Depending on the species there may be specificity for particular sulfur compounds and metabolic pathways are not necessarily restricted to sulfur Kirkwood et al. Desulfurization comes with an associated carbon cost. The viability of BDS depends both on the desulfurization efficiency and the selectivity of sulfur over carbon. If carbon metabolism is high it becomes important to harvest the microorganisms to recover some of the lost carbon. The main advantage of anaerobic desulfurization processes over aerobic desulfurization is that oxidation of hydrocarbons to undesired compounds, such as colored and gum-forming products, is negligible McFarland A sulfate-reducing bacterium SRB , Desulfovibrio desulfuricans M6, was used to desulfurize model sulfur compounds and crude oils of different origins Kim et al.
It was shown that more sulfur can be removed from heavier fractions of petroleum than the total crude and the lighter fractions. Some sulfur compounds were removed completely; while others were not affected, i. Other microorganisms did not perform as well. Other sulfate-reducing bacteria from the Desulfovibrio genus isolated from oil field production facilities, such as Desulfovibrio vulgaris and Desulfovibrio desulfuricans , also performed poorly. Experiments under well-controlled anaerobic conditions did not demonstrate significant desulfurization of dibenzothiophene or a significant reduction in the total sulfur content of vacuum gas oil, deasphalted oil, and oil sands-derived bitumen Armstrong et al.
Alkylation-based desulfurization illustrated by the acid catalyzed alkylation of thiophene with 2-butene to increase the boiling point temperature T b of the product. Alkylation-based desulfurization was designed specifically for upgrading olefinic gasoline rich in thiophenic compounds. This stream also contains olefins and has a high octane number, partly on account of its high olefin content. When the FCC naphtha is desulfurized by HDS, the olefins are saturated and the octane number decreases, which is avoided by alkylation-based separation Song It is impractical to apply this type of desulfurization technology to broad distillation cuts, or heavy distillation cuts.
In both instances separation by distillation is difficult due to boiling point overlap and the need to remove the alkylated sulfur compounds as bottom product. This technology is consequently not suitable for the desulfurization of heavy oil.
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S-Alkylation of thiophenic compounds by iodomethane and silver tetrafluoroborate to produce S-alkylsulfonium salts. It requires good mixing of oil and the chlorine gas and it requires equipment having adequate corrosion resistance to chlorine. At moderate temperature and in the presence of water, chlorinolysis can be followed by hydrolysis and oxidation of the sulfur to produce sulfates. A volumetric ratio of water to oil works best Kalvinskas et al.
This is followed by aqueous and caustic washes to remove the sulfur and chlorine containing by-products. Although the chlorinolysis-based desulfurization method has not been tested with heavy oil or oil sands-derived bitumen, in theory it has some potential to be applied to bitumen production at steam-assisted gravity drainage SAGD sites. In this way the reaction is conducted within the oil sands formation, avoiding much of the cost associated with chlorine-resistant materials. However, there is a safety risk associated with such operation and the volume of chlorine required is considerable.
The effect of supercritical water SCW on desulfurization of oil is marginal Vogelaar et al. Similar findings were reported by Katritzky et al. It was found that thermal free radical-based conversion dominated and not conversion by aqueous ionic pathways. These are indirect benefits. The best desulfurization results with SCW were achieved when conventional hydrotreating catalysts were added to the system, which facilitated HDS Adschiri et al. The experimental results show that SCW alone cannot remove sulfur appreciably, but in combination with H 2 and conventional HDS catalysts, sulfur and metal impurities can be removed.
There are some reports dealing with the conversion of heavy oil in SCW. In the presence of a catalyst desulfurization took place, mainly by the formation of sulfur-rich precipitates. The product contained a higher content of asphaltenes than the feed, except when H 2 was co-fed. More optimistic results can be found in the patent literature. For example, it was reported that when heavy hydrocarbons, such as shale oil, were converted in a SCW and light olefin mixture, the liquid yield from heavy hydrocarbon cracking was improved Paspek However, the overwhelming body of evidence suggests that SCW itself does not react to any appreciable degree with the heavy oil.
The main advantages of SCW water are dilution, precipitation of sulfur-rich species, and H 2 production by water gas shift. SCW is therefore not really responsible for desulfurization.
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The physical properties and fouling nature of heavy oil undermine the efficiency of any desulfurization strategy that depends on a solid absorbent or catalyst to perform primary desulfurization of the feed. This does not imply that such technologies cannot be used. Industrially, hydroprocessing is one of the key desulfurization technologies for heavy oil; however, in application it is very different from hydroprocessing of lighter oils Ancheyta et al.
The service life and per pass desulfurization conversion is lower for heavy oils and the application of fixed bed hydroprocessing is restricted by the fouling nature of the feed Rana et al. The same fate befalls adsorptive desulfurization.
