SANTUCCI ROBERTO (IT)
NOE SERGIO (IT)
SANTUCCI ROBERTO (IT)
| WO2001046159A2 | 2001-06-28 | |||
| WO2002100839A1 | 2002-12-19 |
CLAIMS
1. A process for recovering from the crystallization mother liquor of melamine both melamine and oxyaminotri- azines (OAT) , comprising an ultrafiltration of the flush- ing mother liquor for separating the OAT in the colloidal state from the mother liquor containing them, said ultrafiltration producing a permeate and a retentate in aqueous solution, the permeate impoverished in OAT being recycled to the preparation step of the aqueous solution of raw melamine, said process being characterized in that the retentate is recycled to the synthesis reactor.
2. The process according to claim I 7 characterized in that the retentate is dehydrated before recycling to the synthesis reactor. 3. The process according to claim 2, characterized in that the dehydrated retentate is put in solution/suspension in the molten urea and injected into the synthesis reactor. 4. The process according to claim 1, characterized in that the aqueous retentate as such is joined to the aqueous solution of urea, the joint streams containing the retentate and aqueous solution of urea being dehydrated and recycled to the synthesis reactor. |
IMPROVED PROCESS FOR THE PRODUCTION OF MELAMINE WITH AN INCREASED YIELD
The present invention relates to an improved process for the production of melamine with an increased yield. In particular, the present invention proposes to effect the total recovery of the OAT (oxyaminotriazines) which are generated, as by-product, in plants for the production of melamine from urea.
As is well known, all synthesis processes of mela- mine from urea operate at temperatures higher than 36O 0 C and can be divided into two categories : higher pressure processes in liquid phase, in which the reaction pressure is maintained at over 70 bars and the reactions which lead to the formation of melamine take place without catalysts and low pressure processes in gaseous phase, in which the reaction pressure is lower than 10 bars and the reactions which lead to the formation of melamine take place in the presence of a solid catalyst. For both process categories, the reaction which starting from urea leads to the formation of melamine is represented, in its totality, by the following stoichiometric equation:
6 H 2 NCONH 2 → (CN) 3 (NH 2 ) 3 + 6 NH 3 + 3 CO 2 urea melamine ammonia carbon dioxide
The reaction is endothermic, it is almost totally shifted towards the right (irreversible reaction) and is evidently characterized by the formation of considerable quantities of gaseous by-products . The liquid phase extracted from the reactor (raw melamine) also contains, in addition to not completely converted urea, a certain quantity of non-volatile byproducts consisting of reaction intermediates not completely transformed into melamine and compounds coming from subsequent reactions which involve melamine itself.
Typical examples of the two species of by-products are, ammeline and ammelide (indicated as a whole with the term OAT, an acronym of oxyaminotriazines) and polycon- densates (mainly melam and melem) . These non-volatile by-products accompany the raw melamine leaving the synthesis reactor and form its impurities. They are separated from the main product, i.e. melamine, by means of specific forms of purification treatment which characterize the various production proc- esses.
In most processes currently used, the raw melamine is dissolved in an aqueous solution from which pure melamine is separated by crystallization. The resulting aqueous solution after the separation of the pure melamine, i.e. mother liquor, however, still contains a significant
quantity of dissolved residual melamine. In order to effect a production process characterized by a high yield to melamine, it is therefore necessary for said mother liquor to be either totally or partially recycled to the production process itself.
The recycling of the crystallization mother liquor is therefore conditioned by the non-volatile by-products present in the raw melamine leaving the synthesis reactor which, as they are also dissolved in the mother liquor, can limit the purity of the end-product.
The non-volatile by-products coming from the subsequent reactions involving melamine (typically polyconden- sates) , are eliminated in the purification processes of the mother liquor and, for the most part, transformed again into melamine with a yield recovery, and partially into OAT in proportions depending on the specific purification cycle proposed.
