Methods of producing concentrated urea-sulfuric acid reaction products , United States Patent 4445925
Abstract: | Stable concentrated solutions of urea and sulfuric acid containing mono- and/or diurea sulfates and less than 35 weight percent water, free of sulfamic acid and/or ammonium sulfamate, are produced by a unique process that involves the simultaneous and separate addition of urea, sulfuric acid and optionally water to a reaction zone at controlled rates and in stochiometric proportions equivalent to the composition of the desired product, and maintaining reaction temperatures below the incipient decomposition temperature for the particular composition. The heat generated by the highly exothermic reaction can be removed by cooling the liquid phase during the course of the reaction by direct air heat exchange with only nominal, if any, atmospheric emissions. |
Inventors: | Young, Donald C.; |
Application Number: | 318629 |
Filing Date: | 1981-11-05 |
Publication Date: | 1984-05-01 |
Other References: |
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Primary Examiner: |
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Attorney, Agent or Firm: |
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Claims: |
I claim: 1. A method for producing concentrated solutions of urea and sulfuric acid of predetermined composition which method comprises the steps of: (i) simultaneously adding urea, concentrated sulfuric acid, and optionally water to a reaction zone at relative rates corresponding to the concentration of each respective component in said predetermined composition and within the ranges of about 5 to about 75 weight percent urea, about 5 to 85 weight percent sulfuric acid, and 0 to about 35 weight percent water, in which said urea and sulfuric acid are introduced to said reaction zone separately of each other and, in combination, constitute at least about 65 weight percent of the feed to the reaction zone, (ii) continuously agitating said reacting liquid phase formed by the addition of said urea, sulfuric acid, and water to said reaction zone during the addition of said urea, acid and water to rapidly disperse said urea, acid, and water therein, (iii) cooling said reacting liquid phase during the addition of said urea, acid and water by an amount sufficient to maintain said liquid phase at a temperature of about 176.degree. F. or less and below the incipient decomposition temperature of said predetermined composition, and (iv) recovering from said reaction zone a product solution having said predetermined composition. 2. The method defined in claim 1 wherein (i) said urea, sulfuric acid and water added to said reaction zone are admixed therein with said reacting liquid phase comprising urea, acid, water and reaction product of said predetermined composition, (ii) the quantity of said reacting liquid phase in said reaction zone corresponds to at least about 0.1 times the combined hourly addition rates of said urea, sulfuric acid and water, and (iii) said liquid phase is maintained at a temperature of at least about 120.degree. F. 3. The method defined in claim 2 wherein said urea, sulfuric acid and water are simultaneously added to said reaction zone at rates corresponding to about 10 to about 70 weight percent urea, about 10 to about 80 weight percent sulfuric acid, and about 0 to about 25 weight percent water, the combined weights of sulfuric acid and urea added to said reaction zone correspond to at least about 75 weight percent of the total feed to said reaction zone; and said reacting liquid phase is continuously cooled during the addition of said urea, sulfuric acid and water by an amount sufficient to maintain a temperature of about 170.degree. F. or less. 4. The method defined in claim 2 wherein said predetermined composition has a crystallization temperature of about 77.degree. F. or less and a composition falling on or above the 77.degree. F. isotherm of the ternary phase diagram of FIG. 1, and said reacting liquid phase is maintained at a temperature of about 160.degree. F. or less. 5. The method defined in claim 2 wherein said predetermined composition contains, and the respective feed rates to said reaction zone comprise, 0 to 15 weight percent water and at least about 85 weight percent urea and sulfuric acid taken in combination. 6. The method defined in claim 2 wherein said predetermined composition comprises, and the respective feed rates to said reaction zone correspond to, about 0 to about 25 weight percent water and at least about 75 weight percent urea and sulfuric acid taken in combination, and the weight ratio of sulfuric acid to urea in said predetermined composition and in said feed to said reaction zone is at least about 1. 7. The method defined in claim 2 wherein said reacting liquid phase is maintained at a temperature of about 150.degree. F., or less, and said product solution contains less than about 0.1 weight percent of a member selected from the group consisting of sulfamic acid, ammonium sulfamate and combinations thereof. 8. The method defined in claim 2 wherein said reaction zone comprises at least one reaction vessel and cooling means external of said reaction vessel, and wherein said method further comprises the steps of simultaneously adding said urea, sulfuric acid and water to said reaction vessel and admixing the same with said reacting liquid phase of urea, acid, water and reaction product of said predetermined composition within said reaction zone, circulating at least a portion of said reacting liquid phase from said reaction vessel through said cooling means to cool the same, and returning at least a portion of the cooled reacting liquid phase to said reaction vessel. 