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Introduction to the Low-Temperature Methanol Wash Process · Natural gas is an ideal raw material for producing ammonia and hydrogen; the syngas obtained from it can be produced and purified more efficiently, cleanly, and economically through steam reforming, whereas syngas made from other common raw materials is far less effective. For carbon products made from syngas, such as methanol, carbonyl alcohols, and hydrocarbons produced by the Fischer-Tropsch process, there is one minor drawback: the hydrogen content in the syngas obtained via steam reforming is usually too low. The world’s total reserves of natural gas remain very substantial, but in regions with advanced industries and more mature economic development, natural gas resources are running out, and this trend has become more apparent recently. In the winters of previous years, natural gas prices in the United States reached high levels during peak demand periods, while in the winter of 2006, a decline in natural gas production in the North Sea led to a shortage of natural gas supply in Europe. The natural gas supply in these regions will gradually rely on imports, and from a strategic perspective, this situation is quite unfavorable and even dangerous. Some areas where industry is developing rapidly still lack abundant resources, and some places may never have natural gas at all. Humans have been paying attention to CO2 emissions and global warming, issues that are closely related to \"atmospheric dust\" in the 1980s and 1990s; these issues are also closely linked to the accelerated consumption of natural gas and coal resources in industrially developed countries. Coal contains the most carbon of all mineral resources, and the industry has begun to pay attention to it again, especially in the United States. In China, coal has always been an important fuel and raw material, and it is likely to remain so in the future. 1 Focus on coal gasification to produce syngas. It is obvious that, as a raw material for syngas, coal cannot be processed through steam reforming since it is in solid form; therefore, it cannot be fed into the solid catalysts required for the steam reforming process. However, even if it can be transported, the type and amount of impurities in coal will quickly deactivate the catalysts used for steam conversion as well as other catalysts downstream that are sensitive to toxins. The same is true when using liquid hydrocarbons that are heavier than light naphtha. The solution is to use the gasification method, or partial oxidation, in which coal is burned with an appropriate amount of oxygen or oxygen-rich air along with steam, so as to react with CO or the gaseous hydrocarbons produced during incomplete combustion to form CO2 and excess H2. The combustion process provides sufficient heat for reactions that do not use a catalyst and involve steam, thus preventing damage to the catalyst in the syngas reactor. The syngas feedstock derived from the gasification of coal and heavy hydrocarbon feedstocks contains hydrogen, CO, CO2, and residual steam; in the rare cases where the gasifying agent is not pure oxygen, it also includes nitrogen and inert gases from air, along with hydrogen sulfide, carbonyl sulfide (COS), soot, and ash. After gasification, solid particles are first removed using conventional gas purification methods. Then, CO is further reacted with steam to produce CO2 and H2, in order to adjust the gas composition to make it more suitable for the synthesis of methanol or other products, or to increase the amount of hydrogen in hydrogen or ammonia production facilities. Regardless of the method used to remove CO, efforts should be made to minimize the residual CO. The water-gas shift reaction requires a catalyst; even in the high-temperature shift (HTS) process, the sulfur content in the feed gas is relatively high for the more durable catalysts used. In hydrogen and ammonia production facilities that employ conversion methods, low-temperature shift (LTS) reactions are carried out to further reduce the CO content in the gas, which means that the sulfur concentration in the feed gas becomes even higher for the more sensitive catalysts. Therefore, the sulfur in the gas must be removed to a certain extent before it reaches the HTS catalyst; however, it is impractical to reduce the sulfur concentration to such a low level that it does not damage the LTS catalyst. As a result, even if a gasification-based syngas production facility includes an LTS process, a small amount of sulfur remains. In cases where it is necessary to remove all carbon oxides, in facilities such as ammonia plants and those for producing high-purity hydrogen, large amounts of CO2 are removed after high-temperature conversion using certain wet purification processes; subsequent to this, residual CO2 and CO are removed through physical absorption methods such as pressure swing adsorption (PSA), cryogenic separation, or catalytic methanation. The disadvantage of