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    • 专利摘要:
    The present invention relates to a process for the production of melamine from urea, with the separation and recovery of high-purity CO2 and NH3, obtaining dried melamine crystals with a high degree of purity and an exhausted stream of air containing ammonia; said process requiring a step for the extractive distillation of the stream of off-gas be carried out in the presence of an aqueous extraction solvent, at a pressure ranging from 55 to 250 bar, preferably from 70 to 170 bar.
    公开号:NL2017130A
    申请号:NL2017130
    申请日:2016-07-08
    公开日:2017-01-19
    发明作者:Bogotto Mattia；Merelli Giuseppe；Santucci Roberto
    申请人:Eurotecnica Melamine Luxemburg Zweigniederlassung In Ittigen；
    IPC主号:
    专利说明:

    PROCESS FOR THE PRODUCTION OF MELAMINE WITH SEPARATION AND RECOVERY OF HIGH-PURITY C02 AND NH3
    The present invention relates to a process for the production of melamine with separation and recovery of high-purity C02 and NH3.
    In particular, the present invention relates to a process for the production of high-purity melamine according to the synthesis process based on the high-pressure pyrolysis of urea, with the contemporaneous separate and complete recovery of high-purity NH3 and C02 generated in the various phases of the process.
    The production of melamine from urea is effected, in industrial practice, at a high temperature, in the presence of NH3, at both low pressure, in the presence of a suitable catalyst, and also at high pressure, in the absence of catalyst. In both cases, the transformation of urea into melamine takes place according to the following overall reaction:
    (1) urea melamine off-gas
    In addition to the main product (melamine), NH3 and C02 are also formed together with smaller, but not negligible, quantities of by-products well-known to skilled persons in the field, among which oxy-amino-triazines (OATs) and polycondensates, intermediate products of pyrolysis and subsequent reactions of melamine, respectively.
    From the stoichiometry of the overall reaction (1), it can be deduced that 50% of moles of urea fed to the melamine synthesis process is theoretically transformed into NH3 and C02. Consequently, a quantity of C02 and NH3, equal to at least 1.86 kg per 1 kg of melamine produced, is present in the effluent leaving the synthesis reactor, together with melamine.
    Due to the high economic value of NH3, there is the problem of the recovery of this reaction co-product in all melamine production processes.
    One of the most widely-used processes for the production of melamine is based on the high-pressure pyrolysis reaction of urea, with the recovery and purification of melamine in an aqueous ammonia solution. In this process, the effluent leaving the synthesis reactor consists of a liquid phase, containing most of the melamine produced, and a gaseous phase containing most of the NH3 and CO2 co-products.
    The total of the effluent leaving the reactor (gas and liquid) is put in contact with an aqueous ammonia stream, containing CO2, coming from the sections downstream, in a specific apparatus hereunder called “Quench”. A liquid stream containing all the melamine produced leaves the bottom of the Quench, which, after eliminating the CO2 present, is treated with ammonia to remove the polycondensates, reaction by-products, maintaining the OATs in solution. Melamine having a high degree of purity, with a titer higher than 99.8%, is obtained from the above purified solution, by crystallization and subsequent drying of the solid separated. A water-saturated gaseous stream of NH3 and CO2 (wet off-gas) is extracted from the head of the Quench. This gaseous stream is normally recycled to the urea synthesis plant (Urea Plant) to recover the NH3 and CO2 contained therein, thus allowing them to be immediately re-used through their (re)transformation into urea. An indispensable condition for ensuring that the recovery and re-use of the above gases is economically convenient, is that the Melamine plant be located in the immediate proximity of the Urea plant.
    The presence of water in the wet off-gas stream leaving the Quench (normally higher than 20% by weight), however, makes the re-use of NH3 and CO2 in the Urea plant onerous, contributing to increasing the overall costs for the melamine production, above all when the Urea plant is small-sized with respect to the Melamine plant.
    The above production process of melamine according to the state of the art is illustrated hereunder with reference to the enclosed Figures 1 and 2 which show a simplified block scheme of the main operating phases of the process.
    In Figure 1, the stream of molten urea 101 coming from the adjacent Urea plant, is fed at a temperature of 135-140°C to a pyrolysis reactor (Reactor) together with the gaseous stream 114 of anhydrous NH3. The Reactor operates in continuous and is equipped with a heating system which supplies the reagent mixture with the heat necessary for the synthesis of melamine, maintaining the reaction environment at a temperature of 360-420°C. The reaction pressure is kept at a value higher than 70 bar. The Reactor is a single-step reactor and the reagent mixture is kept in high circulation by the gas formed during the pyrolysis of urea.
    Under the above-mentioned operating conditions the pyrolysis reaction generates a biphasic effluent consisting of a liquid phase containing raw molten melamine, which contains reduced quantities of non- converted urea, and other reaction by-products (among which the most significant are the already-mentioned polycondensates and oxy-amino-triazines (OATs) and a gaseous phase essentially consisting of NH3 and CO2. The biphasic effluent 102 is sent to the Quench, positioned immediately downstream of the Reactor, for the melamine recovery. In the Quench, the biphasic stream 102 is put into contact with the aqueous ammonia stream 103, containing CO2, coming from other sections of the plant, and its temperature is lowered to about 160°C. Under these conditions, all of the melamine, the non-reacted urea and the impurities formed during the process, pass into aqueous solution or remain in suspension. The aqueous stream 104 leaving the Quench is directed towards the subsequent phases of the process, whereas the wet off-gas 105 (at a pressure of about 25 bar), substantially consisting of a stream of NH3 and CO2, saturated with humidity, is recycled to the adjacent Urea plant.
