One of the key problems urea plants face is corrosion. The corrosion problem rests with one chemical produced by the urea process, ammonium carbamate.
Ammonium carbamate is produced by reacting ammonia and carbon dioxide at high pressures (up to 200 bar) in a special reactor. The urea reactor is a reactor vessel inside a protective shell. Should there be a leak in the reactor, the shell can contain the chemicals released long enough for an orderly plant shutdown. The space between the reactor skin and the shell is most often empty and employs various methods of detecting a leak ranging from conductivity measurements to infrared analysis.
The actual reactor has been constructed using a variety of materials over the years that can resist ammonium carbamate corrosion. In the latest plants specialty duplex steels, such as Sandvik’s Safurex, have greatly improved the resistance to ammonium carbamate corrosion. Duplex steels have also been adopted for piping, valves, etc.
Duplex steels though are not completely impervious to an attack by ammonium carbamate. There are two circumstances where a different material, zirconium, is needed. Zirconium is the best material for ammonium carbamate but is expensive, hard-to-work-with, etc., so using zirconium, as a standard material of construction is not feasible. There are two instances in urea plants though where zirconium is the prudent choice.One key problem area is the urea stripper. In this device, carbon dioxide is used to “strip” unreacted ammonia from the effluent of the reactor. One key measurement in operating the urea stripper is the level in the vessel. The level is measured by reading the differential pressure (DP) across the urea stripper.
The high-pressure and corrosive fluid in the stripper doesn’t allow a direct connection to the differential pressure transmitter. Instead, we use diaphragm seals where a thin diaphragm isolates the transmitter from the process. “Thin” is the problem.All metals have corrosion problems with ammonia carbamate. The thin pressure seal in the urea stripper level reading is a weak point. Reactor vessels, valves, etc. have enough material in contact with the ammonium carbamate to make some corrosion tolerable. Not so in the diaphragm seal. Should the seal leak, ammonia and ammonium carbamate would be released causing a major environmental and safety problem. The Rosemount 1199 Diaphragm Seal System has a high-pressure zirconium seal option especially designed for the urea market.
Another specialty product for urea production is Fisher Severe Service valves, which have options for high-pressure, zirconium-lined valve applications for the urea reactor effluent.
Why would you want to use a much more expensive zirconium valve when the industry-standard is a duplex steel valve? As I noted, “All metals have corrosion problems with ammonia carbamate” and the corrosion problems increase with temperature. A ten degree Celsius rise in temperature doubles the corrosion rate to the point where the duplex steel is no longer acceptable.
The reason a urea plant might increase the temperature at the top of the urea reactor is financial. In strong urea markets, there is a large incentive to increase production. To increase production in a urea plant you must run the reactor hotter. This change causes a number of problems in a standard plant’s design, not the least of which is the possibility of a valve failure on the reactor effluent stream. The zirconium-lined valve solves this problem.
As with the zirconium seal for the level reading on the stripper, the zirconium-lined valve is a specialty product developed for the urea market. You can connect with your local Fisher valve and Rosemount measurement specialists to discuss your specific application and process parameters. You can also learn more ways to increase efficiency by downloading the whitepaper, 4 Solutions for Urea Automation Challenges: Instrumentation and insights to increase efficiency.