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Vapor Pressure of Petroleum Products

Introduction

Many problems involving hydrocarbon systems include a specification of some sort of vapor pressure measurement. The vapor pressure of a mixture is commonly defined as the pressure where the first bubble of vapor is formed at a given temperature but there are other definitions based on standardized experimental measurements or correlations. Examples of these definitions are: Reid Vapor Pressure, Vapor Pressure of Crude Oils and Vapor Pressure of Liquefied Petroleum Gases, all of them defined by ASTM (American Society for Testing Materials) standard methods [1, 3, 4].

The objective of this article is to present and discuss the different types of vapor pressure calculations available in VMGSim along with describing their calculation procedures, ranges of validity and units. Validation examples are also included.

Vapor Pressure Definitions

The following vapor pressure definitions can be accessed in VMGSim from the Special Properties unit operation, as well as the Refinery and Natural Gas tabs from the Material Stream. These properties are obtained from flash-based calculations that depend on the selected property package.

Vapor Pressure

Vapor Pressure, also known as bubble point pressure and true vapor pressure, is the pressure where the first bubble of vapor is formed at a given temperature. The composition (in the case of a mixture) also influences this equilibrium pressure.

VMGSim calculates the vapor pressure by flashing the mixture of interest using a vapor fraction equal to zero at the temperature specified in the stream that contains the mixture.

Units

The Vapor Pressure is reported in absolute pressure units.

True Vapor Pressure @ 100 F

This property is similar to the Vapor Pressure calculation except that the flash calculation is performed at a specific temperature of 100 F.

Units

The Vapor Pressure @ 100 F is reported in absolute pressure units.

True Vapor Pressure @ Storage Temperature

It is defined as the vapor pressure of a liquid hydrocarbon calculated based on the ASTM D6377-10 [1] standard method and the Environmental Protection Agency (EPA) publication AP-42 [2].

This property is calculated based on an equation presented in the EPA publication, derived from a nomogram first published by the American Petroleum Institute (API), which is based on the Reid Vapor Pressure Equivalent of non – pressurized vessels (see definition below) and the storage temperature. The storage temperature is defined by the specified temperature in the stream that contains the mixture of interest.

Units

The Vapor Pressure @ Storage Temperature is reported in absolute pressure units.

Reid Vapor Pressure (RVP) (D323)

The Reid Vapor Pressure is an important property of gasolines and jet fuels and it is used as a criterion for blending of products. It is also a useful parameter for the estimation of losses from storage tanks during filling or draining.

The Reid Vapor Pressure is defined as the vapor pressure of a mixture as it was measured according to the experimental method depicted in the ASTM D323-99a standard method [3]. This test method covers the Reid Vapor Pressure determination of gasoline, volatile crude oils, and other volatile petroleum products.

The standard method establishes that the analyzed mixture must be placed in a sample container kept in a cooling bath at 32 F. Air saturation of the sample is performed by vigorously shaking the container and returning it to the cooling bath. The sample is transferred to a test apparatus consisting of two interconnected liquid and vapor chambers and equipped with a suitable pressure-measuring device. The ratio of the volume of the vapor chamber to the volume of the liquid chamber is 4. The entire system is immersed in a water bath at 100 F and once equilibrium is attained the observed pressure from the measuring device is reported as the Reid Vapor Pressure.

VMGSim calculates the Reid Vapor Pressure by simulating the ASTM D323-99a experimental procedure. The mixture from the stream of interest is saturated with air (N2 79%, O2 21% mol) at 32 F and 1 atm. Both N2 and O2 must be present in the component list from the property package for the proper use of this method. The resulting saturated mixture is then flashed at 100 F and a constant vapor/liquid volumetric ratio of 4 to 1 (vapor volume fraction = 0.8), and the calculated pressure is reported as the Reid Vapor Pressure (D323).

Previous calculations in VMGSim did not take into account the air saturation, which caused the results to be lower. This is no longer the case since VMGSim 9.5 includes an option to enable the air saturation of the mixture. This option is activated through a check box that can be found in the Special Properties Unit Operation or the Refinery and Natural Gas tabs from the Material Stream as seen in the following figures. If N2 and O2 are not present in the component list the calculation is performed without air saturation.

VP1.png

VP2.png  

Units

As established in ASTM D323-99a, the Reid Vapor Pressure is the absolute pressure that is necessary to exert on a petroleum product in order to obtain a Vapor/Liquid ratio of 4 at 37.8 C (100 F). There has been widespread confusion related to the pressure units of this property. Since most vapor pressures are reported in absolute pressures, most observers expect the Reid Vapor Pressure to follow this norm.

