Characterization of crude oilPhysical propertiesThermal propertiesElectrical propertiesOptical properties

Chromatographic techniquesSpectroscopic methods (Principles and Applications of UV-Visible, IR, and NMR

Spectroscopy)

Topics Covered

Introduction

Petroleum exhibits a wide range of physical properties and several relationships can be made between various physical properties. Whereas properties such as viscosity, density, boiling point, and color of petroleum may vary widely, the ultimate or elemental analysis varies, as already noted, over a narrow range for a large number of petroleum samples. The carbon content is relatively constant, while the hydrogen and heteroatom contents are responsible for the major differences between petroleum samples. Coupled with the changes brought about to the feedstock constituents by refinery operations, it is not surprising that petroleum characterization is a monumental task.

Petroleum assay is an investigative (analytic) procedure in laboratory for qualitatively assessing or quantitatively measuring the presence or amount or the functional activity of a target entity (the analyte).

Thus, analyses are performed to determine whether each batch of crude oil received at the refinery is suitable for refining purposes. The tests are also applied to determine if there has been any contamination during wellhead recovery, storage, or transportation that may increase the processing difficulty (cost). The information required is generally crude oil dependent or specific to a particular refinery and is also a function of refinery operations and desired product slate. To obtain the necessary information, two different analytical schemes are commonly used and these are: (1) an inspection assay and (2) a comprehensive assay

Petroleum Assay

Inspection assays usually involve determination of several key bulk properties of petroleum (e.g., API gravity, sulfur content, pour point, and distillation range) as a means of determining if major changes in characteristics have occurred since the last comprehensive assay was performed.

On the other hand, the comprehensive (or full) assay is more complex (as well as time consuming and costly) and is usually performed only when a new field comes on stream, or when the inspection assay indicates that significant changes in the composition of the crude oil have occurred. Except for these circumstances, a comprehensive assay of a particular crude oil stream may not be updated for several years. A full petroleum assay may involve at least determinations of (1) carbon residue yield, (2) density (specific gravity), (3) sulfur content, (4) distillation profile (volatility), (5) metallic constituents, (6) viscosity, and (7) pour point, as well as any tests designated necessary to understand the properties and behavior of the crude oil under examination.

Petroleum Assay

Recommended Inspection Data Required for Petroleum and Heavy Feedstocks (Including Residua)

Petroleum Heavy Feedstocks

Density, specific gravity Density, specific gravity

API gravity API gravity

Carbon, wt.% Carbon, wt.%

Hydrogen, wt.% Hydrogen, wt.%

Nitrogen, wt.% Nitrogen, wt.%

Sulfur, wt.% Sulfur, wt.%

-Nickel, ppm

-Vanadium, ppm

-Iron, ppm

Pour point Pour point

Wax content

-Wax appearance temperature

Viscosity (various temperatures) Viscosity (various temperatures)

Carbon residue of residuum Carbon residue

-Ash, wt.%

Distillation profile: Fractional composition:

-All fractions plus vacuum residue -Asphaltenes, wt.%

-Resins, wt.%

-Aromatics, wt.%

-Saturates, wt.%

Petroleum Assay

Characterization of medium and heavy crude oils using thermal analysis techniques :

This study focused on the characterization of heavy and medium grade crude oils in limestone matrix using differential scanning calorimeter (DSC) and thermogravimetry (TG-DTG). DSC and TG-DTG curves produced for two different crude oils+limestone mixtures indicate that the crude oil undergoes two major transitions when subjected to an oxidizing and constant rate environment known as low- and high-temperature oxidation. Kinetic analysis in the low- and high-temperature oxidation regions were performed using two different kinetic methods. Throughout the study, it was observed that the activation energy values of the samples are varied between 2.40–10.62 and 42.3–181.9 kJ/mol in low- and high-temperature oxidation regions respectively.

