Patent Application
20090159503
This invention relates to the electrochemical conversion of dibenzothiophene type molecules of petroleum feedstreams to mercaptans that can then be removed, in one embodiment, by caustic extraction. In another embodiment, the mercaptans can be thermally decomposed, removing sulfur as hydrogen sulfide. The conversion of dibenzothiophenes to mercaptans is performed by electrochemical means without the required addition of hydrogen and in the substantial absence of water.
The sulfur content of petroleum products is continuing to be regulated to lower and lower levels throughout the world. Sulfur specifications in motor gasoline (“mogas”) and on-road diesel have been most recently reduced and future specifications will further lower the allowable sulfur content of off-road diesel and heating oils. Sulfur is currently removed from petroleum feedstreams by various processes depending on the nature of the feedstream. Processes such as coking, distillation, and alkali metal dispersions are primarily used to remove sulfur from heavy feedstreams, such as bitumens which are complex mixtures and typically contain hydrocarbons, heteroatoms, and metals, with carbon chains in excess of about 2,000 carbon atoms. For lighter petroleum feedstreams, such as distillates, catalytic hydrodesulfurization is typically used. The sulfur species in such feedstreams span a range of molecular types including sulfides, thiols, thiophenes, benzothiophenes to dibenzothiophenes in order of decreasing hydrodesulfurization (HDS) reactivity. The most difficult to remove sulfur is found in sterically hindered dibenzothiophene molecules such as diethyl dibenzothiophene. The space velocity, temperature and hydrogen pressures of catalytic HDS units are determined primarily by the slow reaction kinetics of these relatively minor components of the feed. These are the molecules that are typically left in the product after conventional low-pressure hydrotreating. Further removing these molecules often requires higher hydrogen pressure and higher temperature (“deep desulfurization”) which leads to higher hydrogen consumption and shorter catalyst run lengths which are costly results. Therefore, it is desirable to have alternative processes that are capable of removing these refractory sulfur molecules without incurring more severe reaction conditions for catalytic hydrotreating, which can result in significant capital and energy savings.
In accordance with a preferred embodiment of the present invention there is provided a process for removing sulfur from petroleum feedstreams containing sulfur in the form of hindered dibenzothiophene compounds, comprising:
a) passing a sulfur-containing petroleum feedstream to an electrochemical cell;
b) subjecting said feedstream to an effective voltage and current that will result in the conversion of at least a portion of said hindered dibenzothiophene compounds to mercaptan compounds;
c) passing the electrochemically treated petroleum feedstream containing said mercaptans compounds to a caustic treatment zone wherein it is contacted with an aqueous caustic solution wherein mercaptan-containing compounds are extracted by the aqueous caustic solution; and
d) collecting a reduced-sulfur petroleum product stream from the caustic treatment zone;
wherein the reduced-sulfur petroleum product stream has a lower sulfur content by wt % than the sulfur-containing petroleum feedstream.
In a preferred embodiment, the sulfur-containing petroleum feedstream is comprised of a bitumen.
In another preferred embodiment, the feedstream is a distillate boiling range hydrocarbon stream and an effective amount of an electrolyte is mixed with the distillate boiling range stream to be treated.
Also in accordance with another preferred embodiment of the present invention is a process for removing sulfur from petroleum feedstreams containing sulfur in the form of hindered dibenzothiophene compounds, comprising:
a) passing a sulfur-containing petroleum feedstream to an electrochemical cell;
b) subjecting said feedstream to an effective voltage and current that will result in the conversion of at least a portion of said hindered dibenzothiophene compounds to mercaptan compounds;
c) passing the electrochemically treated petroleum feedstream containing mercaptan compounds to a thermal decomposition zone wherein at least a portion of the mercaptans are decomposed to hydrogen sulfide at temperatures from about 302° F. to about 932° F. (150° C. to 500° C.); and
d) collecting a reduced-sulfur petroleum product stream from the thermal decomposition zone;
wherein the reduced-sulfur petroleum product stream has a lower sulfur content by wt % than the sulfur-containing petroleum feedstream.
In a preferred embodiment, the sulfur-containing petroleum feedstream is comprised of a bitumen.
