Fluidized Suspension Crystallizer

Although close clearances between the downcomer and the bottom of the suspension chamber can lead to difficulties with plugging by crystalline growth, this design has been widely used for the production of some coarse materials such as ammonium sulfate. Its best field of application is for crystals of very high settling rate such as nickel sulfate. 

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These concerns lead to the conclusion, referred to above, that it is often necessary to choose a mixing condition (impeller speed, type, etc.) that may not be optimum for every aspect of the crystallization and may actually not be optimum for any of them. In many cases, however, one end result (i.e., PSD, bulk density, uniformity of suspension, and approach to equilibrium solubihty [yield]) may dictate the choice of mixing conditions. In this case, it becomes essential to determine if the negatively affected aspects can be tolerated. If these problems are occurring in operation in a stirred vessel, a different type of crystallizer, such as a fluidized bed, might be used to promote crystal growth and minimize nucleation. Readers can find more information on fluidized bed crystallizers in Section 6.6.2 and Examples 7-6 and 11-6. 

Suspension takes place in a liquid fluidized bed. Crystals are separated from circulated solution by sedimentation, thus the circulation pump only circulates crystal-free solution to avoid secondary nucleation and crystal attrition. Generation of coarse and uniform crystals. 

The Oslo fluidized-bed crystallizer has separate sections for the generation of supersaturation and for crystal suspension and growth (Figure 3.3 (iii)). Supersaturation is created externally e.g. by eooling or evaporation and the liquor pumped into a central downeomer in the main vessel. A elassified bed of crystals is supported in the vessel by an upward flowing stream of supersaturated liquor in the armular spaee surrounding the downcomer. Relatively large crystals of mean size say >1000 pm are withdrawn from the base. In an effort to increase efficiency, however, modem practice is frequently to operate Oslo units in the mixed-suspension mode. 

As mentioned above batch crystallizers are usually simple vessels provided with some means of mechanical agitation or particulate fluidization. These have the effect of reducing temperature and concentration gradients, and maintain crystals in suspension. Baffles may be added to improve mixing and heat exchange or vacuum systems may be added, as appropriate. Various design combinations are available and some are illustrated in Figure 7.1. 

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. 

Designfeed rate exceeded. At design suspension density, this results in a reduced average retention time. Within limits, this can be corrected by increasing the suspension density. An increase in production rate usually requires an increase in circulation rate to handle the additional supersaturation and the heavier fluidized bed of crystal... 

Major Application General purpose. A fluidized bed of solids is present in the suspension chamber of the crystal lizer. 

The slurry density in a crystallizer whose fluidization is maintained by a propeller or external circulating pump can be determined by a differential pressure measurement between two points in the suspension located far enough apart so that suitable differential reading can be obtained. With crystals of relatively high specific gravity, these two points of measurement can be in the range of 6-10 ft (2-3 m) apart. Normally, they would be mounted at two points on... 

In a dynamic process, the mixture is mixed different methods of mixing can be used. Movement of food particles in a stationary solution, mixing of the whole suspension, and the flow of the osmoactive substance through the stationary layer of food pieces are the commonly used designs of the dynamic process. If crystals of the osmoactive substance are used, the fluidized bed is the solution for the dynamic process. It has been shown that the rate of motion has little effect on the rate of osmotic dehydration [71,133]. It is just sufficient to induce motion of particles or solution in the system to have increased mass transfer rates. Moreover, it was shown that the motion of osmotic solution in a turbulent region affected water flux but no difference in solids gain occurred in comparison with laminar flow [134]. 

Utilizing the above factors, theoretical analyses of some possible design procedures for fluidized-bed classifying and mixed-suspension classified product removal crystallizers have been presented together with detailed worked examples by Mullin and N lt (1970) and Nyvlt (1992). 

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