Search

The current kinetic models for polystyrene (PS) pyrolysis contain many simplifications to reduce their size and the corresponding simulation time. Moreover, they are often based on rate coefficients determined using non -ideal experimental data featuring ambiguous process conditions with respect to mixing and temperature uni-formity. The practical interest of PS pyrolysis is the production of styrene monomer to be reused as a feedstock in the polymerization of styrene. In the present work, a lab-scale tree-based kinetic Monte Carlo (kMC) model is presented that differentiates between 18 reaction families and 26 end-group pairs to study the product yield variations for thermal degradation of PS. Model parameters follow from Bayesian optimization to experimental data recorded with an in-house micro-pyrolysis unit coupled with comprehensive two-dimensional gas chro-matography. Low chain length (CL) anionic-made PS is specifically considered to gain an understanding of the role of specific end-groups. The experimental yields of the major products (monomer: 74.7-80.8 wt%, dimer: 5.1-5.5 wt%, trimer: 1.6-7.7 wt%) are well-predicted with the fine-tuned parameters. The main reaction pathway in the formation of styrene monomer is end-chain beta-scission, while mid-chain beta-scission is primarily involved in the formation of the styrene dimer and trimer. Our model shows that the pyrolysis of low CL anionic-made PS leads to better rate coefficients than those obtained from state-of-the-art pyrolysis of long CL PS, in which end-groups play a much smaller role.

ispartof: Progress In Energy And Combustion Science vol:96 status: published

Research in chemical engineering requires experiments, which are often expensive, time-consuming, and labo-rious. Design of experiments (DoE) aims to extract maximal information from a minimum number of experi-ments. The combination of DoE with machine learning leads to the field of active learning, which results in a more flexible, multi-dimensional selection of experiments. Active learning has not yet been applied in reaction modeling, as most active learning techniques still require an excessive amount of data. In this work, a novel active learning framework called GandALF that combines Gaussian processes and clustering is proposed and validated for yield prediction. The performance of GandALF is compared to other active learning strategies in a virtual case study for hydrocracking. Compared to these active learning methods, the novel framework out-performs the state-of-the-art and achieves a 33%-reduction in experiments. The proposed active learning approach is the first to also perform well for data-scarce applications, which is demonstrated by selecting ex-periments to investigate the ex-situ catalytic pyrolysis of plastic waste. Both a common DoE-technique, and our methodology selected 18 experiments to study the effect of temperature, space time, and catalyst on the olefin yield for the catalytic pyrolysis of LDPE. The experiments selected with active learning were significantly more informative than the regular DoE-technique, proving the applicability of GandALF for reaction modeling and experimental campaigns.

The iron and steel industry is a carbon-intensive industry and one of the largest industrial sources of CO2 emissions. In this work, we show how the steel mill gases can be conditioned using three metal oxides to produce a CO/CO2 stream that can be used for the production of chemicals, thereby preventing the emission of carbon to the atmosphere as CO2. Abundant oxides of iron and manganese, characterised by their readiness to capture and release gaseous O2, and calcium oxide, characterised by its capacity to capture and release gaseous CO2 can be deployed in this process. Process analysis indicates that by fully utilising the chemical energy of the carbon-rich blast furnace gas (BFG) of the steel mill, 37% of the associated CO2 emissions can be eliminated. A techno-economic evaluation shows that further reduction of CO2 emissions is viable. Preliminary estimations indicate that the cost for processing BFG through the proposed process is 46 EUR2020 per tonneBFG. The sources of revenue are the product CO/CO2 stream (0.2 tonneproduct per tonneBFG) and electricity constituting 85% and 14% of the total revenue with the remaining 1% obtained by the sale of spent metal oxides used in the process. The technical feasibility of the process was experimentally proven in a fixed bed reactor to produce a CO/CO2 stream and an H2O-rich stream while the metal oxides were periodically regenerated in alternating redox conditions. Thirty executed cycles indicated stable performance of the process. The proposed process concept can be applied to any gas stream containing CO2 and fuel. Catalytic fast pyrolysis of polyolefinic waste streams was investigated to recover valuable base chemicals at high selectivity. HZSM-5 zeolite with different properties, affected by Si/Al, mesoporosity, phosphorus stabilization, and steaming, were tested and thoroughly characterized. Different feeds, catalyst/feed ratios and reaction temperatures were evaluated in a micropyrolysis reactor coupled to two-dimensional gas chromatography. While unmodified HZSM-5 rapidly deactivated, phosphorus-modified and steamtreated HZSM-5 showed almost no deactivation due to its lower coking propensity during 130 runs with stable conversion towards C5+ aliphatics and high C-2-C-4 olefins selectivity (-75%) using post-consumer mixed polyolefins. The performance of this direct olefins production route with unprecedented high olefin selectivity was further evaluated in a plantwide context. It was found that it requires-37% lower energy input than the plastics pyrolysis followed by pyrolytic oil steam cracking, while it results to at least a one order of magnitude lower environmental burden as compared to waste incineration.

