Oil Treatment (Dehydration).pdf

POLPetroleum Open Learning

OPITO

THE OIL & GAS ACADEMY

Oil Treatment(Dehydration)

Part of thePetroleum Processing Technology Series

Petroleum Open Learning

Oil Treatment (Dehydration)(Part of the Petroleum Processing Technology Series)

Contents

Training Targets 4

Introduction 5

Section 1 Emulsions Their Nature and Occurrence 7 What is an emulsion? The Creation of an Emulsion Emulsion Stabiltiy Emulsions and the Problem of Salt

Section 2 Principles of Emulsion Treating 14 The Application of Heat The Application of Electricity The Application of Chemicals DemulsifierSelection DemulsifierBottleTest Equipment Test Procedure Main Test Injection of Chemicals Water Washing Settling

Page Visual Cues

training targets for you to achieve by the end of the unit

test yourself questions to see how much you understand

check yourself answers to let you see if you have been think-ing along the right lines

activities for you to apply your new knowledge

summaries for you to recap on the major steps in your progress




Contents (contd)

Section 3 Dehydration Systems and Equipment 32 Settling Tanks Wash Tanks Free Water Knockout Heater Treaters Electrostatic Treaters Desalting

Section 4 A Typical Dehydration System 45 Separation of Free Water Knockout Crude and Emulsion Heating Electrostatic Dehydrators Dilution Water System

Check Yourself Answers 58

Pages Visual Cues

training targets for you to achieve by the end of the unit

test yourself questions to see how much you understand

check yourself answers to let you see if you have been think-ing along the right lines

activities for you to apply your new knowledge

summaries for you to recap on the major steps in your progress


Training TargetsWhen you have completed this unit on Oil Treatment (Dehydration), you will be able to :

Explain what constitutes an emulsion. Describe how an emulsion is formed. Explain how residual water in oil can cause problems with salt content. Explain the basic principles of emulsion treating.Listthebasicpropertiesofademulsifier.

Explainhowabottletest,usedindemulsifierselection,iscarriedout.

Explain how demulsifying chemicals are injected into a dehydration process. Describe the construction and operation of wash tanks and free water knockouts. Describe the construction and operation of heater treaters and electrostatic treaters.Explainthelayoutandflowthroughatypicaldehydrationplant.

Tick the box when you have met each target


Oil Treatment (Dehydration)IntroductionWater is produced together with oil from most oil fields.Thiswater,whichmaymakeupavery largepercentageof the totalproduction fromafield,cancause considerable problems. These problems include.

In order to minimise the problems I have just described, the water is separated from the oil at the earliest opportunity. This separated water is then treated before being disposed of.

(Another unit in the Petroleum Processing Technology Series covers produced water treatment in detail.)I said that water is separated from the oil at the first opportunity. But how is this done ? If youhave completed previous units in this series you will be aware of the primary separation facilities in a productionprocessingplant.Letslookbrieflyatthesystem to refresh your memory.

The totalproduction fromanoil fieldflows from thewells to the separation system. The function of this system is to separate the production into its individual phases of oil, gas and water. The process is carried out in large vessels - the separators. A typical 3 phase separator is shown in the diagram below.

Typical 3 phase separator

Corrosion - The produced water is very salty. If this water is allowed to remain in the oil it could cause corrosion damage to pipes, vessels and other equipment

Scaling - Salts are initially dissolved in the water present in a reservoir. As conditions change when this water is produced these salts may be precipitated as solids and deposited as scale. This in turn can reduce pipe diameters, plug vessels and equipment and lead to lost production

Transportation - The oil will be transported fromthefieldbypipelineortanker.Eitherway,water in the oil will cause problems. Water in the pipeline leaves less room for oil and results in lossofpipelineefficiency.Waterbeingsentto a refinery can cause serious upsets in thedistillation process. Tankers will not accept a cargo which contains more than a very small percentage of water


The vessel is called a 3 phase separator because it separatesthetotalflowstreamintothethreeseparatestreams of oil, water and gas. A 2 phase vessel would only separate the stream into the liquid and gas streams.

I dont intend to go through the construction and operation of a separator at this point (another unit on oil and gas separation is available from Petroleum Open Learning). For the time being just look at the bottom left hand side of the vessel.

This part of the separator is the liquid accumulation section. The oil, water and gas stream has entered the vessel at the inlet and been deflected at theinletdeflector.Thegashaspassedtowardsthegasoutlet via straightening vanes and mist extractor, and the liquids have fallen into the liquid / accumulation section.

This is where the separation of oil and water takes place. But how does it occur? The water and oilseparate due to a difference in their densities. Providing the oil and water stay in the vessel for a sufficientperiodoftimethebulkofthewatercanbeseparated from the oil. This water is the produced water which has now to be disposed of.

This all seems fairly straightforward. However, there is a potential problem at this point. In order for this separation to occur the water must exist as free water. In other words, the water must be present as a body of water. Or, if the water is present as droplets, these must be large enough to fall through the oil and accumulate as a water layer. Unfortunately, some water may be present in the Oil as very small droplets. These droplets are dispersed throughout the oil and form an emulsion which can be very stable. Further treatment is then required on the oil to break down the emulsion and separate the oil and water from each other.

This is what this unit is all about - the treatment of oil to remove the final amounts of water afterprimary separation. The treatment is often called Oil Dehydration.

I have divided the unit into four sections as follows:

Section 1 covers emulsions. In this section we will look at the nature of emulsions, how and why they form and what affects emulsion stability

In Section 2 we will look at the basic principles of emulsion treatment

In Section 3 we will examine the construction and operation of equipment used in emulsion treatment or oil dehydration

Finally in Section 4 I will take you through a typical dehydration system


Oil Treatment (Dehydration)Section 1 Emulsions Their Nature and Occurrence

What is an Emulsion ?Oil and water do not mix. This is an old saying which is often quoted. In fact this is the basis of oil and water separation. If we were to shake up some oil and some water in a bottle and then let it stand the following would happen; the water would sink tothebottomofthebottleandtheoilwouldfloatontop. When two liquids are not capable of being mixed we say that they are immiscible.

However, oil and water can be made to mix under certain circumstances. This occurs when one of the liquidsisdispersedasfinedropletsthroughouttheother and is stabilised.

Havingsaidthat,wecoulddefineanemulsionasfollows:

An emulsion is a mixture of two liquids which are usually immiscible. One of these liquids is dispersed throughout the other as small droplets and is stabilised by a third substance called an emulsifying agent.

Inoilfieldemulsionsthetwoimmiscibleliquidsareoil and water. Either one could be dispersed in the other. The most common, however, is the situation where the water is dispersed in the oil. This is known as a water in oil emulsion. Occasionally an oil in water, or reverse emulsion will form but these are much rarer. In this unit we will concentrate on the more common one.

The dispersed water droplets are known as the internal or discontinuous phase. The oil surrounding the droplets is the external or continuous phase.

WhenIdefinedanemulsionIsaidthatathirdsubstance is present in the mixture. This is a substance which separates the internal phase from the continuous phase and vice versa. It is known as an emulsifying agent.

So, for an emulsion to form, there must be three components present. ie.

water - which is the internal phase

oil- which is the continuous phase

an emulsifying agent

In addition to the three components being present, they must be agitated for the emulsion to form. The individual components in themselves would neverformanemulsionunlesstherewassufficientagitation to disperse the water through the oil. However, no amount of agitation will form an emulsion without the liquids being immiscible and an emulsifying agent being present. This being so, lets look at the formation of an emulsion.

The Creation of an EmulsionThe two liquids, oil and water, if they are in a pure state could not form an emulsion. You could agitate the two liquids for ever, creating droplets of water in the oil, but as soon as the agitation is stopped the two would separate from each other. The reason for this is that the liquids are not compatible. When they are placed together in the same container they try to findaconditionwhichwillgivetheleastcontactareabetween themselves.

The shape which has the least surface area for a given volume is a sphere. So, a droplet of water within a body of oil will assume a spherical shape. This will ensure the minimum contact area between itself and the surrounding oil. In addition, the droplet will try to make itself as small as possible. This alsowillreducethecontactarea.Butwhathasthe smallest surface area, a lot of small droplets or a single droplet with the same volume as the combined volume of the small droplets? Try the following Test Yourself question which will show you.


From the answer to the question, you can see that the surface area of the larger droplets is smaller than the sum of the surface areas of the combined smaller droplets. We have already said that the water will try tofindaconditiongivingtheleastcontactarea.Thetendency is for all the droplets of water in an oil water mixture to join together to form one body of water.

What I have just said might indicate that we are unlikely to have a problem with emulsions. However, thisiswheretheemulsifyingagent,oremulsifier,comes into the picture. This substance is essential to the creation of an emulsion.

