CN105709675B

CN105709675B - Post-reactor of a plant for the production of melamine and related process

Info

Publication number: CN105709675B
Application number: CN201511036047.4A
Authority: CN
Inventors: R·圣图齐
Original assignee: EUROTECNICA MELAMINE
Current assignee: Proman Ltd
Priority date: 2014-12-11
Filing date: 2015-12-10
Publication date: 2020-04-10
Anticipated expiration: 2035-12-10
Also published as: CN105709675A

Abstract

The invention relates to a post-reactor for treating a crude melamine stream originating from a plant for producing melamine from urea, comprising: -a vertical cylindrical sealed chamber; -a heating jacket arranged outside the vertical cylindrical chamber, preferably with molten salts; -a plurality of plates having through holes, said plates being arranged in a vertically mutually spaced configuration one above the other to define a plurality of portions within said sealed chamber, each portion communicating with an adjacent portion through said through holes; -inlet means of the flow of crude melamine liquid inside said sealed chamber, arranged inside a head of the sealed chamber, said head being defined between an upper plate of said plurality of plates and an upper wall of the chamber; -a rear portion of said sealed chamber, said rear portion being defined between a lower plate of said plurality of plates and a lower wall of the chamber; -one or more intermediate portions between the head and the tail, which define a refining zone; -said head and possible adjacent portions, defining a CO₂A stripping zone; -NH within the post-reactor₃An inlet means for the gaseous stream disposed in any portion below the head.

Description

Post-reactor of a plant for the production of melamine and related process

The present invention relates to a post-reactor of a plant for the production of melamine from urea and to a relative process.

More particularly, the invention relates to a post-reactor and to a process for producing high-purity melamine from urea using a high-pressure process.

The process for the production of melamine from urea can be carried out at low pressure, in the presence of a catalyst, or at high pressure, without the use of a catalyst. In both cases, the reaction is carried out at a temperature of 360-420 ℃ and is highly endothermic. The heat of reaction, which is negative, is about 93000 kcal/Kmol of melamine produced.

Irrespective of the reaction pressure, the conversion of urea into melamine proceeds according to the following overall reaction mechanism:

6CO(NH₂)₂→(CN)₃(NH₂)₃+6NH₃+3CO₂(A)

urea melamine waste gas

The process for the production of melamine from urea at high pressure, the most likely kinetics of the intermediate reactions (which lead to the formation of the final melamine) are believed to be as follows:

3CO(NH₂)₂----------→3HOCN+3NH₃(1)

urea isocyanic acid

3HOCN----------→C₃N₃(OH)₃(2)

Isocyanuric acid cyanuric acid

C₃N₃(OH)₃+NH₃---------→C₃N₃(OH)₂NH₂+H₂O (3)

Cyanuric acid ammelide

C₃N₃(OH)₂NH₂+NH₃----------→C₃N₃(oH)(NH₂)₂+H₂O (4)

Ammelide ammeline

C₃N₃(oH)(NH₂)₂+NH₃----------→C₃N₃(NH₂)₃+H₂O (5)

Cyanuric diamide melamine

3CO(NH₂)₂+3H₂O----------→6NH₃+3CO₂(6)

Urea

(1) The reaction group of (1) to (6) results in the overall reaction (A).

In the final reaction product, in addition to unreacted urea (at conversions of less than 100%), some intermediate products may be present, in particular ammelide and ammeline, hereinafter referred to as OAT (oxyaminotriazine). At the same time, under the conditions of melamine synthesis, the melamine produced reacts with itself to eliminate ammonia, transforming into condensation products (polycondensates) with high molecular weight, which contaminate the melamine itself, reducing its purity and reducing the overall yield of the process.

The polycondensates present in the greatest amount in the reaction product are melam and melem, which are formed by the following overall reaction formula:

the reaction leading to the formation of polycondensates is reversible, taking place in the liquid phase, for which a low partial pressure of ammonia and an extended residence time of the melamine at temperatures (> 354 ℃) at which the melamine is liquid are advantageous. Small amounts of polycondensates are produced in the melamine synthesis conditions, but their amount is still significant in terms of the high purity of the final product currently required by the market for subsequent applications.

In the high-pressure process, molten urea at a temperature of 140 ℃ and 150 ℃ is generally mixed with gaseous NH₃Co-introduced into the synthesis reactor, the temperature of which is maintained at 360-420 ℃ by suitable heating means. The crude melamine leaving the reactor is generally subjected to a purification treatment by dissolution in water and subsequent recrystallization. In this treatment, the gaseous reaction by-products (ammonia and carbon dioxide) are regenerated, the reaction by-products (mainly polycondensates and OAT) and the unreacted urea are removed and/or separated.

