Abstract:
A highly pure cyanuric acid is produced by a process which comprises gradually adding a nitrogenous material capable of producing cyanuric acid, such as urea, to a hot N-methylpyrrolidone solvent. The overall process converts substantially all of the urea to cyanuric acid which, upon drying, is a free flowing product capable of being converted into trichloroisocyanuric acid and similar compounds with a minimum of extra processing while allowing substantially complete recovery of the N-methylpyrrolidone solvent for reuse.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an improved method for making cyanuric acid from urea. 
     It is known that cyanuric acid can be produced by the pyrolysis of urea. This reaction may be expressed by the equation: ##STR1## The resulting product, cyanuric acid, which has the empherical formula, C 3  H 3  O 3  N 3 , is generally expressed structurally either as: ##STR2## 
     Pyrolysis can be carried out at temperatures above about 180° C. either in a dry state that is, in the absence of a solvent, as is described in U.S. Pat. No. 2,943,088 issued to R. H. Westfall on June 28, 1960 or in the presence of various high boiling inert solvents. One of these, N-methylpyrrolidone, is described in U.S. Pat. No. 3,164,591 issued to Wilhelm E. Walles et al on Jan. 5, 1965. However, when attempts are made to apply the method taught by Walles et al, it is found that cyanuric acid is obtained in only about a 60 to 70 percent yield. Moreover, the crude acid obtained contains substantial percentages of the mono- and diamides of cyanuric acid, commonly referred to as ammelide and ammeline as well as other undesirable side reaction products and &#34;color bodies&#34;. Thus, the crude end product of this procedure is frequently not sufficiently pure to be readily converted into a number of chlorinated secondary products of commercial interest. To make it so, it must first be subjected to further processing for such purification. This normally includes a digestion of the crude cynauric acid in a strong acid medium, e.g., 3 to 15 percent sulfuric or hydrochloric acid to selectively hydrolyze the acid soluble cyanuric acid amides to convert them back to cynauric acid. In general, an acid digestion is required whenever the concentration of ammeline or ammelide exceeds about 2 percent by weight of the cyanuric acid product. 
     Still a further problem with the method of Walles et al is that of producing a free flowing product suitable for further processing. 
     It has been found that when the reaction mass is allowed to fall below a temperature of about 100° C., cyanuric acid and N-methylpyrrolidone combine to form an insoluble adduct which has a distinct tendency to &#34;set up&#34; into a hard concrete-like mass which must be broken up before any further use of the product can be made. Should this reaction occur during the processing steps used to remove the cyanuric acid from the reaction mass, such as filtering or centrifuging, drastic steps, sometimes involving disassembling the equipment and often damaging it, are necessary to remove the product and resume production. 
     It has also been found that even if the above adduct is broken up and then washed with water to remove residual solvent, it will often set up again into a concrete-like mass due to hydration of the cyanuric acid so that complete removal of the solvent to produce a purified product is difficult, if not impossible. 
     Lastly, the &#34;color body&#34; content is often sufficiently high that a supplemental bleaching operation is required to achieve a final product having a proper whiteness for commercial use. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a process that produces cyanuric acid product in a highly purified state, substantially free of cyanuric acid amides, so that acid digestion is not required. 
     It is a further object of the present invention to provide an improved process in which urea is converted essentially completely to produce a cyanuric acid product containing minimal amounts of color bodies and other impurities. 
     It is yet a further object of the present invention to describe a process which converts urea to cyanuric acid in high yields and in a form from which a free flowing cyanuric acid product can be readily recovered. 
     These and other objects of the subject invention will become apparent from the following description and the appended claims. 
     It has now been found that the foregoing objects can be accomplished in a process in which a nitrogenous material such as urea or biuret is selectively pyrolyzed to form a cyanuric acid product containing only minimal amounts of impurities and color bodies by the slow, controlled addition of said nitrogenous material to N-methylpyrrolidone heated to a temperature of at least 180° C. When performed in accordance with this invention, N-methylpyrrolidone readily dissolves the urea while only dissolving limited amounts of cyanuric acid. It is found that the acid formed can be readily recovered from the reaction mass in a free flowing form having minimal amounts of impurities which can then be easily and conveniently utilized in a variety of processes. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a flow diagram of a first embodiment of the subject invention, utilizing distillation to recover the cyanuric acid product. 
