Method for detecting bubbles in water and method for adding a degassing agent using the same

Abstract

To provide a method of detecting bubbles in water, which enables appropriate and easy detection of bubbles in water even when the water inside a water pipe contains fibrous substances and bubbles, and to provide a method of adding a degassing agent using the same.SOLUTION: A method of detecting bubbles in water is provided, comprising detecting bubbles in water containing fibrous substances and bubbles by measuring intensity of an ultrasonic wave received by a reception unit of a transmission ultrasonic sensor.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for detecting air bubbles in water containing fibrous substances and air bubbles, and a method for adding a deaerating agent using the same.

Background Art

[0002] Air bubbles generated in the papermaking process and the like in papermaking, particularly air bubbles remaining in water, have an adverse effect on the quality and productivity of paper products. For example, when air bubbles or aggregates formed with air bubbles as nuclei are mixed into the product, a product defect called a bubble spot occurs. In addition, air bubbles in water cause fluctuations in the rotational speed of various pumps, the pressure in the water supply pipe, and the liquid level in the papermaking apparatus, leading to a decrease in productivity due to a decrease in water supply efficiency and a decrease in product quality.

[0003] In order to prevent such adverse effects caused by air bubbles, the air bubbles on the liquid surface are visually observed or the liquid surface height is measured, and an antifoaming agent is added based on this information.

[0004] For example, Patent Document 1 describes that the degree of foaming on the liquid surface is detected by measuring the liquid surface height with an ultrasonic level gauge or a laser distance meter. Further, Patent Document 2 describes that the amount of dissolved gas is measured by pressurizing and depressurizing the inside of the measurement cell and obtaining the relationship between the volume change and the internal pressure.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the method described in Patent Document 1 could not accurately detect bubbles when the liquid level was not uniform, nor could it detect bubbles inside water supply pipes, etc. Furthermore, the method described in Patent Document 2 involves operations to cause fluctuations in the internal pressure within the measurement cell, which makes measurement time-consuming and laborious, and can also make accurate detection difficult due to clogging by fibrous material, etc.

[0007] The present invention was made to solve the above-mentioned problems, and aims to provide a method for detecting bubbles in water and a method for adding a deaeration agent using the same, which can accurately and easily detect bubbles in water even when water containing fibrous material and bubbles is inside a water supply pipe. [Means for solving the problem]

[0008] This invention is based on the discovery that by using a transmissive ultrasonic sensor, it is possible to accurately detect bubbles in water containing fibrous material and bubbles inside a water supply pipe.

[0009] The present invention provides the following means. [1] A method for detecting bubbles in water, which includes fibrous material and bubbles, by measuring the intensity of ultrasonic waves received by the receiver of a transmissive ultrasonic sensor. [2] The method for detecting bubbles in water according to [1], wherein the oscillation frequency of the ultrasonic wave is 0.5 MHz or higher. [3] The water bubble detection method according to [1] or [2], wherein the water containing the fibrous material and bubbles is flowing water, and the intensity of the ultrasonic waves is monitored periodically and continuously. [4] The water containing the fibrous material and bubbles has a sieve residue of 100 mg / L or more, according to any one of [1] to [3], for detecting bubbles in water. [5] The water containing the fibrous substance is an aqueous slurry in the papermaking process, the method for detecting bubbles in water according to any one of [1] to [4]. [6] The water bubble detection method according to any one of [1] to [5], wherein the pressure of the water containing the fibrous material and bubbles at the point where the intensity of the ultrasonic wave is measured is less than 0.15 MPa. [7] The method for detecting bubbles in water according to any one of items [3] to [6], wherein the pressure of the water containing the fibrous material and bubbles at the point where the intensity of the ultrasonic wave is measured is 0.9 to 1.1 times the pressure at the point upstream of the measurement point where the target of monitoring bubbles in water is to be measured. [8] A method for detecting bubbles in water, as described in any of [1] to [7], wherein the ultrasonic reduction rate expressed by the following formula (1) is used as an indicator of the amount of bubbles. Ultrasonic reduction rate [%]=(R0-R1) / R0×100 (1) (In the formula, R0: ultrasonic received intensity measured for water without bubbles) R1: Ultrasonic received intensity measured for water containing fibrous material and air bubbles. (This represents...)

