Gas Flotation Separation

In onshore applications, initial separation of oil from the unwanted solids present when the oil is produced is often accomplished by a gravity process. However, oil and solid particles are often so small that they distribute themselves evenly in the water phase and will not settle readily by gravity alone. To meet the discharge/reinjection specification, additional separation processes may be required which involve some type of mechanical or hydraulic mechanism to remove the oil and solids. Equipment used in this secondary separation process includes induced gas flotation (IGF) and dissolved gas flotation (DGF).

IGF and DGF involve the introduction of fine gas bubbles that attach to the oil and fine solids in the produced water and then float to the surface where they are removed to slop oil systems. Flotation units can remove up to 95% of free oil and are most commonly used as a process step downstream of gravity separation to meet discharge limits or to reduce the load on filtration systems. Inlet oil content should not exceed 1000 ppmv and should preferably be below 300 ppmv.

Produced gas recirculation is generally used to generate the micro bubbles. The introduction of air is not recommended in produced water systems because this makes the water very corrosive to steels, leads to the precipitation of dissolved solids and can result in the creation of flammable gas mixtures.

The most common flotation separators in onshore produced water treatment are horizontal IGF types, generally multi-cell mechanical or hydraulic units. They have lower retention times than DGF and are therefore physically smaller, but generally have a much higher float recycle rate than a comparable DGF unit. IGF is efficient at elevated water temperatures, which are often experienced at the wellhead. Without chemical addition, flotation units should remove particles larger than 25 microns. With chemical addition to coagulate the oil and solids, particles less than 10 microns may be removed.

In an IGF unit, gas is introduced into the water either mechanically or hydraulically.

Mechanical distribution (fig. 1) uses a draft tube and rotating impeller to draw gas from above the liquid level and discharge it into the water. The result is a turbulent mixture of gas and liquid. Rising gas bubbles attach to oil and solids and float these particles to the surface where they are removed into a skimming launder. Retention time is generally between 4 to 8 minutes depending on the method of inducing the gas.

Hydraulic IGF units (fig. 2) use eductors to inject gas into the flotation cell. Gas bubbles are often larger than those generated by mechanical means, resulting in a system that is less turbulent, but providing more retention time for separation. Hydraulic IGF units have fewer moving parts than mechanical units.

For both types of processes, the floating material is watery and needs further thickening or recycle. Subnatant from oil thickening can be recycled upstream of the IGF. Recovered float material may also be recycled to upstream separation equipment, though this can lead to the development of rag layers. Skimming devices exist to reduce the water content of the float and lower recycle rates. Still float recycle can account for 3 to15% of feed flow.

DGF is very effective for removing oil and solid particles. This operates by taking a recycle stream of 10 to 50% of the treated water and dissolving gas into it under pressure. The recycle stream is combined with inlet water prior to, or within the flotation cell. The reduction of pressure as the recycle stream combines with the inlet stream causes gas to come out of solution, liberating very small bubbles of gas. These bubbles attach to and float particles to the surface where they are mechanically skimmed over a beach or weir.

Gas bubbles produced by DGF are much smaller and provide a denser bubble curtain than IGF, allowing DGF to operate with much lower skimming rates than IGF. Despite this advantage, DGF is rarely seen in produced water service as it requires more retention time and is sensitive to temperature, since gas solubility reduces as temperature increases. DGF efficiency tends to reduce as water temperature rises. Given the adaptability of IGF to variations in flow and conditions and its relatively small equipment size compared to DGF, it is generally the preferred flotation solution for produced water.

It should be noted that demulsifying chemicals may be added in the initial gravity separation process to aid removal of water from oil. These chemicals may have a detrimental effect on flotation cell performance. For this reason, some type of foaming agent may be needed to improve performance.