Numerical Study of Oil-water Separation in Inline Axial Hydrocyclone

The fluid mixture hydrocyclone is an excellent tool for downhole oil/water separation (DOWS) for high water cut. In the present work the flow through inline axial inlet hydrocyclone is investigated computationally using ANSYS-FLUENT-19 software. The three dimensional continuity and momentum equations with SST k-omega turbulence model are used to simulate the strongly swirling, turbulent flow. Oil/water two phase mixture treated as the working fluid, water is considered as the primary phase and oil is the secondary phase. The considered oil volume fraction this work is 0.25 and flow split is 0.3 for three different flow rates of (14, 28, 56) m 3 /hr corresponding to Reynold's number range of (1.6 to 6.6)*10 4 . The pressure and velocity fields were analyzed in the whole hydrocyclone with core recirculation. The results are helpful to predict the flow motion inside the cyclone to estimate the oil/water separation process. The present work indicates that the water purity at cyclone exit was 90%, while the obtained oil purity was 38%.


Introduction
The hydrocyclone technique is one of the most efficient and proven oil/water separation technology due to its simplicity, compactness, robustness, and low manufacturing and maintenance costs.
First traditional liquid-liquid hydrocyclone separator (LLHC) design was produced by (Dirkzwager, 1996) to study the flow behavior for single phase and two phase flow in axial cyclone [1]. A numerical model for liquid-liquid turbulent flow called (HAAS) is predicted by [2], its showed that the HAAS model is very time efficient in cyclone design, The design of (LLHC) is further investigated using two commercial CFD package and compared it with the experimental results, It was found that the main features of the flow were qualitatively well represented in the numerical simulations, however large quantitative difference were found between the numerical results and experimental data [3]. The swirl number at any location along the pipe length depends on the swirl number at inlet, Reynolds number, the distance from the pipe inlet, the pipe diameter and the nature of the inlet swirl as found by [4]. Laser Doppler anemometry (LDA) is used to measure and compare the tangential and axial mean velocity components in the cyclone with numerical results by [5]. The sensitivity of flow field and cyclone efficiency with the variation of the swirl vanes geometry is shown experimentally by [6]. The fluid flow in cyclone inserted with different guide vane designs is predicted by [7]. It is compared with that predicted in conventional cyclone.
These vanes consist of three semicircle plates, each plate form 30 o with the pipe axis.
The flow field through this pipe was measured by PIV ( Particle Image Velocimetry).
The results show that the flow pattern inside the current cyclone is improve where its compare with traditional design, also the experimental results predicted that the velocity field is influence by the inlet floe rate and low sensitive to the flow split when its ration less than 13%. One phase laminar flow in axial hydrocyclone is investigated numerically and experimentally to show the flow features and pressure drop under swirl flow condition [8]. 156 separation efficiency is reports as stated by [9] and [10]. The effect of replacing the cylindrical pipe by the conical pipe cyclone is studied by [11]. This Causes a reduction in the recirculation zone and improve the separation efficiency by rises the tangential and axial velocity components. This is considered a worth improvement in the cyclone specifications. The separation of oil/water mixture by filtering hydrocyclone (a porous ceramic membrane) is studied numerically and compared with the conventional cyclone by [12]. Eulerian-Eulerian approach and the turbulence model are applied in this study.
Velocity, pressure drop and separation performance of the hydrocyclone were analyzed.
The results showed that due to the effect of porous wall the oil phase was instable in the cyclone core. The oil concentration and pressure are lower inside the filtering hydrocyclone compared with conventional cyclone. Also the separation efficiency by using porous wall are reduction by about 5%. An axial inlet hydrocyclone with two stages of separator and two light phase outlets is designed by [13]. The effect of the flow rate, oil volume fraction and the split ratio on the predicted cyclone was investigated for water flow rate of (4-7 m 3 /h), and oil fraction (1%-10%). The results show that the split ratio affected the separation efficiency. Also the experimental results appeared that the predicted design is suitable for less than 10% oil fraction, and its structure required optimization to extend its application. The effect of the swirl structure parameters on the velocity field for single phase flow is studied numerically by [14]. The results show that the separator parameters (vane outlet and twist angles as well as number of vanes) have proportional effect on the swirling flow inside hydrocyclone., The tangential velocity is improved and then the separation process. A numerical investigation of flow behavior inside axial flow hydrocyclone is conducted by [15]. The aim of this work is to present the structural cyclone effect on the pressure drop and velocity field using mixture multiphase model and Reynolds stress turbulence model. Three different cyclone structures are used, each contains two swirl generators with two light phase outlets. The test chamber for the first cyclone is a conventional cylindrical tube, second cyclone is a conical diffuser tube and that for the last is a multiple stages of contraction. Results showed that the third cyclone structure have a higher pressure drop compared to the second cyclone, and these two cyclones are more efficient than conventional cylindrical cyclone tube. Despite of understanding the flow field development inside hydrocyclone very limited previous studies treated with oil-water separation for Iraqi specifications in hydrocyclone are found. The small differences in the density between the two fluids required high cyclone design to enhance separation efficiency. The present work concerned with the detection of the flow behavior inside inline axial hydrocyclone that can be used for downhole oil-water separation. Numerical simulation for turbulent swirling flow of oil/water in concurrent axial inlet hydrocyclone is adopted. The purpose of this simulation is to display the flow field inside the hydrocyclone to improve its design parameters under different flow rates and specify the rise in its' performance.

