Oil in Water Conditions

Free Oil

• Droplet size 10 microns and larger
• Will separate in a 6″ high beaker in 90 seconds
• Requires gravity separation such as API type

Dispersed

• Droplet size 149 microns to 20 microns
• Will separate in a 6″ high beaker in 80 minutes
• Requires gravity coalescing such as the OCS separator

Mechanically Emulsified Oil

• Droplet less than 20 microns in size
• Will not separate in a 6″ high beaker in 80 minutes
• Requires chemical treatment and coalescing or chemical treatment and Dissolved Air Floatation or Ultra Filtration

Chemically Emulsified Oil

• Droplet less than 20 microns in size with surfactants or emulsifiers present
• Will not separate in a 6″ high beaker in 80 minutes
• Requires chemical treatment and coalescing or chemical treatment and DAF or UF, the surfactants will pass through with the permeate

Soluble Oil

• No discernible droplet size and the ions are crossed linked in the water
• Will not separate in a 6″ high beaker in 80 minutes
• Requires chemical treatment and coalescing or chemical treatment and DAF or UF, (with solublizers staying with the permeate)

Introduction

Environmental awareness is presently manifesting itself in tightening environmental legislation throughout the world.

Many industries discharge liquid waste contaminated with hydrocarbon or oil-like pollutants. Sources of such waste include petroleum and petrochemicals refining and processing, tramp oils from machine tool coolants, utility operations, sanitary sewage, bilge and ballast water, contaminated surface runoff.

Oil discharges into the environment typically have deleterious effects. Oily waste discharge creates potential safety hazards and consumes dissolved oxygen necessary to aquatic life.

Toxic effects of oil fall into two categories (Laws, 1981):

1. Effects due to smothering or coating of animals or plants with oil. Coating effects are most noticeable when large amounts of free oil are present as in an oil spill. Coating effects are not usually found when only parts per million (ppm) are present, as is the case of an industrial plant effluent.

2. Disruption of animal’s or plant’s metabolism due to the ingestion of oil and incorporation of oil into the organism’s fatty tissues. Generally, toxic compounds are not water-soluble but are oil soluble, thus tending to accumulate in body fat and damaging animals or human beings.

This paper introduces the oil-in-water separation theory and the basic criteria for selection and design of a separation system, presenting an up-to-date overview of technologies currently available and of their capabilities in terms of achievable discharge levels.

2. Background

Oil can exist in water in several forms:

• Free Oil is composed by oil droplets with a diameter exceeding about 30 microns. It rises quickly to the water surface when given a sufficient quiescent settling period.
• Mechanical dispersions are distributions of fine oil droplets ranging in size from less than 1 micron to 30 microns and having stability due to electrical charges and other forces, but not due to the presence of surface active materials.
• Chemical emulsions are distributions of oil droplets similar to mechanical dispersions, but which have additional stability due to chemical interactions typically caused by surface active agents present at the oil/water interface.
• Dissolved oil is dissolved in a chemical sense; the removal by normal physical means is impossible.
• Oil that adheres to the surface of particulate materials is referred to as oil-wet solids.

The degree of an oil/water separation problem depends on the oil droplet size distribution. Separation problems also involve chemicals other than oil, which have an effect on the treatment required.

The degree of the dispersion/emulsion of the oil is difficult to assess, but steps can be taken to discourage its formation by:

1. Preventing from using detergents, which may cause stable chemical emulsions.

2. Ensuring an adequate size piping to avoid the possibility of turbulence and formation of fine droplet dispersions.

3. Avoiding devices such as pumps, especially centrifugal ones, valves, especially globe ones, and other restrictions in flow, such as elbows, tees, etc., or simply unduly small line sizes, which may create shear stresses and enhance the formation of mechanical dispersions. As a matter of fact, where an oil and water mixture has to be presented to the separation equipment, it is of the utmost importance to select a pump which imparts the lowest amount of energy to the mixture, as any excess energy thus created has to be removed by the equipment. For this reason, the use of high speed centrifugal pumps, although relatively cheap and small, should be avoided and pumps of low shear characteristics, such as peristaltic or progressive cavity, used.

Ideal inlet conditions for an oil-water separator are:

1. Gravity flow (not pumped) in the inlet piping.
2. Inlet piping sized for minimum pressure drop.
3. Inlet piping straight for at least ten pipe diameters upstream of the separator (directly into nozzle).
4. Inlet piping containing a minimum of elbows, tees, valves, and other fittings.
5. Inlet piping should be as smooth as possible to avoid turbulence caused by pipe roughness. Smooth PVC is preferable to rough concrete. Nevertheless it must be pointed out that a slight amount of turbulence (i.e. at Reynolds numbers close to translation) is likely to increase the probability to coalescence with other oil droplets.

The importance of viscosity

From Stokes’ Law, the lower the water viscosity, the faster the oil rises in water.

Increasing the temperature will significantly reduce water viscosity. For example, oil droplets in water at 4 degrees C (40 degrees F) will rise at only half the rate as if they were at 32 degrees C (90 degrees F), if other parameters remain the same.

The importance of specific gravity and oil droplets diameter

From Stokes’ Law, the greater the difference in specific gravity between oil and water, the faster the oil droplet rise in water. For example, a fuel oil that has a specific gravity of 0.8 will rise twice as fast as oil with a specific gravity of 0.9.

The importance of both oil droplet diameter and specific gravity is pointed out in the following table where it may be seen the travel time for a 10 cm (4 in.) vertical distance for various values of the two parameters. Time is expressed in hr.min.sec.

Determination of specific gravity, viscosity and oil droplet size

Water viscosities as well as specific gravities of water and oil are readily obtained from literature data. It must be pointed out that, in the design phase of separation devices, a wide variety of temperatures (and therefore of viscosities and specific gravities) have to be considered to take into account summer and winter conditions as well as possible process upsets.

Oil droplet size is much more difficult to be determined. The most common way to measure it is by particle size counters such as laser light scattering analyzers.