API PUBL 2509C-1967 (R1987)

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.

Glasstone ^1a states that if a solution of two or more
components exhibits positive deviation from Raoult's law, the
observed vapor pressure and volume would be greater than if the
components had formed an ideal solution. This he attributes to the
mean attractive forces between the molecules in the mixture being
smaller than for the constituents separately.

Conversely, if a solution should exhibit negative deviation from
Raoult's law, usually there is a decrease in vapor pressure and
volume on mixing. This is attributed to the mean attractive forces
between the molecules in the mixture being greater than for the
constituents separately.

Inasmuch as petroleum components contain molecules of various
sizes and weights, solutions of two separate components are seldom
ideal. Consequently, it is to be expected there may be a change in
volume associated with the mixing or blending of petroleum
components of varying gravities and molecular structure.

All available test data covering blends of lighter and heavier
petroleum components (e.g., butane with crude oil) indicate that
the change in volume is negative in direction resulting in a
shrinkage of the total volume.

^a Referes to REFERENCES ON P.13.