Shekhar Kumar

Modeling of Hydrodynamics in a 25 mm ϕ Pulsed Disk and Doughnut Column

The hydrodynamic parameters, namely, dispersed phase holdup and flooding throughput, have been investigated in 25 mm diameter pulsed disk and doughnut column (PDDC), in no mass transfer conditions. In this work, using existing correlations on plate pulsed columns, the dispersed phase holdup and the flooding throughput are empirically modelled well using the slip velocity concept. A good agreement is observed between experimental values and predicted values obtained from empirical correlation. The experimental data for dispersed phase holdup and flooding throughput has been modelled using the Van Delden model to describe the hydrodynamics characteristics of a PDDC and necessary adjustable parameters for drop size distribution and dispersed phase holdup are updated for 30% TBP-nitric acid system. The model parameters were estimated by minimizing the absolute error between experimental and theoretical values of flooding throughput and holdup data. It was found that the measured values and observed trends could be described accurately using this model after fitting holdup and flooding data. The error between the experimental and theoretical values of flooding throughput and holdup was found to be less than 10%.

Synthesis and characterization of red-oil from tri iso-amyl phosphate/n-dodecane/nitric acid mixtures at elevated temperature

Red-oil is a mixture of nonspecific composition consisting of extractant, degradation products, nitrated solvent and unidentified red-coloured nitro-organics. Red-oil formation is coupled with decomposition of extractant and diluent into gases of explosive nature. If ignited or incinerated, these gases may cause rapid pressurization and endanger the integrity of containment. Such an event occurred at Tomsk-7 facility in 1993. To ensure safe operation, red-oil formation has to be avoided in the fuel cycle facilities by a careful combination of several independent measures like strict control over temperature, limiting organic entrainment in the aqueous streams (which are to be concentrated by evaporation) and control over acidity of aqueous phases. Since tri-iso amyl phosphate (TiAP) has much lower aqueous solubility as compared to TBP, it is visualized as alternate solvent for PUREX process. In this work, TiAP red-oil was synthesized and characterized.

New Data on Activity Coefficients of Potassium, Nitrate, and Chloride Ions in Aqueous Solutions of KNO 3 and KCl by Ion Selective Electrodes

Ion selective electrodes (ISEs) are used to measure the single-ion activity coefficients in aqueous solutions of KNO3 and KCl at 298.15K against a double-junction reference electrode. The EMF responses of ISEs up to 0.01m are plotted to obtain the slope and intercept values. The obtained slopes and intercepts are used in Nernst equation for higher concentrated solutions for calculation of individual ion activity coefficient. The mean ionic activity coefficients are estimated from single ion activity coefficient, and the obtained results are compared with the literature values.

Thermal Decomposition of Nitrated Tri-n-Butyl Phosphate in a Flow Reactor

Tri-n-butyl phosphate (TBP) is a universal nuclear extractant, commercially used in the PUREX process for the last 60 years. However, it is prone to nitration and thermal degradation, and as a consequence a red-oil event may be initiated under several operating conditions resulting in severe pressurization of vessel/cell if venting is inadequate. In this work, an attempt was made to understand the reaction pathway of thermal decomposition of nitrated TBP in a flow reactor at atmospheric pressure. Many reaction products were identified and quantified by instrumental methods like HPLC-RI and GC-TCD. The experimental data was analysed with a power law model and the apparent rate constants were estimated. The activation energy for thermal decomposition of nitrated TBP, assuming an Arrhenius type of temperature dependency, was estimated to be kJ·mol−1. The effect of both varying temperature and concentration of nitric acid on conversion of TBP into degradation products and products distribution was experimentally studied. Based on the experimental observations, a reaction mechanism framework for thermal decomposition of nitrated TBP is proposed.