    The aqueous stream 104 leaving the Quench contains, dissolved, in addition to raw melamine, a consistent quantity of NH3 and CO2, which are removed in the subsequent step of the process by means of stripping with water vapour (CO2-Stripper section). The removal of CO2 is necessary for helping to reach a high degree of purity of melamine in the subsequent purification step. A gaseous stream 106 leaves the head of the CfVStripper, which is sent to an absorption column (NH3-CO2 Absorber) together with the recovery streams 113 and 124. The NH3-CO2 Absorber allows the total recovery of the above streams in a single aqueous stream 103 which is recycled to the Quench.
    An aqueous stream 107, without CO2 (and without NH3), containing melamine in solution at a concentration ranging from 6 to 12% by weight, leaves the bottom of the CfVStripper, together with non-negligible quantities of OATs and polycondensates. Due to their low solubility, the polycondensates must be eliminated before subjecting the aqueous stream containing melamine to the subsequent crystallization phase.
    The elimination of the polycondensates takes place in the Purification Section where the stream 107 is heated to about 170°C in the presence of ammonia which is added (stream 116) until a concentration of 12-15% by weight has been reached. Under the above conditions, the polycondensates are almost completely transformed into melamine.
    The aqueous ammonia stream 108 of melamine, substantially free of polycondensates, leaves the Purification Section and is fed to the Crystallization Section, where the temperature of the stream 108 is lowered to 40-50°C, thus allowing the crystallization of most of the melamine.
    The presence of NH3 in the stream 108 keeps the OATs in solution, allowing the precipitation of high-purity melamine crystals. The stream 109 containing the melamine crystals in suspension, is sent to the Solid/Liquid Separation Section in which a wet cake 110 of melamine and a stream 111 of crystallization mother liquor, are separated. The stream 111 is saturated with ammonia and contains all of the OATs, both those leaving the Reactor and also those formed by the hydrolysis of melamine in the subsequent steps of the production process. The formation of OATs is particularly favoured in the process steps carried out at a high temperature, in the presence of water.
    The wet cake 110 is finally dried by means of hot air in the Drying Section, obtaining a high-purity product having a titer of melamine of over 99.8%. The exhausted air stream 112 used for the drying is purified of the NH3 present therein, by washing with water in the NH3 Scrubber. The air thus purified is discharged into the environment.
    The aqueous washing stream 113, containing absorbed ammonia, is recycled to the process through the NH3-CO2 Absorber.
    The stream 111, consisting of the crystallization mother liquor, containing melamine in a concentration in the order of 0.8-1.0% by weight, is sent to the Ammonia Separation Section. In this section, the following products are separated, through distillation: a stream 115 of anhydrous NH3 which, with the addition of further anhydrous NH3, is recycled to the Purification Section; a purge stream 123 containing CO2 possibly present in the mother liquor 111; an aqueous stream 117, without NH3, substantially containing only melamine and OATs.
    The stream 117 is cooled to a temperature of 60-70°C and neutralized to pH 7, by adding CO2, in order to cause the precipitation of the OATs. The stream 118, containing OATs in a colloidal suspension, is subjected to a separation process of the OATs by ultrafiltration on ceramic candles, according to what is described in patent application WO 01/46159. In the OAT Ultrafiltration section, the OATs are separated as retentate (stream 119) from the corresponding stream of permeate 120. The latter stream is practically free of OATs and is recycled to the Quench for the recovery of the residual melamine contained therein.
    The stream of retentate 119, practically containing all of the OATs, is subjected to thermal treatment in the Decomposer-Stripper section, in which the OATs, the residual melamine and all of the other organic substances present therein are almost totally transformed by hydrolysis into NH3 and CO2 with the formation of: - a purified aqueous stream (121) that can be discharged into the environment: - a wet stream rich in NH3 and CO2 (122) recycled to the NH3-CO2 Absorber.
    The process illustrated above, even if currently applied in numerous chemical plants, is characterized by a high vapour consumption and a conversion yield of urea to melamine which is much lower than the stoichiometric value. Furthermore, the process is jeopardized by an onerous recovery of the gaseous reaction products NH3 and CO2, due to the considerable amount of water present in the stream 105 of the wet off-gas.
    The water present in the wet off-gas recycled to the adjacent Urea Plant, in fact, causes an increase in the urea production costs, which are obviously sustained by the management of the melamine production plant.
    This problem has already been faced in the past, with attempts to find separation methods of NH3 and CO2 from mixtures of off-gas and by-products coming from the synthesis process of melamine, for obtaining pure gases that can be recycled to the Urea Plant, can be stored and/or released into the environment (C02).
    Contrary to what is known in the state of the art which also already describes distillations of CO2 for obtaining gaseous streams of high-purity CO2 (IT 1387832, whose process scheme is shown in the enclosed figure 2), the Applicant has now surprisingly found that particular operating conditions allow a process to be obtained for the preparation of melamine from urea, with the separation and recovery of high-purity CO2 and NH3, which is improved and optimized, operating with reduced quantities of water and with surprising results in terms of energy consumption.
    The objective of the present invention is therefore to overcome the drawbacks revealed by the current state of the art.
