Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Reexamination Certificate
2000-06-02
2004-05-04
O'Sullivan, Peter (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C564S066000, C564S067000, C564S071000, C564S072000, C422S186220
Reexamination Certificate
active
06730811
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 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:
2NH
3
+CO
2
→H
2
N—CO—ONH
4
Dehydration then causes the ammonium carbamate formed to form urea according to the equilibrium reaction:
H
2
N—CO—ONH
4
⇄H
2
N—CO—NH
2
+H
2
O
The degree to which this last conversion proceeds depends on, among other factors, the temperature and the ammonia excess used. As the reaction product a solution is obtained that consists substantially of urea, water, ammonium carbamate and unbound ammonia. The ammonium carbamate and the ammonia must be 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, different methods are used for the preparation of urea. At first urea was prepared in so-called conventional high-pressure urea plants, which were at the end of the 1960s however 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 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. In a conventional high-pressure urea plant the reactants not converted into urea are, after expansion, dissociation and condensation at a pressure of between 1.5 and 10 Mpa, 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 molar NH
3
/CO
2
ratio (=N/C ratio) in the urea synthesis lies between 3 and 5 in a conventional high-pressure urea process.
These conventional urea plants were initially designed as so-called once-through processes, in which the non-converted ammonia was neutralized with acid (for example nitric acid) and converted into ammonium salts (for example ammonium nitrate). Major disadvantages of this process were this large amount of ammonium salt and the low degree of CO
2
conversion. These conventional once-through urea processes were soon replaced by the so-called conventional recycle processes, in which all the non-converted ammonia and carbon dioxide are returned to the urea reactor. This recycling is carried out in two steps. A first recycling step at a medium pressure (1.8-2.5 MPa) and a second recycling step at a low pressure (0.2-0.5 MPa). In the first recycling step the urea synthesis solution coming from the reactor is heated in a heater, upon which ammonium carbamate decomposes into gaseous ammonia and carbon dioxide while further the excess ammonia also evaporates here. This gas mixture is subsequently converted into pure ammonia and a water-containing ammonium carbamate stream in a rectifying column. Both streams are returned to the urea reactor. In the second recycling step the urea solution from the first recycling step is reheated and then separated. The gas stream thus obtained is condensed and subsequently fed to the rectifying column of the first step. Next, urea is released from the urea solution coming from the second recycling step, in the evaporation at reduced pressure, through the evaporation of water. The two recycling steps and the evaporation together constitute the main part of the urea recovery.
A urea stripping plant is understood to be a urea plant in which the greater parts of the decomposition of the ammonium carbamate not converted into urea and the expulsion of the usual ammonia excess take place at a pressure that is essentially almost the same as the pressure in the synthesis reactor. This decomposition/expulsion takes place in a stripper, whether or not with the addition of a stripping medium. In a stripping process, carbon dioxide and/or ammonia can be used as stripping gas before these components are dosed to the reactor. This stripping takes place in a stripper placed downstream of the reactor, the solution coming from the urea reactor, which, in addition to urea, ammonium carbamate and water, also contains ammonia and carbon dioxide, being stripped with the stripping gas with the supply of heat. It is also possible to use thermal stripping here. 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 gas stream containing ammonia and carbon dioxide that is released from the stripper is returned to the reactor via a high-pressure carbamate condenser.
The gas mixture that has not reacted in the urea synthesis is removed from the synthesis section via a blow-down stream. In addition to the condensable ammonia and carbon dioxide, this gas mixture (synthesis off-gas) also contains inert gases such as, for example, nitrogen, oxygen and optionally hydrogen. These inert gases derive from the raw materials and from the make-up air in the carbon dioxide feed to the synthesis to protect the materials from corrosion. This gas stream is blown down from the synthesis section for example downstream of the reactor or downstream of the high-pressure carbamate condensation, depending on the process route chosen. It is however preferable to absorb the condensable components (ammonia and carbon dioxide) in a high-pressure scrubber at synthesis pressure before the inert gases are blown down. In such a high-pressure scrubber the condensable components, ammonia and carbon dioxide, are absorbed from the synthesis off-gas into the low-pressure carbamate stream formed in the further upgrading. This scrubbing process in the high-pressure scrubber can be stimulated by using a heat exchanger that extracts heat from the process. The carbamate stream from the high-pressure scrubber, which contains the ammonia and carbon dioxide absorbed from the synthesis off-gas, is returned to the synthesis via the high-pressure carbamate condenser. The reactor, high-pressure scrubber, stripper and high-pressure carbamate condenser are the most important parts 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 in a stripping plant lies between 2.5 and 4. The synthesis can be carried out in one or two reactors. When use is made of two reactors, 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 described in European Chemical News, Urea Supplement, of Jan. 17, 1969, pages 17-20. In this process the urea synthesis solution formed in the synthesis zone at a high pressure and temperature is subjected to a stripping treatment at synthesis pressure by bringing the solution into countercurrent contact with gaseous carbon dioxide while heat is being supplied. This causes the greater part of the ammonium carbamate present in the solution to be decomposed into ammonia
DSM N.V.
O'Sullivan Peter
Pillsbury & Winthrop LLP
LandOfFree
Process for the preparation of urea does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for the preparation of urea, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for the preparation of urea will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3239230