To design and analyze a fluidized-bed reaction with high selectivity, a new comprehensive reaction model for fluidized-bed consecutive reactions was developed, and mathematical solutions were derived for the case in which effect of axial gas dispersion in emulsion phase can be ignored (VUME model) and for the case of perfect mixing (PME model). The performance of fluidized-bed reaction was analyzed with these models, and basic parameters affecting and controlling the catalytic reaction in the fluidized-bed and the similarity rule of fluidized-bed reactions were clarified. Based on the similarity rule and comparison with the experimental result of a fluidized-bed consecutive reaction, it was found that a mass-transfer controlling region and a back-mixing controlling region exist in a dense-bed reaction, and the experimental result agreed well with the PME model. It was also concluded that direct contact ratio (Ndc), the mass-transfer capacitance coefficient to reaction rate constant ratio (Nmr), and the consecutive reaction rate constant ratio of catalyst particles (Nrr) play important and effective roles in enhancing the reaction performance, especially selectivity, and the cooperative effect of these parameters is essential for designing a highly selective fluidized-bed reaction.
We observed the phenomena occurring in a fluidized catalyst bed by using a reactor made of transparent glass in order to study the decrease in fluidization quality during a reaction involving a decrease in gas volume. It was found that the upper part of the bed agglomerated with the progress of the reaction, and defluidization occurred. This agglomerated part was lifted up by the fluidizing gases passing through the column. Since the difference in density between the gases in the bubble and emulsion phases affected the fluidization behavior, we studied the effect of the gas density on the emulsion-phase contraction. In the present study, this effect was found to be insignificant. The decrease in fluidization quality caused by the reaction involving gas volume reduction was considered to be due to the decrease in gas volume in the emulsion phase and the agglomeration of the bed, which prevented the formation and rise of bubbles in the bed.
Dimethylcarbonate (DMC) has been attracting wide attention as an environmentally benign chemical raw material. To develop a vapor-phase synthesis process of DMC by oxidative carbonylation of methanol, effects of reaction conditions have been investigated over CuCl2/NaOH/activated carbon catalysts. A fluidized-bed reactor is preferable for the vapor-phase DMC synthesis, considering the need for temperature control and continuous catalyst regeneration. Experiments using small-scale (13.3 mmID) and bench-scale (53.5 mmID) fluidized bed reactors were carried out. Bench-scale reaction including catalyst regeneration showed good performance over 200 hours.
A downflow-type reactor, or downer, is expected to provide high gas-solid contact efficiency and reaction selectivity. In a catalyst-circulating process, for example, an HS-FCC process, steady upward transportation of catalyst particles using a riser to the top a downer is very important, together with efficient catalyst regeneration. Particle-transport characteristics from a freeboard to the riser were investigated using a fluidized bed with a cone-type freeboard for a variety of experimental conditions of gas flow rate, bed depth, and bed structure. The applicability of this concept to HS-FCC processes is also discussed.
JFE Group has produced phthalic anhydride at its Chiba plant since 1979 by using a fluidized bed reactor. A ground silica-based catalyst was used at the beginning of fluidized bed operation. By changing to a spherical type catalyst, the duration of catalytic activity was increased and catalyst density in the bed was reduced by around 10%. Addition of Cs to the catalyst enhanced its activity. Non-active silica gel was used as diluent for matching catalyst activity with plant capacity. The active component of Cs added to the catalyst has a lower melting point, the catalyst particles bound with it. The fluidized bed was operated stably by controlling the Cs/K ratio.
Thermal cracking of heavy oil in a fluidized bed was investigated to establish a technology for simultaneously upgrading the heavy oil at a high cracked-oil yield and converting the coke deposited as a cracking by-product into synthesis gas or hydrogen. As a result, liquid-phase thermal cracking at a mild temperature in a fluidized bed, which has long been thought difficult to perform, was found to be possible by use of fine porous particles owing to their excellent fluidity and the “particle capacitance effect”. Since the feed heavy oil was quickly occluded in the pores of the particles by the capacitance effect and the particle surface was kept dry, liquid-phase thermal cracking proceeded stably at a relatively low temperature while smooth fluidization was maintained. It was also found, as another capacitance effect, that cracking was highly enhanced by micro- or meso-pores of the particles. Based on the reaction results, a reaction model for fluidized-bed thermal cracking was developed, and the effect of reaction conditions and particle pore size on the cracking was quantitatively clarified. The enhancement of cracking by reduction of particle pore size could be explained by the reduction of vapor pressure at the meniscus surface of the liquid phase occluded in the pores of 1 to 10 nanometer order diameter.
