Process for purifying melamine-containing ammonia Number:6,870,051 from the United States Patent and Trademark Office (PTO) owispatent

Title: Process for purifying melamine-containing ammonia

Abstract: A process for purifying gaseous melamine containing ammonia is provided. The method comprises partly condensing the gaseous melamine-containing ammonia to form melamine in liquid NH.sub.3 and purified NH.sub.3 gas. The melamine and purified NH.sub.3 gas are taken off.

Patent Number: 6,870,051 Issued on 03/22/2005 to Coufal

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Inventors:	Coufal; Gerhard (Linz, AT)
Assignee:	Agrolinz Melamin GmbH (Linz, AT)
Appl. No.:	416064
Filed:	October 17, 2003
PCT Filed:	November 2, 2001
PCT NO:	PCT/EP01/12691
371 Date:	October 17, 2003
102(e) Date:	October 17, 2003
PCT PUB.NO.:	WO02/38498
PCT PUB. Date:	May 16, 2002

Foreign Application Priority Data

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Nov 08, 2000[AT]

A 1888/2000

Current U.S. Class:	544/201; 544/203
Intern'l Class:	C07D 251//60; C07D 251//62
Field of Search:	544/201,203

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References Cited [Referenced By]

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U.S. Patent Documents

3555784	Jan., 1971	Mohr.
4251235	Feb., 1981	Biermans.
4565867	Jan., 1986	Thomas et al.
5721363	Feb., 1998	Canzi et al.
2001/0005751	Jun., 2001	Coufal	544/200.
Foreign Patent Documents
WO95/01345	Jan., 1995	WO.
WO 95/01345	Jan., 1995	WO.
WO 97/20826	Jun., 1997	WO.
WO99/38852	Aug., 1999	WO.
WO 99/38852	Aug., 1999	WO.
WO 00/39107	Jul., 2000	WO.

Other References

International Search Report of PCT/EP01/12691, dated Mar. 8, 2002. International Preliminary Examination Report of PCT/EP01/12691, dated Apr. 12, 2002.

Primary Examiner: Balasubramanian; Venkataraman
Attorney, Agent or Firm: Christie, Parker & Hale, LLP

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Claims

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What is claimed is:

1. A process for purifying melamine-containing gaseous NH.sub.3 comprising:

partly condensing melamine-containing gaseous NH.sub.3 to obtain melamine enriched in liquid NH.sub.3 and purified NH.sub.3 gas; and

removing the purified NH.sub.3 gas.

2. The process as claimed in claim 1, wherein:

the melamine-containing gaseous NH.sub.3 is passed through liquid NH.sub.3, being cooled and partly condensed to form the purified NH.sub.3 gas and melamine enriched liquid NH.sub.3,

the purified NH.sub.3 gas is taken off, and

melamine-enriched liquid NH.sub.3 is discharged.

3. The process as claimed in claim 2, wherein the melamine-enriched liquid NH.sub.3 contains the melamine in dissolved form.

4. The process as claimed in claim 2, wherein the melamine-enriched liquid NH.sub.3 contains solid melamine.

5. The process as claimed in claim 2, wherein the melamine-enriched liquid NH.sub.3 is recirculated to a plant for solidifying liquid or gaseous melamine.

6. The process as claimed in claim 2, wherein the melamine-enriched liquid NH.sub.3 is recirculated to a fluidized bed that comprises solid melamine or a combination of solid melamine and solid inert matter, and is maintained using NH.sub.3 gas.

7. The process as claimed in claim 1, wherein the melamine-containing NH.sub.3 gas originates from a process for solidifying liquid melamine under NH.sub.3 pressure using liquid, supercritical or gaseous ammonia.

8. The process as claimed in claim 1, wherein the melamine-containing NH.sub.3 gas originates from a process for solidifying gaseous melamine under NH.sub.3 pressure using liquid, supercritical or gaseous NH.sub.3.

9. The process as claimed in claim 1, wherein the melamine-containing NH.sub.3 gas originates from the fluidized bed of a plant for solidifying liquid melamine.

10. The process as claimed in claim 1, wherein the melamine-containing NH.sub.3 gas originates from a fluidized bed of a plant for solidifying gaseous melamine.

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Description

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CROSS REFERENCE TO RELATED APPLICATION

This application is a National Phase Patent Application of International Application No. PCT/EP01/12691, filed on Nov. 2, 2001, which claims priority of Austrian Patent Application No. A 1888/2000, filed Nov. 8, 2000.

FIELD OF THE INVENTION

The application relates to a process for purifying gaseous melamine-containing ammonia. The inventive process is suitable in particular for purifying ammonia gases originating from high-pressure melamine plants using anhydrous melamine workup.

