Hydrocarbon Components and Physical Properties Core COPYRIGHT. Basic Terminology

Transcription

1 3/7/2017 Learning Objectives Hydrocarbon Components and Physical Properties Core Basic Terminology By the end of this lesson, you you will will be be able able to: to: Describe the concept of atomic mass, molecular mass, and the mol Identify the four main hydrocarbon groups Practice the concept of relative density 1 1

2 3/7/2017 Relative Atomic Mass of Common Oil/Gas Elements Avogadro's number: 6 x Relative Molecular Mass (Weight) The relative molecular weight (MW) of a molecule is the sum of the relative atomic weights of the atoms combining to produce the molecule. What is the mol. weight of methane (CH 4 )? Atomic mass of elements: C = 12, H = 1 MW = 12 + (4 x 1) =

3 3/7/2017 The Mol The mol is the amount of substance of a system which contains as many elementary entities as there are atoms in kg (12 g) of Carbon-12 1 mol (gram mol) is based on Avogadro s number (6x10 23 ) of molecules. Mol. wieght of CH 4 Mol. weight = = what?? 16 grams/gram-mol = 16 g/mol = 16 kg/kmol or 16 lbm/lb-mol 1 kmol = 1000 mol 1 kmol = lb-mol 1 lb-mol = 454 mol 3 3

4 Hydrocarbon Groups Paraffins Olefins Naphthenes Aromatics Paraffin Family Methane (CH 4 ) Ethane (C 2 H 6 ) PONA Propane (C 3 H 8 ) Chemical formula for paraffins: C n H 2n+2 4 1

5 Paraffin Isomers Normal Butane (n-c 4 H 10 ) Iso Butane (i-c 4 H 10 ) Paraffin Isomers 2-Methyl Propane The number of isomers increases dramatically with the number of carbon atoms in the molecule 5 2

6 Olefins Family Propene (H 2 C=CHCH 3 ) (also known as Propylene) Ethene (H 2 C=CH 2 ) (also known as Ethylene) Chemical formula for olefins: C n H 2n Naphthenes Family Cyclo-Pentane (C 5 H 10 ) Cyclo-Hexane (C 6 H 12 ) Chemical formula for napthenes: C n H 2n 6 3

7 Aromatics Family (BETX Compounds) Benzene (C 6 H 6 ) Ethylebenzene (C 6 H 5 CH 2 CH 3 ) Toulene Para-Xylene (C 6 H 5 CH 3 ) (C 6 H 4 (CH 3 ) 2 ) Chemical formula for aromatics: C n H n Aromatics Family (BETX Compounds) Since these compounds are unsaturated, they react readily and Benzene may be oxidized to Ethylebenzene form organic acids Toulene Para-Xylene (CThey 6 H 6 also ) promote (Cfoaming 6 H 5 CHand 2 CHother 3 ) operational (C 6 H 5 CH problems 3 ) (C 6 Hin 4 (CH the 3 ) 2 ) production and handling of crude oil and natural gas Aromatics are significantly more soluble in water and water-based solvents Benzene and ethylbenzene are known carcinogens Toluene and xylene are respiratory and digestive irritants As a result they all have environmental emission limits Chemical formula for aromatics: C n H n 7 4

8 Aromatics Family (BETX Compounds) Significant amounts of BETX components may be absorbed in Benzene gas dehydration and Ethylebenzene gas treating contactorstoulene Para-Xylene For glycol units, this stream was historically vented to atmosphere (C 6 H 6 ) (C 6 H 5 CH 2 CH 3 ) (C 6 H 5 CH 3 ) (C 6 H 4 (CH 3 ) 2 ) For amine units, this stream may be vented to atmosphere or fed to a sulfur plant, where hydrocarbons can deactivate the catalyst in the reactor beds Any analysis of crude oil and natural gas should include aromatics Even small amounts can influence physical behavior and affect design and operation of facilities Chemical formula for aromatics: C n H n Hydrocarbon Mixture Composition Hydrocarbons from the three main families have different properties: The hydrocarbon composition of natural gas and crude oils determines both the phase behavior of the fluid, and the fluid physical properties The composition must be known to develop accurate thermodynamic models to predict fluid behavior in processing facilities For liquids, characterization parameters can be developed from compositional analysis and distillation characteristics For crude oils, PNA analysis using test method ASTM D5443, provides the mass % of PNA compounds in crude oils P = Paraffins N = Naphthenes A = Aromatics 8 5

