Page 228 - Fluid Catalytic Cracking Handbook

196 Fluid Catalytic Cracking Handbook

by lowering the top temperature on the main column by increasing
the top pumparound or the top reflux rate.
The LCD distillation range is typically 430°F to 67()°F (221°C to
354°C) ASTM. Undercutting the gasoline end point drops the heavy end
of the gasoline fraction to be withdrawn with LCO. This affects only the
apparent conversion and does not cause changes in the flow rate of other
products. Reducing the gasoline end point usually increases the octane
because of the lower-octane components in the heavy end of gasoline.
A better method of increasing LCO yield is through better frac-
tionation upstream. The removal of the fraction under 650°F from the
feed requires better stripping. The total refinery yield of diesel will
increase when the light ends are fractionated from the feed (Table 6-1).
Some of the catalytic routes to maximize LCO yield are:
» Decrease in the reactor temperature
* Decrease in the catalyst-to-oil ratio
« Decrease in catalyst activity
• Increase in HCO recycle

LCO Quality

The CAAA of 1990 set fuel standards for the new "over-the-road
diesel fuel." It requires a maximum sulfur of 0.05 wt% (500 ppm) and

Table 6-1
Effects of Feed Fractionation on Total Distillate Yield

Feedstock

"Raw" "Fractionated"
Gas Oil Gas Oil
Initial Boiling Point, °F/°C 435/224 660/349
Final Boiling Point, °F/°C 1,080/582 1,080/582
435°F/224°C to 660°F/349°C 8 0
Content, wt%
Conversion, wt% 75.9 75.9
LCO, wt% 15.4 14.0
Potential FCC LCO, wt% 15.4 (0.92 x 14.0) = 12.9
Total Potential Refinery Distillate 15.4 (12.9 + 8.0) = 20.,9
Source: Engelhard [6]