RU2783419C2 - Use of urea containing formaldehyde in method of producing melamine by urea pyrolysis and related method of producing melamine - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method of producing melamine by urea pyrolysis carried out at high pressure using urea containing formaldehyde as starting material.

EFFECT: technical result: disclosed method of producing melamine enables to obtain melamine, from which end products are obtained, having high whiteness and/or brightness, high resistance to aging; wherein said characteristics can be obtained with less consumption of dyes and/or other additives, such as, for example, titanium dioxide TiO₂.

6 cl, 1 dwg, 2 ex

Description

The invention relates to the use of urea containing formaldehyde in a method for producing melamine by pyrolysis of urea and to a related method for producing melamine.

In particular, the present invention relates to the use of formaldehyde-containing urea as a feedstock to a pyrolysis reactor in a melamine production process which includes successive steps of purifying melamine produced in the pyrolysis reactor and separating it as a final product.

It is known that the transformation of molten urea into melamine is described by the following general reaction equation (1):

whereby for every kilogram of melamine, 1.86 kg of NH ₃ and CO ₂ are obtained, generally referred to as off-gas.

Methods for converting urea to melamine can be essentially divided into two groups: in particular, high pressure urea pyrolysis methods and low pressure urea pyrolysis methods.

Surprisingly, applicants have found that when melamine is produced by pyrolysis of urea containing formaldehyde, end products made from the melamine thus obtained have advantageous characteristics.

Thus, an object of the present invention is to use formaldehyde-containing urea in a method for producing melamine by pyrolysis of urea.

Another object of the present invention is to provide a method for producing melamine by pyrolysis of urea, in which urea containing formaldehyde is used as a starting material.

In particular, the amount of formaldehyde contained in the urea supplied to the pyrolysis reactor is from 10 to 5000 wt. hours per million, preferably from 50 to 2000 wt. hours per million, and even more preferably from 100 to 500 wt. ppm

Formaldehyde may enter the urea during the preparation of the urea itself, or it may be added to the urea in a suitable manner; a combination of the above cases is possible, that is, the addition of formaldehyde to urea already containing formaldehyde.

Thus, according to the present invention, the expression "urea containing formaldehyde" means both urea, which is a finished product that already contains formaldehyde that enters the urea during its production, and urea, which is a finished product that does not contain formaldehyde, which add later, as well as urea, which is a finished product that already contains formaldehyde, to which formaldehyde is additionally added.

The urea used to make the melamine can come from one or more urea plants, either as an internal stream of the urea plants themselves (which is withdrawn between the two stages of the urea production process and then dehydrated) or as urea, representing the finished product.

When the urea used to produce melamine is obtained as an internal stream of a urea plant, said stream may contain formaldehyde. This is an advantage over the prior art, according to which only streams containing urea, but not containing formaldehyde, are sent to melamine plants.

When the urea used to make melamine is the final product, such urea can be any type of urea ("grade"), whether or not containing formaldehyde.

For example, such urea may be of a fertilizer grade ("fertilizer grade" containing formaldehyde) or an industrial grade ("technical grade" of formaldehyde free). It is also possible to use different types of urea mixtures. This is an additional advantage of the present invention over the prior art in which only formaldehyde-free ureas, typically "technical grade" urea, are sent to melamine plants. Indeed, this type of urea is commercially available in small quantities and is a more expensive urea grade than "fertilizer grade", as it is usually produced on order and during its lifetime, after plant refurbishment. Greater flexibility in the use of feedstocks gives more options for locating melamine plants as they can be supplied with urea that is commercially available in the region.

In addition, the formaldehyde added to urea can be added as such, or in a compound, or as a mixture containing formaldehyde, for example, in the form of form-urea (an aqueous solution containing high concentrations of urea and formaldehyde, which, for example, in the so-called form-urea 80 (Formurea 80) are 23 and 57 wt.%, respectively).

As mentioned above, the use of formaldehyde-containing urea as a raw material for the production of melamine makes it possible to obtain melamine from which finished products having particularly advantageous characteristics can be obtained.

Indeed, finished products obtained from said melamine have more pronounced characteristics of brightness (regardless of their color) and whiteness (if they are white), as well as greater resistance to aging, and while these characteristics can be obtained with less consumption of dyes and / or other additives such as, for example, titanium dioxide TiO ₂ .

Such an unexpected improvement in the characteristics of the product obtained from the melamine produced in this way is observed in finished products of all types for which surface properties are important: in laminates, molded materials, surface coatings, etc.

In addition, in some cases, melamine obtained by the formaldehyde-containing urea pyrolysis method of the present invention can be visually identified because it fluoresces when illuminated with ultraviolet light. This effect is noticeable if a certain amount of formaldehyde is present in urea.

As already mentioned, the present invention is also to provide a method for producing melamine by pyrolysis of urea, using urea containing formaldehyde as a raw material. Such a method for producing melamine by pyrolysis of urea can be both a high pressure method and a low pressure method, and preferably a high pressure method.

