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The synthesis of ammonia is the final step in an ammonia plant, where the task is to directly convert a refined hydrogen-nitrogen mixture into ammonia under appropriate temperature and pressure conditions in the presence of a catalyst. The resulting gaseous ammonia is then condensed and separated from the mixture of gases that did not undergo synthesis to produce ammonia, thereby yielding liquid ammonia as the product; the hydrogen and nitrogen gases remaining after ammonia separation are reused. I. Characteristics of the ammonia synthesis reaction The chemical equation for ammonia synthesis is as follows: This chemical reaction has the following characteristics: (1) It is a reversible reaction. That is, while hydrogen and nitrogen react to form ammonia, ammonia also decomposes into hydrogen and nitrogen. (2) It is an exothermic reaction. Heat is released while ammonia is generated; the heat of reaction is related to temperature and pressure. (3) It is a reaction involving volume reduction. (4) The reaction requires a catalyst to proceed more rapidly. II. Chemical equilibrium of the ammonia synthesis reaction 1. Equilibrium constant The equilibrium constant for the ammonia synthesis reaction can be expressed as: = Where p(NH3), p(H2), and p(N2) represent the partial pressures of ammonia, hydrogen, and nitrogen at equilibrium. Since the synthesis reaction is a reversible, exothermic reaction that results in a decrease in volume, according to the law of equilibrium shift, lowering the temperature and increasing the pressure cause the equilibrium to shift in the direction of ammonia formation, thereby increasing the equilibrium constant. 2. Equilibrium ammonia content: When the reaction reaches equilibrium, the percentage of ammonia in the mixed gas is referred to as the equilibrium ammonia content, or the equilibrium yield of ammonia. The balanced ammonia content is the maximum level that can be achieved by the synthesis reaction under given operating conditions. The purpose of calculating the equilibrium constant is to determine the equilibrium ammonia content. The relationship between the equilibrium ammonia content and pressure, equilibrium constant, inert gas content, and hydrogen-to-nitrogen ratio is as follows: Where --- is the volume percentage of ammonia at equilibrium; ----- is the volume percentage of inert gases; p is the total pressure; ----- is the equilibrium constant; and r is the hydrogen-to-nitrogen ratio. It can be seen from this equation that as the temperature decreases or the pressure increases, the value on the right side of the equation increases, and consequently the equilibrium ammonia content also increases. Therefore, in actual production, the ammonia synthesis reaction is carried out under pressure. III. Ammonia Synthesis Kinetics (I) Reaction Mechanism Nitrogen and hydrogen approach the catalyst surface from the gas phase; the majority of them diffuse from the outer surface toward the inner surface of the catalyst’s pores, where they undergo active adsorption. The adsorbed nitrogen reacts chemically with the adsorbed hydrogen and gaseous hydrogen to produce NH, NH2, and NH3. The latter enters the gas phase space after desorbing from the surface. The entire process can be represented as follows: In the aforementioned reaction process, when the gas flow velocity is high enough and the particle size of the catalyst is small enough, the effects of external diffusion and internal diffusion on the reaction are minimal. The rate at which nitrogen is adsorbed onto the iron catalyst is numerically very close to the rate of ammonia synthesis; in other words, the step of active adsorption of nitrogen proceeds most slowly, and this is the key factor determining the reaction rate. In other words, the rate of the synthesis reaction is controlled by the adsorption rate of nitrogen. (II) Reaction rate The reaction rate is expressed as the decrease in the concentration of the reacting substance or the increase in the concentration of the product per unit of time. In industrial production, not only is a high ammonia content required, but also a fast reaction rate is needed in order to synthesize more ammonia from hydrogen and nitrogen per unit of time. Based on factors such as the active adsorption of nitrogen on the catalyst surface being the controlling step in the ammonia synthesis process, a moderate coverage of nitrogen on the catalyst surface, and highly uneven adsorption on the surface, Temkin and Pereverzev derived the following rate equation: W ----- the instantaneous overall reaction rate, which is the difference between the rates of the forward and reverse reactions; k_f and k_r ----- the rate constants for the forward and reverse reactions respectively; P_H, P_N, P_A ----- the partial pressures of hydrogen, nitrogen, and ammonia gases. (III) Effect of internal diffusion: When the particle diameter of the catalyst is 1 mm, the rate of internal diffusion is more than a hundred times higher than the reaction rate; therefore, the effect of internal diffusion can be ignored. However, when the radius is greater than 5 mm, the internal diffusion rate becomes slower than the reaction rate, and its influence cannot be ignored. The smaller the diameter of the catalyst pores and the longer the pores (the larger the particle diameter), the greater the influence of internal diffusion. In actual production, as long as the structure of the synthesis tower and the resistance of the catalytic layer permit it, catalysts with a smaller particle size should be used to reduce the impact of diffusion, improve the utilization rate of the inner surface, and accelerate the rate of ammonia production. IV. Various factors affecting the operation of the synthesis tower 1. Conditions affecting the reaction in the