SPE 120899

Viscosity Effect in Cyclone Separators A. Brito, J. Trujillo, PDVSA Intevep

Copyright 2009, Society of Petroleum Engineers This paper was prepared for presentation at the 2009 SPE Latin American and Caribbean Petroleum Engineering Conference held in Cartagena, Colombia, 31 May–3 June 2009. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract The gas liquid cyclone separator is a device that works as a result of the centrifugal force acting on the denser phase of a two-phase flow. This causes the liquid phase which is denser than the gas, to move outward to the cyclone wall. The separated gas comes out by the top, the crude/water blend comes out by the bottom as a two phase stream. The separation time is almost instantaneous and due to its simple design, no moving parts and low cost, the cyclone separators have become an attractive solution for the new oil an gas field developments. The liquid viscosity has a predominant influence in the performance of this kind of separators. As the liquid viscosity increases the shear stresses increase too producing a higher dissipation of the vortex intensity and as a consequence the separation efficiency is diminished. Therefore, the cyclone operational envelope for heavy oils is reduced as compared to light oils. This paper presents the status of the gas-liquid cyclones separation studies related to their performance when used with high viscosity liquids. Experimental and computational studies from different sources, including PDVSA Intevep, are presented for different types of cyclones, with liquid viscosity between 1 and 1500 cP.

Introduction

The cyclones have been successfully used in different industrial applications for several decades from the most common to the most severe conditions as gas-solid, gas-liquid and liquid-liquid separators. However, the oil industry has shown interest in the development and application of cyclonic separation technologies in recent years. Due to the growing energy consumption world wide, the large reserves of heavy oil and the high cost of the bulky conventional vessel-type separators used for heavy oil, the oil industry have been demanding to expand the use of cyclone technologies from light and medium oil-gas separator to heavy oil-gas separators. The main advantages of cyclones are their compactness, simple geometry, low weight, low cost, easy installation and maintenance.

The PDVSA strategic plans consider a strong development of the important heavy and extraheavy oil reserves of the

Orinoco Belt. The high liquid viscosity of these oils imposes technical challenges for their profitable, reliable, long term production. The production streams transport from the wellbore to the receiving facilities is one of those challenges as the friction pressure losses could become a severe constraint. One of the most common solutions so far has been the dilution of the oil in the well to obtain a blend of around 16 °API gravity, using a closed loop for the diluent. As a result, and considering the relative high temperature of these streams, their operation liquid viscosity is reduced to figures normally found in medium oils at ambient temperature. In others words, the liquid viscosity of these heavy and extraheavy oil resources at the operation conditions of the flowlines and receiving facilities is typically reduced to only a few hundred centipoises.

However, the gas liquid gravity separators for these heavy and extraheavy oils are bulky and costly as they have to

provide a liquid residence time over 30 minutes in some cases. The use of any improving separation technology could provide a reduction in size, weight and cost of these equipment. Even though the cyclone technology performance is affected

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by the liquid viscosity, it could be an option to enhance the gas-liquid separation process in the new Orinoco Belt developments.

This paper will present experimental and numerical evidence taken from different sources, including some results

obtained in PDVSA for different kind of cyclones, that considers the effect of the fluid properties in the performance of the cyclones and mainly the liquid viscosity effect. Their main drawback is that the cyclonic separators performance is affected 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. Despite of the separation capacity reduction caused by the higher liquid viscosity of heavier oils, the cyclone performance is still attractive compared to a conventional gravity separator of the same size. Cyclone separator operational envelope

The separation between the liquid and gas phase in the cyclone separator depends on the swirling motion created by the tangential velocity of the fluids that promotes the centrifugal forces acting on the gas liquid phases, and its operational envelope is limited by two undesirable phenomena, known as liquid carry over and gas carry under, that are related to the entrainment of the liquid into the discharged gas stream at the top of the separator and the entrainment of the gas into the discharged liquid stream at the bottom of the separator, respectively. Gomez et al. (1999) presented that high tangential velocities cause gas bubbles to get entrained into the spinning liquid phase below the gas-liquid interface and may be carried downward by the liquid flow; that phenomenon is known as gas carry under and the liquid carry over into the gas stream might occur under high gas and/or liquid flow rates.

