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OPTIMIZATION OF NATURAL GAS DEHYDRATION

REDUCTION IN EMISSIONS AND ENERGY USEAT MAKOWICE NATURAL GAS

DEHYDRATION FACILITY

Artur Ryba

Diploma Thesis

Faculty of Drilling, Oil and Gas

AGH University of Science and Technology in Cracow

Trondheim

June 2005

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OPTIMIZATION OF NATURAL GAS DEHYDRATION ii

Abstract

An approach for reduction of triethylene glycol (TEG) losses and energy consumption in

Makowice Dehydration Facility, Poland is presented. Operating manuals of devices, and

charts showing parts of dehydration facility Makowice were used for creating a steady state

simulation of dewatering process under process engineering program Hysys. Analytical and

mathematical calculations were made and compared with simulation outcome and

experimental data for achieving reliable results. Water content values in natural gas were

obtained from Makowice Treatment Facility operaton manual and calculated with empirical

equations. The values obtained were compared to water amount in natural gas according to

Hysys computation. Subsequently the amount of water necessary to be removed from natural

gas in order to meet the demand for dew point temperature was calculated. The values

obtained show the minimum TEG circulation for gas dehydration. Calculations of minimum

TEG concentrations required for given conditions and dew point temperature required were

made. On basis of the above an attempt was made to find optimum pressure and temperature

work range for gas dehydration from the viewpoint of TEG losses and energy use reduction.

A solution was suggested for limitation of energy and glycol consumption.

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OPTIMIZATION OF NATURAL GAS DEHYDRATION iii

Acknowledgements

I wish to express my sincere appreciation to my supervisor Professor Jon Steinar

Gudmundsson. I am very grateful for the advice, support, guidance, assistance, patience and

enthusiasm.

I wish to thank Dr Hab. In. Stanisaw Nagy, my supervisor from AGH University of Science

and Technology in Cracow, Poland for his support, propositions, help and understanding.

I am grateful to all my teachers who, giving me a small part of their wide knowledge, got me

to the stage when I am writing this thesis.

Special thanks to all contributors that make my Erasmus Link Scholarship possible. I would

like to especially mention here Dr Czesawa Ropa, Professor Danuta Bielewicz, and Professor

Jan Falkus.

Special thanks to Regional Department of Gas Transport in Tarnw (ROP Tarnow) for the

necessary materials, support and technical knowledge.

Last but not least thanks to my family, friends and colleagues who supported me in the time

spent on creating the thesis, and much longer than that. You are always there when I need you

and I appreciate that.

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OPTIMIZATION OF NATURAL GAS DEHYDRATION iv

List of Contents

Abstract.....................................................................................................................................ii

Acknowledgements..................................................................................................................iii

List of Contents........................................................................................................................ iv

List of Tables............................................................................................................................vi

List of Figures........................................................................................................................viii

Abbreviations............................................................................................................................ x

1. Introduction...................................................................................................................... 1

2. Dewatering Technology ................................................................................................... 62.1 Theory of hydrates ........................................................................................................... 6

2.2 Technologies used for dehydration .................................................................................. 9

2.3 Dehydration by absorption.............................................................................................11

2.4 Dehydration by adsorption.............................................................................................18

2.5 Dehydration by permeation ............................................................................................20

2.6 Dehydration by refrigeration..........................................................................................21

3. Makowice Facilities ...................................................................................................... 23

4. Hysys Simulation Package............................................................................................. 26

5. Water Content of Natural Gas...................................................................................... 31

5.1 Water content measurement........................................................................................... 31

5.2 Water content from GPSA diagram ............................................................................... 32

5.3 Water content values obtained from Makowice operation manual .............................. 33

5.4 Water content calculations from empirical equations .................................................... 35

5.5 Water content in natural gas according to Hysys program ............................................37

5.6 Water content results comparison .................................................................................. 40

5.7 Amount of water to remove during dehydration process...............................................42

5.8 Dew point values comparison ........................................................................................44

6. Glycol solutions...............................................................................................................45

6.1 Use of glycol solutions ................................................................................................... 45

6.2 Minimum strong TEG concentration ............................................................................. 47

6.3 TEG circulation in Makowice dehydration facility...................................................... 50

7. Hysys simulations ........................................................................................................... 51

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OPTIMIZATION OF NATURAL GAS DEHYDRATION v

8. Discussion........................................................................................................................ 57

9. Conclusions ..................................................................................................................... 60

References ............................................................................................................................... 62

Tables....................................................................................................................................... 66

Figures.....................................................................................................................................94

Appendices ............................................................................................................................ 117

Appendix A Specification of Aviaterm 6 heating oil...................................................... 117

Appendix B - Water content according to manual [g/Nm3] ..............................................118

Appendix C Water content according to article [g/Nm3] ............................................... 120

Appendix D Water content according to Hysys in g/Nm3.............................................. 121

Appendix E Example of calculation of water content saturating natural gas .................122

Appendix F Real gas law equation use for standard volume calculation........................ 124

Appendix G Amount of TEG necessary to dehydrate gas of given water content ......... 126

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OPTIMIZATION OF NATURAL GAS DEHYDRATION vi

List of Tables

Table 2. 1 Physical Properties of Commercial Glycols (reproduced from Daubert and Danner,

1985)......................................................................................................................................... 66

Table 5. 1 Water contents of gas for given dew points in Makowice dehydration facility

pressure and dew point work-range (Nafta-Gaz, 2004) ........................................................... 67

Table 5. 2 Water content calculation with use of Hysys application (page 1 of 4).................. 68

Table 5. 3 Water content of natural gas after Hysys [gH2O/Sm3]..............................................72

Table 5. 4 Water content of natural gas after Hysys [gH2O/Nm3] ............................................. 72

Table 5. 5 Water content on basis of gas stream flow (after Hysys)........................................ 73

Table 5. 6 Water content comparison between Clapeyron equation based solution and flows

based solution (after Hysys) ..................................................................................................... 73

Table 5. 7 Percent difference of amount of water saturating gas between values obtained from

manual and Hysys package ...................................................................................................... 74

Table 5. 8 Percent difference of amount of water saturating gas between values obtained from

manual and article according to P. Gandhidasan ..................................................................... 75

Table 5. 9 Water amount in dehydrated gas [mgH2O/Sm3] ....................................................... 76

Table 5. 10 Water amount in dehydrated gas [mgH2O/Nm3] .................................................... 77

Table 5. 11 Amount of water in natural gas [mgH2O/Sm3] ....................................................... 78

Table 5. 12 Amount of water in natural gas [mgH2O/Nm3]....................................................... 79

Table 5. 13 Water to remove from natural gas for 10 oC [mgH2O/Sm3] ................................... 80

Table 5. 14 Water to remove from natural gas for 10 oC [mgH2O/Nm3]................................... 81



Table 5. 17 Water to remove from natural gas for 20o

C [mgH2O/Sm3

] ................................... 84






Table 5. 23 Values of dew point temperature for given water content obtained with use of

Hysys package.......................................................................................................................... 90

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OPTIMIZATION OF NATURAL GAS DEHYDRATION vii

Table 5. 24 Values of dew point for given water content achieved from the Makowice

dehydration facility operation manual...................................................................................... 91

Table 5. 25 Values of dew point temperature for given water content calculated with use of

empirical equations ..................................................................................................................92

Table 6. 1 Minimum strong TEG concentration required in given conditions 93

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OPTIMIZATION OF NATURAL GAS DEHYDRATION viii

List of Figures

Figure 2. 1 Hydrate Crystal Unit Structure I (McMullan and Jeffrey, 1965 figure reproduced

from the Journal of Chemical Physics by the American Institute of Physics).........................94

Figure 2. 2 Hydrate Crystal Unit Structure II. Small and large cavities (Behlar et al., 1994) . 94

Figure 2. 3 Hydrate Crystal Unit Structure sH (Figure reproduced from the Journal of

Chemical Physics) .................................................................................................................... 95

Figure 2. 4 Dehydration Unit Using Triethylene Glycol (ATG, 1988).................................... 96

