separation of multiphase, multicomponent systems

5.3 Multi-Component Mixtures 655.4 Multi-Phase Mixtures 70 5.5 Charged Mixtures 75 5.6 The Criteria of Similarity 78 5.7 The State Equations 82 5.7.1 The State Equation for an Ideal Gas

Emmanuil G Sinaiskiand Eugeniy J LapigaSeparation of Multiphase,Multicomponent Systems

Emmanuil G Sinaiski and Eugeniy J Lapiga

Separation of Multiphase,

Multicomponent Systems

E J Lapiga: Oil rig, developed by EITEK

carefully produced Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

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Preface XI

List of Symbols XIII

I Technological Fundamentals of Preparation of Natural Hydrocarbons for

Transportation 1

Introduction 3

1 Technological Schemes of Complex Oil, Gas and Condensate Processing

Plants 7

2 Construction of Typical Apparatuses 13

2.1 Separators, Dividers, and Settlers 13

II Physical and Chemical Bases of Technological Processes 43

4 The Transfer Phenomena 45

4.1 Phenomenological Models 45

4.2 Momentum Transfer 46

4.3 Thermal Conduction and Heat Transfer 51

4.4 Diffusion and Mass Transfer 51

4.5 Electro-Conductivity and Charge Transfer 55

5 Conservation Laws and Equations of State 57

5.1 Isothermal Processes 57

5.2 Non-isothermal Processes 61

V

5.3 Multi-Component Mixtures 65

5.4 Multi-Phase Mixtures 70

5.5 Charged Mixtures 75

5.6 The Criteria of Similarity 78

5.7 The State Equations 82

5.7.1 The State Equation for an Ideal Gas and an Ideal Gas Mixture 825.7.2 The State Equation for a Real Gas and a Real Gas Mixture 865.7.3 Methods of Calculation of Liquid–Vapor Equilibrium 91

5.8 Balance of Entropy – The Onsager Reciprocal Relations 93References 103

III Solutions 105

6 Solutions Containing Non-charged Components 107

6.1 Diffusion and Kinetics of Chemical Reactions 107

6.2 Convective Diffusion 112

6.3 Flow in a Channel with a Reacting Wall 116

6.4 Reverse Osmosis 119

6.5 Diffusion Toward a Particle Moving in a Solution 128

6.6 Distribution of Matter Introduced Into a Fluid Flow 133

6.7 Diffusion Flux in a Natural Convection 140

6.8 Dynamics of the Bubble in a Solution 145

6.9 Evaporation of a Multi-component Drop Into an Inert Gas 1516.10 Chromatography 160

6.11 The Capillary Model of a Low-permeable Porous Medium 164

IV Suspensions and Colloid Systems 195

8 Suspensions Containung Non-charged Particles 197

8.1 Microhydrodynamics of Particles 197

8.2 Brownian Motion 211

8.3 Viscosity of Diluted Suspensions 222

8.4 Separation in the Gravitatonial Field 228

8.5 Separation in the Field of Centrifugal Forces 237

9 Suspensions Containing Charged Particles 245

9.1 Electric Charge of Particles 245

9.2 Electrophoresis 247

9.3 The Motion of a Drop in an Electric Field 253

9.4 Sedimentation Potential 257

10 Stability of Suspensions, Coagulation of Particles, and Deposition of Particles

on Obstacles 259

10.1 Stability of Colloid Systems 259

10.2 Brownian, Gradient (Shear) and Turbulent Coagulation 266

10.3.2 Particles’ Collisions with an Obstacle 278

10.4 The Capture of Particles Due to Surface and Hydrodynamic Forces 28010.5 Inertial Deposition of Particles on the Obstacles 288

