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Process Description
The full process description given below is divided in the process sections:
1.Synthesis
1.1The pool condenser concept
Ammonia and carbon dioxide are introduced in the high-pressure
synthesis by means of a high-pressure ammonia pump and a carbon
dioxide compressor. The ammonia is introduced via an ejector
together with a carbamate solution into the pool condenser.
In the pool condenser two third of the urea formation
takes place. The carbon dioxide, entering the synthesis
in the HP stripper, flows counter-current with the urea
solution leaving the reactor and is there after fed to the pool condenser.
The gases are condensed and the heat released
in the condensation of the gases and in the formation of ammonium
carbamate produces LP steam. The pool condenser is divided into compartments
to create residence time and an excellent mixing behavior.
After the pool condenser the remaining gases and the urea-carbamate
liquid enter the vertical reactor in which the final part
of the urea formation will take place. This reactor is divided
into compartments as well. The urea solution will leave the reactor at the top via
a funnel and is introduced into the stripper. In the stripper
the greater part of the unconverted carbamate is dissociated
and the ammonia and carbon dioxide is stripped off. Stripping
is effected by counter-current contact between the urea solution
and the fresh carbon dioxide.
Ammonia and carbon dioxide conversions in the synthesis section
of a Stamicarbon CO2 stripping plant are high, thus reducing
the need for recycle of unconverted ammonia and carbon dioxide.
The carbon dioxide conversion in the synthesis section as
well as the ammonia conversion is typically as high as 80%.
The gases leaving the reactor are fed to the HP scrubber.
In the scrubber the gases are washed with the carbamate solution
from the low-pressure recirculation stage. The enriched carbamate
solution is fed to the high-pressure ejector and subsequently to the pool condenser. Inert gases
and some ammonia and carbon dioxide are released to the 4 bar
absorber.
The carbon dioxide feed usually originates from an associated
ammonia plant and therefore nearly always contains hydrogen.
This hydrogen is removed by catalytic combustion either at
synthesis pressure or at an interstage pressure of the carbon
dioxide compressor. The air is also needed as passivating
air for the synthesis.
1.2 The pool reactor concept
 Instead
of the combination of a pool condenser and a vertical reactor,
the pool condenser can also be enlarges by adding several
compartments. Now enough residence time has been created to
allow the reaction to reach is optimum conditions and eliminating
the need for a separate vertical reactor. The scrubber can
also be integrated in the pool condenser concept. By placing
the scrubber sphere above the poolreactor and adding the ammonia
to the synthesis in this scrubber, no heat exchanging part
in necessary.
The carbamate from the low-pressure recirculation section
will flow together with the absorbed gases and the ammonia
via a sparger into the pool reactor. Because of the static
height, no ejector is needed.
2. Recirculation
Only one recirculation stage is required due to the low ammonia
and carbon dioxide concentrations in the stripped urea solutions.
In this stage, ammonia and carbon dioxide still present in
the urea solution coming from the stripper are recovered.
Required heat is supplied by the condensation of the produced
LP steam. Because of the ideal ratio between ammonia and carbon
dioxide in the recovered gases, the water dilution of the
resultant ammonium carbamate solution is at a minimum.
After the stripper the urea solution is fed to the dissociation
heater, where most of the ammonia and carbon dioxide is removed.
The ammonia and carbon dioxide are fed to the low-pressure
carbamate condenser, where they are condensed. The resulting
carbamate solution is fed, via a HP carbamate pump, back to
the synthesis, as a scrubbing agent in the high-pressure scrubber.
The temperature of the carbamate solution is 75 °C, so
its corrosiveness is negligible. The vent gas from the recirculation
stage is practically free from ammonia as it is scrubbed in
an atmospheric absorber. Before entering the urea solution
tank, a part of the water present in the solution is removed
in the pre-evaporator.
3. Evaporation and finishing technique
The urea solution, present
in the urea solution tank, must be concentrated
before the final product can be made. Therefore the urea solution
is send to an evaporator where water in the urea solution
is evaporated under vacuum conditions. The remaining
urea melt, with a urea concentration varying from 95
to 99 wt.%, depending on the requirements of the granulation,
will be send to the granulation unit.
An older finishing technique is prilling of the urea melt.
Before entering the prilling tower, the urea solution is concentrated,
under vacuum, in two steps to a 99.7 wt.% urea melt. The resultant
molten stream is prilled with the aid of a rotating prilling
bucket. Using a special technique of seeding when prilling,
impact-resistant prills are obtained, which are very resistant
to degradation.
4. Waste water treatment
The process condensate emanating from the evaporation, together
with other process effluents such as sealing water from stuffing
boxes, contains ammonia and urea. The process condensate is
collected in the process condensate tank. From this tank the
water is fed to the first desorber (top part of one vessel).
In the desorber the ammonia and carbon dioxide are stripped
off with the off-gas from the second (bottom part of same
vessel) desorber. The remaining water still contains urea.
To remove this urea the process condensate is fed to the hydrolyzer.
In the hydrolyzer, the urea, at elevated pressure and temperature,
is transformed back into ammonia and carbon dioxide. Hydrolyzer
feed and hydrolyzer steam are introduced in a counter current
fashion. The remaining ammonia and carbon dioxide are stripped
off with steam in the second desorber. The off gases are being
recycled to the synthesis section after being condensed in
the reflux condenser.
The purity of the remaining water satisfies requirements
for boiler feed water make-up or cooling tower make-up, consequently
Stamicarbon urea plants do not have a waste water stream.
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