Ministry of Higher Education
& Scientific Research
University of Technology
Chemical Engineering Department
A study of the dehydration process of
natural gas in Iraqi North Gas Company
and the treatment methods of molecular
sieve problems
A Research Submitted to the Department of Chemical
Engineering University of Technology in Partial Fulfillment
of the Requirements for the Degree of Higher Diploma in
Chemical Engineering/Petroleum Refining and Gas
Technology
By
Farman Saeed Abd. Zangana
(B.Sc. in Chemical Engineering 2004)
Supervised by
Assist.Prof. Dr.Anaam A.Sabri
2012
CERTIFICATE
We certify that we have read this research entitled " A study of the
dehydration process of natural gas in Iraqi North Gas Company and
the treatment methods of molecular sieve problems " by Farman
Saeed Abd. Zangana and as an Examining Committee examined the
student in its contents and that in our opinion it meets the standard of a
research for the degree of Higher Diploma in Chemical Engineering
/Petroleum Refining and Gas Technology.
Signature:
Assistant Prof. Dr. Anaam A.Sabri
(Supervisor)
Date:
/
/ 2012
Signature:
Signature:
Assist. Prof. Dr.Mohammed F.Abid
Prof. Dr. Mumtaz A. Zablouk
(Member)
Date:
/
(Chairman)
/ 2012
Date:
/
/ 2012
Approved for the University of Technology
Signature:
Prof. Dr. Mumtaz A. Zablouk
Head of Chemical Engineering Department
Date:
/
/ 2012
CERTIFICATION
This is to certify that I have read the research titled " A study
of the dehydration process of natural gas in Iraqi North Gas
Company
and
the
treatment
methods
of
molecular
sieve
problems " and corrected any grammatical mistakes I found. The
research is therefore qualified for debate.
Signature:
Prof. Dr. Mumtaz A. Zablouk
University of Technology
Date: /
/ 2012
Acknowledgments
First of all, praise is to Allah for every thing. Without his great
assistance the work wouldn't have been finished.
I would like to express my sincere appreciation and thanks to my
supervisor Assist.Prof. Dr.Anaam A.Sabri, for his constant guidance and
valuable comments, without which, this research would not have been
successfully completed.
My grateful thanks to Prof. Dr. Mumtaz A. Zablouk, the Chairman
of the Department of Chemical Engineering at the University of
Technology for the provision of research facilities.
My deep thanks go to Assist. Prof. Dr. Mohammed I. Mohammed,
the head of post graduate committee for all the help and encouragement,
also I wish to express my sincere gratitude to my family
for them
encouragement and helpful.
Also I would like to convey my sincere appreciation to any one that
helped me in Iraqi North Cas Company.
Also I would like to convey my sincere appreciation to all staff of
Chemical Engineering Department in the University of Technology and the
workshops unit.
Finally, to all that helped me in one way or another, I wish to express
my thanks.
Farman
Abstract
The purpose of this research is to study the dehydration process of
natural gas by adsorption using molecular sieve as it is in the North Gas
Company. Dehydration of natural gas is needed to remove the water that is
associated with natural gas in vapor form. The natural gas industry has
recognized that dehydration is necessary to insure smooth operation of gas
transmission lines, dehydration prevents the formation of gas hydrates and
reduces corrosion. Unless gases are dehydrated, liquid water may condense
in pipeline and accumulate at low point along the line and reducing its
flow capacity. Several methods have been developed to dehydrate gases in
an industrial scale . The four major methods of dehydration are direct
cooling , indirect cooling , absorption and adsorption. This study focuses
on the adsorption method which is used to dehydrate the natural gas in
North Gas Company and also focuses on the problem of breaking up and
aging of the molecular sieve before ending its real life time .Some testing
were made on the aging and the new molecular sieve to show the difference
between them. Some suggestions are made to overcome the problem of
aging of the molecular sieve, like improve the efficiency of gas
separator,using antifoaming to prevent liquid carryover, choose the
regeneration conditions carefully, using multi bed technology and using
glycol dehydration unit with molecular sieve dehydration unit to reduce the
water content of natural gas.
Contents
Subject
1
Page
Chapter One
Introduction
1-3
1-1
Problem Statement
3
1-2
Scope of the Study
3
2
Chapter Two
4
Literature Review
4
Types of Dehydration of Natural Gas
4
2-1-1
Direct Cooling
4
2-1-2
Indirect Cooling
4-5
2-1-3
Dehydration by Absorption
5-9
2-1-4
Dehydration by Adsorption (Solid Desiccant)
10
2-2
Types of Adsorbents
11
2-2-1
Alumina
11
2-2-2
Silica Gel and Silica-Alumina Gel
11
2-2-3
Molecular Sieves
2-3
Some Types of Molecular Sieves
14
2-3-1
3A Molecular Sieve
14
2-3-2
4A Molecular Sieve
14-15
2-3-3
5A Molecular Sieve
15-16
2-3-4
13X Molecular Sieve
2-1
11-13
16
2-4
Solid Desiccant Adsorption Kinetics
16-17
2-5
Fundamentals of Adsorption
17-18
2-6
Development of Adsorption Isotherms
18-19
2-6-1
Freundlich Isotherms
19- 20
2-6-2
Langmuir Isotherm
20-21
2-7
Some previous Work on Dehydration of Natural Gas
3
Chapter Three
23
3-1
Design and Theoretical Considerations
Design Consideration
23
23
3-1-1
Solid Desiccant Dehydration
23
3-1-1-1
Allowable Gas Velocity
23-24
3-1-1-2
Bed Length to Diameter Ratio
24-25
3-1-1-3
Desiccant Capacity
25
3-1-1-4
MTZ Length
25
3-1-1-5
Breakthrough Time
25
3-1-2
Glycol Dehydration
25
3-1-2-1
Absorber (Contactor)
3-1-2-2
25-27
Still (Stripper)
27
3-1-2-3
Reboiler
27
3-1-2-4
Surge Tank (Accumulator)
28
3-1-2-5
Heat Exchanger
28
3-1-2-5-1
Glycol/Glycol Exchanger
28
3-1-2-5-2
Dry Gas/Lean Glycol Exchanger
28
3-1-2-6
Phase Separator (Flash Tank)
3-1-2-7
Glycol Circulation Pumps
29
3-1-2-8
Filters
29
3-1-2-8-1
Fabric (Particulate) Filters
29
3-1-2-8-2
Carbon Filters
29
Operational Problems
30
3-2-1
Solid Desiccant Dehydration
30
3-2-1-1
Bed Contamination
30
3-2-1-2
High Dew Point
3-2-1-3
Premature Breakthrough
31
3-2-1-4
Hydrothermal Damaging
31-32
3-2-1-5
Liquid Carryover
32
3-2-1-6
Bottom Support
32
3-2-2
Glycol Dehydration
33
3-2
U
28-29
30-31
3-2-2-1
Absorber
33
3-2-2-1-1
Insufficient Dehydration Causes of insufficient
dehydration
33
3-2-2-1-2
Foaming
33
3-2-2-1-3
Hydrocarbon Solubility in TEG Solution
33
3-2-2-2
Still (Stripper)
3-2-2-3
Reboiler
34
3-2-2-3-1
Salt Contamination
34
3-2-2-3-2
Glycol Degradation
34-35
3-2-2-3-3
Acid Gas
35
3-2-2-3-4
Surge Tank
35
3-2-2-3-5
Heat Exchanger
3-2-2-3-6
Phase Separator (Flash Tank)
36
3-2-2-3-7
Glycol Circulation Pump
36
Chapter Four
37
Case Study
37
4
33-34
35-36
4-1
Dehydration Process
37-38
4-2
Regeneration Process
38-39
4-3
The Molecular Sieve Dehydrate
39-42
5
Chapter Five
43
5-1
Result and Discussion
43-49
5-2
Overcome the Breaking Up and Aging Problems of the
Molecular Sieve
Chapter Six
Conclusions and Recommendations
49-50
Conclusions
Recommendations
51-52
53
6
6-1
6-2
51
51-53
Chapter One
Introduction
Introduction
Today, natural gas is one of the most important fuel in our life and one
of the principle sources of energy for many of our day-to-day needs and
activities. It is an important factor for the development of countries that
have strong economies because it is a source of energy for household,
industrial and commercial use, as well as to generate electricity.
Natural gas, in itself, might be considered a very uninteresting gas - it
is colorless, shapeless, and odorless in its pure form, but it is one of the
cleanest, safest ,and most useful of all energy sources. Natural gas is a
gaseous fossil fuel. Fossil fuels are essentially, the remains of plants and
animals and microorganisms that lived millions and millions of years
ago. It consists primarily of methane but including significant quantities of
ethane, propane, butane, and pentane. Methane is a molecule made up of
one carbon atom and four hydrogen atoms, and is referred to as CH4.
Natural gas is considered 'dry' when it is almost pure methane, having
had most of the other commonly associated hydrocarbons removed. When
other hydrocarbons are present, the natural gas is 'wet'.The natural gas used
by consumers is composed almost entirely of methane .However, natural
gas found at the wellhead, although still composed primarily of methane, is
by no means as pure. Raw natural gas comes from three types of wells: oil
wells, gas wells, and condensate wells.
Natural gas that comes from oil wells is typically termed 'associated
gas'. This gas can exist separately from oil in the formation of (free gas),or
dissolved in the crude oil (dissolved gas). Natural gas from gas and
condensate wells ,in which there is little or no crude oil, is termed non
associated gas. Gas wells typically produce raw natural gas by itself, while
condensate wells produce free natural gas along with a semi-liquid
hydrocarbon condensate. Whatever the source of the natural gas, once
Page 1
Chapter One
Introduction
separated from crude oil it commonly exists in raw natural gas or sour
gas.The raw natural gas contains water vapor, hydrogen sulfide
(H2S),carbon dioxide, helium, nitrogen, and other compounds .In order to
meet the requirements for a clean, dry, wholly gaseous fuel suitable for
transmission through pipelines and distribution for burning by end users,
the gas must go through several stages of processing, including the removal
of entrained liquids from the gas, followed by drying to reduce water
content. In order to remove water content, dehydration process is used to
treat the natural gas. Dehydration is the removal of water from an object. In
Physiologic terms, it entails a relative deficiency of water molecules in
relation to other dissolved solutes. Gas dehydration is one of the most
prominent unit operations in the natural gas industry(1).
There are five major reasons for natural gas treating from water contents:
• Liquid water and natural gas can form hydrates which causes plug the
pipelines and other equipments such as valves, collectors etc. (Gas
Hydrate: are solids formed by the physical combination of water and other
small molecules of hydrocarbons. They are icy hydrocarbon compounds of
about 10% hydrocarbon and 90% water).
• Natural gas containing H2S and/or CO2 is corrosive when liquid water is
present (corrosion often occurs when liquid water is present along with
acidic gases, which tend to dissolved and disassociate in the water phase,
forming acidic solutions).
• Water content of natural gas decreases of it is heat value.
