Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
2002-04-05
2003-12-16
Shah, Mukund J. (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Four or more ring nitrogens in the bicyclo ring system
C544S203000
Reexamination Certificate
active
06664392
ABSTRACT:
The invention relates to a process for the preparation of melamine comprising a reaction step, a gas/liquid separation step in which a melamine melt is separated from off-gases, a stripping step and a cooling step.
Such a process is disclosed in WO 97/20826. That publication teaches that melamine is prepared by separating, in a gas/liquid separation step, a melamine melt, which preferably has been prepared from urea in a non-catalytic reaction step in a high-pressure reactor, from released gases, and by subsequently treating the melt with NH
3
in a stripping step in order to reduce the amount of CO
2
dissolved in the melamine melt. Next, the melamine melt with the NH
3
dissolved in it is cooled, at a pressure of between 5 and 40 MPa, in a cooling step to a temperature of between 0 and 60° C. above the melting point at the prevailing ammonia pressure but below 350° C. and then expanded to produce solid melamine and then further cooled. In the examples described in WO 97/20826 in which high-purity melamine is obtained, the process pressure p
1
in the melamine melt during the cooling step is always 20 MPa or higher. The highest purities are obtained in those examples in which the process pressure p
1
is 25 MPa or higher. In the process according to WO 97/20826, the process pressure in the reaction step, the gas/liquid separation step and the stripping step is preferably the same as the pressure in the cooling step.
A drawback of the known process is that the pressure in a large part of the preparation process, in particular in the stripping step, needs to be maintained at a very high level, in the light of the examples preferably higher than 20 MPa, in order to obtain melamine with a high purity. The investment costs of process plants are known to increase with increasing plant operating pressure. Thus, the known process leads to relatively high investment costs as far as the required process equipment is concerned. It is also known that the operating costs of a process plant increase strongly with increasing plant operating pressure. Thus, the known process also leads to relatively high operating costs.
The object of the invention is to largely avoid the aforementioned drawbacks by a process that yet produces melamine of high purity.
Said object is achieved by the stripping step being operated at a pressure of between 5 MPa and 17 MPa and a temperature of between 330° C. and 450° C. and by the melamine melt obtained in the preceding steps being pressurised in the cooling step to a pressure of between 15 MPa and 35 MPa, with the pressure in the cooling step being higher than the pressure in the stripping step and with the temperature in the cooling step being adjusted between the melting point of the melamine melt at the prevailing pressure and 365° C.
Due to the lower pressure in the stripping step than in the process according to WO 97/20826, the process of the invention allows the equipment in which the stripping step is effected to be of less heavy construction.
Despite the relatively low pressures in the stripping step, the eventually obtained melamine is of very high purity: the percentage of undesirable compounds such as ammeline, ammelide, cyanuric acid, melem and melam is very low. This is surprising and cannot be derived from WO 97/20826, which states that it is optimum for the stripping step to be conducted at the same high pressure as the cooling step if high-purity melamine is to be obtained. Surprisingly, the amounts of stripping medium per kg of melamine needed in the stripping step are also lower when the step is carried out at a lower pressure between 5 MPa and 17 MPa.
In the off-gas released in the stripping step there is present, besides the stripping medium and the CO
2
released, an amount of evaporated melamine. Such melamine vapour needs to be removed from the off-gas in a scrubbing step. An added advantage of the process of the invention is that, because of the smaller amounts of required stripping medium, less melamine vapour is released in the stripping step per unit time than in the known process. As a result, less melamine needs to be scrubbed per unit time so that the scrubbing step may be of simpler design.
As a consequence, both the investment costs and the operating costs of a high-purity melamine plant according to the process of the invention are substantially lower than in the known process.
EP-A-0808836 discloses a process for the preparation of melamine which comprises a stripping step and a cooling step in which the stripping step is effected in a CO
2
remover and the cooling step is effected in a mixing vessel. EP-A-0808836 does not teach, however, under what conditions the stripping step should be effected. Nor does EP-A-0808836 teach how the relationship in operating conditions between the stripping step and the cooling step should be chosen. Lastly, EP-A-0808836 fails to show that the advantages of the process of the invention may be achieved.
The preparation of melamine normally starts from urea, in the form of a melt, as a raw material. NH
3
and CO
2
are by-products during the preparation of melamine, which proceeds according to the following reaction equation:
6 CO(NH
2
)
2
→C
3
N
6
H
6
+6 NH
3
+3 CO
2
The preparation may be effected in a high-pressure process known per se, in which melamine is prepared without the presence of a catalyst and with pressure normally being between 5 and 50 MPa. The temperature at which the reaction is effected is between 325 and 450° C. The by-products NH
3
and CO
2
are customarily returned to an adjacent urea plant.
As a rule, a high-pressure process includes a scrubber unit, a reactor, a gas/liquid separator, a stripper and/or an after-reactor or ageing vessel, one or more cooling vessels or mixing vessels and an expansion vessel.
It is possible for some vessels to be combined into a single vessel. Examples are a combination of the reactor with the gas/liquid separator, a combination of the gas/liquid separator with the stripper or a combination of stripper and cooling vessel.
In an embodiment of the high-pressure process, melamine is prepared from urea in for example a plant consisting of a scrubber unit, a reactor for the preparation of melamine, a gas/liquid separator, a stripper, a cooling vessel and an expansion vessel.
For effecting the scrubbing step, a scrubber unit is supplied with urea melt from a urea plant at a pressure of 5 top 50 MPa and at a temperature above the melting point of urea.
In the scrubber unit the liquid urea comes into contact with off-gases from the gas/liquid separator and from the stripper.
The off-gases essentially consist of CO
2
and NH
3
and also contain an amount of melamine vapour. The molten urea scrubs the melamine vapour out of the off-gas and carries this melamine back to the reactor so that the evaporated melamine is not lost. At the same time, the temperature of the urea is increased.
The off-gases are discharged from the top of the scrubber unit and are preferably returned to a urea plant for use as a raw material for urea production.
In order to carry out the reaction step, the preheated urea, which contains the scrubbed melamine, is passed from the scrubber unit to the reactor, which has a pressure of 5 to 50 MPa. The urea melt may be transferred to the melamine reactor with the aid of gravity by placing the scrubber unit above the reactor. An amount of NH
3
in the form of for example a liquid or hot vapour, may be metered to the reactor. The NH
3
added may serve as for example a purifying agent to prevent blockage of the reactor bottom or to prevent the formation of melamine condensation products such as melam, melem and melon or to promote mixing in the reactor.
In the reactor, the molten urea is heated to a temperature of 325 to 450° C. at the above-mentioned pressure, in which conditions the urea is converted into liquid melamine, CO
2
and NH
3
.
In order to carry out the gas/liquid separation step, the reaction product, essentially consisting of melamine and of CO
2
and NH
3
evolving in the reaction, is passed to a gas/
Balasubramanian Venkataraman
DSM N.V.
Shah Mukund J.
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