Hydrodynamics Optimizing Methods and Tools Part 13 pdf

Hydrodynamics – Optimizing Methods and Tools 348 Fig. 9. Speculated mechanisms of hydrate formation in static-mixing type flow reactor (Tajima et al., 2011b) Case C is for strong hydrate shell formation. In this case, the target gas bubbles are rapidly covered with strong hydrate shell because the hydrate formation rate r f is relatively higher Gas Hydrate Formation Kinetics in Semi-Batch Flow Reactor Equipped with Static Mixer 349 than shedding rate r s . The apparent interfacial area between gas and water, a, is considerably restricted and also the dissociation rate r d is considerably decreased (for example, a similar situation have been observed in the case of CO 2 hydrate formation (Ogasawara et al., 2001)). As a result, there is little the further hydrate formation, and thus the overall hydrate formation rate constant aK* is low depending on r d and r s . This hydrate formation occurs under hard thermodynamic conditions (higher pressure and lower temperature) and lower mechanical mixing conditions. Although the additive addition can prevent the strong hydrate shell, sufficient mechanical condition is necessary to form further hydrate with accelerating the hydrate shedding process. Case B is for porous and rough hydrate particle/film formation and the intermediate case between Cases A and C. Hydrate particles and partial hydrate film are formed on bubble surface. The film pore and void channels allow target gas to diffuse into water phase (Sloan & Koh, 2008), and partial hydrate shedding is occurred on bubble surface. The apparent interfacial area between target gas and water, however, is decreased and the dissociation of target gas into water is limited by rough hydrate film formation. As a result, the aK* value (not only a but also K* values) is lower than that for Case A. In another case, higher concentration of additive in water phase will contribute to keep porous and rough hydrate film (Case B) with preventing hydrate growth (Tajima et al., 2010b). That is, additives (like as surfactants) adsorbing on bubble surface can keep the gas dissociation and the hydrate shedding rates. If the solubility in water is very low, the dissociation rate (mass transfer rate) will be low. As a result, the overall formation rate is low. For example, relatively high solubility of CH 2 FCF 3 and CHClF 2 (near CO 2 solubility in water) leads to higher dissociation rate and hydrate formation rate. On the other hand, lower solubility of SF 6 (near CH 4 solubility in water) cause lower dissociation rate. This trend is in agreement with the data obtained in this study (Table 1). The dissociation rate may be a rate-controlling step. Further investigation is necessary for hydrate formation rate equation. 5. Conclusion The gas hydrate formation kinetics is investigated in the semi-batch flow reactor equipped with static mixer, and thus discusses the hydrate formation process based on the experimental data by varying thermodynamic, mechanical, and chemical conditions. In the flow reactor, there are multiple flows with gas-liquid-solid system, and the gas hydrate formation process is overly complicated. There are mainly two hydrate formation patterns in the reactor; hydrate slurry and hydrate plug. According to the experimental observation and results, the gas hydrate formation process consists of the hydrate nucleation, hydrate growth, hydrate shedding, and gas dissociation processes. Especially, the idea of the hydrate shedding from the interface is very important. The balance among these processes is altered under thermodynamic, mechanical, and chemical conditions. For the application of the gas hydrate technologies, it is necessary to not only convert sufficiently (mixture) gas to hydrate but also form hydrate appearance to transport and apply easy. Many researchers have investigated about the thermodynamic and chemical conditions in stirred tank, but the mechanical conditions have been less noticed. The static mixer in the flow reactor improves the mixing function in the reactor. Although it is perhaps difficult to find out the essential hydrate formation rate, the author expects that these results help the