Figure 2. 5 Simplified flow diagram for a glycol dehydration unit (reprinted from GPSA

Engineering Data Book, 11th edition)....................................................................................... 96

Figure 2. 6 Stahl or gas-stripping column (Manning and Thompson, 1991) ........................... 97

Figure 2. 7 Dehydration by adsorption (reprinted from Alexandre Rojey et al, Natural Gas

Production Processing Transport, 1997) .................................................................................. 97

Figure 3. 1 Location of Makowice Dehydration Facility (reprinted from Autoatlas Polski,

2003, reproduced)..................................................................................................................... 98

Figure 3. 2 Water content of imported gas with water content limit under 3900 kPa (ROP,

2005)......................................................................................................................................... 99

Figure 3. 3 Pipeline system with the destinations of gas flow (ROP, 2005).......................... 100

Figure 3. 4 Flowsheet of Makowice dehydration facility (Hysys, 2005)............................. 101

Figure 3. 5 Work range of Makowice dehydration facility (Nafta-Gas, 2004).................... 102

Figure 5. 1 Water content of natural gas (ATG, 1990) .......................................................... 103

Figure 5. 2Correction to water content in presence of brine (Katz et al, 1959)..................... 104

Figure 5. 3Water content of hydrocarbon gas after GPSA .................................................... 105

Figure 5. 4 Water content of natural gas at 10 oC according to manual................................. 106

Figure 5. 5 Water content of natural gas at 15o

C according to manual................................. 106

Figure 5. 6 Water content of natural gas at 20 oC according to manual................................. 107

Figure 5. 7 Water content of natural gas at 10 oC according to equations ............................. 107



Figure 5. 10 Flow sheet of gas saturation system with Hysys ............................................... 109

Figure 5. 11 Water content of natural gas at 10 oC according to Hysys ................................ 109

Figure 5. 12 Water content of natural gas at 15

o

C according to Hysys ................................ 110Figure 5. 13 Water content of natural gas at 20 oC according to Hysys ................................ 110

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OPTIMIZATION OF NATURAL GAS DEHYDRATION ix

Figure 5. 14 Water content comparison at 10 oC ...................................................................111



Figure 5. 17 Dew point comparison....................................................................................... 113

Figure 6. 1 Dew point of a gas in contact with solutions of triethylene glycol after ATG.... 114

Figure 6. 2 Minimum strong TEG concentration for dew point temperatures range between

-18oC and -29oC ..................................................................................................................... 115

Figure 6. 3 Minimum strong TEG concentration for dew point temperatures range between

-18oC and -19oC ..................................................................................................................... 116

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OPTIMIZATION OF NATURAL GAS DEHYDRATION x

Abbreviations

ATG - Association Technique de lIndustrie du Gaz en France

BMP - Best Management Practice

CH4 - Methane

CO2 - Carbon Dioxide

DEG - Diethylene Glycol

EG - Ethylene Glycol

EPA - Environmental Protection Agency

GPSA - Gas Processors Suppliers Assn

h - hour

H2O - Water

H2S - Hydrogen Sulphide

LNG - Liquefied Natural Gas

LPG - Liquefied Petroleum Gas

LTX - Low-Temperature Extraction

MEG - Monoethylene Glycol

Nm3 - Normal Cubic Meter

NMR - Nuclear Magnetic Resonance

PHA - Process Hazards Analysis

PRO-OP - Process Optimization Review

PROs - Partner Reported Opportunities

sI - Structure I (hydrate structure)

sII - Structure II (hydrate structure)

sH - Structure H (hydrate structure)

Sm3 - Standard Cubic Meter

TEG - Triethylene Glycol

TREG - Tetraethylene Glycol

VLE - Vapor Liquid Equilibrium

Xe - Xenon

yr - Year

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OPTIMIZATION OF NATURAL GAS DEHYDRATION 1

1. Introduction

In this paper the author is studying the possibilities of reducing energy use and triethylen

glycol losses during natural gas dehydration process. This is done on the example of

Mackowice Treatment Facility, Poland.

Gas demand increases in Poland, likewise in other countries. Huge part of gas used in Poland

is imported from Russia, through Ukraine. As it is usually off-spec when it arrives, before

getting to the final receiver it has to be processed in order to meet the required conditions

specified in Polish norms. Therefore every year larger quantities of natural gas need to

undergo different processes (ROP, 2005).

One of the specifications of natural gas is the amount of water in gas for sale specified as dew

point temperature of natural gas. The presence of water raises a number of problems for the

production operations depending on the temperature and pressure prevailing in an installation.

If the natural gas is transported by pipeline, the processing installation must be designed to

meet transport or final specifications.

If the gas is to be transported, the main requirement is to prevent the formation of a liquid

phase. If, during transport, the minimum temperature of the gas is for example 0 oC under

7 MPa pressure (typical conditions appearing in high pressure gas pipelines during the winter

season in temperate climate), the dew point must not exceed this temperature at the same

pressure. However, the pressure generally varies considerably in the pipe, as a result of

pressure drop. To avoid possible liquid-phase formation, one condition frequently imposed isto set the dew point temperature at a value not exceeding the minimum temperature during

transport. Therefore one of the processes in natural gas production, processing and

transportation is natural gas dewatering process (Rojey et al., 1994).

With the increase of amount of gas to be processed and from the other side in order to be able

to endure in the competition between natural gas companies there is a growing necessity of

optimization of processes, dehydration process among them, not only in the stage of designing

and building facilities, but also in the exploitation stage.

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The problem of optimization has been known for many years now and recently becomes more

and more important. The importance of optimization is significant. Optimization of processes

brings savings in materials, energy and labor. Optimization can be seen from the

environmental point of view as a tool for environment conservation. It may encompass safety.

It is also considered from economical viewpoint. And the last reason usually is the standpoint

from which the decisions about granting money for optimization research are made.

Nowadays the competition in petroleum and natural gas market is extremely tough and

therefore every corporation in order to compete with others has to minimize the costs

maximizing profits, which is a primary purpose of senior management. Summing up the

crucial part of managers job is to make decisions around capital allocation that will improve

the performance of the corporation.

The oil and gas production, gas processing and petroleum refining industries are faced with

the need to optimize the design of processes and achieve more reliable and stable operations,

The process industries must identify optimum designs quickly with minimum risk of rework

while they remain competitive and maximize the business performance. Process engineers are

challenged with making timely business decisions while meeting the business objectives of

designing and operating efficient, safer and profitable process plants (Aspen Tech, 2004).

Optimization of processes is necessary. As mentioned, nowadays on every stage of projecting,

building and exploiting of any facility optimization has a big part. As presented by Aspen

Tech (2004) there are different approaches towards optimization and the model chosen

depends on the base of optimization. The most powerful technology that enables managersand engineers link critical business objectives to process design is process modeling. The

major business benefits of process modeling include (Aspen Tech, 2004):

a) usage of what-if scenarios and sensitivity analyses to identify the optimal design

based on operating and business targets.

b) ensuring that process equipment is properly specified to deliver desired product

throughput and specifications.

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c) evaluation of the effect of feed changes, upsets, and equipment downtime on process

safety, reliability, and profitability.

d) monitoring of equipment performance against expectations.

e) assessment of equipment deficiencies such as heat exchanger fouling and column

flooding by evaluating the equipment employed in different services or evaluating the

consequences of a design basis change.

According to Pontiff (2005) a typical example of widely used optimization method is called

Process Optimization Review (PRO-OP). PRO-OP is a systematic approach used in

production operations to identify opportunities to increase profitability while reducing

greenhouse gases. It is a systematic approach to assess processes at new and existing facilities

with an emphasis on energy efficiency, natural resource conservation and waste minimization.

This methodology can be used in conjunction with a Process Hazards Analysis (PHA) for new

facilities and prior to modification of en existing facility.