In most of the industrial processes currently in existence, the melamine synthesis reaction is carried out continuously in liquid phase and at high pressure, typically in a single homogeneous reactor, usually in a cylindrical vessel (tank reactor), wherein the reactants (reagentmass) are kept under vigorous stirring by the gases generated during the reaction itself. The heat of reaction (negative) is transferred to the reactants by means of suitable heat-exchange tubes located inside the reactor, in which a heat-carrier fluid circulates at a temperature higher than the reaction temperature.

Due to the intense stirring caused by the formation of gaseous reaction by-products, the concentration of all the chemical substances in the reactor tends to take values almost equivalent to each point of the liquid substance, and the urea fed continuously mixes rapidly with the recycled reactants. The reaction product withdrawn continuously therefore has the same concentration as the concentration of the reactants circulating in the reactor. The use of large volumes of reaction may be required in systems such as those described above, in view of the kinetics involved in the reaction, which also makes the operation very expensive, while the materials forming such reactors and their processing are extremely expensive, since the reactors must withstand the high corrosive action of the reactants and the products produced under very severe conditions of temperature and pressure.

To the skilled person, in a single continuous reactor, which is completely mixed and is characterized by a uniform concentration of chemical substances at each point of the reactor, the residence time of any part of the fluid circulating therein according to a distribution curve is from zero to infinite, which most probably is very close to the mean residence time, which is defined as the ratio of the system volume and the feed rate (in urea).

Under these conditions, the larger the amount of reactants that can be discharged from the reactor before complete conversion of urea into melamine, the shorter the average residence time, i.e. the smaller the ratio of the reaction volume to the feed rate of the dosed urea.

Furthermore, due to the practical difficulty of obtaining a state of complete mixing, bypass phenomena may occur, including leakage of unreacted urea from the reactor before complete incorporation of the recycled mass.

Based on the above description, for the above mentioned hydrodynamic problems, it is necessary to use high reaction volumes to obtain high urea conversion (i.e. high overall yield) in a single continuous homogeneous reactor (tank reactor) under otherwise constant conditions. The reaction volume can be reduced only by appropriately controlling the actual residence time of the mixed reactants and eliminating or reducing the bypass phenomenon of the fed urea.

In the prior art, the method for solving the above problems mainly comprises:

apportioning the degree of urea conversion in a plurality of cascaded homogeneous reactors (multiple biogenic reactors in cassettes), or

Using a tubular reactor (plug flow reactor), or

-providing a post-reactor.

All these methods take into account the chemical thermodynamic characteristics of the reactant system, which is inclined towards the separation of solid products at low urea conversion values. As taught by US6252074 (US' 074), in order to overcome these drawbacks, a configuration may be adopted consisting of a first reaction stage consisting of a homogeneous reactor (tank reactor), characterized by a urea conversion higher than 85%, followed by one or more reaction stages, of a tubular system (plug flow type), wherein a complete conversion of the urea to the desired final value and a complete elimination of the bypass phenomenon are obtained.

However, if a high selectivity of the urea to melamine reaction is not obtained simultaneously, it is not acceptable to obtain only a high level of urea conversion.

The term (urea) conversion, denoted by "c", is the ratio of the amount Up of pyrolysed (and thus lost) urea to the amount Ua of added urea multiplied by 100. It is represented by the following formula:

on the other hand, the selectivity "s" of the pyrolysis reaction means the ratio of the obtained melamine M to the theoretical value Mt obtainable from the converted urea multiplied by 100:

the melamine yield "r" is the molar amount of melamine obtained M/mole of urea addition Ua, multiplied by 100. It is represented by the following formula: :

combining formulas 7-9, the following formula can be obtained:

from the stoichiometric reaction (a) of the overall reaction of conversion of melamine into urea, the molar ratio of the theoretical amount of melamine Mt to the amount of pyrolysed urea Up that can be obtained is:

substitution of formula 11 for formula 10 ultimately results in:

at the limit of 100% conversion and 100% selectivity, the dependency on urea feed per 100mol provides a maximum yield of 16.67mol of melamine, corresponding to 0.35Kg of melamine per Kg of urea, with a urea consumption ratio of 1/0.35, i.e. 2.86Kg of urea are required for the production of 1Kg of melamine.

As described in US3116294 (US' 294), it is not possible to reach values of 100% for a single homogeneous and continuous reactor (i.e.the type of reactor usually employed in most industrial processes), since the CO is present in the reaction system₂Is present. CO 2₂The presence of CO is inevitable₂Is one of the reaction products. According to US' 294, due to CO₂The maximum selectivity achievable in such reactors is about 95%.

It is therefore clear that even if the above-mentioned loss of conversion related to the fluid-dynamic problem is eliminated (i.e. obtaining a complete mixing of the reactants and eliminating the urea by-pass phenomenon), a total maximum yield of reaction of 0.35 x 0.95 to 0.33Kg of melamine per Kg of urea is obtained, equal to the consumption of about 3Kg of urea per Kg of melamine leaving the reactor.

