CN106349174B

CN106349174B - Production of melamine and separation and recovery of high purity CO2And NH3Method (2)

Info

Publication number: CN106349174B
Application number: CN201610450990.8A
Authority: CN
Inventors: 马蒂亚·波格托; 乔治白·梅莱利; 罗伯托·桑图齐
Original assignee: Ouji Melamine Co ltd
Current assignee: Proman Ltd
Priority date: 2015-07-17
Filing date: 2016-06-21
Publication date: 2021-10-15
Anticipated expiration: 2036-06-21
Also published as: RU2016119933A3; BR102016016006A2; BR102016016006B1; NL2017130B1; NL2017130A; RU2016119933A; CN113234035A; DE102016112944A1; ITUB20152258A1; CN106349174A; RU2713178C2

Abstract

The invention relates to a process for the production of melamine from urea with separation and recovery of high purity CO₂And NH₃The method of (1).

Description

Production of melamine and separation and recovery of high purity CO2And NH3Method (2)

The invention relates to a method for producing melamine and separating and recovering high-purity CO₂And NH₃The method of (1).

In particular, the present invention relates to a process for the production of high-purity melamine according to a synthesis process based on high-pressure pyrolysis of urea, with simultaneous separation and complete recovery of the high-purity NH produced at the different stages of the process₃And CO₂。

In industrial practice, the production of melamine from urea is carried out at high temperatures, at NH₃In the presence of a suitable catalyst at low pressure and in the absence of a catalyst at high pressure. In both cases, the conversion of urea into melamine is carried out according to the following overall reaction:

6NH₂CONH₂→(CN)₃(NH₂)₃+6NH₃+3CO₂ (1)

urea melamine waste gas

In addition to the main product (melamine), NH is also formed₃And CO₂And therewith lesser but non-negligible amounts of by-products known to the person skilled in the art, among which are intermediate products of oxy-amino-triazine (OAT) and polycondensates, melamine pyrolysis and subsequent reactions, respectively.

From the stoichiometric ratio of the total reaction (1) it can be concluded that 50% of the moles of urea fed to the melamine synthesis process are theoretically converted into NH₃And CO₂. Thus, a certain amount of CO is present in the effluent leaving the synthesis reactor₂And NH₃(equivalent to at least 1.86kg/1kg of melamine produced), and melamine.

Due to NH₃Has the problem of recovering this reaction by-product in the overall melamine production process.

For generatingOne of the most widely used processes for the production of melamine is based on the high-pressure pyrolysis of urea, with the recovery and purification of melamine in aqueous ammonia solution. In this process, the composition of the effluent leaving the synthesis reactor is a liquid phase (which contains most of the melamine produced) and a gaseous phase (which contains most of the NH)₃And CO₂By-products).

The entire effluent (gases and liquids) leaving the reactor is combined with the CO-containing effluent from the downstream zone in a specific apparatus hereinafter referred to as "Quench" (Quench)₂Is contacted with the aqueous ammonia stream. The liquid stream containing all the melamine produced leaves the bottom of the Quench (Quench), which removes the CO present₂Thereafter, treatment with ammonia removes the condensation polymer, a by-product of the reaction, and maintains OAT in solution. From the above purified solution, melamine is obtained in high purity and with a titre higher than 99.8% by crystallization and subsequent drying of the solid separated.

NH₃And CO₂Is extracted from the top of the Quench (Quench). This gaseous stream is generally recycled to a urea synthesis plant (urea plant) for recovering the NH contained therein₃And CO₂Thus making them immediately reusable by their (re) conversion to urea. An indispensable condition for ensuring the economic convenience of recovery and reuse of the aforementioned gases is that the melamine plant is arranged in the immediate vicinity of the urea plant.

However, the presence of water (typically above 20 wt%) in the wet off-gas stream leaving the Quench (Quench) is such that the NH is present₃And CO₂The reuse in the urea plant is laborious, which results above all in an increase in the total cost of the melamine production when the urea plant is of a smaller scale than the melamine plant.

The above-described prior art melamine production process is illustrated hereinafter with reference to the accompanying figures 1 and 2, which show a simplified block diagram of the main operating phases of the process.

In FIG. 1, a stream of molten urea 101 from an adjacent urea plant is mixed with anhydrous NH₃Together at 135-1A temperature of 40 ℃ was supplied to the pyrolysis reactor (reactor). The reactor is operated continuously and is equipped with a heating system which provides the reactant mixture with the thermal energy necessary for the synthesis of melamine, maintaining the reaction environment at a temperature of 360-420 ℃. The reaction pressure is maintained at a value higher than 70 bar. The reactor is a single-stage reactor and the reactant mixture is maintained at a high circulation by the gases formed during the pyrolysis of urea.

Under the operating conditions described above, the pyrolysis reaction produces a two-phase effluent consisting of a liquid phase containing the crude molten melamine, which comprises a reduced amount of unconverted urea, and other reaction by-products, among which the maximum amounts are the polycondensates already mentioned and oxy-amino-triazines (OAT), and a gaseous phase, the essential composition of which is NH₃And CO₂). The two-phase effluent 102 is sent to a Quench (Quench) located immediately downstream of the reactor to recover the melamine. In this Quench (Quench), the two-phase flow 102 is accompanied by a CO-containing stream from other zones of the apparatus₂And its temperature is reduced to about 160 c. Under these conditions, all the melamine, unreacted urea and impurities formed in the process are fed to the aqueous solution or kept in suspension. The aqueous stream 104 leaving the Quench is directed to the subsequent stages of the process, while the wet exhaust gas 105 (at a pressure of about 25bar, essentially consisting of moisture-saturated NH₃And CO₂Stream) is recycled to an adjacent urea plant.