Justifying and obtaining approval of optimization projects from management often requires

that the projects are cost effective and have a net increase in profits. The PRO-OP technique

divides the oil and gas business into phases: drilling, completion/stimulation, production, and

workover operations. Unlike other optimization techniques, where the focus is typically on

like devices across a whole operation, the PRO-OP technique is a systematic approach

whereby processes and components (separators, heater treaters, compressors, venting/flaring

practices) are evaluated for cost effective natural gas reduction opportunities from the start of

the process to the end. This PRO-OP technique gives the user a structure to the process of

optimization (Pontiff, 2005).

There are many technologies and methods to reduce vent gas emissions that are readily

available to operators. The United States Environmental Protection Agency's (EPA) Natural

Gas STAR Program supplies valuable optimization tools and resources to guide the oil and

gas industry. The Natural Gas STAR Program is a flexible and voluntary program focused on

helping the oil and gas industry to voluntarily and cost-effectively reduce methane emissions,

a potent greenhouse gas. The Natural Gas STAR Program promotes the use of these emission

reduction technologies and practices through the programs Best Management Practices

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(BMPs) and Partner Reported Opportunities (PROs) and in-depth Lessons Learned documents

(Pontiff, 2005; US EPA, 2005).

The PRO-OP approach is analogous to a Process Hazards Analysis (PHA) review. In a PHA

review of an oil and gas production facility, the components and processes of the facility are

evaluated for identifiable hazards. These hazards are then mitigated through elimination,

controls, or other safe guards. The PRO-OP process employs the same thought

process. During the PRO-OP review, each component and process in the facility flow scheme

is evaluated for vent gas (i.e., methane) emission reduction opportunities (Pontiff, 2005; US

EPA, 2005).

Once the optimization opportunities are identified, the reviewer determines the mitigation

techniques that can be used and then determines whether the mitigation can be implemented

cost effectively. The reviewer should ask such questions as, Can I cost-effectively eliminate

the source, or capture for sales, or destroy (e.g., burn in a flare) the vent gas emissions?" Then

the reviewer can perform a cost analysis to determine the effectiveness and profitability of

optimization, which in this example is done through reducing emissions (Pontiff, 2005; US

EPA, 2005).

Mackowice Dehydration Facility was opened on 21st January 2005. The bilding was begun in

April 2004. The necessity of building this facility was caused by high water content in the

imported gas and hydrate problems deriving from it. The imported gas hardly ever met dew

point specifications required by Polish norms.

The author is trying to solve the problem of setting the arrangement of equipment used in gasdehydration facility based on glycol solution in such a way that brings most profits and

minimizes the loss in energy and glycol solvent. In order to do so he compares the data

provided in operating manuals of natural gas dewatering facility Makowice, Poland with

analytical equation-based solution and numerical calculation made with use of petroleum

engineering program Hysys. Having the required results he is comparing them looking for the

possibilities of energy and solvent savings. The author is also taking a general look at

different economical aspects in the final part of this thesis.

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The author shows his approach towards creating an optimization strategy for natural gas

dehydration unit Makowice, Poland. The optimization is to made on the basis of energy

saving and glycol absorbent waste. Operating manuals of devices, and charts showing parts of

dehydration facility Makowice were used for creating a simulation of dewatering process

under petroleum engineering program Hysys. Analytical and mathematical calculations were

made and compared with simulation outcome and experimental data for achieving reliable

results.

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2. Dewatering Technology

2.1 Theory of hydrates

Good reviews on hydrate theory were provided by Sloan, 1997, Rojey et al., 1994, Rosman

1973, Gandhidasan, 2002, Carroll, 2003.

Since the beginning of the century the production of natural gas has encountered difficulties

connected with the plugging of piping by the deposition of crystals, first thought to be ice

crystals. These crystals are in fact hydrates of natural gas. In the mid-1930s Hammerschmidt

studied the 1927 hydrate review of Schroeder, to determine that natural gas hydrates were

blocking gas transmission lines frequently at temperatures above the ice point. This discovery

was pivotal in causing a more pragmatic interest in the gas hydrates, and shortly thereafter led

to the regulation of the water content in natural gas pipelines. This led to limitation of

appearance of hydrates which are inclusion compounds which result from the combination of

water with some of the components of natural gas and primarily methane (Rojey et al.,

1994,Gandhidasan, 2002, Carrll, 2003).

In the presence of light gas, water molecules can form a regular crystalline structure

containing cavities, in which gas molecules are trapped. Owing to this cage structure, the

hydrates belong to the category of inclusion compounds called clathrates. The crystal lattice is

due to hydrogen bonding between water molecules. It is stabilized by gas molecules, which

are themselves held in the cavities by van der Waals forces (Sloan, 1997).

Only molecules having a certain range of diameters can form inclusions. This is because the

diameter of the molecule must be smaller than that of the cavity (or close to it) for the

molecule to enter the cavity, and sufficiently large for the crystal lattice to be stable (Sloan,

1997; Rojey et al., 1994).

In the late 1940a and early 1950s von Stackelberg and co-workers summarized two decades

of X-ray hydrate crystal diffraction experiments at the University of Bonn. The interpretation

of these early diffraction experiments by von Stackelberg and co-workers, Claussen, and

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Pauling and Marsh led to determination of two hydrate structures (sI and sII). Within the last

decade structure H (sH), a third hydrate with a unit cell was discovered by Ripmeester (Sloan,

1997).

In these structures, the water molecules form polyhedra. The pentagonal dodecahedron,

designed by the notation 512, is a basic building block of hydrate structures. It is not possible

to fill space entirely with dodecahedra. Because of to this restriction dodecahedra are

necessarily associated with other types of polyhedron to form the structure of the hydrates

(Sloan,1997; Rojey et al., 1994).

Structure I is composed of two small cavities formed by a dodecahedron and six large cavities

formed by a tetradecahedron with twelve pentagonal faces and two hexagonal faces

referenced as 51262 (Figure 2.1).

Structure II is composed of sixteen small cavities (512) and eight large cavities, formed by a

hexadecahedron with twelve pentagonal faces and four hexagonal faces, referenced as 5 1264

(Figure 2.2).

Each of these polyhedra forms a cavity which can contain a molecule of natural gas

components with which it forms a hydrate. Methane fits into the small cavities (512) of

structures I and II, and in the large cavities (51262) of structure I. Nitrogen, propane and

isobutene form structure-II hydrates (Sloan, 1997).

In the pure state, methane, ethane, carbon dioxide and hydrogen sulfide form structure-I

hydrates. However, since propane and isobutene molecules can enter only the large cavities ofstructure II, a natural gas containing propane and isobutane generally forms structure-II

hydrates. Normal butane does not form hydrates as a pure component. Hydrate formation can

occur when normal butane is mixed with other components (Rojey et al., 1994; Sloan, 1997).

The structure H was determined through diffraction and NMR studies. In this structure, the

512 dodecahedra coexist with 435663 dodecahedra as well as 51268 polyhedra, with twelve

pentagonal faces and eight hexagonal faces, forming large cavities. The small cavities are

stabilized by molecules like Xe, H2S and CH4, and the large cavities by hydrocarbons with

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much higher molecular weights such as adamantine and methylcyclohexane (Figure 2.3)

(Sloan, 1997).

The role that structure-H hydrates may play in natural gas production is still unclear.

However, it has been proven that hydrocarbon molecules commonly found in condensates or

oils, together with methane, can form this new hydrate structure, under pressure and

temperature conditions easily encountered in production and transport facilities (Sloan 1997).

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2.2 Technologies used for dehydration

It is necessary to prevent the condensation of liquid water and hydrocarbons to ensure trouble-

free operation of a natural gas transmission system. Apart from the risk of hydrate formation,

the liquids can reduce the volumetric capacity of the system and interfere with the operation

of pressure regulators and filters. Condensed liquids accumulated in pipelines, which caused

an increase in operating pressures and potential damage to equipment due to liquid carryover.

Many transmission companies impose restrictions on the quality of natural gas acceptable for

transporting, such as water and hydrocarbon dew point limits, in order to reduce operational

problems (Rosman, 1973, Gandhidasan, 2002).