In another preferred embodiment, the feedstream is a distillate boiling range stream and an effective amount of an electrolyte is mixed with the distillate boiling range stream to be treated.
Feedstreams suitable for use in the present invention range from heavy oil feedstreams, such as bitumens to those boiling in the distillate range. In a preferred embodiment the heavy oil feedstream contains at least about 10 wt. %, preferably at least about 25 wt. % of material boiling above about 1050° F. (565° C.), both at atmospheric pressure (0 psig). Such streams include bitumens, heavy oils, whole or topped crude oils and residua. The bitumen can be whole, topped or froth-treated bitumen. Non-limiting examples of distillate boiling range streams that are suitable for use herein include diesel fuels, jet fuels, heating oils, kerosenes, and lubes. Such streams typically have a boiling range from about 302° F. (150° C.) to about 1112° F. (600° C.), preferably from about 662° F. (350° C.) to about 1022° F. (550° C.). Other preferred streams are those typically known as Low Sulfur Automotive Diesel Oil (“LSADO”). LSADO will typically have a boiling range of about 350° F. (176° C.) to about 550° F. (287° C.) and contain from about 200 wppm sulfur to about 2 wppm sulfur, preferably from about 100 wppm sulfur to about 10 wppm sulfur. The process embodiments of the present invention electrochemically treat a sulfur-containing petroleum feedstream resulting in a reduced-sulfur petroleum product stream which has a lower sulfur concentration by wt % than the sulfur-containing petroleum feedstream.
A major of the sulfur contained in heavy oils and distillates are in the form of hindered dibenzothiophene molecules. Although such molecules are difficult to remove by conventional hydrodesulfurization processes without using severe conditions, such as high temperatures and pressures, such molecules are converted by the practice of the present invention to sulfur species that are more easily removed by conventional non-catalytic processes. For example, the electrochemical step of the present invention converts the hindered dibenzothiophene (“DBT”) molecules, which are substantially refractory to conventional hydrodesulfurization, to hydrogenated naphthenobenzothiophene mercaptan molecules that are more readily extracted with use of caustic solution or by thermal decomposition. This capability can significantly debottleneck existing distillate hydrotreating process units by converting the slowest to convert molecules (hindered dibenzothiophenes) into much more readily extractable mercaptan species, preferably alkylated biphenyl mercaptan species.
The process of the present invention does not require the addition of an electrolyte when heavy oil is the feedstream, but rather, relies on the intrinsic conductivity of the heavy oil at elevated temperatures. It will be understood that the term “heavy oil” and “heavy oil feedstream” as used herein includes both bitumen and other heavy oil feedstreams, such as crude oils, atmospheric resids, and vacuum resids. This process is preferably utilized to upgrade bitumens and/or crude oils that have an API gravity of less than about 15. The inventors hereof have undertaken studies to determine the electrochemical conductivity of crudes and residues at temperatures up to about 572° F. (300° C.) and have demonstrated an exponential increase in electrical conductivity with temperature as illustrated in
A 4 mA/cm2 electrical current density at 662° F. (350° C.) with an applied voltage of 150 volts and a cathode-to-anode gap of 1 mm was measured for an American crude oil. Though this is lower than would be utilized in preferred commercial embodiments of the present invention, the linear velocity for this measurement was lower than the preferred velocity ranges by about three orders of magnitude: 0.1 cm/s vs. 100 cm/s. Using a 0.8 exponent for the impact of increased flow velocity on current density at an electrode, it is estimated that the current density would increase to about 159 mA/cm2 at a linear velocity of about 100 cm/s. This suggests that more commercially attractive current densities achieved at higher applied voltages. Narrower gap electrode designs or fluidized bed electrode systems could also be used to lower the required applied voltage.
