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.     

New predictive correlation for mass transfer coefficient in structured packed extraction columns

Developments in the area of packed columns, particularly structured packed columns, are ongoing, specifically in the area of liquid–liquid extractions in different industries. In the present study, mass transfer coefficients have been obtained experimentally in a structured packed extraction column to develop a new correlation for prediction of continuous phase Sherwood number. The experiments were carried out for toluene/acetic acid/water and n-butyl acetate/acetic acid/water systems with counter current flow in different heights of column. A new dimensionless parameter, d₃₂/h, is introduced in proposed equation. This number considers the effect of column height (h) and mean drop diameter (d₃₂) jointly. The main advantage of this approach is that the principal effect of column height is considered in correlation without which the experimental data could not be fitted with a acceptable accuracy.     

Region-dependent mass transfer behavior in a forced circulation airlift loop reactor

The contribution of local regions to global mass transfer holds the key to optimization and scale-up of a reactor. Extensive study has been conducted to investigate gas-liquid mass transfer occurring in the internal airlift loop reactor, but mostly restricted to global mass transfer performance. A cold model forced circulation internal airlift loop reactor was employed and divided into six regions in which dissolved oxygen concentration in slurry and mass transfer interfacial area were measured respectively. Different models were utilized to calculate volumetric mass transfer coefficient. Contributions of individual region to global mass transfer performance were calculated and compared. It was found that mass transfer coefficient and mass transfer interfacial area of individual region increases with increasing superficial gas velocity and slurry feed flowrate. The feed affected region has the greatest mass transfer coefficient and volumetric mass transfer coefficient, contributing more than 30% to global mass transfer in most operating condition. Mass transfer interfacial area is close in the gas distributor region, feed affected region and the gas-slurry separator region. In the present work, circulating bubbles are rare and contribute negligibly to the global mass transfer. Global volumetric mass transfer coefficient is close to that of the gas-slurry separator region, ranging from 0.02 to 0.1 1/s. Comparison of k_La is made between this work and literatures, suggesting a great improvement of mass transfer due to external liquid circulation.     

Solid effects on hydrodynamics and heat transfer in an external loop airlift reactor

The effect of a solid presence on global hydrodynamic parameters and heat transfer in an external loop airlift reactor has been experimentally investigated. Results obtained in both two- and three-phase flow are presented in this study. Two different external loop airlift reactor sizes have been used and local hydrodynamic characteristics including local gas hold-up and bubble velocity have been obtained in two-phase flow. Optical and ultrasound probes have been used to obtain this information, respectively. It was found that an increase of solid hold-up leads to a decrease of liquid velocity and heat transfer coefficient. Measured in a two- and three-phase reactor using a horizontal-heating probe, a correlation of the average gas hold-up and heat transfer coefficient is proposed. Correlation parameters are identified in homogeneous and heterogeneous flow regimes, which have been derived from the gas slip velocity concept. The experimental liquid velocity and gas hold-up in the riser have been represented in a satisfactory way by a hydrodynamic model, either in the absence or in the presence of solid particles.     

Hydrodynamics and mass transfer study of aliphatic alcohols in airlift reactors

Gas holdup and gas–liquid mass transfer coefficient were considered in an external airlift reactor. Air was sparged through some aliphatic alcohols (methanol, ethanol, n-propanol, and n-butanol) with different concentrations (0–1%, v/v). It was observed that gas holdup and mass transfer coefficient increased with increasing the number of carbons in alcohols. Furthermore, an increment in alcohols concentration increased gas holdup and mass transfer coefficient. The same behavior was observed in external and internal loop airlift reactors although gas holdup and mass transfer coefficient values were less than those of internal airlift reactor. According to the experiments, two correlations for gas holdup and mass transfer were developed.     

并流旋转床压降与传质特性的研究

利用水-空气系统对并流旋转床的气相压降进行了研究,并与逆流旋转床气相压降进行了对比。研究结果表明：并流较逆流旋转床的气相压降低;并流旋转床的气相压降随气体流量的增大而增大,随液体流量的增大而减小,随转速的增大明显降低;而逆流旋转床的气相压降随转速的增大明显升高。利用水吸收SO2的实验对并流旋转床的传质特性进行了研究。研究结果表明：并流旋转床填料层内各点的体积传质系数随着气体流量、液体流量和转速的增大而增大;填料层半径由70mm增大至90mm时,并流旋转床的体积传质系数迅速增大,而后并流旋转床的体积传质系数随半径的增大而减小。对并流和逆流旋转床填料层内体积传质系数进行了对比。结果表明：填料层半径由70mm增大至130mm时,并流旋转床的体积传质系数较逆流时大;当半径大于130mm后,逆流旋转床的体积传质系数大于并流旋转床的体积传质系数,且随半径增大而增大。根据研究结果,提出了降低系统压降的设想,即并流与逆流旋转床串联操作。     

