阅读说明：本技术 用于洗涤处理熔融尿素的分离器的方法以及相关的分离器 (Method for washing a separator for the treatment of molten urea and related separator ) 是由 R·圣图西 A·德阿米西斯 G·迪罗科 于 2021-05-13 设计创作，主要内容包括：本申请公开了一种用于洗涤处理熔融尿素的分离器的方法,包括连续地再循环从所述分离器的底部获取的至少一部分熔融尿素,其中,抽取的所述一部分熔融尿素在分离器的圆柱形侧壁的上部部分上连续地冲洗,所述流动沿所述分离器的内部圆柱形侧壁的整个圆周分布。本申请还公开了一种用于处理熔融尿素的分离器。(A method for washing a separator for the treatment of molten urea, comprising continuously recirculating at least a portion of the molten urea taken from the bottom of said separator, wherein said portion of molten urea withdrawn is continuously flushed on the upper portion of the cylindrical side wall of the separator, said flow being distributed along the entire circumference of the internal cylindrical side wall of said separator. The application also discloses a separator for treating molten urea.)

1. A method for washing a separator for the treatment of molten urea, comprising continuously recirculating at least a portion of the molten urea taken from the bottom of said separator, wherein said portion of molten urea withdrawn is continuously flushed over an upper portion of the cylindrical side wall of the separator, said flow being distributed along the entire circumference of the internal cylindrical side wall of said separator.

2. The method of claim 1, wherein: the withdrawn and recirculated molten urea stream is fed above a tangent line connecting the upper portion of the inner cylindrical sidewall of the separator and an upper cover forming the top of the separator.

3. The method according to one or more of the preceding claims, wherein: the ratio between the amount of urea recirculated and the total amount of urea leaving the separator is in a range varying from 66 to 80% by weight.

4. The method according to one or more of the preceding claims, wherein: the ratio between the amount of urea recycled and the amount of urea fed to the melamine synthesis reactor is between 200% and 400% by weight.

5. The method according to one or more of the preceding claims, wherein: the recirculation of urea is performed by distributing a liquid film of urea over the entire circumference of the cylindrical side wall of the separator without free falling droplets and/or without forming a spray phase.

6. A separator (10) for the treatment of molten urea, characterized in that it comprises at least one distributor element (11), said distributor element (11) being suitable for feeding the urea to be recycled to the separator (10), wherein said distributor element (11) is located above a connecting tangent (5), said connecting tangent (5) being between an upper portion of an internal cylindrical side wall (12) of the separator (10) and an upper cover forming the top of the separator (10).

7. The separator of claim 6, wherein: the at least one distributor element (11) is positioned internally or externally with respect to the separator (10).

8. The separator of claim 7, wherein: the externally positioned distributor element (11) has at least one unidirectional tangential inlet (13) or at least one bidirectional tangential inlet (13), or is a coaxial circumferential distributor (11).

9. The separator of claim 7, wherein: the externally positioned distributor element (11) has at least four unidirectional tangential inlets (13), said at least four unidirectional tangential inlets (13) being preferably positioned at 90 ° with respect to each other, or said distributor element (11) has at least three bidirectional tangential inlets (13), said at least three bidirectional tangential inlets (13) being positioned at 120 ° with respect to each other.

10. The separator of claim 8, wherein: the coaxial circumferential distributor element (11) comprises a half-pipe welded on the outer surface of the side wall (12) of the separator (10), wherein on said outer surface of the side wall (12) of the separator (10) there is a series of openings (16), said openings (16) being suitable for feeding and distributing the recycled urea along the entire circumference of the inner cylindrical side wall (12) of the separator (10), said openings (16) being preferably internally provided with deflectors (17).

11. The separator of claim 7, wherein: the internally positioned distributor element (11) comprises a coaxial pipe (14), said coaxial pipe (14) being provided with openings (15), preferably holes, suitable for feeding and distributing the recycled urea along the entire circumference of the internal cylindrical side wall of the separator.