synthesis tower: The catalytic synthesis reaction can be expressed by the following formula. Under the recommended operating conditions, the ammonia content in the gas exiting the synthesis tower is approximately 13.9% (on a molecular basis). The unreacted gas is recycled back to the synthesis tower, where it ultimately becomes part of the product. (1) Temperature: Changes in temperature have two effects on the ammonia synthesis reaction; it affects both the equilibrium concentration and the reaction rate. Since the ammonia synthesis reaction is exothermic, an increase in temperature reduces the equilibrium concentration of ammonia while simultaneously accelerating the reaction. This indicates that at conditions far from equilibrium, the synthesis efficiency is higher when the temperature rises. On the other hand, for systems at equilibrium, the synthesis efficiency is lower when the temperature increases. Without taking catalyst degradation into account, the synthesis efficiency always varies directly with temperature. The synthesis efficiency is defined as the ratio of the actual percentage of ammonia in the gas after the reaction to the percentage of ammonia that could theoretically be obtained under the given conditions. (II) Pressure: During ammonia synthesis, the volume decreases (the number of molecules decreases); therefore, the equilibrium percentage of ammonia increases as pressure rises. Meanwhile, the reaction rate also accelerates with increasing pressure, so raising the pressure will promote the reaction. (III) Space velocity: At higher process gas velocities (space velocity), the reaction time is shorter; as a result, the ammonia concentration at the outlet of the synthesis tower is not as high as it is at lower space velocities. However, the percentage decrease in yield is much smaller than the increase in space velocity. Since more gas passes through the synthesis tower, the increased ammonia production is sufficient to compensate for the yield loss caused by the short residence time and incomplete reaction. Therefore, under normal or reduced production levels, with all other conditions unchanged, increasing the gas flow rate through the synthesis tower will boost the yield. Typically, the air velocity is adjusted by changing the amount of circulating gas. When the circulation volume increases (if possible), the temperature of the catalyst bed drops due to a decrease in the one-pass synthesis efficiency, and the system pressure also decreases as a result of an increase in the total ammonia production. When MIC-22 is shut down, the circulation volume increases, and it reaches its maximum when MIC-22 is completely closed. (IV) Hydrogen-to-nitrogen ratio: The hydrogen-to-nitrogen ratio of the fresh syngas fed to the synthesis section should generally be maintained at around 3.0:1.0, as hydrogen and nitrogen are combined in a 3.0:1.0 ratio to produce ammonia. It should be noted, however, that the hydrogen-to-nitrogen ratio in the synthesis tower is not necessarily 3.0:1.0; it has been found that the highest synthesis efficiency is achieved when this ratio ranges from 2.5 to 3.0:1.0. To achieve the optimal H2:N2 ratio in the gas mixture entering the synthesis tower, the hydrogen-to-nitrogen ratio in the fresh gas can be slightly different from 3.0:1.0. (V) Inert gases: A portion of the gas is continuously discharged from the suction side of the circulator to the exhaust system, in order to control the levels of methane and other inert gases. Otherwise, they would accumulate in the synthesis loop, reducing the efficiency of synthesis, increasing system pressure, and decreasing production capacity. (VI) Fresh gas: Increasing the flow rate of fresh gas alone can produce more ammonia, and it has the following effects on the aforementioned conditions: I. Increase in system pressure ； II. Increase in catalyst bed temperature ； III Increase in inert gas content ； Ⅳ H2:N2 ratio may change. Conversely, if the amount of syngas decreases, the effect is the opposite. Under normal operating conditions, the amount of fresh gas is determined by the production volume; however, an increase in the gas supplied to the synthesis section must be preceded by an increase in the gas output from the plant’s gas generation process. 2. Operational control of the synthesis reaction: The synthesis system begins with the pipeline at the outlet of the synthesis gas compressor. The amount of gas consumed (a mixture with a hydrogen-to-nitrogen ratio of 3:1) depends on the operating conditions, the activity of the catalyst, and the overall production capacity of the synthesis circuit. The gas that is removed or used in the reaction is continuously replaced by gas coming from the compressor. If there is an excess of fresh gas, the production volume increases up to the compressor’s maximum capacity; in such cases, the excess fresh gas is vented from the 104-F suction tank before it enters the compression stage. If the amount of gas is insufficient, the compressor slows down, and the pressure in the circuit decreases until the ammonia production volume is balanced with the amount of gas entering the system. To change the operation of the synthesis loop, one or several conditions can be altered. The more important control conditions are as follows: fresh gas flow rate, inlet temperature of the synthesis tower, recycle gas flow rate, hydrogen-to-nitrogen ratio, high-pressure off-gas flow rate, purity of the fresh gas, and temperature of the catalyst layer. It should be noted that system pressure is not listed as a control condition here, as changes in pressure are often the result of changes in other conditions; it is rare to increase pressure for the sole purpose of doing so without considering any other effects. Synthesis systems are usually operated by keeping the pressure at a