Coal tar refining technology

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1 Modern tar distillation methods The modern approaches to tar distillation generally fall into the following categories: the use of large-scale facilities; for example, the largest tar distillation plant in Germany has a capacity of 500,000 tons per year； Continuous distillation under normal and reduced pressure is employed, with waste heat recovery implemented in all stages, ensuring that the heat consumption per ton of coke is less than 0.879 MJ ； Improve the separation efficiency of the tower, so that the recovery rate of naphthalene exceeds 90% and its concentration also reaches over 90% ； Asphalt with different softening points is obtained through the “flash evaporation” method ； Effectively inhibits polymerization reactions that occur during tar formation, especially to prevent coking ； Microcomputer control is used to achieve automatic regulation of all process parameters for startup, shutdown switching, and normal operation ； Seek to expand resources for tar raw materials. 2 Development trends in coal tar processing 2.1 Centralized processing and scale-up The advantages of centralized processing and scale-up of coal tar are: lower costs ； Low energy consumption, high yield ； Ministry of Investment ； A wide variety of high-quality products is available, which facilitates further processing ； Environmental protection is good. At present, tar processing facilities in China are generally scattered and of small scale (less than 30,000 t/a); the energy consumption per ton of tar is as high as 2.0 MJ. Moreover, there is severe environmental pollution, a limited range of product types, poor quality, high production costs, and low economic efficiency. 2.2 Expanding the resources for raw materials in processing That is, mixing a portion of the \"pyrolysis tar\" produced during the \"steam cracking process for olefin production\" and processing it together with coal tar. This process has been industrialized at Lütgé Company. In China, a large amount of coal tar is still used directly as fuel oil, preventing the comprehensive utilization of these resources. 3 New Technologies for Coal Tar Processing 3.1 Tar Distillation Technology In China, a distillation process that operates at atmospheric pressure, uses a single tower, and involves the separation of two or three different fractions is commonly employed. The introduced coal tar distillation unit features continuous dehydration and light oil removal; the fractionation tower operates under reduced pressure, with phenol oil being extracted from the top of the tower at a pressure of 13.3 kPa, while the bottom product is soft asphalt with a softening point of 65°C ； A square box tube furnace is used, with a tar outlet temperature of 330℃ ； Make good use of the waste heat: soft asphalt exchanges heat with tar, and steam generators are used to produce low-pressure steam at 0.3 MPa for various fractions ； The oil vapor at the top of the distillation column is cooled using an air condenser; moreover, by employing reduced pressure, energy consumption can be reduced by approximately 15% to 50% ； The exhaust gas drawn under reduced pressure and the separated phenol-water are both sent to a tubular furnace for incineration ； The material for the distillation column is corrosion-resistant low-carbon alloy steel. 3.2 Industrial naphthalene distillation technology At present, most coking plants in China produce 95% industrial naphthalene without acid washing; only those plants that recycle quinoline derivatives produce 95% industrial naphthalene after acid washing. Additionally, the raw materials used for producing 95% industrial naphthalene also vary: narrow fraction (i.e., naphthalene oil fraction), four-component mixture fraction (light compounds, phenol, naphthalene, washing agents), three-component mixture fraction (phenol, naphthalene, washing agents), two-component mixture fraction (naphthalene, washing agents), etc. 　　The industrial naphthalene distillation processes can be divided into atmospheric batch still distillation, reduced-pressure batch still distillation, atmospheric continuous distillation with two reactors and two towers, atmospheric continuous distillation with two furnaces and two towers, atmospheric continuous distillation with one furnace and two towers, atmospheric continuous distillation with one furnace and one tower, and high-pressure continuous distillation with one furnace and two towers, among others. In terms of the actual number of trays in the distillation column, it started at 50 trays, and then increased to 63, 64, and 70 trays. The types of distillation columns include packed columns (ceramic rings, Pall ring, corrugated plates, etc.), circular bubble cap columns, strip bubble cap columns, inclined hole plate columns, floating valve columns, etc. Currently, most large coking plants use 70-stage floating valve columns, along with a continuous distillation process using two or three mixed fractions as feedstock, in a double furnace and double column configuration at atmospheric pressure. The normal-pressure single-furnace, dual-tower continuous process is quite common, while Baosteel’s normal- and pressure-driven single-furnace dual-tower continuous process has the lowest energy consumption. With the application of microcomputers, the continuous distillation process using a single furnace or tower holds promise for development. 