Efficient NOx removal from a large volume of exhaust gas with high NOx concentration was investigated using a fluidized bed system with a fine photocatalyst. An ultrafine photocatalyst was coated on carrier particles for easy handling in the fluidized bed process. Two types of fluidized beds were tested for NOx removal; a Bubbling bed of 35 mm inner diameter, and a circulating fluidized bed of 18 mm I.D. with 1140 mm riser height. Simulation was also carried out to clarify the effect of the size of the experimental setup and flow characteristics. The fluidized bed system with a photocatalyst was able to remove NOx continuously from a large volume amount of exhaust gas. The dominant parameters were mass transfer from the main gas stream to the catalyst surface, solid loading rate and maximum capturing capacity of NOx.
A two-stage process was proposed for the chemical recycling of plastics through their conversion into hydrogen and carbon. In the first of the two reactors, fed plastic chips are thermally decomposed into hydrocarbons. In the second reactor, the hydrocarbons are catalytically decomposed into carbon and hydrogen. In this study, in order to obtain basic data for the second reactor, ethane, ethene and propene were catalytically decomposed in a laboratory-scale spouted bed reactor (600 mm high, 21.6 mm in internal diameter, made of SUS304). The effects of hydrocarbon type, hydrocarbon partial pressure, nickel-plating on the spouting medium (alumina balls), temperature, and superficial gas velocity on the decomposition behavior were investigated. The main products were hydrogen and carbon. Nickel plating on the spouting medium contributed a significant increase in hydrogen yield.
It is widely known that shirasu balloons are produced from shirasu. This study, aimed to develop a continuous reactor for producing shirasu microballoons of less than 20 μm in diameter. The effect of the riserbottom temperature on the bulk density and morphology of shirasu microballoons was investigated. It was found that the bulk density of shirasu microballoons reached a minimum at 1175 K and that shirasu microballoons with a particle size of 20 μm or less could be produced.
In the field of heat transfer and fluid flow, the application of numerical simulations using commercial CFD (Computational Fluid Dynamics) software for development of combustion equipment has become widespread with the improvement of power and the fall in price of computers. However, there are few reports concerning the examination of CFD performance through comparison with experimental data or other CFD software. This paper reports the numerical simulation of the turbulent air jet in a cylindrical combustor using commercial CFD software and comparison of the calculated results with the results obtained with our original CFD software and the experimental data reported previously. The velocity profiles estimated with the commercial software were found to differ from those calculated with our original software, though almost the same turbulence model, boundary conditions, numerical grid and discretization scheme were used in both. Moreover, the employment of a higher order discretization scheme and finer grid with the commercial software did not always produce good results compared to our original software. The results reported here will be useful in applying commercial CFD software to the development of combustion equipment.
The quality of dispersion of nanoparticles, which agglomerate strongly, greatly influences the processing process and the quality of final products such as sintered bodies. Therefore, the technology of nanoparticle dispersion is considered as an important element in industry. In the present work, aqueous suspensions of nanoparticles were produced by a high pressure wet-type jet mill, using SiO2 (primary particle diameter: about 20 nm) and Al2O3 (primary particle diameter: about 13 nm) as dispersed particles. The effects of the operating conditions on the mean size of particles, and the relation of the mean particle size to aqueous suspension viscosity were examined. The volumetric mean diameter (dv) decreased with increasing processing pressure (P), number of passages (N) and concentration of particles (C), but did not decrease to the primary particle size. The agglomerated particle size was 5.9-7.4 and 6.5-8.1 times the primary particle size for SiO2 and Al2O3, respectively. Therefore, dispersion became increasingly difficult as primary particle size decreased. For all the present experimental conditions, the relationship between dv and the energy dissipation rate (ε) could be expressed by a straight line with a slope of −0.03 on logarithmic paper. Aqueous suspensions of both types of particles exhibited plastic behavior at N=0, and pseudo-plastic behavior at N≥1. Furthermore, the pseudo-plasticity decreased with increasing N, and at N≥10 the aqueous suspensions apparently exhibited Newtonian behavior.