level below its limit, minimizing the off-gas flow rate, and maintaining the synthesis tower at a sufficiently low temperature to extend the catalyst’s lifespan. Under conditions where the fresh gas and exhaust gas flow rates are normal and the synthesis temperature is appropriate, a lower pressure generally indicates good operation. Below are some factors that affect various conditions in the synthesis loop; operators should pay attention to checking for any abnormal changes during these processes. By understanding all such situations, operators can more easily explain any changes in operating conditions, allowing them to adjust one or several of those conditions as necessary. Pressure in the synthesis tower: The main factors that can increase the pressure in the synthesis loop, either individually or collectively, include: (1) an increase in the amount of fresh gas ； (II) Decrease in the temperature of the synthesis tower ； (III) The gas composition in the synthesis loop deviates from the optimal hydrogen-to-nitrogen ratio (2.5~3.0:1). (IV) The ammonia content in the recycle gas increases ； (5) Increase in the content of inert gases in the circulating gas: (6) Decrease in the volume of circulating gas ； (7) Catalyst poisoning caused by impure syngas (8) Catalyst aging. Conversely, Ming – acting in the opposite direction to those effects – will reduce pressure. The temperature of the catalyst; the main factors that can raise the catalyst temperature, either individually or collectively, are: (1) an increase in the amount of fresh gas ； (II) Reduction in circulating air volume ； (III) The hydrogen-to-nitrogen ratio is relatively close to the optimal range of 2.5–3.0:1 ； (IV) Reduction in ammonia content in the recycled gas ； (V) Increase in pressure of the synthetic system ； (VI) Reduction in the flow rate of cold air taken via a shortcut into the synthesis tower (cold shock) ； (7) Reduction in the content of inert gases in the circulating gas ； (8) After temporary catalyst poisoning caused by impure syngas, the catalyst activity is restored subsequently. Conversely, acting in the opposite direction to those effects will lower the temperature of the catalyst. The optimal temperature for stable operation is the lowest temperature at which ammonia production is highest; however, the temperature must still be high enough to ensure stability during pressure fluctuations. Excessive temperatures can cause the catalyst to age and lead to a rapid decline in its activity. Hydrogen-to-nitrogen ratio: The main factors that can individually or collectively change the H2:N2 ratio in the recycle gas are: (1) changes in the composition of the syngas coming from the conversion and purification systems ； (II) Changes in fresh air volume: (III) Changes in the ammonia content in the recirculated air ； (IV) The content of inert gases in the circulating gas changes. The hydrogen-to-nitrogen ratio in the recycle gas entering the synthesis tower should be maintained at around 2.5–3.0:1.0; rapid changes in this ratio can cause sharp fluctuations in temperature. Ammonia content in the recycle gas: The factors that can cause changes in the ammonia concentration in the feed gas to the synthesis tower, either individually or collectively, include: (1) Changes in the degree of cooling in the ammonia cooler located before the high-pressure ammonia separator 106-F ； (II) System pressure: The expected ammonia concentration in the outlet gas of the synthesis tower is approximately 1.39%. After the recycled gas is mixed with fresh gas, the ammonia concentration rises to 4.15%. Following ammonia cooling and separation using 106-F, the ammonia concentration in the gas mixture entering the synthesis tower is about 2.42%. The content of cyclic gases in the recycled gas: The main components of the inert gases in the recycled gas are argon and methane. These gases accumulate over time, increasing the pressure in the system and thereby reducing the effective partial pressure of the syngas; this results in a decrease in the synthesis efficiency per pass. To control the concentration of inert gases in the system, a portion of the gas is diverted through 125-C and then released into the vent gas separation tank 108-F. The designed concentration of inert gases (methane and argon) in the inlet gas to the synthesis tower is approximately 13.6% on a molecular basis. However, it has been found that maintaining a higher concentration of inert gases can reduce the amount of hydrogen carried away by the vent gas, while also increasing the total yield of nitrogen. As can be seen from the discussion on ammonia synthesis operations above, the efficiency of synthesis is influenced by the various control conditions listed at the beginning of section “(2)”. All these conditions are interrelated; a change in one condition affects the others as well. Therefore, good operation requires a combination of practical experience and an understanding of the various factors that affect system operation. If any of these conditions undergoes a sudden change, experience helps determine what steps should be taken to compensate for that change and thus maintain stable system operation. It may be necessary to make changes gradually in order to avoid significant fluctuations. 