JP2006116544A - Method for neutralizing high alkaline waste water with flue gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for neutralizing high alkaline waste water with flue gas wherein the alkaline waste water can be neutralized using a compact neutralizing apparatus at a low cost. <P>SOLUTION: In the method for dissolving and diffusing gaseous CO<SB>2</SB>in the flue gas in a large volume of the alkaline waste water to neutralize it, two or more tanks 1, 2 are provided for neutralization. The neutralization to pH 9.0-8.0 is carried out in the first tank and the neutralization to ≤pH 8.0 is carried out in the second and subsequent tanks with the flue gas. Since dissolution efficiency remains high up to pH 8.0 even when the concentration of the gaseous CO<SB>2</SB>in the flue gas is 10 vol.% and the neutralization to ≤pH 8.0 is carried out in the second and subsequent tanks, the volume of the flue gas required can be reduced as compared with the case of a single neutralization tank. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for neutralizing a large amount of highly alkaline wastewater discharged from factories such as fiber dyeing, paper pulp, food and linen supply with combustion exhaust gas, and more specifically, to convert carbon dioxide in combustion exhaust gas into highly alkaline wastewater. The present invention relates to a neutralization method using combustion exhaust gas of high alkaline waste water that can be efficiently dissolved and diffused, and can reduce the amount of combustion exhaust gas for neutralization that is neutralized with only one tank, and neutralizes at a low cost.

  A large amount of highly alkaline wastewater discharged from various factories is generally neutralized by injecting sulfuric acid or the like according to the water quality regulations of the wastewater, or after neutralizing as necessary, aerobic or Anaerobic activated sludge treatment. In the neutralization treatment, sulfuric acid or the like is injected into a highly alkaline wastewater having a pH of 10 to 14, and neutralization is performed up to the neutrality of pH 6 to 8. In activated sludge treatment, air aeration is performed to reduce BOD and COD by anaerobic bacteria.

  In addition, as other neutralization treatment methods for highly alkaline wastewater, a method using carbon dioxide or liquefied carbon dioxide as a neutralizing agent, a method using carbon dioxide contained in combustion exhaust gas as a neutralizing agent, and melting sodium sulfate A method of using a solidified powdered solid acid as a neutralizing agent or a method of using the above neutralizing agent in combination has been proposed.

For example, by blowing exhaust gas from a combustion furnace into alkaline wastewater, reducing the hydrogen ion concentration (pH) in the wastewater, and adjusting to a predetermined hydrogen ion concentration (Patent Document 1), alkaline blow drainage from a boiler A method of neutralizing by mixing combustion exhaust gas (Patent Document 2), bringing the flue gas into the entrainment passage by a jet of alkaline waste liquid, then bringing it into the reaction tank liquid as fine bubbles, A method of neutralizing alkali with sulfurous acid gas and carbon dioxide in the gas (Patent Document 3), and reacting carbon dioxide or carbon dioxide-containing gas with alkaline waste liquid to recover a solution mainly composed of alkali hydrogen sulfide and alkali carbonate Method (Patent Document 4), a method of directly mixing liquefied carbon dioxide into alkaline drainage and neutralizing by liquid-liquid reaction (Patent Document 5), saturated carbon dioxide gas pre-concentrated in alkaline wastewater (Patent Document 6), in which alkaline waste liquid is fed from one end of the spiral pipe, and carbon dioxide gas is injected from the alkaline waste liquid feed side for neutralization reaction. (Patent Document 7), monitoring the pH of the flowing alkaline water flow, operating the proportional control valve according to the pH signal, reducing the pressure of the liquid CO ₂ fluid to the pressure in the intermediate region, a considerable amount A method has been proposed in which the gas phase is generated to reduce the density of the CO ₂ fluid and injected into the alkaline water stream (Patent Document 8).

In addition, after adjusting the primary pH of a highly concentrated alkaline solution to 11.5 or less with a strong acid such as hydrochloric acid or sulfuric acid, the secondary pH is adjusted to about pH 7 with carbon dioxide gas from the alkaline solution that has undergone the primary treatment. (Patent Document 9), a primary pH adjustment step of adding a powdered solid acid obtained by melting and solidifying sodium hydrogen sulfate to highly alkaline wastewater to control the pH in the range of 10 to 11, and after the first pH adjustment step Proposed is a method of sequentially performing a second pH adjustment step in which the wastewater is neutralized with carbon dioxide gas or carbonated water to control it at around pH 7 (Patent Document 10).
JP 61-227890 JP JP 59-80390 A Japanese Unexamined Patent Publication No. 55-1814 JP 59-69192 A JP-A-58-101788 JP 50-112272 A JP-A-53-118278 Japanese Unexamined Patent Publication No. 3-89994 JP-A-62-258793 JP-A-6-91275

