Investigation of First-Stage Liquefaction of Coal with Model Plastic

PE was by far the most difficult of the model plastics to convert. In two-component and multicomponent tests, tetrahydrofuran (THF) conversions could ...
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Energy & Fuels 1997, 11, 849-855

849

Investigation of First-Stage Liquefaction of Coal with Model Plastic Waste Mixtures Kurt S. Rothenberger* and Anthony V. Cugini Federal Energy Technology Center, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, Pennsylvania 15236-0940

Robert L. Thompson and Michael V. Ciocco Parsons Power Group, Inc., P.O. Box 618, Library, Pennsylvania 15129 Received November 18, 1996. Revised Manuscript Received March 19, 1997X

A series of liquefaction tests have been conducted using coal, polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), in various combinations and proportions. These tests were done in batch microautoclaves, 1-L semibatch reactors, and a smallscale continuous unit. Results on individual plastics showed that PE and PP were converted primarily into aliphatic hydrocarbons, PS into alkyl benzenes, and PET into benzenes, ethane, and carbon dioxide. PE was by far the most difficult of the model plastics to convert. In twocomponent and multicomponent tests, tetrahydrofuran (THF) conversions could be estimated from the behavior of the individual components under similar conditions. Results were highly sensitive to conditions, especially those of atmosphere and temperature. Higher conversions could be obtained with higher temperatures, provided that retrograde reactions that re-polymerize the products were minimized. Retrograde reactions could be suppressed via the use of a synthesis gas atmosphere and/or the absence of added solvent. Acidic catalysts, such as molecular sieves, also led to higher conversions, but only in systems where coal was absent.

Introduction As part of the U.S. Department of Energy (DOE) Fossil Energy Program, the Federal Energy Technology Center (FETC) has recently initiated research in advanced coal-waste coprocessing. Coal-waste coprocessing involves the conversion to liquid feedstocks of a combination of coal with any or all of the following: rubber, plastics, paper, heavy oil, and waste oil. Advanced coprocessing of coal with solid wastes is a novel approach to recycling that can recover and reuse the inherent value of wastes by producing liquid fuels and other useful byproducts. The prevalent methods for disposing of solid waste include landfilling, combustion, and recycling. Over 120 million tons of solid waste are sent to landfill each year, and that amount will not decrease significantly in the foreseeable future.1 Some waste is incinerated to produce energy, but this method has not gained public acceptance due to air pollution concerns. Approximately 45 million tons of solid waste are recycled each year, but this method can be costly and is typically reserved for plastics which have high recovery value.1 Landfilling is simply too easy and economical to compete with recycling. The current DOE advanced waste research effort has concentrated on the combined processing of coal and waste plastics.2-5 One potential advantage in the coprocessing of plastic materials with coal is the higher * E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, April 15, 1997. (1) Characterization of Municipal Solid Waste in the United States: 1994 Update; U.S. Environmental Protection Agency, U.S. Govt. Printing Office: Washington, DC, 1994; EPA 530-S-94-042.

S0887-0624(96)00207-1 CCC: $14.00

hydrogen-to-carbon ratio in most plastics as compared to coal, which is hydrogen deficient relative to the petroleum-like liquids desired as products. In this study, a series of coprocessing tests has been conducted in microautoclave reactors, semibatch stirred autoclave reactors, and a small-scale continuous unit. All tests employed Black Thunder subbituminous coal with plastic feed streams containing mixtures of polyethylene (PE), polystyrene (PS), polyethylene terephthalate (PET), and polypropylene (PP) in various combinations and proportions. These particular polymers were chosen to be similar to those expected to form the bulk of authentic municipal waste streams.1 Due to the continually evolving nature of the coalwaste coprocessing initiative, the emphasis in selecting experimental conditions was on the identification of potential problem areas for scheduled runs on larger units rather than systematically exploring the chemistry involved in the co-liquefaction of coal and plastics. Nevertheless, insights into both the chemistry and operability of coal-waste coprocessing can be gained from the data. Experimental Section Materials. Liquefaction experiments were conducted using -200 mesh Black Thunder mine coal (Wyodak-Anderson seam, Campbell Co., WY). High-density polyethylene (HDPE; Tm ) (2) Taghiei, M. M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 1228-1232. (3) Anderson, L. L.; Tuntawiroon, W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (3), 816-822. (4) Joo, H. K.; Curtis, C. W. Energy Fuels, 1996, 10, 603-611. (5) Coal and Waste; Huffman, G. P., Anderson, L. L., Eds.; Fuel Processing Technology Vol. 49; Elsevier: Amsterdam, 1996.

© 1997 American Chemical Society

850 Energy & Fuels, Vol. 11, No. 4, 1997

Rothenberger et al.

