Liquid circulation time in concentric-tube airlift columns with non-newtonian fermentation broths

Liquid circulation times were measured in a 40-dm³ concentric-tube airlift fermenter with simulated media over a wide range of rheological properties. Circulation times decreased with increasing shear-thinning, and the influence of draft-tube geometry on circulation time was found to depend on the shear-thinning of the media. A semi-theoretical correlation for both Newtonian and non-Newtonian fermentation media was developed.     

Heat transfer in bubble column and airlift bioreactors: Newtonian and non-Newtonian fermentation broths

A theoretical model has been developed for heat transfer in bubble column and airlift bioreactors, which is applicable for Newtonian and non-Newtonian fermentation media. The proposed model is based on a similarity between heat transfer in gas-sparged pneumatic reactors and turbulent natural convection. The applicability of the proposed model was discussed using a wide range of experimental data, and good agreement was obtained.     

Gas—liauid mass transfer in three-phase stirred tank reagors: Newtonian and non-newtonian fluids

Gas—liquid mass transfer has been investigated in gas—liquid-solid three-phase stirred tank reactors with Newtonian and non-Newtonian liquids. Volumetric mass transfer coefficients and gas hold-ups were measured in a 0.2 m i.d. stirred tank reactor and the effects of low-density polymeric particles (ρ_s, =1030 and 1200 kg/m³; up to 15 vol%) on gas—liquid mass transfer were examined. The volumetric mass transfer coefficients in water were found to decrease due to the presence of solid particles at constant impeller speed and superficial gas velocity. On the other hand, solids loading led to higher mass transfer rates in non-Newtonian carboxymethyl cellulose aqueous solutions. Our previously proposed model for mass transfer in gas—liquid two-phase systems was extended to gas—liquid—solid three-phase systems. Reasonable agreement was found between the predictions of the proposed model and the experimental data.     

Comparison of lipase production by Geotrichum candidum in stirring and airlift fermenters

The production of lipase by Geotrichum candidum in both, stirred tank and airlift bioreactors were compared. G candidum an imperfect filamentous fungus, grows well in liquid medium, and produces a lipase with specific affinity for long‐chain fatty acids with cis‐9 double bonds but, lipase production is generally not efficient because the optimum medium composition and fermentation conditions are not known. Response surface methodology was used to optimize the agitation speed (100–500 rpm) and aeration (0.2–1.8 vvm) for production of lipase by G candidum in a bench‐scale stirred fermenter. A Central Composite Rotatable Design (CCRD) was used to optimize lipase activity and productivity. Lipase production in an airlift fermenter was also studied with aeration ranging from 1 to 3 vvm. A previously optimized culture medium containing 3.58% of peptone, 0.64% of soy oil and an initial pH of 7.0, was used in the experiments, incubating at 30°C. In the stirred reactor the optimum conditions of agitation and aeration for lipase production and productivity were 300 rpm and 1 vvm, leading to an activity of 20 U cm^?3 in 54 h of fermentation and 0.3900 (U cm^?3 h^?1) of productivity. The best aeration condition in the airlift fermenter was 2.5 vvm, which yielded similar lipase activity after 30 h of fermentation, resulting in a productivity of 0.6423 (U cm^?3 h^?1). In the absence of mechanical agitation similar lipase yields were achieved but in less time, resulting in productivity, about 60% greater than in a stirred fermenter; the lower energy demand for the same lipase yield offers economic advantages. Copyright © 2004 Society of Chemical Industry     

Heat transfer to newtonian and non-newtonian liquids in a screw agitator and draft coil system

Heat transfer has been investigated for several Newtonian and non-Newtonian liquids mixed in a flat bottomed vessel equipped with a screw agitator and a coil acting simultaneously as draft tube and heat exchanger. Heat transfer rates from coil to liquid were determined for different coil designs, rheological properties and operating conditions in the heating and cooling modes. A circulation Reynolds number is defined, the characteristic length being the effective height of the heat exchanger (the coil) and the characteristic velocity being the circulation velocity of the fluid which was determined experimentally. Using this Reynolds number, it was possible to establish a single correlation for the Nusselt number as a function of Reynolds and Prandtl numbers. Geometric ratios do not appear in this correlation which could adequately represent more than 200 experimental data. A similar single correlation could not be obtained when using the conventional mixing Reynolds number. This novel system is shown to be very efficient for handling rheologically complex fluids.     