To date, the recycling of polyurethane (PU) and polyisocyanurate (PIR) waste still poses a significant problem. Within this contribution, the thermal degradation of high-resilient ether PU, semirigid PU, rigid PU, and (PIR) with and without TCPP flame retardant was studied at different temperatures to identify and quantify the main products. For this, a tandem micropyrolyzer coupled to GC × GC with FID and ToF-MS detectors was used, and the yield volatiles, light gases, and residue was quantified. The volatile pyrolysis vapours obtained from pyrolyzing high-resilient ether PU and semirigid PU at 600 °C were very similar, and the main products obtained were ethylene and propylene (combined yield of ~13 wt%), ~16 wt% of various oxygenates, and 4-6 wt% organic nitrogen compounds. The oxygenates included mostly poly-ether type compounds with varying molecular weight, and these are attributed to the decomposition of the polyol chains forming the soft segments of the polymer. The main semi-volatile nitrogen compound was 4,4'-methylenedianiline, attributed to the decomposition of the hard segments in the PU structure synthesized from MDI. The pyrolysis vapours from rigid PU contained several ether-type compounds and high yields of heteroatom-free monoaromatics—in particular styrene. The PIRs were more difficult to decompose and volatilize than the PUs, resulting in ~20 wt% higher residue yields. The charring propensity was even higher in the absence of a flame retardant in the formulation. Pyrolysis of TCPP-containing PIR produced Cl-containing pyrolysis vapours such as allyl chlorides. While the yield of H2O was comparable for the different PUs and PIR, pyrolysis of PIR produced higher yields of CO2 and less CO compared to the different PUs. The use of catalysts can help to convert PU waste pyrolysis vapours more efficiently to desirable chemicals.

The effect of H2/CO addition on flame dynamics of propane-air mixtures in a stepped microcombustor with heat recirculation was investigated experimentally. Unstable flame propagation regimes and their transition modes were observed for a range of mixture velocities (5���8 m/s) and equivalence ratios (0.6���0.95). Up to 60% H2 addition to lean propane-air mixtures helps improve the flame stability range in the combustor. The suppression of these flame instabilities with H2 addition is attributed to the increase in the overall mixture reactivity and burning velocity. The enhancement in OH intensity with H2 addition indicated a change in the elementary reactions, thereby affecting the flame dynamics. Contrary to H2 addition, CO addition does not show any noticeable effect on the formation of stable flame modes. The change in mixture properties and burning velocity leads to a change in the flame behavior and alters the heat release rate due to H2 addition to propane-air mixtures. A higher percentage of H2 addition led to the reappearance of the unstable (rotating) flames in the microcombustor. Intrinsic flame instabilities appeared at higher H2 percentages due to differential diffusion and increased thermal-wall coupling effects.

This paper reports the measurement of laminar burning velocities of DME (dimethyl ether)-air mixtures at higher mixture temperatures using planar flames stabilized in a controlled temperature mesoscale diverging channel. The reliability of this technique for flame speed measurement at high mixture temperatures has been established through various numerical studies using detailed 3-D computational model for DME-air mixtures. The distribution of fuel -air mass flux, reaction zone, and flame shape indicate that a planar flame is indeed formed in both transverse and depth directions of the channel. The stabilized flame is independent of any stretch effects except mild hydrodynamic strain due to channel divergence (30-50 s(-1)), and measured values of laminar burning velocities are within 5% of the actual value. The experiments were carried out at various equivalence ratios (phi = 0.7-1.4) and elevated mixture temperatures (350-640 K). The burning velocities and temperature exponents were determined using the planar flames stabilized at different mixture inlet velocities and temperatures. Slightly rich mixtures (phi = 1.1) point to the maximum burning velocity, in good agreement with recent literature at ambient conditions. Temperature exponents for different equivalence ratios increase to both sides of 4) = 1.1. Numerically calculated laminar burning velocities with different chemical kinetic schemes compared well with the measured burning velocities at higher mixture temperatures. (C) 2017 Elsevier Ltd. All rights reserved.

Chemical recycling of plastic waste via thermochemical processes is essential to move to a carbo-circular economy, reducing our dependence of fossil resources. However, recovering monomers from polyolefins in one or multiple steps is challenging due to their chemical inertness. In the present work, a tandem micro-pyrolyzer coupled to comprehensive two-dimensional gas chromatography and FID/ToF-MS detectors was utilized to study the performance of industrial formulations of steam-treated FCC catalysts and HZSM-5 additives for the in-line catalytic upgrading of polyolefin pyrolysis products towards light olefins and aromatics in two steps. When upgrading pyrolysis vapors from LDPE over the steam-treated catalysts at 600 °C, CH4 yields did not exceed 0.5 wt% due to their low acidity. The FCC catalyst formulations obtained higher yields of C5-C11 aliphatics (up to 42 wt%) and were more active in converting C12 + products than the HZSM-5-containing additives. The highest C2-C4 olefin selectivity of 53 wt% was obtained using a bare steam-treated HZSM-5 additive. With this catalyst, at higher catalyst loading and temperature (700 °C), the light olefin yield reached 69 wt% (19% ethylene, 22% propylene, 10% 1,3-butadiene, and 18% other C4 olefins). Importantly, similar yields of light olefins with even higher propylene yields of 31 wt% were obtained when processing real post-consumer mixed polyolefin waste. An adjusted loading of unsteamed catalyst (to obtain a similar level of initial conversion of C12 + products) showed higher coking propensity and deactivated more rapidly than its steam-treated version. The research results show great potential for the pyrolysis of mixed polyolefin waste followed by the direct in-line upgrading of the pyrolysis vapors to produce high value base chemicals such as C2-C4 olefins, aromatics, and naphtha-range aliphatics at high selectivity while limiting formation of coke, CH4, and H2.