Clarifying Oilfield and Refinery Waste Waters by Gas Flotation

Abstract

Gas flotation is a method of removing residual oil and suspended solids from oilfield and refinery waste waters. Because the system has a history of generally effective performance and because the equipment involved requires little space, it appears that flotation could be a desirable way of handling some very big problems. Introduction For more than 20 years the Union Oil Co. of California has used gas flotation as an effective means of removing residual oil and solids from many waste waters before disposing of them. Proper selection of equipment, flotation gas, demulsifiers, coagulants, and other factors makes it possible to tailor the flotation process for optimum treatment of most waste waters. Relatively small space requirements and minimum retention times are important - sometimes essential - features of gas flotation equipment. Increasingly stringent waste-disposal requirements dictate a new look at the advantages and practical limitations of this method. Waste waters can be clarified in many ways, each method having its own advantages and limitations. Surge vessels, skim ponds, API separators, and lagoons are adequate if the oil is not emulsified, if few entrained solids are present, and if there is adequate space or retention time. Coalescing filters can often improve the performance of these simple systems by helping to break emulsions and by trapping oily solids and slimes. Often, chemical demulsifiers or coagulants are also effective. Combinations of these systems have been used for many years with varying degrees of success. For example, a random sampling of coalescing tanks produced the varying results shown in Table 1. The quality of treated waters depends less on coalescer capacity and throughput than upon the presence of emulsions and the type of solids in the water. However, there are many locations where adequate space is not available for conventional equipment, and others where clarification must exceed the levels obtainable by the older methods. In these locations, gas flotation should be considered. Principle of Flotation Principle of Flotation Gas flotation speeds tip the gravity separation of oil droplets and dispersed solids from waste water. According to Stokes' law, the vertical movement of a particle through a fluid is expressed as particle through a fluid is expressed as where V = rate of rise or fall of the particleg = gravity constantpf = density of the fluidpp = density of the particleD = deiameter of the particlemu = viscosity of the fluid Gas flotation modifies two of the variables listed above: particle density and particle diameter. JPT P. 426