Feb 19, 2016 - 1. INTRODUCTION. Different forms of renewable energy were developed by researchers in order to tackle the energy crisis, and those that.
Article pubs.acs.org/IECR
Catalytic Cracking of LDPE Dissolved in Benzene Using NickelImpregnated Zeolites Syieluing Wong,†,‡ Norzita Ngadi,*,† Tuan Amran Tuan Abdullah,‡ and Ibrahim Mohammed Inuwa§ †
Department of Chemical Engineering, Faculty of Chemical and Energy Engineering, ‡Centre of Hydrogen Energy, Institute of Future Energy, and §Department of Polymer Engineering, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia S Supporting Information *
ABSTRACT: Among 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 fixedbed reactor was studied. The catalysts used included three zeolites in original form and nickel (Ni)-impregnated forms. The LDPE conversions achieved were high (>98%) despite the short retention times. The catalytic cracking of LDPE produced liquid in the gasoline range, gases ranged from C1 to C4, and hydrogen gas. The increase in catalyst acidity improved gas yield at the expense of the liquid yield. Although high catalyst acidity had less influence toward 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.
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 granted feed-in tariffs by the government in many countries.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 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, because it is light yet has the strength and durability to be used for the mentioned purposes. In 2012, an 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 rise by 4% every year and is expected to reach 99.6 tonnes in 2018.4 Numerous research reports have indicated polymer pyrolysis and cracking as solutions toward disposal of these plastic wastes after their consumption. These processes have shown the depolymerization 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 because a number of studies indicated its potential in plastic pyrolysis and cracking. The use of zeolites as catalysts in polymer pyrolysis and cracking has © 2016 American Chemical Society
become common place because of 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 In contrast, 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 a 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 1990s, several studies proposed the incorporation of polymer waste in fluid catalytic cracking (FCC) process meant for petroleum refinery because of the similarities between 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 purposes. 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 a significant amount of gasoline and dry gas.13 Since then, catalytic cracking of polymers dissolved in compatible solvents has been 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 Received: Revised: Accepted: Published: 2543
November 27, 2015 February 11, 2016 February 19, 2016 February 19, 2016 DOI: 10.1021/acs.iecr.5b04518 Ind. Eng. Chem. Res. 2016, 55, 2543−2555
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Industrial & Engineering Chemistry Research
Figure 1. Schematic diagram of the LDPE cracking system.
hours, then calcined at 650 °C for 3 h. The catalyst was pressed and sieved to obtain particles in the range of size between 1.0 and 1.4 mm. The impregnated catalysts were named Ni-Z1, NiZ2, and Ni-Y, respectively. The crystallinity of the catalysts was determined by X-ray diffraction (XRD) using the method described by Botas et al.20 using Cu Kα radiation in a Phillips X’PERT MPD diffractometer. XRD patterns at 2θ were recorded in the range of 2−70° using a step size of 0.1° and a counting time of 10 s. 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 a work by Escola et al.21 Sample outgassing was done at 80 °C for 30 min under helium flow. The catalyst reduction was then carried out under the flow of 10 vol % hydrogen in argon while heating at a ramp rate of 10 °C min−1 to 800 °C and then holding for 10 min. The acidity of the catalyst was studied using temperatureprogrammed desorption of ammonia (TPD-NH3) in Micromeritics Autochem II 2920 Chemisorption Analyzer with the method 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−1) for 30 min. After that, the sample was cooled to 180 °C, and ammonia (20 mL min−1) was flown through the sample for 30 min. This was followed by removal of the residual ammonia by flowing helium for 90 min
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 product evolution in thermal and catalytic cracking of low-density polyethylene (LDPE) dissolved in benzene in a fixed-bed reactor. The changes of zeolites properties due to impregnation by nickel (Ni) were also studied in detail. The compositions of liquid products from thermal and catalytic cracking of LDPE−benzene solutions were analyzed using GC/ MSD, whereas the gaseous products were characterized using residual gas analyzer (RGA).