• Liquid water in natural gas pipelines potentially causes slugging flow
conditions resulting in lower flow efficiency of the pipelines.
• In most commercial hydrocarbon processes, the presence of water may
cause side reactions, foaming or catalyst deactivation(2).
Page 2
Chapter One
Introduction
There are several methods of dehydrating natural gas. The
refrigeration (direct cooling and indirect cooling). liquid desiccant (glycol)
dehydration and solid desiccant dehydration, the first two methods employ
cooling to condense the water molecules to the liquid phase with the
subsequent injection of inhibitor to prevent hydrate formation. The other
two methods utilize mass transfer of the water molecular into a liquid
solvent (glycol solution) or a crystalline structure (dry desiccant). However,
the choice of dehydration method is usually between glycol and solid (3).
1-1 Problem Statement
Gas dehydration is a common process in gas treatment plant because
water and hydrocarbons can form hydrates ,which may block valves and
pipelines .Also water can cause corrosion in present of acid compounds in
natural gas .In Iraqi North Gas Company, the molecular sieve of type 4A is
used to remove water from the natural gas, the main problem is the
breaking up and aging of the molecular sieves (1.5-2 year) before ending its
service life time (3years).
1-2 Scope of the Study
The aim of this research is
1.To conduct literature survey of different methods for dehydration of
natural gas .
2.To describe the process of dehydration of natural gas using molecular
sieve as it is in Iraqi North Gas Company and try to study the problem of
breaking up and aging of the molecular sieve which is used in the
dehydration process .
3.Suggesting solutions for this problem (aging of molecular sieve) .
Page 3
Chapter Two
Literature Review
Literature Review
2-1 Dehydration Methods of Natural Gas
Dehydration of natural gas is the process removal of the water that is
associated with natural gases. The natural gas industry has recognized that
dehydration is necessary to ensure smooth operation of gas transmission
lines. Several methods have been developed to dehydrate gases on an
industrial scale.
2-1-1 Direct Cooling
The ability of natural gas to contain water vapor decreases as the
temperature is lowered at constant pressure. During the cooling process, the
excess water in the vapor state becomes liquid and is removed from the
system. Natural gas containing less water vapor at low temperature is
output from the cooling unit. The gas dehydrated by cooling is still at its
water dew point unless the temperature is raised again or the pressure is
decreased. It is often a good practice that cooling is used in conjunction
with other dehydration processes. Glycol may be injected into the gas
upstream ahead of the heat exchanger to reach lower temperatures before
expansion into a low temperature separator(5).
2-1-2 Indirect Cooling
Expansion is a second way of natural gas cooling. It can be achieved
by the expander or Joule-Thomson valve. These processes are
characterized by a temperature drop to remove condensed water to yield
dehydrated natural gas. The principal is similar to the removal of humidity
from outside air as a result of air conditioning. Gas is forced through a
constriction called an expansion valve into space with a lower pressure. As
a gas expands, the average distance between molecules increase. Because
of intermolecular attractive forces, expansion causes an increase in the
potential energy of the gas. If no external work is extracted in the process
and no heat is transfered, the total energy of the gas remains the
Page 4
Chapter Two
Literature Review
same. The increase in potential energy thus implies a decrease in kinetic
energy and therefore in temperature(5).
2-1-3 Dehydration by Absorption
The basis for gas dehydration using absorption is the absorbent; there
are certain requirements for absorbents used in gas treating.
● Strong affinity for water to minimize the required amount of absorbent
(liquid solvent).
● Low potential for corrosion in equipments, low volatility at the process
temperature to minimize vaporization losses.
● Low affinity for hydrocarbons to minimize their loss during the process.
● Low solubility in hydrocarbons to minimize losses during treating.
● Low tendency to foam and emulsify to avoid reduction in gas handling
capacity and minimize losses during regeneration.
● Good thermal stability to prevent decomposition during regeneration and
low viscosity for easily pumping and good contact between gas and liquid
phases. Off course, the major critical property for a good absorbent is the
high affinity for water. The others are used to evaluate potential absorbents
practical applicability in the industry (2) .
There are numbers of liquids that can be used to absorb water from
natural gases such as calcium chloride, lithium chloride and glycols. Glycol
dehydration is a liquid desiccant system for the removal of water from
natural gas. It is the most common and economic means of water removal
from these streams .Glycol, the principal agent in this process, has a
chemical affinity for water. The liquid glycol will absorbs the water content
in the natural gas. This means that, when in contact with a stream of natural
gas that contains water, glycol will serve to 'steal' the water out of the gas
Page 5
Chapter Two
Literature Review
stream. This operation is called absorption
(1)
. There are a few types of
glycol usually used in industry with their advantages and disadvantages
like ethylene glycol (EG), diethylene glycol (DEG), triethyleneglycol
(TEG), and tetraethylene glycol (TREG). Glycols typically seen in industry
include monoethylene glycol (MEG) Table( 2-1) shows the properties of
the glycols (1) .The commonly available glycol and their uses are described
as follows.
1. Monoethylene glycol (MEG);high vapor equilibrium with gas so tend to
lose Monoethylene glycol to gas phase in contactor. Use as hydrate
inhibitor where it can be recovered from gas by separation at temperature
below 50ºF.
2. Diethylene glycol (DEG); high vapor pressure leads to high losses in
contactor. Low decomposition temperature requires low reconcentrator
temperature (315 to 340ºF) and thus cannot get pure enough for most
applications.
3.Triethylene glycol (TEG); most common. Reconcentrate at 340-400ºF,for
high purity. At contactor temperatures in excess of 120ºF, there is a
tendency to high vapor losses. Dew point depressions up to 150ºF are
possible with stripping gas.
4. Tetraethylene glycol (TREG); more expensive than TEG but less loss at
high gas contact temperatures. Reconcentrate at 400 to 430ºF.
TEG is by far the most common liquid desiccant used in natural gas
dehydration. It exhibits most of the desirable criteria of commercial
suitability listed here.
1.TEG is regenerated more easily to a concentration of 98-99% in an
atmospheric stripper because of its high boiling point and decomposition
temperature.
2. TEG has an initial theoretical decomposition
whereas that of diethylene glycol is only 328ºF.
Page 6
temperature of 404ºF,
Chapter Two
Literature Review
Table( 2-1) Shows the properties of the glycols
Page 7
Chapter Two
Literature Review
3. Vaporization losses are lower than monoethylene glycol or diethylene
glycol. Therefore, the TEG can be regenerated easily to the high
concentrations needed to meet pipeline water dew point specifications.
4. Capital and operating costs are lower (3) .
The rational of using TEG or advantages of TEG is ease of
regeneration and operation, minimal losses of drying agent during
operation, high affinity for water (this may be attributed to hydrogenoxygen bonds
which are set up between atoms of the hydroxyl groups
and those of water), chemical stability, high hygroscopicity and low vapor
pressure at the contact temperature
(1)
. TEG, or triethyleneglycol is a
colorless, odourless viscous liquid with molecular formula C6H14O4 and
molecular structure as shown in Figure (2-1).
Figure (2-1): Molecular structure of triethylene glycol (TEG)
Essentially, Dehydration of natural gas by TEG is first outlined by
summarizing the flow paths of natural gas and glycol . Then the individual
components of a typical TEG unit are described in detail. shown in Figure
(2-2), wet natural gas first typically enters an inlet separator to remove all
liquid hydrocarbons from the gas stream. Then the gas flows to an absorber
(contactor) where it is contacted countercurrently and dried by the lean
TEG. TEG also absorbs volatile organic compounds (VOCs²) that vaporize
with the water in the reboiler. Dry natural gas exiting in the absorber
passes through a gas/glycol heat exchanger and then into the sales line.
Page 8
Chapter Two
Literature Review
The wet or "rich" glycol exiting the absorber flows through a coil in
the accumulator where it is preheated by hot lean glycol. After the glycolglycol heat exchanger, the rich glycol enters the stripping column and
flows down the packed bed section in to the reboiler . Steam generated in
the reboiler strips absorbed water and VOCs out of the glycol as it rises up
the packed bed. The water vapor and desorbed natural gas are vented from
the top of the stripper. The hot regenerated lean glycol flows out of the
reboiler into the accumulator (surge tank) where it is cooled via cross
exchanger with returning rich glycol; it is pumped to a glycol/gas heat
exchanger and back to the top of the absorber. Glycol unit decreases the
water content to 60 ppm (3) .
Sweet
gas
inlet
Page 9
Chapter Two
Literature Review
2-1-4 Dehydration by Adsorption (Solid Desiccant)
Adsorption (or solid bed) dehydration is the process where a solid
desiccant is used for the removal of water content from a gas stream. The
solid desiccants commonly used for gas dehydration are those that can be
regenerated and, consequently, used over several adsorption-desorption
cycles. The mechanisms of adsorption on a surface are of two types;
physical and chemical. The latter process, involving a chemical reaction, is
termed "chemisorption". Chemical adsorbents find very limited application
in gas processing. Adsorbents that allow physical adsorption hold the
adsorbate on their surface by surface forces. For physical adsorbents used
in gas dehydration, the following properties are desirable(1) .
1. Large surface area for high capacity. Commercial adsorbents have a
surface area of 500-800 m² /g.
2. Good "activity" for the components to be removed and good activity
retention with time/use.
3. High mass transfer rate or high rate of removal.
4. Easy, economic regeneration.
5. Small resistance to gas flow, so that the pressure drop through the
dehydration system is small.
6. High mechanical strength to resist crushing and dust formation. The
adsorbent also must retain enough strength when "wet".
7. Cheap, non-corrosive, non-toxic, chemically inert, high bulk density, and
small volume changes upon adsorption and desorption of water(1)ℓ(6).
The most widely used adsorbents today are activated alumina, silica
gel, molecular sieves (zeolites) (1) .
Page
10
Chapter Two
Literature Review
2-2 Types of Adsorbents
2-2-1 Alumina
A hydrated form of aluminum oxide (Al2O3), alumina is the least
expensive adsorbent. It is activated by driving off some of the water
associated with it in its hydrated form ((Al2O3.3H2O) by heating. It
produces an excellent dew point depression values as low as -100 ºF, but
requires much more heat for regeneration. Also, it is alkaline and cannot be
used in the presence of acid gases, or acidic chemicals used for well
treating. The tendency to adsorb heavy hydrocarbons is high, and it is
difficult to remove these during regeneration. It has good resistance to
liquids, but little resistance to disintegration due to mechanical agitation by
the flowing gas (6) .
2-2-2 Silica Gel and Silica-Alumina Gel
Gels are granular, amorphous solids manufactered by chemical
reaction. Gels manufactured from sulfuric acid and sodium silicate reaction
are called silica gels, and consist almost solely of silicon dioxide (SiO2).
