Flow through packed bed reactors: 1. Single-phase flow

Single-phase pressure drop was studied in a region of flow rates that is of particular interest to trickle bed reactors . Bed packings were made of uniformly sized spherical and non-spherical particles (cylinders, rings, trilobes, and quadralobes). Particles were packed by means of two methods: random close or dense packing (RCP) and random loose packing (RLP) obtaining bed porosities in the range of 0.37–0.52. It is shown that wall effects on pressure drop are negligible as long as the column-to-particle diameter ratio is above 10. Furthermore, the capillary model approach such as the Ergun equation is proven to be a sufficient approximation for typical values of bed porosities encountered in packed bed reactors. However, it is demonstrated that the original Ergun equation is only able to accurately predict the pressure drop of single-phase flow over spherical particles, whereas it systematically under predicts the pressure drop of single-phase flow over non-spherical particles. Special features of differently shaped non-spherical particles have been taken into account through phenomenological and empirical analyses in order to correct/upgrade the original Ergun equation. With the proposed upgraded Ergun equation one is able to predict single-phase pressure drop in a packed bed of arbitrary shaped particles to within ±10% on average. This approach has been shown to be far superior to any other available at this time.     

Basic viscoelastic fluid flow problems using the Jeffreys model

Using Laplace transformation technique, semi-analytical solutions are obtained for three basic viscoelastic fluid flow problems under the effect of the Jeffreys model. These semi-analytical solutions are not available in the literature. The present work investigates the effect of two types of driving forces on the flow behavior. These two types are the velocity-type and shear-type driving forces. The effect of the relaxation and retardation times on the flow behavior for these two types of driving forces may be viewed well using the obtained semi-analytical solutions. The three fundamental problems are transient Couette flow, transient wind-driven flow over finite domains and the transient Poiseuille flow in parallel-plates channels. It is shown that as the dimensionless relaxation time (λ₁) increases, the flow response to the imposed driving force becomes slower. This implies that the flow needs more time to feel the presence of the driving force and hence needs more time to attain steady-state behavior. On the other hand, the effect of the dimensionless retardation time (λ₂) depends on the type of the driving force imposed on the system. For a velocity-type driving force, the flow response becomes faster as the dimensionless retardation time (λ₂) increases and for a shear-type driving force the flow response becomes slower as the dimensionless retardation time (λ₂) increases.     

Numerical simulation of turbulent solid–liquid two-phase flow and orientation of slender particles in a stirred tank

In this paper, turbulent solid–liquid two-phase flow involving slender particles in a tank stirred by standard Rushton turbines is simulated with two-fluid model using the improved inner–outer iterative method. Standard k–ε model is used to deal with turbulent flow. By comparison with the case of equivalent spherical particles, it is found that the flow field of slender particles is similar to that of spherical particles. The evolution of particle orientation as it follows the liquid flow in a stirred tank is modeled directly from the rigid slender rods revolution equation. Experiments about solid–liquid two-phase flow are also performed in a baffled tank using DPIV (digital particle image velocimetry). All simulation results are compared with experiments. The comparison between simulation and experiments confirms that the results are reliable. The good agreements between simulation and experiments verify the reliability of the methods employed in this paper. The influences of impeller speed on flow field and orientations are also investigated.     

Measurement of local gas holdup in bubble columns via a non-isokinetic withdrawal method

An inexpensive method was developed to measure local gas holdup based on withdrawal of the air–liquid dispersion in bioreactors under non-isokinetic conditions. The method was tested with several suction probe designs and withdrawal pressures using coalescing and non-coalescing model media as well as during yeast fermentations. Simultaneously, gas holdup was measured manometrically. With a straight end probe and vacuum pressure of 3 kPa, local gas holdup measurements presented a relative error <6% compared to the manometric method, independently of the media composition. This method was used to characterize local gas holdup in a bubble column. Two new parameters are proposed to characterize non-gassed volume and gas holdup homogeneity.     

Experimental and computational study of gas–solid fluidized bed hydrodynamics

The hydrodynamics of a two-dimensional gas–solid fluidized bed reactor were studied experimentally and computationally. Computational fluid dynamics (CFD) simulation results from a commercial CFD software package, Fluent, were compared to those obtained by experiments conducted in a fluidized bed containing spherical glass beads of 250– in diameter. A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied in order to simulate the gas–solid flow. Momentum exchange coefficients were calculated using the Syamlal–O’Brien, Gidaspow, and Wen–Yu drag functions. The solid-phase kinetic energy fluctuation was characterized by varying the restitution coefficient values from 0.9 to 0.99. The modeling predictions compared reasonably well with experimental bed expansion ratio measurements and qualitative gas–solid flow patterns. Pressure drops predicted by the simulations were in relatively close agreement with experimental measurements at superficial gas velocities higher than the minimum fluidization velocity, U_mf. Furthermore, the predicted instantaneous and time-average local voidage profiles showed similarities with the experimental results. Further experimental and modeling efforts are required in a comparable time and space resolutions for the validation of CFD models for fluidized bed reactors.     