    An object of the present invention relates, in fact, to a process for the production of melamine from urea, with the separation and recovery of high-purity CO2 and NH3, comprising the following operating steps: a) synthesizing in a Main Reactor melamine by pyrolysis of a stream of molten urea in the presence of NH3, obtaining a stream of molten raw melamine and a stream of off-gas containing NH3 and CO2; b) optionally, treating the stream of molten raw melamine leaving the Main Reactor in a Post Reactor in the presence of gaseous ammonia, for the completion of the pyrolysis reaction of urea, obtaining a stream of molten raw melamine, substantially free of unconverted urea and CO2 and a stream of off-gas mainly consisting of NH3 and containing all the CO2 formed in the reaction; c) subjecting the stream of off-gas coming from step a) and, if present, the stream of off-gas coming from step b), to distillation in the presence of an aqueous extraction solvent, obtaining a gaseous stream of high-purity CO2 and an aqueous ammonia stream containing CO2; d) subjecting the aqueous ammonia stream coming from step c) to stripping to remove the CO2 still present; e) cooling and solubilizing the molten raw melamine stream obtained from step a) or from step b) when present, by putting them in contact with the aqueous ammonia stream coming from step c), obtaining an aqueous ammonia solution containing dissolved melamine, impurities of poly condensates and OATs; f) purifying the aqueous ammonia solution of melamine coming from step e) by remaining in the presence of NH3, transforming the polycondensates into melamine; g) crystallizing the melamine present in the purified aqueous solution leaving step f) and separating the melamine crystals from said solution, obtaining a wet cake of melamine and a stream of crystallization mother liquor containing only OATs, dissolved melamine residues and NH3; h) distilling the mother liquor leaving step g), obtaining a stream of high-purity NH3 and an aqueous stream free of ammonia, containing OATs and melamine residues, subsequently recovered and/or eliminated in specific sections downstream of the process, for example in a Decomposer/Stripper section; i) drying the wet melamine cake coming from step g) with hot air, obtaining dried melamine crystals with a high degree of purity and an exhausted air stream containing ammonia; said process requiring that step c) for the extractive distillation of the off-gas stream coming from step a) and, if present, the off-gas stream coming from step b), be carried out in the presence of an aqueous extraction solvent, at a pressure ranging from 55 to 250 bar, preferably from 70 to 170 bar.
    The aqueous extraction solvent is preferably water or an aqueous ammonia-free solution, even more preferably a recycled aqueous ammonia-free solution.
    The aqueous extraction solvent is present in a quantity ranging from 3 to 6, preferably from 4 to 5 tons of solvent per 1 ton of melamine produced.
    Furthermore, the extractive distillation step c) is preferably carried out in separation equipment (distillation column or Distex), produced with materials suitable for resisting the required operating conditions, such as, for example, an alloy substantially consisting of a combination of nickel, chromium and molybdenum, or metals such as zirconium, titanium or suitable combinations of these materials.
    The process according to the present invention therefore consists of a process for the production of melamine, enhanced and optimized with respect to the state of the art, which, in addition to allowing the production of high-purity melamine with a considerable increase in the yield and with reduced costs, also allows the contemporaneous separation and recovery of high-purity CO2 and NH3 generated/used in the production cycle and purification of melamine; a reduction in the total quantity of water used and its complete recycling in the melamine production process, with the consequent elimination of liquid discharges; an extractive distillation of CO2 effected at high pressures, wherein the use of said pressures allows the direct distillation of off-gas leaving the reactor and possibly the post reactor of the synthesis process of high-pressure melamine, without the need for intermediate treatment for bringing them to the operating pressures of an extraction column which operates at pressures lower than 50 bar, as described in the processes of the state of the art; an extractive distillation of CO2 which, even if carried out at high pressure, has reduced energy consumptions, which can even be halved with respect to operating at lower pressures.
    An essential aspect of the process according to the present invention, moreover, is to envisage a treatment of off-gas containing CO2 and NH3 for obtaining high-purity CO2 and NH3, wherein a gaseous stream of high-purity CO2 is separated in a first step, and a stream of high-purity NH3 is separated in a subsequent step.
    Furthermore, the off-gas streams containing CO2 and NH3 generated/used in the production and purification cycle of melamine that are treated according to the process of the present invention, can be generated in the same plant in which the extractive distillation column is present, or they can come from single or multiple combinations of other melamine production plants, conventionally called high- or low-pressure plants, not equipped with an extractive column or Distex. A further object of the present invention relates to a specific group for the separation and recovery of high-purity CO2 and NH3 from off-gas coming from a plant for the production of melamine, said group comprising the following elements: a first distillation column for the extraction of CO2, suitable for operating with an aqueous extraction solvent, such as water: a second separation column suitable for recovering, from the head, the CO2 and NH3 present in the stream leaving the bottom of the first extractive distillation column; a third separation column suitable for recovering from the head, the NH3 present in the stream leaving the head of the second column, said elements being suitable for effecting the separation and recovery steps of the streams of high-purity CO2 and NH3 from said off-gas.
    Said group can be used in cases of revamping of plants for the production of melamine already existing and/or for integrating the same with plants for the production of urea.
    The group object of the present invention, for example, can be used for the separation and recovery of high-purity CO2 and NH3, generated from a process for the production of melamine from urea, comprising the previously described operating steps a)-i).
    The process proposed with the present invention applies simple but substantial modifications with respect to the processes known in the state of the art.
    The synthesis process of melamine from urea according to the present invention, and the advantages deriving therefrom, can be more clearly understood from the following description, which makes reference to Figure 3 that illustrates a simplified scheme of the main operating steps of a preferred embodiment. The description and relative process scheme should be considered as being illustrative of the present invention and in no way represent a limitation to the scope of the enclosed claims.
    The following distinctive feature of the present invention should be firstly observed, and namely that the proximity of the Urea Plant to the melamine production unit is no longer binding, as the process according to the present invention no longer produces any off-gas mixture containing NH3 and CO2 (either wet off-gas or anhydrous off-gas), whose recovery and conveyance to the Urea plant is economically viable only thanks to this proximity. In addition, the specific pressure conditions in the extractive distillation step of the process according to the present invention allow an optimized process to be obtained, which operates with reduced quantities of water and with surprising results in terms of energy consumptions.
