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Contents

y Project Background

y Problem Statements

y Objective

y

Literature ReviewyGas hydrates

yNatural gas dehydration

yAbsorption process

yEquation of State

y Methodology

y References


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Project Background

y Natural gas is a mixture of naturally occurring hydrocarbon and non-hydrocarbon gasesfound in porous geologic formations beneath the earths surface.

y It occurs with petroleum deposits, principally methane together with varying quantities

of ethane, propane, butane, and other gases as well.

y Mostly it is used as a source of fuel and in manufacture of organic compounds.

y Raw natural gas drilled from the offshore platforms contains other gases as well such as

carbon dioxide, nitrogen, hydrogen sulfide, various mercaptans, and water vapour along

with trace amounts of volatile organic component (VOC) gases, sand and other

compounds as well once it comes out from the underground.


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Methane: 0.88322

Ethane: 0.06755

Propane: 0.01995

i-Butane: 0.00688

n-Butane: 0.00769

i-Pentane: 0.00388

n-Pentane: 0.00183

n-Hexane: 0.00177

n-Heptane: 0.00132

n-Octane: 0.0005

n-Nonane: 0.00012

n-Decane: 0.00005

Water: 0.00121

Nitrogen: 0.00237

Carbon dioxide 0.00166

Hydrogen sulphide 0

Table 1:Sample of Natural Gas Composition in Malaysia (source: Dan Laudal Christensen, February2006, thermodynamic simulation of the water/glycol mixture)


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y

Natural gases commonly are classified according to their liquids content as either lean orrich and according to the sulfur content as either sweet or sour.

y Sweet Gas : Contains negligible amounts of H2S,

y Sour Gas : Unacceptable quantities of H2S

y This H2S in natural gas when present with water, it could be corrosive. Mainly, the

corrosion products are iron sulphides, FeSX, a fine black powder.

y Water at specific temperature and pressure will formed hydrates with hydrocarbon

y Thus it is important for this natural gas to undergo preliminary natural gas processing in

order to remove either thewater ( gas dehydration) or H2Sbefore it being transported

using gas pipeline


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Figure 1 : Natural Gas Processing (source: Arthur J. Kidnay et. al, 2006, fundamental of natural

gas processing)


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Problem Statements

y Natural gas that comes from oil wells is not totally pure but there are contaminants or

mixtures in gas

y These mixtures in natural gas can cause problems for the production operation,

transportation, storage and uses of the gas.

y -H2S c rr si n

y -water hydrates

y Gas hydrates are ice-like clathratesolids that are formed from water and small

hydrocarbons at elevated pressures and at lower temperatures (depends on the

equilibrium between hydrocarbon and water contents). This hydrates can causedproblems especially in gas transportation in pipelines.

y This hydrates formed as a result of reaction that occurred between hydrocarbons with the

water contents available in the natural gas.


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Figure 2 : Pressure-temperature graph for a typical natural gas hydrate. (source: ProductionChemicals for the Oil and Gas Industry)

y The agglomeration of these hydrates in the pipeline may not be the same location as it is

formed. The hydrate can flow with the fluid phase especially the liquid. As the liquid

accumulated, the hydrate would also tend to accumulate in the same location.


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y These accumulati ns f hydrates will f rm bl ckage that will bl ck the line and damage the

equi ment such as the i eline.

Figure 3: Safety Hazard f r m ving hydrate lugs ( Re-illustrated from the author from Sloan,

2000)

y A: A hydrate lug m ves d wn a fl w line at a very high vel city

y B: Where the i e bends, the hydrate lug can ru ture fl w line thr ugh r jectile im act


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y Thus it is important for these natural gases to be treated first in the natural gas processing

before it being transported to other places or used as the reactant for other processes.

Natural Gas Dehydration :

Removing water vapor from the gas

Mostly by two (2) methods that is through Absorption or Adsorption

y Mainly research will involved in the simulation process of natural gas dehydration by

absorption using Triethylene Glycol (TEG) as the absorbents medium or liquid desiccants

using current Aspen HYSYS 2006 software.


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OBJECTIVES

The objectives of the studies are:

y To compare between current technologies for natural gas dehydration.

y To study the effect of operating conditions on the efficiency of the process.

y To investigate the possibility of absorbent modifications.

y To suggest modification of existing technologies.