Gas hold-up and oxygen transfer in three-phase external-loop airlift bioreactors: Non-Newtonian fermentation broths

The effects of solids loading on gas hold-up and oxygen transfer in external-loop airlift bioreactors with non-Newtonian fermentation media are discussed. Experiments were performed in two model external-loop airlift bioreactors with aqueous solutions of carboxymethyl cellulose (CMC) and xanthan gum representing non-Newtonian flows. Low-density plastic particles of 1030 and 1300 kg m⁻³ were used and the solids loading was varied in the range 0–20% (v/v). For the inelastic non-Newtonian CMC aqueous solutions, the presence of low-density solid particles slightly increased the riser gas hold-up, ϕ_gr, but decreased the volumetric mass transfer coefficient, k_La. On the other hand, ϕ_gr decreased but k_La increased with solids loading in the viscoelastic non-Newtonian xanthan gum aqueous solution. The extent of these effects depended on non-Newtonian flow behavior. Theoretical models of riser gas hold-up and volumetric mass transfer coefficient have been developed. The capability of the proposed models was examined using the present experimental data obtained in the model external-loop airlift bioreactors and the available data in the literature. The data were successfully correlated by the proposed correlations except the results for k_La coefficient in the xanthan gum solution.     

Correlations for liquid-phase mass transfer coefficients in bubble column reactors with newtonian and non-newtonian fluids

The earlier work of Calderbank and Moo-Young (1961) dealing with the liquid-phase mass transfer coefficient in gas-liquid dispersions is examined. Their well-known empirical correlations for small and large bubbles which are free to move under gravity are theoretically derived. The analyses are based on the approach for natural convection mass transfer, and include the case where the fluid is non-Newtonian. The predictions of the models are compared with reported experimental data and correlations.     

Salt dissolution in drilling muds — a generalized correlation for mass transfer in non-newtonian fluids

The rate of dissolution of rock salt in drilling mud was measured as a function of flow rate and temperature under conditions simulating those in a well. A water based drilling mud, containing about 10% solids and additives was used, as well as a polymer “mud” composed of a Xanthan gum polymer solution and 5% salt, but no solids. The non-Newtonian muds were pumped through a “slit” flow cell, with a slab of rock salt flush with one side of the cell wall. The data (including water over the same range of conditions) were described by a mass transfer coefficient, and were correlated in dimensionless form based on the theoretical solution of the analogous heat transfer problem, adapted to non-Newtonian fluids. The non-Newtonian properties were described by an effective viscosity, which is consistent with either the power law or Bingham plastic viscosity model.     

Xanthan production in an external-circulation-loop airlift fermenter

A preliminary study of xanthan production by Xanthomonas campestris was performed in an external-circulation-loop airlift (ECL-AL) fermenter. An average of 25 g/L xanthan of good viscosity characteristics was obtained in a total fermentation time of 49 hours. Oxygen limitation was observed after 36 hours of fermentation time. It is conclusively demonstrated that the ECL-AL fermenter can be used to economically produce xanthan. The key operating and design features are the liquid height, downcomer-to-riser cross-sectional area ratio (A_D/A_R) and the orfice diameter of the air sparger.     

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.     

内循环气升式生物流化床水力学与传质特性研究

通过测定内循环气升式生物流化床的停留时间分布与溶解氧浓度，分析流化床内有效工作体积与氧传质系数的变化，以此研究其水力学与传质特性。结果表明，随着载体含量和气流量的增大，流化床内有效工作体积明显减小；氧传质系数则随着载体含量的增加先增后减，随着气流量和液流量的增大而增大。在本实验范围内，得出最适宜操作参数为载体含量为6％左右、气流量为180～240L／h之间、液流量为90L／h。     

Influence of impeller type on hydrodynamics and gas‐liquid mass‐transfer in stirred airlift bioreactor

The influence of impeller type in a mechanically stirred airlift bioreactor was analyzed in relation to the non‐Newtonian viscous fluids. The agitation was carried out through a marine impeller (axial impeller) and a paddle impeller (radial impeller) located along with the gas sparger in the region comprised by the riser. The bioreactor was sparged with air under different velocities (0.036–0.060 m s^?1). Carboxymethylcellulose 1.94% and xanthan 1.80% were used as a fluid model. The gas holdup and volumetric mass‐transfer coefficient increased in up to five and three times, respectively, when compared to a conventional airlift bioreactor; however, better results were obtained when the straight paddle impeller type was used. The results suggest that the studied bioreactor can be used successfully in viscous fluid, and it can be more efficient than conventional airlift bioreactors. The results obtained suggest the use of radial impellers. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3159–3171, 2015     

