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Catalytic Cracking of LDPE Dissolved in Benzene using Nickel-impregnated Zeolites Syie Luing Wong, Norzita Ngadi, TUAN AMRAN TUAN ABDULLAH, and Ibrahim Mohammed Inuwa Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04518 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016

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Industrial & Engineering Chemistry Research

Catalytic Cracking of LDPE Dissolved in Benzene using Nickel-impregnated Zeolites

Syieluing Wong1,2, Norzita Ngadi*1, Tuan Amran Tuan Abdullah2, Ibrahim Mohammed Inuwa3

1

Department of Chemical Engineering, Faculty of Chemical and Energy Engineering, Universiti

Teknologi Malaysia, 81310 Skudai, Johor, Malaysia. 2

Centre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, 81310

Skudai, Johor, Malaysia. 3

Department of Polymer Engineering, Faculty of Chemical and Energy Engineering, Universiti

Teknologi Malaysia, 81310 Skudai, Johor, Malaysia.

1

ACS Paragon Plus Environment


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Abstract Amongst the various processes and reactor designs in polymer cracking, catalytic cracking of polymer dissolved in solvent offers interesting features. In the present research, catalytic cracking of low-density polyethylene (LDPE) dissolved in benzene in a fixed bed reactor was studied. The catalysts used included three zeolites in original form and nickel (Ni)impregnated forms. The LDPE conversion achieved were high (>98%) despite the short retention times. The catalytic cracking of LDPE produced liquid in gasoline range and gases ranged from C1 to C4, and hydrogen gas. The increase in catalysts acidity improved gas yield at the expense of the liquid yield. Although high catalyst acidity had less influence towards liquid product composition, it led to higher degree of cracking of the gaseous products. The dissolution of LDPE in benzene led to high cracking rate despite short retention time, and produced liquid products that can be used as fuels.

Keywords: catalytic cracking, low density polyethylene (LDPE), fixed bed reactor, Nickelimpregnated zeolites, fuel

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1.

INTRODUCTION Different forms of renewable energy were developed by researchers in order to tackle the

energy crisis, and those that were able to be adapted into the national grid were rewarded by the government in many countries in the form of feed-in tariffs.1, 2 Other than the development of renewable energies in the form of electricity, attention is also given to conversion of waste materials, including plastics, to energy in the form of fuels. Studies in this field are important to handle numerous problems brought about by the plastic waste together with other forms of waste after their consumption. Among different types of plastic, polyethylene (PE) is widely used for numerous purposes, including the production of wrapping papers and plastic bags, as it is light, yet having strength and durability enough to be used for the mentioned purposes. In 2012, annual LDPE production of 21 million tonnes was reported, following a steady growth of over 700,000 tonnes over a year.3 The global demand of PE is expected to rises by 4% every year, and is expected to reach 99.6 metric tons in 2018.4 Numerous research reports have indicated polymer pyrolysis and cracking as a solution towards disposal of these plastic wastes after their consumption. These processes have shown the depolymerisation of the plastic waste into the smaller hydrocarbons in liquid and gaseous forms, which could be used as fuels.5 A great number of processes and reactor designs were tested and proved to be useful despite the need for some improvements.6 Among these processes and reactor designs, conical spouted bed reactor (CSBR) seemed to be more promising than the others to be applied in industry, as a number of studies indicated its potential in plastic pyrolysis and cracking. The use of zeolites as catalysts in polymer pyrolysis and cracking has become common place due to their superior catalytic property compared to other catalysts.7, 8 Several studies have shown the increase of polymer conversion by increasing of zeolite acidity.9 On the other hand, impregnation of metals in zeolites is proven to improve the quality of liquid products in pyrolysis of biomass10, as well as pyrolysis of plastic in batch reactor.11, 12 Currently, nickel (Ni) is the most widely used metal in catalyst impregnation, however researchers are also investigating the effects of other metals on zeolitic catalysts.11 In early 90s, several studies have proposed the incorporation of polymer waste in fluid catalytic cracking (FCC) process meant for petroleum refinery due to the similarities between 3