[0010] A method for adding a degassing agent, comprising using the underwater bubble detection method described in any of [9][1] to [7], and controlling the amount of degassing agent added to the water containing the fibrous material and bubbles according to the intensity of the received ultrasonic waves. A method for adding a degassing agent, wherein the amount of degassing agent added to water containing the fibrous material and bubbles is controlled using the water bubble detection method described in

[10] [8], with the ultrasonic reduction rate used as an indicator of the amount of bubbles. [Effects of the Invention]

[0011] According to the underwater bubble detection method of the present invention, underwater bubbles can be accurately and easily detected even when water containing fibrous material and bubbles is present in a water supply pipe. Therefore, the underwater bubble detection method of the present invention is useful in processes such as adding a deaeration agent to raw material pulp slurry or white water in papermaking. [Modes for carrying out the invention]

[0012] The present invention's underwater bubble detection method detects bubbles in water containing fibrous material and other bubbles by measuring the intensity of ultrasonic waves received by the receiver of a transmissive ultrasonic sensor. By using a transmission type ultrasonic sensor, bubbles in water containing fibrous substances and bubbles can be accurately and simply detected.

[0013] The water containing fibrous substances and bubbles is not particularly limited, and examples thereof include aqueous slurries in each process of papermaking that contain bubbles. Examples of the aqueous slurry include raw material pulp slurry, white water, and the like. According to the transmission type ultrasonic sensor, only bubbles can be accurately detected without being affected by fibrous substances such as pulp contained in water or paper fillers.

[0014] The transmission type ultrasonic sensor has an oscillation unit that oscillates ultrasonic waves and a reception unit that receives the transmitted ultrasonic waves arranged to face each other, and a measurement target is arranged between the oscillation unit and the reception unit. The ultrasonic waves oscillated from the oscillation unit are blocked or reflected by the measurement target, and the intensity of the ultrasonic waves received by the reception unit changes. By measuring the intensity of the ultrasonic waves received by this reception unit (hereinafter, also referred to as "ultrasonic reception intensity"), bubbles in water containing fibrous substances and bubbles can be detected, and information about the bubbles can be obtained.

[0015] In the present invention, for example, water containing fibrous substances and bubbles in a container, water containing fibrous substances and bubbles flowing through a water supply pipe, etc. can be used as the measurement target. Taking water without bubbles as a blank, the ultrasonic reception intensity measured for the blank is R0, the ultrasonic reception intensity measured for water containing fibrous substances and bubbles is R1, and { (R0 - R1) / R0 × 100} [%] is obtained, and this value is defined as the ultrasonic reduction rate. The ultrasonic reduction rate is a value with the influence of environmental factors reduced for bubbles in water and can be used as an index of the amount of bubbles. In addition, in the measurement with the transmission type ultrasonic sensor of the present invention, when it is considered that the influence of fibrous substances on the ultrasonic reception intensity is almost negligible, the water without bubbles as the blank may not contain fibrous substances.

[0016] Air bubbles in water are susceptible to the influence of the pressure applied to the water containing the fibrous material and air bubbles to be measured. Therefore, when measuring with a transmission ultrasonic sensor for the purpose of monitoring air bubbles in water under atmospheric pressure, the pressure of the water containing the fibrous material and air bubbles at the measurement part of the transmission ultrasonic sensor, that is, the place where the intensity of ultrasonic waves is measured, is preferably less than 0.15 MPa, more preferably 0.1 MPa or less, still more preferably 0.05 MPa or less, and also 0 MPa or more.

[0017] Also, when the measurement is for the purpose of monitoring air bubbles in water under a pressure higher than atmospheric pressure and the measurement location by the transmission ultrasonic sensor is the downstream part of the flowing water containing the fibrous material and air bubbles, the pressure of the flowing water at the measurement part of the transmission ultrasonic sensor, that is, the place where the intensity of ultrasonic waves is measured, is preferably the same as the pressure at the monitoring location of air bubbles in the upstream water, more preferably 0.8 to 1.1 times the pressure at the monitoring location of air bubbles in the upstream water, still more preferably 0.85 to 1.05 times.