In addition to the crude melamine, the aqueous stream 104 leaving the Quench contains dissolved, in the same amount, NH₃And CO₂Which is stripped by means of steam (CO) in a subsequent step of the process₂Stripper zone). CO 2₂Is necessary to help achieve high purity of the melamine in the subsequent purification steps.

Gaseous stream 106 leaves the CO₂The top of the stripper, which is sent to the absorption column (NH) together with the recovery streams 113 and 124₃-CO₂An absorber). NH (NH)₃-CO₂The absorber allows the above streams to be all recovered in a single aqueous stream 103 (which is recycled to Quench).

Is free of CO₂(and absence of NH)₃) With 6-12% by weight of melamine in solution, leaves the CO with a non-negligible amount of OAT and polycondensate₂-the bottom of the stripper. Due to their low solubility, the polycondensates have to be removed before the aqueous stream containing melamine is subjected to a subsequent crystallization stage.

The polycondensate removal is carried out in a purification zone where stream 107 is heated to about 170 ℃ in the presence of added ammonia (stream 116) until a concentration of 12-15% by weight is reached. Under the above conditions, the polycondensate is almost completely converted into melamine.

An aqueous ammonia stream 108 of melamine (substantially free of condensation polymer) leaves the purification zone and is fed to the crystallization zone, where the temperature of stream 108 is reduced to 40-50 ℃, thus allowing most of the melamine to crystallize.

NH in stream 108₃The presence of (a) keeps the OAT in solution, causing precipitation of high purity melamine crystals. The flow 109 containing the melamine crystals in suspension is sent to a solid/liquid separation section in which a wet cake 110 of melamine and a flow 111 of crystallization mother liquor are separated. Stream 111 is saturated with ammonia and contains all OAT, both those leaving the reactor and those formed by hydrolysis of melamine in the subsequent steps of the production process. The formation of OAT is particularly favored in the process steps carried out in the presence of water at elevated temperatures.

The wet cake 110 is finally dried in the drying zone by means of hot air, obtaining a high purity product with a titre of melamine higher than 99.8%. Exhaust air stream 112 for drying is passed over the NH₃Water scrubbing in a scrubber to purify NH present therein₃. The air thus purified is discharged into the environment.

By NH₃-CO₂The absorber recycles an aqueous purge stream 113 containing the absorbed ammonia to the process.

Stream 111 (consisting of the crystallization mother liquor containing melamine in a concentration of the order of 0.8-1.0% by weight) is sent to the ammonia separation zone. In this zone, the following products are separated by distillationAn object: anhydrous NH₃Stream 115 with additional anhydrous NH added₃Recycling to the purification zone; containing CO possibly present in the mother liquor 111₂The purge stream 123; no NH₃Aqueous stream 117 (which contains essentially only melamine and OAT).

Stream 117 is cooled to a temperature of 60-70 ℃ and by adding CO₂Neutralized to pH 7 to allow precipitation of OAT. Stream 118, which contains OAT in colloidal suspension, is subjected to a process for the separation of OAT on candle ceramics by ultrafiltration according to what is described in patent application WO 01/46159. In the OAT ultrafiltration zone, OAT is separated as retentate (stream 119) from permeate corresponding stream 120. The latter stream is practically free of OAT and is recycled to the Quench (Quench) to recover the residual melamine contained therein.

The retentate stream 119 (containing almost all OAT) is subjected to a thermal treatment in the decomposer-stripper zone, in which OAT, residual melamine and all other organic substances present therein are almost completely converted into NH by hydrolysis₃And CO₂And forming:

-a purified aqueous stream (121) that can be discharged into the environment:

NH enrichment₃And CO₂Is recycled to NH (122)₃-CO₂An absorber.

The above process, even if currently applied to numerous chemical plants, is characterized by high steam consumption and by a conversion of urea to melamine which is much lower than the stoichiometric ratio. Furthermore, the process is affected by the gaseous reaction product NH₃And CO₂Due to the presence of a considerable amount of water in the stream 105 of wet exhaust gases.

In fact, the presence of water in the wet off-gases recycled to the adjacent urea plants results in an increase in the urea production costs, which are obviously maintained by the management of the melamine production plant.

This problem has been faced in the past and attempts have been made to find a way of separating NH from a mixture of off-gases and by-products from a melamine synthesis process₃And CO₂For obtaining a purified gasWhich can be recycled to the urea plant, can be stored and/or released into the environment (CO)₂)。

With the prior art (which has also described CO)₂Distillation to obtain high purity CO₂Contrary to what is known about the gaseous stream (IT1387832, the process scheme of which is shown in figure 2), the applicant has now surprisingly found that the particular operating conditions allow to obtain a process for the preparation of melamine from urea, with separation and recovery of high purity CO₂And NH₃It is improved and optimized, operates with reduced amounts of water, and has surprising results in terms of energy consumption.

The object of the present invention is therefore to overcome the drawbacks disclosed in the prior art at present.