For eliminating CO₂On the negative effect of selectivity, US' 294 suggests the use of a second reactor operating under the same temperature and pressure conditions as the pyrolysis reactor, by bubbling gaseous NH therein₃CO extraction from reactants₂(stripping). With this process it is possible to obtain a reaction yield of up to 99.5%, with a consumption of 2.874Kg of urea per Kg of melamine leaving the reactor (minus losses in the melamine purification and crystallization cycles).

The process claimed in US' 294 greatly increases the yield of the high-pressure synthesis process of melamine which is generally employed. However, since the claimed second reactor is similar to the main reactor (continuous tank reactor), the same hydrodynamic problems already described (homogeneity of the reactants and urea bypass) still exist, albeit to a lesser extent.

By passingA further improvement in the yield is obtained by the solution claimed in the cited patent US' 074. The patent aims at subjecting the mixture leaving the main reactor to subsequent reaction stages (i.e. distribution of residence time of plug flow type) proceeding in the tubular reactor. This approach overcomes the hydrodynamic problems still present in US' 294 and at the same time allows the reaction yield to be increased to 99.62%. Also according to US' 074, if the discharge product downstream of the system consisting of a "tank reactor/tubular reactor" combination is subjected to NH₃Stripping to completely eliminate CO still present in the reactant system₂And subjected to a further final reaction stage operating at elevated pressure, the yield can be further improved.

Both solutions described in US '074 and US' 294 allow to obtain a significant improvement in the yield of the process for the production of melamine from urea compared to processes carried out in a single homogeneous reactor. However, for both solutions there are still quite unsolved problems, such as obtaining maximum yield (especially for US '294) and such as investment costs (especially for US' 074).

Further alternatively, the proposal as a solution to these problems is described in patent IT1391372, which involves the use of a separate post-reactor or a post-reactor integrated into the reactor itself. In the post-reactor described by IT' 172, the two-phase substance comprising a liquid phase (essentially consisting of crude melamine, reaction by-products and unreacted urea) and a gaseous phase (mainly consisting of ammonia and carbon dioxide, saturated with melamine vapor) travels in the post-reactor from the inlet to the outlet according to a radial path from the centre to the edge of the post-reactor and vice versa, or according to a path from the bottom upwards and vice versa, thanks to the presence of a plurality of concentric or radial cylindrical partitions placed in the post-reactor defining a series of interconnected compartments or spaces, which are successively crossed by the two-phase mixture.

This solution, which increases the obtainable yield by increasing the conversion of the unreacted urea present in the liquid phase of the melamine, is however characterized by a high structural complexity of the post-reactor itself, said complexity being such as to ensure a complete separation of the incoming liquid stream from the outgoing liquid stream, to limit the possibility of bypassing the discharge towards the bottom of the incoming and incompletely converted urea, and to reduce the back-mixing of the chemical species present in the liquid stream flowing through the post-reactor. The structural complexity, involving high installation costs, makes such post-reactors virtually unusable in industrial plants.

The object of the present invention is to overcome the drawbacks described in the prior art by means of a post-reactor which allows to use a particularly simple structure and therefore requires a reduced economic investment to obtain a yield comparable to that described in IT' 372.

A first object of the present invention is a post-reactor for treating a crude melamine stream coming from a reactor of a plant for producing melamine from urea, comprising:

-a vertical cylindrical sealed chamber;

-a heating jacket arranged outside the vertical cylindrical chamber, preferably with molten salts;

-a plurality of plates having through holes, said plates being arranged in a vertically mutually spaced configuration one above the other to define a plurality of portions within said sealed chamber, each portion communicating with an adjacent portion through said through holes;

-inlet means of the flow of crude melamine liquid inside said sealed chamber, arranged inside a head of the sealed chamber, said head being defined between an upper plate of said plurality of plates and an upper wall of the chamber;

-a rear portion of said sealed chamber, said rear portion being defined between a lower plate of said plurality of plates and a lower wall of the chamber;

-one or more intermediate portions between the head and the tail, which define a refining zone;

-said head and possible adjacent portions, defining a CO₂A stripping zone;

-NH within the post-reactor₃An inlet means for the gaseous stream arranged in any portion below the head. In the present description, the head refers to the part of the sealed chamber of the post-reactor which is formed by the upper part of a plurality of platesThe plate, the upper wall of the sealed chamber of the post-reactor, and the portion of the cylindrical side wall between the upper plate and the upper wall, are thus delimited by a head arranged above the upper plate having through holes, i.e. the first plate with which the crude melamine liquid stream is in contact.

