Melamine of improved purity produced by high-pressure, non-catalytic process

A high-pressure, non-catalytic process for producing melamine from urea wherein the melamine is recovered directly as a dry powder without washing or recrystallization is described. In the process, liquid melamine is fed to a cooling unit where it is quenched with a liquid medium, preferably liquid ammonia. By increasing the pressure within the cooling unit to a pressure in excess of 600 psi and preferably to 1200 to 1400 psi provides melamine having a purity in excess of 99%.

FIELD OF INVENTION 
The present invention is directed to a high-pressure, non-catalytic, 
non-aqueous process for producing melamine from urea wherein the melamine 
is recovered directly as a dry powder without washing or 
recrystallization. More particularly, the present invention is directed to 
an improvement wherein the melamine as provided has consistently a purity 
of 99.0% or above. 
BACKGROUND OF INVENTION 
Melamine is commercially produced by heating urea to provide melamine and 
ammonia and carbon dioxide as by products. The basic reaction is 
##STR1## 
The commercial processes are either high-pressure, non-catalytic or 
low-pressure and catalytic using a catalyst such as alumina. 
Conventionally, in low-pressure, catalytic processes the melamine is 
recovered in an impure form and, subsequently, recrystallized using a 
chemical treatment to provide melamine which is essentially 100% pure. 
Similar, chemical treatment and recrystallization steps were used in 
producing pure melamine with the high-pressure, non-catalytic process. 
U.S. Pat. No. 4,565,867 issued Jan. 21, 1986 and assigned to the assignee 
of the present application, describes a high-pressure process wherein the 
melamine is recovered at a relatively high purity and used in that form 
without a crystallization step. The process is highly efficient and 
provides a low cost melamine. In carrying out the process, urea melt is 
fed into a scrubber unit at from about 1500 to 2500 psi pressure, 
preferably from about 1700 to 2200 psi, and at a temperature above the 
melting point of urea. In the scrubber unit, the liquid urea contacts 
reaction offgases principally composed of CO.sub.2 and NH.sub.3 and 
containing melamine. The urea, in molten condition, scrubs the melamine 
from the offgas. In the scrubbing process, the offgases are cooled from 
about the temperature of the reactor, i.e., from about 670.degree. to 
800.degree. F. to from about 350.degree. to 450.degree. F. The temperature 
and pressure are interrelated. If the pressure is at the low end of the 
range, i.e., 1500 to 1700 psi, the minimum temperature of the scrubber 
will vary from about 350.degree. to 360.degree. F.; whereas if the 
scrubber is at the high end of the pressure range, i.e., 2000 to 2200 psi, 
the minimum temperature can be increased to about 360.degree. to 
380.degree. F. Below the above minimum temperatures ammonia and CO.sub.2 
condense in the bottom of the scrubber and may form carbamate which can be 
detrimental. As a rule of thumb, the higher the pressure the higher the 
required minimum temperature. Above about 500.degree. F. the urea may 
react to form intermediate products. These intermediate products can be 
detrimental. 
A gas separator is provided in the system wherein liquid melamine is 
separated from offgases, and liquid melamine is collected in the bottom of 
the separator. The separator is held at a temperature above the melting 
point of melamine. The gaseous ammonia and carbon dioxide saturated with 
melamine vapor are removed overhead and fed into the urea scrubber. The 
liquid melamine is removed from the gas separator on level control and 
injected into a product cooling unit. 
A unique feature of the process described in the '867 patent is the cooling 
unit wherein liquid melamine recovered from the separator is depressurized 
and rapidly cooled with a liquid medium which will form a gas at the 
temperature of the liquid melamine. By utilizing the rapid 
depressurization and quenching, the liquid melamine is directly converted 
to a solid powder having a high purity without washing or further 
purification. As is disclosed in the '876 patent, the liquid quenching 
agent is a low boiling liquid which gasifies with the gas being readily 
separated from the solid melamine product. Suitable quenching agents are 
ammonia, water, or a low boiling alcohol. As further disclosed in the '867 
patent, the pressure of the quenching can be atmospheric pressure or a 
pressure up to about 600 psi. According to the '867 patent, it was 
preferred to operate at a pressure of about 200 to 400 psi and a 
temperature of from about 120.degree. to 165.degree. F. In the disclosed 
process the pressure, as above defined, is the same in the scrubber, 
reactor, and gas separator. The offgases removed from the gas separator 
are at the same temperature as the reactor and separator until they reach 
the scrubber where they are cooled in the process of being scrubbed with 
the molten urea. The liquid melamine transferred from the gas separator 
enters the product cooling unit at the same temperature range as the 
reactor and gas separator. 
