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Experimental validation and CFD Simulation of a Gas-Liquid Cylindrical Cyclone (GLCC©)

José de Jesús Serrano Hernández

Chemical Engineering Department. Universidad de los Andes, Bogotá, Colombia

ABSTRACT This article constitutes an experimental and CFD simulation study of the separation performances of a Gas-Liquid Cylindrical Cyclone (GLCC) separator. The Oil & Gas industry has shown great interest in the GLCC because of their versatility and cost effectiveness compared to the conventional vessel-type separator. The aim of this study is to improve the understanding of the effects of the dynamic viscosity and surface tension of the liquid phase in the flow hydrodynamics and performance of the GLCC. To accomplish this, it was used three different liquid substances (mineral oil, ISOPAR L and water). The experimental data have been acquired including flow visualization and the amount of the Liquid Carry-Over (LCO) in the gas outlet stream. In the CFD simulation, the RST turbulence model was chosen for the prediction of the hydrodynamics behavior inside the GLCC because of the good agreement with the LCO experimental data (error < 25%). The increase of the viscosity and the reduction of the surface tension shows a reduction of the GLCC efficiency. The increase of viscosity rises the viscous dissipation energy that deforms the helical shape and generates a strong swirl decay in the downstream region. The reduction of the surface tension generates a weaker and longer wave-length of the helical shape whit less defined region for the upward flow and low-pressure zone in the vortex core.

Keywords: GLCC, tangential velocity, axial velocity, swirl flow, viscosity, surface tension, RST.

NOMENCLATURE

Subscripts 𝐺 𝐺𝑎𝑠 𝐿 𝐿𝑖𝑞𝑢𝑖𝑑 𝑖 𝑖)*𝑝ℎ𝑎𝑠𝑒 𝑎𝑣 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑆𝐺 𝐺𝑎𝑠𝑠𝑢𝑝𝑒𝑟𝑓𝑖𝑐𝑖𝑎𝑙 𝑆𝐿 𝐿𝑖𝑞𝑢𝑖𝑑𝑠𝑢𝑝𝑒𝑟𝑓𝑖𝑐𝑖𝑎𝑙 𝑂𝐸 𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙𝑒𝑛𝑣𝑒𝑙𝑜𝑝𝑒 𝑆 𝑆𝑙𝑢𝑔

Greek letters 𝜇 𝐷𝑦𝑛𝑎𝑚𝑖𝑐𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦, [𝑃𝑎 ∙ 𝑠] 𝜌 𝐷𝑒𝑛𝑠𝑖𝑡𝑦[𝑘𝑔 𝑚G⁄ ] 𝛼 𝑉𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛

Letters 𝑉 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦[𝑚 𝑠⁄ ] 𝑊 𝑇𝑎𝑛𝑔𝑒𝑛𝑡𝑖𝑎𝑙𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦, [𝑚 𝑠⁄ ] 𝕍 𝑉𝑜𝑙𝑢𝑚𝑒, [𝑚G]

OBJECTIVES To study the effect of the fluid properties, surface tension and liquid viscosity, in the hydrodynamic behavior and performance of the GLCC by CFD simulation with experimental validation. The specific goals are:

• To evaluate different turbulence models on CFD simulations and their relevance in the prediction of flow hydrodynamics.

• To compare the flow patterns, tangential and axial velocity profiles inside the GLCC.

1. INTRODUCTION In the past, the petroleum industry has relied mainly on the conventional vessel-type separator, which is bulky, heavy and expensive, to process well-head production of oil-water flow [1]. The hard-economic situation and the increasing number of offshore exploitations force the petroleum industry to seek less expensive, more efficient and compact gas-liquid separators [2]. The Gas-Liquid

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Cylindrical Cyclone (GLCC) is a viable alternative to the conventional gravity-based gas-liquid separator. The GLCC was first developed together by Tulsa University (OK, USA) and Chevron. Nowadays, there are more than 2000 GLCCs in the field and they have become increasingly popular [3]. As compared to vessel-type separators, the GLCC is a simple, low-cost and low-weight separator that is easy to install and operate, especially in offshore applications [4]. In addition, the cylindrical cyclone is more compact, smaller footprint, requires little maintenance and reduces the inventory of hydrocarbons significantly [1].

The GLCC consists on a cylindrical vertical pipe with a middle height tangential inlet and two outlets. The outlet at the top is the gas outlet and the other are for the liquid phase. It has neither moving parts nor internal devices. A schematic view of the separator is shown in Figure 1. The separation is favored due to the tangential inlet, which generates a swirl near the inlet of the GLCC. The swirl produces centrifugal and buoyancy forces on the fluids that are an order of magnitude higher than the force of gravity [5]. The combination of gravitational, centrifugal, and buoyancy forces push the liquid toward the wall and downward, while the gas flows to the cylinder axis, forming a gas filament which rejoins the vortex. Then, the density difference of the fluids makes the gas go up and the liquid go down [6].

The achieved efficiency of the separation in the GLCC is greater in comparison with the conventional separators. However, the cylindrical cyclone operation is limited by two physical phenomena: one is the Liquid Carry-Over (LCO), when some liquid flows in the gas stream and the other is the Gas Carry-Under (GCU), when some gas may be entrained in the liquid stream. Prediction of these two phenomena will allow proper design and operation of the GLCC in industry [7].

The GLCC has a wide range of potential applications in the industry, varying from partial separation to a complete phase separation, like flare gas scrubbing; sub-sea separation and slug flow in offshore platforms [8]. However, the GLCC most common utilization nowadays is the control of gas/liquid ratio upstream of flow meters or other equipment as pumps or de-sanders [2]. Furthermore, due to the growing energy consumption worldwide and the large reserves of heavy oil, the industry has been demanding to expand the use of cyclone technologies from light and medium oil-gas separator to heavy oil-gas separators [9]. The understanding of the

effect of using these substances in current separators has become an engineering challenge.

