Thermal plasma synthesis of SiC from rice hull (husk)
Recently the synthesis of ultrafine powders has drawn considerable attention because these powders can have physical and chemical properties superior to those of bulk specimens. In particular, emphasis is on the preparation of ultrafine ceramic powders for their applications in the development of sintered products having higher density and lower sintering temperature. One such industrially important ceramic material is SiC. In many cases ultrafine SiC has been prepared by thermal plasma processes [1]. However, these processes require one or more reactive/corrosive gases. This letter describes a new method to overcome such problems. For this purpose thermally pretreated rice hull was used as the solid precursor. Rice hull, an agricultural waste product, was first used by Cutler [2] as a starting material for the production of silicon carbide. Since the rice hull route promises to be most economical, much attention has recently been paid to it [3-6]. Rice hull consists of silica in hydrated amorphous form, and cellulose which yields carbon when thermally decomposed. The relative abundance of silica with very high surface area in close proximity to active carbon in rice hull makes it amenable for SiC formation during the pyrolysis process. The possible reaction was given by Lee and Cutler [7] as
Synthesis of ultrafine SiC from rice hulls (husks): A plasma process
Heat-treated rice hulls have been used as precursor material for synthesis of ultrafne SiC in a RF plasma reactor. Rice hulls containing finely distributed silica and active carbon act as a source for SiC formation. The plasma-synthesized powder contained ultrafnc β-SiC with excess carbon and some unreacted silica. Post-treatment processes such as oxidation and acid (HF) treatment appear to be effective in removing the excess carbon and silica.
Influence of pressure and fluidization velocity on the hydrodynamics of a fluidized bed containing horizontal tubes
Abstract An experimental investigation of the influence of horizontal tubes on the bubble behaviour and through-flow velocity of gas inside bubbles was carried out and the influence of pressure and fluidization velocity was studied. The visible bubble flow rate and the gas velocity through the bubbles were measured simultaneously in a pressurized fluidized bed, using capacitance and Pitot-static pressures probes respectively. Silica sand with a mean particle diameter of 0.7 mm was fluidized at pressures between 01. and 1.6 MPa, in a bed of 0.2 × 0.3 m cross-section. Six different horizontal tube configurations were used and comparisons were made with results obtained in the same bed operated without tubes. The results from the bed with tubes show that the influence of pressure and fluidization velocity is basically the same as in the bed without tubes; an increase in pressure and fluidization velocity increases the bubble activity. At low excess gas velocities, the influence of immersed tubes on the bubble behaviour is less pronounced than at high excess gas velocities. Measurements and visual observations show that a small tube pitch gives a more uniform distribution of the visible bubble flow over the bed cross-section. At a combination of high pressure, high excess gas velocity and a large tube pitch, a preferred bubble path was observed in the centre of the bed cross-section. The pressure probe measurements show that the through-flow velocity of gas inside the bubbles is in most cases higher when tubes are present in the bed. The through-flow velocity consistently decreases with increasing pressure in the bed without tubes. The tendency is largely the same at a low excess gas velocity when tubes are present, but at a high excess gas velocity the through-flow velocity first decreases and then increases as the pressure is increased.