12. The separator of claim 11, wherein: the internally positioned distributor element (11) is unidirectional, the opening (15) of the coaxial tube (14) facing the side wall (12) of the separator (10), preferably with a downward angle of 30 ° with respect to the horizontal plane.

13. The separator of claim 11, wherein: the internally positioned distributor element (11) is bidirectional, the openings (15) of the coaxial tubes (14) facing the side wall (12) of the separator (10), preferably with a downward angle of 30 ° with respect to the horizontal plane, more preferably the coaxial tubes (14) have alternating openings or holes (15, 15'), wherein the washing openings or holes (15) of the internal side wall (12) of the separator (10) have a downward angle of 30 ° with respect to the horizontal plane, the washing openings or holes (15') of the upper portion of the distributor (11) have an upward angle of 90 ° with respect to the horizontal plane, the passage surface of the washing openings or holes (15') of the upper portion of the distributor (11) being in the range of 6% to 25% of the passage surface of the washing openings or holes (15) of the internal side wall (12) of the separator (10).

Technical Field

The invention relates to a method for washing a separator for molten urea and to a related separation plant.

More specifically, the invention forms part of the industrial field of urea conversion, in particular for the manufacture of melamine. Even more specifically, the present invention relates to a method of washing a separator for the treatment of molten urea, to a related separation plant and to a related installation, in particular for the manufacture of melamine by high-pressure treatment.

Background

It is known to produce melamine by pyrolysis of urea according to the overall reaction (1):

6NH₂CONH₂→(CN)₃(NH₂)₃+6NH₃+3CO₂ (1)

urea melamine

It is well known that the reaction is highly endothermic.

The treatments used to convert urea to melamine fall into two categories: the process of urea pyrolysis is carried out at high pressure and at low pressure.

Both treatments are generally carried out in a reactor fed with a stream of molten urea. Preferably, the reactor is also fed with an ammonia stream.

In high pressure processing, the reaction chamber is always maintained at a pressure higher than 60 bar and is equipped with heating means that keep the reactant system at a temperature of about 360-450 ℃.

The invention relates in particular to a process for the preparation of melamine at high pressure. More specifically, in order to produce melamine, when urea is supplied to the plant in aqueous solution and must be concentrated inside the plant itself, the urea must be moved and pressurised in liquid state at a concentration higher than 99.8% and then melted, so as to take it out of the vacuum separator, or, when urea is supplied to the melamine plant already concentrated, from a buffer tank maintained at a lower pressure, to supply the reactor operating at a pressure of about 80 bar.

Absolute vacuum separators (in which the concentration of molten urea is higher than 99%) are prone to incrustation, since the urea droplets carried by the ascending vapours remain on the walls of the separator itself, which are not immersed in the urea.

In the calm zone, i.e. in the zone where the urea droplets carried by the steam remain dry, the phenomenon of formation of encrustations is maximized, since they are no longer continuously impacted (lap) by the steam itself; these quiet zones of the separator can be located mainly near the tangent between the cylindrical side wall of the separator and the top cover of the separator, the steam outlet pipe being located in the center of this tangent, as can be seen in fig. 1. In fig. 1, 10 denotes the separator, 1 denotes the quiet zone, 2 denotes the steam outlet pipe 3, 4 denotes the outlet mouth of the separator and 5 denotes the connection point between the cylindrical wall of the separator and the top cover of the separator.

The vapor 3 rising upwards tends to be conveyed towards the outlet pipe 2, increasing their velocity as they approach the outlet 4, forming a region of maximum central velocity and a region of minimum peripheral velocity, tending towards zero when in contact with the wall of the separator.

The droplets thus formed on the hemispherical cover forming the ceiling of the separator 10 slide outwards, reaching the calming zone 1, where they will dry when they are no longer impinged by the steam, remaining stuck to the walls of the separator 10, which in turn creates a bearing point for the subsequent droplets.

