Cracking Of High-density Polyethylene Research Articles

Insights into structure–performance relationship in the catalytic cracking of high density polyethylene

Zeolite is considered a promising catalyst for plastic cracking because of its well-defined acid sites, high product selectivity, and outstanding stability. The relationship between zeolite structure and catalytic performance is still not well understood. Therefore, on either as prepared or purposely modified ZSM-5 zeolites, we made a systematic investigation of how acid density and pore structure affect catalytic performance in this work. Our results demonstrate that Brønsted acid site density had profound impact on the catalyst lifetime and aromatic selectivity. The relationship between Brønsted acid site density and catalyst lifetime displays a tendency that catalyst lifetime declines with acid site density at the beginning, then rises up, instead of a linear correlation. Also, the increase of mesoporosity extends the catalyst lifetime to some extent. This information can give us a better understanding of how to design higher-performance catalysts for chemical upcycling of waste plastics.

Modelling pyrolysis process for PP and HDPE inside thermogravimetric analyzer coupled with differential scanning calorimeter

Plastic pyrolysis is widely studied and implemented at lab-scale but rarely modelled numerically. For the sake of designing efficient industrial reactors, modelling plastic pyrolysis process at particle scale could be a prerequisite. Therefore, the aim of this work is to model the whole plastic pyrolysis process, on particle scale, for polypropylene (PP) and high density polyethylene (HDPE) inside a thermogravimetric analyzer coupled with differential scanning calorimeter (TGA-DSC). First, the kinetic triplet for PP and HDPE pyrolysis, at heating rates 4-10°C/min, are determined according to the isoconversional methods. Secondly, the kinetic triplets are used along with the appropriate conditions to model and simulate the pyrolysis process for PP and HDPE in TG analyzer, using finite element method. The melting sub-process is modelled using the modified apparent heat capacity method, which resulted in a relative error below 10% between the simulated and measured heat flow. Furthermore, the cracking model describes perfectly PP and HDPE cracking, where the average relative error among all calculated and experimental conversions didn't exceed 5%. Furthermore, the heat flow inside the TGA-DSC crucible was modelled and the average error was reduced to less than 8% by dividing the cracking phenomena into a “latent” and an apparent process.

Application of Artificial Neural Networks to Predict the Catalytic Pyrolysis of HDPE Using Non-Isothermal TGA Data.

This paper presents a comprehensive kinetic study of the catalytic pyrolysis of high-density polyethylene (HDPE) utilizing thermogravimetric analysis (TGA) data. Nine runs with different catalyst (HZSM-5) to polymer mass ratios (0.5, 0.77, and 1.0) were performed at different heating rates (5, 10, and 15 K/min) under nitrogen over the temperature range 303–973 K. Thermograms showed clearly that there was only one main reaction region for the catalytic cracking of HDPE. In addition, while thermogravimetric analysis (TGA) data were shifted towards higher temperatures as the heating rate increased, they were shifted towards lower temperatures and polymer started to degrade at lower temperatures when the catalyst was used. Furthermore, the activation energy of the catalytic pyrolysis of HDPE was obtained using three isoconversional (model-free) models and two non-isoconversional (model-fitting) models. Moreover, a set of 900 input-output experimental TGA data has been predicted by a highly efficient developed artificial neural network (ANN) model. Results showed a very good agreement between the ANN-predicted and experimental values (R2 > 0.999). Besides, A highly-efficient performance of the developed model has been reported for new input data as well.

Experimental Investigation on the Reduction of Catalyst Costs in the Polyethylene Pyrolysis Process

In our earlier works, the catalyst USY zeolite was tested for the pyrolysis of High density polyethylene (HDPE). It yielded 71% liquid oil that could be separated into diesel and gasoline-like fractions [3]. However, the high cost of this catalyst led to the present study where the effects of different low-cost catalysts on catalytic cracking of HDPE were investigated. GC-MS analysis were performed to check the carbon distribution of each sample and to make theoretical gasoline/diesel quantification in the pyrolysis liquids. Products where compared by studying carbon distribution, condensable range, branching degrees and reaction speeds. According to all those parameters, none of them outperformed USY zeolite. The Second Part has been conducted to observe the regeneration of USY Zeolite and monitor its performance for its economic value. The regeneration was done 10 times and the performance was monitored after every experiment with respect to % of heavier compound yields, carbon distribution range, char deposition range and gasoline and diesel yields. It was found that the regenerated catalyst loses its capability very slowly and even after 10 regenerations products characteristics were not sensibly altered.

