High Superficial Gas Velocities Research Articles

Probing the differences in CO2 adsorption/desorption behaviors of solid amine sorbents in fixed and fluidized beds

Circulating fluidized bed temperature swing adsorption (CFB-TSA) CO2 capture process using solid amine sorbents has become a widely recognized solution to reduce stationary sources CO2 emissions. The successful design and operation of the adsorber and desorber requires much information on the CO2 adsorption and desorption behaviors of the sorbents in fluidized bed reactors. However, among the existing studies, little attention has been paid to desorption performance, which is equally important as adsorption performance. And almost all of them were conducted in TGAs or fixed beds, the lack of research in fluidized beds making it difficult to guide the design of industrial fluidized bed reactors. In addition, the systematic comparison of the above-mentioned adsorption/desorption behaviors and bed hydrodynamics (e.g. pressure drops) in fixed and fluidized beds can help provide theoretical support for the optimization of adsorbers, desorbers and coolers, which has not been reported yet. Therefore, we for the first time investigated the CO2 adsorption/desorption behaviors as well as hydrodynamics in both fluidized and fixed beds using a solid amine sorbent in a temperature swing adsorption (TSA) experimental rig. The results show that the sorbent exhibits faster adsorption and desorption kinetics in fluidized beds than those in fixed beds, as reflected by the fitted parameters of the Avrami kinetic model and the change of bed temperature during adsorption and desorption tests. And the bed pressure drops in fluidized beds are also much lower than that in fixed beds, especially at high superficial gas velocities and high sorbent loadings. By further discussion, a fluidized bed can also achieve lower equipment investment and less sorbent loading compared to multiple fixed beds in parallel. This study demonstrates the advantages of fluidized beds over fixed beds in large-scale TSA units in view of higher reactor efficiency, lower energy consumption and equipment investment.

Effects of riser height and pipeline length on the transition boundary and frequency of severe slugging in offshore pipelines

Investigating the transition boundary and frequency characteristics of severe slugging under different riser heights and pipeline lengths provides important support for the expansion of offshore oil and gas to deeper waters. The visualization research on air-water two-phase flow is carried out in a system with a horizontal pipeline length of 1687 m and riser heights of 16 m, 22 m, and 29 m, and flow pattern maps are plotted. Based on existing experimental data, the database of the flow pattern and slugging frequency in pipeline-riser systems with riser heights of 3–29 m, pipeline lengths of 10–1687 m, and pipe diameters of 25.4–50.8 mm is established. The long pipeline makes the gas pressure growth rate slower than that of the liquid pressure in the riser, which causes severe slugging occurring at higher superficial gas velocity. On the contrary, increasing the riser height causes the boundary of severe slugging to shift to a lower superficial gas velocity. The severe slugging length is greater in the higher riser and longer pipeline system, causing a smaller severe slugging frequency. A correlation for predicting severe slugging frequency at various riser heights, pipe lengths, and pipe diameters is established by employing gas and liquid Froude numbers.

Euler-Lagrange Simulations of Microstructured Bubble Columns Using a Novel Cutting Model.

In the concept of a microstructured bubble column reactor, meshes coated with catalyst can cut the bubbles, which in turn results in high interfacial area and enhanced interface hydrodynamics. In previous work, we developed a closure model for the fate of bubbles interacting with a wire mesh based on the outcomes of energy balance analysis. In this paper, the model is validated using Euler-Lagrange simulations against two experimental cases of microstructured bubble columns. Before validation of the model, the definition of the deceleration thickness, as used in the calculation of the virtual mass term, is refined to introduce the effects of liquid viscosity and wire thickness. Proceeding with the validation, the inclusion of our cutting closure model results in an excellent match of the bubble size reduction by the wire mesh with the experimental data. Consequently, the simulations produce a more accurate prediction of the reactor performance for the gaseous reaction in view of the pH and gas holdup profiles. The effect of liquid viscosity on the bubble size reduction by the wire mesh is replicated accurately as well. Noticeably, the significance of bubble coalescence and breakup in bubble dynamics overperforms the role of bubble cutting at high superficial gas velocities; thus, further improvement is needed there. Finally, based on the validated cutting model, we share some perspectives on the design of wire meshes to increase the bubble interfacial area.