the last method is that carbon oxides are converted back into methane; in ammonia plants, methane accumulates in the synthesis loop, increasing the requirements for purification. In steam reforming syngas plants that use clean feedstocks, large-scale CO2 removal systems generally employ regenerated chemical scrubbing solutions such as activated thermal potassium alkalis (Benfield, Vetrocoke, Catacarb, Carsol processes) or activated MDEA. However, when heavy feedstocks are used to produce syngas, the impurities present in them tend to undergo irreversible reactions with these chemical washes, which reduces efficiency and may exacerbate corrosion. Therefore, syngas production plants using the gasification process commonly employ reversible physical absorption processes to remove large amounts of CO2. This is particularly applicable to high-pressure gasification units. 2 Low-temperature methanol washing purification process For decades, the acid gas removal process has been dominant in gasification synthesis units, as it is highly suitable for such specific conditions. This is the low-temperature methanol washing purification process, jointly developed by the companies Linde and Lurgi. The industrial low-temperature methanol washing purification process is used to purify hydrogen and syngas from ammonia, methanol, pure CO, or oxygen-containing gases, with the aim of removing acidic gases. The low-temperature methanol washing purification process is a physical acid gas purification system that uses methanol (Grade A, Category 21) as a purification absorbent at operating temperatures below the freezing point of water. The total sulfur content in the purified syngas (H2S and COS) is below 0.1×10—6 (by volume). Depending on the application requirements, the molar concentration of CO2 can be adjusted to a few percent or a few parts per million (by volume). Before the gas is used in final synthesis processes (such as ammonia, methanol, carbonyl synthesis alcohols, and Fischer-Tropsch hydrocarbon synthesis), there is no need to employ upstream COS hydrolysis processes or to pass the gas through additional sulfur protection layers. Compared with other processes, the main advantages of this process are the use of methanol, which is inexpensive and easy to obtain as a solvent, extreme flexibility in process configuration, and very low power consumption, in addition to the extremely low sulfur concentration in the syngas. Furthermore, the sulfur compounds in the feed gas and CO2 are removed during separation and distillation processes, and further processed in a Claus sulfur recovery unit to be produced as pure CO2 products. Table 1 presents the specifications of the main products produced by the low-temperature methanol washing purification process. Figure 1 compares the solubilities of the acid gas components H2S/COS and CO2 with those of the main components of syngas, H2 and CO (the vertical axis is logarithmic). Two phenomena can be observed from this graph. First, methanol exhibits high selectivity for acid gases, so the loss of valuable syngas components is kept at a minimum level ； Secondly, the solubility of acidic gases increases significantly at low temperatures; therefore, the process requires external cooling, usually at –35°C or –40°C. —At 40°C, the solubility of H2S is about 4 times that of CO2. This shows that even when the CO2/H2S ratio in the feed gas is as high as 200, a gas rich in H2S can still be produced. Using other methods (washing with nitrogen under suitable conditions), even when the CO2/H2S ratio in the feed gas is as high as 700, the molar concentration of H2S in the resulting gas remains above 25%. The typical process flow of one-step low-temperature methanol washing purification (RWU) with a pure CO2 recovery process is shown in Figure 2. Since it is physical absorption, low-temperature methanol washing purification is particularly economical for high-pressure gases containing high concentrations of acidic gases. One of the advantages is that the low-temperature methanol washing purification process is compatible with the downstream cryogenic separation system (cryogenic box), as the low-temperature methanol washing unit supplies cooled and dried feed gas to the cryogenic unit, allowing the cryogenic unit itself to cool the methanol solvent used in the low-temperature methanol washing unit. The feed gas to the low-temperature methanol wash purification unit is either shift gas containing a high concentration of CO2 (such as in ammonia synthesis), or unshifted or partially shifted gas containing a low concentration of CO2 (such as CO, oxygen-containing gases, or methanol synthesis gas); it may also be a mixture of these gases. Under the latter condition, the low-temperature methanol washing purification unit is designed to process both converted and unconverted feed gases simultaneously (such as producing CO and methanol at the same time), as shown in Figure 3. Two different purification processes can be adopted, but a common solvent regeneration system must be used. The use of an optimized lean/semi-lean solvent can significantly improve the process economics of this configuration, reduce the amount of solvent that needs to be thermoregenerated, and enable integration of this process with Linde’s cryogenic CO cryocoolers. 