Study of synergistic effects during co-pyrolysis of cellulose and high-density polyethylene at various ratios

The mechanism of synergistic effects occurred during co-pyrolysis of cellulose (CE) and high-density polyethylene (HDPE), mixed at various mass ratios (3:1, 1:1, 1:3 w/w), was studied by thermogravimetric analysis coupled with mass spectrometry (TG-MS) and pyrolysis-gas chromatography coupled with mass spectrometry (Py-GC/MS). The TG analysis showed that the co-pyrolysis process was divided into two-stages: the first stage was the decomposition of CE (at 260 °C–410 °C), and the second stage between 410 °C and 527 °C was mainly the decomposition of HDPE. The experimental mass loss values of the mixtures were greater than the estimated values, which confirmed a synergistic effect in the co-pyrolysis of CE and HDPE. The results of MS and Py-GC/MS indicated that the effects of co-pyrolysis promoted the release of small-molecule volatile (H2O, CO/C2H4, CO2), and the promotion effect was strongest when the ratio was 1:3. Hydrogen transfer from the scission of HDPE could be provided for the decomposition of CE, and the oxygen-containing compounds from CE could promote the chain scission and cracking of HDPE. When the CE/HDPE ratio was less than 1:1, the production of carbohydrate, aldehyde, ketone, and furan groups assigned to CE pyrolysis was suppressed, and the production of alkane and alkene groups was promoted. The decomposition pathways in the co-pyrolysis process of CE and HDPE were proposed.

Effects of recycled HDPE and nanoclay on stress cracking of HDPE by correlatingJcwith slow crack growth

The effects of recycled high density polyethylene (HDPE) and nanoclay on the stress crack resistance (SCR) of pristine HDPE were evaluated using the Notched Constant Ligament Stress (NCLS) test. The test data were analyzed by both linear elastic fracture mechanics (LEFM) and elastic plastic fracture mechanics (EPFM). The LEFM approach uses the stress intensity factorKto define the two failure mechanisms: creep and slow crack growth (SCG). In contrast, using theJ‐integral in EPFM, which emphasizes the nonlinear elastic‐plastic strain field at the crack‐tip, revealed a short‐term failure stage prior to the creep failure. In this article, a power law correlation between the fracture toughnessJcand SCG was found under a plane‐strain condition. Increasing recycled HDPE content lowered the SCG resistance of pristine HDPE by decreasingJc. Adding nanoclay up to 6 wt% also decreasedJcwhile simultaneously, lowering the stress relaxation of nanocomposites, leading to longer SCG failure times at lowJvalues. POLYM. ENG. SCI., 58:1471–1478, 2018. © Published [2017]. This article is a U.S. Government work and is in the public domain in the USA.

Properties of hierarchical Beta zeolites prepared from protozeolitic nanounits for the catalytic cracking of high density polyethylene

