Combination of methanol gas generation and ammonia synthesis processes

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Combination of Methanol Gas Generation and Ammonia Synthesis Processes Author/Source: Zhang Ruquan (Shandong Hongri Akang Chemical Co., Ltd., Linyi 276021, Shandong) 0 Introduction At present, UGI gas generators are still widely used in the domestic production of ammonia and methanol. This gas generation process is characterized by cyclic batch operation, involving processes such as blowing, top blowing, bottom blowing, secondary top blowing, and cleaning. Generally, air is used for blowing instead of oxygen-enriched or pure oxygen, and this has no impact on ammonia synthesis production ； However, for methanol production, reducing the N2 content is crucial, as the presence of N2 not only increases the amount of gas that needs to be vented during methanol synthesis but also **raises energy consumption throughout the process, thereby increasing production costs. How to reduce the presence of inert gases (N2, CH4, etc.) in the process is a key factor in producing methanol using UGI gas generators. Our company built a 100 kt/a ammonia synthesis plant in 1999, equipped with 8 UGI gas furnaces ； In 2006, a 100 kt/a methanol production plant was built and put into operation, equipped with 10 UGI gasifiers. Our company’s technical staff conducted research on how to take advantage of the characteristics of methanol and synthetic ammonia processes to complement each other, during the design phase at the initial stage of building the methanol plant. Given that the inert gas N2 in the methanol system is a feed gas in the ammonia synthesis system, the N2-containing gas (nitrogen-rich gas) accumulated in the equipment and pipelines during the methanol production process is sent to the ammonia synthesis system. This not only reduces the amount of inert gas in the methanol plant but also increases the amount of useful gas in the ammonia synthesis plant, avoiding the uneconomical practices of releasing large quantities of nitrogen-rich gas to reduce inert gas in methanol production or sending it to gas recovery units for combustion. This approach achieves an optimal outcome through the organic integration of the two systems and their complementary use. 1 Main process steps: Each gas production cycle (in the methanol plant) is mainly divided into seven steps, and the specific control and operation procedures (with the corresponding gases at each stage) are as follows. (1) Blowing stage: fan → butterfly valve → blowing valve → gas burner → dust collector → recovery valve → blowing gas recovery device (mixed waste combustion furnace). (2) Early stage of the upward blowing phase (ammonia synthesis removal): Steam storage tank → Main steam valve → Upward blowing valve → Gas furnace → Dust collector → Upward flow valve → Maintenance water seal → Nitrogen-enriched valve → Nitrogen-enriched combined waste boiler → Nitrogen-enriched gas scrubber → Ammonia synthesis gas holder. (3) Later stage of the upward blowing phase (methanol removal): Steam storage tank → Main steam valve → Upward blowing valve → Gas furnace → Dust collector → Upward flow valve → Maintenance water seal → Main coal valve → Combined waste boiler → Gas washing tower → Methanol gas holder. (4) Downward blowing stage: Steam storage tank → Main steam valve → Downward blowing valve → Gas furnace → Downward flow valve → Maintenance water seal → Main coal valve → Combined waste boiler → Gas washing tower → Methanol gas holder. (5) Secondary upward blowing stage (methanol removal): Steam storage tank → Main steam valve → Upward blowing valve → Gas furnace → Dust collector → Upward flow valve → Maintenance water seal → Main coal valve → Combined waste boiler → Gas washing tower → Methanol gas holder. (6) Early stage of the purging phase (methanol supply): fan → butterfly valve → blowing valve → gas furnace → dust collector → upward valve → maintenance water seal → main coal valve → combined waste boiler → gas washing tower → methanol gas tank. (7) Later stage of purging phase (delivery of synthetic ammonia): fan → butterfly valve → blowing valve → gas furnace → dust collector → upward valve → maintenance water seal → nitrogen-enriched waste boiler → nitrogen-enriched scrubber tower → synthetic ammonia gas holder. The water-gas composition of the methanol plant after process integration is shown in Table 1. As can be seen from Table 1, after feeding nitrogen-rich gas into the ammonia synthesis process, the content of effective gases (H2 + CO, excluding CO2) in the water gas is greater than 92%, while the content of inert gases (N2 + CH4) is only about 2%. This figure is very good compared to other gas production methods. Table 1 Composition of water gas in the methanol plant　　　　　　　% The composition of the nitrogen-enriched gas fed to ammonia synthesis is shown in Table 2. For ammonia synthesis, as shown in Table 2, the contents of effective gases (H2 + CO) and N2 in the nitrogen-enriched gas exceed 46% and 39% respectively; the relative volume fraction of N2 is high. During the ammonia synthesis process, the N/H ratio can be adjusted by reducing