Unlike bitumen, performing controlled potential electrolysis on a non-conductive fluid such as a LSADO, or other petroleum distillate streams, requires the introduction of an effective amount of an electrolyte, such as a conductive salt. There is an insufficient concentration of large multi-ring aromatic and heterocyclic molecules in distillate boiling range feedstreams to produce sufficient intrinsic conductivity without the use of an electrolyte. The direct addition of a conductive salt to the distillate feedstream can be difficult for several reasons. The term “effective amount of electrolyte” as used herein means at least than amount needed to produce conductivity between the anode and the cathode of the electrochemical cell. Typically this amount will be from about 0.5 wt. % to about 50 wt. %, preferably from about 0.5 wt. % to about 10 wt. %, of added electrolytic material based on the total weight of the feed plus the electrolyte. Once dissolved in the oil, most salts are difficult to remove after electrolysis. Incomplete salt removal is unacceptable due to product specifications, negative impact on further catalytic processing, potential corrosivity and equipment fouling. Even salts that are soluble in a low dielectric medium are often poorly ionized and therefore unacceptable high concentrations are required to achieve suitable conductivities. In addition, such salts are typically very expensive. However, recent advances in the field of ionic liquids have resulted in new organic soluble salts having melting points lower than about 212° F. (100° C.) that can be used in the present invention. They can be recovered by solvent washing the petroleum stream after electrolysis. Non-limiting examples of such salts include: 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluoro phosphate, 1-butyl-1-methyl pyrrolidinium trifluoro-methyl sulfonated, trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphate and ethyl-dimethylpropyl-ammonium bis(trifluoro-methylsulfonyl)imide.
An alternate solution to the low conductivity problem of distillate boiling range feedstreams to produce a two phase system. Rather than adding an electrolyte to the feedstream, the feedstream can be dispersed in a conductive, immiscible, non-aqueous electrolyte. Such a two-phase system of oil dispersed in a continuous conductive phase provides a suitable electrolysis medium. The continuous conductive phase provides the sufficient conductivity between the cathode and anode of an electrochemical cell to maintain a constant electrode potential. Turbulent flow through the electrochemical cell brings droplets of the feedstream in contact with the cathode, at which point electrons are transferred from the electrode to sulfur containing species on the droplet surface.
After reaction, the immiscible electrolyte from the treated feedstream is separated by any suitable conventional means resulting in a reduced sulfur product stream. The immiscible electrolyte can be recycled. The electrolyte in the immiscible electrolysis medium is preferably an electrolyte that dissolves, or dissociates, in the solvent to produce electrically conducting ions, but that does not undergo a redox reaction in the range of the applied potentials used. Suitable organic electrolytes for use in the present invention, other than those previously mentioned, include quaternary carbyl- and hydrocarbyl-onium salts, e.g., alkylammonium hydroxides. Non-limiting examples of inorganic electrolytes include, e.g., NaOH, KOH and sodium phosphates, and mixtures thereof. Non-limiting examples of onium ions that can be used in the practice of the present invention include mono- and bis-phosphonium, sulfonium and ammonium, preferably ammonium. Preferred carbyl and hydrocarbyl moieties are alkyl carbyl and hydrocarbyl moieties. Suitable quaternary alkyl ammonium ions include tetrabuytyl ammonium, and tetrabutyl ammonium toluene sulfonate. Optionally, additives known in the art to enhance performance of the electrodes can also be used. Non-limiting examples of such additives suitable for use herein include surfactants, detergents, emulsifying agents and anodic depolarizing agents. Basic electrolytes are most preferred. The concentration of salt in the electrolysis medium should be sufficient to generate an electrically conducting solution in the presence of the feedstream. Typically, a concentration of about 1 to about 50 wt % conductive phase, preferably about 5 to about 25 wt % based on the overall weight of the oil/water/electrolyte mixture is suitable. It is preferred that petroleum stream immiscible solvents be chosen, such as dimethyl sulfoxide, dimethylformamide or acetonitrile.
Dispersions are preferred for ease of separation following electrolysis. However, more stable oil-in-solvent emulsions can also be used. Following electrolytic treatment, the resulting substantially stable emulsion can be broken by the addition of heat and/or a de-emulsifying agent.
The electrochemical cell used in the practice of the present invention may be divided or undivided. Such systems include stirred batch or flow through reactors. The foregoing may be purchased commercially or made using technology known in the art. Suitable electrodes known in the art may be used. Included as suitable electrodes are three-dimensional electrodes, such as carbon or metallic foams. The optimal electrode design would depend upon normal electrochemical engineering considerations and could include divided and undivided plate and frame cells, bipolar stacks, fluidized bed electrodes and porous three dimensional electrode designs; see Electrode Processes and Electrochemical Engineering by Fumio Hine (Plenum Press, New York 1985). While direct current is typically used, electrode performance may be enhanced using alternating current or other voltage/current waveforms. The gap between electrode surfaces will preferably be about 1 to about 50 mm, more preferably from about 1 to about 25 mm, and the linear velocity in the electrochemical cell will be in the range of about 1 to about 500 cm/s, more preferably in the range of about 50 to about 200 cm/s.
