Investigation of gas properties, design, and operational parameters on hydrodynamic characteristics, mass transfer, and biomass production from natural gas in an external airlift loop bioreactor

An external airlift loop bioreactor (EALB) was used for production of biomass from natural gas. The effect of riser to downcomer cross sectional area ratio (A_r/A_d), volume of gas-liquid separator, superficial gas velocity (U_sgr), and physical properties of gases and their mixtures [υ_g (μ/ρ) and D_g] were investigated on mixing time, gas hold-up, and volumetric gas liquid mass transfer coefficients (k_La). It was found that A_r/A_d has remarkable effects on gas hold-up and k_La due to its influence on mixing time. Kinematic viscosity (υ_g) showed its significant role on mixing time, gas hold-up and k_La when different gases used (mixing time changes directly whereas gas hold-up and k_La change indirectly). Moreover, it was found that diffusion coefficient of gas in water (D_g) has remarkable effect on k_La. The volumetric mass transfer coefficients for methane and its mixtures with oxygen (three different mixtures) were determined at different geometrical and operational factors. In average, the rate of oxygen utilization is approximately 1.8 times higher than that of methane. A gas mixture of 25 vol% methane and 75 vol% oxygen was the best gas mixture for biomass production in the EALB. The correlations developed for predicting the mixing time, gas hold-up, and k_La in terms of U_sgr, A_r/A_d, volume of gas-liquid separator, and gas phase properties have been found to be encouraging.     

Mixing and chemical reactions: A contrast between fast and slow reactions

The effect of fluid mixing on chemical reactions is analyzed by means of a model which treats simultaneously the complex interactions of mechanical mix diffusion and reaction. The model assumes that reactants diffuse from adjacent sheets of fluid undergoing a stretching motion, contact one another and This stretching motion causes deformation of the intermaterial surface and is designated mechanical mixing. A history of striation thickness s(t) contains all information about fluid mechanical mixing. Numerical methods were employed to solve the simultaneous partial differential equations of the model for mixing and single-step irreversible reactions. A criterion is established for limits of reaction control of bimolecular reactions by mixing, including diffusion, and by chemical kinetics in terms of a single dimensionless group φ  kC_B0S²/D_A. The present model can be applied to reactions in both laminar and turbulent flows.     

Drag coefficients of irregularly shaped particles

The steady-state free-fall conditions of isolated groups of ordered packed spheres moving through Newtonian fluids have been studied experimentally. Measurements of the drag coefficients are reported in this paper for six different geometrical shapes, including isometric, axisymmetric, orthotropic, plane and elongated conglomerates of spheres. From these measurements, a new and accurate empirical correlation for the drag coefficient, C_D, of variously shaped particles has been developed. This correlation has been formulated in terms of the Reynolds number based on the particle nominal diameter, Re, the ratio of the surface-equivalent-sphere to the nominal diameters, d_A/d_n, and the particle circularity, c. The predictions have been tested against both the experimental data for C_D collected in this study and the ones reported in previous works for cubes, rectangular parallelepipeds, tetrahedrons, cylinders and other shapes. A good agreement has been observed for the variously shaped agglomerates of spheres as well as for the regularly shape particles, over the ranges 0.15<Re<1500, 0.80<d_A/d_n<1.50 and 0.4<c<1.0.     

Cinetique de l'adsorption du phenol en couche fluidisee de charbon actif

Phenol adsorption from dilute aqueous solutions was studied using fluidized bed of active carbon. Isotherms adsorption at 14 and 18°C were determined and Langmuir type equation is proposed. For three different particles sizes, d_s = 0,129 cm, d_s = 0,093 cm and d_s = 0,071 cm, and various flow rate the controlling diffusional mechanisms were checked, i.e. external and/or internal diffusion.     

Experimental study of swelling and shrinking kinetics of spherical poly(N,N-diethylacrylamide) gel with continuous phase transition

The kinetics of swelling and shrinking of spherical particles of poly(N,N-diethylacrylamide) (DEAA) gel, prepared with static mixing technology, was investigated experimentally. The shrinking process was separated into late shrinking and initial shrinking by the skin layer effect. When the final temperature of temperature swing experiments was above the lower critical solution temperature (LCST) of polyDEAA, the apparent increase in the response rate constant (k_v) and the polymer diffusion constant (D) were due to reduction of the friction coefficient between polymer chains and water, arising from decrease in the water viscosity. When the final temperature was below the LCST, the main factor determining k_v and D was the change in the network structure of DEAA particles caused by the final temperature. The initial temperature affected the values of k_v and apparent diffusion constant D_app only in initial shrinking, in which the decrease in D with increase in the initial temperature came mainly from the spatial limitation.     