[0018] The oscillation frequency of the ultrasonic waves from the oscillation part of the transmission ultrasonic sensor is preferably 0.5 MHz or more, more preferably 1 MHz or more, still more preferably 2 MHz or more, and even more preferably 3 MHz or more. If the oscillation frequency is 0.5 MHz or more, air bubbles in water can be detected more accurately, and even if fibrous materials or inorganic particulate materials are mixed, it is possible to measure air bubbles in water. Also, from the perspective of the sensitivity to air bubbles in water, the oscillation frequency is preferably 50 MHz or less, more preferably 20 MHz or less, still more preferably 10 MHz or less.

[0019] When the water containing the fibrous material and bubbles is flowing water such as in a water supply pipe, the intensity of the ultrasound can be monitored continuously and periodically. In this case, efficient monitoring is possible by using an instrument that can automatically measure periodically. Based on the accumulated data of the ultrasound decay rate obtained through such continuous monitoring, information corresponding to the number and diameter of bubbles can be obtained, and based on this information, the bubble diameter distribution, volume distribution, and amount of bubbles in the water can be estimated.

[0020] Transmission-type ultrasonic sensors are suitable for periodic, continuous monitoring of water containing fibrous materials and air bubbles, as described above. Transmission-type ultrasonic sensors are also suitable for long-term continuous monitoring when the water being measured has a sieve residue of 100 mg / L or more with a mesh size of 20 μm. Measurement using a volumetric bubble analyzer requires changing the pressure on the fibrous material being measured and the water containing the bubbles, making the measurement procedure complicated and time-consuming. Furthermore, the more fibrous material there is, the more likely clogging is to occur in the measurement cell and other components. In contrast, transmission-type ultrasonic sensors do not come into contact with the fibrous material being measured or with water containing bubbles, allowing for measurement without being affected by the concentration of the fibrous material, thus enabling simpler and longer-term monitoring.

[0021] The underwater bubble detection method according to the present invention, as described above, is useful when adding a degassing agent to water containing fibrous material and bubbles, particularly to aqueous slurries in papermaking processes, and allows for control of the amount of degassing agent added according to the intensity of the ultrasonic waves received by the receiver of a transmissive ultrasonic sensor.

[0022] Furthermore, as mentioned above, since the ultrasonic decay rate serves as an indicator of the amount of bubbles, the amount of degassing agent added to water containing fibrous material and bubbles can be controlled based on the ultrasonic decay rate measured for the aqueous slurry. Such a method of detecting bubbles in water using the ultrasonic decay rate as an indicator of the amount of bubbles is also useful for methods of adding degassing agents. [Examples]

[0023] Hereinafter, the present invention will be described based on examples, but the present invention is not limited to the following examples.

[0024] [Measurement method] The various measurement methods for the physical properties of the samples in the following tests are as follows.

[0025] [Ultrasonic reduction rate] Using a transmission ultrasonic sensor, ultrasonic waves were oscillated in the radial direction of the water supply pipe from the oscillation part contacting the side circumferential surface of the water supply pipe, and the intensity of the ultrasonic waves received by the receiving part contacting the side circumferential surface on the opposite side in the radial direction was measured. Using water without bubbles as a blank, the reduction rate {(R0 - R1) / R0 × 100} [%] of the ultrasonic wave reception intensity R1 of the sample with respect to the ultrasonic wave reception intensity R0 of the blank was defined as the ultrasonic reduction rate. Unless otherwise specified, the oscillation frequency of the ultrasonic wave was 3 MHz, and the ultrasonic reduction rate was obtained as the average value of the values measured 10 times at 1-minute intervals.

[0026] [Amount of air bubbles in water (volumetric air bubble measuring device)] Using a volumetric air bubble measuring device, the sample was sealed in the measurement cell, and the amount of air bubbles in water was measured at 5-minute intervals from the volume changes during pressurization (maximum: 0.5 MPa) and depressurization (minimum: -0.08 MPa).

[0027] [Average bubble diameter] The liquid to be measured was passed through at a flow rate of 10 L / min, and the median diameter (D50) was measured using a laser diffraction / scattering particle size distribution measuring device (“LA-300”, manufactured by Horiba, Ltd.; the same applies hereinafter), and this was defined as the average bubble diameter.

[0028] [SS (Suspended Solids) concentration] It was measured by a method conforming to the measurement method of the sample solid content concentration of JIS P 8225:2003.