9 Hydrocarbon Mixture Composition Hydrocarbons from the three main families have different properties: The hydrocarbon composition of natural gas and crude oils determines both the phase behavior of the fluid, and the fluid physical properties The composition must be known to develop accurate thermodynamic models to predict fluid behavior in processing facilities For liquids, characterization parameters can be developed from compositional analysis and distillation characteristics For crude oils, PNA analysis using test method ASTM D5443, provides the mass % of PNA compounds in crude oils Normalized Weight Percent Profile Carbon Naphthenes Paraffins Aromatics Total Total

10 2/3/2017 Pentane Isomer Data 596 g/l 37.3 lb/ft

11 2/3/2017 Relative Density of Gas MW of air is (or 29) ROT: Relative density of natural gas usually falls between 0.55 and

12 Hydrocarbon Components and Physical Properties Core Relative Density of a Liquid Relative Density of Liquid Density of Liquid Density of Water Standard Conditions = Water at 4 deg. C ρ water = 1000 kg/m 3 [62.4 lb/ft 3 ] Specific gravity = Relative density of the fluid with respect to water API Gravity and Relative Density General API Gravity Ranges for Liquid Hydrocarbons

13 Hydrocarbon Components and Physical Properties Core API Gravity Examples Forties (North Sea) 43.4 Brent (North Sea) 38.5 Bonny Light (Nigeria) 33.4 Cabinda (Angola) 32.8 Mars (GoM) 31.0 Liuhua (South China Sea) 23.0 Captain (North Sea) 19.1 Kern River (California) 13.0 Athabasca (Alberta) 8-10 Learning Objectives Light Medium Heavy You are now able to: Describe the concept of atomic mass, molecular mass, and the mol Identify the four main hydrocarbon groups Practice the concept of relative density 13 2

14 3/3/2017 Learning Objectives Hydrocarbon Components and Physical Properties Core Hydrocarbon Analysis and C 6 + Characterization By the end of this lesson, you you will will be be able able to: to: Discuss how a gas chromatograph works, the limitations of various analysis methods, and the difference between an extended analysis and a standard gas chromatographic analysis Recognize the uncertainties involved with characterizing the C 6 + components in a natural gas, condensate or crude oil stream, and describe the relationship of these factors with hydrocarbon liquid composition 14 1

15 3/3/2017 Gas Chromatograph Schematic 15 2

16 Standard and Extended Analysis In a standard natural gas analysis, the following compounds are detected: C 1 C 5 paraffins, and a C 6 + composite May include an air composite, (nitrogen, oxygen, and argon), and carbon dioxide. Extended analysis reports the concentration of C 6 + components In extended gas analysis the following information must be specified: The extent of the analysis for the C 6 + components (for example C 5 through C 16 ) For crude oils, this is often specified through C 20 + If BETX compounds need to be detected. Optional H 2 S and trace sulfur components Standard Gas Chromatograph Analysis Air Composite Methane Carbon Dioxide Ethane C 6 + Backflush Propane Isobutane n-butane Isopentane n-pentane 16 1

17 Extended Gas Analysis C 6 + and light ends Extended Analysis of the C 5 C

18 Chromatogram of an NGL Mixture Example Analysis of a Hydrocarbon Mixture C 6 = nc 6 C 7 = nc 7 Mol. weight of nc 7 =

19 Analysis Guidelines Many operating problems are the result of inadequate analysis. Analyze for CO 2 and H 2 S. Analyze for aromatics. Select a reasonable heavy hydrocarbon extended analysis. Inadequate sampling and analysis is a major cause of system problems. 19 4

20 Distillation Characteristics Two types of distillations for characterization of a liquid ASTM D86 ASTM Distillation ASTM D2892 (True Boiling Point (TBP) method) Uses a non-refluxed, single stage unit The temperature at which the first droplet of liquid is collected overhead is recorded as the initial boiling point (IBP) The temperature is raised at a steady rate and recorded when several mass of volume percentages of the sample have been recovered The end point temperature is recorded, usually when 95% of the sample volume has been recovered ASTM D-86 Distillation Curve Uses a column with 15 theoretical stages Follows the true boiling point (TBP) of the sample and is the preferred approach The apparatus contains multiple theoretical stages, and a high reflux rate is used to obtain a sharper distillation of the fractions distilled For purposes of predicting physical and thermodynamic properties, each subfraction may be treated as a single component The sub-fractions consist of many different types of hydrocarbon compounds and are referred to as pseudo-components 20 1

