Part #1
Separators in oil and gas industry - Part #1 why we need separation and treatment in oil and gas fields ?
https://youtu.be/zZN_IFRY-9A
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Introduction
Petroleum Gases and Liquids are Called Hydrocarbon fluid because the main component of the petroleum fluid is the hydrogen and the carbon atoms.
Oil and gas wells produce a mixture of hydrocarbon gas, condensate or oil, salty water,
Also the mixture contains inorganic gases, such as nitrogen, carbon dioxide (CO2), and possibly hydrogen sulfide (H2S),
In addition to few solids, including sand from the reservoir, scale, and corrosion products from the tubing.
These mixtures are very difficult to handle, meter, or transport. In addition to the difficulty, it is also unsafe and uneconomical to ship or to transport these mixtures to refineries and gas plants for processing.
Further, hydrocarbon shipping tankers, oil refineries, and gas plants require certain specifications for the fluids that each receive. Also, environmental constraints exist for the safe and acceptable handling of hydrocarbon fluids and disposal of produced salt water.
It is therefore necessary to process the produced fluids in the field to yield products that meet the specifications set by the customer and are safe to handle.
The goal is to produce oil that meets the purchaser’s specifications that define the maximum allowable amounts of water, salt, and sulfur. In addition to the maximum allowable value of Reid vapor pressure and maximum allowable pour point temperature.
Similarly, the gas must be processed to meet purchaser’s water vapor maximum allowable content (Water dew point), hydrocarbon dew point specifications to limit condensation during transportation, in addition to the maximum allowable content of CO2, H2S, O2, Total Sulfur, Mercaptan, Mercury, and maximum gross heating value.
The produced water must meet the regulatory requirements for disposal in the ocean if the wells are offshore, or to meet reservoir requirements for injection into an underground reservoir to avoid plugging the reservoir or underground water contamination.
The specifications for the above requirements may include maximum oil in water content, total suspended solids to avoid formation plugging, bacteria counts, toxicity in case of offshore disposal, and oxygen content.
Phases that include (Gas- Oil – Water) in oil and gas production lines are not naturally 100% separated.
Part of the gases is free in field production or transportation lines, and another part is dissolved in the oil stream.
In the same time, part of the liquid produced is trapped in the gas stream.
Part# 2
1- The necessity of separation in oil field.
2- Separator Types (Horizontal – Vertical)
3- Separator function for 2& 3 phase separators.
4- Theory and technique of separation
5- Retention time (residence time ) for separation of each phase.
https://www.youtube.com/watch?v=27Vn9bVmmOI
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Separator Configurations
separators may be horizontal
Or
Vertical
A comparison between Horizontal and Vertical separators, will be presented later in this presentation, where, Advantages and disadvantages of each configuration will be discussed.
Terminology and general aspects of separation
Separators may be classified as
2 phase separators , in which the separator is designed to separate gases from liquid, and liquid from gases.
The two phases in this case are :
Gas and liquid.
Or
Separators can go further to separate water from oil , and oil from water
In the last case separators are called three phase separators,
the three phases are:
Gas
– oil – and water.
Separation whether 2 or 3 phase separation is achieved by one or more of the following techniques:
Gravity separation, which depends on the density difference between phases
Coalescences , which depends on the growth of droplets caused by the
collision of the small droplets at a surface of a metal.
or growth after collision of small droplets in an electrical field.
Centrifugal force, which accelerate the gravity separation
Impingement, which depends on the separation caused by a collision of phases with a plate or a deflector.
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The most important technique in separation is the
Gravity separation which depends on the difference in density between the phases to be separated as follows:
Crude oil is a complex mixture of hydrocarbons; its density usually ranges from 40 to 55 pounds per cubic foot.
