A comparison of oil-in-water analyzers for VOC emission control
Leaks of hydrocarbons from heat exchangers can emit large quantities of hydrocarbons and Volatile Organic Carbons (VOCs). In addition to posing a flammable hazard, leaks can result in atmospheric emissions. Approximately 99.9 to 100% of VOCs are stripped to the atmosphere from cooling towers. For example, a typical cooling tower with 10 ppm by weight hydrocarbon leak in the heat exchanger can emit a 15,000 lbs. per day loss of product at 1,500 million lbs. per day flow rate. Table 1, below, shows chemicals and their lower explosive limit.
Unfortunately, airflow through a cooling tower dilutes the hydrocarbons, allowing higher concentrations than what would be registered in an ambient air monitor.
Continuous monitoring of hydrocarbon leaks directly in cooling water at the return pipe (at a point downstream of the exchanger but upstream of the tower) provides the earliest warnings of leaks. An oil/VOC in water analyzer can provide warnings of tiny leaks, far before they become an issue, saving lost product and processing time.
Fluorescence Technology Faces Drawbacks in Monitoring Hydrocarbon Leaks
Many oil/VOCs in water monitors in service today utilize fluorescence technology that detects a fraction of hazardous hydrocarbons in water samples, leaving many hydrocarbons un-monitored and contributing to pollution issues around the world.
The popular fluorescence technology-based oil/VOC in water monitors emit light on the water sample, which fluoresces in presence of some oil or VOCs. The light is typically in the ultra-violet light spectrum or the IR spectrum. A detector, typically a spectrometer or photometer, detects the excitation of the aromatic hydrocarbons and attempts to quantify the concentration, typically using ppm engineering units.
Drawbacks exist with the fluorescence method of detection.
1. Many non-polluting elements found in water sources may be fluoresced, causing false positives, false high readings, and false high alarms. Particulate in water may include liquids and solids such as clay, wood, dirt, plankton, algae, and bacteria.
2. Optional filters to correct the problem of interference produce little results as particulates are difficult to filter completely and effectively. Clay particles can even pass through a one-micron filter. Filtering also results in the ‘scrubbing' of critical oil components that need quantification.
3. Issues with inorganic materials and gases absorbing light giving erroneous results. Naturally-occurring bubbles in the water also give erroneous readings. Besides, straight-chain VOCs and aliphatic compounds typically do not respond to the UV Fluorescence methods, causing undervalued readings (false negative) that can cause more problems than false positives.
4. While using a large surface sensing area to sample water to increase accuracy, the fluorescence-based oil and water analyzer has problems with turbidity. Oil hydrocarbons may still hide behind solids and evade measurement. This results in inaccurate readings.
5. While selective light filtering techniques, which use specific wavelengths, are used in an attempt to increase accuracy and sensitivity, the technique is highly sensitive to anything in addition to VOCs and oil molecules. As a result, the UV Fluorescence method suffers from cross-sensitivity with components in the water not intended to be measured, such as debris and contamination. Output readings including zero may be unstable. This filtering technique often blocks out important oil molecules critical for accurate and representative measurements.
Membrane-Sensor Methods Overcomes Fluorescence Limitations
Oil/VOCs in water monitors incorporating membrane-sensor technology offers a total hydrocarbon measurement - including aromatic and aliphatic hydrocarbons - for more accurate results in readings.
Instead of light refraction that is the key mechanism of the UV methods, this method combines proprietary membrane technology with a solid-state sensor housed in a temperature-controlled analyzer cabinet. (See photo 1) The water sample flows continuously into the heated Sample Transfer Stripper (containing ASK Membrane Technology) unit that effectively strips the VOCs/hydrocarbons from water and into a vapour phase. Carrier air then sweeps the vapor phase VOCs/hydrocarbons into the metal-oxide sensor for quantitative analysis in ppb or ppm levels. See Graphic 2.
The sensor measures aliphatic and aromatic hydrocarbons including important carcinogenic compounds BTEX like benzene, toluene, and xylene. UV Fluorescence methods cannot measure aliphatic compounds.
Turbidity never reaches the solid-state sensor. The proprietary membrane technology only allows molecules with a hydrocarbon bond that boil at or below 400°C to pass through, causing conductivity to increase across the metal oxide sensor. The sensors can output and display values on a local LCD or provide data into a remote or web-based monitoring and control system.
The membrane-sensor oil in water monitor can continuously monitor the discharge of produced water required by the Environmental Protection Agency and IMO to produce water discharge to limit the concentration to less than 29 ppm oil and less than 42 ppm grease for a 30 days average and daily maximum. The monitor is capable of low ppb level analysis.
The use of the membrane-sensor technologies overcomes many of the interferences, inaccuracies and maintenance requirements of the fluorescence and turbidity methods. Utilizing an exclusive sample transfer stripper and solid-state sensor, the water analyzer measures VOCs directly in the water as opposed to the air around the water, a method that misses VOCs and results in non-alarm events. It also measures very low levels (parts per billion) to detect a small leak very early before becoming an environmental issue.
Thomas Freeman is with Analytical Systems Keco.