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IV Journeys in Multiphase Flows (JEM 2017) March 27-31, 2017, São Paulo, SP, Brazil Copyright © 2017 by ABCM Paper ID: JEM-2017-0026 EXPERIMENTAL MEASUREMENTS OF HORIZONTAL THREE-PHASE SOLID-LIQUID-GAS SLUG FLOW WITH HYDRATE-LIKE PARTICLES Luis M.M. Rosas a , [email protected] Carlos L. Bassani a , [email protected] Moisés A.M. Neto a , [email protected] Marco J. da Silva a , [email protected] Rigoberto E.M. Morales a , [email protected] Amadeu K. Sum b , [email protected] a Multiphase Flow Research Center (NUEM), Federal University of Technology – Paraná (UTFPR), R. Deputado Heitor Alencar Furtado, 5000, Bloco N, Campo Comprido, Curitiba/PR, CEP 81280-340. b Hydrates Energy Innovation Laboratory, Chemical and Biological Engineering Department, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401, USA. Abstract. Gas hydrate is a main flow assurance concern for worldwide oil companies due to the high risk of pipe blockages. Those blockages represent a global obstacle to the successful production of deep-water hydrocarbons. Hydrates are crystals formed by the trapping of gas molecules into cages formed by hydrogen-bonded water molecules and may form at the water-gas interface. Right after the hydrate formation onset, when the volumetric fraction and the size of the particles are still small, the particles flow homogeneously dispersed in the liquid phase. However, the particles may interact with the gas-liquid flow, changing the flow hydrodynamics. For offshore operations, the phases are assumed as flowing predominantly in the slug flow pattern due to the range of gas and liquid superficial velocities in those operations. Understanding the effects of the solid particles introduction in the slug flow is essential to improve the efficiency and safety of oil and gas facilities. The purpose of the present work is to experimentally characterize solid-liquid-gas slug flow with the presence of homogeneously dispersed hydrate-like particles. Experimental tests were carried out with polyethylene particles of 0.5 mm-diameter with density similar to the hydrates (938 kg/m 3 ). The test section comprised a 26 mm-ID, 9 m-long horizontal duct made of transparent Plexiglass. High Speed Imaging was used to analyze the bubbles shape behavior due to the introduction of the solid particles. Resistivity sensors were placed in the test section to measure the unit cell translational velocity, the slug flow frequency and the bubble and slug region lengths. Two distinct solid particles concentrations were tested (6 and 8 g/dm 3 ) and compared to a similar case of liquid-gas slug flow. Keywords: flow assurance, hydrates, three-phase solid-liquid-gas slug flow, high speed imaging, resistivity sensor. 1. INTRODUCTION The slug flow pattern is often assumed as the prevailing one in oil and gas offshore exploitation processes due to the characteristic operation ranges of the superficial velocities of the phases. Slug flows are characterized by the intermittent succession of two bodies: a liquid slug, which may or may not contain dispersed gas bubbles in it, and an elongated bubble sliding over a thin liquid film. Together, those two structures constitute that what is known as a unit cell (Shoham, 2006). The slug and the elongated bubble possess characteristic velocities and geometric features such as lengths and phase fractions. Those characteristics depend on time and space and their prediction is relevant in the design of offshore facilities. Besides oil and gas, offshore operations may also contain water (brine) and solid particles dispersed in the mixture, such as sand. High water cut, high pressure and low temperature conditions – often found in offshore exploitation scenarios – favor the formation of a new phase, known as hydrate. Hydrates are crystals formed by the trapping of gas molecules into cages formed by hydrogen-bonded water molecules (Sloan and Koh, 2008) and are related to pipe blockage risks, with consequent income losses caused by production interruptions. Hydrates may form at the gas-water interface, where a more effective contact between the phases occurs (Sloan et al., 2011), thus forming a dispersion. The dispersion may be homogeneous or heterogeneous, depending on the concentration and size of the particles (Peker and Helvaci, 2007). At some point, the particles may agglomerate due to capillary and intermolecular forces (Camargo et al., 2000), forming: (i) a moving or stationary bed, related to flow restriction; or (ii) a massive plug, which will block the production. Pipe blockage due to hydrate formation is one of the main challenges faced by oil and gas production companies, especially when dealing with deep and cold waters (Cardoso et al., 2015). Efforts have been made on experimentally characterizing and modelling gas-liquid slug flows (Bassani et al., 2016; Castillo, 2013; Kjeldby et al., 2013; Rogero, 2009; Shoham, 2006; Taitel and Barnea, 1990) and solid-liquid flows (Doron et al., 1987; Doron and Barnea, 1993; Peker and Helvaci, 2007). However, studies on three-phase solid-liquid-gas flows are scarce and focused in the flow
Transcript
Page 1: EXPERIMENTAL MEASUREMENTS OF HORIZONTAL THREE …eventos.abcm.org.br/jem2017/content/uploads/2017/03/JEM-2017-0026… · IV Journeys in Multiphase Flows ... EXPERIMENTAL MEASUREMENTS