To prevent pipe plugging, production and transport installations must be protected from the

risks of hydrate formation. One way to achieve this is to dry the natural gas. If this is not

feasible, temperature and pressure conditions must be created to prevent formation of

hydrates. Operating outside the thermodynamic conditions of hydrate formation can be

achieved either by raising temperature at a given pressure, or by lowering the pressure at a

given temperature. In both instances inhibitor must be introduced. They are generally selected

from solvents miscible in the aqueous phase, which, by altering the fugacity of the water,

lower the hydrate formation temperature (Rosman, 1973, Gandhidasan, 2002).

Dehydration of natural gas is the removal of water that is associated with natural gas in vapor

form. It is necessary to prevent the corrosion and erosion problems in pipelines and equipment

particularly when CO2 and H2S are present in the gas. Water is removed from the gas to meet

water dew point requirements of a sales pipeline condition. For these reasons one specifies

upper limits for both the water and hydrocarbon dew points of natural gas. Onshore thenatural gas conditioning process employs a dehydration process for control of the water dew

point, and a refrigeration plant is used for control of the hydrocarbon dew point (Carroll,

2003).

The water present in natural gas may, depending on the temperature and pressure prevailing in

an installation, condense and cause the formation of hydrates, solidify, or favor corrosion if

the gas contains acid components. To avoid such situations, natural gas must be dehydrated.

Four types of processes are used (Rojey at al., 1997):

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a) absorption

b) adsorption

c) gas permeation

d) refrigeration

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2.3 Dehydration by absorption

The most common method for dehydration in the natural gas industry is the use of a liquid

desiccant contactor-regeneration process. In this process, the wet gas is contacted with a lean

solvent (containing only a small amount of water). The water in the gas is absorbed in the lean

solvent, producing a rich solvent stream (one containing more water) and a dry gas

(Campbell, 1992; ATG, 1990; Arnold and Steward, 1989; ATG, 1988; Kumar, 1987;,

Maddox and Erbar, 1982; Ikoku, 1980; C.R. Sivalls,1976; Tannehill at al, 1994; Trent, R.E.,

2001).

In case of absorption based natural gas dehydration processes the gas is dried by

countercurrent scrubbing with a solvent that has a strong affinity for water. The solvent is

usually a glycol, although other liquid desiccants are met which are calcium chloride, lithium

chloride, zinc chloride, etc. The dehydrated gas leaves at the top of the column. The glycol

leaving the bottom is regenerated by distillation and recycled (Carroll, 2002; Rojey et al.,

1994).

Several liquids possess the ability to absorb water from a gas stream. Few liquids, however,

meet the criteria for a suitable commercial application. A suitable solvent should have the

following properties (Carroll, 2002; Rojey et al., 1994, Campbell, 1992):

a) strong affinity to water (the absorbing liquid should be highly hygroscopic)

b) low cost

c) noncorrosive to the selected metallurgy of the hydrocarbon equipment, especially the

reboiler vapor space, the stripping column of the regenerator, and the bottom of thecontactor

d) low affinity for hydrocarbons and acid gases

e) thermal stability, particularly in the high temperature ranges found in the reboiler

f) easy regeneration to higher concentration for reuse, usually by the application of heat,

which drives off the absorbed water

g) low viscosity

h)

low vapor pressure at the contact temperature to reduce the amount of solvent losses

due to vaporization

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i) low solubility in hydrocarbons (low solubility in the solvent minimizes the loss of

desired product and reduces hydrocarbon emissions)

j) low tendency to foam and emulsify, nor to chemical reactions with any of the natural

gas constituents, including carbon dioxide and sulfur compounds

The organic compounds known as glycols approximate the properties that meet the

commercial application criteria. Glycols have a higher boiling point than water and a low

vapor pressure. Glycols will, however, decompose at elevated temperatures. The

decomposition temperature limits the maximum temperature at which the process operates,

particularly in the reboiler. Several glycols have been found suitable for commercial

application (Rejoy, 1997, Carroll, 2002).

The most common glycols for dehydration applications are (Rojey et al., 1994):

a) Monoethylene glycol (MEG) which is commonly known as simply ethylene glycol

(EG)

b) diethylene glycol (DEG)

c) triethylene glycol (TEG)

d) tetraethylene glycol (TREG)

Table 2.1 lists the main physical properties of commercial glycols. They can be obtained in

the pure state by fractionation by vacuum distillation.

The heaviest glycols are most hygroscopic. Triethylene glycol (TEG) offers the best

cost/benefit compromise, and is the most widely used. It exhibits most of the desirablecharacteristics listed earlier and has other advantages compared to other glycols (Rojey et al.,

1994; Carroll, 2002).

By comparison, DEG is marginally lower in cost than TEG, however because DEG has a

larger vapor pressure, it has larger losses. TEG has less affinity to water and thus has less dew

point depression. Tetraethylene glycol is higher in cost and is more viscous than TEG. High

viscosity translates into higher pumping costs. On the other hand TREG has a lower vapor

pressure, which reduces losses (Gandhidasan, 2003; Rojey et al., 1994)

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Before undergoing the actual dehydration process any free liquids in the natural gas stream

are removed. A separator should be included upstream of the contactor to separate any

hydrocarbon liquids and free water. The separator could be a two-phase or three-phase

separator depending on the amount of free water expected. The inlet separator can be free

standing with interconnecting piping to the contactor or it can be an integral part of the

contactor, usually at the base of the contactor with a chimney tray between the contactor

bottom and the separator vessel. The separator should be equipped with a high-efficiency wire

mesh mist extractor in the top part to remove any liquid entrainment and particulates from the

gas stream before entering the absorber section. Integral separators are usually outfitted with a

heating coil to prevent water from freezing. Hot solvent from the accumulator is circulated

through this heating coil to provide the required heat. When the stream is devoid of free

liquids and mist the actual dehydration process starts (Rojey et al., 1994; Carroll, 2002;

Gandhidasan, 2003).

Figures 2.4 and 2.5 show the flow schemes of a typical glycol units. The descriptions of these

figures are provided by John Carroll, 2003 and Alexandre Rojey et al., 1994. Basically, the

liquid desiccant process is a two-step process. In the first step, the water is absorbed from the

gas in the staged tower. The solvent is regenerated in a second column. The solvent is then

returned to the first column to remove water from more feed gas. The absorption step is

carried out in a plate or packed column. The actual stages could be either trays like bubble

caps, valve trays, or sieve trays, or a suitable packing material. The number of plates is

usually between 6 and 8. For small diameters, packings are generally used, while the larger

columns are equipped with the bubble-cap or valve trays. For very large diameters, the use of

structured packing is currently spreading, finding more acceptance in glycol contactors(Carroll, 2003; Rojey et al., 1994).

The temperature at which the absorption step is carried is usually limited to 38 oC to avoid

excessive glycol losses. A lower temperature helps to reduce the losses as well as the water

content in the processed gas. However, due to the higher viscosity of the glycol, temperature

of about 10 oC is considered as a lower limit (Carroll, 2003; Rojey et al., 1994).

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The contactor (also called an absorber) is the workhorse of the dehydration unit. The gas and

liquid are mixed in the contactor, and the actual water removal takes place there. The

contactor is a typical absorber tower properly sized for the process objective(Carroll, 2003;

Rojey et al., 1994).

The feed gas flow rate is the most significant factor in determining the diameter of the

contactor. The outlet gas water content specification is the key to determining the contactor

height, although other factors contribute as well. The contactor consists of several equilibrium

stages, enough to ensure mass transfer from the gas phase to the liquid so that the outlet gas is

at the desired water specification (Carroll, 2003; Rojey et al., 1994).

The flow of streams is countercurrent. Feed gas enters the bottom of the contactor and flows

upward. Lean solvent enters the top of the contactor and flows downward. The solvent

absorbs water as it travels downward through the column and the gas transfers the water to the

solvent as it travels upward. The contactor pressure is set by the feed gas pressure, which is

normally in the range of 4000 to 8500 kPa. The contactor is essentially isothermal (the

temperature profile is essentially uniform throughout the contactor) (Carroll, 2003; Rojey et

al., 1994).

