Review on Recent Patents in Sulfur Removal from ...

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Recent Patents on Chemical Engineering, 2010, 3, 30-37

Review on Recent Patents in Sulfur Removal from Liquid Fuels by Oxidative Desulfurization (ODS) Process Maoqi Feng* Chemical Engineering Department, Division of Chemistry and Chemical Engineering, Southwest Research Institute, San Antonio, TX 78238, USA Received: May 12, 2009; Accepted: August 3, 2009; Revised: August 13, 2009

Abstract: Oxidative desulfurization (ODS) process, a non-hydrogen consuming method to remove sulfur from liquid fuels, has received a lot of attention recently for desulfurization of some liquid fuel feedstocks. This paper reviewed the recent patents in the ODS process. Generally Hydrogen peroxide (H2O2) is used as an oxidant in organic acids (e.g., acetic acid or formic acid) medium for the ODS process. Compared with the traditional hydrodesulfurization (HDS) process, ODS process has two main advantages: 1) the ODS process can be carried out in liquid phase under very mild operating conditions, thus much less capital intensive; 2) this process shows high reactivity and selectivity for some sulfide compounds which are hard to be removed by the HDS process, e.g. dibenzothiophene (DBT) and its derivatives. After the oxidation, the sulfur compounds are oxidized to their corresponding sulfoxides and subsequently sulfones. Afterward, these highly polarized products can be removed by solvent extraction or sorbent adsorption. Other oxidants, such as t-Butyl hydroperoxide, air, and solid catalyst were also reviewed. Ultrasound greatly facilitates the ODS process. The ODS process is quite useful for small and medium refineries, and also a good fit as a finishing process for downstream of an existing HDS unit producing 300-500 ppm sulfur.

Keywords: Desulfurization, oxidation, liquid fuel, hydrogen peroxide. INTRODUCTION Recently, regulations for sulfur content in liquid fuels are becoming more and more stringent worldwide, since sulfur compounds in liquid fuels lead to emission of SOx and sulfate particulate matter (PM) after engine combustion, which endanger public health. The degree of sulfur reduction for liquid fuels has been increasing continuously. In the U.S., the specification for the sulfur content of diesel oil was reduced from 500 ppm to 50 ppm in 2004 and further down to 15 ppm in September 2006. Ultra-low sulfur regulation for off-road diesel fuel will take effect in 2010. Significant cost is needed to meet the Ultra-Low Sulfur Diesel (ULSD) or low sulfur gasoline regulations. The conventional method to remove sulfur from liquid fuels is catalytic hydrotreating. For ULSD production, in most cases, either a new high pressure hydrotreating unit or a major revamp of low/moderate pressure hydrotreating unit is required. However, hydrotreating desulfurization (HDS) process requires severe reaction conditions, such as high pressure and high temperature, which is both extremely energy and capital intensive. Other alternative desulfurization routes, e.g., oxidative desulfurization become attractive, especially for smaller refineries since it is more flexible with scale and feedstock. This paper summarizes the latest patents and patent applications on oxidative desulfurization. Oxidative desulfurization (ODS) process is a sulfur removal method which uses oxidative reactions instead of *Address correspondence to this author at the Chemical Engineering Department, Division of Chemistry and Chemical Engineering, Southwest Research Institute, San Antonio, TX 78238, USA; Tel: 210-522-5008; Fax: 210-522-3967; E-mail: [email protected] 1874-4788/10 $100.00+.00

hydrogenation in HDS process [1]. In a typical ODS process, oxidants such as hydrogen peroxide (H2O2) or oxygen from air are used as reactant rather than H2 in HDS process, which is supplied by a capital intensive hydrogen plant. After the oxidation, the sulfides in liquid fuels are oxidized to sulfones in Eq. (1) with higher polarity. The polar sulfones are then removed by extraction with organic solvent or by adsorption with solid sorbent like alumina or silica. Sulfide +

2H2O2

40-100°C H+

Sulfone +

2H2O

(1)

The most difficult sulfur streams to remove by HDS process are dibenzothiophene (DBT) and its alkyl group substituted dibenzothiophenes. ODS has the advantage of oxidizing DBT and its analogues efficiently, as shown in Eq. (2), which show high oxidation rate in the ODS process [1]. 40-100°C + 2H2O2 S

+ 2H2O H+

S O

O

(2)

Typically, ODS process comprises two steps, as shown in Fig. (1), the liquid fuel feedstock is contacting with an oxidant to selectively oxidize the sulfur-containing compounds in liquid fuels into sulfones. In this process, nitrogencontaining compounds are also oxidized into nitrogen oxides, and 2) removing the sulfones byproducts with solvent extraction or sorbent adsorption. Comparing with the HDS process, ODS process is less capital intensive, and it is especially interest to existed small © 2010 Bentham Science Publishers Ltd.

