Cascade-type plates

Because acid cbncentratims are favored by low-temperature absorption, several different cooling methods have also been developed, e.g., by external units of the plate, drum, or cascade type by water curtains outside the tower and also by cooling coils strategically located inside the absorption column. In some plants using low-pressure ammonia oxidation, vaporization of the ammonia is used to prechiQ the absorber feedwater and c.Qoling wEiter. ... 

FIGURE 9.2 Nozzles for sprays and units for aeration or stripping (a-d) nozzle types (e) inclined apron that may be studded with riffle plates (f) perforated plates (g) spray tower and (h) cascade. 

Many different bubble column designs have been used over the years, all adapted to particular practical needs [133]. Figure 8.6 depicts a few of the modifications frequently used. A special case is the down-flow bubble column. In the column part of this reactor flow of the two phases is co-current from top to bottom as shown in Fig 8.6. The advantages of this type of reactor is that the relatively short gas residence times obtained in up-flow reactors, can be increased since the residence time is determined by the bubble rise velocity [28]. Furthermore, incorporation of additional perforated plates transforms the simple column, sketched in Fig 8.1, into a multistage cascade version, Fig 8.6. This redistribution of gas over the perforated plates intensifies mass transfer,... 

BAFFLE TOWERS. These extraction towers contain sets of horizontal baffle plates. Heavy liquid flows over the top of each baffle and cascades to the one beneath light liquid flows under each baffle and sprays upward from the edge through the heavy phase. The most common arrangements are disk-and-doughnut baffles and segmental, or side-to-side, baffles. In both types the spacing between baffles is 1(K) to 150 mm (4 to 6 in.). 

A multichannel plate (MCP) is a type of CDEM in which a series of microchannels on a disk-shaped device are coated with an electron-emissive material to generate 10 to 10" amplification as the electrons cascade through the microchannels. MCPs can be stacked to increase amplification or focused onto a fluorescent surface for ion-beam imaging. Because of the short electron pulse widths ( 1 ns) obtained with MCPs, they are the ion detector of choice for time-of-flight mass spectrometers. 

The Daly detector uses a photomultiplier rather than an electron multiplier. Ions leaving the analyzer are directed onto a conversion dynode, and the ejected electrons are accelerated onto a plate coated with a fast-acting scintillant. Each electron releases a photon from the scintillant. The photons then enter a photomultiplier tube and impact on a photocathode, producing electrons (photoelectric effect) and initiating an electron cascade (Pigure 2.40). The output from the photomultiplier is further amplified electronically, similarly to the output of dynode type electron multipliers. The level of amplification is similar to that of electron multipliers. Photomultiplier tubes last longer than electron multipliers, but the scintillant-coated plates require replacement every few years. 

At near atmospheric pressures, oxidation and absorption rates are slow, and earlier atmospheric and low-pressure plants used between five and ten very large towers. Maximum strengths attainable were in the range of 42% to 52% HNO3. To reduce capital investment and to obtain increased operating efficiencies, absorption under pressure was developed, using various types of equipment, e.g., cascade coolers, packed columns, spray towers, and columns incorporatir bubble plates, sieves, and such special devices as the Kuhlman tray. 

In contrast to the two previous experiments, calcite-type polarizers were used with transmission efficiencies cmi = 0.910 0.001, m2 = 0.880 0.001, Emi, e 2< lO . This type of polarizer has a much better extinction ratio than pile-of-plates polarizers, but the values of Cmi and m2 are not particularly high. What is more, since a 1-1-0 cascade is being used, the factor F(< ) takes on the relatively low value 0.951 even with an acceptance half-angle of only 13°. 

The polarizers used in this experiment were of some interest, being of the pile-of-plates variety, with each polarizer consisting of two sets of seven plates symmetrically arranged so as to cancel out transverse ray displacements. A typical time correlation spectrum for this type of experiment using an atomic cascade is shown in Figure 10. 

After collection and collimation by a lens system which subtended a half-angle of about 32° at the source, the photons from the cascade were analyzed by polarizers and filters in much the same way as in previous experiments. The polarizers were of the pile-of-plates type, each consisting of 10 optically flat plates set nearly at Brewster s angle, with efficiencies measured to be 6 1 = 0.971 0.005, e , = 0.029 0.005, Sm2 = 0.968 0.005, and e 2 = 0.028 0.005. 

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