Patented July 20, 1937 2,087,325 UNITED STATES PATENT OFFICE 2,087,325 PROCEss FoR THE PRODUCTION of UREA AND AMMONUM SATS Charles K. Lawrence, Syracuse, N. Y., and Her man A. Beekhuis, Jr., Petersburg, Va., assign ors, by mesne assignments, to The Solvay Proc ess Company, New York, N. Y., a corporation of New York Application September 13, 1933, serial No. 689,246 15 Claims, (C. 260-125) This invention relates to a process for the method for the production of valuable am- . production of urea and ammonium salts such monium salts from the unconverted ammonia in as annonium carbonates, ammonium chloride, the urea synthesis effluent which obviates the ammonium nitrate or ammonium sulfate. necessity of recovering both the ammonia and 5 It is known to produce urea by heating am carbon dioxide for re-use in the urea Synthesis monia and carbon dioxide under pressure to process itself and in which the value of the an 5 convert the materials into urea in accordance monium salt compensates for the cost involved with the following equation: in its production. It is furthermore an object 0. of this invention to provide a novel method for This reaction under the conditions employed for the effective elimination of unconverted am O carrying it out does not go to completion so that monia and carbon dioxide from a urea. Synthesis the reaction product recovered from an autoclave effluent and the recovery from the gaseous mix in which Synthesis of urea is carried out contains ture obtained in the treatment of the urea. Syn in addition to the urea and water (the products thesis effluent of ammonia in the form of an am of the above reaction) a considerable proportion gasmonium which salt may and be a readily concentrated and economically carbon dioxide re 5 of ammonia and carbon dioxide which may be turned to the urea Synthesis. present principally in the form of ammonium In carrying out the process of this invention, carbamate or carbonate. It is frequently desir ammonia and carbon dioxide are heated under able, furthermore, to employ in the synthesis of pressure to a temperature promoting the forma urea an excess either of ammonia or of carbon tion of urea. The resulting product is passed 20 dioxide over that corresponding to the stoi in contact with an inert gas such as air to strip chiometric proportions for the formation of urea unconverted ammonia and carbon dioxide from and, in Such a case, the reaction product from the product. The mixture of gases thus ob 25 the urea. Synthesis may contain uncombined am tained is then treated to separate the ammonia, monia or carbon dioxide in addition to am and carbon dioxide from the inert gas. This 25 monium carbamate or carbonate. separation may be accomplished by cooling the One of the major problems in developing a gases to a temperature at which the ammonia commercially satisfactory method for the syn and carbon dioxide combine to form ammonium thesis of urea, from ammonia and carbon dioxide carbonate-carbamate; i. e., a product which may has been the provision of satisfactory methods consist principally of normal ammonium car 30 for recovering the unconverted ammonia and car bonate, ammonium bicarbonate or of ammonium bon dioxide from the effluent of the urea, auto carbamate, together with more or less of the clave. Heretofore it has been proposed to heat other named compounds, the exact composition 35 the effluent to drive out the unconverted am of the product depending upon the ratio of am monia and carbon dioxide and return these ma monia and carbon dioxide in the cooled gas and 35 terials to the autoclave for the production of the proportion of water combined with the am further quantities of urea. In one process, the monia and carbon dioxide. This cooling of the ammonia and carbon dioxide distilled from the gases to form ammonium carbonate-carbamate 40 autoclave effluent are directly compressed with may be accomplished by passing the gases in out separation for return to the autoclave. This contact with a solution of ammonium carbonate, 40 compression, however must be carried out at Or the gases may be passed in contact with a high temperatures to avoid the formation of solution of an ammonium salt such as am Solid ammonium carbamate, which high tem monium nitrate, ammonium sulfate, and the 45 perature compression involves mechanical dif like, to, form a solution of ammonium carbonate. ficulties and low efficiencies of the compression It is apparent that in order to accomplish this 45 step. It has also been proposed to separate the separation of the ammonium carbonate-car ammonia from the carbon dioxide as, for ex bamate or ammonium carbonate from the inert ample, by selective absorption of the ammonia, gas, the inert gas used is one which, like air, is 50. and then recovering the gases separately and not condensed at temperatures