Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Reexamination Certificate
2002-06-12
2003-03-25
O'Sullivan, Peter (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C564S066000, C564S067000
Reexamination Certificate
active
06538157
ABSTRACT:
BACKGROUND OF THE INVENTION
commercial urea processes consist overall of three process steps, that is to say synthesis; prilling, granulation or crystallisation; and effluent treatment and circuits for recycling carbamate to the reactor.
The synthesis step usually comprises two half-)reactions i.e.:
1) Reaction of ammonia with carbon dioxide, which reaction proceeds rapidly and completely to give carbamate in accordance with the reaction equation:
2NH
3
+CO
2
→NH
2
CO
2
NH
4
2) The reaction for the dehydration of carbamate to give urea in accordance with the equation
NH
2
CO
2
NH
4
⇄(NH
2
)
2
O+H
2
O
The latter reaction is an equilibrium reaction and in the usual urea processes achieves approximately 50-60% conversion. The carbamate reaction is highly exothermic and the urea reaction is endothermic.
In modern processes the major proportion of the unconverted carbamate is decomposed in a steam stripper and, via a condensation step in which steam is recovered, is recycled to the reactor. The economy of these known processes is highly linked to the yield from the urea synthesis reaction because the latter to a large extent determines how large the recirculation steams are.
In the 1960s and 70s substantial progress was made in the economy of conventional urea processes by installing a high pressure stripper in the urea process. With this arrangement, in this stripper a substantial proportion of the unconverted carbamate in the reactor discharge is recycled, with a limited water content, via a high pressure condensation step directly to the reactor with the feed (CO
2
or NH
3
) to the urea process.
The most important developments for further improvement of the yield in the urea process the 1980s and 90s can be summarised in the following modifications to the reactor section:
produce more plug flow in the reactor
combination of the urea reactor with other process steps
allow the reaction to take place in temperature zones
install a second reactor
remove water via condensation of gas between the reactors.
In Netherlands Patent 1 000 416 the condensation step by means of which the high-pressure recycle stream is usually recycled to the reactor is combined with the reactor. With this arrangement the reactor is oriented horizontally. The NH
3
is fed into the cooled section of the reactor. The gas from the stripper is distributed transversely to the liquid stream in the reactor. The intention is that as much urea as possible should already be formed in the cooled section of the reactor. Furthermore, the urea equilibrium in the reactor is better approached by fitting baffles. These prevent back-mixing and give a better approach to plug flow than the screen plates in a conventional vertical reactor, as a result of which the synthesis reaction proceeds more rapidly.
EP 0 751 121A2 presents a process in which the urea reaction is distributed over two temperature zones. One of the zones is at relatively low temperature, as a result of which the carbamate equilibrium shifts to higher values, and the other temperature zone is at relatively high temperature, as a result of which the urea equilibrium shifts to higher value. Between the zones water is also separated off by condensation.
EP 0 727 414A1 a process is employed which has an additional reactor under high pressure and temperature in order to achieve a higher conversion.
In EP 0 624 571 A1 a urea process with high yield is described which has an additional reactor, water being removed from the feed to the second reactor by means of condensation. A separate feed control to this reactor makes it possible to maintain an optimum temperature and NH
3
/CO
2
ratio.
The process improvements in the urea process have to date been directed towards achieving an equilibrium for conversion to urea that is as advantageous as possible and approaching this as closely as possible by, within the limitations which apply for this high pressure and temperature process, feeding a minimum amount of water to the reactor(s) and optionally carrying out interim water removal between two reactors, choosing the process conditions pressure, temperature and residence time to be as optimum as possible, choosing a high NH
3
concentration and as far as possible approaching a plug flow regime in the reactor.
However, during the reaction for the formation of urea a quantity of water which is equimolar to the urea produced is formed at the same time. This substantial quantity of water formed still always ensures that the equilibrium for conversion to urea in the reaction is 20-30% below the maximum conversion of 100%.
SUMMARY OF THE INVENTION
The aim of the present invention to improve the known processes and the invention relates in particular to the removal of water from the urea reactor during the synthesis in order to improve the yield. Specifically, by removing water the equilibrium of the reaction for the formation of urea is shifted towards more extensive conversion to urea.
The removal of this quantity of water formed during the reaction, in order substantially to increase the conversion to urea in the urea process reactor and to reduce the recycle streams, has to date not proved possible in practice. This is a consequence of the high degree of difficulty associated with such a water separation step. An adequate selective water separation step was a technology which did not yet exist for the very high pressure ad temperature conditions in the urea process and the reactor environment, which is highly aggressive from the corrosive standpoint.
According to the invention a water-selective membrane, in particular a pervaporation membrane, is now used to remove the water formed during the formation of urea-that is to say during the abovementioned reaction step 2)—from the reactor, optionally in combination with a pressure drop over said membrane.
In first aspect the invention therefore relates to a method for the preparation of urea in a reactor using ammonia and carbon dioxide as starting materials, which method comprises;
a) bringing the ammonia and carbon dioxide into contact in the reactor under conditions for the formation of carbamate; and
b) dehydrating the carbamate thus formed to give urea and water, characterised in that the water formed during step b) is removed from the reaction mixture by the us of a water-selective membrane.
To this end the reaction mixture (or at least the reaction mixture in step b)) is brought into contact in the reactor with one side of the selective membrane, the water formed during step b) being removed from the reaction mixture trough said membrane to the other side of the membrane, where it is caught/collected and from where it is removed formed the reactor. During this operation a pressure difference is preferably maintained or applied over the membrane.
The reaction is preferably carried out in the liquid phase, that is to say the reactants (in particular ammonia and carbon dioxide in step a) and carbamate in step b)) are mainly, and preferably essentially exclusively, in the liquid phase.
As a rule the reaction will be carried out as if in a cyclic process. With is arrangement said cyclic process can comprise a high-pressure cycle and a low-pressure cycle is described in more detail below. (A possible advantage of the invention could be that the high-pressure cycle can optionally be omitted, as explained in more detail below),
The invention can—with suitable modifications of the equipment used (reactor)—be employed with virtually all known urea processes, including the Stamicarbon processes. Reference is made to the known handbooks, such as the “Encyclopedia of Chemical Technology”, Ed. Kirk-Othmer, Wiley Interscience, 3
rd
Ed (1983), vol. 23, pages 515-561, for descriptions of these known urea processes.
According to one embodiment (the carbon dioxide stripping process), the carbon dioxide is fed via a stripper and carbamate condenser to the reactor. The ammonia is fed together with the low-pressure recycle via the condenser to the reactor.
According to another embodiment (using a so-called pool reactor),
De Wit Jacobus Johannes
Goorden Josephus Johannes Petrus Maria
Continental Engineering B.V.
O'Sullivan Peter
Young & Thompson
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