  The methods disclosed in Patent Documents 1 to 3 use carbon dioxide in combustion exhaust gas as a neutralizing agent, diffuse the combustion exhaust gas into alkaline drainage by an ejector method, or dissolve carbon dioxide, or neutralize one tank The combustion exhaust gas is blown into the tank and the carbon dioxide gas is dissolved, but the carbon dioxide gas in the combustion exhaust gas is not efficiently dissolved, so it is necessary to circulate the alkaline drainage or the combustion exhaust gas or to inject a large amount of the combustion exhaust gas. However, there is a problem that the size is increased and the neutralization cost is increased.

  In addition, since the methods disclosed in Patent Documents 4 to 8 use liquefied carbon dioxide gas and pure carbon dioxide gas as a neutralizing agent, the cost is higher than neutralization with a strong acid such as sulfuric acid, but the handling is safe. It has been put to practical use in small volume wastewater treatment. However, in the neutralization treatment of large-capacity wastewater, there is a problem that the costs of liquefied carbon dioxide gas and pure carbon dioxide gas are high and are not practical from an economic viewpoint.

  Further, the methods disclosed in Patent Documents 9 to 10 use a strong acid and carbon dioxide gas or a powdered solid acid and carbon dioxide gas as a neutralizing agent. There are problems such as bad odor caused by hydrogen sulfide generation due to water and corrosion of sewer pipes.

  An object of the present invention is to provide a neutralization method using combustion exhaust gas of high alkaline wastewater that can eliminate the above-mentioned drawbacks of the prior art and can neutralize alkaline wastewater at a low cost using a compact neutralization facility.

  In the method of neutralizing carbon dioxide in combustion exhaust gas by dissolving, diffusing into large-capacity high alkaline wastewater, the present invention has two or more tanks used for neutralization, and the first tank uses combustion exhaust gas. By neutralizing to pH 9 to 8.0 and neutralizing to pH 8 or lower with combustion exhaust gas in the second and subsequent tanks, the total amount of combustion exhaust gas for neutralization is continuously neutralized with combustion exhaust gas in only one tank. It can be further reduced as compared with the method.

  The neutralization method of the highly alkaline waste water of the present invention with combustion exhaust gas is made up of two or more tanks used for neutralization, and by neutralizing to pH 9 to 8.0 with combustion exhaust gas in the first tank, about pH 8.0 Until the carbon dioxide concentration in the combustion exhaust gas is around 10% by volume, the dissolution efficiency is high, so the required amount of combustion exhaust gas can be reduced compared to the case where one tank is used for neutralization. .

  In addition, according to the present invention, compared with conventional sulfuric acid neutralization, sulfuric acid or the like is not used, so that neutralization costs can be greatly reduced, sulfate ions in the treated wastewater are reduced, and adverse effects on the ecosystem are mitigated. At the same time, it is possible to prevent the bad smell and corrosion of the sewer, and to fix the carbon dioxide in the combustion exhaust gas that causes global warming.

In order to examine the neutralization method with combustion exhaust gas, alkaline drainage of pH 11.6 to 12.8 from a raw water tank with a width of 600 mm, a depth of 300 mm, a height of 1000 mm, and a volume of 180 l, 42 l / hr, a width of 600 mm, a depth of 300 mm, and a height of 600 mm , While continuously supplying to a neutralization tank with a volume of 108 liters, a mixed gas of carbon dioxide and air (CO ₂ concentration: 10%, 20%, 30%) was installed in the neutralization tank at the bottom of the neutralization tank Neutralization tank pH: 7.2, 7.5, 7.8, 8.0, 8.5, 9.0 was blown through the device (400w, rotation speed 1800rpm), neutralization tank pH and carbon dioxide gas concentration and carbon dioxide in the off-gas rising from the water surface. The relationship between the ratio of gas dissolved in wastewater was investigated. The results are shown in FIG.

