JP2006110526A - Sterilized aqueous urea solution and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aqueous urea solution capable of suppressing propagation and decomposition of microorganisms such as mold and fungi during storage and usage fitted as an aqueous urea solution used for SCR denitration for an on-vehicle or installation type apparatus and preventing generation of a harmful ammonia gas and odor, and its production method. <P>SOLUTION: In the production method for the sterilized aqueous urea solution, after carbon dioxide is dissolved in the aqueous urea solution having a urea concentration of 30-50 mass% to adjust a pH value to 7.5 or lower, hydrogen peroxide is added to the aqueous solution. In the obtained aqueous urea solution, pH is maintained to 8.0 or lower under airtight storage at 40°C or lower for one year. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a sterilized aqueous urea solution. More specifically, the present invention, for example, suppresses the growth and alteration of microorganisms such as mold and bacteria during storage and use of an aqueous urea solution used for selective catalytic denitration (SCR) using urea as a reducing agent. The present invention relates to a technology for preventing generation of harmful ammonia and odor.

  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). FIG. 1 is an example showing such a state of pH change, and the pH of the urea aqueous solution changes with time and reaches pH 9 or more.

  This reaction is the same as the natural ureolytic action of enzymes such as urokinase. As a result, in actual use, the decomposition of urea by urokinase and the like mixed in urea water from the environment is strongly related to the increase in pH. ing.

  When the urea water for SCR is produced industrially and transported, stored, or used in an on-vehicle state, it is inconvenient from the viewpoint of corrosion and odor to generate ammonia from the urea aqueous solution.

  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, for stationary devices, matters that have not been a major problem must be resolved in the in-vehicle environment, including the severe temperature conditions for urea aqueous solutions and contamination from the environment. It has emerged as an issue.

  Against this background, no prior art has been proposed for various problems such as chemical stability and sterility 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, it does not impair the characteristics as a reducing agent for SCR denitration, and further within the practical restrictions of mounting on diesel vehicles. There is a problem of preventing generation of excess ammonia from an aqueous urea solution caused by contamination with an enzyme such as urokinase or a microorganism having a urea decomposing action. It is also necessary to prevent various foreign substances from being generated in the aqueous urea solution by mixing and growing various mold spores in the aqueous urea solution from the external environment. In order to inactivate enzymes such as urokinase and microorganisms having ureolytic activity, commonly used chlorine-based disinfectants and disinfectants are effective. However, in SCR denitration, from the influence on denitration catalyst Components such as chlorine cannot be used. Also, sterilization by heat treatment cannot be applied to the reducing agent for SCR denitration. Furthermore, under a general environment, urokinase and the like easily enter urea and an aqueous urea solution to cause contamination. In particular, urea (granular), which is easily available, is handled in bulk in an open system for fertilizers, and is very likely to be mixed with other fertilizer components such as phosphoric acid and potassium. Urea (granular) is in a favorable nutritional condition for the growth of microorganisms, and ammonia generated by these already adheres in the state of urea raw material.

  Accordingly, an object of the present invention is to provide a method for producing a stabilized and sterilized urea aqueous solution. The present invention further suppresses the growth and alteration of microorganisms such as mold and bacteria during storage and use, which are suitable as an aqueous urea solution used for in-vehicle SCR denitration, and prevents generation of harmful ammonia gas and odor. It is an object of the present invention to provide an aqueous urea solution and a method for producing the same.

  In the present invention for solving the above-mentioned problems, carbon dioxide is dissolved in a urea aqueous solution having a urea concentration of 30 to 50% by mass to adjust the pH value to 7.5 or less, and then hydrogen peroxide is added to the aqueous solution. This is a method for producing a sterilized aqueous urea solution.

  The present invention that solves the above-mentioned problems also provides a urea aqueous solution in which urea is dissolved using water in which carbon dioxide is previously dissolved, the urea concentration is 30 to 50% by mass, and the pH value is adjusted to 7.5 or less. On the other hand, it is a method for producing a sterilized aqueous urea solution characterized by adding hydrogen peroxide.

  The present invention for solving the above-mentioned problems is a sterilized urea aqueous solution characterized by being produced by the production method described above.

  The present invention also shows the urea aqueous solution that is used for selective catalytic denitration.

  The present invention further shows the urea aqueous solution characterized in that the pH is maintained at 8.0 or lower under sealed storage for 1 year at 40 ° C. or lower.

