Analysis of the theoretical basis and technical advantages of the full low-temperature transformation process reform

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The Hubei Provincial Institute of Chemistry began researching the spherical sulfur-resistant shift catalyst, namely B302Q, in 1984. This catalyst was first applied to the medium-string low process in small and medium-sized fertilizer plants, and in 1988, the transformation of the medium-string low process was completed in China’s first medium-sized plant. Regarding the next step of deciding whether to use a process with 2 low-variation medium-low-low stages or a process with 3 low-variation full-low-variation stages, after comparison it was determined that the medium-low-low approach is essentially still a medium-low variation process; therefore, the next research goal was set as adopting a full-low-variation process. Industrial trials for this research began in 1989; through 18 years of development and improvement, the full low-variation process technology evolved from an immature stage to a mature one. It has now been successfully applied in more than 200 units of various sizes both domestically and internationally, making positive contributions to energy savings and consumption reduction in industries such as fertilizers, as well as to the development of the methanol industry. As one of the inventors of this technology, the author intends to summarize the theoretical basis and technical advantages of this process modification on the occasion of the successful implementation of 200 cases using the full low-variation process, for reference by industry decision-makers and technicians. 1 Development process of the all-low-variation process. The research and development process of the all-low-variation process can be roughly divided into the following 4 stages. (1) Phase 1 consisted of theoretical research, research on protective agents, and industrial experimentation; it took place in 1989, with the first industrial test being conducted at the Hubei Yunxian Fertilizer Factory, yielding partial results. (2) The second phase was the initial stage of industrial application, spanning from 1990 to 1991. The first industrial application was carried out at the fertilizer factory in Lichuan City, Hubei, and in the same year, industrial applications were also conducted at 11 other fertilizer factories in Wuhan and Qichun in Hubei, as well as in Huaiyin, Xuyi, and Jinhu in Jiangsu. All the aforementioned plants were upgraded through simple modifications based on the original medium-string low or medium-transform process. Although it passed the technical evaluation, there were still 3 issues with this technology at that time: ① The equipment was not compatible ； ②Imperfect process ； ③The catalyst suffers from deactivation issues. (3) Phase 3 is the technical completion phase, which took place in 1992, when the full low-variation process based on the first set of standard designs was implemented at the Danyang Fertilizer Plant in Jiangsu. The plant has one catalyst furnace that has been in use continuously for 12 years, with a steam consumption per ton of ammonia of ≤250 kg. The successful implementation of the full low-temperature conversion process at Jiangsu Danyang Fertilizer Factory indicates that all issues related to this technology – from process design and equipment selection to catalyst deactivation – have been resolved. It also proves that the full low-temperature conversion technology has evolved from an immature state to a more mature one. (4) Phase 4 is the technology dissemination phase, spanning from 1993 to the present, during which the full low-temperature conversion technology began to be widely adopted by small and medium-sized nitrogen fertilizer manufacturers. Over the past 5 years, with the development of the methanol industry, full low-temperature shift technology has been widely adopted in this field. Through numerous applications in various processes, this technology has continued to improve, with advancements such as the elimination of humidifying systems in temperature-regulating heaters, the use of processes without saturation towers, and the adoption of warm-water hydrolysis techniques in certain conversion sequences. The main problems at this stage are: ① Catalyst manufacturers that do not possess the technology for full low-volatility conversion have also begun to adopt this process. According to incomplete statistics, a total of 5 companies that produced sulfur-resistant shift catalysts attempted to implement the full low-temperature shift process in more than 10 small and medium-sized enterprises, all of which ended in failure ； ②Following these failure cases, a few technicians, without understanding the true reasons for those failures, gave unscientific evaluations of the all-low voltage conversion technology, which had a negative impact within the industry ； ③Some enterprises do not pay enough attention to certain technical details of the full low-temperature conversion process, and lax operation control also affects the economic efficiency of this technology. 