Organic compounds -- part of the class 532-570 series – Organic compounds – Four or more ring nitrogens in the bicyclo ring system
Reexamination Certificate
2001-04-10
2003-04-15
Ford, John M. (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
06548669
ABSTRACT:
The invention relates to a method for preparing melamine by causing liquid urea to react and form a melamine melt, separating the melamine melt from gaseous products formed in the reaction, and treating the melamine melt with ammonia in order to remove a majority of the dissolved carbon dioxide.
Such a method is described in, inter alia, U.S. Pat. No. 3,116,294 which discloses preparing melamine by converting urea into melamine, carbon dioxide, and ammonia, with the bulk of the melamine forming a liquid phase, and the bulk of the carbon dioxide and ammonia forming a gas phase. The gas phase is then separated from the liquid phase, with the melamine melt then being treated with ammonia to remove dissolved carbon dioxide. Although, according to U.S. Pat. No. 3,116,294, the disclosed method is suitable for producing high purity of melamine, the disclosure does not provide a specific method of implementing the disclosed method on an industrial scale.
A common method for purifying a liquid stream is to use a stripping process in which the impurities are removed with the aid of a gas passed through the liquid. Stripping processes are often carried out in gas-filled packed columns or gas-filled tray columns.
Packed columns are columns filled with bodies (packings) that promote the contact between the different phases present in the column. These bodies may be obtained in a wide variety of shapes, such as spheres, rings, and saddles, sizes, and compositions. The bodies may be deployed in the packed column as random packings or may comprise specific units that are stacked on top of one another (structured packings). In a gas-filled packed column, the gas phase is the continuous phase, with the liquid phase typically flowing down through and along the packing as a film, thereby providing a large contact area between the liquid and gas phases.
A tray column, however, typically includes a number of trays or plates that divide the column into a number of distinct compartments. Commonly the trays will be positioned parallel to each other and spaced evenly along an axis of the column. In operation, a tray column will typically maintain a thin layer of liquid on each tray with a large gas-filled space between the trays in order to suppress entrainment and flooding. The term entrainment refers to a condition in which liquid droplets are dragged or blown from one compartment to the next compartment by the gas flow. In countercurrent operation, the entrained liquid droplets will be forced in a direction opposite from the bulk liquid flow. The term flooding refers to a condition in which the liquid phase fills a compartment, eliminates the desired gas space between compartments, and begins to flow into an adjacent compartment. Uncorrected, a flooding condition will fill the column with the liquid phase flowing in a direction opposite the desired flow. It has been found that using gas-filled packed columns and tray columns for stripping a melamine melt, particularly operating at high pressures, achieves unsatisfactory results either in terms of purity or energy and gas consumption.
The applicant has now found that it is possible, without increasing the size of the stripping unit or increasing the volume of ammonia employed, to achieve much better stripping of carbon dioxide from a melamine melt. This improvement is obtained by using a liquid-filled stripping column, also referred to as a bubble column, to remove dissolved carbon dioxide from the melamine melt. According to the present invention, the bubble column uses between 0.02 and 3 tons of ammonia per ton of melamine melt while operating at a pressure of between 1 and 40 MPa, at a temperature between the melting point of melamine at the operating pressure and 450° C., and providing a residence time of between 1 minute and 10 hours for the melamine melt. Preferably, a bubble column according to the present invention will use between 0.1 and 1 ton of ammonia per ton of melamine while operating at a pressure of between 4 and 25 MPa and provide a residence time for the melamine melt of between 10 minutes and 3 hours within the stripper.
This method of operating a bubble column for stripping a melamine melt differs significantly from methods using a gas-filled packed column in that the ammonia gas, rather than being the continuous phase, exists as bubbles in melamine melt that forms a continuous liquid phase. The bubble column can be operated with crosscurrent, cocurrent, or countercurrent flows of the respective gas and liquid phases. The ammonia gas throughput and the column diameter are preferably chosen such that the superficial gas velocity based on the total column cross-section is between 0.001 and 0.2 m/sec, and more preferably between 0.003 and 0.1 m/sec. The term superficial gas velocity refers to the volume flow of the gas (in m
3
/sec) at the operating pressure divided by the column diameter (in m
2
).
A bubble column according to the present invention is preferably provided with a packing. If present, the packing can either be a random or a structured packing, so long as the packing provides sufficient open space for the flow of the desired amount of ammonia gas. Preferably, the gas flow will be distributed evenly across the column cross-section. If a packing is used, it is preferable to select a packing having a specific surface area of between 10 and 3000 m
2
/m
3
, and more preferably, between 25 and 600 m
2
/m
3
.
More preferably, the bubble column, rather than employing a packing, is divided by a number of plates into a plurality of compartments in which the liquid is treated with ammonia gas, with the gas being incorporated into the melamine melt in a crosscurrent, countercurrent, or cocurrent flow pattern. A combination of different flow patterns, a combination of a packed column and a tray column, or a combination of packed and tray regions within a single column are obviously also possible.
If the bubble column is divided into compartments by a series of trays or plates, the number of compartments is preferably less than 500, and more preferably 100 or less. The plates used to separate the compartments may be solid or perforated. If the plates are perforated, the perforations may be sized and configured to allow or promote the flow of the melamine melt, the ammonia gas, or both. The term plate should not be construed as limited to a generally planar structure but rather should be understood to encompass a variety of structural configurations that may be used to divide a column into a plurality of compartments.
In a first embodiment, the compartments are situated next to one another and are separated by one or more plates, either solid or perforated, to define a plurality of separate pipes that at least partially filled with the melamine melt. In another embodiment, the compartments are situated underneath one another in a stacked configuration and are separated by one or more plates, either solid or perforated, positioned generally horizontally across the bubble column.
If a plate is perforated in a manner designed to permit gas flow through at least some of the perforations, a certain gas velocity will be achieved through those perforations. According to the present invention, the number and size of such perforations should be chosen to produce a gas velocity of between 0.01 and 100 m/sec, preferably between 0.2 m/s and 20 m/s, through the perforations. In particular, it has been found that perforations between 0.1 and 200 mm, and more preferably, between 0.5 mm and 100 mm, can be used to achieve such gas velocities. For a given plate, the total area of perforations designed for gas flow is preferably between 0.02% and 30% of the bubble column cross-sectional area. Alternatively, if the plates are not perforated, sufficient space must be provided between the wall of the column and the plates for the required liquid and the gas flows. Although gas and liquid can flow through the same perforations, it is preferable to provide separate perforations for the gas and liquid flows. Also, it is preferable to distribute the perforat
Balasubramanian Venkataraman
DSM N.V.
Ford John M.
Pillsbury & Winthrop LLP
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