    These specific features make the process of the present invention unique with respect all of the high-pressure technologies used and/or proposed so far for the production of melamine from urea. A stream of molten urea 301 is fed to a first reactor (Main Reactor) where the high-pressure pyrolysis process takes place under standard conditions, i.e. in the presence of anhydrous ammonia (stream 319), at a temperature ranging from 360 to 420° and a pressure higher than 70 bar. Contrary to the melamine production process according to the state of the art illustrated in Figure 2, the two components of the effluent generated by the pyrolysis reaction, i.e. the anhydrous off-gas stream 302 consisting of NH3 and CO2 and the liquid stream 303 consisting of molten raw melamine, are extracted separately from the Main Reactor, or from a suitable phase separator situated downstream, not shown in Figure 3, and treated separately.
    In the embodiment of the present invention represented in Figure 3, the stream 303 is sent to a second reaction step (Post Reactor) in which, operating under the same temperature and pressure conditions as the Main Reactor and in the presence of a continuous flow of anhydrous ammonia (stream 304), the conversion of the non-reacted urea still present in the stream 303 leaving the Main Reactor, is completed, together with the intermediate reactions that lead to the formation of melamine. At the same time, the conditions present in the Post Reactor, in which the partial ammonia pressure is much higher with respect to that of the Main Reactor, also lead to the reduction of polycondensates and OATs, in the molten raw melamine produced by the Main Reactor. A further beneficial effect of the presence of the Post Reactor is a reduction in the quantity of CO2 present in the liquid phase, i.e. in the stream of molten raw melamine, also due to the stripping effect of the gaseous stream of NH3 entering in continuous. Step b) of the process, however, is optional. The process according to the present invention, in fact, allows the production of high-purity melamine with the separate recovery of NH3 and CO2 also without the help of the Post Reactor.
    Two separate streams also leave the Post Reactor, both free of water: an offgas stream 305 mainly consisting of NH3 and a liquid stream 306 consisting of molten raw melamine, practically free of urea and CO2, containing very limited quantities of high-boiling by-products such as OATs and polycondensates.
    The off-gas stream 305 leaving the Post Reactor or from a suitable separator positioned downstream, not shown in Figure 3, is combined with the off-gas stream 302 leaving the Main Reactor to be treated in a specific extractive distillation column (hereinafter “Distex”) as illustrated hereunder. The stream of molten raw melamine 306 leaving the Post Reactor is, on the other hand, sent to the recovery step of the melamine by dissolution in an aqueous ammonia solution. This step is carried out in a specific apparatus (hereinafter “Quench”), inside which the stream of molten raw melamine 306 comes into contact, at a temperature ranging from 110 to 180°C, preferably from 150 to 170°C, with the aqueous ammonia stream prevalently consisting of the aqueous ammonia stream leaving the CO2 stripper (stream 315a), containing a significant concentration of NH3, but almost free of CO2. Under these conditions, a preliminary demolition of the polycondensates takes place in the Quench apparatus. The melamine solution leaving the Quench is then sent to the Purification Section (stream 330) for the complete demolition of the poly condensates.
    The purification of the aqueous solution 330 containing melamine, leaving the Quench, is effected under the pressure and concentration conditions known in the art, but at a lower temperature. The aqueous ammonia solution of purified melamine 307 leaving the Purification Section is then fed to the Crystallization Section where, analogously to the process of the state of the art of Figure 2, the temperature of the solution is lowered to 40-50°C, obtaining the separation of extremely pure melamine crystals. The stream 308, containing the melamine crystals in suspension, is sent to the Solid/Liquid Separation section with the separation of a wet cake of melamine 309 and an aqueous stream 310 consisting of crystallization mother liquor. The anhydrous crystallized melamine with a high degree of purity degree (titer higher than 99.8% by weight), is recovered from the Drying Section.
    The stream of crystallization mother liquor 310, containing ammonia, is partially recycled directly to the Quench (stream 310a) and partially fed to the Ammonia Separation Section (stream 310b) whose main function is to recover all of the NH3 having a high degree of purity. The separation of NH3 in this section is effected by means of distillation, analogously to the melamine production processes of the state of the art.
    The gaseous stream 312, deriving from the combination of the two anhydrous off-gas streams 302 and 305, coming from the Main Reactor and from the Post Reactor respectively, and saturated with melamine vapours, is subjected to extractive distillation in the specific distillation column (Distex), in the presence of an aqueous extraction solvent selected from water and an aqueous ammonia-free solution, which proceeds from the top downwards in the column, in countercurrent with respect to the rising gaseous stream.
    The aqueous extraction solvent is preferably a recycled aqueous ammonia-free solution, coming from the section downstream of the plant (streams 313 and 325) from the Decomposer/Stripper section.
    The gaseous stream 314 consisting of high-purity CO2 (NH3 content lower than 10 ppm) is discharged from the head of the Distex, at the operating pressure of the column, whereas an aqueous ammonia stream 315, with a low content of CO2, containing the melamine present in the stream 312, which is thus recovered, is discharged from the bottom of the Distex. The whole of the aqueous stream 315 leaving the Distex is sent to the CO2 stripping column for the complete removal of CO2 from the bottom solution, and the bottom solution is sent from here to the Quench as solvent for the recovery of melamine leaving the Post Reactor. The stream 316, coming from the NH3 Scrubber, containing the ammonia which has developed as a result of the drying of the wet melamine cake, is fed to the Quench as aqueous solvent. A stream of pure NH3 320 is also fed to the Quench to obtain the concentration of NH3 necessary for the purification step. Consequently, the aqueous ammonia stream 307 which flows downstream from the Purification Section, contains all the NH3 produced by the process, i.e. that deriving from the pyrolysis of urea (see reaction (1)) and that deriving from the hydrolysis reactions (in particular from the Decomposer/Stripper Section described hereunder). The total quantity of ammonia contained in the stream 310b is separated at a high purity (317) by distillation in the Ammonia Separation Section.
    The aqueous ammonia-free stream 322 containing OATs and residues of dissolved melamine in solution, is also separated from the Ammonia Separation Section.