Hierarchical Beta zeolites have been prepared employing two strategies of synthesis for the generation of mesopores with different size distribution. In both cases, the starting gel was subjected to pre-crystallization to induce the formation of protozeolitic nanounits. The first strategy involves the reorganization of the protozeolitic nanounits by incorporation of a surfactant (cetyltrimethylammonium bromide, CTAB), whereas the second one is based on the use of a silanization agent (phenylaminopropyl-trimethoxysilane, PHAPTMS) that it is grafted to the outer part of the nanounits to avoid their total merge during the crystallization process. In addition, conventional Beta zeolite and ordered mesoporous aluminosilicates have been prepared and employed as reference samples. All materials have been characterized by a number of techniques, whereas their catalytic behavior has been evaluated in the cracking of high density polyethylene (HDPE). The h-Beta (CTAB) sample shows a narrower mesopore size distribution, more uniform and stable tetrahedral Al species and a higher acid strength than the material obtained from silanized nanounits, although the latter exhibits a larger accessibility as denoted by its higher mesopore/external surface area (351m2/g in h-Beta (PHAPTMS) versus 228m2/g in h-Beta (CTAB)). While the ordered mesoporous aluminosilicates were almost inactive in HDPE cracking (<5wt% of conversion) due to its weak acidity, high plastic conversions were obtained over the zeolite Beta samples (>50wt% of conversion). The results obtained indicate that the overall catalytic activity depends mainly on the accessibility to the active sites, whereas the product selectivity is rather determined by the acid strength of the zeolite catalysts. Thus, h-Beta (CTAB) sample promoted in a great extension both end-chain cracking and aromatization reactions due to its high acid strength.

Kinetic analysis for thermal cracking of HDPE: A new isoconversional approach

Converting waste plastic into marketable hydrocarbons is a promising way to protect the environment and earn financial gain. While several kinetic studies have examined the cracking of waste plastic, they are often oversimplified or inappropriate for designing an industrial reactor. This work proves the necessity of using isoconversional methods by using Differential Scanning Calorimetry (DSC). DSC verified that cracking of waste plastic involves the production of several intermediates, indicating that the kinetics is more complex and requires further studies. Several isoconversional methods such as Friedman, Ozawa, Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS) and model-free were applied to estimate the apparent activation energy and frequency factor of thermal cracking of high-density polyethylene (HDPE) using nonisothermal and isothermal Thermogravimetric Analysis (TGA). The determined kinetic parameters from each method were evaluated against the experimental data. As none of these methods were found to have an acceptable agreement with the experimental data, a new isothermal isoconversional model was proposed and good matching results was were observed.

Thermo-catalytic pyrolysis of waste polyethylene bottles in a packed bed reactor with different bed materials and catalysts

Plastic waste is an increasing economic and environmental problem as such there is a great need to process this waste and reduce its environmental impact. In this work, the pyrolysis of high density polyethylene (HDPE) waste products was investigated using both thermal and catalytic cracking techniques. The experimental work was carried out using packed bed reactor operating under an inert atmosphere at 450°C. Different reactor bed materials, including sand, cement and white clay were used to enhance the thermal cracking of HDPE. In addition, the catalytic effect of sodium hydroxide, HUSY and HBeta zeolite catalysts on the degradation of HDPE waste was also investigated. The reactor beds were found to significantly alter the yield as well as the product composition. Products such as paraffins (⩽C44), olefins (⩽C22), aromatics (⩽C14) and alcohols (C16 and C17) were obtained at varying rates. The highest yield of liquid (82%) was obtained over a cement powder bed with a paraffin yield of 58%. The yield of paraffins and olefins followed separate paths, for paraffins it was found to increase in the order or Cement>White clay>Silica Sand, whereas for the olefins it was in the reverse order Silica Sand>White clay>Cement. The results obtained in this work exhibited a higher P/O ratio than expected, where the amount of generated paraffins was greater than 60% in most cases. Less olefin was generated as a consequence. This indicates that the product generated is more suited to be used as a fuel rather than as a chemical feedstock. The carbon chain length was narrowed to C10–C28 when the zeolitic catalysts were employed, as well as a significant yield of aromatics was obtained mainly naphthalene and d-limonene as an indication that the products obtained are fuel-like products.

Effect of Temperature and Catalyst Loading on Product Yield in Catalytic Cracking of High Density Polyethylene (HDPE)

We have studied the effect of temperature and catalyst loading on the product yield in cracking of high density polyethylene (HDPE). We have identified the optimal temperature and catalyst loading that results in maximum conversion. We have constructed a kinetic model of the process and have determined the activation energy.

