______________________________________
             Incipient          Heat of Reaction
    Composition
             Decomposition Temperature
                                BTUs per Ton
    ______________________________________
    29-0-0-9 158.degree. F.      73,600
    9-0-0-25 176.degree. F.     149,500
    10-0-0-19
             176.degree. F.     195,500
    ______________________________________

The heats of reaction reported in the foregoing table were determined calorimerically using the reaction of prilled urea with 98 percent sulfuric acid and the amount of water required for the designated formulation.

Incipient decomposition temperatures can be determined by very gradually increasing the temperature of a solution of the designated composition until gas evolution is first observed. The incipience decomposition temperature of any formulation can be determined by this procedure.

The evolved gas is carbon dioxide and, in the absence of unreacted sulfuric acid, may also comprise ammonia. In the presence of unreacted sulfuric acid, a condition that exists in the reaction zone, the ammonia would react very exothermically with sulfuric acid to increase solution temperature and heat load at a rate even faster than that occasioned by the sulfuric acid-urea reaction. This mechanism may be partially responsible for the observed autocatalytic decomposition of the more concentrated compositions at elevated temperatures.

As a general rule, incipient decomposition temperatures ragne from about 155.degree. F. to about 176.degree. F. with the higher decomposition temperatures being associated with products having higher acid-to-urea ratios.

Gross system temperature is not an adequate indication of incipient decomposition at localized points within a relatively large volume of solution, e.g., in a commercial reactor. For instance, the direct addition of concentrated sulfuric acid to a large volume of urea as described in U.S. Pat. No. 4,116,664 referred to above, will invariably result in localized overheating and temperatures in excess of incipient decomposition temperatures even though the average temperature for the bulk of urea may be somewhat lower. This was confirmed by the fact that samples of materials produced in the process described in that patent were found to contain as much as 5 to 6 weight percent of the decomposition products ammonium sulfamate and/or sulfamic acid.

Continuous processing is preferred, particularly in the manufacture of the more concentrated solutions, since it improves process stability and the control of composition, reaction temperature, crystallization point and corrosivity. All of these factors are important for different reasons.

Accurate control of reaction phase composition is closely related to temperature control due to the highly exothermic nature of the sulfuric acid-urea reaction. It is also closely related to crystallization point and corrosivity; minor variations in product composition can significantly affect both properties. Significant variation in crystallization point can result in solids formation or complete "setting up" of the product in lower temperature treating, storage or application facilities. Increased corrosivity occasioned by composition changes can dramatically increase the corrosion of the reactor and processing facilities, particularly at elevated processing temperatures, unless sufficient inhibitor is present in the system.

These several characteristics can be controlled by gradually and simultaneously adding urea, concentrated sulfuric acid and water to the reaction zone at relative rates corresponding to the concentration of each component in a predetermined product and cooling the resultant reacting liquid phase sufficiently to maintain it at a temperature below its incipient decomposition temperature and below 176.degree. F., at all times. As pointed out above, bulk system temperature will not accurately indicate the presence of absence of localized overheating unless the reacting liquid phase is adequately agitated and thoroughly mixed during the course of the reaction.

Although the reaction will proceed at relatively low temperatures, it becomes too slow to be economically desirable at temperatures much below 120.degree. F. Accordingly, the reaction is usually run at temperatures of at least 120.degree. F., preferably at least about 130.degree. F., and below 176.degree. F., preferably below about 160.degree. F., and most preferably about 150.degree. F. or less. The lower temperatures, e.g., of about 150.degree. F.-160.degree. F., or less, are particularly preferred.

The feed rates of all three components, and the composition of the reacting liquid phase, should be maintained as closely as possible to the stoichiometric proportions of each respective component in the predetermined product. Thus, the concentration of each component should be maintained within about 2 percent, preferably within 1 percent or less, of its stoichiometric value in the product. In the preferred method in which a portion of the reaction phase is removed from the reaction zone and cooled by direct air contact heat exchange, some water is lost from the system and must be made up by increasing the water feed to the reaction zone by an amount proportional to the rate of water loss in the cooler. These close tolerances on reactant composition and temperature will generally allow control of product crystallization temperature within 10.degree. F., preferably within 5.degree. F. or less, of the desired crystallization temperature.