About My Paper

Characteristics of Petroleum Products

Refinery Coke

Vacuum Resid

Atmospheric Resid

Crude Oil

Gas Oil

Jet Fuel/Diesel

Gasoline

LPGAverage Carbon Number (Atoms per Molecule)

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/Carb

on

Ato

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Characteristics of Petroleum Products

Petroleum Fractions

Hydrocarbon Range Boiling Range (°C) Boiling Range (°F)

Light gases C2-C4 -90 ~ 1 -130 ~ 30

Gasoline (light and heavy) C4-C10 -1 ~ 200 30 ~ 390

Naphtha (light and heavy) C4-C11 -1 ~ 205 30 ~ 400

Jet fuels C9-C14 150 ~ 225 300 ~ 490

Kerosene C11-C14 205 ~ 225 400 ~ 490

Diesel fuel C11-C16 205 ~ 290 400 ~ 550

Light gas oil C14-C18 255 ~ 315 490 ~ 600

Heavy gas oil C18-C28 315 ~ 425 600 ~ 800

Wax C18-C26 315 ~ 500 600 ~ 930

Lubricating oil >C25 >400 >750

Vacuum gas oil C28-C55 425 ~ 600 800 ~ 1100

Residuum >C55 >600 >1100

Physical PropertiesELEMENTAL (ULTIMATE ) ANALYSIS :

The analysis of petroleum for the percentages of carbon, hydrogen, nitrogen, oxygen, and sulfur is perhaps the first method used to examine the general nature, and perform an evaluation, of a feedstock.

Of the data that are available, the proportions of the elements in petroleum vary only slightly over narrow limits:

• Carbon 83.0 to 87.0%

• Hydrogen 10.0 to 14.0%

• Nitrogen 0.1 to 2.0%

• Oxygen 0.05 to 1.5%

• Sulfur 0.05 to 6.0%

• Metals (Ni and V) <1000 ppm

DENSITY AND SPECIFIC GRAVITY :

The density and specific gravity of crude oil are two properties that have found wide use in the industry for preliminary assessment of the character and quality of crude oil.

Density, specific gravity, or API gravity may be measured by means of a hydrometer, a pycnometer, by the displacement method, or by means of a digital density meter and a digital density analyzer.

The specific gravity of petroleum usually ranges from about 0.8 (45.30 API) for the lighter crude oils to over 1.0 (less than 100 API) for heavy crude oil and bitumen.

One percent by weight solids in the sample can raise the density by 0.007 g/cm3.

Physical Properties

VISCOSITY :

Viscosity is the force in dynes required to move a plane of 1 cm2 area at a distance of 1 cm from another plane of 1 cm2 area through a distance of 1 cm in 1 sec.

Viscosity is derived from the equation:

µ=πrrP/8nl,

where r is the tube radius, l the tube length, P the pressure difference between the ends of a capillary, n the coefficient of viscosity, and m the quantity discharged in unit time.

In the early days of the petroleum industry, viscosity was regarded as the body of petroleum, a significant number for lubricants or for any liquid pumped or handled in quantity.

Physical Properties

SURFACE AND INTERFACIAL TENSION :

Surface tension is a measure of the force acting at a boundary between two phases. If the boundary is between a liquid and a solid or between a liquid and a gas (air) the attractive forces are referred to as surface tension, but the attractive forces between two immiscible liquids are referred to as interfacial tension.

The values of surface tension vary over a narrow range of approximately 24–38 dyne/cm for such widely diverse materials as gasoline (26 dyne/cm), kerosene (30 dyne/cm), and the lubricating fractions (34 dyne/cm)

Physical Properties

THERMAL PROPERTIESVOLATILITY :

The volatility of a liquid or liquefied gas may be defined as its tendency to vaporize, that is, to change from the liquid to the vapor or gaseous state.

For some purposes, its necessary to have further more information regarding vaporization.

The flash point of petroleum or a petroleum product is the temperature to which the product must be heated under specified conditions to give off sufficient vapor to form a mixture with air that can be ignited momentarily by a specified flame. The fire point is the temperature to which the product must be heated under the prescribed conditions of the method to burn continuously when the mixture of vapor and air is ignited by a specified flame.

Thermal PropertiesANILINE POINT :

The aniline point of a liquid was originally defined as the consolute or critical solution temperature of the two liquids, that is, the minimum temperature at which they are miscible in all proportions. The term is now most generally applied to the temperature at which exactly equal parts of the two are miscible.