at which the separately recompressing them and returning gaseous ammonia, and carbon dioxide combine 50 them to the urea Synthesis autoclave. Such a to form solid ammonium carbonate-carbamate mode of operation, however, is costly and adds Or, in the presence of Water, a solution of am- : appreciably to the cost of the urea synthesis. monium carbonate. The Solution of ammonium 55 It is an object of this invention to provide a carbonate, or the ammonium carbamate-car 55 2 .. a,087,825 bonate solid obtained as described, is treated top of a stripper 4, where it is passed in direct with an acid such as nitric acid, sulfuric acid, contact with air which is introduced into the phosphoric acid, or the like, to form an aim bottom of stripper 4. Stripper 4 is operated at monium salt and to liberate a concentrated car a pressure materially below that of autoclave 3; bon dioxide gas, which may be returned to the for example, a pressure of a few pounds gauge, urea synthesis step. and as the effluent from the autoclave at an ele Instead of treating with an acid a solution wated temperature is introduced into the stripper of ammonium carbonate obtained as described at the lower pressures, ammonia and carbon di oxide are evolved from the melt. The liquor or above, or prepared by dissolving the solid am 0. O monium carbamate-carbonate recovered from melt in the top of the stripper may have a tem the gases in water, a salt such as sodium chlor perature of, for example 100° C., which is below ride, sodium nitrate or sodium sulfate may be the boiling point of the liquor. The air intro dissolved in the solution and it is then carbon duced into the stripper may be preheated or the ated by absorbing carbon dioxide in it to pre stripper may contain heating coils whereby the 5 cipitate sodium bicarbonate. The sodium bicar heatrequired for the evolution of the uncon bonate may be filtered from the Solution COn verted carbon dioxide and ammonia in addition taining ammonium chloride, ammonium nitrate to the heat already contained in the autoclave or ammonium sulfate and heated to evolve a effluent may be supplied. Also, steam may be concentrated carbon dioxide gas and form the introduced with the air entering the bottom of . 20 normal sodium carbonate. In case the am the stripper to Supply a desired amount of heat. 20 monium carbonate solution in which the Sodium If desired, this steam may be introduced into salt is dissolved originally contains ammonium the stripper at an intermediate point instead of bicarbonate, the carbonating of the Solution ob into the bottom of the stripper. tained after dissolving the sodium salt may be A solution of urea substantially free from am 25 Omitted or materially curtailed. monia and carbon dioxide foWS off from the bot 2 5 Ammonium Sulfate may also be prepared in tom of stripper 4 and may be treated in any accordance with this invention by mixing the desired manner for the recovery of the urea. A ammonium carbonate solution obtained as de gaseous mixture of air, ammonia and carbon di scribed above with finely divided calcium Sulfate, oxide is drawn off from the top of the stripper 30 whereby calcium carbonate is precipitated and and conducted to a cooler or absorber 5 while 30 a Solution of ammonium sulfate formed. After maintaining the gases at a temperature of, for separation of the calcium carbonate from the example, 75°C., which is above the temperature solution the ammonium sulfate may be recovered at which the ammonia and carbon dioxide would by concentrating and crystallizing the ammo combine to precipitate a Solid ammonium car 35 nium sulfate from the Solution. The calcium bamate-carbonate in the pipes leading from the 3. 5 carbonate may then be decomposed with an acid, Stripper to the absorber. Absorber 5 contains an for example, nitric acid, to recover the carbon aqueous solution of ammonium carbonate which dioxide. is maintained at a temperature of about 40 C. The process of this invention further comprises This temperature regulation may be accom 40 the modifications of processes described above plished by circulating cooling water through coils 40 which involve the direct absorption of the ammo submerged in the ammonium carbonate Solution. nia and carbon dioxide in the gases obtained by The ammonia, and carbon dioxide are absorbed distilling the effluent from a urea, Synthesis auto in the ammonium carbonate Solution and water clave in Solutions of Sodium Salts such as Sodium is continuously added at a rate sufficient to Sup chloride, Sodium nitrate or sodium sulfate, or a ply the desired proportion of Water for combi 45 Suspension of Calcium sulfate whereby ammo nation with the ammonia, and carbon dioxide to nium chloride, ammonium nitrate.or ammonium form ammonium carbonate and to give a solution Sulfate is directly formed in the solution.