Combining production water treating devices boosts efficiency

Coalescence, gravity skimming, hydrocyclone, flotation
equipment used on alternative system

Kenneth E. Arnold, Michael Hale
Paragon Engineering Services

• Process block flow diagram for the triple combined cycle water treating system [10,834 bytes]
• S.P. grow a larger drop size distribution on the inlet to gravity settler [45,734 bytes]
• Typical size requirements [7,841 bytes]

There are many different methods of water treating commonly used to attain environmentally safe discharge water. Most often a series of different water treating methods is used in order to attain the required oil levels in the discharge water.

Each method for water treating has its own unique advantages and disadvantages, and the selection of a particular method or combination of methods is highly dependent on the overall process, the local environmental requirements, and considerations of operability.

A recent US Patent acquired by Paragon Engineering Services describes a combination of currently available water treating devices into a single package that will treat water from several low pressure sources and take advantage of the interaction between these devices in order to mitigate the operability problems associated with some of the individual items.

This arrangement can be referred to as a triple-combined-cycle (TCC) system. In this article, we will first discuss the three water treating methods included in the TCC water treating device (gravity settling with an SP pack, hydrocyclones, and gas flotation), and then we will describe how they are combined into a single unit for maximum efficient operation.

Gravity settling/SP pack

A gravity settling device relies on the driving force of gravity to separate the two immiscible phases and thereby effect a separation of the phases. This is the least effective of the listed methods of water treatment as there is little turbulence within the vessel to promote coalescence. An SP pack can be introduced in the inlet to a gravity settler to provide this coalescence and allow better separation associated with larger droplet size. The SP pack promotes turbulence by forcing the water to flow through a combination of pipe and specially designed turns, which is sized in order to create turbulence of sufficient magnitude to create the desired turbulence but not so great as to shear the droplets.

Since the SP pack is sized for a specific flow rate and a desired level of turbulence, the design is dependent on the Reynolds number of the inlet flow. The SP pack becomes ineffective at flow rates both below and above its design operating range. Therefore, for maximum effectiveness, it is desirable to maintain a nearly constant Reynolds number, i.e., flow rate, in the inlet.

Hydrocyclone separation

Hydrocyclone separation effects separation between phases by the tangential flow introduced at the inlet. The spiral flow forces the heavier phase (water) to the outside and the lighter phase (oil) toward the axis of rotation. The lighter phase is forced through the reject orifice at the inlet end of the cone, and the heavy phase spirals out the opposite end. Through the spiral motion of the fluids, a centrifugal force many times greater than that of gravity is imposed, thereby effecting a better phase separation than that of a simple gravity settling separation.

Experience has shown that hydrocyclones work very effectively as coalescing agents. A small amount of the oil in the inlet stream adheres to the surface of the cone and exits as very large oil droplets with the water underflush. This oil can be easily separated in a downstream gravity settler or flotation unit if no shearing device such as a level control valve is installed between the hydrocyclone and the downstream unit.

The hydrocyclone separation has many advantages, including high separation efficiency and small physical size. However, for effective operation, the hydrocyclone requires a constant throughput, significant inlet pressures, and the absence of droplet shearing devices before the inlet. Because a droplet shearing device (such as a control valve or centrifugal pump) positioned before the hydrocyclone will seriously affect the outlet water quality, it is common to provide a hydrocyclone for each water source, further complicating operability and increasing the cost of the system.

Manufacturers have designed hydrocyclones to work at low inlet pressures (50 psia), but field experience has shown that they are more efficient at recovering oil if the inlet pressure is on the order of 100 psig. In many production facilities, there are several streams of low-pressure water. If hydrocyclones are to be used, they require pumping through either low-shear progressive cavity pumps, which are relatively costly to purchase and maintain, or through specially designed banks of low-head centrifugal pumps in series.

Gas flotation

Gas Flotation is a method of water treating designed to coalesce and float oil droplets to the surface, where they may be more easily skimmed and separated from the water phase.

Gas flotation is effective in removing oil droplets as small as 15-20 microns from water by introducing very small bubbles of gas, which help to raise the oil to the surface in a froth that can easily be removed. Gas flotation is often used as the final stage of treating in order to remove the smallest oil droplets that have not been removed by previous stages of separation. However, gas flotation has proved ineffective in removing droplets less than 15-20 microns in diameter.