Prediction of the volumetric mass transfer coefficient in pneumatic contactors

Semi-theoretical expressions are developed for the prediction of the volumetric mass transfer coefficient, (K_La_D)_T, in pneumatic contactors, using the correlation of Calderbank and Moo-Young Chem. Engng Sci.16, 39 (1961) for the mass transfer coefficient and the local isotropic turbulence theory for predicting bubble diameter. A direct proportionality of (K_La_D)_T to the gas hold-up is predicted, with an exponent of 1.2 on ε_G, while, in terms of physical parameters, (K_La_D)_T is predicted to be proportional to the 0.8 power of the superficial gas velocity for both bubble column and airlift contactors, and proportional to (1 + A_d/A_r)⁻² for airlift contactors, where A_d/A_r is the downcomer-to-riser cross-sectional area ratio. Experimental results obtained in bubble column and airlift contactors (external-loop and concentric-tube) of pilot-plant scale (ca. 50L liquid capacity), with water and 0.15 kmol m⁻³ NaCl solution as liquid media, were used to test the proposed expressions.     

Piezoelectric properties of langasite group based on the ionic size of cation

As langasite A₃BC₃D₂O₁₄ compounds group with piezoelectric properties has no phase transition up to the melting point of 1400–1500 °C, they have been applied for the combustion pressure sensor. As they also have a larger electromechanical coupling factor compared to quartz and nearly the same temperature stability as quartz, the surface acoustic wave (SAW) filters are expected based on the digital transformation of wider bandwidth and higher-bit rates. In the case of three-element compounds such as [R₃]_A[Ga]_B[Ga₃]_C[GaSi]_DO₁₄ (R=La, Pr and Nd), the piezoelectric constant increases with the ionic radius R. In the case of four-element compounds such as [A₃]_A[B]_B[Ga₃]_C[Si₂]_DO₁₄ (A=Ca or Sr, B=Ta or Nb), |d₁₁| and k₁₂ values as a function of A_L/B_L ratio showed a linear relationship completely. There are two effects for the substitutions of A- and B-site cations: the substitution of Sr for Ca brings expansion toward [100] and enlargement in |d₁₁| and k₁₂. On the other hand, as the substitution of Ta for Nb brings not much change, the properties are similar. In this study, five-element compounds such as La_3?xSr_xTa_yGa_5?x+ySi_1+x?2yO₁₄ (0≤x≤3, 0≤y≤1) solid solutions analysed by a single crystal X-ray diffraction are compared with the three- and four-element compounds on the coordinates of oxygen ions. As the coordinates positions are located on the extended line of coordinate positions on the three-element compounds as increasing ionic radius of R-ions in A-site, the piezoelectric properties |d₁₁| and k₁₂ are expected become large.     

Water sorption and diffusion in starch/polyolefin blends

The kinetics of water sorption by starch filled polyolefins has been investigated using blends of commercial starch masterbatch with low density polyethylene (LDPE), high density PE (HDPE), and copolymers of ethylene with methyl acrylate (EMA), ethyl (EEA), AND n-butyl acrylate (EBA). Transient state diffusion coefficients (D) of water in the blends were determined using Fickian analysis of the initial stages of the sorption isotherms. Measured D values were several orders of magnitude lower than values reported for either pure starch of LIDPE by various investigators. Resin effects on D were on the order D_HDPE < D_LDPE ? D_EBA < D_EEA < D_EMA. The resin effects diminished as the starch content increased. The lower magnitude of D in the blends relative to pure LDPE was attributed to the sorption of water by the dry starch particles, in a manner similar to dual-mode sorption in glassy polymers. Copolymer effects were consistent with reduction in crystallinity caused by the comonomers. The dependence of the diffusion coefficient on starch content was not described by mixing models of heterogeneous systems of percolation theory.     

Cooperative and self-diffusion of polymers in semidilute solutions by dynamic light scattering

The quasielastic light scattering from semidilute solutions of polystyrene in tetrahydrofuran has been measured and we observe two distinct exponential decays separated by several orders of magnitude. The angular dependence of the decay constants is indicative of diffusive processes which we identify with the cooperative diffusion coefficient, D_c and the self diffusion coefficient D_s. It is found that D_c, identified with the fast decay, increases with polystyrene concentration and is independent of molecular weight. However, D_s decreases sharply with concentration and molecular weight. An explanation is given for the light scattering detection of these two diffusion coefficients which is based only on the assumptions inherent in the reptation model. In a limited region of molecular weight and concentration the experimental results appear to be consistent with the predictions of scaling theory.     

