Modification of the synthesis process flow

LZ’s idea is very bold; I wonder if they have seen the internal structure of Lengjiao? If the internals of the cold exchanger are not changed, there are several issues with this approach: 1. By eliminating the primary ammonia separation stage after water cooling, the flow enters the cold exchanger from the original secondary outlet, with the cold exchanger taking over the function of that primary separation stage. The separator in the lower part of the cold exchanger was designed with the material flow direction in mind; therefore, I don’t think the separation efficiency will be better when the flow goes in the opposite direction (not to mention the additional load imposed by the original primary ammonia separation stage) ； 2. The heat exchange in the upper part of the cold exchanger, which was originally counter-current heat exchange, has been changed to co-current heat exchange, resulting in a decrease in heat exchange efficiency ； 3. The temperature of the gas exiting the cooler is lower than before, resulting in an increase in the ammonia content entering the tower. Furthermore, according to this modification plan, it seems that the goals of improving ammonia utilization efficiency and reducing the ammonia cooling load cannot be achieved; a detailed explanation is requested.

The idea mentioned above is creative, but it has the following problems: 1. The secondary ammonia coolant flows in the opposite direction; the structure of the ammonia component needs to be modified, otherwise the separation efficiency will be poor； 2. After the primary ammonia is converted to secondary ammonia, the cooling capacity used for ammonia cold separation is utilized only to cool the inlet gas from the primary separation stage in the heat exchange section; as a result of this temperature drop, some ammonia gets separated. If this separation does not take place, liquid ammonia will end up in the gas entering the heat exchange stage, which has a significant impact on the operation of the synthesis tower. 　　Please share your opinions fully.

The original idea is good, but there are a few points that need to be modified: 1. After ammonia cooling, the gas should enter from the outlet of the original cooler; by doing this, countercurrent heat exchange can be ensured. 2. The cooler can definitely meet the separation requirements after modification. 3. Using ammonia for secondary separation will certainly yield better results. 4. If the original process was the conventional one involving ammonia separation before entering the tower, then using ammonia cooling will result in a lower ammonia content in the gas entering the tower. 5. Since the operating temperature of the ammonia separation equipment decreases, its performance must be taken into consideration. 6. As the temperature of the liquid ammonia product decreases, the user’s requirements must also be taken into account. 7. This affects the quality of gas purification before it enters the tower

Many of the newly built synthesis systems these days use the process mentioned by the original poster; our DN1200 synthesis system follows this process as well. However, it should be noted that: 1. The gas flow direction in the separation section of the cold exchanger in the newly designed systems has been adjusted to ensure effective separation. In older systems that use this process, careful consideration must be given to the gas flow direction in the cold exchanger. 2. As the first stage of separation, the cold exchanger is used to remove approximately 60-70% of the ammonia; the amount of ammonia remaining after the second stage of separation is significantly reduced, which may result in a weaker washing effect on the supplementary gas. It is necessary to install a bypass line between the water-cooling outlet and the hot gas outlet of the cold exchanger, so that the amount of ammonia separated in the second stage can be increased if needed. We use DN32 pipes for this purpose, but we have not used them yet.

It can be modified in this way; that’s exactly what we do. However, the swirl plates of the separator on the cold exchanger need to be installed in reverse order. Additionally, the packing on the cold exchanger must be secured to prevent air leakage and movement of the internal components of the cold exchanger.

Thank you; it’s just an idea, and I hope an example can be found in actual production. Here’s how I think about it: Redesign the swirl plates in the lower separation section of the cold exchanger, so that the gas flows downward along the annular gap of the cylinder, rises upward through the swirl separation mechanism, and then exits downward along the central tube at the bottom of the original cold exchanger ； The upper pressure cylinder of the cold exchange internals is secured with a large cover in order to eliminate the stress caused by the upward movement of the internals (after the process modification, the pressure outside the pipes is higher than that inside the pipes) ； The sealing between the cold junction and the tube still uses the original packing seal. But I’m not sure about the actual performance; could you give some details? QQ408368917

Agree. However, after the process modification, the separation temperature for cold separation might be around 10 degrees, while the separation temperature for secondary ammonia separation is -5 degrees; thus, there should still be sufficient ammonia available to meet the requirements for minor washing. I was wondering if you have encountered any issues of slight overruns in such a process? Thank you

I think the original poster should first describe in detail the internal process of your cold outreach before bringing it up for discussion; otherwise, it’s impossible to have a proper discussion, as the process for each cold outreach might not be the same, right?