3.2.1 Double-reactor, double-column atmospheric continuous distillation process: This is the 95N production process most commonly used in small and medium-sized coking plants in China, and it is characterized by stable operation and easy control. The process is shown in Figure 1. Figure 1 Process flow of continuous distillation under atmospheric pressure with two reactors and two columns 3.2.2 Continuous distillation process under atmospheric pressure with two furnaces and two columns The process is shown in Figure 2. The characteristic of this process is that the heating at the bottom of both the initial distillation tower and the rectification tower is provided by their respective dedicated tubular furnaces, in order to control the temperature at the tower bottom ； The top temperatures of both towers are controlled by adjusting their reflux rates, and each has its own independent temperature control system, thus making operation convenient and easy to manage. Figure 2 Process flow of continuous distillation under atmospheric pressure with two reactors and two columns. 3.2.3 Process of continuous distillation under atmospheric pressure with one reactor and two columns; the process flow is shown in Figure 3. The characteristic of this process is that a tubular furnace supplies heat to the bottoms of the two towers, and the distribution of the area of the tubular furnace is particularly important ； The temperatures of the two tower tops are still controlled by their respective reflux flows ； The temperature control at the bottom of the two towers is achieved by adjusting the tubular furnace (such as gas flow rate, air flow rate, secondary air flow rate, flue dampers, etc.). If the bottom temperatures of the two towers cannot be adjusted to the desired values, it indicates that the area allocation design of the tubular furnace is unreasonable and must be modified. Figure 3 Continuous distillation process under atmospheric pressure with a single reactor and two columns. 3.2.4 Continuous distillation process under atmospheric pressure with a single reactor and a single column: see Figure 4. Figure 4: Process flow for continuous distillation under atmospheric pressure with a single furnace and single tower. 3.2.5 Baogang’s process for continuous distillation using a single furnace and two towers (with the distillation tower under pressure); the process flow is shown in Figure 5. The characteristics are as follows: the distillation tower operates under pressure; the raw naphthalene oil exchanges heat with the oil at the bottom of the distillation tower, reaching a temperature of 120–125°C. It then exchanges heat with 95N vapor coming from the reflux tank, reaching a temperature of 180–200°C before entering the primary distillation tower. Part of the bottom oil from the primary distillation tower is pumped to the distillation column, while another part goes to the reboiler. Using the 95N vapor from the top of the distillation column as a heat source, its temperature is raised to 235–245°C before it returns to the bottom of the primary distillation tower. A portion of the oil at the bottom of the distillation tower is discharged after heat exchange with the feed naphthalene oil, while the remaining oil is heated to 319°C using a tubular furnace and then returned to the distillation tower to provide heat for the bottom of the tower. The bottom layer is methylnaphthalene oil, containing less than 1% naphthalene. The heating at the bottom of the initial distillation tower does not rely on a tubular furnace; this is the fundamental reason why the naphthalene distillation tower operates under pressure. The waste heat from each fraction is used to generate low-pressure steam in a steam generator, with a pressure of 0.3 MPa. Figure 5: Process flow of Baogang’s single-reactor, double-tower continuous distillation process. 3.3 Washing technology for the fractions obtained from tar distillation: This refers to devices for alkaline extraction of phenol or acidic extraction of quinoline, which enable the production of phenolate and quinoline sulfate, respectively. Generally, phenol is removed first, followed by quinoline. It is also possible to remove only phenol without removing quinoline. The raw materials vary depending on the different fractions obtained from tar distillation, and can be classified as narrow fractions or wide fractions. The washing process can operate intermittently or continuously. Washing equipment comes in various types such as air agitation, mechanical agitation, pump mixing, static mixers, and jet mixers. The latter two types of scrubbers are more advanced, offer better cleaning performance, and are suitable for continuous operation and automatic control. The main controlling factors for alkali-eluted phenol include: alkali concentration, washing temperature, separation time, and the number of washing stages. The washing requirement for each fraction is that its phenol content be less than 0.5%. 　　Baosteel has adopted a fully continuous alkaline washing and phenol removal process, with a low concentration of alkali solution, ranging from 8% to 10% ； Both light oil and phenol oil undergo primary defenolization, with defenolization efficiencies of ~38% and 88%, respectively. Its light oil defenolization serves to purify phenoxide salts. Naphthalene oil is subjected to three-stage defooling, with a defooling efficiency of 79% ； Static mixers are used in the defoaming equipment. Furthermore, quinoline removal was carried out via continuous acid washing only on dephenolated phenol oil and methylnaphthalene oil respectively, using an acid concentration of 30%–39%, with efficiencies of 38.5% and 52.2% respectively. Static mixers are also used in the equipment. 