Unit 1 Electrostatic Desalter

Page 1/20

Module No. 15 : Electrostatic desalter

Unit No. 1- Electrostatic desalter

UNITS IN THIS COURSE

UNIT 1

ELECTROSTATIC DESALTER

Page 2/20

Page 3/20



TABLE OF CONTENTS Para

Page

1.0

COURSE OBJECTIVES

3

1.1

SALT AND CRUDE OIL

4

1.2

TERMINOLOGY

4

1.3

THE BASIC SALT REMOVAL METHOD

10

1.4

THE DESALTING PROCESS

11

1.4.1

Chemical Injection

11

1.4.2

Preheating

12

1.4.3

Mixing Valve

12

1.4.4

Desalter Vessel

13

1.5

BREAKING THE EMULSION

13

1.6

ELECTROSTATIC DESALTER OPERATION

14

1.7

DESALTER INTERNAL PARTS

17

1.7.1

Dispersion Inlet

17

1.7.2

Distributor

17

1.7.3

Electrodes

18

1.7.4

Vortex Breakers

19

1.7.5

Multi-ported Collecting Headers

19

1.8

EMULSION/WATER INTERFACE CONTROL

19

1.9

DESALTER SHUTDOWN SYSTEM

20

Page 4/20

1.0

COURSE OBJECTIVES

Page 5/20



This course describes electrostatic desalters. The course progresses from basic terminology to the principles of operation of electrostatic desalters and their associated safety systems. On completion of the course the student will be able to explain: •

The terminology used in the electrostatic desalting process.

•

The reasons for removing salt from crude oil and how it is don

•

Why the crude feed and wash water are pre-heated before the enter the desalter.

•

The function of the mixer valve, how it operates and how it is controlled.

•

Where demulsifier is injected and why it is needed.

•

How the 'water-in-oil' emulsion is broken.

•

The functions of the desalter internals.

•

How the emulsion/water interface is controlled.

•

How the desalter safety systems operate.

Page 6/20

1.1

SALT AND CRUDE OIL

Page 7/20



When oil is produced, there is almost always some salt dissolved in it or in the water mixed with the oil. If the salt is not removed it can damage heaters, heat exchangers and other process equipment. The salt drops out of the crude oil and sticks to heating coils and the internal parts of other equipment. Because of the damage salt can cause, refineries will only accept crude oil as feedstock if the salt content is less than 4.5 kg (10 lb) of salt in 1000 barrels of crude oil. An electrostatic desalter is used to remove the salt from the crude oil. 1.2

TERMINOLOGY Some special words are used when talking about electrostatic desalters. The following terminology will be explained: •

Desalting

•

Feedstock

•

Specific Gravity

•

Emulsion

•

Suspension

•

Coated

•

Paraffin Wax

•

Viscosity

•

Pressure Drop*

•

Coalescing

•

Demulsifier

•

Surface Tension

•

Electrical Potential

•

Commingle

•

Straightening Vanes

•

Laminar Flow

•

Elongated

•

Interface

•

Carry-over

•

Interlock

Desalting Desalting is the process of removing 'water soluble' salts from a crude oil stream. Something 'water soluble' can be dissolved in water. For example, sugar is water soluble but sand is not.

Page 8/20

Feedstock

Page 9/20



Feedstock is the mixture of hydrocarbons sent to a refinery for processing. The refinery will only accept feedstock which meets certain conditions. One of these conditions is the amount of salt the mixture contains. Specific Gravity The Specific Gravity (SG) of something compares the density of that thing with the density of pure water, when both are at the same temperature and pressure. Specific gravity is a ratio so it does not have any units of measurement. The specific gravity of pure water is 1.00. Anything with a lower specific gravity than this will float on water. Anything with a specific gravity more than 1.00 will sink in water. For example, crude oil has a specific gravity of between 0.80 and 0.97. Its specific gravity is less than water so it will float on water. Emulsion An emulsion is a suspension of small droplets of one liquid in a second liquid. The liquids in an emulsion can only be separated by a treating process. This means that the liquids cannot be separated by allowing the emulsion to stand in a settling tank or other vessel.

Figure 1-1 Water-in-Oil Emulsion

Page 10/20

Unit No. 1- Electrostatic desalter Module No. 15 : Electrostatic desalter

How hard it is to break an emulsion depends on the stability of the emulsion. Page 11/20

Emulsion stability is affected by: •

The gravity of the crude oil.

•

The surface tension of the water.

•

Any chemical impurities and contaminants

•

Any waxy materials in the crude oil.

•

Organic acids.

