Energy & Fuels 1997, 11, 801-812
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Effect of Reaction Time on the Coprocessing of Low-Density Polyethylene with Coal and Petroleum Resid Hyun Ku Joo and Christine W. Curtis* Chemical Engineering Department, Auburn University, Alabama 36849-5127 Received September 16, 1996X
The effect of reaction time on the reactivity, conversion, and product distribution of coprocessing reactions in systems containing LDPE (low-density polyethylene), coal, and heavy petroleum resid was evaluated by performing reactions at 30-360 min. Individual reactions of LDPE and binary combinations of LDPE and coal were also performed. LDPE reactions with reaction times that increased from 60 to 360 min resulted in conversions that increased from 39.5% at 60 min to 90.2% at 300 min. After 360 min the conversion decreased to 70.9%. Similar results were obtained with the LDPE and coal reactions; increased reaction time resulted in increased LDPE conversion as well as increased overall conversion. In both reaction systems, lighter reaction products and more gases were produced as reaction time increased. The conversions and hexane solubles produced for ternary systems, containing either Maya or Manji resid, also increased with increased reaction time. The reaction solids produced in these systems at different reaction times were analyzed by Fourier transform infrared spectroscopy and differential scanning calorimetry. Increased crystallinity that directly correlated with longer reaction times was observed in reaction solids from the LDPE reactions. The reaction solids from the LDPE/coal and LDPE/coal/resid systems also increased in crystallinity after reaction, but the crystallinity was not correlated directly with reaction time. The combination of higher catalyst loading and increased retention time to 90 and 120 min resulted in higher conversions as well as the production of lighter products.
Introduction Plastics are being produced and utilized worldwide at an increasing rate with each subsequent year.1 Plastics are manufactured for various uses including, but not limited to, consumer packaging, wires, pipes, containers, bottles, appliances, electrical/electronic parts, and automotive parts. Plastics are produced from petroleum and are composed primarily of hydrocarbons but also contain antioxidants and colorants.2 Plastics once used are not effectively recycled3 and are difficult to collect from the consumer and then to separate into specific types.4 Postconsumer plastics are disposed of by landfilling, thereby removing a potential hydrocarbon fuel or chemical feedstock source from the market. Tertiary recycling of the waste plastics produces fuels and chemical feedstocks from mixed waste plastics and offers an alternative to primary recycling where the plastics must be carefully separated in order to recover the monomer.5,6 The coprocessing of other hydrocarbon resources such as our most abundant U.S. hydrocarbon resource, coal, to the tertiary recycling of plastics will provide an additional source of hydrocarbon fuels and chemical feedstocks. Abstract published in Advance ACS Abstracts, June 1, 1997. (1) Resin Report. Mod. Plastics 1996, January, 70. (2) Leidner, J. Plastics Waste; Marcel Dekker: New York, 1991. (3) Porter, J. W. National Recycling Goal Met, But... Chemunique 1996, April. (4) Huffman, G. P.; Anderson, L.; Shah, N. Report on a Trip to Ascertain the Status of Feedstock Recycling of Waste Plastics in Europe. Consortium for Fossil Fuel Liquefaction Science, October, 16, 1995. (5) Leaversuch, R. D. Chemical Recycling Brings Real Vesatility to Solid-Waste Management. Mod. Plastics 1991, July, 40. (6) Miller, A. Back to Basics. Chem. Ind. 1994, 8 (2), 1. X
S0887-0624(96)00158-2 CCC: $14.00
The direct coprocessing of coal and waste plastics has been studied7-14 and has been shown to be difficult because of their chemical and processing incompatibility. Typical household plastic waste consists of ∼63% high- and low-density polyethylene (HDPE, LDPE), 11% polypropylene (PP), 11% polystyrene (PS), 7% polyethylene terephthalate (PET), and 5% polyvinyl chloride (PVC),15 causing these wastes to be highly aliphatic. By contrast, coal is highly aromatic. These differences in their chemistry cause the two materials to be incompatible during simultaneous coprocessing. (7) Luo, M.; Curtis, C. W. Thermal and Catalytic Coprocessing of Illinois No. 6 Coal with Model and Cominingled Plastics. Fuel Process. Technol. 