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）酸洗精馏法

　　(1) Acid pickling and rectification

　　酸洗精馏法一般用浓硫酸为原料，使工业萘中的硫杂茚、其他不饱和化合物与硫酸发生磺化反应，聚合成树脂（酸焦油），酚类经碱洗而去除。碱洗后的液体萘经真空蒸馏，从塔顶采出精萘。在酸洗过程中，由于发生磺化反应而使萘的损失率较高，一般高达10%以上。酸洗精馏法对硫杂茚的去除率并不高，所生产的精萘一般只能达到国家二级精萘质量标准。酸洗设备因腐蚀严重需要采用特殊钢材，投资较大。另外，酸洗产生的废液和污水难以处理。

　　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）溶剂法

　　(2) Solvent method

　　溶剂法是利用硫杂茚与萘在溶剂中的溶解度差异而加以分离，完成萘的提纯过程。溶剂法需要选择一种选择性良好的溶剂，一般通过二次萃取就可以得到二级精萘。若需进一步精制，还需要进行精馏或白土处理。其缺点是所采用的溶剂具有一定的毒性，生产设备较庞杂，精制效果较差。

　　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）催化加氢法

　　(3) Catalytic hydrogenation

　　催化加氢法是由美国环球石油公司和联合石油公司开发的联合精制工艺，是石油化工中常用的方法，也可用于萘的精制。目前在美、英等国建有这种装置。

　　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.

　　催化加氢法是将工业萘气化，在高温、常压下与氢混合并通过催化剂层进行催化加氢，萘中的主要杂质硫杂茚发生氢解反应，转化为硫化氢与烃类，其它非萘杂质在加氢过程中，也转化为易分离的氨、水和烃类。同时萘也发生副反应，生成四氢萘等副产物。该工艺的特点是：产品质量高，含硫低于300ppm，最低可达10ppm左右，但由于工艺较复杂，基建及操作费用较高，推广应用较困难。生产过程中有副产物四氢萘生成，限制了这种萘产品应用的范围。

　　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）区域熔融结晶法

　　(4) Zone melting crystallization

　　区域熔融法制取精萘主要是以工业萘为原料，利用固体萘与其他杂质熔点的差别，于精制机内用区域熔融法进行提纯，再将已提纯的萘送蒸馏塔去精馏，进一步除去高沸点及低沸点杂质后，即得精萘产品。该方法为连续生产过程，产品质量稳定。但是，因其基建投资和操作费用高，操作条件要求较严，所以在中国还没有得到普遍应用。

　　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）分步结晶法

　　(5) Fractional crystallization

　　分步结晶法是利用熔融的粗萘在冷却结晶时发生组分在液固两相间重新分布的原理，经过多次熔融结晶来提纯萘的工艺过程，是对萘油馏分加工得到的粗萘进行精制的工艺之一。粗萘中的杂质一般会降低萘的熔点，所以当液态粗萘缓慢冷却时萘首先结晶析出，而杂质则聚集在液相中。若析出的萘再次熔融和结晶，则再次析出的萘结晶纯度又进一步提高。严格控制熔融和结晶过程的温度和速度，最终可得到高纯度的精萘产品。萘分段结晶精制可以连续进行，也可半连续或间歇进行。工业萘中的主要杂质是硫茹，硫茹的沸点与萘的沸点只差2℃，很难用蒸馏方法除去，而它们的结晶点相差达48℃，所以更宜用分段结晶法把它们分离而使萘得到精制。分步结晶法制取精萘的特点如下：

　　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）原料单一，不需要辅助原料。

　　a) The raw material is single, and no auxiliary raw material is needed.

　　b）工艺流程、设备及操作都比较简单，设备投资少。

　　b) The process flow, equipment and operation are relatively simple, and the equipment investment is less.

　　c）操作时仅需泵的压送、冷却结晶、加热熔融，操作费用和能耗都比较低。

　　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）产品质量可用结晶循环次数加以调节，灵活性较大。

　　d) The product quality can be adjusted by the number of crystallization cycles.

　　e）生产工艺较成熟，产品质量稳定。

　　e) The production process is mature and the product quality is stable.