore size). The carrier gas flow rate was adjusted to 14 mL/min for SimDis GC analysis, and SimDis Expert 6.3 software was used to calculate the percent of the fractions. The temperature program for the SimDis GC was 40 °C (4 min), programmed from 40 to 350 °C at 15 °C/min, and then held at 350 °C for an additional 10 min. Prior to injection, the samples were diluted with CS2 to about 3 wt %. The injection volume was 1.0 μL. The mean average boiling point (MeABP) was calculated from the boiling cuts using equations given in the API Technical Data Book.10 Specific gravity (SG) of the feedstocks and liquid products were measured in a pycnometer, by adding the sample to a known volume, then taking the mass of the sample, and recording the temperature of the sample. The API gravity was calculated using the following equation:
DECS-38
petrographic composition (vol %, dmmf)
a
vitrinite
85.5
90.2
exinite
2.8
6.8
inertinite
11.7
3.0
ASTM rank was determined per ASTM D388.
Table 2. Composition and Characteristics of the Medium Oil and Heavy Oil medium oil
heavy oil
ash
0.00
0.00
volatile matter fixed carbon
99.90 0.10
96.76 3.24
proximate analysis (wt %)
ultimate analysis (wt %, daf) carbon
88.30
89.10
hydrogen
8.74
8.02
nitrogen
0.03
0.03
total sulfur
0.38
0.70
oxygen (by difference)
2.55
2.15
11.4
0.5
API gravity (60 °F)
2.3. Liquefaction Procedure. All liquefaction reactions were performed in microreactors (tubing bombs) of 93 mL capacity. Unless described otherwise, the typical liquefaction procedure was as follows: 10 g of Dietz coal (8590% at 200 mesh), 30 g of solvent, and 0.3 g of ferrous sulfide as the catalyst were charged to the tubing bomb. The loaded reactor was flushed 3 times with hydrogen and then pressurized with hydrogen to 1000 psi (6.9 MPa). The reactor was immersed in a fluidized sand bath preheated to 430 °C and shaken vertically for 1 h at 160 times/min. During the reaction, the maximum pressure was approximately 3 times that of the initial hydrogen pressure. At the end of an experiment, the reactor was then removed from the sand bath and quenched using cold water. The pressure dropped rapidly to 1000 ( 50 psi. After the gases were released in the reactor, liquid and solid products were transferred to a dried Soxhlet extractor thimble and extracted with tetrahydrofuran (THF) for 48 h until the extraction solvent appeared clear. THF was removed from the extract by rotary vacuum evaporation. The residue remaining in the thimble was vacuum-dried at 100 °C for 6 h. The THF solubles (THFS) were refluxed in toluene for 24 h, followed by filtration. Toluene was removed by rotary vacuum 2120
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evaporation, and the filter cake was vacuum-dried at 100 °C for 6 h to obtain pre-asphaltenes (PA). The toluene solubles (TS) were separated from the oil and asphaltenes (A) by adding hexane. The mixture was refluxed for 24 h, followed by filtration. Hexane was removed by rotary vacuum evaporation, and the filter cake was vacuum-dried at 100 °C for 6 h to obtain asphaltenes. Conversion was calculated on the basis of the weight of dry THF insolubles relative to the weight of the coal used as follows: conversion ðwt %, on a daf coal basisÞ ¼
weight of coal charged weight of THF insolubles 100% weight of coal
Yields and concentrations in THFS of pre-asphaltenes and asphaltenes were calculated as follows: weight of PA 100%, weight of coal weight of A A yield ðwt %Þ ¼ 100% weight of coal
PA yield ðwt %Þ ¼
weight of PA 100%, weight of THFS weight of A 100% A concentration ðwt %Þ ¼ weight of THFS
Figure 1. Change of THFS viscosity with the analysis temperature and pressure. Bars represent error in duplicate (coal, Dietz; coal particle size, 200 mesh; solvent, medium oil; and reaction temperature/pressure, 430 °C/1000 psi).
Table 3. Values of the Fitting Parameters in eq 1 and the Correlation Coefficient
PA concentration ðwt %Þ ¼
The oil [hexane solubles (HS)] þ gas was calculated by the difference from the overall conversion, pre-asphaltene yield, and asphaltene yield. The gas produced in the liquefaction experiments, under the present conditions, was less than 2% of the initial coal sample, and therefore, the hexane solubles are referred to as oil in the following text for the sake of simplicity. The repeatability of experimental data is within 3.9%. 2.4. Viscosity Measurements. The viscosities of the samples at ambient pressure were measured using a rotational rheometer (Bohlin Gemini HR Nano) manufactured by Malvern Instruments with parallelplate geometry (40 mm diameter, 1 mm gap size). The flow behavior for the three solvents and CDLs generated from these three solvents was determined with varying shear rates from 0 to 300/s at 40 °C. It was found that tetralin and the heavy oil solvent exhibited Newtonian behavior, and the medium oil was a shear thinning fluid, whose apparent viscosity decreases with increasing shear rate. The CDLs generated from tetralin and the medium oil exhibited shear thinning behavior, while those generated from the heavy oil behaved as Newtonian fluids. Because viscosities of some CDLs change with the shear rate, all of the viscosity measurements were performed at a fixed shear rate of 100/s to compare the process parameters. The analysis temperature was varied from 30 to 100 °C and was controlled by an extended temperature cell. The temperature was kept below 100 °C because of the concern that some lower boiling components may be lost by evaporation and a certain degree of oxidation may take place at high temperatures, leading to changes in viscosity. The viscosity at different pressures was measured using a high pressure cell with bob-cup geometry at a fixed shear rate of 100/s. The accuracy of the rheometer was examined with a viscosity standard (Cannon N35). Deviations between the measured and reported values were within 4%.