1996, 49, 91. (8) Luo, M.; Curtis, C. W. Effect of Reaction Parameters and Catalyst Type on Waste Plastics Liquefaction and Coprocessing. Fuel Process. Technol. 1996, 49, 177. (9) Taghiei, M. M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Coliquefaction of Waste Plastics with Coal. Energy Fuels 1994, 8, 1228. (10) Taghiei, M. M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Coliquefaction of Waste Plastics with Coal. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (4), 810. (11) Huffman, G. P.; Feng, Z.; Mahajan, V.; Sivakumar, P.; Jung, H.; Tierney, J. W.; Wender, I. Direct Liquefaction and Coliquefaction of Coal-Plastic Mixtures. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (1), 35. (12) Anderson, L. L.; Tuntawiroon, W. Coliquefaction of Coal and Waste Plastic Materials to Produce Liquids. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (4), 816. (13) Palmer, S. R.; Hippo, E. J.; Tandon, D.; Blankenship, M. CoConversion of Coal/Waste Plastic Mixtures Under Various Pyrolysis and Liquefaction Conditions. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (1), 29. (14) Rothenberger, K.; Cugini, A. V.; Ciocco, M. V.; Anderson, R. R.; Veloski, G. A. Investigation of First Stage Liquefaction of Coal with Model Plastic Waste Mixtures. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (11), 38. (15) Erwin, L.; Healy, L. H., Jr. Packing and Solid Waste Management Strategy; American Management Association: New York, 1990.
© 1997 American Chemical Society
802 Energy & Fuels, Vol. 11, No. 4, 1997
The conditions and catalysts that are optimal for liquefying coal and for converting waste plastics are different,6,7 requiring some additional component or components in the reaction to improve the compatibility. Coprocessing of petroleum resid with coal has been studied extensively.16-21 Only in recent years have waste plastics been used as a material to coprocess with coal, resid, or other petroleum products.7-14,22-24 Luo and Curtis7,8 have investigated the conversion of waste plastics individually and in various mixtures with coal. Catalytic reactions, by use of fluid catalytic cracking catalysts and zeolite HZSM-5, were most effective for converting the solid reactants to tetrahydrofuran (THF) soluble material and producing a liquid product slate. However, the presence of LDPE and HDPE, which are the predominant postconsumer plastic waste, resulted in conversions lower than those of the other plastic materials. Ng24 has studied the conversion of polyethylene when it was blended at 5 and 10% levels with vacuum gas-oil (VGO) using thermal and catalytic cracking. Thermal cracking at 510 °C resulted in substantial conversion of polyethylene but in low overall conversion to gasoline. By contrast, high conversion to gasoline products was obtained by catalytic cracking, although the weight percent of the blend affected the overall product slate. Joo and Curtis22,23 have shown that adding heavy petroleum resid as a solvent in coprocessing reactions with waste plastics and coal has a beneficial effect on the reactivity of the system. The resid acts as an effective bridging solvent that, when added to coal and waste plastics, provides a medium for their mutual dissolution. The presence of resid in the coprocessing of coal and LDPE increased conversion and the amount of hexane soluble material produced. The conversion and product distributions obtained from coprocessing reactions of plastics with coal and resid showed that the composition and chemistry of the plastic used strongly affected the overall reactivity of the system. Robbins and co-workers25,26 have investigated the composition and character of waste streams from coprocessing reactions of coal and HDPE. A substantial amount of unconverted HDPE was recovered from an ash-free waste stream but no unconverted coal, indicating that HDPE was less reactive than coal (16) Curtis, C. W.; Pass, M. C.; Guin, J. A.; Tsai, K. J. Coprocessing of Coal in Heavy Petroleum Crudes and Residua. Fuel Sci. Technol. Int. 1987, 5, 245. (17) Guin, J. A.; Curtis, C. W.; Tsai, K. J. Effect of Solvent Composition on Coprocessing, Coal with Petroleum Residua. Fuel Process. Technol. 1987, 16, 71. (18) Curtis, C. W.; Guin, J. A.; Tsai, K. J. Catalytic Coprocessing: Effect of Catalyst Type and Sequencing. Ind. Eng. Chem. Res. 1987, 26, 12. (19) Monnier, J. Canmet Report 84-E, March 1984. (20) Speight, J. G.; Moschopedis, S. E. The Co-Processing of Coal with Heavy Fractions. Fuel Process. Technol. 