Table 1. Summary of Single-Component Microautoclave Reaction Conditions and Results run

coal

% feed mixture PE PS PET

PP

solvent

catalyst

time, min

temp, °C

gas

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

100 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 100 100 100 100 100 100 100 100 100 100 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 100

L-814 L-814 L-814 L-814 none none none 1-MN 1-MN none 1-MN L-814 L-814 1-MN 1-MN

AO-60 AO-60 AO-60 13X none none 13X none 13X 13X 13X none AO-60 MoS3b 13X

60 60 30 60 60 60 60 60 60 60 60 60 60 30 60

430 430 465 460 430 445 430 445 430 430 430 430 430 425 430

H2 H2 H2 H2 H2 H2 H2 H2 H2 N2 N2 H2 H2 H2 H2

0 0 0 0 0 0 0 0 0 0 0 100 100 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 100 0

% conversion THF hept 83 -19a 79 69 56 98 72 35 14 64 46 100 99

consumed H2, mmol

51 -20a 76 32

50 28 75 48 6 18 8 15 15

77 97 100 99

18 54 76 17

a Negative solvent conversions indicate that oil has been sequestered by molten PE. b The MoS used in run 14 was obtained from 3 Aldrich Chemical Co. and used as received.

Table 2. Summary of Two-Component Microautoclave Reaction Conditions and Results run

coal

16 17 18 19 20 21 22 23

33 50 50 50 0 0 0 0

% feed mixture PE PS PET

PP

solvent

67 50 50 50 67 50 67 50

0 0 0 0 0 0 0 50

L-814 1-MN none 1-MN L-814 1-MN L-814 1-MN

0 0 0 0 33 50 0 0

0 0 0 0 0 0 33 0

catalyst

time, min

temp, °C

gas

AO-60 none AO-60 AO-60 AO-60 13X AO-60 13X

60 60 60 60 60 60 60 60

430 460 430 430 430 430 430 430

H2 H2 H2 H2 H2 H2 H2 H2

% conversion THF hept 28 82 34 47 36 70 35 58

26 81 28 31 36 35 58

consumed H2, mmol 38 28 35 56 16 46 11

predicted % conversiona THF hept 28

17

42 33

25 33

33

33

a Predicted conversions were calculated assuming coal(THF) ) 83%, coal(hept) ) 51%, PE(THF) ) PE(hept) ) 0%, and PS(THF) ) PS(hept) ) PET(THF) ) PET(hept) ) PP(THF) ) PP(hept) ) 100%.

135 °C, d ) 0.96 g/mL) was obtained from Solvay Polymers. Polystyrene (PS; Tm ) 95 °C, d ) 1.0 g/mL) was obtained from BASF. Polyethylene terephthalate (PET; Tm ) 215 °C, d ) 1.4 g/mL) was obtained from Hoechst Celanese. Polypropylene (PP; Tm ) 176 °C, d ) 0.94 g/mL) was obtained from Amco Plastics. All plastics were obtained by FETC through Hydrocarbon Technologies, Inc. (HTI) as 3.2 mm (0.125 in.) extruded pellets. Two different vehicles were used in the microautoclave coprocessing tests: 1-methylnaphthalene (1-MN), which was used as received from Aldrich Chemical Co., and L-814, a mildly hydrogenated (9% hydrogen) fluid catalytic cracking decant oil, obtained as the 340-510 °C fraction from run POC-1 of the proof-of-concept liquefaction unit at HTI. Both the semibatch experiments and continuous unit exclusively employed L-814 as the solvent. Catalysts. Three different catalysts were employed in the microautoclave, semibatch, and continuous unit experiments. Molecular sieves (13X; pore size ) 10 Å) were obtained from Fisher Scientific Co. and were oven dried and crushed to -60 mesh before use. AO-60, an aged Ni-Mo catalyst from run POC-1, was supplied by HTI. It was manufactured by Akzo using an alumina support in the form of 1/16-in. extruded pellets. One semibatch run used a 2% SiO2 in Al2O3 cocatalyst obtained from Engelhard. A fresh sample of alumina-supported Ni-Mo catalyst (Amocat-1A), used in some of the continuous unit experiments, was obtained from Amoco. Reactions. Microautoclave reactions were conducted in 43mL cylindrical, stainless steel batch reactors constructed at FETC.6 The base conditions of the autoclave tests were 2:1 ratio of vehicle to feed, 60 min at 430 °C, 7 MPa (1000 psi) of cold H2 pressure, and 0.7 g of molecular sieves, although variations in most of the conditions were studied. Detailed (6) Rothenberger, K. S.; Cugini, A. V.; Schroeder, K. T.; Veloski, G. A.; Ciocco, M. V. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39 (3), 688-694.