BACKGROUND

In high-pressure processes for preparing melamine, urea is converted to melamine in an endothermic liquid-phase reaction. After removal of NH.sub.3 and CO.sub.2, a melamine melt under high pressure is obtained which is then solidified in what are called wet workup processes by quenching with water. In what are called the dry processes, as described, for example, in U.S. Pat. No. 4,565,867, WO 95/01345, WO 97/20826 and WO 99/38852, the melamine melt is solidified by quenching with ammonia, by expanding the NH.sub.3 -saturated melamine melt or a melamine/NH.sub.3 suspension at a temperature just above the melting point of melamine, by sublimation with subsequent desublimation or by cooling in a fluidized bed. However, in these processes, sometimes very large amounts of gaseous NH.sub.3 are produced which, depending on pressure and temperature of the gas produced, comprise greater or lesser amounts of melamine. This already CO.sub.2 -free gas can be returned to the melamine plant only in part and sometimes only with difficulties. The melamine present in the NH.sub.3 gas causes difficulties and faults, in particular in the compressors, if the NH.sub.3 gas needs to be compressed for liquification. A need therefore exists to free melamine-containing NH.sub.3 gas from melamine in a simple process.

SUMMARY OF THE INVENTION

The invention is directed to a method comprising partly condensing the melamine-containing gaseous NH.sub.3. This produces firstly melamine-containing liquid NH.sub.3 and, secondly, purified melamine-free NH.sub.3 gas which is separated off from liquid NH.sub.3.

More particularly the invention relates to a process for purifying melamine-containing gaseous NH.sub.3, which comprises partly condensing melamine-containing gaseous NH.sub.3 by cooling, the melamine accumulating in the liquid NH.sub.3 and the purified NH.sub.3 gas being taken off.

DESCRIPTION OF THE DRAWING

A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawing, wherein:

FIG. 1 is a schematic view of an exemplary plant in accordance with the invention.

DETAILED DESCRIPTION

The melamine-containing gaseous NH.sub.3 can be cooled using any cooling device, for example in a cooling column or by means of liquid NH.sub.3, liquid NH.sub.3 being mixed with the NH.sub.3 to be purified, for example by spraying liquid NH.sub.3 into the NH.sub.3 to be purified or by passing the NH.sub.3 to be purified through liquid NH.sub.3. Preferably, the melamine-containing gaseous NH.sub.3 is passed through liquid NH.sub.3, in which case it cools and partly condenses. The resultant purified NH.sub.3 gas is taken off and can if appropriate be condensed, and the melamine-enriched liquid NH.sub.3 is discharged. The amount of liquid NH.sub.3 discharged is preferably replaced by the condensing NH.sub.3 gas, but it can also be replaced by feeding fresh NH.sub.3.

The melamine-containing NH.sub.3 gas to be purified is already CO.sub.2 -free and preferably originates from dry melamine processes as set forth above, for instance from processes for solidifying liquid or gaseous melamine under NH.sub.3 pressure or a melamine/NH.sub.3 suspension using liquid supercritical or gaseous NH.sub.3. In the dry melamine processes the solidification is performed, for example, by quenching with NH.sub.3, with NH.sub.3 and liquid or gaseous melamine being sprayed into a quencher. In an exemplary embodiment, the melamine-containing NH.sub.3 gas to be purified originates from the fluidized bed of a plant for solidifying melamine, into which the liquid or gaseous melamine is introduced into a fluidized bed made up of solid melamine, or solid inert particles and solid melamine, and maintained using NH.sub.3 gas.

Preferably, the process is carried out continuously. The gaseous melamine-containing NH.sub.3 is, for example, partly condensed using an air- or water-cooled cooler or via direct contact with liquid NH.sub.3. Preferably, the gaseous, melamine-forming NH.sub.3 here is passed through liquid NH.sub.3, with melamine precipitating or dissolving and accumulating in the liquid NH.sub.3. Depending on from which plant for the dry melamine workup the gas to be purified comes, the gas can be of varying pressure and varying temperature and, as a function thereof, differing melamine content. The lower limit of temperature is preferably just above the respective condensation temperature at the respective pressure, and the upper limit at the operating temperatures of the plant section from which the NH.sub.3 to be purified originates.