A well known example of an emulsion which you wouldfindinthekitchen,ismayonnaise.Thebasic ingredients for making mayonnaise are vegetable oil and vinegar. If these two liquids are whiskedtogethertheytendtomix.But,assoonasthe whisking is stopped, the oil and vinegar would immediately separate. If eggs are slowly added during the Whisking however, the mixture soon takes on the familiar thick creamy appearance of mayonnaise an emulsion.

In this case the emulsion is formed from two immiscible liquids vegetable oil and vinegar, then subjecting them to violent agitation whisking in the presence of an emulsifying agent eggs.

Test Yourself 1.1

A sphere with a diameter of 15 mm has a volume of 1767 mm3. Five smaller spheres each having a diameter of 8.77 mm have the same volume in total. Determine what has the smallest surface area, the single dropletorthefivesmallerdroplets.

The formula for the surface area of a sphere is d2

YouwillfindthecorrectanswerinCheck Yourself 1.1 on Page 58.


Emulsifying agents are always present in crude oils produced from the reservoir. They include the following substances.

asphaltines a term given to a variety of compounds containing sulphur, nitrogen, oxygen, etc

resins

organic acids

metallic salts

silts

clays and many others

These agents are known as surface active agents, which means that they tend to alter the nature of the interface between the water droplets and the oil. Theemulsifier,whichispresentintheoil,migratestotheinterfaceandconcentratesthere.Emulsifiersarefairlyneutralasfarasanaffinityforoilorwateris concerned. They neither like nor dislike the two liquids. They tend to form a barrier between the water droplets and the surrounding oil. They form a type of skin round each droplet which prevents them from joining together. It is useful to imagine each water droplet being wrapped in a substance rather like clingfilm. Since the emulsifiers are movingaround in the oil, they tend to carry the surrounded water droplets with them and keep the droplets floatingintheoil.

So, in the oilfield we have all the conditionsnecessary for the creation of an emulsion. The two immiscible liquids, the presence of an emulsifying agent but what about agitation? The very process of producing reservoir fluids ensures that there isagitation. Imagine the fluids flowing up the welltubings,throughchokes,viaflowlinesandheadersinto processing equipment. That certainly agitates thefluids.

Emulsion StabilityThe stability of an emulsion is a measure of its resistance to being broken down into the separate components of oil and water. We can refer to an emulsion as being tight(difficulttobreak)orloose (more easy to break). Whether the emulsion is tight or loose depends on a number of factors and we can look at some of these now.

Amount of water present - As the quantity of water present in the mixture increases, more and more agitation is required to completely emulsifyit.Ifcompleteemulsificationoccurshowever, there will be a greater number of water droplets present in a given volume. Therefore, there will be a greater number of collisions between the droplets, which gives them a better chance of uniting, and then separatingfromtheoil.Byandlarge,waterin oil emulsions with a high water content tend to form less stable emulsions.

Oil viscosity In a thick viscous oil, water droplets cannot move around very easily. This means that there will be less chance of the droplets meeting each other. Even if the dropletswhichformduringemulsificationarerelatively large, they will not be able to sink through the oil and separate out. Therefore the oil will be able to hold the water droplets in suspension more easily.

Emulsifying agent The type of emulsifierwill dramatically affect the stability of the emulsion. However, an emulsifier whichcreates a stable emulsion in one situation could form a very loose emulsion under different circumstances. There are so many variables in the conditions under which an emulsion is produced, that it is impossible to state which agent creates the most stable emulsion.

Age of the emulsion - When the water and oilarefirstmixedtogether theemulsifyingagent is evenly distributed throughout the oil. It takes time for the ernulslfier tomigrate to the interface between the oil and the water droplets. So initially the emulsion is relatively unstable. As time goes on and themigrationproceeds, thefilm is formedaround the water droplets. With increasing timethefilmbecomesthickerandtoughermaking it more difficult for the dropletsto combine. This results in a more stable emulsion.


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Size of water droplets - In general the size of the dispersed water droplets is a measure of its stability. If the agitation is such that very small droplets are produced, the emulsion will tend to be tight.

Figure 1.1 shows the difference between a tight and loose emulsion with respect to their droplet size.

Figure 1.1

Now try the following Test Yourself question before we go on to discuss the problems of salt in crude oil.

Test Yourself 1.2

Are the following statements True or False ?

a. Mayonnaise is an example of an unstable emulsion.

b. If a mixture of oil and water is violently agitated a tight emulsion will form.

c. If, after agitation of oil and water in the presence of an emulsifying agent small droplets of water are produced, the resulting emulsion will tend to be a tight emulsion.

d.Emulsifiersaresurfaceactiveagentswhichmigratetotheinterface between oil and water and form a barrier between the droplets and the surrounding oil.

TRUE FALSE

YouwillfindthecorrectanswersinCheck Yourself 1.2 on page 58


Emulsions and the Problem of SaltThe water of the internal phase of an emulsion is invariably salt water, The salt content or salinity of the water is expressed in ppm NaCI. This means parts per million sodium chloride which is common salt.Thissalinitycanvaryfromfieldtofieldbutcouldbe as much as 200 000 ppm.

Refineriescannotacceptoilwhichhasahighsaltcontentasthesaltbreaksdownduringtherefiningprocess and causes considerable problems. Severe corrosion, scaling and fouling of equipment and pipework are just some of the undesirable effects of saltinrefineryoperations.

The saltwater is removed as far as possible before thecrudeoil getsas farasa refinery. Of coursethis unit is all about the breaking down of emulsions and the removal of the water. However, no matter howefficient thedehydrationprocess is, therewillusually be a very small amount of residual water in the oil. This is expressed as the amount of base sediment and water ( BS&W ) as a percentage of the total liquids. This residual water will vary with the efficiency of the dehydration equipment but couldrangefrom0.1to0.3%BS&W.

Since the residual water is the salt carrier, the actual amount of salt being transported in the crude oil will depend on the salinity of the water and the amount remaining after dehydration.

The amount of salt in oil is usually quoted at a refinery inunitsofpounds per thousand barrels ( PTB ).A limitofsalt incrudeof50PTBmaybeestablished by the refinery, and any salt contentabove this would be unacceptable.

If we know the salinity of the residual water and the percentageBS&WwecandeterminethesaltcontentofthecrudeinPTB.ThegraphillustratedinFigure 1.2 can be used to determine salt in oil content, if the residual water percentage is just 0.1%.

Figure 1.2 Salt in oil when 0.1% water remains


Using Figure 1.2 try the following Test Yourself question on salt contents.

Test Yourself 1.3 After dehydration, 0.1 % water remains in a

certain crude oil, and the salinity of this water is 140 000 ppm NaCl. Would this crude be acceptable toa refinerywhoseupper limit forsaltincrudeis50PTB?

Wouldtherefineryacceptthecrudeifthewatersalinity is 100 000 ppm NaCI ?

Youwillfindthecorrectanswersin Check Yourself 1.3 on Page 58.

You can see from the answer to the Test Yourself that evenwithanefficientdehydrationsystemitmaybenecessary to reduce the salinity of the residual water in crude in order to be able to export it for sale. In Section 3 we will look at ways of doing this.


Summary of Section 1

In this section we have been looking at emulsions in general, what they are and howtheyareformed.Idefinedanemulsionasamixtureoftwonormallyimmiscibleliquids in which one of the liquids is dispersed throughout the other as small droplets. It is stabilised by an emulsifying agent. I pointed out that in a water in oil emulsion the water is the internal phase and the oil is the continuous phase. You saw that in addition to the two immiscible liquids and an emulsifying agent being present, the mixture must also be agitated for an emulsion to form.

We then went on to look at the way in which an emulsion is formed and I gave an example of mayonnaise as a well known emulsion. In this case eggs form the emulsifying agent. You saw what types of substances form emulsifying agents in the oilfieldandhowtheagitationoccurswhenreservoirfluidsareproduced.

We considered the difference between a tight and a loose emulsion and the various factors which affect its stability.

Finally we looked at the problems of salt in crude oil. You saw that even if the residual amount of water in oil is reduced to very low percentages, if the salinity of that water is very high then there could be problems with the total amount of salt in the oil.


Oil Treatment (Dehydration) Section 2 Principles of Emulsion Treating

In theory, if an emulsion was allowed to remain in a vessel for an unlimited period of time it would eventually separate into oil and water. The droplets of water would fall through the oil and form a layer of water at the bottom of the vessel. In fact the settling process is the basis of all emulsion treating systems. The time required for this to happen we could call the settling time. Unfortunately in petroleum producing operations we just do not have this time, so, in order to separate the two liquids in an emulsion and allow them to settle, we must assist the process.

