JP2006111602A - Stabilized aqueous solution of urea and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stabilized aqueous solution of urea for an SCR (selective catalytic reaction) denitrification apparatus for automobile or stationary use and free from the problems of the generation of ammonia gas and ammonia odor and metal corrosion of piping, etc., during transportation or storage. <P>SOLUTION: A stabilized aqueous solution of urea is produced by dissolving carbon dioxide in an aqueous solution of urea having a urea concentration of 30-50 mass% to lower the pH to ≤7.5. The aqueous solution of urea produced by the method has stabilized properties to keep the pH to ≤7.5 after the storage at 60°C for 2 hr and to ≤7.5 after the storage at ≤40°C for 30 days. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a stabilized urea aqueous solution and a method for producing the same. More specifically, the present invention, for example, suppresses the generation of ammonia during storage and use of an aqueous urea solution used for selective catalytic denitration (SCR) using urea as a reducing agent, thereby increasing the pH. The present invention relates to a technique for suppressing and preventing ammonia odor and at the same time preventing metal corrosion in a urea water tank, piping, an injection device, and the like.

  As a method for detoxifying and removing nitrogen oxides (NOx) contained in exhaust gas of diesel engines, SCR denitration using ammonia as a reducing agent has been put into practical use as shown in the following formula (1).

  In flue gas denitration of small diesel generators and diesel vehicles, as shown in the following formula, ammonia is mainly generated by spraying urea water into the exhaust pipe and generating it by thermal decomposition and hydrolysis. (For example, Patent Document 1, Patent Document 2, etc.).

  As for the urea aqueous solution used for such SCR denitration, in order to avoid the influence of various cation components as a catalyst poison for the SCR catalyst, standards regarding purity have been studied, and urea aqueous solution for in-vehicle SCR of diesel vehicles (32. 5 mass%), DIN 70 070 exists as a standard.

  The purity of the aqueous urea solution depends on the purity of the raw material urea and the raw material water, and it is technically easy to produce high-purity urea water if the cost is not taken into consideration.

  On the other hand, as a chemical characteristic of urea water, hydrolysis of urea gradually proceeds even at room temperature in a solution state, and the pH gradually increases according to the chemical formula (2). SCR denitration is based on the principle that the urea aqueous solution sprayed on the exhaust pipe of the engine is decomposed according to the chemical formula (2) to generate ammonia as a reducing agent as described above, but before being sprayed on the exhaust pipe, Decomposing is not preferable for the following reason.

  That is, it is inconvenient from the viewpoint of alkali corrosion, odor, etc. of the metal material to produce ammonia from the urea aqueous solution when the urea water for SCR is industrially manufactured, transported, stored, or used in a vehicle. .

  So far, SCR denitration is a technology that has been mainly used for flue gas denitration of stationary devices such as diesel generators, and on-vehicle SCR for diesel vehicles has been recently studied in accordance with the trend of environmental regulations. For this reason, matters that have not been a major problem for stationary devices have emerged as issues that must be solved in the in-vehicle environment, including the problem of severe temperature conditions for urea aqueous solutions. It is a thing.

  Against this background, no prior art has been proposed for problems such as the chemical stability of aqueous urea solutions.

Japanese Patent Laid-Open No. 53-112273 JP-A-63-190623

  As described above, in applying SCR denitration to exhaust gas denitration of diesel vehicles, urea aqueous solutions defined in the DIN standards as shown in Table 1 are particularly selected as long as the characteristics as a reducing agent for SCR denitration are not impaired. As long as the characteristic value can be maintained, it is necessary to chemically stabilize the urea aqueous solution as described above. Furthermore, in severe conditions such as in-vehicle temperature environment for diesel vehicles, excessive ammonia is generated from the aqueous urea solution to increase pH and prevent odor from being generated, It is necessary to establish a technique for preventing metal corrosion in a urea water tank, piping, and an injection device.

  Accordingly, an object of the present invention is to provide a stabilized urea aqueous solution and a method for producing the same. Further, the present invention is suitable as an aqueous urea solution used for SCR denitration for in-vehicle use or for stationary devices, and is suitable for generation of excessive ammonia gas and generation of odor due to this, during transport and storage, and for piping, etc. It is an object of the present invention to provide a stabilized urea aqueous solution capable of preventing the problem of metal corrosion and a method for producing the same.