Bubble dynamics properties of B-particles in a quasi-2D gas-solid fluidized bed: Computational particle fluid dynamics numerical simulation and post-processed by digital image analysis technique

Bubble dynamics properties play a crucial and significant role in the design and optimization of gas-solid fluidized beds. In this study, the bubble dynamics properties of four B-particles were investigated in a quasi-two-dimensional (quasi-2D) fluidized bed, including bubble equivalent diameter, bubble size distribution, average bubble density, bubble aspect ratio, bubble hold-up, bed expansion ratio, bubble radial position, and bubble velocity. The studies were performed by computational particle fluid dynamics (CPFD) numerical simulation and post-processed with digital image analysis (DIA) technique, at superficial gas velocities ranging from 2umf to 7umf. The simulated results shown that the CPFD simulation combining with DIA technique post-processing could be used as a reliable method for simulating bubble dynamics properties in quasi-2D gas-solid fluidized beds. However, it seemed not desirable for the simulation of bubble motion near the air distributor at higher superficial gas velocity from the simulated average bubble density distribution. The superficial gas velocity significantly affected the bubble equivalent diameter and evolution, while it had little influence on bubble size distribution and bubble aspect ratio distribution for the same particles. Both time-averaged bubble hold-up and bed expansion ratio increased with the increase of superficial gas velocity. Two core-annular flow structures could be found in the fluidized bed for all cases. The average bubble rising velocity increased with the increasing bubble equivalent diameter. For bubble lateral movement, the smaller bubbles might be more susceptible, and superficial gas velocity had a little influence on the absolute lateral velocity of bubbles. The simulated results presented a valuable and novel approach for studying bubble dynamics properties. The comprehensive understanding of bubble dynamics behaviors in quasi-2D gas-solid fluidized beds would provide support in the design, operation, and optimization of gas-solid fluidized bed reactors.

Effect of inlet gas and liquid velocity profiles on one-group interfacial area transport equation in a vertical large rectangular channel

Bubbly flows appear in many heat and mass transfer equipment. The interfacial area concentration is a critical parameter in determining the performance of the heat and mass transfer systems. The one-dimensional one-group interfacial area transport equation (IATE) was developed based on area-averaged bubbly flow data collected under uniform inlet flow boundary conditions. The applicability of the IATE to the flow systems with non-uniform inlet flow boundary conditions was not comprehensively examined. This study investigated the effect of inlet liquid and gas velocity profiles on measured interfacial area concentration changes along the flow direction. The data used in this study were collected for vertical upward air-water two-phase flow in a rectangular channel with a gap length of 10 mm and width of 200 mm. The test conditions were the superficial gas velocity ranging from 0.0870 to 0.288 m/s and the superficial liquid velocity ranging from 0.515 to 2.53 m/s. The interfacial area concentrations with uniform and non-uniform inlet flow boundary conditions measured in a large vertical rectangular channel were compared under the same area-averaged flow rate conditions. Then, the predictive capability of the IATE to the bubbly flow test data with uniform and non-uniform inlet flow boundary conditions was assessed. Some non-uniform inlet flow boundary conditions, including channel-center peaking of liquid flow (CPF), channel-center peaking of gas flow (CPG), single-side-wall peaking of liquid flow (SPF), and single-side-wall peaking of gas flow (SPG), significantly affected the interfacial area concentration changes along the axial location at relatively high superficial gas or liquid velocity conditions. The IATE's predictive capability deteriorated for several non-uniform inlet flow boundary conditions, including double-side-wall peaking of gas flow (DPG) and single-side-wall peaking of liquid flow (SPF), specifically at relatively high superficial liquid velocity conditions. The maximum percentage error of the interfacial area concentration prediction by the IATE for these non-uniform inlet flow boundary conditions reached 75%.

Effects of superficial gas velocity on the performance of an air-lift internal circulation partial nitrification-anammox granular sludge reactor

The airlift internal circulation reactor for partial nitrification-anammox (PNA-ALR) has the advantages of a small footprint, high mass transfer efficiency, and the ease of formation of granular sludge, thus making it an effective biological treatment for ammonia-containing wastewater. Although superficial gas velocity (SGV) is an essential parameter for PNA-ALR, it is unclear how the magnitude of SGV impacts nitrogen removal performance. In this study, the nitrogen removal efficiencies of five PNA-ALRs with different SGV were measured during feeding with synthetic municipal wastewater. At an optimal SGV of 2.35 cm s−1, the PNA-ALR consistently maintained the total inorganic nitrogen (TIN) removal efficiency at 76.31% and the effluent TIN concentration was less than 10 mg L−1. By increasing or decreasing the SGV, the nitrogen removal efficiency decreased to a range between 30% and 50%. At lower SGV, the dead space in the PNA-ALR was increased by 21.15%, and the feast/famine ratio of sludge increased to greater than 0.5, which caused a disruption in the structure, and a large loss of, granular sludge. Computational fluid dynamics (CFD) simulations showed operation at a higher SGV, resulting in excessive shear stress of 3.25 N m−2 being generated from bubble rupture in the degassing section. Fluorescent staining determined a decrease of 26.5% in viable bacteria. These results have improved our understanding of the effects of SGV on a PNA-ALR during mainstream wastewater treatment.