Effects of phase behavior on mutual diffusion at polymer layers interface

Using oscillation mode of rheology and theoretical calculation, we have observed for the first time the crossover of mutual diffusion coefficient, D_m, from high to low temperatures at the multiple layers interface of polymer films. A model which reflects a more realistic terminal state has been proposed to fairly fit the experimental data, by which the mutual diffusion coefficient D_m can be determined. It is substantially found that the diffusion keeps proceeding for the multilayer system at the temperature lower than the critical temperature due to the requirement of a period of time for binodal compositions to reach. Moreover, it is found that the apparent activation energy, E_d, derived from the Arrhenius relation of D_m versus 1/T, increases surprisingly when the welding temperature is below 150 °C, which relates closely to the effects of the phase behavior occurring in the two-phase region of the blend.     

Magnetic field influence on mass transport phenomena

The oxidation reactions of hexacyanoferrate(II) and hydroquinone in KCl media were studied on disk platinum electrodes using chronoamperometry under a strong magnetic field (1.74 T). The limiting current measured under magnetic field i_B can be expressed as a function of parameters that control the mass transfer phenomenon by i_B = KC^aD^bd^cν^e?^fB^gn^h. C represents the electroactive species concentration, D the diffusion coefficient of the electroactive species, d the working electrode diameter, ν the kinematic viscosity of the electrolyte, ? the dielectric constant of the solution, B the magnetic field strength, n the number of electrons involved in the redox process and K is a proportionality constant. Contribution of B to the limiting current is well established (g = 1/3), whereas the contribution of D has to be confirmed (b = 1). The aim of this work was to specify the influence of the other parameters for which various results have been published in recent literature. We concluded that i_B = KC^4/3Dd^5/3ν^−2/3?^−7/4B^1/3n, quantifying for the first time, to our knowledge, the drastic influence of the electrolyte dielectric constant.     

Quasi-elastic light scattering from branched polymers: 1. Polyvinylacetate and polyvinylacetate—microgels prepared by emulsion polymerization

Emulsion polymerization of vinylacetate leads to branched polymers which at high monomer conversions form microgels of the shape and size of the latex particles. Quasielastic light scattering measurements from samples in the pre-gel state give at small q² a linear angular dependence of $\text{D}{}_{\mathrm{app}}\text{=}\text{Г}\text{q}{}^2$ which resembles that of randomly branched chain molecules, where Г is the decay constant of the time correlation function. Extrapolation of D_app towards zero scattering angle yields the translational diffusion constant D_z. The diffusion constant follows the molecular weight dependence D_z = 9.78 10^?5M_w^?0.478. The diffusion constant of the microgels, i.e. at molecular weights M_w > 14 10⁶, remains constant because of the finite and constant size of the latex particles. The coefficients k_f and k_D in the concentration dependence of the frictional and diffusion coefficients are related according to the equation $\text{k}{}_{\mathrm{D}}\text{= k}{}_{\mathrm{f}}\text{? 2A}{}_2\text{M}{}_{\mathrm{w}}\text{?}\text{v}\text{?}$ where A₂ is the second virial coefficient and v? the partial specific volume of the particle. The coefficient k_f is calculated from the experimentally determined quantities k_D, A₂ and M_w, and the result is compared with the theory by Pyun and Fixman. Accordingly the branched coils in the pre-gel state resemble soft spheres, but the microgels behave more like spheres of some rigidity.     

Vapor barrier properties of polycaprolactone montmorillonite nanocomposites: effect of clay dispersion