Vice versa, as the non-volatile by-products consisting of the reaction intermediates (typically OAT) , cannot be retransformed into melamine in the aqueous treatment medium, they must be continuously extracted from the cycle to prevent their accumulation which would finally lead to the saturation threshold. This must be prevented as, if the concentration of OAT in the circulating aque- ous solution were to reach the saturation value, the OAT
would precipitate together with the melamine in the crys- tallizer with the consequent polluting of the end- product .
The extraction of OAT from the aqueous purification cycle is one of the major causes of yield losses for synthesis processes of melamine starting from urea.
The discharging as such of an aqueous flushing stream containing OAT to be eliminated does in fact inevitably cause a loss of melamine as said flushing is normally effected by the solution from the melamine- saturated crystallization mother liquor.
In the various melamine production processes, the crystallization mother liquor (and therefore also the above flushing) generally contains a melamine residue which varies from 5 to 8 g per litre approximately, whereas the OAT are normally present in a quantity ranging from 3 to 4 g per litre.
Consequently, if the OAT were simply extracted from the mother liquor through the flushing stream, without any treatment, there would be an additional much higher loss in yield than that attributed to the OAT alone. The treatment of the flushing stream containing OAT, aimed at reducing the loss of melamine, however, is neither a simple nor economical operation. In order to limit melamine losses, it is in fact
necessary for the OAT to be concentrated as much as possible in the flushing stream, or even better, for the OAT to be separated from the aqueous stream by precipitation and subsequent filtration. When this operation is applied in some plants, it can be extremely difficult as a result of the colloidal nature of the OAT which requires resort to filtration auxiliaries (such as for example the use of infusorial earth) with a not completely satisfactory result, as described hereunder. When this process is applied, the OAT are separated as a filtration panel containing them in a quantity of about 30% by weight, inevitably associated however with a certain quantity of solid, precipitated melamine. In industrial practice the quantity of melamine precipitated and present in the filtration panel is normally lower than 50% of the quantity of OAT on a dry base, and therefore considerably reduced with respect to the quantity present in the flushing as such, again with respect to the quantity of OAT. The presence in the panel of the filtration auxiliary, however, makes the recovery or upgrading both of OAT and melamine associated therewith, difficult.
Using this separation method of OAT, in addition to the delicacy and inherent difficulties in the operation, there is the problem of how to eliminate the filtration
panel without polluting the environment.
In this respect, it is particularly advantageous to use the technique described in WO 01/46159 which illustrates a process for separating OAT in colloidal suspen- sion from aqueous streams containing melamine . According to the process described in WO 01/46159, the separation of the colloidal suspension of OAT is effected according to the ultrafiltration technique using porous ceramic membranes, without the addition of any foreign substance. According to the process cited in WO 01/46159, two separate aqueous streams are obtained: a "permeate" impoverished in OAT, characterized by a content of OAT of less than 100 ppm; a "retentate" which contains practically all the OAT in the form of a pumpable colloidal suspension, in which the quantity of OAT can exceed 11% by weight.
By applying this technique to the flushing stream of the mother liquor, a permeate is obtained which can be totally recycled to the aqueous purification and separa- tion cycle of the melamine, with the recovery of most of the melamine contained in said flushing. According to what is indicated in WO 01/46159, the OAT can be recovered from the retentate with means known in the art.
In commonly applied practice, the retentate is nor- mally completely demolished by hydrolysis at 270-280 0 C so
— 1 —
that all the organic molecules therefore including OAT and melamine, are transformed into ammonia and carbon dioxide, suitable for being possibly recovered as raw material for the production of urea. This treatment, on the one hand allows the organic material contained in the retentate to be recovered, and on the other, discharges clean water into the environment, thus solving the ecological problem. This operation, however, is not only costly from the point of view of energy consumption, but it also causes a significant loss in yield to the detriment of the melamine production process .
An objective of the present invention is therefore to find a process which overcomes the drawbacks of the known art.
