Phase Separation and Coalescence

• Phase separation is a process where molecules in a solution spontaneously separate into at least two distinct phases with different compositions.
• Phase separation commonly occurs between two immiscible liquids, like oil and water, and this is known as liquid-liquid equilibrium.
• Coalescers, decanters, and centrifugal separators are methods for liquid-liquid separation.
• Coalescence is the process of coming together or agglomerating
• During coalescence, fluid molecules agglomerate to form a larger mass that is then separated as particulate components.
• A coalescing filter, or coalescer, is an industrial device that separates mixtures (liquid or solid) from a gas or liquid medium by inducing them to merge or coalesce.
• A coalescer is a vessel that accelerates the joining of two or more dispersed particles to form larger particles using any possible technique.
• Coalescers are used to separate emulsions or impurities from various processes.
• Coalescers are commonly used in oil refineries to separate oil and water from hydrocarbon gases.
• Coalescers are used in downstream oil and gas, petrochemical, and chemical industries for liquid-liquid or liquid-gas separation.
• In the oil and gas industry, high-speed centrifugal compressors can be damaged, and this damage is often linked to slugs of liquid entering the compressor.
• Knockout drums are used to separate liquid droplets from a gas stream.
• Gravity vapor-liquid separators are installed in the majority of process vessels throughout the industry.
• Coalescers play a vital role in separating impurities from mixtures using coalescence, and they are phase separation techniques.
• Industrial process fluids contain impurities such as sulfur, ethane, carbon dioxide (CO2), water vapor, and methane which must be removed to maintain product quality.

Coalescer Function

• A coalescer consists of baffle walls or screens inside the device, screening components from a mixture, trapping them in different sections, using each component's molecular weight and density.
• At the same time, water vapor molecules diffuse through the filter element to agglomerate and drain out gravitationally, and its baffles coalesce droplets into larger ones
• The primary function is to separate mixtures or emulsions components using various methods, including homogeneous or heterogeneous mixtures.
• Coalescing filters are used independently or as components of larger separation units and are widely used as oil-treating equipment.

Factors Affecting Coalescer

• Factors that influence the settling rate of droplets in a coalescer.
• The settling rate of droplets refers to the speed at which these droplets separate from the surrounding fluid phase.
• Settling rate increases as the droplet size and liquid density increase, but decreases as the vapor density, vapor velocity, and vapor viscosity increase.
• Water droplets settle out of hydrocarbon phases due to being denser than liquid hydrocarbon.
• Liquid droplets settle out of gas phases because of gravity.
• Two primary types: electrostatic and mechanical, classified by working mechanisms.

Electrostatic Coalescers

• Characteristics: Use AC, DC, or both currents.
• Electric fields rapidly move droplets in a water-in-oil emulsion to increase their collision potential, causing coalescence.
• Working mechanisms:
  • Droplets become polarized by an electric current and align with the electric force lines.
  • Positive and negative poles are brought close together, attracting the droplets and leading to agglomeration.
  • Charge-induced droplets may attract to an electrode.
  • The inertia in AC vibrated droplets promoting coalescence over a larger distance.
  • In DC, droplets collect on electrodes, form large drops, and fall by gravity.

Mechanical Coalescers

• Characteristics: uses elements or barriers to separate, water droplets are directed or intercepted by the barriers.
• The oil thins when baffle fibers are passed through.
• Water and oil separate.
• Plate Coalescers are typical examples.
• Working Mechanisms:
  • Plate coalescers rely on gravity separation, allowing oil droplets to rise to a plate surface for coalescence and capture.
  • Flow splits by parallel plates spaced 0.5–2 inches apart, sometimes inclined horizontally.
  • It promotes oil droplets into films, guiding them to the top for entrapment and prevents reblending with water.
  • The plates provide surface area for oil droplets and solid particle collection.
• Recommended for:
  • Plate separators which are suitable with steady water flow rates, no size or weight constraints, negligible solid contaminants(less than 110 ppm), availability of utilities and equipment for plate pack cleaning, and high influent level/ concentration with a reduction to 150 mg/l.
• Not Recommended when:
  • Size/weight are primary, droplet sizes is less that 30mm, and sand particles are less the 25mm, and solids removal is the primary purpose

Methods of Coalescer

• Electrically Precipitation
• Electric Precipitation in Mist Removal
• Static Coalescer

Electrostatic Precipitation

• Speeds up the settling rate of water using electricity
• Water: This is a polar molecule, meaning its sides are positively and negatively charged, making it a good conductor of electricity.
• Hydrocarbons: It is non-polar molecules, are less affected by electricity.
• An electric precipitator uses electric plates with a high voltage of 20,000V.
• Water droplets electrically attract onto these plates, increasing the amount and settling rate.

Electrostatic Precipitation in Mist Removal

• Another type of electrostatic precipitator removes liquid–gas phase products
• This can be seen in liquid sulfuric acid with a gas stream; it operates like a liquid–liquid precipitator.
• Electrodes/grids are tubes of lead wires are not parallel plates, which are used in singlepass shell and tube heat exchangers.
• If a leadcoated wire breaks, the precipitator arcs, and with the amps jumping around because the worn wire is flapped around in the tube.

Static Coalescers

• A coalescer works the same as a demister.
• Accelerating the removal of droplets from a heavier liquid in a lighter liquid.
• An example includes removing entrained caustic from a flowing isobutane stream by impacting a pad.
• A droplet of caustic would have a heightened surface tension relative to isobutane.
• The caustic adheres to the surface of the fibers and collects, draining by the fiber into the boot.

Design

• A coalescer can be designed in vertical or horizontal configurations, and must be sized to have adequate residence time for coalescence.
• Properly, internal parts, like baffles/ distribution plates, are needed for uniform flow distribution.
• Designed to withstand process operating pressure and temperatures, and the feed stream characteristics.
• Consider: gas/liquid flow rates and gas compressibility, as well as density and viscosity relating to settling velocity/separation efficiency.
• The type and design/type, purity, and grade depends on coalescing media and droplet size distribution.
• Types of media options include fibrous or membrane, which is determined by a specific function/ nature of the fluids that separates.
• Media velocity needs to be optimized as it passes the coalescing media affects the efficiency.
• It should promote long-term reliability, easy access, and simple maintenance, and made of select compatible materials with fluid and resistant to corrosion/ fouling.
• Designed to meet specific requirements and reliability after carefully considering factors.