Hydraulic modelling of liquid-solid fluidisation in drinking water treatment processes

In the Netherlands annually 400 million m3 drinking water is softened in treatment plants applying fluidised bed pellet reactors. Generally, sand is used as seeding material and calcium carbonate pellets are produced as a by-product. To improve sustainability calcite pellets are grained and sieved and re-used as seeding material.

Theoretical knowledge of perfect round spheres in liquids is generally accepted and applied to predict the fluidisation behaviour. Regarding natural particles numerous semi-empirical models have been published. However, there is no general agreement regarding which equation is the most accurate. In many cases shape factors are introduced for the particle diameter to improve the numerical results.
In the transition from a fixed to a fluidised bed state, after increasing the water throughput, the drag force existing on particles increases. This research will show the dependency of the drag on the actual particles size and change in orientation. It will be demonstrated that not the shape of the particle will decline, but the re-orientated will cause the drag force the decrease with 50%. This revised approach will result in a better understanding and prediction of the fluid bed state.
In the well know Moody chart the friction factor is plotted against the Reynolds number, in which the emphasis is made on the turbulent flow. In liquid-solid systems the flow regime is in general assumed to be laminar. In an improved approach the friction factor is represented not using the default log-log method. The frequently cited and applied Richardson-Zaki fitting model is revised and replaced by an improved hydraulic model. The improved prediction model is more accurate and based on thoroughly carried out pilot plant experiments.

The performance of the chemical process in pellet softening reactors is proven dependent on the state of the fluidised particle bed.

The aim of the research is to obtain substantial more knowledge regarding the hydraulics of the liquid-solid fluidisation phenomena which will optimize the softening process in fluid bed reactors,

The research will take place at the Weesperkarspel facility in Amsterdam.

Project partners
TU Delft, Waternet

Funded by
Waternet

Project coordinator
Jan Peter van der Hoek

Period
2016-2020