Different compositions of poly(ε-caprolactone) (PCL) and (organo-modified) montmorillonite were prepared by melt blending or catalyzed ring opening polymerization of ε-caprolactone. Microphase composites were obtained by direct melt blending of PCL and sodium montmorillonite (MMT-Na⁺). Exfoliated nanocomposites were obtained by in situ ring opening polymerization of ε-caprolactone with an organo-modified montmorillonite (MMT-(OH)₂) by using dibutyltin dimethoxide as an initiator/catalyst. Intercalated nanocomposites were formed either by melt blending with organo-modified montmorillonite or in situ polymerization within sodium montmorillonite. The barrier properties were studied for water vapor and dichloromethane as an organic solvent. The sorption (S) and the zero concentration diffusion coefficient (D₀) were evaluated for both vapors. The water sorption increases with increasing the MMT content, particularly for the microcomposites containing the unmodified MMT-Na⁺. The thermodynamic diffusion parameters, D₀, were compared to the value of the parent PCL: both microcomposites and intercalated nanocomposites show diffusion parameters very near to PCL. At variance exfoliated nanocomposites show much lower values, even for small montmorillonite content. In the case of the organic vapor, the value of sorption at low relative pressure is mainly dominated by the amorphous fraction present in the samples, not showing any preferential adsorption on the inorganic component. At high relative pressure the isotherms showed an exponential increase of sorption, due to plasticization of the polyester matrix. The D₀ parameters were also compared to those of the unfilled PCL; in this case, both the exfoliated and the intercalated samples showed lower values, due to a more tortuous path for the penetrant molecules.     

Prediction of single particle settling velocities through liquid fluidized beds

Single particle settling velocities through water fluidized beds of mono-sized glass spheres (d_p = 0.645, 1.20, 1.94, 2.98 and 5 mm in diameter) were studied experimentally using a column, 40 mm in diameter. The settling spherical particles (D_p = 10 and 19.5 mm) had different densities (1237 to 8320 kg/m³), while the settling particles (D_p = 5 and 2.98 mm) were glass spheres. The pseudo-fluid model, which considers a liquid fluidized bed as a homogenous pseudo-fluid, predicts single particle settling velocities quite well if the ratio D_p/d_p is larger than about 10. With decreasing ratio D_p/d_p, the overall friction between the settling particle and the fluidized media increases. A method for predicting single particle settling velocities through a liquid fluidized bed is proposed and discussed. Following the approach of Van der Wielen et al. [L.A.M. Van der Wielen, M.H.H Van Dam, K.C.A.M. Van Luyben, On the relative motion of a particle in a swarm of different particles, Chem. Eng. Sci. 51 (2006) 995-1008], the overall friction is decomposed into a particle-fluid and a particle-particle component. The effective buoyancy force is calculated using the transition function proposed by Ruzicka [M.C. Ruzicka, On buoyancy in dispersion, Chem. Eng. Sci. 61 (2006) 2437-2446]. A simple model for predicting the collision force is proposed, as well as a correlation for the collision coefficient. The mean absolute deviation between the experimental and calculated slip velocities was 5.08%.     

Modelling the post treatment process of model implants prepared by in situ polymerized poly(ε-caprolactone) using a BF3-glycerol catalyst system

The post treatment process of a poly(ε-caprolactone) (PCL) model implant prepared using a boron trifluoride (BF₃) catalyst and glycerol initiator by in situ polymerisation process for craniofacial and maxillofacial treatment is modelled using a ‘moving-boundary’ diffusion model. A numerical method was used to solve a system of diffusion equations of the model. The variable diffusion coefficient (D) was correlated with crystallinity (x_c) of the polymer which is a function of its molecular weight (M_w) and its degradation rate constant (k_d), D=f(x_c(M_w,k_d)). The post treatment time and the molecular weight retained after post treatment can be obtained using this model. The modelling results show that the process is potentially suitable for manufacturing thin model implants of complex shape.     

Diffusion of disperse dye in atactic polystyrene

Sublimative desorption experiments were carried out on atactic polystyrene containing p-nitroaniline, p-aminoazobenzene, or C.I. Disperse Yellow 7 at 114°–170°C (above the T_g of the polymer). The diffusion coefficient of each dye in the polymer increased monotonically with rise in the desorption temperature. The mode of this change was exactly expressed by a WLF relation having the universal parameters given for amorphous polymers. The value of B_d, defined as the ratio of diffusional penetrant volume to that of a segment of the chain molecule, varied from 0.37 to 0.70 for the different dyes used. It is also shown that the B_d value is related to the logarithmic rotational volume of the dye molecule estimated from a molecular model.     

