JPH0785797B2 - Condensate filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a hollow fiber module installed in a thermal power plant, an electronic power plant or the like for removing a clad due to iron oxide contained in condensate. The present invention relates to a filtering device used.

<Prior art> As a condensate treatment device for thermal power plants and nuclear power plants,
In recent years, in a filtration tower using a hollow fiber module, first, the clad caused by iron oxide fine particles in the condensate is removed by filtration, and then the filtered water is a mixed-bed desalting tower of a cation exchange resin and an anion exchange resin. There has been proposed a condensate treatment device of a type in which impurity ions such as dissolved Fe ²⁺ ions are removed by treatment with. FIG. 2 shows a flow of power generation equipment of a boiling water (BWR) nuclear power plant including such a condensate treatment device, and steam generated in the BWR reactor 1 is a turbine. It is sent to the condenser 3 via 2 and condensed in the condenser 3 to be condensed water. During the condensate,
Since clad and trace amount of impurity ions due to iron oxide fine particles caused by corrosion of the turbine 2 or piping are contained, they are passed through the condensate pump 4 to the filtration tower 5 using the hollow fiber module. First, the clad in the condensate is removed by feeding, and then the filtered water is fed to the mixed bed type desalting tower 6 of the cation exchange resin and the anion exchange resin to remove the impurity ions. The condensate from which the clad and the impurity ions have been removed is injected with a predetermined amount of oxygen by an oxygen injection device 7 including, for example, an oxygen cylinder and a regulator, and then circulated through the heater 8 to the BWR reactor 1 again. The reason for injecting oxygen into the condensate treated in the mixed bed desalination tower 6 is to suppress the above-mentioned corrosion of the heater 8 or the pipes. That is, oxygen is originally the heater 8
Since it promotes corrosion of carbon steel and stainless steel, which are materials such as pipes and pipes, in the condensate of the power plant,
It is necessary to minimize the dissolved oxygen in the condensate,
On the contrary, when the dissolved oxygen concentration becomes too low, corrosion is accelerated. Figure 3 shows the relationship between the corrosion rate of carbon steel and the dissolved oxygen concentration. As can be seen, when the dissolved oxygen concentration exceeds approximately 0.7 ppm, the corrosion rate is significantly high.
It is natural that it is smaller than that, but when the dissolved oxygen concentration becomes extremely low such as approximately 0.02 ppm (20 ppb) or less, the corrosion rate increases again. However, in the above-mentioned BWR type nuclear power plant, since the steam sent to the condenser 3 via the turbine 2 is condensed under reduced pressure to form condensate, the dissolved oxygen concentration in the condensate is concerned. Is 2
It will be degassed to an extremely low concentration of ~ 10 ppb. That is, as can be seen from FIG. 3, this concentration is not preferable because it belongs to the region where the corrosion rate is high. Therefore, as described above, the dissolved oxygen concentration in the condensate falls within the region where the corrosion rate is low in FIG. Thus, a predetermined amount of oxygen is supplied to the condensate.

The filtration tower 5 using the hollow fiber module used in the condensate treatment apparatus as described above will be described in detail.
01μ to 1μ or a large number of fine pores with a molecular weight cutoff of 500 to 1,000,000 and an outer diameter of 0.3 to 2 mm, an inner diameter
A large number of hollow fibers of 0.2 to 1.5 mm are bundled to form a hollow fiber module, and one side of each hollow fiber is arranged from the outside to the inside or from the inside to the outside of the hollow fiber. To pass the condensate to the other side to filter the iron oxide fine particles on one side of each hollow fiber, and to collect the filtered water obtained from the other side to flow out from the filtration tower 5. Passing condensate from the outside to the inside of each hollow fiber,
So-called external pressure type filtration is the mainstream.

