No-Solvent Oil-in-Water Analysis - A Robust Alternative to Conventional Solvent Extraction Methods
Authors: Brost Dale F. , Foster Amy , Holmes Michael 
Published: May 2011

Most oil-in-water analysis methods for produced water require the oil to be extracted into an organic solvent prior to measurement. Many of the organic solvents used for extraction are either extremely flammable, hazardous to human health or both. The chlorinated hydrocarbons are very expensive and must be either recycled or disposed of as hazardous waste. Volatile hydrocarbon solvents such as pentane and hexane are extremely flammable and present a serious fire and explosion risk. All major airlines and many helicopter services consider the risk so serious that they will not transport flammable solvents.

The methodology presented here makes it possible to perform oil-in-water analyses by making measurements directly on the produced water sample. No organic solvents are required. The method is based upon the addition of a detergent surfactant to a produced water sample. The surfactant converts the dispersed oil in the sample into an optically clear microemulsion that is ideal for direct fluorescence measurements using the TD-500D Oil-in-Water Analyzer. The surfactant is safe to handle with a minimum of personal protective equipment and is only slightly flammable even under a direct flame. The US Department of Transportation does not consider it to be a hazardous material. It can be shipped without hazardous identification labels and can be carried on commercial airlines and helicopters without declaration.

Two samples (" Background?? and " OIW??) are collected to perform an analysis. The Background sample is untreated produced water. It is filtered into a measurement cuvette through an ultra-filter to remove suspended solids and dispersed oil. Only water-soluble substances pass through the filter into the cuvette. The OIW sample is collected into a bottle containing surfactant. The sample is then heated to the cloud point of the surfactant and allowed to cool until the cloudiness disappears. This converts the dispersed oil into a stable microemulsion. The dispersed oil is located inside micelles that are small enough to pass through an ultra-filter. The converted OIW sample is then filtered into a cuvette. The TD-500D readings for the Background and OIW cuvettes are then recorded. The dispersed oil concentration is the calculated difference between the OIW and Background readings. The Background reading itself provides additional information. Since it is proportional to the concentration of fluorescent water-soluble organics in the produced water sample, the Background reading can be used to track changes in the concentrations of these substances.

The TD-500D has two measurement channels, A and B. Channel A makes fluorescence measurements using ultraviolet light and is used when the highest sensitivity is required. Channel B uses visible light for reduced sensitivity and extended dynamic range. With the " No-Solvent?? method, channel A detects most crude oils at dispersed oil concentrations less than 1 ppm. The linear range is from 0 to at least 100 ppm. The dynamic range can typically be extended to 750 ppm or greater with a non-linear calibration function. When the instrument is set to channel B, the " No-Solvent?? method can measure dispersed oil concentrations up to 10,000 ppm, eliminating the need to dilute highly concentrated oil-in-water samples.