For oils of a given type, it increases slightly with molecular weight; for those of given molecular weight it increases rapidly with increasing paraffinic character.

SPECIFIC HEAT :

Specific heat is defined as the quantity of heat required to raise a unit mass of material through one degree of temperature.

It can be given by this general equation:

C = 1/d (0:388 + 0:00045t)

C is the specific heat at t0F of an oil whose specific gravity 60/600F is d; thus, specific heat increases with temperature and decreases with specific gravity.

Thermal Properties

HEAT OF COMBUSTION :

The gross heats of combustion of crude oil and its products are given with fair accuracy by the equation:

Q = 12,400 - 2100d2

where d is the 60/608F specific gravity. Deviation is generally less than 1%, although many highly aromatic crude oils show considerably higher values; the ranges for crude oil is 10,000 to 11,600 cal/g and the heat of combustion of heavy oil and tar sand bitumen is considerably higher.

Heats of combustion of petroleum gases may be calculated from the analysis and data of the pure compounds.

Thermal Properties

Electrical PropertiesCONDUCTIVITY :

From the fragmentary evidence available, the electrical conductivity of hydrocarbons is quite small. For example, the normal hydrocarbons (from hexane up) have an electrical conductivity smaller than 1016 V/cm; benzene itself has an electrical conductivity of 4.4 1017 V/cm, and cyclohexane has an electrical conductivity of 7 1018 V/cm. It is generally recognized that hydrocarbons do not usually have an electrical conductivity larger than 1018 V/cm. Thus, it is not surprising that the electrical conductivity of hydrocarbon oils is also exceedingly small, of the order of 1019 to 1012 V/cm.

Available data indicate that the observed conductivity is frequently more dependent on the method of measurement and the presence of trace impurities than on the chemical type of the oil. Conduction through oils is not ohmic; that is, the current is not proportional to field strength: in some regions it is observed to increase exponentially with the latter. Time effects are also observed, the current being at first relatively large and decreasing to a smaller steady value. This is partly because of electrode polarization and partly because of ions removed from the solution. Most oils increase in conductivity with rising temperatures.

DIELECTRIC CONSTANT :

The dielectric constant, ε, of a substance may be defined as the ratio of the capacity of a condenser with the material between the condenser plates C to that with the condenser empty and under vacuum C0:

ε = C/C0

The dielectric constant of petroleum and petroleum products may be used to indicate the presence of various constituents, such as asphaltenes, resins, or oxidized materials. Further, the dielectric constant of petroleum products that are used in equipment, such as condensers, may actually affect the electrical properties and performance of that equipment. The dielectric constant of hydrocarbons, and hence most crude oils and their products, is usually low and decreases with an increase in temperature. It is also noteworthy that for hydrocarbons, hydrocarbons fractions, and products, the dielectric constant is approximately equal to the square of the refractive index. Polar materials have dielectric constants greater than the square of the refractive index.

Electrical Properties

STATIC ELECTRIFICATION :

Dielectric liquids, particularly light naphtha, may acquire high static charges on flowing through or being sprayed from metal pipes. The effect seems to be associated with colloidally dispersed contaminants, such as oxidation products, which can be removed by drastic filtration or adsorption. Since a considerable fire hazard is involved, a variety of methods have been studied for minimizing the danger.

For large-scale storage, avoidance of surface agitation and the use of floating metal roofs on tanks are beneficial. High humidity in the surrounding atmosphere is helpful in lowering the static charge, and radioactive materials have been used to try to induce discharge to ground. A variety of additives have been found that increase the conductivity of petroleum liquids, thus lowering the degree of electrification; chromium salts of alkylated salicylic acids and other salts of alkylated sulfo-succinic acids are employed in low concentrations, say 0.005%.

Electrical Properties

Optical PropertiesREFRACTIVE INDEX :

The refractive index is the ratio of the velocity of light in a vacuum to the velocity of light in the substance. The measurement of the refractive index is very simple, requires small quantities of material, and, consequently, has found wide use in the characterization of hydrocarbons and petroleum samples.

