Advance Cracking Course.zip
Advance Cracking Course.zip ===> https://urlin.us/2tkrhK
Through this order, my Administration is taking a critical step in what must be part of a larger effort to strengthen our democracy and advance the principles of equality and dignity. While we can make policing safer and more effective by strengthening trust between law enforcement officers and the communities they serve, we must also reform our broader criminal justice system so that it protects and serves all people equally. To be clear, certain obstacles to lasting reform require legislative solutions. In particular, system-wide change requires funding and support that only the Congress can authorize. But my Administration will use its full authority to take action, including through the implementation of this order, to build and sustain fairness and accountability throughout the criminal justice system.
(ii) Within 365 days of the date of this order, the Working Group shall assess practices and policies governing the acquisition, use, and oversight of advanced surveillance and forensic technologies, including commercial cyber intrusion tools, by Federal, State, Tribal, local, and territorial law enforcement, and shall include in the report referenced in subsection (c)(i) of this section recommendations based on this assessment that promote equitable, transparent, accountable, constitutional, and effective law enforcement practices.
Designed for the climber with some rock experience. Your guide will customize the class to your skill level and interest. Topics may include multi-pitch skills, placing and removing gear, anchors, lead climbing and more advanced crack technique needed for more athletic desert routes. This is a great opportunity to advance your movement and technical skills with expert instruction from our professional desert guides. This class will also help you prepare you for a day at Indian Creek or a Tower.
As part of the Kurdish and rebel advance on Tal Abyad, the U.S.-led coalition carried out five airstrikes near the town Monday, hitting five Islamic State tactical units and three Islamic State vehicles.
The subject of the cracking mechanism was discussed from the early days of catalytic cracking.33 It is now generally accepted that catalytic cracking involves the formation of carbenium ions.34 As depicted in Fig. 9, there is variety of ways these can be created:35,36
(1) Brønsted acid sites can donate a proton to an alkene. This alkene must than have been formed by thermal cracking beforehand. Dupain et al. describe that the initial stages of the FCC process involve mostly thermal (radical) cracking on the outer surface.32
While FCC catalyst testing is already complicated, the protocol will also have to take into account the deactivation of the catalyst during its lifetime of cracking and regeneration cycles. The deactivation of the catalyst is caused by steaming during the regeneration and assisted by the presence of metals like Ni and V (but also Fe, Na and Ca). Deactivated commercial catalysts may contain thousands of ppms of Ni and V, depending on the operation. Mitchell Impregnation (MI)45 is used to deposit Ni and V on the catalyst particle, usually prior to steaming. The metals are impregnated throughout the catalyst particle, which is maybe (in part) correct for V, but certainly not for Ni. Simple steaming of the catalyst (with or without metals) at increased temperatures mimics the effect of the regenerator in vary crude way.
The addition of solid acids to the catalyst improves both the conversion as well the product selectivity towards gasoline. The original FCC catalyst contained clay, and later amorphous silica-alumina and silica-magnesia. The advent of zeolite-based catalytic cracking was seen shortly after their discovery at Union Carbide,20,21 in the early 1960's. Zeolite Y combines high surface area/pore volume solid acidity (both Brønsted and Lewis) with sufficient room to allow bimolecular (carbenium ion) cracking. The preparation of the zeolite is relatively simple, no organic Structure Directing Agents (SDAs) or even autoclaves are required. However, the as-prepared zeolite is not very stable towards hydrothermal conditions. The stability can be improved by controlled steaming and washing/leaching cycles (to make the so-called ultra stable Y, or US-Y).
On the one hand, conventional feedstocks are becoming heavier. Resid cracking in FCC gained popularity in the early 1990's, and has gained importance since. Heavier feedstocks imply that larger, more aromatic molecules need to be cracked, which calls for improved accessibility and improved metals tolerance. At the same time, there is a drive to increase activity, but at the same time limit the amount of coke produced to the absolute minimum required for heat balance of the unit. This is a continuous challenge in FCC since the early days, and various improvements have been made over the decades, as illustrated in Fig. 11.
The development of specific FCC-propylene capacity follows the demand for olefins.66 It illustrates the clear expected increase in the propylene demand, which cannot be absorbed by steam cracking alone, and has to come from the FCC unit. On the other hand, as shown in Fig. 13, the world market for gasoline seems to flatten out, and developing countries and even the USA show an increasing demand for diesel as a transportation fuel.
