Urea was first discovered in urine in 1773 by the French
chemist Hilaire Rouelle. In 1828,
the German chemist Friedrich Wöhler obtained
urea by treating silver cyanate with ammonium chloride in a failed
attempt to prepare ammonium cyanate:
- AgNCO + NH4Cl → (NH2)2CO
This was the first time an organic compound was artificially
synthesized from inorganic starting materials, without the involvement
of living organisms. The results of this experiment implicitly
discredited vitalism: the theory that the
chemicals of living organisms are fundamentally different from
inanimate matter. This insight was important for the development of
organic chemistry. His
discovery prompted Wöhler to write triumphantly to Berzelius:
"I must tell you that I can make urea without the use of kidneys,
either man or dog. Ammonium cyanate is urea." For this discovery,
Wöhler is considered by many the father of organic chemistry.
Detail Updated 2010-06-21
The production of urea proceeds by a two-stage reaction. Ammonia and
carbon dioxide react to form ammonium carbamate.
CO2 <---------> NH4O--CO--NH2
H = -117kj/mol
strongly exothennic reaction very rapidly reaches equilibrium. The
reaction system shown above will hereinafter be referred to as the
carbamate equilibrium. In the liquid phase, ammonium carbamate is next
dehydrated to urea and water:
NH2--CO--NH2 + H2O
= +15.5 kj/mol
equilibrium reaction is rather slow compared with the first one; the
system will hereinafter be called the urea equilibrimn.
smnmary data on equilibrium and kinetics enable the essence of the
layout of the urea plant to be derived. Ammonia and carbon dioxide have
to be contacted in a device capable of removing large quantities of
reaction heat: the cooling part of the Reactor.
part of the Pool Reactor not only removes heat but is dimensioned in
such a way, that also residence time is available to perfonn a great
part of the slower second reaction. In other words, a part of the urea
reaction is performed in the so called cooling part of
From this part of the Reactor the mixture flows to
the adiabatic part of the Urea reactor, where the seeond
equilibrium is established. From the fact that the dehydration reaction
does not proceed to completion, it follows that the unconverted
reactants must be removed from the reactor solution. The way in which
this is done characterizes most urea processes. The process pressures
and temperatures are dictated, by the compositions, by the phase
behaviour of the four-component mixture consisting of ammonia, carbon
dioxide, water and urea, by the inert percentage, and by the desired
utility consumption or the steam production of the plant.
a good understanding of the process, knowledge of phase behaviour of a
mixture containing ammonia, carbon dioxide, urea and water is
essential. The phase behaviour of such a quaternary system is far from
ideal and rather difficult to understand. The behaviour of mixtures
from the properties of their components will be explained by
application of basic principles from phase theory.
The four components mentioned above are classified into three groups:
The light components, ammonia and carbon dioxide, which in pure form
occur in the liquid phase only at high pressures and/or low
2. The medium-weight component, water, which occurs both in liquid and
in the gas phase.
3. The heavy component, urea, which occurs in the gas phase only at
very low pressures and/or high temperatures.
The phase transitions are schematically represented as follows:
phase behaviour of this four-component system is largely determined by
the behaviour of the binary system ammonia-carbon dioxide.
and carbon dioxide:
simplest description of the behaviour of the four-component mixture,
therefore, is obtained by considering the behaviour of this binary
system and then to add the properties of water and urea. According to
the first reaction equation, ammonia and carbon dioxide react to form
the salt, ammonium carbamate. For the ammonia-carbon dioxide system
this has two important consequences:
a) The reaction of two
light components results in the formation of the heavy component
ammonium carbamate. The pressure of the
will therefore be lower
than the pressure ofthe individual components
b) The strong
interaction between ammonia and carbon dioxide results in an azeotrope.
