三相逆流湍动床气液传质性能的研究

由空气-水(清水/废水)-中空玻璃珠构成三相体系,在表观气速0·53~10mm·s-1、固含率为0~0·3、表观液速0~0·2mm·s-1的条件下,采用溶氧仪研究了三相逆流湍动床的气液传质性能,考察了操作参数和液体性质对液相容积传质系数kLa的影响。结果表明,在所试条件下,kLa为0·0456~1·414min-1。kLa随着表观气速和表观液速的增加而增加,随着固含率的增加先增加后减小,0·05~0·08为反应器传质的最优固含率条件。液体性质对kLa有重大影响,高浓度模拟废水和工业废水中的kLa比清水中的kLa分别减小39·0%和50·9%。研究结果可为后续逆流湍动床废水生物处理过程分析与模拟提供传质基础数据。     

Scale-up of enzymatic production of lactobionic acid using the rotary jet head system

Enzymatic oxidation of lactose to lactobionic acid (LBA) by a carbohydrate oxidase from Microdochium nivale was studied in a pilot-scale batch reactor of 600 L working volume using a rotary jet head (RJH) for mixing and mass transfer (Nordkvist et al., 2003, Chem Eng Sci 58:3877-3890). Both lactose and whey permeate were used as substrate, air was used as oxygen source, and catalase was added to eliminate the byproduct hydrogen peroxide. More than 98% conversion to LBA was achieved. Neither enzyme deactivation nor enzyme inhibition was observed under the experimental conditions. The dissolved oxygen tension (DOT) was constant throughout the tank for a given set of operating conditions, indicating that liquid mixing was sufficiently good to avoid oxygen gradients in the tank. However, at a given oxygen tension measured in the tank, the specific rate of reaction found in the RJH system was somewhat higher than previously obtained in a 1 L mechanically stirred tank reactor (Nordkvist et al., 2007, in this issue, pp. 694-707). This can be ascribed to a higher pressure in the recirculation loop which is part of the RJH system. Compared to mechanically stirred systems, high values of the volumetric mass transfer coefficient, k(L)a, were obtained when lactose was used as substrate, especially at low values of the specific power input and the superficial gas velocity. k(L)a was lower for experiments with whey permeate than with lactose due to addition of antifoam. The importance of mass transfer and of the saturation concentration of oxygen on the volumetric rate of reaction was demonstrated by simulations.     

Flow and oxygen transfer in a plunging water jet system using inclined short nozzles and performance characteristics of its system in aerobic treatment of wastewater

For the plunging water jet system using inclined short nozzles, the flow characteristics such as the bubble penetration depth and the gas entrainment rate, which changed depending on the jet velocity, the nozzle diameter, the jet length, and the jet angle were first evaluated in an air-water system. A comparable investigation between our results and those of existing studies used the long nozzles on those characteristics revealed that both the bubble penetration depth and the gas entrainment rate differed depending on the nozzle length; that is, the nozzle-length-to-diameter ratio L(N)/D(N) and that of these characteristics the gas entrainment rate affected considerably by its magnitude and tended to be high when the nozzle of a large L(N)/D(N) ratio was used. It was also confirmed from the oxygen transfer experiments that the transfer efficiency at low jet velocities in the present water jet system was not inferior to the ones of other types of existing aeration systems; that is, the utilization of this jet aeration system to a high rate reactor for wastewater treatment or fermentation was sufficiently possible. The applicability of the plunging jet aeration method to microbial processes was then examined. As a typical example of microbial processes to be tested, the continuous treatment of an organic wastewater using activated sludge microorganisms was carried out, and the performance and related problem when this type of aeration system was applied to such a microbial process were investigated. Experimental results showed that, when viewed from the removal ability of dissolved organic matters, the plunging jet aeration system was capable of treating a wastewater of considerable high loading without the rate of oxygen transfer becoming the biooxydation-rate-limiting factor. Special attention was necessary for the choice of the liquid pump to be employed, however, due to the increased amount of fine suspended solids in the treated water caused by the shearing action between sludge flocks and pump blades.     