Lets start by having a look at a physical law regarding the speed at which a suspended particle would fall through a continuous medium. It can be described by an equation known as Stokes equation. This is written as :

V =

Where : V = velocity g = gravitational constant r = radius of particle d1 = density of continuous medium d2 = density of particle N = viscosity of continuous medium

2g r 2 (d 2- d 1)9N

Dont worry about this equation if your maths are a bit rusty, this is the last you will see of it. However what it means is, that to increase the speed of set-tling we must do one of two things. We must either increase the value of the factors on the top line of the equation, or decrease the value of the factor on the bottom line. How can we do this?

Lets examine each of these factors in turn.

Firstly g the gravitational constant. This, as its name states, is a constant and we can do nothing at all about this

Secondly r which is the radius of the particle, in our case the water particle. We could try to increase the radius of the particles by causing the droplets to join together thus increasing their size and hence their radius

The expression ( d 2- d 1) represents the difference in density between the water and the oil. We could try to increase this

Finally N is the viscosity of the oil. To in-crease the speed of settling, this must be reduced. We could certainly try to do that

It would appear therefore that our emulsion treating problem can be overcome if we can achieve the following :

1. Decrease the viscosity of the oil

2. Increase the difference in density between the water and the oil

3. Cause the water droplets to join together and form larger droplets

There is in fact a fourth factor we could add to this list time. If it were possible we could try to increase the settling time available.

Beforewemoveonfromheretrytheactivityonthenext page.


This is simply an activity to get you thinking about ways to decrease the viscosity of a liquid.

Imagine working in the kitchen and having to mix some treacle with dry ingredients in order to prepare a particular dish. If you took the treacle straight out of the tin it might prove difficulttostir-itisthickorveryviscous.Whatcould you do to make the treacle thinner? One thing you might think about doing is to warm up the treacle in a pan on the stove. As the treaclegetshotter itwill start to flowmore easily. It gets thinner or its viscosity is reduced.

From what you have just been thinking about, it would appear that item one on our list can be achieved by heating the emulsion, so lets consider this now.

The Application of HeatIn fact heating the emulsion can assist not only in item one in our list but in items two and three also.

You have already seen that increasing the temperature of the oil reduces its viscosity. This allows the water droplets to sink more rapidly through the oil.

As the temperature is increased the difference in density between the water and the oil also increases. This occurs up to a temperature of about 8OC. After that the effect of heat on density difference diminishes.

Finally, heating the emulsion promotes the combining together or coalescing of the droplets. It does this in two ways. Firstly, having a hot emulsion means that the water droplets move around much more freely and collide with each other far more frequently. If these collisions are forceful enough, thefilmsurroundingthedropletscanberupturedandthey will coalesce. Secondly, as the water droplets are heated they will expand. This will stretch the surroundingfilmsandmakethemweakerenablingthem to be ruptured more readily.

In view of all this, most emulsion treating plants use heat. You should note however, that heating causes some vaporisation of the lighter components of the oil. If this is not contained, a reduction in gravity with a corresponding reduction in volume will occur. This of course means loss in revenue. Also, as the temperature is increased, the likelihood of maintenance problems occurring in the plant and equipment will increase. This being so, other methods are used to assist the application of a reasonable amount of heat in the treatment process.

We can now go on to look at some of these other methods.

Activity


The Application of Electricity The application of electricity in emulsion treating is an attempt to promote coalescenceofthewaterdroplets.Letshavealookathowthisworks.Beforewe do this, try the following activity.

Activity

All you need to perform this activity is a plastic ball point pen a piece of woollen cloth and a source of running water e.g. the kitchen tap.

Run the water from the tap as a small, thin continuous stream. Make sure that the stream is not breaking up into droplets.

Hold the blunt end of the pen against the stream of running water. Observe what happens to the stream.

Now rub the end of the pen against the piece of woollen cloth a few times. (The blunt end of the pen not the metal ball point end).

Hold the end of the pen close to the stream of running water again and observe what happens.


What you should have noticed during the activity you have justperformed is the following; thefirst timethat you held the pen against the stream nothing happened. The second time however, after rubbing the pen against the wool, the stream of water bent towards the pen.

Obviously some force acts between the pen and the water after the pen has been rubbed. It is in fact an electrostatic force. It occurs because in rubbing the pen against the wool you charge the pen electrically.

Buthowdoesthatattractthewater?Theanswertothis lies in the way that the water itself behaves.

The water droplets in the emulsion are made up of molecules which themselves are neutrally charged electrically. However within the molecules is an arrangement of charges which is known as an electric dipole. This has a positive and a negative end and is shown very simply in Figure 2.1.

Figure 2.1 : Electric Dipole

Normally the dipoles are arranged randomly within the molecules as shown in Figure 2.2.

Figure 2.2 : Electric Dipoles in Water Molecules Randomly Arranged

When the charged pen is placed near to the water stream the molecules line up with their negative ends being attracted towards the positively charged end of the pen. We can say that they become polarised. This has the effect of pulling the water towards the pen. Figure 2.3 shows this.

Figure 2.3 : Water Modules Attracted to the Positively Charged Pen

The phenomenon we have just been looking at can be used in the problem of treating emulsions.

If theemulsion ispassed throughanelectric fieldbetween two electrodes, the water droplets are polarised. They are then stretched due to the polar attractions which weakens the surrounding film.They are also attracted towards one or other of the electrodesandtendtospeedtowardsit.Becauseoftheweakenedfilmandthegreatercollisionforceasthey hurtle through the oil, the droplets unite more readily to form the larger droplets necessary for faster settling.


Although the application of heat and the application of electricity are both commonly used in dehydration they are rarely used by themselves. In order to assist in the process or to speed it up, chemicals are invariably injected into the emulsion. We can look at this now but before we do, try the following Test Yourself question.

Test Yourself 2.1Are the following statements true or false? If they are false give the reasons why.

a) The speed at which a suspended particle would fall through a continuous medium is described by Stokes equation.

b) Decreasing the difference in density between water and oil in an emulsion would assist in allowing the water to settle during treating.

c) Increasing the temperature of oil reduces its viscosity.

d) Electric dipoles in water molecules are normally arranged with their negative ends all pointing in the same direction.

e)Ifanemulsionispassedthroughanelectricfieldbetweentwoelectrodesthe water droplets are polarised. This causes them to be stretched due to polarattractionswhichweakensthesurroundingfilm.

YouwillfindthecorrectanswersinCheckYourself2.1onPage59.

TRUE FALSE REASON


The Application of Chemicals The addition of a chemical to the emulsion helps to cause coalescence of the water droplets. It does this bybreakingthefilmsurroundingthewaterdroplets.Before it cando this it has toget to the interfacebetween oil and water droplets. It then has to gather sufficient droplets together prior to coalescence.This gathering together is called flocculation. In addition to the above it must be able to remove any solid particles from the interface and carry them away with the separated water. Chemicals which are able to do this are called demulsifiers.

Wecouldsaytherefore,thatgooddemulsifiershavefour basic properties:

They are strongly attracted to the water / oil interface

Theycauseflocculationofthewaterdroplets

Theyhelptorupturethefilmsurroundingthe droplets, promoting coalescence

They cause solid particles to be attracted to the water so that they can be removed with the water

Demulsifiersareinfactverysimilartotheemulsifyingagentswhichcausetheemulsiontoforminthefirstplace. They are surface - active chemicals which, when added to the emulsion, diffuse rapidly to the interface. Once there, they attempt to neutralise the effect of the emulsifying agent.

Having arrived at the interface, the demulsifiergathers together droplets of water by the action of flocculation. The demulsifier, which is nowconcentrated on a droplet, has a strong attraction for other water droplets in the same condition. The droplets tend to join up rather like a bunch of grapes. If theycollidewithsufficientforcetheskinmayberuptured and coalescence takes place. Sometimes however they just nestle together and further action is needed for coalescence.

Thenext actionof thedemulsifier is toattack thefilmssurroundingthedropletsiftheyarestillintact.It does this by causing eruptions at the interface whichconsequently ruptures thefilm.Withnofilmto prevent coalescence, the water tries to find acondition giving the least contact area with the oil. The water droplets, which are close together because of the flocculation, unite and form largerand larger droplets.

The action of flocculation and coalescence isillustrated in Figure 2.4.

Figure 2.4


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YousawinSectionOnethatemulsifierscancontainsolid particles such as sand, silt and clays etc. They may come from the reservoir or be residues from the drilling mud and so on. These particles help to increase the stability of the emulsion and must be removed if successful demulsification is to beachieved.Thedemulsifierdoesthisbywetting the particles. At this point I will say a little more about wetting and wettability.

