JP2006218405A - Method and apparatus for treating waste liquid containing hardly decomposable hazardous substance and nitrogen compound - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for treating a waste liquid which can efficiently decompose a hardly decomposable hazardous substance while suppressing the formation of nitric acid derived from a nitrogen compound when the waste liquid containing both the hardly decomposable hazardous substance and the nitrogen compound is treated, and an apparatus. <P>SOLUTION: The method and the apparatus for treating the waste liquid containing the hardly decomposable hazardous substance and the nitrogen compound comprises carrying out a supercritical water oxidation reaction at temperature required to decompose the hardly decomposable hazardous substance after carrying out a hydrothermal oxidation reaction for a predetermined time at intermediate temperature without reaching the temperature required to decompose the hardly decomposable hazardous substance. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a treatment method and apparatus for waste liquid containing both a hardly decomposable hazardous substance represented by PCB (polychlorinated biphenyl) and dioxin and a nitrogen compound, and in particular, while suppressing nitric acid production from the nitrogen compound, The present invention relates to a method and an apparatus for treating a waste liquid containing a hardly decomposable hazardous substance and a nitrogen compound, which can efficiently decompose the hardly decomposable harmful substance.

  In recent years, attention has been paid to attempts to decompose and detoxify environmental pollutants using a hydrothermal oxidation reaction in the presence of high-temperature and high-pressure water, particularly supercritical water. In particular, supercritical water oxidation utilizing the high reactivity of supercritical water decomposes harmful and hardly decomposable organic substances, such as PCB, dioxin, and organic chlorinated organic substances, which were difficult to be decomposed by the prior art. Attempts to convert to harmless products such as carbon dioxide, water and inorganic salts are drawing attention. The supercritical water means water in a supercritical state, that is, water in a state exceeding the critical point of water. Specifically, it is at a temperature of 374.1 ° C. or higher and under a pressure of 22.04 MPa or higher. Says water in a state. Supercritical water has a high ability to dissolve organic substances, and can completely dissolve non-polar substances that are abundant in organic compounds. Conversely, the ability to dissolve inorganic substances such as metals and salts is extremely low. Supercritical water can be mixed with a gas such as air, oxygen, or nitrogen at an arbitrary ratio to form a single phase.

  In such supercritical water oxidation of organic matter, when a nitrogen compound is treated using air or oxygen as an oxidant, nitric acid is generated at a high reaction temperature, and there is a concern about apparatus corrosion. Therefore, Patent Document 1 discloses means for suppressing the production of nitric acid by setting the reaction temperature to 580 to 620 ° C.

On the other hand, in the supercritical water oxidation treatment of refractory hazardous substances represented by PCB and dioxin, the reaction temperature is used in order to perform the reliable treatment while taking advantage of the high reaction rate that is the advantage of supercritical water oxidation. There is a case where it is processed at about 630 ° C. (Non-patent Document 1).
Japanese Patent Laid-Open No. 11-226583 Annual Meeting of Chemical Engineering Society 34th Autumn Meeting, 2001, 2001

  However, when supercritical water oxidation treatment is performed on a waste liquid containing nitrogen compounds mixed with hardly decomposable harmful substances, if the reaction temperature is set to about 630 ° C. in order to reliably treat the hardly decomposable harmful substances, the nitrogen compound-derived Nitric acid may be generated, causing equipment corrosion. In this case, the corrosion of the apparatus may be suppressed by adopting a structure and material that give corrosion resistance, but this causes a problem that the equipment cost increases. Further, even when the equipment cost is allowed to increase, the problem that the generated nitric acid must be processed remains. On the other hand, if the reaction temperature is lowered in order to suppress nitric acid production, there arises a problem that undecomposed and hardly decomposable harmful substances remain.