The applied cell voltage, that is, the total voltage difference between the cathode and anode will vary depending upon the cell design and electrolytes used. What is critical, however, is that the cathode be polarized sufficiently to achieve electron transfer to the dibenzothiophene molecules, which occurs at reduction potentials more negative than −2.3 Volts versus a standard calomel electrode. Normal electrochemical practices can be used to ensure that the cell is operated under these conditions. In preferred embodiments, the voltage across the electrochemical cell will be about 4 to about 500 volts, preferably from about 100 to about 200 volts, with a resulting current density of about 10 mA/cm2 to about 1000 mA/cm2, preferably from about 100 mA/cm2 to about 500 mA/cm2.
At least a portion of the hindered dibenzothiophene compounds in the feedstream are converted to the corresponding alkylated biphenyl mercaptan compounds in the electrochemical cell. The mercaptans containing treated feedstream is passed to a caustic wash step wherein it is contacted with an aqueous caustic solution for extraction of the mercaptan species. Any suitable caustic wash technology can be used in the practice of the present invention. The most preferred caustic wash would be an aqueous solution of sodium hydroxide having a strength from about 0.5 M to about 5 M and mixing the mercaptan-containing stream with air and the caustic solution to remove the mercaptan species in the caustic solution. Non-limiting examples of caustic extraction processes that can be used in the practice of the present invention include the UOP® MEROX® process and the Merichem® THIOLEX® and EXOMER® processes. The MEROX® Process was announced to the industry in 1959. The Oil & Gas J. 57(44), 73-8 (1959) contains a discussion of the MEROX® Process. In the MEROX® oxidation process, mercaptan compounds are extracted from the feed and then oxidized by air in the caustic phase in the presence of the MEROX® catalyst, which is typically an iron group chelate (cobalt phthalocyanine) to form disulfides which are then redissolved in the hydrocarbon phase, leaving the process as disulfides in the hydrocarbon product. The disulfides, which are not soluble in the caustic solution, can be separated and recycled for mercaptan extraction. The treated stream is usually sent to a water wash in order to reduce the sodium content.
All of these processes take advantage of the acidity of the mercaptan species. By contacting a petroleum stream that contains acidic mercaptan species with an aqueous base solution, the mercaptans are de-protonated, converted to salts and are now more soluble in the aqueous stream and thus can be extracted nearly quantitatively from the petroleum stream. Such an extraction is ineffective with the original, non-acidic dibenzothiophenic sulfur species. The desulfurized petroleum stream is then separated from the resulting mercaptide containing caustic solution. The caustic solution can then be regenerated and the mercaptides isolated in a variety of conventional ways depending on the process design. Such mercaptan extractions are widely used in the petroleum refining industry and it is likely that every refinery has at least one such unit. The extracted mercaptans can be readily oxidized to disulfides, separated from the caustic stream, and recycled for more mercaptan extraction. The hindered dibenzothiophene (“DBT”) species which are removed from the feedstream are converted to a relatively small substantially pure stream of disulfides that can be disposed of via combustion. They can also be fed to a coking unit for thermal decomposition. Being able to target hindered DBT molecules can also enable the disposition of Light Catalytic Cycle Oil (“LCCO”), which is rich in DBTs, to distillate hydrotreaters.
In a second embodiment of the present invention, following, or simultaneous with the electrochemical conversion of the dibenzothiophenic species to mercaptans, a thermal decomposition reaction of the mercaptans is performed to decompose them with loss of hydrogen sulfide from the mercaptan molecule. This thermal decomposition can be performed at temperatures from about 302° F. to about 932° F. (150° C. to 500° C.), preferably from about 482° F. to about 932° F. (250° C. to 500° C.) and at ambient to autogenous pressure. Subsequent removal of this hydrogen sulfide from the petroleum stream will produce a reduced sulfur product stream that is lower is sulfur content by wt % than the sulfur-containing petroleum feedstream treated by the current process.
