239
W
Table 23.2. Input Parameters and Results of Calculations with Dispersion Model
Protactinium columns Top section
33.4 16.7 37.3 0.404
Bottom section
0.01600 0.03199 0.01432 1.325
17.6 17.6 17.6 17.6
2-5 2-5 2-5 2-5
0.94 0.94 0.94 0.94
7-17 7-17 7-17 7-17
Rareearth columns
Top section
Bottom section
1.2 1.5 2.0 1.2 1.5 2.0
1.988 1.589 1.182 0.8297 0.6718 0.5038
1.5 1.5 1.5 0.61 0.61 0.61
3-5 3-5 3-5 25 25 25
<O.lO 0.10-0.20 0.27 <0.20 <0.20 "0.15
-
>95
48-160 35-60 350 >350 "500
>
Rareearth-thorium separation factor.
I
.
-
t
Jtcsd
that the length of the column be increased by less than 10% over that of a column in which no backmixing occurs. The values of F were computed from flowsheet calculations of McNeese? The estimate of HTU (3.2 ft) used in calculating NPe and L* was taken from the experimental results of Johnson et Q L A ~ value of 3.5 cm2/sec was used for E in calculating N P e .This number is based on measurements made with columns packed with '&-in. Raschig rings (described in Sect. 23.3). The column height is acceptably low and is dependent only upon the number of transfer units desired. For the rareearth columns, extraction factors corresponding to rareearth-thorium separation factors, a, of 1.2, 1.5, and 2.0 were calculated from the flowsheet calculations of McNeese.' The same values for HTU (3.2 ft) and axial diffusivity (3.5 cm2/sec) were assumed as for the protactinium columns (see above). In the upper column, efficiencies were quite low and were strongly dependent on NPe and NTU. It appears likely that column heights will be excessive in this section. In the lower part of the rareearth system, low efficiencies and very long columns are a certainty since the Peclet number is low and the number of transfer units required is high. Because of the detrimental effect of backmixing on column performance, the effect of segmenting the rareearth column at intervals with backflow preventers was investigated. The model used in this study was a staged column featuring salt-phase backflow between adjacent stages. As in the previous calculations the rareearth extraction system was simplified in order to
permit consideration of only one component transferring between immiscible phases with constant molar flow rates. The equilibrium distribution coefficient of this component between the two phases was assumed to remain constant through the column. Multicomponent masslbalance equations with variable distribution coefficients have been solved by McNeese for staged columns with no backflow (see Sect. 22.2). These results constitute the basis of the reference flowsheet for MSBR fuel processing and indicate that 5 and 19 theoretical stages will be required in the upper and lower columns, respectively, of the rareearth extraction system. Metal and salt flow rates and rareearth concentrations in the feed and product salt and metal were taken from McNeese's calculations. It was also necessary to select the constant distribution coefficient, 0,' which, with no salt backflow, would produce the same degree of extraction as that obtained with variable distribution coefficients. Once the proper value of D had been obtained, solution of the problem with various degrees of salt backflow was straightforward. The column efficiency (i.e., the ratio of the number of theoretical stages to the number of mechanical stages) was found to decrease approximately linearly with the percentage of salt throughput that returns through the backflow preventers: To achieve stage efficiencies of 75% or greater, the return flow through the preventer must be less than 17.5% of the salt flow in the upper column and less than 25% in the lower column. As noted earlier, the need for backflow preventers is most critical in the lower portion of the