Dynamic stability of cantilevered piping system conveying slug flow

In this paper, the dynamics and stability of a cantilevered piping system conveying slug flow are examined using the stable slug flow model and hysteretic model. The truncation technique of Galerkin and eigenvalue analysis are used to discrete and solve the model equations. The Argand diagrams and stability maps are employed to investigate the effects of superficial gas and liquid velocities and structural damping on the dynamics and stability of the system. The results showed that the increase of superficial gas velocities have a negative influence on the stability under the condition that the dimensionless liquid velocity is caused by the pipe length. In the process of the increase of the superficial gas velocity, the critical flutter velocity reduced and the system underwent several phenomena such as instability-restabilization-instability and mode exchange. However, the superficial liquid velocities have a positive effect on the stability, and the critical flutter velocities increased with it, which resulted in the stability boundary moving upward. It should be pointed out that its increase made the stability of the 4th mode worse and the vibration became more complex. Additionally, the structural damping could improve the stability of the high superficial gas velocity system. This work is helpful to design operating parameters to ensure the safety of the piping system.

Study of sparger design effects on the hydrodynamic and mass transfer characteristics of a D-shape hybrid airlift reactor

This study investigated the effect of sparger designs on a D-shape hybrid airlift reactor (DHALR) performances. The designs of airlift and torus reactors were combined to get the hybrid configuration. Experiments were carried out over a range of superficial gas velocities from 0.86 to 5.10 × 10−2 m/s, using three different spargers: single orifice (S-SO), multiple orifices (S-MO) and flexible membrane (S-FM). The findings highlighted that the S-FM sparger performed much better than the other spargers. An improvement in the gas holdup, εG, volumetric mass transfer coefficient, kLa, liquid circulation velocity, UL, and mixing time, tm, was observed and was attributed to the bubble size distribution enhancement. The S-FM sparger generates smaller and evenly distributed bubbles. However, it increases the axial dispersion coefficient and therefore the backmixing extent. It was demonstrated that even if S-FM leads to more pressure drop at the sparger level, it remains the most efficient in terms of power saving for a given mass transfer rate. The results demonstrated that the sparger has less effect at high superficial gas velocities in the DHALR. The flow inside the DHALR was modelled by the Axial Dispersion and the Tank in Series Models. The results show that the DHALR has a plug-flow-like behaviour.

The compensation of micro-carriers on hydraulic shear force during aerobic sludge granulation

Hydraulic shear force (HSF) is of great importance in granulation and treatment performance of aerobic granular sludge (AGS) process. High HSF has been shown to be conductive to granulation, which could be obtained by high superficial gas velocity (SGV). Micro-carriers could influence the flow regime in reactors. Thus, AGS system with micro-carriers addition could obtain higher HSF. In this paper, impact of SGV on granulation under micro-carriers enhancement was comparatively evaluated by keeping SGV at 0.6 cm/s (R1) and 1.2 cm/s (R2), respectively. Results showed that sludge granulation and nutrient removal performance was greatly enhanced by micro-carriers under low SGV. However, microorganisms were suppressed inside micro-carriers under high SGV, although the extracellular polymeric substances (EPS) contents in R2 were higher. The gas holdup in R2 was 1.32 times higher than traditional AGS system, resulting in excessive collision and velocity field difference. Overall, addition of micro-carriers could compensate the HSF under low SGV, which is of great significant in reducing the energy consumption of AGS system.

