JP2016169236A - Method for producing high-purity urea water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing high-purity urea water, in which the biuret concentration of the urea water can be reduced by using the inexpensively-produced urea and by an inexpensive method.SOLUTION: The method for producing high-purity urea water for a urea SCR system, which urea water is obtained by treating an urea water undiluted solution having 2,500 ppm or higher biuret concentration with activated carbon and has the reduced biuret concentration, comprises: a biuret adsorbing/removing step of treating the urea water undiluted solution of 100 ml with the activated carbon of 1.0-3.0 g; and a biuret desorbing/removing step of desorbing the biuret from the biuret-adsorbed activated carbon. At the moment that the biuret concentration exceeds a regulation value of 2,400 ppm when the biuret adsorbing/removing step is carried out, the biuret desorbing/removing step is carried out.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing high-purity urea water.

  As a method for reducing and purifying NOx contained in the exhaust gas of an internal combustion engine such as a diesel engine, a method has been proposed in which NOx is greatly reduced by urea water and a catalytic converter of an SCR (Selective Catalytic Reduction) method. (Patent Document 1). This urea SCR system is a system in which urea water is used as a reducing agent and this is mixed with exhaust gas immediately before entering the SCR catalytic converter. Urea changes into ammonia in the exhaust gas, and the exhaust gas in the SCR catalytic converter. Since NOx in it is combined with ammonia and decomposed into water and harmless nitrogen, it is considered a promising technology for exhaust gas cleaning.

  By the way, the piping for introducing the urea water into the SCR catalyst is required to be a thin tube because it is necessary to introduce the urea water in the form of spray. Therefore, urea water from which impurities such as guanidine and biuret have been removed has been proposed as a substance causing blockage of the introduction pipe (Patent Document 2). And here it teaches how to adsorb and separate guanidine by ion exchange resin with regard to guanidine concentration, and how to control the purification process of urea process liquid to suppress the reaction from urea to biuret with respect to biuret concentration. Has been.

JP 2004-290835 A JP 2007-14596 A

  However, it is not always easy to reduce the concentration of biuret in urea water to 2000 ppm or less, for example, by controlling the purification conditions of the urea process liquid and suppressing the production reaction from urea to biuret. The biuret concentration in urea water may exceed 3500 ppm.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing high-purity urea water that uses urea produced at low cost and can reduce the biuret concentration by an inexpensive method. It is in.

  That is, the gist of the present invention is a method for producing high-purity urea water for a urea SCR system with reduced biuret concentration by treating a urea water stock solution having a biuret concentration of 2500 ppm or more with activated carbon. It includes a biuret adsorption / removal process for treating the urea aqueous solution with 1.0 to 3.0 g of activated carbon per 100 ml and a biuret desorption / removal process for desorbing biuret from the activated carbon that has adsorbed biuret. Then, the biuret desorption removal step is performed when the biuret concentration in the urea water derived from the step exceeds a regulation value of 2400 ppm or less.

  According to the present invention, the above problems are solved.

  Hereinafter, the present invention will be described in detail.

Examples of urea used in the urea aqueous solution include various urea raw materials such as fertilizers manufactured at low cost. When urea is heated above its melting point (132.7 ° C.), biuret (urea condensate) is produced. This biuret is represented by the chemical formula NH ₂ CONHCONH ₂ and is a substance that can be produced by separating one molecule of ammonia from the condensate of two molecules of urea, and it is difficult to completely remove it from the method for producing fertilizer urea. Increased when urea is granulated. Biuret is a harmful substance to crops, and the content of biuret in urea for fertilizer is the content of “biuret nitrogen 0.02% per 1.0% of total nitrogen content” according to the official standard stipulated by the Fertilizer Control Law. Is allowed. Therefore, the upper limit of the biuret content of fertilizer urea is 2.4%.

  On the other hand, it is preferable to use water having an electric conductivity of 50 to 100 μS / cm as water used in the urea aqueous solution. In general, the electrical conductivity of industrial water or tap water falls within this range. The urea concentration is usually 30-50% by weight.

The activated carbon is not particularly limited, and is based on wood (derived from wood, sawdust, coconut shell, etc.), coal (derived from lignite, cutlet charcoal, Dee coal, coal, etc.), petroleum pitch, phenol resin Any of activated carbons of animal type (cow bone, blood-derived, etc.) can be appropriately selected and used. Examples of the activated carbon include powdered coal, crushed coal, granulated coal, honeycomb coal, and sheet coal. As the crushed coal, for example, palm crushed coal having a particle size of about 4 to 8 mesh is suitable. This crushed coal is obtained by crushing and sizing raw coal such as coconut shell charcoal and then activating it. On the other hand, as the granulated charcoal, for example, pellet charcoal having a particle size of about 4 to 6 mesh, spherical charcoal having a particle size of about 3 to 7 mm, and the like are preferable. This granulated coal is obtained by adding a binder such as bituminous material to a finely pulverized raw coal, granulating it into pellets (columnar shape) or spherical shape, and carbonizing and activating it. . In general, activated carbon has an average pore diameter of 1.5 to 2.0 nm and a specific surface area of 600 m ² / g or more.