10.6 The Kinetics of Coagulation 289

10.7 The Filtering and a Model of a Highly Permeable Porous Medium withResistance 293

10.8 The Phenomenon of Hydrodynamic Diffusion 296

References 297

V Emulsions 301

11 Behavior of Drops in an Emulsion 303

11.1 The Dynamics of Drop Enlargement 303

11.2 The Basic Mechanisms of Drop Coalescence 312

11.3 Motion of Drops in a Turbulent Flow of Liquid 317

11.4 Forces of Hydrodynamic Interaction of Drops 325

11.5 Molecular and Electrostatic Interaction Forces Acting on Drops 330

11.6 The Conducting Drops in an Electric Field 333

11.7 Breakup of Drops 338

12 Interaction of Two Conducting Drops in a Uniform External Electric

Field 347

12.1 Potential of an Electric Field in the Space Around Drops 347

12.2 Strength of an Electric Field in the Gap Between Drops 355

12.3 Interaction Forces of Two Conducting Spherical Drops 361

12.4 Interaction Forces Between Two Far-spaced Drops 367

12.5 Interaction of Two Touching Drops 370

12.6 Interaction Forces Between Two Closely Spaced Drops 379

12.7 Redistribution of Charges 388

13 Coalescence of Drops 393

13.1 Coalescence of Drops During Gravitational Settling 393

13.2 The Kinetics of Drop Coalescence During Gravitational Separation of anEmulsion in an Electric Field 410

Contents VII

13.3 Gravitational Sedimentation of a Bidisperse Emulsion in an ElectricField 416

13.4 The Effect of Electric Field on Emulsion Separation in a GravitationalSettler 419

13.5 Emulsion Flow Through an Electric Filter 423

13.6 Coalescence of Drops with Fully Retarded Surfaces in a TurbulentEmulsion Flow 430

13.7 Coalescence of Drops with a Mobile Surface in a Turbulent Flow of theEmulsion 436

14 Formation of a Liquid Phase in a Gas Flow 465

14.1 Formation of a Liquid Phase in the Absence of Condensation 46614.2 Formation of a Liqid Phase in the Process of Condensation 469

15 Coalescence of Drops in a Turbulent Gas Flow 481

15.1 Inertial Mechanism of Coagulation 483

15.2 Mechanism of Turbulent Diffusion 484

15.3 Coalescence of a Polydisperse Ensemble of Drops 488

16 Formation of a Liquid Phase in Devices of Preliminary Condensation 49516.1 Condensation Growth of Drops in a Quiescent Gas–Liquid Mixture 49516.2 Condensation Growth of Drops in a Turbulent Flow of a Gas–LiquidMixture 505

16.3 Enlargement of Drops During the Passage of a Gas–Liquid MixtureThrough Devices of Preliminary Condensation 514

16.4 Formation of a Liquid Phase in a Throttle 519

16.5 Fomation of a Liquid Phase in a Heat-Exchanger 531

18 Efficiency of Gas-Liquid Separation in Separators 581

18.1 The Influence of Non-Uniformity of the Velocity Profile on the EfficiencyCoefficient of Gravitational Separators 584

18.2 The Efficiency Coefficient of a Horizontal Gravitational Separation 58718.3 The Efficiency Coefficient of Vertical Gravitational Separators 593

18.4 The Effect of Phase Transition on the Efficiency Coefficient of a

18.7 The Influence of a Distance Between the Preliminary Condensation

Device and the Separator on the Efficiency Coefficient 604

19 The Efficiency of Separation of Gas–Liqid Mixtures in Separators with DropCatcher Orifices 607

19.1 The Efficiency Coefficient of Separators with Jalousie Orifices 608

19.2 The Efficiency Coefficient of a Separator with Multicyclone Orifices 61019.3 The Efficiency Coefficient of a Separator with String Orifices 618

19.4 The Efficiency Coefficient of a Separator with Mesh Orifices 629

20 Absorption Extraction of Heavy Hydrocarbons and Water Vapor from NaturalGas 635

20.1 Concurrent Absorption of Heavy Hydrocarbons 635

20.2 Multistage Concurrent Absorption of Heavy Hydrocarbons 641

20.3 Counter-Current Absorption of Heavy Hydrocarbons 646

20.4 Gas Dehydration in Concurrent Flow 650

20.5 Gas Dehydration in Counter-Current Absorbers with High-Speed

Separation-Contact Elements 659

21 Prevention of Gas-Hydrate Formation in Natural Gas 667

21.1 The Dynamics of Mass Exchange between Hydrate-Inhibitor Drops andHydrocarbon Gas 671

21.2 Evolution of the Spectrum of Hydrate-Inhibitor Drops Injected into a

Turbulent Flow 682

References 695

VII Liquid–Gas Mixtures 699

22 Dynamics of Gas Bubbles in a Multi-Component Liquid 701

22.1 Motion of a Non-Growing Bubble in a Binary Solution 702

22.2 Diffusion Growth of a Motionless Bubble in a Binary Solution 706

22.3 The Initial Stage of Bubble Growth in a Multi-Component Solution 71022.4 Bubble Dynamics in a Multi-Componenet Solution 713

