CHARACTERIZATION OF CRUDE OIL

What is Crude oil?

Crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights, and other organic compounds, that are found in geologic formations beneath the earth's surface.

Characterization Knowing what your crude is worth begins by having good crude assay data. The identification of chemical and physical properties of crude oil provides the basis for economic valuation, engineering design and refinery processing.

Why Characterize Crude Oil?

Crude grades vary considerably from each other - in yield and properties. Crude characterization is essential to estimate feedstock properties for refinery units, produce an optimal amount of finished products, meet product quality specifications and to provide an economic assessment for crude oils.

Crude Oil Characterization

•TRADING : Density, API Gravity

•TRANSPORTATION:RVP, Pour Point, KV,Wax content

•CONTAMINATION : Salt content, BS&W

•PROCESSABILITY: Sulfur, Nitrogen, TAN, Asphaltene, MCR

•CRACKING POINT: ASTM Distillation

•LPG POTENTIAL: Light hydrocarbons (GC)

•CLASSIFICATION: Characterization factor

Crude Oil Constituents

•Crude oil contains hundreds of thousands of distinct chemical compounds.

•Crude petroleum from the wellhead generally contains brine.

•All crude oils are emulsions.

•Crude oil consists primarily of hydrocarbons, but also includes compounds containing sulphur, nitrogen, oxygen and metals.

•Every crude petroleum contains about the same kinds of compounds, but in different proportions.

•Crude petroleum also contains colloidal particles, sediment and water(S & W) or, as formerly called, basic sediment and water (BS & W), and solids.

Hydrocarbons

The three main building blocks of the hydrocarbons are the alkyl, or paraffin, chain, the saturated ring (naphthene ring), and the aromatic ring.

•The desirable constituents of crude oil are the hydrocarbons. These range from methane(CH4) at the low-molecular weight end all the way up to paraffins or alkanes with 33 carbon atoms and polynuclear aromatic hydrocarbons with 20 or more carbon atoms. •Natural gas is mostly methane.

• Natural gas that contains hydrocarbon condensate is said to be wet.

• If no condensate forms when the gas is produced to the surface, the gas is called dry.

•If no gas is evolved when a crude oil is reduced to atmospheric pressure, it is said to be dead oil.

•If the quantity of dissolved gas is not large, the oil is said to be low shrinkage oil. •If the GOR is between 2000 to 3300, the oil is said to be a high shrinkage or volatile oil.

Sulphur content

Sulphur is one of the most significant contaminants present in crude oils on account of the problems it can cause during transport and refining, and this is reflected in the terminology.

• Sweet crude• Sour crude

Sweet Crude • Contains less than 0.5% sulphur

• Contains small amounts of hydrogen sulphide and carbon dioxide.

Some of the producers include:• The North Sea area of Northern Europe: Norway and United Kingdom(Brent Crude).•North Africa: Libya and Algeria

Sour Crude

• The total sulphur level in the oil is > 0.5 %

• The majority of the sulphur in crude oil occurs bonded to carbon atoms, with a small amount occurring as elemental sulphur in solution and as hydrogen sulphide gas.

• Processing is essential.

Some of the producers include:•North America: Alberta (Canada), U.S. Gulf of Mexico and Mexico.• South America: Venezuela, Colombia and Ecuador.• Middle East: Saudi Arabia, Iraq, Kuwait, Iran, Syria and Egypt.

•Most of the sulphur is present in the higher boiling fractions of the crude(i.e. the large molecules), but in cases significant amount of hydrogen sulphide and lower-boiling sulphur compounds are present. When this is the case, the strong smell with the toxic and corrosive properties of these compounds will cause the crude to be classed as sour even when the weight content of sulphur is relatively low.

•Sweet crudes attract a premium price, as sour crudes require heavy additional expenditure in refining operations in order to reduce the sulphur content of the product streams to acceptable levels.

Sulphur Compounds

• Thiols• Sulphides• Cyclic sulphides• Disulphides• Thiophenes• Benzothiophenes• Dibenzothiophenes•Naphthobenzothiophenes

Effect of sulphur compounds

•Great environmental concern, contribute to particulate matter, emission of obnoxious odoured sulphur oxide gases

•Receding sulphur level in all petroleum products

•Refining & Technology Focus shifted to sulphur removal from hydrocarbons

Nitrogen Compounds

•Nitrogen compounds occur in relatively small concentration, usually less than 0.1 weight percent as N2.

