This application claims priority to Italian Application No. MI2010A 000810, filed on May 6, 2010, the text of which is also incorporated by reference.
The present invention relates to a low-energy-consumption process for the production of high-purity melamine, through the high pressure pyrolysis of urea, and the relative equipment.
More specifically, the invention relates to a process which comprises the collection and purification in aqueous solution of the melamine produced in the pyrolysis reactor, and its separation by crystallization.
It is known that the transformation of molten urea into molten melamine is described by the following over-all reaction (1):
6 NH2CONH2→(CN)3(NH2)3+6 NH3+3 CO2 (1)
urea melamine
according to which, for each kilogram of molten melamine 1.86 kg of NH3 and CO2 are formed, called as a whole off-gas.
One of the most widespread industrial processes based on the pyrolysis of urea at high pressure, is that described in patent U.S. Pat. No. 3,161,638 in the name of Allied. In this process, the whole biphasic effluent coming from the melamine synthesis reactor is cooled and collected in an aqueous ammonia medium.
For a detailed illustration of the process according to patent U.S. Pat. No. 3,161,638 mentioned above and in order to appreciate the improvements and advantages provided by the present invention, FIG. 1 shows the simplified block scheme of an embodiment of the above process according to the state of the art.
According to the scheme of FIG. 1, the urea (stream 2) produced in an adjacent synthesis plant (not shown in FIG. 1) is sent in the liquid state, at a temperature of 135-145° C., to a reaction section R consisting of a pyrolysis reactor where an appropriate heating system maintains the reagent system at a temperature of about 360-420° C.; the pressure is kept at a value higher than 70 barrel. Anhydrous gaseous NH3 (stream 12) is also preferably introduced into the reactor together with the molten urea. The reactor has one single step and the reagent mass is maintained under strong circulation by the gases formed during the pyrolysis of the urea.
The whole reacted mass, consisting of a biphasic liquid/gas effluent (stream 4), is discharged into a quench section Q where, by contact with an aqueous ammonia solution (stream 31), its temperature is lowered to about 160° C. Under the above operating conditions, all the melamine, the non-reacted urea and various impurities formed during the synthesis (for example oxyaminotriazines OATS and polycondensates) pass into solution and are sent to the subsequent treatment (stream 5), whereas the remaining gaseous phase, substantially consisting of NH3 and CO2 coming from the reactor and saturation water vapour, is separated and sent to the urea plant (damp off-gas stream 19), after undergoing possible treatment (not indicated in FIG. 1), such as for example, condensation by absorption in aqueous solution.
The stream 5 contains a certain quantity of dissolved NH3 and CO2, which are eliminated in the subsequent stripping section with vapour StrS. The elimination of the CO2 is necessary for obtaining a high purity of the melamine in the subsequent treatments; the elimination of the NH3 is not necessary, but takes place due to the nature of the liquid/gas equilibria in the water—NH3-CO2 system.
Two streams leave the StrS section: a gaseous stream (stream 33) comprising the NH3 and CO2 extracted from stream 5; and an aqueous stream comprising melamine and the remaining impurities (stream 6).
The stream 33 is sent to an absorption section Abs where it comes into contact with an aqueous stream 30, forming an aqueous stream 31 comprising the CO2 and NH3 extracted from stream 5. The aqueous stream 31 is in turn sent to the quench section Q, where, as already mentioned, it is used for cooling and dissolving stream 4 leaving the reactor.
The aqueous stream leaving the bottom of the section StrS (stream 6) contains residual CO2 for about 0.3-0.5% in mass, melamine for about 6-12% in mass, impurities of OATs and polycondensates and also the urea leaving the reactor and non-hydrolyzed to NH3 and CO2 in the quench Q and in the stripping StrS. This urea hydrolyzes in the sections of the plant downstream of the stripping StrS, as far as the deammoniation section AR (see further on), leading to the undesired formation of additional CO2.