22.5 The Effect of Surfactants on Bubble Growth 716

Contents IX

23 Separation of Liquid–Gas Mixtures 721

23.1 Differential Separation of a Binary Mixture 723

23.2 Contact Separation of a Binary Mixture 727

23.3 Differential Separation of Multi-Componenet Mixtures 72923.4 Separation of a Moving Layer 736

24 Separation with Due Regard of Hinderness of Floating Bubbles 74324.1 Separation in the Periodic Pump-out Regime 744

24.2 Separation in the Flow Regime 748

25 Coagulation of Bubbles in a Viscous Liquid 751

25.1 Coagulation of Bubbles in a Laminar Flow 752

25.2 Coagulation of Bubbles in a Turbulent Flow 758

25.3 Kinetics of Bubble Coagulation 761

References 764

Author Index 765

Subject Index 769

This book sets out the theoretical basis underpinning the separation of phase, multi-component systems with application to the processes used to pre-pare hydrocarbon mixtures (oil, natural gas, and gas condensate) for transporta-tion The text is divided into seven sections

multi-Section I provides an introduction to the basic processes, the technologicalschemes, and the components of the equipment employed in systems for thefield preparation of oil, natural gas, and gas condensate The emphasis is onthe designs and the principles of operation of separators, absorbers, and coolingdevices Mathematical modeling of the processes in these devices is covered insubsequent sections of the book

The media with which one has to deal when investigating preparation cesses of hydrocarbon systems are invariably multi-phase and multi-componentmixtures Section II thus covers the aspects of the hydromechanics of physicaland chemical processes necessary for an understanding of the more specializedmaterial contained in following sections Among these are transfer phenomena

pro-of momentum, heat, mass, and electrical charge; conservation equations for thermal and non-isothermal processes for multi-component and multi-phasemixtures; equations of state, and basic phenomenological relationships

iso-Natural hydrocarbon systems exist as solutions, suspensions, colloidal systems,emulsions, gas-liquid and liquid-gas mixtures Accordingly, Sections III–VII aredevoted to each of the aforementioned kinds of systems

Section III covers the theory and methods for investigating the behavior ofmulti-component charged and uncharged solutions Considering non-chargedsolutions, the main focuses of attention are on diffusion processes with and with-out the possibility of chemical reactions, the flow of solutions in channels andpipes, processes on semi-permeable membranes (return osmosis), and massexchange of particles, drops, and bubbles with the ambient media For chargedsolutions, consideration is given to processes in electrolytic cells, electrodialysis,the structure of electrical double layers, electrokinetic phenomena, and electro-osmosis

The behavior and stability of suspensions and colloidal systems, including charged and charged suspensions, along with the coagulation and sedimentation

non-of particles and their deposition on obstacles, are considered in Section IV

Chap-XI

ter 8 (devoted to non-charged suspensions) provides an introduction to the hydrodynamics of particles, covering the fundamentals of Brownian motion, theviscosity of dilute suspensions, and the separation of suspensions in a gravita-tional field or under centrifugal forces Chapter 9, devoted to charged suspen-sions, deals with the definition of particle charge, electrophoretic effects, the mo-tion of conductive drops in an electric field, and sedimentation potential Chapter

micro-10 deals with the problem of colloidal system stability, various mechanisms ofparticle coagulation, and the capture of particles by obstacles when a suspension

is passed through a filter

The behavior of emulsions is considered in Section V in connection with theprocess of oil dehydration Actual problems of drop integration in emulsions arediscussed It is shown that this process occurs most effectively if the emulsion issubjected to an electric field In this context, the behavior of conducting drops inemulsions, the interaction of drops in an electric field, and the coalescence ofdrops in emulsions are examined in detail In terms of applications, processes ofemulsion separation in settling tanks, electro dehydrators, and electric filters areconsidered

