Low Gas Velocities Research Articles

Foam flow in vertical gas wells under liquid loading: Critical velocity and pressure drop prediction

Foam lift is one of the most cost effective methodologies for unloading gas wells. The surfactants are either injected intermittently or continuously to lift the liquid to the surface. By reducing the gravitational gradient and increasing the frictional gradient, the critical velocity at which liquid loading occurs is shifted to lower gas velocities. Currently, we do not have a methodology to predict the critical velocity (at the transition boundary of annular and intermittent flow) and the pressure drop under foam flow conditions.To address this, we measured several foam flow characteristics in both small scale and large scale facilities. Small scale facility involved measurement of foam carryover capacity as a function of time and surfactant concentration. Large scale facility involved measurement of liquid holdup, pressure drop, fraction of gas trapped in foam and foam holdup in 40-ft 2-in. and 4-in. tubing.We developed closure relationships for liquid hold up, foam holdup, fraction of gas trapped in the foam and interfacial friction factor by combining the small scale data with the data collected in the large scale experiments. These closure relationships are applicable to four different surfactants tested. A new transition criterion was developed and successfully used to predict onset of liquid loading under foam flow. Using a force balance over the gas core in annular flow, we developed a new procedure to calculate the pressure drop under foam flow conditions. We compared our model results with actual measurements in the large scale facility. Our model was reasonably able to predict the pressure drop within ±30%. The reason for such a large variance is that the small scale facility was not able to capture all the characteristics of the foam which were observed in the large scale facility. It is very difficult to reproduce the foam characteristics exactly in two different experiments. This is discussed further in this paper.The procedure developed is the only one currently available to calculate the pressure drop under the foam flow conditions using the small scale data. It is superior to conventional annular flow pressure drop prediction models which are currently available in the literature.

DEM-CFD analysis of contact electrification and electrostatic interactions during fluidization

Contact electrification and electrostatic interactions often occur in the fluidization process, which can significantly influence the dynamic behaviour of particles and the fluidization performance. In this study, a discrete element method coupled with computational fluid dynamics (DEM-CFD) is developed by implementing contact electrification and electrostatic interaction models and the combined effects of contact electrification and electrostatic interaction on fluidization are analysed. It is found that the charge of the particle system increases with the superficial gas velocity. Particles of different material properties (especially work function) can be bi-charged and form agglomerates. At low superficial gas velocities, the particle bed cannot be fully fluidized and the pressure drop tends to be stable rather than fluctuating as the gas flows through the micro-channels of the formed agglomerates. However, at high superficial gas velocities, the agglomerates can break, inducing strong fluctuation of the pressure drop. This implies that the electrostatic phenomena and fluidization behaviour can mutually influence each other during the process.

Characterization of gas–liquid–solid fluidized beds by S statistics

The hydrodynamics of a gas–liquid–solid fluidized bed was investigated by applying the S statistics method to pressure fluctuations measured under various operating conditions in a laboratory-scale bed. S statistics tests reveal the existence of three transition velocities, especially at low gas velocities. Four distinct fluidization regimes, namely, the compacted bed, agitated bed and coalesced and discrete bubble regimes were detected. A comparison of reconstructed attractors of pressure fluctuations measured at different axial positions along the riser and with various solid loadings showed significant differences in the signals compared before fluidization, especially at minimum liquid agitation velocity. Close to the minimum liquid fluidization velocity and high liquid velocities, the variation in particle size has an insignificant effect on the bed hydrodynamics. Therefore, S statistics is a reliable method to demarcate different fluidization regimes and to characterize the influence of various operating conditions on the hydrodynamics of gas–liquid–solid fluidized beds. The method is applicable in large-scale industrial installations to detect dynamic changes within a bed, such as regime transitions or agglomeration.

Influence of interparticle forces on solids motion in a bubbling gas-solid fluidized bed

This article presents some observations of the effect of interparticle forces (IPFs) on solids motion in a gas-solid fluidized bed operated in the bubbling fluidization regime and at atmospheric pressure. The radioactive particle tracking (RPT) technique was adopted to observe the solids flow pattern and quantify spherical equivalent bubble size, distributions of upward and downward-moving clusters and idle and bubble-induced times, cycle frequency, and axial/radial solids diffusivities. The level of cohesive IPFs was increased and controlled in the fluidized bed with a polymer coating approach. Experimental results showed that the presence of IPFs could effectively modify the solids flow pattern in a bubbling gas-solid fluidized bed. The influence was more pronounced at low gas velocity, where the ratio of the magnitude of IPFs to hydrodynamic forces was high. At constant superficial gas velocity, beds with IPFs contained smaller bubbles indicating a higher tendency of gas entering the dense phase compared to the bubble phase. The different characteristic parameters of solids mixing showed that the favorable effect of IPFs on the division of the fluidizing gas between the bubble and dense phases was accompanied by reductions in the quality of global and local solids mixing.

