Natural Gas Liquids Recovery.pdf

POL Petroleum Open Learning

Natural Gas Liquids Recovery Part of the Petroleum Processing Technology Series

OPITO THE OIL & GAS ACADEMY

POL Petroleum Open Learning

Natural Gas Liquids Recovery Part of the Petroleum Processing Technology Series

OPITO THE OIL & GAS ACADEMY

Petroleum Open Learn-

Designed, Produced and Published by OPITO Ltd., Petroleum Open Learning, Minerva House, Bruntland Road, Portlethen, Aberdeen AB12 4QL

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© OPITO 1993 (rev.2002)

ISBN 1 872041 85 X

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Petroleum Petroleum Open Open Learning Learn-

Natural Gas Liquids Recovery (Part of the Petroleum Processing Technology Series)

Contents

Page

BOOK1 *

Training Targets

4

*

Introduction

5

*

Section 1

7

Chemistry Fundamentals Hydrocarbons and Chemical Bonding The Physical Properties of Hydrocarbons Boiling Point of Alkanes Dew Point Curve Vapour Pressure Mulit-Component Mixtures Absorption

- The Theory of NGL Recovery

*

Section 2

- NGL Recovery Using Compression & Cooling

A Typical Compression and Cooling System

23

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

1


2


Contents (cont'd) *

Section 3 - NGL Recovery by Refrigeration

*

*

32

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

Mechanical Refrigeration A Typical Mechanical Refrigeration NGL Recovery System Auto Refrigeration

Section 4

Page

- NGL Recovery Using Absorption Process

47

A Typical Absorption System Gas Flow Absorption Oil Flow

Check Yourself - Answers

57

3


Training Targets When you have completed this unit on Natural Gas Liquids Recovery, you will be able to :

• State the reasons for recovering natural gas liquids • Explain in simple terms the behaviour of hydrocarbons with respect to changes in energy level • Define the terms bubble point, dew point and vapour pressure • Describe the construction and operation of equipment used in a typical compression and cooling NGL recovery plant • Explain the operation of a refrigeration process • Differentiate between mechanical refrigeration and auto refrigeration • Explain what is meant by the Joules Thompson effect • Describe two types of NGL recovery plant which use refrigeration systems • List the equipment used in an NGL recovery plant using the absorption process • Outline the flow through an absorption system used to recover NGLs Tick the box when you have met each target.

4

Oil and Gas Separation Natural Liquids Recovery Systems


Introduction Natural Gas is produced from petroleum reservoirs as associated gas ( associated with an oil accumulation) or non-associated gas ( produced independently of oil). This natural gas is a complex mixture of hydrocarbons which, in the main, belong to a family of hydrocarbons called the alkane or paraffin series. We will talk about this hydrocarbon family in more detail in Section 1. The first ten members of this alkane series are listed below. • Methane • Ethane • Propane • Butane • Pentane • Hexane • Heptane • Octane • Nonane • Decane At normal temperatures and pressures the first four members of the series exist as gases. The other components are liquid at these conditions.

A natural gas stream is capable of being partially or totally liquefied, using various types of processing equipment. We will look at why we might want to do this in a moment. Total liquefaction of natural gas is only possible using processing techniques involving extremely low temperatures. These are called cryogenic techniques. The processes are normally carried out in large, shore-based installations, similar to oil refineries. The liquid produced is referred to as liquefied natural gas ( LNG) Partial liquefaction can be achieved with less complicated processes than those required for total liquefaction. These processes are often incorporated into the facilities on an oil and gas production platform. In these systems, the components of the natural gas stream are liquefied with the exception of the methane and most of the ethane. The resulting liquid is called natural gas liquid ( NGL ) Whilst I am quoting abbreviations regarding gas liquids, it is worth while mentioning another one which you will come across. This is the expression LPG which means liquefied petroleum gas. LPG, which consists of liquefied propane and/or butane, is extensively used for domestic heating and cooking.

Finally you should be aware of the expression condensate. This is a hydrocarbon mixture which exists as a gas in the reservoir but condenses to a liquid as the pressure and temperature is reduced when the gas is produced. In the Petroleum Processing Technology series of units, we are concerned with the processing of oil and gas in the oilfield or on the production platform. Therefore, in this unit, we will concentrate on the recovery of natural gas liquids or NGLs. But why bother recovering the liquids? There are a number of reasons for this, and I think we should look at them in some detail before starting the unit. • If the associated gas is being re-injected after separation, valuable components are being lost. There is a strong case for recovering as much as possible from the gas stream before re-injection, and the NGLs are the easiest fraction to recover. For example, from a total of 25 million cu ft of gas produced daily,5 000 bbl of liquid might be recovered with a potential value somewhere in the region of $100,000.

. 5


• Gas being marketed has to meet certain specifications regarding water content, calorific value (heating capability), composition etc. Processes designed to remove water vapour and recover the gas liquids, will ensure that the sales contract specifications are met. • Gas is usually transported via a pipeline to the sales point. If the heavier liquefiable components are not removed they could condense in the pipeline as free liquids. This would result in loss of pipeline efficiency. The liquid accumulation would necessitate frequent pipeline cleaning or pigging. So you can see that it makes sense in most cases to recover the gas liquids and in this unit we will look at how this is done. Liquefaction of gas, whether it be partial or total, always involves control of pressure and temperature. In addition, the composition of the gas and recovered liquid streams may be controlled to ensure the correct type and amounts of liquid are obtained.

Composition can be controlled by using an adsorption or absorption process. In an adsorption process the components are deposited on the surface of a solid material then regenerated off. In an absorption process the gas is contacted with liquid. The heavier components of the gas stream are absorbed into the liquid. These can then be recovered from the liquid and the absorbing liquid used again. The unit is divided into four sections: • Section 1 will concentrate on the theory of NGL recovery. • In Section 2 we will be looking at NGL recovery by compression and cooling. • Section 3 will cover NGL recovery by refrigeration. • Finally in Section 4 we will be looking at NGL recovery by absorption techniques.

Pressure is controlled directly. Temperature is controlled by adding or removing heat.

6

Natural Gas Liquids Recovery


Section 1 - The Theory of NGL Recovery In this section we are going to start by looking at the chemical and physical properties of hydrocarbons. This will enable us to see how the recovery processes work. We will do this in a rather simple way so don’t get concerned about the science involved. Let’s start by having a look at some basic fundamentals of chemistry.

Chemistry Fundamentals All substances are matter. This can be described as anything which occupies space and can be seen and felt etc. Chemistry is the science which investigates the composition and structure of substances and the changes which substances may undergo.

ELEMENT

CHEMICAL SYMBOL

Aluminium

AI

Carbon

C

Hydrogen

H

Table 1 : Elements with English names

ELEMENT LATIN NAME

CHEMICAL SYMBOL

Copper

Cuprum

Cu

Iron

Ferrum

Fe

Silver

Argentum

Ag

Pure substances can be elements or compounds. These are basic terms used extensively in chemistry and we must understand these before we carry on.

Table 2 : Elements with symbols from Latin names

Elements are the building blocks of all matter. They are substances which cannot be broken down into simpler substances. There are about 90 naturally occurring elements, including things like Hydrogen, Oxygen and Carbon.

Compounds are pure substances which consist of two or more elements which are chemically combined to form a new substance.

Instead of using the full name to describe an element, symbols are used. These are usually the first one or more letters of the full name, either English or foreign ( usually Latin). I have listed some common examples of each in the following tables.

The compound may not bear any resemblance to the elements from which it is made. For instance water is a compound which is normally a liquid. It is made from the chemical linking of the elements hydrogen and oxygen which are normally gases.

Another term with which you should be famiiar at this point is mixture. Mixtures are not pure substances in theselves. They are made up from two or more pure substances which are not chemically combined. These pure substances may be elements or compounds which retain their own properties when forming the mixture. A combination of iron filings and sand is a mixture. They could be separated from each other quite easily. A magnet would attract the iron but not the sand. Both the iron and sand are pure substance elements. As a combination they are a mixture but they retain their own properties. ( I.e. iron is magnetic, sand is not ). Each element is made up from even simpler building blocks, called atoms. These are the smallest particles that can be identified as an element. An atom is also the smallest particle of an element that can combine chemically with another atom. When atoms combine chemically, they form molecules. These are the smallest particles of a combination of atoms which can exist and still retain the characteristics of the combination. 7


Atoms of a single element can combine to form molecules of that element. For instance the gas oxygen is a pure substance. It is composed solely of atoms of the element oxygen combined into molecules. Water on the other hand is also a pure substance. It is a compound composed of atoms of oxygen and hydrogen combined to form water molecules. A word of warning at this point. I said earlier that chemical symbols are used to describe elements. This same symbol is also used to describe one atom of the element. Therefore the symbol C can mean the element carbon or one atom of carbon. This is important when we are looking at the way atoms combine to form molecules. We will be looking at the way this happens later. Before we move on however, have a go at the following Test Yourself Question on elements and compounds.

The following list of substances are either elements or compounds. Indicate which. nitrogen - (N)

...............................................................

sodium chloride - (NaCl)

...............................................................

water - (H2O)

...............................................................

helium - (He)

...............................................................

sulphuric acid - (H2SO4)

...............................................................

carbon - (C)

...............................................................

iron - (Fe)

...............................................................

You will find the correct answers in Check Yourself 1.1 on Page 57.