The ASTM D323-99a method establishes that the Reid Vapor Pressure is read from a pressure-measuring device that is attached to an ambient air filled chamber. The ASTM D323-99a method reports that the Reid Vapor Pressure must be reported in units such as "psi" or "kPa”; and, no place in the ASTM D323-99a standard states that units such as "psia" or “kPa absolute” should be used.

Previous versions of VMGSim reported the Reid Vapor Pressure with absolute pressure units. This is no longer the case since it has been decided to report the Reid Vapor Pressure using “psi” and “kPa” units in order to honor the ASTM D323-99a method. Although the units have changed, the calculated values are the same as in previous versions since the internal flash calculations are still the same and based on absolute pressures.

Vapor Pressure of Liquefied Petroleum Gas (LPG) (D1267)

The Vapor Pressure of Liquefied Petroleum Gases (LPG) is measured according to the ASTM D1267-12 standard method [4], which differs from D323-99a in that the latter is used to measure vapor pressure of gasoline and volatile petroleum products.

The ASTM D1267-12 standard method follows a similar procedure than the D323-99a method, except that the ratio of the volume of the vapor chamber to the volume of the liquid chamber is 2. The sample is similarly saturated with air at 32 F and the vapor pressure form the system is also read from a pressure-measuring device attached to the apparatus.

VMGSim calculates this property by simulating the ASTM D1267-12 experimental procedure. The mixture from the stream of interest is saturated with air (N2 79%, O2 21% mol) at 32 F and 1 atm. The resulting saturated mixture is then flashed at 100 F and a constant vapor/liquid volumetric ratio of 2 to 1 (vapor volume fraction = 0.666), the calculated pressure is reported as the Vapor Pressure of LPG (D1267).

The air saturation can be activated through the same check box as in the Reid Vapor Pressure calculation. If N2 and O2 are not present in the component list the calculation is done without the saturation part.

Previously, VMGSim labeled this property as Reid Vapor Pressure D1267 but this is no longer the case since it has been changed to Vapor Pressure of LPG (D1267).

VP3.png 

VP4.png

Units

Like in the D323-99a method, there is confusion about the units used for this property. The ASTM D1267-12 method states that the Vapor Pressure of the LPG must be reported in “kPa” or “psi”. Similar to the Reid Vapor Pressure, it has been decided to report the Vapor Pressure of the LPG (D1267) with “psi” and “kPa” units in order to honor the ASTM D1267-12 method.

Vapor Pressure of Crude Oil (VPCRX) (D6377)

The ASTM D6377-10 method [1] covers the experimental procedure to obtain the vapor pressure of crude oils. The method is very similar to D323-99a but in this case the air saturation is not required. The vapor pressure is determined by this method at a vapor/liquid ratio of X to 1 (where X varies from 0.02 to 4) at 100 F.

VMGSim calculates the Vapor Pressure of the Crude Oils (D6377-10) by simulating the experiment described in the ASTM D6377-10 method. The mixture of interest is flashed at 100 F and a constant vapor/liquid volumetric ratio of X to 1 (X can be specified by the user and its default value is 4 as seen in the following figures). The resulting calculated pressure is reported as the Vapor Pressure of Crude Oils (D6377-10).

VP5.png

VP6.png

In previous versions of VMGSim this property was labeled as the Reid Vapor Pressure Equivalent (RVPE), which is obtained by a different procedure. Now, the RVPE is obtained through a new property as shown below.

Units

This ASTM D6377-10 method also reports that measurements must be reported in “psi” or “kPa”. Therefore, in order to comply with the method, it was decided to report the Vapor Pressure of Crude Oils (D6377-10) with “psi” and “kPa” units.

Reid Vapor Pressure Equivalent (RVPE)

The Reid Vapor Pressure Equivalent (RVPE) for pressurized and non-pressurized vessels are values calculated by the defined correlations from the ASTM D6377-10 standard method (Appendix XI) [1] using a vapor/liquid ratio of 4 and a temperature of 100 F.

These properties are new to VMGSim and are labeled as:

  • RVPE (Pressurized Vessels)
  • RVPE (Non-Pressurized Vessels)

The non-pressurized value is used in a correlation from the EPA Publication AP-42 [2] to obtain the True Vapor Pressure at Storage Temperature (see description above).

VP7.png

VP8.png

Units

In order to be consistent with the previous vapor pressure calculations from ASTM methods the Reid Vapor Pressure Equivalent has units of “psi” and “kPa”.

Summary of Vapor Pressure Definitions

The following tables show the principal characteristics of the previously described vapor pressure definitions.