[0029] [Residue on a sieve with a mesh size of 20 μm] The sample was placed on a stainless steel sieve with a mesh size of 20 μm, thoroughly washed with tap water, and the residue on the sieve was collected. The weight (oven-dry weight) of this residue was measured after drying at 105°C for 3 hours. The residue content on the 20 μm sieve [mg / L] was calculated using the formula: oven-dry weight [g] / sample volume [mL] × 1,000,000.

[0030] [Test 1] (Confirmation of bubble detection using ultrasonic intensity) Various samples, as shown in Table 1 below, were prepared using water containing bubbles with an average bubble diameter of 100 μm, an aqueous dispersion (slurry) of LBKP (Leaf Bleached Kraft Pulp) with a CSF (Canadian Standard Freeness) of 420 mL, or an aqueous dispersion of calcium carbonate with an average particle size of 4 μm. The term "water containing bubbles with an average bubble diameter of 100 μm" as used here refers to tap water to which a surfactant (a mixture of sodium α-olefin sulfonate and polyoxyethylene fatty acid alkanolamide) is added. Air is then introduced at a flow rate of 0.05, 0.1, or 0.15 L / min, and the water is passed through a stirring pump at a flow rate of 10 L / min (bubble volume of 0.5, 1.0, or 1.5 vol%), with the amount of surfactant added adjusted so that the average bubble diameter is 100 μm. In Examples 1-8 and 1-9, the LBKP or calcium carbonate dispersion was prepared by adding the surfactant in the same amount as in Example 1-6 (1.0 vol%), and in Examples 1-10 to 1-13, the surfactant was added in an amount equivalent to 0.01 mass% of the amount used in Example 1-6 (1.0 vol%), and the mixture was then pumped to a stirring pump at a flow rate of 10 L / min. Each sample, after being passed through a stirring pump, was supplied to a water supply pipe at a flow rate of 10 L / min, and the ultrasonic decay rate was measured using an ultrasonic underwater bubble measuring device. The measurement results are shown in Table 1. The blank sample is tap water that has not had any surfactants added and has been pumped without introducing air.

[0031] [Table 1]

[0032] As shown in Table 1, it was confirmed that the ultrasonic attenuation rate increases as the amount of bubbles increases (Examples 1-1, 1-6, and 1-7). Furthermore, it can be said that the higher the ultrasonic oscillation frequency, the greater the ultrasonic attenuation rate and the easier it is to detect bubbles in water (Examples 1-2 to 1-6). On the other hand, aqueous dispersions of LBKP or calcium carbonate (Examples 1-10 to 1-13) showed a small decrease in ultrasonic intensity, indicating that the dispersion did not significantly reduce the ultrasonic intensity. Furthermore, when the aqueous dispersion of LBKP or calcium carbonate contained air bubbles (Examples 1-8 and 1-9), the detection of air bubbles was approximately the same as that of water containing air bubbles (Example 1-6). From this, it can be said that selective detection of air bubbles in water is possible even in pulp slurries whose main components are pulp fibers such as LBKP and fillers such as calcium carbonate during the papermaking process.

[0033] [Test 2] (Correlation between ultrasonic decay rate and bubble volume and bubble diameter) A surfactant (a mixture of sodium α-olefin sulfonate and polyoxyethylene fatty acid alkanolamide) was added to aerated tap water in adjusted amounts, and the water was supplied to a water supply pipe to achieve the bubble volumes shown in Table 2. Air was then introduced into this water supply pipe, and the water was supplied at a flow rate of 10 L / min to a transmission-type ultrasonic sensor and a laser diffraction / scattering particle size distribution analyzer connected in series. The ultrasonic decay rate and bubble diameter were measured at 0.1-second intervals for 10 minutes. Furthermore, if the ultrasonic reduction rate was less than 0.5%, it was considered to be within the detection error range and no bubbles were present. Table 2 shows the occurrence rate and average value of bubbles with an ultrasonic reduction rate of 0.5% or higher, as well as the measurement results of bubble diameter. Furthermore, the blank was used to pump water without introducing air.

