UREA

History  Updated 2010-06-23

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:[2]

AgNCO + NH4Cl → (NH2)2CO + AgCl

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.

Process Detail  Updated 2010-06-21

The production of urea proceeds by a two-stage reaction. Ammonia and carbon dioxide react to form ammonium carbamate.

        2NH3 + CO2 <---------> NH4O--CO--NH2         H = -117kj/mol

This 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:

        NH4O--CO--NH2  <-------------> NH2--CO--NH2 + H2O         H = +15.5 kj/mol

This endothennic equilibrium reaction is rather slow compared with the first one; the system will hereinafter be called the urea equilibrimn.
The above 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.
The cooling 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 the" Reactor".
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.

PHASE BEHAVIOUR:

For 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:
1. The light components, ammonia and carbon dioxide, which in pure form occur in the liquid phase only at high pressures and/or low temperatures.
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:

The phase behaviour of this four-component system is largely determined by the behaviour of the binary system ammonia-carbon dioxide.
.
Ammonia and carbon dioxide:
The 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             mixture 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             carbamate 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 gas-liquid equilibrium of the system at a pressure and temperatures at which ammonium carbamate
    does not occur in the solid state.


Graph-1

When 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
composition is 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.
The 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.

Ammonia, carbon dioxide and water:
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
fraction 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.


Graph-2






This figure also shows the projections of the top-ridge liquid line and of a number ofliquid isotherms. A liquid isotherm represents the
compositions 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.
Graph-3


If 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.

Ammonia, carbon dioxide, water and urea:
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.
The 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 unity.
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
equilibrium.
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.

Uses  Updated 2010-06-20

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.