Agitated vessels draft tubes

The DTB, crystallizer has a relatively slow-speed propeller agitator located within a draft-tube which draws a fine-crystal suspension up to a boiling zone of wide cross-sectional area, as shown in Figure 3.3(i). The fine-crystal magma then passes through an annular zone in which an additional baffle is located. Liquor flow continues upwards at low velocity while crystals settle out and fall to the base of the vessel. Liquor from the external pumped loop provides an up-... 

Jones, A.G. and Mullin, J.W., 1973. Crystallization kinetics of potassium sulphate in a draft-tube agitated vessel. Transactions of the Institution of Chemical Engineers, 51, 362-368. 

Figure 1. Slurry reactors classified by the contacting pattern and mechanical devices (a) slurry (bubble) column (b) countercurrent column (c) co-current upflow (d) co-current downflow (e) stirred vessel (C) draft tube reactor (g) tray column (h) rotating disc or multi-agitated column reactor (i) three-phase spray column — liquid flow —> gas flow.

Solids suspension is usually carried out in mechanically agitated vessels with or without draft tubes, as shown in Figs. 5 and Dished heads (ASME dished, elliptical, or torispherical heads) are preferred. A single impeller is usually sufficient for off-bottom solids suspension in vessels with dished heads and 7 < 1.3. Dual impellers are recommended for vessels with 13 < H/T < 2.5, which are used for uniform suspension of fast-settling solids. Three or more impellers... 

Fig. 6 Schematic representation of a mechanically agitated vessel with a draft tube. (From Ref...

The flow and dissipation fields are determined in turn by the agitator(s), its (their) placement and rotational speed(s), the placement and dimensions of baffles and draft tubes, and geometry of the vessel. 

The important design parameters for a draft-tube agitated vessel are the diameters of the vessel and draft tube, the height of the draft tube and its position in the vessel, and the type and speed of the agitator. The liquid emerging from a down-flow draft tube under the action of the impeller has a radial component of velocity which may create an undesirable swirling motion in the annulus and induce a vortex at the free liquid surface. Vertical baffles located in the vessel wall can overcome this difficulty and produce a relatively stable suspended bed of particles. 

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

It is also important to control the degree of supersaturation in order to produce crystals of a useful size. This involves heavy recirculation of a slurry, usually maintained at about 20% solids in suspension. Draft-tube crystallizers with mechanical agitation are a commonly used design. Mother liquor is then withdrawn from a baffled zone and crystal slurry from the bottom of a vessel. Since separation is not perfect, the mother liquor transferred from stage to stage contains some quantity of fine crystal that acts as... 

Figure 10-8 Mechanically agitated vessel with a draft tube.

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