3. RESULTS AND DISCUSSION 3.1. Viscosity Dependence upon Analysis Temperature and Pressure. The effect of the analysis temperature and pressure on
viscosity was studied by varying the temperature from 30 to 90 °C and pressure from 0 to 2000 psi (13.8 MPa). As shown in Figure 1, the viscosity of the THFS generated with the medium oil decreases with the analysis temperature. The curves of viscosity change with
analysis pressure (psi) 0
A
B
R2
6241
0.0019
0.99533
1000
1792.3
0.0015
0.99504
2000
1206.6
0.0013
0.98233
temperature was made to fit the Andrade equation (eq 1) η ¼ AeB=T
ð1Þ
where T is temperature in K and A and B are fitting constants. Table 3 lists the values of the fitting parameters and correlation coefficients. The correlation coefficients, R2, are g0.98, indicating that change in viscosity with the measurement temperature is satisfactorily described by the Andrade equation. Figure 1 shows that, when the analysis pressure was varied at a given temperature, the viscosity increased slightly with the analysis pressure. The dependence of viscosity upon pressure was determined using the Krishnamoorthy correlation,9 as shown in eq 2 ηp ¼ ηsl ð1 þ ðP P0 Þ 0:0016Þ
ð2Þ
where ηp is viscosity at pressure P in mPa s, ηsl is viscosity of the saturated liquid measured in mPa s, P is the pressure at which viscosity is being predicted in bar, and P0 is pressure at which the single viscosity data was measured in bar. Average absolute deviations (AAD, % = ∑(|ηcal ηexp| 100/ηexp)/number of data points) of 4.41 and 7.10% were obtained at 1000 and 2000 psi, respectively. The AADs of less than 10% indicate that the Krishnamoorthy correlation is a good model to predict the viscosity dependence upon pressure. 3.2. Effect of Pre-asphaltenes and Asphaltenes. The CDLs in this study consisted of three major components based on their solubility in THF, toluene, and hexane. These components are referred to as pre-asphaltenes (THF-soluble and toluene-insoluble), asphaltenes (toluene-soluble and hexane-insoluble), and oil (hexane-soluble). Asphaltenes and pre-asphaltenes have broad chemical compositions because they are soluble fractions. Asphaltenes are considered the principle intermediates in the conversion of coal to oil products, and pre-asphaltenes are the intermediates between coal and asphaltenes. The effect of the presence of pre-asphaltenes and asphaltenes on the viscosity of 2121
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CDLs was studied. As shown in Figure 2, the viscosities of the three fractions were in the order of THFS > TS > HS and the viscosity of THFS decreased as the temperature was increased to a greater extent than those of TS and HS. The viscosities of CDLs at 30 °C decreased from 67 to 25 mPa s with the removal of preasphaltenes and from 25 to 7 mPa s with the removal of asphaltenes, respectively, indicating that the viscosity of CDLs was significantly influenced by the presence of pre-asphaltenes and asphaltenes. The great effect of pre-asphaltenes and asphaltenes on viscosity is very likely associated with the strong intermolecular bonding between liquid products. To verify this speculation, pre-asphaltenes and asphaltenes separated from THFS produced with Illinois No. 6 coal were well-mixed physically with the corresponding HS and the viscosity of this mixture was measured. It can be seen from Figure 2 that the viscosity of the mixture of PA þ A þ HS is comparable to that of HS and is much lower than that of THFS. The viscosity was dramatically reduced from 67 to 7 mPa s with breaking the molecular connections of various components. This fact indicates that the presence of intermolecular bonding considerably increased the viscosity of CDLs. Because both pre-asphaltenes and asphaltenes increase the viscosity of liquid products, their removal can lead to the decreased viscosity of the liquid products. 3.3. Effect of the Solvent. In coal liquefaction processes, solvents can act as heat-transfer media, coal dissolution media, hydrogen transport media, additional reactants along with the coal, and media to transport coal liquefaction products away from the coal matrix. Three solvents, tetralin, the medium oil, and the heavy oil, were used to explore the solvent effect coal conversion and viscosities of CDLs. For comparison, naphthalene was used as a solvent in some cases. The results are summarized in Table 4
Figure 2. Viscosity of different fractions of CDLs: (a) THFS, (b) TS, (c) HS, and (d) mixture of PA þ A þ HS (coal, Illinois No. 6; coal particle size, 100 mesh; solvent, tetralin; and reaction temperature/ pressure, 430 °C/1000 psi).
and shown in Figure 3. Tetralin was found to be the most effective for coal conversion (71.0%), while the conversions using naphathalene (39.1%), the medium oil (42.0%), and the heavy oil (44.7%) are comparable to each other and much lower than tetralin. Among the three solvents, the heavy oil resulted in the highest PA þ A yield (36.7%), lowest oil yield (8%), and highest viscosity of the THFS. The viscosity of the THFS obtained by the different solvents is in the order of the heavy oil > the medium oil > tetralin (Figure 3a), being consistent with that of solvent viscosity (Figure 3b). It is interesting to observe that the THFS, TS, and HS generated by naphthalene were yellowish particles, looking similar to naphthalene at room temperature. Their viscosities are considered as infinite and were not measured in this study. Apparently, the above-mentioned facts demonstrate that the type of solvent has a significant effect on the viscosity of CDLs and coal conversions. To gain more insight into this aspect, the residues obtained from different solvents, as well as the parent coal, were characterized by FTIR. As shown in Figure 4, the strong
Figure 3. Viscosity of (a) THFS generated with different solvents and (b) the solvents (coal, Dietz; coal particle size, 200 mesh; and reaction temperature/pressure, 430 °C/1000 psi).
