What is the relationship between fresh gas temperature and ammonia synthesis yield? What are the effects of high temperatures?

This is a very imaginative question. First of all, an increase in the temperature of the make-up gas makes it more difficult to separate oil from water (this might be less of an issue if there is a air separation unit). Oil and water are naturally toxic substances in ammonia synthesis; assuming that they can be completely separated, let’s consider the process flow. If the make-up gas is introduced after water cooling but before ammonia introduction, then changes in the temperature of this gas have little impact on the ammonia synthesis process. After ammonia is introduced, trace amounts of oil, water, and other toxic substances are dissolved in the liquid ammonia, which helps to separate them. Although this does have some effect on the temperature required for ammonia cooling, since the temperature variations of the make-up gas are limited, the impact is minimal, and this issue can be addressed by increasing the cooling capacity. What if the make-up gas is introduced directly into the synthesis tower? In that case, the temperature of the gas entering the tower increases, allowing the ammonia synthesis reaction to reach equilibrium more quickly, thus bringing it closer to the optimal conditions for the reaction – which is beneficial for ammonia production. However, in practice, it may be necessary to use additional cooling systems to prevent an increase in the temperature of the catalyst bed, and it’s unlikely that this approach will yield significant benefits. Unless the circulation rate can also be increased, then ammonia production will benefit from this. The extent to which the temperature of the make-up gas can be increased is limited; if it becomes too high, it will make oil-water separation more difficult and increase corrosion in the pipelines. Therefore, I am not in favor of raising the temperature of the make-up gas.

We use a natural gas steam reforming process, where the syngas is fed into a compressor to be pressurized and then sent to the synthesis tower. After being pressurized and dehydrated in the first and second stages of the compressor, the fresh gas enters the third stage of the compressor. There, it mixes with the recycled gas coming from the recycling stage within the cylinders of this stage, and is pressurized to 14.5 MPa while the temperature remains around 70 degrees. After passing through the heat exchangers for inlet and outlet gases, it enters the synthesis tower to undergo the reaction. Therefore, a high temperature of the fresh gas is unfavorable for water removal in the compressor. At the methanation outlet, in addition to the boiler water heat exchanger, boiler feedwater preheater, and circulating water cooler, an ammonia cooler has been added to reduce the temperature of the gas entering the compressor in the synthesizer. Lowering the temperature is very helpful for improving the efficiency of the compressor’s work. We require the fresh air temperature to be between 8-12 degrees

Generally speaking, a high temperature of the make-up gas is unfavorable for the operation of the synthesis tower: 1. A high make-up gas temperature increases the load on the oil-water separator, resulting in poor separation efficiency; this can lead to the presence of oil and water in the synthesis tower, thereby poisoning the catalyst； 2. If the make-up gas is mixed with the circulating gas after the ammonia cooler but before the heat exchanger, it raises the temperature of the heat exchanger, which is unfavorable for the formation and separation of liquid ammonia in the heat exchanger. This increases the ammonia content at the inlet of the synthesis tower as well as the cooling load ； 3. The saturated mist-like oil-water in the high-temperature make-up gas, after cooling, enters the ammonia storage tank along with the ammonia released during cooling, thereby reducing the quality of ammonia. The addition of an ammonia cooler affects the cooling efficiency, and it becomes more difficult to separate syngas from liquid ammonia ； 4. The temperature of the supplementary air is high; it mixes with the circulating air and enters the circulator, resulting in a high inlet temperature for the circulator, poor air pumping capacity, and increased power consumption.

In the synthesis process, the fresh gas generally needs to undergo an ammonia wash in order to remove trace amounts of CO and CO2 from it, thereby preventing fluctuations in the synthesis reaction. Therefore, the point at which the fresh gas is introduced should be before the second ammonia separation step (since the amount of ammonia separated in the first step is large, introducing the fresh gas before that step would dilute the ammonia concentration, which is not conducive to ammonia separation). This implies that a lower temperature is preferable, in order to avoid an increase in the temperature during the second ammonia separation step as a result of mixing with the recycled gas, which could affect the efficiency of ammonia separation and lead to an increase in the ammonia content entering the tower.

Friend on the 3rd floor, could you send us your PFD diagram for reference? Email: zxzxx321@163.com

It’s mainly the temperature; it’s not good for the unit as it causes high bearing oil temperatures. Secondly: it affects the product quality of ammonia, as well as the cooling load and the load on the inter-stage heat exchangers.