Gaseous diffusion stage

Fig. 5. Schematic of a gaseous-diffusion stage showing a single converter in center.

For operational efficiency a number of gaseous diffusion stages are operated together in units referred to as cells and buildings. Cells and buildings. Cells and buildings can be removed from operation for routine maintenance and bypassed without disturbing the diffusion cascade. 

A Back-Pressure Efficiency Factor. Because a gaseous diffusion stage operates with a low-side pressure p which is not negligible with respect to there is also some tendency for the lighter component to effuse preferentiahy back through the barrier. To a first approximation the back-pressure efficiency factor is equal to (1 — r), where ris the pressure ratiopjpj. 

Sepa.ra.tive Capacity. An expression for the separative capacity of a single gaseous diffusion stage where the upflow rate is C mols per unit time, given ia equation 7, can be written as... 

A second type of apparatus based on the pressure diffusion effect is the separation nozzle. Pressure gradients in a curved expanding jet produce an isotopic separation similar to that in a centrifuge. The separation effect obtained with a single jet is relatively small, and separation nozzle stages, similar to gaseous diffusion stages, must be used in a cascade to realize most of the desired separations. 

Because this value is so close to unity, to obtain a useful degree of separation the process must be repeated many times in a countercurrent cascade of gaseous diffusion stages, such as was shown in Fig. 12.2. 

Partial descriptions of the type of equipment used in the gaseous diffusion plants of the U.S. DOE are given in references [Ul] and [U2]. Figure 14.1 is a schematic plan view of three gaseous diffusion stages. The separating unit on each stage, called a converter, contains... 

Figure 14.1 Arrangement of gaseous diffusion stages. Courtesy of U.S. Energy Research and Development Administration.)...

Figure 14.7 Cross-flow gaseous diffusion stage. Stage separation factor a = > (l — )/ ( — y) stage efficiency E = (y — x)l(y — Xo) Np, Ni, N,M = molar flow rates Xp, Xf, x,y,v= mole fraction light component.

Stage design variables. The principal independent variables involved in designing a gaseous diffusion stage to serve in an ideal cascade are as follows ... 

Stage separation factor. For a cross-flow gaseous diffusion stage with a cut 6 of the stage separation factor a is given by... 

The appropriate criterion to optimize the design of a gaseous diffusion stage is that the... 

Table 14.9 Design conditions and characteristics of gaseous diffusion stage designed for minimum unit cost of separative work ...

The corresponding expression for a cross-flow gaseous diffusion stage, from Eqs. (14.92) and (14.93), is... 

Because the separation obtainable in a mass diffusion stage is even smaller than in a gaseous diffusion stage, a practical degree of separation requires either a multistage cascade, such as the 48-stage cascade used by Hertz [H3] to separate neon isotopes, or a mass diffusion column. 

This is to be compared with 0.168 kW/(kg SWU/year) for the rate of loss of availability associated with pressure drop through the diffusion barrier of the optimized gaseous diffusion stage of Table 14.9. 

In qualitative terms, how would the optimum design of the gaseous diffusion stage of Sec. 

How does the separative capacity of a cross-flow gaseous diffusion stage vary with the cut At what cut is the separative capacity highest What is the ratio of separative capacity at a cut of I to the maximum separative capacity ... 

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