8


In this unit we are concerned with the recovery of natural gas liquids. But what is natural gas? In the introduction I defined it as a complex mixture of hydrocarbons.

Of course, each arm of the carbon atom need not necessarily grasp the arm of a hydrogen atom. Two atoms of carbon could ‘hold hands’ leaving the remaining six carbon atom arms grasping hydrogen arms. This could be represented as shown in Figure 1.3.

Hydrocarbons are compounds of the elements hydrogen and carbon. These elements have the chemical symbols, H for hydrogen and C for carbon. We can now look at the formation of these compounds.

Hydrocarbons and Chemical Bonding When atoms combine to form molecules they do so in a special way called chemical bonding. The easiest way to visualise this bonding is to imagine each atom to be a ball having a number of arms. For chemical bonding to occur, each arm of an atom must grasp the arm of another atom leaving no arm free. The actual number of arms on an atom will depend on the specific element.

Methane

Figure 1.1 : Methane Molecule The simple molecule shown in Figure 1.1 is the hydrocarbon molecule methane. A simple method of representing the structure of methane is shown in Figure 1.2 Figure 1.3 : Ethane Molecule

For instance, carbon has four arms whilst hydrogen has only one. So, for hydrogen and carbon atoms to combine into a simple hydrocarbon molecule, each of the four arms of the carbon atom must grasp the single arm of each of four hydrogen atoms. This is shown pictorially in Figure 1.1.

This is the hydrocarbon molecule ethane. Using small drawings to represent molecules is somewhat clumsy. So chemists have developed a system of shorthand, in which a chemical formula is used to describe the number and type of atoms which make up a molecule. Figure 1.2 : Methane Molecule 9


In the alkane series, which starts with metane, each subsequent member has one extra cabon atom and two extra hydrogen atoms.

For instance ethane, which has two carbon atoms and six hydrogen atoms, can be written:

C 2H 6 Note that when there is only one atom of a particular element in the molecule, the subscript “1” is left off. The chemical formula for methane therefore is

CH4 As I pointed out in the Introduction, methane and ethane are the first two members of a family of hydrocarbons called the alkanes. In such a family each of the members are related by a general formula. The general formula for the alkane family is:

Test Yourself 1.2 Using the general formula for alkanes, determine the chemical formula for octane which has eight carbon atoms and nonane which has nine.

CnH(2n+2) In the formula, ‘n’ represents the number of carbon atoms in the molecule. The greater the number of carbon atoms, the larger and, therefore heavier, the molecule.


So a hydrocarbon molecule of the alkane family which has 4 carbon atoms would have the chemical formula

C4H(2x4)+2 i.e. C4H10 This is the fourth member of the alkane family -Butane.

Scientists do not use the word family’ to describe a related group of compounds such as the alkanes. They call them an homologous series. This is from the Greek ‘homos’ meaning ‘same’, and ‘logos’ meaning ‘speech’. In other words related or similar.

In the Introduction, I listed the first ten members of the series: •

Methane

•

Ethane

•

Propane

•

Butane

•

Pentane

•

Hexane

•

Heptane

•

Octane

•

Nonane

•

Decane

You will also remember from the Introduction that, at normal temperatures and pressures, the first four members of the series exist as gases. The heavier components are liquid under these conditions. Although I have just said that pentane and the heavier components are liquids at normal temperatures and pressures, a natural gas stream may contain some of these components in very small quantities. This constituent of the gas stream is usually referred to as pentanes plus (pentanes + ) or C5+ 10


A typical gas stream leaving a separator in a production facility could have the following analysis.

Component Methane (C1) Ethane (C2) Propane (C3) Butane (C4) Pentanes + (C5+)

mol % 83.9% 11.6% 3.3% 1.0% 0.2%

These components are often referred to by abbreviating the chemical formula as indicated. i.e. Butane C 4H10 is referred to as C4. Mol % stands for molar percentage. It is just one way of expressing the proportion of each substance in a mixture of several substances. It is used extensively in chemical calculations but we don’t need to go any further into this at this point. In addition to the alkane series, other hydrocarbons can be produced from a petroleum reservoir. Components of the cycloparaffln series or the aromatic series can form a small proportion of the reservoir fluids. You can see then that the gas stream is a mixture of hydrocarbons in varying proportions. This is of course how I defined natural gas.

However, other chemicals may also be present in the natural gas stream. Compounds of oxygen, nitrogen and sulphur, together with water vapour, can occur in greater or lesser amounts. These substances can cause problems in the production and processing of natural gas. However for the present we will ignore these impurities and concentrate on the hydrocarbons of the alkane series. Each of the components of the hydrocarbon mixture has its own unique physical properties. I think it would be worthwhile having a look at these now.

The Physical Properties of Hydrocarbons I said earlier that all substances are referred to as matter. Matter is all around us, from the air that you breathe to the food that you eat and the liquids that you drink. Matter can exist in three different physical states. They are: •Solid - which has a definite shape and volume. •Liquid - which has a definite volume but no specific shape. •Gas - which has neither definite shape nor volume.

I’m quite sure that you are fully aware of the state of everyday matter. For instance at normal room temperature and pressure a steel bar is in the solid state, water is liquid and the air you are breathing is a gas. You will note, however, that I said at normal temperature and pressure. If the temperature and / or the pressure is changed, the state of the matter may change. We would actually be adding or removing energy from the matter by changing its temperature or pressure. It is the energy possessed by the matter which determines what state it is in. A very familiar example of changing the state of a substance is that of adding or removing energy from water. Water is a pure substance which is a compound of hydrogen and oxygen i.e. H2O. At atmospheric pressure and room temperature water is a liquid. If however we raise the temperature of the water by adding energy in the form of heat to the water, it will eventually boil and turn to steam - a gas or vapour. Similarly, if we were to remove heat energy, the water would eventually freeze and turn to ice - a solid. This process of changing the state of water is easily represented on a simple graph and I have included one on the next page. 11


As we continue to add heat you will notice that the temperature stops increasing and remains constant until point C is reached. The heat energy added between points B and C is called latent heat, and is used to convert solid ice to liquid water. When all the ice has turned to water, a further addition of heat energy causes the temperature to start rising again until point D is reached. Once again the heat added between C and D is sensible heat. At point D the first bubble of steam appears, as the water starts to boil. The liquid has started to change to a vapour or gas. Between point D and point E, the temperature again remains constant as further latent heat converts liquid water to steam. When all the water has turned to steam, a further addition of sensible heat causes the temperature to climb again towards point F.

Figure 1.4 : Changing State of Water

In Figure 1.4 the vertical axis of the graph is the temperature of the water and the horizontal axis is the heat input. Note that I have not indicated any units for either temperature or heat input. You should also note that the whole process takes place at a constant pressure, i.e. atmospheric pressure. Take a look at the graph and try to visualise what is happening in this representation. At the starting point A, the water is in its solid state of ice. As we add heat energy the temperature of the ice would start to rise and would continueto rise until point B is reached. The heatwhich is added during this time is called sensible heat. At point B the ice would start to melt.

Of course this process would work in reverse. If we started at point F where all the water is steam and began to remove heat energy we would follow the graph back to point A, passing through temperatures corresponding with the boiling point and freezing point of water. We have talked so far about the three states of matter. You will often find these states referred to as the three phases of matter: the solid, liquid and vapour (gas) phases. I will continue to use the terms vapour and gas to mean the same thing. Before leaving this exampleof changing the state, or phase, of matter have a go at Test Yourself 1.3. 12


Test Yourself 1.3 If the substance in Figure 1.4 is water at atmospheric pressure, what would be the temperature at points B, C, D and E.


So both pressure and temperature have an affect on the way a substance changes its phase. In other words its phase behaviour. This being so, it is useful to prepare diagrams to illustrate this effect. These diagrams are called phase diagrams. The simplest type of phase diagram is one which shows the behaviour of a pure substance. That is, a substance where all the molecules are identical, such as pure water or pure ethane. The diagram is in the form of a graph with the vertical axis representing pressure and the horizontal axis representing temperature.

Figure 1.5 : Pure Substance Phase Diagram

You will remember that, in the example shown in Figure 1.4, the phase changes were achieved by changing the amount of heat energy in the system, at constant pressure (that is, at a constant level of pressure energy). We can, however, affect phase behaviour by changing the level of pressure energy, as an alternative to changing the level of heat energy. For example, the boiling point of water is 100°C (212°F) at atmospheric pressure. However, if the pressure was decreased, for example by conducting the experiment on top of a mountain, the boiling temperature would also decrease.

You can see in the figure that there are three regions. A solid region, a liquid region and a gaseous region. The lines separating these regions are called lines of equilibrium or saturation. For example, along the line B D the substance could be all solid, all liquid or a mixture of both. Similarly, along line A B the substance could exist as a solid, a gas or a mixture of both. The point B is called the triple point. At this point all three phases could exist together. It is difficult to imagine ice, water and steam existing together, and ot course this condition is very seldom seen.

13


If changing conditions of temperature and pressure took a substance across line B D, the phase change would be from a solid to a liquid. Across line B C there would be a phase change from liquid to gas. However if changing condltions of temperature and pressure took a substance through line A B, the phase change would be from a solid directly into a vapour. No liquid would be formed. A common substance which does this is solid carbon dioxide, or dry ice. It sublimes from a solid to a fog like vapour and is often used on stage to give the mist effect you see at pop concerts.

