Fluid cracking erosions

In support of the power recovery expander market for fluid catalytic cracking units in refineries, some turboexpander manufacturers have an ongoing program to improve the solid particle erosion characteristics of the machine. Improved erosion characteristics will result in longer blade life, less downtime, and consequently greater profits for the users. 

The purchaser shall specify any erosive or corrosive agents (including trace quantities) present in the process fluids and in the site environment, including constituents that may cause stress-corrosion cracking or attack elastomers. 

Edges or surface irregularities on particulate catalysts used in fluid- or fixed-bed applications are susceptible to attrition and erosion during reaction. The morphology of a typical fluid-bed cracking catalyst is shown in Fig. 9. The surface of spheres, extrudates, and tablets is relatively free of topological features, and thus physical losses are usually not a serious problem. 

Wear impact plastic deformation makes some constituents more susceptible to corrosion. Cracks brittle constituents, tears apart ductile constituents to form sites for crevice corrosion, hydraulic splitting. Supplies kinetic energy to drive abrasion mechanism. Pressurizes mill water to cause splitting, cavitation, and jet erosion of metal and protective oxidized material. Pressurizes mill water and gases to produce unknown temperatures, phases changes, and decomposition or reaction products from ore and water constituents. Heats ball metal, ore, fluids to increase corrosive effects. 

Chemical fluidized-bed reactors can have diameters of up to 10 m. Their scale-up is associated with major risks (erosion of reactor walls and especially cooling tubes, bubble formation, solids entrainment, formation of flnes), so that today the main application is syntheses with large heats of reaction (e.g., acrylonitrile, maleic anhydride, fluid cat cracking) and fluidized combustion. 

Coupons may be used to determine the average fluid corrosivity by measurement of weight loss (See Figure 8.1). Susceptibility to pitting, bimetallic corrosion, stress corrosion cracking, crevice corrosion, corrosion in weldments or heat affected zones (HAZ), hydrogen embrittlement, scaling, erosion, and cavitation may also be determined. The method facilitates an assessment of the corrosivity of an environment with respect to the specific material of construction of that part of the plant. 

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