Gas flotation can be further differentiated as mechanical flotation or hydraulic flotation. Mechanical flotation occurs through the generation of bubbles introduced through mechanical agitation, commonly with mechanical paddles. Hydraulic flotation occurs with gas bubbles introduced via an inductor or sparger device. Both methods of flotation have specific advantages and disadvantages. Mechanical flotation has proven more effective in removing oil than hydraulic flotation while at the same time displaying higher maintenance requirements. Hydraulic flotation requires pump rates varying from 50% - 125% of the design rate.

Varying throughput through standard flotation units causes level changes over the water outlet weir or interface level, greatly affecting the oil skimming rate. At low water throughput, the oil is not skimmed, the oil pad builds, and oil is re-entrained in the water. At high water throughput, a large amount of water is skimmed over the oil weir; this situation can cause problems in the oil sump system.

Combined cycle

The TCC package is designed with a horizontal vessel containing three separate internal compartments and a centrifugal pump and hydrocyclone external to the vessel. The produced water influent first enters the vessel and resides in the inlet surge section. This section is sized based on the expected surges in the water production and effects no oil/water separation. The internals are designed in such a manner as to purposely keep an oil pad from building.

The water/oil mixture is then pumped into the gravity settling chamber in the vessel. Due to the fact that the pumping action will result in shearing and reduced oil droplet sizes, the fluids are first passed through an SP pack designed to increase droplet size and enhance oil/water separation. Residence time in the gravity settling chamber allows some additional coalescence to take place before water enters the hydrocyclone.

The oil level in the liquid-packed gravity settling section of the vessel is controlled by interface level control. The rejected oil is sent to the slop oil tank, and the water is sent to the second stage of treatment in the hydrocyclone.

The third stage, downstream of the hydrocyclone, treats the water by gas flotation. Sparge gas is introduced into the compartment through the sparger, which produces gas bubbles that help raise the oil droplets so that they may be removed. The gas flotation cell is controlled with interface level control on the water outlet and an oil bucket with level control. Treated water leaves the unit from the gas flotation cell.

The gas in the vessel, both from the sparge gas introduced and from gas dissolved in the inlet, forms a gas blanket for the first and third compartments in the vessel and is controlled by a pressure control valve on the outlet gas.

Design benefits

The triple-combined-cycle water treating device is an application of existing technologies, described above, that takes advantage of the favorable interactions among these technologies.

The arrangement of these technologies in an integral packaged product provides a single piece of water treating equipment that can be located on a platform or in an onshore production facility without the need for large interconnect piping for water lines, the expense of individual hydrocyclones for each water source, and without problems associated with surging and varying water production rates. Water can be gathered to the package from several sources which have different operating pressures without the need for special-purpose pumps.

A great advantage of this arrangement is the ability to take very low pressure water sources (near atmospheric) and effectively treat that water in three separate treatment steps. The usual hindrance to this result is the shearing of oil droplets due to the pumping action. This factor has been mitigated by the addition of the SP pack integral to the vessel.

Another advantage is gained by the interaction of the system curves of the centrifugal pump and the hydrocyclone. This interaction maintains the pressure of the liquid packed second compartment at a constant pressure and also maintains the water flow through the SP pack, hydrocyclone, and flotation section at a constant rate. The interaction of the pump and hydrocyclone curves results in a constant flow through the unit with no net pressure drop. The level control of the inlet surge section operates a control valve that returns clean water to the inlet to maintain the constant water rate through the package, thereby ensuring correct operating characteristics in the SP pack and the hydrocyclone.

The interaction of the pump and hydrocyclone curves allows the unit to work without the need for a liquid control valve on the hydrocyclone outlet. Thus, oil droplets that coalesce in the hydrocyclone and flow out with the water are much easier to separate in the flotation compartment of the unit. This factor is a major advantage of this design over other similar designs and process patents.

The triple-combined-cycle water treating device is a collection of commonly used technologies designed to benefit from the interactions among these technologies and eliminate common water treating problems such as those associated with varying and surging flow rates. It should be noted that this design is a new development. No packages are currently in use, and the device has never been fully tested. However, as the design is based on frequently used and tested components, the reliability of this concept is firmly established.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.