An (N+2)-field model proposed in our previous study (Shimada and Tomiyama, 2005) was modified for application to compressible bubbly flows with chemisorption by taking into account the dependence of phase densities on components. Then, a numerical method for solving the modified (N+2)-field model was developed. The proposed method was verified through a sample calculation, i.e., a simulation of a bubbly flow with chemisorption in a rectangular vessel. Predictictions were compared with experiments. As a result, it was confirmed that the proposed method gave good predictions for the time evolutions of pH and gas holdup and for natural convection induced by the heat of reaction.
In three-phase agitated contactors, the dispersion of gas and the suspension of solid particles are required simultaneously to promote both gas–liquid mass transfer and solid–liquid mass transfer. A new type of mechanically agitated three-phase contactor with a large ring sparger and a downflow pitched blade turbine was proposed, and its solid–liquid mass transfer performance was investigated experimentally with 4.75 vol% solids in comparison with the conventional types. The new type of contactor prevents the concentration of inflow gas around the impeller, resulting in efficient discharge flow from the impeller and efficient suspension of solid particles. The solid–liquid mass transfer coefficients under the regime of complete suspension of particles were correlated with the agitation power input to gassed liquid, independently of the types of contactor and the conditions of gassing. Under the regime of partial suspension of particles, the mass transfer coefficients depended strongly on the fraction of solid particles suspended.
The flow pattern and the solid-liquid and gas-liquid mass transfer coefficients in figure-eight shaking vessels were observed by experimental methods. The flow patterns in the figure-eight shaking vessels changed irregularly with increase in the shaking frequency. The region of the optimum operating condition of the figure-eight shaking was larger than that of the reciprocal shaking. The solid-liquid mass transfer coefficient was correlated with the same correlation as that of the rotary shaking vessel. The gas-liquid mass transfer coefficient of the figure-eight shaking vessel was also correlated with the same type of correlation as that of the rotary shaking vessel.
Fluoride ion removal by calcium fluoride crystallization was studied. This study aimed to find the range of fluoride ion concentration which would be applicable to calcium fluoride crystallization. First, batch experiments without a seed crystal were conducted to obtain the relationship between initial fluoride ion concentration and calcium ion concentration where fluoride ion concentration would begin to decrease in the process of calcium fluoride precipitation. Batch experiments using a seed crystal were then conducted under the same conditions as those without a seed crystal to obtain the relationship between initial fluoride ion concentration and calcium ion concentration where fluoride ion concentration would begin to decrease due to crystallization of calcium fluoride on the seed crystal. Supersolubility curves of calcium fluoride were obtained from both sets of experiments. From these, the metastable region of calcium fluoride between the solubility curve and the supersolubility curve was obtained. The range of fluoride ion concentration applicable to calcium fluoride crystallization was found to be less than 3 mmol/l. XRD of the grown calcium fluoride crystals was investigated. The main peaks of the calcium fluoride used in the experiments were found to agree with the literature values.
To understand the effect of soot on heat transfer in a combustion furnace, a combined heat transfer analysis in a methane-air diffusion combustion furnace including soot was carried out. Nongray radiative properties of the combustion gas were evaluated using the Exponential Wide-Band Model, which was applied to the gray band model. The Discrete-Ordinates Method was used to solve the radiation transport equation. As the soot concentration increases, the radiative heat flux to the combustion furnace walls increases, whereas the convective heat flux decreases. The temperature in the furnace falls with an increase in the soot concentration, and both peaks of the temperature and the total wall heat flux shift to the upstream. This is because net irradiation of the radiation energy from the combustion gas–soot mixed medium increases with the soot concentration. Since the medium radioactivity decreases with the reduction in temperature accompanying the increase in the soot concentration, the wall surface thermal radiation flux decreases in the downstream of the furnace.
