Catalyst, general poisoning

In most ammonia plants, there are facilities to remove CO from the feed because CO will poison the catalyst. Generally, the technique used is to react the CO with water to produce CO2 and H2. The CO2 is removed by solvent extraction, and the H2 is recycled. (In case you were wondering, typical solvents used to remove CO2 are ethanolamine or an aqueous solution of potassium carbonate.)... 

The characterization of petroleum cracking catalysts, with which a third of the world s crude oil is processed, presents a formidable analytical challenge. The catalyst particles are in the form of microspheres of 60-70 micron average diameter which are themselves composites of up to five different micron and submicron sized phases. In refinery operation the catalysts are poisoned by trace concentrations of nickel, vanadium and other contaminant metals. Due to the replacement of a small portion of equilibrium catalyst each day (generally around 1% of the total reactor inventory) the catalyst particles in a reactor exist as a mixture of differing particle ages, poisoning levels and activities. 

Poisoning. Specific components present in the reactant feed can adsorb selectively onto active catalytic sites rendering them inactive, in much the same way as CO can react with Fe-hemoglobin in the blood. For heterogeneous catalysts sulfur compounds are the most universal poisons for both base metal catalysts and to a lesser extent precious metals. Sulfur compounds present in petroleum, chemical, and environmental streams adsorb on the surface of Ni, Cu, Co, etc. forming metal sulfides that have little or no activity. In general poisoning by sulfur compounds is irreversible. For this reason upstream processes are used to reduce the sulfur to acceptable levels. 

Sulfur, Phosphorus, and Arsenic Compounds. Sulfur, occasionally present in synthesis gases from coal or heavy fuel oil, is more tightly bound on iron catalysts than oxygen. For example, catalysts partially poisoned with hydrogen sulfide cannot be regenerated under the conditions of industrial ammonia synthesis. Compounds of phosphorus and arsenic are poisons but are not generally present in industrial synthesis gas. There are... 

In general, the effect of coke and poisons cannot be cleanly separated. On zeolite catalysts, trace amounts of heavy metal poisons such as V and Ni not only decrease the number of available active sites, they also increase the amount of coke formed. On supported metal catalysts, some poisons such as sulfur can decrease the amount of coke formed, as well as its distribution between metal and surface. 

These different deactivation behaviors could probably be related to the formation of surface sulfates and sulfites. The formation of such compounds is generally much less pronounced on platinum surfaces than on palladium [10]. There has even been shown that the activity of platinum could increase with small additions of SO2 [16,17]. Hence, the platinum catalysts were poisoned in a much less degree than the other catalysts. 

Different model reactions were used in order to study the interaction between the modifier and the parent metal. It was observed that an inert additive introduced by a redox reaction generally poisons, more or less, the activity of the parent metal or strongly modifies the selectivity of the reaction, which indicates a deposition of the additive on the parent metal. For example, a decrease in activity for structure insensitive reactions, such as toluene hydrogenation [41] or cyclohexane dehydrogenation [43, 78] proves the existence of bimetallic nanoparticles. Likewise, in the case of the 2,2-dimethylpropane reaction, the modification of both the selectivity and the apparent activation energy, demonstrates an interaction between Pd and Au introduced by direct redox reaction. Conversely, no modification was observed on the catalysts prepared by incipient wetness co-impregnation [75]. 

The concentrations of CO (10%-50%) are different in the synthesis gases produced from different feedstock. CO must be removed because it is a poison for ammonia synthesis catalysts. Generally, CO is converted via reaction with steam to form CO2 and H2 over a catalyst, and then CO2 is removed. The reaction between CO and steam over a catalyst is called CO shift reaction as shown in Eq. (1.16). 

It is seen from the Table 8.33 that the catalyst of plant A had a much shorter life, which is caused by poisoning of sulfur. The catalyst of plant D was used for more than two years, and the decrease of activity was due to the accumulation of sulfur. The catalyst of plant E was forced to be replaced because of leakage in process equipment. The table shows that poisoning of sulfur on catalyst generally occurs in the upper part of the catalyst bed. 

The refining industry generally seeks either to eliminate asphaltenes or to convert them to lighter materials because the presence of heteroelements cause pollution problems, e.g., sulfur and nitrogen, catalyst poisoning, and corrosion (formation of metal vanadates during combustion). 

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