Figure 1. Schematic plot of a GLCC [10]

Despite its simplicity and its numerous advantages, the GLCC has not known yet a widespread success. In fact, a lack of understanding of the complex multiphase hydrodynamic flow behavior and the variability of the operating conditions inside the GLCC inhibits complete confidence in its design and its prediction of performances become very difficult. [1]. The progress in computer technology and the development of turbulence models, the Computational Fluid Dynamics (CFD) has gained more popularity in recent years [11]. Due to the complex hydrodynamics inside the cyclone, CFD can be a useful tool to understand the flow patterns and the efficiency of the separator. As it can be seen in the literature review, CFD modelling of the GLCC can be possible an it obtains reliable results in various investigations. One of main advantages of CFD modelling is feasibility to study different operational conditions, two-phase flow inter characteristics without the expenses of experimental setup and measurements.

2. FLOW HYDRODYNAMICS IN THE GLCC The hydrodynamics present in the GLCC have been studied separately for each regions of the cyclonic separator. In the upper part, liquid droplets contained in the gas stream are centrifuged toward the walls, and coalesce into a liquid film [12]. Therefore, the liquid from

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the wall film falls by gravity into the liquid vortex. Additionally, an amount of liquid in the shape of an annular film is encountered just above the entrance nozzle [12]. This liquid film results from the impact between the inlet jet and the wall, this is refereed as Upper Swirling Liquid Film, USLF (Figure 2).

In the GLCC lower part, if the swirl intensity is high enough, the free gas-liquid interface gets carved out and the liquid vortex can be observed. The liquid flows from the inlet in a thin swirling film (Lower Swirling Liquid Film, LSF) as shown in Figure 2. In the vortex, large bubbles move toward the free interface due to buoyancy. Smaller bubbles are dragged downward by the liquid, while getting pushed radially toward the vortex center due to centrifugal forces, forming a bubbly filament, like the schematic representation in Figure 2 [12].

Figure 2. Schematic representation of the GLCC and it's entrance pipe in a full separator configuration [2]

The characteristics of the hydrodynamics inside the GLCC depend mainly on the two-limiting phenomena (LCO and GCU), the swirl flow, the viscosity effect, the flow regime in the inlet and upper section and the surface tension of the liquid phase with the air.

2.1. The two-limiting phenomena The LCO is defined as the amount of liquid that flows with the gas and move up toward the gas leg through the top outlet [13]. This phenomenon can occur as the form of droplets or stratified flow in the gas leg [7], when the liquid and gas flow rates couple exceed the limit tolerated by the system, which is the operational envelope line [12].

The mechanism responsible for LCO are the churn and annular flow occurring in the upper part of the GLCC, as shown in Figure 3. At relatively high liquid and low gas flow rates (𝑉NO < 3𝑚/𝑠), the liquid churns up and down in the upper part of the cylindrical cyclone. On other hand, at relatively high gas (𝑉NO > 6𝑚/𝑠), and low liquid flow rates, the flow pattern in the upper part of the GLCC is annular [7].

The percent of LCO is the percentage of the inlet liquid volumetric flow rate, which is carried over with the gas stream. In the churn region, large quantities of liquid can be carried over relatively easily, because of the lines are very close to each other, as shown in Figure 3 [7].

Figure 3. Operational envelope and flow pattern regions of churn and annular flow in a GLCC. [7]

On the other hand, under churn flow condition, the liquid level in the GLCC is above the inlet and the liquid is carried over through the gas leg in form of stratified flow. Moreover, under annular flow condition, the liquid level is below the inlet and the liquid is carried over in form of droplets, as shown in Figure 4 [7].

The other phenomena, the GCU, is defined as the amount of bubbles in the discharge liquid stream at the bottom. The magnitude of the GCU is affected by several factors such as the length of the lower segment, the dimensions of the inlet and the force vortex [14]. Furthermore, three mechanisms have been identified as possible ways by which the GCU occurs: presence of shallow radial trajectory of small individual bubbles preventing from coalescing with the gas-core filament; gas-core filament instability; and a bubble swarm instability [8].

L

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Figure 4. LCO in churn (A) and annular (B) flow conditions [7]

2.2. The swirl flows Swirl flows are generated by imparting a tangential component to an axial flow, resulting in a helical winding of the streamlines [10]. In this case, the tangential injection generates the swirl flow, which decreases due to viscous dissipation downward the cyclonic separator [12].

The swirl characteristics depend on the Reynolds number and on the swirl intensity (tangential and axial velocity, see the section 2.2.1 and 2.2.2) [10]. If the flow enters the GLCC body with a mid-high velocity, then a swirling motion begins promoting an inertia-dominated separation process. The centrifugal and buoyancy forces drive the gas toward the body centerline and upward to the top, while impelling the liquid toward the wall and bottom of the cylindrical body. However, the excessive inflow velocity may decrease the separator performance since it also promotes the turbulent mixing of the phases [15].

2.2.1. Tangential velocity The separation between the liquid and gas phase inside the GLCC depends on the tangential velocity of the fluids that promotes the centrifugal forces acting on the gas liquid phases [9].

The typical mean tangential velocity profile is shown in Figure 5. There are divided in three regions. First one, a

core region where the tangential velocity profile corresponds to a forced vortex, this velocity distribution has a strong stabilizing effect. Second one, an annular region where the profile corresponds to a free vortex, it is separated from the core region by a transition zone. The centrifugal action is destabilizing in this region, the skewness of the flow is very high, and the turbulence is highly anisotropic [10]. Finally, the wall region consists in a small layer near the wall where tangential velocity decreases with a steep gradient to be equal to zero at the wall.

