CN1295220C

CN1295220C - Method and device for the recovery of melamine by expansion

Info

Publication number: CN1295220C
Application number: CNB038151480A
Authority: CN
Inventors: M·帕梅贾尼; R·圣图奇
Original assignee: EUROTECNICA MELAMINE LUXEM Zweig in Ittigen
Current assignee: Ouji Melamine Co ltd
Priority date: 2002-06-27
Filing date: 2003-06-26
Publication date: 2007-01-17
Anticipated expiration: 2023-06-26
Also published as: WO2004002966A1; AU2003246347A1; ITMI20021422A1; EP1515957A1; CN1665794A; ITMI20021422A0

Abstract

Treatment of the pyrolysis product of urea to melamine, in one or more expansion and cooling steps to separate the offgas and recover the melamine in solution/suspension, carried out by expanding the product under a liquid seal, to prevent the melamine crystals in formation from adhering to the internal walls of the equipment.

Description

Method and apparatus for the recovery of 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 recent and mainstream industrial practice, melamine is generally 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 amounts of carbon dioxide and ammonia, commonly referred to as off-gas. The entire reaction requires a supply of heat and interconnects a complex series of reactions in one or more steps; according to the current industrial technique, the reaction is carried out in a liquid phase at high pressure and without catalyst, and in a vapor phase at low pressure and with heterogeneous catalyst.

Non-catalytic high pressure techniques typically operate at about 400 ℃ and pressures of 7-15 megapascals, producing products in a mixed gas-liquid phase. The catalytic technique generally employs heterogeneous catalysts based on alumina, operating in the gas phase, also at about 400 ℃, but at pressures ranging from 0.5 to 10 relative bar (0.05 to 1 relative megapascal) giving the gaseous products.

In order to expand and cool the melamine, liquid, gaseous or in solution, until it is separated off as crystals in a subsequent step, said industrial techniques generally consist in reducing the temperature and pressure of the reaction products, even operating under different temperature and pressure conditions, by treatments commonly known as quenching and stripping.

Usually, but not necessarily, after these expansion operationsA step of mixing and/or adsorption in an aqueous ammonia solution of melamine, which solution may also contain CO₂And reaction by-products. The process is carried out resulting in a mixed phase wherein melamine is present in the solution/suspension during the recovery and separation process. These solutions/suspensions are then treated to obtain melamine within specification.

An important technical problem in the production of melamine relates to the phase transition of the melamine to be separated from the reaction raw product, since the crystals that grow during the transition to the crystalline phase that occurs during expansion are particularly viscous and therefore they often adhere and encrust firmly to solid surfaces immediately downstream of the expansion device, generally a laminar valve of the reaction effluent, such as the walls and the inside of the plant that receives the expanded flow of reaction products from urea to melamine. This technical problem is very common in both the non-catalytic and high-pressure technologies of urea to melamine and the low-pressure catalytic technologies of pyrolysis. These shells cause disturbances to the reaction steps and frequent shutdowns for cleaning, with consequent increase in costs and reduction in the operating factor of the whole plant.

The scheme of figure 1 shows the first step of a process for the production of melamine according to the known art, which is a high-pressure pyrolysis technique, operating in liquid phase in thepresence of excess ammonia and without the use of a catalyst, by feeding liquid urea at 135-. In the schematic of FIG. 1, the urea to melamine pot pyrolysis reactor is operated continuously at a temperature of 360-420 ℃ and a pressure of 7-15 MPa. The reactor a is a one-stage reactor in which molten urea is fed from the bottom together with gaseous ammonia. The reaction mass is kept in vigorous circulation by the gases formed in the reaction itself; the reacted materials (liquid and gas) are discharged from reactor a through valve C and expanded to vessel B. The reaction for the pyrolytic conversion of urea into melamine is carried out in said pyrolysis reactor. A heating system using molten salts provides the heat required to sustain the reaction.

A mixed phase is produced in the pyrolysis reactor A, which consists essentially of a coarse liquidMelamine, off-gas from the reaction and excess ammonia fed from the bottom together with the molten urea. This mixed phase is fed to a cooling column B, usually called a "quench" column, where it is expanded, usually at a pressure of 0.1 to 3 relative MPa, in a valve C, cooled to 180 ℃ and contacted with a recycled ammonia solution. This operation is carried out in a quench column B into which the reaction product containing melamine is injected from above, together with a recycled ammonia solution which cools and dissolves the melamine, injected from one or more distributors. The melamine and ammonia solution both descend in the tower; under such conditions, all melamine and unreacted urea, together with high molecular weight impurities, go into solution/suspension and are sent to the subsequent treatment process. Essentially from NH coming from reactor A₃And CO₂And the gas phase consisting of the equilibrium water vapour rises from column B, is discharged at the top thereof and is returned to the synthesis of urea. 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 a subsequent stripping column E, which is usually operated at a pressure of 0-15 relative bar (0-1.5 relative mpa) by means of a further expansion in a valve D. The aqueous stream leaving the bottom of the stripping column E comprises6-12% w/w melamine, and impurities to be removed in the subsequent steps, mainly oxy-amino-triazine (usually indicated by the acronym OAT) and polycondensation products.