Which is 0.65 to 0.9 kg/ liter
While, produced Water usually has a density about 65 pounds per cubic foot
which is about 1.05 Kg/ liter
density of gases is strongly changes due to temperature and pressure
for example
volume of 1 cubic feet of gas at 60 degree Fahrenheit ( 15 degree celiesius ) and pressure 35 psig ( 2.5 Barg)
is about 0.3 and its density is about 0.2 pounds per cubic foot ( which is 3.5 Kg per cubic meter)
While at the same temperature, and at pressure 100 psig ( 7 Barg) the gas volume is reduced to 0.1 cubic foot and the density is increased to 0.6 pounds per cubic foot ( which is 9 Kg per cubic meter)
Further increase in pressure to 500 psig (33 barg) will lead to more reduction in gas volume to reach 0.02 cubic foot , and the density will be 3 bounds per cubic foot (46 kg per cubic meter).
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Whatever, the increase in gas pressure, still there is a big difference in densities between gases ( 3 lb/ cubic foot) and oil or water .
We can notice, the difference in density between oil and water is less than the difference in densities between gases and any of the produced liquids.
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The targets of the 2 phase separation are:
to remove liquid droplets from Gas- and remove gas bubbles from Liquid.
The liquid droplets entrained in gas stream must fall down to the liquid surface
It will take less than 5 seconds in small diameter separators, at low pressure.
and up to 15 seconds in case of high diameter separators, at high pressure
the gas stream, has to stay in the separator a residence time more than the time of liquid droplets fall.
Calculations for the time required for droplet separation, are usually based om 100 or 140 micron droplet diameter.
Larger droplet fall down faster, hence consumes less time to reach the liquid level.
The only restriction for the liquid droplet , while falling down is the viscosity of gas, which is of a low value, and will not make a remarkable effect on the droplet velocity.
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For the Gas bubble entrained in the liquid,
due to the density difference, the gas bubble will ascend leaving the liquid zone to the gas area,
Due to the viscosity of surrounding liquid, the gas bubble will take much more time to ascend, than the time required for the liquid droplet to fall down.
The gas bubble, may need up to 4 minutes to ascend and leave the liquid surface, depending on the viscosity of the liquid.
Therefore, liquid residence time in two phase separator, usually ranges from 1 minute for light crude oil ( low viscos liquid) to about 4 minutes for high viscos – low API crude oil.
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in horizontal separators
Gravitational force is perpendicular to the drag force caused by fluid movement.
The falling direction of the droplet will be in an angle between gravitational force and drag force.
Also the ascending direction of the gas bubble will be in angle between drag force caused by liquid movement and buoyancy or gravitational force.
in vertical two phase separators:
The same concept is applied as in horizontal separators.
The only difference is that :
The flow of gas stream, and the drag force caused by the gas flow, is in the opposite direction of the gravitational force ( or the falling path of the liquid droplet), hence there is a limit for speed for the gas flowing upward, to allow liquid droplet separation by gravity force.
In other words, The drag force, caused by gas movement, must be less than the gravity force downward, for separation to occur.
Therefore, in a vertical separator design, there is a minimum diameter used for each calculation.
A bigger Diameter – increase flow area – hence reduce flow velocity, consequently reduces the drag force.
The same concept is applied for a gas bubble ascending in liquid stream, which is affected by drag force caused by liquid movement downward.
let’s remember that: this is not the same in the horizontal separators, because the drag force is perpendicular to the falling direction – not opposite to it.
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For 3-phase separation
In addition to removal of liquid droplets from Gas- and removal of gas bubbles from Liquid as in 2 phase separation it is also design to Remove Water from Oil and Oil from Water.
Removing liquids from gas stream will consume 5:15 seconds, as in 2 phase separation.
Removing gas bubbles from liquid stream will consume also from 1 to 4 minutes as in 2 phase separation
The difficulty, or the critical path is to remove water from oil, where it will consume from 20 to 30 minutes.
depending on the oil viscosity, and the density of oil (API degree).
The more the difference in densities between oil and water the faster the separation will be.
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in vertical separators 3 phase separation.
The same concept of horizontal 3 phase separation is applied, except we have to consider the maximum flow speed – or – the minimum diameter required – as we mentioned for the two phase separators.
