WO2004002966A1

WO2004002966A1 - Method and device for the recovery of melamine by expansion

Info

Publication number: WO2004002966A1
Application number: PCT/EP2003/006990
Authority: WO
Inventors: Massimo Parmegiani; Roberto Santucci
Original assignee: Eurotecnica Group S.A.
Priority date: 2002-06-27
Filing date: 2003-06-26
Publication date: 2004-01-08
Also published as: CN1665794A; ITMI20021422A0; CN1295220C; ITMI20021422A1; EP1515957A1; AU2003246347A1

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 DEVICE FOR THE RECOVERY OF MELAMINE BY EXPANSION

The present invention relates to the method for the production of melamine, with particular reference to the treatment of the reaction product.

In the most advanced and prevailing industrial practice, melamine is normally produced by pyrolysis of urea, according to the general reaction:

6 moles of urea > 1 mole of melamine + 3CO₂ + 6NH3

As can be seen, the pyrolysis of urea to melamine is accompanied by the development of a considerable amount of carbon dioxide and ammonia, normally called off-gas. The overall reaction requires the supply of heat and articulates in a complex series of reactions in one or more steps; the reaction is carried out, depending on the present industrial technologies, both in liquid phase, at high pressure and without catalysts, and in vapour phase at lower pressures and with heterogeneous catalysts. The non-catalytic, high pressure technologies generally operate at around 400°C and with pressures in the range of 7-15 Mpascal, giving rise to a product in mixed gas-liquid phase. The catalytic technologies normally op- erate in gas phase with heterogeneous catalysts based on alumina, again at around 400°C, but with pressures in the range of 0.5-10 relative bar (0.05-1 relative Mpascal) giving a gaseous product.

The industrial technologies in question generally comprise the lowering of the temperatures and pressures of the reaction product, through treatment usually called quenching and stripping, even when operating under different temperature and pressure conditions, in order to obtain the expansion and cooling of the melamine, either liquid, gaseous or in solution, until it separates as crystals in the subsequent steps.

These expansion operations are generally, but not necessarily, followed by mixing and/or adsorption steps in aqueous ammonia solutions of melamine, which can also contain C0₂ and the reaction by-products. The process evolves giving rise to mixed phases in which melamine is present in solution/suspension during the recovery and the 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 change of the melamine to be separated from the reaction raw product, as in the transition to the crystalline phase, which occurs during expansion, the growing crystals are particularly sticky: they therefore tend to strongly adhere to and encrust the solid surfaces immediately downstream of the expansion device, generally a lamination valve of the reaction effluent, such as the walls and inside of the apparatuses which receive the expanded flow of the reaction product from urea to melamine. This technical problem is common in the two pyrolysis technologies of urea to melamine, both the non-catalytic and high pressure technology, and also the catalytic technology at a lower pressure. These crusts cause disturbances and frequent stops in the reac- tion step for cleaning, with consequent costs and loss of the running factor of the overall plant.

The scheme of figure 1 shows the first step of the production process of melamine according to the known technique, with specific reference to the high pressure pyrolysis technology, that which operates in liquid phase, by feeding liquid urea at 135-140°C to a pyrolysis reactor A heated in the presence of an excess of ammonia and without the use of catalysts. In the scheme of figure 1, the vat pyrolysis reactor of urea to melamine operates in continuous at reaction temperatures of 360-420°C, with pressures in the range of 7-15 Mpascal. The reactor A is a one-step reactor, in which the molten urea is fed from the bottom, together with gaseous ammonia. The reaction mass is maintained in vigorous circulation by the gases which are formed in the reaction itself; the reacted mass (liquid and gas) is discharged and expanded from the reactor A to the vessel B through the valve C. The pyrolysis transformation reaction of urea to melamine takes place in the pyrolysis reactor. A heating system with molten salts provides the heat necessary for sustaining the reaction.

