Steam Reforming of the Oils Produced from Waste Plastics

Steam reforming of the oils produced from waste plastics 148 Toshiro Tsuji, Satoshi Okajima, Akira Sasaki, Teruoki Tago, and Takao Masuda Division of Materials Science and Engineering Graduate School of Engineering, Hokkaido University Kita-ku N13W8, Sapporo, Japan 060-8628 E-mail:[email protected] Tel & Fax +81-11-706-6551

ABSTRACT

The steam reforming to produce hydrogen from the oils derived from the waste plastics such as polyolefins has been investigated. The polyethylene oil and the polystyrene oil were produced by the decomposition of each plastic pellets at relatively low temperature (350-450℃). The experiments of steam reforming of these oils were carried out at the temperature from 650℃ to 850℃ using commercial Ni/Al2O3 catalyst. Gas yield, gas composition, carbon conversion and coking characteristic were investigated. Both oils from polyethylene and polystyrene were well gasified with very high carbon conversions and low coking rates at the temperatures higher than 800℃. The gas compositions were well agreed with the calculated equilibrium compositions. The coking rate of polyethylene oil was less than that of polystyrene oil and the coking rate was the lowest at 800℃.

KEYWORDS steam reforming, waste plastics, hydrogen

INTRODUCTION

In Japan, with the enforcement of Packaging Recycling Law, several waste plastic liquefaction plants have been constructed together with other recycling processes such as reduction in blast furnace (Plastic Waste Management Institute Home Page). The liquid products from the plant are used for fuel oil at the present time. Gasification plant of waste plastics by partial oxidation is also constructed and is considered to be one of the promising methods of feed stock recycling. However, the partial oxidation is operated at very high temperature; more efficient energy recovery is desirable. In this study, the method of production of hydrogen from waste plastics by steam reforming is discussed which enables recovery of energy with very high efficiency combining with the use of fuel cell. The production of hydrogen containing gases by reacting hydrocarbons with steam in the presence of a catalyst is well known process which has been established since the 1930s. Many studies have been reported for the steam reforming of natural gas, naphtha, and other hydrocarbons. (Ooki, and Morita, 1965) However the reforming of the oil derived from waste plastics is not reported hitherto. Although the waste plastics oil contains heavy molecules as it contains much oligomers, their monomers are light hydrocarbons and seem to react easily with steam in the presence of catalyst. However plastic oil contains many components and their heavy components may become a cause of coking which reduces the activity of catalyst. In this work, the performance of the steam reforming of the oils produced from polyethylene and polystyrene was investigated with a small scale reactor, as these two plastics are the primary component of waste plastics. MATERIALS AND METHODS

Preparation of oil Feedstock oils were made by the thermal decomposition at the temperature 380-450 ℃ from virgin pellets of low density polyethylene (Mitsubishi Kasei , M430) and polystyrene (A&M Styrene, G8102 K). Thus produced polyethylene oil contains primary paraffins and 1-olefins from C5 to C25 around. Polystyrene oil contains mostly styrene monomer and some dimer and trimer. More detail of these oils is elsewhere (Tsuji et al. 1999, 2001). Hexane and styrene monomer (purchased from Wako Junyaku) were also used without any pretreatment for comparison with plastic oils.

Preparation of Catalyst Catalyst used in this experiment is commercial Ni/Al2O3 catalyst (N134) purchased from Nikki Kagaku. This catalyst has originally cylindrical shape (17mmφx 17mm) with a hole. It was crashed and sieved with screens of opening sizes 2.38mm and 4mm. Then the catalyst was pre reduced in hydrogen stream for three hours at the temperature of 600℃.

Experimental apparatus and method of steam reforming Fig.1 shows the experimental apparatus for steam reforming. ← Oil The reactor is a quartz tube with inside diameter of 10mm and Thermo- ← Steam length 600mm. The reactor tube is heated by tubular electric couple Quartz tube heater. Three grams of catalyst was packed in the reactor tube. The catalyst was held in the tube by being sandwiched between Electric heater quartz wool. Feedstock oil and steam were fed continuously to Quartz wool this upper packing wool. Feed rate of oil was fixed at about LHSV= 1/hr. Steam ratio R which is defined in eq.(1) was Catalyst varied from 3 to 5.

moles of steam R = (1) atoms of carbon in the feedstock oil

The reaction temperature was in the range 650- 850 ℃. Product gas was cooled in the ice water trap and flowed into the gas bag after being separated by water. The gas bag was changed at given time. The volume of gas and its component Ice Water were measured by gas meter and gas chromatography trap Gas bag respectively. The coking weight was measured from the weight difference of the quartz tube + catalyst before and after the experiment. All experiments were carried out under the pressure of 1atm.