Liquid Fuels Oxidative Desulfurization

Recent Patents on Chemical Engineering, 2010, Vol. 3, No. 1

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Fig. (1). Process flow diagram for the ODS process.

and medium-sized refineries and new refineries. The advantages of the oxidative desulfurization process can be summarized as follows: 1) No using hydrogen which is capital intensive to supply; 2) Mild operating conditions, no high pressure or high temperature; 3) Easy to oxidize dibenzothiophene, complementary to hydrodesulfurization; 4) Use conventional reaction and separation refinery equipments. In recent years, many patents and papers have been claimed and published on the ODS process. These patents can be classified as five categories according to the oxidants: 1) H2O2 as oxidant; 2) O2 from air as oxidant; 3) Oxidants are generated in situ; 4) solid catalysts; 5) ultrasonic ODS. A brief description for each of these categories is as follows. H2O2 AS OXIDANT H2O2 is a green oxidant; 30-50% H2O2 is often used as oxidant for the ODS process. US Patent 6,402,940 describes a process for desulfurizing diesel fuel to achieve a sulfur level of 2 to 15ppm [2]. The oxidant is H2O2 in a formic acid solution with no more than 25 wt% water. Since H2O2 is in an aqueous phase, the formic acid functions as the phase transfer agent that transfers the hydrogen peroxide to the oil phase. There are also some other patents using H2O2 and formic acid for liquid fuel desulfurization [3-7]. US Patent 6,160,193 reported a method for removing sulfur and nitrogen compounds from petroleum distillates, such as light gas oil (diesel) by oxidation with a selective oxidant [6]. In this patent, the oxidants are divided into three categories: (1) hydrogen peroxide based oxidants, (2) ozone based oxidants, and (3) air or oxygen based oxidants. The preferred oxidant is formed by oxidizing glacial acetic acid with 30-50% aqueous hydrogen peroxide. Since the peroxide is in the aqueous phase, a phase transfer agent is required to carry the peroxide from the aqueous phase to the oil phase where it oxidizes the sulfur and nitrogen compounds. The phase transfer, which is the rate-limiting step, significantly slows down the reaction rates. In this case, acetic acid is the phase transfer agent for the oxidation of the sulfur and nitrogen compounds in the light gas oil. A small but not insignificant amount of acetic acid remains in the oil phase in the reactor effluent.

The rate of DBT oxidation reaction was found to increase with the reaction temperature increase. At room temperature, the reacted DBT was about 30% after 90 min; at 50°C, about 80% of DBT was reacted; while at 80°C, 94% of DBT was reacted for the same period of reaction time [6]. Reaction temperature at above 80°C may lead to oxidation of olefins in fuels. Besides formic acid, other acids such as acetic acid and HCl were also used as catalyst. Studies on the acid effect showed that formic acid had better oxidation effect than HCl and acetic acid [7]. Since HCl cannot form peracid, its oxidation rate is lower. The oxidation rate is related to the pKa of the acid. Compared with acetic acid, formic acid is better, because the pKa of formic acid (pKa = 3.5) is smaller than that of acetic acid (pKa = 4.75) [7]. Another possible reason is that formic acid is a more efficient phase transfer agent than acetic acid. Mass transfer between aqueous phase and oil phase might be the rate-limiting step. During the oxidation with H2O2 and organic acid, a peroxyacid is formed as an intermediate, which was observed in literature [8]. The formation of performic acid is shown in Eq. (3), and the reaction rate varies moderately with the concentrations of the reactants and the temperature [8]. HCOOH + H2O2 HCOOOH + H2O

(3)

Dimethyl sulfide (Me2S) is much easier to be oxidized than DBT. In aqueous solution, even at ambient temperatures, H2O2 converts Me2S quantitatively to Me2SO, as shown in Eq. (4). CH3SCH3 + H2O2 (CH3)2SO + H2O

(4)

At higher temperatures and with excess H2O2, the sulfoxide Me2SO is further oxidized to sulfone Eq. (5): (CH3)2SO + H2O2 (CH3)2SO2 + H2O

