Benzene series produced by coking coal in the dry distillation process of coke oven escape with raw gas. Crude benzene is an important raw material in organic chemical industry, and its recovery has high economic benefits.
The content of crude benzene in coke oven gas is generally 25 ~ 40g/m3. The yield of crude benzene is related to coal charging quality, coking temperature and headspace temperature of coke oven. That is, the yield of crude benzene increases with the increase of volatile matter in coal, and decreases with the increase of coking temperature and furnace top space temperature. Usually 0.9% ~ 1.3% dry coal.
There are two main technical requirements for crude benzene products: one is moisture, which requires that there is no visible insoluble water at room temperature; The second is the determination of distillation range of crude benzene products. When crude benzene is used as solvent, the distillation yield before 180℃ should be greater than 9 1%, and when crude benzene is used for refining, the distillation yield before 180℃ should be greater than 93%.
The main components of crude benzene are aromatic hydrocarbons such as benzene, toluene, xylene and trimethylbenzene, in addition to unsaturated compounds, sulfur-containing compounds, aliphatic hydrocarbons, naphthalene, phenols and pyridine compounds.
Generally, tar washing oil is used as absorbent to recover crude benzene from coke oven gas, and its process includes washing and distillation.
1, crude benzene washing
At 25 ~ 27℃, the coke oven gas passes through a series of benzene washing towers, and contacts with tar washing oil sprayed at the top of the tower in countercurrent, and is discharged from the top of the tower after crude benzene is removed. The rich oil with crude benzene content of about 2.5% discharged from the bottom of the tower is sent to the distillation unit for benzene removal. The lean oil after debenzylation contains 0.2% ~ 0.4% benzene, which is cooled to 27 ~ 30℃ and then sent to the benzene washing tower for recycling.
2. Crude benzene distillation
Before 1980s, most coking plants in China used the steam-heated oil-rich debenzylation process of the former Soviet Union, which was characterized by large steam consumption, huge equipment, poor product quality and backward technology.
During the period of 1976 ~ 1979, China's self-designed benzene removal units were built in Xin (Xinyu) Iron and Steel Coking Plant and Jigang Coking Plant respectively. After tackling key problems, debugging and trial operation, 30-story single tower benzene removal and 45-story single tower benzene removal were put into production.
At present, the process of heating oil-rich benzene with tube furnace is widely used in coking plants in China. The process can include two towers to produce light benzene and heavy benzene, and one tower to produce rich oil from crude benzene washing process. The rich oil enters the dehydration tower after being heated by oil-steam heat exchanger and oil-oil heat exchanger, and the oil-gas-water mixture escaping from the top of the tower is sent to the oil-water separator after passing through the condenser.
The rich oil discharged from the bottom of the dehydration tower is pumped into the tube furnace, heated to 180 ~ 190℃ and then enters the benzene removal tower.
1% ~ 1.5% rich oil pump enters the oil washing regenerator, and superheated steam heated by tube furnace is directly steamed and blown. Oily steam is discharged from the top of the regenerator to the lower part of the benzene removal tower, and the residue is discharged from the bottom of the regenerator.
Crude benzene vapor escaping from the top of benzene stripper is cooled by oil-gas heat exchanger, condensed by condenser and separated from oil and water, and then sent to crude benzene intermediate tank.
A part of crude benzene is sent to the top of the benzene stripper for reflux, and the rest is used as raw materials for two benzene stripper.
The hot lean oil discharged from the bottom of the benzene stripper enters the hot lean oil tank under the tower through the oil-oil heat exchanger, is cooled by the extraction lean oil cooler, and then enters the washing system for recycling.
Crude benzene is fractionated by two benzene towers, and the light benzene vapor escaping from the top of the tower enters the reflux tank through the condenser and oil-water separator.
Part of light benzene is sent to the top of diphenyl tower for reflux, and the rest is products. Refined heavy benzene is led out from the side line of the tower, and naphthalene solvent oil is discharged from the bottom of the tower.
Benzene and naphthalene can be removed simultaneously by heating rich oil in tube furnace. The temperature of oil and gas outlet at the top of the benzol stripper is kept at 103 ~ 105℃. In the process of oil-rich benzene removal, naphthalene is also evaporated and dissolved in crude benzene, and discharged from the bottom of the benzene removal tower in the form of naphthalene solvent oil. The naphthalene content of lean oil is less than 2%, which can greatly reduce the naphthalene content of gas after benzene washing tower.
(2) Oil-rich debenzylation process for producing crude benzene by single tower heating in tubular furnace. A single tower for crude benzene production has 30 layers of benzene removal towers. Crude benzene is produced by reflux at the top of the tower, naphthalene oil is extracted from the side line, and the bottom of the tower is lean oil.
There are three methods to produce light benzene and heavy benzene from crude benzene and single tower.
(2) Crude benzene refining technology
At present, the refining of crude benzene is generally divided into two categories: pickling and hydrogenation.
(1) pickling method. Pickling is a traditional method for refining crude benzene in China, which is purified by sulfuric acid washing. Because this method has the advantages of simple process flow, flexible operation, simple equipment, easily available raw materials and operation at normal temperature and pressure, it is still a feasible method for refining crude benzene for small and medium-sized coking enterprises. Therefore, at present, most coking plants in China still use pickling as the refining method of crude benzene. However, compared with hydrogenation, this method has many fatal shortcomings that are difficult to overcome. Because unsaturated compounds and sulfides generate dark brown deep polymer (acid tar) under the action of sulfuric acid, there is no effective treatment method so far. In addition, the product quality and product yield can not be compared with hydrogenation, and it is gradually being replaced by hydrogenation.
(2) Hydrorefining. Based on many shortcomings of pickling method, China has been engaged in the research and development of hydrotreating of coking crude benzene since the early 1970s, and developed medium-temperature hydrogenation method and low-temperature hydrogenation method. The advantage of medium temperature hydrogenation is that high purity benzene can be obtained without extractive distillation. 1976, Beijing Coking Plant adopted the medium-temperature crude benzene hydrogenation technology of Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, and built the first domestic industrial test device with an annual processing capacity of 25,000 tons of crude benzene. In 1980s, the research and development of crude benzene hydrofining process at low temperature (300 ~ 370℃) was carried out. In 1990s, China introduced Japanese Litol process to produce high-purity benzene in Baosteel Phase I Project and Shenma Project in Henan Province. Shijiazhuang Coking Plant and Baosteel Phase III Project successively introduced German K.K. technology. Zhejiang Yangmei International Petrochemical and Pharmaceutical Engineering Design Co., Ltd. successfully developed domestic gas-phase hydrogenation technology in 2004 on the basis of digesting and absorbing similar foreign devices, and aimed at this problem, it was obtained in the near 10 crude benzene hydrofining unit of Shanxi Taihua Co., Ltd., the15,000 t/a crude benzene hydrofining project of Shandong Zaokuang Group and the 80,000 t/a crude benzene hydrofining project of Shandong Lihai Chemical Co., Ltd. The hydrogenation reaction of light benzene reduced the coking and blocking problem of hydrogenation system caused by unsaturated hydrocarbons in crude benzene, improved the adaptability to raw materials, reduced the hydrogenation load and optimized the hydrogenation process.