Adhesion is the property by which particles of a given substance stick together. Liquids will stick to some solid substances more than others. For example, if you were to dip a glass rod in a beaker of water and then remove it, the rod would be wet. If however you do the same thing in a beaker of mercury, when you remove the rod no mercury would be clinging to the rod. This shows that some water is more adhesive to the glass than to water itself. Mercury however sticks to itself rather than the glass. We could say that the glass is water wettable but not mercury wettable. If you coated the glass rod with a greasy substance however, the glass rod would not be wet by water. Figure 2.5 shows this in a simple diagram.

The demulsifer makes the particles water wet. it has one end which is strongly attracted to the solild particle and forms a coating on the particle. The other end is strongly attracted to the water and will carry the particle in the water. This means that when the water droplets coalesce and sink, the soild particles will be carried out of the oil and can be disposed of with the water.

Youcandeducefromallthisthatthedemulsifierhasseveral jobs to do. It would be almost impossible to find a single chemicalwhich could accomplish alltheseactions.Therefore,Demulifiersarecocktailsof chemicals which are blended to give the best possible results for the type of emulsion being treated.

Figure 2.5


Demulsifler Selection

Just as there are many different types of crude oil there can be many different types of emulsions formed. To obtain optimum dehydration the most effectivedemulsifierforeachtypeofemulsionmustbeselected.Whatiseffectiveinonefieldmaynotwork in another. In fact, the addition of the wrong type of demulsifier could aggravate the situationand cause the emulsion to become more stable.

The selection of the demulsifier depends on anumber of factors, including:

The type of crude oil produced The nature and composition of the produced water The type of dehydration process The point of chemical injection The temperature Whether other chemicals will be used whichmayreactwiththedemulsifier

The above list is not exhaustive but it serves to showjusthowdifficultthechoiceofdemulsifiercanbe. Service companies who specialise in supplying oilfield chemicals producea rangeof demulsifiersfor the different situations encountered. Even so a considerable amount of work must be done in the fieldtoensurethatthecorrectchoiceismade.

When all the details regarding the type of crude and the nature of the produced water is known, the search for the most effective demulsifier can benarroweddown. Anumberofdemulsifiers fromasuppliers range would be chosen and subjected to field tests.Themost common typeof test carriedout is known as the bottle test.

Demulsifier Bottle Test

The bottle test is used to help to determine which chemical can most effectively treat an emulsion from agivenfield.Theresultsofthetestcanalsoindicatetheoptimumamountofdemulsifiertobeadded,i.e,the ratio of chemical to emulsion. Adding too much can be as bad as adding too little.

The basic procedure for carrying out the test involves taking a representative sample of the emulsion to be treated from a point in the process plant. The sample is placed in a calibrated bottle andaspecifiedamountofdemulsifiersadded.Thesample is agitated, allowed to stand whilst settling takes place and separation of water is observed and measured. After a time, a sample from the oil layer above the water is taken. This is processed in a centrifuge so that any emulsion, water and solids remaining in the oil can be determined. Lets expand this procedure and go through the basics of a bottle test.

When carrying out the test several points must be adhered to regarding the sample of emulsion. These are :

thesamplemustbefreeofanydemulsifier chemical

the sample must be truly representative of the total production

the sample must be tested as soon as possible. Ageing of the emulsion sample could affect its reaction to the treatment

EquipmentThe following list of equipment needed for the test is fairly straightforward, however I have given a brief description of the items which you may not be completely familiar with.

12 Calibrated bottles. These are similar to medicine bottles with graduations marked in millilitres (ml)

A 100 ml pipette. A glass instrument with which accurate amounts of emulsion can be taken


Micro pipettes calibrated in 0.001 ml divisions. Used for dispensing the demulsifying chemical in very small but accurately known quantities

Graduated glass syringes of 50 and 100 ml

A water bath with thermostatic control

A centrifuge with calibrated centrifuge tubes. This is a machine which spins a number of tubes at a very high speed. The centrifugal force causes the samples in the tubes to separate into oil and water

An agitator. This is capable of agitating the samples of emulsion in the calibrated bottles.(Sometimesinthefieldthebottles are shaken by hand.)

In addition to the equipment I have just listed, demulsifier chemical and solvents are required.Thedemulsifierisusuallyusedintestinginadilutedformcalled a solution. A typical 5% solution would be preparedbymixing1mlofconcentrateddemulsifierwith 19 ml of demulsifying solvent.

Test ProcedureA representative sample of the emulsion to be treated is taken in a suitable container capable of holding at least 2 Iitres.

Beforeconductingthemaintest,thetotalamountofwater and emulsion in the oil must be determined. This is done in the following manner:

Fill the centrifuge tubes with a solvent such as xylene up to the 50% level then top up to 100% with the emulsion

Agitate the tubes to mix the contents thoroughly

Centrifuge the tubes for 10 minutes

Note the emulsion and free water content

Add a few droplets of a slugging compound (this is a chemical which does not over treat the emulsion and cause the formation of a stable emulsion even if excessive amounts of it are used)

Agitate to mix and heat in the water bath for 10 minutes at 60C

Centrifuge again for 10 minutes. This should totally break the emulsion. If not, repeat adding more slugging compound

Record the total water content. This gives afigurewhichcanbeused tocompare thedemulsifying chemicals under test

Main TestWith the total amount of water and emulsion in the sample known, the main test can be carried out as follows :

Fill the calibrated bottles with 100ml of sample

Label the bottles with details of type of demulsifierandamountsused

Heat the bottles in the water bath to the sametemperatureasthatof thedemulsifierinjectionpointinthefield

Add the demulsifying chemicals in exact amounts using the micro pipette

Screw the tops on the bottles and ensure there is no leakage

Agitate the bottles either by hand or using an agitator, for a period of time which relates to theintensityofagitationinthefield


After agitation, immediately place the bottles in the water bath where the temperature has been adjusted to that of the settling temperatureinthefield

Record the start of the settling time and allow the samples to settle

Record the amount of separated water and emulsion at regular intervals

From the results, select the best performing samples

From the best samples, remove the oil from just above the interface using a syringe. Leave an equal amount of oil above the oil water interface in each sample

With this oil conduct a centrifuge test, which I described earlier, to determine the amount of residual water in the oil if any

From this test the best performing chemical can be determined to treat the particular emulsion problem.

Of course we not only want to know which type of demulsifierworksbestforaparticularemulsionbutalso, what is the optimum dosage.

The amount of chemical added in a treating system is usually quoted in parts per million ( ppm ). This means the volume of chemical used per million volumes of emulsion throughput.

In fact the treatment dosage is determined before doing the main test to determine the most suitable demulsifier.

If we use a demulsifier which is known to bereasonably effective, then the test which I have just described is carried out using different amounts of thesamedemulsifierinsteadofdifferentchemicals.This time 6 bottles would be used. Knowing a typical dosing ratio, the bottles would have chemicals added at 0.5, 0.75, 1, 1.5, 2, and 4 times this amount. From the results of the test the optimum dosage is determined, and this figure is used forfurther testing.

The bottle test will then indicate which demulsifying chemical is going to be best for our particular dehydration problem and what the optimum dosage rate will be.

I now want to look at the actual injection of the demulsifierintotheprocessstream.However,beforedoing so, try the following Test Yourself question.


Test Yourself 2.2

The steps taken in carrying out a bottle test are listed below in the wrong order. List the steps in the correct order starting with :

a. Fill the calibrated bottles with 100 ml of sample.

b. Add the demulsifying chemicals in exact amounts using the micropipette.

c. Agitate the bottles for a period of time which relates totheintensityofagitationinthefield.

d.Labelthebottleswithdetailsoftypeofdemulsifierandamountsused.

e. Heat the bottles in the water bath to the same temperature as that of thedemulsifierinjectionpointinthefield.

f. Record the amount of separated water and emulsion at regular intervals.

g. From the results select the best performing samples.

h. Record the start of the settling time and allow the samples to settle.

i. Screw the tops on the bottles and ensure there is no leakage.

j. After agitating immediately place the bottles in the water bath where the temperaturehasbeenadjustedtothatofthesettlingtemperatureinthefield.

YouwillfindthecorrectanswersinCheak Yourself 2.2 on page 59.

1


Whereisthebestplacetointroducethedemulsifierinto the emulsion ? The answer to this question is that there is no single best chemical injection location. Each process system must be carefully evaluated to determine the most effective point of injection. We can look at a typical system and identify some possibilities.

The following drawing Figure 2.6,isasimplifiedlayout of a typical production process. Study the drawing for a few minutes and mark on it the points where you think we could inject the demulsifying chemical.

