In coking

A new countercurrent continuous centrifugal extractor developed in the former USSR (214) has the feature that mechanical seals are replaced by Hquid seals with the result that operation and maintenance are simplified the mechanical seals are an operating weak point in most centrifugal extractors. The operating units range between 400 and 1200 mm in diameter, and a capacity of 70 m /h has been reported in service. The extractors have been appHed in coke-oven refining (see Coal conversion processes), erythromycin production, lube oil refining, etc. 

Many valuable chemicals can be recovered from the volatile fractions produced in coke ovens. Eor many years coal tar was the primary source for chemicals such as naphthalene [91-20-3] anthracene [120-12-7] and other aromatic and heterocycHc hydrocarbons. The routes to production of important coal-tar derivatives are shown in Eigure 1. Much of the production of these chemicals, especially tar bases such as the pyridines and picolines, is based on synthesis from petroleum feedstocks. Nevertheless, a number of important materials continue to be derived from coal tar. 

In 1990, U.S. coke plants consumed 3.61 x 10 t of coal, or 4.4% of the total U.S. consumption of 8.12 x ICf t (6). Worldwide, roughly 400 coke oven batteries were in operation in 1988, consuming about 4.5 x 10 t of coal and producing 3.5 x 10 t metallurgical coke. Coke production is in a period of decline because of reduced demand for steel and increa sing use of technology for direct injection of coal into blast furnaces (7). The decline in coke production and trend away from recovery of coproducts is reflected in a 70—80% decline in volume of coal-tar chemicals since the 1970s. 

The conversion of coal to gas on an industrial scale dates to the early nineteenth century (14). The gas, often referred to as manufactured gas, was produced in coke ovens or similar types of retorts by simply heating coal to vaporize the volatile constituents. Estimates based on modem data indicate that the gas mixture probably contained hydrogen (qv) (ca 50%), methane (ca 30%), carbon monoxide (qv) and carbon dioxide (qv) (ca 15%), and some inert material, such as nitrogen (qv), from which a heating value of approximately 20.5 MJ/m (550 Btu/fT) can be estimated (6). 

The main difference between gas works and coke oven practice is that, in a gas works, maximum gas yield is a primary consideration whereas in the coke works the quality of the coke is the first consideration. These effects are obtained by choice of a coal feedstock that is suitable to the task. For example, use of lower volatile coals in coke ovens, compared to coals used in gas works, produces lower yields of gas when operating at the same temperatures. In addition, the choice of heating (carbonizing) conditions and the type of retort also play a principal role (10,35). 

Methyl- and dimethylnaphthalenes are contained in coke-oven tar and in certain petroleum fractions in significant amounts. A typical high temperature coke-oven coal tar, for example, contains ca 3 wt % of combined methyl- and dimethylnaphthalenes (6). In the United States, separation of individual isomers is seldom attempted instead a methylnaphtha1 ene-rich fraction is produced for commercial purposes. Such mixtures are used for solvents for pesticides, sulfur, and various aromatic compounds. They also can be used as low freezing, stable heat-transfer fluids. Mixtures that are rich in monomethyinaphthalene content have been used as dye carriers (qv) for color intensification in the dyeing of synthetic fibers, eg, polyester. They also are used as the feedstock to make naphthalene in dealkylation processes. PhthaUc anhydride also can be made from m ethyl n aph th al en e mixtures by an oxidation process that is similar to that used for naphthalene. 

Of the total tar bases in U.K. coke-oven and CVR tars, pyridine makes up about 2%, 2-methyl pyridine 1.5%, 3- and 4-methylpyridines about 2%, and ethylpyridine and dimethylpyridines 6%. Primary bases, anilines and methylanilines, account for about 2% of the bases in coke-oven and CVR tars and 3.5% of the bases in low temperature tars. The main basic components in coke-oven tars are quinoline (16—20% of the total), isoquinoline (4—5%), and methyl quinolines. These dicycHc bases are less prominent in CVR and low temperature tars, in which only a minority of the basic constituents have been identified. 

The demand for electrode binder pitch has grown as aluminium output has expanded and the requirement for aluminium smelting is now between 1.5 and 2 x 10 t /yr. In Japan pitch is used for mixing with coal for carbonization in coke ovens to make metallurgical coke. 

Material and Energy Requirements. Material requirements per metric ton of carbide vary within moderate limits. On the basis of 95% available CaO in the lime and 88% fixed carbon in coke about 865 kg of lime and 494 kg of coke is required to produce a metric ton of calcium carbide of 80% purity. 

The principal binder material, coal-tar pitch, is produced by the distillation of coal tar. Coal tar is obtained primarily as a by-product of the destmctive distillation of bituminous coal in coke ovens during the production of metallurgical coke. Petroleum pitch is used to a much lesser extent as a binder in carbon and graphite manufacture. Because of its low sohds content, petroleum pitch is used as an impregnant to strengthen carbon artifacts prior to graphitization. 

