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MicroMist Venturi scrubber used to further cool and humidify the gas flow. This
TM
process is a very important step to assure that re-
The MicroMist™ Venturi (MMV) scrubber is a maining submicron particulate is exposed to satu-
multi-stage process for the effective and efficient rated gas, where particles can substantially grow
scrubbing of urea particulate matter and ammo- in size through condensation. Inside the scrubber
nia gaseous residues from the urea vessel several Dual-Orifice Conditioning (DOI) trays
melt and the granulation plant to can optionally be installed to further condition the
meet the more stringent particulate gas stream. Multiple parallel venturi tubes are in-
matter emission regulations. During stalled vertically on a diaphragm in the vessel.
the granulation process, submicron The diaphragm forces the gas flow to accelerate
dust is generated which is mainly re- through the tubes (figure 3). Each venturi tube in-
sponsible for the higher emission va- cludes a converging conical section (the inlet) whe-
lues. While older technology scrub- re the gas is accelerated to throat velocity, a cylin-
bers easily scrub larger particles, the drical throat, and the diffuser outlets of the MMV
presence of a high degree of submi- tubes are aerodynamically designed to reduce the
cron dust requires a new capture overall pressure drop by slowing down the gas and
approach, and to efficiently remove recovering the energy. In the tubes, gases interact
ammonia, an acid solution needs to with the particulates and droplets twice (accelera-
be injected. This newly introduced tion and deceleration).
scrubber technology is successfully Downstream the MMV stage, the ammonia aci-
implemented and proven in other in- dic scrubbing takes place. The DOI tray is flooded
dustrial applications, but is comple- from above with acidified water and the acid flow
tely new for the urea industry. The rate is controlled by a pH measurement. Typical-
MMV scrubber contains five stages ly sulfuric acid or nitric acid are used to neutralize
that progressively treat and cle- ammonia, forming an ammonium salt. Depending
an the off-gas (figure 2). With this on the type of acid used, ammonium sulphate or
technology emissions of less than ammonium nitrate salts are formed. They can be
Fig. 2 - MMV Scrubber stages layout: 10 mg/Nm for dust and 20 mg/Nm fed as a feedstock to the granulation plant in case
3
3
1) Quenching; 2) Subcooling; for ammonia can be achieved. of ammonium sulphate to produce urea with tra-
3) Atomization; 4) Collection element At the first stage, the exhaust gas is ces of sulphur. Alternatively they can be used in a
cooled down, saturated and most of secondary process such as a UAN plant in case of
the coarse particulate urea dust is collected from ammonium nitrate, or being supplied to other pro-
the gas stream At this point in the process the con- cesses, depending on the specific process layout
centrated urea solution is purged and available for chosen. By further addition of ammonium sulphate
further (re)processing. The concentrated urea solu- in the granulation plant, urea ammonium sulphate
tion is typically between 35 and 45 wt-% urea, and (UAS) compounds are made, satisfying the growing
can be fed to the urea melt plant. Downstream of need for sulphur, which has meanwhile become the
the first quench zone, a secondary quench is used. fourth macro nutrient.
In this second quench, a dilute solution of urea is Remaining suspended water droplets are removed
from the gas stream in the mist eliminator before
the gases leave the scrubber. Fresh (clean) water
is continuously sprayed on the mist eliminator to
catch and wash away dirty particles. For prilling
plants a similar scrubber is available. An optional
Wet ElectroStatic Precipitator (WESP) can be inte-
grated on top of the MMV scrubber to further redu-
ce overall emissions.

Waste Water Treatment
In the waste water treatment section liquid effluents
from the granulation and urea melt plants are tre-
ated (figure 4). The process condensate coming
from the evaporation section, together with other
process effluents such as sealing water from stuf-
fing boxes, contain ammonia and urea. All of the
process condensates are collected in the ammonia
water tank. From this tank, the waste water is fed
to the top part of the desorber. In the top part of the
desorber, the bulk of the ammonia and carbon dio-
Fig. 3 - Micro Mist™ Venturi xide are stripped off from the water phase by using

42 Impiantistica Italiana - Maggio-Giugno 2018