The term "plurality of plates" means at least two plates, while the term "plurality of portions" means at least three portions, the number of portions defined by a plate always being one greater than the number of plates.

The tail refers to the portion of the sealed chamber of the post-reactor defined by the lower plate of the plurality of plates, the lower wall of the sealed chamber of the post-reactor and the portion of the cylindrical side wall between said lower plate and the lower wall, so that said tail is arranged below the lower plate having the through-hole.

Within the sealed chamber of the post-reactor, the plates thus define a series of successive sections through which the liquid stream flows from the top downwards, while the NH flows₃The gas stream flows upward from the bottom.

The post-reactor is located downstream of the first reaction stage of the process for the production of melamine from urea, which is the pyrolysis reaction of urea, generally carried out in a single-tank reactor according to procedures known in the prior art, as described for example in US '074 and US' 294. The liquid stream of raw melamine produced in the first stage of the process for the production of melamine from urea flows into the post-reactor.

In particular, the post-reactor according to the invention comprises an outlet for the liquid melamine stream and the gaseous phase, which maintains a constant level of the liquid phase and a pressure of the gaseous phase.

Preferably, NH in the post-reactor₃The gas phase inlet means may be provided with distributor means.

The post-reactor according to the invention can also be located inside the melamine synthesis reactor, i.e. an integrated system is realized.

A heating jacket located outside the vertical cylindrical sealed chamber of the post-reactor, preferably with molten salt, has inlet and outlet means for the heating fluid and is characterized by the presence of a Δ T between the inlet temperature and the outlet temperature of the heating fluid, corresponding to the heat supplied to the post-reactor.

One or more intermediate sections between the head and the tail of the post-reactor chamber define a finishing zone in which the conversion of the pyrolysis intermediates (mainly OAT) into melamine (resulting in improved selectivity) and the conversion of the unreacted urea into melamine (resulting in improved yield) take place simultaneously.

The head of the sealed chamber of the post-reactor, alone or optionally in combination with its adjacent parts, defines the CO₂A stripping zone.

The post-reactor has 3-40 perforated plates and sections which themselves define the sealed chamber of 4-41 post-reactors, preferably 4-20 perforated plates and 5-21 sections in the sealed chamber of the post-reactor.

Another object of the present invention is a process for the production of melamine from urea, in which the liquid flow of raw melamine coming from the first reaction stage of a process for the production of melamine from urea flows continuously from top to bottom, preferably at least four, in at least three successive portions, defined by perforated plates, communicating with each other through said holes and arranged in succession with respect to the flow of the liquid flow of the vertical cylindrical sealed chamber of the post-reactor maintained at a temperature which can vary from 360-420 ℃ and at a pressure of at least 7000kPa, or in either case at a pressure equal to or slightly lower than that of the reactor, said flow of the liquid flow being in succession with NH flowing from bottom to top₃Counter-current to the gaseous stream of (a).

The first reaction stage of a process for the production of melamine from urea is the pyrolysis reaction of urea, which is generally carried out by a single tank reactor.

The liquid flow of raw melamine produced in the first stage of a process for the production of melamine from urea flows into the post-reactor.

When the reactor and the post-reactor are operated at the same pressure, the crude melamine liquid stream can be fed from the reactor to the post-reactor using overflow. The crude melamine liquid stream can be fed from the reactor to the post-reactor by level control when the reactor is operated at a higher pressure than the post-reactor.

When NH is present₃When the stream is fed to the end of the seal chamber of the post-reactor, the lower plate is brought into contact with NH₃A first plate to which a gaseous stream is contacted, said NH₃The gaseous stream is distributed uniformly in the melamine stream through continuously provided through-openings in the sealed chamber of the post-reactor.

These through holes are in fact capable of optimizing the gas phase redistribution, creating liquid-gas contact and minimizing the backflow.

Due to NH₃The feeding of the gaseous stream may still produce agitation and cause reflux, and in a preferred embodiment, the post-reactor NH₃The inlet means of the flow are arranged in the middle part adjacent to the tail part or in a suitable position in the tail part, which allows to have a smooth situation in the tail part, minimizing the reflux in the outlet melamine flow. The proper positioning of the inlet device into the tail, which helps to prevent backflow phenomena in said section and is suitable for creating a plateau, is not described in detail, as it is known to the person skilled in the art.

Such a solution at the same time allows to prevent backflow phenomena in the part of the outlet device in which the purified melamine liquid stream is provided, which phenomena can lead to the entrainment of part of the gaseous ammonia incorporated out of the liquid melamine stream, and at the same time allows to use the entire volume of the post-reactor for the contact between the gaseous ammonia stream and the liquid melamine stream.