Influence of dissolved hydrocarbons on volumetric oxygen mass transfer coefficient in a novel airlift contactor

Hydrocarbon compounds are sparsely soluble in aqueous systems but, nonetheless, their presence can influence significantly mass transfer behavior in gas-liquid systems. water-p-xylene and water-p-xylene-naphthalene mixtures were employed in order to determine the influence of dissolved hydrocarbons on mass transfer of oxygen from air bubbles to water. The surface renewal-stretch model has been modified for predicting the volumetric mass transfer coefficient, (K_La)_h, in the presence of surface contaminant molecules, including hydrocarbon compounds and surfactants. Theory and experimental oxygen transfer results were found to be in satisfactory agreement with average absolute deviation of 15%. Pendant drop and contact angle measurements by axisymmetric drop shape analysis were carried out to determine the reduction in surface tension of water due to the addition of p-xylene and naphthalene. Molecular orientation caused by instantaneous attraction of the polar moieties of the organic compounds toward the water interface has been found to be the main cause of reduction in surface tension. It was predicated that changes in gas-liquid mass transfer behavior resulted from surface contamination and that the significant parameter was the reduction in surface tension.     

Gas–liquid mass transfer in a new three-phase stirred airlift reactor

The gas–liquid mass transfer performances of a novel three-phase reactor involving both airlift and mechanical stirring have been tested using aqueous solutions of glucose. Stirring in addition to classical airlift leads to an importance increase of k_La. The absolute increase depends mainly on the stirrer speed are not on the gas velocity. A slight effect of the solid loading with a maximum at about 2% (w/v) was observed. Two correlations that show the influence of physical parameters are proposed for both water and glucose solutions. © 1998 SCI     

Gas-liauid mass transfer in non-newtonian fluids in a tank stirred with a helical ribbon screw impeller

Oxygen transfer in aqueous solutions of CMC, xanthan gum, and polyacrylamide in a vessel equipped with a helical ribbon screw impeller and a multi-orifice ring sparger was studied. The K_La values obtained were comparable to those reported for radial flow impellers, but they are more representative of the oxygen transfer in the bulk of the liquid because of a more homogeneous mixing and absence of dead zones. Dimensionless correlations including the effect of the physical properties of the fluids, and those of operating conditions were developed. A dimensionless correlation for each fluid is proposed.     

环流反应器的流动、混合与传递特性

由于环流反应器内存在定向循环,与鼓泡塔反应器相比,其混合性能大幅提升,已广泛应用于许多工业过程,如发酵、反应器结晶等工业过程,近年来成为国内外学者研究的热点。针对环流反应器内操作条件下的流动形态、流体力学（包括相含率分布、循环液速、混合时间以及离集指数等）及传质/传热特性,总结了其最新研究进展,分析了相含率尤其是固含率变化对反应器中关键参数如循环液速和化学反应速率的影响,展望了从机理上研究相互耦合的多相流动、传质/传热和化学反应规律,为进一步推动其工业应用提供参考。     

Mixing length equation for high schmidt number mass transfer at solid boundaries

A simple modification of the Van Driest damping factor for mixing length calculations of mass and/or heat transfer coefficients at solid boundaries is proposed to increase mass eddy diffusivities in the near-wall region, without affecting velocity predictions. Calculated mass transfer rates with a constant Sc_t = 0.85 are in good agreement with present and previous experimental results obtained in pipe and duct flows at high Schmidt numbers.     

Influence of geometry and solids concentration on the hydrodynamics and mass transfer of a rectangular airlift reactor for marine sediment and soil bioremediation

Hydrodynamics and mass transfer characteristics of a three-phase airlift reactor were studied in a rectangular split-vessel reactor and using an air-seawater-marine sediment system. Experiments were conducted over a range of downcomer to riser cross-sectional area ratios (A_D/A_R = 0.65 to 1.0) for two-phase systems and for five sediment concentrations (5 to 25% w/v) using marine sediments. The influence of higher sediment concentrations (30 to 50% w/v) was examined for A_D/A_R = 1. The presence of fine sediment particles in the system had little effect on hydrodynamic and mass transfer parameters compared to the two-phase systems up to 25% loading, decreasing at higher loadings. The airlift reactor was found to meet the dissolved oxygen demand needed for a contaminated sediment treatment process. Axial distribution of the particles was uniform along the riser and the downcomer. Correlations were developed that described the hydrodynamic and mass transfer behaviour for all experimental conditions examined.     

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.