A novel rectangular airlift reactor with mesh baffle-plates

An experimental rectangular airlift reactor having mesh baffle-plates has been fabricated out of Perspex and compared with conventional airlift and bubble column in terms of gas holdup, volumetric mass transfer coefficient and mixing time. Mesh baffle-plates improved mass transfer and mixing with the mass transfer coefficient of the proposed reactor being up to 12% higher than that in a conventional airlift reactor under the same operating condition. The mixing time of the proposed reactor can be 95% lower than that of the airlift reactor.     

Modeling and simulation of oxygen transfer in airlift fermentors

A mathematical model for simulation of oxygen transfer in airlift fermentors is presented. The airlift fermentor is represented by a number of interconnected compartments, each of which is assumed to be well mixed. In the annular region, the model includes both upflow and downflow for the gas phase. The model contains several adjustable parameters through which important hydrodynamic effects affecting oxygen transfer are incorporated. The effect of hydrostatic pressure is also included in the model. The model is simple enough to be used in design studies and it can be easily adapted to other airlift system configurations. The simulation results show good qualitative agreement with available experimental results.     

Oxygen transfer in an airlift reactor with multiple net draft tubes

A pilot scale airlift reactor with multiple net draft tubes was developed to improve oxygen transfer in the reactor. The reactor was 0.29 m in diameter and 2 m height. A steadystate sulfite oxidation method was applied to determine an overall volumetric mass transfer coefficient. Oxygen transfer of the proposed airlift reactor can be 60–100% higher than that of bubble columns under the same operating conditions.List of Symbols C ^* mol·dm^–3 saturated concentration of dissolved oxygen - C _L mol·dm^–3 bulk concentration of dissolved oxygen - G mol/min nitrogen flow rate - k _L a hr^–1 the volumetric gas-liquid mass transfer coefficient - Mo ₂ g/mol molecular weight of oxygen - OTR g/min the oxygen transfer rate - U _g cm/s superficial air velocity - V _L dm³ volume of the liquid phase - _in oxygen mole ratio in the inlet gas - _out oxygen mole ratio in the outlet gas     

Hydrodynamic characteristics and gas-liquid mass transfer in a biofilm airlift suspension reactor

The hydrodynamics and mass transfer, specifically the effects of gas velocity and the presence and type of solids on the gas hold-up and volumetric mass transfer coefficient, were studied on a lab-scale airlift reactor with internal draft tube. Basalt particles and biofilm-coated particles were used as solid phase. Three distinct flow regimes were observed with increasing gas flow rate. The influence of the solid phase on the hydrodynamics was a peculiar characteristic of the regimes. The volumetric mass transfer coefficient was found to decrease with increasing solid loading and particle size. This could be predominantly related to the influence that the solid has on gas hold-up. The ratio between gas hold-up and volumetric mass transfer coefficient was found to be independent of solid loading, size, or density, and it was proven that the presence of solids in airlift reactors lowers the number of gas bubbles without changing their size. To evaluate scale effects, experimental results were compared with theoretical and empirical models proposed for similar systems.     

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.     

Gas holdup and overall volumetric oxygen transfer coefficient in airlift contactors

The two major types of airlift contactors, concentric-tube and external-loop, were investigated for their gas holdup (riser and downcomer) and overall mass transfer characteristics. Results obtained in batch charges of tap water and 0.15 kmol/m(3) NaCl solution are reported for external-loop airlift contactors having downcomer-to-riser cross-sectional area ratios, A(d)/A(r), ranging from 0.11 相似文献   

Influence of dissolved oxygen concentration on nitrite accumulation in a biofilm airlift suspension reactor

The biofilm airlift suspension (BAS) reactor can treat wastewater at a high volumetric loading rate combined with a low sludge loading. Two BAS reactors were operated, with an ammonium load of 5 kg N/(m(3) d), in order to study the influence of biomass and oxygen concentration on the nitrification process. After start-up the nitrifying biomass in the reactors gradually increased up to 30 g VSS/L. Due to this increased biomass concentration the gas-liquid mass transfer coefficient was negatively influenced. The resulting gradual decrease in dissolved oxygen concentration (over a 2-month period) was associated with a concomitantly nitrite build-up. Short term experiments showed a similar relation between dissolved oxygen concentration (DO) and nitrite accumulation. It was possible to obtain full ammonium conversion with approximately 50% nitrate and 50% nitrite in the effluent. The facts that (i) nitrite build up occurred only when DO dropped, (ii) the nitrite formation was stable over long periods, and (iii) fully depending on DO levels in short term experiments, led to the conclusion that it was not affected by microbial adaptations but associated with intrinsic characteristics of the microbial growth system. A simple biofilm model based on the often reported difference of oxygen affinity between ammonium and nitrite oxydizers was capable of adequately describing the phenomena.Measurements of biomass density and concentration are critical for the interpretation of the results, but highly sensitive to sampling procedures. Therefore we have developed an independent method, based on the residence time of Dextran Blue, to check the experimental methods. There was a good agreement between procedures.The relation between biomass concentration, oxygen mass transfer rate and nitrification in a BAS reactor is discussed. (c) 1997 John Wiley & Sons, Inc.     