IV Journeys in Multiphase Flows (JEM 2017) March 27-31, 2017, São Paulo, SP, Brazil

Copyright © 2017 by ABCM Paper ID: JEM-2017-0026

EXPERIMENTAL MEASUREMENTS OF HORIZONTAL THREE-PHASE SOLID-LIQUID-GAS SLUG FLOW WITH HYDRATE-LIKE PARTICLES

Luis M.M. Rosasa, [email protected] Carlos L. Bassania, [email protected] Moisés A.M. Netoa, [email protected] Marco J. da Silvaa, [email protected] Rigoberto E.M. Moralesa, [email protected] Amadeu K. Sumb, [email protected] aMultiphase Flow Research Center (NUEM), Federal University of Technology – Paraná (UTFPR), R. Deputado Heitor Alencar Furtado, 5000, Bloco N, Campo Comprido, Curitiba/PR, CEP 81280-340. bHydrates Energy Innovation Laboratory, Chemical and Biological Engineering Department, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401, USA. Abstract. Gas hydrate is a main flow assurance concern for worldwide oil companies due to the high risk of pipe blockages. Those blockages represent a global obstacle to the successful production of deep-water hydrocarbons. Hydrates are crystals formed by the trapping of gas molecules into cages formed by hydrogen-bonded water molecules and may form at the water-gas interface. Right after the hydrate formation onset, when the volumetric fraction and the size of the particles are still small, the particles flow homogeneously dispersed in the liquid phase. However, the particles may interact with the gas-liquid flow, changing the flow hydrodynamics. For offshore operations, the phases are assumed as flowing predominantly in the slug flow pattern due to the range of gas and liquid superficial velocities in those operations. Understanding the effects of the solid particles introduction in the slug flow is essential to improve the efficiency and safety of oil and gas facilities. The purpose of the present work is to experimentally characterize solid-liquid-gas slug flow with the presence of homogeneously dispersed hydrate-like particles. Experimental tests were carried out with polyethylene particles of 0.5 mm-diameter with density similar to the hydrates (938 kg/m3). The test section comprised a 26 mm-ID, 9 m-long horizontal duct made of transparent Plexiglass. High Speed Imaging was used to analyze the bubbles shape behavior due to the introduction of the solid particles. Resistivity sensors were placed in the test section to measure the unit cell translational velocity, the slug flow frequency and the bubble and slug region lengths. Two distinct solid particles concentrations were tested (6 and 8 g/dm3) and compared to a similar case of liquid-gas slug flow. Keywords: flow assurance, hydrates, three-phase solid-liquid-gas slug flow, high speed imaging, resistivity sensor.

1. INTRODUCTION

The slug flow pattern is often assumed as the prevailing one in oil and gas offshore exploitation processes due to the characteristic operation ranges of the superficial velocities of the phases. Slug flows are characterized by the intermittent succession of two bodies: a liquid slug, which may or may not contain dispersed gas bubbles in it, and an elongated bubble sliding over a thin liquid film. Together, those two structures constitute that what is known as a unit cell (Shoham, 2006). The slug and the elongated bubble possess characteristic velocities and geometric features such as lengths and phase fractions. Those characteristics depend on time and space and their prediction is relevant in the design of offshore facilities.

Besides oil and gas, offshore operations may also contain water (brine) and solid particles dispersed in the mixture, such as sand. High water cut, high pressure and low temperature conditions – often found in offshore exploitation scenarios – favor the formation of a new phase, known as hydrate. Hydrates are crystals formed by the trapping of gas molecules into cages formed by hydrogen-bonded water molecules (Sloan and Koh, 2008) and are related to pipe blockage risks, with consequent income losses caused by production interruptions. Hydrates may form at the gas-water interface, where a more effective contact between the phases occurs (Sloan et al., 2011), thus forming a dispersion. The dispersion may be homogeneous or heterogeneous, depending on the concentration and size of the particles (Peker and Helvaci, 2007). At some point, the particles may agglomerate due to capillary and intermolecular forces (Camargo et al., 2000), forming: (i) a moving or stationary bed, related to flow restriction; or (ii) a massive plug, which will block the production.

