Operating charts for continuous sedimentation I: Control of steady states

The industrial process of continuous sedimentation of solid particles in a liquid takes place in a clarifier-thickener unit, which is a large tank with a feed inlet somewhere in the middle and outlets at the top and bottom. For half a century the constitutive assumption by Kynch has provided a platform from which steady-state mass-balance considerations have been used to obtain rules and graphical tools for prediction of steady-state situations, design and control. This is often referred to as the 'solids-flux theory' containing such key words as the 'operating line', 'state point' and 'limiting flux'. The basic assumptions of the solids-flux theory yield a nonlinear partial differential equation that models the entire process. Since unique physically relevant solutions can now be obtained, the knowledge of these is used to establish and extend the solids-flux theory. Detailed information on all steady-state solutions and the control of these by adjusting a volume flow is presented by means of operating charts. Most of these are concentration-flux diagrams with information on, for example, how to perform a control action to fulfil a certain control objective formulated in terms of the output variables in steady state.