According to Hoffmann and Stein (2002) the cyclones inlet velocities usually vary from 15 to 18 m/s for heavily loaded

cyclones and increase from 23 to 26 m/s for most lightly-loaded or second-stage units. However, depending on the application, cyclones can operate reasonably well with inlet velocities as low as 5 m/s.

Gomez et al. (1999) and Barbuceanu and Scott (2001) recommended to design the inlet nozzle area small enough to

obtain mixture tangential velocity of 4.57 to 6.1m/s (15 to 20 ft/s). Also, they determined an acceptable range of G-force for separation between 56 and 100 gravitational force; this range of G force is difficult to maintain. Commonly, there is a significant fluctuation of the pressure, liquid and gas flow rates in the separator during the field life. Therefore, if the tangential velocities fall out of the recommended range of G force some problems could arise, e.g. longer vortex length than expected and premature appear of liquid carry over or gas carry under.

Fluid viscosity effect in cyclone separators

It is well known that the liquid viscosity affects the performance of the cyclone separators, because when the viscosity increases the tangential velocity decays faster along the axial direction in the cyclone affecting the centrifugal forces and increasing the gas carry under in the liquid stream at the bottom of the separator, diminishing the efficiency of the cyclone. However, it is important to know if the cyclone is still a good option for a heavy oil field and if its performance will be less or equal than a conventional vertical separator used for the gas liquid separation for heavy oil. In some cases, the cyclone could be a good option as inlet device separator, as a pre-separator or for high gas oil ratio operational condition in a field station, obtaining better results than with a single conventional separator.

According to Hoffmann and Stein (2002), defoaming or foam breaking cyclones have been commercially successful in

recent years, especially in refinery and drilling installations handling heavy crude oil. However, when the liquid phase viscosity is too high, greater than approximately 50 cP, the dispersed foam bubbles may not respond well to the applied centrifugal field within the cyclone.

Vale et al. (1998) studied the performance of three prototypes of the VASPS (Vertical Annular Separation and Pumping

System) with high viscosity liquids. The system consists of a two phase subsea separation and pumping system without moving parts, that induces a helical flow generating centrifugal forces promoting the gas liquid separation. The difference between the VASPS prototypes is that the model A is the original one and models B and C have an expansion chamber at the top of the unit. Tests were performed with high viscosity oils from 5 to 65 cP. They said that VASPS is viable as a liquid-gas separator when processing crude oils with viscosity up to 46 cP. The results presented for Vale et al. show that the prototype C has the best performance considering the viscosity effect over the liquid separation capacity (Figure 1). For the VASPS type A when the liquid viscosity increases from 14 cP to 46 cP the oil flow rate capacity of the separator is reduced from 55 m3/h to around 40 m3/h. Therefore, there is a clear tendency to decrease the separation performance as the oil viscosity increases; however, the VASPS model forecasted separation performances were compatible and reproduce those experimentally obtained.

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Rosa et al. (2001) studied the Cyclone Separator (CS), through a joint development by Petrobras and the State University of Campinas. In this technology the separation process combines the centrifugal and gravitational forces acting on the gas-liquid mixture. Three different liquid fluids were used during the tests with liquid viscosity from 1 to 150 cP, at 25°C. As expected the capacity of the separator to handle the flow rate of liquid decreases as the liquid viscosity increases. The separator can handle 417 m3/h for liquids with 4 cP, and only 105 m3/h for liquids with 15 cP. Also they found that the diameter of the critical gas bubble is proportional to the liquid viscosity increases.

Marrelli et al. (2000) developed software models for conventional and compact separation, which allow comparison in

cost and size for equal separator performance. They found using the program GOSPSIM and the GLCC© model for a liquid fluid of 70 cP, that a GLCC© operating at 5% gas carry under is 99% less in volume than a conventional separator sized for zero gas carry under. That is an important issue, because it is well known the effect of the liquid viscosity over the cyclone performance. If the liquid viscosity increases the performance of the cyclone separator decreases, however the cyclone is still economically attractive for primary separation with liquids of high viscosity (70 cP).

During the year 2.000 Intevep studied the performance of the GLCC (Figure 2). The GLCC© separator resulted from a

joint development by Chevron Petroleum Technology and Tulsa University. This separator was tested in the multiphase flow experimental loop of PDVSA Intevep, with liquids of 1, 132 and 480 cP @ 15 Psig and 72°F. It was found a clear trend to decrease of separation performance when the oil viscosity increases (over approximately 100cP), the gas bubbles are entrained in the liquid core at the inlet section, as the dissipation of the centrifugal forces is higher due to the viscous stresses compared to a less viscous oil, then, the bubble separation is more difficult.

