Separation of oil and gas is a critical field processing operation.
As producing pressure is increased and lighter condensates are produced, efficient separation has become more critical than ever.
As gas transmission lines raise their standards, separation becomes a part of the overall field processing necessary to condition the gas.
Selecting gas/liquid separation technologies requires not only knowledge of the process conditions, but knowledge of the characteristics of the liquid contaminants.
Selection should be made based on the droplet size, concentration, and whether the liquid has waxing or fouling tendencies.
Three principles used to achieve physical separation of gas and liquids or solids??:
Any separator may employ one or more of these principles; however, the fluid Phases must be immiscible and have different densities for separation to occur.
Momentum force: is utilized by changing the direction of flow and is usually employed for bulk separation of the fluid phases.
The gravitational force: is utilized by reducing velocity so the liquid droplets can settle out in the space provided. Gravity segregation is the main force that accomplishes the separation, which means the heaviest fluid settles to the bottom and the lightest fluid rises to the top.
Coalescing: However, very small droplets such as mist cannot be separated practically by gravity. These droplets can be coalesced to form larger droplets that will settle by gravity.
Gravity separators is??
Gravity separators are pressure vessels that separate a mixed-phase stream into gas and liquid phases that are relatively free of each other.
In a gravity separator, gravitational forces control separation, and the efficiency of the gas/liquid separation is increased by lowering the gas velocity.
Because of the large vessel size required to achieve settling, gravity separators are rarely designed to remove droplets smaller than 250 μm .
This limitation could be rephrased as gravity separators can only separate free oil/water droplet out of the raw gas stream.
Gravity separator Classify by:
Their geometrical configuration (vertical, horizontal)
And by their function (two-phase/three-phase separator).
“Two phase” if they separate gas from the total liquid stream
“Three phase” if they also separate the liquid stream into its crude oil and water-rich phases.
According to their operating pressure.
Low-pressure units handle pressures of 10 to 180 psi.
Medium-pressure separators operate from 230 to 700 psi.
High pressure units handle pressures of 975 to 1500 psi.
Separators are sometimes called “scrubbers” when the ratio of gas rate to liquid rate is very high.
These vessels usually have a small liquid collection section and are recommended only for the following items;
• Secondary separation to remove carryover fluids from process equipment such as absorbers and liquid dust scrubbers.
• Gas line separation downstream from a separator and where flow lines are not long.
• Miscellaneous separation where the gas–liquid ratio is extremely high.
All gravity separators normally have the following components or features and in compliance to API Spec 12J, 1989:
• A primary gas/liquid separation section with an inlet divertor to remove the bulk of the liquid from the gas.
• A gravity-settling section providing adequate retention time so that proper settling may take place.
• A mist extractor at the gas outlet to capture entrained droplets or those too small to settle by gravity.
• Proper pressure and liquid-level controls.
Horizontal Gravity Separator
The fluid enters the separator and hits an inlet diverter.
This sudden change in momentum generates the initial bulk separation of liquid and gas.
The inlet diverter contains a downcomer that directs the liquid flow below the oil/water interface.
This forces the inlet mixture of oil and water to mix with the water continuous phase in the bottom of the vessel and rise through the oil/water interface.
This process is called “water washing” and promotes the coalescence of water droplets that are entrained in the oil continuous phase.
The inlet diverter assures that little gas is carried with the liquid, and the water wash assures that the liquid does not fall on top of the gas/oil or oil/water interface, mixing the liquid retained in the vessel and making control of the oil/water interface difficult.
The liquid-collecting section of the vessel provides sufficient time so that the oil and emulsion form a layer or “oil pad” at the top.
The free water settles to the bottom. The produced water flows from a nozzle in the vessel located upstream of the oil weir.
An interface level controller senses the height of the oil/water interface. The controller sends a signal to the water dump valve, thus allowing the correct amount of water to leave the vessel so that the oil/water interface is maintained at the design height.
The gas flows horizontally and outs through a mist extractor (normally known as a demisting device) to a pressure control valve that maintains constant vessel pressure.
Vertical Gravity Separator:
The flow enters the vessel through the side as in the horizontal separator
The inlet diverter separates the bulk of the gas from the liquid.
The gas moves upward, usually passing through a mist extractor to remove suspended mist, and then the dry gas flows out.
A downcomer is required to transmit the liquid collected through the oil–gas interface (oil layer) so as not to disturb the oil-skimming action taking place.