9. The method defined in claim 8 wherein said cooling means comprises a direct contact countercurrent air heat exchanger, and said method further comprises the steps of passing hot reacting liquid phase from said reaction vessel downwardly through said heat exchanger, passing air upwardly through said heat exchanger into direct heat exchange contact with said liquid phase and cooling said liquid phase, returning at least a portion of the resulting cooled liquid phase to said reaction vessel, and discharging said air from said heat exchanger to the atmosphere. 10. The method defined in claim 9 wherein said urea, sulfuric acid and water are continuously added simultaneously to said reaction vessel at a combined rate equivalent to a predetermined production rate, product solution of said predetermined composition is continuously removed from said reaction zone at said predetermined production rate, and the volume of said reacting liquid phase in said reaction zone corresponds to at least said predetermined production rate. 11. The method defined in claim 10 which further comprises determining (i) the amount of water removed from said liquid phase in said heat exchanger and (ii) the concentration of urea, sulfuric acid and water in a member selected from the group consisting of said liquid phase, said product solution, and combinations thereof, and adjusting the urea, sulfuric acid and/or water feed rates to said reaction vessel responsive to the determinations of items (i) and (ii) as required to maintain the composition of said product solution at said predetermined composition. 12. The method defined in claim 2 wherein said urea, sulfuric acid and water are continuously added simultaneously to said reaction vessel at a combined rate equivalent to a predetermined production rate, product solution of said predetermined composition is continually removed from said reaction zone at said predetermined production rate, and the volume of said reaction liquid phase in said reaction zone corresponds at least to the value of V.sub.o determined in accordance with the expression V.sub.o =k/u(14.3 d.sup.2 -1) wherein k is the first order rate constant for said predetermined product in reciprocal minutes, d is the diameter of the largest urea feed particles in millimeters, V.sub.o is the volume of said reaction liquid phase within said reaction zone in gallons, and u is the production rate from the reaction zone in gallons per minute. 13. The method defined in claim 1 wherein said urea, sulfuric acid and water are continuously added simultaneously to said reaction vessel at a combined rate equivalent to a predetermined production rate, product solution of said predetermined composition is continuously removed from said reaction zone at said predetermined production rate, and the volume of said reacting liquid phase in said reaction zone corresponds to at least about one-half said predetermined production rate. 14. The method defined in claim 13 wherein said predetermined composition has a predetermined crystallization temperature of about 80.degree. or less, and the feed rates of said urea, sulfuric acid and water to said reaction vessel are continuously controlled within predetermined limits sufficient to maintain the concentration of each of said urea, sulfuric acid and water in said product solution within 2 weight percent of their predetermined concentrations and to maintain the crystallization temperature of said product solution within 10.degree. F. of said predetermined crystallization temperature. 15. The method defined in claim 1 wherein the weight ratio of said sulfuric acid to said urea in said product solution is at least about one, said product solution comprises 0 to about 25 weight percent water, said urea and said sulfuric acid, in combination, constitute at least about 75 weight percent of said product solution, and said product solution is free of reaction by-products resulting from the decomposition of a member selected from the group consisting of said urea, said sulfuric acid, and combinations thereof. 16. The method defined i claim 1 wherein said cooling of said reacting liquid phase during the addition of said urea, acid and water, is achieved, at least in part, by directly contacting said reacting liquid phase with air. 17. A method for continuously producing concentrated, stable solutions of urea and sulfuric acid of predetermined composition which method comprises the steps of (i) continuously and simultaneously adding urea, sulfuric acid and water to a reacting liquid phase, hereinafter defined, in a reaction zone at relative rates corresponding to the concentration of each respective component in said predetermined composition and within the ranges of about 5 to about 75 weight percent urea, about 5 to 85 weight percent sulfuric acid, and 0 to about 35 weight percent water, in which said urea and sulfuric acid constitute at least about 65 weight percent of the feed to said reaction zone and are added separately to said reaction zone, (ii) admixing said urea, sulfuric acid and water in said reaction zone with a reacting liquid phase comprising urea, sulfuric acid, water and said reaction product of said predetermined composition, the quantity of said reacting liquid phase in said reaction zone corresponding to at least about one half the combined hourly addition rates of said urea, acid and water to