SA05260342B1 - Apparatus for preparing dry melamine powder - Google Patents

Abstract

Abstract: The present invention relates to a process for preparing melamine from urea by means of a high-pressure process whereby dry melamine powder is obtained by transferring the resulting melamine melt from the reactor into a vessel where the melamine melt is cooled with ammonia. Melamine melt is sprayed Inside a cooling vessel and cooled by drops of evaporated liquid ammonia sprayed inside the cooling vessel itself at a pressure of more than 0.1 MegaPascal and a temperature ranging between 50 °C and the melting point of melamine in the cooling vessel.

Description

Z Z Apparatus for preparing dry melamine powder Solid melamine is obtained by transferring the resulting melamine melt from the reactor to a vessel where the melamine melt 0 is cooled by ammonia. Such a process has been described in; Other Alen patents US Patent Z A does not include a Jaina process that describes a highly efficient process for the preparation of melamine from urea: and as duals describes US Patent 575851;-a-pyrolysis of urea in a reactor at a pressure of 0.01” to 17.4 MPa and a temperature of 54 to 477 °C to form the reaction product. This reaction product contains liquid melamine; 60, 10, and pressure induced locks in the form of a mixed stream; To a separator separator, and in this separator, which is kept at approximately the same pressure and temperature as the aforementioned Celia, the said reaction product is separated into a gaseous stream, By liquid. The gaseous stream Z !: contains off-gases of (NH; 3 9 CO;) and melamine vapor as well. The liquid stream E 1 consists essentially of liquid melamine. The gaseous product is transported. To unit 0: escrubber unit washed while the liquid melamine is conveyed to the product cooling unit.In the scrub unit the off-gases from the above-mentioned 02© 5 NH containing melamine vapor 061800106 are scrubbed at a pressure approximately the same as that of the reactor”; > Molten urea to pre-heat the urea and cool the said off-gases and remove the existing melamine from the off-gases.Then the pre-heated molten urea is then fed;

z z z z which contain the aforementioned melamine; to the reactor. In the product cooling unit, the pressure of the liquid melamine is lowered and cooled with a liquid refrigerant medium to produce a solid melamine product without washing or additional preserving. And in US Patent 487.58177-a- it is preferable to use liquid ammonia as a liquid refrigerant. One disadvantage of this method is that in a commercial scale production facility the melamine product obtained is inconsistent in particle size and purity. Important specificities include color, reactivity, type, and concentration of impurities. In the production of melamine for the preparation of melamine-based resins, the purity and consistency of the product are important factors, and it is necessary to maintain a low and reproducible level of impurities; for example melem and cammelide from Jal Transparency of melamine-based resins: General disadvantage of the invention: The present invention aims to obtain an improved high-bia process for the preparation of: melamine from urea Cus urea is obtained On de gu melamine (pb) a consistent, homogeneous 1 product is produced in the form of a dry powder directly from the liquid melamine melt. The applicants have found that it is possible to obtain a melamine powder of the desired product quality by using a process where the melamine melt is sprayed melamine melt into dua cooling vessel cools down very quickly by contacting it with small droplets of 0a © ammonia that spray at the same time to the same cooling vessel.and the cooling vessel has a pressure of more than 100 MPa and a temperature greater than S00 and below the melting point of melamine. The dry melamine powder produced according to the present process is suitable for applications requiring high purity melamine without the need for additional purification. It is preferable that the pressure in the cooling container be less than 10 MPa, and the best is less than no

