Process for manufacturing melamine from urea

An improved process for manufacturing melamine from urea simplifies the recovery of melamine, carbamate and ammonia from a fluidized bed reactor effluent stream by operating the process at a pressure between 1.4 and 2 MPa. In such a manner, a carbamate solution can be produced at a sufficiently high concentration for use in a urea plant without an intervening concentration step. In addition, ammonia recycled as a fluidizing gas can be condensed against cooling water to permit easy separation of noncondensables such as oxygen which used in the process as a passivator for carbamate corrosion inhibition. The melamine product is produced as an aqueous solution free of melamine solids. Heat is recovered from the carbamate condensation and used for the vaporization of ammonia which is recycled to the reactor for fluidization.

FIELD OF THE INVENTION 
The present invention relates to an improved process for manufacturing 
melamine from urea. 
BACKGROUND OF THE INVENTION 
Melamine is generally produced by heating molten urea in an 
ammonia-fluidized catalytic reactor above 350.degree. C. Known as the 
Stamicarbon process, the fluidized catalytic preparation of melamine from 
urea is described in U. S. Pat. Nos. 3,598,818 to Krekels and 3,682,911 to 
Kaasenbrood et al. Typically, the melamine in the reaction effluent is 
quenched with an aqueous liquid to extract the melamine. Carbon dioxide 
and water vapor remaining in the reaction effluent are absorbed by an 
ammonia absorbent to produce an aqueous ammonium carbamate solution useful 
as a reactant for urea production. Excess ammonia from the absorption step 
is recycled to the melamine reactor as a catalyst fluidizing gas and to 
suppress by-product reactions. Currently, fluidized bed melamine reactors 
are operated at a relatively lower pressure (less than about 1.0 MPa) to 
permit ammonia recovered from the reaction effluent gas to be recycled 
directly to the melamine reactor. 
Several drawbacks to the prior art are evident. The use of lower pressure 
in the melamine reactor produces an effluent stream having a relatively 
high water partial pressure following melamine recovery. Upon CO.sub.2 
absorption (and carbamate condensation), the carbamate solution produced 
is generally too dilute for recycling to the urea plant without an 
intervening concentration step. Further, the pressure in the ammonia 
recovery is too low to permit economical ammonia condensation without 
expensive refrigeration. Thus, the use of oxygen passivation for 
inhibiting corrosion in the quenching and stripping equipment is limited 
since there is no provision for separating the oxygen from the ammonia 
recycled to the reactor. In addition, melamine recovery in the Stamicarbon 
process produces a slurry containing melamine solids. The presence of 
solids can lead to plugging problems and require the use of solids 
separation equipment such as cyclones. 
SUMMARY OF THE INVENTION 
A process for manufacturing melamine from urea is enhanced by incorporation 
of several improvements. Improvements include increasing the system 
pressure to a range of from about 1.4 to 2 MPa. Thus, the partial pressure 
of water vapor in a CO.sub.2 absorption tower is lowered to increase the 
concentration of ammonium carbamate solution produced for recycle to urea 
production. A higher system pressure also enables an oxygen or air 
passivating gas to be conveniently used to inhibit carbamate corrosion 
because passivating gas can be subsequently separated as a noncondensable 
stream following an ammonia condensation step wherein cooling water is 
conveniently employed. Ammonia condensate can then be vaporized for 
recycle to the reactor as a catalyst fluidizing gas using heat supplied by 
carbamate condensation. Melamine product is recovered as an unsaturated 
aqueous solution following quench and wash steps using a sufficient volume 
of aqueous mother liquor from a melamine purification unit to eliminate 
formation of melamine solids and obviate solids separation equipment. 
