Production of benzene/toluene/ethyl benzene/xylene (BTEX) via multiphase catalytic pyrolysis of hazardous waste polyethylene using low cost fly ash synthesized natural catalyst
Introduction
Silica and alumina (Si/Al) based ZSM-5 is widely used catalyst in cracking, isomerization and aromatization of larger hydrocarbon molecules due to its excellent catalytic properties, thermal stability and acidity (Gaurh and Pramanik, 2018). ZSM-5 is commercially available costly catalyst and thus, the use of this catalysts in selective catalytic cracking of high molecular petroleum products and different types of polymers e.g., polyethylene (PE), poly propylene (PP), and poly styrene (PS) will add cost to entire process in manufacturing of valuable marketable products like benzene, toluene, ethyl benzene and xylene (BTEX). However, many scientists have tested laboratory synthesized catalyst e.g., mordenite, clay, red mud, natural zeolite for the catalytic pyrolysis of wastes plastics (Chen et al., 2014). As we know that the selection of catalyst and raw material both are very important to get the desired products by means of catalytic pyrolysis. Here, our research focuses on the utilization of hazardous waste materials, like fly ash as a catalyst and waste polyethylene as feed material for the catalytic pyrolysis, which eventually reduces the environmental pollution problem and thus, protects mother earth.
Catalytic pyrolysis is a chemical recycling method that involves the conversion of polymers to recover useful products could be the appropriate route (Malkow, 2004, Shent et al., 1999). So far, many articles have been published on catalytic conversion of different types of plastics and polymers to different products, including chemicals and fuels on different types of catalysts (Lin et al., 1998, Lin et al., 2010). Among them, mostly focused on the catalytic conversion of plastic wastes using different types of catalysts to improve the quality of liquid oil (Wang and Wang, 2011). The catalysts include red mud (Lopez et al., 2011a), FCC (Lee, 2009), ZSM-5 (Gaurh and Pramanik, 2018, Lopez et al., 2011a), HZSM-5 (Hernandez et al., 2007), Y-zeolite (Lee, 2012), Fe2O3 (Sarker and Rashid, 2013), Al2O3, Ca(OH)2 (Sarker et al., 2011) and natural zeolite (Miandad et al., 2017, Syamsiro et al., 2014). The key role of catalysts was to increase the lighter fractions in the liquid oil such as gasoline (Lerici et al., 2015), and decrease the overall process energy inputs (Lopez et al., 2011a). For instance, the use of ZSM-5 catalyst decreased the impurities such as solid residue, sulphur, nitrogen, and phosphorous in the produced liquid oil (Miskolczi et al., 2009). It is well known that the use of catalysts with a high BET surface area provides more interaction between reactants and the catalyst surface, resulting in an increased rate of cracking reaction to produce more gases than liquid oil (Syamsiro et al., 2014). Substantial research is underway to explore different types of catalysts and their dynamic role in the pyrolysis process and its products. Different types of catalysts such as FCC (Achilias et al., 2007), spent FCC (Lee, 2009), HZSM-5 (Lee, 2012), ZSM-5 (Lopez et al., 2011b, Miskolczi et al., 2009), Cu-Al2O3 (Adnan et al., 2014), CoMo/Z (Sriningsih et al., 2014), Zeolite-ß (Ojha and Vinu, 2015), natural zeolite (NZ) (Syamsiro et al., 2014), Red Mud (Lopez et al., 2011a), Al(OH)3 Ca(OH)2 (Sarker et al., 2011) and Fe2O3 (Sarker and Rashid, 2013) have been used extensively to enhance the yield and quality of the products.
It is clear from the above literature survey that there is a still scope of manufacture catalyst from many natural solid wastes for the catalytic cracking of plastic wastes, mainly polyethylene (PE) due to its uncontrolled and excessive use. In this context, hazardous solid waste of thermal power plant i.e., “fly ash” could be used as catalyst material which is a rich source of natural silica and alumina those are trapped in the fuel coal from its origin.
Fly ash is the main combustion by-product of coal fired power plants and a huge amount of fly ash is produced by the almost all developed and developing countries as reported by Earth science reviews (Ram and Masto, 2014) (Table 1). Unfortunately, more than half of fly ash is disposed of in land filling because it finds no other suitable and economical application. The huge production of fly ash is extremely worrying because of the unplanned disposal. In India, fly ash (FA) is being generated at the rate of nearly 132 million tonnes per annum (MTPA) from thermal power plants in 2011–12 (Table 1). There are serious environmental health hazards associated with fly ash. In addition, the land requirement envisaged for disposal of fly ash is about 50,000 acre, with an annual expenditure of about Rs. 500 million for transportation. These problems clearly spell out the fact that utilization of fly ash is absolutely essential. As a consequence, several investigations have been carried out in order to exploit this waste material into value added material. Over the last few years, fly ash has been gaining ground in finding solutions to environmental problems using it as an active ingredient in cement manufacturing and brick making, which is not sufficient (Table 2) (The Gazette of India, 2016).
Due to unique composition of fly ash (Table 3) (Malik et al., 2016) i.e., quartz, mullite, subordinately hematite and magnetite, carbon, and a prevalent phase of amorphous alumino silicate (Bayat, 1998, Hall and Livingston, 2002, Hower et al., 1996, Koukouzas et al., 2006, Kukier et al., 2003, Mishra et al., 2003, Sokol et al., 2000) makes fly ash an important source material in zeolite synthesis.
