Crystal Growth and Viscosity Behaviors of Ammonium Alum Hydrate Solution with PVA in Shear Flow

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1 Article Nihon Reoroji Gakkaishi Vol.42, No.4, 219~226 (Journal of the Society of Rheology, Japan) 2014 The Society of Rheology, Japan Crystal Growth and Viscosity Behaviors of Ammonium Alum Hydrate Solution with PVA in Shear Flow Takafumi Toyoda *, Ruri Hidema **, Hiroshi Suzuki *, and Yoshiyuki Komoda * * Department of Chemical Science and Engineering, Kobe University ** Organization of Advanced Science and Technology, Kobe University 1-1 Rokkodai-cho, Nada-ku, Kobe , Japan (Received : October 11, 2013) Aluminum ammonium sulfate dodecahydrate (ammonium alum hydrate) slurry is a promising latent heat medium for high-temperature systems. However, there exist the significant issues with this medium at high temperature with respect to crystal agglomeration, growth, and sedimentation. Therefore, the effects of adding drag-reducing surfactants and poly vinyl alcohol (PVA) on the formation process of ammonium alum hydrates crystals under a controlled shear flow have been investigated in this study. In addition, in order to investigate the impact of the surface hydrophilicity and hydrophobicity of cooling walls on the crystal growth, a copper plate and a silica coated copper plate were used as the substrates of the wall. From the results, it was found that the mean crystal size of the ammonium alum hydrate decreased with increasing shear rates and with the addition of surfactants and PVA. In addition, the crystals on the silica coated copper plate were smaller than those on the copper plate without a coating. Thus, the addition of dragreducing surfactants and PVA and use of a hydrophilic surface prevented the crystal agglomeration and crysal growth of the ammonium alum hydrate under a shear flow. Furthermore, the viscosity of each solution was also measured, as affected by the various additives and by crystal agglomeration and growth. Key Words: Ammonium Alum / Crystal Growth / Viscosity / Coating / PVA 1. INTRODUCTION Latent heat transportation systems are operated by the circulation of phase change slurries, which consist of fluids containing fine particles that have high latent heat capacities. Such fine particles are called phase change materials. Since the phase change material sustains a temperature at its fusion temperature, the phase change slurries can transfer high amounts of heat. Therefore, the flow rate of the heat media is reduced, which minifies industrial systems and reduces the energy required for operation. Latent heat transportation systems using phase change slurries have great potential for various applications 1-4) ; however, the slurries have some disadvantages. One significant disadvantage is their low fluidities; due to the presence of particles, phase change materials have higher viscosities compared with solutions without particles. In addition, the particles agglomerate and disperse in the slurry, imparting non-newtonian characteristics to the slurry. 5-9) To increase slurry fluidity, Corresponding Author Tel and Fax: , hero@kobe-u.ac.jp the addition of surfactants that form drag-reducing rod-like micelles 10-15), polymers 3,16), and some types of brines 1,9,17,18) have been tested to prevent particle agglomeration. These additives also enhance the non-newtonian behavior of the slurries. The elucidation of the rheological properties of slurries is important to achieve effective control of fluid flows in industrial latent heat transportation systems. Different types of latent heat slurries are used in industry depending on a particular situation and the temperature of the process (Table I). For example, ice/water slurries have been previously employed in lower temperature applications. 1-3,5,16,18,19) Since ice has a large latent heat of 334 kj kg -1 at 0 ºC, the ice slurries are used for food cold chains. Much research has been performed on the heat transfer characteristics of ice slurries, on techniques for preventing the agglomeration of ice particles, and on techniques to increase fluidity. In these studies, surfactants, some types of brines, and poly vinyl alcohol (PVA) have been tested to prevent agglomeration. 3,16-18,20) Surfactants and PVA are effective as stabilizers for preventing crystal agglomeration and growth even with the PVA concentrations of only several thousand ppm. 3,20,16) Phase change materials whose fusion 219