Due to their low solubility, the polycondensates must be eliminated before sending said stream 6 to the crystallization section Cr for the recovery of the melamine.
In order to eliminate the polycondensates, NH3 (stream 34) is added to stream 6 until 12-15% in mass is reached. The resulting stream remains at about 170° C. in an ammonolysis section AL, in an apparatus called “ammonolyser”, where the polycondensates are almost totally eliminated transforming most of them to melamine.
The aqueous ammonia solution leaving the section AL (stream 7) is sent to the finishing filtration section F and then to the crystallization section Cr (stream 8), where the temperature is lowered to about 40-50° C. and most of the melamine crystallized. The high concentration of NH3 and low concentration of CO2 in the crystallization allow the OATs, whose solubility in a base increases considerably with the pH increase, to be maintained in solution, thus separating a high-purity product (higher than 99.8% in mass). The control of the CO2 concentration in the crystallization is therefore a critical point of the process.
The aqueous ammonia suspension comprising the crystallized melamine leaving the section Cr (stream 9) is sent to the solid/liquid separation section SLS, where the crystallized melamine (stream 10) and a stream of mother liquor (stream 23) comprising OATs—formed during the pyrolysis reaction and also deriving from the hydrolysis of melamine in the various apparatuses where it has resided in warm aqueous phase—are separated from stream 9.
The stream of mother liquor 23, where the residual melamine is present at a concentration of 0.8-1% in mass, cannot be directly recycled to the quench section Q: this direct recycling would cause an increase in the concentrations of OATs, exceeding the saturation concentration in crystallization, where it would precipitate together with the melamine, polluting the product. The mother liquor stream 23, on the other hand, cannot be discharged directly into the environment due to environmental and economical problems linked to the strong content of NH3 and organic compounds, among which melamine.
In order to overcome the problems connected to the direct recirculation of the crystallization mother liquor, the process according to patent U.S. Pat. No. 3,161,368 envisages the deammoniation of the mother liquor in a section AR where three streams are separated by distillation: a stream of NH3 substantially free of CO2, to be recycled to the ammonolysis section AL (stream 26); a stream rich in CO2, which is recycled to the absorber Abs (stream 27); an aqueous stream almost free of NH3 and practically comprising only melamine and OATs (stream 28).
The stream 28 of deammoniated mother liquor is sent to a section OE for the elimination of the OATs and to obtain an aqueous solution to be recycled to the quench section Q (stream 30). The section OE can be obtained in two different ways, both applied in the state of the art:
The process illustrated above is currently applied industrially in numerous plants, but is jeopardized by a high consumption of raw materials and utilities, such as for example, vapour, cooling water, fuel gas, electric energy). In particular, there is a high vapour consumption, due to the vapour stripping StrS and to the treatment of all the mother liquor in the deammoniation AR by distillation.
The solution of some of the drawbacks of the state of the art described above is contained in patent U.S. Pat. No. 7,125,992 in the name of the same Applicant, which envisages a decrease in energy consumptions, investment and specific consumption of urea necessary for the production of melamine.
The process according to U.S. Pat. no. 7,125,992 is illustrated in FIG. 2, where the streams and sections corresponding to those of the scheme of FIG. 1 are identified with the same numbers or abbreviations as FIG. 1 and will not be further described.
Unlike the state of the art represented by the process according to U.S. Pat. No. 3,161,368, the process according to U.S. Pat. No. 7,125,992 can be distinguished by:
The stripping StrN eliminates the CO2 present in the raw melamine leaving the reaction through anhydrous gaseous NH3 (stream 13); said NH3 also favours the completion of the pyrolysis reaction, increasing the yield and eliminating the residual urea which would hydrolyze down-stream giving further CO2.
