Page 73 - Fluid Catalytic Cracking Handbook

FCC Feed Characterization 51

Viscosity

Viscosity indicates the chemical composition of an oil sample. As
the viscosity of a sample increases, paraffins increase, hydrogen
content increases, and the aromatic fraction decreases.
Viscosity is normally measured at two different temperatures: typically
tOO°F (38°C) and 210°F (99°C). For many FCC feeds, the sample is
too thick to flow at 100°F and the sample is heated to about 130°F,
The viscosity data at two temperatures are plotted on a viscosity-
temperature chart (see Appendix 1), which shows viscosity over a
wide temperature range [4]. Viscosity is not a linear function of tem-
perature and the scales on these charts are adjusted to make the
relationship linear.
Viscosity is a measurement of resistance to flow. Although the unit
of absolute viscosity is poise, its measurement is difficult. Instead,
kinematic (flowing) viscosity is determined by measuring the time for
a given flow through a capillary tube of specific diameter and length.
The unit of kinematic viscosity is the stoke. However, in general
practice, centistoke is used. Poise is related to stoke by the equation:

„ . . Centipoise
Centistokes =
Density

Saybolt viscosity (ASTM D-88) is the most popular method of
measuring the viscosity of oils such as FCC feed. This method covers
two procedures:

* Saybolt Universal Viscometer (SUV) for light oils
• Saybolt Furol Viscometer (SFV) for heavy oils

Both procedures measure the time for a fixed volume of the sample
to flow though a calibrated tube at a controlled temperature.
The difference between the two instruments is the inside diameter
(ID) of the outlet flow tube. The SUV uses a 0.176 centimeter ID and
the SFV uses a 0.315 centimeter ID. The SFV is used for samples that
have a flowing time greater than 600 seconds. For most conventional
gas oils, the flowing time is short enough that the Universal Visco-
meter is frequently used. The tube dimensions in the two procedures