Synthetic ammonia: increase in space velocity

Based on the catalyst’s activity, the cold shock gas is adjusted so that the temperature inside the tower remains near the value indicated by the reaction curve; the circulation rate is such that it does not cause a drop in temperature (the temperature can be maintained at a stable level). This represents the maximum space velocity within the tower.

Under certain synthesis conditions, as the space velocity increases and the contact time between the gas and the catalyst decreases, the ammonia content in the gas exiting the synthesis tower decreases. For example, when the reaction takes place at 29.4 MPa and 475°C, the space velocity increases from 10,000 h-1 to 20,000 h-1 and then to 40,000 h-1; meanwhile, the ammonia content in the gas exiting the tower decreases from 25% to 21.5% and further to 16.2%. This seems detrimental to production, but due to the increased space velocity, more gas passes through the catalyst per unit of time, resulting in an increase in the actual ammonia output. Under certain conditions, as the space velocity increases from 10,000 h-1 to 20,000 h-1, 30,000 h-1, and 40,000 h-1, the catalyst’s production capacity rises from 1,950 kgNH3/(h•m3) to 3,340, 4,280, and 5,040 kgNH3/(h•m3), respectively. It can be seen from this that increasing the space velocity can improve ammonia production. However, due to the increase in space velocity, the amount of recycle gas required to produce one ton of ammonia, as well as the resistance that must be overcome to transport the gas, all increase, resulting in higher energy consumption. Especially when the space velocity is too high, the heat carried away by the gas exiting the synthesis tower increases, making it difficult to control the temperature of the catalyst bed and preventing ammonia in the recycle gas from condensing easily. In small-scale ammonia synthesis, the space velocity is generally controlled at 15,000–25,000 h-1.

It must be made clear here: an increase in space velocity simply means that the production capacity per unit volume of catalyst per unit of time increases. This is somewhat different from an increase in ammonia production. For a fixed ammonia synthesis plant, if only the space velocity is increased while all other conditions remain unchanged, will the ammonia production increase? (Of course, this is assuming that the current space velocity is already sufficient.) If no other production conditions are changed (especially the amount of make-up gas), and only the space velocity is increased, then due to the increased catalyst production capacity, ammonia production may rise in the short term. However, since there is no increase in the amount of make-up gas, the pressure in the system will definitely drop, and as the pressure decreases, the space velocity will also decrease (according to the gas law, gas volume is proportional to pressure). As a result, the overall ammonia production does not increase. In other words, any increase in ammonia production is only temporary; without an additional supply of make-up gas, there is no possibility of an increase in ammonia production over the long term. As for the choice of space velocity, it depends on whether you are involved in the design or operation of ammonia synthesis. If you are working on the design aspect, various factors need to be taken into consideration, such as the amount of make-up gas (i.e., the production capacity), the selection of reactors and catalysts, the process layout, and the operating pressure. It can only be determined after completing a full material balance. If it is for ammonia synthesis operations. The main factors to consider are the pressure in the synthesis loop and the bed temperature. During operation, what is mainly adjusted is the circulation rate; if the pressure is high, the circulation rate must be increased. Of course, the bed temperature also needs to be taken into account; when the temperature is high, the circulation rate can be increased. In actual production, the amount of freezing required also needs to be taken into account. As the air velocity increases, more cooling capacity is required to maintain the ammonia cooling temperature. In short, increasing the circulation volume as much as possible and reducing the opening degree of the cold bypass is highly beneficial for ammonia synthesis production.