Emulsions

Crude Oil Dehydration - TopicsTrainers:
Oil Treatment
The production of crude oil unfortunately provides us with a few chemistry related problems. These are in addition to those health, safety, environmental and engineering problems, with which you are all probably familiar.
© Baker Petrolite 2002
Introduction to Paraffins and Related Problems
Asphaltene Chemistry and its Application to Crude Oil Production
This section considers the nature of oilfield emulsions and how they might be resolved into separate oil and water layers. This process then facilitates the physical separation of the two fluids.
The following two sections will then address the problems encountered due to two natural components in some oils.
The presence of water can lead to corrosion
In shipping lines and process equipment
In Refinery process systems
Transport Costs
Transporting a by-product with no value is wasteful
Entrained oilfield water (brine) has little value and leads to refining problems if present in too high a quantity. Hence, the water content of the oil must be reduced in order to achieve shipping specification.
The presence of water in the system can also lead to corrosion and scaling problems.
Disposal of oily separated water
Standards from as high as 40ppm hydrocarbon (offshore) to as low as 5ppm hydrocarbon (onshore)
Standards often exceeded due to operational problems
Solids build up in production systems
Do you know what the oil in water (O/W) limit is for the discharge of separated water from your oilfield (or nearest one if not directly involved in an oilfield operation)?
Definition:
A mixture of two immiscible liquids, one of which is dispersed as droplets in the other, this dispersion being stabilised by an emulsifying agent.
If pure oil and water are mixed they will quickly separate, with the lighter oil as the upper layer. In oilfield systems there are various materials which migrate to the interfaces of the dispersed droplets and stabilise these in the dispersed phase. These materials are termed ‘emulsifying agents’.
Types of Emulsion
“Normal” Emulsion
“Reverse” Emulsion
Continuous phase - Water
Dispersed phase - Oil
Within the oil industry, water droplets dispersed in the bulk oil phase is called a ‘regular (or ‘normal’) emulsion’. ‘Reverse emulsions’ are associated with the separated water phase which carries entrained oil droplets.
“Normal” Emulsion - Photomicrograph
This photograph is taken through a microscope and shows a magnified section of water droplets dispersed in oil. Some bridging between droplets can be seen as the droplets join together to form bigger droplets.
Bigger water droplets separate from the oil quicker than smaller ones. This is the principal which is utilised in helping to dehydrate (remove water) from crude oil - discussed later.
Two Immiscible Liquids
Oil and Water
Formation water
Water Coning in a Waterdrive Reservoir
At some stage in the life of every oil-well, water is produced alongside the oil. For some wells it starts immediately with the first production.
Oil and Water
A Source of Mixing Energy / Shear
Well Bore, Pumps, Choke, Valves, Bends in Pipework, Flow Regime (Turbulent Flow)
Obviously, it is not possible to produce the well fluids without the equipment mentioned above, so mixing energy is unavoidable.
Potential Shear Sources
Oil Bearing Formation
Flow Regime
This slide shows a free-flowing well. If the well is pumped, then the pump itself is another source of mixing energy.
Oil and Water
A Source of Mixing Energy / Shear
Well Bore, Pumps, Choke, Valves, Bends in Pipework, Flow Regime (Turbulent Flow)
An Emulsifying Agent(s)
Treatment Chemicals - Production and Drilling
Natural Surfactants - e.g. Paraffins, Naphthenic Acid Salts
As mentioned earlier, there would be less problems if only oil and clean water were produced. However, many of the emulsifying agents listed above are naturally occurring with the crude oil. Among these are naphthenic acids and paraffin waxes.
Emulsifying agents tend to increase the “interfacial tension” at the water droplet / oil interface.
Emulsifying Agent - Surfactant
Water soluble
surfactant head
Oil soluble
surfactant tail
The organic emulsifying agents can be represented by the simplified structure shown in this slide.
Emulsifying Agent - Surfactant
Head has affinity
for water phase
Tail has affinity
for oil phase
Interface
Note: Emulsifying agents exhibit limited solubility in both the oil and water phases
The curved blue line represents the surface of the water droplet.