RESULTS AND DISCUSSION Fig.1 Experimental apparatus

Comparison of Pyrolysis and Steam reforming If no catalyst is packed in the reactor, the reaction turns to only pyrolysis instead of steam reforming. Both reactions are expressed by following equations. Where, CmHn is designated as the component of plastic oil. Pyrolysis;

→ + + + +⋅⋅⋅ (2) CmHn aH2 bCH4 cC2H4 dC2H6

Steam reforming; (3) 1 + → + ⎛ n + ⎞ C m H n H 2O CO ⎜ 1⎟H 2 m ⎝ 2m ⎠ + ←⎯→ + (4) CO H 2O CO 2 H 2 + ←⎯→ + (5) CO 3H 2 CH 4 H 2O

Fig.2 shows the comparison of gaseous Pyrolysis products both of the pyrolysis and the steam reforming of polyethylene oil (PE Steam oil). reforming In the case of the pyrolysis, gas yield was only 60 wt %( the rest was oil 0 0.2 0.4 0.6 0.8 1 product) and the gas components were Gas yield (wt/wt) various hydrocarbons as described in eq.(2). The experimental result showed CH4 C2H6 C3H6 the primary components were methane, ethylene and propylene. Pyrolysis2 C2H4 Whereas, in the case of steam reforming, gas yield exceeds 95wt% H2 CO CH4 and the gas components are primarily Steam CO2 hydrogen, carbon monoxide, carbon reforming1 dioxide and methane. This means the steam reforming reaction (eq.(3)) is very fast in the presence of catalyst and 0 0.2 0.4 0.6 0.8 1 plastic oil can be easily gasified. And Gas composition (wt/wt) produced CO reacts with steam to become CO2 according to eq.(4). Fig.2 Comparison of gas yield and gas composition for pyrolysis and steam reforming (PE oil,700℃,R=3.5)

Steam reforming of polyethylene oil and polystyrene oil We define the carbon conversion for this reaction as follows.

moles of CO + CO + CH in the product gas Carbon conversion CC = 2 4 (6) atoms of carbon in thefeedstock oil

Fig.3 shows the carbon conversion for low density polyethylene oil (PE oil). The carbon conversion increased with increasing temperature and at higher temperature, the carbon conversion exceeded more than 90% and no deactivation of catalyst was observed in a few hours of reaction. Fig.4 shows the carbon conversions 1 of polystyrene oil (PS oil) at various 850℃ temperatures. The carbon conversion 800℃ 0.9 ℃ of PS oil is lower compared to that of 750 PE oil. (Fig.3) 0.8 700℃ Fig.5 shows the comparison of the carbon conversion and coking weight CC 0.7 650℃ at 1 hr between PE oil and PS oil. Coking weight is the amount of carbon coked for 1hour per unit mass 0.6 of catalyst. The carbon conversion of PS oil is less than PE oil at any 0.5 temperature. This means aromatic oil 0 20406080 is less reactive than aliphatic oil. Reaction time [min] However coking weight was not so Fig.3 Carbon conversion of the steam reforming of much different between PE oil and polyethylene (PE oil, R=3.5) PS oil. At the temperatures higher than 800 ℃ , the carbon 1 conversions of both oils were very high and both weights of coking 800℃ 0.9 were very low. 850℃ At 650 ℃, both carbon conversion were very low (less than 75wt%) 0.8 CC 750℃ and coking weights were increased largely. This is because the 0.7 700℃ decomposition of PE and PS oil is lowered at low temperature and the 0.6 amount of unreacted oil increases 650℃ in addition to the decreasing of steam reforming reaction rate. 0.5 020406080 Reaction time [min] Gas component Fig.6 shows the experimental Fig.4 Carbon conversion of the steam reforming of product gas components for both polystyrene (PS oil, R=3.5) PE oil (open mark) and PS oil (close mark). The solid lines and the dotted lines are calculated values based on the equilibrium constants of eq.(1) and eq.(2). The solid lines are the values for PS oil and the dotted lines are those for PE oil. As PE oil contains more hydrogen than PS oil, the resulted hydrogen fraction of PE oil was larger than PS oil. Moreover, by the water gas shift reaction (eq.(2)), higher hydrogen content will result lower CO2 content and higher CO content. As a result, CO2 fraction of PE oil was less than that of PS oil. Thus, the experimental results had good agreement with calculated ones as shown in Fig.6.

