Liquid Cross-flow Velocity Research Articles

Evaluation of Gas-to-Liquid Transfer with Ceramic Membrane Sparger for H2 and CO2 Fermentation.

Hydrogen and carbon dioxide fermentation to methane, called bio-methanation, is a promising way to provide renewable and easy-to-store energy. The main challenge of bio-methanation is the low gas-to-liquid transfer of hydrogen. Gas injection through a porous membrane can be used to obtain microbubbles and high gas-to-liquid transfer. However, the understanding of bubble formation using a membrane in the fermentation broth is still missing. This study focused on the impact of liquid pressure and flow rate in the membrane, gas flow rate, membrane hydrophobicity, surface, and pore size on the overall gas-to-liquid mass transfer coefficient (KLa) for hydrogen with gas injection through a porous membrane in real fermentation conditions. It has been shown that KLa increased by 13% with an increase in liquid pressure from 0.5 bar to 1.5 bar. The use of a hydrophilic membrane increased the KLa by 17% compared to the hydrophobic membrane. The membrane with a pore size of 0.1 µm produced a higher KLa value compared to 50 and 300 kDa. The liquid crossflow velocity did not impact the KLa in the studied range.

Mass transfer and residence time distribution in an electrochemical cell with an air-diffusion electrode: Effect of air pressure and mesh promoters

Electrochemical advanced oxidation processes (EAOPs) have shown excellent capabilities to the abatement of recalcitrant organic pollutants. The Fenton-based electrochemical systems have shown great performance for in-situ generation of H2O2, allowing an efficient ●OH production for the mineralization of organic pollutants. These systems have been widely applied at bench scale; however, studies to scale-up to a higher technology readiness level (TRL) are lacking. One of the main scale-up challenges of the Fenton-based systems is the implementation of air diffusion electrodes (ADE) in flow-by electrochemical cells. The ADE adds additional complexity with respect to mass transfer effects due to hydraulic reactor design and ADE gas pressure control. Therefore, this work experimentally investigated residence time distribution and platinum-sheet electrode mass transfer effects due to (a) the liquid cross-flow velocity through the electrochemical cell, (b) the gas pressure of the air-diffusion electrode (ADE), and (c) the presence of mesh sheet mass transfer promoters between the electrodes. Analysis of experimental results revealed a synergistic improvement of mass transfer with the ADE gas flow and the presence of mesh promoters. Engineers could exploit this synergistic effect to design electrochemical cells with significantly lower capital cost.

Mathematical modeling of CO2 separation using different diameter hollow fiber membranes

This research presents a 2D mass-transfer simulation model using computational fluid dynamics (CFD) for separation of CO2 from a binary gas mixture of CO2/CH4 by means of polytetrafluoroethylene (PTFE) hollow fiber membrane contactor (HFMC). Governing equations with their corresponding boundary conditions are solved using COMSOL Multiphysics and the results are validated against reported experimental data. Convection and diffusion flux vectors and concentration gradient of CO2 species in the radial and axial directions of the HFMC are investigated. This study provides an opportunity to investigate the effects of gas and liquid cross flow velocities on the overall performance of membrane contacting system. The results demonstrate that increasing the liquid phase velocity improves the CO2 absorption performance of the membrane system, while increasing the gas mixture velocity deteriorates the CO2 separation of the system. The impact of hollow fiber geometry on the removal of CO2 is investigated and the results indicate that hollow fibers with smaller inner diameter provides higher effective mass-transfer area and therefore superior CO2 removal performance. Furthermore, the modeling predictions for the CO2 removal are in good agreement with the experimental data under various ratios of gas to liquid velocity. The PTFE hollow fiber membrane contacting system showed a great potential for separation of CO2 from gas mixtures.