2. MATERIALS AND METHODS 2.1. Feedstock and Catalysts. The polymer used was a commercial 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 previous studies.15,16 For catalytic cracking of LDPE, two ZSM5 types zeolites and one USY type zeolite were used because of the excellent catalytic properties of such zeolties in polymercracking process.17 The zeolites are coded Z1, Z2, and Y in this paper for simplicity. Silicon carbide (SiC) with a particle size of 1−1.4 mm was used as received as a catalyst diluent. For impregnation of nickel on zeolites, incipient wetness technique was used, modified from methods described by Aguado et al.18 and Dueso et al.19 The amount of Ni added was 10 wt % referring to the mass of catalyst. Prior to the impregnation, the parent zeolite was calcined and dried at 500 °C for 3 h. Then, nickel(II) nitrate hexahydrate with the calculated mass was added to the support in aqueous solution as precursor, and the mixture was stirred under heating at 90 °C until a slurry was formed. The slurry was dried at 110 °C for 2544
DOI: 10.1021/acs.iecr.5b04518 Ind. Eng. Chem. Res. 2016, 55, 2543−2555
Article
Industrial & Engineering Chemistry Research at 180 °C. The temperature was then increased at the rate of 10 °C min−1 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, and the reactor consists of syringe pump, tube furnace, mass flow controller, and condenser. The reactor was located in a tube furnace. The reactor was made of 1/2 in. 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 downwardflowing 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. Prior to the reaction, the reactor was purged with nitrogen for 5 min 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−1. LDPE−benzene solution (0.02 g mL−1) was fed by a single-syringe pump (Cole Parmer 74900 series) at 2 mL min−1 into a tee then mixed with nitogen gas as a carrier gas. Under such reaction conditions, the LDPE solution dopped vertically from the mixing tee onto the catalyst bed and underwent a cracking reaction. Because of the short path 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). In contrast, 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 ±0.05 g. After weighing, the sample bottles were 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, 0.2 g of zeolite and 0.3 g of silicon carbide (SiC) were mixed and located on the catalyst bed. A total of 10 mL of LDPE solution was used for each experiemntal run. The yield of liquid product collected in each run was calculated using eq 1. 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 gaseous yield was then calculated using eq 2. Because the mass of coke formed on catalysts was very small (as discussed in section 3.3), it is assumed that all of the LDPE solution forms liquid and gases products, within experimental error.
using Fourier-transformed infrared (FTIR; IR-Prestige-21, Shimadzu). FTIR was used in this study because long-chain LPDE is difficult to analyze 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. Because of the unavailability of such an instrument, FTIR was used in this study. Beer’s law was applied to calculate the wavelengthdependent absorptivity coefficient a(λ), as shown in eq 3, to generate the calibration curve of sample’s absorbance against concentration. A = a(λ) bc
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 analyzed by gas chromatography coupled with mass selectivity detector (GC/ MSD) using Agilent 6890N Network GC system through the method described by Ates et al.23 The GC/MSD was equipped with a 30 m × 0.25 mm capillary column coated with a 0.25 μm thick film of 5% phenyl-methylpolysiloxane (HP-5). Helium was employed as a carrier gas at constant flow rate of 1.2 mL min−1. The initial oven temperature was 45 °C held for 2 min, ramping from 45 to 290 °C at 5 °C min−1 and then holding for 10 min. Prior to injection, the liquid samples were filtered using syringe filters with pore size of 0.22 μm. Chromatographic peaks were identified by means of NIST standard reference database. The gas product was analyzed using a residual gas analyzer (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 temperature-programmed oxidation (TPO-TGA) using a 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 to 850 °C at a heating rate of 10 °C min−1 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 = 7.9, 8.8, 14− 17, and 23−25° were observed. Such peaks are the characteristic peaks for MFI type framework in ZSM5 zeolites.25 In contrast, for zeolite Y, peaks were observed at 2θ = 6.2, 10.3, 12.1, 15.9, 18.9, 20.7, 24.0, and 27.5°. Such peaks are the characteristic peaks for FAU framework in USY zeolites.26 All the reduced catalysts show reflections at 44.5 and 51.9° (marked by the arrows), which correspond to metallic nickel.27 However, the peaks corresponding 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 peaks at the same 2θ values, however with lower intensity. This observation indicates a slight decrease in crystallinity of the impregnated zeolites caused by the nickel
Liquid yield (%) mass of liquid collected after cracking = × 100% mass of LDPE solution fed into the reactor (1)
Gas yield (%) = 100 − liquid yield
(3)
(2)
2.3. Characterizations of Products and Used Catalysts. Conversion of LDPE during the cracking process was determined by eq 1. The LDPE in the liquid was analyzed 2545
DOI: 10.1021/acs.iecr.5b04518 Ind. Eng. Chem. Res. 2016, 55, 2543−2555
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Industrial & Engineering Chemistry Research
Figure 2. Diffratograms of zeolites Z1, Ni-Z1, Z2, Ni-Z2, Y, and Ni-Y.