Pipe blockage due to hydrate formation is one of the main challenges faced by oil and gas production companies, especially when dealing with deep and cold waters (Cardoso et al., 2015). Efforts have been made on experimentally characterizing and modelling gas-liquid slug flows (Bassani et al., 2016; Castillo, 2013; Kjeldby et al., 2013; Rogero, 2009; Shoham, 2006; Taitel and Barnea, 1990) and solid-liquid flows (Doron et al., 1987; Doron and Barnea, 1993; Peker and Helvaci, 2007). However, studies on three-phase solid-liquid-gas flows are scarce and focused in the flow

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L.M.M. Rosas, C.L. Bassani, M.A. Marcelino Neto, M.J. da Silva, R.E.M. Morales, A.K. Sum Experimental Analysis of Horizontal Three-Phase Solid-Liquid-Gas Slug Flows with Hydrate-Like Particles

effects on the transportation of the solid particles (Goharzadeh et al., 2010; Goharzadeh and Rodgers, 2009; Stevenson and Thorpe, 2003, 2002), but do not comprise the effects of the particles in the flow hydrodynamic characteristics, such as the slug flow frequency and the unit cell geometry. Those works also assume particles with density higher than water, such as sand. However, this is not the case of hydrates, which are often lighter than water and thus tend to fluctuate (Sloan and Koh, 2008).

The present study aims to experimentally characterize the gas-liquid-solid three-phase flow in a horizontal pipe. The slug flow pattern was chosen, since it is the prevailing one in offshore petroleum operations. To avoid experimentation with real hydrate particles – which would demand a long distance flowloop, with a precise temperature and pressure monitoring system – polyethylene particles were chosen to represent the hydrates, keeping a similar density and size (938 kg/m3 with 0.5 mm diameter). The objective is to understand the effects introduced by the solid particles on the main parameters of the slug flow (namely, unit cell translational velocity, slug flow frequency and unit cell region lengths) by means of resistive sensors and High Speed Imaging in a 9 m-length transparent flowloop with 26 mm-ID. 2. EXPERIMENTAL FLOWLOOP

An experimental flowloop was assembled to characterize three-phase solid-liquid-gas horizontal slug flows using High Speed Imaging and resistive sensors. Table 1 presents the experimental flowloop characteristics and the range of parameters covered by the measurements. The fluids used were air and water at ambient conditions (approximately 100 kPa and 298 K). A total of 8 liquid-gas superficial velocities combinations was measured for two different solid particles concentration (6 and 8 g/dm3). Two-phase liquid-gas flow – that is, without the solid particles – was measured at the same conditions for comparison purposes. Each measure was made three times to assure repetition. All the jL/jG combinations fall within the slug flow pattern, visually confirmed since the pipeline is made of transparent Plexiglass.

Table 1. Experimental flowloop characteristics and range of measured parameters. Fluids Air and water, at slug flow pattern

Particles material/density/diameter Polyethylene / 938 kg/m3 / 0.5 mm Pipe wall material Plexiglass, transparent

Pipeline inclination Horizontal Flowloop length 9 m (~346D)

Flowloop pressurization Ambient Flowloop temperature Ambient, isothermal

Liquid superficial velocity range 0.5 ≤ jL ≤ 1.5 m/s Gas superficial velocity range 0.25 ≤ jG ≤ 1.5 m/s Solid particles concentration 0 / 6 / 8 g/dm3

Mixture superficial velocity range 1 ≤ J ≤ 2 m/s

Number of measures 8 jL/jG combinations x 3 particles concentration

x 3 repetitions each = 72 measurements

Figure 1 shows a scheme of the experimental flowloop. Water is stored in a tank (i) of 250 dm3, where the solid particles are added at the desired concentration. A mixer (ii) is placed inside the tank to assure a homogeneous dispersion. The solid-liquid dispersion passes through a centrifugal pump (iii) for up to 200 kPa, which is fed by a 3 hp electric motor (iv). The water flow rate is controlled by a variable-frequency drive (v) actuating on the electric motor. A Coriolis-type flowmeter (vi) is positioned downstream of the pump and upstream of the mixing location with the gas phase. In addition to the water flow rate, the Coriolis flowmeter returns the water density and its temperature. The water dynamic viscosity is estimated using the temperature value via an empirical correlation (Fox et al., 2011).