Experimental and modeling studies of gas-liquid flow in vertical pipes at high superficial gas velocities

In many oilfield applications, gas-liquid (GL) flow modeling is required. One of these applications is predicting the uncontrolled hydrocarbon flow through a wellbore after a blowout, referred to as the worst-case discharge rate (WCD rate). It is common to see annular flow patterns in a wellbore during a blowout, especially when the superficial gas velocity (Vsg) is high and the superficial liquid velocity (Vsl) is low to moderate. As a result, accurate predictions of WCD rates require a better understanding of annular flow characteristics. Thus, this study is conducted with an annular flow pattern, maintaining Vsg between 8 and 120 m/s and Vsl between 0.25 and 0.96 m/s.The analysis of the experimental measurements shows the establishment of an annular flow pattern with pressure drop (Δp) that increases with the in-situ Vsg at a constant Vsl. Moreover, the liquid holdup (HL) reduced sharply with the Vsg at a constant liquid rate. Furthermore, a new mechanistic GL flow model is formulated based on an existing model and correlations. Finally, measurements are compared with the forecasts of the new and existing models and commercial software. It is shown that the new model provides a more precise forecast of Δp than the existing software and models.

Experimental investigation on drying performance of pharmaceutical granules in a pulsation‐assisted fluidized bed dryer

AbstractFluidized bed drying has been widely employed in pharmaceutical manufacturing processes. Due to the considerable cohesiveness of wet pharmaceutical granules, channelling phenomena pose significant challenges to fluidization and drying. In this work, the drying performance of pharmaceutical granules in a pulsation‐assisted fluidized bed dryer was experimentally investigated. The drying rate and energy efficiency were investigated with representative pharmaceutical powders, including active pharmaceutical ingredients (APIs). It is found that the pulsed airflow is effective in enhancing the drying rate at higher superficial gas velocity. A lower pulsation frequency is more favourable to improve the drying rate. During the constant rate period, energy efficiency is between 60% and 45% for the drying process, while the energy efficiency decreases to 10% during the falling rate period. A pulsed fluidized bed dryer has shown a higher energy efficiency than a conventional one with a constant air flow. Among nine thin‐layer drying models examined in this work, the Midilli and Kucuk model has shown the best agreement between the experimental data and the predicted results.

Effect of Oil Viscosity on Flow Pattern Transition of Upward Gas-Oil Two-Phase Flow in Vertical Concentric Annulus

Summary Gas-oil two-phase flow usually occurs during gas influx into the wellbore annulus of deepwater drilling with oil-based drilling fluid or gas-oil production from the wellbore annulus. A flow pattern transition often is used in well control or in production parameter design. Related studies primarily have focused on gas-water two-phase flow, which is significantly different from gas-viscosity oil two-phase flow. We conducted experiments on a facility that included a flow section that was 13 000 mm long, with a 60-mm inner pipe and a 100-mm outer pipe in the annulus. A range of oil viscosities from 16 to 39 mPa∙s has been studied. The superficial gas and oil velocities varied from 0.55 to 17.077 m/s and from 0 to 0.414 m/s, respectively. A flow pattern was identified based on visualized analysis using electrical capacitance volume tomography (ECVT) and void faction wave analysis. Flow pattern maps of different viscosities were plotted based on measured data. Transition criteria based on Froude number and Archimedes number were established. The new model was compared with gas-water two-phase flow transition models and validated with experimental data. The effect of viscosity on flow pattern transition was revealed. The transition boundary of the bubble to slug flow and slug to churn flow both moved in the direction of smaller superficial gas velocity with an increase in oil viscosity. There may exist a critical viscosity value when churn flow transited to annular flow. When the viscosity was lower than this critical point, the above result was the same for churn to annular flow when the superficial oil velocity was low. With an increase in the superficial oil velocity, however, the boundary gradually changed to high superficial gas velocity as the oil viscosity increased. When the viscosity was larger than the critical point, the oil viscosity had a slight influence on the transition.

CFD-simulation of gas-solid flow in bubbling fluidized bed reactor

The hydrodynamic behaviour of gas-solid flow has been studied in a two-dimensional bubbling fluidized bed reactor filled with particles of 275 µm in size, using computational fluid dynamics (CFD). The two-fluid model (TFM) approach based on the concept of Eulerian–Eulerian in combination with Kinetic Theory of Granular Flow (KTGF) is used to represent the fluid mechanics involved in the flow. The computational implementation was realized by the commercial software ANSYS FLUENT. Interphase momentum exchange between gas and solid phases were calculated using the modified Syamlal O’Brien drag force function. The simulation predictions have been comprehensively validated with experimental data measurements of bed expansion rate, pressure drops and average gas volume fraction profiles. The pressure drops predicted by the simulations were in comparatively strong agreement with the experimental data at higher superficial gas velocities. The bed expansion ratio of the simulation results is closer to that of the experimental data. This work provides a scalable means to assist in the design and operation of fluidised bed reactors and fluidisation processes.