As shown in FIG. 1, even when the carbon dioxide concentration in the mixed gas to be blown changed to 10%, 20%, and 30%, the carbon dioxide concentration in the off-gas was substantially constant with respect to the neutralization tank pH. During this experiment, the pH of the raw water varied from 11.6 to 12.8, but the carbon dioxide concentration in the off-gas did not change. When the neutralization tank pH is 8 or more, 70% or more of the injected carbon dioxide dissolves.However, when the neutralization tank pH is 8 or less, the dissolution efficiency of the injected carbon dioxide deteriorates (especially CO _{2 in the} blown gas). It was confirmed that the concentration of carbon dioxide in the off-gas increased. That is, it was confirmed that the lower the neutralization tank pH, the lower the proportion of carbon dioxide dissolved.

From the above results, the principle of pH decrease by carbon dioxide gas is that carbon dioxide dissolved in water changes to H ₂ CO ₃ , HCO ₃ ⁻ , CO ₃ ²⁻ in water to form carbonate, and in carbonate aqueous solution Then, the following dissociation equilibrium is established.
CO ₂ + H ₂ O ← → H ₂ CO ₃ , H ₂ CO ₃ ← → H ⁺ + HCO ₃ ⁻ , HCO ₃ ⁻ ← → H ⁺ + CO ₃ ²⁻
Since the abundance ratio of H ₂ CO ₃ , HCO ₃ ⁻ and CO ₃ ^{2− in} the total carbonic acid is determined depending on the pH of water, the pH reduction effect of dissolved carbon dioxide gas is also different.

At pH <4, carbon dioxide dissolves slightly and becomes H ₂ CO ₃ but does not contribute to pH reduction. At 4 <pH <9, as shown below, the dissolved carbon dioxide gas becomes H ₂ CO ₃ , part of which is ionized and becomes HCO ₃ ⁻ to produce 1 mole of H ⁺ per mole of carbon dioxide gas. Contributes to pH reduction.
CO ₂ + H ₂ O ← → H ₂ CO ₃ ← → H ⁺ + HCO ₃ ⁻
The higher the pH of water, the higher the ratio of being HCO ₃ ^−, and at pH 8, 98% of the dissolved carbon dioxide is ionized to become HCO ₃ ⁻ .
At pH> 9, as shown below, all dissolved carbon dioxide gas is ionized to become HCO ₃ ⁻ and CO ₃ ²⁻ . In particular, CO ₃ ²⁻ produces 2 moles of H ⁺ per mole of carbon dioxide gas, so that the pH reduction efficiency is good. The higher the pH, the higher the CO ₃ ²⁻ ratio.
CO ₂ + H ₂ O ← → H ₂ CO ₃ ← → 2H ⁺ + CO ₃ ²⁻

That is, when pH <9, even if carbon dioxide is blown into the alkaline drainage, the concentration of carbon dioxide in the off-gas is increased because the blown carbon dioxide contributes to lowering the pH as CO ₃ ²⁻ , HCO ₃ ⁻ This is because H ₂ CO ₃ ← → (CO ₂ + H ₂ O) and the ratio of escape from waste water by CO ₂ (gas) increases.

  The basic technical idea of the present invention is that two or more tanks are used for neutralization, the first tank is neutralized to pH 9 to 8.0, and the second and subsequent tanks are neutralized to pH 8 or less. In other words, neutralization to pH 9 to 8.0 in the first tank requires high dissolution efficiency up to about pH 8.0 regardless of the concentration of carbon dioxide in the combustion exhaust gas, compared to the case where one neutralization tank is used. The amount of combustion exhaust gas can be reduced. As a tank used for neutralization, an aeration tank of an activated sludge treatment apparatus can be used as the second and subsequent tanks.

  The neutralization method of the highly alkaline waste water of the present invention using combustion exhaust gas will be described with reference to FIG. 2 which is a system diagram of the embodiment. The apparatus shown in FIG. 2 is configured to continuously feed alkaline waste water 3 into two neutralization tanks 1 and 2, preceding neutralization tank 1 and neutralize the wastewater neutralized to about pH 8.5 to the subsequent neutralization tank 2 To flow into. An underwater diffuser (not shown) is provided at the bottom of each of the neutralization tanks 1 and 2, and the flue gas 4 containing about 10% by volume of carbon dioxide gas is guided by the pipe 5 to generate fine bubbles of the flue gas 4, and the alkali The gas-liquid contact between the waste water 3 and the combustion exhaust gas 4 is enhanced to efficiently dissolve and diffuse the carbon dioxide gas. Waste water neutralized to about pH 7.5 in the subsequent neutralization tank 2 is sent to an aeration tank of an activated sludge treatment apparatus (not shown) through a pipe 6.

