Operational considerations catalytic cracking

Since the first fluid-bed catalytic cracking unit was commissioned in 1942, more than 300 additional units have been built. During this time, the process has evolved and has seen considerable improvement in mechanical constmction, reflabiUty, and process flow. A modern FCCU typically operates continuously for three to four years between turnarounds, during which time 10 kg of feedstock are processed and 7 x 10 ° kg of catalyst circulated. Early FCCU designs, (53) were complex compared with the compact configuration of more recent design (Fig. 1). 

While this unit is considerably cheaper, it also has certain disadvantages. For example, changes or upsets in any one unit may be felt throughout the refinery because of the changes in fractionator operation. However, the considerable cost saving possible with the combination type unit has permitted many small refineries to finance a catalytic cracking unit when they could not afford a conventional model. 

Anyone who is seriously involved in catalytic cracking, whether as an operator, a catalyst manufacturer, or a researcher, soon learns how severely sodium, vanadium, nickel, iron, and copper act as poisons. In the past, FCC feedstock preparation via vacuum distillation was to a considerable extent, determined by metal carryover. Generally, metal carryover to the fluid unit was limited to 0.1 ppm or less of each of these metals. 

A model for the riser reactor of commercial fluid catalytic cracking units (FCCU) and pilot plants is developed This model is for real reactors and feedstocks and for commercial FCC catalysts. It is based on hydrodynamic considerations and on the kinetics of cracking and deactivation. The microkinetic model used has five lumps with eight kinetic constants for cracking and two for the catalyst deactivation. These 10 kinetic constants have to be previously determined in laboratory tests for the feedstock-catalyst considered. The model predicts quite well the product distribution at the riser exit. It allows the study of the effect of several operational parameters and of riser revampings. 

The distillation of tarry. dues in a vacuum for the production of either catalytic cracking plant feedstock or asphalt differs little in principle from the operations already described except that the vacuum tower can be much simplified (Fig. 7-27) because of the following considerations. 

ABSTRACT As a step in the application of the cracking of tar in fuel gas amelioration the characteristics of the endothermic reaction potential of tar was studied experimentally and theoretically. In this context, however, due to the structural complexity of tar and/or tany constituents in fuel gas well defined hydrocarbons as tar model compounds were applied with inexpensive and readily available materials (dolomites, dolomitic magnesium oxide [MgO], quicklime [CaO]). The effects of operation condition on extent of hydrocarbon conversion, gas product composition, and corresponding endotherai of the reaction potential have been explored. The results obtained in this work provide a basis for ture considerations of catalytic tar cracking. 

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