Catalytic reduction cooling issue

With a constant circulation rate, the effects of these two control methods may be the same； Firstly, in terms of the heating rate, there is no difference between the two methods; both must be carried out at the rate specified in the heating plan ； Looking at the space velocity aspect as well, since the circulation rate cannot be increased, the space velocity for the catalyst section that is currently under reduction is the same for both methods ； When the upper layer temperature exceeds the limit due to adjustment using an electric furnace, the cold shock line should be activated to maintain the upper layer temperature.

At present, we use the method of reducing the load on the electric furnace to lower the temperature in the upper layer. People from the catalyst manufacturer say that during reduction, heat exchange cannot occur in the catalyst layer, and allowing cold air to be used for regulation is not advisable, as it is harmful to the catalyst. I just don’t understand why the two methods yield the same results; what effect does applying a cold shock have on the catalyst?

The ammonia synthesis catalyst has been reduced to the lowest level, so the system pressure does not drop; the synthesis reaction is already in full swing, and due to the heat generated by the reaction, reduction occurs simultaneously as the reaction takes place. The manufacturer is concerned that by applying a cold shock, the catalyst surrounding the cold tubes may not be fully reduced due to the cold wall effect. I think as long as the amount of cold shock air isn’t excessive, the catalyst surrounding the cold tube doesn’t determine the outcome in any case (this has been taken into account in the design of the internal components); using a moderate level of cold shock for temperature control is completely fine.

The pressure is currently around 8.5 MPa. I also feel that a slight cold shock shouldn’t have much impact, because the temperature around the cold shock tube during production isn’t very high either, and the utilization rate of the catalyst is low as well. But the manufacturers don’t think so. Let’s take a look at others’ insightful views together

There is a reason why applying cold shock can cause the cold tube effect; therefore, it is best to follow the manufacturer’s recommendations in order to reduce the load on the electric furnace. Moreover, wouldn’t it be better to reduce the load on the electric furnace a bit to ensure its safe operation?

We encountered this problem before during catalytic temperature elevation for reduction as well; in those cases, electric furnaces were used for control. If cold gas is used, there should be water present. The gas coming out of the synthesis tower isn’t cooled by an ammonia cooler, nor is the water separated from it. When this gas enters the tower again after passing through the gas-to-gas heat exchanger, it can cause temporary poisoning of the synthesis catalyst. Repeated reduction leads to an increase in crystal size, which affects the catalyst’s activity. Moreover, using an electric furnace allows for precise control of the zero-degree temperature, making it easy to manage.

Can’t we raise the system pressure to address the issue of low circulation volume? This would allow the upper temperature to be reduced, increase the space velocity, and at the same time raise the lower temperature.

Thank you to the original poster for their active participation. The ammonia synthesis catalyst has been reduced to the lowest level; the system pressure is 8.5 MPa. The synthesis reaction is proceeding very vigorously, and the heat generated by the reaction is substantial. As a result, the circulation rate is not sufficient to keep the temperature under control. If the pressure is increased, it will undoubtedly promote the ammonia synthesis reaction; the heat of reaction increases, and the temperature rises. This post was last edited by snowdfr on 2009-4-13 15:44]

Our process involves feeding the gas supplement at the ammonia inlet, where it passes through an ammonia cooling exchanger; from there it goes to the circulation machine. After exiting the circulation machine, it passes through an oil cooling exchanger before entering the synthesis tower. There are four cooling stages in between. At the exit of the synthesis tower, the fluid goes to the waste heat boiler, then to another heat exchanger, followed by a cold exchanger, yet another heat exchanger, and finally an ammonia cooler. After cooling, it returns to the ammonia inlet – that’s roughly how it works. The gas exiting the synthesis tower must undergo two stages of cooling and separation before it can enter the tower again. We analyze the vapor concentration at both the inlet and outlet every half hour. The temperature for ammonia cooling is always below –10°C, and the vapor concentration at the inlet is 0. Therefore, the situation described by the original poster – that there should be water in the gas coming out of the tower, as this gas has not been cooled by an ammonia cooler nor has its water content been removed, and then enters the tower again through a gas-to-gas heat exchanger – should not occur. Such a situation could cause temporary poisoning of the synthesis catalyst, repeated reduction, enlargement of crystal particles, and an impact on its activity. Moreover, since the electric cooker mentioned to the original poster can detect the temperature at zero degrees, there is no reason to return it – otherwise the temperature of the rice will drop. Last edited by snowdfr on 2009-4-13 15:57.]

“My assumption that there is moisture in the cold gas was wrong; the power reduction to the furnace causes a drop in temperature at zero degrees, which in turn lowers the temperature of the entire upper layer. There shouldn’t be any problem with that, right?

Since the second method is not approved by catalyst manufacturers, the first method can achieve the goal of temperature control. Then use the first method of adjusting the electric furnace load to control it. Of course, the best approach is to use the electric furnace at its maximum capacity and control the temperature through the flow rate. How to focus on adding up the circulation amounts.

