CN1665794A - Method and device for the recovery of melamine by expansion - Google Patents

Abstract

Treatment of pyrolysis products of urea to melamine in one or more expansion and cooling steps for separation of off-gas and recovery of melamine in solution/suspension by expanding the product under a liquid seal to prevent sticking of melamine crystals formed Attached to the inside wall of the device.

Description

Method and apparatus for recycling melamine by expansion

The present invention relates to a process for the production of melamine and in particular to the treatment of the reaction product. In the latest and mainstream industrial practice, melamine is usually produced by pyrolysis of urea according to the following overall reaction:

          

It can be seen that the pyrolysis of urea to melamine is accompanied by the formation of large quantities of carbon dioxide and ammonia, commonly referred to as tail gas. The entire reaction requires heat supply and interconnects a complex series of reactions in one or more steps; according to the existing industrial technology, the reaction is carried out at high pressure and in the liquid phase without a catalyst, and at low pressure and with a heterogeneous catalyst in the vapor phase.

Non-catalytic high-pressure techniques typically operate at about 400°C and pressures of 7-15 MPa, producing products in a mixed gas-liquid phase. Catalytic technology usually employs heterogeneous catalysts based on alumina, in the gas phase, also operating at about 400 °C, but at a pressure of 0.5-10 relative bar (0.05-1 relative MPascal), Gives gaseous products.

To expand and cool melamine as a liquid, gas, or in solution until it is isolated as crystals in a subsequent step, the industrial technique generally involves reducing the reaction product by processes commonly referred to as quenching and stripping. temperature and pressure, even when operating under different temperature and pressure conditions.

These expansion operations are usually but not necessarily followed by a step of mixing and/or adsorption in an ammonia solution of melamine, which solution may also contain _CO2 and reaction by-products. The performance of the method results in a mixed phase where the melamine is present in solution/suspension during the recovery and separation process. These solutions/suspensions are then processed to obtain melamine within specification.

An important technical problem in the production of melamine concerns the phase transition of the melamine to be separated from the reaction crude product, since during the transition to the crystalline phase that occurs during expansion, the growing crystals are particularly viscous, so that they are often firmly attached to the and incrustation to solid surfaces immediately downstream of the expansion device (usually a lamination valve of the reaction effluent), such as the walls and interior of a device receiving an expanded flow of reaction product from urea to melamine. This technical problem is very common in both types of pyrolysis technologies from urea to melamine, non-catalytic and high-pressure technologies, and low-pressure catalytic technologies. These shells lead to disturbances in the reaction steps and frequent shutdowns for cleaning, resulting in increased costs and reduced operating coefficients of the entire plant.

The scheme of Figure 1 shows the first step of a process for the production of melamine according to known technology, which is a high-pressure pyrolysis technique, by feeding liquid urea at 135-140°C to a heated pyrolysis reactor A, in excess of It operates in the liquid phase in the presence of ammonia and without the use of catalysts. In the schematic diagram of Figure 1, the urea to melamine tank pyrolysis reactor was operated continuously at a temperature of 360-420 °C and a pressure of 7-15 MPa. The reactor A described is a one-step reactor in which molten urea is fed from the bottom together with gaseous ammonia. The reaction mass is kept in vigorous circulation by the gas formed in the reaction itself; the reacted mass (liquid and gas) is discharged from reactor A through valve C and expanded into vessel B. The pyrolysis conversion reaction of urea into melamine is carried out in the pyrolysis reactor. A heating system using molten salts provides the heat needed to maintain the reaction.

