Oil Treatment (Dehydration).pdf

POL Petroleum Open Learning

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

OPITO THE OIL & GAS ACADEMY

POL Petroleum Open Learning

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

OPITO THE OIL & GAS ACADEMY

Petroleum Open Learning

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

Contents

Page

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 The Application of Heat The Application of Electricity The Application of Chemicals Demulsifier Selection Demulsifier Bottle Test Equipment Test Procedure Main Test Injection of Chemicals Water Washing Settling

14

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 thinking 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 (cont’d) Section 3 – Dehydration Systems and Equipment Settling Tanks

32

Wash Tanks Free Water Knockout Heater Treaters Electrostatic Treaters Desalting

Section 4 – A Typical Dehydration System

Pages

Separation of Free Water Knockout Crude and Emulsion Heating Electrostatic Dehydrators Dilution Water System

Check Yourself – Answers

Visual Cues training targets for you to achieve by the end of the unit test yourself questions to see how much you understand

45

58

check yourself answers to let you see if you have been thinking 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 Targets When 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.

• List the basic properties of a demulsifier. • Explain how a bottle test, used in demulsifier selection, is carried out. •

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.

• Explain the layout and flow through a typical dehydration plant.

Tick the box when you have met each target

Oil Treatment (Dehydration)


Introduction

Water is produced together with oil from most oil fields. This water, which may make up a very large percentage of the total production from a field, can cause considerable problems. These problems include. • 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

(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 you have completed previous units in this series you will be aware of the primary separation facilities in a production processing plant. Let’s look briefly at the system to refresh your memory.

The total production from an oil field flows from the wells 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.

• 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 from the field by pipeline or tanker. Either way, water in the oil will cause problems. Water in the pipeline leaves less room for oil and results in loss of pipeline efficiency. Water being sent to a refinery can cause serious upsets in the distillation process. Tankers will not accept a cargo which contains more than a very small percentage of water 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.

Typical 3 phase separator


The vessel is called a 3 phase separator because it separates the total flow stream into the three separate streams of oil, water and gas. A 2 phase vessel would only separate the stream into the liquid and gas streams. I don’t 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 the inlet deflector. The gas has passed towards the gas outlet 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 oil separate due to a difference in their densities. Providing the oil and water stay in the vessel for a sufficient period of time the bulk of the water can be separated 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 after primary 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


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 to the bottom of the bottle and the oil would float on top. When two liquids are not capable of being mixed we say that they are immiscible.

The dispersed water droplets are known as the internal or discontinuous phase. The oil surrounding the droplets is the external or continuous phase. When I defined an emulsion I said that a third substance 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.

However, oil and water can be made to mix under certain circumstances. This occurs when one of the liquids is dispersed as fine droplets throughout the other and is stabilised.

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

•

water - which is the internal phase

Having said that, we could define an emulsion as follows:

•

oil- which is the continuous phase

•

an emulsifying agent

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. In oilfield emulsions the two immiscible liquids are oil 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.

In addition to the three components being present, they must be agitated for the emulsion to form. The individual components in themselves would never form an emulsion unless there was sufficient agitation 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, let’s look at the formation of an emulsion.


The Creation of an Emulsion The 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 find a condition which will give the least contact area between 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 also will reduce the contact area. But what has the 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.


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 droplet or the five smaller droplets. The formula for the surface area of a sphere is πd2

You will find the correct answer in Check Yourself 1.1 on Page 58.

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 to find a condition giving the least contact area. The tendency 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, this is where the emulsifying agent, or emulsifier, comes into the picture. This substance is essential to the creation of an emulsion. A well known example of an emulsion which you would find in the kitchen, is mayonnaise. The basic ingredients for making mayonnaise are vegetable oil and vinegar. If these two liquids are whisked together they tend to mix. But, as soon as the 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.


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. The emulsifier, which is present in the oil, migrates to the interface and concentrates there. Emulsifiers are fairly neutral as far as an affinity for oil or water is 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 moving around in the oil, they tend to carry the surrounded water droplets with them and keep the droplets floating in the oil.

So, in the oilfield we have all the conditions necessary 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 is agitation. Imagine the fluids flowing up the well tubings, through chokes, via flowlines and headers into processing equipment. That certainly agitates the fluids.

