Effect of liquid-phase surface tension on hydrodynamics of a three-phase airlift reactor with an enlarged degassing zone

The effect of the addition of ethanol (10?g/l) to the liquid-phase on gas and solids holdup, circulation and mixing times and interstitial liquid velocity in a three-phase airlift reactor was investigated. The airlift reactor (60?l) is of the concentric draught-tube type with an enlarged degassing zone. Ca-alginate beads were used as solid-phase and airflow rate (from 1.9 to 90.2?l/min) and solids loading (0–30% (v/v)) were manipulated. Riser and downcomer gas holdup were found to increase with the addition of ethanol, leading to a decrease on the relative solids holdup. The presence of ethanol seems to have no influence on the circulation time. On the other hand, mixing time variation depends on the solids loading and airflow rate. Riser and downcomer interstitial liquid velocity are lower for ethanol solution than for water.     

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.     

Flow rate dependent kinetics of urease immobilized onto diverse matrices

Gas holdup and liquid circulation velocity meassurements were made for a range of liquid viscosities in a 22 l external loop airlift column and 250 l pilot-scale concentric cylinder airlift bioreactor. The results showed that for a fixed superficial gas velocity, liquid circulation velocity decreased monotonically with increasing liquid viscosity. The gas holdup for a fixed gas flow rate showed an initial increase with liquid viscosity followed by a decrease when liquid viscosity increased beyond a critical value. These observations could not be described satisfactorily using the available models of gas holdup and liquid circulation.List of Symbols U _sg m/s Superficial gas velocity - U _sl m/s Superficial liquid velocity in the riser Greek Letters Pas Liquid viscosity - _g Gas holdup in the riser     

Concentric-tube airlift bioreactors

Gas holdup investigations were performed in three concentric-tube airlift reactors of different scales of operation (RIMP: 0.070 m³; RIS-1: 2.5 m³; RIS-2: 5.2 m³; nominal volumes). The influences of the top and bottom clearances and the flow resistances at the downcomer entrance were studied using tap water as liquid phase and air as gaseous phase, at atmospheric pressure. It was found that the gas holdup in the individual zone of the reactor: riser, downcomer and gas-separator, as well as that in the overall reactor is affected by the analyzed geometrical parameters in different ways, depending on their effects on liquid circulation velocity. Gas holdup was satisfactorily correlated with Fr, Ga, bottom spatial ratio (B), top spatial ratio (T), gas separation ratio (Y) and downcomer flow resistance ratio (A _d /A _R ). Correlations are presented for gas holdup in riser, downcomer, gas separator and for the total gas holdup in the reactor. All the above stressed the importance of the geometry in dynamic behaviour of airlift reactors.     

Characterization of a pilot plant airlift tower loop reactor: III. Evaluation of local properties of the dispersed gas phase during yeast cultivation and in model media

The local properties of the dispersed gas phase (gasholdup, bubble diamater, and bubble velocity) were measured and evaluated at different positions in the riser and downcomer of a pilot plant reactor and, for comparison, in a laboratory reactor. These were described in Parts I and II of this series of articles during yeast cultivation and with model media. In the riser of the pilot plant reactor, the local gas holdup and bubble velocities varied only slightly in axial direction. The gas holdup increased considerably, while the bubble velocity increased only slightly with aeration rate. The bubble size diminished with increasing distance from the aerator in the riser, since the primary bubble size was larger than the equilibrium bubble size. In the downcomer, the mean bubble size was smaller than in the riser. The mean bubble size varied only slightly, the bubble velocity was accelerated, and the gas holdup decreased from top to bottom in the downcomer. In pilot plant at constant aeration rate, the properties of the dispersed phase were nearly constant during the batch cultivation, i.e., they depended only slightly on the cell concentration. In the laboratory reactor, the mean bubble sizes were much larger than in the pilot plant reactor. In the laboratory reactor, the bubble velocities in the riser and downcomer increased, and the mean gas holdup and bubble diameter in the downcomer remained constant as the aeration rate was increased.     