  Accordingly, an object of the present invention is to efficiently decompose a hardly decomposable harmful substance while suppressing the production of nitric acid derived from the nitrogen compound when treating a waste liquid containing both the hardly decomposable harmful substance and the nitrogen compound. It is an object of the present invention to provide a method and apparatus for treating the waste liquid.

  In order to solve the above problems, in the course of completing the present invention, first, it was examined whether there is a method for suppressing nitric acid production in a supercritical water oxidation reaction at a high temperature. As a result of air oxidation in supercritical water oxidation so far, waste liquid with high concentration, that is, high calorific value, is treated, so the temperature of the measuring part is around 600 ° C, but the temperature of the reaction center is exothermic reaction. It was expected that the temperature was higher than the measured temperature. Therefore, in order to examine the influence of the reaction center temperature, the final control temperature was constant at 630 ° C., and the experiment was performed by changing the heat generation amount of the waste liquid to be treated. The results shown in Table 1 were obtained. It was.

  In Table 1, the reaction center assumed temperature is the fluid temperature reached by the amount of heat generated when the supercritical water oxidation reaction starts at 400 ° C. and the entire amount is oxidized. Table 1 also shows the relationship between the amount of heat generated and the expected reaction center temperature. As described above, it was confirmed that the nitric acid conversion rate greatly changed due to the different calorific value of the waste liquid. As a result, it was considered that the temperature reached at the reaction center was high, affecting the nitric acid production.

Here, in each of the experiments described above, the final reached temperature was 630 ° C., and although it was higher than the expected reaction center portion reached temperature in Experiment No. 1-2 where nitric acid was generated, Experiment No. 1- In 1, no nitric acid was substantially generated. This was considered as follows.
(1) The general TOC (total organic carbon) component disappears at the reaction center expected temperature because the reaction occurs relatively quickly, but the nitrogen compound remains when the temperature is low. There is.
(2) Nitric acid is generated not by a single reaction of the nitrogen compound at a high temperature but by a combined reaction of the nitrogen compound and the TOC component.
Therefore, in order to confirm whether or not the coexisting TOC component has an influence on the nitric acid production, an experiment was conducted by changing the TOC / TN (total nitrogen) molar ratio, and the results shown in Table 2 were obtained. It was.

  As shown in Table 2, the experiment numbers 2-1 to 2 having a low TOC / TN molar ratio (C / N molar ratio) despite the above-described reaction center temperature reaching the temperature at which the nitric acid was generated as described above. In -3, nitric acid was not substantially produced, and it was confirmed that the presence of the TOC component was involved in the nitric acid production.

From the above results, it was concluded that nitric acid generation from nitrogen compounds in the supercritical water oxidation reaction occurs when the following requirements overlap.
(1) A large amount of organic carbon exists in the presence of a nitrogen compound.
(2) The temperature is high.
That is, even if the temperature is high, nitric acid is not generated if only a small amount of organic carbon is present. On the other hand, when organic carbon and nitrogen compounds are compared, in general, organic carbon is more likely to be oxidized in an intermediate temperature range (temperature condition that does not reach the temperature required to decompose hardly decomposable harmful substances). Therefore, once the organic carbon is reduced by the hydrothermal oxidation reaction at an intermediate temperature, the remaining hardly decomposed harmful substance can be completely decomposed by supercritical water oxidation at high temperature. Ascertaining that it is possible to suppress the production of nitric acid, the present invention has been completed.

  That is, according to the present invention, a method for treating a waste liquid containing a hardly decomposable harmful substance and a nitrogen compound is used after performing a hydrothermal oxidation reaction for a predetermined time at an intermediate temperature that does not reach the temperature required for decomposing a hardly decomposable harmful substance. It comprises a method characterized in that a supercritical water oxidation reaction is carried out at a temperature required to decompose a hardly decomposable harmful substance.

  In this waste liquid treatment method, the intermediate temperature can be maintained for a predetermined time by balancing the heat generated by the reaction and the heat released from the apparatus. The intermediate temperature is preferably in the range of 350 ° C to 550 ° C.

