Crystallizers draft-tube

Fiq. 28. Vacuum crystallizer. Draft tube and baffle type. After Newman and Bennett (N3). 

Besides the production of large crystals, draft tube baffle crystallizers have several other advantages ... 

Fig. 23. Schematic diagram of draft-tube-baffle crystallizer.

Figure 3.3 Agitated crystallizers (/) Swenson DTB Draft tube and baffle), (ii) Forced circulation (a) with and (b) without external heat exchange, in) Krystal Oslo a) circulating liquor b) circulating slurry...

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-... 

Figure 7.1 Some agitated hatch crystallizers, a) Simple unhaffied, (h) draft-tube baff led, (c) jacket or coil heat exchange, (d) external heat exchange, and (e) vacuum or evaporation...

Figure 7.4 Microcomputer programming of a hatch cooling crystallizer. A, crystallization vessel, B, control heater, C, control cooler. surrounding the draft-tube), D, contact thermometer, E, discharge plug and conical baffle), F, recorder, G, relay, H, temperature programmer, I, cooling water pump, J, cooling water reservoir, K, water inflow L, water outflow after Jones and Mullin, 1974)...

The reaction engineering model links the penetration theory to a population balance that includes particle formation and growth with the aim of predicting the average particle size. The model was then applied to the precipitation of CaC03 via CO2 absorption into Ca(OH)2aq in a draft tube bubble column and draws insight into the phenomena underlying the crystal size evolution. 

Figure 9.18 Continuous crystallizers, a) draft-tube and baffle (DTB), (b) single effect forced-circulation evaporative, (c) Oslo or Krystal type after Rohani, 2001)...

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 11. Standard Dupont Draft tube crystallizer design which was developed in the early sixties.

In this section the model for a continuous evaporative crystallizer is discussed. The crystallizer is of the draft tube baffled (DTB) type and is equiped with a fines removal system consisting of a large annular zone on the outside of the crystallizer (see Figure 1). In order to vary the dissolved fines flow without changing the cut-size of the fines removal system, the flow in the annular zone is kept constant and the flow in the dissolving system is varied by changing the recycle flow rate. The model assumptions are ... 

Other studies have tried to relate sizes of draft tubes, locations and sizes of baffles, circulation rate, and so on to crystallization behavior. So far the conclusions are not general enough to do a designer much good. A possibly useful concept, the separation index (SI), is mentioned by Mullin (1976, p. 293) ... 

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... 

FIGURE 14 Draft-tube-baffle crystallizer. [Courtesy of Signal Swenson Division.]... 

A simple method for implementation of classified-fines removal is to remove slurry from a settling zone in the crystallizer. The settling zone can be created by constructing a baffle that separates the zone from the well-mixed portion of the vessel—recall, for example, the draft-tube-baffle crystallizer described in Section V—or, in small-... 

Newman and Bennett applied dimensional analysis in order to compare forced-circulation and draft-tube crystallizers. The first variables... 

Various equipment and process improvements have been introduced in the industrial practice of crystallization from solution (S15, S16). Saeman (S14) has published a comprehensive discussion of crystallizer design principles, extending some of his earlier work cited above (Si, S4). A companion paper by Garrett (G9) considers the application of theory to the selection and operation of commercial equipment, Design and operation of draft tube and baffle type crystallizers is reviewed by Caldwell (C6),... 

Draft-Tube (DT) Crystallizer This crystallizer may be employed in systems in which fines destruction is not needed or wanted. In such cases the baffle is omitted, and the internal circulator is sized to have the minimum nucleating influence on the suspension. 

The classifying crystallizer (Fig. 18-70) requires approximately the same control of the fines-removal stream and, in addition, requires control of the fluidizing flow circulated by the main pump. This flow must be adjusted to achieve the proper degree of fluidization in the suspension chamber, and this quantity of flow varies as the crystal size varies between start-up operation and normal operation. As with the draft-tube-baffle machine, a considerably higher degree of skill is required for operation of this equipment than of the forced-circulation type. 

---