Alumina gels consist primarily of some hydrated form of Al2O3. Silicaalumina gels are a combination of silica and alumina gel. Gels can
dehydrate gas to as low as 10 ppm, and have the greatest ease of
regeneration of all desiccants. They adsorb heavy hydrocarbons, but release
them relatively more easily during regeneration. Since they are acidic, they
can handle sour gases, but not alkaline materials such as caustic or
ammonia. Although there is no reaction with H2S, sulfur can deposit and
block their surface. Therefore, gels are useful if the H2S content is less than
(5-6%) (6) .
2-2-3 Molecular Sieves
Molecular Sieves
are crystalline alkali metal alumino
silicates
comprising a three dimensional interconnecting network of silica and
alumina tetrahedral (3). These tetrahedras are the basic building blocks for
Page
11
Chapter Two
Literature Review
various zeolite structures, such as zeolites A and X, the most common
commercial adsorbents (figure.2-3). (7)
Molecular sieve is the most versatile adsorbent because it can be
manufactured for a specific pore size, depending on the application. It is:
● Capable of dehydration to less than 0.1 ppm water content.
● The overwhelming choice for dehydration prior to cryogenic processes
(especially true for LNG ).
● Excellent for H2S removal, CO2, dehydration, high temperature
dehydration, heavy hydrocarbon liquids, and highly selective removal.
● More expensive than silica gel ,but offers greater dehydration.
Figure (2-3): Zeolite structure
● Requires higher temperatures for regeneration, thus has a higher
operating,cost(3).
.
The water adsorption in zeolites functions on the basis of physisorption. the
main driving force for adsorption is the highly polar surface within the
pores. This unique characteristic distinguishes zeolites from other
commercially available adsorbents, enabling an extremely high adsorption
capacity for water and other polar components even at very low
concentrations (figure 2-4) (7) .
The structure of molecular sieve is an array of cavities connected by
uniform pores with diameters ranging from about 3 to 10ºA, depending on
Page
12
Chapter Two
Literature Review
the sieve type. In addition, the pore size plays a significant role, allowing or
prohibiting the entrance of molecules to the pore system (3) .Some types of
molecular sieves and it application in table (2-2).
Figure( 2-4) Water adsorption curve versus relative humidity.
Table(2-2) Technical data & application of some kind of molecular
sieve.
Bulk
Water Loss on
Type density adsorption abrasion
Application
3
(kg/m )
(%)
(%)
0.60Drying agent for pyrolysis gas and
3A
19-20
0.3-0.6
0.70
alkene
0.60Drying agent for natural gas and
4A
20-21
0.3-0.5
0.70
absorbent for paraffin separation
Dehydration, desulfurization and
0.65purification of air, natural gas and oil
5A
21-21
0.3-0.5
0.70
during oxygen making and hydrogen
making by PSA processes
High-pow absorbents for removal of
0.5010X
23-24
0.8 H2O, H2S and CO2, in liquid and gas, as
0.60
well as for paraffin separation
0.55Drying, desulfurization and purification
13X
23-24
0.4
0.65
of oil and gas
Page
13
Chapter Two
Literature Review
2-3 Some Types of Molecular Sieves
2-3-1 3A Molecular Sieve
The pore size of 3A molecular sieve is 3A. It does not adsorb any
molecular larger than 3A. According to the industrial application
specialties, 3A molecular sieves have the characters of higher adsorption
speed, stronger crushing and anti-contaminative resistance, more cyclic
times and longer work-span. All these advantages have made it come to be
the most essential and necessary desiccant in the fields of the deep drying,
refinery polymerization for cracked gases (pyrolysis gas), ethylene,
propylene and any other non-acidic gasses of liquids in petroleum and
chemical
industrials.
Formula
0.4K2O · 0.6Na2O · Al2O3 · 2.0SiO2 · 4.5H2O
Applications
Dehydration of many kinds of liquids (such as ethanol), dehydration
of air, dehydration of refrigerant, dehydration of natural gas or methane,
dehydration of cracked gas (pyrolysis gas), ethylene, propylene or
butadiene(8).
.
2-3-2 4A Molecular Sieve
Molecular Sieve type 4A is an alkali alumino silicate, obtained
through synthesis of type A zeolite in sodic form. It is the sodium form of
Type A crystal structure. 4A molecular sieve has an effective pore opening
of about 4 angstroms (0.4nm). Galaxychem type 4A molecular sieve will
adsorb most molecules with a kinetic diameter of less than 4 angstroms and
exclude those larger. Regeneration: Molecular sieve 4A can be revived
for reuse and the required factors for revival are temperature and pressure.
To remove the adsorbed moisture and other materials, the revival condition
Page
14
Chapter Two
Literature Review
is set up considering the features of target material and processing
environment.The general revival temperature of Molecular Sieve 4A is
about200-300℃(8).
Formula
Na2O . Al2O3 . 2SiO2 . 4.5 H2O
Applications
Deep drying of air, Natural gas, alkane and refrigerant. Generation and
purification of argon. Static dehydration of electronic element,
pharmaceutical and unstable materials. Desiccant for paint, dope and foul
etc. Type 4A molecular sieve is typically used in regenerable drying systems
to remove water vapor or contaminants which have a smaller critical
diameter than four angstroms.
2-3-3 5A Molecular Sieve
Molecular Sieve type 5A is an alkali alumino silicate; it is the calcium
form of the Type A crystal structure. The pore size of 5A molecular sieve is
about 5 Å . It can adsorb all kinds of molecular smaller than this size, it is
mainly used in separation of the normal and isomerous alkane,pressure
swing adsorption (PSA) for gases,co-adsorption of moisture and carbon
dioxide.Regeneration: molecular sieve Type 5A can be regenerated by
pressure swing or purging, usually at elevated temperatures. The purge gas
temperature must be sufficiently high to bring the molecular sieve to a level
of 200°C - 350°C (8) .
Formula
0.75 CaO . 0.25 Na2O . Al2O3 . 2 SiO2 . 4.5H2O
Applications Oxygen/hydrogen PSA (Pressure Swing Adsorption) process
for removal of water and carbon dioxide from air. Type 5A separates normal
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15
Chapter Two
Literature Review
paraffins from branched-chain and / cyclic hydrocarbons through a selective
adsorption process. Ethanol refinement (moisture, CO2, etc.).
2-3-4 13X Molecular Sieve
Molecular Sieve 13X is the sodium form of the type X crystal and has a
much larger pore opening than the type A crystals.The pore size 13X
molecular sieve is about 10 Å . It can adsorb any molecular smaller than 10
Å , mainly used as catalyst carrier, co-adsorption of CO2 and H2O, H2O and
H2S, as desiccant for medical and air compressor system, and can also be
adjusted to fit other various applications. Regeneration : molecular sieve
Type 13X can be regenerated by evacuating or purging, usually at elevated
temperatures. The purge gas temperature must be sufficiently high to bring
the molecular sieve to a level of 250°C - 350°C (8) .
Formula
Na2O . Al2O3 . (2.8±0.2) SiO2 . (6~7)H2O.
Applications
Air refining (removing CO2 and H2O) removal of mercaptans and
hydrogen sulphide from natural gas, removal of mercaptans and hydrpogen
sulphide from hydrocarbon liquid streams (LPG, butane, propane etc.) ,
oxygen PSA (Pressure Swing Adsorption) process special double glass
(removing solvent and grease).
2-4 Solid Desiccant Adsorption Kinetics
Figure (2-5) illustrates the movement of an adsorption zone front as a
function of time. In a dry desiccant bed, the adsorbtcate components are
adsorbed at different rates. A short while after the process has begun, a series
of adsorption zones appear, as shown in Figure (2-5)b. The distance between
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16
Chapter Two
Literature Review
successive adsorption zone fronts is indicative of the length of the bed
involved in the adsorption of a given component. Behind the zone, all of the
entering component is removed from the gas; ahead of the zone, the
concentration of that component is zero. Note the adsorption
sequence: C1
and C2 are adsorbed almost instantaneously, followed by the heavier
hydrocarbons, and finally by water that constitutes the last zone. Almost all
the hydrocarbons are removed and dehydration begins. Water displaces the
hydrocarbons on the adsorbent surface if enough time is allowed. At the start
of dehydration cycle, the bed is saturated with methane as the gas flows
through the bed. Then ethane replaces methane, and propane is adsorbed
next. Finally, water will replace all the hydrocarbons. For good dehydration,
the bed should be switched to regeneration just before the water content of
outlet gas reaches an unacceptable level. The regeneration of the bed consists
of circulating hot dehydrated gas to strip the adsorbed water, then circulating
cold gas to cool
the bed down (6).
SAT
ADSORBATE
1
BED SATURATION
2
3
direction OF FLOW
5
0
t1
BED LENGTH
(a)
SAT
ADSORBATE BED
H2O
C6+
C5
C4
SATURATION
0
1
BED LENGTH
(b)
Figure (2-5) Schematic representation of the adsorption process (6)
2-5 Fundamentals of Adsorption
The adsorption process, takes place in four more or less definable steps.
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17
Chapter Two
Literature Review
1.Bulk solution transport.
2.Film diffusion transport.
3.Pore transport.
4.Adsorption (or sorption).
Bulk solution transport involves the movement of the organic material
to be adsorbed through the bulk liquid to the boundary layer of fixed film of
liquid surrounding the adsorbent, typically by advection and dispersion in
adsorbent contactors. Film diffusion transport involves the transport by
diffusion of the organic material through the stagnant liquid film to the
entrance of the pores of the adsorbent. Pore transport involves the transport
of the material to be adsorbed through the pores by a combination of
molecular diffusion through the pore liquid and/or by diffusion along the
surface of the adsorbent. Adsorption involves the attachment of the material
to be adsorbed to adsorbent at an available adsorption site. Because the
adsorption process occurs in a series of steps, the slowest step in the series is
identified as the rate limiting step. In general, if physical adsorption is the
principal method of adsorption, one of the diffusion transport steps will be
the rate limiting, because the rate of physical adsorption is rapid. Where
chemical adsorption is the principal method of adsorption, the adsorption
step has often been observed to be rate limiting. When the rate of sorption
equals the rate of desorption, equilibrium is achieved and the capacity of the
adsorbent is reached. The theoretical adsorption capacity of the molecular
sieve for a particular contaminant can be determined by developing its
adsorption isotherm as described below (9).