Moreover, it is seen in the Table 1 that for the countries like India, China, USA, Japan, Canada etc. a large amount of fly ash is unutilized. Thus, this paper focuses on the effective use of waste fly ash as a natural catalyst for the conversion of solid waste mainly polyethylene (PE) to valuable hydrocarbons like benzene, toluene, ethyl benzene and xylene (BTEX). In addition, plastic waste polyethylene is nightmare for all the developing countries. This paper explores the scope of waste polyethylene management using waste fly ash as a natural catalyst resulting in valuable aromatics/BTEX.
As we know, used polyethylene accounts major portion of the plastic wastes which is the key fraction of municipal solid wastes (Gaurh and Pramanik, 2018). World’s oldest living holy city Varanasi, India is severely affected by this waste polyethylene (Srivastava et al., 2014). The polyethylene creates several problems which are (i) water logging due to blockage in city drainage system (ii) animals mainly Cow in many occasions dyes as they eat polyethylene when it is thrown with waste food stuff and (iii) contaminate soil and water body as polyethylene is non-biodegradable. In a nutshell, polyethylene is a hazardous material for the environment and living being (Al-Salem et al., 2009). Keeping in mind the problems of holy city Varanasi, India due to the dumping of huge amount of solid wastes polyethylene, it was selected as a raw material for the conversion of waste PE to valuable aromatics BTEX.
Park et al., 2002 derived hydrocarbons (HC) using pyrolysis of different plastic wastes such as HDPE, LDPE. PP (Hwang et al., 2002, Hwang et al., 2014) and PS (Bagri and Williams, 2002a, Kim et al., 2002) contains low carbon chain compounds including, gasoline range HCs in comparison to thermal pyrolysis (Scott et al., 1990, Park et al., 1999, Aguado et al., 2000). Moreover, the catalysts increase the gaseous fraction and reduce the liquid oil yield when compared to thermal pyrolysis (Park et al., 1999, Beltrame et al., 1989). Furthermore, the conversion rate was increased at low temperature in catalytic conditions in comparison to thermal pyrolysis (Park et al., 1999, Ding et al., 1997, Lee, 2001). Chung et al., 2003 studied on pyrolysis of polyethylene (PE) and polypropylene (PP) using fly ash treated with NaOH as a catalyst. No such detail study was performed in terms of catalysts preparation, characterization and product yield analyzes etc.
Till date no research has been carried out on the BTEX production from waste polyethylene using fly ash as a cheap and natural catalyst. The BTEX were chosen as a target/ideal product here, since they have numerous applications, like benzene is primarily used as raw material for ethyl benzene to styrene and cumene to phenol production. The third largest use of benzene is in the production of cyclohexane, a nylon precursor. Toluene, the second largest aromatic in BTEX/Hydrotreated Pygas (HPG), is used in refinery streams such as gasoline blending for improvement of octane value. Ethyl benzene is widely used in industrial processes for the manufacture of styrene, which is then used for polystyrene manufacture. Ethyl benzene is also present as a solvent in inks, dyes and in petrol. Xylene is widely used in the production of plastic bottles and polyester clothing and as a solvent with a range of applications from circuit board cleaning to thinning paints and varnishes. Xylene may either be used in refinery streams for gasoline blending or further separated by isomers for chemical applications (Thongplang, 2016). This research explores the opportunities to utilize hazardous waste material polyethylene and fly ash both in the effective and efficient way. Thus, the main aim of the present study was to synthesize low cost silica-alumina based catalyst from fly ash and systematically investigate the effect of different process parameters e.g., operating temperature, reaction time and batch by batch catalytic and thermal pyrolysis to achieve maximum aromatics BTEX yield. The synthesized catalysts were thoroughly characterized and the pyrolysis products were systematically analyzed using GC-FID, flash and fire point, calorific value (CV) test and carbon residue to check the suitability of pyrolysis oil for the IC engine and other commercial uses.
Section snippets
Raw materials
Municipal solid waste of Varanasi city, India comprises maximum food waste (31.9%) followed by plastic (22%), textile (10.6%), paper (9.6%), glass (6.7%), cardboard (6.2%), ash (5.3%), leather (5.7%) and minimum metals waste (2.8%). Per capita MSW waste generation rate is 800 MT per day, 0.217 kg/person/day in Varanasi city, India (Srivastava et al., 2014). The published literature shows that the plastic waste polyethylene contributes nearly 41.5 wt% of municipal plastic waste. In view of this,
SEM-EDX analysis
Surface morphology of the treated (calcined) and untreated fly ash samples are shown in Fig. 4. It is evident that most of the particles were spherical in shape with high porosity. The fly ash consists of spherical, vitreous particles of different sizes. These particles are usually porous in nature and some pores may contain other smaller particles in their interior. The surface texture of fly ash particles appears to be smooth and also some vitreous, unshaped fragments or quartz particles can
Conclusions
The in-situ pyrolysis and aromatization of plastic waste PE on synthesized FA catalyst using two different types of catalyst arrangements (A-type and B-type) was studied for the first time and was not reported before in the available literature. The optimum calcination temperature of fly ash for catalyst synthesis was 800 °C, as FA-800 catalyst showed excellent performance for aromatization of pyrolysis product in the reactor. The maximum surface area and (Si/Al) ratio of FA-800 catalyst were
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