Simulation of entrained air separation from paper machine circulation water using CFD
Multi-scale and/or multi-disciplinary approach to process-product innovation
CFD & Multiscale Modelling in Chemical Engineering (T3-4P)
Keywords: entrained air, paper machine, degassing, CFD
Entrained air is a common problem in paper and pulp industry, which can cause detrimental effects, for instance, on product quality, machine runnability, and pumping and washing efficiency. Different methods are used in paper mills to remove entrained air including active methods, such as vacuum deaeration tanks and deaeration pumps. Passive deaeration methods are used in wire pits and flumes. Also, chemical deaeration is frequently used by addition of degassing chemicals.
In this work gas removal efficiency is studied both experimentally and by CFD simulations. An experimental device to study air separation was constructed from transparent polyacrylate plates. The length of the rectangular air separation channel is 1.5 m, its height is 0.35 m and depth 0.05 m. The studied solution is circulated by pump from the circulation tank back to the air separation channel through a 50 mm pipe connection. Before the entrance, a controlled volumetric flow of gas is fed and dispersed into the pumped solution. The size of the dispersed bubbles is measured by photometrical method. Flow fields of the liquid and gas bubbles are studied by Particle Image Velocimetry (PIV).
CFD simulations were carried out by using ANSYS CFX 10.0. A 2D model was used to decrease the computational load. The upper boundary of the gas removal area is defined as degassing boundary. This makes it possible for the gas bubbles to escape from the liquid through this boundary. Runs were simulated using different feed rates, bubble sizes and volume fractions. The results were compared to experimental results for flow fields and gas removal efficiency. A reasonable agreement between the measured and calculated values was found to exist.
The developed CFD model will be further developed to include physical properties of pulp suspension, like viscosity, interfacial tension and bubble drag force. Also, the solution of population balances will be implemented. This makes it possible to study the effect of bubble coalescence and presence of degassing chemicals on the air removal efficiency.
See the full pdf manuscript of the abstract.
Presented Tuesday 18, 13:30 to 15:00, in session CFD & Mutliscale Modelling in Chemical Engineering (T3-4P).
