EP1504045A1

EP1504045A1 - Process for the production of melamine from urea, and particularly to obtain melamine-free off-gas in the first separation step

Info

Publication number: EP1504045A1
Application number: EP03749861A
Authority: EP
Inventors: Roberto Santucci; Massimo Parmegiani; Sergio Noe'
Original assignee: Eurotecnica Group SA
Current assignee: Eurotecnica Melamine Luxemburg Zweigniederlassung
Priority date: 2002-05-14
Filing date: 2003-04-29
Publication date: 2005-02-09
Also published as: CN1659201A; ITMI20021026A0; CN1300122C; ITMI20021026A1; AU2003232234A1; WO2003095516A1

Abstract

Treatment of off-gas deriving from the pyrolysis reaction of urea to produce melamine, carried out in liquid phase at a high pressure, wherein the melamine content present in the off-gas is separated by washing the off-gas with a cooling liquid stream which rapidly brings the gaseous mixture to de-sublimation temperatures of the gaseous melamine which separates in solution in said stream, thus obtaining melamine-free off-gas on the one hand, and a fraction of high purity de-sublimated melamine in solution, on the other.

Description

PROCESS FOR THE PRODUCTION OF MELAMINE FROM UREA, AND PARTICULARLY TO OBTAIN MELAMINE-FREE OFF-GAS IN THE FIRST SEPARATION STEP

The present invention relates to a process for the production of melamine, with particular reference to the separation phase and treatment of the reaction product.

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

6 moles of urea -→ 1 mole of melamine + 3 CO₂ + 6NH3

The carbon dioxide and ammonia which develop from this pyrolysis are commonly called off-gas. The overall pyrolysis reaction requires a heat supply and is carried out, depending on the technologies available, both in liquid phase, at a high pressure and without catalysts, and in vapour phase, at lower pressures and with heterogeneous catalysts .

In these technologies, the selection of the operat- ing parameters allows the reaction to be specifically shifted towards the exhaustion of the urea fed. On the basis of the molecular weights of the chemical species involved, for every 126 kg of melamine produced in the reaction, 234 kg of off-gas are formed, consisting of a mixture of C0₂ and NH₃, . with a gas/melamine weight ratio equal to 1.85.

The technical problem which arises therefore relates to the separation of the melamine within specification as well as the recovery and re-use of the considerable amount of off-gas which derives from the main reaction. The efficiency and economy of the separation and treatment of the effluents of the reaction step are crucial for the commercial success of the production process of melamine. It is specifically in the separation and recov- ery steps that there are the major differences between the various competing processes. In the current industrial practice prevalently adopted, the production plants of melamine from urea are coupled, or even integrated, with the production plants of urea, so that the off-gas can be re-used in the overall cycle.

In the description of the present invention, reference is made to the high pressure pyrolysis technology, which operates in liquid phase by feeding a heated reactor with molten urea, in the presence of an excess of am- monia and without the use of catalysts. In general, the pyrolysis reactor of urea to melamine operates at temperatures between 380 and 450°C, at pressures within the range of 80-150 bar.

Several methods have been proposed in the known art for the treatment of off-gas, after the separation and recovery of the melamine still contained therein in vapour phase. For example, the off-gas can be absorbed in water, forming ammonium carbamate or carbonate, it can be condensed and fractionated to separate the ammonia from the carbon dioxide, or it can be used for the production of ammonium nitrate or sulfate, which can be used as fertilizers. These methods on the whole have considerable drawbacks due to both the high investments and high energy costs. A more recent system for the treatment of off-gas, relating to the synthesis of melamine at high pressure, in liquid phase, resorts to a washing of the gas phase, which is separated from the reaction effluent, with molten urea, in order to separate the melamine present in the off-gas. Its content is not negligible, as it can reach 10-20% of the total melamine produced, depending on the operating conditions. The above method allows this part of the melamine to be recovered and to recycle the off-gas in the anhydrous state to the urea plant section which has a pressure compatible with that of the off-gas thus made available. The molten urea, which contains the recovered melamine, is then fed to same melamine synthesis reactor.

These recovery systems with molten urea are de- scribed, for example, , in US patents 3,700,672 and 4,565,867. This washing method with molten urea operates on the gaseous phase separated from the reaction effluent, at the same synthesis pressure and at temperatures normally around 180°C. The selection of the washing tem- perature represents a compromise between the necessity of preventing the condensation of the ammonium carbamate and limiting the degradation of the urea to undesired products, such as biuret, triuret or cyanuric acid, which cause great drawbacks in the operating units. Under the operating conditions of said adsorption with molten urea, the condensable substances in the off-gas separated from the liquid phase, essentially melamine, are condensed and solidified, remaining dispersed in the molten urea, which is then fed to the reaction. Under these conditions, how- ever, there is also the parallel phenomenon of a significant dissolution of the same off-gas in the molten urea. Data available in the state of the art, show, for example, that the molten urea coming from the separation apparatus of melamine, contains around 5% by weight of dis- persed melamine, and about 20% by weight of off-gas in solution, thus causing an undesired recycling of the off- gas which should instead be separated and sent for subsequent treatment .