If the melamine-containing NH.sub.3 originates, for example, from a process for solidifying gaseous melamine using, or in the presence of, ammonia, the pressure is, for example, about 1-20 barand about 1.5-15 bar, respectively, and the temperature is, for example, about 290-520.degree. C. If the melamine-containing NH.sub.3 originates, for example, from a process for solidifying liquid melamine, lower temperatures down to room temperature are also possible, preferably from about 100.degree. C. to just below the pressure-dependent melting point of melamine, for example from about 100 to 340.degree. C., more preferably from about 200 to 320.degree. C. Higher pressures are also possible up to about 500 bar, preferably from about 5 to 250 bar, more preferably from about 10 to 100 bar.

Pressure and temperature in the partial condensation must be chosen in such a manner here that the NH.sub.3 can be condensed using the coolant available. For example, when water is used as the coolant, the pressure shall not be less than 10 bar, in accordance with the ammonia condensation curve.

The process can be preferably and particularly simply carried out in such a manner that the melamine-containing gaseous NH.sub.3 is passed through liquid NH.sub.3 and partly condensed. The NH.sub.3 gas thus purified can then be taken off and the melamine-enriched liquid NH.sub.3 can be discharged. The amount of melamine-containing liquid NH.sub.3 discharged is continuously replaced by condensing NH.sub.3. However, it is also possible to feed in part fresh liquid NH.sub.3. The partial condensation of the gaseous NH.sub.3 can take place either directly on its introduction into the liquid NH.sub.3, or else by condensation of the purified NH.sub.3 continuously forming, for example using a downstream cooler.

On passing the NH.sub.3 to be purified through the liquid NH.sub.3, melamine-enriched liquid NH.sub.3 is formed which, depending on pressure and temperature, contains greater or lesser amounts of melamine. It can be saturated with melamine or subsaturated, or else solid melamine can already have formed in other words, melamine-containing liquid NH.sub.3 is to be taken to mean not only a solution, but also a suspension, of melamine in NH.sub.3.

In a continuous process in which the melamine-enriched NH.sub.3 is continuously discharged via an overflow, the melamine-enriched liquid NH.sub.3 is customarily recirculated to a suitable section of the melamine preparation process. This can be the reactor or a subsequent separator. Advantageously, the recirculation is into a device in which liquid melamine is allowed to stand under ammonia pressure ("aging") or into a device in which already-solidified melamine is allowed to stand under ammonia pressure ("tempering").

Preferably, the melamine-containing liquid NH.sub.3 is recirculated to a plant in which liquid melamine is quenched (solidified) under expansion using liquid ammonia or the gaseous ammonia formed on expansion of the liquid ammonia. Preference is likewise given to recirculating the melamine-containing liquid NH.sub.3 to a plant in which a mixture of gaseous NH.sub.3 and gaseous melamine is quenched and solidified using ammonia.

In another embodiment, the melamine-containing liquid NH.sub.3 is recirculated to a fluidized bed that comprises solid melamine, or solid melamine and solid inert matter, and is maintained by NH.sub.3 gas.

A possible plant for carrying out the inventive process is shown diagrammatically in FIG. 1. In the figure are shown the following:

(1) fluidized bed,

(2) pressure vessel,

(3) cooler,

(4) intermediate vessel,

(5) pump,

(6) melamine-enriched liquid NH.sub.3,

(7) gaseous melamine-free NH.sub.3,

(8) gaseous melamine-containing NH.sub.3.

EXAMPLE 1

A melamine-containing gaseous NH.sub.3 stream 8 of 424 kg/h coming from the fluidized bed 1 of a melamine solidification plant is passed, at a temperature of 280.degree. C. and a pressure of 10 bar, from the bottom through the liquid NH.sub.3 in a pressure vessel 2 equipped with a cooler 3 which is filled with liquid NH.sub.3. A portion of the NH.sub.3 condenses and the melamine separates out in the liquid NH.sub.3 6. To maintain a constant liquid level in the pressure vessel, 384 kg/h of liquid melamine-containing NH.sub.3 6 are discharged and recirculated to the melamine plant. The purified NH.sub.3 gas 7 vaporizing in the pressure vessel partly condenses in the attached cooler, and the non-condensed portion is taken off as pure NH.sub.3 gas 7 at a rate of 40 kg/h.

EXAMPLES 2-4

In a similar manner to example 1, melamine-containing NH.sub.3 gas was purified at various pressures and temperatures.

The values for Examples 1 to 3 are listed in Table 1.

    TABLE 1
                            NH.sub.3 to be
     Exam-   Temp.  Pressure purified  Recirculated
      ple   (.degree. C.)  (bar)   (kg/h)   NH.sub.3 (kg/h) Pure NH.sub.3
     (kg/h)
       1      280     10       424        384           40
       2      280     40       740        650           87
       3      340     10       380        340           40

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