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polymer and crude oil in terms of their chemical composition. Incorporation of polymer recycling into petroleum refinery avoids the need of setting up a new plant solely for plastic recycling purpose. Furthermore, such combination adds value to the existing petroleum refinery plant in term of environment conservation.12 Ng et al. performed catalytic cracking on HDPE pellets dissolved in an automated microactivity test (MAT) unit, and produced significant amount of gasoline and dry gas.13 Since then, catalytic cracking of polymers dissolved in compatible solvents is studied by several research groups, and these studies are well documented in some reviews.16, 17 This process is characterized by high conversion (~100%) despite low retention time. The possibility of upgrading the products from thermal pyrolysis of polymer is also another interesting option.14 There are no detailed studies on product evaluation during catalytic cracking of polymer dissolved in solvent using different types of parent zeolites, as well as nickel-impregnated zeolites. The aim of this study is to study the products evolution in thermal and catalytic cracking of low density polyethylene (LDPE) dissolved in benzene in fixed bed reactor. The changes of zeolites properties due to impregnation of nickel (Ni) were also studied in detail. The compositions of liquid products from thermal and catalytic cracking of LDPE-benzene solutions were analysed using GC/MSD, while the gaseous products were characterized using residual gas analyser (RGA). 2.

MATERIALS AND METHODS

2.1

Feedstock and Catalysts The polymer used was a commercial low density polyethylene (LDPE) purchased from

Titan Chemicals©. The benzene used was of reagent grade, purchased from Qrec. LDPE was dissolved in hot benzene to produce LDPE-benzene solution. The dissolution process was adopted from the previous studies.15,

16

For catalytic cracking of LDPE, two ZSM-5 types

zeolites and one USY type zeolite were used, due to the excellent catalytic properties of such zeolties in polymer cracking process.17 The zeolites are coded Z1, Z2 and Y in this paper for simplicity. Silicon carbide (SiC) with particles size of 1-1.4 mm used as catalyst diluent as received. For impregnation of nickel on zeolites, incipient wetness technique was used as modified from methods described by Aguado et al.18 and Dueso et al. 19. The amount of Ni added was 10 4


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wt.% referring to the mass of catalyst. Prior to the impregnation, the parent zeolite was calcined and dried at 500 °C for 3 hours. Then, nickel (II) nitrate hexahydrate with calculated mass was added to the support in aqueous solution as precursor, and the mixture was stirred under heating at 90 °C until slurry was formed. The slurry was dried at 110°C for hours, then calcined at 650 °C for 3 hours. The catalyst were pressed and sieved to obtain particles in the range of size between 1.0-1.4 mm. The impregnated catalysts were named as Ni-Z1, and Ni-Z2 and Ni-Y respectively. The crystallinity of the catalysts was determined by X-ray Diffraction (XRD) using method as described by Botas et al.

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using Cu-Kα radiation in a Phillips X’PERT MPD

diffractometer. XRD patterns at 2θ was recorded in the range of 2o to 70o using a step size of 0.1° and a counting time of 10 seconds. Morphology of the catalysts was studied by a scanning electron microscope (SEM, JEOL, JSM-6390). The assessment of the specific surface area of the samples was carried out using nitrogen adsorption–desorption isotherms at -196 °C in a Micromeritics Gemini 2360 apparatus. The samples were previously outgassed at 350 °C for 4 h. The surface areas of the samples were calculated by means of the BET multipoint equation using five data points obtained during the test. Temperature-programmed reduction of hydrogen (TPR-H2) of the catalyst was carried out using Micromeritics ChemiSorb 2720 system equipped with TCD detector, using the method adapted from work by Escola et al..21 Sample outgassing was done at 80 °C for 30 minutes under helium flow. The catalyst reduction was then carried out under the flow of 10 % vol. hydrogen in argon, while heated at a ramp rate of 10°C/min to 800 °C, then hold for 10 minutes. The acidity of the catalyst was studied using temperature-programmed desorption of ammonia (TPD-NH3) in Micromeritics Autochem II 2920 Chemisorption Analyser with method as described by Aguado et al.