Figure 3. TPR-H2 result for Ni-Z1, Ni-Z2, and Ni-Y.
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 °C. At higher temperature, zeolites start to experience slight framework dealumination and structural collapse. Because of the dealumination, the aluminum species in the framework experience migration to extra-framework sites.30 The effects become more pronounced at 850 °C and lead to the total structural collapse at 900 °C. Likewise, 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 Because thermal treatment causes only a slight effect to the zeolite structure at 550 °C, it is believed that high metal loading of nickel (10 wt %) causes a 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 that of their parent zeolites. In contrast, USY zeolite is less thermally stable than ZSM5-type zeolites, as suggested by its lowest relative crystallinity among the three impregnated zeolites (61.6%). Figure 3 shows the reduction peaks during TPR of NiO impregnated on the zeolites. Ni-Z1 produced two reduction peaks (at 434.8 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. A 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 and 564.8 °C, with a difference in the peaks’ height, suggesting a 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, a tiny
fraction of the nickel deposited in the mesopore or micropore system. An interesting observation was made on Ni-Y, where three reduction peaks were observed. The first and second peak (at 441.9 and 509 °C) indicated that almost equal amounts 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 and by the peak temperatures, were stronger compared to those of Ni-Z1 and Ni-Z2. 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 the reduction peak at temperature below 500 °C is related to reduction of superficial NiO, whereas the peak at range of 500− 630 °C can be assigned to reduction of smaller Ni particles and Ni oligomeric species. The peak above 630 °C is most probably due to the reduction of Ni species in zeolites. SEM analysis was carried out for the parent zeolites and Niimpregnated 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. Zeolites Y and Z1 are formed by irregularly shaped crystals within the size range of 0.2−1.0 μm. In contrast, zeolite Z2 is formed by crystal in more distinct 3D shapes, which are mostly oval or cubic. The particles for Z2 are bigger than those of Z1 and Y, which fall in the range of 1.2−2.0 μm. This is in accordance with the observation by Vichaphund et al.10 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 μm because of the pressing action during catalyst preparation. From Figure 4b,d,f, we conclude that the structures of all Niimpregnated zeolites are similar to those of their parent zeolites; therefore, it is inferred that the slight structural degradation of Ni-impregnated 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 a phenomenon does not occur on Ni-Z1 and NiY. 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 amounts of Ni 2546
DOI: 10.1021/acs.iecr.5b04518 Ind. Eng. Chem. Res. 2016, 55, 2543−2555
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Industrial & Engineering Chemistry Research
It is also observed that nickel impregnation decreased the surface area of all zeolites because of the dispersion of Ni particles on the external surface of zeolites as well as blockage of zeolites micropores by Ni particles. Such phenomenon is often reported, although less pronounced compared to impregnation of other metals on zeolites.20 However, it is interesting to note that Ni impregnation led to different degrees of reduction of surface area in each zeolite. Y has highest reduction in surface area (24.4%) after nickel impregnation, compared to that for Z1 (23.4%) and Z2 (17.5%). To explain such an observation, two important facts have to be considered: (i) The surface area of zeolites as calculated using BET equation is a sum of external surface area and micropore area of the zeolite. During metal impregnation, the introduced metal can either deposit on the external surface, or inside the micropores of the zeolites, thus causing a reduction of their respective areas.35 (ii) Although these properties were not measured in this study, it is generally shown that micropore area of zeolites is larger than its external surface area.43,44 Thus, when the same amount of metal is deposited on the external surface and in the micropores of zeolite, the former case results in a high reduction of total surface area compared to that in the latter case. From TPR-H2 results, it is shown that a large proportion of nickel species is deposited in the micropores of Z1, followed by Y and finally Z2. Thus, this sequence can also be applied to the reduction of micropore areas and hence the total areas of the impregnated zeolites. Although Ni-Z2 experienced higher reduction in external area after impregnation, its effect toward reduction of total surface area is less compared the that on the reduction of micropore areas after impregnation. The acidity of zeolite is one of the contributors toward its performance in catalytic cracking of polymer. The acidity of the catalysts before and after nickel impregnation is indicated by the result of TPD in Figure 5. Z1, Z2, and Y zeolites exhibit
Figure 4. Micrographs of zeolites (a) Z1, (b) Ni-Z1, (c) Z2, (d) NiZ2, (e) Y, and (f) Ni-Y.
deposit on the external surface and pores system, respectively, during Ni impregnation. Therefore, the concentration of Ni particles on the 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,
