Mass transfer in external-loop airlift bioreactors using static mixers

The influence of static mixers on the overall gas-liquid volumetric mass transfer coefficient (K_La_L) was examined in an external-loop type airlift bioreactor (approximately 15 L volume, 1.8 m static liquid height, A_r/A_d = 0.444). The study was conducted with aqueous salt solution (0.15 kmol ? m^?3 NaCl) and with pseudoplastic solutions of carboxymethyl cellulose (0.2 ? 0.6 wt./vol. % (g/100 mL) CMC). Over a broad range of power law parameters K (10^?3 ? 10 Pa ? sⁿ) and n (0.5 ? 1.0), the presence of static mixers in the riser was found to enhance the K_La_L relative to mixer-free mode of operation. The extent of increase in K_La_L depended on the fluid “thickness”, K: the higher the K, the greater the K_La_L intensification due to static mixers. For otherwise identical conditions, the presence of static mixers improved K_La_L by 30-500%, depending on the fluid. The boost in K_La_L was associated with increased gas holdup and gas-liquid interfacial area, which arose due to bubble breakup accomplished by the static mixing elements. Potential advantages of static mixers in upgrading the performance of oxygen-limited fermentations were pointed out.     

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.     

Gas holdup,bubble size distribution,and mass transfer in an airlift reactor with ceramic membrane and perforated plate distributor

The airlift reactor is one of the most commonly used gas–liquid two-phase reactors in chemical and biological processes. The objective of this study is to generate different-sized bubbles in an internal loop airlift reactor and characterize the behaviours of the bubbly flows. The bubble size, gas holdup, liquid circulation velocity, and the volumetric mass transfer coefficient of gas–liquid two-phase co-current flow in an internal loop airlift reactor equipped with a ceramic membrane module (CMM) and a perforated-plate distributor (PPD) are measured. Experimental results show that CMM can generate small bubbles with Sauter mean diameter d₃₂ less than 2.5 mm. As the liquid inlet velocity increases, the bubble size decreases and the gas holdup increases. In contrast, PPD can generate large bubbles with 4 mm < d₃₂ < 10 mm. The bubble size and liquid circulation velocity increase as the superficial gas velocity increases. Multiscale bubbles with 0.5 mm < d₃₂ < 10 mm can be generated by the CMM and PPD together. The volumetric mass transfer coefficient k_La of the multiscale bubbles is 0.033–0.062 s⁻¹, while that of small bubbles is 0.011–0.057 s⁻¹. Under the same flow rate of oxygen, the k_La of the multiscale bubbles increases by up to 160% in comparison to that of the small bubbles. Finally, empirical correlations for k_La are obtained.     

Thermal expansion and P-V-T equation of state of cubic silicon nitride

We performed in-situ X-ray diffraction measurements of polycrystalline cubic silicon nitride samples at high temperatures under atmospheric pressure and at simultaneous high-pressure-temperature conditions. In air, cubic silicon nitride survives metastably up to 1733 K without oxidation. The temperature dependence of the thermal expansion coefficient was determined to be α(T) = a₁ + a₂T – a₃T⁻² where a₁ = 1.34(6) × 10⁻⁵ K⁻¹, a₂ = 5.06(44) × 10⁻⁹ K⁻², and a₃ = 0.20(10) K. Using all the experimental data obtained under atmospheric and high pressures, a complete set of parameters of the high-temperature third-order Birch Murnaghan equation of state was obtained: K₃₀₀,₀ = 303(5) GPa, K′_300,0 = 5.1(8), and (∂K_T,0/∂T)_P = –0.017(1) GPa K⁻¹, where K₀, K′₀, and (∂K_T,0/∂T)_P are the isothermal bulk modulus, its pressure derivative, and its temperature derivative, respectively. These parameters are necessary to calculate the equilibrium phase boundary between the β and cubic phases in silicon nitride.     