Increase in time
With time, more emulsifying agent concentrates in the water droplet interfaces with the oil. The extending ‘tails’ of the surfactant molecules inhibit the water droplets from approaching each other. This stabilises the water dispersion.
Solids such as sand or fine particles of formation rock or corrosion by-products, also stabilise the emulsion.
In effect, the emulsifying agents form a ‘skin’ around each water droplet. This ‘skin’ (increased ‘interfacial tension’) hinders their coalescence (joining together).
Degree of Agitation (mixing)
Density Differential of Produced Fluids - Stokes’ Law
Disperse Phase Content (Water)
Emulsion Age
Temperature
emulsion stability decreases with increase in temperature
Some emulsifying agents form more stable emulsions.
The greater the mixing, the smaller the water droplet size and hence the easier it is to remain is suspension (i.e more stable emulsion).
With a viscous (thick) oil, it is harder for the water droplets to separate (lower settling velocity).
dw = Density of water
do = Density of oil
u = Viscosity of oil
Where,
It is NOT important to remember the Stokes’ Law formula. The important issue is that the droplet radius parameter is a squared function. Hence, any increase in radius has the greatest effect on settling velocity.
Our efforts are focussed on resolving the emulsion (demulsification) -- breaking down the stabilising ‘skins’ -- thereby allowing the water droplets to coalesce, so that the bigger droplets more easily separate from the oil.
General Aim
Water droplet radius is a squared function in Stoke’s Law
Therefore, water droplet settling velocity is most easily increased by increasing the radius of the droplets
Hence, any means of coalescing the water droplets will increase settling velocity and reduce the settling time needed for water separation.
Resolving “Normal” Emulsions
Defined as:-
“The resolution of crude oil emulsions and the subsequent removal of the separated water phase (dehydration)”
Note the difference in definitions:
Demulsification - resolving the water in oil emulsion.
Dehydration - removing the separated water from the oil.
The “Mechanism” of Demulsifiers
To Break (or destabilise) a “Normal” Emulsion a Demulsifier Must Achieve the Following:-
Rapid Migration to the oil/water interface
Flocculation
Coalescence
The demulsifier displaces the natural emulsifying agents from the interface, thereby reducing the ‘interfacial tension’. This effectively overcomes the ‘skin’ problem, allowing water droplets to coalesce.
Choice of demulsifier is important, since some chemistries can give adverse side effects such as causing oil carryover with the separated water.
Factors In Demulsification
Droplet Remains Intact when
a Collision Occurs
Water
Droplet
Water
Droplet
Water
Droplet
Water
Droplet
The first step is breaking down the film of emulsifying materials.
“EB” in the above diagram means “Emulsion Breaker”, another name for demulsifier.
Droplets have
takes place
Viewed under the microscope, the flocculation process sees the water droplets come together in groups that resemble bunches of grapes. As the demulsifier takes further effect, these ‘bunches’ coalesce into a bigger droplet.
Solids Removal
Removal of solids from the interface may completely resolve some emulsions
Types of Solids:-
Organic
Oil wetting (dispersion in oil)
Water wetting (dispersion in water)
Some Nigerian crude oils are good examples of solids stabilised emulsions. One or two of these are stabilised by calcium carbonate and similar solids and are most easily resolved by the application of acid based demulsifiers. Several others appear to be stabilised by paraffin wax solids. Some heating to melt the wax is usually very effective with this type of emulsion, but it is not always practical, or desirable (for the oil company), to install heating facilities.
easier refining
Erosion of valves, pumps, hydrocyclones
There will be less operational problems overall, if solids removal is done as part of the water separation process. Unfortunately, treatment systems are often designed without taking such solids into consideration.
Chemical treatment can both facilitate solids removal and assist in deoiling these solids prior to disposal.
Hence, chemical selection (see later) seeks to maximise demulsification and dehydration, while minimising oil-in-separated water.