0 MPa. It is preferable that the temperature in the cooling vessel be less than 1077 °C, and the best is “el z z l” and to increase the strength of the resulting solid melamine to the maximum, it is preferable to cool the melamine melt as quickly as possible through rapid and vigorous mixing With '6 cool ammonia spray. This method freezes the molten melamine very quickly, thus preventing the molten melamine from touching the wall of the cooling container. The eal between the molten melamine and the walls of the cooling vessel leads to the formation of large blocks of melamine: they contain different levels of impurities that will determine the purity and degree of consistency of the melamine product that can be obtained ade 1 has been found The applicants further state that it is necessary to minimize any contact between liquid ammonia and the walls of the refrigeration vessel. and when the liquid ammonia mist has not completely evaporated before reaching the wall of the cooling vessel; The liquid ammonia itself catalyzes the formation of JS from melamine containing different levels of 'impurities' which will determine the purity and degree of consistency of the melamine product that can be obtained. To minimize the possibility of liquid ammonia reaching the wall of the cooling vessel; The present process sprays liquid ammonia into a Cada melamine melt spreader in small droplets fast enough to provide a fast and vigorous mixing of the ammonia! ammonia and sprinkle melamine towards the center of the cooling bowl. And also the size increases ; © Little Ammonia Drops Jed! ammonia at which the melamine is cooled by evaporation of ammonia and to obtain the benefits of rapid cooling provided according to the present process; 1 ammonia spray devices should be placed near the melamine inlet in the cooling bowl with the direction, speed and amount of spray perforation to achieve intense and rapid mixing of ammonia and melamine spray to obtain rapid freezing and cooling of melamine without Deposition of ve blocks of melamine on the walls of the refrigeration sles. And to achieve mixing melamine spray bel

! z melamine | z ] This need to place spray nozzles ammonia Lisl and the inlet of melamine z melamine © _ close to each other in the cooling equipment used as the sole in the current technology for the production of melamine and in the application of the current processes for cooling melamine slurries or melamine melts emelamine melts The nature, location, and rate of feeding of the cooling or drying medium to the cooling vessel are not important factors; This allows such processes to be run in vessels of a wide range of physical shapes. is that; in applying the current process; The distance between the melamine inlet and the ammonia spray nozzle becomes critical for a successful operation. And in practical application; The distance is preferably less than two meters; the best is less than fia and a half; This ensures adequate operation at reasonable ammonia feeding conditions. A larger separation distance between the melamine inlet and the ammonia spray nozzles causes an undesirable delay in cooling the melamine, which requires more stringent ammonia feeding conditions; or ve both. : is ' the application of the current operation; Liquid ammonia spray and melamine melt spray should be mixed at sufficient speeds, rates and directions to produce rapid mixing of the ammonia and melamine drops and to obtain such smooth mixing. Preferably, the velocity of liquid ammonia should be at least + m/s. This µb© determines the velocity in fia per Aol by dividing the flow of a volume of liquid (m”/s) by the smallest flow cross-sectional area (in Tp) at the spray nozzle. Similarly, it is preferable to spray magma Melamine melts at a high speed.: Although an ammonia spray nozzle (or nozzles) may be formed to spray liquid ammonia in a wide variety of directions, it is preferred that the nozzles be directed at

A vo to spray ammonia drops directly into the melamine dropper with

0 Z 1 Place the central axes of the ammonia nozzles so that they intersect with the central axis of the melamine nozzle. To reduce the distance to the minimum, the ammonia spraying device should be moved to reach the emelamine melting device. It is preferable to direct the ammonia nozzles so that their central axes are perpendicular to the central axis of the melamine melt nozzle approximately 3 and by measuring along the central axis of the ammonia spray nozzles relative to the intersection with the central axis of the ammonia nozzle cmelamine melt This figure adjusts the distance of the ised! ammonia sprayer: equal to the separation distance between the nozzles; Preferably less than two metres. He will realize that ammonia nozzles can also be oriented to angle an angle of intersection less than 0.90 degrees to produce a longer ammonia spray distance; However, less than five jad is preferred, as long as other conditions are chosen to ensure the necessary rapid and intense mixing of the ammonia spray and the melamine melt: It is preferable to use at least two ammonia spray nozzles to provide sufficient cooling for the ammonia melt: melamine melt Although only dag is the theoretical maximum number of ammonia ve spray nozzles that may be used in the application of the present process; However, it is expected that physical and economic considerations do not encourage the use of excessive numbers of ammonia spray nozzles. liquid ammonia to adjacent spray nozzles causing larger droplets to form; > The larger ammonia droplets which are less likely to evaporate completely are more likely to reach the wall of the cooling vessel and give the above negative results. In light of these considerations; The applicants believe that it is usually necessary to apply the current method; Less than Yo Nozzle for spraying ammonia .ammonia aul Mixing nature of melamine melt spray 33.5 ammonia Ye It is useful to take into account the impulse flow value YrVY | | |