The gas stripped in column E is separated and scrubbed in column L with recycled water to give a recycled ammonia solution which is used as quench in column B. A cooling water condenser is located at the bottom of the column L. The heat of absorption removed from the column L is equal to about 1100Kcal/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, a crust will be present in the process, substantially downstream of the two expansion valves C and D, at the inlet of the two columns B and E, into which the stream of expanded liquid in mixed phase enters at a fairly high speed, due to the crystallization of the melamine. These growing crystals reach the walls or the interior of the two columns and cause encrustations which interfere with the operation of the plant and require frequent stops for cleaning. In terms of the running factor, these shutdowns cause losses of 6-8%, which means production losses of the same order of magnitude and considerable imbalances in the entire production line. In the known art, the plant is operated with a greater dilution of the melamine solution/suspension in response to this drawback, which results in a considerable increase in the costs and energy consumption required for the treatment of the solutions, their transfer, heat exchange and stripping operations. 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 weight range of 8-10%.

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

The object of the present invention is to provide an apparatus and a process for the expansion and cooling of the crude effluent of the pyrolysis reaction of urea to melamine which make it possible to avoid the said drawbacks caused by the crusting of melamine which crystallizes during the said expansion.

This object is achieved by means of the apparatus and the method according to the invention, which is most generally defined in claim 1, while preferred embodiments or possible variants of the method according to the invention are defined in the dependent claims 2 to 7. The most general meaning of the device according to the invention is defined in claim 8, while preferred embodiments or possible variants of the device according to the invention are defined in the dependent claims 9-12.

The characteristics and advantages of the apparatus and process according to the invention for the expansion and crystallization of the melamine contained in the reaction stream, as well as its sending to the subsequent plant sections, will be more apparent from the following description of an exemplary and non-limiting scheme with reference to the attached drawings.

From the foregoing discussion, FIG. 1 represents known technology. Fig. 2 illustrates a simplified embodiment of the invention, while fig. 3 to 8 show examples of embodiments of the expansion devicewhich allow the separation and crystallization of melamine, avoiding or significantly limiting encrustation.

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

Again according to the scheme of figure 2, a mixed phase is obtained from the upper part of the reactor, comprising the off-gases of the reaction and melamine, small amounts of unreacted urea and by-products. This mixed phase is expanded in a valve C located on the connecting conduit between the reactor and the quench column, just 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 surface as they are formed and separated from the liquid phase. This result is therefore obtained by having the expansion take place in the lower part of the quench column B, in a zone immersed in the liquid. It has in fact been found that the adhesive properties of the melamine crystals, which once separated in the liquid phase no longer have the same crusting behaviour, lead to crusting during their formation and separation from the parent solution/suspension.

In the above case, ammonia or alkaline solution is also injected from above with a special distributor and descends from top to bottom, washing the off-gas rising inside the column, cooling and dissolving the melamine introduced in the lower mixed phase after expansion in the valve C, similarly to what has been illustrated in connection with the scheme of figure 1. A liquid phase comprising a melamine solution/suspension is thus formed.

In the following process step, the solution/suspension of melamine obtained at the bottom of the quench column B is sent to the stripping section. This stream is expanded in a valve D to reduce the pressure, typically from atmospheric to 15 bar (0-1.5 relative megapascals), before it is introduced into the stripping column 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, a considerable amount of encrustation occurs in the column E (also downstream of the valve D).

According to the invention, in the solution of figure 2, this drawback is therefore eliminated by carrying out the expansion under a liquid seal in the vessel F located upstream of the following stripping column E, in which stripping column E most of the gases still present in the melamine solution/suspension coming from B are stripped and removed. Vessel F is maintained at the same pressure as the subsequent stripping column and may consist of the space obtained in column E, in which a seal of liquid solution is maintained, said stream being sent to it after expansion. An alternative to this configuration is described later with reference to fig. 5 and 6. The vessel F is 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 from 20 to 1200 seconds, preferably from 120 to 300 seconds, is sufficient to achieve an operation without any significant encrustation.