Miniaturized devices referred to as microchannels produce special and outstanding chemical reaction behavior; however their mechanism and their fluidic behavior, which is indispensable to analysis of the mechanism, have not been elucidated. This work performs three-dimensional observation of fluid using confocal fluorescence microscopy and computational fluid dynamics simulation to assess the fluidic behavior in curved microchannels. Results showed that a complicated differential medium interface between aqueous liquid/liquid is produced by the laminar secondary flow at the curve of the microchannel. The interface area increased more than three times over that of the flat simple interface. Density and viscosity of the fluid also affect interface configuration, and the results suggest that only the Reynolds number is insufficient to account for the fluid behavior in curved microchannels. Drastic variation in the interface area increases diffusive mixing: it also shows that the understanding of fluid motion and mixing behavior is necessary to design and manufacture microreactors.
Using coal-water slurry with fine coal of mean diameter 28 μm, the heat transfer coefficient at the annular outer wall of a double tube bubble column was determined based on measurement of the lateral temperature profile and heat transfer rate in the annular section. Heat transfer behavior of the slurry system without bubbling was similar to that of liquid single phase and, it was found that the heat transfer coefficient could be estimated by introducing the viscosity ratio of coal slurry to water into the estimation equation for a liquid single phase. In the gas-liquid and gas-slurry systems, the effect of gas holdup on the heat transfer is more remarkable than that of liquid or slurry velocity. Consequently, a major factor affecting the heat transfer at the annular wall for the gas-coal slurry is the gas holdup.
Yamamoto’s equation (Yamamoto, 1961) was modified to estimate the vortex profiles in baffled agitated vessels by assuming that the power input varied with the radius of the cylindrically rotating zone under the baffled conditions. The equation obtained is a function of agitator dimensions, baffle type and operating conditions. Under industrial conditions (b/D<0.16, b: height of impeller blade, D: vessel diameter), the radius of the cylindrically rotating zone calculated in this work agrees with Kamei’s experimental data (Kamei et al., 1999) measured in a baffled agitated vessel with a flat paddle. The vortex profile in a commercial agitated vessel can be calculated by the proposed modification of Yamamoto’s equation.
The steam reforming of methane with addition of a small amount of oxygen was simulated in a co-current, isothermal and plug-flow-type membrane reactor with selective permeation of hydrogen. The effect of operating conditions on the conversion of methane and the permeated hydrogen yield was investigated using two dimensionless numbers, the Damkohler number and the permeation number, θ. Methane conversion, XCH4, shows approximately the same dependency on permeation number in terms of the permeability ratio of hydrogen to nitrogen, while hydrogen purity in the permeate increased with an increase in hydrogen selectivity. As for heat balance of the membrane reactor, the addition of oxygen, which generates heat of combustion, in feed at O/C of 0.3 to the endothermic steam-reforming reaction, is required to maintain the membrane reactor autothermal. For the case of heat-balance of membrane reactor system, which considers the heat balance of the membrane reactor and the heat of combustion of the retentate stream, the system can be autothermal for the case of methane conversion of 0.7-0.8.
A novel ion-exchange moving bed has been developed which uses resin-liquid slurry tanks, thereby allowing fermentation broth to be fed directly to the resin process without pretreatment. A previous study using L-Lysine fermentation broth demonstrated that clogging by bacteria did not occur, but such problems as poor performance in the elution process and instability of the equipment remained. To overcome these problems, the experimental apparatus was improved by using a new type of resin separator. Stable operation was achieved, and the production of L-Lysine reached 15.4 ton in 12 days. The elution process was especially improved compared to the previous work. The amount of the eluent could be reduced and the concentration of L-Lysine in the eluate increased. The novel method also produced a smaller amount of waste water than the conventional fluidized bed. In addition, a numerical simulation model was constructed in order to optimize the operation. The predictions from the simulation agreed well with the experimental data.
A method has been developed for removing humic substances by hybrid ultrafiltration combined with both flocculation and adsorption treatments. Flocculation by use of poly aluminum chloride (PACl) was particularly effective for the removal of humic acids, which constitute the relatively high molecular weight fractions of humic substances, whereas adsorption by use of powdered activated carbon (PAC) was able to remove fulvic acids of relatively low molecular weight effectively, which could not be fully flocculated by PACl. Consequently, the combined operation of flocculation and adsorption was extremely effective for the treatment of humic substances. It was shown that both flocculation and adsorption characteristics of humic substances were strongly influenced by the solution pH. Hybrid ultrafiltration in combination with flocculation and adsorption treatments exhibited high permeate flux with high permeate quality. It was found that the dosages of both PACl and PAC exert large effects on the filtration performance, suggesting that there exist optimum dosages of PACl and PAC.