Comprising CO formed during the pyrolysis reaction₂And NH₃In the gas phase (off-gas), which is separated from the liquid stream of melamine, in CO₂Collected in a stripping zone, said CO₂The stripping zone corresponds to the head of the post-reactor, or to the head of the post-reactor and to the adjacent part of the head, the gas phase being conveyed outwards at a flow rate that keeps the pressure inside the post-reactor constant.

A melamine liquid stream is withdrawn from the post-reactor at a flow rate which maintains a constant liquid level in the post-reactor.

The plant according to the invention is particularly advantageous not only because it allows an optimization of the yield and selectivity with a post-reactor of very simple structure, but in particular this result is particularly evident in the case of an increase in the capacity of existing plants, where the production of melamine has a higher level of intermediate products and of unreacted urea as a result of the increased rate of entry of raw materials into the reactor and the resulting reduction in the residence time of the raw materials in the reactor. Using the post-reactor and the process according to the invention, a plant with increased capacity with high yield and selectivity can be realized in a cost-effective manner starting from existing plants.

The invention is described with reference to the following schematic drawings, which are non-limiting examples of the scope of the appended claims.

FIG. 1: a schematic diagram of a post-reactor according to a first embodiment of the invention, in which a sequential order of a main reactor (not shown) in which a first stage of the pyrolysis reaction of urea takes place and a post-reactor in which a second stage takes place is provided;

FIG. 2: according to a schematic representation of the post-reactor of the first embodiment of the invention, integrated in a single element of the reactor, the reactor and the post-reactor are at the same pressure;

FIG. 3: according to a schematic of the process of the invention, the reactor and the post-reactor are at the same pressure;

FIG. 4: according to a schematic of the process of the invention, the reactor and the post-reactor are at different pressures.

According to the invention, the post-reactor is arranged downstream of the main reactor of the type commonly used in most of the existing industrial processes, which allows to solve both the hydrodynamic problems, which limit the conversion of urea, and the chemical thermodynamic problems (CO)₂Presence) negatively affects the selectivity of the reaction for converting urea into melamine and at the same time achieves a very simple structure of the post-reactor.

The post-reactor according to the invention operates under the same temperature conditions as the main reactor, whereas it can operate under the same (fig. 3) or different pressure conditions (fig. 4), which can be slightly lower but never exceed the value of the pressure in the reactor.

The post-reactor receives a liquid phase from the main reactor, consisting of crude melamine, NH, containing unreacted urea in solution₃，CO₂And impurities consisting essentially of OAT and condensation polymer. The gases (exhaust gases) formed during the pyrolysis reaction are separated in the main reactor or in a dedicated device connected to the main reactor and sent downstream for their treatment. Typically, the off-gas is then recycled to the urea production plant.

The post-reactor can also be operated continuously and preferably consists of a vertical cylindrical vessel with a height to diameter ratio of 0.5 to 20, preferably 5 to 15.

The post-reactor has a first opening (inlet) through which the liquid melamine stream leaving the main reactor is fed and a second opening (outlet), from which the liquid melamine stream leaves the post-reactor in high purity and continues to the subsequent stages of the production process once the conversion of the unreacted urea and of the OAT present in the feed stream are completed.

In the passage through the post-reactor from the inlet to the outlet, the melamine liquid stream is forced to pass continuously through successive sections (i.e. cascades) of the post-reactor, which successive sections are defined by perforated plates, through which holes communicate with each other.

As mentioned, each section of the post-reactor is in communication with only two adjacent sections.

In a first preferred embodiment, see fig. 1, the post-reactor 1 has a sealed chamber with a continuous plate 2, said continuous plate 2 having holes 3, said plate 2 forming horizontal sections 4-8 through which the liquid substance (melamine stream) entering the inlet 9 passes according to a path from top to bottom until reaching the outlet conduit 10.

The flow of the melamine stream coming from the post-reactor 10 is regulated by means of a specific level controller.

Fig. 1 schematically shows the internal structure of a first embodiment of the post-reactor according to the invention, consisting of, as an example, 5 sections (identified by the numbers 4-8) defined by perforated plates 2, arranged horizontally with respect to the vertical axis of the reactor.

According to this configuration, the crude melamine liquid stream coming from the main reactor enters the post-reactor through the nozzle 9 and the feeding means connected thereto. In particular, the crude melamine liquid stream enters the head 4, i.e. the upper part, and travels by gravity from top to bottom until it reaches the tail 8 and the bottom of the post-reactor. The liquid substance of the crude melamine stream travels from the head 4 to the lower part 5 through the holes 3 in the perforated plate 2 separating the part 4 from the part 5. Similarly, the liquid substance of the melamine stream travels up to the tail 8 and thus leaves the post-reactor through the outlet conduit 10.

In the particular configuration depicted in fig. 1, a gaseous ammonia stream is fed to the post-reactor through a nozzle 11.

