Steam Reformers Catalyst Poisoning

Sulphur is the most severe poison for steam reforming catalysts. A detailed study of sulphur contamination is provided in [7], On the other hand, sulphur may have a positive effect too, because it may depresse coke formation on nickel catalysts [16],... 

Sulphur Poisoning. Sulphur is the most common poison for steam reforming catalysts. Sulphur is a natural component of all hydrocarbon feedstocks, but the sulphur contents of the feed is reduced to a few ppb by hydro-desulphurization followed by absorption over zinc oxide. The remaining sulphur, normally below the analytical detection limit, will slowly poison the catalyst (7). The mechanisms of sulphur poisoning are described in detail in the literature 2,8), The sulphur compounds are chemisorbed dissociatively on the nickel surface equation 4. 

Gum formation, which is a polymerization of hydrocarbons (especially aromatic compounds) on the catalyst surface, is a deactivation phenomenon that takes place at low temperature. Therefore, an investigation of the appearance of gum on steam reforming catalysts used at prereforming conditions is very relevant. Deactivation by gum formation can proceed several times faster than ordinary sulphur poisoning. 

Carbon deposition is one of the luost serious problems of the steam reforming catalyst process (ref 1). The deposition of carbon on naphtha steam reforming catalysts depends ori the chemical composition of the hydrocarbon oil, the steam/carbon ratio in the feedstock, as well as the pi ocesa temperature and pressure, it is also affected by tlie presence of sulfur poisons Our past research of SNG catalysts ejiamined the nature of the carbon deposits as a function of the sulfur level on the catalyst (refs, 2 4). A small amount of sulfur was found to promote the formation of carbon that is non-reactive with steam and hydrogen under steam reforming reaction conditions. The continuous accumulation of this less reactive carbon [continuous carbon deposition (CCD)l on the catalyst surface leads to coke fouling Studies of the occurrence of CCD in our laboratory tests allow ua to predict, that coke fouling is likely to occur on the same catalyst used in real Indusl.rlal applications. 

Steam-reforming catalysts may deactivate because of sintering, poisoning, or by carbon formation. 

Steam-reforming catalysts are susceptible to sulfur poisoning. At reforming conditions, all sulfur compounds are converted to hydrogen sulfide, which is chemisorbed on the metallic surface... 

From consideration of the thermodynamics of sulfur chemisorption on ruthenium (ref. 6), the gas phase sulfur activities (llgS/l ) of the lightly and moderately sulfur-poisoned Ru catalysts in equilibrium with the adsorbed sulfur at the process temperature (190 C), were approximately 0.02 and 1 ppb+ respectively. On the basis of these results, the equivalent partial pressure ratio for critical sulfur coverage is about 1 ppb at 490 C. This level is well below that attainable by conventional sulfur removal methods. Thus our result confirms the need for high performance desulfurization technology (ref 3) that can reduce sulfur contaminants in feedstocks to a sufficiently low sulfur level to avoid carbon fouling cf Ru/A Og steam reforming catalysts. 

The coking tolorance of the Ni/Al Oj SNG steam reforming catalyst, both clean and moderately poisoned, was clearly inferior to thaL of the fiu/AljO SNG catalyst. With the clean catalyst, a steam/carbon of 1.5 marked the boundary for CCD with light naphtha at 25 atm and 500 "C- This steam/carbon level is well above the performance of the Ru/A CLj at stcam/carbon=0.6. With the sulfur-poisoned Ni/A Og catalyst, t.he steam/carbon boundary for... 

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]. 

Metal catalysts are poisoned by a wide variety of compounds, as is evidenced by Fig. 5.2.a-l. The sensitivity of Pt-refonning catalysts and of Ni-steam reforming catalysts is well known. To protect the catalyst, guard reactors are installed in industrial operation. They contain Co-Mo-catalysts that transform the sulfur... 

Siloxanes also have a poisoning effect on SOFCs performances. Segregated silica can deposit in porous cermet anodes [7, 38, 39], also on steam reforming catalysts and FC anodes, causing their silication and consequent deactivation. [17]. Moreover it also affects many other components of the fuel cell system, such as heat exchangers, catalysts, and sensors [3,40]. 