Experimental study of submerged gas jets in liquid cross flow

Gas jets submerged in the liquid cross flow are prevalent in the nature and essential in numerous industrial applications. The interaction between the gas jet and liquid cross flow is not well characterized for the difficulties in experimental research. In the present work, a new experimental method was designed and the evolution of the gas jet in liquid cross flow was captured by the high speed camera. Five experimental cases with different initial liquid cross flow velocities (0.35 m/s, 0.7 m/s, 1.0 m/s, 1.5 m/s, 2.0 m/s) were performed where the momentum ratio of the initial gas jet to liquid cross flow varies to highlight the effects of the liquid cross flow velocity on the flow characteristics. The summation and statistical methods were adopted to analyze the experimental sequences, and the results from the two methods showed good agreement. The gas jet morphology and the gas liquid interface were clearly identified from the experimental results, which showed that the gas jet deflected in a less pronounced manner with less unsteadiness generated at the gas liquid interface as the liquid cross flow velocity decreases. The gas jet penetration length in the momentum region was measured and it was found to be severely influenced by the liquid cross flow, and a power relationship was found between the gas jet penetration length and the cross flow velocity. The expansion angle of the gas jet in the upstream was also shown to grow in a power function with the liquid cross flow velocity. The gas jet diameter indicates the ambient liquid entrainments and the gas liquid interface evolution. In the region where the gas jet momentum is dominant, the evolution of the gas jet diameter followed a similar pattern with the submerged gas jets without liquid cross flow. Finally, empirical correlations were developed to predict the characteristic parameters of the submerged gas jet in liquid cross flow.

Parametric Study of the Frequency of Bubble Formation at a Single Orifice With Liquid Cross-Flow

The bubble formation frequency from a single-orifice nozzle subjected to the effects of a crossflowing liquid was investigated using high-speed shadowgraphy, combined with image analysis and signal processing techniques. The effects of the nozzle dimensions, orientation within the conduit, liquid cross-flow velocity, and gas mass flow rate were evaluated. Water and air were the working fluids. Existing expressions in the literature were compared to the experimental values obtained. The expressions showed modest agreement with the experimental mean average frequency magnitude. It was found that increasing the gas injection diameter could decrease the bubbling frequency approximately 12% until reaching a certain value (0.52 mm). Further increasing the nozzle dimensions increase the frequency by around 20%. Bubbling frequency is more sensitive to the liquid velocity where changes up to 63% occurred when the velocity was raised from 3.1 to 4.3 m/s. Increasing gas mass flow rates decreased the gas jet breakup frequency in all cases. This phenomenon was primarily attributed to changes in the bubbling mode from discrete bubbling to pulsating and jetting modes. The nozzle orientation plays a role in modifying the bubbling frequency, having a higher magnitude when oriented against gravity.

Simultaneous absorption of carbon dioxide and nitrogen dioxide from simulated flue gas stream using gas-liquid membrane contacting system

Membrane gas contactors are a promising alternative to conventional post-combustion carbon capture technologies. However, residuals of the other acid gas compounds can exist in the flue gas streams emitted from industrial facilities, having a notable impact on the absorption performance of the membrane system. Simultaneous removal of CO2 and NO2 from a simulated flue gas stream was carried out in a polytetrafluoroethylene (PTFE) hollow fiber gas-liquid membrane contacting (GLMC) system using different scrubbing solutions. A series of experiments were conducted to study the effects of operating conditions such as gas and liquid cross flow velocities, concentration of feed gas, absorbent nature and concentration, and long-term performance of the GLMC system on the removal efficiencies as well as mass transfer rates of CO2 and NO2. Experimental results indicated that simultaneous absorption of CO2 and NO2 were enhanced with increasing the liquid-phase cross flow velocity, decreasing gas-phase cross flow velocity, and using chemical stripping absorbents. Moreover, it was shown the sodium hydroxide to be a superior absorbent as compared to alkanolamine solutions for the co-capture of CO2 and NO2 species. It was observed that low concentrations of NO2 in the feed gas had a minimal impact on the decarbonization of GLMC system. The durability of the membrane system was also evaluated by running the simultaneous gas removal experiments over a 24-h period. The consistency of the absorption efficiency results confirmed the potential of using PTFE membrane system for the simultaneous absorption of CO2 and NO2 gases.