(5)

Literature study shows that the reaction is first order with respect to both H2O2 and Me2S and was subject to catalysis by strong acids [9]. The catalytic effect of HCl was found to be twice that of H2SO4 at 20°C. US Patent 6,402,940 assigned to Unipure describes a cost-effective approach, which is carried out at nearatmospheric pressure and less than 250°F, with a residence time of under 5 min [6]. The oxidant, an aqueous mixture of hydrogen peroxide and formic acid as a catalyst, is mixed with the fuel in a stirred-tank reactor. Following the reaction, the two phases are separated. Sulfones in the aqueous phase are recovered and the catalyst recycled. The sulfones in the liquid fuel are removed by sorbent adsorption. For the ODS

32 Recent Patents on Chemical Engineering, 2010, Vol. 3, No. 1

process, in most cases, the solid sorbents are alumina and silica. Sorbent regeneration is conducted in a column by organic solvent wash. For US Patent 6,402,940, after sulfones are adsorbed onto alumina, the alumina sorbent is regenerated by methanol wash. The recovered sulfones may be fed to a coker or used to make surfactants. After the sulfides in liquid fuels are oxidized to sulfones, they can be also removed by solvent extraction. In most cases, organic solvents, such as acetonitrile, methanol, or ethanol, are used for sulfone extraction. Several patents, for example, US Patent 6,160,193, US Patent 6,274,785, US Patent 6,406,616, and EP0565324A1, have reported many solvents for the sulfones extraction, including dimethyl sulfoxide (DMSO), formic acid, nitromethane, dimethyl formamide (DMF) and trimethyl phosphate, etc. One of the drawbacks for solvent extraction is that the solvents used to extract sulfones from the oil phase in the process, as described in US Patent 6,160,193, tend to extract appreciable amounts of oil along with the sulfones and organic nitrogen oxides. A different case is US Patent 6,245,956, which describes a method using water as solvent to extract sulfones from hydrocarbon streams [10]. Other similar patents are: 1) WO/2008/079195A1, “oxidative desulfurization and denitrogenation of petroleum oils”. This patent describes an improved oxidative process that employ a robust, nonaqueous, and oil- soluble organic peroxide oxidant for effective desulfurization and denitrogenation of hydrocarbon fuels. 2) U.S. Patent Application US20040007501, “hydrocarbon desulfurization with preoxidation of organosulfur compounds”. This patent describes a system for removing sulfur from liquid hydrocarbon streams in which desulfurization is enhanced by oxidizing certain organosulfur compounds of the liquid hydrocarbon streams. 3) U.S. Patent Application US20070051667, “diesel oil desulfurization by oxidation and extraction”. This patent describes the reduction in the sulfur-containing content of diesel fuel achieved by oxidation in the presence of a catalyst followed by a liquid-liquid countercurrent extraction. 4) U.S. Patent Application 20020035306, “method of desulfurization and dearomatization of petroleum liquids by oxidation and solvent extraction”. This patent describes a multi-step process for desulfurization of liquid fuels that also removes nitrogen-containing compounds and aromatics. There are two process steps: thiophene extraction and thiophene oxidation. 5) U.S. Patent 6,596,914, “method of desulfurization and dearomatization of petroleum liquids by oxidation and solvent extraction”. This patent describes a multi-step process for desulfurization of liquid petroleum fuels, and also nitrogen-containing compounds and aromatics are removed. t-BUTYL HYDROPEROXIDE AS OXIDANT H2O2 is miscible with water. When H2O2 is used as oxidant, it is in the aqueous phase. The oxidant must be

