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 steadystate 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 non-linear partial differential equation that models the entire process. Since unique physically relevant solutions now can 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.