Pyrolysis fluidised bed

The results of research into the fluidised bed pyrolysis of plastic wastes are reported, with reference to determining the optimum process conditions for the process with respect to the reactor behaviour. The study investigates the effects of process variables such as bed temperature, polymer feed rate, bed hold-up, fluidising velocity, and size of inert material. Findings illustrate the importance of the knowledge of the hydrodynamics of the fluidised bed and of the interactions between bed and polymer particles in the design and operation of the reactor. 15 refs. 

The use of pyrolysis for the recycling of mixed plastics is discussed and it is shown that fluidised bed pyrolysis is particularly advantageous. It is demonstrated that 25 to 45% of product gas with a high heating value and 30 to 50% of an oil rich in aromatics can be recovered. The oil is found to be comparable with that of a mixture of light benzene and bituminous coal tar. Up to 60% of ethylene and propylene can be produced by using mixed polyolefins as feedstock. It is suggested that, under appropriate conditions, the pyrolysis process could be successful commercially. 23 refs. 

The BP-led feedstock recycling consortium recently unveiled its new larger-scale fluidised bed pyrolysis pilot plant, located on the BP refinery site at Grangemouth. The 2 tonne/day plant will take mixed plastics waste from a variety of sources to provide more extensive trial results, to be used in the conceptual design of a 25,000 t/y semicommercial demonstration plant. The consortium envisages a series of plants, of around 25,000-50,000 t/y, scattered across Europe. 

P.T. Williams, E.A. Williams, Fluidised bed pyrolysis of low density polyethylene to produce petrochemical feedstock, J. Anal. Appl. Pyrolysis 51 (1999) 107-126. 

H. Nishizaki, K. Yoshida and K. Endoh, Material recovery from plastic waste by fluidised bed pyrolysis.. In M. Moo-Young and G. J. Farquhar (eds). International Symposium on Waste Treatment Utilisation and Processing, Pergamon Oxford, 1979. 

M. Ponte, M. L. Mastellone, F. Perugini and U. Arena, Fluidised bed pyrolysis of plastic mixtures, Joint Meeting of Italian and Greek Sections of The Combustion Institute, paper 4-section 4, Corfu, 17-19 June 2005. 

P. T. Williams and A. J. Brindle, Fluidised bed pyrolysis and catalytic pyrolysis of scrap tyres. Environmental Technology, 24, 921-929 (2003). 

Some polymers, (e.g. polymethylmethacrylate, polystyrene) lead to good yields of monomer on pyrolysis. Similarly, poly(ethylene terephthalate), PET results in good recovery of monomers on hydrolysis. There is also a case for pyrolysing contaminated mixed plastics packaging to obtain a mixture of hydrocarbons. It can be seen from Table that at moderate temperatures in a fluidised bed pyrolysis unit, ethene and propene are major products with methane and ethane in almost equal amounts. The latter and the scores of other compounds that have been detected in small amount have fuel value. 

Studies on thermal degradation of PE have reported hydrogen production in the fluidised-bed pyrolysis of a low-density polyethylene (LDPE) [a.54]. Hydrogen may be produced from the polymer melt and/or from thermal dehydrogenation of volatile products in the gas phase as a secondary reaction at the reactor temperature. From the results, it seems more possible that hydrogen is formed in the polymer melt, because, if more hydrogen were produced from thermal dehydrogenation of volatile products in the gas phase, increased... 

Macromolecular Symposia Vol.152, March 2000, p. 191-9 RECYCLING OF MIXED PLASTICS BY PYROLYSIS IN A FLUIDISED BED... 

The thermal cracking of a light ffaction of mixed plastics waste was carried out in a fluidised bed reactor and the fractions obtained were analysed by elemental analysis, gas chromatography and ashing. The main components of the waste were PE and PP with a small amount of PS and the bed was fluidised by pyrolysis gas, nitrogen or preheated steam. Experiments conducted at different temperatures and residence times were compared by calculating the crack severity for each experiment. The results obtained revealed that the amounts of ethene and propene obtained by pyrolysis with steam were comparable with those obtained using a commercial steam cracker. 

The application of a selective pyrolysis process to the recovery of chemicals from waste PU foam is described. The reaction conditions are controlled so that target products can be collected directly from the waste stream in high yields. Molecular beam mass spectrometry is used in small-scale experiments to analyse the reaction products in real time, enabling the effects of process parameters such as temperature, catalysts and co-reagents to be quickly screened. Fixed bed and fluidised bed reactors are used to provide products for conventional chemical analysis to determine material balances and to test the concept under larger scale conditions. Results are presented for the recycling of PU foams from vehicle seats and refrigerators. 12 refs. 