In one embodiment, the present invention provides a process for 
manufacturing melamine from urea. The process includes the following 
steps: (a) urea and a fluidizing amount of ammonia are supplied to a 
reactor at a pressure from about 1.4 MPa to about 2.0 MPa and a 
temperature effective to substantially convert the urea in the presence of 
a catalyst to melamine and form an effluent stream comprising melamine, 
ammonia and carbon dioxide; (b) the effluent stream is quenched to form a 
vapor-liquid mixture essentially free of solids; (c) the vapor-liquid 
mixture is separated into a concentrated aqueous melamine product stream 
essentially free of solids, ammonia and carbon dioxide, and a high 
pressure vapor stream essentially free of urea and melamine; (d) the high 
pressure vapor stream is contacted, preferably in a single stage with an 
aqueous ammonia stream in an absorption zone refluxed with liquid ammonia 
to form a concentrated aqueous ammonium carbamate stream and an overhead 
ammonia vapor stream essentially free of carbon dioxide; (e) ammonia is 
condensed from the overhead vapor stream to form a liquid ammonia stream; 
and (f) a portion of the liquid ammonia stream is vaporized to form the 
fluidizing ammonia for supply to the reactor. The absorption zone is 
preferably cooled to recover heat for the ammonia vaporization step. 
In a preferred embodiment, a relatively cold heat transfer fluid medium is 
circulated in heat exchange with the absorption zone to form a relatively 
hot fluid medium, and the hot fluid medium is circulated in heat exchange 
with the vaporizing ammonia to form the cold fluid medium. A passivation 
gas is preferably introduced to the quenching or stripping step wherein 
the passivation gas includes oxygen or air. The passivation gas is 
preferably removed in an overhead stream of noncondensables from the 
ammonia condensation step. The noncondensables stream is preferably 
contacted with water to remove ammonia therefrom, and form a 
noncondensables stream essentially free of ammonia and the aqueous ammonia 
stream for the contacting step (d). Preferably, the reactor temperature is 
from about 380.degree. to about 430.degree. C. and the reactor pressure is 
from about 1.5 to about 1.8 MPa. The concentrated aqueous ammonium 
carbamate stream preferably comprises from about 65 to about 80 percent 
carbamate by weight. The separation step (c) preferably includes stripping 
the melamine solution to reduce the carbon dioxide and ammonia content in 
the melamine product stream to less than 0.5 percent by weight. 
In another embodiment, the present invention provides apparatus for 
manufacturing melamine from urea. A reactor is provided for converting 
urea and a fluidizing amount of ammonia in the presence of a catalyst at a 
pressure from about 1.4 MPa to about 2 MPa and an effective temperature 
into melamine and forming a reactor effluent stream comprising melamine, 
ammonia, carbon dioxide and water. A quench zone is provided for mixing 
the effluent stream with an aqueous quench stream to form a vapor-liquid 
mixture essentially free of solids. A separation zone is provided for 
separating the vapor-liquid mixture into a concentrated aqueous melamine 
product stream essentially free of solids, ammonia and carbon dioxide, and 
a high pressure vapor stream essentially free of urea and melamine. An 
absorption zone is provided for contacting the high pressure vapor stream 
with aqueous ammonia and refluxing the high pressure vapor stream with 
liquid ammonia to form a concentrated aqueous ammonium carbamate stream 
and an overhead ammonia vapor stream essentially free of carbon dioxide 
and water vapor. A condenser is provided for condensing the overhead 
ammonia vapor from the absorption zone to form a liquid ammonia stream. A 
heat exchanger is provided for vaporizing the liquid ammonia stream to 
form a fluidizing ammonia stream. A line is provided for supplying the 
fluidizing ammonia stream to the reactor. 
In a preferred embodiment, a heat transfer medium is provided for 
circulating in heat exchange with the absorption zone to form a relatively 
hot fluid medium for circulation in heat exchange with the vaporizing 
ammonia to form the cold fluid medium. A passivation gas including oxygen 
or air is preferably introduced to the quench or separation zones to 
inhibit corrosion. The passivation gas is preferably removed in an 
overhead stream of noncondensables from the ammonia condenser. A 
noncondensables wash zone is preferably provided to remove ammonia 
therefrom, and form a noncondensables stream essentially free of ammonia 
and the aqueous ammonia stream for the absorption zone. Preferably, the 
reactor is operable at a temperature from about 380.degree. to about 
430.degree. C. and a pressure from about 1.5 to about 1.8 MPa. The 
concentrated aqueous ammonium carbamate stream preferably comprises from 
about 65 to about 80 percent carbamate by weight. The melamine separation 
zone is preferably operable to strip the melamine with steam to reduce the 
carbon dioxide and ammonia content in the melamine product stream to less 
than 0.5 percent by weight.

