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%. Plant layout
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%. Plant layout
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 12oC
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 75oC
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 40oC 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.
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| Flow sheet of Urea prilling system |
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| Block diagram of urea prilling section |
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