Impinging stream crystallizer

As described in previous chapters in Part II, liquid-continuous impinging streams (LIS) has the feature of very efficient micromixing and can provide a uniform and controllable supersaturation environment for the crystallization process, favoring the production of uniformly large-sized crystalline also, it has been proved experimentally that, to an extent, LIS can enhance crystal-growth rate. For industrial application, Wu [237] designed and patented the impinging stream crystallizer, the structure of which is shown in Fig. 17.3. 

The essential parts included in the impinging stream crystallizer are the cylindrical vessel (1) the upper and lower drawing tubes (2) co-axially fixed in the vessel (1) and the upper and lower propellers (3) and (4) set up inside the upper and lower drawing tubes, respectively, which are driven by the same motor and have the spiral blades with opposite screw directions, i.e., one is the left screw and the other is the right. The vessel 1 contains the process medium, crystals-in-solution suspension, with the level over the 

In addition to continuous operation, the impinging stream crystallizer can also be operated in batch mode to suit the crystallization of substances with very low crystal-growth rates, e.g., some of the organic compounds. 

While in batch operation, the fresh solution is fed into the crystallizer all at once the crystallization process carries on for a certain period, then the suspension is discharged from the crystallizer and is conveyed to the device for solid-liquid separation. For such operation the classifying leg (9), the overflow cofferdam (7), and the pump A are not needed. 

In comparison with existing crystallizers, including DTB, DP and Standard-Messo 

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Y. Wu, Y. Zhou, X. He and C. Bao, Crystal-growth rate of di-sodium phosphate in impinging stream crystallizer, a paper to be published. 

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). 

Figure 12.3 Scheme of experimental system with impinging stream crystallizer. 1-cooling tank ... 

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... 

Because LIS itself has the nature of efficient micromixing, the circulation of the suspension inside the crystallizer in large amounts is not necessary, but in a small amount, i.e., the cycling ratio in the impinging stream crystallizer is very small (< 2), yielding lower power consumption ... 

The special flow configuration of the perfect mixing-plug flow in series in the impinging stream crystallizer is more suitable to crystallization process and so it is much easier to prepare uniformly large-sized crystalline particles and... 

Wu, Yuan, Wuhan Institute of Chemical Technology (2003). The impinging stream crystallizer. CNP ZL03235520.3. 

Actually, the mechanism for the formation of HAP crystalline during the process under consideration is very complex and some suggestions proposed for the mechanism cannot be supported by experimental evidence so details of the chemical aspect will not be discussed here. In any case, precipitation, i.e., crystallization from a solution, must be involved in the process so the fact that liquid-continuous impinging streams cause strong micromixing and pressure fluctuation would favor the synthesis of nano HAP. 

Impinging jets, routinely used in the plastics industry, have been used to crystallize small particles with a narrow particle size distribution [27,28].The impinging jet crystallizer is a cylindrical chamber with two spray nozzles that enter from opposite sides of the chamber. A solution of the material to be crystallized is fed into one nozzle, and the agent to induce crystallization is fed into the other nozzle. The smaller the nozzle apertures, the faster the velocity of the solutions into the chamber, and the faster the two streams mix and reach a uniform environment. In this manner a stream of finasteride (Figure 11.10) in AcOH and water was fed into one nozzle, and a stream of water (the crystallizing agent) was fed into the other nozzle. Crystallized finasteride had an average particle size of 10-15 pun. 

Figure 9-27 Schematic of the impinging jet crystallizer showing the option to recycle around the mixing device. The recycle loop provides additional turbulence for the exit stream after the impingement.

Highly oriented fibers may be crystallized from solution when two streams of dilute polyethylene solution impinge upon one another at appropriate speeds. 

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