JP2013075973A - Cleaning agent for hardly soluble fouling, centrifugal thin-film dryer, and method of cleaning the centrifugal thin-film dryer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for cleaning a hardly soluble fouling, which successfully cleans the scales of calcium salt attached to a metal surface of an apparatus, piping or the like.SOLUTION: A cleaning agent for the hardly soluble fouling contains, as a component, at least formic acid, and cleans the scales of the calcium salt attached to the metal surface. A supply tank 31 for supplying the cleaning agent is connected to the centrifugal thin-film dryer 20. For example, when a waste liquid to be dried is a suspension liquid consisting mainly of calcium borate, the concentration of the formic acid in the cleaning agent is adjusted to 0.5 to 10 wt.%, preferably to 1 to 5 wt.%.

Description

  The present invention relates to a technique for cleaning a metal surface to which a calcium salt scale that is hardly soluble is attached.

  Scales adhering to metal surfaces such as boilers and heat exchangers cause a decrease in the flow rate of a heat medium or the like, a deterioration in thermal conductivity, and the like, leading to a reduction in device performance and failure. In order to maintain the performance of the equipment, it is necessary to periodically clean these deposits, and a cleaning technique that can be performed in a short time without damaging the equipment material is required.

In many cases, the main component of the scale to be removed in general industrial equipment is iron oxide or calcium carbonate.
In nuclear power facilities, for example, in a pressurized water reactor (PWR), a waste liquid containing boric acid is generated, and a centrifugal thin film dryer is used to dry and pulverize the waste liquid. Since a treatment for adding calcium to the waste liquid to insolubilize boric acid is performed, a scale mainly composed of calcium borate adheres to the centrifugal thin film dryer.

  Thus, the calcium salt scale adheres to the metal surface of equipment such as boilers, heat exchangers, and centrifugal thin film dryers. These scales not only reduce the heat transfer performance of these devices, but for devices with rotating bodies, the scales adhere to the weight, which deteriorates the weight balance of the rotating bodies and increases the rotational resistance. Vibrations and noises are generated in the equipment, the motor load increases, and the stable operation is hindered.

For this reason, many techniques have been studied so far to effectively remove the attached scale without corroding the metal surface of the device.
With respect to the centrifugal thin film dryer, a scale removing technique is disclosed in which a step of dissolving and washing the deposit by spraying hot water on the axially central portion where the powder adheres most during the rated operation period is disclosed (for example, patents). References 1, 2)
A technique for removing scales of calcium salts by spraying high-pressure water with respect to an evaporator is disclosed (for example, Patent Document 3).

Regarding a cleaning agent for hardly soluble deposits, a technique is disclosed in which an isoprene sulfonic acid-based agent is applied to the removal of calcium carbonate scale (for example, Patent Document 4).
Further, as an acidic detergent not belonging to a strong acid, a mixture of glycine-type amphoteric surfactant, lactic acid, nonionic surfactant, hydroxyalkyl cellulose, xanthan gum and the like is disclosed (for example, Patent Document 5), in addition, phosphonic acid, A mixture of an organic acid having two or more carboxyl groups and a chelating agent (for example, Patent Document 6) and a mixture of an organic acid citric acid and an inorganic acid sulfamic acid (for example, Patent Document 7) are disclosed.

Japanese Patent Publication No.8-24802 Japanese Patent Publication No. 6-79641 JP-A-6-269602 JP-A-6-63590 JP 2010-84087 A JP-A-6-154790 JP 2006-305509 A

However, the cleaning methods disclosed in Patent Documents 1 to 3 described above are effective when the deposit shows high solubility in hot water or cold water, but if the solubility is low, the removal effect of the scale is insufficient. Become.
In the case of centrifugal thin film dryers used in nuclear facilities, the cleaning method described above is implemented from the standpoint of maintaining performance during rated operation, reducing the radiation exposure received by workers from residual deposits during equipment overhaul. Is not considered.

The cleaning agents disclosed in Patent Documents 4 to 6 described above contain inorganic compounds that cannot be decomposed, difficult-to-decompose chelating agents, etc., generate secondary waste by waste liquid treatment, or are released to the environment. There is concern that it may adversely affect the situation.
In the case of centrifugal thin film dryers used in nuclear facilities, we want to avoid an increase in radioactive secondary waste. In addition, when solidifying waste with concrete, the chelating agent may adversely affect the trapping property of the nuclide, so it is desirable to avoid the use of a cleaning agent containing the chelating agent.

