Extractive Desulfurization Processes

The removal of AAT sulfur compounds from the diesel fuel feedstock by extractive desulfurization (EDS), either adsorption or solvent extraction, is a viable alternative to HDS. This approach is based on the polarity difference between the AAT family of compounds and the hydrocarbons found in the diesel fraction. Because AATs comprise several dozen different compounds, they represent a range of solvent polarities, in some cases quite similar to the aromatic compounds found in diesel fuel. The critical polarity difference between individual hydrocarbon and thiophenic compounds is therefore variable, and in some cases may be insufficient to allow a functional separation by extraction. Polarity difference can, however, be increased by oxidizing the thiophenic sulfur to the corresponding more polar mono- and dioxides, the alkylated-aryl-thiophene-sulfoxide or sulfone (AATS). This in turn facilitates the separation, and several EDS processes therefore are designed to oxidize the AAT before extraction of the sulfur.  

An examination of the chemical reactivity of AAT shows that it correlates with the electron density on the sulfur atom. Interestingly, the affinity for oxidation is directly proportional to the electron density on the sulfur atom, while the affinity for hydrogenation is inversely proportional. Alkyl-groups positioned near the sulfur atom are incremental electron donors and they also are responsible for steric hindrence that has been shown to bloek aeeess of the thiophene sulfur to the active site on the surface . This obviously is an important consideration, because AAT compoimds that are refractory to 

HDS are more readily oxidized and more amenable to EDS, which can provide the basis for synergistic application of these two processes for ULSD. 

During the past several years, several new desulfurization processes have been announced that utilize the solvent polarity differences between AAT, AATS and hydrocarbons. While several of these EDS processes are designed around a combination of oxidation and solvent extraction, at least two are based on solid-phase adsorption of the AAT. One of these, the ConocoPhillips S Zorb process for gasoline, has commercial experience with a 6,000 b/d facility that started up in mid 2001.  

S Zorb / ConocoPhillips Solid adsorbent 6,000 b/d plant started in 2001 

---

Desulfurization processes are absolutely necessary for producing clean fuels. Possible strategies to realize ultradeep suffiirization currently include adsorption, extraction, oxidation, and bioprocesses. Oxidative desulfurization (ODS) combined with extraction is considered one of the most promising of these processes [13]. Ultradeep desulfurization of diesel by selective oxidation with amphiphilic catalyst assembled in emulsion droplets has given results where the sulfur level of desulfurized diesel can be lowered from 500 ppm to about 0.1 ppm without changing the properties of the diesel [12]. 

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. 

Time will show if there is need for desulfurization of high speed Diesel fuels. There is no doubt that many companies are actively studying desulfurization processes so as to be in a position to make the best choice. Of those for which published information is available for full scale operation, the most attractive seemed to be sulfur dioxide and furfural extraction, the latter having merit because it has successfully handled high sulfur catalytically cracked recycle stocks. The previously mentioned extraction process using anhydrous hydrogen fluoride also seems attractive because of the low treatment and high yield of refined product, but, until the results of commercial operation are fully known its merits cannot be established. 

In contrast to the previously reported work utilizing supercritical solvent extraction of coal, the major objective of our research effort is to develop a desulfurization process that will result in a solid product suitable for combustion in existing coal fired utility boilers. 

CED [Conversion Extraction Desulfurization] A process for reducing the sulfur content of diesel fuel. Peroxyacetic acid oxidizes the organic sulfur compounds to sulfones, which are removed by solvent extraction. Developed in 2000 by Petro Star. 

Sulfur-X A process for removing sulfur compounds (principally thiophene) from naphtha by solvent extraction with sulfolane. Developed by UOP and announced in 2002. This process does not require hydrogen, which gives it an advantage over competing desulfurization processes. 

The high solubility of aromatic compounds in ionic liquids can also be used for the removal of the different dibenzothiophenes using liquid-liquid extraction. These are the compounds that are usually difficult to convert by hydro-treating in desulfurization processes. 