Test Yourself 1.4 If the phase diagram in Figure 1.5 is for water, explain what would happen as conditions change along the line from Y to X.

Look at the figure again and try to imagine what is happening as conditions of temperature and pressure change. Then have a go at Test Yourself 1.4.

How many phase changes would there be?


14


Of course we are not really interested in the solid phase when we are discussing Natural Gas Liquids Recovery. So the most important line as far as we are concerned is the line B C, the line which separates the liquid and vapour regions. Along this line the substance would exist as a liquid, a vapour, or a mixture of the two. This line is known by a number of names. It is called the bubble point curve, the dew point curve or the vapour pressure line. We will have a look at the meaning of these terms shortly. Take a look at the following figure which shows the line B C from a phase diagram for a pure hydrocarbon of the alkane series.

Once again I have not included any actual values of temperature or pressure so we are not specifying which alkane it is. In the region to the left of the line B C the substance is all liquid, whilst to the right it is all gas. The point C is called the critical point of the substance. At this point the properties of the liquid phase and the vapour phase become identical. The actual temperature and pressure at this point for a particular substance are called the critical temperature and the critical pressure. For a pure substance the critical temperature is the temperature above which a vapour cannot be liquefied no matter how much pressure is applied. Similarly the critical pressure is the minimum pressure necessary to liquefy a vapour at its critical temperature. Don’t worry about these terms. I have included them for interest only and we will not be looking at them in any detail.

Fig 1.6 : Pure Hydrocarbon Phase Diagram

15


Boiling Point of Alkanes If we increased the temperature of the alkane along the line X Y the following would happen. When the temperature (T) corresponding to point Z is reached, the first bubble of gas would appear and the liquid will start to boil. You can probably see now why the line BC is called the bubble point curve. The actual temperature at this point is the boiling point of this alkane at the pressure (P) which also corresponds to point Z. Note that I said the boiling point at a specific pressure. If we crossed the line B C at a different position the liquid would start to boil at a different pressure and temperature. The boiling point is one of the physical properties of hydrocarbons. But to compare one with another we must use the same pressure. The commonly used standard is atmospheric pressure. So, the boiling point of our alkane Is the temperature at which the liquid bolls at atmospheric pressure. These boiling points vary quite considerably and I have listed the first 8 members of the alkane series in the following table.

ALKANE BOILING POINT AT ATMOSPHERIC PRESSURE Methane -162°C -259°F Ethane

-89°C

-128°F

Propane

-42°C

-43.7°F

Butane

-0.5°C

Pentane

36°C

Hexane

69°C

Heptane

99°C

209°F

Octane

126°C

258°F

31.1°F 97°F

156°F

Table 3: Boiling Points of Alkanes You will notice from the table that the boiling point increases as the alkanes. get heavier. In other words the more carbon atoms, the higher the boiling point.

Vapour Pressure Closely related to boiling point is the property of vapour pressure. It is the pressure which would be exerted by a liquid in a closed container if there is a vapour space above the liquid level. That may seem rather complicated, so let me try to explain it. When a liquid is contained in a closed vessel, some of the liquid molecules will leave the liquid and enter the vapour space. This is evaporation. These molecules strike the sides of the container causing the pressure in the container to rise. Some of the molecules in the vapour space will strike the surface of the liquid and re-enter the liquid. This is the opposite of evaporation, i.e. condensation. As long as more molecules are leaving the liquid than entering it the pressure in the vessel will continue to rise. Figure 1.7 shows this situation in a simple way.

Dew Point Curve Now go back to Figure 1.6. If we were to start at point Y and reduce the temperature the opposite would happen. When we reached the bubble point curve the gas would start to condense. At this point the first dew drop of liquid would appear. You can now see why this line B C is also called the dew point curve.

Figure 1.7 : Vapour pressure Rising 16


At a certain temperature however, a state of equilibrium is reached between liquid and vapour. This occurs when exactly the same number of molecules are leaving the liquid as are reentering it. At this temperature the pressure in the vessel would remain constant. This pressure is the vapour pressure of the liquid at that temperature. This is shown again pictorially in Figure 1.8.

Figure 1.8 : Phase Equillibrium

If the liquid in the vessel is propane at a temperature of 38°C (100°F), the pressure would be 12 bar (175 psi). If, however, heat was added to the vessel, the temperature would increase. More molecules wouId start to leave the liquid and the equilibrium would be lost. The pressure in the vessel would rise once more. The pressure would increase until equilibrium was again established at a new temperature. Figure 1.9 shows this happening. So the vapour pressure is dependent on temperature. In order then that the vapour pressures of different hydrocarbons may be compared, they must be quoted at a standard temperature. This temperature is 38°C or (100°F).

Figure 1.9 : Vapour Pressure Rise - Heat Induced

17


ALKANE

VAPOUR PRESSURE AT 38°C 100°F

Methane

344.8 bar

5 000 psi

Ethane

5.4 bar

780 psi

Propane

12 bar

175 psi

Butane

2.5 bar

36.9 psi

Pentane

0.06 bar

0.9 psi

Table 4 : Vapour Pressure of Alkanes You can probably see now why the line B C in Figures 1.5 and 1.6 can also be called the vapour pressure line.

So by reducing the temperature, increasing the pressure or a combination of the two, any of our hydrocarbons can be liquefied. Some are more difficull than others, of course. It is relatively simple to liquefy butane at atmospheric pressure. We just have to cool it below -0.5°C (31.1°F). However liquefying methane is much more difficult. At atmospheric pressure, the temperature here would have to be reduced below minus -162°C (-259°F), a very low temperature indeed.

Multi-Component Mixtures The phase behaviour we have been looking at so far has been for a pure substance (or a single component). The trouble with the gas leaving a separator is that it is a mixture of hydrocarbons. In other words a multi-component mixture. Each of these components has a different boiling point. We can no longer represent the behaviour of this mixture with a single vapour pressure line. So, the phase diagram now becomes a phase envelope. The dew point curve and bubble point curve are now different lines.

pressure

In the following table I have listed the vapour pressures of the first five alkanes.

Look at Figure 1.10. It is a phase diagram for a multi-component mixture of hydrocarbons with a fixed composition. In the diagram the line A C is the bubble point curve and line B C is the dew point curve. At conditions to the left of the bubble point curve we would have a mixture which is all liquid. To the right of the dew point curve we would have all gas. Within the envelope we would have a mixture of gas and liquid. The dotted lines within the envelope are lines of quality. Each one represents a constant percentage of liquid. At point X, we have a mixture composed of 50% liquid and 50% gas. Let’s see what happens if we remove heat at constant pressure from a mixture whose phase diagram is represented by Figure 1.10. Follow this along the line Z V.

temperature Figure 1.10 : Hydrocarbon Phase Envelope

If the hydrocarbon mixture is all gas at point Z and we begin to take away heat energy, the following will happen: The mixture would cool until point Y is reached. At this point the first drop of liquid would appear from the component with the highest boiling point. This is the dew point for this mixture at the corresponding pressure. 18


When more heat is removed, more and more liquid is formed as the temperature is reduced to the boiling points of lighter hydrocarbons. When the temperature corresponding to point W is reached the mixture would be all liquid. Point W is of course the bubble point of the mixture. Although the behaviour of a hydrocarbon mixture is more complicated than that of a single component, what I said earlier still applies. We can liquefy some or all of the components of a natural gas stream by reducing the temperature, raising the pressure or a combination of both. The actual amounts of liquid obtained will depend on the composition of the stream and the degree to which we can cool or compress the gas. All of this is a rather roundabout way of showing how we can liquefy natural gas. •If we can create conditions of temperature and pressure so that we cross the vapour pressure line from the gas to the liquid state, the problem is solved

Test Yourself 1.5 Assume that the substance indicated in Figure 1.10 is a mixture of Heptane hexane and pentane. Determine the temperatures at points W and Y if there was a temperature rise from V to Z at atmospheric pressure.

NOTE: The actual shape of the graph will vary depending on the composition of the hydrocarbon mixture. I am using this general shape for the purpose of the Test Yourself.

Compression and cooling, or refrigeration, are by far the most common methods used in the recovery of gas liquids. There are other methods however, and in the introduction I mentioned adsorption and absorption. Of these the adsorption process is more commonly used in gas dehydration operations. A separate unit in the petroleum processing technology series covers this. Its use in natural gas liquids recovery is not very widespread so we will disregard it in this unit. However the absorption process is often used so we must consider the theory behind this before looking at some of the processes in more detail.

Absorption Absorption is a process which involves contacting the gas with a liquid called lean oil or absorption oil. When this is done some of the components of the gas will dissolve in the oil. The heavier components, the gas liquid components, will dissolve more readily but some of the lighter gas components will also be absorbed. It is quite simple to see that gas can in fact dissolve in a liquid.

Before I move on to another method of recovering some of the lighter hydrocarbons from a gas stream, have a go at the following Test Yourself question.

If you take a sealed bottle of soda water and stand it on a table, you will see a bottle of clear liquid with no apparent activity within it.