3. Performance of synthetic catalysts: Activation of catalysts: Synthetic catalysts are made from molten iron oxides; they contain oxides of potassium, calcium, and aluminum as stabilizers and promoters, and these are introduced into the synthesis tower in their oxidized state. Before ammonia production can take place, the catalyst must be activated, with the iron oxide being reduced to essentially pure elemental iron. The reduction of the catalyst takes place under such conditions: hydrogen is passed over the catalyst in its oxidized state, and the pressure and temperature are gradually increased. Hydrogen reacts with the oxides in iron oxide to form water; this water must be removed as much as possible before it can circulate back to the catalyst bed. The amount of water produced during the activation process is a good indicator of the progress of catalyst reduction. At the beginning of the reduction process, little water is generated; as the catalyst continues to be reduced, the amount of water produced increases. To facilitate catalyst reduction, a relatively high temperature is required along with pressure control, and the amount of water generated reaches a peak before gradually decreasing until the reduction is complete. The reduction temperature should always be kept below the operating temperature of the catalyst, in order to avoid deactivation due to the following reasons: (1) high water vapor concentration in the recycle gas, (2) overheating; however, if the temperature is too low, the reduction of the catalyst proceeds too slowly, and if it drops too low, the reduction will cease. During the reduction of the catalyst, the effect of pressure and/or pressure changes is crucial; as the reduction proceeds, if the activation of the different layers of catalyst is uneven, increasing the pressure can lead to \"channeling\": that is, in certain areas of the catalyst bed where the catalyst is more thoroughly reduced, the reaction between hydrogen and nitrogen to form nitrogen occurs, and the heat released by this reaction can raise the temperature of that local area of the catalyst to such an extent that it becomes difficult to control. During catalytic reduction, the pressure should be maintained such that even if the reduction occurs uniformly and with minimal temperature differences across the same level of the catalyst bed, increasing the pressure accelerates the ammonia production reaction, while decreasing the pressure slows it down. The reduction of the catalyst can be carried out at quite low space velocities, but the higher the space velocity, the shorter the reduction time; moreover, channeling can be eliminated at higher space velocities. During catalytic reduction, syngas is circulated through the synthesis tower. Once the reaction has begun, it is crucial to cool the circulating gas as much as possible (without the risk of freezing in the equipment), so that the water content in the gas can be condensed and removed before it re-enters the synthesis tower. Otherwise, gas with a high water vapor concentration will enter the already reduced catalyst bed, and the water vapor can reduce the activity or cause poisoning of the reduced catalyst. Once the ammonia synthesis reaction starts, the ammonia produced will lower the freezing point, allowing the water content in the gas stream to be removed at lower temperatures. Carefully controlling the conditions during catalyst activation allows for uniform reduction, which helps to extend the catalyst’s service life. The reduction of the synthetic catalyst is carried out during the initial startup of the plant; the recommended procedure is provided in Section 3. Thermal stability of the catalyst: Even when pure syngas is used, ammonia catalysts cannot maintain their activity indefinitely. Some data indicate that when pure gas is used, temperatures below 550°C have no effect on the catalyst; however, at higher temperatures the catalyst is damaged. These data also show that a catalyst that has been slightly overheated experiences a decrease in activity at 400°C, while its activity remains unchanged at 500°C. It should be emphasized, however, that there is no fixed temperature limit below which the catalyst remains unaffected – even at a constant temperature, harsh conditions regarding pressure and space velocity can cause the catalyst’s activity to decline rapidly. The aging of a catalyst is first manifested as a decrease in efficiency when it operates under conditions of lower temperature, higher pressure and/or higher space velocity. It has been found that the greater the decline in the catalyst’s activity compared to its initial level, the longer it will take for further damage to occur, or the more severe the conditions required for that to happen. Catalyst poisons: Compounds in syngas that can reduce the activity or lifespan of a catalyst are known as poisons. These substances are able to form compounds with the active components of the catalyst to varying degrees of stability. Permanent poisons cause an irreversible and permanent decline in the catalyst’s activity; they can form stable surface compounds with the catalyst’s active parts. Other poisons, on the other hand, only lead to a temporary reduction in activity, and the catalyst can return to its original level of activity within a relatively short time once these poisons are removed from the gas. The main poisons for ammonia synthesis catalysts are oxygen compounds. These compounds cannot be considered either temporary or permanent poisons. When the syngas contains small amounts of oxides, such as CO, some of the active surfaces of the catalyst bind with oxygen, resulting in a reduction in the catalyst’s activity. Once these oxygen compounds are removed from the syngas, the catalyst is restored to its full activity level once again; however, not all of its active sites can return to their original state or regain their initial activity. Therefore, oxygen compounds can cause severe temporary poisoning as well as mild permanent poisoning. Common oxygen compounds that can poison catalysts include water vapor, H2, CO2, and molecular O2. Other important poisons include H2S (permanent) and deposits of oil mist; the latter is not a true poison, but it can reduce the catalyst’s activity by covering and clogging its surface. Mechanical strength of the catalyst: The mechanical strength of synthetic catalysts is