All these problems are in fact overcome by the present invention which allows the losses of melamine associated with the flushing stream containing OAT to be completely eliminated, with the recovery of the OAT them- selves and a further yield recovery, contemporaneously also solving the ecological problem.
An object of the present invention therefore relates to a process for recovering from the crystallization mother liquor of melamine, both the residual saturation melamine and the oxyaminotriazines (OAT) , comprising an
ultrafiltration of the flushing mother liquor to separate the OAT in the colloidal state from the mother liquor containing them, said ultrafiltration producing a permeate and a retentate in aqueous solution, the permeate im- poverished in OAT being recycled to the preparation step of the aqueous solution of raw melamine, said process being characterized in that the retentate is recycled to the synthesis reactor.
The main advantage of the process according to the present invention is to allow a further yield recovery.
As the retentate is not polluted by any foreign substance, it can in fact be sent back to the synthesis reactor, where the melamine contained therein is directly and totally recovered and the OAT and other organic mole- cules are in turn transformed into melamine.
In particular, the process according to the present invention envisages that the retentate be dehydrated before being recycled to the synthesis reactor.
The retentate, recycled to the melamine synthesis reactor, allows an increase in the overall yield of the plant which passes from 88.5% of the process according to the present state of the art to the 94.4% of the process according to the present invention. This increase in yield corresponds to a lower specific consumption of urea which passes from 3.23 to 3.03 Kg of urea per Kg of mela-
- S -
mine produced.
With the same annual consumption of urea and other conditions, the plant consequently increases its production capacity by 6.6%. Before sending the retentate to the synthesis reactor, however, the water solution must be removed as, under the reactor temperature conditions, it would destroy the melamine transforming it into ammonia and carbon dioxide in a ratio of a mole of melamine destroyed for every six moles of water:
(CN) 3 (NH 2 ) 3 + 6 H 2 O -> 6 NH 3 + 3 CO 2 The dehydration of the retentate can be effected for example in a conventional scraped-wall dryer.
The dehydrated retentate is then put in solu- tion/suspension in the molten urea, obtaining an extremely fluid pumpable slush, which is injected into the synthesis reactor by means of the same pumps which feed urea to the reactor.
The composition of the feeding stream to the reac- tor, containing the dehydrated retentate, normally consists of 96-97% by weight of urea and 3-4% by weight of dehydrated retentate, the latter in turn essentially consisting of OAT and melamine.
As an alternative to the separate dehydration of the retentate, in the case of plants which start from urea in
aqueous solution and dehydrate the urea in situ (as occurs in most cases) , the process according to the present invention envisages that the aqueous retentate as such be joined to the aqueous solution of urea, thus effecting the contemporaneous dehydration of both streams mixed together .
Without limiting the applicability of the process according to the present invention to all plants and processes for the recovery and purification of melamine in aqueous solution, whether they be at high or low pressure, the present invention will be better understood and the advantages evident from the following description with an embodiment example and with the help of the enclosed figures. The description and figures should not be considered as limiting the scope of the invention.
Figure 1 is a block scheme showing a production process of melamine according to the state of the art .
Figure 2 is a block scheme showing how the recycling of the retentate containing OAT according to the present invention is inserted in a production process of melamine.
We will now describe in detail the block diagram of figure 2. This description however can also apply to the block diagram represented in figure 1 for the parts of the process in common.
The block diagram of figure 2 refers to a melamine production plant starting from urea in aqueous solution. It illustrates a process based on a high-pressure non- catalytic reaction with the recovery and purification of raw melamine in aqueous solution and separation of melamine by crystallization.
The stream of urea in aqueous solution, together with the retentate, also in aqueous solution, enters section 1 (Urea Dehydration) . In this section, all the water is separated and an anhydrous molten mass is produced, which is pumped to section 2 (Reaction) .
