Ashwin W. Patwardhan

CFD modelling and mixing in stirred tanks

In order to develop rational design procedures for stirred vessels, several important points have to be addressed. Firstly, it is necessary to relate the impeller-vessel geometry to the flow field produced. Secondly, it is necessary to establish energy balance in stirred vessels. Thirdly, it is necessary to understand the linkage between the flow field and the design objective. All the three aspects have been addressed here. Five different designs of axial flow impellers have been studied. The zonal modelling concept used by Sahu et al. (1998 Ind. Engng Chem. Res. 37, 2116–2130.) has been extended to predict the flow field generated by these impellers. It is observed that the predictions of turbulent kinetic energy (k) were significantly improved by using zonal modelling. A new method has been proposed to estimate the values of turbulent energy dissipation rate (e). This new method gave excellent comparison between the estimated values and CFD simulations. The CFD simulations have been extended to predict the mixing time for different impellers. The predicted mixing times are in excellent agreement with the experimental measurements of Rewatkar and Joshi (1991, Chem. Engng commun. 91, 322–353).

CFD modeling of jet mixed tanks

Jet mixing is one of the simplest methods to achieve mixing. There have been a number of experimental studies concerned with jet mixing. Most of these studies end up in empirical correlations. In the past, there have been only a few attempts towards computational fluid dynamics (CFD) modeling of jet mixed tanks. Further, these CFD models have not been validated by detailed comparison with experimental measurements. In view of this, the present work deals with development of a CFD model and a detailed comparison with experimental measurements. From the results, it is clear that the CFD model predicts overall mixing time fairly well. However, the concentration profiles as a function of time at various locations in the vessel are not predicted well. The reasons for this are explored and attempts have been made to improve the predictions of concentration profiles.

MIXING IN TANKS AGITATED BY JETS

Experiments have been carried out to study the effects of various parameters such as nozzle diameter, angle of inclination and jet velocity, on mixing time. The dependence of mixing time on the nozzle angle has been explained based on the flow patterns and concentration profiles. It has been observed that an increase in the nozzle diameter reduces the mixing time at a given level of power consumption. The energy efficiency of the jet mixers has been compared with impeller stirred tanks. Attempts have been made to identify the correlation (from previous literature) that could satisfactorily predict the experimental results obtained.

The role of convection and turbulent dispersion in blending

Blending is an important operation in process industries. Under turbulent conditions, it occurs via convection and turbulent dispersion. In the present work, attempts have been made to identify the controlling mechanism, by comparing the blending performance of three different equipments: stirred tanks, jet mixers and ultrasound mixers. It is seen that jet mixers offer lower mixing time as compared with impeller mixers, whereas ultrasonic horns show higher mixing time as compared with impeller mixers, when compared at the same level of power consumption in tanks having the same size.

Relation between flow pattern and de-activation of enzymes in stirred reactors

Enzymes and microbial cells are increasingly being used for a variety of biochemical transformations. During the process the enzymes/micro-organisms are subjected to hydrodynamic stresses which can lead to their de-activation. This has enormous economic impact due to their high cost. In the present work, attempts have been made to relate the extent of de-activation with the hydrodynamic stresses produced in impeller stirred reactors. The hydrodynamic stresses produced by a variety of impellers have been compared. The extent of de-activation of cellulase enzyme has been co-related with the average hydrodynamic stress within the stirred reactor.

Measurement of Gas Hold-up Profiles in Stirred Tank Reactors by Gamma Ray Attenuation Technique

Gas hold-up is one of the most important hydrodynamic characteristics that is needed for performance estimation, design and scale-up of reactors. The performance of large-scale reactors can be reliably evaluated by measuring the gas hold-up in such reactors. In the present work, local gas hold-up in an industrial reactor has been measured with the help of gamma ray tomography. Based on these measurements, an empirical equation has been proposed for the estimation of the gas hold-up distribution. The proposed equation has been validated by comparison with laboratory scale data.