2 Disadvantages of Fe-Cr shift catalysts Over the past 20 years, in the industrial use of shift catalysts and shift technology, companies have frequently reported various issues with Fe-Cr-based shift catalysts. The most common problem is that many small and medium-sized fertilizer plants replace these catalysts by 1/3 to 1/2 of their original amount during annual maintenance; this not only requires a lot of labor but also increases the costs associated with purchasing catalysts. Research findings show that whether it is the medium-string low-process or the medium-low-low process, Fe-Cr-based catalysts exhibit the following 4 prominent disadvantages: ① High activity temperature, resulting in significant heat loss, high steam consumption, and relatively high resistance ； ②Compared to sulfur-resistant low-temperature shift catalysts, it is prone to powdering, susceptible to poisoning by toxins such as sulfur, and has a short service life ； ③Under the same production capacity, the use of Fe-Cr catalysts requires larger equipment; as a result, both the initial investment and maintenance costs are higher than those of the all-low-temperature shift process ； ④Fischer-Tropsch reaction problems. The author analyzes the causes of these 4 problems as follows. 2.1 Sheet forming of Fe-Cr catalysts: The Fe-Cr catalysts used in industry today are all formed into sheets by physical methods, and the problem of \"good activity but poor strength\" has always existed. Although various catalyst grades have emerged, this long-standing problem has not yet been completely resolved, posing a risk of catalyst pulverization. 2.2 Fertor side reactions and the effect of sulfur: When the CO/CO2 molar ratio is above 1.6 and the inlet vapor-to-gas ratio is less than 0.6, the amount of by-products generated can reach several hundred parts per million (see Table 1). Although the situation improves at 0.8 MPa, sufficient attention should still be paid. For domestically produced medium-temperature catalysts, the inlet gas-to-catalyst ratio is generally not supposed to be less than 0.4. http://www.nmtech.com.cn/jishuwang/upload1/070903825155424.jpg This problem has occurred in various methanol manufacturers in China, manifesting itself as a catalyst lifespan of ≤0.5 years along with significant powdering. When sulfur is present in the gas (as in fixed-bed batch gas production in China), the iron formed through excessive reduction undergoes a chemical reaction: Fe + H2S → FeS + H2. The equilibrium H2S for this reaction is very low, making the formation of FeS extremely easy, as shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload1/070903825516355.jpg At this point, the reduced iron reacts with H2S in the reforming gas to form low-activity FeS. In this case, no hydrocarbons are produced, but it affects the catalyst’s activity; many domestic companies operate under such conditions. The resulting FeS undergoes the following chemical reaction with steam: http://www.nmtech.com.cn/jishuwang/upload1/070903826282764.jpg When the levels of H2O and H2S fluctuate, especially around the equilibrium point, the catalyst undergoes repeated phase transitions between FeS and Fe3O4. Although the catalyst activity is good (between that of FeS and Fe3O4), it severely affects the catalyst’s strength, leading to pulverization which increases the bed resistance; moreover, the broken dust can enter the low-temperature conversion zone, causing deactivation of the low-temperature conversion catalyst. 2.2.1 Low sulfur content in the gas: When the sulfur content in the gas is low, it is possible for the iron that has been reduced in the upper layer to act as a \"desulfurizing agent,\" while the catalyst in the lower layer operates under conditions of \"sulfur-free gas.\" At this point, F-T side reactions may occur, typically manifested by an increase in the methane content after conversion; in severe cases, acetylene is also produced, leading to copper-contaminated liquid. 2.2.2 Too low steam-to-gas ratio When the steam-to-gas ratio is too low (≤0.3), it may directly trigger the following chemical reactions. Fe3O4 + 6CO → Fe3C + 5CO2; 5Fe3O4 + 32CO → 3Fe3C2 + 26CO2. These reactions occur in processes involving monoalcohol or dialcohol, or when the load is very low at the beginning of operation, such as during the heating and reduction of the synthesis catalyst or to increase the copper content in the copper melt. At this point, the carbon monoxide content at the outlet of the converter increases, and the vapor-to-gas ratio is very low, which easily leads to \"excessive reduction\" and the formation of carbides that can initiate the Fischer-Tropsch reaction. Once carbides that can initiate the Fischer-Tropsch reaction are formed, this process is irreversible under normal operating conditions; this is why some users end up producing copper wash liquor over time. 2.2.3 Transformation reactions at higher pressures The active component of iron-chromium catalysts, Fe3O4, undergoes the following chemical reactions with CO: Fe3O4 + 6CO → Fe3O4 + 5CO2; 5Fe3O4 + 32CO → 3Fe3O2 + 26CO2. These are reactions that result in a decrease in volume. When the reaction pressure is high (≥2.0 MPa) and the ratio of n(CO) to n(CO2) is large, iron-carbon compounds tend to form, leading to F-T reactions that affect both the catalyst’s activity and its strength. 