Injection of Chemicals

Activity

Figure 2.6


Thedrawingshowstheflowoffluidsfromthereservoirthroughthe separators. In such a system there are several locations which seem to be suitable as chemical injection points. I would suggest the following :

Down hole in each individual well

Atthesurfaceintoeachflowline

Into the main header

Into the separators

Lets consider each of these locations.

In general the chemical should be introduced as far upstream as possible. Doing this ensures that there is a minimum of time for the emulsifying agent to create and stabilise an emulsion. It also meansthatthedemulsifierhasmaximumtimetodoitswork.Theturbulenceasthefluidsflowupthewellborethroughthesurfacevalves and pipework ensures the dispersal of the chemical.

Having said that, it would appear that injection downhole is the most effective location. Many wells are equipped with facilities for chemical injection. The most common method would be to have a chemical injection valve installed in a side pocket mandrel in the tubing string.

Figure 2.7 showspart of a simplifiedwell completiondrawingwith a chemical injection valve in a side pocket mandrel.

Figure 2.7


The mandrel is a piece of tubing with a bulbous side to it. A small tube, the pocket, is incorporated into this. The chemical injection valve sits in this pocket. You canseefromthefigurethatasmalllineisfedtotheside pocket mandrel down the tubing / casing annulus from the surface. Chemical being pumped down this line is contained between the two seals which straddle the inlet port in the pocket.

The valve itself looks like the one shown in Figure 2.8.

The chemical enters the valve through the inlet port and is pressurised against the valve and seat. The valve is being held on its seat by a spring. At a predetermined pressure the valve will open allowing thechemicaltoflowroundthevalvesinternalpathwaysand out through the outlet port and into the well tubing. Pumping the chemical under pressure through such an injector ensures that it sprays into the produced fluidsandisthoroughlymixeddownhole.

Although downhole injection is certainly carried out in many locations it does present certain problems. Each well has to be completed in such a way that chemical injection valves can be installed in the tubing string. This adds to the cost of the well completion and introduces extra possibilities for mechanical failure in the well. Maintaining the many injectors is time consuming and costly.

Figure 2.8


A more practical solution then, may be to inject the chemical at the surface into each wells flowlinenear the wellhead. There would be less time for the demulsifier toworkandmore timeforstabilisationthan downhole but the costs would be less.

Injecting the chemical into the main header ensures that it is introduced continuously into the total production. It could be a relatively low cost installation having a single injection point as opposed to multipoint injection downhole or in the flowlines.However therewillbe lessagitationandless time for the chemical to work.

Ifchemicalisinjectedatthesurfaceintotheflowlinesor the header, it will be injected via an injection quill. This is designed to ensure that mixing is as complete as possible between the chemical and the emulsion. Figure 2.9 shows a simple injection quill arrangement.

A non-return valve fitted in the injection line willprevent back flow and protect the line from wellfluids.

Injectingthedemulsifierintotheseparatorsisrarelyconsidered.By the time thefluids reach thispointthere is very little time left for the chemical to work effectively.Thefluidflowthroughthevesselismuchless turbulent thus the chemical is less effectively dispersed. The emulsion has also had more time to stabilise.

The actual point is often a compromise which de-pends on the type of operation. Sometimes a few wellswouldbetreateddownholeorattheflowlinewith additional injection at a single point in the main header. The character of each wells production must be determined so that the wells which contrib-ute most to the emulsion problem can be selected for downhole treatment.

Before we finish this section on the principles ofemulsion treating we should consider two other points, i.e. settling and water washing. Lets look at waterwashingfirst.


Water WashingThis process is a mechanical means of reducing the water content of a destabilised emulsion. In such a system the emulsion is introduced to, or made to pass through, a large body of water. As it does so, each water droplet in the emulsion may be absorbed by contact with this large volume. This is referred to as water washing. For most effective absorption, the wash water should be exactly the same water as the droplets. In fact the wash water is often free water which has already been removed from the emulsion. In awaterwashingfacilitytheemulsionflowsunderabaffleinthetreatingvesselthusensuringthatitpassesthrough the wash water. Figure 2.10 illustrates this.

SettlingI have already said that settling is common to all types of treatment of emulsions. At the beginning of this section I said that if an emulsion could be left for asufficientlengthoftimethewaterdropletswouldsink and form a water layer at the bottom of any vessel. So, in addition to heat, electricity, addition of chemicals and water washing, there must be some time allowed for the water to form a layer from where it can be drained separately from the oil.

We have covered quite a lot in this section but before I summarise for you, try the following Test Yourself question.


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Test Yourself 2.3

Readthroughthefollowingstatementsandfillinthemissingword/wordsfromthelistgivenbelow:

a. Heating oil tends to reduce its

b. The speed at which a suspended particle would fall through a continuous medium can be described by equation.

c. An electric dipole has a and a end.

d.Ifanemulsionispassedthroughanelectricfieldbetweentwo . the water droplets become

e. When water droplets gather together we could say that occurs.

f.Ademulsifierhelpstoremovesolidparticlesfromtheemulsionby the particles.

g. A chemical injection valve could be situated in a side mandrel in the tubing string.

h. An injection is designed to ensure that mixing is as complete as possible between the chemical and emulsion.

LIST OF WORDS

POLARISED, QUILL, NEGATIVE, ELECTRODES, HEADER, VISCOSITY, STOKES, WETTING,

POSITIVE,ELECTROSTATIC,BOTTLETEST,FLOCCULATION,POCKET.

YouwillfindthecorrectanswersinCheck Yourself 2.3 on Page 59.


In this section I have taken you through the principles of emulsion treating in a logical manner. We started by having a look at Stokes equation which is written as :

We saw from the equation that to increase the speed of settling of droplets of water through a continuous medium we must do three things :

1. decrease the viscosity of the oil

2. increase the density difference between the water and the oil

3. cause the water droplets to coalesce and form larger droplets.

We saw that heating the oil helps to decrease its viscosity and also increase the density difference between the oil and water.


V = 2g r2 ( d2 - d1 )

9N

We further saw that the application of electricity can help in promoting coalescence of the water droplets. It does this by polarising the droplets. This has the effect ofweakeningthesurroundingfilmandcausingthemtobeattractedtowardstheelectrodes.Becauseoftheweakenedfilmandthecollisionswhichoccurasthedroplets move rapidly through the oil, the droplets unite and form the larger droplets necessary for faster settling.

We then moved on to look at the application of chemicals which help to promote coalescence. These demulsifying chemicals are surface active chemicals which have four basic properties. They :

are strongly attracted to an oil / water interface

causeflocculationofthewaterdroplets

helptorupturethefilmsurroundingthewater droplets

cause solid particles to be attracted to the water so that they can be removed along with the water

You saw that there are many different types of demulsifiersavailableandcarefulselectionofthebest one must be made for a particular emulsion treating application. I described for you the bottle test which is used to help determine which demulsifiercanmosteffectivelytreatagivenemulsion. From there we moved on to look at the injection of the chemical. We saw that there are several options for injection points. These could bedownhole,inthewellflowlines,intheheaderor in the vessels. We looked at the advantages and disadvantages of each of these.

To end the section we had a brief look at water washing and settling. In water washing the emulsion is made to pass through a large body of water which absorbs the water droplets from the oil. Time for settling as I have mentioned on several occasions is necessary for any oil dehydration process, but speeding up the settling time is what most of this unit has been about.


Oil Treatment (Dehydration)Section 3 Dehydration Systems and EquipmentHaving looked at the basic principles of treating emulsions lets now go on to the practical application oftheseprinciplesinthefield.Thedehydrationequipment will rarely use just one of the principles covered in the last section, but will use a combination of them. We can look at several types of treating vessels. I intend to start with some rather basic equipment and follow on with some which is slightly more complicated.

Settling TanksAll treating systems involve settling. In some situations a simple settling tank used in conjunction with chemical injection could be all that is necessary. In this case the tank must be big enough toallowsufficientretentiontimeforthewater droplets to sink to the bottom.

A typical example of such a system is atank farm at a shore terminal. Here the totalproductionfromanoffshorefieldistransported via a subsea pipeline to very large tanks at the terminal. Although the tanks are principally used for storage, because of the amount of time that the oil remains in the vessels, settling can take place.

At least three tanks would be used. In operation, one ofthetankswouldbeintheprocessofbeingfilled,andone would be settling. The third, having had the settled water drained off, would be having its clean oil pumped toatankerorarefinery.Figure 3.1showsasimplifiedversion of such a system.

Note that there is provision for injecting chemical on the offshore platform and also into the pipeline before the tankfarm.

Figure 3.1


Wash TanksA wash tank is more likely to be found on older land installations than offshore and can only be used with relatively low throughputs. It is basically a settling tankwithafewrefinements.Althoughyoumaynotcome across one of these vessels, it is worth having a look at its construction and operation as it uses some basic principles of emulsion treating. A typical wash tank is shown in Figure 3.2. Look at this now and identify the various components.