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.

SIFT-MS a novel tool for monitoring and evaluating a biofilter performance

Biofilters are suitable to treat industrial emissions polluted with volatile organic compounds. This study investigates the selected ion flow tube mass spectrometer (SIFT-MS) as a new and fast analysis apparatus to determine the performance, the biokinetic parameters and the external porosity of a biofilter. The SIFT-MS was also used to obtain more information about mass transfer resistance and reaction limitation which can occur in bioreactors. Online analyses were performed on a biofilter packed with a mixture of compost and wooden dowels, treating an air stream contaminated with dimethyl sulphide (DMS), hexane and toluene. Measurements to determine the biokinetic parameters were performed in less than three days to keep the biomass about constant. The Michaelis–Menten half saturation constant, Km, and the maximal volumetric elimination rate, rm, were calculated for the removal of DMS based on measurements at 35, 60 and 90s empty bed residence time (EBRT). Using pulse injections, the external porosity of the biofilter, 40.2±0.3% could be determined online and information about mass transfer resistance and reaction limitation could be retrieved. Mass transfer resistance becomes significant for compounds with a high Henry coefficient at lower gas velocities.

Impacts of dense heat exchanging internals on gas holdup cross-sectional distributions and profiles of bubble column using gamma ray Computed Tomography (CT) for FT synthesis

The effect of heat exchanging internals that represent Fischer–Tropsch (FT) synthesis in bubble column has been investigated for the first time using gamma ray Computed Tomography (CT) in a 5.5″ (0.14m) inner diameter Plexiglas bubble column operated at atmospheric condition with air water system. Thirty vertical Plexiglas rods of 0.5″ outer diameter were used which covered ∼25% of the total cross-sectional area and were arranged in a triangular pitch of 0.84″ (2.14cm). The superficial gas velocities applied were based on both total cross-sectional area as well as free cross-sectional area available for the flow and were in the range of 5–45cm/s covering the bubbly through churn-turbulent flow regimes.New knowledge and findings have been obtained which have not been reported in previous studies. In churn turbulent flow regime, the overall gas holdup and the profiles of gas holdup obtained in bubble columns without internals can be extrapolated to those with internals in the central region of the column if the superficial gas velocity is based on the free cross-sectional area (CSA) available for the flow of the phases provide that symmetric time averaged gas holdup cross-sectional distributions are achieved. The results show that a significant increment in overall and local gas holdup are obtained upon inserted of the internals when the velocity measured based on the total cross sectional area of the column. The gas holdup distribution over the cross-sectional area (CSA) of the column in the presence of internals has a symmetrical shape at low gas velocities and asymmetry at higher ones for the honeycomb configuration and its installation used in this study. At a high superficial gas velocity that is based on free CSA for the flow, the influence of dense internal becomes insignificant at the central region of the column. However, the profiles of the gas holdup in the column with internals become less steeper compared to those that are like parabolic shape at high gas velocity in the column without internals due to higher gas holdup was obtained in the region near the wall with internals.