The ultradeep desulfurization of the current infrastructure fuels has become a bottleneck in the production of H2 for fuel cell applications. It is urgent to develop a more efficient and environmentally friendly process and technology for the ultradeep desulfurization of the hydrocarbon fuels. Consequently, many approaches have been conducted in order to improve the conventional HDS process and to develop new alternative processes. These approaches include catalytic HDS with improved and new catalysts, reactor and/or process, adsorptive desulfurization,27 oxidative desulfurization (ODS), extractive desulfurization (EDS), and biodesulfurization (BDS) by using special bacteria and others. Some of these works were reviewed recently by Topsoe et al.,15 Whitehurst et al.,28 Kabe et al.,29 Cicero et al.,30 Babich and Moulijn,31 Dhar et al.,32 Song,33 Song and Ma,16,34 Bej et al.,35 Mochida and Choi,36 Hemandez-Maldonado and Yang,37,38 Hemandez-Maldonado et al.,39 Topsoe,40 Brunet et al.,41 Gupta et al.,42 and Ito and van Veen.43... 

Zannikos, F, Lois, E., and Stournas, S. Desulfurization of petroleum fractions by oxidation and solvent-extraction. Fuel Processing Technology, 1995, 42, 35. 

Metallic sodium, or sodium hydroxide and sulfur, may also be extracted from flue gas by electrolysis of molten sodium sulfide (produced in the gas desulfurization process) by application of the charging reaction of the sodium-sulfur battery. This could conceivably be converted to a power-producing system if oxygen can be reduced at the cathode without severe polarization. Again, a beta-alumina diaphragm must be used to separate the sodium sulfide from the sodium hydroxide. 

Properties for processes can be calculated from thermodynamic quantities for individual species. A sample (Table II) from the NBS Interim Report (4), A Report on Some Thermodynamic Data For Desulfurization Processes, shows typical values for selected quantities of some chemical species extracted from the NBS Technical Note 270-series (5). A sample (Table III) from the NBS Interim Report illustrates a set of processes for a few reactions related to the flue gas washing process. This reaction table can be constructed from the data on individual species. 

Uses Curing agent for epoxy resins solvent for extraction of benzene, toluene, other aromatic hydrocarbons from oil refinery streams cosolvent in Sulfinol gas desulfurization process aprotic process solvent (chem. synthesis) solvent in surf, coatings, pharmaceuticals, polymerization selective solvent separation of low boiling alcohols, mineral oils, tars plasticizer dielec, in elec, equipment component in hydraulic fluids medicine (antibacterial) adjuvant in prod, of polyatylsulfone resins for food pkg. 

Shiraishi et al. " explored a new two-stage oxidative desulfurization process of light oil, effected by a combination of photochemical reaction and organic two-phase liquid-liquid extraction. The first consists of the transfer of the sulfur-containing compounds from the light oil to an aqueous-soluble... 

Shiraishi Y, Hirai T, Komasawa I. A deep desulfurization process for light oil by photochemical reaction in an organic two-phase hquid-hquid extraction system. Industrial Engineering Chemistry Research, 1998, 37 (1), 203-211. 

The physical basis for this S5mergism in process application is based in the chemistry of the alkylated aromatic thiophenic compounds in diesel fuel feedstock those most resisitant to conversion to hydrocarbons by HDS are those most readily amenable to oxidation and extractive desulfurization. Refiners expanding their processing capacity or adjusting to the new ULSD rule may want to consider the new extractive desulfurization (EDS) alternatives to high severity HDS, including debottelnecking HDS units by feedstock pretreatment or post-treatment, or parallel operation to reduce the load on their units. 

Extractive desulfurization (EDS) probably will be economically attractive as a stand-alone process in some refinery applications. However, it seems likely that many refineries also will look to employ a synergistic combination... 

There are several reasons why refinery engineers will want to consider using the new extractive desulfurization (EDS) processes. First, EDS can be considered for stand-alone desulfurization to produce ultra-low-sulfur diesel fuel (ULSD), similarly to HDS processing. Second, EDS may be considered for tandem operation with HDS, either as HDS feed pretreatment or effluent post-treatment to achieve the new ULSD specifications. In these cases EDS promises to be an alternative for very-high severity HDS processing. 

---