One object of the present invention relates in fact to a process for the production of melamine from urea, with separation and recovery of high purity CO₂And NH₃The method comprises the following operation steps:

a) at NH₃In the presence of a stream of molten urea, a melamine is synthesized in a main reactor by pyrolysis, obtaining a stream of molten raw melamine and a stream containing NH₃And CO₂The exhaust gas stream of (a);

b) optionally, the crude melamine stream leaving the main reactor is treated in a post-reactor in the presence of gaseous ammonia in order to complete the pyrolysis reaction of urea, obtaining a product substantially free of unconverted urea and CO₂And a crude melamine stream of molten, and consisting essentially of NH₃Composed of and containing all CO formed in the reaction₂The exhaust gas stream of (a);

c) distilling the offgas stream from step a), and if present the offgas stream from step b), in the presence of an aqueous extraction solvent to obtain high-purity CO₂And a gaseous stream containing CO₂An aqueous ammonia stream of (a);

d) stripping the aqueous ammonia stream from step c) to remove CO still present₂；

e) Obtaining an aqueous ammonia solution containing dissolved melamine, polycondensate impurities and OAT by contacting the crude molten melamine stream obtained from step a) and, when present, the crude molten melamine stream from step b) with the aqueous ammonia stream from step c), cooling and dissolving the crude molten melamine stream obtained from step a) and, when present, the crude molten melamine stream from step b);

f) by reaction at NH₃Maintaining the conversion of the polycondensate into melamine in the presence of an aqueous ammonia solution of melamine coming from step e);

g) crystallizing the melamine present in the aqueous solution purified leaving step f) and separating the melamine crystals from said solution, obtaining a melamine wet cake and a cake containing only OAT, dissolved melamine residues and NH₃The crystallization mother liquor stream of (a);

h) distilling off the mother liquor of step g) to obtain high-purity NH₃Streams and an ammonia-free aqueous stream containing OAT and melamine residues which are subsequently recovered and/or removed in a dedicated downstream processing zone of the process, for example in a decomposer/stripper zone;

i) drying the melamine wet cake coming from step g) with hot air obtaining dry melamine crystals with high purity and an effluent air stream containing ammonia;

the process requires that step c) for the extractive distillation of the offgas stream from step a) and, if present, of the offgas stream from step b) is carried out in the presence of an aqueous extraction solvent at a pressure of from 55 to 250bar, preferably from 70 to 170 bar.

The aqueous extraction solvent is preferably water or an ammonia-free aqueous solution, and even more preferably a recycled ammonia-free aqueous solution.

The aqueous extraction solvent is present in an amount of 3 to 6, preferably 4 to 5, tonnes of solvent per 1 tonne of melamine produced.

Furthermore, the extractive distillation step c) is preferably carried out in a separation unit (distillation column or Distex) which is produced from a material suitable for withstanding the required operating conditions, such as, for example, an alloy whose essential composition is a combination of nickel, chromium and molybdenum, or a metal such as zirconium, titanium or a suitable combination of such materials.

The process of the invention therefore consists of a process for the production of melamine which is enhanced and optimized with respect to the prior art and which, in addition to allowing the production of high-purity melamine with considerable increase and decrease in yield and cost, also allows

-simultaneous separation and recovery of high purity CO produced/used in the production cycle and purification of melamine₂And NH₃；

-reducing the total amount of water used and completely recycling it in the melamine production process and thus eliminating liquid discharge;

-CO₂the extractive distillation of (a) is carried out at high pressure, wherein the use of said pressure allows direct distillation of the off-gases leaving the reactor and possibly the post-reactor of the high-pressure melamine synthesis process, without intermediate treatments to bring them to the operating pressure of the extraction column as described in the processes of the prior art (which operates at a pressure lower than 50 bar);

-CO₂has reduced energy consumption even when operated at high pressure, which can be halved even when operated at low pressure.

Furthermore, a fundamental aspect of the process of the invention is the design to treat the gas containing CO₂And NH₃To obtain high purity CO₂And NH₃Wherein the CO is of high purity₂Is separated in a first step and the NH is of high purity₃The streams are separated in a subsequent step.

In addition, the CO-containing substances produced/used in the production and purification cycles of melamine₂And NH₃The offgas streams of (a) which are treated in accordance with the process of the invention can be produced in the same apparatus in which the extractive distillation column is present, or they can originate from a single or a plurality of combinations of other melamine production apparatuses, generally referred to as high-pressure or low-pressure apparatuses, which are not equipped with an extraction column or Distex.

Another object of the present invention relates to the separation and recovery of high purity CO from the off-gases coming from plants for the production of melamine₂And NH₃In particular, the system comprises the following elements: extraction of CO₂A first distillation column suitable for operating with an aqueous extraction solvent, such as water; second separationA column adapted to recover from the top the CO present in the stream leaving the bottom of the first extractive distillation column₂And NH₃(ii) a A third separation column adapted to recover from the top the NH present in the stream leaving the top of the second column₃Said element being suitable for carrying out the separation and recovery of high purity CO from said flue gas₂And NH₃The step of flowing the stream of (a).

The system can be used in the context of retrofitting an existing plant for the production of melamine and/or for integrating an existing plant for the production of melamine with a plant for the production of urea.

