PE5623- Lecture 05- Natural Gas Hydrates.pdf

7/24/2019 PE5623- Lecture 05- Natural Gas Hydrates.pdf

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Copyright 2003 by Faruk Civan 1

Natural Gas Processing

Dr. Faruk Civan, Ph. D.Professor, The University of Oklahoma


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CopyrightsThis presentation contains copyrighted

material as indicated in the attributions

on individual slides, or by F. Civan

2003. This material is provided insupport of class presentation and for no

other use. Permission for any other

use, duplication or distribution must be

obtained from the copyright holder.


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Important Notice All of Dr. Faruk Civans lecture

notes, course syllabuses, handouts,

homeworks, and exams are

copyright material. They cannot be reproduced,

recorded and copied in any way orform without the written permission

from Dr. Faruk Civan. All rights

reserved.


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Natural GasHydrates


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H2O Vapor Presence

Critical values for water:

Critical Pressure = 3,208 psia

Critical Temperature = 705 oF

Reservoir pressures are muchhigher, therefore, gases aresaturated with H

2

O vapor


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Estimating Water

Content

oVapor-

Pressure

For H2O

Temperature


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Estimating Water

ContentDaltons Law:

Mole (Vol.) Fraction:

H2O partial

Pressure:T

vap

ww

T

ii

n

i

iT

P

PY

P

PY

PP

=

=

= = 1


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Estimating Water

ContentIf the laboratory analysis shows the

molar concentration of the species Yiin the dry gas analysis, then the

corrected analysis for the water vaporsaturated gas can be obtained

from the following equation:


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Estimating Water

Content

( ) ( ) ( )wLiCi YYY = 1

Mole of dry

gas/mole

of saturated gas

Mole of i/mole

of dry gas

(from lab)

Mole of i/mole

of saturated

gas


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Estimating Water

ContentThus the water content of a gas is:( )

( ) ( )

MMscf

lbmW

Y

MYWor

Y

MYW

SCF

lbmole

Wlbmole

WlbmM

gaslbmole

Wlbmole

Y

YW

HC

w

wwHC

w

wwHC

w

w

wHC

=

=

=

=

169.380

10

169.380

9.380

1

_

_

_

_

1

6

Industrial Practice of reporting

water content

(Sales Gas Specification is 7 lbm/MMscf)


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Estimating Water

ContentLooking back to the previous equationand substituting the value of Y

w

( ) 69.38010 6

=

=

vapw

w

vap

w

HC

T

vap

ww

PP

MPW

P

PY


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Vapor-

PressureFor H2O

Temperature

P1P2

P3

P4

P1


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W

aterContent,lb/

MMscf

Temperature, T

Pres

sure(

Isob

ars)

,

WHC!

WHC2

ps

ia

Mcketta and WeheChart


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Most Common chart used in the

processing industry

This chart is for hydrocarbongases only

It has been used in design since1958

Source: GPSA Figure-20.3

Mcketta and WeheChart


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Estimating Water

ContentHydrogen sulfide and carbon dioxideare very common contaminants innatural gas.

Correction factors are applied tocorrect the water content estimation

in natural gas


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Estimating Water

ContentGeneral Formula (GPSA):

SHSHCOCOHCHCtotal WYWYWYW2222 ++=

where, Y is the molar volumeconcentration and W is the water

content in lbm/MMscf (read from

charts)


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Correction for CO2


Isobars

Temperature

Effective

Water

Con

tent,L

b/MMscf

o


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Isobars

TemperatureEffective

Water

Con

tent,L

b/MMscf


o

Correction for H2S


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Hydrate As defined by GPSA: Hydrate is a

physical combination of water andother small molecules to produce asolid which has an ice-likeappearance but possesses a

different structure.


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Hydrate

Why hydrate is not desired ?Creates various transmissionproblems such as plugging:

Pipeline Equipment

Instrumentation

and hence restricts the flow


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HydrateWhen gas is produced to the surface,

there are two hydrate inducing

factors:

Reduction in temperature

Reduction in pressure


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Hydrate

SpecificGravity

1.0

HydrateFormation

Pressure

Temperature


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PT Chart forHydrate Formation

Metha

ne

0.6Sp

.Gr.

0.7

0.8 Hydrate-Free

RegionP

ressure


Temperature


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Gas Expansion andJoule-Thomson EffectThe Joule-Thomson coefficient

> 0, then P and T

H

JP

T

=

< 0, then P and T

Pi and

Ti

Pfand

Tf


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Hydrate Formation forcertain gravity gas

Source: GPSA

Figure-20.16

Temperature

o

o

Tinitial

isotherm

Tfinal

isothermHydrateformation

No

hydrates

Intersection

with the 45o

line gives the

finaltemperature

to be reached

after

expansionInitialPres

sure

Pinitial

Final PressurePfinal


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Example 2 Expansion /Hydrate FormationGiven:

Initial P = 3000 psia, T=160 oF; and gas

specific gravity is 0.7Required:

What is the minimum pressure towhich the gas can be expanded

without forming hydrate and to what

temperature will the gas be cooled ?