2.1 One-step and two-step low-temperature methanol washing purification processes In the traditional design of the two-step low-temperature methanol washing purification process (see Figure 4), H2S and carbonyl sulfide are removed in the first step upstream of the shift reaction, while CO2 is removed in the second step downstream of the shift reaction. Each step has its own separate washing tower. Since the gas has been removed to a high degree, the ferrochrome catalyst can be used safely during the conversion process. The operating pressure is generally below 5.5 MPa. Recently, Linde has developed an advanced one-step low-temperature methanol wash purification process, as shown in Figure 5. Sulfur compounds and CO2 are selectively removed in different sections of a separate unit, respectively. Since the low-temperature methanol wash purification process is located after the shift reaction, a sulfur-resistant (acidic) shift catalyst must be used. To maintain catalyst activity, it is required that the H2S concentration in the gas be kept at a minimum. The pressure is generally 8 MPa. Since the one-step low-temperature methanol washing purification process requires only one purifier and one gas cooling unit, it significantly reduces investment costs. 2.2 Industrial Applications Currently, there are approximately 40 units around the world that have been built or are in the design/construction phase and utilize Linde’s cryogenic methanol cleaning technology. In addition to the low-temperature methanol washing units that Luzzi Company has been supplying, this purification process is used to clean 75% of the syngas produced worldwide from petroleum residues, coal, and waste materials, as well as 90% of the syngas used for non-IGCC applications through gasification. Linde’s low-temperature methanol washing purification process is primarily used for coal and petroleum feedstocks in the gasifiers of GE/Texaco and Shell companies, as well as in industrial facilities utilizing asphalt and Eureka asphalt, along with high-temperature Winkler and Cooper gasification processes. It is worth noting in particular that among the GE/Texaco gasification units put into operation in the 1980s, 5 Linde low-temperature methanol washing purification units are operating reliably, 2 of which operate at a pressure of around 6 MPa. In 2004, Linde signed a contract to build 3 low-temperature methanol washing units downstream of Shell’s gasification plant. Two of them are used for syngas with an annual production of over 550 kt of ammonia in Yunnan Province, China. Another set is a dual-series unit used to purify hydrogen in a coal liquefaction unit in Inner Mongolia. In 2005, Linde signed three more contracts to build low-temperature methanol washing processes for purifying syngas produced by GE/Texaco gasification units. Two of these units are designed for the joint production of low-temperature CO and methanol synthesis gas, while the third unit is used for a mixture of three different types of synthesis gas: hydrogen, oxygen-containing synthesis gas, and methanol synthesis gas. 2.3 Comparison of methanol and PEGE absorbents Since both H2S and CO2 have high solubility in methanol, the low-temperature methanol washing purification process operates at a relatively low solvent circulation rate; this rate is 25% lower than that of other physical absorption processes using PEGE as a solvent (such as UOP—SelexoR, BASF—SepasolvR, Clariant GenosorbR, etc.). Figure 6 compares the solubilities of CO2 and H2S in PEGE and methanol; the PEGE solvent exhibits higher selectivity for H2S than for CO2. Therefore, the absorption process using PEGE desulfurization can only remove a very limited amount of CO2 from the process gas, generally around 20%. But in methanol plants this can be an advantage, as CO2 is a desirable component in syngas there; whereas in ammonia plants CO2 must be completely removed, requiring additional process steps. This is pressure swing adsorption (PSA). However, if PSA is used to remove large amounts of CO2, much H2 will be discharged into the exhaust gas along with CO2. The H2 recovery rate in PSA is at most 90%; although the use of liquid nitrogen washing may increase this rate by a few percentage points, there are still limitations, especially under high-pressure conditions. In summary, by increasing all upstream equipment (raw material preparation, syngas production/adjustment, air separation units, etc.) and raising the corresponding raw material demand by 10% each, the high loss rate of H2 can be compensated for. This approach of increasing equipment size is much more effective than reducing capital and operational costs in downstream acid gas removal and syngas purification