Steam reforming of hexane and styrene monomer For comparison with the plastic oils, the experiments of steam reforming of hexane and styrene monomer were carried out. Fig.7 shows the result of PE oil and hexane. At 650℃, the carbon conversion of PE oil decrease extremely compared to that of hexane. This is because the PE oil contains more heavy oil and is hardly decomposed at low temperature which reduces steam reforming reaction and results increasing of unreacted oil and also 1 0.2 increasing of coking. However, CC ]

at higher temperature than ）

0.9 h

800℃, the carbon conversion of 0.15 ・ PE oil exceeds than that of LDPE hexane. The reason of this is 0.8 maybe that the decomposed PS 0.1 [g/(g-cat gases of PE oil are likely to 0.7 t reactive than those of hexane. CC The coking weight of hexane Coking 0.05 is slightly lower than that of PE 0.6 oil. Especially at 800 ℃ , the coking of hexane oil was 0.5 0 negligibly small. Coking weigh 600 650 700 750 800 850 900 Fig.8 shows the comparison between PS oil and styrene Temperature [℃] monomer. PS oil contains much styrene monomer because of Fig.5 Carbon conversion of the steam reforming and deploymerization reaction. coking weight of plastic oils (R=3.5)

0.8 0.7 H2(PS) CO2(PS) 0.6 CH4(PS) CO(PS) 0.5 H2(PS,exp) CO(PS,exp) 0.4 CH4(PS,exp) CO2(PS,exp) 0.3 H2(PE)

Molar fraction Molar CO2(PE) 0.2 CH4(PE) 0.1 CO(PE) H2(PE,exp) 0 CO(PE,exp) CH4(PE,exp) 500 600 700 800 900 CO2(PE,exp) Temprature[℃]

Fig.6 Product gas components for PE oil and PS oil and comparison with calculated equilibrium values in eqs.(4) and (5) The carbon conversion of PS oil was higher than that of styrene at 1 0.2

] low temperature and lower at high CC ） temperature. Both coking weight h 0.9 Hexane ・ had similar trend. There is not 0.15 good explanation of this. For (g-cat

further discussion more data would 0.8 ・ LDPE oil CC be required. 0.1 [g t 0.7 CONCLUSION 0.05 0.6 Though the oils derived from Coking plastics contains a large amount of high molecular weight components, 0.5 0 Coking weigh they decomposed easily at higher 600 650 700 750 800 850 900 temperature than 800℃ and the ℃ carbon conversion of the steam Temperature[ ] reforming was very good with Fig.７Comparison between PE oil and Hexane small coking rate. Therefore production of hydrogen from waste 1 0.2 plastics by steam reforming will be h)] one of the promising process of 0.9 Styrene ・ recycling or recovery of energy. 0.15 Collection and transportation of PS oil waste plastics are very difficult 0.8 CC problems because bulk density of 0.1 waste plastics is very small. So, the 0.7 efficient small scale energy CC recovery is desirable for waste Coking 0.05 plastics. In this sense, the 0.6 production of hydrogen and power generation with fuel cell would be Coking weight [g/(g-cat one of the most suitable energy 0.5 0 recoveries from waste plastics. 600 650 700 750 800 850 900 Temperature [℃] REFERENCES Fig.8 Comparison between PS oil and Styrene

1) Plastic Waste Management Institute Home Page, http://www.pwmi.or.jp (accessed April 2004) 2) Ooki,T. and Y. Morita; Catalytic Reactions (4) , A Series of Monographs on the Science and Engineering of Catalysis 9,pp. 195-247, Chijinshokan, Tokyo(1965) 3) Tsuji, T., O. Uemaki and H. Itoh; ”Two-stage thermal gasification of polyethylene (in Japanese),” Chemical Engineering, 44 , 697-701(1999) 4) Tsuji T., Y. Tanaka and H. Itoh; ”Two-Stage Thermal Gasification of Polyolefins,” J Mater Cycles Waste Manag, 3, 2-7 (2001) 5) Tsuji, T., K. Hasegawa and T. Masuda; ”Thermal Cracking of Oils from Waste Plastics,” J Mater Cycles Waste Manag, 5, 102-106 (2003)