In addition to naphthalene, industrial naphthalene also contains azaindene, phenols, quinolines and other unsaturated compounds. In order to obtain refined naphthalene, industrial naphthalene needs to be further refined. The traditional industrial naphthalene refining process is pickling distillation. 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) Pickling distillation

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。

Pickling distillation generally uses concentrated sulfuric acid as raw material to sulfonate azaindene and other unsaturated compounds in industrial naphthalene with sulfuric acid to polymerize into resin (acid tar), and phenols are removed by alkali washing. The liquid naphthalene after alkali washing is vacuum distilled to extract refined naphthalene from the tower top. In the pickling process, the loss rate of naphthalene is high due to sulfonation reaction, which is generally more than 10. The removal rate of azaindene by pickling distillation is not high, and the refined naphthalene produced can only meet the national secondary refined naphthalene quality standard. The pickling equipment needs to use special steel due to serious corrosion, and the investment is large. In addition, the waste liquid and sewage produced by pickling are difficult to treat.

（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 uses the solubility difference 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 required, rectification or clay treatment is also required. The disadvantage is that the solvent used has certain toxicity, complex production equipment and poor refining effect.

（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 global petroleum company and United Petroleum Company. It is a common method in petrochemical industry and can also be used for the refining of naphthalene. At present, such devices are 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，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 it with hydrogen at high temperature and atmospheric pressure, and conduct catalytic hydrogenation through the catalyst layer. The main impurity of naphthalene, azaindene, 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 to produce by-products such as tetrahydronaphthalene. The process is characterized by high product quality, sulfur content is lower than 300ppm and can be as low as about 10ppm. However, due to the complex process and high capital construction and operation costs, it is difficult to popularize and apply. Tetrahydronaphthalene, a by-product, is produced in the production process, which limits the application range 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。

Refined naphthalene produced by regional melting method mainly takes industrial naphthalene as raw material, uses the difference in melting point between solid naphthalene and other impurities, purifies it in the refining machine by regional melting method, and then sends the purified naphthalene to the distillation tower for rectification. After further removing high boiling point and low boiling point impurities, refined naphthalene products are 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 cost 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。

The step-by-step crystallization method is a process of purifying naphthalene through multiple melt crystallization based on the principle that the components of molten crude naphthalene are redistributed between liquid and solid phases during cooling crystallization. It is one of the processes for refining crude naphthalene obtained from naphthalene oil fraction. Impurities in crude naphthalene generally reduce the melting point of naphthalene, so when the liquid crude naphthalene cools slowly, naphthalene crystallizes first, and impurities accumulate in the liquid phase.

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，终可得到高度的精萘产品。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：

If the precipitated naphthalene is melted and crystallized again, the crystallization purity of the precipitated naphthalene is further improved. By strictly controlling the temperature and speed of melting and crystallization process, a high degree of refined naphthalene products can be obtained. Naphthalene fractional crystallization refining 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, which is difficult to be removed by distillation, and the phase difference of their crystallization point is up to 48 ℃, so it is better to separate them by segmented crystallization to refine naphthalene. The characteristics of refined naphthalene prepared by step crystallization are as follows:

a）原料单一，不需要辅助原料。

a) Single raw materials, no auxiliary raw materials.

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) During operation, only pump pressure, cooling crystallization, heating and melting are required, 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.

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