Characterization of a pilot plant airlift tower loop bioreactor. I: evaluation of the phase properties with model media

Investigations were carried out in a 9 m high, 4 m(3) volume, pilot plant airlift tower loop bioreactor with a draft tube. The reactor was characterized by measuring residence time distributions of the gas phase using pseudostochastic tracer signals and a mass spectrometer and by evaluating the mixing in the liquid phase with single-pulse tracer inputs. The local gas holdup and the bubble size (piercing length) were measured with two-channel electrical conductivity probes. The mean residence times and the intensities of the axial mixing in the riser and downcomer and the circulation times of the phases as well as the fraction of the recirculated gas phase were evaluated. The gas holdup in the riser is nearly uniform along the reactor. In the downcomer, it diminishes from top to bottom. The liquid phase dispersion coefficients, D(L), are smaller than those measured in the corresponding bubble columns. In the pilot plant with tap water the following relationship was found: D(Lr) = cw(SG) (n); with c = 203.4; n = 0.5;D(Lr)(cm(2) s(-1);) and W(SG)(cm s(-1)) where D(Lr) is the longitudinal dispersion coefficient in the riser and W(SG) is the superficial gas velocity. The gas phase dispersion coefficients in the riser of the pilot plant, D(Gr), are also enlarged with increasing superficial gas velocity, W(SG), however, no simple relationship exists. Parameter D(Gr) is the highest in the presence of antifoam agents, intermediate in tap water, and the smallest in ethanol solution.     

Liquid circulation velocity and overall gas holdup in a modified jet loop bioreactor with low density particles

Effect of low density particles on the apparent liquid circulation velocity and overall gas holdup was studied in a modified reversed flow jet loop bioreactor. Experiments were conducted using polyurethane beads, polystyrene particles which are comparable to bioparticles found in biological applications and glass beads. Influence of gas and liquid flow rates, draft tube to reactor diameter ratio and solids loading on these hydrodynamic properties were studied. The liquid circulation velocity was found to increase with an increase in liquid flow rate but decrease with an increase in gas flow rate or solids loading. The overall gas holdup increased with an increase in gas or liquid flow rate but decreased with an increase in solids loading. The range of optimum draft tube to reactor diameter ratio was found to be 04–0.5. The results obtained with low density particles were comparatively better than those with glass beads. Correlations were proposed to evaluate liquid circulation velocity and overall gas holdup in terms of operational and geometrical variables.     

Mass transfer in an airlift reactor with a net draft tube.

Gas-liquid mass transfer in an airlift reactor with net draft tube is investigated. The effects of both the ratio of draft tube to reactor diameter and the reactor pressure on oxygen transfer are considered. The value of the volumetric mass transfer coefficient, kLa, increases with a decreasing diameter ratio at higher air flow rates. The correlation of volumetric mass transfer coefficient with respect to the true superficial air velocity under different reactor pressures is determined. The kLa value decreases with increasing reactor pressure.     

High oxygen transfer rate in a new aeration system using hollow fiber membrane

A new type of bubble aeration column called a hollow fiber membrane (HFM) aeration column was proposed, which was featured in the use of hollow fiber membranes and gave a high bubble density in the column. The value of k(L)a was increased by modifying the membrane surface for making the pore size smaller. The Sauter mean diameter of bubbles (D(vs)) was 2.0 +/- 0.1 mm in the range of the superficial gas velocity from 0.02 m s(-1) to 0.065 m s(-1), while that obtained for the bubbles near the membrane was 811 mum at the superficial gas velocity of 4.0 x 10(-4) m s(-1). The difference was ascribed to the effect of coalescence of bubbles. The value of K(L)a increased in proportion to the superficial gas velocity up to 0.02 m s(-1), and was almost constant above 0.03 m s(-1). The maximum value of k(L)a, 2.5 s(-1), was higher than those of the other aeration columns reported previously. The pneumatic power consumption per unit liquid volume (P(v)) for obtaining the same k(L)a was the smallest in the HFM aeration columns. P(v), for obtaining the same interfacial area of bubbles per liquid volume, was also lower than those for other types of aeration columns. It was suggested from the measurement of bubble diameter that the larger interfacial area generated in the HFM aeration column ascribes to the larger gas holdup than the smaller D(vs). (c) 1992 John Wiley & Sons, Inc.     

The influence of vessel height and top-section size on the hydrodynamic characteristics of airlift fermentors

Fermentations of the yeast Saccharomyces cerevisiae were carried out in a 90 to 250-L working volume concentric tube airlift fermentor. Measurements of liquid circulation velocity, gas hold-up, and liquid mixing were made under varying conditions of gas flowrate, vessel height, and top-section size. Both liquid circulation velocity and mixing time increased with vessel height. Liquid velocity varied approximately in proportion to the square root of column height, supporting a theoretically based relationship. The effect of vessel height on gas hold-up was negligible. The height of the top-section had a significant effect on liquid mixing. Mixing time decreased with increasing size of the top-section up to a critical height. As the top-section was expanded beyond this height, little improvement in mixing was seen. This indicated the presence of a two-zone flow pattern in the top-section. Liquid velocity and gas hold-up were essentially independent of top-section height. (c) 1994 John Wiley & Sons, Inc.     

