Low Gas Flow Rates Research Articles

Gas transfer in a bubbly wake flow

The present work reports simultaneous bubble size and gas transfer measurements in a bubbly wake flow of a hydrofoil, designed to be similar to a hydroturbine blade. Bubble size was measured by a shadow imaging technique and found to have a Sauter mean diameter of 0.9 mm for a reference case. A lower gas flow rate, greater liquid velocities, and a larger angle of attack all resulted in an increased number of small size bubbles and a reduced weighted mean bubble size. Bubble-water gas transfer is measured by the disturbed equilibrium technique. The gas transfer model of Azbel (1981) is utilized to characterize the liquid film coefficient for gas transfer, with one scaling coefficient to reflect the fact that characteristic turbulent velocity is replaced by cross-sectional mean velocity. The coefficient was found to stay constant at a particular hydrofoil configuration while it varied within a narrow range of 0.52-0.60 for different gas/water flow conditions.

Uniform, fast, high concentration delivery of bone marrow stromal cells and gingival fibroblasts by gas-brushing

Delivering multipotent stem cells in adequate milieu directly to site of injury is important for successful tissue regeneration. Accelerated wound healing has been reported in recent publications when large area wounds were treated with sprayed cells. However, little is known about the basic physics of cell spraying in hydrogel free medium, causes of concomitant cell damage, and whether it can be applied for craniofacial tissue regeneration. To answer some of these questions, a simple co-axial CO2 gas operated brush (g-brush) was used to deliver human bone marrow stromal (hBMSC) and mouse gingival fibroblast (ESK-1) cells at low gas flow rates and in a small volume of medium. Live/dead, metabolic activity (MTS-1) and total DNA quantification assays were utilized to evaluate cell cultures after g-brush and typical pipette deposition. Viable hBMSC and ESK-1 cells were uniformly delivered at higher cell concentrations (≥60 000), in a short period of time (15 and 9 s), and in a small fluid volume (0.25 ml). Significantly, after the hBMSC were g-brushed and cultured up to 21 d, the cell multipotency was found to be preserved and comparable to the pipetted cells. Fluid dynamic calculations highlighted importance of low interfacial shear stress (<35 Pa) and gas velocity (<160 m s−1) in delivering concentrated, viable cells. Results of this research support applicability of the g-brushing technique for the quick, large number delivery of viable cells for tissue regeneration applications.

Characteristics of Atmospheric Pressure Rotating Gliding Arc Plasmas**supported by National Natural Science Foundation of China (No. 51576174), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120101110099) and the Fundamental Research Funds for the Central Universities (No. 2015FZA4011)

In this work, a novel direct current (DC) atmospheric pressure rotating gliding arc (RGA) plasma reactor has been developed for plasma-assisted chemical reactions. The influence of the gas composition and the gas flow rate on the arc dynamic behaviour and the formation of reactive species in the N2 and air gliding arc plasmas has been investigated by means of electrical signals, high speed photography, and optical emission spectroscopic diagnostics. Compared to conventional gliding arc reactors with knife-shaped electrodes which generally require a high flow rate (e.g., 10–20 L/min) to maintain a long arc length and reasonable plasma discharge zone, in this RGA system, a lower gas flow rate (e.g., 2 L/min) can also generate a larger effective plasma reaction zone with a longer arc length for chemical reactions. Two different motion patterns can be clearly observed in the N2 and air RGA plasmas. The time-resolved arc voltage signals show that three different arc dynamic modes, the arc restrike mode, takeover mode, and combined modes, can be clearly identified in the RGA plasmas. The occurrence of different motion and arc dynamic modes is strongly dependent on the composition of the working gas and gas flow rate.

Catalytic membrane microreactor with Pd/γ-Al2O3 coated PDMS film modified by dopamine for hydrogenation of nitrobenzene