Chemical savings can often be achieved by injecting the demulsifier into the system as early (as far upstream) as possible.
Free Water Knockout
When high quantities of free water are produced with the crude oil, a ‘Free Water Knockout’ (FWKO) vessel is usually employed. This reduces the load on the separators downstream.
Water
Outlet
Siphon
In this system, all of the crude oil passes through the lower separated water layer. This assists in the demulsification / dehydration process.
Horizontal 3-Phase Separator
Produced
Fluids
A three phase separator of this design is typical of offshore installations.
A more typical onshore tank separator can be seen on page 17 of the Shell book “Process Chemistry 2000”.
Horizontal 3-Phase Separator
Production
Rate
Demulsifier
Demulsification and dehydration tend to improve at higher temperatures.
Water, oil and interface levels, need to be controlled within the design specifications of the separator. These units are designed for oil treatment and not for water treatment.
Should gas evolution (due to a high gas:oil ratio) be too high, then the resultant foaming will disrupt the oil/water interface and adversely affect separation. Antifoam is then needed.
Solids build up in the vessel can interfere with flow patterns and reduce residence time (settling for the water droplets).
Insufficient demulsifier will not resolve all of the emulsion, but too much can ‘overtreat’ and cause re-stabilised emulsion.
Distributor Header
Electrical dehydrators are used in some oilfields to enhance water separation. The oil is passed through an electric field which aids flocculation of the water droplets.
In some systems there is also a salt specification for the shipped crude oil. Then, a wash-water (low in chloride salt concentration) is mixed with the crude oil prior to entry into the electrical dehydrator. After treatment, the residual water in the oil will contain less salt.
Chemical treatment is nearly always still required to resolve the emulsion and enhance the water separation when the electric field brings about droplet collisions.
Basic Theory of Electrostatic Separation
When a water in oil mixture is subjected to an AC electrostatic field the following things happen:-
Collisions occur between the relatively conductive brine droplets
Coalescence occurs
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Brine Droplet in an AC Field (Induced Dipole)
The charge separation shown in the diagram, occurs due to the dissolved ions migrating towards the oppositely charged electrodes.
Basic Theory of Electrostatic Separation
Now, the two droplets are attracted to each other (unlike charges attract), bringing them into contact.
Emulsified Drops
Elongated Drops
Coalescing Drops
In the lower photograph (via a microscope), the formation of a bridge and the onset of coalescence can be clearly seen.
Population Density
Population Density
Research has shown that a minimum of 2.5 to 3 percent water is needed to achieve adequate droplet collisions.
Critical Voltage Gradient
(Electric Field Strength)
Upper Limit
Crude Oil Desalting
Essentially refinery based (some exceptions)
Lower water content of crude feed
wash water added
98% (two stage)
Solids removal 50% - 80%
Oil in effluent water 100ppm - 1.0%
Wash water rate 3% min. - 8% of volume of crude feed
Operating temperature 100 -150oC / 212 - 301°F
Typical Desalter Performance
Production rates
Vessel residence times
System specifications and problems
A number of companies have spent much time (over the past 50 years), effort and many hundreds of thousands of dollars, attempting to bring demulsification down to being a strict science. To date those efforts have not been successful.
In this work, the secret is in designing the test to duplicate the system and achieving a demulsifier use rate in the bottles that almost coincides with that used in the system. It is easier said than done, unless the tester has considerable experience of duplicating systems.
The “Bottle Test”
Looking For
Low levels of residual emulsion
Good interface quality
Cost effective treatment levels
Obviously, if a particular oilfield has long residence time tank separators, then a fast water drop may not be necessary. The oil quality leaving the tank is the deciding factor.
Oil in Water Problems
Once the formation brine is separated from the oil, it is either discharged (to a water course or the sea) or reinjected into the ground (secondary recovery or disposal). No matter where its destination, as much oil as possible must be removed from that water.
