The agitation of pulp suspensions with a jet nozzle agitator

The agitation of pulp suspensions in various types of tanks is one of the m ost common unit operations in a pulp and paper mill. Despite this, there is a lack of basic understanding of the flow behaviour of pulp suspensions un der these conditions. Thus. the purpose of this work has been to investigat e the impact of fibre suspension rheology on flow behaviour at low velociti es in large-scale, agitated vessels. The study is focused on jet nozzle agi tation, which is widely used in the pulp and paper industry due to its favo urable equipment and maintenance costs.learn more about agitation in tanks and the parameters that control the process. Theoretical CFD-calculations (Computational Fluid Dynamics), describing the pulp suspension as a Bingham fluid with a measured yield stress, were comp ared with experimental data of the flow field in an industrial-scale pilot tank equipped with a jet nozzle mixer. The velocity was measured with a son ar doppler velocity meter at four positions in the tank. At three of these points, the velocity meter was fixed in two directions and movable in one. Results from CFD-calculations show good agreement with measured velocities close to the inlet source, i.e. the impeller. The calculated flow field dev iates increasingly from the measurements as the distance from the impeller increases. This suggests that a Bingham fluid doer not fully describe the r heology of the pulp suspension, probably due to a shear-thinning behaviour. Additional calculations substantiate this assumption. The CFD-model shows that the yield stress affects the flow field in a dramatic way. A small cha nge in consistency, e.g. by 5 %, will result effect the in a large change i n the yield stress of about with 20 %. This change will then result in a ch ange of the volume of the agitated zone, i.e. the "cavern", of 20 %.