The liquid into which the mixed phase containing raw melamine expands can be an ammonia solution and/or an alkaline solution and/or water, depending on the catalytic or non-catalytic method of pyrolysis of urea to melamine.

The exemplary embodiments of fig. 3 and 4 show the process of forming the vessel in which the expansion of the melamine-containing liquid stream in solution/suspension under a liquid seal is carried out.

In the embodiment of fig. 3, the container F has a vertical cylindrical shape and is completely filled with the mixed phase after expansion. A pressure-reducing valve, for example a valve D located upstream of the stripper E, is preferably fitted directly on the bottom of the vessel F. According to an embodiment variant of this construction, the valve D can be mounted on a truncated cone-shaped shutter connecting the above-mentioned valve and the bottom of the container. As indicated in fig. 3, two separate zones are formed within the solution/suspension: an expansion zone and a circulation zone. An expansion zone, indicated by a dotted line, is formed centrally and immediately downstream of the inlet of the stream through the expansion valve; in this zone, there is a mixed phase with a lower density, accompanied by the growth of bubbles, sudden cooling and separation of the growing melamine crystals. Instead, the circulation zone is located on the side around the expansion zone; in this region, there is a greater density because part of the gas phase in the bubble has separated from the suspension and rises at a higher velocity towards the outlet. The gas-poor liquid phase falls again towards the bottom, as indicated by the arrows, creating a circulation of the cooler liquid, which circulation thus favours the formation of a suspension of melamine crystals before they contact the walls of the apparatus. The discharge from the upper outlet takes place in the form of a 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. The depressurization valve, for example the 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 construction, the valve D can be mounted on a truncated-cone-shaped shutter connecting said valve with the cylindrical wall of the container. During operation, the liquid level in the expansion vessel is maintained above the installation point of the expansion valve, with a wide margin to ensure that the expansion takes place under a liquid seal.

According to a preferred embodiment of the invention, the ejection outlet of the valve D is oriented tangentially so as not to collide directly with the shaft of the stirrer, but so as to spread the expansion zone around it. Also, in the embodiment shown in fig. 4, two separate regions are formed in the bulk of the solution/suspension: an expansion zone and a circulation zone. The expansion zone marked by a dotted line is formed immediately downstream of the outlet of the stream flowing through the expansion valve and often has an annular movement as a result of the effect of the rotary movement generated by the stirrer; the mixed phase with the lower density is present in said annular zone, in which said flow is cooled and the growing melamine crystals are separated. Instead, 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 stirrer propeller is installed, which brings said liquid phase into intimate contact with the expanded flow coming from the valve D, thus favouring the formation of a suspension of melamine crystals before they come into contact with the surfaces not wetted by the liquid. The discharge from the further rear upper outlet takes place in the form of a mixed phase.

Figures 5 to 8 below show constructive variants of the vessel F with respect to the embodiment of figures 3 and 4, which comprise the separation of the liquid and gaseous streams inside the vessel, and their discharge through respective outlets, and/or comprise the insertion of the vessel F into the column E.

Figures 5 and 6 schematically illustrate the manner in which the expansion vessel F is formed under a liquid seal inside the stripper E. In the embodiment of fig. 5, the expansion vessel F is located above the tray section of column E. For the communication of the volumes between the vessel F and the column E, the two conduits are concentrically located in the centre of the vessel. 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. As indicated previously, the progressive formation of the gas phase and the recirculation of the heavy liquid phase take place in the remaining space. Threshold s of outer catheter s₁The liquid phase is allowed 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 rising of the vapour coming from the column E, which combines with the gas phase formed in the vessel F and is discharged from the upper conduit t. In the embodiment shown in fig. 6 in side and top views, expansion vessel F is located above the tray section of column E and is equipped with an agitator similar to the embodiment described with reference to fig. 4. A dividing septum p is located in the upper part of the vessel F, fitted with a lower peripheral overflow opening p allowing the liquid to descend towards the column E₁And a higher peripheral opening p allowing the vapor phase from the column E to rise₂The steam isThe phase combines with the gas phase that gradually forms in the vessel F. The melamine solution/suspension is introduced from the side with the inlet of the valve D and expands in the zone indicated by the circular dots with an annular trend due to the movement of the stirrer driven by the motor M. The steam from the column E is passed through the opening p₂Collected, gradually brought into gas phase association with the container F and discharged together from the upper conduit t.

