Prediction of Petroleum Fraction Properties
This article presents the definition and estimation methods of physical properties that are used to determine the quality of petroleum products. The list of properties considered in this publication contains: Reid Vapor Pressure, Flash and Pour Points, Bromine Number, Cetane Index, Carbon Residue, Aniline and Smoke points, Octane Numbers, Stoichiometric Air to Fuel Ratio, Methane number and CO2, SO2 and NO2 equivalents and emission factors.
Some of these properties like Reid Vapor Pressure, Flash Point or Pour Point are employed for safety consideration or storage and transportation of products. Other properties apply only to a specific petroleum fraction or product; for example, Octane Numbers apply to gasoline and engine type fuels, Methane Number and Stoichiometric Air to Fuel Ratio apply to natural gas, Cetane Indexes are related to diesel fractions and, Carbon Residue and Pour Point are characteristic of heavy fractions, residues, and crude oils.
Predictive methods for some of these properties are obscure and limited accuracy. VMGSim makes use of the best reported methods to estimate the property values and in some cases those methods have been tuned and improved to better match experimental data. All the properties discussed in this communication can be found in VMGSim’s Special Properties unit operation under the Refinery tab.
Reid Vapor Pressure
One of the most important properties of petroleum products related to volatility after the boiling point is vapor pressure. For petroleum fractions, vapor pressure is measured by the method of Reid. Reid Vapor Pressure is the absolute pressure exerted by an air saturated mixture at 37.8 C (100 F) at a vapor-to-liquid volume ratio of 4 . The Reid Vapor Pressure is one of the important properties of gasolines and jet fuels and it is used as a criterion for blending of products, it is also a useful parameter for estimation of losses from storage tanks during filling or draining.
The details of the Reid Vapor Pressure calculation as well as for other types of vapor pressures have been discussed in a previously published VMG Newsletter article .
Flash Point of petroleum fractions is the lowest temperature at which vapors arising from the oil will ignite, or in other words flash, when exposed to a flame under specified conditions. Therefore, the Flash Point of a fuel indicates the maximum temperature that it can be stored without serious fire hazard.
There are several methods for determining Flash Points of petroleum fractions. The most widely used relation for estimation of Flash Point is the API (American Petroleum Institute) method 2B7.1 , which was developed by Riazi and Daubert . This method is used in VMGSim to perform the estimation of this property. The estimation method is a correlation between Flash Point and the petroleum fraction’s distillation temperature at 10 vol% vaporized (ASTMD86 at 10%). This correlation should be applied to fractions with normal boiling points from 65 to 590 C (150 to 1100 F).
The 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.
Riazi and Daubert  proposed a generalized correlation to estimate the Pour Point of petroleum fractions based on viscosity, molecular weight, and specific gravity. This equation was developed with data from pour points of more than 300 petroleum fractions with molecular weights ranging from 140 to 800 and API gravities from 1 to 50 with an average absolute deviation of 3.9 C [3, 4]. This method is also accepted by the API (API method 2B8.1) and it is included in VMGSim as the standard method to estimate Pour Point of petroleum fractions.
Bromine Number is the amount of bromine in grams absorbed by 100 grams of a sample. This number indicates the degree of unsaturation. The Bromine Number is usually determined by electrochemical titration, where bromine is generated in situ with the redox process of potassium bromide and bromate in an acidic solution to ensure the complete bromination of all olefins .
Every unit of Bromine Number is roughly equivalent to twice of the mass olefin content in a sample . VMGSim calculates this property through an in-house developed correlation based on a fair number of literature data. The following plot shows a direct comparison between experimental and bromine numbers calculated from VMGSim.
The Cetane Number is a relative measure of the time delay between the injection of fuel into the chamber and the start of combustion. Fuels for compression ignition engines must auto-ignite readily. If ignition does not occur promptly when the fuel is injected into the cylinder, premixed fuel and air accumulate such that when ignition occurs, the rate of burning is too fast. The fast burning produces high pressure rates that can result in engine knock that decreases efficiency and can damage the engine. Without adequate fuel ignition quality (a high enough Cetane Number), the engine will start with difficulty and run poorly . Thus, the ability to rate the ignition quality of compression ignition fuels is important to diesel fuel formulation.
The procedure to determine the Cetane Number is similar to the octane rating method for gasoline using two reference hydrocarbon fuels: 1-hexadecene (Cetane Number 100) and α-methylnaphthalene (Cetane Number 0).
Since determination of Cetane Number is difficult and costly, this property can be estimated using a correlation from the ASTM D-976 method . This estimation method is a way for directly estimating the Cetane Number of distillate fuels from API gravity and mid-boiling point . The calculated value, as computed from the formula, is called Cetane Index. VMGSim has implemented and validated this correlation with a fair number of experimental data; the validation showed that the average absolute error of this correlation is around 0.8 %.
The calculated Cetane Index formula is particularly applicable to straight run fuels, catalytically cracked stocks, and blends of the two. The expected deviation of the calculated Cetane Index is ±2 Cetane Numbers for 75 % of the distillate fuels evaluated by the ASTM method .
When a petroleum fraction is vaporized in the absence of air at atmospheric pressure, the non-volatile compounds have a carbonaceous residue known as Carbon Residue . Therefore, heavier fractions with more aromatic contents have higher Carbon Residues while volatile and light fractions such as naphthas and gasolines have no Carbon Residue. Higher Carbon Residue values indicate low-quality fuel and less hydrogen content.
