Crude Oil Desalter/Dehydration

After primary oil/water separation there is often a small amount of unwanted salts in residual water in the crude oil that needs to be reduced to a concentration around 5-10 PTB (Pounds per Thousand Barrels) of salt. This salt needs to be removed so that the crude oil can be processed in a refinery or stabilizer plant without fouling heaters, exchangers and other equipment.

Crude Oil Desalting technology is utilised to remove residual salt to meet these requirements. The type and size of the Desalter that is used is dependent upon a number of fundamental factors such as pressure, temperature, fluid viscosity and flow-rate, as well as customer requirements relating to maximum salt allowed in the product oil stream.

Howe-Baker has developed a range of crude oil dewatering and desalting technologies to be applied in appropriate situations.

Desalting

Depending on the downstream process, a limit of between 1- 10 PTB of salt is usually specified which can require additional treatment beyond dehydration.

This can be performed by a desalter unit.

Clean dilution or wash water is injected into the crude oil feed to the desalter through a mixing device to dilute the brine to a level where the target salt content can be achieved by the downstream Dehydration Unit.

In difficult applications this wash water can be recovered and recycled in a 2 stage dehydration and desalting process.

Electrostatic Dehydration

The basic principle behind electrostatic dehydration is that an electrical field is used to excite droplets of brine within the bulk oil phase so that they collide with other droplets and coalesce into larger globules that separate under gravity.

Traditional electrostatic coalescers employ two horizontal grids connected to a high voltage AC supply and the emulsion flows upwards through the grids.

The water coalesces and flows downwards whilst the clean oil leaves via the top of the vessel.

Distributors are used to ensure even ‘plug’ flow up the vessel and grids.  The 2 grid system is still widely used, however more modern and improved Low Salt Treaters are available in various other proven styles – 3 grids and 1, 2 or 3 phase power supplies to achieve oil purity of 0.1% BS&W or better.

Design Expertise

Howe-Baker can provide a wide range of electrostatic dehydrators and desalters to effectively and economically treat the range of crude oils produced today.

By providing different vessel sizes, different numbers and types of power units together with several different electrode grid arrangements, Howe-Baker’s custom designs meet the most stringent performance and code requirements.

Each system is designed to give a high performance for a given electrode grid area, resulting in a smaller, more cost effective pressure vessel for a given oil throughput. The unit consists of three parallel horizontal electrode grids within the vessel with each layer divided into three sections.

Using three single-phase AC power units (each power unit using a different phase) and cross-connecting the grids, a balanced electrical load is achieved and full utilisation of the grid system is assured in the unlikely event of a bushing or power unit failure. The grid spacing is optimized to give a high voltage gradient and to optimize this deep field design.

The design has the added security that if one power unit should fail, an electric field is still maintained throughout the deep field region. This means that the unit can still operate to produce the same specification oil though at a reduced throughput or lower specification oil at the same throughput.

How AC Electrostatic Dehydrators/Desalters Work

The wet crude oil is fed into the dehydrators/desalters through Howe-Baker’s specially designed distribution headers and spreaders which ensure even flow through the unit such that no section of the unit is over or under-loaded and the longest retention time for the crude oil inside the treater with no recirculation inside the treater. This gives the maximum removal of free water and therefore dissolved salts from the crude oil in the fastest possible time, which results in smaller and lower cost vessels.

The headers are positioned in the oil phase below the electrical grid assembly, that is the heart of the electrostatic process.

The wet crude oil flows evenly up through the grid assembly and is subjected to an intense AC electrostatic field created by the high voltage charge imposed on the grid electrodes. The electrostatic field aids the small water droplets to coalesce to form larger droplets and thus be able to separate under gravity from the crude oil.

The water droplets fall to the oil water interface below the inlet distribution headers and the separated water is discharged through a separate pipe under level control. The dehydrated dry oil rises to the top of the vessel where it is collected and discharged through the crude oil outlet piping.

Howe-Baker locates the electrical grids to ensure optimum power consumption and maximize the residence time of the crude oil in the electrostatic field, thus allowing the water droplets more time to coalesce and separate from the crude oil.

The arrangement of header positions, electrode layout and interface level position has proven, in the field, the capability of Howe-Baker’s systems to handle and resolve fully emulsified crude oils with water cuts in excess of 30%. The high voltage imposed on the grids is achieved by transforming the voltage of a standard AC supply up to a voltage of 14,000 to 22,000 volts using power units and high voltage connection assemblies, electrically certified for use in hazardous areas.

The power unit electrical connections and the unique electrode grid connections ensure a balanced electrical load on all three phases of the electric supply. The power units are specifically designed for dehydrating/desalting applications where the grids may be short-circuited by excessive water slugs in the crude oil. 100% reactance in the power unit ensures that under such upset circumstances the units can stay on line safely, until the upset is cleared, without shutting down the process. Depending on the required quality of the crude oil and the salinity of the feed, more than one stage of desalting may be required.

The Howe-Baker desalting systems are capable of reducing the salt content to as low as 1 ppm weight. Multistage desalting systems have two or more desalting vessels in series. The system works by introducing the fresh wash water into the crude stream immediately upstream of the last vessel in the system.

The product crude exits this vessel and the effluent water from this vessel is recycled and is mixed with the crude entering the vessel immediately upstream in the system. This is repeated with the effluent water from each vessel being recycled to the upstream vessel.