A Fluidized Catalyst Cracking Unit (FCCU) operating at a near stack opacity limitation potentially can be the primary refinery production limitation. Refiners can overcome opacity, which are flue gas particulate emissions, limitations in several ways.

1. Identifying variables that affect opacity measurements
2. Optimizing operating conditions
3. Modifying catalyst physical properties
4. Considering abatement hardware options

Familiarity with the procedure to measure stack opacity allows refiners to identify variables, which affect opacity. This knowledge coupled with operating and catalyst data, can help trouble shoot an opacity problem. As a last resort several hardware options exist for controlling or eliminating stack opacity losses.

In the US flue gas particulate emission are limited to 1 lb of solids to 1000 lbs of coke burned, or about 6.5 X 10-6 lbs / scf of flue gas. In Europe, this limit ranges from 100 to 500 mg / ccm. Typically, opacity is measured as a relative index based on a base number that varies among the states. Usually the maximum opacity limit is 20, with measurements of 10 to 12 being desirable.