Table_1.png

Comments on Vapor Pressure

If True or Reid Vapor Pressure values are calculated for oils or its fractions, the oil pseudo-components should be modeled as accurately as possible with respect to the oil characterization data. The calculation of the oil pseudo-components physical properties is very important since they define the computation of fugacities and equilibrium ratios that are part of the flash calculations used in vapor pressure determination.

The chemical nature of the oil pseudo-components also affects these physical properties and one way to take this into account is by using VMGSim’s PIONA Characterization (see Example 3 below).

Examples

The following examples, taken from literature, are used to demonstrate VMGSim’s capability on calculating vapor pressures. All examples were calculated using VMGSim 9.5 (build 68).

Example 1 – Air Saturated Pure Component

The ASTM 6377-10 standard method [1] publishes acceptable reference Reid Vapor Pressure (RVP) values for air saturated reference fluids. The following table shows the comparison of those values with those calculated from VMGSim:

Table_2-0001.png

VMGSim’s values were calculated using the Advanced Peng-Robinson property package. VMGSim reports accurate results that are within, or close to, the reported uncertainty.

VP9.png

Example 2 – Pure Hydrocarbons Mixture

Campbell [5] and McKetta [6] give the RVP and TVP @ 100 F values for the hydrocarbon mixture shown in the next table. Campbell [5] reports that the air was removed from the cell where the RVP was measured. Because of this the D6377 method will be used to calculate this value.

The reported RVP is 18 psi and the TVP is 19.5 psia. Using VMGSim, the values for RVP (actually the Vapor Pressure of Crude Oils D6377) and TVP were found to be 18.45 psi and 19.21 psia, respectively, as it can be seen in the next figure. Calculations were done using the Advanced Peng-Robinson property package.

Table_3.png

VP10.png

Example 3 – Gasoline Assay

Gardiner [7] showed an ASTM D86 distillation curve and an RVP D323 value for a Light Isocrackate (LIC). The following table shows the gasoline’s ASTM D86 distillation data, the LIC has a reported RVP value of 11.5 psi. 

 Table_4.png

To characterize this fluid, VMGSim’s PIONA Characterization was used in order to take advantage of the physical properties calculation based on the chemical nature of the PIONA Slate.

This example has been done using the Advanced Peng-Robinson property package. The following pure components were added to the components list: Nitrogen, Oxygen, Methane, Ethane, Propane, iso-Butane, n-Butane, iso-Pentane and n-Pentane and the PIONA Slate was built with the following characteristics:

VP11.png

Note that Nitrogen and Oxygen were added to the component list in order to be able to perform air saturation in the sample. Inside the flowsheet environment an Oil Source unit operation was added and the D86 distillation curve was entered using the Direct Calculation method (the IBP value from the adjustment factors was set to 0.16 and the EBP value to 0.03). Once this was done, a close match between calculated and experimental values of the distillation curve was obtained.

VP12.png

To inspect the RVP a Material Stream was connected to the Oil Source and a Special Properties unit operation was then attached to the Material Stream. In the Refinery Tab from the Special Properties unit operation, the Reid Vapor Pressure (D323) was selected and the air saturation was included to obtain the RVP value:

VP13.png

VP14.png

The calculated RVP value is 11.87 psi, which is very close to the experimental value (11.5 psi). Note that this value has been obtained with “out of the box” calculations from VMGSim, with no regressions involved other than the oil characterization.

Herbert Loria, Ph.D, P.Eng., VMG Calgary

Please contact your local VMG office for more information.

References

[1] ASTM International. ASTM D6377-10 Standard Test Method for Determination of Vapor Pressure of Curde Oil VPCRX (Expnasion Method) ASTM International, West Conshohocken, PA, 2010

[2] U. S. Environmental Protrection Agency Compilation of Air Pollutant Emissions, Ch. 7 Publication AP-42 5th Ed., 2006

[3] ASTM International. ASTM D323-99a Standard Test Method for Vapor Pressure of Petroleum Products (Reid Method) ASTM International, West Conshohocken, PA, 1999

[4] ASTM International. ASTM D1267-12 Standard Test Method for GageVapor Pressure of Liquefied Petroleum (LP) Gases (LP-Gas Method) ASTM International, West Conshohocken, PA, 2012

[5] Campbell, J. R. Gas Conditioning and Processing 7th Ed., Campbell Petroleum Seires, Norman, OK, 1992

[6] McKetta Jr, J. J. Encyclopedia of Chemical Processing and Design: Vol. 47 Marcel Dekker Inc., New York, 1994

[7] Gardiner,D., Bardon, M. and Pucher, G. An Experimental and Modeling Study of the Flammability of Fuel Tank Headspace vapors from Hight Ethanol Content Fuels Nexum Research Corporation, Mallorytown, Canada, 2008

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