Page 229 - Fluid Catalytic Cracking Handbook

Products and Economics 197

a maximum aromatics of 35 vol%. The California Air Resources Board
(CARB) has imposed a tighter restriction on diesel aromatics of 10
vol% for larger refineries (over 50,000 bpd capacity) and 20 vol% for
smaller refineries. A minimum cetane number (CN) is also required.
The aromatics content of a diesel fuel affects its cetane number; the
following section discusses the cetane number and the principal factors
affecting it.

Cetane. Like octane number, cetane number is a numerical indica-
tion of the ignition quality of a fuel. However, the two numbers work
in reverse. A gasoline engine is spark-ignited and an important fuel
quality is to prevent premature ignition during the compression stroke.
A diesel engine is compression-ignited and it must ignite when com-
pressed. Unfortunately, components that increase octane will decrease
cetane. For example, normal paraffinic hydrocarbons have a low octane
number but a very high cetane number. Aromatics have a high octane
number but a very low cetane number. The adjustments in the reactor
yield mentioned above to improve LCO yield and quality will all lower
gasoline yield and quality.
Cetane number is measured in a single-cylinder laboratory engine,
but cetane index (CI) is more commonly used. Cetane index is a
calculated value and correlates adequately with the cetane number.
Most refiners use the ASTM equation (Method D-976-80) to calculate
the cetane index. The equation uses the 50% boiling point and °API
gravity (Example 6-1).
Example 6-1
Cetane Index Equation

2 2
CI = 65.01 (log T) + [0.192(°API) x log T] + 0.16(°API)
2
- 0.0001809 (T) - 420.34
2 2
= 65,01 (log 550) + [0.192(19)(/0g 550) 0.16(19)
2
- 0.0001809 (550) - 420.34
2
= 65.01(2.74) + [0.192(19)(2.74)J + 0.16(361) -
0.0001809(302,500) - 420.34
= 488.2 + 10.0 + 5.8 - 54.7 - 420.34

CI = 28.9