The production of a mixed phase takes place in the pyrolysis reactor A, substantially consisting of the liquid raw melamine, off-gas produced by the reaction and the excess ammonia fed from the bottom together with the molten urea. This mixed phase is fed to a cooling column B, normally called "quench" column, in which it is expanded in the valve C, generally at pressures ranging from 0.1 to 3 relative Mpascal, cooled to 120-180°C, and put in contact with a recycled ammonia solution. This operation is carried out in the quench column B, into which the reaction product containing melamine is sprayed from above together with the recycled ammonia solution, injected from one or more distributors, which cools and dissolves the melamine. Both the melamine and ammonia so- lution descend the column; under these conditions, all the melamine and non reacted urea, together with the high molecular weight impurities, pass in solution/suspension and are sent to the subsequent processing. A gaseous phase, substantially consisting of NH₃ and C0₂ coming from the reactor A, with the water vapor corresponding to the equilibrium, rises up column B, is evacuated at its top and returns to the synthesis of urea. The aqueous solution/suspension obtained from the quench column B con- tains a certain amount of ammonia and carbon dioxide in solution which must be eliminated in the subsequent stripper E, which normally operates at a. pressure of 0-15 relative bar (0-1.5 relative Mpascal), by means of a further expansion in the valve D. The aqueous strea leaving the bottom of the stripper E contains 6-12% w/w of melamine, together with the impurities to be eliminated in the subsequent steps, mainly oxy-amino-triazines (normally indicated with the acronym OAT) and poly-condensed products . The gases stripped in column E are separated and washed with recycled water in the column L, obtaining the recycled ammonia solution which is used as quenching agent in column B. A cooling water condenser is situated at the bottom of column L. The absorption heat to be re- moved from column L is equal to about 1100 Kcal/kg of am- monia and carbon dioxide to be condensed.

According to the scheme of figure 1, crusts are present in the process, essentially downstream of the two expansion valves C and D, at the inlet of the two columns B and E, into which the streams of the expanded fluid in mixed phase enter at a considerably high speed, during the segregation of melamine from the liquid phase, due to its crystallization. These growing crystals reach the walls or internal parts of the two columns and are re- sponsible for the encrusting, which disturbs the plant operations and requires frequent stops for cleaning. In terms of the running factor, these stops cause a loss of 6-8%, which means a production loss of the same order of magnitude and considerable unbalances in the overall pro- duction line. This drawback is faced, in the known art, by running the plants with greater dilutions of the melamine solutions/suspensions, with a considerable increase in both costs and energy consumptions necessary for the treatment of the solutions, for their transfer, heat ex- change and stripping operations. This stratagem limits the concentration of the melamine in the solutions/suspensions in the quenching and stripping equipment - with reference to the high pressure pyrolysis technology- in the range of 8-10% by weight. The problem of the expansion of the strea of raw melamine and the formation of crusts during its solidification into crystals, is also present when the catalytic pyrolysis technology in gas phase of urea to melamine, is used. The objective of the present invention is to provide a device and a process for the expansion and cooling of the raw effluent of the pyrolysis reaction of urea to melamine which can avoid said drawbacks due to the crusts of crystallized melamine during said expansion. This objective is achieved by means of the device and process according to the present invention, of which claim 1 is the most general definition, of the process, whereas its preferred embodiments or possible variants are defined in the dependent claims 2 to 7. The device according to the present invention is defined, in its most general meaning, in claim 8, whereas its preferred embodiments or possible variants are defined in the dependent claims 9-12.

The characteristics and advantages of the device and process according to the present invention for the expansion and crystallization of the melamine contained in the reaction stream and its sending to the subsequent plant sections, will appear more evident from the following illustrative but non-limiting description, which refers to the schemes of the figures. Figure 1 represents the known technique according to the previous discussion. Figure 2 illustrates a simplified embodiment of the invention, whereas figures 3 to 8 show examples of the embodiment of the expansion device which allows the segregation and crystallization of melamine, avoiding or considerably limiting encrusting-

The pyrolysis reaction is carried out in the reactor A, with the method already mentioned with reference to figure 1. A mixed phase, containing the liquid melamine produced, is formed in the pyrolysis reactor A, which rises up the reactor and is discharged from its upper outlet.

