Coking principle

Early work on the solvent extraction of coal was focused on an attempt to separate from coal a coking principle (i.e., the constituents believed to be responsible for coking and/or caking properties). But solvent extraction has actually been used to demonstrate the presence in coal of material that either differed from the bulk of the coal substance or was presumed to be similar to the bulk material. 

When certain bituminous coals are heated, they soften and become fluid commensurate with the evolution of gas and tar (Van Krevelen et al., 1956 Waters, 1962). The plastic behavior is transient and the mass eventually thickens, swells, and fuses to form a porous solid or coke. This phenomenon is of the utmost importance to the production of metallurgical coke, and in other processes sensitive to caking and agglomeration of coal. Because of the impact of plastic behavior on industrial processes, the solvent extraction of coal has been studied in the past in an attempt to isolate the coking principle from the coal (Burgess and Wheeler, 1911 Illingworth, 1922 Dryden and Pankhurst, 1955). 

Reaction (13.4) is exothermic and reversible, and begins at about 700 K by Le Chatelier s Principle, more iron is produced higher up the furnace (cooler) than below (hotter). In the hotter region (around 900 K), reaction (13.5) occurs irreversibly, and the iron(II) oxide formed is reduced by the coke [reaction (13.6)] further down. The limestone forms calcium oxide which fuses with earthy material in the ore to give a slag of calcium silicate this floats on the molten iron (which falls to the bottom of the furnace) and can bo run off at intervals. The iron is run off and solidified as pigs —boat-shaped pieces about 40 cm long. 

The FCC process is used worldwide in more than 300 installations, of which about 175 are in North America and 70 in Europe. Figure 9.10 shows the principle of an FCC unit. The preheated heavy feed (flash distillate and residue) is injected at the bottom of the riser reactor and mixed with the catalyst, which comes from the regeneration section. Table 9.5 gives a typical product distribution for the FCC process. Cracking occurs in the entrained-flow riser reactor, where hydrocarbons and catalyst have a typical residence time of a few seconds only. This, however, is long enough for the catalyst to become entirely covered by coke. While the products leave the reactor at the top, the catalyst flows into the regeneration section, where the coke is burned off in air at 1000 K. 

The necessary heat may be supplied by passing the hydrocarbon gas or vaporised oil through a tube of refractory material which is externally heated, or the ingenious Rincker-Wolter method may be used. In this process the rough principle is to use a generator similar to a blue-gas generator filled with coke. By means of an air blast the temperature of the coke is raised to about 1200° C., then, when this temperature has been reached, the air supply is stopped and crude oil is blown in at the bottom of the hot coke. 

For either plant type, incineration, or fuel type, these factors must be empirically determined and controlled. Because dioxins as effluents are concerned, it is possible to reduce I-TE values from about 50 ng/m to about 1 ng/m. Additional secondary measures (filter techniques) are therefore necessary for obtaining the lower limit value of 0.1 ng/m. Secondary measures are special filter techniques for pollutants formed in nongreen processes, also called end-of-pipe technology. The main part of technical incineration plants consists of filter devices, mostly coke as adsorbent is used, which must be decontaminated later by itself by burning in hazardous-waste incinerators. The inhibition technology, discussed later, is related on principles of primary (green) measures for a clean incineration method. 

Some of the principles used in Thermofor catalytic cracking have been applied to a coking operation. Coke itself, instead of catalyst, is the solid circulated. The coke is heated in the regenerator of the unit and more coke is deposited on the hot moving solid in the reactor of the unit. Appropriate proportions of the coke are removed continuously as the process proceeds (84). 

The principles of fluidized solids so successfully used in catalytic cracking are applicable to the coking operation. Again the coke itself is the solid circulated. It is heated in the regenerator and coke is deposited on it in the reactor (24). 

The principle Zr ore, zircon (Zr silicate) is processed by caustic fusion or by direct chlorination of milled coke and zircon mixts. Washing of the Na fusion cake leave an acid soluble hydrated Zr oxide, whereas chlorination yields mixed Si and Zr tetrachlorides which are separated by distillation. Removal of the Hf from the Zr takes place through counter current liq-liq extraction (Ref 33), For this purpose the oxide or the tetrachloride is dissolved in dil hydrochloric acid to which ammonium thiocyanate is added as a complexing agent. The organic extracting phase is methyl isobutylketone... 

On similar principles great attention is required in selecting the materials employed in the construction of furnaces. To prevent the escape of the caloric, and thus (o economise fuel, the hearth and walls of the furnace are invariably compounded of some non-conducting substance, as Stourbridge clay or powdered brick, mixed with a large quantity of powdered charcoal or coke. For the same reason, boilers are imbedded in bricks or day, and in many boilers the fire is literally enclosed within them, that none of the heat may be lost, but that all may be rendered available for raising the temperature of Hie water,... 

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