DETAILED DESCRIPTION OF THE INVENTION 
An improved process for synthesizing melamine from urea simplifies, in 
part, the recovery of melamine, carbamate and ammonia from a fluidized bed 
reactor effluent by increasing the pressure of the system. Operating at a 
higher pressure reduces the partial pressure of water in a carbamate 
recovery stream and produces a carbamate solution suitable for recycle to 
urea production without an intervening concentration step. In addition, 
cooling water can be used to condense ammonia for separation of 
noncondensables such as an oxygen passivating gas. The condensed ammonia 
stream can be vaporized for recycle to the reactor as a catalyst 
fluidizing gas by the heat of carbamate condensation. The melamine product 
can be formed as an aqueous solution free of solids for easier handling. 
Referring to the FIGURE, a melted urea stream 12 above 140.degree. C. is 
directed to a melamine reactor 14 in an improved melamine manufacturing 
process 10 according to the present invention. As is well known in the 
art, urea is converted into melamine, carbon dioxide and ammonia in the 
presence of a bed of fluidized catalyst such as alumina silicate at an 
elevated temperature and pressure. The molten urea stream 12 is typically 
sprayed into the catalyst bed fluidized by a sufficient flow of gaseous 
ammonia fed to the bottom of the reactor 14 through line 16. The catalyst 
fluidization gas also assists in the suppression of melamine by-product 
reactions. 
In the practice of the present invention, the reactor 14 preferably 
operates at a temperature of from about 380.degree. C. to about 
430.degree. C. and a pressure sufficiently high to substantially convert 
urea into melamine and recover an ammonium carbamate solution having a 
suitably high carbamate concentration for recycle to a urea plant (not 
shown) without intervening carbamate concentrating steps. (Ammonium 
carbamate being the well known product of CO.sub.2 absorption by an 
aqueous ammonia sorbent.) Typically, reactor 14 has an operating pressure 
of from about 1.4 MPa to about 2.0 MPa, preferably from about 1.5 MPa to 
about 1.8 MPa. 
Reaction heat for the reactor 14 can be supplied by any acceptable 
external, indirect heating source employing a suitable high temperature 
heat transfer fluid stable at reaction conditions. A molten metal or 
inorganic salt which is non-corrosive to the heat transfer equipment is 
conveniently employed. The present molten salt heat source includes an 
closed loop system wherein a molten salt heating fluid is circulated 
between a heater 18 such as a fired furnace and the reactor 14. A hot 
molten salt stream is fed through line 20 from the heater 18 to heat 
exchange tubes 22 in the reactor 14 wherein heat is exchanged with the 
fluidized catalyst bed to maintain the catalyst bed at the desired 
reaction temperature. A cooled molten salt stream is withdrawn from the 
reactor tubes 22 through line 24 for return to the heater 18 via a 
reservoir tank 26. Molten salt from the tank 26 is withdrawn by pump 27 
and circulated through line 28 to heat exchange tubes 30 of the furnace 14 
wherein the molten salt becomes reheated. 
Following conversion in the reactor 14, a reaction effluent vapor stream 
containing melamine, ammonia, carbon dioxide and essentially free of urea 
is removed through line 32 for melamine and CO.sub.2 removal. A purified 
ammonia stream is then recycled to the reactor as the catalyst fluidizing 
gas as mentioned above. The hot effluent vapor 32 is first quenched in a 
quench pipe 34 by contact with a quench stream comprising a sufficient 
quantity of an aqueous-based mother liquid from a melamine purification 
unit (not shown) fed through line 36 to form a vapor-liquid mixture. In 
the quench pipe 34, melamine in the reactor effluent vapor 32 becomes 
dissolved in the quench mother liquid. The vapor-liquid mixture cooled to 
a temperature on the order of 140.degree. C. is then phase separated to 
produce a liquid melamine-rich product stream. The melamine-rich solution 
is preferably separated from the vapor phase in a combined 
quench/stripping tower 38 receiving the vapor-liquid mixture at a feed 
zone above a stripping zone 40. 
