Fluid cracking operation control

For industrial fluid cracking units most of the modeling work in the literature is based upon a highly empirical approach that helps in building units and in operating them, but does not elucidate the main features and characteristics of the units in order to help improve the design and control of such units, or to optimize their output. 

Instrumentation and control. Operation of a fluid cracking unit is simplified by the use of automatic controls. As a further aid, graphic panel-boards are sometimes employed which utilize small indicating instruments located at the appropriate positions in a simplified flow diagram of the process (68,309). Audible and visual alarms, as well as automatic controls for emergency shutdown of the unit, are often provided (202). 

Control of catalyst particle losses from both the cracker and regenerator of fluid catalytic cracking units is achieved by two cyclones operating in series right inside each unit. This is usually followed by an electrostatic precipitator for fine particle control, working on the exhaust side of the catalyst regenerator [62]. The metal content of spent catalysts may be recovered for reuse [63]. 

The unit operations are as applicable to many physical processes as to chemical ones. For example, the process used to manufacture common salt consists of the following sequence of the unit operations transportation of solids and liquids, transfer of heat, evaporation, crystallization, drying, and screening. No chemical reaction appears in these steps. On the other hand, the cracking of petroleum, with or without the aid of a catalyst, is a typical chemical reaction conducted on an enormous scale. Here the unit operations— transportation of fluids and solids, distillation, and various mechanical separations—are vital, and the cracking reaction could not be utilized without them. The chemical steps themselves are conducted by controlling the flow of material and energy to and from the reaction zone. 

In this section a Fluid Catalytic Cracking (FCC) process case study is examined. The aim is to compare the alternative methodologies for regulatory control structure selection presented in sections 2 and 3. The FCC process is particularly suited for this purpose. The process d3mamics described by a low order but highly non-linear set of DAEs. The actual operation of the process is dominated by economics and a small number of disturbances that affect significantly its economics has been identified. Furthermore, the most appropriate control structure for this process is a matter of some controversy, with the conventional structure being criticized in a number of recent publications. 

The analyses that have been used by the commercial nuclear power industry, subject to NRC approval on a case by case basis, make use of the LBB concept to justify exclusion of the dynamic effects of postulated pipe rupture. This concept is based on the ability to detect a fluid system leak- and perform an orderly and controlled plant shutdown before any potential exists for catastrophic pipe failures. Thus, the object of applying the LBB concept is to establish that a postulated crack remains stable under normal operating plus faulted loads or that significant margin exists against unstable crack growth if the postulated crack is predicted to grow with the applied loads. 

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