2 Nihon Reoroji Gakkaishi Vol temperatures are approximately 15 ºC are appropriate for airconditioning systems. On this basis, clathrate hydrates such as tetrabutylammonium bromide (TBAB) hydrate 6,7,21,22) and trimethylolethane (TME) hydrate 12-14,23) are suitable for use in air-conditioning systems. Thus, their flow and heat transfer characteristics have been reported by many researchers. For example, to increase fluidity, Suzuki et al. added dragreducing surfactant micelles to clathrate hydrate slurries and examined their rheological properties. 14,23) In contrast, only a small amount of literature is available for higher temperature heat transportation systems that operate at approximately 30 ºC. Hence, our group has focused on the applicability of phase change materials in this higher temperature range. Suzuki et al. proposed the applicability of inorganic disodium hydrogen phosphate dodecahydrate slurry, which has a fusion temperature of 35 ºC, in the cooling system of an absorption chiller. 4) For even higher temperatures, i.e., approximately 50 ºC, we propose the use of aluminum ammonium sulfate dodecahydrate (ammonium alum hydrate) as a latent-heat transportation media. Ammonium alum is an inorganic material whose fusion temperature is controlled by its concentration in solution. At 35 wt% in water, the fusion temperature of the slurry is 51 ºC 24,25) ; and this temperature is appropriate for waste heat transportation, as more than 50 % of waste heat streams from industrial plants are at temperatures higher than 50 ºC. Furthermore, the latter temperature is sufficiently high for domestic tasks such as air heating and the supply of household hot water. Heating energy households accounts for more than 50 % of the total household energy consumption. Therefore, we believe that the use of the phase change slurry at 51 ºC for district heating has Table I. Typical phase change materials. the potential to save much energy. However, specific problems are associated with these higher temperature slurries, including sedimentation, fouling, and pipe blockage. Fouling is the accumulation and fixing of phase change materials on industrial equipment surfaces. Fouling leads to a reduction in the efficiency of heat exchangers due to the low heat transfer coefficients of the fixed phase change material on heat transfer surfaces. Moreover, fouling causes pipe blockages. Sedimentation is due to the high density of ammonium alum hydrate (1630 kg m 3 ), which leads to phase separation and contributes further to fouling. Since the solubility of latent heat media such as ammonium alum hydrate is very low at the room temperature, it is not possible to dissolve the sedimentation hydrates at ambient conditions. Therefore, once sedimentation and fouling occur in the system, they are difficult to resolve. To address these problems, we added drag-reducing surfactants, which form rod-like micelles, and PVA to the ammonium alum hydrate slurry. 25) The sedimentation and crystal growth phenomena associated with this ammonium alum hydrate slurry were then observed under still-standing conditions. The drag-reducing surfactants and PVA were assumed to interact with each other in the slurry, and clearly prevented sedimentation, crystal growth, and crystal agglomeration over several days. In addition, the flow and heat transfer characteristics of the slurry containing both surfactants and PVA were measured in a pipe. While the viscosity of the slurry was slightly increased by the additives, the friction coefficients of the slurry were decreased due to the effects of the drag-reducing surfactants. 25) The fluidity and fouling problems of the slurry also depend on the nature and properties of the inner surface of the pipewall. To date, much research has been conducted in this area, and both the surface roughness and the hydrophilicity have been found to be important factors that must be controlled to prevent fouling ) In our previous studies, adsorption forces between calcium carbonate particles known to be the main fouling materials and walls having different hydrophilicity values were measured. 30,31) The adsorption force at the hydrophilic wall was found to be smaller than that at the hydrophobic wall. The same tendency has been observed in the case of ammonium alum hydrate particles under stillstanding conditions. 30,31) The crystal growth of calcium carbonate particles on the hydrophilic and hydrophobic walls was also observed. As expected, fewer fouling particles adsorbed to the hydrophilic wall. Moreover, the mean crystal size was smaller on the hydrophilic wall than on the hydrophobic wall. Therefore, it was concluded that the crystal growth of ammonium alum hydrate particles, which influence 220