Scope of studies:

y focuses on the mechanism of natural gas dehydration by absorption in the aspects of

process simulation

y Analysis parameters that involved in the simulation

y To come out with the best solution of natural gas dehydration process by using the

existence technology of natural gas absorption process as the main reference.


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LITERATURE REVIEW

Gas Hydrates

y solid crystalline material formed from cages of water molecules containing voids that are

occupied by gas molecules whose presence stabilizes the crystal structure through Van der

Waals bonding

Figure 4 : Gas Hydrate structure (source: Dan Laudal Christensen, February 200, thermodynamic simulation of the

water/glycol mixture)

y The hydrogen bonding causes water molecules to align in regular orientations.

y The presence of certain compounds in natural gas causes the aligned molecules to stabilize and

solid mixture will precipitate.


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Gas hydrates formation requires three conditions which are:

y Right combination of temperature and pressure ( formation is favoured under low temperature

and high pressure)

y Hydrate formers (guest molecules)

y Sufficient amount of water

Currently there are three (3) structures of gas hydrates:

y Structure 1 : Type I

yStructure 2 : Type II

y Structure H: Type H

y All these hydrates structures can be differentiated in the size of cavities and the ratio of

different sizes of cavities


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Figure 5: Cages of water molecules bond building different gas hydrates structures (source:http://www.science.org.au)

cavities


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Before Gas Dehydration Process

After Gas Dehydration Process

Figure 6: Dew points Depressions (source: Dr. Mahmood Moshfeghian, Process

Simulation in Gas Conditioning and Processing)


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Purpose of Gas Dehydrations :

y To prevent hydrate prevention.

hydrates can plug lines and retard the f low of gaseous hydrocarbon streams.

Reduce effectiveness gas transportation

y To avoid from any corrosion to occurred.

The water vapour will dissolve in hydrogen sulphide in the natural gas to form an

acidic solution. This acidic solution will reacts with carbon steel in the pipeline to

caused corrosion.

y Downstream process requirements.

presence of water may cause side reactions, foaming, or catalyst deactivation.

Consequently, purchasers typically require that gas and liquid petroleum gas (LPG)

feedstocks meet certain specifications for maximum water content.


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Natural Gas Dehydration by

Absorption

y Liquid desiccant dehydrator serves to absorb water va our from the gas stream.

y Essentially, glycol dehydration involves using a glycol solution, usually either DEG or TEG, which

is brought into contact with the wet gas stream in a contractor.

Table 2: Glycols used in Dehydration ( source: Kohl and Nielsen (1997) )


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y The OH groups in glycol solution has high affinity to water since it will form hydrogenbonding with the water molecules.

y It will absorb water from the wet gas. Once absorbed, these glycol particles becomeheavier and will sink to the bottom of the contactor where they are removed while the gasthat are light in density will rise upward to the top of the contactor and will come out as adry gas.


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y Generally, in absorption, net mass is transferred from the vapour to the liquid, resulting in the

recovery of certain components from the vapour feed by the liquid.

y On each stage, the intermediate components distribute themselves between the vapor and

liquid or until:

Yi = KiXi, where;

y Ki : the vapour-liquid equilibrium distribution coefficient

y Yi : mole fractions in the vapour of component i and

y Xi : mole fractions in liquid of component i.

y In ideal solutions this relationship simplifies to Raoults law;

y PYi = pioXi, where;

y P : the stage pressure

y Pi : vapour pressure of component i at the stage temperature


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PYi = pio

Xi

y Referring to Raoults law, the vapour pressure of the light components is too high to allow

them to move in any appreciable amounts to the liquid. (If pi is large, Xi must be small to keep

their product equal to the partial pressure of i, PYi)

y Conversely, the vapour pressure of the heavy components is too low to allow it to be

transferred significantly to the vapour.

y Hence, in absorption, net mass is transferred from one phase to the other and as for the case of

natural gas dehydration the mass transfer of water vapour is from gas phase (natural gas) to

liquid phase (absorbent).


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y Figure 7: Absor tion column com osition data from the simulation


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y The OH groups in glycol solution has high affinity to water since it will form hydrogen

bonding with the water molecules.

y Thus it will absorb water from the wet gas. Once absorbed, these glycol particles become

heavier and will sink to the bottom of the contactor where they are removed while the gas

that are light in density will rise upward to the top of the contactor and will come out as a

dry gas.

