Trace elements catalyst poisoning

Crude oil and products of the petroleum industry frequently are subjected to spectroscopic analysis to determine trace metal concentrations. It is important to the industry to know trace element concentrations in crude oil since some trace elements can poison the catalysts used in the cracking process. Some of the particularly critical trace elements are vanadium, copper, nickel, and iron. 

Other substances decrease or annihilate, even in traces, the catalytic properties of iron. Such catalyst poisons had already been known as a nuisance in the catalytic oxidation of sulfur dioxide. With the ammonia catalysis several elements, particularly sulfur proved to be harmful, even in amounts of 4oo of one per cent. Chlorine, phosphorus and arsenic showed a similar behavior (30), just as certain metals, such as lead, tin and zinc. 

Crude oil contains a number of metallic elements which are of interest either due to the undesirable effects they cause in the refining process or as an indication of the origin of the oil. The concentration levels encountered will vary with the type of crude oil. Crudes originating from different oil fields may vary markedly in trace-metal content. Also some crude oils will become contaminated in transport from the oil well to the refinery by, for example, pipeline material or seawater. The levels of such metals as Ni, V and Na must be carefully controlled in order to reduce production problems such as plant corrosion or catalyst poisoning. 

A special group of materials analyzed are crade and lubricating oils. Ni, V, and Fe act as poisons on cracking catalysts (see Catalysis), and the wear of mechanical parts of engines can be indicated by the content of trace elements (see Trace Element) in lubricating oils. Their determination is therefore an important task. To eliminate the tedious mineralizatiou of the samples, they can be analyzed directly after dilution with some nonpolar solvent, usually xylene, or methyl isobutyl ketone (MIBK). 

However, when we wish to prolong the catalyst life, it is necessary to pay attention not only to thermal deactivation but also to the deactivation by trace amounts of poisonous elements which are not problematie for automobiles due to the shorter life time required. 

Cullis and Willatt carried out Auger electron spectroscopy on catalysts poisoned or inhibited by HMDS (Table 5). This treatment was found to destroy the methane oxidation activity completely. The results show that silicon and carbon were only present on the outer surface and that removal of 1.5 nm of the surface layer by argon ion bombardment left little trace of these elements on the Pd/Sn02 catalyst. The results also show that the silica had penetrated below the surface of the bead in the case of Pd/(Th02 + VAI2O3) catalyst, but its concentration decreased as successive atomic layers were removed. This is different from the case of Pd/Sn02 bead, in which the silica had only penetrated in the first few atomic layers and argon ion bombardment for 15 seconds removed all the silica as well as carbon. ... 

These trace elements are released during gasification from pollutants in the biomass. Some of them can poison Ni catalysts and the anodes of fuel cells. How important this could be for power generation applications and catalytic system performance has yet to be subjected to detailed analysis and experimental investigation. 

The anhydrous ammonia and process air used must be free from catalyst poisons, dust, and oil. Platinum catalysts can be poisoned by such elements as As, Bi, P, Pb, S, Si, and Sn. Fortunately, synthetic ammonia is normally of high purity unless it is accidentally-contami -nated. However, since air can be contaminated by dust or many other pollutants, thorough air cleaning is necessary. Location of the air intake in an area relatively free from contaminants will help. If poisoning by impure ammonia or air ould arise, deep penetration may occur, leading to the formation of inactive compounds in the catalyst wires and, perhaps, to the extent of ruining the catalyst, fri other instances, contamination by traces of Cr, Fe, or Ni may temporarily reduce conversion efficiency, but this can often be restored by treatment with hydrochloric acid or certain sails. 

Note finally that, as mentioned in the Introduction, the corrosion of the substrate may also damage irreversibly a microstructured device under the severe conditions of fuel processing reactions. For example, under water vapor pressure, many detrimental effects can occur, such as surface migration of Ni in stainless-steel alloys, surface oxidation of metals (Fe to Fe203), surface enrichment with trace elements able to alloy/react with the coated catalyst (Sn, Pb, Cl ions) and poison it or surface substrate restructuring. 

In the production of petrochemicals and related products, it is critical for refineries and chemical plants to closely monitor trace element contamination levels at various stages of the manufacturing process. For example, in the refining of crude oil, some elements such as Ni and V, even at ppb levels, can act as catalyst poisons and cause enormous problems owing to the volumes of hydrocarbons that are processed. In addition, if the final product is intended for use by the food industry or the manufacture of electronic devices, the specifications for trace element contamination are even more stringent. 

One strong point of SIMS is its ability to detect elements that are present in trace amounts, and as such the technique is highly suited to the detection of poisons on a catalyst caused by contaminants in the reactor feed. Chlorine, for example, poisons the iron catalyst used in ammonia synthesis because it suppresses the dissociation of nitrogen molecules. Plog et al. [18] used SIMS to show that chlorine impurities may coordinate to potassium promoters, as evidenced by a KCI2- signal, or to iron, visible by an FeCh- peak. The SIMS intensity ratio... 

For Fe, Zn and Pb that may cause chemical poisoning, model poisoned catalysts were prepared by dipping catalysts into aqueous solutions of metal nitrates at various concentrations, and the catalyst carrying the nearest amount of each element was selected for the selectivity measurement. The uniform distribution of the loaded metals in the catalyst layer was confirmed by EPMA line analysis. While B. E. T. surface areas of the model poisoned catalysts differ little, the amoimt of CO adsorption decreases with the increase in the concentration of the poisonous metal, and it is noteworthy that the amount greatly decreases by loading a trace amount of Zn or Pb. 

Finally, deactivation of the catalyst by poisoning elements should be mentioned. Precious metal based catalysts are poisoned by sulfur oxides which mainly originate from the combustion of sulfur-containing fuel constituents, by phosphorus and zinc which mainly originate from some additives in the engine lubricating oil, and by silicium which was sometimes present in some engine seals (Table 21). Also, traces of lead, present in the fuel because of contamination of the fuel supply chain, made an important contribution to the deactivation of the catalyst in the past. 

Purification of synthesis gas. The sulfur- and carbon-containing compounds in synthesis gas must be removed in order to avoid the poisoning of catalysts in the following processes. Sulfur and carbon containing compounds are absorbed by different solvents. The used solvents are regenerated by desorption and H2S (or element S) and carbon dioxide are recovered. The trace amounts of carbon monoxide and carbon dioxide which remained in synthesis gas is removed via the reaction of methanation or other methods. After a series of purifications, the content of carbon monoxide and carbon dioxide in the synthesis gas are on the levels of ppm (1 ppm = 1 ml m ). 

Anode If pure hydrogen is used as fuel, the performance of the anode is excellent with pure Pt catalyzing the oxidation of hydrogen. Unfortunately, in most practical systems, the fuel stream contains certain traces of elements or compounds such as CO, S, and NH3. All of these substances can to a greater or lesser extent poison the anode catalysts. 

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