Separation of cohesive particulate suspensions under stagnant conditions in horizontal pipes

• We study separation of cohesive suspensions under stagnant flow in horizontal pipes. • Sedimentation and consolidation properties determined from vertical cylinder tests. • Sedimentation in horizontal pipes is accurately predicted from these properties. • Consolidation in horizontal pipes not accurately predicted due to wall effects. • Resolution of wall effects in circular geometries is required to close this gap. Separation of cohesive particulate suspensions in horizontal pipes can lead to formation, growth and consolidation of a solid-like bed that can severely retard pipeline performance. As stagnant flow conditions frequently arise during processing, critical operational questions include: (i) at what rate and extent does sedimentation proceed, and (ii) can separation dynamics be predicted from conventional suspension characterisation methods? These questions are addressed by characterising the sedimentation and consolidation properties of an aqueous kaolin suspension via batch settling tests and comparing numerical predictions from 1D theory for sedimentation and consolidation in arbitrary-shaped containers with separation experiments in a horizontally oriented cylindrical pipe. Under assumptions of negligible gravity currents and horizontal solids concentration gradients, it is shown that particulate sedimentation in a stagnant pipe can be accurately predicted to within 5% error, indicating that these assumptions are valid and sedimentation properties estimated from batch settling experiments are representative material properties. Conversely, it is found that the consolidation of the kaolin suspension in the horizontally oriented pipe is not well predicted by 1D theory due to shear stresses arising from adhesion of the consolidated suspension to the horizontal pipe walls. Although wall adhesion is accounted for in the batch settling experiments, solution of the consolidation problem with wall adhesion in horizontally oriented cylinders is an unsolved problem which requires closure of the governing visco-plastic constitutive model. These findings provide a basis for the development of methods to predict pipeline sedimentation under more general (laminar and turbulent) flow conditions.