Figure : Schematic diagram of an absorption column


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y As for the material balance, since the liquid and gas components do not change phases, their

flow rates will remain constant in the column.

Lx0 +Vyj+1 = Lxj+Vy1

y L : Liquid flow

y V: VaporFlow

y x:mole fraction of component in liquid phase

y y: mole fraction of component in vapour phase

y the composition of the respective component in the vapour phase or liquid phase coming out

from the absorption column can be determine if the balance amount of liquid and vapour

entering the absorption column are given.


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Equation of State

y First step requirements in simulating a process in ASPEN Hysys that is to choose suitable

package for the fluid involved in the process

y As for the case of natural gas dehydration simulation, the Equation of state used must

able to encompass wide ranges of pressure and temperature in order to represent the PVT

behaviour of both vapours and liquids.

Peng-Robinson Equation of State could actually predict better values for both liquid andgas densities.

At low temperature and high pressure


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y

Peng-Robinson Equation of State

y Where -p is the pressurey Tis the temperature

y V is the molar volume

y n is the number of moles

y R is the gas constant


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METHODOLOGY

Literature Review

Gas HydratesNatural Gas Dehydrations

Absor tion rocess

hase Envelopes

Data Collection

Natural Gas Composition

Absorption Column arameters

Operating Conditions

rocess Descriptions

HYSYS Simulation

Absorption Simulation

Stripping Simulation

Simulation of Recycle Stream

Separators Sequencing

Data Gathering from Simulation Results

hase Envelopes

Dew oint Depressions


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Simulation Result Analysis

Temperature and ressure and Temperature Differentiation

towards Rate ofAbsorption

Temperature and ressure and Temperature Differentiation

towards Rate of Distillation (Regenaration)

Developing Complete Flowsheet

Improved Rate ofAbsorption

Improve Rate of Regeneration

TEG recovery

Report Writting

Summarised Analysis Result From Simulation

Listing All Related arameters

Establish Complete rocess Descriptions


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Natural Gas composition before Dehydration process

Natural Gas composition after Dehydration process


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Natural Gas composition before

Dehydration process for TEG

Natural Gas composition after

Dehydration process for TEG


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Ghantt Chart

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15TopicSelection

Mid Semeter

Break/ Hari

Raya Holiday

Proposal Submission

Preliminary Research Work (Basis

theory of project)

LiteratureReview

Submission of Progress Report

Seminar (Oral Presentation)

Second Phase Research Work

( Data Gathering for ICON

simulation)

ICON SimulationTrial

Analysis ICON Simulation Result

Summarized all analysis data into

presentableformed

PreparePresentationSlides

SubmissionInterimReport

Oral Presentation


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Key Milestone

Activity Date

Proposal Submission 6/08/2010

Progress Report Submission 3/09/2010

Seminar (Oral Presentation) 3/09/2010

Interim Report Submission 1/11/2010

Oral Presentation 1/11/2010


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References

y Kermit E. Woodcock. Gas, Natural, Vol. 12. pp1: Gas Research Institute

y Kh. Mohamadbeigy(2008). Studying of the effectiveness parameters on gas dehydration plant.pp1: ResearchInstitute of etroleum Industry Tehran

y H.K.Abdel-Aal and Mohamed Aggour(2003). etroleum and gas field processes: Marcel Dekker Inc.

y (2009). roduction Chemical for the Oil and Gas Industry: Taylor and Francis Group

y Dr. James G. Speight (2006). The Chemistry and Technology of etroleum. 4th Ed: Taylor & Francis Group

y Myron Gottlib (2003). Natural Gas. Gasoline and other Motor Fuels. Vol 12. pp 1

y (2009). What are gas hydrates?http://www.centreforenergy.com/AboutEnergy/ONG/GasHydrates/Overview.asp?page=2

y Fouad M. Khoury(2005). Multistage Separation rocesses. 3rd Edition: CRC ress

y Geankoplis (2003). Transport rocess and Separation rocess rinciples. 4th Ed: earson Education InternationalFouad M. Khoury(2005). Multistage Separation rocesses. 3rd Edition: CRC ress

y E.G. LARIONOV*, YU.A. DYADIN, F.V. ZHURKO (2005). hase Diagrams of the Ternary Gas Hydrate Forming Systems

at High ressures. art II. EthaneMethaneWater System. p1

y John M. Campbell (1994). Gas Condition and rocessing. Vol 1: Campbell etroleum Series

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