CO2 separation performance of different diameter polytetrafluoroethylene hollow fiber membranes using gas-liquid membrane contacting system

Commercially available polytetrafluoroethylene (PTFE) hollow fiber membranes were investigated for the selective separation of carbon dioxide from a binary gas mixture of CO2/CH4. A series of experiments were carried out to evaluate the separation performance of different diameter PTFE hollow fiber membranes using physical and chemical absorbents from a gas mixture containing 2.5% CO2 balanced in methane. DI-water and aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA) and triethylamine (TEA) were employed as the absorbents. The effects of gas and liquid cross flow velocities, temperature, nature of absorbent, packing density, module configuration and flow pattern on the CO2 mass transfer rate and separation efficiency of the membrane modules were examined. The experimental results indicated that CO2 separation performance of the system enhanced with increasing the liquid phase velocity, decreasing gas phase velocity, and using chemical scrubbing absorbents. For the physical absorbent, the CO2 separation efficiency decreased with increasing the temperature, whereas increasing the temperature was in favor of system performance when using chemical absorbents. The module with smaller size hollow fibers showed the best performance for separation of CO2 from the gas mixture.

Theoretical Analysis of CO<sub>2</sub> Bubble Formation in Anode Flow Field of μDMFC

A mathematical model was established and validated to predict the microbubble diameter when it departing from the carbon paper and moving into the channel of μDMFC. Single bubble behaviors were studied using the model, which took the gas velocity, liquid cross-flow velocity, micro porous diameter and other parameters into account. Results indicate that the microbubble departure diameter decreases with the increasing liquid velocity, and increases with the increasing micro porous diameter and increasing gas velocity.

Comparison of four types of membrane bioreactor systems in terms of shear stress over the membrane surface using computational fluid dynamics

Membrane bioreactors (MBRs) have been used successfully in biological wastewater treatment to solve the perennial problem of effective solids-liquid separation. A common problem with MBR systems is clogging of the modules and fouling of the membrane, resulting in frequent cleaning and replacement, which makes the system less appealing for full-scale applications. It has been widely demonstrated that the filtration performances in MBRs can be greatly improved with a two-phase flow (sludge-air) or higher liquid cross-flow velocities. However, the optimization process of these systems is complex and requires knowledge of the membrane fouling, hydrodynamics and biokinetics. Modern tools such as computational fluid dynamics (CFD) can be used to diagnose and understand the two-phase flow in an MBR. Four cases of different MBR configurations are presented in this work, using CFD as a tool to develop and optimize these systems.

Analytical and numerical modelling of Newtonian and non-Newtonian liquid in a rotational cross-flow MBR

Fouling is the main bottleneck of the widespread use of MBR systems. One way to decrease and/or control fouling is by process hydrodynamics. This can be achieved by the increase of liquid cross-flow velocity. In rotational cross-flow MBR systems, this is attained by the spinning of, for example, impellers. Validation of the CFD (computational fluid dynamics) model was made against laser Doppler anemometry (LDA) tangential velocity measurements (error less than 8%) using water as a fluid. The shear stress over the membrane surface was inferred from the CFD simulations for water. However, activated sludge (AS) is a non-Newtonian liquid, for which the CFD model was modified incorporating the non-Newtonian behaviour of AS. Shear stress and area-weighted average shear stress relationships were made giving error less that 8% compared with the CFD results. An empirical relationship for the area-weighted average shear stress was developed for water and AS as a function of the angular velocity and the total suspended solids concentration. These relationships can be linked to the energy consumption of this type of systems.

Flux and transmission characteristics of a vibrating microfiltration system operated at high biomass loading

Conventional, crossflow microfiltration systems rely on high liquid crossflow velocities to generate shear at the liquid-membrane surface. Shear is necessary to maintain acceptable permeate fluxes and solute transmission especially at high biomass concentrations. In contrast, vibrating membrane filtration (VMF) technology uses mechanical energy generated by vibration to create these shear fields thus decoupling shear from the liquid crossflow velocity. This allows the maintenance of permeate fluxes and solute transmission at much lower crossflow velocities and transmembrane pressures. In this work, we report experimental data for a commercial VMF system (PallSep PS10) for protein recovery from a model biological feed stream containing 200–500 g l −1 Saccharomyces cerevisae wet weight and 0.75 g l −1 bovine serum albumin (BSA). For operation in total recycle mode, basic permeate flux and transmission characteristics of the VMF system are reported for operation at these high biomass loadings as a function of the frequency and peak to peak amplitude of vibration. At a biomass concentration of 500 g l −1, for example, flux and transmission levels of 45 l m −2 h −1 and 67% (w/w), respectively, were maintained over extended operating periods. For operation in concentration mode, a theory is proposed that allows prediction of the maximum solids handling capacity of the VMF unit. We show how this value, and hence the maximum volumetric concentration factor that can be achieved, is influenced by gap width between the membranes. The model is then verified experimentally using S. cerevisae feedstreams showing that the predictions are accurate within 14%. The influence of increasing membrane gap width is also shown using real industrial fermentation broths from Bacillus and Aspergillus cultures. The results for both real and model system studies demonstrate the benefits of VMF systems in terms of both permeate flux and protein transmission especially when operating at high solids loadings.