Assessment of mass transfer coefficients in coalescing slug flow in vertical pipes and applications to tubular airlift membrane bioreactors

One of the operational challenges associated with membrane bioreactors (MBRs) is the fouling of the membranes. In tubular side-stream MBRs, fouling reduction can be achieved through controlling the hydrodynamics of the two-phase slug flow near the membrane surface. The two-phase slug flow induces higher shear stresses near the membrane surface, which generate high mass transfer coefficients from the surface to the bulk region. However, measuring the mass transfer coefficient is difficult in complex heterogeneous mixtures like activated sludge and existing techniques (e.g. electrochemical methods) cannot be applied directly. As an alternative, in this work, a multidisciplinary approach was selected, by exploiting dimensionless analysis using the Sherwood number. Mass transfer coefficients were measured at various superficial velocities of gas and liquid flow in a tubular system. Due to the variability of the mass transfer coefficient obtained for each experimental condition, the results were compiled into, mass transfer coefficient histograms (MTH) for analysis. A bimodal MTH was observed, with one peak corresponding to the mass transfer induced by the liquid flow, and the other peak induced by the gas flow. It was noted that coalescence of bubbles affects the MTH. Coalescence increased the “width” of the peaks (i.e. the estimate of the variability of the mass transfer coefficient) and the height of the peak (i.e. amount of time that a mass transfer coefficient of a given value is maintained). A semi-empirical relationship based on the Lévêque relationship for the Sherwood number (mass transfer coefficient) was formulated for the laminar regime. A test case comparison between water and activated sludge was performed based on full-scale airlift MBR operational conditions. It was found that the Sherwood number in the non-Newtonian case is 8% higher than that in the Newtonian case.     

Hydrodynamic and mass transfer characteristics of external-loop airlift reactors without an extension tube above the downcomer

The effects of the horizontal connection length (0.1≤L_c≤0.5 m), the downcomer-to-riser cross-sectional area ratio (0.11≤A_d/A_r≤0.53) and the superficial gas velocity (0.02≤U_G≤0.18 ms^-1) on gas holdups in the riser and downcomer, the circulation liquid velocity, the mixing time, and the overall volumetric mass transfer coefficient were determined in external-loop airlift reactors without an extension tube above the downcomer [configuration (a)]. For otherwise fixed conditions, the absence of the extension tube strongly affected the hydrodynamic and mass transfer characteristics of external-loop airlift reactors. In contrast with the external-loop airlift reactor with the extension tube [configuration (b)], a large air pocket formed in the top horizontal connection and the surface aeration took place in the external-loop airlift reactor without the extension tube [configuration (a)]. As a result, the riser circulation liquid velocity in configuration (a) was slower than that in configuration (b). The riser and downcomer gas holdups, the mixing time and the overall volumetric mass transfer coefficient in configuration (a) were larger than those in configuration (b), respectively.     

Heat transfer of gas flow through a packed bed

This paper reports an experimental study of both the transient and steady-state heat transfer behaviour of a gas flowing through a packed bed under the constant wall temperature conditions. Effective thermal conductivities and convective heat transfer coefficient are derived based on the steady-state measurements and the two-dimensional axial dispersion plug flow (2DADPF) model. The results reveal a large temperature drop at the wall region and the temperature drop depends on the axial distance from the inlet. The 2DADPF model predicts the axial temperature distribution fairly well, but the prediction is poor for the radial temperature distribution. Length-dependent behaviour of the effective heat transfer parameters and non-uniform flow behaviour are proposed to be responsible. A comparison with previously published correlations and data in the literature shows that the relationships proposed by Bunnell et al. and Demirel et al. agree well with the measured effective radial thermal conductivity, whereas the wall-fluid heat transfer coefficient is better represented by the Li-Finlayson correlation.     