Separation processes of gas-liquid (gas-condensate) mixtures are considered inSection VI The following processes are described: formation of a liquid phase in

a gas flow within a pipe; coalescence of drops in a turbulent gas flow; tion of liquid in throttles, heat-exchangers, and turboexpanders; the phenomenarelated to surface tension; efficiency of division of the gas-liquid mixtures ingas separators; separation efficiency of gas-condensate mixtures in separatorsequipped with spray-catcher nozzles of various designs – louver, centrifugal,string, and mesh nozzles; absorbtive extraction of moisture and heavy hydrocar-bons from gas; prevention of hydrate formation in natural gas

condensa-Section VII is devoted to liquid-gas (oil-gas) mixtures The topics discussed arethe dynamics of gas bubbles in multi-component solutions; the separation ofliquid-gas mixtures in oil separators both neglecting and taking into account thehindrance due to the floating-up of bubbles; and the coagulation of bubbles inliquids

A list of literature is given at the end of each section

All of the considered processes relate to the separation of phase, component media, hence the title of the book It should be noted that in the prep-aration technology for the transportation of oil, natural gas, and gas condensates,the term separation is traditionally understood only as the process of segregation

multi-of either a condensate and water drops or multi-of gas and gas bubbles (occluded gas)from an oil The concept of separation used herein can mean any segregation ofcomponents in multi-component mixtures or of phases in multi-phase systems

List of Symbols

ai Activity of i-th component

a Radius of tube, pipe, capillary,

Acyl Parameter of stream function at flow

around cylinder

Ai Dimensionless parameters of charged

particles, of jalousie separator

As Parameter of stream function at flow

around sphere

Arav Archimedean number calculated by

average radius of particles

b Ellipsoid semi-axis; radius of cell

boundary, of collision section ofparticles with cylinder

m

B Constant of reaction of n-th order mole1nm3n2s1

ccap Capillary wave velocity ms1

ci Inflow of energy to i-th phase due to

work of external forces

Jm2s1

cn

i Work of external surface forces Jm2s1

cp Specific heat capacity at constant

Ccr Critical concentration of electrolyte molem3

Cij Pair interaction factor of molecules of

i-th and j-th components

Cs Saturation concentration of dissolved

de Hydraulic diameter of microchannel

in porous environment medium

Dcr Critical diameter of drop to be broken m

Dmax Maximal drop diameter behind

atomizer

m

E Activation energy Jmole1

Ecr Critical strength of electric field Wm1

Ecyl Capture efficiency of particles by

fk Dimensionless parameter of k-th

component of electric force ofinteraction between two chargedparticles

f0

component of electric force ofinteraction between two far-spacedcharged particles

f1

component of electric force ofinteraction between two far-spacedcharged particles, found with greateraccuracy

