Phase Separation & Coalescence

Phase Separation

• Molecules in solution spontaneously separate into at least two phases and different compositions
• Phase separation is most common between two immiscible liquids, such as oil and water
• This form of phase separation leads to liquid-liquid equilibrium
• Coalescers, decanters, and centrifugal separators serve as methods for liquid-liquid separation

Coalescence

• Coalescence is coming together or agglomerating
• Fluid molecules agglomerate, forming a larger mass that will be separated as particulate components

Coalescer

• A coalescing filter/coalescer is an industrial device
• It separates mixtures (liquid or solid) from gas/liquid by merging or coalescing them
• This vessel uses a technique to accelerate dispersed particles to form larger particles

Coalescer Applications

• Coalescers separate emulsions/impurities from various processes
• Oil refineries use coalescers to separate oil and water from hydrocarbon gases
• Downstream oil, gas, petrochemical, and chemical industries conduct liquid-liquid or liquid-gas separation using coalescers

How Coalescers Work

• A coalescer contains baffle walls or screens at different points inside
• The separation device's baffles screen out the mixture's components, by trapping them in different sections
• Screening mechanism uses the molecular weight and density of individual components
• Water vapor molecules diffuse through the filter element to agglomerate, then are drained gravitationally

Function of Coalescer

• The primary function of a coalescer is to separate mixtures/emulsions into individual components using various methods
• They separate homogenous or heterogeneous mixtures
• A coalescing filter is used independently or as part of a larger separating unit
• Coalescers are used as oil-treating equipment

Factors Affecting Coalescer: Settling Rate of Droplets

• The bigger the droplet size, the faster the settling rate
• The less dense the vapor, the faster the settling rate
• The more dense the liquid droplet, the faster the settling rate
• The slower the vapor's vertical velocity, the faster the settling rate
• The lower the vapor viscosity, the faster the settling rate

Water vs. Liquid Droplets

• Water droplets settle out of a lighter liquid hydrocarbon phase due to gravity
• Water is denser than liquid hydrocarbon
• Liquid droplets settle out of a lighter gas phase because of gravity

Types of Coalescers

• Two primary types of coalescers: electrostatic and mechanical
• Types are classified based on their working mechanisms

Electrostatic Coalescers

• Use AC or DC current
• Mixture (water-in-oil emulsion) is subjected to a high-voltage electrical field
• The electric field forces droplets to move rapidly and randomly
• Rapid, random movement increases the collision potential with another droplet, causes coalescence

Electrostatic Coalescers: Working Mechanisms

• Droplets are polarized by electric current and align with the lines of electric force
• Positive/negative poles of droplets are brought together
• Electrical attraction brings droplets together to agglomerate
• Due to an induced charge, droplets may be attracted to an electrode
• Due to inertia in AC field, small water droplets vibrate over a larger distance, promoting coalescence
• In a DC field, droplets collect on electrodes, form larger drops, which fall by gravitational force

Mechanical Coalescers

• Employ a series of filter elements or barriers for separation
• Barriers direct or intercept water droplets
• Oil thins when the baffle fibers pass through
• Water and oil separate from one another
• A plate coalescer is a typical example

Mechanical Coalescers: Working Mechanisms

• depends on gravity separation
• Allows oil droplets to rise to a plate surface where coalescence and capture occur
• Flow splits via parallel plates spaced 0.5–2 inches apart
• Plates are sometimes inclined horizontally
• Promotes oil droplet coalescence into films, guiding oil to channels, preventing remixing with water
• Plates allow oil droplets to collect and solid particles to settle

Mechanical Coalescers: Recommended Use

• With a steady water flow rate
• No size and weight constraints
• Solid contaminants are not significant in the waste stream; sand content is less than 110 ppm
• When utilities and equipment are available for periodic cleaning of plate packs
• Influent oil content is high, oil concentration must be reduced to 150 mg/l

Mechanical Coalescers: Not Recommended

• If size and weight is a primary consideration
• if influent droplet sizes are below 30 mm
• If sand particle diameters are less than 25 mm and solids removal is a primary objective

Methods

• Electrically precipitation
• Electric precipitation in mist removal
• Static coalescer

Electrostatic Precipitation

• Electricity speeds up the settling rate of water
• Water is a polar molecule - one side of the water molecules is positive and the other negative
• The water's characteristics make it a good conductor of electricity
• Hydrocarbons are nonpolar molecules, so they are not affected by electricity
• Electrical precipitators contain sets of electric plates where high voltages (20,000V) of electric current are applied
• Droplets of water are electrically attracted to the plates, thus coalescing into larger, heavier droplets, and rapidly fall to the bottom of the vessel

Electrostatic Precipitation in Mist Removal

• Electrostatic precipitators are used to remove a liquid–gas phase product
• Liquid sulfuric acid with gas stream is an example
• Electrodes or grids are not parallel plates, but are lead tubes and lead coated wires
• Works the same as a liquid–liquid precipitator

Static Coalescers

• These work the same as de-misters
• They accelerate the removal of heavier liquid passing through a lighter liquid
• An example is removing entrained caustic from a flowing isobutane stream
• Velocity of liquid isobutane would impact the coalescer pad at 1-2 ft/mom
• Caustic droplets have higher surface tension than isobutane
• Caustic adheres to the surface of the coalescer fibers and grows larger and drains via the fiber of the pad into the boot

Coalescer Design: Configuration

• A coalescer has a vertical or horizontal configuration
• Properly designed internals (baffles, distribution plates) can enhance separation by promoting uniform flow distribution
• Vessel size and shape dimensions should be designed to provide adequate residence time for coalescence to occur.

Coalescer Design: Operating Conditions

• The coalescer must be designed to withstand operating temperatures & pressures
• Tendencies of the feed stream are helpful in designing a coalescer that can handle variations

Coalescer Design: Flow Rates

• Minimum, average, and peak flow rates of gas and liquids should be considered

Coalescer Design: Fluid Properties

• Density and Viscosity affect the settling velocity of droplets and the overall separation efficiency
• Compressibility must be considered for gas-liquid coalescers

Coalescer Design: Separation Requirements

• The degree of separation will dictate the design and type of coalescer used
• Droplet Size Distribution Knowing helps in selecting coalescing media

Coalescer Design: Coalescer Media

• Choice between fibrous and membrane coalescers depends on the specific application/fluids being separated
• Media Velocity The velocity at which the fluid passes affects the coalescence efficiency, which must be optimized

Coalescer Design: Maintenance and Durability

• The coalescer should be designed for easy access and maintenance
• Material Selection must be compatible with process fluids, as well as being resistant to corrosion and fouling