CPFD simulation of a pilot-scale CFB riser for sugar cracking to glycolaldehyde and other oxygenates: Coupling hydrodynamics and reaction kinetics

Cracking of sugars to glycolaldehyde and other value-added oxygenates has shown potential in lab-scale fluidized beds with high yield and selectivity towards glycolaldehyde. In this work, a homogeneous gas-phase kinetics model for sugar cracking available in literature is adopted and implemented into the Computational Particle Fluid Dynamics (CPFD) framework in order to simulate sugar cracking in a pilot-scale circulating fluidized bed riser operated at conditions relevant for continuous production of glycolaldehyde in the industry. An Eulerian-Lagrangian approach is applied to model the three-phase flow where liquid feed droplets are injected into the hot gas-solid fluidized bed. A modified gas-to-liquid ratio dependent Rosin-Rammler droplet size distribution is proposed to accurately represent the gas-liquid jet in the simulation. Simulated temperature and pressure distributions are in good agreement with measured ones. Prediction of the yield of glycolaldehyde at 60.9 wt.%-C and other oxygenates using the the CPFD model corresponds closely to the results obtained from the same kinetic model implemented in an isothermal plug flow reactor model. This suggests that the CFB riser behaves essentially like a plug flow reactor, as hydrodynamic and thermal deviations from plug flow conditions do not have a considerable effect on the predicted yields. Comparing the model predictions to the measured yield of glycolaldehyde, a 50% overprediction is observed, suggesting that the hydrodynamic and cracking kinetics submodels need to be more closely coupled to more accurately predict the product yields, by means of e.g. heterogeneous cracking reactions.