In a parallel branch of the flowloop, air is compressed (vii) up to 800 kPa. The air is stored in two pressure vessels (viii) with working pressure of 1.4 MPa and combined capacity of 600 dm3. The gas flow rate is measured through an orifice plate (ix). A differential pressure transducer (x), designed for a measurement range of 0 to 60 kPa, evaluates the pressure drop in the orifice plate. The pressure difference is correlated to the volumetric flow rate via calibration with a rotameter, with a maximum deviation of 2.9%. Additionally, the gas density and dynamic viscosity are evaluated using empirical correlations (Fox et al., 2011) as a function of the local air temperature and gauge pressure measured at the orifice plate inlet (xi). The gas flow rate is controlled by a manual valve positioned downstream of the orifice plate (xii).

The phases are mixed at the inlet of the horizontal pipeline via a parallel plate mixer (xii), which locally generates a stratified flow pattern. This pattern should naturally evolve to slug flow if the phases superficial velocities are propitious, which was indeed the case for all the jL/jG combinations tested in this work. In order to ensure this flow pattern transition, a development section length of 5.2 m (~200D) was used before the measurement points. Resistivity sensors (xiii) were positioned at 5.2, 6.5 and 7.8 m (~200 / 250 / 300D) of the pipe inlet. Gauge pressure transducers (xiv) were inserted near each resistive sensor. A High Speed Camera (xv) was positioned at 6.45 m (~248D) from the pipe inlet. A pipe length of 1.2-m (~46D) was inserted after the last resistive sensor to avoid any reverse

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IV Journeys in Multiphase Flows (JEM 2017)

upstream effects. The flowloop has a total length of approximately 9 m. The fluids are discharged back to the water tank, where the separation of the phases occurs naturally by gravity. The air is released to the ambient, while the solid-liquid dispersion is reinjected on the flowloop.

Figure 1. Experimental flowloop.

The High Speed Camera is used to visualize the flow pattern. The acquisition rate can be set for up to 3600 frames/s at its higher resolution of 1024x1024 pixels. To avoid light refraction, a box of Plexiglas – full with water – was positioned so as to wrap the pipe, as shown in Figure 2. A light source was used to illuminate the ambient. A diffuser surface was placed between the light source and the Plexiglas box to enhance image contrast.

The resistive sensors measurements follow the method proposed by Machado et al. (2013). The resistive sensor, shown in Figure 3a, is composed by a printed circuit board (PCB) made of glass fiber, containing two wires made of stainless steel, with 0.12 mm of diameter and separated by 3 mm each to another. One wire acts as an electric current transmitter, while the other acts as a receiver. The transmitter wire is fed by a squared electrical signal with amplitude of ±5V and frequency of 1.75 kHz. The electrical resistance has a linear dependence on the amount of liquid between the wires, thus allowing measuring the liquid height hL in the cross section as (Castillo, 2013):

LtL

G L

V Vh

D V V

(1)

being V(t) the electric tension signal of the resistive sensor and VL and VG the electric tensions when the pipe cross section is full of liquid and gas, respectively. Assuming a flat liquid-gas interface along the cross sectional area, as shown in Figure 3b, the gas fraction RG can be related to the liquid height hL as:

22 2 21

1 arccos 1 1 1 1L L LG

h h hR

D D D

(2)

Pump (iii)

Variablefrequencydrive (v)

Coriolis-typeflowmeter (vi)

Orifice plate(ix)

Parallel platemixer (xii)

Particles mixer (ii)

High SpeedCamera (xv)

Resistive sensors(xiii)

Differential pressuretransducer (x)

Gauge pressuretransducer (xi)

Water tank (i)

Gauge pressuretransducers (xiv)

Electric motor(iv)

Gas valve(xii)

5.2 m (~200D)

6.5 m (~250D)

7.8 m (~300D)

9 m (~346D)

1.2 m(~46D)

6.45 m (~248D)

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L.M.M. Rosas, C.L. Bassani, M.A. Marcelino Neto, M.J. da Silva, R.E.M. Morales, A.K. Sum Experimental Analysis of Horizontal Three-Phase Solid-Liquid-Gas Slug Flows with Hydrate-Like Particles

Figure 2. High Speed Camera positioned at the flowloop, with a Plexiglas box to avoid light refraction and a light-diffuser surface to enhance image contrast.