1986, 13, 215. (21) Huang, J. S.; Curtis, C. W. The Reactivity of Different Coal and Residuum Combinations in Coprocessing. Fuel Process. Technol. 1992, 30, 47. (22) Joo, H. K.; Curtis, C. W. Coprocessing of Waste Plastics with Coal and Petroleum Resid. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (1), 92. (23) Joo, H. K.; Curtis, C. W. Catalytic Coprocessing of Plastics with Coal and Petroleum Resid Using NiMo/Al2O3. Energy Fuels 1996, 10 (3), 603. (24) Ng, S. H. Conversion of Polyethylene Blended with VGO to Transportation Fuels by Catalytic Cracking. Energy Fuels 1995, 9, 216. (25) Robbins, G. A.; Winschel, R. A.; Burke, F. P. Determination of Unconverted HDPE in Coal/Plastics Co-Liquefaction Stream Samples. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (3), 1069. (26) Robbins, G . A.; Winschel, R. A.; Burke, F. P. Characterization of Coal-Waste Coprocessing Samples from HRI Run POC-2. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (1), 92.
Joo and Curtis
under the liquefaction conditions used. The researchers recovered the HDPE by washing the solids with THF, filtering the solids, and washing the filter cake with THF. In streams where unconverted coal, HDPE, and coal ash were present, HDPE was more difficult to recover. Robbins developed an analytical procedure using a hot Decalin extraction that was applied in conjunction with THF extractions to remove coal materials from the sample. Rothenberger et al.27 examined polyethylene (PE) degradation under coal liquefaction conditions and developed a method to recover incompletely reacted polyethylene from coprocessing product streams. The method involved removing coal-derived material by a cold THF wash. The remaining solid that contained PE was subjected to a hot decane wash and was then filtered and concentrated. Then a dichloromethane wash was used to remove any coal-derived materials and to assist in producing a beige, powdery solid. Analysis of the recovered solids by nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) showed that the separation procedure method was effective and, although the PE was unconverted to THF solubles in the reaction, PE had undergone reaction. This study is on the evaluation of the effect of reaction time on the coprocessing reaction of LDPE with coal and heavy petroleum resid. The reaction times ranged from 30 to 360 min with the intent of determining if a longer reaction time was beneficial to LDPE conversion. Three systems, composed of LDPE, LDPE/coal, and LDPE/coal/ resid, were examined. After these three reaction systems were reacted at times ranging from 30 min to 6 h, each reaction system was subjected to solvent fractionation, simulated distillation of the hexane solubles, and analysis of the solid products remaining after reaction. The conversion of the solids to THF soluble material and the amount of hexane soluble material produced increased generally with reaction time. The unconverted solids from each of the reaction systems were analyzed by Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) in order to determine the effect of reaction time on their composition and crystallinity. The crystallinity of the solid reaction products was studied to determine if any changes occurred in the unconverted LDPE during reaction or if the unconverted LDPE essentially remained the same as in the starting material. Experimental Section Materials. The model plastic low-density polyethylene (LDPE) used in this study was obtained from Aldrich Chemical Co. as a powder of ∼40 mesh and was used as received (MW ≈ 55 000). The coal used was Blind Canyon bituminous coal (DECS-17) obtained from the Penn State Coal Sample Bank. The resids used were Manji and Maya obtained from Amoco. The resids contained different contents of metal and amounts of asphaltenes: 25.2% for Maya and 5.2% for Manji. Analyses of both the coal and resids are given in Table 1. The catalyst used in this study was a commercial grade NiMo/Al2O3 obtained from Shell and was composed of 2.72 wt % Ni and 13.16 wt % Mo. The catalyst, ground to ∼60 mesh, was presulfided external to the reactor prior to use. The procedure for presulfiding NiMo/Al2O3 began by predrying (27) Rothenberger, K. S.; Cugini, A. V.; Thompson, R. L. Polyethylene Degradation in a Coal Liquefaction Environment. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (3), 1062.