    The process object of the invention therefore also allows the complete recovery, and at a high degree of purity, of the whole ammonia generated in the melamine production section, after the complete recovery and at a high degree of purity, of all the CO2 generated in the melamine production process. The pure NH3, in the liquid state, forming the stream 317, is partially used in various points of the production process, such as, for example, the Main Reactor (stream 319), Post Reactor (stream 304) and the Purification Section (stream 320), whereas the excess NH3, which represents the most relevant portion, is delivered to the battery limit of the plant (stream 321) and can be transferred to the Urea Plant or recovered as a commercial high-value co-product, which can be used as raw material in other industrial processes.
    The aqueous ammonia-free stream 322, discharged as bottom product of the Ammonia Separation Section, is sent to the Decomposer/Stripper section, in which the OATs and residual melamine, in addition to the other organic molecules present, are decomposed by means of high-temperature thermal hydrolysis, into ammonia and carbon dioxide. The NH3 and CO2 are separated, by stripping, in the same section, as head product and recycled (stream 318) to the Distex, whereas a stream of almost pure water is recovered from the bottom and is either recycled to the Distex as solvent of the extractive distillation (325) or sent to the Scrubber for washing of the exhausted air coming from the Dryer (stream 313) or drying section.
    The drying step can in fact be carried out with hot air or other means known to skilled persons in the field.
    The small quantities of CO2 possibly present in the stream 310 (crystallization mother liquor) are separated in the Ammonia Separation Section in the form of an aqueous ammonia mixture of CO2, as known in the state of the art, and conveyed (purge stream 329) to the Distex for their recovery. All of the CO2 formed in the production cycle (by pyrolysis and hydrolysis) is then separated at a high purity in the Distex, as head product, and made available at the battery limit of the plant, at the preferred pressure ranging from 70 to 170 bar (stream 314). Similarly to NH3, the CO2 can also be transferred to the Urea Plant or recovered as raw material to be used in other industrial processes.
    According to the embodiment illustrated herein, the process for the production of melamine from urea with separate recovery and with a high purity of the co-products ammonia and carbon dioxide, comprises the following basic operational step: - subjecting the off-gas stream leaving the melamine synthesis reactor, to distillation in an extractive distillation column using water as extraction solvent or an aqueous solution, and preferably a recovery aqueous stream in a quantity ranging from 3 to 6, preferably from 4 to 5 tons of solvent per ton of melamine, and operating at a pressure ranging from 55 to 250 bar, preferably from 70 to 170 bar, obtaining a gaseous stream of high-purity CO2 and an aqueous ammonia stream used for the recovery and purification of the melamine produced in the reaction, with particularly reduced energy consumptions.
    The aqueous ammonia stream resulting from the Solid/Liquid Separation step, downstream of the Crystallization, is then distilled, obtaining a high-purity ammonia stream and an aqueous ammonia-free stream containing residual melamine, OATs and other minor organic impurities to be sent for recovery or elimination.
    In a preferred embodiment of the process according to the present invention, the high-purity NH3 and CO2 streams recovered, are suitably fed separately to the Urea Plant, with particular advantages with respect to the feeding stream of these substances contained, not separated, in the off-gas stream.
    Alternatively, one or both of the above-mentioned streams are suitably used as raw materials in other industrial production processes.
    The present invention therefore relates to an innovative cycle organized so as to obtain the separation and recovery of high-purity NH3 and CO2 in a melamine production process through the pyrolysis of urea, said cycle comprising: a reaction area consisting of a Main Reactor and a Post Reactor in which the pyrolysis is effected with a high conversion of urea into melamine with the separation of a stream of anhydrous off-gas consisting of NH3, CO2 and melamine vapours; an extractive distillation column (Distex) connected to said reaction area from which it receives said off-gas stream, said Distex separating a head product consisting of high-purity CO2 and an aqueous bottom stream containing NH3 and residual melamine, using, as extraction solvent, preferably aqueous ammonia-free solutions coming from the recovery and purification cycle of melamine; the high-purity CO2 stream is distilled at the preferred pressure of 70-170 bar, i.e. at the same pressure at which it leaves the reactor, without any need for intermediate treatment; a stripping section for removing the residual CO2 from the ammonia-free aqueous solution; a Quench section and a Purification Section for the recovery and purification of melamine by means of aqueous ammonia streams coming from the production cycle, possibly with the addition of NH3; a precipitation area by crystallization of the purified melamine, associated with the separation and drying sections of the product; an Ammonia Separation Section from the crystallization mother liquor with the production, by distillation, of high-purity ammonia used for the internal requirements of the cycle, the excess NH3 being delivered, at a high purity, to the battery limit for transfer to a Urea Plant or for sale on the market; a Section for the treatment of the ammonia-free mother liquor leaving the Ammonia Separation Section whereby a congruent recovery of the melamine contained therein is effected, and/or the thermal demolition of all the other organic substances present with the total recovery of the resulting NH3 and CO2; the aqueous stream deriving from this series of operations forming most of the extraction solvent requirements of the Distex.
    The production process of high-purity melamine, by the high-pressure pyrolysis of urea, object of the present invention, allows the following advantages to be obtained with respect to the processes known in the state of the art: 1) the complete recovery, at a high purity, and separately, of the gaseous co-products CO2 and NH3 generated in the synthesis reaction of melamine (reaction (1)) or formed in the recovery and purification cycle of melamine. The innovative recovery mode of CO2 and NH3, object of the present invention, with respect to the recycling practices of off-gas (wet or anhydrous) to the Urea Plant, known from the state of the art, allows all the problems relating to this latter operation to be overcome.
