The Snamprogetti Urea Process Description

The Snamprogetti Urea Process is basically a total recycle stripping process using ammonia as self stripping agent. The Snamprogetti process used the excess ammonia present in the urea solution leaving the reactor to strip CO₂ from urea solution in a falling film steam heated heat exchanger operated at the urea reactor pressure. The separated CO₂ and NH₃ are recombined as ammonium carbamate in the carbamate condensers, also operating at urea reactor pressure and then returned to the reactor for conversion to urea.

The overall result of this scheme is that an internal recycle of both NH₃ and CO₂ in the urea reactor system is established without having to pump either component , as in previous total recycle process in which NH₃ and CO₂ were separated from the solution at lower pressure. This substantially reduces the high pressure pumping requirements for both the NH₃ and ammonium carbamide solution, since in the Snamprogetti stripping process approximately 80% of the CO₂ fed is converted to urea within high pressure synthesis loop and only about 20% of it must be pumped back to the reactor as ammonium carbamate solution from lower pressure. In addition to the reducing pumping requirements, this process permits a substantial saving in steam requirement, since the operating temperature level of the ammonium carbamate condenser by utilizing the heat released by the condensing vapor.

The NH₃ to CO₂ ratio used in Snamprogetti process is 3.3-3.6 : 1which combines with a temperature of 186-189deg C and pressure of 155 Kg/cm²g approximately, permits a conversion yield in the reactor of 62 to 65%.

Block diagram of total recycle of ammonia stripping in urea production:

 

Urea production takes place through the following main operations:

• Urea synthesis and high pressure recovery.
• Urea purification in the medium, low pressure decomposers, pre-vacuum concentrators. Urea concentration.
• Urea Prilling.

The liquid ammonia coming from the battery limit at a temperature of 12^oC and 18 Kg/cm²(g) is collected in a ammonia receiving vessel which is operated at a medium pressure(17Kg/cm2). From the ammonia receiver, the liquid ammonia is pumped to the reactor by two pumps. The first pump is the ammonia booster pump which is centrifugal type and supplies the liquid ammonia at 22Kg/cm2 to second pump suction. The second pump is the high pressure ammonia pump which is a reciprocating-plunger type. Due to the reciprocating motion of second pump pulsating takes place in the discharge of booster pump and those cause the change of loads on booster pump to avoid these booster pump and those cause the change of loads on booster pump to avoid these pulsations a damper is provided in the suction of second pump. In the H.P. ammonia pump, the ammonia pressure is increased to 239Kg/cm² and then goes to a ammonia pre heater where it is preheated to 75^oC by L.P. decomposer outlet gases, H.P. ammonia is used as pressure section contains first and second stage another one is the high pressure section contains third and fourth stage. First and second stages contains three, third stage contains four and fourth stage two numbers of closed type impellers. Carbon dioxide gas enters first stage suction at 1.5kg/cm2 and 40^oC and compressed to 160 Kg/cm² in four stages. Intercoolers are provided after first, second and third stages to cool the carbon dioxide using cooling water. Along with coolers knock out drums are provided between each stage in which moisture gets separated. Lube oil system provides the lubrication of the rotating parts. The superheated steam and saturated LS steam is used as a driving fluid for steam turbine. Superheated HS steam is extracted and part of steam is condensed in condenser. Condensate is then pumped to D.M. plant.