quite good, but operators should not handle them carelessly; improper handling can cause rapid temperature fluctuations, which in turn may lead to the breakdown of the catalyst. During the catalyst reduction process, any sudden temperature changes must be carefully avoided, as it is believed that the catalyst is particularly sensitive to mechanical stress and sudden temperature changes during this period. During the initial startup of the plant, the reduction of the synthetic catalyst was carried out only when the processes ahead in the plant had approached the designed conditions and flow rates. For detailed catalyst loading procedures, please refer to the catalyst manufacturer’s instructions on the method of loading catalyst into the ammonia synthesis tower. It is extremely important to conduct certain tests before loading the catalyst. Contact between chlorides and stainless steel catalysts can cause stress corrosion cracking in the components of the synthesis tower; therefore, the chlorine content in each batch of catalyst must be checked before loading. The maximum allowable level of water-soluble chlorine in the catalyst is 10.0 ppm. If the containers used for holding the catalyst are damaged, impurities may be introduced, so each container should be inspected. III. Separation of anhydrous liquid ammonia from syngas The ammonia produced in the synthesis tower quickly reaches levels that are unfavorable for the reaction; therefore, it must be continuously removed from the syngas flowing into the tower. This is achieved by using a series of coolers and ammonia coolers to cool the syngas, thereby cooling the net ammonia product generated with each pass through the synthesis tower. The temperature of the syngas entering the high-pressure ammonia separator is -21.3°C, and at a pressure of -11.7 MPa, the ammonia in the gas stream in the synthesis circuit condenses and is supercooled to -23.3°C. As a result, the ammonia content in the syngas drops to 2.42%. The condensed liquid ammonia is collected in the high-pressure ammonia separator (106-F), and after being regulated by the level controller (LC-13), it is sent on for final product purification. V. Main Equipment for Ammonia Synthesis 1. Synthesis Tower 1.1 Structural Features The ammonia synthesis tower is a key piece of equipment in ammonia production; its function is to enable the mixture of hydrogen and nitrogen to be converted into ammonia within the catalyst layer located in the tower. Since the reaction takes place under high temperature and pressure, the synthesis tower is required to not only have high mechanical strength but also the ability to resist creep and relaxation at high temperatures. At the same time, under high temperature and pressure, hydrogen and nitrogen have a significant corrosive effect on carbon steel, further complicating the operating conditions of the synthesis tower. The corrosion effects of hydrogen on carbon steel include hydrogen embrittlement and hydrogen corrosion. Hydrogen embrittlement refers to the phenomenon where hydrogen dissolves into the metal lattice, causing brittle failure in steel under slow deformation. Hydrogen corrosion refers to the penetration of hydrogen into steel, which causes the carbides to decompose and methane to be generated: FeC + 2H2 → 3Fe + CH4 + Q. The methane produced by this reaction accumulates in the existing micro-pores at the grain boundaries, resulting in excessive local pressure and stress concentration. This leads to the formation of cracks, and the accumulation of methane within the steel causes bulging, thereby damaging the steel’s structure and reducing its mechanical strength. Under high temperature and pressure, nitrogen forms hard and brittle nitrides with iron and many other alloying elements in steel, thereby reducing the mechanical properties of the steel. To adapt to the conditions of the ammonia synthesis reaction and properly resolve the existing contradictions, the ammonia synthesis tower consists of an internal component and an outer cylinder, with the internal component placed inside the outer cylinder. The gas entering the synthesis tower (at a lower temperature) first passes through the annular gap between the internal components and the outer cylinder; an insulation layer is provided around the internal components to reduce heat loss to the outer cylinder. Therefore, the outer cylinder is primarily subjected to high pressure (the difference between the operating pressure and atmospheric pressure), but not to high temperatures; it can be made from ordinary low-alloy steel or high-quality carbon steel. The internal components operate at high temperatures of around 500°C, but they are only subjected to the pressure difference between the circumferential airflow and the airflow within the components; this pressure difference is usually only 1–2 megapascals. In other words, the internal components are exposed to high temperatures but not to high pressures, which reduces the requirements regarding the materials and strength of these components. The internal components are generally made of alloy steel; those for towers with a smaller diameter can also be made of pure iron. The internal components consist of three main parts: a catalyst basket, a heat exchanger, and an electric heater. Large ammonia synthesis towers generally do not have electric heaters; instead, they are heated by a furnace located outside the tower. 