There are two older different test methods to measure Carbon Residues: Ramsbottom (ASTM D 524) and the Conradson (ASTM D189). There is a more recent test method (ASTM D 4530) that requires smaller sample amounts and is often referred as microcarbon residue (MCR) and as a result it is less precise . In most cases Carbon Residues are reported in wt% by the Conradson method, which is designated by CCR.
In the case of the estimation of Carbon Residues, it has been observed that this property can be correlated to a number of other properties like carbon to hydrogen ratio, sulfur content, nitrogen content, asphaltene content or viscosity. However, the most precise relation is between Carbon Residue and hydrogen content . The hydrogen content is expressed in terms of hydrogen to carbon atomic ratio, Altgelt et al.  proposed a useful correlation to estimate carbon residue based on the hydrogen to carbon ratio, this formulation is the one used by VMGSim. The correlation has been improved inside VMGSim in order to better match values for lighter feeds with high hydrogen content.
The Aniline Point of a petroleum fraction is defined as the minimum temperature at which equal volumes of aniline and the oil are completely miscible. Aniline Point indicates the degree of aromaticity of the fraction, the higher the Aniline Point the lower aromatic content.
The estimation method that VMGSim uses to calculate the Aniline Point is based on Linden’s method . This relation is a mathematical representation of an earlier graphical method and uses a blending index that is calculated from a relation developed by Chevron Research .
Smoke Point is a characteristic of aviation turbine fuels and kerosenes and indicates the tendency of a fuel to burn with a smoky flame. High amounts of aromatics in a fuel causes a smoky characteristic for the flame and energy loss due to thermal radiation . The Smoke Point is a maximum flame height at which a fuel can be burned in a standard wick-fed lamp without smoking. It is expressed in length units (millimeters) and a high Smoke Point indicates a fuel with low smoke-producing tendency.
The Smoke Point can be estimated from the Aniline Point and specific gravity. Jenkins and Walsh  proposed an estimation method to calculate the Smoke Point based on the ASTM D 1322 method, this is the method used by VMGSim. This method is expected to perform better than other methods based on the Paraffin, Naphthene and Aromatic content, because the Smoke Point is very much related to the aromatic content of the fuel which is expressed in terms of Aniline Point in the Jenkins and Walsh method. In addition, the specific gravity, which is an indication of molecular type, is also used in the equation.
Octane Numbers are important characteristics of spark engine fuels such as gasoline and jet fuel or fractions that are used to produce these fuels (i.e., naphthas) and they represent antiknock characteristics of a fuel . Isooctane (2,2,4-trimethylpentane) has octane number of 100 and n-heptane has octane number of 0 on both scales of the Octane Numbers. There are two types of Octane Numbers: research octane number (RON) measured under city conditions and motor octane number (MON) measured under road conditions.
VMGSim has developed in-house methods for the calculation of Octane Numbers. The following figures show VMGSim’s calculated values compared to experimental data for some fuel mixtures.
The details of the Octane Numbers calculation for pure components and mixtures have been discussed in a previously published VMG Newsletter article .
The Methane Number is an important parameter to optimize natural gas fueled internal combustion engines. It is analog to the Octane Numbers which are used for gasoline fueled engines. Methane Number is a measure of the gaseous fuels propensity to cause knock in an engine. The Methane Number scale was defined by setting the knock rating of a high knock resistance fuel (Methane) to 100 and a fuel with low knock resistance (hydrogen) to 0.
The Methane Number estimation method used in VMGSim is based on the work of Kubesh et al. , in this method the Methane Number is a function of the molar hydrogen to carbon ratio of the fuel, VMGSim has tuned this correlation to better match better the experimental data presented by Kubesh et al. .
VMGSim’s modified estimation method was validated with the experimental data provided by Kubesh et al.  and Malenshek and Olsen ; the average absolute error between VMGSim calculations and experimental data was about 4%. A graphical comparison between model and experimental results is presented in the figure below.
Stoichiometric Air to Fuel Ratio
The Stoichiometric Air to Fuel ratio is an important parameter to optimize natural gas fuels internal combustion engines for power emissions. It is defined as the ratio of the air to the fuel (by mass) such that there is just enough oxygen to burn all the fuel . Adjusting an engine to the proper air / fuel ratio based on the fuel composition is the most important control to avoid knock and maintain emissions.
The method that VMGSim uses to calculate the Stoichiometric Air to Fuel ratio is based on a balance of the individual gas components that participate in a combustion reaction.
CO2, SO2 and NO2 Combustion Equivalents
VMGSim can perform the calculation of CO2, SO2 and NO2 combustion equivalents which are defined as the produced amount of CO2, SO2 or NO2, if all the carbon, sulfur or nitrogen content in a fuel is completely burned. The values are reported as kg of CO2/SO2/NO2 generated per kg of fuel.
The calculation of these properties is based on the mass balance of the combustion reaction of the fuel.
CO2, SO2 and NO2 Combustion Emission Factors
The CO2, SO2 and NO2 combustion emission factors are defined as the released amount of CO2, SO2 or NO2 when a fuel is completely burned (combustion equivalent) per energy unit from the combustion of the mixture. It is calculated by dividing the combustion equivalent by the Gross Heating Value (in mass basis) of the mixture.
Herbert Loria, Ph.D, P.Eng., VMG Calgary
Please contact your local VMG office for more information.
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