Emulsion Stability The 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 (difficult to break) or loose (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 emulsify it. If complete emulsification occurs however, 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 separating from the oil. By and large, water in oil emulsions with a high water content tend to form less stable emulsions.

•

O il 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 droplets which form during emulsification are relatively 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 emulsifier will dramatically affect the stability of the emulsion. However, an emulsifier which creates 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 oil are first mixed together the emulsifying agent is evenly distributed throughout the oil. It takes time for the ernulslfier to migrate to the interface between the oil and the water droplets. So initially the emulsion is relatively unstable. As time goes on and the migration proceeds, the film is formed around the water droplets. With increasing time the film becomes thicker and tougher making it more difficult for the droplets to combine. This results in a more stable emulsion.


•

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.

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

Test Yourself 1.2

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

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.

TRUE

FALSE

d. Emulsifiers are surface active agents which migrate to the interface between oil and water and form a barrier between the droplets and the surrounding oil.

You will find the correct answers in Check Yourself 1.2 on page 58 Figure 1.1

10


Emulsions and the Problem of Salt The 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. This salinity can vary from field to field but could be as much as 200 000 ppm. Refineries cannot accept oil which has a high salt content as the salt breaks down during the refining process and causes considerable problems. Severe corrosion, scaling and fouling of equipment and pipework are just some of the undesirable effects of salt in refinery operations. The saltwater is removed as far as possible before the crude oil gets as far as a refinery. Of course this unit is all about the breaking down of emulsions and the removal of the water. However, no matter how efficient the dehydration process is, there will usually 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 could range from 0.1 to 0.3% BS&W.

The amount of salt in oil is usually quoted at a refinery in units of pounds per thousand barrels ( PTB ). A limit of salt in crude of 50 PTB may be established by the refinery, and any salt content above this would be unacceptable. If we know the salinity of the residual water and the percentage BS&W we can determine the salt content of the crude in PTB. The graph illustrated in Figure 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

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.

11


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

Test Yourself 1.3 fter dehydration, 0.1 % water remains in a A certain crude oil, and the salinity of this water is 140 000 ppm NaCl. Would this crude be acceptable to a refinery whose upper limit for salt in crude is 50 PTB ? Would the refinery accept the crude if the water salinity is 100 000 ppm NaCI ?

You will find the correct answers in Check Yourself 1.3 on Page 58. You can see from the answer to the Test Yourself that even with an efficient dehydration system it may be necessary 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.

12


Summary of Section 1 In this section we have been looking at emulsions in general, what they are and how they are formed. I defined an emulsion as a mixture of two normally immiscible liquids 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 oilfield and how the agitation occurs when reservoir fluids are produced. 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.

13



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. Let’s 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= 2 g r 2 ( d 2- d 1) 9N Where :

V g r d1 d2 N

= = = = = =

velocity gravitational constant radius of particle density of continuous medium density of particle viscosity of continuous medium

Don’t 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 settling 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? Let’s examine each of these factors in turn. •

•

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

•

F inally N is the viscosity of the oil. To increase 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. Before we move on from here try the activity on the next page.

14


Activity 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 difficult to stir - it is thick or very viscous. What could 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 treacle gets hotter it will start to flow more 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 let’s consider this now.

The Application of Heat In 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 8O°C. After that the effect of heat on density difference diminishes.

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.

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, the film surrounding the droplets can be ruptured and they will coalesce. Secondly, as the water droplets are heated they will expand. This will stretch the surrounding films and make them weaker enabling them to be ruptured more readily.

15


The Application of Electricity The application of electricity in emulsion treating is an attempt to promote coalescence of the water droplets. Let’s have a look at how this works. Before we 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.

•

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

•

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

•

N ow 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).

•

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

16


What you should have noticed during the activity you have just performed is the following; the first time that 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.

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

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. But how does that attract the water? The answer to this 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

Figure 2.3 : Water Modules Attracted to the Positively Charged Pen Figure 2.2 : Electric Dipoles in Water Molecules Randomly Arranged

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

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.

If the emulsion is passed through an electric field between 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 electrodes and tend to speed towards it. Because of the weakened film and the greater collision force as they hurtle through the oil, the droplets unite more readily to form the larger droplets necessary for faster settling.

17


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 2.1 Are 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)

D ecreasing 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.