2-6 Development of Adsorption Isotherms
The quantity of adsorbate that can be taken up by an adsorbent is a
function of both the characteristics and concentration of adsorbate and the
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18
Chapter Two
Literature Review
temperature. The amount of material adsorbed is determined as a function of
the concentration at a constant temperature, and the resulting function is
called an adsorption isotherm. Adsorption isotherms are developed by
exposing a given amount of absorbate in a fixed volume of liquid to varying
amounts of molecular sieve. At the end of the test period, the amount of
absorbate remaining in solution is measured. The absorbent phase
concentration after equilibrium is computed using Eq.(2-1). The absorbent
phase concentration date computed using Eq. (2-1) are then used to develop
adsorption isotherms as described below(9).
qe =
(𝐶𝐶𝑜𝑜−𝐶𝐶𝐶𝐶)𝑉𝑉
𝑚𝑚
(2-1)
where qe= adsorbent (i.e., solid ) phase concentration after equilibrium, mg
adsorbate/g adsorbent
Cо=initial concentration of adsorbate, mg/L
Ce=final equilibrium concentration of adsobate after absorption has occurred,
mg/L
V= volume of liquid in the reactor,L
m= mass of adsorbent, g
2-6-1 Freundlich Isotherms
Equations that are often used to describe the experimental isotherm
date were developed by Freundlich, Langmuir , and Brunauer, Emmet, and
Teller (BET isotherm) . the Freundlich isotherm is used most commonly to
describe the adsorption characteristics of the adsorbent used in water
treatment. The Freundlich isotherm is defined as follows:
𝑥𝑥
𝑚𝑚
1
𝑛𝑛
= Kƒ𝐶𝐶𝑒𝑒
(2-2)
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19
Chapter Two
Where
𝑥𝑥
𝑚𝑚
Literature Review
= mass of adsorbate adsorbed per unit mass of adsorbent, mg
adsorbate/g molecular sieve
Kƒ = Freundlich capacity factor, (mg absorbate/g adsorbent)(L water/mg
1
adsorbate)(L water/mg adsorbate) 𝑛𝑛
Ce = equilibrium concentration of adsorbate in solution after
adsorption,mg/L
1/n = Freundlich intensity parameter
The constants in the Freundlich isotherm can be determined by plotting
log(x/m) versus log Ce and making use of Eq. (2-2) rewritten as(9) :
1
Log ﴾ 𝑚𝑚𝑥𝑥 ﴿ = log Kƒ + log Ce
𝑛𝑛
(2-3)
2-6-2 Langmuir Isotherm
Derived from rational considerations, the Langmuir adsorption isotherm
is defined as:
𝑥𝑥
𝑚𝑚
=
𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
1+𝑏𝑏𝑏𝑏𝑏𝑏
(2-4)
a, b = empirical constants
The Langmuir adsorption isotherm was developed by assuming:
1. A fixed number of accessible sites are available on the adsorbent surface,
all of which have the same energy,
2. Adsorption is reversible.
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20
Chapter Two
Literature Review
Equilibrium is reached when the rate of adsorption of molecules onto
the surface is the same as the rate of desorption of molecules from the
surface. The rate at which adsorption proceeds is proportional to the driving
force, which is the difference between the amount adsorbed at a particular
concentration and the amount that can be adsorbed at that concentration. At
the equilibrium concentration, this difference is zero correspondence of
experimental data to the Langmuir equation does not mean that the stated
assumptions are valid for the particular system being studied, because
departures from the assumptions can have a canceling effect. The constants
in the Langmuir isotherm can be determined by plotting Ce /(x/m) versus Ce
and making use of eq.(2-5)rewritten as:
𝐶𝐶𝐶𝐶
(𝑥𝑥 ⁄𝑚𝑚 )
=
1
𝑎𝑎𝑎𝑎
1
+ Ce
(9) .
(2-5)
𝑎𝑎
2-7 Some Previous Work on Dehydration of Natural Gas
Theoretical and experimental works on dehydration of natural gas have
been extensively reported. Ahmad
(1)Studied
pressure and volumetric
flowrate of natural gas on the effect of operating the removal of water from
natural gas by using triethylene glycol (TEG). And he focused on the effect
of operating pressure and volumetric flowrate of natural gas. From the result
of experiments he found that, the highest amount of water was removed
when the operating pressure at lowest and the volumetric
highest. Pezhman and Roya
(2)
flowrate at
presented a comprehensive study of software
simulator and investigate the effectiveness parameters in a TEG-dehydration
plant in Persian Gulf region. Henry and Julie
(19)
developed molecular sieve
uI-94 adsorbent to overcome the problems of accelerated particle breakup. So uI-94 adsorbent increase the operational life of molecular sieves in
natural gas dehydration units and doubled it compared to standard 4A-type
molecular sieve by minimizing pressure drop increase over the life of the
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21
Chapter Two
Literature Review
adsorbent. Siti (4) studied, the dehydration of natural gas by adsorption using
silica gel as a solid desiccant and he focuses on designing, fabrication,
hydrostatic test and experimental part . Lukás
(5)
focused on dehydration of
natural gas by absorption drying using triethylene glycol (TEG) its benefits
and disadvantages. Then the dehydration is simulated with Aspen Hysys
software. The minimum glycol mass flow and specific glycol circulation rate
is calculated. Mohamadbeigy
(20)
presented a comprehensive study on gas
drying unit and investigated the effectiveness parameters such as glycol flow
rate, stages number of absorption tower and stripping gas rate on water
content in glycol dehydration units. Mohamadbeigy,et al
(7)
investigated
effectiveness parameter of water adsorption on molecular sieve to find
optimum operating condition. The obtained experimental breakthrough
curves were fitted to theoretical models in order to establish the main
mechanisms of mass transfer. Mohamadbeigy , Kasir and Hormozdi
(21)
investigated the effect of some influential factors such as temperature and
flow rate of TEG entering the tower number of equilibrium stages and liquid
fractional entrainment in gas phase on performance of absorption tower in
natural gas dehydration process. Vincente et. Al.
(22)
studied the effects of
varying the glycol flow rate, number of stages in the contactor, reboiler
temperature, and stripping gas rate on water content in glycol dehydration
units and presented the effect of high carbon dioxide composition in the feed.
Gholaml
(23)
developed a comprehensive mathematical model to investigate
the simplifying assumptions in modeling of natural gas dehydration by the
adsorption process. Axens company
(11)
used multi bed technology in the
drying of natural gas by combination of activated alumina with molecular
sieve .This technology improved adsorption capacity , purification capability
and life time of the adsorbent.
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22
Chapter Three
Design and Theoretical Consideration
Design and Theoretical Considerations
3-1 Design Considerations
3-1-1 Solid Desiccant Dehydration
The following considerations are a good approximation for estimation
of the solid desiccant dehydration behavior. This information serves only as
a basis for performing preliminary design calculations based on a given cycle
length, number of vessels and their configuration, and a given desiccant.
Therefor, it is highly recommended to refer to desiccant vendors. For
designing a solid desiccant dehydration unit, as the useful capacity of a
desiccant is highly dependent on its aging behavior. To take aging into
account, experience is very important and no published correlations exist
intellectual property and know how. To use only literature data will result in
either uneconomical units or potentially nonworking units (3) .
3-1-1-1 Allowable Gas Velocity
Generally, as the gas velocity during the drying cycle decreases the ability of
the desiccant to dehydrate the gas increases. Low velocities require towers
with large cross sectional areas to handle a given gas flow and allow the wet
gas to channel through the desiccant bed with incomplete dehydration. In
selecting the design velocity, therefore,a compromise must be made between
the tower diameter and the maximum use of the desiccant. Smaller velocities
may be required due to pressure drop considerations. An alternative method
for determining superficial gas velocity in a molecular sieve bed is using the
Ergun equation
(12),
which relates ∆P to V sG , µ, p,and desiccant size, as
follows(3) .
∆P/L = Bµ VsG +C pG V²sG
(3-1)
Where ∆P/L is pressure drop per length of bed ,psi/ft.
µ is gas viscosity, cP.
pG is gas density,Ib/ft³. and VsG is superficial gas velocity,ft/min. Constants
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23
Chapter Three
Design and Theoretical Consideration
for Equation (3-1) given in Table (3-1) for molecular sieve materials.
Table (3-1) Parameters used in Equation (3-1)
Particle type
1/8-in. beads
1/8-in. extrudate
1/16-in. beads
1/16-in. extrudate
coefficients
B
0.0560
0.0722
0.152
0.238
C
0.0000889
0.000124
0.000136
0.000210
The design pressure drop across the entire bed should be about 5 psi;
values higher than approximately 8 psi are not recommended. Most designs
are based on∆P/L of about 0.31 -0.44 psi/ft , and typical superficial gas
velocities of 30-60 ft/min(3).
3-1-1-2 Bed Length to Diameter Ratio
After superficial gas velocity is determined, then the diameter and
length of the bed can be calculated from geometry of the adsorber. The
adsorber is normally a cylindrical tower filled with a solid desiccant. The
depth of the desiccant may vary from a few feet to 30 ft or more. The
minimum bed internal diameter for a specified superficial gas velocity is
given by the following equation (Ergun,) (12).
D²= 25(QG)(T)(Z)/(P)(VsG)
(3-2)
Where D is bed diameter,ft, QG is gas flow rate, MMscfd;T is inlet gas
temperature, ºR; P is inlet gas pressure, psia; Z is compressibility factor and
VsG is superficial gas velocity, ft/min.
Also, the bed length, can be determined by the following equation (Collins,)
(13) .
LB= 127.3(W)/(pb)(D²)(X)
(3-3)
Where LB is bed length, ft; W is weight of water adsorbed, Ib per cycle,pb is
bulk density of desiccant,Ib/ft³;and X is maximum desiccant useful capacity,
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24
Chapter Three
Design and Theoretical Consideration
Ib water/100 Ib desiccant. A bed length to diameter ratio of higher than 2.5 is
desirable.
3-1-1-3 Desiccant Capacity
The maximum desiccant useful capacity can be calculated as equation.
(X)(LB) = (Xs)( LB) –(0.45)(Lz)(Xs)
(3-4)
Where X is desiccant useful capacity, Ib water per 100-Ib desiccant;Xs is
dynamic capacity at saturation, Ib water per 100-Ib desiccant; Lz is MTZ
length, ft;and LB is bed length, ft (3) .
3-1-1-4 MTZ (Mass Transfer Zone) Length
The MTZ length, LMTZ , depends on gas composition, flow rate,RS
For silica gel the MTZ length may be estimated from the following equation
(simpson and cummings) (14) .
LMTZ = 375 [mw^0.7895 / VsG^0.5506 (RS)^0.2646]
(3-5)
Where LMTZ is MTZ length, inch; mw is water loading, Ib/(hr.ft²);VsG is gas
superficial velocity, ft/min; RS is percentage relative saturation of inlet gas.
The relevant equation for calculation of the water loading (mw ) can be
written as follows (Ledoux) (15) .
mw = 0.053[QG(W)/D²]
(3-6)
where QG is gas flow rate , MMscfd; D is bed diameter, ft; and w is water
content of gas, Ib/MMscf.
3-1-1-5 Breakthrough Time
The breakthrough time for the water zone formed, tb in hours, can be
estimated as follows (3) (McCabe et al) (16) .
tb = (0.01)(X)(pb)(LB)/mw (3-7)
3-1-2 Glycol Dehydration
More detailed information about each equipment operation and
design used in a TEG dehydration unit is described as follow.
3-1-2-1 Absorber (Contactor)
The incoming wet gas and the lean TEG are contacted countercurrently
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25
Chapter Three
Design and Theoretical Consideration
in the absorber to reduce the water content of the gas to the required
specifications (3) .