Although the process of the '867 patent has produced melamine in the range 
of 96 to 99.5% melamine which contains low levels of melem and melam, the 
process of the '867 patent in commercial operation has only produced 
melamine in the range of about 97.5% with the main impurities being melem, 
melam, urediomelamine and ammeline. Although this product is usable in 
most melamine markets, it is limited in some because of the impurities. In 
high pressure melamine technologies other than as described in the '867 
patent, the impurities are removed by using a chemical treatment and then 
a recrystallization process. The impurities are converted to components 
that can be removed from the product by either filtration or sedimentation 
or both. This reduces the yield of melamine from urea by removing these 
impurities and also increases the disposal cost by having to dispose of 
the filter cake. 
There is a need, therefore, to produce a more pure melamine, namely 99+ 
percent, on a commercial basis without either recrystallizing or having to 
dispose of the by products. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a primary object of the present invention to produce high purity 
melamine directly from an anhydrous high-pressure melamine synthesis 
process without the need to dispose of the impurities or to require an 
expensive recrystallization step. 
The aforesaid and other objectives of this invention are accomplished by 
use of a continuous product cooling unit in conjunction with a high 
pressure, non-catalytic system, particularly the Melamine Chemicals, Inc. 
Anhydrous High-Pressure Melamine Synthesis disclosed in the '867 patent, 
incorporated herein by reference. 
Thus, according to the present invention melamine melt, or liquid melamine, 
is removed from the gas separator and flowed into the product cooling 
unit. In the product cooling unit the liquid is rapidly cooled and 
depressurized using a liquid medium that is a vapor at the conditions of 
the cooling unit, preferably ammonia. Dry melamine powder is formed. The 
melamine powder is removed from the bottom of the product cooling unit. 
The product cooling unit is preferably maintained at a temperature below 
the melt point of melamine and the reaction temperature of urea. The 
minimum temperature is the vapor temperature equilibrium of the cooling 
liquid. 
The pressure of the cooling unit is the critical feature of the present 
invention along with the rapid cooling of the melamine. Thus, 
surprisingly, it has been found that rather than keeping the cooling unit 
at atmospheric or up to about 600 psi, a pressure of at least 600 psi is 
essential to prevent the formation of melam, melem, and the other 
impurities to obtain melamine of 99+ percent purity. The preferred 
pressure is a pressure from about 1200 to about 1600 psi. It is theorized 
that the high pressure prevents side reactions and deamination as the 
rapid quench and cooling occurs, and in this way prevents the formation of 
any unwanted impurities.

The flow diagram of FIG. 1 diagrammatically illustrates the present 
invention. Urea is fed through line 20 to scrubber unit 22 at a 
temperature above the melting point of urea, and preferably at about 
280.degree. F.; and at a pressure of from about 1700 to 3500 psi. In the 
continuous process, scrubber unit 22 is also fed through line 23 with 
offgases from separator 24. The offgases, consisting primarily of ammonia, 
carbon dioxide and melamine, will be at a temperature of approximately 
700.degree. to 800.degree. F. and at a pressure of from about 1700 to 3500 
psi, i.e., the reaction conditions of the reactor and separator unit. The 
stream composition from the separator unit will be approximately 45 to 65% 
ammonia, 30 to 50% carbon dioxide, and 3 to 10% melamine. The molten urea 
will be used to "scrub" the melamine from the offgases, giving off heat 
energy to preheat the urea and reducing the temperature of the offgases to 
about 350.degree. to 450.degree. F. The urea containing the melamine will 
settle at the bottom of the scrubber 22. The purified ammonia and carbon 
dioxide gases at the reduced temperature is fed through line 26 to a urea 
plant for utilization in producing urea, or any unit that utilizes the 
NH.sub.3 /CO.sub.2 blend or separates it so that the ammonia can be used. 