Biofilm reactors: an experimental and modeling study of wastewater denitrification in fluidized-bed reactors of activated carbon particles

A fluidized-bed biofilm reactor using activated carbon particles of 1.69 mm diameter as the support for biomass growth and molasses as the carbon source is used for wastewater denitrification.The start-up of the reactor was successfully achieved in 1 week by using a liquor from garden soil leaching as the inoculum and a superficial velocity u(0) = 5u(mf). Typical biofilm thickness is 800 mum; therefore covered activated carbon particles have 3.3 mm in diameter.Reactor hydrodynamics was studied by tracer (KCl solution) experiments. The analysis based on residence time distribution theory involved a model with axial dispersion flow and tracer diffusion with linear adsorption inside the biofilm. Peclet numbers higher than 100 were found, allowing the plug flow assumption for the reactor model.Experimental profiles of nitrate and nitrite species were explained by a kinetic model of two consecutive zero-order reactions coupled with substrate diffusion inside the biofilm. Under the operating conditons used thick biofilms were obtained working in a diffusion-controlled regime.Comparison is made with results obtained in the same reactor with sand particles as the support for biomass growth. Activated carbon as the support has the following advantages: good adsorptive characteristics, homogeneous biofilm thickness along the reactor, and easy restart-up of the reactor. (c) 1992 John Wiley & Sons, Inc.     

Hydrodynamic and Mass Transfer Characteristics of Three-Phase Gaslift Bioreactor Systems

ABSTRACT:?

This review focuses on the hydrodynamic and mass transfer characteristics of various three-phase, gaslift fluidized bioreactors. The factors affecting the mixing and volumetric mass transfer coefficient (k_La), such as liquid properties, solid particle properties, liquid circulation velocity, superficial gas velocity, bioreactor geometry, are reviewed and discussed. Measurement methods, modeling and empirical correlations are reviewed and compared. To the authors' knowledge, there is no 'generalized' correlation to calculate the volumetric mass transfer coefficient, instead, only 'type-specific' correlations are available in the literature. This is due to the difficulty in modeling the gaslift bioreactor, caused by the variation in geometry, fluid dynamics, and phase interactions. The most important design parameters reported in the literature are: gas hold-up, liquid circulation velocity, 'true' superficial gas velocity, mixing, shear rate, aeration rate and volumetric mass transfer coefficient, k_La.     

Flow regimes in a two phase reversed flow jet loop bioreactor

Reversed flow jet loop bioreactors (RFJLB) have been used extensively for 2 or 3 phase biochemical reactions. From visual observations and gas holdup data, 3 distinct flow regimes are identified in RFJLB, namely: (1) Bubble free regime (BFR), where bubbles are observed in the draft tube only; (2) Transition regime (TR), where bubbles are observed in both the draft tube and the annulus, but without circulation; and (3) Complete bubble circulation regime (CBCR), where bubbles circulate in both the draft tube and annulus. CBCR is the most desirable regime, since the reactor operation in this regime gives a higher gas holdup and mass transfer rate than in the other two regimes. In the present study, the hydrodynamic behavior of RFJLB was investigated under various operational and geometrical conditions, such as gas and liquid velocity and nozzle configuration. Factors affecting the critical liquid circulation velocity (CLCV) above which the CBCR is established were identified and evaluated quantitatively.     

Liquid-phase dispersion in an airlift reactor with a net draft tube

Liquid-phase dispersion in an airlift reactor with a net draft tube was considered. Four net tubes with different ratios of draft tube to reactor diameters and superficial air velocities ranged from zero to 6.05 cm/s were investigated. The sparger was a porous plate. The parameter of the dispersion effect, axial dispersion coefficient, was characterized by measuring the residence time distribution in the liquid phase with single-pulse tracer input. The values of the dispersion coefficient of the proposed airlift reactor were much higher than those of the bubble column under the same operating conditions.     

