JP3925778B2 - Method for separating volatile organic compounds in wastewater - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for efficiently separating volatile organic compounds from waste water when waste water such as groundwater or industrial waste water contains volatile organic compounds such as trichlorethylene or tetrachloroethylene.
[0002]
[Prior art]
Conventionally, in the treatment of wastewater such as groundwater or industrial wastewater, volatile organic compounds such as trichlorethylene or tetrachloroethylene contained therein are separated from the wastewater by using a gas such as air. Bubbling (aeration) is performed by blowing a large amount, and the volatile organic compound in the wastewater is volatilized in the gas blown into the wastewater to separate it from the wastewater.
[0003]
[Problems to be solved by the invention]
However, in this bubbling method, in order to increase the separation rate from volatile organic compounds, the amount of bubbling gas blown into the waste water must be remarkably increased. In order to handle a large amount of gas, There is a problem that not only an increase in the size of the entire apparatus cannot be avoided, but also an increase in noise and power consumption due to an increase in the size of a blower that pumps air.
[0004]
Moreover, in the conventional bubbling method, the volatile organic compound separated from the waste water is diluted with a large amount of gas blown in order to separate the volatile organic compound from the waste water, thereby discharging the waste water from the waste water. Since the concentration of the volatile organic compound contained in the gas is extremely low, it is very troublesome to decompose the volatile organic compound having a very low concentration. There was also a problem that running costs were necessary.
[0005]
In order to improve these problems, the present inventors introduced waste water containing a volatile organic compound into a vacuum evaporator from the bottom in a previous patent application (Japanese Patent Application No. 2001-93962). A method for separating volatile organic compounds from waste water by discharging the waste water from the higher portion and boiling and evaporating the waste water in the evaporator from a portion of the waste water having a deep liquid depth from the water surface. Proposed.
[0006]
In this separation method by boiling and evaporation, the exhaust gas can be reduced and the concentration of volatile organic compounds can be increased. However, in order to further promote the separation of volatile organic compounds, the amount of waste water must be increased. However, in order to increase the amount of evaporation, it was necessary to raise the temperature of the wastewater, and as a result, the heat energy added to the wastewater increased significantly.
[0007]
It is a technical object of the present invention to provide a separation method that solves these problems.
[0008]
[Means for Solving the Problems]
In order to achieve this technical problem, claim 1 of the present invention provides:
“Gas is absorbed into wastewater containing volatile organic compounds, and then this wastewater is placed in a degassing vessel maintained at a reduced pressure below atmospheric pressure, below or below the packed bed provided in the degassing vessel. It is introduced from the part and discharged from the water surface at the upper part of the packed bed or at a higher part so as to be stored at a suitable liquid depth. The pooled waste water is lower than the upper surface of the packed bed in the water below the water surface. The gas is deaerated and foamed so that the gas bubbles are generated in the portion. "
It is characterized by that.
[0009]
Claim 2 of the present invention includes:
“In the first aspect of the present invention, the waste water discharged from the deaeration container is supplied into an evaporator maintained at a reduced pressure below atmospheric pressure, stored at an appropriate liquid depth, and then discharged from the water surface. ”The accumulated waste water boils and evaporates so that steam bubbles are generated in the submerged portion of the water .”
It is characterized by that.
[0010]
Claim 3 of the present invention provides:
“In claim 2, waste water discharged from the evaporator is led into a stripping vessel where it is brought into direct contact with the steam generated in the evaporator .”
It is characterized by that.
[0011]
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0013]
FIG. 1 shows a first embodiment.
[0014]
In this figure, reference numeral 1 denotes a gas absorption container having a packed bed 2 such as Raschig ring, and a waste water supply line 3 having a pump 4 is provided above the packed bed 2 in the gas absorbing container 1. The waste water containing a volatile organic compound such as trichlorethylene or tetrachloroethylene is sprayed and the air compressed by the compressor 5 is gasified below the packed bed 2 in the gas absorption container 1. By blowing through the supply pipe 6, the waste water absorbs air.