You will find the correct answers in Check Yourself 1.5 on Page 58. 19


If you now gently remove the cap of the bottle what happens? You should see the contents start to fizz and gas escape from the surface of the liquid. The gas must have been dissolved within the liquid. In fact the gas is carbon dioxide which is dissolved in plain water. The process of removing the cap reduced the pressure in the bottle causing the gas to be liberated from the liquid. In order for the water to absorb the gas, they would need to be in contact with each other under pressure. You have probably seen this in practice. The equipment which is sold for making fizzy drinks at home uses this principle.

The process is made more efficient at lower temperatures. (In general, gases are more soluble in liquids at lower temperatures). Also, if the contact area between gas and liquid can be increased the process will again be more efficient. On an offshore oil production platform the recovered gas liquid components may be sent ashore still dissolved in the absorbing oil. This oil is injected into the main crude oil transportation pipeline. Of course, when it arrives at the reception terminal, further processing is necessary to recover the natural gas liquids.

The gas ( carbon dioxide) is supplied in small pressurised cartridges. When the cartridge is mated with a bottle of water and the cartridge punctured the carbon dioxide is forced into the water under pressure.

An altemative to this is to remove all the NGLs as gases from the absorbing oil at the processing location. These will then have to be liquefied again for onward transportation or storage.

You can see then that this process can be used to recover the gas liquids components from a gas stream by dissolving them into a liquid under pressure.

The absorption process then is only part of the story. In order to recover natural gas liquids by this method a two stage process is involved.

In addition to pressure two other factors influence the efficiency of the process. These are :

First the NGL components of the total gas stream are absorbed into the absorption oil. Then the components are removed from the oil as gas. The simple block diagram Figure 1.11 shows the principle of this.

• temperature

• contact area between gas and liquid

Figure 1.11 : Absorption Process Block Diagram

20


The terminology associated with such a process I have listed below : • Rich gas

Gas containing absorbable ( NGL ) components.

• Dry gas

Gas having given up its absorbable components.

• Rich oil

Oil having absorbed the NGL gases.

• Lean oil

Oil having had the absorbed NGL gases removed.

b)

• Absorber

Process vessel where absorption takes place.

• Stripper

Process vessel where removal of NGL gases from lean oil takes place.

Of course the actual process is somewhat more complex than the simple example I have just given you. In Section 4 we will look at one such process in more detail.

Before moving on to the next section though, have a go at the final Test Yourself in this section.

Test Yourself 1.6 a)

c)

Classify the following substances as either solid, liquid or gases at room temperature.

steel - water - acetylene - copper - oxygen. Match the following symbols with the list of element names.

CI - a - Sl - He - Fe - C.

Carbon - Hydrogen - Iron - Oxygen - Chlorine - Sodium - Silicon - Helium. Using Figure 1.10 match the letters in the multi-component phase diagram with the descriptions below. 1. The 50% liquid line. 2. The gas region. 3. The bubble point curve. 4. The dew point curve. 5. The liquid region.


21


Summary of Section 1 In this section we have been looking at some of the basic theory behind natural gas liquids recovery. Having defined natural gas in the introduction, we went on to consider the simple chemistry of hydrocarbons. You saw how the elements hydrogen and carbon can bond together to form the hydrocarbon compounds of natural gas. You also saw that these compounds form the hydrocarbon series called the paraffin or alkane series. Although members of other hydrocarbon series, together with impurities, can be present in natural gas, in this unit we concentrated on the alkanes. Having covered the very basic chemistry of hydrocarbons we went on to look at the physical properties of these compounds.

Using the example of water, you saw that adding or removing energy from a substance can change the state of the substance. Ice can be turned to water and then to steam and vice versa. Heat energy and pressure energy are responsible for these changes of state.

We defined boiling point and vapour presure. We then showed how this idea of phase behaviour could be applied to a multi-component mixture. We used a modified phase diagram to illustrate this, which we called a phase envelope.

You saw that the three states of matter, solid, liquid and gas are referred to as phases. The effects that temperature and pressure have on a substance is called its phase behaviour.

All this helped to show that, by decreasing the temperature and increasing the pressure of natural gases, certain of the components can be liquefied. The actual amounts of liquid recovered from a gas of a certain composition will depend on the degree to which these energy levels are altered.

We prepared diagrams to illustrate this effect called phase diagrams. Firstly we concentrated on pure substances or single components. The region which we were most interested in was the change in state from liquid to gas and from gas to liquid. You saw that the line on the phase diagram which separates these regions can be called the bubble point curve, the dew point curve or the vapour pressure line.

At the end of the section we looked at an alternative method of recovering natural gas liquids by absorption. This is a process which involves contacting the gas with an oil under pressure. The NGL components of the gas are absorbed by the oil and then recovered by stripping. In the next section we will look at the simplest of the recovery processes, that of compression and cooling.

22



Section 2 - NGL Recovery Using Compression and Cooling As we showed in Section 1, NGL recovery by compression and cooling is the simplest and cheapest method available. In fact, natural gas liquids are often produced as a by-product during petroleum gas compression operations. Imagine an oil production plant where a considerable amount of associated gas is also produced. This gas is then available for sale. Let’s say that the separation facilities on this plant involve 2 stages of separation operating at pressures of 25.9 bar (375 psi) and 5.2 bar (75 psi). Gas from the separators will require to be dehydrated and then routed through gas export compressors into the pipeline for transportation to the point of sale. If we say that a gas export pressure of 138 bar (2 000 psi) is required, then it is obvious that the gas from each stage of separation will have to be compressed in order to achieve this. You will remember from Section 1 that compressing the gas is likely to promote a phase change of some components. So, the very fact that we have to compress the gas from the separators will cause NGLs to be produced.

Cooling the gas also promotes a phase change of some of the gas components. More liquids are produced! So, in a gas compression facility we are going to get some liquids produced whether we want them or not. But these liquids are valuable, so in some instances, it is worth recovering them for sale. Unfortunately, this type of process is relatively inefficient in recovering the maximum amount of NGL. Where large quantities of gas or NGL are involved it is more common to increase the recovery of gas liquids by reducing the temperature much further than is necessary simply to protect the compression equipment. This is done by refrigerating the gas rather than just cooling it. We will look at some plants which do this in the next section. However, in this section let’s look at a typical Compression and Cooling System which is used to recover NGL from the gas stream leaving a crude oil separation plant.

Test Yourself 2.1 Ethane, Propane and Pentane, are leaving a separator at atmospheric pressure and 38°C (100°F). By how much would each have to be cooled in order to liquefy them?


Before you move on, have a go at the following Test Yourself question.

The process of gas compression also causes its temperature to rise. If the gas is being compressed in a number of stages this increase in temperature can cause damage to the compression machinery. (The gas compression programme in the petroleum processing technology series covers this in more detail.) To prevent damage to the compressors the gas must be cooled after each stage of compression. 23


A Typical Compression and Cooling System Take some time to study Figure 2.1 which shows a typical compression and cooling NGL recovery system. We will follow the flow through the system.

Figure 2.1 : Compression and Cooling System 24


In our example the gas to be treated is from a two stage Separation System. The gas is compressed in two stages by two centrifugal compressors. The compressors are positioned on a single shaft and driven by a single motor. The temperatures and pressures which I will use are purely hypothetical and are not meant to represent a specific process plant. Let’s follow the first part of the process now. Look at Figure 2.2. It is simply the first part of the overall system which I have reproduced for greater clarity. First of all identify where the gas from the low pressure or second stage separator enters the system. The gas leaves the 2nd stage Separator at a pressure 5.2 bar (75 psi) and temperature of, say, 80°C (176°F) and enters the 1st stage compressor suction knockout drum. This drum is a vessel whose job it is to remove any droplets of free liquid from the gas. It is an essential part of any gas compression facility. If any free liquids reach the compressor, serious damage could be done to the equipment. You may also come across the term suction gas scrubber to describe the knockout drum. Figure 2.3, on the next page, shows a typical suction knockout drum.

Figure 2.2 : 1st Stage Gas Compression 25


The drum in our system is a vertical steel cylinder which has an inlet connection approximately three quarters of the way up. Gas enters the vessel and impinges on a baffle plate which is positioned opposite the inlet nozzle. This causes a change in flow direction and reduction of gas velocity. At this point liquid droplets separate from the gas and fall to the bottom of the vessel. These liquid drops will be a mixture of condensed gas liquids and water. The liquid accumulates at the bottom and is taken out of the drum under level control. The recovered liquid is dumped to a Closed Drain System. From there liquids will be pumped back to the 2nd stage Separator. Already some small amounts of gas liquids have been recovered. The gas rises towards the outlet of the drum and passes through a mist extractor. As its name suggests this unit is there to extract any very fine droplets of liquid which may remain in the gas in the form of mist. It consists of a number of knitted wire mesh pads through which the gas must flow. As the gas flows through, any mist droplets impinge on the wire mesh and stick to it. Further small droplets coalesce until they are large enough to fall down to the bottom of the vessel. The gas, now completely free of liquid, leaves the 1st stage Compressor Suction K.O. Drum through the dry gas outlet, at the top of the vessel. Figure 2.3 : Suction Knockout Drum

26


The liquid level in the knockout drum is controlled via a level controller (LC) and control valve. A sight glass or level gauge (LG) is also incorporated in the level control instrumentation. Further instrumentation protects the system against loss of level control. A level switch low ( LSL ) activates an alarm if the level should go too low. Similarly a level switch high ( LSH) activates a high level alarm.

