Patent Application
20120273393
The present invention relates generally to methods for removing sulfur from liquid fuels. More particularly, the invention includes a system and a process for removing elemental sulfur compounds from hydrocarbons including liquid fuels that have previously undergone reformate hydro-desulfurization (HDS).
The removal of sulfur from hydrocarbon fuels reduces the sulfur dioxide emissions that result from combustion of those fuels and prevents the poisoning of expensive catalysts that utilize these fuels for other purposes. In addition, the proper operation of various technologies such as fuel cells requires the use of hydrocarbon fuels with very low sulfur concentrations. Therefore a variety of methodolgies have been employed to economically and efficiently reduce sulfur concentrations in hydrocarbon fuels. Industrial hydrodesulfurization processes typically focus upon the conversion of organic sulfur to H2S and capture and removal of hydrogen sulfide (H2S) gas. However, these processes are still less than fully effective in removing all of the sulfur from these fuels. HDS treatment of high sulfur fuel can result in elemental sulfur compounds remaining in the fuel after one stage of treatment and in the case of very high sulfur contents in the fuel, contamination levels of up to 100 ppmw of elemental sulfur can still exist. This level of sulfur contamination makes the use of such fuels unsuitable in a steam reformer. While other approaches including a second HDS treatment or high levels of hydrogen relative to fuel may remove/reduce this sulfur contamination, the high pressure and temperature of HDS process makes this an expensive option. What is needed is a method or system for elemental sulfur removal that provides efficient enhanced sulfur removal capability and enables JP-8 and other kerosene based fuels to be sufficiently desulfurized so as to be used in steam reformers and other very low sulfur applications.
The present invention is a method for removing sulfur from hydrocarbon fuels by a stepwise process wherein hydrosulfurization reaction products are condensed at relatively low temperatures (between 50 and 200 degrees C.) to separate a liquid product from a H2S containing gas in the vapor phase. This liquid product contains elemental sulfur which is reacted with a hydrogen containing gas in the presence of a catalyst to react the elemental sulfur with hydrogen to form H2S which is then separated as a gas from the hydrocarbon liquid fuel. The remaining hydrocarbon liquid fuel is then sufficiently desulfurized so as to be suitably utilized in a steam reforming reaction. This low temperature desulfurization process provides an energy efficient way to remove sulfur to very low concentrations. This method is directly contrary to many of the methodologies in the prior art which reduce pressure on the reactant products following reaction to allow H2S to be separated along with hydrogen gas, remove hydrogen sulfide from the hydrogen and then recompress the hydrogen in order to recycle back to the high pressure reactor. The need for this approach is to prevent an increase in HDS residual sulfur levels associated with H2S accumulation in the recycle loop. However, this increase in residual sulfur levels is associated not with unreacted organic sulfur molecules but rather the formation of elemental sulfur molecules. The present invention, by acting contrary to the conventional wisdom in the art, obtains efficiencies and lowered sulfur levels previously unobtainable without extensive processing and cost. These systems and processes are particularly useful for desulfurization of high sulfur (e.g., ˜3000 ppm) hydrocarbon fuels (e.g., JP-8).
Depending upon the needs and necessities of the user, a variety of materials may be utilized for performing the aforementioned method steps. For example, the hydrogen containing gasses utilized for reaction with the elemental sulfur may include any of a variety of materials including, but not limited to, e.g., reformate product gases (shifted or unshifted) produced using steam reforming, autothermal reforming or partial oxidation reaction; retentate gases representing the hydrogen lean stream from a hydrogen separation process including, e.g., a hydrogen permeable membrane; or any other type of hydrogen containing gas. Preferably such gasses have lowered concentrations of water to avoid water contamination of the fuel. In addition, the catalyst utilized to bind the elemental sulfur to the hydrogen to form H2S may be any of a variety of catalysts including catalysts used to conduct the HDS reaction including NiMo and CoMo catalysts which use a molybdenum disulfide active phase as well as HDS catalysts that use a tungsten disulfide active phase, as well as a ZnO catalyst. Such catalysts may be provided as a packed bed of catalyst pellets, as catalyst coatings on random or structured packings, or any other configuration sufficient to allow the designated reactions to take place. Optimal efficiency is provided by arranging the column in a countercurrent configuration with liquid containing the elemental sulfur flowing down and hydrogen containing gas flowing up through the column. In this arrangement the hydrogen containing gas provides the dual function of enabling reaction of elemental sulfur to form H2S and of stripping the H2S into the gas phase to yield a desulfurized liquid product.