The incoming liquid melamine stream encounters, during the transit path of the post-reactor, from the inlet 9 to the outlet 10, a stream of gaseous ammonia uniformly distributed in said liquid stream through the holes 3 provided in the plate 2.

The introduction of gaseous ammonia has the stripping of the CO present in the melamine liquid phase₂To reduce and maintain its concentration at a value close to 0. The gas released into the head 4 (substantially NH saturated with melamine vapour)₃And CO₂) Collected in the head 4 of the post-reactor and eventually conveyed out through the nozzle 12 to be combined with the exhaust gases from the main reactor. For this purpose, suitable means (not shown in FIG. 1) known to the person skilled in the art are used, which allow the gas to be conveyed out at a rate which keeps the pressure in the post-reactor constant. More specifically, the adjustment may be made using a single device (fig. 3) or two separate devices (fig. 4).

CO is absent from the liquid stream circulating in the post-reactor in the sections below the stripping section (sections 6 and 7)₂Allowing the process of conversion reaction of the pyrolysis intermediate (substantially OAT) to melamine with consequent improved selectivity.

Furthermore, the arrangement of the plates inside the post-reactor ensures the separation of the incoming liquid stream from the outgoing liquid stream, limiting the possibility of bypassing the incoming urea, which is discharged towards the bottom, together with this stream and not completely converted.

Backmixing of chemicals present in the liquid stream flowing through the post-reactor is also reduced due to the separation caused by the presence of the plates, which cause a continuous cascade flow between the post-reactor sections. The greater the reduction in back-mixing, the smaller the distance between the plates.

In the last stage of the pyrolysis process of urea, the conversion of OAT by a weakly exothermic reaction in melamine is ubiquitous. The heat formed after these reactions contributes to maintaining the reaction temperature in the post-reactor. However, in order to increase the conversion of unreacted urea into melamine and thus to provide the pyrolysis heat of the urea and the subsequent reactions, there is a negative balance and therefore additional heat must be provided from the outside: for this reason, the post-reactor 1 has an external heating jacket 13 for limiting the heat losses and for providing the heat needed for the conversion of the unreacted urea while keeping the temperature of the post-reactor at almost the same value as the main reactor (360-.

The jacket 13 is provided with a heating fluid inlet nozzle 14 and an outlet nozzle 15 of the fluid itself.

A second embodiment of the invention, which allows to further reduce the investment costs, is shown in figure 2, which shows a post-reactor incorporated in the main reactor for the synthesis of melamine.

In particular, FIG. 2 shows that the post-reactor at the top included in the main reactor has the same features as shown in FIG. 1, forming a single apparatus.

For clarity of description, two distinct regions are identified within the device: a "pyrolysis zone" consisting of the main reactor body and a "refining zone" II consisting of the post-reactor. In the pyrolysis zone I, which is maintained at a temperature between 360 and 420 ℃ and at a high pressure (for example, greater than 7000kPa) using suitable heating means, a flow of molten urea enters through the bottom nozzle 16, is pyrolyzed into melamine according to the reaction (a) described above, forming a two-phase mass comprising a liquid phase, consisting essentially of raw melamine, reaction by-products and unreacted urea, and a gaseous phase, consisting essentially of ammonia and carbon dioxide saturated with melamine vapor.

From pyrolysisThe two-phase material of zone I passes upwardly through intermediate conduit 18 into refining zone II. In fig. 2, the refining zone II has, for example, a post-reactor with five sections (4, 5, 6, 7, 8) defined by perforated plates 2. In the upper part or head of the post-reactor 4, which is not occupied by the two-phase material, the gas phase separated from the liquid phase is collected. The gas phase is then conveyed outside the post-reactor through an exhaust gas outlet nozzle 12, the opening of said nozzle 12 being controlled by a dedicated control device 17 capable of allowing the exit of the gradually-formed gas phase, maintaining a constant pressure both inside the post-reactor and inside the reactor itself. In zone II, the liquid phase is fed into the head 4 through the intermediate conduit 18 and from the head 4 it flows from the top through the continuations 5-7 to the bottom up to the tail 8, due to the holes 3 of the perforated plate 2. Eventually, it passes to a discharge conduit 19 which communicates with a discharge valve 20. The valve has special means 21 allowing to control the rate of discharge of the reaction products from the device, keeping the level of liquid inside the device constant. In zones 4 and 5, the liquid melamine stream coming from the pyrolysis zone I is contacted with gaseous ammonia, which is introduced uniformly into the stream itself through the nozzles 11 and through the distribution of the holes 3 of the perforated plate 2, to separate the CO present therein by means of a stripping function₂This is similar to that shown in figure 1 for the post reactor. Gaseous ammonia fed through nozzle 11 and CO stripped from a melamine stream₂Together in the head 4 of the post-reactor to be discharged through the exhaust gas outlet nozzle 12 together with the exhaust gas entering through the intermediate conduit 18 from the lower pyrolysis zone I, as shown.