The water removal rate, assuming the inlet gas is water saturated, can be
determined as:
Wr = QG (Wi –wo)/24
(3-8)
Where Wr is water removed, Ib/hr; Wi is water content of inlet gas,
Ib/MMscf; Wo is water content of outlet gas, Ib/MMscf; and QG gas flow
rate,MMscfd.
The glycol circulation rate is determined on the basis of the amount of
water to be. Higher circulation rates provide little additional dehydration
while increasing reboiler fuel and pumping requirements. Problems can arise
if the TEG circulation rate is too low (3) .
The minimum glycol circulation rate can then be calculated as
QTEG,min = G * Wr
(3-9)
Where QTEG,min is the minimum TEG circulation rate (gal TEG/hr) and G is
the glycol-to-water ratio (gal TEG/Ib water removed).
The diameter of the contactor (absorber) can be estimated from the
Souders and Brown (17) . correlation as follows.
Vmax = KSB [pL – pG/ pG] = [4QG/∏D²]
(3-10)
Where D is internal diameter of glycol contactor,QG is gas volumetric flow
rate,ft³/hr;Vmax is maximum superficial gas velocity,ft/hr; KSB is Souders
and Brown coefficient, ft/hr; pL is glycol density, Ib/ft³; and pG gas density at
column condition, Ib/ft³. Traditionally , the glycol absorber contains 6-12
trays. The standard tray spacing in glycol contactors is 24 inches;closer
spacing increases glycol losses if foaming occurs because of greater
entrainment. The total height of the contactor column will be based on the
number of trays required plus an additional 6-10 ft to allow space for a vapor
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26
Chapter Three
Design and Theoretical Consideration
disengagement area above the top tray and an inlet gas area at the bottom of
the column (3).
3-1-2-2 Still (Stripper)
The still or stripper column is used in conjunction with the reboiler to
regenerate the glycol. The diameter of the still is based on the liquid load
(rich glycol and reflex) and the vapor load (water vapor and stripping gas).
(Caldwell,; Sivalls) (18). Alternatively, the following approximate equation is
used:
D = 9(QTEG)º·⁵
(3-11)
Where QTEG is TEG circulation rate, gal/min ;and D is inside diameter of
stripping column,inch. Smaller diameter towers (less than 2 ft in diameter)
are often packed with ceramic intalox saddles or stainless steel pall rings
instead of trays. Larger-diameter towers may be designed with 10 to 20
bubble cap trays or structured packing.
3-1-2-3 Reboiler
The reboiler and still are typically a single piece of equipment. The
reboiler supplies heat to regenerate the rich glycol in the still by simple
distillation. Reboiler duty can be estimated by the following equation
(Sivalls,) (18).
QR = 900 + 966(G) (3-12)
Where QR is regenerator duty,Btu/Ib H2O removed; and G is glycol-towater ratio,gal TEG/Ib H2O removed.
The reboiler normally operate at a temperature of 350 to 400 ºF for a
TEG system. This temperature controls the lean glycol water concentration.
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27
Chapter Three
Design and Theoretical Consideration
3-1-2-4 Surge Tank (Accumulator)
Lean glycol from the reboiler is routed through an overflow pipe or weir
to a surge tank or accumulator. Because this vessel is not insulated in many
cases, the lean glycol is cooled to some extent via heat loss from the shell.
3-1-2-5 Heat Exchanger
Two types of heat exchangers are found in glycol plant: glycol/glycol and
gas/glycol.
3-1-2-5-1 Glycol/Glycol Exchanger
A Glycol/Glycol exchanger cools the lean glycol while preheating the
rich glycol. It may be an external exchanger or may be located within the
surge tank (an integral exchanger). For small standard designs, the integral
exchanger is economical to fabricate but may not heat the rich glycol above
200 ºF. (3) .
3-1-2-5-2 Dry Gas/Lean Glycol Exchanger
This type of exchanger uses the exiting dry natural gas to control the
lean glycol temperature to the absorber. High glycol temperature relative to
the gas temperature reduce the moisture absorption capacity of TEG (3) .
3-1-2-6 Phase Separator (Flash Tank)
Many glycol dehydration units contain an emissions separator and a
three-phase vacuum separator. An emissions separator removes dissolved
gases from the warm rich glycol and reduces VOC emissions from the still.
The wet or rich glycol is flashed at 50-100 psia and 100-150ºF.
A three-phase vacuum separator is desirable if liquid hydrocarbons are
present. This allows the liquid hydrocarbon to be removed before it enters
the still, where it could result in emissions or cause excess glycol losses from
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28
Chapter Three
Design and Theoretical Consideration
the still vent. Because the hydrocarbons are collected under a vacuum,they
are stable and no vapor losses or weathering occurs. The design of the threephase separator is similar to that of two-phase separator except that it has a
second control valve and liquid level controller to drain the accumulated
hydrocarbon phase.
3-1-2-7 Glycol Circulation Pumps
A circulation pump is used to move the glycol through the unit. A wide
variety of pump and driver types are used in glycol systems, including
gas/glycol-powered positive displacement or glycol balance pumps (e.g.,
kimray pumps) and electric motor-driven reciprocating or centrifugal pumps.
3-1-2-8 Filters
Two types of filters are commonly used in glycol system.
3-1-2-8-1 Fabric (Particulate) Filters
The suspended solids content of glycol should be kept below 0.01 wt%
to minimize pump wear,plugging of exchangers,fouling of absorber trays and
stripper packing,solids deposition on the firetube in the reboiler, and glycol
foaming. Solid filters are selected to remove particles with a diameter of 5
µm and larger. A common design is a 3-inch diameter by 36-inch-long
cylindrical element in housing,sized for a flow rate of 1 to 2 gallon per
minutes per element. The filters are sized for a 12 – to 15-psi pressure drop
when plugged. This filter will remove any foreign solid particles picked up
in the absorber before entering the glycol pump.
3-1-2-8-2 Carbon Filters
Activated carbon filters are used to remove dissolved impurities in the
glycol,
such
as
high-boiling
hydrocarbons,surfactants,well-treatin
chemicals,compressor lubricants,and TEG degradation products.
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29
Chapter Three
Design and Theoretical Consideration
3-2 Operational Problems
3-2-1 Solid Desiccant Dehydration
Operational problems that may occur because of poor design,
operation, and maintenance in a solid desiccant unit are described in this
section.
3-2-1-1 Bed Contamination
The most frequent cause is incomplete removal of contaminants in the
inlet gas separator. Also, if the regeneration gas leaving the separator is
commingled with the feed gas to the dehydrators,then a separator
malfunction can dump liquid hydrocarbons and water onto the desiccant
regenreration separators should usually be equipped with filtration levels
similar to the inlet gas to prevent recontamination (3) .
3-2-1-2 High Dew Point
High dew point is one of the two common problems that can cause
operating trouble. Possible causes include the following.
1."wet" inlet gas bypasses the dehydrator through cracks in the internal
insulation. Cracks in a liner or in sprayed-on insulation can be detected by
"hot spots" and peeling paint on the outer shell. Other symptoms are fast
water breakthrough and an unusually rapid rise in the effluent gas
temperature during regeneration.
2.Leaking valves also permit wet gas to bypass the dehydrators. Even a
slight leak of hot gas usually produces a detectable temperature rise in what
should be the cold side of the valve. Ultrasonic translators are also useful.
3.Incomplete desiccant regeneration will lead to a sudden loss in adsorption
capacity and a significantly premature breakthrough. To be sure to well
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30
Chapter Three
Design and Theoretical consideration
regenerate the adsorbents the inlet and outlet temperatures of the adsorber in
regeneration should be analyzed. At the end of the heating step the outlet
temperature should be almost contact during a certain time (30 minutes to 2
hours) depending on the design of the adsorber,and the temperature
difference between inlet and outlet should not be more than 59-68ºF
depending on the quality of the heat insulation.
4.Excessive water content in the wet feed gas due to increased flow
rate,higher temperatures,and lower pressure. It is very important to respect
the inlet temperature (feed temperature) of the adsorbers in case of saturated
gas. Small variations in temperature will lead to significant increases in the
water content (3) .
3-2-1-3 Premature Breakthrough
Satisfactory dew points are observed at the beginning but not for the
entire duration of the drying cycle. Desiccant capacity should decrease with
use but should stabilize at 55-70% of the initial capacity Ballard,
(24).However,
premature symptoms of "old gas" are caused by an
unrecognized increase in inlet water loading, an increase in heavy
hydrocarbons (C4+) in feed gas, methanol vapor in feed, desiccant
contamination, or incomplete regeneration.
3-2-1-4 Hydrothermal Damaging
Heating up the adsorber without using a heating ramp or an
intermediate heating step leads to a strong temperature difference in the
vessel. At the bottom, the molecular sieve will be very hot and will desorb
rapidly the adsorbed water while the layers at the top of the adsorber will be
still at adsorption temperature. The water desorber in the bottom layer will
condense in the top layer. Hydrothermal damaging will appear in
consequence, which is different depending on the type of molecular sieve. In
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31
Chapter Three
Design and Theoretical Consideration
order to prevent hydrothermal damaging of molecular sieves it is not only
important to choose the right formulation of the molecular sieve (binder and
zeolite) but the operating conditions; the regeneration conditions should be
determined carefully. In fact, the higher the regeneration temperature and the
higher the amount of liquid water present on the sieves, the heavier the
damaging of the molecular sieves. In an industrial unit it is also important to
limit the quantity of water appearing in liquid phase (condensing water due
to oversaturation of the gas phase), as this decreases the temperature where
hydrothermal destruction may occur with water acting as a stabilizer for
intermediates formed by dissolution of the zeolites.
3-2-1-5 Liquid Carryover
Liquid (particularly amines) carryover in the molecular sieve bed has a
bad effect on the drying process. In order to reduce the liquid carryover in
adsorbers, separators must be modified to improve their efficiency. The
regeneration procedure should also be changed so as to have a moderate
temperature increase to avoid water recondensation. Although on-site
mechanical changes of the drying unit can improve the performance,
however, the right corrective action can be found by using a more resistant
molecular sieve, SRA. The SRA adsorbent offers a better mechanical
resistance than the regular one to severe operating conditions simulating the
thermal regeneration step of a natural gas purification unit.
3-2-1-6 Bottom Support
Sometimes operators have problems with the support grid and leakage
of molecular sieves through the support grid. As a result, they have to
replace the whole bed. Important point here is the good mechanical design of
the support bed, putting three wire mesh on the support grid (4, 10,20 mesh)
and installing the correct quantity and size of ceramic balls.
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32
Chapter Three
Design and Theoretical Consideration
3-2-2 Glycol Dehydration
The operating problems associated with each equipment in the TEG
dehydration unit are described individually in the following sections.
3-2-2-1 Absorber
The main operating problems associated with the absorber are
insufficient dehydration, foaming and hydrocarbon solubility in glycol,
which are discussed next.