Figure 5. Typical average tangential velocity profile in swirl flows. [10]

2.2.2. Axial velocity The axial velocity profile more likely to be found in the GLCC lower part can be described by the regime presented in Figure 6. If the swirl intensity is high enough the flow reverses near the vortex center. In fact, due to centrifugal forces, a radial pressure gradient develops in a low-pressure region around the vortex center. [10]. Moreover, going down-stream, the pressure in the core region increases due to swirl decay.

Figure 6. Average mean axial velocity profiles in the GLCC lower part, including a single flow reversal [2]

V

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This flow field is considered beneficial for bubbles separation in the GLCC because the centrifugal forces push the bubbles radially toward the vortex center. Once the bubbles reach the flow reversal region, the upward moving liquid carries them back toward the free interface where they can disengage [10]. However, that principle does not hold for the high swirl intensity cases, where bubbles in the bubbly filament are brought toward the GLCC lower outlet and cause GCU [12].

2.3. The viscosity effects The increase of viscosity generates a rise of the equilibrium liquid level in the GLCC. This is due to the increase in frictional losses in the liquid level [8]. It is observed a continuous reduction of operational envelope with increase in liquid viscosity, causing earlier LCO at lower gas and liquid rates, by the increase of the shear stress which produces a higher dissipation of the vortex intensity. Therefore, diminishing the effect of the centrifugal forces over the phase separation [9].

Additionally, when the viscosity increases, the cyclonic separators performance is affected because the faster decay of the tangential velocity along the axial direction in the cyclone, affecting the centrifugal forces and increasing the GCU [9].

2.4. Flow regimes at the inlet and upper part The gas-liquid flow regimes in the inlet channel may influence significantly the hydrodynamics inside the GLCC. The flow patterns more common are shown in the Figure 7.

Figure 7. Sketches of flow regimes for gas-liquid mixtures in a horizontal pipe [16]

The slug flow regime consists of an intermittent flow regime, with large gas pockets entrapped between liquid plugs, and a continuous liquid phase in the lower part of the channel [2]. On the other hand, an annular flow regime is characterized by the presence of liquid film flowing on

the channel wall and with the gas flowing in the gas core. Furthermore, the stratified smooth regime has been identified when the two phases are completely segregated, and has been identified as wavy when bubbles and/or droplets are present [2].

Otherwise, the typical flow regimes present in the upper part of the GLCC are the churn and annular flow [7], as shown in Figure 8. The annular flow in a vertical pipe has the same characteristics in a horizontal pipe. Moreover, the churn flow is distinguished with irregular and relatively unstable slugs of gas move up to the center of the pipe, usually carrying droplets of the liquid phase with them. If the gas velocity increases, it changes to annular flow.

Figure 8. Typical gas-liquid flow regime in a vertical pipe [16]

2.5. Surface tension Surface tension is a fundamental property by which the gas-liquid interfaces are characterized and is defined as the force at right angle to any line of unit length in the surface [17]. The cohesive forces between the molecules are responsible for this phenomenon. These interactions happen between bulk liquid’s particles like attractive forces in all directions, creating a uniform field of force [17]. Nevertheless, the molecules of the liquid phase at the surface do not have neighbors in the upper part of them, therefore the direction of the net force field is directed towards the bulk, pulling them inwards and gaining resistance to being stretched of broken [18].

Furthermore, the inward net force contracts the surface and minimizes the superficial area of the volume of liquid. Consequently, the energy state of the liquid bulk is reduced, because the energy of the molecules on the interior is much lower than ones on the exterior region [18].

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3. LITERATURE REVIEW – PREVIOUS STUDIES

The GLCC has been specially studied by the University of Tulsa. In 1997, Erdal, Shirazi, Shoham, & Kouba, performed CFD simulations of single-phase and two-phase flow in several GLCC configurations, to compare with experimental data including tangential velocity profiles and tangential velocity decay [13]. They found that the high tangential velocity dissipates significantly at the inlet region and the decay of the tangential velocity continues at a lower rate downstream in the axial direction [13]. Then, in 1998, Shoham & Kouba, summarized experimental and simulations studies [5]. They found that the inlet section determines the incoming gas/liquid distribution and the initial tangential-inlet velocity in the GLCC [5].

Additionally, they found that LCO is largely dependent on the flow pattern in the upper part. About the GCU, they identified three mechanisms as possible contributors to that phenomena, but they are not studied at all. After that, in 2000, Movafaghian, Jaua-Marturet, Mohan, Shoham, & Kouba, developed an experimental setup to study the effects of geometry, fluid properties and pressure on the flow hydrodynamics [8]. Additionally, the data obtained were used to refine an existing mechanistic model [8].

On the other hand, Gomez, Mohan, Shoham, & Kouba, enhanced a mechanistic model for prediction of the hydrodynamic flow behavior in a GLCC [1]. The main enhancement that they incorporated was a flow pattern dependent nozzle analysis of the cylindrical cyclone inlet prediction of the gas and liquid tangential velocities at the GLCC entrance. Besides, Chirinos developed and mechanistic model for the prediction of the percent LCO beyond operational envelope for churn flow conditions [7]. Furthermore, they extended an existing model to LCO under high-pressure conditions. They compared the new mechanistic model with the experimental data, obtained good agreement [7].