All of these things affect the size of the water droplets. Smaller water droplets make a stronger emulsion. A strong emulsion is more difficult to break than a weak emulsion formed by larger Water droplets. Suspension Crude oil contains small quantities of small, solid objects such as particles of sand or grit as well as droplets of water. The solid objects and the droplets of water are in 'suspension' in the oil. Coated Coated means that something is completely covered by something else. When crude oil comes out of the well some of the salt particles in the oil are coated (completely covered) with paraffin wax. Paraffin Wax Paraffin wax is a hydrocarbon wax which is found in crude oil. Paraffin wax is waterproof so if the wax coats something, water will not be able to get to that thing. This is important in desalting crude oil because the salt is washed out using water. If the particles of salt are coated with paraffin wax, the wash water will not be able to reach them. Viscosity Viscosity is a measure of the resistance of a liquid to flow. Viscosity is affected by temperature. If the temperature is increased the viscosity will be lowered. A liquid with a low viscosity will flow more easily. So liquids with high temperatures flow more easily than those with low temperatures.

Page 12/20

Pressure Drop

Page 13/20



Pressure drop may also be called pressure differential (Ap or dp). When a fluid (a liquid or a gas) flows through something, the pressure of the fluid gets less. Imagine a pipeline with a valve in it. The pressure of the fluid at the entry to the valve will be more than the pressure of the fluid when it leaves the valve. The difference between the two pressures is called the pressure drop. We usually talk about the pressure drop across a piece of equipment. For example, the pressure drop across a valve is 5 psi, the pressure drop across a filter is 10 psi. Coalescing Coalescing means joining together to make larger. If very small droplets of water are suspended in oil they will either stay like that or take a very long time to separate out from the oil. If two or more of these very small droplets are made to join together (to coalesce) they will form one larger droplet. If two or more of these larger droplets are also made to coalesce they will form an even larger drop. Eventually the drop of water will be large enough for it to separate out from the oil by gravity. Figure 1-2 shows how coalescing takes place.

Page 14/20

Page 15/20



Figure 1-2 Coalescing Demulsifier Demulsifier is also known as emulsion breaker. It is a chemical which helps to separate the liquids in an emulsion. Surface Tension Surface tension is an invisible 'skin' that holds a liquid in droplets. In an emulsion it is surface tension which holds the water in droplets in the oil. When the surface tension is reduced by using chemicals it is easier for the water droplets to coalesce. Electrical Potential Electrical potential is the electrical force between any two points in an electrical circuit. It is measured in volts.*

Page 16/20

Commingle

Page 17/20



When two or more insoluble substances are mixed together they are said to commingle. Water and oil commingle when an emulsion is formed. Straightening Vanes Straightening vanes are devices fitted in pipelines so that the liquid in the pipeline will have laminar flow (see below). Laminar Flow If a liquid in a pipeline is flowing smoothly, steadily and in a straight line it has 'laminar' flow. The opposite of laminar flow is turbulent flow. Figure 1-3 shows both laminar and turbulent flow of a liquid in a pipeline. Compare the two.

Figure 1-3 Laminar and Turbulent Flow Carry-over In an electrostatic desalter there are three different fluids - water at the bottom, emulsion in the middle and oil at the top. If the emulsion level rises too much, emulsion can get into the oil stream and flow out of the desalter. If this happens there is emulsion 'carry-over'.

Page 18/20

Elongated

Page 19/20



Something is 'elongated' when it is longer than it would normally be. In an electrostatic desalter, the water droplets in the emulsion are elongated by an electrostatic field so that they coalesce more easily.

Page 20/20

Page 21/20

Figure 1-4 Elongated Interlock An interlock may be electrical or mechanical. It is used to link together two or more devices which are often safety devices. An electrostatic desalter has several safety devices. These safety devices are interlocked so that if any one device detects a fault the desalter will be shut down. Interface An interface is the place in a vessel where two liquids meet. In a desalter there are three liquids and so there are two interfaces. At the bottom of the vessel there is water. On top of the water there is emulsion, and on top of the emulsion there is oil. These two interfaces are called the oil/emulsion interface and the emulsion/water interface. The emulsion/water interface is used to detect the water level in the desalter. 1.3

THE BASIC SALT REMOVAL METHOD The following steps are taken to remove salt from crude oil. Each step is described in more detail later in this Unit •

The wet crude oil is preheated to a temperature between 95°C and 120°C (200°F and 250°F) upstream of the desalter.

•

The wash water is preheated upstream of the desalter.

•

Demulsifier (emulsion breaker) chemical is added to the crude oil.

Page 22/20


•

The wash water is injected into the crude oil at about 3% to 5% by volume. This Page 23/20

forms an emulsion and dissolves the salt in the wash water.

1.4

•

The emulsion enters the desalter.

•

Inside the desalter the demulsifier chemical and the electrostatic field make the emulsion break down into oil and water.