Oxygen transfer and mixing in mechanically agitated airlift bioreactors

Gas holdup, mixing, liquid circulation and gas–liquid oxygen transfer were characterized in a large (∼1.5 m³) draft-tube airlift bioreactor agitated with Prochem^® hydrofoil impellers placed in the draft-tube. Measurements were made in water and in cellulose fiber slurries that resembled broths of mycelial microfungi. Use of mechanical agitation generally enhanced mixing performance and the oxygen transfer capability relative to when mechanical agitation was not used; however, the oxygen transfer efficiency was reduced by mechanical agitation. The overall volumetric gas–liquid mass transfer coefficient declined with the increasing concentration of the cellulose fiber solids; however, the mixing time in these strongly shear thinning slurries was independent of the solids contents (0–4% w/v). Surface aeration never contributed more than 12% to the total mass transfer in air–water.     

Hydrodynamic performance of a three-phase airlift bioreactor with an enlarged degassing zone

The hydrodynamics of biotechnological processes is complex. So far, few studies were made with bioreactors of the airlift type with an enlarged degassing zone.In this work, the influence of solids loading, solids specific gravity and draught tube dimensions on mixing and circulation times and critical air flow rate for an internal loop airlift bioreactor with an enlarged sedimentation/degassing zone is studied.The results indicate that the critical air flow rate as well as the mixing time increase with an increase in solids loading in the bioreactor. Circulation time presents a maximum for a solids load between 5 and 10% (v/v). It is also shown that small variations in solids specific gravity, for values close to that of the liquid, have a significant influence on the critical air flow rate and on the mixing time.An optimal (minimal) value for the circulation time and for the critical air flow rate was obtained for a riser to down comer diameter ratio of 0.46. The minimum mixing time was obtained for a riser to down comer height ratio of 0.80.This work was supported by J.N.I.C.T. (Junta Nacional de Investigação Cientifica e Tecnológica).     

Two- and three-phase mixing in a concentric draft tube gas-lift fermentor

Two- and three-phase mixing studies were carried out in a 44-L concentric draft tube gas-lift fermentor. It was proposed to use the fermentor for the production of solvents using immobilized bacteria. Bubble size, gas holdup, liquid velocities, circulation, and mixing times were determined for various superficial gas velocities in distilled water, starch, carboxymethyl cellulose, and ethanol solutions. The observed trends for two phase mixing were similar to other studies but the results were found to be more sensitive to liquid properties. This was possibly due to the large value of downcomer to riser area used in this study. Mixing in three phases highlighted the difficulty in predicting the effect of adding solids to the gas-liquid system. Results showed that the gas-lift fermentor was ideally suited to dealing with three phases but more work is necessary before accurate models can be developed to account for the effect of solids.     

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.     

Concentric-tube airlift bioreactors

Liquid circulation velocity was investigated in three concentric-tube airlift reactors of different scales (RIMP, V _L =0.07 m³; RIS-1, V _L =2.5 m³; RIS-2, V _L =5.20 m³). The effects of top and bottom clearance and resistance in flow pathway at downcomer entrance on the riser liquid superficial velocity, the circulation time, the friction coefficient and flow radial profiles of the gas holdup and the liquid superficial velocity in riser, using water-air as a biphasic system, were studied. It was found that the riser liquid superficial velocity is affected by the analyzed geometrical parameters in different ways, depending on their effects on the pressure loss. The riser liquid superficial velocity, the friction coefficient and the parameters of the drift-flux model were satisfactorily correlated with the bottom spatial ratio (B), gas separation ratio (Y) and downcomer flow resistance ratio (A _d /A _D ), resulting empirical models, with correlation coefficients greater than 0.85.     

Digestion of sand-laden manure slurry in an upflow anaerobic solids removal (UASR) digester

Studies on the performance of a laboratory scale upflow anaerobic solids removal (UASR) digester were carried out using sand-laden cow manure slurries having total solids (TS) concentration as 50 and 100 g/l. Hydraulic retention time (HRT) was maintained as 32.4 days, which resulted in the volatile solids (VS) loading rates of 1 and 1.64 g/l d. The UASR system was designed to remove sand from the manure slurry, while anaerobically digesting biodegradable solids inside a single reactor. To enhance the contact of microorganisms and substrate, the liquor from the top of the digester was recirculated through the bed of settled solids at its bottom. Volatile solids reduction through this process was observed to be 62% and 68% in the case of feed slurries having TS concentration as 50 and 100 g/l (referred in the text as 5% and 10% feed slurries), respectively. The methane production rates were observed to be 0.22 and 0.38 l/l d, while methane yield was 0.21 and 0.27 l CH₄/g VS loaded, for 5% and 10% feed slurries, respectively. This indicates that the increase in the VS loading had a positive impact on methane production rate and methane yield. It would be of interest to study the performance of a UASR digester at higher solids loadings and with longer solids retention times. Nonetheless, the presented study showed that sand-laden manure slurries can be successfully digested in a UASR digester producing methane energy equivalent to 4 kW h per m³ of digester volume per day.     