FIGS. 7 and 8 showThe manner in which the expansion vessel F, similar to those of figures 3 and 4, but with separation of the streams inside, is formed under a liquid seal is schematically illustrated. In both cases there is a vertical conduit s, for use as a threshold s₁Discharge of liquid phase, threshold s₁The liquid phase is maintained at a level and the separated liquid phase is allowed to overflow, descend in a conduit and go to column E to be stripped. Instead, the gaseous phase which is gradually formed in the vessel F can be discharged from the upper conduit t directly to the absorption column L, thus reducing the gas stream and the cross-sectional dimensions of the column E.

The process for separating melamine from a melamine solution/suspension by expansion under a liquid seal represents a considerable advance over the known art.

The invention makes it possible to recover 6-8% of the coefficient of operation of the plant which would otherwise be lost by encrustation, and to ensure better continuity and balance throughout the production line. Furthermore, there is no longer any reason to limit the concentration of melamine in the solution/suspension in the quenching and stripping apparatus to the range of 8-10% by weight: these concentrations can be increased by 13-15% by weight for the same apparatus and other conditions remaining unchanged. The processing capacity increases by about 50% for the same device. The specific consumption of steam, electricity and refrigeration, and the cost of the equipment that affects the overall production cost, are correspondingly reduced.

Claims

1. A process for the treatment of the reaction products of the pyrolysis of urea to melamine, consisting of a mixture of crude melamine also comprising off-gas consisting of carbon dioxide and ammonia, comprising one or more expansion and cooling steps of said reaction products, to separate said off-gas and to separate the melamine separated in crystalline form from the liquid phase, characterized in that the expansion of said reaction products is carried out in a zone immersed in a liquid so that the melamine crystals do not contact the walls or other internal parts of the apparatus and therefore do not adhere to them during their formation and separation from the liquid or gaseous phase.

2. Process for the treatment of products of the pyrolysis reaction of urea to melamine according to claim 1, characterized in that the expansion of the reaction products is carried out in a valve (C) placed close to the outlet of the lower part of the quenching column (B), operating at a gauge pressure of 0.1-3 mpa, by expansion in a zone immersed in the liquid in the lower part of the quenching column (B), and by injection of ammonia or an alkaline solution from above to wash the off-gases, cooling and dissolving the melamine of the mixed phase introduced from below, forming a solution or suspension of melamine crystallized in the liquid phase.

3. Process for the treatment of products of the pyrolysis reaction of urea to melamine according to claim 2, characterized in that, in the subsequent treatment step, the melamine solution or suspension obtained at the bottom of the quenching column (B) is sent to the column (E) for stripping after expansion in the valve (D) to reduce the pressure to a gauge pressure of 0-15 bar (0-1.5 mpa) and the second expansion is carried out under a liquid seal in the vessel (F) downstream of the stripping column (E).

4. Process for the treatment of products of the pyrolysis reaction of urea to melamine according to claim 3, characterized in that said container (F) is constituted by the space obtained in column E, which space maintains a liquid seal of the solution or suspension, said stream being sent to said space after expansion in valve (D).

5. Process for the treatment of products of the pyrolysis reaction of urea to melamine according to claim 4, characterized in that the residence time of the solution or suspension of melamine in the vessel (F) is comprised between 20 and 1200 seconds.

6. Process for the treatment of products of the pyrolysis reaction of urea to melamine according to claim 5, characterized in that the residence time of the solution or suspension of melamine in the vessel (F) is comprised between 120 and 300 seconds.

7. Process for the treatment of the products of the pyrolysis reaction of urea to melamine according to claim 1, characterized in that the expansion of the reaction products is carried out under a liquid seal.

8. Plant for the implementation of the treatment of the products of the urea-to-melamine pyrolysis reaction according to one or more of the preceding claims, characterized in that it consists of a vertical cylindrical vessel with pressure relief valves (C, D) fitted directly to the bottom or lower wall of the cylindrical surface, while a mixed phase discharge gate is located at the top or upper part of the cylindrical surface, so as to create two separate expansion and circulation zones, respectively, in the body of solution or suspensionbeing treated.

9. The apparatus of claim 8, characterized in that said valve (D) is mounted on a truncated cone shutter connecting said valve directly to the bottom of the container or to the lower part of the cylindrical wall of the container.

10. The apparatus of claim 8, wherein the apparatus consists of a vertical cylindrical vessel equipped with an overflow separation device which separates the liquid and gas streams within the vessel and provides for their discharge through separate outlets.

11. The apparatus according to claim 8, characterized in that it consists of a vertical cylindrical vessel equipped with an agitator with one or more propellers, said decompression valve (C, D) being fitted directly on the cylindrical wall of said vessel, forming an expansion zone and a circulation zone in the body of solution or suspension being treated, said expansion zone with annular motion being formed downstream of the outlet of the flow from the expansion valve (C, D).

12. The 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 stirrer.