Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Reexamination Certificate
2002-04-22
2003-10-14
O'Sullivan, Peter (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C068S069000, C068S070000, C068S071000, C068S072000
Reexamination Certificate
active
06632967
ABSTRACT:
Urea can be prepared by introducing ammonia and carbon dioxide into a synthesis zone at a suitable pressure (for example 12-40 MPa) and a suitable temperature (for example 160-250° C.), which first results in the formation of ammonium carbamate according to the reaction:
nNH
3
+CO
2
→H
2
N—CO—ONH
4
+(n−2)NH
3
Dehydration of the ammonium carbamate formed then results in the formation of urea according to the equilibrium reaction:
H
2
N—CO—ONH
4
⇄H
2
N—CO—NH
2
+H
2
O
The theoretically attainable conversion of ammonia and carbon dioxide into urea is determined by the thermodynamic position of the equilibrium and depends on for example the NH
3
/CO
2
ratio, the H
2
O/CO
2
ratio and temperature, and can be calculated with the aid of the models described in for example Bull. of the Chem. Soc. of Japan 1972, Vol. 45, pages 1339-1345 and J. Applied Chem of the USSR (1981), Vol. 54, pages 1898-1901.
In the conversion of ammonia and carbon dioxide to urea there evolves as a reaction product a urea synthesis solution which consists essentially of urea, water, ammonium carbamate and unbound ammonia. In a urea process, the concentrations of the various components in this reaction product are determined and the measurement results are used for controlling the process. In particular the molar NH
3
/CO
2
ratio (N/C ratio) is determined for controlling the NH
3
and/or the CO
2
feed to the synthesis. The N/C ratio is calculated as follows:
N
/
C
=
2
moles
urea
+
1
mole
NH
3
1
mole
urea
+
1
mole
CO
2
Besides the aforementioned urea synthesis solution, there may evolve in the synthesis zone a gas mixture of unconverted ammonia and carbon dioxide along with inert gases. Ammonia and carbon dioxide are removed from this gas mixture and are preferably returned to the synthesis zone. The synthesis zone may comprise separate zones for the formation of ammonium carbamate and urea. These zones may, however, also be united in a single apparatus.
The conversion of ammonium carbamate into urea and water in the reactor can be effected by ensuring a sufficiently long residence time for the reaction mixture in the reactor. The residence time will in general be longer than 10 min, preferably longer than 20 min. The residence time will in general be shorter than 2 hours, preferably shorter than 1 hour.
The conversion of ammonium carbamate into urea is an equilibrium reaction whose position is adversely effected by the water present in the reactor.
An important water source is the low-pressure carbamate stream which evolves during the further recovery of ammonia and carbon dioxide from the urea synthesis solution. This carbamate stream is rich in water and has an adverse effect on the conversion of ammonia and carbon dioxide into urea. This carbamate stream is, however, an important source of feedstocks, for which reason one chooses in most urea plants to return this carbamate stream to the synthesis zone all the same.
In practice, various processes are used for the preparation of urea. Initially, urea was prepared in so-called conventional high-pressure urea plants, which at the end of the 1960s were succeeded by processes carried out in so-called urea stripping plants.
A conventional high-pressure urea plant is understood to be a urea plant in which the decomposition of the unconverted ammonium carbamate into urea and the expulsion of the customary excess ammonia take place at a substantially lower pressure than the pressure in the synthesis reactor itself. In a conventional high-pressure urea plant the synthesis reactor is usually operated at a temperature of 180-250° C. and a pressure of 15-40 MPa. In a conventional high-pressure urea plant, following expansion, dissociation and condensation at a pressure of between 1.5 and 10 MPa, the reactants that are not converted into urea are returned to the urea synthesis as a carbamate stream. In addition, in a conventional high-pressure urea plant, ammonia and carbon dioxide are fed directly to the urea reactor. The N/C ratio in the urea synthesis in a conventional high-pressure urea process is between 3 and 5 and CO
2
conversion between 64 and 68%.
Initially, such conventional urea plants were designed as so-called ‘Once-Through’ processes. Here, non-converted ammonia was neutralised with acid (for example nitric acid) and converted into ammonia salts (for example ammonium nitrate). It did not take long until these conventional Once-Through urea processes were replaced with Conventional Recycle Processes, in which all non-converted ammonia and carbon dioxide are recycled to the urea reactor as carbamate streams. The water percentage of these carbamate streams is determined. The result of this measurement is used for controlling the process. It is essential here that the amount of water be controlled such that the carbamate streams are just above the crystallisation point. This is essential in order to limit as much as possible the adverse effect of the amount of water on the synthesis. In the recovery section, non-converted ammonia and carbon dioxide are removed from the urea synthesis solution obtained in the synthesis reactor, in which process a urea in water solution evolves. Next, this urea in water solution is converted into urea in the evaporation section by evaporating water at reduced pressure. The urea concentration of the feed to the evaporation is determined for optimum control of evaporation. Especially steam consumption can be optimised in this way.
A urea stripping plant is understood to be a urea plant in which the decomposition of the ammonium carbamate that is not converted into urea and the expulsion of the customary excess ammonia largely take place at a pressure that is essentially almost equal to the pressure in the synthesis reactor. This decomposition/expulsion takes place in a stripper with or without addition of a stripping agent. In a stripping process, carbon dioxide and/or ammonia may be used as stripping agent before these components are added to the reactor. Such stripping is effected in a stripper installed downstream of the synthesis reactor; in it, the urea synthesis solution coming from the urea reactor, which contains urea, ammonium carbamate and water as well as ammonia, is stripped with the stripping agent with addition of heat. It is also possible to use thermal stripping. Thermal stripping means that ammonium carbamate is decomposed and the ammonia and carbon dioxide present are removed from the urea solution exclusively by means of the supply of heat. Stripping may also be effected in two or more steps. In a known process (IDR process) a first, purely thermal stripping step is followed by a CO2 stripping step with addition of heat. The gas stream containing ammonia and carbon dioxide exiting from the stripper is returned to the reactor whether or not via a high-pressure carbamate condenser.
In a urea stripping plant the synthesis reactor is operated at a temperature of 160-240° C. and preferably at a temperature of 170-220° C. The pressure in the synthesis reactor is 12-21 MPa, preferably 12.5-19.5 MPa. The N/C ratio in the synthesis in a stripping plant is between 2.5 and 4 and CO
2
conversion between 58 and 65%. The synthesis can be carried out in one or two reactors. When use is made of two reactors, the first reactor, for example, can be operated using virtually fresh raw materials and the second using raw materials entirely or partly recycled, for example from the urea recovery.
A frequently used embodiment for the preparation of urea by a stripping process is the Stamicarbon CO
2
stripping process as described in European Chemical News, Urea Supplement, of Jan. 17, 1969, pages 17-20. The greater part of the gas mixture obtained in the stripping operation is condensed and adsorbed in a carbamate condenser, after which the high-pressure ammonium carbamate stream formed is returned to the synthesis zone for the formation of urea. The stripping of th
Scholten Jacob F.
Van Laak Franciscus A. L.
DSM N.V.
O'Sullivan Peter
Pillsbury & Winthrop LLP
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