Thermal And Catalytic Cracking Ppt Slides![]() Deactivation of heterogeneous catalysts is a ubiquitous problem that causes loss of catalytic rate with time. This review on deactivation and regeneration of. Catalysts . For example, a catalyst solid may be poisoned by any one of a dozen contaminants present in the feed. Its surface, pores, and voids may be fouled by carbon or coke produced by cracking/condensation reactions of hydrocarbon reactants, intermediates, and/or products. In the treatment of a power plant flue gas, the catalyst can be dusted or eroded by and/or plugged with fly ash. Catalytic converters used to reduce emissions from gasoline or diesel engines may be poisoned or fouled by fuel or lubricant additives and/or engine corrosion products. If the catalytic reaction is conducted at high temperatures, thermal degradation may occur in the form of active phase crystallite growth, collapse of the carrier (support) pore structure, and/or solid- state reactions of the active phase with the carrier or promoters. In addition, the presence of oxygen or chlorine in the feed gas can lead to formation of volatile oxides or chlorides of the active phase, followed by gas- phase transport from the reactor. ![]() Similarly, changes in the oxidation state of the active catalytic phase can be induced by the presence of reactive gases in the feed. Thus, the mechanisms of solid catalyst deactivation are many; nevertheless, they can be grouped into six intrinsic mechanisms of catalyst decay: (1) poisoning, (2) fouling, (3) thermal degradation, (4) vapor compound formation and/or leaching accompanied by transport from the catalyst surface or particle, (5) vapor–solid and/or solid–solid reactions, and (6) attrition/crushing. As mechanisms 1, 4, and 5 are chemical in nature while 2 and 6 are mechanical, the causes of deactivation are basically threefold: chemical, mechanical, and thermal. Updated: July 5, 2017 Copyright © 2017 John Jechura ([email protected]) Implementation of Technologies 30 Ref: http://www.osha.gov/dts/osta/otm/otm Amines and amine derivatives are the most diverse group of epoxy curing agents. The fully polymerized epoxy resins exhibit a very wide range of thermal and mechanical. Updated: July 5, 2017 Copyright © 2017 John Jechura ([email protected]) Feed Preheat Train & Desalter Feed Preheat Train Initial heat exchange with. Check out our extensive selection of exhaust parts and emissions products at JCWhitney.com. We offer an industry leading 30-day Guarantee on all car exhaust parts. Corrosion Short Courses: API 571 Damage Mechanisms Affecting Fixed Equipment in the Refining and Petrochemical Industries, presented by NACE certified Corrosion. Each of the six basic mechanisms is defined briefly in Table 1 and treated in some detail in the subsections that follow, with an emphasis on the first three. Mechanisms 4 and 5 are treated together, since 4 is a subset of 5. Poisoning. Poisoning . Thus, poisoning has operational meaning; that is, whether a species acts as a poison depends upon its adsorption strength relative to the other species competing for catalytic sites. For example, oxygen can be a reactant in partial oxidation of ethylene to ethylene oxide on a silver catalyst and a poison in hydrogenation of ethylene on nickel. In addition to physically blocking of adsorption sites, adsorbed poisons may induce changes in the electronic or geometric structure of the surface . Finally, poisoning may be reversible or irreversible. An example of reversible poisoning is the deactivation of acid sites in fluid catalytic cracking catalysts by nitrogen compounds in the feed. Although the effects can be severe, they are temporary and are generally eliminated within a few hours to days after the nitrogen source is removed from the feed. Similar effects have been observed for nitrogen compound (e. Fischer- Tropsch catalysts, although these surface species require weeks to months before the lost activity is regained . However, most poisons are irreversibly chemisorbed to the catalytic surface sites, as is the case for sulfur on most metals, as discussed in detail below. Regardless of whether the poisoning is reversible or irreversible, the deactivation effects while the poison is adsorbed on the surface are the same. Many poisons occur naturally in feed streams that are treated in catalytic processes. For example, crude oil contains sulfur and metals, such as vanadium and nickel, that act as catalyst poisons for many petroleum refinery processes, especially those that use precious metal catalysts, like catalytic reforming, and those that treat heavier hydrocarbon fractions in which the sulfur concentrates and metals are almost exclusively found, such as fluid catalytic cracking and residuum hydroprocessing. Coal contains numerous potential poisons, again including sulfur and others like arsenic, phosphorous, and selenium, often concentrated in the ash, that can poison selective catalytic reduction catalysts as discussed later in Section 4. As a final example, some poisons may be added purposefully, either to moderate the activity and/or to alter the selectivity of fresh catalysts, as discussed as the end of this section, or to improve the performance of a product that is later reprocessed catalytically. An example of this latter case is lubricating oils that contain additives like zinc and phosphorous to improve their lubricating properties and stability, which become poisons when the lubricants are reprocessed in a hydrotreater or a fluid catalytic cracking unit. Mechanisms by which a poison may