The recovery treatment of melamine from off-gas with molten urea, making the. anhydrous off-gas available for possible re-use, appears to be relatively interesting, but is not without complications and inconveniences . The washing section of the molten μrea requires machinery and equipment which operate almost under the same pressure conditions as the pyrolysis reactors of urea to melamine, and with substances which cause significant corrosion phenomena, even in the case of stainless steel materials; it is therefore necessary to resort to the use of valuable materials, such as alloys called INCONEL and HASTEL- LOY, and extremely costly constructions.

The efficiency itself of the pyrolysis unit is substantially diminished by the recycling of both the recovered melamine which, under regime conditions, recirculates on itself for about 15-20%, and also of the off-gas which recirculates for about 40-50% of that produced by the "useful" pyrolysis reaction. This recirculation not only requires an increase in the reaction volumes, but also disturbs the circulation inside the reactor 'and diminishes the efficiency of the thermal exchange due to the unfavourable change in the gas/liquid ratio in the reactor. A substantial increase in the thermal exchange surfaces is therefore necessary for the same net production of melamine.

There are also considerable drawbacks with respect to the reduced reliability of the whole system, in which the pyrolysis section is integrated with the washing section with molten urea, which forms the pyrolysis feed. A malfunctioning in the washing section - which operates under extremely delicate conditions - generally causes the immediate stoppage of the primary pyrolysis section and subsequent interruption in production. Furthermore, stoppages in the primary section of the pyrolysis, whether programmed or accidental, involve complex procedures such as emptying the reactor, normally effected by sublimating the melamine through an injection of ammonia at high temperature. When the washing unit with molten urea is integrated with the reaction step, this method of emptying by sublimation can no longer be adopted, as the molten urea used for the washing cannot be disposed of by feeding it to the reactor, which is not operating: it is therefore necessary to install an additional specific section which can receive the sublimation products and which is activated only in such occurrences.

The known art also proposes a system for separating off-gas from the raw effluent of the pyrolysis reaction - melamine and off-gas still in gaseous phase - in a rapid cooling column - normally called a quench column - by separation with water or cold aqueous ammonia or ammonia solutions. This separation system is illustrated in the scheme of figure 1.

The reaction is carried out in the reactor A fed with molten urea through line 1 and with additional ammonia through line 2. The raw reaction effluent produced in step A is discharged from the top through line 3 and is rapidly cooled in the quench column B to a temperature at which there is the complete recovery of melamine, which passes in solution or in suspension in the aqueous separation stream. The column B is fed with the aqueous stream of the bottom of column C in which the gases pro- duced in the stripping column D are separated. Condensed melamine is obtained in the aqueous liquid phase leaving column B, together with ammonia and saturation carbon dioxide. The remaining off-gas is released, rising up the column, with a certain water vapour content but substan- tially free of melamine. The column B can operate within an extremely wide pressure range, from the reaction pressure up to 1-2 bar, depending on the requirements of the subsequent system which receives the off-gas. The operating temperatures, in turn, correspond to the pressure and composition of the gaseous phase. The quenching operation is generally carried out at 20-30 bar (about 2-3 10⁶ Pascal) and at 155-165°C. This system, however, also has its disadvantages. It leads to the formation of an aqueous solution of melamine containing however an unacceptable quantity of C0₂ in the steps downstream. It must be removed before going to the purification phase to obtain a product with a low content of polycondensates (among which those generally called melam, elem and elom are prevalent) , as described for example in WO 01/36397A1, causing a removal of the C0₂ in a stripping column D. The solution/suspension of melamine is subjected therein to stripping and produces a vapour phase, containing NH₃ and C0₂, which is sent through line 7 for separation carried out in the absorption column C with recycled water coming from line 8. The product at the bottom contains stripped C0₂ and is recycled through line 5 to the quench column B. The product at the bottom of column D contains melamine and is subsequently sent for purification, indicated as E for short. It can be seen how the removal of C0₂ re- quires an additional stripping and separation step with an increase in investment costs and energy consumption. The vapour consumption alone in the stripping column D is in the order of 1 kg of vapour per kg of NH₃ and C0₂ stripped. An objective of the present invention is to provide a treatment process of off-gas which overcomes the disadvantages of the processes according to the known art in a simple and economic way. The process according to the invention is aimed at recovering the content of melamine, still in vapour phase, _.from off-gas, after it has been directly separated in the upper part of the same reactor or in the separator immediately downstream.

This objective is achieved with the process according to the present invention of which claim 1 forms the most general definition; its preferential embodiments or possible variations are defined in the dependant claims.

