Process for the production of concentrated cyanuric acid slurries

Concentrated slurries of cyanuric acid are produced in a process which comprises feeding urea to a hot solvent. In the hot solvent, the urea is pyrolyzed to produce a reaction mixture of cyanuric acid particles in the solvent. Agitation of the reaction mixture forms a suspension of cyanuric acid particles. At cyanuric acid concentrations of from about 20 to about 50 percent by weight, the cyanuric acid particles are settled from the suspension to produce a concentrated slurry phase and a supernatent solvent phase. The concentrated slurry phase is then separated from the solvent phase. The process produces concentrated cyanuric acid slurries which minimize the complexity and cost of solvent recovery, reduces energy requirements for heating the reaction mixture and recovering the solvent, and maximizes heat transfer efficiency during the pyrolysis reaction.

This invention relates to the production of cyanuric acid. More 
particularly, this invention relates to the production of cyanuric acid 
from the pyrolysis of urea in a solvent. 
Cyanuric acid can be produced by adding urea to a hot solvent medium. 
During the process, solid particles of cyanuric acid are formed in the hot 
solvent. As solvent losses or elaborate solvent recovery methods add 
significantly to the production cost, it is advantageous to produce 
concentrated slurries of cyanuric acid while minimizing the amount of 
solvent required and subsequent processing for its recovery. 
Production of concentrated slurries of cyanuric acid by feeding large 
quantities of urea into the hot solvent however results in extensive 
scaling of reactor surfaces. The scaling causes losses in heat transfer 
efficiency and requires frequent cleaning of the reactor surfaces. 
Previous descriptions of solvent processes for producing cyanuric acid are 
silent on methods of obtaining concentrated cyanuric acid slurries in 
which solvent losses or processing costs are minimized. They do, however, 
teach solvent recovery methods in which, for example, a second solvent is 
added to the cyanuric acid reaction mixture thereby complicating both the 
product and solvent recovery procedures. 
It is an object of the present invention to provide a process for producing 
concentrated cyanuric acid slurries in which the amount and cost of 
solvent recovery is minimized. 
Another object of the present invention is to provide a process for 
producing concentrated cyanuric acid slurries in which heat transfer 
losses are reduced while reactor productivity capacity is increased. 
These and other objects of the invention are accomplished in a process for 
the production of concentrated cyanuric acid slurries which comprises: 
(a) feeding urea to a hot solvent to pyrolyze the urea to produce a 
reaction mixture comprised of solid particles of cyanuric acid in the 
solvent, 
(b) agitating the reaction mixture at a first agitation rate during said 
pyrolysis to form a suspension of solid particles of cyanuric acid in the 
solvent, 
(c) settling the solid particles of cyanuric acid to form a concentrated 
lower slurry phase and an upper solvent phase, and 
(d) separating the concentrated lower slurry phase from the upper solvent 
phase.

FIG. 1 represents a flow diagram of the process of the present invention. 
Hot solvent from heat exchanger 14 is charged to reactor 10 through line 
11. Molten urea is fed to reactor 10 through line 13. During the pyrolysis 
reaction in which cyanuric acid particles and ammonia gas are formed, 
agitator 12 stirs the reaction mixture to keep cyanuric acid particles in 
suspension. A gaseous mixture of ammonia and solvent vapors is passed from 
reactor 10 to scrubber 16 through line 15. The scrubbed gaseous mixture 
passes through line 17 to condenser 18. Condensed solvent liquid returns 
to scrubber 16 through line 19 and from scrubber 16 to reactor 10 through 
line 21. Ammonia gas is removed from condenser 18 through line 23. 
Following completion of the reaction, agitator 12 is slowed or turned off 
and cyanuric acid particles settle to the bottom of reactor 10 as a 
concentrated slurry. The hot concentrated slurry is discharged through 
outlet 20 to flash dryer 24, the flow through line 25 being regulated by 
valves 22. Solvent vapors are recovered from flash dryer 24 through line 
27 and fed to condenser 26. Liquid solvent is returned from condenser 26 
through line 29 to scrubber 16 or alternately to heat exchanger 14. Dry 
cyanuric acid product is discharged from flash dryer 24 through line 31. 
In an alternate embodiment illustrated in FIG. 2, cyanuric acid is 
continuously produced by feeding hot solvent through line 11 and urea 
through line 13 to reactor 30. Pyrolysis of the urea in the hot solvent 
produces cyanuric acid particles and ammonia gas in a reactor mixture. 
Agitator 12 stirs the reaction mixture to form a suspension of cyanuric 
acid particles. A portion of the cyanuric acid suspension continuously 
flows through line 41 into heated settling tank 40. Cyanuric acid 
particles settle to the bottom of tank 40 as a concentrated slurry. A 
solvent layer forms above the slurry and solvent is continuously charged 
through line 43 to surge tank 42 and is returned to reactor 10 through 
line 45. The concentrated slurry in settling tank 40 is charged to flash 
dryer 24 and dried as described above. Condensed solvent is returned to 
surge tank 42 through line 47. 
