Fluidized beds, crystallization

M. Belcu, D. Turtoi. Simulation of the fluidized-bed crystallizers (I). Influence of parameters. Cryst Res Technol 57 1015, 1996. 

A comparative study [10] is made for crystal-growth kinetics of Na2HP04 in SCISR and a fluidized bed crystallizer (FBC). The details of the latter cem be found in [11]. Experiments are carried out at rigorously controlled super-saturations without nucleation. The overall growth rate coefficient, K, are determined from the measured values for the initial mean diameter, t/po, masses of seed crystals before and after growth. The results show that the values for K measured in ISC are systematically greater than those in FBC by 15 to 20%, as can be seen in Table 2. On the other hand, the values for the overall active energy measured in ISC and FBC are essentially the same. 

Huang, C Pan, J.R., Lee, M. and Yen, S. (2007) Treatment of high-level arsenic-containing wastewater by fluidized bed crystallization process. Journal of Chemical Technology and Biotechnology, 82(3), 289-94. 

For comparison, the experiments for measuring the overall crystal-growth rate coefficient are carried out in an impinging stream crystallizer (ISC) and a fluidized bed crystallizer (FBC). 

The structure of a experimental fluidized bed crystallizer (FBC) is shown in Fig. 12.4, where the crystallizer is actually a universal equipment for the measurement of crystal-growth rate. The solution enters the FBC at its bottom, and leaves the FBC by overflow. All the other parts of the experimental system are the same as shown in Fig. 12.3, and so are not shown in Fig. 12.4. The operation procedure for the FBC is the same as for the ISC. For convenience of comparison, the corresponding conditions, temperature and concentration of the solution, operated in the ISC and the FBC are rigorously controlled to be the same, with the deviation of the operating temperature no greater than 0.05 °C. 

The mean observed active energies obtained by regression of the experimental data for various crystal seeds with different mean diameters and in different crystallizers are listed in Table 12.4, where, similarly, the subscripts IS and FB denote the parameters in the impinging stream crystallizer and the fluidized bed crystallizer, respectively. ... 

Except for a few questionable data, the values for the observed active energy measured in the two crystallizers of different types, EiS and EFB, show little difference and can be considered to be more or less identical. On the other hand, the values measured in the impinging stream crystallizer for the overall crystal-growth rate coefficient, KIS, are obviously and systematically larger than those in the fluidized bed crystallizer, A pe. Therefore it can be affirmed without the need for further analysis that, with the observed frequency factors, there must... 

Figure 4-21 Fluidized bed crystallizer growth rate test apparatus.

The fluidized bed crystallizer typically has a screen support (as in a commercial chromatography column) but can be operated without one by use of a tubing clamp on the column inlet hne. (The clamp must be closed quickly whenever the pump is stopped.)... 

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. 

The second, (b), shows clear liquid overflow, as in a well-behaved fluidized bed crystallizer or in the semicontinuous stirred tank (SCST) operation illustrated in Example 7-5. The SCST has proven to be a versatile and practical method of maintaining control in critical separation and polymorph control, and can be operated in small and large conflgurations. 

Fluidized bed crystallizers operated in particulate mode (usually the case with hquids) can also come very close to plug flow operation. An example of such crystallizers will be shown in Example 7-6. 

The pure fluidized bed crystallizers described later in Example 7-6, while designed for a particular purpose, have many similarities with the Oslo commercial surface-cooled crystallizer shown in Fig. 7-10. A fluidized magma in the crystallizer body (E) carries out the crystallization. Feed enters the clear overflow stream (at G), is cooled within the metastable super-saturation region in the cooler (H), and then enters as a fluidizing stream at the bottom of (E). 

Example 7-6 illustrates the applicability of good crystallization practice to achieve continuous production of large-volume pharmaceutical compounds. It also illustrates a crystallization process that is inherently unfeasible by any method other than continuous operation. When carried out using fluidized bed crystallizers, ultrasonic crystal disraption is used, even at factory scale, to maintain a steady-state population of seed particles in this all-growth system. 