engineering application of gas hydrate. Hydrodynamics – Optimizing Methods and Tools 350 6. Acknowledgment The author is greatly thanks Professor Akihiro Yamasaki (Seikei University, Japan), Dr. Fumio Kiyono (AIST, Japan), and Professor Kazuaki Yamagiwa (Niigata University, Japan) for variable discussions. A part of this work was supported through the Grant-in-Aid for Young Scientists B (No.21710074), Japan, and Sasaki Environment Tec. Found, Japan. The author appreciates student's cooperation, Mr. Yasuhiro Oota, Mr. Hiroki Yoshida, Mr. Toshinao Furuta (graduated from Niigata University, Japan), Mr. Yosuke Nakajima (graduated from Kogakuin University, Japan), and Mr. Toru Nagata (finished Graduate School of University of Tsukuba, Japan). 7. References Daimaru, T.; Yamasaki, A. & Yanagisawa, Y. (2007). Effect of Surfactant Carbon Chain Length on Hydrate Formation Kinetics, Journal of Petroleum Science and Engineering, Vol.56, No.1-3, (March 2007), pp.89-96, ISSN 0920-4105 Englezos, P.; Kalogerakisa, N.; Dholabhaia, P.D. & Bishnoi, P.R. (1987) Kinetics of Formation of Methane and Ethane Gas Hydrates, Chemical Engineering Science, Vol.42, No.11, (November 1987), pp.2647-2658, ISSN 0009-2509 Fukumoto, K.; Tobe, J.; Ohmura, R. & Mori, Y.H. (2001). Hydrate Formation Using Water Spraying in a Hydrophobic Gas: A Preliminary Study, AIChE Journal, Vol.47, No.8, (August 2001), pp.1899-1904, ISSN 0001-1541 Godfrey J. C. (1997). Static Mixer, In: Mixing in the process industries, Harnby, N.; Edwards, M. F.; Nienow, A. W. (Eds.), 225-249, Butterworth-Heinemann, ISBN 0-7506-3760-9, Oxford, UK. Gudmundsson, J. S. & Børrehaug A. (1996). Frozen Hydrate for Transport of Natural Gas, Proceedings of 2nd International Conference on Natural Gas Hydrates, pp439-446, Toulouse, France, June2-6, 1996. Hashemi, S.; Macchi, A. & Servio, P. (2009) Gas-Liquid Mass Transfer in a Slurry Bubble Column Operated at Gas Hydrate Forming Conditions. Chemical Engineering Science, Vol.64, No.19, (October 2009), pp.3709-3716, ISSN 0009-2509 Huo, Z.; Freer, E.; Lamar, M.; Sannigrahi, B. ; Knauss, D. M. & Sloan E. D. (2001). Hydrate Plug Prevention by Anti-Agglomeration, Chemical Engineering Science, Vol.56, No.17, (September 2001), pp.4979-4991, ISSN 0009-2509 Kang, S P. & Lee, H. (2000). Recovery of CO 2 from Flue Gas Hydrate: Thermodynamic Verification Through Phase Equilibrium Measurements, Environmental Science and Technology, Vol.34, No.20, (October 2000), pp.4397-4400, ISSN 0013-936X Lee, H. ; Lee, J. W.; Kim, D. Y.; Park, J.; Seo, Y. T.; Zeng, H.; Moudrakovski, I. L.; Ratcliffe, C. I. & Ripmeester, J. A. (2005). Tuning Clathrate Hydrates for Hydrogen Storage, Nature, Vol.434, 7 April, (April 2005), pp.743-746, ISSN 0028-0836 Li, S.; Fan, S.; Wang, J.; Lang, X. & Liang, D. (2009). CO 2 Capture from Binary Mixture via Forming Hydrate with the Help of Tetra-n-Butyl Ammonium Bromide, Journal of Natural Gas Chemistry, Vol.18, No.1, (March 2009), pp.15-20, ISSN 1003-9953 Lo, C. ; Zhang, J.S.; Somasundaran, P.; Lu, S.; Couzis, A. & Lee, J.W. (2008). Adsorption of Surfactants on Two Different Hydrates, Langmuir, Vol.24, No.22, (November 2008), pp.12723-12726, ISSN 0743-7463 Gas Hydrate Formation Kinetics in Semi-Batch Flow Reactor Equipped with Static Mixer 351 Luo, Y T.; Zhu, J H.; Fan, S S. & Chen, G.J. (2007). Study on the Kinetics of Hydrate Formation in a Bubble Column, Chemical Engineering Science, Vol.62, No.4, (February 2007), pp.1000-1009, ISSN 0009-2509 Nagata, T.; Tajima, H.; Yamasaki, A.; Kiyono, F. & Abe, Y. (2009). An Analysis of Gas Separation Processes of HFC-134a from Gaseous Mixtures with Nitrogen- Comparison of Two Types of Gas Separation Methods, Liquefaction and Hydrate- Based Methods, in Terms of the Equilibrium Recovery Ratio, Separation and Purification Technology, Vol.64, No.3, (January 2009), pp.351-356, ISSN 1383-5866 Ogasawara, K.; Yamasaki, A. & Teng, H. (2001). Mass transfer from CO 2 Drops Traveling in High-Pressure and Low-Temperature Water, Energy & Fuels, Vol.15, No.1, (January 2001), pp.147-150, ISSN 0887-0624 Sloan, E. D.; Koh, C. A. (2008). Clathrate Hydrates of Natural Gases, 3rd Ed., CRC Press, ISBN 978-0-8493-9078-4, Boca