The dissociation pressure of the ammonium
remains low, also at
higher temperatures. It has the surprising effect that ammonium
carbamate may be present in
liquid form while
the components constituting it are supercritical. In combination with
the azeotrope, this leads to the situation illustrated
in Graph I, which describes the
equilibrium of the system at a pressure and temperatures at which
does not occur in the solid state.
a gas mixture having the composition Xaz is cooled, it will condense at
temperature Taz, the azeotropic condensation temperature. During the
condensation the temperature remains constant. The mixture behaves like
a unary system. When a gas mixture with a different
cooled, it will behave as a binary mixture and show a condensation
range, not a point. The beginning and the end of this range will lie at
temperatures lower than the azeotropic condensation temperature.
boiling range starts at the lowest temperature, at which gas bubbles
are formed. This temperature is therefore called the bubble point. The
start of the condensation range is the highest temperature at which
liquid is still present, and is therefore called the dew point.
carbon dioxide and water:
is a medium-weight component compared with ammonia and carbon dioxide.
However, the latter two components differ widely in their behaviour
from water. Ammonia is very readily soluble in water, carbon dioxide
only very poorly. This means that a liquid phase can only contain
appreciable amounts of carbon dioxide in the bound form, e.g. as
ammonium carbamate or, possibly, as ammonium carbonate and only to a
very low degree as free carbon dioxide.
The presence of water makes the azeotrope disappear, but a maximum dew
point and a bubble point remain. The location of these points is
determined by the water fraction. As water is a medium-weight
component, the maxima will be at a higher temperature as the water
increases. The relation between the composition and the maximum
temperatures of dew and bubble points is given by the so-called
top-ridge lines. The phase diagram for a pressure such as occurs in the
synthesis section is shown schematically in Graph 2.
figure also shows the projections of the top-ridge liquid line and of a
number ofliquid isotherms. A liquid isotherm represents the
of mixtures which at the isotherm temperatures are exactly at their
bubble points. The isothelms are shown in more detail in Graph 3. It is
seen that on the cm'bon-dioxide side of the top-ridge liquid line the
liquid isotherms are much closer together than on the ammonia side.
there is an excess of ammonia or carbon dioxide with respect to the
top-ridge composition, the pressure in the mixture can be maintained by
applying, for equal disturbances, a lower temperature if the excess is
on the carbon dioxide side. In other words, carbon dioxide is less
soluble in the mixture than ammonia.
carbon dioxide, water and urea:
is the heaviest of the four components. This means that, like water, in
a liquid phase urea can act as a solvent. As the urea fraction becomes
greater, the vapour pressure of the mixture will become lower, assuming
the temperatures to remain constant. The affInity between ammonia and
urea is lower than that existing between ammonia and water, however.
graphical representation of the phase behaviour of a quaternary mixture
is very difficult. Therefore, use is mostly made of a quasi-ternary
representation, urea and water, occurring in a given ratio, are taken
to constitute one component. The result is that one can consider a
system having only three components: ammonia, carbon dioxide, and
water/urea. If no extra water is pumped to the synthesis section, the
water/urea ratio would, by the urea equilibrium equation, be equal to
Graph 3 gives the projection of the quasi-ternary
equilibrium at constant pressure of this situation. The figure further
shows the course of the isobar at chemical equilibrium, representing
the compositions and temperatures at a given pressure, with the mixture
being in chemical
On the intersection of this
isobar and the top-ridge liquid line lies the equilibrium composition
at which the temperature of the liquid mixture is maximum at this
pressure. The fornation reaction of urea is endothennic, the conversion
into urea will under these conditions be maximum, too. For this
equilibrium composition to be reached, a mixture has to be used that
has a composition lying on the line representing the reaction path to
the point of intersection mentioned.
From the gross reaction
equation of the formation of urea from ammonia and carbon dioxide and
the molar weight of these substances it follows that the use of a
mixture containing 56.4 % wt. carbon dioxide and 43.6 % ammonia will
lead exactly to the water/urea corner point. All reaction paths are
parallel to this line.
Urea is a popular solid nitrogen fertilizer because of its high
nitrogen content (46%), with nearly 90% of output going into this
application. Most world output is in a solid form, either prills or
granules, or crystalline for specialised small-volume uses. In a number
of industrialised countries, a growing volume of liquid product is
consumed in the production of nitrogen solution fertilizers, and in
liquid cattle feeds.