Reaction kinetics versus transport mechanisms: Assessing the rate-limiting factor in high-temperature PMMA pyrolysis for chemical recycling

Closed-loop chemical recycling of poly(methyl methacrylate) (PMMA) via pyrolysis to recover the high-value methyl methacrylate (MMA) monomer is critical for a circular economy. The overall conversion process is highly sensitive to the complex interplay of transport processes, intrinsic pyrolysis reaction kinetics, and secondary gas-phase decomposition. This study investigates PMMA pyrolysis in a small-scale fluidized bed reactor under flash conditions, utilizing detailed ex-situ Fourier-transform infrared (FTIR) gas analysis to achieve robust mass balance closure. Apart from the MMA monomer, its fragments (especially CO , CO X 2 , CH X 4 , and C X 3 H X 6 ) are present in the product spectrum. Regarding the reaction rate, two distinct regimes are identified: At temperatures below 698 K, the reaction is primarily controlled by intrinsic reaction kinetics, accurately modeled by a single first-order reaction (SFOR) with pre-exponential factor A = 1.85 ⋅ 10 9 s −1 and an activation energy E a = 144 kJ mol −1 . For higher temperatures, the apparent reaction rate is significantly reduced. A one-dimensional shell model is used to correlate the reduction with a limitation due to insufficient heat transport in relation to the highly endothermic pyrolysis reaction. However, the experimental results show an even lower apparent reaction rate than the model prediction, suggesting that reduced intra-particle thermal conductivity – likely caused by monomer vapor bubble formation – and the mass transport resistance play a significant role under the flash pyrolysis conditions.