After the absorption step, the glycol solution is sent to a three-phase separator in which the

stripped hydrocarbon liquids and the dissolved gas are separated, followed by a cartridge filter

to trap solid particles, and finally an activated-charcoal filter to retain the chemical impurities

(Carroll, 2003; Rojey et al., 1994).

In some cases this process is divided in parts (Figure 2.5). The rich glycol is withdrawn fromthe bottom of the contactor, usually on level control. Typically, the lean glycol is preheated,

often by passing it through tubes in the overhead condenser at the top of the still column.

Then it is flashed at low pressure in a flash tank, where most of the volatile components

(entrained and soluble) are vaporized. Flash tank pressures are typically in the range of

300 kPa to 700 kPa (Carroll, 2003; Rojey et al., 1994).

The glycol leaves the flash tank, again usually on level control, then passes through a filter.

Then the rich glycol enters the lean-rich heat exchanger, whose basic purpose is to conserve

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energy. In the lean-rich exchanger, hot, lean glycol from regeneration is cooled with rich

glycol from the contactor. The lean glycol entering the contactor should be cool, and rich

glycol to regeneration should be warm (Carroll, 2003; Rojey et al., 1994).

The solvent is regenerated by reboiling. The still column, usually filled with packing, is

cooled at the top by a coil in which circulates the glycol solution. The reflux thus generated

helps to reduce glycol losses. A basic regeneration unit consists of a combination of a fired

boiler, located at the lower section of a horizontal vessel with a vapor space above the tube

bundle, a distillation column (still column) connected vertically to the vapor space of the

reboiler vessel, and a surge tank located below the reboiler. Also included in the regeneration

unit is a condensing coil added to the top of a still column to provide reflux to improve

solvent/water separation. This coil often performs the dual purpose of preheating the rich

glycol ahead of the flash tank (Carroll, 2003; Rojey et al., 1994).

The size of the regenerator is determined by a balance between the solvent circulation rate,

the amount of water vapor in the gas stream and the reboiler temperature. The standard TEG

dehydration unit operates effectively at the reboiler temperature around 175 oC, or about

20 oC below the decomposition temperature of TEG. Trays are sometimes used in very large

units (Carroll, 2003; Rojey et al., 1994, Gandhidasan, 2003).

In the regenerator, separation of water from glycol takes place by fractionation. Water and

glycol have widely varying boiling points (100 oC for water, 288 oC for TEG). Furthermore

the two substances can be easily separated by fractional distillation. This is accomplished in

the still column mounted directly on the top of the reconcentration vessel (Rojey et al., 1994).

Within the column, water-rich vapor rises in intimate contact with descending glycol-rich

liquid. Between the two phases, a continuous exchange of material and heat takes place. The

temperature difference causes the glycol vapor (heavy component) to condense and liquid

water (light component) to vaporize. At the top of the column the vapor is virtually pure water

whereas there is very little water in the glycol in the bottom. A small portion of the vapor

mixture(mainly water) at the top condenses at the overhead condenser to provide sufficient

reflux that will aid in the process of fractionation (Carroll, 2003; Rojey et al., 1994).

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Stripping gas is used to increase the lean glycol concentrations. This is deeply dried natural

gas taken usually from the main stream of dehydrated gas. With the use of stripping gas the

glycol solvents can have the concentrations up to 99,6 %. The stripping gas is sparged directly

into the reboiler. The typical example of a TEG regenerator is Stahl column, called also a gas-

striping column (Figure 2.6) (Carroll, 2003; Rojey et al., 1994, Manning and Thompson,

1991).

The main purpose of the still column is to effect final separation between the absorbed water

and the absorbing TEG, to vent the separated water to the atmosphere, and to recover the

glycol vaporized by the reboiler. The glycol-rich liquid, now becoming lean glycol, leaves the

bottom of the packed still column and enters the reboiler vessel. Heat is applied in the reboiler

to raise the temperature and cause partial vaporization. In a normal TEG dehydration unit, this

temperature level has been found to cause no noticeable thermal decomposition of the TEG

(Carroll, 2003; Rojey et al., 1994).

The hot, lean glycol leaves the reboiler vessel and overflows by gravity to the surge tank, a

vessel normally located below the reboiler vessel. The hot lean glycol passes to the lean rich

exchanger, where it is cooled. Ultimately it is returned to the contactor and the cycle is

complete (Carroll, 2003; Rojey et al., 1994).

The TEG natural gas dehydration unit operates, as noted before, at relatively high pressure on

the contactor side and low pressure on the regeneration side. The high-pressure side consists

of the glycol contactor and the inlet separator.

Intensive dehydration of natural gas demands high purity of the recycled solvent. This purityis improved by lowering the pressure and raising the temperature during the regeneration step.

Thus the low-pressure side consists of the regenerator, the flash tank and associated

equipment.

In addition to water, the solvent selectively absorbs H2S and aromatic compounds such as

benzene, toluene, ethylbenzene and xylenes present in the natural gas. These components are

removed with the water on completion of the regeneration step. They are frequently released

directly to the atmosphere, but, as they are toxic, this incurs risks for the operating personnel .

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The installation of a condenser improves the situation, but is generally not sufficient to

eliminate the problem of aromatics releases completely. Complete elimination requires the

incineration of the nonocondensable flare gas in the reboiler fire tube. Difficulties in burning

noncondensable vapors in low-pressure burners were reported (Carroll, 2003; Rojey et al.,

1994).

To prevent any air from entering, the pressure must be kept slightly above atmospheric. The

regeneration temperature must also remain below an acceptable limit for glycol

decomposition. This temperature is 177 oC for diethylene glycol, 204oC for thiethylene glycol

and 224 oC for tetraethylene glycol (Rojey et al., 1994).

These regeneration conditions lead to a water content of about 35 g/1000 Sm3 in the processed

gas. By increasing solvent circulation, the purity of the processed gas can be further improved

to reach water contents in the range of 20 g / 1000 Sm3. To drop to even lower contents in the

range of a few parts per million the purity of the recycled solvent must be even further

increased. Two techniques are available for this (Gandhidasan, 2003; Rojey et al., 1994):

a) the already dehydrated gas is sent to the reboiler, to lower the water partial pressure by

stripping with natural gas. As an example the injection of 45 Sm3 of gas per m3 of

triethylene glycol helps to purify the solvent to 99,0 or 99,4 %, according to whether

the gas is simply injected into the reboiler or introduced into an additional stripping

section after the reboiler

b) a hydrocarbon (toluene, octane) is injected into the reboiler, forming a heteroazeotrope

with water. This heteroazeotrope rises to the top of the column and, after condensation

of the vapor phase, the hydrocarbon is separated by simple settling and recycled. Thetriethylene glycol is thus obtained with a purity that may be higher than 99,9 %,

without any consumption of carrier gas. This method is called Drizo process

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2.4 Dehydration by adsorption

Separation processes by adsorption uses a solid phase with large surface area, which

selectively retains the components to be separated. The adsorbents are generally characterized

by a microporous structure which affords a very large specific surface (Campbell, 1992; ATG,

1990; Arnold and Stewart, 1989; ATG, 1988; Kumar, 1987; Maddox and Erbar, 1982; Sivalls,

C.R., 1976; Tannehill, C.C., 1994, Trent, R.E., 2001).

Adsorption processes are generally applied when a high purity is required for the processed

gas. Adsorbents are naturally unsuitable for continuous circulation, owing to mechanical

problems and also due to the risks of attrition (erosion of adsorbent particles due to friction

and collisions during movement). This is why adsorbents are normally used in fixed beds with

periodic sequencing (Carroll, 2003; Rojey et al., 1994).

The flow scheme of a dehydration operation by adsorption in a fixed bed is shown in

Figure 2.7. The process is conducted alternately and periodically, with each bed going

through successive steps of adsorption and desorption (Rojey et al., 1994).

During the adsorption step, the gas to be processed is sent on the adsorbent bed which

selectively retains the water. When the bed is saturated, hot natural gas is sent to regenerate

the adsorbent (Rojey et al., 1994).