Maoqi Feng

transported to the oil phase to react with the sulfide compounds therein. Efficient mass transfer between aqueous phase and oil phase is a challenge for the ODS process. Organic peroxide, t-butyl hydroperoxide (TBHP), can ease this problem. Unlike hydrogen peroxide, TBHP is completely fuel soluble, making the mass transfer more efficient in the ODS process. Lyondell Chemical Company first demonstrated the use of TBHP for oxidative desulfurization in the early 1970’s [11]. TBHP is not as corrosive as peroxyacetic acid, the use of TBHP avoids the need to recycle corrosive organic acid catalysts. The fuel and TBHP are cofeed over a fixed bed catalyst, at mild temperatures and pressures, less than 90°C and less than 100 psig [12]. Minimal pressure is required to keep TBHP/fuel mixture in a liquid state and to maintain the hydraulic pressure required to move the fuel stream over the fixed bed catalyst. The residence time for the oxidation is less than 10 minutes and the sulfides are nearly quantitatively converted to sulfones. After the oxidation, t-butyl alcohol is produced as a coproduct, which can be easily removed from the fuel. The tbutyl alcohol byproduct can be also used as fuel without separation. After the reaction, any unreacted TBHP is decomposed. One concern for this process is that, the unreacted TBHP might decrease the storage stability of liquid fuels. To maintain good fuel stability, all unreacted TBHP must be either completely decomposed or removed from the fuel. In the process flow diagram shown in Fig. (2), TBHP decomposes in the oxidation reactor, and unreacted TBHP will be removed in the separation step. The removal of the sulfones from the liquid fuel can be affected either through solvent extraction or adsorption. Figure 2 shows the solvent extraction method. The extraction solvent can be recovered by distillation and reused. The recycled solvent is collected from overhead and the sulfones are concentrated as a heavy stream. For 10,000 barrels per day (BPD) plant, which processes diesel fuel with a sulfur concentration of 350 - 500ppm, the sulfone stream is about 17 - 33BPD [12]. There are several methods to dispose this sulfone stream, including processing in the coker or sending for bioprocessing. For safety reason, TBHP is usually diluted with t-butyl alcohol during shipment and handling. After the oxidation, tbutyl alcohol is produced as a co-product. For a 10,000BPD plant processing diesel fuel with a sulfur level of 500ppm, the two t-butyl alcohol streams combined will amount to approximately 50BPD. The t-butyl alcohol can be added to gasoline to boost octane, converted to MTBE or isooctane, or used as fuel within the refinery. After the oxidation, the color of the liquid fuel becomes darker. However, the final product is nearly colorless after the sulfones and other polar species are removed. AIR AS OXIDANT In the traditional ODS process, hydroperoxide H2O2 or organic hydroperoxide is used as a source of oxidant for sulfide oxidation. The high cost of these hydroperoxides makes the process less economic competitive than HDS process. Also, large-scale storage and use of such peroxides



33

Fig. (2). Process flow diagram for the Lyondell ODS method [12].

are dangerous. ODS process will be more economic viable if a cheap oxidant can be used to replace the hydroperoxides. Air has been reported to be used as oxidant for the ODS process.

Therefore, peracids produced might be real active species for oxidation of sulfide compounds in liquid fuels Eq. (8) and Eq. (9). R’SR’ + RCOOOH R’SOR’ + RCOOH

(8)

Murata et al. reported a method to oxidize substituted DBT to corresponding sulfone with molecular oxygen in the presence of cobalt catalysts and an aldehyde [13]. At a reaction temperature of 40°C, 15 min. of residence time, octanal with Co(OAc)2 as catalyst, 97% of dimethyl dibenzothiophene was oxidized to corresponding sulfone. Comparing with the case of H2O2 as oxidant, the oxidation yield is very high at this temperature.

R’SOR’ + RCOOOH R’SO2 R’ + RCOOH

(9)

Photooxidation with molecular oxygen in the presence of sensitizers such as cyanoarenes is also reported in literature [13,14]. Transition metals like Co(acac)2, Mn(OAc)2, Ni(OAc)2, etc, catalyze co-oxidation of aldehydes and organic compounds with molecular oxygen Eq. (6), which includes oxidation of aldehydes with molecular oxygen to the corresponding peroxy acids Eq. (6) and oxidation of organic sulfide compounds with the peroxy acids produced Eq. (7). This method may avoid the danger of peroxy compounds [15,16]. RCHO + O2 RCOOOH

(6)

Sulfide + RCOOOH Sulfone + RCOOH

(7)

In this reaction, both aldehydes and molecular oxygen were used instead of peracids, both of which are not so dangerous reagents in general. Co-oxidation of aldehydes and alkenes proceeds smoothly to give the corresponding carboxylic acids and oxiranes in the presence of transition metal salts or complexes, respectively [14]. Acylperoxy radicals react with aldehydes to give peracids and to regenerate acyl radicals Eq. (6). It was reported that peroxy acids could oxidize sulfides to sulfones [17].