The present invention will be better understood with reference to the following examples which are presented for illustrative purposes and are not to be taken as limiting the invention in anyway.
The following three examples were performed using a 300-cc autoclave (Parr Instruments, Moline, Ill.) was modified to allow two insulating glands (Conax, Buffalo, N.Y.) to feed through the autoclave head. Two cylindrical stainless steel (316) mesh electrodes are connected to the Conax glands, where the power supply (GW Laboratory DC Power Supply, Model GPR-1810HD) is connected to the other end. The autoclave body is fitted with a glass insert, a thermal-couple and a stirring rod. The autoclave can be charged with desired gas under pressure and run either in a batch- or a flow-through mode.
To the glass insert was added 1.0 g dibenzothiophene (“DBT”), 3.87 g tetrabutylammonium hexafluorophosphate (TBAPF6), and 100 milliliter (“ml”) anhydrous dimethyl sulfoxide (DMSO, Aldrich). After the content was dissolved, the glass insert was loaded into the autoclave body, the autoclave head assembled and pressure tested. The autoclave was charged with 70 psig of N2 and heated to 212° F. (100° C.) with stirring (300 rpm). A voltage of 5 Volts was applied and the current was 0.8 Amp. The current gradually decreased with time and after two hours, the run was stopped. The autoclave was opened and the content acidified with 10% HCl (50 ml). The acidified solution was then diluted with 100 ml of de-ionized (“DI”) water, extracted with ether (50 ml×3). The ether layer was separated and dried over anhydrous Na2SO4, and ether was allowed to evaporate under a stream of N2. The isolated dry products were analyzed by GC-MS. A conversion of 12% was found for DBT and the products are as the following.
This example shows that the electrochemical reduction of DBT under N2 resulted in: 12% DBT conversion after 2 h at 212° F. GC-MS revealed that the products consisted of 35% 2-phenyl benzenethiol, 8% tetrahydro-DBT, and 57% of a species with a mass of 214. The assignment of this peak as 2-phenyl benzenethiol was done by comparing with an authentic sample. The mass 214 species was tentatively assigned as 2-phenyl benzenethiol with two methyl groups added. Addition of methyl groups to DBT indicates that decomposition of solvent DMSO occurred since it is the only source of methyl groups in this system. No desulfurization product biphenyl was observed in this run.
To the glass insert was added 0.5 g dibenzothiophene (“DBT”), 3.87 g tetrabutylammonium hexafluorophosphate (TBAPF6), and 100 ml anhydrous dimethyl sulfoxide (DMSO, Aldrich). After the content was dissolved, the glass insert was loaded into the autoclave body, the autoclave head assembled and pressure tested. The autoclave was charged with 300 psig of H2 and heated to 257° F. (125° C.) with stirring (300 rpm). A voltage of 4.5 Volts was applied and the current was 1.0 Amp. The current gradually decreased with time and after three and half (3.5) hours, the run was stopped. The autoclave was opened and the content acidified with 10% HCI (50 ml). The acidified solution was then diluted with 100 ml of DI water, extracted with ether (50 ml×3). The ether layer was separated and dried over anhydrous Na2SO4, and ether was allowed to evaporate under a stream of N2. The isolated dry products were analyzed by GC-MS. A conversion of 16.5% was found for DBT and the products are as the following.
To the glass insert was added 1.0 g 4,6-diethyl dibenzothiophene (“DEDBT”), 3.87 g tetrabutylammonium hexafluorophosphate (TBAPF6), and 100 ml anhydrous dimethyl sulfoxide (DMSO, Aldrich). After the content is dissolved, the glass insert was loaded into the autoclave body, the autoclave head assembled and pressure tested. The autoclave was charged with 200 psig of H2 and heated to 100° C. with stirring (300 rpm). A voltage of 7 Volts was applied and the current was 1.0 Amp. The current gradually decreased with time and after two and half (2.5) hours, the run was stopped. The autoclave was opened and the content acidified with 10% HCl (50 ml). The acidified solution was then diluted with 100 ml of DI water, extracted with ether (50 ml×3). The ether layer was separated and dried over anhydrous Na2SO4, and ether was allowed to evaporate under a stream of N2. The isolated dry products were analyzed by GC-MS. A conversion of 16% was found for DEDBT and the products are as the following.
