Experimental study of catalytic pyrolysis of polyethylene and polypropylene over USY zeolite and separation to gasoline and diesel-like fuels

Highlights

• Non-catalytic and catalytic cracking of PE and PP over USY catalyst in a batch reactor was studied.

• Non-catalytic pyrolysis of PP contained branched hydrocarbons showing a preferential cracking position.

• The liquid product derived from catalytic pyrolysis of PP and that of PE was respectively rich in C₅-C₁₁ and C₁₀-C₁₃ compounds.

• After separation at 170 °C, Light and heavy fractions of pyrolytic oils of PE and PP met the standards of gasoline and diesel fuels.

Abstract

Thermal catalytic and non catalytic degradation of polyethylene (PE) and polypropylene (PP) were carried out in a batch reactor at 450 °C. The major product obtained from the non-catalytic pyrolysis of PE was 80 wt% of wax, while the degradation of PP without any catalyst produced 85.5% of liquid. In catalytic degradation, ultra-stable Y (USY) zeolite was used in a ratio of 1:10, with respect to PE and PP. In both cases, polymers were converted to liquid products with high yields (71 and 82 wt% respectively) and a low amount of coke deposit has appeared. The liquid fraction derived from the catalytic pyrolysis of PE was a mixture of C₅-C₃₉ compounds and that of PP was a mixture of C₅-C_30. This wide range of components suggests the separation of liquids into two phases in order to use them as diesel-like and gasoline-like fuels. Three temperatures of separation (130, 150, 170 °C) were tested and the physical properties were compared. Among the tested temperatures, 170 °C was the optimal temperature of separation where the distillation curves and physical properties of the light and heavy phases fitted completely those of gasoline and diesel fuels, respectively. After separation, the gasoline-like fuel which represents 60.6% of the total oil of PP and 57% of that of PE had high octane numbers (RON = 96 and RON = 97, respectively), while the diesel-like fuel which accounts for 36.5% of the total oil of PP and 35.3% of that of PE have high calculated cetane numbers (52 and 53, respectively).

Introduction

Due to rapid urbanization and economic development, the world’s annual production of plastic materials has increased from around 1.7 million tons in the 1950s to nearly 311 million tons, of which 59 million tons are accounted for by Europe alone [1]. Due to the increase in generation, waste plastics are becoming a major stream in solid wastes, generating heavy environmental problems.

There are six main plastic components in European municipal solid waste which are high-density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinylchloride (PVC) and polyethyleneterephthalate (PET). Polyolefin materials (PE and PP) constitute the majority of plastic wastes and represent around 68% of the municipal solid wastes [2].

A major part of post consumer plastic wastes are currently landfilled or incinerated. Since these wastes are non biodegradable and the areas for landfill sites are limited, landfilling is not a suitable option for disposing plastic wastes. In addition, the use of incinerator have high cost consumption, and generates some pollutants to the air like nitrous and sulfur oxides, dusts, dioxins and furan [3]. Therefore, in order to reduce environmental charge, recycling and recovering methods should be employed. Pyrolysis is a recovery method that consists on upgrading plastic wastes into fuel oil and valuable chemicals by heating at temperature range of 400–600 °C in the absence of oxygen [4].

Thermal and catalytic degradation of waste plastics are two kinds of chemical valorization processes. The presence of catalysts in pyrolysis reduces reaction temperature and residence time, decreases energy consumption, promotes decomposition reaction and improves selectivity and the quality of the products [5].

Pyrolysis of waste polyethylene has been investigated by many researchers who found that waxes are the main products in thermal cracking. Waxes derived from thermal cracking of PE are long chain made up of linear hydrocarbons having a carbon number heavier than C₂₀ and which solidify at room temperature [6]. Kiran et al. [7] studied the pyrolysis of waste polyethylene in a fixed bed reactor, at a low heating rate and reaching 600 °C. Gas and wax were the two main products obtained during the pyrolysis experiments. The wax formed caused an operating problem by plugging the product lines and condenser tubes.