Hydrodynamic effect on sludge accumulation over membrane surfaces in a submerged membrane bioreactor

The influence of hydrodynamic conditions on membrane fouling was investigated. Ten experiments were designed and operated over a wide range of suspended solid (SS) concentrations (2–20 g l −1), filtration fluxes (4.5–27 l m −2 h −1) and aeration intensities (10–100 m 3 m −2 h −1). In each experiment, the liquid cross-flow velocity was measured and the membrane filtration resistance was monitored. A modified model was set up for calculating the cross-flow velocity of the activated sludge over the membrane surface. The increasing rate of filtration resistance was deduced as a power function of the cross-flow velocities, filtration fluxes and SS concentrations, and a critical curve corresponding to a group of critical fluxes and critical aeration intensities was found. The membrane filtration resistance sharply increased at a flux over the critical flux or at an aeration intensity below the critical aeration intensity.

A model for bubble formation and weeping at a submerged orifice with liquid cross-flow

A new theoretical model to predict bubble frequencies and weeping rates at a submerged orifice with liquid cross-flow has been developed. The model predicts a significant influence of liquid cross-flow velocity on bubble formation frequency, and especially on liquid weeping. Simulated values of weeping rates for different orifice diameters, gas flowrates and liquid cross-flow velocities show good agreement with experimental data.

Prediction of bubble diameter at detachment from a wall orifice in liquid cross-flow under reduced and normal gravity conditions

Bubble formation and detachment is an integral part of the two-phase flow science. The objective of the present work is to theoretically investigate the effects of liquid cross-flow velocity, gas flow rate embodied in the momentum flux force, and orifice diameter on bubble formation and detachment in a wall-bubble injection configuration. A two-dimensional one-stage theoretical model based on a global force balance on the bubble evolving from a wall orifice in a cross liquid flow is presented in this work. In this model, relevant forces acting on the evolving bubble are expressed in terms of the bubble center of mass coordinates and solved simultaneously. Relevant forces in low gravity included the momentum flux, shear-lift, surface tension, drag and inertia forces. Under normal gravity conditions, the buoyancy force, which is dominant under such conditions, can be added to the force balance. Two detachment criteria were applicable depending on the gas to liquid momentum force ratio. For low ratios, the time when the bubble acceleration in the direction of the detachment angle is greater or equal to zero is calculated from the bubble x and y coordinates. This time is taken as the time at which all the detaching forces that are acting on the bubble are greater or equal to the attaching forces. For high gas to liquid momentum force ratios, the time at which the y coordinate less the bubble radius equals zero is calculated. The bubble diameter is evaluated at this time as the diameter at detachment from the fact that the bubble volume is simply given by the product of the gas flow rate and time elapsed. Comparison of the model's predictions was also made with predictions from a two-dimensional normal gravity model based on Kumar–Kuloor formulation and such a comparison is presented in this work.

Experimental study on the enhancement of yeast microfiltration with gas sparging

AbstractGas‐sparged microfiltration experiments with baker's yeast suspensions were performed using a 0.15 µm PCI multi‐tubular membrane module. The permeate flux and the enhancement from gas sparging were measured over the following operational parameter ranges: yeast suspension concentration 0.01–10% (wt), transmembrane pressure 0.5–4.0 bar, liquid crossflow velocity 0.36–1.8 ms−1, gas superficial velocity 0.18–1.02 ms−1. It was demonstrated that injecting air to create a gas–liquid two‐phase crossflow operation can significantly increase the permeate flux. The observed enhancement, resulting from injecting a relatively small amount of gas with a superficial velocity of 0.18 ms−1, ranges from 10% when fouling is insignificant to 135% when fouling tendency is severe. Controlled pulse injection leads to further increase in permeate flux. Examining the dependence of permeate flux on transmembrane pressure, it is clearly demonstrated that the injection of gas will elevate the value of the limiting flux. There is also evidence to show that the value of critical flux is increased when gas injection is applied. Further quantitative analysis will be performed in our future work.© 2001 Society of Chemical Industry