TRUE

FALSE

REASON

e) If an emulsion is passed through an electric field between two electrodes the water droplets are polarised. This causes them to be stretched due to polar attractions which weakens the surrounding film.

You will find the correct answers in Check Yourself 2.1 on Page 59.

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The Application of Chemicals The addition of a chemical to the emulsion helps to cause coalescence of the water droplets. It does this by breaking the film surrounding the water droplets. Before it can do this it has to get to the interface between 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. We could say therefore, that good demulsifiers have four basic properties: •

hey are strongly attracted to the water / oil T interface

• They cause flocculation of the water droplets • They help to rupture the film surrounding the droplets, promoting coalescence •

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

Demulsifiers are in fact very similar to the emulsifying agents which cause the emulsion to form in the first place. 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 demulsifier gathers together droplets of water by the action of flocculation. The demulsifier, which is now concentrated 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 they collide with sufficient force the skin may be ruptured and coalescence takes place. Sometimes however they just nestle together and further action is needed for coalescence. The next action of the demulsifier is to attack the films surrounding the droplets if they are still intact. It does this by causing eruptions at the interface which consequently ruptures the film. With no film to prevent coalescence, the water tries to find a condition giving the least contact area with the oil. The water droplets, which are close together because of the flocculation, unite and form larger and larger droplets. The action of flocculation and coalescence is illustrated in Figure 2.4.

Figure 2.4

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You saw in Section One that emulsifiers can contain solid 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 be achieved. The demulsifier does this by wetting 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.

Figure 2.5 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.

You can deduce from all this that the demulsifier has several jobs to do. It would be almost impossible to find a single chemical which could accomplish all these actions. Therefore, Demulifiers are cocktails of chemicals which are blended to give the best possible results for the type of emulsion being treated.

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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 effective demulsifier for each type of emulsion must be selected. What is effective in one field may not work in another. In fact, the addition of the wrong type of demulsifier could aggravate the situation and cause the emulsion to become more stable. The selection of the demulsifier depends on a number 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 which may react with the demulsifier The above list is not exhaustive but it serves to show just how difficult the choice of demulsifier can be. Service companies who specialise in supplying oilfield chemicals produce a range of demulsifiers for the different situations encountered. Even so a considerable amount of work must be done in the field to ensure that the correct choice is made.

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 be narrowed down. A number of demulsifiers from a supplier’s range would be chosen and subjected to field tests. The most common type of test carried out is known as the bottle test.

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

Demulsifier Bottle Test

• 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

The bottle test is used to help to determine which chemical can most effectively treat an emulsion from a given field. The results of the test can also indicate the optimum amount of demulsifier to be added, 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 and a specified amount of demulsifiers added. The sample 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. Let’s expand this procedure and go through the basics of a bottle test.

• the sample must be free of any demulsifier chemical

Equipment The 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

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•

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. (Sometimes in the field the bottles are shaken by hand.)

In addition to the equipment I have just listed, demulsifier chemical and solvents are required. The demulsifier is usually used in testing in a diluted form called a solution. A typical 5% solution would be prepared by mixing 1 ml of concentrated demulsifier with 19 ml of demulsifying solvent.

Test Procedure

• Record the total water content. This gives a figure which can be used to compare the demulsifying chemicals under test

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

Main Test

Before conducting the main test, the total amount of water and emulsion in the oil must be determined. This is done in the following manner:

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

•

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 60°C

•

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

•

• Fill the calibrated bottles with 100ml of sample • Label the bottles with details of type of demulsifier and amounts used • Heat the bottles in the water bath to the same temperature as that of the demulsifier injection point in the field • 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 the intensity of agitation in the field

22


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

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.

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

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

• 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 demulsifier works best for a particular emulsion but also, what is the optimum dosage.

If we use a demulsifier which is known to be reasonably effective, then the test which I have just described is carried out using different amounts of the same demulsifier instead of different chemicals. 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 for further 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 demulsifier into the process stream. However, before doing so, try the following Test Yourself question.

23


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.

1

c. Agitate the bottles for a period of time which relates to the intensity of agitation in the field. d. Label the bottles with details of type of demulsifier and amounts used. e. Heat the bottles in the water bath to the same temperature as that of the demulsifier injection point in the field. 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 temperature has been adjusted to that of the settling temperature in the field. You will find the correct answers in Cheak Yourself 2.2 on page 59.

