The present invention relates to the preparation of biuret and more particularly is concerned with a pyrolysis process employing a falling film reactor for preparing biuret with good urea conversion.
The preparation of biuret by pyrolysis of urea, as well as other methods, has long been known in the art. Many of these preparations are summarized in an article "Biuret and Related Compounds" published in Chemical Reviews, 56 p. 95-197 (1956). Of the various methods for preparing biuret set forth in this review article it was indicated that although difficulties are present, large scale preparations have been developed based on the pyrolysis of urea.
Olin (U.S. Pat. No. 2,370,065) teaches a process for preparing biuret wherein urea is heated to a temperature above its melting point but below the decomposition temperature of the biuret and by-product ammonia formed by the condensation of urea is swept from the reaction zone with a stream of a hydrocarbon gas. In the practice of the Olin process, the hydrocarbon gas, preferably toluene, is introduced during the reaction period below the surface of the molten urea and the ammonia-hydrocarbon gas mixture rapidly removed from the reaction zone. The ammonia is removed from the resulting hydrocarbon sweep gas-ammonia mixture and the ammonia depleted hydrocarbon gas returned to the reaction zone for removal of further quantities of ammonia.
Harmon (U.S. Pat. No. 2,145,392) teaches a process for preparing biuret by heating urea at a temperature of 130.degree. to 205.degree.C. at a pressure of not substantially greater than 200 mm. of mercury. This allegedly provides a mixture of urea and biuret from which the biuret is subsequently separated.
Kamlet (U.S. Pat. No. 2,768,895) lists a number of references directed to the preparation of biuret by pyrolysis of urea and teaches a process for directly pyrolyzing urea in the absence of a catalyst at a temperature between 120.degree. and 205.degree.C. This effects substantial autocondensation of the urea to produce a mixture of unreacted urea and an admixture of urea autocondensation products, the total mixture consisting of 30 to 70 percent urea with biuret being a predominant component of the autocondensation products. The Kamlet process further includes extracting urea from the resulting product with a selective solvent for urea, e.g. preferably water, to leave a product containing 60 to 90 percent of the admixed autocondensation products with the remainder being urea. The so-extracted product mass is taught to be suitable for use as a protein supplement for ruminant feeds.
Formaini et al (U.S. Pat. No. 3,057,918 ) teaches a cyclic process for preparing biuret in which urea is heated at from 135.degree. to 200.degree.C. and the resulting crude pyrolytic product quenched and digested in hot aqueous ammonia until no triuret remains. The liquid mass is then cooled to fractionally crystallize biuret which is removed. The ammonia is stripped from the residual solution whereupon cyanuric acid crystallizes. This solid product is separated from the residual aqueous solution and the solution concentrated by removal of water. The resulting concentrate is recycled with additional urea for subsequent pyrolysis.
Colby (U.S. Pat. No. 2,861,886), Kamlet, (referenced hereinbefore) and other publications attest to the utility of biuret as a feed composition additive for ruminants. This additive provides usable nitrogen to supplement the protein content of feeds from natural sources.
Each of the hereinbefore listed processes for pyrolyzing urea into biuret in general consists of reacting urea in a molten reaction mass over prolonged periods of time to achieve appreciable conversion to the desired biuret product. Correspondingly, the co-production of other urea condensation products, e.g. triuret, ammelide and cyanuric acid in detrimental amounts many times results. Formation of such undesired by-products is favored at the higher temperatures, which at the same time are preferred for maximum production of biuret.
It is a principal object of the present invention to provide a urea pyrolysis process for production of biuret in good yields and with a short reaction time.
It is another object of the present invention to provide a process for production of biuret which assures for good control of reaction conditions and provides for ready recovery of any urea based reactant which might be lost from the system during the processing.
It is also an object of the present invention to provide a high temperature-short reaction time process for preparing biuret by pyrolysis of urea wherein autocondensation by-product formation, particularly cyanuric acid, is held to a low level.
It is another object of the present invention to provide a process for preparing a biuret product by pyrolysis of a urea feedstock wherein there is direct rapid conversion of urea.
These and other objects and advantages of the process of the present invention readily will become apparent from the detailed description presented directly hereinafter.