The urea constituting these droplets therefore remains in contact with the walls of the separator for a long time, thus degrading and forming by-products which solidify, thus retaining new urea again by growth of the layer in contact with the walls of the separator, until partially or sometimes totally clogging the upper part of the separator itself, as can be clearly seen in figure 2, which is a photograph of the upper part of the separator in figure 2.

When the clogging of the separator reaches a rate that impairs the functioning of the separator itself, resulting in a reduction in the vacuum, the system must be stopped in order to clear the encrustations, as shown in fig. 5.

Detachment of the hard shells, which fall into the lower region of the separator but block the outlet or, when they are of limited size, reach the suction part of the pump, causing cavitation, as shown in figures 3 and 4, is also a problem.

Disclosure of Invention

It is an object of the present invention to overcome the previously mentioned disadvantages and problems that characterize the prior art.

In fact, the applicant has surprisingly found a method for washing a separator for the treatment of molten urea which prevents the formation of urea encrustations on the inner surface of the wall of the separator itself.

The applicant has also found a separator for the treatment of molten urea which makes it possible to wash the walls of the separator itself in a particularly effective manner.

The present invention therefore relates to a method for washing a separator for the treatment of molten urea, comprising continuously recirculating at least a portion of the molten urea taken from the bottom of said separator, wherein the portion of molten urea withdrawn is continuously flushed over the upper portion of the cylindrical side wall of the separator, said flow being distributed along the entire circumference of the internal cylindrical side wall of said separator.

The flow of this portion of molten urea withdrawn and recycled or the feed of the recycle is preferably located above a tangent line connecting the upper part of the internal cylindrical side wall of the separator and the upper cover forming the ceiling of the separator.

The ratio between the amount of urea recycled and the total amount of urea leaving the separator is preferably a weight percentage in the range of 66% to 80%.

This ratio makes it possible to generate a flow of recycled urea so as to guarantee complete coverage of the entire circumference of the cylindrical wall of the separator, thus achieving an effective continuous washing of the wall, but without a greater amount of urea recycling than necessary.

Moreover, the ratio between the amount of urea recycled and the amount of urea fed to the melamine synthesis reactor is preferably between 200% and 400% by weight.

In practice, the amount of urea to be recycled (i.e. the degree of recycling) can also be calculated on the basis of the flow rate/amount of urea supplied or to be supplied to the reactor, considering that the size of the separator is determined with respect to the flow rate of the upwardly discharged steam, which in turn is related to the concentration and flow rate of the diluted urea supplied to the separator, which in turn is related to the amount of urea supplied to the reactor.

Furthermore, the recirculation of urea is performed by distributing a liquid film of urea over the entire circumference of the cylindrical side wall, without free falling droplets and/or without forming a spray phase.

The invention therefore relates to a separator for the treatment of molten urea, characterized in that it comprises at least one distributor element suitable for feeding recycled urea to said separator, wherein said distributor element is located above a tangent line connecting an upper portion of the internal cylindrical side wall of the separator and an upper cover forming the top plate of the separator.

The at least one distributor element can be positioned internally or externally with respect to the separator.

When positioned externally, the at least one distributor element can provide at least one unidirectional tangential inlet, at least one bidirectional tangential inlet or can be produced by a coaxial circumferential distributor.

The term "coaxial" refers to the position of the distributor relative to the separator.

Said coaxial circumferential distributor can consist of a half-pipe welded on the outer surface of the separator, wherein on said outer surface of the separator there is a series of openings suitable for feeding and distributing the recycled urea along the entire circumference of the cylindrical side wall inside the separator.

Said opening can be internally provided with a deflector suitable to prevent the urea exiting from the opening from separating from the inner wall of the separator.

When positioned internally, said at least one distributor element can consist of a coaxial pipe provided with openings, preferably holes, suitable for feeding and distributing the recirculated urea along the entire circumference of the internal cylindrical side wall of the separator.

In a first embodiment of the separator according to the invention, the externally positioned distributor element can comprise at least four unidirectional tangential inlets.