[0015]
In addition, since the absorption rate of the air with respect to this wastewater is proportional to a pressure, it is preferable to carry out in the state of pressure higher than atmospheric pressure.
[0016]
The gas absorbed in the wastewater is not limited to the air, and other gases such as carbon dioxide, nitrogen gas and oxygen gas which are not harmful may be used.
[0017]
As described above, the waste water that has absorbed the gas such as air in the gas absorption container 1 is sent to the bottom of the sealed deaeration container 8 through the pipe line 7, and the degassing container 8 has an appropriate depth H. And then discharged from the surface of the water through the drain line 9.
[0018]
At this time, by making the inside of the deaeration container 8 in a reduced pressure state below atmospheric pressure by suction by a vacuum generation source such as a vacuum pump 10, gas bubbles absorbed in the wastewater are contained in the wastewater. It deaerates and foams so as to occur in the part below the surface of the water, that is, the part of the water under the surface of the water.
[0019]
In this way, the waste water that has absorbed the gas is degassed and foamed in the deaeration container 8 that is kept in a decompressed state so that bubbles of the gas are generated in a portion of the waste water under water. Thus, a volatile organic compound such as trichlorethylene or tetrachlorethylene can be reliably separated from the waste water at a high rate.
[0020]
The reason is considered to be as follows.
[0021]
In other words, volatile organic compounds such as trichlorethylene contained in wastewater collect on the bubbles generated by the degassing of the wastewater, and when these bubbles burst, they move to the gas phase (vaporize) and are separated from the wastewater. However, when degassing of wastewater is performed in a state where gas bubbles absorbed in this wastewater are generated only at or near the surface of the wastewater, this degassing / foaming occurs. The time for the bubbles to contact the wastewater is very short, and therefore the trichloroethylene collected in the bubbles is small, so that the separation rate of the volatile organic compound from the wastewater is low.
[0022]
On the other hand, when the degassing / foaming of the wastewater is performed such that gas bubbles absorbed in the wastewater are generated in the submerged portion of the wastewater, the degassing / foaming is performed. Bubbles due to foaming are generated in the portion of the waste water below the surface of the water, and the bubbles rise toward the water surface. In the middle of the rise, a large amount of volatile organic compounds such as trichlorethylene in the wastewater gather in the bubbles and gradually vaporize in the bubbles to increase the size of the bubbles. When the bubbles burst on the surface of the water, the volatile organic compounds collected in the bubbles move (vaporize) into the gas phase and separate from the wastewater, so the volatile organic compounds in the wastewater are separated from the wastewater. This can be done with a high separation rate.
[0023]
In this case, a packed bed 11 filled with a filler such as Raschig ring is provided in the degassing container 8, and waste water is fed to the lower part of the degassing container 8 or a lower portion of the packed bed 11. By supplying the water, it is appropriately stored at a depth H, and the water level in the stored waste water is set at the upper part of the packed bed 11 or higher than this, and the waste water stored in the deaeration container 8 is drained from the water surface. While being configured to discharge through the conduit 9, the deaeration / foaming is performed so that bubbles of the gas are generated in a portion of the waste water below the surface of the waste water that is lower than the top surface of the packed bed 11. To be done.
[0024]
If comprised in this way, the bubble of the gas which floats on the water surface after it generate | occur | produces by deaeration and foaming in the part lower than the upper surface of the said packed bed 11 among the water under the surface of wastewater in the said packed bed 11 will be said. When passing through the part located above, the bubble area increases and stays without rising rapidly, so that the time until the bubbles rise to the surface of the water becomes longer. Since the contact time becomes longer, more volatile organic compounds can be collected by the gas bubbles.
[0025]
On the other hand, the concentration of the volatile organic compound in the waste water in the deaeration container 8 is the lowest on the water surface due to deaeration and foaming in which gas bubbles are generated in the portion of the water below the water surface. Otherwise, wastewater with a low concentration of volatile organic compounds on the surface of the water will be mixed into the underwater part due to convection caused by air bubbles rising above the surface. Therefore, the waste water having the lowest concentration of the volatile organic compound cannot be discharged from the drain pipe 9.