The schematic representation of the reactor and the post-reactor of the process according to the invention at the same pressure or at different pressures is shown in fig. 3 and 4. In the embodiment in fig. 3, in which the reactor (R) and the post-reactor (PR) are operated at the same pressure, a liquid stream essentially consisting of crude melamine, reaction by-products and unreacted urea leaves the reactor in 22 and is fed directly to the post-reactor in 9, while gaseous ammonia is introduced into the post-reactor in 11. The main reactor (R) is fed with urea and ammonia in 16.

During the passage through the post-reactor path, the liquid melamine stream entering the post-reactor encounters, from the inlet 9 to the outlet 10, a stream of gaseous ammonia uniformly distributed in said liquid stream by means of a perforated plate (not shown in the figure). The gases released as a result of the stripping action are conveyed in 12 to the outside and combined in 24 with the exhaust gases exiting from the main reactor (R) in 23.

The leakage of gas from the post-reactor is controlled by a dedicated control device 17 capable of allowing the leakage of the gas phase to develop, maintaining a constant pressure inside the post-reactor and the reactor operating at the same pressure.

The level of liquid entering the post-reactor is kept constant by regulating 10 the flow of the melamine stream leaving the post-reactor by means of a dedicated level controller 21 which allows to control the discharge rate of the reaction products from the post-reactor.

In the embodiment of fig. 4, in which the reactor (R) and the post-reactor (PR) are operated at different pressures, the liquid stream leaving the reactor in 22, consisting essentially of crude melamine, reaction by-products and unreacted urea, is fed to the post-reactor in 9, and the flow of the melamine stream leaving the reactor in 22 is regulated by a dedicated level controller 25, which allows to control the discharge rate of the reaction products from the reactor, keeping the liquid level in the reactor constant.

The gaseous ammonia stream is introduced into the post-reactor in 11. The main reactor (R) is fed with urea and ammonia at 16.

During its path through the post-reactor, the liquid melamine stream entering the post-reactor, from the inlet 9 to the outlet 10, encounters a gaseous ammonia stream uniformly distributed in said liquid stream by means of a perforated plate (not shown in the figure). The gases released as a result of the stripping action are conveyed to the outside in 12 and the gas outlet is controlled by a dedicated control device 17 which allows the gas phase to escape, keeping the pressure in the post-reactor constant.

The exhaust gases leaving the main reactor (R) in 23 are sent to the outside and the gas outlet from the reactor is controlled by a dedicated control device 26 allowing gas phase leakage, keeping the pressure inside the reactor constant.

The method and the device according to the invention have many advantages, in particular:

-CO₂removal from the crude melamine stream at various points in the post-reactor is uniform, resulting in increased selectivity of the melamine synthesis process;

-completely eliminating the by-pass phenomenon of the unreacted urea entering the post-reactor, ensuring its complete conversion into melamine, thus increasing the conversion of urea as raw material feed;

the investment costs are greatly reduced in a particularly simple plant due to the lack of OAT to produce high-purity melamine;

the apparatus is particularly simple compared to other post-reactors present in the prior art;

regardless of the type of construction used, the device can be very easily integrated even onto existing equipment.

The following embodiments are provided for illustrative purposes only of the present invention and should not be construed as limiting the scope of protection defined by the appended claims.

Example 1

In a plant for the production of 30,000t/a melamine 12.345Kg/h of molten urea and 615Kg/h of ammonia were used as first reaction stage feed to a tank reactor operated at a pressure of 8,000kPa and a temperature of 385 ℃.

8,473Kg/h of off-gas was withdrawn from the reactor head and consisted of:

3,880Kg/h NH₃

4,263Kg/h CO₂

330Kg/h melamine vapor

Sent to the recovery of the melamine contained therein, and 4,489Kg/h of crude melamine in the liquid phase, having the composition:

3,672Kg/h of melamine (100%)/h

617Kg/h of unreacted urea

OAT 23Kg/h

80Kg/h of polycondensate

NH₃60Kg/h (in solution)

CO₂37Kg/h (in solution)

Since 80Kg/h of polycondensate are completely converted into melamine and are thus recovered in the purification section placed downstream of the reactor, the melamine yield (also including that leaked with the gas phase, totally recovered in the other sections of the plant) is, at the outlet from the first reaction stage:

3,672+330+800.331Kg/Kg urea feed

12,345

I.e. 94% of theory, not counting the melamine recovery and purification cycle losses.