     FIG. 2 is a flow diagram of a second embodiment of the subject invention, utilizing water quenching and filtration to recover the cyanuric acid product. 
     FIG. 3 is a flow diagram of a third embodiment of the subject invention, utilizing hot filtration to recover the cyanuric acid product. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIGS. 1-3 show the basic method for producing and three different embodiments for recovering cyanuric acid from a reaction mass produced by pyrolyzing a nitrogenous product, such as urea or biuret, with urea being preferred, dissolved in a solvent. It has been shown that thermal condensation of urea to cyanuric acid can be carried out when urea is dissolved in a solvent of the general formula ##STR3## wherein R is hydrogen or a lower alkyl group containing 1-4 carbon atoms, R&#39; is hydrogen, a lower alkyl group containing 1-4 carbon atoms, or phenyl, A is an oxygen atom or CR 2 , R being defined as above, and n is 0 or 1 when A is oxygen and n is 0 when A is CR 2 . The general formula shown therefore includes 2-pyrrolidones, 2-oxazolidones, and tetrahydro-1,3-oxazin-2-ones, all of which are revealed as being substantially substitutable for each other for urea pyrolysis. However, for reasons of availability and price, N-methylpyrrolidone is preferred. 
     In the process of this invention, as shown in these drawings, the urea is gradually added, preferably in the form of prills having a minimum of &#34;fines&#34; or other powdery material, or as a molten stream to a heated pyrolysis vessel 10 containing N-methylpyrrolidone. To provide a satisfactory reaction rate, this should be heated to a temperature of at least 180° C. and preferably to a temperature in the range of about 190° to 210° C. The urea charge is carefully controlled so as to provide a urea to solvent ratio of from about 0.5:1 to about 2:1 and preferably from about 0.9:1 to about 1.5:1. This should be done with a uniform addition rate of between about 0.1 and about 1.25 and preferably from between about 0.2 to about 0.8 pounds of urea/hour/pound of solvent. When the preferred rate of addition is used, it is found that the cyanuric acid product will contain less than 2% ammelide and ammeline impurities so that for most uses a supplementary acid digestion will not be required. 
     At the end of this addition, the reaction mass comprises a hot slurry of about 25 to about 60% and preferably from about 30 to about 50% by weight of cyanuric acid suspended in liquid N-methylpyrrolidone. As will be shown below, it is this controlled gradual addition of urea which produces the improved quality and yield of cyanuric acid end product achieved by the present invention. 
     The condensation reaction produced by pyrolysis is endothermic so that this rate of addition smooths out and simplifies the problem of providing the heat needed to start and maintain the condensation reaction. Further, by keeping the instantaneous concentration of urea low at all times, the condensation reaction forming the cyanuric acid tends to proceed quickly and smoothly with essentially all of the urea reacting and with a minimum of unwanted by-products being produced. In so doing, the formation and emission of NH 3  occurs at a relatively low but steady rate. Such a situation permits significant economies in the design and implementation of any units which might be used to recover this valuable by-product. Further, it is a significant factor in the reduction in the ammelide and ammeline content in the product of this reaction. 
     The pyrolysis and condensation reaction described above can be performed under either subatmospheric, atmospheric or superatmospheric pressure conditions. Each mode of operation offers particular advantages and disadvantages and the use of any of them is within the ambit of this invention. With subatmospheric pressure operation, the ease of removing or purging the by-product ammonia from the pyrolysis vessel is greatly enhanced due to a somewhat slower reaction rate. However, for practical purposes it is not desirable to go much below a 500-550 torr pressure since the boiling point of N-methylpyrrolidone will fall to below 180° C. At these conditions of temperature and pressure, the combined amount of ammelide or ammeline which is produced in the final reaction product is normally no greater than about 1 percent and is usually much less. Furthermore, the conversion of urea to cyanuric acid is substantially complete with total yields as high as 99 percent by weight or higher frequently being obtained. 