Dynamics of droplets and mass transfer in a rotating packed bed

To do further research on the mass‐transfer mechanism in rotating packed bed (RPB), dynamics of droplets in a RPB are studied by an analytical approach combined with a series of laboratory measurements. Based on the results of the fluid dynamics, mathematical models of mass‐transfer coefficient and mass‐transfer process in RPB are proposed, respectively. Mass‐transfer experiments in RPB are also carried out using ethanol–water solution. By comparison, the results of simulation agree well with that of the experiment, which demonstrate that both hydrodynamic model and mass‐transfer models can better describe the real conditions of RPB. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2705–2723, 2014     

Continuous bioremediation of phenol‐polluted air in an external loop airlift bioreactor with a packed bed

An external loop airlift bioreactor with a small amount (99% porosity) of stainless steel mesh packing inserted in the riser section was used for bioremediation of a phenol‐polluted air stream. The packing enhanced volatile organic chemical and oxygen mass transfer rates and provided a large surface area for cell immobilization. Using a pure strain of Pseudomonas putida, fed‐batch and continuous runs at three different dilution rates were completed with phenol in the polluted air as the only source of growth substrate. 100% phenol removal was achieved at phenol loading rates up to 33 120 mg h^?1 m^?3 using only one‐third of the column, superior to any previously reported biodegradation rates of phenol‐polluted air with 100% efficiency. A mathematical model has been developed and is shown to accurately predict the transient and steady‐state data. Copyright © 2006 Society of Chemical Industry     

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

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

Mass transfer and mixing characteristics in an airlift‐driven fibrous‐bed bioreactor

An internal loop airlift‐driven fibrous bed bioreactor (ILALFBB) was designed and developed with a high degree of flexibility to handle genetically engineered and fragile shear‐sensitive cells. The mixing and oxygen mass transfer characteristics have been investigated. A cotton fibre was set in the downcomer of the ILALB to represent the fibrous packed bed and the outcome results were compared with those of the polyurethane foam (PUF) packed and unpacked ILALB systems. The effects of fibre, packing height, bed top and bottom clearances, spacing between adjacent fibre surfaces, and superficial gas velocity were investigated. The liquid phase mixing output variables included the liquid circulation velocity (U_Lc), circulation time (t_Lc), mixing time (t_Lm), Bodenstein number (Bo_L), and axial dispersion coefficient (E_L), whereas the mass transfer out variable was the K_La. Bo_L and E_L in the riser and downcomer regions of all packed systems increased with increasing in packing height, packing top clearance, and superficial gas velocity, except the overall Bo_L was independent of gas velocity at low gas velocities. The Bo_L was found highest in the riser of the large cotton and small PUF packed system with large spacing and the E_L in the downcomer of PUF packed systems with smaller spacing between fibre surfaces. Increased amounts of packing in the ILALB, whether in the form of cotton or PUF decreased the U_Lc in the bioreactor because of the increased frictional resistance and tortuosity. The reduction in U_Lc was significant for large packing with smaller spacing between fibre surfaces and increased bottom clearances of the cotton packed system. High circulation times (t_Lc) and shorter mixing times (t_Lm) were achieved using small PUF packing with large top clearance. Relatively high K_La values were obtained using large packing with large top clearances and spacing between fibre surfaces. The boost in K_La was associated with increased gas holdup and/or interfacial area, due to bubble breakage by the shearing action of the fibrous‐bed. Empirical correlation proposed for E_L, Bo_L, and K_La gave a good fit of the experimental data.     

Mass transfer characteristics in a rotating packed bed with split packing

The rotating packed bed (RPB) with split packing is a novel gas–liquid contactor, which intensifies the mass transfer processes controlled by gas-side resistance. To assess its efficacy, the mass transfer characteristics with adjacent rings in counter-rotation and co-rotation modes in a split packing RPB were studied experimentally. The physical absorption system NH3–H2O was used for characterizing the gas volumetric mass transfer coeffi-cient (kyae) and the effective interfacial area (ae) was determined by chemical absorption in the CO2–NaOH sys-tem. The variation in kyae and ae with the operating conditions is also investigated. The experimental results indicated that kyae and ae for counter-rotation of the adjacent packing rings in the split packing RPB were higher than those for co-rotation, and both counter-rotation and co-rotation of the split packing RPB were superior over conventional RPBs under the similar operating conditions.     

Hydrodynamics and mass transfer characteristics in an internal loop airlift reactor with sieve plates

Effects of the sieve plate on hydrodynamics and mass transfer in an annulus sparged airlift reactor (0.08 m³, 1.3 m tall, and 0.284 m in diameter) were investigated. It is found that the sieve plate can significantly enhance gas holdup and volumetric mass transfer coefficient. The sieve pore plays an important role in breaking up bubbles. With a given free area ratio, the sieve plate with a larger sieve pore diameter is more efficient in increasing the volumetric mass transfer coefficient. Four different free area ratios between 37% and 73% are tested, and then an optimal free area ratio is determined. The effect of the sieve plate is found to be related to sparger types. The sieve plate leads to a larger increase of volumetric mass transfer coefficient with the O-ring distributor as compared to the 4-orifice nozzle. Empirical correlations and a hydrodynamic model are proposed to predict gas holdup, volumetric mass transfer coefficient and liquid velocity in airlift reactors with sieve plates.     

