Flow and Mixing of Pulp Suspensions

Doctoral thesis, 2004

When producing pulp and paper, pulp suspensions are treated in a number of unit operations at medium consistency (MC) concentrations (3-15% by weight). Pulp suspensions at these concentrations behave as solids at low shear rates, but at sufficiently high shear rates they start to flow and can even be made to act as water. Even though these concentrations have been used for some time, there is still a lack of fundamental understanding of their flow behaviour. The flow behaviour of the suspensions determines, e.g. how well mixing, screening or pumping are performed and, consequently, how the equipment for these unit operations should be developed and optimised. For instance, if mixing is executed in an optimal manner chemical and/or energy consumption, as well as fibre damage can be reduced, meaning higher profits and lower impact on the environment. Mixing fibre/liquid/gas occurs in bleaching with gaseous chemicals. The yield stress, which governs the flow behaviour at low shear rates, is measured in a three-phase system. A model suspension of glass fibres is used, both for its gas containing properties and for its transparency, thus allowing visible control of gas homogenisation and gas bubble size. The presence of gas was found to decrease yield stress. Turbulence, i.e. the characteristic property at higher shear rates, is investigated in the three-phase system using Laser Doppler Anemometry (LDA) in a stirred tank with fibre concentrations in the MC range. It was found that introducing gas into the suspension decreases the system’s ability to transport momentum and that the gas bubbles also have a direct influence on the turbulent properties of the suspension by acting as turbulence dampers. The LDA technique is proved valid in pulp suspensions by simultaneously measuring velocity profiles in the pipe flow using the two independent techniques, LDA and Ultrasonic Velocimetry Profiling (UVP). Measurements are performed in pulp suspensions up to 7.8% fibres by weight with a penetration depth of a few millimetres. Using the LDA technique in pipe flow, the near wall region is investigated by measuring mean and RMS velocity profiles together with the pressure drop. A distinct plug is found with a distance between the plug and the wall of sometimes much less than a millimetre. Also, efforts are made to relate the data-rate of the LDA to fibre concentration, thus indicating a dewatering region 1-2 millimetres from the wall and into the suspension. A method for defining and measuring mixing quality of mixers for pulp suspensions is developed. The method uses small amounts of dyed pulp fibres in uncoloured fibres and image analysis. The method is tested in a stirred tank and the mixing quality is studied as a function of mixing intensity and mixing power.