Although the considerable heat of reaction theoretically can be dissipated by essentially any cooling means, such as cooling coils within the reactor, heat dissipation and temperature control are facilitated by assuring that the reaction zone into which the urea-sulfuric acid and water are introduced, contains an amount of a mixture of reactants and reaction product corresponding to at least about 0.1, and preferably at least about 0.2 times the hourly feed rate. Larger inventories of at least about 0.5, usually at least about 1, and often at least about 2 times the hourly feed rate are preferred in the continuous process. Although somewhat lower reactor volumes are adequate to control temperature in the continuous process in some cases, they are not adequate to assure complete reaction of the customary forms of urea feeds, i.e., prills and/or granules. Longer holding times, and thus larger reactor inventories relative to product withdrawal rate and reactant feed rate, are preferred in the continuous process to assure that the withdrawn product does not contain unreacted urea.

The minimum reactor volume required to prevent the discharge of unreacted urea during continuous operation can be defined by the following expression which is unique to this reactor system: ##EQU1## where k is the first order rate constant in reciprocal minutes, d s the diameter of the largest urea particles in millimeters, v.sub.o is the volume of the liquid phase within the reaction zone in gallons, and u is the production rate from the reaction zne in gallons per minute. From this relationship, it can be seen that theoretically very small reactor volumes could be used with very small diameter urea feeds, e.g., urea dust. As a practical matter, however, minimum volumes of about one-half the hourly production rate are required to provide sufficient inventory for adequate cooling to prevent incipient decomposition and for more effective process control.

The first order rate constant can be determined from the expression unique to this system: ##EQU2## where t is the time in minutes required for dissolution of the type of urea feed, e.g., prills, pellets, granules, etc.

The dissolution rate varies with urea type, e.g., prilled urea or granular urea. Prilled urea is usually less dense and somewhat more porous than is granular urea, and is produced by forming droplets of molten urea in a prilling tower of sufficient height to allow the urea droplets to solidify during their descent. Granular ureas are usually produced by spraying molten urea onto urea "seeds" or dust in granulating apparatus such as pan or drum granulators.

The reaction rate constant can be determined experimentally for any given product composition and urea feed type by determining the rate at which the urea particle dissolves in the given formulation.

The reaction rate is first order and varies markedly with temperature. Experimentally observed values for the rate constants for 29-0-0-9 and 10-0-0-19, and the effect of temperature on the rate constant for each product using prilled urea and granular urea, are graphically illustrated in FIGS. 5 and 6, respectively.

The rate of urea dissolution can be determined by any one of several means. The data illustrated in FIGS. 5 and 6 were obtained by suspending urea prills or granules of known diameter in the selected solution at a predetermined temperature. The solution was agitated by the minimum amount required to suspend the urea particles. The elapsed time within which the urea particles disappeared was determined by visual observation and was taken as the value of t for that combination of urea type and product solution. As discussed above, particle diameter is taken as the diameter of the largest urea particles in the feed. The largest particles in most prilled ureas have diameters of at least about 1 millimeter, usually about 2 millimeters. Granular ureas may be somewhat larger.

Knowing the value of k, the minimum reactor volume required for continuously producing any product at a given temperature can be determined from the expression for V.sub.o. The same procedure can be used to determine the value of t, and thus the values of k and V.sub.o for any combination of urea type and reactant phase composition.

As can be seen from FIGS. 5 and 6, the reaction rate constant diminishes markedly with temperature. Thus, from the relationship between V.sub.o and reaction constant discussed above, it can be seen that larger reactor volumes are required to obtain the same production rate of the same product at lower reaction temperatures.

Adequate control of the factors discussed above, particularly heat load, solution temperature, composition, crystallization point and corrosivity, is particularly important in industrial scale reactors of relatively large volume in which the excess heat associated with decomposition cannot be rapidly dissipated. This is especially true in the production of higher acid content compositions, e.g., those having H.sub.2 SO.sub.4 /urea weight ratios above 1. Most commercial systems will have reaction zone volumes of at least about 50 gallons, usually at least about 100 gallons, and most often in excess of 500 gallons. The reaction phase is relatively viscous even at reaction temperatures, and that factor, combined with the relatively low heat capacity of these compositions, makes adequate temperature control and rapid heat exchange even more difficult in the large volumes associated with commercial production.

The process can be better understood by reference to FIG. 7 which is a schematic illustration of a preferred continuous method employing countercurrent direct air heat exchange.

Solid urea, water and sulfuric acid are simultaneously and continuously added through pipes 1, 2, and 3 to reactor 4 provided with efficient agitating means, such as impeller 5 driven by motor 6 or other means. The reacting liquid phase is continuously passed from reactor 4 through pipe 7 to spray nozzles 9 in the direct air heat exchanger 8. Ambient air or cooled air is introduced to the lower portion of the heat exchanger through pipe 11 or other means, and passes upwardly through packed section 10 into direct contact with downward flowing liquid phase. Following contact with the acid, the warmed air passes through demister section 14 and can be emitted directly to the atmosphere. Even at elevated temperatures very little sulfate is present in the effluent air. Cooled product is removed from heat exchanger 8 through pipe 16 and is either passed to storage via pipe 13 or is returned as cooling medium to reactor 4 by pipe 12.