Thank you for participating. The manufacturer recommends it; of course, there are their reasons for adopting their method. The key is that I want to know why cold shock can’t be used. All 4 of our circulation pumps are running, but the circulation volume is still not sufficient. The design team said that 3 pumps would be enough during the design phase; I’m not sure how they arrived at that figure

The quality of catalytic heating reduction directly determines its service life and activity; therefore, it is essential to pay attention to every detail. Objectively speaking, both of LZ’s approaches can address the issue of excessive heat buildup at the upper levels. It can be understood in this way: the first approach reduces the supply of heat, while the second approach disperses the heat. Catalytic oxidation requires a process; to determine whether the upper layer has been fully reduced, we generally analyze indicators such as water vapor concentration and the amount of water produced, before deciding whether to proceed with reduction in the lower layer. In fact, even after the reduction of the entire tower is complete, the reduction of the catalyst is likely not yet sufficient; this requires maintaining low-load operation for a period of time at the beginning of production in order to further enhance the reduction effect. By introducing cold air directly, and since it is impossible for the mixer to achieve complete homogeneous mixing, local overcooling or overheating may occur (once the temperature rises, it is generally better not to lower it again to avoid repeated oxidation and reduction), resulting in incomplete reduction. Therefore, methods of reducing heat supply are more effective than directly introducing cold air to control the upper layer; I do not recommend using these two approaches to control temperature. Under the conditions described by the original poster, the reduction in the upper layer is essentially complete and it has gained activity; neither of the two methods mentioned should be used. Instead, it is necessary to control the gas composition (for example, by removing hydrogen – when the hydrogen-to-nitrogen ratio is not appropriate, the heat generated by the reaction is reduced). Pressure can be increased to boost the circulation rate and thus raise the temperature of the upper layer, while a high circulation rate is used to carry heat downward, thereby improving the reduction effect of the catalyst in the lower layer.

By introducing cold air directly, and since it’s impossible for the mixer to achieve complete homogeneous mixing, local overcooling or overheating may occur (once the temperature rises, it’s better not to lower it further to avoid repeated oxidation-reduction reactions), which can result in inadequate reduction. I agree; thank you. What I want to say is that we have two towers connected in parallel, with one tower operating under normal conditions. In order to accommodate the production process in that tower, the hydrogen level cannot be raised too high; during reduction, the hydrogen level is kept between 70% and 75%. Furthermore, increasing pressure can raise the space velocity, but at this point the pressure increase also leads to an increase in temperature; therefore, pressure alone cannot keep the temperature under control. This post was last edited by snowdfr on 2009-4-16 20:07.]

When the two towers operate in parallel, to ensure the normal operation of the other tower, the hydrogen level cannot be set too high. In this situation, I approve of any method used to control the temperature at the upper layer; the problem is that the temperature at the lower layer may not increase, resulting in a rather satisfactory reduction process throughout the entire tower. It needs to be gradually restored in future light-load production. Of course, the utilization rate of the catalyst throughout the entire tower is itself a concept that is difficult to determine accurately, and it is also unclear whether the quality of reduction at the bottom layer will have a significant impact on the production capacity of the tower.

Thank you; the original poster thought of everything very carefully. We have completed the restoration, and overall it’s quite good. At the end of the reduction, at a pressure of 10.0 MPa, the electric furnace also completely disappeared. During reduction, the temperature at the bottom reached over 490 degrees, and it took 5 hours for the pressure to rise to 15.0 MPa before switching to low load operation.

Increase pressure + reduce hydrogen partial pressure to boost circulation volume and suppress reaction heat?

We generally adopt measures such as reducing power supply while maintaining a constant temperature and increasing hydrogen supply, in order to raise the temperature at the bottom of the catalyst. If a cold shock bypass is used, a cold wall effect can occur, resulting in the catalyst surrounding the cold tubes not being fully reduced; this leads to large radial temperature differences within the catalyst. This phenomenon will be particularly evident in future actual production processes. So, I still ask the original poster to think carefully.

I’m glad you’re about to successfully complete the restoration. May I ask the original poster: are the four cooling stages inside the tower individually adjustable? What is the model of the catalyst? Is its activity at low temperatures good? How long does light-load production take? After some thought, the safest approach is to gradually increase the pressure while raising the air velocity (with the circulator filled to capacity), and to combine this with a gradual reduction in voltage, thereby achieving a better pressure difference. Even if it has nothing to do with tower production, once a series of processes are involved, it becomes very troublesome to adjust the gas composition.

The cooling air in each section of the tower is adjusted independently: Section 1 is cooled by zero-degree cooling air, Section 2 by #1 cooling air, Section 3 by #2 cooling air, and Section 4 by #3 cooling air. The catalysts are A202Q and A310W; according to the manufacturer, they are low-temperature catalysts, but I’m not very familiar with them. The requirement for light-load production is 48–72 hours.