Further, when the differential pressure in the filtration tower 5 increases due to the continuation of such filtration, the raw water side membrane surface of the hollow fiber to which iron oxide is attached is bubbled with a gas such as air, or the hollow fiber is blown. The iron oxide is peeled from the hollow fiber by backflowing a fluid such as gas or water from the other side to the one side of the hollow fiber, and a cleaning drainage solution containing a large amount of iron oxide is obtained. Is sequentially repeated to perform processing.

As described above, since the filter tower using the hollow fiber module directly filters the condensate with each hollow fiber, it is compared with the pre-coated filter tower using the filter aid such as the conventional fine powder ion exchange resin. In addition, the amount of solids contained in the cleaning effluent is extremely small, and it is particularly suitable for removing iron oxide containing radioactive substances like condensate of a boiling water nuclear power plant.

That is, in the case of a precoating type filtration tower, the cleaning wastewater containing an overwhelmingly large amount of the used precoating agent than the iron oxide is discharged together with the iron oxide removed by filtration during the cleaning. Since the cleaning drainage in the filtration tower to be used does not contain such a used precoating agent at all, it has an advantage that the solid matter to be treated for radioactive waste can be significantly reduced.

<Problems to be Solved by the Invention> When the condensate treatment is performed for a relatively long time in the filtration tower using the hollow fiber module as described above, even if the cleaning is performed using the gas or water, the differential pressure is increased. May not return to the original state. This is particularly remarkable in a filtration tower using an external pressure type filtration type hollow fiber module for filtering condensed water from the outside to the inside of the hollow fiber.

Therefore, when an external pressure type filtration type hollow fiber module which has reached such a state was investigated, iron oxide was strongly adhered to the inside of the hollow fiber which should not originally adhere, that is, the membrane surface on the filtered water side. Therefore, it became clear that the inside of the hollow fiber, which is the flow path of the filtered water, was narrowed to increase the differential pressure.

The reason for this is not clear at present, but it is presumed as follows.

That is, the clad contained in the condensate contains Fe ₃ O ₄ and Fe.
_It consists of iron oxides in the form of ₂ O ₃ , FeOOH, etc., and their composition is not constant and difficult to grasp, but in general, during steady operation of power generation equipment (so-called hot water circulation), Fe ₃ O ₄ (magnetite) Will increase. In magnetite, Fe ²⁺ and Fe ³⁺ are 1: 2
In the condensate under a condition where the pH is neutral and the dissolved oxygen concentration is extremely low due to degassing in the condenser, the Fe ^{2+ in the} magnetite is extremely high. It becomes easy to dissolve. As a result, in the condensate after passing through the condenser,
Fe ²⁺ ions are present in relatively large numbers. Further, as is clear from the above explanation, the condensate that has passed through the condenser is highly corrosive because of its low dissolved oxygen concentration, and therefore promotes the corrosion of piping etc. between the condenser and the filtration tower. However, this point also causes an increase in Fe ²⁺ ions in the condensate supplied to the filtration tower. That is, in FIG. 2, during the condensate fed to the filtration tower 5 via the condenser 3 and the condensate pump 4,
Not only iron oxide fine particles but also Fe ²⁺ ions are relatively contained in a large amount.

When such condensate is filtered through a module composed of hollow fibers having fine pores with a pore size of 0.01 to 0.1 μ, fine particles of iron oxide are captured outside the hollow fibers.
^{The 2+} ions pass through the membrane, reach the inside of the hollow fiber, and remain in the filtered condensate. On the other hand, since dissolved oxygen exists in the filtered condensate water (usually 2 to 10 ppb) as described above, it oxidizes a part of Fe ²⁺ ions that have passed inside the hollow fiber. It is thought that Fe ³⁺ ions are generated. Unlike Fe ²⁺ ions, Fe ³⁺ ions have a very low solubility, so they are immediately precipitated as iron oxides such as FeOOH. Although it is considered that the dissolved oxygen concentration in the condensate does not change before and after passing through the hollow fiber membrane, why the Fe ²⁺ ion is oxidized to Fe ³⁺ ion after passing through the hollow fiber membrane. Although it is not clear whether or not the oxidation reaction is promoted when the condensate passes through the hollow fiber membrane, for example.

