Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Reexamination Certificate
2002-09-26
2004-01-20
O'Sullivan, Peter (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C564S066000, C564S067000, C564S069000, C564S070000, C564S071000, C422S149000, C422S187000, C422S186220, C422S198000, C422S198000, C422S202000, C422S203000, C422S204000
Reexamination Certificate
active
06680407
ABSTRACT:
The invention relates to an installation for the preparation of urea. The invention also relates to a process for the preparation of urea in this installation.
Urea can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150 and 250° C. into a urea synthesis zone. The resulting urea formation can be represented best in the form of two consecutive reaction steps, in the first step ammonium carbamate being formed according to the exothermic reaction:
2NH
3
+CO
2
→H
2
N—CO—ONH
4
after which the ammonium carbamate formed is dehydrated in the second step to give 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 take place depends amongst other things on the temperature and the ammonia excess used. The reaction product obtained is a urea synthesis solution substantially consisting of urea, water, unbound ammonia and ammonium carbamate. The ammonium carbamate and the ammonia are removed from the solution and are preferably returned to the urea synthesis zone. In addition to the above-mentioned solution in the urea synthesis zone a gas mixture is formed which consists of unconverted ammonia and carbon dioxide together with inert gases, the so-called reactor off-gas. Ammonia and carbon dioxide are removed from this gas mixture and are preferably also returned to the urea synthesis zone. The urea synthesis zone may comprise separate zones for the formation of ammonium carbamate and urea. These zones may, however, also be combined 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. At the end of the 1960s, however, this process was 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 ammonia excess 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 zone lies between 3 and 6. Depending on the extent to which the unconverted ammonia and carbon dioxide are returned to the urea synthesis section in conventional urea plants, a distinction is made between Once Through (no recycle), Partial Recycle (only partial recycle of ammonia and/or carbon dioxide) or Total Recycle (both ammonia and carbon dioxide recycle) plants.
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 ammonia excess 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, preferably 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 use is made exclusively of the supply of heat to decompose ammonium carbamate and remove the ammonia and carbon dioxide present from the urea synthesis solution. The gas stream containing ammonia and carbon dioxide that leaves the stripper is condensed in a high-pressure condenser and then returned to the urea synthesis zone.
The gas mixture that has not reacted in the urea synthesis zone of a urea stripping plant is removed from the urea synthesis zone and absorbed at synthesis pressure, for example in a high-pressure scrubber. In such a high-pressure scrubber the condensable components, ammonia and carbon dioxide, are preferably absorbed from the reactor off-gas into a 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 off-gas, is returned to the urea synthesis zone, optionally via the high-pressure carbamate condenser. The reactor, high-pressure scrubber, stripper and high-pressure 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 urea synthesis zone 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, for example, 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 for example described in European Chemical News, Urea Supplement, of Jan. 17, 1969, pages 17-20. The high-pressure condenser in a Stamicarbon CO
2
stripping process is preferably designed as a submerged high-pressure condenser a so called poolcondensor, as described in NL-A-8400839.
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 urea synthesis zone operating at synthesis pressure.
In a particular embodiment of a urea stripping process the functions of reactor and poolcondenser are combined in a single high-pressure vessel with the functionalities of these process steps being separated by partition walls designed for small pressure differences in this high-pressure vessel. 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 is placed in a horizontal position.
The disadvantage of this horizontal position is that the horizontally placed poolreactor takes up a great deal of space and must also be placed at a greater height in order to enable the urea synthesis solution to be transferred to the stripper by gravity. This necessitates high investments.
The aim of the present invention now is to provide an installation comprising an improved reactor for the preparation of urea which requires lower investment costs. The aim of the present invention is also to provide an improved process for the preparation of urea in an installation comprising this reactor.
The applicant has found an improved installation for the preparation of urea from ammonia and carbon dioxide, which is characterized in that the installation comprises two reactor sections in a vertically placed combined reactor and a high-pressure condenser section.
In particular, the installation in a first embodiment comprises a vertically placed combined reactor comprising two reactor sections that are separated by a high-pressure condenser section. In a second embodiment the installation comprises a vertically placed combined reactor comprising two reactor sections and a high-pressure condenser section placed outside the combined reactor.
More in particular the applicant has found improved installations in which in the first embodiment the vertically placed combined reactor consists o
DSM N.V.
O'Sullivan Peter
Pillsbury & Winthrop LLP
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