Coke-Benzene Production

COKE-BENZENE PRODUCTION, the production of coke by the dry distillation of coal with the recovery of by-products. The production process can be divided into the following three stages: a) production of coke and gas, b) recovery of useful products from the gas (tar, ammonia, crude benzene), and c) production of corresponding products from the tar and benzene. Bituminous coals are subjected to coking in coke ovens without access to air at a temperature of 700°, reaching 800–1,000° by the end of the coking process. Upon completion of coking, the coke is pushed out of the oven chamber and quenched with water, while the gas is sucked out by exhausters to the coke-benzene plants throughout the entire coking period. From each oven chamber, the gas enters the collector, or "barillet," through ascending pipes, and from there it travels through a gas pipeline to the plant. The gas pipeline serves not only for the transport of gas but also for the recovery of tar and the partial recovery of NH₃. The temperature of the gas leaving the barillet is approximately 200°, while at the end of the gas pipeline it reaches 50–66°, resulting in the condensation of tar and water vapors, which partially capture NH₃, resulting in the formation of so-called "gas liquor." Further along the path of the gas, there are air and water coolers, after which the gas temperature reaches 25–30°, and finally, the gas passes through an impact condenser for the mechanical retention of tar. This concludes the condensation of tar and partially NH₃; then, due to the difference in specific gravities, the gas liquor and tar are separated from the condensate. Ammonia is captured by water in so-called scrubbers based on the counter-current principle: the gas moves in the scrubbers from bottom to top against the flow of liquid in the presence of a large absorption surface (e.g., wooden packing). Another method of capturing NH₃ consists of obtaining ammonium sulfate (NH₄)₂SO₄ directly from crude coke oven gas by saturating it with sulfuric acid at a concentration of 42–45° Bé. The process is carried out in a saturator equipped with a steam or air ejector, by means of which the liquid (acid saturated with ammonia) is fed into a wooden trough, then into a wooden box, and again into the saturator. This cycle continues until saturation is complete; then the mixture goes to a centrifuge and from there to storage. After the saturators or ammonia scrubbers, depending on the NH₃ recovery methods, the gas enters benzene scrubbers for the recovery of crude benzene (benzene and its homologs). The principle of benzene recovery is exactly the same as the principle of NH₃ recovery in scrubbers, but various oils are used instead of water (in the Donbas plants, almost exclusively petroleum solar oil). After absorbing the benzene, the coke oven gas travels through a return gas pipeline to the coke ovens or boiler rooms, etc., and serves as fuel. The oil that has absorbed the benzene is subjected to distillation, during which the distilled benzene vapors are condensed in appropriate equipment, and the crude benzene condensate is collected in storage tanks. The oil freed from benzene goes back to absorb benzene vapors from the gas. The crude benzene is subjected to washing and then distillation in stills. The first fraction is light oil, the second is benzene (at 76°), the third is toluene (at 82°), and the fourth is xylene (at 111°). "Solvent naphtha" remains in the distillation still. The processing of crude tar also consists of distillation. The vapors are condensed in water coolers, and the condensate is collected fractionally in separate storage tanks. First, light oil is distilled at 170–210°, then medium oil at 210–250°, heavy oil at 250–300°, and anthracene oil at 300–350°. Light and medium oils are sent for processing to a phenol plant, while heavy and anthracene oils are pumped into crystallizers (open iron boxes), where, due to natural cooling, crystals of naphthalene and anthracene precipitate. The mother oils are drained from the crystallizers into storage tanks and subsequently serve as material for impregnating railroad ties, while naphthalene and anthracene are removed from the crystallizers by hand. After centrifugation, the naphthalene is melted again and then washed in special apparatuses with compressed air and caustic alkali to remove phenols, and with sulfuric acid to remove pyridine and other bases. After washing, the naphthalene undergoes a second distillation, and after the condensation of vapors in a cooler, it is drained into vats, where it crystallizes. The main occupational hazards of coke-benzene production stem from the course of the production process. 1. Harmful gases. At the temperature at which coking is performed, almost all products of the dry distillation of coal are in a gaseous state and, therefore, can be present in the atmosphere surrounding the worker. There are an extremely large number of these products. In general, they can be divided into the following 5 groups: 1) aliphatic hydrocarbons, 2) aromatic hydrocarbons, 3) other neutral substances [(CN)₂, CO, CS₂, COS, etc.], 4) acidic compounds (H₂S, HCN, phenol, acetic acid, etc.), 5) bases (NH₃, acridine, etc.). According to production analysis data, the composition of coke oven gas is approximately as follows: aromatic hydrocarbons 5% (of which benzene is 0.5–0.8%), CO 6.2%, CS₂ traces, H₂S up to 0.5%, SO₂ traces. The content of sulfur compounds depends, of course, on the amount of S in the coal. As the gas cools while passing through the barillet, gas pipeline, and coolers, substances with a high boiling point condense; therefore, the percentage ratio of individual gas components changes due to the reduction of the condensed components. However, in the atmosphere of the cooler rooms, machine room, ammonia scrubbers, and in the production of sulfate by the direct and semi-direct method, the presence of all the above-mentioned gas components should be expected, with the most important from an occupational-hygiene point of view being benzene, toluene, xylene, CO, sulfur dioxide, H₂S, carbon disulfide, pyridine bases, and phenols. In the ammonia liquor concentration departments, the presence of NH₃ and H₂S in the room atmosphere can be expected. In the crude benzene distillation and rectification departments, the air contains benzene, toluene, xylene, and carbon disulfide. In the air of the tar distillation plant, aromatic hydrocarbons (benzene, toluene, xylene, etc.), carbon disulfide, thiophene, pyridine, phenols, and naphthalene are of occupational-hygiene significance. The question of the possibility of the presence of hydrogen cyanide in the air is unclear. In naphthalene production, the air contains aromatic hydrocarbons, sulfur compounds, pyridine bases, etc. In the washing department, one should expect the presence of naphthalene, phenols, pyridine bases, and also SO₂ in the air during the washing of naphthalene with sulfuric acid. In the naphthalene bottling area, apparently, only naphthalene vapors and crystals are present in the air. 2. Another harmful aspect of coke production is the direct contact of the worker with all the above-described products of the dry distillation of coal, the contamination of the skin with oils that are part of the tar, and the relatively high air temperature in the rooms due to the presence of large heat-radiating surfaces of the apparatus for distillation and heating of liquids. This mainly applies to the benzene and rectification departments and partly to the ammonia department. So far, there are no statistical data on the morbidity of workers in coke-benzene production. There are indications in the literature of morbidity in industries that use coal tar distillation products, e.g., benzene, carbon disulfide, tar, and partially toluene, xylene, and naphthalene. In coke-benzene production, there is the effect of the sum of these components, the combined toxic effect of which is likely much more significant. A prominent place among these diseases of coke-benzene production workers is occupied by skin changes, namely, lesions of the hair follicles and sebaceous glands—dermatitis, hyperpigmentation, verrucous growths, and carcinomatous neoplasms. It was assumed that the lesion of the hair follicles and sebaceous glands is the result of mechanical blockage of the follicle openings and sebaceous gland ducts; however, it is certain that chemical irritation also plays a role in this case. Among workers employed with tar, cancer is observed rarely, but the possibility of its induction by exposure to high-boiling fractions of coal tar has been experimentally proven. Dermatitis affects the exposed parts of the body most of all, and more during work in the daytime than at night; more in summer and spring than in winter, which is explained by the sensitization of the skin's sensitivity to light when exposed to substances possessing photodynamic action. Such substances include anthracene, acridine, phenanthrene, and their derivatives. Measures for improving labor conditions in coke-benzene production. Old coke ovens are designed in such a way that almost all operations, with the exception of pushing out the coke with a special mechanical coke pusher, are performed manually. Coke ovens operate under a pressure of 150–200 mm of water column; therefore, a large amount of gas continuously escapes into the surrounding atmosphere through all leaks (doors, hatches).

