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.

　?。?）酸洗精餾法

　　(1) Acid pickling and rectification

　　酸洗精餾法一般用濃硫酸為原料，使工業(yè)萘中的硫雜茚、其他不飽和化合物與硫酸發(fā)生磺化反應(yīng)，聚合成樹脂（酸焦油），酚類經(jīng)堿洗而去除。堿洗后的液體萘經(jīng)真空蒸餾，從塔頂采出精萘。在酸洗過程中，由于發(fā)生磺化反應(yīng)而使萘的損失率較高，一般高達(dá)10%以上。酸洗精餾法對硫雜茚的去除率并不高，所生產(chǎn)的精萘一般只能達(dá)到國家二級精萘質(zhì)量標(biāo)準(zhǔn)。酸洗設(shè)備因腐蝕嚴(yán)重需要采用特殊鋼材，投資較大。另外，酸洗產(chǎn)生的廢液和污水難以處理。

　　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.

　?。?）溶劑法

　　(2) Solvent method

　　溶劑法是利用硫雜茚與萘在溶劑中的溶解度差異而加以分離，完成萘的提純過程。溶劑法需要選擇一種選擇性良好的溶劑，一般通過二次萃取就可以得到二級精萘。若需進(jìn)一步精制，還需要進(jìn)行精餾或白土處理。其缺點(diǎn)是所采用的溶劑具有一定的毒性，生產(chǎn)設(shè)備較龐雜，精制效果較差。

　　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.
              

　?。?）催化加氫法

　　(3) Catalytic hydrogenation

　　催化加氫法是由美國環(huán)球石油公司和聯(lián)合石油公司開發(fā)的聯(lián)合精制工藝，是石油化工中常用的方法，也可用于萘的精制。目前在美、英等國建有這種裝置。

　　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.

　　催化加氫法是將工業(yè)萘氣化，在高溫、常壓下與氫混合并通過催化劑層進(jìn)行催化加氫，萘中的主要雜質(zhì)硫雜茚發(fā)生氫解反應(yīng)，轉(zhuǎn)化為硫化氫與烴類，其它非萘雜質(zhì)在加氫過程中，也轉(zhuǎn)化為易分離的氨、水和烴類。同時萘也發(fā)生副反應(yīng)，生成四氫萘等副產(chǎn)物。該工藝的特點(diǎn)是：產(chǎn)品質(zhì)量高，含硫低于300ppm，最低可達(dá)10ppm左右，但由于工藝較復(fù)雜，基建及操作費(fèi)用較高，推廣應(yīng)用較困難。生產(chǎn)過程中有副產(chǎn)物四氫萘生成，限制了這種萘產(chǎn)品應(yīng)用的范圍。

　　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.

　?。?）區(qū)域熔融結(jié)晶法

　　(4) Zone melting crystallization

　　區(qū)域熔融法制取精萘主要是以工業(yè)萘為原料，利用固體萘與其他雜質(zhì)熔點(diǎn)的差別，于精制機(jī)內(nèi)用區(qū)域熔融法進(jìn)行提純，再將已提純的萘送蒸餾塔去精餾，進(jìn)一步除去高沸點(diǎn)及低沸點(diǎn)雜質(zhì)后，即得精萘產(chǎn)品。該方法為連續(xù)生產(chǎn)過程，產(chǎn)品質(zhì)量穩(wěn)定。但是，因其基建投資和操作費(fèi)用高，操作條件要求較嚴(yán)，所以在中國還沒有得到普遍應(yīng)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）分步結(jié)晶法

　　(5) Fractional crystallization

　　分步結(jié)晶法是利用熔融的粗萘在冷卻結(jié)晶時發(fā)生組分在液固兩相間重新分布的原理，經(jīng)過多次熔融結(jié)晶來提純萘的工藝過程，是對萘油餾分加工得到的粗萘進(jìn)行精制的工藝之一。粗萘中的雜質(zhì)一般會降低萘的熔點(diǎn)，所以當(dāng)液態(tài)粗萘緩慢冷卻時萘首先結(jié)晶析出，而雜質(zhì)則聚集在液相中。若析出的萘再次熔融和結(jié)晶，則再次析出的萘結(jié)晶純度又進(jìn)一步提高。嚴(yán)格控制熔融和結(jié)晶過程的溫度和速度，最終可得到高純度的精萘產(chǎn)品。萘分段結(jié)晶精制可以連續(xù)進(jìn)行，也可半連續(xù)或間歇進(jìn)行。工業(yè)萘中的主要雜質(zhì)是硫茹，硫茹的沸點(diǎn)與萘的沸點(diǎn)只差2℃，很難用蒸餾方法除去，而它們的結(jié)晶點(diǎn)相差達(dá)48℃，所以更宜用分段結(jié)晶法把它們分離而使萘得到精制。分步結(jié)晶法制取精萘的特點(diǎn)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）原料單一，不需要輔助原料。

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

　　b）工藝流程、設(shè)備及操作都比較簡單，設(shè)備投資少。

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

　　c）操作時僅需泵的壓送、冷卻結(jié)晶、加熱熔融，操作費(fèi)用和能耗都比較低。

　　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）產(chǎn)品質(zhì)量可用結(jié)晶循環(huán)次數(shù)加以調(diào)節(jié)，靈活性較大。

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

　　e）生產(chǎn)工藝較成熟，產(chǎn)品質(zhì)量穩(wěn)定。

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