24


Injection of Chemicals Where is the best place to introduce the demulsifier into 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.

Activity The following drawing Figure 2.6, is a simplified layout 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.

Figure 2.6

25


The drawing shows the flow of fluids from the reservoir through the 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 At the surface into each flowline

•

Into the main header

•

Into the separators

Let’s 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 means that the demulsifier has maximum time to do its work. The turbulence as the fluids flow up the wellbore through the surface valves 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 shows part of a simplified well completion drawing with a chemical injection valve in a side pocket mandrel. Figure 2.7

26


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 can see from the figure that a small line is fed to the side 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 the chemical to flow round the valves internal pathways and 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 fluids and is thoroughly mixed downhole. 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

27


A more practical solution then, may be to inject the chemical at the surface into each well’s flowline near the wellhead. There would be less time for the demulsifier to work and more time for stabilisation than 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 there will be less agitation and less time for the chemical to work. If chemical is injected at the surface into the flowlines or 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 will prevent back flow and protect the line from well fluids. Injecting the demulsifier into the separators is rarely considered. By the time the fluids reach this point there is very little time left for the chemical to work effectively. The fluid flow through the vessel is much less 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 depends on the type of operation. Sometimes a few wells would be treated downhole or at the flowline with additional injection at a single point in the main header. The character of each well’s production must be determined so that the wells which contribute most to the emulsion problem can be selected for downhole treatment.

Before we finish this section on the principles of emulsion treating we should consider two other points, i.e. settling and water washing. Let’s look at water washing first.

28


Water Washing

Settling

This 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 a water washing facility the emulsion flows under a baffle in the treating vessel thus ensuring that it passes through the wash water. Figure 2.10 illustrates this.

I 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 a sufficient length of time the water droplets would sink 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.

29


Test Yourself 2.3 Read through the following statements and fill in the missing word / words from the list given below : 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

c.

An electric dipole has a

and a

d. If an emulsion is passed through an electric field between two e.

When water droplets gather together we could say that

f. A demulsifier helps to remove solid particles from the emulsion by g.

A chemical injection valve could be situated in a side

h.

An injection

equation.

end. . the water droplets become occurs. the particles. mandrel in the tubing string.

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, BOTTLE TEST, FLOCCULATION, POCKET. You will find the correct answers in Check Yourself 2.3 on Page 59.

30


Summary of Section 2 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 : V=

2g r2 ( d2 - d1 ) 9N

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.

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 of weakening the surrounding film and causing them to be attracted towards the electrodes. Because of the weakened film and the collisions which occur as the droplets 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

• cause flocculation of the water droplets • help to rupture the film surrounding the water 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 demulsifiers available and careful selection of the best one must be made for a particular emulsion treating application. I described for you the bottle test which is used to help determine which demulsifier can most effectively treat a given emulsion. 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 be downhole, in the well flowlines,in the header or 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.

31


Section 3 – Dehydration Systems and Equipment Having looked at the basic principles of treating emulsions let’s now go on to the practical application of these principles in the field. The dehydration equipment 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.

At least three tanks would be used. In operation, one of the tanks would be in the process of being filled, and one would be settling. The third, having had the settled water drained off, would be having its clean oil pumped to a tanker or a refinery. Figure 3.1 shows a simplified version of such a system.


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

Settling Tanks All 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 to allow sufficient retention time for the water droplets to sink to the bottom. A typical example of such a system is a tank farm at a shore terminal. Here the total production from an offshore field is transported 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.

Figure 3.1

32


Wash Tanks

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.

A 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 tank with a few refinements. Although you may not come 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.

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 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 body of the tank. The gas-free emulsion then flows down 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.

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 :

Figure 3.2

•

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.

33


Free Water Knockouts Strictly speaking, free water is produced water which will settle out of the oil within five minutes if the fluids are at rest. As such it is not 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

34


This vessel is basically a horizontal 3 phase separator. The incoming fluids impinge on an inlet diverter where 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 outlet line. The oil and emulsion flows over a weir into the 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 at something a little more complicated. Before we do, 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 You will find the correct answer in Check Yourself 3.1 on Page 60.