2.2.4 Sulfur poisoning at low steam-to-gas ratios The active component of ferrochromium catalysts, Fe3O4, can undergo the following chemical reaction with H2S: Fe3O4 + 3H2S + H2 → 3FeS + 4H2O; ΔH0298 = –75 kJ/mol. This reaction is clearly dependent on the steam content in the gas (i.e., the steam-to-gas ratio). The equilibrium concentrations of H2S at various steam-to-gas ratios are shown in Table 3 and Figure 1. http://www.nmtech.com.cn/jishuwang/upload1/070903827105033.jpg http://www.nmtech.com.cn/jishuwang/upload1/070903827497156.jpg The product specifications for medium-temperature catalysts include sulfur tolerance values, which are generally around 400 mg/m3; however, there are no corresponding requirements regarding the vapor-to-gas ratio. As can be seen from Table 3 and Figure 1, there is a strong relationship between H2S and the vapor-to-gas ratio; when the vapor-to-gas ratio is less than 0.5, the equilibrium H2S level decreases sharply. The sulfur poisoning (or sulfur tolerance) criteria for domestic catalysts are meaningful only at a certain gas-to-vapor ratio. Currently, the gas-to-stoichiometric ratio for activity testing according to corporate or industry standards is 1.0 or 2.0, with a test temperature of 300°C. As can be seen from Table 1, its sulfur balance value ranges from 300×10⁻⁶ to 700×10⁻⁶, that is, 455 to 1,000 mg/m³. When testing activity, there is no problem with tolerating 400 mg/m³ of sulfur; therefore, this sulfur tolerance index is meaningless for practical applications. Some medium-temperature catalysts can tolerate high sulfur levels during activity testing, but this is only temporary; of course, catalysts that contain other active components such as MoS2 are another matter. 2.2.5 Sulfur poisoning under actual operating conditions Most organic sulfides are rapidly converted to H2S during the CO conversion reaction. Nearly all of COS and CS2 were converted into H2S. COS + H2 → CO + H2S, ΔH0298 = 9.2 kJ/mol; COS + H2O → CO2 + H2S, ΔH0298 = –34.7 kJ/mol; CS2 + 2H2O → CO2 + 2H2S, ΔH0298 = –60.4 kJ/mol. The sulfur content refers generally to the total sulfur content. Large fluctuations in H2S levels above or below the equilibrium level are dangerous, as the catalyst can break down due to repeated phase changes between Fe3O4 and FeS. As the reaction proceeds, the vapor-to-solid ratio in the bed gradually decreases, and the H2S content at equilibrium also drops, meaning that the sulfur resistance declines. As a result, the catalyst in the lower part becomes sulfur-poisoned while the catalyst in the upper part remains relatively intact. 2.2.6 Safe operating sulfur concentration at various gas-to-gas ratios Similar to other catalysts, due to the chemical adsorption of H2S on the catalyst surface, the catalyst’s activity decreases even though the concentration is not yet high enough to cause the formation of FeS on the catalyst. Therefore, the H2S concentration that actually causes poisoning should be lower than the equilibrium H2S concentration. Furthermore, as the conversion reaction proceeds, although the temperature gradually increases, the vapor-to-gas ratio in the bed becomes increasingly low; more organic sulfur is converted into inorganic sulfur, and the H2 content rises. As a result, the maximum allowable H2S concentration decreases, meaning that the resistance to sulfur becomes poorer. Due to the lack of maximum H2S poisoning experiment data for various Fe-Cr conversion catalysts, the equilibrium temperature is taken as 50°C using traditional empirical methods. The total S concentration for safe operation at various gas-to-vapor ratios is shown in Table 4. http://www.nmtech.com.cn/jishuwang/upload1/070903828364758.jpg The highest sulfur tolerance in the medium-low-low process: At present, the volume fraction of CO at the outlet of this process is 1.5%, the inlet temperature of the medium-temperature catalyst is around 300°C, and the steam-to-gas ratio is approximately 0.45; therefore, the total sulfur content should be ≤50×10‑6, or ≤76 mg/m3. This value corresponds to a transformed H2S concentration of ≤60 mg/m3, which obviously cannot be achieved by all users. When the process settings are not appropriate, the CO content in semi-water gas is low; with a vapor-to-gas ratio at the inlet of the medium-shift catalyst of around 0.35, the total sulfur content should be ≤38×10‑6, that is, ≤57 mg/m3. This value corresponds to a transformed H2S concentration of ≤44 mg/m3, which is clearly difficult for most users to achieve. When the hydroformylation process is used, the volume fraction of CO at the outlet is 7%–5%, and the steam-to-gas ratio at the inlet of the medium-temperature converter is approximately 0.30; therefore, the total sulfur content should be ≤30×10‑6, or ≤45 mg/m3. This value corresponds to a transformed gas H2S concentration of ≤36 mg/m3. When the process settings are not appropriate and the gas-to-vapor ratio in the converter becomes lower, the total sulfur content also decreases. This is difficult to achieve in the case of desulfurizing