Noncorrosive urea-sulfuric acid reaction products , United States Patent 4404116
|Abstract:||Improved liquid, urea-sulfuric acid compositions noncorrosive to stainless steel contain about 5 to about 75 weight percent urea, about 5 to about 85 weight percent sulfuric acid, 0 to about 75 weight percent water and a corrosion inhibiting amount of a cupric ion-containing compound, in which the urea and sulfuric acid together comprise at least about 25 weight percent of the composition, and in which at least a portion of the urea and sulfuric acid are present as monourea sulfate, diurea sulfate or combinations thereof. The compositions may also contain amounts of selected dialkylthioureas sufficient to reduce their corrosivity to carbon steel.|
|Inventors:||Young, Donald C.;|
|Assignee:||Union Oil Company of California (Los Angeles, CA)|
|US Patent References:||
|Attorney, Agent or Firm:||
1. 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 about 75 weight percent water in which said urea and sulfuric acid, in combination, constitute at least about 25 weight percent of said composition and 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, containing a corrosion inhibiting amount of a cupric ion-containing compound sufficient to reduce the corrosivity of said composition to stainless steel.
2. The composition defined in claim 1 containing less than about 50 weight percent water wherein said sulfuric acid and said urea, in combination, constitute at least about 50 weight percent of said composition.
3. The composition defined in claim 1 containing less than about 25 weight percent water, wherein said sulfuric acid and said urea, in combination, constitute at least about 75 weight percent of said composition.
4. The composition defined in claim 1 wherein the weight ratio of said sulfuric acid to said urea is about 1.5 or less and said composition contains an amount of said cupric ion-containing compound corresponding to a cupric ion concentration of at least about 25 ppm.
5. The composition defined in claim 1 wherein the weight ratio of said sulfuric acid to said urea is greater than about 1.5 and said composition contains an amount of said cupric ion-containing compound corresponding to a cupric ion concentration of at least about 250 ppm.
6. The composition defined in claim 1 wherein said cupric ion-containing compound is selected from the group consisting of cupric oxide, cupric sulfate, cupric nitrate, cupric halides, and combinations thereof.
7. The composition defined in claim 1 containing a corrosion inhibiting amount of cupric sulfate.
8. The composition defined in claim 1 containing a corrosion inhibiting amount of a dialkylthiourea in which each alkyl group has from one to about four carbon atoms sufficient to reduce the corrosivity of said composition to carbon steel.
9. The composition defined in claim 8 having a urea/sulfuric acid mole ratio greater than 2 or less than 1.
10. The composition defined in claim 8 wherein each of said alkyl groups in said dialkylthiourea have from 1 to 3 carbon atoms.
11. A composition of matter relatively noncorrosive to stainless steel comprising a liquid urea-sulfuric acid reaction product containing 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 in which said urea and sulfuric acid, in combination, constitute at least about 75 weight percent of said composition and in which at least a portion of said urea and said sulfuric acid are present as a member selected from the group consisting of monourea sulfate, diurea sulfate, and combinations thereof, containing a corrosion inhibiting amount of a cupric ion-containing compound corresponding to a cupric ion concentration of at least about 50 ppm sufficient to reduce the corrosivity of said composition to stainless steel.
12. The composition defined in claim 11 having a urea/sulfuric acid mole ratio greater than 2 or less than 1 and comprising a corrosion inhibiting amount of a dialkylthiourea in which each alkyl group has from 1 to about 4 carbon atoms sufficient to reduce the corrosivity of said composition to carbon steel.
|BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of liquid urea-sulfuric acid compositions, and particularly to such compositions having reduced corrosivity to stainless steel. The compositions are stable homogeneous solutions of urea-sulfuric acid reaction products of significantly reduced corrosivity toward stainless steels even at elevated temperatures and under flow conditions. Thus, they enable the use of such urea-sulfuric acid reaction products at elevated temperatures and/or under flow conditions during their manufacture, transport or use in stainless steel equipment.
2. Description of the Prior Art
Both urea and sulfuric acid are widely used for a variety of purposes in numerous industries as fertilizers, soil adjuvants, chemical treating agents, chemical precursors and reactants. They are sometimes useful in combination, particularly in the agricultural industry, when the simultaneous addition of urea and sulfur to the soil is desired.
