Akihiro Yamasaki

Formation experiments Of CO2 hydrate from the two-phase flow of liquid CO2 and water were carried out using static mixers with different types of mixing elements to elucidate the effects of mixing functions of the static mixer on CO2 hydrate formation. The mixing principle of the Kenics type static mixer is composed of three mixing functions, flow division, flow reversal, and radial mixing. In this study, three types of mixing elements in addition to the basic Kenics type static mixer were applied for CO2 hydrate formation; each mixing element lacked at least one mixing function among three mixing functions. The observed behaviors Of CO2 hydrate formation could be classified into six patterns: (1) hydrate chunks, (2) CO2 drops agglomerated through the hydrate film, (3) dispersed liquid CO2 drops covered with hydrate film, (4) mixture of tiny liquid CO2 drops and hydrate particles, (5) mixture of hydrate chunks and agglomerated CO2 drops, and (6) mixture of agglomerated and dispersed CO2 drops. Occurrences of the above patterns depended on the type of the mixing element as well as the flow rates of liquid CO2 and water. From the observations, flow division was suggested to be important for the formation of hydrate chunks but not essential, and flow reversal was essential for the formation of dispersed hydrate particles. The formation mechanisms Of CO2 hydrate with the static mixer are discussed according to the results.

Process design for a new injection method of liquid CO2 using a static mixer was conducted based on laboratory experimental results on the formation process of liquid CO2 drops covered with hydrate film by a Kenics-type static mixer, and numerical simulation of the liquid CO2 drops at 500 and 1500 m. The Sauter Mean Diameter (SMD) of the liquid CO2 drops covered with hydrate film was dramatically decreased with the use of the static mixer; empirical equations were obtained for the SMD, and also the maximum and minimum diameters of the liquid CO2 drops for a given flow velocity (Weber number, We). The ascending and dissolving behavior of a liquid CO2 drop with hydrate released in the ocean at an intermediate depth was numerically simulated, and the maximum drop diameter to avoid evaporation of the drop before complete dissolution was estimated. Based on these results, scaling up of the static mixer was conducted by assuming a disposal process Of CO2 emitted from a 100-MW thermal power plant, and the mixer diameter was determined as a function of the given SMD. Moreover, the power consumption of the static mixer was evaluated and found to be almost negligible. (c) 2005 Elsevier B.V. All rights reserved.

A method for predicting the location of a dissociation condition on an H-L-w-V line under isochoric operation was presented. To establish the method, the governing equations for the H-L-w-V coexistence under isochoric conditions were derived. Here, a liquid and a vapor phase were expressed by the PR EOS + MHV2 model and a hydrate phase by the van der Waals-Platteeuw model. The molar volume of the vapor phase was calculated from the equation of state, and a simple expression for the molar volume of the hydrate phase was derived. Then, to prove the validity of the proposed method, experimental studies about the dissociation process of the hydrates were performed. The temperature and pressure traces in the hydrate dissociation process, including the location of the dissociation condition, were successfully predicted by the proposed method. In addition, the thermodynamic consistency among the phase models was discussed. It was pointed out that agreement between the calculated and experimental results about the H-L-w-V equilibrium line did not ensure thermodynamic consistency among the phase models. (c) 2005 Elsevier B.V. All rights reserved.

A method for determining quadruple points of a two-component system containing a simple hydrate phase is proposed. This method utilizes the quasi-static change of the system along three-phase equilibrium lines and was proved to be able to determine the quadruple points as accurately as the conventional method. By using this method, even though some preparation is necessary, a quadruple point can be determined in just a single experimental run. The behavior of the system near the quadruple points was also examined experimentally, for both the quasi-static and the irreversible change cases. At the quadruple points, the temperature and pressure of the system were kept constant for a while, as at the triple point of water. In both cases, the representative point of the state of the system passed through the quadruple point on a p-T diagram. © 2004 Elsevier B.V All rights reserved.