descriptions of the reaction conditions for the microautoclave runs are listed in Tables 1-3. During workup, the contents of the reactor were sonicated in THF for 30 min and subsequently filtered through a 0.45 µm filter under 0.28 MPa (40 psi) N2 pressure. The solvent was evaporated from the THF-soluble material using a rotary evaporator, and the product was subsequently extracted with heptane to produce a heptane-soluble fraction. Conversions to THF and heptane solubles were calculated from the measured mass of insoluble material, adjusted for catalyst and coal mineral matter, based on the mass of feed plastic and MAF coal. Neither PE, PP, nor PET showed any significant solubility in either THF or heptane under the workup conditions used. PS showed appreciable solubility in THF, rendering the THF conversion calculations of PS experiments meaningless, but PS was not soluble in heptane. A series of semibatch (batch slurry, flow-through gas) tests was performed in a 1-L stirred-tank reactor system.7 The feed charge consisted of 350 g of a slurry that typically consisted of a 2:1 ratio of vehicle to feed with 30 g of AO-60 catalyst. The feed compositions are listed in Table 4. The semibatch tests were performed using L-814 and under 17.5 MPa (2500 psi) of a 97% H2/3% H2S gas mixture, flowing at a rate of 1.9 L/min (4 SCF/h). The products were characterized in terms of gas yield and composition, conversions to heptane and THF solubles, and conversion of material distilling above 450 °C (including MAF coal, plastics, and oil) into material distilling below 450 °C (450 °C+ conversion). Continuous mode catalytic liquefaction experiments were conducted in a 1-L bench-scale unit.7 The unit is a oncethrough system without recycle. A typical charge consisted of a vehicle:feed mixture of 70:30 at an overall slurry feed rate of 146 g/h. The catalyst employed (35 g) was contained in an annular basket surrounding the stirrer to simulate the action (7) Cugini, A. V.; Krastman, D.; Lett, R. G.; Balsone, V. Catal. Today 1994, 19, 395-408.

First-Stage Liquefaction of Coal

Energy & Fuels, Vol. 11, No. 4, 1997 851

Table 3. Summary of Multiple-Component Microautoclave Reaction Conditions and Results run

coal

24 25 26 27 28 29 30 31 32 33 34 35 36

70 70 50 50 50 50 50 50 0 0 0 0 0

% feed mixture PE PS PET 15 15 25 25 25 25 25 25 50 50 50 50 50

10 10 16 16 16 16 16 16 35 35 35 35 35

5 5 9 9 9 9 0 0 15 15 15 0 0

PP

solvent

catalyst

time, min

0 0 0 0 0 0 9 9 0 0 0 15 15

L-814 L-814 L-814 L-814 1-MN 1-MN 1-MN 1-MN none none 1-MN none 1-MN

AO-60 AO-60 AO-60 AO-60 none none 13X 13Xb AO-60 AO-60 13X 13X 13X

60 120 60 60 30 60 60 60 60 60 60 60 60

temp, °C

gas

445 445 430 430 460 460 430 430 430 445 430 430 430

H2 H2 H2 H2 H2 H2 H2/CO H2/CO H2 H2 H2 H2/CO H2/CO

% conversion THF hept 71 48 65 63 71 72 63 67 59 84 75 97 87

37 13 57 56 66 68 59 84

consumed H2, mmol 73 71 54 48 20 29 46 102 38 43 18 12 12

predicted % conversiona THF hept

67 67

51 51

a Predicted conversions on runs at 430 °C for 60 min using AO-60 catalyst were calculated assuming coal(THF) ) 83%, coal(hept) ) 51%, PE(THF) ) PE(hept) ) 0%, and PS(THF) ) PS(hept) ) PET(THF) ) PET(hept) ) PP(THF) ) PP(hept) ) 100%. b Catalyst in run 31 included an equivalent amount of AO-60.

Table 4. Summary of Semibatch Reaction Conditions and Conversionsa % feed mixture

% conversion

run

coal

PE

PS

PET

PP

temp, °C

gas

THF

hept

450 °C+

1 2b 3 4 5 6 7 8 9 10 11 12

50 50 50 50 50 50 50 50 0 0 0 0

25 25 25 25 25 50 50 50 100 100 100 0

17 17 17 17 17 0 0 0 0 0 0 0

8 8 8 8 8 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 100

430 430 430 445 460 430 445 460 430 445 460 445

H2 H2 H2/H2S H2/H2S H2/H2S H2/H2S H2/H2S H2/H2S H2/H2S H2/H2S H2/H2S H2/H2S

55 61 54 70 75 14 36 62 0 0 39 85

53 60 50 70 69 17 40 69 0 0 42 82

45 55 32 65 62 11 26 50 0 12 45 73

a

Base run conditions: L-814 solvent, 60 min, AO-60 catalyst. b Run 2 catalyst was 1:16 SiO2:AO-60. Table 5. Summary of Continuous Unit Reaction Conditions and Results

run period

%plastics in feed

1 2 3 4 5 6

0 0 7.5 7.5 10 15

catalyst

temp, °C

450 °C+ conversiona (%)

