On the role of micro- and mesomixing in a continuous Couette-type precipitator

Precipitation of barium sulphate in the annular gap of a continuous Couette reactor with two unpremixed feeds has been experimentally investigated. The experiments have been carried out in the turbulent regime with an axial Re(x) varying from 100 to 240; the hydrodynamics can be described with the plug flow plus dispersion model, and in the condition tested the Peclet number varied from 2 to 6. The initial supersaturation has the strongest effect on both the particle size and the crystal morphology. The average size of the crystals is also influenced in some cases by the diameter of the feed tube and by the feed velocity, indicating the relevance of mesomixing. The rotation velocity has a relatively weak effect on crystal size and CSD, but a maximum has been observed increasing the rotation velocity. The performances of the system have been predicted in the limit cases of minimum and maximum age mixedness as a function of the Peclet number. (C) 1999 Elsevier Science Ltd. All rights reserved.