Reyes et al. (2001), used computational fluid dynamics (CFD) simulations to investigate the hydrodynamic flow behavior

in a cylindrical cyclone (GLCC©), for a wide range of liquid viscosities (1 cP, 10 cP, 100 cP, 223 cP and 480 cP). Part of those results were compared with the experiments carried out in Intevep. As shown in Figures 3 and 4, for a mixture tangential velocity in the inlet around 6 m/s, the tangential velocity in the separator decreased with the increment of the liquid viscosity and the gas void fraction in the lower part of the GLCC© increases too, because the velocities are not enough to promote the gas liquid separation. Reyes et al. used as acceptable separation performance reference a gas void fraction (GVF) of 10% or less into the discharged liquid stream at the bottom of the separator. Using this criteria they found that the liquid capacity of certain GLCC© was approximately 23 m3/h for the liquid with a viscosity of 230 cP, and was reduced to 13 m3/h for another liquid with a higher viscosity of 450 cP. They concluded that the liquid capacity in the separator is diminished with the liquid viscosity increasing because the vertical length of the swirling flow promoted by the inlet velocity is reduced by the viscous dissipation.

Reyes et al. (2001) found when the viscosity increased from 1 to 10cP the liquid capacity of the separator was reduced

10%, when the viscosity increased from 1 to 100 cP the liquid capacity was reduced 30% approximately, when the viscosity increased from 1 to 223 cP the liquid capacity was reduced 40% and finally from 1 to 450 cP the liquid capacity was reduced 70% (Figure 5). Reyes et al. proposed a correlation to predict a critical separation gas bubble diameter. Using this correlation and the hydrodynamic equations used in the CFD code they were able to predict the optimum operation envelope of the GLCC© separator when the liquid viscosity increased.

Méndez (2002) studied the effect of the angle in three different cyclones with conical section of 0° or GLCC©, 9° and 18°

or CYCINT™ of 9° and 18° developed by Intevep, for four different liquid viscosities (86, 172, 344 and 428 cP), using computational fluid dynamics (CFD). The results show that for liquid viscosity less than 200 cP, the GLCC© and 9° conic cyclone efficiencies are better than the 18° conic cyclone. However, for liquid viscosities higher than 200 cP, the 18°conic cyclone maintain an efficiency around 80% meanwhile the GLCC© and 9°conic cyclone efficiencies drop sharply around 20%. It is possible that stronger separator radius reduction in the 18° conic cyclone tends to compensate the tangential velocity decreasing caused by viscous stress, therefore maintaining an acceptable centrifugal acceleration compared to other geometries that have cylindrical body or a smaller cone angle. These finds suggest the possibility of usage for a conic cyclone with an optimum angle to be determined as a separation option in a heavy oil crude field development.

In the year 2003, Intevep tested the cyclone CYCINT™ (Figure 6) in the Centro Experimental de Producción de PDVSA

(CEPRO) in Western Venezuela, with a mixture of natural gas and crude oil of 22 and 14 °API, with viscosity between 80 cP and 1600 cP at the operation temperature. The liquid capacity of the CYCINT™ cyclone is reduced as expected when the oil viscosity increases (Figure 7), having a separator liquid capacity reduction of 40% with a 14 °API heavy oil when compared to the results obtained with the same CYCINT™ cyclone operating with a 22 °API oil. However, when the CYCINT™ liquid capacity is compared to the theoretical liquid capacity of a conventional vertical gravity separator of the same size, for the 14 °API heavy oil, the CYCINT™ liquid capacity is approximately 270% higher (Figure 8). This capacity increasing against the vertical gravitational separator is significant and suggests that even though the cyclone separator performance is affected by the viscosity increase, it could be used instead of a conventional separator or together with other

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separation technologies to improve the separation performance in heavy oil developments, contributing to a size, weight and cost reduction of these traditionally bulky heavy equipment.