With respect to the process according to U.S. Pat. No. 3,161,368, the process described in U.S. Pat. No. 7,125,992 offers the following advantages:
In the process described in U.S. Pat. No. 7,125,992, there is the necessity of recovering the melamine vapour contained in the anhydrous off-gas, before sending the latter to the urea plant. The concentration of melamine vapour in the off-gas is not negligible, possibly amounting to 2-5% in mass, depending on the operating conditions of the reactor and efficiency of the gas/liquid separation.
The above recovery is effected in the OGQ washing section using the same molten urea fed to the melamine plant (stream 1).
The use of the molten urea allows practically all of the melamine to be recovered and the anhydrous off-gas (stream 19) to be sent to a point of the urea plant which is at a compatible pressure. The molten urea which contains the melamine recovered is sent (stream 2) to the reactor.
Equipment for washing off-gas with molten urea is described, for example, in patents U.S. Pat. No. 3,700,672 in the name of Nissan Chemical and U.S. Pat. No. 4,565,867 in the name of Melamine Chemicals.
The OGQ section operates at approximately the same reaction pressure and temperatures generally around 180° C. The choice of the washing temperature derives from a compromise between the necessity of preventing the formation of solid ammonium carbamate from NH3 and CO2 and the necessity of limiting the degradation of the urea to undesired solid products (such as biuret, triuret or cyanuric acid). All these solid products cause considerable operative problems.
In the washing with molten urea, the condensable substances (essentially melamine) contained in the off-gas are condensed and solidified, remaining in solution/suspension in the molten urea. A significant dissolution of the same off-gas in the molten urea also takes place in parallel: data available in the state of the art indicate for example that the molten urea leaving the washing can contain 3-6% in mass of melamine in solution/suspension and about 20% in mass of off-gas in solution/emulsion. This generates an undesired recycling of the same off-gas towards the reactor, which, on the other hand, should be entirely sent to the urea plant.
Washing with urea consequent has various disadvantages:
An objective of the present invention is to overcome the drawbacks indicated in the state of the art.
A first object of the present invention relates to a low-energy-consumption process for the production of high-purity melamine, through the pyrolysis of urea, comprising the following operative steps:
A second object of the present invention relates to the equipment for applying the above process, comprising:
In its essence, the process, object of the present invention, envisages separately treating the two components of the biphasic liquid/gas effluent which is generated during the pyrolysis reaction of urea, i.e. the gaseous phase consisting of anhydrous off-gas and the liquid phase consisting of raw melamine.
In particular, the separate removal of the CO2 from the anhydrous off-gas and CO2 from the raw melamine requires a much lower energy consumption with respect to the removal of CO2 from the whole biphasic effluent collected in an aqueous ammonia solution as is the case in the state of the art (FIG. 1).
The process according to the present invention and the advantages deriving therefrom can be better understood from the following description of one of its embodiments, illustrated in FIG. 3.
The description of this embodiment and relative process scheme should not be considered as limiting the protection scope defined by the enclosed claims.
The block diagram of FIG. 3 shows the main sections of a melamine production plant and the main material streams according to the process of the present invention.
A stream 2 of liquid urea at a temperature higher than the melting point (equal to about 133° C.) and a stream 12 of gaseous, anhydrous NH3 are sent to a reaction section R. The section R comprises a reactor equipped with a suitable heating system which maintains the reagent system at a temperature of about 360-420° C.; the pressure is maintained at a value higher than 70 barrel.
Inside the reactor, or in one or more separators positioned downstream, (not shown in FIG. 3), or again in the stripping section with NH3 (StrN) downstream of the reactor (described hereunder), the biphasic liquid/gas effluent produced by the urea pyrolysis reaction, is separated in a liquid stream 3 of raw melamine comprising non-reacted urea, NH3, CO2 and impurities such as OATs and polycondensates and a first stream of anhydrous off-gas 15, comprising NH3, CO2 and melamine vapour.
When the separation is effected in section StrN, phases a) and b) of the process are effected contemporaneously in this section, from which a single anhydrous off-gas stream exits, comprising NH3, CO2 and melamine vapour, which is sent to phase c) to be washed with water, and a stream of raw melamine impoverished in CO2.
