Development of High-speed Gas Dissolution Device Yoichi Nakano*, Atsushi Suehiro**, Tetsuhiko Fujisato***, Jun Ma**** Kesayoshi Hadano****, Masayuki Fukagawa***** *Ube National College of Technology, Tokiwadai 1-14-2,Ube,755-8555,Japa-n **Shinko Industrial Co. Limited, Kiwanami 1440, Ube, 759-0297, Japan ***Bubble Tank Co. Limited, Nishihirabaracho 4-10-30, Ube, Japan ****Graduate School of Science and Engineering, Yamaguchi University, Tokiwadai 2-16-1, Ube, 755-8611, Japan *****Station for Fostering Industrial Creative, Kawakami 582, Ube, 755-0084, Japan 1. Introduction Dissolution and stripping of gas into/from liquid are both extremely important processes to the problems in the natural environments or in the field of industry. In order to explain the phenomena of dissolution and stripping various models such as the double film model, the infiltration model and the surface update model have been proposed, and a fairly good result has been obtained. These models tried to deal with the mass transfer of a gas through the interface of gas-liquid from a viewpoint of the film resistance and the unsteady diffusion process. Since the diffusion in the liquid is hard to occur compared with the diffusion in the gas, the mass transfer in the process considered here is chiefly discussed to the film resistance in the liquid side. On the other hand the authors recently invented a method of the high speed dissolution or stripping of gas into/from liquid. This paper presents a discussion for this technique with experimental result. It is possible if we can simultaneously produce the situations that (1) the thickness of the liquid film becomes thin as soon as possible, (2) a gas to be dissolved is sufficiently supplied, and (3) the liquid film of bubble is created one after another and destroyed at large amount. We have succeeded in producing the situation in the laboratory. This paper presents the principle, the experimental set up, and some experimental results for this phenomenon. 2.Principle The most noticeable feature of the authors method is to produce the bubble cluster by the liquid to be treated, and the element of liquid treated all becomes the component of the thin surface film of the bubble. In this system gas is efficiently dissolved or stripped into/from liquid. By the supplying of the liquid to be treated, a great many bubbles are continuously created, then form the bubble cluster, and finally break as the gas dissolved or stripped liquid. The above mentioned process contains multiple phenomena, so the phenomena are described individually. 2-1 Formation of liquid film and the rate of gas dissolution Generally, Henry's law is applied to the relation between the pressure and the concentration of gas dissolved in a liquid in equilibrium condition. The practically important point is the time to be taken for the gas dissolution to become equilibrium or saturated condition, and the shorter time is preferable. Theoretically liquid film formed in the gas where liquid film has been thinned to the limited thickness will lead to saturated condition most promptly. Figure 1 shows this situation schematically by a part of liquid film of small thickness δ cut in short length where pressure of the gas both sides the film is put to P. Since the thickness of film δ is small, the concentration of the dissolved gas in the liquid film will attain the saturated value in a shortly after - 103 -
the liquid film is formed. In order to make a simple image of this situation let us consider a soap bubble indicated in Fig.2. When a soap bubble is formed in the gas, the bubble has the following features: (1) liquid component constituting the liquid film will descend along the film due to the greater gravity force exerting the liquid component, thus the film becomes the thicker near the bottom of it, (2) due to the surface tension of the film the inside pressure is slightly higher than the outside pressure, (3) bubble is hard to break when the surface tension is small. Through the above process (1) the liquid elements contacting the gas will exchange each other, thus further efficient dissolution or stripping is carried out. Concerning (3), if the film is thickened, soap bubbles will become stable further. However, it is necessary to enlarge the viscosity to thicken the surface film. Anyway it is considered that the exchange of gas substance between liquid and gas phases is stimulated by the formation of bubble cluster compared with the situation where babbles exist in the liquid apart each other. δ P P Fig.1 Dissolution of gas into liquid Fig.2 Movement of liquid element at the liquid film of bubble In order to make clear the difference, the situation in which a bubble is in the liquid under the free surface is shown in Figure 3. The dissolution velocity of the gas to the liquid is explained by the double film model, the surface element exchange model, and the infiltration model, etc. so far. However, when the bubble exists in the gas with very thin films as shown in Figs.2 and 3, Figure 3 Bubble in the liquid body we do not have to consider the existence of the very thin liquid film neighborhood of the interface explained by double film model or surface element exchange model and only have to consider the gas film. Anyway it is considered that the existing models are applicable to the case in which both gas and liquid bodies extend to fairly distant position from the boundary dividing them. It is difficult to apply these model to the situation indicated in Figs.1 and 2. Among the existing models, double film model treats the nearest phenomenon to the situation indicated in Figs.1 and 2. So the limitation for applying the model is briefly described here. By the double film model, the diffusion coefficient D m 2 /s, and film thickness δ L m, and mass transfer coefficient K L m/s are related by the following form: KL= D δl (1) By the above equation, when D is constant,δ L is inversely proportional to K L, andδ L is decreases with increase of K L. It should be noticed that the double film model has no power when the thickness of the film is sufficiently thin. 2-2 Generation and destruction of many bubbles As was described before, key point of this system is to produce the bubbles of the liquid to be treated, and the liquid element all becomes the component of the thin surface film of the bubble. By the supplying of the liquid to be treated, a great many bubbles are formed one after another, then the bubble cluster is formed, and finally the bubbles break, thus the gas is efficiently dissolved or stripped into/from the liquid. - 104 -