said reaction zone and being sufficient to prevent the discharge of unreacted urea in the product from said reaction zone, (iii) agitating said reacting liquid phase continuously during the addition of said urea, sulfuric acid and water to rapidly disperse the same therein, (iv) cooling at least a portion of said reacting liquid phase by an amount sufficient to maintain the temperature of said reacting liquid phase at a level of about 120.degree. F. to about 176.degree. F., and below the incipient decomposition temperature of said predetermined composition, and (v) continuously withdrawing product solution of said predetermined composition from said reaction zone. 18. The method defined in claim 17 wherin (i) said urea is selected from the group consisting of prilled urea and granular urea, and combinations thereof, (ii) the volume of said reacting liquid phase in said reaction zone corresponds to at least about said combined hourly addition rate of said urea, sulfuric acid and water, (iii) said reacting liquid phase is maintained at a temperature below about 170.degree. F., (iv) said concentrations of said urea, sulfuric acid and water in said predetermined composition are within the ranges of about 10 to about 70 weight percent urea, about 10 to about 80 percent sulfuric acid and about 0 to about 25 weight percent water, and (v) said urea and sulfuric acid constitute at least about 75 weight percent of said predetermined composition. 19. A composition of matter comprising about 5 to about 75 weight percent urea, about 5 to about 85 weight percent sulfuric acid, and about 0 to 35 weight percent water based on the combined weight of urea, sulfuric acid and water, and less than about 0.1 weight percent of a member selected from the group consisting of sulfamic acid, ammonium sulfamate, and combinations thereof, in which at least a portion of said urea and sulfuric acid are present as a member selected from the group consisting of monourea sulfate, diurea sulfate, and combinations thereof. 20. The composition defined in claim 19 comprising about 10 to about 70 weight percent urea, about 10 to about 80 weight percent sulfuric acid, and about 0 to about 25 weight percent water, wherein the weight ratio of sulfuric acid to urea is at least about 1. 21. The composition defined in claim 20 containing no detectable amount of a member selected from the group consisting of sulfamic acid, ammonium sulfamate and combinations thereof. 22. The composition defined in claim 19 containing less than about 0.05 weight percent of said member selected from the group consisting of sulfamic acid, ammonium sulfamate, and combinations thereof. 23. The composition defined in claim 19 containing no detectable amount of a member selected from the group consisting of sulfamic acid, ammonium sulfamate and combinations thereof. 24. The method of improving the quality of agricultural soils including the steps of applying to said soil an agronomically effective amount of the composition defined in claim 23. 25. The method of fertilizing crops including the steps of applying to said crop an agronomically effective amount of the composition defined in claim 23. 26. The method defined in claim 25 wherein said composition is applied to said crop at a rate equivalent to a nitrogen dosage of at least about 40 lbs. of nitrogen per acre of said crop. 27. A method for producing concentrated solutions of urea and sulfuric acid of predetermined composition, which method comprises the steps of: (i) simultaneously adding urea, concentrated sulfuric acid, and optionally water, to a reaction zone at relative rates corresponding to the concentration of each respective component in said predetermined composition, and within the ranges of about 10 to about 70 weight percent urea, about 10 to about 80 weight percent sulfuric acid, and 0 to about 35 weight percent water, in which said urea and said sulfuric acid, in combination, constitute at least about 65 weight percent of the feed to said reaction zone and are introduced into said reaction zone separately of each other, and in which the weight ratio of said sulfuric acid to said urea is at least about 1, (ii) continuously agitating the resulting liquid phase in said reaction zone during the addition of said urea, acid, and optionally water, to rapidly disperse said urea, acid, and water therein, (iii) cooling said liquid phase during said addition of said urea, acid, and water, by an amount sufficient to maintain said liquid phase at a temperature of about 176.degree. F. or less, and below the incipient decomposition temperature of said predetermined composition, and (iv) recovering from said reaction zone a product solution having said predetermined composition free of reaction byproducts resulting from the decomposition of a member selected from the group consisting of urea, sulfuric acid, and combinations thereof. 28. The method defined in claim 27 wherein said product solution comprises 0 to about 25 weight percent water, said urea and said sulfuric acid, in combination, constitute at least about 75 weight percent of said product solution, and said product solution is free of reaction by-products resulting from the decomposition of said urea. 29. A composition of matter comprising the reaction product of urea and sulfuric acid, which composition contains 0 to about 35 weight percent water, about 10 to about 80 weight percent sulfuric acid, and about 10 to about 70 weight percent urea, wherein said urea and said sulfuric acid, in combination, constitute at least about 65 weight percent of said composition, the weight ratio of said sulfuric acid to said urea is at least about one, and wherein said composition is free of urea-sulfuric acid reaction by-products resulting from the decomposition of a member selected from the group consisting of urea, sulfuric acid, and combinations thereof. 