UREA process

UREA
Urea accounts for almost 50% of world nitrogen fertilizer production (in terms of N content, and including
multi-nutrient products), compared with only 30% a decade previously. It is produced by combining
ammonia and carbon dioxide at high pressure (140-200 bar) and high temperature (180-190°C) to form
ammonium carbamate, which is then dehydrated by heat to form urea and water, according to the following
reaction:
(1) 2NH3 + CO2 ————> NH2COONH4 —————> CO(NH2) 2 + H2O
The first stage of the reaction is exothermic and proceeds to virtual completion under industrial conditions.
The second stage is endothermic, and conversion is only partial (50-80%, CO2 basis). The conversion is
increased by increasing the temperature, increasing the NH3/CO2 ratio and/or decreasing the H2O/CO2
ratio. Process design is mainly concerned with the most efficient ways to separate product urea from the
other reaction components, to recover excess NH3, and to decompose the residual carbamate to NH3 and
CO2 for recycling.
3.1 Processes
There are three main types of process:
– Once-through process: unconverted CO2 and NH3 are discharged to other plants, where the NH3 is
used for the production of fertilizers such as ammonium sulphate and ammonium nitrate;
– Partial recycle process: unconverted CO2 and NH3 are partially separated in the decomposition section
of the first stage and are then recovered in an absorber, the remainder being delivered to other plants
as in the once-through process;
– Total recycle process: unconverted CO2 and NH3 are totally separated in multi-stage decomposers,
recovered in corresponding multi-stage absorbers, and recycled to the reactor.
These possibilities are illustrated in figure 3.5.

If the residual NH3 and CO2 cannot be used in downstream plants, a total recycle process is necessary. In
early urea technology, this was achieved in a series of loops with decreasing pressure, which cooled,
condensed and recombined the gases to form carbamate liquor, which was then recycled to the synthesis
section. This increased the NH3/CO2 ratio, and hence the yield of urea. Better than this, however, processes
were developed which decompose the carbamate in the reactor effluent without reducing the system
pressure. This requires a “stripping” gas, which can be either CO2 or NH3, or both.

Carbon Dioxide Stripping
If CO2 is used as the stripping agent, urea conversion occurs at about 140 bar and 180-185°C, with a molar
NH3/CO2 ratio of 2.95. This gives a conversion of about 60% CO2 and 41% NH3. At or about system
pressure, CO2 is added to the reactor effluent, and the stripped NH3 and CO2 are then partially condensed
and recycled. Resultant heat is used to produce steam, some of which provides heat for the downstream
sections of the process, and some goes to drive the turbine of the CO2 compressor. NH3 and CO2 in the
stripper effluent are first vaporized and then condensed to carbamate solution, which is recycled. The
process urea solution is further concentrated in an evaporation section., producing a melt of 99.7% urea,
which is then prilled or granulated.

Ammonia Stripping
If NH3 is used for carbamate stripping, the pressure and NH3/CO2 ratio are somewhat higher in the
synthesis section, giving a CO2 conversion rate of 65%. Excess NH3 is introduced to the reactor effluent,
decomposing a large part of the unconverted carbamate. Residual carbamate and CO2 are then recovered
in a two-stage process. Gas vapours from the top of the stripper are mixed with the recovered carbamate
solution, condensed and recycled to the reactor. Resultant heat is used to produce steam. The urea
solution is evaporated to a melt, and then prilled or granulated.
Advanced Cost and Energy Saving (ACES) Process
The so-called ACES process is essentially a CO2 stripping process which operates at somewhat higher
pressure (175 bar) and NH3/CO2 ratio (4), as well as a slightly higher temperature, compared with the
conventional process. Stripper gases are passed into two parallel carbamate condensers. Steam is
generated for downstream heating, and the carbamate solution and non-condensed gaseous mixture are
recycled to the reactor. The urea solution passes through a vacuum concentrator and is then further
evaporated to about 99% urea melt.
Isobaric Double Recycle (IDR) Process
This process uses both CO2 and NH3 as stripping agents. Operating at 200 bar and 185-190°C, with an
NH3/CO2 ratio of 4.5, a CO2 conversion rate of 71% is obtained, with 35% for NH3. The reactor effluent
then passes into a first stripper, which uses NH3, and the remaining NH3 is then separated in a second
stripper, using CO2. Gases from the first stripper go directly to the reactor, and those from the second
stripper pass first through the carbamate condenser. Two vacuum evaporators concentrate the urea
solution to a melt for prilling or granulation.
Prilling is achieved by conveying the urea melt to the top of a tall tower and spraying it down the tower
through an up-draft of air, which can be either natural or forced. As it falls, the liquid droplets solidify to
prills with diameters of 1.6-2.0 mm. Granulation is achieved by spraying the melt on to recycled seed
particles or prills rotating in the granulator. Granules grow larger, and the product is simultaneously
solidified and dried. Traditional granulation processes involve recycling, the ratio of recycled to final
product varying between 0.5-1.0. However, prill granulation has a very small recycle ratio, typically 2-4%.
3.2 Inputs, Outputs and Emission Levels
Typical inputs into modern urea plants are shown in table 3.2, although it should be noted that actual
figures can vary considerably.
Urea plant sizes are normally commensurate with the amounts of NH3 and CO2 available from associated
NH3 plants. A typical output in a new BAT plant is 1500 t/d of urea. Water outputs include the process
condensate (up to 0.5 t/t urea) and the steam and/or turbine condensates (0.4 and 0.2 t/t urea respectively).
They are normally treated and re-used as boiler feed. Low pressure steam may be used for process
heating, or in turbines, or exported for other activities.