In order to maintain appropriate efficiencies, the separation of hydrogen containing gases from a liquid hydrocarbon takes place at a pressure drop of less than 10 psi. This reduces the cost of compressing the hydrogen containing gas for recycle to the HDS reactor. In mobile applications the ability to allow elemental sulfur formation in the HDS reaction enables the use of a smaller split stream of reformate for the HDS reaction resulting in increased efficiency. In other applications, use of a low hydrogen-containing gas:hydrocarbon ratio enables faster processing due to higher residence time in the reactor for a given hydrocarbon feed rate.
In one embodiment of the invention, optimum results were achieved when an unshifted reformate gas was introduced to the liquid hydrocarbon in the feed at a ratio at or below 1 standard liter of reformate gas per cubic centimeter (cm3) of the liquid hydrocarbon, preferably less than 0.02 standard liters of reformate gas per cubic centimeter (cm3) of the liquid hydrocarbon.
The method of the present invention is particularly effective on fuels such as JP-8, or other fuels that contain high concentrations of organic sulfur compounds. The ability to tolerate formation of elemental sulfur in the HDS reaction with subsequent removal downstream under less severe conditions allows the liquid HDS liquid product to be condensed and separated from the hydrogen containing gas at near full HDS pressure while the recycle of hydrogen containing gases can be accomplished with a low power compressor operating at a pressure differential of less than 10 psi. HDS reaction conditions will be feed specific but conditions of 270 psig and 380° C. were found effective for HDS of JP8 fuel. In contrast, the reaction of elemental sulfur in the liquid can occur near ambient pressure and ˜150° C. The elemental sulfur reaction can utilize lower hydrogen content gases to drive the reaction compared to the HDS reaction and if used in a recycle arrangement the H2S can be removed from the loop and the majority of the hydrogen in the loop recycled. If reformate gases are used as the hydrogen source, the H2S removal can be configured to also remove CO2 to prevent buildup of CO2 in the recycle loop.
In various configurations, alterations to the basic method can be had. In particular, for a mobile system in which a hydrogen recycle on the HDS reactor is not implemented, the hydrogen-containing gas may be routed first to the reactor where elemental sulfur is converted to H2S and then carry this H2S into the HDS reactor, eliminating the need for a second hydrogen containing gas. The contacting step in which the elemental sulfur is reacted may include sparging the hydrogen-containing gas through the liquid hydrocarbon while the liquid hydrocarbon is in contact with the catalyst. The contacting step may also be performed in a countercurrent stripping column that is packed with the catalyst in a pelletized form so as to be in contact with the liquid hydrocarbon introduced to the column, wherein the liquid hydrocarbon flows down and the hydrogen-containing gas flows up. The countercurrent stripping column may be loaded with a packing (random or structured) coated with the catalyst such that the liquid hydrocarbon is in contact with the catalyst when introduced to the column. The reaction of elemental sulfur compounds in the hydrocarbon preferably occurs at a temperature between 50° C. and 150° C., a pressure between about 1 atmospheres (atm) absolute and 4 atmospheres (atm) absolute, and for a time sufficient to convert the elemental sulfur to H2S.
In one embodiment of the invention, a dehydrodesulfurization (HDS) treated reaction product is condensed to form a liquid, this liquid is then reacted in the presence of a dehydrodesulfurization (HDS) catalyst and a hydrogen containing gas such as reformate to convert elemental sulfur compounds in the liquid to H2S. The reaction is preferably performed at temperatures below about 200° C., preferably in the range from about 50° C. to about 150° C., well below those employed in typical HDS reactors.