processes. From Figure 6, the following conclusions can be drawn: a) Since the solvent circulation rate is much lower, the power consumption indices (such as electrical energy, steam, and cooling water) decrease significantly. The low-temperature methanol washing purification process generally does not require an expansion turbine to generate electricity. b) For large-scale synthesis plants, the acid gas removal unit of the low-temperature methanol washing process can be designed as a single unit. c) The dimensions of most equipment (heat exchangers, pumps, and specific containers) are determined by the liquid circulation rate, so they are smaller than those in the PEGE process. Compared to the PEGE system, the other advantages of the low-temperature methanol washing purification process are as follows: a) there is no need for an upstream hydrogenation step, such as for COS, which can be completely removed from the gases produced by coal or oil gas conversion. The PEGE solvent can only remove part of the COS. b) Methanol has very stable chemical properties. The PEGE solvent gradually degrades as the operating time increases. c) The process gas from the low-temperature methanol washing purification unit is very dry, containing only a small amount of methanol. When cryogenic processes (liquid nitrogen washing or H2/CO separation processes) are placed immediately downstream of the low-temperature methanol washing purification process, the temperature of the process gas is extremely low; the low-temperature methanol washing purification process improves the efficiency of the cryogenic units, simplifies their design, and thereby reduces equipment costs. The gas from PEGE is saturated with water vapor and has a relatively high temperature, which is unfavorable for the feed to the cryogenic process. d) Methanol is non-corrosive, cheaper than PEGE solvents, and has a wide range of applications. e) Trace components do not accumulate in the solvent, as the low-temperature methanol washing purification process is equipped with a device for long-term purification of water. f) No foaming has ever occurred. g) Good syngas purification parameters. Figure 7 compares the power consumption of the low-temperature methanol washing purification process with that of the PEGE purification process. PEGE was designed and optimized to remove large amounts of CO2 and generate power through solvent evaporation, but only 30% of the PEGE is thermoregenerated. The operating pressure of a 1,200 t/d ammonia plant using the GE/Texaco gasification process in China is 3.3 MPa. The specific power cost shown in Table 2 indicates that the power cost of the PEGE process is about 50% higher than that of the low-temperature methanol washing purification process. 2.4 Comparison between the low-temperature methanol washing purification process and the chemical (amine) absorption process. Figure 8 shows the relationship between the CO2 capacity of physical and chemical solvents and the CO2 partial pressure in the feed gas. When the latter exceeds 0.8 MPa, and cold methanol is used in the low-temperature methanol washing purification process, the CO2 load can reach its maximum value. This indicates that in the low-temperature methanol washing purification process and when MDEA is used for syngas produced from coal or oil gasification, a much smaller amount of purification solvent is required in the former cycle when the CO2 partial pressure is high. Compared to physical low-temperature methanol wash purification units, activated MDEA purification units used for desulfurization and the removal of large amounts of CO2 have the following characteristics: a) Due to their lower capacity to handle CO2 (in more complex two-step activated MDEA processes), the circulation rate of the activated MDEA solvent is significantly higher than that of the solvent downstream of high-pressure gasification; as a result, activated MDEA units are larger in size compared to low-temperature methanol wash units, which offsets the complexity associated with the low-temperature methanol wash purification process. b) The dimensions of the equipment and pipelines limit the capacity of individual units. c) Due to frequent solvent circulation, the energy consumption of the equipment (electricity, steam, cold water) is high. d) When cryogenic processes such as liquid nitrogen washing units are located after the CO2 removal unit, a cooling unit is required in the activated MDEA process to pre-cool the feed gas; however, this unit is smaller than the cooling unit used in the low-temperature methanol washing purification process. In addition, the size of the cryostat for the liquid nitrogen washing purification process will be increased. e) Activated MDEA produces low concentrations of H2S, which is not suitable for Claus units. Therefore, given the high CO2 load, to design a sulfur recovery unit capable of handling large volumes, special technologies such as liquid redox sulfur recovery units with high operating costs (requiring the addition of chemical agents) are needed; if these are expensive, additional concentration processes should be avoided. f) When the low-temperature methanol washing purification process is used, there is no need to desulfurize the syngas to high standards. g) The purity of the CO2 product is low. h) The cost of solvent replenishment is high. i) It needs to be tested to prevent foaming and corrosion. 