[0034] [Table 2]

[0035] As the amount of surfactant added increased, the average ultrasonic decay rate tended to decrease, and the average bubble diameter tended to decrease (Examples 2-1 to 2-4). From this, it can be said that the ultrasonic decay rate can be an indicator of bubble diameter (bubble size). Furthermore, the occurrence rate of bubbles with an ultrasonic falloff rate of 0.5% or higher was 0% when there were no bubbles (blank), and increased with increasing bubble volume when the average bubble diameter was similar (Examples 2-5, 2-3, and 2-6). From this, it can be said that the occurrence rate of bubbles with an ultrasonic falloff rate of 0.5% or higher can be an indicator of the number of bubbles in water. Therefore, based on the data distribution of the ultrasonic decay rate, it is possible to estimate the bubble diameter distribution, volume distribution, and amount of bubbles in water.

[0036] [Test 3] (Continuous measurement test) Various samples, as shown in Table 3 below, collected during the papermaking process, were supplied to a water supply pipe at a flow rate of 10 L / min. The blockage status of the water supply path, equipped with a transmission-type ultrasonic sensor or a volumetric underwater bubble measuring device, was observed for 7 days. Furthermore, to prevent blockage due to slime generation, 50 mg / L of 5-chloro-2-methyl-4-isothiazolin-3-one was added at the start of the test. Table 3 shows the time it took for each sample to become blocked when using a transmission-type ultrasonic sensor and a volumetric bubble analyzer, respectively.

[0037] [Table 3]

[0038] As shown in Table 3, the clear water recovered from the white water (Example 3-6) had a low SS concentration and low residue on a 20 μm sieve. No blockage was observed for more than 7 days, regardless of whether a transmission-type ultrasonic sensor or a volumetric bubble analyzer was used. Other samples (Examples 3-1 to 3-5) with high SS concentrations and a large amount of residue after sieving with a 20 μm mesh size showed no blockage for more than 7 days when using a transmission-type ultrasonic sensor. On the other hand, when using a volumetric bubble analyzer, blockage was observed within 2 days near the switching valves for water flow and sealing, and for pressurization and depressurization. This suggests that, compared to volumetric bubble analyzers, the transmission-type ultrasonic sensor can continuously detect water bubbles containing fibrous material for a longer period of time.

[0039] [Test 4] (Addition of degassing agent) A degassing agent (an emulsion of a mixture of higher alcohol and paraffin) was added to the inlet slurry of a corrugated cardboard base paper manufacturing apparatus in the amounts shown in Table 4. The slurry was then passed through a stirring pump at a flow rate of 10 L / min and degassed by remaining in a degassing tank for 30 seconds. The amount of bubbles in the inlet slurry was adjusted so that when the filtrate obtained by filtering the inlet slurry through a 5 μm pore size filter was supplied to the stirring pump at a flow rate of 10 L / min, bubbles with an average bubble diameter of 100 μm accounted for 2 volume percent. The degassed slurry was supplied through a water pipe at a flow rate of 10 L / min, and the ultrasonic degradation rate and bubble volume were measured using a transmission-type ultrasonic sensor or a volumetric underwater bubble measuring device. The measurement results are shown in Table 4.

[0040] [Table 4]

[0041] It was confirmed that the greater the amount of degassing agent added, the smaller the ultrasonic decay rate and the lower the amount of bubbles. Since the degassing effect of the degassing agent is reflected in the ultrasonic decay rate, it can be said that the amount of degassing agent added can be adjusted based on the ultrasonic decay rate measured with a transmission-type ultrasonic sensor.

[0042] [Test 5] (Effect of pressure (1)) The raw material slurry from the fan pump outlet (pressure 0.4 MPa) of a corrugated cardboard base paper manufacturing apparatus was supplied to a water supply pipe at a flow rate of 10 L / min, and the pressure at the measurement section of a transmission-type ultrasonic sensor was changed to measure the ultrasonic reduction rate. The measurement results are shown in Table 5.

[0043] [Table 5]

[0044] As shown in Table 5, the ultrasonic reduction rate decreased as the pressure at the measurement site increased, and it tended to decrease significantly above 0.15 MPa (Examples 5-4 to 5-7). Therefore, for example, when the purpose is to measure the amount of bubbles in a slurry on a wire at atmospheric pressure, it is preferable to measure at a pressure of less than 0.15 MPa, and more preferably at 0.1 MPa or less.

[0045] [Test 6] (Effect of pressure (2)) The raw material slurry from the fan pump outlet (pressure 0.25 MPa) of a high-quality paper manufacturing apparatus was supplied to a water supply pipe at a flow rate of 10 L / min. The ultrasonic decrease rate and bubble volume were measured by varying the pressure at the measurement point using a transmission-type ultrasonic sensor or a volumetric underwater bubble measuring device. The measurement results are shown in Table 6.