3 TOYODA HIDEMA SUZUKI KOMODA : Crystal Growth and Viscosity Behaviors of Ammonium Alum Hydrate Solution with PVA in Shear Flow fouling phenomena, could be affected by the hydrophilicity of the inner pipe-wall surface. These wall conditions can affect the flow properties of the slurries by affecting the propensity of pipe walls to foul and restrict the cross-sectional flow area of the pipe. To investigate the effects of the surfactants and PVA on the crystal growth and fouling properties of the ammonium alum hydrates, the crystallization and crystal growth phenomena of the ammonium alum hydrates in solution with or without the additives under controlled shear flow will be observed in this study. As the viscosity of the slurry is a function of the amounts of particles and their degree of agglomeration, the viscosity will be measured using a rheometer. In addition, a small amount of the slurry will be applied to a constant shear stress for a certain time during which the crystals in the slurry will be observed using a microscope. To analyze how the wall surface characteristics (hydrophilic vs. hydrophobic) affect the crystal growth behaviors of the ammonium alum hydrates in shear flow, hydrophilic and hydrophobic plates will be used during the application of shear and the measurement of viscosity. These experiments will be designed to quantify the relationship between fluidity and the crystal growth characteristics of the ammonium alum hydrates under a shear flow, which has not yet been established. 2. EXPERIMENTAL METHODS 2.1 Materials Aluminum ammonium sulfate dodecahydrate (ammonium alum hydrate: AlNH 4 (SO 4 ) 2 12H 2 O) was used as an inorganic latent heat material. This hydrate has a latent heat of 251 kj kg -1. According to our previous studies 24,25), its fusion temperature is 51 ºC at a concentration of 35 wt% in water. Behenyltrimethyl ammonium chloride was used as a cationic surfactant for forming drag-reducing rod-like micelles. This surfactant exhibits effective drag reduction in water in from 40 ºC to 80 ºC. Sodium salicylate was used as the source of counter-ions required for forming rod-like surfactant micelles. The surfactant concentration was set to 2000 ppm, and the molar ratio of counter-ions to surfactant was fixed at 1.5. At this molar ratio, the surfactant solution shows strong viscoelasticity due to the rod-like micelles. Completely saponified PVA with a degree of polymerization of 500 (Wako pure chemical, ) was added to the solution as a stabilizer to prevent particle sedimentation and agglomeration. The concentration of PVA used was 1000 ppm. 2.2 Surface Coating of a Substrate To investigate the influence of surface hydrophilicity on the crystal growth of ammonium alum hydrate particles and the fluidity of the slurry, hydrophilic and hydrophobic plates were used as testing plates for the rheometer (Anton Parr, MCR301). Since copper is widely used for heat exchanger pipes of transportation systems, we selected this material as the hydrophobic substrate. A copper plate coated with silicon dioxide (silica: SiO 2 ) was used as the hydrophilic substrate. The silica coating method was relatively simple. After a small amount of the perhydropolysilazane solution was applied to a copper plate, the plate was dried for 1 h at 500 ºC. The perhydropolysilazane reacted with the moisture in the air, and was converting it into silica glass. Eq. (1) shows the conversion reaction of the perhydropolysilazane. SiH 2 NH + 2H 2 O SiO 2 + NH 3 + 2H 2 (1) The values of the surface roughness (R a ) for the copper plate and that coated with silica were measured by atomic force microscopy (AFM) to be 33 nm and 0.79 nm, respectively. Figure 1 shows the surface profiles of the bare copper and silica coated copper measured by AFM. 2.3 Viscosity Measurements and Crystal Growth Observation To investigate how crystal agglomeration and growth affect the fluidity of the slurries, the viscosities of the solutions/ slurries with and without surfactants or PVA were measured during crystal growth under controlled shear flow using a rheometer (Anton Paar, MCR-301). The experimental set up of the test section is shown in Figure 2(a); a cone-and-plate device with a diameter of 50 mm and an angle of 1º was used for the measurements. The circular copper plate and the silicacoated circular copper plate were used as hydrophobic and hydrophilic wall surfaces, respectively. Every sample solution Fig. 1. Surface profiles of (a) Bare copper and (b) Silica coated copper. 221