The steam reforming catalysts are very sensitive to some impurities in the feedstock such as sulfur, arsenic, halogens, phosphorous and lead etc., even with very low contents. Generally, sulfur content is required to be below 0.5 ml m . Halogen such as chlorine, poisoning role is similar to sulfur, has the same limited content. Arsenic poisoning is permanent and irreversible. Thus, the restriction for arsenic is very strict. The steam reforming catalysts must be replaced when they are seriously poisoned by arsenic. 

Natural gas and petroleum liquids contain organic sulphur compounds that normally have to be removed before any further fuel processing can be carried out. Even if suphur levels in fuels are below 0.2 ppm, some deactivation of steam reforming catalysts can occur. Shift catalysts are even more intolerant to sulphur (Farrauto, 2001), and to ensure adequate lifetimes of fuel processors the desulphurisation step is very important. Even if the fuel processor catalysts were tolerant to some sulphur, it has been shown that levels of only 1 ppb are enough to permanently poison a PEM anode catalyst. 

Good current collection Can reform methane Steam reforming catalyst Not redox stable Formation of carbon fibres Low sulphur tolerance Triple phase boundary easily poisoned... 

Carbon produced by these latter reactions is formed in the catalyst pores, making it much more difficult to remove, and potentially causing physical breakage. Operating steam to carbon ratios are chosen above the minimum required in order to make carbon formation by these reactions thermodynamically impossible (3). Steam is another potential source of contaminants. Chemicals from the boiler feedwater or the cooling system are poisons to the reformer catalyst, so steam quality must be carefully monitored. 

Steam reforming is the reaction of steam with hydrocarbons to make town gas or hydrogen. The first stage is at 700 to 830°C (1,292 to 1,532°F) and 15-40 atm (221 to 588 psih A representative catalyst composition contains 13 percent Ni supported on Ot-alumina with 0.3 percent potassium oxide to minimize carbon formation. The catalyst is poisoned by sulfur. A subsequent shift reaction converts CO to CO9 and more H2, at 190 to 260°C (374 to 500°F) with copper metal on a support of zinc oxide which protects the catalyst from poisoning by traces of sulfur. 

Figure 8.3.1 is a typical process diagram for tlie production of ammonia by steam reforming. Tlie first step in tlie preparation of tlie synthesis gas is desulfurization of the hydrocarbon feed. Tliis is necessary because sulfur poisons tlie nickel catalyst (albeit reversibly) in tlie reformers, even at very low concentrations. Steam reforming of hydrocarbon feedstock is carried out in tlie priiiiiiry and secondary reformers. 

The operation of a large synthetic ammonia plant based on natural gas involves a delicately balanced sequence of reactions. The gas is first desulfurized to remove compounds which will poison the metal catalysts, then compressed to 30 atm and reacted with steam over a nickel catalyst at 750°C in the primary steam reformer to produce H2 and oxides of carbon ... 

Remaining trace quantities of CO (which would poison the iron catalyst during ammonia synthesis) are converted back to CH4 by passing the damp gas from the scmbbers over a Ni methanation catalyst at 325° CO -t- 3H2, CRt -t- H2O. This reaction is the reverse of that occurring in the primary steam reformer. The synthesis gas now emerging has the approximate composition H2 74.3%, N2 24.7%, CH4 0.8%, Ar 0.3%, CO 1 -2ppm. It is compressed in three stages from 25 atm to 200 atm and then passed over a promoted iron catalyst at 380-450°C ... 

Natural gas consists mainly of methane together with some higher hydrocarbons (Tab. 8.1). Sulfur, if present, must be removed to a level of about 0.2 ppm prior to the steam reforming process as it poisons the catalyst. This is typically done by cata-lytically converting the sulfur present as thiols, thiophenes or COS into H2S, which is then adsorbed stochiometrically by ZnO, at 400 °C, upstream of the reactor. 

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