Operation problems and countermeasures of catalytic desulfurization and denitration equipment – Corrosion protection for HH flue gas desulfurization towers

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Catalytic cracking is a major unit in oil refining plants. As environmental regulations become stricter, the levels of NOx, SOx, and dust in flue gas containing catalytic agents no longer meet the environmental emission standards; therefore, measures must be taken for desulfurization, denitration, and dust removal. To this end, in recent years, catalytic units at petrochemical refining companies in China and Japan have successively been equipped with desulfurization and denitration sections to meet environmental emission requirements. However, the newly installed desulfurization and denitration equipment suffers from immature technology and low reliability; equipment failures occur frequently during operation, making it difficult to meet the requirements for long-term operation. 1. Operating conditions under standby mode for desulfurization and denitrification in catalytic units: The feed oil used in catalytic cracking units typically contains sulfur-containing compounds such as thiol, thioether, cyclic thioether, thiophenol, and thienophenol [1]. During the catalytic cracking process, some of these sulfur-containing compounds are converted into condensates with larger molecular weights that remain attached to the coke. These sulfur-containing coals undergo oxidation reactions with oxygen present in the main airflow in the regenerator, resulting in the formation of flue gas; therefore, flue gas contains Sox ； The nitrogen-containing compounds in the catalytic feedstock result in nitrogen oxides N○x being present in the flue gases after combustion ； Since catalytic cracking is a reaction that occurs in the presence of a catalyst, the flue gas also contains dust particles. Currently, the concentrations of NOx, SOx, and dust in the flue gases from catalytic cracking vary significantly. Under standard conditions, the SOx concentration ranges from 700 to 4500 mg/m3, the NOx concentration ranges from 50 to 400 mg/m3, and the dust particle concentration ranges from 150 to 200 mg/m3. After desulfurization and denitration treatments, it is necessary to meet the requirements of the current **emission standards, namely the ‘Emission Standards for Pollutants in the Petroleum Refining Industry’. The SO2 concentration in the flue gas must be less than 100 mg/m3 under standard conditions, the dust concentration must not exceed 50 mg/m3, and the NOx concentration must be less than 200 mg/m3. In accordance with the newly established emission standards GB31570‑2015 \"Emission Standards for Pollutants from Petroleum Refining Industries\", as of July 1, 2017, the allowable limits for air pollutant emissions (under standard conditions) are as follows: SO2 concentration must be less than 50 mg/m3, dust concentration must not exceed 30 mg/m3, and NOx concentration must be less than 100 mg/m3 ； Environmental requirements for desulfurization and denitration will become stricter. 1.1 Desulfurization and denitrification technologies for catalytic units 1.1.1 Desulfurization technology for catalytic units At present, 27 enterprises under Sinopec are equipped with a total of 46 sets of catalytic regenerative flue gas desulfurization and denitrification units. In terms of desulfurization technology, 20 units employ the EDV wet scrubbing desulfurization method ； The 16-set capacity utilizes a dual-circulation turbulent venturi desulfurization technology ； 3 sets of units use sulfur transfer agents for desulfurization ； 3 The desulfurization process uses the sodium-calcium double-alkali method ； 2 sets of units use regenerative wet flue gas desulfurization ； 1 set of the units uses the wet limestone/quicklime-gypsum process for pneumatic desulfurization ； One set of the units uses the ammonia desulfurization method, as shown in Figure 1. 1.1.2 Denitration technology in catalytic devices: Among the 46 desulfurization and denitration units, 14 of them do not have a denitration unit, as the NOx emissions in the flue gas meet the specified standards ； The 32 sets are equipped with separate denitration units; 21 of them use the SCR ammonia-based selective catalytic reduction denitration technology, while 11 use the ozone oxidation denitration technology. Technical statistics are shown in Figure 2. Chimneys can be made of all-stainless steel plates. For existing equipment, continuing to use anti-corrosion coatings sprayed inside HH flue gas desulfurization towers is an option; there are also successful and unsuccessful cases of using materials such as plastic linings and polyurea. Regardless of the approach chosen, it is essential to ensure high construction quality. For the impellers and pipelines of the slurry circulation pump, stainless steel can be used in place of carbon steel lined with PTFE, in order to improve wear resistance and construction quality. For instruments in corrosive environments, the corrosion resistance of the material must be considered; stainless steel can be used as a substitute for carbon steel. The anti-dew point corrosion coating technology specifically designed for HH flue gas desulfurization towers has been used by more than 20 companies in China’s petrochemical industry, and it enjoys a good reputation. (In brief) In summary, the desulfurization and denitrification units of the catalytic device have not been in operation for long; there are inherent defects in their design and manufacturing, and coupled with a lack of experience in operating and maintaining such equipment, the failure rate is relatively high. Only by strictly controlling the construction quality during manufacturing and installation, continuously analyzing the causes of frequent problems that arise during operation, and summarizing the effective experiences in resolving such issues, can it be possible to continuously improve and enhance the long-term operational performance of the equipment.