In pyrolysis reactor A a mixed phase is produced which consists essentially of liquid crude melamine, off-gases resulting from the reaction and excess ammonia fed from the bottom together with molten urea. This mixed phase is fed into cooling tower B, commonly called a "quench" tower, where it is expanded in valve C, typically at a pressure of 0.1 to 3 relative MPa, cooled to 120-180 ℃, and contact with recycled ammonia solution. This operation is carried out in quench column B into which the melamine-containing reaction product is sprayed from above together with recycled ammonia solution injected from one or more distributors which cools and dissolves the melamine. Both the melamine and the ammonia solution descend in the column; under these conditions all the melamine goes into solution/suspension together with unreacted urea and high molecular weight impurities and is sent to the subsequent treatment process. A gas phase consisting essentially of _NH3 and _CO2 from reactor A and equilibrated with water vapor rises from column B, exits at its top, and returns to the urea synthesis. The aqueous solution/suspension obtained from the quench column B contains in solution a certain amount of ammonia and carbon dioxide, which must be removed in the subsequent stripper E, usually at 0-15 relative bar (0- 1.5 relative MPa) by means of further expansion in valve D. The aqueous stream leaving the bottom of stripper E contains 6-12% w/w melamine, and mainly oxy-amino-triazines (commonly referred to by the acronym OAT) to be removed in later steps. ) and impurities of polycondensation products.

The gas stripped in column E is separated and scrubbed with circulating water in column L, resulting in recycled ammonia solution which is used as quenchant in column B. There is a cooling water condenser located at the bottom of column L. The heat of absorption removed from column L is equal to about 1100 Kcal/kg of ammonia and carbon dioxide to be condensed.

According to the scheme of Figure 1, during the separation of melamine from the liquid phase, due to the crystallization of melamine, hard shells will exist in the process, basically downstream of the two expansion valves C and D, in the two columns B and At the inlet of E (the stream of expanded liquid in the mixed phase enters the column at a rather high velocity). These grown crystals reached the walls or interior of both towers and resulted in encrustations which interfered with the operation of the plant and required frequent shutdowns for cleaning. In terms of operating factors, these shutdowns result in a loss of 6-8%, which means a loss of production of the same order of magnitude and a considerable imbalance in the entire production line. In the known technology, for this disadvantage, the installation is operated at a large dilution of the melamine solution/suspension, which results in costs and energy required for the treatment of the solutions, their transfer, heat exchange and stripping operations Consumption is greatly increased. This strategy limits the concentration of melamine in the solution/suspension in the quenching and stripping equipment - in the case of high pressure pyrolysis technology - to a range of 8-10% by weight.

The problem of the expansion of the crude melamine stream and the formation of shells during the solidification of the melamine into crystals also exists when using the gas-phase catalytic pyrolysis technology of urea to melamine.

It is an object of the present invention to provide a plant and a method for the expansion and cooling of the crude effluent of the pyrolysis reaction of urea to melamine, which avoids the resulting melamine shell crystallized during said expansion. the shortcomings.

This object is achieved by means of the device and the method of the invention, claim 1 of the present invention is the most general definition of said method, and preferred embodiments or possibilities of the method of the invention are defined in dependent claims 2 to 7 The change. The most general meaning of the inventive device is defined in claim 8, while preferred embodiments or possible variants of the inventive device are defined in the dependent claims 9-12.

From the following illustrative and non-limiting description of the scheme with reference to the accompanying drawings, the apparatus of the invention for the expansion and crystallization of the melamine contained in the reaction stream and its delivery to the subsequent parts of the plant and The features and advantages of the method will be more apparent.

From the previous discussion, Figure 1 represents known technology. Figure 2 illustrates a simplified embodiment of the present invention, while Figures 3 to 8 show examples of embodiments of the expansion device that allow separation and crystallization of melamine, avoiding or significantly limiting encrustation.

The pyrolysis reaction is carried out in reactor A with the method already mentioned with reference to FIG. 1 . A mixed phase comprising the liquid melamine produced is formed in the pyrolysis reactor A, which rises in the reactor and is discharged from an outlet above it.

