Water softening using caustic soda: privileges and restrictions

The aims of the current study is to investigate the constraint of using caustic soda in water treatment and evaluating its performance in water softening, compared to other chemical group, including lime and sodium carbonate. Based on mass balance of reactants in the caustic softening process, a mathematical relation for expressing the constraint of using caustic soda in water softening was derived. To evaluate caustic soda performance in water softening and proving the derived relation as well, some experimental works on some water sources including well water and clarifier’s inlet water in two oil refineries were performed. The results showed that compared to lime- sodium carbonate, the caustic soda is the best choice for water softening, however, using caustic soda in water softening, while restrictive mathematical relation doesn’t verify the chemical characteristics of water, could lead to an extreme increase in alkalinity

Large-eddy simulation of turbulent flow in a stirred tank driven by a Rushton turbine

Large-eddy simulation (LES) of mixing process in a baffled tank was presented. The impeller rotation was modeled using the sliding mesh technique In this study the CFD code was used for simulation of a standard vessel agitated by a 6-blade Rushton turbine and results were evaluated in terms of the predicted flow field, power number, mean velocity components, mixing time, turbulent kinetic energy and turbulent dissipation rate using published experimental data Subsequently, the effects of varying injection position of the passive scalar have been investigated The results show that LES is a reliable tool to investigate the unsteady behavior of the turbulent flow in stirred tank Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved

A mixing study in a double-Rushton stirred tank

Computational and experimental methods have been used to investigate the flow field, power and mixing time in a fully baffled stirred vessel with two six-blade Rushton turbines. Flow in a stirred tank involves interactions between flow around rotating impeller blades and stationary baffles. In computational fluid dynamics (CFD), the flow field was developed using the sliding mesh (SM) approach. The large eddy simulation (LES) was used to model the turbulence. For validation of simulation results two series of experiments were performed: (i) velocity measurements of the liquid phase using particle image velocimetry (PIV) and (ii) concentration measurements of the determining tracer in the liquid phase using the planar laser-induced fluorescence (PLIF) technique. In each series three different rotational speeds of impellers: 225, 300 and 400 rpm were employed. The stirring power input was also calculated based on the PIV results. A considerable reduction in mixing time was achieved and stirring power input was increased by increasing the impeller speed. The satisfactory comparisons indicate the potential usefulness of this CFD approach as a computational tool for designing stirred reactors. (C) 2009 Elsevier Ltd. All rights reserve