Chemical Engineering July 2017 - 32

Technology Profile
Urea Production from NH3 via a Self-Stripping Process
By Intratec Solutions
Ammonia,
carbon dioxide
U
rea is a nitrogenous compound
consisting of a carbonyl
group attached to two
amine groups. Urea plays an
important role in many biological processes,
but is also an important material
in synthesis (for example, resins
and plastics), as well as in fertilizers
and animal feed.
The process
The process described is similar to
Saipem's (formerly Snamprogetti) selfstripping
process, based on formation
of ammonium carbamate from liquid
NH3 and gaseous CO2, followed by
dehydration of the ammonium carbamate
to urea. Figure 1 presents a simplified
flow diagram of this process.
Reaction. Initially, CO2 and a liquid
mixture of ammonia and carbamate,
recovered downstream, are fed to the
urea reactor, part of the high-pressure
synthesis loop. Here, the ammonia
and the CO2 react, yielding ammonium
carbamate and urea.
Stripping. The reactor effluent, containing
carbamate, is fed to a fallingfilm
stripper, where excess NH3 strips
out the carbamate from the effluent.
The off-gas from the stripper is absorbed
in a liquid carbamate stream
recovered downstream, and fed to
a kettle-type boiler to be condensed
and recycled to the reactor. The heat
exchanged is used to produce steam.
Carbamate decomposition. The urea
solution is directed to two successive
decomposers for the removal of residual
carbamate and CO2. After decomposition
of carbamate and evaporation
of ammonia, a urea solution, substanAmmonia
ST
4
BFW
3
10
Carbon
dioxide
1
2
5
6
8
FIGURE
1. This diagram shows prilled urea production from ammonia via a self-stripping process
30
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Urea pills
Process
condensate
7
ST
9
CW
11
12
Naptha, air
Hydrogen,
nitrogen
Urea synthesis
Steam reform./
NH3 synthesis/
urea synthesis
NH3 synthesis/
urea synthesis
FIGURE 2. Several possible production pathways exist for urea
n Raw material n Pathway n Main product
tially free of carbamate, is obtained.
The off-gas from the decomposers
is rectified in a medium-pressure absorber,
from which gaseous ammonia
is obtained as the top product and a
liquid ammonium carbamate stream is
obtained as bottom product. The ammonia
is condensed, mixed with fresh
ammonia and routed to urea synthesis,
while the ammonium carbamate is
directed to the condenser.
Urea concentration. At this point, the
urea-water mixture is concentrated
in a two-stage evaporator, forming a
urea melt suitable for prilling (pelletizing).
The vapor obtained is condensed
and directed to a condensate treatment
unit. The treatment consists of
stripping and hydrolysis steps, and the
condensate obtained is used as process
water and boiler feed water, while
the off-gas, containing ammonia and
CO2, is recycled to urea synthesis.
Finishing. Here, the urea melt is
sprayed at the top of a prilling tower,
forming spheroidal urea particles
(called prills) which are packed in bags
and stored.
Urea production pathways
Urea was first produced in 1828 from
ammonia and cyanic acid in aqueous
solution. Modern commercial urea
manufacture occurs exclusively from
NH3 and CO2, in such a way that different
production routes are related to
different sources of those materials.
Figure 2 shows the different pathways
by which urea is produced.
Economic performance
The total operating cost (raw materials,
utilities, fixed costs and depreciation
costs) estimated to produce urea
is about $220 per ton of urea. The
analysis is based on data from the
second quarter of 2013 from a plant
with capacity to produce 1.3 million
metric tons of urea per year. It was assumed
that the plant is integrated to
an ammonia unit that supplies ammonia,
at its cost of production.
This column is based on " Prilled
Urea from Ammonia via Self-Stripping
Process - Cost Analysis, " a report
published by Intratec. It can be
found at: www.intratec.us/analysis/
urea-production-cost.
n
Edited by Scott Jenkins
Editor's note: The content for this column is supplied by Intratec
Solutions LLC (Houston; www.intratec.us) and edited by
Chemical Engineering. The analyses and models presented
are prepared on the basis of publicly available and nonconfidential
information. The content represents the opinions
of Intratec only. More information about the methodology for
preparing analysis can be found, along with terms of use, at
www.intratec.us/che.
1. C02
compressor
2. Urea reactor
3. Stripper
4. Carbamate condenser
5. Medium-pressure decomposer
6. Low-pressure decomposer
7. MP absorber
8. Vacuum evaporators
9. Condensate treatment
10. Prilling tower
11. Steam boiler
12. Cooling tower
CW Cooling water
ST Steam
BFW Boiler feed water
Urea
Steam reform./
NH3 synthesis/
urea synthesis
Partial oxidation/
NH3 synthesis/
urea synthesis
Electrolysis/
NH3 synthesis/
urea synthesis
Water, nitrogen
Natural gas, air
Coal, air
http://www.intratec.us/analysis/ http://www.intratec.us http://www.intratec.us/che http://WWW.CHEMENGONLINE.COM
Chemical Engineering July 2017

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