Simulation of mixing dynamics in agitated pulp stock chests using CFD

Agitated‐pulp chests function as low‐pass filters to reduce high‐frequency variability in pulp properties (mass concentration, freeness, and so on) ahead of many pulping and papermaking operations. Tests on both industrial and scale‐model chests have shown that their dynamic performance is far from ideal, with a significant extent of nonideal flow (short circuiting, recirculation and stagnation) possible. The flow field of a 1:11 scale‐model pulp chest was modeled using a commercial computational fluid dynamics (CFD) software package (Fluent) with the pulp suspension treated as a modified Bingham plastic. A multiple reference frame approach was used with coupling between reference frames made using a velocity transformation. The flow profiles predicted by the simulation agreed qualitatively with those observed in the experiments. The power input predicted by the simulations was slightly higher (about 12%) than that measured. The velocity field obtained from the CFD model was used to obtain the system's dynamic response to a frequency‐modulated random binary input signal. These data were then used as input to a dynamic model that treated flow within the chest as following two streams: one bypassing the mixing zone and one entering it. For both streams, the fraction of suspension passing through each zone was determined and a time constant and delay time computed. These parameters were then compared to those measured experimentally under identical operating conditions. The CFD simulation provides detailed information on the velocity profile within the chest and allows the location(s) of poor mixing regions to be identified. © 2006 American Institute of Chemical Engineers AIChE J, 2006