Contaminant poisons, distribution

Two kinds of poison distributions must be distinguished. One distribution is that along the catalyst bed, the other one is within the porous system of the catalyst. It may be reasonably anticipated that under most conditions there will be a gradient of contaminant concentration which decreases in the direction from inlet to outlet also that there will be a decreasing concentration of contaminants from the outer confines of each separate catalyst body inwards into the pore system. The contaminant distribution will, however, differ for different types of catalysts and contaminants. 

Application Upgrade natural gas condensate and other contaminated streams to higher-value ethylene plant feedstocks. Mercury, arsenic and lead contamination in potential ethylene plant feedstocks precludes their use, despite attractive yield patterns. The contaminants poison catalysts, cause corrosion in equipment and have undesirable environmental implications. For example, mercury compounds poison hydrotreating catalysts and, if present in the steam-cracker feed, are distributed in the C2-C5+ cuts. A condensate containing mercury may have negative added-value as a gas field product. 

Lead and sulphur are derived from the fuel and there is a complex equilibrium dependent upon temperatures and gas composition controlling the absorption/desorption of these poisons. In the case of lead, extended trials have demonstrated the feasibility (ref. 20) of successful operation of oxidation catalysts on leaded fuel. However, it has been noted that in the decade since introduction of lead-free fuel in the USA, residual lead levels have fallen dramatically. In that market, where leaded and unleaded fuels are both available, incidents of poisoning reflect contamination of distribution equipment or deliberate misfuelling (refs. 21,22). Sulphur may also be derived from lube oil but its impact in the sense of poisoning is low on PGM catalysts. Interaction with catalyst components can, however, influence secondary/unregulated emissions of... 

The spatial distributions of catalytic metals and contaminant poisons in auto exhaust catalysts were delineated by electron probe line scans. Element concentrations were characterized by element sensitivities, i.e. in counts per second (cps). The electron probe microanalyses (EPM) were qualitative or semiqualitative in nature. Accurate correlation between element sensitivity and element concentration requires rather sophisticated instrument calibration. A quantitative evaluation of the EPM findings is beyond the scope of this paper. In general, it can be stated that element concentration is directly proportional to element sensitivity. Furthermore, the proportionality constant between element concentration and element sensitivity varies greatly from element to element. 

Shelef et ah (2) reported the following representative contaminant retention values for monolithic noble metal HC-CO oxidation catalysts lead, 15% phosphorus, 9% zinc, 3% and sulfur, 0.05%. Furthermore, for monolithic noble metal HC-CO catalysts which had been subjected to 30,000 miles of vehicle testing, the ratios of front to rear contaminant poison concentrations were lead, 7 phosphorus, 16 and zinc, 11. Because NO, and HC-CO catalysts are normally operated under different ambient conditions, i.e. net reducing vs. net oxidizing atmosphere, it is expected that the nature, distribution, and retention of contaminant poisons will differ for these two types of auto exhaust catalysts. 

Microdistribution of Contaminant Poisons. The distributions of lead and phosphorus in the washcoat of a front quarter core section of the catalyst are revealed by the EPM line scans in Figure 10. Throughout the interior portion of the washcoat, the distributions of lead and phosphorus were parallel. This suggests the presence of lead phosphates, e.g. 

Under FCCU operating conditions, almost 100% of the metal contaminants in the feed (such as nickel, vanadium, iron and copper porphyrins) are decomposed and deposited on the catalyst (2). The most harmful of these contaminants are vanadium and nickel. The deleterious effect of the deposited vanadium on catalyst performance and the manner in which vanadium is deposited on the cracking catalyst differ from those of nickel. The effect of vanadium on the catalyst performance is primarily a decrease in catalyst activity while the major effect of nickel is a selectivity change reflected in increased coke and gas yields (3). Recent laboratory studies (3-6) show that nickel distributes homogeneously over the catalyst surface while vanadium preferentially deposits on and reacts destructively with the zeolite. A mechanism for vanadium poisoning involving volatile vanadic acid as the... 

The distribution of contaminants within the porous layer again has to be considered separately for monolithic and pelleted catalysts. Gradients of the contaminant concentration in both cases can be very steep or relatively flat. Some inferences on the poison-carrying species can be deduced from such gradients. 

There are situations in which crystallites are readily visible, especially on supports which do not offer excessive electron scatter. In these cases, metal content can be quantitatively determined for areas which have highly dispersed metal and agglomerated metal. This information in conjunction with the crystallite size distribution provides the microscopist with the information required to make an estimate of metal dispersion (13). These estimates are valuable especially in situations where conventional gas adsorption measurements cannot be made on the metal, i.e., when the crystallites are contaminated, have multiple oxidation states, or are poisoned. 

B. cereus food poisoning occurs year-round without any particular geographic distribution and all people are believed to be susceptible. The emetic type of food poisoning is most often associated with rice products that have been cooked and then held at warm temperatures for several hours other starchy foods such as potato, pasta, and cheese products have also been implicated. The emetic form is characterized by nausea and vomiting with 0.5-6 h after consumption of contaminated foods, symptoms that parallel those of Staphylococcus aureus food poisoning. The diarrheal type of food poisoning is frequently associated with foods (meats, milk. 

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