C1-C4 gas production (%)

AO-60 Amo-1Ab AO-60 AO-60 AO-60 AO-60

404 430 430 460 430 430

42 58 49 90 61 32

1 10 2 6 3 2

a Calculation of 450 °C+ conversion based on feed only. Solvent conversion has been factored out of calculation. b Amo-1A indicates Amocat-1A catalyst (data reported are from a separate continuous run).

in an ebullated bed. The coprocessing tests were done at a reactor temperature of 430 °C under 17.5 MPa (2500 psi) of a 97% H2/3% H2S gas mixture, flowing at a rate of 2.4 L/min (5 SCF/h). The system was run at the conditions listed in Table 5. Continuous unit products were characterized on the basis of their 450 °C+ conversion. Gas and Pressure Analyses. Microautoclave reactor gas samples were collected at the completion of each run. Product gases were analyzed at FETC by a previously published method8 and corrected for molar compressibility. Hydrogen consumption was calculated, based on the difference between initial and final (cold) gas pressure as adjusted for product gas composition.9 Semibatch unit gas samples were slowly (8) Hackett, J. P.; Gibbon, G. A. In Automated Stream Analysis for Process Control; Manka, D. P., Ed.; Academic Press: New York, 1982; Vol. 1, pp 95-117. (9) Ciocco, M. V.; Cugini, A. V.; Rothenberger, K. S.; Veloski, G. A.; Schroeder, K. T. Proceedings of the 11th Annual International Pittsburgh Coal Conference, September 12-16, 1994; Vol. 1, pp 500-505.

collected once during the run (tail gas) and at its completion (flash gas). Hydrogen consumption was calculated, based on the assumption that the tail gas sample was representative of the gas composition throughout the run.

Results HDPE Reactions. The THF conversion, heptane conversion, and hydrogen consumption results for the microautoclave reactions with single-component feeds are listed in Table 1. The experiment using only coal as feed yielded 83% THF solubles and 51% heptane solubles using the supported Ni-Mo catalyst (AO-60) in L-814 at 430 °C for 60 min (run 1). The set of experiments performed on HDPE alone (runs 2-11) illustrates the effect of temperature, catalyst, solvent, and atmosphere on THF conversion. Under conditions that gave extensive conversion of coal, HDPE is not broken down into THF-soluble material (run 2); however, improved THF conversion of HDPE is observed when higher temperatures and molecular sieves are used (run 4). The HDPE is also converted into THFand heptane-soluble material using the supported NiMo catalyst at higher temperature, yielding a greaselike mixture of wax and oil vehicle in the heptane solubles (run 3). However, about one-half of the HDPE ends up in the form of C1 to C4 gases with production of approximately 10-15 mmol each of methane, ethane, propane, and butanes. Increasing the temperature from 430 to 445 °C (runs 5 and 6) demonstrated that thermal conversion of HDPE to THF solubles increases significantly with temperature. This result is expected because a higher reaction temperature provides more