The melamine-rich solution flows downward through the stripping zone 40 
wherein dissolved ammonia and CO.sub.2 are essentially stripped therefrom 
by steam generated by a reboiler 42. A melamine product stream at a 
temperature on the order of 200.degree. C. is removed from the stripping 
zone 40 for feed through line 44 to the melamine purification unit (not 
shown). The melamine product stream 44 should have less than about 0.5 
weight percent dissolved ammonia and carbon dioxide gas, more preferably 
less than 0.1 percent by weight of dissolved gas. The concentration of 
melamine in the product stream 44 is less than the melamine saturation 
point to prevent precipitation of melamine solids. A portion of the 
melamine solution is removed from the product stream 44 through line 46 to 
the reboiler 42 where it is heated and returned to the stripping zone 40 
through line 48. The reboiler 42 is preferably heated using high pressure 
stream. 
Vapor components (primarily ammonia, carbon dioxide, water and residual 
melamine) from the mixed vapor-liquid stream, and stripped vapor from the 
stripping zone 40 flow upward into an upper wash zone 50 of the tower 38. 
The lower and upper zones 40, 50 are separated by an appropriately 
designed withdrawal tray 52. In the wash zone 50, the vapor is scrubbed of 
residual melamine by the mother liquid stream (from melamine purification) 
introduced through line 54. Mother liquid can be introduced by means 
suitable for enhancing liquid-gas contact (e.g. through a spray nozzle 
56). Following liquid-gas contacting, mother liquid accumulating in the 
tray 52 is withdrawn though line 58 and pumped through line 60 via pump 62 
as the quench stream for contact with the hot reaction effluent stream 32 
in the quench pipe 34. A portion of the mother liquid stream in line 60 is 
preferably sprayed into the wash zone 50 through line 64 as an additional 
wash stream. 
In a preferred embodiment, a small amount of passivation gas comprising 
oxygen or air is preferably introduced through line 66 at the quench pipe 
inlet and through line 68 at the bottom of the stripping zone 40 for 
corrosion protection of the entire carbamate system (i.e. those portions 
of the process wherein ammonium carbamate is produced). It is understood 
that while the above detailed points for passivation gas injection are 
preferred, additional and/or alternative locations for passivation gas 
introduction could also be used. 
An essentially melamine-free vapor stream comprising ammonia, carbon 
dioxide, water vapor and passivation gas is withdrawn from the wash zone 
50 through line 70 for feed below an absorption zone 71 of a carbamate 
absorber 72. In the carbamate absorber 72, the melamine-free vapor is 
contacted with an absorbent, preferably comprising liquid ammonia and 
aqueous ammonia, to absorb carbon dioxide and water and produce a 
concentrated ammonium carbamate condensate stream. The absorber 72 
generally includes vapor-liquid contacting elements and nozzles to enhance 
the absorption process. A spent absorbent solution comprising primarily 
condensed ammonium carbamate is removed from the absorber bottom through 
line 74 for feed via pump 76 to a urea production unit (not shown). The 
spent absorbent solution, preferably after a single-stage condensation, 
comprises from about 65 to about 80 percent by weight ammonium carbamate, 
and more preferably above 70 percent by weight ammonium carbamate, so that 
no additional process steps are necessary (prior to use in the urea plant) 
to enhance the concentration of the carbamate solution from the line 74. 