After regeneration and before the adsorption step, the bed must be cooled. This is achieved by

passing through cold natural gas. After heating, the same gas can be used for regeneration. Inthese conditions, four beds are needed in practice, two beds operating simultaneously in

adsorption, one bed in cooling and one bed in regeneration (Carroll, 2003; Rojey et al., 1994).

The desorption step is carried by different methods (Carroll, 2003; Rojey et al., 1994):

a) lowering the pressure, sometimes even under vacuum

b)

sweeping by an inert natural gas to lower the partial pressure of the component to be

desorbed

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c) sweeping by a displacement agent, which, by being adsorbed, allows more effective

desorption than with a simple elution gas

d) heating, in which the temperature rises facilities desorption: in a fixed-bed operation, a

significant variation in temperature between the adsorption and desorption steps is

practical only if the cycle time is relatively long, because of the thermal inertia of the

adsorbent bed.

An adsorbent must have the following properties (Carroll, 2003; Rojey et al., 1994):

a) high adsorption capacity at equilibrium

b) reversible adsorption allowing regeneration of the adsorbent

c) fast adsorption kinetics

d) low pressure drop

e) attrition resistance

f) chemical inertness

g) no significant volume change with temperature and saturation

The most widely used adsorbents today are the following (Carroll, 2003; Rojey et al., 1994):

a) activated alumina a low residual-water content of about 1 ppm vol can be achieved

by using activated alumina. The heavy hydrocarbons are adsorbed but cannot then be

desorbed during regeneration. Therefore if such heavy hydrocarbons are present in the

gas, they have to be removed before the adsorption step

b) silica gel the water content of the gas processed by adsorption on silica gel is about

10 ppm vol. Silica gel is easily regenerated at a temperature between 120 and 200o

C.It adsorbs water from the hydrocarbons, which are then desorbed during regeneration.

It can be used therefore to separate simultaneously the water and the condensate

fraction of the gas processed, provided a number of precautions are observed

c) molecular sieves (zeolites) used for gas processing are silicoaluminates, in which the

crystal structure forms cavities making up a microporous network on a molecular

scale. This structure has cations that play the role of charge compensation. Depending

on the type of zeolite, the size of the access cavities varies

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2.5 Dehydration by permeation

In the process of dehydration by permeation, the dried natural gas is going through a

membrane leaving particles of water and impurities on its surface. Industrial applications of

dehydration by gas permeation are currently very limited. However, many investigations have

demonstrated the potential value of such a process which, in comparison with a glycol

dehydration unit, could prove to be more economical and more compact, which is extremely

important for offshore production (Fournie and Agostini, 1984). These advantages only

appear clearly in the case of single-stage operation without recycle or recompression of the

permeate (Carroll, 2003; Rojey et al., 1994, Deschamps et al. 1981).

For the separation to be effective, the membrane must be very permeable with respect to the

contaminant to be separated, which passes through the membrane driven by pressure

difference, and it must be relatively impermeable to methane. The permeability of methane

must be accepted to avoid an excessively large membrane area nevertheless means a

significant loss of methane in the permeate (Deschamps et al, 1981).

Membrane separation processes require large membrane areas, which are generally expressed

in thousands of square meters. The membrane surface is dependent on the amount of gas

permeating through it. Compact permeation modules with a high membrane area are therefore

needed. The most widely used industrial modules belong to two types (Rojey et al., 1994):

a) modules with plane membranes wound spirally around a collector tube

b) modules with a bundle of hollow fibers

For a gas-permeation unit processing 1107 Nm3/d of gas at 7 MPa and required to reduce the

water content from 1040 to 170 ppm vol, the loss of gas in the permeate is estimated at 4,2 %

and the membrane area is estimated 1430 m2 (Deschamps et al., 1989).

Under these conditions, to make this process economically viable, it is either necessary to find

an application compatible with the production of gas low pressure, or to reduce the gas loss

substantially, by improving membrane performance (Deschamps et al., 1989).

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2.6 Dehydration by refrigeration

If a natural gas contains a relatively large fraction of hydrocarbons other than methane

(condensate gas or associated gas), it may be necessary to separate of at least part of these

hydrocarbons to avoid the formation of a liquid phase during transport (Rojey et al., 1997).

This separation is usually performed by lowering the temperature with the formation of a

liquid phase. It can also be achieved by adsorption or absorption.

The following liquid fractions can be obtained in succession by lowering the temperature

(Rojey et al., 1994):

a) natural gasoline or condensate which is a light gasoline representing the C5+ fraction

b) the LPG fraction which includes propane and butanes (normal butane and isobutene);

the mixture of natural gasoline and LPG (which also contains ethane) obtained by

lowering the temperature of the natural gas up to the LPG liquefaction point but

without separation between natural gasoline and LPG, is called natural gas liquids

c) by lowering the temperature to about -160 oC, it becomes possible to liquefy the

methane: the natural gas can thus be transported at atmospheric pressure in the form of

liquefied natural gas (LNG), which is mainly formed of methane, and generally

contains ethane; it may include an LPG fraction if this fraction has not been separated

in the liquefaction plant.

In most cases refrigeration is used for cases of a previously dehydrated gas to avoid hydrateformation during refrigeration. Examples are: process of liquids recovery by refrigeration,

refrigeration by isenthalpic expansion and expansion through a turbine which is similar to

isenthalpic expansion but much more effective, as the process operates at low temperature

thorough dehydration and carbon dioxide removal is needed to prevent formation of crystals

though (Rojey et al., 1994).

If the gas is not dehydrated before the refrigeration step the injection of an inhibitor is often

the simplest and most economical solution. In this way, refrigeration simultaneously yields a

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condensate and an aqueous phase consisting of the mixture of water and inhibitor(Carroll,

2003; Rojey et al., 1994).

The use of glycol as inhibitor allows relatively easy regeneration by distillation. This

regeneration may, however, become very costly if the water content is high, especially in the

presence of free water. Methanol is also used, but is generally not recycled. Refrigeration in

the presence of methanol helps to control water and heavy-hydrocarbon contents

simultaneously, and the solution of water and methanol is regenerated without a distillation

step (Rojey et al., 1997).

One of main ways of natural gas dehydration through refrigeration is carried through

expansion refrigeration. This process is also known as low-temperature extraction (LTX). It

employs Joule-Thompson expansion (isothermal expansion) to dry the gas and recover

condensate. Joule-Thompson expansion requires large pressure drops. Because of large

pressure drops, LTX is used only when the prime objective is condensate recovery (Manning

and Thompson, 1991). This method is used at Lollsnes, Norway to remove water from natural

gas. Kollsnes is one of the largest systems in the world. Kollsnes receives the gas from Troll

A, the largest gas field in Norway.

Cool gas holds less water than hot gas. Therefore the process of refrigeration removes also

water. The cold temperatures in a refrigeration process result in water removal. In order to

prevent the formation of ice and hydrates, the cold gas is mixed with a polar solvent, usually

ethylene glycol. A typical refrigeration process can easily reduce the water content of a gas

stream down to 1,60*10-5 kg/m3 level (Carroll, 2003).

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3. Makowice Facilities

The gas drying unit Makowice, Poland is located 25 km from the Ukrainian Border, about 10

km from Przemyl (Figure 3.1). It is located in the neighborhood of compressor unit used for

compressing gas imported from Ukraine. It is owned and operated by Regional Department of

Gas Transport (ROP), Tarnow. It is used for drying natural gas flowing from Ukraine. The

gas may be previously compressed in neighboring compressor unit.

The dehydrating facility was built in this location deliberately. The main reasons were

(Stosur, 2005):

a) the possibility of drying two times larger amount of gas under higher pressure thanks

to the neighboring gas compressor unit, in comparison to the drying capability under

lower pressure range

b) possibility of drying not only imported, but also polish gas,

c) closeness to power plant solves the problem of energy delivery

d) pressure loss up to 0,2 MPa acceptable due to proximity of compressor unit.

The facility was opened on 21st January 2005. The building was begun in April 2004. The

necessity of building a dehydration facility was caused by high water content in the imported

gas and hydrate problems deriving from it. The imported gas hardly ever met dew point

specification required by Polish norms (Figure 3.2)

The system of gas pipelines in the region of Makowice dehydration facility is shown onFigure 3.3. The natural gas coming from the direction of Ukrainian border is metered and

compressed in Hermanowice compressor station. Some of the gas is then sent to Strachocin.