Previous investigators have observed that urea, sulfuric acid and, optionally water, can be reacted to form concentrated solutions of urea and sulfuric acid reaction products in which the urea is present as mono- and/or diurea sulfate. However, they did not recognize that the reaction product solutions of different urea/sulfuric acid ratio exhibit markedly different corrosivity towards stainless steel particularly at elevated temperatures and/or under flow conditions, or that the corrosivity of all of the solutions towards stainless steel could be markedly reduced by the addition of corrosion inhibiting amounts of cupric ion-containing compounds.
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.). "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 Fertilizer," 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 disclosed what is referred to therein as a tortuous, multistage process for 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 200.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 and that the products can be used as fertilizers.
A wide variety of compounds, including cupric sulfate and dialkylthioureas, are known to reduce the corrosivity of sulfuric acid to stainless steels and carbon steels.
Although these investigators disclosed several characteristics of urea-sulfuric acid combinations and methods of making those combinations, and that the products were useful soil adjuvants and/or fertilizers, they did not recognize that the corrosivity of the urea-sulfuric acid reaction products in either concentrated or diluent form to stainless steel varies dramatically as a function of the urea/sulfuric acid weight ratio, or that the corrosivity of all solutions to stainless steel at elevated temperatures and/or under fluid flow conditions can be dramatically reduced by the addition of corrosion inhibiting amounts of cupric ion-containing compounds.
Previous investigators also were not aware that the corrosion characteristics of the urea-sulfuric acid compositions of this invention differ markedly from those of sulfuric acid. For instance, all of the sulfuric acid inhibitors, with the exception of cupric ion, have little or no beneficial effect on the urea-sulfuric acid compositions, and may significantly increase corrosion rate. Furthermore, none of the known inhibitors have any significant beneficial effect on carbon steel corrosion by compositions having urea/sulfuric acid molar ratios between 1 and 2. Carbon steel corrosivity is relatively low within that composition range but is intolerably high with compositions having urea/sulfuric acid molar ratios above 2 or below 1. Thus, carbon steel corrosivity increases dramatically as sulfuric acid concentration is either increased or decreased relative to urea concentration outside this range of molar ratios. Another anomalous characteristic of these compositions is that some, but not all of them are significantly more corrosive to stainless steel than they are to carbon steel. Thus, 10-0-0-19 corrodes AISI C-1010 carbon steel at a rate of 56 mils per year under static conditions at 150.degree. F. and corrodes AISI type 316 stainless steel at 300 mils per year under identical conditions. These properties are not characteristic of sulfuric acid.
It is therefore one object of this invention to provide improved, liquid urea-sulfuric acid compositions.
It is another object of this invention to provide liquid urea-sulfuric acid reaction product compositions which are noncorrosive to stainless steel even at elevated temperatures or under fluid flow conditions.
Another object is the provision of urea-sulfuric acid compositions of reduced corrosivity to both stainless steel and carbon steel.
Other objects, aspects and advantages of this 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 liquid urea-sulfuric acid compositions which are noncorrosive to stainless steel under either quiescent or fluid flow conditions, even at elevated temperatures. These compositions contain corrosion inhibiting amounts of one or more cupric ion-containing compounds and can be used in stainless steel equipment with considerably less corrosion than would result from the use of similar compositions in the absence of cupric ion-containing compounds. The invention also relates to compositions containing cupric ion and one or more dialkythioureas which are relatively noncorrosive to both stainless steel and carbon steel.
In accordance with one embodiment of this invention, homogeneous, liquid urea-sulfuric acid compositions noncorrosive to stainless steel comprise about 5 to about 75 weight percent urea, about 5 to about 85 weight percent sulfuric acid, 0 to about 75 weight percent water and a corrosion inhibiting amount of one or more cupric ion-containing compounds. The urea and sulfuric acid, in combination, comprise at least about 25 weight percent of the composition, and at least a portion of the urea and sulfuric acid are present as monourea sulfate, diurea sulfate, or combinations thereof.
In accordance with another embodiment of this invention, the compositions have sulfuric acid to urea weight ratios of about 1.5 or less and contain an amount of a cupric ion-containing compound corresponding to a cupric ion concentration of at least about 25 ppm.
In accordance with another embodiment of this invention, the urea-sulfuric acid compositions have sulfuric acid/urea weight ratios of at least about 1.5 and contain an amount of a cupric ion-containing compound corresponding to a cupric ion concentration of at least about 250 ppm.
