Page 72 - Fluid Catalytic Cracking Handbook

50 Fluid Catalytic Cracking Handbook

In the laboratory, RI is measured using a refractometer. The instru-
ment has two prisms and a light source. The technician spreads a small
amount of sample on the faces of both prisms in the refractometer.
The light is then directed at the sample and the scale is read. The
observed scale is then converted to a refractive index with tables
supplied with the instrument and corrected for the sample temperature.
Both refractive index and aniline point tests qualitatively measure
the aromaticity of a stock. With dark and viscous samples, both
methods have their limitations. For darker samples, the aniline point
test is slightly more accurate because of its larger scale over the
same range of aromatics. The industry does not agree as to which
method is more accurate. The three published correlations that will
be discussed later, use the refractive index at 68°F (20°C) for
calculating feed composition. However, at 68°F most FCC feeds are
solid and their refractive indexes cannot be determined accurately.
Both the TOTAL and API [31 correlations predict Rl values using
feed properties such as specific gravity, molecular weight, and
average boiling point.

Bromine Number and Bromine Index

Bromine number (ASTM D-1159) and bromine index (ASTM D-
2710) are qualitative methods to measure the reactive sites of a sample.
Bromine reacts not only with olefin bonds, but also with basic nitrogen
molecules and with some aromatic sulfur derivatives. Nevertheless,
olefins are the most common reactive sites and the bromine number
is used to indicate olefinicity of the feed.
The bromine number is the number of grams of bromine that will
react with 100 grams of the sample. Typical bromine numbers are:

» Less than 5 for hydrotreated feeds
• 10 for heavy vacuum gas oil
* 50 for coker gas oil

A general rule-of-thumb is that the olefin fraction of the sample is
one-half of its bromine number.
Alternatively, the bromine index is the number of milligrams of
bromine that will react with 100 grams of the sample, and is most
often used by the chemical industry for stocks that have very low
olefin contents.