Analysis and treatment of high pressure difference in cobalt-molybdenum hydrogenation reactor

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1 Analysis of the reasons for the increase in pressure difference: After a 50% capacity expansion and renovation of the ammonia synthesis unit at the Fertilizer Plant of CNPC Daqing Petrochemical Branch on October 2005, the cobalt-molybdenum hydrogenation reactor (101-D) operated for 7 months, during which the pressure difference across the catalyst bed reached 0.22 MPa (the design value being 0.05 MPa). At 85% load, the pressure at the outlet of the feed gas compressor reached the design value; however, the high pressure difference in reactor 101-D became a bottleneck in production. 1) By comparing the components of oilfield gas before and after the renovation in 2005, as well as the temperature rise during the 101—D reaction and the concentrations of the reactants H2S and S, it can be seen from the data that the catalyst in this furnace exhibits good reaction efficiency with normal activity ； However, after the 50% capacity expansion and renovation in 2005, the CO2 content increased significantly, as shown in Table 1. Moreover, every 1% increase in CO2 raises the bed temperature by 30°C, and the high temperature in the bed leads to the hydrodecomposition of hydrocarbons and carbon deposition. http://www.nmtech.com.cn/jishuwang/upload1/070816902165945.jpg 2) Oil field gas contains small amounts of olefins, CO, N2, H2O, and high-carbon hydrocarbons; these olefins polymerize on the surface of the catalyst to form polymeric compounds, which block the pores in the catalyst and increase the resistance of the bed. CO and H2O may react to produce carbon black, while a small amount of N2 undergoes competitive adsorption, affecting the catalyst’s activity ； A small amount of high-carbon hydrocarbons decomposes at the top of the bed, forming carbon deposits. Analysis was conducted for this purpose, and the data are shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload1/070816903137180.jpg 3) The raw material gas coil suffers from uneven heating, resulting in high local temperatures and carbon deposition; the carbon black is carried by the raw material gas to the top of 101-D where it is captured by the carbon black catcher, thereby increasing the pressure difference across the bed. In previous years, when replacing the 101-D catalyst, it was found that a large amount of carbon black was adsorbed on the top carbon black catcher and in the upper part of the catalyst bed. 4) After the 50% capacity expansion renovation, the operating pressure of the process increased and the gas flow rate rose. Since the catalyst used in this furnace is a new product utilized for the first time in our factory, it is possible that fluctuations in the process gas led to the breakdown of the catalyst. To this end, a comparative analysis of the old and new catalysts was conducted, with the data shown in Table 3. The results showed that the strength of both the new and old catalysts was higher than the industry standards. http://www.nmtech.com.cn/jishuwang/upload1/070816904062288.jpg 5) Pipeline corrosion: iron powder is captured by the catcher, creating resistance. The capacity expansion and renovation took 2 months; there were many points where the equipment and pipelines in the front section needed to be modified, and it was possible that welding slag ended up in vessel 101-D. Additionally, no protective measures were taken during the maintenance period, resulting in corrosion of the pipelines in the front section as well as the raw material gas coils. After the plant was put back into operation, the volume and flow rate of the process gas increased, causing rust to accumulate on top of the catalysts. 2. Treatment for high pressure difference: Treatment was carried out on September 7, 2006; it was found that the interior and exterior of the collector were filled with black powder, and this powder gradually decreased as one moved downward from the top of the catalyst, with no obvious signs of catalyst fragmentation. Collect powder samples, sieve them through a screen, and analyze the weight of dust contained in the samples. The calculated bulk density is 1.21 kg/L, and the true density is 1.25 kg/L. The components were analyzed using X-ray fluorescence spectroscopy as shown in Table 4. http://www.nmtech.com.cn/jishuwang/upload1/070816904386089.jpg Upper-layer catalyst samples were collected, and XRD spectral comparisons were conducted with the unused catalyst samples, as shown in Figures 1 and 2. It can be seen that the crystal structure of the catalyst has not changed. http://www.nmtech.com.cn/jishuwang/upload1/070816905164292.jpg Therefore, based on the above data, the causes are corrosion of the front pipelines, accumulation of iron oxide and sulfur, as well as carbon deposition on the catalyst; the catalyst’s activity is not significantly affected. It was decided to replace the top catalyst by 1.5m, which resolved the issue of high pressure difference. 3 Effect analysis: By replacing the catalyst at a height of 1.5 m according to the dust content, unit 101-D operated well on September 20, 2006, with a bed pressure difference of 0.04 MPa, thereby overcoming the bottleneck that prevented the device from operating at full capacity. Furthermore, a cyclone separator is designed at the 101-D inlet to completely eliminate the factors causing high pressure differences, which can extend the catalyst’s service life by 2–3 years and generate significant economic benefits.