Ref: M.R. Riazi, Characterization and Properties of Petroleum Fractions, ASTM, 2005

Petroleum Definitions

Petroleum is a complex mixture of hydrocarbons that occur in the sedimentary rocks in the form of gases (natural gas), liquids (crude oil), semisolids (bitumen), or solids (wax or asphaltite).

An underground reservoir that contains hydrocarbons is called petroleum reservoir and its hydrocarbon contents that can be recovered through a producing well is called reservoir fluid.

Petroleum Definitions Paraffins

Generally, hydrocarbons are divided into four groups: paraffins, olefins, naphthenes and aromatics.

Paraffins are also called alkanes and have the general formula of CnH2n+2, where n is the number of carbon atoms. Paraffins from C1 to C40 usually appear in crude oil and represent up to 20% of crude by volume. Since paraffins are fully saturated (no double bond), they are stable and remain unchanged over long periods of geological time.

Petroleum Definitions Olefins

Olefins are another series of noncyclic hydrocarbons but they are unsaturated and have at least one double bond between carbon-carbon atoms. Compounds with one double bond are called monoolefins or alkenes. Monoolefins have a general formula of CnH2n.

Olefins are uncommon in crude oils due to their reactivity with hydrogen that makes them saturated; however, they can be produced in refineries through cracking reactions.

Petroleum Definitions Naphthenes

Naphthenes or cycloalkanes are ring or cyclic saturated hydrocarbons with the general formula of CnH2n. Cyclopentane (C5H10), cyclohexane (C6H12), and their derivatives such as n-alkylcyclopentanes are normally found in crude oils.

Thermodynamic studies show that naphthene rings with five and six carbon atoms are the most stable naphthenic hydrocarbons. The content of cycloparaffins in petroleum may vary up to 60%.

Petroleum DefinitionsAromatics

Aromatics are an important series of hydrocarbons found in almost every petroleum mixture from any part of the world. Aromatics are cyclic but unsaturated hydrocarbons that begin with benzene molecule (C6H6) and contain carbon-carbon double bonds.

Some of the common aromatics found in petroleum and crude oils are benzene and its derivatives with attached methyl, ethyl, propyl, or higher alkyl groups. This series of aromatics is called alkylbenzenes and have a general formula of CnH2n-6 (where n ≥ 6).

Petroleum Definitions Sulfur content

Sulfur is the most important heteroatom in petroleum and it can be found in cyclic as well as noncyclic compounds such as mercaptanes (R-S-H) and sulfides (R-S-R’), where R and R’ are alkyl groups.

Sulfur in natural gas is usually found in the form of hydrogen sulfide (H2S). Some natural gas contain H2S as high as 30% by volume. The amount of sulfur in a crude may vary from 0.05 to 6% by weight.

Petroleum Definitions Gas-to-Oil Ratio

The most important characteristic of a reservoir fluid in addition to specific gravity (or API gravity) is its gas-to-oil ratio (GOR), which represents the amount of gas produced at SC in standard cubic feet (scf) to the amount of liquid oil produced at the SC in stock tank barrel (stb). Generally, reservoir fluids are black oil, volatile oil, gas condensate, wet gas, and dry gas.

Petroleum Definitions Crude Oil

The crude oil produced from the atmospheric separator has a composition different from the reservoir fluid from a producing well. Two important characterisitics of a crude that determine its quality are the API gravity (specific gravity) and the sulfur content.

Generally, a crude with the API gravity of less than 20-22 is called heavy crude and with API gravity of greater than 33-40 is called light crude. Similarly, if the sulfur content of a crude is less than 0.5 wt% it is called a sweet oil.

Petroleum Fractions

Some of the petroleum fractions produced from distillation columns with their boiling point ranges are given in Table below. These fractions may go through further processes to produce desired products.

Petroleum Standards There are a number of international standard organizations that recommend specific characteristics or standard measuring techniques for various petroleum products. Some of these organizations are as follows:

1. ASTM (American Society for Testing and Materials) in the United States

2. ISO (International Organization for Standardization), which is at the international level

3. IP (Institute of Petroleum) in the United Kingdom

4. API (American Petroleum Institute) in the United States

Petroleum Properties Distillation curves

For a crude oil or a petroleum fraction of unknown composition, the boiling point may be presented by a curve of temperature versus vol% (or fraction) of mixture vaporized.