In one embodiment, (i.e., hydrogen-rich) reformate gas containing a preselected minimum partial pressure of hydrogen is delivered (e.g., bubbled) through the liquid phase reformate fuel at a preselected flow rate, reacting with the elemental sulfur compounds present in the HDS reformate to form H2S gas, leaving a low (e.g., sub-ppm) residual sulfur concentration in the condensed phase product. In other embodiments, the hydrogen-containing gas is a shift gas, or a retentate gas. In one embodiment, reformate fuel in the feed exiting the HDS reactor is also cooled to, e.g., ˜150° C. However, any temperature that condenses the reformate fuel to a liquid can be used. Preferred temperatures are in the range from 50° C. to about 150° C.
Once the condensed reformate fuel is separated from the hydrogen-rich reformate gas, the hydrogen-rich reformate gas is then used to accomplish the subsequent removal of elemental sulfur compounds present in the initial reformate fuel product. In one embodiment, the condensed liquid reformate fuel is contacted with an HDS catalyst (e.g., NiMo), while being sparged with a supply of hydrogen-rich gas. In a preferred embodiment, the hydrogen-rich gas is reformate gas collected from the HDS reactor earlier in the treatment process. The sparging of the reformats gas at a preselected rate into the condensed liquid reformats fuel product, reacts in the presence of the catalyst to convert elemental sulfur compounds present in the fuel to H2S gas.
The purpose of the foregoing abstract is to enable the United States Patent and Trademark Office and the public generally, especially scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.
Various advantages and novel features of the present invention are described herein and will become further readily apparent to those skilled in this art from the following detailed description. In the preceding and following descriptions, the preferred embodiment of the invention is shown and described by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the drawings and following descriptions of the embodiments of the present invention set forth hereafter should be seen as illustrative and not limiting in any way. Additional advantages and novel features of the present invention will be set forth as follows and will be readily apparent from the descriptions and demonstrations set forth herein. A more complete appreciation of the invention will be readily obtained by reference to the following description of the accompanying drawings in which like numerals in different figures represent the same structures or elements.
The following description includes the preferred modes of various embodiments of the present invention. It will be clear from this description of the invention that the invention is not limited to these illustrated embodiments but that the invention also includes a variety of modifications and embodiments thereto. Therefore the present description should be seen as illustrative and not limiting. While the invention is susceptible of various modifications and alternative constructions, it should be understood, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims.
To maintain pressure in the system, the gas phase initially separated from the liquid following the reduction in pressure is compressed back to HDS pressure, and then combined with additional high pressure hydrogen. In conventional processing, H2S is presumed to act against the desulfurization of the product, therefore it is removed from the recycled gases prior to compression back to HDS pressure. The aim of this entire system is to attempt to keep all sulfur in the H2S phase and avoid the formation of any elemental sulfur. The present invention acts in direct contradiction to these prior art principles, recirculating H2S containing gas and driving the formation of elemental sulfur to be subsequently reacted with a hydrogen containing gas for removal.
The invention in various embodiments includes systems and methods for reduced-temperature removal of elemental sulfur compounds from condensed hydrocarbon reformate fuels (e.g., JP-8) obtained after HDS desulfurization of the starting feedstock (i.e., untreated) fuels. The term “HDS product” as used herein refers to an HDS treated reactor product obtained from an original sulfur-contaminated hydrocarbon fuel (e.g., JP-8). When desulfurizing high (>1500 ppm up to about 3000 ppm) sulfur fuels in a hydrodesulfurization (HDS) reformate process, elemental sulfur compounds can form in the HDS reaction vessel. That HDS treated fuels contain elemental sulfur compounds formed during the HDS process has not been previously reported or recognized in the prior art. However, chemiluminescent GC tests for sulfur show these elemental sulfur compounds to be absent in the starting feedstock fuels. However, when the HDS reformate product fuel is condensed, elemental sulfur compounds are dissolved in the fuel. At typical processing conditions, these elemental sulfur compounds can provide 100 ppm of sulfur contamination when processing high sulfur fuels (up to 3000 ppmw), which is sufficient to require a second desulfurization treatment of the hydrocarbon.
