The influence of air velocity, pressure, and temperature on ammonia synthesis

Synthesis certainly requires energy, such as for amino acids to form peptide chains； However, decomposition does not necessarily release energy; for example, the breakdown (hydrolysis) of proteins also requires energy, though only in small amounts. Process factors affecting the ammonia synthesis reaction and the selection of process conditions: The ammonia synthesis reaction is an exothermic, volume-reducing reversible reaction, and temperature, pressure, and gas composition have a certain influence on the extent of the reaction and the rate at which it proceeds. In industrial production, the extent of the ammonia synthesis reaction depends directly on various process conditions. The selection of parameters for this ammonia synthesis process must take into account factors such as the net yield of ammonia, reaction rate, catalyst performance, as well as raw material and energy consumption. Therefore, the choice of process conditions has significant technical and economic implications. 1. Pressure: Pressure is an important factor in determining the configuration of a synthesis loop. From the perspective of chemical equilibrium, increasing pressure helps to raise the equilibrium ammonia concentration, accelerates the reaction rate, and increases the production capacity of the facility ； Furthermore, increasing the pressure facilitates ammonia separation and reduces refrigeration power consumption. However, an increase in pressure raises the requirements regarding the material of the equipment and its manufacturing process, and it also increases the power consumption required for compressing the feed gas; therefore, factories must consider both technical and economic factors when choosing the pressure level. It depends on power consumption, as well as infrastructure investment and factory costs. Currently, the synthesis pressure in the country ranges from 15.0 to 32.0 MPaG, while this unit operates at 13.8 MPaG. 2. Temperature: The ammonia synthesis reaction is an exothermic reaction; therefore, reducing the reaction temperature helps to shift the equilibrium in favor of ammonia production. However, the reaction speed slows down as the temperature decreases. At the same time, the ammonia synthesis reaction uses active catalysts, so when selecting the reaction temperature, the active temperature range suitable for the catalyst must be taken into account first; this range is generally between 380–525°C. Furthermore, for reversible reactions, the temperature at which the overall reaction rate is maximum for a given composition of reactants is referred to as the optimal temperature for that composition. Therefore, there exists an optimal temperature for the entire reaction process in the synthesis tower, meaning that there is an optimal temperature curve corresponding to the whole reaction process in that tower. The optimal temperature continues to decrease as the reaction progresses. To this end, for the exothermic ammonia synthesis reaction, different methods are employed simultaneously to remove the heat generated during the reaction, so as to keep the reaction temperature as close as possible to the optimal value and thereby achieve better technical and economic benefits. In actual production, the temperature at the inlet to the reactor is generally kept slightly higher than the active temperature of the catalyst; the maximum temperature should not exceed the catalyst’s heat tolerance. At the beginning of use, the temperature is set a bit lower to prevent premature aging of the catalyst, while later on, higher temperatures are utilized to take advantage of the catalyst’s high-temperature activity. 3. Composition of gas entering the tower: The composition of the feed gas at the inlet of the synthesis tower includes aspects such as ammonia content, inert gas content, and hydrogen and nitrogen contents. The composition of the gas entering the tower has a significant impact on the synthesis reaction. (1) Effect of the hydrogen-to-nitrogen ratio (H2/N2): In the ammonia synthesis reaction, a hydrogen-to-nitrogen ratio of 3 yields the highest equilibrium ammonia concentration. Moreover, when all other conditions are constant, a hydrogen-to-nitrogen ratio of 3 maximizes the instantaneous rate of the ammonia synthesis reaction. In the fresh gas, hydrogen and nitrogen are partially combined to form ammonia; a small amount dissolves in liquid ammonia, some is lost due to leaks and venting, while the remainder is recycled within the system. However, the ratio of hydrogen to nitrogen lost in these cases is not necessarily 3:1 ； Furthermore, in actual production, the pressures of the feed gas and the nitrogen supply also fluctuate, which affects the H2/N2 ratio. Therefore, to maintain a hydrogen-to-nitrogen ratio of 3:1 in the feed gas at Jinta, it is necessary to adjust the ratio of hydrogen to nitrogen in the fresh gas according to changes in the gas composition within the circuit, so as to keep it at an optimal level. Practice has shown that when the ratio of H2 to N2 is 3, the reaction rate is fastest and the yield is highest. (2) Effect of ammonia content: According to chemical equilibrium theory, a low ammonia content in the recycle gas at the inlet of the synthesis tower results in a higher catalytic reaction rate and greater amounts of ammonia produced; simultaneously, more cooling energy is required to separate the ammonia. Synthesis towers operating at different pressures use ammonia with varying inlet concentrations. For synthesis towers operating at higher pressures, both the equilibrium ammonia content and the catalytic reaction rate are higher; it is thus possible to maintain a higher inlet ammonia concentration, which in turn allows the ammonia cooling temperature to be set at a higher level as well ； For