The refractive and specific dispersion, as well as the molecular and specific refraction, have all been advocated for use in the characterization of petroleum and petroleum products. The refractive dispersion of a substance is defined as the difference between its refractive indices at two specified wavelengths of light. Two lines, commonly used to calculate dispersions are, the C (6563 Å, red) and F (4861 Å, blue) lines of the hydrogen spectrum. The specific dispersion is the refractive dispersion divided by the density at the same temperature:

Specific dispersion = nF – nC/d

This equation is of particular significance in petroleum chemistry because all the saturated hydrocarbons, naphthene and paraffin, have nearly the same value irrespective of molecular weight, whereas aromatics are much higher and unsaturated aliphatic hydrocarbons are intermediate.

OPTICAL ACTIVITY :The occurrence of optical activity in petroleum is universal and is a general phenomenon not restricted to a particular type of crude oil, such as the paraffinic or naphthenic crude oils. Petroleum is usually dextrorotatory, that is, the plane of polarized light is rotated to the right, but there are known leavo-rotatory crude oils, that is, the plane of polarized light is rotated to the left, and some crude oils have been reported to be optically inactive. Examination of the individual fractions of optically active crude oils shows that the rotatory power increases with molecular weight (or boiling point) to pronounced maxima and then decreases again. The rotatory power appears to be concentrated in certain fractions, the maximum lying at a molecular weight of about 350 to 400; this maximum is about the same for all crude oils. The occurrence of optically active compounds in unaltered natural petroleum has been a strong argument in favor of a rather low temperature origin of petroleum from organic raw materials.

A magnetic field causes all liquids to exhibit optical rotation, usually in the same direction as that of the magnetizing current; this phenomenon is known as the Faraday effect, θ and it may be expressed by the relation:

θ = pth

θ is the total angle of rotation, t is the thickness of substance through which the light passes, and h is the magnetic field; the constant p is an intrinsic property of the substance, usually termed the Verdet constant (minutes of arc/cm per G); there have been some attempts to use the Verdet constant in studying the constitution of hydrocarbons by physical property correlation.

Optical Properties

Chromatographic TechniquesGAS CHROMATOGRAPHY :

Gas–liquid chromatography (GLC) is a method for separating the volatile components of various mixtures. It is, in fact, a highly efficient fractionating technique, and it is ideally suited to the quantitative analysis of mixtures when the possible components are known and the interest lies only in determining the amounts of each present.

A gas chromatograph with a headspace

sampler

Thin layer chromatography (TLC) :

It is a chromatography technique used to monitor the progress of a reaction, identify compounds present in a given substance, determine the purity of a substance and separate mixtures of compounds.

Thin layer chromatography is performed on a sheet of glass, plastic, or aluminum foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminum oxide, or cellulose. This layer of adsorbent is known as the stationary phase. TLC plate can be made by dissolving silica gel in chloroform or methanol and glass plate should be dip into that.

After that sample has been applied on the plate, a solvent or solvent mixture (known as the mobile phase) is drawn up the plate via capillary action because different compounds run on the TLC plate at different rates due to different polarity of each compound so that separation is achieved.

Separation of black ink on a TLC plate

Chromatographic Techniques

ION-EXCHANGE CHROMATOGRAPHY :

Ion-exchange chromatography is widely used in the analyses of petroleum fractions for the isolation and preliminary separation of acid and basic components.

This technique has the advantage of greatly improving the quality of a complex operation, but it can be a very time consuming separation.

Cation-exchange chromatography is now used primarily to isolate the nitrogen constituents in a petroleum fraction

Anion-exchange chromatography is used to isolate the acid components (such as carboxylic acids and phenols) from petroleum fractions.

Chromatographic Techniques

High-Performance Liquid Chromatography(HPLC) :

HPLC, particularly in the normal phase mode, has found great utility in separating different hydrocarbon group types and identifying specific constituent types.

The general advantages of high-performance liquid chromatography method are: (1) each sample is analyzed as received; (2) the boiling range of the sample is generally immaterial; (3) the total time per analysis is usually of the order of minutes; and (4) the method can be adapted for on-stream analysis.

A modern self contained HPLC.