Identification of the coke deposited on an HZSM-5 zeolite catalyst during the sequenced pyrolysis–cracking of HDPE

The pyrolysis–cracking of high-density polyethylene (HDPE) has been studied in two sequenced steps: (1) flash pyrolysis in a conical spouted bed reactor, and (2) catalytic cracking of the volatiles (waxes) in a fixed bed reactor containing a HZSM-5 zeolite catalyst, aiming light olefins as final products. The pyrolysis and cracking have been carried out isothermally at 500°C, with a continuous feed of HDPE (1gmin−1) for up to 5h (300g of HDPE fed). We have correlated the catalytic deactivation by coke (carbonaceous deposits), in terms of amount and composition, with the profiles of gas composition along time on stream and space time. The amount and composition of coke in three axial positions of the catalytic bed have been elucidated using thermogravimetric (TG-TPO) and spectroscopic techniques (13C CP-MAS NMR, Raman, FTIR, FTIR-TPO-MS and FTIR-pyridine). Our results show that there are two pathways of coke formation: (i) initiation, during the first hour on stream and particularly in the inlet of the catalytic reactor; and (ii) steady coke formation, after the first hour on stream which is more severe in the last axial position of the catalytic reactor. The initiation step stems from the degradation of the waxes produced in the pyrolysis of HDPE and causes a dropping in the mesopore area of the catalyst. The steady coke formation step is caused by the condensation of light olefins and causes the degradation of the micropore area and the Brønsted acidity of the catalyst.

Polyethylene Cracking on a Spent FCC Catalyst in a Conical Spouted Bed

The catalytic cracking of HDPE (high density polyethylene) at 500 °C using a spent FCC catalyst agglomerated with bentonite (50 wt %) has been studied in a conical spouted bed reactor. The reaction is carried out in continuous regime (1 g min–1 of HDPE is fed) with no bed defluidization problems. The results obtained, namely, total conversion, and high yields of gasoline (C5–C11 fraction) (50 wt %) and C2–C4 olefins (28 wt %), are explained by favorable reactor conditions and good catalyst properties. These results are compared with those for a catalyst prepared in the laboratory by agglomerating a commercial HY zeolite (SiO2/Al2O3 = 5.2). The conical spouted bed is a suitable reactor for enhancing the physical steps of melting the polymer and coating the catalyst with the melted polymer. Furthermore, high heat and mass transfer rates promote devolatilization, and short residence times minimize secondary reactions from olefins by enhancing primary cracking products. The meso- and macroporous structure of the spent FCC catalyst matrix enhances the diffusion of long polymer chains, whereas the zeolite crystals have micropores that give a proper shape selectivity to form the lumps of gasoline and light olefins. Because of long use in reaction–regeneration cycles, the moderate acidity of the spent FCC catalyst minimizes the secondary reactions of hydrogen transfer, and so restricts the formation of aromatics and paraffins, as well as the reactions of overcracking and condensation and, therefore, the coke formation. The spent FCC catalyst exhibits a low deactivation rate and is regenerable by coke combustion with air at 550 °C. Consequently, the use of a catalyst with the sole cost of a simple agglomeration and the production of value added product streams make the process of polyolefin catalytic cracking a promising option for refinery integration.

Light olefins from HDPE cracking in a two-step thermal and catalytic process

The continuous catalytic pyrolysis of high density polyethylene (HDPE) has been carried out in a two-step reaction system involving a pyrolysis conical spouted bed reactor followed by a catalytic fixed bed reactor. The good performance of the conical spouted bed reactor has allowed using a low temperature in thermal cracking (500°C) without defluidization problems, obtaining a volatile stream with a 90wt.% overall yield of C12–C20 and waxes in the first step. The effect of the second-step operating conditions on product yields and composition has been studied using a catalyst based on a HZSM-5 (SiO2/Al2O3=30) zeolite. The influence of temperature in the 350–550°C range and space–time in the 0–8gcat min gHDPE−1 range has been studied. An increase in temperature or space–time gives way to an increase in the yield of light olefins, reaching a value of 62.9wt.% at 550°C and 8 gcat min gHDPE−1, with the individual yields of ethylene, propylene and butenes being 10.6, 35.6 and 16.7wt.%, respectively. Although the yield of single-ring aromatics increases when both variables studied are increased, the maximum yield obtained was lower than 13wt.%. The yield of waxes (the main product in the first step) is negligible even at low temperatures or spaces-times, which evidences the efficiency of the catalytic step.