  The predetermined time is preferably a time sufficient for oxidizing most of the organic carbon in the waste liquid at the intermediate temperature. The predetermined time is preferably 3 seconds or longer. That is, when only water and air are heated without a reaction, heating from 350 ° C. to 550 ° C. takes about 5 seconds even if it is performed efficiently. However, even in the same heating method, when the waste liquid and air are heated, there is a temperature at which the organic matter in the waste liquid starts to react rapidly (rapid reaction start temperature, for example, 400 ° C.). Rises in a shorter time. When the calorific value is high, the heat generated by the reaction is larger than the heat released from the apparatus, and the temperature rapidly rises to 550 ° C. or more even in about 2 seconds from the rapid reaction start temperature. Therefore, when the waste liquid is supplied, it is necessary to keep the temperature within a temperature range lower than the rapid reaction start temperature, so that the reaction of organic matter is gradually advanced without performing the rapid reaction. At the time of such a reaction, it is easy to balance the heat generated by the reaction and the heat released from the apparatus, so that a rapid rise in temperature can be suppressed and the TOC component can be reduced. Although this temperature range and required retention time vary depending on the reactivity and concentration of the waste liquid, the temperature range is generally between 350 ° C and 550 ° C, and it takes approximately 3 seconds or more to reduce the TOC component in that temperature range. And However, when the reactivity of the waste liquid is high, the temperature range is low, and when the concentration of the waste liquid is high, the necessary holding time is long.

  The temperature required for decomposition of the hardly decomposable harmful substance is preferably 600 ° C. or higher.

  The apparatus for treating a waste liquid containing a hardly decomposable hazardous substance and a nitrogen compound according to the present invention comprises a means for performing a hydrothermal oxidation reaction for a predetermined time at an intermediate temperature that does not reach the required decomposition temperature of the hardly decomposable harmful substance, And a means for performing a supercritical water oxidation reaction at a temperature required for decomposition of a hardly decomposable hazardous substance after the thermal oxidation reaction.

  In this waste liquid treatment apparatus, as shown in an embodiment described later, a first preheater, an intermediate reactor, a second preheater, and a high temperature reactor, each of which can be independently adjusted in temperature in the flow direction of the waste liquid, are provided in this order. It can be set as the structure currently provided. In addition, the first preheater and the intermediate reactor can be collectively configured as a Bessel reactor for intermediate temperature. Further, when the apparatus is started up, the waste liquid can be circulated in the order of the second preheater, the intermediate temperature vessel reactor, and the high temperature reactor, thereby reducing the incidental facilities.

  According to the method and apparatus for treating a waste liquid containing a hardly decomposable hazardous substance and a nitrogen compound according to the present invention, after a hydrothermal oxidation reaction is performed at an intermediate temperature for a predetermined time, a supercritical water oxidation reaction is performed at a high temperature. Thus, the hardly decomposable harmful substance can be completely decomposed, and the production of nitric acid can be suppressed, whereby the corrosion of the apparatus can be suppressed and the equipment cost can be reduced. In addition, even when the organic carbon concentration is high, the facility cost and the operating cost can be reduced.

Hereinafter, the present invention will be described in detail together with preferred embodiments.
As described above, in a conventional hydrothermal reactor, the reactor is controlled at a single temperature, and when processing waste liquid containing nitrogen compounds and persistent decomposable substances, a temperature suitable for either one is used. However, nitric acid formation or persistent decomposable harmful substances had occurred. For example, as shown in FIG. 5, a conventional hydrothermal reaction apparatus has a preheater 101 that raises the liquid to be treated (waste liquid) supplied together with an oxidant (for example, air) to a reaction temperature, and a predetermined temperature at the reaction temperature. It consists of a reactor 102 for holding time. Although not shown, the apparatus is provided with a waste liquid and oxidant supply device, a cooler for a treated fluid, a pressure reducing valve, and the like. In this apparatus configuration, there is only one point of control of the reaction temperature, and there is a problem that nitric acid is generated when the reaction temperature is raised, and hardly decomposable harmful substances remain when the reaction temperature is lowered.

