Numerical modeling of circulating fluidized bed for cracking of sugars into value-added chemicals

Sugar cracking is a promising technology for renewable chemical production of glycolaldehyde and other oxygenate products including pyruvaldehyde, acetol, and formaldehyde. Glycolaldehyde is a renewable platform molecule that can be converted into other useful chemicals such as ethylene glycol. Production of ethylene glycol by means of sugar cracking would present an alternative to the conventional fossil-based production route, thus contributing to the green transition.

In this work, efforts to support scale-up of the sugar cracking technology from lab-scale to industrial-scale using computational fluid dynamics (CFD) are undertaken. The technology has shown potential in lab-scale fluidized beds with high yield and selectivity towards glycolaldehyde. However, more sophisticated means of designing the fluidized bed system is needed on an industrial scale for continuous production of glycolaldehyde. The use of a circulating fluidized bed for energy-efficient and low-cost production of the glycolaldehyde requires detailed understanding of the hydrodynamics of the system as well as the effect of the fluidization state of operation on the yields of the valuable product species. Several steps to build and validate a CFD model to simulate the system and improve upon this understanding are carried out in this work.

The Eulerian-Lagrangian multiphase particle-in-cell (MP-PIC) model, implemented in the computational particle flfluid dynamics (CPFD) code, Barracuda Virtual Reactor (BVR) ®, is adopted in this study to simulate the circulating flfluidized bed (CFB) system for cracking of sugar. The overall BVR model consists of a number of submodels, including submodels for hydrodynamics, gas-liquid injection, liquid evaporation, and reaction kinetics. Several steps are taken along the way to ensure that each submodel is tested and validated against experimental data.

The hydrodynamic model is set up and tested on a cold-flflow dual-circulating flfluidized bed using a novel segregated approach, in which the experimental solids circulation rate is specifified as an input parameter. A riser-only simulation approach is also proposed to reduce computational time when simulating only the reactor riser of the CFB system. Model predictions in terms of pressure and solids distribution compare well with measurements using either of the two approaches, suggesting that the riser-only approach for the system in question can be used to reduce computational time without compromising on accuracy.

In the sugar cracking process, an aqueous solution of sugar, specifically glucose, is sprayed into the fluidized bed reactor undergoing a fast pyrolysis process. The injection of a gas-liquid spray into a gas-solid fluidized bed is simulated using BVR to investigate the mechanisms in a generic bubbling fluidized bed for which measurements have been reported in the literature. This submodel accounts for evaporation of volatile liquid in the form of individually moving droplets and thin films distributed on the surface of the solid bed particles. The model tends to capture overall features of the gas temperature distributions within the injection region under varying operating conditions of the spray nozzle. Furthermore, increasing the gas-to-liquid mass flow rate ratio of the spray nozzle is found to improve the liquid distribution among bed particles, consistent with experimental observations in the literature.

A reaction kinetics submodel for simulating sugar cracking in a pilot-scale circulating fluidized bed riser operated at conditions relevant for continuous production of glycolaldehyde in the industry is added to the BVR model framework using a gas-phase kinetics model available in literature. To handle the glucose solution and corresponding atomization, 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 riser temperature and pressure distributions are compared to measured ones. Glycolaldehyde yield is predicted at 60.9 wt.%-C, 20 wt.%-C higher than the measured value. Yield predictions of a plug flow reactor model correspond to those of the CFD model. This suggests that the CFB riser behaves essentially like a plug flow reactor, as deviations in hydrodynamics as well as mass and heat transfer from plug flow conditions do not have a considerable effect on the predicted yields.

Improvements to the understanding of the hydrodynamics of the CFB and especially the reactor riser are made, along with how it pertains to the sugar cracking kinetics. The proposed CFD model is fully capable of supporting scale-up of the sugar cracking technology with respect to hydrodynamics and operation of the CFB based on e.g. maldistribution of temperatures, solids, and velocities across the entire domain of the CFB system. Additional fundamental research on the sugar cracking kinetics and its dependency on e.g. solids and liquid concentrations, temperatures, and residence time is needed to further improve the kinetic part of the CFD model, thus providing a broader basis for numerically supporting scale-up of the sugar cracking technology.