35


Heater Treaters We 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, flows through the shell as shown. The heating medium flows through the tubes. This could be hot oil which 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 of the treater then we will follow the flow through the vessel.

Figure 3.4: Shell / Tube Heat Exchanger

36


Figure 3.5 Heater Treater Vessel

Figure 3.6 Mist Extractor

Flow, which is a mixture of oil, some gas, emulsion and free water enters the separation section of the treater, where initial separation takes place. Any gas in the fluids is flashed off at this point and flows to the gas outlet line. Before leaving the vessel the gas passes through a mist 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 3.6 shows a simplified version of a mist extractor.

37


The liquids fall down into the bottom of the separation section and are deflected round the heating element whilst 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 may be simply a tube coil through which heating fluid is being circulated. On some land locations a directly fired heating system may be used. The heated oil and emulsion then flows over a weir into the oil surge chamber. From here it flows through 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.

As the heated fluids rise the water droplets coalesce and 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 pipe and flows to the next part of the production process system. You saw in Section 2 that passing the emulsion through an electric field can help in the coalescence of water droplets. We can now see how this is done in practice.

The spreader ensures that the flow is distributed evenly throughout the length of the section. If this were not used the liquid flow could channel towards the outlet and reduce the efficiency of the treater.

38


Electrostatic Treaters Electrostatic treaters are very similar in construction to the heater treater we have just been looking at. The main difference is that they incorporate high voltage AC and / or DC electrostatic field in the coalescing 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

39


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. The initial flow through the vessel is the same as that described earlier. However when the heated emulsion rises through the coalescing section it has to pass through the electric field created by the electrodes. 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 weakens the film around the droplets so that as they collide 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 meet the requirements of the specific emulsion being treated.

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.

Desalting As I pointed out in Section 1, crude oil which is contaminated with salt is unacceptable to a refinery. 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 be sufficient to accomplish the desalting. In some cases 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 system commonly found at a terminal where a fired pre-heater is used. Look at Figure 3.8 on the next page, which shows this system as a simple block diagram.

40


Figure 3.8 : Desalting System

41


The salt - contaminated oil passes firstly through the 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

• firebox with burner • flow tubes The heat is generated by burning fuel gas or oil in a burner. The hot flue gases flow through fire tubes and are exhausted through the stack. This flow of hot gases heats up a body of water contained in the shell of the heater. The water in turn heats up the oil which is flowing through the flow tube bundle. Figure 3.9 : Pre-heater

42


After being heated the oil flows towards the dehydrator. In this case it is an electrostatic dehydrator which operates in the manner described earlier in this section. Before it gets to the dehydrator, fresh water is mixed with the oil. The fresh water is pumped from a storage tank to a spray injector in the flowline. The salt in the oil is thus diluted by this fresh water which mixes with the very salty emulsion water. The dilution water plus the emulsion water is finally removed 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

Heater Treater

Electroststic Treater

Mist Extractor Spreader Equalising Line Weir Electrodes Conductor Pipe Heating Element Transformer You will find the correct answers in Check Yourself 3.2 on Page 61.

43


Summary of Section 3 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. 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

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

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

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, create an electric field. The water droplets passing through this field are polarised which causes them to speed towards the electrodes, colliding as they do so. The polarisation also weakens the film surrounding the droplets so that when they collide they coalesce more readily and sink to the water layer.

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

44


Section 4 – A Typical Dehydration System In this final section we are going to look at a complete dehydration 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.


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 used and the flow paths through the system. We will now follow the flow through the system in more detail. Let’s consider the process, section by section.

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

45


Figure 4.1 : Typical Dehydration System

46


Separation and Free Water Knockout Look at Figure 4.2 which shows this part of the system. It is a straightforward separation process. The reservoir fluids which are a mixture of oil, gas, free water and emulsion, flow to the first vessel in the system, the first stage separator. This is a 3 phase separator which in our system is operating at a pressure of 10 barg. The first chemical injection point is into the line entering the first stage separator. Demulsifier is injected here to give it as much time as possible to take effect before the emulsion reaches the electrostatic dehydrators.

Figure 4.2 : Typical Separation and Free Water Knockout System

47


In the first stage separator, the free water is separated and forms a water layer at the bottom with oil floating on top. The level of the interface between 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 water flows to the FWKO drum where it is used as wash water. The oil plus emulsion flows to the second 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).