A non-spherical model for bubble formation with liquid cross-flow

A non-spherical model for bubble formation at an orifice with liquid cross-flow has been developed. The interface element approach is applied to describe the dynamics of bubble formation. The effect of liquid cross-flow on the bubble formation process is modelled by a combination of tilting of the bubble axis and liquid pressure analysis of each element on the bubble interface. Model predictions compare well with the experimental results available in the literature for different conditions of gas flow rate, orifice diameter and liquid cross-flow velocity. Simulated bubble shapes are highly non-spherical, especially at high liquid velocities, and bear a striking resemblance with the experimental high-speed video sequences.

Bubble formation from wall orifice in liquid cross-flow under low gravity

Two-phase flows present a wide variety of applications for spacecraft thermal control systems design. Bubble formation and detachment is an integral part of the two-phase flow science. The objective of the present work is to experimentally investigate the effects of liquid cross-flow velocity, gas flow rate, and orifice diameter on bubble formation in a wall–bubble injection configuration. Data were taken mainly under reduced gravity conditions but some data were taken in normal gravity for comparison. The reduced gravity experiment was conducted aboard the NASA DC-9 Reduced Gravity Aircraft. The results show that the process of bubble formation and detachment depends on gravity, the orifice diameter, the gas flow rate, and the liquid cross-flow velocity. The data are analyzed based on a force balance, and two different detachment mechanisms are identified. When the gas momentum is large, the bubble detaches from the injection orifice as the gas momentum overcomes the attaching effects of liquid drag and inertia. The surface tension force is much reduced because a large part of the bubble pinning edge at the orifice is lost as the bubble axis is tilted by the liquid flow. When the gas momentum is small, the force balance in the liquid flow direction is important, and the bubble detaches when the bubble axis inclination exceeds a certain angle.

Bubble formation from cylindrical, flat and concave sections exposed to a strong liquid cross-flow

The results of an experimental study on gas bubble formation from a submerged orifice on a cylindrical, flat or concave blade section, which is exposed to a strong liquid cross-flow, are presented. The effects of the gas velocity in the orifice (3–45 m s -1), the liquid cross-flow velocity (0.5–4 m s -1) and the blade configuration on the mode of bubble formation and the detached bubble size are investigated using high-speed flash photography. The results show that when the orifice is positioned in, or close to, an unseparated flow region the predominant bubbling mode is jetting. In contrast, when the orifice is positioned within the wake region behind the blade, the bubbles are generated individually at the orifice. For a fixed gas velocity, the detached bubble size decreases significantly with increasing liquid velocity, by approximately 50%, between 1 and 3 m s -1; at liquid velocities greater than 3 m s -1, the bubble size decreases more slowly. For a fixed liquid velocity, the bubble size increases approximately linearly with increasing gas velocity through the orifice.

Flux enhancements with gas sparging in downwards crossflow ultrafiltration: performance and mechanism

In this study gas-liquid two-phase crossflow ultrafiltration was studied in downwards flow condition. A commercially available tubular membrane module was installed vertically and the feed solution and the injected gas bubbles flow downwards inside the membrane tubes. The permeate flux achieved in such an operation was compared to conventional single phase ultrafiltration and also to gas sparged upwards crossflow operation. The effect of operating parameters, including liquid and gas flow rates, transmembrane pressure (TMP) and feed concentrations, was examined with dextran solutions. Flux increases of up to 320% were achieved with gas sparging in the experimental study compared to single liquid phase crossflow ultrafiltration. This enhancement is much more profound when the liquid phase is in laminar flow, particularly when the concentration polarisation is more severe, for example, at a high TMP, a higher feed concentration, and a low liquid crossflow velocity. The enhancing effect is not very significant when liquid flow itself is turbulent. The flow pattern proved to be an important parameter and the operation could be optimised to achieve maximum enhancement with minimum gas injection.