University of British Columbia
Vancouver, British Columbia, Canada
Modelling fluid flow and associated phenomena in industrial processes is a powerful tool in process and equipment design and optimization. Advances in numerical methods as well as progress in computer speed and memory have created the possibility of using more scientific methods for process and equipment optimization.
Some two-phase processes occurring in the pulp and paper industry are discussed. Issues of problem definition, governing equations and boundary conditions are addressed. The computational method used is presented. The computer code uses a curvilinear rectangular co-ordinate system and the equations are discretized using a control volume method. The solution procedure is based on a modified Vanka method.
Examples of two-phase flow modelling from the pulp and paper area are given. The first describes the flow in a continuous digester. Digesters are large installations used in kraft pulping. The process in the digester involves chemicals which are in contact with the wood chip. The digester is a pressure vessel constructed of carbon steel so that the chips and the liquor descend through an impregnation zone into a heating zone, where the temperature is increased to the final processing temperature. After the chips are heated, they pass through the so-called cooking zone and, at the end of this zone, the temperature is rapidly lowered to stop the pulping reaction by quenching it with cold chemical. Then the chips reach the bottom of the digester, the pressure is suddenly lowered and the rapid temperature drop causes the liquid inside the chip to begin to boil and evaporate. The steam then forces the chips to explode and the fibres are separated into pulp. The process in the digester is three- dimensional, two-phase, liquid-solid flow. The liquid phase is present both as free-phase and as entrapped in the chips, and mass exchange has to be accounted for. Energy equations, including heat generation through chemical reactions are solved and transport equations are solved for lignin, alkali and carbohydrates. The second example given is flow in hydrocyclones in which the objective is to investigate the flow and particle separation occurring in hydrocyclones. The flow in hydrocyclones is turbulent, three-dimensional and is affected by strong body forces. The turbulence, therefore, is very non-isotropic and curvature correction is used in the turbulence equations. The particle separation in the hydrocyclone is analyzed using a Lagrangian method for tracking particles of different shapes. Flow patterns, pressure distribution and turbulence parameters are presented. Three-dimensional effects on particle separation are analyzed.