     Pyrolysis can also be performed at a higher pressure, such as 760 torr (1 atmosphere). While the reaction rate is faster and the final product tends to have a slightly higher level of ammelide or ammeline than with subatmospheric pressure operation, it has been found that the product obtained is quite satisfactory with essentially 100 percent urea conversion being obtained and no particular difficulty in recovering the by-product ammonia. Thus, an atmospheric pressure reaction, which requires no special facilities for producing and holding a vacuum is the preferred mode of operation for this invention. In superatmospheric (i.e. greater than 760 torr) operation, ammonia removal is less complete so that ammeline and ammelide impurity levels tend to be higher than with atmospheric operation; sometimes reaching a level where an acid hydrolysis operation is required to remove them. 
     Where the cyanuric acid is intended as feedstock for subsequent chlorination reactions to produce products such as sodium dichloroisocyanurate or trichloroisocyanuric acid, lower levels (i.e. about 1 percent or less) of ammelide and ammeline do not present a significant quality problem since this treatment causes them to eliminate the amine groups and replace them with carbonyl oxygen so that the same end product is made but with some NCl 3  also being given off as a by-product. 
     At the conclusion of the pyrolysis, the reaction mass of crude cyanuric acid suspended in the hot liquid N-methylpyrrolidone may be maintained under a purge of inert gas such as nitrogen or ammonia for a post-reaction time of from about 5 to about 30 minutes at temperature. Where quenching or filtration means are used to recover the cyanuric acid this will allow the final pyrolysis of any unreacted urea present after which the cyanuric acid suspensate can be separated easily from the reaction mass. It is important that the reaction mass not be allowed to cool below about 100° C. and preferably not below 120° C. to avoid any tendency to form a 1:1 cyanuric acid N-methylpyrrolidone adduct which will solidify or &#34;set up&#34; in the pyrolysis vessel if cooled much below this temperature. The final cyanuric acid product is generally at least 99 percent pure with the total amount of ammelide and ammeline being considerably less than 1 percent by weight. As is shown by the examples below, this is true regardless of the recovery method and whether the final product is free flowing or caked. 
     The recovery of the crude cyanuric acid from the reaction mass and its subsequent purification can be performed by a variety of methods. In one such method, illustrated in FIG. 1, the hot cyanuric acid slurry is directly distilled in still 12 to recover the N-methylpyrrolidone, giving a relatively pure cyanuric acid containing only minor amounts of other reaction products. With the normally low by-product content, it is usually quite practical to use the crude product directly as a feed for the production of chloroisocyanurates. 
     The product obtained by this procedure ranges in color from an off-white to a light tan. It has been found that the use of Hastelloy C as the material of construction for the reactor seems to provide a whiter product (but not a more efficient conversion) than either stainless steel or glass. Where a purer whiter colored cyanuric acid is needed, it can simply be obtained by slurrying the crude product in water vessel 14, to permit hydration, treating with a chlorinating agent (Cl 2 , NaOCl, Ca(OCl) 2  or a chloroisocyanurate) to bleach any color bodies present, filtering in filter 16, and drying in dryer 18. If the crude cyanuric acid is only slightly off color, bleaching to an acceptable whiteness can be done by simply treating the crystalline product in the dry state with gaseous chlorine in a suitable vessel such as a fluidized bed or a rotary drum reactor. The ability to essentially completely remove such color bodies by such a simple operation is an unexpected and surprising result of the process of this invention. This is a matter of considerable importance where the product is tobe used for swimming pool and similar treatment. Here a minimum whiteness value of 65 is normally specified and, as shown in the Examples below, salvaging of off color material now becomes a relatively simple and low cost procedure as compared to prior art methods involving treating a slurried product. 