The liquid stream 3 of raw melamine and the stream of anhydrous off-gas are subjected to two different types of treatment for the recovery of the melamine contained therein.
The liquid stream of raw melamine 3 is sent to a stripping section StrN, which preferably operates at the same temperature and pressure conditions as section R, wherein it is put in contact with a stream 13 of anhydrous, gaseous NH3. The mass ratio between said stream 13 and said stream 3 ranges from 0.06 to 0.60, and is preferably equal to 0.20.
The anhydrous gaseous stream of NH3 flows in close contact with the liquid stream of raw melamine and extracts the CO2 dissolved therein, until a residual concentration of about 200 ppmw is reached, lower than that obtained in the stripping section StrS of the state of the art represented by the procedure of FIG. 1.
Furthermore, the residence of the raw melamine under the stripping conditions of CO2 with NH3 offers advantageous side-effects:
A liquid stream of melamine (stream 4) therefore leaves the section StrN, which is not only practically free of CO2, but also partially purified as it is practically free of urea and contains reduced quantities of OATs and polycondensates. A second stream of anhydrous off-gas (stream 16) also leaves the section StrN, mainly comprising NH3, a little CO2 and a certain quantity of melamine vapour.
The anhydrous off-gas streams coming from the sections R and StrN are subjected, together or separately, to a recovery treatment of melamine by washing with water, in a single washing section OGQ or in separate washing sections (not represented in FIG. 3). The second anhydrous off-gas stream 16 is preferably joined to the first stream 15 of anhydrous off-gas, forming a single stream 17 of anhydrous off-gas to be subjected to the same treatment in a single washing section OGQ.
The stream 17 can contain up to 10% in mass of the total melamine vapour produced in the sections R and StrN. Said melamine is recovered in the section OGQ by putting the stream 17 in contact with an aqueous washing stream, preferably consisting of one or more streams collected from suitable points of the same melamine plant (in FIG. 3 these streams are represented by a single stream 32), with the formation of aqueous stream (stream 20), comprising melamine, NH3 and CO2.
The section OGQ operates at temperatures ranging from 125-190° C., preferably 160-175° C., and pressures within the range of 20-30 barrel., preferably at about 25 barrel; the mass ratio between the stream 32 and the stream 17 ranges from 0.3 to 2.0, preferably from 0.4 to 0.7.
Some possible ways for forming the washing section of the off-gas with aqueous solution are illustrated in the patents CN 1300122C and in the international patent application WO 03/095516 A1 in the name of the Applicant, which specifically and exclusively relate to this operation.
The gaseous stream leaving the section OGQ after washing (stream 19) consists of damp off-gas substantially including NH3, CO2 and saturation water vapour; it is sent to the urea plant as such or after undergoing treatment (not shown in FIG. 3), such as, for example, condensation by absorption in aqueous solution.
The aqueous stream leaving the section OGQ (stream 20) is sent to a section OGS for the separation of CO2, where a part of the CO2 contained therein is removed, preferably by flash, and even more preferably by vapour stripping, produced, for example, by a reboiler at the bottom of the stripper itself. A gaseous stream rich in CO2 (stream 22) is recovered from the section OGS and sent to a point of the process where it can no longer contribute to lowering the crystallization pH, together with an aqueous stream comprising melamine and impoverished in CO2 (stream 21).
As already mentioned, the liquid stream 4 of melamine, substantially without CO2, leaves the stripping section with NH3, StrN. The same is sent to a quench-ammonolysis section QAL, where the quench (dissolution in water of the raw melamine) and ammonolysis (elimination of the polycondensates) treatments are effected, corresponding to the treatments effected in the sections Q and AL, respectively, of the state of the art (FIG. 1).