Later, Erdal, Shirazi, Mantilla, & Shoham, investigated the behavior of small gas bubbles in the lower part of the GLCC, below the inlet and the related GCU phenomena by CFD simulations [19]. They quantified the effects of important parameters on bubble carry-under, like bubble size, viscosity, Reynolds number and inlet tangential velocity [19]. Next, in 2004, Erdal & Shirazi, conducted local measurements of tangential velocities and turbulent

kinetic energy across the cylinder diameter to understand the swirling flow behavior [6]. They found that the axial downward flow has a helical path near the wall and a forced vortex occurs near the center of the cylinder [6]. Furthermore, the data obtained were compared with CFD simulations with different turbulence models with good agreement.

Furthermore, in 2006, Reyes M. A., Rojas, Marín, Meléndez, & Colmenares, developed a numerical analysis for air-water mixtures, while both liquid and gas flow rates were changed [15]. The two-phase flow behavior was modeled using an Eulerian-Eulerian approach. They obtained a satisfactory agreement between the vortex free surface in the lower part and the mean angular velocities [15]. Then, in 2008, Molina, developed and studied experimentally a GLCC with an annular film extractor under low and high-pressure conditions [3]. Additionally, they found that for low pressures, the modified GLCC can remove all the liquid from the gas stream and for high pressure the GLCC had a good separation efficiency [3]. After that, in 2009, Brito & Trujillo, developed a bibliographical research to analyze the effect of viscosity in the performance of separators [9]. They found that the separation efficiency is diminished with higher viscosities [9].

In addition, in 2011, Hreiz, Gentric, & Midoux, developed a CFD model to study the swirling flow. The results were validated with experimental data of Erdal & Shirazi, experiments developed in 2004 [10]. They found that the 𝑘 − 𝜀 model performs the best for predicting the local mean axial and tangential velocities [10]. Afterwards, in 2014, Hreiz, Gentric, Midoux, Lainé, & Fünfschilling, developed an experimental analysis of the flow hydrodynamics. They found very complex results as the vortex core varies between laminar and turbulent state, and this flow structure appears for high swirl intensities and is associated with good separation performance of the cyclone [12]. As a conclusion, they propose the use of multiple tangential inlets to improve the separation efficiency [12].

Furthermore, in 2016, Kanshio, Yeung, & Lao, studied the structure of phase distribution and liquid holdup in gas discharge section of a GLCC using a wire mesh sensor (WMS) [20]. They found a significant variation in liquid holdup during LCO phenomenon due to the intermittent nature of the liquid flow in the gas outlet [20]. Afterwards, Sy, studied the changes of flow pattern with respect to different inclined angles and flow conditions [21].

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Furthermore, the distribution of radial, axial and tangential velocity profiles and their maximum magnitudes with respect to the change of inlet angle were carefully considered in his study [21]. He found that the maximum radial velocity is increased when the inlet angle is increased, however, increasing excessively the inlet angle results the GCU phenomena. Appendix A summarizes the previous works reported before.

4. MATERIALS AND METHODS

4.1. Experimental setup The experimental data used in this project were collected at Universidad de los Andes, to validate the CFD model under full separation configuration. The GLCC loop is used to bring together data of the quantity of LCO, the visualization of flow patterns in the inlet section and the pressure values at both outlets [22].

Figure 9. Schematic experimental setup

The liquid substance was stored in a 100 L tank, which is connected to a 0.5 HP Pedrollo centrifugal pump. Then, it passes through a gate valve to regulate the flow and it is measured by a Hedland flowmeter (FS102), with a range of 2 to 15 LPM. The air comes from a pressurized line and it is measured and regulated by a Dwyer flowmeter (FS101), with a range of 10 to 100 LPM. After that, the air and the water are mixed in a T-shape mixer.

The liquid-air flow goes into the GLCC through a transparent acrylic pipe of 2 m of length and 1.4 cm of inner diameter with a 0° angle of inclination, ensuring a tangential inlet. The inlet is located at the middle height of the separator which has a height of 1.6 m and an inner diameter of 4.2 cm. Then, the two-phase flow is separated

in the GLCC and the liquid goes to another storage tank through the bottom outlet. At the top outlet, there is a liquid trap which contains a grid to avoid that small liquid droplets leave the system with the air [22].

The gate valve installed at the bottom outlet is used to control the pressure and the liquid level inside the GLCC. Furthermore, there are two pressure sensors at the outlets of the GLCC. These are Rosemount 3051 pressure transmitters (PS101 and PS102) that are connected to a Keysight triple output DC power supply that works at 24 V. These pressure transmitters are used to define the outlet boundary conditions used in the CFD simulation. Images of the experimental setup are shown in Appendix B.

As the main goal of the experimental setup is to obtain data for the validation with simulation results, the LCO was selected as the variable of interest, because the simplicity in its measurement. The LCO data are collected with different values of viscosity and surface tension, three different liquid phases were used: water, mineral oil and ISOPAR L. Furthermore, in order to obtain a measurable quantity of LCO, values of superficial velocities of the liquid phase are defined, and the superficial gas velocity is slowly increased until LCO is evidenced. Meanwhile, the liquid level inside the GLCC is maintained approximately equal with the different liquid phases owing to the bottom valve opening control.

In addition, the conditions determined experimentally are shown in Table 1. The deviations of the inlet flows were taken from the flowmeters manuals and the pressure deviations were calculated with the observed experimental values. The gas outlet pressure is equivalent to the atmospheric pressure in Bogota, Colombia.

Table 1. Inlet and outlet conditions.

Liquid inlet flow [LPM] 15 ± 1 Air inlet flow [LPM] 20 ± 1

Gas outlet pressure [psia] 10.82 ± 0.03 Liquid outlet pressure [psia] 12.2 ± 0.05

4.2. Flow pattern visualization A transparent GLCC made of acrylic was used to observe the flow patterns at the inlet and throughout the separator. Moreover, the flow profiles were captured and recorded with a high-speed camera with 240 and 500 fps video format.