Conventional gas–liquid–solid reactors usually face the gaseous reactant transport issues in association with the interface and liquid phase to immobilized solid catalysts. To resolve this problem, a catalytic membrane microreactor (CMMR) with the catalytic membrane formed by coating the catalysts on a thin PDMS film modified by dopamine was developed in this study, which not only eliminated the issues in the conventional gas–liquid–solid reactors but also provided a large surface-area-to-volume ratio to enhance the mass transport. The performance of the developed CMMR was evaluated by hydrogenation of nitrobenzene. It was found that the nitrobenzene conversion and operation stability was greatly improved by the CMMR as a result of enhanced mass transport of hydrogen. Parametric study was also performed in this work to explore how the operation conditions and reactor design affected the performance. It was shown that low liquid flow rate and high gas flow rate benefited for the improvement in the nitrobenzene conversion due to the increased residence time and hydrogen permeation rate through the membrane. The results on the effect of the inlet nitrobenzene concentration showed that an increase in the inlet nitrobenzene concentration increased the aniline concentration because of enhanced catalytic reaction rate but lowered the nitrobenzene conversion because of the overloaded liquid reactant. Besides, it was also shown that thinner membrane could enhance the hydrogen permeation through the membrane, thereby yielding higher nitrobenzene conversion. This study fully demonstrates the feasibility and superiority of the developed CMMR for the gas–liquid–solid reaction system.

The Effect of Surfactants on Vertical Air/Water Flow for Prevention of Liquid Loading

Summary From field experience in the gas industry, it is known that injecting surfactants at the bottom of a gas well can prevent liquid loading. To better understand how the selection of the surfactant influences the deliquification performance, laboratory experiments of air/water flow at atmospheric conditions were performed, in which two different surfactants (a pure surfactant, sodium dodecyl sulfate, and a commercial surfactant blend) were added to the water. In the experiments, a high-speed camera was used to visualize the flow, and pressure-gradient measurements were performed. Both surfactants increase the pressure gradient at high gas-flow rates and decrease the pressure gradient at low gas-flow rates. The minimum in the pressure gradient moves to lower gas-flow rates with increasing surfactant concentration. This is related to the transition between annular flow and churn flow, which is shifted to lower gas-flow rates because of the formation of an almost stagnant foam substrate at the wall of the pipe. At high surfactant concentration, it appears that the churn flow regime is no longer present at all and that there is a direct transition from annular flow to slug flow. The results also show that the critical micelle concentration, the equilibrium surface tension, the dynamic surface tension, and the surface elasticity are poor predictors of the effect of the surfactant on the flow.

Optimization of carbon dioxide supply in raceway reactors: Influence of carbon dioxide molar fraction and gas flow rate

Influence of CO2 composition and gas flow rate to control pH in a pilot-scale raceway producing Scenedesmus sp. was studied. Light and temperature determined the biomass productivity whereas neither the CO2 molar fraction nor the gas flow rate used influenced it; because pH was always controlled and carbon limitation did not take place. The CO2 molar fraction and the gas flow rate influenced carbon loss in the system. At low CO2 molar fraction (2–6%) or gas flow rate (75–100l·min−1) the carbon efficiency in the sump was higher than 95%, 85% of the injected carbon being transformed into biomass. Conversely, at high CO2 molar fraction (14%) or gas flow rate (150l·min−1) the carbon efficiency in the sump was lower than 67%, 32% of the carbon being fixed as biomass. Analysis here reported allows the pH control to be optimized and production costs to be reduced by optimizing CO2 efficiency.

Modeling of Liquid Level and Bubble Behavior in Vacuum Chamber of RH Process

In the Ruhrstahl-Heraeus (RH) refining process, liquid steel flow pattern in a ladle is controlled by the fluid flow behavior in the vacuum chamber. Potassium chloride solution and NaOH solution saturated with CO2 were respectively used as a tracer to investigate the liquid and gas flow behaviors in the vacuum chamber. Principal component and comparative analysis were made to show the factors controlling mixing and circulation flow rate. The liquid level and bubble behavior in the vacuum chamber greatly affect fluid flow in RH process. Experiments were performed to investigate the effects of liquid steel level, gas flow rate, bubble residence time, and gas injection mode on mixing, decarburization, and void fraction. The results indicate that the mixing process can be divided into three regions: the flow rate-affected zone, the concentration gradient-affected zone, and their combination. The liquid steel level in the vacuum chamber of 300 mm is a critical point in the decarburization transition. For liquid level lower than 300 mm, liquid steel circulation controls decarburization, while for liquid level higher than 300 mm, bubble behavior is the main controlling factor. During the RH process, it is recommended to use the concentrated bubble injection mode for low gas flow rates and the uniform bubble injection mode for high gas flow rates.

Dense bubble traffic in microfluidic loops: Selection rules and clogging.

We study the repartition of monodisperse bubbles at the inlet node of an asymmetric microfluidic loop for low to high bubble densities. In large loops, we evidence a new regime. Contrary to the classical belief, we point out that bubbles are directed not towards the arm having the higher total flow rate but towards the arm with the higher water flow rate at low and moderate relative gas flow rates. At higher rates, they enter the longer arm when they reach close packing in the shorter arm. In small loops, we evidence a clogging regime at high relative gas flow rates. Collisions between bubbles coming from the two arms at the outlet clog the longer arm. We propose a comprehensive analysis allowing us to explain these results.