3.4 Technology for producing crude anthracene 　 Domestic plants all use a batch processing method, with rotary drum crystallizers as the equipment. To improve the yield of crude anthracene, a two-stage crystallization method was developed. The process introduced by Baosteel employs fully continuous program-controlled operation, including the following steps: I. Loading of anthracene oil → Cooling and crystallization → Discharging → Centrifugation, for a total of 44 hours. Later, it was improved by combining natural and forced cooling, reducing the time to 35 hours while resulting in larger crystal particles ； The equipment uses a vertical cooling crystallizer, which facilitates continuous operation ； The crude anthracene obtained contained as much as 38% anthracene, with very low oil content. 3.5 Sodium phenoxide decomposition technology 　In China, the sulfuric acid decomposition method is mostly used; its drawback is that it generates concentrated phenol water, which is difficult to handle. The flue gas decomposition method was developed in the 1970s, but secondary pollution remains a problem. Baosteel’s adopted process uses the blast furnace gas decomposition method, operating in two stages, with a decomposition rate of 98% ； It is also equipped with a causticization unit, which enables the production of caustic alkali solution with a concentration of 8%–10%, and a causticization rate of 77% ； There is no issue of secondary pollution. 3.6 Technology for producing refined naphthalene 　In China, the concentrated sulfuric acid purification method has been used traditionally; its drawback is that it generates large amounts of waste acid that are difficult to handle, resulting in high energy consumption and low yields. The batch-step crystallization method was developed in the 1980s and has been widely applied. Baosteel once adopted the zone melting method, which is characterized by continuous operation, but the yield of refined naphthalene is low, at only 56%. In recent years, the “Praobd” process technology has been adopted, which involves batch crystallization in a tank; the yield of high-purity naphthalene is 90% ； And everything is automatically controlled and operated continuously in accordance with the procedures. 3.7 Rough phenol refining technology 　In China, the processes of atmospheric pressure dehydration – vacuum deslagging and distillation are commonly used, resulting in phenol products of poor quality. The introduced process utilizes 5 towers for continuous operation of dehydration, deslagging, and distillation, with the 6th tower operating in a batch mode. All towers operate under reduced pressure, achieving a phenol recovery rate of up to 42%, which is about 10% higher than that in China ； The product quality is excellent; it includes special-grade phenol (with a melting point of over 40°C), o-cresol (with a melting point of over 29°C), m-cresol, p-cresol, and dimethylphenol, among others. 3.8 Purification techniques for crude pyridine and crude quinoline In China, caustic soda solution is used to neutralize and decompose quinoline sulfate, while abroad, liquid ammonia is more commonly used for this purpose. The purification of crude pyridine and crude quinoline both employs a process flow of batch operation, azeotropic dehydration, and vacuum distillation. Unlike domestic systems, the introduced unit uses 6-tower batch dehydration and vacuum distillation for operation ； An air cooler is also used, which helps save cooling water. 3.9 Production technologies for pure anthracene, pure carbazole, and anthraquinones In China, crude anthracene is used as the raw material; it is processed via a solvent-distillation method to obtain pure anthracene, which is then catalytically oxidized to produce anthraquinones. Baosteel has adopted the “Praobd” technology, which involves using anthracene oil as raw material, adding a solvent for stepwise crystallization (referred to as solvent crystallization method), and then performing vacuum distillation to obtain pure anthracene (with over 95% anthracene content) and pure carbazole (with a purity of over 90%). The anthraquinone production process is a technology from the Swiss company Ciba Geigy; through multiple stages of fixed-bed catalytic oxidation and cooling, anthraquinone with a purity of over 99% is obtained. Compared to domestic methods, the level of this process and the equipment used are roughly on par. Its feature is that very little waste liquid is generated during the entire production process, which can be sent to an activated sludge system for treatment ； The amount of waste gas generated is large, but it can be recovered and filtered, and then destroyed in a waste gas combustion device before being released, so it does not pose a threat to the environment ； The DCS control system from the American company Foxboro was also used. 3.10 Utilization and Modification Technologies of Asphalt At present, coal tar asphalt is mainly used in the production of asphalt coke, binders for electrodes and anode pastes (modified asphalt), binders for briquetted coal, road construction asphalt, and various asphalt-based anti-corrosion coatings. Abroad, production technologies for \"producing needle coke for ultra-high power electrodes from coal tar pitch, as well as carbon fibers for aircraft and spacecraft\" have been successfully developed, which represents the future direction for the utilization of coal tar pitch. 