Computer Analysis on Steady State Separation Characteristics of Hydrogen Isotope Separation System by Cryogenic Distillation

A computer code is developed for effective and exact simulation of cryogenic distillation columns. The simulation model incorporates the differences of latent heat of vaporization among the six molecular species, pressure drop, decay heat of tritium, heat transfer through the column wall, Raoult's law deviation of the solutions of hydrogen isotopes, multiple feeds and multiple sidestreams. The calculational procedure developed by Tomich for general equilibrium stage processes is further modified to match this complicated simulation model.

A computer simulation study is performed for the typical cryogenic distillation column cascade and separation characteristics of each column are clarified. The effects of pressure drop are found not significant. If the tritium concentration in the column is considerably high, the lower performance caused by decay heat effects cannot be neglected any longer. Increase of the reflux ratio or refrigeration of the stripping section, is required for improvement.

The present study provides a great help to development of exact simulation models and simulation procedures of cryogenic distillation columns.     

Numerical Analysis on Heavy Water Separation Characteristics for Pair of Bithermal Trickle-Bed Type H2/H2O-Exchange Columns

In order to study how the operating pressure and temperature have influence on the heavy water separation characteristics for a pair of bithermal trickle-bed type H₂/H₂O-exchange columns packed with the hydrophobic Pt catalyst, the authors utilized the analytical expressions derived from the differential equations expressing the material balance of the three components, hydrogen, water vapor and water in the exchange column.

The cases where the operating temperature in a hot column is 200°C, the deuterium enrichment factor is 3, the hydrogen superficial velocity is (0.2)×(Operating pressure (atm)) (Nm/s) and the column efficiency is high and low were studied. As the results, the optimum operating pressure is about 50 atm in both cold and hot column, the optimum operating temperature in a cold column is 100°C and other important parameters such as the optimum length of a cold and a hot column, the optimum hydrogen to feed water molar flow ratio and the optimum degradation ratio were also obtained.     

Solvent Extraction Calculation Model for Purex Process in Pulsed Sieve Plate Column

A new calculation code PULCO has been developed to simulate the Purex extraction process in a pulsed column. The model for the PULCO code is based on the mass transfer taking place at the interface between dispersed drops and continuous phase fluid. The following physical quantities of the moving drop are included in the model for each period of drop motion in a pulsed column:

1. Mass transfer coefficient from liquid drop to continuous phase

2. Diameter and velocity of moving drop in the pulsed column

3. Holdup of dispersed and continuous phases in the pulsed column

4. Longitudinal dispersion coefficient in the pulsed column.

To establish the validity of the PULCO code, experiments were performed using a 50 mm diameter pulsed column which had a sieve plate section height of 2 m and plates spaced 50 mm apart. The experiments were performed by CY tests with use of unirradiated U and Pu. The experimental concentration profiles fitted well with the calculated concentration profiles.     

Effect of Changes in Feed Rate and Cut on Circulating Flow within Thermal Diffusion Column for Isotope Separation

A two-dimensional axisymmetric convectional flow field of an isotope separating thermal diffusion column with continuous feed and draw-offs is analyzed through the Newton iterative numerical solution of the equations of change without the Boussinesq approximation. Computations are performed for Ar gas within an inner hot radius of 0.2 mm and an outer cold radius of 5 mm, between which the temperature differs by 300 K. The rate of feed F, supplied into the middle point of the column, is varied from 1 to 5 cm³/s, while the cut θ, or the ratio of the upper drawing-off to feeding rates, from 0.1 to 0.9. Comparison of flow vectors, temperature and density profiles among various sets of (F, θ) makes it clear that the flow fields agree with one another for the same value of the total transport; that is, θF_pc (pc: feed gas density) in the upper section and -(1-θ)pc in the lower section of the column. Observations on the mass flow vectors near the feed slit lead to the finding that the multiplicative effect of separation cannot be expected in the column with the inner radius r_c of 5 mm at the feed rate F exceeding 2 cm³/s(i.e F/ΔTr_c ⁴ )>0.1 [1/(s·K·cm)]), because the feed flow interrupts the whole-length circulation of the natural convection.     

