A study of the mechanism of gas absorption into oil-water emulsions

The mechanism of oxygen and argon transfer into aqueous emulsions of n-alkans and of oleic acid was studied both theoretically and experimentally. An apparatus allowing to measure the coefficient of mass transfer from individual bubbles into the turbulent medium of either a single- or multi-phase liquid has been designed and constructed. The effect of the oil phase on the bubble-to-emulsion mass transfer coefficient was investigated. In o/w type emulsions (oil as the dispersed and water as the continuous phase) of n-alkans (the system with negative spreading coefficient) the mass transfer coefficient k_L^w is not affected by the content of the oil phase, and is equal to the coefficient of mass transfer into a pure aqueous phase. In the w/o type emulsion the k_L^w value increases proportionately to the volume fraction of n-alkans. In the oleic acid-in-water emulsion (the system with positive spreading coefficient) k_L^w initially decreases and then increases proportionately to the oil fraction. The initial decrease of k_L^w is attributed to surface activity effects of oleic acid. The data suggest that the mechanism of gas transfer to the emulsions is as follows: No direct contact between the oil and the gas phase exists in o/w type emulsions with negative spreading coefficient, and the transfer path is gas-water-oil. In w/o type emulsions (both with negative and positive spreading coefficient), however, there is a direct contact between the gas and both the continuous oil phase and the dispersed aqueous phase; there is a parallel transfer of gas to both the phases.     

Oxygen mass transfer and hydrodynamics in a multi-phase airlift bioscrubber system

The addition of select polymer beads to stirred tank bioscrubber systems has been shown to greatly enhance the removal and treatment of toxic VOCs via the capture and sequestration of poorly soluble compounds such as benzene, and the release of these materials, based on equilibrium partitioning, to microorganisms in the aqueous phase. In this study, oxygen volumetric mass transfer coefficients were determined for an 11 L airlift vessel containing tap water alone, tap water with Nylon 6,6 polymer beads (10% v/v), and tap water with silicone rubber beads (10% v/v), over various inlet gas flow rates, with the aim of initially characterizing a low-energy pneumatically agitated reactor (concentric tube airlift). In addition, oxygen transfer rates into the airlift with and without polymers with high oxygen affinity were determined. To further characterize this reactor system, a residence time distribution analysis was completed to determine hydrodynamic parameters including the Peclet number (Pe), circulation time (t_c) and mixing time (t_m) over various gas flow rates for the airlift containing tap water with and without silicone rubber. It was found that the addition of silicone rubber beads, which has a high affinity for oxygen, reduced the measured volumetric mass transfer coefficient relative to a system without polymers due to oxygen sorption during the dynamic period of testing, but increased the overall amount of oxygen that was transferred to the system during the dynamic period. The addition of Nylon 6,6, which has very low oxygen uptake, allowed for estimation of the physical effect of solids addition on gas-liquid mass transfer and it was found that there was no effect on the measured volumetric mass transfer coefficient relative to a system without polymers. However, hydrodynamic parameters revealed that the addition of silicone rubber into an airlift vessel improves liquid phase mixing. This investigation has defined key operational features of a low-energy three-phase airlift bioscrubber system for the treatment of toxic VOC substrates.     

Mass transfer performance of a rotating packed bed equipped with blade packings in removing methanol and 1-butanol from gaseous streams

The removal of methanol and 1-butanol from gaseous streams by absorption with water was investigated in the RPB equipped with blade packings. The overall volumetric gas-phase mass transfer coefficient (K_Ga) for methanol and 1-butanol absorption was observed to increase with the rotational speed, the gas flow rate, and the liquid flow rate. Also, the local volumetric gas-phase mass transfer coefficient (k_Ga) was estimated, and then the portion of the total resistance to mass transfer in gas phase was determined. The result indicated that more than 90% of the total resistance to mass transfer in methanol and 1-butanol absorption was found to be due to the gas phase. Comparison with the conventional packed tower demonstrated that mass transfer efficiency in the RPB equipped with blade packing was higher than that in the conventional packed tower. Consequently, the RPB equipped with blade packings would be an excellent absorber for the removal of alkanols from the exhausted gases.     