There are several methods of measuring and reporting boiling points curves of crude oil and petroleum fractions:

1- ASTM D 86

2- True Boiling Point (TBP)

3- Simulated Distillation by GC (ASTM D 2887)

4- Equilibrium Flash Vaporization (EFV)

5- Distillation at Reduced Pressures (ASTM D 1160)

Petroleum Properties ASTM D 86

ASTM D 86 is one of the simplest and oldest methods of measuring and reporting boiling points of crude oil and petroleum fractions. The test is conducted at atmospheric pressure with 100 mL of sample and the result is shown as a distillation curve with temperatures at 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, and 100% volume vaporized.

For crudes and heavy products, temperatures are reported at maximum of 90, 70, or even 50% volume vaporized. This is due to the cracking of heavy hydrocarbons at high temperatures.

Petroleum Properties ASTM D 86

Petroleum Properties True Boiling Point

ASTM D 86 distillation data do not represent actual boiling point of components in a petroleum fraction. Atmospheric true boiling point (TBP) data are obtained through distillation of a petroleum mixture using a distillation column with 15-100 theoretical plates at relatively high reflux ratios (1-5 or greater).

The high degree of fractionation in these distillations gives accurate component distributions for mixtures. The lack of standardized apparatus and operational procedure is a disadvantage, but variations between TBP data reported by different laboratories for the same sample are small.

Petroleum Properties True Boiling Point

The IBP from TBP curve is less than the IBP from ASTM D 86 curve, while the FBP of TBP curve is higher than that of ASTM curve. Therefore, the boiling range based on ASTM D 86 is less than the actual true boiling range.

Petroleum Properties ASTM D 2887

A distillation curve produced by GC is called a simulated distillation (SD) and the method is described in ASTM D 2887 test method. Simulated distillation method is simple, consistent, and reproducible and can represent the boiling range of a petroleum mixture without any ambiguity.

Distillation curves by SD are presented in terms of boiling point versus wt% of mixture vaporized because in gas chromatography composition is measured in terms of wt% or weight fraction. SD curves are very close to actual boiling points shown by TBP curves.

Petroleum Properties ASTM D 1160

For products such as heavy gas oils that contain heavy compounds and may undergo a cracking process during vaporization at atmospheric pressure, distillation data are measured at reduced pressures, 1, 2, 10, or 50 mmHg. The experimental procedure is described in ASTM D 1160 test method.

ASTM D 1160 distillation data are measured more accurately than ASTM D 86 since it is conducted at low pressure. For this reason ASTM D 1160 curves are closer to TBP curves at the same pressure base.

Petroleum Properties The Watson characterization factor (Kw) is one of the oldest characterization factors originally defined by Watson et al. of the Universal Oil Products (UOP) in mid 1930s. For this reason the parameter is sometimes called UOP characterization factor and is defined as

The naphthenic hydrocarbons have Kw values between paraffinic and aromatic compounds. In general, aromatics have low Kw values while paraffins have high values.

RTSG

TK b

bw

o3/1

point boiling normal where

Petroleum Properties Reid vapor pressure (RVP) is the absolute pressure exerted by a mixture at 100 oF and a vapor-to-liquid volume ratio of 4. The RVP is one of the important properties of gasolines and jet fuels. The standard test of RVP is ASTM D 323.

For a pure compound the freezing point is the temperature at which liquid solidifies at 1 atm pressure. Similarly the melting point is the temperature that a solid substance liquefies at 1 atm. A pure substance has the same freezing and melting points; however, for petroleum mixtures, there are ranges of melting and freezing points versus percent of the mixture melted or frozen.

Petroleum Properties Pour point of a petroleum fraction is the lowest temperature at which the oil will pour or flow when it is cooled without stirring under standard cooling conditions.

Pour point represents the lowest temperature at which an oil can be stored and still capable of flowing under gravity. When temperature is less than pour point of a petroleum product it cannot be stored or transferred through a pipeline. Test procedures for measuring pour points of petroleum fractions are given under ASTM D 97 and ASTM D 5985 methods.

Petroleum Properties Cloud point is the lowest temperature at which wax crystals begin to form by a gradual cooling under standard conditions. At this temperature the oil becomes cloudy and the first particles of wax crystals are observed. The standard procedure to measure the cloud point is ASTM D 2500.

Low cloud point products are desirable under low-temperature conditions. Wax crystals can plug the fuel system lines and filters, which could lead to stalling aircraft and diesel engines under cold conditions. Cloud points are measured for oils that contain paraffins in the form of wax and therefore for light fractions (naphtha or gasoline) no cloud point data are reported.