The section QAL can consist of one or more stationing apparatuses, preferably a single apparatus. The stream 4 enters the bottom of said equipment, maintained under vigorous stirring, and is put in close contact with an aqueous ammonia solution (stream 36), where it is completely dissolved at a temperature of 160-180° C., preferably 170-172° C. The stirring and contact between streams 4 and 36 can be obtained by means of distributors, static mixers, fillings, internal stirrers, external circulation pumps or any other system normally used in chemical industry for favouring the complete mixing of various fluid streams.
The aqueous stream 36 is formed by the joining of an aqueous stream (stream 31) coming from the subsequent phases of the melamine process and a stream of NH3 (stream 34), formed, in turn, by the joining of a stream of NH3 26 also coming from the subsequent phases of the melamine process and the make-up stream of NH3 11; a direct recycling aqueous stream of the crystallization mother liquor (stream 25) is also sent to the quench-ammonolysis section.
The mass ratios, at the inlet of the quench-ammonolysis, between the aqueous streams 31, 34, 25 and the stream of molten melamine 4, are selected so as to have at the exit, in the aqueous ammonia solution comprising purified melamine (stream 35), concentrations of NH3 ranging from 10 to 17% in mass (preferably from 12 to 15% in mass) and concentrations of melamine ranging from 5 to 19% in mass (preferably from 7 to 15% in mass, more preferably from 9 to 11% in mass).
The residence time in the quench-ammonolysis equipment is sufficient for almost completely eliminating the polycondensates present in the stream 4, transforming most of them into melamine. As far as the OATs are concerned, the unification of the quench and ammonolysis sections and the suppression of the intermediate vapour stripping section (section StrS in FIG. 1) reduce the residence time of the melamine in aqueous phase under heat, causing a lower formation of OATs through hydrolysis (from about 10 to about 20% less with respect to the state of the art represented in FIG. 1, according to the reduction degree applied to the various residence times in the various apparatuses).
The stream 21 coming from the section OGS, is preferably joined with the stream 35 leaving the quench-ammonolysis to form the stream 7; from this point on, the melamine leaving the reaction section R and the stripping section with NH3 StrN in vapour phase, joins the melamine which has left the same sections in liquid phase, and undergoes the same treatment.
We note incidentally that the stream 21 comprising the melamine coming, as vapour, from the reaction and stripping with NH3, can also have different destinations with respect to the destination illustrated in FIG. 3:
The stream 7 is sent to the filtration section F, consisting of finishing filters which also complete the ammonolysis treatment. The stream leaving the section F (stream 8) is sent to the crystallization section Cr, where the melamine is precipitated by lowering the temperature to a value of about 40-50° C., obtaining an aqueous suspension of high purity melamine (stream 9).
The low content of CO2 in the stream 21 obtained by means of the section OGS and the low content of CO2 in the stream 35 obtained through the section StrN, allow the crystallization pH to be kept high, thus operating under more unfavourable conditions for the precipitation of OATs. Unlike the state of the art (FIG. 1), these advantageous conditions for the crystallization of pure melamine are obtained: in the section OGS, by separating the CO2 not from the whole stream of melamine produced (stream 5, FIG. 1), but only from the stream of melamine recovered from the off-gas (stream 20, FIG. 3), which is quantitatively much less; in the section StrN, by stripping the CO2 without using vapour as in the section StrS (FIG. 1), but using only NH3 coming from the urea plant (stream 13, FIG. 3) and being returned to this urea plant with the damp off-gas (stream 19 of FIG. 3). Furthermore, in the section StrN, only CO2 is removed, whereas, as already mentioned, in the section StrS, the NH3 must also be removed.
The stream 9 leaving the section Cr is subjected to a solid/liquid separation in the section SLS, where the melamine crystals (stream 10) are separated from the crystallization mother liquor (stream 23) and sent to the drying and packaging sections (not indicated in FIG. 3).
