Summary
An extensive experimental program has been conducted using a 2-in. hydrocyclone. The inlet flow conditions are: total flow rates between 18 to 27 GPM, oil-cut up to 10%, median droplet size distributions from 30 to 180 µm, and inlet pressures between 60 and 90 psia. The acquired data include the flow rate, oil-cut, and droplet size distribution in the inlet and in the underflow; the reject flow rate and oil concentration in the overflow; and the separation efficiency. Additional data were taken from the literature, especially from the Colman and Thew1 study. This experimental investigation provided insight into the hydrodynamic flow behavior in the liquid/liquid hydrocyclone (LLHC), and helped develop and refine a mechanistic model for the LLHC, which is presented in Part 2 of this paper.
Introduction
The petroleum industry traditionally has relied on conventional gravity-based vessels that are bulky, heavy, and expensive to separate multiphase flow. The growth of the offshore oil industry, in which platform costs to accommodate these separation facilities are critical, has provided the incentive for the development of compact separation technology. Hydrocyclones have emerged as an economical and effective alternative for produced water deoiling and other applications. The hydrocyclone is simple in design with no moving parts, easy to install and operate, and is inexpensive to purchase and maintain.
Hydrocyclones have been used in the past to separate solid/ liquid, gas/liquid, and liquid/liquid mixtures. For the liquid/liquid case, both dewatering and deoiling hydrocyclones have been used in the oil industry. This study focuses only on the latter case, namely using the LLHC to remove dispersed oil from a water continuous stream.
Oil is produced with a significant amount of water and gas. Typically, a set of conventional gravity-based vessels are used to separate most of the multiphase mixture. The small amount of oil remaining in the water stream, after the primary separation, has to be reduced to a legally allowable minimum level for offshore disposal. LLHCs have been used successfully to achieve this environmental regulation.
There is a large quantity of literature available on the LLHC, including experimental data sets, computational fluid dynamic (CFD) simulations, and modeling. However, there is still a need for more comprehensive and detailed data sets, including measurements of the inlet and underflow droplet size distributions, utilizing appropriate sampling procedures. The objective of this study is to conduct detailed experimental investigation to provide insight into the hydrodynamic flow behavior in the LLHC and to help develop and refine a mechanistic model for the LLHC (to be presented in the second part of this paper).
LLHC Hydrodynamic Flow Behavior.
The LLHC, shown schematically in Fig. 1, uses centrifugal force to separate the dispersed phase from the continuous fluid. The swirling motion is produced by the tangential injection of pressurized fluid into the hydrocyclone body. The flow pattern consists of a spiral within another spiral moving in the same circular direction (Seyda and Petty2). There is a forced vortex in the region close to the LLHC axis and a free-like vortex in the outer region. The outer vortex moves downward to the underflow outlet, while the inner vortex flows in a reverse direction to the overflow outlet. Moreover, there are some recirculation zones associated with the high swirl intensity at the inlet region. These zones, with a long residence time and very low axial velocity, have been found to be diminished as the flow enters the low-angle taper section (see Fig. 1).
An explanation of the characteristic reverse flow in the LLHC is presented by Hargreaves.3 With high swirl intensity at the inlet region, the pressure is high near the wall region and very low toward the centerline, in the core region. As a result of the pressure gradient profile across the diameter, which decreases with downstream position, the pressure at the downstream end of the core is greater than at the upstream, causing flow reversal.
As the fluid moves to the underflow outlet, the narrowing cyclone cross-sectional area increases the fluid angular velocity and the centrifugal force. It is due to this force and the difference in density between the oil and the water that the oil moves to the center, where it is caught by the reverse flow and separated, flowing into the overflow outlet. Instead, if the dispersed phase is the heavier, like solid particles, it will migrate to the wall and exit through the underflow. Thus, for these two different separation cases, two different geometries are needed (Seyda and Petty2). In the deoiling case, usually 1 to 10% of the feed flow rate goes to the overflow.
Another phenomenon that may occur in a hydrocyclone is the formation of a gas core. As Thew4 explained, dissolved gas may come out of solution because of the pressure reduction in the core region, migrating fast to the LLHC axis, and eventually emerging through the overflow outlet. An experimental study on the effect of gas on the LLHC performance is found in Smyth and Thew.5
LLHC Geometry.
The deoiling LLHC consists of a set of cylindrical and conical sections. The Colman and Thew6 design has four sections, as shown in Fig. 2. The inlet chamber and the reducing section are designed to achieve higher tangential acceleration of the fluid, while reducing the pressure drop and the shear stress to an acceptable level. The latter has to be minimized to avoid droplet breakup leading to reduction in separation efficiency. The tapered section is where most of the separation is achieved. The low angle of this segment keeps the swirl intensity with high residence time. An integrated part of the design is a long tailpipe cylindrical section in which the smallest droplets migrate to the reversed flow core at the axis and are being separated flowing into the overflow exit. This configuration gives a very stable smalldiameter reversed flow core, utilizing a very small overflow port.