[0026]
However, as described above, when the packed bed 11 is provided in the deaeration vessel 8, the presence of the packed bed 11 can suppress the convection phenomenon. Only the low-concentration waste water in the surface portion of the waste water can be discharged.
[0027]
As a result, the separation rate of the volatile organic compound can be greatly improved by the presence of the packed bed 11.
[0028]
On the other hand, since the exhaust gas discharged from the vacuum pump 10 that maintains the inside of the deaeration container 6 in a depressurized state contains a gasified volatile organic compound, the exhaust gas is converted into a gas purification device using activated carbon, ultraviolet rays, and the like. After being decomposed into harmless gas or the like by the gas purifying device 12 such as a gas purifying device or a combustion type gas purifying device, it is released into the atmosphere.
[0029]
In this case, the exhaust gas from the vacuum pump 8 is approximately equal to the amount of gas initially absorbed into the waste water, much less than the conventional bubbling, in addition to the concentration of volatile organic compounds. Therefore, the gas purification device 12 can be greatly simplified and miniaturized as compared with the case where a large amount of exhaust gas having a low concentration is handled as in the prior art.
[0030]
Next, FIG. 2 shows a second embodiment.
[0031]
The second embodiment is a case where an evaporation type separation device is used in combination with the first embodiment.
[0032]
In this figure, reference numeral 13 denotes a decompression type evaporator, and waste water discharged from the drain line 9 in the deaeration vessel 8 is introduced into the bottom of the evaporator 13 through a transfer line 14. Then, by discharging from the outlet line 15 provided at a position where it is appropriately higher by the height H0, it is supplied from the lower part into the evaporator 1 and is always stored for a predetermined liquid depth H0, and then exits from the water surface. It is configured to discharge through the pipe line 15.
[0033]
After the waste water discharged from the outlet pipe 15 in the evaporator 13 is sprayed by the nozzle 18 on the upper surface of the packed bed 17 such as Raschig ring provided in the sealed stripping container 16, While discharging from the lower side through the outlet pipe 19, the steam generated in the evaporator 13 is introduced below the packed bed 17 through the steam duct 20, so that the waste water is discharged in the stripping container 16. And direct vapor-liquid contact with vapor.
[0034]
On the other hand, an indirect heat exchange type feed water heater 21 is provided in the middle of the waste water transfer line 14 from the deaeration container 8 to the evaporator 13, and the feed water heater 21 is provided in the stripping container 16. The water vapor in the tank is sucked into a compressor 22 such as a blower that is rotationally driven by an electric motor through a duct 23, compressed and then introduced through a duct 24. The waste water sent to 13 is heated (heated water heating).
[0035]
Condensed water and non-condensable gas in the feed water heater 21 are sucked by a liquid-sealed vacuum pump 25 through a pipe line 26 so that the inside of the evaporator 13 and the stripping container 16 are at atmospheric pressure. Maintain the following reduced pressure.
[0036]
In this case, the waste water stored in the evaporator 13 at a predetermined liquid depth H0 can be obtained by appropriately setting the degree of decompression in the evaporator 13 and the temperature at which the waste water flows into the evaporator 13. , Boiling and evaporation are performed so that steam bubbles are generated in the underwater portion of the water.
[0037]
In this way, the waste water in the evaporator 13 is boiled and evaporated so that steam bubbles are generated in the portion of the water below the surface of the water, so that the waste water is boiled and evaporated as in the case of the deaeration container 8. In the middle of the rising of the vapor bubbles toward the water surface, a large amount of trichlorethylene in the waste water gathers in the vapor bubbles, gradually evaporates into the bubbles, and the bubbles become larger. When ruptured on the water surface, the trichlorethylene collected in the vapor bubbles is transferred to the gas phase (vaporized) and separated from the wastewater, so that the volatile organic compounds in the wastewater are also separated from the wastewater in the evaporator 13. Can be separated.
