Surfactant-assisted oil-in-water monitoring

A variety of common surfactants have been used to reduce fouling and enhance the performance of fluorescence-based oil-in-water monitors. Among the most effective are the alcohol ethoxylates with HLB values close to 12. Examples are SurfonicTM L24-9 (Hunstman, USA, HLB 13) and TergitolTM 15-S-7 (HLB 12.1). These surfactants readily convert heterogeneous oil/water mixtures into homogeneous, optically clear, microemulsions. They also emit no detectable fluorescence and therefore do not interfere with the fluorescence emitted by dispersed oil and water-soluble organics. Fluorescent surfactants, such as the alkylphenol ethoxylates (e.g. TergitolTM NP-9, HLB 12.9), also efficiently create microemulsions. They can be used without interference if the oil can be detected at analysis wavelengths that ignore the surfactant (i.e Excitation >325 nm).

Field studies have shown that detergent surfactants can significantly reduce the accumulation of fouling deposits from crude oil and suspended solids, often keeping monitors clean for months without operator intervention.1 Surfactants also condition the water sample for better measurement by releasing oil from suspended solids and breaking up large oil droplets (particle diameter 1 to 200 μm) into tiny micelles (particle diameter 0.003 to 0.2 μm) that naturally disperse themselves evenly throughout the water stream. The increased homogeneity of the sample and the radical reduction in the size distribution of the dispersed oil particles amplifies the amount of fluorescence emitted by the dispersed oil. The degree of amplification depends on the oil type. Amplification factors of ~2X have been observed with light oils (>20°API). Amplification factors for heavy crudes are typically 20X or greater. The fluorescence emitted by water-soluble organics (WSO) is not affected.

In practice, the surfactants are injected at constant rate into a side-stream carrying produced water to the instrument, at a concentration in excess of the surfactant’s critical micelle concentration (CMC). Since they are either viscous liquids or waxy solids in their 100% active forms, they must be diluted with water before use. Additional ingredients (e.g. methanol, acetic acid, etc.) may also be added to prevent the blend from freezing or to help break up precipitated solids that trap oil droplets.

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