The relationship between the occupational exposure limits (OEL) and the lethal dose 50 (LD50) Values of rats or mice for metals and metallic compounds was statistically analyzed by a stepwise multivariate regression method. The OEL values were predicted from LD50 values and metallic compensation coefficients (MCC), which were developed as the regression coefficients of dummy variables that represented the metallic element contained in the substance of interest. The value of the MCC indicated the extent of the adverse health effects of the metal in the substance. Smaller values of the MCC were assigned to metals that would have the more severe adverse health effects, such as carcinogenesis, while larger values were given to the less toxic metals. The Health Index (HI) based on the OEL values was proposed as a convenient measure of the toxicity of industrial products. The prediction method could be applied to toxicity risk assessments by using the HI when a designer of consumer products wants to use substances for which OEL values have not been determined. Two case studies were conducted to estimate the potential toxicity of materials used in solders and in rechargeable batteries.

We propose a new sequestration process for anthropogenic carbon dioxide (CO2) that uses waste cement. The proposed process consists of two main reactions. The first is the extraction of calcium ions (Ca2+) from waste cement particles by pressurized carbon dioxide (several megapascals of pressure). The second is the precipitation of calcium carbonate (CaCO3). Ca2+ extracted from waste cement is deposited as CaCO3 when the pressure is reduced. CaCO3 is disposed of directly, or recycled as a raw material for cement production. In the latter case, the same amount Of CO2 is considered to be sequestered because the net amount of virgin limestone mined can be reduced. The power consumption and cost of the proposed sequestration process for CO2 emitted from a 100 MW thermal power plant were evaluated, on the basis of laboratory-scale experimental results. The power consumption for the operating process strongly depended on the operating conditions such as the cement/water ratio, the CO2 pressure, and the average cement diameter. The minimum power consumption was 25.9 MW/100 MW of power generation when optimized within the operating conditions studied experimentally, and the sequestration cost associated with the power consumption (excluding capital and maintenance) would be about $22.6/t of carbon dioxide. This result indicates that the present process is highly competitive with previously reported CO2 sequestration scenarios such as ocean sequestration. Sensitivity analysis of the operating parameters was carried out on the operating power consumption, and it was found that a smaller ratio of waste cement to water and a lower CO2 pressure will decrease the operating power consumption.

A new separation method using gas hydrate formation is proposed for separating HFC-134a from gas mixtures containing N-2 and HFC-134a. The feasibility of this separation method was investigated from various points of view. First, to determine the mixed hydrate stability region, three-phase equilibria of hydrate (H), liquid water (L-W), and vapor (V) for HFC-134a + N-2 + water mixtures with various HFC-134a vapor compositions were closely examined in the temperature and pressure ranges of 275-285 K and 0.1 - 2.7 MPa, respectively. Second, the compositions of the hydrate and vapor phases at a three-phase equilibrium state were analyzed for identical mixtures at 278.15 and 282.15 K to confirm the actual separation efficiency. Third, kinetic experiments were performed to monitor the composition change behavior of the vapor phase and to determine the time required for an equilibrium state to be reached. Furthermore, X-ray diffraction confirmed that the mixed HFC-134a + N-2 hydrates were structure II. Through an overall investigation of the experimental results, it was verified that more than 99 mol % HFC-134a could be obtained from gas mixtures after hydrate formation and subsequent dissociation processes. Separation of HFC-134a using hydrate formation can be carried out at mild temperature and low-pressure ranges. No additive is needed to lower the hydrate formation pressure.