30. The composition defined in claim 29 comprising 0 to about 25 weight percent water, wherein said urea and said sulfuric acid, in combination, constitute at least about 75 weight percent of said composition, and said composition is free of reaction by-products resulting from the decomposition of said urea. 31. A composition of matter comprising the reaction product of urea and sulfuric acid, about 10 to about 80 weight percent sulfuric acid, and which composition contains 0 to about 35 weight percent water, about 10 to about 70 weight percent urea, wherein said urea and said sulfuric acid, in combination, constitute at least about 65 weight percent of said composition, and wherein said composition is free of urea-sulfuric acid reaction by-products resulting from the decomposition of a member selected from the group consisting of urea, sulfuric acid and combinations thereof. 32. The composition defined in claim 31 comprising 0 to about 25 weight percent water, wherein said urea and said sulfuric acid, in combination, constitute at least about 75 weight percent of said composition, and said composition is free of reaction by-products resulting from the decomposition of said urea. 33. The method defined in claim 1 wherein said product solution recovered from said reaction zone is free of decomposition products of urea, sulfuric acid, and combinations therof formed by the reaction of said urea and said sulfuric acid in said reactio zone. 34. A composition of matter formed by the reaction of urea and sulfuric acid to convert at least a portion of said urea and sulfuric acid to a member selected from the group consisting of the monourea adduct of sulfuric acid, the diurea adduct of sulfuric acid, and combinations thereof, which composition comprises 0 to about 35 weight percent water, about 10 to about 80 weight percent sulfuric acid, and about 10 to about 70 weight percent urea, wherein said urea and sulfuric acid, in combination, constitute at least about 65 weight percent of said composition, and wherein said composition is free of decomposition products of urea, sulfuric acid, and combinations thereof, formed by said reaction of said urea and sulfuric acid. 35. The composition of matter produced by the method which comprises the steps of reacting urea, sulfuric acid, and optionally water, in proportions within the ranges of about 10 to about 70 weight percent urea, about 10 to about 80 weight percent sulfuric acid, and 0 to about 35 weight percent water, in which said urea and said sulfuric acid, in combination, constitute at least about 65 weight percent of the combination of said urea, sulfuric acid, and water, and maintaining the temperature of said combination of said urea, sulfuric acid and water below its incipient decomposition temperature. 36. The composition defined in claim 35 wherein the temperature of said combination of said urea, sulfuric acid, and water, is maintained below 176.degree. F. 37. The method defined in claim 35 wherein the temperature of said combination of said urea, sulfuric acid, and water, is maintained below about 160.degree. F. 38. The composition defined in claim 35 wherein the temperature of said combination of said urea, sulfuric acid, and water, is maintained below about 155.degree. F., and the weight ratio of said urea to said sulfuric acid is at least about 1. 39. A method for producing reaction products of urea and sulfuric acid, which method comprises the steps of combining urea, sulfuric acid, and optionally water, in proportions corresponding to about 10 to about 70 weight percent urea, about 10 to about 80 weight percent sulfuric acid, and 0 to about 35 weight percent water, wherein said urea and said sulfuric acid, in combination, constitute at least about 65 weight percent of said reaction product, reacting said urea with said sulfuric acid in said combination to produce said reaction products, and, at all times during said reaction, maintaining the temperature of said combination at a level below the incipient decomposition temperature of said combination. 40. The method defined in claim 39 wherein said temperature of said combination is maintained at a level below about 176.degree. F. 41. The method defined in claim 39 wherein said temperature of said combination is, at all times during reaction, maintained at a temperature below about 160.degree. F. 42. The method defined in claim 39 wherein said temperature is maintained at a level of about 155.degree. F. or less and the weight ratio of said urea to said sulfuric acid in said combination is at least about 1. 43. The method of improving the quality of agricultural soils including the step of applying to said soil an agronomically effective amount of the composition defined in claim 29. 44. The method of fertilizing crops including the step of applying to said crop an agronomically effective amount of the composition defined in claim 29. 45. The method of improving the quality of agricultural soils including the step of applying to said soil an agronomically effective amount of the composition defined in claim 31. 46. The method of fertilizing crops including the step of applying to said crop an agronomically effective amount of the composition defined in claim 31. 