Cascade operations

Successful operation of the gaseous diffusion process requires a special, fine-pored diffusion barrier, mechanically rehable and chemically resistant to corrosive attack by the process gas. For an effective separating barrier, the diameter of the pores must approach the range of the mean free path of the gas molecules, and in order to keep the total barrier area required as small as possible, the number of pores per unit area must be large. Seals are needed on the compressors to prevent both the escape of process gas and the inflow of harm fill impurities. Some of the problems of cascade operation are discussed in Reference 16. 

The relations indicated by these two equations are shown graphically in Figure 8.12 for two alternative configurations. In both cases the cascade operates between the same initial and... 

Fig. 8.9 (a) A four stage thermal diffusion cascade for argon isotope separation (Modified from Spindel, W. ACS Symp. Ser. 11, 82 (1975)). (b) The thermal diffusion cascade operated by K. Clusius and collaborators at the University of Zurich during the 1950s (Photo credit Archives of the Institute of Physical Chemistry, University of Zurich)... 

A comparison for the different cases of the production costs and dependence on the annual capacity is given in Fig. 8.1-4. The results are based on the cascade-operation mode, with three extractors, extraction at 280 bar and 65°C, cycle-times of 7.5 hours, and a separation pressure of 60 bar for the non-isobaric process. 

Comparison of modified cascade operation with Stratco performance... 

When the separation potential satisfies (12.172), the separative capacity of a single stage in a close-separation cascade operated at a cut of (M = 1V) from Eq. (12.170) is... 

Squared-off cascade. The preceding treatment of a water distillation plant as an ideal cascade operated at uniform vapor velocity has required that the steam flow rate be varied continuoutiy as its deuterium content changes and that the number of towers in parallel, or the tower area. 

In Fig. 13.15, the first m—2 stages constitute a simple cascade, operated without recycle, with constant heads separation factor 0. The recovery of deuterium from a simple cascade of m—2 stages operated at constant 0 is... 

After the cascade improvement and cascade operating programs planned 6y U.S. DOE have been completed, their power consumption will be increased to 7,380,000 kW and their separative capacity to 27,700,000 kg SWU/year, equivalent to a specific power consumption of 0.266 kW/(kg SWU/year). 

Because of its importance, only the continuous countercurrent method is discussed here. Also, since the stage method is normally used, the differential-contact method is not considered. In common with other stage cascade operations, leaching may be considered, first, from the standpoint of ideal stages and, second, from that of stage efficiencies. 

For continuous cascade operation, the friction plate is lowered in the bowl so that a volume of material always remains inside while excess overflows. The residual volume can be either measured experimentally or calculated, assuming that the cross-section of the rope may be approximated by a fourth of a circle (quarter torus). To obtain a particular spheronization effect, an overall residence time must be maintained. The processing time in each machine can be calculated as the ratio of residual volume divided by the volumetric feed rate (= volumetric throughput). Since bowl diameters are predetermined and fixed by the design, the position of the friction plate in the bowl is the only variable which can be modified to match a certain feed rate or system capacity to the desired or necessary residence time. 

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