  Since the urea aqueous solution produced by the production method of the present invention is stably maintained for a long time in a pH range where hydrogen peroxide effectively acts, unlike the urea aqueous solution produced by the conventional method, urokinase is stored during storage. Since the generation of ammonia and the accompanying increase in pH due to such actions are avoided, the generation of odor is suppressed. Specifically, it can be suppressed to a pH of 7.5 or less immediately after production and a pH of 8.0 or less after long-term storage for one year. As a result, corrosion of metals such as aluminum due to high pH can also be prevented. This is advantageous in handling the urea aqueous solution in the in-vehicle SCR for a diesel vehicle having a particularly severe temperature environment, and does not cause corrosion of tanks and pipes, so that stable operation can be performed for a long period of time. Also, microorganisms typified by black mold and the like grow and solid foreign matter does not increase, and the insoluble component standard value (<5 mg / kg) of the DIN standard is not exceeded, and urea water in the denitration apparatus The filter is not clogged, and the maintenance is good.

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

  Due to the characteristics required of an aqueous urea solution as a reducing agent for SCR denitration, it is necessary to select a method for sterilizing urokinase, mold spores, and the like by using as a bactericide a component that does not leave decomposition residues. Table 1 shows the characteristics of DIN 70 070 standard urea water (32.5%).

  The cationic component is limited to 0.2 to less than 0.5 mg / kg, and the foreign matter (insoluble component) is limited to less than 5 mg / kg. Therefore, those containing chlorinated compounds or sodium salts as disinfectants cannot be used because chlorine will eventually remain in the urea aqueous solution. In fact, oxygen-based substances such as hydrogen peroxide and ozone Limited to bactericides. Here, ozone has a strong sterilizing power, but is difficult to adopt because it requires an ozone gas generator and a separate dissolving device in an aqueous urea solution. Therefore, hydrogen peroxide (aqueous solution) is considered to be the most preferred disinfectant for the convenience of use.

  However, in order to use hydrogen peroxide for sterilization of urea aqueous solution, it is necessary to satisfy the chemical conditions under which hydrogen peroxide works effectively. Since hydrogen peroxide decomposes immediately (ineffective decomposition) in a high pH state where free ammonia exists, it is difficult to obtain a sustained bactericidal effect.

  For this reason, as a result of intensive studies, the present inventors have used the principle of chemical equilibrium of urea hydrolysis to keep the reaction equilibrium of the above chemical formula (2) on the left side and prevent carbon dioxide gas, That is, the method of injecting carbon dioxide into the solution is used in combination to adjust the chemical environment in which the action of the hydrogen peroxide solution near the neutral pH is continued, and the aqueous urea solution is sterilized.

  That is, in the method for producing an aqueous urea solution according to the present invention, carbon dioxide is dissolved in an aqueous urea solution having a urea concentration of 30 to 50% by mass, and the pH value is adjusted to 7.5 or less. This is characterized by adding hydrogen peroxide, which suppresses the decomposition of urea due to the action of urokinase or the like during storage and use of an aqueous urea solution. Also, this method is an effective sterilization method because mold spores, microorganisms and the like contained in urea water are also killed.

  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.

  The reason why the pH of the aqueous urea solution is adjusted to 7.5 or less by dissolving carbon dioxide at the time of production is to continue the sterilization action by hydrogen peroxide as described above. This is also desirable from the viewpoint of suppressing the above problem and suppressing metal corrosion in storage containers, pipes and the like. 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. Accordingly, it is more desirable to use water having an electric conductivity of about 2 to 8 μS / cm by reverse osmosis membrane treatment or the like.

  Further, the urea aqueous solution according to the present invention is manufactured by dissolving urea using water in which carbon dioxide is dissolved in advance, instead of the method of dissolving carbon dioxide in the urea aqueous solution as described above. You can also. That is, the second production method according to the present invention uses urea in which carbon dioxide is dissolved in advance to dissolve urea, the urea concentration is 30 to 50% by mass, and the pH value is adjusted to 7.5 or less. Hydrogen peroxide is added to the aqueous solution. 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.

