Permanent Catalyst Poisons

Organo-metallic compounds contained in the feed will be decomposed and the metals will be retained on the catalyst, thus decreasing its activity. Since metals are normally not removable by oxidative regeneration, once metals have poisoned a catalyst, its activity cannot be restored. Therefore, metals content of the feedstock is a critical variable that must be carefully controlled. The particular metals which usually exist in vacuum gas oil type feeds are naturally occurring nickel, vanadium and arsenic as well as some metals which are introduced by upstream processing or contamination such as lead, sodium, silicon and phosphorous. Iron naphthenates are soluble in oil and will be a poison to the catalyst. Iron sulfide as corrosion product is normally not considered a poison to the catalyst and is usually omitted when referring to total metals. 

The tolerance of the catalyst to metals is difficult to quantify and is somewhat dependent upon the type of catalyst being employed and the severity of the operation, i.e., the higher the severity, the lower will be the metals tolerance since any impairment of activity will affect the ability to make the desired conversion. It is recommended to keep the total metals in the feedstock as low as possible and certainly not higher than 2 wt-ppm. 

The amount of catalyst loaded into the reactors is based upon the quantity and quality of design feedstock and the desired conversion level. The variable 

Total Feed to Reactor Inlet Total Catalyst Volume 

Most Hydrocrackers are designed to recycle unconverted feed from the product fractionator bottoms back to the reactors. This stream is normally material distilled above the heaviest fractionator side eut product. For a distillate producing Hydrocracker, the recycle stream is normally a 600-700°F (315-370°C) heavy diesel plus material. 

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The resistance of new zeolitic catalysts to temporary and permanent catalyst poisons is essential to the economic and commercial success of a zeolitic based cumene process. The following commercial data obtained using beta zeolite as a catalyst illustrates the outstanding ability of beta zeolite to cope with a wide range of feedstock contaminants ... 

Permanent catalyst poisoning occurs when some material reacts with the catalyst to form a chemically altered surface that no longer retains catalytic... 

Hydrogen chloride is a permanent irreversible poison to the metha-nation activity of C150-1-03 even though most of it is not picked up by the catalyst but is observed in the effluent gas. Only 0.02-0.04% was found on the discharged catalyst, but any amount of chloride in the feed gas is detrimental to catalyst activity. 

Catalyst poisoning. The more or less permanent deactivation of a catalyst by chemical reaction with a contaminant. Sulfur will poison platinum catalysts vanadium will poison zeolyte catalysts. 

The hydrogenation of 2-ethyl-5,6,7,8-tetrahydroanthraqumone (THEAQ) at the oxygen in the presence of a palladium supported catalyst is a key step in the industrial production of hydrogen peroxide. In industrial plants, the performance of the catalyst slowly decreases because of deactivation. Two types of catalyst poisoning are operative, a reversible one, related to the presence of water, and a permanent one, probably due to the condensation of two or more anthraquinone molecules on the palladium surface. The kinetic data obtained from laboratory runs are used to simulate the performance in industrial plants. 

Electronic promoters, for example, the alkali oxides, enhance the specific activity ofiron-alnmina catalysts. However, they rednce the inner snrface or lower the thermal stability and the resistance to oxygen-containing catalyst poisons. Promoter oxides that are rednced to the metal during the activation process, and form an alloy with the iron, are a special group in which cobalt is an example that is in industrial use. Oxygen-containing compounds such as H2O, CO, CO2, and O2 only temporarily poison the iron catalysts in low concentrations. Sulfur, phosphorus, arsenic, and chlorine compounds poison the catalyst permanently. 

R. F. Brill (Polytechnic Institute of Brooklyn) Professor de Boer mentioned in his lecture that the effect of AI2O3 in the well-known NHs catalyst consists in preventing sintering of iron and, thus, in stabilizing the activity (Lecture 16). This is true only under the conditions of the technical process. If a catalyst is mn with a very pure mixture of N2+3H2, its activity is permanent even if no activator (or stabilizer) is present, i.e., if it consists of pure a-iron. Such a nonstabilized catalyst is much more sensitive to catalyst poisons than an activated one. 

