Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Reexamination Certificate
2001-05-18
2003-07-01
Ford, John M. (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C544S201000, C544S203000
Reexamination Certificate
active
06586629
ABSTRACT:
The invention relates to the introduction of melamine off-gases into a urea plant by means of ejectors.
The off-gases obtained in the melamine synthesis and comprising in particular NH
3
and CO
2
are usually used for the preparation of urea. Advantageously, the off-gases from the melamine plant are transferred directly into the urea plant, where, for example, they are absorbed in a carbamate stream and transported further into the reactor. An improved, more efficient and more economical method of off-gas introduction is described in K. Abe et al., Kagaku Kogaku 40, 298-302 (1976), in which the off-gases, optionally after removal of residual melamine in a urea scrubber, are introduced directly and unchanged, in the dry state, into the high-pressure zone of the urea plant. This process variant is also described in SU 899538 or WO 98/08808. According to WO 98/32731, the melamine off-gases are first condensed at the pressure of the melamine reactor, the ammonium carbamate formed then being transported into the high-pressure zone of the urea plant.
The disadvantage of the known methods is in particular that in some cases energy losses occur through pressure relief or cooling of the off-gases, with the resulting additional process steps and apparatus components, or that, in the case of direct transfer of the off-gases into the urea plant, the pressure of the off-gases or the pressure in the melamine reactor has to be greater than the pressure in the urea reactor. This necessitates an inflexible and rigidly specified procedure in the two reactors, with the result that the reactors often cannot be operated under the conditions optimal for the respective process.
It has now been found, unexpectedly, that these disadvantages can be eliminated when the melamine off-gases are introduced directly into the high-pressure zone of the urea plant by means of ejectors.
The invention accordingly relates to a method for producing urea, in which the gases (melamine off-gases) which originate from a melamine plant and substantially comprise NH
3
and CO
2
are introduced directly into the high-pressure zone of a urea plant by means of one or more ejectors. The high-pressure zone of the urea plant includes in particular the reactor, the stripper and the carbamate condenser, as well as lines and apparatus parts present in this region of the plant.
The present method is suitable for any desired melamine plants and urea plants. Such plants are known, for example, from “Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., vol. A16 (1990), pages 171-185 (Melamine) and vol. A27 (1996), pages 333-365 (Urea), and from K. Abe et al., Kagaku Kogaku 40 (1976), pages 298-302, and from EP-727,414 A, WO 98/08808 and WO 98/32731.
The melamine off-gases used in the present method preferably originate from a melamine high-pressure plant in which melamine is obtained at temperatures of about 300-500° C. and pressures of about 80-800 bar from urea with elimination of NH
3
and CO
2
. The off-gases eliminated and substantially comprising NH
3
and CO
2
are passed, according to the invention, directly into a urea plant, preferably after being passed through molten urea (urea scrubber) for removing residual melamine, as described, for example, in Ullmann. Particularly suitable urea plants are those which are based on the “urea-stripping process”, the gases being converted into urea at about 150 to 350° C., preferably at about 170 to 200° C., and about 125 to 350 bar, preferably at about 140 to 200 bar. NH
3
and CO
2
not converted into urea is expelled in a downstream stripper and then condensed in a condenser, NH
3
- and CO
2
-containing ammonium carbamate being formed, which is recycled into the urea reactor. The urea solution emerging from the stripper is concentrated in further downstream decomposers at decreasing pressures, for example in a medium-pressure decomposer, then in a low-pressure decomposer and in a vacuum evaporator, by further decomposition of the carbamates and carbonates present and expulsion of NH
3
and CO
2
. The gases obtained during the concentration are condensed and then recycled into the urea process.
In a preferred method according to the present invention, the ejectors for transporting the melamine off-gases into the high-pressure zone of the urea plant are operated with one or more of the following streams as driving media:
a) liquid NH
3
,
b) gaseous NH
3
or CO
2
,
c) aqueous solutions containing substantially NH
3
and CO
2
.
The aqueous solutions containing substantially NH
3
and CO
2
are preferably obtained in a urea plant but they may also originate from other processes in which NH
3
and CO
2
are obtained, for example from a melamine plant. Aqueous solutions which originate in the preparation of urea, in particular from the working-up, contain substantially NH
3
and CO
2
and originate, for example, from the carbamate condenser or from the low-pressure zone of the urea plant, for example from the medium-pressure absorber, can be used according to the invention, as a driving medium for the ejectors for introducing the melamine off-gases into the high-pressure zone of the urea plant.
A particular advantage of using the solutions originating from the bottom of the medium-pressure absorber and also containing carbonates in addition to NH
3
, CO
2
, H
2
O and carbamates is that they have a low expulsion pressure of the dissolved gases. In fact, it proves advantageous if the gas expulsion pressure of the propellants, optionally also the temperature, are lower than the pressure and optionally the temperature of the melamine off-gases to be introduced. This prevents expulsion of dissolved gases in the ejector.
Depending on the type and amount of the driving medium, the pressure of the driving media used is such that as far as possible the total amount of off-gases obtained can be transported, depending on the suction conditions and the amount of the off-gases to be conveyed and on the respective counter-pressure in the high-pressure zone of the urea plant. The pressure and the amount of the driving media must accordingly be correspondingly high in order to ensure that the off-gases to be transported can be introduced in the respective amount at the respective pressure in the high-pressure zone of the urea plant. The pressure of the driving media is higher than the pressure in the high-pressure zone of the urea plant and is preferably from 1.1 to 3 times as high, particularly preferably from 1.3 to 2.5 times as high, as the pressure in the high-pressure zone of the urea plant. The temperature of the driving medium is in particular dependent on the respective procedure and is preferably in a range from about 10° C. to 200° C. In the case of the use of NH
3
, for example of the synthesis NH
3
for the urea preparation, a temperature of from about 10° C. to 80° C., particularly preferably from about 20° C. to 65° C., proves advantageous. In the case of the use of synthesis CO
2
, the temperature is preferably slightly higher, from about 115° C. to 140° C. NH
3
- and CO
2
-containing aqueous solutions as driving media which originate, for example, from the carbamate condenser of the urea plant, for example from 3 to 4 bar steam simultaneously being generated, preferably have temperatures of from about 150° C. to 160° C. Recycled carbonate solutions, for example from the medium-pressure absorber of the urea plant, can have temperatures of from about 65° C. to 100° C., preferably from about 65° C. to 70° C.
The molar ratio of NH
3
to CO
2
in the melamine off-gases to be introduced and originating from the melamine plant depends on the type of melamine process used and is preferably from about 2.5 to 5. The pressure of the melamine off-gases originating from the melamine plant corresponds substantially to the pressure in the melamine reactor and is preferably from about 50 to 250 bar, particularly preferably from about 70 to 200 bar. The temperature of the melamine off-gases is preferably from about 175 to 250° C., particularly preferably from about 180 to 210° C.
Accordingly, the
Agrolinz Melamin GmbH
Balasubramanian Venkataraman
Ford John M.
Wenderoth , Lind & Ponack, L.L.P.
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