HYDRODYNAMICS OF DENSE HYDRATE SUSPENSIONS IN PRECIPITATORS

In the last twenty years, precipitator technology has significantly evolved. Mixing of the suspension of hydrate and Bayer liquor was originally mainly ensured by forced circulation in a draft tube, with small diameter and fast propellers. Nowadays, solid suspension is often carried out with large diameter impellers rotating at low speed.

The present study aims at developing a process analysis methodology, based on numerical simulations (CFD), that is able to predict the flow patterns and the solid distribution in precipitators. A two fluid model with Eulerian approach has been chosen and checked using measurements of solid content distribution carried out on pilot facility.

A batch mode has been first investigated to evaluate the ability of the agitator system alone. This operating mode can be encountered if the vessel must be isolated due to process or operation constraints. The influence of rotational speed, mean solid concentration and particle diameter on homogeneity has been studied. The effect of PSD on sedimentation and risk of solid accumulation in the bottom part of precipitator has been assessed. In the case of high solid content slurries, the positive effect of solid concentration increase on the homogeneity in the vessel has been quantitatively determined.

Experimental and modelling investigation of the continuous mode confirms and quantifies the positive effect of slurry flow through a series of precipitators on homogenisation. This effect can be taken into account for the global design of precipitation facility.

CFD has shown to successfully predict the performances of a mixing system in high concentrated media, and consequently it can be used to optimise mixer design. A higher performance and less power-demanding mixing system is thus proposed.