The characteristics and advantages of the process according to the present invention for the treatment of off-gas for the recovery of the melamine contained therein and for sending it to the subsequent sections of the plant, will appear more evident from the following description, which is illustrative and non-limiting, referring to the schemes of the figures.

Figure 1 represents the known art according to what is described above. Figure 2 illustrates an embodiment of the invention, whereas figures 3, 4 and 5 show its possible variations.

The pyrolysis reaction takes place in the reactor A, consisting of a vertical vessel into which the molten urea is fed from the bottom through line 1, together with a stream of anhydrous ammonia, preferably in gas phase, fed through line 2. The pyrolysis reaction for the transformation of urea to melamine takes place in the reactor A, operating at 380-450°C and at 80-150 bar, and provid- ing the heat necessary for sustaining the pyrolysis reaction.

A mixed phase is formed in the pyrolysis reactor A, substantially consisting of the liquid melamine formed, the off-gas produced in the reaction and the excess am o- nia injected from the bottom through line 2. This mixed phase is maintained in constant circulation due to the hydraulic pressure of the gases in formation and to the particular internal geometry of the pyrolysis reactor.

The reaction gases containing melamine in vapour phase in a ratio of 6-8% by weight are separated by the upper part of the reactor A as illustrated in the scheme of figure 2, or by a gas/liquid separator A' immediately downstream as shown in the scheme of figure 3, and are sent through line 3 to the gas washing apparatus B. The molten raw melamine, which contains a small part of dissolved gases in a ratio of 2-3% by weight of NH₃ and C0₂ with respect to the total, is sent to the dissolution and quench equipment C through line 4. Before being charged into the dissolution and quench apparatus C, this molten raw melamine can be subjected to enhancing purification treatment, as described in U.S. patent 6,252,074 and Italian patent application MI 01A001216 filed by the same Applicant. A stream of aqueous ammonia is fed into the apparatus C from the bottom through line 6, as illus- trated in the schemes of figures 2, 3, 4 and 5.

A special feature of the invention, as illustrated in the schemes of figures 2-5, consists in the fact that - with respect to the schemes of the known art as shown in figure 1 - the quench column C operates in liquid phase and in the presence of a reduced concentration of C0₂, due to its low solubility in the mixture of raw melamine fed through line 4. Under these conditions, the treatment consisting of the injection of ammonia through line 6, operating according to what is described in pat- ent application WO 001/36397A1, allows an effective elimination of the polycondensates, without having to resort to the preventive stripping of C0₂, as is required, on the contrary, in the scheme of figure 1.

A stream of purified melamine is extracted from the upper part of the apparatus C, which is sent to the crys- tallizer through line 7 together with the melamine separated in the apparatus B, discharged through line 9.

The total separation of the melamine takes place in the apparatus B, with a modest quantity of reintegration water, generally called make-up, fed from the top through line 8, together with a recirculation stream of the aqueous solution of melamine through line 9' , taken from the stream of aqueous solution of melamine separated in the column B which is discharged through line 9. The reinte- gration or make-up stream can consist of demineralized water, supplied to the plant from an external source, or it can be taken from a suitable stream inside the plant, having a composition compatible with its use in column B. According to a preferred embodiment of the inven- tion, the stream 9' is cooled with an exchanger F before being recycled to the upper part of the column B, which allows the quantity of make-up to be limited, with the same separation efficiency. In general practice, the exchanger F substantially supplies the cooling necessary in column B for separating the condensed components from the gaseous reaction effluent, and which are discharged through line 9. The quantity of make-up water necessary is normally extremely reduced, in a weight ratio with the off-gas treated in column B within the range of 0.2-1.5, preferably ranging from 0.3 to 0.5, whereas the recirculation of the melamine solution through line 9' is in a weight ratio with the off-gas treated within the range of 4-40, preferably ranging from 10 to 15. The separation of the melamine in column B is generally carried out at tem- peratures within the range of 120-180°C, preferably 155- 165°C, at pressure within the range of 2-30 bar, preferably around 25 bar.

The melamine recovered in the gaseous phase with the stream of line 9 has a very high purity, as contains nei- ther oxyaminotriazines, generally called OAT, nor poly- condensates. It consists of a suspension/solution which can be sent directly to the crystallization section.