A novel circulating loop bioreactor with cells immobilized in loofa (<Emphasis Type="Italic">Luffa cylindrica</Emphasis>) sponge for the bioconversion of raw cassava starch to ethanol

A circulating loop bioreactor (CLB) with cells immobilized in loofa sponge was constructed for simultaneous aerobic and anaerobic processes. The CLB consists of an aerated riser and a non-aerated downcomer column connected at the top and bottom by cylindrical pipes. Ethanol production from raw cassava starch was investigated in the CLB. Aspergillus awamori IAM 2389 and Saccharomyces cerevisiae IR2 immobilized on loofa sponge were placed, respectively, in the aerated riser column and non-aerated downcomer column. Both alpha-amylase and glucoamylase activities increased as the aeration rate was increased. Ethanol yield and productivity increased with an increase in the aeration rate up to 0.5 vvm, but decreased at higher aeration rates. The CLB was operated at an aeration rate of 0.5 vvm for more than 600 h, resulting in an average ethanol productivity and yield from raw cassava starch of 0.5 g-ethanol l(-1) x h(-1) and 0.45 g ethanol/g starch, respectively. In order to increase ethanol productivity, it was necessary to increase the dissolved oxygen (DO) concentration in the riser column and decrease the DO concentration in the downcomer column. However, increasing the aeration rate resulted in increases in the DO concentration in both the riser and the downcomer columns. At high aeration rate, there was no significant difference in the DO concentration in the riser and downcomer columns. The aeration rate was therefore uncoupled from the liquid circulation by attaching a time-controlled valve in the upper connecting pipe. By optimizing the time and frequency of valve opening, and operation at high aeration rate, it was possible to maintain a very high DO concentration in the riser column and a low DO concentration in the downcomer column. Under these conditions, ethanol productivity increased by more than 100%, to 1.17 g l(-1) x h(-1).     

Hydrodynamic study of an internal airlift reactor for microalgae culture

Internal airlift reactors are closed systems considered today for microalgae cultivation. Several works have studied their hydrodynamics but based on important solid concentrations, not with biomass concentrations usually found in microalgae cultures. In this study, an internal airlift reactor has been built and tested in order to clarify the hydrodynamics of this system, based on microalgae typical concentrations. A model is proposed taking into account the variation of air bubble velocity according to volumetric air flow rate injected into the system. A relationship between riser and downcomer gas holdups is established, which varied slightly with solids concentrations. The repartition of solids along the reactor resulted to be homogenous for the range of concentrations and volumetric air flow rate studied here. Liquid velocities increase with volumetric air flow rate, and they vary slightly when solids are added to the system. Finally, liquid circulation time found in each section of the reactor is in concordance with those employed in microalgae culture.     

Hydrodynamic behaviour of animal cell microcarrier suspensions in split-cylinder airlift bioreactors

Hydrodynamic characteristics of suspensions of microcarriers used for culturing anchorage dependent animal cells are reported in split-cylinder internal-loop airlift bioreactors. Cell culture media are simulated using salt solutions that duplicate the ionic strengths of typical media. Effects of solids loading (0–30 kg·m^–3), microcarrier particle size (150–300×10^–6 m diameter) and density (1030–1050 kg·m^–3) on gas induced circulation of the slurry, mixing time, gas holdup and gas velocity requirements to attain complete suspension of solids are discussed for two reactors with aspect ratios of 7.6 and 14.5, but equal riser-to-downcomer cross-sectional area ratios of 1.0, aerated at low air flow rates (0–8×10^–6 m³·s^–1) through a sintered glass sparger with 110×10^–6 m diameter pores. The study covers the ranges of solids concentrations, types, densities, particle sizes and aeration rates that are of relevance in animal cell culture applications.Airlift bioreactors displayed suitable hydrodynamic characteristics for potentially supporting anchorage dependent cell cultures on microcarriers at carrier loadings similar to those that are currently used in stirred tank bioreactors. The minimum gas flow rates and the induced liquid circulation rates necessary to achieve and maintain suspension of the heaviest and the largest microcarriers were well within practicable limits, limits which have been shown to be withstood by animal cells in non-anchorage dependent suspension culture in airlift bioreactors. No floatation problems were encountered with the carriers, nor was sedimentation a problem so long as the identified minimum suspension criteria were met. Chisti's liquid circulation equation, originally intended for two-phase flow, applied to the three-phase gas-liquid-microcarrier systems.     

