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Abstract: In this work, we model a countercurrent tubular reactor where an automotive shredder residue (ASR) is subject to pyrolysis. An inert gas is adopted to convey the pyrolysis products; the solid entering the reactor undergoes an endothermic chemical process which is very sensitive with respect to the temperature. Pressure effects and friction loss of the gas phase are taken into account to better match the real operative conditions. The reaction is modelled by a system of boundary-value differential-algebraic equations (DAEs) where the thermodynamic variables of both phases are investigated with respect to the axial coordinate of the tube. This algorithmic strategy has shown a greater robustness and reliability in comparison with a more traditional approach relying upon well-known ODE solver codes. In particular, the effects of different gas-solid ratios on conversion and the relevant configurations in the inner phase geometries have been investigated. [Copyright &y& Elsevier]