  The present invention was made in consideration of such circumstances, and an object of the present invention is to provide a technique for cleaning hardly soluble deposits that satisfactorily clean scales of calcium salts adhering to metal surfaces such as equipment and piping. To do.

The cleaning agent for hardly soluble deposits of the present invention is characterized by cleaning scales of calcium salts having at least formic acid as a component and adhering to a metal surface.
In the centrifugal thin film dryer, a supply tank for supplying the cleaning agent is connected.

  ADVANTAGE OF THE INVENTION By this invention, the washing | cleaning technique of the hardly soluble deposit | attachment which wash | cleans the scale of the calcium salt adhering to metal surfaces, such as an apparatus and piping, favorably is provided.

The flowchart which shows embodiment of the washing | cleaning method of the centrifugal thin film dryer which concerns on this invention. Schematic which shows embodiment of the drying process which pulverizes waste liquid in the centrifugal thin film dryer which concerns on this invention. Schematic which shows embodiment of the simple washing | cleaning process in a rated operation period in the centrifugal thin film dryer which concerns on this invention. Schematic which shows embodiment of the thorough washing process implemented prior to a decomposition | disassembly process in the centrifugal thin film dryer which concerns on this invention. Schematic which shows other embodiment of a thorough washing process. The comparative experiment graph for demonstrating the effect of embodiment of the cleaning agent of the hardly soluble deposit | attachment which concerns on this invention. The comparative experiment graph for demonstrating the effect of embodiment of the cleaning agent of the hardly soluble deposit | attachment which concerns on this invention. The table which shows the dissolution rate of the scale of the calcium salt with respect to the initial concentration of a formic acid solvent, and the hydrogen ion concentration of the solution of the produced calcium salt. The graph which shows the dissolution rate of the scale with respect to the molar ratio of the formic acid contained in a cleaning agent, and the calcium contained in a scale. The graph which shows the dissolution rate of the scale with respect to the temperature of a formic acid solvent.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
As shown in FIG. 1, the centrifugal thin film dryer cleaning method according to the embodiment includes a powdering step (S11) in which waste liquid is charged into the centrifugal thin film dryer and pulverized. A step of simply washing the centrifugal thin film dryer (S12, S13; NO) and a step of thoroughly washing the centrifugal thin film dryer with a cleaning agent (S13; YES, S14) prior to the decomposition step (S15) are included.
And after completion | finish of a thorough washing process (S14), a centrifugal thin film dryer is disassembled (S15), and inspection processes, such as inspection of a component, repair, exchange, and assembly, are carried out (S15, S16).

The waste liquid that is the subject of the drying process is, for example, a poorly soluble solution in which calcium hydroxide is added to sodium borate produced by neutralizing boric acid waste liquid discharged from a pressurized water reactor (PWR) with sodium hydroxide. It is a suspension mainly composed of calcium borate (Ca (BO ₂ ) ₂ ).

A drying process (FIG. 1; S11) is demonstrated based on FIG.
The waste liquid supplied from the waste liquid supply unit 12 is input to the input port 21 of the centrifugal thin film dryer 20 through the valve 14. The waste liquid thrown into the centrifugal thin film dryer 20 is heated while descending while wetting the heat transfer surface 23, the liquid in the waste liquid evaporates, and the solid content adheres to the heat transfer surface 23.
The heat transfer surface 23 is continuously supplied with heat as the heated steam 17 flows from the inlet 24 and is discharged from the outlet 25.

A rotary blade 27 is provided on the rotary shaft 26 that rotates at the center of the centrifugal thin film dryer 20. Since the interval between the rotary blade 27 and the heat transfer surface 23 is about 0.5 mm, the solid content (Ca (BO ₂ ) ₂ ) adhering to the heat transfer surface 23 is scraped off by the rotary blade 27. .
The solid content thus scraped off becomes a dry powder and is discharged from the lower discharge port 22 of the centrifugal thin film dryer 20 and is accumulated in the powder accumulation unit 42 via the valve 16. The dry powder accumulated in the powder accumulation unit 42 is solidified with concrete as radioactive waste.

  Further, the vapor obtained by evaporating the liquid in the waste liquid is discharged from the upper discharge port 28, becomes a liquid in the condensing unit 15, and is stored in the liquid storage unit 43. The liquid stored in the liquid storage unit 43 can be reused in the cleaning liquid supply unit 13.