Methane biodegradation in a two‐phase partition internal loop airlift reactor with gas recirculation

BACKGROUND: The potential of organic liquid solvents and solid polymers to enhance CH₄ mass transfer was studied in a two‐phase partition internal loop airlift reactor operated with gas recirculation under biotic and abiotic conditions. A preliminary screening of the most common liquid solvents (silicone oil 20 cSt, silicone oil 200 cSt and 2,2,4,4,6,8,8‐heptametilnonane) and solid polymers (Kraton® G6157, Desmopan® DP9370A and Elvax® 880) resulted in the selection of silicone oil 200 cSt (S200) and Desmopan DP9370A (D9370) for further investigation based on their high affinity for CH₄, biocompatibility and nonbiodegradability. RESULTS: Under abiotic conditions, the increase in gas recirculation from 0 to 1 vvm in the absence of a transfer vector increased the overall mass transfer coefficient for oxygen (k_La) by 195%. The presence of S200 and D9370 at 10% (v/v) under operation at 1 vvm of gas recirculation rate mediated an increase in k_La of 100% and 136%, respectively. Likewise, the increase in gas recirculation from 0 to 1 vvm in the absence of a transfer vector and in the presence of S200 during the continuous biodegradation of methane at 3% (v/v) and 7.3 min empty bed residence time resulted in increases in CH₄ removal and CO₂ production rates of 47% and 36%, respectively. Nevertheless, no significant enhancement in CH₄ removal due to the presence of 10% of Desmopan or silicone oil was recorded under operation at 1 vvm. CONCLUSIONS: These results suggest that microbial activity rather than mass transport could be the limiting step in biological CH₄ abatement in this system, contrary to that observed in previous studies with stirred tank reactors, where the organic phase addition increased methane biodegradation. Copyright © 2010 Society of Chemical Industry     

Oxygen mass transfer coefficient in bubble column slurry reactor with ultrafine suspended particles and neural network prediction

The gas–liquid volumetric mass transfer coefficient was determined by the dynamic oxygen absorption technique using a polarographic dissolved oxygen probe and the gas–liquid interfacial area was measured using dual‐tip conductivity probes in a bubble column slurry reactor at ambient temperature and normal pressure. The solid particles used were ultrafine hollow glass microspheres with a mean diameter of 8.624 µm. The effects of various axial locations (height–diameter ratio = 1–12), superficial gas velocity (u_G = 0.011–0.085 m/s) and solid concentration (ε_S = 0–30 wt.%) on the gas–liquid volumetric mass transfer coefficient k_La_L and liquid‐side mass transfer coefficient k_L were discussed in detail in the range of operating variables investigated. Empirical correlations by dimensional analysis were obtained and feed‐forward back propagation neural network models were employed to predict the gas–liquid volumetric mass transfer coefficient and liquid‐side mass transfer coefficient for an air–water–hollow glass microspheres system in a commercial‐scale bubble column slurry reactor. © 2012 Canadian Society for Chemical Engineering     

Model development,parameter identification,and simulation of SCP production processes in air lift tower reactors with external loop—II: Extended culture modelling and quasi steady state parameter identification

Yeast was cultivated in extended culture in a bench-scale 275 cm high air lift tower reactor 15 cm dia. with an external loop. Longitudinal dissolved oxygen concentration profiles, substrate and cell mass concentrations in the medium, O₂ and CO₂ concentrations in the gas phase, as well as gas flow rates and liquid recirculation rates were measured. A distributed parameter model was used to describe the cultivation process variation along the column, cell mass, substrate and oxygen balances in the medium, O₂ and CO₂ balances in the gas phase, variation of the volumetric mass transfer coefficient along the column due to bubble coalescence, as well as double substrate Monod kinetics. Based on simulation runs it was assumed that under non limited and oxygen transfer limited growth conditions, the cell mass and substrate concentrations are uniform in the reactor. The simulation was carried out by a hybrid computer. The unknown model parameters (volumetric mass transfer coefficient at the gas entrance, k_La^E, and coalescence factor K_ST) and two kinetic parameter R_Omax and K_O were identified by means of experimental results with quasi steady state simulation methods.     

