experimental analysis of liquid carry-over in gas-liquid cylindrical cyclone...

International Journal of ISSN 0974-2107

Systems and Technologies

Vol.1, No.2, pp 1-14

1

IJST

KLEF 2008

EXPERIMENTAL ANALYSIS OF LIQUID CARRY-OVER IN

GAS-LIQUID CYLINDRICAL CYCLONE SEPARATORS

Srinivas Swaroop Kolla

*, Ram S. Mohan

†,

Ovadia Shoham‡, Shoubo Wang

§, Luis Gomez

**

ABSTRACT: PREDICTION OF THE OPERATIONAL ENVELOP FOR LIQUID

CARRY-OVER IS ESSENTIAL FOR PROPER OPERATION OF GAS-LIQUID

CYLINDRICAL CYCLONE (GLCC) COMPACT SEPARATORS. A SERIES OF

EXPERIMENTS WERE CONDUCTED TO EVALUATE THE PERFORMANCE OF

GLCC LIQUID CARRY-OVER FOR THREE-PHASE GAS-OIL-WATER FLOW.

EXPERIMENTAL DATA WERE ACQUIRED IN A 3” DIAMETER GLCC FOR THE

OPERATIONAL ENVELOP FOR LIQUID CARRY-OVER, UNDER THREE-P††

HASE

FLOW. BOTH LIGHT OIL AND HEAVY OIL WERE UTILIZED, WITH WATERCUTS

RANGING FROM 0 TO 100 %. THE LIQUID LEVEL WAS CONTROLLED AT 6”

BELOW THE GLCC INLET. A SIGNIFICANT EFFECT OF WATERCUT ON THE

OPERATIONAL ENVELOP FOR LIQUID CARRY-OVER FOR THREE-PHASE FLOW

HAS BEEN OBSERVED. AS THE WATERCUT REDUCES, THE OPERATIONAL

ENVELOP FOR LIQUID CARRY-OVER REDUCES, TOO. ALSO, THE

OPERATIONAL ENVELOP FOR HEAVY OIL REDUCES AS COMPARED TO LIGHT

OIL WHICH COULD BE PRIMARY DUE TO THE EFFECT OF VISCOSITY.

FINALLY, THE ANNULAR MIST VELOCITY INCREASES WITH SURFACE

TENSION.

INTRODUCTION

The Gas-Liquid Cylindrical Cyclone (GLCC) Separator technology has been an

emerging technology in the petroleum industry. Its rise has been very promising to meet the

ever increasing demands of petroleum industry, thus providing an attractive alternative to the

conventional separator which has been in industry for more than 100 years. The GLCC

separator is a vertically installed pipe mounted with a downward inclined tangential inlet,

with outlets for gas and liquid provided at the top and bottom respectively. The two phases of

the incoming mixture are separated due to the centrifugal/ buoyancy forces caused by the

swirling motion. The liquid is forced radially towards the wall of the cylinder and is collected

from the bottom, while the gas moves to the center of the cyclone and is taken out from the

top of the GLCC. Significant advantages of the GLCC’s are its compact

lower weight, ease of operation, and lower cost when compared to conventional separators.

Experimental analysis of liquid

2

Due to the wide variety of potential applications ranging from partial separation to complete

phase separation, GLCC is used as an alternative to vessel-type separators. GLCC is not only

used for bulk separation but also used for enhancing the performance of multiphase meters,

multiphase flow pumps and de-sanders through the control of gas-liquid ratio. Other

applications of the GLCC are as automated well testing units, gas knock out and

pre-separation devices, flare gas scrubbers, slug catchers, downhole separators, and primary

separators. Performance of the GLCC is limited by two phenomena, namely the liquid

carry-over into the gas stream, termed as LCO (Liquid Carry-Over), and gas carry-under into

the liquid stream, termed as GCU (Gas Carry-Under). These phenomena are strongly

dependent on the flow patterns existing in the upper part, above the inlet for LCO and in the

lower part of the GLCC for GCU. It is necessary to predict these two phenomena for

optimum design and proper operation of the GLCC in the field. This paper presents

experimental investigations on the flow behavior in the upper part of the GLCC and

mechanisms associated with the LCO phenomena.