Synthesis of ceria nanoparticles by flame electrospray pyrolysis

A flame electrospray pyrolysis is presented for synthesizing CeO₂ nanoparticles with a dense morphology, high crystallinity and nanometer size. Hydrated cerium nitrate precursor dissolved in an ethanol/diethylene glycol butyl ether mixture was injected into a CH₄/air premixed flame using an electrospray method. The number size distributions of the as-prepared particles were trimodal. It is suggested that the particles for the fine mode were formed by a Rayleigh disintegration of the charged precursor droplets during the droplet evaporation. The particles for the coarse and middle modes are surmised to come from primary and secondary droplets, respectively, which were formed simultaneously during the atomization processes. The CeO₂ nanoparticles for the coarse mode were nonspherical and composed of few crystallites. The nanoparticles for the fine and middle modes were nearly spherical and nonagglomerated. The as-prepared CeO₂ nanoparticles showed highly crystallinity.     

Water effect on the conductivity behavior of NH4PO3-based electrolytes for intermediate temperature fuel cells

The electrical conductivity at intermediate temperature of 150–250 °C and the activation energy for conductivity of composite proton conductors, 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ and 2NH₄PO₃–(NH₄)₂SiP₄O₁₃, were investigated as a function of water vapor pressure, P_H2O. The activation energy decreased linearly with the natural logarithm of P_H2O, indicating that water is chemically adsorbed to the electrolytes. The decrease in activation energy is possibly caused by formation of hydrogen bonds between the adsorbed water and the electrolytes. In addition, the pre-exponential factor of Arrhenius equation, σ₀, increased with P_H2O. This suggests that the adsorbed water may generate additional mobile protons for the composite electrolyte. Therefore, the enhancement in the electrical conductivity of a NH₄PO₃-based electrolyte in a water-vapor rich atmosphere originates possibly from the decrease in activation energy as well as the increase in mobile proton concentration.     

The contributions of “minimum primary emissions” and “new particle formation enhancements” to the particle number concentration in urban air

In an urban site affected by fresh vehicle exhaust emissions, the ambient air number concentrations of particles coarser than 3 nm (N) was split into two components, N=N1+N2. This was done using a method based on the high correlation between black-carbon (BC) and number (N) concentrations which is typically observed in ambient air and is the result of vehicle exhaust emissions. The component N1 accounts for “those aerosol components directly emitted in the particle phase” and “those components nucleating immediately after emission”. The component N2 accounts for the new particle formation enhancements during the “dilution and cooling of the vehicle exhaust” and is also influenced by “in situ new particle formation in ambient air”. The contribution of N1 to N exhibits a maximum of 55% during the morning rush hours (07:00–08:00). The contribution of N2 to N exhibits a daily evolution with a broad maximum during daylight (as solar radiation intensity), while for about 7 h (11:00–17:00) the N2 contribution to N is about 70%. During some “afternoon N2 events”, N2 contributions exceeded 90%. Enhancements in the new particle formation processes may increase the N/BC concentrations ratio in one order of magnitude, from 4.82×10⁶ particles/ng BC to 47×10⁶ particles/ng BC and during some events up to 97×10⁶ particles/ng BC. The results show evidence of the high potential of the vehicle exhausts and of the urban atmosphere to trigger new particle formation if the ambient air conditions are favourable. The method used in this study is useful in assessing future changes in the number to BC relationship due to forthcoming regulations in the vehicle exhaust emissions.     

Prediction of the pH and the temperature-dependent swelling behavior of -alginate hydrogels by artificial neural networks

This paper considers the possibility of using artificial neural network models to identify model for swelling behavior as new techniques. Multi-layer feed-forward, radial basis function and generalized regression neural network models were employed to predict the swelling behaviors of Ca²⁺-alginate hydrogels under different environmental conditions of pH and temperature. The results show that an excellent correlation between the experimental and predicted swelling ratios was obtained by the artificial neural networks. Generalized regression neural network has a better performance than the other neural network models. The absolute mean error, the determination coefficient and the standard error of prediction were used as performance criteria. In addition, the performances of the neural network models are significantly superior compared with those of second-order swelling kinetics, quadratic and cubic models of response surface methodology.     

