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Ind. Eng. Chem. Prod. Res. Dev. 1980, 79, 442-446
Molten Salt Hydrocracking of Lignite. Screening of Viscosity Reducers and Hydrogen Sources Warren P. Scarrah Department of Chemical Engineering, Montana State University, Bozeman, Montana 597 17
Bench-scale screening tests were made to investigate the effects of viscosity reducers and hydrogen sources on the molten salt hydrocracking of lignite to chemical feedstocks. Variables included (1) the addition of alkali metal (Li, Na, K) halides (Cl, Br, I) to the ZnCI, catalyst, (2) reducing gases (H, CO, syngas), and (3) water sources (wet lignite, dry lignite, dry lignite plus added water). Yields ranged up to 0.18 g/g of MAF coal for vapors plus oils and 0.28 g/g of MAF coal for asphaltols. Potassium gave the highest vapors plus oils yields; the iodide, hydrogen, and dry coal plus added water were used. Both sodium and potassium gave significantly higher asphaltols yields than lithium; however, potassium was less sensitive to the levels of the other variables than sodium.
Introduction Alternate sources of chemical feedstocks must be found to replace the natural gas and petroleum currently being used (Chem Systems, 1975; Van Antwerpen, 1978). Montana lignite is an attractive alternative because of its abundance, accessiblilty, lower sulfur content, and chemical reactivity relative to other coals. Although complete coal conversion to chemical feedstocks is appealing, a more realistic approach is a coal refinery producing many types of feedstocks and fuels. Coal liquefaction to produce chemical feedstocks differs significantly from liquefaction to fuels in that the chemical structure is the primary concern rather than energy content and convenience. In particular, a very desirable product is one retaining the aromatic characteristics of the starting coal. This project investigated a liquefaction process using rather mild operating conditions that could be an initial process to strip off the more tractable aromatic compounds from the coal. The use of massive quantities of molten zinc chloride catalyst with coal and coal extracts retains their aromatic structure because the catalyst has little hydrocracking activity after the coal components have been reduced to single-ring aromatic compounds (Zielke et al., 1966; U S . Department of the Interior, 1969). Separation of the hydrocarbons from the molten salt was impractical until potassium chloride was added to reduce the viscosity of the molten salt and thus produce distinctive hydrocarbon and salt phases upon cooling (Berg and Malsam, 1973). The hydrocracking activity of the zinc chloride was not lost due to the addition of potassium chloride as coal conversions into benzene-soluble compounds as high as 90 wt 70 were still attained. It was then discovered that mixtures of zinc chloride with other viscosity reducers (lithium chloride, potassium bromide, potassium iodide) all showed hydrocracking activity and good separation properties; however, the products were distinctly different and ranged in appearance from tars to light, naphtha-like materials (Scarrah, 1973). In the solvent refining liquefaction process the organic material in the coal is heated and dissolved in an organic solvent; the depolymerized coal fragments are stabilized by hydrogen. The conversion and product distribution are influenced by the source of hydrogen which can be (1) added directly, (2) supplied by a donor solvent, or (3) generated by adding carbon monoxide and water to react via the water gas shift reaction; lignite conversions varied from 48 to 94 wt 70 depending on the combination of 0196-4321/80/1219-0442$01 .OO/O
Table I. Fractional Factorial Experimental Design treatment level& experimen tQ
1 2 3 4
A
B
C
0 0 0 1
0
0 2 1 1 0 2 2 1 0
5
1 2 0
1 1 6 1 2 7 2 0 8 2 1 9 2 2 The sequence of experimental runs was randomly selected. A = halide: 0 = chloride, 1= bromide, 2 = iodide; B = reducing gas: 0 = hydrogen, 1= carbon monoxide, 2 = syngas; C = water source: 0 = wet lignite, 1= dry lignite, 2 = dry lignite + added water.