The emulsion to be treated enters the unit through the inlet line and passes to a larger diameter pipe, the conductor. This vertical pipe may be mounted either inside the vessel as shown in the drawing, or outside. Gas may be liberated from the emulsion at this point. The conductor acts as a vertical separator within the wash tank. Any gas is taken from the top of the conductor and passes through an equalising line into the top of the wash tank. This equalises pressure between the conductor and the main bodyofthetank.Thegas-freeemulsionthenflowsdown the conductor and is spread out through the water layer at the bottom of the tank. A spreader arrangement at the bottom of the conductor helps to do this.

Inside the main body of the tank a water layer is maintained. This is the wash water. The spreader is designed so that the emulsion exits as several small streams which rise independently through the wash water. As these small streams rise, a certain amount of de-emulsifying takes place as the water droplets in the emulsion contact the large body of water. The clean oil will continue to rise whilst the water droplets remain in the wash water.

Any emulsion which has not broken down during its passage through the water will form a layer on top of the water. Clean oil will form a further layer on top of the emulsion. Further breakdown of the emulsion will take place within the layer on top of the water. This layer will remain in the vessel for a relatively long time so a certain amount of settling will take place. Water will sink into the water layer with oil rising to join the oil layer.

The wash water level in the tank is maintained by a level control valve in the water outlet line.

In the system we have just looked at, the breaking down of the emulsion is achieved in two parts :

water washing

settling

A variation of these principles can be found in the type of vessel known as the Free Water Knockout which we will look at now.

Figure 3.2


Free Water KnockoutsStrictlyspeaking,freewaterisproducedwaterwhichwillsettleoutoftheoilwithinfiveminutesifthefluidsareatrest.Assuchitisnot part of the emulsion and can be removed by gravity separation in a simple separator. It is important that this free water is removed before the emulsion is further treated in a system such as the heater treater which we will look at shortly. A free water knockout drum, as illustrated in Figure 3.3, will do this.

Figure 3.3 Free Water Knockout Drum


This vessel is basically a horizontal 3 phase separa-tor.Theincomingfluidsimpingeonaninletdiverterwhere initial separation of gas from liquids takes place. The liquids fall to the bottom of the vessel and the diverter ensures that they pass through a water layer which is maintained in the vessel. Thus the liquids are water washed. The separated free water plus any water which has been washed out of the emulsion settles into the water layer. The level of the water layer is maintained by an interface level controller, operating a level control valve in the water outletline.Theoilandemulsionflowsoveraweirintothe oil accumulation section from where it is taken under level control to the emulsion treating facility.

The treating facilities we have just been looking at are fairly simple systems. We can now go on to look atsomethingalittlemorecomplicated.Beforewedo,try the following Test Yourself question.

Test Yourself 3.1

The following terms apply to a wash tank, a free water knockout drum, both of these or neither. Mark with a which.

Terms Wash Tank Free Water Knockout Drum

Both Neither

Inlet diverter

Gas Equaliser

Spreader

Water Layer

Weir

Conductor Pipe

Injection Quill

Water Level Control Valve

YouwillfindthecorrectanswerinCheck Yourself 3.1 on Page 60.


Heater TreatersWe can now move on to a treatment facility which uses the application of heat to assist in the process. The heat may be applied prior to treating the emulsion in a simple wash tank. In this case the heating could be carried out in a heat exchanger similar to the one shown in Figure 3.4.

This heat exchanger is of the shell and tube type. In our case the medium to be heated, the emulsion, flowsthroughtheshellasshown.Theheatingmediumflowsthroughthetubes.Thiscouldbehotoilwhich has been heated using waste heat from power generation turbine exhausts.

Although the application of heat via an external heating source which I have just described is perfectly feasible, it is more common to incorporate this into a vessel called a heater treater.

This vessel can include a number of elements such as :

separation section

heating elements

oil surge section

mist extractor

coalescing section

spreader

oil collector

There are a number of different styles of heater treaters so I will describe just one which is typical. It is a horizontal vessel which looks rather like a separator. Its internal features however are completely different. Look at Figure 3.5 on the next page, which shows the internals of a horizontal heater treater vessel. Identify the internal features ofthetreaterthenwewillfollowtheflowthroughthevessel.

Figure 3.4: Shell / Tube Heat Exchanger


Flow, which is a mixture of oil, some gas, emulsion and free water enters the separation section of the treater, whereinitialseparationtakesplace.Anygasinthefluidsisflashedoffatthispointandflowstothegasoutletline.Beforeleavingthevesselthegaspassesthroughamist extractor. This is a device which ensures that any small droplets of liquid which may have been retained in the gas stream are removed. A common type of mist extractor is in the form of a knitted wire mesh. The droplets of liquid impinge on the mesh, form larger droplets then fall into the liquid below. Figure 3.6showsasimplifiedversionofamistextractor.

Figure 3.5 Heater Treater Vessel Figure 3.6 Mist Extractor


The liquids fall down into the bottom of the separation sectionandaredeflectedroundtheheatingelementwhilst doing so. Free water is separated here and the water accumulates as a layer at the bottom of the vessel. The water layer forms a water wash section which helps to remove unstabilised water from the emulsion. An interface between water and oil / emulsion is maintained by an interface level controller. This operates a level control valve in the free water outlet line.

The oil and emulsion then rise past the heating element where the temperature is increased to the optimum treating temperature. The heating element maybesimplyatubecoilthroughwhichheatingfluidis being circulated. On some land locations a directly firedheatingsystemmaybeused.

Theheatedoilandemulsionthenflowsoveraweir intotheoilsurgechamber.Fromhereitflowsthrough a spreader arrangement into the coalescing section of the vessel. The coalescing section is kept completely full of liquid. Unlike the separators which you are probably familiar with, the oil outlet is at the top of the vessel rather than at the bottom.

Thespreaderensuresthattheflowisdistributedevenly throughout the length of the section. If this werenotusedtheliquidflowcouldchanneltowardstheoutletandreducetheefficiencyofthetreater.

Astheheatedfluidsrisethewaterdropletscoalesceand when they become large enough they fall through the rising continuous phase. The water droplets accumulate at the bottom of the section and form another water layer. The height of this layer is maintained by a further interface level controller. This operates a level control valve in the water outlet line from the coalescing section.

The liquid which reaches the top of the vessel is treated oil, which should be free from any water or emulsion. This is taken from the treater via a collector pipeandflowstothenextpartoftheproduction process system.

You saw in Section 2 that passing the emulsion throughanelectricfieldcanhelpinthecoalescenceof water droplets. We can now see how this is done in practice.


Electrostatic TreatersElectrostatic treaters are very similar in construction to the heater treater we have just been looking at. The main difference is that they incorporate high voltageACand/orDCelectrostaticfieldinthecoalescing section. Figure 3.7 illustrates a typical electrostatic treater. Study this for a while and note the differences between this and the heater treater.

Figure 3.7 Electrostatic Treater Vessel


0

In this vessel you will notice that the major difference between it and the heater treater is the pair of electrodes in the electric coalescing section.

Theinitialflowthroughthevesselisthesameasthat described earlier. However when the heated emulsion rises through the coalescing section it hastopassthroughtheelectricfieldcreatedbytheelectrodes. As it does so, the water droplets are given an electric charge. The polarised droplets are attracted to one or other of the electrodes and race towards it. As they move rapidly through the emulsion they collide with each other. The polarisation also weakensthefilmaroundthedropletssothatastheycollide they readily coalesce. When the droplets are large enough they sink to the bottom of the vessel forming a water layer. The oil / water interface level is controlled by a level controller, operating a level control valve in the water outlet line.

The electrical system in an electrostatic treater consists of a transformer and the two electrodes which are suspended one above the other in the coalescing section. In some types of electrostatic treater the distance between the electrodes can be adjusted. This allows the voltage to be varied to meettherequirementsofthespecificemulsionbeingtreated.

Damage to the electrical system could occur if the level in the vessel were to go low enough to uncover the electrodes. To prevent this happening a low level shut down switch is incorporated into the emergency shutdown system for the vessel.

DesaltingAs I pointed out in Section 1, crude oil which is contaminatedwithsaltisunacceptabletoarefinery.In production systems where salt in oil is a problem, something must be done to desalt the oil. Often the dehydration process of chemical injection coupled with heater treaters and / or electrostatic treaters will besufficienttoaccomplishthedesalting.Insomecases however, it may be necessary to inject fresh water into the emulsion. This will dissolve the salt so that it can be removed together with the water in a treating vessel.