It is presumed that the iron oxide deposited as described above adheres to the membrane surface inside the hollow fiber and gradually accumulates inside the hollow fiber to increase the differential pressure. Regarding the adhesion of the precipitated iron oxide to the inner surface of the hollow fiber membrane, the following may be considered. That is, a fluid such as condensed water having a very low content of impurities and therefore a large specific resistance flows at a relatively high flow rate inside the hollow fiber having a very small inner diameter, and static electricity due to friction is accumulated on the membrane surface. Therefore, it is presumed that the adhesion of the precipitated iron oxide to the film surface may become stronger.

The above is presumed to be the reason why the iron oxide adheres strongly to the inner surface of the external pressure type filtration type hollow fiber, that is, the membrane surface on the filtered water side. Since it is inside the hollow fiber, it cannot be removed even by performing the above-mentioned cleaning using gas or water.

Therefore, the hollow fiber module cannot be used again unless the iron oxide is removed by washing the hollow fiber module in which the inner membrane surface of the hollow fiber is contaminated with iron oxide with a suitable cleaning chemical.

A hollow fiber module contaminated with such iron oxide is treated with a reducing agent solution such as hydrosulfite or an oxalic acid solution, a reducing acid such as a citric acid solution, a chelating acid, or a chemical solution such as hydrochloric acid. By washing, even iron oxide strongly adhered to the film surface can be dissolved and removed.

However, in filtration etc. using a hollow fiber module for removal of iron oxide containing radioactive substances such as condensate of boiling water nuclear power plant, cleaning drainage with the aforementioned chemical liquid is subject to radioactive waste treatment, The reducing agent contained in the cleaning drainage, the salt produced by neutralizing the acid, and the like are not preferable in that they increase the solid matter in the radioactive waste treatment.

The present invention has been made in view of such a problem in a filtration tower using a hollow fiber module, and provides a filtration device having means for preventing iron contamination on the membrane surface of the hollow fiber on the filtered water side. The purpose is.

<Means for Solving the Problems> The present invention is based on the above-described reasoning about the reason why iron oxide adheres to the membrane surface on the filtered water side of the hollow fiber in the filtration tower using the hollow fiber module. It is a thing. That is, by injecting an oxidizing agent into the ascites before being supplied to the filtration tower, the Fe ²⁺ ions dissolved in the condensate are pre-deposited by oxidation, and the condensate is supplied to the filtration tower.
The present invention has been made based on the idea that the precipitation of iron oxide on the membrane surface of the hollow fiber on the filtered water side can be prevented if Fe ²⁺ ions are not present.

The present invention is based on this idea, a filtration tower using a hollow fiber module for treating condensate containing iron oxide,
A condensate filtering device comprising an oxidant adding means for adding an oxidant to the condensate, which is attached in the middle of a condensate inflow pipe connected to the filtration tower.

Examples of the oxidizing agent added to the condensate in the present invention include oxygen (including oxygen-containing gas such as air), ozone, hydrogen peroxide, chlorine, sodium hypochlorite, etc. Is preferably one that does not increase the ion load of the condensate demineralization tower installed in the latter stage of the filtration tower, and in this respect, oxygen, ozone, and hydrogen peroxide are particularly suitable. Usually, these may be used alone, but two or more of them may be used at the same time without any problem.

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is an explanatory view of a flow showing an example of an embodiment of the present invention. In a filtration tower 5 shown in this embodiment, a condensate inflow pipe 9 is connected to the lower part thereof and a filtered water outflow pipe 10 is connected to the upper part thereof.
A partition plate 11 is horizontally installed in the filtration tower 5 connected to the above, and a large number of hollow fiber modules 12 are vertically installed on the partition plate 11, which is conventionally known. The hollow fiber module 12
As disclosed in Japanese Patent Application No. 59-273579 previously disclosed by the applicant of the present application, a large number of hollow fibers are bundled by opening the upper end of each hollow fiber and closing the lower end thereof. , Also has a skirt portion 13 at the lower end of each hollow fiber module 12,
Distributor 14 when cleaning the module 12
Bubbles generated from the skirt are floated on the skirt portion 13. In addition, the lower end of the module 12 is provided with a through hole for allowing the bubbles received by the skirt portion 13 to pass to the outer surface of each hollow fiber.