2.5 Material: The low-temperature methanol washing purification process is actually a non-corrosive process. All pipes and almost all equipment are made of carbon steel. Only the following components are made of stainless steel: a) all tray plates whose operating temperature is higher than the ambient temperature. b) Reboiler pipes. c) Raw material heater pipes for the methanol/water separator. The widespread use of many devices proves the correctness of the material selection. The choice between carbon steel, low-temperature carbon steel, or alloy low-temperature carbon steel should be based on the design temperature of the equipment and the operating conditions. 2.6 Trace Components Apart from the removal of acidic gases such as H2S+COS and CO2, the feed gas generally does not contain other trace components, such as ammonia, hydrogen cyanide, nitrogen oxides, benzene, naphthalene, organic sulfur compounds (mainly carbon disulfide and thiol compounds), nickel carbonyl, and iron carbonyl. Advantageously, cold methanol can completely remove trace components from the feed syngas ； Disadvantageously, they can accumulate to harmful levels in a circulating methanol solvent. This may be detrimental to process gas purity and exhaust emissions. In addition, corrosion and blockage of equipment and pipes can occur. The properties and quantities of trace components in the feed gas are related to the feed to the gasification unit, the type and design of the gasification unit, the process design, as well as the process design and sequence of gasification and low-temperature methanol washing. Based on experience, regarding trace components and shifted gas, more attention should be paid to the unshifted feed gas, as the shift catalyst only removes certain identified trace components to specified limits. Linde has extensive experience in dealing with trace components; it can provide special methods to prevent these trace components from entering the solvent (water or the methanol pre-washing section upstream of the main absorption tower), or remove them from the solvent through purification or filtration/washing, thereby ensuring that the product meets the specified requirements and allowing the plant to operate reliably over the long term. It is worth noting that the operating rate of Linde’s low-temperature methanol wash purification unit is 98% or even higher. 2.7 Spiral heat exchangers: A design feature of Linde’s low-temperature methanol washing purification process is the use of multiple spiral heat exchangers. The advantages of this heat exchanger are as follows: a) the total investment cost is reduced, as one spiral heat exchanger can replace a tubular heat exchanger. Reducing auxiliary equipment (connection pipelines, equipment, steel structures, vertical equipment, etc.) will result in additional cost savings. b) Power consumption is reduced, as the spiral heat exchanger can operate with a small temperature difference, thereby minimizing the methanol circulation rate. In this way, consumption of electricity, cooling, and steam is reduced. c) Operational advantages include mature technologies such as injecting methanol into the feed gas and reduced pressure drop. Linde’s cryogenic methanol wash purification units generally consist of 4 to 6 spiral heat exchangers. Heat exchangers are used in the following situations: a) when it is necessary to cool or heat multiple gases simultaneously, allowing for heat exchange between them (multi-gas flow heat exchanger). b) If the temperature difference is small, a large amount of heat must be removed. c) When a large amount of undissolved gas is removed from methanol (heating methanol containing undissolved gas releases a large amount of water vapor, and the vapor/liquid flow rate ratio is 20 times that at the outlet). 3 Liquid nitrogen washing process: Linde is a leading company in the design, engineering, and manufacturing of liquid nitrogen washing processes. No other company in the world has such a long history in cryogenic technology as this one. Linde’s expertise is built on the operational experience and feedback related to these systems; the company has made many significant developments and filed a series of patents, thereby greatly advancing the development of the liquid nitrogen washing process. The liquid nitrogen washing process is mainly used to produce ammonia synthesis gas. It is usually the final step in the production and purification process. This process has two main functions: a) it uses the high-pressure nitrogen that enters to remove impurities such as CO, argon, and methanol from the hydrogen feedstock, by