Reactor optimum pyrolysis

A Systems Approach to Optimum Pyrolysis Reactor Design... 

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. 

Significant amounts of CH4 and C2H2 are also formed but will be ignored for the purposes of this example. The ethane is diluted with steam and passed through a tubular furnace. Steam is used for reasons very similar to those in the case of ethylbenzene pyrolysis (Section 1.3.2., Example 1.1) in particular it reduces the amounts of undesired byproducts. The economic optimum proportion of steam is, however, rather less than in the case of ethylbenzene. We will suppose that the reaction is to be carried out in an isothermal tubular reactor which will be maintained at 900°C. Ethane will be supplied to the reactor at a rate of 20 tonne/h it will be diluted with steam in the ratio 0.3 mole steam 1 mole ethane. The required fractional conversion of ethane is 0.6 (the conversion per pass is relatively low to reduce byproduct formation unconverted ethane is separated and recycled). The operating pressure is 1.4 bar total, and will be assumed constant, i.e. the pressure drop through the reactor will be neglected. 

As stated previously, optimal temperatures and residence times are needed for maximal bio-oil production, which could be achieved in so-called fast or flash pyrolysis, when the residence time of the pyrolysis vapors and the optimal temperature are 0.1-2 s and between 400°C and 650°C, respectively [12, 13], with the optimum being usually approximately 500°C [14], Reactors where the optimal pyrolysis conditions can be achieved include the following [11, 13, 15, 16] ... 

Computer modeling of hydrocarbon pyrolysis is central to an optimum design of reactors for olefins production. Approaches to pyrolysis model development can be classified into four groups mechanistic, stoichiometric, semikinetic, and empirical. Selection of approaches to meet minimum development cost must be consistent with constraints imposed by such factors as data quality, kinetic knowledge, and time limitations. 

Pyrolysis temperature has the greatest influence on the quality and quantity of pyrolysis liquids. Earlier studies indicate the optimum temperature for a high oil yield between 475 and 500 °C [6] [15], Table I shows the experimental conditions for three different feedstock investigated by lab, scale pyrolysis (glass-pyrolysis-reactor). The experimental conditions for the different feedstock pyrolysed with the PDU-scale equipment are presented in Table 2. 

During the last six years a fluidized bed fast pyrolysis process for biomass has been developed at the University of Waterloo (The Waterloo Fast Pyrolysis Process). This process gives yields of up to 70% of organic liquids from hardwoods or softwoods, which are the highest yet reported for a non-catalytic pyrolytic conversion process. A fluidized sand bed is used as a reactor and optimum liquid yields are normally obtained in the range of 450 to 550 C at about 0.5 seconds gas residence time with particles of about 1.5 mm diameter or smaller. Two units are in use, one with a throughput of 20 to 100 gms/hr, and another with a throughput of 1 to 4 kg/hr. 

In addition to the thermogravimetry experiments, batch pyrolysis experiments continuously monitoring HCl formation were performed with PVC to determine the optimum temperature for HCl formation. Under conditions of maximum HCl formation, PVC was pyrolyzed with and without oxygen in a fluidized bed reactor and the formation of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) measured. Avoiding the formation of these highly toxic compounds would be a critical element in any waste stream processing scheme. 

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