  In the present invention, the reason why activated carbon is selected as the biuret adsorbent is as follows. That is, as shown in the following adsorption test results, it is because only activated carbon has an effect as a biuret adsorbent among the conventionally known general adsorbents.

<Adsorption test>
First, a urea water stock solution was prepared by the following method. That is, industrial urea was added to pure water heated to 60 ° C. under a stirring condition (using a magnetic stirrer) to obtain 32.5% by weight of urea water, and a reagent biuret (special grade reagent manufactured by Kanto Chemical Co., Ltd.) was added thereto. The biuret concentration was 0.55% by weight.

  Next, using various adsorbents shown in Table 1, the urea water stock solution was adsorbed in accordance with the treatment method shown in the same table, and biuret in the water to be treated was measured. Moreover, the measurement about triuret was also performed. The concentration of biuret and triuret was determined by a liquid chromatography method (detection limit 10 ppm). The results are shown in Table 1.

  In the present invention, a urea aqueous solution having a biuret concentration of 2500 ppm or more is treated with activated carbon. This is because urea water having a biuret concentration equal to or higher than the above concentration is likely to block the piping introduced into the SCR catalyst. The biuret concentration of the urea aqueous solution to which the present invention is applied is 2500 ppm or more, preferably 3500 ppm or more.

  The present invention includes a biuret adsorption / removal step of treating a urea aqueous solution with activated carbon and a biuret desorption / removal step of desorbing biuret from the activated carbon that has adsorbed biuret. These steps may be performed alternately using two facilities or sequentially using one facility.

  As a method of adsorbent treatment in the biuret adsorption removal step, various methods shown in Table 1 can be adopted. Usually, a column type or a stirring and mixing type is used. In industrial scale implementations, for example, stainless steel columns and mixing vessels are used.

  The biuret desorption removal step is performed when the biuret concentration in the urea water derived from the biuret adsorption removal step exceeds a regulation value of 2400 ppm or less. A preferable regulation value is 2000 ppm or less. The detection of whether or not the regulation value of the biuret concentration has been reached can be easily performed by periodically measuring the biuret concentration in the urea water derived from the biuret adsorption and removal step, for example, by a liquid chromatography method.

The method for removing and removing the biuret is not particularly limited, and a method of dissolving with hot water, a method of thermally decomposing, a method of dissolving with a solvent, and the like can be appropriately employed. Examples of the solvent include an aqueous solution of an alkali metal hydroxide or an alkaline earth metal hydroxide, for example, an aqueous solution of NaOH, KOH, Ca (OH) ₂ , and its concentration is usually 10 to 50% by weight. .

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded. In addition, since it is clear that there is no problem in the timing of switching between the biuret adsorption / removal process and the biuret desorption / removal process, in the “Example” described below, the biuret adsorption / removal process and the biuret desorption / removal process are separated. I went.

<Preparation of urea aqueous solution>
In the same manner as in the adsorption test described above, a 32.5 wt% urea water stock solution having a biuret content of 0.35 wt% was prepared.

<Biuret adsorption removal process>
The activated carbon described in Table 2 was used, and a biuret adsorption removal process by a batch method was performed. That is, 100 ml of urea water stock solution and 1.0 g to 3.0 g of activated carbon are put into a 200 ml glass beaker, and the mixture is stirred and mixed with a magnetic stirrer for 1 hr. And after removing activated carbon with a 0.2 micrometer filter, the amount of biuret is measured by the liquid chromatography method. The results are shown in Table 3.

<Biulet desorption removal process>
Following the biuret adsorption / removal step using “WH2c8 / 32” manufactured by Nippon Enviro Chemicals, the biuret desorption / removal step described below was performed.

  The activated carbon removed by the 0.2 μm filter was transferred to a 200 ml glass beaker, 100 ml of 25 wt% NaOH aqueous solution was added, and the mixture was stirred and mixed with a magnetic stirrer to dissolve and remove biuret from the activated carbon.

  Then, in order to confirm the biuret desorption rate, the activated carbon was removed with a 0.2 μm filter, the removed activated carbon was transferred to a 200 ml glass beaker, and urea water for washing (32.5 wt% urea aqueous solution prepared without adding biuret) ) 100 ml was added and stirred and mixed with a magnetic stirrer for 1 hr. And the amount of biuret in the urea water for washing | cleaning was measured by the liquid chromatography method. As a result, it was below the detection limit (10 ppm) of the liquid chromatography method, and it was confirmed that the biuret desorption rate was substantially 100%.

Claims (1)

A method for producing high-purity urea water for urea SCR system with reduced biuret concentration by treating a urea water stock solution having a biuret concentration of 2500 ppm or more with activated carbon, wherein 1.0 to It includes a biuret adsorption / removal process in which the urea aqueous solution is treated with 3.0 g of activated carbon and a biuret desorption / removal process in which biuret is desorbed from the activated carbon that has adsorbed biuret. First, a biuret adsorption / removal process is performed. A method for producing high-purity urea water, characterized in that a biuret desorption removal step is performed when the concentration of biuret in the derived urea water exceeds a regulation value of 2400 ppm or less.