Weekly topic: How to properly adjust the hydrogen-to-nitrogen ratio? (10/7----16/7)

A few days ago, there was a popular post discussing the hydrogen-to-carbon ratio; I’ll summarize it as follows: The selection of this ratio varies depending on whether normal or abnormal operating conditions prevail. Under normal conditions, the choice of the hydrogen-to-carbon ratio depends first on the quality of the feed gas – if the quality is excellent, there is no need for venting after the tower, in which case the ratio should be strictly controlled at 3:1. If venting is necessary, the ratio can vary depending on the specific circumstances of each plant; some plants use a ratio of 2.8–2.9, while others use 3.1–3.3. Personally, I prefer a ratio of 2.8–2.9. Under abnormal operating conditions, such as changes in load or abnormal hydrogen content in the gas entering the tower, the hydrogen-to-carbon ratio can be changed significantly on a temporary basis; the principle is to adjust the hydrogen content in the gas entering the tower back to normal levels ； In more severe situations such as high levels of fresh gas, ammonia present in the synthesis tower, or temperature differences, it is necessary to temporarily shut off the fresh gas flow, enable self-circulation of the system, or even activate an electric heater for heat retention.

Reference answer: You can take a look at it; it’s just for reference. The hydrogen-to-nitrogen ratio in semi-water gas is an important control parameter in ammonia synthesis. The equilibrium concentration of ammonia is highest when the volume ratio of hydrogen to nitrogen is 3:1. If either hydrogen or nitrogen is in excess, the percentage of the hydrogen and nitrogen gases that participate in the reaction decreases in the total gas volume. Under normal circumstances, hydrogen and nitrogen account for about 80% of the gas fed into the synthesis tower. If the inert gases and ammonia in the recycled gas are excluded, and the remaining hydrogen and nitrogen are used in a synthesis reaction in a 3:1 ratio, then hydrogen should make up 75% of it, while nitrogen accounts for 25%. If there is a 3% excess of hydrogen, the hydrogen content in the hydrogen-nitrogen mixture will be 78% while the nitrogen content will be 22%. Of this hydrogen, only 66% reacts with the 22% nitrogen; thus, the hydrogen and nitrogen gases that participate in the reaction account for 88% of the total gas volume. If the nitrogen content in the hydrogen-nitrogen mixture is 3% too high, its composition becomes 28% nitrogen and 72% hydrogen; of this 72% hydrogen, only 24% can combine with nitrogen, meaning that 96% of the hydrogen and nitrogen participate in the reaction. Obviously, an excess of nitrogen is much better than an excess of hydrogen. Furthermore, in terms of the reaction rate of ammonia synthesis, under non-equilibrium conditions, appropriately increasing the nitrogen partial pressure is beneficial for the rate at which nitrogen is adsorbed by the catalyst, as the active adsorption of nitrogen is the controlling step in the ammonia synthesis process. A hydrogen-to-nitrogen ratio slightly lower than \"3\" can increase the partial pressure of nitrogen in the gas, allowing more nitrogen to diffuse to the catalyst surface and increasing the chances of adsorption, thereby raising the synthesis rate. Due to the increased synthesis rate, the power consumption of the compressor also decreases accordingly. A too high hydrogen-to-nitrogen ratio is highly detrimental to the synthesis reaction. For example, when the cycle gas consists of 75% hydrogen, 20% inert gas, and 5% nitrogen, 5 parts of nitrogen can only react with 15% of the hydrogen; in other words, only 20% of the components participate in the reaction, while the remaining 80% does not take part in it. As a result, the synthesis rate decreases significantly, the synthesis pressure rises, and the amount of gas vented increases. This not only leads to the loss of large amounts of hydrogen, affecting ammonia production, but also causes an increase in various consumption metrics. Therefore, it is appropriate to maintain a hydrogen-to-nitrogen ratio in the cycle gas between 2.4 and 2.8. However, hydrogen and nitrogen gases are combined in a 3:1 ratio to form ammonia, so the hydrogen-to-nitrogen ratio of the supplementary gas should also be “3”.

According to the online analyzer: check whether the hydrogen-to-nitrogen ratio in the recycled gas is within the range of 2.8–2.9. If it is not within this range, adjust the hydrogen-to-nitrogen ratio of the fresh gas to bring the ratio of the recycled gas to the required level. If it is within the range, then keeping the fresh gas hydrogen-to-nitrogen ratio at 3:1 is sufficient! If the ammonia content entering the tower is too high, first maintain the bed temperature by adjusting the bypass line or reducing the circulation volume. At the same time, monitor whether the liquid level in the separation tank, as well as the liquid level, temperature, and pressure of the ammonia cooler, are within the specified ranges; ensure that they remain within those limits. If the pressure of the ammonia cooler is not within the acceptable range, increase the capacity of the ice machine!

1) Control Scheme: Based on the on-site process conditions and the requirements set by the factory, a three-loop cascade control scheme was proposed. The main loop (ARC6) is a hydrogen-to-nitrogen ratio control loop; the input is the actual value of this ratio, while the set value for the hydrogen-to-nitrogen ratio is provided by the DCS via communication. The output serves as the set value for the secondary loop ； The secondary loop (ARC3) is a control loop for regulating the hydrogen content at the low-temperature transformer outlet; its input is the hydrogen content at that outlet, while its output serves as the setpoint for the tertiary loop ； The secondary feedback circuit (FRC3) is an air flow control circuit; its input is the air flow rate, and the output value is sent directly to the DCS via communication, from where it is then transmitted to the on-site air flow control valve. At the same time, three quantities that have a significant impact on the feedback path are proposed as feedforward quantities. Based on the actual conditions on site, the delay for the first loop is approximately 10–15 minutes, the delay for the second loop is about 20 minutes, while the delay for the third loop is very small. Therefore, the first two control loops should be adjusted stably while ensuring no overshoot, and feedforward should be applied to the secondary loop to ensure that the air flow can be adjusted in a timely manner. The control scheme is shown in Figure 1. Figure (4) Control Scheme: All three loops employ model-free control methods. The three feedforward signals applied to the sub-loops are: the flow rate of raw oilfield gas (FRC!), the Prissman reflux flow rate, and the change in circulating gas temperature (TD106).