3.10.1 Asphalt delayed coking production technology: The technology for producing delayed coker from petroleum asphalt has been in use in the petrochemical industry for a long time, while the technology for producing delayed coker – asphalt coke from coal asphalt was first introduced in the 1980s. Asphalt coke is typically produced using the chamber coke oven method and delayed coking method; recently, Germany has developed a rotary furnace method as well, but it has not been put into industrial use. The chamber-type coke oven method causes severe pollution, so it has been phased out in China; Baosteel uses the delayed coking method. 3.10.2 Granular asphalt production technology: In the 1980s, China developed its own production process for granular asphalt and brought it into industrial use. Its production principle involves using a pump to atomize the asphalt through an atomization nozzle into fine droplets, which are then cooled and shaped in a stream of cold air; the surface tension of the asphalt itself causes them to form into granular asphalt. 3.10.3 Road asphalt production technology In the past, road asphalt was produced by melting and mixing 60%–80% of coal-based medium-temperature asphalt with 40%–20% anthracene oil. Germany has currently developed a tar-petroleum mixed asphalt, which is a mixture of 30% coal tar pitch and 70% petroleum asphalt. It exhibits good miscibility, with a uniform distribution of the binder components, allowing it to be used in the production of asphalt concrete for highway pavements. Using this type of asphalt for paving offers the advantages of petroleum asphalt, such as rapid curing during construction and reduced deformation of the pavement in summer ； It also possesses the advantages of tar asphalt, such as strong adhesion to stones, resistance to oil erosion, ease of processing and use, and a durable pavement surface. In recent years, bituminous road asphalts modified with rubber, waste rubber, waste plastics, etc., have been successfully developed, reducing the cost of road asphalts. 3.10.4 Asphalt carbon fiber production technology The process for preparing carbon fibers from coal tar asphalt is as follows: (tar asphalt) → heat treatment → (modified asphalt) → melt spinning → (asphalt fibers) → non-fusing treatment → carbonization (heating under inert gas conditions) → carbon fibers. The most important step in this process is pretreatment, which involves modifying the raw asphalt to give it sufficient spinnability ； A non-melting treatment is applied to render the surface of the asphalt fibers non-meltable. 　　Pre-treatment generally involves dry distillation or vacuum dry distillation in an inert gas flow to remove the low-molecular-weight components from the raw asphalt and increase its molecular weight. Extraction can also be used to remove free carbon and polymer-insoluble substances from the raw material. Direct pressure filtration at a temperature about 100°C higher than the softening point of asphalt can remove certain quinoline-insoluble substances or mineral components that degrade fiber quality. 　The softening point of the fibers obtained by melt spinning is lower than their decomposition temperature, which makes further heat treatment of the asphalt fibers difficult; this issue can only be overcome through non-melting treatments. The non-melting treatment is an oxidation process whose purpose is to introduce oxygen-containing functional groups with high thermal reactivity into aromatic compounds with low thermal reactivity, thereby forming oxygen bridge bonds that interconnect the condensation rings and create a non-melting film on the surface. 　　Due to advantages such as a high carbon content and ease of undergoing non-fusion treatment, a mixture of coal tar pitch and petroleum tar pitch to which air has been blown can be used as a raw material for producing melt-pyrolyzed pitch carbon fibers, and the quality of the resulting products meets the requirements for ordinary carbon fibers. The preparation process is as follows: The mixture of the two aforementioned substances is dry-distilled at 380°C for 1 hour, and the residual product is then heat-treated under vacuum at 270–340°C. If necessary, diisopropylphenyl peroxide can be added to the residue, and the mixture is heat-treated at 280°C under a stream of dried nitrogen. The resulting residue exhibits good spinability, and the fibers formed oxidize into substances that are difficult to melt at temperatures below 290°C; finally, carbonization at 1000°C yields melt-pyrolyzed pitch carbon fibers with good mechanical properties. 3.10.5 Modified asphalt production technology 　　(1) Oxidation and thermopolymerization methods. A batch heating distillation vessel is used; medium-temperature asphalt is placed at the bottom of the vessel, after which compressed air is introduced to carry out heating and oxidation. Substances such as pyrene, chrysene, and fluorene, which are produced by cracking during the oxidation process, are recovered through a distillation column and then via a condensation cooler; the temperature of the liquid in the distillation vessel is generally maintained at 340–350°C. This process can increase the softening point of asphalt, thereby enabling the production of \"hard asphalt\". However, it is difficult to obtain high-quality, qualified electrode asphalt. 　　