PREPARATION METHOD FOR REMOVING TRIURET CAUSING TURBIDITY IN UREA WATER

Abstract

The present disclosure relates to a preparation method for removing triuret causing turbidity in a urea solution and to a urea solution prepared using the method, and more particularly, to a preparation method, in which urea is introduced to ultrapure water at 15 C. to 24 C. in a magnetic tank with a size of 1 m.sup.3 to 20 m.sup.3, and the mixture is stirred by using a magnetic mixer and filtered through a hollow-fiber type ultrafiltration membrane (having a pore size in a range of 0.01 m to 0.3 m) such that a 32.5% automotive urea solution has a turbidity of 0.02 NTU to 0.2 NTU, and the triuret causing turbidity in a urea solution and reducing conversion efficiency to NH.sub.3 in selective catalytic reduction (SCR) is removed. To achieve the purpose of the present disclosure, there is provided a method of preparing a urea solution by dissolving urea in ultrapure water, wherein the 32.5% automotive urea solution is provided by a preparation method including the steps of: a step of preparing ultrapure water at 15 C. to 24 C.; a step of moving the ultrapure water into a magnetic mixer tank; a step of introducing urea to the ultrapure water contained in the magnetic mixer tank; a step of operating a magnetic mixer to induce vortex effect for mixing and stirring the ultrapure water with urea to prepare a urea solution; and a step of filtering by passing the urea solution including triuret through a hollow-fiber type ultrafiltration filter (U/F) to filtrate 20 L of triuret and impurities per 1,000 L of the urea solution, while maintaining a low temperature of the urea solution resulting from the stirring (S5).

Claims

1-3. (canceled)

4. A preparation method of a urea solution having a turbidity of 0.02 NTU or greater and 0.2 NTU or less and a pH in a range of 9 to 11 by more than 5 tons per hour by dissolving urea in ultrapure water, the preparation method being characterized by removing triuret that causes turbidity in the urea solution and reducing conversion efficiency to NH.sub.3 in SCR, and the preparation method comprising: a step of preparing ultrapure water at 15 C. to 24 C. (S1); a step of moving the ultrapure water into a magnetic mixer tank having a size of 1 m.sup.3 to 20 m.sup.3 (S2); a step of introducing urea to the ultrapure water contained in the magnetic mixer tank (S3); a step of operating a magnetic mixer to induce vortex effect for mixing and stirring the ultrapure water with the urea to prepare a urea solution having a temperature reduced to 2 C. or lower and 20 C. or higher due to an endothermic reaction caused by the mixing (S4); and a step of filtering the urea solution comprising triuret by passing the urea solution through a hollow-fiber type ultrafiltration filter (U/F) while maintaining a low temperature of the urea solution reduced by the stirring (S5).

5. The preparation method of claim 4, further comprising: a step of washing the hollow-fiber type ultrafiltration filter either in a forward direction or a reverse direction with hot water when a filtration capacity per hour is less than a given capacity (S6) in the step of filtering the urea solution comprising triuret by passing the urea solution through a hollow-fiber type ultrafiltration filter (having a pore size in a range of 0.01 m to 0.3 m), while maintaining a low temperature of the urea solution reduced by the stirring (S5).

6. The preparation method of claim 5, characterized by alternately using several hollow-fiber ultrafiltration filters to change a flow from a hollow-fiber ultrafiltration filter with a decreasing filtration capacity, which is currently used, to another hollow-fiber ultrafiltration filter and then wash the hollow-fiber ultrafiltration filter with a decreasing filtration capacity with hot water to allow preparation of the urea solution not to be interrupted in the step of washing the hollow-fiber type ultrafiltration filter (having a pore size in a range of 0.01 m to 0.3 m) either in a forward direction or a reverse direction (S6).

Description

BRIEF DESCRIPTION OF DRAWINGS

[0028] FIG. 1 is a flow chart illustrating a preparation method according to an embodiment of the present disclosure;

[0029] FIG. 2 is a front view of a magnetic mixer used in an embodiment of the present disclosure; and

[0030] FIG. 3 is a perspective view of a hollow-fiber ultrafiltration filter in an embodiment of the present disclosure.

BEST MODE

[0031] Hereinafter, preferred embodiments of the present disclosure will be described with reference to the accompanying drawings. In the following, the thicknesses of lines and the sizes of components shown in the drawings may be exaggerated for clarity and convenience of explanation. In addition, the terms described herein are defined in consideration of functions of the present disclosure, which may vary depending on the intention or custom of a user or an operator. Therefore, definitions of these terms should be understood based on the contents throughout the specification.

[0032] In addition, the following embodiments are not intended to limit the scope of the present disclosure, and these are illustrative purposes only. Various embodiments may be made within the spirit of the present disclosure.

[0033] A first step (S1) is preparing ultrapure water at 15 C. to 24 C. (S1) in the case of an automotive urea solution.

[0034] Ultrapure water may be prepared by various methods, however, in a preferred embodiment of the present disclosure, ultrapure water is obtained by mixing a carbon filter filtration system, a reverse osmosis filtration system, and an ion exchange filtration system.

[0035] A second step (S2) is moving the ultrapure water into a magnetic mixer tank.

[0036] A third step (S3) is introducing urea to the ultrapure water contained in the magnetic mixer tank.

[0037] A fourth step (S4) is operating a magnetic mixer for mixing and stirring the ultrapure water with urea.

[0038] Due to an endothermic reaction of urea, a stirring temperature may be dropped to 2 C. to 20 C. depending on an amount to be stirred.