•

The oil flows upwards and leaves the desalter. The oil is free of salt. This is called dry crude.

•

The water collects in the bottom of the desalter and leaves through the base of the vessel. The water now has the salt in it.

THE DESALTING PROCESS

Figure 1-5 Simplified Desalting Process 1.4.1

Chemical Injection Demulsifier chemical is injected into the oil stream. This will help to break the emulsion later in the process. The best place for the chemical to be injected is just upstream of the crude oil feed pump. The action of the pump will make sure that the demulsifier chemical is well mixed with the oil. On some process plants the demulsifier may be injected into the crude oil stream at another point, or there will be more than one injection point.

Page 24/20


If the demulsifier is not injected upstream of the crude oil feed pump, it must be Page 25/20

injected upstream of the mixing valve. 1.4.2

Preheating The crude oil and the wash water are both preheated upstream of the desalter. The crude oil is preheated to melt the paraffin wax which coats most of the salt crystals. Heating the oil also reduces its viscosity which makes it easier to break the emulsion later in the process. Care in heating the crude oil is necessary because if the temperature is too high the crude will vaporise (become a vapour) in the desalter. The wash water is preheated so that it maintains the oil temperature.

1.4.3

Mixing Valve It is necessary to thoroughly mix the crude oil and the wash water so that the wash water can absorb the salt in the crude oil. This is to make sure that the wash water gets spread all through the crude oil so that maximum contact between the salt and the wash water takes place. This mixing is done through the mixing valve. There are two inlets to the valve - one for oil, the other for wash water - and one outlet which passes the resulting emulsion to the desalter vessel. The mixing of the crude oil and the wash water is due to a pressure drop across the mixing valve. The pressure drop is in the range of 20 psi to 30 psi. A higher pressure drop causes better mixing. The amount of water added to the crude oil is between 3% and 5% by volume of crude oil feed.

Page 26/20

1.4.4

Desalter Vessel

Page 27/20



Page 28/20

Page 29/20

Figure 1-6 Electrostatic Desalter Inside the desalter vessel the emulsion is 'broken' into oil and water. The oil is then salt free and passes out of the top of the desalter to the next stage in the process. The water then contains all the salt and leaves through the base of the desalter under level control. 1.5 BREAKING THE EMULSION The emulsion in the desalter is a water-in-oil emulsion. That is, there are very small droplets of water suspended in the oil. The water will not settle out of the oil using gravity only because the water droplets are not large enough. To break the emulsion we must make the water drops become larger so that they will settle out. We do this by making the water droplets coalesce.

Page 30/20

Two methods are used to coalesce the water droplets:

Page 31/20



•

By using chemicals (demulsifiers).

•

By using electricity (electrostatic field).

With the chemical method, demulsifier chemicals (sometimes called emulsion breakers) are injected into the oil stream at a point where they will mix thoroughly with the oil. This could be upstream of the crude feed pumps or upstream of the mixing valve. Upstream of the crude feed pumps is better. The demulsifiers reduce the surface tension of the water droplets. When the surface tension is reduced it is easier. for the droplets of water to join together to make bigger, heavier droplets. With the electrical method, a high voltage electrical current is passed through the emulsion. This high voltage potential is made between two electrodes immersed in the emulsion. The high voltage potential polarises the water droplets making them act like small magnets and causing them to come together. The water droplets also become elongated and this makes them touch each other and join together (coalesce). Large droplets or droplets close together coalesce more easily. 1.6

ELECTROSTATIC DESALTER OPERATION An electrostatic desalter works like this: •

The emulsion enters the desalter through a dispersion inlet. The dispersion inlet contains straightening vanes to produce laminar flow.

•

Smooth flow inside the desalter is important otherwise the oil and water will not come out of the emulsion.

•

The emulsion is delivered into a distributor inside the desalter. The distributor is raised above the bottom part of the desalter.

•

The emulsion leaves the distributor through holes along the walls of the distributor. This makes sure that the incoming emulsion is evenly distributed throughout the desalter.

•

The emulsion is moving very slowly and forms 'fingers' in the other liquids.

Page 32/20

Page 33/20



Figure 1-7 Fingers of Emulsion •

The fingers of emulsion pass through the falling water drops without affecting the water drops.

•

The rising emulsion then passes between the electrodes. The electrical field causes the water droplets in the emulsion to coalesce with each other until they are large enough to fall towards the bottom of the vessel.

Page 34/20

Page 35/20



Figure 1-8 Electrostatic Treating The oil continues to flow upwards and leaves through the top of the desalter. The oil outlet is connected to a multi-ported collecting header inside the desalter.