Subsequently part of the main gas stream from the direction of Ukrainian border, or whole the

amount of gas imported is carried through dehydration process in Makowice dewatering unit.

After dehydration the gas is sent to Jaroslaw compressor and metering station where the

stream is split and sent to receivers. The internal diameters of gas pipelines are given on the

figure.

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The process of dehydration is led with use of TEG absorption in typical way described in

previous chapter (see Chapter 2.3). The facility contains two independent drying units. The

gas coming from the direction of Ukrainian border is split equally between them and,

depending on the strategy chosen, there may be a third gas stream led directly to the transport

pipelines. The dehydration process scheme for Makowice dehydration facility is shown on

Figure 3.4.

The first step of dehydration process is removing any free liquids from the natural gas stream.

Separators are placed upstream of the absorption columns. The separators are free standing,

vertical two-phase separators. The internal diameter is 1500 mm. They are equipped with a

high-efficiency wire mesh mist extractor to remove any free liquids and mist.

After separating the free water the gas stream is directed through an oil propelled heater to

absorbtion column where the actual dehydration takes place.The oil used for heater propelling

is Aviaterm 6 (see Appendix A). The heater should keep the gas temperature between 10 oC

and 38oC depending on chosen strategy. The gas enters the bottom part of the absorber and

flowing upward meets countercurrent flow of lean TEG stream. The column is filled with

Mellapak structured packing provided by Sulzer company.

The pressure and temperature range for the dehydration facility is suggested by the Nafta-Gaz

Company the designer of Makowice facility. The pressure of gas can be in the range from

2700 kPa to 4000 kPa for gas coming directly from Ukraine, and in the range from 4700 kPa

to 5500 kPa for gas going through compressor unit. Under different pressures, natural gas of

different range of temperatures can be dryed (Figure 3.5). Depending on the selected pressure

75 000 [Nm3

/h] to 280 000 [Nm3

/h] per one contactor can be dehydrated.

After leaving the absorber the natural gas stream goes through heat exchanger cooling down

the TEG stream going into the dehydration column. Finally about 20 [Nm3/h] of the dry gas is

directed to glycol regenerator as stripping gas and the remaining part, as the sales gas, flows

to system pipelines. The stripping gas is heated to the temperature of 104 oC and

depressurized to the regenerator pressure.

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The rich glycol leaves the contactor under the contactor pressure (depending of gas inlet

pressure) and goes through valve decreasing the pressure to 400 kPa, which allows TEG to

lose most of the entrained and soluble volatile components while in the flash tank. Then the

TEG stream flows through a heat exchanger in which it is heated before getting to the

regeneration column. In the regenerator it is further heated to the temperature of 180 oC to

200 oC. The separation of water from TEG takes place by fractional distillation. The still

column is filled with packing and cooled at the top by a coil in which circulates the glycol

solution (condenser part). This helps to reduce the glycol losses. A fired boiler and surge tank

are located at the lower section of the vessel. Aforementioned stripping gas is put in the upper

part of the column in order to regenerate the glycol solution to concentration of 99,5 % TEG

mole fraction.

After leaving the regenerator, TEG stream is directed through heat exchanger where it warms

up rich TEG flowing towards the regenerator. Then the lean TEG is mixed with TEG makeup

stream in order to compensate the glycol losses. Subsequently it goes through a pump where

the pressure is increased in order to surpass the pressure in absorber tower. After compression

the lean TEG stream goes through a heat exchanger where it is cooled down by the dry gas

going out of absorber.

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4. Hysys Simulation Package

Aspen Hysys 3.2 is a process modeling tool for steady state simulation, design, performance

monitoring, optimization and business planning for oil and gas production, gas processing and

petroleum refining industries. The program is built upon proven technologies, with more than

25 years experience supplying process simulation tools to the oil, gas and refining industries.

It proves an interactive process modeling solution that enables engineers to create steady state

models of plant design, performance monitoring, troubleshooting, operational improvement,

business planning and asset management. Hysys helps process industries improve

productivity and profitability throughout the plant lifecycle. The powerful simulation and

analysis tools, real-time applications and the integrated approach to the engineering solutions

enable the user to improve designs, optimize production and enhance decision-making (Aspen

Tech, 2004).

Hysys offers a high degree of flexibility because there are multiple ways to accomplish

specific tasks. This flexibility combined with consistent and logical approach to how these

capabilities are delivered makes Hysys a versatile process simulation tool (Aspen Tech,

2004).

Another Hysys feature is that modular operations are combined with non-sequential solution

algorithm, so not only is information processed as it is supplied, but the results of any

calculation are automatically produced throughout the flowsheet, both forwards and

backwards. The modular structure of the operation means they can be calculated in either

direction, using information in an outlet stream to calculate inlet conditions (Aspen Tech,2004).

In Hysys, all necessary information pertaining to pure component flash and physical property

calculations is contained within the Fluid Package, therefore choosing the right Fluid Package

for given compounds is substantial. For the given composition of natural gas flowing through

Makowice dehydration unit different Fluid Packages were checked, but finally the Peng-

Robinson equation of state was chosen, as an ideal model for VLE calculations as well as

calculating liquid densities for hydrocarbon systems. In the used property package several

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enhancements to the original Peng-Robinson model were made by the creators of Hysys

program in order to extend the range of applicability and to improve its predictions in some

non-ideal systems. The results achieved with use of Peng-Robinson equation of state were

found to be most similar to empirical calculations of all used Fluid Packages. The values were

also compared to analytical results and only insignificantly differed (Aspen Tech, 2003).

Once the Fluid Package for given compounds was chosen, the author prepared a detailed

scheme of Makowice dehydration facility along with surrounding pipelines in order to be

able to simulate dehydration and glycol regeneration processes. The process was

reconstructed in as much detail as it was possible (Operating Manual of Makowice

Dehydration Facility, 2004). All the known dimensions were inserted, likewise gas and glycol

temperatures and pressures. The author tried to avoid using simplified and non-physical units

but failed by little as balance units for stripping gas getting into regenerators had to be used

(Figure 3.4). This does not yet influence the simulation results as the amount of energy

necessary to heat up the stripping gas stream is known.

Hysys offers an assortment of utilities which can be attached to process streams and unit

operations. These tools interact with the process and provide additional information. The unit

operations are used to assemble flow sheets. By connecting the proper unit operations and

streams the user can model a wide variety of oil, gas, petrochemical and chemical processes

(Aspen Tech, 2004).

Included in the available operations are those which are governed by thermodynamics and

mass/energy balances, such as heat exchangers, separators, compressor, and the logical

operations like adjust, set, and recycle (Aspen Tech, 2004).

All unit operations and utilities are connected by material and energy streams. Multiple

properties pages are connected with every streams. Examples are conditions and composition

pages. The properties pages display the property correlations of the inlet and outlet streams of

the unit operations (Aspen Tech, 2004).

Material streams are used to simulate the material traveling in and out of the simulation

boundaries and passing between unit operations. For the material stream the user has to define

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their main properties and composition so Hysys can solve the stream. The parameters

necessary are the temperature, pressure, flow based for example on molar flow, and

composition (Aspen Tech, 2003).

Energy streams are used to simulate the energy traveling in and out of the simulation

boundaries and passing between unit operations. The energy stream property view contains of

fields allowing user to define stream parameters, view objects to which the stream is attached

and specify dynamic information. The main parameter for energy streams is heat flow (Aspen

Tech, 2003).

In the next part of this chapter units used for building Makowice dehydration facility will be

briefly described. The sequence in which the description are provided reflects the sequence of

TEG solution and natural gas flow.

Separator is an unit with one or multiple feeds, one vapor and one liquid product stream. The

separator divides the vessel contents into its constituent vapor and liquid phases. Every

separator may be provided with some common features like for example the geometry of the

vessel and heat loss model which accounts for the convective and conductive heat transfer

that occurs across the vessel wall. The user can choose between various heater types, which

determine the way in which heat is transferred to the vessel operation (Aspen Tech, 2003).

