Pneumatically controlled downcomer

For slow reactions, the shallow fluid beds have been organized into a cocurrent multistage fluid bed (MSFB) reactor as shown in Fig. 33 (Yan, Yao, Wang, Liu and Kwauk, 1983). In this reactor, solids are carried up by the flowing gas stream, and once they reach the top, they are collected through a funnel and recirculated to the bottom by means of a pneumatically controlled downcomer. 

Various configurations have been developed for muitistaging of fluidized beds using the pneumatically controlled downcomer, as shown in Fig. 38. Figure 38A shows the fundamental concept of multistaging of fluidized beds, with a downcomer between two adjacent beds, each downcomer being supplied with a separate stream of control gas. Consequently, there are as many control gas streams as there are stages. 

Fig. 38. Configurations for multistage fluidized beds using the pneumatically controlled downcomer. [After Kwauk, 1974 Liu and Kwauk, 1980 Liu et al., 1981.]...

Figure 1 presents an isometric view of the pneumatically controlled downcomer. Solids drop by gravity into a fully fluidized dipleg and pass via... 

In the light of the preceding separate studies on the spout and the dipleg, the overall operation of the pneumatically controlled downcomer will now be analyzed. 

Normally the pneumatically controlled pulse feeder operates with limited pressure difference between the hopper and the stop plate. When the pressure difference is in the direction opposite to that of the downward solids flow, the downcomer needs to be lengthened in order to seal the solids, or else other modes of downcomer design must be resorted to. 

Fig. 3 shows the experimental apparatus. The feed tank had a 50 gallon capacity and was equipped with a variable speed mixer. The feed pump was a flexible impeller, positive-displacement pump to minimize shearing of the feed emulsion. The pumping rate was regulated by a Graham Variable Speed Transmission. Each flotation tank was 11.5 in. ID with 6.5 in. liquid depth maintained by a CE IN-VAL-CO conductometric level controller with a pneumatically actuated control valve in the effluent line. The fourth cell was not equipped with an air inducer. The outer diameter of the air downcomers was 1.5 in. The rotor in each air inducer was a turbine taken from a 2 in. turbine flow meter. Each rotor was belt driven by a 10,000 rpm, 1/30 hp motor and all three motors were governed by the same variable transformer. Another pulley on each rotor shaft was attached to a non-powered belt connecting all three shafts to ensure that each rotor turned at the same speed. 

Thirdly, the pneumatically actuated valve, such as the V-valve installed at the lower end of the downcomer, only serves as a loop seal rather than a control, thus resulting in a free system, that is, it does not influence gas-solids flow ... 

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