     FIG. 2 shows a second embodiment for recovering the cyanuric acid from the pyrolysis step. In this case, the reaction mass is cooled by feeding it to quench vessel 20 containing sufficient water at ambient temperature to cool the slurry to 60°-90° C. The wet slurry is then filtered in filter 16 with the separated filter cake being washed with an additional amount of water at 60°-80° C. to further remove any retained N-methylpyrrolidone and/or soluble impurities. The filtration and washing must be performed at above about 60° C. to prevent hydration of the cyanuric acid in the filter. It has been found, however, that if the product is stirred it is possible to reduce the cyanuric acid solubility losses in the solvent by cooling it down to a temperature of about 25° to about 30° C. before filtering. This appears to be due to impurities in the mother liquor which act to retard the hydration and set up of the cyanuric acid at this temperature. Care must be used when this is done since uncontrolled hydration will occur if the filter cake is washed with cold water. To avoid this, the filter cake should be reslurried in water to allow a controlled hydration and then refiltered. 
     The filter cake may go to a flash dryer to produce an anhydrous product or it may go to a trough cooler (a jacketed screw feeder) (not shown) where the residual water may be absorbed without having the cyanuric acid set up thus providing a free flowing hydrated cyanuric acid suitable for use in preparation of chloroisocyanurates. The filtrate is processed in a multi-stage distillation train to separate the water from the N-methyl-pyrrolidone and dissolved solids. The pot residue from distillation comprises primarily a residue of cyanuric acid crystals mixed with some small amount of &#34;color bodies&#34; or tars which must be removed before it can be used for chlorinated isocyanurate production. 
     To eliminate the need for reslurrying the filter cake to achieve hydration, the initial slurry in quench tank 20 can be treated with an amount of caustic soda solution up to about 0.1 mol NaOH/mol cyanuric acid to initiate a controlled hydration at this stage. When treated this way, the cyanuric acid hydrates completely in about 2-3 hours without &#34;setting up&#34;. After hydration, the slurry is neutralized with a hydrochloric or sulfuric acid solution and then filtered, washed, and dried as described above. 
     FIG. 3 shows a third embodiment of the process of this invention. In this case, the hot reaction mass from pyrolysis vessel 10 is filtered in filter 16 and washed with distilled N-methylpyrrolidone to produce a crude cyanuric acid filter cake and an N-methylpyrrolidone filtrate. The filtrate can be distilled to recover a substantially pure N-methylpyrrolidone for reuse. However, ot is often found that the levels of N-methylsuccinimide and color bodies are sufficiently low for the recovered filtrate to be recycled directly without further treatment. 
     To avoid problems with setting up of the slurry during filtration, the temperature is maintained above 100° C. and preferably about 120° C. in the slurry hold tank and in the filter. The filter cake is vacuum dried in dryer 18 with the small amount of N-methylpyrrolidone still on the product being recovered. 
     Whichever of these procedures is used, with proper handling procedures total solvent recovery generally in the range of 97-99 percent or even higher can be obtained. Furthermore, reuse of the solvent for as many as five or even more additional pyrolysis reactions shows that essentially no buildup of N-methylsuccimide, the main decomposition product, occurs. This capability adds a significant economic advantage to this process. 
     The highly selective nature of the present process in producing a cyanuric acid product containing only minimum amounts of impurities is extremely important in commercial manufacture. The very low levels of amide impurities produced by slow addition of urea obviates the need for digesting the present cyanuric acid product in a strong mineral acid. Since this digestion step is a relatively long procedure, requiring several hours, and further since it requires special, acid resistant holding tanks and centrifuges to hold the acid and recover the digested cyanuric acid, the process of this invention provides still another marked advantage over many prior art processes by eliminating the need for this costly and time-consuming operation. 
     The following Examples are given to illustrate the invention and are not deemed to be limiting thereof. All parts and percentages are by weight unless otherwise specified. 
     EXAMPLES 1-4 
     A quantity of N-methylpyrrolidone solvent was charged into a 3-neck 1-liter glass flask fitted with a stirrer, thermometer, condenser, and urea addition funnel. The solvent was heated to reflux (about 202° C.) under an NH 3  purge flowing at a rate of about 500 cc/min. When reflux was achieved, urea was added intermittently, in small portions, over periods of time lasting from 0.55 to 3.5 hours. After an 0.5 hour post reaction hold time at temperature, the reaction mass was quenched in 400 ml of water. After cooling to room temperature, the resultant slurry was filtered and the filter cake washed with an additional 300 ml of water. On washing with water, the cyanuric acid hydrated, releasing some heat and forming a hardened mass which was broken up prior to drying. After drying, the crude cyanuric acid product was analyzed for amide impurities. The quantities of materials and rates of urea addition along with the results obtained are summarized in Table I. 
