Earl S. Snavely

Chemical removal of oxygen from natural waters

Dissolved oxygen usually increases the corrosion rate of steel in natural waters. The effect is particularly severe in brines, because the presence of dissolved oxygen causes pitting. Therefore, various methods are employed to remove oxygen from oil-field waters, both fresh and brine, to protect the enormous investments in steel pipe and process equipment. Both mechanical and chemical methods are used to remove dissolved oxygen from water. Probably the most widely used mechanical method in the oil field is to strip the dissolved oxygen from water by countercurrent contact with a gas. This process requires a source of oxygen-free gas, usually methane. The most common chemical way of removing oxygen from oil-field waters is to add sulfur dioxide or sodium sulfite, because very little capital investment is required. A relatively simple apparatus was used to measure the rate of reaction of dissolved oxygen with hydrogen sulfide and with added chemical scavengers in oil-field waters. Results show that natural waters may catalyze or retard the reaction of sulfur dioxide or sodium sulfite with dissolved oxygen. Kinetic data for the sulfite-oxygen reaction can be obtained rapidly and accurately with a polarographic oxygen sensor.

Offshore oily-water separation

Various oily-water separation devices were evaluated for deoiling drilling-rig deck drainage. A loose-media coalescer selected for the application averaged 92% removal of free oil over a 1-year period of operation when it was fitted for solids removal and media cleaning. Dispersed-air flotation cells were found to remove the same amount of oil from water that could be removed by filtration with a filter retaining particles with diameters greater than 8 microns. Filtration is a simple and accurate method for determining the oil-removal efficiency that can be expected from a flotation separator for a particular application.