852 Energy & Fuels, Vol. 11, No. 4, 1997

thermal energy for productive bond breaking; however, increasing temperature also increases the yield of unwanted C1-C4 gases. In many of the microautoclave and semibatch experiments, sheetlike deposits of unconverted HDPE were observed in the residual material, indicating the HDPE had resolidified upon cooling. This phenomenon led to the negative conversions reported for microautoclave run 2, which is attributed to the fact that the molten HDPE encapsulated the catalyst, along with some of the product oil, thus increasing the mass of the insoluble extraction residue. Comparison of runs performed with and without 1-MN as a vehicle (runs 6-11) shows that THF conversions of HDPE are decreased in the presence of solvent, regardless of catalyst or atmosphere used. If the dominant reaction mechanism is thermal bond scission, then the solvent may serve to deactivate the thermally excited HDPE molecules through collisions, resulting in less fragmentation when solvent is present. The decrease in THF conversion due to solvent was larger for the reactions performed under H2 than for those performed under N2, suggesting that the solvent may facilitate retrograde polymerization reactions, perhaps by keeping the depolymerized fragments in proximity of each other for longer periods of time or by preventing cracking reactions. However, the physical effect of dispersing an aliphatic material into an aromatic solvent cannot be disregarded. Experiments performed on HDPE under atmospheres of H2 and N2 (runs 7 and 10) show that in the presence of molecular sieves, but no solvent, use of H2 results in higher THF conversions. Conversely, the use of both molecular sieves and solvent (runs 9 and 11) demonstrated that the use of N2 results in considerably higher THF conversions. In the presence of solvent, H2 may quench the free radical transfer reactions which are necessary to propagate the fragmentation of the HDPE. Analysis of the product from run 11 by low-voltage, high-resolution mass spectrometry (LVHRMS) revealed that olefins were formed when N2 atmosphere was used. Comparison of runs 5 and 7 demonstrates that the use of molecular sieves increases the THF conversion of HDPE compared to a thermal run (neglecting reactor wall effects) using the same conditions. Acidic catalysts such as molecular sieves are known to catalyze cracking reactions in hydrocarbons by way of intimate contact between the hydrocarbon and acidic sites in the pores of the sieves. The effects of solvent and catalyst on the THF conversion of just the HDPE in the microautoclave reactor at 430 °C for 60 min under H2 are illustrated in Figure 1. The THF conversions of HDPE in coalcontaining systems were calculated on the basis of the conversion of coal-only feed under the same conditions and the assumption that coal conversion is unaffected by the presence of HDPE. Figure 1 clearly shows the effectiveness of molecular sieves as a catalyst and the improved conversions of HDPE observed in the absence of solvent. The acidic molecular sieve catalyst is far more effective at converting HDPE than is supported Ni-Mo (AO-60). However, its effect on feeds containing both HDPE and coal is diminished by the tendency of coal to poison acidic catalyst sites. Supported Ni-Mo is able to facilitate the conversion of coal but has no such

Rothenberger et al.

Figure 1. Effect of solvent, coal, and catalyst on HDPE conversion in autoclave runs using 2:1 1-MN:solids, 430 °C, 60-min reaction time, and either 13X or AO-60 catalyst. In multicomponent runs, the HDPE conversion was calculated by assuming the equivalent conversion of non-HDPE components from separate runs employing single components and using similar reaction conditions.

effect on HDPE. In fact, the THF conversion of HDPE using supported Ni-Mo was less than that with no catalyst at all. This may be due to the hydrogenation ability of the supported Ni-Mo catalyst, resulting in the capping of radical fragments formed by thermal bond scission of HDPE. In the results discussed above, extraction by THF was used to measure the extent of conversion of HDPE. This crude method does not provide any information on the extent of breakdown that may have occurred in the THF insolubles. Analysis of the unconverted HDPE in the THF insolubles in POC-2 runs at HTI by GPC determined that the molecular weight of the HDPE is reduced by 1 order of magnitude.10 This was not enough of a molecular weight reduction to render the HDPE soluble in THF or heptane. Because all of the C-C bonds in the HDPE chain are equivalent, HDPE is expected to show random fragmentation along the chain to produce a distribution of aliphatic hydrocarbons of varying length.11 PS, PET, and PP Reactions. Autoclave runs were performed separately on the rest of the polymers studied in this work (runs 12-15), employing conditions similar to those used for PE. PS is expected to depolymerize to styrene,12 which would subsequently hydrogenate to alkylbenzenes in the reducing environment of the reactor. Alkylbenzenes (C1 to C3) have indeed been observed by LVHRMS and gas chromatography-mass spectrometry (GC-MS) analyses of products from PS-containing feeds. In fact, PS showed high conversion to heptane solubles even in the absence of catalyst (run 12). Under the reaction conditions studied, PET showed nearly quantitative heptane conversion (run 14). PET was expected to undergo facile scission at the C-O linkages because they have a lower bond strength than the C-C bond.13 Under hydrogenation conditions, degradation of PET was expected to yield 2 mol of CO2, 1 mol of C2H6, and 1 mol of C6H6, all requiring 2 mol of (10) Rothenberger, K. R.; Cugini, A. V.; Thompson, R. L. Manuscript in preparation. (11) Madorsky, S. L. Thermal Degradation of Organic Polymers; John Wiley and Sons: New York, 1964; Chapter 4. (12) Reference 9, Chapter 3. (13) Reference 9, Chapter 14.