Following absorption of the carbon dioxide and water, an ammonia-rich vapor 
stream which is essentially CO.sub.2 -free and water-free is removed 
overhead through line 78. The ammonia-rich stream 78 is preferably 
condensed by water cooled condenser 80 to form a vapor-liquid stream 
containing liquid ammonia and noncondensable gas. The vapor-liquid ammonia 
stream is fed through line 82 to the bottom of a vapor-liquid separator 84 
wherein liquid ammonia is separated from the noncondensable gas. The 
noncondensable stream passes upward through a withdrawal tray 86 to a 
water wash zone 88. A liquid ammonia makeup stream is introduced as needed 
through line 90. Ammonia in the noncondensable stream is washed therefrom 
by contact with water condensate introduced to the wash zone 88 through 
line 92. An essentially ammonia-free noncondensable stream is vented from 
the wash zone 88 through line 94. An aqueous ammonia stream accumulating 
on the tray 86 is preferably drawn off through line 96 as a liquid 
absorbent for CO.sub.2 absorption in the carbamate absorber 72 as 
mentioned previously. 
One portion of the ammonia condensate accumulating at the bottom of the 
ammonia separator 84 is pumped via pump 100 through line 98 as a liquid 
ammonia reflux to the carbamate absorber 72 above the aqueous ammonia 
feed. A second portion thereof is taken off through line 102 to ammonia 
vaporizer 104 for recycle via line 16 to the reactor 14 for catalyst 
fluidization as previously mentioned. Ammonia vaporization is conveniently 
effected by a closed-loop circulating pump using heat generated by 
carbamate condensation in the absorber 72. The ammonia stream 102 is fed 
shell side to a high pressure vaporization heat exchanger 104. Ammonia is 
vaporized by an exchange of heat against a hot heat exchange liquid such 
as water on the tube side and the heat exchange liquid is cooled. The 
ammonia vapor for catalyst fluidization is directed to the reactor 14 
through line 16 as mentioned previously. The cooled heat exchange liquid 
is withdrawn through line 106 and circulated via a pump 107 to a heat 
exchanger 108 receiving a side stream 110 of the hot carbamate solution 
74. The cooled heat exchanger liquid is reheated in the exchanger 108 by 
an exchange of heat against the hot carbamate stream 110. A cooled 
carbamate stream is returned to the absorber 72 via line 112. The reheated 
heat exchange fluid is circulated back to the ammonia vaporization 
exchanger 104 through line 114. As is well understood, the amount of the 
carbamate circulated through the line 110 will depend on the heat balance 
or cooling required for the carbamate absorber 72. 
The present invention can be further illustrated by reference to the 
following example. 
EXAMPLE 
The simplified melamine process of the present invention as shown in the 
FIGURE is evaluated by standard process design techniques to estimate the 
condition of selected process streams and vessels. The evaluation assumes 
a melamine formation rate of 30-34 weight percent of the incoming urea 
(the remainder being converted to ammonia and CO.sub.2). The carbamate 
stream 74 from the carbamate absorber 72 comprises 75 weight percent 
ammonium carbamate and 25 weight percent water. Results are presented in 
the Table. 
TABLE 
______________________________________ 
Temp. Press. 
Process Stream/Equipment 
(.degree.C.) 
(MPa) 
______________________________________ 
Molten salt heater 440 0.4 
(line 20) 
Molten salt tank 26 400 0 
Molten salt pump 400 0.4 
(line 28) 
Reactor (line 32) 410 1.7 
Quench pipe 34 140 1.68 
Stripping zone (line 44) 
203 1.65 
Reboiler 42 &gt;210 -- 
Wash zone 50 140 1.64 
Wash pump (line 60) 140 1.9 
Absorber, overheads 40 1.62 
(line 78) 
Absorption zone 71 100 1.64 
Carbamate pump (line 74) 
100 1.75 
Carbamate cooler 80 1.75 
(line 112) 
Ammonia condenser 40 1.62 
(line 82) 
Ammonia separator 84 
40 1.61 
Ammonia pump (line 98) 
40 1.9 
Ammonia vaporizer 48 1.9 
(line 16) 
______________________________________ 
The present melamine synthesis process and apparatus are illustrated by way 
of the foregoing description and examples. The foregoing description is 
intended as a non-limiting illustration, since many variations will become 
apparent to those skilled in the art in view thereof. It is intended that 
all such variations within the scope and spirit of the appended claims be 
embraced thereby.