Table 4. Conversion and Yields of Pre-asphaltenes (PA), Asphaltenes (A), and Oil Obtained under Different Conditions coal
particle size (mesh)
solvent
overall conversion (%)
PA yield (%)
A yield (%)
oil yield (%)
Dietz
60
tetralin
69.3
17.5
12.4
39.4
Dietz
100
tetralin
66.6
15.0
5.7
45.9
Illinois No. 6
100
tetralin
93.9
19.9
13.9
60.1
Dietz Dietz
200 200
tetralin naphthalene
71.0 39.1
13.2 12.9
14.4 17.2
43.4 9.0
Dietz
200
medium oil
42.0
10.3
11.7
20.0
Dietz
200
heavy oil
44.7
27.7
9.0
8.0
2122
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the coal with its conversion to naphthalene as demonstrated by the following equation:
Figure 4. FTIR spectra of (a) Dietz coal, (b) tetralin residue, (c) medium oil residue, and (d) naphthalene residue (the conditions are the same as in Figure 3).
band near 3420 cm1 for the parent coal sample and the tetralin residue is caused by the stretching vibration of OH groups.12 This band is not observed in the spectra of the naphthalene residue and the medium oil residue. The peak at 3054 cm1 is due to the aromatic CH stretching vibration mode. The peaks at 2920 and 2851 cm1 are assigned to the aliphatic CH stretching vibration mode and are reduced in intensity after the coal was reacted with the medium oil or naphaltene. The band intensities of the aliphatic and aromatic CH stretching modes of the coal sample and the tetralin residue are stronger than those of the medium oil residue and the naphthalene residue. The band at 1600 cm1 is assigned to the aromatic ring stretching vibration, indicating more polyaromatic rings or heterocyclic compounds in the parent coal. The band at 1440 cm1 is attributed to aliphatic CH2 and CH3 bending modes. The intensity ratio of the band at 1600 cm1 to the band at 1440 cm1 in the tetralin residue is lower than that in the parent coal. These changes imply that, for the parent coal, aromatic rings were opened and aromatic hydrogen was converted to aliphatic hydrogen during coal liquefaction. In general, hydrogen present in coal lies in aromatic CH, aliphatic CH, and OH groups. Aliphatic and aromatic hydrogen in coals can be quantitatively determined through analyzing hydrogen peak areas in FTIR spectra.13 The bands related to hydrogen at 3420, 3054, and 2920 cm1 in the spectrum of the parent coal deceased or disappeared after liquefaction when the medium oil or naphthalene was used as the solvent, indicative of significant consumption of the hydrogen in the parent coal during the liquefaction reaction. As we have known, the hydrogen required in the liquefaction process may come from different sources: the hydrogen donor solvent, the gaseous hydrogen, and the intrinsic hydrogen-rich components of the coal. This suggests that the hydrogen components in the coal would be reduced to a greater extent if the solvent could not effectively provide hydrogen to the coal. In Figure 4, the spectrum of the residue from the medium oil is comparable to that from the naphthalene. Because naphthalene is not a hydrogen-donor solvent and hydrogen in coal is significantly consumed in the liquefaction process, it can be deduced that the medium oil, such as naphthalene, is not able to efficiently donate hydrogen to stabilize radical fragments. Gas chromatographymass spectrometry (GCMS) results depicted that the major component in HS obtained from tetralin is naphthalene at 76.20 wt %, with other components such as butyl hydroxy toluene at 14.53 wt %, phenol at 4.53 wt %, 1-methyl indane at 1.70 wt %, and some minor aromatic components. It is suggested that tetralin donated hydrogen to
During coal liquefaction, highly reactive free radicals can be formed through the rupture of chemical bonds in the coal. A good solvent, such as tetralin, can donate hydrogen to stabilize free radicals and increase the mobility of the small molecules that are rich in hydrogen, which could facilitate internal hydrogen selfdonation reactions. Thus, the initial regressive reactions are suppressed, and the coal conversion is enhanced. If using naphthalene that cannot donate hydrogen, hydrogen consumed during liquefaction is only supplied by the gas-phase and hydrogen-rich components in the coal. Therefore, the free radicals generated during liquefaction reactions cannot be effectively stabilized by hydrogen, so that they prefer to combine with surrounding molecules to form high-molecular-weight substances, such as pre-asphaltenes and asphaltenes. This would terminate free-radical reactions and then result in low coal conversion and high viscosity of products.14 It was reported that using waste oils as coal liquefaction solvents is beneficial to liquefaction chemistry and economics;15 however, the present study revealed that the medium oil and heavy oil could not effectively enhance the conversion of coal. Specifically, the viscosities of the THFS generated from the heavy oil were extremely high, which is undesirable for the liquefaction process. 