47. The method of improving the quality of agricultural soils including the step of applying to said soil an agronomically effective amount of the composition defined in claim 34. 48. The mehtod of fertilizing crops including the step of applying to said crop an agronomically effective amount of the composition defined in claim 34. 49. The method of improving the quality of agricultural soils including the step of applying to said soil an agronomically effective amount of the composition defined in claim 35. 50. The method of fertilizing crops including the step of applying to said crop an agronomically effective amount of the composition defined in claim 35. 51. The method of improving the quality of agricultural soils including the step of applying to said soil an agronomically effective amount of the composition defined in claim 36. 52. The method of fertilizing crops including the step of applying to said crop an agronomically effective amount of the composition defined in claim 36. |
Description: |
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of urea-sulfuric acid compositions, and particularly to improved methods of producing stable, highly concentrated urea-sulfuric acid compositions which are useful in a variety of applications. The methods allow sufficient control of reaction parameters to consistently maintain a predetermined product composition and crystallization temperature while avoiding incipient product and/or reactant decomposition and potentially explosive autocatalytic decomposition associated with the highly exothermic reaction. The invention also relates to a unique direct air heat exchange process for cooling the reacting liquid phase without significant emissions to the atmosphere. 2. Description of the Prior Art Urea is widely used as a topical, sub-surface and foliar fertilizer. Sulfuric acid has also been widely used in the agricultural industry and in other industries for numerous purposes. It is known to be highly corrosive both to metals and animal tissue, including human skin. In the agricultural industry, sulfuric acid has been used as a soil adjuvant, a water penetration improving agent, a herbicide for a wide variety of undesired vegetation, and as a selective herbicide on resistant crops such as onions and garlic. Previous investigators have observed that urea, sulfuric acid and water can be reacted to form solutions containing mono- and diurea sulfates. This reaction is so exothermic, that it is difficult to control reaction temperature in large volume production plants and it is essentially impossible to control reaction temperature during formulation of the higher acid content compositions, e.g., where the ratio of sulfuric acid to urea is about 1 or greater, with available methods. Furthermore, previous investigators did not recognize either the magnitude or importance of incipient product and/or reactant decomposition or the temperatures at which such decomposition occurs for products having different urea/sulfuric acid ratios. Their methods were not adequate to avoid incipient decomposition, particularly in the higher acid compositions, and they did not recognize the effect of such decomposition on process control or product quality. D. F. du Toit found that urea formed certain compounds with oxalic, acetic, hydrochloric, nitric and sulfuric acids, and that the resulting compounds were stable in contact with their solutions at 20.degree. C. Verslag Akad. Wetenschappen, 22, 573-4 (abstracted in Chemical Abstracts, 8, 2346, 1914). L. H. Dalman expanded on du Toit's work by developing the phase relationships between the solid phase and saturated solutions at 10.degree. C. (50.degree. F.) and 25.degree. C. (77.degree. F.) but, as in the case of du Toit, did not develop or disclose methods capable of handling the high heat of reaction involved in large scale industrial processing. "Ternary Systems of Urea and Acids. I. Urea, Nitric Acid and Water. II. Urea, Sulfuric Acid and Water. III. Urea, Oxalic Acid and Water"; JACS 56, 549-53 (1934) In the article "Adding Plant Nutrient Sulfur to Fertlizer," Sulfur Institute Bulletin No. 10 (1964), the Sulfur Institute discussed the addition of nutrient sulfur to fertilizers and mentioned that urea reacts with sulfuric acid to form two complexes of urea sulfate which are useful fertilizers. Jones, U.S. Pat. No. 4,116,664 discloses what is referred to therein as a tortuous, multistage process of producing combinations of urea and sulfuric acid in which portions of the sulfuric acid are incrementally added to and reacted with the total amount of urea to be reacted in each of several stages until the total amount of sulfuric acid has been reacted with the urea. The resulting product is unstable and requires further processing. Jones preferably adds water later as required to obtain stability and the desired composition. He discloses that the reaction can be carried out at temperatures of 100.degree. to 225.degree. F. and that if the sulfuric acid is added to the total amount of urea at a rate which is too fast, the temperature goes to about 200.degree. to 225.degree. F. and that a gas is emitted that causes changes in product characteristics such as solidification. The patent states that temperatures of 160.degree. to 200.degree. F. are preferred. Other writers have discussed methods for controlling the heat of reaction in highly exothermic systems such as the urea-sulfuric acid reaction described by du Toit, Dalman and Jones. For instance, William Lohry, in "Techniques of