LITERATURE REVIEW

Compared to conventional separators, only few

publications are available on the optimal experimental design of the GLCC separator. Below

is a brief overview and latest information of pertinent literature on some important aspects of

the compact separation technology studies. Detailed literature review on compact separators

technology was given by Arpandi et al. (1995). Shoham and Kouba (1998) presented the

state-of-the art of GLCC technology. Mohan and Shoham (1999) presented the design and

development of GLCC for three-phase flow. Davies (1984), Davies and Watson (1979) and

Oranje (1990) studied compact separators for offshore production with respect to weight,

cost and separation efficiency when compared to conventional separators. Bandyopadhyay et

al. (1994), at the Naval Weapons Research Laboratory, considered the use of cyclone type

gas-liquid separators to separate hydrogen bubbles from liquid sodium hydroxide electrolyte

in aqueous aluminum silver oxide battery systems. The cyclone separator used for gas-oil

separation developed by Nebrensky et al. included a tangential rectangular inlet, equipped

with a special vane


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and shroud arrangement to change the inlet area, which allowed control of the inlet velocity

independent of the throughput, and extended the operating range of the separator. A hollow

gas-liquid separator with rectangular tangential inlet near the bottom of the separator has

been developed by Zhikarev et al. (1985). A cylindrical cyclone with spiral vane internals

called auger separator was developed by ARCO (Kolpak, 1994) and exhibited 2% to 18% gas

carry under when tested in Alaska. Weingarten et al. (1995) explored alternatives to

conventional methods of controlling liquid level inside separators by using throttling floats

and throttling diaphragm valves operated by the vessel hydrostatic head. Arato and Barnes

(1992) used an in-line free vortex separator downstream of a centrifugal multiphase pump for

gas-liquid separation. Davies and Watson (1979) developed miniaturized compact separators

for offshore platforms which require less space than conventional separators. Chevron has

successfully built and operated several GLCC’s in low GOR flow metering applications. Liu

and Kouba (1994) and Kouba (1995) from Chevron conducted various studies for the

development of multiphase metering loop incorporating the Net Oil Computer, where gas and

liquid phases are separated by means of a GLCC separator and separately metered by gas and

liquid flow meters prior to recombination for transport.A 6-inch diameter and 12-ft high

single inlet GLCC at Texaco Humble test facility was used (Kouba, 2002) to measure gas

carry-under for various combinations of crude oil, water and natural gas using nuclear

densitometers located at the inlet vertical riser and pipe section of the GLCC liquid exit.

Colorado Engineering Experimental Station Inc. (CEESI) tested (Wang et al., (2002a) a

6-inch dual inlet GLCC at pressures of 200 to 1000 psi, with natural gas and decane. Gas and

liquid flow rates ranging from 34 Mscfd and 2000 bbld, respectively, were used to test a

6-inch dual inlet GLCC multiphase metering system at Daqing oil field experiment station

with natural gas and crude oil for watercuts from 0 to 100 % (Wang et al., 2006). A 60 in. ID

and 20 ft tall GLCC, largest in the world was employed at Minas for bulk separation/metering

(Marrelli et al., 2000). This GLCC operated at 170 psia and 260oF, handling liquid and gas

production rates of 160,000 bpd and 70 MMscfd, respectively and is equipped with control

valves on the gas and liquid legs and a sophisticated control system for liquid level control.

GLCC’s were designed for Duri, Indonesia field to handle both sand production and terrain

slugging.Sensitivity Analysis of conventional separators vs. GLCC demonstrated that its

application for Duri Area-10 alone was estimated to improve the metering accuary

considerable and save about $3.2 million over conventional separators. A 12-inch diameter


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and 12-ft high dual inlet wet gas configuration of the GLCC was installed for metering

application by CNOOC on an offshore platform in China (Wang and Zhang, 2005). The first

subsea GLCC application designed and constructed by Curtiss Wright (Campen et al., 2006)

has been developed by joint industry project led by Petrobras and is located downstream of

the multiphase pump, separating and recalculating liquid from its liquid outlet back in to the

pump suction.

EXPERIEMENTAL PROGRAM

The Experimental data were acquired using advanced state-of-the-art instrumentation and

data acquisition system in a three-phase experimental flow loop as shown in Figure

Experimental Facility

The experimental flow loop comprises of a metering section to measure the single phase gas and

liquid flow rates and a GLCC test section.

Metering Section

The metering section consists of three parallel, single phase feeder lines for

measuring the incoming single-phase gas and liquid flow rates. Three phase mixture is

formed at the mixing tee and delivered to the GLCC test section. Air is used as the gas phase,

which is supplied to a gas tank by an air compressor with a capacity of 250 cfm at 108 psia.