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Gravity separation depends on the difference in density between phases
For example
@ 100 Psig the densities of the 3 phases are as follows:
Water 65 lb/cu ft.
Oil 50 lb/cu ft. (Average)
Gas 0.6 lb/cu ft. @ 100 psig
Therefore, it’s much easier and faster to separate gases from liquids, than to separate water from oil.
Gravity Separation also, depends on the Viscosity of the continuous phase.
For example
To separate gas trapped in a liquid phase, the gas bubble has to move upward through the viscous liquid.
While, Separation of liquid from the gas phase is much easier and faster, since the viscosity of the gas phase will not restrict the dropping of liquid drop downward.
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According to Stok’s Law, and similar equations usually used in gravity separation
Velocity of separated droplets
(V) α is directly proportional to the (Density difference)
and is indirectly proportional to the diameter of the droplet or bubble,
and indirectly proportional to the viscosity of the continuous phase (in which the droplet or bubble will move through),
Therefore, We can conclude that :
1- Increase in Density difference will enhance separation.
examples
( Gas is easily separated from Liquids,
It is easier to separate Water from light crude oil than from heavier crude oil “Lower API).
2- Viscous crude oil, is harder in water separation.
3- Small droplet sizes are harder to be separated.
4- Increasing temperature, will reduce viscosity and therefore enhance separation.
5- three phase separator size is bigger than 2 phase separators for the same liquid fluid, because, an oil room is added, and more residence (retention) time will need to separate water from oil.
6- Reducing the separator operating pressure, will increase gas volume, consequently, reduce retention time of gas inside separator.
7- Separators working at higher pressure, will treat more gas than the same size at low pressure, due to reduction in gas volume.
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Time required for
Gravity separation
The required time to separate a 150 micron droplet of liquid entrained in the gas phase is less than 15 seconds at 1000 psig – large diameter separators.
and less than 5 seconds @ 100 Psig- small separators.
Therefore, the residence or retention time of gas in separators must be more than the time required for separation process.
Liquid required residence time to separate a bubble of 150 micron gas from liquid phase is ranging from 1 : 4 Minutes (Depends of temperature, and viscosity of liquid).
Liquid residence time to separate water from oil is usually 20 – 30 minutes.
depends on oil density, temperature, and viscosity.
For the above residence time consideration
API 12 J
presented tables for recommended retention time in separators as we can see:
Liquid retention time range is from 1: 4 minutes – to separate gases from liquid.
To separate free water from oil the retention time ranges extend to 30 minutes.
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All separated water, are the free water existing, not emulsified water; which require more treatment through heating and electrostatic heaters, or desalting.
Sizing of separators usually, is based on 20-30 minutes; liquid residence time for 3-phase separators.
And about 1:4 minutes for 2-phase separators.
Additional residence time in separators, will not be effective in reducing the water content in oil.
After about 20 : 30 minutes, an equilibrium is reached and further water separation will not be achieved.
As it is clear from the graph
in the beginning
separation rate of free water from oil is very high
then the rate is decreased with time
till reaching an equilibrium after about 30 minutes, where more residence time, will result in a very small increase in separated water.
further processing is required in oil and gas processing, to achieve the required crude oil specs.
a heater treater usually exists Downstream the separator,
to heat the emulsion, hence, reducing the crude oil viscosity, and allow more water separation.
Then a fresh or low salinity water usually added to the crude oil ( wash water) , and the crude oil is pumped to a desalting vessel, which separate the small water droplets, by applying an electrical field.
Electrical voltage affect the water droplets, and help in coalescing droplets together to enhance separation.
Further gas treatment may be required which include
acid gas treatment which is known as gas sweetening
Dehydration of natural gas
Condensate recovery
In the previous session
We explained :
1- The necessity of separation in oil field.
2- Separator Types (Horizontal – Vertical)
3- Separator function for 2& 3 phase separators.
4- Retention time (residence time ) for separation of each phase.
Next
Separation Internal Components.
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