Outdoor helical tubular photobioreactors for microalgal production: modeling of fluid-dynamics and mass transfer and assessment of biomass productivity

The production of the microalga Phaeodactylum tricornutum in an outdoor helical reactor was analyzed. First, fluid dynamics, mass-transfer capability, and mixing of the reactor was evaluated at different superficial gas velocities. Performance of the reactor was controlled by power input per culture volume. A maximum liquid velocity of 0.32 m s(-1) and mass transfer coefficient of 0.006 s(-1) were measured at 3200 W m(-3). A model of the influence of superficial gas velocity on the following reactor parameters was proposed: gas hold-up, induced liquid velocity, and mass transfer coefficient, with the accuracy of the model being demonstrated. Second, the influence of superficial gas velocity on the yield of the culture was evaluated in discontinuous and continuous cultures. Mean daily values of culture parameters, including dissolved oxygen, biomass concentration, chlorophyll fluorescence (F(v)/F(m) ratio), growth rate, biomass productivity, and photosynthetic efficiency, were determined. Different growth curves were measured when the superficial gas velocity was modified-the higher the superficial gas velocity, the higher the yield of the system. In continuous mode, biomass productivity increased by 35%, from 1.02 to 1.38 g L(-1) d(-1), when the superficial gas velocity increased from 0.27 to 0.41 m s(-1). Maximal growth rates of 0.068 h(-1), biomass productivities up to 1.4 g L(-1) d(-1), and photosynthetic efficiency of up to 15% were obtained at the higher superficial gas velocity of 0.41 m s(-1). The fluorescence parameter, F(v)/F(m), which reflects the maximal efficiency of PSII photochemistry, showed that the cultures were stressed at average irradiances within the culture higher than 280 microE m(-2) s(-1) at every superficial gas velocity. For nonstressed cultures, the yield of the system was a function of average irradiance inside the culture, with the superficial gas velocity determining this relationship. When superficial gas velocity was increased, higher growth rates, biomass productivities, and photosynthetic efficiencies were obtained for similar average irradiance values. The higher the superficial gas velocity, the higher the liquid velocity, with this increase enhancing the movement of the cells inside the culture. In this way the efficiency of the cells increased and higher biomass concentrations and productivities were reached for the same solar irradiance.     

A novel split-cylinder airlift reactor for fed-batch cultures

Conventional airlift reactors are not adequate to carry out variable volume processes since it is not possible to achieve a proper liquid circulation in these reactors until the liquid height is higher than that of the downcomer. To carry out processes of variable volume, the use of a split-cylinder airlift reactor is proposed, in the interior of which two multi-perforated vertical baffles are installed in order to provide several points of communication between the reactor riser and downcomer. This improves the liquid circulation and mixing at any liquid volume. In fed-batch cultures, it is important to know how liquid height affects the hydrodynamic characteristics and the volumetric oxygen transfer coefficient since this impacts on the kinetic behavior of any fermentation. Thus, in the present work, the effect of the liquid height on the mixing time, the overall gas hold-up, and the volumetric oxygen transfer coefficient of the proposed airlift reactor were determined. The mixing time was increased and the volumetric oxygen transfer coefficient decreased due to the increase of the liquid height in the reactor in all the superficial gas velocities tested, which corresponded to a pseudohomogeneous flow regime. The experimental values of the mixing time and the mass-transfer coefficient were properly described through correlations in which the U_GR/H_L ratio was used as the independent variable. Thus, this variable might be used to describe the hydrodynamic behavior and the oxygen transfer coefficient of airlift reactors when such reactors are used in processes where the liquid volume changes with time. However, these correlations are useful for the particular device and for the particular operating conditions at which they were obtained. These empirical correlations are useful to understand some factors that influence the mixing time and volumetric oxygen transfer coefficient, but such correlations do not have a sufficient predictive potential for a satisfactory reactor design. The overall gas hold-up values were not significantly affected when the liquid height was lower than the downcomer height. However, the values decreased abruptly when the reactor was operated with liquid heights over the downcomer height, especially at high superficial gas velocities.     

Steroid transformation in a laboratory-scale glass air-lift fermenter

A glass airlift fermenter, 1550 ml working volume, was used for microbiological transformation of phytosterols. A gas hold-up of 1.6% was observed with the lowest superficial gas velocity (1.89 cm/s). The volumetric liquid circulation rate remained relatively constant (0.21 l/s to 0.23 l/s) for superficial gas velocity values up to 11.37 cm/s. A 72% conversion of sitosterol to 1,4-androstadiene-3,17-dione was obtained.     