Investigation of mass transfer characteristics under turbulent condition in adsorption separation process for CO2 capture

Turbulence caused by high superficial gas velocity has significant impact on the CO2 capture process by adsorption. However, the poor consideration of turbulent diffusion in existing methods limits the further analysis of mass and heat transfer in CO2 adsorber. In this study, a novel computational fluid dynamics (CFD) model was introduced for CO2 capture with fixed bed, which is based on the methodology of computational mass transfer (CMT)[1]. Compared with traditional CFD models, it rigorously considers turbulent diffusion and can more accurately predict the mass and heat transfer behavior of CO2 capture processes at high gas superficial velocity while reducing computer resources. Through the comparison of simulation with breakthrough experiment, the feasibility of proposed approach was verified. In addition, the simulated results indicated that the turbulent mass diffusion causes a more uniform distribution of CO2 in the radial direction of adsorption bed. As Reynolds number decreases, turbulent mass diffusivity gradually weakens, and it could be neglected when the Reynolds number is below 10. Besides, adsorbent size and adsorber size was found to have significant influences on turbulent mass diffusion. They affect turbulent diffusion by changing the condition of fluid flow, then having influences on the mass transfer characteristics in CO2 capture process. Our results can provide a valuable guide for the structural design and optimization of industrial CO2 adsorber.

Quantitative comparison of measurement quality of cross-correlation based particle velocity instruments in different gas fluidization regimes

Particle velocity measurement based on cross-correlation algorithm is used widely in various multiphase flow systems. In this study, an optical fiber probe, i.e. Labasys® 100 from MSE Meili was used to systematically compare the qualities of its measured particle velocities in a pair of hydrodynamically distinct regimes, i.e. a fast fluidized bed (FFB) and a bubbling fluidized bed (BFB). Compared to FFBs, the measured particle velocities in BFBs show lower data repeatability, higher percentage of invalid data and higher sensitivity to the selection of the threshold value of maximum cross-correlation coefficient, which corresponds to much lower measurement qualities in this regime. Comparatively, higher measurement qualities were observed in the FFB based on the same evaluating methods. The highest measurement qualities appear in the core region of the riser and under high superficial gas velocities. A matchup relationship between the measurement quality and the shape of the probability density function (PDF) curves of maximum cross-correlation coefficient is found. Narrower PDF curves correspond to higher measurement quality and vice versa.

Microbubbles intensification and mechanism of wet air oxidation process of MDEA-containing wastewater

ABSTRACT In order to the intensification of gas–liquid mass transfer of MDEA-containing wastewater during wet air oxidation (WAO) process, the microbubbles and millimetre bubbles were applied by fine-pore sparger (5 and 20–30 μm) and single pore sparger (6.35 mm), respectively. Effect of the superficial gas velocity on the average microbubble size, gas holdup and oxygen mass transfer coefficient (KLa) of MDEA-containing wastewater at the ambient conditions was studied. The results showed that the microbubbles (less than 1 mm) were beneficial to enhance mass transfer process and had a higher dissolved oxygen concentration during WAO process of MDEA-containing wastewater owing to higher gas holdup and larger oxygen mass transfer coefficient. The COD removal ratio was 66% at low superficial gas velocity (ug = 0.3 cm/s) in WAO process by microbubbles, while it achieved at high superficial gas velocity (ug = 3.0 cm/s) by millimetre bubbles. The critical oxygen mass transfer coefficient KLa was 0.183 min−1 of MDEA-containing wastewater by 20–30 and 5 μm fine pore sparger, which was 2∼5 times more than that of single pore sparger (<0.1 min−1). The microbubbles could improve dissolved oxygen concentration and enhance the formation of hydroxyl radical at short time with atmospheric pressure. During the WAO process, the MDEA would be converted into intermediates including formic acid, acetic acid, ammonium, nitrite and nitrate. The WAO process with microbubbles could significantly improve the gas–liquid mass transfer performance at low superficial gas velocity and greatly reduce air consumption for MDEA-containing wastewater.

