Received: February 14, 2012 / Accepted: March 28, 2012 / Published: June 20, 2012.
　　Abstract: Using novel catalyst the pyrolysis of mixed plastics has been considered as an effective way to convert waste plastics into environmental friendly and industrially useful hydrocarbon gas and liquid products. Catalytic cracking is a promising alternative for plastic wastes recycling. More than 99% of a polymer mixed converted into combustible hydrocarbon in a catalytic converting reaction. The products are mainly middle distillates. In this work equally weighted mixed HDPE (high density polyethylene), LDPE(low density polyethylene) and Polypropylene were degraded. The reaction occurred in a semi batch reactor at several temperatures and catalyst/polymer ratios in search for an optimum operating condition. The products are liquid and gaseous hydrocarbons with minor of residue. The liquid and gas products were in the range of middle distillate cuts of gasoline, kerosene and gas oil. Finally, with a metallic base, yielded 99.5% of given mixed to valuable middle distillate products that include 86% liquid hydrocarbon and 13.5% gas, ranging between C1 and C5 with less percent of residue. The optimum condition for this yield reports at a temperature of 450 oC and 10% of catalyst w/w at atmosphere pressure.
　　Key words: Environment, degradation, fuel, polyethylene, polypropylene, catalyst.
　　 1. Introduction
　　Plastic materials have a profound contribution towards the advancement in the recent technologies and new scientific achievements. It is unavoidable to use plastic materials because of their multiples applications in our daily life [1]. The increased use of different types of plastics has also increased its waste release into the environment. Used LDPE (low density polyethylene), HDPE (high density polyethylene) and PP from household and industries are recognized to be a major environmental problem [1]. Serious environmental problems are caused by these waste plastics because of their low density and large volume and so their non-biodegradability [2].
　　There are several methods for disposal of municipal and industrial plastic waste, i.e., landfill, incineration, true material recycling, and chemical recovery. Landfill treatment and incineration destruction are quite expensive and may raise problems with unacceptable emissions [3]. Mechanical recycling of the waste plastics has also been practiced but the recycled plastics are generally of low grade and less market value [2].
　　A promising solution to the waste plastics is chemical recovery where the polymers are thermally and catalytically converted into useful products that can be used as fuel oil [1-7].
　　The main drawbacks of thermal degradation are wide product distribution and requirement of high temperatures, typically more than 500 oC and even up to 900 oC. There is a growing interest in chemical recycling of waste polymers, because they can cause serious environmental problems, but can be converted to useful materials.
　　Some researchers have been done about degradation of the Polyethylene and Polypropylene thermocatalytically, using various catalysts. Different types of catalysts like H-ZSM-5, silica/alumina, FCC, CRT, HUSY and etc., have been used for the conversion of HDPE, PP (polypropylene) and LLDPE(linear low density polyethylene) into liquid fuel[2-6].
　　Kim has been studied thermogravimetric analysis of HDPE and PP thermal degradation. Kato et al. [7] have been studied the effect of two kind of catalysts on degradation of PP and PE. He selected the more effective catalyst with the production of the 86.6% liquid fuel at 500 oC [7].
　　Lopez has been studied the degradation of mixed polymers PE, PP, PS, PET, PVC with the composition of 40%, 35%, 18%, 4% and 3%, respectively. The degradation process has done in the presence of Red-Mud and ZSM-5 catalysts. Maximum yield of liquid was obtained in the temperature of 425 oC and presence of ZSM-5 and reported as 61.2%. Also Red-Mud at 440 oC yield was 79.3% liquid [4].
　　Their works also include gaseous production investigation. The maximum yield of the gas was obtained 41.3%, under 500 oC and ZSM-5 condition. All of the experiments have been done in the presence of 10 percent catalyst [4].
　　Ghoshal [8] has been studied the conversion of PP and LDPE mixture with different ratios. His research carried out in non-isothermal condition and the results were reported using thermogravimetric analysis. It reported some marked differences in the weight loss behaviors due to the interaction between the polymers.
　　Also it reported the quality of the products. He compared the yield of hydrocarbons in the range of C11-C13 for the different mixtures of polymers. Experiments indicate that the trend was as following:
　　LDPE > (PP(20%) + LDPE(80%)) > (PP(65%) + LDPE(35%)) > (PP(50%) + LDPE(50%)) > PP >(PP(80%) + LDPE(20%)) > (PP(35%) + LDPE(65%)).
　　Sakata et al. [9] also studied the catalytic degradation of the polyethylene and polypropylene. He used acid (SA-1, SA-2) and non-acid catalyst(FSM) and effect of each catalyst on conversion factor of the polymers was investigated [9].
　　Wang et al. [10] also investigated the catalytic degradation of LDPE (low-density polyethylene) and PP using a modified ZSM-5 zeolite, DeLaZSM-5.