The hydrogen and H2S gas are recycled through the condenser, reheated and returned to the HDS reactor while the gases are maintained near the reaction pressure (e.g., 270 psig). This eliminates energy costs associated with compressing the hydrogen recycle stream, as a differential pressure of less than 10 psi is maintained. As shown in the figure, liquid reformate fuel is heated to temperature in a heat exchanger and heater just prior to entry into HDS reactor. Make-up of the hydrogen supply to the high pressure circulation loop need only be sufficient to satisfy the hydrogen consumed in saturating hydrocarbons and to replace hydrogen that leaves as dissolved hydrogen or dissolved H2S gas in the feed. The fraction of sulfur leaving in elemental form does not carry additional high pressure hydrogen from the system.
Low pressure hydrogen in contact with elemental sulfur containing liquid and the catalyst converts elemental sulfur back to H2S and this H2S can be carried with the hydrogen containing gas from the low pressure elemental sulfur reactor and then captured by an amine scrubber or other type of H2S capture device. The reaction of the elemental sulfur is preferably but not exclusively done via counter-current contact with hydrogen on a packing coated with an HDS catalyst, e.g., a nickel-molybdenum (NiMo) catalyst. Elemental sulfur compounds in the hydrocarbon liquid are converted to H2S gas by reaction with the catalyst at a low pressure of about 1 atm and a preferred temperature of about 150° C. on average, which is not limited. The ultimately desulfurized fuel product is removed, e.g., from the bottom of low-pressure elemental sulfur reactor. The sub-ppm sulfur level provided by this flowsheet means that the final fuel product can be steam-reformed, e.g., for use in fuel cell Auxiliary Power Units (APUs). This flowsheet is less energy intensive to remove H2S than that for distillation or steam stripping of the fuel.
In the particular embodiment shown in
The fuel is then passed to vapor liquid separation stage where the fuel mixture now having a lower temperature and pressure (e.g., ˜150° C. and a pressure of 40 psi) can be separated into a gas and a liquid phase. In such an arrangement, water vapor leaves in the gas phase and is not retained in the liquid fuel. Hence, condensation of reformate water becomes optional, or can be omitted as a step in the flowsheet while still avoiding a need for fuel/water separation following condensation. After separation, the liquid fuel is passed through a low pressure reactor where the elemental sulfur is reacted with a hydrogen containing gas in the presence of an HDS catalyst at a temperature of about 150° C. H2S is removed from the reactor in the gaseous phase and the liquid fuel is cooled and sent to a clean fuel tank. In a system using a palladium membrane to purify hydrogen for a fuel cell, the membrane retentate stream containing residual hydrogen may provide hydrogen for reaction with elemental sulfur to produce H2S and strip the H2S from solution. The H2S containing gases from the vapor-liquid separation and the reaction of elemental sulfur are routed to a combustor to recover fuel value. In embodiments such as mobile systems where H2S containing gas is combusted, temperatures downstream of the burner are maintained above the sulfuric acid dew point, typically 125° C. to 200° C., to avoid corrosion.
HDS systems and flowsheets according to these embodiments provide various routes for sulfur removal. Some of these eliminate the need for solid sorbents such as ZnO to remove H2S gas. Tests performed in conjunction with the invention have shown removal of elemental sulfur compounds from previously HDS desulfurized fuels to sub-ppm concentration levels suitable for various domestic and military uses. In other embodiments, retentate gas from a hydrogen permeating membrane can be used as an alternative to reformate gas. Hydrogen content of retentate gases is lower than for reformate gases, but is usually sufficient for use as a hydrogen-containing gas for reaction with elemental sulfur in the various flowsheets presented herein.