In the pyrolysis process, urea is fed to a reactor containing a body of 
solvent. The solvent is maintained at temperatures sufficient to pyrolyze 
the urea, for example, in the range of from about 150.degree. to about 
300.degree. C. During the pyrolysis process, the urea is converted to 
cyanuric acid in a reaction which is believed to be expressed by the 
following equation: 
EQU 3H.sub.2 NCONH.sub.2 .fwdarw.HN--CO--NH--CO--NH--CO+3NH.sub.3 
Ammonia gas formed combines with solvent vapors to form a gaseous mixture 
which is removed from the reaction mixture. Solvent vapors are condensed 
to liquid solvent which is returned to the reactor or the solvent storage 
vessel. Ammonia gas is recovered by known procedures. 
The solvent selected is preferably one in which the cyanuric acid has a low 
order of solubility. Solid particles of cyanuric acid produced are kept in 
suspension by agitating the reaction mixture using, for example, 
mechanical or gas agitation means. Agitation also contributes to the 
formation of suitably sized cyanuric acid crystals. 
Suitable rates of agitation are employed to keep the cyanuric acid 
particles suspended in the solvent. For example, agitator 12 is operated 
at from about 100 to about 200 revolutions per minute. 
Increasing concentrations of cyanuric acid in the suspension can result in 
loss of heat transfer efficiency and substantial deposits of cyanuric acid 
particles on reactor surfaces. In the process of the present invention, 
this is prevented by maintaining the concentration of cyanuric acid in the 
suspension at levels which maintain good heat transfer rates and minimize 
scaling of reactor surfaces. The suspensions produced have a concentration 
in the range of 20 to about 50 and preferably from about 25 to about 40 
percent by weight of cyanuric acid. When the suspensions have reached 
these concentration levels, agitation is stopped or reduced to permit the 
solid particles of cyanuric acid to settle to form a concentrated slurry 
phase at the bottom of the reactor. The cyanuric acid crystals settle 
rapidly, for example, at a rate of from about 0.5 to about 50 feet and 
preferably from about 5 to about 30 feet per minute. During the settling 
period a low rate of agitation is preferably employed to prevent cyanuric 
acid crystals from packing against the reactor bottom and inhibiting or 
preventing removal of the slurry from the reactor. This rate of agitation 
is from about 1 to about 20 revolutions per minute. The concentrated 
slurry phase at the bottom of the reactor contains from about 40 to about 
70 and preferably from about 50 to about 65 percent by weight of cyanuric 
acid. Solvent content of the concentrated slurry phase is in the range of 
from about 30 to about 60 and preferably from about 35 to about 50 percent 
by weight. 
The upper solvent phase which is retained in the reactor contains from 
about 30 to about 60 and preferably from about 35 to about 50 percent of 
the total amount of solvent charged to the reactor. 
Separation of the concentrated cyanuric acid slurry from the solvent phase 
may be accomplished in several ways. In a preferred embodiment, the 
concentrated slurry is removed by gravity flow or pumping means from the 
bottom of the reaction vessel. The hot solvent phase is retained in the 
reactor and both solvent and heat loss is minimized. 
Level control means may be employed to prevent the removal of undesired 
amounts of hot solvent along with the slurry. These control means monitor, 
for example, the percent of suspended solids in the slurry phase and in 
the solvent phase and, by operating valve means, stop the flow or pumping 
of the slurry when the solvent phase-slurry phase interface is reached. 
For concentrated cyanuric acid slurries in the range of from about 40 to 
about 70 percent by weight of cyanuric acid, a nuclear control system is 
preferred. In a nuclear control system, particles are emitted from a 
radioactive element, for example, cesium, cobalt, or radium through the 
slurry (and liquid) phase to a detector which is similar to a Geiger 
counter. Detection of the radioactive particles is inhibited by solid 
particles of cyanuric acid in the slurry phase. Removal of the slurry from 
the reactor permits increased particle detection to a predetermined level 
at which valve means are activated and slurry removal is stopped. 
In another embodiment, the hot solvent is removed, for example, by 
decanting, from the reactor and introduced into a heated storage vessel or 
another reactor. 
The hot concentrated slurry which has been separated from the solvent phase 
is further processed to recover a cyanuric acid product. For example, the 
concentrated slurries produced by the process of the present invention may 
be fed to a flash dryer as shown in FIG. 1. Sufficient heat is available, 
for example, in hot slurries having a cyanuric acid concentration of about 
60-70 percent by weight to completely evaporate the solvent present under 
suitable vacuum conditions without requiring additional heat to the dryer. 
The evaporated solvent is recovered, for example, as shown in FIG. 1. 
Where the production of cyanuric acid is continuous, the suspension of 
cyanuric acid is discharged continuously to, for example, a holding tank. 
The rapid settling rate of cyanuric acid particles from the solvent 
suspension permits the concentrated cyanuric acid slurry to be formed and 
removed quickly from the holding tank. 
Any solvent may be used in the pyrolysis process including, for example, 
alkyl cyclohexanols, methoxy ethoxy isopropanols, tetrahydrofurfuryl 
alcohol, alkyl sulfones, dialkyl sulfones, dialkyl ethers of polyalkylene 
glycols, alkyl pyrrolidones, cycloalkyl pyrrolidones, diphenyl oxide, and 
alkyl oxazolidinones. 