     
                                           TABLE I__________________________________________________________________________Cyanuric Acid Purity As A Function Of Urea Addition Time                   Grams of UreaUrea Addition Time          Urea              Solvent                   Per Hour Per                            Total Ammelide andExample(hrs.)    (gms.)              (gms.)                   Gram of Solvent                            Ammeline %__________________________________________________________________________1    0.55      169.8              266.4                   1.15     2.342    0.90      270 360  0.83     2.223    1.02      270 360  0.73     1.924    3.5       335 375  0.26     0.46__________________________________________________________________________ 
    
     EXAMPLES 5-9 
     375 g of N-methylpyrrolidone solvent was charged into a 1-liter, 316 stainless steel Parr Autoclave fitted with a stirrer, condenser, thermometer, and urea addition funnel. The assembled system was purged with NH 3  flowing at about 500 ml/min. while the solvent was heated to reflux (about 202° C.) by means of an oil bath. 335 g of urea prills were then added intermittently, in small portions, over a 3.5-hour period. At the conclusion of the reaction, the condenser was reoriented for distillation. About 80-85% of the N-methylpyrrolidone solvent distilled off at atmospheric pressure with the remaining solvent being removed under a reduced pressure in the range of 1 to 5 torr. About 240 g of crude cyanuric acid (approximately 100% yield) was obtained with the recovered solvent distillate was returned to the reactor for reuse. This procedure was followed in 5 consecutive test runs with the results shown in Table II. At the conclusion of the fifth run (Example 9), the solvent quality seemed unaffected and no significant buildup of N-methylsuccinimide or other decomposition products was observed. 
     
                                           TABLE II__________________________________________________________________________Effect Of Solvent Recycle           %   Analysis (Wt. %)ExampleColor     Whiteness           Yield               CA Ad &amp; Am                        NH.sub.3                             Urea Biuret                                      NMP &amp; NMS__________________________________________________________________________5    light tan  100.0               99.2                  0.56  0.01 0.03 &lt;0.1                                      0.026    light tan  100.0               99.3                  0.49  --   --   &lt;0.1                                      0.107    off-white     37    100.0               99.3                  0.43  0.02 0.02 &lt;0.1                                      0.128    off-white     47    100.0               99.5                  0.43  &lt;0.01                             0.01 &lt;0.1                                      0.069    off-white     46     99.2               99.4                  0.40  0.01 &lt;0.01                                  &lt;0.1                                      0.07__________________________________________________________________________ CA = Cyanuric Acid Ad = Ammelide Am = Ammeline NMP = N--methylpyrrolidone MNS = N--methylsuccinimide 
    
     EXAMPLE 10 
     375 g of N-methylpyrrolidone solvent was charged into the apparatus of Examples 5-9 and heated as described therein. 335 g of urea prills were added in small portions over a 3.5-hour period at the conclusion of which the reaction mass was held at a temperature of between 200° and 210° C. for about 15 minutes. The hot slurry of crude cyanuric acid in solvent was then quenched with 485 g of water at about 25° C. This reduced the temperature of the total mass to between 80° and 90° C. from which point, under constant stirring, it cooled to room temperature. Observation showed no significant cyanuric acid hydration after about 2 hours of stirring but essentially complete hydration after 16 hours, such a condition being achieved with complete dispersion of the cyanuric acid crystals in the still liquid water/N-methylpyrrolidone mixture. This slurry was filtered and washed 3 times with 100 ml of water. The wet crystals were dried to produce a  95% yield of anhydrous cyanuric acid contaning 0.44% combined ammelide and ammeline. 
     The N-methylpyrrolidone was recovered from the filtrate by fractional distillation. Analysis of the pot residue remaining after distillation showed it to contain an additional 4% to 5% yield of cyanuric acid raising the total yield to over 99%. 