The stream of molten urea leaving section 1, enters section 2 (Reaction) where the urea is transformed into melamine and other volatile and non-volatile by-products . The whole product leaving section 2 enters the aqueous circuit for the recovery, purification and separation of the melamine, which comprises sections 3 (Melamine Recovery and Dissolution) , 4 (Purification Aqueous Solution) , 5 (Melamine Crystallization) and 6 (Solid/Liquid Separation) .
The mother liquor circulates through these four sections in which the following operations are effected: dissolution and total recovery of the molten raw melamine coming from section 2, with the contempora- neous separation of the off-gases consisting of NH 3 ,
CO 2 and water vapour (section 3) ; treatment of the aqueous solution with ammonia to eliminate the polycondensates (section 4) ; crystallization of pure melamine from the solution purified as above, by cooling it to approximately 5O 0 C (section 5) ; separation of the solid crystallized mass of melamine according to the known filtration or decanting techniques (section 6) . The mother liquor separated in section 6 is recycled to section 3, after extraction of a flushing stream. The removal of the flushing stream has the main function of maintaining the concentration of the OAT in the crystal- lizer below the saturation threshold, to prevent them from precipitating together with the melamine.
The humid panel of crystallized melamine leaving section 6 is dried in section 7 (Panel Drying) and the dry melamine is sent for storage and packaging for selling. The flushing stream removed from the mother liquor leaving section 6 is subjected to the following treatment : section 8 (Ammonia Separation and Recovery) totally recovers the ammonia which is reused in the plant, mainly in sections 2 and 4; the resulting ammonia-
free solution is cooled and acidified with gaseous CO 2 in section 9 (OAT Precipitation) where the OAT are separated from the solution in the form of a colloidal suspension; - the colloidal suspension is separated in section 10 (Ultrafiltration with Membranes) , into two streams, permeate and retentate, by means of ceramic membranes, as described in WO 01/46159; the retentate practically contains all the OAT in colloidal sus- pension in a ration of over 11% by weight, whereas the permeate is almost totally free of OAT which are present in the solution in a quantity of less than 100 ppm.
The permeate is totally recycled to section 3 to- gether with reintegration demineralized water, thus closing the mother liquor cycle.
In this way, the concentration of the OAT is kept constant in the whole aqueous cycle and, in particular, the concentration of OAT in the solution entering section 5 is controlled, ensuring that said concentration is significantly below the saturation threshold under crystallization conditions.
The quantity of flushing water, separated from the stream of crystallization mother liquor, is calculated so that the quantity of OAT contained in the retentate leav-
ing section 10 is equal to the sum of the OAT leaving section 2 with the molten melamine plus the quantity of OAT formed by hydrolysis in the aqueous circuit.
As observed above, according to the state of the art, the retentate is normally demolished by hydrolysis at 270-280 0 C so that all the organic molecules, therefore including OAT and melamine, are transformed into ammonia and carbon dioxide, suitable for being optionally recovered as raw material for the production of urea. This treatment allows the water contained in the retentate to be discharged into the environment, however, even if NH 3 and CO 2 are recovered, this operation is not only costly from the point of view of energy consumption, but it also causes a considerable loss in yield with re- spect to the production process of melamine.
In the process according to the present invention, on the other hand, the retentate is recycled to section 2 (Reaction) according to two alternative procedures, both illustrated in figure 2. When the melamine production plant is fed with an aqueous solution of urea and is therefore equipped with section 1 (Urea Dehydration) , the retentate is simply added to the feeding stream of said section which it enters mixed with the aqueous solution of urea. No modifi- cation is applied to the plant scheme: it is sufficient
to potentiate the evaporative capacity of section 1 itself for the additional thermal charge corresponding to the evaporation of the water present in the retentate.
In this way, the dehydrated retentate is introduced into section 2, together with the molten anhydrous urea, using the same high-pressure pumps and therefore without any modification of the plant.