3-2-2-1-1 Insufficient Dehydration Causes of insufficient dehydration (i,e,
wet sales gas) include excessive water content in the lean glycol, inadequate
absorber design, high inlet gas temperature, low lean glycol temperature, and
overcirculation /undercirculation of glycol (3) .
3-2-2-1-2 Foaming
Foaming causes glycol to be carried out of the absorber top with the
gas stream, resulting in large glycol losses and decreased glycol unit
efficiency. Foaming can normally be traced to mechanical or chemical
causes. High gas velocity is usually the source of mechanical entrainment.
Chemical foaming is caused by contaminants in the glycol, liquid
hydrocarbons, well-treating chemicals, salts, and solid. Adequate inlet
separation and filtration system (cartridge filter and activated carbon bed) are
therefore needed to prevent foaming due to chemical contamination.
3-2-2-1-3 Hydrocarbon solubility in TEG solution Aromatic hydrocarbon
solubility in glycol is a significant issue in gas dehydration technology due to
the potential release of aromatics to the atmosphere at the regenerator.
3-2-2-2 Still (Stripper)
Excessive glycol vaporization is more of a problem for finned
atmospheric condensers than for water-cooled or glycol-cooled condensers.
Page
33
Chapter Three
Design and Theoretical Consideration
Although finned atmospheric condensers are simple and inexpensive,
they are sensitive to extremes in ambient temperature. For example, during
cold winter periods, a low temperature at the top of the still column causes
excessive condensation and floods the reboiler. This prevents adequate
regeneration of the glycol and reduces the potential dew point depression of
the glycol causing insufficient dehydration in the absorber. Also, excessive
glycol losses may occur as the reboiler pressure increases and blows the
liquids out the top of the column. During the summer months, inadequate
cooling may allow excessive glycol vaporization losses.
3-2-2-3 Reboiler
Operational problems associated with the reboiler include salt
contamination,glycol degradation, and acid gas-related problems.
3-2-2-3-1 Salt Contamination
Carry over of brine solution from the field can lead to salt
contamination in the glycol system. Sodium salt(typically sodium chloride,
NaCl) are a source of problems in the reboiler, as NaCl is less soluble in hot
TEG than in cool TEG; NaCl will precipitate from the solution at typical
reboiler temperature of 350-400ºF. The salt can deposit on the fire tube,
restricting heat transfer. If this occurs,the surface temperature of the fire tube
will increase,causing hot spots and increased thermal degradation of the
glycol. The deposition of salt may also result in corrosion of the fire tube.
Dissolved salt cannot be removed by filtration. As a general rule, when the
salt content reaches 1%, the glycol should be drained and reclaimed. If the
level of salts is allowed to increase beyond 1% both severe corrosion and
thermal degradation threaten the system.
3-2-2-3-2 Glycol Degradation
Glycol degradation is caused primarily by either oxidation or thermal
Page
34
Chapter Three
Design and Theoretical Consideration
degradation. Glycol readily oxidizes to form corrosive acid. Oxygen can
enter the system with incoming gas, from unblanketed storage tanks or
sumps, and through packing glands. Although oxidation inhibitors [such as a
50-50 blend of monoethanolamine (MEA) and 33% hydrazine solution] can
be used to minimize corrosion, a better approach to controlling oxidation is
blanketing the glycol with natural gas, which can be applied to headspace in
storage tank and any other area where glycol may contact oxygen. Thermal
degradation of the glycol results from the following conditions: high reboiler
temperature, high heat flux rate, and localized overheating. The reboiler
temperature should always be kept below 402ºF to prevent degradation, and
a good fire tube design should inherently prevent a high heat flux rate. In
addition, thermal degradation of the glycol produces acid degradation
products that lower the PH and increase the rate of degradation, creating a
destructive cycle.
3-2-2-3-3 Acid Gas
Some natural gas contains H2S and/or CO2, and these acid gases may
be absorbed in the glycol. Acid gases can be stripped in the reboiler and still.
Mono-,di-, or triethanolamine may be added to the glycol to provide
corrosion protection from the acid gases(3) .
3-2-2-3-4 Surge Tank
When surge tanks also serve as glycol/glycol heat exchanger, the level
must be monitored to ensure that the lean glycol covers the rich glycol coil.
Otherwise, inadequate heat exchange will occur, and the lean glycol will
enter the absorber at an excessively high temperature.
3-2-2-3-5 Heat Exchanger
The primary operational problem with heat exchangers is poor heat
transfer, which results in lean glycol that is too warm. When this occurs,
Page
35
Chapter Three
Design and Theoretical Consideration
poor dehydration and insufficient dew point depression can result. Also,
glycol vaporization losses to the product gas may be higher with increased
lean glycol temperature. Poor heat transfer and the resulting high lean glycol
temperature are usually caused by fouled heat exchangers. Exchangers may
be fouled by deposits such as salt, solids,coke,or gum. Corrosion of the coil
in surge tank heat exchangers can also present operating problems, as it can
lead to cross-contamination of rich and lean glycol.
3-2-2-3-6 Phase Separator (Flash Tank)
Inadequate residence time in the phase separator may result in a large
quantity of glycol being included in the hydrocarbon stream and vice versa.
3-2-2-3-7 Glycol Circulation Pump
Major problems associated with the circulation pump and rates are
related to reliability,pump wear, and overcirculation or undercirculation(3) .
Page
36
Chapter Four
Case Study
Case Study
4-1 Dehydration Process
In the north gas company the treated gas stream from the sweetening
unit is cooled to approximately 38ºC through EA-1305 to condense out
excess water.This cooling reduces significantly the required amount of
molecular sieve .The water condensate ,after separation from the gas stream
in the K.O.(knock-out) drum which is a vapor-liquid separator. It is a vertical
vessel used in several industrial applications to separate a vapor-liquid
mixture. There is a barrier inside K.O drum to separator the gas by bang to
it .Gravity causes the liquid to settle to the bottom of the vessel ,where it is
withdrawn and there is a clip upper the drum to prevent crossing of the liquid
with gas . (FA-1304), is returned to the sweetening unit for re-use. The gas
is normally not cooled enough to condense hydrocarbons. Any condensed
hydrocarbons ,if present in the K.O. drum, are sent to the Burning pit (BT0603). The gas from K.O. drum flows to the charge-Gas Dryer (FF-1301
A/B) before proceeding to the downstream , low-temperature separation
section. The gas is dried to prevent plugging due to freeze-ups and hydrate
formation at low temperatures . Two vessels containing molecular sieve
(type 4A) desiccant are provided ; these alternate between on-stream drying
service and regeneration , on an 8-hour cycle (cycle length will be longer
with new desiccant and progressively decline with time). An inter-bed
moisture probe monitors operation and detects water break-through from the
bed .The dried gas is sent to the activated carbon Bed (FA-1305), where any
trace amounts of mercury in the gas stream are removed .The gas then flows
through the filter (FD-1301 A/B), to remove desiccant dust and other solids
which may cause plugging in the down-stream equipment. Then the gas is
sent to section drum (surge drum)(FA-1302) where its given enough time to
process. The gas is then compressed to approximately 46 kg/cm²G with
Page
37
Chapter Four
Case Study
a sidestream draw off at about 32.6 kg/cm²G
for dryer regeneration
gas.Before entering the chilling section, the compressor (GB-1301)
discharge is cooled to 40ºC with cooling water. The vapor, separated in the
compressor discharge Drum (FA-1303), is sent to the chilling section, while
the hydrocarbon condensate is directed to the Deethanizer (DA-1501). figure
(4-1) illustrates this process.
Figure (4-1) Dehydration process of natural gas in North Gas Company.
4-2 Regeneration Process
The sidestream from the charge-Gas compressor (GB-1301) , is first
heated in the fired heater (BA-1301) to 343ºC and then used as the
regeneration gas for all the dryers in the plant. The regeneration gas, after
dryer regeneration , is cooled with cooling water to 38ºC where it is used for
cooling the dryer to 38 ºC after heating. The regeneration gas is then sent to
Page
38
Chapter Four
Case Study
the K.O. drum (FA-1308) where water is removed . The water separated
from the K.O. (knock-out) drum is degassed in the degassing Drum (FA1306) before being discharged to the waste water treatment . The
regeneration gas from the K.O. (knock-out) drum (FA-1308) is recycled
back to the Absorber in the sweetening unit , for acid gas removal (DA1201) (10) . Anti surge out from the top of the vessel FA-1303 and enter the
path to enter the gas to the compressor GB-1301. Figure (4-1) illustrates this
process, and table (4-1) shows the composition of natural gas in dehydration
unit.
Table (4-1) The composition of natural gas in dehydration unit
4-3 The Molecular Sieve Dehydrator
In the North Gas Company, the molecular sieve bed for dehydration of
natural gas as shown in figure (4-2) consists of the following:1. The upper ceramic balls layer used as a support to the molecular sieve
and facilitates the passage of gas to the molecular sieve. The diameter of
single ball is (19mm) and the height of ceramic balls layer is (250mm) .
Page
39
Chapter Four
Case Study
2.The wire mesh layer is used to isolated molecular sieves from ceramic
balls and keeps the molecular sieves from coming up with gas.
4700 mm I.D
Sweet gas inlet
1. Ceramic balls
250mm
4800mm
2. Wire mesh
6800mm
3. Molecular sieves
4. Ceramic balls
100mm
Dry gas outlet
5.Bottom support grid
Figure (4-2) Molecular sieve bed
Page
40
Chapter Four
Case Study
3.The molecular sieves layer of type 4A represents the main part for
adsorbing the water from the inlet gas. The height of the molecular sieve
layer is (4800mm).
4.The lower ceramic balls layer is used to support the molecular sieve and
facilitates the passage of gas to the molecular sieve passage of gas to the
molecular sieve. The diameter of single ball is (6.2mm) and the height of
ceramic balls layer is (100mm) .
5.Bottom support grid, used to support the heavy weight of molecular sieve
(53.325ton).
Two beds are used in cyclic operation to dry the natural gas on a
continuous basis , one bed operates in adsorption , while the second operates
in desorption , and both beds are switched periodically. For dehydration , of
natural gas 4A type of molecular sieve is used in the North Gas
Company.This is a good choice because of its high selectivity and affinity
for water but the problem is after a certain number of cycles , molecular
sieve beds show broken particles and dust ,leading to aging of the molecular
sieve after (1.5-2) years instead of three years (the real life time of the
molecular sieve).
Many tests are made for the aging molecular sieve and the new
molecular sieve to show the difference between them.
1.(XRD) (X-ray Diffraction) tested in the state company of Geological
Survey and Mining.
2. (FT-IR) (Fourier Transform Infra Red) (BRUKER . TENSOR 27) tested
in Chemical Engineering Department at the University of Technology.
Figure (4-3) shows the FT-IR device.
3. Crystal structure analysis was carried out by microscope at the Materials
Page
41
Chapter Four
Case Study
Engineering Department at the University of Technology.
