Liquefaction and Dechlorination of Hydrothermally Treated Waste

Feb 23, 2011 - Motoyuki Sugano†‡*, Takayuki Shimizu†, Akihiro Komatsu§, Yusuke ... Naonari Shitara , Yuka Saitoh , Yusuke Kakuta , Katsumi Hira...
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Liquefaction and Dechlorination of Hydrothermally Treated Waste Mixture Containing Plastics with Glass Powder. Motoyuki Sugano,†,‡,* Takayuki Shimizu,† Akihiro Komatsu,§ Yusuke Kakuta,† and Katsumi Hirano† †

Department of Materials and Applied Chemistry, College of Science and Technology, Nihon University, Chiyoda-ku, Tokyo, Japan. Center for Creative Materials Research, Research Institute of Science and Technology, College of Science and Technology, Nihon University, Chiyoda-ku, Tokyo, Japan. § Glass Resourcing Inc., Choushi-shi, Chiba, Japan ‡

ABSTRACT: Additive effects of glass powder upon the product yields and chlorine distribution after liquefaction of hydrothermally pretreated mixed waste (HMW) are compared with liquefaction of HMW with any one of water, quartz sand, or glass powder plus water. As a result, addition of either water or quartz sand did not affect liquefaction and dechlorination of HMW. Further, water (5 g) addition did not enhance liquefaction and dechlorination of HMW with glass powder. On the other hand, after liquefaction of HMW with glass powder, the yields of chlorine in the gas and water insoluble constituents decreased and the chlorine yield in the watersoluble constituent increased significantly. Because sodium in glass powder dissolved in a small amount (0.5 g) of water resulted from dehydration of HMW during liquefaction. Further, hydrogen chloride derived from polyvinylchloride in HMW was neutralized by ion exchange between Hþ and Naþ dissolved in a small amount of water forming NaCl in the Residue (watersoluble) constituent. Therefore, most of chlorine in HMW was removed easily by water extraction of the Residue constituent after liquefaction of HMW with glass powder. Further, upgrading of HMW into the oil constituent was enhanced due to inhibition of production of chlorine containing organic compounds. Accordingly, it was clarified that glass powder was the most effective additive for liquefaction and dechlorination of HMW.

1. INTRODUCTION In FY 2007 in Japan, 73% of plastic waste was processed by means of thermal recovery, mechanical and feedstock recycling. However, 13% of plastic waste (1.2 Mt) was landfilled.1 Most of the landfilled plastic waste is a mixture with incombustible wastes (such as glass, metals, and ceramics) because plastic waste containing metals cannot be handled by the conventional recycling processes. For example, household appliances are a typical mixed waste (MW) composed of both plastic and inorganics. Separation of these waste mixtures into organic and inorganic substances is considered to be difficult and hazardous for the workers because medical waste is contained in the mixture. In FY 2003 in Japan, the remaining lifetime of final disposal sites for the municipal solid waste was estimated as 13.2 years.2 Therefore, an effective method for the separation of MW into the organic and inorganic substances is required. One of the effective methods, a hydrothermal treatment, is proposed for separation of MW into organic and inorganic substances. The steam-explosion process based on a sudden decompression of the contents of a hydrothermal reactor down to atmospheric pressure, follows after a preliminary hydrothermal treatment. After hydrothermal treatment of MW, organic substances become powdery, so that separation of MW into the organic and inorganic substances becomes easy. Organic substances in hydrothermally pretreated mixed waste (HMW) mainly consist of plastics; however, wastepaper and woody chips are also contained in HMW. Therefore, oil production by liquefaction is considered as a suitable means of utilizing the organic substances in HMW. It was reported that effects of liquefaction conditions upon liquefaction of polypropylene,3 polyethylene (PE),3,4 polystyrene r 2011 American Chemical Society