Gas holdup, foaming and oxygen transfer in a jet loop bioreactor with artificial foaming media and yeast culture

A concentric draft tube jet loop bioreactor (10.5 m3) was used to study the influence of aerated liquid height (above the draft tube) on the amount of surfactant addition allowable without foaming. Sodium lauryl sulfate (SLS) and defatted soybean flour in tap water were used as model artificial media. The amount of surfactant required to develop foaming and the maximum gas holdup achieved prior to foaming were notably influenced by aerated liquid height. Decreasing the aerated liquid height from 1.50 to 0.05 m increased the amount of SLS allowed without foaming from 2.2 to 12.1 g, the gas holdup in the riser from 0.18 to 0.31 and the gas holdup in the downcomer from 0.12 to 0.25. Similar behavior was observed for soybean flour. Decreasing the aerated liquid height from 1.45 to 0.05 m increased the amount of soybean flour allowed without foaming from 822 to 3200 g, the gas holdup in the riser from 0.17 to 0.26 and the gas holdup in the downcomer from 0.10 to 0.19. Data from a representative continuous yeast culture are reported to show that operation at low aerated liquid heights (0.5 m) can also be used to produce a culture broth with large gas holdup and oxygen transfer but without foaming.     

Residence time distribution of liquid phase in an external-loop airlift bioreactor

The residence time distribution analysis was used to investigated the flow behaviour in an external-loop airlift bioreactor regarded as a single unit and discriminating its different sections. The experimental results were fitted according to plug flow with superimposed axial dispersion and tank-in-series models, which have proved that it is reasonable to assume plug flow with axial dispersion in the overall reactor, in riser and downcomer sections, as well, while the gas separator should be considered as a perfectly mixed zone. Also, the whole reactor could be replaced with 105-30 zones with perfect mixing in series, while its separate zones, that is the riser with 104-27, the downcomer with 115-35 and the gas separator with 25-5 perfectly mixed zones in series, respectively, depending on gas superficial velocity, A_D/A_R ratio and the liquid feed rate.List of Symbols A _D cross sectional area of downcomer (m²) - A _R cross sectional area of riser (m²) - A ₁ A ₂ length of connecting pipes (m) - Bo Bodenstein number (Bo=vL·L/D _ax (-) - C concentration (kg m^–3) - C residence time distribution function - C ₀ coefficientEquation (12) - C _r dimensionless concentration - D _D diameter of downcomer (m) - D _R diameter of riser column (m) - D _ax axial dispersion coefficient (m²s^–1) - H _d height of gas-liquid dispersion (m) - H _L height of clear liquid (m) - i number of complete circulations - L length of path (m) - m order of moments - N _eq number of perfectly mixed zones in series - n _c circulating number - Q _c recirculating liquid flow rate (m³ s^–1) - q _F liquid feed flow rate (m³s^–1) - Q _G gas flow rate (m³s^–1) - Q _T total liquid flow rate (m³s^–1) - r recycle factor - s exponent inEquation (12) regarded as logarithmic decrement of the oscillating part of RTD curve - t time (s) - t _C circulation time (s) - t _s mean residence time (s) - t ₉₉ time necessary to remove 99% of the tracer concentration (s) - V _A volume of connecting pipes (m³) - V _D volume of downcomer (m³) - V _L liquid volume in reactor (m³) - V _R volume of riser (m³) - V _LD linear liquid velocity in downcomer (m s^–1) - V _LR linear liquid velocity in riser (m s^–1) - V _SLD superficial liquid velocity in downcomer (m s^–1) - V _SLR superficial liquid velocity in riser (m s^–1) - x independent variable inEquation (1) - ¯x mean value of x - z axial coordinate - _GR gas holdup in riser - _m(x) central moment of m order - ² variance - dimensionless time     