the amount of nitrogen supplied. Table 2 Composition of Nitrogen-Rich Gas Sent to the Ammonia Synthesis Unit　　　％ After the methanol plant was successfully put into operation for the first time, there was still some capacity remaining in the methanol purification and synthesis sections. Meanwhile, the gas generation furnaces in the ammonia synthesis unit produced insufficient gas due to the use of briquetted coal as fuel. Therefore, it was decided to add two new gas generation furnaces to this unit, and to modify one of the existing furnaces so that it could operate together with the two new furnaces as a single unit ； The gas production process was adjusted with reference to the methanol plant; during the later stage of upper blowing, the lower blowing phase, and the secondary upper blowing phase in this set of gas generation furnaces, the gas produced was sent to the methanol production plant, while the nitrogen-rich gas from the early stage of upper blowing and the cleaning phase was sent to the ammonia synthesis plant. After making process adjustments tailored to the characteristics of each of the two production units, the quality of the water gas became very high; this reduced unnecessary losses. The amount of air released during alcohol synthesis could be reduced from 383 m3 per ton as originally designed to around 230 m3 per ton. The methanol production facility, whose original design capacity was 120 kt/a, was able to achieve an actual production capacity of 140 kt/a. 2 Benefit Estimation 2.1 Comparison of Benefits from Using Nitrogen-Rich Gas to Produce Steam vs. Synthesizing Ammonia 2.1.1 Using Nitrogen-Rich Gas to Produce Steam If 7500 m3/h of nitrogen-rich gas is used in a blast gas recovery system for combustion to generate steam, the steam produced and its associated benefits can be calculated by taking into account the calorific value of the blast gas as well as the components of that gas, as detailed below. CO + 0.5O2 === CO2 + 12650 kJ/m3; H2 + 0.5O2 === H2O + 10805 kJ/m3; CH4 + 2O2 === CO2 + 2H2O + 35960 kJ/m3. The heat values in these equations are expressed per cubic meter of gas. Based on the average contents of each gas listed in Table 2, the heat value per hour of combustion is: Q = QCO + QH2 + QCH4 = 7500 × (21.83% × 12650 + 25.66% × 10805 + 1.42% × 35960) = 41505435 kJ/h. The amount of water at 20 °C (84.0 kJ/kg) that can be converted into steam at 160 °C (2767.2 kJ/kg) using this heat is given by g = 41505435 / (2767.2 – 20) = 15468 kg/h. At a cost of 90 yuan per ton, and ignoring other expenses such as power consumption, material costs, depreciation, and wages, the profit generated can be approximately 1400 yuan/h. 2.1.2 Production of ammonia using nitrogen-rich gas: If this nitrogen-rich gas is used to produce ammonia, the cost of all materials other than coal is approximately 1000 yuan per hour. 7500 m3 of nitrogen-rich gas can be used to produce 7500×(21.83%+25.66%) = 3562 m3 of ammonia. After CO is converted into H2, it can combine with N2 to form NH3 in an amount of 1781 m3; converted to mass, this amounts to 1781 m3×0.771 kg/m3 = 1373 kg ； After processes such as purification, conversion, and decarburization, and taking into account the losses due to off-gases, the overall yield is 88%. Therefore, the amount of ammonia produced is 1373 kg × 88% = 1208 kg. At a price of 3100 yuan per ton for liquid ammonia, the profit generated is (3100 – 1000) × 1.208 = 2536.8 yuan per hour. Compared to using nitrogen-rich gas to generate steam, using it in ammonia synthesis plants to produce ammonia can yield an additional profit of 2536.8 – 1400 = 1136.8 yuan per hour; over the course of a year, this amounts to 1136.8 × 7200 = 8,184,960 yuan. 2.2 Production of methanol using nitrogen-rich gas: If nitrogen-rich gas is used in methanol production facilities, an additional 7500×(40.24%–1.3%) = 2920.5 m3/h of N2 will be introduced. Given that 2750 m3 of feed gas are required to produce 1 ton of methanol, with an electricity consumption of 900 kW·h per ton and an electricity cost of 0.62 yuan/kW·h, this unnecessary N2 will result in wasted electricity costs amounting to 2920.5/2750×900×0.62 = 593 yuan/h. Over a year, this amounts to 593×7200 = 4269600 yuan. In addition, it also increases the losses in the bleed gas portion (whose composition is shown in Table 3). Along with this nitrogen-rich gas, an additional 2920.5/19.4%×(59.3%+5.3%+5.4%) = 10538 m3/h of effective gas must be released ； Although about 90% of H2 can be recovered through the hydrogen extraction process, around 10% of H2 is still lost. Compressing this gas requires 90 kW of electricity, which amounts to approximately 90×0.62 = 55.8 yuan per hour; over the course of a year, this amounts to 55.8×7200 = 401,760 yuan. Table 3 Composition of methanol off-gas　　　　　　　％3 Conclusion In summary, the combination of methanol production and ammonia synthesis processes yields significant benefits, which is highly advantageous for enterprises that use UGI gas generators to produce methanol and ammonia. This is especially important in an environment where there is a focus on energy conservation, cost reduction, and efficiency improvement.