The simple cleaning process (FIG. 1; S12) will be described with reference to FIG.
A part of the solid content contained in the waste liquid is not discharged from the lower discharge port 22 but adheres to the rotating shaft 26 and the rotating blade 27, and the amount of adhesion is the progress of the pulverization step (FIG. 1; S11). It grows with it. Then, the stability of the operation of the centrifugal thin film dryer 20 gradually decreases.
Therefore, a step (S12) of simply cleaning the centrifugal thin film dryer with a cleaning liquid (water or warm water warmed with water) is executed at an interval of the powdering step (FIG. 1; S11).

The cleaning liquid supplied from the cleaning liquid supply unit 13 is input to the input port 21 of the centrifugal thin film dryer 20 through the valve 14. The cleaning liquid charged into the centrifugal thin film dryer 20 wets the heat transfer surface 23, the rotary shaft 26, and the rotary blade 27, and peels off the attached solid content.
The peeled solid content is discharged from the lower discharge port 22 of the centrifugal thin film dryer 20 together with the cleaning liquid, and is stored in the cleaning / draining liquid storage unit 41 via the valve 16.
The accumulation in the washing and draining liquid storage unit 41 is fed back to the waste liquid supply unit 12 and reprocessed by the centrifugal thin film dryer 20 in the next powdering step (S11).

The thorough cleaning process (FIG. 1; S14) will be described based on FIG.
When the rated operation of the centrifugal thin film dryer 20 is completed, a periodic inspection of the centrifugal thin film dryer 20 is performed next. On the surfaces of the rotary shaft 26 and the rotary blades 27 of the centrifugal thin film dryer 20 immediately before the periodic inspection, solids that cannot be removed by the simple cleaning process (FIG. 3) are adhered as scales.
Since the periodic inspection involves the work of disassembling and assembling the equipment (FIG. 1; S15), the scale attached to the surface of the rotary shaft 26 and the rotary blade 27 is thoroughly cleaned in order to reduce the exposure dose of the worker as much as possible. . The thorough cleaning is a cleaning using a cleaning agent having a higher ability to dissolve the calcium salt scale than the cleaning liquid used for the simple cleaning described above.

  In the thorough cleaning process (S14), the cleaning agent circulates in a circulation path 30 having both ends connected to the upper and lower parts (the inlet 21 and the lower outlet 22 in FIG. 1) of the centrifugal thin film dryer 20. This cleaning agent is accumulated in the supply tank 31 and is supplied to the circulation path 30 via the pump 32 and the valve 33.

The cleaning agent contains at least formic acid as a component, and cleans the scale of calcium salt adhering to the metal surfaces of the rotating shaft 26 and the rotating blade 27.
Here, the concentration of the formic acid is adjusted to a range of 0.5 to 10 wt%, preferably 1 to 5 wt%.
If the formic acid concentration of the cleaning agent is less than 0.5 wt%, a large amount of cleaning agent is required to remove the scale adhering to the centrifugal thin film dryer 20 and the processing load increases. When the formic acid concentration of the cleaning agent exceeds 10 wt%, the working environment is deteriorated due to volatilization.

The composition of the cleaning agent used includes not only the aqueous solution of formic acid alone but also the mixed acid solvent of at least one of lactic acid, citric acid, ascorbic acid and gluconic acid and formic acid. These organic acids have high scale dissolving ability without corroding the metal surface, and can be easily decomposed after use.
The amount of the cleaning agent used is desirably set so that the molar ratio of the organic acid is twice or more with respect to the assumed amount of calcium in the scale attached to the metal surface.

The circulation path 30 includes a pump 34 for circulating the cleaning agent, a pH detection unit 35 for detecting the hydrogen ion concentration of the cleaning agent, a filter 36 for filtering solid matter in the circulating cleaning agent, and a cleaning agent. A heating unit 37 for heating and setting the temperature is provided.
Since the solubility of the calcium salt scale increases as the temperature of the aqueous formic acid solution increases, the temperature of the cleaning agent is preferably set to 60 ° C. or higher.

In addition, as the dissolution of the calcium salt scale progresses, the formic acid concentration of the cleaning agent decreases, so that the progress of scale removal can be grasped by monitoring the hydrogen ion concentration detected by the pH detector 35. Can do.
Further, based on the detection value of the pH detection unit 35, formic acid (organic acid) can be newly introduced to control the cleaning agent to an appropriate concentration.