The impact that variations in coke content and burning conditions can have on the overall heat of coke combustion is shown in Table 2. Because the heat balance dictates the amount of heat that is required from burning coke, the heat of combustion then determines the amount of coke that must be burned. 

The hydrogen contained in coke bums at a higher rate than carbon. Hydrogen-burning rates are four to five times greater than carbon-burning rates. 

Most coal chemicals are obtained from high temperature tar with an average yield over 5% of the coal which is carbonized. The yields in coking are about 70% of the weight of feed coal. Tars obtained from vertical gas retorts have a much more uniform chemical composition than those from coke ovens. Two or more coals are usually blended. The conditions of carbonization vary depending on the coals used and affect the tar composition. Coal-tar chemicals include phenols, cresols, xylenols, benzene, toluene, naphthalene, and anthracene. 

Worldwide demand for blast furnace coke has decreased over the past decade. Although, as shown in Figure 1, blast furnace hot metal production (pig iron) increased by about 4% from 1980 to 1990, coke production decreased by about 2% over the same time period (3). This discrepancy of increased hot metal and decreased coke production is accounted for by steady improvement in the amounts of coke required to produce pig iron. Increased technical capabihties, although not universally implemented, have allowed for about a 10% decrease in coke rate, ie, coke consumed per pig iron produced, because of better specification of coke quaUty and improvements in blast furnace instmmentation, understanding, and operation methods (4). As more blast furnaces implement injection of coal into blast furnaces, additional reduction in coke rate is expected. In some countries that have aggressively adopted coal injection techniques, coke rates have been lowered by 25% (4). 

Fig. 3. Development of plastic layer movement during coking (a), about midway in coking cycle (b), convergence of plastic envelope and (c), end of coking cycle. The thick, dark, soHd line represents the plastic envelope, defining the boundary between coal and semicoke and M represents the plasticized...

Knowledge of the composition of coal ash is usehil for estimating and predicting coal performance in coke making and, to a hmited extent, the folding and corrosion of heat-exchange surfaces in pidverized-coal-fired furnaces. 

Sihca bricks are used extensively in coke ovens, the roofs and walls of open-hearth furnaces, and the roofs and sidewalls of glass tanks and as linings of acid electric steel furnaces. Although sihca brick is readily spalled (cracked by a temperature change) below red heat, it is very stable if the temperature is kept above this range and for this reason... 

As an example, a tank farm that is to be cathodically protected by this method is shown schematically in Fig. 11-4. As can be seen in the figure, injection of the protection current occurs with two current circuits of a total of about 9 A, via 16 vertically installed high-silicon iron anodes embedded in coke. These are distributed over several locations in the tank farm to achieve an approximately uniform potential drop. The details of the transformer-rectifier as well as the individual anode currents are included in Fig. 11-4. Anodes 4, 5 and 6 have been placed at areas where corrosion damage previously occurred. Since off potentials for 7/ -free potential measurements cannot be used, external measuring probes should be installed for accurate assessment (see Section 3.3.3.2 and Chapter 12). 

Fig. 19-4 Older anode system for roadway plates (a) Silicon iron anodes in coke breeze bed, (b) noble metal lead in conducting bedding in a trench.

Installation of an anode bed with 4 FeSi anodes in coke backfill, cable and connecting sleeves, including earthworks DM 6000... 

The transition resistance between the surface of the metal and the electrolyte with uncoated iron anodes in coke backfill, the transition resistance is usually low. With metals in soil, it can be increased by films of grease, paint, rust or deposits. It contains in addition an electrochemical polarization resistance that depends on the current [see Eq. (2-35)]. 

Blended coal is first heated in coke ovens to produce coke. This process is known as carbonization. The gas produced during carbonisation is extracted and used for fuel elsewhere in the steelworks. Other by-products (such as tar and benzole) are also extracted for further refining and sale. Once carbonised, the coke is pushed out of the ovens and allowed to cool. 

That imports of coke have increased in the United States is mostly due to the high cost of transportation within the United States and the lower cost of shipping by foreign firms. It is cheaper to extract coal in China, coke it and ship it to California than to buy coke from U.S. producers and transport it within the United States. The trade in coke doesn t hurt the U.S. economy any more than the trade in jet aircrafts to China hurts China. Mutual trade is always beneficial to trading partners. 

Data for 1-in. ceramic Raschig rings, correlates reasonably well with 3-in. spiral die and 1-in. coke. 

One area of cat cracking not fully understood is the proper determination of carbon residue of the feed and how it affects the unit s coke make. Carbon residue is defined as the carbonaceous residue formed after thermal destruction of a sample. Cat crackers are generally limited in coke burn capacity, therefore, the inclusion of residue in the feed produces more coke and forces a reduction in FCC throughput. Conventional gas oil feeds generally have a carbon residue less than 0,5 wt for feeds containing resid, the number can be as high as 15 wt lf. 

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