hydrogen sources (Severson, 1975b). When the water gas shift reaction is used to generate hydrogen, product characteristics are also affected by whether the lignite is used (1)in its natural moisture-containingstate or (2) dried and an equivalent amount of water added; lignite conversions were 89 w t % for the former and 58 wt % for the latter (Severson, 1975a). Experimental Section Objective and Experimental Design. The innovations investigated were the effects on product distribution of viscosity reducers and hydrogen sources. Specific variables included the addition of (1)alkali metal (lithium, sodium, potassium) halides (chloride, bromide, iodide) to the zinc chloride catalyst, of (2) reducing gases (hydrogen, carbon monoxide, syngas (50 mol % H2and 50 mol % CO)), and of (3) water sources (wet lignite, dry lignite, dry lignite plus added water. Because of the exploratory nature of the project, it was decided to screen the process variables to determine which were most significant rather than to attempt to optimize the process. For each alkali metal, fractional factorial experimental designs were selected to investigate the combinations of halides, reducing gases, and water sources. A one-third fractional factorial design was selected so that the effects of the variables could be evaluated using only nine runs for each alkali metal; in this design the effect of each variable is confounded with the interactions of the other two variables (Davies, 1967). By choosing the same fractional factorial for each alkali metal (Table I), the 0 1980 American Chemical Society
Ind. Eng.
ANALYTICAL TECH 11I Q U E
SEPARATION
SOURCE
Reactor Gases -m a n d Vapors
Cold Traps
Chem. Prod. Res. Dev., Vol. 19, No. 3,
Gases
1
Condensed
';Eaniase c
+
I
c
Reactor Liquids a n d Solids
Water
--
Phase
Water Extraction
Water Solub:es
Gas Chromatography
-
Extraction
1
CH4,C2H6,C3H8,
-
w
ridine
Solubles
*
Benzene, Toluene, Xylenes
No Analyses
No Analyses
Sol ids Pyridine Soxhlet
A
CO,C02,H2>92,N2
Vapors
Decanter
443
PRODUCTS
Gas Chromatography
____t
1980
Liquid
Chromatography
-
Oils, Asphaltenes, Asphal to1 s
Solids
t
No Analyses
Figure 1. Product characterization schematic.
experimental design allowed the alkali metals to be compared through the use of hypotheses testing of the means; each of the three possible combinations of alkali metals taken two a t a time was treated as paired observations (Ostle, 1963). Analysis of variance calculations were made to determine significance levels, i.e., the probability of rejecting the hypothesis that a variable does not affect product yields when that is actually the situation. Materials and Reagents. Lignite (61.2 wt 7'0 moisture-and-ash free (MAF) coal, 31.7 wt % HzO, 7.1 wt % ash) was obtained from the Savage, Mont., mine of the Knife River Coal Co.; a single batch large enough for all the experimental runs was ground to -140 mesh (U.S. sieve series) using a ball mill. The zinc chloride and alkali metal halides were reagent grade. Apparatus and Procedures. The reactor was a 500mL stainless steel rocking bomb (Parr Series 4000 pressure reaction apparatus). The gas was collected in a stainless steel drum after passing through a sequence of three cold traps: a filter flask at room temperature, a glass cold trap immersed in a Dewar llask containing ice and water, and a glass cold trap immersed in a Dewar flask containing dry ice and isopropyl alcohol. The experimental conditions held constant for all runs (Table 11) were selected after reviewing previous molten salt investigations (Scarrah, 1973). Because the total gas pressure was 3000 psig the hydrogen partial pressure varied with the reducing gas composition; however, for process variable screening this approach was satisfactory for identifying significant effects (the same procedure had been used in the previously mentioned solvent refining tests). The experimental procedure started with the drying of the salts and lignite (if necessary) overnight at 105 "C. The lignite, salts, water (if required), and reducing gas/ gases were added to the reactor, and it was inserted in the rocking-heating apparatus; the desired temperature was reached after about 50 min. At the end of the run the gases and vapors were bled through the cold traps into the evacuated drum; after overnight cooling the liquids and solids were removed from the reactor.