Spurious potential dependence of diffusion coefficients in Li insertion electrodes measured with PITT

Diffusion coefficients of Li⁺ in insertion electrodes determined by the potentiostatic intermittent titration technique (PITT) have been reported in the literature as being potential-dependent (and thus dependent on the degree of insertion). With this PITT method the diffusion coefficients D^PITT are determined from the current response of potential-step experiments using an approach based on the Cottrell equation. This equation assumes as boundary conditions infinitely fast kinetics and a linear system with semi-infinite thickness. Using the example of a spinel-type Li_δMn₂O₄ electrode it will be demonstrated that this potential dependence of D^PITT is not real but occurs, because the inadmissible boundary condition of infinitely fast kinetics which is not fulfilled has been adopted. This will be demonstrated with the help of numerically simulated PITT data calculated with a constant diffusion coefficient D. Diffusion coefficients D^PITT calculated from these simulated PITT data are potential dependent, even though the PITT experiments were simulated with a constant diffusion coefficient D. The inadmissible boundary condition of a linear system with semi-infinite thickness applied to a bed of spherical active particles with finite radii as represented by a Li_δMn₂O₄ electrode leads to further deviations of D^PITT which will be investigated.     

Morphological and fractal studies of polypropylene/poly(ethene‐1‐octene) blends during melt mixing using scanning electron microscopy

BACKGROUND: Polymer blending creates new materials with enhanced mechanical, chemical or optical properties, with the exact properties being determined by the type of morphology and the phase dimension of the blend. In order to control blend properties, morphology development during processing needs to be understood. The formation and evolution of polypropylene/poly(ethylene‐1‐octene) (PP/POE) blend morphology during blending are qualitatively represented by a series of time‐dependent scanning electron microscopy (SEM) patterns. The area diameter and its distribution of dispersed phase domains are discussed in detail. In order to characterize the formation and evolution of phase morphology quantitatively, two fractal dimensions, D_s and D_d, and their corresponding scaling functions are introduced to analyze the SEM patterns. RESULTS: The evolution of the area diameter indicates that the major reduction in phase domain size occurs during the initial stage of melt mixing, and the domain sizes show an increasing trend due to coalescence with increasing mixing times. The distribution in dispersed phase dimension obeys a log‐normal distribution, and the two fractal dimensions are effective to describe the phase morphology: D_s for dispersed phase dimension and D_d for the distribution in it. CONCLUSIONS: The fractal dimensions D_s and D_d can be used quantitatively to characterize the evolutional self‐similarity of phase morphology and the competition of breakup and coalescence of dispersed phase domains. It is shown that the fractal dimensions and scaling laws are useful to describe the phase morphology development at various mixing times to a certain extent. Copyright © 2007 Society of Chemical Industry     

Shrinking-core model for systems with facile heterogeneous and homogeneous reactions

The shrinking-core equation for pore diffusion control has been extended to the case of a facile heterogeneous reaction coupled to a facile homogeneous reaction occurring within the pores of the product layer and in the bulk solution. The model considered is very general in that the simultaneous transport of all reacting species is included. The resulting equation is identical to the standard one for diffusional control by a single species except that the parabolic rate constant k_d is considerably more complex and contains useful information concerning the system. Analysis of its dependence on the system parameters yields important criteria that determine the identity of the rate-controlling species and the direction of the heterogeneous reaction. Depending upon the relative values of the equilibrium constants of the two reactions, the dependence of k_d on the bulk reactant concentration can vary significantly from the linearity expected from the standard model.     

Diffusion of low molecular weight substances into a laminar film. I: Rigorous solution of the diffusion equation in a composite film of multiple layers

A rigorous solution of the diffusion of penetrant into a laminar film comprised of multilayers of m components is presented by an orthogonal-expansion method. As the simplest practical cases of m ? 2 and m ? 3, with stepwise distribution of both diffusion coefficients and solubility coefficients at the boundary between respective layers, the diffusion properties in the transient state are analyzed in detail. That is, changes in the penetrant concentration distribution within the laminar film and the total amount of penetrant sorbed within the film both with time after exposing the film to an atmosphere of fixed penetrant concentration are calculated for A–B as well as A–B–A type layer arrangements. The calculation is performed while keeping (L₂/L₁) at a constant value of 2.0 but varying the diffusion coefficient ratio of (D₁/D₂) from 10² to 10^?2, and/or varying the solubility coefficient ratio of (S₁/S₂) from 1 to 10, where L₁ and (L₂ ? L₁) are the thickness D₁ and D₂ are the diffusion coefficients of penetrant, and S₁ and S₂ are the solubility coefficients in the A-component and B-component, respectively. The sorption curves deviate considerably from those of Fickian curves of homogeneous film with (D₁/D₂) ? 1 in their respective ways. The results obtained here can be applied to the diffusion in a single component polymer film having a surface layer with different diffusion properties from that of the inner side of the film caused by differing distributions in molecular orientation or degrees of crystallinity.