First-Stage Liquefaction of Coal

H2/mol of monomer. Significant quantities of both CO2 and C2H6 were observed in the gas analysis of the product; however, the molar ratio was considerably below the 2:1 expected for complete decomposition. Also, in other runs (not shown), a substantial amount of terephthalic acid, representing some 10% of the mass of starting PET, was recovered from the THF-soluble product. This indicates that the mechanism of PET degradation is sequential, consisting of initial cleavage along C-O bonds with liberation of ethylene (which hydrogenates to ethane), followed by cleavage or reduction of the remaining carboxyl groups. The experiment performed on PP alone resulted in quantitative THF and heptane conversions in the presence of molecular sieves (run 15). In PP, every other carbon atom in the polymer chain is tertiary, whereas in linear PE, all carbon atoms are secondary. As a result, the C-C bonds in the PP chain are weaker than those in PE, causing PP breakdown to be more facile than PE.11 Supporting evidence of this can be found in kinetics studies on the thermal degradation of polyolefins, which have estimated the activation energies of PP and PE at 58 and 72 kcal/mol, respectively.14 Multicomponent Reactions. The THF and heptane conversions of reactions with two-component and multiple-component feeds are listed in Tables 2 and 3, respectively. The THF and heptane conversions of those reactions conducted with solvent and AO-60 catalyst at 430 °C and 60 min were compared with those predicted, based on the assumption that no interaction existed between the plastics and coal in the feed, so that each converted independently of the other. The predicted THF and heptane conversions were calculated on the basis of the assumptions that coal conversions were the same as in run 1. Conversion of HDPE to both THF and heptane solubles is assumed to be negligible as indicated in run 2, whereas PS, PET, and PP conversion to both THF and heptane solubles is assumed to be quantitative as supported by runs 13-15 in Table 1. This method was able to predict unexpectedly well the THF conversions of the two-component and multicomponent runs of similar time, temperature, and catalyst with absolute deviations in THF conversion averaging less than 3%. The heptane conversions are not as well predicted by this method, with absolute deviations in conversion ranging from 2 to 9%. It is noteworthy that all of the heptane conversions are better than those predicted from the assumption of a noninteracting system, and the greatest deviations occurring in those runs which included coal (runs 16, 19, 26, and 27). Deviations in the runs containing PS in the feed (runs 26 and 27) may be due to alkylbenzenes, which would be present in the heptane solubles. Residual alkylbenzenes in the heptane solubles would increase the polarity of the heptane and thus dissolve more coal-derived material than heptane alone. With the exception of the noted runs, the evidence does not support any beneficial effect on the co-conversion of coal with plastics, at least in the presence of hydrogen, solvent, and supported Ni-Mo catalyst. The exceptional heptane conversions appear to be artifacts of the analytical procedure. (14) Wall, L. A.; Madorsky, S. L.; Brown, D. W.; Straus, S.; Simha, R. J. Am. Chem. Soc. 1954, 76, 3430-3437.

Energy & Fuels, Vol. 11, No. 4, 1997 853

Experiments were performed on feeds consisting of coal and mixed plastics to investigate the effects of temperature, catalyst, and reaction time (runs 24-31). Many reaction parameters were varied during these runs, but the THF and heptane conversions did tend to increase with increasing temperature. When reaction time was extended from 60 to 120 min at 445 °C on a coal and mixed plastics feed using supported Ni-Mo catalyst (runs 24 and 25), THF and heptane conversions were cut in half, indicating that catalyzed retrograde reactions were occurring. When reaction time was extended from 30 to 60 min at 460 °C using similar feed but no catalyst (runs 28 and 29), THF and heptane conversions were not affected. Experiments were also performed on a feed consisting of a mixture of 50% PE, 35% PS, and 15% of either PET or PP by weight (without coal), where temperature, solvent, and atmosphere were varied. With PET and supported Ni-Mo catalyst, THF conversion increased with temperature (runs 32 and 33). With PP, no solvent, and molecular sieves, THF conversion was nearly quantitative when an atmosphere of a 1:1 mixture of H2 and CO was employed rather than H2 (run 35). When this same reaction was conducted in 1-MN (run 36), the THF conversion decreased by more than 10%, illustrating the adverse effect solvent has on the conversion of plastics feeds. Semibatch Tests. The THF, heptane, and 450 °C+ conversions for the semibatch experiments are listed along with the experimental conditions in Table 4. Except for runs 1 and 2, the atmosphere used in the semibatch experiments was 97% H2/3% H2S; each run employed L-814 solvent and supported Ni-Mo catalyst. Experiments varying feed composition and temperature were performed. Increasing the temperature from 430 to 445 °C resulted in substantial increases in all conversions, regardless of feed. A further increase in temperature to 460 °C resulted in further substantial increases in THF, heptane, and 450 °C+ conversions, except for the coal and mixed plastics feed (run 5), where heptane conversion was essentially the same and 450 °C+ conversion decreased slightly. The decreased conversions in run 5 may be due to retrogressive polymerization reactions. The effect of temperature on conversion of these feeds to distillable products is illustrated in Figure 2, which shows the progressive increase in conversion for all feeds as the temperature is increased. It is important to note the sensitivity of conversion to temperature when the feed is pure HDPE because HDPE is consistently the most difficult feed material to convert. The difficulty with which HDPE is converted is reflected by the trend of lower overall conversions as the amount of HDPE in the feed is increased. As the HDPE content increases from 25% to 50% to 100% at 445 °C (runs 4, 7, and 10), the 450 °C conversions drops from 65% to 26% to 12%. Comparison of semibatch experiments using PP and HDPE alone as feeds at 445 °C (runs 10 and 12) reveal that PP gives considerably higher THF, heptane, and 450 °C+ conversions than HDPE, as expected due to the more extensive branching in the PP backbone. One semibatch run (run 2) employed a small amount of SiO2 on Al2O3 as a cocatalyst with supported Ni-Mo