According to the scheme of Fig. 2 again, a mixed phase comprising tail gas of reaction and melamine, a small amount of unreacted urea and by-products is obtained from the upper part of the reactor. This mixed phase is expanded in a valve C located on the connecting conduit between the reactor and the quench column a little before or even directly in contact with the outlet of the lower part of the quench column B. A liquid seal is maintained in the lower part of the column to prevent the crystals from coming into contact with and thus adhering to the solid surfaces as they form and separate from the liquid phase. Thus, this result is obtained by allowing the expansion to take place in the lower part of the quench column B, in a region submerged in the liquid. It has in fact been found that the adhesive nature of the melamine crystals leads to encrustation during their formation and separation from the parent solution/suspension, whereas said melamine crystals no longer exhibit the same encrustation behavior once separated in the liquid phase .

In the above case, similar to the scheme illustrated in Figure 1, the ammonia or alkali solution is also injected from above with a special distributor, and descends from the top to the bottom, and the tail gas rising in the washing tower is cooled and dissolved in the valve. C after expansion introduces the melamine in the lower mixed phase. This forms a liquid phase containing the melamine solution/suspension.

In the following process steps, the solution/suspension of melamine obtained at the bottom of quench column B is sent to a stripping section. This stream is expanded in valve D to reduce the pressure, usually from atmospheric pressure to 15 bar (0-1.5 relative MPa), before it is introduced into stripper E. This expansion results in significant evaporation of the liquid and further separation of solids beyond the saturation of the remaining liquid. In the process of the known art, in column E (also downstream of valve D) a large number of crusts occurs.

According to the invention, in the scheme of FIG. 2 , this disadvantage is thus eliminated by carrying out the expansion under a liquid seal in the vessel F located upstream of the subsequent stripper E, still present in the melamine solution from B/ Most of the gas in the suspension is stripped and removed in stripper E. Vessel F is kept at the same pressure as the subsequent stripping column and may consist of the space obtained in column E, in which the liquid solution is kept sealed, said stream being sent after expansion. Alternatives to this configuration are described below with reference to FIGS. 5 and 6 . Vessel F should be sized to ensure that the residence time under the liquid seal is sufficient to separate the crystals before they come into contact with the walls or other internal parts of the apparatus. In general, a residence time of the melamine solution/suspension in the vessel of 20 to 1200 seconds, preferably 120 to 300 seconds, is sufficient to achieve an operation without any significant crust formation.

Depending on the catalytic or non-catalytic process of the pyrolysis of urea to melamine, the liquid into which the crude melamine-containing mixed phase expands can be ammonia solution and/or alkaline solution and/or water.

The exemplary embodiments of Figures 3 and 4 show the process of forming said vessel in which the expansion of a liquid stream comprising melamine in solution/suspension under liquid seal takes place.

In the embodiment of Figure 3, vessel F has a vertical cylindrical shape and is completely filled with the expanded mixed phase. A pressure relief valve, for example valve D upstream of the stripping column E, is preferably mounted directly on the bottom of the vessel F. According to an embodiment variant of this structure, the valve D can be mounted on a frusto-conical gate connecting said valve to the bottom of the container. As indicated in Figure 3, two separate zones are formed within the solution/suspension: the expansion zone and the circulation zone. The expansion zone, indicated by the dotted line, forms in the center and immediately downstream of the inlet of the stream flowing through the expansion valve; in this zone, there is a mixed phase with a lower density, accompanied by the growth of gas bubbles, sudden cooling and growing melamine crystals separation. On the contrary, the circulation zone is located on the edge around the expansion zone; in this zone, there is a greater density, because part of the gas phase in the bubbles has separated from the suspension and rises towards the outlet at a higher velocity. As indicated by the arrows, the gas-lean liquid phase descends again towards the bottom, creating a circulation of the cooler liquid, which thus favors the suspension of the melamine crystals before they come into contact with the walls of the apparatus. The discharge from the upper outlet is carried out in the form of mixed phase.

