DTB crystallizer

FIG. 18 65 Draft-tiibe-haffle (DTB) crystallizer, (Swenson Process Equipment, Inc.)... 

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-... 

Table III. Stationary values of G and for a DTB crystallizer operated with a fines r oval system and a size dependent growth regime. The values are given before and 35000 second after a step in the heat input of the crystallizer from 120 to 170 kW. The simulation has been performed with kOO gridpoints...

The origin of this concept is that the fraction of the total area under a normal distribution curve between the 16 and 84% points is twice the standard deviation. The smaller CV, the more nearly uniform the crystal sizes. Products of DTB crystallizers, for instance, often have CVs of 30-50%. The number is useful as a measure of consistency of operation of a crystallizer. Some details are given by Mullin (1972, pp. 349, 389). 

Natural circulation evaporators like those shown on Figure 8.16 may be equipped for continuous salt removal and thus adapted to crystallization service. For large production rates, however, forced circulation types such as the DTB crystallizer of Figure 16.10(g), with some control of crystal size, are the most often used. The lower limit for economic continuous operation is l-4tons/day of crystals, and the upper limit in a single vessel is 100-300 tons/day, but units in parallel can be used for unlimited capacity. 

Special devices for classification of crystals may be used in some applications. Figure 14 shows a draft-tube-baffle (DTB) crystallizer that is designed to provide preferential removal of both fines and classified product. As shown, feed is introduced to the fines circulation line so that any nuclei formed upon introduction of the feed can be dissolved as the stream flows through the fines-dissolution heat exchanger. The contents of the crystallizer are mixed by the impeller, which forces the slurry to flow in the indicated direction. A quiescent zone is formed between the... 

OSLO crystallizers because MSMPR crystallizers operate with much higher slurry densities and lower levels of supersaturation. Forced circulation and DTB crystallizers are examples of MSMPR crystallizers. An OSLO crystallizer operating at high slurry densities with the slurry circulated to the vaporizer will perform like an MSMPR crystallizer. 

Although such a feature can be included on many types of classified-suspension or mixed-suspension crystallizers, it is most common to use this feature with the forced-circulation evaporative-crystallizer and the DTB crystallizer. 

The third, (c), is clear liquid overflow recycling to an elutriating leg which prevents undersized particles from escaping prematurely. Draft tube baffle (DTB) crystallizers contain elutriation legs, but DTB units are more commonly evaporative rather than cooling crystallizers. 

DRAFT TUBE-BAFFLE CRYSTALLIZER. A more versatUe and effective equipment is the draft tube-bafiie (DTB) crystallizer shown in Fig. 27.11. The crystallizer body is equipped with a draft tube, which also acts as a baflie to control the circulation of the magma, and a downward-directed propeller agitator to provide a controllable circulation within the crystallizer. An additional circulation system, outside the crystallizer body and driven by a circulating pump, contains the heater and feed inlet. Product slurry is removed through an outlet near the bottom of the conical lower section of the crystallizer body. For a given feed rate both the internal and external circulations are independently variable and provide controllable variables for obtaining the required CSD. 

Figure 5.5 Calculated size distribution of (NH4)2S04 in a DTB crystallizer as a function of Lf.

Figure 5.8 Particle size measurement and control of a DTB crystallizer.

Figure 5.15 Swenson atmospheric up-pumping reaction-type DTB crystallizer.

---