焦化除焦系统腐蚀原因分析及防护措施

Analysis of Causes of Decoking System Corrosion in Delayed Coker and Countermeasures

惠州炼油4200kt/a延迟焦化装置引进美国Foster Wheeler公司的工艺包,采用"两炉四塔"大型化工艺,生焦周期为18h,设计循环比为0.3,焦炭塔顶盖和底盖采用自动卸盖装置。该装置采用单井架水力除焦,冷焦水和切焦水采取一体化处理方式。切焦水补水部分采用新鲜水,净化后的含硫酸性水回用焦池作切焦水补水。由于延迟焦化装置的高压水管道和设备周期性运行,介质压力高,管件壁厚较厚,管件材质较高,运行一段时间后,出现了除焦系统管道和设备腐蚀现象,腐蚀形态主要为应力腐蚀。经分析,回用的净化水氯离子含量偏高及除焦水温相对较高是主要的影响因素。应对措施方面,通过限制不合格净化水和氯离子含量高的净化水排入焦池、降低净化后的酸性水和含油污水的回用水量等,可降低除焦水氯离子浓度。同时,应考虑工艺条件的改进,包括降低除焦水水温、限制管道流速以及设备材质的提高,加强设备检测等,降低除焦设备腐蚀对装置长周期运行的影响。

CNOOC Huizhou Refinery ′ s 4200kt/a delayed coking unit uses Foster Wheeler′s expertise and a 'two-furnace and four-column' process.The coke deposition cycle of the unit is 18 hours and its designed recirculating ratio is 0.3.Both head cover and bottom cover of the coking column are equipped with an automatic cover removing mechanism.Single-derrick hydraulic decoking technology is used in the unit and coke cooling water and coke cutting water are treated in an integrated manner.Fresh water is used to replenish the coke cutting water.Purified sulfur-containing water is circulated back to the coking tank for use to replenish coke cutting water.Since the high-pressure water pipe and equipment of the delayed coking unit operate periodically,medium pressure is high,the wall of pipe fittings is thick and pipe fittings are made of high-quality materials,after some time of operation,corrosion took place in the pipes and equipment of the decoking system.The form of the corrosion is primarily stress corrosion.Assay shows that the higher than normal content of chloridion in recycled purified water and the relatively high temperature of decoking water are the primary influencing factors.On countermeasures,limiting the discharge of substandard purified water and purified water with high chloridion contents into the coking tank and reducing the use of recycled purified acidic water and oily sewage can lower the concentration of chloridion in decoking water.In addition,improvement of process conditions should be considered,including reducing the temperature of decoking water,limiting the flow rate in pipes and improving the quality of materials used for making equipment.Equipment testing should also be strengthened to reduce the effect of corrosion in decoking equipment on the long-term operation of the delayed coking unit.