Currently, ovens are being built that differ sharply from the old ones in terms of working conditions. Coal delivery is carried out in mechanically hauled wagons. At the end of the wagon's cone, there is a telescopic tube, which is lowered into a special hatch socket, forming a tight connection with it that prevents gas from escaping outward. A steam injector, which operates during the entire time the oven is being loaded and removes all gases into the barillet, also serves this purpose. Opening the doors on the hot side of the ovens is performed mechanically or with the help of winches or a special device on the loading wagons. On the cold side, the doors are opened by the coke pusher. The oven doors are equipped with so-called self-sealing packing. If one also provides for the loading hatch covers to be gas-tight and for the possibility of opening and closing them from the loading wagon, then it is possible to achieve almost absolute cessation of gas leakage. Coke quenching is also performed mechanically. The coke is pushed directly into a mechanically hauled wagon and then quenched with water, either in a closed chamber with subsequent utilization of the resulting water gas for heating, or in a special tower that vents vapors and gases. The room for the oven control system is made spacious and bright with sufficiently favorable working conditions. Such an oven design significantly reduces the labor force, prevents gas leakage, and improves working conditions.

From the point of view of occupational hygiene, the maximum conversion of equipment to vacuum is extremely desirable in Coke-Benzene Production. When washing liquids, compressed air can be replaced by a mechanical stirrer, thereby eliminating increased pressure in the equipment. Regarding the remaining apparatus, for now, the question can only be about eliminating leaks, which is connected with the material for gaskets of pipe flanges and distillation columns. "Klingerit," which is currently used, is quickly corroded by liquids, resulting in a very large number of leaks through which harmful vapors and gases are released into the working atmosphere. Poor covering of equipment or completely open equipment, which is completely unacceptable, is also of great importance. In some cases, local exhaust ventilation with injector-induced draft can be of great importance for improving working conditions, for example, in sulfate production—at the troughs, in tar distillation—at the points where condensed oils drain, and in naphthalene production—at the naphthalene pouring area. A general issue for Coke-Benzene Production is the question of placing equipment in rooms and isolating one room from another. It is necessary to mechanize the unloading of naphthalene and anthracene from crystallizers, their delivery to the centrifuge, and to eliminate manual pitch chipping, i.e., crushing pitch in pitch pits. During manual chipping, work must be performed at night (absence of sunlight); light-type helmets-masks must be issued, and the face must be smeared with fats or clay. This measure justifies itself, as the effect of pitch on the skin is apparently connected with the photodynamic property of certain substances contained in pitch. A very important point in the issue of improving working conditions is the installation of supply and exhaust ventilation, primarily in the machine, benzene, rectification, and tar distillation departments, if the process is conducted without a vacuum. In the field of labor legislation directly concerning Coke-Benzene Production, there is only the decree of the People's Commissariat of Labor of the Ukrainian SSR and the People's Commissariat of Health of the Ukrainian SSR dated 12/VIII 1926, No. 270, on the mandatory periodic medical examination of workers in hazardous industries, which provides for a medical examination of coal gas workers once a year, and for workers in coke-benzene and naphthalene production—once every 6 months.