4 Nihon Reoroji Gakkaishi Vol was first heated at 80 ºC to completely dissolve the ammonium alum hydrates. The solution was then taken and placed on the substrate in a cone plate rheometer configuration. The substrate was heated to 50 ºC, gradually cooling the sample from 80 ºC to 50 ºC. Ammonium alum hydrate particles appeared on the surface of the substrate, while particles were also partially removed from the surface by the shearing of the fluid. Thus, the fluid was converted from a solution to a slurry state, and partially hydrate particles remained on the surface of the substrate. The interaction between the slurry and the hydrates on the surface and/or the influence of the hydrates in the slurry were indicated by the viscosity data. The viscosities of the sample solutions were measured for 30 minutes, at four shear rates of 10, 100, 500, and 1000 s -1 were used. After viscosity measurement, the sample slurries were moved to a petri dish heated at 50 ºC. The petri dish was set under a microscope to observe the crystals. A schematic of the experimental set up is shown in Figure 2(b). Images of the crystals were taken by a digital camera attached to the microscope, and the crystal size of each sample was calculated by analyzing more than 100 crystals. As the crystal shapes were sufficiently varied, each crystal was characterized by its characteristic length, as shown in Figure RESULTS AND DISCUSSION 3.1 Crystal Growth Characteristics Figures 3 and 4 show the ammonium alum hydrate crystal images after the viscosity experiments on the copper and silica coated copper plates, respectively. In Figure 4, the crystal sizes of ammonium alum hydrate without the surfactants or PVA are large and nonuniform at low shear rates (10 s -1 ). The mean crystal size decreases with increasing the shear rate to 1000 s -1. When surfactants are added to the ammonium alum solution, the mean crystal size is slightly further decreased. Moreover, the mean crystal size becomes much smaller when both surfactants and PVA are added to the solution. The difference in the mean crystal size between slurries with and without additives is large at a low shear rate. By increasing the shear rate, the mean crystal size difference between slurries with and without additives becomes smaller. Therefore, the degree of crystal growth is small for solutions under high shear conditions, such as those in the operation of an apparatus, whether with or without additives. The addition of surfactants and PVA to the slurries is effective at preventing the crystal growth under low shear conditions where sedimentation tends to occur. The crystal growth behavior on a silica coated copper plate is the similar as shown in Figure 5. However, the mean crystal size is smaller than that on the copper plate. This indicates that hydrophilic surfaces are effective at preventing crystal growth. Figure 6 shows the median diameter of the crystals observed in Figures 3 and 4. In the case of ammonium alum hydrate solution without additives on the copper plate, the median diameter is large at the low shear rate but much smaller at the high shear rate. The same solution on the silica coated copper plate shows the same tendency, but the median crystal size is smaller even at the low shear rate. At the high shear rate of 1000 s -1, the median diameter is the same value as that observed on the copper plate. Therefore, coating effects are negligible when the shear rate is high. When surfactants are added to the solution, the median diameter is decreased at low shear rates, but the difference in median diameter between slurries with and without surfactants is much smaller at high shear rates. When surfactants and PVA are added to the solution, the median diameter is much smaller even at the low shear rate; however, surfactants-added solutions and surfactants-pva-added solutions show equivalent median crystal size diameters at high shear rates. Surface effects, that is, the differences in behavior observed between slurries Fig. 2. Schematic of viscosity measurement and crystal observation system. Fig. 3. Typical shapes of ammonium alum hydrates and the definition of the crystal size. 222

5 TOYODA HIDEMA SUZUKI KOMODA : Crystal Growth and Viscosity Behaviors of Ammonium Alum Hydrate Solution with PVA in Shear Flow Fig. 4. Ammonium alum hydrate crystals generated on copper plates. Fig. 5. Ammonium alum hydrate crystals generated on silica coated copper plates. Fig. 6. Median diameters of crystals depending on additives, substrate hydrophilicity, and shear rate. 223