When the distributor element comprises four unidirectional tangential inlets, the inlets are positioned at 90 ° with respect to each other.

In different embodiments of the separator according to the invention, the externally positioned distributor element can comprise at least three bidirectional tangential inlets.

When the distributor element comprises three bidirectional tangential inlets, said inlets are positioned at 120 ° with respect to each other.

In a further embodiment of the separator according to the invention, it can preferably comprise at least one unidirectional distributor element positioned internally and coaxially with respect to the separator, wherein said distributor element can be a tube with openings or perforations, said openings or even more preferably the holes of said tube preferably facing the side wall of the separator, even more preferably having a downward angle of 30 ° with respect to the horizontal plane, so that the urea to be recycled is directed towards the lower portion of the separator.

Horizontal plane refers to a plane perpendicular to the side wall of the separator, which is considered a vertical plane.

In a different embodiment of the separator according to the invention, the separator can preferably comprise at least one bidirectional distributor element, positioned internally and coaxial with respect to the separator. Wherein the distributor element can be a pipe provided with openings or holes, the openings or even more preferably the holes of the pipe preferably facing the side wall of the separator, more preferably having a downward angle of 30 ° with respect to the horizontal plane, so that a portion of the urea to be recirculated is directed towards the lower part of the separator, even more preferably having alternating openings or holes, wherein the washing openings or holes of the inner wall of the separator have a downward angle of 30 ° with respect to the horizontal plane, so that a portion of the urea to be recirculated is directed directly towards the lower part of the separator, the washing openings or washing holes of the upper part of the distributor having an upward angle of 90 ° with respect to the horizontal plane, so that a portion of the urea to be recirculated is directed towards the upper part of the distributor itself, so as to keep the upper surface of the distributor continuously flushed and avoid urea from depositing on its surface, the passage surface of the washing openings or holes in the upper part of the distributor is 6% to 25% of the passage surface of the washing openings or holes of the inner wall of the separator.

The lower part of the separator refers to the part of the separator from which concentrated urea is discharged, while the upper part of the separator refers to the part of the separator where the steam outlet pipe is arranged.

The distributor element according to the invention is particularly advantageous because it prevents urea to be recirculated, which is supplied to the scrubber separator, from being entrained upwards by the steam released from the dilute urea during its concentration, thereby blocking the vapour outlet area.

A further advantage of the distributor element of the separator according to the invention is that it is capable of feeding urea so as to form a liquid film over the entire circumference without free falling droplets and/or without forming a spray phase.

Moreover, the distributor element can form a surface which in turn can create a support for the formation of urea encrustations which should be avoided or in any case reduced as much as possible.

The invention also relates to a plant for the production of high-pressure melamine, comprising a separator for the treatment of molten urea, which separator comprises at least one distributor element suitable for feeding recycled urea to the separator, wherein the distributor element is located above a tangent line connecting an upper portion of the cylindrical side wall of the separator and an upper cover forming the ceiling of the separator.

Drawings

Further features and advantages of the method and the separator according to the invention will become more apparent from the following detailed description of some preferred embodiments of the separator with reference to the attached drawings.

Figures 1-5 show a separator of the prior art;

figure 6 is a schematic view of a part of a plant for the high-pressure synthesis of melamine comprising a separator for the treatment of molten urea according to the invention;

FIG. 7 is a schematic view of an externally positioned distributor element with a unidirectional tangential inlet;

FIG. 8 is a schematic view of an externally positioned distributor element, similar to that of FIG. 7, but with a tangential inlet of the bidirectional type;

figure 9 is a schematic view of a separator with a coaxially internally positioned distribution element: figures 9a-9c show details of the dispensing element of figure 9:

FIG. 9a shows a schematic view of the distributor element of FIG. 9, which is manufactured from a tube having openings or holes;

FIG. 9b shows a schematic view of the holes in the distributor element of FIG. 9, which is manufactured by means of a tube provided with openings or perforations for unidirectional distribution;

