Impinging jet mixing

OPTION 3 IMPINGING JETS MIXING/SIMULTANEOUS ADDITION... 

Figure 9-19 Microscopic photo of crystals for Example 9.4, Option 3 (impinging jet mixing/simultaneous addition). Needle-like crystals are formed under rapid mixing and high supersaturation instead of crystals with a third dimension. In addition, these needle crystals are a new solvate.

Benet, N., L. Falk, H. Muhr, and E. Plasari (1999). Experimental study of a two-impinging-jet mixing device for application in precipitation processes. Int. Symp. Ind. Cryst. 14th (computer optical disc), 1007-1016 (IChemE, Rugby, UK). 

Figure12.3 (a and b) Design examples of impinging jet mixing devices, where the two streams are not confined.

This parallel reaction set was used, for example, by Johnson and Prud homme (2003a) to investigate the quality of mixing in a confined impinging-jets reactor. 

Unger, D.R. 8t Muzzio, F.J., Laser-induced fluorescence technique for the quantification of mixing in impinging jets AIChE J. 1999, 45(12), 2477-2486. 

Becker, H. A., Cho, S. H., Ozum, B. and Tsujikawa, H. (1988). Turbulent mixing in the impingement zone of dual opposed free jets and of the normal wall-impinging jet. Chem. Eng. Commn., 67 291-313. 

Arrays with multiple oblique impinging jets are generated via 3-D channel networks which feed fluid from a reservoir via the outlet nozzles of the network into a mixing chamber (see Figures 1.196 and 1.197) [54], Perforated plates contain such arrays. 

Figure 1.199 Determination of the mixing quality of the different impinging jet micro mixers using a competing reactions approach based on measuring UV-Vis absorption ([53] source IMM).

Figure 1.200 Dilution-type fluorescence imaging for visualization of the mixing process in the impinging-jet array micro mixer under offset conditions for the arrays at various times [54] (by courtesy of IOP Publishing Ltd.).

The second type of mixer uses two impinging jets exiting from two borings to contact the reactants (for more detailed information, see Section 1.3.33, Jet Collision Turbulent or Swirling-flow Mixing). 

Experimentally it has been found that the Recrit for impingement mixing for onset of turbulence is around 140. This value seems to be insensitive to the impinging jet angle of incidence (82). Furthermore, the Re 3/4 dependence seems to hold until the range 140 < Re < 250 - 500 (83). 

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. 

Much of this chapter assumes operation in stirred vessels. Several alternative designs (fluidized bed and impinging jet crystallizers) are summarized early (Table 6-1) for comparison with stirred tanks, and are described later in this chapter and in other parts of this book. An alternative feed addition geometry (mixing elbow) is described in Example 7-1. 

Although many variations of mixing systems have been used for crystallization processes, the three primary types that are discussed in this book are the stirred vessel, fluidized bed, and impinging jet devices. Each of these utilizes different mixing environments to achieve the desired local and global conditions. 

The predominant system in the pharmaceutical industry is the stirred vessel. Fluidized beds (Chapters 7 and 11) and impinging jets (Chapter 9) fill specific mixing requirements, as indicated in Table 6-1. 

Clearly, the key to a successful scale-up of this process is to maintain the mixing time at the pilot plant and factory scale below the threshold which would cause the process to move down the curve of Fig. 6-2. This can be best accomplished by using a special mixing device, such as an impinging jet, to approach the same mixing time at all scales. The reader can find more information on impinging jets and crystallization in Section 6.6.3 and Examples 9-5 and 9-6. 

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