1.2 Classification and structure Since the optimal temperature for the ammonia synthesis reaction gradually decreases as the ammonia content increases, cooling measures must be taken in the catalyst layer as the reaction proceeds. Based on the cooling method, ammonia synthesis towers can be divided into the following three categories: (1) Cold-tube type. Cooling tubes are installed in the catalyst layer; the feed gas, which has a lower temperature before the reaction, flows through these cooling tubes, thereby removing the heat generated by the reaction and lowering its temperature. At the same time, this process preheats the feed gas to the reaction temperature. Depending on the structure of the cold tube, it can be further divided into various types such as double-tube, triple-tube, and single-tube. The cold-tube type synthesis tower has a complex structure and is generally used in small ammonia synthesis towers. (2) Cold-shock type. The catalyst is divided into multiple layers; as the gas passes through each layer, its temperature rises due to adiabatic reactions, after which it is mixed with cold feed gas, causing its temperature to drop before it proceeds to the next layer of catalyst. The cold shock type has a simple structure, but the addition of unreacted cold feed gas reduces the ammonia synthesis rate; it is generally used in large-scale ammonia synthesis towers. (3) Intermediate heat exchange type. The catalyst is divided into several layers, with heat exchangers placed between them; the high-temperature gas resulting from the reaction in the upper layer enters the heat exchanger to cool down before proceeding to the next layer for further reaction. 2. Synthetic compressors: In large ammonia plants, synthetic compressors use centrifugal compressors driven by steam turbines. Such units are primarily composed of the compressor main body, the drive machine, the lubricating oil system, the seal oil system, and anti-surge devices. 2.1 Working principle of centrifugal compressors The working principle of centrifugal compressors is similar to that of centrifugal pumps: gas flows into the impeller from the center, and under the action of the rapidly rotating impeller, it rotates at high speed along with the impeller and is thrown out in a radial direction. Driven by the driving machinery, the impeller rotates and transfers the resulting mechanical energy to the gas flowing past it through the impeller; in other words, the centrifugal compressor does work on the gas via the impeller. On the one hand, the gas experiences rotational centrifugal force, which increases its own pressure; on the other hand, it gains a large amount of kinetic energy. After the gas leaves the impeller, this portion of kinetic energy can be converted into pressure energy as it passes through the diffuser and the return bend, further increasing the pressure of the gas. In centrifugal compressors, the increase in pressure of the gas after being compressed by an impeller is limited. Therefore, when a higher pressure rise is required, multiple stages of impellers are typically used to compress the fluid one after another in sequence, until the outlet of the final stage reaches the desired pressure. The more impellers a compressor has, the greater the total pressure head it can generate. The temperature of the gas rises after compression. When a high degree of compression is required, the gas is often compressed to a certain pressure and then drawn out of the cylinder, cooled in an external cooler, and subsequently fed back in for further compression. In this way, the compressor is divided into several sections based on the number of cooling cycles, with each section being either one stage or multiple stages. 2.2 Surge phenomenon in centrifugal compressors and preventive measures Surge in centrifugal compressors is an abnormal condition that occurs due to improper operation and an insufficient inlet gas flow rate. When the flow rate of the incoming gas is reduced to an inappropriate level, the velocity of the gas as it enters the impeller becomes too low; the gas no longer flows along the impeller, and a large vortex region forms on the back side of the blades. This vortex region may even fill the entire blade passage, blocking it off, so that the gas can only swirl within this vortex area and cannot flow out. At this point, the gas in the system flows back from the compressor outlet into the compressor, temporarily compensating for the insufficient intake volume. Although the compressor seems to have resumed normal operation and started compressing gas again, once the gas is compressed, the aforementioned backflow phenomenon occurs repeatedly due to the ongoing shortage of incoming gas. Such a flow pattern of air that alternates between being drawn in and expelled at the outlet causes low-frequency, high-amplitude flow fluctuations in the outlet duct, which quickly affect the impellers at all stages; as a result, the entire compressor generates noise and vibration. This phenomenon is known as surge. Surge is very detrimental to machinery; excessive vibration can cause localized overheating, and over time it may even lead to serious accidents such as the destruction of the impeller. When surge occurs, efforts should be made to immediately increase the inlet gas flow rate. The method involves using an anti-surge device to return a portion of the gas at the compressor outlet back to the compressor inlet through a bypass valve, or opening the outlet vent valve to reduce the outlet pressure. 