SEPARATOR FUNCTIONS

SEPARATOR FUNCTIONS


A SEPARATOR is a vessel in which a mixture of immiscible fluids are separated; e.g. Crude oil, Natural gas and Water. A separator may be a 'Horizontal', 'Vertical' or 'Spherical' vessel and generally consists of the following :
1. A primary separation section to remove the bulk of the liquid from the gas.
2. Sufficient liquid capacity to handle surges of liquid from the line.
3. Sufficient length or height to allow the small droplets to settle out by gravity (to prevent undue entrainment).
4. A means of reducing turbulence in the main body of the separator so that proper settling may take place.
5. A mist extractor to capture entrained droplets or those too small to settle by gravity.
Where a vessel is simply separating total liquid from gas, it is called a 'Two-Phase Separator' When the process requires the separation of two liquids and a gas, the separator is called a 'Three-Phase Separator'. 'Two or Three Phase separation', refers to the number of streams leaving the vessel and not the inlet fluid stream.

Petroleum as produced from a reservoir is a complex mixture of hundreds of different compounds of hydrogen and carbon, all with different densities, vapour pressures, and other physical characteristics. A typical well stream is a high velocity, turbulent, constantly expanding mixture of gases and hydrocarbon liquids, intimately mixed with water vapour, free water, solids, and other contaminants.

As it flows from the hot, high pressure petroleum reservoir, the well stream is undergoing continuous pressure and temperature reduction. Gases evolve from the liquids, water vapour condenses, and some of the well stream changes in character from liquid to free gas. The gas is carrying liquid mist droplets, and the liquid is carrying gas bubbles.

The function of field processing is to remove undesirable components and to separate the well stream into sellable gas and petroleum liquids, recovering the maximum amounts of each at the lowest possible overall cost.

Field processing of natural gas actually consists of four basic processes:
1. Separation of the gas from free liquids such as crude oil, hydrocarbon condensate, water, and entrained solids.
2. Processing the gas to remove condensable and recoverable hydrocarbon vapours.
3. Processing the gas to remove condensable water vapour which, under certain conditions, might cause hydrate formation.
4. Processing the gas to remove other undesirable components, such as Hydrogen Sulphide and / or Carbon Dioxide.

WELL FLUIDS & WELL CLASSIFICATION
Fluid flow from a well can include gas, free water, condensable vapours (water or hydrocarbons), crude oil, and solid debris (basic sediment). The proportion of each component varies in different well streams.

When water is produced with crude oil, it is mixed in either or both of the following forms:
1. FREE WATER: Water mixed with the oil but will separate easily into a clear layer when the mixture is allowed enough time to settle.
2. EMULSION: Water can also be mixed with the oil in the form of very small droplets of water coated with oil. A mixture like this is called an EMULSION.
Water in this case cannot be easily separated from oil. As for the gas, it can be found in the well as:
1. SOLUTION GAS: Gas dissolved in the well fluids under the effect of pressure of the reservoir. As the fluids flow from the reservoir into the well and up to the surface, the pressure of the fluid decreases. The capacity of the liquid to hold gas in solution also decreases and gas starts to separate out of the oil.
2. FREE GAS: Gas that is NOT held in the oil under reservoir conditions.
3. ASSOCIATED GAS: Total gas produced with the oil in a crude oil well.
Wells are generally classified according to the type of fluid they produce in the greatest quantity. The main three types of well are:
1. CRUDE OIL WELL: A well that produces mostly crude oil with varying proportions of water, solution gas, possibly free gas and some solid debris.
2. DRY GAS WELL: A well that produces mostly gas with no crude oil (or liquid hydrocarbon). The produced gas can contain some water.
3. GAS CONDENSATE WELL: A well that produces both gas and light liquid hydrocarbon (condensate) and maybe some water, but no crude oil.
Much of the hydrocarbon condensate is very light, and changes from liquid to vapour at near atmospheric conditions. Therefore, when they are produced from high-pressure reservoirs to a surface line at near atmospheric pressure, they vaporise.

Gas, which is produced from a well together with oil, is called 'CASING HEAD GAS' or 'ASSOCIATED GAS'.

Gas produced alone or with water is called 'NON ASSOCIATED GAS'. This gas is produced from both dry gas wells and gas condensate wells.