The desalting system which I have used to illustrate such a process, utilises a pre-heater, a fresh water storage tank, a fresh water injection pump and an electrostatic desalter/dehydrator. It is the type of systemcommonlyfoundataterminalwhereafiredpre-heater is used.

Look at Figure 3.8 on the next page, which shows this system as a simple block diagram.


Figure 3.8 : Desalting System


Thesalt-contaminatedoilpassesfirstlythroughthe heater where the temperature is raised to the optimum treating temperature. The heater itself is called an indirect heater. This is because the heat from the burning fuel is not transferred directly to the oil. It is transferred indirectly through a water bath in the body of the vessel to the oil being heated as it passes through tubes in the heater body.

Figure 3.9 shows such a heater.

The heater itself consists of the following items.

heater shell

fireboxwithburner

flowtubes

The heat is generated by burning fuel gas or oil in aburner.Thehotfluegasesflowthroughfiretubesandareexhaustedthroughthestack.Thisflowofhotgases heats up a body of water contained in the shell of the heater. The water in turn heats up the oil which isflowingthroughtheflowtubebundle.

Figure 3.9 : Pre-heater


Afterbeingheatedtheoilflowstowardsthedehydrator. In this case it is an electrostatic dehydrator which operates in the manner described earlierinthissection.Beforeitgetstothedehydrator,fresh water is mixed with the oil. The fresh water is pumped from a storage tank to a spray injector in the flowline.Thesaltintheoilisthusdilutedbythisfreshwater which mixes with the very salty emulsion water.

Thedilutionwaterplustheemulsionwaterisfinallyremoved in the dehydrator and led off for disposal. The oil leaving this vessel should be clean in terms of salt content and water.

This completes Section 3, but before I summarise what we have looked at in this section try the following Test Yourself question.

Test Yourself 3.2

The following pieces of equipment could be found in a wash tank, a heater treater, an electrostatic treater or all of them. Fill in the boxes with a to show which piece of equipment goes where.

Equipment Wash Tank

Mist Extractor

Spreader

Equalising Line

Weir

Electrodes

Conductor Pipe

Heating Element

Transformer

Heater Treater Electroststic Treater




We began this section by considering some basic equipment used for treating emulsions. Firstly, I pointed out that a simple settling tank could be used, and illustrated this by showing you a tank farm system.

We then looked at facilities used to wash an emulsion. The simple wash tank was explained in detail and you saw that the breaking of the emulsion is achieved in two parts i.e. water washing and settling. I also showed you a variation of the wash tank which is used to remove any free water prior to emulsion treating. The vessel doing this is called a free water knockout drum.

You saw how the treater vessel uses these elements to break down the emulsion so that the water can be removed leaving clean oil.

We similarly went through the operation of an electrostatic emulsion treater vessel. You saw that in operation it is very similar to the heater treater vessel. The essential difference is the inclusion of a pair of electrodes. These, when connected to a power supply,createanelectricfield.Thewaterdropletspassingthroughthisfieldarepolarisedwhichcausesthem to speed towards the electrodes, colliding as theydoso.Thepolarisationalsoweakensthefilmsurrounding the droplets so that when they collide they coalesce more readily and sink to the water layer.

Finally we saw that fresh water maybe injected into an emulsion to reduce the amount of residual salt in a produced oil stream.

In the next section we are going to combine some of these treatment systems and look at an overall dehydration process.

Heater treaters vessels came next and we looked at a typical treater vessel containing the following elements :

separation section

heating elements

oil surge section

mist extractor

coalescing section

spreader

oil collector


Oil Treatment (Dehydration) Section 4 A Typical Dehydration System Inthisfinalsectionwearegoingtolookatacompletedehydration and desalting system.

The system I will use as an illustration includes two separators and a free water knockout (FWKO) drum for initial separation. From the FWKO drum the crude and emulsion is pumped via a pre-heater through a water bath heater and two stages of dehydration to storage. As a means of reducing the residual salt content, water is injected prior to dehydration. This system is typical and does not represent any particular system.

The function of this system is to :

Separate free water from the incoming well stream

Treat the remaining emulsion and reduce the residual water content to an acceptably low level

Reduce the residual salt content to within the limits set by the purchaser

On the next page, I have included a simple block diagram to show the system in its entirety. Study this for a while and familiarise yourself with the equipment usedandtheflowpathsthroughthesystem.

Wewillnowfollowtheflowthroughthesysteminmore detail. Lets consider the process, section by section.


Figure 4.1 : Typical Dehydration System


Separation and Free Water KnockoutLook at Figure 4.2whichshowsthispartofthesystem.Itisastraightforwardseparationprocess.Thereservoirfluidswhichareamixtureofoil,gas,freewaterandemulsion,flowtothefirstvesselinthesystem,thefirststageseparator.Thisisa3phaseseparator which in our system is operating at a pressure of 10 barg.

Thefirstchemicalinjectionpointisintothelineenteringthefirststageseparator.Demulsifierisinjectedheretogiveitasmuchtime as possible to take effect before the emulsion reaches the electrostatic dehydrators.

Figure 4.2 : Typical Separation and Free Water Knockout System


Inthefirststageseparator,thefreewaterisseparated and forms a water layer at the bottom withoilfloatingontop.Theleveloftheinterfacebetween the two is maintained by an interface level controller (LC 01) which controls the interface level control valve (LCV 01) in the water outlet line. This waterflowstotheFWKOdrumwhereitisusedaswashwater.Theoilplusemulsionflowstothesecond stage separator under the control of the level controller (LC 02) operating LCV 02 in the oil outlet line. The separated gas is taken from the top of the vessel through pressure control valve (PCV 01) operated by pressure controller (PC 01) which maintains the correct pressure in the vessel.

The second stage separator is a 2 phase vessel. It is operating at a pressure of 3.5 barg maintained by a pressure controller (PC 02) and a pressure control valve (PCV 02).

Beinga2phasevesselthisseparatorhasnooilwater interface control. All the liquids leave the vessel viatheoiloutletandflowtotheFWKOdrum.Theliquid level in the separator is maintained by the level controller (LC 03) operating LCV 03 in the liquid outlet line.

TheinletflowintotheFWKOdrumconsistsoftheliquids from the second stage separator plus the water whichhasbeenremovedinthefirststageseparator.It may seem strange removing water from the liquid stream then recombining them at a later stage. You will remember the reason for doing this if you think back to our discussion on free water knockout facilities in Section 3. To make sure that you can recall the process try the following Test Yourself question.

Once again the interface level in the FWKO drum is controlled by an interface controller (LC 04) operating LCV 04. The water which is removed in this vessel consists of the water removed in the firststagevesselplusanyfurtherfreewaterwhichhas been washed out of the emulsion. This water is routed to a produced water clean up facility prior to disposal.

Test Yourself 4.1Without referring to the notes make a sketch of a simple free water knockout drum

YouwillfindthecorrectanswerinCheck Yourself 4.1 on page 62


The pressure in the vessel is maintained just above atmospheric by PCV 03 operated by PC 03.

The oil level in the vessel is maintained by a control valve which is located downstream of the feed pump to the heater section. This is operated by LC 05. We will look at this shortly.

We can now move on to the next part of the plant whichincludestheheaters.Beforewedosohowever, read through the last few paragraphs and makesurethatyouunderstandtheflowthroughtheseparation section.

Crude and Emulsion HeatingYou will remember that heating an emulsion helps to enhance the dehydration process. In this section of theplanttheliquidsareheatedintwostages,firstina shell and tube heater then in a water bath heater.

Figure 4.3 shows this small section.

PickinguptheflowfromtheFWKOdrumyouwillsee that there is further provision for the injection of demulsifierintotheflowlineupstreamofthecrudepump. This ensures good mixing as the crude and emulsionflowthroughthepump.Downstreamofthepump is the control valve LCV 05. This valve controls the oil level in the FWKO drum via LC 05.

Figure 4,3 Typical Crude and Emulsion Heating System


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The pump transports the crude onwards through the pre-heater. This heater is of the shell and tube type which we looked at in Section 3. The heating medium is the treated crude which comes from the second stage of the dehydration plant. Using this crude as a heating medium means that energy is recovered which would otherwise have been wasted.

Fromthepre-heaterthecrudeflowsthroughthewater bath heater. Gas from the plant is used as fuel tofiretheheater.

The temperature inside the heater is controlled by a temperature controller (TC 01) which regulates the fuel supply through TCV 01.

A safety shutdown system also protects the heater ifthereshouldbeaflamefailureattheburner.This stops supply of fuel gas, ensuring that there is no dangerous build up of gas escaping from unlit burners.

We can now look at the dehydrators themselves, but before we do, try the following Test Yourself question.