Raton, Florida, USA. Szymcek, P.; McCallum, S.D.; Taboada-Serrano, P. & Tsouris, C. (2008). A Pilot-Scale Continuous-Jet Hydrate Reactor, Chemical Engineering Journal, Vol.135, No.1-2, (January 2008), pp.71-77, ISSN 1385-8947 Tajima, H.; Yamasaki, A. & Kiyono, F. (2004). Continuous Formation of CO 2 Hydrate via a Kenics-type Static Mixer, Energy & Fuels, Vol.18, No.5, (September 2004), pp.1451- 1456, ISSN 0887-0624 Tajima, H.; Yamasaki, A. & Kiyono, F. (2005). Effects of Mixing Functions of Static Mixers on the Formation of CO 2 Hydrate from the Two-Phase Flow of Liquid CO 2 and Water, Energy & Fuels, Vol.19, No.6, (November 2005), pp.2364-2370, ISSN 0887-0624 Tajima, H.; Nagata, T.; Yamasaki, A.; Kiyono, F. & Masuyama, T. (2007) Formation of HFC- 134a Hydrate by Static Mixing, Journal of Petroleum Science and Engineering, Vol.56, No.1-3, (March 2007), pp.75-81, ISSN 0920-4105 Tajima, H.; Nagata, T.; Abe, Y.; Yamasaki, A.; Kiyono, F. & Yamagiwa, K. (2010a). HFC-134a Hydrate Formation Kinetics During Continuous Gas Hydrate Formation with a Kenics Static Mixer for Gas Separation, Industrial and Engineering Chemistry Research, Vol.49, No.5, (March 2010), pp.2525-2532, ISSN 0888-5885 Tajima, H.; Kiyono, F. & Yamasaki, A. (2010b). Direct Observation of the Effect of Sodium Dodecyl Sulfate (SDS) on the Gas Hydrate Formation Process in a Static Mixer, Energy & Fuels, Vol.24, No. 1, (January 2010), pp.432-438, ISSN 0887-0624 Tajima, H.; Oota, Y. & Yamagiwa, K. (2011a). Effects of “Promoter” on Structure I Hydrate Formation Kinetics, In: Physics and Chemistry of Ice 2010, Y. Furukawa, G. Sazaki, T. Uchida, N. Watanabe (Ed.), pp.253-259, Hokkaido University Press, ISBN 978-4- 8329-0361-6, Sapporo, Japan. Tajima, H.; Oota, Y.; Yoshida, H. & Yamagkiwa, K. (2001b). Experimental Study for Gas Hydrate Formation and Recovery of Fluorine-Containing Compound in Static Mixing-type Flow Reactor, Proceedings of 7th International Conference on Gas Hydrate, Edinburgh, Scotland,UK, July 17-22, 2011. Warzinski, R. P.; Riestenberg, D.E.; Gabitto, J.; Haljasmaa, I.V.; Lynn, R.J. & Tsouris, C. (2008). Formation and Behavior of Composite CO 2 Hydrate Particles in a High- Pressure Water Tunnel Facility, Chemical Engineering Science, Vol.63, No.12, (June 2008), pp.3235-3248, ISSN 0009-2509 Hydrodynamics – Optimizing Methods and Tools 352 Zhang, J.S.; Lo, C.; Somasundaran, P.; Lu, S.; Couzis, A. & Lee, J.W. (2008). Adsorption of Sodium Dodecyl Sulfate at THF Hydrate/Liquid Interface, Journal of Physical Chemistry C, Vol.112, No.32, (August 2008), pp.12381-12385, ISSN 1932-7447 Zhong, Y. & Rogers, R. E. (2000). Surfactant effects on gas hydrate formation, Chemical Engineering Science, Vol. 55, No.19, (October 2000), pp. 4175-4187, ISSN 0009-2509 16 Study of the Mass Transport on Corrosion of Low Carbon Steel Immersed in Sour Solution Under Turbulent Flow Conditions R. Galvan-Martinez 1 , R. Orozco-Cruz 1 , J. Mendoza-Flores 2 , A. Contreras 2 and J. Genesca 3 1 Unidad Anticorrosión, Instituto de Ingeniería Universidad Veracruzana, Veracruz 2 Instituto Mexicano del Petróleo, San Bartolo Atepehuacan 3 Departamento de Ingeniería Metalúrgica, Facultad de Química Universidad Nacional Autónoma de México México 1. Introduction A corrosion process can be influenced, in different ways, by the relative movement between the metal and the corroding environment. This relative movement can increase the heat and mass transfer of reactants towards and from the surface of the corroding metal, with a consequent increase in the corrosion rate. Also, if solid particles are present, removal of protective films, erosion and wear on the metallic surface can occur. The corrosion of the metallic structure under turbulent flow is complex, but this problem has been studied mainly in the oil industry (Garnica-Rodriguez et al., 2009; Genesca et al., 2010; Mora-Mendoza et al., 2002; Papavinasam et al., 1993; Poulson, 1993), where, the flow and some gases are very important in the behaviour of the phenomenon processes. This oil industry has processes that involve the movement of corrosive liquids in metallic structures, for example, the transport of mixtures of liquid hydrocarbons and gas with water through pipes. Therefore the influence of