Pump

Liquid

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4.3. CFD Simulation The CFD simulation program used was STAR-CCM+ v12.04 [23], and in the next sections the main features about the configuration used for the simulation are described:

4.3.1. Geometry

For this study, a diameter of 4.2 cm and a total height of 1.6 m is proposed for the cyclonic separator. The tangential inlet pipe used includes the liquid-air mixture section and it has a total length of 2.213 m. The radius of the outlets is 2.17 cm and the length of each is 33 cm. The final geometry used with different views are shown in Figure 9.

Figure 10. Lateral view, frontal view and top view of the geometry for the GLCC

4.3.2. Mesh

A polyhedral grid is selected in STAR-CCM+. This grid is relatively easy and efficient to build and contain five times fewer cells than a tetrahedral mesh for a given starting surface [23]. To improve the accuracy of the flow solution near to the wall, the prism layer model is selected. This layer is critical in determining not only the forces on walls, but also drag and pressure drop. Accurate prediction of these flow features depends on resolving the velocity gradients normal to the wall [23]. This model, generates orthogonal prismatic cells next to wall surfaces of boundaries [23]. With the purpose of improving the overall quality of the existing surface and optimize it for the volume mesh models, the surface remesher model is selected [23]. Based on the grid independence test done by Berrio, [24] the mesh generated has 798307 cells (see Figure 11) and it was not repeated on this study.

Figure 11. Mesh done for the experimental GLCC

4.3.3. Boundary conditions The inlets before the mixing zone are defined as a velocity inlet. The velocity and the volume fraction are defined in each phase. It is assumed that both phases input with a volume fraction equal to 1. The values of velocity inlet for each phase were determined from the measurements of the respective flowmeters and the value of the cross-sectional area of the inlet pipe. The gas and liquid outlet are modelled as pressure outlets. In each exit, the gauge pressure measured is stablished with the values obtained by the pressure sensors in the experimental setup. The other boundaries are settled as walls with no-slip condition.

4.3.4. Physical models Due to the characteristics of the phenomenon in study, a three-dimensional case is established. The implicit unsteady model is selected since the properties in the system variate with time and is the only model available with segregated flow models. This model solves each of the momentum equations, one for each dimension, with a predictor-corrector approach as a linkage with the continuity equations [25]. When the implicit unsteady model is used, it is important to assure that the Courant-Friedrich-Levy number is below to 1 to obtain reliable results. This number associate the time-step, the velocity and the space between cells. Furthermore, the gravity is enabled as it plays an important role in the separation principle of the equipment. On the other hand, to reduce the computational effort and due to low velocities presented inside the pipe, both phases were modeled with constant density.

Additionally, to model the two-phase flow phenomenon, the Volume of Fluid (VOF) is used. This model requires

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the Eulerian multiphase model, which is the most common mathematical model used to describe a multiphase flow [24], to examine the effects of the interaction and interface between the gas and liquid phases [13]. In this method, a single set of momentum and turbulence equations are considered for the continuous phase, and the dispersed phase is modeled using transport equation for its volumetric fraction [26]. The VOF model is used for immiscible fluids on numerical grids capable of resolving the interface between the phases of the mixture. The model assumes that all the phases share velocity and pressure fields [23]. The equations that describes the VOF model are shown in equations 1 to 4.

𝜌 =`𝜌a𝛼aa

(1)

𝜇 =`𝜇a𝛼aa

(2)

𝛼a =𝕍a𝕍 (3)

𝑑𝑑𝑡c𝛼a𝑑𝕍𝕍

+c𝛼ae𝑣 − 𝑣Of ∙ 𝑑𝛼g

= ch𝑠ij −𝛼a𝜌a𝐷𝜌a𝐷𝑡

k𝑑𝕍𝕍

(4)

From previous equations, the sub index 𝑖 represents the 𝑖)* phase, 𝜌 the density, 𝜇 the viscosity, 𝛼 the volume fraction, 𝑠i the source or sink and 𝐷𝜌a/𝐷𝑡 is the material derivative of the phase density [23]. For the VOF model, the phase interaction must be specified. In this case, the surface tension effects are considered.

The Continuum Surface Force (CSF) model is used to calculate the surface tension force, using as primary phase the liquid and a secondary phase the air [27]. However, the problem with CSF model is the generation of parasitic currents; these are unphysical currents generated by a discontinuity in the domain, such as an interface, and cannot be solved using finer mesh [28]. The Interface Momentum Dissipation (IMD) model is available to reduce those currents adding an extra dissipation near the free surface; this dissipation is proportional to the velocity gradients and an interface artificial viscosity [23]. In the present study, a value of 0.1 is used because the good results obtained by Pineda [27].

Another parameter of the VOF model that is going to take in account is the sharpening factor. This number is used to reduce the numerical diffusion in the domain adding an extra term in the VOF transport equation [23]. This factor varies from 0.0 to 1.0, so the highest value is going to be use in the present work, to obtain a sharper interface [27].

As the flow inside the separator is in turbulent regime, the turbulence model used has a significant importance. The models more used are 𝑘 − 𝜖, 𝑘 − 𝜔 and Reynolds Stress Transport (RST). For the strong swirling flow in a cyclone, the 𝑘 − 𝜖 model based on eddy-viscosity approach fails to predict the flow behaviors well. Therefore, the RST is selected to capture the anisotropic character of the turbulence in the cyclone [9]. This type of model has the potential to predict complex flows more accurately than eddy-viscosity models because the transport equations for the Reynolds stresses naturally account for the effects of streamline curvature, swirl rotation and high strain rates [23]. However, the use of RST showed only modest improvement over the 𝑘 − 𝜖 model prediction of the flow field in the GLCC [19]. Nevertheless, most of the previous studies worked with a two-phase flow of water and air, therefore in the present study is going to be tested the three models due to the use of higher viscosity liquids.