Page 36/20


•

The water leaves the desalter through the bottom of the desalter. The water Page 37/20

outlet is connected to a multi-ported collecting header inside the desalter. Each port on the water collecting header is covered by a vortex breaker. The vortex breaker prevents emulsion being drawn down and leaving with the water. 1.7

DESALTER INTERNAL PARTS An electrostatic desalter is fitted with the following internal parts: •

Dispersion inlet.

•

* Distributor.

•

Electrodes.

•

Vortex breaker.

•

Multi-ported collecting headers - one for oil, one for water.

Figure 1-9 shows these internal parts. 1.7.1

Dispersion Inlet The dispersion inlet is fitted with straightening vanes. The purpose of the straightening vanes is to produce laminar flow in the emulsion as it enters the desalter. Laminar flow is a smooth, steady flow which will not cause waves or ripples in the liquids which are already ' inside the desalter. Keeping the liquids still helps the emulsion to separate out. The dispersion inlet feeds the incoming emulsion into the distributor.

1.7.2

Distributor The distributor is a box-like channel with a pointed top which runs the full length of the desalter vessel. Along both sides of the distributor are many small holes which allow the emulsion to enter the main part of the desalter. The holes in the distributor: •

Allow the incoming liquid to enter the desalter slowly so that the liquids already in the desalter are not mixed up.

Distribute the incoming emulsion evenly throughout the desalter vessel.

Page 38/20

1.7.3

Electrodes

Page 39/20



Page 40/20

Page 41/20

Figure 1-9 Desalter Internal Parts There are two electrodes, each made of steel rods. One elect rode is supplied with electricity from the transformer, the other one is grounded (earthed). The transformer is called a step-up transformer. This means that it receives electricity at a low voltage (480 volts) and changes it (transforms it) to a high voltage (20 000 volts) for use inside the Desalter

Page 42/20


When the electric current is turned on it flows from one electrode to the other so Page 43/20

that the space between the electrodes becomes an electrostatic field. 1.7.4

Vortex Breakers A vortex is like a small whirlpool. It happens when a large amount of liquid is trying to leave a vessel through a small hole. The vortex makes suction so in a desalter a vortex in the water could pull emulsion down into the water. Vortex breakers are fitted over the water outlets to prevent a vortex forming. By preventing a vortex the emulsion will not be pulled down into the water and so it will not leave the vessel with the water. Each vortex breaker is like a flat plate which is mounted over the water outlets. The flat plate prevents a vortex from forming.

1.7.5

Multi-ported Collecting Headers A multi-ported collecting header is a pipe, closed at each end, with several holes along its length. The liquid enters the header through the holes. By having several holes the liquid can be evenly collected along the length of the desalter vessel. This stops any flow currents from being set up which could mix up the liquid layers in the vessel. In a desalter there is one multi-ported collecting header for the oil and one for the water.

1.8

EMULSION/WATER INTERFACE CONTROL The oil/emulsion interface is difficult to detect so the emulsion/water interface is used for automatic level control within the desalter vessel. Control of the water level is important because the water must not be allowed to get into the area of the electrodes. The emulsion/water interface level is controlled by a differential level controller. The differential level controller signals a level control valve in the water outlet pipeline to open or close to keep the correct interface level inside the vessel. Some desalters may be fitted with a set of small valves which are called trycocks. The trycocks can be manually opened to find out the approximate positions of the oil/emulsion and emulsion/water interfaces.

Page 44/20

1.9

DESALTER SHUTDOWN SYSTEM

Page 45/20



Electrostatic desalters are fitted with a number of safety devices. The safety devices are interlocked to the desalter shutdown system. This means that if any one safety device goes off, the desalter will be shutdown even if everything else is working normally. The system safety devices cannot be manually bypassed or cancelled. The safety devices fitted on a desalter will protect against: •

Too much pressure in the vessel. The desalter is a pressure vessel. Like all other pressure vessels it is protected from too much pressure by pressure safety valves. The pressure safety valves vent any excess pressure to the drains.

•

A leak from the vessel into the transformer. The transformer of the desalter is fitted with a pressure sensor. An increase in pressure inside the transformer shows there is a leak from the desalter vessel into the transformer.

•

An electrical fault. The transformer of the desalter is fitted with a temperature sensor. An increase in temperature inside the transformer shows there is an electrical fault.

•

Water level too high. An increase in the emulsion/water interface must be detected before the water reaches the electrodes. Level alarms are fitted at the emulsion/water interface to warn of interface high level.

•

Emulsion carry-over with the crude oil. A drop in the voltage of the electrical supply to the transformer will cause a drop in the transformer output voltage. The electrostatic field will not work properly and there will be carry-over of emulsion with the crude oil. Low voltage alarms are fitted on the electrical supply to the transformers.

Page 46/20

Unit 1 Electrostatic Desalter

Recommend Documents