Of cokes

Coking units are operated to optimize the light products produced, coke being considered as a by-product. Its quality is not too important. Generally speaking, the quality of coke produced varies widely according to the feed, the operating conditions, and the process. 

The major portion of sait is found in residues as these streams serve as the bases for fuels, or as feeds for asphalt and petroleum coke production, the presence of salt in these products causes fouling of burners, the alteration of asphalt emulsions, and the deterioration of coke quality. Furthermore, calcium and magnesium chlorides begin to hydrolyze at 120°C. This hydrolysis occurs rapidly as the temperature increases (Figure 8.1) according to the reaction i. ... 

The visbreaking process thermally cracks atmospheric or vacuum residues. Conversion is limited by specifications for marine or Industrial fuel-oil stability and by the formation of coke deposits in equipment such as heaters and exchangers. 

Cracking reactions are endothermic the energy balance is obtained by the production of coke that deposits on the catalyst and that is burned in the regenerator. 

Fig. 4.23 Adsorption isotherms of butane vapour at difTerent temperatures on a sample of carbon (prepared by heating a mixture of coke and pitch at 600°C), burnt off by 0.27%.

Coke oven light oil is a by-product of the manufacture of coke for the steel industry. When coal is subjected to high temperature carbonization, it yields 16—25 Hters /tonne of light oil that contains 3—6 vol % of mixed xylenes. 

The first process for manufacture of calcium carbide [75-20-7] and acetylene [74-86-2] involved the reaction of coke and lime. The carbide process operates at a temperature of about 2000°C according to the following reaction ... 

Essentially all the ammonium sulfate fertilizer used in the United States is by-product material. By-product from the acid scmbbing of coke oven gas is one source. A larger source is as by-product ammonium sulfate solution from the production of caprolactam (qv) and acrylonitrile, (qv) which are synthetic fiber intermediates. A third but lesser source is from the ammoniation of spent sulfuric acid from other processes. In the recovery of by-product crystals from each of these sources, the crystallization usually is carried out in steam-heated sa turator—crystallizers. Characteristically, crystallizer product is of a particle size about 90% finer than 16 mesh (ca 1 mm dia), which is too small for satisfactory dry blending with granular fertilizer materials. Crystals of this size are suitable, however, as a feed material to mixed fertilizer granulation plants, and this is the main fertilizer outlet for by-product ammonium sulfate. 

The original process of heating coal in rounded heaps, the hearth process, remained the principal method of coke production for over a century, although an improved oven in the form of a beehive was developed in the Durham-Newcastie area of England in about 1759 (2,26,28). These processes lacked the capabiHty to collect the volatile products, both Hquids and gases. It was not until the mid-nineteenth century, with the introduction of indirectiy heated slot ovens, that it became possible to collect the Hquid and gaseous products for further use. 

High Temperature Carbonization. When heated at temperatures in excess of 700°C (1290°F), low temperature chars lose their reactivity through devolatilization and also suffer a decrease in porosity. High temperature carbonization, at temperatures >900° C, is, therefore, employed for the production of coke (27). As for the low temperature processes, the tars produced in high temperature ovens are also sources of chemicals and chemical intemiediates (32). 

Documented efforts at cokemaking date from 1584 (34), and have seen various adaptations of conventional wood-charring methods to the production of coke including the eventual evolution of the beehive oven, which by the mid-nineteenth century had become the most common vessel for the coking of coal (2). The heat for the process was suppHed by burning the volatile matter released from the coal and, consequently, the carbonization would progress from the top of the bed to the base of the bed and the coke was retrieved from the side of the oven at process completion. 

F. M. Fess, EListory of Coke Oven Technology, Gluckauf Vedag, Essen, Germany, 1957. 

The furnace (Fig. 2) maybe divided into four zones (from bottom to top). (/) Hearth and raceway as the coke descends through the furnace, it is heated by the ascending gases to about 1370°C. When it reaches the raceway in front of the tuyeres, it reacts immediately with the oxygen in the hot blast air. Equation 1, however, is actually the combination of coke combustion (eq. 6) and coke gasification (eq. 7, also referred to as solution loss). 

Another thermally efficient kiln is the modem mixed-feed vertical kiln ia which coke is admixed with 8.5—20 cm lump limestone and charged into the top of the vertical kiln by a mobile, overhead charging system. However, use of this kiln is waning since the quaUty of mixed-feed kiln lime does not equal that of the other three kilns described above, owing to ash contamination from the coke and poorer reactivity and to the higher cost of coke in most areas. 

Ferromanganese is produced iu blast fiimaces and electric smelting fiimaces. Economics usually determine which smelting process is chosen for ferromanganese. Both methods require about the same amount of coke for reduction to metal, but iu the case of the blast fiimace, the thermal energy required for the smelting process is suppHed by the combustion of additional coke, which iu most countries is a more expensive form of energy than electricity. 

The Iron Bla.stFurna.ee, The reduction of iron oxides by carbon in the iron (qv) blast furnace is the most important of all extractive processes, and the cornerstone of all industrial economies. Better understanding of the reactions taking place within the furnace has made possible a more efficient operation through better preparation of the burden, higher blast temperature, and sometimes increased pressure. Furnace capacity has doubled since the 1800s, whereas coke consumption has been reduced by about half The ratio of coke to iron produced on a per weight basis is ca 0.5 to 1. 

