ScienceAsia 32 (2006): 231-239 |doi: 10.2306/scienceasia1513-1874.2006.32.231

Flow Fields in a Mixed-Suspension Mixed-Product-Removal Crystallizer: A Numerical Analysis

Jaruwan Tangtonsakulwong^a, Adrian Flood^b* and Tawit Chitsomboon^a

 
ABSTRACT: Isokinetic withdrawal of solution from research crystallizers is a key factor in obtaining a representative crystal-size distribution and is critical for correct analysis of the crystallization process to accurately determine crystallization kinetic parameters such as crystal growth rates, nucleation rates, breakage rates, and agglomeration rates. Isokinetic withdrawal, in turn, depends upon the characteristics of the flow field in the crystallizer. This research numerically simulated the fluid flow field in a small-scale (experimental) cylindrical round-bottomed, continuous-flow, cooling crystallizer, which was set up for analysis of kinetic parameters in sugar production. The commercial software “CFX 5.5.1” was employed to perform the 3 dimensional simulation with the finite volume method using an unstructured mesh. The impeller used for increasing the gross velocity of the solution inside the crystallizer was modeled using a momentum source. Seven momentum source strength values were used to perform the simulation. The results showed that the momentum source strongly increased the axial flow velocity but only slightly influences the overall flow pattern, except the flow near the outlet tube. The isokinetic withdrawal condition was achieved at a momentum source strength of about 25,000 kg/m²/s². The isokinetic condition for the nuclei crystals was best for the research MSMPR, as it would make the particle size distribution in the product stream most accurate for the analysis of the nucleation and growth rates in the crystallizer. For larger particles, the settling velocity was required for determination of isokinetic withdrawal. The power transmitted by impeller shaft was estimated as 0.6 W, which allowed the impeller rotational speed to be estimated. Furthermore, the velocity at the center of the tank’s bottom was lower than that of the surrounding region. This could cause a collection of settled crystals, which was problematic for the operation and analysis of the crystallizer.

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a School of Mechanical Engineering, Institute of Engineering, Suranaree University of Technology, 111 University Ave., Muang District, Nakhon Ratchasima, 30000 Thailand.
b School of Chemical Engineering, Institute of Engineering, Suranaree University of Technology, 111 University Ave., Muang District, Nakhon Ratchasima, 30000 Thailand.

* Corresponding author, E-mail: adrianfl@sut.ac.th

Received 27 Jun 2005, Accepted 31 Mar 2006