Andreas Hornung

Kinetic study on the thermal degradation of polypropylene and polyethylene

Abstract In this work, a kinetic study on the thermal degradation of polyethylene and polypropylene is presented. The thermal degradation of these polymers is investigated under isothermal conditions using a gradient free reactor with on-line mass spectrometry. Apparent kinetic parameters for the overall degradation are determined. In the case of polyethylene degradation, a change of the apparent reaction order with temperature is observed, whereas in the case of polypropylene degradation, a constant apparent order of reaction within the investigated temperature range was found. Fractional pyrolysis under dynamic conditions of polypropylene performed at temperatures below the isothermal measurements reveal a decreasing amount of alkenes and alkanes and an increasing amount of dienes with temperature. On the basis of these data and corresponding literature, a detailed discussion of the reaction mechanisms is possible. Rate equations are formulated and kinetic models for polyethylene and polypropylene degradation are discussed, which are consistent with the measured rate coefficients.

Mechanisms and kinetics of thermal decomposition of plastics from isothermal and dynamic measurements

Abstract The kinetics of decomposition of plastics are of interest from different points of view, i.e. evolution of harmful substances during fires or waste incineration, recovering of chemical raw materials from plastic refuses and designing of recycling procedures. To measure the formal kinetic parameters of the degradation of polymers isothermal and dynamic methods are applied in this work. Dynamic measurements are performed by combined thermogravimetry mass spectrometry (TG-MS), the isothermal measurements are carried out with a new closed loop-type reactor. To evaluate consistent kinetic data from isothermal and dynamic measurements, the energy balance for the sample in dynamic measurements has to be considered to obtain the true sample temperature and heating rate. Subject of this investigation is the exploitation of dynamic and isothermal methods for measuring and interpreting the kinetics of thermal decomposition of plastics. Results for commodity plastics polyethylene and poly(vinyl chloride) (PVC) are presented. The combined application of TG–MS, isothermal experiments in the closed loop-type reactor and DSC leads to new results for the decomposition kinetics of PVC. The dehydrochlorination mechanism at moderate temperature can be distinguished in an endothermal and exothermal part. The benzene formation is identified as a second order reaction. A great advantage of the isothermal method is, that changes in the mechanisms are detectable, i.e. changes in the apparent order of the reaction and the apparent activation energy. From that, new mechanistic aspects of the decomposition kinetics of polyethylene were obtained.

Environmental engineering: Stepwise pyrolysis of plastic waste

Abstract Kinetic data obtained from micro thermogravimetry and gradient free reactor experiments confirm that different molecular structures of commodity plastics bring about different reaction mechanisms of thermal decomposition, different reaction rates, and different temperature dependencies of the decomposition rates. From that, stepwise pyrolysis of mixtures of plastics seems to be reasonable where the different components of the mixture are pyrolysed at different temperatures. To perform a stepwise pyrolysis in laboratory scale a cascade of well stirred reactors has been developed where mixing of the reactor contents occurs by circulating of stainless steel spheres. Examples for the separation of single plastics by stepwise thermal decomposition of mixtures of poly(vinyl chloride), polystyrene and polyethylene are presented. In the first step hydrogen chloride from poly(vinyl chloride) is released, in the second step styrene from polysytrene is formed and in the third step aliphatic compounds from polyethylene decompositon are trapped. Differences in the thermal degradation of single polymers and mixtures of polymers, e.g. in the apparent activation energies and preexponential factors, are investigated using mixtures and blends of polyethylene and polystyrene.

Dehydrochlorination of plastic mixtures

Abstract Dehydrochlorination of plastic mixtures from domestic waste as well as from other chlorine containing mixtures such as electronic scrap is an essential reaction step in waste incineration, pyrolysis and chemical recycling of polymers. For designing pyrolysis procedures, controlled combustion processes and to control the emissions from incinerators, the behaviour of polymers in thermal decomposition with regard to decomposition products and the kinetics of decomposition must be known. The kinetic data, for thermal decomposition of commodity plastics, confirms that in mixtures of different plastics the dehydrochlorination of, e.g. poly (vinyl chloride) (PVC) can be conducted at moderate temperatures and prior to the thermal degradation of the polymer skeleton. In stepwise low temperature pyrolysis mixtures of, e.g. PVC, polystyrene and polyethylene have been separated into hydrogen chloride, the monomer of polystyrene and aliphatic compounds from polyethylene decomposition. The degree of conversion of chlorine from PVC into hydrogen chloride in the low temperature (330°C) first step is about 99.6%. A similar behaviour for dehydrochlorination is obtained during the thermal degradation of electronic scrap. The hydrogen chloride evolution from PVC occurs in the same way as in mixtures of commodity plastics with a maximum rate of HCl loss at 280°C, when heated at 2 K min −1 . Brominated flame retardants are decomposed or evolved at higher temperatures (>300°C). A possibility to fix bromine in the residue is to add calcium carbonate to the electronic scrap before pyrolysis.