cooling and liquefying the high-pressure nitrogen in a cryogenic tank using the Joule-Thomson effect. b) Upon calculation, nitrogen was added to hydrogen to achieve a final hydrogen-to-nitrogen ratio of 3:1 in the ammonia synthesis gas. CO must be completely removed from the syngas, as it poisons sensitive ammonia synthesis catalysts. Argon and methane are inert in the ammonia synthesis loop; if they remain in the inlet gas without being removed, they will accumulate in the ammonia synthesis loop. This will result in a loss of syngas and increase the costs of gas purification and separation equipment. As shown in Figure 9, liquid nitrogen washing purification includes: a) 1 absorption tower, where trace amounts of solvents (such as methanol, water), small quantities of CO2, or other compounds coming from the upstream acid gas washing process are completely removed by the molecular sieve layer, thereby preventing condensation in the low-temperature process, whose operating temperature is –190°C. b) Liquid nitrogen cooling box. If the installed equipment exceeds the transportation limits, all cryogenic process equipment should be pre-assembled as a unit or installed on-site. The advantages of liquid nitrogen washing equipment are: a) since it is mainly used in cryogenic separation processes, there is no loss of hydrogen product. The hydrogen recovery rate is approximately 99.5%, making it more efficient than other hydrogen purification processes such as PSA or membrane processes. As mentioned above, one set of PSA units can increase the size of the upstream syngas production and regulation equipment by about 10%, while also increasing the raw material supply accordingly. b) The high purity of the ammonia synthesis gas results in excess purge gas, extending the service life of the downstream synthesis catalyst. c) There is no need to use rotating equipment such as turbine expanders. d) The device is easy to operate and can withstand fluctuations in flow rate and composition. e) All devices and pipes/valves in the liquid nitrogen cooling box are made of aluminum or stainless steel and are welded together as a whole, thereby preventing gas leakage from the flanges. The valves can be repaired outside the cold box. f) Linde’s own workshops produce and assemble all cryogenic cabinets. g) Relying on Linde’s many years of technical expertise, design and supply absorption towers. h) No additional external cooling equipment is required, as the upstream low-temperature methanol washing purification process serves as a pre-cooling mechanism. Apart from the fact that nitrogen is required for the ammonia synthesis process itself (but only 3%–4% of the nitrogen is lost through the tower), the liquid nitrogen washing process has very low energy consumption, and no special requirements are placed on the equipment. For the regeneration of the absorption tower, only a small amount of steam and cooling water is required. The high-pressure nitrogen used as regenerative gas is not wasted; it is simply used for desorption in the later stages of the low-temperature methanol washing purification process. Unlike during driving, only liquid nitrogen from the air separation unit is required; at this time, the liquid nitrogen washing process operates under conditions of insufficient pressure, utilizing the Joule-Thomson effect to provide the necessary cooling. If necessary, the liquid nitrogen washing can also be designed to suit \"warmer\" feed gas resulting from the purification of MEA or MDEA (such as methanol-purified gas). Therefore, the absorption tower must be designed to remove water rather than methanol. The residual CO2 concentration is also high. It is generally required to use external condensation to pre-cool the feed gas to facilitate absorption. As the heat transfer area of the heat exchanger increases, the size of the cold box must be enlarged. 4 The low-temperature methanol washing purification process, combined with the liquid nitrogen washing process, is used to produce ammonia synthesis gas. As mentioned earlier, there are clear advantages to combining the low-temperature methanol washing purification process with downstream cryogenic processes such as liquid nitrogen washing or CO production processes (cold boxes), as this process serves as a pre-cooling and drying step for the subsequent cryogenic separation processes. When producing ammonia synthesis gas, the one-step selective low-temperature methanol washing purification process works best in combination with the downstream liquid nitrogen washing process. The pressure of the feed gas is usually 3–8 MPa, which is relatively high. The raw gas from the gasification unit is cooled through multiple steps, during which CO is converted into hydrogen and CO2. The gas contains a large amount of CO2. The molar concentration of CO remaining in the transformed gas is 1.5%, the molar concentration of CO2 in the gas derived from oil gasification is 34%, whereas the molar concentration of CO2 in the gas derived from coal