The system object of the invention can be used, for example, for separating and recovering high purity CO produced in a process for the production of melamine from urea₂And NH₃Comprising the aforementioned operating steps a) -i).

The method proposed by the present invention is simple to use, but is significantly modified with respect to the methods known from the prior art.

Drawings

Figure 1 is a diagram of a process for the production of melamine according to the prior art.

Figure 2 is a diagram of a process for the production of melamine according to the prior art.

Figure 3 is a diagram of the process for the synthesis of melamine from urea according to the invention.

The process for the synthesis of melamine from urea according to the invention and the advantages deriving therefrom can be more clearly understood from the following description, which refers to figure 3, which shows a simplified version of the main operating steps of a preferred embodiment. The description and the related method schemes should be considered as examples of the present invention and in no way represent limitations to the scope of the appended claims.

It should be observed first of all that the characteristic feature of the invention that the urea plant is no longer restricted from being adjacent to the melamine production unit, since the process of the invention no longer produces a product containing NH₃And CO₂Any exhaust gas mixture (wet exhaust gas or anhydrous exhaust gas) of (a), solely due to such proximity, contains NH₃And CO₂Is economically feasible to recover and deliver to the urea plant.In addition, the specific pressure conditions in the extractive distillation step of the process of the invention allow an optimized process to be obtained which operates with reduced amounts of water and which has surprising results in terms of energy consumption.

These particular characteristics make the process of the invention unique with respect to all the high-pressure technologies used and/or proposed hitherto for the production of melamine from urea.

The molten urea stream 301 is fed to the first reactor (main reactor) where the high pressure pyrolysis process is carried out under conventional conditions, i.e. in the presence of anhydrous ammonia (stream 319), at a temperature of 360-. In contrast to the prior art melamine production process shown in fig. 2, the two components of the effluent resulting from the pyrolysis reaction (i.e., from NH)₃And CO₂The combined anhydrous off-gas stream 302 and the liquid stream 303 consisting of molten raw melamine) are extracted separately from the main reactor or from a suitable downstream-located phase separator, not shown in fig. 3, and treated separately.

In the embodiment of the invention shown in fig. 3, stream 303 is sent to the second reaction step (post-reactor), in which the conversion of the unreacted urea still present in stream 303 leaving the main reactor is completed, operating under the same conditions of temperature and pressure as the main reactor and in the presence of a continuous flow of anhydrous ammonia (stream 304), with the completion of the intermediate reactions leading to the formation of melamine. At the same time, the conditions present in this post-reactor, in which the partial pressure of ammonia is much higher than in the main reactor, also lead to a reduction in polycondensates and OAT in the molten raw melamine produced in the main reactor.

Another advantageous effect of the presence of the post-reactor is the reduction of the CO present in the liquid phase (i.e. in the flow of molten raw melamine)₂Also due to continuously entering NH₃The stripping effect of the gaseous stream. However, step b) of the process is optional. The process of the invention allows in fact to produce melamine with high purity and to recover the NH separately without the aid of a post-reactor₃And CO₂。

Two separate streams also leave the post-reactor, neitherWater content: exhaust gas stream 305 (consisting essentially of NH)₃Composition) and a liquid flow 306 (consisting of molten raw melamine, almost free of urea and CO₂Containing a very limited amount of high boiling by-products such as OAT and condensation polymers).

The offgas stream 305 leaving the post reactor or from a suitable separator located downstream, not shown in fig. 3, is combined with the offgas stream 302 leaving the main reactor for treatment in a specific extractive distillation column (hereinafter "Distex") as described below. On the other hand, the crude melamine stream 306 leaving the post-reactor is sent to the melamine recovery step by dissolution in an aqueous ammonia solution. This step is carried out in a specific apparatus (hereinafter referred to as "Quench") in which the crude melamine melt stream 306 is at a temperature of 110-₂Aqueous ammonia stream (containing a significant concentration of NH) consisting of the aqueous ammonia stream of the stripper₃But almost no CO₂) (stream 315 a). Under these conditions, the initial destruction of the polycondensate is carried out in a Quench apparatus. The melamine solution leaving the Quench is then sent to a purification zone (stream 330) for the complete destruction of the polycondensate.

The purification of the aqueous melamine-containing solution 330 leaving the Quench is carried out under pressure and concentration conditions known in the art, but at a lower temperature. The aqueous ammonia solution 307 of purified melamine leaving the purification zone is then fed to a crystallization zone where, similarly to the process of the prior art of figure 2, the temperature of the solution is reduced to 40-50 c, obtaining the separation of the very pure melamine crystals. The flow 308 containing the melamine crystals in suspension is sent to a solid/liquid separation section and separates a wet cake 309 of melamine and an aqueous flow 310 consisting of crystallization mother liquor. High purity anhydrous crystalline melamine (titre higher than 99.8% by weight) is recovered from the drying zone.

The crystallization mother liquor stream 310 containing ammonia is partly recycled directly to the Quench (stream 310a) and partly fed to the ammonia separation zone (stream 310b), its main function being to recover all the high purity NH₃。NH₃The separation in this zone is carried out by means of distillation, similar to the melamine production process of the prior art。

The gaseous stream 312, which originates from the combination of two anhydrous waste gas streams 302 and 305, respectively coming from the main and post-reactor and saturated with melamine vapor, is subjected to extractive distillation in a specific distillation column (Distex) in the presence of an aqueous extraction solvent selected from water and ammonia-free aqueous solutions, which travels in countercurrent from the ascending gaseous stream in the column from the top downwards.