A chimney is needed to equalize gas pressure between the lower section and the gas section.
The spreader or downcomer outlet is located at the oil–water interface.
From this point as the oil rises any free water trapped within the oil phase separates out.
The water droplets flow countercurrent to the oil. Similarly, the water flows downward and oil droplets trapped in the water phase tend to rise countercurrent to the water flow.
It should be clear that the principles of operation (such as oil/water interface level controlling) of three-phase vertical separators are the same as the three-phase horizontal separators.
Essentially, the only difference is that horizontal separators have separation acting tangentially to flow, whereas vertical separators have separation acting parallel to flow.
In the vertical separator, level control is needed in spite of the fact that the liquid level can fluctuate several inches without affecting operating efficiency. However, liquid level can affect the pressure drop for the downcomer pipe (from the demister), therefore affecting demisting device drainage.
Gravity Separators Selection
• Large volumes of gas and/or liquids.
• High-to-medium gas/oil ratio (GOR) streams.
• Foaming crudes.
• Three-phase separation.
• Require smaller diameter for similar gas capacity as compared to vertical vessels.
• No counterflow (gas flow does not oppose drainage of mist extractor).
• Large liquid surface area for foam dispersion generally reduces turbulence.
• Larger surge volume capacity.
• Only part of shell available for passage of gas.
• Occupies more space unless “stack” mounted.
• Liquid level control is more critical.
• More difficult to clean produced sand, mud, wax, paraffin, etc.
• Small flow rates of gas and/or liquids.
• Very high GOR streams or when the total gas volumes are low.
• Plot space is limited.
• Ease of level control is desired.
• Liquid level control is not so critical.
• Have good bottom-drain and clean-out facilities.
• Can handle more sand, mud, paraffin, and wax without plugging.
• Less tendency for re-entrainment.
• Has full diameter for gas flow at top and oil flow at bottom.
• Occupies smaller plot area
• Require larger diameter for a given gas capacity.
• Not recommended when there is a large slug potential.
• More difficult to reach and service top-mounted instruments and safety devices.
3.3 Filter Separators
Filter separators are designed to remove small liquid and/or solid particles from gas streams.
Contaminants of this small size can be removed most effectively by passing the gas through a fine high-quality filtering medium.
Several configurations of filter separators are used, depending upon the required efficiency and on whether liquids or solids or both are to be removed.
Some filter elements have collection efficiencies of 98 percent of the 1-micron particles and 100 percent of the 5-micron particles when operated at rated capacity and recommended filter-change intervals.
A simple dry-gas filter separator: Where removal of dry solids is the only requirement, this configuration is used.
A dry-gas filter separator is used in pipelines carrying under-saturated gas.
When both liquid and solid contaminants must be removed from the gas, a two-compartment vessel; Liquid is coalesced, and solid particles are filtered from the gas by a glass-fiber element. Solids are retained by the element, while liquids are agglomerated by wetting the surface of the fiber glass.
A standard filter element is made of a perforated metal cylinder with gasketed ends for compression sealing.
3.4 Separators Design Basis
a- Factors affecting design:
1- Liquid flow rate. 2- Gas flow rate. 3- Oil, gas and water densities and viscosities.4- Temperature and pressure. 5- Two phase or three phase.
6- Particulate impurities. 7- Extent of foaming. 8- Required liquid retention time.
b- Sections of separators:
1- Primary separation section (centrifugal).
2- Secondary separation section (gravity).
3- Liquid accumulation section.
4- Mist extraction section (Impingement).
Liquid level control to-prevent discharging liquid out of the gas line or gas out of the liquid line
Pressure controlled by a gas backpressure regulating valve.
Temperature within most separators is usually not controlled although there are exceptions, such as low-temperature separation systems.
Safety and protection against overpressure is provided by a pressure-relief valve.
3.5 Practical Separator Design
The most important areas to ensure a separator performs to design are as follow:
• Correct inlet nozzle sizing and a good inlet device (momentum breaker).
• Primary fluid distribution plates to translate the reduced but still high velocities from the inlet device into quiescent flows in a liquid–liquid separator body.
• Intermediate fluid distribution when necessary.
• Exit devices: vortex breakers.
3.6 Separators Operating Problems
Foamy Crudes, Paraffin, Sand, Liquid Carryover and Gas Blowby and Emulsions