Fluidized bed conversion

The most significant development has been the introduction of fluidized bed conversion. One fluid bed replaces the three beds before intermediate absorption in the double absorption set-up. In a fluidized bed converter the gas flow is reversed and the catalyst becomes fluid or movable. The heat transfer properties are then much greater than for fixed beds and internal boiler tubes can control the temperature to within a degree in the 460-500 °C range. This technique has not been used except as a prototype because of the parlous state of the industry following recession. 

The dense fluidized bed conversion conditions include temperature in the range 540 "C to 650 C and pressure from 0.10 to 0.45 MPa, catalyst/oil weight ratio of 0.1 to 10, and a weight hourly space velocity (WHSV) of 1 to 10 h. Because the catalyst used in the process has lower cracking activity relative to conventional FCC catalysts, a higher temperature compared with that for conventional FCC may be used in order to achieve a higher conversion to the desired light olefins. 

Whereas Geldart s classification relates fluidized-bed behavior to the average particle size in a bed, particle feed sizes maybe quite different. For example, in fluidized-bed coal (qv) combustion, large coal particles are fed to a bed made up mostly of smaller limestone particles (see Coal conversion processes). 

Another hydrogenation process utilizes internally generated hydrogen for hydroconversion in a single-stage, noncatalytic, fluidized-bed reactor (41). Biomass is converted in the reactor, which is operated at about 2.1 kPa, 800°C, and residence times of a few minutes with steam-oxygen injection. About 95% carbon conversion is anticipated to produce a medium heat value (MHV) gas which is subjected to the shift reaction, scmbbing, and methanation to form SNG. The cold gas thermal efficiencies are estimated to be about 60%. 

In TBP extraction, the yeUowcake is dissolved ia nitric acid and extracted with tributyl phosphate ia a kerosene or hexane diluent. The uranyl ion forms the mixed complex U02(N02)2(TBP)2 which is extracted iato the diluent. The purified uranium is then back-extracted iato nitric acid or water, and concentrated. The uranyl nitrate solution is evaporated to uranyl nitrate hexahydrate [13520-83-7], U02(N02)2 6H20. The uranyl nitrate hexahydrate is dehydrated and denitrated duting a pyrolysis step to form uranium trioxide [1344-58-7], UO, as shown ia equation 10. The pyrolysis is most often carried out ia either a batch reactor (Fig. 2) or a fluidized-bed denitrator (Fig. 3). The UO is reduced with hydrogen to uranium dioxide [1344-57-6], UO2 (eq. 11), and converted to uranium tetrafluoride [10049-14-6], UF, with HF at elevated temperatures (eq. 12). The UF can be either reduced to uranium metal or fluotinated to uranium hexafluoride [7783-81-5], UF, for isotope enrichment. The chemistry and operating conditions of the TBP refining process, and conversion to UO, UO2, and ultimately UF have been discussed ia detail (40). 

Oxychlorination of Ethylene or Dichloroethane. Ethylene or dichloroethane can be chlorinated to a mixture of tetrachoroethylene and trichloroethylene in the presence of oxygen and catalysts. The reaction is carried out in a fluidized-bed reactor at 425°C and 138—207 kPa (20—30 psi). The most common catalysts ate mixtures of potassium and cupric chlorides. Conversion to chlotocatbons ranges from 85—90%, with 10—15% lost as carbon monoxide and carbon dioxide (24). Temperature control is critical. Below 425°C, tetrachloroethane becomes the dominant product, 57.3 wt % of cmde product at 330°C (30). Above 480°C, excessive burning and decomposition reactions occur. Product ratios can be controlled but less readily than in the chlorination process. Reaction vessels must be constmcted of corrosion-resistant alloys. 

Exxon was the first to investigate the suitabiUty of a wide range of different U.S. coals for conversion. Operation of the EDS process was demonstrated in a 230 t/d unit in Baytown, Texas that had a start-up in May of 1980. Data on the response of a variety of coals to once-through and bottoms recycle operations are shown in Eigure 4. Eigure 5 presents typical Hquefaction product distributions for the system operated both with and without the Elexicoking (fluidized-bed coking) option. 

At least two catalytic processes have been used to purify halogenated streams. Both utilize fluidized beds of probably noimoble metal catalyst particles. One has been estimated to oxidize >9000 t/yr of chlorinated wastes from a vinyl chloride monomer plant (45). Several companies have commercialized catalysts which are reported to resist deactivation from a wider range of halogens. These newer catalysts may allow the required operating temperatures to be reduced, and stiU convert over 95% of the halocarbon, such as trichlorethylene, from an exhaust stream. Conversions of C-1 chlorocarbons utilizing an Englehardt HDC catalyst are shown in Figure 8. For this system, as the number of chlorine atoms increases, the temperatures required for destmction decreases. 

The Fischer-Tropsch reaction is highly exothermic. Therefore, adequate heat removal is critical. High temperatures residt in high yields of methane, as well as coking and sintering of the catalyst. Three types of reac tors (tubular fixed bed, fluidized bed, and slurry) provide good temperature control, and all three types are being used for synthesis gas conversion. The first plants used tubular or plate-type fixed-bed reactors. Later, SASOL, in South Africa, used fluidized-bed reactors, and most recently, slurry reactors have come into use. 

This reaction is carried out in tall fluidized beds of high L/dt ratio. Pressures up to 200 kPa are used at temperatures around 300°C. The copper catalyst is deposited onto the surface of the silicon metal particles. The product is a vapor-phase material and the particulate silicon is gradually consumed. As the particle diameter decreases the minimum fluidization velocity decreases also. While the linear velocity decreases, the mass velocity of the fluid increases with conversion. Therefore, the leftover small particles with the copper catalyst and some debris leave the reactor at the top exit. 

Since the catalyst is so important to the cracking operation, its activity, selectivity, and other important properties should be measured. A variety of fixed or fluidized bed tests have been used, in which standard feedstocks are cracked over plant catalysts and the results compared with those for standard samples. Activity is expressed as conversion, yield of gasoline, or as relative activity. Selectivity is expressed in terms of carbon producing factor (CPF) and gas producing factor (GPF). These may be related to catalyst addition rates, surface area, and metals contamination from feedstocks. 

In the fluid coking process, part of the coke produced is used to provide the process heat. Cracking reactions occur inside the heater and the fluidized-bed reactor. The fluid coke is partially formed in the heater. Hot coke slurry from the heater is recycled to the fluid reactor to provide the heat required for the cracking reactions. Fluid coke is formed by spraying the hot feed on the already-formed coke particles. Reactor temperature is about 520°C, and the conversion into coke is immediate, with... 

In the moving bed processes, the preheated feed meets the hot catalyst, which is in the form of beads that descend by gravity to the regeneration zone. As in fluidized bed cracking, conversion of aromatics is low, and a mixture of saturated and unsaturated light hydrocarbon gases is produced. The gasoline product is also rich in aromatics and branched paraffins. 

Preparation. Its prepn was first carried out by the action of metals as F acceptors on N trifluoride in a flow reactor at 375°, conversion 42—62%, yield 62—71% (Ref 1). It has also been prepd by the passage of N trifluoride thru a fluidized bed of powd C at 400—500°, conversion 38%, yield 77% (Refs 3 3a) or by the oxidn of difluor-amine (Ref 6)... 

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