In addition, the properties of the fluids used in the simulation are described in the Table 2. The density data of the liquid phases is not considered in the subsequent analysis because do not correspond with the objectives of this study.

Table 2. Physical properties of the fluids used in this study

Properties of fluids

Water Mineral Oil

ISOPAR L Air

Density [𝒌𝒈/𝒎𝟑] 997.6 863.1 770 1.18

Dynamic viscosity [𝟏𝟎𝟒𝑷𝒂 ∙ 𝒔]

10 318.8 13.5 0.186

Surface tension [𝑵/𝒎]

0.073 0.033 0.025 -

5. RESULTS AND ANALYSIS

5.1. Selection of the more accurate turbulence model

Axial velocity profile

The turbulence model’s comparison was made between the realizable 𝜅 − 𝜖, SST 𝜅 − 𝜔 and the RST. The first variable analyzed was the axial velocity profile in the region below the inlet, to see the vortex core and the strong change in the downward and upward flow because of the tangential inlet. The positive values of the axial

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velocity indicate the upward flow and the negative values the downward flow.

Figure 12. Axial velocity profile below the inlet. a) realizable 𝜅 − 𝜖, b) SST 𝜅 − 𝜔, c) RST

As shown in Figure 12, the realizable 𝜅 − 𝜖 and the SST 𝜅 − 𝜔 models predicts approximately the same axial velocity profile. However, the RST model predicts higher values for the downward flow and upward flow in the right side of the cyclone too. Besides, the RST model show a helical shape of the upward flow in the axial axis that the other models cannot predict. In addition, RST model predicts much stronger decay in the upward flow region. Flow pattern in the inlet pipe The pattern at the entrance of the cyclone presents several differences according to the model chosen for the simulation (Figure 13). The realizable 𝜅 − 𝜖 predicts an intermediate between a wavy and slug flow pattern.

Figure 13. Flow inlet pattern (A) Realizable 𝜅 − 𝜖, (B) SST 𝜅 −𝜔, (C) RST

The SST 𝜅 − 𝜔 model show a wavy pattern and the RST model displays an annular-slug flow pattern, which agrees with the experimental behavior, as shown in Figure 14.

Figure 14. Experimental Flow inlet pattern.

Amount of LCO and flow pattern in the top outlet

Another variable to compare with the experimental data is the amount of LCO. The experimental results obtained of the LCO by using mineral oil as the liquid phase are shown in Table 3. The last row is the average and the standard deviation of each data column. It can be seen that the data of the pressures and LCO are replicable because the relative low variation in the measurement. Table 3. Experimental data of LCO

Pressure top outlet

[psia]

Pressure bottom

outlet [psia]

Delay time to first

LCO [s]

LCO [mL/s]

10.83 12.21 40 13.4 10.86 12.22 28 15.4 10.78 12.24 34 15.1 10.84 12.19 63 15.3 10.81 12.20 38 14.3 10.82 12.21 68 14.3 10.83 12.18 48 15.6 10.83 12.24 40 14.1 10.82 12.23 44 14.2 10.84 12.22 40 14.1

10.83±0.02 12.21±0.02 44±12 14.6±0.7 The comparison between the different turbulence models used in the CFD simulation with the experimental data are shown in Table 4. Table 4. LCO data of CFD simulation with different turbulence models

Turbulence model

LCO [mL/s] %Error

𝜿 − 𝝐 0.02 100 𝜿 −𝝎 0.02 100 RST 18.4 26

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The difference between the accuracy in the prediction of the hydrodynamics inside the GLCC is remarkable. The RST model predicts better than the other models with more viscous liquid because of the high anisotropic turbulence and the complex turbulent viscosity. However, the RST model requires more computer-time to solve the physics, but with the increase in the CPU power by the technology development it should not be an issue. Moreover, the 𝑘 − 𝜖 and 𝑘 − 𝜔 models need less computer-time but the gap between the simulation and experimental data are very large, therefore the accuracy of the hydrodynamic behavior estimation is insufficient. On the other hand, the three turbulence models predict the churn flow condition in the upper part of the GLCC, pattern that was expected according to the operational envelope done by Chirinos [7] and the velocity inlets values of each phase used. The experimental flow visualization shows similar irregular slugs of gas move up to the center of the separator, validating the agreement with simulated data, as shown in Figure 15.

Figure 15. Simulated churn flow in the GLCC upper part with (A) Realizable 𝜅 − 𝜖, (B) SST 𝜅 −𝜔, (C) RST and (D) Experimental visualization.

Therefore, the churn flow pattern was proved in the upper part of the GLCC, the work done by Chirinos demonstrate that LCO stratified regime happens [7]. Nevertheless, the RST model was the only that predicts the expected behavior, as shown in Figure 16. This one of the most important variables that need to agree with the experimental data and literature review, with the aim of predicts correctly the hydrodynamics behavior and the outlets conditions of the cyclonic separator.

Figure 16. LCO flow pattern prediction in the top outlet by the RST turbulence model.

