User:Sayyidthwahap/sandbox

SPOUTED BEDSayyidthwahap (talk) 06:20, 18 January 2015 (UTC) The spouted bed was originally known as a modified version of fluidized bed. This modification arose from the poor fluidization quality of fluidization associated with coarse particles. The spouted bed has the special characteristic of performing certain cyclic operations on solid particles which cannot be performed in a fluidized bed. If fluid is injected vertically through a small opening at the base of the spouted bed, the fluid jet causes a stream of particles to rapidly rise in a hollowed central core within a bed of solids. These particles rain back onto the annular region between the hollowed core and the column wall, where they slowly travel downward and the column wall, where they slowly travel down ward. A cyclic pattern of solids movement is thus established Due to their unique hydrodynamic system, spouted beds have been widely used in many applications such as drying, gasification, combustion, pyrolysis and coating. A recent application of spouted beds has been mechanical extraction of natural dye extract. Despite the wide applications and advantages, conventional spouted bed has some operating limitations such as the maximum spoutable bed height and the formation of dead zones in the annulus. Many modifications have been proposed to improve its operability. One modification is the introduction of a draft tube. This modification allows the flow rates of gas and solids to be controlled independently and makes it possible to operate in higher bed as well as to handle even small particles. CSB's never appeared commercially. The main problems were: 1) high blower capacity requirements related to pressure drop before and after spouting, unequal grain residence times due to entrainment along the height of the spout well-above the bed bottom, compromise of air flow parameters for drying due to requirements for stable spouting, and limited drying capacity related to scale-up problems One approach to overcoming these problems was to install a draft tube above the air inlet. This development resulted in better grain circulation, reduced pressure drop, lower air velocity requirements and increased maximum spoutable bed height, albeit at the penalty of retarded mixing. However, this led to more complex designs while leaving the problems of drying capacity and scale-up unresolved. Spouted beds were also studied by the Russians (Mathur and Epstein, 1974b). They used units with rectangular cross-sections of small aspect ratio and sloped sides. Although they managed to achieve stable spouting using bed width to air entry slot width ratios of 10 to 20, wall effects, or "three-dimensionality", prevented them also from overcoming the drying capacity and scale-up problems of CSB's. The idea of a "fully two-dimensional" spouted bed (2DSB) was then proposed by Mujumdar (1982), principally as an approach to resolving scale-up problems. A 2DSB has plane wa1ls with slanting bases on both narrow sides and has an air entry slot at the centre of the bed width running parallel to the bed length. The rectangular geometry makes the 2DSB easier to fabricate than the cylindrical bed and is readily adapted to scale-up studies. A laboratory scale unit can be considered to be a thin vertical slice of the proposed large scale industrial unit (i.e. just shorter along the length axis). The behaviour observed in the laboratory scale model should then be applicable to the large scale model if the wall effects are quantified. A 2DSB exhibits peak pressure drop before the onset of spouting and spouting pressure drop after spout formation as is the case in CSBs. Minimization of these pressure drops is important in design since they relate to required blower capacity. The peak pressure drop exhibited during spout formation is thought to be influenced partially by the vertical pressure of grains. The vertical pressure of grains during spout formation is difficult to measure, but can be if the bed is static and non-aerated; in this situation, the bulk storage theories can be used for prediction. Draft plates installed on both sides of the air entry slot in a 2DSB act in a similar manner a draft tube in the CSB. This configuration favours plug flow of grains which helps to ensure uniform drying, this in contrast to beds without draft tubes, where significant variation in moisture content of the drying particles has been reported. When the draft plates are present in a bed of given dimensions, spouting pressure drop, air flow through the bed, and solids circulation rate vary when either slant angle, spout width, or separation distance is changed. In scaling-up a 2DSB by increasing the aspect ratio between the sides of bed (i.e. increasing bed length), it is assumed that spouting pressure drop, air inlet flux and particle velocity vary linearly from the laboratory scale models if the width and height of the bed are held constant. The particles in the dilute phase are fully exposed to the spouting air. In the dense phase, grains are exposed to lower air flow rates, but for a much longer time. This resuIts in the tempering of grains, moisture migration towards the surface of grains, and some evaporation due to the sensible heat gained while passing through the dilute phase. Slotted 2DSB's with draft plates are therefore seen to operate in an alternating two-stage mode with respect to heat transfer.