3.4. Effect of the Reaction Temperature and Pressure. The effect of reaction conditions was investigated using the medium oil of specific interest as the solvent in the initial temperature range of 370450 °C and in the initial hydrogen pressure range of 8001200 psi (5.58.3 MPa). The conversion data are presented in Table 5. It can be seen that the overall conversion increases from 32.8% at 370 °C/1000 psi to 42.0% at 430 °C/ 1000 psi and then decreases to 33.4% at 450 °C/1000 psi. The oil yield increases from 3.4% at 370 °C/1000 psi to 20.0% at 430 °C/1000 psi. The conversion increases slightly with an increase of the initial reaction pressure from 800 to 1200 psi at 450 °C. It implies that the reaction temperature had a stronger effect on the conversion than the reaction pressure. One definite trend shown by the pre-asphaltene and asphaltene yields is that a higher temperature and pressure favor the conversion of preasphaltenes and asphaltenes to oil. This is because a high liquefaction temperature is effective at reducing the molecular weight and the polar compounds in the pre-asphaltenes.16 Figure 5 shows the viscosity of THFS generated at 370 °C/ 1000 psi, 430 °C/1000 psi, 450 °C/1000 psi, and 450 °C/1200 psi. It can be seen that the viscosity of THFS decreased with an increasing reaction temperature and pressure. The viscosity of THFS generated at 450 °C/1000 psi was lower than that generated at 430 °C/1000 psi but higher than that generated at 450 °C/1200 psi. In addition, the total concentration of preasphaltenes and asphaltenes in the THFS, as listed in Table 5, decreased with an increasing temperature and pressure. When the viscosity results were combined with the total concentration of pre-asphaltenes and asphaltenes in the THFS, it was found 2123
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Table 5. Conversion and Yields and Concentrations in THFS of Pre-asphaltene (PA), Asphaltene (A), and Oil Obtained at Different Temperatures and Pressures overall
PA
A
oil
PA
A
PA þ A
oil
T (°C)
P (psi)
conversion (%)
yield (%)
yield (%)
yield (%)
concentration (%)
concentration (%)
concentration (%)
concentration (%)
370 400
1000 1000
32.8 38.0
19.3 14.8
10.1 11.0
3.4 12.2
3.5 3.0
3.7 2.2
7.2 5.2
92.8 94.8
430
1000
42.0
10.3
11.7
20.0
2.3
2.6
4.9
95.1
450
1000
33.4
9.2
11.7
12.5
1.3
2.8
4.1
95.9
450
800
32.3
7.5
13.5
11.3
2.4
4.4
6.8
93.2
450
1200
34.5
10.2
13.5
10.8
1.0
1.3
2.3
97.7
Figure 5. Viscosity of THFS generated at different temperatures and pressures (coal, Dietz; coal particle size, 200 mesh; and solvent, the medium oil).
that the liquids produced at a higher temperature and pressure contained lower concentrations of pre-asphaltenes and asphaltenes and, therefore, lower viscosity. Coal liquid viscosity decreases with an increase in both the reaction temperature and pressure, because the high reaction temperatures facilitate the cracking of the coal molecules and high hydrogen pressures help to stabilize the coal radicals to prevent their recombination by retrogressive reactions.17 However, at extremely severe conditions, such as 450 °C in this study, recombination of radicals generated in thermal cracking will take place and the possibility of breaking the coal macromolecule into smaller molecules will be reduced by retrogressive reactions, resulting in a decrease in conversion. It has been documented by Thomas and Granoff that the effect of the pre-asphaltenes and asphaltenes on the product viscosity could be expressed as a linear relation.18 On the basis of the viscosity obtained at different reaction temperatures, the following equation was obtained: log η ¼ 0:82 þ 0:074½PA þ 0:02½A
ð3Þ
where η is the viscosity measured at 60 °C and [PA] and [A] are the pre-asphaltene and asphaltene concentrations (wt %), respectively. Equation 3 was used to fit the viscosity data of CDLs at different reaction pressures and Illinois No. 6 CDLs as well, yielding an absolute average deviation of 12.9%. The coefficient values in eq 3 indicated that (i) there is a very strong correlation between pre-asphaltenes and asphaltenes and viscosity and (ii) pre-asphaltenes has a greater effect on viscosity than asphaltenes. The major contributors to the high viscosity of coal conversion products were found to be in pre-asphaltenes,18,19 probably
Figure 6. Viscosity of THFS generated with Illinois No. 6 coal and Dietz coal (coal particle size, 100 mesh; solvent, tetralin; and reaction temperature/pressure, 430 °C/1000 psi).