Improving gas absorption efficiency using a novel dual membrane contactor

A dual membrane contactor containing two types of membranes (polypropylene membrane and silicone rubber membrane) was designed and constructed for gas absorption process. The module performance was evaluated based on permeation flux experiments. The experimental results were compared with the predictions from a numerical model developed in our previous studies. Furthermore, the mass transfer resistance in the fabricated module was investigated using resistance-in-series model. An empirical correlation was developed to describe mass transfer coefficient under different operational conditions.The performance tests completed with the proposed novel dual membrane design shows that by simultaneously stripping gaseous component from the solvent through nonporous membrane along with absorption process, permeation flux was improved by 12–78%, compared to conventional modules containing only one type of porous membrane. The enhancement can be further improved by optimizing the membrane design and operational conditions. The effect of various operational conditions on absorption performance of dual membrane module was also examined. The experimental results were found in good agreement with those generated by simulation model. It was found that mass transfer resistance in the dual membrane module existed mainly in the liquid phase while membrane's resistance contribution to the total mass transfer resistance was about 7%. Higher permeation flux was achieved when the module was operated with lower gas flow rates, higher liquid flow rates and a low vacuum pressure in nonporous membrane. When compared to dilute feed gas (solute concentration ≤5%), removal efficiency with concentrated gas is much higher when other operational conditions were kept the same. The proposed novel membrane design holds great potential for gas processing at remote locations such as offshore platforms.

Electrochemically assisted synthesis of fuels by CO2 hydrogenation over Fe in a bench scale solid electrolyte membrane reactor

The electrochemically assisted synthesis of fuels by CO2 hydrogenation was studied over a cheap, widespread and non-precious Fe catalyst in a bench scale oxygen ion conducting membrane (YSZ) reactor. The studies were performed under conditions representative of potential practical application i.e., under atmospheric pressure, at relatively low temperatures and high gas flow rates, with varying H2/CO2 ratios and using gas compositions typical for postcombustion CO2 capture exit streams and easily scalable catalyst-electrode configurations.The Fe-TiO2 catalyst film was deposited by “dip-coating” and characterised both after pre-reduction and after testing.CO2 conversion and selectivities to CH3OH and C2H6O can be enhanced up to 4, 50 and 1.7 times, respectively, by electrochemically controlled migration of O2− promoting ions to or from the catalyst surface.The optimum temperature for the process was 325°C. Lower gas flow rates favoured the synthesis of CH3OH and C2H6O. CO2 conversion and selectivities to CH3OH and C2H6O showed a maximum for a stoichiometric H2/CO2 ratio of 3. Formation of C3H6 and CO is strongly favoured for a H2/CO2 ratio of 4 and 2, respectively, as a result of the increased and decreased hydrogen availability in the reaction system.

On the importance of gel rigidity and coverage in a smart water shutoff treatment in gas wells

This paper addresses a conceptual study on different aspects of a novel smart treatment method for a relatively new challenge of saline water production from low permeability lenses of high flow gas wells. Selective sealing of such lenses along with minor effect on gas productivity usually faces practical difficulties due to the inherent permeability contrast. Engineered application of salt sensitive gellan biopolymer through a smart treatment scenario that includes a protective gas flow proved its ability for treating such challenge. This paper investigates the importance of the gel rigidity and its coverage in the smart treatment scenario through considering the brine salinity and the injected biopolymer concentration. It is illustrated that during low gas flow rates, the performance of the smart water shutoff treatment is mainly governed by the aptitude of the biopolymer for covering the low permeability medium while at higher gas flow rates the importance of the gel rigidity is noticeably increased. It is concluded that for appropriate treatment, the biopolymer concentration should be determined based on the production strategy and brine salinity.

Impact of gas flow rate on breakdown of filamentary dielectric barrier discharges

The influence of gas flow rate on breakdown properties and stability of pulsed dielectric barrier discharges (DBDs) in a single filament arrangement using a gas mixture of 0.1 vol. % O2 in N2 at atmospheric pressure was investigated by means of electrical and optical diagnostics, accompanied by fluid dynamics and electrostatics simulations. A higher flow rate perpendicular to the electrode symmetry axis resulted in an increased breakdown voltage and DBD current maximum, a higher discharge inception jitter, and a larger emission diameter of the discharge channel. In addition, a shift of the filament position for low gas flow rates with respect to the electrode symmetry axis was observed. These effects can be explained by the change of the residence time of charge carriers in the discharge region—i.e., the volume pre-ionization—for changed flow conditions due to the convective transport of particles out of the center of the gap.