Fluidized bed crystallizers to avoid the need for heavy-magma, high-flux filtration equipment. 

The astute observer will see that the fluidized bed crystallization system shown in Fig. 7-27 has an unusual feature. Flow sonication units are in position to operate on pumped slurry from the seed beds. In other systems, the sonicators are located internally in the bottom of the column. The sonicators are... 

Sonication is the means used in the fluidized bed crystallizers to maintain the number of seed particles in the magma to replace those removed in the product and at the same time to prevent the formation of overly large crystals. Excessive particle size starves the seed bed of crystal surface area for growth and, in the case of fluidized bed crystaUizers, causes sluggish solids movement, which can cause the particles to grow together. 

Figure 7-30 shows a pair of factory fluidized bed crystallizers, one for each stereoisomer, in construction. Internal sonicators were installed in the column bottoms. The blowers shown at the bottom of each column were used to cool the sonication units. Calculated values of localized solids concentration and PSD in these columns were in good agreement with predicted values from the calculation procedure described above. 

Design of fluidized bed crystallizers requires estimates of the required seed bed volume and the quality of fluidization. Fluidization behavior of seed was measured in the laboratory for monosized cuts. For a given volumetric flow rate through any given fluidized bed crystallizer, there is a minimum and maximum particle size which will result. The minimum is that below which the particles will elutriate out the top of the column. The maximum is determined by the size at which removal or controlled attrition (see below) takes place at the bottom. 

The crystallizers described in this section were designed with a 3 1 diameter ratio (9 1 ratio in linear velocity). Plug flow approximation for these fluidized bed crystallizers was confirmed by point insertion studies with a radioactive tracer. 

The fluidization studies noted above for monosized crystal populations showed that the minimum particle size in the fluidized hed crystaUizers should be 150 microns and the maximum 600 microns. In an all-growth process (absence of nucleation), the smallest should grow to become the largest with a distribution like that of the IdeaUzed curve in Fig. 11-23. The histogram plot of actual column data (Actual) in the same figure shows that the fluidized bed crystallizer run under the conditions of that experiment, corresponds to an all-growth situation. 

This also implies that the main mechanism for crystal breakage is individual crystal breakage in the cavitation field rather than particle-particle interaction, which is more common for most wet-milling operations. The breakage constant of about 0.03 gm/hr per (gm/liter, watt) was found to hold for a number of small molecule organic compounds tested in the fluidized bed crystallizer system. 

As noted above in the section on fluidization, it is appropriate to minimize the angle of the conical section from the vertical to minimize backmixing. Different relative scales of some actual fluidized bed crystallization operations are shown in Fig. 11-32. 

In a growth-dominated fluidized bed crystallizer, the interface between the fluidized bed and the mother liquors should be sharp, as the particles either grow larger or wash out at the top. Figure 11-33 shows a sight glass view of such an interface in a factory-scale crystallizer. 

Allegreth, J.E. and M. Midler (1970). Separation of stereoisomers by fluidized bed crystallization. Presented at AIChE annual meeting, Cleveland, OH, November. 

Midler. M. (19751. Process for production of crystals in fluidized bed crystallizers. U.S. Patent 3,892,539. Midler. M. (19761. Crystallization system and method... 

Figure 2.39 Fluidized bed crystallizer glass tube (1), stock vessel (2), cooler (3), thermostat (4), centrifugal pump (5), infrared lamp (6), power source (7), contact thermometer (8), mercury thermometer (9), and stirrer (10). (Reproduced with permission from Nyvlt et al. 1985.)...

A batch fluidized bed crystallizer is frequently used for precise measurements of the crystal growth kinetics (Karpinski 1981). The technique is particularly suitable for systems with the density of crystals exceeding that of the solution by more than 10%, and for seed crystals in the 1 x 10 — I x 10 m size range. Most inorganic salts fall into that category. An example of an experimental... 

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