Reactor entrained bed

SASOL has pursued the development of alternative reactors to overcome specific operational difficulties encountered with the fixed-bed and entrained-bed reactors. After several years of attempts to overcome the high catalyst circulation rates and consequent abrasion in the Synthol reactors, a bubbling fluidized-bed reactor 1 m (3.3 ft) in diameter was constructed in 1983. Following successflil testing, SASOL designed and construc ted a full-scale commercial reac tor 5 m (16.4 ft) in diameter. The reactor was successfully commissioned in 1989 and remains in operation. 

Pressure drop Fixed-, moving-bed, and BFB G/S reactors Liquid-phase reactors Generally all deep beds Entrained-bed reactors... 

For reactor design calculations it is necessary to know the total devolatilization rate as well as the species production rates. Therefore, one needs to include in the reactor model all the reaction rates that are available for the devolatilization of the particular coal. Kayihan and Reklaitis (8) show that the kinetic data provided by Howard, et al. (5,6) can be easily incorporated in the design calculations for fluidized beds where the coal residence times are long. However, if the residence time of pulverized coal in the reactor is short as it is in entrained bed reactors, then the handling of ordinary differential equations arising from the reaction kinetics require excessive machine computation time. This is due to the stiffness of the differential equations. It is found that the model equations cannot be solved... 

The Model. The physical system considered is an entrained bed reactor. Pulverized coal, carried by a gas stream, mixes with hot gas at the reactor entrance. As coal particles are carried their temperatures increase and devolatilization takes place. For practical purposes the particle size distribution was approximated by 10 discrete cuts. Since the devolatilization kinetics are highly temperature dependent and the temperature transient of a particle is affected by its size and mass separate account must be taken of each of the 15 reactions in each of the 10 different size particles. Without any detail on the derivation of the model, the equations can be summarized as ... 

This is a well-established technology, and has been replaced today by the fluidization of fine parddes (grains of sand, carbonization products, ash) using a controlled flow rate gas stream. As for entrained-bed reactors, they have not yet been employed to treat lignoceilulose wastes. 

The Fischer-Tropsch process has several variants one of which (a high temperature process) produces significant volumes of olefins. This particular variant, known as the Synthol Process, is used to produce fuels from both coal and natural gas in South Africa. A recent development of this process (The Sasol High Temperature Process ) has replaced the original entrained-bed reactors with fluid-bed reactors. The product breakdown is shown in Table 11.3. 

Due to their excellent mechanical properties and resistance toward abrasion, fused catalysts are suited to entrained bed reactors where tlie hi reaction temperature limits product formation toCi Cn (c.f. Table I). 

After generation of the synthesis gas, conversion to liquid hydrocarbons, waxes, alcohols, and ketones is achieved using an iron or a cobalt catalyst (Table 19.10) in fixed-bed or entrained-bed reactors. A variety of catalysts, among them magnetite (iron oxide), have been proposed and used for the Fischer-Tropsch synthesis (Kugler and Steffgen, 1979 Cooper et al., 1984 Hindermann et al., 1984 Mirodatos et al, 1984 Moser and Slocum, 1992). Magnesium oxide (MgO) is frequently added as a structural, or surface, promoter, and potassium oxide (or other alkali metal oxide) is often added as a chemical promoter (Dry and Ferreira, 1967,1968). 

Koppers-Totzek Entrained bed reactor 1800°C 1 atm 34 51 12 Can handle all coals high temperatures destroy heavy organic tars. Shell-Koppers pressurized version (15-30 atm) under development... 

Winkler Texaco Fluidized bed reactor -900°C latm Entrained bed reactor - 1200°C 20-80 atm 3 42 36 18 Higher pressure process (15 atm) under development Commercially successful process for partial oxidation of fuel oil to synthesis gas being developed to handle coal as coal/water or coal/oil slurries... 

The fixed bed of ARGE and the entrained bed reactors were steadily optimized and modernized to present-day technology. The three-phase FT processes were realized only in pilot and demonstration plants. They had actually no chance to prove their effectivity and superiority over a longer period of operation and were not optimized in larger-scale equipment. Although the three-phase processes were a full success from an engineering point of view they were a failure in view of the economic requirements in those days. 

---