6 Nihon Reoroji Gakkaishi Vol tested on the copper and silica coated copper plates, become negligible once more when surfactants and/or PVA are added to the solutions. Figure 7 (a) shows the cumulative distributions of crystal sizes for the ammonium alum solution without additives. The silica coating was found to be effective in decreasing the median crystal diameters. The median diameters also decreased with increasing shear rate. Moreover, in the lower shear rate condition, the crystal sizes show a broad distribution. The similar charts for the solution with the surfactant and with the surfactant and PVA are displayed in Figures 7(b) and Figure 7(c), respectively. The additives significantly affected the distributions of the crystal sizes. In particular, the distributions showed steeper slopes for the combined use of the surfactant and PVA with silica coating. The effects of the surface characteristics and the effects of the additives are assumed to be different. In general, crystal nucleation is more dominant than crystal growth on a rough surface. In contrast, crystal growth is more dominant than crystal nucleation on a smooth surface. Considering these tendencies, the mean crystal size of the ammonium alum hydrate on the bare copper surface is expected to be smaller than that of the hydrate on the silica coated copper surface. However, the observed crystal sizes on the silica coated copper surface were smaller than those on the bare copper surface. The adsorption force between a hydrate and substrate might be significantly lower when a substrate is hydrophilic. 31,32) Therefore, crystals generated on the substrate will be easily removed into the slurry, resulting in the generation of a new crystal on the substrate, before it detaches from the surface. Thus, the crystals cannot grow to large sizes on the hydrophilic substrate. When the shear rate is high enough to remove new crystals on the substrate, the crystal sizes are relatively small. In addition, the surfactants and PVA prevent crystal agglomeration and growth. Therefore, crystal sizes remain smaller even at low shear rates. The reason why the surfactants and PVA prevent crystal agglomeration and growth is still under debate. In our previous study, we focused on the interaction between surfactants and PVA. 25) The PVA used in this study can form a network structure, which might be further entangled by surfactants. In another explanation for the effects of PVA, the Fig. 7. Cumulative distribution of each sample as affected by substrate hydrophilicity and shear rate. Each figure shows (a) Ammonium alum hydrate crystal, (b) Ammonium alum hydrate crystal with surfactants and (c) Ammonium alum hydrate crystal with surfactants and PVA. 224

7 TOYODA HIDEMA SUZUKI KOMODA : Crystal Growth and Viscosity Behaviors of Ammonium Alum Hydrate Solution with PVA in Shear Flow Kelvin effect was proposed. 3,20) That is, PVA can attach and orient on crystals, consequently preventing crystal growth. In our previous study, however, the use of PVA additives alone could not prevent crystal agglomeration and growth. 25) Therefore, the proposed explanation based on the Kelvin effects has yet to be verified. 3.2 Viscosity Characteristics Figure 8 shows the effects of surfactants and PVA on the viscosities of ammonium alum solutions on both the copper and silica coated copper plates. The viscosities decrease with increasing shear rate in all samples. It is well known that solutions with drag-reducing surfactants show shear-thinning effects due to the formation of rod-like micelles. In addition, the shear-thinning effect of the surfactants is not disturbed by the additional presence of PVA additives. 25) In our previous study, we confirmed that the slurry without surfactants showed a shear-thinning effect at low shear rates; however, the viscosity had already reached a 40 s -1 constant value. 25) Therefore, the shear-thinning behavior was attributed mainly to the rod-like micelles. The slurry viscosity measured on the silica coated copper plate is slightly higher than that observed on the copper plate, as shown in Figure 8. As mentioned above, in the case of the hydrophilic substrate, a part of the crystalized hydrates on the substrate are removed into the solution due to their low adherence to the substrate. In the case of the hydrophobic substrate, the crystalized hydrates on the substrate are not easily removed into solution due to the higher degree of adherence. Therefore, the volume fraction of the slurry on the hydrophilic substrate is larger than that on the hydrophobic substrate, and the viscosity of the slurry on the hydrophilic substrate is higher. Based on this fact, the influence of the volume fraction of the slurry on the viscosity data is assumed to be larger than that of the crystals remaining on the substrate. Thus, the ease of hydrate crystal removal from the substrate was quantified using the viscosity. In general, the quality of adherability of a crystal to a surface decreases with increasing of a crystal size. However, the results indicate that crystal adhesion to the hydrophilic surface is weak even when the crystal size is small. In addition, the viscosity of the slurry without additives is higher than those with surfactants and/or PVA. Both crystal growth and agglomeration increase the apparent volume fraction. When crystals agglomerate with each other, some solvent can be trapped inside the agglomerate, including an increase in the apparent volume fraction and consequently increasing viscosity. Since the surfactants and PVA prevent crystal growth and agglomeration, the solutions with these additives show lower viscosities. 