  However, in addition to being able to suppress nitric acid generation when the reaction temperature is lowered, we have discovered new findings that nitric acid generation can be suppressed if organic carbon does not coexist even at high temperatures. As a result, once the organic carbon is reduced by a hydrothermal oxidation reaction at an intermediate temperature, the remaining hardly decomposable hazardous substance is supercritically hydroxylated at a high temperature to completely decompose the hardly decomposable harmful substance. At the same time, it was found that nitric acid production can be suppressed.

  Therefore, in the present invention, in order to suppress the production of nitric acid, a method is adopted in which a hydrothermal oxidation reaction is performed at an intermediate temperature for a predetermined time and then a supercritical water oxidation reaction is performed at a high temperature. When the conventional method and the method of the present invention are compared, the temperature distribution (temperature transition) and the TOC residual ratio at each part of the reactor are as shown in FIG. 1 (method of the present invention) and FIG. 2 (conventional method). It is estimated that That is, in the conventional method, the TOC residual rate is high when reaching the temperature range (above the two-dot chain line) where the nitric acid production rate increases (FIG. 2), but in the method of the present invention, the TOC residual rate in the temperature range. Is low (FIG. 1). Therefore, according to the method of the present invention, there is no coexistence of organic carbon and a nitrogen compound at a high temperature, and nitric acid production can be suppressed. 1 and 2, the accumulated time in the apparatus indicates an accumulated value of the time that has existed in the apparatus after the waste liquid as the liquid to be treated is supplied into the apparatus.

  Table 3 shows an example of the results when supercritical water oxidation treatment is performed on a waste liquid containing a nitrogen compound in comparison with the conventional method. Except for the presence or absence of intermediate reaction temperature, the waste liquid properties, the final reaction temperature, and the reaction time at the final reaction temperature are the same. In the conventional method according to this example, the treatment is performed at 630 ° C. for the purpose of complete decomposition of the hardly decomposable harmful substance. As a result of the formation of nitric acid, the pH of the treated water is lowered, and there is a concern about the corrosion of the apparatus. However, in the method of the present invention, although the final supercritical water oxidation temperature is the same, nitric acid production is suppressed and there is a slight pH drop, but the apparatus corrosion is within an acceptable range.

  In the above examples, the intermediate holding reaction temperature was 385 ° C. (set temperature, which may be raised to the set temperature or higher by reaction heat), and the intermediate holding reaction time was 5 seconds. When organic carbon that tends to occur is present and the intermediate holding reaction temperature is high, or when the concentration of organic carbon is high, the temperature rise due to reaction heat may exceed the amount of heat released from the apparatus. In such a case, it is difficult to maintain the intermediate holding reaction temperature at a temperature within a certain range, and nitric acid may be generated due to the temperature rise, so the holding reaction temperature needs to be lowered. The intermediate holding reaction temperature thus varies depending on the substance in the waste liquid, but generally falls within the range of about 350 ° C to 550 ° C.

  When the TOC concentration in the waste liquid is high, it is necessary to lengthen the intermediate holding reaction time in order to sufficiently reduce the organic carbon concentration at the intermediate holding reaction temperature. Although the holding reaction time depends on the TOC concentration in the liquid as described above, it is generally preferable to secure 3 seconds or more as described above.

  As for the apparatus, a conventional hydrothermal reaction apparatus is, as shown in FIG. 5, for example, a preheater that raises the liquid to be treated (waste liquid) supplied together with an oxidizing agent (for example, air) to the reaction temperature. 101. It is comprised with the reactor 102 for hold | maintaining for predetermined time at reaction temperature. TICA indicates a temperature sensor or a temperature controller. Although not shown, the apparatus is provided with a waste liquid and oxidant supply device, a cooler for a treated fluid, a pressure reducing valve, and the like. In this apparatus configuration, there is only one point of control of the reaction temperature, and there is a problem that nitric acid is generated when the reaction temperature is raised, and hardly decomposable harmful substances remain when the reaction temperature is lowered.