  In solving the above-mentioned problems, for example, it is necessary to stabilize the aqueous urea solution under practical restrictions as a reducing agent for SCR denitration, so the method of adding various chemicals is adopted. Can not do it. For example, the method of adding an organic acid to an aqueous urea solution is effective in preventing ammonia generation and controlling the pH, but these acids are oxidized and discharged as aldehyde etc. from the exhaust pipe during SCR denitration. Therefore, it is not preferable and cannot be adopted. A method of removing generated ammonia by blowing nitrogen gas or the like is also effective, but ammonia is generated again from urea due to chemical equilibrium, which is not a fundamental solution.

  From such a viewpoint, the present inventors, as a result of diligent investigation, use the principle of chemical equilibrium of urea hydrolysis, keep the reaction equilibrium of the above chemical formula (2) on the left side, and as a means not to proceed to the right side, By adopting a method of blowing carbon dioxide into a solution, it was found that an aqueous urea solution is very stabilized, and the present invention has been achieved.

  That is, the present invention that solves the above problems is stabilized by dissolving carbon dioxide in a urea aqueous solution having a urea concentration of 30 to 50% by mass to lower the pH value to 7.5 or less. It is a manufacturing method of urea aqueous solution.

  The present invention that solves the above-mentioned problems is also that water is previously saturated with carbon dioxide, urea is dissolved, the urea concentration is 30 to 50% by mass, and the pH value is adjusted to 7.5 or less. This is a method for producing a stabilized aqueous urea solution.

  The method for producing an aqueous urea solution according to the present invention preferably further includes a step of filtering out a carbonate precipitate formed in the aqueous urea solution by carbon dioxide saturation.

  The present invention for solving the above-mentioned problems is further a stabilized urea aqueous solution produced by the above production method.

  The aqueous urea solution according to the present invention is suitably used for selective catalytic denitration.

  The present invention further shows an aqueous urea solution having a Ca concentration of 0.5 mg / kg or less and an insoluble component of 5 mg / kg or less.

  The present invention also shows an aqueous urea solution characterized in that the pH is maintained at 7.5 or lower under storage conditions at 60 ° C. for 2 hours.

  The present invention also shows an aqueous urea solution characterized in that the pH is maintained at 7.5 or lower under storage conditions for 30 days at 40 ° C. or lower.

  The urea aqueous solution produced by the production method of the present invention is different from the conventional urea aqueous solution, during the long-term storage for several months at a temperature of 30 ° C. or lower, and even under a temperature rising condition of about 40 to 60 ° C. Since the generation is suppressed, there is little increase in pH and the odor of ammonia can be suppressed. Since ammonia is highly toxic to the human body, the allowable concentration in the working environment is limited to 25 mg / kg (ACGIH), but the urea aqueous solution according to the present invention suppresses the generation of ammonia as described above. It is effective in maintaining the environment below the regulation value.

  In general, light metals such as aluminum, particularly aluminum, are extremely corroded at a pH of 9.5 or more, and copper metal is also corroded greatly, which is a problem when used in a contact portion with a conventional aqueous urea solution. . The urea aqueous solution according to the present invention has a pH of 7.5 or less, and practically alkaline corrosion of the metal material is greatly suppressed. This effect is particularly advantageous in handling the SCR urea aqueous solution for diesel automobiles, which has a severe temperature environment, and is unlikely to cause corrosion of tanks, pipes, and injection devices. There is an advantage that it can be used.

  In summary, the urea aqueous solution according to the present invention provides an effect that enables stable operation of the SCR device over a long period of time, for example.

  Hereinafter, the present invention will be described more specifically based on embodiments.

  The urea aqueous solution production method according to the present invention is characterized in that the pH value is lowered to 7.5 or less by dissolving carbon dioxide in a urea aqueous solution having a urea concentration of 30 to 50 mass%.

  In the method for producing an aqueous urea solution according to the present invention, the urea concentration is set to 30 to 50% by mass. By using such a high concentration, for example, when used for SCR denitration, water at the time of vaporization decomposition is used. This is to reduce the latent heat as much as possible and to improve the ammonia conversion rate. Note that when the concentration is extremely high, urea hydrolysis does not proceed sufficiently and the ammonia conversion rate may decrease, so it is desirable that the urea concentration be in the above range.

  Moreover, the reason why the pH of the urea aqueous solution is adjusted to 7.5 or less by dissolving carbon dioxide at the time of production is to suppress the problem of odor due to free ammonium and the like, and to suppress metal corrosion in storage containers, piping, etc. Is desired. More preferably, the pH is desirably 5.5 to 7.5.

  The method for dissolving carbon dioxide in the urea aqueous solution may be a method in which carbon dioxide gas is blown into the urea aqueous solution or a method in which dry ice is added to the urea aqueous solution. A method of blowing gas, for example, a method of blowing until carbon dioxide is saturated and dissolved in an amount of about 50 to 200 ml / min for 20 liters of solution is desirable.

