Regarding the commissioning of urea plants using the CO2 stripping method
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I had previously read articles about urea CO2 stripping units being operated at low pressure; why isn’t this approach mentioned anymore? I only came across one article stating that operating at low pressure causes greater corrosion to the static equipment in high-pressure systems. I’m not sure of the exact details – what are everyone’s opinions? At what pressure do the companies you work for start feeding material into their systems?
Our device feeds materials at atmospheric pressure; before feeding, the high-pressure system is heated to 160 degrees, with a pressure of 0.3 MP, after which the materials are fed directly. Commissioned in September 2003, using the carbon dioxide stripping method, with an annual production capacity of 800,000 tons. There is an article about it in “Da Dangui,” you can check it out.
Starting up at low pressure is difficult to control, as overpressure tends to occur quite easily. This is because the lower the pressure, the worse the reaction performance becomes..
Yes, the dissociation pressure and temperature of methylammonium should be above 7.5 MP and 120 degrees, right? Feeding at low pressure results in an extremely low conversion rate of the product after synthesis. As mentioned on the third floor, it is very difficult to control the system pressure when feeding at low pressure, and this also poses a significant threat to the safe operation of high-pressure equipment. For example, due to the differences in the material used for the synthesis towers, deformation can occur quite easily The above are my personal opinions!
Our plant uses a feeding pressure of 8.0 MPA. As for the negative effects on the equipment mentioned by those upstairs, I really want to learn more about them; the more, the better!
When feeding materials into our plant, the pressure is usually increased to around 8–10 Mpa; at this pressure the temperature also rises above 120 degrees. As mentioned by someone on the 4th floor, this brings the conditions above the dissociation pressure and temperature of methylammonium, thereby increasing the conversion rate. It also makes it easier to control the pressure in the high-pressure system. Overpressure is one aspect; the main issue is still corrosion of the equipment.
Our unit uses normal-pressure feeding. As for the issue mentioned by some that pressure is difficult to control and overpressure can occur easily, we have not encountered such a problem in our unit. The main reason is that the unit uses a horizontal tank-type high-pressure ammonium methoxide condenser; before feeding materials, water is pumped into the tank-type condenser for 2.5 hours, which amounts to approximately 27 cubic meters of water. This reduces pressure fluctuations. If you have any questions, feel free to ask; I will answer them one by one. I’ll find some information on this topic and share it with you.
It seems that starting up our full-circulation process for aqueous solutions is much simpler than for you
Too low a pressure leads to severe corrosion of the equipment; this is because at such low pressures, oxygen cannot dissolve in the liquid phase, and as a result, the inner walls of the equipment cannot be oxidized and passivated, which accelerates corrosion. In existing plants, the feed pressure is generally not lower than 8 MPa. However, in some new installations, especially those using large-capacity shell-and-tube condensers for carbon dioxide stripping, it is possible to use a feed pressure of 0 MPA without any problems.
You can find a summary of experiences with full-low-pressure startup in the first issue of Fertilizer Design published in 2000. I was the first manufacturer to use this low-pressure startup technique, and it was also recommended by Starmic Company. By using this method, it is easier to control the ammonia-to-carbon ratio in the system, and the time required for startup and product discharge is reduced. Carbon dioxide is first introduced via a small feed line, and at a system pressure of 70 kilograms, it is transferred to the main line
Our carbon dioxide stripping unit was put into operation in June 2007. At that time, it was intended to operate at atmospheric pressure, and analysis showed that this was feasible; however, due to concerns regarding corrosion, conversion efficiency, and environmental issues related to ammonia water recovery, this approach was not adopted. In my opinion, aside from the issues mentioned above, low-pressure feeding should be safer and more reliable.
We operate at atmospheric pressure; the only requirement is that the temperature be above 150°, with a corresponding pressure of 0.3 MP. A horizontal methamine condenser is used to ensure that ammonia enters the system at low pressure and vaporizes, thereby preventing the system’s temperature from dropping below 150°. Rinse the high-pressure system before feeding. Before the system pressure reaches 7.0 MPa, ammonia and CO2 are fed via a bypass line (to protect moving equipment). Tests have shown that the degree of corrosion on the high-voltage equipment is within acceptable limits.
The biggest problem with operating at low pressure is that it requires a great deal of experience in determining the ammonia-to-carbon ratio and water-to-carbon ratio in the high-pressure system after feeding materials in. Otherwise, if the material ratios become unbalanced when material is discharged from the urea tower, the high-pressure system will experience overpressure; in such cases, it’s necessary to empty the high-pressure section before feeding materials again, which results in significant losses. When starting up using the high-pressure method, such risks are much lower.
For the temperature-raising passivation in the low-pressure startup method, only steam needs to be added; this requires materials of grade 252 or higher. Currently, the old equipment used in large fertilizer plants in China is made of 316 material, so conventional temperature-raising passivation and startup procedures must be followed.
We’re all using old-type urea production equipment. Operating at low pressure essentially means relying on the equipment itself; since the pressure must be higher than the dissociation pressure of ammonium carbamate before discharge, the pressure increase rate is 50–60 kilograms. As for corrosion issues, many new installations use duplex steel, which requires no oxygen addition or passivation, and it’s incomparable to older materials. Back in the day, 316L steel was already considered quite good for urea production. There is also discharge at low pressure, as well as high loads that need to be recovered by the subsequent system; with old installations, the only option is to release those loads, which is not worth it. So that’s something played by young people born in the 1990s; if the old folks from the 1970s and 1980s try to imitate it, it’s like old people taking arsenic.
Haha, funny – seems like an old pro at using urea! Indeed, there is little need for heating for passivation or operating at low pressure; only with new installations, new materials (with an expansion coefficient similar to that of carbon steel and greater corrosion resistance), and pool-type ammonium methoxide condensers is it feasible to operate at low pressure. Let the old guys honestly rise to 8MPa and above 130 degrees.