OXIDANT GENERATED IN SITU ENI S.p.A. and UOP LLC patented a new ODS process in which the hydroperoxide is produced in situ to reduce ULSD production costs [18]. The process scheme for the UOP/ENI Oxidative Desulfurization Process is shown in Fig. (3). In this process, the first step is an oxidation reactor in which a suitable hydrocarbon refinery stream is mixed with atmospheric air to produce hydroperoxide. The hydroperoxide rich stream is mixed with hydrotreated diesel to prepare the feed stream for the sulfur oxidation section. In the second stage, a proprietary oxidation catalyst oxidizes sulfur compounds at low pressure (< 8 bar) and low temperature (< 180°C). In the third section, all oxidized compounds are separated from the diesel stream producing ULSD. Either oxidized products separation by solvent extraction or by adsorption is effective. However, extraction is more expensive from both the capital and the operating cost point of view. Separation of all oxidized compounds results in a diesel yield loss. Hence, the upgrade of this oxidized hydrocarbon stream affects the process economics. This stream can be blended into the heating oil pool or treated in either a coking or hydrocracking unit to recover valuable products. US Patent Application 20060108263 assigned to Chinese Petroleum Corporation describes a similar method [19]. The non-aqueous oxidant is prepared by oxidizing an organic acid with an aqueous hydrogen peroxide solution to produce a peroxy organic acid in solution and thereafter dehydrating the solution to yield the peroxy organic acid.


Maoqi Feng

Fig. (3). UOP/Eni Oxidative Desulfurization Process Scheme [18].

In the above process, aldehyde such as acetaldehyde and the organic acid is acetic acid. The non-aqueous oxidant is prepared by mixing acetaldehyde in ketone to form a mixture and then oxidizing the aldehyde with molecular oxygen to produce peracetic acid. U.S. Patent 7,030,255 assigned to Lyondell describes a method using in situ generated H2O2 as oxidant, which is produced by some solid catalysts. The solid catalyst, titanium zeolite catalyst, and with Pd support on TS-1, is encapsulated in a thin layer of polymer, e.g., polystyrene, and polyesters, etc. H2O2 is generated in situ with H2 and O 2 catalyzed by Pd/TS-1 with the presence of propylene [20]. Thiophene is oxidized to thiophene sulfoxide at 60°C and a pressure of 1292 psig with 25% conversion. However, the disadvantage for this process is, hydrocarbons can also be oxidized to alcohols and ketone at this condition [20]. ODS WITH SOLID CATALYST The drawback of using aqueous oxidant and organic acid as catalyst is that, the presence of water in the reactor makes it harder for phase separation of oil from the aqueous acid when the oil feed is vacuum gas oil, atmospheric residual oil, crude oil, or other heavy hydrocarbons. The sulfones generated in the oxidation reactor also function as surfactants to inhibit phase separation. Without phase separation, the spent acid, which is equivalent to 7 to 10 wt% of the oil feed, cannot be effectively removed from the oil for recycling. The presence of water can also cause a significant portion of the sulfones and organic oxides to precipitate from the reactor effluent. Solids precipitation may cause the valves, pumps, and even the adsorbent bed to malfunction. This occurs when the distillate contains more than 500 ppm sulfur and nitrogen compounds. Solid acid catalysts avoid the disadvantage of spent liquid organic acid recovery. Solid acid catalysts, polyoxometallates, which have a Keggin structure such as H3PM12O40 [M = Mo(VI), W(VI)], can be activated with H2O2 [21]. After activation, polyoxoperoxo complexes will be produced, which are more effective and selective oxidants, such as PO4[MO(μ-O2)(O2)2]43, for the oxidation of sulfides [22]. US Patent Application 2005/0109677 assigned to Lyondell is an example of using solid catalyst for sulfides oxidation. Titanium zeolite together with Pd/TS-1 is used as the solid catalyst, and Pd/TS-1 is encapsulated in a thin layer