A comprehensive judgment is made by using an online analyzer, manual analysis, as well as the hot spot temperature of the synthesis tower and system pressure

As we all know, the process from gas production to synthesis is quite lengthy; by the time the hydrogen ratio resulting from synthesis has changed, making adjustments through gas production is often too delayed. Therefore, it is much better to detect the hydrogen content after transformation (which we called transformed hydrogen at that time) and adjust the hydrogen-to-nitrogen ratio based on this data.

The methods for adjusting the hydrogen-to-nitrogen ratio should also vary depending on the different process flows. In our plant, we use processes such as shift reaction, carbonization, methanol production, and copper washing. Under normal conditions, as long as the carbon monoxide levels at the exit of the shift reaction and methanol production units are kept stable, the hydrogen-to-nitrogen ratio in the synthesis process will also remain stable, without any significant fluctuations. This is mainly related to the control mechanisms used in gas production； However, once the carbon monoxide output exceeds the specified level, it increases the load on methanol production, as the reaction for synthesizing methanol requires a large amount of hydrogen. This usually manifests itself first as an elevated temperature of the methanol, followed by a sharp drop in the hydrogen-to-nitrogen ratio, resulting in a deterioration in the synthesis process ； During the adjustment process, hydrogen recovery is used as an auxiliary method ； If the conditions in the gas generator are poor, it is very difficult to adjust the hydrogen-to-nitrogen ratio to within the specified range.

The hydrogen-to-nitrogen ratio is adjusted automatically, with hydrogen meters installed at the gas holder outlet, desulfurization outlet, purification outlet, synthesis inlet, and synthesis tower outlet. The adjustment method involves modifying the recovery time during the blowing phase through the aforementioned parameters. The operation is generally stable; when large errors occur, the gas production operator should pay attention to making corrections. The automatic hydrogen-to-nitrogen ratio adjustment operates stably and normally when the entire production system is stable; however, it is prone to deviations when significant fluctuations occur. For the synthesis unit, adjusting the hydrogen recovery flow rate is considered only when there is a significant deviation in the hydrogen-to-nitrogen ratio (by adjusting the amount of recycle gas vented to modify the gas composition).

The methods for adjusting the hydrogen-to-nitrogen ratio should also vary depending on the different process flows. In our plant, we use a process that involves shift reaction, decarburization, methanol production, and methane generation. Under normal conditions, as long as the carbon monoxide levels at the outputs of the shift reaction and methanol production processes are kept stable, the hydrogen-to-nitrogen ratio in the synthesized hydrogen will also remain stable, without any significant fluctuations. This is mainly related to the control mechanisms used in gas production； However, once the carbon monoxide output exceeds the specified level, it increases the load on methanol production, as the reaction for synthesizing methanol requires a large amount of hydrogen. This usually manifests itself first as an elevated temperature of the methanol, followed by a sharp drop in the hydrogen-to-nitrogen ratio, resulting in a deterioration in the synthesis process ； During the adjustment process, hydrogen recovery is used as an auxiliary method

Reply to 1# TH373637: The principle for adjusting the hydrogen-to-nitrogen ratio is to primarily recover hydrogen and nitrogen by shutting down the chimney in advance during the blowing stage (i.e., recovery). For temporary adjustment, manual purging is primarily used, or a combination of nitrogen injection and purging followed by recovery is employed, with flexible application based on the specific circumstances. When adjusting nitrogen, the addition and reduction of blowback recovery should not be concentrated on a single furnace. Gas stoves with unstable performance should be avoided as hydrogen-to-nitrogen ratio furnaces. When adjusting hydrogen, it is necessary to be aware of the external production load and the time it takes for changes to take effect. One must be able to anticipate the impacts of increasing or decreasing the load, as well as of shutting down equipment or furnaces; adjustments should be made early and in a proper manner. Only by keeping track of the operation of the gas furnace and any changes in raw materials, and by maintaining stable furnace conditions, can the stability of semi-water gas be ensured. To control the hydrogen-to-nitrogen ratio of the recycled hydrogen, it is necessary to estimate the changes in recycled hydrogen based on the changes in supplementary hydrogen; finding a stable axis for the supplementary hydrogen and operating within that axis ensures the stability of the recycled hydrogen. Of course, this axis is relative; it changes depending on factors such as the alcohol-to-ammonia ratio, the production load, and the synthesis of inert gases. There are many factors that affect the drop in hydrogen level; a low furnace temperature, poor combustion layer quality, abnormal operation of the gas burner, deformation or overturning of the fuel layer, as well as issues with the air supply holes or localized excess oxygen can all cause a rapid decrease in hydrogen levels. Furthermore, a reduced blowing load, unstable steam consumption, interference between blowing and purging processes, and equipment failures – such as chimney valves that fail to open or close, secondary air valves that fail to close or open, or the main steam supply being unavailable – can all lead to instability in hydrogen levels. There is also the change in the carbon monoxide conversion rate of the converter in the latter stage; the level of carbon dioxide in the decarburization gas also affects the stability of hydrogen. Therefore, controlling the hydrogen-to-nitrogen ratio requires taking various factors into account and operating in a proactive manner to maintain it at a relatively stable level.