because the pre-asphaltenes have larger molecular weights and functionalities and a stronger ability to form multiple hydrogenbonding systems in comparison to asphaltenes.20,21 Asphaltenes and oil fractions are intermolecularly associated through hydrogen bonding and are partly responsible for the increase in the viscosity of the CDLs.22 It must be noted that the chemical and physical properties of pre-asphaltenes and asphaltenes, such as component concentration, molecular weight, and molecular interaction, strongly depend upon their source. Pre-asphaltenes and asphaltenes in CDLs dramatically vary with parent coal types and solvents. For this reason, viscosity comparisons based on pre-asphaltenes and asphaltenes alone are reasonable only when they are generated from the same coal and solvent source. 3.5. Effect of the Coal Rank/Composition. The effect of the coal rank on coal conversion and viscosity of liquid products was studied using two coals (100 mesh particle size): Dietz subbituminous coal and Illinois No. 6 bituminous coal. A comparison of conversion and yields of pre-asphaltenes and asphaltenes is provided in Table 4. The conversion of Illinois No. 6 coal (93.9%) is higher than that of Dietz coal (66.6%), and the Illinois No. 6 coal had a higher PA þ A yield. Figure 6 clearly shows that the viscosity of THFS generated by Illinois No. 6 bituminous coal is higher than that produced by Dietz subbituminous coal. It is known that vitrinite and exinite are susceptible to liquefaction, whereas inertinite is more resistant. As presented in Table 1, the Illinois No. 6 coal contains more vitrinite and exinite and less inertinite than the Dietz coal. The composition difference is a major reason that bituminous Illinois No. 6 coal resulted in higher conversion than subbituminous Dietz coal. Fisher et al.23 found that high-volatile bituminous coals appeared to be the 2124
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Table 6. MeABP of the CDLs Obtained at Different Conditions particle size solvent
coal
(mesh)
MeABP T (°C)
tetralin
most desirable rank for liquefaction and low-rank coals, such as subbituminous coals, resulted in lower liquid yields than highvolatile bituminous coals. The same phenomenon was observed by Given et al.24 They evaluated a number of coals and concluded that coals of high-volatile bituminous rank resulted in the highest conversions. The chemical and physical characteristics of coal, which depend upon its rank and petrographic composition, have a strong effect on coal liquefaction behavior. Bituminous coals have a higher proportion of PA/A (w/w) than subbituminous coal, and the bituminous CDLs contain less aromatics, more non-hydrocarbons, and more pre-asphaltenes and asphaltenes than the subbituminous CDLs because the higher rank coals are harder to hydrogenate.25,26 As previously mentioned, more preasphaltenes and asphaltenes result in higher viscosities of CDLs. As a result, the bituminous THFS have higher viscosity than the sub-bituminous THFS. The viscosity of the liquid product is a function of the coal rank, with the viscosity increasing with an increasing coal rank.27 3.6. Effect of the Coal Particle Size. Because coal particle size distributions have a considerable effect on the viscosity of coal/ solvent mixtures that are fed with a process reactor, it is of interest to determine the effect of the coal particle size on the viscosities of the liquid products. The liquefaction behavior of Dietz coal with three different particle sizes, 60 mesh, 100 mesh, and 200 mesh, was investigated. The conversion and pre-asphaltene and asphaltene yields are given in Table 4. The conversions of 60 mesh, 100 mesh, and 200 mesh are 69.3, 66.6, and 71.0%, respectively. There is no significant difference between the conversions obtained from different coal particle sizes; however, the pre-asphaltene yield increases with the coal particle size. In addition, it can be seen from Figure 7 that the viscosity of THFS decreases with a decreasing coal particle size. This may be due to the larger surface area in the smaller sized coal accelerating the diffusion of the coal radicals during liquefaction. In the initial stages of liquefaction, solvent accessibility to the reactive sites in coal micropores is diffusion-limited.28 To overcome this limitation, contact between solvent and coal surface should be maximized. Apparently, smaller sized coal particles are beneficial in increasing this contact than larger sized coal particles, thereby improving the diffusion of the radicals formed from bond rupture. The high degree of radical mobility could limit the condensation to produce heavy molecular-weight products, such as residue, pre-asphaltenes, and asphaltenes.29 Apparently, the coal with the smaller particle size generates less pre-asphaltenes and asphaltenes,
solvent
207.0a
60
430
THFS
204.9
Dietz
200
430
THFS
207.5
Illinois No. 6
100
430
THFS
205.8
solvent
290.8
Dietz
200
370
THFS
295.5
Dietz
200
370
TS
288.7
Dietz Dietz
200 200
370 430
HS THFS
288.3 282.8
solvent
410.0
Dietz
200
430
THFS
379.1
Dietz
200
430
HS
381.1
heavy oil
a
(°C)
Dietz
medium oil
Figure 7. Viscosity of THFS generated by Dietz coal with different coal particle sizes (coal, Dietz; solvent, tetralin; and reaction temperature/ pressure, 430 °C/1000 psi).
fraction
Boiling point.
therefore resulting in liquid products with lower viscosity. The viscosity of 60 mesh THFS is higher than that of 100 mesh THFS at analysis temperatures below 80 °C, while this difference gradually decreases with an increasing temperature. Above 80 °C, the viscosity of 60 mesh THFS is the same as that of 100 mesh THFS. This viscosity difference change is the result of some hydrogen bonds in the 60 mesh THFS breaking during heating, and the intermolecular force was weakened.30 3.7. Correlation of Viscosity with the Boiling Point and API Gravity. The boiling temperature is an important physical property of CDLs. In this study, the mean average boiling point (MeABP) is used to illustrate boiling point distribution of CDLs. The MeABP of the solvents and the CDLs is listed in Table 6. It was found that the MeABP of the THFS is similar to that of the corresponding liquefaction solvent. It is not surprising to observe that the boiling point ranges of fractions obtained from the heavy oil are much higher than that of equivalent fractions obtained from the medium oil and tetralin. The fractions obtained from tetralin have the lowest and narrowest boiling range. When the same solvent was used, the MeABPs of the liquid products obtained at different process conditions, such as coal type, reaction time, and temperature, are comparable. In addition, as shown in Table 6, the MeABPs of different fractions, THFS, TS, and HS, generated at 370 °C with the medium oil solvent have MeABPs that are similar to that of the medium oil. The carbon number distribution in coal liquid products was identified by means of SimDis GC analysis. In Figure 8, the peaks before n-C7 and at n-C7 are attributed to the analysis solvent CS2 and the fraction extraction solvent (e.g., hexane), respectively. Note that the carbon number distribution curve was obtained on the basis of the boiling temperature range of straight-chain hydrocarbon standards. Most hydrocarbons in the medium oil distribute in the carbon number range of n-C9n-C24, while in the heavy oil, the range is n-C9n-C32. THFS produced using the medium oil as the solvent has similar carbon number distributions as the medium oil, as shown in panels a and b of Figure 8. The same result was obtained when heavy oil was used, as shown in panels c and d of Figure 8. The carbon distributions in CDLs generated from different coals and different coal particle sizes are identical if the same solvents are used. This result is supported by 2125
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Figure 8. Carbon number distribution in (a) the medium oil, (b) THFS generated from the medium oil, (c) THFS, and (d) HS generated from the heavy oil (the conditions are the same as in Figure 3).