flow on the corrosion processes is an important issue to be considered in the design and operation of industrial equipment. This influence is complex and many variables are involved. Many observations of flow- accelerated corrosion problems have been documented (Dean, 1990; Garverick, 1994; Poulson, 1993). One aim that has been so much studied in the petroleum industry is the effect of flow and dissolved gases, such as hydrogen sulphide (H 2 S) and carbon dioxide (CO 2 ). The most common type of flow conditions found in industrial processes is turbulent and according to increasing of the necessity to describe the corrosion of metals in turbulent flow conditions some laboratory hydrodynamic systems have been used with different degrees of success (Poulson, 1983, 1993, 1994). Among these hydrodynamic systems, rotating cylinder electrodes (RCE), pipe segments, concentric pipe segments, submerged impinging jets and close-circuit loops have been used and have been important in the improvement of the Hydrodynamics – Optimizing Methods and Tools 354 understanding of the corrosion process taking place in turbulent flow conditions (Liu et al., 1994; Lotz, 1990; Schmitt et al., 1991; Silverman, 1984, 1988, 1990). The use of the RCE, as a laboratory hydrodynamic test system, has been gaining popularity in corrosion studies (Nesic et al., 1995, 2000). This popularity is due to its characteristics, such as, it operates mainly in turbulent flow conditions; it has a well understood mass transfer properties and it is relatively easy to construct and operate (Gabe, 1974; Schlichting & Gersten, 1979; Gabe & Walsh, 1983; Poulson, 1983). The critical Reynolds number, Re, for the transition from laminar to turbulent flow is 200 approximately, for a smooth surface laboratory RCE (Gabe, 1974; Gabe & Walsh, 1983; Poulson, 1983, 1993; Galvan-Martinez et al., 2010). This Reynolds value will be equivalent to a rotation rate  38 rpm, for a cylinder of 0.01 m of diameter immersed in a fluid of ν = 1.0E-06 m 2 s -1 (e.g. pure water). When the RCE is immersed in a fluid and rotated at a very low rotation rate the fluid moves in concentric circles around the cylinder (laminar conditions). As the rotation rate of the cylinder increases the flow pattern is disrupted, cellular flow patterns, known as “Taylor vortices”, appear and the turbulent condition develops. These vortices enhance the mass, momentum and heat transfer at the rotating electrode (Gabe, 1974; Gabe & Walsh, 1983). In 1954, some researchers published what it is now considered as the basic study on the mass transfer characteristics of the RCE (Eisenberg et al., 1954). The Reynolds number for a RCE is given by the following expression u u RCE RCE RCE RCE RCE d ρ d Re μ   (1) Where u RCE is the peripheral velocity of the RCE, d RCE is the diameter of the RCE,  and µ are the density and viscosity of the environment, respectively. It is clear from this equation that there is a linear relationship between the Reynolds number and the rotation rate of the electrode. Figure 1 shows the correlation between the rotation rate of the electrode and the equivalent Reynolds number. The RCE in corrosion laboratory studies is an useful tool for the understanding of mass transfer processes, effects of surface films, inhibition phenomena, etc., (Galvan-Martinez et al., 2010; Mendoza-Flores et al., 2002) taking place in turbulent flow conditions. However, the use of the RCE has been questioned by some researchers (Efird et al., 1993), due to the differences found between the values of corrosion rates measured on pipe flow electrodes and on the RCE. The reasons for these differences are still not well understood. However, some works have provided ideas on the explanation of this apparent difference (Mendoza- Flores, 2002; Mendoza-Flores & Turgoose, 2002; Turgoose et al., 1995). One of the main objectives of using hydrodynamic test systems in laboratory studies of turbulent flow is to obtain a series of criteria, aimed to help in the explanation and prediction of real life situations. In order to attain this, the data measured in one hydrodynamic system has to be compared, somehow, with the data measured in other hydrodynamic systems or