•As with sulphur compounds, the nitrogen tend to concentrate in the higher boiling fractions.

•Nitrogen compounds may poison noble metal and nickel catalysts in refining processes.

Basic Nitrogen Compounds

Pyridines

Quinolines

Isoquinolines

Acridines

Non-Basic Nitrogen Compounds

Pyroles

Indoles

Carbazoles

Porphyrin

Nitrogen compounds in petroleum

Oxygen compounds

•Most of the oxygen compounds are organic acids, which because of their acidity can be removed by neutralization.

•In addition, phenols are contained in crude oil.

Acidic

Aliphatic Carboxylic Acids

Branched Aliphatic Carboxylic AcidsMonocyclic Naphthenic AcidsBicyclic Naphthenic AcidsPolynuclear Naphthenic AcidsAromatic AcidsBinuclear Aromatic AcidsPolynuclear Aromatic AcidsPhenolCresols

Non- Acidic

EstersAmidesKetonesBenzofuransDibenzofurans

Oxygen compounds in petroleum

Metallic Compounds

Two groups of metal compounds occur in crude oil.

•Light metals, mainly sodium with lesser amount of calcium and magnesium.

•The second group of metal includes vanadium, nickel, cobalt and iron which occur in the higher boiling fractions of crude oils, apparently in the form of metallic porphyrins.

•Metal compounds concentrate in the heavy ends of the crude. Upon combustion vanadium compounds form vanadium pentoxide (V2O5), which catalyzes the oxidation of sulphur compounds to SO3.

•Vanadium and nickel poison catalytic cracking catalysts, causing activity loss and increased coke and hydrocarbon formation.

Particles

•Crude oils are colloidal systems, rather than homogenous solutions.

•Solid particles of two types of materials are suspended in the main liquid, namely asphaltenes and resins.

•Asphaltene particles remain suspended in the oil because they are peptized by the resins.

•The non hydrocarbon constituents of petroleum are highly concentrated in the asphaltene and resin particles.

Asphaltenes

•Asphaltenes contain polycyclic ring compounds that are insoluble in paraffinic solvents, such as n- pentane, but are soluble in aromatic solvents.

•Asphaltene tend to contain the oxygen and sulphur compounds.

Resins

•Resins also contain polycyclic ring compounds that are insoluble in the bulk of the crude oil but are soluble in n- paraffins.

•Resins are mainly hydrocarbons along with nitrogen compounds.

•Both asphaltenes and resins contribute to emulsion stabilization in field processing if they settle out at interfaces.

•They may also cause foaming.

Asphaltene and Wax deposition

•Operating problems are generally caused by asphaltenes and paraffin wax.

•Both materials can precipitate as solids during the field handling of crude oil.

•The deposition of such solids can foul equipment surfaces.

Factors causing wax deposition

•Lowering of the crude oil temperature.

•Flashing of crude oil in flow equipment.

Arsenic and Mercury

•Cause problems in crude oil processing.

•Cause corrosion and catalyst poisoning.

Separating based on characters

Based on Sulphur content

•Sweet crude

• Sour crude

API GRAVITY

The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water.

The specific gravity of oil, measured relative to (fresh) water and given the symbol γo or d, is defined by the following expression, using a standard temperature of 60°F:

γo = Density of crude at 60o F Density of water at 60o F

It is assumed that both the oil and the water are at atmospheric pressure.

The oil industry uses the API gravity to specify the densities of crudes and of liquid petroleum products. This is defined by:-

oAPI= 141.5- 131.5 γo

• Crudes with low specific gravities have high API gravities (up to 50° API); these are referred to as light crudes and will contain high proportions of hydrocarbons with low molecular weights and relatively low boiling points.

• Condensate liquids may have API gravities up to 70°.

Heavy crudes have high specific gravities corresponding to low API gravities between 20° and 10° (often with high viscosities). Very heavy crudes (often called bitumens or tars) have API gravities less than 10° and will consist mostly of very high molecular weight compounds, giving 40% or more of residuum on distillation.

Crude Viscosity oAPI

Conventional crude

<10,000 cP >20

Heavy crude <10,000 cP 10<oAPI<20

Extra Heavy crude

<10

Characterization factor

Characterization factor= (TB) 1/3

Sp. Gravity @ 60oF/60oF

TB- Normal boiling point, oF

Watson and Nelson introduced characterization factor as an index of the chemical character of pure hydrocarbons and petroleum fractions.