  Next, the method for adding hydrogen peroxide to the urea aqueous solution added with carbon dioxide and having a pH of 7.5 or less is not particularly limited, but an appropriate concentration, for example, It is desirable to add 30% hydrogen peroxide to the urea aqueous solution so that the concentration is about 50 to 5000 mg / kg, more preferably 500 to 2000 mg / kg in terms of 100% concentration hydrogen peroxide. The added hydrogen peroxide is decomposed over time, but by adding the above-mentioned predetermined amount during production, sterilization stabilized for a long period of time can be maintained.

  The aqueous urea solution according to the present invention thus produced is stabilized over a long period of time and is maintained in a pH range where hydrogen peroxide water effectively acts. The associated increase in pH is avoided. Therefore, the sterilized state can be maintained for a long time. Specifically, for example, the pH is maintained at 8.0 or lower for at least one year under storage conditions of 40 ° C. or lower. Become.

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

Example 1
Urea for fertilizer was dissolved in reverse osmosis treated water to produce a 40 mass% urea aqueous solution, then carbon dioxide gas was blown into this, and the result of adding hydrogen peroxide after saturation with carbon dioxide gas was shown together with a comparative example. It is shown in 2. Carbon dioxide was blown at a rate of 100 ml / min into 20 liters of an aqueous urea solution and treated for about 25 minutes until the pH became substantially constant. The pH at this stage was 6.49 as shown in Table 2. Further, 67 g of 30% high-purity hydrogen peroxide solution (equivalent to 1000 mg / kg in terms of 100% hydrogen peroxide) was added to the solution into which carbon dioxide was blown. At this stage, neither pH nor appearance change was observed (Sample 4). Furthermore, after storing at room temperature (10 ° C. ± 20 ° C.) for 12 months, the pH becomes 7.64 (Sample 5), and is maintained at a low value compared to pH 9.45 of the comparative example after storage for 12 hours. It had been. Odor (sensory test) and ammonia concentration in the upper space of the container (measurement of gas detector tube) also confirmed the change in pH.

  The same test was performed on a sample group of urea aqueous solution prepared in the same manner as described above except that the concentration was 32.5% by mass, but almost the same result as that of the 40% by mass urea aqueous solution was obtained. .

Example 2
As a result of experimenting by incorporating mold in the environment into urea aqueous solution, the reproducibility was poor and it was difficult to compare the effect of the hydrogen peroxide solution of the present invention numerically. A comparative experiment was performed by adding to an aqueous solution. The obtained results are shown in Table 3 together with comparative examples. In the case of the urea aqueous solution to which black mold fungus 100 mg / kg was added without adding hydrogen peroxide (samples 1 and 2), turbidity and precipitation were observed after 12 months, and about 2000 mg / kg insoluble matter was contained. This could not be completely removed by subsequent filtration. On the other hand, in sample 3 (black mold 100 mg / kg) and hydrogen peroxide added sample 4 (black mold 1000 mg / kg), there was no growth of mold in 12 months, and the insoluble matter after filtration was 3 It was -4 mg / kg and was within the range of the DIN standard.

  The same test was performed on a sample group of urea aqueous solution prepared in the same manner as described above except that the concentration was 32.5% by mass, but almost the same result as that of the 40% by mass urea aqueous solution was obtained. .

  According to the present invention, a stabilized and sterilized urea aqueous solution can be easily and inexpensively manufactured by dissolving carbon dioxide and further adding hydrogen peroxide. This makes it possible to provide useful products for SCR denitration for in-vehicle use of diesel vehicles and stationary devices such as diesel generators.

It is a graph which shows an example of pH change by the heat processing (2 hours) after urea melt | dissolution of urea aqueous solution.

Claims (5)

  Sterilized urea characterized by adding carbon dioxide to a urea aqueous solution having a urea concentration of 30 to 50% by mass to adjust the pH value to 7.5 or less and then adding hydrogen peroxide to the aqueous solution. A method for producing an aqueous solution.   Hydrogen peroxide is added to an aqueous urea solution in which urea is dissolved using water in which carbon dioxide is previously dissolved, the urea concentration is 30 to 50% by mass, and the pH value is adjusted to 7.5 or less. A method for producing a sterilized aqueous urea solution.   A sterilized urea aqueous solution produced by the production method according to claim 1 or 2.   The aqueous urea solution according to claim 3, which is used for selective catalytic denitration.   The aqueous urea solution according to claim 3 or 4, wherein the pH is maintained at 8.0 or lower under sealed storage for 1 year at 40 ° C or lower.

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