Steam reforming catalysts are poisoned by sulfur, arsenic, chlorine, phosphorus, copper and lead. Poisoning results in catalyst deactivation however, sulfur poisoning is often reversible. Reactivation can be achieved by removing sulfur from the feed and steaming the catalyst. Arsenic is a permanent poison therefore, feed should contain no more than 50 ppm of arsenic to prevent permanent catalyst deactivation by arsenic poisoning 13]. 

Fuel Starvation—Starvation of fuel can improve performance of a PEMFC exposed to CO [130]. Periodic fuel starvation causes the anode potential to increase and allows the oxidation and removal of the catalyst poisons from the anode catalysts, improving fuel cell performance. The preferred method has successive localized portions of the fuel cell anode momentarily and periodically fuel starved, while the remainder of the fuel cell anode remains elec-trochemically active and saturated with fuel so the fuel cell can continually generate power. However, when the cell is deprived of fuel, cell voltages can become negative as the anode is elevated to positive potentials and the carbon is consumed (carbon corrosion) instead of the absent fuel [131]. When this happens, the anodic current is generally provided by carbon corrosion to form carbon dioxide, resulting in permanent damage to the anode catalyst layer. 

In the case of gaseous catalyst poisons, a distinction can be made between permanent poisons causing an irreversible loss of catalytic activity and temporary poisons which lower the activity only while present in the synthesis gas. This distinction is fully discussed in the book by Nielsen. Permanent poisons such as sulfur accumulate upon the catalyst surface and may be detected by chemical and spectroscopic analysis, while temporary poisons do not interact nearly as strongly with the catalyst. It is very difficult to detect temporary poisons by means of post-analytical methods. The principal temporary poisons are oxygen, carbon oxides, and water. Since the catalyst also contains percent amounts of oxygen... 

Poisoning is operationally defined. Often catalysts beheved to be permanently poisoned can be regenerated (5) (see Catalysts, regeneration). A species may be a poison ia some reactions, but not ia others, depending on its adsorption strength relative to that of other species competing for catalytic sites (24), and the temperature of the system. Catalysis poisons have been classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites (24). 

These metals permanently poison the FCC catalyst by lowering the catalyst activity, thereby reducing its ability to produce the desiretl products. Virtually all the metals in the FCC feed are deposited on the cracking catalyst. Paraffinic feeds tend to contain more nickel than vanadium. Each metal has negative effects. 

Vanadium also promotes dehydrogenation reactions, but less than nickel. Vanadium s contribution to hydrogen yield is 20% to 50% of nickel s contribution, but vanadium is a more severe poison. Unlike nickel, vanadium does not stay on the surface of the catalyst. Instead, it migrates to the inner (zeolite) part of the catalyst and destroys the zeolite crystal structure. Catalyst surface area and activity are permanently lost. 

The residual portion of feedstocks contains a large concentration of contaminants. The major contaminants, mostly organic in nature, include nickel, vanadium, nitrogen, and sulfur. Nickel, vanadium, and sodium are deposited quantitatively on the catalyst. This deposition poisons the catalyst permanently, accelerating production of coke and light gases. 

If the catalyst surface is slowly modified by chemisorption on the active sites by materials which are not easily removed, then the process is frequently called poisoning. Restoration of activity, where possible, is called reactivation. If the adsorption is reversible then a change in operating conditions may be sufficient to reactivate the catalyst. If the adsorption is not reversible, then we have permanent poisoning. This may require a chemical retreatment of the surface or a complete replacement of the spent catalyst. 

Other factors indicated m the data of Tables 1 and 2 include Pour Point—defined as the lowest temperature at which the material will pour and a function of the composition of the oil in terms of waxiness and bitumen content Salt Content—which is not confined to sodium chloride, but usually is interpreted in terms of NaCl Salt is undesirable because of the tendency to obstruct fluid flow, to accumulate as an undesirable constituent of residual oils and asphalts, and a tendency of certain salt compounds to decompose when heated, causing corrosion of refining equipment Metals Content—heavy metals, such as vanadium, nickel, and iron, tend to accumulate in the heavier gas oil and residuum fractions where the metals may interfere with refining operations, particularly by poisoning catalysts. The heavy metals also contribute to the formation of deposits on heated surfaces in furnaces and boiler fireboxes, leading to permanent failure of equrpment, interference with heat-transfer efficiency, and increased maintenance. 

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