•Paraffinic: >12.1

•Mixed based: 12.1 -11.5

•Naphthenic :<11.5

UOP Characterization factor

Compound TB ,oF SG @ 60oF/60oF

UOP K

n- Hexane 155.72 0.66383 12.81

Cyclohexane 177.29 0.78347 10.98

Benzene 176.18 0.88448 9.72

2-Methylhexane 194.09 0.68310 12.59

Methylcyclohexane 213.68 0.77400 11.11

Toluene 231.13 0.87190 9.86

Characterization Factors of Hydrocarbons

Characterization factor

Viscosity index of Lube

Fractions12.1- 12.6 80-100

11.9-12.2 60-80

11.7-12.0 40-60

11.5- 11.8 20-40

11.3- 11.6 0-20

Crude Oil Analysis

Crude oil composition is discussed in terms of the three main needs for composition characterization.

• Crude Assay

• Reservoir Simulation

• Surface Processing

Crude Oil Assay

• Crude assay is a detailed report which describes the properties of the whole crude, as well as the major fractions into which a crude is distilled at the refinery -gasoline, naphtha, kerosene, jet fuel, middle distillates, gas oils and resid.

• Typically, the data contained in a crude assay includes yields generated from the physical distillation & Distillate/resid properties

Crude Oil Evaluation Scheme

TBP analysis

• A TBP analysis is performed in a packed batch distillation column having a large number of theoretical stages at a high reflux ratio.

• The distillation is termed a 15-5 distillation because of the use of approximately 15 theoretical stage equivalents and a reflux ratio of 5:1.

•The charge sample is large: 1 to 5 liters.

Combined TBP curves of Basrah Light Crude oil

Reservoir Simulation

• Assay quality analysis is required

• Proper characterization of the high boiling fractions is essential.

• A great deal work has been done to represent the so called C7+ fraction accurately for the phase- equilibrium calculations required in reservoir simulations.

Surface Processing

For surface processing, the need for an analysis is to provide a composition for thermodynamic property calculations, including density, enthalpy, entropy, and phase equilibrium ratios (K-values).

Laboratory Distillation

•ASTM D86 •ASTM D1160 •TBP Distillation –ASTM D2892•High vacuum Distillation, ASTM D5236•Simulated Distillations based on gas chromatography principle

Test methods-Density

•Density is defined as the mass per unit volume of a substance. (Units:g/mLor g/cm3or kg/m3). Density is temperature-dependent.

•Density is measured using an Anton Parr DMA 48/Kyoto digital density meter, and following ASTM method D 5002 -Density and Relative Density of Crude Oils by Digital Density Analyzer (ASTM, 1996a).

•Density of crude oils : 0.83 to 0.90 g/ml. Densities can be measured to 0.0001 g/ml with a repeatability of "0.0005 g/ml.

Flash Point

• There are several ASTM methods for measuring flash points. The minimum flash point that can be determined by method D93/IP34 is 10oC. Method D 56 is intended for liquids with a viscosity less than 9.5 cSt at 25oC. The flash points and fire points of lubricating oils can be determined by ASTM method D 92/IP 36

• Many fresh crude oils have flash points below 10oC and/or viscosities above 9.5 cSt at 25oC

Pour Point

• The pour point of an oil is the lowest temperature at which the oil will just flow, under standard test conditions. The pour point of the oils is therefore an indication, and not an exact measure, of the temperature at which flow ceases.

• ASTM method D 97 -Standard Test Method for Pour Point of Petroleum Oils (ASTM, 1996a) for pour point determinations.

• Heavy & waxy oil transportation is a challenge

Viscosity

• There are several ASTM Standard Methods for measuring the viscosity of oils. Of these, only methods D 445 -Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) and D 4486 -Standard Test Method for Kinematic Viscosity of Volatile and Reactive Liquids, will yield absolute viscosity measurements (ASTM, 1996a).

• Both of these methods make use of glass capillary kinematic viscometers and will produce absolute measurements in units of centistokes (cSt) only for oils that exhibit Newtonian flow behavior (viscosity independent of the rate of shear).

Flow curve of waxy crude at different temperatures

Factors Effecting the Selection of Crude oil

Choice of crude oil for a refinery depends on:• Product mix• Product quality• Refinery configuration• Product treatment facilities• Refinery design spec-Metallurgy, desalter etc• Environment stipulation for pollution control• Pipeline design for inland refineries

•Proper crude selection is necessary for optimisation of refinery margin.

Yield Structure of Various Crudes