The content of naphthalene in coal tar is about 8% ~ 12%. Besides naphthalene, industrial naphthalene also contains thioindene, phenols, quinolines and other unsaturated compounds. In order to obtain refined naphthalene, it is necessary to further refine industrial naphthalene. The traditional refining process of industrial naphthalene is acid pickling rectification. The gradually developed process methods include solvent method, catalytic hydrogenation method, regional melting crystallization method and step-by-step crystallization method.
(1) Acid pickling and rectification
In acid pickling and rectification process, concentrated sulfuric acid is used as raw material to sulfonate thioindene and other unsaturated compounds in industrial naphthalene with sulfuric acid to polymerize into resin (acid tar), and phenols are removed by alkali washing. After alkali washing, the liquid naphthalene is distilled in vacuum and refined naphthalene is extracted from the top of the tower. In the process of acid pickling, the loss rate of naphthalene is high due to sulfonation reaction, which is generally higher than 10%. The removal rate of thioindene by acid pickling and distillation is not high, and the refined naphthalene produced can only reach the national secondary refined naphthalene quality standard. The pickling equipment needs to use special steel because of serious corrosion, so the investment is large. In addition, it is difficult to treat the waste liquid and sewage produced by pickling.
(2) Solvent method
Solvent method is to use the difference of solubility between azaindene and naphthalene in solvent to separate and complete the purification process of naphthalene. Solvent method needs to choose a solvent with good selectivity. Generally, secondary refined naphthalene can be obtained by secondary extraction. If further refining is needed, rectification or clay treatment is needed. The disadvantage is that the solvent used has certain toxicity, the production equipment is complex, and the refining effect is poor.
(3) Catalytic hydrogenation
Catalytic hydrogenation is a combined refining process developed by universal oil company and United Petroleum Company. It is a common method in petrochemical industry and can also be used for naphthalene refining. At present, such devices have been built in the United States, Britain and other countries.
The catalytic hydrogenation method is to gasify industrial naphthalene, mix with hydrogen under high temperature and normal pressure, and conduct catalytic hydrogenation through the catalyst layer. The main impurity in naphthalene, thioindene, is hydrogenated and converted into hydrogen sulfide and hydrocarbons. Other non naphthalene impurities are also converted into easily separated ammonia, water and hydrocarbons in the hydrogenation process. At the same time, naphthalene also has side reactions, which produce tetralin and other by-products. The characteristics of this process are: the product quality is high, the sulfur content is less than 300ppm, and the lowest is about 10ppm. However, due to the complexity of the process and the high cost of infrastructure and operation, it is difficult to popularize and apply it. Tetralin is a by-product in the production process, which limits the application of this naphthalene product.
(4) Zone melting crystallization
Refining naphthalene by regional melting method mainly takes industrial naphthalene as raw material, uses the difference of melting point between solid naphthalene and other impurities, purifies the refined naphthalene in the refining machine, and then sends the purified naphthalene to the distillation tower for rectification, and further removes the impurities with high boiling point and low boiling point, the refined naphthalene product is obtained. The method is a continuous production process and the product quality is stable. However, it has not been widely used in China because of its high infrastructure investment and operation costs and strict requirements for operation conditions.
(5) Fractional crystallization
Fractional crystallization is a process of refining crude naphthalene from naphthalene oil by multiple melting crystallization based on the principle of redistributing components between liquid and solid during cooling crystallization. The impurities in crude naphthalene generally reduce the melting point of naphthalene, so when the liquid crude naphthalene is cooled slowly, the naphthalene crystallizes first, and the impurities gather in the liquid phase. If the precipitated naphthalene is melted and crystallized again, the purity of the precipitated naphthalene will be further improved. High purity naphthalene can be obtained by strictly controlling the temperature and speed of melting and crystallization. The fractional crystallization of naphthalene can be carried out continuously, semi continuously or intermittently. The main impurity in industrial naphthalene is thioru. The boiling point of thioru is only 2 ℃ different from that of naphthalene. It is difficult to remove it by distillation. However, the phase difference of their crystallization points is up to 48 ℃. Therefore, it is better to separate them by fractional crystallization method to refine naphthalene. The characteristics of preparation of refined naphthalene by fractional crystallization are as follows:
a) The raw material is single, and no auxiliary raw material is needed.
b) The process flow, equipment and operation are relatively simple, and the equipment investment is less.
c) The operation only needs the pump's pressure feeding, cooling crystallization, heating and melting, and the operation cost and energy consumption are relatively low.
d) The product quality can be adjusted by the number of crystallization cycles.
e) The production process is mature and the product quality is stable.