Introduction

Deoiler Hydrocyclones were developed for the offshore oil industry in the 1980’s and rapidly became established as standard equipment used for recovery of oil from Produced Water streams.
Deoiler Hydrocyclones exhibit the following benefits:

• Compact design, replace substantially larger equipment
• Cost effective with low OPEX costs,
• No moving parts, little maintenance required
• Highly consistent performance,
• Proven technology.
• Suitable for use on FPSO’s, etc.

Description

Deoiler Cyclones are used to recover liquid hydrocarbons from oily-water streams. The Cyclones are mostly installed in a pressure vessel in a cluster, with the required number (1-270) of Cyclones Liners to match the water flow rate.

Cyclone Liners are made in a range of sizes, (40-70 mm diameter) and are constructed from Duplex Stainless Steel to provide maximum durability and corrosion resistance.
As shown below, the Deoiler Cyclone Liners are installed into the vessel by hand, and can be removed for inspection, or Liners or Liner Blanks added with simple hand tools. (Liner Blanks are used where the water rate may begin at a low rate, and increase over time. Liners can be added or removed to match changing water rates).

Operating Principles

Deoiler cyclones are driven by inlet water pressure and utilise a pressure drop across the Cyclone to provide the energy or driving force to cause oil-water separation.
Normally, system pressure is used to provide the driving pressure, but if too low (<60 psi / 4 Bar), a pump can be used to boost the feed pressure. Typically, single stage centrifugal pumps are used, where pressures are too low.

View of Deoiler Hydrocyclone Vessel

Each Cyclonixx® Deoiler Cyclone Liner includes a tangential Inlet Section where the water enters, and is forced to spin rapidly, generating high centripetal forces. These forces, combined with the tapering shape of the internal profile, accelerate the spinning. This effectively forces the water away from the centre axis to the outer walls, and forces the lower density oil to the central core that forms along the axis of the Deoiler Cyclone.

The water spirals down the tapered section of the Cyclone and exits via the clean Water Outlet nozzle. The central oil core is forced in the reverse direction by back-pressure on the Water Outlet, and exits via a small orifice as the Oil Reject Outlet. The Oil Reject stream flow is largely controlled by this orifice size, but can also be regulated by an outlet control valve, and is typically set to allow a flow at 2-4% of the inlet flow.

The gravitational forces generated within each Cyclonixx® Deoiler Cyclone Liner are very high, and therefore these Deoiler Cyclones can be installed vertically, horizontally or on moving structures (eg: FPSO’s).

Technical

There are a few important factors involved in the process design and selection of a Deoiler Cyclone Liner type, and these are:

• Available pressure/Pressure Drop,
• Oil droplet size & distribution,
• Liquid Viscosity/Temperature,
• Cyclone diameter & required Oil Recovery,

Available Pressure /Pressure Drop
It is usually preferred to utilise the full system pressure to drive Deoiler Cyclones to maximise oil recovery and throughput. The preferred site to locate Cyclones in a process is on the water outlet line from the Separator or Water Knock-out Vessel, upstream of the Level Control Valves. This usually provides the highest capacity with minimal droplet shearing.

The operating pressures for a Cyclonixx® Deoiler Cyclone are defined by the Pressure Differential Ratio (PDR). The PDR is defined as:

Where:
P.Inlet  = Pressure at inlet of Deoiler Cyclone,
P.Reject = Pressure of Reject oil stream outlet,
P.Outlet = Pressure of Water Outlet stream.

PDR should be maintained in the range of 1.7 – 1.8.

Pressure Drop vs. Flow for one XL-40 Cyclone Liner
A graph depicting the effect of increasing pressure drop is shown below, with Pressure Drop vs. Flowrate for an XL-40 Deoiler Cyclone Liner.

Note: the above graph is for one type of Deoiler

Process Group manufactures a number of cyclones as Spare Parts for most common brands of Deoiler Cyclones.

Oil Droplet Size /Distribution
The Oil droplet size range has a major impact on the Deoiler Cyclone’s performance, and it is important to maintain as little shearing of oil droplets as possible prior to the Cyclone, by installing upstream of control valves to avoid excessive droplet shearing.

Liquid Viscosity/Temperature
Temperature has a direct effect on the water viscosity, which has a significant effect on Cyclone’s performance. At higher temperatures the water viscosity is reduced, creating less resistance to the separation of oil droplets. Higher temperatures will result in higher levels of Oil/water separation.

Cyclone Diameter/Performance
The effect of the diameter of the Deoiler Cyclone Liner on oil & water separation is very important:. Smaller diameter Cyclones provide higher levels of oil removal, but they have a lower capacity.  To cater for this, multiple Cyclone Liners are packaged inside each vessel to treat large flows, while maintaining high oil-removal levels.

Deoiler Unit Sizing:
Process Group provides a range of Deoiler Cyclone Vessel sizes, but the following provides a guide to typical vessel capacities.

Deoiler Cyclone Vessel Size on Flowrate