Particle generation by laser ablation during surface decontamination

Laser ablation allows significant number of particles to be generated from the surfaces of cement, chromium-embedded cement, stainless steel, or alumina. The number concentrations and size distributions of the particles were experimentally investigated with respect to applied laser fluence (mJ cm^-2) and wavelength. Based on the measurements, 266-nm laser ablation generates particles most efficiently. Of the three materials tested, cement was the most favorable for material removal, stainless steel was the next, and alumina was the least. The removal of particles from chromium-embedded cement by 532- and 1064-nm-wavelength lasers was less effective than from stainless steel, but more effective than from alumina. For ablation with a 266-nm laser, chromium enhanced the removal above 20 J cm^-2. Comparisons of other characteristics such as the size and removal rate of these particles are also discussed in this paper.     

Inverting cascade impactor data for size-resolved characterization of fine particulate source emissions

In this paper, the inversion processing of cascade impactor data to construe continuous size distributions within fine particulate matter (PM_2.5) is examined for residential oil furnace and fireplace appliance emissions. Impactor data from tests with these emissions sources are selected for the challenges they pose to comprehending the size distributions of aerosol mass and chemical species. In specific, the oil furnace aerosol offers an opportunity to apply data inversion to study a bimodal lognormal distribution in which much of the aerosol mass is impactor-penetrating nanoparticles . The fireplace emissions on the other hand cover the issue of a chemical size distribution, which is subject to particle loss and characterized by a single lognormal, accumulation mode peak. Computational steps relevant to the application of the data inversion are illustrated in detail. Evaluation of correlation coefficients (0.992) indicates that the inversion model predictions fit the impactor data well. Simulations of systematic measurement error (±10%) at each impactor stage are shown to have a negligible impact on the inversion results for test data. It is concluded that data inversion can be effective when (i) source emissions contain a portion of particles that falls outside the measurement range of cascade impactors, (ii) a mass size distribution of an individual species is determined without the knowledge of the total mass concentration for that species, or when (iii) losses in the particle charger system are significant.     

Cl electrosorption on Ag(1 0 0): Lateral interactions and electrosorption valency from comparison of Monte Carlo simulations with chronocoulometry experiments

We present Monte Carlo simulations using an equilibrium lattice-gas model for the electrosorption of Cl on Ag(1 0 0) single-crystal surfaces. Fitting the simulated isotherms to chronocoulometry experiments, we extract parameters such as the electrosorption valency γ and the next-nearest-neighbor lateral interaction energy ϕ_nnn. Both coverage-dependent and coverage-independent γ were previously studied, assuming a constant ϕ_nnn [I. Abou Hamad, Th. Wandlowski, G. Brown, P.A. Rikvold, J. Electroanal. Chem. 554–555 (2003) 211]. Here, a self-consistent, entirely electrostatic picture of the lateral interactions with a coverage-dependent ϕ_nnn is developed, and a relationship between ϕ_nnn and γ is investigated for Cl on Ag(1 0 0).     

Novel zirconia-supported catalysts for low-temperature oxidative steam reforming of ethanol

Three zirconia-supported platinum group metal (Pt, Ru and Pt–Ru) catalysts were prepared by impregnation. The activity of these catalysts toward the oxidative steam reforming of ethanol (OSRE) was examined in a fixed-bed reactor in the temperature range of 260–380 °C. The catalysts were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and nitrogen adsorption at −196 °C. Activity results indicated that the optimized experimental conditions involved a reforming temperature of close to 300 °C and the molar ratios of O₂/EtOH and H₂O/EtOH of 0.44 and 4.9, respectively. An ethanol conversion (C_EtOH) approaching 100% and a hydrogen yield (Y_H2) exceeding 3.0 mole/mole ethanol were noticed at 280 °C over all the catalysts. Among these catalysts, the Pt–Ru/ZrO₂ catalyst was an excellent OSRE catalyst at low temperature. The maximum Y_H2 was 4.4 and the CO distribution was 3.3 mol% at 340 °C.     

About randomness of aerosol size distributions

All aerosol formation and evolution processes, such as nucleation, condensation, fragmentation, etc., are understood and rationalized via fundamental probabilistic concepts such as probabilities of collision, coagulation, dispersion, etc. Therefore all theoretical size distribution functions (lognormal, modified gamma distribution, self-preserving particle size distribution for Brownian coagulation, etc.) are in fact size probability density functions pdf(r). Any (e.g., measured) size distribution f(r) of an aerosol system is some random realization of its pertinent size probability density function pdf(r). When pdf(r) is viewed as a continuous function, the corresponding size distribution vanishes almost everywhere excluding some randomly set of sizes where f(r)=1. We investigate proximity between f(r) and pdf(r) in finite size intervals and derive expressions for estimation of the standard deviations of several aerosol size-dependent properties arising from randomness of f(r).     