of polymer, e.g., polystyrene, and polyesters, etc. Here Pd/TS-1 functionalizes as catalyst for H2O2 generation in situ with the reaction of H2 and O2 and the presence of propylene [20]. Propylene oxide is an intermediate for the H2O2 in situ production. Since the catalyst is a solid, it can be easily recovered for reuse. Another example is the US Patent 6,673,236, which was assigned to the Minister of Natural Resources of Canada in Jan. 2004. In this patent, ethanol is used as solvent, and the oxidant is H2O2, oxygen, ozone, or air, with solid catalyst vanadium/tungsten/TiO2 supported on cordierite. During the reaction, part of the ethanol is oxidized to peracetic acid and the peracetic acid oxidizes sulfides to sulfones. The patent application WO 2005/061675 also uses solid catalyst as oxidant [21]. Mesoporous silica functionalized with regenerable peroxycarboxylic groups is used as solid oxidizing reactant for oxidation and subsequent removal of organic sulfur compounds with alumina sorbent or silica bed [23]. After the reaction, mesoporous silica is regenerated with 50% H2O2 in acid solution. Hot nitrogen (90°C) is used to strip hydrocarbons from the sorbent. U.S. Patent 7,309,416 reported a method for desulfurization of hydrocarbon fuels with transition metal oxides as desulfurization agent, such as molybdenum oxide impregnated into a porous support, e.g., alumina, with a surface area of more than 150 m2/g and pore volume of greater than 0.45cm2/g [24]. The sulfide compounds are removed by reacting with the solid metal oxides at 300-600°C and 0.793.5MPa pressure via the following reaction Eq. (10). Table 1 lists the different solid catalysts and their sulfur removal yields. MOx + x CH3SCH3 MSx +C2x H4x +x H2O

(10)

Regeneration can occur through the Eq. (11): MSx + 3x 2O2 MOx +x SO2

(11)

The desulfurization agent is then regenerated (wherein sulfur is released) by exposing to oxygen at 300 to 600°C. ODS UNDER ULTRASONIC CONDITION It has been reported that ultrasound can significantly improve the reaction efficiency under phase transfer conditions [25]. Ultrasound increases liquid–liquid interfacial area through emulsification, which is essential for



Table 1. Sulfur Removal Yields for Different Solid Catalysts [24] Absorbent

Sulfur Removal (%)

19 wt % MoO3 /Al2O 3

65

29 wt % Ta 2O 5 /Al2O3

35

22 wt % WO3 /Al2O3

31

16 wt % ZnTiO 3 /Al2O3

28

21 wt % ZnO/Al2O3

11

11 wt % MnO2 /Al2O 3

8

viscous films containing gas-filled bubbles and cavitation bubbles. Cavitational bubbles spray solvent on the film that covers the pulsing gas bubble; therefore, the gas bubble is disrupted by the pulsing action, leading to highly dispersed emulsions. Ultrasonic process has the advantages of low investment cost and without use of the expensive hydrogen for hydrotreating. The treatment of crude oil fractions, fossil fuels, and organic liquids by sonic energy with the presence of an oxidizing agent was reported [26]. The oxidative treatment with ultrasound reduced the concentrations of sulfur and nitrogen. SulphCo, Inc. patented an ultrasound process for crude oil desulfurization. As shown in Fig. (4) [27], the ultrasound used in the patent comprises of sound waves with frequency from 20kHz to 200kHz, and preferably within the range of 20kHz to 50kHz, and the energy of the sound waves is between 30w/cm2 to 300w/cm2. The ultrasound unit can be put in a trailer; this is also a big advantage over the HDS process and other ODS processes.

35

Figure 5 shows the process flow diagram of the ultrasonic ODS process [28]. In this process, sulfides in diesel fuel are oxidized to sulfones by hydroperoxide in an aqueous organic medium with the aide of ultrasound. After ultrasonication, the sulfur compounds in diesel fuels are easier to be removed by solvent extraction, because sulfones have much higher polarity and thus easily removed from the liquid fuels by polar extraction. SulphCo is trying to scale up the reaction, and focus on process optimization to minimize costs associated with the overall material and mass balance for future commercialization [27]. In a recent patent application 20090008293 [29], ultrasonic transducer and horn are used in fossil fuel desulfurization. SOME CONCERNS FOR THE ODS PROCESS For the ODS process, the oxidation reactions depend on the hydrocarbon types as well as on the severity of the conditions. The process can proceed at low temperature, also the oxidation reactions are accelerated by higher temperatures at higher efficiency. However, higher temperatures result in slow darkening and increasing viscosity of the treated fuels [28]. The oil viscosities increase is due to accumulation of asphaltenes by metal oxide contaminants accelerated oxidations. Sludge formation is also a problem for the ODS process. It was observed that atmospheric oxidation during liquid fuels storage usually causes progressive quality deterioration, and this can pose significant environmental hazards. The reason is probably the atmospheric oxidation initiates O2-promoted polymerization which generates sludges with increasingly high asphaltene contents [30]. Nitrogen and sulfur compounds contribute to the sludge formation. Efforts to avoid sludge formation have so far not been very successful. For the patents using organic solvent for sulfones removal from oxidized liquid fuels, recycle of the used solvent is necessary to make the process more cost effective. This still needs concept-proof in large scale. The ODS process is not applicable to FCC gasoline. Since the high olefin content that tends to react with hydrogen peroxide to form epoxide products [7], hydrogen peroxide consumption will be much higher. Also, the epoxides are not stable which decreases the fuel stability. Disulfides are easily hydrodesulfurized, but they are oxidized slowly. Thus, ODS is better used as a second stage for HDS streams post-treatment [18].