Hwang’s study, which showed that coal source little affected the volatility of CDLs, provided that the liquids have a similar boiling point distribution.31 The boiling point range and composition of CDLs are very significantly dependent upon the liquefaction solvent. In various viscosity correlations, the boiling point is introduced as an important parameter. Our study shows that the viscosity of CDLs differs with altering liquefaction conditions; however, the boiling point distribution of CDLs generated using the same solvent does not significantly change when varying other process conditions. Hence, we believe that there must be other important properties, such as density, which appreciably affect viscosity of CDLs. The density is one of the important characteristics of CDLs. The crude oil industry convention uses API gravity. There is an inverse relationship between the API gravity and density; the higher the density, the lower the API gravity. Figure 9 shows the API gravities and the viscosities of THFS produced at different conditions measured at 37.8 °C. A clear trend can be seen from Figure 9, where THFS with a lower API gravity has a higher viscosity.
From the above discussion, it can be inferred that process parameters have a strong effect on CDL chemical composition and, therefore, physical properties, such as viscosity, boiling point, and API gravity. Because CDLs are considered as an alternative for petroleum, some correlations developed for petroleum products might be used for coal-derived liquids. To accurately predict CDL viscosity and identify how it varies with the process parameters, the viscosity data obtained at different conditions were correlated with two easily measured physical properties, boiling point and API gravity, using three commonly used petroleum viscosity correlations: (1) Abbott correlation: Abbott et al. proposed a petroleum fraction liquid viscosity correlation, as shown in eq 4.32 It was used by Hwang et al.8 to predict the viscosity of liquids produced from the Exxon donor solvent process log ν ¼ AðK, APIÞ þ
BðK, APIÞ API þ CðKÞ
ð4Þ
where ν is kinetic viscosity, A, B, and C are functions of K and API. API gravity = 141.5/SG 131.5 (SG = specific gravity at 2126
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where ν is the kinematic viscosity, r1 and r2 are reference fluids, and k is the Watson factor. (3) Abu-Eishah correlation: Abu-Eishah36 suggested a viscositytemperature correlation for liquid petroleum fractions, as shown eq 7. The only characterization properties required for estimation are the API gravity and the 50% boiling point temperature ln ν ¼ ln a þ b lnðAPIÞ þ c lnðTb =TÞ þ dðTb =TÞ þ e=T ð7Þ 6
Figure 9. Viscosity and API gravity of THFS produced at different conditions.
60/60 °F), and K is the Watson characterization factor. K ¼
ðaverage boiling point, °FÞ1=3 SG
ð5Þ
(2) Twu correlation: Twu33,34 proposed an internally consistent correlation for the calculation of liquid viscosities of petroleum fractions and subsequently developed a generalized correlation. The Twu correlation employs MeABP and specific gravity as the parameters. The accuracy of the generalized correlation was improved using petroleum fractions as reference fluids. The Twu generalized correlation was modified by Sharma and Goel35 to predict coal liquid viscosities. The Twu generalized correlation is shown as eq 6 !ðk 10Þ=2 ðr2 Þ ðr1 Þ ν2 ð6Þ ν¼ν ν1 ðr1 Þ
2 1
where ν is the kinematic viscosity (1 cSt = 10 m s ), a, b, c, d, and e are fitting parameters; ln a = 6.489 436, b = 0.614 916, c = 7.285 711, d = 7.448 011, and e = 251.945 53. The above three correlations were used to predict kinematic viscosity; however, only dynamic viscosity could be directly obtained from the present study. The density is required for converting kinematic viscosity to dynamic viscosity. The density of the CDLs was obtained from their specific gravity and corrected to the temperature at which the viscosity was predicted by means of API/ASTM Petroleum Measurement Tables.11 Comparisons of the experimental viscosities of CDLs with the predicted values are presented in Table 7. For most of the CDLs and for all of the correlations, the predicted values were lower than the experimental values. In a few cases, the comparisons were good, while in others, the deviations were too large for the petroleum correlations to be used for predicting coal liquid viscosity. The Abbott correlation, the Twu correlation, and the Abu-Eishah correlation yielded an overall AAD of 74, 75, and 48%, respectively. Of the three correlations, the Abu-Eishah correlation gave the least errors for all of the CDLs. The highest errors were obtained using the Twu method. The large deviation can be explained by the differences between CDLs and petroleum fractions. CDLs have higher levels of organic oxygen and nitrogen compounds compared to petroleum fractions, and more hydrogen bondings (OH 3 3 3 N) are formed between oxygen and nitrogen compounds. The increased concentration of large complexed ions formed in the transition from the hydrogen-bonded complexes (OH 3 3 3 N) to the protontransfer complexes (O 3 3 3 HþN) is partially responsible