In accordance with yet another embodiment of this invention, urea-sulfuric acid compositions relatively noncorrosive to both stainless steel and carbon steel comprise corrosion inhibiting amounts of one or more cupric ion-containing compounds and one or more dialkylthioureas.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more readily understood by reference to the drawings of which:
FIG. 1 is a ternary phase diagram for the urea, sulfuric acid, and water system illustrating isotherms at several different temperatures, the existence of three prominent eutectics along these isotherms, and the urea-sulfuric reaction products encompassed by this invention;
FIG. 2 is a correlation of corrosion rate of AISI 316 stainless steel at 15 feet per second fluid velocity and 150.degree. F. versus cupric ion concentration for the three designated products;
FIG. 3 is a correlation of AISI C-1010 carbon steel versus diethylthiourea concentration for the three designated compositions;
FIG. 4 is a correlation of the first order reaction rate constant versus temperature for the three designated urea-sulfuric acid reaction products using prilled urea feed;
FIG. 5 is a correlation of the reaction rate constant with temperature for the three designated urea-sulfuric acid reaction products using granular urea feed; and
FIG. 6 is a schematic illustration of an apparatus and process system suitable for use in the manufacture of concentrated urea-sulfuric reaction products.
DETAILED DESCRIPTION OF THE INVENTION
The compositions of this invention comprise liquid urea-sulfuric acid reaction products containing 5 to about 85, preferably about 10 to about 80 weight percent sulfuric acid, about 5 to about 75 weight percent, preferably about 10 to about 70 weight percent urea, and 0 to about 75, usually 0 to about 50, and preferably 0 to about 25 weight percent water. Urea and sulfuric acid, in combination, constitute at least about 25, usually at least about 50, and preferably at least about 75 weight percent of the composition. The more concentrated compositions are particularly preferred during manufacture and transportation due to the lower volumes and weights of materials that must be handled and transported. At least a portion of the urea and sulfuric acid are present as mono- or diurea sulfates or combinations thereof.
The four-digit composition designations used herein, 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 O), 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) and sulfur (32) 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 38.6 weight percent urea and 52.1 weight percent sulfuric acid. By difference, the solution contains 9.3 weight percent water. The concentrations of sulfuric acid and urea in all other compositions can be determined by the same procedure.
The compositions and some of their physical properties are illustrated by the ternary phase diagram of FIG. 1. The phase diagram 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, the composition at the urea apex is 100 percent urea and dimishes linearly to 0 along a straight line from the urea apex to the mid-point of the H.sub.2 O-H.sub.2 SO.sub.4 boundary line, i.e., the side of the triange 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 diagram. Each isotherm defines compositions which, if cooled below the temperature indicated for the respective isotherm, will precipitate components 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 eutectics 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 solute concentration, and indicates the points of maximum solute concentration for a given isotherm in the regions of the phase diagram associated with those eutectics.
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 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 10-0-0-19.
The bold lines within the diagram generally define the boundaries of the preferred reaction products of this invention.
Bold lines parallel to one side of the phase diagram define a fixed concentration of the component designated at the apex of the triangle opposite the side to which that line is parallel. Thus, the higher horizontal line in FIG. 1 borders the area of formulations containing 75 percent water or less and varying amounts of urea and sulfuric acid. The area below the horizontal line across the center of the diagram defines formulations containing 50 weight percent water or less.
Vertical line A in the center of the diagram intersecting the urea-sulfuric acid boundary at the half-way point defines compositions having a one-to-one weight ratio of sulfuric acid to urea. If extended vertically, line A would intersect the water apex. Line B represents reaction products having H.sub.2 SO.sub.4 /urea weight ratios of 1.5 and intersects the urea-sulfuric acid boundary at the point corresponding to 40 weight percent urea and 60 weight percent sulfuric acid. If extended, line B would pass through the water apex of the diagram. All formulations to the right of line B have H.sub.2 SO.sub.4 /urea weight ratios of 1.5 or greater.
The compositions of this invention contain corrosion inhibiting amounts of cupric ion-containing compounds in concentrations sufficient to reduce the corrosion of the composition to stainless steel. The corrosion inhibiting compounds are usually present in amounts corresponding to cupric ion concentrations of at least about 25 ppm, and generally at least about 50 ppm.
Different cupric ion concentrations are required to achieve comparable corrosivity in different compositions due to the dramatic differences in corrosivity and inhibitor effect between different compositions encompassed by this invention. For instance, compositions having sulfuric acid/urea weight ratios of less than 1.5 should contain at least about 25 ppm cupric ion, while compositions having sulfuric acid/urea weight ratios greater than about 1.5 should have cupric ion concentrations of at least about 250 ppm.