In the embodiment of FIG. 4 , the vessel still has a vertical cylindrical shape and is equipped with a conventional stirrer consisting of a vertical shaft with one or more propellers and a motor M. A pressure relief valve, for example valve D upstream of the stripping column E, is preferably fitted directly on the cylindrical wall of the vessel. According to an embodiment variant of this structure, the valve D can be mounted on a frusto-conical gate connecting said valve to the cylindrical wall of the container. During operation, the liquid level of the expansion vessel is kept above the installation point of the expansion valve with a wide margin to ensure that the expansion is carried out under the liquid seal.

According to a preferred embodiment of the invention, the injection outlet of the valve D is oriented tangentially so as not to directly collide with the axis of the stirrer, but to spread the expansion zone around it. Also, in the embodiment shown in Figure 4, two separate zones are formed in the solution/suspension body: an expansion zone and a circulation zone. As a result of the action of the rotary motion produced by the agitator, the expansion zone marked by the dotted line forms immediately downstream of the outlet of the stream flowing through the expansion valve and often has a circular motion; The mixed phase is present in the annular region in which the stream is cooled and the growing melamine crystals are separated. On the contrary, the circulation zone is located outside the annular expansion zone; in this zone again there is a greater density and the gas-depleted liquid phase circulates towards the bottom of the vessel where the agitator propeller is installed, which causes said liquid phase to The expanded stream of valve D is in intimate contact, thereby facilitating the formation of a suspension of melamine crystals before they come into contact with surfaces not wetted by the liquid. The discharge from the further rear upper outlet takes place in the form of mixed phase.

Figures 5 to 8 below show constructional variants of the embodiment container F with respect to Figures 3 and 4, which include the separation of the liquid and gaseous streams inside the container, and their discharge through the respective outlets, and/or include the container F is inserted into column E.

Figures 5 and 6 illustrate schematically the manner in which the expansion vessel F is formed under a liquid seal inside the stripper E. In the embodiment of Figure 5, said expansion vessel F is located above the tray section of column E. For the communication of the volumes between vessel F and column E, two conduits are located concentrically in the center of the vessel. The melamine solution/suspension is introduced from the side with the inlet of valve D and expands in the area indicated by the dots. As previously indicated, the gradual formation of the gas phase and the recirculation of the heavy liquid phase take place in the remaining space. Threshold s ₁ of outer conduit s allows the liquid phase to overflow, descend from the conduit and flow through the trays of column E. In contrast, the inner conduit r acts as a riser for the rise of the vapor from the column E which, combined with the gas formed in the vessel F, exits the upper conduit t together. In the embodiment shown in side view and top view in FIG. 6 , the expansion vessel F is located above the tray section of column E and is equipped with a stirrer similar to the embodiment described with reference to FIG. 4 . The separating spacer p is located in the upper part of the vessel F and is fitted with a lower peripheral overflow opening _p1 allowing the liquid to descend towards the column E, and a higher peripheral opening _p2 allowing the vapor phase from the column E to rise, the vapor phase Combine with the gas phase developing in Vessel F. The melamine solution/suspension is introduced from the side with the inlet of the valve D and expands in the area indicated by the dots with a circular tendency due to the movement of the stirrer driven by the motor M. Vapor from column E is collected through opening _p2 , combines with the gas gradually formed in vessel F, and is discharged together through upper conduit t.

Figures 7 and 8 illustrate schematically the manner of forming an expansion vessel F under a liquid seal, similar to those of Figures 3 and 4, but with internal separation of the streams. In both cases there is a vertical duct s to discharge the liquid phase with a threshold s ₁ which maintains the level of the liquid phase and allows overflow of the separated liquid phase which descends in _the duct and goes to to column E to be stripped. Conversely, the gaseous phase developing in vessel F can be withdrawn from upper conduit t directly to absorption column L, thereby reducing the gas stream and the cross-sectional dimensions of column E.

The separation of melamine from melamine solutions/suspensions by expansion under a liquid seal represents a considerable improvement over known techniques.