Investigations of nanoparticle generation during surface decontamination by laser ablation at low fluence

Through laser ablation processes, significant amounts of particles can be generated from a surface of cement, stainless steel, or alumina. The minimal laser fluence (mJ cm^-2), or threshold energy, required to produce a detectable amount of particles (100 particles cm^-3) was investigated experimentally. The threshold energy was wavelength-dependent and was found to be the greatest for a pure material, alumina, then for a complex mixture, cement, and least for a simple mixture, stainless steel. The threshold energy requirement for three tested materials was found to be significantly higher for the IR (1064-nm) laser; it was 2.4–10.1 times higher than for the UV (266-nm) laser and 9.1–15.2 times higher than for the Vis (532-nm) laser. Interestingly, the UV laser has a higher threshold energy (1.5–4.0 times higher) than the Vis does. A log–log linear model was found to correlate particle production with the laser fluence of all three wavelengths. Of the three materials tested, stainless steel produced the most particles at a given fluence while alumina produced the fewest. Hypotheses of the particle generation mechanisms based upon the observations are also given here.     

Reactivity and stability of Ni/Al2O3 oxygen carrier for chemical-looping combustion (CLC)

Chemical-looping combustion (CLC) is a technology that reduces the carbon dioxide emissions from fossil fuel power stations. A nickel supported on -alumina oxygen carrier is investigated in this study, for use in a CLC process. Oxygen carriers with various nickel loadings on alumina are prepared according to the incipient wetness technique. The reactivity and stability of the prepared oxygen carrier samples, during repeated reduction–oxidation cycles, is demonstrated using temperature programmed reduction and oxidation. Pulse chemisorption results show that the dispersion and active crystallite diameter of the nickel particles remain constant over multiple reduction–oxidation cycles, indicating that no agglomeration occurs up to a nickel loading of 20 wt% supported on alumina. The stability and reactivity of the oxygen carriers, under industrial relevant conditions, are also investigated using the CREC fluidized bed riser simulator. It is observed that a 20 wt% nickel supported on alumina oxygen carrier is stable under industrial relevant fluidized bed reaction conditions, converting 76% of methane to carbon dioxide and water vapor, the combustion products. The metal support interaction is assessed by H₂ temperature programmed desorption, which shows that the metal-support interaction decreases as more nickel is loaded onto the alumina support.     

An experimental system for evaluation of well-defined catalysts on nonporous electrodes in realistic DMFC environment

This paper reports on an experimental setup wich enables us to investigate planar model catalysts in an environment closely resembling the environment found in an actual direct methanol fuel cell. The working electrodes were nano-structured catalyst particles immobilised on planar supports, reducing many of the commonly present non-catalyst related effects in conventional porous electrodes. Colloidal lithography was used for nano-structuring the samples. Nafion was used as electrolyte. Results are presented for the oxidation of methanol, formaldehyde, formic acid and carbon monoxide at temperatures between 30 and 70 ° C on Pt particles supported on glassy carbon disks.     

Improving mixing in microbioreactors

This paper describes a possible active mixing method for a microbioreactor that was designed, simulated and tested. Pressure based recycle flow was investigated in a cylindrical microreactor for mixing efficiency. Based on the computational fluid dynamics (CFD) simulation results and the requirements of the application, the recycle flow mixing method proved to be suitable as a method to induce sufficient mixing in the microbioreactor. This was verified experimentally using image analysis of dye distribution behavior.     

On heat conduction between laser-heated nanoparticles and a surrounding gas

Uncertainties in modeling heat conduction in connection with the application of laser-induced incandescence (LII) to primary particle sizing are discussed. Comparing two models widely used in this context, namely those of Fuchs [(1963). On the stationary charge distribution on aerosol particles in a bipolar ionic atmosphere. Pure and Applied Geophysics 56, 185–193] and McCoy/Cha [(1974). Transport phenomena in the rarefied gas transition regime. Chemical Engineering Science 29, 381–388], it is demonstrated that arising differences may be accounted for by the choice of a proper “effective” thermal accommodation coefficient _eff. In experiments on a large number of carbon blacks an overally good agreement between LII results and specified values for particle sizes based on electron-microscopy (EM) is obtained with a choice of _eff=0.25 (based on the McCoy/Cha-model). As aggregate size is expected to influence heat transfer from primary particles, the experimental data are analyzed by a model for an effective heat transfer surface of fractal aggregates. Based on values for the average number of primary particles per aggregate as derived from photocentrifuge measurements the data yield an extrapolated value for the physical accommodation coefficient for isolated particles of ₁=0.43.