semi-water gas, and it is also contrary to the sulfur requirements of the subsequent sulfur-resistant low-temperature conversion catalysts. To address the excessive reduction of ferrochrome catalysts, low gas-to-solid ratios for such catalysts have been developed abroad; the key to these catalysts is to kinetically prevent or reduce the rate of the excessive reduction reaction between Fe3O4 and CO, with a low gas-to-solid ratio generally being ≥0.8. There are few reports abroad on the poisoning of ferrochrome catalysts by S; in such cases, a full low-temperature shift process is generally used, or a medium-series low-temperature process is employed at high gas-to-steam ratios (≥1.0). In summary, the “excessive reduction” that occurs in medium-temperature catalysts in the presence of sulfur-containing gases and at low gas-to-vapor ratios is prevented by the presence of sulfur, which inhibits the Fischer-Tropsch reaction. In a sense, the low steam-to-gas ratio of sulfur-free gases leads to the Fischer-Tropsch reaction, making the shift reaction difficult to proceed, whereas the presence of sulfur ensures the possibility of operating at a low steam-to-gas ratio. However, a sulfur content that is neither high nor low not only leads to a decrease in catalyst activity but also severely affects the catalyst’s strength, increases the pressure drop across the catalyst bed, and even allows broken dust to enter the low-temperature conversion section, resulting in the deactivation of the low-temperature conversion catalyst and disrupting normal production. The solution is to move the reaction away from the equilibrium line where FeS is oxidized by H2O, thereby preventing fragmentation due to repeated phase transitions between Fe3O4 and FeS. The same applies to solutions for sulfur poisoning at low gas-to-vapor ratios. 2.2.8 Operation of the Fe3O4 active phase: In terms of process design, a shift process with a high gas-to-vapor ratio is employed; methaneation purification or decarburization methods such as MDEA, which ensure an outlet CO level of ≤0.3%, can help avoid excessive reduction of the catalyst. To prevent sulfur poisoning, semi-water gas desulfurization can be carried out using the S concentration criteria for safe operation provided in this article, thereby reducing the total sulfur content to the required level to ensure proper operation of the Fe3O4 active phase. 2.2.9 Operation of the FeS active phase: To maintain the strength of the medium-temperature catalyst and prevent catalyst degradation caused by repeated phase transformations resulting from operational fluctuations, it is recommended to increase the total sulfur content and keep it away from the equilibrium line (at a temperature 50°C above the equilibrium temperature), so as to use stable FeS as the active phase of the medium-temperature catalyst; however, this requires increasing the amount of catalyst used by about a factor of two. For the medium-low-low process with a 1.5% change in the CO volume fraction at the outlet, and an inlet steam-to-gas ratio of 0.45, the total sulfur content should be ≥200 mg/m3 ； For the medium-low-low hydroxyalkane process with a CO volume fraction at the outlet of about 5%, and an inlet steam-to-gas ratio of 0.35, the total sulfur content should be ≥150 mg/m3. This is why some factories that use high-sulfur coal for a long time have a relatively high total sulfur content in the conversion process, yet their catalysts still maintain a long service life. From medium-low-low process to the so-called “medium-medium-low-low process,” what actually happens is an increase in the amount of medium-temperature catalyst used, and that is precisely why this change has its effect. It is important to note that the operating conditions must remain stable; fluctuations in the gas-to-vapor ratio and total sulfur content can cause the reaction Fe3O4+3H2S+H2→3FeS+4H2O to occur repeatedly, severely affecting the strength of the catalyst. 2.3 Excessive steam consumption leads to increased resistance. Some people have conducted calculations by comparing 800 kg and 300 kg of steam per ton of ammonia consumed; at an ammonia production rate of 5 t/h, this results in an additional 2.5 t of steam being used per hour. With a standard flow meter, the system volume increases by 3111 m3 per hour, which represents an increase of over 20% on top of a flow rate of 15,000 m3/h; as a result, the system resistance will increase significantly. When the oxygen content rises, production practices typically involve increasing the amount of steam used to lower the temperature. As the operating time of the medium-temperature shift catalyst increases, its activity gradually declines. To maintain the process parameters, it is common to raise the inlet temperature of the medium-temperature shift reactor and increase the steam consumption, which can lead to a vicious cycle in the operational conditions. 