FIG. 9c shows a schematic view of the openings or holes of the distributor element of FIG. 9, which is manufactured by means of a tube provided with openings or perforations distributed bidirectionally;

fig. 10 is a schematic view of a coaxial distributor element positioned externally with respect to the separator, made by means of welded half-tubes: fig. 10a shows a special embodiment of the distributor element of fig. 10:

fig. 10a shows a schematic view of a distributor element positioned externally with respect to the separator, made by means of welded half-tubes, wherein the distribution is achieved by means of an inlet or hole formed directly on the separator wall and an internal deflector.

Detailed Description

In the following description, for the purpose of illustrating the drawings, the same reference numerals are used to designate structural elements having the same functions. Moreover, for clarity of illustration, some reference numbers may not be repeated in all of the figures.

With reference to figure 6, a part of a plant for the synthesis of melamine at high pressure is schematically represented, comprising a separator for the treatment of molten urea according to the invention: the separator, indicated with 10, is provided with an outlet conduit 6 for concentrated urea, wherein the concentrated urea flow is directed locally to the synthesis 8 of melamine by means of a suitable pump 7 and locally to a recirculation 9 for the washing treatment of the separator 10 according to the invention. The urea to be recirculated is fed in 5 to the distributor element 11.

Fig. 7 shows a distributor element 11 positioned externally with respect to the wall 12 of the separator 10, having four tangential inlets 13 of the unidirectional type, each inlet 13 being positioned at an angle of 90 ° with respect to the direction of the other inlet 13.

Fig. 8 shows a detail of the bidirectional tangential inlet 13 of the distributor element 11 (not shown in the figures).

Fig. 9 shows a separator 10, which separator 10 has a distributor element 11 positioned internally with respect to the wall 12 of the separator. More specifically, fig. 9a shows a distributor element 11, which distributor element 11 comprises a perforated tube 14 with holes 15 or with alternating holes 15 and 15' (in the case of a bidirectional distributor). Fig. 9b shows a detail of the holes 15 of the distributor element 11 of fig. 9, which distributor element 11 is manufactured by means of perforated pipes 14 having a unidirectional distribution, wherein said holes 15 face the side wall 12 of the separator 10 at a downward angle of 30 ° with respect to the horizontal plane, so that the urea to be recirculated is directed towards the lower part of the separator 10. Fig. 9c shows a detail of the holes 15 and 15' of the distributor element 11 of fig. 9, this distributor element 11 being made by means of a perforated pipe 14 with a bidirectional distribution, in which said perforated pipe 14 is provided with alternate holes 15 and 15', the washing holes 15 of the inner wall 12 of the separator 10 having a downward angle of 30 ° with respect to the horizontal plane, so that a portion of the urea to be recycled is directed towards the lower part of the separator 10, and the washing holes 15' of the upper part of the distributor 11 having an upward angle of 90 ° with respect to the horizontal plane, so that a portion of the urea to be recycled is directed towards the upper part of the distributor 11, thus avoiding the urea to remain on the upper surface of the distributor itself and thus forming crusts.

Fig. 10 shows a distributor element 11 positioned externally with respect to the separator 10 and coaxial to the separator 10, the distributor element 11 being made by a half-pipe welded on the external surface of the separator 10, where 16 denotes the feed orifice of the urea to be recycled and an internal deflector is present in the wall 12 of the separators 10 and 17. Fig. 10a shows a detail of an external distributor 11 manufactured by welding half-tubes, where the distribution is achieved by means of inlet holes 16 and internal deflectors 17.

Other variations from the above-described embodiments are possible without departing from the disclosure of the invention.

Finally, it is clear that the high-pressure melamine manufacturing plant comprising the separator according to the invention is capable of numerous variations and modifications, all of which are included in the present invention. Moreover, all the details can be replaced by technically equivalent elements. In practice, the materials used, as well as the dimensions, can vary according to technical requirements.