S


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Example 2Temperature

Final Pressure

InitialPressure 160oF

59o

F

158 psia

Source:

GPSA

Figure-

20.17

SpecificGravity = 0.7

3,000 psia


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Example 2Given:

A 0.7 gravity natural gas is saturatedwith water vapor at 150 oF and 3,000

psia. This gas is expanded through achoke and its pressure is reduced to a

pressure of 1000 psia.

Required:Will hydrate be formed at the outlet of

the choke?


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Example 2 (Solution)Determine first the final temperaturefrom Mollier-Diagram

Entropy, Btu/lbmoleoF

Pressure, psia

IsobarsEn

thalpy

Btu

/lbmole

Temperature, oF

Isotherms


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Example 2 (Solution)

Second check the hydrate region

No hydrate will be formed

Metha

ne

0.6Sp

.Gr.

0.7

0.8 Hydrate-Free

Region

Temperature

P

ressure


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Vapor-SolidEquilibriumReference: SPE15306 and SPE 50749

Solidhydrate

Vapor


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Vapor-SolidEquilibriumLet:

Xi = Mole fraction of component i in

the solid hydrate phase on a water-

free (or dry) basis

Yi = Mole fraction of component i in

the vapor phase on a water-free (dry)

basis


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Vapor-Solid

EquilibriumThe vapor-solid equilibrium ratio forspecies i is given by:

),( TPKXYK

ii vs

i

ivs ==

Similar to dew-point calculation, forthe first hydrate phase formed, it is true

that

====

11

1),(

1i vs

i

i i TPK

YX

i


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Vapor-Solid

EquilibriumApplication:

yi is normally given or measured

Solve for P if T is given, or Solve for T if P is given.


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Vapor-SolidEquilibriumIso-bars

IncreasingPressureX

YK=

Temperature

Note: Each component has its own chart


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Hydrate PreventionHydrate formation can be avoided by

using the following methods:

Operating outside the thermodynamic

condition (P&T) of hydrate formation.This is done by adjusting the values of

temperature and pressure

Using dehydrating processes to

remove free water

Adding hydrates inhibitors


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Kinetic Hydrate

InhibitorsA polymeric material that delays thehydrate crystal growth

N-vinylpyrrolidone

N-vinylcaprolactam

Saccarides


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Anti-agglomerates forHydrate Inhibition

Prevents agglomerations of hydrate

crystals from growing into large

size

Alkyl aromatic sulphonate Quaternary ammonium salt

Alkyl glycoside surfactant


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ThermodynamicHydrate Inhibitors

Methanol

Ethanol

Iso-propanol

Ethylene glycol

Propylene glycol

Diethylene glycol


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Controlling OperatingConditions

a. Controlling hydrate temperature.

b. Controlling of hydrate formationpressure.


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Controlling hydrateformation temperatureKeeping gas above hydrate formation

temperature.

a. Heating the transmission line

continuously by means of electricalheater. Temperature normally has

limitation to protect pipeline integrity.


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Controlling hydrateformation temperatureb. Heating can also be accomplished by

an exothermic chemical reaction.

NaNO2 + NH4NO3 N2 + 2H2O

+NaNO3 + Heat

Risk: N2 gas can overpressure the

system


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Controlling hydrateformation pressure

Rapid pressure reduction causes

overcooling and hydrate formation Lower the pressure gradually at

isothermal conditions Avoid sudden pressure reduction


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DehydrationProcessesRemove free water by two means:

Using solid adsorbent

Using liquid absorbent

Detailed evaluation of dehydration

processes will be discussed later.


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Hydrates

Inhibitors Salts Alcohols

Ammonia

Monoethanolamine


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Salts InhibitorsAqueous solutions salts are:

Electrolytes are very effective

inhibitors. These prevent formation of lattice

around the gas molecules.

Salts also cause corrosion.


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Salts InhibitorsChlorides: Effectiveness sequenceAl+3 > Mg+2 > Ca+2 > Na+ > K+

AlCl3 MgCl2 CaCl2 NaCl KCl

Preferred because of low cost

Sulfates: Na2SO4, MgSO4, Al(SO4)3

Phosphates: Na3PO4


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Alcohols InhibitorsTypes:

Glycol base (Ethylene glycol is the

preferred)

Methanol base


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Alcohols

InhibitorsApplications for cryogenicprocesses:

Methanol amine is preferred

because the glycol viscositymakes separation difficult at

cryogenic conditions.