Effect of contaminants on mass transfer coefficients in bubble column and airlift contactors

In this work, the effects of surface-active contaminants on mass transfer coefficients k_La and k_L were studied in two different bubble contactors. The oxygen transfer coefficient, k_L, was obtained from the volumetric oxygen transfer coefficient, k_La, since the specific interfacial area, a, could be determined from the fractional gas holdup, ε, and the average bubble diameter, d₃₂. Water at different heights and antifoam solutions of 0.5- were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k_La and k_L values over a span of almost 400%. A theoretical interpretation is proposed based on modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant cap model. Model results match experimental k_L data within ±30%.     

The specific interfacial area in external-circulation-loop airlifts and a bubble column-I. Aqueous sodium sulphite solution

In order to contribute to general knowledge of the performance and design of external-circulation-loop type airlift fermentors, the specific gas-liquid interfacial area a was studied. Three different measurement methods were applied, namely sulphite oxidation, light scattering and photographic. Two sizes of fermentors having downcomer-to-riser cross-sectional area ratios (A_d/A_r) of 0.111 and 0.444 were employed. Parallel studies in a bubble-column-type fermentor (A_d/A_r = 0) were carried out for comparison. Experimental data on the liquid circulation velocity, gas hold-ups in the riser and the downcomer and the average bubble diameter necessary for evaluation of a by the chemical and photographic methods were also measured and are reported. While the data for a obtained by the chemical and light scattering methods agree well, a significant discrepancy was found with the results obtained using the photographic method. The specific interfacial area is shown to be mainly dependent on the downcomer-to-riser cross-sectional area ratio and the superficial gas velocity. However, the effects of liquid velocity, dispersion height and sparger hole diameter have also been taken into account to facilitate comparison with other works.     

Mass transfer in bubble columns packed with motionless mixers

The cocurrent upward mode was employed to absorb pure oxygen into water in bubble columns packed with Koch (Sulzer) motionless mixers. The liquid-side volumetric mass transfer coefficient, K_La, in the packed bubble column was found to be always larger than that in the unpacked bubble column. In the range of liquid velocities from 6.7 cm/sec to 39.9 cm/sec, the value of K_La in the packed bubble column increased with the increasing liquid velocity while that in the unpacked bubble column was almost independent of the liquid velocity. The equation of the formK_La= mν_l^β? was successfully adopted to correlate the K_La data.     

Effect of ultrasonic waveforms on gas–liquid mass transfer in microreactors

This study aims to investigate the effect of ultrasonic waveforms on the gas–liquid mass transfer process. For a given load power (P), continuous rectangular wave yielded stronger bubble oscillation and higher mass transfer coefficient (k_La) than continuous triangular and sinusoidal wave. For pulsed ultrasound, the k_La decreased monotonically with decreasing duty ratio (D), resulting in weak enhancement at low D (≤33%). For a given average load power (P_A), concentrating the P for a shorter period resulted in a higher k_La due to stronger cavitation behavior. For a given P_A and D, decreasing the pulse period (T) led to an increase in k_La, which reached a constant high level when the T fell below a critical value. By optimizing the D and T, a k_La equivalent to 92% of that under continuous ultrasound was obtained under pulsed ultrasound at a D of 67%, saving 33% in power consumption.     

Circulation liquid velocity,gas holdup and volumetric oxygen transfer coefficient in external-loop airlift reactors

The effects of the horizontal connection length (0.1 ± L_e ± 0.5 m), the cross-sectional area ratio of downcomer-to-riser (0.11 ± A_d/A_r± 0.53), and the superficial gas velocity on gas phase holdups in the riser and downcomer were studied. The circulation liquid velocity, the mixing performance and the volumetric mass transfer coefficient in the external-loop airlift reactors were also measured. The horizontal connection length and A_d/A_r were major parameters which strongly affected the performance of external-loop airlift reactors. Useful correlations in the external-loop airlift reactors were obtained for gas holdups, the volumetric mass transfer coefficient, the circulation liquid velocity, and the mixing time.     