Petroleum Properties Flash point for a hydrocarbon or a fuel is the minimum temperature at which vapor pressure of the hydrocarbon is sufficient to produce the vapor needed for spontaneous ignition of the hydrocarbon with the air with the presence of an external source, i.e., spark or flame. The standard procedure to measure the Flash point is ASTM D 93.

Flash point is an important parameter for safety considerations, especially during storage and transportation of volatile petroleum products (i.e., LPG, light naphtha, gasoline). The surrounding temperature around a storage tank should always be less than the flash point of the fuel to avoid possibility of ignition.

Petroleum Properties Flash point should not be mistaken with fire point, which is defined as the minimum temperature at which the hydrocarbon will continue to burn for at least 5 s after being ignited by a flame.

Autoignition temperature is the minimum temperature at which hydrocarbon vapor when mixed with air can spontaneously ignite without the presence of any external source. Values of autoignition temperature are generally higher than flash point. This is particularly important from a safety point of view when hydrocarbons are compressed. Standard test is ASTM D 2155.

Petroleum Properties

To have a combustion, three elements are required: fuel (hydrocarbon vapor), oxygen (i.e., air), and a spark to initiate the combustion. One important parameter to have a good combustion is the ratio of air to hydrocarbon fuel. The combustion does not occur if there is too much air (little fuel) or too little air (too much fuel). This suggests that combustion occurs when hydrocarbon concentration in the air is within a certain range. This range is called flammability range and is usually expressed in terms of lower and upper volume percent in the mixture of hydrocarbon vapor and air.

Petroleum Properties Octane number is a parameter defined to characterize antiknock characteristic of a fuel (gasoline and jet fuel) for spark ignition engines. Octane number is a measure of fuel's ability to resist auto-ignition during compression and prior to ignition. Higher octane number fuels have better engine performance.

The octane number of a fuel is measured based on two reference hydrocarbons of n-heptane with an assigned octane number of zero and isooctane (2,2,4-trimethylpentane) with assigned octane number of 100.

Petroleum Properties There are two methods of measuring octane number of a fuel in the laboratory; motor octane number (MON) and research octane number (RON). The MON is indicative of high-speed performance (900 rpm) and is measured under heavy road conditions (ASTM D 357). The RON is indicative of normal road performance under low engine speed (600 rpm) city driving conditions (ASTM D 908). The arithmetic average value of RON and MON is known as posted octane number (PON).

Isoparaffins and aromatics have high octane numbers while n-paraffins and olefins have low octane numbers.

Petroleum Properties Generally there are three kinds of gasolines: regular, intermediate, and premium with PON of 87, 90, and 93, respectively. Improving the octane number of fuel would result in reducing power loss of the engine, improving fuel economy, and a reduction in environmental pollutants and engine damage. There are a number of additives that can improve octane number of gasoline or jet fuels. These additives are tetra-ethyl lead (TEL), alcohols, and ethers.

Petroleum Properties For diesel engines, the fuel must have a characteristic that favors auto-ignition. The ignition delay period can be evaluated by the fuel characterization factor called cetane number (CN). The shorter the ignition delay period the higher CN value.

The cetane number is defined as:

CN = vol% n-cetane + 0.15(vo1% HMN)

Where n-cetane (n-C16H34) has a CN of 100, and heptamethylnonane (HMN) has a CN of 15. The cetane number of a diesel fuel can be measured by the ASTM D 613 test method.

Petroleum Properties Higher cetane number fuels reduce combustion noise and permit improved control of combustion resulting in increased engine efficiency and power output. Higher cetane number fuels tend to result in easier starting and faster warm-up in cold weather and can cause reduction in air pollution.

The product distributed in France and Europe have CN in the range of 48-55. In the United States and Canada the cetane number of diesel fuels are most often less than 50. Cetane number of diesel fuels can be improved by adding additives such as 2-ethyl-hexyl nitrate or other types of alkyl nitrates.

Petroleum Properties Aniline point for a hydrocarbon or a petroleum fraction is defined as the minimum temperature at which equal volumes of liquid hydrocarbon and aniline are miscible.

The aniline point is important in characterization of petroleum fractions and analysis of molecular type. The aniline point is also used as a characterization parameter for the ignition quality of diesel fuels. It is measured by the ASTM D 611 test method. Aromatics have very low aniline points in comparison with paraffins, since aniline itself is an aromatic compound (C6H5-NH2) and it has better miscibility with aromatic hydrocarbons.