  In the present invention, as the urea raw material, it is desirable to use various urea raw materials for fertilizer use from an industrial viewpoint.

  As water, industrial water or tap water generally having an electric conductivity of 50 to 100 μS / cm can be used as it is. In this case, in the obtained urea aqueous solution, Ca, Fe, Na And ionic impurities such as K are present to some extent, for example, about 1 to 10 mg / kg. Therefore, it is desirable to clean the water in advance when a filtration process step as will be described later is not provided. For example, water having an electrical conductivity of about 2 to 8 μS / cm by reverse osmosis membrane treatment or the like is used. It is more desirable.

  Further, the urea aqueous solution according to the present invention is produced by dissolving urea using water saturated with carbon dioxide in advance, instead of the method of dissolving carbon dioxide in the urea aqueous solution as described above. You can also. That is, in the second production method according to the present invention, urea is dissolved using water saturated with carbon dioxide in advance, and the urea concentration is adjusted to 30 to 50% by mass and the pH value is adjusted to 7.5 or less. It is characterized by this.

  Also in the second production method, the carbon dioxide addition method, the urea raw material to be used, and water are the same as described above.

  Furthermore, in the production method of the present invention, carbon dioxide is dissolved in the urea aqueous solution. When the ionic impurity, typically Ca, is contained in the urea aqueous solution, the addition of the carbon dioxide is performed. Depending on the case, ionic impurities may be precipitated as carbonates. Therefore, in the production method of the present invention, the carbonate precipitate formed in the urea aqueous solution after the treatment as described above is applied to an appropriate filter having a large filtration area and less likely to be clogged, such as a polyester wind cartridge filter ( It is desirable to add a step of filtering with a long-fiber wound type cartridge filter such as Toyo Filter Paper Co., Ltd. As a result, the concentration of ionic impurities contained in the urea aqueous solution can be reduced and insoluble matters can be removed.

  The urea aqueous solution according to the present invention thus produced is stabilized over a long period of time. Specifically, for example, under storage conditions of 40 ° C. or lower, the pH is 7. It will be maintained below 5. In addition, the temperature is stabilized even under a higher temperature condition. Specifically, for example, the pH is maintained at 7.5 or lower for at least 2 hours under storage conditions at 60 ° C. It will be a thing.

  In addition, since the aqueous urea solution according to the present invention dissolves carbon dioxide in the solution as described above, ionic impurities such as Ca derived from the raw material contained in the aqueous urea solution are precipitated as carbonates. By adding a step of filtering this out, the Ca concentration of 0.5 mg / kg or less and the insoluble component of 5 mg / kg or less can satisfy the DIN standard as described above. .

  Hereinafter, the present invention will be described more specifically based on examples.

Example 1
Industrial urea was dissolved in reverse osmosis membrane-treated water having an electric conductivity of 5 μS / cm to prepare urea aqueous solutions of 32.5% by mass and 40.0% by mass. Each sample is divided into two parts, one is blown with carbon dioxide gas at 100 ml / min for 30 liters for 20 liters to saturate the carbon dioxide gas, and then stored at room temperature (20 ° C ± 10 ° C) for 30 days did. On the other hand, the measured values of the respective pH values stored in the same manner as untreated are shown in Table 2.

  As shown in Table 2, the untreated sample had a pH of 9 or more, whereas the urea aqueous solution saturated with carbon dioxide had a pH of 6.21 and 6.49, and the pH did not increase. The urea aqueous solution saturated with carbon dioxide did not feel the odor of ammonia. The specific gravity of the urea aqueous solution is proportional to the concentration, and is 1.08 g / ml at a concentration of 32.5% by mass and 1.10 g / ml at a concentration of 40.0% by mass. It was confirmed that there was no change in specific gravity after standing for 30 days, and there was no change in the quality of the urea aqueous solution.

Example 2
Industrial ureas (A) and (B) having different production locations were dissolved in reverse osmosis membrane treated water having an electric conductivity of 5 μS / cm to prepare two types of 32.5 mass% urea aqueous solutions. Each sample was divided into two parts, one of which was blown with carbon dioxide gas at 100 ml / min for 60 minutes for 20 liters to saturate the carbon dioxide gas and then at room temperature (20 ° C. ± 10 ° C.) for 12 hours. After storage, it was stored in an environment of 60 ° C. for 2 hours. The measured values of each pH are shown in Table 3.