Evaluation of a rotating packed bed equipped with blade packings for methanol and 1-butanol removal

Absorption removal of methanol and 1-butanol from gaseous streams with water was investigated in the RPB equipped with blade packings. The removal efficiency (E) of methanol and 1-butanol was found to increase with the RPB speed and the liquid flow rate but decrease with the gas flow rate. Also, the overall volumetric gas-side mass transfer coefficient (K_Ga) for methanol and 1-butanol absorption was observed to increase with the RPB speed, the gas flow rate, and the liquid flow rate. According to the obtained dependence of K_Ga on the gas and liquid flow rates, the mass transfer in methanol and 1-butanol absorption was observed to be controlled primarily by the gas-side mass transfer. Furthermore, the height of a transfer unit (HTU) for methanol and 1-butanol absorption decreased with the RPB speed and the liquid flow rate but increased with the gas flow rate. The obtained results demonstrated that mass transfer efficiency of the RPB equipped with blade packing was comparable to that of a hollow fiber absorber. Consequently, the RPB equipped with blade packings has a great potential in the removal of alkanols from the exhausted gases.     

Gas–liquid mass transfer in oil–water emulsions with an airlift bio-reactor

Biodesulfurization reaction must be performed in oil–water emulsions with an aerobic biocatalyst which demands oxygen. Different reactor configurations can be used for this purpose, but the bubble column bio-reactors with internal recirculation loop are usually not used. In the present work, the absorption of oxygen in water–dodecane emulsions was studied in a bubble column bio-reactor with internal recirculation loop, in operative conditions normally used for biodesulfurization. The K_La for oxygen was determined for several organic fractions from 0 to 100%, as well as at different gas flow rates. Estimation of K_La was done according to a fluid dynamic model based on an energy balance which takes into account the energy dissipated at the interfaces and on a mass transfer model based on the fluid dynamic model, the Higbie's penetration theory and Kolmogoroff's theory of isotropic turbulence. Experimental data of mass transfer coefficient were simulated with satisfactory accuracy, and differences were less than 20% for most cases. Mayor deviations were obtained for emulsions with 30 and 70% dodecane fraction. To obtain good agreement, assumptions of higher bubble diameter and slip velocity were done, evincing the effect of surface tension and liquid viscosity on the mass transfer coefficient.     

Nonsteady-state simulation of extended cultures in tower loop reactors

Hansenula polymorpha was cultivated in extended culture in an air lift tower loop reactor with the substrates enthanol and glucose, respectively. The spacial and time variations of process variables were simulated by a hybrid computer by means of a distributed parameter (dispersion) model, taking into account the spacial variations of pressure, volumetric mass transfer coefficient, dissolved oxygen concentration, oxygen gas phase mole fraction and gas velocity. The volumetric mass transfer coefficient at the gas entrance, k_La^E, its variation near the gas distributor, described by a coalescence factor, K_ST, and the oxygen saturation constant, K_O, were identified by quasi-steady-state methods.The kinetic parameters, maximum specific growth rate, μ_max, and oxygen yield coeflicient, Y_X/O, were identified during nonstationary simulation. The distributed parameter cultivation processes were simulated in a z (dimensionless longitudinal distance)—t (cultivation time)- The agreement between calculated and measured process variables is excellent.     

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.     

Carbon Dioxide Absorption into Stirred Emulsions of n‐Alkanes

Volumetric mass transfer coefficients k_La for CO₂ absorption into n‐alkane/water emulsions were determined at oil volume fractions of 0–100 % in a stirred tank at a stirring speed of 1000 min^?1. The oil was n‐heptane, n‐hexadecane, or n‐dodecane. The decrease of k_La with increasing volume fraction of dispersed oil can be uniformly correlated to the emulsion viscosity with the power of ?0.72. Only the addition of n‐heptane caused a strong increase of the mass transfer coefficient. Upon addition of the surfactant sodium dodecyl sulfate to n‐heptane emulsions, k_La decreased as for the other oils. The increase can therefore be attributed to the spreading of n‐heptane on the bubble surface enabling gas‐oil contact, whereas spreading is inhibited by the ionic surfactant.     