Pathways of coke formation on an MFI catalyst during the cracking of waste polyolefins

A study has been carried out on the deposition kinetics of carbonaceous-species on an acid catalyst (containing an MFI zeolite) in the cracking of high-density polyethylene and polypropylene. The initiation of coke deposition occurs on the acid sites, followed by aromatic and aliphatic growth in the micro- and mesopores, respectively.

Effect of the acidity of the HZSM-5 zeolite catalyst on the cracking of high density polyethylene in a conical spouted bed reactor

The catalytic cracking of high density polyethylene (HDPE) has been carried out at 500 °C in a conical spouted bed reactor with two catalysts prepared with HZSM-5 zeolites with SiO 2/Al 2O 3 ratios of 30 and 80. The polyethylene has been fed continuously (1 g min −1) over 10 h to a 30 g catalyst bed. The results show the good performance of the conical spouted bed reactor in minimising the limitations of the physical steps of the process. The deactivation of the catalysts is very low and it is demonstrated that the moderation of the acidity is useful in modifying the product distribution. The SiO 2/Al 2O 3 ratio increment involves a decrease in the total acidity and in the acid strength, resulting in a higher yield of C 2–C 4 olefins and that of the non-aromatic C 5–C 11 fraction, and a decrease in the yields of aromatic components and C 1–C 4 paraffins. The yield of the C 2–C 4 olefins obtained with the HZSM-5 zeolite catalyst with a ratio of Si/Al 2O 3 = 80 is 59.8 wt% (that of propylene is 29.6 wt%) and the yield of the gasoline fraction (C 5–C 11) accounts for 32.1 wt%. The coke deposited on the catalyst has a heterogeneous nature and is constituted by two types of coke, which are deposited on the exterior and the interior of the crystalline channels of the HZSM-5 zeolite. The evolution of the coke is attenuated as the SiO 2/Al 2O 3 ratio of the zeolite is increased.

The effect of ZSM-5 zeolite acidity on the catalytic degradation of high-density polyethylene using simultaneous DSC/TG analysis

In the present work, simultaneous thermogravimetric (TG) and differential scanning calorimetric (DSC) analyses were used to investigate the effect of the acidity of HZSM-5 zeolites on the catalytic degradation of high-density polyethylene (HDPE). The acidity of the zeolite was modified by ion exchange with sodium. The results obtained using HZSM-5 zeolites of varying acid strengths show the effect of increasing the acidity on the reduction of the degradation temperature. The addition of sodium to HZSM-5, by ion exchange, results in a decrease of the acid strength of the catalyst and an increase in the observed degradation temperature from 402 to 465 °C. The simultaneous use of the signals from the TG and DSC allowed the development of a kinetic model that is able to accurately describe all the runs performed, both the thermal and the catalytic degradation of the polymer. The kinetic parameters obtained clearly revealed the reduction in the activation energy due to the presence of the catalyst and its relation to the overall acidity of the samples. The gases evolved from the pyrolysis of polyethylene were also analysed using gas chromatography, and it was also found that volatile product selectivity changes with the catalyst acidity. The fact that the overall activity for the catalytic cracking of HDPE on these zeolites varies with internal acid strength of the zeolite indicates that the inner surface of these zeolites participates in this reaction despite the fact that the reactant molecules are extremely large.