Petroleum Properties When a petroleum fraction is vaporized in the absence of air at atmospheric pressure, the nonvolatile compounds have a carbonaceous residue known as carbon residue (CR). Therefore, heavier fractions with more aromatic contents have higher carbon residues while volatile and light fractions such as naphthas and gasolines have no carbon residues.

There are three different test methods to measure carbon residues, Ramsbottom (ASTM D 524), the Conradson (ASTM D 189) and microcarbon (ASTM D 4530). In most cases carbon residues are reported in wt%.

Petroleum Properties The smoke point (SP) is a maximum flame height at which a fuel can be burned in a standard wick-fed lamp without smoking. It is expressed in millimeters and a high smoke point indicates a fuel with low smoke-producing tendency. Measurement of smoke point is described under ASTM D 1322.

Smoke point is a characteristic of aviation turbine fuels and kerosenes and indicates the tendency of a fuel to burn with a smoky flame. Higher amount of aromatics in a fuel causes a smoky characteristic for the flame and energy loss due to thermal radiation.

Quality of Petroleum Products The quality of a petroleum product depends on certain specifications or properties of the fuel to satisfy required criteria set by the market demand. These characteristics are specified for best use of a fuel (i.e., highest engine performance) or for cleaner environment.

These specifications vary from one product to another and from one country to another. Standard organizations such as ASTM give such specifications for various products. For example: ASTM D 4814 for gasoline, ASTM D 975 for diesel fuel, ASTM D 3699 for kerosene and ASTM D 6615 for jet fuel.

Crude Oil Distillation Ref: R. Smith, Chemical Process Design and Integration, Wiley, 2005.

In the first stage of processing crude oil, it is distilled under conditions slightly above atmospheric pressure. A range of petroleum fractions are taken from the crude oil distillation.

Designs are normally thermally coupled. Most configurations follow the thermally coupled indirect sequence as shown in Figure (a). However, rather than build the configuration in Figure (a), the configuration of Figure (b) is the one normally constructed. Notice that the two arrangements are equivalent.

Crude Oil Distillation

Crude Oil Distillation Unfortunately, a practical crude oil distillation cannot be operated in quite the way shown in Figure (b), because: Extremely high temperature sources of heat would be required. Steam is usually not distributed for process heating at such high temperatures. High temperatures in the reboilers would result in significant fouling of the reboilers from decomposition of the hydrocarbons to form coke.

Therefore, in practice, some or all of the reboiling is substituted by the direct injection of steam into the distillation. The steam is condensed in the overhead and is separated in a decanter from the hydrocarbons.

Crude Oil Distillation Another problem with the arrangement in Figure (b) is that as the vapor rises up the main column, its flow rate increases significantly.

This problem can be solved by removing heat from the main column at intermediate points by pumparound. This corresponds with introducing some condensation of the vapor at the top of intermediate columns.

Crude Oil Distillation Pumparound

In a pumparound, liquid is taken from the column, sub cooled and returned to the column at a higher point.

By choosing the most appropriate flow rate and temperature for the pumparound, the heat load to be removed can be adjusted to whatever is desired.

The trays between the liquid draw and return in a pumparound have more to do with heat transfer than mass transfer. In addition to returning a sub cooled liquid to the column, mixing occurs as material is introduced to a higher point in the column.

Crude Oil Distillation Furnace

The crude oil entering the main column needs to be preheated to around 400 ◦C. This is down by a furnace (fired heater). Note that this temperature is higher than decomposition limit, but a high temperature can be tolerated in the furnace if it is only for a short residence time.

All of the material that needs to leave as product above the feed point must vaporize as it enters the column. In addition to this, some extra vapor over and above this flowrate must be created that will be condensed and flow back down through the column as reflux. This extra vaporization to create reflux is known as overflash.

Crude Oil Distillation

The distillation of crude oil under conditions slightly above atmospheric pressure is limited by the maximum temperature that can be tolerated by the materials being distilled, otherwise there would be decomposition.

The residue from the atmospheric crude oil distillation is usually reheated to a temperature around 400◦C or slightly higher and fed to a vacuum column, which operates under a high vacuum (about 50 mmHg) to allow further recovery of material from the atmospheric residue, as shown in the next Figure.

Crude Oil Distillation