18

Prior to the test, outgassing of the samples was performed by

heating at 550 °C under helium flow (20 mL/min) for 30 minutes. After that, the sample was cooled to 180 °C, and ammonia (20 mL/min) was flown through the sample for 30 minutes. This was followed by the residue ammonia by flowing helium for 90 min at 180 °C. The temperature

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was then increased at the rate of 10 °C/min to 550 °C. A thermal conductivity detector was utilized to measure the concentration of desorbed ammonia in effluent stream. 2.2

Catalytic Cracking in the Reactor The schematic diagram of the fixed bed reactor system is shown in Figure 1, consists of

syringe pump, tube furnace, mass flow controller, and condenser. The reactor was located in a tube furnace. The reactor was made of ½ inch 316 SS tubing with length of 27 cm. The catalyst bed was located 15 cm from top, and supported by X 316 SS mesh. The LDPE solution was allowed to pass through the fixed catalyst bed in a down-flow direction. The temperature of the catalyst bed was monitored by a K-type thermocouple located right above the catalyst bed. The column was heated by a cylindrical refractory heater.

Figure 1. Schematic diagram of the LDPE cracking system.

Prior to the reaction, the reactor was purged with nitrogen for 5 minutes to ensure an inert atmosphere for cracking process. Then, the reactor was heated at 550 °C, and the flow rate of nitrogen gas was adjusted to be 60 mL/min. 0.02 g of LDPE/ml benzene was fed by a single syringe pump (Cole Parmer 74900 series) at 2 mL/min into a tee then mixed with nitogen gas as a carrier gas. Under such reaction condition, the LDPE solution dopped vertically from the mixing tee onto the catalyst bed, and underwent cracking reaction. Due to the short period taken by the polymer solution to reach the catalyst bed, it is assumed that most of the cracking process took place on the catlayst bed. By adopting the calculation methods by Artetxe et al.

22

, the

space-time used in this study was 50 gcat /(gLDPE.min-1). On the other hand, the residence time was 0.3 s-1, and the reaction time was 600 s. The reaction products formed during the polymer cracking flowed from the reactor into a glass condenser, which is cooled at 0.5 °C using a circulating chiller (FIRSTEK, MODELB401L), and then into a gas liquid separator. All the liquid products were collected and weighed using electronic balance with precision up to ±0.05g. After weighing, the sample bottles were 6


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kept at -5 °C in a refrigerator to prevent the loss of highly volatile compound of hydrocarbon molecules during storage. The gaseous product was collected in Tedlar bag. For catalytic cracking of LDPE solution, 0.2g of zeolite and 0.3g of silicon carbide (SiC) were mixed and located on the catalyst bed. A total of 10mL LDPE solution was used for each experiemntal run. The yield of liquid product collected in each run was calculated using Equation (1). In order to ensure repeatability of the result, each run was repeated once, and the average value of the liquid yield from the two runs was taken to represent the result. The gases yield was then calculated using Equation (2). As the mass of coke formed on catalysts was very small (as discussed in Section 3.3), it is assumed that all the LDPE solution form liquid and gases products, within experimental error.

(%) =

!" #

× 100%

(1)

'() (%) = 100 −

2.3

(2)

Characterizations of Products and Used Catalysts Conversion of LDPE during the cracking process was determined by equation (1). The

LDPE in the liquid were analysed using Fourier-transformed infrared (FTIR), IR-Prestige-21 (Shimadzu). FTIR was used in this study because long chain LPDE is difficult to analyse using normal chromatography, for example, GC or HPLC. Typical determination of LDPE chain distribution is by using a high temperature gel permeation chromatography (HT-GPC) with IR detector. Due to the unavailability of such instrument, FTIR was used in this study. Beer’s law was applied to calculate the wavelength-dependent absorptivity coefficient a(λ), as shown in Equation (3) to generate the calibration curve of sample’s absorbance against concentration.

+ = ((,) ∗ . ∗ /

(3)

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where A is the calculated absorbance for the peak, a(λ) is the wavelength-dependent absorptivity coefficient, in the unit of ml.g-1.cm-1, b is the path length, and c is the analyte concentration.