(2) Heating polymerization method. An intermittent heating kettle is used, with direct heating by gas; medium-temperature asphalt is added to the kettle and heated while kept at a constant temperature for a certain period of time. The process can be carried out at atmospheric pressure or under a certain pressure. However, without introducing air for oxidation, the goal is to raise the softening point of asphalt through thermal polymerization and the evaporation of low-boiling-point substances; only \"hard asphalt\" can be produced, and modified asphalt of good quality cannot be obtained. Due to its relatively simple process, it is currently widely used in China as a binder for producing \"anode paste\". 　　(3) Pressurized heat polymerization treatment method. The process involves using a pump to feed the melted medium-temperature asphalt into a box-type heating furnace, where it is heated to 420–430°C, and then it flows sequentially into 5 parallel reaction vessels with a volume of 2 m3 each. The temperature in the reactor is maintained at 1.0–1.2 MPa and 420–430°C; the hot asphalt is kept at these conditions for 4–6 hours to undergo thermal polymerization. Afterwards, the asphalt is pumped from the bottom of the reactor to a flash tower, and its softening point is adjusted using other oils. The modified asphalt at the bottom of the tower flows by gravity to the intermediate tank, where it is periodically sent to an asphalt cooler or an elevated asphalt tank for further cooling and solidification. The reaction gas and oil vapor escaping from the top of the reactor and flash tower are condensed into liquids in the condensation coolers, after which they flow automatically into the flash oil tank. The exhaust gas is washed in two stages through sequentially operated scrubbers before being sent to the heating furnace. 　　(4) Lutghe polymerization method. This method uses medium-temperature asphalt as raw material; ordinary medium-temperature asphalt is first continuously pumped into a reaction vessel, where it undergoes a thermal polymerization reaction under stirring conditions to form \"electrode asphalt\". The distilled volatile gases are cooled by condensation and then discharged into a storage tank ； The electrode asphalt is continuously discharged into the product asphalt tank. The gas that has not yet condensed can be used as fuel; for medium-temperature asphalt, this amounts to approximately 4 m3/h per ton, with a calorific value of around 25,000 kJ/m3. The specifications of the electrode asphalt can be adjusted by changing the heating temperature and the reaction time in the reactor. The softening point of electrode pitch can be altered by adding adjusting oils (usually tower top distillates or anthracene oil). The quality parameters of the ordinary-grade electrode asphalt obtained are: softening point (Hg method) of 80–90°C, BI of 25%–35%, QI of 6%–14%, β-resin content greater than 19%, and ash content of 0.3%. 　　(5) Cherry–T method (abbreviated as C–T method). This method is a comprehensive process developed by Osaka Gas Company in Japan, which uses heavy residual oil as raw material for modification and refinement; it enables the production of high-quality modified asphalt with a softening point of 80°C and a β-resin content of over 32%, achieving a yield 10% higher than that of the thermal polymerization method. The process involves: the raw coal tar being pumped into a dehydration tower, where almost all of the water is removed; the coal tar is then drawn out from the bottom of the tower and sent to a low-pressure dehydration tower, where the remaining water and light oils are evaporated. Then, the tar is pumped into a tubular furnace and heated to 400–410°C before entering the reaction vessel. The reaction vessel is equipped with a stirring device of special design; the unstable components in coal tar undergo condensation under high temperature and pressure, and the condensable parts from these fractions combine with asphaltenes, thereby modifying the asphalt. The light oil vapor exiting the reactor is cooled by water in a condensation cooler, and the power oil, gas, and condensed water are separated in an oil-water separator. The tar resulting from the reaction is fed into a flash tower under reduced pressure to separate the distillate oils. Superheated steam is introduced from the bottom of the flash tower to adjust the softening point of the asphalt. A side stream is taken from the upper part of the flash tower to separately extract the light oil fraction and the heavy oil fraction. Asphalt is pumped from the lower part of the flash tower and sent to the overhead tank. The liquid asphalt coming out of the sump is sent to the granulator, where it cools and is granulated through direct contact with circulating water, and then is transported to the warehouse by a conveyor. 　　(6) Production technology of needle coke. Depending on the raw material, needle coke is divided into petroleum-based and coal-based types. Petroleum-based needle coke was successfully developed and put into industrial production by Continental Oil Company in the 1960s. Coal-based needle coke was successfully developed and industrialized in the early 1980s by two Japanese companies, Nippon Steel Chemical and Mitsubishi Kasei. There are many methods for producing coal-based needle coke, but only Japan has achieved industrial production, using the solvent method. Research on the production of coal-based acicular coke in China has not yet made any breakthroughs. A brief overview is shown in Table 1. Table 1 Methods for producing coal-based needle coke at home and abroad % Item Overseas Domestic Vacuum extraction method Two-stage method Solvent method Centrifugation method Modification method Solvent method Type of pretreatment method QI value of refined material Yield of refined oil to soft asphalt Technology owner Ⅰ