4. CONCLUSIONS The effects of drag-reducing surfactants and PVA on the formation process of ammonium alum hydrate crystals in solution under shear flow were investigated. In addition, the manner in which hydrophilic and hydrophobic substrates affect the crystal growth was observed. The viscosities of various slurries, which reflect the crystal growth and agglomeration in the slurries during phase changes, were also measured. The crystal sizes in ammonium alum hydrate slurries became much smaller and more uniform when both surfactants and PVA were added to the slurry. These additives Fig. 8. Viscosity of each sample as affected by substrate hydrophilicity and shear rate. (a) Viscosities measured on the hydrophilic surface. (b) Viscosities measured on the hydrophobic surface. 225

8 Nihon Reoroji Gakkaishi Vol prevented crystal agglomeration and growth in the slurry. In the case without additives, the crystal sizes had larger median diameter and a broader distribution at lower shear rates. However, the mean crystal size remained small and more uniform at greater shear rates. Hydrophilic and hydrophobic substrates affected the crystal growth process. The hydrophilic substrate was associated with a small adsorption force between the crystal and the substrate. Since small crystals generated on the wall were easily removed into the slurry, many small crystals existed in the slurry. These crystal formation processes affected slurry viscosities. When surfactants and PVA were introduced to prevent crystal agglomeration and growth, the apparent volume fraction along with the viscosity of the slurry were small. Furthermore, when the substrate was hydrophilic, the apparent volume fraction increased due to the many small particles removed from the wall into the slurry; the viscosity was slightly increased in this case. However, to prevent fouling phenomena, a hydrophilic surface is optimal. These results provide useful knowledge to facilitate the design of an effective latent transportation system for higher temperatures using ammonium alum hydrate slurries. REFERENCES 1) Bellas J, Chaer I, Tassou SA, Appl Term Eng, 22, (2002). 2) Melinder A, Granryd E, Int J Refrig, 28, (2005). 3) Inaba H, Inada T, Horibe A, Suzuki H, Usui H, Int J Refrig, 28, (2005). 4) Suzuki H, Kishimoto T, Komoda Y, Usui H, J Chem Eng Jpn, 43, (2010). 5) Egolf PW, Kauffeld M, Int J Refrig, 28, 4-12 (2005). 6) Darbouret M, Cournil M, Herri JM, Int J Refrig, 28, (2005). 7) Ma ZW, Zhang P, Wang RZ, Furui S, Xi GN, Int J Heat Mass Trans, 53, (2010). 8) Rensing PJ, Liberatore MW, Sum AK, Koh CA, Sloan ED, J Non-Newtonian Fluid Mech, 166, (2011). 9) Clain P, Delahaye A, Founaison L, Mayoufi N, Dalmazzone D, Fürst W, Chem Eng J, , (2012). 10) Suzuki H, Fuller GG, Nakayama T, Usui H, Rheol Acta, 43, (2004). 11) Suzuki H, Nguyen H-P, Nakayama T, Usui H, Rheol Acta, 44, (2005). 12) Suzuki H, Itotagawa T, Indartono YS, Usui H, Wada N, Rheol Acta, 46, (2006). 13) Indartono YS, Usui H, Suzuki H, Komoda Y, Nakayama K, J Chem Eng Jpn, 39, (2006). 14) Suzuki H, Wada N, Usui H, Komoda Y, Ujiie S, Int J Refrig, 32, (2009). 15) Różański J, Int J Heat Mass Trans, 55, (2012). 16) Suzuki H, Konaka T, Komoda Y, J Chem Eng Japan, 43, (2010). 17) Nørgaard E, Sørensen TA, Hansen TM, Kauffeld M, Int J Refrig, 28, (2005). 18) Suzuki H, Nakayama K, Komoda Y, Usui H, Okada K, Fujisawa R, J Chem Eng Jpn, 42, (2009). 19) Egolf PW, Kitanovski A, Ata-Caesar D, Stamatiou E, Kawaji M, Bedecarrats JP, Strub F, Int J Refrig, 28, (2005). 20) Lu SS, Inada T, Yabe A, Zhang X, Grandum S, Int J Refrig, 25, (2002). 21) Ma ZW, Zhang P, Int J Refrig, 35, (2012). 22) Ma ZW, Zhang P, Int J Therm Sci, 68, (2013). 23) Suzuki H, Tateishi S, Komoda Y, Int J Refrig, 33, (2010). 24) Suzuki H, Konaka T, Komoda Y, Ishigami T, Int J Refrig, 36, (2013). 25) Hidema R, Tano T, Suzuki H, Fujii M, Komoda Y, Toyoda T, J Chem Eng Jpn, In press (2013). 26) Bansal B, Chen XD, Müller-Steinhagen H, Chem Eng Process, 47, (2008). 27) Gunn DJ, J Cryst Growth, 50, (1980). 28) Herz A, Malayeri MR, Müller-Steinhagen H, Energ Convers Manag, 49, (2008). 29) Albert F, Augustin W, Scholl S, Chem Eng Sci, 66, (2011). 30) Yamanaka S, Ito N, Akiyama K, Shimosaka A, Shirakawa Y, Hidaka J, Adv Powder Tech, 23, (2012). 31) Toyoda T, Suzuki H, Komoda Y, Shibata Y, 2012 AIChE Annual Meeting, Pittsburg (2012). 32) Toyoda T, Suzuki H, Komoda Y, Hidema R, Rfirig Sci Tech Proc, 2, (2012). 226