Similarly, desulfurization was also observed for sterically hindered Diethyl Dibenzothiophene (DEDBT) under H2. A conversion of 16% of the DEDBT was observed and the products contained 53% desulfurized compounds, 46% dihydro-DEDBT and a trace amount of tetrahydro-DEDBT. Solvent decomposition also occurs in this case. Although electrochemical desulfurization of DBT and hindered DBT has been achieved under H2 in the 212° F. to 257° F. (100° C. to 125° C.) temperature range, the conversion is still quite low. Increased conversions were attempted by extending the run time by operating within this temperature range or by running at higher temperature of about 392° F. (200° C.) to about 482° F. (250° C.).
The first example illustrates that DBT's can be readily converted into alkylated biphenyl mercaptans electrochemically without the addition of hydrogen or water. The mercaptans can be removed by caustic extraction. For example, standard MEROX® caustic treatment could be used to remove these molecules from the electro-treated LSADO producing ultra-low sulfur distillate without the need for additional hydrotreatment. Due to the low concentration of these molecules in the LSADO, the power consumption should be minimal. The comparative examples demonstrate that, electrochemical reduction in the presence of hydrogen leads to production of hydrogenated naphtheno dibenzothiophenes and not biphenyl mercaptans. These species are not caustic extractable. By limiting the availability of hydrogen sources by eliminating the hydrogen or water content, the products of the electrolysis can be controlled. The chemistry of conversion to biphenyl mercaptans and subsequent extraction processes are as follows:
A volume of 1.5 ml of a tetralin solution containing 0.1 M of 2-phenylthiopheol was placed into 3 ml stainless-steel mini-bomb inside a dry-box. The mini-bomb was heated at 400° C. in an oven for a certain period of time and the content analyzed by GC/MS. Results in Table 1 below indicate desulfurization of 2-phenylthiophenol, giving biphenyl as the major product.
A volume of 1.5 ml of a tetralin solution containing 0.1 M of 2-phenylthiopheol was placed into 3 ml stainless-steel mini-bomb inside a dry-box. The mini-bomb was heated at 375° C. in an oven for a certain period of time and the content analyzed by GC/MS. Results in Table 1 below indicate desulfurization of 2-phenylthiophenol, giving biphenyl as the major product.
A volume of 1.5 ml of a tetralin solution containing 0.1 M of 2-phenylthiopheol was placed into 3 ml stainless-steel mini-bomb inside a dry-box. The mini-bomb was heated at 350° C. in an oven for a certain period of time and the content analyzed by GC/MS. Results in Table 1 below indicate desulfurization of 2-phenylthiophenol, giving biphenyl as the major product. Based on the thermal decomposition rates at various temperatures, the activation energy for 2-phenylthiophenol thermal decomposition was determined to be ˜29.2 kcal/mol.
A volume of 1.5 ml of a tetralin solution containing 0.1 M of phenyl disulfide (PhS—SPh) was placed into 3 ml stainless-steel mini-bomb inside a dry-box. The mini-bomb was heated at 572° F. (300° C.) in an oven for 4h and the content analyzed by GC/MS. All disulfide is converted into thiophenol. By analogy, biphenyl disulfide (Ph-Ph-S—S-Ph-Ph) can be converted into 2-phenylthiophenol, which can be desulfurized at higher temperature as shown in Examples 2 through 4 herein. Equation 5 illustrates the thermal conversion of 2-phenylthiophenol to biphenyl and hydrogen sulfide.
As Examples 2 through 5 clearly demonstrate, the biphenyl mercaptan can be desulfurized by thermal treatment. This reaction could occur simultaneously with electrochemical processing if conducted at sufficiently elevated temperatures or may require a separate thermal soak step.
This Application claims the benefit of U.S. Provisional Application No. 61/008,413 filed Dec. 20, 2007.
| Number | Date | Country | |
|---|---|---|---|
| 61008413 | Dec 2007 | US |