towers with lower operating pressures, in order to maintain a certain production capacity and ammonia yield, the inlet ammonia concentration is kept low, which requires more cooling energy for ammonia separation. When the circulation volume remains constant, the higher the ammonia content in the inlet gas, the slower the reaction rate in the synthesis tower; moreover, less ammonia is separated with each cycle of the gas, resulting in a lower production capacity. The volume of recycle gas that must pass through the synthesis tower for each ton of ammonia produced can be calculated using the following formula: V1=1318×(1+Z2)/(Z2-Z1) (Nm3/TNH3). Here, Z1 and Z2 represent the ammonia concentrations at the inlet and outlet of the synthesis tower, respectively (as volume fractions). It can be seen that the higher the imported ammonia concentration Z1, the greater the circulation gas volume V1. However, its impact largely depends on the ammonia concentration Z2 of the off-gas. In addition, the selection of ammonia concentration entering the tower also needs to take into account factors such as separation power consumption. The ammonia content in the ammonia entering our plant is controlled at 3–5%. (3) Effect of inert gases: The level of inert gases in the synthesis system affects the concentration of the active gas components used in synthesis. A lower content of inert gases allows for an increase in the partial pressures of hydrogen and nitrogen, which is beneficial for maintaining the equilibrium of the ammonia synthesis reaction as well as for increasing its rate. This, in turn, helps to raise the yield of ammonia produced in the synthesis tower. However, excessive emissions increase the consumption of fresh gas and result in a loss of the active components in the feed gas. The inert gas content in the synthesis system depends on the amount of inert gas required in the fresh gas fed into the process for the Zhiqu synthesis gas process, as well as on the gas volume exiting the synthesis system. In the presence of inert gases, the equilibrium ammonia concentration can be calculated using the following formula: Z = Z0 × (1 – i0) / (1 + i0). Here, Z0 is the equilibrium ammonia concentration in the absence of inert gases, and i0 is the concentration of inert gases without ammonia. Furthermore, adjusting the inert gas content can change the temperature distribution in the catalyst bed and the overall system pressure; when the conversion rate is too high and causes the temperature at the outlet of the synthesis tower to become excessive, increasing the inert gas content can help address this issue of high temperature. Furthermore, under system pressure operation, in order to maintain a certain output, it is necessary to determine the appropriate inert gas content, thereby selecting the right discharge volume. The syngas produced by this device is obtained through gasification; the synthesis circuit is essentially a circuit without inert gases, with helium being the main inert gas present, and amounts of other inert gases being very small. 4. Space velocity: Also referred to simply as velocity, space velocity is the volume of fluid that passes through a unit volume of catalyst per unit of time. The value of the space velocity indicates the length of time that the gas is in contact with the catalyst; numerically, space velocity and contact time are inversely related to each other. Generally speaking, the higher the catalyst activity, the shorter the contact time required for the same production load, and the higher the space velocity. The size of the space velocity selected for synthesis is related to factors such as the net ammonia yield of the synthesis reaction, the production capacity of the synthesis tower, the volume of the recycle gas, and the system pressure drop, as well as the efficient utilization of reaction heat. For a synthesis tower with a fixed structure, when the pressure and composition of the gas entering the tower remain constant, increasing the space velocity will reduce the ammonia content in the outlet gas; in other words, the net ammonia content decreases. The difference between the ammonia content at a specific location in the catalyst bed and the equilibrium ammonia concentration increases, and as a result, the reaction rate also increases. Since the degree of decrease in the ammonia net value is smaller than the multiple by which the space velocity increases, the production capacity of the synthesis tower improves as the space velocity rises; therefore, the space velocity can be increased to boost ammonia production. However, in actual production, the space velocity cannot be too high; otherwise, a series of problems will arise. First: increasing the space velocity means an increase in the amount of circulating gas, which raises the resistance throughout the system and thus increases the power consumption in the compressor’s circulation section. Second: As the ammonia content in the exhaust gas decreases, higher temperatures are required for separating liquid ammonia; this means that the freezing temperature used for separation must be lowered. Additionally, due to the increased volume of circulating air, the load on the refrigeration unit also increases. Third: The ammonia synthesis reaction is an exothermic reaction, and the bed temperature is maintained by the heat generated from this reaction. Therefore, as the air velocity increases, the heat of reaction generated per unit volume of gas decreases as the net ammonia value falls. If the space velocity is too high, it becomes difficult to maintain the catalyst temperature; if the synthesis tower cannot sustain its own heat, the temperature may drop even without using a heater. Furthermore, from the perspective of heat recovery, the value of heat utilization also decreases, thereby affecting the overall heat balance.