Normally however the gas flows forward to the 1st stage Compressor. This raises the pressure from 5.2 bar (75 psi) to 25.9 bar (375 psi). During compression the temperature of the gas increases from 80°C (176°F) to 127°C (260°F). If the gas is not cooled at this point the efficiency of the next compressor would be affected. So this hot, medium pressure gas, is cooled by passing it through a heat exchanger. In our system, the heat exchanger is of the shell and tube type. Figure 2.4 shows an exchanger of this type.

If, despite the activation of a high level alarm the level of liquid in the vessel continues to rise, a dangerous situation could occur. Too high a level could result in carry over of liquid into the compressor with possible serious damage being done. This situation is prevented by having a shutdown incorporated into the vessels instrumentation. A level switch high high ( LSHH ) activates a signal which shuts down the system, preventing potential hazardous situations arising. A pressure controller on the top of the drum controls the pressure of gas in the vessel. If the pressure is too high and the compressor is not pulling the pressure down for whatever reason, then the pressure controller opens the valve and allows excess gas to escape to the flare.

Figure 2.4 : Shell and tube Heat Exchanger

27


The hot gas passes through the inside of the tubes where it is cooled by sea water passing through the shell. The temperature of the gas leaving the heat exchanger is controlled by a temperature controller (see Figure 2.2). The temperature controller opens and closes a valve to control the flow of cooling water through the unit. If the gas temperature leaving the exchanger is too high the temperature controller will open the valve and allow more sea water to flow through. If the gas temperature is too low the temperature controller will close the valve and allow less sea water to flow through the heat exchanger. Go back over the process so far and satisfy yourself that you understand it before having a go at the following Test Yourself question.

Test Yourself 2.2 insert the correct word or words from the list provided into the blanks in the following sentences. 1.

The process of gas compression causes the gas ................................................... to rise.

2.

The suction gas scrubber removes ................................................... from the gas.

3.

Within a suction K.O. drum wire mesh pads form a .....................................................

4.

In a .................................and tube heat exchanger, gas flows through the tubes and water flows through the shell.

List of words.

wier - mist extractor - solids - temperature - liquids - shell - filter

You will find the correct answers in Check Yourself 2.2 on Page 59. 28


Medium pressure gas leaves the 1st stage Separator at a pressure 25.9 bar (375 psi) and a temperature of, say, 82°C (180°F). The two streams, now mixed together, enter the 2nd stage Compressor Suction K.O. Drum.

We have now increased the pressure and cooled the gas. More gas liquids will have formed. At this point the mixture is joined by the gas which is leaving the 1st stage Separator.

This drum removes any liquid which is in the gas coming from the 1st stage separator, plus any liquids which have been formed during the compression and cooling of the original gas from the 2nd stage separation. The 2nd stage Compressor Suction K.O. Drum operates at a higher pressure than the 2nd stage Separator. (Remember, it operates at 5.2 bar (75 psi)) The recovered liquid can therefore be piped straight back to the 2nd stage Separator. The gas, again completely free of liquid, leaves the 2nd stage Compressor Suction K.O. Drum where a pressure controller controls the pressure of gas in the vessel. It operates in a similar manner to the one previously described. The gas then enters the 2nd stage Compressor. The 2nd stage Compressor compresses the gas to a pressure 50 bar (725 psi) and the temperature will rise to, say, 138°C (280°F).

Figure 2.5 shows this part of the process. You can see that it is almost identical to part 1. Let's quickly go through this part now.

Figure 2.5 : 2nd Stage Gas Compression 29


The hot, high pressure, gas is then cooled by passing it across a shell and tube heat exchanger similar to the one used to cool the 1st stage discharge gas.

The gas and liquid mixture enters the NGL Knockout Drum which separates the two phases. This vessel is yet another separator which works in the same way as the Compressor Suction K.O. Drum. In this case however the vessel is a horizontal cylinder.

The temperature of the gas leaving this heat exchanger is again controlled by a temperature controller.

The gas leaving the NGL Knockout Drum is routed to dehydration. The liquids accumulate at the bottom of the vessel where the level is controlled by a level control system.

The pressure of the gas is maintained at 50 bar (725 psi) as it passes the heat exchanger but the temperature of the gas is lowered to, say, 29°C (85°F). This combination of high pressure and relatively low temperature causes the heavier hydrocarbons to turn into a liquid. All that remains now is to separate these liquids from the gas before the gas is dehydrated, further compressed and exported.

In our case, the NGLs will be transported from the plant via the main oil pipeline operating at, say, 69 bar (1 000 psi). This injection of NGLs into the crude is often referred to as spiking the crude with NGLs. The Knockout Drum operates at a lower pressure than the main oil pipeline. In order to get the NGLs into the line, the pressure must be boosted by an NGL Booster Pump.

There will be no NGL knockout after the export compressor discharge gas is cooled, as the pressure will be above the point at which any liquid could exist.

The level controller on the NGL Knockout Drum is different to the previous ones we looked at. It works in conjunction with the booster pump. The control valve is positioned downstream of the pump. If the level in the vessel goes down the controller opens the control valve and allows liquid to circulate back to the drum. If the level goes too high all the NGLs are pumped into the crude line.

This brings us to the final part of this simple system which is shown in Figure 2.6.

Figure 2.6 : NGL Knockout Drum 30


Summary of Section 2 In this short section we have looked at a simple process used to recover liquids from natural gas. In the section we looked at a typical Compression and Cooling System and saw:

• what equipment was used

• how the NGL was recovered

• how the process was controlled

• what happened to the NGL after it had been recovered

As I pointed out in this section the simple compression and cooling process recovers a limited amount of liquids. To make the process more efficient, the degree of cooling must be much greater. We need to refrigerate the gas. In the next section we will look at some methods of doing this.

31



Section 3 - NGL Recovery by Refrigeration In this section we are going to take the recovery process one step further. More NGL recovery can be achieved if the temperature is reduced to a greater degree than that obtained by simply cooling. In order to reduce the temperature of the gas even more, it is necessary to refrigerate it. There are two methods commonly used to do this. The first method can be called mechanical refrigeration and the second, auto refrigeration. We can look at these two processes in turn . Let’s start with mechanical refrigeration.

When a substance is liquefied by compression, heat energy is liberated. In a refrigerator this heat is dissipated through a condenser which resembles a car radiator. The liquid then passes through an expansion valve where it vapourises. The necessary heat for vapourisation comes from the surroundings (in this case, the refrigerator contents) which are thereby cooled. The refrigerant is then recycled. Figure 3.1 is a block diagram which shows this process very simply.

Mechanical Refrigeration Mechanical refrigeration systems use a similar process to that found in a domestic refrigerator. Before we look at a refrigeration process to recover NGL let’s take a look at an ordinary domestic refrigerator, and see how it works. We have already seen that many substances that are gases at normal temperatures can be liquefied by increasing their pressure. The liquids can then be vapourised again by increasing the temperature. In a refrigerator a substance which is liquefied then vapourised is called a refrigerant.

Fig 3.1 : Mechanical refrigeration Block Diagram 32


In the next figure Figure 3.2, the main components of an ordinary domestic refrigerator are shown. Take a look at the figure and identify the following components.

On the outside of the refrigerator we have •An electric power supply which drives an electric motor. •A compressor which is driven by the electric motor. •A compressor suction line which takes refrigerant gas into the compressor. •A compressor discharge line with a check valve which stops anything flowing back through the compressor. •A set of condensing coils where the heat in the refrigerant is dissipated and the refrigerant turns from a gas into a liquid. •An accumulator which stores a small amount of liquid refrigerant. Inside the refrigerator, in the freezer section, we have: •An expansion valve where the liquid refrigerant ‘flashes off’, changing into a liquid/gas mixture. •A set of chilling coils where the refrigerant is boiled off, completing the change from a liquid to a gas.

Fig 3.2 : Domestic Fridge Schematic Diagram 33


Inside the refrigerator, in the cooler section, there is a temperature switch which will switch off the electric power to the electric motor when the temperature falls to a preset level, and, switch on the electric power to the electric motor when the temperature rises to a pre-set level. Now that you have identified the refrigerator components we can now look at what is going on inside the unit. Starting from the discharge of the compressor Figure 3.3 shows what is happening as follows: The refrigerant leaving the compressor is a hot, high pressure, gas. As the refrigerant gas passes through the condensing coils it loses heat and turns into a liquid. It arrives in the accumulator as a relatively cool, high pressure liquid. From the accumulator it flows through the expansion valve where the pressure is reduced. This expansion causes the refrigerant liquid to start to vapourise and become a cold, low pressure, gas/liquid mixture. This mixture now enters the chiller coils. The remaining liquid is boiled off by heat entering the chiller coils from the air in the freezer section of the refrigerator. This completes the vapourisation of the liquid refrigerant. The refrigerant gas is now at low pressure and temperature.

Fig 3.3 : Domestic Fridge Schematic Diagram 34


This gas enters the compressor. The compressor raises the pressure and the temperature of the gas and the process starts all over again. The system will keep running, circulating a limited amount of refrigerant around the system, until the temperature falls to the point where the temperature switch operates and stops the electric motor.

Test Yourself 3.1 Starting with ‘Refrigerant leaves the compressor’, put the following steps of a refrigeration process in the correct order.

It will stay switched off until the temperature in the cooler section rises and starts the electric motor again.

1.

Refrigerant leaves the compressor.

..............................

2.

