Fcc surfaces

Figure 4. Possible atomic configurations for monomers, dimers and triangular trimers on (111) FCC surfaces at normal (N) and fault (F) sites. Trimers of type A have their center above an atom in the surface layer, for type B the center is at an adsorption site...

More recently, Reppenhagen and Werther (1997) have conducted a comprehensive study ofthe attrition mechanism in a cyclone. Their experimental set-up is shown in Fig. 16. The 0.09 m ID cyclone is operated in the suction mode. The solids (spent FCC, surface mean diameter 105 microns) are introduced via a vibrating feeder into the cyclone inlet tube which allows an independent variation of cyclone inlet velocity Ue and solids loading ll... 

Figure 5. The effects of V levels on an FCC surface area after heating for 10 h at A) 540 °C in air, B) 730 °C in air and C) 730 °C will 95% steam. Reprinted with permission from Ref. 13. (Copyright 1985 Academic Press)...

Fig.4 Examples for adsorbate lattice structures on (100) and (111) fcc-surfaces. The choices of unit vectors for adlattice and substrate lattice are indicated. Adatoms are shown in grey. Instead of the four-fold and three-fold coordination sites for the adsorbates, the same adlayer periodicities can result for sites with different coordination e.g., on top or bridge sites...

Figure 14. A) Large scale AFM image showing a micropore on the FCC surface. B) Details of a micropore wall. C) Opening or pores formed by missing plates. D) Narrow slits, or cracks, 6.0-9.0nm wide are often observed on the FCC surface. E) Large scale AFM image of steamed GRZ-1 containing 2%V. F) Vanadia filled slits representing pore blockage [47,48].

FIGURE 9.2 Side and top views of a typical starting configuration for an isothermal-isobaric simulation of dodecane confined between FCC surfaces. (From Cummings, P. T., et al. AIChE J. 56 842, 2010. With permission.)... 

Fig. VIII-8. Surface structures (a) (1 x 1) structure on the (100) surface of a FCC crystal (from Ref. 76) (b) C(2 x 1) surface structure on the (100) surface of a FCC ciystal (from Ref. 76). In both cases the unit cell is indicated with heavy lines, and the atoms in the second layer with pluses. In (b) the shaded circles mark shifted atoms. (See also Ref. 77.)...

Tochihara H and Mizuno S 1998 Composite surface structures formed by restructuring-type adsorption of alkali-metals on FCC metals Prog. Surf. Sc/. 58 1... 

Figure Bl.21.1. Atomic hard-ball models of low-Miller-index bulk-temiinated surfaces of simple metals with face-centred close-packed (fee), hexagonal close-packed (licp) and body-centred cubic (bcc) lattices (a) fee (lll)-(l X 1) (b)fcc(lO -(l X l) (c)fcc(110)-(l X 1) (d)hcp(0001)-(l x 1) (e) hcp(l0-10)-(l X 1), usually written as hcp(l010)-(l x 1) (f) bcc(l 10)-(1 x ]) (g) bcc(100)-(l x 1) and (li) bcc(l 11)-(1 x 1). The atomic spheres are drawn with radii that are smaller than touching-sphere radii, in order to give better depth views. The arrows are unit cell vectors. These figures were produced by the software program BALSAC [35]-...

Figure Bl.21.2. Atomic hard-ball models of stepped and kinked high-Miller-index bulk-temiinated surfaces of simple metals with fee lattices, compared with anfcc(l 11) surface fcc(755) is stepped, while fee...

Bed-to-Surface Heat Transfer. Bed-to-surface heat-transfer coefficients in fluidized beds are high. In a fast-fluidized bed combustor containing mostly Group B limestone particles, the dense bed-to-boiling water heat-transfer coefficient is on the order of 250 W/(m -K). For an FCC catalyst cooler (Group A particles), this heat-transfer coefficient is around 600 W/(600 -K). 

A parameterization of many different surface potentials, ranging from (100) surfaces of FCC crystals to graphite surfaces, has been given by Steele [146-148]. Since most of the systems discussed below are adsorbed layers on graphite surfaces, we consider the graphite substrate in detail. The interaction potential between an adsorbate particle at the position r = (x,y, z) and all other substrate particles consists of two contributions,... 

The single crystal of a polymer is a lamellar structure with a thin plateletlike form, and the chain runs perpendicular to the lamella. The crystal is thinner than the polymer chain length. The chain folds back and forth on the top and bottom surfaces. Since the fold costs extra energy, this folded chain crystal (FCC) should be metastable with respect to the thermodynamically more stable extended chain crystal (ECC) without folds. 

Here, we will first briefly recall the principles of this method in the case of transition metals. Then we will apply it to two illustrative examples the surface segregation energy of an impurity is a pure host and the growth of adislands on FCC(lll) surfaces of the same chemical species. 

Crystal growth has, apart from its basic surface science interest, important applications in technology for instance in microelectronics, optoelectronics, recording... However, even the simplest case of homoepitaxy is not perfectly understood. In particular, growth on FCC (111) transition metal surfaces raises some interesting questions. 

Here, we address the more general question of the relative stability of monomers, dimers and triangular trimers on the (111) surface of FCC transition metals of the same chemical species as a function of the d band filling Nd. All possible atomic configurations of the systems are considered monomers and dimers at sites N and F, triangles with A and B borders at sites N and F (Fig. 4). The d band-filling includes the range of stability of the FCC phase (Nd > 7.5e /atom). The densities of states are obtained from... 

It is seen that there exists a domain of d band-fillings Nchemical species. This domain narrows very rapidly when the cluster size increases. Consequently, outside this domain and in the range of stability of bulk FCC, i.e., when N 8.2e /atom, we predict that cluster adatoms sit always at normal sites irrespective of the size of the... 

S. Papadia, B. Piveteau, D. Spanjaard and M. C. Desjonqudres, Structural Stability of Adislands on FCC(lll) Transition Metal Surfaces, submitted... 

Another approach used to reduce the harmful effects of heavy metals in petroleum residues is metal passivation. In this process an oil-soluble treating agent containing antimony is used that deposits on the catalyst surface in competition with contaminant metals, thus reducing the catalytic activity of these metals in promoting coke and gas formation. Metal passivation is especially important in fluid catalytic cracking (FCC) processes. Additives that improve FCC processes were found to increase catalyst life and improve the yield and quality of products. ... 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

The flow of spent catalyst to the regenerator is typically controlled by a valve that slides back and forth. This slide valve is controlled by the catalyst level in the stripper. The catalyst height in the stripper provides the pressure head, which allows the catalyst to flow into the regenerator. The exposed surface of the slide valve is usually lined with refractory to withstand erosion. In a number of earlier FCC designs, lift air is used to transport the spent catalyst into the regenei ator (Figure 1-10). 

Zeolite lattices have a network of very small pores. The pore di uneter of nearly all of today s FCC zeolite is approximately 8.0 angstroms (°A). These small openings, with an internal surface area of roughly 600 square... 

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