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Alcohols

InhibitorsApplications for non-cryogenicconditions:

Glycol is desired because of low

cost.

Ethylene glycol is also used as a

car antifreeze.


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Ammonia & Mono-Ethanol-Amine InhibitorsAmmonia: Very effective but has undesirable

properties also. May cause corrosion problems

Toxic

Forms carbonates with CO2Monoethanolamine:

Very effective


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Hydrate

Inhibitors EffectAlCl3

C

aCl2

CH

3OH

EGor

TEG

Wt % inwater

oT


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Hydrate Inhibitor Effecton Hydrate Formation

Temperature Depression

Hammerschmidt (1939) equation:

WW

MKT

w

H

=

100

Hydrate Inhibitor Effect


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on Hydrate FormationTemperature Depression

T = oF

Mw = Inhibitor molecular weight

lbm/lb-moleW = weight percent of inhibitor

KH = Empirical factor 1,297 formethanol and 2,222 for ethylene

glycol


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ExerciseAnswer the following questions:

1. What are the basic methods used in

hydrate prevention?2. Describe the various methods

available for prevention of hydrateformation and their operatingprinciples.

3. List the primary hydrate inhibitors.


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ExerciseEthylene glycol (C2H6O2) will be usedas a hydrate inhibitor at a 25 wt. %

concentration in water. Answer the

following questions:

1. What is the molecular weight of ethyleneglycol?

2.How much will the ethylene glycolsolution lower the temperature for

hydrate formation?


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ExerciseA reservoir contains a 0.65 specificgravity natural gas at 200 oF and4,000 psia conditions. The wellhead

conditions are 130 oF and 2,000psia. The wellhead gas is expandedthrough a choke to reduce itspressure to 1,200 psia. Determine:


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Exercise1. The amount of free water present

in the fluid system entering thechoke.

2. The amount of additional waterthat will be separated at the outletof the choke.

3. Will hydrate be formed at theoutlet conditions of the choke?


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ExerciseDevelopment and Demonstration of aHydrate Prediction Program.

Carry out the following project basedon Paper SPE 15306, Hydrate

Prediction on a Microcomputer byB.K. Berge, 1986. However, you canalso use other relevant references.


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HydratePrediction Determine the mathematical

equations leading to the model for

hydrate prediction forcompositional and non-

compositional gases in SPE 15306.Summarize the equations in a

consistent unit system, such as SI

or FPS.


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Hydrate

PredictionPresent the equations separatelyfor:(a) Compositional gases

(b) Non-compositional gases


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Hydrate

Prediction Prepare a step-by-step

computational algorithm required tocarry out calculations for hydrateprediction for compositional and

non-compositional gases in alogically sequenced manner.


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HydratePrediction Prepare an information flow chart to

implement the above algorithm.

Prepare a spreadsheet program to

implement the above algorithm.


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Hydrate

Prediction Describe the capabilities of your

program by presenting a list oftasks that it can perform.


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Hydrate Prediction Carry out the following

applications:(a) Prepare typical charts for

vapor-solid ratios of various lighthydrocarbon components, similar

to typical vapor-liquid equilibrium

ratio charts for compositionalgases. You may present charts in 2-

and 3-variables forms.


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Hydrate

Prediction(b) Prepare typical charts forhydrate prediction for non-compositional gases. You may

present charts in 2- and 3-variablesforms.


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HydratePrediction(c) Demonstrate several applications of

your program using typical datasimilar to those presented in SPE15306. Decide and present

representative applications, whichbest illustrate the capabilities ofyour program.

Hydrate


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Hydrate

Prediction Submit a written report in the

form of a technical paper by

December 3, 2002 preparedaccording to the SPE papersubmission guidelines (see SPE

web page for instructions).Present the details of calculationsand results in an appendix.


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References K. Arnold and M. Stewart, Surface

Production Operations- Design ofGas-handling Systems and Facilities,Second Ed., Volume 2, Gulf

Publishing Company, 1999.

Y.E. Makogan and S. A. Holditch,

Experiments Illustrats Hydrate,Oil&Gas Journal, Feb. 12, 2001, pp.45-50.


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References Y.E. Makogan and S. A. Holditch, Lab

Work Clarifies Gas Hydrate, Oil&GasJournal, Feb. 5, 2001, pp. 47-52.

Berge, B.K., Hydrate Predictions on a

Microcomputer, SPE Paper 15306, SPESym. On Petroleum Industry Applicationsof Microcomputers held in Silver Creek,

CO, June 18-20, 1986. SPE Paper 50749

PE5623- Lecture 05- Natural Gas Hydrates.pdf

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