Table 11. Standard Experimental Conditions condition
level
catalyst temperature, C pressure, psig time, min mixing, reactor agitation salt:coal, weight ratio ZnC1,:alkali metal halide, weight ratio coal size, U.S. sieve series lignite charge, g of MAF
ZnC1, 450 3000 15 rocking 2: 1 1:1 - 140 25
The separation and analytical scheme used to characterize the reaction products is presented in Figure 1. For all runs material balances indicated the total reactor outputs were greater than 90 w t % of the reactor inputs. Analytical Methods. More extensive chemical analysis is required when the objective is the production of chemical feedstocks rather than fuels. However, for this type of exploratory investigation the process is the prime consideration and an effort was made to use simple rather than elaborate analytical techniques. Product chracterization included the identification of gaseous hydrocarbons, simple aromatic liquids (benzene, toluene, xylenes), and the major fractions of the heavier liquids (oils, asphaltenes, and preasphaltenes). The gas composition was determined using a dual column Aerograph Model 202 gas chromatograph with a thermal conductivity detector. A Porapak Q-Scolumn was used to detect COz, C2H6,and C3H8while 13-X molecular sieve column was used to detect HS,0 2 N2, , CH,, and CO. A gas density bulb was used to determine the molecular weight of the gas and check the value calculated using the composition determined by gas chromatography. Of the vapors collected in the cold traps, only the filter flask liquids separated into two phases-an organic layer on top of a water layer. An Aerograph Model 660 gas chromatograph with a flame ionization detector was used to analyze the organic phase. The column was 5 wt % diisodecylphthalate and 5 wt 7'0 Bentone 34 on Chroma-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
Table I11 eluent 1. chloroform
fraction oils (saturated and aromatic hydrocarbons and heteroatom compounds) 2. ethyl ether/3% ethanol asphaltenes (monofunctional compounds) 3. pyridine asphaltols (multifunctional compounds)
sorb W; benzene, toluene, and xylenes were detected. The liquidsolid mixture from the reactor was extracted with water to remove the inorganic salts (water-soluble organics were also removed). The mixture was placed in a Waring blender, water was added, and the contents were thoroughly mixed. The suspension was then filtered and the precipitate further washed until no chloride ion was indicated in the wash water. Finally, the precipitate was dried at 105 " C for a t least a day. The water-insoluble precipitate was then extracted with pyridine using a Soxhlet extractor until the pyridine percolating through the thimble was colorless. The product-containing pyridine was then evaporated at 105 "C to obtain the pyridine-solubles. To characterize the water-insoluble, pyridine-soluble product a liquid chromatographic procedure called sequential elution with specific solvents chromatography (SESC) was selected that chemically fractionates the product into different classes of compounds (Whitehurst et al., 1976). The advantage of this technique is that it separates fractions relative to their chemical functionality rather than their solubility. Solubility fractionation suffers from the interaction of soluble compounds that act as cosolvents and can cause a compound to be separated into several fractions depending on the presence or absence of other species in the mixture. The standard SESC procedure using nine eluents was modified in this study to use only three; this simplification corresponds to typical coal liquefaction characterizations and was based on the thin layer chromatography studies used to develop the SESC procedure that showed preceding fractions being eluted while the subsequent fractions were not affected. The sequence of the three eluents and their relationship to the classical solubility fractions are given in Table 111. Although oils and asphaltenes are common terms, the asphaltols (sometimes referred to as preasphaltenes) were so named to indicate they had multiple-OH groups per molecule. About 0.25 g of the dried pyridine-solubles