for the high viscosity.22 Therefore, the presence of the stronger association effect in CDLs than in petroleum fractions leads to inaccurate estimations of viscosities for CDLs. The results showed that the existing correlations are unsatisfactory for the viscosity of CDLs; therefore, modification should be made when a petroleum model is applied to CDLs. Driven by the need of reliable viscosity correlation, a simple and reliable technique for predicting viscosity of CDLs is expected to be developed. In this study, the Abu-Eishah correlation was further modified to improve its accuracy. On the basis of Sharma’s study,35 a correction factor, R, was introduced to “convert” the viscosities of petroleum fractions obtained by the Abu-Eishah correlation to those of CDLs, as shown in eq 8. ηexp ð8Þ R¼ ηcal The correction factors for viscosities of CDLs generated with tetralin were calculated by following Sharma’s method. The correction factor at 2.45 reduces the AAD from 65 to 28%, indicative of the effectiveness of this simple modification. Further efforts will be devoted to expending the applicability range of petroleum viscosity correlations to CDLs. A reliable method has 2127
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Table 7. Comparison of the Experimental Viscosities of the CDLs with the Predicted Values Abbott coal Dietz
particle size (mesh) 60
solvent tetralin
T (°C) P (psi) fraction T (°C) ηexpt (mPa s) 430
1000
THFS
37.8
11.46
98.9 HS Dietz
Illinois No. 6
Dietz
Dietz
100
100
200
200
tetralin
tetralin
tetralin
medium oil
430
430
430
370
1000
1000
1000
1000
THFS
6.483
Dietz
Dietz
200
200
200
medium oil
medium oil
medium oil
430
450
450
1000
1000
1200
200
heavy oil
430
1000
% error
ηcalc
% error
0.8209
93
1.423
88
3.454
70
0.8064
88
0.5707
91
1.614
75
6.638
0.9833
85
1.280
81
2.833
57
4.557
0.7849
83
0.5811
87
1.352
70
37.8
7.901
0.8390
89
1.382
83
3.375
57
98.9
5.841
0.8065
86
0.5741
90
1.591
73
HS
37.8
4.025
1.256
69
1.165
71
2.245
44
THFS
98.9 37.8
3.024 57.43
0.7585 0.658
75 99
0.6229 1.397
79 98
1.067 3.778
65 93
98.9
13.32
0.7956
94
2.462
82
1.811
86
HS
37.8
1.046
84
1.259
81
2.696
59
98.9
5.247
0.7813
85
0.5887
89
1.285
76
THFS
37.8
5.580
0.9592
83
1.344
76
3.069
45
6.568
98.9
3.302
0.8052
76
0.5871
82
1.447
56
HS
37.8
4.888
1.118
77
1.230
75
2.542
48
THFS
98.9 37.8
2.834 20.88
0.7759 4.022
73 80
0.5988 10.74
79 49
1.211 10.90
57 48
98.9
11.56
1.735
85
1.2063
90
3.693
68
HS
37.8
10.96
3.780
66
4.872
56
9.087
17
THFS
37.8 98.9
6.180
1.747
71
1.215
80
3.577
HS
37.8
7.896
3.601
54
4.204
47
8.108
3
THFS
98.9 37.8
3.547 14.59
1.597 4.186
55 71
1.146 5.789
68 60
2.766 10.34
22 29
98.9
5.557
1.756
68
1.221
78
3.506
37
HS
37.8
7.023
3.795
46
4.042
42
7.507
7
98.9
3.558
1.603
55
1.176
67
2.558
28
THFS
4.576 19.16
37.8
10.44 5.154
1.671
63
1.175
74
4.124
78
5.836
70
3.110 10.56
32 45 42
4.389
58
5.715
45
9.893
5
1.774
66
1.237
76
3.325
35
HS
37.8
7.257
3.841
47
3.999
45
7.362
1
THFS
98.9 37.8
3.774 1097
1.604 158.5
58 86
1.184 60.82
69 94
2.509 113.5
34 90
HS
37.8
252.1
98.9
32.2
98.9
53.44
3.541
93 29
178.1 3.782
AADa (%) a
ηcalc
37.8
98.9
Dietz
% error
Abu-Eishah
98.9
98.9 Dietz
ηcalc
Twu
88 74
2.422 64.00 2.484
95 75 92 75
25.55 104.4 23.66
52 59 27 48
Average absolute deviation (%) = ∑(|ηcal ηexp| 100/ηexp)/number of data points predicted.
been proposed by our group for predicting viscosities of coal liquid oils and fractions. This method is proven to be more accurate than other models available in the literature and is discussed in more detail elsewhere.9
4. CONCLUSION The present work demonstrated the effect of various process parameters on rheological properties of CDLs. The results showed that the viscosity of CDLs is significantly determined by the concentrations of pre-asphaltenes and asphaltenes in the CDLs. A strong positive correlation was found between the viscosity of CDLs and that of the solvent used. Increases in both the reaction temperature and initial hydrogen pressure can
decrease the viscosity of CDLs most likely because a higher temperature and pressure favor the conversion of pre-asphaltenes and asphaltenes to oil. It was also found that using lower rank coal in a smaller particle size was beneficial for the production of CDLs with lower viscosity. It is clear that coal liquefaction process parameters can have a pronounced effect on the chemical compositions of CDLs and, therefore, their physical properties as well. Three correlations commonly used in the petroleum industry (the Abbott correlation, the Twu correlation, and the Abu-Eishah correlation) were applied to correlate the viscosity of CDLs with their boiling point and API gravity, which however yielded large deviations. The prediction results imply that the presence of stronger hydrogen bonding in CDLs compared to petroleum products is responsible for the higher 2128
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Energy & Fuels viscosities of CDLs. A simple correction of the petroleum correlations was proven to be effective in improving the predictive accuracy for their applications to CDLs.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: (814) 865-2964. Fax: (814) 863-7432. E-mail:
[email protected].