A mixed phase containing the off-gas of the reaction and melamine, the small amount of non-reacted urea and by-products, is obtained from the upper part of the reactor, again according to the scheme of figure 2; this mixed phase is expanded in the valve C situated on the connection duct between the reactor and the quench column, immediately before, or even in direct contact with, the outlet in the lower part of the quench column B. A liquid seal is maintained in said lower part of the column to prevent the crystals, at the moment of their formation and separation from the liquid phase, from coming into contact with a solid surface and consequently adher- ing to the same. This result is therefore obtained by making the expansion take place in a zone immersed in a liquid, in the lower part of the quench column B. It has been found, in fact, that the adhesion characteristics of melamine crystals cause encrusting during their formation and segregation from the mother solution/suspension and that the melamine crystals, once segregated within the liquid phase, no longer have the same encrusting behavior.

In the case described above, and analogously to what is illustrated for the scheme of figure 1, an ammonia or alkaline solution is again injected from above by specific distributors and descends from above, washing the off-gas rising up the column, and cooling and dissolving the melamine of the mixed phase introduced into the lower part, after its expansion in the valve C. A liquid phase containing 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 phase. This stream, before being introduced into the stripping column

E, is expanded in the valve D, in order to reduce the pressure, normally from atmospheric pressure to 15 bar

(0-1.5 relative Mpascal). This expansion causes a consid- erable evaporation of the liquid and a further segrega- tion of the solid, which exceeds the saturation of the remaining liquid. In the processes of the known art, considerable encrusting occurs in the column E, also downstream of the valve D. This drawback is therefore eliminated, according to the present invention, in the scheme of figure 2, by making the expansion take placed under a liquid seal, in a vessel F, situated upstream of the subsequent stripping column E, in which most of the gases still present in the solution/suspension of melamine coming from B, are stripped and removed. The vessel F is kept at the same pressure as the subsequent stripper and .can consist of a space obtained in column E, in which a liquid solution seal is maintained, into which the stream is sent after expansion. Said construction alternative is illustrated below with reference to figures 5 and 6. The dimensions of the vessel F are such as to ensure a residence time under the liquid seal sufficient for segregating the crystals before they come into contact with walls or other internal parts of the equipment. In general, a residence time of the melamine solution/suspension in the vessel ranging from 20 to 1200 seconds, preferably between 120 and 300 seconds, is sufficient for effecting the operation without the formation of any significant encrusting. The liquid in which the mixed phase containing raw melamine is expanded, can be ammonia solutions and/or alkaline solutions and/or water, according to the catalytic or non-catalytic processes of the pyrolysis of urea to melamine.

The illustrative embodiments of figures 3 and 4 show procedures for producing the vessels in which the expansion of the liquid stream containing melamine in solution/suspension, takes place, under a liquid seal. In the embodiment of figure 3, the vessel F has a vertical cylindrical shape and is completely filled with the mixed phase after expansion. The pressure reduction valve, for example valve D placed upstream of the stripping column E, is preferably assembled directly on the bottom of the vessel F. According to an embodiment variant of this configuration, the valve D can be installed on a truncated-conical gate which connects the above valve to the bottom of the vessel. As indicated in figure 3, two separate zones are formed within the solu- tion/suspension: an expansion zone and a recirculation zone. The expansion zone, shown with a dotted line, is formed in the center and immediately downstream of the inlet of the stream which has passed through the expansion valve; in this zone, there is a mixed phase, having a lower density, with the development of gas bubbles, a sudden cooling and the segregation of the growing melamine crystals. The recirculation zone, on the contrary, is situated on the border, around the expansion zone; in this zone there is a greater density, as part of the gas phase in bubbles has separated from the suspension and rises at a higher rate towards the outlet. The liquid phase, impoverished of gas, redescends, as shown by the arrows, towards the bottom, creating a recirculation of the cooler liquid which thus favors the formation of the suspension of melamine crystals, before touching the walls of the equipment. The discharge occurs in mixed phase, from the upper gate.

In the embodiment of figure 4, the vessel still has a vertical cylindrical shape and is equipped with a con- ventional stirrer, consisting of a motor M with a vertical shaft carrying one or more propellers. The pressure reduction valve, for example valve D, upstream of the stripping column E, is preferably assembled directly on the cylindrical wall of the vessel. According to an em- bodiment variant of this configuration, the valve D can be installed on a truncated-conical gate which connects said valve to the cylindrical wall of the vessel. During its functioning, the liquid level on the expansion vessel is maintained above the assembly point of the expansion valve, with a wide margin, to guarantee that the expan- sion takes place under a liquid seal.

