Characterizing Flow and Solid Suspension in Optimax Crystallization Workstation

Characterizing Flow and Solid Suspension in OPTIMAX Crystallization Workstation

  M. Sardeshpande¹, A. Pandit¹, S. Vedantam² and V.V. Ranade^{1* 1}Industrial Flow modeling Group Chemical Engineering and Process Development Division National Chemical Laboratory, Pune 411008, INDIA ²Chemical Engineering Division, Indian Institute of Chemical Technology, Hyderabad – 500007, INDIA *Email: vv.ranade@ncl.res.in  

OPTIMAX® (of Mettler-Toledo Ltd.) is one of the widely used crystallization workstations. OPTIMAX® is an automated reactor, offering heating and cooling electrically on the basis of Peltier technology. It is usually equipped with Focused Beam Reflectance Measurement (FBRM) probe for measuring on-line Chord Length Distributions (CLD). A schematic of the workstation is shown in Figure 1. In order to appropriately interpret the data obtained from OPTIMAX® workstation, it is essential to quantitatively understand flow, heat transfer and solid suspension in it. In this work we have developed a CFD model to characterize flow and solid suspension in OPTIMAX® workstation.

Figure 1: Schematic of OPTIMAX® crystallization workstation

In order to characterize solid suspension in OPTIMAX® workstation, we have used the approach proposed by Sardeshpande et al. (2010). Sardeshpande et al. (2010) reported hysteresis in cloud height observed experimentally in a stirred tank reactor while increasing and then decreasing the impeller speed. The hysteresis was then captured using a CFD model which served as a rigorous validation for the model. In the present work we wish to capture the hysteresis on increasing and then decreasing the impeller using the FBRM measured particle counts. A sample of the particle counts measured by the FBRM for increasing impeller speeds is shown in Figure 2. This relies on measurement of hysteresis in particle volume fraction (or number of particle counts) with respect to rotational speed of impeller.  Experiments were carried out with baffled, hemi-spherical bottom stirred tank reactor. 1000ml of this OPTIMAX reactor contents Focused Beam Reflectance Measurement (FBRM) probe (used for estimating the Particle Size Distributions (PSD)). Hysteresis measurements were carried out for three different particle volume fractions using glass beads.

Figure 2: Particle Counts measured by FBRM as a function of impeller speed while increasing impeller speed

CFD model of the considered configuration was developed. Multiple reference frame approach was used to model the rotating impeller and multiple phases have been simulated using the Eulerian - Eulerian approach. Appropriate care was taken to ensure that simulated results are independent of numerical parameters (grid size, discritization scheme, time step and so on). The simulated results of hysteresis in particle counts are compared with the experimental data. Critical analysis of obtained results was carried out.

The presented approach, model and results will be useful to characterize flow and solid suspension in the OPTIMAX® workstation. The developed CFD model will be useful for eventual simulations of crystallization process occurring in OPTIMAX® workstation as also in crystallizer scale up studies.

References   Sardeshpande M. V., Juvekar V. A.  and Ranade V.V., 2010 Hysteresis in Cloud Heights During Solid Suspension in Stirred Tank Reactor: Experiments and CFD Simulations, AIChE Journal, 56(11), 2795-2804