As already mentioned, thanks to the lower residence time of the melamine in hot aqueous phase, the stream 23 of mother liquor contains a lower quantity of OATs with respect to the corresponding stream 23 in the process of the state of the art represented in FIG. 1; it can therefore be partially recycled directly to the quench-ammonolysis section QAL without any treatment, in such a quantity as to remain in any case below the saturation of the OATs in the crystallization.
The stream 23 leaving the separator SLS is therefore divided into two streams (streams 24 and 25): stream 25 is recycled directly to the quench-ammonolysis section QAL, whereas stream 24 is sent to the deammoniation section AR.
The lower the quantity of OATs in the mother liquor, the greater the fraction of mother liquor which can be recycled directly to the section QAL, and the greater the savings on the vapour consumption and investment in the section AR and the direct recovery of melamine in the section QAL will be. With the process according to the present invention, the amount of mother liquor which can be recycled directly to the section QAL (stream 25) is 10% higher than the total amount of mother liquor (stream 23); it is preferably 20% higher.
The fraction of mother liquor which cannot be recycled directly (stream 24) is subjected to deammoniation in the section AR, which separates three streams: a stream of NH3 substantially free of CO2, to be recycled to the ammonolysis section QAL (stream 26); a stream rich in CO2 (stream 27); an aqueous stream comprising melamine, OATs and substantially free of CO2 and NH3 (stream 28 of deammoniated mother liquor). The deammoniation AR can operate with any known method for the separation of water-NH3-CO2 mixtures.
The stream of NH3 26 is extracted from the section AR preferably in the gaseous state and then mixed with a part of the aqueous stream 31 (not shown in FIG. 3); in this way, the condensation enthalpy of the NH3 heats the resulting stream, with a positive effect on the thermal balance of the section QAL and consequently on the vapour consumption of the whole plant. The recovery of the condensation heat of the NH3 cannot be effected in the state of the art (FIG. 1), where the stream 26 goes to the section AL which does not need the above contribution to its thermal balance.
The stream rich in CO2 27 can be sent directly to the urea plant (FIG. 3), or recycled to a point in the melamine plant; it is preferably recycled to a point where the CO2 contained therein cannot lower the crystallization pH, and can ultimately leave the melamine plant, taking less water with it than if it were sent directly to the urea plant (for example, after treatment which reduces its water content).
The stream of deammoniated mother liquor 28 is sent to a section OE to eliminate the OATs and obtain an aqueous solution to be recycled to the quench-ammonolysis section QAL (stream 31) and off-gas washing section OGQ (stream 32).
The section OE can be obtained in many different ways. The two preferred ways are already applied in the state of the art and are the following:
The process according to the present invention provides the following advantages, relating to the consumption of utilities (especially energy as vapour and cooling water), investment and urea consumption:
The following embodiment example is provided for purely illustrative purposes of the present invention and does not limit its protection scope defined by the enclosed claims.
16.0 t/h of molten urea and 1.0 t/h of gaseous NH3 are sent to the reactor in a melamine plant having a nominal capacity of 40,000 t/y; the reactor operates at 380° C. and 80 barrel.
11.8 t/h of anhydrous off-gas comprising melamine and 5.2 t/h of liquid raw melamine, are separated from the reactor.
Said liquid melamine is treated under the same conditions as the reactor in a stripper with 1.1 t/h of anhydrous gaseous NH3, obtaining 5.1 t/h of liquid melamine comprising only 0.02% in mass of dissolved CO2; this stream also contains 3.4% in mass of polycondensates, 0.6% in mass of OATs and is substantially free of non-reacted urea.
1.2 t/h of anhydrous off-gas comprising melamine vapour are separated from the stripper, which, when joined with the off-gas leaving the reactor, form a stream of 13.0 t/h comprising 3.8% in mass of melamine vapour.
The liquid melamine leaving the stripper is sent to the quench-ammonolyser for purification in aqueous solution with NH3, under operative conditions (temperature 172° C., pressure 25 barrel and concentration of NH3 of 14% in mass) which are as such as to allow the almost total elimination of the polycondensates, mostly transformed into melamine.