In order to improve the quality of the liquid fuel produced from the degradation of plastics, a range of catalysts have been utilized in the catalytic degradation of polymers. The most commonly used catalysts are solid acid ones such as zeolites and silica–alumina [8]. Lerici et al. [9] studied thermocatalytic degradation of polyethylene, polypropylene (PP) and polyestyrene (PS) using the H-Y zeolite in a batch reactor at 500 °C. They reported that thermal cracking of polyolefins produced waxes and the use of catalyst yielded higher percentages of gaseous products which ranged between 42 wt% and 45 wt%, while PS yielded a greater amount of liquids (∼72 wt%).

A number of authors have reported promising results on the cracking of polyolefins over several catalysts. Onu et al. [10] compared the degradation of polyethylene and polypropylene over HZSM-5 zeolite catalyst and a catalyst modified with orthophosphoric acid (PZSM-5) in a fixed bed reactor at 460–480 °C. According to their study, the quantities of liquid products with PZSM-5 are smaller than those obtained over HZSM-5 for both PP (43.3%) and PE (34.6%), and all of the liquids had high proportions of aromatic hydrocarbons.

The cracking ability of a catalyst is related to the acidity strength and the pore size which represent its main characteristics [11]. Strong acid catalysts enhance the cracking of heavier hydrocarbons into more lighter or gaseous hydrocarbons comparing to the weak acid catalysts. This was reported by Sakata et al. [12] who studied the catalytic degradation of HDPE into fuel oil over mesoporous silica (KFS-16) catalyst at 430 °C. The Product yields and composition were compared with those obtained over solid acid catalyst (silica–alumina and ZSM-5). According to their study, ZSM-5 possessing strong acid sites produced less liquid products and more gaseous products than the silica-alumina catalysts. On the other hand, KFS-16, having no acid sites, produced liquid hydrocarbons with a high yield as 71 wt%.

Sakata et al. [13] also investigated the effects of various types of solid acid catalysts such as silica–alumina, zeolite and non-acidic mesoporous silica catalysts on the degradation of polyethylene and polypropylene at 430 and 380 °C respectively in batch operation. The degradation of PP and PE into gases was accelerated when using catalysts with strong acid sites such as zeolite ZSM-5.

A number of authors have investigated the use of USY zeolite in the catalytic degradation of polyethylene and polypropylene [2], [14], [15]. They reported that large-pore USY zeolite produced the highest amount of liquid fraction compared to other tested catalysts like ZSM-5 zeolite which favored gas production. Moreover, the hydrocarbons formed over USY zeolite were heavier than those formed with medium-pore zeolites and the main products were alkanes with less alkenes and aromatics.

The main disadvantage of using zeolite catalyst is coke formation which is due to the deposit of heavy by-products on the catalyst surface. Coke formation occurs during catalytic reactions and causes the gradual deactivation of the catalyst which also affects the product distribution and selectivity [16]. Several authors investigated the deactivation of zeolites in polymer cracking.

Marcilla et al. [17] studied the deactivation of HZSM5 and HUSY zeolites during the catalytic pyrolysis of polyethylene and reported that HUSY showed a higher initial relative activity as it presents large pore size but it lead to a fast deactivation, while small pore size in HZSM5 prevented coke deposition.

Previous studies have compared the thermal and catalytic degradation of polyethylene and polypropylene, but most of these works were performed using catalysts that favored gas production [18] and did not focused on the quality of the liquid fraction produced. In the present work, the main objectives were to produce useful fuels from PE and PP over USY zeolite which enhance liquid formation and to investigate the quality of the liquid fraction produced. Furthermore, the influence of USY zeolite on the yield and composition of the derived oils from catalytic cracking of PP and PE was studied. Moreover, the separation of the pyrolytic oils of PP and PE into light, gasoline-like and heavy, diesel-like fractions was studied in order to find a single temperature where the boiling range distributions and physical properties of each fraction meet the gasoline and diesel fuels standards.