Lactate Metabolism and Microbiome Composition Are Affected by Nitrogen Gas Supply in Continuous Lactate-Based Chain Elongation

Chain elongation reactor microbiomes produce valuable medium-chain carboxylates (MCC) from non-sterile residual substrates where lactate is a relevant intermediate. Gas supply has been shown to impact chain elongation performance. In the present study, the effect of nitrogen gas (N2) supply on lactate metabolism, conversion rates, biomass growth, and microbiome composition was evaluated in a lactate-fed upflow anaerobic reactor with continuous or intermittent N2 gas supply. Successful MCC production was achieved with continuous N2 gas supply at low superficial gas velocities (SGV) of 0.22 m∙h−1. Supplying N2 at high SGV (&gt;2 m∙h−1) either continuously (2.2 m∙h−1) or intermittently (3.6 m∙h−1) disrupted chain elongation, resulting in production of short-chain carboxylates (SCC), i.e., acetate, propionate, and n-butyrate. Caproiciproducens-dominated chain-elongating microbiomes enriched at low SGV were washed out at high SGV where Clostridium tyrobutyricum-dominated microbiomes thrived, by displaying higher lactate consumption rates. Suspended growth seemed to be dominant regardless of SGV and gas supply regime applied with no measurable sludge bed formed. The highest MCC production from lactate of 10 g COD∙L−1∙d−1 with electron selectivities of 72 ± 5%was obtained without N2 gas supply at a hydraulic retention time (HRT) of 1 day. The addition of 5 g∙L−1 of propionate did not inhibit chain elongation, but rather boosted lactate conversion rates towards MCC with n-heptylate reaching 1.8 g COD∙L−1∙d−1. N2 gas supply can be used for mixing purposes and to steer lactate metabolism to MCC or SCC production.

Investigation of cold flow hydrodynamics in a dual fluidized bed for gasification of high-ash coal

The hydrodynamics of a compact dual fluidized bed (DFB) gasifier, consisting of a short riser and a bubbling fluidized bed (BFB), designed for high-ash coal, has been investigated. The effects of fluidization numbers from various aeration ports, and secondary aeration (SA) in the riser have been examined on solids circulation rate and solids holdup in the DFB. The performance of solids recycle systems has been investigated for on-bed and in-bed solids discharge from the BFB to the riser resulting in a low- and high- bed density operation in the riser. For high superficial gas velocity and high-density operation of the riser, an intense solids backmixing is noticed near the riser-exit, which decreases when SA is used. SA is found to be more effective in maintaining a proper balance between solids holdup and solids circulation rate when the bottom riser bed is operated in the fast fluidized bed regime for in-bed solids discharge.

Numerical analysis of bubble characteristics in a pressurized bubble column using CFD coupled with a population balance model

The effects of the coalescence coefficient and bubble breakup models used in the numerical modeling of bubble behavior are investigated in this study. Simulations show that variations in the system pressure and superficial gas velocity affect the gas phase density and distribution of “gas holdup” in the form of large and small bubbles in the bubble column. In a pressurized bubble column with a high superficial gas velocity, owing to the churn turbulence in the bubble column, the pressure is the primary parameter that causes bubble breakup; however, previous studies have not sufficiently described the treatment of bubble coalescence under this condition. Therefore, in this work, gas holdup in the form of small bubbles is introduced into Luo’s coalescence coefficient to improve the bubble coalescence model. This optimized bubble coalescence model was found to improve the accuracy of bubble size prediction in a pressurized bubble column.

An experimental study of liquid unloading in the curve section of horizontal gas wells

Liquid unloading is a very common and important issue in horizontal gas wells, and the presence of curve sections increases the complexity of the phenomenon and its study. Liquid loading in a gas well will sharply reduce production, therefore, the liquid-unloading onset of different curved pipes is essential to gas production. In this work, liquid-unloading onset experiments were conducted in curved pipes with different curvatures. Then, the critical gas velocityVsgCRcan be determined according to the measured pressure gradients, liquid holdup, and liquid film reversal. This work analyzes the factors which will lead to the liquid unloading and explores the trend of the pipe curvature’s influence on the liquid unloading under laboratory conditions. The experimental results show that the critical gas velocity rises with the increase of pipe curvature, the increase is mainly due to the centrifugal force. The present work also compares the predicted results of the OLGA model and Beggs–Brill model with experimental data. The comparison results indicate that both models fit relatively well to the experimental data at the low superficial gas velocity, and both models have poor performance at high superficial gas velocity. The OLGA model fits the experimental data better than the Beggs–Brill model at high superficial gas velocity. The error analysis shows that most of the predicted data is not in good agreement with experimental data. Some errors between experimental data and calculation results are out of the range of 50%.