Mass transfer and liquid mixing in an airlift reactor with a net draft tube

Mass transfer and liquid mixing in an airlift reactor with a net draft tube were experimentally investigated. Four different column diameters were considered. The mass transfer was measured using the volumetric gas-liquid mass transfer coefficient which was determined by the dynamic method. The mass transfer coefficients in the airlift reactors with different column diameters were not always higher than those in the bubble columns. The liquid mixing was measured using mixing time which was determined by a pulse technique. Under the same superficial gas velocity, the mixing times of the airlift reactors with a net draft tube were always less than those of the bubble columns.List of Symbols C mol·dm^–3 bulk concentration of dissolved oxygen - C ₀ mol·dm^–3 initial concentration of dissolved oxygen - C _e mol·dm^–3 saturated concentration of dissolved oxygen - ¯C dimensionless dissolved oxygen concentration - D _c cm diameter of column - D _N cm diameter of the nozzle hole - D _T cm diameter of the net draft tube - H _L cm static liquid height - H _T cm height of the net draft tube - k _L a hr^–1 volumetric mass transfer coefficient - L _T cm length of the net draft tube - t _M sec mixing time of the liquid phase - t ₀ sec mixing time of the liquid phase in a bubble column - V _L dm³ volume of the liquid phase - U _g cm/s superficial air velocity     

Mass transfer phenomena in an airlift reactor: effects of solids loading and temperature

Gas holdups and volumetric mass transfer coefficients were measured in a concentric tube airlift reactor designed for the microbial desulfurization of coal. The solutions studied were comprised of an acidified basal salts solution containing thirteen different weight percentages (0 to 40) of coal (74 mum Ohio #1) at three different temperatures (30, 50, and 72 degrees C). Gas holdup epsilon(G) decreased with solids loading for the entire range studied. An enhancement in the volumetric mass transfer coefficient K(L)a with respect to that in pure solution was observed from zero to approximately 5 wt % (solids volume fraction epsilon(s) = 0.035), the maximum enhancement occurring at approximately 2 wt % (epsilon(s) = 0.014). At higher solids fractions, the mass transfer coefficient decreased with further solids additions. Gas holdups and the mass transfer coefficients increased with temperature over the studied range. The K(L)a and epsilon(G) were correlated to three process variables separately and the separate correlations combined to yield generalized correlations for the mass transfer coefficient and gas holdup for this system. The correlations may be used for design, operation, and ost estimation of such systems.     

Bubble characteristics and mass transfer in an airlift reactor with multiple net draft tubes

A pilot scale airlift reactor with multiple net draft tubes was developed. The reactor, 29?cm in diameter and 300?cm height, had four modules of double net draft tubes. Bubble size, bubble number, gas holdup, and volumetric mass transfer coefficient were measured under different superficial air velocities. The air velocity had little effect on bubble size but had significant influence on bubble number. A bubble column was also investigated for comparison. The airlift reactor had a higher gas holdup and volumetric mass transfer coefficient than those in the bubble column. The draft tubes in the airlift reactor substantially improved the reactor performance.     

Mixing time and oxygen transfer characteristics of double draft tube airlift fermentor

Despite the increasing importance of airlift fermentors, very little published information is available on how the geometric configurations of the draft tubes and the air-sparging system affect the mixing and oxygen transfer characteristics of the fermentor. A 14-L air-lift fermentor was designed and build with a fixed liquid height to diameter ratio of 1.5 utilizing four equally spaced air jets at the bottom. Two jet orifice sizes were used, 1.27 and 3.81 mm i.d., and for each jet size the following four geometric configurations were used: Single inner concentric draft tube, single outer concentric draft tube, two concentric draft tubes, and no draft tubes where the fermentor was operated as a shallow bubble column. It was found that the presence of draft tubes stabilized liquid circulation patterns and gave systemically higher mixing times than those obtained in the absence of draft tubes. In addition, the double draft tube geometry resulted in higher mixing times than the single draft tubes. For the power unit volume range 20 to about 250 W/m³ the larger 3.81-mm orifices gave systemically higher k_L a values than the smaller 1.27-mm i.d. orifices. At 200 W/m³ the use of a single outer draft tube with the 3.81-mm orifices resulted in 94% increase in k_L a values over that obtained with no draft tubes. However, the effect of draft tube geometry on k_L a values when the 1.27-mm orifices were used was not significant. The air bubble formation characteristics at the jet orifices were found to be different, which reflected the differences observed in mass transfer and mixing characteristics. The power economy for oxygen transfer was found to be depend strongly on the orifice size and less on the geometric configuration of draft tubes.     

Liquid circulation velocity in the inverse fluidized bed airlift reactor

Results of the investigations of the liquid circulation velocity in the external loop airlift reactor with the fluidized bed in the downcomer are reported. Densities of solid particles are much lower than the liquid density. It is found that the curve of the liquid circulation velocity dependence on the gas flow rate consists of two different parts. A slow linear increase of the liquid circulation velocity is observed if the gas velocity is lower than the minimum fluidization velocity. An exponential course of the liquid circulation velocity curve is observed if the gas flow rate exceeds the minimum fluidization velocity. A theoretical equation for the prediction of liquid circulation velocity for gas velocities higher than the minimum fluidization velocity is presented.     