21 Some Representative TBP Distillation Curves 21 2

22 Watson Characterization Factor Watson Characterization Factor Average Boiling Point or NBP Where T b = γ = A = Average Boiling Point or NBP Relative Density Conversion Factor Relative Density (sp. gr.) You may want to PAUSE a moment and review the equations and following terms Relative Density vs. MW for Generalized Hydrocarbon Fractions 22 1

23 Relative Density vs. Boiling Point for Generalized Hydrocarbon Fractions Learning Objectives You are now able to: Discuss how a gas chromatograph works, the limitations of various analysis methods, and the difference between an extended analysis and a standard gas chromatographic analysis Recognize the uncertainties involved with characterizing the C 6 + components in a natural gas, condensate or crude oil stream, and describe the relationship of these factors with hydrocarbon liquid composition 23 2

24 3/8/2017 Learning Objectives Hydrocarbon Components and Physical Properties Core Gas and Liquid Physical Properties By the end of this lesson, you will be able to: By the end of this lesson, you will be able to: Describe an Equation of State, its purpose and uses Define standard (normal) conditions for SI and FPS units, and calculate the molar volume at these conditions Describe the gas compressibility factor, and use it to calculate gas density Define the property viscosity, list applications where it is used, and describe correlations that can be used to predict its value Estimate the density of a hydrocarbon liquid at a specified temperature and pressure 24 1

25 3/8/2017 Hydrocarbon Properties Why should I care about properties of hydrocarbons? Physical Properties Density Viscosity Used to size separators, piping, columns, etc. Hydrocarbon Components and Physical Properties Core Equations of State Phase Behavior Vapor Pressure K-values Used to calculate the amount and composition of vapor and liquid in the system Qualitative Phase Behavior and Vapor Liquid Equilibrium Core Thermodynamic Properties Enthalpy Entropy Heat Capacity Used to calculate energy changes in compressors, pumps, heat exchangers, etc. Thermodynamics and Application of Energy Balances Core For gases, density is a function of pressure, temperature and mixture composition For liquids, pressure and temperature have less effect compared to gases The interaction of molecules in mixtures has a significant effect on physical properties Modelling this interaction requires quantifying the attractive and repulsive forces between molecules Much easier to do for mixtures of similar molecules, like methane, ethane and propane than for mixtures, which include different types of molecules such as water, hydrogen sulfide, carbon dioxide and different hydrocarbon types such as naphthenes and aromatics 25 2

26 3/8/2017 Equations of State Any equation correlating Pressure (P), Volume (V) and Temperature (T) is called an equation of state (EOS) Equations of state are important because physical and thermodynamic properties can be written as a function of P, V and T The simplest EOS is the Ideal Gas Law The ideal gas law applies up to about 400 kpa or 60 psi Ideal Gas Law PV nrt V n RT v P Wh ere: or Where: P = absolute pressure V = volume n = number of mols of gas in volume V at P and T T = absolute temperature v = molar volume R = ideal gas law constant (see table below) P V T R v kpa m 3 K kpa. m 3 /kmol, K m 3 /kmol MPa m 3 K kpa. m 3 /kmol. K m 3 /kmol bar m 3 K bar. m 3 /kmol. K m 3 /kmol psia ft 3 o R psia. ft 3 /lb-mol.o R ft 3 /lb-mol lbf/ft 2 ft 3 o R 1545 psf. ft 3 /lb-mol.o R ft 3 /lb-mol You may want to PAUSE a moment and review the equations and following terms 26 3

27 3/8/2017 Some Common Equations of State Molecular forces Van der Waals P V RT - b - a 2 V Soave Redlich Kwong (SRK) Peng Robinson (PR) Where P = R = T = V = a = b = Pressure Ideal gas constant Temperature Volume Molecular forces Volume of molecules P V RT - b Molar Volume at Standard Conditions m kpa K kmol K v kPa 3 v 23.64m / kmol Where At standard conditions (SI Units) V n P - V RT - b - a V V t b b b(v b) V V a & b = f(t, P, composition, critical properties, acentric factor, mixing rules) At standard conditions, the ideal gas law equation applies P = R = T = V = v = v RT P Pressure Ideal gas constant Temperature Volume Molar volume m kpa K kmol K v kpa 3 v 22.41m / kmol At normal conditions (SI Units) You may want to PAUSE a moment and review the equations and following terms a t ft psi 520 R lb mol R v psi v 379.5scf / lb mol At standard conditions (FPS Units) 27 4