Fluid coking (Fig. 4) is a continuous process that uses the fluidized soflds technique to convert atmospheric and vacuum residua to more valuable products (12,13). The residuum is converted to coke and overhead products by being sprayed into a fluidized bed of hot, fine coke particles, which permits the coking reactions to be conducted at higher temperatures and shorter contact times than they can be in delayed coking. Moreover, these conditions result in decreased yields of coke greater quantities of more valuable Hquid product are recovered in the fluid coking process. 

The catalyst is employed in bead, pellet, or microspherical form and can be used as a fixed bed, moving bed, or fluid bed. The fixed-bed process was the first process used commercially and employs a static bed of catalyst in several reactors, which allows a continuous flow of feedstock to be maintained. The cycle of operations consists of (/) the flow of feedstock through the catalyst bed (2) the discontinuance of feedstock flow and removal of coke from the catalyst by burning and (J) the insertion of the reactor back on-stream. The moving-bed process uses a reaction vessel, in which cracking takes place, and a kiln, in which the spent catalyst is regenerated and catalyst movement between the vessels is provided by various means. 

Ammonium Phosphates. In the manufacture of ammonium phosphates, an atmosphere of ammonia may need to be maintained because the partial pressure of ammonia rises rapidly as either the temperature or the NH2/P20 mole ratio of the reaction mass increases. Phosphoric acid reacts quickly with ammonia vapor and is used in multistage reactor systems as a scmbber fluid to prevent NH emissions and recover ammonia values. For example, H PO scmbbing of coke-oven off-gases produces ammonium phosphates of relatively good purity. 

The dominant commercial form of elemental phosphoms is the a-white aHotrope. a-White phosphoms is often designated simply as because the soHd consists of tetrahedral P molecules. In its pure form, it is a white soHd that forms a clear Hquid when melted. However, the commercial product is generally somewhat yellow, both as a soHd and as a Hquid, owing to the presence of small amounts of a ted phosphoms aHotrope. Commercial white phosphoms may also be slightly gray in color because of incomplete separation of coke dusts and other impurities generated in the manufacturing process. 

Until 1960—1970, in countries where natural gas was not available, large amounts of coal were carbonized for the production of town gas, as well as a grade of coke which, although unsuitable for metallurgical use, was satisfactory as a domestic fuel in closed stoves. The early cast-iron and siUca horizontal retorts used at gasworks were replaced by continuous vertical retorts. These operated at flue temperatures of 1000—1100°C. The volatile products were rapidly swept from the retort by the introduction of steam at 10—20% by weight of the coal carbonized. 

In the case of low temperature tar, the aqueous Hquor that accompanies the cmde tar contains between 1 and 1.5% by weight of soluble tar acids, eg, phenol, cresols, and dihydroxybenzenes. Both for the sake of economics and effluent purification, it is necessary to recover these, usually by the Lurgi Phenosolvan process based on the selective extraction of the tar acids with butyl or isobutyl acetate. The recovered phenols are separated by fractional distillation into monohydroxybenzenes, mainly phenol and cresols, and dihydroxybenzenes, mainly (9-dihydroxybenzene (catechol), methyl (9-dihydtoxybenzene, (methyl catechol), and y -dihydroxybenzene (resorcinol). The monohydric phenol fraction is added to the cmde tar acids extracted from the tar for further refining, whereas the dihydric phenol fraction is incorporated in wood-preservation creosote or sold to adhesive manufacturers. Naphthalene Oils. Naphthalene is the principal component of coke-oven tats and the only component that can be concentrated to a reasonably high content on primary distillation. Naphthalene oils from coke-oven tars distilled in a modem pipe stiU generally contain 60—65% of naphthalene. They are further upgraded by a number of methods. 

Chloride Process. In the chloride process (Fig. 3), a high grade titanium oxide ore is chlorinated in a fluidized-bed reactor in the presence of coke at 925-1010°C ... 

Of the alkaline-earth carbonates, BaCO requires the greatest amount of heat to undergo decomposition to the oxide. Thus carbon in the form of coke, tar, or carbon black, is added to the carbonate to lower reaction temperature from about 1300°C in the absence of carbon to about 1050°C. The potential for the reverse reaction is decreased by removing the CO2 as shown in equation lb. 

Until 1960, coal was the source material for almost all benzene produced in Europe. Petroleum benzene was first produced in Europe by the United Kingdom in 1952, by Erance in 1958, by the Eederal Republic of Germany in 1961, and by Italy in 1962. Coal has continued to decline as a benzene source in Europe, and this is evident with the closure of coke ovens in Germany (73). Most of the benzene produced in Europe is now derived from petroleum or pyrolysis gasoline. In Europe, pyrolysis gasoline is a popular source of benzene because European steam crackers mn on heavier feedstocks than those in the United States (73). 

Dehydrogenation of /i-Butane. Dehydrogenation of / -butane [106-97-8] via the Houdry process is carried out under partial vacuum, 35—75 kPa (5—11 psi), at about 535—650°C with a fixed-bed catalyst. The catalyst consists of aluminum oxide and chromium oxide as the principal components. The reaction is endothermic and the cycle life of the catalyst is about 10 minutes because of coke buildup. Several parallel reactors are needed in the plant to allow for continuous operation with catalyst regeneration. Thermodynamics limits the conversion to about 30—40% and the ultimate yield is 60—65 wt % (233). 

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