Crystallizers draft-tube-baffle evaporator-crystallizer

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 ... 

Draft-Tube-Baffle (DTB) Evaporator-Crystallizer Because mechanical circulation greatly influences the level of nucleation within the crystallizer, a number of designs have been developed that... 

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. 

The process system consists of two long-tube vertical evaporators, a draft-tube baffled crystallizer, a rotary-drum vacuum filter, and a direct-heat rotary dryer. Also, pumps are needed to move the solution from evaporator 1 to evaporator 2, to recycle the filtrate from the filter to the crystallizer, and to move the magma from the crystallizer to the filter and a heat exchanger is needed to heat the recycle filtrate. However, the purchase costs for the three pumps and the heat exchanger are not considered here because examples for these types of equipment are presented in Section 16.5. For the equipment considered here, assume fabrication from stainless steel, with a material factor of 2 for the ratio of stainless steel cost to carbon steel cost. For the process, using the following size factors and the equations in Table 16.32, the estimated f.o.b. equipment purchase costs at a CE index of 394 are included in the following table. 

Figure 7.1 Some agitated batch crystallizers, a) Simple unbeffled, (b) draft-tube baffled, (c) jacket or coil heat exchange, (d) external heat exchange, and (e) vacuum or evaporation...

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)...

Recirculating crystals return to the agitator through the annulus between the draft tube and the baffle. The latter as shown here is an extension of the shell of the upper chamber. The volume between the baffle and the outer wall allows the slurry to separate. The crystals fall toward the bottom, and the clarified mother liquor overflows from the top of this section. This mother liquor can go on to further processing or return to the bottom of the crystallizer. The rate of withdrawal of mother liquor determines the clarifying efficiency behind the baffle. It can be varied to control the amount or size of fine crystals that leave the crystallizer. These crystals can redissolve in a fines killer if the circulating mother liquor is heated or if an unsaturated feed stream is introduced. The latter is the normal practice in the recovery of Glauber s salt from caustic evaporators. This action keeps the fines out of the final crystal product and makes the particle size distribution narrower. 

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