Spouting mechanism

Figure 11.10(b) can be modeled as a piston flow reactor with recycle. The fluid mechanics of spouting have been examined in detail so that model variables such as pressure drop, gas recycle rate, and solids circulation rate can be estimated. Spouted-bed reactors use relatively large particles. Particles of 1 mm (1000 pm) are typical, compared with 40-100 pm for most fluidizable catalysts. 

Lefroy, G. A., and Davidson, J. F., The Mechanics of Spouted Beds, Trans. Inst. Chem. Engrs., 47 T120 (1969)... 

The classic mechanism of particles being lifted up through the bed in the bubble wake and in the spout behind a bubble (see Chapter 1, p 18) still operates when the bed is composed of two distinct layers jetsam at the bottom and flotsam above. However, Rowe, Nienow and co-workers also showed that bubbles are responsible for segregation... 

Lead azide is manufactured on a technical scale by the action of sodium azide on an aqueous solution of lead nitrate. According to a description of manufacture in the Wolfratshausen factory in Germany [109], the reaction is conducted in an open reactor of stainless steel, provided with a jacket warmed by hot water and a stirrer which may be lifted out of the reactor (Fig. 49). The reactor is emptied by tilting. Its upper edge is therefore fitted with a spout so that the contents pour easily. The size of the reactor is such that 4.5 kg of lead nitrate in the form of a 9-10% solution can be used in each batch. This solution is poured into the reactor, warmed to 50°C and neutralized with sodium hydroxide to a pH of about 4.0 (in the presence of methyl orange) and 150 g of dextrin mixed with a small amount of water, is added. The suspension or solution of dextrin in water should be decanted before use to separate mechanical impurities, such as sand. 

Stable fluidization requires a distribution of particle sizes, preferably in the range of a few hundred microns. Normally a size of 4 mm or so is considered an upper limit, but the coal dryers of Tables 9.15(a) and (b) accommodate sizes up to 0.5 in. Large and uniformly sized particles, such as grains, are dried successfully in spouted beds [Fig. 9.13(f)] Here a high velocity gas stream entrains the solid upward at the axis and releases it at the top for flow back through the annulus. Some operations do without the mechanical draft tube shown but employ a naturally formed central channel. 

Szentmarjay, T., A. Szalay, E. Pallay, in [94], 341-350 Scale-up of the Mechanically Spouted Bed Dryer with Inert Particles... 

The downcomer (or dipleg, or standpipe) through which solids move from bed to bed is a key component of a circulating or a multi-stage fluidized bed. It is often connected at the lower end to a mechanical or non-mechanical valve for controllable transport. The so-called V-valve is a conical (tapered) spout installed at the lower end of a fully fluidized dipleg for controlling... 

Flnidized-bed processes are either bnbbling or internally circulating. The fluidized-bed reactor is very versatile for the pyrolysis of polyolefins. Nevertheless one of the problems with fluidized-bed pyrolysis of post-consumer plastics relates to the stickiness of the sand particles (the fluidization medium) that becomes coated with fused plastic. In order to solve these problems, new reactors have been proposed, snch as the conical spouted bed, the conical rotary reactor, a sphere circnlation reactor and a reactor with mechanical particle stirring. 

The mechanism by which a bed of particulate solids transforms to the spouted state is best explained by following the changes that occur in the pressure drop across the bed, with varying rate of gas flow (Fig. 4). 

A more detailed study of the sequence of transition to the onset of spouting has been reported by Volpicelli and Raso (Vl), who carried out instantaneous pressure measurements with an electric transducer near the base of the bed at varying gas flow rates in a tw o-dimensional column. They also recorded the development of the internal spout and the particle trajectories with a motion-picture camera. The average pressure, together with records of instantaneous pressures published by these workers, are reproduced in Fig. 6. Their results broadly confirm the mechanism already described (Figures 4 and 5), and provide the following additional information ... 

---