Improvements in the production of refined naphthalene using static step crystallization

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The crystallization method relies on the differences in the melting points (crystallization points) of materials to achieve separation that is difficult to accomplish using other separation methods. The production process of pure naphthalene from industrial naphthalene involves using crystallization to remove thiophenanthrene from naphthalene and improve its purity. Currently, the two methods widely used in China are dynamic falling film crystallization and static crystallization. Ansteel Chemical Plant uses the dynamic falling film crystallization method, while Baosteel, Shanghai Coking Plant, and others employ the static crystallization method. 1 Problem Analysis Our company uses the static step-wise crystallization method to produce refined naphthalene. The process generally involves loading the raw material, industrial naphthalene, into a crystallization tank and then rapidly cooling it; once the temperature reaches 82°C, cooling is carried out more gradually at a rate of 2°C per hour until the temperature drops to 60°C. The first batch of naphthalene oil containing thiophene is then extracted and used as an intermediate fraction for further processing. Then, the material in the crystallization tank is heated at a rate of 4–5°C/h, with samples taken every half hour to determine its crystallization point. Depending on this crystallization point, the material is sent to the corresponding fractionation tank. By carrying out 3–4 such steps of fractional crystallization, naphthalene of higher purity can be obtained. However, the colorimetric value of the refined naphthalic acid produced by our company remains consistently high, often around level 10. Shanghai Coking Plant, whose production processes and equipment are roughly similar to those of our company, has a pickling color value for refined naphthalene that does not exceed level 2. Actual investigations have shown that during the crystallization of naphthalene, a layer of colloidal impurities adheres to the crystal surface; if this is not removed, it will inevitably be carried into the corresponding distillation tanks or product tanks. We overlooked this process, assuming that these colloidal substances could be separated through multiple stepwise crystallizations, resulting in a higher colorimetric value for the pure naphthalene product. 2 Improvement measures: When improving the refined naphthalene process, we made use of the existing equipment without making any changes to the pipelines or devices; instead, we adopted a three-step or two-step crystallization method, with each step consisting of six stages: feeding, pre-cooling, crystallization, liquid removal, sweating, and melting. (1) Feeding. The raw material, industrial naphthalene, is pumped into the crystallization tank. (2) Pre-cooling. By adjusting the temperature of the circulating medium in the crystallization tank, the naphthalene inside it is cooled to its crystallization point. At this point, impurities such as thienoindene accumulate in the liquid phase, while naphthalene gradually crystallizes on the crystal tube, with the crystals growing larger over time. (3) Crystallization. As the crystallizer temperature was further reduced, extensive crystallization began to occur; naphthalene formed a network of columnar crystals, while thienoindene existed in the liquid phase surrounding these crystals. At this time, the density of the network crystals should not be too high, in order to facilitate the removal of the liquid phase within the crystals. (4) Drainage. The liquid phase enriched with impurities such as thienoindene flows out of the crystallization tank on its own and is discharged into the residual liquid tank. (5) Inducing sweating. Also known as partial melting, it refers to the gradual increase in the temperature of the heat transfer medium in the crystallization tank, which causes the crystals within the tank to be heated and partially melted. As the temperature rises, the low-melting-point impurities contained within the crystal melt first and gradually penetrate to the outer surface of the crystal; a small amount of naphthalene also melts, and the film of impurities on the crystal surface is gradually replaced. The melted liquid flows out of the crystallization tank on its own and is discharged into the corresponding fractionation tank. (6) Melting. Once the crystallization point of the sample meets the requirements specified in the procedure, heating is increased rapidly until all the material in the crystallization chamber melts; feed can be added again as needed. After these improvements, the rate of first-class pure naphthalene reached 100%, with a product yield of around 90%.