Test Yourself 4.2

Ihavelistedtheitemsofequipmentandinjectionpointsintheinitialflowpathofourtypicaldehydration system. These items are in the wrong order. Place them in the correct order, start-ing with inlet.

a) Inlet

b) 1st stage separator

c) Injection of water from 1st stage separator

d)Seconddemulsifierinjectionpoint

e) FWKO drum

f) Crude pump

g) 2nd stage separator

h) Water bath heater

i) LCV for FWKO drum oil level

j) Pre heater

k) Dehydrators


1


ElectrostaticDehydrators

Figure 4.4 : Electrostatic Dehydrators

The crude enters this part of the processfrom the water bath heater. Identify thispoint on the drawing of this part of thesystem in Figure 4.4.

Thefirstthingyouwillseeasyoutracetheflowisaninjectionpointforwater. This water is the reject waterfrom the second stage dehydrator.It is injected at this point to help reducethe salinity of the incoming water in theemulsion and to water wash the emulsion.Although the water used here is itself salty,it is less saline than the incoming water.The water is injected through nozzleswhichensurethatitentersthemainflowasfinedroplets.Thesedropletsmustthen combine with the water in theemulsion which requires some formof agitation. A mixing valve takes careof this. The valve is a differential pressurecontrol valve (DPCV 01). Its controller(DPC 01) maintains a pressure dropletacross the valve and this, together withtheplugandseatprofileofthevalveitself,providesthe necessary surface and energy for the agitation to take place.


Aftermixing,theflowofliquidsentersthefirststageelectrostatic dehydrator. This dehydrator works in almost the same way as the one we looked at in Section 3. The essential difference is that no heating element is included in the dehydrator itself.The heating of the emulsion is done priorto the treater as we have just seen.

Theflowatthispointbecomesmore complex so we will divideitupandfollowtheflowsofoil / emulsion and waterseparately. Lets start with theoil / emulsion.

These liquids enter thedehydrator and follow asimilarflowpathtotheonedescribed in Section 3.The instrumentation on thevessel can be quite complexbut we can look at some ofthe more important instruments.To minimise the complexity ofFigure 4.4 we can look at this inisolation in Figure 4.5.

Figure 4.5 : Electrostatic Dehydrator


Lookfirsttotherighthandsideofthevessel.Thereyou will see LG 01. This is a sight glass which gives a visual indication of the interface level between water and emulsion. An interface level controller (LC 06) controls the water level through its control valve (LCV 06) which is situated downstream of the dilution water heater. Alarms are incorporated into the interface level control instrumentation. These are designated level alarm high and level alarm low (LAH&LAL)andwillwarntheoperatorifthelevelisreaching potentially serious points. Separate level switches(LSHH&LSLL)aretiedintotheshutdownsystem of the plant. If the interface level should reach the set points of these instruments a shut down will automatically be activated.

Becauseitwouldbedangerousiftheoilleveldropped and uncovered the electrodes in this section of the dehydrator, further level instrumentation protects against this. A level transmitter in this section activates an electrical power shut down if the oil level drops below a pre determined minimum. This is shown on the drawing as (LT 01).

We can now go back to Figure 4.4 again and continuetotracetheflow.Theoil/emulsionfromthefirststagedehydratorpassestothesecondstagevessel.Beforeenteringthisvesselmorewaterisinjected into the stream. This water is dilution water which is often supplied from specially drilled water wells. The heated dilution water is injected through nozzles again, and a second mixing valve (DPCV 02) controlled by DPC 02 ensures correct agitation. The second stage electrostatic dehydrator works in the samemannerasthefirst.Itisalsoprotectedbythesame type of instrumentation.

The hot treated crude from this dehydrator, prior to beingroutedtostorage,flowsthroughthepreheaterwhere it acts as the heating medium to raise the temperature of the crude before it enters the main water bath heater.

TracingthewaterflowsthroughFigure4.4webeginwiththerejectwaterfromthefirststagedehydrator.Thiswaterflowsfirstlythroughthedilutionwaterheater where it acts as the heating medium for the water from the wells. After passing through the level control valve (LCV 06), the water is routed to a produced water clean up facility prior to disposal.

The dilution water is heated in the heater and then joins the oil entering the second stage dehydrator. The reject water from this vessel is recycled to the firststagedehydratorwhereithelpstodilutetheincoming water. It is pumped by the recycle pump through level control valve (LCV 07) which together with level controller (LC 07) maintains the water level in the second stage dehydrator.

We have only one small section to look at now, the dilutionwatersystem.Beforewegoontothis,trythefollowing Test Yourself question.


Test Yourself 4.3

Complete the following sentences with an appropriate word or phrase.

a) Upstream of the 1st stage dehydrator there is an injection point for reject water which comes from the

b) The injection water and water in the emulsion require agitation. This is taken care of by a mixing valve which is a valve.

c) Dilution water is passed through a before joining the oil entering the 2nd stage dehydrator.

d) It would be dangerous if the oil level dropped and uncovered the in the dehydrator.

e) The dilution water heater uses from the as its heating medium.

Youwillfindthecorrectanswersin Check Yourself 4.3 on Page 63.


Dilution Water SystemLookatthefinaldrawinginSection4Figure 4.6 which shows the dilution water system.

In this system the dilution water is obtained from specially drilled water wells. In some areas where fresh water sources are scarce, slightly salty brackish water could be used.

The water is produced from the wells to a storage tank. It is pumped using submersible pumps which are driven by an electric motor. The level in the tank is maintained by the on / off operation of the water wellpumpsusinglevelswitches(LSH02&LSL02).

Figure 4.6 : Dilution Water System


From the tank the water is pumped by the dilution water pump to the second stage dehydrator. Upstream of the pump, there is provision for injecting chemicals. Scale inhibitor is added to the water to prevent scale building up in the pipework and vessels. An oxygen scavenger is also injected to reduce the dissolved oxygen content of the water and reduce its corrosiveness.

The ratio of dilution water to crude / emulsion throughputiscarefullycontrolled.Atypicalfigurecould be 1 : 20. The actual amount of dilution water addediscontrolledbyaflowcontrolvalve(FCV01).Thisisregulatedbyaflowcontroller(FC01)takingitssignalfromaflowtransmitter.

Beforebeinginjectedintothefeedtothesecondstage dehydrator, the dilution water is heated. This is done in a shell and tube type heat exchanger which uses the produced water as its heating medium.

This completes this unit on dehydration of crude oil.BeforeIsummariseSection4,trythelastTest Yourself question.

Test Yourself 4.4 Without referring to Figure 4.1 sketch a block diagram, illustrating the typical dehydration system which we have just studied.

Youwillfindthecorrectanswerin Check Yourself 4.4 on Page 64.



In this section I have taken you through a typical dehydration and desalting plant. It is an entirely hypothetical plant which is not intended torepresent any existing installation. It has simply been used to illustrate the principles which we discussed in the preceding sections.

Inthisplantwesawthattheinitialseparationofwaterwasaccomplishedusingathreestageseparationprocess.Thefirststageremovedfreewater which was used as wash water in the third stage or free water knockout. All the water removed in the FWKO drum was taken to a producedwater clean up facility prior to disposal.

Thecrudeoilandremainingemulsionwasthenheatedinapre-heaterandawaterbathheaterbeforeenteringthefirststageofatwostagedehydrationanddesaltingprocess.Thesevesselswereelectrostaticunits.Priortothefirststagetherejectwaterfromthesecondstagewasaddedtothefeed.Thishelpedtodilutethesaltcontentoftheproducedwater.Therejectwaterfromthefirststagewascombinedwiththewaterfrom the FWKO drum and sent to disposal via the produced water clean up facility.

Beforeenteringthesecondstagedehydrator,dilutionwaterwasaddedtothefeed.Thiswatercanbeobtainedfromwaterwellsandheatedbytherejectstreamfromthefirststageinaheatexchangerlocatedupstreamoftheinjectionpoint.

Inthesecondstagedehydrator,thecrudestreamwasfinallytreatedtoachievethecorrectspecification.Thetreatedcrudewasthenusedasaheating medium in the pre-heater prior to being sent to storage facilities from where it would be transported to the purchaser.


Check Yourself Answers

Check Yourself 1.1 Check Yourself 1.2 Check Yourself 1.3

Using the formulasurface area = d2

the S.A. of a single droplet is 152 = 707 mm2

the S.A. of each small droplet is 8.772 = 241.6 mm2

total of 5 small droplets is5 x 241.6 = 1208 mm2

single droplet has smaller surface area.

a. False Mayonnaise once formed is verystableandisdifficulttobreakdown.

b. False An emulsifying agent is also required.

c. True.

d. True.

If the salinity of

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