[0046] [Table 6]

[0047] The amount of bubbles measured by the positive displacement underwater bubble measuring device was approximately the same regardless of the pressure in the measuring section. This is thought to be because the positive displacement underwater bubble measuring device involves pressurization and depressurization during measurement, so dissolved gases become bubbles regardless of the pressure in the measuring section, and the volume-based bubble measurement value includes the dissolved gases. In contrast, the ultrasonic degradation rate was almost the same when the pressure at the measurement point was 0.20 MPa or higher (Examples 6-1 to 6-3), but tended to be significantly larger when it was 0.15 MPa or lower (Examples 6-4 to 6-7). This is presumed to be because, when measuring with a transmission-type ultrasonic sensor, dissolved gas is detected as bubbles when it falls below a certain pressure below the fan pump outlet pressure. Therefore, by measuring the ultrasonic degradation rate under pressures equivalent to those near the outlet of the water pump, where the pressure is higher than atmospheric pressure, it becomes possible to accurately monitor bubbles, which are a factor that reduces the water pumping efficiency.

Claims

1. A method for detecting bubbles in water containing fibrous material and air bubbles by measuring the intensity of ultrasonic waves received by the receiver of a transmissive ultrasonic sensor, The oscillation frequency of the aforementioned ultrasonic wave is 0.5 MHz or more and 50 MHz or less. The pressure of the water containing the fibrous material and bubbles at the point where the intensity of the ultrasound is measured is set to less than 0.15 MPa. A method for detecting bubbles in water, wherein the ultrasonic decay rate expressed by the following formula (1) is used as an indicator of the amount of bubbles, and the appearance rate of bubbles with an ultrasonic decay rate of 0.5% or more is used as an indicator of the number of bubbles in water. Ultrasound reduction rate [%] = (R 0 - R 1 ) / R 0 ×100 (1) (In the formula, R0: ultrasonic received intensity measured for water without bubbles) R1: Ultrasonic received intensity measured for fibrous material and water containing air bubbles. (This represents...)

2. A method for detecting bubbles in water containing fibrous material and air bubbles by measuring the intensity of ultrasonic waves received by the receiver of a transmissive ultrasonic sensor, The oscillation frequency of the ultrasonic wave is 0.5 MHz or more and 3 MHz or less. A method for detecting bubbles in water, wherein the ultrasonic decay rate expressed by the following formula (1) is used as an indicator of the amount of bubbles, and the appearance rate of bubbles with an ultrasonic decay rate of 0.5% or more is used as an indicator of the number of bubbles in water. Ultrasound reduction rate [%] = (R 0 - R 1 ) / R 0 ×100 (1) (In the formula, R0: ultrasonic received intensity measured for water without bubbles) R1: Ultrasonic received intensity measured for fibrous material and water containing air bubbles. (This represents...)

3. The method for detecting bubbles in water according to claim 1 or 2, wherein the water containing the fibrous material and bubbles is flowing water, and the intensity of ultrasonic waves is monitored periodically and continuously.

4. The water containing the fibrous material and bubbles has a sieve residue of 100 mg / L or more, according to claim 1 or 2, for detecting bubbles in water.

5. The water containing the fibrous material and bubbles is an aqueous slurry used in the papermaking process, as described in claim 1 or 2, for detecting bubbles in water.

6. The method for detecting bubbles in water according to claim 3, wherein the pressure of the water containing the fibrous material and bubbles at the point where the intensity of the ultrasonic wave is measured is set to 0.9 to 1.1 times the pressure at the point upstream of the measurement point where the target of monitoring bubbles in water is to be measured.

7. The method for detecting bubbles in water according to claim 1 or 2, wherein the ultrasonic reduction rate is used as an indicator of bubble diameter.

8. A method for adding a degassing agent, using the underwater bubble detection method according to claim 1 or 2, wherein the amount of degassing agent added to the water containing the fibrous material and bubbles is controlled according to the intensity of the received ultrasonic waves.

9. A method for adding a degassing agent, using the underwater bubble detection method according to claim 1 or 2, wherein the amount of degassing agent added to the water containing the fibrous material and bubbles is controlled using the ultrasonic reduction rate as an indicator of the amount of bubbles.

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