854 Energy & Fuels, Vol. 11, No. 4, 1997

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Discussion

Figure 2. Effect of temperature on distillable products (450 °C+) conversion in semibatch studies of HDPE, 1:1 HDPE:coal, and 1:1 mixed plastics:coal using 2:1 1-MN:solids, AO-60 catalyst, and 60-min reaction time.

(1:16 by weight SiO2:AO-60), using the same conditions as in run 1. The SiO2 cocatalyst increased THF, heptane, and 450 °C+ conversions by nearly 10%, presumably by improving the fragmentation of the HDPE relative to run 1. Acidic catalysts are known to facilitate the cracking of aliphatic molecules. Continuous Unit Tests. The run conditions and distillation results for the continuous coprocessing runs are given in Table 5, during which a plastics mixture of 50% PE, 35% PP, and 15% PS was used. Continuous unit experiments performed at 430 °C by varying the plastics content in the feed from zero to 10% had very little effect on the 450 °C+ conversions. The 450 °C+ conversion for the coal-only run (run 2) was 58%, and the presence of plastics in the feed at levels of 7.5% and 10% (runs 3 and 5) did not significantly affect the 450 °C+ conversion. Increasing the plastics content in the feed to 15% did result in a decrease in 450 °C+ conversion from 58% to 32% (run 6). Increasing the reaction temperature of the continuous unit from 430 to 460 °C increased 450 °C+ conversion dramatically, to 90% (run 4). Successful operation of a continuous unit introduces another important requirement, that of pumping the feed slurry. In separate experiments with the continuous unit (not shown), the plastic component was increased to as high as 50% of the feed. During these experiments, it was found that successful operation of the continuous unit required careful regulation of the temperature of the feed line in order to control the slurry viscosity. At a given temperature, the viscosity of coalplastics mixtures was found to increase significantly as the plastics concentration was increased.15 When the slurry viscosity was too high, the mixture could not be pumped; when it was too low, the coal would settle out of solution, causing plugging. In these continuous unit tests,15 a 3:1 ratio of coal to plastics feed could be pumped at a temperature of 120 °C. However, a 1:1 ratio of coal to plastics feed became extremely viscous at 150 °C, and the feed system needed to be heated to 180 °C before the pump could manage the feed mixture. (15) Ciocco, M. V.; Cugini, A. V.; Wildman, D. J.; Erinc, J. B.; Staymates, W. J. Powder Technol., in press.

To access the hydrogen content of the plastics, unsaturated products must be formed from them, but these would then quickly be saturated under liquefaction conditions, thus defeating the original purpose of including the plastics. However, the hydrogen demand to produce low molecular weight liquids from plastics is much less that required for coal. Furthermore, most common plastics do not consume any H2 to eliminate heteroatoms as is required in coal liquefaction. So while no synergism was observed in the co-liquefaction of coal and plastics, the presence of plastics did not hinder the conversion of coal so long as the amount of plastics was limited. To realize an advantage in the coprocessing of coal with plastics from the higher hydrogen content of plastics, the selectivity of conversion must be controlled. Under typical liquefaction conditions, each C-C bond scission in the polymer chain would consume one molecule of H2 as the olefinic products formed by depolymerization are hydrogenated. This is particularly true with HDPE, which fragments randomly along the polymer chain. If light hydrocarbon (C1-C4) gas production from HDPE relative to the yield of liquid products from HDPE can be reduced, less H2 will be consumed by the HDPE, which otherwise offsets the advantage of higher hydrogen content in the plastics stream. The results from earlier studies had shown good conversions for autoclave liquefaction of plastics;2,3 however, there were conflicting reports of whether coprocessing led to better or worse conversions. Huffman and co-workers reported higher conversions when plastics were coprocessed with coal, especially when zeolite catalysts were used, and that PE could play the role of hydrogen donor solvent when no solvent was added.2 Anderson and Tuntawiroon reported that lower conversions were obtained when plastics and coal were coprocessed, contradicting any synergistic effects of plastics on coal.3 More recently, a study by Joo and Curtis supports the latter result.4 They studied similar plastics as coprocessed with Blind Canyon coal and petroleum resids but examined each plastic individually. They found that introduction of coal into a plastic/resid system or of resid into a coal/plastic system increased conversions but introduction of plastic into a coal/resid system decreased conversions. They also determined that the coprocessing of plastic with resid was much more compatible than with coal, which would be expected considering the mutually aliphatic character of the plastics studied and petroleum resid. The results of the current study also tend to support the lack of synergy between the coal and plastic, as illustrated by the decreasing conversions of HDPEcontaining runs in Figures 1 and 2. In agreement with the work of Joo and Curtis,4 PE was found to be the most difficult plastic to convert and large gas production in PET-containing runs was observed. The traditional solvent extraction methods used in evaluating coal conversion can be misleading when plastics, particularly HDPE, are included in the feed. One deficiency of the traditional solvent conversions is the inability to account for reactions which occur but do not lead to products which are more soluble. A significant reduction in the molecular weight of HDPE