Characteristics of jet droplet produced by bubble bursting on the free liquid surface

Droplet formation that follows gas bubble bursting at a free liquid surface, being known as “entrainment”, is a common phenomenon in many fields. Though there were a couple of investigations focusing on entrainment, specific characteristics of jet drops produced by bubble bursting on free surface are still unknown in the current stage. Efforts were performed to try to determine the expression of critical bubble size, above which the bubble does not produce any drops by bursting on liquid surface. Dimensionless analyses show that in air–liquid systems, ReFr^?0.5, ReWe^?0.5 and WeFr^?0.5Re^?1 can be readily correlated to Mo. Error analysis demonstrated the following expression: D_cr=10^0.1914σ_L^0.5517ρ_L^?0.4830g^?0.5170μ_L^?0.0688 can give the most accurate prediction of D_cr as a universal expression. Further, for correlating the number of droplets to bubble diameter in air–water and air–iron system, D_cr was used as the length scale to normalize the bubble diameter. Through curve fitting, a universal expression has been obtained to predict the number of droplets produced by bubble bursting on free liquid surface in any air–liquid system: N_dr=7.9 exp(?D_B/(0.338D_cr))?0.41. Comparison shows that this expression is able to give more accurate prediction than the previous empirical expressions. The most significant innovation is that it is a universal model that can be used in various gas–liquid systems.     

The specific interfacial area in external-circulation-loop airlifts and a bubble column-II. Carboxymethyl cellulose/sulphite solution

Specific gas-liquid interfacial areas and gas hold-ups in the riser and downcomer sections as well as the liquid circulating velocity were studied using highly viscous pseudo-plastic solutions of carboxymethyl cellulose (CMC). The specific interfacial area was measured by the chemical reaction method (sulphite oxidation). The measurements were carried out in two sizes of external-circulation-loop airlift (ECL-AL) having downcomer-to-riser cross-sectional area ratios (A_d/A_r) of 0.111 and 0.444. Parallel studies in a bubble-column-type fermentor (A_d/A_r = 0) were performed for comparison. All the parameters studied are greatly affected by A_d/A_r and the gas superficial velocity, u_GR, while the influence of the effective viscosity, η_eff, varies from parameter to parameter. A correlation for the specific interfacial area considering the effect of u_GR, A_d/A_r and η_eff is proposed.     

Application of the fourier transform to the measurement of KLaL in non-mechanically agitated contactors

The authors recently proposed (André et al., 1981) a new method of measuring the overall volumetric mass transfer coefficient (K_La_L) in gas-liquid-solid contactors of the bubble-column type. The present paper deals with improved implementation of this method. The previous work showed that prior knowledge of the extent of backmixing in the gas phase was required in order to obtain reliable estimates of K_La_L. It is shown here that parameter fitting in the frequency domain, using the Fourier transform of the response curves, may be used to estimate both K_La_L and the extent of backmixing in the gas phase simultaneously. The main features of this particular approach are described in detail and are discussed in the light of experimental data obtained in two bench-scale contactors (a bubble column and the novel inverted conical bottom contactor). In addition, the effect of contactor geometry and paper pulp addition on K_La_L also is presented. It is shown that both the conventional bubble column and the novel contactor yield comparable gas-liquid mass transfer rates.     

Synthesis,structure and single-molecule magnet characterization of Dy2 dinuclear complexes with an 8–hydroxyquinoline carbohydrazone ligand

