The best process would be to avoid the butane adjustment by blending the fuels in such a way that butane loss is avoided. Practically, this means having all fuel samples at or below 32 degrees Fahrenheit before their containers are opened for blending. In practice, however, some loss may occur. Three assumptions are made in the adjustment procedure.

The first of these is that RVP blends linearly with volume. This is not true if ethanol fuels are included, so ethanol blended fuels must be treated separately. If ethanol blended fuels are to be composited, a separate composite must be maintained for them. If different ethanol blended fuels are to be produced, such as 2.0% oxygen and 3.7% oxygen fuels, these must be composited separately. In operation, the maintenance of several composites may be necessary, one containing all hydrocarbon fuels, one with 2.0% oxygen from ethanol, and one with 3.7% oxygen from ethanol. In general, any single ethanol fuel composite may span a range of up to 0.5% oxygen. In other words, samples containing from 3.3% to 3.8% oxygen may be composited. If other samples are produced, they will require a separate composite. Since fuels containing MTBE and similar ethers do not affect RVP as dramatically, they may be combined with the hydrocarbon fuels.

The second assumption is that all the loss in RVP observed is due to evaporation of n-butane. In reality, this is not true, as virtually all of the isobutane and some of the pentanes will be lost. However, considering the difficulty of ascertaining the exact species lost, this is a reasonable approximation.

The final assumption is that the RVP of n-butane is 51.6 psi. This value was taken from a Phillips Petroleum Reference Data circular (bull.no.521).

The technique for producing a composite sample would require that additions to any composite be of consistent volume, 100 ml for example. Using this method, the final expected RVP of the composite would be the simple arithmetic average of the included samples. If the measured RVP of the composite is different than this calculated value, any measured property should be adjusted for the volume loss due to butane. This is done by calculating the quantity of n-butane required to bring the composite to its original RVP. As an example, the following calculation would result from a composite of 80 samples at 100 ml each. The calculated average RVP should be 7.20 psi, and the measured RVP of the composite is 6.60 psi.

6.6(8000 - z) + 51.6(z) = 7.2(8000) (I)

52,800 - 6.6(z) + 51.6(z) = 57,600 (II)

45.0(z) = 4,800 (III)

z = 106.7 (IV)

106.7/(8000) - 106.7) = 1.35% (V)

This means that 1.35 volume percent butane must be added to bring the composite sample to its original RVP. This is the volume correction that must be applied to all other measured properties. For intrinsic properties, such as the concentration of benzene, this correction is applied as a ratio, so that a measured concentration of benzene in the composite would be reduced by 1.35%. As an example of this, where the measured benzene concentration is 0.925%

0.925(8000 - 106.7) = (b)8000 (VI)

b = 0.913 (VII)

The corrected benzene concentration 0.913% by volume. This type of correction would also be applied to oxygenate, sulfur, aromatic, and olefin measurements.

This is a simplified version of the correction calculation, and assumes a consistent product batch size. In cases where the batch size varies, the sample removed for composite must be proportional to the size of the batch. For example, if batches totaling 50,000, 30,000, and 80,000 bbls. are produced, one might remove 100, 60, and 160 ml from the respective batches. This amounts to volume weighting the composite for batch sizes. The calculated average RVP value is a weighted average of the original values:

RVP1(Vol1) + RVP2(Vol2) + RVP3(Vol3)....= RVP(avg)(Vol(total))

The volume compensation is calculated exactly as in the simplified case.

Distillation measurements require a different type of correction, since any evaporative loss due to butane would affect the initial portion of the distillation curve. In fact, during the test, there is loss, and this loss is assumed to be due to the inability of the still to recover butane. The most appropriate way to apply the correction here would be to begin the distillation with only 98.6 ml of fuel. The result will be a larger loss. This measured loss will be the correction for butane loss, and will yield corrected values for the distillation. If E200 and E300 values are needed, they are taken from the corrected curve. (1/9/95)

This question and answer was posted at Consolidated List of Reformulated Gasoline and Anti-Dumping Questions and Answers: July 1, 1994 through November 10, 1997 (PDF)(333 pp, 18.17 MB, EPA420-R-03-009, July 2003, About PDF)