Manufacturing Hot Mix Suspensions," National Fertilizers Solutions Association "Round-Up Papers," pages 34-38 (1968), disclosed that the exothermic heat of reaction of ammonia with concentrated phosphoric acid can be controlled by either internal or external cooling of the reactants in the reaction vessel, and that it is usually desirable to provide a heel of reaction product in the vessel before adding reactants to prevent drastic variations in product pH. In U.S. Pat. No. 1,884,105, H. C. Moore disclosed a method for producing salts of sulfuric acid, e.g., by reacting concentrated sulfuric acid with liquid anhydrous ammonia, in which control of the highly exothermic reaction is assisted by adding an initial inventory of product to the reaction zone before reactant addition. In Moore's process, the total amount of sulfuric acid to be reacted is mixed with a quantity of ammonium sulfate previously produced and that mixture is then reacted with liquid anhydrous ammonia. Similarly, in U.S. Pat. No. 3,459,499, G. C. Mullen, Jr. disclosed a process for ammoniating superphosphoric acid in which, according to Mullen, temperatures are effectively controlled, and diminished product quality associated with excessive reaction temperatures is avoided, in part, by adding a large body of ammonium phosphate product solution to the reaction zone followed sequentially by the introduction of the relatively small quantities of ammoniating fluid and phosphoric acid. Although these investigators disclosed several characteristics of urea-sulfate combinations, methods of making those combinations and, in general, methods of controlling exothermic reactions, they did not recognize either the magnitude or significance of the incipient decomposition temperature in large volumes of reacting urea and sulfuric acid in the presence of reacted urea-sulfate. Nor did they appreciate that incipient decomposition temperature varies with the urea-sulfate composition, i.e., with the ratio of urea, sulfuric acid and water in the reaction phase, or the effect that exceeding the incipient decomposition temperature has on product composition. They also did not devise or appreciate the need for process conditions required to achieve acceptable reaction rates in large volumes of reacting urea and sulfuric acid while preventing either gross or localized overheating to temperatures in excess of the incipient decomposition temperature. Nor did they recognize several characteristics of urea-sulfuric acid reaction products that make them particularly attractive for certain utilities. It is therefore one object of this invention to provide an improved method for producing concentrated solutions of urea and sulfuric acid. It is another object to provide a method for maintaining the temperature at all points in a large volume of reacting urea and sulfuric acid at a level below the incipient decomposition temperature. Another object is to provide a method for producing concentrated solutions of urea and sulfuric acid in which the reacting liquid phase is cooled by direct air heat exchange without polluting the atmosphere. Yet another object of this invention is the provision of a method for producing concentrated urea-sulfuric acid solutions containing little or no toxic by-products such as ammonium sulfamate and/or sulfamic acid Another object is the provision of a continuous method for producing urea-sulfuric acid reaction products in relatively large volumes in which reaction parameters, including reactant composition, reaction temperature, product composition and crystallization temperature are continuously maintained at predetermined values. Another object is the provision of an improved method for the production of concentrated, stable solutions of urea and sulfuric acid having H.sub.2 SO.sub.4 /urea weight ratios of at least about 1. Another object of this invention is the provision of improved compositions comprising urea and sulfuric acid reaction products essentially or completely free of sulfamic acid and/or ammonium sulfamate. Yet another object of this invention is the provision of an improved method for fertilizing crops with improved compositions comprising reaction products of urea and sulfuric acid essentially or completely free of sulfamic acid and/or ammonium sulfamate. Other objects, aspects and advantages of this new invention will be apparent to one skilled in the art in view of the following disclosure, the drawings, and the appended claims. SUMMARY OF THE INVENTION This invention relates to methods of producing concentrated solutions of urea and sulfuric acid in which reaction temperature is continuously maintained below incipient decomposition temperature to prevent explosive autocatalytic decomposition or, in milder cases, the formation of one or more undesirable reaction by-products. It also relates to methods capable of continuously maintaining predetermined product composition, crystallization temperature, and concentrations of urea, sulfuric acid and, optionally, water in the reaction zone, and to the production of products free of reaction by-products such as ammonia, ammonium sulfate, carbon dioxide, ammonium sulfamate, and/or sulfamic acid In accordance with one embodiment of this invention, urea, sulfuric acid and, optionally, water are simultaneuously added to a reaction zone at constant relative rates that are controlled to achieve a concentration of each respective component corresponding to a predetermined product composition within the ranges of about 5 to about 75 weight percent