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The properties of oil and water are sufficiently different to make distinguishing them relatively easy. This article describes some of the state-of-the-art sensors used for oil in and on water detection, including interfaces.
Oil on water
Outfalls from ships and municipal or industrial waste treatment plants must be monitored because if the emissions contain hydrocarbons they’ll float on the water’s surface and form a barrier, which will prevent the oxygenation of the water and cause fish to suffocate. On-off, oil-on-water detectors are capable of measuring even a few drops of petroleum floating on the surface, so these alarm devices are used downstream of plant outfalls and at oil loading and unloading stations of tankers and trucks.
Laser nephelometers measure light scattered by particles contained in all oils. They operate by focusing a laser beam onto the surface of the water. A second, receiving lens refocuses the reflected (scattered) light onto a photocell. When there’s no oil on the water’s surface, only a minimum amount of reflection occurs. When floating oil is present, reflected light intensity increases substantially due to light scattering caused by particles in the oil. The measurement is based on the differential between the outputs of the measurement photocell and a reference photocell, which measures the output of the light source itself.
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The sensing head is usually mounted on pontoons or floats in the water. An s-shaped baffle can be provided to direct the flowing water past the sensing head. Accurate detection requires regular, conscientious maintenance for continuous and reliable performance. The capacitance approach used for monitoring of oil film thickness on the water, which I will discuss next, requires less maintenance, but is limited to detecting larger quantities of floating oil.
This device requires maintenance, as do all optical measurements because the windows must be kept clean. If an air column is provided between the water surface and the window, that column will reduce fouling caused by splashing. Therefore, these sensors are mounted above the water. This instrument must also have optical filters to eliminate effects of sunlight or other stray light sources.
Oil slick thickness can be detected by capacitance sensors that detect the thickness of the oil layer by generating a DC signal for transmission. The generated signal is in proportion to the inverse of the measured capacitance because total capacitance between the plates drops as the thickness of the low dielectric constant oil rises (Figure 1).
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The dielectric constant of water (80) is so high relative to that of oil (1.9 to 2.1) that the capacitance contribution of water can be neglected and the oil thickness can be calculated (estimated) based only on that of the oil (κoil) as shown in the equation below:
toil = κoilA/C
where:
- A = effective area of one capacitor plate
- C = measured capacitance
- toil = oil thickness
- κoil = dielectric constant of oil
Oil in water
Water discharged into lakes or rivers should not contain oil because it contributes to the biological oxygen demand (BOD) of the discharged water and can also be toxic to the aquatic biota or to the fish themselves.
Ultraviolet radiation can be used to detect the oil contamination of water. When UV radiation is sent through an oil-contaminated water sample at a peak intensity of 365 nm, visible radiation (400-800 nm) is emitted. The intensity of this radiation can be measured by a photocell. The intensity of this emitted radiation increases as the concentration of oil (the fluorescent substance) rises.
The most common configuration is to pass the process sample through the sensing head in an up-flow direction (Figure 2). The head is equipped with two windows that are set at right angles to each other to minimize the intensity of direct radiation from the source striking the photocell, and also to reduce the effect of multiple scattering of the visible radiation. Optical filters at the incident and at the emergent windows are used (not shown) to reduce this effect to a negligible level.
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The UV analyzer used in this system is a single-beam, dual-wavelength analyzer. This is superior to single-wavelength designs because it’s able to compensate for variations in sample sediment content, turbidity and algae concentration, and also for window coatings.
Oil-water interface detection
The capacitance of water is much higher (its dielectric constant is about 80) than that of oil (about 2), so measuring the dielectric constant is a convenient way to tell them apart. In addition to conventional capacitance probes, special dual-concentric designs are also available to detect the interface between water and oil in tanks. In addition, flow-through sensors are also available for in-pipeline applications.
The flow-through version of the dual-concentric electrode consists of two concentric pipes that are insulated from each other, thereby forming the capacitor through which the process stream flows. The unit is available both for switching or for transmitting applications, and can be used for both oil-in-water and water-in-oil measurement.
Radio-frequency (microwave) sensors use the fact that shortwave RF energy is absorbed much more efficiently by water than by oil. A radio-wave detector produces waves of a constant amount of energy. The more this energy is absorbed by the process fluid (the more water is in the mixture), the lower will be the voltage at the detector. The advantages of this design (relative to capacitance sensors) include wider range (0% to 100%), lower sensitivity to buildup, insensitivity to temperature and salinity variations, and suitability for higher-temperature operations (up to 450 °F or 232 °C).
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Water dump control can use RF interface detectors for “free water knockout” (Figure 3). The probe is installed horizontally at one-third of the diameter of the separator vessel, and is set to open the water dump valve when the emulsion concentration drops below 20% oil. This way, the emulsion (rag layer) will build up above the probe. These instruments can detect water concentration within about 5%.
Rag layer profiler portable tank profilers are also available, using RF principle of operation. Here, the tape-supported radio frequency element is gradually lowered into a tank, which can be up to 100 feet (30 m) tall. As the sensor is lowered, it measures both the location of the interface (within an error of 0.12 in. or 3 mm), and it also measures the emulsion concentration throughout the tank height (from 0% to 100% within an error of 1%).
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