(340a) Development of a Liquid Bridge Model for Particle Agglomeration and Defluidization in Plastic Pyrolysis

Pyrolysis is an important process for recycling and upscaling of plastics and municipal solid waste whereby the polymeric residues are reduced to basic raw materials that can be used in refineries or in the petrochemical industry. Fluidized bed reactors are commonly used for pyrolysis because they allow for the high value products to be extracted easily. Several recent studies have reported issues associated with agglomeration of the bed material when coated with fused (melted) plastic and consequent defluidization of the fluidized bed. The mechanism for the agglomeration is the cohesive forces arising from the formation of liquid bridges between the inert particles by the melted plastic. Previous research in the literature modeling the liquid bridge interactions using computational fluid dynamics (CFD) did not accurately account for the capillary forces between inhomogeneous particle pairs or the modified collision properties of coated particles. The implementation of the liquid bridge model also considered a fixed volume of liquid, which is inapplicable to a system with evolving liquid layer thickness as the melted plastic pyrolyzes. In this work, a novel liquid bridge model capability is implemented into the open-source CFD solver MFiX developed at the National Energy Technology Laboratory that explicitly models the mass, volume, and species of the liquid layer such that the evolution of the liquid bridge forces during pyrolysis can be accurately modeled.

The capillary force and normal and tangential viscous forces associated with the liquid bridge were developed from first principles and implemented into MFiX through a user-defined function. The implementation of the liquid bridge model without pyrolysis was quantitatively validated against the experimental pseudo-2D spouted bed of Tang et al. (Ind. Eng. Chem. Res., 58, 2019) using glass beads coated with silicone oil and showed excellent match across a range of different liquid volume fractions and viscosities. The validated model was used to predict the agglomeration and consequent defluidization, identified by a decrease in the pressure drop, of sand coated with melted low-density polyethylene (LDPE) in inert conditions by updating the liquid viscosity and surface tension to representative values for LDPE based on literature. When the pyrolysis reaction was enabled, the key result was the recovery of the fluidization performance as measured by the pressure drop to the pre-agglomeration state as the LDPE layer pyrolyzed and the cohesive forces associated with the liquid bridge dissipated. The defluidization and subsequent recovery matched the evolution of pressure drop reported in literature. Finally, the sensitivity of the defluidization to the solids holdup, material size, and fluidizing velocity were investigated to optimize the operating envelop for minimal impact on the performance of the pyrolysis reactor.