Hydrodesulfurization catalyst poisoning

The catalysts used in the steam reforming process are poisoned by trace components in the hydrocarbon feed - particularly sulfur, chlorine, and metal compounds. The best way to remove sulfur compounds is to convert the organic sulfur species to H2S over a hydrodesulfurization catalyst. The next step is sulfur removal with an absorbent. The same catalyst can usually convert any organochlo-ride species to give HC1 and also act as an absorbent for most problematic metal species. A second absorbent is used for chloride removal.70... 

A study on the residue hydrodesulfurization catalysts used in the commercial reactors has suggested that there exists two deactivation mechanism such as metal-controlled deactivation and coke-controlled deactivation, depending on a residue conversion level. In the second and third bed, the deactivation is controlled by metal deposition. However, in the fourth bed, a coke-controlled deactivation appears at a high residue conversion. We also have proposed that there exist two stages in the metal-controlled deactivation. During the first stage, metal sulfides partially poison the active sites and... 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

Removal of the metal contaminants is not usually economical, or efficient, during rapid regeneration. In fact, the deposited metals are believed to form sulfates during removal of carbon and sulfur compounds by combustion that produce a permanent poisoning effect. Thus, if fixed-bed reactors are to be used for residuum or heavy oil hydrodesulfurization (in place of the more usual distillate hydro-desulfurization) it may be necessary to first process the heavier feedstocks to remove the metals (especially vanadium and nickel) and so decrease the extent of catalyst bed plugging. Precautions should also be taken to ensure that plugging of the bed does not lead to the formation of channels within the catalyst bed which will also reduce the efficiency of the process and may even lead to pressure variances within the reactor because of the distorted flow patterns with eventual damage. 

In summary, the hydrodesulfurization of the low-, middle-, and highboiling distillates can be achieved quite conveniently using a variety of processes. One major advantage of this type of feedstock is that the catalyst does not become poisoned by metal contaminants in the feedstock since only negligible amounts of these contaminants will be present. Thus, the catalyst may be regenerated several times and onstream times between catalyst regeneration (while varying with the process conditions and application) may be of the order of 3 to 4 years (Table 6-6). 

In contrast to the lighter feedstocks that may be subjected to the hydrodesulfurization operation, the heavy oils and residua may need some degree of pretreatment. For example, the process catalysts are usually susceptible to poisoning by nitrogen (and oxygen) compounds and metallic salts (in addition to the various sulfur-compound types) that tend to be concentrated in residua (Chapter 3) or exist as an integral part of the heavy oil matrix. 

It follows that regeneration may consist of either (i) removal of IS sometimes poisons, most often inhibitors or fouling agents, e.g., coke (hydrogenation catalysts, e.g., selective hydrogenation of pyrolysis gasoline) or (ii) redispersion of the active species (platinum catalysts) or (iii) both (hydrodesulfurization or catalytic reforming catalysts). 

In the steam-reforming process, any sulfur compounds present in the hydrocarbon feedstock have to be removed because the nickel-containing catalysts are sensitive to poisons. This is either achieved by hydrodesulfurization (see Hydrodesulfurization Hydrodenitrogenation), generally with a combination of cobalt-molybdenum and zinc oxide... 

The raw synthesis gas produced by steam reforming of natural gas and light hydrocarbon feedstocks is free of sulfur. Any sulfur contained in the feedstock has to be removed of upstream of gasification to avoid poisoning of the sensitive reforming catalysts. This is usually performed by hydrodesulfurization and adsorption of the H2S by ZnO. As this is an essential part of the steam reforming process, it was already treated in Section 4.1.1. 

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