The inlet flow into the FWKO drum consists of the liquids from the second stage separator plus the water which has been removed in the first stage separator. 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 first stage vessel plus any further free water which has been washed out of the emulsion. This water is routed to a produced water clean up facility prior to disposal.

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

Being a 2 phase vessel this separator has no oil water interface control. All the liquids leave the vessel via the oil outlet and flow to the FWKO drum. The liquid level in the separator is maintained by the level controller (LC 03) operating LCV 03 in the liquid outlet line.

You will find the correct answer in Check Yourself 4.1 on page 62

48


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 which includes the heaters. Before we do so however, read through the last few paragraphs and make sure that you understand the flow through the separation section.

Crude and Emulsion Heating You will remember that heating an emulsion helps to enhance the dehydration process. In this section of the plant the liquids are heated in two stages, first in a shell and tube heater then in a water bath heater. Figure 4.3 shows this small section.

Picking up the flow from the FWKO drum you will see that there is further provision for the injection of demulsifier into the flowline upstream of the crude pump. This ensures good mixing as the crude and emulsion flow through the pump. Downstream of the pump 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

49


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. From the pre-heater the crude flows through the water bath heater. Gas from the plant is used as fuel to fire the heater. 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 if there should be a flame failure at the burner. 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 I have listed the items of equipment and injection points in the initial flow path of our typical dehydration system. These items are in the wrong order. Place them in the correct order, starting with inlet. a)

Inlet

b)

1st stage separator

c)

Injection of water from 1st stage separator

1

d) Second demulsifier injection point 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


50


Electrostatic Dehydrators The crude enters this part of the process from the water bath heater. Identify this point on the drawing of this part of the system in Figure 4.4. The first thing you will see as you trace the flow is an injection point for water. This water is the reject water from the second stage dehydrator. It is injected at this point to help reduce the salinity of the incoming water in the emulsion 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 nozzles which ensure that it enters the main flow as fine droplets. These droplets must then combine with the water in the emulsion which requires some form of agitation. A mixing valve takes care of this. The valve is a differential pressure control valve (DPCV 01). Its controller (DPC 01) maintains a pressure droplet across the valve and this, together with the plug and seat profile of the valve itself, provides the necessary surface and energy for the agitation to take place. Figure 4.4 : Electrostatic Dehydrators

51


After mixing, the flow of liquids enters the first stage electrostatic 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 prior to the treater as we have just seen. The flow at this point becomes more complex so we will divide it up and follow the flows of oil / emulsion and water separately. Let’s start with the oil / emulsion. These liquids enter the dehydrator and follow a similar flow path to the one described in Section 3. The instrumentation on the vessel can be quite complex but we can look at some of the more important instruments. To minimise the complexity of Figure 4.4 we can look at this in isolation in Figure 4.5.

Figure 4.5 : Electrostatic Dehydrator

52


Look first to the right hand side of the vessel. There you 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) and will warn the operator if the level is reaching potentially serious points. Separate level switches (LSHH &LSLL) are tied into the shut down system of the plant. If the interface level should reach the set points of these instruments a shut down will automatically be activated. Because it would be dangerous if the oil level dropped 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 continue to trace the flow. The oil / emulsion from the first stage dehydrator passes to the second stage vessel. Before entering this vessel more water is injected 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 same manner as the first. It is also protected by the same type of instrumentation. The hot treated crude from this dehydrator, prior to being routed to storage, flows through the preheater where it acts as the heating medium to raise the temperature of the crude before it enters the main water bath heater.

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 first stage dehydrator where it helps to dilute the incoming 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 dilution water system. Before we go on to this, try the following Test Yourself question.

Tracing the water flows through Figure 4.4 we begin with the reject water from the first stage dehydrator. This water flows firstly through the dilution water heater 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.

53


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

c)

Dilution water is passed through a

valve. 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.


54


Dilution Water System Look at the final drawing in Section 4 Figure 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 well pumps using level switches (LSH 02 & LSL 02).

Figure 4.6 : Dilution Water System

55


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.

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

The ratio of dilution water to crude / emulsion throughput is carefully controlled. A typical figure could be 1 : 20. The actual amount of dilution water added is controlled by a flow control valve (FCV 01). This is regulated by a flow controller (FC 01) taking its signal from a flow transmitter. Before being injected into the feed to the second stage 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. Before I summarise Section 4, try the last Test Yourself question.

