Page 168 - Fluid Catalytic Cracking Handbook

142 Fluid Catalytic Cracking Handbook

cut points isolates the reactor system from the distillation sys-
tem performance.
Conversion is defined as the volume or weight percent of feedstock
converted to gasoline and other lighter products, including coke.
However, conversion is typically calculated by subtracting the volume
percent or weight percent of liquid products heavier than gasoline from
fresh feed, and dividing by the volume or weight of fresh feed. This
is shown as follows:

~ , m Feed - (light cycle oil + heavy cycle oil + decanted oil) . ,,„
Conversion % = ^-^ —- x 100

Depending on seasonal demands, the gasoline end point can range
from 380°F to 450°F (193°C to 232°C). Undercutting of gasoline
increases the LCO product and can appear as low conversion. There-
fore, it is necessary to distinguish between the apparent and true
conversion. The apparent conversion is calculated before the gasoline
end point adjustment is made, and the true conversion is calculated
after the adjustment.

Testing Methods

The material balance around the riser requires the reactor effluent
composition. Two techniques are used to obtain this composition. Both
techniques require that the coke yield be calculated.
The first technique is to draw an envelope with the reactor effluent
as the inlet stream and the product flows as the outlet streams. Streams
from other units must be included. The flow rates and composi-
tions of the entering and leaving streams are then totaled. The net is
the reactor effluent. This is the method practiced by most refiners.
The second technique involves direct sampling of the reactor effluent
(Figure 5-2). In this technique, a sample of reactor effluent is collected
in an aluminized polyester bag for separation and analysis.
There are several advantages and disadvantages to reactor effluent
sampling;

Advantages of Reaction Mix Sampling
• Allows data gathering on different sets of conditions without
waiting for the recovery side to equilibrate.