The scrubber bottoms are removed from the bottom of the scrubber and fed 
through line 27 by means of a pump 28 at a temperature of from about 
350.degree. to 450.degree. F., and a pressure of from about 1700 to 3500 
psi into reactor 29. Ammonia from a suitable ammonia source is pumped 
through line 32 into the urea stream from the scrubber. The hot ammonia 
which is injected into the line carrying the scrubber bottoms acts as a 
purge to keep the bottom of the reactor from plugging and supplies excess 
ammonia to react with any deammoniation product which may be present. The 
reactor will also be maintained at an operating temperature of from about 
700.degree. to 800.degree. F., and a pressure of from 1700 to 3500 psi. 
The reactor, which is resistant to corrosion, i.e., titanium-clad carbon 
steel; preferably includes means to circulate the reactant within the 
reactor. The preferred reactor temperature is about 770.degree. F. and the 
preferred pressure is 2000 psi. The reactor is temperature controlled 
using conventional heat control systems including thermocouples. 
The product of the reactor, comprised primarily of ammonia, carbon dioxide 
and melamine, is fed to gas separator 24. The reaction product is released 
into the top of the separator. In the separator the gaseous by-products 
consisting of ammonia, carbon dioxide and melamine which are fed to the 
scrubber unit 22 through line 23 are removed from the top of the 
separator. Liquid melamine is removed from substantially the bottom 
one-third of the separator controlled by level indicator at a temperature 
of approximately 700.degree. to 800.degree. F., and a pressure of about 
1700 to 3500 psi, and fed through line 36 to the product cooling unit 38. 
The product cooling unit comprises three compartments 45, 46 and 47. The 
liquid melamine from line 36 is let down through valve 44 into compartment 
45 with liquid ammonia being simultaneously flushed into compartment 45 
through line 40 to quench and rapidly cool the liquid melamine to provide 
solid melamine. Ammonia is cooled and recirculated from compartment 45 
through valve 51. The pressure within compartment 45 after quenching is at 
approximately 1200 psi and at a temperature of 350.degree. to 450.degree. 
F. The pressure of compartments 45 and 46 is equalized by feeding ammonia 
vapor through valve 52 into compartment 46. When the two compartments are 
at an equal pressure, the solid melamine is let down from compartment 45 
into compartment 46 through valve 50. After the solid melamine is in 
compartment 46 at the pressure of approximately 1200 psi, valve 50 is 
closed and the pressure within compartment 46 is reduced to atmospheric by 
releasing ammonia through valve 53 for recirculation. Thereafter, valve 54 
is opened and the solid melamine is dropped into compartment 47. The solid 
melamine product is continuously removed from compartment 47 through a 
rotary valve 60 controlled by a level control 64. The melamine product is 
released through rotary valve 60 onto a suitable conveyor 66 for 
subsequent bagging or the like. 
The efficiency of the present process is established from the data set 
forth in the Table. Thus, in the Table, Sample No. 1 is a process as 
defined in the '867 patent where the cooling unit is at a pressure of 
about 600 psi and at a temperature of about 150.degree. F. Sample Nos. 2 
through 6 are produced according to the present invention, where the 
pressure within the cooling unit is in the range of 800 to 1400 psi and at 
a temperature of about 350.degree. to 450.degree. F. As is apparent, the 
level of impurities is substantially reduced. This level of reduction of 
impurities is surprising and is unexpected, providing an enhanced 
commerical system. 
TABLE 
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Analysis 
Sample No. 
1 2 3 4 5 6 
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Ammelide 0.0 0.0 0. 0.0141 
0.0 0.0 
Ammeline 0.11 0.0098 0.0225 
0.0470 
0.0225 
0.0183 
Melamine 97.5 99.96 99.54 99.84 99.04 99.51 
Ureido 0.65 0.0 0.0 0.0 0.0 0.0 
Melem 0.139 0.0132 0.417 0.3182 
0.1295 
0.4507 
Melam 1.92 0.0145 0.013 0.1354 
0.8009 
0.0140 
______________________________________ 
The present invention is not directed to any specific high temperature 
process but, preferably, is a high pressure process such as described in 
the '867 patent. The invention can be utilized, however, with other 
high-pressure, non-catalyst systems. 
As will be apparent to one skilled in the art, various modifications can be 
made within the scope of the above description. Such modification being 
within the ability of one skilled in the art form a part of the present 
invention and are embraced by the appended claims.