urea, about 5 to 85 weight percent sulfuric acid and 0 to about 35 weight percent water. The urea and sulfuric acid are introduced separately of each other and, in combination, constitute at least about 65 weight percent feed to the reaction zone. The reactants produce a reacting liquid phase which is continuously agitated during the addition of urea, acid and water to evenly disperse all of the reactants throughout the reacting liquid phase. During the process, the temperature of the reacting liquid phase is maintained at a level of at least about 120.degree. F. and less than about 176.degree. F., and below its incipient decomposition temperature, as shall be more specifically described hereinafter. In accordance with another embodiment, the reacting liquid phase is continually cooled in a direct contact air heat exchanger in which the liquid phase is countercurrently contacted with air and returned to the reaction vessel while heated air containing little or no contaminants is released to the atmosphere. In accordance with another embodiment of this invention, there are provided reaction products of urea and sulfuric acid essentially or completely free of toxic reaction by-products such as sulfamic acid and/or ammonium sulfamate In accordance with yet another embodiment, soils and/or crops are fertilized or treated with solutions comprising urea and sulfuric acid reaction products essentially or completely free of toxic reaction products such as sulfamic acid or ammonium sulfamate. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more readily understood by reference to the drawings of which: FIG. 1 is the ternary phase diagram for the urea, sulfuric acid and water system illustrating isotherms at several different temperatures and the existence of three prominent eutectics; FIG. 2 is a ternary phase diagram for the system urea, sulfuric acid and water generally illustrating the static corrosion rate of various compositions on mild steel at 75.degree. F.; FIG. 3 illustrates the correlation between mild steel corrosion rate and temperature for three urea-sulfuric acid products; FIG. 4 is a correlation of the first order reaction rate constant versus temperature for the three designated urea-sulfuric acid products using prilled urea feed; FIG. 5 is a correlation of the first order rate constant with temperature for the designated urea-sulfuric acid products using granular urea feed; and FIG. 6 is a schematic illustration of an apparatus and process flow system suitable for use in one embodiment of this invention. DETAILED DESCRIPTION OF THE INVENTION The compositions of this invention are clear, homogeneous reaction products of urea, sulfuric acid and, optionally, water. Water may be present in only minor amounts or may be omitted entirely in some compositions as illustrated in the ternary phase diagram of FIG. 1. These compositions usually comprise about 5 to about 75 weight percent, preferably at least 10 to about 70 weight percent urea; about 5 to about 85 weight percent, preferably about 10 to about 80 percent sulfuric acid; and about 0 to about 35, usually less than about 25, and preferably less than about 15 weight percent water. Urea and sulfuric acid, in combination, will usually comprise at least about 65 weight percent, generally at least about 75 weight percent, and preferably at least about 85 weight percent of the product solution. Four digit product designations, e.g., 18-0-0-17, are conventionally used in the agricultural industry to designate the concentration of nitrogen, phosphorus (as P.sub.2 O.sub.5), potassium (as K.sub.2), and a fourth component--in this case sulfur expressed as the element. Thus, the composition 18-0-0-17 contains 18 weight percent nitrogen derived from urea and 17 weight percent sulfur derived from sulfuric acid. Using the atomic weights for nitrogen (14.01) and sulfur (32.07) and the molecular formulas and molecular weights for urea (60.06) and sulfuric acid (98.08), it can be readily determined that this formulation contains 38.58 weight percent urea and 51.99 weight percent sulfuric acid. By difference, the solution contains 9.43 weight percent water. The composition of all other products can be determined by the same procedure. The ternary phase diagram of FIG. 1 defines the relative proportions in weight percent for each of the three components-- urea, sulfuric acid, and water--at any point within the diagram. At each apex of the triangle, the system consists completely of the indicated component. Thus, urea concentration at the urea apex is 100 percent urea and diminishes linearly to 0 percent urea along a straight line from he urea apex to the midpoint of the H.sub.2 O--H.sub.2 SO.sub.4 boundary line, i.e., the side of the triangle opposite the urea apex. The same is true of the remaining two components; water and sulfuric acid. The diagram also illustrates the isotherms for the system at 14.degree. F., 32.degree. F., 50.degree. F., 77.degree. F., and 150.degree. F. The 150.degree. F. isotherm is illustrated only partially at the lower left-hand portion of the phase diagram. Each isotherm defines compositions which, if cooled below the temperature indicated for the respective isotherm, will precipitate components or reaction products of the system. However, the solutions will super-cool dramatically, e.g., by as much as 50.degree. F., or more, under quiescent conditions in the absence of seed crystals, impurities, etc. that promote crystallization. As indicated by the pattern of the isotherms, systems having a fixed ratio of