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Overview: Before the 50% capacity expansion renovation of the ammonia synthesis unit at the Fertilizer Plant of CNPC Daqing Petrochemical Company in 2005, the demister mesh in the gas-liquid separator at the outlet of the decarburization system was damaged, causing some potassium carbonate solution to be carried into the methanation reactor (106-D) along with the process gas, which resulted in an increase in the pressure difference within the methanation reactor. The decarburization system uses an energy-efficient phenolite process to remove CO2 from the process gas. The decarburized gas passes through a demisting layer at the top of the absorption tower to have its moisture removed, and after the entrained liquid is separated in a separation tank, it enters 106-D via the methanation furnace inlet heat exchangers (136-C, 104-C). After passing through a nickel-based catalyst, it exits from the bottom, where the reaction of CO, CO2, and H2 to form CH4 takes place, reducing the levels of CO and CO2 to below 10 mL/m3, thereby producing purified gas that meets the required standards. The methanation catalyst was loaded in August 2000; it is of model J103H, a pre-reduced catalyst with a total nickel content of ≥12% (of which the percentage of reduced Ni is ≥5), and the amount loaded was 16.91 tons. Because the decarburization system entrains solution, potassium carbonate deposits at the top of the methanation reactor, increasing the bed resistance. In July 2002, the 4t catalyst at the top of the furnace was replaced. In 2005, the pressure difference of the methanation catalyst increased gradually for the same reason, reaching up to 0.2 MPa (referring to the pressure difference between the 106-D inlet and the outlet of the separator tank at the syngas compressor inlet; the same applies hereafter). Since there were no spare catalysts in stock, it was decided to wash the catalyst inside the furnace in order to reduce the pressure difference across the methanation bed. 2 Water washing treatment 2.1 Preparations before water washing 1) Provision of blind plates and valves. After the unit was shut down in 2005, the methanation system was purged with nitrogen to replace the process gas; a blind flange was installed at the inlet of the methanation reactor, and temporary flanges were added at the outlet. No. 1 and No. 2 exhaust valves were installed on top of the methanation reactor, while No. 4 and No. 5 drain valves were added at the outlet. The water washing of the methanation catalyst is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload1/070816908017969.jpg 2) Cool down the catalyst. The methanation catalyst is in a reduced state, and it undergoes an oxidation reaction when in contact with deionized water; therefore, the catalyst temperature must be lowered. This capacity expansion and renovation involves the replacement of the outlet pipeline of the methanation reactor; a blind flange needs to be installed at the outlet flange of the methanation reactor. When the catalyst cools down, nitrogen gas is introduced via tape at valve #5, valve #4 is closed, while valves #5, #3, #2, and #1 are opened. Release is carried out at valves #1 and #2. On September 20, 2005, the temperature of the catalyst bed dropped to room temperature. 3) Water quality analysis. Methanation catalysts are highly susceptible to poisoning, with sulfur, chlorine, or decarburization fluids being particularly toxic to them. Before washing the methanation catalyst, it is necessary to analyze the quality of the water used. To ensure the accuracy and representativeness of such analyses, samples are taken from the top of the large tank containing deionized water in order to examine the impurities present in it. The analysis results for the deionized water sample taken on September 22 showed a pH value of 7.46 ； Cl—, Fe2+, and SO4 2— are all zero. 2.2 Water washing process: On September 23, 2005, tape was attached at the pressure gauge at the outlet of the desalination water pump. Water was fed into 106-D from the No. 5 drain outlet; feeding started at 19:00. At 19:16, a leak occurred at the blind flange at the outlet of the methanation reactor, so the desalinated water that had entered the reactor was drained to replace the gasket. At 15:00 on the 24th, water filling into the methanation reactor resumed; at 19:15, the tape and hoses connected to valves No. 1 and No. 2 at the top of the methanation reactor began to drain water. Water filling was stopped at 20:25, and drainage was initiated from drain No. 4 at the bottom. To increase the drainage rate and prevent air from entering the methanation reactor, nitrogen gas was introduced through valve No. 2 at the top, and drainage was completed at 21:00. Continue to fill nitrogen at the top of 106-D and drain from the bottom. During the water washing period, the potassium ion concentration in the water was analyzed every 15 minutes; the analysis results are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload1/070816908382877.jpg On the 25th at 15:00, the 106-D access hole was inspected, and no lumps of potassium carbonate were found. Seal the access hole, continue to feed nitrogen at the top of 106-D, and vent it at the bottom to displace water vapor. 3 Washing Effect 3.1 Impact on Catalyst Reactivation When a methanation catalyst is washed, it reacts with water, releasing heat that causes the temperature of the catalyst bed to rise slightly. This leads to slow oxidation of the catalyst, and a passivation layer forms on its outer surface, protecting the catalyst from further contact with water and subsequent oxidation reactions. At the start of operation, only the outer layer of the catalyst needs to be reduced; no special reduction process is required – it is sufficient to feed in the process gas that has been degassed and made suitable for use. After the decarburization and low-variation systems were brought back online on October 26, 2005, the methanation system was pressurized at 19:20; the methanation processes were connected at 20:50, and the gas from the methanation outlet met the required standards at 23:00. 3.2 Effect on the pressure difference across the catalyst bed: After the plant was put into operation, the pressure difference in the methanation reactor decreased by 50%, achieving the desired outcome and meeting the requirements for capacity expansion. The comparison of pressure differences before and after washing the methanation catalyst under the same production load is shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload1/070816909185235.jpg 3.3 Effect on catalyst activity: The potassium carbonate solution enters the methanation reactor and forms a hard layer on the top surface of the catalyst; the potassium carbonate powder covers the outer surface of the catalyst, preventing gases from entering its pores. This increases the pressure difference across the catalyst bed, thereby reducing its activity. The catalyst was washed with water to remove most of the potassium carbonate powder covering its surface, thereby restoring its activity; the performance of the methanation catalyst after washing is shown in Table 3. http://www.nmtech.com.cn/jishuwang/upload1/070816909527696.jpg In September 2006, the methanation catalyst was replaced for other reasons; upon inspecting the removed catalyst, it was found that the catalyst particles were largely intact. Before the replacement, the pressure difference across the methanation bed at high loads remained at 0.12–0.14 MPa, indicating that washing had little effect on the strength of the catalyst. 4 Conclusion The water washing of the methanation catalyst was successful; after washing, the catalyst’s activity improved compared to before, and the pressure drop across the catalyst bed was significantly reduced, meeting the production requirements following the capacity expansion upgrade.