Determination of the Overall Heat Transfer Coefficient in a Vacuum Pyrolysis Moving and Stirred Bed Reactor

A novel moving and stirred bed reactor with a high heat transfer capacity has been operated to achieve the thermal decomposition of used tyre particles under vacuum. The overall heat transfer coefficient determined in this reactor reaches 200–250 W m−2K−1, a value exceeding the levels obtained in conventional rotary kilns and multiple hearth furnaces. In order to design large scale stirred bed vacuum pyrolysis reactors, both experimental and theoretical studies were carried out to understand the heat transfer mechanism and to determine the heat transfer coefficient in the reactor as a function of the operating conditions. In this work, the heat transfer coefficients under different agitation speeds up to 22.5 rpm were measured. The heat transfer coefficient was found to increase with the agitation speed, proportionally to (1/tmix)1/2. A Schliinder's modified model was used to describe the correlation between the heat transfer coefficient and the operating conditions. Calculation of the partial heat transfer coefficients during the three pyrolysis evolution periods revealed the influence of the chemical reactions, the phase change and the feedstock thermal property variation on the overall heat transfer coefficient during the vacuum pyrolysis of tyre particles.