We investigated a continuous process for recycling of silane cross-linked polyethylene by use of supercritical alcohol. Conditions for the recycling silane cross-linked polyethylene to yield polyethylene were investigated by a batch process using an autoclave. It was found that the use of supercritical alcohol at around 10 MPa and 320°C was optimal for the plasticization of silane cross-linked polyethylene. For the continuous process, twin-screw extruder was adopted, into which the supercritical alcohol was injected. As a result of the optimal reaction between silane cross-linked polyethylene and alcohol, the silane cross-linked polyethylene could be recycled at the speed of 8 kg/h. The recycled Polyethylene showed good mechanical properties and satisfied the standards for its use in the insulation of cable. Thus the twin-screw extruder is potentially useful as a reactor for supercritical fluid.
The effect of humidity on decomposition of ethylene by a spiral-wound wire-net photocatalyst was investigated to obtain kinetic information for the design of an efficient photocatalytic reactor. Photocatalytic decomposition of ethylene by titanium dioxide was found to proceed more rapidly the lower the humidity. Kinetic analysis was performed on the assumption that ethylene and water vapor molecules were adsorbed competitively on the same active site, and photocatalytic decomposition was controlled by the surface reaction step of adsorbed ethylene. At water vapor concentrations from 10,200 to 28,300 ppm, and the ethylene concentrations from 30 to 100 ppm, the reaction rate equation is expressed as follows:
where CC2H4 [ppm] is the concentration of ethylene, CH2O [ppm] is the concentration of water vapor, and r [ppm·min−1] is the decomposition rate of ethylene. The decomposition rates evaluated from experimental data are in fair agreement with the calculated ones.
Solid Oxide Fuel Cells (SOFC) power plant systems have operational constraints on state variables, such as fuel cell temperature and pressure. In this research, a dynamic model of a SOFC power plant was derived and its accuracy was evaluated by using experimental plant data. The optimal operations to minimize start-up and shutdown time under various operational restrictions were also derived. Data on the start-up and shutdown of a real plant were used to evaluate the modeling accuracy. The results show that the modeling errors of SOFC inlet temperatures could be kept within 5.0% of the sensor ranges by including the heat-leak and the heat-pass of the apparatus in the model. The optimization results for start-up time and shutdown time showed that they could be reduced to 28% and 67% of the conventional operation times, respectively. Furthermore, the optimal operation to minimize energy consumption was derived for the periodic operation mode of weekly start-stop.
Introduction of advanced control has provided improvements in product quality and production efficiency in the petrochemical field. In batch processes, applying advanced control is difficult because of the complicated recipes and the generally limited production. This paper presents a design method for a temperature control system using Generic Model Control (GMC) for batch reactor temperature control. Although the conventional GMC algorithm has a position form, the present algorithm was given a velocity form for simpler implementation. Moreover, a new compensation term regarding the thermometer position was proposed. A correspondence formula between GMC tuning parameters and the conventional PID was derived. To verify its validity, an examination using actual liquids was carried out in a polymerization reaction plant. As a result, compared with an established PID control, GMC was found to be suitable for variation control of reaction temperature deviation, and to be robust to process characteristic changes in accordance with advances of batch recipes.
The optimum condition for the cyclic steady state is of central importance in the design and operation of a pressure-swing adsorption (PSA) process. Numerical simulation by use of a mathematical model of the process is suitable for optimizing the operating condition. This paper describes the direct method for determining the cyclic steady state based on the mathematical model and the optimizing method for the PSA process. The proposed method was proved through application to a laboratory-scale PSA system for air separation. The results showed that the optimum operating condition was obtained by the proposed method in a short calculation time.
Bubble diameter and gas holdup were measured in a gas-liquid dispersion system under microgravity. The results show that an optimal airflow rate exists in this experimental system. The mean bubble diameter was in the order of 10–20 mm and the bubble diameter decreased with increasing airflow rate. Gas holdup was 0.22 under microgravity, which is greater than that at 1 G.
