Heat transfer, mass transfer and kinetics study of the vacuum pyrolysis of a large used tire particle

Waste Refinery: The Valorization of Waste Plastics and End-of-Life Tires in Refinery Units. A Review

This review collects a wide range of initiatives and results that expose the potential of the refineries to be converted into waste refineries. Thus, they will use their current units for the valorization of consumer society wastes (waste plastics and end-of-life tires in particular) that are manufactured with petroleum derivatives. The capacity, technological development, and versatility of fluid catalytic cracking (FCC) and hydroprocessing units make them appropriate for achieving this goal. Polyolefinic plastics (polyethylene and polypropylene), the waxes obtained in their fast pyrolysis, and the tire pyrolysis oils can be cofed together with the current streams of the industrial units. Conventional refineries have the opportunity of operating as waste refineries cofeeding these alternative feeds and tailoring the properties of the fuels and raw materials produced to be adapted to commercial requirements within the oil economy frame. This strategy will contribute in a centralized and rational way to the recycling of the consumer society wastes on a large scale. Furthermore, the use of already existing and, especially, depreciated units for the production of fuels and raw materials (such as light olefins and aromatics) promotes the economy of the recycling process.

Waste Tyres Pyrolysis for Obtaining Limonene

This review deals with the technologies of limonene production from waste tyre pyrolysis. Thermal decomposition is attractive for tackling the waste tyre disposal problem, as it enables both: energy to be recovered and limonene to be obtained. This material management recycling of tyres is environmentally more beneficial than the burning of all valuable products, including limonene. Given this recoverability of materials from waste tyres, a comprehensive evaluation was carried out to show the main effect of process conditions (heating rate, temperature, pressure, carrier gas flow rate, and type of volatile residence and process times) for different pyrolytic methods and types of apparatus on the yield of limonene. All the results cited are given in the context of the pyrolysis method and the type of reactor, as well as the experimental conditions in order to avoid contradictions between different researchers. It is shown that secondary and side reactions are very sensitive to interaction with the above-mentioned variables. The yields of all pyrolytic products are also given, as background for limonene, the main product reported in this study.

Modeling kinetics and transport phenomena during multi-stage tire wastes pyrolysis using Comsol®

This paper is devoted to the modeling of the pyrolysis process in order to predict mass and heat loss profiles of a used tire sample and ultimately prevent eventual difficulties in pyrolysis reactors. Once assumptions are made, the thermal balances and kinetics of each reaction were written. The resolution of the differential equations allowed us to present the profile of heat variation within the sample as a function of temperature in a fixed bed reactor. The modeling is based on the energy behavior of each reaction, the rate conversion of which was also modeled and compared with that obtained experimentally. There is a satisfactory agreement between the theoretical and the experimental results in one hand and a good fit with experimental and regression results of other researchers in another hand. It was shown in particular, that some exothermic reactions intervene during the pyrolysis of the used tires. Indeed, the exothermic heat at the center of 2 cm particle exceeds 1500 kJ/kg which presents an economic and energetic payoff for the plant. It has also been noted that the small particle size can result in faster heat transfer and shorten the process completion time. At the same time, rapid heat transfer can trigger more endothermic reactions, increasing thus the overall energy consumption of the process.

Scrap tyre pyrolysis: Modified chemical percolation devolatilization (M-CPD) to describe the influence of pyrolysis conditions on product yields

This paper attempts to develop a modified chemical percolation devolatilization (M-CPD) model that can include heat transfer, primary pyrolysis and the secondary cracking reactions of volatiles together to describe the pyrolysis of waste scrap tyre chip, as well as to examine the influence of operating conditions on the scrap tyre pyrolysis product yields. Such a study has yet to be conducted in the past, thereby leading to a large knowledge gap failing to understand the pyrolysis of the coarse feedstock appropriately. To validate the developed model, a number of operating parameters including reactor configurations, carrier gas compositions (argon and argon blended with CO₂ and/or steam), scrap tyre chip size (0.5-15.0 mm), terminal pyrolysis temperature (400-800 °C) and heating rate (10 °C/min and 110 °C/min) were examined in a lab-scale fixed-bed pyrolyser, with a particular focus on the secondary cracking extents of the liquid tar. Through both experimental investigation and modelling approach, it was found that significant secondary cracking extent occurred upon the increase in the feedstock size, heating rate and residence time. Upon the fast pyrolysis, the average temperature gap between the centres of the coarse particle and reactor wall could reach a maximum of 115 °C for the tyre chips of 6-15 mm. Consequently, its primary volatiles underwent the secondary cracking reaction at an overall extent of 17% at a terminal temperature of 600 °C and a fast heating rate of 110 °C/min. Consequently, the yield of light gases including methane was increased remarkably. The flow rate of inert carrier gas was also influential in the secondary cracking, in which a maximum tar yield (54 wt%) was reached at a carrier gas flow rate of 1.5  L/min. This indicates the occurrence of secondary cracking has been largely minimised. At a pyrolysis temperature of 600 °C, the addition of CO₂ in the carrier gas had an insignificant effect on the product yield distribution under the slow heating scheme. In contrast, the addition of steam resulted in a slight increase of carbon monoxide, presumably due to the occurrence of gasification reaction.