Urea can be fed in any available form, such as prills, granules, powder and the like. The minor variations in the purity of commercial ureas can be sufficient to significantly affect process conditions, even though nitrogen content usually varies only from about 46 to about 46.6 weight percent. The urea feed is preferably periodically analyzed for nitrogen content and its feed rate adjusted accordingly in view of the stoichiometry of the desired product.

The sulfuric acid feed can be concentrated sulfuric acid, usually 92 to 98 weight percent H.sub.2 SO.sub.4, or it can be diluted with water before introduction into the reaction zone. Fuming sulfuric acid can also be used. Essentially any acid source is suitable. Spent alkylation acid can also be used in this process. Concentrated sulfuric acid is presently preferred due to commercial availability, and to the higher corrosivity of more dilute acid solutions.

If dilute acid feeds are employed, the amount of water added with the acid feed should not exceed the amount permitted in the product. This amount varies substantially from product to product. For instance, the minimum acid concentration that can be employed in the manufacture of 10-0-0-19 is 74.2 weight percent H.sub.2 SO.sub.4 (unless water is removed from the system).

Once the continuous process is commenced, it can be run indefinitely provided that sufficient provision is made to control corrosion and that changes in product composition are not required.

In starting up either a batch or continuous process, a product inventory can be manufactured in the reaction vessel by gradual addition of the reactants in stoichiometric proportions, provided that sufficient cooling is available to maintain the reacting mixture at a temperature below the incipient decomposition temperature. In the alternative, an inventory of material produced in a previous operation can be used. In either event, the initial inventory, or heel, must be of the same composition as that of the desired product so that deviations in composition, crystallization point, heat of reaction or corrosivity do not occur during the process.

During startup, the initial inventory may be of lesser volume than that ultimately maintained in the reaction zone to assure complete reaction. The initial heel enables more adequate control of reaction temperature developed by the exothermic reaction during start-up.

The heat of reaction involved in the formation of any product can be determined calorimetrically by reacting the selected urea and sulfuric acid feeds and water, when required, under closely controlled temperature conditions sufficient to prevent incipient decomposition. The heat of reaction for a given product can then be used to calculate the total heat load on a given system, and thus the cooling capacity required for a given production rate. In the alternative, the heat of reaction can be calculated from the following expressions: ##STR1##

The overall reaction is illustrated by the equation: ##STR2##

Applying these expressions to the amount of solid urea and sulfuric acid added to the reaction zone will yield the amount of heat to be expected in the reaction. That value in turn allows the determination of production rates permissible in any system assuming the process is limited by cooling capacity, or conversely, the cooling capacity that must be provided for the production of that product at a given rate.

The high viscosity, low specific heat, low maximum allowable temperature, high corrosivity to conventional alloys at high fluid velocity in the absence of inhibitors, and low water content of these products, place severe limitations on conventional heat exchangers. Nevertheless, conventional designs such as shell and tube, coil, etc., can be used, although they must be designed in view of the product characteristics mentioned above.

The reacting liquid phase can be adequately cooled by direct contact countercurrent heat exchange with ambient air making use of a relatively simple cooling unit design such as that illustrated in FIG. 7. This approach mitigates the problems associated with the low specific heat, high viscosity and corrosivity of uninhibited urea-sulfuric acid compositions. The direct air heat exchange method adequately cools the reacting liquid phase even though it has very low vaporizable water content, and does so without introducing or removing uncontrollable amounts of water to or from the reaction phase or polluting the atmosphere.

The contact section of the cooler can consist of any corrosion and heat resistant shell, e.g., stainless steel, and an adequate quantity of acid-resistant packing of any one of numerous types. Acceptable packing materials include plastic or ceramic saddles and the like.

The design of the direct contact exchanger for any particular operation should be based upon the highest heat load anticipated which is a function of production composition and production rate, and can be established by testing different combinations of packing material, packing section design, product flow rate and air flow rate through the exchanger.

As a practical matter, the packing section should have a height to diameter ratio of at least about 1 and , for most packing materials, should be operated at liquid flow rates of about 25 to about 200 pounds per hour per cubic foot of packing and air flow rates of about 25 to about 100 cubic feet per minute per cubic foot of packing material. Significantly higher liquid flow rates should be avoided to avoid flooding the cooler while higher air flow rates should be avoided to prevent excessive resistance to downward liquid flow and product carryover into the demister section.

