Selective Adsorption for Removing Sulfur

Velu, S., Ma, X., and Song, C. Selective adsorption for removing sulfur from jet fuel over zeolite-based adsorbents. Industrial Engineering Chemistry Research, 2003, 42, 5293. 

Turaga, U.T. Song, C. Am. Chem. Soc. Div. Fuel Chem. Preprints, 2002,47 (2), 457. Vein, S., Ma, X. Song, C. Selective Adsorption for Removing Sulfur fiom Jet Fuel over Zeohte-Based Adsorbents, fnd. Eng. Chem. Res. 2003,42, 5293-5304. 

Sulfur Removal by Selective Adsorption. The ConocoPhillips S Zorb process uses selective adsorption to remove sulfur from FCC gasoline." The feed is combined with a small amount of hydrogen, heated, and injected into an expanded fluid-bed reactor, where a proprietary sorbent removes sulfur from the feed. A disengaging zone in the reactor removes suspended sorbent from the vapor, which exits the reactor as a low-sulfur stock suitable for gasoline blending. 

An alternate process is being explored at Pennsylvania State University for deep desulfurization of distillate fuels (diesel, gasoline and jet fuels) based on selective adsorption for removal of sulfur compounds (PSU-SARS) at ambient conditions without using H2 Figure 7 illustrates the known... 

Ceramic membranes with adsorptive properties is a new field of application for multifunctional membranes. Single or mixed reactive oxides can be used for removing toxic gas from gas mixtures by selective chemisorption. For instance, sulfur compounds like hydrogen sulfide, H2S, or SO2 can be chemisorbed as sulfide or... 

In ODS, sulfur compounds present in fuels are oxidized to more polar sulfones / sulfoxides to facilitate their removal by solvent extraction or adsorption. Various oxidation systems have been reported in the literature for this transformation. Among these oxidants like hydrogen peroxide (H2O2) and carboxylic acid as catalyst3"5. For the chemical industry, it becomes more and more important to develop cleaner technologies. Solvent extraction processes are used to separate sulfones / sulfoxides from oxidized fuels. These processes required suitable and selective solvents for separation of oxidized sulfur compounds from petroleum feedstocks. 

When type X is utQized, in any of its ion exchange forms, for dehydration or possibly for sweetening (sulfur removal), there is little likelihood that the intracrystalline diffusion will be the dominant resistance to mass transfer. Large aromatic sulfurs would of course be an exception. When type X is used for adsorption of hydrocarbons or aromatics then it is possible that the micro-pore diffusion might dominate. When type A is used there is always a distinct possibility that intra-crystalline diffusion will be slow and may dominate the mass transfer, even for relatively small molecules. This is especially true when the chosen structure is a K A or type 3A. Selection of other small pore structures, for separations or purification applications can also create situations where the dominant resistance is found in the crystaUites. 

There are many methods for the desulfurization of nature gas, which can be classified into dry desulfurization, wet desulfurization, and catalytic adsorption. In the dry desulfurization, some solid sorbents, such as iron oxide, zinc oxide, activated carbon (AC), zeolites, and molecular sieves, are used. In wet desulfurization method, liquid-phase chemical/physical solvent absorption systems are usually used for scrubbing H2S amine-based processes are subject to equipment corrosion, foaming, amine-solution degradation, and evaporation, and require extensive wastewater treatment. As a result, this sulfur removal technology is complex and capital intensive,44 although the processes are still employed widely in the industry. The desulfurization of coal gasification gas will be reviewed in detail in Section 5.5. In the catalytic-adsorption method, the sulfur compounds are transformed into H2S by catalytic HDS or into elemental sulfur or SOx by selective catalytic oxidation (SCO), and then, the reformed H2S and SOx are removed by the subsequent adsorption. 

The ODS of hydrocarbon fuels consists of the conversion of sulfur compounds in the fuels by oxidation to element sulfur, sulfur oxides, sulfoxides, and/or sulfones followed by adsorption or abstraction separation of the oxidized sulfur compounds from the hydrocarbon fuels. The potential advantages for ODS are (a) the process does not need to use H2 gas (b) ODS takes place at relatively mild operating conditions in comparison with HDS or even at ambient conditions and (c) ODS is able to remove some refractory sulfur compounds, such as 4,6-DMDBT, that are difficult to be removed by HDS or selective adsorption. 

However, fractional separation has been the basis for most asphalt composition analysis (Fig. 15.5). The separation methods that have been used divide asphalt into operationally defined fractions. Three types of asphalt separation procedures are now in use (a) chemical precipitation in which n-pentane separation of asphaltenes is followed by chemical precipitation of other fractions with sulfuric acid of increasing concentration (ASTM D-2006) (b) adsorption chromatography with a clay-gel procedure in which, after removal of the asphaltenes, the remaining constituents are separated by selective adsorption/desorption on an adsorbent (ASTM D-2007 and ASTM D-4124) and (c) size exclusion chromatography in which gel permeation chromatographic (GPC) separation of asphalt constituents occurs based on their associated sizes in dilute solutions (ASTM D-3593). 

Efficiency of the desulfurization is mainly determined by the sorbent properties its adsorption capacity, selectivity for the sulfur compounds, durability and regenerability [11]. There has been an ongoing effort to develop new sorbents to remove the sulfur compounds from liquid fuels as summarized in Table 2. 

The potential of MIL-47 and MIL-53(A1) for adsorption of other types of aromatic adsorbates has also been explored, for instance, of dichlorobenzene, cresol, or alkylnaphthalene isomers [17, 98]. The removal of sulfur-containing aromatics from fuels via physisorption on MOFs has been investigated on several instances in literature, for instance, via the selective removal of thiophene from a stream of methane gas by MIL-47 [99], the removal of tetrahydrothiophene from methane by... 

Obviously to remove large amounts of asphaltic materials, substantial quantities of clay would be required in both the percolation and hot contacting methods, and the adsorption process then may become uneconomical in the treating of raw residua. With the exception of residual oils containing low concentrations of asphalts, oils to be treated with clay generally receive some pretreatment—for example, with sulfuric acid followed by neutralization of the acid oil, or selective solvent extraction. 

In the study of adsorptive desulfurization of a model diesel fuel over a nickel-based sorbent in a fixed-bed adsorption system at ambient temperature and atmosphere pressure, Kim et al.123 found that the direct interaction between the sulfur atom in the sulfur compounds and the surface nickel plays an important role, indicating that the supported nickel adsorbent is good for the selective removal of the sulfur compounds, which have no alkyl steric hindrance, from hydrocarbon streams. However, the sorption affinity of the alkyl DBTs with alkyl groups at 4- and/or 6-positions is reduced due to the steric hindrance. The sorption selectivity of the nickel-based sorbent for various compounds at room temperature increases in the order of Nap 1-MNap < 4,6-DMDBT < DBT < quinoline < indole, as shown in Figure 5.14.123... 

When producing hydrogen as the final product, impurities such as CO, sulfur compounds, and other trace contaminants must be removed, particularly for application in fuel cells. Currently, pressure swing adsorption (PSA) is commonly used for the separation and purification of hydrogen from mixed gas streams. PSA systems are based on selective adsorbent beds. The gas mixture is introduced to the bed at an elevated pressure and the solid adsorbent selectively adsorbs certain components of the gas mixture, allowing the unadsorbed components, in this case hydrogen, to pass through the bed as purified gas. 

---