The aqueous extraction solvent is preferably a recycled ammonia-free aqueous solution (streams 313 and 325) from the decomposer/stripper zone from a zone downstream of the apparatus.

Will consist of high purity CO₂Composed of gaseous stream 314 (NH)₃Less than 10ppm) is withdrawn from the top of the Distex at the operating pressure of the column, while an aqueous ammonia stream 315 (having a low content of CO)₂Containing the melamine present in the stream 312, which is thus recovered) is discharged from the bottom of the Distex. Sending the entire aqueous stream 315 leaving Distex to CO₂Stripping column for complete removal of CO from the bottom solution₂And from there the bottom solution is sent to Quench as solvent for the recovery of the melamine leaving the post-reactor. Will be derived from NH₃Scrubber stream 316, which contains ammonia, which has been formed as a result of the drying of the wet melamine cake, is fed to Quench as an aqueous solvent. Pure NH₃Stream 320 is also fed to Quench to obtain the NH required for the purge step₃And (4) concentration. Thus, the ammonia water stream 307 flowing downstream from the purification zone contains all of the NH produced by the process₃I.e. it originates from the urea pyrolysis (see reaction (1)) and it originates from the hydrolysis reaction (in particular from the decomposer/stripper zone described below). The total amount of ammonia contained in stream 310b is separated in high purity (317) by distillation in an ammonia separation zone.

An ammonia-free aqueous stream 322 containing OAT and melamine residues dissolved in solution is also separated from the ammonia separation zone.

The process object of the invention therefore also allows to fully recover all the CO produced in the melamine production process₂After being in high purity, the total ammonia produced in the melamine production zone is completely recovered(at high purity). Pure NH in liquid state forming stream 317₃Partially used in different points of the production process, such as, for example, the main reactor (stream 319), the post-reactor (stream 304) and the purification zone (stream 320), while excess NH is present₃(which represents the most relevant portion) is passed to a plant limit of the plant (stream 321) and may be transferred to a urea plant or recovered as a high commercial value by-product, which may be used as raw material in other industrial processes.

The ammonia-free aqueous stream 322 discharged as the bottom product of the ammonia separation zone is sent to a decomposer/stripper zone where OAT and residual melamine are decomposed to ammonia and carbon dioxide by means of pyrolysis, in addition to other organic molecules present. Reacting NH₃And CO₂An almost pure water stream is recovered from the bottom by stripping separation in the same zone as the overhead and recycled (stream 318) to Distex and recycled to Distex as solvent for extractive distillation (325) or sent to a scrubber to purge the vent air from the dryer (stream 313) or drying zone.

This drying step may be carried out in practice with hot air or by other means known to the person skilled in the art.

Minor amounts of CO that may be present in stream 310 (crystallization mother liquor)₂Is carried out in an ammonia separation zone with CO₂As known in the art, and sent (purge stream 329) to the Distex to recover them. All CO formed in the production cycle₂It is then separated off in high purity as overhead product in Distex (by pyrolysis and hydrolysis) and is available in the battery limits at a preferred pressure of 70 to 170bar (stream 314). Similar to NH₃，CO₂It can also be transferred to a urea plant or recycled as raw material for use in other industrial processes.

The process for the production of melamine from urea, with separation recovery of the high purity by-products ammonia and carbon dioxide, according to the embodiments illustrated herein, comprises the following basic operating steps:

-subjecting the off-gas stream leaving the melamine synthesis reactor to extractionDistillation in a distillation column using water as extractive solvent or aqueous solution, and recovering an aqueous stream preferably in an amount of 3-6, preferably 4-5, tonnes solvent/tonne melamine, and operating at a pressure of 55-250bar, preferably 70-170bar, to obtain high purity CO₂A gaseous flow and an aqueous ammonia flow for recovering and purifying the melamine produced in the reaction and, in particular, reducing the energy consumption.

The aqueous ammonia stream formed by the solid/liquid separation step downstream of the crystallization is then distilled, obtaining a high-purity ammonia stream and an ammonia-free aqueous stream containing residual melamine, OAT and other minor organic impurities (sent to recovery or to removal).

In a preferred embodiment of the process according to the invention, the NH is recovered in high purity₃And CO₂The streams are suitably fed separately to the urea plant, with particular advantages over feed streams containing these substances in the exhaust stream, but without separation.

Alternatively, one or both of the streams are suitable for use as raw materials in other industrial processes.

