The influence of paste feed on the minimum spouting velocity

Abstract

Due to the lack of reliable mathematical models for predicting the minimum spouting velocity in spouted bed dryers, in this work a new model is proposed that takes into account the effects of adhesive forces acting on particle agglomerates due to the presence of pastes. The results are compared with experimental data reported by Spitzner and Freire (2001), obtained by the continuous feeding of egg paste into a spouted bed, and with additional data obtained for batch feeding of glycerol into the same bed. The equipment was a conical spouted bed with a 0.05 m inlet diameter, a 60° cone angle, and a 0.21 m cone height. Glass spheres with a diameter of 2.6x10-3 m were used as inert particles. It was observed that as saturation increases, the minimum spouting velocity increases for egg paste and decreases for glycerol. The results indicate that there are different forces acting on the bed of particles depending on the paste and operating conditions. The mathematical model provided good predictions only for egg paste, although the values for minimum spouting velocity were slightly overestimated

Analysis of pressure fluctuations during water evaporation in spouted bed

“…However, the feeding of liquids into SBDI may cause important changes in its hydrodynamic behaviour, as pointed out by several authors (Strumillo, 1981;Pham, 1983;Passos and Mujumdar, 2000;Spitzner Neto et al, 2001, 2002. The liquid interfacial tension and viscosity exert significant resistance to the inert bodies' movement, which is increased as paste moisture content decreases.…”

“…Significant effort was directed to determine the effects of liquid feeds on SBDI hydrodynamics. Theoretical studies on the effect of liquid film cohesive forces on SBDI and fluidized beds was conducted using the concepts of soil mechanics and mixture theory (Passos et al, 2000) and the agglomeration tensile strength theory (Spitzner Neto et al, 2001). The effect of paste presence on key spouting features like the minimum spouting velocity, maximum and spouting pressure drop has been studied using glycerol (Schneider and Bridgewater, 1993;Passos et al, 2000;Spitzner Neto et al, 2001), but also aqueous alumina suspension (Schneider and Bridgewater, 1993) as model pastes for batch operation and egg homogenate, animal blood, and xantham gum (Spitzner Neto et al, 2001) as models for continuous operation.…”

“…Theoretical studies on the effect of liquid film cohesive forces on SBDI and fluidized beds was conducted using the concepts of soil mechanics and mixture theory (Passos et al, 2000) and the agglomeration tensile strength theory (Spitzner Neto et al, 2001). The effect of paste presence on key spouting features like the minimum spouting velocity, maximum and spouting pressure drop has been studied using glycerol (Schneider and Bridgewater, 1993;Passos et al, 2000;Spitzner Neto et al, 2001), but also aqueous alumina suspension (Schneider and Bridgewater, 1993) as model pastes for batch operation and egg homogenate, animal blood, and xantham gum (Spitzner Neto et al, 2001) as models for continuous operation. In batch operation studies the SBDI were loaded with different saturation indexes, that is, volumetric ratios of paste model to inert bodies, and as saturation is increased the spouting pressure drop decreases (Schneider and Bridgewater, 1993;Passos et al, 2000;Spitzner Neto et al, 2001) but contradictory results were found for minimum spouting velocity (Schneider and Bridgewater, 1993;Passos et al, 2000;Spitzner Neto et al, 2001, 2002 which is supposed to depend on paste nature (Schneider and Bridgewater, 1993) and the type of bridges it forms between particles.…”

“…In batch operation studies the SBDI were loaded with different saturation indexes, that is, volumetric ratios of paste model to inert bodies, and as saturation is increased the spouting pressure drop decreases (Schneider and Bridgewater, 1993;Passos et al, 2000;Spitzner Neto et al, 2001) but contradictory results were found for minimum spouting velocity (Schneider and Bridgewater, 1993;Passos et al, 2000;Spitzner Neto et al, 2001, 2002 which is supposed to depend on paste nature (Schneider and Bridgewater, 1993) and the type of bridges it forms between particles. In continuous operation, similar trends were observed for the spouting pressure drop and minimum velocity (Spitzner Neto et al, 2001, 2002Barcelos et al, 2005).…”

“…Although it is well known that the hydrodynamics of SBs is markedly affected by the liquid presence (Schneider and Bridgewater, 1993;Reyes et al, 1998;Passos et al, 2000;Spitzner Neto et al, 2001, 2002Nagahashi et al, 2006), most researches did not consider the bed structure. Recently, many researchers have focused their attention to the use of pressure fluctuations signals, PFS, for gas-solid or gas-liquid-solid systems characterization (Fan et al, 1983;Bai et al, 1999).…”