Reference Mean Temperature for Evaluation of Performance of Thermal Diffusion Column for Isotope Separation

In order to evaluate separative performance of a thermal diffusion column, a simplification is usually made in which the temperature dependence of the relevant properties such as thermal diffusion constant is ignored and some proper mean values evaluated at a specific “mean” temperature are used. Adoption of weighted average of temperature distribution is common for the “mean” temperature, but there exists no definite way of determining mean temperature. The present paper proposes a new reference mean temperature determined by the equation governing the free convection. It is based on the fact that the multiplication effect of free convection is essential to separation by thermal diffusion column. The reference mean temperature is related to pressure difference between top and bottom of column and is higher than a mass-averaged temperature (due to gravitational force) by a contribution of viscous force. The reference mean temperature was calculated, as a reference, for an Ar isotope separating column with an inner hot radius of 0.2 mm and an outer cold radius of 5 mm. The results confirmed the validity of an approximate formula expressing effects of temperature difference and ratio of inner and outer radii of column explicitly for the temperature. The reference mean temperature calculated from pressure difference given by axisymmetric solution of equations of change was in good agreement with the analytical solution.     

Equilibrium and kinetic studies of selective adsorption and separation for strontium using DtBuCH18C6 loaded resin

A crown ether loaded resin was prepared by successive impregnation and fixing the 4′,4′(5″)-di(tert-butylcyclohexano)-18-crown-6 (DtBuCH18C6) and its molecule modifier, 1-dodecanol, onto the porous silica/polymer composite support (SiO₂-P particles). The characterization of DtBuCH18C6 loaded resin was examined by thermal gravimetry and differential thermal analysis and electron probe microanalysis. The adsorption behavior of Sr(II), Cs(I), Ru(III), Pd(II), La(III), Nd(III), Sm(III), Gd(III), Zr(IV), and Mo(VI) was investigated by the batch method. Furthermore, the column test for Sr (II) was performed. The batch experiments were carried out by varying the shaking times, HNO₃ concentration, and initial concentration of metal ions. A relatively large K _d value above 182 cm³/g for Sr(II) was obtained in the presence of 3 M HNO₃. In contrast, the K _d values of Cs(I), Ru(III), Pd(II), La(III), Nd(III), Sm(III), Gd(III), Zr(IV), and Mo(VI) were considerably lower than 10 cm³/g. The adsorption of Sr(II) was found to be controlled by chemisorption mechanism, and followed a Langmuir-type adsorption equation. The breakthrough curve of Sr(II) had S-shaped profile, and the elution percentage was estimated to be 99.9% by using the eluent of H₂O.     

Optimum Cut of Separating Element as a Function of Total Separation Factor

Calculation of one of the SIMBATH experiments was performed using the SIMMER-II code. The experiments were intended to simulate the fuel pin disintegration, the molten materials relocation and following materials redistribution that could occur during core disruptive accidents assumed in fast breeder reactors. The calculation by SIMMER-II showed that the incorporated step-wise fuel pin disintegration model and the modified particle jamming model were capable of reproducing the course of materials relocation within the identified ranges of the parameters which governed the blockages formation, i.e. the characteristic radius of solid particles jamming and/or sieving out in the flow and the effective particle viscosity. In particular the final materials redistribution calculated by SIMMER-II very well reproduced the experiment. This fact made it possible to interpret theoretically the mechanisms of flow blockages formation and related materials redistribution.     

Separation of Heat-Generating Nuclides from High-Level Liquid Wastes through Zeolite Columns

Heat-generating nuclides Cs and Sr were separated from simulated high-level liquid wastes (HLLW) by successive adsorption on columns of ferrierites (F) and zeolite A. Adsorbed Cs and Sr were efficiently eluted with NH₄NO₃ and EDTA solutions, respectively, yielding the recovery over 96%. A simulated waste solution containing 29 components was denitrated with a formic acid up to pH 7.92. The amounts of nuclides adsorbed from this denitrated solution were experimentally estimated to be 0.33 mmol Cs/g·F and 0.19 mmol Sr/g·A, respectively.     

Elution Behavior of Cesium from Ferrierite Column by Using Inorganic Salt and Acid Solutions

The distribution coefficient (K_d) of Cs on ferrierite decreased with increasing concentration of the coexisting inorganic cations in the order of Na⁺>H⁺>K⁺>NH₄ ⁺. A linear relationship with a slope of about —1 was obtained between log K _d and log[M⁺] above 0.1M (=mol/dm³). The retention volume (V _R ) of Cs in chromatography also decreased in a similar manner to K _d , depending on the kind of inorganic cations. The V _R value can be predicted from the K _d value based on the linear relation. The column efficiency was improved with fine particles of ferrierite, yielding the elution percentage above 95%.