The invention therefore relates to a new cycle for obtaining high-purity NH by urea pyrolysis in a melamine production process₃And CO₂The recycling comprising:

a reaction zone consisting of a main reactor and a post-reactor, in which pyrolysis takes place and urea is highly converted to melamine, with separation of NH₃、CO₂And melamine vapor;

-an extractive distillation column (Distex) connected to said reaction zone and receiving said off-gas stream from this reaction zone, said Distex preferably using an ammonia-free aqueous solution from the melamine recovery and purification cycle as extraction solvent, separating CO of high purity₂Top product of composition and containing NH₃And an aqueous bottom stream of residual melamine; the high purity CO₂The stream is distilled at the preferred pressure of 70-170bar (i.e. at the same pressure as it leaves the reactor) without any intermediate treatment; stripping zone for stripping from an ammonia-free aqueous solutionRemoval of residual CO from liquids₂；

-a Quench zone and a purge zone for the possible addition of NH by means of a feed from said production cycle₃To recover and purify melamine;

-a crystalline precipitation zone of purified melamine connected to a separation and drying zone of the product;

-an ammonia separation zone, producing high purity ammonia by distillation from the crystallization mother liquor, for the internal requirement of the cycle, excess NH₃Passed to battery limits in high purity for transfer to a urea plant or for marketing;

a zone for treating the ammonia-free mother liquor leaving the ammonia separation zone, thereby carrying out a suitable recovery of the melamine contained therein, and/or thermal destruction of all the other organic substances present, and total recovery of the NH formed₃And CO₂(ii) a The aqueous stream from this series of operations forms the majority of the extraction solvent requirement of Distex.

The object of the present invention, a process for the production of melamine with high purity by high-pressure pyrolysis of urea, allows to obtain the following advantages with respect to the processes known from the prior art:

1) with high purity and complete recovery, respectively, of the gaseous by-product CO generated in the melamine synthesis reaction (1)) or formed in the recovery and purification cycle of melamine₂And NH₃. CO, object of the invention, in contrast to the practice of recycling exhaust gases (wet or anhydrous) to a urea plant known from the prior art₂And NH₃Can solve all the problems associated with the operation of prior art practices.

As an alternative to conventional exhaust gas stream recirculation, high purity NH is introduced₃And CO₂Are separately recycled to the urea plant, which results in:

a)CO₂the considerable increase in conversion of the various pathways to urea (conversion for reach passage) and therefore the increase in production capacity with the same investment, and the reduction in the production costs of urea;

b) the energy consumption, such as steam and/or electric energy, necessary for operating the urea plant is significantly reduced, and in addition the energy recovery mode from the same urea plant is considerably improved;

c) reduction of the compression costs of a feed stream to a urea plant, in particular CO₂Which is available in the battery limits of the melamine plant at a preferred pressure of 70-170 bar;

d) the reduction of the intermediate treatment costs, which bring the waste gases leaving the melamine reactor to a pressure of less than 50bar, where the distillation column operates according to the prior art;

e) less corrosion problems of the equipment and production lines involved in recycling;

f) overall improved operational reliability and availability of the urea synthesis process;

2) the process for the production of melamine according to the invention does not require the supply of additional NH₃And CO₂And (4) streaming. The process requires that the NH is actually recovered by the by-production and recovery of the NH at different process steps₃Partially re-used/recycled integrated and specifically satisfied;

3) the OAT formed by the hydrolysis of melamine is reduced not only due to the effect of reducing the number and volume of high temperature operating equipment, but also as a result of the reduction in temperature itself. The purification can be carried out at temperatures substantially 5-10 degrees lower than in the prior art. This leads to a further increase in the overall yield of the process, in addition to the yield increase obtained as a result of the higher conversion of urea (due to the insertion of a post-reactor);

4) use of NH in industrial processes other than urea production₃And CO₂Both possibilities; since both compounds are recovered in high purity and separately, they can be used for different purposes and with potentially higher added value;

5) the total amount of water used is reduced and it is totally recycled to the melamine production process, with the result that liquid discharge is eliminated;

6)CO₂the extractive distillation of (a) has reduced energy consumption, although carried out at high pressure;

7) the melamine production process of the invention can be combined with a process for the production of urea carried out in any known prior art urea plant, which significantly reduces the downtime and the intervention technical changes necessary for the modernization and the revamping of said plant.

The following provides an example of an embodiment of the process of the present invention for illustrative and non-limiting purposes of the present invention.

Example 1

In a plant for the production of melamine with high purity constructed according to the present invention, as shown in figure 3, a flow of molten urea 301 is sent to the main reactor at a flow rate of 22650kg/h, together with an anhydrous ammonia flow 319 in an amount of 1500 kg/h.

The liquid stream 303 leaving the main reactor is then sent to a subsequent post-reactor where there is little CO₂The synthesis of melamine is completed in an environment of 1600kg/h, also due to the continuous introduction of gaseous NH on the bottom₃The stripping effect of stream 304.

From this post-reactor a liquid effluent is then obtained (stream 306), equal to 7600kg/h of crude melamine, containing reduced amounts of OAT and polycondensate (0.6 and 1.0% by weight, respectively). Both reactors were operated at a temperature of 380 ℃ and a pressure of 80 bar.

NH to be saturated with melamine vapour₃And CO₂The composed gas phases 302 and 305 are separated from the two reactors and a single stream 312 is synthesized, which is sent to an extractive distillation column (Distex) for melamine recovery and high purity CO₂And (5) separating.

The Distex was used at a pressure of 75bar, using 30000kg/h of an aqueous stream 325 from the subsequent operating stage as extraction solvent.

In particular, the amount of aqueous stream used as extraction solvent in the process of the invention is less than 280% with respect to an equivalent process in which the distillation in Distex is carried out at a pressure of 35 bar.

8950kg/h of very pure and essentially anhydrous CO₂(and NH)₃Content less than 10ppm) from the top of DistexSeparated, forming a gaseous stream 314, which is available at 75bar pressure in the battery limits.

