Extraction sulfur

One more variation to the many methods proposed for sulfur extraction is the fire-flood method. It is a modem version of the Sickian method, by which a portion of the sulfur is burned to melt the remainder. It would be done in situ and is said to offer cost advantages, to work in almost any type of zone formation, and to produce better sweep efficiency than other systems. The recovery stream would be about 20 wt % sulfur as SO2 and 80 wt % elemental sulfur. The method was laboratory-tested in the late 1960s and patents were issued. However, it was not commercially exploited because sulfur prices dropped. 

SULF-X [Sulfur extraction] A regenerable flue-gas desulfurization process in which the sulfur dioxide is absorbed by aqueous sodium sulfide in a bed packed with pyrite. Ferrous sulfate is produced this is removed by centrifugation and calcined with coke and fresh pyrite. Sulfur vapor is evolved and condensed, and the residue is re-used in the scrubber. Piloted in the mid-1980s. Not to be confused with Sulfex or Sulph-X. 

Direct hydration, of ethylene, 10 538 Direct hydrogenation, 6 827 Direct immunosensors, 14 154 Direct ingot (dingot) method, 25 409 Direct initiation, 14 270 Direct injection (DI) diesel engines, 12 421 Direct inlet injection, gas chromatography, 6 383, 415-416 Directional couplers, 17 446 Directional drilling techniques, in sulfur extraction, 23 572 Directive 89/107/EEC (EU), 12 36 Direct liquefaction, 6 827 Direct marketing, technical service personnel and, 24 343 Direct metal nitridation, 17 211-213 aerosol flow reactor, 17 211-212 Direct methanol fuel cells (DMFC),... 

Frank-Wen flickering cluster model, of liquid water, 26 15, 16 Frascati Manual, 21 610 Frasch sulfur extraction process, 23 564, 570-573... 

Arsenic pentasulfide is prepared by precipitation from an acidic solution of orthoarsenic acid, H3ASO4, or arsenic pentachloride, AsCls or any other soluble As(V) salt by passing hydrogen sulfide. It may be also prepared by heating a mixture of arsenic and sulfur, extracting the fused mass with ammonia solution and reprecipitating arsenic pentasulfide at low temperature hy addition of HCl. 

Some general trends may be observed in this table. First, it can be seen that the direct sulfur extraction rate constant (ko0) is much more sensitive to the presence of methyl substituents adjacent to the sulfur atom than is either the hydrogenation rate of the sulfur-containing compound (kHSl) or the hydrogenation rate of the corresponding biphenyl derivative (kHPj). In fact, the rate of hydrogenation of 4-MDBT is a little higher than that of... 

In the case of 4,6-DMDBT, it was possible to determine the rate constants for direct extraction of sulfur from the fully saturated sulfur-containing ring system (k0l) and for the secondary hydrogenation of the tetrahydro-dibenzothiophene intermediate (fcHs2)- As might be expected, the rate constants for direct sulfur extraction follow a clear trend in which A Dq < < d2 The reverse trend is observed in the aromatic ring hydrogenation rates, /cHs, > kHS, and kHPl > kw , which is consistent with the literature (see Fig. 10) (5, 35). 

One curious observation is that high activities for direct sulfur extraction from thiophene derivatives are only exhibited by metal sulfides that form stacked lamellar crystallites, similar to graphite structures (1-3). MoS2 is classic in this regard and has found applications as a high-temperature lubricant with properties very similar to those of graphite. The other widely used metal sulfide in HDS is WS2, which also forms lamellar crystal struc-... 

This matter is more important than one of nomenclature, in that Topspe postulated that the edge sites (of his definition) are responsible for hydrogenation activity (kHSl). Chianelli, on the other hand, concluded that the rim sites are responsible for hydrogenation and his edge sites are active for direct sulfur extraction. 

Topspe proposed that corner sites are responsible for direct sulfur extraction (A Do) (53-60), but the exact nature of corner sites is not known. What is known is that the active sites for sulfur removal constitute only about 10% of all of the Co(Ni)-Mo-S sites as identified by Mossbauer emission spectroscopy (MES) (57). Thus, there is something special about some of the Co-Mo-S sites. Further study in this area is greatly needed to clarify this issue, and it is recommended that, in the future, authors use terminology in a uniform manner. Some suggestions for standardization are made in later discussions. 

Throughout the previous discussions, HDS catalysts were described as containing two different types of catalytic sites, one that facilitates direct sulfur extraction and another that facilitates hydrogenation. This could easily be rationalized in catalysts of a few years ago wherein the distribution of the promoter in the catalyst surface was uncertain, the crystals of MoS2 were large, and the composition of the support was variable. However, as catalysts have been improved, the crystallite sizes have been reduced to as small as seven Mo atoms in a cluster, and the stoichiometry of promoter to Mo is optimized at 1/2. The surface of the support is now carefully controlled, and the stacking of MoS2 can be dictated with reasonable accuracy. With such improved catalysts, it now becomes difficult to surmise how two different types of sites can exist, each with a different composition and function. 

The sequence of these phenomena may not necessarily occur in the order shown. As discussed earlier, it is believed that there are two different types of sites which have different functions, direct sulfur extraction and hydrogenation. Certainly, a site capable of extracting sulfur from the parent aromatic molecule can also extract sulfur from its hydrogenated derivative, although the rate may be higher. In the following discussion, we refer to these phenomena as steps in various mechanistic pathways. 