Processes for the pyrolysis of urea in these solvents are described, for 
example, in U.S. Pat. No. 3,008,961, issued Nov. 14, 1961, to B. H. 
Wojcik; U.S. Pat. No. 3,065,233, issued Nov. 20, 1962, to T. R. Hopkins et 
al; U.S. Pat. No. 3,117,968, issued Jan. 14, 1964, to K. Merkel et al; 
U.S. Pat. No. 3,164,591, issued Jan. 5, 1965, to W. E. Walles et al; U.S. 
Pat. No. 3,563,987, issued Feb. 16, 1971, to S. Berkowitz; U.S. Pat. No. 
3,635,968, issued Jan. 18, 1962, to H. Goelz et al; U.S. Pat. No. 
3,810,891, issued May 14, 1974, to J. M. Lee as well as Canadian Pat. No. 
687,279, issued May 26, 1964, to B. H. Wojcik; Canadian Pat. No. 729,190, 
issued Mar. 1, 1966, to R. M. Thomas; and Canadian Pat. No. 740,444, 
issued Aug. 9, 1966, to R. E. Bailey et al. 
The process of the present invention is able to substantially reduce: 
(1) the amount of solvent inventory required in the production of cyanuric 
acid, 
(2) the complexity and cost of solvent recovery, 
(3) the energy requirements for heating and recovering the solvent, and 
(4) the size of reaction vessels for producing cyanuric acid. 
The novel process of the present invention is further illustrated by the 
following examples. 
EXAMPLE 1 
Molten urea was fed to a jacketed reaction vessel containing N-cyclohexyl 
pyrrolidone as the solvent and pyrolyzed to produce a reaction mixture 
containing cyanuric acid crystals. The reaction mixture was maintained at 
a temperature of about 200.degree. C. and was agitated at a rate 
sufficient to keep the cyanuric acid particles in suspension. When 
sufficient urea had been pyrolyzed to form a suspension containing about 
40 percent by weight of cyanuric acid, a portion of the suspension was 
charged to a conical bottom settling tank. The suspension was allowed to 
settle to form a concentrated slurry of cyanuric acid at the bottom of the 
reactor having a supernatant solvent layer. The concentrated slurry was 
discharged from the bottom of the reactor through manipulation of a ball 
valve. A grab sample of the solids discharging was analyzed and found to 
contain 56 percent by weight of cyanuric acid and 44 percent by weight of 
solvent. The concentrated slurry was charged to a dryer in which the 
remaining solvent was evaporated and the vapor fed to a condenser for 
recovery. A cyanuric acid product was recovered from the dryer containing 
about 0.1 percent by weight of solvent. 
This example illustrates the production of a solvent-free product using the 
process of the present invention. 
EXAMPLE 2 
Urea was pyrolyzed in N-cyclohexylpyrrolidone solvent using the procedure 
of Example 1. Following completion of the reaction, the hot suspension was 
allowed to settle to form a concentrated slurry of cyanuric acid at the 
bottom of the reactor with a supernatant solvent layer. Cyanuric acid 
particles settled from the suspension at a rate of about 10 feet per 
minute. Following settling, the hot solvent layer was removed from the 
reactor by decanting and pumped to a heated storage vessel. The 
concentrated slurry containing about 55 percent by weight of cyanuric acid 
and 45 percent by weight of solvent was removed from the bottom of the 
reactor by gravity flow. The decanted hot solvent (65 percent of the total 
solvent charged) was pumped back into the reactor for use in additional 
cyanuric acid production. 
This example shows an embodiment of the process of the present invention in 
which a major portion of the solvent employed is readily separated from 
the cyanuric acid slurry which is to be further processed. Solvent 
recovery procedures and energy requirements are substantially reduced. 
EXAMPLE 3 
Molten urea is fed to a jacketed reaction vessel containing N-cyclohexyl 
pyrrolidone as the solvent. The reaction vessel is equipped with a 
variable speed, dual impeller agitator. During the pyrolysis reaction, the 
agitator runs at 150 rpms to provide the necessary mixing action to insure 
complete reaction and good heat transfer. At the end of the pyrolysis 
reaction when the solid cyanuric acid concentration is 40 percent by 
weight, the agitator speed is reduced to 5 rpms to allow cyanuric acid 
crystals to settle and form a hot concentrated slurry phase containing 57 
percent by weight of cyanuric acid. After about 5 minutes has passed to 
allow settling of the crystals, the bottom outlet valve is opened to allow 
the concentrated slurry to discharge by gravity to the next processing 
vessel. A nuclear level detection device is used to determine when the 
concentrated cyanuric acid slurry transfer is complete. After discharge of 
the concentrated slurry, the bottom outlet valve is closed and the hot 
solvent phase containing 50 percent of the total solvent used is retained 
in the reaction vessel. 
This example illustrates the production of a concentrated cyanuric acid 
slurry where solvent handling is minimized and energy costs reduced.