The employment of an 8-hydroxyquinoline carbohydrazone tetradentate ligand, 8-hydroxyquinoline-2-carboxaldehyde-(benzoyl)hydrazone (H₂L), DyCl₃·6H₂O and Et₃N has led to the formation of dinuclear Dy(III) complex [Dy₂(HL)₂Cl₄(CH₃OH)₂], (1) and [Dy₂(HL)₂((PhO)₂PO₂)₄]·2(CH₃OH) (2) in the presence of excess amount of bridging diphenyl phosphate anion, (PhO)₂PO^2 −. X-ray crystallography data revealed that two Dy^III atoms in 1 are doubly bridged by the two phenolato oxygen atoms of HL⁻ ligands, however, the Dy^III atoms are bridged by additional two deprotonated μ_1,3-phosphate anions in 2. Each of the two lanthanide ions is eight-coordinated and is located in the different symmetry with triangular dodecahedron (D_2d) for Dy^III in 1 and elongated triangular bipyramid (D_3h) for Dy^III in 2. Dc magnetic susceptibility studies in the 2–300 K range reveal the weak antiferromagnetic interaction for 1 and 2. Both of them showed field-induced slow magnetic relaxation behavior with the U_eff for 1 of 46.0 K higher than 36.8 K for 2.     

Azocoupling products. V. Electronic spectroscopy study of two azocoupling products of 1-(4-hydroxy-6-methyl-pyrimidin-2-yl)-3-methyl-pyrazolin-5-one

A systematic study of the effect of solvent, sample concentration, time and acid or alkali addition, respectively of pH′, upon the electronic absorption spectra of two dyes 7 and 8, as well as the pK_a′ determination of these dyes was carried out. In C₆H₆, CHCl₃, DMSO, and acidic media (AcOH, acidified EtOH) these dyes that theoretically may be involved in azo–hydrazone tautomerism have been detected only as hydrazone tautomers 7c and 8c. In exchange, in neutral and alkalized EtOH, respectively in DMF, in concentration range 10⁻⁴–10⁻⁶ mol L⁻¹ these tautomers are in equilibrium with monodeprotonated (9, 10) and twice deprotonated (13, 14) species. By increase of pH, or by dilution, the equilibrium shifts increasingly towards the deprotonated species. The monodeprotonated species, correspond also to a hydrazone structure 9, 10, whilst the doubly deprotonated species represent a common dianion that should be predominantly azo in character (13, 14).     

Delicate structural regulation and stepwise phase transformation of Cu(II) coordination assemblies directed by inorganic counterion

Assembly of 3-(2-pyridyl)-4-(4-pyridyl)-5-(3-pyridyl)-1,2,4-triazole (L²⁴³) with Cu(NO₃)₂ or Cu(ClO₄)₂ affords a mononuclear complex [Cu(L²⁴³)₂(H₂O)₂](NO₃)₂ (1) or a 1D linear coordination polymer {[Cu(L²⁴³)₂(H₂O)](ClO₄)₂(CH₃OH)}_n (3). Incorporating NaN₃ into such assembled systems under similar conditions will produce two distinct 2D layered networks with the formulas {[Cu(L²⁴³)(N₃)](NO₃)(H₂O)₂}_n (2) or {[Cu(L²⁴³)(N₃)](ClO₄)(H₂O)}_n (4), both of which contain dual inorganic-anions as the counterparts (NO₃⁻/N₃⁻ for 2 and ClO₄⁻/N₃⁻ for 4). Further, with a subtle change of reaction condition for 2 or 4, a 1D linear array [Cu(L²⁴³)(N₃)₂]_n (5) is obtained with only N₃⁻ equilibrating the positive charges. Significantly, stepwise structural transformations between such crystalline phases can also be achieved by the direction of azide anion.     

Mass Transfer of Ozone Using a Microporous Diffuser Reactor System

An ozone reactor was constructed using a tubular gas diffuser made of microporous stainless steel to significantly reduce gas bubble size and increase overall mass transfer area. Overall mass transfer coefficient, K_La [s ^?1], was correlated with gas (G) and liquid (L) flow rates using K_La = AL^αG^β , with A = 3.96 × 10 ⁸ [s^?1], α = 1.53, and β = 0.40, with L and G in [m ³s^?1]. The reactor is essentially plug flow at high G or L. This system achieves one of the highest ozone mass transfer rates observed in the literature.     