Chromatographic Techniques

Column Chromatography :

Proteins are often fractionated by column chromatography. A mixture of proteins in solution is applied to the top of a cylindrical column filled with a permeable solid matrix immersed in solvent. A large amount of solvent is then pumped through the column. Because different proteins are retarded to different extents by their interaction with the matrix, they can be collected separately as they flow out from the bottom. According to the choice of the matrix, proteins can be separated according to their charge, hydrophobicity, size or ability to bind to particular chemical groups.

Spectroscopic MethodsINFRARED SPECTROSCOPY :

Conventional infrared spectroscopy yields information about the functional features of various petroleum constituents. For example, infrared spectroscopy will aid in the identification of N–H and O–H functions, the nature of polymethylene chains, the C–H out-of-place bending frequencies, and the nature of any polynuclear aromatic systems.

With the recent progress of Fourier transform infrared (FTIR) spectroscopy, quantitative estimates of the various functional groups can also be made. This is particularly important for application to the higher molecular weight solid constituents of petroleum (i.e., the asphaltene fraction). It is also possible to derive structural parameters from infrared spectroscopic data and these are: (1) saturated hydrogen to saturated carbon ratio; (2) paraffinic character; (3) naphthenic character; (4) methyl group content; and (5) paraffin chain length.

In conjunction with proton magnetic resonance, structural parameters such as the fraction of paraffinic methyl groups to aromatic methyl groups can be obtained.

Absorption bands in IR Spectroscopy

Spectroscopic Methods

Infrared Spectroscopy

Infrared Spectrum of Diethyl Ether

NUCLEAR MAGNETIC RESONANCE(NMR) SPECTROSCOPY :

Nuclear magnetic resonance has frequently been employed for general studies and for the structural studies of petroleum constituents. In fact, proton magnetic resonance (PMR) studies (along with infrared spectroscopic studies) were, perhaps, the first studies of the modern era that allowed structural inferences to be made about the polynuclear aromatic systems that occur in the high molecular weight constituents of petroleum.

In general, the proton (hydrogen) types in petroleum fractions can be subdivided into five types : (1) aromatic hydrogen; (2) substituted hydrogen next to an aromatic ring; (3) naphthenic hydrogen; (4) methylene hydrogen; and (5) terminal methyl hydrogen remote from an aromatic ring. Other ratios are also derived from which a series of structural parameters can be calculated.

Spectroscopic Methods

Example 1H NMR spectrum of ethanol plotted as signal intensity vs. chemical shift. There are three different types of H atoms in ethanol regarding NMR. The hydrogen (H) on the -OH group is not coupling with the other H atoms and appears as a singlet, but the CH3- and the -CH2- hydrogens are coupling with each other, resulting in a triplet and quartet respectively.

Spectroscopic Methods

MASS SPECTROMETRY :

Mass spectrometry can play a key role in the identification of the constituents of feedstocks and products.

The principal advantages of mass spectrometric methods are: (1) high reproducibility of quantitative analyses; (2) the potential for obtaining detailed data on the individual components and carbon number homologues in complex mixtures; and (3) a minimal sample size is required for analysis. The ability of mass spectrometry to identify individual components in complex mixtures is unmatched by any modern analytical technique. Perhaps, the exception is gas chromatography.

However, there are disadvantages arising from the use of mass spectrometry and these are: (1) the limitation of the method to organic materials that are volatile and stable at temperatures up to 3000C (5700F); and (2) the difficulty of separating isomers for absolute identification. The sample is usually destroyed, but this is seldom a disadvantage.

Spectroscopic Methods

Schematics of a simple mass spectrometer with sector type mass analyzer. This one is for the measurement of carbon dioxide isotope ratios (IRMS) as in the carbon-13 urea breath test

Spectroscopic Methods

Mass spectrum of a peptide showing the isotopic distribution

Spectroscopic Methods

ULTRAVIOLET–VISIBLE SPECTROSCOPY :

Ultraviolet–visible spectroscopy (UV-Vis) refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent (near-UV and near-infrared (NIR)) ranges. The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transitions.

UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as transition metal ions, highly conjugated organic compounds, and biological macromolecules. Spectroscopic analysis is commonly carried out in solutions but solids and gases may also be studied.

Spectroscopic Methods

Spectroscopic Methods

An Example of an UV-Vis sample readout.

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