Another embodiment of the thorough cleaning process (FIG. 1; S14) will be described based on FIG.
FIG. 4 shows a thorough cleaning process in which the cleaning agent is circulated. FIG. 5 shows a thorough cleaning process in which the cleaning agent flows in one pass.
In this case, it is desirable that the inside of the centrifugal thin film dryer 20 is filled with a cleaning agent, and the rotating shaft 26 and the rotating blades 27 are intermittently rotated at a low speed in a state where the rotating shaft 26 and the rotating blades 27 are attached.
Accordingly, the cleaning agent is brought into fluid contact with the metal surface to which the scale is attached, and the same cleaning effect as that obtained when the cleaning agent is circulated can be obtained.

Next, examples of the beaker level in which the effects of the present invention have been confirmed were described. A scale sample was prepared as follows. First, a boric acid solution was neutralized with sodium hydroxide to form sodium borate, and calcium hydroxide was further added to prepare a simulated waste liquid mainly composed of calcium borate Ca (BO ₂ ) ₂ .
The prepared simulated waste liquid was put into a mock-up of a centrifugal thin film dryer and a drying process was performed, and scale was adhered to the metal surface. Then, the adhered scale was peeled off to obtain an experimental scale sample.

Formic acid (Example 1), sulfamic acid (Comparative Example 1), EDTA · 2Na [ethylenediaminetetraacetic acid disodium salt] (Comparative Example 2), 3-hydroxy-2, Tetrasodium 2'-iminodisuccinate (Comparative Example 3) was used.
100 mL of these various solvents were set to 60 ° C., and stirred with a stirrer for 10 hours together with 3 g of the scale sample. FIG. 6 shows the results of measuring the calcium concentration of the solution sampled through a syringe filter having a pore diameter of 0.45 μm after completion of stirring and calculating the dissolution rate of the scale sample.

The solvent selected in FIG. 6 is formic acid as an organic acid, sulfamic acid as an inorganic acid, EDTA · 2Na as a chelating agent (ethylenediaminetetraacetic acid disodium salt), biodegradation, among compounds having solubility in calcium salts. 4-hydroxy-2,2′-iminodisuccinic acid tetrasodium was selected as the sex chelating agent.
From the result of FIG. 6, it can be said that formic acid, which is an organic acid, has the best solubility of the calcium salt scale.

Since organic acids can be decomposed into carbon dioxide and water by an oxidizing agent such as hydrogen peroxide, secondary waste due to waste liquid treatment does not occur even when used at high concentrations. Therefore, a dissolution test of the scale sample was performed using a higher concentration organic acid as a solvent.
Formic acid (Example 2), citric acid (Comparative Example 4), and L-ascorbic acid (Comparative Example 5) each having a concentration set to 2.5 wt% were used as organic acid solvents, and an aqueous solvent was also used as a reference example.
FIG. 7 shows the results of calculating the dissolution rate by dissolving the scale sample in these various solvents under the same conditions as in FIG.

The solution selected in FIG. 7 is an organic acid having a solubility for calcium salts. Oxalic acid is generally used as an organic acid as a decontamination agent for nuclear facilities, but oxalic acid reacts with calcium to produce calcium oxalate with low solubility, so detergency cannot be expected and excluded from consideration. .
From the results of FIG. 7, formic acid was most excellent in the dissolving ability of the calcium salt scale, and it was confirmed that the entire scale sample was dissolved.

FIG. 8 shows the dissolution rate of the calcium salt scale with respect to the initial concentration of the formic acid solvent, and the hydrogen ion concentration (pH) of the solution of the produced calcium salt. The experimental conditions are the same as those in FIGS.
FIG. 8 shows that formic acid is consumed as dissolution of the calcium salt scale progresses, and the pH of the solution shifts to the alkali side as the amount of formic acid required to dissolve all the scale is insufficient. I understand.
Thus, by monitoring the pH of the calcium salt solution (cleaning agent), it is possible to grasp the excess or deficiency of the formic acid amount with respect to the dissolution reaction of the scale. Then, the amount of formic acid is appropriately controlled based on this excess / deficiency information, and the centrifugal thin film dryer can be efficiently cleaned.

FIG. 9 is a graph showing the dissolution rate of the scale with respect to the molar ratio of formic acid contained in the cleaning agent and calcium contained in the scale.
From FIG. 9, it was confirmed that when the molar ratio of formic acid / calcium was 2 or more, the entire amount of the scale sample put into the beaker was dissolved. From this, it is possible to dissolve the whole scale by using a cleaning agent containing formic acid at a molar ratio of 2 times or more with respect to the calcium equivalent of the scale attached to the device.
However, even if the entire amount of the scale is not dissolved, it will be removed from the surface of the device if it is dissolved to some extent and its structure is broken, so that it is not necessary that the molar ratio is twice or more depending on circumstances.