Thank you, Idea 1 is better. After the process modification, the separation temperature should remain above 0, and no special requirements should be placed on the material.

1. The lower separation section needs to be redesigned, still using a swirl plate structure. 2. I don’t quite understand. After being cooled with ammonia, the cold gas flows through the tube side, where it exchanges heat with the water-cooled gas in the shell side. The reduced temperature of the hot gas in the shell side facilitates the separation of ammonia (which is also one of the objectives of this process modification); this allows for an increased amount of ammonia to be separated in the first stage, thereby reducing the load on the ammonia cooler. The efficiency of ammonia separation in the second stage is higher as well.

Thank you for following! 1. The separation structure under cold conditions definitely needs to be modified ； 2. Countercurrent heat exchange can still be achieved, as discussed above ； 3. At the low temperature of ammonia separation in the first stage of cold separation, increasing the amount of ammonia separated can reduce the volume of circulating gas, thereby lowering the ammonia cooling load. The separation efficiency increases at lower secondary ammonia temperatures. I’m not sure if there’s any mistake in our acquaintance.

Original process: First, it enters the upper inlet pipe; there, ammonia is used for cooling and separation. Then it enters the lower separation section, where the separated gas exchanges heat with the water-cooled gas inside the pipes before being discharged from the upper part.

The idea is good, but there are no practical examples to show whether it’s feasible. Could you provide an example of a transformation?

For two-stage condensation separation, such modifications are no problem. But there is actually another issue: regarding the location of the circulation pump and the location where fresh gas is introduced, how did the original poster consider this? Everyone can take a look together.

Currently, the temperature at the hot gas outlet of our cooling system is 12°C, while the temperature at the ammonia cooling outlet is –5°C. We are cold-jet ammonia synthesis towers; the ammonia content in the output gas is about 12%, and the net ammonia content is less than 10%. I did the calculations (the data is based on tons of ammonia; it’s not entirely precise, but it helps to illustrate the situation): the flow rate of hot gas entering the cold exchanger is approximately 15,500 Nm3/h. The ammonia content in the gas at the inlet and outlet is 12% and 5%, respectively. The amount of ammonia separated in the first stage is 1,085 Nm3/h. The volume of ammonia-containing gas entering is approximately 17,300 Nm3/h; the ammonia concentration at the inlet is 4.5% (with supplemental gas being introduced). The ammonia concentration corresponding to the outlet temperature is 2.9%, and the amount of ammonia separated in the second stage is 277 Nm3/h. In this way, the amount of ammonia separated at the first stage accounts for 80% of the total ammonia amount, and this situation is worth considering. Since we have been using this system for not too long, no signs of minor poisoning have been observed yet. This post was last edited by *aoye613 on 2009-3-12 11:38.]

In the original process, heat is first transferred out before the fluid enters the water cooler; once the temperature drops to room temperature, it proceeds to the cold exchange section where heat is recovered. After further temperature reduction, it moves on to the lower part of the cold exchanger, then to the horizontal ammonia cooler, where the temperature is lowered to around -10°C. Thereafter, it enters the ammonia separator to separate liquid ammonia, and after heat exchange within the cold exchange tubes, the fluid reaches a temperature of around 25°C before being sent to the compressor for pressurization, and this cycle repeats itself. If we follow LZ’s idea and have the water-cooled fluid follow the original path after being separated from ammonia, entering the heat exchanger, then coming out of the heat exchanger without ammonia separation will inevitably result in liquid entering the ammonia cooler, which affects its operation. At the same time, it increases the load on the ammonia separation process, impacting its efficiency. Eventually, the separation task is shifted to the lower part of the heat exchanger (where the ammonia-separated fluid follows the original water-cooled path before entering the heat exchanger and then passing through the separator there). The separator located at the lower part of the original heat exchanger must be modified; otherwise, proper separation cannot be ensured, leading to the presence of ammonia. Moreover, the gas emerging from the ammonia separation may contain some liquid ammonia, which first enters the original heat exchanger where it exchanges heat with the hot gas coming from the water cooling process, causing the ammonia to evaporate. As a result, less ammonia remains for separation, increasing the ammonia content in the fluid entering the tower. Such a modification does not improve the heat exchange process; it merely delays the ammonia separation step, and the efficiency of both heat exchange and separation ends up being worse than with the original process. (It’s just my personal opinion!) ）

The original process is the latter one you mentioned, and it is precisely the original process that we wish to modify. The problem is that after the process modification, it is unclear whether the original internal components of the cold exchanger can still be retained; only the separation section of the cold exchanger is modified.