Details are given of the pyrolysis of plasties waste with emphasis given to the use of a heated fluidised bed reaetor. Data are given for the pyrolysis eonditions of mixed plasties as well as gas eomposition and high- and low-boiling point fraetions. 20 refs. 

This paper diseusses in detail the option of fluidised-bed reaetors to eraek mixed plastics waste into valuable raw materials, under the headings thermal cracking for feedstocks, pyrolysis of polyolefins, and other options. 7 refs. 

R. Bagri and P. T. Williams, Fluidised-bed catalytic pyrolysis of polystyrene, Journal of the Institute of Energy, 75, 117 (2002). 

M. Predel and W. Kaminsky Pyrolysis of Mixed Polyolefins in a Fluidised-Bed Reactor and on a Pyro-GC/MS to Yield Aliphatic Waxes. Polym. Degrad. Stabil, 70, 373 (2000). 

W. Kaminsky, M. Predel and A. Sadiki, Feedstock recycling of polymers by pyrolysis in a fluidised bed. Polymer Degradation and Stability, 85, 1045-1050, (2004). 

B. J. Milne, L. A. Behie and F. Berrnti, Recycling of waste plastics by ultrapyrolysis using an internally circulating fluidised bed reactor. Journal of Analytical and applied Pyrolysis, 51, 157-166, (1999). 

W. Kaminsky and H. Sinn, Pyrolysis of plastic wastes and scrap tyres using a fluidised bed process.In J. L. Jones and S. B. Radding(eds) Thermal Conversion of Solid Wastes and Biomass.ACS Syposium Series 130, American Chemical Society, Washington,D.C (1980). 

W. Kaminsky H. Schmidt and C. M. Simon, Recycling of mixed plastics by pyrolysis in a fluidised bed, Macromoiecuiar Symposia, 152, 191-199 (2000). 

J. G. Schoeters and A. Buekens, Pyrolysis of plastics in a steam fluidised bed. In ... 

For an experimental determination of the pyrolysis time, a fluidised bed of sand and a large volume hot vessel have been used [20]. The method for reaction rate measurements in the fluidised bed is partly explained in Fig. 7. The pyrolysis gases in the fluidising nitrogen gas stream are combusted after 02 addition downstream from the vessel and the CO2 plus some CO are monitored with NDIR-analysers. Thus, the course of a slow pyrolysis has been followed with a time resolution of several seconds. After pyrolysis, the O2 is added upstream from the fluidised bed and the combustion of the pyrolysis char can be followed in the same way. The equipment is calibrated by injection of a known C02-volume to the bed. Area and time response of the resulting calibration signal are needed for data analysis. 

Fig. 7 Concept for pyrolysis and combustion rate measurement in a fluidised bed of...

Pyrolysis kinetics for cylindrical 12 mm diameter straw pellets in a fluidised bed of sand at different temperatures are shown in Fig. 8. The long pyrolysis times around 100 s are a consequence of the large particle dimension. This demonstrates, that pelletisation is not an advantage it is expensive and destroys the high reactivity of untreated straw with thin ca. 0.5 mm thick walls. 

Fig. 8 Pyrolysis kinetics for cylindrical 12 mm straw pellets in a fluidised bed [20]...

Prior pyrolysis studies on alkaline black liquor from pulping of straw have been developed in our work research group [3], In those studies the processed material was black liquor with 37 % in weight of solids. The experimental system used in that case was a fluidised bed reactor, and the results shown that the main gas product was H2 (about 65% in volume) followed by CO (about 30% in volume). 

This pyrolysis step is very fast. For biomass particle diameters of 1 to 10 mm and when pyrolysis is made at 800 C in fluidised bed, it has been recently calculated (37) to be of 5 to 22 seconds. 

There are several hundreds of papers studying the kinetics of reactions involved in the pyrolysis step (27). Unfortunately for the gasifier modelling there is neither a unified approach nor an overall kinetic equation(s) describing the pyrolysis step for all possible biomass under all possible circumstances. Besides, most of the kinetic studies have been made in thermobalances or related apparatuses with low heating rates (around 25 "C/min), far from the high heating rates in fluidised beds of up to a claimed 1000 C/s or even 10000 C/s. Kinetic equations obtained in thermobalances would provide results very different from the ones in fluidised bed, which is the present case. 

The kinetics for the pyrolysis step are very complex thus. Besides, only data generated in fluidised beds should be used in this modelling but there are only a few of works (i.e. refs. 32, 35, 39) made with such a fast heating process and, even more, such papers do not provide kinetic equations. They would have been very useful for the present modelling. 

Tests have been made feeding smalt pine wood chips at the bottom of a fluidised bed. Fluidising agents have been Ni (pyrolysis tests) and air with an equivalence ratio (ER) of 0.24 (gasification tests). Gasification tests were of two types ... 

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