First-Stage Liquefaction of Coal

takes place, yet goes unnoticed because those products would still be insoluble in THF and heptane. The problem goes beyond simply being an analytical problem. Separation and recovery methods used in larger scale units (i.e., pressure filtration, solvent deashing, vacuum distillation, etc.) could be less efficient when handling partially degraded HDPE. For example, this material could become molten in pressure filters and pass through them, even though the HDPE has not yet been converted. The presence of high molecular weight but only partially degraded HDPE could interfere with subsequent processing and be mistakenly considered as liquid product, when in fact it was not. Improved characterization of polyolefin degradation products, especially the “unconverted” fraction of HDPE and PP, is needed to understand the chemistry taking place in these complex systems. A process for removing incompletely converted HDPE from coprocessing residues has been developed in our laboratory and molecular weight analyses of the HDPE-containing material extracted from a series of continuous-scale runs suggests that HDPE is initially broken down at its branching points along the polymer chain.16 A manuscript is currently in preparation which will discuss these issues in greater detail.10 Conclusions Degradation of the individual plastics that would likely make up the bulk of a typical municipal waste stream under coal liquefaction conditions was studied. The PS was converted into alkylbenzenes, the PP and HDPE into aliphatic hydrocarbons, and the PET into CO2, ethane, and benzenes. The formation of up to 3 mol of gaseous CO2 and C2H6 per every mole of PET monomers and consumption of 2 mol of H2 indicate that PET is not a good candidate for co-conversion with coal. Aside from being an inherently valuable plastic due to the ease with which it is recycled, PET consumes costly H2 rather than helping to provide H2 to the coal under liquefaction conditions. The conversion of HDPE to low molecular weight species required higher temperatures than did the other plastics, and under those conditions, the production of unwanted gases becomes significant. (16) Rothenberger, K. S.; Cugini, A. V.; Thompson, R. L. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (3), 1062-1068.

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The increased resistance of HDPE toward conversion relative to the other plastics is consistent with the lack of reactive sites in its structure. In the two-component and multicomponent microautoclave tests, THF conversions could be estimated from the behavior of the individual components under similar conditions. The heptane solubles were consistently greater than that predicted by the assumption of individual behavior; however, this may have been an artifact of the analytical procedure. The results on two-component and multicomponent mixtures were highly sensitive to reaction conditions, especially those of atmosphere and temperature. Results indicate that higher conversions are favored at higher temperatures, provided that retrograde reactions which re-polymerize the products are minimized. Results from the semibatch and continuous unit experiments supported these trends. Other conditions which seem to help suppress retrograde reactions in the autoclave reactor are the use of a synthesis gas atmosphere and forgoing the use of solvent. The use of molecular sieves, similar to the catalysts used in petroleum cracking, also led to higher conversions compared to supported Ni-Mo catalyst, provided coal was not present. During continuous unit experiments, it was found that successful operation required careful regulation of the temperature of the feed line in order to control the slurry viscosity. The feed slurry viscosity was directly dependent on plastics content and inversely dependent on temperature. When the slurry viscosity was too high, the mixture could not be pumped; when it was too low, the coal would settle out of solution, causing plugging. Acknowledgment. This research was supported in part by an appointment to the U.S. DOE Fossil Energy Postgraduate Research Training Program at the Federal Energy Technology Center administered by the Oak Ridge Institute for Science and Education. Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favor by the U.S. DOE. EF9602077