[0038]
As in the case of the deaeration vessel 8, the inside of the evaporator 13 is provided with a packed bed 13 a by Raschig ring or the like to boil and evaporate the waste water in the water below the surface of the packed bed 13 a. By adopting a configuration in which vapor bubbles are generated in a portion lower than the upper surface, it is possible to improve the separation rate of the volatile organic compound.
[0039]
The waste water discharged from the outlet pipe 15 in the evaporator 13 is brought into direct contact with the vapor generated in the evaporator 13 in the stripping container 16, thereby allowing the waste water in the waste water. Since the volatile organic compound moves to the vapor side, the separation rate of the volatile organic compound from the wastewater can be further improved, and the wastewater after the separation treatment is discharged via the outlet pipe 19 and is separated. Served for use.
[0040]
In the second embodiment, the inventors set the pressure in the deaeration vessel 8 to 14 mmHg, the pressure in the evaporator 13 to 25 mmHg, and the boiling / evaporation in the evaporator 13. An experiment was conducted in which waste water having a volatile organic compound concentration of 8.2 ppm was supplied to the gas absorption container 1 when the temperature difference was 15 ° C.
[0041]
In this experiment, the concentration of the volatile organic compound in the waste water at the outlet from the deaeration vessel 8 is reduced to 2.8 ppm, and the concentration of the volatile organic compound in the waste water at the outlet from the evaporator 13 is A reduction to 0.08 ppm was observed.
[0042]
That is, the separation of the volatile organic compound from the wastewater uses gas absorption and degassing under reduced pressure in the wastewater together with boiling / evaporation under reduced pressure of the wastewater. The concentration of can be reduced from 8.2 ppm to 0.08 ppm.
[0043]
On the other hand, in the case where the gas absorption and degassing / foaming under reduced pressure are not applied as pretreatment for separation by boiling / evaporation in the evaporator 13, when the heat energy applied to boiling / evaporation is the same, The concentration of volatile organic compounds could only be lowered from 8.2 ppm to 0.27 ppm.
[0044]
Further, the waste water from the evaporator 13 can be brought into direct contact with the evaporated vapor in the stripping vessel 16 so that the concentration of the volatile organic compound can be reduced to about half.
[0045]
On the other hand, condensed water and non-condensable gas discharged from the liquid-sealed vacuum pump 25 are introduced into the gas-liquid separation container 27, the condensed water is pumped out by the circulation pump 28, and then the waste water transfer pipe 14 The liquid seal is configured so as to repeat the circulation of supplying to the indirect heat exchange type sub-feed water heater 29 and returning the sub-feed water heater 29 to the suction side of the liquid-sealed vacuum pump 25. The heat energy applied at the location of the vacuum pump 25 is recovered by heat for the feed water heating of the waste water supplied to the evaporator 13 by the auxiliary feed heater 29 provided in the waste water transfer pipe 14 to the evaporator 13. .
[0046]
Further, surplus condensed water and non-condensable gas in the gas-liquid separation container 27 are introduced into the next gas-liquid separation container 30, and this condensed water is introduced into the decomposition container 31 equipped with the ultrasonic transmission means 32. Or by radiating ultrasonic waves from the ultrasonic wave transmitting means 32 in the decomposition vessel 31 to decompose the volatile organic compound contained in the condensed water and then take it out from the conduit 33; A part thereof is mixed with waste water supplied into the deaeration container 8 through a pipe 34.
[0047]
Since the non-condensable gas in the gas-liquid separation container 30 and the exhaust gas from the decomposition container 31 contain gasified volatile organic compounds, this exhaust gas is used as in the first embodiment. Further, after being decomposed into harmless gas or the like by a gas purification device 12 such as a gas purification device using activated carbon, a gas purification device using ultraviolet rays, or a combustion type gas purification device, it is released into the atmosphere.