    The separate recycling of high-purity NH3 and CO2 to the Urea Plant, as an alternative to the traditional recycling of the off-gas stream, in turn leads to: a) a considerable increase in the conversion for each passage of CO2 to urea, with a consequent increase in the production capacity, with the same investment, and a decrease in the production costs of urea; b) a significant decrease in the energy consumption, such as vapour and/or electricity, necessary for running the Urea Plant, in addition to a considerable improvement in the energy recovery modes from the same Urea Plant; c) a decrease in the compression costs of the feeding stream to the Urea Plant, in particular of the CO2 which is made available at the battery limit of the Melamine Plant at the preferred pressure of 70-170 bar; d) a decrease in costs for intermediate treatment which brings the offgas leaving the melamine reactor to pressures lower than 50 bar, at which the distillation columns operate according to the state of the art; e) fewer corrosion problems of the equipment and lines involved in the recycling; f) an overall improved operational reliability and availability of the synthesis process of urea; 2) the production process of melamine according to the invention does not require the feeding of additional NH3 and CO2 streams. The process requirement is, in fact, integrally and exclusively satisfied by the partial reuse/recycling of the NH3 co-produced and recovered in the various steps of the process; 3) a reduction in the formation of OATs by hydrolysis of the melamine, due not only to the effect of the decrease in the number and volume of the equipment operating at a high temperature, but also as a result of the reduction in the temperature itself. The purification can, in fact, be carried out at a temperature from 5 to 10 degrees lower than that of the state of the art. This leads to a further increase in the overall yield of the process in addition to that obtained as a consequence of the higher conversion of urea thanks to the insertion of the Post Reactor; 4) the possibility of using both NH3 and CO2 in industrial processes other than that for the production of urea; as both compounds are recovered with a high purity and separately, they are available for different uses and with a possible higher added value; 5) a reduction in the overall quantity of water used and its complete recycling to the production process of melamine, with the consequent elimination of the liquid discharges; 6) an extractive distillation of CO2 which, although carried out at a high pressure, has reduced energy consumptions; 7) the melamine production process according to the present invention can be coupled with processes for the production of urea effected in existing urea plants of any known technology, drastically reducing the shutdown periods and invasive technological modifications necessary for the modernization and adaptation of said plants.
    An embodiment example of the process according to the present invention is provided hereunder for illustrative and non-limiting purposes of the present invention. EXAMPLE 1
    In a plant for the production of high-purity melamine constructed according to the present invention, as illustrated in Figure 3, a stream of molten urea 301 was sent to the Main Reactor with a flow-rate of22,650 kg/h, together with a stream of anhydrous ammonia 319 in a quantity of 1,500 kg/h.
    The liquid stream 303 leaving the Main Reactor was then sent to the subsequent Post Reactor, where the synthesis reactions of melamine were completed in a practically CCVfree environment, also thanks to the stripping effect effected by the stream 304 of 1,600 kg/h of gaseous NH3, continuously introduced onto the bottom. A liquid effluent was then obtained (stream 306) from the Post Reactor, equal to 7,600 kg/h of raw melamine, containing reduced quantities of OATs and polycondensates (0.6 and 1.0% by weight, respectively). Both reactors were run at a temperature of 380°C and a pressure of 80 bar.
    The gaseous phases 302 and 305 consisting of NH3 and CO2, saturated with melamine vapours, were separated from the two reactors and were joined in a single stream 312 which was sent to the extractive distillation column (Distex) for the melamine recovery and separation of high-purity CO2.
    The Distex was used at a pressure of 75 bar, adopting, as extraction solvent, 30,000 kg/h of the aqueous stream 325 coming from the subsequent operating phases.
    In particular, with respect to an identical process, in which the distillation in the Distex was carried out at a pressure of 35 bar, the quantity of aqueous stream used as extraction solvent in the process according to the present invention, is less than 280%. 8,950 kg/h of extremely pure and substantially anhydrous CO2 with a NH3 content lower than 10 ppm, were separated from the head of the Distex, forming the gaseous stream 314 which was made available at the battery limit, at a pressure of 75 bar.
    The aqueous stream 315, discharged from the bottom of the Distex, contains all the ammonia and melamine entering the process with the streams 318, 325 and 329 and has a minimum quantity of residual CO2. The aqueous stream 315 was sent to the CO2 stripper for the total removal of CO2 from the aqueous stream containing ammonia and melamine. The stream at the head of the column (315b), containing all the CO2 and part of the ammonia of the stream 315, is sent to the separation section of ammonia; the bottom stream 315a is sent to the Quench for the recovery of the raw melamine coming (306 stream) from the Post Reactor.
    The whole product leaving the Quench (stream 330) was sent to the Purification Section which operates at a pressure of 25 bar and a temperature of 165°C. A further quantity of ammonia equal to 9,000 kg/h (stream 320) was added in the Purification Section in order to obtain the complete transformation of the polycondensates into melamine.
    The aqueous stream 307 of purified melamine, containing less than 100 ppm of polycondensates, was cooled to 40-50°C and decompressed to atmospheric pressure in the Crystallization section in which the precipitation of melamine crystals was obtained.
    The suspension of the melamine crystals in the mother liquor (stream 308) was then sent from the crystallizer to a centrifuge decanter, thus separating a cake of wet crystals from the crystallization mother liquor. The cake was then dried in the specific Drying Section with the production of 7,500 kg/h of anhydrous melamine with a high degree of purity (titer higher than 99.8% by weight and humidity lower than 0.1% by weight).
    The stream 310 of the crystallization mother liquor leaving the centrifuge decanter was then sent to the Ammonia Separation Section where, operating at a pressure of 20 bar, 18,800 kg/h of extremely pure ammonia were recovered and partially used in the same production process of melamine (streams 304, 319 and 320) and for the most part made available at the battery limit (stream 321) as a coproduct of the plant (6,700 kg/h of NH3 in the liquid state).
