By Bruce J. Artuso
In the current climate of significantly reduced refining margins, it is critically important to maintain perspective and emphasis on the areas that still produce a good financial return. Fluid Catalytic Cracking (FCC) has been, and remains, one of the most important and most profitable processes in a refinery operation.
The basic process (illustrated in the attached sketch) is one in which a relatively low value feedstock is converted into a wide range of much more valuable products. These include yields of about 60% of high-octane gasoline product, about 20% of a diesel-boiling-range product, and significant amounts of lighter gases, some of which become feedstocks for other downstream processes, as well as a major contribution to the refinery fuel gas system. In addition, the total volume of products represents an actual overall increase over the volume of feedstock. The process also has great flexibility to handle a wide range of feedstocks, and operating conditions can be manipulated to satisfy differing product requirements. Thus, optimizing FCC operation has a very significant bearing on overall refinery profitability.
Because of its great complexity and flexibility, a Cat Cracking unit has a variety of components which can contribute to a profitable result. Some of these are: Unit Design Features, Catalyst Selection, Operating Conditions, Mechanical Reliability, and Operator Capabilities. These are briefly discussed below.
Some of the most important features of most modern FCC unit designs are:
Modern catalyst technologies can tailor FCC catalysts for most feedstock, operating conditions, and emissions combinations. Selection of the best catalyst for a specific refining and FCC application can be critical to effective and profitable unit operation. The selection process is usually enhanced by utilizing the expertise available in catalyst suppliers, who have access to extensive pilot unit and testing facilities, and a broad background of experience.
It is obvious that the profitability of an FCC unit is very dependent on the conditions under which that unit operates. The choice of optimum conditions must, of course, recognize actual unit constraints. Within those constraints, optimization can be determined externally by studies involving availability of multiple FCC yield structures within a refinery linear program. More commonly, however, that optimization is currently developed on a continuous basis, utilizing unit supervisory control within an overall refinery economic plan. This yields more immediate, continuous, and probably more accurate economic benefits.
The FCC reactor/regenerator section is complex, with large vessels that operate at fairly high temperature under an erosive, severe environment for long process runs before turnaround, unless problems dictate otherwise. Planned turnarounds can be 4-5 years apart; so, ensuring the mechanical reliability of equipment, piping and refractory linings, etc., for the duration of the run is a major consideration.
The unit must process feedstock safely and reliably, and minimizing down-time is paramount to achieving optimum refinery economics. This also includes advance turnaround and maintenance planning.
Even on a unit which has supervisory control, continuous and effective monitoring of the operation by the operator is still extremely important. In addition to routine tasks such as changing unit objectives as required, the operator has a critical role in proper handling of non-standard operations such as upsets and emergency conditions.
As mentioned above, most units have at least several basic emergency systems (ESD) to put the unit initially in a safe condition when an emergency situation occurs. However, it is critically important for the operator to implement the necessary follow-up actions to:
To ensure the necessary degree of knowledge and expertise by the operator, both for monitoring normal unit operations, as well as effective handling of abnormal conditions, the refinery must provide adequate and extensive training programs.
Achievement of these goals necessarily requires a high degree of expertise within a refinery’s engineering staff. Alternatively, and especially for smaller organizations on tight budgets, that expertise can be secured by utilizing “outside” technical specialists in the various areas (e.g., design, computer control, catalyst selection, optimization of operating conditions, mechanical reliability, and effective training of process operating personnel). “Outside” specialists can similarly be effectively utilized in studying and resolving operating problems such as unsteady catalyst circulation and regenerator afterburning, or even perform broader data analysis and process troubleshooting in areas that may impact the FCC feed quality and ultimately unit economy.
In many cases, combining that outside expertise with the knowledge and unit familiarity that is available within a refinery will yield very beneficial and economically attractive results.