X X Figure4. Profile of a large mount of bubbles generation Here the process that liquid to be treated efficiently becomes the element of the surface film of the bubbles will be explained employing Fig.4. If the liquid to be treated is injected into a vessel, the bubble cluster is formed in the upper half of the vessel. This is like the situation that is occurred in the glass when the beer is poured into a glass. The concentration of the liquid body in the bubble region in the upper half reduces upward due to the separation of liquid and gas by the gravitational force, the separation process has also been explained from the view point of the spatial variation of the surface tension employing the concept of the pressure difference balancing with the curvature of the liquid film. If the liquid is injected from a nozzle, the injected liquid will entrain the surrounding gas, and will plunge into the liquid in the vessel with gas bubble to some depth which depends on the injected velocity, the cross-sectional area ratio of nozzle and vessel etc. Though the formulation of the quantitative relationship for the preferable bubble cluster formation to the purpose of this system is beyond the scope of this paper, a particular depth is required to the injection condition. The important thing is to form the stable bubble cluster in the upper half region for fairly large amount of the flow rate of the liquid to be treated. 2-3 Generation of saturation water Since the forming the stable bubble cluster in the upper half region of the vessel is the key of this technique, we would like to give a preliminary physical consideration for forming the bubble cluster. Firstly the velocity of appropriate value at which the liquid to be treated is injected is needed in order that the bubble cluster stably exists in the upper half region of the vessel, because the bubble cluster of low mass density exists with a significant depth against the static pressure of the liquid. The depth of the bubble cluster D b in the upper half region of the vessel will depend on the velocity head formed by the injection velocity V 2 /2g. The depth will also depend on the ratio between the cross-sectional area of the nozzle and that of the vessel A 1 /A 2. The quantity of the gas entrained into the jet of the liquid to be treated injected is also important. The quantity will depend on the injected velocity V, length of the outer edge of the jet flow of the liquid S, and the vertical distance H from the nozzle to the upper edge of the vessel or to the interface dividing the upper bubble cluster region and the lower region occupied by the liquid body. Therefore, D b will be a function of V 2 /2g, A 1 /A 2, V, S and H whose functional form should be determined in the future study. Secondly the depth of the vessel should be greater than D b. The other restrictive conditions may exist, for which we have to say that it left for future work. 3 High-speed gas dissolution device 3-1 Outline of device Outline of the high-speed gas dissolution device is shown in Figure5. A vessel whose upper part is opened is set up in the airtight and pressure container. The water supplied from the upper part of the airtight container kept pressured constant becomes a lot of flux and it collides with water in the vessel while entraining a large amount of gas. A large amount of air that collides with water becomes a lot of bubbles and they rise in the vessel. The bubble is destroyed even if the bubble in the uppermost part reaches the upper part of the vessel, it becomes saturated solution, it overflows, - 105 -
5 2 Journal of Water and Environment Technology, Vol.5, No.2, 2007 1 4 3 1 Container sealed up 2 Overflowing water 3 Vessel 4 Introduction tube 5 Pressure-regulating and it goes out the water that overflows and came out becomes saturation water corresponding to pressure from the lower side of the airtight container and is discharged. Figure5.Profile of the device 3-2 Technology that makes it rolls a large amount of air and collides with surface of water Flux of the liquid injected from the nozzle. It is important to entrain a large amount of gas that we want to dissolve in the liquid, and to make bubble cluster in the upper half region of the vessel. If the jet is normal one with sooth surface as shown in Fig.6 the amount of entrained gas is little. On the other hand, if the jet is disturbed at the nozzle to form irregular edge of jet surface as shown in Fig.7, entrainment of a large amount of gas will be expected. Since this method does not use a Air Flux Rolled air Air Fluxes Rolled air Water Water Figure 6. Liquid jet with regular surface Figure7. Liquid jet disturbed at nozzle Figure8. Figure indicating the jet disturbed at nozzle lot of minute holes but only disturbs the jet, and easily reserved to the required values. generating many bubbles, becomes a large container. (Figure4). the loss of energy and momentum are a little The air introduced in water rises while amount of bubble group, and rises in the 4. Examination and Discussion Experiment was conducted using the tap water as a liquid and air was used in the device shown - 106 -