Movafaghian (1997) studied experimentally and theoretically the effects of geometry, fluid properties and pressure on the hydrodynamics flow behavior in a GLCC©, for four different liquid viscosities (1, 2.5, 5 and 10 cP). They used a closed experimental loop configuration, where the equilibrium liquid level in the GLCC© is determined by a pressure balance from the inlet to the recombination point, between the gas leg and the liquid leg across the GLCC©. They found that the effect of increase in viscosity is to increase the equilibrium liquid level in the GLCC©, due to the increase in frictional losses in the liquid leg. All this effect promotes a continuos reduction of the GLCC© operational envelope, causes earlier liquid carry over at lower gas and liquid flow rates as the liquid viscosity increases. From the Movafaghian (1997) studies is possible to determine the reduction of the separator liquid capacity caused by the increasing of the liquid viscosity. When the viscosity increased from 1 to 10 cP the gas capacity of the separator was reduced approximately 50% and the liquid capacity was reduced 20%.

Recent studies carried out in Intevep evaluated the performance of two different cyclones, one is a cyclone cluster

separator and the other is a modified CYCINTTM, using two different liquid viscosities (1 cP and 80 cP). It was found a reduction of the operational envelope in both cyclones of around 30% due to the earlier liquid carry over occurrence when the liquid viscosity is increased from 1 cP to 80 cP, using the criteria that the gas void fraction (GVF) into the discharged liquid stream at the bottom of the separator were less than 10%.

Conclusions It is a fact the reduction of the cyclone performance when the liquid viscosity increases; however, there are different mechanistic models and fluid dynamic simulators which have proven to be reliable tools to validate the performance of the cyclones and predict the behavior of the separator with high viscosity of the liquid phase. Because of that is possible to design cyclone separators in a operational envelope where they can work without a presence of undesirable phenomena as a liquid carry over or gas carry under and still having a good efficiencies/capacity ratio compared to conventional separators. Now a days there are several unknown questions related to the behavior of the cyclonic separator with high liquid viscosity, the range of applicability of the cyclonic separators and what items need to be considered for a proper selection of the cyclone technology in heavy oil fields and some of them are: - It is possible to design a reliable cyclone separator to be used as a gas-liquid separator for heavy crude oil, but it is

recommended to validate this design with computational fluid dynamics tools or some mechanistic models that have been proved with high liquid viscosity.

- It is important to consider the oil viscosity at the operational conditions for the cyclone separator design, because this property is strongly affected by the operational temperature, and in some cases a high temperature heavy oil could have a behavior similar to a medium crude oil.

- In most of the Venezuelan’s heavy oil fields the liquid stream coming to the separator is a blend between light and heavy oil and at the operational separation conditions the liquid viscosity is similar to a medium oil liquid viscosity, and it is a good option to evaluate the applicability of cyclones in these cases.

Acknowledgment The authors thanks the support of PDVSA Intevep during all the phases of the development program of the Cyclone Separators and to Simon Bolivar University for their support with the CFD simulations. We also acknowledge the active participation of: - Trallero, J., García, S., Colmenares, J., Vielma, J., Pereyra, E., Cabello, R and Teran, V. on the experimental

definition of GLCC© operational envelope with viscous liquids during 2001. - Colmenares,J., Perdomo,Y., Ortega, P., Corrales,F., Zapata,N., Chacon,L., Flores,R., Lara,J and Castillo,F. On the

experimental evaluation of fluid properties effect over the CYCINTTM separator during 2003. - Caliz,L., Valdez,J., Cabello,R., Lopez,J. On the experimental evaluation of the modified separator CYCINTTM and

multicyclone of Intevep with water-air and oil-air systems during 2008.

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Figure 1. Effect of the liquid viscosity over the VASPS separator liquid capacity

Figure 2. Gas Liquid Cylindrical Cyclone Separator. (GLCC©)

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Figure 3. Tangential velocity in the axial direction for the location 4 in the GLCC©.

Reyes et al. (2001)

Figure 4. Gas void Fraction in the GLCC© for different liquid viscosity at a mixture inlet

tangential velocity of 6 m/s. Reyes et al. (2001)

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Figure 5. Effect of the liquid viscosity over the GLCC© separator liquid capacity

Figure 6. CYCINTTM Cyclone Separator.

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Figure 7. Effect of the liquid viscosity over the CYCINTTM separator liquid capacity

Figure 8. Comparison of the liquid capacity of the CYCINTTM separator and a conventional vertical separator of the same size

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