Tube bundle conversion

The composition of the gas mixture, which is introduced into the tube bundle reactor (tubes of 6-12 m length and 20-50 mm diameter, filled with the Ag catalyst) consists of 15-50 vol % ethylene, 5-9% oxygen, as much as 60% methane as dilution gas, and 10-15% carbon dioxide. The reaction therefore proceeds above the upper explosion limit. The ethylene conversion runs up to 10% per cycle through the reactor. The ethylene oxide selectivity amounts to 75-83 % maximum. The formed ethylene oxide is recovered by scrubbing with water and the newly formed carbon dioxide is separated from the cycle gas, e.g., by hot potash washing process. 

It is possible to convert the two-pass tube bundle shown in Fig. 19,1 to the four-pass tube bundle shown in Fig. 19.6. This conversion is effected as follows ... 

To accomplish the catalytic conversion of SO2 to SO3 (using platinized asbestos), the critical step in production of concentrated H2SO4, Knietsch invented a tube-bundle reactor in which the feed for the fixed bed inside the tubes was preheated by the countercurrent flow of hot reactants outside the tubes. Haber must have had this model in mind when he designed his preheating arrangement. 

Uranium oxide [1344-57-6] from mills is converted into uranium hexafluoride [7783-81-5] FJF, for use in gaseous diffusion isotope separation plants (see Diffusion separation methods). The wastes from these operations are only slightly radioactive. Both uranium-235 and uranium-238 have long half-Hves, 7.08 x 10 and 4.46 x 10 yr, respectively. Uranium enriched to around 3 wt % is shipped to a reactor fuel fabrication plant (see Nuclear REACTORS, NUCLEAR FUEL reserves). There conversion to uranium dioxide is foUowed by peUet formation, sintering, and placement in tubes to form fuel rods. The rods are put in bundles to form fuel assembHes. Despite active recycling (qv), some low activity wastes are produced. 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

The reaction is exothermic and so to avoid serious temperature excursions the reactor consists of a bundle of narrow tubes, each a few centimeters in diameter, surrounded by a heat transfer medium. The catalyst consists of relatively large silver particles on an inert a-Al203 support. The surface area is below 1 m g". Promoters such as potassium and chlorine help to boost the selectivity from typically 60% for the unpromoted catalysts to around 90%, at ethylene conversion levels of the order of50%. 

Hollow fiber modules, or permeators, are precisely machined units containing bundles of hollow fine fibers positioned parallel to and around a perforated center feed-water tube, with only one or two bundles per pressure vessel. They are widely used for brackish and seawater applications. Hollow fiber modules exhibit a low flux rate (permeate flow per unit membrane per unit time) and can foul easily but tend to have high conversion factors (the percentage of feed flow converted to permeate), typically 50 to 55%. This makes them suitable for both small and large RO systems. They are easy to troubleshoot, and bundles are easy to change in the field. 

We may now recall the fundamental equations for calculating chemical conversion in the limiting states of Min Mix and Max Mix. The BPT model provides a convenient picture of the two situations (figure 3). If the bundle is arranged in such a way that the particles of same residual lifetime are situated on the same vertical line, then minimal mixedness corresponds to a perfect insulation between tubes and the conversion for a single reaction is written... 

Whereas the laboratory fluidized bed is generally operated with no internals, plant equipment often must contain bundles of heat-exchanger tubes. Screens, baffles, or similar internals are frequently used to redisperse the bubble gas in industrial reactors. The mass-transfer area is thus increased relative to the fluidized bed without internals the extra area can be utilized to partially offset the conversion-reducing effects of bed diameter and gas distributor [122]. 

Enriched UF is shipped to the plant for fabricating reactor fuel elements in monel cylinders whose size is determined from the content, so as to prevent accumulation of a critical mass. At the fuel fabrication plant UF is converted to UO or other chemical form used in reactor fuel. For light-water reactors the UOj is pressed into pellets, which are sintered, ground to size, and loaded into zircaloy tubing, which is filled with helium and closed with welded zircaloy end plugs. These individual fuel rods are assembled into bundles, constituting the fuel elements shipped to the reactor. Conversion of UFj to UO2 is described in Chap. 5. Extraction of zirconium from its ores and separation of zirconium from its companion element hafnium is described in Chap. 7. 

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