2.3 Structure of centrifugal compressors A centrifugal compressor consists of two main parts: the rotor and the stator. The rotor consists of components such as the main shaft, impeller, shaft sleeve, and balance disk. All rotating components are mounted on the main shaft; except for the shaft sleeves, the other components are fixed to the main shaft using keys. The spindle is mounted on radial bearings to facilitate rotation. The impeller is the main component of a centrifugal compressor, and it is equipped with several blades used to compress gas. As the gas is compressed by the blades, its pressure increases; as a result, the gas pressure acting on each side of the blades differs, generating an axial thrust directed toward the low-pressure side. This thrust can cause the rotor to move toward the low-pressure side, and in severe cases, it can lead to friction and collision between the rotor and the stator. To eliminate axial thrust, a balance disc and a thrust bearing are installed on the outside of the high-pressure side. One side of the balance disk is in contact with high-pressure gas, while the other side is in contact with low-pressure gas; the thrust generated by the pressure difference between the two sides is used to balance the axial thrust. The stator of a centrifugal compressor consists of components such as the cylinder, diffuser chamber, bend, return vessel, partition, seal, and bearings. Cylinders, also known as housings, come in two types: horizontally split and vertically split. Horizontal splitting involves dividing the casing into upper and lower sections, with the upper cover being removable; this design is commonly used in low-pressure applications. A vertical split structure is a tubular design, consisting of a cylindrical body and end caps, and is commonly used in high-pressure applications. There are several partitions inside the cylinder, which separate the blades and form diffusers, bends, and recirculators. To prevent cross-ventilation between stages or leakage outward, inter-stage seals and shaft seals are provided. The auxiliary equipment of a centrifugal compressor includes intercoolers, gas-liquid separators, and oil systems. 2.4 Working Principle of Turbines A turbine, also known as a steam turbine, is a rotary prime mover that uses steam to generate power. The high-pressure, high-temperature steam entering the turbine is ejected through nozzles; after expansion and pressure reduction, the resulting high-speed airflow pushes against the moving blades on the turbine rotor in a specific direction, causing the rotor to rotate at a constant speed and thus converting the energy of the steam into mechanical energy. Due to different methods of energy conversion, turbines are divided into impulse type and reaction type. In the impulse type, steam expands only within the nozzle, and the moving blades are subjected only to the force of the high-speed airflow. In reaction turbines, steam expands not only in the nozzles but also within the blades; the moving blades are subjected to both the thrust from the high-speed airflow and the reaction force generated by the expansion of steam within those blades. Depending on the number of impeller stages in the turbine, they can be divided into single-pole or multi-pole types. Based on different thermodynamic processes, steam turbines can be classified into back-pressure type, condensing type, and extraction-condensing type. In a back-pressure steam turbine, the steam does work through expansion and is then discharged from the turbine at a certain temperature and pressure, where it can be used again in the process ； After expanding to perform work, the inlet steam of the condensing steam turbine is entirely discharged into the condenser where it condenses into water ； In an exhaust-condensing steam turbine, as the steam expands to perform work, a portion of it is extracted for other uses, while the remaining part continues to do work inside the cylinder before being discharged to the condenser to be condensed. Chapter 2: Equipment Overview Section 1: Brief Description of the Process Flow I. Synthesis System The fresh gas derived from methanation (40°C, 2.6 Mpa, H2/N2 = 3:1) first passes through the pre-compression separator tank (104-F) before entering the low-pressure stage of the syngas compressor (103-J). In the low-pressure cylinder of the compressor, the fresh gas is compressed to about half of the final pressure required for synthesis. The gas exiting the low-pressure stage is first cooled to 93.3°C using the methanation feed gas in vessel 106-C, then cooled to 38°C via a water cooler (116-C), and finally cooled to 7°C using an ammonia cooler (129-C). After that, it is mixed with the hydrogen recovered from other processes and sent to the intermediate separator tank (105-F). The hydrogen and nitrogen gases emerging from the intermediate separator tank then proceed to the high-pressure stage of the syngas compressor. The recycle gas from the synthesis loop is mixed with the hydrogen and nitrogen gases that have been compressed in the high-pressure section and then fed into the compressor circulation section; the syngas that emerges from this circulation section goes to the water cooler (124-C) of the synthesis system. After emerging from the final cooler 124-C, the high-pressure syngas is cooled further in two separate paths. The first path passes through the tube sides of the primary and secondary ammonia coolers 117-C and 118-C for the feed gas and recycle gas, with liquid ammonia used as the cooling medium. The other path undergoes throttling via the on-site MIC-23 before being cooled on the shell side of the reactor inlet and recycle gas heat exchanger 120-C. After these two paths converge, they are cooled once more in the tertiary ammonia cooler 119-C, using liquid ammonia from the tertiary liquid ammonia flash tank 112-F, to a temperature of -23.3°C. The cooled gas enters the high-pressure ammonia separator (106-F) through horizontal distribution pipes; the ammonia present in the circulating gas that has condensed in the preceding ammonia coolers is separated out in 106-F, and the separated liquid ammonia is sent to the refrigeration intermediate flash tank (107-F). After exiting the ammonia separator, the recycle gas enters the tube side of the reactor inlet – fresh gas and recycle gas heat exchanger 120-C, where it absorbs heat from the process gas on the shell side. It then goes to the tube side of the reactor inlet – outlet heat exchanger (121-C), and under the control of HCV-11, it