Deactivation dynamics of a Ni supported catalyst during the steam reforming of volatiles from waste polyethylene pyrolysis

https://doi.org/10.1016/j.apcatb.2017.02.015Get rights and content

Highlights

  • Ni catalyst used for producing H2 from waste polyethylene suffers deactivation.

  • Deactivation occurs due to the simultaneous sintering and encapsulation (by coke).

  • The formation mechanism of encapsulating coke and its precursors are investigated.

  • Filamentous coke forms and grows from the carbonization of encapsulating coke.

Abstract

The valorization of waste high density polyethylene (HDPE) for hydrogen production has been studied in a two-step process, comprising pyrolysis and subsequent steam reforming of the volatiles produced in the first step. Particularly, this work focuses on the deterioration mechanisms (sintering and coke deposition) of the Ni commercial catalyst used in the second step, as it conditions the overall process performance. Pyrolysis of HDPE has been performed in a conical spouted bed reactor at 500 °C, and the catalytic steam reforming of the pyrolysis volatiles, in a fluidized bed reactor at 700 °C. Deactivated catalyst samples were recovered at different values of time on stream, and characterized using XRD, N2 adsorption-desorption, SEM and TEM electronic microscopies, temperature programmed oxidation (TPO), Raman, FTIR and LDI-TOF MS spectroscopies. The results show that the deactivation is due to the sintering and encapsulation -by coke- of Ni. The former is inevitable within the current conditions, and the latter can be ascribed to the condensation of adsorbed precursors that evolve over time. Encapsulating coke is partially carbonized into filamentous coke with lower effect on catalytic deactivation and higher economic interest.

Introduction

Plastic wastes are a great source of chemicals and fuels due to the following facts: (1) their world production shows a continuous increasing trend, registering a global production of 311 million tons in 2014 [1], (2) the subsequent dumping, generating a serious environmental problem due to their low biodegradability [2], and (3) their favorable chemical composition. Europe reached a plastic waste generation of 25.8 million tons in 2014, from which 30.8% went to landfill and 69.2% was used for recycling or incineration [1]. Polyolefins involved approximately 49% of the plastic demand, consisting of high density polyethylene, low density polyethylene and linear low density polyethylene (HDPE, LDPE and LLDPE, respectively) and polypropylene (PP). The USA registered a municipal plastic waste generation of almost 32 million tons in 2012 (63% polyolefins), from which 8.8% was recovered [3]. Taking this all into account, the valorization of waste plastics, and polyolefins in particular, turns out to be essential, in order to counteract this increasing worldwide consumption and its effects in the environment within the sustainable development [4].

Tertiary recycling of waste plastics by means of pyrolysis is considered the most attractive strategy to valorize these residues on a large scale in the consumer society [5], [6]. Consequently, the pyrolysis of plastics has received great attention and acquired notable technological development [7], [8]. Furthermore, pyrolysis units are relatively simple, respectful with environment and may be placed next to the collection and classification points of urban solid waste. On the other hand, the composition of products can be modified by incorporating acid catalysts to the pyrolysis reactor [9].

The studies on thermal or catalytic pyrolysis of waste polyolefins have been mainly targeting the monomer recovery [10], [11], [12], [13] or the production of diesel fuel [14], [15], [16]. Pyrolysis of polyolefins at low temperature (500 °C) allows the selective production of pyrolysis waxes (C21+) which can be fed into refinery units such as the catalytic cracker or the catalytic reformer, alone or together with the regular feed [17], [18], [19]. Besides, intending the production of monomers, the volatiles of the pyrolysis can be further treated with an in-line second step, such as secondary thermal cracking [20] or catalytic cracking [21], [22], [23], [24]. The advantages of the sequenced two-step process are: (i) higher versatility when operating at the optimum temperature in each step; (ii) lower catalyst deactivation in the second transformation, without the problems inherent to the plastic melting on the catalyst. These advantages of the sequenced two-step process are attractive for H2 production, by means of in-line catalytic reforming of the volatiles exiting from the pyrolysis reactor. In a pioneer work with this strategy, Czernik and French [25] studied the continuous and sequenced polypropylene pyrolysis and volatile reforming, in separated fluidized beds, yielding 80% H2 (with respect to the theoretical maximum) using a naphtha reforming commercial catalyst in the second reactor. Wu and Williams [26], [27], [28], [29], [30], [31] and Acomb et al. [32] studied the performance of different reforming catalysts and conditions in this kind of sequenced pyrolysis-reforming of polyolefins. Furthermore, several authors [33], [34] have employed fixed bed reactors in both steps. Erkiaga et al. [35], operating in continuous regime, proved the advantages of the conical spouted bed reactor (CSBR) in the first step of HDPE pyrolysis, due to the fact that the vigorous cyclic movement of sand particles covered by plastic avoids the bed defluidization. Moreover, the fast rate of mass and heat transport between phases allows obtaining a homogeneous volatiles stream consisting of C5+ hydrocarbons at low temperature (500 °C). These authors emphasize the limitation of the process due to the deposition of carbonaceous material (coke) in the reforming fixed-bed catalytic reactor. Barbarias et al. [36] have improved the system performance, using a fluidized bed reactor in the catalytic reforming step.