Continuous monitoring and compensation for water removal from the system is preferred and is necessary for precise control of composition, temperature, and corrosion. This can be achieved by monitoring product or reactor phase composition or water removal rate in the cooler and adding water as required to the reaction zone.

Even small variations in sulfuric acid feed concentration, or minor excursions in product composition, can produce sharp changes in the water removal rate in the direct air heat exchanger. This occurrence has a feedback effect on the overall process which alters reaction temperature and cooler efficiency and can result in unacceptable swings in product and reactant phase composition. This problem can be mitigated by monitoring the rate of water removal from the system and by precise control of product composition.

Product composition can be determined by periodically sampling the product effluent or reaction phase, analyzing for sulfuric acid, urea and water, and gradually modifying reactant feed rates as necessary to maintain specification product composition. Acid content can be determined by standard acid titration techniques, and both acid and urea concentrations can be determined by mass spectrographic analysis, high precision infrared or liquid chromatographic analysis, or by standard wet chemical test procedures for urea and sulfuric acid. Having determined sulfuric acid and urea concentration, water can be determined by difference. Product composition is also reflected by specific gravity and refractive index. Thus, one or both of these tests can be used in combination with total acidity to determine urea and the sulfuric acid concentration while water, again, can be determined by difference.

The thiourea compounds, either with or without the described cupric ion-containing compounds, can be added to the urea-sulfuric acid system either during or after the reaction. However, it is presently preferred that the inhibitors be added directly to the reaction zone or be combined with one of the feed components to the reaction zone when required to minimize corrosion to any carbon steel or stainless steel components to which the reaction phase and product are exposed during manufacture, storage or use.

EXAMPLE

The inhibitors listed in Table 1 were evaluated for their effectiveness in reducing the corrosion of carbon steel (AISI C-1010) by 29-0-0-9 at 130.degree. F. under static conditions and the corrosion of stainless steel (AISI type 316) by 10-0-0-19 at 170.degree. F. and 15 feet per second fluid velocity as indicated in the table. Sample steel coupons of known weight and surface area were suspended in a large excess of the designated compositions having the inhibitor concentrations shown in the table. Each solution was maintained at the indicated temperature throughout the test period. A 15 fps. fluid velocity was maintained in the 10-0-0-19 tests by attaching the 316 SS coupons to a rotating rod immersed in each sample solution. After 72 hours exposure, the coupons were removed from their respective solutions, cleaned and weighed to determine weight loss and corrosion rate in mils per year (MPY). The results are listed in Table 1.

                  TABLE 1
    ______________________________________
                Con-  Corrosion Rate, MPY
                  centra- 29-0-0-9   10-0-0-19
                  tion,   Carbon Steel
                                     Stainless Steel
    Inhibitor     ppm.    130.degree. F., Static
                                     170.degree. F.; 15 fps.
    ______________________________________
    None                  220        625
    Ammonium
    Thiocyanate   10,000  565        691
    Thiomalic Acid
                  10,000  882        817
    Potassium
    Dichromate    10,000  712        410
    Potassium
    Permanganate  10,000  735        356
    Thiourea      10,000  993        615
    1,3-Dibutylthio-
    urea          10,000  4          610
    Diethylthiourea
                  10,000  31         709
    Diethylthiourea
                  400     38         NA
    Potassium Chlorate
                  10,000  1200       950
    Dimethylsulfoxide
                  10,000  291        575
    Tetramethylammonium
    chloride      175     260        655
    Cupric Ion
    (As CuSO.sub.4)
                  300     375        <1
    Cupric Ion
    (As CUSO.sub.4)
                  250     NA         152
    Sodium Sulfate
                  10,000  NA         510
    Sodium Sulfide
    Nonylhydrate  5,000   830        685
    Ammonium Nitrate
                  2,000   1465       898
    Ammonium Phos-
    phate (10-34-0)
                  10,000  231        NA
    ______________________________________

Of all the inhibitors tested on carbon steel, only the alkylthioureas had any beneficial effect, and that effect was dramatic. The other inhibitors either had no effect or significantly increased conversion rate.

Similar results were observed in the 10-0-0-19 tests on stainless steel. Cupric ion was the only agent that significantly reduced corrosion at a practically low concentration. Dibutylthioureas had no significant effect on stainless steel corrosion and diethylthiourea slightly increased the corrosivity of 10-0-0-19 toward stainless steel. Similarly, cupric ion increased the corrosivity of 29-0-0-9 to carbon steel.

Numerous variations and modifications of the concepts of this invention will be apparent to one skilled in the art in view of the aforegoing disclosure, drawings, and the appended claims, and are intended to be encompassed within the scope of this invention as defined by the following claims.