A new plate static micromixer interconnected with σ-type unit elements in series has been developed, which has systematic mixing functions of division and multiplayer formation of fluid layers by combining splitting, realignment and recombination of fluid streams. The number of unit elements required for complete mixing n was measured by conducting the decolorizing reaction of iodine with sodium thiosulfate for various Reynolds numbers Re and flow ratios of two fluids to be mixed. At low Re, n increased with Re and its variation with operating variables and device dimensions were well correlated by use of a functional relation derived from a model analysis for the diffusion process in a fluid layer whose thickness is reduced by half after the fluid passes through each unit element. When Re exceeds 10, n begins to decrease with Re. Under the condition of large Reynolds number, CFD analysis shows that a large deformation and stretch of the fluid interface takes place due to the bending and winding channel structure of σ-type element, and the mixing rate is accelerated through the deformed and stretched interface. n was correlated experimentally with Re and a variable representing Schmidt number derived from the model. In addition, the effects of aspect ratio were investigated by CFD analysis. In order to promote ideal fluid mixing of splitting and recombination and efficient removal of heat generated by reaction in σ-type plate static mixer, it is recommended to adopt channels whose cross-section is as close as possible to a square.
A batch small-scale fluidized bed was placed in a pressure vessel to measure CO2 gasification rate of coal char under almost same conditions as in the entrained bed coal gasifiers under development in Japan. This apparatus allowed the rate to be measured under various conditions of CO2 concentration, temperature up to 1773 K and pressure up to 0.8 MPa for char produced at high heating rate, without cooling of the produced char. The rate increased with increased total pressure at constant CO2 concentration as well as with increased concentration at constant total pressure. Thus the gasification rate at a given temperature and its time variation of the char from the coal used in the present study were affected only by the partial pressure of CO2, regardless of the total pressure and CO2 concentration.
To reduce heavy metal content in molten slag discharged from the melting process of municipal solid waste, the volatilization behavior of lead from simulated molten slag was studied. Experimental studies were conducted to determine the apparent volatilization rate of lead from simulated molten slag samples composed of CaO–SiO2–Al2O3 at temperatures of 1673 and 1773 K. The effects of slag composition, melting temperature and the atmosphere of O2–N2 or CO–CO2–N2 mixed gas on the volatilization rate were investigated. It was found that the volatilization rate of lead was higher from the slag with a high content of CaO and low contents of SiO2 and Al2O3 in an oxidizing atmosphere. The rate of volatilization was closely related to the slag viscosity, which depended on slag composition. At the higher temperature, the rate of volatilization was enhanced with a decrease in slag viscosity caused by temperature rise. It was also found that the rate of volatilization of lead was significantly increased in a reducing atmosphere compared to an oxidizing atmosphere, with an increase in the concentration ratio of CO to CO2 in CO–CO2–N2 mixed gas.
Phytoremediation, which is a plant-based technology for the removal of toxic contaminants from soil, has been receiving renewed attention as an environmentally friendly purification method. In this work, phytoremediation of soil contaminated with heavy metals and a novel recovery method of valuable metals were investigated. Athyrium yokoscense was used as a hyperaccumulating plant. The plants were separated into water-soluble material, holocellulose, methanol-soluble lignin (low-molecular-weight lignin), and Klason lignin (high-molecular-weight lignin) using steam-explosion and Wayman's extraction method, and the concentrations of metals, i.e., copper and iron, were measured. Levels of 15 mg of copper and 2.6 mg of iron were found in 1 g of the water-soluble material component of aboveground parts, i.e., leaf blades and petioles; and 41 mg of copper and 4.2 mg of iron in 1 g of the Klason lignin (high-molecular-weight lignin) component of underground parts, i.e., rhizomes and roots. This suggests that these components can be used for recovery of metals. The amounts of heavy metals in contaminated soil and plants were estimated. It was found that 1 g of copper and 0.1 g of iron were recovered from 500 g of contaminated soil, and the removal ratios of copper and iron in the contaminated soil were 82 and 95%, respectively.
This study was performed to develop a method for removing polychlorinated biphenyls (PCBs) from soil using hydrogen peroxide under high pressure (0.5–2.5 MPa) and high temperature (150°C) in an aqueous phase (wet peroxide oxidation). The level of PCBs in highly contaminated soils could be reduced below environmental standards by performing this oxidation process repeatedly. The ability of this process to remove PCBs was almost the same among PCB homologue groups. Although removal of PCBs from black soil with high organic content (26%) was only 72.6%, removal from soils with lower organic content (4–16%) was more than 90%. These results indicated that the wet peroxide oxidation is effective for removal of PCBs from soil.