    The stream 322, produced at the bottom of the Ammonia Separation Section, consists of 71,000 kg/h of ammonia-free mother liquor. The stream 322 contains all the OATs produced in the sections upstream of the plant and is saturated with melamine under the crystallization conditions (about 0.8 - 1.0% by weight). The stream 322 was sent to the final sections of the plant (Decomposer/Stripper) for the destruction, through the high-temperature hydrolysis of OATs, of the residual melamine and other organic by-products.
    The stream 322, in fact, was first heated to 280°C and then sent to the Decomposer/Stripper section where all the organic material (OATs, melamine, other by-products) was destroyed by hydrolysis and transformed into NH3 and CO2 leaving, as liquid residue, after the separation of these gases in the Stripper, an almost pure aqueous stream (stream 313) containing less than 100 ppm of total solid products and less than 10 ppm of free NH3, which was used for washing the exhausted stream of air in the Scrubber.
    The gases formed in the Decomposer and those separated at the head of the Stripper of the same section (stream 318) were subjected to extraction distillation in the Distex for the recovery of CO2 and then NH3.
    Part of the ammonia-free aqueous stream leaving the decomposer is used as extraction solvent in the Distex.
    The overall yield of the production process proved to be equal to 0.33 kg of melamine per 1 kg of urea, corresponding to a specific consumption of 3.02 kg of urea per 1 kg of high-purity melamine produced.
    The following streams were also obtained from the process: - a stream of CO2 having a purity higher than 99.9%, on a wet basis, and at 75 bar of pressure, in a quantity equal to 0.395 kg per 1 kg of urea consumed; - a stream of anhydrous and liquid NH3 with a titer of 99.9%, in a quantity of0.296 kg per 1 kg of urea consumed.
    Furthermore, the energy balance of the process according to the present invention, with respect to an identical process, in which the distillation in the Distex was carried out at a pressure of 35 bar and with a quantity of extraction solvent equal to 85,000 kg/h, is the following:
    EXAMPLE 2
    The process described in Example 1 was repeated with the following operating pressures of the Distex: 55 bar - 170 bar.
    In Table 1 below, the values measured at these pressures are also compared with the corresponding values measured during the test carried out at 35 bar according to the teaching of WO 01/46159 and at 75 bar according to Example 1.
    Table 1
    The results indicated in the above table show the following reductions in the total vapour consumption in the case of the process carried out with the Distex at 55 bar, 75 bar and 170 bar, with respect to the Distex carried out at 35 bar according to the teaching of WO 01/46159: 55 bar = - 4,000 kg/h 75 bar = - 14,400 kg/h 170 bar = - 17,300 kg/h
    Furthermore, in order to also take into account, in the energy balance, the energy consumption for introducing the solvent and recycled streams into the extractive distillation column at a higher pressure, the values indicated in Table 2 were measured.
    Table 2
    The contents of Table 2 show that even considering the energy consumed for reaching a higher pressure in the extractive distillation column, considerable reductions in the total consumption of vapour are obtained in the case of the process carried out according to the present invention, with the Distex operating at 55 bar, 75 bar and 170 bar, with respect to the Distex carried out at 35 bar according to the teaching of WO 01/46159.
    权利要求:
    Claims (17)
    [1]
    Process for the preparation of melamine from urea, with separation and recovery of very pure CO2 and very pure NH3, comprising the following operating steps: a) synthesizing melamine in a main reactor by pyrolysis of a stream of molten urea (301) in presence of NH 3 (319), with obtaining a stream of molten crude melamine (303) and a stream of off-gas containing NH 3 and CO 2. (302); b) optionally treating the molten crude melamine stream (303) exiting the main reactor in a post-reactor in the presence of gaseous ammonia (304) to complete the urea pyrolysis reaction with a molten stream crude meiamine (306) that is substantially free from unreacted urea and CO2 and a stream of off-gas (305) consisting essentially of NH3 and CO2 formed in the reaction; c) subjecting the waste gas stream (302) from step a) and, if present, the waste gas stream (305) from step b), to distillation in the presence of an aqueous extraction solvent (329, 318), to obtain a gas stream of high purity CO2 (314) and an aqueous ammonia stream (315) containing CO2; d) stripping the aqueous ammonia stream from step c) (315) to remove the remaining CO2; e) curing and opising the molten crude mayamine stream (303) obtained from step a) and (306) from step b), if present, by contacting the aqueous ammonia stream (315) from step c) with obtaining an aqueous ammonia solution (330) containing dissolved meiamine, polycondensate impurities, and OAT; f) purifying the aqueous ammonia solution of meiamine (330) from step e) by a stay in the presence of NH3s that transforms polycondensates into meiamine; g) crystallizing the meiamine in the purified aqueous solution (307) from step f) and separating meiamine crystals from the solution, obtaining a wet cake (309) of meiamine and a stream of crystallization mother liquor (310) containing only OAT, dissolved melamine residues and NH3; b) distilling the mother liquor from step g), obtaining a stream of very pure NH 3 (317) and an aqueous stream without ammonia (322) containing OAT and melamine residues, which are subsequently recovered and / or eliminated are in specific downstream process sections, for example in a decomposition / strip section; i) drying the meiamine wet cake (309) from step g) with hot air, obtaining high purity meiamine crystals and an outlet air stream comprising ammonia; which method requires that step c) of extractive distillation of the waste gas stream (302) from step a) and, if present, of the waste gas stream (305) from step b) be carried out in the presence of an aqueous extraction solvent, at a pressure ranging from 55 -250 bar, preferably from 70-170 bar.
    [2]
    The method according to claim 1, wherein the aqueous extraction opioids is water or an aqueous solution free of ammonia, preferably a recycled solution without ammonia (329, 318).
    [3]
    Method according to one or more of the preceding claims, wherein the aqueous extraction solvent is present in an amount ranging from 3 to 6, preferably 4 to 5 tons of solvent per 1 ton of melamine produced.