In the configuration as described above, the feature of the present invention resides in that, in the middle of the condensate inflow pipe 9 connected to the filtration tower 5 using the hollow fiber module 12, an oxidizer is added to the condensate flowing in the inflow pipe 9. This is because an oxidant adding means 15 for adding is added, and in FIG. 1, the condensate containing iron oxide is provided in the filter tower 5 and the inflow pipe 9 in the middle thereof. Between the condensate inflow pipe 9 connected to the condensate pump 4 for supplying, an oxidant adding means 15 is attached so as to inject the oxidant into the inflow pipe 9. Needless to say, the oxidizing agent adding means 15 may be attached to the condensate inflow pipe 9 on the suction side of the condensate pump 4. Further, 16 is a reaction tank provided in communication with the condensate inflow pipe 9 on the downstream side of the oxidizing agent adding means 15, and the oxidizing agent added into the condensate by the oxidizing agent adding means 15 and Fe ²⁺ ions in the condensate It is installed in order to complete the reaction with. Although the capacity of the reaction tank 16 varies depending on the oxidizing agent used, the capacity may be such that the residence time is generally about 1 to 5 minutes. It is not necessary to install the reaction tank 16 if the length of the inflow pipe 9 can be configured so as to have the above-mentioned time in the condensate inflow pipe 9 from the position where the oxidant is added to the filtration tower 5. Absent. When a peroxide such as ozone or hydrogen peroxide is used as an oxidant, such a reaction time allows a very small amount to remain in the condensate after reacting these oxidants. It is convenient because the decomposition of the peroxide is also performed.

<Operation> The operation of the device of the present invention will be described below.

In the present invention, when the condensate containing iron oxide is filtered, first, a predetermined amount of oxidizing agent is added to the condensate sent from the condensate pump 4 through the condensate inflow pipe 9 by using the oxidizing agent adding means 15. Then, the condensate containing the oxidizing agent is fed to the reaction tank 16, and Fe ²⁺ ions in the condensate are sufficiently oxidized in the reaction tank 16 to be precipitated as iron oxide, and then the lower part of the filtration tower 5 is supplied. It is made to flow into the said filtration tower 5 through the condensate inflow pipe 9 connected to.

The oxidizing agent adding means 15 used in the present invention may be one that is generally used. For example, when a gas such as oxygen or ozone is used as the oxidizing agent, it can be obtained by an oxygen cylinder or an ozone generator. These gas bodies are supplied to the inflow pipe 9
It may be configured such that the gas is directly injected into the inlet pipe 9 or directly after being dissolved in pure water or the like. Further, it may be configured such that an oxidation tank having the gas body diffuser therein is provided in communication with the condensate inflow pipe 9. Further, if the oxidant is one that is usually used as an aqueous solution, such as hydrogen peroxide, the pump may be used to inject the oxidant into the inflow pipe 9.

Condensed water that has flowed into the filtration tower 5 is used for each hollow fiber module 12
Each hollow fiber module 12 from the through hole or the side of
Introduced in, the iron oxide is filtered on the outside of each hollow fiber, the filtered water obtained on the inside of each hollow fiber is collected above the partition plate 11,
Further, it is made to flow out from the filtered water outflow pipe 10 and fed to a mixed bed type demineralization tower (not shown) in the subsequent stage.