urea to sulfuric acid become more stable at lower temperatures as the water concentration is increased. This is true throughout most of the phase diagram with the exception of the region in the vicinity of the higher acid eutectic in the lower right-hand portion of the phase diagram. Three prominent eutectcs are apparent within the region of the illustrated isotherms. Each eutectic represents a discontinuity in the response of the system, e.g., of crystallization point, to changes in the solute concentration, and indicates the point of maximum solute concentration for a given isotherm in the region of the phase diagram associated with each eutectic. As indicated in the legend on FIG. 1, the left-hand eutectic on the 50.degree. F. isotherm corresponds to the formulation 29-0-0-9. The middle eutectic on the same isotherm corresponds to the composition 18-0-0-17. The right-hand eutectic on the 14.degree. F. isotherm corresponds to the formulation 9-0-0-25, and the formulation intermediate the 50.degree. F. and the 77.degree. F. isotherms between the middle and right-hand eutectics indicated by a triangular designation corresponds to the formulation 10-0-0-19. The dramatic discontinuities in these isotherms and their proximity to each other confirm the observation that minor variations in reactant phase or product composition can result in dramatic changes in crystallization temperature. The bold lines within the diagram generally define the boundaries for formulations for which the methods of this invention are uniquely suited. Bold lines parallel to one side of the triangular diagram define a fixed concentration of the solute designated at the apex of tne triangle opposite the side to which that line is parallel. Thus, the higher horizontal line in FIG. 1 borders the area of formulations containing 35 percent water or less and varying amounts of urea and sulfuric acid. The area below the lower horizontal line defines formulations containing 25 weight percent water or less. Vertical line A in the center of the diagram intersecting the urea-sulfuric acid at the half-way point defines compositions having a one-to-one weight ratio of urea to sulfuric acid. The area to the right of that vertical line defines formulations having H.sub.2 SO.sub.4 /urea weight ratios of 1 or greater. The angular line B in the lower right-hand portion of the diagram intersects the urea-H.sub.2 SO.sub.4 boundary at the 90 percent sulfuric acid-10 percent urea point and, if extended, would intersect the water apex. Thus, it represents formulations having a 9:1 weight ratio of sulfuric acid to urea. A very limited range of compositions having crystallization temperatures over 150.degree. F. can be produced by the methods described herein when the temperature required to maintain the reaction product in solution form does not exceed the incipient decomposition temperature for that composition. However, the products of this invention preferably have crystallization temperatures below about 150.degree. F., usually less than about 80.degree. F., and preferably less than about 50.degree. F. The lower crystallization temperatures are particularly preferred if the composition is to be used directly after production without dilution. One unique advantage of the methods described herein is that they can consistently produce compositions containing precisely predetermined proportions of urea, sulfuric acid and water. This feature is of particular importance due to the fact that minor deviations in composition can dramatically affect crystallization temperature and can result in product solutions that crystallize at intolerably high temperatures. For instance, compositional differences as little as 2 percent in urea and sulfuric acid can change crystallization temperature by as much as 20.degree. F. This difference could result in complete solidification of a product solution in transportation, processing or storage facilities. accordingly, very accurate and continuous control of product quality is an essential aspect of the methods of this invention, particularly in continuous processes discussed hereinafter. Even minor decomposition of any of these compositions results in the formation of known toxic materials including ammonium sulfamate and sulfamic acid. Furthermore, incipient decomposition temperatures vary from product to product. For instance, the 29-0-0-9 formulation incipiently decomposes, i.e., begins to decompose, at temperatures of about 159.degree. F. and above, and thus should not be exposed to any temperature in excess of that point. If it is, system temperatures may become uncontrollable and the product will contain minor, if not significant, amounts of sulfamic acid or ammonium sulfamate. Thus, adequate temperature control is imperative to prevent decomposition which, once commenced in a large volume of inadequately cooled material, can lead to very rapid temperature escalation, e.g., up to 600.degree. F. and higher, and to the literal explosion of the reactor and associated processing facility. The magnitude of the reaction exotherm and incipient decomposition temperature variations are illustrated in the following table. ______________________________________ Incipient Heat of Reaction Composition Decomposition Temperature BTU's per Ton ______________________________________ 29-0-0-9 158.degree. F. 73,600 18-0-0-17 176.degree. F. 173,400 9-0-0-25 176.degree. F. 149,500 10-0-0-19 176.degree. F. 195,500 ______________________________________
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