A novel sensor measuring local voidage profile inside a fluidised bed reactor

O.J.I Kramer,C Van Schaik,J.J Hangelbroek,P.J De Moel,M.G Colin,M Amsing,E.S Boek,W.P Breugem,J.T Padding,J.P Van Der Hoek

doi:10.1016/j.jwpe.2021.102091

Abstract

Liquid-solid fluidisation is frequently encountered in drinking water treatment processes, often to obtain a large liquid-solid interfacial surface area. A large surface area is crucial for optimal seeded crystallisation in full-scale softening reactors. Due to crystallisation, particles grow and migrate to a lower zone in the reactor which leads to a stratified bed. Larger particles adversely affect the surface area. To maintain optimal process conditions in the fluidised beds, information is needed about the distribution of particle size, local voidage and available surface area, over the reactor height.In this work, a sensor is developed to obtain the hydraulic state gradient, based on Archimedes’ principle. A cylindrical heavy object is submerged in the fluidised bed and lowered gradually while its weight is measured at various heights using a sensitive force measuring device.Based on accurate fluidisation experiments with calcite grains, the voidage is determined and a straightforward empirical model is developed to estimate the particle size as a function of superficial fluid velocity, kinematic viscosity, suspension density, voidage and particle density. The surface area and specific space velocity can be estimated accordingly, which represent key performance indicators regarding the hydraulic state of the fluidised bed reactor. The prediction error for voidage is 5 ± 2 % and for particle size 9 ± 4 %.The newly developed soft sensor is a more time-effective method for obtaining the hydraulic state in full-scale liquid-solid fluidised bed reactors.

Highlights

A novel sensor has been developed, exploring the possibility of reasonably quick measurement of the hydraulic gradient in a liquidsolid fluidised bed applied in full-scale drinking water treatment plants
The preliminary test in a full-scale pellet-softening reactor at normal conditions was successfully performed to examine the existence of a gradient in the stratified bed
A mass loss of 10 % was measured over the reactor bed, corresponding to 0.7 mm difference in particle size

Summary

Introduction

In the Netherlands, more than 400 million m3 water is softened annually in drinking water treatment plants using fluidised bed pellet reactors [5]. In these reactors, sand is traditionally dosed as seeding material. By carefully dosing caustic soda, su persaturated conditions are created in the fluidised bed [8], leading to calcium carbonate crystallisation on the surface of calcite grains. To maintain optimal process conditions i.e. fast calcium carbonate crystallisation in the fluidised beds, information is needed about particle size, local voidage and spe cific surface area (SSA), over the reactor height [9]

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