The liquid phase is sent to the post reactor of the embodiment shown in fig. 1 and operated at the same pressure and temperature as the first reaction stage. The post-reactor had a diameter of 0.9m and a total height of 11m, and the liquid phase occupied a total volume of 6.04m³And has 17 perforated plates therein. In this way, 18 annular cylindrical sectors are defined, at the bottom of which gaseous ammonia is introduced and uniformly distributed and superheated, in a total amount of 1,100 Kg/h.

4,004Kg/h of purified melamine were collected from the bottom discharge of the post-reactor, with a composition of:

3863Kg/h of melamine (100 percent)

Absence of unreacted urea

OAT 4Kg/h

60Kg/h of polycondensate

NH₃77Kg/h (in solution)

CO₂(in solution) is absent

1585Kg/h of off-gas which is almost entirely composed of ammonia and contains 62Kg/h of melamine vapor are discharged from the post-reactor head.

The off-gas of the post-reactor is combined with the off-gas of the tank reactor and sent to a treatment for the total recovery of the melamine present therein.

In the purification section downstream of the reactor, considering that 60Kg of polycondensate is completely converted and thus recovered as melamine, equal to 64Kg of melamine, the melamine yield is thus changed from 0.331Kg/Kg (94% of theory) at the outlet of the first reaction stage to:

(3863+330+62+64)/12,345 to 0.3498 Kg/Kg of urea feed, corresponding to 99.94% of theoretical values, still not counting the melamine recovery and purification cycle losses.

Claims

1. Post-reactor for treating a crude melamine stream coming from a plant for the production of melamine from urea, comprising:

-a vertical cylindrical sealed chamber;

-a heating jacket arranged outside the vertical cylindrical chamber;

-a plurality of plates with through holes arranged in a vertically mutually spaced configuration one above the other to define a plurality of portions within the sealed chamber, each portion communicating with an adjacent portion through the through holes;

-said head and optionally adjacent parts, defining a CO₂A stripping zone;

-NH within the post-reactor₃An inlet means for the gaseous stream, arranged in any part below the head,

wherein a horizontal controller is disposed corresponding to the head of the post-reactor, and

wherein the conversion reaction of the pyrolysis intermediate product with increased selectivity into melamine and the conversion reaction of the unreacted urea with increased yield into melamine are carried out simultaneously in the refining zone.

2. The post reactor of claim 1 wherein the heating jacket utilizes molten salt.

3. The post-reactor according to claim 1, wherein the head portion is a portion of the sealed chamber of the post-reactor defined by an upper plate of the plurality of plates, an upper wall of the sealed chamber of the post-reactor, and a portion of the cylindrical sidewall included between the upper plate and the upper wall, the portion being disposed above the upper plate having the through-hole, and the tail portion is defined by a lower plate of the plurality of plates, a lower wall of the sealed chamber of the post-reactor, and a portion of the cylindrical sidewall included between the lower plate and the lower wall, the portion being disposed below the lower plate having the through-hole.

4. The post-reactor according to any preceding claim, wherein NH within the post-reactor₃The inlet means for the gaseous stream is arranged in the middle part adjacent to the tail section or in the tail section.

5. Post-reactor according to any of claims 1-3, comprising an outlet device for the liquid melamine stream and a reactor containing CO₂And NH₃Of the gas phase formed during the pyrolysis reaction.

6. The post-reactor of any of claims 1-3, wherein NH is in the post-reactor₃An inlet means for the gaseous stream is arranged in the tail section, before the distributor means.

7. The post-reactor according to any of claims 1-3, wherein the perforated plates are 3-40 and define 4-41 sections of the sealed chamber of the post-reactor.

8. The post-reactor according to any of claims 1-3, wherein the perforated plates are 4-20 and define 5-21 sections of the sealed chamber of the post-reactor.

9. Plant for the production of melamine from urea, comprising a post-reactor for treating a crude melamine stream according to any one of the preceding claims.

10. The plant according to claim 9, wherein the post-reactor is arranged downstream of the reactor for synthesizing melamine from urea or it is inserted into the reactor for synthesizing melamine in an integrated system.

11. Process for the production of melamine from urea, in which a liquid flow of raw melamine from a first reaction stage of a process for the production of melamine from urea continues from top to bottom in at least three successive sections defined by perforated plates, communicating with each other through holes in said perforated plates and arranged in succession with respect to the flow of the liquid flow of the vertical cylindrical chamber of a post-reactor maintained at a temperature variable at 360-₃The gaseous streams are counter-current.

12. The process of claim 11, wherein the separated liquid stream comprising CO from the crude melamine comprises₂And NH₃Is collected in the upper part or head space of the post-reactor and is conveyed out at a flow rate which keeps the pressure in the post-reactor constant.

13. Process according to any one of claims 11-12, wherein the melamine liquid stream is discharged from the post-reactor at a flow rate that keeps the liquid level in the post-reactor constant.