affect catalytic activity are multifold, as illustrated by a conceptual two- dimensional model of sulfur poisoning of ethylene hydrogenation on a metal surface shown in Figure 1. To begin with, a strongly adsorbed atom of sulfur physically blocks at least one three- or fourfold adsorption/reaction site (projecting into three dimensions) and three or four topside sites on the metal surface. Second, by virtue of its strong chemical bond, it electronically modifies its nearest neighbor metal atoms and possibly its next- nearest neighbor atoms, thereby modifying their abilities to adsorb and/or dissociate reactant molecules (in this case H2 and ethylene molecules), although these effects do not extend beyond about 5 atomic units . A third effect may be the restructuring of the surface by the strongly adsorbed poison, possibly causing dramatic changes in catalytic properties, especially for reactions sensitive to surface structure. In addition, the adsorbed poison blocks access of adsorbed reactants to each other (a fourth effect) and finally prevents or slows the surface diffusion of adsorbed reactants (effect number five). Figure 1. Reproduced from . Copyright 2. 00. 6, Wiley- Interscience. Reproduced from . Copyright 2. 00. 6, Wiley- Interscience. By: Robert A. Meyers Abstract: This unique reference is the only one-stop source for details on licensed petrochemical processes for. Catalyst poisons can be classified according to their chemical makeup, selectivity for active sites, and the types of reactions poisoned. Table 2 lists four groups of catalyst poisons classified according to chemical origin and their type of interaction with metals. It should be emphasized that interactions of Group VA–VIIA elements with catalytic metal phases depend on the oxidation state of the former, e. Thus, the order of decreasing toxicity for poisoning of a given metal by different sulfur species is H2. S, SO2, SO4. 2. Toxicity also increases with increasing atomic or molecular size and electronegativity, but decreases if the poison can be gasified by O2, H2. O, or H2 present in the reactant stream . It is apparent that organic bases (e. Metal compounds (e. Ni, Pb, V, and Zn) are poisons in automotive emissions control, catalytic cracking, and hydrotreating. Acetylene is a poison for ethylene oxidation, while asphaltenes are poisons in hydrotreating of petroleum residuum. Table 3. If sites of lesser activity are blocked initially, the poisoning is “antiselective”. If the activity loss is proportional to the concentration of adsorbed poison, the poisoning is “nonselective.” An example of selective poisoning is the deactivation of platinum by CO for the para- H2 conversion (Figure 3a) . For nonselective poisoning, the linear decrease in activity with poison concentration or susceptibility (. Several other important terms associated with poisoning are defined in Table 4. Poison tolerance, the activity at saturation coverage of the poison, and resistance (the inverse of deactivation rate) are important concepts that are often encountered in discussions of poisoning including those below. Figure 2. Reproduced from . Copyright 2. 00. 6, Wiley- Interscience. Reproduced from . Copyright 2. 00. 6, Wiley- Interscience. Table 4. Measure of a catalyst’s sensitivity to a given poison. Toxicity. Susceptibility of a given catalyst for a poison relative to that for another poison. Resistance. Inverse of the deactivation rate. Property that determines how rapidly a catalyst deactivates. Tolerance (a(Csat))Activity of the catalyst at saturation coverage (some catalysts may have negligible activity at saturation coverage)Figure 3. These assumptions, however, are rarely met in typical industrial processes because the severe reaction conditions of high temperature and high pressure bring about a high pore- diffusional resistance for either the main or poisoning reaction or both. In physical terms, this means that the reaction may occur preferentially in the outer shell of the catalysts particle, or that poison is preferentially adsorbed in the outer shell of the catalyst particle, or both. The nonuniformly distributed reaction and/or poison leads to nonlinear activity versus poison concentration curves that mimic the patterns in Figure 2 but do not represent truly selective or antiselective poisoning. For example, if the main reaction is limited to an outer shell in a pellet where poison is concentrated, the drop in activity with concentration will be precipitous. Pore diffusional effects in poisoning (nonuniform poison) are treated later in this review. As sulfur poisoning is a difficult problem in many important catalytic processes (e. Fischer–Tropsch synthesis, steam reforming, and fuel cell power production), it merits separate discussion as an example of catalyst poisoning phenomena. Studies of sulfur poisoning in hydrogenation and CO hydrogenation reactions have been thoroughly reviewed . Much of the previous work focused on poisoning of nickel metal catalysts by H2. S, the primary sulfur poison in many important catalytic processes, and thus provides some useful case studies of poisoning. Previous adsorption studies . The high stability and low reversibility of adsorbed sulfur is illustrated by the data in Figure 4 . The solid line corresponds to the equilibrium data for formation of bulk Ni. S2. Based on the equation . Most of the adsorption data lie between the dashed lines corresponding to . Indeed, extrapolation of high temperature data to zero coverage using a Tempkin isotherm .
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