~ffk Dimensionless parameter of k-th

component of electric force ofinteraction between two touchingcharged particles

fsr, fsy, fer, fey, fey1 Correction factors of hydrodynamic

List of Symbols XV

a Molecular attraction force between

two spherical particles

R Electrostatic repulsion force between

two spherical particles

G Absolute value of vorticity vector ms2

h Distance between particle centre and

wall

m

h0 Factor of hydrodynamic resistance at

motion of non-hindered (free)particle

kgs1

hcr Critical thickness of liquid film on

im Limiting density of electric current A

I Nucleation rate in a unit volume m3s1

j Mass flux through a unit surface kgm2s1

j0 Non-hindered (free) diffusion flux of

particles

m3s1

jrw Diffusion flux of particles through a

unit surface of solid angle

m2s1

js Entropy flux through a unit surface Jm2s1

ji Individual mass flux of i-th

JA Diffusion flux of drops with regard to

molecular attraction force

s1

JAþR Diffusion flux of drops with regard to

both molecular attraction force andelectrostatic repulsion force

Ji Moment of inertia of i-th particle kgm2

JiðrÞ Rate of i-th chemical reaction molem3s1

Jji Mass-exchange rate between j-th and

i-th phases in a unit volume

Ji Relative mass flux of i-th component kgm2s1

Ji Relative mole flux of i-th component molem2s1

l Distance between centres of particles m

L Mole fraction of liquid phase

L0 Distance between device of

preliminary condensation (DPC) andseparator

m

LB Distance from the point of jet outflow

up to the place of jet disintegration

Leq Length of equilibrium establishment m

mC kþ Relative amount of extracted

components of fraction Ckþ

mk Distribution moment of k-th order m3k3

^

mk The dimensionless moment of k-th

order

nðR; t; PÞ Distribution of drops over radiuses m4

nðm; t; PÞ Distribution of bubbles over mass kg1m3

nðV; t; PÞ Distribution of drops over volumes m6

ndðDÞ Distribution of drops over diameters

behind atomizer

m4

ni Number of moles of i-th component mole

nm3 Cubic metre of gas under normal

conditions

m3

List of Symbols XIX

N Numerical concentration of particles m3

Nad Adhesion parameter of cylinder

Nadsph Adhesion parameter of sphere

Nd Numerical concentration of drops

pðeqÞy Established pressure above solution

pvt Partial pressure of saturated vapor

above drop surface

Pa

pvy Partial pressure of saturated vapor

above flat surface

Pa

P Probability of particle displacement

PðV; oÞ Probability of drop formation

Pji Intensity of momentum exchange

between j-th and i-th phases

kgm2s2

PeT Temperature Peclet number

qa Specific flow rate of absorbent 103kgm3

Q Heat brought to a unit mass of gas Jkg1

Qh Amount of hydrocarbons extracted

Qin Specific heat released by

condensation

Jm3s1

Qs Specific heat due to heat transfer

through pipe wall

Jkg1

ri Rate of mass formation of i-th

component in a unit volume

Ri Factors of resistance (components of

resistance tensor) along principalaxes of ellipsoid

m

List of Symbols XXI

RðsÞi Specific mole rate of heterogeneous

chemical reaction with formation ofi-th component

molem2s1

RðvÞi Specific mole rate of homogeneous

chemical reaction with formation ofi-th component

molem2s1

Rms Minimal radius of drops settling with

SA Parameter of molecular interaction

SR Parameter of electrostatic interaction

te Residence time in separation-contact

element

s

tin Characteristic time of inertial

tm Characteristic time of drops mass

exchange with gas

s

tmono Characteristic time of coagulation

(coalescence) in monodisperseemulsion

s

tpoly Characteristic time of drop

coagulation (coalescence) inpolydisperse emulsion

ud Dynamic velocity of gas ms1

ul Velocity of turbulent pulsation of

V Total potential energy of interaction

between two particles

J

VS

A Potential of molecular attraction force

between two spherical particles

J

VP Potential of molecular attraction force

between two infinite parallel planes

Jm2

Vav Average volume of drops m3

R Potential of electrostatic repulsion

force between two particles

W Work of drop done on the change of

volume in a unit time

xcr Critical distance from top end of the

string up to the point of liquid filmdetachment

yi Mole fraction of i-th gas phase

zi Charge of ion of i-th component

b1 Asymmetry square of distribution

repulsion force energy

gI Activity factor of inhibitor

gw Activity factor of water

gj Dimensionless parameter of cyclone

G Surface concentration of surfactant molem2

Gy Limiting surface concentration of

dv Thickness of viscous boundary layer m

dD Thickness of diffusion boundary layer m

between two spherical particles

Dyi Difference between mole fractions of

i-th component at the interface and

in gas bulk flow

Dr Difference of densities of bordering

e0 Dielectric permittivity in vacuum CV1m1

e0 Specific energy dissipation of

er Relative dielectric permittivity

eij Components of strain rate tensor ms2

ev Void fraction of mesh layer

hG Effectiveness coefficient of mesh

droplet capture

hh Effectiveness coefficient of horizontal

separator

hi Mole fraction of i-th component