The melamine contained in the retentate is thus recovered directly, whereas the OAT are transformed in the reactor mostly into melamine, as indicated by the following stoichiometric equations :
6 (CN) 3 (NH 2 ) 2 OH -> 5 (CN) 3 (NH 2 ) 3 + 3 CO 2 ammeline melamine
6(CN) 3 (NH 2 )(OH) 2 → 4 (CN) 3 (NH 2 ) 3 + 6 CO 2 ammelide melamine
A second procedure for recovering the retentate is indicated in figure 2 with the dashed lines and comprises sections 11 (Retentate Dehydration) and 12 (Mixer) . This procedure is the only possible solution when the melamine production plant is not fed with urea in solution, but with anhydrous urea (solid or molten) .
According to this alternative solution, the retentate is dehydrated in section 11 by means of a standard evaporator of the scraped-wall type, for example, avail- able on the market in various types.
τhis equipment produces an anhydrous product in powder form which is englobed in the molten anhydrous urea in a normal mixer (Section 12) . The urea-retentate mixture in powder form thus obtained is fed to section 2 , using the same high-pressure feeding pumps of molten anhydrous urea . EXAMPLE
An experimental run was effected with recycling of the retentate on a 20,000 t/a melamine production plant, fed with an aqueous solution at 75% by weight of urea, operating according to the high-pressure technology comprising the mother liquor ultrafiltration section.
The plant is fed with 10,940 Kg/h of aqueous solution of urea at 75% by weight which is dehydrated in a two-step concentrator from which a stream of molten urea is emitted, which is sent directly to the reactor by means of high-pressure pumps.
A permeate and a retentate are produced in the ultrafiltration section of the mother liquor. The permeate is recycled to the plant quench, i.e. the recovery and dissolution section of the raw melamine, and therefore totally recovered.
The retentate consists of a colloidal suspension of
OAT in water. In particular, the retentate stream has an hourly flow-rate of 850 Kg and contains 180 Kg of OAT in
colloidal suspension and 8 Kg of melamine in solution. At the outlet of the ultrafiltration area, it has a pressure of 3 bars and can therefore be recycled to the urea concentrator without the necessity of installing specific pumps. In the plant, it was sufficient to connect, by means of suitable piping, the ultrafiltration section to the tank containing urea in solution and add an evaporating unit to the first step of the concentrator.
With this simple modification the plant was able to run with the total recovery of the retentate.
The data collected during the test were the following:
Total flow-rate to the first step of the urea concentrator: 10,940 + 850 = 11,790 Kg /h
Melamine production
2707 Kg/h As the urea fed was equal to :
10,940 x 0.75 = 8,205 Kg/h the overall specific consumption of the plant, including the recycling of the retentate, proved to be:
8,205/2,707 = 3.03 Kg urea/Kg of melamine equal to a yield of 94.4%, with respect to the theoretical value of 2.86. With the same operating factor, considered equal to
7,874 hours/year, the modified plant with recycling of the retentate produces
2,707 x 7,874/1,000 = 21,315 t/a of melaraine, with the same consumption of urea (i.e. the plant increased its production capacity by 6.58%) . COMPARATIVE EXAMPLE
The plant of the previous example was run according to a configuration in according to the state of the art wherein the retentate leaving the ultrafiltration sec- tion, consisting of a colloidal suspension of OAT in water, is sent to the demolisher/stripper for destruction. It is in fact transformed into a gaseous phase, containing NH 3 and CO 2 (the only demolition products) , returned to the adjacent urea plant, and a residual liquid phase consisting of water which can be discharged into the outside environment.
Consequently only 10,940 Kg/h of aqueous solution at 75% by weight of urea enter the urea concentrator and the plant produces, according to the purchase specification, 2,540 Kg/h of melamine which, for an operating factor of 7,874 hours/year, correspond to the production capacity of 20, 000 t/a. The consumption of 100% urea is:
10,940 x 0.75 = 8,205 Kg/h which compared with the hourly production of melamine of
2,540 Kg gives a specific consumption of
8,205/2,540 = 3.23 Kg of 100% urea per Kg of product, equal to a yield with respect to the theoretical value of 88.5%.