Tangential velocity and swirl flow

Finally, the swirl flow is evaluated in a cross-section of the GLCC at 3 cm below the inlet section, as shown in Figure 17. Positive values of the tangential velocity represent the flow moving against clockwise and the negatives represent clockwise flow direction. It is important to remember that the tangential inlet is closer to the front wall of the cylindrical separator. The realizable 𝜅 − 𝜖 and the SST 𝜅 − 𝜔 predicts a similar swirl flow, with concentric streamlines around the axial axis. The tangential velocity is high near the wall region and it decays towards the center. This flow pattern evidences the desirable swirl flow, but the works done by Erdal, et al. demonstrate with experimental measures of tangential velocity that these models make an overestimation of that variables [13] [6] [19]. Instead, the RST model does not predict that behavior, and show two vortex sections with higher positive values near the axial axis and in the left side, where the inlet flow hit with the wall of the cyclone. That swirl pattern may be due to underestimation of the velocity field by the RST turbulence model (Figure 17), which is consistent with the results obtained by Erdal [6]. However, the range of values with the RST turbulence model is smaller than with the others, which agrees to the high viscous dissipation expected.

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Figure 17. Cross section at 3 cm below the inlet. (A) Realizable 𝜅 − 𝜖, (B) SST 𝜅 −𝜔, (C) RST

5.2. Effect of the viscosity and surface tension

Axial velocity

The axial velocity profile in the region below the inlet shows several differences between the three liquid phases used (mineral oil, ISOPAR L and water) with air in the two-phase simulations (Figure 18). In the mineral oil axial velocity profile is not clearly observed the helical shape of the upward flow and it blurs downstream. In addition, as shown the Figure 18, the position of the vortex core with the mineral oil is not located in the axis center, indicating that the flow is not axisymmetric. Otherwise, the helical shape in the ISOPAR L and water is totally defined, with strong regions of flow reversal in the region near the inlet. The sections close to the walls near the inlet

have high negative axial velocities which help to form the helical shape. Therefore, the increase in the viscosity affects the helical shape of the upward flow (reversal flow) by the viscous energy dissipation downstream.

On the other hand, the effect of surface tension could be seen by the differences of the helical shape between the water-air and ISOPAR L-air simulation. The increase in the surface tension generates a more curved and wider helical shape with more visible demarcated areas of downward and upward flow, as shown in the water-air simulation (Figure 18). Therefore, the wavelength of the vortex is shorter in the water-air simulation. This effect can be explained due to the greater cohesion forces between the water molecules compared to those of ISOPAR L, which generates a clearly zone of water downward and air upward flow.

Figure 18. Axial velocity profile with RST turbulence model. (A) Mineral Oil, (B) ISOPAR L, (C) Water

Tangential velocity and swirl decay

The tangential velocity curved vectors display a strong dependency in the increase of viscosity, as show in Figure 19. The swirl decay in the axial direction is stronger for the mineral oil, even at 40 cm below the inlet section, the direction of the swirl is reversed, with values of tangential velocity close to zero, little swirl motion. Additionally, it is not observed significant variation between the tangential velocity values between the water and ISOPAR L simulations (Figure 19). However, it is clearly that the tangential velocity decays in the axial direction as well.

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Consequently, the increase of the viscosity promotes the increase of the shear stress which produces a higher dissipation of the vortex intensity, as show in the work done by Brito [9]. However, it is necessary to highlight that the region with zero or low tangential velocity has a helical path like the one observed in the axial velocity contours. The water-air simulation shows a more significantly downstream variation in the position of the vortex core than the mixture of ISOPAR L and air, as noted above.

In the case of surface tension, it can be observed that the vortex core region in the mixture water-air is bigger than in the case of ISOPAR L-air mixture, as show Figure 20. It can be observed that the vortex core position is the same for the mixtures of water and ISOPAR L with air. Nevertheless, the width of the region of tangential equal to or greater than zero is larger in the case of water. That region is related with the low-pressure zone and the upward flow. Therefore, an increase in the surface tension generates a more defined region for the upward flow, with

a lower pressure zone in the vortex core. Therefore, the interface region between the air and the liquid phase is more defined, which enhances the separation between the liquid and gas phases.

Figure 20. Tangential profile at a cross-section 20 cm below the inlet

Figure 19. Tangential velocity at a cross-section at 3, 20 and 40 cm below the inlet. (A) Mineral Oil, (B) ISOPAR L, (C) Water

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Amount of LCO and GCU

The three liquid phases (mineral oil, ISOPAR L and water) where simulated and validated experimentally at the same boundary conditions of velocity inlets of each phase and pressure at the outlets. In the case of the water-air mixture, no measurable amount of LCO could be seen. Therefore, a new case for the water-air mixture was added with a higher pressure at the bottom outlet (12.6 psi instead of 12.2). Moreover, the GCU was evaluated only by the CFD simulation. The results are shown in Figure 21.

Figure 21. Amount of LCO and GCU obtained by CFD simulation for each liquid phase. (*) A new case was created for water because at the original conditions no measurable LCO obtained.

Figure 22. Volume fraction in the lower part of the GLCC for each liquid phase. (A) mineral oil, (B) ISOPAR L, (C) Water.

The increase of the viscosity makes an increment in the amount of LCO (21%) and GCU (700%), which the last one has the largest increment related to other liquids phases with less viscosity (Figure 21). Moreover, as shown in Figure 22, the air volume fraction in the mineral oil is higher on average in the lower part of the separator. Besides, values of volume fraction of air greater than 0.2 are close to the bottom outlet. This large change of the amount of GCU is mainly caused by the weak swirl flow and the small values of axial velocity that cannot generates a defined helical shape for the upward flow in the downstream region. Additionally, the tangential velocity decays faster along the axial direction in the GLCC, as shown before, affecting the centrifugal forces and diminishing the efficiency of the cyclone.