4. The surface area and the pore volume of the aging and the new molecular
sieve were measured at the Petroleum Research and Development Center.
Figure (4-3) FT-IR device
Page
42
Chapter Five
Results and Discussion
5-1 Results and Discussion
The results of testing the aging and the new molecular sieve are shown
in figure (5-1) and figure (5-2) . The results of X-ray Diffraction test showed
no large and influential different between the new and the aging molecular
sieve as shown in figures (5-1) and (5-2). The results of (FT-IR) (Fourier
Transform Infra Red) as in figure (5-3) showed no difference between the
new and the aging of molecular sieve. The crystal structure of the aging and
the new molecular sieve were tested by microscope with
magnification
power (100X ) (BEL. Italy ) and a picture was taker for the result of the
tested samples, the microstructure of each sample is different as shown in
figures (5-4) and (5-5).From these figures, it can be seen that the dark
regions in the aging molecular sieve are less than in the new molecular
sieves. This means that most of the pores (dark regions) in the aging
molecular sieve are blocked by contaminants which causes the decreasing in
the adsorption capacity of the molecular sieves. Also the surface area and the
pore volume of the aging and the new molecular sieve were measured as
shown in table (5-1). It can be seen that the surface area and the pore volume
for the aging molecular sieve are lower than for the new molecular sieve.
This confirms the above result about the blocking of the molecular sieve.
Table (5-1) Values of Surface Area and Pore volume as means the new
and aging molecular sieve
Test
1
Surface Area (m²/gm)
New molecular Aging
sieve
molecular
sieve
46.2643
7.3170
2
Pore volume (cm³/gm)
0.0569
NO
Page
43
0.0184
Chapter Five
Result and Discussion
Figure (5-1) New molecular sieve
Page
44
Chapter Five
Result and Discussion
Figure (5-2) Aging molecular sieve
Page
45
Chapter Five
Result and Discussion
Figure (5-3) FT-IR of new and aging molecular sieve
Page
46
Chapter Five
Result and Discussion
Figure (5-4) New molecular sieve
Figure (5-5) Aging molecular sieve
Page
47
Chapter Five
Result and Discussion
So the most frequent cause of the aging , blocking and breaking up of
the molecular sieve may be the following :1.The incomplete removal of free liquid water in the inlet gas separator , so
the natural gas enters the dehydrator with a large amount of water
(2800ppm). This is a very important part of the system because free water
can cause the adsorbent materials to break down.This leads to higher
pressure drop and channeling , reducing the overall performance of the unit.
2. Liquid (particularly amines from sweetening unit) may carry over in the
molecular sieve bed which has a negative impact on the drying process (i.e.
poor gas flow distribution due to fines formation as a consequence of
chemical attack causing an increase of the pressure drop and a decrease of
adsorption time).
3. Hydrocarbons are adsorbed on molecular sieve just as water is but less
preferentially. That portion of the bed at a given time which is not
dehydrating gas is retaining hydrocarbons. The heavier hydrocarbon are
retained preferentially over the light hydrocarbons, where dehydration stops
short of the break through point , the portion of the bed not used for
dehydration will have retained hydrocarbons. Many dehydration units
produce hydrocarbon liquid in the regeneration condensate. So aging of
molecular sieve may be caused by coal sorption of hydrocarbons and coke
formation on the active surface of the adsorbent. This phenomenon is not
completely
reversible,
and
carbon
deposits
increase
with
each
regeneration.This will block the pores of molecular sieve.
4. The aging of molecular sieve may be hydrothermal aging which is an
irreversible change of adsorbent structure caused by hydrothermal damaging
during regeneration. At the bottom of the bed , the molecular sieve will be
very hot and will desorb rapidly the adsorbed water while the layers at the
Page
48
Chapter Five
Result and Discussion
top of the a desorber will be still at adsorption temperature (38ºC) . The
water adsorbed in the bottom layer will condense in the top layer . This
phenomenon is called refluxing . The heating going on will heat up the liquid
water and boil the molecular sieves in liquid water. So certain components of
the binders used in standard 4A molecular sieve are some what soluble in
liquid water . Some of the soluble components can ion exchange with the
zeolite and /or combine with anions in the liquid water to form solid salts
(i.e.N2CO3,CaCO3, MgCO3 , NaNO3 , etc ) when the water evaporates . This
condition leads to hydrothermal damaging. Also the intimate contact of
molecular sieve with the rapidly vaporizing liquid water in the liquid reflux
zone results in accelerated particle break-up ; leading to(non – uniform )
pressure drop build-up through the molecular sieve bed and ultimately
channeling and premature water break through. on inspection , such
molecular sieve beds have shown broken particles and dust that has
compacted in a layer that can very in depth from several tens of centimeters
to, in extreme case , the majority of the bed.
5-2 Overcoming the Breaking Up and Aging Problems of the Molecular
Sieve
1. Gas separator must be modified to improve their efficiency to maximize
water droplet removal. Also a filter separator is required if the adsorption
unit is downstream from an amine unit. Although there is a K.O drum , some
time amine cross with the natural gas.
2. Liquid (particularly amines) carry over in the molecular sieve bed must be
prevented by using antifoaming material, in the previous unit , and the gas
flow rate must be controlled carefully because at excessive velocities, amines
can be lifted up of the sweetening unit with the gas to the molecular sieve
bed.
Page
49
Chapter Five
Result and Discussion
3. It is important to choose the regeneration conditions carefully. In fact , the
higher the regeneration temperature and the higher amount of liquid water
present on the sieves, the heavier the damaging of the molecular sieve . This
leads to shorter desiccant life , and lower regeneration temperatures lead to
insufficient dehydration capacities.
4. A layer of less expensive desiccant (activated alumina) in the upper
section of the molecular sieve dehydrator will catch contaminants, such as
amines, this is called multi bed technology (11) . The results of this technology
are improved adsorption capacity , purification capability , life time of the
adsorbent and extend the bed life . Figure (5-6) shows the applications of the
multi bed technology.
5. Solid desiccant dehydrators are typically more effective than glycol
dehydrators , as they can dry a gas to less than 0.1 ppm v (0.05 Ib/MMCF).
However , in order to reduce the high amount of water in the inlet natural gas
and the size of the solid desiccant dehydrator , it is better to use glycol
dehydration unit to reduce the water content to around 60 ppm v and then
use solid desiccant for final drying.
Figure (5-6) Applications of the Multibed™ Technology(11)
Page
50
Chapter Six
6-1
Conclusions and Recommendations
Conclusions
From this research it can be concluded that the reason for breaking
up,blocking and aging the molecular sieve, is one or more of the following:
1. Amine carry over.
2. Water condensed.
3. The temperature of regeneration.
4. Inlet gas velocity.
So the problem can be treated by one or more of the following:
1. Gas separator must be modified to improve their efficiency to maximize
water droplet removal.
2. Filter separator is required if the adsorption unit is downstream from an
amine unit.
3. Liquid (particularly amines) carry over in the molecular sieve bed must be
prevent by using antifoaming material.
4. It is important to choose the regeneration conditions carefully. Where the
higher the regeneration temperature and the higher amount of liquid water
present on the sieves, the heavier the damaging of the molecular sieve.
Which lead to shorter desiccant life and lower regeneration temperatures
lead to insufficient dehydration capacities.
5. A layer of less expensive desiccant (activated alumina) in the upper
section of the molecular sieve dehydrator will catch contaminants, such as
amines, this is called multi bed technology. The results of this technology
improve adsorption capacity , purification capability , life time of the
Page
51
Chapter Six
Conclusions and Recommendations
adsorbent and extend the bed life.
6. It is better to use glycol dehydration unit to reduce the water content to
around 60 ppm v and then using solid desiccant for final drying.
Page
52
Chapter Six
Conclusions and Recommendations
6-2 Recommendations
After finishing this study there are some recommendations that
may
be useful for research in the future work.
1. An experimental study of the problem of breaking up, blocking and
therefore the aging of molecular sieve must be done on-site of molecular
sieve dehydration unit of the Iraqi North Gas Company in order to
investigate the real variables that may affect the aging of molecular sieve and
try to treat it. Some of these variables which may have effects, are the inlet
gas velocity to molecular sieve dehydrator, the composition of the inlet and
the outlet of the natural gas, the temperature of regeneration ,and then try to
find a way to treat it .
2. Study the feasibility when glycol dehydration unit is used at the first to
reduce the water content of natural gas to around 60 ppm v and then use
molecular sieve for final drying in steal of using only molecular sieve for
dehydration of natural gas . In this case , molecular sieve problems which
may occur because of high water content of natural gas.
Page
53
Page
54
References
1. Ahmad Syahrul Bin Mohamad "Natural gas dehydration using
triethyleneglycol (TEG)" Faculty of Chemical & Natural Resources
Engineering University Malaysia Pahang . April 2009.
2. Pezhman Kazemi and Roya Hamidi "Sensitivity analysis of a natural
gas triethylene glycol dehydration plant in Persian gulf region" Received
November 9, 2010, Accepted February 1, 2011.
3.Saeid Mokhatab William A. Poe James G. Speight "Hand Book of natural
gas transmission and processing".
4. Siti Suhaila Bt Mohd Rohani "natural gas dehydration using silica gel"
fabrication of dehydration unit. APRIL 2009.
5. Luká Polák "Modelling absorption drying of natural gas" May 2009
Trondheim.
6. Chapter 7. The dehydration and sweetening of natural gas. From the
internet (the dehydration of natural gas).
7. Kh. Mohamadbeigy , Kh.Forsat , R.Binesh. Experimental studying on
gas dewatering by molecular sieve. Available online at www.vurup.sk/pc
U
U
Petroleum & Coal 49 (1), 41-45, 2007. Received December 21, 2006;
accepted June 25, 2007.
8.From the internet (the chemical structure of molecular sieve types).
9.Metcalf and Eddy., "wastewater engineering treatment and reuse" 4th
edition. 2003.
10.Iraqi North Gas project process plant operating manual vol 1.
11.Activated alumina &Molecular Sieves. Quality and advanced
technology.Company Axens , Procatalyse Catalysts & Adsorbents.
12. Ergun, S ., Fluid flow through packed column. Chem. Eng. prog. 48, 2
1952. Cited in reference (3).
13. Collins,J.J., AIChE Symp. Ser., No. 74, 63: 31, 1967. Cited in
reference (3).
14.Simpson,E.A., and Cummings, W.P., A practical way to predict silica
gel performance."chem. Eng. Prog. 60(4), 57-60 ,1964. Cited in reference
(3).
15. Ledoux, E., Chem. Eng. (March 1948). Cited in reference (3).
16. McCabe, W.L., Smith, C.J., and Harriott, P., "Unit Operations of
Chemical Engineering," 4th Ed. McGraw-Hill, New York (1985). Cited in
reference (3).
17. Souders, M., and Brown, G.G., Fundamental design of absorbing and
stripping columns for complex vapours Ind. Eng. Chem. 24, 519. Cited in
reference (3).