  The air diffuser used in the present invention is a gas-liquid diffuser capable of efficiently dissolving and diffusing gas in water, and extending a vent pipe that also serves as an air diffuser nozzle and a rotating shaft from the liquid to the outside of the liquid. A gas-liquid mixing device rotated by a rotary drive provided outside the liquid, or a carbon dioxide-containing gas and a lower alkali during a negative pressure generated on the back side in the rotation direction of an impeller fixed to the rotary shaft of the submerged rotary drive Water is introduced, gas bubbles are generated by the vortex generated by the rotation of the impeller and the shearing force of the impeller, and the gas-liquid contact between the alkaline drainage and the gas is enhanced to efficiently neutralize with carbon dioxide. Although an underwater diffuser etc. can be used, it is effective to use an underwater diffuser.

The combustion exhaust gas used in the present invention is a combustion exhaust gas from a boiler, a heating furnace or the like containing about 10% by volume of carbon dioxide gas. The combustion exhaust gas after desulfurization treatment has a low SO ₂ concentration that causes equipment corrosion and a small amount of dust. It is a good idea to use.

Example 1
The test equipment is 600mm wide, 300mm deep, 1000mm high, 180l capacity raw water tank, 600mm wide, 300mm deep, 600mm high, 108l neutralization tank (2 tanks), 600mm wide, 300mm deep, 1000mm high , An aeration tank (4 tanks) with a capacity of 720 l, a sedimentation tank with a diameter of 700 mm, a cylinder length of 400 mm, a conical portion length of 400 mm, and a capacity of 205 l.

The pH 13 alkaline waste water discharged from the dyeing factory was used, and a part of the combustion exhaust gas with a carbon dioxide concentration of 10% by volume discharged from the boiler in the factory was taken out as a combustion exhaust gas. And combustion exhaust gas was sent to the underwater diffuser installed in the neutralization tank or the aeration tank, and the neutralization process of the alkaline waste water was performed on the following conditions. Then, the amount of combustion exhaust gas required for neutralizing 1 m ³ of alkaline waste water (calculated as carbon dioxide concentration of 10% by volume) was determined. The results are shown in Table 1.

Condition 1: Neutralization tank with neutralization tank to pH 7.5 with combustion exhaust gas Condition 2, Neutralization tank with neutralization tank to pH 8.5, neutralization tank with neutralization tank to pH 7.5 with neutralization tank

As shown in Table 1, the condition 2 was neutralized tank and 2 tank, compared with the case of condition 1 neutralization tank 1 tank, a combustion exhaust gas amount necessary to neutralize the alkaline waste water 1 m ³ 5.0 Nm ³ could be reduced.

  In addition, the BOD and COD values of the wastewater from the sedimentation tank of the activated sludge treatment equipment were the same as those obtained with conventional sulfuric acid neutralization, and it was also confirmed that there was no adverse effect of activated sludge bacteria caused by using combustion exhaust gas It was. Furthermore, since the sulfuric acid ion concentration in the treated wastewater was neutralized with carbon dioxide in the combustion exhaust gas instead of sulfuric acid, the raw water was 500 to 1000 ppm, and from 2000 to 3000 ppm in the case of sulfuric acid neutralization, 500 to 1000 ppm. It was confirmed that it did not rise.

  As apparent from the examples, the present invention efficiently dissolves and diffuses the carbon dioxide gas in the combustion exhaust gas in the highly alkaline waste water, thereby continuously using the total amount of the combustion exhaust gas for neutralization in only one tank. Compared with the method of neutralizing with combustion exhaust gas, it can be further reduced and neutralization can be performed at low cost.

It is a graph which shows the relationship between the neutralization tank pH and the carbon dioxide gas concentration in off-gas, the ratio which a carbon dioxide melt | dissolves in waste water, and the carbon dioxide gas concentration in blowing gas in the alkaline wastewater neutralization experiment in a water flow state. FIG. 2 is a system diagram of an apparatus used in the invention according to claim 1 of the present application.

Explanation of symbols

1,2 Neutralization tank
3 Alkaline drainage
4 Combustion exhaust gas
5,6 Piping

Claims (1)

  In the method of dissolving and diffusing carbon dioxide in combustion exhaust gas into high-capacity, highly alkaline wastewater, neutralize it with two or more tanks for neutralization, and in the first tank up to pH 9 to 8.0 by combustion exhaust gas Neutralization method, neutralizing to pH 8 or less with combustion exhaust gas in the second and subsequent tanks, and reducing the total amount of the combustion exhaust gas with neutralization with only one tank, using a highly alkaline drainage combustion exhaust gas.