The following table shows well classifications, fluid compounds, and processing methods.
CLASS OF WELL FLUIDS IN RESERVOIR FLUIDS IN FLOW LINE PROCESSING STEPS WHICH MAY BE REQUIRED
DRY GAS GAS, POSSIBLY WATER GAS, POSSIBLY WATER SEPARATION, GAS DEHYDRATION
GAS CONDENSATE GAS, POSSIBLY WATER GAS CONDENSATE, POSSIBLY WATER SEPARATION, GAS & CONDENSATE DEHYDRATION
CRUDE OIL CRUDE OIL, POSSIBLY GAS POSSIBLY WATER CRUDE OIL, POSSIBLY GAS, POSSIBLY WATER SEPARATION, GAS DEHYDRATION
Oil well fluids are produced normally in two phases - vapour and liquid. These two phases require entirely different handling, measuring, and processing methods. Therefore, it is necessary to separate the phases as soon as practical after leaving the wellhead. The basic equipment used for this purpose is the 'OIL & GAS SEPARATOR'.

Reservoir pressures are generally much higher than atmospheric pressure. As well fluids reach the surface, pressure on them is decreased. The liquid ability to hold gas in solution decreases, and the liquids begin to release 'Solution Gas'.
Light fluids begin to separate naturally when the pressure on them is lowered.
The solution gas released as Free Gas is held by the surface tension of the oil.
This free gas is released from the oil when the well fluids are warmed to reduce the surface tension of the oil. Gravity alone will eventually cause heavy components to settle out and light components to rise.

In summary, there are variables which aid in the separation of a fluid stream.
1. Temperature of the fluids.
2. Pressure on the fluids.
3. Density of the components.
In addition to using the force of gravity, modern separators make use of other forces to get the best possible separation of oil and gas. The way in which each of these forces is used can be better understood by following the flow of a mixture of oil and gas through a separator.

SEPARATOR FUNCTIONS
A wellstream separator must perform the following:
1. Cause a primary phase separation of the liquid hydrocarbon from those that are Gas.
2. Refine the primary separation by removing most of the entrained liquid mist from the gas.
3. Further refine the separation by removing the entrained gas from the liquid.
4. Discharge the separated gas and liquid from the vessel and ensure that no re-entrainment of one into the other takes place.
If these functions are to be accomplished, the basic separator design must:
1. Control and dissipate the energy of the well stream as it enters the separator.
2. Ensure that the gas and liquid flow rates are low enough so that gravity segregation and vapour-liquid equilibrium can occur.
3. Minimise turbulence in the gas section of the separator and reduce velocity.
4. Control the accumulation of froth and foam in the vessel.
5. Eliminate re-entrainment of the separated gas and liquid.
6. Provide an outlet for gases, with suitable controls to maintain the required operating pressure.
7. Provide outlets for liquids, with suitable liquid-level controls.
8. If necessary, provide clean-out ports at points where solids may accumulate.
9. Provide relief for excessive pressure in case the gas or liquid outlets should be plugged.
10. Provide equipment (Pressure gauges, Thermometers, and Liquid Level gauge assemblies), to check visually for proper operation.
Most platforms have a series of production separators, starting with a high-pressure separator, which separates the (HP) gas from the liquids. Liquids are then piped to a medium pressure (MP) separator, which removes more gas and then passes the liquids to a low pressure (LP) separator that removes even more gas and then separates water from the oil.

The water from the low-pressure separator is piped to a skim tank or to a drain pit, with the oil being piped to a metering and pumping station to be piped to other processes or storage tanks.

Well fluid separation depends on the composition of the fluids, and on their pressure and temperature.

The pressure of the fluids is controlled by the back - pressure regulator and the temperature may be regulated by expanding the fluids through a choke, by addition of heat in a furnace or by heating or cooling in a heat exchanger. Therefore, separators can be designed to handle fluids according to the fluid composition.

Separators are built in various designs, such as horizontal and vertical. The internal structures of the vessel, to aid in the mechanical separation of the gas and liquids, are of a spherical design, depending upon the manufacturer.

Although most separators are two -phase in design, separating the gas and total liquids, three - phase vessels can be built to separate natural gas, oil or other liquid hydrocarbons, and free water.

The main principles used to achieve physical separation of gas and liquids are: GRAVITY SETTLING and COALESCING Any separator may employ one or more of these principles, but the fluid phases must be 'Immiscible' (cannot mix), and have 'Different Densities' for separation to occur.

GRAVITY SETTLING:
During the separation process, the gas is moving in an upward direction into the vapour section of the separator and the liquid particles are tending to fall to the vessel bottom under the influence of gravity.