Removal of SO2from Flue Gas Using Basic Aluminum Sulfate Solution with the Byproduct Oxidation Inhibition by Ethylene Glycol

This paper presents a novel wet flue gas desulfurization (FGD) technology based on a basic aluminum sulfate (BAS) desorption regeneration process, in which ethylene glycol (EG) was first employed to inhibit the byproduct oxidation. The operating parameter effect on SO2 absorption efficiency and oxidation efficiency of sulfite was thoroughly examined in a lab-scale bubbling column. The results indicated that both the amount of aluminum and basicity play important roles on the desulfurization time with above 90% absorption efficiency. The BAS-based desulfurization process was more suitable for a low temperature and low gas flow rate. High inlet SO2 concentrations may contribute to the mass-transfer rate of SO2, and the SO2 absorption efficiency remained above 90% when the pH value was over 3.10. With the addition of 1% (v/v) EG in BAS solution, the oxidation efficiency dropped dramatically from 86 to below 10% (in 120 min). On the basis of the two-film theory, a model of the SO2 absorption process was developed and the mass-transfer characteristics were analyzed. The calculation results indicated that the SO2 absorption process for this system was decided by a combination of both the gas- and liquid-phase diffusion controls.

Influence of tube wall longitudinal heat conduction on temperature measurement of cryogenic gas with low mass flow rates

Longitudinal heat conduction is an important parameter in the cryogenic field, especially in cryogenic heat exchangers. In the present study, the parasitic effect of tube wall longitudinal heat conduction on temperature measurement within the tube has been studied for cryogenic gas with low mass flow rates by finite element method and experimental tests. The effects of various parameters such as tube outlet temperature, tube wall thermal conductivity, mass flow rate, and tube wall thickness have been investigated. Axial positioning errors of temperature sensor due to tube wall longitudinal heat conduction were higher for lower gas flow rates. The results showed that the tube wall thermal conductivity leads to axial heat conduction within the tube wall, but the higher tube wall thermal conductivity does not lead to bigger axial positioning error of temperature sensor at tube outlet. According to data obtained from simulations and experiments, sensor with distance of 5mm from tube outlet had 14.92% and 8.51% temperature measurement error (with respect to gas flow temperature at tube outlet) for tube wall thermal conductivities of 16 and 400Wm−1K−1, respectively.

A temporary immersion plant propagation bioreactor with decoupled gas and liquid flows for enhanced control of gas phase.

Temporary immersion bioreactors (TIBs) are being used to propagate superior plant species on a commercial scale. We demonstrate a new TIB design, a Hydrostatic-driven TIB (Hy-TIB), where periodic raising and lowering the media reservoir maintains the advantages of temporary immersion of plant tissues without requiring large amounts of gas to move the media that is a characteristic of other TIB designs. The advantage of utilizing low volumes of gas mixtures (that are more expensive than air) is shown by a doubling of the growth rate of plant root cultures under elevated (40%) oxygen in air, and with CO2 supplementation showing improved phototrophic and photomixotrophic growth of seedless watermelon meristem cultures. The development of this bioreactor system involved overcoming contamination issues associated with utilizing very low gas flow rates and included utilizing microchip pressure sensors to diagnose unexpected changes in internal bioreactor pressure (± 20 Pa ∼0.0002 atm) caused by flexing of non-rigid plastic bag vessels. The overall design seeks to achieve versatility, scalability and minimum cost such that bioreactor technology can play an increasing role in the critical need to improve plant productivity in the face of increasing demand for food, reduced resources, and environmental degradation. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:337-345, 2016.

磁性流体を用いた気体流量制御

A novel method of controlling gas flow rate using magnetic fluid is proposed. Magnetic fluids under non-uniform magnetic field have strong dynamic pressure distribution along the magnetic field gradient at the gas-liquid interface with bulk magnetic body force. The principle of proposed gas flow rate control technique is based on controlling the pressure distribution, which can be controlled by magnetic field. In this report, controlling technique is introduced and the accuracy of the flow rate control with resolution of the control technique is reported. From measured data (giving a reference for measuring the small gas flow rate), it was found that the flow rate decreases with increasing magnetic flux intensity, and is largely dependent on the magnetic pressure which acts on magnetic fluid, particularly to the interface of gas-liquid phase. The flow rate and the magnetic pressure have a definite relation with the flow coefficient, so that, the flow rate can be controlled only by the magnetic pressure. It was satisfactory verified from the proposed controlling technique that the proposed gas flow controlling method is useful to control the low gas flow rate as low as 0.3×10-5m3/s with the excellent resolution.