[0048]
In this case as well, the amount of exhaust gas to the gas purification device 12 is approximately equal to the amount of gas initially absorbed into the wastewater, much less than conventional bubbling, and in addition to volatile organics. Since the concentration of the compound is also high, the gas purification device 12 can be greatly simplified as compared with the case where a large amount of low concentration gas is handled as in the prior art.
[0049]
[Operation and effect of the invention]
Thus, claim 1 in the present invention is a method in which waste water containing a volatile organic compound is absorbed with a gas such as air, and then the waste water is submerged under the surface of water in a deaeration container maintained at a reduced pressure. In this part, the volatile organic compound is separated from the wastewater by degassing and foaming so that gas bubbles are generated. In this separation, a large amount of air is blown and evaporation is performed. Therefore, it is possible to greatly reduce the size of the device, reduce noise and power consumption, and thus reduce the running cost, and achieve high concentration of volatile organic compounds. Because it can be separated from wastewater as exhaust gas, it is easier to decompose the separated volatile organic compounds than in the conventional case With the results.
[0050]
In addition, since the separation of the volatile organic compound in the deaeration container can be promoted by the packed bed, the above effect can be further promoted.
[0051]
On the other hand, according to claim 2 of the present invention, the separation of the volatile organic compound from the wastewater is divided into two stages of gas absorption and degassing / foaming under reduced pressure and boiling / evaporation under reduced pressure of the wastewater. As a result, the concentration of volatile organic compounds in the treated wastewater can be greatly reduced by the two-stage separation without increasing the exhaust gas, and the heat applied for boiling and evaporating the wastewater. The energy can be reduced by an amount corresponding to the separation of the volatile organic compound by gas absorption into the wastewater and degassing / foaming under reduced pressure before the boiling / evaporation.
[0052]
According to claim 3 of the present invention, the separation of volatile organic compounds from wastewater is carried out by gas absorption and degassing / foaming under reduced pressure, boiling / evaporation under reduced pressure of wastewater, and subsequent stripping. By performing in three stages with steam contact in the container, the concentration of volatile organic compounds in the treated wastewater can be significantly reduced by the above three-stage separation without increasing exhaust gas, Before the boiling / evaporation, the heat energy applied for boiling / evaporation of the wastewater is separated by volatile organic compounds by gas absorption into the wastewater and degassing under reduced pressure, and after the boiling / evaporation. In this case, the amount of the volatile organic compound separated by vapor contact can be greatly reduced.
[Brief description of the drawings]
FIG. 1 is a flow sheet showing a first embodiment of the present invention.
FIG. 2 is a flow sheet showing a second embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Gas absorption container 3 Waste water supply line 6 Gas supply line 8 Deaeration container 10 Vacuum pump 11 Packing layer 12 Gas purification device 13 Evaporator 16 Stripping container 17 Packing layer 21 Water heater 22 Compressor 25 Liquid seal type vacuum pump

Claims (3)

Gas is absorbed in waste water containing volatile organic compounds, and then this waste water is placed in a deaeration container maintained at a reduced pressure below atmospheric pressure, below the packed bed provided in the deaeration container or in a lower part thereof. By introducing more and discharging from the water surface at the upper part or higher part of the packed bed, the liquid is appropriately stored at a liquid depth, and the pooled waste water is a part of the water below the upper surface of the packed bed that is lower than the upper surface of the packed bed. And separating the volatile organic compound in the wastewater, wherein the gas is deaerated and foamed to generate gas bubbles. In the description of claim 1, the waste water discharged from the deaeration container is supplied into an evaporator maintained at a reduced pressure of atmospheric pressure or less, and is stored at an appropriate liquid depth, and then discharged from the water surface. the pooled effluent, a method of separating volatile organic compounds in the waste water, characterized in that a vapor bubble in water at portions under the water surface boils and evaporation to occur. The waste water discharged from the evaporator according to claim 2, wherein the waste water discharged from the evaporator is introduced into a stripping container, where the waste water is brought into direct contact with the steam generated in the evaporator. For separating the organic compound.

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