Effect of surfactants on liquid loading in vertical wells

Most gas wells produce some amount of liquid. The liquid is either condensate or water. At high rates, the gas is able to entrain liquid to the surface; however, as gas well depletes, the liquid drops back in a gas well (called liquid loading) creating a back pressure on the reservoir formation. Addition of surfactants to the well to remove liquid is one of the common methods used in gas wells. Liquid loading in vertical gas wells with and without surfactant application was investigated in this study. Anionic, two types of amphoteric (amphoteric I and amphoteric II), sulphonate and cationic surfactants were tested in 2-inch and 4-inch 40-feet vertical pipes. Pressure gradient and liquid holdup are measured. Visual observation with a high speed camera was used to gain insight into the direction of foam flow in intermittent flow and foam film flow under annular flow conditions.Liquid loading is initiated when the liquid film attached to the wall in annular flow starts flowing downwards. Introduction of foam causes the gas velocity at which film reversal occurs to decrease; this shift increases with increasing surfactant concentration and it is more pronounced in 2-inch pipe than in 4-inch pipe. That is, the benefit of surfactants is much more pronounced in 2-inch pipe than in 4-inch pipe. The reason for postponement of liquid loading is reduction in the liquid holdup at low gas velocities which reduces the liquid holdup in foam flow compared to air-water flow. However, at higher gas velocities, the pressure drop in 2-inch compared to 4-inch pipe increases rapidly as the surfactant concentration increases. The selection of optimum concentration of the surfactant is a balance between the reductions in the gas velocity at which liquid loading occurs compared to increase in the frictional loss as the concentration increases. We provide guidelines about the selection of the surfactant concentration.Visual observations using high speed camera show differences in the behavior under foam flow conditions. Unlike air-water flow, the liquid film attached to the wall is replaced by thick foam capturing the gas bubbles. The type of roll waves which carry the liquid in 2-inch pipe is different than what was observed in 4-inch pipe. Compared to 4-inch pipe, the roll waves in 2-inch pipe are much thicker. This partly explains the differences in 2-inch versus 4-inch pipe behavior.

De-mixing characteristics of fine coal in an air dense medium fluidized bed

An air dense medium (magnetite) fluidized bed (ADMFB) provides an alternative method for dry coal beneficiation. The de-mixing (segregation) characteristics can significantly affect the quality of the final products. In this study, the segregation characteristics of fine coal (1–3mm) in an ADMFB were compared to those in an ordinary fluidized bed (FB). Specifically, the pressure drop, density-segregation behaviors, segregation degree, motions of the coal/magnetite mixture, and bubble size were determined with various gas velocities. Density-segregation occurred at a low gas velocity in the ADMFB. However, the particles started to mix when the gas velocity was increased. There was an optimum gas velocity to produce the maximum amount of segregation. The comparative analysis (ash segregation degree and bubble size characteristics) between ADMFB and FB suggested that the bubble-drive flotsam-jetsam mechanism can explain the particle segregation. Furthermore, adding dense medium to a fluidized bed led to a higher amount of segregation because of the reduction in bubble size.

Effect of agitation on the characteristics of air dense medium fluidization

In order to study the effect of agitation on the characteristics of air dense medium fluidization, we designed and constructed an agitation device. Analyses were then conducted on the fluidization characteristics curves, the bed density stability and the average bubble rise velocity Ua under different agitation conditions. The results indicated that a lower bed pressure drop (without considering lower gas velocity in a fixed bed stage) and higher minimum fluidized velocity are achieved with increasing agitation speed. The height d (distance between the lower blades and air distribution plate) at which the agitation paddle was located had a considerable effect on the stability of the bed density at 9.36cm/s<U<10.70cm/s. The higher the value of d, the better the stability, and the standard deviation of the bed density fluctuation σ dropped to 0.0364g/cm3 at the ideal condition of d=40mm. The agitation speed also had a significant influence on the fluidization performance, and σ was only 0.0286g/cm3 at an agitation speed of N=75r/min. The average bubble rise velocity decreased significantly with increasing agitation speed under the operating condition of 1.50cm/s<U–Umf<3.50cm/s. This shows that appropriate agitation contributes to a significant improvement in the fluidization quality in a fluidized bed, and enhances the separation performance of a fluidized bed.

Mechanism of metal dusting corrosion by pitting of a chromia-forming alloy at atmospheric pressure and low gas velocity

FeNiCr samples (800HT) were exposed at 570°C, 1bar to a 47.25CO-47.25H2-5.5H2O atmosphere (ac=33) flowing at 18μm/s. Pitting corrosion was observed. Pits showed a flattened morphology and a constant pit diameter growth rate. Corrosion rings appeared successively at the surface during pit growth. A four-step mechanism is proposed which includes internal oxidation of carbides, graphitisation and localised enhanced graphitisation. Gas velocity and thermal cycling play key roles in pit morphology. Thermal cycling induces circular cracks. Low gas velocity induces the gas to evolve in crevices, due to local oxygen consumption.

