What are the methods for separating oil and water?

Separating oil and water can be achieved through various methods, mainly leveraging the differences in their physical properties such as density and chemical incompatibility. Here are some commonly used methods:

    Gravity Separation

        Principle: Utilizing the density difference and immiscibility between oil and water, separation occurs either in a static or flowing state.

        Process: Oil droplets dispersed in water slowly rise to the surface due to buoyancy. The rise in the speed of oil droplets depends on factors such as droplet size, density difference between oil and water, flow state, and fluid viscosity.

    Centrifugal Separation

        Principle: A container filled with oily wastewater is spun at high speeds to create a centrifugal force field. Due to the different densities of solid particles, oil droplets, and wastewater, they experience different centrifugal forces, enabling separation.

        Application: This method is more thorough and requires only a short residence time and a small equipment volume. However, it is currently only used in laboratory analysis equipment and places where footprint reduction is desired due to the moving parts and the complexity of routine maintenance.

    Air Flotation (or Floatation Separation)

        Principle: Tiny bubbles are generated in the water, causing fine suspended oil droplets and solid particles to attach to the bubbles and rise to the surface together, forming a scum layer (oil-containing foam layer) that can then be skimmed off.

        Technological Advancement: This water treatment technology is being actively researched and promoted domestically and internationally.

    Filtration

        Principle: Wastewater passes through a device with holes or a filter layer composed of particulate media, using interception, screening, inertial collision, and other mechanisms to remove harmful substances such as suspended matter and oil from the wastewater.

    Biological Oxidation

        Principle: Utilizing the biochemical action of microorganisms to purify wastewater. Oil, as a hydrocarbon organic compound, can be decomposed into carbon dioxide and water through microbial metabolic activities.

    Chemical Methods

        Principle: Adding chemicals to convert pollutants in the wastewater into harmless substances through chemical reactions, thereby purifying the wastewater.

        Common Methods: Neutralization, precipitation, coagulation, oxidation-reduction, etc. Coagulation is primarily used for oily wastewater. It involves adding a certain proportion of flocculant to the oily wastewater, which hydrolyzes to form positively charged colloidal groups that electrically neutralize negatively charged emulsified oil, causing oil particles to aggregate, increase in size, and form floc that adsorbs fine oil droplets. Separation is then achieved through sedimentation or air flotation.

    Adsorption

        Principle: Utilizing oleophilic materials to adsorb dissolved oil and other dissolved organics in wastewater.

        Common Adsorbent: Activated carbon, which can adsorb dispersed oil, emulsified oil, and dissolved oil in wastewater.

    Coarsening

        Principle: Based on the significant difference in affinity between oil and water for coalescing materials, oil droplets are captured by the material and retained on the surface and within the pores of the material to form an oil film. When the oil film thickens to a certain extent, it detaches, merges, and coalesces into larger oil droplets.

    Acoustic, Microwave, and Ultrasonic Dewatering Techniques

        These techniques can accelerate the coalescence of water droplets, improve crude oil dewatering efficiency, reduce energy consumption and demulsifier usage, and further promote the coalescence of water droplets by heating the emulsion while reducing emulsion stability.

Each method has its specific application scenarios and advantages. The choice of method depends on factors such as the type of oil-water mixture, the required separation efficiency, and the available equipment and conditions.