You will find the correct answer in Check Yourself 4.4 on Page 64.

56


Summary of Section 4 In this section I have taken you through a typical dehydration and desalting plant. It is an entirely hypothetical plant which is not intended to represent any existing installation. It has simply been used to illustrate the principles which we discussed in the preceding sections. In this plant we saw that the initial separation of water was accomplished using a three stage separation process. The first stage removed free water 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 produced water clean up facility prior to disposal. The crude oil and remaining emulsion was then heated in a pre-heater and a water bath heater before entering the first stage of a two stage dehydration and desalting process. These vessels were electrostatic units. Prior to the first stage the reject water from the second stage was added to the feed. This helped to dilute the salt content of the produced water. The reject water from the first stage was combined with the water from the FWKO drum and sent to disposal via the produced water clean up facility. Before entering the second stage dehydrator, dilution water was added to the feed. This water can be obtained from water wells and heated by the reject stream from the first stage in a heat exchanger located upstream of the injection point. In the second stage dehydrator, the crude stream was finally treated to achieve the correct specification. The treated crude was then used as a heating medium in the pre-heater prior to being sent to storage facilities from where it would be transported to the purchaser.

57

Check Yourself – Answers

Check Yourself 1.1

Check Yourself 1.2

Using the formula

a. False – Mayonnaise once formed is very stable and is difficult to break down.

surface area =

d2

the S.A. of a single droplet is 152 = 707 mm2 the S.A. of each small droplet is 8.77 = 241.6 mm 2

b.

False – An emulsifying agent is also required.

c.

True.

d.

True.


Check Yourself 1.3 If the salinity of water is 140 000 ppm and 0.1% water remains in the oil, from the graph (Figure 1.2) the equivalent salt would be 57 PTB. Therefore it would not be acceptable. If the water salinity was reduced to 100 000 ppm the equivalent salt would be 39 PTB. This would fall in the acceptable range.

2

total of 5 small droplets is 5 x 241.6 = 1208 mm2 single droplet has smaller surface area.

58


Check Yourself 2.1

Check Yourself 2.3

a)

True

a.

Heating oil tends to reduce its VISCOSITY.

b)

False, we need to increase the density difference

b.

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

c)

True

c.

An electric dipole has a POSITIVE and a NEGATIVE end.

d)

False, they are arranged randomly

e)

True

d. If an emulsion is passed through an electric field between two ELECTRODES the water droplets become POLARISED.

Check Yourself 2.2 The correct order is :

e.

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

f. A demulsifier helps to remove solid particles from the emulsion by WETTING the particles. g.

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

h.

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

a, d, e, b, i, c, j, h, f, g

59


Check Yourself 3.1 Your answer should look like the following: Terms

Wash Tank

Inlet Diverter

Free Water Knockout Drum

4

Spreader

4

Water Layer

4

Weir

4 4

Injection Quill Water Level Control Valve

Niether

4

Gas Equaliser

Conductor Pipe

Both

4 4

60


Check Yourself 3.2 Your answer should look like the following: Equipment

Wash Tank

Mist Extractor Spreader

4

Equalising Line

4

Weir

Heater Treater

Electrostatic Treater

4

4

4

4

4

4

Electrodes Conductor Pipe Heating Element Transformer

4 4 4

4 4

61


Check Yourself 4.1

Check Yourself 4.2

Your sketch should look similar to Figure 3.3, which is reproduced below.

a, b, g, c, e, d, f, i, j, h, k

62


Check Yourself 4.3 a)

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

the 2nd STAGE DEHYDRATOR.

b)

The injection water and water in the emulsion require agitation, This is taken care of by a mixing

valve which is a DIFFERENTIAL PRESSURE CONTROL valve.

c)

Dilution water is passed through a HEATER before joining the oil entering the 2nd stage

dehydrator.

d)

It would be dangerous if the oil level dropped and uncovered the ELECTRODES

in the dehydrator.

e)

The dilution water heater uses WATER from the 1st STAGE DEHYDRATOR as its heating

medium.

63


Check Yourself 4.4 Your answer should look like the following.

64

Oil Treatment (Dehydration).pdf

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