Three resistive sensors are always coupled together in a set at each measurement point of the test section, as shown

in Figure 3c. The center plate acts as grounding so as to avoid interferences. The signals of the other two plates permit estimating the unit cell translational velocity – here assumed equal to the bubble nose velocity – knowing that the plates are separated by a distance of 50 mm and measuring the delay on detecting the elongated bubble front between the two plates. Figure 4a shows the signal captured by the two plates, already transformed to the gas fraction using Eqs. (1) and (2). A binary function is applied to the signal so as to detect the elongated bubble and the slug regions (Bertola, 2003):

,

,,

0 ;

1 ;G

G G crit

R tG G crit

if R Ru

if R > R

(3)

being RG,crit a critical gas fraction above which an elongated bubble is assumed as passing through the cross section. This critical gas fraction depends on the gas and liquid superficial velocities and is shown in Figure 4a by a dashed line.

Figure 3. (a) Printed circuit board of the resistive sensor with the transmitter and receiver wires. (b) Cross sectional area during the passage of an elongated bubble, assuming a flat interface between the phases. (c) Set of three resistivity

sensors for measuring the unit cell translational velocity.

Light source

Plexiglasvisualization box

Flow direction

High SpeedCamera

Diffuser surface

Transmitter Receiver

Flow direction

Wires

hLFilm

Elongated bubble

(a) (b) (c)

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IV Journeys in Multiphase Flows (JEM 2017)

Figure 4. Gas fraction from two resistivity sensors separated by 50 mm: (a) as measured and (b) after applying the binary function of Eq. (3) to recognize the elongated bubble and slug regions.

After applying Eq. (3) over the resistive sensor signal, it becomes a squared wave, as shown in Figure 4b. The

period for an elongated bubble and for a liquid slug to pass through the resistive sensor are indicated by TB and TS, respectively. The delay in capturing the elongated bubble front between the two distinct plates is named BT . Since the

distance between the two resistive sensors dS is known, the elongated bubble translational velocity can be estimated as shown in Eq. (4). The elongated bubble and the slug lengths can be estimated assuming that the entire unit cell translates with the same velocity of the elongated bubble front UT (Taitel and Barnea, 1990), as presented in Eq. (5). The slug flow frequency is estimated as the inverse of the unit cell period of passage, Eq. (6).

T S BU d T (4)

;B T B S T SL U T L U T (5)

1

B Sf T T (6)

Near each set of resistive sensors, a gauge pressure transducer is positioned to estimate the local gas superficial

velocity, which is accomplished by comparing the gauge pressures at the test section inlet and at the orifice plate. Using

the volumetric gas flow rate measured by the orifice plate G,orifice plateQ and dividing by the cross sectional area of the

pipe:

2

4orifice plate G,orifice plateG,test section

test section

P Qj =

P D

(7)

3. RESULTS AND DISCUSSIONS

Before starting the analysis of the results, it is interesting to understand the main aspects brought by the introduction of the solid particles in the flow. These aspects can be divided into three groups:

(A) Increase in the mixture superficial velocity: the fixed parameters during the measures were the liquid (jL) and gas (jG) superficial velocities. However, the presence of a slurry flow is associated to the insertion of a solid superficial velocity (jS). Shoham (2006) shows that, for an ideal case of non-slip flow, the volumetric fraction of the particles inside the liquid can be estimated as the ratio between the solid and the liquid superficial velocities, /S L S LR j j . This is a reasonable assumption for homogeneous solid-liquid flows and for small

differences between the densities of the phases, which is, indeed, the case of the present study. Upon isolating the solid superficial velocity, it can be written as /S S L Lj R j . Shoham (2006) also affirms that the mixture

superficial velocity is the sum of the superficial velocities of all the phases, L G SJ j j j . Since the gas and

liquid superficial velocities remained constant, but the solid concentration changed, then the mixture superficial velocity increases in /S S L LJ j R j .