(PS),4 polyethylene terephthalate (PET),4 and mixture of PE/ PET/PS.4 Huffman et al. clarified that over 85% of commingled waste plastic 3,5 or post consumer plastic5 was converted into the oil constituent after liquefaction with HZSM-5 zeolite or several kinds of catalyst under the pressurized H2 gas at 445 °C for 60 min. Liquefaction of the municipal waste plastics from a town in Saitama, Japan was discussed by using a single-screw extruder.6 On liquefaction of HMW in a semibatch type reactor, the authors clarified that conversion of HMW to oil was enhanced by the increases both the liquefaction temperature to 350 °C and the holding time to 60 min.7 However, during liquefaction of HMW, corrosion on the surface in the reactor, and production of chlorine containing organic compounds were caused by formation of hydrogen chloride from HMW, because polyvinylchloride (PVC) is contained in MW. One of the recovery processes of chlorine in PVC, liquefaction of PVC with metal sorbents, such as lime,8 Fe,9 Zn,9 Ca/Zn carbonate,9 CuO,9,10 TiO2,9 Co3O4,10 MgO,10 iron oxides,11,12 red mud,13 carbon composites of Ca14 and FeCa,15 aluminum foil,16 NaOH,17 LiOH,17 Ca(OH)2,18 Al-Zn,19 and Al-Mg20 composites, were reported. In comparison with the chlorine content (398 ppm) in the oil constituent after pyrolysis (600 °C) of post consumer plastic, Huffman et al. clarified that the content was decreased to 54 ppm by addition of Na2CO3.5 On these liquefactions, chlorine in PVC was stabilized as metal chloride after the evolved HCl was reacted with the Received: October 29, 2010 Accepted: February 7, 2011 Revised: December 31, 2010 Published: February 23, 2011 2493

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Figure 2. Experimental scheme for liquefaction of HMW with water. Figure 1. Experimental without water.

scheme

for

liquefaction

of

HMW

added metal sorbents. During gasification (450-550 °C) of PVC with powdery glass, it was reported that most of chlorine generated from PVC was neutralized by sodium in the glass particles forming NaCl.21,22 Therefore, the authors are trying fixation of chlorine from hydrogen chloride as sodium chloride during liquefaction of HMW with glass powder. In this study, additive effects of glass powder upon the product yields and chlorine distribution after liquefaction of HMW are compared with liquefaction of HMW with any one of water (5 or 10 g), quartz sand or glass powder plus water (5 g).

2. EXPERIMENTAL SECTION 2.1. Samples. Hydrothermal treatment of MW was carried out in a similar manner reported in the previous paper.7 Hydrothermal treatment of MW discharged from the offices of our laboratory and Glass Resourcing, Inc. was carried out in the hydrothermal reactor located at Glass Resourcing, Inc. A sample was heated to 200 °C with water vapor, and the pressure in the reactor rose to 2.0 MPa. After the reactor reached 200 °C, the contents were discharged from the reactor and collected in a water tank. At this time, the steam-explosion process based on a sudden decompression of the contents took place. The products were pulverized less than 250 μm and then dried for 3 h under vacuum at 110 °C before the following liquefaction process. The waste glass sample was pulverized less than 75 μm. The quartz sand was commercially purchased from Wako pure chemical Industries, Ltd. and the particle size is between 600 and 850 μm. A part of quartz sand was pulverized less than 75 μm. Upon hydrothermal reaction, polyethylene (PE, high density, Mw: less than 125 000), polypropylene (PP, isotactic, average Mw: less than 190 000, average Mn: less than 50 000), polystyrene (PS, Mw: less than 280 000), and polyvinylchloride (PVC, Mw: less than 80 000, Mn: less than 47 000) were used without further purification. They were commercially purchased from Sigma-Aldrich. 2.2. Liquefaction of HMW without Water. The experimental scheme for liquefaction of HMW without water is summarized in Figure 1. Mixture of HMW (10 g) and 6 g of additive

