Removal rates exceeding 90% should be easily attained. Stripping is relatively cheap to operate when few control systems are required for nitrogen blanketing or H2Scontrol. The process does not require much retention time but runs under pressure. Free oil can contaminate the packing material in the stripper and cause a loss of stripping performance until it is washed off or removed with surfactants. Suspended solids that pass through with the water will generally foul the packing which must then be cleaned either with acid or mechanically. Volatile hydrocarbons and oxygen can form an explosive mixture. Nitrogen blanketing may be necessary to avoid explosive atmospheres. Lastly, H2S is an extremely toxic compound often found in produced water. It will be stripped with dissolved hydrocarbons. Controlling it is critical and may add significant expense to stripping operations.
Air stripping is similar to steam stripping. Dissolved aromatics removal is accomplished by moving large quantities of air through produced water. A packed column constructed of material that is resistant to the water temperature can be used. Produced water entering at the top above the packing is brought into countercurrent contact with air drawn through the column from the bottom by a fan. Upon contact dissolved hydrocarbons will, as a result of their concentration difference and partial pressure, be transported to the gaseous phase. Air strippers are generally designed for the compound with the lowest vapor pressure at the lowest expected operating temperature.
Air stripping efficiency is improved by distributing the water through a spray nozzle above the packing in order to maximize contact between water and air. Also, air is often heated to promote higher transfer rates. Air and stripped hydrocarbons are generally sent to an emission control device such as a catalytic oxidizer, thermal oxidizer or activated carbon.
As with steam stripping, free oil and solids may foul the packing material and cause poor performance. Introducing air into hydrocarbon may create a hazardous atmosphere. Often nitrogen is used for stripping. If activated carbon is used to remove stripped hydrocarbons, the nitrogen can be recycled through the stripping tower. Control of H2S must be taken into account during design.
Dissolved hydrocarbon may also be removed using activated carbon. This adsorption technique is able to remove most hydrophobic compounds from the water. A pressure vessel filled with granular activated carbon is fed top down with produced water. As the water runs through the carbon bed, solutes diffuse into the pores of the carbon grains and adsorb to the large surface available. When the carbon is saturated, adsorption can occur no further and components will break through. Benzene, generally the one with the least affinity, should break through first.
Activated carbon can be regenerated by steam stripping the hydrocarbons off. For remote operations, like offshore platforms, this is done in-situ. In-situ removal of hydrocarbons is not comparable to a full regeneration the carbon suppliers can do ex-situ, so over time the quality of the carbon will deteriorate and must be replaced. Apart from regeneration, the carbon bed must be backwashed from time to time to remove solids and carbon fines from deterioration.
Removal of aromatic hydrocarbons is better than 99% and absolute removal from 4000 mg/l to < 1 mg/l is possible. Biological growth inside the filter may extend the bed life for biodegradable compounds. Degradation of organic substances by microbial growth on carbon may contribute to improved removal of compounds including TOC and benzene. The process is contained within a pressure vessel, so contact with air is avoided. Activated carbon adsorption is a reliable technique that is applied throughout the world with success.
Activated carbon is not without its disadvantages though. Free oil poisons the active carbon as the fine droplets plug the pores. Quick exhaustion will thus occur if free oil is present in the water. Concern exists about the regenerability of the active carbon. Some organic compounds may not fully strip off the carbon, thus decreasing overall capacity. Adsorption capacity differs quite a lot per contaminant. The component adsorbing the worst determines capacity. The application requires heavy units with respect to the amount of water it treats. For offshore application this is especially a drawback. Units are typically sized for a 2 hour liquid retention time, so a 10 m3/h (44 gpm) installation weighs approx. 20 tons. Presence of inorganic oxides cause oxidation of the active carbon, so feed should kept as free of air as possible.
Condensed water from gas production can, after the hydrate inhibitor glycol is recovered out of it, be treated for hydrocarbon recovery by liquid-liquid extraction setup very similar to ion exchange. Produced water is fed through an extraction column where a packed bed of small balls of polymer filled with an extraction fluid extracts hydrocarbons in the water. Treated water can be discharged immediately. When the extraction fluid in the column is nearly saturated, indicated by increasing TOC concentration in the outlet, produced water needs to be switched to a standby extraction column and the depleted column needs to be regenerated.
Regeneration is done by introducing low pressure (LP) steam counter-currently through the drained column. Steam and stripped hydrocarbons are condensed and can be easily gravity separated due to the high hydrocarbon concentration. Subnatant from the separator is recycled back to the treatment process. Cycle times can be chosen based on bed volumes. The system is typically executed in duplex to enable continuous extraction. The steam generator can be a small electrically driven unit.
Removal of hydrocarbon is higher than 99% and absolute removal from 2000 mg/l to less than 1 mg/l is possible. Presence of non-scaling salts does not affect the process. Presence of corrosion-inhibitors, demulsifiers and H2S do not appear to affect performance. Presence of dispersed oil up to 150 mg/l does not affect BTEX removal efficiency. However, periodic resin replacement is required. The unit requires retention time similar to activated carbon; therefore, the total unit weight is a drawback. The bed does not like salt deposition; therefore, air contact should be avoided and scaling potential of the water should be fully understood. The resin is susceptible to clogging by longer chain (greater than C20) hydrocarbons.