  On the other hand, the processing apparatus according to an embodiment of the present invention includes a first preheater 1, an intermediate reactor 2, a second preheater 3, and a high temperature reactor 4, for example, as shown in FIG. . The first preheater 1 raises the temperature to the reaction temperature in the intermediate reactor 2, and most of the organic carbon is oxidized in the intermediate reactor 2. Next, the temperature is raised to the reaction temperature in the high temperature reactor 4 by the second preheater 3, and the high temperature reactor 4 decomposes the hardly decomposable harmful substance, thereby suppressing the generation of nitric acid and the remaining of the hardly decomposable harmful substance. It becomes possible. That is, in the first preheater 1 and the intermediate reactor 2, in particular, the intermediate reactor 2 causes the hydrothermal oxidation reaction to be performed for a predetermined time at an intermediate temperature that does not reach the required decomposition temperature of the hardly decomposable hazardous substance in the present invention. Means, and the second preheater 3 and the high temperature reactor 4, particularly the high temperature reactor 4, after the hydrothermal oxidation reaction in the present invention, the supercritical hydroxylation reaction at the temperature required for decomposition of the hardly decomposable hazardous substance. The means to perform is comprised. In FIG. 3, each of the preheaters 1 and 3 and each of the reactors 2 and 4 is configured, but is not limited to this as long as the temperature can be maintained for a predetermined time within the range of the intermediate reaction temperature. The temperature may be controlled so that the preheater can hold the temperature for a predetermined time by using one preheater and one reactor and adjusting the amount of external heating and the amount of liquid to be processed. Moreover, in FIG. 3, although each preheater 1, 3 and each reactor 2, 4 are shown as a tube type, it is not limited to this, Use a Bessel reactor for a preheater or a reactor. You can also. Further, in each reactor, as long as the reaction temperature can be maintained, only the heat retention is performed, and the heater may not be provided. Alternatively, in the conventional apparatus shown in FIG. 5, one or more temperature sensors or temperature controllers are installed in the preheater portion, and when the intermediate reaction temperature rises, heat is dissipated, heat is removed, and cooling is performed. The temperature may be maintained. In addition, in order to suppress the temperature rise associated with the start of the reaction in the preheater and maintain the intermediate temperature range in the preheater, a mechanism that actively dissipates heat, removes heat, and cools may be installed in that part. Good.

  If the apparatus shown in FIG. 3 is used to decompose most organic carbon at an intermediate temperature when treating a waste liquid having a high organic matter concentration and an extremely high oxidation reaction heat, the first preheating is performed. The vessel 1 becomes larger. In such a case, for example, the apparatus shown in FIG. 4 is suitable as an apparatus according to another embodiment of the present invention. The apparatus shown in FIG. 4 eliminates the first preheater and the intermediate reactor in FIG. 3 and combines them into an intermediate temperature vessel reactor 11, followed by a second preheater 12 and a high temperature reactor 13 in that order. Connected. The vessel reactor 11 here is a vessel reactor (for example, as shown in Japanese Patent Laid-Open No. 10-314768) that maintains the reaction temperature by utilizing the oxidation reaction heat of the waste liquid. It is preferable that supercritical water is not supplied and heating is not performed. In this type of reactor, after the inside of the reactor is brought into a supercritical state at the time of starting up the apparatus, even if the waste liquid is supplied at room temperature by using the energy of the oxidation reaction of the waste liquid, the reactor is kept at an intermediate temperature. (Temperature at which most organic carbon is oxidized). As a specific temperature maintaining method, for example, there is a method of detecting the temperature of the intermediate temperature vessel reactor and adjusting the amount of waste liquid supplied. In this method, when the temperature is high, the flow rate of the waste liquid is reduced, and when the temperature is low, the total heat value entering the reactor is adjusted by increasing the flow rate of the waste liquid to balance the heat value and the heat dissipation value. This makes it possible to maintain the temperature. Therefore, since heating of the intermediate temperature vessel reactor 11 is not required during operation, there is an advantage that both the equipment cost and the operation cost can be reduced.

