What Are The Oil-Water Separation Technologies for Oily Wastewater?
For oil-water separation, commonly used technologies include:
1. Dissolved Air Flotation (DAF)
Dissolved air flotation relies on the formation of tiny air bubbles in water, which carry flocculated particles to the surface, thereby purifying the water. The condition is that the air bubbles attach to the oil droplets, forming oil-gas particles.
The presence of bubbles increases the density difference between water and particles. Since the particle diameter is larger than the oil droplet diameter, replacing the oil density with the particle density significantly increases the rising speed. In other words, when one (or more) air bubble(s) attaches to an oil droplet, it enhances the vertical rising speed, allowing the removal of oil droplets with diameters much smaller than 50 μm.
2. Gravitational Separation
Due to the different relative densities of oil, gas, and water, when an oil-water mixture is under specific pressure and temperature conditions, it will form a certain proportion of oil, gas, and water phases when the system reaches equilibrium. When the relatively lighter components are in a laminar flow state, the heavier component droplets settle according to the motion laws described by Stokes' law. Gravitational sedimentation separation equipment is designed based on this fundamental principle.
According to Stokes' law, the settling velocity is directly proportional to the square of the radius of the water droplets in the oil, and the density difference between water and oil. It is inversely proportional to the viscosity of the oil. By increasing the density of the water, enlarging the density difference between oil and water, and reducing the viscosity of the oil, the settling speed can be improved, which in turn increases separation efficiency.
Further exploration led to the "Shallow Pool Theory" proposed by Hazen in 1904, based on practical experience. This theory states that during gravitational sedimentation, the settling effect of dispersed (rather than flocculated) particles is measured as a function of particle settling speed and pool area. It is independent of pool depth and settling time. In other words, there are two ways to improve the capacity of a settling tank: one is to expand the settling area, and the other is to increase the settling speed of water droplets. The measures to increase the settling speed can be derived from Stokes' law, while expanding the settling area can be achieved by adding multiple horizontal baffles in the container.
Based on this theory, in 1950, Shell USA developed the first parallel plate separator, which can remove oil droplets as small as 60 μm from water. In the 1970s, Fram Company developed the V-type plate separator, and in the 1980s, CE-NATCO Company developed the plate-type coalescer. This is a cross-flow combination corrugated plate, and after continuous improvements, this equipment has been widely applied in oil and gas separation, oil-water separation, and oily wastewater purification.
Further research into the oil-water separation mechanism led to the development of high-efficiency evaporation equipment, which is generally divided into three parts based on the separation process: pre-separation chamber, sedimentation chamber, and oil and water chambers. The pre-separation chamber typically has disc-shaped deflectors and homogenizing liquid distribution plates. The principle is to enhance mechanical de-emulsification by repeatedly changing the direction and flow velocity of the oil-water emulsion, thus further accelerating the oil-water separation rate.
Active water washing can significantly reduce the strength of the emulsion interface. The shear and friction between the emulsion and the water layer cause the interface to break, promoting droplet coalescence, increasing droplet size, and accelerating oil-water separation. The sedimentation separation chamber primarily serves to further purify and separate, with the oil-water separator being the key component in the design.
3. Coarse-Particle Evaporation of Emulsified Water
By utilizing the different affinities of oil and water for solid materials, various evaporation devices are commonly made using hydrophilic, oleophobic solid materials. The solid materials used for oil-water separation should have good wettability. Suitable materials for this purpose include: ceramics, sawdust, fibrous materials, walnut shells, and others.
For example, the ceramic pellet evaporator used in China's Dagang Oilfield employs ceramic pellets as packing. When the oil-water mixture flows through the ceramic pellet layer, it is forced to continuously change its flow speed and direction, increasing the chances of droplet collisions and coalescence, thereby enabling small droplets to quickly coalesce and settle.
4. Centrifugal Separation
Centrifugal separation utilizes the difference in density between oil and water to generate varying centrifugal forces in the high-speed rotating oil-water mixture, thereby separating the oil from the water. Because centrifugal equipment can achieve extremely high rotational speeds, generating centrifugal forces that are hundreds of times greater than the force of gravity, it can effectively separate oil and water in a very short residence time and with a smaller equipment footprint. However, since centrifugal equipment contains moving parts, routine maintenance is more challenging, so it is currently mainly used in laboratory analysis equipment and locations where space needs to be minimized.
A key device that works on the principle of centrifugal separation is the hydrocyclone, which is used for the physical separation of liquid as the continuous phase and solid particles, droplets, or bubbles as the dispersed phase. The greater the density difference between the dispersed phase and the continuous phase, the easier the two phases are to separate. Similar to the situation in a gravitational field, under certain conditions where the density difference between the two phases is fixed, the larger the diameter of the dispersed phase particles, the greater the velocity difference between the two phases when they reach equilibrium in a gravitational field. This makes the separation easier.
5. Electro-separation
Electro-evaporation, as a final method for oil-water treatment, is widely used in oilfields and refineries. The principle involves placing emulsified liquid in a high-voltage alternating or direct current electric field. The electric field weakens the interfacial strength of the emulsion, promoting the collision and coalescence of water droplets, which ultimately combine into larger water droplets that separate from the crude oil.
However, when treating crude oil emulsions with high water content using electro-evaporation, electrical breakdown may occur, preventing the necessary electric field strength from being established between the electrodes. Therefore, electro-separation cannot be used independently and is only applied as a subsequent process in conjunction with other treatment methods.