(glass powder or quartz sand) was placed in a 100 cm3 autoclave with a magnetic drive agitator. After the autoclave was sealed, the autoclave was not purged by any pressurized gas. The autoclave was heated to 300 °C in an external electric furnace and the liquefaction process was maintained at the temperature for 60 min. After the liquefaction process was finished, the autoclave was air-cooled. The products remaining in the autoclave were recovered, filtered and rinsed with n-hexane under an ultrasonic irradiation. The n-hexane insoluble (Residue) material was prepared from the residue by drying for 3 h under vacuum at 110 °C. After n-hexane was evaporated from the filtrate, the nhexane soluble (Oil) constituent was obtained. The gas yield was calculated from the difference between the weight of the feed HMW and that of the recovered contents (oil and residue) on a dry ash free basis. 2.3. Liquefaction of HMW with Water. The experimental scheme for liquefaction of HMW with water is summarized in Figure 2. Mixture of HMW (10 g) with water (5 or 10 g) or glass powder (6 g) plus water (5 g) was placed in a 100 cm3 autoclave with a magnetic drive agitator. After liquefaction was carried out the same as in section 2.2, the products remaining in the autoclave were recovered, filtered, and rinsed with n-hexane under an ultrasonic irradiation. The Residue material was prepared the same as in section 2.2. The filtrate separated into the two phases. From the upper and bottom phases, the watersoluble (WS) and the n-hexane soluble (Oil) materials were obtained respectively. The gas yield was calculated from the difference between the weight of the feed HMW and that of the recovered contents (WS, Oil, and Residue) on a dry ash free basis. 2.4. Reaction of Glass Powder with Plastics. The experimental scheme for reaction of glass powder with plastics is summarized in Figure 3. An amount of 6 g of glass powder with either PVC (0.29 g) or mixture of PE (4.53 g), PP (3.09 g), and PS (2.09 g) was placed in a 100 cm3 autoclave with a magnetic drive agitator. The autoclave was heated to 400 °C in an external electric furnace and the reaction process was maintained at the temperature for 60 min. After the reaction process was finished, the autoclave was air-cooled. The Residue material in the product was prepared the same as in section 2.2. The reaction written above was carried out at 400 °C because a part of plastics did not melt on reaction at 300 °C. The mixed ratio of PVC, PE, PP, and 2494

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Figure 4. Product yield after liquefaction of HMW with additives. Figure 3. Experimental scheme for reaction of glass powder with plastics.

PS was determined as same as the ratio of breakdown of total plastic waste by resin type.23 2.5. Quantification of Chlorine. The chlorine contents in the products were quantified as follows.24 The Residue material was extracted with water under an ultrasonic irradiation to separate the water-soluble (WS) and insoluble (WI) constituents. The chlorine content in the Residue-WS constituent was directly quantified by ion chromatography. A portion of either Oil or WI constituent was burned at 1030 °C and absorbed by a NaOH aqueous solution containing 3% H2O2. The chlorine present in the solution was also quantified by ion chromatography. The chlorine contents in the constituents of the ResidueWI and Residue-WS were represented as water-insoluble chlorine (organic chloride) and water-soluble chlorine (inorganic chloride), respectively. The difference between the chlorine content in HMW and the sum of the chlorine contents in the products (Oil, Residue-WS, and Residue-WI) was estimated as chlorine in gas, such as the vaporized HCl and the other gaseous and volatile components. 2.6. Quantification of Sodium. The concentration of sodium in WS was determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES), SEIKO model SPS1200A (SEIKO Instruments Inc.). The following operating conditions were employed: RF power 1.3 kW; observing height 10.0 mm; plasma gas flow rate 18 L/min; carrier gas flow rate 3.0 mL/min; nebulizer flow rate 0.65 L/min. 2.7. Ultimate Analysis. The ultimate analysis (contents of C, H, and N) of HMW was carried out at the Center for Creative Materials Research in Nihon University Research Institute of Science & Technology.