The gas flow rate into the loop is controlled by a control valve and metered utilizing Micro

Motion®

mass flow meter. The liquid phases are mineral oil of specific gravity 0.854 and

water. The two liquid phases are supplied from 400 gallon storage tanks at atmospheric

pressure, and pumped to the liquid feeder lines with centrifugal pumps. Similar to the gas

phase, liquid rate is controlled by separate control valves and metered using respective Micro

MotionR

mass flow meters. The single phase gas and liquid streams are combined at the

mixing tee. Check valves located downstream of each feeder are provided in order to prevent

probable backflow. The three phase mixture downstream of test section is separated utilizing

a conventional three-phase separator. Gas is vented into the

atmosphere and liquid is returned to the storage tank to complete the cycle.

GLCC Test Section: The test section, as shown in Figure 1, comprises of a GLCC separator, in

a multiphase flow metering loop configuration. The test section is divided as follows


5

Figure 1. Schematic of GLCC Test Section

1. Inlet Section: The Inlet of the GLCC consists of an Inlet pipe section, 3” diameter

connected to the GLCC with an inlet having a sector-slot/plate configuration, with a nozzle

area of 25% of the inlet pipe cross-sectional area. The inlet section of the GLCC is shown in

Figure 2.

2. GLCC Body, Gas Leg and Liquid legs: The GLCC body is 3” diameter and 8’ tall as

shown in Figure 3.5. The gas leg is a 2” diameter pipe and it has a gas control valve (GCV).

On the other hand, the liquid leg consists of 2” diameter pipe sections. The Coriolis Micro

Motion® mass flow meters are located on both gas leg and liquid leg to measure the gas and

liquid outflow rates respectively.

8

”

24”

48

”

6

”

24”

THREE-PHASE

INLET

THREE-PHASE

OUTLET

GLCC

3’’


6

Figure 2. GLCC Inlet Section and Body

3. Control System: The main objective of the control system is to maintain the liquid level in

the GLCC by using the Control Valve. There are two simple control strategies and two

integrated control strategies mentioned below. Each of the control strategies explained below

has one final aim i.e. to control the liquid level in the GLCC. An integrated liquid level and

pressure control by LCV and GCV i.e. the third kind is used to conduct experiments.

Physical Phenomena

The Performance of GLCC is limited by two undesirable physical

phenomena namely Liquid Carry-Over (LCO) in the gas outlet stream and Gas Carry-Under

(GCU) in the liquid outlet stream. The ability to predict these two phenomena will ensure

optimum design parameters for the operation of the GLCC.

Liquid Carry-Over (LCO)

Initiation of liquid entrainment into the discharged gas stream at

the top of GLCC is called as Liquid Carry-Over (LCO). LCO plays an important role in the

analysis of the performance of the GLCC. The current study is concentrated on capturing the

effect of oil properties and the effect of watercut on the liquid carry over operational envelop

of a GLCC in which the liquid level and pressure are controlled.


7

Operational Envelope: The operational envelop for liquid carry-over is defined as the loci of

vsl and vsg for which the liquid starts to get carried into the gas leg. It occurs under extreme

operating conditions of high gas and/or high liquid flow rates. Plotting the loci of the liquid

and gas flow rates at which LCO is initiated provides the operational envelop for liquid carry

over as illustrated in the Figure 3.

Figure 3. Operational Envelop for Light Oil with Different Watercuts

The area below the operational envelope (OPEN) is the region of normal operating

condition (NOC). In this region, there is no liquid carry over in the separator. The region

above the OPEN represents the flow conditions for continuous LCO. Point (a) in the figure

represents NOC in the GLCC. Point (b) marks the initiation of the LCO phenomena in the

GLCC. This point represents the minimum gas flow rate required to initiate LCO for a given

liquid flow rate. For higher gas flow rate at point (c), the liquid is carried over in to the gas

stream continuously.

Flow regimes in GLCC: There are two distinct flow regimes responsible for liquid carry over

in the upper part of the GLCC. At relatively high liquid and low gas flow rates, the liquid

churns up and down in the upper part of the GLCC which is termed

as Churn Flow. Under this condition, liquid is carried over in to the gas leg in the form of

churn flow. This phenomenon is presented in Figure 4. At relatively high gas and low liquid

flow rates, the flow pattern in the upper part of the GLCC is annular flow. Under this

condition liquid is carried over into the gas stream and through the gas leg in the form of

droplets as shown in Figure 5.

LCO REGION

(a)

(b)

(c)

vsg

vsl


8

LIQUID IN GAS

LIQUID

LIQUID IN GAS

LIQUID

LIQUID IN GAS

LIQUID

LIQUID IN GAS

LIQUID

Fig 4. Schematic of Churn Flow in GLCC Fig 5.Schematic of Annular Flow in GLCC

EXPERIEMENTAL RESULTS AND DISCUSSION

The experimental results of effect of fluid properties and watercut are presented along

with discussion.