As an illustrative example, a high-pressure process that can be carried out using formaldehyde-containing urea according to the present invention can be mentioned. One of the most widely used industrial processes based on high pressure pyrolysis of urea is that described in US Pat. To illustrate the method discussed in the above US patent 3161638, figure 1 shows a simplified block diagram of an exemplary implementation of the above method.

According to the scheme shown in Figure 1, the urea (stream 2), obtained in a nearby synthesis unit (not shown in scheme 1), is sent in liquid form, at a temperature of from 135 to 145 ° C, to the reaction section P, consisting from a pyrolysis reactor in which the temperature of the reaction system is maintained by means of a suitable heating system in the range from about 360 to 420° C. and the value of the maintained pressure exceeds 7 MPa (rel.) (70 bar (rel.)). Preferably, anhydrous NH ₃ gas is also introduced into the reactor along with the molten urea (stream 12). The reactor is a one-stage reactor, and the reaction mass is vigorously mixed due to the release of gases formed during the pyrolysis of urea.

In one embodiment of the present invention, the urea (stream 2) obtained in a nearby synthesis unit contains formaldehyde.

The entire reacted mass, consisting of a two-phase recoverable stream containing liquid and gas (stream 4), is discharged into the reaction stop section OR, in which, upon contact with an aqueous ammonia solution (stream 31), its temperature is lowered to approximately 160 ° C. Under these operating conditions, all melamine, unreacted urea, and various contaminants formed during the synthesis (for example, hydroxy-amino-triazines (OAT) and polycondensates) go into solution, and they are sent for further processing (stream 5), while while the remaining gaseous phase, essentially consisting of NH ₃ and CO ₂ , extracted from the reactor and from the saturated steam, is separated and, after possibly carried out processing (not shown in scheme 1), for example, condensation by absorption with an aqueous solution, is sent to urea plant (wet exhaust gas stream 19).

Stream 5 contains a certain amount of dissolved NH ₃ and CO ₂ , which are recovered in the next OTS stripper with steam. Removal of CO ₂ is necessary to obtain high purity melamine in the subsequent steps; removal of NH ₃ is not necessary, but proceeds due to the nature of the liquid/gas equilibria in the water-NH ₃ -CO ₂ system.

Two streams are recovered from the OTC section: a gaseous stream (stream 33) containing NH ₃ and CO ₂ recovered from stream 5, and an aqueous stream containing melamine and residual impurities (stream 6).

Stream 33 is sent to absorption section Abs where it is brought into contact with water stream 30 to form water stream 31 containing the CO ₂ and NH ₃ recovered from stream 5. In turn, water stream 31 is sent to reaction stop section OP, wherein said stream is used to cool and dissolve stream 4 withdrawn from the reactor.

The water stream leaving the bottom of the OTS section (stream 6) contains approximately 0.3-0.5% wt. residual CO ₂ approximately 6-12% wt. melamine, OAT contaminants and polycondensates, as well as urea removed from the reactor and not hydrolysed to NH ₃ and CO ₂ in the OP section of the reaction stop and the OTS stripper section. This urea is subjected to hydrolysis at plant sites located downstream of the OTS stripper section, but before it enters the DA deammonization section (see below), resulting in an undesirable additional amount of CO ₂ .

Due to their low solubility, the polycondensates must be removed before said stream 6 is fed into the crystallization section Kp in which the melamine recovery is carried out.

To remove polycondensates, NH ₃ is added to stream 6 (stream 34) to obtain a concentration of 12-15% wt. In the AL ammonolysis section, the resulting stream is kept at a temperature of approximately 170°C in a device called "ammonolysis device", in which the polycondensates are almost completely removed and most of them are converted to melamine.

Aqueous ammonia solution extracted from the AL section (stream 7) is sent to the filtration section F for final processing and then to the crystallization section Kp (stream 8), in which the temperature of the stream is lowered to approximately 40-50 ° C, and most of the crystallization occurs. melamine. The high concentration of NH ₃ and the low concentration of CO ₂ during crystallization make it possible to keep the OAT in a dissolved state, the solubility of which in an alkaline medium increases strongly with increasing pH, which thus allows the product to be isolated with a high degree of purity (more than 99.8% wt. ).

An aqueous ammonia slurry containing crystallized melamine, extracted from section Kr (stream 9), is sent to the OTJ section for separating solids from liquid, in which stream 9 is separated into crystallized melamine (stream 10) and a stream of mother liquors (stream 23) containing OAT obtained in the pyrolysis reaction, and OAT obtained by the hydrolysis of melamine in various devices, in which the OAT remains in the heated aqueous phase.

The mother liquor stream 23, in which the residual concentration of melamine is 0.8-1% wt., cannot be recycled directly to the reaction stop section OR: such direct recycling would lead to a gradual increase in the concentration of OAT to a saturation concentration and above, leading to crystallization, that is, the accumulated OAT would precipitate along with the melamine, contaminating the product. On the other hand, the mother liquor stream 23 cannot be directly discharged to the environment since its high content of NH ₃ and organic compounds including melamine would cause environmental and economic problems.

