PRODUCED WATER TREATMENT

PRODUCED WATER TREATMENT

Produced Water treatment to meet the increasingly more onerous environmental discharge/disposal criteria has required the development of new innovative technology. Typically maximum Produced Water handling facilities are required towards the end of field life and with up to 95% of fluids processed being Produced Water compared with 5-10% at the start of field life so large turndown capabilities and operating flexibility of the Produced Water treatment equipment is required.

Fraser Uniquips Produced Water treatment technology portfolio includes :-

• Deoiling Hydrocylones
• Enhancements Units – Degassers and Induced Gas Floatation
• CPI’s and TPI’s
• Media Absorption

Enhancement Units

Enhancement Units vary in size, shape and complexity depending on the application and are typically installed downstream of a Hydrocyclone Unit for final polishing of the Produced Water to meet the required discharge/disposal criteria. Enhancement Units utilise Induced Gas Floatation technology which allow the small gas bubbles to attach to the oil particles increasing the buoyancy of the oil particles and enhancing gravity separation and coalescence.

The Fraser Uniquip Enhancement Vessel(s) comprise of the following features :-

• Deoiling Hydrocylones
• Enhancements Units – Degassers and Induced Gas Floatation
• CPI’s and TPI’s
• Media Absorption

1. Directional Inlet

The Produced Water Inlet Nozzle is tangential to the vessel resulting in incoming plug flow of Produced Water being set in a swirling motion. This effect maximises the flow path length of the Produced Water inside the vessel mitigating dead zones and allowing maximum time for the oily water to contact with evolved gas bubbles which are liberated for solution after entry of the Produced Water into the Enhancement Vessel. Indirectly, this swirling motion also results in the distribution of gas bubbles around the full cross section area of the Vessel. Internal directional baffles are used to ensure the swirling motion is maintained through the desired regions in the vessel. In large Enhancement Vessels hydraulically balanced multiple Produced Water inlet nozzles are sometimes used to ensure uniform flow and flux rate.

2. Central Outlet Skimmer

Continuous skimming of oil inside the Enhancement Vessel is necessary to mitigate re-entrainment of oil and to avoid an excessive hydrocarbon layer that can result in erroneous level readings. Hydrocarbon liquid is skimmed from the centre of the vessel via the internal Skimmer Tube which is important for Vessels installed on FPSO/FSO/FPU’s since the liquid level at the centre of the vessels remains constant. The Skimmer Tube is effectively an internal cylinder inside the Enhancement Vessel and also provides the “core” for the directional inlet to set up the swirling motion inside the vessel. Collected hydrocarbon liquids overflow for discharge on a continuous basis – typically the Skimmer Tube is hydraulically designed for 1-2% of the Produced Water inlet flow rate at the top of the Skimmer Tube into the central column. Skimming can be carried out manually or under level control. This arrangement is preferable to a Skimmer design which relies on an internal pipe “skimming” at the oil/gas interface as it mitigates constant gas blowby to downstream equipment.

3. Gas Sparging Assembly

The use of sintered porous gas spargers in the lower part of the Enhancement Vessel produces <50 micron gas bubbles necessary for correct Floatation. Dissolved gas in the Produced Water inlet evolves from solution relatively quickly after the upstream Hydrocyclone level control valve resulting in only a small zone of the Enhancement Vessel having sufficient gas bubble population to be effective. The sparging gas flow rate is typically set as a function of the incoming (or outgoing) Produced Water flow rate.

4. Outlet Collector

A hydraulically designed Outlet Collector is provided at the Produced Water Outlet nozzle to ensure an even draw down of Produced Water from the vessel as conventional vortex breakers have been shown to result in undesirable short circuiting. The tangential inlets to the Outlet Collector continue to maintain the swirling motion of the Produced Water at the bottom of the Vessel ensure that micro gas bubbles from the gas Spargers are properly distributed.

5. Inlet Mixer

The amount of dissolved gas in the Produced Water can vary significantly depending on the upstream processes, chemical Injection and Produced Water characteristics and as such where required an in-line Dynamic Sparger type Static Mixer located upstream of the Enhancement Vessel is installed. Gas injection is through a porous sintered pipe to ensure maximum gas absorption into solution. Produced Water flows inside the tube with gas entering externally. This arrangement avoids blockage of the porous tube due to solid particles in the Produced Water since the higher pressure clean gas flow is from the outside to the inside. The Dynamic Sparger temporarily super saturates the produced water as gas is dissolved into solution which is then released in the Enhancement Vessel. The use of a porous tube static mixer provides significant advantages over vane type static mixers which are very inefficient in absorbing gas into solution.

Media Absorption

For applications requiring the removal of dissolved hydrocarbons Fraser Uniquip is able to provide a Absorbent Media Units.