The aqueous stream 315 exiting the Distex bottom contains all of the ammonia and melamine entering the process with

streams

318, 325 and 329 and has a minimal amount of residual CO₂. Sending the aqueous stream 315 to the CO₂A stripper for the total removal of CO from the aqueous stream comprising ammonia and melamine₂. The top stream (315 b) of the column, which contains all of the CO of stream 315₂And a portion of the ammonia) to an ammonia separation zone; the bottom stream 315a is sent to Quench to recover the crude melamine from the post-reactor (stream 306).

The entire product leaving the Quench (stream 330) is sent to a purge zone, which operates at a pressure of 25bar and a temperature of 165 ℃. An additional amount of ammonia (stream 320) equal to 9000kg/h was added to the purification zone to obtain a complete conversion of the polycondensate to melamine.

An aqueous stream 307 of purified melamine, which contains less than 100ppm of polycondensate, is cooled to 40-50 c and depressurized to atmospheric pressure in a crystallization zone, where a precipitation of melamine crystals is obtained.

The suspension of melamine crystals in the mother liquor (stream 308) is then sent from the crystallizer to a centrifuge decanter, thus separating the wet crystal cake from the crystallization mother liquor. The cake was then dried in a special drying section, producing 7500kg/h of anhydrous melamine with high purity (titre higher than 99.8% by weight and humidity lower than 0.1% by weight).

The flow 310 of the mother liquor of crystallization leaving the centrifuge decanter is then sent to an ammonia separation zone, operating here at 20bar pressure, where 18800kg/h of very pure ammonia are recovered and partly used in the same process for the production of melamine (flows 304, 319 and 320) and mostly available as by-product of the plant in the battery compartment (flow 321) (6700kg/h of liquid NH)₃)。

The stream 322 produced at the bottom of the ammonia separation zone consisted of 71000kg/h of ammonia-free mother liquor. Stream 322 contains all the OAT produced in the upstream section of the plant and is saturated with melamine (about 0.8-1.0 wt%) under crystallization conditions. Stream 322 is sent to the final section of the plant (decomposer/stripper) for destruction of residual melamine and other organic by-products by pyrohydrolysis of OAT.

Stream 322 is first heated to substantially 280 ℃ and then sent to a decomposer/stripper zone where all organic material (OAT, melamine, other by-products) is destroyed by hydrolysis and converted to NH₃And CO₂After these gases are separated in the stripper, an almost pure aqueous stream (stream 313) is left as a liquid residue, containing less than 100ppm of total solid products and less than 10ppm of free NH₃Which is used to clean the air stream discharged in the scrubber.

The gases formed in the decomposer and those separated at the top of the stripper of the same zone (stream 318) are subjected to extractive distillation in a Distex for the recovery of CO₂And then recovering the NH₃。

The part of the ammonia-free aqueous stream leaving the decomposer was used as extraction solvent for Distex.

The overall yield of the production process proved to be equal to 0.33kg of melamine per 1kg of urea, which corresponds to a specific consumption of 3.02kg of urea per 1kg of high-purity melamine produced.

The following streams are also obtained from the process:

CO with a purity of more than 99.9% (based on wet matter) and at a pressure of 75bar₂A flow in an amount equal to 0.395kg/1kg of urea consumed;

anhydrous and liquid NH₃The flow, the titre of which was 99.9%, was measured as 0.296kg/1kg of urea consumed.

Furthermore, with respect to an equivalent process in which the distillation in Distex is carried out at a pressure of 35bar and the quantity of extraction solvent is equal to 85000kg/h, the energy balance of the process of the invention is as follows:

example 2

The process described in example 1 was repeated with the following Distex operating pressure:

-55bar

-170bar。

in table 1 below, the values measured at these pressures are also compared with the corresponding values measured during tests carried out according to the teaching of WO01/46159 at 35bar and according to example 1 at 75 bar.

TABLE 1

Pressure of	35	55	75	170	bar
						Water flow Rate to Distex (flow 325)	85000	57500	30000	11250	kg/h
Flow rate of liquid stream 318	4200	4200	4200	4200	kg/h
						Hourly flow rate of vapor required for Distex	23000	16500	9900	13000	kg/h
CO₂Hourly flow rate of steam required for stripper	12000	11500	10700	4700	kg/h

The results shown in the table above show the following reductions in total steam consumption in the case of processes carried out with Distex at 55bar, 75bar and 170bar, relative to Distex carried out at 35bar according to the teaching of WO 01/46159:

55bar＝-4000kg/h

75bar＝-14400kg/h

170bar＝-17300kg/h

furthermore, in the energy balance, in order to also take into account the energy consumption for introducing the solvent and the recycle stream into the extractive distillation column at higher pressure, the values shown in table 2 were measured.

TABLE 2

The content of table 2 shows that even when considering the energy consumed in order to reach higher pressures in the extractive distillation column, a considerable reduction in the total consumption of steam is obtained in the case of the process of the invention carried out with Distex at 55bar, 75bar and 170bar, relative to Distex carried out at 35bar according to the teaching of WO 01/46159.