2.4 Investment Issues (1) Investment costs (all calculated at the prices prevailing at the time of implementation in the industrial sector): Taking Danyang Fertilizer Plant as an example, if a medium-voltage to low-voltage conversion process is used, an investment of 3.5 million yuan is required; whereas a full low-voltage process with equivalent capacity only requires an investment of 2 million yuan, resulting in a savings of 1.5 million yuan. (2) Low maintenance costs: The all-low-temperature conversion process not only features a low reaction temperature and a small heat exchange area required, but also boasts a high activity and long lifespan of the CO-MO catalyst system, resulting in fewer catalyst replacements. Therefore, its maintenance costs are much lower than those of the medium-temperature process. (3) Low operating costs: Huaiyin County Fertilizer Factory is a small plant with an annual ammonia production capacity of 20,000 tons, and it produces 84,000 tons of fertilizers per year. Compared with the medium-string low-process, the full-low-variation process offers significant advantages in the following aspects: ① Steam consumption savings (0.911－0.484)×40×2=3.416 (10,000 yuan) ； ②The resistance is reduced, resulting in 8 kW·h of electricity savings per ton of ammonia. At a cost of 0.15 yuan/kW·h, this amounts to 24,000 yuan in annual savings ； ③Driving and parking 10 times per year, with a time savings of 10 hours per trip, results in an increase in fertilizer production of 84,000 × 10 × 10 / 7,200 = 1,166 tons ； The fixed cost per ton of fertilizer is set at 100 yuan, and the profit and tax amount is 30 yuan. 1,166×130／10,000 = 15.16 (10,000 yuan) ； ④The cost savings in electricity, ammonia, chemicals, etc., resulting from avoiding one desulfurization process amount to approximately 80,000 yuan. The total annual value of these benefits amounts to about 600,000 yuan, with even greater benefits in factories that are well-managed. The Danyang Fertilizer Factory can achieve economic savings of up to 660,000 yuan in just half a year solely from steam savings. The economic benefits obtained from implementing the all-low-voltage process are very significant. 3 Advantages of the full low-temperature shift process: This process reduces the peak temperature in the original medium-temperature shift system by over 100°C, which greatly facilitates the equilibrium of the carbon monoxide shift reaction. The actual steam consumption per ton of ammonia produced is only around 250 kg, and the area required for heat recovery equipment is small. The benefits brought by this process are obvious, with the following specific advantages. (1) The amount of catalyst used in the original shift reactor was reduced by more than 1/2, which lowered the bed resistance and increased the capacity of the shift furnace. (2) The bed temperature drops by 100–200°C, the gas volume decreases by 25%, which reduces system resistance and lowers compressor power consumption. (3) There are no F-T side reactions, which eliminates the reaction that produces acetylene, reduces the consumption of copper melt, and thus prevents the occurrence of \"liquid carryover\". (4) The heat exchange area is reduced by about 1/2. (5) It fundamentally solved the problem of powdering in medium-temperature catalysts, improving the labor and health conditions associated with their handling. (6) It improves the conversion capacity of organic sulfur; under the same operating conditions, the full low-temperature shift process achieves a 5% higher conversion rate of organic sulfur compared to the medium-high or medium-low-low processes. (7) It is easy to operate and starts quickly, increasing the effective operating time. (8) The material requirements for the converter have been reduced. (9) The catalyst has a long service life; it can generally be used for about 5 years, with the longest recorded usage period being 12 years ； (10) The desulfurization quota can be relaxed once, thereby reducing desulfurization costs. The advantages of the fully low-temperature shift process using unsaturated towers, which have been developed in recent years, are even more evident: it completely eliminates equipment corrosion, reduces shutdowns caused by corrosion related to the shift reaction, results in fewer pieces of equipment, lowers system resistance, increases the output capacity of compressors, saves energy used by the hot water pumps in the traditional saturated tower cycles as well as the energy required for hot water discharge, and reduces the requirements regarding the material quality of the equipment. More importantly, it improved the conversion capacity of organic sulfur. Because in the traditional saturated hot water tower process, various organic sulfur compounds in the gas are dissolved by the circulating hot water and then released through the shifted gas; the circulating hot water serves as a ‘green pathway’ for these organic sulfur compounds. This is why some plants have high levels of \"non-COS organic sulfur\" in their reformate gas. A non-saturated tower process can solve this problem; not only can the portion of organic sulfur that is converted during desulfurization be removed, but also the non-COS organic sulfur present in the gas – which is difficult to remove using conventional hydrolysis methods – will not reach subsequent stages. This is highly beneficial for methanol production or synthetic catalysts.