Multiple draft tubes

Several laboratory scale devices which include internal elements to enhance mass transfer rates have appeared. These include draft-tubes, multiple sieve plates staged along the length of the column, and static mixing elements. 

Multiple Draft Tubes. Studies in the past always concentrate on beds with a single draft tube. A literature survey failed to uncover any reference on operation of multiple draft tubes. Even in the area of conventional spouted beds, the references on multiple spouted beds are rare. Foong et al. (1975) reported that the multiple spouted bed was inherently unstable due to pulsation and regression of the spouts. Similar instability was also... 

The design methodology proposed earlier for beds with a single draft tube is still applicable here for beds with multiple draft tubes. 

Some typical fluid bed capacities used in the pharmaceutical industry are given in Table 2. From this information, it can be seen that for beds up to 450 mm (18") in diameter, one draft tube is typically used. The diameter of the draft tube is nominally one-half the bed diameter. For larger beds, the use of multiple draft tubes, 225 mm in diameter, is common. Also shown in Table 2 are the heights ofthe draft tubes used in each bed. Since the solids in the bed are filled close to the top of the draft tube, the capacity of the equipment is determined by the diameter of the bed, the height of the draft tube, and the density of the bed solids. An example illustrating the scale-up procedure is given below ... 

Tong, C. C., and Fan, L. S., Concentration Multiplicity in a Draft Tube Fluidized-Bed Bioreactor Involving Two Limiting Substrates, Biotechnol. Bioeng., 31 24 (1988)... 

Agitated contactor with multiple stirrers and a central draft tube 11 20... 

Agitated crystallizer (single mixed vessel or horizontal crystallizer with multiple mixed cells) Draft tube crystallizer... 

The basic heat-transfer surface is a bank of 1 to 20 (but usually 3 to 12) rows of high-tinned tubes. The fluids enter the tubes by box headers (pipe headers at higher pressures), with the tubes welded into one wall of the header internal dividers are used in the headers to create multiple tube passes. The header cover may be bolted to the header, or the header may be welded, with threaded plugs opposite the tube ends to facilitate assembly and tube cleaning. The tubes are slanted downward toward the exit (about 1/4 in. per ft) to facilitate drainage. Occasionally, especially for turbine steam condensation, an A-frame configuration is employed, with the tubes slanted about 30° from the vertical and the fans mounted in forced draft at the base of the A, thereby reducing the plan area required. 

In Fig. 1.2, a draft of a reactor for methane steam reforming with multiple membrane tubes for hydrogen separation, operating in co-current mode, is shown. 

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