FIG. 10 is a graph showing the dissolution rate of the calcium salt scale with respect to the temperature of the formic acid solvent. In the experiment, 3 g of the scale sample was immersed for 90 minutes in each of 100 mL of 2.5 wt% formic acid solvent set at 23 ° C., 45 ° C., and 60 ° C. with and without liquid flow by stirring. It was.
As a result, even in the presence of a sufficient amount of formic acid to completely dissolve the scale sample, it was confirmed that the dissolution rate tends to increase as the temperature rises under no-flow conditions. Furthermore, in the condition with flow, dissolution of the entire scale sample was confirmed without depending on the temperature.

The reason why the temperature dependence of the dissolution rate was observed under the condition of no flow was considered to be due to the fact that the solubility and the dissolution rate were improved by increasing the temperature, and the flow was generated by thermal convection. On the other hand, under the condition with flow, it is considered that the dissolution reaction is accelerated because the scale comes into contact with a fresh cleaning agent (has few impurities), and the entire amount is rapidly dissolved.
From this, it was demonstrated that dissolution of the scale is promoted by flowing or warming the cleaning agent. From these results, in the centrifugal thin film dryer, sufficient cleaning performance can be obtained without heating the cleaning agent by circulating the cleaning liquid by a pump or rotating the rotating shaft at a low speed. Moreover, even if it is not made to flow a washing | cleaning liquid, a washability can be improved by heating to 60 degreeC or more.

  According to the cleaning agent for hardly soluble deposits of at least one embodiment described above, the scale of calcium salt can be satisfactorily cleaned without corroding the metal surface. The attached radioactive scale can be thoroughly removed to reduce the exposure of inspection workers.

In the embodiment, the formic acid solvent cleaning agent is used in the thorough cleaning process of the centrifugal thin film dryer. However, the formic acid solvent cleaning agent can also be used in the simple cleaning process.
In the embodiment, the method for cleaning the scale attached to the centrifugal thin film dryer has been exemplified. However, the application object of the invention is not limited to the centrifugal thin film dryer, and is applied to general industrial equipment such as a boiler and a heat exchanger. be able to.
In addition, the scope of protection of the present invention also extends as a cleaning agent for cleaning the scale of calcium salt without specifically limiting the application target.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 12 ... Waste liquid supply part, 13 ... Cleaning liquid supply part, 15 ... Condensing part, 17 ... Heated steam, 20 ... Centrifugal thin film dryer, 21 ... Input port, 22 ... Lower discharge port, 23 ... Heat-transfer surface, 26 ... Rotating shaft , 27 ... Rotating blade, 28 ... Upper discharge port, 30 ... Circulation path, 31 ... Supply tank, 35 ... pH detection part, 36 ... Filter, 37 ... Heating part, 41 ... Washing waste liquid storage part, 42 ... Powder storage Part, 43... Liquid accumulation part.

Claims (7)

  A cleaning agent for hardly soluble deposits, characterized by cleaning scales of calcium salts having at least formic acid as a component and adhering to a metal surface.   A centrifugal thin film dryer, to which a supply tank for supplying the cleaning agent according to claim 1 is connected. The centrifugal thin film dryer according to claim 2,
The centrifugal thin film dryer is characterized in that the detergent has a formic acid concentration adjusted to a range of 0.5 to 10 wt%. In the centrifugal thin film dryer according to claim 2 or 3,
A centrifugal thin film dryer comprising a circulation path for circulating the cleaning agent. The centrifugal thin film dryer according to any one of claims 2 to 4,
A centrifugal thin film dryer comprising a heating unit for heating the cleaning agent. In the centrifugal thin film dryer according to claim 4 or 5,
A centrifugal thin film dryer comprising a pH detection unit for detecting a hydrogen ion concentration of the circulating cleaning agent. A pulverization step of charging the waste liquid into the centrifugal thin film dryer according to any one of claims 2 to 6 and pulverizing it,
A step of simply washing the centrifugal thin film dryer in the interval of the powdering step;
And a step of thoroughly cleaning the centrifugal thin film dryer with the cleaning agent prior to the decomposition step.

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