18. Caldwell, R.E., "Glycol Dehydration Manual,"NATCO Group,
Tulsa,OK (Jan 30, 1976) (Sivalls, C.R., "Glycol Dehydration Design
Manual." Sivalls, Inc., Odessa, TX (June 1976). Cited in reference (3).
19 . Abu Phabi "Middle east adsorbents and gas processing Technology
conference" uAE , 2008.
20. Kh. Mohamadbeigy"Studying Of The Effectiveness Parameters On Gas
Dehydration Plant" Received December 15, 2007, accepted May 15, 2008.
21. N.Kasiri , Sh .Hormozdi "Improving performance of absorption tower
in natural gas dehydration process .
22. Vincente n. Hernandez-Valencia and et al"design glycol units for
maximum efficiency".
23. M. Gholaml and et al. Ind . engineering. Chemical .Res. 2010 , 49(2) ,
pp838-846 ).
24. Ballard, D., "How to Improve Cryogenic Dehydration."petroenergy,
83,Houston, TX (sept. 12-16, 1983). Cited in reference (3).
‫‪U‬‬
‫ﺍﳋﻼﺻﺔ‬
‫اﳍﺪف ﻣﻦ ﻫﺬا اﻟﺒﺤﺚ ﻫﻮ دراﺳﺔ ﻋﻤﻠﻴﺔ اﻟﺘﺠﻔﻴﻒ ﻟﻠﻐﺎز اﻟﻄﺒﻴﻌﻲ ﻋﻦ ﻃﺮﻳﻖ اﻻﻣﺘﺰاز ﺑﺎﺳﺘﺨﺪام اﳌﻨﺎﺧﻞ‬
‫اﳉﺰﻳﺌﻴﺔ ﻛﻤﺎ ﻫﻮ اﳊﺎل ﰲ ﺷﺮﻛﺔ ﻏﺎز اﻟﺸﻤﺎل ‪.‬ﲡﻔﻴﻒ اﻟﻐﺎز اﻟﻄﺒﻴﻌﻲ ﺿﺮوري ﻹزاﻟﺔ اﳌﺎء اﳌﺼﺎﺣﺐ ﻟﻠﻐﺎز‬
‫اﻟﻄﺒﻴﻌﻲ ﻋﻠﻰ ﺷﻜﻞ ﲞﺎر‪.‬ﺻﻨﺎﻋﺔ اﻟﻐﺎز اﻟﻄﺒﻴﻌﻲ أدرﻛﺖ ان اﻟﺘﺠﻔﻴﻒ ﻫﻮ ﺿﺮوري ﻟﻀﻤﺎن اﻟﺘﺸﻐﻴﻞ اﻟﺴﻬﻞ‬
‫ﳋﻄﻮط ﻧﻘﻞ اﻟﻐﺎز‪ ,‬اﻟﺘﺠﻔﻴﻒ ﳝﻨﻊ ﺗﻜﻮﻳﻦ ﻫﻴﺪرات اﻟﻐﺎز‪ ,‬وﻳﻘﻠﻞ ﻋﻤﻠﻴﺔ اﻟﺘﺄﻛﻞ ‪.‬‬
‫ﰲ ﺣﺎﻟﺔ ﻋﺪم اﺟﺮاء‬
‫ﻋﻤﻠﻴﺔ اﻟﺘﺠﻔﻴﻒ ﻗﺪ ﻳﺘﻜﺜﻒ اﳌﺎء اﻟﺴﺎﺋﻞ ﰲ اﻷﻧﺒﻮب وﻳﺘﺠﻤﻊ ﰲ ﻧﻘﻄﺔ ﻣﻨﺨﻔﻀﺔ ﻋﻠﻰ ﻃﻮل اﳋﻂ وﻳﻘﻠﻞ ﻣﻦ‬
‫ﺪرﻬﺗﺎ ﻋﻠﻰ اﻟﺘﺪﻓﻖ ‪ .‬وﻗﺪ وﺿﻌﺖ اﻟﻌﺪﻳﺪ ﻣﻦ اﻟﻄﺮق ﻟﺘﺠﻔﻴﻒ اﻟﻐﺎزات ﻋﻠﻰ ﻣﺴﺘﻮى ﺻﻨﺎﻋﻲ ‪.‬وﻫﻨﺎك ارﺑﻌﺔ‬
‫ﻃﺮق رﺋﻴﺴﻴﺔ ﻟﻠﺘﺠﻔﻴﻒ ﻫﻲ اﻟﺘﱪﻳﺪ اﳌﺒﺎﺷﺮ‪ ,‬واﻟﺘﱪﻳﺪ ﻏﲑ اﳌﺒﺎﺷﺮ ‪,‬اﻣﺘﺼﺎص‪ ,‬اﻣﺘﺰاز‪ .‬وﺗﺮﻛﺰ ﻫﺬﻩ اﻟﺪراﺳﺔ ﻋﻠﻰ‬
‫ﻃﺮﻳﻘﺔ اﻻﻣﺘﺰاز واﻟﺬي ﻳﺴﺘﺨﺪم ﻟﺘﺠﻔﻴﻒ اﻟﻐﺎز اﻟﻄﺒﻴﻌﻲ ﰲ ﺷﺮﻛﺔ ﻏﺎز اﻟﺸﻤﺎل‪ ،‬وﻳﺮﻛﺰ أﻳﻀﺎ ﻋﻠﻰ ﻣﺸﻜﻠﺔ‬
‫ﺗﻔﺘﻴﺖ واﻧﺘﻬﺎء ﻛﻔﺎءة اﳌﻨﺎﺧﻞ اﳉﺰﻳﺌﻴﺔ ﻗﺒﻞ أن ﻳﻨﺘﻬﻲ ﻋﻤﺮﻩ اﻟﺘﺸﻐﻴﻠﻲ ‪.‬‬
‫ﺑﻌﺾ اﻟﺘﺠﺎرب اﺟﺮﻳﺖ ﻟﻠﻤﻨﺎﺧﻞ‬
‫اﳉﺰﻳﺌﻴﺔ ﻟﻠﺠﺪﻳﺪ واﻟﺘﺎﻟﻒ ﻷﻇﻬﺎر اﻟﻔﺮق ﺑﻴﻨﻬﻤﺎ ‪ .‬ﺑﻌﺾ اﳌﻘﱰﺣﺎت وﺿﻌﺖ ﻟﻠﺘﻐﻠﺐ ﻋﻠﻰ ﻣﺸﻜﻠﺔ اﻧﺘﻬﺎء ﻛﻔﺎءة‬
‫اﳌﻨﺎﺧﻞ اﳉﺰﻳﺌﻴﺔ ‪ ,‬ﻣﺜﻞ ﺗﺤﺴﻴﻦ ﻛﻔﺎءة وﻋﺎء ﻓﺼﻞ اﻟﻐﺎز ‪ ,‬اﺳﺘﺨﺪام ﻣﺎﻧﻊ اﻟﺮﻏﻮة ﻟﺘﺠﻨﺐ ﻋﺒﻮر اﻷﻣﲔ ﻣﻊ‬
‫اﻟﻐﺎز‪,‬اﺧﺘﻴﺎر ﻇﺮوف اﻋﺎدة اﻟﺘﻨﺸﻴﻂ ﺑﺪﻗﻪ ‪ ,‬اﺳﺘﺨﺪام ﺗﻜﻨﻮﻟﻮﺟﻴﺎ ﺛﻨﺎﺋﻲ اﳌﺎدة واﺳﺘﺨﺪام وﺣﺪة اﻟﺘﺠﻔﻴﻒ‬
‫ﺑﺎﻟﻜﻼﻳﻜﻮل ﺑﺎﻷﺿﺎﻓﺔ اﱃ وﺣﺪة اﻟﺘﺠﻔﻴﻒ ﺑﺎﳌﻨﺎﺧﻞ اﳉﺰﻳﺌﻴﺔ ﳋﻔﺾ ﳏﺘﻮى اﳌﺎء ﻣﻦ اﻟﻐﺎز اﻟﻄﺒﻴﻌﻲ‪.‬‬
‫ﻭﺯﺍﺭﺓ ﺍﻟﺗﻌﻠﻳﻡ ﺍﻟﻌﺎﻟﻲ ﻭﺍﻟﺑﺣﺙ ﺍﻟﻌﻠﻣﻲ‬
‫ﺍﻟﺟﺎﻣﻌﺔ ﺍﻟﺗﻛﻧﻭﻟﻭﺟﻳﺔ‬
‫ﻗﺳﻡ ﺍﻟﻬﻧﺩﺳﺔ ﺍﻟﻛﻳﻣﻳﺎﻭﻳﺔ‬
‫ﺩﺭﺍﺳﺔ ﻋﻣﻠﻳﺔ ﺍﻟﺗﺟﻔﻳﻑ ﻟﻠﻐﺎﺯﺍﻟﻁﺑﻳﻌﻲ ﻓﻲ ﺷﺭﻛﺔ ﻏﺎﺯ ﺍﻟﺷﻣﺎﻝ ﺍﻟﻌﺭﺍﻗﻳﺔ‬
‫ﻭﻁﺭﻕ ﻣﻌﺎﻟﺟﺔ ﻣﺷﺎﻛﻝ ﺍﻟﻣﻧﺎﺧﻝ ﺍﻟﺟﺯﻳﺋﻳﺔ‬
‫رﺴﺎﻟﺔ ﻤﻘدﻤﺔ اﻟﻰ‬
‫ﻗﺴم اﻟﻬﻨدﺴﺔ اﻟﻛﻴﻤﻴﺎوﻴﺔ ﻓﻲ اﻟﺠﺎﻤﻌﺔ اﻟﺘﻛﻨوﻟوﺠﻴﺔ ﻛﺠزء ﻤن ﻤﺘطﻠﺒﺎت‬
‫ﻨﻴﻝ درﺠﺔ اﻟدﺒﻠوم اﻟﻌﺎﻟﻲ‬
‫ﻓﻲ اﻟﻬﻨدﺴﺔ اﻟﻛﻴﻤﻴﺎوﻴﺔ ‪/‬‬
‫ﺘﺼﻔﻴﺔ اﻟﻨﻔط‬
‫وﺘﻛﻨوﻟوﺠﻴﺎ اﻟﻐﺎز‬
‫إﻋداد‬
‫ﻓرﻤﺎن ﺴﻌﻴد ﻋﺒداﷲ زﻩ ﻨﻛﻨﻪ‬
‫)ﺒﻛﺎﻟورﻴوس ﻫﻨدﺴﺔ ﻛﻴﻤﻴﺎوﻴﺔ ‪(2004‬‬
‫ﺒﺈﺸراف‬
‫أ‪.‬م‪.‬د‪ .‬أﻨﻌﺎم أﻛرم ﺼﺒري‬
‫ﺼﻔر ‪1433‬ه‬
‫ﺸﺒﺎط ‪2012‬‬