1. Temperature: While ensuring the temperature required for catalyst activation, maintaining a relatively low temperature is beneficial for the ammonia synthesis equilibrium. 2. The higher the pressure, the more the equilibrium shifts toward ammonia formation, which facilitates the production and condensation of ammonia ； However, the higher the pressure, the greater the power consumption for synthesizing and compressing the gas; therefore, for ammonia synthesis in the range of 21–25 MPa, the overall energy consumption is lowest. It is more scientific for Topsoe and Hunan Amuchun to design the system at 22 MPa. 3. Air velocity: An increase in air velocity helps to boost production, but it is affected by factors such as the temperature of the catalyst layer, tower resistance, and water cooling load, so a comprehensive balance is necessary.

1. The higher the pressure, the more favorable it is for the synthesis reaction. 2. The lower the temperature, the more favorable it is for the synthesis reaction; however, considering catalyst activity, the temperature is generally kept at a higher level. 3. The lower the space velocity, the higher the conversion rate and the higher the net ammonia yield, but this is not the most economical approach; therefore, space velocity needs to be balanced carefully. 4. When it comes to inert gases, there is no ideal option in terms of inert gas theory – hydrogen and nitrogen are the effective gases that result in higher conversion rates. But inert gases involve two factors: first, temperature control, as inert gases are needed to maintain the temperature in the reactor; the amount of inert gas required varies depending on the lifespan of the catalyst. Second, there is the issue of vent volume in the synthesis system; economic considerations must be taken into account, as lower levels of inert gases lead to a larger vent volume. Thus, the control of inert gases is achieved by balancing these two factors

When the reaction is constant, increasing the space velocity is equivalent to increasing the concentration of the reactants, causing the reaction to proceed in the forward direction; as a result, increasing the space velocity leads to a decrease in temperature and pressure. And due to pressure increases caused by various reasons, its circulation volume also increases accordingly, resulting in an increased space velocity.

Advantages of increasing the synthesis pressure: 1: It favors the shift of the reaction equilibrium in the direction of ammonia production; at the same time, the reaction rate increases as pressure rises. Due to the increase in equilibrium concentrations and reaction rates, this contributes to an improved synthesis yield and a reduced need for catalysts； 2: As pressure increases, the ammonia concentration in the gas rises; accordingly, the condensation temperature of ammonia increases as well. The demand for cooling capacity decreases, which facilitates the separation of ammonia from the circulating gas and simplifies the process. The effect of temperature on ammonia synthesis: 1. Ammonia synthesis is a reversible exothermic reaction that requires a catalyst to proceed; lowering the temperature favors the reaction equilibrium ； 2: As the temperature increases, the reaction rate rises, but the equilibrium concentration decreases; moreover, a catalyst can exhibit its catalytic activity only within a certain temperature range ； 3: Low temperatures help extend the catalyst’s lifespan and reduce hydrogen corrosion of the equipment ； 4: At the early stage of the reaction, when the ammonia concentration is far from equilibrium, the focus should be on increasing the reaction rate; at the later stage, when the ammonia concentration approaches equilibrium, the focus should be on raising the equilibrium concentration of the reaction. The most suitable temperature should be changed from a higher value to a lower one. Effect of space velocity on synthesis: 1: An increase in space velocity reduces the contact time with the catalyst, thereby lowering the ammonia content in the gas leaving the tower ； 2: Increasing the space velocity raises the gas ammonia concentration and the equilibrium concentration difference, thereby accelerating the reaction rate; this can compensate for the decrease in the ammonia content leaving the tower and help increase production ； 3: Increasing the air velocity raises resistance, and constraints imposed by the synthesis tower and synthesis compressor lead to higher power consumption in the circulation section ； 4: Due to the reduced ammonia content, more cooling capacity is required to separate the ammonia ； 5: The ammonia synthesis reaction is an exothermic reaction; the reaction heat is used to maintain the temperature of the bed. An increase in space velocity carries away more heat, making it difficult to maintain the desired temperature. 6: Increasing the space velocity can boost the yield per unit of catalyst, but it should not be too high; an optimal space velocity should be selected under a certain pressure

The explanation on the 2nd floor was very professional; I’ve learned a lot, thank you