     EXAMPLE 11 
     The procedure of Example 10 was repeated but with the quenched reaction mass being filtered after it reached room temperature and the filter cake being reslurried in about 300 ml of water. When this was done, the cyanuric acid hydrated almost immediately. This slurry was filtered and the wet crystals dried to produce about a 92% yield of anhydrous cyanuric acid. 
     EXAMPLE 12 
     The procedure of Example 10 was repeated with the hot slurry being split into two portions. The first portion was quenched with the results of Example 10 being obtained, i.e. the slurried cyanuric acid being unhydrated after about 2 hours, but hydrating completely after 16 hours of additional stirring. The second portion, while still hot, was treated with about 7.5 g of 50% NaOH solution. This hydrated completely while being stirred over a 2.5 -hour period. 
     EXAMPLE 13 
     The procedure of Example 10 was repeated with a 1-liter glass reactor and a nitrogen purge prior to the urea addition. After a two-hour stirring period, the crude cyanuric acid in the cooled slurry showed no sign of hydration at which time the slurry was vacuum filtered. The filter cake was then washed with water at which time the cyanuric acid hydrated. The hardened hydrated product was crushed and dried in an oven to produce a 93% yield of anhydrous cyanuric acid having an assay of 99+% purity and an ammelide content of 0.39% and an ammeline content of only 0.02%. Analysis of the filtrate showed an additional 6% yield of cyanuric acid. 
     EXAMPLE 14 
     The procedure of Example 13 was repeated but with a 1-liter, 316 stainless steel reactor and a 45-minute post reaction time before quenching. After cooling to about 50° C., 15 g of 50% NaOH solution was added to the stirred slurry, which resulted in the cyanuric acid hydrating over a period of about an hour. The caustic was neutralized with aqueous HCl prior to filtration. A 92% yield of dried cyanuric acid was obtained. 
     EXAMPLE 15 
     The procedure of Example 8 was repeated in a Hastelloy C reactor giving a cyanuric acid product having a whiteness of 62 as measured on a Photovolt Reflectance meter in which MgO has a whiteness value of 100. While commercial grade cyanuric acid has a whiteness value ranging from about 55 to 75 material designated for swimming pool use generally has a minimum value of 65. 
     EXAMPLE 16 
     Approximately 100 grams of the product of Example 15 after being crushed and screened to -50 mesh were placed in a stoppered 250 ml flask which was then filled with gaseous chlorine. After magnetically stirring for about 2-3 hours, the chlorine was purged with dry air. The cyanuric acid charge showed an increase in whiteness from a value of 62 to 69. 
     EXAMPLE 17 
     Using the method of Example 16, the product of Example 7 was treated with dry gaseous chlorine resulting in an increase in whiteness from 37 to 66. 
     EXAMPLE 18 
     The product from Example 8 was slurried in water and treated with gaseous chlorine. After filtration and drying, the product had a whiteness of 82. 
     The above examples show that the controlled gradual addition of urea to a heated N-methylpyrrolidone solvent produces essentially a quantitative conversion to a high purity cyanuric acid product which is readily adaptable to further processing. To distinguish the product and process of this invention from the prior art, Comparative Example A, which is the procedure disclosed by Walles et al as Example 1 of U.S. Pat. No. 3,164,591, supra, was performed. 
     COMPARATIVE EXAMPLE A 
     300 g of urea prills and 495 g of N-methylpyrrolidone were charged into a 1-liter glass reactor fitted with stirrer, condenser and thermometer. The reaction mixture was heated with a mantle and maintained at reflux for a 2-hour period during which the reaction temperature increased from 185° to 212° C. On cooling, the reaction mixture solidified at about 35° C. The reaction mixture was reheated to about 160° C. to make it stirrable, cooled to 40°-50° C. and filtered. The cake set up in the filter funnel and had to be broken up. On washing with water, the filter cake solidified again into a hard cement-like solid with release of some heat. The solid was broken up, washed further, and dried giving 129 g (60% yield) of impure cyanuric acid containing 5.6% ammelide and 0.4% ammeline. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.