(B) Properties variation: the homogeneous solid-liquid dispersion can be treated as a liquid phase with equivalent properties, dependent on the particles concentration. That is, the three-phase solid-liquid-gas flow can be

1.665 1.67 1.675 1.680

0.1

0.2

0.3

0.4

0.5

0.6

Time (s)

Gas

fra

ctio

n [-

]

,G critR

BT STBT

Time (s)(a) (b)

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L.M.M. Rosas, C.L. Bassani, M.A. Marcelino Neto, M.J. da Silva, R.E.M. Morales, A.K. Sum Experimental Analysis of Horizontal Three-Phase Solid-Liquid-Gas Slug Flows with Hydrate-Like Particles

treated as a two-phase dispersion-gas flow. The dispersion density can be calculated via an homogeneous model (Shoham, 2006) as / /1L S S L L S LR R . The notation ( )L represents the dispersion, being the

index L an indication that it can be treated as homogeneous liquid phase. The dispersion viscosity can be estimated by Krieger and Dougherty’s (1959) correlation for spherical particles with maximum package factor

of 0.63, 1.575

/1 0.63L L S LR .

(C) Particles interference on the liquid velocity profile: the particles are submitted to different forces, such as weight, buoyancy, drag and lift (Peker and Helvaci, 2007). Depending on the magnitude of those forces, movement may be generated between a single particle and the liquid continuous phase. Even though in the average all particles are carried out by the liquid at approximately the same velocity – as previously assumed that no-slippage occurs – locally there is a relative movement between the particle and the liquid phase. The drag induced by the liquid over the particle results in a terminal velocity Vt. If the particle Reynolds number is sufficiently high ( 100)P L t P LRe V d (Crowe et al., 2012), being dP the particle diameter, then vortices

will be generated at the particle’s wake zone, which by their turn will disturb the liquid velocity profile. Next, the results will be discussed in means of the aforementioned effects brought by particles introduction in the

gas-liquid slug flow. The results will be divided in two subsections: a qualitative analysis, based on the High Speed Camera results; and a quantitative analysis, based on the resistive sensors results. 3.1. Flow visualization

Figure 5 presents an assembly of different images along the same elongated bubble. The chosen pair of superficial velocities for this analysis is jG = 1 m/s and jL = 0.5 m/s. The two particle concentration cases are shown together with the case of gas and liquid flow only. It is important to notice that, for all measured data, the solid particles stayed homogeneously dispersed in the liquid. Heterogeneous distributions and moving and/or stationary beds are not presented in this study.

The presence of 8 g/dm3 /( 0.0085)S LR of solid particles in the jG = 1 m/s and jL = 0.5 m/s flow generated: (i) a

mixture superficial velocity variation of 0.008J m s (approximately 0.5% of mixture acceleration); (ii) a dispersion

density of 3994L kg m (approximately 0.1% smaller than the water density); (iii) a dispersion viscosity of 48.9 10 .L Pa s (approximately 2% higher than the water viscosity); and (iv) a particle Reynolds number of

2.5PRe – using Zigrang and Sylvester (1981) correlation for the terminal velocity.

Figure 5. Image assembly of different parts of the elongated bubble for jG = 1 m/s and jL = 0.5 m/s for particles concentrations of: (a) 0 g/dm3, (b) 6 g/dm3 and (c) 8 g/dm3.

From Figure 5, it can be noticed that the elongated bubble has an aerodynamic shape, and its nose travels a little bit

displaced from the upper wall. This displacement is slightly pronounced when the solid particles are present. Taitel and Barnea (1990) discuss the elongated bubble shape in means of a gas and liquid combined momentum equation. By their theory, the density variation due to the introduction of the solid particles – effect (B) – would affect the inertial terms of

Flow direction(a) Without solid particles

Slug

(b) Solid particle concentration: 6 g/dm3

(c Solid particle concentration: 8 g/dm3)

Wake zone

Elongated bubble

FilmBubble nose

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IV Journeys in Multiphase Flows (JEM 2017)

the momentum balance. In the same way, the viscosity variation would change the friction terms – also effect (B). Furthermore, the liquid velocity profile at the rear of the slug body is related to the elongated bubble nose shape due to local shear stresses, and the variation in the liquid velocity profile due to the introduction of the solid particles – effect (C) – could be another explanation for the nose shape change. However, the particle Reynolds number seems too low for effect (C) to be present.