  In the apparatus shown in FIG. 4, the intermediate temperature vessel reactor 11 can be provided with a heater for heating and keeping warm at the time of starting, and the temperature controllability can be further improved. 2 The preheater 12 is used, and the starter heater necessary for the intermediate temperature vessel reactor 11 can be obtained by flowing the second preheater 12 → the intermediate temperature vessel reactor 11 → the high temperature reactor 13 in this order only at the time of activation. Since it becomes unnecessary, the equipment cost can be reduced.

  The present invention can be applied to treatment in all fields as long as treatment of waste liquid containing a hardly decomposable harmful substance and a nitrogen compound is required.

It is a characteristic view which shows an example of the relationship between the temperature distribution (temperature transition) and the TOC residual ratio in the method of the present invention. It is a characteristic view which shows an example of the relationship between the temperature distribution (temperature transition) and the TOC residual ratio in the conventional method. It is a schematic equipment block diagram of the processing apparatus concerning one embodiment of the present invention. It is a schematic equipment block diagram of the processing apparatus which concerns on another embodiment of this invention. It is a schematic equipment block diagram which shows an example of the conventional processing apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st preheater 2 Intermediate reactor 3, 12 2nd preheater 4, 13 High temperature reactor 11 Bessel reactor for intermediate temperature

Claims (10)

  After the hydrothermal oxidation reaction is performed for a predetermined time at an intermediate temperature that does not reach the temperature required for decomposition of the hardly decomposable hazardous substance, the supercritical water oxidation reaction is performed at the temperature required for decomposition of the hardly decomposed hazardous substance. A method for treating a waste liquid containing a hardly decomposable harmful substance and a nitrogen compound.   The waste liquid treatment method according to claim 1, wherein the intermediate temperature is maintained for a predetermined time by balancing heat generation by reaction and heat radiation from the apparatus.   The waste liquid treatment method according to claim 1 or 2, wherein the intermediate temperature is in a range of 350C to 550C.   The waste liquid treatment method according to any one of claims 1 to 3, wherein the predetermined time is a time sufficient to oxidize most of the organic carbon in the waste liquid at the intermediate temperature.   The processing method of the waste liquid in any one of Claims 1-4 whose said predetermined time is 3 second or more.   The waste liquid treatment method according to any one of claims 1 to 5, wherein the temperature required for decomposition of the hardly decomposable harmful substance is 600 ° C or higher.   Means for carrying out hydrothermal oxidation reaction for a predetermined time at an intermediate temperature that does not reach the temperature required for decomposition of the hardly decomposable hazardous substance, and after the hydrothermal oxidation reaction, a supercritical water oxidation reaction is performed at the temperature for decomposition of the hardly decomposable hazardous substance And a waste liquid treatment apparatus containing a hardly decomposable harmful substance and a nitrogen compound.   The waste liquid processing apparatus according to claim 7, wherein a first preheater, an intermediate reactor, a second preheater, and a high temperature reactor, each of which is capable of independently adjusting the temperature in the flow direction of the waste liquid, are provided in this order.   The waste liquid treatment apparatus according to claim 8, wherein the first preheater and the intermediate reactor are combined and configured as an intermediate temperature vessel reactor.   The waste liquid processing apparatus according to claim 9, wherein at the time of startup, the waste liquid is circulated in the order of the second preheater, the intermediate temperature vessel reactor, and the high temperature reactor.

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