Gas will separate more quickly from a liquid when it is flowing 'HORIZONTALLY'. In a 'VERTICAL' separator, the gas is moving vertically upwards and the liquid droplets, due to gravity, are falling vertically downwards. The contra-flow of the two fluids therefore interferes with the flow paths and separation is slower.

Generally, because of the above factors, the vapour section of a Horizontal separator will be of a smaller volume than that of a Vertical vessel.
COALESCING:
Very small droplets such as fog or mist cannot be separated practically by gravity. However, they can be coalesced to form larger droplets that will separate out.

Coalescing devices in separators force gas to follow a tortuous path. The momentum of the droplets causes them to collide with other droplets or with the coalescing device, forming larger droplets. These can then separate out of the gas phase due to the influence of gravity.
Wire mesh screens, Vane elements, and Filter cartridges are typical examples of coalescing devices.

Separation vessels usually contain four major sections, plus the necessary pressure and liquid level controls. These sections are:
1. Primary Separation Section:
For removing the bulk of the liquid from the inlet stream. For example, free liquids, slugs and large droplets. This is usually accomplished by a change in the direction of fluid flow, either by baffles or deflection plates near the inlet nozzle or by using a tangential inlet nozzle as in 'Tangential Feed' or 'Cyclone' separators which operate by centrifugal force being set up within the vessel.
2. Secondary Separation Section:
For removing the maximum amount of small liquid droplets without an elaborate design. The major separation principle in this section is by gravity settling of the liquid droplets from the vapour stream.
3. Mist Extraction Section:
For removing the maximum amount of tiny liquid droplets remaining in the gas stream. The mist extractor may be of the impingement type; (mesh pads) and/or may use the centrifugal force principle; (the vane type).
4. Liquid Accumulation Section:
For receiving and disposing of the liquid collected. Sufficient volume and proper level control equipment should be provided to handle surges that may occur during operations.

The length of a horizontal separator has a greater effect on capacity than the height of a vertical type. In the horizontal vessel the path of any droplet ideally has a trajectory similar to that of a shell from a gun. Therefore, the length required depends on:
1. Droplet size.
2. Gas velocity.
3. Droplet density.
4. Vessel diameter.
5. Degree of turbulence

The 1st stage on the right, is the Low Pressure suction KO drum to a LP compressor, the 2nd stage (in the middle), is the Medium Pressure separator - discharge from the LP compressor and, suction to the HP compressor. The 3rd drum, on the left, is the final separation stage for the HP discharge gas. Cooling stages are installed after each discharge.

Separated water is usually dumped to a disposal pit. The gas condensate will then be metered and pumped to further treatment facilities and, the gas will be metered and go on to further processing units.

FLORENCE – (goal.com) Thiago Motta is currently with the Italy squad preparing for the Azzurri's forthcoming Euro 2012 qualifier against Slovenia, but he took some time out to give an interview to goal.com. The first thing he talked about was the Nerazzurri's dramatic win at the Allianz Arena and in particular how Leonardo reacted: "Naturally he congratulated us all but he too was delighted. For him it was a new experience in a way, and he revelled in it with the rest of us. He hasn't been working as a coach for long so it was an intense moment for him, as it was for the rest of us."

The Inter midfielder feels the victory over Bayern Munich was an important result for Italian football as a whole: "It was a massive win for Inter and Italian football because it came at a time when everybody was writing us off. For a while it looked as though all the Italian clubs were going to get knocked out and that the Serie A could no longer compete with the other big European leagues, but by winning we proved that's not the case."
On the subject of the Champions League, Thiago Motta believes Inter are right up there with the other big teams and says they are aiming to win it again: "We're in the quarter-finals and we're happy to be involved. Teams like Barcelona and Real Madrid are always going to be there or thereabouts but we're in the mix too. We've done superbly in the last year and a half and we want to win the Champions League again this year."

However, Inter's No. 8 doesn't want that to be at the expense of the league title: "We can win the Scudetto as well. We're up there with AC Milan and the derby is just around the corner so we still have a good chance of winning it."

Asked about that fact that Ibrahimovic is suspended for the derby, Motta replied: "It won't make much of a difference. He's a great player of course, and important for them, but personally I would have preferred it if he had been playing."

Finally, the Italy midfielder revealed that pulling on the Italy shirt was "an amazing feeling, a huge moment" and doesn't seem disappointed that he was never called up to the Seleçao: "I've never thought about it. I know it's supposed to be the dream of every Brazilian but it isn't for me." (goal.com)