It is well stablished in the literature that deactivation of Ni catalysts in reforming reactions takes place due to coke deposition and Ni sintering. Several authors studied the evolution of coke content and nature in the reforming of hydrocarbons, such as methane [37], [38] and propane [39], identifying two coke types: (i) Ni-encapsulating amorphous coke; and (ii) structured filamentous coke. The latter is sometimes called fibrillar or whisker-like coke and if the conditions are appropriate, it can be classified as carbon nanotubes (CNTs). Latorre et al. [40], [41], [42] proposed a mechanism of CNTs formation in which methane forms a metastable carbide, which releases carbon atoms that are diffused through the interphase of metallic nanoparticles, forming the nanotube after a nucleation stage. In the reforming of oxygenates, such as ethanol [43], [44], [45], [46], dimethyl ether [47], [48], and bio-oil [49], two main coke types were determined as well. In these works, the condensation of oxygenated reaction intermediates plays a major role in the rapid formation of amorphous and encapsulating coke, while the formation of structured coke is mainly due to CH4 dehydrogenation and Boudouard reaction. All these works highlight the relevance of the feed composition and reaction conditions (temperature and steam/carbon ratio) on the content and nature of the deposited coke. These types of coke have been detected in the spent catalyst used in the production of hydrogen from polyolefins too [27], [29], [30], [31], [32]. On the other hand, it is well stablished that sintering is a noticeable deactivation cause in Ni catalysts, above temperatures in the range of 600–700 °C, depending on the catalyst structure, the operating conditions and the reaction medium composition [49], [50], [51], [52], [53].

This work focuses on the mechanisms of catalyst deactivation used in the second step of the sequenced pyrolysis-reforming of HDPE for hydrogen production. For this aim, we have used the same system used by Barbarias et al. [36] analyzing the evolution of catalyst and coke morphology at different stages of deactivation (values of time on stream). The dynamics of catalytic deactivation has been studied by means of X-ray diffraction (XRD), N2 adsorption-desorption, scanning and transmission electron microscopies (SEM and TEM, respectively), temperature programmed oxidation (TPO), Raman spectroscopy, Fourier transformed infra-red (FTIR) spectroscopy and laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS). The optimized operational conditions for minimizing the impact of catalyst deactivation are suggested, together with a simplified mechanism of catalyst deactivation.

Section snippets

Materials

The HDPE was provided by Dow Chemical (Tarragona, Spain) in the form of chipping (4 mm). The main properties are: average molecular weight, 46.2 kg mol−1; polydispersity, 2.89; and density, 940 kg m−3. The higher heating value, 43 MJ kg−1, was determined by differential scanning calorimetry (Setaram TG-DSC-111) and isoperibolic bomb calorimetry (Parr 1356).

The reforming catalyst was provided by Süd Chemie (G90LDP catalyst) and its chemical formulation is based on NiO, CaAl2O3 and Al2O3. The catalyst

Evolution with time on stream of reaction indices

Catalyst deactivation leads to a decrease in the rates of reactions involved in the reforming process: (1) hydrocarbon steam reforming and (2) water gas shift (WGS):CnHm + n H2O  n CO + (n + 0.5 m) H2CO + H2O  CO2 + H2

In order to quantify the deactivation, the following reaction indices were established, corresponding to the reforming step: conversion (X), yield of each i product (Yi) and hydrogen yield (YH2)X=CarbonunitsinthegasstreamCarbonunitsintheHDPEfeed100Yi=CarbonunitsintheiproductCarbonunitsintheHDPE

Discussion

The aforementioned results show the existence of two phenomena which contribute to catalyst deactivation during the steam reforming of HDPE pyrolysis volatiles: (i) an increase in the Ni crystal size, and; (ii) coke formation, whose content and nature evolve significantly with time on stream. It is noteworthy that the decrease of the reaction indices with time on stream has an increasing rate (Fig. 1), which may be related with both Ni sintering and coke formation-growth, with two

Conclusions

The deactivation of the Ni catalyst in the steam reforming of a sequenced HDPE pyrolysis-steam reforming process is a consequence of a series of structural and compositional changes of the catalyst: Ni sintering and the deposition of coke. The combination of analytical techniques of coke characterization (SEM and TEM, TPO, Raman, FTIR and LDI-TOF MS) has proved to be successful to analyze the evolution with time on stream of the formation and growth of coke on the catalyst. It has been

Acknowledgments

This work was carried out with the support of the Ministry of Economy and Competitiveness (Spain), some cofounded with ERDF funds (CTQ2013-46172-P and CTQ2013-45105-R), the Basque Government (Spain, IT748-13), and the University of the Basque Country (UPV/EHU, Spain, UFI 11/39)). A. Ochoa is grateful for his predoctoral grant from the Department of Education, Language Policy and Culture of the Basque Goverment (Spain, PRE_2016_2_0129). The help of Dr. Antonio Veloso (LDI-TOF MS technician,

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