Axial dispersion of suspended sediments in vertical upward pipe flow

The increasing pressure on earth׳s raw material resources forces the search for alternatives. An alternative source of raw materials is deep sea deposits. The mining of deep sea deposits comes with many challenges, one of them being the vertical transport of excavated material from the seafloor to a floating production platform at the surface. Material can be transported vertically by means of suspending the material in an upward flow of water in a riser. When the riser is fed irregularly by intermittent batches of solids, accumulation of solids can occur that in turn can result in riser blockage. The accumulation process is counteracted by axial dispersion of the batches. In this paper the vertical hydraulic transport of batches is experimentally explored to get insight in the influence of solids on the axial dispersion process. By analysis of the decrease in volume fraction, the axial dispersion coefficient for the vertical transport of batches of sand, gravel and plastic grains relative to the Taylor dispersion coefficient for dissolved matter in turbulent pipe flow is determined. The analysis shows that the presence of solids attenuates axial dispersion such that it plays a minor role in the transport process, particularly for coarse sediments.