Treatment water [L/min] in Figure 5. The vessel located in the pressure tank has inside size of 25cm deep and 16cm diameter which has a bottle neck (10cm in the diameter) at the place upper 1/3 of it. Inside the vessel, the region above the bottle neck was occupied by bubble cluster. Volume below the bottle neck is about 3.5 L. The nozzle is fixed at 3.5cm above from the upper edge of the vessel. Diameter of the nozzle was set at 4 sizes: 3mm, 4.5mm, 6mm, and 8mm. Pressure before the outlet valve of the device which is almost the same as the pressure in the pressure tank was set 0.1-0.3MPa in gage pressure. Control of the pressure inside the pressure tank was done by operating the valve, also the injection velocity was controlled by another valve. Measurement was done for the flow rate of water to be treated, the amount of the oxygen supply, and the DO concentration for various values of nozzle diameter and pressure. When the pressure inside the pressure tank is high, and the DO value obtained inside the tank is higher than the saturated DO value at the atmospheric pressure water poured from the outlet of the device will reduce due to the foam, and the value of DO after poring from the outlet of the device is reduced from that inside the pressure container, i.e. DO value will be wrong. In order to avoid of this problem, DO sensor was installed before outlet valve and the DO value was measured. The water temperature was measured at each experiment, and was 26~27.4. The saturation DO value is in the range of 7.99~7.81mg/L in this range of the temperature variation for the pressure of 0.1MPa, thus the variation is fairly small. Figure 9 shows the DO value against nozzle: :3mm :4.5mm :6mm :8mm 30.0 the pressure inside the tank for various values of nozzle diameter. The saturation 25.0 concentration of the dissolved oxygen is 20.0 about 8mg/L in 0.1MPa as previously stated. Henry's law has been approved in 15.0 the range of this pressure and temperature. 10.0 The DO concentrations almost reach 5.0 saturation about all measurement point under the actual experiment condition, 0.0 and they are approximated by the straight 0.000 0.050 0.100 0.150 0.200 0.250 0.300 line. This shows that the DO value DO[mg/l] nozzle: 30.000 25.000 20.000 15.000 10.000 5.000 Pressure gauge[mpa] Figure 9 Relation of DO and gage pressure :3mm :4.5mm :6mm :8mm 0.000 0.000 0.050 0.100 0.150 0.200 0.250 0.300 Pressure gauge[mpa] Figure 10 Relation of the quantity of the treated liquid and gage pressure reaches the saturation one in the pressure tank of this device. Figure 10 shows the relation between the flow rate of the treated water and the pressure in the pressure tank for 4 sizes of the nozzle diameter. Flow rate of the treated water decreases with increase of the pressure. Also the flow rate of the treated water increase with increase of the nozzle diameter. The reason for the decrease of the flow rate with increase of the pressure is that the valve controlling the nozzle was fixed and outlet valve was changed and therefore the pressure difference related to the injection velocity will decreases with increase of the pressure in the pressure tank. The increase of the flow rate with increase of - 107 -
SupliedDO[mg/m nozzle: :3mm :4.5mm :6mm :8mm the nozzle diameter would have reflected the situation of small energy loss and 300.0 250.0 large cross-sectional area. But it should be noticed that the indicated in figure may have included the effects of other factors. 200.0 150.0 The inclination of the straight line was small for the caliber of the nozzle to become small though the flow rate of the 100.0 50.0 treated water changed in each caliber in proportion to pressure. The flow rate of water to be treated has changed between 0.0 5-27L/min in the actual experiment 0.000 0.050 0.100 0.150 0.200 0.250 0.300 depending on the condition. Therefore, Pressure gauge[mpa] the residence time was 7.5 second in short Figure 11 Relation of the amount of the dissolved oxygen and gage pressure time and was 40 second in long time. It is necessary to select the caliber and pressure by the supply purpose of oxygen because there is a big difference in the quantity of treatment water by the caliber of the nozzle and pressure. Finally the amount of the dissolved oxygen during the process which is the most descriptive quantity from the view point of the potential of water quality improvement is considered. It is the quantity evaluated as the product of DO increment and the flow rate of the treated water. This is the quantity of the oxygen really supplied to the water per unit time, i.e,. (oxygen supplied per unit time)=(do increment) (flow rate of treated water) If the DO value attains the saturated value in all experiment the DO increment will be almost the same, which is the same value under the same pressure and the temperature. Therefore, the value of the above equation is chiefly governed by the flow rate of the treated water,if the bubble cluster is preferably formed and the supplied oxygen is abundant. From the figure data obtained by the experiment done using the nozzle with diameter smaller than 4.5mm do not show good results. Therefore, it is understood not to be necessarily excellent when aiming at the supply of oxygen. Enlarging the caliber of the nozzle is necessary raise pressure, and 6mm or more is demanded under the actual experiment condition. 5. Conclusion So far a new technique to dissolve gas into a liquid, and the experimental results have been introduced. Though the systematic measurement has not been carried out so the universal relation can not be recognized, the present result implies that the presented system is promising. We have a plan to do systematic experiment to obtain the universal relationship between the quantities concerning the gas dissolving ability, which leads to the design of the high performance device. Especially the condition for forming the preferable bubble cluster in the upper half of the receiving container is mote important, so we will concentrate on this condition. - 108 -