Insights into the coke deposited on HZSM-5, Hβ and HY zeolites during the cracking of polyethylene

The effect of the zeolite structure (HZSM-5, Hβ and HY) on coke deposition during the cracking of high-density polyethylene has been studied by combining the results of multiple spectroscopic and analytical techniques: FTIR, Raman, UV–vis, 13C NMR and coke extraction, followed by GC-MS and 1H NMR analysis. In addition, by combining FTIR and temperature programmed oxidation (TPO) analysis we obtained information on the coke: properties, burn-off, and changes in composition during catalyst regeneration. Samples of the spent catalysts were obtained in a state-of-the-art pilot plant (conical spouted bed reactor) after the continuous treatment of 900g (1gmin−1, 15h) of high-density polyethylene at 500°C, using 30g of catalyst. The results show that as the pore diameter of the zeolite is increased, bimolecular reactions (hydrogen transfer and oligomerizations), condensations and cyclizations are enhanced, yielding more aromatic coke. Furthermore, the pore topology of the HZSM-5 zeolite improves the flow of coke precursors (also favored by the high flow rate of N2) to the outside of the catalyst; viz. HZSM-5 catalyst preserves its activity for longer.

Role of pore structure in the deactivation of zeolites (HZSM-5, Hβ and HY) by coke in the pyrolysis of polyethylene in a conical spouted bed reactor

The deactivation of three different catalysts used in the cracking of high density polyethylene (HDPE) has been compared. The catalysts used are HZSM-5, Hβ and HY zeolites agglomerated with bentonite and alumina. The reactions have been carried out in a conical spouted bed reactor at 500°C, and plastic (high density polyethylene) has been fed in continuous mode (1gmin−1) for up to 15h of reaction. The HZSM-5 zeolite catalyst gives way to high yields of C2–C4 olefins (57wt%) and, moreover, it is the one least influenced by deactivation throughout the run, which is explained by the lower deterioration of its physical properties and acidity. The results of temperature program combustion and transmission electron microscopy show that coke growth is hindered in the HZSM-5 zeolite pore structure. The high N2 flow rate used in the conical spouted bed reactor enhances coke precursor circulation towards the outside of the zeolite crystal channels.

Waste catalysts for waste polymer

Catalytic cracking of high-density polyethylene (HDPE) over fluid catalytic cracking (FCC) catalysts (1:6 ratio) was carried out using a laboratory fluidized bed reactor operating at 450 °C. Two fresh and two steam deactivated commercial FCC catalysts with different levels of rare earth oxide (REO) were compared as well as two used FCC catalysts (E-Cats) with different levels of metal poisoning. Also, inert microspheres (MS3) were used as a fluidizing agent to compare with thermal cracking process at BP pilot plant at Grangemouth, Scotland, which used sand as its fluidizing agent. The results of HDPE degradation in terms of yield of volatile hydrocarbon product are fresh FCC catalysts ≫ steamed FCC catalysts ≈ used FCC catalysts. The thermal cracking process using MS3 showed that at 450 °C, the product distribution gave 46 wt% wax, 14% hydrocarbon gases, 8% gasoline, 0.1% coke and 32% nonvolatile product. In general, the product yields from HDPE cracking showed that the level of metal contamination (nickel and vanadium) did not affect the product stream generated from polymer cracking. This study gives promising results as an alternative technique for the cracking and recycling of polymer waste.

Recycling of Polymer Waste with Fluid Catalytic Cracking Catalysts

Feedstock recycling of high-density polyethylene (HDPE) over fluid catalytic cracking (FCC) catalysts (1:6 ratio) was carried out using a laboratory fluidized bed reactor operating at 450°C. Fresh and steam deactivated commercial FCC catalysts with different levels of rare earth oxide (REO) were compared as well as used FCC catalysts (E-Cats) with different levels of metal poisoning. Fresh FCC catalysts gave the highest results of HDPE degradation in terms of yield of volatile hydrocarbon product. Meanwhile, steamed FCC catalysts and used FCC catalysts showed similar but lower yields. Overall, the product yields from HDPE cracking showed that the level of metal contamination (nickel and vanadium) did not affect the product stream generated from polymer cracking. This study gives promising results as an alternative technique for the cracking and recycling of polymer waste.