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Accessibility and desorption of heavy molecules from solid surfaces are inherently problematic. The prognosis for a breakthrough increase in desulfurization efficiency of heavy oil using either hydrodesulfurization, or adsorption desulfurization on its own is not good. Extractive desulfurization becomes increasingly difficult and unselective as the heaviness of the oil increases. Solvent loss and recovery are important detractors when desulfurizing heavy oil. The sulfur compounds are high boiling and the heavy oil is viscous.
It is unlikely that a solvent can be found that will be sulfur-selective based purely on a physical extraction. It is anticipated that any breakthrough in extractive desulfurization of heavy oil will, out of necessity, be in reactive extractive desulfurization, i. Even so, this does not eliminate the problems associated with solvent recovery, which must still be addressed.
Technology for oxidative desulfurization involves two steps.
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These steps must be considered separately. Removing the oxidized sulfur compounds from the heavy oil requires an extractive or decomposition step. The viability of extractive desulfurization has already been discussed. Decomposition has clear advantages over extraction, even though it requires processing at more severe conditions. Industrially, thermal processing of heavy oil is already practiced on large scale and desulfurization of oxidized heavy oil by thermal decomposition removes the sulfur as SO 2.
The use of catalysts acidic and basic to assist desulfurization of the oxidized product led only to a minor increase in desulfurization Sundaraman et al. Irrespective, it is important to retain the hydrocarbon portion of sulfur-containing compounds. Even when the oxidized sulfur is removed by extraction or precipitation, the sulfur is still associated with a significant amount of hydrocarbon material.
Thermal treatment is therefore still desirable in order to liberate the sulfur as SO 2. Even though thermal processing of oil predates other conversion processes, there may be unexplored opportunities for heavy oil thermal treatment in combination with oxidation. The combination of autoxidation and thermal decomposition for the ODS of heavy oil seems likely to be a viable pathway for a breakthrough in desulfurization. However, this would require a strategy to limit free radical addition and hardening of the bitumen due to the oxidation, which is a formidable obstacle.
In nature there are many examples of microorganisms that metabolize sulfur. The challenge for biodesulfurization is to find appropriate microorganisms. It is desirable that the microorganisms have a high metabolic selectivity for sulfur in general. Establishing and maintaining a viable culture that is capable of a reasonable desulfurization rate is challenging.
Desulfurization of heavy oil | SpringerLink
Heavy oil is viscous and immiscible with water and BDS is inherently transport limited. Yet, there are opportunities for breakthrough desulfurization technology, despite some of the technical challenges associated with bio-conversion in general. Alkylation and chlorinolysis-based desulfurization strategies suffer from the same drawback as ODS with hydrogen peroxide Fig.
The mass of chemicals required for desulfurization is considerable, even if a high selectivity can be achieved. In addition to this, alkylation also has other technical issues that were discussed before, which preclude application to heavy oil. Supercritical water does not result in desulfurization Vogelaar et al.
The oxidation of sulfur in sulfur containing compounds not only provides an oxidative pathway for sulfur removal, but also produces a product that can more efficiently be desulfurized in combination with other technologies. Three specific examples of synergy were noted:. Oxidation increases the polarity of the sulfur-containing species, which changes the partitioning behavior in contact with water. The solubility of the oxidized sulfur species in water is increased relative to the unoxidized sulfur and hydrocarbon species.
This has productivity and selectivity advantages for BDS. One of the metabolic pathways for BDS is also improved by pre-oxidizing the sulfur. Thiophenic sulfur is activated for oxidation and liquid-phase oxidation can readily convert sulfur species that are sterically hindered for adsorption on a catalytic surface. ODS e. Various methods were suggested for the desulfurization of oils and refinery streams. These strategies include hydrodesulfurization, extractive desulfurization, oxidative desulfurization, biodesulfurization, alkylation-based desulfurization, chlorinolysis-based desulfurization, and desulfurization using supercritical water.
Despite the variety of methods reported in literature, few of the strategies are viable for the desulfurization of heavy oil. The following specific observations were made based on a review of desulfurization literature and the applicability of different desulfurization strategies for heavy oil:.
This holds true even when the sulfur molecules are selectively converted by alkylation, oxidation or chlorinolysis prior to separation. Microorganisms with high sulfur specificity are required, as well as ways to overcome the transport limitations. Heavy oil has a high sulfur content and the amount of reagent required for desulfurization is very high.
Chemicals that are more expensive on a molar basis are likely too expensive. This disqualifies alkylation, chlorinolysis, and many of the chemical oxidation processes for desulfurization. Desulfurization that was reported in conjunction with supercritical water can be ascribed to other forms of desulfurization.