Minimum fluidization velocity in gas‐liquid‐solid minifluidized beds

The initial fluidization characteristics of gas‐liquid‐solid minifluidized beds (MFBs) were experimentally investigated based on the analyses of bed pressure drop and visual observations. The results show that ULmf in 3–5 mm MFBs can not be determined due to the extensive pressure drop fluctuations resulting from complex bubble behavior. For 8–10 mm MFBs, ULmf can be confirmed from both datum analyses of pressure drop and Hurst exponent at low superficial gas velocity. But at high superficial gas velocity, ULmf was not obtained because the turning point at which the flow regime changes from the packed bed to the fluidized bed disappeared, and the bed was in a half fluidization state. Complex bubble growth behavior resulting from the effect of properties of gas‐liquid mixture and bed walls plays an important role in the fluidization of solid particles and leads to the reduction of ULmf. An empirical correlation was suggested to predict ULmf in MFBs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1940–1957, 2016

Improved CO2 Absorption in a Gas–Liquid Countercurrent Column Using a Ceramic Foam Contactor

Solid foams are porous, monolithic materials with higher specific surface areas than the random packings that are commonly used in amine-based CO2 capture processes. In this work, the hydrodynamic characteristics (e.g., pressure drop, flooding point, and liquid holdup) and CO2 absorption performance of α-Al2O3 ceramic foam packings of different porosities were investigated experimentally in a gas–liquid countercurrent column. With a 30 wt % diglycolamine (DGA) solvent as the CO2 absorbent, the foams allowed higher flow rates of gas and liquid than a random packing before undesirable flooding was reached. Ceramic foams with lower porosities have larger operating capacities than those with higher porosities. A parametric study of a one-dimensional flow model was performed by investigating the effects of gas velocity, liquid velocity, and CO2 solvent loading on the CO2 removal performance. Lower gas velocities and higher liquid velocities increased the CO2 removal efficiency. The CO2 removal efficiency decre...

Hydrodynamics and particle mixing/segregation measurements in an industrial gas phase olefin polymerization reactor using image processing technique and CFD-PBM model

Particle size distribution (PSD) has a significant impact on the performance of fluidized bed reactors due to uneven distribution in the segregation and mixing phenomena. This paper develops a new method of digital image processing that investigates the hydrodynamics of an industrial gas phase olefin polymerization reactor and studies the fluidization structure of a wide range of particle size distribution in an industrial gas phase polymerization reactor by means of a CFD-PBM coupled model, where the direct quadrature method of moments (DQMOM) was implemented to solve the population balance model. It was shown that the applied parameter assumptions and closure laws were appropriately chosen to satisfactorily predict the available operational data in terms of pressure drop and bed height. The transient CFD-PBM/DQMOM coupled model and image analysis technique are then implemented extensively to analyze bubble fluidization structure and segregation phenomena at different velocities. The particle segregation indicates that the small bubbles present in the bed are unable to induce vigorous mixing at low superficial gas velocity while particle mixing improves at a velocity above the minimum fluidization velocity. Further, the predicted results show higher axial segregation phenomena when compared to the radial direction.

Power consumption, gas holdup, and mass‐transfer coefficient of triple‐impeller configurations in a stirred vessel with vertical tubular coils

AbstractThree kinds of impeller combinations (3 × 4WHd, HB + 2 × 4WHd, 2 × HB + 4WHd) were used to investigate the effects of coil baffles on system power consumption, gas holdup, and mass transfer coefficient. The experiments were performed in a dish‐bottomed vessel with a diameter of 0.29 m. Results of the power consumption experiments show that at the same rotation speed and an increasing rate of gas flow, the power consumptions of 3 × 4WHd and 2 × HB + 4WHd with coil baffles are almost the same as that of a system with four planar baffles, but there is a clear power draw for the HB + 2 × 4WHd combination. At rotational speeds of 250–350 rpm, the largest power draws for the 2 × HB + 4WHd, HB + 2 × 4WHd, and 3 × 4WHd impeller combinations equipped with coil baffles are 27, 25, and 15 %, respectively. The gas holdup experiment shows that with installation of coil baffles in the vessel, axial and radial impeller combinations can achieve gas dispersion identical to that attained with planar baffles. At the same power input and superficial gas rate over a wide range of gas velocities, the kLa values of the coil baffles for the 3 × 4WHd and 2 × HB + 4WHd combinations are almost identical to those of a conventional system with four baffles. However, in the HB + 2 × 4WHd system, kLa values of the coil baffles are 15 % higher than for planar baffles at low gas velocities and 30 % higher at high superficial gas rates. Finally, all data from the experiments on gas holdup and kLa values may be correlated through the equations and .

The impact of gas slug flow on microfiltration performance in an airlift external loop tubular membrane reactor

Under low gas-velocity, the cake layer gradually formed. Then high gas-velocity scoured the cake layer, which obstructed the cake layer's formation.