The compositions of liquid products were analysed by gas chromatography coupled with mass selectivity detector (GC/MSD) using Agilent 6890N Network GC system through method described by Ates et al. 23. The GC/MSD was equipped with a 30m x 0.25 mm capillary column coated with a 0.25µm thick film of 5% phenyl-methylpolysiloxane (HP-5). Helium was employed as a carrier gas at constant flow rate of 1.2 mL/min. The initial oven temperature was 45 °C held for 2 min, ramping from 45 to 290 °C at 5°C/min and then held for 10 min. Prior to injection, the liquid samples were filtered using syringe filters with pore size of 0.22µm. Chromatographic peaks were identified by means of NIST standard reference database. The gas product was analysed using a Residual Gas Analyser (RGA) (Cirrus 2 by MKS Instruments). The total pressure of a particular gas sample was calculated by summing up the partial pressure of each gas measured by the instrument. Then, the percentage of each gas was calculated by dividing their partial pressures with the total pressure of the sample. The amount of coke formed on the catalysts was quantified through temperatureprogrammed oxidation (TPO-TGA) in TG 209 F3 Tarsus, manufactured by Netzsch, according to the method described by Singhal et al.

24

The used catalysts were collected after the LDPE

cracking, and 10 mg of sample was picked from each catalyst. The samples were heated in the thermobalance from 30°C to 850 °C at a heating rate of 10 °C/min in the presence of air.

3.

RESULTS AND DISCUSSION

3.1

Characterization of Zeolites The XRD diffractogram of the parent zeolites, as well as impregnated zeolites is shown

in Figure 2. For zeolite Z1 and Z2, peaks at 2θ= 7.9º, 8.8º, 14-17º, 23-25º were observed. Such peaks are the characteristic peaks for MFI type framework in ZSM5 zeolites.25 On the other hand, for zeolite Y, peaks were observed at 2θ= 6.2º, 10.3º, 12.1º, 15.9º, 18.9º, 20.7º, 24.0º, and 27.5º. Such peaks are the characteristics peaks for FAU framework in USY zeolites.26 All the 8


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reduced catalysts show reflections at 44.5° and 51.9° (marked by the arrows), which correspond to metallic nickel.27 On the other hand, the peaks correspond to NiO are absent on the reduced catalysts. These observations indicate that all NiO species on impregnated zeolites were converted to pure nickel under the reduction condition used.

Compared with their parent zeolites, the nickel-impregnated zeolites exhibited the peaks at the same 2θ values, however with lower intensity. This observation indicates slight decrease in crystallinity of the impregnated zeolites caused by the nickel impregnation, which is in agreement with the result obtained by Luengnaruemitchai et al.

28

. This result is most probably

due to two effects: framework dealumination at high temperature, and presence of metal species on support. Work by Millini et al.

29

indicated that zeolite is relatively stable up to 550 ºC. At

higher temperature, zeolites start to experience slight framework dealumination and structural collapse. Due to the dealumination, the aluminium species in the framework experience migration to extra-framework sites.30 The effects become more pronounced at 850 ºC, and leads to the total structural collapse at 900 ºC. On the other hand, introduction of metal species into zeolites tends to decrease the crystallinity of the zeolites.26 This effect increases with the amount of impregnated metal on zeolites.31 Structural degradation of support other than zeolites upon metal impregnation was also reported.32 As thermal treatment causes only slight effect to the zeolite structure at 550 ºC, it is believed that high metal loading of nickel (10 wt.%) causes greater effect to the zeolite structure degradation. The relative crystallinity of the impregnated zeolites with respect to their parent zeolites was calculated using Procedure B (Peak height method) in method ASTM-D5758-01. It involves division of height of the specific peak in the associated XRD diffractogram of impregnated zeolite by that of the parent zeolite, which is then expressed in percentage crystallinity.33 It was showed through calculation that ZSM5 type zeolites experienced appreciable decrease in crystallinity (82.6% for Z1, and 82.8% for Z2) after nickel impregnation when compared to their parent zeolites. On the other hand, USY zeolite is less thermally stable than ZSM5 type zeolites, as suggested by its lowest relative crystallinity among the three impregnated zeolites (61.6%).

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Figure 2.

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Diffratograms of zeolites Z1, Ni-Z1, Z2, Ni-Z2, Y and Ni-Y.

Figure 3 shows the reduction peaks of impregnated NiO on the zeolites during TPR. NiZ1 produced two reduction peaks (at 434.8 °C and 634.5 °C) with almost the same height. This indicates that almost same amount of nickel species was located on the zeolite surface and inside the pore system respectively. Similar result was obtained by Sarkar et al.