To overcome the above problems, the process described in US Pat. No. 3,161,368 involves deammonizing the mother liquors in the DA section, in which separation is carried out by distillation to produce three streams: an NH ₃ stream substantially free of CO ₂ , which is recycled to the AL ammonolysis section (stream 26); a stream rich in CO ₂ that is recycled to the Abs absorber (stream 27); and an aqueous stream substantially free of NH ₃ and almost exclusively containing melamine and OAT (stream 28).

After ammonia removal, mother liquor stream 28 is sent to the OE section, in which OAT is removed and an aqueous solution is obtained, which is recycled to the reaction stop section OP (stream 30). The OE section can have one of two possible designs used according to the prior art:

a) in one case, the OATs crystallize upon cooling and neutralization with CO ₂ and are then separated from the stream 28 by ultrafiltration. With this division, you get:

- an aqueous stream 30 essentially free of OAT and enriched in melamine, which is recycled to the reaction stop section OP (via the Abs absorber), thus also recovering the melamine contained therein;

- a stream enriched with OAT in suspension, directed to decomposition in order to extract organic compounds in the form of NH ₃ and CO ₂ , with the formation of an aqueous stream (stream 29), essentially free of pollution, which can be released into the environment or used again in a suitable plant section, for example instead of demineralised water;

b) otherwise, the entire stream 28 is decomposed to recover organic compounds in the form of NH ₃ and CO ₂ , resulting in an aqueous stream (stream 29) substantially free of contaminants that can be released to the environment or reused in a suitable section of the plant, and an aqueous stream 30 which is recycled to the reaction stop section OP (via the Abs absorber).

To illustrate the present invention, the following are exemplary embodiments of the invention, without limiting the scope of the invention indicated in the appended claims.

EXAMPLE 1

In accordance with the formulation known from the prior art, a molding composition was obtained:

initial mixture 97.09% wt. zinc stearate 0.97% wt. catalyst 0.49% wt. titanium dioxide 1.46% wt.

The initial mixture consisted of alpha-cellulose and melamine-formaldehyde resin (in this case, the mass ratio of cellulose/resin in the mixture was 30/70, and the molar ratio of formaldehyde/melamine in the resin was 2/1);

the melamine-formaldehyde resin was obtained from melamine coming from a plant that was charged with formaldehyde-free urea;

zinc stearate was a plasticizing and lubricating additive;

the catalyst was a crosslinking catalyst;

titanium dioxide was a bleaching, matting and coating agent that eliminated the translucency of the product (which otherwise has a porcelain-like appearance) and provided it with whiteness and brightness (in the absence of dyes, which is the case in this Example 1) or coloring and brightness (in case of adding dyes).

EXAMPLE 2

The molding mass was obtained in accordance with the following formulation according to the present invention:

initial mixture 98.04% wt. zinc stearate 0.98% wt. catalyst 0.49% wt. titanium dioxide 0.49% wt.

The original mixture had the same composition as the mixture of Example 1;

melamine-formaldehyde resin was obtained from melamine coming from the installation, which was loaded with urea containing 250 wt. hours per million formaldehyde.

Products obtained from the composition of Example 2, always prepared in the absence of dye, were white and had greater whiteness and brightness than products obtained from the composition of Example 1, despite the fact that approximately 1/3 of the titanium dioxide added to the composition of Example 2 was added in the composition of Example 1.

Claims (8)

1. The use of urea containing formaldehyde in a method for producing melamine by pyrolysis of urea, characterized in that formaldehyde is added to urea coming from urea plants before supplying said urea to the melamine plant, while formaldehyde is present in urea in an amount comprising from 10 to 2000 wt. parts per million. 2. Use according to claim 1, characterized in that formaldehyde is present in urea in an amount ranging from 50 to 2000 wt. parts per million, more preferably from 100 to 500 wt. parts per million. 3. Use according to claim 1, characterized in that the urea is any type of urea, for example fertilizer grade or technical grade, or a mixture of different types of urea. 4. Use according to claim 1, characterized in that the formaldehyde is added as such or as a compound or mixture containing formaldehyde, for example form-urea. 5. A method for producing melamine by pyrolysis of urea carried out at high pressure, characterized in that it is carried out using urea containing formaldehyde as a starting material, and the formaldehyde is added to the urea coming from the urea plants before feeding said urea into the plant for obtaining urea, while formaldehyde is present in urea in an amount ranging from 10 to 2000 wt. parts per million. 6. The method according to claim 5, characterized in that formaldehyde is present in urea in an amount ranging from 50 to 2000 wt. parts per million, more preferably from 100 to 500 wt. parts per million. 7. The method according to claim 5, wherein the urea is any type of urea, such as fertilizer grade or industrial grade, or a mixture of different types of urea. 8. The method according to claim 5, characterized in that the formaldehyde is added as such or in the form of a compound or mixture containing formaldehyde, for example form-urea.

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