[0039] When the temperature is dropped, mixing time may be excessively long when a propeller type mixer, i.e., a mixing method in the related art, is used. In this case, productivity may be poor, and dissolution may only be possible without ice formation by incubation at a temperature at 3 C.

[0040] However, the magnetic mixer applied to the present disclosure has a strong turning force and may effectively mix liquids that have strong viscosity even at the dropped stirring temperature without the incubation.

[0041] When the magnetic mixer is applied, vortex effect was generated in the mixing and stirring of the urea solution.

[0042] Accordingly, a 31.8% to 33.2% urea solution was produced within 10 minutes due to 360 strong rotation of the urea solution in a stirrer. In a preferred embodiment of the present disclosure, the time required to reach a required concentration for complete dissolution and the corresponding temperature change are shown in Experimental Example 1 in Table 1.

TABLE-US-00001 TABLE 1 Experimental Example 1 Time (seconds) Temperature ( C.) Concentration (%) Start 18.8 Urea injection 90 11 15.8 150 9 17.9 210 6 23.5 270 3 27.7 330 2 31.5 390 2 32.5

[0043] When 743 liters (L) of ultrapure water is mixed with 358 kilograms (Kg) of urea at a water temperature of 19 C.

[0044] A fifth step (S5) is passing the urea solution (including triuret) through a hollow-fiber type ultrafiltration filter (having a pore size in a range of 0.01 m to 0.3 m) while maintaining a low temperature of the urea solution resulting from the stirring.

[0045] The urea solution prior to filtration has a turbidity in a range of 100 NTU to 300 NTU due to triuret having an enlarged grain size by association.

[0046] When the triuret not removed from the urea solution, the following problems may occur.

H.sub.6N.sub.4C.sub.3O.sub.3 (triuret)+250 C. (exhaust gas temperature)=(base) NH.sub.3+(acid) H.sub.3N.sub.3C.sub.3O.sub.3 (cyanuric acid)

[0047] A decomposition point of cyanuric acid is in a range of 320 C. to 360 C. When a temperature of exhaust gas is 250 C., cyanuric acid is not decomposed and remains as a salt. Accordingly, cyanuric acid may remain in SCR, which may cause failure of SCR.

[0048] A natural conversion of a urea solution not including triuret to NH.sub.3: 2 mol of NH.sub.3 per 1 mol of the urea solution

CO(NH.sub.2).sub.2.fwdarw.NH.sub.3+HNCO

HNCO+H.sub.2O.fwdarw.NH.sub.3+CO.sub.2

[0049] An unnatural conversion of a urea solution including triuret to NH.sub.3 results in generation of 1 mol of NH.sub.3 per 1 mol of the urea solution

[0050] When the urea solution is not completely decomposed due to inclusion of triuret, decomposition of the urea solution proceeds at downstream of a SCR catalyst, and the ammonia slip in which NH.sub.3 is discharged is likely to occur.

[0051] A hollow-fiber ultrafiltration filter (having a pore size in a range of 0.01 m to 0.3 m) may be a general hollow-fiber ultrafiltration filter (U/F). A urea molecule (having a molecular weight of 60.06 g/mol) is smaller than a biuret molecule (C.sub.2H.sub.5N.sub.3O.sub.2, having a molecular weight of 103.081 g/mol) and triuret molecule (C.sub.3H.sub.6N.sub.4O.sub.3, having a molecular weight of 146.11 g/mol). In addition, triuret is agglomerated at a low temperature in the present disclosure, and thus, triuret molecules may easily be filtered by a filter. In a preferred embodiment of the present disclosure, by using a filter having a filter size similar to that of a triuret particle, triuret is filtered, and the filtration speed is fast. In a preferred embodiment of the present disclosure, by using several cylindrical hollow-fiber ultrafiltration filters having a height of 2,275 millimeters (mm) and a radius of 216 mm, 16,000 L per hour was filtered at maximum capacity.

[0052] A high-purity urea solution that underwent such filtration process has a turbidity in a range of 0.02 NTU to 0.2 NTU.

[0053] On the other hand, impurities, aldehydes, insoluble substances, and heavy metals that entered during a mixing process are also separated and removed in the filtration process.

[0054] Accordingly, improved conversion efficiency to NH.sub.3 in SCR results in a high-purity urea solution.

[0055] A sixth step (S6) is washing the hollow-fiber type ultrafiltration filter (having a pore size in a range of 0.01 m to 0.3 m) either in a forward direction or a reverse direction with hot water when a filtration capacity per hour thereof is less than a given capacity.

[0056] As triuret is easily dissolved in hot water, triuret may be easily removed by such washing method.

[0057] Such washing method may be performed by automatic control. In an embodiment of the present disclosure, a flow is changed from a hollow-fiber ultrafiltration filter with a decreasing filtration capacity, which is currently used, to another hollow-fiber ultrafiltration filter, and the hollow-fiber ultrafiltration filter with a decreasing filtration capacity is then washed such that preparation of the urea solution is not interrupted.

[0058] A urea solution prepared as described above may be stably used without any chemical reaction due to impurities regardless of distribution in winter and summer and a temperature of a place of usage. For the above reasons, the turbidity is very low, and impurities are not visible to a naked eye. Also, conversion efficiency to ammonia gas is improved in SCR. In addition, since triuret is removed, expensive SCR systems such as a catalyst, an injector, and a urea solution filter are protected to thereby prevent replacement loss. In addition, since the automobile urea solution is not heated and is stirred at room temperature, boiler is not required to be used for heating. Therefore, energy cost loss per year may be prevented, resulting in an expected effect on greenhouse gas reduction and carbon emission.