hk Effectiveness coefficient of horizontal

separator with regard to coagulation

of drops

hs Effectiveness coefficient of separator

with string droplet capture

y Fraction of surface occupied by

molecules of adsorbed substance

l Correction to minimal radius of drop

on condensation growth of drops

lD Thickness of electric double layer m

lG Heat conductivity factor of gas Wm1K1

lh Ablation factor of horizontal

separator or settler

lv Ablation factor of vertical separator or

settler

mð0Þi Chemical potential of pure i-th

Pi Mass percentage of i-th component

rvG Mass concentration of water vapor in

rwG Equilibrium mass concentration of

water vapor in gas above watersolution of DEG

gm3

chromatography

si Relative deviation of exact value fk

from approximate values f0

k

SSG Coefficient of surface tension of

interface solid body – gas

Nm1

SSL Coefficient of surface tension of

interface solid body – liquid

tikl Stress components of i-th phase Nm2

vi Mole volume of i-th component m3mole1

vji Rate of energy exchange between j-th

and i-th phases

Jm3s1

vm Mole volume averaged over solution

composition

m3mole1 List of Symbols XXIX

vr Reduced mole volume

ji Central angle of i-th corrugation of

jalousie droplet capture

jm; i Solution of Laplace equation in

confluent bispherical coordinates

jm; n Solution of Laplace equation in

bispherical coordinates

F Viscous dissipation, dissipation

function

Wm3

Fij Potential of molecular interaction

between i-th and j-th particles

JFðpÞ Correction factor of molecular

c Ratio of ablation factors of horizontal

and vertical settler

cl Stream function on limiting

oijl Collision frequency of collisions of

particles i with particles j

Bottom Indexes

0 Initial value

G Parameter appropriate to positive or negative ion

y Value of parameter far from boundary or in the infinity

c Value of parameter at cathode

e Parameter of external medium

eff Effective parameter

eq Equilibrium value

f Parameter of liquid phase

G Parameter of gas phase

i Parameter of internal medium

i, j Parameter components

in Parameter of gel pore

L Parameter of liquid phase

m Maximal value; value averaged over composition

 Ordinary time derivative

0 Dimensionless size, parameter of internal medium, transformed variable

y Value of parameter far from boundary or in the infinity

ði; j; kÞ Basis vectors in cartesian system of coordinatesðx; y; zÞ Cartesian coordinates

ðr; y; zÞ Cylindrical coordinates

ðir; iy; izÞ Basis vectors in cylindrical system of coordinatesðr; F; zÞ Cylindrical coordinates in narrow gab between particlesðr; y; jÞ Spherical coordinates

ðh; m; FÞ Bispherical coordinates

EiðxÞ Integral exponential function

Gijðx  yÞ Green Function

JmðxÞ Bessel function of m-th order

InðxÞ Modified Bessel function of 1-st order

KnðxÞ Modified Bessel function of 2-nd order

LðrÞq Lagrange polynomial factor

pðx; yÞ Joint distribution density of random variables x and y

Pm

gða; xÞ Incomplete gamma – function

Poij Tensor with zero-trace matrix, that isPoii¼ 0

PðaÞ Antisymmetric part of tensor

a  b ¼ aibi Scalar product of vectors

a  b ¼ eijkajbj Vector product of vectors

T  n ¼ ðTjiniÞej Scalar product of tensor and vector

T : E ¼ TijEij Full scalar product of tensors

D=Dt ¼ q=qt þ uiq=qxi Substancial derivative

‘j ¼ ðqj=qxiÞei Gradient of scalar

‘  u ¼ qui=qxi Divergence of vector

‘  T ¼ ðqTij=qxjÞei Divergence of tensor

‘u ¼ qui=qxk Gradient of vector

‘y Gradient in tangential direction to axisymmetric body

e ¼ 1:602  1019 Elementary electric charge C

F ¼ 9:648  104 Faraday constant Cmole1

e0¼ 8:854  1012 Dielectric permittivity in vacuum CV1m1

Technological Fundamentals of Preparation of Natural Hydrocarbons for Transportation

The product obtained from wells on petroleum, natural gas and gas-condensatefields is invariably a multi-phase, multi-component mixture The raw hydrocar-bon material produced needs to be processed before it can be transported by pipe-line and delivered to gasoline plants, oil refineries, and fractionating plants Inthis context, engineers widely employ technological processes based on the prin-ciple of division (separation) of the native mixture into liquid and gaseous phases

as a result of the action of intrinsic forces such as gravity or inertia

Gas-oil and gas-condensate systems consist of petroleum and gas or gas andcondensate, respectively The state and properties of these systems are deter-mined by various parameters, the most important of which are pressure, temper-ature, specific volumes and composition of the phases The pressure and temper-ature change continuously during movement of the hydrocarbon systemthroughout the production chain, i.e from bed to well to the system of gatheringand preparation and thence to the pipeline As a result, the phase condition ofsystem as well as the composition of the phases change accordingly Besides,some components of the mixture (liquid, gas, or solid phases) may be introducedinto or removed from the system This also results in a change of both the phasecondition and the composition of the mixture

