Tertiary Oil/Water Separation
Technical Profile
The objective of tertiary separation is removal of fine particles and oil that might clog or foul subsurface injection formations. Filtration equipment is placed after flotation. Inlet oil content should be below 75-100 mg/L as frequent backwash cycles may reduce unit capacity. Oil and solids are adsorbed on the filtration media. Filtration units should remove particles down to 5 microns. Suspended solids are removed through the captive capacity of absorbed oil on the media. As a rule of thumb, no more suspended solids may be removed than oil. Oil and solids are generally removed down to 2 mg/L.
All particles greater than one-third the average formation pore size should be removed for subsurface injection. Typically, this is between 2 and 5 microns. Filtration media includes sand, walnut shells and multimedia consisting of anthracite, sand and garnet. In some instances multimedia may filter slightly smaller particle sizes, but walnut shell filters appear to be less susceptible to fouling. Filtered material is removed via a backwash or scrubbing cycle.
A vertical packed filter of crushed walnut shells is very effective for free oil take up. The principle feature of adsorptive walnut shell filtration is the joint removal of suspended oil and suspended solids down to a particle size of less than 5 microns. Suspended solids are removed because of the capturing capacity of the absorbed oil. So, when too little oil is present in the feed suspended solids may pass trough. Oil/solids removal is often greater than 90% down to 2 mg/L concentration.
Nutshell filtration most often precedes subsurface injection or disposal. For the purpose of produced water injection, residual oil or solids need to comply with particle size constraints. The actual residual hydrocarbon content of the treated effluent is less relevant than the physical size of the residual oil droplets. Particle sizes are often specified to be less than one-third the average pore size of the receiving formation, which generally places them in the 2 - 5 micron range in order to prevent plugging of the injection wells. Chemicals may be added upstream of nutshell operations to break oil in water emulsions and coagulate/flocculate particles for removal.
Filtration is accomplished by downflow operation over a bed of granulated nutshells. The process can be carried out at the temperature at which water is delivered. The filter medium adsorbs the suspended oil and retains it in the filter bed, together with the intercepted suspended solids. The filter bed is cleaned through a scrubbing device that removes oil and solids from the media using shear force. Once removed from the media, the nutshell media is separated from impurities and returned to the filter vessel. Impurities are removed from the filter using a backwash stream. The effectiveness of the cleaning cycle is critical to good nutshell filter operation. Nutsell filters are generally designed with media bed depths of 4 to 6 feet and flux rates of 10 to 15 gallons per minute per square foot of cross sectional area.
Sand or multimedia filtration may be used in place of, or in addition to, nutshell filtration. Contaminants are captured on the filtration media in downflow operation similar to nutshell filters. Contaminants are removed using backwash and in some instances gas scouring. Filters may contain sand or a combination of filter media. Multimedia filters generally utilize separate layers of anthracite, sand and garnet to progressively clean water. Anthracite helps boost oil removal properties of sand alone. Multimedia filters are generally designed with flux rates of 3 to 5 gallons per minute per square foot of surface area. Sand and multimedia filters reportedly filter to a slightly smaller solid particle size; however, nutshell filters appear to be less susceptible to fouling.