Coprocessing of Polyethylene
Energy & Fuels, Vol. 11, No. 4, 1997 803
Table 1. Properties of Blind Canyon DECS-17 Coala and Manji and Maya Residsb Properties of Blind Canyon DECS-17 Coal proximate (wt %) fixed carbon volatile matter ash moisture
45 45 6.3 3.7
ultimate (wt %) C H S N Cl
82.1 6.2 0.4 1.4 0.12
Properties of Resids C, wt % H, wt % N, wt % S, wt % V, ppm Ni, ppm V, ppm
Manji
Maya
85.1 10.8 0.7 2.6 231 220 23
84.1 9.9 0.7 5.1 550 100 17
a Obtained from the Penn State Coal Sample Bank. b Obtained from Amoco Oil Co.
NiMo/Al2O3 in flowing N2 for 1 h at 300 °C. Then a 10 vol % H2S/H2 gas mixture was flowed over the catalyst at 225 °C for 1 h followed by 315 °C for 1 h and 370 °C for 2 h. The final step involved flowing N2 at 370 °C over NiMo/Al2O3 for 1 h after which the catalyst was then allowed to cool to room temperature under a N2 flow. The solvents used for extraction analyses were HPLC grade hexane, toluene, and THF from Fisher Scientific. Spectranalyzed grade carbon disulfide (CS2) from Fisher Scientific was the solvent of choice for simulated distillation of hexane solubles. A column resolution test mix, paraffin calibration mix, and gas-oil mix from Supelco were used as calibration standards for simulated distillation. Fluorolube and Nujol obtained from Fisher Scientific and Aldrich, respectively, were used for making mulls of the solid materials produced in the reactions for FTIR analysis. Analyses of the LDPE before reaction were performed and used for comparison. Reactions and Procedures. Reactions were performed for systems of LDPE, coal/LDPE, and coal/LDPE/resid to evaluate the reactivity, product distribution, and conversion in the three reaction systems. The effect of reaction time on these measures was evaluated by increasing the reaction time from 30 to 360 min using 30 to 60 min intervals. Unless otherwise noted, all reactions were performed using 1 wt % presulfided NiMo/Al2O3 powder on a total charge basis. The reactions were conducted in ∼20 cm3 stainless steel tubular microreactors at 430 °C with 8.3 MPa of H2 introduced at ambient temperature. The microreactors were agitated horizontally at 450 rpm in a heated fluidized sand bath and were immediately quenched in water after reaction. The reactant loadings for individual reactions were 1.0 g of coal, 1.0 g of polymer, or 1.5 g of resid. For the two- and three-component systems, combinations of the aforementioned amounts were used. A total loading of 2.0 g was used for the LDPE/coal and 2.5 g for the coal/resid and LDPE/resid reactions. For the ternary system of LDPE/coal/resid a total loading of 3.5 g was used in each reaction. The coal was stored in a vacuum desiccator before being used. Averages of these data and their standard deviations are presented in the tables. Additional replications were performed on a random basis. After the entire set of reactions was completed, the reactions at 300 and 360 min were replicated, since these reactions produced marked changes in the conversion of the reactants and in the amount of hexane solubles produced in the reactions. A second set of replications of the 300 and 360 min reactions were performed 1 month after the first replications of 300 and 360 min experiments were performed. All these data are averaged in the tables and figures except for the two sets of data points at 300 min presented in Figures 2 and 3.