　　In this work, catalytic degradation of LDPE, HDPE and PP mixed which represent the majority polyolefins, were carried out over the AIT100 catalyst that was manufactured and patented in the fuel research center of Abadan institute of technology. Maximum conversion of this mixed polymers and maximum yield of the liquid fuel product will be reported.
　　 2. Experimental
　　
　　2.1 Materials
　　The catalytic and isothermal decomposition were carried out for mixtures of PP, HDPE and LDPE. PP was supplied by Marun petrochemical company with the melt flow index of 16 Dg/min. HDPE and LDPE were supplied by Bandar Imam Petrochemical Complex with the melt flow index of 0.43 and 2.2 g/10min. The main properties of the catalyst (AIT100) shown in Table 1.
　　2.2 Experimental Procedure and Product Analysis
　　Each plastic sample was mixed with proportional amounts of waste HDPE: LDPE:PP = 42%:16%:42% based on 100 g of reactant. The proportion of plastics in the mixture was obtained from American chemistry Council Resin Statistics Summary 2010 vs. 2009 U.S. production, sales & captive use [11].
　　The degradation experiments were carried out in a semi-batch reactor (volume 1 L) at a constant degradation temperature under inert atmospheric pressure. The experimental conditions were as follows: a reactant amount of 100 g, a nitrogen stream of 25 cm3·min and heating rate of about 20 oC/min. Initially, the sample weighed and charged into the reactor. The reactor was sealed, purged with a nitrogen stream to remove the oxygen from the reactor, and then heated quickly at a determined temperature program. During the reaction, the gas products were vented after cooling by a condenser to 9 oC. The liquid products were measured by weight, as a function of lapse time of reaction.
　　The experimental set up has shown in Fig. 1.
　　 3. Results and Discussion
　　Different parameters like temperature and amount of catalyst were optimized for maximum conversion into fuel products and the parameter of temperature is discussed below.
　　Thermal and catalytic degradation reaction of polymers generally occurs through the most accepted radical chain mechanism where the polymer is converted into radicals at high temperatures [2].
　　
　　In the present study AIT100 has been used for the catalytic cracking of HDPE, LDPE and PP as a catalyst. The catalyst influences on the reaction condition and also on the products quality [12]. With the use of the catalyst, the temperature of the reaction decreases and C₆-C₁₀ products increases[7].
　　It means that catalytic converting of the polymers is more valuable and appropriate for producing more and qualitative liquid products in comparison on the thermal cracking of them [13, 14].
　　Degradation of polymers has been investigated by many researchers using fluidized bed reactor system[6, 12-15] and using acidic catalysts for comparison of the effect of catalysts [4, 8, 9, 16, 17].
　　On the other hand, some researchers decomposed polymer blends [4-9, 11, 18, 19] at different reaction conditions like pressure, temperature, polymer/catalyst ratio, type of catalyst and reaction time.
　　The innovative aspect of present work is the use of new catalyst (AIT100). Catalytic degradation of the sample was carried out for 100g polymer and 10 g of the catalyst at different temperatures (Fig. 2).
　　From Fig. 2, this is clear that by increasing the temperature, the conversion of the polymers is increased. On the other hand the coke production decreased with increasing of the temperature. Current results are close to report of other researchers [1, 5].
　　A further increase up to 450 °C caused a profound total percent conversion associated with the production of increased oil yield, wax and gases. Thus 450 °C temperature at which maximum conversion and maximum oil yield were achieved was considered as optimum reaction temperature under the given experimental setup.
　　
　　
　　Fig. 3 shows the evolution of the polymer conversion with time in the isothermal experiments carried out with mixed polymers.
　　As observed, an increase of temperature led to greater weight loss rate. When temperatures below 420 °C, even for long times, the solid conversion is low. However, the reaction is fast when temperatures of 440 °C and above, the maximum solid conversion being 99.5% at the 450 oC.
　　Other researchers reported the same results for degradation of polymers. Encinar also attempt to degrade the waste polymers and specifically polyethylene. The same experiments have been done by Ordedairo for analysis of benzene ethylation using ZSM-5 catalyst in a fluidized bed reactor. They were reported the same results for degradation of waste LDPE and Benzene respectively [20].
　　 4. Conclusion
　　The pyrolysis of mixed plastics has been considered as an effective way to convert waste plastics into environment friendly and useful hydrocarbon liquid products. Catalytic Cracking is a promising alternative for plastic wastes recycling. Poor quality mixed plastics can be converted into valuable liquids like automobile fuel. They might be an alternative source of chemicals for petrochemical processes. Additionally, rich gaseous products were generated, which can be used for power generation for the process and/or for external applications. The use of catalysts in pyrolysis of plastic wastes has a significant influence in both products yields and reaction condition. The main purpose of using catalysts in pyrolysis study is to improve the yield up to more than 99 percent. The study shows that the use of appropriate catalyst can cause to achieve more desired conversion percent of the waste plastics into useful products, which are nuisance materials for the environment.
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