Sulfur catalyst selection

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

Esomeprazole (Nexium, 13.45), a proton-pump inhibitor, is marketed as a singleenantiomer drug under the name Nexium (Scheme 13.7).17 The diethyl ester of (+) -tartaric acid (13.43, R = ethyl) serves as a chiral ligand for the titanium catalyst, and hydroperoxide is the stoichiometric oxidant. Because of the chiral environment created by the (+)-tartrate ligand, the catalyst selectively adds an oxygen atom to just one of the lone pairs to form a new stereocenter at the sulfur atom. 

Influence of Sulfur Compounds on Nickel Catalyst Selectivity... 

Poisoning of transition metals by sulfur is a serious problem encountered in many industrial processes [1-4]. In the Fisher-Tropsch synthesis of hydrocarbons from CO/H2 mixtures, the presence of a few ppm volume ratio of a sulfur containing gas can have a drastic effect on the life time of an iron catalyst [1]. On the other hand, a partial and well controlled treatment of a metal by sulfur can result in desirable effects, particularly with regard to the manipulation of catalyst selectivity for certain reactions [5]. 

Catalyst selectivity differences have been found for sulfur and metals removal in residuum hydroprocessing (19). Mass transfer limitations are believed to be important (20). The data reported here show that the metal-containing molecules are larger consequently, they should be more subject to diffusion restrictions than the sulfur-containing molecules. Therefore, it will be more difficult for a small pore catalyst to demetallate a residuum than to desulfurize it. 

Catalyst selectivity is somewhat meaningless unless the term is defined. There also are selective catalysts that do not meet the technical or practical definition of hydrogen selectivity. Such catalysts are sulfur-poisoned catalyst. Sulfided nickel catalyst produces high trans-isomers, has lower activity than conventional nickel, exhibits longer reaction times, and is used for specialty applications (e.g., coating fats and hard butters). 

Reviewing the tracer studies on the mechanism of catal3dic hydrodesulfu-risation, it can be concluded that at this time, a general mechanism for these reaction cannot be given. Possibly, it does not exist at all. It is well seen, however, that sulfur-catalyst interaction influences the catalysts behaviour substantially the activity, to a large extent, depends on the Smob/Sirr correlation, S-uptake affects the number of vacancies and consequently the value of the vacancy empty sites ratio. It was seen that H2S treatment affects the catalyst selectivity. There are a number of examples on this, mostly for HDS/hydrogenation selectivity ratio, not referred here. 

The concentration of sulfur in the feed can be up to 1500 ppm in the form of organic sulfldes. In the reforming reactor, it is transformed in H2S. Sulfur adsorbs strongly on the metal surface and affects the catalyst activity. However, it has to be emphasized that sulfur in low concentrations on the catalyst has a beneficial effect. Particularly, on the bimetallic systems Pt-Re and Pt-Ir, if no presulfldation is carried out, a runaway in the start up occurs. Sulfur is used as a selective poison for hydrogenolysis, to improve catalyst selectivity. 

Donor ligands added to the system TiCl4/Et AlCl3 (n = 1, 2) increase the catalyst selectivity to linear a-olefins. These catalysts, modified by an addition of ketones, amines, nitriles, phosphines, and sulfur compounds, yield 70-80% Cg-Cjo olefins by ethylene oligomerization. 

The catalytic oxidation/electrochemical membrane process consists of an upstream commercial sulfuric acid catalyst to convert SO2 to SO3 followed 1 a molten salt electrochemical cell using a sulfur oxide selective membrane. Removal efficiencies of 95% have been simulated. Projected economics for a 300 MW power plant burning 3.5% sulfur coal are 96/kW capital cost and 3.24 mills/kWh operating cost. Capital cost includes the catalytic converter and oleum plant and assumes cell replacement twice over a 30-year life (McHenry and Winnick, 1991). The process is in a very early stage of development, and no cortunercial or demonstration operations have been reported. 

When treating Claus unit tail gas, the process is capable of overall sulfur recoveries of up to 99.6% The number of reactors included is dependent on the feed gas concentration and the required sulfur recovery. Catalyst selectivity is maximized at a MODOP reactor outlet temperatures of 250°-270°C (482°-518 F). As the oxidation reaction is highly exothermic, additional stages must be employed to limit the reactor outlet temperature to below 320°C (608°F) at high concentrations of H2S in the feed gas. 

Conditions of hydrogenation also determine the composition of the product. The rate of reaction is increased by increases in temperature, pressure, agitation, and catalyst concentration. Selectivity is increased by increasing temperature and negatively affected by increases in pressure, agitation, and catalyst. Double-bond isomerization is enhanced by a temperature increase but decreased with increasing pressure, agitation, and catalyst. Trans isomers may also be favored by use of reused (deactivated) catalyst or sulfur-poisoned catalyst. 

Boron trifluoride catalyst may be recovered by distillation, chemical reactions, or a combination of these methods. Ammonia or amines are frequently added to the spent catalyst to form stable coordination compounds that can be separated from the reaction products. Subsequent treatment with sulfuric acid releases boron trifluoride. An organic compound may be added that forms an adduct more stable than that formed by the desired product and boron trifluoride. In another procedure, a fluoride is added to the reaction products to precipitate the boron trifluoride which is then released by heating. Selective solvents may also be employed in recovery procedures (see Catalysts,regeneration). 

In the three-step process acetone first undergoes a Uquid-phase alkah-cataly2ed condensation to form diacetone alcohol. Many alkaU metal oxides, metal hydroxides (eg, sodium, barium, potassium, magnesium, and lanthanium), and anion-exchange resins are described in the Uterature as suitable catalysts. The selectivity to diacetone alcohol is typicaUy 90—95 wt % (64). In the second step diacetone alcohol is dehydrated to mesityl oxide over an acid catalyst such as phosphoric or sulfuric acid. The reaction takes place at 95—130°C and selectivity to mesityl oxide is 80—85 wt % (64). A one-step conversion of acetone to mesityl oxide is also possible. 

Several processes are available for the recovery of platinum and palladium from spent automotive or petroleum industry catalysts. These include the following. (/) Selective dissolution of the PGM from the ceramic support in aqua regia. Soluble chloro complexes of Pt, Pd, and Rh are formed, and reduction of these gives cmde PGM for further refining. (2) Dissolution of the catalyst support in sulfuric acid, in which platinum is insoluble. This... 

Another sulfur dioxide appHcation in oil refining is as a selective extraction solvent in the Edeleanu process (323), wherein aromatic components are extracted from a kerosene stream by sulfur dioxide, leaving a purified stream of saturated aHphatic hydrocarbons which are relatively insoluble in sulfur dioxide. Sulfur dioxide acts as a cocatalyst or catalyst modifier in certain processes for oxidation of o-xylene or naphthalene to phthaHc anhydride (324,325). 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

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