The effect of catalyst loading on selected reaction systems was also evaluated. Catalyst loadings of 1, 2, 3, and 10 wt % were used selectively for reaction times of 60, 90, and 120 min. High gas make limited the catalyst loading at the longer times and precluded using loadings higher than 1 wt % at reaction times greater than 120 min. Analysis. After the reaction, gaseous reaction products were determined by weighing the tubular microreactor before and after the release of gaseous products; this fraction in the product distribution is called gas. The liquid and solid products were analyzed by solvent fractionation, fractionating the reaction products by using a series of solvents of increasing polarity and strength. The fractions obtained were hexane soluble materials (HXs), toluene soluble and hexane insoluble materials (TOLs), THF soluble and toluene insoluble materials (THFs), and THF insoluble material or IOM, which is defined as insoluble organic matter that is calculated on an ash-free basis. The solid reaction product (IOM) from LDPE varied in consistency with reaction time, ranging from being waxy to being in small pieces or clumps of material; when resid was added to the reaction, the reaction product was no longer waxy. The solids from the reactions of LDPE and LDPE/coal were difficult to separate by centrifuging and decanting the liquids from the solids according to the fractionation procedure typically used in our laboratory. Consequently, these solids were filtered using a Whatman no. 2 filter paper. The masses of the HXs, TOLs, and THFs fractions were determined after the fractionating solvent was removed by rotary evaporation, and the fractions were air-dried for 15 h. The weight percentage of each of these fractions was determined and used in the recovery calculation. Then the weight percentage of the HXs fraction was determined by difference and was defined as 100 - (gas + TOLs + THFs + IOM). Although the actual weight of the HXs fraction was determined and used in the recovery calculation, the HXs calculated by difference was used in the product distribution because of the large loss of volatiles that occurred during product workup.23 The recovery from these reactions was defined as
% recovery )
grams of output × 100 grams of input
(1)
The solubility of the unreacted materials was also determined. The coal, a solid at room temperature, was essentially insoluble in THF. LDPE, also a solid at room temperature, has limited solubility (4.5%) in THF, and resids, semisolids at room temperature, are totally soluble in THF and are also quite soluble in hexane, yielding 87.1% HXs for Manji and 63.0% HXs for Maya. The recoveries from the unreacted materials were greater than 100%, which was most likely caused by incorporation of the extraction solvents into the systems. The definition for conversion used in this study is the conversion of the total reactants of each reaction on a moisture and ash-free (maf) basis to THF soluble material:
[
% conversion ) 1-
]
grams of IOMmaf × 100 (2) grams of total chargedmaf
The reactions and analyses were at least duplicated. Simulated Distillation Analysis of Hexane Solubles. The boiling point range of the HXs fractions from the different reactions was obtained by simulated distillation using the ASTM D-2887 method and a Varian 3700 gas chromatograph equipped with a DB-5 fused silica capillary column from J&W Scientific and an HP3396A integrator from Hewlett-Packard. The system was connected to a personal computer equipped with Peak9600 software to collect data from the integrator. Boiling point distribution data were calculated using Microsoft Excel software. The simulated distillation was performed by injecting 0.4 µL of solutions of HXs material dissolved in CS2 onto the column and analyzed by calculating the accumulated weight percentage of the sample versus the boiling point.
804 Energy & Fuels, Vol. 11, No. 4, 1997
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Excel software was used for data analysis, which consisted of 9601 data points, since five data points were collected per second for 32 min. The temperature program used for the simulated distillation started at an initial temperature of 50 °C that was held for 1 min and then increased at a rate of 17 °C/min to 330 °C and held at the final temperature for 15 min. The column resolution obtained using this column and these conditions was 6.8 as calculated by ASTM D-2887 and was in the acceptable range 3-8. A comparison of the retention times obtained from the standard gas-oil mixture data supplied by Supelco has been reported previously.23 A calibration line relating the boiling point to the retention time was obtained using a calibration gas-oil mixture obtained from Supelco, and the equation for that line is given as
bp(°C) ) 19.91953 × RT(min) + 55.03439
(3)
where bp is the boiling point and RT is the retention time. The R2 value for the calibration line was 0.9987 and the reproducibility of the retention time was within 0.07 and usually within 0.02. A comparison of the weight percentage for several selected boiling points obtained from the standard gas-oil mixture data supplied by Supelco with that obtained in our laboratory showed good agreement. Weight percentage differences of 0.3-5.8 wt % were obtained between the two sets of data. These values tended to increase at higher boiling points. Recovery of the HXs fraction from the column was calculated by the method described in our previous work.23 Reproducibility of the system was established by analyzing CS2 and biphenyl solutions repeatedly. The areas obtained for CS2 varied within (1.2% while those for biphenyl were within (3.1%. Since linearity is required for the recovery calculation, the linearity of the chromatographic system was determined using biphenyl solutions of varying concentrations. An assumption was made that biphenyl eluted completely. The line produced from plotting area versus concentration represented 100% recovery. The HXs from the different unary, binary, and ternary reactions were analyzed using the assumption that the HXs fraction was predominately composed of hydrocarbons and that the response factors for the compounds present were similar. These assumptions were reasonable considering that the hexane solubles were derived primarily from LDPE and resid and because the GC response of the HXs showed a good fit to a linear relation between area and concentration. Lines of area versus concentration obtained from the HXs showed lower slopes than that from biphenyl and represented samples with less than 100% recovery. Therefore, the ratio of the sample slope to the biphenyl slope was considered to be the recovery of the sample:
% recovery )
slope of sample line × 100 slope of biphenyl line
Figure 1. Schematic diagram of mass balance calculation for total boiling point distribution.