Computational model
The concurrent axial inlet hydrocyclone adopted in this work is shown in Figure   (

Mathematical model
The flow behavior inside the cyclone is investigated by the fundamental principles of classical fluid mechanics expressing the conservation of mass and momentum. The continuity and momentum equations for turbulent, isothermal, incompressible flow are: [16] Continuity equation for phase q The momentum equation where: P is the pressure shared by all phases.
α q is the volume fraction of phase q.
⃗ is the mean velocity.
is the viscous stress tensor given by: R pq is the interface force, this force depends on the friction, pressure, cohesion, and other effects, and is subject to the conditions: and ⃗ This force defined as : where K pq = (K qp ) is the interphase momentum exchange coefficient.
Momentum transfer between the phases is depend on the value of the fluid-fluid exchange coefficient K pq . For liquid-liquid mixture the exchange coefficient K pq is indicated to how the secondary phase (droplet or bubble ) do affect the predominant fluid. It can be defined as following: where f is the drag function.
, the particulate relaxation time and is defined as: d p is the diameter of the bubbles or droplets of phase p. Nearly all definitions of (f) include a drag coefficient (C D ) that is based on the relative Reynolds number (Re). For the model of [17]: C D =0.44 for Re number up to 1000 Here Re is the Reynolds number based on the relative velocity: The turbulence model (Shear Stress Transport ) SST k-omega is used since it has low computational cost and took low iteration time compared to its sensitivity to the complex flow field inside the cyclone. The turbulence governing differential equations are [18] and [19]: Specific Dissipation Rate F 1 is blending Function: where y is the distance to the next surface and CD kω is the positive portion of the crossdiffusion term: Kinematic eddy viscosity (14) where Ω is the absolute value of the vorticity and F 2 is second blending function: The SST constants are: ; ; ; ; ; √

Mesh Generation and Computational Algorithm
A computational mesh is generated with 2.9 million numbers of grids, extra work and care was taken to get a good mesh quality of skewness 0.2. Figure (3) presents the mesh adopted on the surface and on the plane through the axis of the separator. The mesh is refined near wall and on the separator surface in order to capture the flow behavior near the wall. SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm is used for pressure-velocity coupling, Diffusive and convective terms were discretized using a hybrid scheme.

Grid independency
Grid independencey is performed in which three different grids are used, coarser mesh 2.7 million elements, medium mesh 2.9 million elements and fine mesh 3 million elements. The tangential velocity distribution at x= 0.5m after separator is presented for these grid systems as shown in Figure (4). It can be shown that the difference in the tangential velocity between the coarser and medium mesh is relatively large, whereas, the difference between medium and fine mesh is less than 3%. Hence the medium system estimate is selected in this work. Tangential velocity is the dominant velocity component in the separator and the key factor which responses for the centrifugal force. It has a direct effect on the separation deteriorate and on the cyclone efficiency. When the tangential velocity increases, the centrifugal force and separation efficiency will increase [20]. The reported values for tangential velocity was 9.93 m/s near the separator middle section while at its nose there were a reverse tangential oil flow. As shown in Figure (6a), the swirl intensity is maximum at the region after separator nose and decays along the axial direction as well as when it approaches the pipe center and pipe wall. Figure (6b) shows the axial flow distribution along the axial hydrocyclone. The axial velocity reaches its peak value in the annular region, this indicated to the favorable forward positive velocity. Its value is rabidly decreases toward the wall, where it falls to zero on the wall (no slip condition).
As proceed to the pipe core its rate decline to the negative value, where the unfavorable backward velocity appeared. As fluid flows toward the pipe outlet the axial velocity increases in the core region and reaches to fully developed flow after 1.5m from the separator, at this position the pickup tube is located for oil collection. The inlet flow rate is considered as the main parameter that affects the separation process. Figure (7a) shows an improve in the axial velocity as flow rate increases. The influence of increasing the inlet flow rate on the swirling intensity increase and then tangential velocity is shown in Figure (7b).
As the flow leaves the separator, the velocity vector shows that the backward flow have maximum value at the pipe core due to the adverse pressure gradient causes a recirculation zone as illustrated in Figure ( with results obtained by [21].

Oil volume fraction
The distribution of oil volume fraction presented in Figure (

Conclusion
In this work, a numerical investigation of oil-water separation based on inline axial-flow hydrocyclone technique to contribute in recovering oil production and minimize water extra producing. This investigation is mainly attempted to exposure the performance of this oil/water separator for Iraqi oil specifications. The results show that: 1-The swirl intensity and the centrifugal force have highest value at cyclone segment after the separator, then decreases along the cyclone axis. The tangential velocity is directly affected by this force, its profile consist of combination of free and force vortex distributions.

2-
The oil vortex is close to the pipe core and its strength decays obviously after half of the pipe.
3-Pressure has a minimum value in the core region after separator and it has a main effect on the adverse axial flow.
4-Water purity at cyclone exit was 90% for 25% oil volume fraction received at its inlet, while the obtained oil purity was 38%.