Formation of CO2 hydrate using a Kenics-type static mixer was studied experimentally. The flows of liquid CO2 and water were mixed in the static mixer, and CO2 hydrate was formed continuously from the two-phase flow. The patterns of hydrate formation were found to be dependent on the flow velocities of liquid CO2 and water. The flow of agglomerated hydrate chunks in water occurred under relatively CO2-rich conditions, while dispersed flow of tiny particles of CO2 hydrate with small liquid CO2 drops was observed under relatively water-rich conditions. These effects could be explained by two mechanisms occurring in the static mixer, namely, continuous shedding of hydrate films from the interface between liquid CO2 and water induced by the shearing force and breakup of the CO2 drops. The energy consumption by the static mixer for the hydrate formation process was estimated, and it was significantly less than that for a stirring vessel type reactor. A continuous hydrate formation process could be achieved using the static mixer.

The process energy consumption was estimated for gas separation processes by the formation of clathrate hydrates. The separation process is based on the equilibrium partition of the components between the gaseous phase and the hydrate phase. The separation and capturing processes of greenhouse gases were examined in this study. The target components were hydrofluorocarbon (HFC-134a) from air, sulfur hexafluoride (SF6) from nitrogen, and CO2 from flue gas. Since these greenhouse gases would form hydrates under much lower pressure and higher temperature conditions than the accompanying components, the effective capturing of the greenhouse gases could be achieved by using hydrate formation. A model separation process for each gaseous mixture was designed from the basis of thermodynamics, and the process energy consumption was estimated. The obtained results were then compared with those for conventional separation processes such as liquefaction separation processes. For the recovery of SF6, the hydrate process is preferable to liquefaction process in terms of energy consumption. On the other hand, the liquefaction process consumes less energy than the hydrate process for the recovery of HFC-134a. The capturing Of CO2 by the hydrate process from a flue gas will consume a considerable amount of energy; mainly due to the extremely high pressure conditions required for hydrate formation. The influences of the operation conditions on the beat of hydrate formation were elucidated by sensitivity analysis. The hydrate processes for separating these greenhouse gases were evaluated in terms of reduction of global warming potential (GWP). (C) 2004 Elsevier Ltd. All rights reserved.

A carbonyl sampler originally designed for the active sampling method (Sep-Pak XPoSure) was used for long-term passive sampling, and its applicability as a passive sampler was examined through field experiments. The uptake rates of passive sampling were determined experimentally from collocated passive and active samplings for various sampling periods. The obtained uptake rates of formaldehyde, acetaldehyde, and acetone were 1.48, 1.23, and 1.08 mL/min, respectively. These uptake rates were consistent for a wide range of the sampling term (12 hr-2 weeks). Uptake rates of each carbonyl were proportional to the diffusion coefficients of each. Therefore, the ratios of diffusion coefficients were used to calculate the uptake rates of carbonyls for which the rates were not determined experimentally. Lower limits of determination were 2.16-17.5 mug/m(3) for 2-week sampling. It was confirmed that 2-week monitoring of carbonyl concentrations up to 118229 mug/m(3) was possible. Relative standard deviations of the passive method generated from the repeatability test were 2-12.3% error for five samplings, and the recovery efficiencies were larger than 90%. Thus, the passive sampler was found to be highly suitable for long-term monitoring of carbonyl compounds.

The effect of the casting solvent on the structure of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) membranes was investigated experimentally. The PTMSP membranes were cast from solutions of cyclohexane, toluene, and tetrahydrofuran; the membranes were characterized by the positron annihilation lifetime spectroscopy (PALS) technique and by gas-permeation measurements of O-2, N-2, and CO2. The decay curves from the positron annihilation lifetime spectroscopy gave the best fit when two long-life components (tau(3) and tau(4), tau(3) < tau(4)) were employed. This suggests that two types of free volume existed in the PTMSP membranes. The size and number density of tau(4), which was characteristic for PTMSP, decreased in the following order of the casting solvents: cyclohexane > toluene > tetrahydrofuran. The order was consistent with the order of gas permeability. A good correlation was observed between the permeability and the structural parameter that denoted the free-volume size and the number density of tau(4). (C) 2002 Wiley Periodicals, Inc.