    [4]
    Method according to one or more of the preceding claims, wherein the step c) of extractive distillation is carried out in a separation device, preferably in a distillation column or Distex, made from materials suitable for functioning under the required conditions, such as, for example, a alloy consisting essentially of a combination of nickel, chromium and molybdenum, or metals such as zirconium, titanium, or associations of such materials.
    [5]
    The method of any one of the preceding claims, wherein the high purity NH 3 stream (317) from step h) is supplied to the synthesis reactor (319) in step a), to the post-reactor (304) in step b) and to the purification step e) (320).
    [6]
    The method of any one of the preceding claims, wherein the high purity NH 3 stream (317) from step h) and the high purity CO 2 stream (314) from step c) are fed to a urea synthesis process.
    [7]
    The method according to one or more of the preceding claims, wherein the high purity NH 3 stream (317) from step h) and the high purity CQ 2 stream (314) from step c) are recovered as raw materials for use in other industrial production processes.
    [8]
    A method according to any one of the preceding claims, wherein a portion of the gaseous high purity CO 2 stream (314) from step c) is used to neutralize the aqueous stream without ammonia (322) from step h).
    [9]
    The method of any one of the preceding claims, wherein step h) further comprises separating a purge stream (329) comprising CO 2 from the crystallization mother liquor stream, said purge stream being subjected to extractive distillation in step c).
    [10]
    The method of any one of the preceding claims, wherein the outlet air stream containing ammonia from step i) is contacted with an aqueous wash stream in a gas washer, and forms an aqueous ammonia stream (316) and a purified air stream.
    [11]
    The method of claim 10, wherein the aqueous ammonia stream (316) is supplied from the gas washer to the Cow section of step e) for cooling and dissolving the molten crude melamine stream.
    [12]
    A method according to any one of the preceding claims, wherein CO2 possibly present in the crystallization mother liquor is separated as an aqueous ammonia mixture (329) from CO2 in the Ammonia Separation section and transferred for recovery to the extractive distillation of step c ), together with the waste gas stream (302) from step a) and the waste gas stream (305) optionally from step b),
    [13]
    A method according to any one of the preceding claims, wherein the distillation of step c) is fed with stream of waste gases from processes for the production of meiamine conducted in other installations for the production of melamine.
    [14]
    14. Group devoted to the separation and recovery of very pure CO2 and very pure NH3 from waste gas streams from a melamine production plant, said group characterized in that it comprises the following components: a first column of distillation, which contains CO2 extracts and is suitable for use with an aqueous extraction solvent, for example water; a second separation column, suitable for recovery from the head, the CO2 and NH3 present in the stream verifying the bottom of the first column of extractive destruction; a third separation separation, suitable for recovering from the head, the NH3 present in the stream coming from the head of the second communication, the said elements being suitable for carrying out the steps of separation and recovery of very pure CO2 and very pure NH3 from the aforementioned off-gas streams.
    [15]
    Group according to claim 14, for the renewal of existing installations for the production of melamine and / or the integration of institutions for the production of meiamine with installations for the production of urea.
    [16]
    16. Cycle for the separation and recovery of very pure CO2 and very pure NH3 from waste gas streams from an installation for the production of meiamine by urea pyrolysis, said cycle comprising: - A reaction section consisting of a main reactor and a post-reactor wherein pyrolysis occurs with a high conversion of urea to meiamine with separation of anhydrous off-gas stream made from NH 3, CG 2 and meiamine vapors; - An Extractive Destination kiaom or Distex associated with the reaction section and receiving the anhydrous off-gas stream, said Distex separating a top product consisting of very pure CO2 by distillation at a pressure preferably ranging from 70-170 bar and a bottom water stream consisting of NH3 and remaining meiamine, wherein as extraction solvent preferably aqueous solutions without ammonia from the cycle of purification and recovery of meiamine are used; a stripping section for removing residual CO 2 from the aqueous solution without ammonia; - A cooling section and a purification section for the recovery and purification of meiamine fed with ammonia aqueous streams from the production cycle, optionally with the addition of NH3; - A precipitation section by crystallization of purified meiamine associated with the separation and drying of the melamine product; - An Ammonia Separation section for the production of ammonia at high purity by removal from the crystallization mother eyes; - A Section for the treatment of mother liquors without ammonia, leaving the section of ammonia separation.
    [17]
    A cycle according to claim 16, wherein the Extractive Distillation Koiorn or Distex is the first column of the group according to claim 14 dedicated to the separation and recovery of highly pure CO 2. and very pure NH 3 flows from waste gas from an installation for the production of melamine.
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    同族专利:
    公开号 | 公开日
    NL2017130B1|2018-04-16|
    RU2713178C2|2020-02-04|
    BR102016016006B1|2021-06-08|
    ITUB20152258A1|2017-01-17|
    CN106349174B|2021-10-15|
    BR102016016006A2|2018-02-20|
    DE102016112944A1|2017-01-19|
    RU2016119933A|2017-11-29|
    RU2016119933A3|2019-09-18|
    CN106349174A|2017-01-25|
    CN113234035A|2021-08-10|
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    法律状态:
    2021-04-07| PD| Change of ownership|Owner name: EUROTECNICA MELAMINE AG; CH Free format text: DETAILS ASSIGNMENT: CHANGE OF OWNER(S), CHANGE OF LEGAL ENTITY; FORMER OWNER NAME: EUROTECNICA MELAMINE, LUXEMBURG, ZWEIGNIEDERLASSUNG IN ITTIGEN Effective date: 20210311 |
    优先权:
    申请号 | 申请日 | 专利标题
    ITUB2015A002258A|ITUB20152258A1|2015-07-17|2015-07-17|Process for the production of melamine with separation and recovery of high purity CO2 and NH3|
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