Figure 6 presents a similar assembly of images, but for the slug region. An increase in the dispersed bubbles released in the elongated bubble tail can be noticed when introducing the solid particles. This region, known as bubble wake, presents a high liquid recirculation due to the shock of two different velocities: the slug and the film ones. Usually, the slug travels faster than the film, which results in a liquid mass exchange at the bubble rear, phenomenon known as scooping (Shoham, 2006). The liquid captured by the slug causes a local change on the velocity field, with a consequent recirculation zone and a hydraulic jump. The introduction of the particulate material will intensify this recirculation due to the interaction of the particles with the liquid velocity profile – effect (C) – if the particle Reynolds number is sufficiently high, thus causing higher shear stresses and higher amount of gas dispersed bubbles release – which is indeed observed in Fig. 6. The particle Reynolds number, as evaluated in the present work, may not be representative for the elongated bubble wake zone, due to the characteristic recirculation at this region and since ReP as evaluated in the present work does not take into account this case. Higher particle Reynolds numbers are expected to be present at this region; however, it is hard to establish if they are sufficiently high to be a cause of the higher amount of dispersed bubbles release.

Some of the dispersed bubbles recirculate, reaching again the elongated bubble tail and re-coalescing. Others just travel in the wake zone, with a velocity approximately equal to the elongated bubble rear. However, some of the dispersed bubbles detach from the wake zone, reaching the slug body. The slug body does not present high recirculation as in the wake zone, and thus the dispersed bubbles tend to agglomerate in the upper wall due to buoyancy forces. Since these bubbles are near the wall, they travel at a smaller velocity in comparison to the liquid in the central part of the slug body. As a consequence, they are left behind to be captured by the next elongated bubble.

Once the dispersed bubbles reach the tail of the slug body, they can either: (i) coalesce to the next elongated bubble or (ii) accumulate at the upper part of the elongated bubble nose. Mechanism (i) was observed as being predominant in the absence of the solid particles, while mechanism (ii) was predominant when the particles were introduced. This points out that the solid particles may retard bubble coalescence, which by its turn can be related to: (i) surface tension variation – effect (B); or (ii) capillary interaction between particle and gas-liquid interface. Further analysis should be made to understand which is causing the bubble coalescence to retard – if surface tension or capillary forces – and closer images of the particles and interface interaction will probably be required.

Figure 6. Image assembly of different parts of the slug region for jG = 1 m/s and jL = 0.5 m/s for particles concentrations of: (a) 0 g/dm3, (b) 6 g/dm3 and (c) 8 g/dm3.

3.2. Effects of the solid particles in the slug flow parameters

In this subsection, the results obtained with the resistivity sensors will be shown as to understand the effects of the solid particles presence in the main slug flow parameters. The slug flow parameters analyzed are: the unit cell translational velocity, the slug flow frequency and the elongated bubble and slug lengths. The left column of Figure 7 presents the average results of these parameters in means of the solid particles concentration for all the gas-liquid

Dispersed bubble coaslescing with elongated oneWake zone

Dispersed bubble accumulation

(a) Without solid particles

(b) Solid particle concentration: 6 g/dm3

(c Solid particle concentration: 8 g/dm3)

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L.M.M. Rosas, C.L. Bassani, M.A. Marcelino Neto, M.J. da Silva, R.E.M. Morales, A.K. Sum Experimental Analysis of Horizontal Three-Phase Solid-Liquid-Gas Slug Flows with Hydrate-Like Particles

Figure 7. Results obtained with the resistive sensor positioned at 250D from the pipe inlet for: (a) unit cell translational velocity, (b) slug flow frequency, (c) elongated bubble length and (d) slug length. Left column: average results for all

gas-liquid superficial velocities in means of the solid particles concentration. Right column: Probability Density Function (PDF) for the different particles concentration when jG = 1 m/s and jL = 0.5 m/s.

0 2 4 6 8Particles concentration [g/dm3]

1

1.5

2

2.5

UT [

m/s

]

jG = 0.25 ; jL = 0.75 [m/s]jG = 0.50 ; jL = 0.50 [m/s]jG = 0.50 ; jL = 1.00 [m/s]

jG = 0.75 ; jL = 0.75 [m/s]jG = 1.00 ; jL = 0.50 [m/s]jG = 0.50 ; jL = 1.50 [m/s]

jG = 1.00 ; jL = 1.00 [m/s]jG = 1.50 ; jL = 0.50 [m/s]

0 2 4 6 8Particles concentration [g/dm3]

0

2

4

6

8

10

12

Fre

quen

cy [

Hz]

0 2 4 6 8Particles concentration [g/dm3]

0

0.5

1

1.5

2

2.5

LB [

m]

0 2 4 6 8Particles concentration [g/dm3]