Fig. (4). SulphCo’s ultrasound cracking unit [27], courtesy of Chem Info.

To be more cost-effective than the HDS process, the ODS process must operate at minimum loading of the hydrogen peroxide to reduce cost. High sulfur liquid fuel feedstock will drive the oxidant cost much higher, thus the ODS process is more suitable for desulfurization of liquid fuels with 350-500ppm of sulfur to liquid fuels with less than 10ppm of sulfur. The economic feasibility of the ODS process for high sulfur liquid fuel desulfurization will depend on the price of the H2O2 market.


Maoqi Feng

Fig. (5). Process flow diagram for ultrasonic ODS process [28].

CURRENT & FUTURE DEVELOPMENTS During the past 15 years, ODS process development is in progress. Table 2 lists the chronology of the ODS process development. Table 2.

Chronology of ODS Process Development

Patent Number, Year

Development Status

JP 05286869 (1993)

H2O2 and formic acid for liquid fuel desulfurization

US6,160,193 (2000)

Gas oil desulfurization and denitrogenation

US6,245,956 (2001)

Water as solvent to extract sulfones from hydrocarbons

US20026500219 (2002)

Sonochemical treatment for heavy gas oil desulfurization

US20040035753 (2004)

Ultrasonic process for crude oil desulfurization

US 7,030,255 (2005)

Solid catalyst for ODS process

US20077309416 (2007)

Solid catalyst for ODS process

US20090008293 (2009)

Ultrasonic transducer and horn for desulfurization

Currently, ODS technology is being pursued by Unipure, Petrostar, Lyondell, and SulphCo., etc [31]. For the ODS processes used by theses companies, the chemical oxidation desulfurization of liquid fuels is accomplished by first forming organic acid emulsion with the liquid fuels, then the sulfides in the liquid fuel are oxidized to sulfone by peracid formed from the reaction of organic acid and H2O2. The sulfone molecules are polar and hydrophilic and then move into the aqueous phase and removed. To commercialize the ODS process, the process must be proved to be scalable to a certain level. UniPure did a 35bbl/day pilot plant test in a refinery in 2003 [31]. It was estimated that a 25,000-bbl/d plant to produce 5-ppm-S diesel fuel would cost about $1,000/bbl of installed capacity, which is less than half the cost of a new, high-pressure hydrotreater. Petrostar did a bench scale pilot plant test and they had intended to demonstrate their technology with a commercial demonstration unit. In 2003, Lyondell had demonstrated a pilot plant test, in which t-butyl hydroperoxide oxidant was used to convert sulfur species to sulfones, and 500ppm sulfur diesel fuel was desulfurized successfully to below 10ppm [12]. Lyondell had achieved over 3000 hours of continuous operation in a laboratory-scale continuous pilot unit, with near quantitative oxidation of the sulfur to sulfones. Either


solvent extraction or adsorption of the oxidized fuel stream produces a fuel with less than 10ppm of sulfur. SulphCo did field trials on commercial scale (10,000 barrels per day) and promising results have been obtained [32]. Sonochemical treatment of heavy gas oil (HGO) derived from Athabasca bitumen had also been studied [33, 34]. An observation needs to be mentioned is, during the ultrasound treatment of HGO, low volatile hydrocarbon gases such as methane, ethylene, ethane, and propylene were produced, and the concentration of the lighter gases increased with increasing amplitude and sonofication time. The mechanism for ultrasonification is still not very clear. Currently, SulphCo is commercializing its technology at a scale of 30,000 barrels per day [32]. CONFLICT OF INTEREST The author declares no conflict of interest. ACKNOWLEDGMENT The author thanks Professor Feng Liu of Beijing University of Chemical Engineering for his helpful discussion.

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