In the present invention, as described above, the oxidizing agent is added to the condensate before being supplied to the filtration tower 5 using the hollow fiber module 12,
The Fe ²⁺ ion in the condensate is pre-oxidized and precipitated, and the condensate containing almost no Fe ²⁺ ion flows into the filtration tower 5, whereby the inside of the hollow fiber, that is, the membrane on the filtered water side. It is intended to prevent iron oxide from depositing and adhering to the surface, and the reason for reaching such an idea is as already inferred. In fact, as shown in Examples below, iron contamination on the membrane surface of the hollow fiber on the filtered water side can be reliably prevented in the present invention.

When the pressure loss increases due to the continuation of the filtration as described above, the following washing is performed.

That is, the flow of water is interrupted, while the inflowing condensate is filled in the lower part of the partition plate 11 of the filtration tower 5 and the filtered water is filled in the upper part of the partition plate 11 from the air introduction pipe 17A attached to the lower part of the filtration tower 5. To introduce. The air rises in the form of bubbles from the distributor 14, and the raised bubbles flow into the inside of each hollow fiber module 12 from the through-hole provided at the lower end via the skirt portion 13 of each hollow fiber module 12 described above. , Each hollow fiber is vibrated, and the iron oxide adhering to the outer membrane surface of each hollow fiber is peeled off. Note that the air bubbles are in each hollow fiber module.
It is discharged from the upper part of 12 and then discharged from an air vent pipe 18 attached to the upper part of the filtration tower 5.

After sufficiently vibrating the air bubbles of each hollow fiber as described above, air is press-fitted from the air introducing pipe 17B branched and connected to the filtered water outflow pipe 10, and the filtered water existing above the partition plate 11 is hollowed out. The washing waste liquid containing a large amount of iron oxide is made to flow back from the inside to the outside of the yarn, is made to flow out from the blow pipe 19 connected to the lower part of the filtration tower 5, and is transferred to the wastewater treatment system.

After performing such washing, the above-mentioned filtration is continued again, and thereafter the filtration and washing are repeated, but in the present invention, as shown in Examples described later, a conventional filtration tower using a hollow fiber module is used. Unlike the case, iron oxide does not adhere to the inner membrane surface of each hollow fiber, while iron oxide that adheres to the outer membrane surface of each hollow fiber is reliably removed by the above-mentioned washing operation, so long Stable processing can be performed.

In addition, when the filter of the present invention is applied to the condensate treatment of a BWR type nuclear power plant as shown in FIG. 2 and oxygen is used as an oxidant, the conventional mixed bed desalination is used. Tower 6
The oxygen injection device 7 (see FIG. 2) installed in the latter stage can be omitted.

Further, in the above-described embodiment, the filtering device using the external pressure type hollow fiber module has been described, but the present invention passes water containing iron oxide through the inside of the hollow fiber to obtain filtered water from the outside, so-called. It goes without saying that the present invention can also be applied to a filtration device using an internal pressure type hollow fiber module.

<Effect> As described above, in the device of the present invention, the deposition and adhesion of iron oxide on the membrane surface on the filtered water side of the hollow fiber, which should not be originally adhered, which is observed in the conventional hollow fiber module, is ensured. Since it can be prevented, iron oxide adhering to the raw water side membrane surface of the hollow fiber can be stably treated for a long period of time only by periodically removing it by an ordinary washing method using water or air.

Further, conventionally, the iron oxide adhered to the film surface could not be removed except by washing with a chemical such as a reducing agent or an acid, but according to the present invention, the adhesion of the iron oxide is prevented. Since it can be prevented in advance, there is no need to perform cleaning with these chemicals. Therefore, the device of the present invention is extremely useful in the case where the cleaning effluent containing the chemical must be treated as radioactive waste, such as in a BWR type nuclear power plant.

<Example> An example will be described below in order to further clarify the effect of the present invention.