Investigation on a micro-pin-fin based membrane separator

We proposed a micro-pin-fin based membrane separator. An enclosed membrane with micron scale holes was symmetrically populated in a rectangular duct. When gas phase interacts with the membrane, the gas–liquid interface cannot break through the pin-fin holes due to the increased surface energy. A two-dimensional numerical model simulated the separation process. The volume of fluid (VOF) method tracked the gas–liquid interface. Multiscale grids were used. When a bubble attacks the pin-fin membrane, strong liquid circulation occurs at the membrane entrance. Pressures in the side region are larger than those in the core region. Liquid plugs are shortened due to the pressure driven flow from side region to core region to cause the bubble coalescence. The separation length was shortened while increasing the gas flow rates. The bubble lengths were weakly influenced by gas flow rates. Liquid plugs are quickly shortened following the membrane entrance. The frictional pressure drop of the two-phase mixture in the side region was larger than that of liquids in the core region, even at low gas flow rates. The ultra-large gas flow rates yielded quite large bubble pressure to exceed the capillary pressure limit, causing the separator failure. Ultra-low and large gas flow rates specified the separator operation range.

Effects of gas flow rate on deposition rate and number of Si clusters incorporated into a-Si:H films

The suppression of cluster incorporation into a-Si:H films is the key to better film stability, because incorporated clusters contribute to the formation of SiH2 bonds and hence lead to light-induced degradation of the films. To deposit stable a-Si:H solar cells at a high deposition rate (DR), we studied the effects of the gas flow rate on DR and the number of Si clusters incorporated into a-Si:H films with discharge power as a parameter, using a multihollow discharge-plasma chemical vapor deposition method. We succeeded in depositing high-quality a-Si:H films with the incorporation of few clusters at DR of 0.1 nm/s. We also found that, under a low gas flow rate and a high discharge power, high-density clusters exist in plasma and hence DR is reduced as a result of radical loss to the clusters.

Permeability of high-Kn real gas flow in shale and production prediction by pore-scale modeling

Although shale gas has been commercially exploited, the gas transport mechanism in shale is still unclear. Because nanoscale pores are dominant in shale, the Knudsen number of the flow is relatively high so that the conventional Darcy's law fails. What is more, the shale gas in situ is under high pressure and high temperature so that the real gas (or non-ideal gas) effect is significant. Aiming at these two challenges, we did a pore-scale modeling by using lattice Boltzmann method in this work. We developed a pore-field-iteration (PFI) method to bridge up the pore-scale modeling results with the field-scale concerns, such as inflow performance relationship and decline curve analysis. Our results show that the high Knudsen effect leads to a higher gas flow rate, while the real gas effect causes lower gas flow rate. The gas production may be overestimated at early stage due to the real gas effect, while underestimated at late stage because of the high Knudsen number effect. These results may be very helpful for better understanding of gas transport mechanism in shale and for possible process optimization of shale gas developments in future.

Comparison of electrical and optical characteristics in gas-phase and gas-liquid phase discharges

This paper presents an AC-excited argon discharge generated using a gas-liquid (two-phase) hybrid plasma reactor, which mainly consists of a powered needle electrode enclosed in a conical quartz tube and grounded deionized water electrode. The discharges in the gas-phase, as well as in the two-phase, exhibit two discharge modes, i.e., the low current glow-like diffuse mode and the high current streamer-like constrict mode, with a mode transition, which exhibits a negative resistance of the discharges. The optical emission spectral analysis shows that the stronger diffusion of the water vapor into the discharge region in the two-phase discharges boosts up the generation of OH (A–X) radicals, and consequently, leads to a higher rotational temperature in the water-phase plasma plume than that of the gas-phase discharges. Both the increase of the power input and the decrease of the argon flow rate result in the increase of the rotational temperature in the plasma plume of the water-phase discharge. The stable two-phase discharges with a long plasma plume in the water-phase under a low power input and gas flow rate may show a promising prospect for the degradation of organic pollutants, e.g., printing and dyeing wastewater, in the field of environmental protection.