The inhibitor concentration required to obtain a certain level of corrosivity with a given composition at a particular temperature and flow velocity can be readily determined by exposing a standard sample of the stainless steel to be employed to different samples of the given composition containing various inhibitor concentrations at the temperature and flow velocity involved. The results of this test will provide a correlation similar to that illustrated in FIG. 2 which is a correlation of the corrosion rate of 316 stainless steel at 150.degree. F. and 15 feet per second fluid velocity at various cupric ion concentrations (added as cupric sulfate) for the compositions 18-0-0-17, 29-0-0-9, and 10-0-0-19. These correlations illustrate that the corrosivities of 18-0-0-17 and 29-0-0-9 are remarkably similar under these test conditions but that they differ dramatically in corrosivity and in their response to cupric ion concentration from 10-0-0-19.
The corrosivity of both 18-0-0-17 and 29-0-0-9 at 0 cupric ion concentration was approximately 24 mils per year (MPY) under the test conditions illustrated in FIG. 2, and decreased rapidly with increased cupric ion concentration to essentially zero at a cupric ion concentration of 50 ppm. In contrast, the corrosivity of 10-0-0-19 was approximately 190 mils per year at 150.degree. F. and 15 feet per second (fps.) fluid velocity in the absence of cupric ion, and decreased only gradually as cupric ion concentration was increased up to about 250 ppm. After that point the corrosivity of the 10-0-0-19 composition decreased dramatically to essentially zero mils per year at a cupric ion concentration of 300 ppm.
Correlations similar to those illustrated in FIG. 2 can be obtained for any composition encompassed by this invention under any conditions of temperature, fluid flow velocity and cupric ion concentration by the test procedure discussed above.
Reduced corrosivity is dependent only upon the presence of sufficient cupric ion regardless of the form in which it is added. Thus, any organic or inorganic copper-containing compounds can be employed which, when added to the composition, either cause the formation of or introduce cupric ion into the composition. However, inorganic compounds such as cupric oxide, sulfate, nitrate, halides, and the like are presently preferred due to their ready availability and relatively low cost. Cupric sulfate is particularly preferred in many formulations since the sulfate anion does not introduce additional ions into the composition. Organic or inorganic compounds other than the sulfates may be converted to the sulfate in solution, particularly in compositions containing higher proportions of sulfuric acid. Nevertheless, such compounds can be effectively used within the scope of this invention since they effectively introduce cupric ion into the composition.
The corrosivity of these compositions either with or without the cupric ion inhibitors, is a function of temperature, flow velocity and composition. Corrosion rate increases significantly with temperature for all compositions. For instance, in the absence of corrosion inhibitors, 10-0-0-19 exhibits a corrosion rate of approximately 190 miles per year at 150.degree. F. and 15 feet per second fluid velocity as illustrated in FIG. 2 as compared to a corrosion rate of approximately 630 mils per year at a temperature of 170.degree. F. under otherwise identical conditions. Similarly, the 18-0-0-17 eutectic composition has a corrosion rate of about 24 mils per year on 316 stainless steel at 150.degree. F. and 15 feet per second fluid velocity in the absence of the cupric ion inhibitor as illustrated in FIG. 2, as compared to a corrosion rate of about 44 mils per year at 170.degree. F. under otherwise identical conditions.
The corrosivity of these compositions to stainless steel is significantly affected by fluid velocity. Under quiescent conditions, i.e., zero fluid velocity, the corrosion rates for stainless steels are very low, possibly due to surface passivation. However, any significant fluid flow, i.e., 0.5 feet per second or higher, negates the surface passivating effect and promotes significant corrosion. Thus, in the absence of corrosion inhibitors, these compositions become corrosive to stainless steels under any conditions that produce relative movement between the composition and the steel such as fluid flow through valves, pipelines, pumps and the like, agitation in reactors or mixing vessels, etc.
The effect of composition on stainless steel corrosion rate is referred to generally above and is illustrated in FIG. 2. Stainless steel corrosion rate generally increases with H.sub.2 SO.sub.4 /urea weight ratio and the amount of inhibitor required to produce a significant reduction in corrosivity also generally increases significantly as the H.sub.2 SO.sub.4 /urea weight ratio increases. Thus, higher cupric ion concentrations are required for the higher H.sub.2 SO.sub.4 /urea weight ratio compositions, e.g., compositions having ratios of about 1.5 or greater.