An experimental filtration tower filled with one hollow fiber module (total filtration area 0.38 m ² ) that bundles 100 external pressure type polyolefin hollow fibers with an inner diameter of 0.8 mm, an outer diameter of 1.2 mm and a length of 1,000 mm, and an oxygen cylinder. Oxidizing agent adding means comprising an oxygen water adjusting tank for obtaining oxygen water by dissolving oxygen gas from the water in pure water and a metering pump for injecting the oxygen water in the adjusting tank into the condensate inflow pipe of the filtration tower Using the filtration apparatus of the present invention having the flow shown in FIG. 1 equipped with and, a filtration experiment of condensate (dissolved oxygen concentration 2 to 5 ppb) of a power plant containing iron oxide was conducted. In order to accelerate iron contamination of the hollow fiber, the amount of condensate supplied to the hollow fiber module was set to twice the standard value (0.1 m ³ / hr), that is, 0.2 m ³ / hr. Oxygen as an oxidant was injected so that the dissolved oxygen concentration in the condensate would be 50 ppb. Further, in the condensate inflow pipe after the oxygen injection point, a reaction tank having a capacity of about 2 minutes was provided in order to completely ^{carry out} the oxidation reaction of Fe ²⁺ ions.

After the condensate was filtered under the above conditions, the cycle of repeating vibration by bubbles and backwashing of the filtered water and repeating the above filtration was repeated. pressure 0.83kg / cm ² at0.2m ³ / hr, and the most elevated pressure difference as compared to the water passing the initial pressure difference new module was 0.82kg / cm ² at0.2m ³ / hr It was not observed, and it was thought that the filtration treatment could be carried out without problems even if further filtration cycles were continued.

After the above cycle, washing was performed, and then the hollow fiber module was taken out from the filtration tower and disassembled, and the hollow fiber was observed. As a result, a very slight adhesion of iron oxide was observed on the outside of the hollow fiber, that is, on the raw water side. However, no adhesion of iron oxide was found inside the hollow fiber, that is, on the filtered water side.

For comparison, a conventional filtration tower using a hollow fiber module of the same specifications was installed side by side, oxygen was not added to this, and other conditions were all the same. Despite regular cleaning, the differential pressure in the early stages of water flow gradually increased, and in the third cycle the differential pressure in the early stages of water flow was 1.2 kg / cm ^{2 at} 0.2 m ³ / hr. . After the completion of the cycle, washing was performed, and then the hollow fiber module was disassembled in the same manner as above, and the hollow fiber was observed.As a result, in the same way as in the present invention, only a slight amount of iron oxide adhered to the outside of the hollow fiber. On the other hand, it was observed that a considerable amount of iron oxide adhered to the inside of the hollow fiber.

[Brief description of drawings]

FIG. 1 is an explanatory view of a flow showing an example of an embodiment of the present invention, and FIG. 2 is an explanatory view of a flow of power generation equipment of a BWR type nuclear power plant including a filtration tower using a conventional hollow fiber module. FIG. 3 is a graph showing the relationship between the corrosion rate of carbon steel and the dissolved oxygen concentration, in which the vertical axis represents the corrosion rate and the horizontal axis represents the dissolved oxygen concentration. 1 …… BWR type reactor, 2 …… turbine 3 …… condenser, 4 …… condensate pump 5 …… filter tower, 6 …… mixed bed type demineralization tower 7 …… oxygen injection device, 8 …… Heater 9 …… Condensate inflow pipe, 10 …… Filtered water outflow pipe 11 …… Partition plate, 12 …… Hollow fiber module 13 …… Skirt part 14 …… Distributor 15 …… Oxidizer addition means 16 …… Reaction tank, 17 …… Air inlet pipe 18 …… Air vent pipe, 19 …… Blow pipe

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location G21C 19/307

Claims (2)

[Claims] 1. A filtration tower using a hollow fiber module for treating condensate containing iron oxide and a condensate inflow pipe connected to the filtration tower, wherein an oxidizing agent is added to the condensate. A condensate filtration device comprising: 2. The condensate filter according to claim 1, wherein the oxidizing agent is one or more selected from oxygen, ozone and hydrogen peroxide.