Mass Transfer Studies in Stirred AirLift Reactor

In the present work, water and three phase compositions of Solka-Floc, a cellulose fiber for simulating the biomass in bacteria, yeast, and fungal fermentation were studied in a 1.4?m³ stirred airlift reactor. The fractional dispersed phase holdup and the overall volumetric mass transfer coefficients were measured. The dispersed phase riser gas holdup and overall volumetric mass transfer coefficients both increased with increasing riser superficial dispersed phase velocity (0.02–0.1?ms^?1) and agitator speed in the range of 0–5?rs^?1. An increase in the Solka-Floc concentration (1–3% w/v) was found to reduce ?_GR and K _L a _L . Empirical correlations have been developed for fractional dispersed phase gas holdup and overall volumetric mass transfer coefficients.     

Mass transfer and hydrodynamic characteristics of a high aspect ratio self-ingesting reactor for gas-liquid operations

The mass transfer performance of a gas-liquid self-ingesting stirred reactor is reported both for coalescing and non-coalescing systems. The vessel features are a high aspect ratio and a rather narrow multiple-impeller draft tube, through which the gas phase is ingested and led down to the vessel bottom, where it is finely dispersed into the liquid rising in the annular portion of the vessel. Comparison is made between k_La values determined by several variants of the dynamic method, among which pure oxygen absorption in a previously de-gassed liquid phase. Results show that the gas-liquid mass transfer coefficient values obtained with the last approach are remarkably larger than those measured with all other techniques in which nitrogen is initially dissolved in the liquid phase. Possible reasons behind this discrepancy are discussed.The gas-liquid mass transfer performance of the investigated gas inducing contactor is finally compared with literature data on other self-inducing/ingesting devices. Comparison results encourage further development of the investigated apparatus.     

Numerical simulation of the mass transfer process of CO2 absorption by different solutions in a microchannel

The flow and mass transfer characteristics of CO₂ absorption in different liquid phases in a microchannel were studied by numerical simulation. The mixture gas phase contained 5 vol% CO₂ and 95 vol% N₂ , and the different liquid phases were water, ethanol solution, 0.2 M monoethanolamine solution, and 0.2 M NaOH solution, respectively. Based on the permeation theory, the distribution of velocity and concentration in the slug flow was obtained by local simulation of flow and mass transfer coupling and was described in depth. The influence of contact time and bubble velocity on the mass transfer of the whole bubble was highlighted. The volumetric mass transfer coefficient on the bubble cap and liquid film, CO₂ absorption rate, and enhancement factor were calculated and analyzed. The results showed that the volumetric mass transfer coefficients of chemical absorption were ~3 to 10 times that of physical absorption and the CO₂ was absorbed more completely in chemical absorption. The new empirical correlations for predicting the mass transfer coefficient of the liquid phase were proposed respectively in physical absorption and chemical absorption, which were compared with the empirical formulas in the literature. The volumetric mass transfer coefficients obtained by predictive correlations are in good agreement with those obtained by simulation in this paper. This work made a basic prediction for CO₂ absorption in microchannel and provides a foundation for later experimental research.     

Effects of emulsification variables on fuel properties of two- and three-phase biodiesel emulsions

Biodiesel has attractive fuel properties such as excellent biodegradability and lubricity, almost no emissions of sulfur oxides, PAH and n-PAH, reduced CO₂, PM and CO emission, superior combustion efficiency, etc. However, burning of biodiesel generally produces higher levels of NO_x emissions, primarily due to its high oxygen content. In this study, the emulsification technology has been considered to reduce the NO_x emission level of fossil fuel. Biodiesel, produced by means of transesterification reaction accompanied with a peroxidation process, was emulsified to form two-phase W/O and three-phase O/W/O emulsions. The effects of the emulsification variables such as hydrophilic lipophilic balance (HLB), and water content on the fuel properties and emulsion characteristics of W/O and O/W/O emulsions were investigated in this study. The experimental results show that the surfactant mixture with HLB = 13 produced the highest emulsification stability while HLB = 6 produced the lowest emulsification stability and the most significant extent of water–oil separation among the various HLB values for O/W/O biodiesel emulsion. The kinematic viscosity, specific gravity and carbon residual of the biodiesel emulsions were larger than those of the neat biodiesel. In addition, the W/O biodiesel emulsion was found to have a smaller mean droplet size, lower volumetric fraction of the dispersed phase than the O/W/O biodiesel emulsion, and the highest heating value among the test fuels, if the water content is deducted from the calculation of the heating value.     