The heater operations are one-sided heat exchangers. The inlet stream is heated to the required

outlet conditions, and energy stream provides the enthalpy difference between the two

streams. These operations provide information on how much energy is required to heat a

process stream with a utility (Aspen Tech, 2003).

The column is a special type of sub-flow sheet in Hysys. A sub-flow sheet contains equipment

and streams, and exchanges information with the parent flow sheet through the connected

internal and external streams. In general the column appears as multi-feed multi-product unit.

Depending on demands the user can choose one of the predefined columns, or build his own

column along with side equipment such as pump arounds, side strippers and side rectifiers

(Aspen Tech, 2003).

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The column used by the author for separating water from natural gas is a typical absorber

column with two inlet and two exit streams. One of the inlet streams is natural gas saturated

with water in given conditions, the other is lean TEG glycol. The column is designed in such a

way that it should allows to dry the gas to the content of water in gas below 0,050 [g/Nm3]

which determines the dew point of -18 oC under the pressure of 3900 kPa (Aspen Tech,

2003).

A valve is used to decrease the pressure of dry natural gas exiting from the TEG contactor to

the value of 400 kPa. Hysys performs a material and energy balance on the inlet and exit

streams of the valve. The calculations are based on equal material and enthalpy between the

two streams. It is assumed that the valve operation is isenthalpic. The variable specified by the

user is outlet pressure. The rest of variables necessary for solving the valve operation is taken

from the stream flowing out of the contactor (Aspen Tech, 2003).

The separator used for removing vapor part from the rich TEG stream under lower pressure is

similar to the one separating free water from rich gas stream. The entering stream contains

particles of vapor and liquid. On the exit the vapor which is composed of volatile gases and a

small quantity of water is taken out at the top part of the separator, and the liquid part

composed of glycol and water is carried to the heat exchanger (Aspen Tech, 2003).

Heat exchanger performs two-sided energy and material balance calculations. The heat

exchangers calculations are based on energy balances for the hot and cold fluids on the basis

of temperatures of inlet and outlet streams. In the considered case the TEG stream is heated

up to the temperature of approximately 100 oC by lean TEG stream exiting regenerator

(Aspen Tech, 2003).

The regenerator is an example of distillation column with two inlet and two exit streams.

Warm rich glycol flows into the regenerator where it is heated up and losses water. In order to

dry the absorbent to higher concentration stripping gas in the quantity of 20 Nm3/h is injected

into the regenerator. Fully refluxed condenser is built at the top of the column, and a reboiler

in the lower part of the column is added for heating up bottom liquid to the temperature range

of 180 oC to 200 oC (Aspen Tech, 2003).

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The lean glycol flowing out of the regenerator is mixed with stream of additional TEG in the

makeup mixer. The additional TEG is put into the circulation in order to make up glycol

losses due to solution in natural gas and vaporization. The mixer combines the two streams to

produce a single outlet stream, which subsequently gets to the TEG pump. The properties of

both streams entering the mixer are known, just as the amount of lean TEG from the

regenerator and amount of TEG going out of the pump. On this ground Hysys calculates the

amount of glycol necessary to compensate the TEG losses and the properties of absorbent

entering the pump.

The pump operation is used to increase the pressure of an inlet liquid stream. The outlet

pressure, the inlet pressure and the pump efficiency are known. The heat flow necessary for

compression is calculated by Hysys. The dynamics pump operation is similar to the

compressor operation in that it increases the pressure of its inlet stream. The pump operation

assumes that the inlet fluid is incompressible though (Aspen Tech, 2003).

After compression the lean TEG stream goes through another heat exchanger where it gives

some of its energy to dry gas stream flowing out of TEG contactor. The glycol is cooled down

while the dry gas is warmed up.

Before getting to the contactor lean TEG stream goes through recycle operation. The recycle

operation is a theoretical block in process stream. This block gives Hysys the ability to back-

calculate through many operations in a non-sequential manner. All material recycles, where

downstream material mixes with upstream material, require a recycle operation. Hysys uses

the assumed values and solves the flowsheet around the recycle, then it compares the assumed

values in the attached streams to the calculated values in the opposite stream. Based on thedifference between the assumed and calculated values Hysys generates new values to

overwrite the previous assumed values. The calculation process repeats until the calculated

values match the assumed values within specified tolerances. The given values are the amount

of TEG going into the contactor given with relative tolerance, and internal absolute tolerances

(Aspen Tech, 2003).

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5. Water Content of Natural Gas

5.1 Water content measurement

The water content of a natural gas at saturation conditions depends essentially on the

temperature and pressure. Corrections can be made for the sake of the composition of gas and

the salinity of the water. The Figure 5.1 shows the water content at saturation point of

nitrogen-free natural gases as a function of pressure and temperature. Dissolved salts reduce

the partial pressure of water in the vapor phase, and the water content of the gas is

accordingly decreased. The Figure 5.2 helps to correct the water contents given by the

Figure 5.1, as a function of salinity of the aqueous phase (after Katz, 1962).

The water content of natural gas can be measured by three different methods (Rojey et al.,

1994):

a) by observation of the dew point

b) by water retention on an adsorbent

c) by absorption in liquid.

Alexandre Rojey et al., 1994 provides a short description of these methods. In the dew point

method, a cooler mirror is used to observe the water condensation temperature. The water

dew point is sometimes difficult to distinguish from the hydrocarbon dew point. The water

content can also be measured by adsorption on magnesium perchlorate. The quantity of water

adsorbed is determined by gravimetric method. In the widely used absorption based Karl-

Fischer method, the water is absorbed in a solution, and the water content is measured fromthe amount of gas required to neutralize the reagent (solution of iodine, pyridine and sulfur

dioxide in methanol, called the Karl-Fischer reagent).

If the variation of temperature and pressure in an installation is known, the water dew point

curve of the natural gas can be used to determine the zone where water may condense. The

amount of condensed water released from gas can be then calculated from the difference

between the water content in gas at saturation point at the inlet and outlet respectively.

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5.2 Water content from GPSA diagram

Natural gases containing significant quantities of acid gas are encountered frequently in the

world. Estimates of the water content of these sour gases are required for the design of plant

and pipelines facilities. Three methods are currently available for estimating the water content

of sour natural gases.

The most commonly used procedure is prepared by the Gas Processors Suppliers Association

(GPSA). In the procedure outlined by GPSA, the estimated water content of a sour gas is a

molar average of the solubility of water in the hydrocarbons, hydrogen, sulfide, and carbon

dioxide. The water content curves for H2S and CO2 are based on experimental data for the

binary mixtures H2O-H2S and H2O-CO2, respectively. Both these binaries display liquid-

liquid equilibria at temperatures and pressures common in processing applications, and the

water content read for the acid gas components often corresponds to the solubility of water in

nonaqueous liquid phase rather than in vapor phase. In general, the predicted water content of

sour natural gas is high when based on these experimental curves (Robinson et al., 1977).

A chart was prepared containing aheadmentioned curves for temperatures from -50 oC to

140 oC under pressures of 100 kPa, 250 kPa, 500 kPa, 750 kPa, 1000 kPa, 1500 kPa,

2000 kPa, 3000 kPa, 4000 kPa, 5000 kPa, 8000 kPa, 20 000 kPa, 30 000 kPa, 40 000 kPa,

50 000 kPa, and 60 000 kPa (Figure 5.3). The figure shows the amounts of water saturating

natural gas in given temperature and pressure along with charts for correction for gas relative

density and for salinity.

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5.3 Water content values obtained from Makowice operation manual

The designer of Makowice natural gas dehydration facility provided an operating manual.

An attachment to the manual shows water content of natural gas within the range of pressures

for which the facility was designed (Figure 3.5, Appendix B). The table in Appendix B shows

water content in natural gas in [g/Nm3] within the range of pressures between 2700 kPa and

5500 kPa, for the temperatures between 10 oC and 40 oC with stress to points within the

te

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