Summary of experience in temperature-raising passivation of urea using 300,000 tons of CO2 stripping method

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The 300,000-ton urea plant of our plant was completed and put into operation in 2005. It has been operating at full load, long-term, safe and environmentally friendly for two years. During this period, it was overhauled twice, and the maintenance time is generally between 7-10 days. Since the unit was put into operation, it has been shut down three times for various reasons and stopped six times (including the trial production stage). Summary of the temperature rise and passivation of the 300,000-ton urea unit through several starts.: Two routes can be adopted for temperature rise and passivation of the 13,000-ton urea plant.: The first is to add steam and instrument air to the CO2 inlet pipe of the stripping tower, and the second is to add steam and instrument air to the liquid outlet pipe of the synthesis tower. Our factory uses the second method. 2. Several principles that should be mastered during temperature rise passivation: a. Control the heating rate to be less than or equal to 12 degrees per hour ； b. Control the speed of pressure increase (system pressure. steam pressure) ； c. There must be a certain oxygen content, H2O (g) is less than the instrument air pressure ； d. Ensure that there is condensation in every place and prevent the accumulation of condensate. ； e. The instrument air pressure is greater than or equal to 0.6Mpa. 3. The temperature rise rate must be controlled for temperature-raising passivation, because the high-pressure system equipment includes a stripping tower, a synthesis tower, a high-pressure condenser, and a high-pressure scrubber. Except for the synthesis tower, which is a carbon steel shell lined with stainless steel, the rest are carbon steel shells with stainless steel tubes. Because the thermal expansion coefficient of stainless steel is 1.8mm/100 degrees, while that of carbon steel is 1.2mm/100 degrees. If the temperature rises too fast, it will inevitably cause a large temperature difference between the inside and outside of the entire tower, which can easily cause deformation of the lining or tubes. Therefore, the heating rate must be strictly controlled at 10 to 12 degrees. 4. Temperature-raising passivation requires raising the wall temperature of the synthesis tower to 130 to 150 degrees. The purpose is to increase the urea production and conversion rate in the synthesis tower after feeding. 5. The valve on the outlet pipe from the synthesis tower to the high-pressure scrubber must have a certain opening. (Unfinished)