An extended Setschenow model for Pu(IV) third phase formation in 20% tri-n-butyl phosphate based nuclear solvent extraction system

Abstract Formation of a third phase during liquid–liquid extraction is observed when solvate concentration in the organic phase exceeds its solubility limit in the diluent. This phenomenon is also observed in certain reaction systems like phase‐transfer catalysis. Although this phenomenon has been known since the early fifties, quantitative models have not been reported in the open literature. Recently, a Setschenow type model for Pu(IV) third phase formation in BESO/diluent system has been proposed. In the present work, the salt‐effect model is extended to cover Pu(IV) third phase formation in 20% TBP/diluent based nuclear solvent extraction system. Since it is an extended form of the Setschenow model for salting, it provides a basic understanding of salting‐in/out during third phase formation and related phenomena.

Speciation Studies in Third Phase Formation: U(IV), Pu(IV), and Th(IV) Third Phases in TBP Systems

Abstract Conventionally, composition of the actinide‐tributylphosphate (TBP) solvate is assumed to be the same in the unpartitioned organic phase and the formed third phase. For example, if a 1:2 solvate has formed during extraction, the solvate is expected to be in the same state even during the third phase condition. However contemporary analysis, based on the spectroscopy, has indicated the presence of an extended solvate. In this study, a possible mechanism is described and validated using the published data on U(IV), Pu(IV), and Th(IV) third phase in TBP systems. The proposed mechanism can be readily extended to speciation in U(VI) third phase formation in TBP systems.

Development of a miniature Taylor-Couette extractor column for nuclear solvent extraction

Miniature annular centrifugal contactors are nearly perfect for shielded hot-cell applications during flowsheet evaluation but these contactors require complex maintenance of electrical drive-motors during radioactive experiments. To reduce the number of electrical drives in the shielded cell, an indigenous design of miniature Taylor Couette (TC) mixing based countercurrent differential extraction column has been developed. In this paper, results of mass transfer experiments for an indigenously developed TC column with 30% TBP/aqueous nitric acid solutions are reported. The developed device worked perfectly in counter–current differential mode and demonstrated equivalence to multiple-extraction stages while working with a single electrical drive. The developed TC unit demonstrated operation with a reduced efficiency without flooding even in absence of rotor rotation. This observation is a vital step towards designing of robust contactors, which do not flood during temporary power failure or failure of drive mechanism.

Application of COSMO-RS Method for the Prediction of Liquid-Liquid Equilibrium of Water/n-Dodecane/1-Butanol

The liquid-liquid equilibrium (LLE) for the system water-dodecane-butanol was estimated using the UNIQUAC model. In the UNIQUAC model interaction parameters were estimated from the vapor-liquid equilibrium (VLE) and LLE data of their constituent binary pairs. The water-dodecane-butanol LLE was experimentally measured at 298.15 K. Phase stability constraints were taken into account while calculating the binary interaction parameters from the mutual solubility data. The COSMO-RS method was used to estimate the activity coefficient in the miscible binary pair. The ternary LLE composition was predicted using the experimental VLE data as well as using the COSMO-RS calculated activity coefficient data along with the experimental mutual solubility data. In the latter case the root mean square deviation (RMSD) for the distribution of butanol between aqueous and organic phase is 0.24%. The corresponding UNIFAC model prediction is 7.63%.

Experimental Measurements of Drop Size Distributions in 30 mm Diameter Annular Centrifugal Contactor with 30% TBP-Nitric Acid Biphasic System

For design and development of liquid-liquid extraction systems, it is essential to have an accurate estimation of hydrodynamic and mass transfer characteristics of the employed contactor. In the present study, experimental evaluations consisted primarily of determining the maximum solution throughput that could be processed without cross-phase contamination at a given rotor speed, O/A flow ratio, and organic-aqueous solution pair in a 30 mm bowl diameter centrifugal contactor. In addition, analysis included experimental drop size determinations as well as holdup determination. The experimental drop size distributions are expected to be helpful for modeling work.

Dissolution of intact UO2 pellet in batch and rotary dissolver conditions

Comparative dissolution of intact un-irradiated UO2 pellet of PHWR fuel dimensions was performed in batch and dynamic rotary dissolver conditions in aqueous nitric acid solutions at elevated temperatures. The extent of dissolution was estimated by determining the uranium concentration of the resulting aqueous solution. It was observed that rate of dissolution was much faster in dynamic conditions as compared to static batch conditions.