with data obtained in real life systems. It has been suggested that the comparison among the results obtained in different hydrodynamic systems can be made by means of the wall shear stress ( w ). This suggestion considers that, when two hydrodynamic systems are at the same value of  w , at the same flow regime (turbulent or laminar), the same flow velocities near the surface and mass transfer conditions, prevail (Silverman, 1990). Study of the Mass Transport on Corrosion of Low Carbon Steel Immersed in Sour Solution Under Turbulent Flow Conditions 355 Fig. 1. Equivalence of rotation rate (rpm) and peripheral velocity (m/s) of the electrode and the calculated Reynolds number. Dimensionless analysis using mass transfer concepts showed that the corrosion when controlled by diffusion of one of the species between the bulk fluid and the surface could be modelled completely by the rate of mass transfer of the rate limiting species and the Reynolds (Re), Sherwood (Sh) and Schmidt numbers (Sc) (Dean & Grab, 1984; Ellison & Schmeal, 1978; Ross et al., 1966). In general, the effect of flow can be used to determine if corrosion is under activation, diffusion or mixed control. 2. Experimental 2.1 Test environment All experiments were carried out at 60°C, under static conditions (0 rpm) and turbulent flow conditions and, at the atmospheric pressure of Mexico City (0.7 bars). Two aqueous solutions were used as test environment: NACE brine (National Association of Corrosion Engineers, 1996) and a 3.5 % NaCl solution. These test environments were selected due to the fact that most of the H 2 S corrosion laboratory tests are carried out in this solutions. The solutions were prepared using distilled water and reagent grade chemicals. In order to remove oxygen from the solution, N 2 gas (99.99%) was bubbled into the test solution for a period of 30 minutes before each experiment was carried out. After oxygen removal, H 2 S gas (99.99%) was bubbled into the test solution until saturation was reached. H 2 S bubbling was maintained during all the experimentation. The measured saturation pH was 4.4 for the NACE brine and a pH of 4.5 for the 3.5% NaCl solution. In order to determine the purging time needed to remove all O 2 from the solution, a rotating cylindrical platinum electrode was cathodically polarized in a 1 M sodium sulphate solution, at room temperature and at different rotation rates. It was established that Hydrodynamics – Optimizing Methods and Tools 356 the region associated to the mass transfer reduction of oxygen, on the cathodic polarization curve, disappeared after 30 minutes of purging time. 2.2 Experimental set up All electrochemical measurements were carried out in an air-tight three-electrode electrochemical glass cell. Cylindrical working electrodes were used in all experiments. These cylinders were made of API X52 steel (American Petroleum Institute, 2004). The working electrode (WE) was machined from the parent material API X-52 and it had a diameter of 0.0012 m. The total exposed area of the working electrodes was 5.68E-04 m 2 and 3.4E-04 m 2 for static and dynamic conditions respectively. As reference electrode (RE) a saturated calomel electrode (SCE) was used and a sintered graphite rod was used as auxiliary electrode (AE). The experimental set up is schematically shown in Figure 2. Fig. 2. Experimental set-up used in the electrochemical measurement. Prior to each experiment, the steel working electrode was polished up to 600 grit SiC paper, cleaned in deionised water and degreased with acetone. All electrochemical tests were carried out on clean samples. Hydrodynamic conditions were controlled using a Perking-Elmer EG&G Model 636 Rotating Cylinder Electrode system. In dynamic conditions or turbulent flow conditions, the [...]...  