The limited room in the pore system of zeolite ZSM-5 compared to the supercages in zeolite Y implies that it is much more difficult to accommodate the larger bimolecular transition states. As a result, the secondary cracking of gasoline range molecules in ZSM-5 will produce more olefins. This is illustrated in Fig. 15.
Just like the primary cracking zeolites, also zeolite ZSM-5 is unstable towards the harsh environments of the FCC process. Dealumination by repeated contact with steam in the regenerator dislodges the aluminum from its framework position, thus removing the active acid sites, and in the process destroying the zeolite lattice. Although a partial destruction of the zeolite lattice may improve the diffusion characteristics of the zeolite by creating access to the interior through mesopores, this also creates larger pores, and hence the opportunity for bimolecular cracking.
Park et al.103 describe ZSM-5 based catalysts with hierarchical pore systems prepared with soft templating. When compared to normal ZSM-5 catalysts in the cracking of gas oil, they observe higher overall activity, and higher yield of lower olefins like propylene and butylene. The catalysts contain intracrystalline mesopores. The author assume that pre-cracking of larger molecules inside the mesopores provides the molecules that can be cracked inside the MFI micropores to give the desired products. Normal ZSM-5 would require conversion of gasoline range molecules to form the desired olefins, whereas the mesoporous catalysts described by the authors have similar or better gasoline yields compared to normal ZSM-5. However, the catalytic performance was tested on pure zeolite samples. The addition of matrix and binder, as well as the presence of a main Y-zeolite based FCC catalyst in the catalyst system, may cause the observed benefits to change, among others because this would supply a large concentration of gasoline molecules. The conversion and selectivity to propylene observed for the hierarchical ZSM-5 samples described by the authors is not high enough to warrant use by itself (see e.g. the performance characteristics of the DCC process104).
Early work by Derouane and co-workers109 explains this effect. The authors describe the role of the curvature of the zeolite pore surface and explain that the interaction between molecules and the zeolite surface is strongest when the radius of the molecule and the surface curvature are similar. At this exact fit, a number of phenomena are described that have a direct effect on the performance, e.g. a supermobility instead of Knudsen diffusion. The increased interaction leads to increased concentration of reactants near the acid sites, and expresses itself macroscopically as increased apparent acid strength. This implies that the 3D structure of the zeolite and its effect on sorption equilibria can play a large role in reaction kinetics; they directly influence the observed rate of reaction, especially when the sorption energetics are magnified by the surface curvature.110 This implies that the decreased rate of cracking of n-hexane in MCM-41 as compared to zeolite US-Y does not necessarily mean that the acid sites in MCM-41 are weaker than those in zeolite US-Y.
Fig. 20 gives an overview of some of the new zeolites tested in FCC as a function of their pore diameters. When examining the medium pore size zeolite MCM-22,114 Corma et al. observed little activity in the cracking of larger molecules. When using it in an additive similar to zeolite ZSM-5 additives, zeolite MCM-22 produces less gases (lower loss in gasoline yield), but with higher olefinicity (so higher propylene and butylene selectivity than ZSM-5). ZSM-5 is more active, though. ITQ-13115 with a 3D 9-MR 10-MR pore system, presents acid sites that are similar in strength to those of ZSM-5, or stronger. The specific pore structure induces an increased yield of propylene in VGO cracking.
Zeolite ITQ-7116 has a pore system similar to zeolite Beta, yet a higher gasoline yield and improved olefin selectivity are observed in FCC cracking, where an ITQ-7 containing additive was used.117 The authors conclude that the specific structure and tortuosity of the pore system favors β-scission over protolytic cracking and limits hydrogen transfer reactions.
Zeolite ZSM-20118 and ITQ-21119 both have structures that resemble zeolite Y, and pore openings that are similar in size to zeolite Y. Their cracking characteristics are similar to zeolite Y, except for a higher gas (LPG) and propylene yield but lower gasoline olefinicity in ITQ-21. Zeolite ZSM-20 shows good thermal stability compared to zeolite Y, but this does not directly translate into higher activity. 59ce067264
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