Refrigerant flows through accumulator.

..............................

Think about the operation of a fridge and have a go at the following Test Yourself question.

3.

Refrigerant enters condensing coils.

..............................

4.

Refrigerant enters compressor.

..............................

5.

Refrigerant starts to vapourise and becomes cold. ..............................

6.

Refrigerant loses heat and liquefies.

..............................

7.

Refrigerant completely vapourises.

..............................

8.

Refrigerant mixture enters chiller coils.

..............................

9.

Refrigerant flows through expansion valve.

..............................

Now let’s see how we can apply the theory of operation of a domestic refrigerator by comparing it with a typical natural gas liquids recovery plant.

You will find the correct answers in CheckYourself 3.1 on Page 60.

35


A Typical Mechanical Refrigeration NGL Recovery System Figure 3.4 shows a typical Mechanical Refrigeration System which uses Propane as the refrigerant.

Figure 3.4 : Mechincal Refrigeration NGL Recovery System 36


The boiling point of Propane is -42°C (-43.7°F). It is often used as a refrigerant in gas processing. It is not used in domestic refrigerators because it is flammable. If leaked, it could cause a fire or explosion. Within this system, there are actually two interconnected sub-systems. They are:

• the refrigeration system which reduces the temperature of the refrigerant

• the natural gas system where the reduction in temperature of the refrigerant is used to recover the NGL

Activity Take a close look at Figure 3.4. See if you can determine where these two points of contact are.

The first point of contact is the chiller. It is in this vessel that the low temperature created by the refrigeration system is used to refrigerate the natural gas. The second one is the refrigerant condenser where the heat generated during refrigerant compression is dissipated.

Let’s now work our way through the two systems starting with the refrigeration system. We can go through this rather quickly as it is very similar to the domestic refrigerator we looked at earlier.

There are two points of contact between the two systems. Take a look at Figure 3.5 on the next page. It is the refrigeration system from Figure 3.4 which I have isolated from the rest of the drawing.

37


Some of the liquid refrigerant leaves the acumulator and flows across the expansion valve, where partial vapourisation takes place. This gas/ liquid mixture now flows into the chiller. Warm natural gas is passing through the coils in the chiller, as we will see shortly. This is the gas which needs to be refrigerated in order to form NGL. The heat content of this gas is transferred to the refrigerant in the chiller. The net effect of this is to vapourise some of the liquid refrigerant, whilst reducing the temperature of the natural gas. The amount of natural gas flowing through the chiller will vary. Therefore, the amount of liquid refrigerant vapourised in the chiller will vary, affecting the liquid level. This level is maintained by a level controller operating the expansion valve. The refrigerant leaves the chiller as a cold, low pressure gas and enters the compressor suction knockout drum. This drum removes any entrained liquids which may be carried over by the refrigerant gas from the chiller.

Figure 3.5 : Refrigeration System We will start at the same point as we did with the domestic refrigerator, that is, at the discharge of the compressor.

As it passes over the refrigerant condenser the refrigerant exchanges heat with cold gas coming from the gas/gas heat exchanger.

The refrigerant leaving the compressor is a high pressure, high temperature gas.

The refrigerant condenses to high pressure, relatively cool liquid as it enters the refrigerant accumulater,

The refrigerant gas leaves the compressor suction knockout drum and enters the compressor as a cold, low pressure gas. After compression the refrigerant leaves the compressor as a hot, high pressure gas and the process starts again.

38


Now let’s go through the actual gas liquids recovery part of the system. Figure 3.6 is once again the relevant part of Figure 3.4 which has been isolated from the rest of the drawing. The natural gas enters the NGL recovery system after it has been dehydrated. This is to remove any chance of hydrate formation as the gas passes through the refrigeration process. (A separate unit in the Petroleum Processing Technology Series covers dehydration). The incoming natural gas stream enters the gas / gas heat exchanger. It is called a gas / gas exchanger because both the medium to be cooled and the cooling medium are gas.

Figure 3.6 : NGL Recovery System

39


As the warm incoming natural gas enters the system it passes through the exchanger tubes and is cooled by the cold natural gas leaving the system through the exchanger shell. This pre-cools the incoming gas and pre-warms the outgoing gas. The pre-cooled gas leaves the gas/gas exchanger and enters the refrigerant chiller. As it flows through the tubes in the chiller, it is refrigerated to a temperature of say -15°C (-59°F). As the natural gas is refrigerated, the heavier hydrocarbons turn into natural gas liquids. This mixture of residual cold gas and liquids then enters the NGL/gas separator where the NGL is separated from the gas. The cold gas leaves the separator from the top of the vessel and flows through the gas/gas heat exchanger where, as we have seen, ~ cools down the incoming warm gas.

The NGLs accumulate at the bottom of the NGLs gas separator, where a liquid level is maintained by a level controller. From there they flow to an NGL flash separator. In this vessel any small amounts of light hydrocarbon gases which may still be entrained in the NGLs are separated. These gases are usually disposed of via a flare system. From there, the NGLs are pumped via a level control valve to a transport system for sale. The system we have just been looking at is a very common system which you could find on an offshore oil and gas production platform. However there are other systems. I mentioned one earlier where refrigeration is achieved using what is called an auto refrigeration system. We will look at one of these systems shortly. Before you move on to this however have a go at the following Test Yourself question on mechanical refrigeration.

Next the gas flows through the refrigerant condenser. Here it condenses the refrigerant and is itself warmed up further to a level where it can be put into a pipeline as a sales gas. Note : Depending on operating pressures, this gas may require to be further compressed prior to being exported as sales gas.

40


Auto Refrigeration In this part of Section 3 we are going to look at an alternative method of reducing the temperature of gas in order to recover NGLs. It is a method which uses one of the properties of the gas itself. This property can be stated as follows :

Test Yourself 3.2 Are the following statements true or false ? 1.

Heat generated during refrigerant compression is dissipated in the refrigerant condenser.

2. .

The chiller is used to cool down the refrigerant.

3.

As liquid refrigerant flows across the expansion valve partial condensation takes place.

4.

Incoming natural gas flows through a heat exchanger which uses liquid refrigerant as the cooling medium.

5.

In the NGL flash separator, small amounts of entrained light hydrocarbon gases are removed from the NGL stream.

TRUE

FALSE

If a natural gas is rapidly expanded by reducing its pressure, its temperature will drop. This temperature drop associated with rapid gas expansion is known as the Joules/Thompson Effect. The greater the pressure drop, the greater the temperature reduction. You can see this effect illustrated on next page in Figure 3.7.

You will find the correct answers in Test Yourself 3.2 on Page 60.

41


This chart is only by way of an example, it will not be accurate for all gases. Also, temperature and pressure are shown in imperial units, however it serves to illustrate the use of such charts. You can use this chart for SI Units by converting them to imperial units, then use the graph to find your answer, and then convert your answer back into SI Units. In order to find a temperature drop associated with a given pressure drop, just follow the steps I have listed here for you. • Find the point on the horizontal axis of the graph which corresponds to the initial pressure before expansion takes place. • From this point move vertically upwards to where this pressure intersects the pressure drop curve. (Note that on the graph the pressure drop is indicated as A P i.e. initial pressure - final pressure.) • From the intersection point, move horizontally to the left hand vertical axis of the graph. • Read off the temperature at this point. Figure 3.7 : Typical Natural Gas Expansion - Temperature Reduction Curves

Let's do that with some actual figures.

Note: take care to note that pressure and temperature on this graph are given in psig and fahrenheit only. 42


Work through with me, the following example, where I will start of with SI Units and convert them to imperial to use the graph and then convert my answer back to SI Units. : If gas at an initial pressure of 207 bar (3000 psi) is expanded to a final pressure of 69 bar (1 000 psi), what will be the temperature drop? • First calculate the pressure drop. The pressure drop will be (3000-1 000) = 2 000 psi. • Find the point on the horizontal axis of the graph of an initial pressure of 3 000 psi.

Test Yourself 3.3 If a gas at an initial pressure of 310 bar (4 500 psi) is expanded 10 34.5 bar (500 psi), what will be the drop in temperature?

• You should have found that the temperature drop is 70°F, which you can convert to 21°C. Of course, the actual pressure drop will not always coincide exactly with one of the curves. In such a case a curve parallel to the nearest printed curve needs to be drawn or imagined. Now have a go at Test Yourself 3.3.

What could be the advantage in using the turbine ?

(Use Figure 3.7) Check your answer in Check Yourself 3.3 on Page 60.

• Move vertically upwards to intersect the AP = 2 000 curve. • From the intersection point move horizontally to the left to read off the temperature drop on the left hand vertical axis.

Test Yourself 3.4

We have seen that a pressure drop will reduce the temperature of a gas. But how do we create the drop in pressure? Basically there are two methods in common use.

Check your answer in Check Yourself 3.4 on Page 60.

We will now take a look at an Auto-Refrigeration system which uses both the valve and the turbine in the system.