Mathematical Model for the Extraction of Neodymium from Nitrate Media using Hollow Fiber Supported Liquid Membrane Operated in a Recycling Mode

A mathematical model for facilitated extraction of Neodymium (Nd3+) ions from nitrate media using microporous hollow fiber supported liquid membrane (HFSLM) operated in a recycling mode is presented. Extractant N,N,N′, N′-tetraoctyl diglycolamide (TODGA) diluted with n-dodecane was used as the membrane phase. Di-n-hexyl octanamide (DHOA) has been used as a phase modifier for the extractant. The model developed is not specific to the case considered and has a more general and wide applicability. The model has been developed using equilibrium-based approach. The complexation and de-complexation reactions were assumed to be fast and at equilibrium. Mass balance equations for both acid (HNO3) and TODGA were also incorporated in the model. It was observed that the model results are in good agreement with the experimental data when diffusivity of metal-complex (D m ) and acid-complex (D hm ) through the membrane phase in the pore is 6 × 10−12 m2/s and 1.2 × 10−10 m2/s. Once the values of D m and D hm are estimated by simulation for one set of data, there are no further fitting parameters in the model. The model can then be used in a truly predictive mode for all the remaining data sets.

Mathematical Model for the Extraction of Metal Ions using Hollow Fiber Supported Liquid Membrane Operated in a Recycling Mode

The role of cations H+ and Na+ on the transport of Nd3+ ions was investigated using microporous Hollow Fiber Supported Liquid Membrane (HFSLM) contactor. Extractant N,N,N′,N′-tetraoctyl diglycolamide (TODGA) diluted with n-dodecane was used as the membrane phase. Di-n-hexyl octanamide (DHOA) was used as a phase modifier for the extractant. Transport of Nd3+ ions and HNO3 was studied at varying concentrations of NaNO3 and HNO3 while keeping total nitrate ( ) concentration nearly constant at 3 M. The extraction equilibrium constant (K ex ) for the complexation reaction was experimentally measured for various conditions. The maximum rate of extraction of Nd3+ ions was observed at an equimolar concentration (1.5 M each) of HNO3 and NaNO3 in feed solution under otherwise identical conditions. A mathematical model has also been developed to simulate this system. Mass balance equations for both acid (HNO3) and TODGA were also incorporated in the model. It was observed that the model results are in good agreement with the experimental data when diffusivity of metal-complex (D m ) and acid-complex (D hm ) through the membrane phase in the pore is 6 × 10−12 m2/s and 1.2 × 10−10 m2/s. Once the values of D m and D hm are estimated by simulation for one set of data, there are no further fitting parameters in the model. The model can then be used in a truly predictive mode for all the remaining data sets.

Comparison of Different HFSLM Configurations for Separation of Neodymium and Uranium

Different feed and strip operating modes have been compared for the transport of neodymium (Nd3+) and uranyl (UO22+) ions in nitric acid media. The flux values of UO22+ for feed in recycling and once through mode with fresh strip were 2.26 × 10−9 kmol/m2-s and 2.33 × 10−9 kmol/m2-s, respectively. So, this resulted in low separation factors. The flux values of Nd3+ for feed and strip—both in recycling and in once through mode—were 2.21 × 10−9 kmol/m2-s and 2.14 × 10−9 kmol/m2-s, respectively. The operating modes—feed and strip both recycling and feed and strip both in once through—were found to be the best configurations.

Simultaneous Extraction of Neodymium and Uranium using Hollow Fiber Supported Liquid Membrane

Simultaneous extraction of neodymium and uranium ions from aqueous nitrate media was investigated using hollow fiber supported liquid membrane (HFSLM). The organic phase supported in the membrane pores consisted of extractant N,N,N’,N’-tetraoctyl diglycolamide (TODGA), phase modifier isodecanol, and diluent n-dodecane. Experimental results suggest that there is competition between neodymium and uranium ions for complexation with TODGA. The initial rate of extraction of Nd3+ ions was found to be approximately six times to that of UO22+ ions. Experimental data was explained by a mathematical model for simultaneous transport of two metal ions. The model results were found to be in good agreement with the experimental data when the diffusivities of neodymium-TODGA complex (Dnm) and uranium-TODGA complex (Dum) in the membrane pore are 1.1 x 10−11 and 4 x 10−12 m2/s, respectively.