3. RESULTS AND DISCUSSION 3.1. Additive Effect of Either Water (5 or 10g) or Quartz Sand upon Liquefaction of HMW. The ultimate analysis (dry

ash free basis) for HMW are as follows; C: 74.2, H: 9.1, N: 1.1, Cl: 1.7, O (diff.): 13.9, Ash (dry basis): 15.9%, H/C: 1.46, O/C: 0.14. The product yield after liquefaction of HMW with either quartz sand or water (5 or 10 g) is shown in Figure 4. There was little difference between the product yields of HMW only and HMW with water (5 or 10 g). With the increases of additive

Figure 5. Chlorine distribution after liquefaction of HMW with additives.

amount and the particle size of quartz sand, the yield of Oil decreased in compensation with the increase of the Gas yield. It was considered that gasification of a part of oil constituent occurred because the oil constituent adsorbed on the surface of quartz sand particles was heated locally through the particles of quartz sand during liquefaction. Chlorine distribution after liquefaction of HMW with quartz sand is shown in Figure 5. With the increases of additive amount and the particle size of quartz sand, the yield of chlorine in Residue-WS decreased in compensation with the increase of that of Gas. It was anticipated that absorption of hydrogen chloride on the surface of quartz sand particles was enhanced during liquefaction. After that, chlorination of a part of oil constituent occurred on the surface of quartz sand particles, followed by gasification of the chlorinated oil constituent as written above. Accordingly, it was clarified that addition of either water (5 or 10 g) or quartz sand did not enhance liquefaction and dechlorination of HMW. 3.2. Additive Effect of Glass Powder and Water (5 g) upon Liquefaction of HMW. The product yield after liquefaction of HMW with glass powder and/or water (5 g) is shown in Figure 4. In comparison with the product yield after liquefaction of HMW only, the Residue yield of HMW decreased by addition of glass powder and/or water (5 g). However, after liquefaction of HMW with glass powder plus water (5 g), the Gas yield increased significantly in compensation with the decrease of Oil yield. 2495

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Environmental Science & Technology Chlorine distribution after liquefaction of HMW with glass powder and/or water (5 g) is shown in Figure 5. In comparison with chlorine distribution after liquefaction of HMW only, the yield of chlorine in the Residue-WI constituent decreased significantly by addition of glass powder. Further, the yield of chlorine in the Gas constituent also decreased and that of the Residue-WS constituent increased significantly by addition of glass powder. After liquefaction of HMW with glass powder, 0.1, 1.5, and 0.1% of chlorine was observed in the Gas, Residue-WS and Residue-WI constituents. After liquefaction of HMW with glass powder plus water (5 g), the yield of chlorine in the Residue-WI constituent was extremely low, however, the yield of chlorine in the Gas yield increased significantly in compensation with the decreases of the yields of chlorine in the other constituents. Therefore, upon liquefaction of HMW with glass powder and/ or water (5 g), it was considered that hydrogen chloride from HMW immediately reacted with Naþ from glass powder, followed by formation of NaCl. The formed NaCl was separated as the Reside (WS) constituent after liquefaction. Further, the nonchlorinated organic constituents from HMW decomposed into the Oil and Gas constituents during liquefaction. As a result, in comparison with liquefaction of HMW only, the decreases of the Residue yield and the chlorine yield in Reside (WI) constituent were observed after liquefaction of HMW with glass powder and/or water (5 g). On the other hand, in comparison with liquefaction of HMW with glass powder, the yield of chlorine in Gas increased significantly after liquefaction of HMW with glass powder plus water (5 g). The pressure in the autoclave during liquefaction of HMW with glass reached to 1.0 MPa, while that with glass powder plus water (5 g) reached to 5.0 MPa. Therefore, upon liquefaction of HMW with glass powder plus water (5 g), it was anticipated that excess decomposition of the Oil constituent into the Gas constituent occurred under the high pressure liquefaction. As a result, in comparison with liquefaction of HMW with glass powder, the decreases of the Oil yield and the sum of the yields of chlorine in Residue-WS and WS constituents, and the increases of the Gas yield and the chlorine yield in the Gas constituent were observed after liquefaction of HMW with glass powder plus water (5 g). Accordingly, it was expected that water (5 g) addition did not enhance liquefaction and dechlorination of HMW with glass powder. The chlorine contents in the Oil constituent after liquefaction of HMW only, HMW with glass powder, HMW with glass powder plus water (5 g), and HMW with quartz sand were 120, 40, 80, and 100 ppm, respectively. Therefore, upon liquefaction of HMW with glass powder, it was clarified that the chlorine content in the Oil constituent is extremely low, and the Oil yield was high. Further, most of chlorine in HMW was fixed as NaCl in the Residue-WS constituent. Therefore, it was also clarified that most of chlorine in HMW was removed easily by water extraction of the Residue constituent after liquefaction of HMW with glass powder. Accordingly, it was clarified that glass powder was the most effective additive for liquefaction and dechlorination of HMW. 3.3. Amounts of Sodium and Chlorine in The Residue-WS Constituent after Liquefaction of Glass Powder with Plastics. The amounts of sodium and chlorine in the Residue-WS constituent after liquefaction of glass powder with plastics are shown in Figure 6. These amounts after liquefaction of glass powder only and that of glass powder with the mixture of PE, PP,

