Desalting of Crude oil means the removal of the dissolved salt in the crude oil and increasing the grade of the crude oil. Crude oil dehydration is the process of removing the water present in crude oil to meet the purchaser’s limit. Since salt is dissolved in the water so that dissolved salt is also removed in the process.
Why desalting and dehydration of Crude Oil?
Crude purchasers place limits on the salt and water contents of the crude they buy, typically:
- Water 0.2 to 0.5% vol.
- Salt 70 g/m3
Since salt is dissolved in the water phase, dehydration is also effectively desalting.
During production, the oil and water are mixed and one phase disperses as droplets (dispersed phase) in the other (continuous phase). Maximum mixing occurs at the points of high energy dissipation, e.g. at flow beans, valves, and pumps.
Theories behind Desalting and Dehydration of crude oil
Water and oil are separated by virtue of their different densities; gravity is the driving force. For laminar flow conditions the settling velocity (VT) of an unhindered dispersed phase droplet is given by Stokes Law:
This can be increased by the use of centrifuges, cyclones, or other such devices, although this is unusual in oilfield practice.
Density Difference and Viscosity
Large density differences between oil (ρo) and water (ρw) and low oil viscosities are favorable for easy separation and therefore light crudes separate faster than heavy crudes. An increase in temperature causes a reduction in viscosity, therefore the application of heat can be used to accelerate the separation of heavy crude.
Droplet Size (d)
The settling rate depends on the square of the droplet diameter. Drop to drop coalescence increases droplet diameter and for this reason, coalescence is a key factor in successful dehydration. Unfortunately, however, many oilfield dispersions resist coalescence and are known as ‘stable dispersions’ or ‘stable emulsions.
Oil and water mixtures would be highly unstable were it not for naturally occurring surface-active agents (surfactants) and finely divided particles that are absorbed in oil/water interfaces to form rigid films that resist coalescence. In this way, surfactants stabilize fine droplets which then accumulate to form emulsions.
Water in oil emulsions is known as ‘normal’; most oilfield emulsions are in this category. Oil in water emulsions are termed ‘reverse’.
Crude Oil desalting and dehydration Procedure
Successful dehydration of crude oil is carried out in three steps
- Destabilization of the emulsion
- Coalescence of small drops into large drops
- Settling out of large drops and separation of the two phases.
Destabilization of the Emulsion
Stable inter-facial films can be broken down by:
- Chemical demulsifiers
- Heat treatment
- pH treatment
- Increased salinity.
The use of chemical demulsifiers is now the main approach for treating stable oilfield emulsions, often in combination with heat treatment. Demulsifiers are most effective when added prior to the formation of an emulsion.
Separators in Crude oil Dehydration
Separators are prefabricated pressure vessels, which are suitable for separating oil, water, and gas.
Separators can be designed for ‘free water knock-out’ (FWKO) or ‘dehydration’ service. Both look similar in appearance, but for a given throughout the dimensions of a vessel in dehydration, service is necessarily larger.
A separator designed for free water knockout service will generally only remove free water from the feed stream. The rest of the water will remain dispersed in the crude, typically 5 to 10% vol. for light crudes (ρo < 850 kg/ m3) and 10 to 20% vol. for heavy crude (ρ o > 900 kg/ m3).
A separator designed for ‘dehydration service’ will dehydrate crudes down to low water content levels, typically 1 to 3% vol. for a liquid-liquid separator and 1 to 5% vol. for a three-phase separator.
Wash Tank/ Concentric Wash Tank for Crude Oil dehydration
Wash tanks are usually the preferred choice for general-purpose dehydration of light and medium density crude Oils (ρo < 900 kg/ m3) on land. The water content of the crude at the outlet is typically 1 to 3% vol. Operating temperatures above 85 ºC are not usually because of unacceptable loss of light ends.
A conventional wash tank is shown below:
Note that ‘free gas’ is removed upstream of the tank in a gas boot. If deeper dehydration is required then either a separate degassing tank should be installed instead of the gas boot, or consideration should be given to using a concentric wash tank.
Concentric wash tanks are a relatively recent development and are particularly suitable for the dehydration of heavy/viscous crudes.
A typical concentric wash tank is above.
Electrostatic Coalescers for Crude Dehydration
Electrostatic coalescers are pressure vessels fitted with electric grid internals and are suitable for deep dehydration of crude oil. The water content of the crude oil at outlet is typically in the range 0.1 to 0.5% vol. The units are relatively compact and therefore suitable for use offshore.