To prevent formation blockage re water use in secondary recovery
Free oil
Dispersion
tiny droplets of oil, emusified, or that separate slowly without agitation
Water is external phase
Oil is dispersed phase
Definition of an emulsion
Charge
OIL PARTICLE
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counter ions
Nett negative charge
Particle Charge - 4
LONG RANGE REPULSION
Particle Charge - 5
Larger density difference between particles and water - faster separation
Again Stokes’ Law is involved. This time it is the radius of the oil droplets which is important. It affects the velocity at which the oil droplets rise through the bulk water phase.
Increased ionic strength will destabilise emulsion
Water clarification processes
API separators(onshore)
API separators are usually concrete basins.
TPI or TPS - tilted plate interceptor, or separator - is the generic name for this piece of equipment. That is, the functional plates are always tilted at an angle. Another name is parallel plate (PPI) and another variation is corrugated plate (CPI).
Flotation units can be of ‘induced gas’ or ‘dissolved gas’ design. We will not be addressing the differences in this course.
Baffle
API separators are generally rectangular, with an entry box which normalises the flow into the unit and a scraper which skims free floating oil .
The unresolved oily layer then passes over baffles (reducing any turbulance ) into the main separator section.
Here oil droplets rising to the surface are skimmed off by a series of paddles, into an oil trough .
The sludge settling on the bottom is scraped to a trough or hopper.
PPI (Parallel Plate
Sludge Pit
Plate separators are primarily designed for removal of free oil. They are not very good at resolving reverse emulsions.
Oily water enters a basin and passes a series of parrallel plates set at an angle to the flow . The oil droplets then have a short distance to reach the upper coalescing surface. The amount of oil on the plate increases till the oil breaks away to the surface as a large oil droplet. Sludge coalesces and drains to the bottom of the basin.
Unit efficiency is reduced if blocking occurs between the plates or if the system is overloaded i.e. throughput is too great. Blocking may also be due to excess solids or micro-organisms.
Outlet
Inlet
Adjustable
Oil
Globules
22
This diagram shows a little more detail. A CPI only differs from a PPI in that the parallel plates are corrugated (like corrugated iron used in some house roofing). This increases the available surface area, within a given volume of unit (especially important with offshore space / weight restrictions).
Flotation Units
By attaching gas particles (approximately 20-100microns in diameter) to each particle, the apparent density of the particle becomes less than that of the surrounding liquid.
Because of this the oil and particulate matter rises in relation to the water.
Look back at Stokes’ law .
Apart from the droplet / particle size being important, the next most important term is (d1-d2) i.e. the density difference between the bulk phase and the dispersed phase.
By attaching gas particles to the oil and therefore decreasing the particle density, you can increase the rate of separation.
Dirty water
Oil
Solids
A typical unit would consist of four of these cells in sequence. Each cell is equipped with a motor driven self-aerating rotor system.
As the rotor spins it acts as a pump, forcing water through a disperser, thus creating a vacuum in the gas intake.
Gas bubbles lift oil and solids to the surface
Oil
Solids
Gas bubbles
The vacuum pulls gas into the stand pipe and throughly mixes it with wastewater.
As the gas/water mixture travels through the disperser at high velocity, the shearing force causes the gas to form bubbles.
Oil droplets and suspended solids attach to the gas bubbles as they rise to the surface .
Gas bubbles lift oil and solids to the surface
Clarified water
Induced air flotation cell
The oil and solids that gather in a froth on the surface, are skimmed by paddles into external launders on the side of the cells .
showing vortex flow
Seen here is the static hydrocyclone.
These have been adapted to remove bulk water from mixtures containing over 60% water.
The entry of oily water sets up a vortex inside the hydrocyclone. The resultant centrifugal forces affect the oil and water in different ways. As oils are usually less dense than water, the oil will be pulled to the centre of the vortex. Oil-free water flows downwards and reject oil out through top.
Again, only free-oil is separated, so chemical treatment may still be necessary to initially resolve the oil-in-water emulsion.
Baker Petrolite
Water droplets
(Dipole Attraction)

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Emulsions