Traditional Techniques and Equipment for Oil-Water Separation in Wastewater Treatment
Oil-Water Separator
Oil-water separators are commonly used for oil-water separation treatment. The main principle is based on the difference in density between oil and water, utilizing methods such as filtration, sedimentation, and flotation to separate the oil from water.
Dissolved Air Flotation (DAF)
The DAF method relies on the formation of tiny air bubbles in water, which carry flocs to the surface for removal, thereby purifying the water. The condition for effective separation is that the air bubbles attach to oil droplets, forming oil-air particles. The appearance of these bubbles increases the density difference between water and particles, and since the particle diameter is larger than that of oil droplets, the upward velocity is significantly increased. This allows for the removal of oil droplets as small as 50μm in diameter.
Centrifugal Separation
Centrifugal separation utilizes the difference in density between oil and water, with high-speed rotation generating centrifugal forces that separate the oil from the water. Due to the high rotational speed achievable by centrifugal equipment, which can create centrifugal forces hundreds of times that of gravity, this method is very efficient at separating oil from water, requiring only short residence times and compact equipment sizes. However, because centrifugal equipment contains moving parts and requires regular maintenance, it is generally used only in laboratory analysis and in areas where space is limited.
A key device utilizing centrifugal separation is the hydrocyclone, which physically separates the continuous liquid phase from the dispersed phase, such as solid particles, droplets, or bubbles. The greater the density difference between the dispersed and continuous phases, the easier it is to separate them. Similar to a gravitational field, when the density difference between the two phases is constant, the larger the particle diameter of the dispersed phase, the greater the speed difference between the two phases in equilibrium, making separation easier.
Electrostatic Separation
Electrostatic separation, used as the final method for oil-water treatment, is widely applied in oilfields and refineries. The principle involves placing emulsified liquids in high-pressure AC or DC electric fields, where the electric field weakens the strength of the interface film on water droplets, promoting collisions and merging of droplets. This causes the droplets to coalesce into larger ones, separating from the crude oil. However, when dealing with highly emulsified crude oil with high water content, electrical breakdown may prevent the necessary electric field strength from being established, so electrostatic separation cannot be used independently and must be followed by other treatment methods.
Coarse Particle Evaporation of Emulsified Water
Using solid materials that have different affinities for oil and water, various evaporators are created for oil-water separation. The solid materials should have good wettability. Suitable materials include ceramics, wood chips, fiber materials, walnut shells, etc. For example, the ceramic granule evaporator used in Daqing Oilfield uses ceramic granules as packing. When the oil-water mixture passes through the ceramic layer, it is forced to change speed and direction, increasing the likelihood of collision and coalescence of water droplets, allowing small droplets to quickly coalesce and settle.
Heated Oil-Water Separation Method
The oil-containing degreased liquid is fed into a heated oil separator. The heating breaks the emulsion, causing the oil to float to the surface, where it is collected through the oil suction port and flows into the oil storage tank. The de-oiled liquid in the separator is returned to the working tank after heavier precipitates are removed by a baffle at the liquid level.
Ultrafiltration (UF) Regeneration of Degreased Liquids
After pre-filtration to remove coarse waste, iron powder, and slag, the oil-containing degreased liquid is purified and concentrated by ultrafiltration membranes. The UF liquid (oil-free degreased liquid) is returned to the working tank.
Adsorption Oil Removal
This method uses materials with oleophilic and hydrophobic properties. These materials continuously contact the oil-contaminated degreased liquid to capture and adsorb the oil in the liquid. The principle involves rotating devices that move oil-adsorbing belts at a specific speed in a secondary tank. The adsorbed oil is then squeezed out through an oil outlet and collected in an oil storage barrel.