Determination of Enthalpy of Pyrolysis from DSC and Industrial Reactor Data: Case of Tires

This study was motivated by the fact that differential scanning calorimetry (DSC)/differential thermal analysis (DTA) results in literature showed significant exothermic peaks while in overall, pyrolysis is an endothermic phenomenon. The specific heat of the decomposing tires has been determined with a new methodology: instead of assuming constant char properties throughout pyrolysis, the specific heat of evolving solids (char) was evaluated with increasing temperature and conversion. Measured specific heat values were observed to increase until pyrolysis was triggered at 250°C. Then, the specific heat of the solids decreased continuously until 400°C at which point they started to increase. This unexpected trend pointed out that the exothermic peak observed with DSC is an artefact generated by the control system of the apparatus. To overcome this limitation, the energy balance was performed over industrial data and the newly found heat capacity values. The enthalpy of pyrolysis was found to have a term dependent on the weight loss derivative, with a constant value of 410 kJ/kg tires. Two other terms for the enthalpy of pyrolysis have been identified, which were independent of weight loss. The first one is believed to correspond to the sulphur cross-link breakage at low temperature (65 kJ/kg), while the second one, at the final stage of pyrolysis, should correspond to charring reactions approaching the thermodynamic equilibrium (75 kJ/kg). Ultimately, this work proposes a new methodology to determine the enthalpy of pyrolysis with larger scale experimental data.

Kinetics of scrap tyre pyrolysis under vacuum conditions

Scrap tyre pyrolysis under vacuum is attractive because it allows easier product condensation and control of composition (gas, liquid and solid). With the aim of determining the effect of vacuum on the pyrolysis kinetics, a study has been carried out in thermobalance. Two data analysis methods have been used in the kinetic study: (i) the treatment of experimental data of weight loss and (ii) the deconvolution of DTG (differential thermogravimetry) curve. The former allows for distinguishing the pyrolysis of the three main components (volatile components, natural rubber and styrene-butadiene rubber) according to three successive steps. The latter method identifies the kinetics for the pyrolysis of individual components by means of DTG curve deconvolution. The effect of vacuum in the process is significant. The values of activation energy for the pyrolysis of individual components of easier devolatilization (volatiles and NR) are lower for pyrolysis under vacuum with a reduction of 12K in the reaction starting temperature. The kinetic constant at 503K for devolatilization of volatile additives at 0.25atm is 1.7 times higher than that at 1atm, and that corresponding to styrene-butadiene rubber at 723K is 2.8 times higher. Vacuum enhances the volatilization and internal diffusion of products in the pyrolysis process, which contributes to attenuating the secondary reactions of the repolymerization and carbonization of these products on the surface of the char (carbon black). The higher quality of carbon black is interesting for process viability. The large-scale implementation of this process in continuous mode requires a comparison to be made between the economic advantages of using a vacuum and the energy costs, which will be lower when the technologies used for pyrolysis require a lower ratio between reactor volume and scrap tyre flow rate.

An algorithm for the kinetics of tire pyrolysis under different heating rates

Tires exhibit different kinetic behaviors when pyrolyzed under different heating rates. A new algorithm has been developed to investigate pyrolysis behavior of scrap tires. The algorithm includes heat and mass transfer equations to account for the different extents of thermal lag as the tire is heated at different heating rates. The algorithm uses an iterative approach to fit model equations to experimental data to obtain quantitative values of kinetic parameters. These parameters describe the pyrolysis process well, with good agreement (r(2)>0.96) between the model and experimental data when the model is applied to three different brands of automobile tires heated under five different heating rates in a pure nitrogen atmosphere. The model agrees with other researchers' results that frequencies factors increased and time constants decreased with increasing heating rates. The model also shows the change in the behavior of individual tire components when the heating rates are increased above 30 K min(-1). This result indicates that heating rates, rather than temperature, can significantly affect pyrolysis reactions. This algorithm is simple in structure and yet accurate in describing tire pyrolysis under a wide range of heating rates (10-50 K min(-1)). It improves our understanding of the tire pyrolysis process by showing the relationship between the heating rate and the many components in a tire that depolymerize as parallel reactions.

Vacuum pyrolysis of waste tires with basic additives

Granules of waste tires were pyrolyzed under vacuum (3.5-10 kPa) conditions, and the effects of temperature and basic additives (Na2CO3, NaOH) on the properties of pyrolysis were thoroughly investigated. It was obvious that with or without basic additives, pyrolysis oil yield increased gradually to a maximum and subsequently decreased with a temperature increase from 450 degrees C to 600 degrees C, irrespective of the addition of basic additives to the reactor. The addition of NaOH facilitated pyrolysis dramatically, as a maximal pyrolysis oil yield of about 48 wt% was achieved at 550 degrees C without the addition of basic additives, while a maximal pyrolysis oil yield of about 50 wt% was achieved at 480 degrees C by adding 3 wt% (w/w, powder/waste tire granules) of NaOH powder. The composition analysis of pyrolytic naphtha (i.b.p. (initial boiling point) approximately 205 degrees C) distilled from pyrolysis oil showed that more dl-limonene was obtained with basic additives and the maximal content of dl-limonene in pyrolysis oil was 12.39 wt%, which is a valuable and widely-used fine chemical. However, no improvement in pyrolysis was observed with Na2CO3 addition. Pyrolysis gas was mainly composed of H2, CO, CH4, CO2, C2H4 and C2H6. Pyrolytic char had a surface area comparable to commercial carbon black, but its proportion of ash (above 11.5 wt%) was much higher.