The present invention can recover 6-8% of the operating coefficient of the device that would have been lost due to encrustation, and ensures better continuity and balance in the entire production line. Furthermore, there is no longer any reason to limit the concentration of melamine in the solution/suspension in the quenching and stripping plant to 8-10% by weight: for the same plant and other conditions held constant , these concentrations can increase by 13-15% by weight. For the same device, the processing capacity is increased by about 50%. The specific consumption of steam, electrical energy and refrigeration, as well as the installation costs affecting the overall production costs are correspondingly reduced.

Claims (12)

1. A process for the treatment of the pyrolysis reaction product of urea to melamine consisting of a mixture of crude melamine also comprising a tail gas consisting of carbon dioxide and ammonia, the process comprising one or more expansions and a cooling step to separate the off-gas and to separate the melamine separated in crystal form from the liquid phase, the process is characterized in that the expansion of the reaction product is carried out in a zone immersed in the liquid, so that melamine crystals are formed therein and separated from the liquid phase The liquid or gas phases do not contact the walls or other internal parts of the device during separation and thus do not adhere to said walls or other internal parts of the device. 2. the method for processing the pyrolysis reaction product of urea to melamine according to claim 1, is characterized in that, the expansion of described reaction product is carried out in valve (C), and this valve (C) is close to quench tower (B ) lower outlet, operating at 0.1-3 relative MPa, by expanding in the area submerged in the lower liquid of the quench tower (B), and injecting ammonia or alkaline solution from above to scrub the tail gas, cooling and dissolving The mixed phase melamine introduced from below forms a solution/suspension of melamine crystallized in the liquid phase. 3. the method for processing the pyrolysis reaction product of urea to melamine as claimed in claim 2, it is characterized in that, in the subsequent treatment step, the melamine solution/suspension obtained from the bottom of the quench tower (B) is in After expansion in the valve (D) to reduce the pressure to 0-15 relative bar (0-1.5 relative MPa), it is sent to stripping in the column (E), and the second expansion is in the stripping column (E ) under a liquid seal in the downstream vessel (F). 4. The method for treating the pyrolysis reaction product of urea to melamine according to claim 3, characterized in that said container (F) is formed by the space obtained in the column E, said space keeps the solution/suspension The liquid seal of said stream is sent into said space after expansion in valve (D). 5. Process for treating pyrolysis reaction products of urea to melamine according to claim 1, characterized in that the residence time of the solution/suspension of melamine in the vessel (F) is from 20 to 1200 seconds. 6. Process for treating pyrolysis reaction products of urea to melamine according to claim 5, characterized in that the residence time of the solution/suspension of melamine in the vessel (F) is 120 to 300 seconds. 7. The method for treating the pyrolysis reaction product of urea to melamine according to claim 1, characterized in that the expansion of the reaction product is carried out under a liquid seal. 8. A plant for carrying out the treatment of the pyrolysis reaction products of urea to melamine as described in one or more of the preceding claims, characterized in that it consists of a base or Composed of a vertical cylindrical vessel with pressure relief valves (C, D) on the lower wall, and a mixed phase discharge gate at the top or upper part of the cylindrical surface to create two separate phases in the bulk of the solution/suspension being processed expansion and circulation area. 9. Apparatus according to claim 8, characterized in that said valve (D) is mounted on a frusto-conical gate which connects said valve directly to the bottom of the vessel or to the cylindrical wall of the vessel lower part. 10. The apparatus of claim 8, characterized in that it consists of a vertical cylindrical vessel equipped with an overflow separation device which separates the streams of liquid and gas in said vessel, and They are provided with discharge through separate outlets. 11. The device according to claim 8, characterized in that it consists of a vertical cylindrical vessel equipped with an agitator with one or more propellers, said pressure relief valves (C, D) are directly mounted on On the cylindrical wall of the vessel, an expansion zone and a circulation zone are formed in the body of the solution/suspension being treated, said expansion zone with annular movement being formed downstream of the outlet of the stream from the expansion valve (C, D). 12. Apparatus according to claim 11, characterized in that the ejection outlets of the valves (C, D) are oriented tangentially so as to avoid direct collision with the shaft of the agitator.

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