enters the reactor (105-D). The gas composition is analyzed at the exit on the tube side of 121-C. SP-35 is a specialized two-way burst pressure relief device designed to protect the heat exchanger of 121-C, preventing damage caused by excessive pressure differences resulting from pressure release on one side of the heat exchanger. The syngas feed enters the synthesis tower 105-D at its bottom and moves upward through the tower, reaching directly the inlet of the first catalyst layer via MIC-13, which is used to control the temperature there. This shortcut includes a cold shock line as well as two inter-layer heat exchanger branches that allow for the control of the inlet temperatures of the second and third layers; adjustments can be made using MIC-14, 15, and 16 as needed. After passing through the catalyst bed at the very bottom, the gas is guided upward to the tube side of the internal heat exchanger, where it transfers heat to the incoming gas, before being discharged from the top outlet of 105-D. The gas exiting the synthesis tower enters the tube side of the synthesis tower–boiler feedwater heat exchanger 123-C, transferring heat to the boiler feedwater; it then exchanges heat with the gas entering the tower on the shell side of 121-C, thereby being further cooled, and finally returns to the high-pressure cylinder circulation section 103-J (the last impeller), thus completing the entire synthesis cycle. A portion of the gas coming out of the synthesis tower is sent to the hydrogen recovery unit or to the fuel gas system of the first-stage converter, after passing through the high-pressure blowout gas separation cylinder 108-F, being regulated by MIC-18, and having its flow rate monitored by Fl-63. The gas discharged from the synthesis loop is used to control the concentrations of methanation and argon in the gas; an excessive accumulation of methane and argon in the system reduces the ammonia synthesis rate. The blown gas is cooled in the ammonia cooler 125-C before entering the separation tank 108-F, and the liquid ammonia separated from 108-F is sent to the low-pressure ammonia separator 107-F for recovery. The synthesis tower is equipped with a start-up heater (102-B), which is used to raise the temperature of the synthesis tower to the reaction temperature at startup. The flow rate of the feed gas to this start-up heater is indicated by FI-62. In addition, it has a low-flow alarm, FAL-85, which works in conjunction with FI-62, while MIC-17 is used to regulate the amount of fuel gas supplied to 102-B. II. Refrigeration System: The synthesized liquid ammonia enters the intermediate flash tank (107-F), and the non-condensable gases that emerge from the flash are discharged via PICA-8 to be used as fuel gas in the first-stage furnace for combustion. Separator 107-F is equipped with a level indicator LI-12. After being depressurized, liquid ammonia is regulated by the level controller LICA-12 and fed into the third-stage flash tank (112-F) for further flashing; thereafter, it enters the system as liquid ammonia for use in freezing purposes. The operating pressures of the frozen first, second, and third flash tanks are 0.4 MPa(G), 0.16 MPa(G), and 0.0028 MPa(G), respectively. These three flash tanks correspond to the first, second, and third ammonia coolers in the synthesis system, and they operate in a cryogenic evaporation cycle based on the principle of thermosiphon action. Liquid ammonia flows from the various flash tanks into the corresponding ammonia coolers; the liquid ammonia, after absorbing heat and evaporating, forms a gas-liquid mixture that returns to the flash tanks for gas-liquid separation. The gaseous ammonia goes into the cylinders of each stage of the ammonia compressor (105-J), while the liquid ammonia goes into the respective ammonia coolers. The liquid ammonia coming from the liquid ammonia receiving tank (109-F) is depressurized step by step before being fed into each flash tank. The liquid ammonia coming out of the primary flash tank (110-F) is sent to the first ammonia cooler (117-C); another portion of it serves as the cooling medium for the ammonia cooler at the exit of the first stage of the syngas compressor (103-J), as well as for the flash tank ammonia cooler (126-C). The evaporated ammonia from the ammonia coolers (129-C) and (126-C) enters the secondary flash tank (111-F), while the excess liquid ammonia from 110-F is sent to 111-F. Apart from being supplied to the second ammonia cooler (118-C) and the vent gas ammonia cooler (125-C) as a refrigerant, the liquid ammonia from 111-F is sent to the third-stage flash tank (112-F). In addition to being supplied to 119-C, the liquid ammonia from 112-F can also be sent by the cold ammonia product pump (109-J) to the liquid ammonia storage tank as cold ammonia product. The gaseous ammonia coming out of the three-stage flash tank (112-F) enters the ammonia compressor (105-J) for stage 1 compression; the output from this stage merges with the gaseous ammonia from 111-F to proceed to stage 2 compression. The gaseous ammonia exiting stage 2 is first cooled by the compressor intercooler (128-C), after which it merges with the gaseous ammonia from 110-F to enter stage 3 compression. The gaseous ammonia exiting stage 3 is condensed in the ammonia condensers (127-CA, CB), and the resulting liquid ammonia flows into the receiving tank (109-F). The vapor in 109-F goes to the ammonia cooler (126-C) of the flash tank; the liquid ammonia obtained through condensation returns to 109-F, while the non-condensable gas is used as fuel gas for combustion in the first-stage furnace. Part of the liquid ammonia in 109-F is sent to the primary flash tank (110-F) after being depressurized, while another part is sent to the urea plant as hot ammonia product via the hot ammonia product pump (1-3P-1,2).