63.0 t/h of an aqueous solution at 8% in mass of melamine, comprising about 3,000 ppm in mass of OATs and about 800 ppm in mass of polycondensates, leave the quench-ammonolyser.
The gaseous stream obtained by joining the anhydrous off-gas streams from the reactor and stripper, is sent to a washing column which operates at 169° C. and 25 barrel, where it is put in contact in countercurrent with 7.0 t/h of an aqueous recycling solution.
The washed off-gas leaving the head of the washing column, containing 18% in mass of water and substantially free of melamine, is sent to the adjacent urea plant for the recovery of the NH3 and CO2 contained therein.
6.4 t/h of an aqueous solution comprising 0.5 t/h of melamine entering as vapour with the off-gas, and 4.5% in mass of CO2, leave the bottom of the washing column. This stream is sent to a vapour stripping column, which operates at the bottom at 160° C. and 7 barrel. 4.6 t/h of an aqueous solution leave the bottom of the stripping column, where the CO2 is reduced to 0.2% in mass.
This solution is joined to that leaving the quench-ammonolysis section, and the whole mixture is sent to filters from which a solution with less than 100 ppm in mass of polycondensates exits; the latter solution is then sent to the crystallizer, operating at a temperature of 45° C., a pressure of 0.5 barrel, a concentration of CO2 of about 0.15% in mass and a pH of about 11.5.
The suspension leaving the crystallizer is sent to a solid/liquid separator (centrifuge), which separates the crystallization mother liquor from the high-purity melamine (titre of over 99.9% in mass with respect to the dry product).
The approximately 62.6 t/h of mother liquor are divided into two streams. One stream of 7.5 t/h is recycled directly to the quench-ammonolysis section, without undergoing any treatment; the remaining 55.1 t/h are sent to the deammoniation section for the recovery of the NH3 dissolved therein.
The deammoniation section is composed of a distillation column, from whose head almost anhydrous gaseous NH3 exits. A liquid stream of CO2, NH3 and water leaves an intermediate plate of the column, and it is recycled to a suitable point of the process.
The aqueous solution leaving the bottom of the column is sent for treatment to eliminate the OATs by thermal decomposition alone. In this way, an aqueous solution is separated, which is recycled to the quench-ammonolysis and off-gas washing, together with a further aqueous solution which can be discharged or re-used as water for the utilities (for example make-up water for the cooling towers) or as process water.
A part of the solution recycled to the quench-ammonolysis section is mixed with gaseous NH3 leaving the head of the distillation column of the deammoniation section, condensing it and recovering its condensation heat.
The consumptions of urea and vapour of the whole process prove to be respectively 3.2 t/t and 4.3 t/t of melamine produced, against the corresponding values of 3.4 t/t and 12.5 t/t necessary with the process of the state of the art represented in FIG. 1. If the elimination of the OATs takes place by ultrafiltration followed by decomposition of the retentate, the lower decomposition of melamine lowers the consumptions of urea to 3.05 t/t and 3.2 t/t (instead of 3.2 t/t and 3.4 t/t), for the process, object of the invention, and for the state of the art of FIG. 1, respectively.
The higher efficiency of the process according to the present invention derives in particular from the choice of eliminating the CO2 present in the reaction effluent by separate treatment of the anhydrous off-gas and raw melamine. The separate treatment, in fact, makes it possible to operate in crystallization at the same pH of about 11.5 with respect to the process of FIG. 1, but consuming much less vapour to remove the CO2, to specifically keep its concentration low in crystallization. A further advantage is provided by the possibility of recycling about 12% of the mother liquor stream leaving the solid/liquid separation section directly to the quench-ammonolysis section.
Number | Date | Country | Kind |
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MI2010A 000810 | May 2010 | IT | national |