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Figure 6. Amounts of sodium and chlorine in the Residue-WS constituent after liquefaction of glass powder with plastics.

Figure 7. Amounts of sodium and chlorine in the Residue-WS constituent after liquefaction of HMW.

and PS were negligible. On the other hand, these amounts were very high after liquefaction of glass powder with PVC. Therefore, it was considered that elution of sodium from glass powder was enhanced by formation of hydrogen chloride from PVC. The sodium content of the glass powder was 12.1%. Estimating from the amount of sodium in the Residue-WS constituent after liquefaction of glass powder with PVC, the recovered sodium as the Residue-WS constituent was 11%. 3.4. Amounts of Sodium and Chlorine in the Residue-WS Constituent after Liquefaction of HMW. The amounts of sodium and chlorine in the Residue-WS constituent after liquefaction of HMW are shown in Figure 7. These amounts after liquefaction of HMW only and that of HMW with quartz sand were negligible. On the other hand, these amounts were very high after liquefaction of HMW with glass powder. Calculating from the amount of sodium in the Residue-WS constituent after liquefaction of HMW, the recovered sodium as the Residue-WS constituent was 10%. The dechlorination mechanism of HMW during liquefaction with glass powder in this study is considered as shown in Figure 8. During steam gasification (450-550 °C) of PVC with glass powder, it was reported that most of chlorine generated from PVC was neutralized by sodium in the glass particles forming NaCl.21 In the previous paper,7 water resulted in 5% yield after liquefaction (300 °C, 60 min.) of HMW only with a semibatch type reactor, because wastepaper and woody chips are also contained in HMW. Therefore, in this study, during liquefaction, sodium in glass powder dissolved in a small amount (about 0.5 g of water for 10 g of HMW) of water resulted from HMW as same as the previous paper.7 Moreover, hydrogen chloride derived from PVC in HMW was neutralized by ion exchange between Hþ and Naþ dissolved in a small amount of water forming NaCl. Therefore, the yields of chlorine in Gas and Residue-WI decreased and the chlorine yield in the Residue-WS increased 2496

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Figure 8. Dechlorination mechanism of HMW during liquefaction with glass powder.

significantly. Accordingly, most of chlorine in HMW was removed easily by water extraction of the Residue constituent. Further, upgrading of HMW into the Oil constituent was enhanced due to inhibition of production of chlorine containing organic compounds, which resulted the decrease of the Residue yield and the increases of the yields of the Oil and Gas constituents. Accordingly, glass powder was the most effective additive for liquefaction and dechlorination of HMW.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ81-3-3293-7572; e-mail: [email protected].

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