’ ACKNOWLEDGMENT Staff from the EMS Energy Institute are acknowledged for instrument training, constructing tubing bombs, coal collection from the mine site, and coal processing and archiving.
ARTICLE
(28) Joseph, J. T. Fuel 1991, 70, 139–144. (29) Sato, Y.; Kamo, T.; Shiraishi, M. Energy Fuels 2002, 16, 388–396. (30) Ren, Y.; Wei, A.; Zhang, D.; Zhao, J.; Lin, C.; Gao, J. J. Fuel Chem. Technol. 2007, 35, 262–267. (31) Hwang, S.; Wilson, G. M.; Tsonopoulos, C. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 636–639. (32) Abbott, M. M.; Kaufmann, T. G.; Domash, L. Can. J. Chem. Eng. 1971, 49, 379–384. (33) Twu, C. H. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 1287–1293. (34) Twu, C. H. AIChE J. 1986, 32, 2091–2092. (35) Sharma, R.; Goel, S. Ind. Eng. Chem. Res. 1997, 36, 3999–4007. (36) Abu-Eishah, S. I. Int. J. Thermophys. 1999, 20, 1425–1434.
’ REFERENCES (1) Okuma, O.; Mae, K.; Hirano, T. Fuel Process. Technol. 1988, 19, 165–178. (2) Gould, K. A.; Gorbaty, M. L.; Miller, J. D. Fuel 1978, 57, 510–511. (3) Argasinski, J. K.; Jones, M. B. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1987, 32, 604–610. (4) Jones, M. B.; Argasinski, J. K. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1985, 30, 250–261. (5) Brush, S. G. Chem. Rev. 1962, 62, 513–548. (6) Monnery, W.; Svrcek, W. K.; Mehrotra, A. K. Can. J. Chem. Eng. 1995, 73, 3–40. (7) Mehrotra, A. K.; Monnery, W. D.; Svrcek, W. Y. Fluid Phase Equilib. 1996, 117, 344–355. (8) Hwang, S.; Tsonopoulos, C.; Cunningham, J. R.; Wilson, G. M. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 127–134. (9) Krishnamoorthy, V.; Falcone Miller, S.; Miller, B. G. Energy Fuels 2010, 24, 5624–5633. (10) Amercian Petroleum Institute (API). Procedure 2B1.3—Characterizing boiling points from simulated distillation data (ASTM D2887). API Technical Data Book; API: Washington, D.C., 2008. (11) Amercian Petroleum Institute (API). API/ASTM Petroleum Measurement Tables: Volume Correction Factors, 1st ed.; API: Washington, D.C., 1980; Vol. 1. (12) Li, J.; Yang, J.; Liu, Z. Catal. Today 2008, 130, 389–394. (13) Solomon, P. R.; Carangelo, R. M. Fuel 1988, 67, 949–959. (14) Neavel, R. C. Fuel 1976, 55, 237–242. (15) Orr, E. C.; Shi, Y.; Shao, L.; Liang, J.; Ding, W.; Anderson, L. L.; Eyring, E. M. Fuel Process. Technol. 1996, 49, 233–246. (16) Masuda, K.; Okuma, O.; Kanaji, M.; Matsumara, T. Fuel 1996, 75, 1065–1070. (17) Handwek, J. G.; Baldwin, R. M.; Golden, J. O.; Gary, J. H. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1975, 20, 26–46. (18) Thomas, M. G.; Granoff, B. Fuel 1978, 57, 122–123. (19) Kutta, H. W.; Burk, E. H.; Beckberger, L. H.; Chao, T. S. Physical Property Improvement of Coal Liquefaction Products, Final Report; March 1977; EPRI AF-392. (20) Bockrath, B. C. Fuel Process. Technol. 1977/1978, 1, 217–226. (21) Krzyzanowska, T.; Marzeca, A. Fuel 1978, 57, 804–805. (22) Tewari, K. C.; Hara, T.; Shiau Young, L.; Li, N. C. Fuel Proc. Technol. 1979, 2, 303–311. (23) Fisher, C. H.; Sprunk, G. C.; Eisner, A.; O’Donnell, H. J.; Clarke, L.; Storch, H. H. Tech. Pap.U. S. Bur. Mines 1942, 642, 1–27. (24) Given, P. H.; Cronauer, D. C.; Spackman, W.; Lovell, H. L.; Davis, A.; Biswas, B. Fuel 1975, 54, 34–39. (25) Chen, C.; Gao, J.; Yan, Y. Fuel Process. Technol. 1998, 55, 143–151. (26) Ruberto, R. G.; Jewell, D. M.; Jensen, R. K.; Cronauser, D. C. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1974, 19, 258–290. (27) Davis, A.; Spackman, W.; Given, P. H. Energy Sources 1976, 3, 55–81. 2129
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