0

0.1

0.2

0.3

0.4

0.5

LS

[m]

(a)

(b)

(c)

(d)

1.4 1.6 1.8 2 2.2UT [m/s]

0

2

4

6

8

PD

F

0 g/dm3

6 g/dm3

8 g/dm3

0 0.5 1 1.5 2 2.5 3Frequency [Hz]

0

0.5

1

1.5

2

PDF

0 0.5 1 1.5 2 2.5 3LB [m]

0

0.5

1

1.5

PDF

0 0.2 0.4 0.6 0.8 1LS [m]

0

1

2

3

4

5

PD

F

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IV Journeys in Multiphase Flows (JEM 2017)

superficial velocities. The right column shows the Probability Density Function (PDF) for the different solid particles concentration for fixed gas and liquid superficial velocities of jG = 1 m/s and jL = 0.5 m/s.

Figure 7a presents the results obtained for the unit cell translational velocity. It can be observed that the average value of the unit cell increases as the solid particles are introduced in the flow, Figure 7a (left). This unit cell acceleration may be caused by the increase in the mixture superficial velocity due to the introduction of the solid particles – effect (A). From Figure 7a (right), it can be seen that the peak of the PDF displaces to the right as the solid particles are introduced, indicating an average acceleration of the elongated bubble. However, the unit cell velocity distribution is larger for higher particles concentration. That is, in the average, the solid particles introduction accelerates the elongated bubble, but some elongated bubbles may present smaller values of translational velocity due to the intermittence of the flow. For example, the lower value of velocity for 8 g/dm3 found was near 1.45 m/s, while for the case of none solid particles was approximately 1.55 m/s.

Figure 7b presents the results for the slug flow frequency. The average value of the slug flow frequency tends to increase with the introduction of the solid particles, as shown in Figure 7b (left). This is a consequence of the unit cell acceleration, which causes the unit cell structures to have lower periods of passage. The PDF is slightly displaced to the right when the solid particles are introduced. The probability peaks are lower, showing a higher dispersion of the frequency results in the presence of the particles.

Figures 7c,d present the results for the elongated bubble and slug region lengths. It is noticeable that their average values tend to decreases (Figures 7c,d – left). When analyzing the PDF’s, the elongated bubble (Figure 7c – right) presents a peak value reduction from approximately 1.3 m to 1 m when the solid particles are introduced. In the other hand, the variation on the PDF of the slug length (Figure 7d – right) is less sensitive to the particles introduction, being the peak displaced from approximately 0.35 m to 0.28 m. The region lengths are strictly related to the elongated bubble shape (Bassani et al., 2016; Taitel and Barnea, 1990), and since the properties variations due to the formation of a dispersion – effect (B) – affect the bubble shape, than it would also affect the region lengths. 4. CONCLUSIONS

This study presents an experimental analysis of three-phase solid-liquid-gas slug flows using particles with similar properties of gas hydrates. High Speed Imaging and resistive sensors techniques were used to characterize the slug flow pattern in the presence of solid particles. The main effects introduced by the solid particles can be split into: (A) a mixture superficial velocity increase due to the particles superficial velocity, (B) properties variations due to the formation of a dispersion and (C) the induction of vortices in the wake of the particles, which will disturb the liquid velocity profile. Effect (A) is related to the unit cell acceleration, with consequent slug flow frequency increase. Effects (B) is related to variations on the elongated bubble shape, especially at the elongated bubble nose, which tends to travel more displaced from the upper wall at the presence of the particles. Effect (C) may be related to higher shear stresses on the elongated bubble wake zone, thus causing a higher release of dispersed bubbles; however, it is hard to estimate the particle Reynolds number at this region and therefore further analysis should be made on verifying cause-effect relation. 5. ACKNOWLEDGEMENTS

The authors acknowledge the financial support of ANP and FINEP through the Human Resources Program for Oil and Gas Segment PRHANP (PRH 10-UTFPR), TE/CENPES/PETROBRAS (0050.0068718.11.9) and National Council for Scientific and Technological Development (CNPq). AKS thanks PETROBRAS for sponsoring his sabbatical leave at UTFPR during the time part of this study was performed. 6. REFERENCES Bassani, C.L., Pereira, F.H.G., Barbuto, F.A.A., Morales, R.E.M., 2016. Modeling the scooping phenomenon for the

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1043–1044. 7. RESPONSIBILITY NOTICE

The authors are the only responsible for the printed material included in this paper.


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