Interstage flow

The simple box-type mixer—settler (113) has been used extensively in the UK for the separation and purification of uranium and plutonium (114). In this type of extractor, interstage flow is handled through a partitioned box constmction. Interstage pumping is not needed because the driving force is provided by the density difference between solutions in successive stages (see Plutoniumand plutonium compounds Uraniumand uranium compounds). 

Insofar as the consumption of chemicals is concerned, it is obvious that the total consumption of reflux-producing chemicals is proportional to the interstage flows, or width of the cascade, but independent of the number of stages in series, or length of the system. 

Figure 5 shows an ideally tapered enricher that has been replaced by three square cascade sections, a process called squaring-off the cascade (3—6). During the squaring-off process, two essential requirements must be kept in mind The interstage flow in all-square sections must always exceed the local value of E at all points in the cascade, and the squared-off cascade must contain a total number of stages which exceeds N. In order for the... 

See also Miyauchi et al. (loc. cit.), who express the axial mixing in terms of interstage flow. For the continuous phase with no dispersed-phase flow, see Bibaud and Treybal, and Cutoff (loc. cit.). 

For contactors in which discrete well-mixed compartments can be identified, for example sieve-plate columns, axial mixing effects are incorporated into the stagewise model by means of the backflow ratio a which is defined as the fraction of the net interstage flow of one phase which is considered to flow in the reverse direction. For a contactor in which there are many compartments, the axial dispersion coefficient and the backflow ratio, a, are interrelated as follows ... 

In the material balance equations given above, rj is the ratio of side-stream flow to interstage flow ... 

The matrices A and B are fixed by the interstage flow pattern and together with the feed matrix F are assumed to be given in the problem statement. 

Here L is the interstage flow (gram atoms N/min.) of exchangeable nitrogen in the liquid phase and L is the same quantity per cm. of column area. For a given transport and feed concentration, the area A required will be proportional to 1/L ( — 1) so that the size (volume) of the system required will be proportional to HETP/L (a — 1). These considerations show the importance of even small increases in (a — 1) or of a decrease in HETP. Similar considerations to be discussed later will show that the equilibrium time is also dependent on 1/ a — 1). ... 

The interstage flow could also be calculated from material balance considerations at the top end of the system. When no product is being withdrawn, the mg. atoms of 4-f and 2+ nitrogen leaving as waste must be equal to the total feed flow L/ in mg. atom N/minute as NO2 or N2O4. The mg. atoms N/minute of 2-f nitrogen as NO in the waste stream is then Lf(l — x ) which must be equal to the 4-f nitrogen that entered the refluxer and was reduced to NO thus. 

Values of a were interpolated from plots of available data and the interstage flows ... 

The relative values of S, HETP, and fc a for the experiments in Table V-B indicate that volumetric flows below 2 cc./min. or mole fractions of oxides of nitrogen in the solvent below 0.6 (concentration <— 9 moles/liter), result in poorer performance of this exchange column. Normally, according to Equation 26, an increase in interstage flow L results in an increase in HETP. However, for the two experiments with no solvent where this equation should apply, HETP decreased when the interstage flow increased from 18 to 35 mg. atoms N/min. The lower flow apparently does not wet the packing effectively as is also indicated by the low value for fc a. 

Interstage flow L, mg. atoms N/min. Interstage flow L mg. atoms 101 112 --80... 

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