(4)
Each sample was injected immediately after it had been prepared. After the simulated distillation recovery was determined, the weight percentages of HXs materials boiling in the ranges ibp-100 (where ibp is the initial boiling point), 500 °C portion. Deviations of hexane solubles were mostly within (1.5 wt %, simulated distillation recoveries ( 5.0 wt %, and toluene solubles, THF solubles, and IOMs were within (2.0 wt %. b 430 °C, 8.3 MPa; 1 wt % of presulfided NiMo/Al2O3 used; 1 g of coal, 1 g of LDPE, and 1.5 g of resid were charged. c ibp ) initial boiling point.
amount of gas that was produced increased substantially with higher catalyst loading so that for the 120 min reaction the highest catalyst loading used was reduced to 2 or 3 wt % depending on the reaction system. The effect of the catalyst loading in terms of conversion and product distribution is given in Table 9. At 60 min reaction time, the effect of increased catalyst loading on the LDPE/coal and LDPE/coal/Manji system was minimal as was previously reported.23 The conversion to THFs material decreased slightly with increased loading. Only small variations were observed
in the different product distributions. After 90 min of reaction, substantial increases in conversion and corresponding increases in gas production were observed. In the LDPE/coal and LDPE/coal/Maya systems, large increases in gas make were observed with increased catalyst loading of 3 and 2 wt %, respectively. At 120 min of reaction, the effect of increased catalyst loading on conversion depended on the reaction system. The LDPE conversion more than doubled from 42.7% to 94.7% when the catalyst loading was increased from 1 to 3 wt %, while the LDPE/coal conversion increased
812 Energy & Fuels, Vol. 11, No. 4, 1997
from 60.9 to 96.2% at the same level of increase. In both reaction systems the amount of gas produced increased substantially, though the LDPE/coal system gave the largest increase. In the coprocessing systems of LDPE/ coal/resid, increasing the catalyst loading from 1 to 2 wt % had little effect on the conversion in either system but resulted in a large increase in gas production, particularly in the Manji coprocessing system. Combined Product Distribution and Simulated Distillation Analysis of Catalytic Coprocessing Reactions at Different Catalyst Loading. The effect of catalyst loading in conjunction with increased reaction time was evaluated in terms of the total boiling point distribution given in Table 10. The total boiling point distribution is a combination of the solvent fractionation and simulated distillation of the HXs fraction as discussed previously. The effect of catalyst loading on the boiling point distribution was strongly affected by reaction time. At 60 min, the three catalyst loadings of 1, 3, and 10 wt % had little effect on the boiling point distributions from either the LDPE/coal or LDPE/coal/Manji systems. Some fluctuations occurred in the amount of each boiling point range. However, no trends were apparent with loading level. By contrast, at longer reaction times of 90 and 120 min, increasing the catalyst loading from 1 to 2% substantially increased the amount of gas produced. In the LDPE/coal system at reaction time of 90 and 120 min, the amount of material boiling
