Entrained-Bed Process

Source Braunstein, H.M. et al., Environmental health and control aspects of coal conversion An information overview, Report ORNL-EIS-94, Oak Ridge National Laboratory, Oak Ridge, TN, 1977. 

An entrained-bed system uses finely sized coal particles blown into the gas steam prior to entry into the reactor and combustion occurs with the coal particles suspended in the gas phase the entrained system is suitable for both caking and noncaking coals. The molten salt system employs a bath of molten salt to convert the charged coal (Cover et al., 1973 Howard-Smith and Werner, 1976 Koh et al., 1978). 

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Still another process, called BI-GAS, was developed by Bituminous Coal Research in a 73 t/d pilot plant in Homer City, Peimsylvania. In this entrained-bed process, pulverized coal slurry was dried and blown into the second stage of the gasifier to contact 1205°C gases at ca 6.9 MPa (1000 psi) for a few seconds residence time. Unreacted char is separated and recycled to the first stage to react with oxygen and steam at ca 1650°C to produce hot gas and molten slag that is tapped. 

Results for the Synthol entrained-bed process (16) are plotted in Figure 8. The available C to C15 data follow the conventional Flory plot with a equal to 0.7. The Synthol process uses a fused Fe catalyst of low surface area and porosity and operates at high temperatures ( 590K). The products in the reactor are mainly gaseous, wax formation is minimal, and the pellet pore structure remains free of liquid products therefore, diffusion-enhanced a-olefin readsorption is much less likely than in the ARGE process. Whereas the product selectivity in the ARGE process is altered by diffusion-enhanced a-olefin readsorption, that in the Synthol process is not. 

Meanwhile, Saso) II went on stream (1980) using the entrained-bed process with a capacity of approximately 2 million t/a and Sasol III with the same capacity is under construction and its production startHip is scheduled for 1982. By then, roughly 40% of the motor fuels consumed in South Africa will be produced by Fischer-Tropsch synthesis [2,12]. 

Flow direction. This must be examined from two points of view. First we consider the flow direction of the reactants in relation to one another. In an entrained-bed process the reactants flow by definition in co-current flow, but in a moving-bed reactor one has a choice. The most well-known moving-bed reactor, the Lurgi-Sasol dry bottom gasifier, operates in counter-current flow, but there are a number of smaller gasifier designs that use cocurrent flow to reduce the tar make. 

There are currently three industrially-proven methods for coal gasification, namely fixed-bed gasification (e.g. Lurgi), fluidized bed reactors (e.g. Fritz Winkler (BASF)) and entrained bed processes (e.g. Texaco, Koppers-Toizt, Other processes are being developed, e.g. coal gasification in an iron bath. 

The gasification plant is equipped with two Texaco gasifiers, each capable of producing all of the synthesis gas required for operation of the complex. Eastman chose an entrained-bed gasification process for the Chemicals from Coal project because of three attractive features. The product gas composition using locally available coal is particularly suitable for production of the desired chemicals. Also, the process has excellent environmental performance and generates no Hquids or tars. EinaHy, the process can be operated at the elevated pressure required for the downstream chemical plants. 

Temperature and Product Yields. Most oil shale retorting processes are carried out at ca 480°C to maximize liquid product yield. The effect of increasing retort temperature on product type from 480 to 870°C has been studied using an entrained bed retort (17). The oil yield decreased and the retort gas increased with increased retorting temperature the oil became more aromatic as temperature increased, and maximum yields of olefinic gases occurred at about 760°C. Effects of retorting temperatures on a distillate fraction (to 300°C) are given in Table 6. 

Fluidized-bed process incinerators have been used mostly in the petroleum and paper industries, and for processing nuclear wastes, spent cook liquor, wood chips, and sewage sludge disposal. Wastes in any physical state can be applied to a fluidized-bed process incinerator. Au.xiliary equipment includes a fuel burner system, an air supply system, and feed systems for liquid and solid wastes. The two basic bed design modes, bubbling bed and circulating bed, are distinguished by the e.xtent to which solids are entrained from the bed into the gas stream. 

The operational window of a fluidised bed process is defined by the minimum fluidisation velocity, Uml, at which a settled bed of adsorbent beads starts to fluidise and the terminal velocity ( /t) at which the bed stabilises and adsorbent beads are entrained from the bed. 

In the Texaco process, there are two steps, an initial liquefaction step followed by treatment in an entrained bed gasifier. The liquefaction step involves heating the plastic scrap in such a way that partial depolymerisation occurs, generating a heavy oil and some gas fractions. Some of the gas is recycled as fuel for the process. 

Texaco gasification is based on a combination of two process steps, a liquefaction step and an entrained bed gasifier. In the liquefaction step the plastic waste is cracked under relatively mild thermal conditions. This depolymerisation results in a synthetic heavy oil and a gas fraction, which in part is condensable. The noncondensable fraction is used as a fuel in the process. The process is very comparable to the cracking of vacuum residues that originate from oil recycling processes. 

Figure 9.10. Scheme of an FCC Unit. Cracking ofthe heavy hydrocarbon feed occurs in an entrained bed, in which the catalyst spends only a few seconds and becomes largely deactivated by coke deposition. Coke combustion in the regenerator is an exothermic process that generates heat for the regeneration and for the endothermic cracking process. 

Koppers-Totzek A coal gasification process using an entrained bed. The coal is finely ground and injected in a jet of steam and oxygen into a circular vessel maintained at 1,500°C. Reaction is complete within one second. The ash is removed as a molten slag. The process was invented by F. Totzek at Heinrich Koppers, Essen, and further developed by Koppers Company in Louisiana, MO, under contract with the U.S. Bureau of Mines. The first commercial operation was at Oulu, Finland, in 1952 by 1979, 53 units had been built. Most of the plants are operated to produce a hydrogen-rich gas for use in ammonia synthesis. Developed by Lurgi. See also PRENFLO. 

The different gasification processes can be characterized 1) by the type of coal used and 2) by the coal s physical and chemical properties. On the other hand, the processes differ in the technology involved. For example the heat supply can be allothermal (i.e., external heating) or autothermal (i.e selfheating) and the reactor can be fixed-bed, fluidized-bed or entrained-bed. Furthermore, the actual gasification reaction and the gas composition are determined by46 ... 

In entrained beds, the three-phase mixture flows through the vessel and is separated downstream. These reactors are used in preference to fluidized beds when catalyst particles are very fine or subject to disintegration or if the catalyst deactivates rapidly in the process. 

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