were placed in the bottom of a 15 x 150 mm glass column packed with silica gel. About 200 mL of chloroform was pumped into the column at 1mL/min followed by 200 mL of ethyl ether/ethanol a t 2 mL/min and finally 200 mL of pyridine at 2 mL/min. Each solvent fraction was collected separately and evaporated at 105 "C to determine the weight of each product. Criteria for Evaluation. Yields were determined in terms of grams per gram of MAF lignite. Hydrocarbon gas yields (methane, ethane, and propane) were so small (generally less than 0.007 g/g of MAF coal) that they could not be used to distinguish any effects of experimental variables. The organic phase of the condensed vapors showed small quantities of benzene, toluene, and xylenes but most of the components were unidentified. Therefore, it was decided to lump these organic vapors with the oil phase determined by the SESC technique. Using the SESC method yields of oils and asphaltols appeared reasonable but only small amounts of asphaltenes were detected. In the early experiments the liquid-solid
Table IV. Yields of Vapors Plus Oils yields, g/g of MAF coal treatmenta A B C Li Na K 0 0 0 1 1 1 2 2 2
0 1 2 0 1 2 0 1 2
0 2 1 1 0 2 2 1 0
0.0671 0.0160 0.0688 0.0922 0.0160 0.0669 0.0808 0.0903 0.0239
0.0596 0.0477 0.0786 0.0948 0.0269 0.1015 0.0473 0.01 29 0.0758
0.1305 0.0426 0.0509 0.0617 0.0392 0.0786
0.1783 0.0253 0.0839
a A = halide: 0 = chloride, 1= bromide, 2 = iodide; B = reducing gas: 0 = hydrogen, 1 = carbon monoxide, 2 = syngas; C = water source: 0 = wet lignite, 1 = dry lignite, 2 = dry lignite + added water.
Table V. Yields of Asphaltols yields, g/g of MAF coal treatmentn A
B
C
Li
Na
K
0 0 0
0 1 2 0 1 2 0 1 2
0 2 1 1 0 2 2 1 0
0.0459 0.0446 0.0188 0.0444 0.0201 0.1636 0.0268 0.0057 0.0297
0.1226 0.0882 0.1466 0.1716 0.0414 0.2752 0.1240 0.0437 0.1704
0.1253 0.0538 0.1107 0.0421 0.1166 0.2348 0.2423 0.0313 0.1244
1 1 1 2 2 2
a A = halide: 0 = chloride, 1= bromide, 2 = iodide; B = reducing gas: 0 = hydrogen, 1 = carbon monoxide, 2 = syngas; C = water source: 0 = wet lignite, 1= dry lignite, 2 = dry lignite + added water.
mixture from the reactor had been extracted directly with pyridine. When conversions exceeding 100% were obtained, it was discovered that some of the inorganic salts were soluble in pyridine. They were removed from the pyridine solubles using water extraction; water extraction was used prior to pyridine extraction for subsequent experiments. Only after the water from the extractions had been discarded was it realized that no analyses had been made for the water-soluble phenols. Since these phenols were not recovered, it was then decided not to analyze the water phases of the vapors for phenols. As the phenols are part of the asphaltenes, this explains the low yields of the latter. Consequently, the effects of the process variables on the asphaltenes yields could not be evaluated. Therefore, the yields of (1) organic vapors plus oils and of (2) asphaltols were used to evaluate the effects of the process variables.
Results and Discussion Yields of vapors plus oils and asphaltols will be discussed in the following sequence: (1)the effects of the experimental variables (halides, reducing gases, water sources) and (2) the effects of the alkali metals (lithium, sodium, potassium). The yield data are presented in Tables IV and V. Graphs have been prepared to represent the effects of the experimental variables; each point on the graphs represents the average yield of the three runs made at that particular level of the variable being illustrated (the combination of levels of the other two variables differed for each of the three runs being averaged). Statistically significant effects will be noted where they were detected. Halides. Figure 2 shows vapors plus oils yields (Table IV) were not very sensitive to halides for any of the alkali metals. The iodide gave slight yield increases with potassium and decreases with sodium.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
445
e
_----
---
I
LI NA
K
I
I
I
0.11
a%
dz
0.00
-
U
CL
I
1
BR
I
0.10
0.05
I
0.00
HZ
Figure 2. Yields of vapors plus oils as a function of halides.