y | » for atomized ammonia and molamine The value of the pulse flow for atomized ammonia and melamine is calculated by multiplying the mass flow through the ammonia nozzle (in kg per second) by the ammonia flow rate liquid ammonia (LS) as above) (in meters j per second) through ammonia spray nozzles and in the present invention; Preferably, the value of oe | pulsating flow of ammonia atomizer should be at least 11 ammonia kg.m/s, and preferably at least 0.7 kg.m/s'z. Similarly, the pulsating flow value should be preferably For spraying melamine melt © kg.m/s' and the best is at least 01 kg.m/s'. Its nozzle was evaluated for spraying ammonia Lisl suitable for use in the current method (using water at mass flow) The expected flow of liquid ammonia (liquid ammonia and appropriate atmospheric pressure) was found to provide a droplet size distribution of .. > 0.1 mm and a droplet pressure through the spray nozzle ranging between 1 kiloPascal and 1 kiloPascal. One type of spray nozzle found to do this is the SK SprayDiy® (trademark) spray nozzles of Spraying Systems Company | of Wheaton, Illinois. During the cd process, the droplets from the melamine melt spray device are cooled and frozen into melamine powder by contacting the melamine melt spray with a small liquid ammonia spray. The volume of ammonia used may be in excess of the volume required to freeze the melamine melt: to provide additional cooling for the solid melamine. To maximize the purity and consistency of the resulting melamine; Preferably, the cooling time (the length of the ammonia spraying period (as measured above) divided by the sum of the velocities of the feed streams of liquid ammonia and melamine melt is less than vod sec, and preferably less than 0 The advantage of the method according to the present invention is that melamine vo powder can be obtained on a commercial scale with a purity greater than 747.5 by weight and a constant level of z|rye i several common impurities, i.e. a stable product quality. The level of purity and the degree of cohesion of impurities leads to approximately. Preparation of melamine (CO, NH) as a molten form. It produces secondary compounds that occur according to the following reaction equation: melamine © 6CONHy) — C3NgHg+ 6NH; + 3CO,

Yo and 1 and the priming process can be carried out at high pressure; preferably between and preferably S800 MPa; Without a catalyst. The reaction temperature ranges between 8" and the production unit of urea CO; and NH by-products from Bale ranges between 70 and 7680 and is returned to #adjacent melamine. The above-mentioned invention objective is achieved in A combined unit for the preparation of melamine; a suitable unit for use in the present invention may comprise a washing unit; urea and a reactor in combination with a gas-liquid separation unit or with a separate gas-liquid separation unit; and a reactor may suffix; and a cooling vessel and/or expansion vessel. , in a unit comprising urea of urea melamine and in an embodiment of the method; prepare melamine | Vo z Je and may be in combination with a separating unit melamine reactor on a unit Washing and Meldmine reactor: a spost-reactor for a liquid or with a separate gas-liquid separation unit; a subsequent reactor may be attached to a urea pressure washing unit from a urea unit and a cooling vessel. MPa, and at a temperature of Yo to A MPa, preferably from Yo to 17 Z ranging from and preferably between 1770-7970 C. This scrubber may supply a curea higher than the melting point of urea © : To provide additional cooling in the washing unit The washing unit may also be fitted with jacket bodies with a cover !' The liquid with urea and in the scrubbing unit the urea comes into contact with internal cooling bodies or from a separate gas-liquid separation unit melamine Reaction gases from the melamine reactor constructed after the reactor or from the post-reactor. separate gas-liquid separation unit ve pressure and temperature may differ from the pressure and temperature in the melamine reactor oo | | yap