Investigation of the Kinetics of Thermal Degradation of Commodity Plastics

Incineration or burning of plastics involves the complicated interaction of thermal degradation, pyrolysis, combustion of pyrolysis products, mass and energy transport. Essential steps in the over-all process are the thermal degradation and formation of gaseous products that determine amongst others the effective rate of combustion. This paper reports some new results for the kinetics and mechanisms of the thermal decomposition of PVC and polyamide 6. The over-all reaction rate for thermal decomposition is measured applying thermo-gravimetry (TGA) with simultaneous analysis of the evolved gases by mass spectrometry. The kinetic parameters are determined by fitting a kinetic model 10 the measured decomposition rates with the help of a direct search method. For PVC different mechanisms during the dehydrochlorination with different reaction rates may occur depending on the sample size for TGA. For polyamide 6 a single mechanism for decomposition is found.

Modelling of isothermal and dynamic pyrolysis of plastics considering non-homogeneous temperature distribution and detailed degradation mechanism

Abstract The kinetics of pyrolysis of plastics are important to predict the formation of gaseous compounds from plastic waste. A common method to determine kinetic parameters is to adapt kinetic models to conversion curves gained from either isothermal or dynamic experiments. Recent studies about the pyrolysis of polystyrene showed considerable discrepancies of parameters derived from isothermal and dynamic experiments. Explanation for this may be supplied by heat transfer limitations or by complex degradation mechanisms that are not in agreement with the simple kinetic model. In the first part of this investigation the non-stationary heat transfer inside the sample is numerically simulated. The comparison of the results from simulating isothermal and dynamic cases reveal no significant shift in the adapted overall kinetic parameters if modest heating rates and sample sizes are applied. Therefore, in the second part a model is considered including a more detailed kinetic scheme which comprises statistic scission of the polymer chain and subsequent depolymerization of the polymer radical. The rate equations for the polydispers species are solved via the method of moments. The parameters derived from isothermal simulations show good correspondence with the rate coefficients of the initiation reactions. However, when simulating dynamic experiments considerable changes in the adapted parameters are found for a certain range of reaction rates. It is demonstrated that degradation mechanisms exist for which dynamic experiments yield kinetic parameters that differ up to 100% from the input parameters due to the inapplicable simplification of the kinetic model.

Detoxification of brominated pyrolysis oils

The development of an innovative technology for the pyrolytic conversion of brominated phenols in a reductive medium aimed at product recovery for commercial use is discussed in this paper. Brominated phenols are toxic products, which contaminate pyrolysis oil of wastes from electronic and electrical equipment (WEEE). The pyrolysis experiments were carried out with 2,6-dibromophenol, tetrabromobisphenol A, WEEE pyrolysis oil and polypropylene or polyethylene in encapsulated ampoules under inert atmosphere in quasi-isothermal conditions (300-400 °C) with a different residence time (10-30 min). Optimal conditions were found to be the use of polypropylene at 350 °C with a residence time of 20 min. The main pyrolysis products were identified as HBr and phenol. A radical debromination mechanism for the pyrolytic destruction of brominated phenols is suggested.

Pyrolysis of polyamide 6 under catalytic conditions and its application to reutilization of carpets

e-caprolactam is a monomer of high value. Therefore, the chemical reutilization of polyamide 6 containing carpets for e-caprolactam recovery offers some economic benefit and is performed on a technical scale with the help of the Zimmer-process. By this process polyamide 6 is depolymerized with steam and phosphoric acid. An alternative to this process is the thermal depolymerization - catalyzed or non-catalyzed. To investigate this alternative in more detail, the formal kinetic parameters of (i) the thermal depolymerization of polyamide 6, (ii) the thermal depolymerization in presence of sodium/potassium hydoxide, and (iii) the thermal depolymerization in presence of phosphoric acid are determined in this work. Based on the kinetics of the catalyzed or non-catalyzed depolymerization a stepwise pyrolysis procedure is designed by which the formation of e-caprolactam from polyamide 6 can be separated from the formation of other pyrolysis products.

Kinetic study on the non-catalysed and catalysed degradation of polyamide 6 with isothermal and dynamic methods

Abstract The catalysed or non-catalysed thermal decomposition of polyamide 6 could be an alternative to current recycling processes, where the depolymerization of polyamide 6 to the monomer ϵ-caprolactam is performed with steam and phosphoric acid (Zimmer AG). Therefore, in this work the formal kinetic parameters of the thermal degradation of polyamide 6 are determined by means of isothermal and dynamic measurements. The functional amide group of polyamide 6 enables catalysis of the decomposition. Therefore, also the formal kinetic parameters of thermal degradation in presence of phosphoric acid and sodium/potassium hydroxide are determined. A mechanistic interpretation is given for the thermal degradation of polyamide 6 and the catalysed decomposition reactions.

Intermediate pyrolysis of biomass energy pellets for producing sustainable liquid, gaseous and solid fuels

This work describes the use of intermediate pyrolysis system to produce liquid, gaseous and solid fuels from pelletised wood and barley straw feedstock. Experiments were conducted in a pilot-scale system and all products were collected and analysed. The liquid products were separated into an aqueous phase and an organic phase (pyrolysis oil) under gravity. The oil yields were 34.1 wt.% and 12.0 wt.% for wood and barley straw, respectively. Analysis found that both oils were rich in heterocyclic and phenolic compounds and have heating values over 24 MJ/kg. The yields of char for both feedstocks were found to be about 30 wt.%, with heating values similar to that of typical sub-bituminous class coal. Gas yields were calculated to be approximately 20 wt.%. Studies showed that both gases had heating values similar to that of downdraft gasification producer gas. Analysis on product energy yields indicated the process efficiency was about 75%.