Local hydrodynamic parameters of bubble column reactors operating with non‐Newtonian liquids: Experiments and models development

The effects of liquid phase rheology on the local hydrodynamics of bubble column reactors operating with non‐Newtonian liquids are investigated. Local bubble properties, including bubble frequency, bubble chord length, and bubble rise velocity, are measured by placing two in‐house made optical fiber probes at various locations within a bubble column reactor operating with different non‐Newtonian liquids. It was found that the presence of elasticity can noticeably increase the bubble frequency but decreases the bubble chord length and its rise velocity. The radial profiles of bubble frequency, bubble chord length, and bubble rise velocity are shown to be relatively flat at low superficial gas velocity while they become parabolic at high superficial gas velocity. Moreover, the bubble size and gas holdup are correlated with respect to dimensionless groups by considering the ratio between dynamic moduli of viscoelastic liquids. The novel proposed correlations are capable of predicting the experimental data of bubble size and gas holdup within a mean absolute percentage error of 9.3% and 10%, respectively. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1382–1396, 2016

Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations

The gassed power demand and volumetric mass transfer coefficient (kLa) were investigated in a fully baffled, dished-base stirred vessel with a diameter of 0.30m agitated by five triple-impeller combinations. Six types of impellers (six-half-elliptical-blade disk turbine (HEDT), four-wide-blade hydrofoil impeller (WH) pumping down (D) and pumping up (U), parabolic-blade disk turbine (PDT), and CBY narrow blade (N) and wide blade (W)) were used to form five combinations identified by PDT+2CBYN, PDT+2CBYW, PDT+2WHD, HEDT+2WHD and HEDT+2WHU, respectively. The results show that the relative power demand of HEDT+2WHU is higher than that of other four impeller combinations under all operating conditions. At low superficial gas velocity (uG), kLa differences among impeller combinations are not obvious. However, when uG is high, PDT+2WHD shows the best mass transfer performance and HEDT+2WHU shows the worst mass transfer performance under all operating conditions. At high uG and a given power input, the impeller combinations with high agitation speed and big projection cross-sectional area lead to relatively high values of kLa. Based on the experimental data, the regressed correlations of gassed power number with Froude number and gas flow number, and kLa with power consumption and superficial gas velocity are obtained for five different impeller combinations, which could be used as guidance for industrial design.

Numerical simulation of terrain-induced severe slugging coupled by hydrodynamic slugs in a pipeline–riser system

A numerical study based on a one-dimensional two-fluid model is carried out to describe the transient hydrodynamic slugging and terrain-induced severe slugging in a pipeline–riser system. The system of equations is rendered well-posed by interfacial pressure model for the riser. The selected flow conditions are restricted in the well-posed region for the horizontal and the downward inclined pipes to ensure the hydrodynamic slug characteristics are predicted correctly. The validity of the model is examined by water faucet problem and horizontal slug flow experiments. Simulations with and without slug capturing are conducted to address the effect of hydrodynamic slugs on severe slugging. It has been found that more accurate predictions are obtained by taking hydrodynamic slugs into account. At low superficial gas velocity, the simulation without slug capturing tends to overestimate the severe slugging period. When hydrodynamic slugs are captured, the upstream gas expansion is suppressed by the hydrodynamic slugs. At relatively high superficial gas velocity, the simulation without slug capturing tends to underestimate the severe slugging period. When hydrodynamic slugs are captured, the upstream compressible volume is greatly enlarged by the blowout of the hydrodynamic slugs. In both situations, the influences of the hydrodynamic slugs can reduce the errors of the predicted severe slugging characteristics.

Numerical study of gas–solid flow behavior in a two-stage high-density riser using EMMS-based drag model

The two-fluid model coupled with the EMMS/Matrix drag model was employed to investigate the gas–solid flow behavior in the novel two-stage high-density riser with an enlarged bottom section. The choking phenomenon in the bottom zone due to the low superficial gas velocity has been predicted. Through the comparison of flow behaviors in the constant-diameter and the novel risers under high solids circulating flux (660kg/m2s), the characteristics of turbulent fluidization was found in the bottom zone, and the dense suspension upflow regime was achieved in the upper zone of the novel riser, but it held much more solids inventory than the typical riser. The larger diameter of the bottom zone made the flow regime transition from fast fluidization to turbulent fluidization, as revealed from probability density distributions. The larger height of the bottom zone extended the horizontal segment of the choking curve. With the increase of the height or the diameter of the bottom zone, it needed more solids inventory to achieve dense suspension upflow in the upper zone, but the solids residence time in the novel riser increased significantly.