The Use of Optimized Restitution Coefficients to Improve Residence Time Prediction in Computational Fluid Dynamics-Discrete Parcel Method Models for Counter-Current Spray Dryers

Borja Hernandez,Luis Martin De Juan,Mark Crosby,Victor Francia,Prashant Gupta,Hossein Ahmadian,Mariano Martin

doi:10.1021/acs.iecr.1c02415

Borja Hernandez, Luis Martin De Juan + Show 5 more

Open Access

https://doi.org/10.1021/acs.iecr.1c02415

Copy DOI

Abstract

This work studies the dispersion of solids in the cold isothermal operation of swirl counter current spray dryers. Residence time distributions (RTDs) of glass beads and detergent powder are obtained in a semi-industrial unit under varying Reynolds and injection positions and validated with the results of a novel Reynolds average Navier Stokes-discrete parcel method (DPM) framework. The simulations stress that the particle RTD is governed to a large extent by the interaction of the solids with the walls, which is usually simplified with the assumption of a particle-wall restitution coefficient. Since this is often unavailable experimentally, here, we propose an alternative combined model that integrates the computational fluid dynamics (CFD)-DPM model with reinforcement learning using a training set of experimental RTDs to extract an “effective pair of wall restitution coefficients”. The method improves the accuracy of existing CFD platforms, reducing the errors in the mean residence time from 30–100% to <25%.

Highlights

Spray drying is used for producing powder by drying an atomized solution with a hot gas
This is in line with the study of fouling that points out the importance of the breakage of deposit structures[12,43] and its effects on the overall solid Residence time distributions (RTDs) in a counter current swirl dryer under full operation
The mean residence time has been related to the operating Reynolds number of the dryer, the Archimedes number of the discrete phase, and to a lower degree, the particle Stokes and Reynolds number referred to its terminal velocity

Summary

Introduction

Spray drying is used for producing powder by drying an atomized solution with a hot gas. Depending on how the contact takes place between the discrete phase and the air, spray dryers are classified into co-current and counter-current units.[1] The most sensitive materials (i.e., food and pharmaceuticals) are produced in co-current spray dryers; thermally stable products like ceramics or detergents are manufactured at higher temperatures in counter-current units to improve heat and mass transfer. Mass transfer rates are usually controlled by the internal diffusion of the solvent and the residence time of the discrete phase in the unit.[1] An excessive particle residence time (RT) can result in charring and thermal degradation of the product and may lead to safety hazards and an unexpected shutdown of the plant. Insufficient RT results in insufficient drying, favoring agglomeration and caking of the detergent powder on the walls.[1,2]

Objectives

Results

Conclusion