Fungal bioremediation of phenolic wastewaters in an airlift reactor

Of the various types of industry-generated effluents, those containing organic pollutants such as phenols are generally difficult to remediate. There is a need to develop new technologies that emphasize the destruction of these pollutants rather than their disposal. In this work the white rot fungus, Trametes pubescens, was demonstrated to be an effective bioremediation agent for the treatment of phenolic wastewaters. An airlift loop reactor was optimized, in terms of volumetric oxygen transfer rate (K(L)a = 0.45 s(-1)), to provide an environment suited to rapid growth of T.pubescens (mu = 0.25 day(-1)) and a particularly efficient growth yield on glucose of 0.87 g biomass.g glucose(-1). The phenolic effluent was shown to be a paramorphogen, influencing fungal pellet morphology in the reactor, as well as increasing laccase enzyme activity by a factor of 5 over the control, to a maximum of 11.8 U.mL(-1). This increased activity was aided by the feeding of nonrepressing amounts (0.5 g.L(-1)) of glucose to the reactor culture. To our knowledge the degradation results represent the highest rate of removal (0.033 g phenol.g biomass(-1).day(-1)) of phenolic compounds from water reported for white rot fungi.     

N-removal in a granular sludge sequencing batch airlift reactor

The removal of N-compounds in the sequencing batch airlift reactor (SBAR) containing granular sludge was studied under conditions of decreased dissolved oxygen (DO). A simulation model was developed to describe and evaluate the effects of several process conditions in the SBAR on N-removal performance. The model described the experimental data reasonable well. It has been shown that nitrification, denitrification, and removal of chemical oxygen demand (COD) can occur simultaneously in a granular sludge SBR. It has also been shown that the exact location of the autotrophic biomass influences the net N-removal. The distribution of the autotrophic biomass is influenced by the DO in the reactor. The optimal DO value is expected to be around 40% air saturation. It was shown that storage and subsequent degradation of poly-beta-hydroxybutyrate (PHB) benefit the denitrification. In particular, PHB is stored in bacteria situated in deeper layers of the granules below where the autotrophic activity occurs, serves as a C-source for denitrification.     

Mass transfer in a rectangular air-lift reactor: effects of geometry and gas recirculation

The interrelationships between the three parts of the air lift reactor, the riser, the downcomer, and gas-liquid separator, were examined with relation to the overall mass transfer in the reactor. This involved studying the mass transfer of oxygen from the gas phase to the liquid phase for 20 different reactor geometries. Both one- and two-sparger systems were studied. It was demonstrated that the gas-liquid separator plays a major role in reactor behavior and must be considered in reactor design. It was found that the overall reactor mass transfer coefficient KLA could be correlated to the pneumatic power of gas input per total dispersion volume (P/VD) and to the true riser superficial gas velocity JGR for all experimental conditions examined. The KLA is directly related to the P/VD with an exponent of approximately 1. "Two-sparger" systems, where an auxiliary gas sparger is placed near the downcomer entrance, have higher ab solute values for KLA than single-sparger systems.     

Kinetics of biodegradation of p-xylene and naphthalene and oxygen transfer in a novel airlift immobilized bioreactor

The scope of this study included the biodegradation performance and the rate of oxygen transfer in a pilot-scale immobilized soil bioreactor system (ISBR) of 10-L working volume. The ISBR was inoculated with an acclimatized population of contaminant degrading microorganisms. Immobilization of microorganisms on a non-woven polyester textile developed the active biofilm, thereby obtaining biodegradation rates of 81 mg/L x h and 40 mg/L x h for p-xylene and naphthalene, respectively. Monod kinetic model was found to be suitable to correlate the experimental data obtained during the course of batch and continuous operations. Oxygen uptake and transfer rates were determined during the batch biodegradation process. The dynamic gassing-out method was used to determine the oxygen uptake rate (OUR) and volumetric oxygen mass transfer, K(L) a. The maximum volumetric OUR of 255 mg O(2)/L x h occurred approximately at 720-722 h after inoculation, when the dry weight of biomass concentration was 0.67 g/L.     

Process optimization of a continuous airlift tower-loop reactor

Based on the experimental investigations with H. polymorpha and Methylomonas M 15 in bench-scale airlift tower-loop reactors, a general distributed parameter model was developed and used to simulate to cultivation process in a 40-m-high production reactor. This general model was simplified with regard to the gas phase and loop balances and was employed to optimize cell productivity and/or profit in a 20-m-high pilot-plant airlift tower-loop reactor. Maximum cell productivity always occurs in the oxygen-transfer-limited growth range. In case of a high "penalty factor" for nonconsumed substrate, maximum profit is attained at the boundary between substrate and oxygen-transfer-limited growth. Oxygen-transfer limitation exists in the lower half of the tower, whereas in the upper half, substrate limitation prevails. The longitudinal dissolved oxygen concentration passes a minimum in this case as has been determined experimentally in the bench-scale column. The simulation results agree fairly well with the data measured in the pilot plant.     

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.