i lim ,H   ilim,H 2S (8) Where ilim,H+ and ilim,H2S are the limiting current densities for the H+ and H2S under turbulent flow condition In order to obtain the ilim,H+ and ilim,H2S Mendoza and Schmitt (Mendoza-Flores, 1997; Schmitt & Rothmann, 1977) proposed that the theoretical ilim for H2S and H+ reduction 362 Hydrodynamics – Optimizing Methods and Tools could be compared with the experimentally... in a water tank, and pumped into the water jacket between the two 376 Hydrodynamics – Optimizing Methods and Tools concentric tubes of the WSA by a circulating pump, and finally sprayed into the airflow field Waste gas exits out through the center gas tube in the WSA and tangentially enters the gas-liquid separator, in which liquid droplets taken out by the waste gas are separated and flow back into... geometries Then, for the same alloy and environment, laboratory simulations allow duplicating the velocity- sensitivity mechanism found in the industrial geometry The shear stress is a measure of the interaction between metallic surface and fluid The shear stress at the wall can be estimated by the following equation (Bolmer, 1965): 364 Hydrodynamics – Optimizing Methods and Tools  LAB   PLANT (12) Then,... polarization curve shows a ba with values from 115 to 135 V vs SCE approximately, where these values correspond to an activational or charge transfer process 368 Hydrodynamics – Optimizing Methods and Tools Figure 15 shows the estimated anodic Tafel slopes (ba) as a function of ReRCE, on cylindrical X52 steel electrodes immersed in NACE brine and 3.5% NaCl solution saturated with H2S The slopes were... Effects of H2S and H2S/CO2/CH4/C3H8 Mixtures Corrosion Vol 42, Issue 7, (July-1986) pp 398-408, ISSN 0010-9312 Papavinasam, S., Revie, R.W., Attard, M., Demoz, A., & Michaelian, K (2003) Comparison of Laboratory Methodologies to Evaluate Corrosion Inhibitors for Oil and Gas Pipelines Corrosion, Vol 59, Issue 12, (December-2003) pp 897-912, ISSN 0010-9312 372 Hydrodynamics – Optimizing Methods and Tools Poulson,... the density and viscosity of the fluid It is a characteristic dimension in order to define the type of flow At low velocities, i.e at low Re, a stable or laminar flow is encountered Assuming the fluids under consideration to be Newtonian and incompressible in nature, the shear stress () at any point in a laminar flow is given by:   du dy (2) 358 Hydrodynamics – Optimizing Methods and Tools If the... for the gas-liquid operation (Bokotko et al., 2005) Because 374 Hydrodynamics – Optimizing Methods and Tools ammonia is a soluble gas with a small Henry’s law constant, the overall mass transfer resistance in the air stripping largely lies on the gas film side (Matter-Muller et al., 1981) Therefore, decreasing the gas film resistance and increasing the gas-liquid contact area will accelerate the mass... mass transfer coefficient kL and specific mass transfer area a were also investigated As a new gas-liquid mass transfer equipment, the WSA was used to simultaneously remove NH3-N, total P and COD from anaerobically digested piggery wastewater using cheap Ca(OH)2 as the precipitant for PO43- and some organic acids, and as pH adjuster for NH3-N stripping 2 Experimental setup and methods 2.1 Design of the... 2004) the measured mass-transfer coefficient could be converted to the Sherwood number and plotted as a function of the Reynolds number (Galvan-Martinez, 2004) The Sherwood number for the RCE (ShH+) is given by the expression: Sh H   i lim,H  nF D d RCE H C H (15) 366 Hydrodynamics – Optimizing Methods and Tools Where: dRCE is the outside diameter of the rotating cylinder, DH+ is the diffusion... increases The behaviour shown in figures 11 and 12 can suggest that the mass transfer coefficient (ShH+ and kH+) is flow dependent, because it increases as the rotation rate also increases In general, the behaviour presented by ShH+ and kH+ indicates that the cathodic current is controlled by the mass transfer rate On the other hand, the behaviour of ShH+ and kH+, in NACE brine, confirm the behaviour . porous and rough hydrate particle/film formation and the intermediate case between Cases A and C. Hydrate particles and partial hydrate film are formed on bubble surface. The film pore and void. submerged impinging jets and close-circuit loops have been used and have been important in the improvement of the Hydrodynamics – Optimizing Methods and Tools 354 understanding of the corrosion. Hydrodynamics – Optimizing Methods and Tools 350 6. Acknowledgment The author is greatly thanks Professor Akihiro Yamasaki (Seikei University, Japan), Dr. Fumio Kiyono (AIST, Japan), and