Zone-heating technique

Synthesis of Zeolite X Using a Zone-Heating Technique.141... 

Fig. 3.2. Needle-eye technique for large FZ crystals on bottom the growing crystal, on top the melting feed rod, in between the molten zone heated by the inductor schematic cross section (a) and photograph (b) of an FZ process (100 mm crystal diameter)...

Zone melting techniques are slow, very slow. Rates of zone movement vary from 1 to 20 cm/h. Figure 2-8 shows a more efficient arrangement. Once the heated zone has refrozen there is no reason why it can t be remelted. In this case, three heaters are used, and the charge is moved. Figure 2-9 shows a helical type where you can get many molten zones. This is more appropriate for organic compounds, which are lower melting than metals. 

Figure 20.8 Zone-refining technique for purifying metals. Top to bottom An impure metal rod is moved slowly through a heating coil. 4s the metal rod moves forward, the impurities dissolve in the molten portion of the metal while pure metal crystallizes out in front of the molten zone.

Steam can also be injected into one or more weUs, with production coming from other weUs (steam drive). This technique is effective in heavy oil formations but has found Httle success during appHcation to tar sand deposits because of the difficulty in connecting injection and production weUs. However, once the flow path has been heated, the steam pressure is cycled, alternately moving steam up into the oil zone, then allowing oil to drain down into the heated flow channel to be swept to the production weUs. 

As in the case of many metal—ahoy systems, weld ductihty is not as good as that of the base metal. Satisfactory welds can be made in vanadium ahoys provided the fusion zone and the heat-affected zone (HAZ) are protected from contamination during welding. Satisfactory welds can be made by a variety of weld methods, including electron-beam and tungsten-inert-gas (TIG) methods. It is also likely that satisfactory welds can be made by advanced methods, eg, laser and plasma techniques (see Lasers Plasma technology). 

Approximately 1 kg of biphenyl per 100 kg of benzene is produced (6). Because of the large scale, HD A operations provide an ample source of cmde biphenyl from which a technical grade of 93—97% purity can be obtained by distillation (35). Zone refining or other crystallization techniques are requited to further refine this by-product biphenyl to the >99.9% purity requited for heat-transfer appHcations. 

As shown in Fig. 13-92, methods of providing column reflux include (a) conventional top-tray reflux, (b) pump-back reflux from side-cut strippers, and (c) pump-around reflux. The latter two methods essentially function as intercondenser schemes that reduce the top-tray-refliix requirement. As shown in Fig. 13-93 for the example being considered, the internal-reflux flow rate decreases rapidly from the top tray to the feed-flash zone for case a. The other two cases, particularly case c, result in better balancing of the column-refliix traffic. Because of this and the opportunity provided to recover energy at a moderate- to high-temperature level, pump-around reflirx is the most commonly used technique. However, not indicated in Fig. 13-93 is the fact that in cases h and c the smaller quantity of reflux present in the upper portion of the column increases the tray requirements. Furthermore, the pump-around circuits, which extend over three trays each, are believed to be equivalent for mass-transfer purposes to only one tray each. Bepresentative tray requirements for the three cases are included in Fig. 13-92. In case c heat-transfer rates associated with the two pump-around circuits account for approximately 40 percent of the total heat removed in the overhead condenser and from the two pump-around circuits combined. 

Zone electrophoresis is defined as the differential migration of a molecule having a net charge through a medium under the influence of an electric field (1). This technique was first used in the 1930s, when it was discovered that moving boundary electrophoresis yielded incomplete separations of analytes (2). The separations were incomplete due to Joule heating within the system, which caused convection which was detrimental to the separation. 

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