Flow characteristics of a pilot-scale airlift fermentor

The effects of aeration on the flow characteristics of water in a glass pilot-scale airlift fermentor have been examined. The 55-L capacity fermentor consisted of a 15.2-cm-i.d. riser column with a 5.1-cm-i.d. downcomer tube. It was found that the average bubble size diminished with increased aeration. Typically, average bubble sizes ranged from 4.32 mm at a superficial gas velocity of 0.64 cm/s to 1.92 mm at 10.3 cm/s. A gas holdup of 0.19 was attained with superficial gas velocities (v_s) on the order of 10 cm/s, indicating the highly gassed nature of the fluid in the riser section of the fermentor. Circulation velocities of markers placed in the fermentor decreased with increasing aeration rates due to increased turbulence and axial liquid back mixing within the riser section. Actual volumetric liquid circulation rates remained relatively constant (0.36–0.49 L/s) for values of (v_s) up to 10 cm/s. Based on theoretical calculations, the ascending velocity of bubbles in a swarm reached 54 cm/s in the range of (v_s) values studied.     

Hydrodynamic and mass transfer characteristics of three-phase gaslift bioreactor systems.

This review focuses on the hydrodynamic and mass transfer characteristics of various three-phase, gaslift fluidized bioreactors. The factors affecting the mixing and volumetric mass transfer coefficient (k(L)a), such as liquid properties, solid particle properties, liquid circulation velocity, superficial gas velocity, bioreactor geometry, are reviewed and discussed. Measurement methods, modeling and empirical correlations are reviewed and compared. To the authors' knowledge, there is no 'generalized' correlation to calculate the volumetric mass transfer coefficient, instead, only 'type-specific' correlations are available in the literature. This is due to the difficulty in modeling the gaslift bioreactor, caused by the variation in geometry, fluid dynamics, and phase interactions. The most important design parameters reported in the literature are: gas hold-up, liquid circulation velocity, 'true' superficial gas velocity, mixing, shear rate, aeration rate and volumetric mass transfer coefficient, k(L)a.     

内循环颗粒污泥床硝化反应器临界曝气强度的研究

内循环颗粒污泥床硝化反应器是一种新型高效硝化反应器 ,在反应器运行过程中 ,液体循环临界曝气强度和颗粒污泥流化临界曝气强度是两个重要操作参数。建立了升流区表观液速Ulr与曝气强度Ugr之间的关系 ,并测定了有关的模型参数 ,得到了具体的数学表达式 :U_lr=(2.613-0.024 )U^0.871_gr 0.276U^0.871_gr-0.28。根据该模型 ,计算得到的液体循环临界曝气强度为1.017cm/min ,颗粒污泥流化临界曝气强度为 2.662cm/min。实测结果证明 ,求得的两个临界曝气强度具有较高的准确性 ,能够用于指导内循环颗粒污泥床硝化反应器的操作优化.     

Flow pattern visualization in a mimic anaerobic digester using CFD

Three-dimensional steady-state computational fluid dynamics (CFD) simulations were performed in mimic anaerobic digesters to visualize their flow pattern and obtain hydrodynamic parameters. The mixing in the digester was provided by sparging gas at three different flow rates. The gas phase was simulated with air and the liquid phase with water. The CFD results were first evaluated using experimental data obtained by computer automated radioactive particle tracking (CARPT). The simulation results in terms of overall flow pattern, location of circulation cells and stagnant regions, trends of liquid velocity profiles, and volume of dead zones agree reasonably well with the experimental data. CFD simulations were also performed on different digester configurations. The effects of changing draft tube size, clearance, and shape of the tank bottoms were calculated to evaluate the effect of digester design on its flow pattern. Changing the draft tube clearance and height had no influence on the flow pattern or dead regions volume. However, increasing the draft tube diameter or incorporating a conical bottom design helped in reducing the volume of the dead zones as compared to a flat-bottom digester. The simulations showed that the gas flow rate sparged by a single point (0.5 cm diameter) sparger does not have an appreciable effect on the flow pattern of the digesters at the range of gas flow rates used.     

Studies on protein adsorption in a vortex flow reactor

The performance of a vortex flow reactor (VFR) with suspended particles for protein adsorption was studied under varying operating conditions, and resin volume fractions. The VFR behaved as an expanded bed in the regimen of laminar vortices flow. Streamline DEAE was used for bovine serum albumin (BSA) adsorption. Expanded bed VFR experiments were performed with varying geometric aspect ratios (14.6, 28.6 and 40.0) and axial superficial velocity (100–300 cm h⁻¹) to investigate their influence on productivity and dynamic capacity. The results are compared with literature data on an expanded bed column (EBC). Adsorption breakthrough curves were fitting to a simple two-parameter model.