A numerical and theoretical investigation were carried out to find the possibility of appearance of a regular regime (RR) curve on the drying with a constant material temperature of a hygroscopic porous slab with mass-transfer resistance on the slab surface. The investigation afforded the following results: first, in the case of constant resistance a RR curve appeared with the same axi parameters as in the case of no resistance, while no curve appeared in the case of increasing resistance. Second, when the dependence of the moisture transfer coefficient on the moisture content is known, the relation between drying rate and mean moisture content can be predicted for the two cases of resistance from the drying characteristic function derived from the flux ratio method.
Highly efficient silica products are widely used in the semiconductor industries. Generally this type of silica is produced by a liquid-phase method, which entails various limitations. In this communication, we report a method for manufacturing silica in the gaseous phase by hydrolyzing alkoxysilane. The operation conditions of the gaseous-phase reactions during the synthesis of silica were optimized. The influences of temperature, gas flow rate and concentrations on the reaction kinetics (particle production rate) were also investigated. Products were characterized by SEM. Results indicated that the silica particle diameters could be controlled by changing reaction conditions.
A new powder simulation method that permits the large-scale of simulation of powder phenomena was developed using cellular automata. The automaton rule consists of the transition rule of constituent particles and the interaction rule between particles. The interaction rule is divided into three rules: the collision rule, the static contact rule between particles, and propagation rule of impulsive force through the particle bed. The interaction rule, which is important in the proposed simulation method, was derived quantitatively on the basis of microscopic information on the interactive force between flowing particles under gravity obtained by particle element simulation. The evolution of powder phenomena with time can be simulated by defining the equivalent time to one step in the automaton simulation, and the state variables in granular flow can be obtained by the proposed simulation method. The validity of the proposed powder cellular automaton method was confirmed by comparison of the simulated results with experimental findings: the simulated flow patterns, contact force and velocity distribution in discharging flow from a hopper agreed well with the experimental ones. The proposed powder automaton simulation method has the feasibility to simulate large-scale powder phenomena.
Mica powders were pulverized by two methods yielding respectively granular and flaky particles. The mica particles were made hydrophobic by using a silane-coupling agent, and their properties as emulsifying agents were investigated. The hydrophobic mica particles formed stable O/W type droplets but not W/O type droplets. The droplet size decreased from several hundred millimeters to below one hundred micrometers with increase in the added amount and decrease in the size of mica particles. Further, when added in the same amount, flaky particles formed smaller droplets than granular particles. Droplets of styrene were formed using the mica particles and polymerized by UV irradiation. SEM photographs of the polystyrene beads suggested that the mica particles are adsorbed on the organic-aqueous interface with their flat plane lying on the interface.
The void fraction inside granules prepared by spray drying is an important factor influencing the strength, dissolution, and other properties of the granules. Void fraction is usually measured with a mercury porosimeter, but this requires elaborate operations and the equipment is not widely available. Moreover, this method is not suitable for broken granules, granules with surface holes, or weak granules. In this paper, a method to estimate the void fraction inside a granule was proposed on the basis of the measured size distribution of granules and the estimated size of initial droplets. It was found that the size distribution based on weight basis provided better results, since the number-based distribution yielded serious errors by ignoring small particles in estimating the size distribution of initial droplets. As a result, the median diameter on a weight basis was found useful to estimate the void fraction inside granules over a wide range from hollow to fully packed granules.
Adsorption by activated carbon is used for removing gas-phase toxic materials. However, because of their high toxicity and the large number of isomers, difficulties exist in testing these materials, and basic knowledge on adsorption in gas phase is limited. We have constructed a lab-scale on-column adsorption experimental apparatus in an attempt to clarify the adsorption of gaseous PCBs by activated carbon. Breakthrough curves showed that the breakthrough proceeded from di- to tri- and to tetra-chlorobiphenyls. Curves for different isomers revealed clear differences in breakthrough time among them. Breakthrough times were shorter for isomers with higher vapor pressures.