Cupric ion does not significantly inhibit the corrosivity of these compositions to nonstainless carbon steels and the dialkylthiourea carbon steel inhibitors discussed hereinafter do not reduce corrosivity to stainless steels. Furthermore, numerous compounds other than dialkylthioureas and cupric ion-containing compounds, known to inhibit the corrosivity of sulfuric acid to stainless steels and/or carbon steels do not significantly reduce corrosion rate as illustrated in the example.
The compositions of this invention may also contain amounts of one or more dialkylthioureas sufficient to reduce their corrosivity to carbon steel as discussed in copending application Ser. No. 331,001, filed Dec. 15, 1981, the disclosure of which is incorporated herein by reference. As pointed out in that copending application, compositions having urea/sulfuric acid molar ratios greater than 2 or less than 1 are substantially more corrosive to carbon steel than are compositions having molar ratios within the range of 1 to 2. Furthermore, the corrosivity of compositions having urea/sulfuric acid molar ratios within the range of 1 to 2 is influenced only to a very minor extent by the dialkylthiourea compounds under all conditions investigated. Also, compositional and inhibitor effects on carbon steel corrosion differ markedly from the effects of the same variables on stainless steel corrosion. The reasons for these anomalies have not been defined with certainty.
These factors are illustrated, in part, in FIG. 3, which is a correlation of carbon steel (AISI C-1010) corrosion rate at 130.degree. F. and 15 feet per second fluid velocity, versus diethylthiourea concentration in parts per million. As illustrated in FIG. 3, 10-0-0-18, having a urea/sulfuric acid molar ratio of 0.633, corroded carbon steel at a rate in excess of 8000 miles per year. However, the corrosivity of that formulation decreased dramatically as diethylthiourea (DETU) concentration was increased, and was reduced to a level of approximately 18 mils per year at a DETU concentration of 400 ppm. The corrosivity of 10-0-0-18 would be even lower at higher DETU concentrations.
Although the 28-0-0-9 composition had a lower corrosivity in the absence of DETU than did 10-0-0-18, its corrosion reate of 2600 mils per year was still excessive. Nevertheless, the corrosivity of 28-0-0-9 (urea/sulfuric acid of 3.57) was reduced to approximately 450 mils per year at 400 ppm DETU. The corrosivity of this formulation could be reduced to acceptable levels (at the defined temperature and flow rate) with higher DETU concentrations.
In contrast to 10-0-0-18 and 28-0-0-9, the corrosivity of the 17-0-0-17 composition (urea/sulfuric acid molar ratio of 1.143), was only 31 mils per year at 130.degree. F. and 15 feet per second fluid velocity in the absence of diethylthiourea. This corrosion rate is acceptable in many situations involving temporary or intermittent exposure. Furthermore, the corrosivity of 17-0-0-17 was reduced only nominally to a level of approximately 28 mils per year at a diethylthiourea concentration of 400 ppm. At that point, the corrosivity of 17-0-0-17 actually exceeded that of 10-0-0-18.
These distinctions in carbon steel corrosion rate and inhibitor response persist under all conditions of temperature and flow rate and over a wide range of dilution. The corrosivity of all formulations to carbon steel increases with temperature, flow rate and dilution with water in the presence or absence of the thiourea inhibitors. For instance, the corrosion rate for 10-0-0-18 at 80.degree. F. without dilution under static conditions in the absence of corrosion inhibitor was only 16 mils per year compared to 1483 mils per year with 40 percent dilution. Similarly, increasing fluid velocity from zero to 15 feet per second increased corrosion rate from 16 MPY to 4489 MPY in the absence of inhibitor at zero dilution. Similar relative effects were observed with 17-0-0-17 and 28-0-0-9.
In marked contrast to the dissimilar corrosivity of 17-0-0-17 and 28-0-0-9 on carbon steel, the close analogs of those compositions--18-0-0-17 and 29-0-0-9--exhibit remarkably similar corrosivity toward stainless steel at 150.degree. F. and 15 feet per second fluid velocity, as illustrated in FIG. 2. They also exhibit remarkably similar response to inhibitor (cupric ion) concentration. For instance the corrosivity of 18-0-0-17 to AISI 316 stainless steel decreases to approximately zero at 50 ppm cupric ion concentration. By comparison, the corrosivity of 17-0-0-17 to carbon steel was relatively uneffected by diethylthiourea as illustrated in FIG. 3.