Figure 5. Yields of asphaltols as a function of reducing gases.
o'20
CL
I
I
BR
I
HALIDE
---- - N A LI
.
I
K
',
2
I
0.00
w
co
0.00
I
2 WET
DRY
DRY PLUS H 2 0
WATER S O U R C E
Figure 3. Yields of asphaltols as a function of halides.
---
I SYNGAS
REDUCING G A S
HALIDE
0.00
1
co
SYNOAS
REDUCING G A S
Figure 4. Yields of vapors plus oils as a function of reducing gases.
Asphaltols yields ((Table V) were highest with the bromide for lithium and sodium; either the bromide or iodide gave good yields with potassium (Figure 3). It is apparent that sodium and potassium resulted in much higher yields than lithium. Reducing Gases. Vapors plus oils yields (Figure 4) were lowest when carbon monoxide was used; this effect was significant at a Significance level of 25% for sodium
Figure 6. Yields of vapors plus oils as a function of water sources.
and 10% for potassium. Hydrogen gave higher yields with potassium and lithium while syngas was better with sodium. In Figure 5 it is obvious that carbon monoxide significantly decreased the asphaltols yields; this was significant for sodium at about a 10% significance level. Syngas gave higher yields than hydrogen for all the alkali metals. The sodium and potassium again gave much higher yields than lithium. Water Sources. Vapors plus oils yields (Figure 6) were decreased using dry coal with potassium (significant a t a significance level of 25%1, were increased with dry coal using lithium, and the water source had little effect using sodium. When some form of water was present potassium gave the best yields and lithium the worst. Figure 7 indicates that asphaltols yields were best for all the alkali metals when the coal was dried and then charged to the reactor with water added; possibly the salts dissolved in the water and coated the coal particles to give more intimate contact when the salts became molten. Dry coal decreased yields appreciably using potassium and slightly using lithium. Again the yields using lithium were considerably lower than when sodium and potassium were used. Lithium. Using lithium, the highest yield of vapors plus dry coal) and oils was 0.092 g/g of MAF coal (bromide, H2, of asphaltols was 0.164 g/g of MAF coal (bromide, syngas, dry coal plus added water).
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
---__
LI NA K
-__
0.00
i
1 9
0.15
I
w ET
I DRY
1 DRY PLUS H 2 0
WATER SOURCE
Figure 7. Yields of asphaltols as a function of water sources.
The sixth highest vapors plus oils yield was still as high as 0.067 g/g of MAF coal; this accounts for the fairly flat curves (Figures 2, 4,and 6) and lack of statistical significance of the variables on yields when lithium was used. However, asphaltols yields (Figures 3, 5, and 7) showed that the bromide, hydrogen, and dry lignite should be used. The highest asphaltols yield was about 3.6 times as large as the next highest yield. Therefore, this single high yield influenced the graphical data presentation but was not statistically significant. However, the lower asphaltols yields observed in the graphs for lithium compared to sodium and potassium were statistically significant at a significance level of 0.5%. Sodium. The highest sodium yields of 0.102 g/g of MAF coal of vapors plus oils and of 0.275 g/g of MAF coal of asphaltols resulted from the same levels of variables: the bromide, syngas, and dry coal plus added water. For three of the four lowest vapors plus oils yields (0.0134.048 g/g of MAF coal) carbon monoxide was used as a reducing gas. Also syngas was used in three of the four runs giving the highest yields (0.076-0.102 g/g of MAF coal). This explains the statistical significance of reducing gases. The statistical significance of reducing gases for asphaltols yields was also due to high yields when using syngas and low yields when using carbon monoxide. From Figure 3 it appears that the halides have a large effect on asphaltols yields but this is because the highest yield was about 1.6 times higher than the next highest yield and disproportionately influenced the graph. Potassium. With