Effects of water on steam rectification in a packed column

The effects of water on steam rectification, i.e., multi-stage saturated steam distillation, were investigated in a packed column. N-octane-p-xylene and 1,3,5-trimethylbenzene-1,2,4-trimethylbenzene were used as test systems. Both binary systems are nearly ideal systems and insoluble in water, thus the effects of water in steam rectification can be clearly and definitely revealed. Such unpolar organic liquid is named as “oil”. The water/oil at column top can be separated and refluxed at different ratio. Compared with conventional rectification, there are some peculiar phenomena in steam rectification. Water greatly enhances the flooding vapor velocity of the rectification, in addition, water plays a predominant role in pressure drop of the packed bed near flooding point. It is clear that liquid water in the packed bed can promote mass transfer of steam rectification, especially for materials with higher viscosity. In a word, steam rectification can be operated at low temperature with good mass transfer.     

The heat transfer coefficient between a particle and a bed (packed or fluidised) of much larger particles

The heat transfer coefficient has been measured for a heated phosphor-bronze sphere (diam. 2.0, 3.0 or 5.56 mm) added to a bed of larger particles, through which air at room temperature was passed. The bronze heat transfer sphere was attached to a very thin, flexible thermocouple and was heated in a flame to before being immersed in the bed. The cooling of the bronze sphere enabled the heat transfer coefficient, h, to be measured for a variety of U/U_mf, as well as diameters of both the particles in the bed and the heat transfer sphere. It was found that before the onset of fluidisation, h rose with U, but h reached a constant value for U?U_mf. These measurements indicate that in this situation (of a relatively small particle in a bed of larger particles) all the heat transfer is between the hot bronze sphere and the gas flowing over it. Consequently, a Nusselt number, based on the thermal conductivity of the gas, is easy to define and for U?U_mf (i.e. a packed bed), Nu is given by

     

Hydrodynamics and mass transfer in a novel multi-airlifting membrane bioreactor

A novel multiple-airlifting membrane bioreactor is built with four sintered stainless steel tubular filters as the risers and downcomers. This work investigates the hydrodynamics including gas holdup, liquid velocity, liquid circulation and mixing times by aerating different number of risers (one to three) at superficial gas velocities of 0.02-0.07 m/s The mass transfer phenomena, including oxygen mass transfer (k_La) and effective molecular diffusivity of lactic and acetic acids through the walls of tubular filters, are also investigated. It is found that gas holdup in individual risers increases linearly with the superficial gas velocity, and performs independently under multiple-airlifting conditions. The vessel-based gas holdup and liquid velocity in downcomer(s) increase with aeration rate of individual risers as well as the number of risers. The liquid velocity in downcomers reaches an upper limit (about 0.6 m/s), because of flow resistance or energy loss of liquid circulation. The oxygen mass transfer coefficient (k_La) is primarily affected by gas holdup and the number of risers, and to some extent influenced by liquid velocity. The novel airlifter configuration results in good liquid mixing in the bioreactor that quickly reaches new steady state in response to a sudden pH change from acid addition.     

Effect of contaminants on mass transfer coefficients in bubble column and airlift contactors

In this work, the effects of surface-active contaminants on mass transfer coefficients k_La and k_L were studied in two different bubble contactors. The oxygen transfer coefficient, k_L, was obtained from the volumetric oxygen transfer coefficient, k_La, since the specific interfacial area, a, could be determined from the fractional gas holdup, ε, and the average bubble diameter, d₃₂. Water at different heights and antifoam solutions of 0.5- were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k_La and k_L values over a span of almost 400%. A theoretical interpretation is proposed based on modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant cap model. Model results match experimental k_L data within ±30%.     

Numerical determination of fluid-to-particle mass and heat transfer coefficients in packed bed reactors

A method based on particle-resolved CFD is built and validated, to calculate the fluid-to-particle mass and heat transfer coefficients in packed beds of spheres with different tube-to-particle diameter ratios (N) and of various particle shapes with N = 5.23. This method is characterized by considering axial dispersion. The mass and heat transfer coefficients increase by 5%–57% and 9%–63% after considering axial dispersion, indicating axial dispersion should be included in the method. The mass and heat transfer coefficients are reduced as N decreases. The catalyst particles without inner holes show higher mass and heat transfer coefficients than the ones with inner holes, because of unfavorable fluid flow in inner holes. The bed of trilobes has the highest mass and heat transfer coefficients, being 85% and 95% higher than the one of spheres. This work provides a versatile method and some useful guidance for the design of packed bed reactors.