According to a preferential embodiment of the invention, the outlet of the spurt of valve D is oriented in a tangential direction so as not to directly collide with the shaft of the stirrer, but to evolve the expansion zone around it. Two separate zones are formed in the solution/suspension body, also in the embodiment illustrated in figure 4: an expansion zone and a recirculation zone. As a result of the effect of the rotating movement created by the stirrer, the expansion zone, marked by a dotted line, is formed immediately downstream of the outlet of the stream which has passed through the expansion valve and tends to have a toroidal movement; a mixed phase with a lower density is present in said toroidal zone, in which the stream is cooled and the growing melamine crystals are segregated. The recirculation zone, vice versa, is situated outside the toroidal expansion zone; in this zone there is again a greater density, and the liquid phase, impoverished of gas, recirculates to- wards the bottom of the vessel, where the stirrer propellers are installed, which put said liquid phase in close contact with the expanded stream coming from valve D, thus favoring the formation of the suspension of melamine crystals, before coming into contact with surfaces not wetted by the liquid. The discharge is effected in mixed phase, from the later upper gate.

The following figures 5 to 8 show constructive variants of the vessel F with respect to the embodiments of figures 3 and 4, which comprise the separation of the liquid and gaseous streams inside the vessels and their evacuation through separate outlets and/or which comprise the insertion of the vessel F in the column E.

Figures 5 and 6 schematically illustrate ways for producing the expansion vessel F, under a liquid seal in- side the stripping column E. In the embodiment of figure 5, the expansion vessel F is situated above the tray section of column E. Two ducts are concentrically positioned at the center of the vessel, for communication between the volumes of vessel F and the column E. The melamine solution/suspension is introduced from the side with the inlet of valve D and expands in the zone indicated by dots. The development of the gas phase and the recycling of the heavier liquid phase take place in the remaining volume, as previously shown. The threshold s of the ex- ternal duct s, allows the overflow of the liquid phase descending the duct and running through the trays of column E. The internal duct r, on the contrary, acts as a riser for the ascent of the vapors coming from column E, which join the gas phase developing in the vessel F and are evacuated together from the superior duct t. In the embodiment of figure 6, in a side and plan view, the expansion vessel F is situated above the tray section of column E and is equipped with a stirrer, analogously to the embodiment illustrated with reference to figure 4. A separating septum p is situated in the upper part of the vessel F, equipped with lower peripheral overflow openings pi which allow the descent of the liquid towards the column E and upper peripheral openings p₂ which allow the ascent of the vapor phase coming from column E, which joins the gas phase that develops in the vessel F. The melamine solution/suspension is introduced from the side with the inlet of valve D and expands in the zone indicated by dots, with an annular trend, due to the movement of the stirrer driven by the motor M. The vapours coming from column E are collected through the opening p₂, join the gas phase developing in the vessel F and are evacuated together from the upper duct t.

Figures 7 and 8 schematically illustrate ways for producing the expansion vessel F, under a liquid seal, similar to those of figures 3 and 4, but with separation of the streams inside. In both cases, a vertical duct s is present for discharging the liquid phase with a threshold Si which maintains the level of the liquid phase and allows the overflow of the separated liquid phase which descends the duct and goes to column E to be stripped. The gaseous phase developing in the vessel F can, on the contrary, be evacuated from the upper duct t and sent directly to the absorption column L, thus reducing the gas streams and the transversal dimensions of column E.

The separation method of melamine from its solutions/suspensions, by expansion under a liquid seal, allows considerable progress to be reached with respect to the known art. The invention allows the recovery of a 6-8% share of the plant running factor, which would have been lost as a result of encrusting, and guarantees a greater continuity and equilibrium in the entire production line. There is no longer any reason, moreover, for limiting the melamine concentration in the solutions/suspensions in the quenching and stripping equipment within the range of 8-10% by weight: these concentrations can be increased by 13-15% by weight, for the same plant and with the other conditions remaining unaltered. For the same plant, the treat- ment capacity increases by about 50%. The specific consumption of vapor, electric energy and refrigeration is correspondingly diminished, as well as the plant cost incidence on the overall production costs .

