Although induced gas flotation is routinely practiced on fixed platforms, there are special challenges for practicing this technology on deep water platforms. This paper presents an overview of the physical and chemical principals of how induced gas flotation actually works, reviews the special challenges involved in applying induced gas flotation technology on a floating platform, and outlines the process and vessel design requirements for the effective use of compact induced gas flotation on a deep water platform.

The challenges for induced gas flotation on a floating platform include designing compact systems that minimize space and weight requirements, dealing with water that may contain hydrate inhibitors such as methanol, cleaning water with very small (and relatively stable) oil droplet size distributions, cleaning water below the Wax Appearance Temperature, and designing compact flotation cells that are tolerant of slosh-motion. The negative impact of process recycle streams on flotation will be illustrated.

The successful practice of compact induced gas flotation on a deep water platform requires a combination of good overall process design, an understanding of the water chemistry involved, and well designed equipment. An example is given to illustrate how process design considerations can affect the performance of flotation as a water cleaning technology.

Flotation Mechanisms

There are three mechanisms by which oily contaminants are removed by induced gas flotation. These mechanisms operate singly or in combination depending upon the specific characteristics of the produced water and the level of attraction and wetting between the gas bubbles and the oily contaminants. The mechanisms are as follows:

  1. Oil coats the rising gas bubbles. This mechanism is operable when the contact angle between the oil and the gas bubble is low (wetting is high).

  2. Oil droplets and oily contaminants stick to a gas bubble and rise with it to the surface. This mechanism is operable at moderately low contact angles (modest attraction of oil to gas bubble) and is most efficient when contaminants and gas bubbles are of similar size.

  3. Oil droplets and oily contaminants are dragged behind in the wake of a rising gas bubble to the water's surface. This is the weakest interactive mechanism and does not require a low contact angle.

Some authors believe that the third mechanism, although being the weakest, is dominant in most oilfield flotation systems.1,2

Mechanical Requirements for Efficient Flotation

For all of the listed flotation mechanisms, efficiency is enhanced by maximizing the number of interactions between the contaminants and the gas bubbles. Interactions are maximized by the following:

  1. Uniform distribution of oily water throughout the flotation cell

  2. Uniform distribution of gas bubbles throughout the flotation cell

  3. Minimizing the size of the gas bubbles

  4. Maximizing the size of the contaminants to be removed

  5. Maximizing the tendency of the contaminants to remain in contact with rising gas bubbles

Vessel Hydraulics.

The first points are related to flotation cell hydraulics.

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