American Petroleum Institute

Volumetric Shrinkage Resulting from Blending Volatile Hydrocarbons with Crude Oils - SECOND EDITION
 N BULL 2509C

Annotation

Introduction and Theory

In recent years, crude oil pipelines have been used for transporting an increasing amount of light products such as butane, natural gasoline, and high-gravity produced distillates. Because of the physical nature of the crude oil system and the widely dispersed receipt points of the light components, such products usually are blended into the crude oil stream. Such blending actually is encouraged to reduce losses by evaporation of the lighter components and to minimize pump suction difficulties.

As a part of the conservation effort of the pipeline industry, the causes of oil losses in transit are being investigated continuously. Such an investigation of the volume loss experienced in systems handling the light components blended into a common crude oil stream indicated that the losses did not result entirely from increased evaporation but were caused partly by a phenomenon associated with the blending of the lighter components and heavier crude oil. That is, when a lighter product such as butane or natural gasoline is mixed with crude oil, the resulting volume is less than the sum of the individual component volumes. This loss or shrinkage is only an "apparent loss" on a volume basis for there is, of course, no loss of weight as a result of the mixing operation.

In the blending of petroleum components having different physical properties, volumetric shrinkage occurs because the components do not form ideal solutions. In an ideal solution, the total solution volume is equal to the sum of the volumes of the components. In order for a solution to approach ideality, the molecules of the materials blended together must be similar in size, shape, and properties. If the nature of the molecules of the components differs appreciably, then deviation from ideal behavior may be expected. This deviation may be either positive or negative; that is, the total volume may increase or decrease when the components are blended.