Furthermore, it can be observed that the reduction of surface tension affects in an important increase in 42% of the LCO amount (Figure 21), which can be related with the less cohesive forces between the molecules of ISOPAR L. Hence, when the liquid droplets of ISOPAR contained in the gas stream in the upper part are centrifuged toward the walls, cannot coalesce properly into a liquid film. Therefore, this liquid from the wall cannot reach an enough thickness, thus only a small amount of ISOPAR L falls by gravity into the liquid vortex. The other fraction of molecules of ISOPAR L that could not coalesce have a great probability to exit in the gas stream and generates LCO.

In addition, with the comparison between the water and ISOPAR L data obtained, it can be observed that the amount of GCU has an increase in 360% with the reduction of the surface tension (Figure 21). That rise of GCU could be explained by the longer wave-length of the helical path and the narrow vortex core region showed previously, which reduce the separation performance in this downstream section of the GLCC. However, the higher amount of GCU with water/ISOPAR L-air could be show clearly with the experimental bubbly filament visualization, as shown in Figure 23.

The bubbly filament corresponds to bubbles that gather around the vortex center, permitting to visualize it. This complex hydrodynamics has been characterized by direct flow visualization and by photos taken with high speed camera. As show the Figure 23, the bubbles of air with water are larger and form a clearly bubble filament. Conversely, the bubbles of air with ISOPAR L are more

15

dispersed in the liquid and are smaller. The reasons of that differences are supported by the strength of cohesive forces of the liquid molecules. In the water, with stronger cohesion, the molecules are closer and the superficial area in the interface water-air is less. Thus, the air bubbles can group easily in the vortex core and shape larger bubbles. On the other hand, the molecules of ISOPAR L have less cohesion, and the air bubbles can disperse in the bulky, reducing the separation performance.

Figure 23. Experimental bubbly filament visualization in the lower section of the GLCC. (A) Water, (B) ISOPAR L

6. Conclusions

The RST turbulence model makes a better prediction of the hydrodynamics inside the GLCC and with that could be obtained more accurate results of the LCO amount and flow patterns in the inlet and downstream region of the cyclonic separator. Additionally, this model can produce a good estimation of the influence of viscous dissipation in the swirl flow decay. However, it is suspected that the RST turbulence model makes an overestimation of the tangential velocities values inside the GLCC, for this reason it is advisable to do an experimental validation as future work.

The increase of the dynamic viscosity reduces the performance of the GLCC significantly, whit both larger amount of LCO and GCU, with a relative increment of 21% and 700% respectively. This reduction is caused by the viscous dissipation energy that deforms the helical shape and generates a strong swirl decay in the

downstream region. For this reason, it is advisable to use pumps with greater power with the applications with heavy oils.

The reduction of the surface tension decreases the performance of the GLCC too, because of the generation of a weaker and longer wave-length of the helical shape whit less defined region for the upward flow and low-pressure zone in the vortex core. Additionally, the less cohesive forces prevent a properly liquid film in the upper part, thus a larger fraction of the liquid phase will have a great probability to exit in the gas stream.

As future work, it is recognized the importance of the understanding the bubbly filament hydrodynamics and his bubble’s size in order to add another variable of interest different of LCO, in order of show more clearly the effect of the surface tension in the amount of GCU. Similarly, the simulation of liquid phases with “invented” physical properties, should prove more properly the effect of the viscosity and the surface tension whit equal values of density of the liquid phases.

References

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Experimental Data," SPE Annual Technical Conference and Exhibition, pp. 1-16, 2015.

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APPENDIX A.

Author Study Type Objective Fluids Turbulence model

Erdal F. M., Shirazi, Shoham, & Kouba, (1997)

CFD simulations with experimental validation

Study single phase and two-phase flow

Water and air

𝜅 − 𝜀

Shoham & Kouba, (1998) State of art of experimental and computational studies

Summarizes main experimental and computational studies done

- -

Movafaghian, Jaua-Marturet, Mohan, Shoham, & Kouba, (2000)

Experimental and mechanistic model

Study of the effects of geometry, fluid properties and pressure

Water and air

-

Gomez, Mohan, Shoham, & Kouba, (2000)

Mechanistic model and field application

Study of the flow pattern dependent nozzle inlet

-

Chirinos, et al., (2000) Mechanistic model and experimental validation

Study of LCO Water and air

-

Erdal F., Shirazi, Mantilla, & Shoham, (2000)

CFD simulations Study the behavior of small bubbles in the lower part

Water and air

𝜅 − 𝜀

Erdal & Shirazi, (2004) CFD simulations with experimental validation

Study of the swirling flow behavior

Water and air

𝜅 − 𝜀, RSM

Reyes Gutiérrez, Rojas Solórzano, Marín Moreno, Meléndez Ramírez, & Colmenares, (2006)

CFD simulations with experimental validation

Study of Eulerian-Eulerian approach of the flow behavior

Water and air

Eulerian-Eulerian

Molina, et al., (2008) Experimental modified GLCC

Study of the effect of an annular film extractor in the efficiency

Wet gas -

Brito & Trujillo, (2009) State of art Study of the effect of viscosity

- -

Hreiz, Gentric, & Midoux, (2011) CFD simulations with experimental validation

Study of swirling flow Water and glycerin

Spalart-Allmaras, 𝜅 − 𝜀, 𝜅 − 𝜔 and RST

Hreiz, Gentric, Midoux, Lainé, & Fünfschilling, (2014)

Experimental setup Study flow hydrodynamics

Water and air

-

Kanshio, Yeung, & Lao, (2016) Experimental setup Study of phase distribution and liquid holdup

Water and air

-

18

Sy, (2016) CFD simulations with experimental validation

Study the influence of inlet angle on flow pattern

Water and air

𝜅 − 𝜀

APPENDIX B

Figure B1. Left Lateral view of the experimental setup performed

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Figure B2. Right lateral view of experimental setup performed