Effect of Fluid Properties

All the previous studies concentrated on the effect of fluid properties on GLCC

performance by adding additives to the liquid phase, such as surfactants, thereby reducing or

increasing the surface tension. This paper presents, different tests that have been carried out

with oil, water, air as three fluids with different viscosities, surface tensions and densities.

The physical properties of the different liquid phases used for the present study are shown in

the Table 1.


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Table 1: Fluid Properties of Different Fluids

API

Gravity

Viscosity

(cp, at 68 oF)

Surface Tension

(dyne/cm, at 77 oF)

Interfacial Tension

(dyne/cm, at 77 oF)

Lab Water

10.2

1.3 70

Light Oil

(Tech80)

35 31.7 25.5 37.5

Heavy Oil

(LubsOil

Lubsnap1200)

22

750

33

16.5

In the churn region, as the viscosity of the fluid increases, there is a significant effect on the

liquid carry-over, and it can be seen that liquid carry-over occurs much earlier. In a similar way, as the

surface tension reduces, the liquid carry-over occurs earlier. The tests conducted in this regard confirm

the physical phenomena. Figure 6 illustrates the effect of fluid properties on the operational envelop.

The tests were conducted at 25 psia.

Effect of Watercut

Figure 7 presents the operational envelop for the light oil, as a function of watercut. The tests

were conducted at 0, 25, 50, 75 and 100% watercuts. As can be seen, the operational envelop

increases as the watercut increases. It is the lowest for pure oil and highest for pure water. The other

observation that can be made is that the operational envelop of the 75% and 100% watercuts are

similar and the dominance of oil properties is only below the 50% watercut. This emphasizes that if

the fluid is water dominant, the operational envelop will be increased and the effect of oil properties is

not that significant. Similar behavior is observed with the operational envelop for the heavy oil as

shown in Figure 8.

Effect of Watercut on Annular Mist Velocity:

The annular mist velocity ( annv ) is that gas velocity where there is onset of annular mist

flow in top section of GLCC regardless of liquid flowrate. The acquired experimental data

demonstrated that watercut significantly affects the annular mist velocity as shown in Figure

9. In the case of 0% watercut, i.e. pure oil, the annular mist velocity is at 19.2 ft/sec (light oil)


10

and 20.8 ft/sec (heavy oil), and in the case of pure water, i.e. 100% watercut, operating at the

same conditions the annular mist velocity Vann is at 24.2 ft/sec.

EFFECT OF FLUID PROPERTIES

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30

vsg(ft/sec)

vs

l(ft

/sec)

water

light oil

Light Oil

Viscosity = 31.7 cp

Surface Tension =

25.5 dyne/cm

Water

Viscosity = 1.3 cp

Surface Tension = 70.0 dyne/cm

Figure 6: Effects of Fluid Properties

LIGHT OIL DIFFERENT WATERCUTS (25PSIA)

LL=6 INCHES BELOW INLET

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30

vsg(ft/sec)

vs

l(ft

/sec)

100 wc

75 wc

50 wc

25 wc

0 wc

Figure 7. Effect of Watercut on the Operation Envelop with Light Oil

Table 2. Annular Mist Velocities at Different Watercuts

Type / wc 0 25 50 75 100

Light Oil 19.2 20.8 21.9 22.8 24.2

Heavy Oil 20.8 22.5 23.5 24.2


11

HEAVY OIL DIFFERENT WATETCUTS (30 PSIA)

LL=6 INCHES BELOW INLET

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25

vsg(ft/sec)

vs

l(ft

/se

c)

75 exp

50 exp

25 exp

0 exp

Figure 8. Effect of Watercut on the Operational Envelop with Heavy Oil

Effect of watercut on vann

15

16

17

18

19

20

21

22

23

24

25

0 20 40 60 80 100

Watercut (%)

va

nn (

ft/s

ec)

LIGHT OIL EXP (25 psia)

HEAVY OIL EXP (30 psia)

Figure 9. Effect of Watercut on the Annular Mist Velocity


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NOMENCLATURE

ann = annular mist

co = carry-over

eqv = equivalent

GCV = gas control valve

LCV = liquid control valve

sg =superficial gas

sl = superficial liquid

v = velocity

ft =feet

sec = second

experimental analysis of liquid carry-over in gas-liquid cylindrical cyclone...

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