Interfacial area and volumetric mass transfer coefficient in a bubble reactor at elevated pressures

The present study deals with the pressure effects on mass transfer parameters within a bubble reactor operating at pressures up to . The gas-liquid systems are N₂/CO₂-aqueous solution of Na₂CO₃-NaHCO₃ and N₂/CO₂-aqueous solution of NaOH. A sintered powder plate is used as a gas distributor. Three parameters characterizing the mass transfer are identified and investigated with respect to pressure: the gas-liquid interfacial area a, the volumetric liquid side mass transfer coefficient k_La and the volumetric gas side mass transfer coefficient k_Ga. The gas-liquid absorption with chemical reaction is used and the mass transfer parameters are determined by using the model reaction between CO₂ and the aqueous solutions of Na₂CO₃-NaHCO₃ and NaOH. For a given gas mass flow rate, the interfacial area as well as the volumetric liquid mass transfer coefficient decrease with increasing operating pressure. However, for a given pressure, a and k_La increase with increasing gas mass flow rates. The mass transfer coefficient k_L is independent of pressure. Furthermore, the pressure increase results in a decrease of k_G and k_Ga for a given gas mass flow rate. The values of the interfacial area, which are obtained from both chemical systems are found to be different. These discrepancies are attributed to the choice of the liquid system in the absorption reaction model.     

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.     

Oxygen Transfer in a Baffled Rectangular Air‐Lift Loop Reactor

The objective of this study is to characterize mass transfer in a rectangular air‐lift loop reactor in two‐phase flow. In a previous work, it has been shown that the reactor presents a complex gas flow pattern. Therefore, first, the global mass transfer volumetric coefficient k_La was measured in two‐phase flow, by three methods (two based on the liquid phase mass balance, one based on the gas phase mass balance). Then, second, a localized analysis was implemented in order to obtain more information about the phenomena governing the gas phase flow.     

Spreading of emulsions on a solid substrate

The wettability of emulsions is a prominent factor with a broad impact on an extensive variety of industrial applications ranging from the petroleum to the cosmetic industries. Surprisingly, there is no comprehensive study of emulsion spreading to date. In this work, the spreading of water/silicone oil emulsions on glass substrates was investigated. The emulsions were prepared with varying volume fractions of water dispersed in silicone oil, with addition of small amounts of surfactant to stabilize the emulsion structure. The time-dependent variation of dynamic contact angle, base diameter, and the spreading rate of the emulsion droplets were studied. The effect of water/silicone oil weight percentage as well as the droplet size and dispersed phase bubble size were also investigated. The weight percentage of water/silicone oil emulsion and droplet size did not have a significant impact on the spreading dynamics; however, the dispersed phase bubble size affected the spreading dynamics substantially. The coarsening of the dispersed phase bubbles was the key factor in the distinct spreading behavior of emulsions compared to pure liquids.     

Mass transfer behaviour of gas-liquid jet loop reactors

A special type of jet loop reactor (JLR), designed for continuous operation and short residence times was investigated with regard to its mass transfer behaviour, described by the volumetric mass transfer coefficient k_La. The jet stream and superficial gas velocities are varied in two JLRs of different sizes, equipped with different nozzles. Fully desalinated water, 0.08 molar NaCI solution and solutions of different concentration of carboxymethyl cellulose (CMC) are used as the liquid phase. A steady-state physical method is employed to determine k_La: air oxygen is purged from the liquid phase by gaseous nitrogen. The measurements show that the reactor is characterized by high power density and high mass transfer performance. No limit of mass transfer capacity was observed in the chosen ranges of volumetric gas and liquid flow rates, i.e. at a given jet stream velocity, the relationship between k_La and the superficial gas velocity is nearly linear. The investigations show that the mass transfer contributed by the jet stream largely depends on liquid phase composition.