• expand the gas across a valve • expand the gas across a turbine In fact expanding the gas across a turbine makes the gas do work as it expands and the cooling effect is greater. Think about using the turbine to create the pressure drop and have a go at the following Test Yourself. 43


First of all, you should appreciate that a gas must be at a high enough pressure initially in order that the required pressure drop can take place. It is seldom worth compressing the gas and raising its pressure just to drop it again as part of an Auto-Refrigeration system. The cost involved in purchasing and running compressors for such a system is far too high under normal circumstances. Because of this, auto-refrigeration systems are less common than mechanical refrigeration systems. Figure 3.8 is an illustration of an auto refrigeration system which uses both forms of expansion to recover NGL. Take a look at the drawing and identify the following components:

•

liquid / gas separator

•

Joules Thompson (JT) valve

•

turbo-expander driving a compressor

•

recovered NGL knockout drum

•

make up compressor

Fig 3.8 : Auto Refrigeration NGL Recovery System

44


Now let’s follow the flow through the system and see how the process works. In this case the gas entering the system comes from a high pressure pipeline, although in other cases it may come from gas dehydration. It first of all enters a liquid / gas separator. Here, any liquids which may have condensed in the pipeline are separated. These liquids which may consist of water and NGLs are dumped to the NGL Knockout Drum. If there is any water in the gas at this stage, methanol will require to be injected upstream of the expansion process to prevent hydrate formation caused by the resultant chilling effect. The gas leaves the separator and, normally enters the turbo-expander. As it passes across the turboexpander it is made to do work by driving the compressor which is attached by a shaft.

The gas leaving the recovered NGL knockout drum flows to the suction of the compressor connected to the turbo-expander. As the gas is compressed it is heated due to the compression process and is warm enough, depending on operating pressures, to leave the system as a sales gas. If the turbo-expander is out of commission then the JT valve acts as a secondary expansion valve. This system will not recover as much NGL as the Turbo expander but it will allow the process to keep running and will recover some of the NGL. When the JT valve is in operation, the make-up compressor will be required to boost the pressure of the gas leaving the recovered NGL knockout drum to sales gas pressure. This is a typical example of an Auto-Refrigeration system. You may find that some systems have an export compressor to compress the gas up to gas pipeline pressure.

As the gas is expanded across the turbine its temperature is reduced and NGLs are condensed. The NGL is separated from the gas in the recovered NGL knockout drum and, under level control, is despatched to NGL Sales. Any water which might be present, is also separated and sent to disposal.

45


Summary of Section 3 In this section we have looked at what is probably the most common natural gas liquids recovery system found in offshore petroleum producing operations. i.e. NGL recovery by refrigeration. As you worked through the section you first of all saw how a simple domestic refrigerator worked. You saw that when a substance is liquefied by compression, heat energy is liberated. In a refrigerator this heat is dissipated through a condensor. The liquid then passes through an expansion valve where it vapourises. The necessary heat for vapourisation comes from the surroundings which are thereby cooled. The refrigerant is then recycled.

I pointed out that there are two points of contact between the two systems. The first one is the chiller. It is in this vessel that the low temperature created by the refrigeration system is used to chill the natural gas. The second one is the refrigerant condenser where the heat generated during refrigerant compression is dissipated by the chilled gas. We then worked through the plant using a typical flow diagram.

We then applied this knowledge to a mechanical refrigeration system used to recover NGLs.

We then looked at an alternative method of reducing the temperature of gas in order to recover NGLs. A method which uses one of the properties of the gas itself. This property can be stated as follows:

You saw that within this system, there are two interconnected sub systems which are:

If a natural gas is rapidly expanded by reducing it’s pressure, it's temperature will drop.

• the refrigeration system which reduces the temperature of the refrigerant

• the natural gas system where the reduction in temperature of the refrigerant is used to recover the NGLs

This temperature drop associated with gas expansion is known as the Joules/Thompson Effect. The greater the pressure drop, the greater the temperature reduction.

We performed some simple calculations to show this effect using a graph of gas expansion / temperature reduction curves. A system which uses this effect to achieve temperature reduction is known as an auto refrigeration system. You saw that there are two methods used to achieve this auto refrigeration. • expand the gas across a valve • expand the gas across a turbine Finally we followed the flow diagram of a simple plant which uses the two types of expansion to achieve refrigeration and hence NGL recovery. In the next section we will look at one more method of recovering the natural gas liquids. This uses an absorption process.

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Section 4 - NGL Recovery Using an Absorption Process In Section 1 you saw that contacting gas with a lean oil causes the heavier gas components to be absorbed into the oil. These can then be recovered and transported as NGLs. In this section we are going to look at a typical system which uses this type of process. As usual we will follow a drawing of a system and see how the plant is constructed and how it works. Before we start however, see if you can remember the simple process which I described in Section 1.

Test Yourself 4.1 Figure 4.1 is the same figure that you saw in Section 1. Where I described a simple absorption system. You will see that there are no labels Complete the figure with the correct labels.

Now have a go at the following Test Yourself question.

You will find the correct answer in Check Yourself 4.1 on page 6.1

Figure 4.1 : Absorption system

47


A Typical Absorption System Figure 4.2 is a line drawing of a typical system. Study the drawing for a while and identify the various components.

Figure 4.2: Absorption System 48


You will probably have noticed that the system is, in essence, the same as the very simple system which you labelled in your answer to Test Yourself 4.1. At the heart of the system are the two towers, the absorber tower and the stripper tower. You will notice that there is a continuous circulation of oil round the system. This is the oil which absorbs the NGLs as the gas is contacted with it. We will refer to it as Absorption Oil. It is a hydrocarbon liquid and in some cases it can be dead crude oil. In our example the absorption oil is classed as lean oil or rich oil as it flows around the circuit. The lean oil is oil which has been stripped free of NGL gases. The rich oil is oil which has absorbed the NGL gases. Before we start to follow the process flow, let’s take a look at the absorber tower which as I said earlier is at the heart of the system. Look at Figure 4.3 which shows a typical absorber tower. The figure is a simplified cutaway view which shows the internal features. As you can see, the absorption oil enters the tower through the inlet pipe near the top. It is spread out through spray bars and a distribution plate before flowing down the tower. It flows down through two sections packed with devices which increase the surface area of the liquid. (You will remember from Section 1 that if the contact area between gas and absorption oil can be increased the process will be more efficient).

Figure 4.3 : Absorption Tower 49


The selection of the packing devices for any type of contactor is dependent upon:

• • • •

the gas/liquid to be contacted the flow rate requirements the size of the column the materials of manufacture for each type of packing

Figure 4.4 is an illustration of a few of the different kinds of packing devices. When the liquid flows over them it forms a thin film and increases its surface area.

The reason for having two separate sections is to ensure that the liquid and gas flows can be evenly re-distributed. If all the liquid went down one side of the tower and all the gas went up the other side there would be very little contact between the two. You will notice that halfway down the tower there is a redistribution plate. After the oil has passed the upper section of packing it accumulates on the plate. It then flows through small caps which spread the oil out over the next section of packing. The lower section of packing is supported on a plate located above the gas inlet. When the oil has percolated through all the packing and the support plate, it accumulates in the bottom of the tower. A liquid level is maintained and the oil exits the tower from the outlet under level control. The gas enters the absorber tower just beneath the lower packed section. It flows upwards through the two packed sections and finally leaves the tower through the gas outlet at the top of the vessel. During its passage up the tower the gas is forced to intimately mix with the downcoming oil. It is this mixing under the right conditions which cause the NGLs to be absorbed into the oil. Let’s now go back to Figure 4.2 and work our way through the process. We will start with the gas flow through the system then follow the flowpath of the absorption oil.

Figure 4.4 : Absorption Tower Packing Devices 50


Gas Flow Once again I have isolated the gas flow section from the rest of Figure 4.2 in order to simplify the explanation.

The first feature you should note is that the absorber can be by-passed. This may be required when the absorption system is out of action for repair or maintenance. Under normal operating conditions the gas enters the tower at the bottom and flows upwards. As the gas rises through the absorber tower, the heavier gases (the NGLs) are absorbed into the absorber oil which is flowing down the tower. The gas, stripped of NGL, leaves the tower at the top. If there is sufficient gas, it may be transported for sale. If the amounts of gas are too small to justify collection for sale, it may be used as fuel with the surplus being flared. Under normal operating conditions there is a small differential pressure from top to bottom across the packed sections. If the packing becomes dirty, the differential pressure could rise and the gas flow could cause the packing to be dislodged. This may cause channelling through the packing resulting in a loss of efficiency. To guard against this the differential pressure is monitored by a differential pressure transmitter PDT 01. This will give an alarm before the packing begins to lift.

Figure 4.5 : Absorption System Gas Flow

51


Before we continue, try Test Yourself 4.2.

Test Yourself 4.2 We said that increasing the contact area between gas and liquid increases the efficiency of the absorption process. State two other factors which influence the efficiency.

From the answer to the Test Yourself you can see that it is necessary to maintain a pressure in the tower. This is done by a back pressure controller PC 01 opening and closing pressure control valve PCV 01 in the gas outlet line. The flow of gas is reasonably straightforward. We can now go on to look at the flowpath of the absorption oil as it circulates round the system.


52


Absorption Oil Flow Look at Figure 4.6 which shows the route taken by the absorption oil flowing round the system. We are going to start at the oil circulation pumps so locate these on the drawing. At this point the oil is lean. You will remember that this means that the absorption oil has had all the NGLs stripped from it. The pumps are used to increase the pressure of the lean oil which enables it to enter the high pressure absorber tower. Downstream of the pumps a flow element FE 01 monitors the flow of lean oil to the absorption tower and switches on a stand-by pump in the event of a flow failure. It is important to ensure that a constant ratio of gas to lean oil flow rates is maintained within the absorber tower. Instrumentation is provided to do this. If the gas flow increases the absorption oil circulation rate will increase. Conversely if the gas flow decreases the absorption oil circulation rate will decrease. Note that I have omitted this instrumentation from the drawing for the sake of simplicity.