Claims

1. A process for the treatment of the reaction product of the pyrolysis of urea to melamine, consisting of a mixture of raw melamine also comprising off-gas consist- ing of carbon dioxide and ammonia, including one or more expansion and cooling steps of the reaction product to separate the off-gas and segregate the melamine which separates from the liquid phase in crystalline form, characterized in that the expansion of the reaction prod- uct is effected in a zone immersed in a liquid, so that the melamine crystals, during their formation and separation from the liquid or gaseous phase, do not touch the walls or other internal parts of the equipment and consequently cannot adhere to the same.

2. The process for the treatment of the reaction product of the pyrolysis of urea to melamine according to claim 1, characterized in that the expansion of the reaction product is effected in a valve (C) situated immediately before the outlet in the lower part of the quench column (B) , operating at 0.1-3 relative Mpascal, by effecting the expansion in a zone immersed in the liquid in the lower part of the quench column (B) and injecting an ammonia or alkaline solution from above, to wash the offgas, cooling and dissolving the melamine of the mixed phase introduced from below into a solution/suspension of melamine which crystallize within the liquid phase.

3. The process for the treatment of the reaction product of the pyrolysis of urea to melamine according to claim 2, characterized in that, in the subsequent proc- essing step, the melamine solution/suspension which is obtained from the bottom of the quench column (B) , is sent for stripping in column (E) , after expansion in the valve (D) to reduce the pressure to 0-15 relative bar (0- 1,5 relative Mpascal) and said second expansion is carried out under a liquid seal in a vessel (F) downstream of the stripping column (E) .

4. The process for the treatment of the reaction product of the pyrolysis of urea to melamine according to claim 3, characterized in that the vessel (F) consists of a space obtained in column E, in which a solution/suspension liquid seal is maintained, into which the stream is sent after expansion in the valve (D) .

5. The process for the treatment of the reaction product of the pyrolysis of urea to melamine according to claim 1, characterized in that the residence time of the solution/suspension of melamine in the vessel (F) ranges from 20 to 1200 seconds.

6. The process for the treatment of the reaction product of the pyrolysis of urea to melamine according to claim 5, characterized in that the residence time of the solution/suspension of melamine in the vessel (F) ranges from 120 to 300 seconds.

7. The process for the treatment of the reaction product of the pyrolysis of urea to melamine according to claim 1, characterized in that the expansion of the reaction product takes place under a liquid seal.

8. A device for the embodiment of the treatment of the reaction product of the pyrolysis of urea to melamine according to one or more of the previous claims, character- ized in that it consists of a vertical cylindrical vessel with the pressure reduction valve (C,D) directly assembled on the bottom or on the lower wall of the cylindrical surface, whereas the discharge gate in mixed phase is situated on the ceiling or on the upper part of the cy- lindrical surface, to produce in the solution/suspension body under treatment, two separate expansion and recirculation zones, respectively.

9. The device according to claim 8, characterized in that the valve (D) is installed on a truncated-conical gate which connects said valve directly to the bottom or the lower part of the cylindrical wall of the vessel.

10. The device according to claim 8, characterized in that it consists of a vertical cylindrical vessel equipped with overflow separating devices which separate the liquid and gaseous streams inside the vessel and pro- vide for their evacuation through separate outlets.

11. The device according to claim 8, characterized in that it ^■ consists of a vertical cylindrical vessel equipped with a stirrer with one or more propellers, the pressure reduction valve (C, D) is assembled directly on the cylindrical wall of said vessel, forming, in the body of the treated solution/suspension, an expansion and a recirculation zone, the expansion zone being formed downstream of the outlet of the stream coming from the expan- sion valve (C, D) , with a toroidal movement.

12. The device according to claim 11, characterized in that the outlet of the spurt of the valve (C, D) is oriented in a tangential direction to avoid directly colliding with the shaft of the stirrer.