Effective thiourea compounds include dialkylthioureas in which each alkyl group has from 1 to about 4 carbon atoms. Combinations of these compounds can be used. Exemplary are dimethylthiourea; dipropylthiourea; dibutylthiourea; methyl,propylthiourea; ethyl,propylthiourea; and the like. The higher cupric ion concentrations markedly reduce the inhibiting effect of butyl-substituted thioureas for reasons that are not fully understood. Thus, the methyl, ethyl and propyl substituted thioureas are presently preferred for use in compositions containing 50 ppm or more of cupric ion. Yet, cupric ion has little, if any, detrimental effect on the lower alkylthioureas.
The thiourea compounds are usually employed at concentrations of at least about 25 ppm, generally at least about 50 ppm, and up to 1000 ppm or more. Different inhibitor concentrations are required to achieve comparable corrosivity with different compositions or under different conditions of temperature, flow rate or degree of dilution. Optimum inhibitor concentration will also vary from one composition to the next due to the dramatic differences in corrosivity and inhibitor effect between different compositions as illustrated in FIG. 3.
The inhibitor concentration required to obtain a certain carbon steel corrosion rate with a given composition at a particular temperature and flow velocity can be readily determined by the test described above for determining optimum cupric ion concentration. Standard samples of the carbon steel can be exposed to different samples of the given composition containing various inhibitor concentrations at the temperature and flow velocity involved. The results of this test will provide a correlation similar to that illustrated in FIG. 3.
Stainless steels are usually classified in three different categories--austentic, ferritic, and martensitic steels--which have in common the fact that they contain significant amounts of chromium and resist corrosion and oxidation to a greater extent than do ordinary carbon steels and most alloy steels. Austenitic stainless steels are the most common alloys of this group and are characterized, in part, by minimum chromium contents of about 16 percent and minimum nickel contents of about 7 percent. AISI types 302, 303, 304, and 316 are several of the more extensively used austenitic stainless steels.
Ferritic stainless steels are generally characterized, in part, by the fact that they contain chromium only (in addition to the other components of carbon steel) or only very minor amounts of other alloying elements. Martensitic stainless steels are also characterized by the fact that they contain only chromium as the primary alloying element and minor, if any amounts, of other alloys, and by their characteristic of being hardenable by heat treatment.
Carbon steels, as that term is used herein, include alloys of iron and 0.02 to about 1.5 weight percent carbon, and less than 4, usually less than 2 weight percent of alloying elements such as cobalt, nickel, molybdenum, boron, manganese, copper, tungsten, cobalt, silicon and the rare earth elements. Carbon steels are conventionally produced from pig iron by oxidizing out the excess carbon and other impurities such as phosphorous, sulfur, and silicon with an oxygen-rich gas and iron ore in any one of several processes such as the open hearth, Bessemer, basic oxygen, or electric furnace processes.
The urea-sulfuric acid compositions of this invention can be produced by the reaction of urea and sulfuric acid and, optionally water, by either batch or continuous processes. The more concentrated solutions, i.e., those containing less than 25 weight percent, preferably less than 15 weight percent water are particularly preferred, and these are preferably produced by the reaction of solid urea and concentrated sulfuric acid by the methods described in my copending application Ser. No. 318,629 filed Nov. 5, 1981, which is incorporated herein by reference.
Those processes can be used to consistently produce urea-sulfuric acid reaction products of predetermined composition and crystallization temperature essentially or completely free of decomposition products such as sulfamic and/or ammonium sulfamate. Generally, the reaction products can be produced by separately and simultantously feeding urea, sulfuric acid and, optionally, water, as required, into a reacting liquid phase contained in a reaction zone in proportions corresponding to the relative proportion of each respective component in a predetermined product composition within the ranges discussed above. The urea and sulfuric acid react within the reaction zone under controlled conditions in which reaction temperature is always maintained at a point below about 176.degree. F. and below the incipient decomposition temperature of the pre-determined product.
Even minor decomposition of the reactants and/or product during manufacture or otherwise results in the formation of known toxic materials including ammonium sulfamate and/or sulfamic acid. 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 facilities.
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 ______________________________________
TABLE 1 ______________________________________ CORROSION RATE, MPY 29-0-0-9 10-0-0-19 Carbon Stainless Steel Steel Concentration 130.degree. F., 170.degree. F., INHIBITOR ppm static 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-Dibutylthiourea 10,000 4 610 Diethylthiourea 10,000 31 709 Potassium Chlorate 10,000 1,200 950 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 1,465 898 Ammonium Phosphate (10-34-0) 10,000 231 NA ______________________________________
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