Saccharomyces cerevisiae fermentations in a pilot scale airlift bioreactor: Comparison of air sparger configurations

Hydrodynamic and oxygen transfer comparisons were made between two ring sparger locations, draft tube and annulus, in a concentric pilot scale airlift reactor with a baker's yeast suspension. Sectional hydrodynamic measurements were made and a mobile DOT probe was used to characterise the oxygen transfer performance through the individual sections of the reactor. The hydrodynamic performance of the reactor was improved by using a draft tube ring sparger rather than the annulus ring sparger. This was due to the influence of the ratio of the cross sectional area of the downcomer and riser (A _D/A_R) in conjunction with the effect of liquid velocity and a parameter,C ₀, describing the distribution of the liquid velocity and gas holdup across the riser on the bubble coalescence rates. The mixing performance of the reactor was dominated by the frequency of the passage of the broth through the end sections of the reactor. An optimum liquid height above the draft tube, for liquid mixing was demonstrated, above which no further improvement in mixing occurred. The liquid velocity and degree of gas entrainment showed little dependency on top section size for both sparger configurations. Extreme dissolved oxygen heterogeneity was demonstrated around the vessel with both sparger configurations and was shown to be detrimental to the oxygen uptake rate of the baker's yeast. Dissolved oxygen tensions below 1% air saturation occurred along the length of the riser and then rose in the downcomer. The greater oxygen transfer rate in the downcomer than in the riser was caused by the combined effects of a larger slip velocity in the downcomer which enhancedk _La and gas residence time, high downcomer gas holdup, and the change in bubble size distribution between the riser and downcomer. The position of greatest oxygen transfer rate in the downcomer was shown to be affected by the reactor from the influence on downcomer liquid linear velocity. UCL is the Biotechnology and Biological Sciences Research Council sponsored Advanced Centre for Biochemical Engineering and the Council's support is greatly acknowledged.     

Effects of geometrical design on hydrodynamic and mass transfer characteristics of a rectangular-column airlift bioreactor

Gas holdup and gas–liquid mass transfer coefficients were measured in a 21-L rectangular-column airlift bioreactor with aspect ratio of 10 and working volumes ranging from 10 to 16 L. The effect of the bottom and top clearances was investigated using water and mineralized CMC solutions and covering a range of effective viscosity from 0.02 to 0.5 Pa s and surface tension from 0.065 to 0.085 N m⁻¹. The gas holdup and mass transfer results were successfully correlated using expressions derived via dimensional analysis. The separator gas holdup was found to be similar to the total gas holdup in the airlift bioreactor. The downcomer gas holdup (ɛ_d) increased two-fold when the bottom clearance (h_b) was increased from 0.014 to 0.094 m while the top clearance (h_t) had no effect. Increasing h_b decreased the mass transfer by 50% compared to 31% when the top clearance (h_t/D_hr) was increased. It was found that the gas–liquid separator diameter ratio (D_hs/D_hc) exerted the maximal influence of over 65% on mass transfer as compared to both clearances.     

Local overall volumetric gas-liquid mass transfer coefficients in gas-liquid-solid reversed flow jet loop bioreactor with a non-Newtonian fluid

The local overall volumetric gas-liquid mass transfer coefficients at the specified point in a gas-liquid-solid three-phase reversed flow jet loop bioreactor (JLB) with a non-Newtonian fluid was experimentally investigated by a transient gassing-in method. The effects of liquid jet flow rate, gas jet flow rate, particle density, particle diameter, solids loading, nozzle diameter and CMC concentration on the local overall volumetric gas-liquid mass transfer coefficient (K(L)a) profiles were discussed. It was observed that local overall K(L)a profiles in the three-phase reversed flow JLB with non-Newtonian fluid increased with the increase of gas jet flow rate, liquid jet flow rate, particle density and particle diameter, but decreased with the increase of the nozzle diameter and CMC concentration. The presence of solids at a low concentration increased the local overall K(L)a profiles, and the optimum of solids loading for a maximum profile of the local overall K(L)a was found to be 0.18x10(-3)m(3) corresponding to a solids volume fraction, varepsilon(S)=2.8%.     

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.