Natural gas contains hydrocarbon components – methane, ethane, propane,butane, and heavier components (which are designated C1, C2, etc.), sour gases– carbon dioxide, hydrogen sulfides, thiols, and other components Besides thelisted components, natural gas also contains water vapor and inorganic admix-tures that are removed together with the extracted from wells The composition

of natural gas from a given source does not remain constant, but changes in thecourse of exploitation as the reservoir pressure falls Table I.1 gives an overviewconcerning rates of change of reservoir pressure through data relating to a realgas-condensate field The numbers in the third row are the projected values ofreservoir pressure

Natural gas and the products of its processing, namely ethane, propane, butane

or the wide fraction of light hydrocarbons (WFLH), as well as the condensate, resent fuels for industry and household consumers and initial raw materials forgas-processing plants If natural gas and the products of its processing are to beused as fuels and raw materials, then they must meet certain requirements, con-

rep-3

cerning on the one hand the quality of commodity output and, on the other hand,the restriction of levels of possible environmental contaminations Specificationsand standards concerning natural gas are dependent on where it is delivered.Basic requirements concerning natural gas supplied through a pipeline, and sa-lient quantitative data are presented in Table I.2.

The restriction on the dew point for hydrocarbons is stipulated for natural gaswith contents of hydrocarbons C5þnot less than 1 g m3

Limitations on the dew point for moisture and hydrocarbons are caused by therequirement that hydrates do not form and the condensate does not precipitate asthe gas is transported at low temperature The moisture content in a gas is deter-mined by the given values of dew point temperature and pressure, using nomo-

Reservoir pressure, MPa 26.6–30.1 23.2–25.6 19.6–21.5 16.3–17.8 10–11.4

grams or empirically-obtained calculating formulas [1] The dew point for carbons depends not only on the pressure of the gas, but also on its composition.

hydro-To find the dew point, we either use special tables or carry out the vapor-liquidbalance calculation for our multi-component system [2]

Heavy hydrocarbons condensed from a gas during its extraction form a densate enriched with a group of hydrocarbons C5þ This by-product of gas-producing and gas-processing industries is an important commodity The con-densate is used as a raw material by oil refineries and in the production of naturalgasoline The fractional composition of condensates varies from one gas field toanother One should make a distinction between a stable condensate containing

con-C5þ, and an unstable one, containing the lighter components excluding C5þ.The condensate grade is characterized by the vapor pressure psatand by the evap-oration of up to 25–85% at the temperature of 50C and the atmospheric pres-sure The vapor pressure of the stable condensate should be such as to assure itsstorage in the liquid state at 37.8C

Thus, both natural gas and gas condensate must be thoroughly processed inorder to achieve the required conditions before we can feed it into a pipeline,whether on a gas-transporting factory or in the communal distribution network.This processing includes the following procedures:

1) Isolation of mechanical admixtures from the gas,

precipitation of moisture and condensate This process is

called the separation;

2) Removal of water vapors from the gas This process is called

gas dewatering (dehydration) Since dehydration causes a

decrease in the threshold temperature of hydrate formation,

this procedure often includes additional steps intended to

prevent the formation of hydrates

3) Extraction of heavy hydrocarbons from the gas

The procedures detailed here are performed by special devices for the complexpreparation of gas (DCPG), which are located at the gas field In their heavy con-centration of machinery and the complexity of technological processes involved,DCPG’s come close to industrial plants

Petroleum is a heavier liquid than gas condensate because it contains muchmore oils, paraffin and other high-molecular compounds Many types of petro-leum are more than 99% hydrocarbons, of which paraffin and the naphthenicseries are most widely submitted Other classes of organic compounds – oxygencompounds, sulphurous compounds, asphalt-tars and others – are also present

in small amounts The majority of sulphurous and oxygen-containing pounds are surface-active compounds They are aggressive with respect to metaland cause heavy corrosion Yet another common admixture in petroleum is min-eralized water, which causes significant complications for its collection and trans-portation The harmful feature of oil-field brines is their ability to form water-oilemulsion that complicates preparation and refining of oil, as well as the move-ment of petroleum in pipelines (water can accumulate in bends and then freeze,

com-Introduction 5