Numerical prediction of the performance of horizontal sand filters as the pitch of the spiral protrusion changes

The height of vertical sand filters are limited due to their structural cost. A horizontal sand filter may offer a better alternative, however flow channels to the least resistance zone generated at the top of the filter as the sand gets wet and settles. A horizontal sand filter, internally baffled with spiral protrusion, is numerically modelled to study the effect of these spirals in reducing the channelling and enhancing filter's effectiveness. Three different spiral pitches, 1.0 m, 0.75 m, and 0.5 m have been numerically modelled using Ansys Fluent software. The parameters investigated were the power needed to run a flow rate through the horizontal filter and the residence time. The results show that as the spiral pitch decreases the channelling reduces while the power increases. The power needed to pump a given flow rate of water in a 10m long horizontal filter in all three cases investigated was less than the power needed to pump the same flow rate to the top of a 10 m long vertically-standing sand filter. Results also showed that the time required traversing the flow through the sand filter increases in a nonlinear fashion as the pitch size decreases, however the effectiveness of the filter increases

Carbothermal reduction of silica in nitrogen and nitrogen-hydrogen mixture

Carbothermal reduction of silica was investigated in a fixed bed reactor at 1300-1650 °C in nitrogen at 1-11 atm pressure and in hydrogen-nitrogen mixtures at atmospheric pressure. Samples were prepared from silica-graphite mixtures in the form of pellets. CO evolution in the reduction process was monitored using an infrared sensor; oxygen, nitrogen and carbon contents in reduced samples were determined by LECO analyses. Phases formed in the reduction process were analysed by XRD. Silica was reduced to silicon nitride and silicon carbide; their ratio was dependent on reduction time, temperature and nitrogen pressure. Reduction products also included SiO which was removed from the pellet with the flowing gas. In the temperature-programmed experiments, reduction of silica started below 1300 °C; the reduction rate increased with increasing temperature. Silicon carbide was the major product at the early stage of reduction; a fraction of silicon nitride increased with increasing reaction time. Maximum silicon nitride to carbide ratio (68.0/9.8 specifically for 720 min) in the reduction of silica in nitrogen at atmospheric pressure was observed at 1450 °C. Further increase in temperature decreased Si3N4/SiC ratio. When nitrogen pressure was 11 atm, maximum Si3N4/SiC ratio of 71.4/13.3 was observed at 1550-1600 °C. Increasing nitrogen pressure increased reduction and nitridation rates and suppressed SiO loss under otherwise the same conditions

Numerical modeling of flow in a horizontal sand filter

Problem statement: Horizontal sand filters may offer some advantage over vertical sand filters as they could be used for in-line treatment of wastewaters. Horizontal pipelines of tens and if not hundreds of meters length, filled with ordinary sand or permeable reactive media such as activated carbon or natural zeolite or iron filings, may be used to remove impurities from mine drainage waters, sewer and storm waters. Approach: However, in reality, in industrial-scale applications, the sand filled horizontal structures are almost always avoided due to the fact that water seeks out the path of least resistance. Once such a path is created, the vast majority of the water channels towards the least resistance zone and very small percentage of the water will go through the sand. Results: This study, applying numerical modeling using FLUENT software, which is based on the integral control volume approach, explores a number of geometries to identify a design that helps the inlet water to sweep the entire sand with the least channeling. The Navier Stokes equations for laminar and incompressible flow through porous media (i.e., the sand) including the viscous resistance were solved. Retention time of a fluid with properties similar to water, called tracer, were also estimated using Eulerian unsteady two phase flow analysis. Conclusion: The results of the models showed that the geometries involving a pipe with spiral protrusions or plate (baffles) inside, would be partially successful in drawing the water away from the channeling zone and moving it through the sand, however the geometry with the spiral protrusions uses much less power than the one with the baffles