Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Reexamination Certificate
2001-07-11
2002-05-21
O'Sullivan, Peter (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C564S067000, C564S068000, C564S069000, C564S071000, C564S072000, C564S073000
Reexamination Certificate
active
06392096
ABSTRACT:
The invention relates to a process for the preparation of urea from ammonia and carbon dioxide.
Urea can be prepared by introducing ammonia and carbon dioxide into a synthesis zone at a pressure of between 12 and 40 MPa and at a temperature of between 150 and 250° C. Urea formation might best be represented here by two consecutive reaction steps with ammonium carbamate being formed in the first step according to the exothermic reaction:
nNH
3
+CO
2
→H
2
N—CO—ONH
4
+(n−2)NH
3
Dehydration in the second step of the ammonium carbamate formed then results in the formation of urea according to the endothermic equilibrium reaction:
H
2
N—CO—ONH
4
→H
2
N—CO—NH
2
+H
2
O
The extent to which these reactions proceed depends on, among other factors, the temperature and the excess ammonia used. A solution that consists essentially of urea, water, unbound ammonia and ammonium carbamate is obtained as reaction product. The ammonium carbamate and the ammonia are removed from the solution and are preferably returned to the synthesis zone. In addition to the aforementioned solution, a gas mixture is formed in the synthesis zone, which consists of non-converted ammonia and carbon dioxide plus inert gases. Ammonia and carbon dioxide are removed from this gas mixture and are preferably also 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.
In practice, various processes are used for the preparation of urea. At first, 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.
The conventional high-pressure urea plants that are currently still operating are understood to be urea plants in which the decomposition of the ammonium carbamate not converted into urea and the expulsion of the usual 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. Furthermore, in a conventional high-pressure urea plant ammonia and carbon dioxide are fed directly to the urea reactor. In a conventional high-pressure urea process the molar NH
3
/CO
2
ratio (=N/C ratio) in the urea synthesis lies between 3 and 6.
As regards the recycling of unconverted ammonia and carbon dioxide to the synthesis section, one can distinguish Once Through conventional urea plants(no recycle), Partial Recycle conventional urea plants (only a proportion of ammonia and/or carbon dioxide is recycled) and Total Recycle plants (both ammonia and carbon dioxide are recycled).
A urea stripping plant is understood to be a urea plant in which the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the usual excess ammonia largely take place at a pressure that is essentially almost equal to the pressure in the synthesis reactor. This decomposition and expulsion take place in one or more stripper(s) installed downstream of the synthesis reactor with the aid of a stripping gas, such as, for example, carbon dioxide and/or ammonia, and with addition of heat. It is also possible to apply 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. The ammonia is and carbon dioxide-containing gas stream exiting from s the stripper is condensed in a high-pressure carbamate condenser.
The gas mixture that has not reacted in the urea synthesis is vented from the synthesis section. In addition to the condensable ammonia and carbon dioxide, this gas mixture (reactor vent gas) also contains inert gases. The condensable components (ammonia and carbon dioxide) can be absorbed, for example, in a high-pressure scrubber at synthesis pressure, before the inert gases are vented. In such a high-pressure scrubber the condensable components, ammonia and carbon dioxide, are preferably absorbed from the reactor vent gas into the low-pressure carbamate stream formed in the further recovery. The carbamate stream from the high-pressure scrubber, which contains the ammonia and carbon dioxide absorbed from the reactor vent gas, is returned to the synthesis whether or not via the high-pressure carbamate condenser. The reactor, high-pressure scrubber, stripper and high-pressure carbamate condenser are the most important elements of the high-pressure section of a urea stripping plant.
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 MPa. The N/C ratio in the synthesis of a stripping plant lies between 2.5 and 5. The synthesis can be carried out in a single reactor or in a plurality of reactors arranged in parallel or in series. When use is made of two reactors in parallel, the first reactor 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 according to 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 high-pressure carbamate condenser in a Stamicarbon CO2 stripping process is preferably designed as a so-called submerged condenser as described in NL-A-8400839. The submerged condenser can be installed in horizontal or vertical position. It is, however, particularly advantageous to carry out the condensation in a horizontal submerged condenser (a so-called pool condenser; see for example Nitrogen No 222, July-August 1996, pp. 29-31), because, in comparison with other designs of this condenser, the liquid generally has a longer residence time in the pool condenser. This results in the formation of extra urea in the pool condenser. The amount of urea formed in the pool condenser is higher than 30% of the theoretically possible amount of urea formed.
After the stripping treatment, the pressure of the stripped urea synthesis solution is reduced in the urea recovery and the solution is evaporated, after which urea is recovered. This produces a low-pressure carbamate stream in the recovery. This low-pressure carbamate stream is preferably returned via the high-pressure scrubber to the section operating at synthesis pressure.
In the high-pressure carbamate condenser the gas stream from the stripper condenses in the carbamate stream coming from the high-pressure scrubber. The carbamate solution coming from the high-pressure carbamate condenser is preferably passed to the synthesis reactor together with the ammonia needed for the reaction.
In a particular embodiment of a urea stripping process the functions of the reactor, pool condenser and high-pressure scrubber are combined in a single high-pressure vessel with the functionalities of these process steps being separated in this high-pressure vessel by low-pressure internals designed for small pressure differences. An example of such an embodiment is described in Nitrogen No. 222, July-August 1996, pages 29-31, which describes the poolreactor, as does U.S. Pat. No. 5,767,313. This poolreactor preferably is placed in horizontal position.
The aim of the present invention is to provide an improved process for the preparation of urea, which also entails lower investment costs.
The applicant has found an improved process for the preparation of urea from ammonia and carbon dioxide, which is characterised in that the preparation takes place wholly or partly in a vertical combi-reactor.
This combi-reactor is comprised of a condenser section, reactor section and scrubber section, with the condenser section most usually being located beneath the reactor sectio
Jonckers Kees
Mennen Johannes H
DSM N.V.
O'Sullivan Peter
Pillsbury & Winthrop LLP
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