A Study In Material Selection And Failure Mechanism For Cyanuric Chloride Plant Polymerization Reactor

To accommodate industrial development and upgrade the out-of-date the cyanuric chloride production process in China, a new gas-phase one-step cyanuric chloride process technology of high output, low consumption, environmental-friendly and high product quality is developed. The development activities rely on inhouse resources, and with eight years efforts, the process technology was industrialized, a pilot plant was built and on-spec product was produced.To keep pace with process development, material selection for the main process equipment was started early at the beginning of conceptual design. During single tube test, such materials as pure nickel, Incoloy 800H, 2RK65, 2RE60 and lCrl8Ni9Ti are selected and a coupon test is performed at different positions of the single tube test facility and similar facility. Preliminary conclusion of the coupon test is that: pure nickel is the optimum material to fabricate chlorination and polymerization reactors in the one-step cyanuric chloride process; Incoloy800H/2RK65 may be used in the fabrication of chlorination and polymerization reactors especially for the polymerization reactor, control of water content in the medium is helpful for reducing corrosion rate of the material and extending equipment lifetime. Considering the pilot plant progress and bearing in mind the principle of reliability and safety, it is decided to use nickel as the material of construction for chlorination and polymerization reactors in the pilot plant. Further material test will be performed after the pilot plant is brought on stream and in normal operation. Whereas the operating temperature of thechlorination and polymerization reactor is at 340℃~400℃, pure nickel has a tendency of graphitization at this temperature as per ASME code. To prevent graphitization from occurring, low carbon nickel with 0.02% carbon content, i. e. N201 in ASME code, should be selected as the material for the tubes in chlorination and polymerization reactor in commercial production plant.After 8000 hours intermittent operation producing 160 tons of product, routine maintenance of the pilot plant discovered that there was molten salt bleeding at the joint between upper tube sheet and tubes in the polymerization reactor. Sampling at the position of damage indicates that crystal boundary S enrichment is the main cause for the damage of polymerization reactor, which is caused directly by stress corrosion. Source of crystal boundary S may be the medium, catalyst or contamination by impurity element in the fabrication process. Crystal boundary S enrichment makes the crystal boundary easy to be corroded, reduces the corrosion resistant performance of nickel, resulting in severe intergranular corrosion. Stress accelerates the occurrence of rupture. Intergranular sulfur damages continuity of the metal, resulting in intergranular weakening, making the material fragile, thus the strength and specific elongation are decreased. Under the action of stress, the crystal boundary is easy to be broken, which accelerates the expansion rate of intergranular corrosion crack.Therefore, medium to the reactor should be purified, making it free of sulfur and sulfides as well as water vapor. Quality control in the fabrication process should be enhanced, and contamination should be minimized. Equipment should be operated strictly in accordance with operating instructions, the number of shutdowns should be minimized, and water vapor should be prevented from entering into the equipment. Depth of fusion welding between tube sheet and tubes should be deeper.