gasification exceeds 40%. The main characteristics of the low-temperature methanol washing purification process in this manner are as follows: a) complete removal of H2S/COS, with CO2 removed to a few parts per million (by volume). b) In high-concentration gases suitable for further treatment (such as in Claus units), all sulfur compounds are provided within the interfacial zone. c) The by-product CO2 is free of impurities and suitable for urea synthesis. The sulfur concentration is at most 2–4 mg/m3 (under standard conditions); the pressure can be very high, and the quality remains stable, making it suitable for converting ammonia completely into urea. d) The flue gas meets the specified environmental standards. The flowchart in Figure 10 illustrates the optimal matching between low-temperature methanol washing purification and liquid nitrogen washing processes. If the designers of these two processes perform their respective tasks, such a combination offers significant advantages, as it is only then that the necessary process cycles can be carried out most effectively. Since Linde is expert in designing and providing low-temperature methanol washing and liquid nitrogen washing processes, the two processes can be integrated seamlessly, without any conflicts regarding process optimization or cost efficiency. 4.1 Material balance The material balance for the low-temperature methanol washing and liquid nitrogen washing units in the 1 kt/d ammonia plant of GE/Texaco Gasification is shown in Table 3. 4.2 Power Consumption The main power consumption indicators for the low-temperature methanol washing and liquid nitrogen washing processes are shown in Table 4. Liquid nitrogen washing requires only a small amount of high-pressure steam, cooling water for the regeneration of the absorber, and low-pressure nitrogen for the purification of the cryogenic tank. 4.3 Operation records of low-temperature methanol washing/liquid nitrogen washing process matching Over the past 10 years, Linde has been responsible for the design and commissioning of 30 liquid nitrogen washing industrial units, 14 of which are equipped with a low-temperature methanol washing purification process. However, in the past, Linde Corporation combined the low-temperature methanol wash process with the liquid nitrogen wash process mainly for use in GE/Texaco oil refining or gasification units; the two most recent units (installed in 2004) were used to process syngas derived from Shell’s gasification process, at lower operating pressures. Both of these units are located in Yunnan Province, China, and supply ammonia to Tianan Company and Zhanhua Company; their designed ammonia production capacity ranges from 560 to 600 kt/year (1,700 to 1,800 t/day). It is planned to be operational by the end of 2006 or early 2007. Contrary to the operating pressure of 7–8 MPa in earlier high-pressure liquid nitrogen washing processes, the designed operating pressure for the liquid nitrogen washing process in these devices is slightly above 3 MPa. Under such pressure, cooling in the cryostat of the liquid nitrogen washing process using the Joule-Thomson effect is insufficient; therefore, some liquid nitrogen must be supplied from the air separation unit to maintain a cold balance. Due to the low operating pressure and large scale of the units, the dimensions of their cold boxes (6m×4.5m×22m) exceed transportation limits; therefore, Linde Company must assist with on-site manufacturing. Linde has experience in on-site manufacturing and is capable of installing cryogenic tanks that are twice the size of those used in air separation units; therefore, for future large-scale ammonia plants, there is great potential for the development of processes that combine low-temperature methanol washing with liquid nitrogen washing. 5 Conclusion The combination of low-temperature methanol washing and the downstream liquid nitrogen washing process provides a reliable process solution for the purification of ammonia synthesis gas. Both the low-temperature methanol washing and liquid nitrogen washing processes perform excellently in the efficient utilization of hydrogen and the purification of syngas. Linde is able to provide specialized acid gas removal systems, built on long-term design and operational experience as well as specialized expertise in matching low-temperature methanol washing with liquid nitrogen washing purification processes, to accommodate various mixed feed gases and gasification conditions, adjust specific pressures, and handle trace impurities. Compared to other physical or chemical absorption methods, the low solvent circulation rate, along with Linde’s patented spiral heat exchanger in its low-temperature methanol washing purification process, provides economic and capacity advantages for one-step processes. If Linde Corporation can install large cryogenic tanks on-site for air separation units, then the liquid nitrogen washing process also holds great potential for use in large-scale installations in the future.