Design and Operation of Dissolved-Gas Flotation Equipment for the Treatment Of Oilfield Produced Brines
Authors: Sport M.C. 
Published: December 2007

To clean up oilfield brine before dumping it into public waters, a major oil company uses gas flotation. Two methods have been successful - vacuum flotation and pressure flotation - both based upon fundamental laws governing the solubility of gases in liquids at varying pressures.

Introduction

To minimize the possibility of polluting State or Federal waters in the Gulf Coast Area, Shell Oil Co. is currently using dissolved-gas flotation as a method of treating produced oilfield brine for the removal of residual oil prior to disposal. Currently, Shell has six dissolved-gas flotation systems with a total design capacity of 250,000 BWPD operating in the Gulf of Mexico. Five more systems to provide treatment for approximately 200,000 BWPD are currently being installed at both onshore and offshore locations, and the installations of 12 additional systems is planned for the immediate future to treat an estimated 365,000 BWPD.

Theory of Gas Flotation

Gas flotation is a process used in separating dispersed solids and immiscible liquids from a continuous liquid phase. Separation is facilitated by finely dispersed phase. Separation is facilitated by finely dispersed bubbles that result from adding a gas phase to the system. These finely dispersed bubbles are attached to the suspended particle either because of the intermolecular attraction existing between the gas and suspended particle, or as a result of physical capture. The rate of rise of a particle moving through a fluid may be described by the use of Stokes' law,

(1)

where

D = particle diameter, g = gravity constant, L = density of the supporting medium, p = density of the suspended particle, = viscosity of the suspended medium, and V = rate of rise of the suspended particle.

This has been shown to be valid for Reynolds numbers equal to or less than 0.1. During flotation the attachment of dispersed gas bubbles to the suspended particle reduces the particle density, thereby particle reduces the particle density, thereby producing a faster rise rate and accelerating the separation producing a faster rise rate and accelerating the separation of suspended particles, either solid or liquid, from the continuous liquid phase. The accelerated use rates obtainable in flotation make the process applicable in installations where adequate surge capacity for an acceptable degree of separation cannot be provided because of economic or installation requirements. In installations where the process is applicable, retention times are far less than those required for surge vessels to obtain comparable results. The two methods of gas flotation that have been used most successfully in treating waste water are vacuum flotation and pressure flotation. Both processes are based upon fundamental laws governing the processes are based upon fundamental laws governing the solubility of gases in liquids at varying pressures. Vacuum flotation is accomplished by first subjecting the solution to be treated to a period of atmospheric aeration and then applying a vacuum that promotes the precipitation of air from solution. In promotes the precipitation of air from solution. In the process of pressure flotation, gas is contacted at an elevated pressure with the solution to be treated and is precipitated from solution by an instantaneous reduction in system pressure.

JPT

P. 918