Pressure-mole fraction phase diagrams for the CO2-water system at temperatures between 278.15 and 298.15 K and pressures up to 30 MPa, which correspond to those of ocean waters at depths to 3000 m, are developed based on the literature data and experimental results obtained by the authors. The resultant phase diagrams can serve as a basis for analyzing the phase behavior of liquid CO2 and CO2 hydrate disposed of in the ocean as a means to mitigate global warming.

A new process to fix anthropogenic carbon dioxide via treatment of waste concrete was proposed. The main reaction of the process is the extraction of calcium ions from waste concrete particles by use of pressurized carbon dioxide (several MPa in pressure). The extracted Ca2+ is deposited as calcium carbonate (CaCO3) particles when the pressure is reduced to the atmospheric pressure. The CaCO3 particles can either be disposed of directly or reused industrially for cement production. The former case is a direct disposal of CO2 the latter case is an indirect disposal of CO2, because the net mining amount of virgin limestone could be reduced. Energy consumption and cost for the fixation process of CO2, emitted from a 100-MW thermal power plant by the proposed process was evaluated based on laboratory scale experimental results. It was found that energy consumption for the operation was 3.5 MW per 100-MW power generation, and the fixation cost was about JPY 2100 per metricton of carbon. This result indicates that the present process is highly competitive with the previous CO2 fixation scenarios such as ocean sequestration.

A poly(vinyl alcohol)/cyclodextrin (PVA/CD) membrane was utilized in the pervaporation of ethanol/water mixtures. The membrane was prepared by casting an aqueous solution of PVA and β-cyclodextrin oligomer. The CD oligomer was successfully immobilized in the membrane by the crosslinking of PVA with glutaraldehyde for 1 h. The content of CD was up to 33%. The effect of CD on the pervaporation performances of water/ethanol was investigated by measurements of the sorption equilibrium and the calculations of the diffusion coefficient of the permeants in the membrane. The addition of CD increased the water selectivity of the pervaporation of water/ethanol, especially at lower (&lt 40%) and higher (90%) ethanol concentrations in the feed. The water selectivity through the sorption equilibrium, on the contrary, decreased by the addition of CD this indicated that the effect of CD on the diffusion coefficient was predominant on the pervaporation performance. At lower ethanol concentrations, the addition of CD increased the permeation rate of water because of the large increase in the diffusion coefficient. The permeation rate of ethanol decreased due to the large decrease in the diffusion coefficient. At higher ethanol concentrations, the permeation rate of water slightly decreased the diffusion coefficient of water increased but the water solubility decreased. The permeation rate of ethanol largely decreased because both the diffusion coefficient and solubility of ethanol decreased. © 1994.

A poly(vinyl alcohol)/cyclodextrin (PVA/CD) membrane was utilized in the pervaporation of ethanol/water mixtures. The membrane was prepared by casting an aqueous solution of PVA and β-cyclodextrin oligomer. The CD oligomer was successfully immobilized in the membrane by the crosslinking of PVA with glutaraldehyde for 1 h. The content of CD was up to 33%. The effect of CD on the pervaporation performances of water/ethanol was investigated by measurements of the sorption equilibrium and the calculations of the diffusion coefficient of the permeants in the membrane. The addition of CD increased the water selectivity of the pervaporation of water/ethanol, especially at lower (&lt 40%) and higher (90%) ethanol concentrations in the feed. The water selectivity through the sorption equilibrium, on the contrary, decreased by the addition of CD this indicated that the effect of CD on the diffusion coefficient was predominant on the pervaporation performance. At lower ethanol concentrations, the addition of CD increased the permeation rate of water because of the large increase in the diffusion coefficient. The permeation rate of ethanol decreased due to the large decrease in the diffusion coefficient. At higher ethanol concentrations, the permeation rate of water slightly decreased the diffusion coefficient of water increased but the water solubility decreased. The permeation rate of ethanol largely decreased because both the diffusion coefficient and solubility of ethanol decreased. © 1994.