Cyclone devices partition two or more phases based on their weights and separate gas-liquid-solid components in a variety of applications. In the petroleum industry, cyclones are used to remove grit or droplets from gas pipelines, segregate sand or oil from produced water, debottleneck gravity settling vessels, or even split mixed gases with different densities. Fig. 1 presents the two main types of cyclones, with examples of cyclone devices.
The continuous phase flowing through the unit designates the cyclone as a pneumatic (gas) or hydraulic (liquid) cyclone.
A cyclone converts the potential energy (pressure) of the transport fluid into rotating kinetic energy (vortex flow) using only the shape of the device to partition two or more phases in that fluid into the respective number of concentrated outlet streams. Key factors for this technology include – the absence of moving parts (static device), a tangential or involute inlet, a cylinder-cone body (see central graphic in Fig. 1), a combined free-forced vortex flow pattern, and a classifying operation (i.e., separation based on particle weight not size). Other swirling devices provide efficient phase separation, such as centrifuges with motor driven rotating discs, but if they are without the key factors listed, they are not characterized as cyclones.
The most attractive feature of cyclones to the upstream oil & gas industry is the equipment size. A cyclone has the highest throughput-to-size ratio of any partitioning device. Thus, for a given flowrate, a cyclone will have the smallest footprint and weight of any separating technology.
Hydrocyclones versus Desanders
The term hydrocyclone indicates a liquid continuous classification device (i.e., Fig. 1). Hydrocyclone is a contraction of hydraulic cyclone. The hydrocyclone is used widely in process industries, with its predominant use and research in mineral processing. There, the technology is used in closed-circuit grinding of ore to partition particles in a highly concentrated slurry stream (solid-solid classification). These devices were primarily developed in the 1940s (the first patent was filed in 1891), and they are found in every mining region. The mineral processing hydrocyclone operates at a low inlet pressure (1030 psig) and a high solids concentration (40-60 wt.%) with an atmospheric discharged underflow and overflow.
The desanding hydrocyclone – simply termed the “desander” – is a derivative technology from mining hydrocyclones. They were initially developed in the 1960s to keep sand out of agriculture irrigation nozzles. The original desanders had a simple design for operation at a low inlet pressure (<20 psig) and a low sand concentration (<50 ppm). They were first used in the oil & gas industry in 1964 with Saudi Aramco.
The primary difference between a hydrocyclone and a desander is that the latter operates with an enclosed underflow chamber, as shown in Fig. 2. Both units have continuous inlet and overflow streams, but the desander has a batch underflow. Whereas the mining hydrocyclone processes fluids with a high solids content, the enclosed underflow on a desander limits the unit to a low solids concentration (<<1 wt.%). Enclosing the underflow also prevents air-core formation in the center of the vortex flow. The air core, which is present in mining hydrocyclones, is a key feature in their modeling and operation but is absent in a desander. The desander is properly termed a “flooded-core hydrocyclone”. The first research on the flow pattern and particle trajectory within a flooded-core static hydrocyclone was recently published.
The Purpose Of Hydrosyclone Desander
- Separate: to partition solid particles from liquid, gas, or multiphase flow to a separate stream. Unit processes include a hydrocyclone desander (liquid or multiphase), production separator, or filter.
- Collect: to gather partitioned solids into one central location and remove from process pressure and flow. This is accomplished using a hydrocyclone accumulator, vessel drain, or sump tank.
- Clean: to remove adsorbed hydrocarbon contaminants from sand particles using an attrition scrubbing system. This is an optional step, which is only used to treat sand for overboard discharge.
- Dewater: to remove free water from sand slurry to minimize the disposal volume. This can reduce the disposal volume by >90%. The simplest method involves a hanging mesh bag or screen lined bin.
- Transport: to bring solids to the disposal location. The design of the transport system depends on the
facility location (subsea/onshore/offshore) and environmental requirements.