As discussed in a later section, H2S is an inhibitor for the catalytic site responsible for direct sulfur extraction. Thus, if the H2S partial pressure could be lowered in the reactor, the desulfurization rate could be increased. The simplest means to achieve this goal is through increased hydrogen recycle rates or increasing the hydrogen/feed ratio. Such changes are expensive and can in some instances lower the overall thoughput of the feed. 

It has also been reported that, in HDS reactions of dibenzothiophenes, the inhibition kinetics of the direct sulfur extraction route (kDf) require a different mathematical treatment from those of the hydrogenation route (kHS ) (104). A possible explanation for this observation could be that the rate-limiting steps in the two processes may involve the interaction of different numbers of adsorbed intermediates [n in Eq. (2)]. Vanrysselberghe and Froment proposed that the rate-limiting step in direct sulfur extraction from dibenzothiophene involves the simultaneous interaction of an adsorbed dibenzothiophene intermediate with two adsorbed hydrogen atoms (104). 

An attempt has been made to summarize the available literature for comparison of adsorption constants and forms of the equations used. Table XV presents a number of parameters reported by different authors for several model compounds on CoMo/A1203 in the temperature range 235-350°C (5,33,104,122,123,125-127). The data presented include the adsorption equilibrium constants at the temperatures employed in the studies and the exponential term (n) of the denominator function of the 0 parameter that was used in the calculation. The numbers shown in parentheses, relating to the value of n, indicate that the hydrogen adsorption term (Xh[H2]) is expressed as the square root of this product in the denominator. Data are presented for both the direct sulfur extraction site (cr) and the hydrogenation site (t). 

Another approach is to conduct competitive experiments with binary mixtures in which the complete reaction pathway is developed according to a reaction scheme like that of Scheme 1 described in the beginning of this review or like those shown in Figs. 12-15. Much of the confusion found in past reports of the kinetics of dibenzothiophene and its alkylated derivatives has come from incomplete deconvolution of the reaction network. Selectivity is often reported as the ratio of the yields of biphenyls (direct sulfur extraction) to the yields of cyclohexylbenzenes (hydrogenative route). As discussed in Section IV, cyclohexylbenzenes are produced via two different routes and, unfortunately, even low-conversion studies do not circumvent this confusion. To illustrate how conclusions can often be confused if the wrong model is used, some examples of reported competitive inhibition experiments will be discussed. 

These results show that, in equimolar concentrations, naphthalene would not be considered as a strong inhibitor toward direct sulfur extraction (A Do) for PASCs. However, as discussed earlier, the content of di- and trinuclear aromatics in diesel fuels and gas oils can be as high as 20-30%, whereas the level of sulfur compounds in today s diesel fuels is only 0.2% sulfur, or about 1 wt% PASCs. So the competition for the active site by aromatic hydrocarbons is very strong. Their effect on the direct desulfurization route will lower the rate to about one-third of the noninhibited rate in the case of dibenzothiophene and would lower that of 4,6-DMDBT even more. 

Inhibition by aromatic hydrocarbons is most severe for dialkyldiben-zothiophenes as these materials are preferentially desulfurized by hydrogenation of the aromatic ring prior to sulfur extraction and as aromatics are more strongly adsorbed on the hydrogenation site of Co(Ni)-Mo-S catalysts than are dibenzothiophenes. 

Inhibition by H2S severely inhibits both the hydrogenation site and the sulfur extraction site of Co(Ni)-Mo-S catalysts, but the inhibition is greater for the sulfur extraction site. The degree of inhibition is less for alkyl-substituted dibenzothiophenes than for the unsubstituted ring system, but the absolute rates of desulfurization of alkyldibenzo-thiophenes are so low that any inhibition is a major problem when attempting to meet the new 0.05% S specifications. 

As described in Section IV.B, dibenzothiophenes, when substituted in positions adjacent to the sulfur atom, have reduced activity for direct sulfur extraction. As a result, catalysts that promote aromatic ring hydrogenation offer another route to desulfurization, as the partially hydrogenated ring presents much less steric restrictions to adsorption via r -S type bonding (17,21) or to oxidative addition to form a metallathiabenzene intermediate, as discussed in Section IV.E.3. In addition, the metal-S coordination bond strength is increased by increasing the electron density on sulfur, and the C-S bonds in hydrothiophenes are much weaker. 

In a coal desulfurization study, Narayan et al. (50) were able to extract an appreciable amount of elemental sulfur (36% of total sulfur) with perchloroethylene at 120 °C from weathered coal, but not from fresh coal. Hackley et al. (51) determined the isotopic composition of elemental sulfur extracted by perchloroethylene and obtained results consistent with the interpretation that the elemental sulfur originates from the oxidation of pyrite. 

In the United States, the situation was in many ways different. With its large sulfur, natural gas, phosphate, and even potash resources, America s fertilizer industry rested on a sound base. It was an exporter of minerals and fertilizers, and did not have to worry to the same extent as Europe s industry about competing imports from Socialist countries. But reserves of sulfur extracted by the Frasch process have been depleted in Louisiana and Texas, and President Ronald Reagan s payment in kind (PIK) farm-acreage cuts reduced the fertilizer requirement of American farmers. These farmers are also much in debt and are having trouble selling their products on saturated markets. 

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