In the embodiments illustrated in figures 4 and 5, the part of the stream of line 9, which is not recircu- lated with line 9' to the separation of column B, is fed to the bottom of the purification column C together with the ammonia solution coming with line 6. This alternative embodiment of the invention ensures a significant improvement in the constancy and qualitative homogeneity of the melamine produced. As there is no longer a significant content of C0₂ in the solution, the apparatus C can act as an effective purifier of the product from the polycondensates . EXAMPLE With reference to the scheme of figure 2, the melamine is produced with a non-catalytic pyrolysis process of molten urea in a reactor with internal recirculation operating at 390°C and 80 relative bar (about 8-10⁶ Pascal) . The reaction product consists of a liquid phase (a) and a gaseous phase (b) with the following weight compo- sitions :

Phase ( » Phase (b) melamine 92.5% 6.9% NH₃ 1.5% 44.5% C0₂ 1.0% 48.6% Urea 5.0%

The gaseous phase (b) is cooled and washed with water, which can be recycled water, in order to de- sublimate and dissolve the melamine contained therein. Two streams consequently leave the cooling and washing apparatus of phase (b) , which operates at 25 relative bar and 160°C: a liquid phase (c) and a gaseous phase (d) with the following weight compositions:

Phase (c) Phase (d) melamine 20 . 0%

NH₃ 15.0% 36.3% C0₂ 4.0% 43.5% H₂0 61.0% 20.2%

The solution of phase (c) contains pure desublimated melamine and is sent directly for crystallization.

Phase (a) mainly consists of liquid melamine which is sent for quenching where it is cooled and dissolved with water. As there is not a significant quantity of carbon dioxide present in the quench apparatus, the lat- ter is fed with a quantity of ammonia which is such as to have a phase (e) in solution with an NH₃ content of at least 13% by weight, with the following composition:

Phase (e) melamine 10.10% urea 0.55%

NH₃ 13.10%

C0₂ 0.11%

H₂0 76.14%

The operating conditions are 25 bar and 172°C and correspond to those necessary for the purification as indicated in WO 01/36397A1.

As a comparison, if the same reaction product is fed, according to the scheme of figure 1, as mixed liquid/gas phase directly to the quench apparatus B, with the other conditions remaining unvaried, the resulting liquid phase contains 3.8% by weight of C0₂ and must therefore be subjected to stripping in the column D to remove it before being sent for subsequent purification with ammonia. A comparison of the two schemes of figure 1, which illustrates the known art, and figure 2, which on the other hand, illustrates an embodiment of the invention, shows that the investments necessary are much lower for the scheme according to the invention, due to the fewer number of apparatuses operating at high pressures and re- quiring valuable materials; the energy consumption and relative costs are greatly reduced as a result of the elimination of the consumption for the stripping of C0₂ and NH₃ in the stripping column D of the scheme of figure 1, which is no longer necessary, thus avoiding a high consumption of both vapour for the stripping and cooling water for the absorption in column C of the scheme of figure 1.

Claims

1. A process for the treatment of off-gas deriving from the pyrolysis reaction of urea to produce melamine, consisting of carbon dioxide and ammonia which develop from this reaction carried out in liquid phase, at a temperature of 380-450°C and a pressure between 80 and 150 bar, comprising a preliminary separation step in which the liquid effluent of the reaction containing the reaction raw melamine is separated from the gaseous effluent of the reaction, essentially consisting of the off-gas developed from the pyrolysis, characterized in that the off-gas separated from the effluent of the reactor is treated in a separation column (B) of melamine with a stream of reintegration water, fed from above through line (8), together with a recirculation stream of the melamine solution fed with line { 9' ) , taken from the stream of the solution of melamine separated in the column B which is discharged with line (9) .

2. The process for the treatment of off-gas according to claim 1, characterized in that the preliminary separation step of the liquid effluent from the gaseous effluent is carried out in the upper part of the reactor (A) .

3. The process for the treatment of off-gas according to claim 1, characterized in that the preliminary separa- tion step of the liquid effluent from the gaseous efflu- ent is carried out in the gas/liquid separator (A' ) immediately downstream of the reactor (A) .

4. The process for the treatment of off-gas according to claim 1, characterized in that the quantity of make-up water fed with line (8) is in a weight ratio with the off-gas treated in column (B) within the range of 0.2- 1.5, whereas the recirculation of the solution of melamine through line (9') is in a weight ratio with the off- gas treated within the range of 4-40. 5. The process for the treatment of off-gas according to claim 4, characterized in that the quantity of make-up water fed with line (8) is in a weight ratio with the off-gas treated in column (B) within the range of 0.3 - 0.

5, whereas the recirculation of the solution of mela- mine through line (9' ) is in a weight ratio with the off- gas treated within the range of 10-15.

6. The process for the treatment of off-gas according to claim 1, characterized in that the recirculation stream of the melamine solution fed through line (9') is cooled with an exchanger (F) before being recycled to the upper part of the column (B) .

7. The process for the treatment of off-gas according to claim 1, characterized in that the separation of the melamine in the column (B) is carried out at temperatures within the range of 120-180°C and at pressures within the range of 2-30 bar.

8. The process for the treatment of off-gas according to claim 7, characterized in that the separation of the melamine in the column (B) is carried out at temperatures within the range of 155-165°C and at pressures around 25 bar.