Figure 4.6 : Absorption System Absorption Oil Flow

53


You saw in Section 1 and the answer to Test Yourself 4.2 that the colder the absorption liquid the more gas it can absorb. So, before passing into the absorber tower, the lean oil flows through a lean oil cooler. The cooler is a heat exchanger which uses sea water to cool the lean oil to as Iow a temperature as possible. The cooled lean absorption oil enters the Absorber Tower near to the top. It flows downwards and spreads out over the packing contacting the gas flowing upwards through the tower. The oil accumulates at the bottom of the tower. It is now rich oil and has absorbed as much NGL as it can. The liquid level in the tower is controlled by a level controller LC 01 through a level control valve LCV 01 located in the liquid outlet line from the tower. From the absorber tower the rich oil flows to a rich oil flash drum. Small amounts of the light hydrocarbon gases may have been absorbed into the oil. The rich oil flash drum acts as a separator to remove the gas which is mainly methane. This gas joins the gas leaving the absorber tower and is sold, used as fuel gas or sent to the flare.

The rich oil flash drum is filted with pressure controller PC 02 which controls the pressure by opening and closing pressure control valve PV 02. The level in the rich oil flash drum is controlled by level controller LC 02 in conjunction with LCV 02. This control valve is positioned downstream of the next equipment in the system, the rich oil filters. As the oil passes across LCV 02, the pressure falls. Because of this reduction in pressure, some of the gases which are absorbed into the oil will begin to separate. This is often called flashing off. If the valve were situated before the filters, there would be a mixture of liquid and free gas flowing through them. This would affect the efficiency of the filters. Placing the control valve downstream, helps prevent separation taking place before we want it to. So, the total flow of rich oil from the rich oil flash drum is passed through a set of filters. These filters remove any fine solids which may have entered the system with the gas. Stop for a moment here and think about the situation in our NGL recovery plant.We have reached a point where we have removed the NGLs from the gas stream and they are dissolved within the absorption oil. It would be possible to transport the NGLs to another location still dissolved in the oil and recover them in a separate processing plant.

In fact this is what happens on some offshore oil producing platforms. The rich oil is injected into the main oil transport line going ashore. Further processing plant at the shore terminal is used to remove the NGLs which are then further refined into pure products of propane, butane etc. But we are following a complete plant so let’s go back to the system which we were following. The flow of oil plus flashed gas is now routed to the Stripping Tower. This tower is almost identical to the absorption tower in size and design. It is used to remove the rest of the absorbed components from the absorption oil. The gas/oil mixture enters the Stripper Tower through a set of spray bars located above the top of the packed section. The spray bars do two things. They: • immediately Increase the surface area of the rich oil to assist in removing the gas. • evenly distribute the oil across the top of the packing. The rich oil falls down the tower. and the large surface area generated over the packing plus the drop in pressure allows the absorbed gas to be released. 54


In order to assist in the stripping process heat is often added to the tower. Also a small amount of dry lean gas may be introduced at the bottom. This flows up and helps to vapourise the absorbed components.

A feature of most absorption systems is that the interplay of : • gas flow rates

Test Yourself 4.3

• tower level control The gas which is released comprises the NGLs in gaseous form which have been recovered from the original gas stream. This gas flows upwards and leaves the tower at the top. The pressure in the tower is controlled by pressure controller PC 03 in conjunction with pressure control valve PCV 03 on the gas outlet line. The absorption oil accumulates at the bottom of the tower where it can now be called lean oil. The level of the oil is controlled by level controller LC 03 operating level control valve LV 03. The next piece of equipment in the system is the lean oil surge drum. This keeps a sufficient amount of oil in the system to ensure that a continuous flow of absorption oil is maintained around the circuit

• flash drum level control • absorption oil flow rates

Place the following items in the correct order in the absorption oil circulation path starting with absorber tower.

these can all add up to result in a slight slugging of flow. The lean oil surge drum evens out this slugging effect by providing a buffer in the system.

a) absorber tower

.................

b) filters

.................

The drum is fitted with:

c) stripper tower

.................

d) lean oil cooler

.................

e) rich oil flash drum

.................

f) lean oil surge drum

.................

g) oil circulation pumps.

.................

• level switch high-high (LSHH 01) • level switch low-low (LSLL 01). These switches will operate to shutdown the system if the slugging becomes too severe. This completes our brief section on gas liquids recovery using an absorption system. Before I summarise the section have a go at the final Test Yourself in this unit.


55


Summary of Section 4 In this section we looked at a system which uses the principle of contacting gas with a lean oil. This causes the heavier gas components to be absorbed into the oil which can then be recovered and transported as NGLs. You saw that at the heart of the system are the two towers, the absorber tower and the stripper tower.

Next we followed the path of the natural gas through the absorber tower. You saw that the gas flowing upwards is contacted with a downcoming stream of absorbtion oil. As the two streams comingle the oil absorbs the NGLs and the residual gas flows from the tower to sales or flare. Within the oil circulation system we looked at the following pieces of equipment.

There is a continuous circulation of oil round the system. This is the oil which absorbs the NGLs as the gas is contacted with it. We referred to it as Absorption oil. It is a hydrocarbon liquid and in some cases it can be dead crude oil. The oil is classed as lean oil or rich oil as it flows around the circuit. The lean oil is oil which has been stripped free of NGL gases. The rich oil is oil which has absorbed the NGL gases.

•

oil circulation pumps

•

lean oil cooler

•

absorber tower

•

rich oil flash drum

•

filters

•

stripping tower

We looked at the construction of a typical packed column type of absorber tower and saw how it worked

•

lean oil surge drum

You have now completed this unit on natural gas liquids recovery. Before you move on to another unit in the Petroleum Processing Technology Series I would like to make some final comments. The unit you have just completed relates to NGL recovery in general. It is not meant to apply to any specific plant or process. If you are involved with process plant operation you must be completely familiar with the specific plant under your control. You must always follow laid down procedures and operational guidelines and adopt safe working practices at all times.

I explained the function of each of these pieces of equipment and pointed out how they are controlled.

56


Check Yourself 1.1 nitrogen - (N)

- element

sodium chloride - (NaCl)

- compound

water - (H20)

- compound

helium - (He)

- element

sulphuric acid - (H2SO4) - compound carbon - (C)

- element

iron - (Fe)

- element

Check Yourself 1.2 a) C8 H (2 x 8) +2

i.e.C8H18

b) C9 H (2 x 9) +2

i.e. C9H20

Check Yourself 1.3 B 0ºC (32ºF) C 0ºC (32ºF) D 100ºC (212ºF)

Check Yourself 1.4 At point Y the substance would exist as steam. If heat energy is removed at constant pressure the temperature of the substance would fall towards point X. When the line BC is reached the steam would start to condense and the first drop of liquid would form. At this point the temperature would remain stable until all the steam had condensed to liquid water. The temperature would then fall again until line BD is reached. Once again the temperature would remain constant until all the liquid freezes into solid ice. When the substance is solid, the temperature again falls to point X. So, there have been 2 phase changes. From gas ( steam) to liquid and from liquid to solid.

E 100ºC (212ºF)

57


Check Yourself 1.5

Check Yourself 1.6

W

36°C (97°F). i.e. the temperature at which pentane ( having the lowest boiling point) starts to boil.

a)

Solid - liquid - gas - solid - gas.

b)

CI- Chlorine, O - Oxygen, Si - Silicon, He - Helium, Fe - Iron, C - Carbon,

Y

99°C (209°F). I.e. the temperature at which Heptane ( having the highest boiling point) boils.

c)

1 - X, 2 - Z, 3 - W, 4 - Y, 5 - V,

58


Check Yourself 2.1 Ethane

Check Yourself 2.2

228ºF

(100 - (-128))

1.

The process of gas compression causes the gas temperature to rise.

Propane

143.7°F

(100 - (- 43.7))

2.

The suction gas scrubber removes liquids from the gas.

Pentane

3°F

(100 - 97)

3.

Within a suction K.O. drum wire mesh pads form a mist extractor.

4.

In a shell and tube heat exchanger, gas flows through the tubes and water flows through the shell.

59


Check Yourself 3.1 The correct order is :

Check Yourself 3.3 110°F (43.3°C)

1-3-6-2-9-5-8-7-4

Check Yourself 3.2

Check Yourself 3.4

1.

True

2.

False - It is used to refrigerate the natural gas.

If the gas is being used to power a turbine as it expands, the turbine could be used to drive some other machinery. The most common would be a compressor. However, a pump or generator could be driven by the turbine.

3.

False - Partial vapourisation takes place.

4.

False - The cooling medium is cold gas from the NGL / gas separator

5.

True

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Check Yourself 4.1

Check Yourself 4.2 a) Pressure

Contact must be made under pressure.

b) Temperature

The process is made more efficient at lower temperatures.

We discussed this in Section 1. Go back and refresh your memory of this if you had any difficulty.

Check Yourself 4.3 The correct order is : a) - e) - b) - c) - f) - g) - d)

61


62

Natural Gas Liquids Recovery.pdf

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