melamine The reaction gases consist mainly of R,0© and 1111 and also contain _ amount of melamine vapor and the molten urea washes the melamine vapor from the off-gases and carries this melamine back to the reactor . In the scrubbing process, the off-gases are cooled from the reactor temperature. any of 76*-40am; To 1 171-7 L, thus the urea is heated to 270-701 C. The exhaust gases are removed from the top of the washing unit and returned, for example, to the urea unit to be used as raw materials for the production of urea, and the urea is withdrawn preheated urea from washing unit with melamine: washed and fed to the resulting stream; For example; by means of a high-pressure pump; Z to the reactor” whose pressure ranges from 7-75 MPa; Preferably - from A to 70 MPa. Gravity can also be used to transfer the melamine melt to the melamine reactor by placing a scrubber over the reactor. In the reactor the molten urea wren is heated to a temperature of FTE: £00 m; preferably from about 770 to air at a pressure equal to the above; Under these conditions 1 urea is converted to melamine NH; 5 CO; 5 melamine: it can. add a standard amount of ammonia to the reactor; In the form of a hot liquid or vapor for example. Additive ammonia (eg Jud) may prevent the formation of melamine condensation products such as melam, melem and melon or enhance mixing in the reactor. The amount of feed ammonia varies Zero to 01 moles per mole of urea is added to the eT reactor, and it is preferable to use zero-e moles of ammonia, especially from Y = hn, a mole of ammonia per mole of urea and collect the NHys COps gases formed in the reaction. In addition to the additional supply of ammonia (A ammonia) the separation fraction eg at the top of the reactor but a separate gas-liquid separation unit can also be used below reactor; separate

In the form of a gas, liquid melamine is obtained. The resulting gas mixture is directed to the scrubbing unit to remove melamine gas and to pre-heat the urea melt. Cun ual: liquid melamine melts with ammonia at a temperature between the melting point of melamine 5 + £ How much and the residence time of the melamine melt in the cooling vessel ranges between two minutes and ten hours, and preferably between ten minutes and five hours. MPa; it is preferable to maintain this pressure during the production of adsorbed ammonia. - 1 The liquid melamine (Wy) of the present invention is transferred to a cooling vessel whereby a solid melamine powder is produced. During the process of cooling with ammonia. And explain the invention in more detail by referring to the following example: oo Detailed description: } For acidity, melamine melt having a temperature of 798 °C was fed through a spray tool to a high-pressure vessel and cooled with liquid ammonia. The spray was used similarly to the pot. To the refrigeration vessel and the point of intersection of the central axis of the ammonia spray cone © with the central axis of the ammonia spray cone © 08 m. The temperature in the vessel was between 176 and 87°C. The ammonia pressure in the vessel was between TA and 9.7 MPa. After two minutes, cool the product further to ambient temperature. The final product contained less than 001 wt. 0 l

Claims (1)

Claims 1-1: A device used to prepare ila melamine powder from melamine; Y molten dwg: on daddy used to react urea and 1011 to produce melamine ¥ molten; a cooling vessel wherein said cooling vessel ¢ has a ctop upper part cbottom and side walls connecting said upper and lower parts; So that the aforementioned cooling container is placed relative to the aforementioned “side-walls” basically vertically; aforementioned melamine 9 on the se spray head 43 so that the spray of Ve [ve] molten melamine is directed towards the bottom of said cooling vessel; 1 set of liquid ammonia inlets ; Cus the mentioned ammonia woo inlets include a group of heads. Cus of) The aforementioned group (Vy) of ammonia funnels is also placed around the aforementioned melamine inlet and directs vg so that de sens Angi from ammonia sprays to melamine 33. D 1e said melt to produce solid melamine; And a means used to remove said solid melamine 5 from the said cooling vessel so that the said method VY | is placed next to the bottom of the said cooling vessel. :,! Z oY 0 device according to Clause Cum 1 The distance of each of the aforementioned groups of Z Y ammonia inlets from the aforementioned melamine inlets is about two metres. oY is a device according to Clause 1 where each of the aforementioned groups of Z provides spray heads when tested with water at an atmospheric pressure approximately equal to the pressure A YYVY | The liquid VY ammonia is approximately equivalent to the atmospheric mass flow of ammonia and the mentioned v mass flow; Pressure drop melamine melt used to cool and freeze melamine melt ¢ 1 | (KPa) KiloPascal.