28 3/8/2017 Real Gas Equation Equation : Where: P = absolute pressure V = volume n = number of mols of gas in volume V at P T = absolute temperature R = ideal gas law constant z = compressibility factor Natural Gas Compressibility Factors, MW = (SI)

29 3/3/2017 Gas Density Equation Gas density Where P = R = T = Pressure Ideal gas constant Temperature ρ = Gas density MW = Molecular weight z = Compressibility factor 29 1

30 Viscosity Characteristics and uses Internal resistance of a fluid to flow against itself Sometimes referred to as internal friction Most light hydrocarbons and gases are Newtonian fluids, i.e. the viscosity is independent of the force applied to make the fluid flow Many heavy and/or waxy crudes are non-newtonian Absolute Viscosity, Units: Centipoise, cp Kinematic Viscosity, Units: Centistokes, cs Kinematic Viscosity rel. Absolute Viscosity Fluid Density Kinematic viscosity is equal to the absolute viscosity in centipoise divided by the fluid density in grams per cubic meter (g/m 3 ). The usual unit used is the centistoke. Viscosity Comparisons Fluids Approximate Viscosity at Room Substance Temperature in Centipoises Water 1 Olive Oil 100 Glycerin 500 Motor Oil Honey Molasses Heinz Ketchup Peanut Butter

31 Viscosity Gases Viscosity used in fluid flow, heat and mass transfer calculations Viscosity of medium to high pressure gases varies from about cp, cp is a good approximation for most calculations Gas viscosity increases with pressure and temperature Lee et al. Correlation for Hydrocarbon Gas Viscosity Viscosity, cp Temperature, Degrees F Gas MW = 19.7 Temperature, Degrees C 31 2

32 Viscosity Liquids Viscosity used in fluid flow, heat and mass transfer, and phase separation calculations Viscosity of hydrocarbon liquids varies from <0.2 cp for NGLs to >1000 cp for heavy crude oils Crude oil viscosity correlations based on the density of the oil are generally unreliable Abu Dhabi 39 API C Beatrice, UK 39 API C Lucina, Gabon 39 API C Note: The viscosity reference figures can be found in the PetroCore Gas Conditioning and Processing Volume 1 reference book: Figure 3.15 for NGLs Figure 3.16 for crude oils Kinematic Viscosity vs. Temperature, and Other Properties, for a Wide Range of Traditional Crude Oils 32 3

33 Hydrocarbon Liquid Density Correlations Nomographs based on empirical data Usually correlated against temperature and characterization parameters such as K w or boiling point Corresponding States Methods API Method Modified Rackett Equation Hankinson-Brobst-Thomson [COSTALD] Corresponding States Liquid Densities Equation of State methods Cubic EOS such as SRK and PR are not normally reliable BWRS EOS is more accurate Most simulation packages recommend COSTALD Correlations Relating Various Characterization Variables and Liquid Properties API = 40 K w = 11.9 MABP = 255 o C MW =

34 Relative Density of Liquid Hydrocarbon Mixtures Relative Density of Paraffin Liquid Implicit Assumption: Fluid at Bubble Point Pressure 34 2

35 Learning Objectives You are now able to: Describe an Equation of State, its purpose and uses Define standard (normal) conditions for SI and FPS units, and calculate the molar volume at these conditions Describe the gas compressibility factor, and use it to calculate gas density Define the property viscosity, list applications where it is used, and describe correlations that can be used to predict its value Estimate the density of a hydrocarbon liquid at a specified temperature and pressure PetroAcademy TM Gas Conditioning and Processing Core Hydrocarbon Components and Physical Properties Core Introduction to Production and Gas Processing Facilities Core Qualitative Phase Behavior and Vapor Liquid Equilibrium Core Water / Hydrocarbon Phase Behavior Core Thermodynamics and Application of Energy Balances Core Fluid Flow Core Relief and Flare Systems Core Separation Core Heat Transfer Equipment Overview Core Pumps and Compressors Overview Core Refrigeration, NGL Extraction and Fractionation Core Contaminant Removal Gas Dehydration Core Contaminant Removal Acid Gas and Mercury Removal Core 35 3