34

in nickel

impregnation on ZSM-5 zeolites. Ni-Z2 also produced two reduction peaks at 365.7 °C and 564.8 °C, with the difference in the peaks height, suggesting higher proportion of nickel species on the catalyst surface, with comparatively weak interaction with the support, as indicated by lower peak temperature compared to other peak. Meanwhile, tiny fraction of the nickel deposited in the mesopore or micropore system. On the other hand, interesting observation was made on Ni-Y, where three reduction peaks were observed. The first and second peak (at 441.9 °C and 509 °C) indicated that almost equal amount of Ni species were located on the surface and in the pore system. The interactions of Ni with zeolites, as indicated by these two peaks, were stronger compared to Ni-Z1 and NiZ2, and by the peak temperatures. Meanwhile, the third peak with higher reduction temperature (677.0 °C) is related to the nickel species that were exchanged with the ions in the zeolite framework.35, 36 It was suggested by Mohan et al. 27 that reduction peak at temperature below 500 °C is related to reduction of superficial NiO, while peak at range of 500-630 °C can be assigned to reduction of smaller Ni particles and Ni oligomeric species. On the other hand, the peak above 630 °C is most probably due to the reduction of Ni species in zeolites.

Figure 3.

TPR-H2 result for Ni-Z1, Ni-Z2 and Ni-Y.

SEM analysis was observed for the parent zeolites and Ni-impregnated zeolites (after hydrogen reduction) to study their morphology. The morphologies of Z1, Z2 and Y zeolites, as well as the corresponding Ni-impregnated zeolites, are shown in Figure 4 respectively. Zeolites Y and Z1 are formed by irregular-shaped crystals within the size of 0.2-1.0µm. On the other 10


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hand, zeolite Z2 is formed by crystal in more distinct three dimensional shapes, which are mostly oval or cubic. The particles for Z2 are bigger than that of Z1 and Y, which ranges in 1.2-2.0 µm. This is in accordance with the observation by Vichaphund et al.

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on the ZSM5 zeolites.

Formation of large clusters due to agglomeration of particles is also observed for all parent zeolites with particles less than 1 micron due to the pressing action during catalyst preparation.

Figure 4. Micrographs of zeolites (a) Z1; (b) Ni-Z1; (c) Z2; (d) Ni-Z2; (e) Y, and (f) Ni-Y. From Figure 4(b), (d) and (f), the structure of all Ni-impregnated zeolites are similar to their parent zeolites, therefore it is inferred that the slight structural degradation of Niimpregnated zeolites, as indicated by XRD result, has little influence to their morphologies. However, a large number of white spots are observed on the surface of Z2 zeolite. Such phenomenon does not occur on Ni-Z1 and Ni-Y. One possible explanation for this observation is the agglomeration of Ni particles on the surface of parent zeolite. It is indicated by the TPR-H2 result that most of the impregnated Ni particles deposit on the external surface of Z2. In contrast, for Z1 and Y, almost equal amount of Ni deposit on the external surface and pores system respectively during Ni impregnation. Therefore, the concentration of Ni particles on their surface is below the observation limit of SEM. In works related to metal impregnation on catalyst supports, the metals are usually not observable at low concentration on their support, for example, less than 1 wt.%10, 1 wt.%37 and 2.4 wt.% 38. However, the observation of impregnated species at high concentration is possible, as indicated by the morphology of 8 wt.% Ni/ZSM-5 observed in the work by Vafaeian et al. al.

40

39

which is similar to Figure 4(d). In addition, Zhang et

observed the agglomeration of 40 wt.% of TiO2 on ZSM-5 zeolite. The surface area of the catalysts as obtained from BET method is shown in Table 1. For

zeolite Y and zeolite Z1, the measured surface area (653 m2/g for zeolite Y, and 311 m2/g for zeolite Z1) is slightly lower than the ones given by the manufacturer (780 m2/g for zeolite Y, and 425 m2/g for zeolite Z1), which is most probably due to the effect of pelletizing, crushing and calcination prior to the surface area measurement. As the measurement method used by the producer company for the surface area is not known, it might also be different from the method 11



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used in this study. It is found that the surface area of the parent zeolites is arranged in the order Z1< Z2

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