Draft tube crystallizer

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

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

Continuous crystallizers frequently employ draft tubes to enforce mixing up and down the vertical axis. Even with draft tubes, however, the solids will not be uniformly distributed in general. Figure 8.3 shows solids distribution in a laboratory model of a draft tube crystallizer (Green and Robertson 1993). Notice that there are variations with vertical position inside the crystallizer. In addition, the average concentration inside the draft tube is different from that outside because the net particle velocity and therefore particle hold-up is different where the particle settling velocity is opposite the net flow, i.e., in up flow than where they settle in the net direction of the flow, i.e., down flow. This is discussed in more detail in Mullin (1994), Jones and Mullin (1973), and the experimental modeling section below. 

Green, D.A., and Robertson, D.C. (1993). Suspension Dynamics and Mixing in Draft Tube Crystallizers, In Proceedings of the I2th Symposium on Industrial Crystallization, vol. 2, (Rojkowski, Z.H., ed ), Politechnika Warszawska, Warsaw. 

Type 2. Draft Tube Crystallizer—Well Mixed... 

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

X X X X t E Draft-tube crystallizer with FCD Vacuum cooling... 

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 Mother liquor is then withdrawn from a baffled zone and crystal slurry from the bottom of a vessel.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... 

Silicon dissolves at temperatures prevailing in PAL operations. During pressure letdown, solubility decreases considerably, giving the potential for scale formation on vessel walls. AMAX devoted considerable effort to understanding and controlling the phenomena [33]. The draft tube crystallizer concept adapted by AMAX from the geothermal industry was incorporated at the Cawse and Ravensthorpe operations. 

Table 10.3 Scale-up rules in a draft tube crystallizer.

Use of the FC crystallizer is preferred for evaporative crystallization. The directed and controlled recirculation is carried out with an axial flow pump (elbow-type). This design can also be used for vacuum cooling crystallization, so that then the heat exchanger is no longer required ((J)). The use of this version is recommended if due to the lack of barometric cover in the draft tube crystallizer the incoming solution would boil out of the feed-in point. 

In these simple crystallizers, the crystal content of the suspension can be determined directly from the mass balance. Higher suspension densities can only be adjusted if parallel to the suspension discharge clear liquor is also removed. This is necessary when the mass balance shows suspension densities (crystal surface area) that are not sufficient for the desupersaturation (Figure 11.25). In this case, it is possible to install clarification surfaces even in these simple crystallizers. If this is carried out with the draft tube crystallizer, the result is a typical crystallizer of the DTB group as already pointed out above. 

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