Diffusion and equilibrium properties of hydrogen in palladium measured by the stripping potentiostatic method

Theory and experiments on the (non-equilibrium) stripping potentiostatic method for evaluation of the diffusion coefficient and desorption isotherms of hydrogen in α-Pd are discussed. Diffusion coefficient of hydrogen D can be evaluated either under transient conditions, from the anodic stripping current (I_a) or under steady-state condition, from the anodic charge corresponding to the stripping of hydrogen held initially in Pd membrane (Q_a). Following data are reported: D = (3.60±0.06) × 10⁻⁷ cm² s⁻¹ (transient) and D = 3.41 × 10⁻⁷ cm² s⁻ (steady-state) at 20°C in 0.1 N H₂SO₄. Equilibrium concentration and equivalent pressure of hydrogen at the entrance side of Pd membrane can be calculated from the steady-state permeation current (I^∞_a or the anodic stripping charge (Q_a) and its open-circuit potential just after stopping hydrogen generation, E⁰_c. Reliable Sieverts' constants (K_s) have been calculated within 15 and 60°C, and the following thermodynamic values obtained: ΔH^o = −(2.6±0.1) kcal (mol H)⁻¹ and ΔS^o = −(14.5±0.4) cal K⁻¹ (mol H)⁻¹. These data agree well with those obtained in gas phase measurements.     

Mass transfer in liquid-liquid (horizontal) pipeline contactors

The mass transfer characteristics of liquid-liquid (horizontal) pipeline contactors were studied for the bubble and plug flow regimes. The contactors were operated co-currently. The theory of mass transfer with chemical reaction was used to obtain values of physical mass transfer coefficient (k_La) and effective interfacial area (a) for a number of systems. In all the experiments the resistance to mass transfer was confined to the continuous phase. The effect of the flow rates and viscosities of the two phases, interfacial tension, contactor length, pipe diameter, the presence of solids, etc. on both, k_La and a was studied. The values of k_La and a were found to vary from 1 to 50 × 10^?3 sec^?1, and 0.4 to 25 cm^?1, respectively.     

Syntheses,characterization and properties of manganese,cobalt and copper complexes from chelate N-donor ligands

2-[(S)-4-isopropyl-2-oxazolyl]quinoline (L¹) and 2-[(S)-4-phenyl − 2-oxazolyl]quinoline (L²) react with manganese, cobalt and copper salts to yield four new complexes: [Mn₂(L¹)₂]Cl₂ (1), {[Co(L¹)₂]₂(OH)}(ClO₄)₃ (2), [Cu₂I₂(L¹)₂] (3), and {[Cu(L²)₂]₂(OH)}(BF₄)₃·H₂O (4), which were fully characterized by single-crystal and powder X-ray diffraction, IR, elemental and thermogravimetric analyses. Complexes 1–4 all show dinuclear structure. Two metal ions are bridged by Cl⁻ in 1, by OH^– in 2 and 4, and by I⁻ in 3. Moreover, non-linear optical, and ferroelectric and magnetic properties have been investigated.     

Calix [4]-bis-monothiacrown-5 as a versatile building block for homo- and heterometallic coordination polymers

Exocoordination-based homonuclear and heteronuclear coordination polymers of calix [4]-bis-monothiacrown-5 (L) are reported. L reacts with HgCl₂ and affords a one-dimensional (1D) coordination polymer [L(μ-Hg₂Cl₄)]_n (1) linked by neutral square-type Hg₂Cl₄ clusters. A straightforward one-pot reaction of L with a mixture of HgCl₂ and KCl resulted in the isolation of a heteronuclear species [(K₂L)(μ-Hg₃Cl₈)]_n (2) adopting a 1D polymeric structure in which the endocyclic dipotassium(I) complex cation units of L are linked by anionic open double-square type (μ-Hg₃Cl₈)^2 − clusters. Both products form a pseudo 3D framework via interchain H-bonds and their topological analyses were also carried out.