potassium the iodide, hydrogen, and dry coal plus added water gave the highest yields: vapors plus oils of 0.178 g/g of MAF coal and asphaltols of 0.242 g/g of MAF coal. The statistical significance of reducing gases on vapors plus oils yields was influenced by carbon monoxide being used in three of the four lowest yielding runs and hydrogen in the two highest yielding runs. Likewise, water source was significant because wet coal was used in two of the three runs with the highest yields and the runs with dry coal gave yields ranking fifth or lower. Vapors plus oils yields with potassium were statistically higher than with lithium at a significance level of 30% and higher than with
sodium at a significance level of 40%. The two highest asphaltols yields were practically the same: 0.242 g/g of MAF coal with the iodide, hydrogen, and dry coal plus added water and 0.235 g/g of MAF coal with the bromide, syngas, and dry coal plus added water. These were about 1.9 times larger than the next highest yields. The differences in halides and reducing gases in these two dominating runs account for their lack of statistical significance on asphaltols yields. The other run in which dry coal plus added water was used only had a yield of 0.054 g/g of MAF coal (seventh highest); this probably was the cause for water source not being statistically significant. Conclusions The conclusions drawn from these process variable screening tests for the production of chemical feedstocks from lignite using massive quantities of molten zinc chloride catalyst can be summarized as follows. (1)Vapors plus oils yields are higher when adding potassium halides than when adding lithium or sodium halides. (2) Asphaltols yields are higher when adding potassium or sodium halides than when adding lithium halides. (3) Lignite conversion to hydrocarbon gases is small using massive molten salt catalysts. (4) The highest asphaltols yields can be obtained at the following variable levels: (a) potassium bromide or iodide, hydrogen or syngas, and dry lignite plus added water; (b) sodium bromide, syngas, and dry lignite plus added water. ( 5 ) Carbon monoxide is a poor reducing gas even when hydrogen can be generated via the water gas shift reaction. (6) Dry lignite plus added water might be effective because the water may dissolve the salts and provide better lignite-salt contact. Acknowledgment The author wishes to acknowledge the laboratory assistance of Joseph Althouse, Daniel Ellig, and Arthur Woods. Literature Cited Berg, L., Malsam, J. S.,U.S. Patent 3 736 250 (1973). Chem Systems, Inc., "Chemicals from Coal and Shale Feedstocks". New York, 1975. Davies, 0. L., "Design and Analysis of Industrial Experiments", 2nd ed, pp 487-8, Hafner Publishing Co., New York, 1967. Ostle, E., "Statistics in Research", 2nd ed, p 121, The Iowa State Unlversity Press, Ames, Iowa, 1963. Scarrah, W. P., 9:D. Thesis, Montana State Unlversity, Bozernan, Mont., 1973. Severson, D. E., Project Lignite Quarterly Technical Progress Report No. 1, April, May and June, 1974", U S . Department of the Interior, OCR, Washington, DlC., 1975a. Severson, D. E., "Project Lignite Quarterly Technical Progress Report No. 6, July, August, and September, 1975", ERDA, Washington, D.C., 1975b. U.S. Deoartment of the Interior. OCR Research and DeveloDment ReDOrt 39. Vol. i I I , Book 1. 1969. Whitehurst, D. D., Farcaslu, M., Mitchell, T. O., "The nature and Origin of Asphaltenes in Processed Coals", (AF-252), RP-410-1 to EPRI (1976). Van Antwerpen, F. J., Proceedings of Conference on Chemical Feedstock Alternatives", A.I.Ch.E., New York, 1978. Zielke, C. W., Struck, R. T., Evans, J. M., Costanza, C. P., Gorin, E., I d . Eng. Chem. Process Des. Dev., 5 , 158-64 (1966).
Received for review August 27, 1979 Accepted May 12, 1980 This work was prepared with the support of the U S . Department of Energy Grant No. ET-78-G-01-2825. However, any opinions, findings, conclusions or recommendations expressed herein are those of the author and do not necessarily reflect the views of DOE.
