Laboratory Results of Cleaning Produced Water by Gas Flotation

Abstract

Abstract Laboratory studies were conducted to determine the factors governingflotation-cell performance. It was learned that oil removal is increased byincreasing collision and attachment efficiencies and gas/liquid contact time.Collision efficiency is increased by increasing oil-drop size and gasconcentration and by decreasing gas-bubble size. Several factors (crude type, pH, temperature, etc.) were found to change attachment efficiency. Nocorrelation between basic system properties and attachment efficiency wasfound. Some of these important parameters are determined by the cell design, whereas others are characteristics of the feed. Thus, the same type (design)cell will produce different effluent oil concentrations for different feeds.Also, the effluent from a given cell will change when the feed characteristicschange. Introduction Flotation cells are used widely throughout industry to remove oil fromoil/water mixtures produced from underground reservoirs. This is becauseflotation cells have proved to be a practical and reliable means of oilremoval. Small-scale pilot tests have been attempted to predict the performanceof a full-scale unit, but these have not been too successful. Therefore, laboratory studies were initiated to increase the knowledge of this process.The process of flotation consists of four basic steps:bubble generation inoily water,contact between a gas bubble and an oil drop suspended in thewater,attachment of the oil drop to the gas bubble, andrising of thegas/oil combination to the water surface where the oil can be removed byskimming. Research was conducted to investigate these basic steps and todetermine the fundamental mechanisms and parameters that govern the process.Both the theoretical and experimental results are discussed in this paper. Theory Bubble Generation There are three common methods of bubble generation:dissolution of gasfrom a supersaturated solution (dissolved gas flotation),mechanical mixingof gas and liquid (dispersed gas flotation), anddirect gas injection bymeans of a sparger. The method of bubble generation and important because itdetermines the bubble size and gas concentration for a given feed. As shown inthe section on bubble-drop contact, bubble size and gas concentration influencethe collision efficiency and, thus, the rate of oil recovery. Bubble/Drop Contact Gas bubbles and oil drops must come into contact for flotation to occur.This contact process is basically one of hydrodynamics. Since both oil and gasare less dense than water (with the exception of a few very heavy crudes), theyboth tend to rise relative to water. Gas bubbles are generally larger than oildrops (gas bubbles are usually larger than 100 m and oil drops in producedwater are usually smaller than 30 m), and the density difference between gasand water is much greater than between oil and water. For these two reasons, gas bubbles rise more rapidly than the oil drops and overtake them. This leadsto the possibility of bubble-drop contact. On the other hand, a fluid flowpattern is established around a moving gas bubble that tends to deflect oildrops, which reduces the possibility of contact. SPEJ P. 175