Claims

1. Production of melamine from urea and separation and recovery of high purity CO₂And high purity NH₃The method comprises the following operation steps:

a) at NH₃(319) In the presence of a main reactor, melamine is synthesized by pyrolysis of a stream of molten urea (301), obtaining a stream of molten raw melamine (303) and a stream containing NH₃And CO₂The exhaust gas stream (302);

b) optionally, the crude melamine stream (303) leaving the main reactor is treated in a post-reactor in the presence of gaseous ammonia (304) to complete the pyrolysis reaction of urea, obtaining a stream substantially free of unconverted urea and CO₂And a crude melamine stream (306) of molten, and consisting essentially of NH₃Composed of and containing all CO formed in the reaction₂The exhaust gas stream (305);

c) distilling the offgas stream (302) from step a), and if present the offgas stream (305) from step b), in the presence of an aqueous extraction solvent (329, 318) to obtain high-purity CO₂And a gaseous stream (314) containing CO₂An aqueous ammonia stream (315);

d) stripping the aqueous ammonia stream (315) from step c) to remove CO still present₂；

e) Obtaining an aqueous ammonia solution (330) containing dissolved melamine, polycondensate impurities and OAT by contacting the crude molten melamine stream (303) obtained from step a) and, when present, the crude molten melamine stream (306) from step b) with the aqueous ammonia stream (315) from step c), cooling and dissolving the crude molten melamine stream (303) obtained from step a) and, when present, the crude molten melamine stream (306) from step b);

f) by reaction at NH₃Maintaining the condensation polymer converted into melamine in the presence of an aqueous ammonia solution (330) of melamine coming from step e);

g) crystallizing the melamine present in the purified aqueous solution (307) coming from step f) and separating the melamine crystals from said solution, obtaining a wet cake (309) containing melamine and a solution containing melamineWith OAT only, dissolved melamine residue and NH₃The crystallization mother liquor stream (310);

h) distilling off the mother liquor of step g) to obtain high-purity NH₃Stream (317) and an ammonia-free aqueous stream (322) containing OAT and melamine residues, which are subsequently recovered and/or removed in a dedicated downstream processing zone;

i) drying the melamine wet cake (309) coming from step g) with hot air obtaining dry melamine crystals with high purity and an effluent air stream containing ammonia;

the process requires that step c) of the extractive distillation of the offgas stream (302) from step a) and, if present, the offgas stream (305) from step b) is carried out in the presence of an aqueous extraction solvent at a pressure of from 55 to 250 bar.

2. The process according to claim 1, wherein the dedicated downstream processing zone is a decomposition/stripping zone.

3. The process according to claim 1, wherein the pressure is from 70 to 170 bar.

4. The process according to claim 1, wherein the aqueous extraction solvent is water or an ammonia-free aqueous solution.

5. The process of claim 1, wherein the aqueous extraction solvent is an ammonia-free recycled aqueous solution (329, 318).

6. A process according to any one of claims 1 to 5, wherein the aqueous extraction solvent is present in an amount of from 3 to 6 tonnes of solvent per 1 tonne of melamine produced.

7. A process according to any one of claims 1 to 5, wherein the aqueous extraction solvent is present in an amount of from 4 to 5 tonnes of solvent per 1 tonne of melamine produced.

8. The process according to any one of claims 1 to 5, wherein the extractive distillation of step c) is carried out in a separation device made of materials suitable to withstand the required operating conditions.

9. The process according to claim 8, wherein the extractive distillation of step c) is carried out in a distillation column or Dis tex.

10. The method of claim 8, wherein the material adapted to withstand the required operating conditions is an alloy consisting essentially of a combination of nickel, chromium and molybdenum, or a metal selected from zirconium, titanium or a combination of zirconium and titanium.

11. The process according to any one of claims 1 to 5, wherein the high purity NH from step h) is subjected to₃Stream (317) is fed to synthesis reactor (319) in step a), post reactor (304) in step b) and purification step e) (320).

12. The process according to any one of claims 1 to 5, wherein the high purity NH from step h) is subjected to₃Stream (317) and high purity CO from step c)₂Stream (314) is fed to a process for synthesizing urea.

13. The process according to any one of claims 1 to 5, wherein the high purity NH from step h) is subjected to₃Stream (317) and high purity CO from step c)₂Stream (314) is recovered as raw material for other industrial processes.

14. Process according to any one of claims 1-5, wherein a portion of the gaseous high purity CO from step c) is subjected to₂Stream (314) is used to neutralize the ammonia-free aqueous stream (322) from step h).

15. The process according to any one of claims 1 to 5, wherein step h) further comprises subjecting the mixture to a reaction comprising CO₂Is separated from the crystallization mother liquor stream, said purge stream having been subjected to the extractive distillation in step c).

16. A process according to any one of claims 1 to 5, wherein the ammonia-containing vent air stream from step i) is contacted with an aqueous purge stream in a scrubber to form an aqueous ammonia stream (316) and a purified air stream.

17. Process according to claim 16, wherein the aqueous ammonia stream (316) coming from the scrubber is fed to the quenching section of step e) for cooling and dissolving the molten raw melamine stream.

18. Process according to any one of claims 1 to 5, wherein CO possibly present in the crystallization mother liquor₂As CO₂Is separated in an ammonia separation zone and is passed to the extractive distillation of step c) for recovery together with the off-gas stream (302) from step a) and possibly the off-gas stream (305) from step b).

19. Process according to any one of claims 1 to 5, wherein the distillation of step c) is fed with an off-gas stream from a process for the production of melamine carried out in a further plant for the production of melamine.