-Gasoline Reid Vapor Pressure (RVP) prediction from Distillation data

-Gasoline  Reid Vapor Pressure (RVP) prediction from Distillation data

Predicted RVP values from Initial Boiling Point (IBP) is useful as an easy way for using the same for volatility calculations like emission product loss, product handling at different temperature etc.

Gasoline vapor pressure is due to light hydrocarbons or more volatile part of Gasoline. As per definition RVP is the vapor pressure expressed at 37.8 deg C ( 100 deg F) in specified equipment conditions. Incidentally, this is the temperature around which ASTM D 86 Initial Boiling Point (IBP) is observed for Gasoline samples.

Luckily for us, the smooth performance of gasoline fuel in internal combustion engine and environment restriction of volatile emissions desires typical lighter hydrocarbon components in gasoline.  These aspects are controlled by Gasoline specification by having minimum and maximum Vapor pressure,  minimum lighter hydrocarbon controlled by front end distillation parameters (min 10% evaporation at 70 deg C), achieving min Octane rating number with type of hydrocarbons having higher Octane number (isopentane) in the absence of Tetra Ethyl Lead (TEL) and alcohol blending contributing to Vapor pressure. 

All the above mentioned factors have contributed today so that most of  gasolines will have Initial boiling temperature  (IBP) in a range of  30 to 44 deg C, with exception of D & E class Gasoline having higher vapor pressure for winter months for colder geographical areas ( Vapor pressure max 13.5 and 15 psa for D & E as per ASTM D 4814 specification).  These higher Vapor gasoline have IBP on lower side about 26 to 30 deg C.

Thus Gasoline IBP range  ( about 26 to 44 ) can be used for predicting RVP with a high degree of accuracy. The following prediction equation is only for Motor Gasoline without ethanol. It is observed that the equation holds for small ( 3-4 %V) amount of ethnaol, MTBE, as well. Higher percentage of oxidants, equation give result on lower side.

Predicted RVP (psi) from ASTM D86 Auto IBP (deg C)

RVP = 23.68 – 0.446*IBP  .................................................(Eq-1)


Predicted RVP (psi) from ASTM D86 Manual IBP (deg C)

RVP = 25.24 – 0.464*IBP  .................................................(Eq-2)

Comparison has been done with  the D5191 RVP values of ASTM PTP round robin programs robust mean  and predicted as per above equations.  In ASTM PTP Gasoline programs about 200 global laboratories participate in proficiency testing.

Accuracy of above prediction will depend off course on accurate determination of IBP. (Reproducibility of D86  IBP is about 6 deg C and for D5191 RVP is about 0.5 psi.)

Following table gives comparison between Predicted RVP as per Eq-1,
and ASTM D86 Auto Initial Boiling Point (IBP). 

ASTM PTP	Auto 86	RVP
Round Robin	IBP	D5191	Calc, Eq-1	difference
( 2010-2016)	Deg C	psi	psi	psi
MG1012	34.6	8.8	8.2	0.6
MG1212	31.5	9.9	9.6	0.3
MG1304	40.1	5.8	5.8	0.0
MG1308	37.8	6.4	6.8	-0.4
MG1404	28.5	11.6	11.0	0.6
MG1408	30.8	9.2	9.9	-0.7
MG1504	32.2	8.7	9.3	-0.7
MG1508	36.5	7.0	7.4	-0.4
MG1512	35.6	7.3	7.8	-0.5
MG1604	28.3	11.4	11.0	0.3
MG1608	40.8	6.2	5.5	0.7

Following table gives comparison between Predicted RVP as per Eq-2,
and ASTM D86 Manual Initial Boiling Point (IBP).

ASTM PTP	Manual D86	RVP
Round Robin	IBP	D5191	Calc, Eq-2	Difference
( 2010-2016)	Deg C	psi	psi	psi
MG1012	36.0	8.8	8.5	0.3
MG1212	32.9	9.9	10.0	-0.1
MG1304	41.8	5.8	5.9	0.0
MG1308	40.0	6.4	6.7	-0.3
MG1404	31.1	11.6	10.8	0.7
MG1408	33.4	9.2	9.7	-0.5
MG1504	35.2	8.7	8.9	-0.3
MG1508	38.8	7.0	7.2	-0.2
MG1512	37.6	7.3	7.8	-0.5
MG1604	30.0	11.4	11.3	0.1
MG1608	42.6	6.2	5.5	0.7

References : ASTM PTP
ASTM D 5191

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