  As shown in Table 3, the untreated material has a pH of 9 or more, and the pH is further increased by the treatment at 60 ° C. On the other hand, the pH of the urea aqueous solution saturated with carbon dioxide is 7.15 and 6.58 after 60 ° C. treatment, and the increase in pH is suppressed, and the urea aqueous solution saturated with carbon dioxide feels the odor of ammonia. There wasn't.

Example 3
Nitrogen gas and carbon dioxide gas were blown at a rate of 100 ml / min into 20 liters of reverse osmosis membrane treated water having an electric conductivity of 5 μS / cm, and the blowing was stopped 40 minutes after the pH became almost constant. Thus, nitrogen gas saturated water (N2) and carbon dioxide saturated water (two types having the same contents of X2 and Y2) were prepared. In addition, reverse osmosis treated water (blank) without blowing gas was also prepared for comparison. The respective pH values are shown in FIG. The pH of the reverse osmosis treated water into which carbon dioxide has been blown is 4.49. The results of measuring 32.5% by weight of urea in these waters and measuring the pH value immediately after that are shown in FIG. 1, but the blank and N2 have high pH values of 9.82 and 9.81. Indicates. On the other hand, X2 and Y2 saturated with carbon dioxide have a low pH of 5.52 and 5.35 even after dissolution of urea. The above four urea aqueous solutions were further treated for 2 hours in an environment at 80 ° C. During this time, the container containing the urea aqueous solution was in a state in which the stopper was lightly placed, and when gas or the like was generated, the container was volatilized. The results of measuring the pH value after the treatment at 80 ° C. are also shown in FIG. 1. The pH after the treatment at 80 ° C. is 9 or more, although the blank and N2 slightly decrease due to the volatilization of ammonia. On the other hand, X2 and Y2 dissolved in water saturated with carbon dioxide increased in pH to 7.84 and 7.51, and almost no odor of ammonia was felt.

Example 4
Using a general-purpose urea raw material with a high Ca content as an impurity, a urea aqueous solution meeting the DIN standard (DIN 70 070) was produced. The implementation progress is shown in Table 4. The 40 mass% urea aqueous solution (sample 1) immediately after urea dissolution contains 24.1 mg / kg of Ca. On the other hand, for the sample 2 saturated with carbon dioxide in the same manner as in Example 1, and further for the sample 3 filtered with a polyester wind cartridge filter having a nominal filtration performance of 3 μm, the Ca concentration, pH value and insoluble matter amount Was measured respectively. The results are shown in Table 4. The decrease in pH value is the same effect as in Examples 1 to 3, but in Sample 2, Ca is precipitated as insoluble calcium carbonate, and in Sample 3 after filtration, the Ca concentration is further decreased. . Since most of the insoluble matter contained in the urea aqueous solution is calcium carbonate, the insoluble matter is reduced to 0.1 mg / kg (5.0 mg / kg in the DIN standard) by filtration.

  According to the present invention, a stabilized urea aqueous solution can be easily and inexpensively manufactured by dissolving carbon dioxide. As a result, it is possible to provide useful products as SCR denitration for stationary vehicles such as on-board diesel vehicles and diesel generators, or cosmetic raw materials.

It is a graph which shows the pH value measured in the Example which concerns on this invention immediately after urea melt | dissolution by various gas saturated water, and after 2 hours processing at 80 degreeC.

Claims (8)

  A method for producing a stabilized urea aqueous solution, wherein carbon dioxide is dissolved in a urea aqueous solution having a urea concentration of 30 to 50% by mass to lower the pH value to 7.5 or less.   Production of a stabilized aqueous urea solution characterized in that urea is dissolved using water saturated with carbon dioxide in advance, the urea concentration is adjusted to 30 to 50% by mass, and the pH value is adjusted to 7.5 or less. Method.   The production method according to claim 1, further comprising a step of filtering out a carbonate precipitate formed in the urea aqueous solution by saturation of carbon dioxide. Method.   A stabilized urea aqueous solution produced by the production method according to claim 1.   The aqueous urea solution according to claim 4, which is used for selective catalytic denitration.   The urea aqueous solution according to claim 4 or 5, wherein the Ca concentration is 0.5 mg / kg or less and the insoluble component is 5 mg / kg or less.   The aqueous urea solution according to any one of claims 4 to 6, wherein the pH is maintained at 7.5 or lower under storage conditions at 60 ° C for 2 hours.   The aqueous urea solution according to any one of claims 4 to 6, wherein the pH is maintained at 7.5 or lower under storage conditions at 40 ° C or lower for 30 days.

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