In this configuration a hydrogen-containing reformate gas is used to react with elemental sulfur contained in a reformate HDS product. The reaction is performed over ZnO which catalyzes the reaction to form H2S and the reacts with H2S to immobilize the sulfur as ZnS. The reformate and desulfurized product then pass through a ˜ambient temperature condenser and the reformate is separated from the liquid desulfurized fuel product. With residual H2S removed, the hydrogen-containing reformate gas is largely composed of hydrogen (H2) and other residual gases including, e.g., H2O and CO2. In this configuration the hydrogen-containing reformate gas is then routed to the HDS reactor at a suitable HDS reactor temperature by first passing through a heat exchanger. The reformate gas is then reacted with incoming fuel in the HDS reactor, condensed and separated into an H2S containing gas stream which is burned and a liquid fuel containing elemental sulfur which is preheated and then reacted over ZnO to remove elemental sulfur as described previously and then condensed in an ˜ambient temperature condenser. The desulfurized fuel product exiting, e.g., from the bottom of condenser is suitable for use as a liquid fuel where sub-ppm sulfur levels are required.
Embodiments of the invention described herein have the advantage that the majority of the H2S formed in the HDS reactor is removed prior to contact with solid desulfurization sorbents that remove elemental sulfur. These processes increase the lifetime of the ZnO sorbent used to remove residual H2S. When processing high sulfur feed containing 3000 ppm sulfur, 100 ppm of elemental sulfur may remain in the fuel following the HDS reaction step. This represents a 97% reduction in the sulfur that must be adsorbed by the sorbent. Other advantages include: 1) Savings in H2 compression costs for the HDS reactor due to the elimination of the requirement to drop the mixture pressure to release H2S; 2) Additional savings on H2 compression costs due to the fraction of sulfur leaving the high pressure section as elemental sulfur rather than H2S; 3) Relaxation of H2:liquid feed ratios to the HDS reactor since formation of elemental sulfur compounds do not degrade performance because the present invention provides a way to remove the elemental sulfur from the system. 4) Ability to desulfurize very high sulfur-containing (>3000 ppm) streams to very low sulfur levels (sub-ppm) in a single HDS reaction step since the performance in a single stage is not limited by H2S buildup and subsequent formation of elemental sulfur compounds; 5) Ability to directly use reformate or low content hydrogen streams at a low pressure of about 1 atm to form the H2S rather than pure hydrogen at high pressure; and 6) a potential reduction in energy usage is achieved when H2S is removed as part of the elemental sulfur reaction via stripping by recirculated H2, making a subsequent steam stripping or distillation of the desulfurized fuel unnecessary.
The following example provides a further understanding of the invention in one or more aspects.
100 mL of an HDS-treated JP-8 fuel initially containing a high concentration of sulfur (˜3000 ppm) prior to HDS treatment at a pressure of 250 psig and a temperature of 360° C. was added to a sealed round bottom flask. The reformate fuel contained ˜68 ppm elemental sulfur as determined from elemental sulfur peaks (peak areas reported in TABLE 1). Fuel in the flask was mechanically stirred. Reformats gas was sparged into the reformate fuel in the flask at a rate of 100 cc per minute (ccm). An initial sample (Sample 1) was collected ˜19 minutes before catalyst addition (labeled “−15 minutes” in TABLE 1). Fuel in the flask was heated to a temperature of 150° C. and maintained. 10 g of HDS catalyst taken from the first of three reaction cylinders of an HDS test reactor which had been operated for ˜300 hrs was added to the flask. The catalyst was not size-reduced prior to addition to the vessel.
Sample 2 was taken within ˜30 seconds of catalyst addition. Sample 3 was taken 10 minutes after addition of the catalyst. Sample 4 was taken 30 minutes after addition of the catalyst. Results show treatment to remove elemental sulfur compounds from the reformate fuel liquid to be very effective. Test data also shows the reaction to be very rapid, with nearly complete removal of elemental sulfur occurring within the first 30 seconds of contact with the catalyst. One elemental sulfur compound identified by combined GC/MS analysis has been positively identified as cyclo-octo sulfur (i.e., S8), a principal elemental sulfur compound. Behavior of other peaks in the chromatograms (also elemental sulfur compounds) correlate with those observed for S8. Volatile compound peaks and other light gas peaks with retention times between 1.3 minutes and 1.6 minutes dissipate upon sparging with the reformate or other gases. H2S gas, initially absent, increases at 0 min and holds nearly steady at 10 minutes and then dissipates to very low levels. This observation correlates with the conversion of elemental sulfur compounds to H2S gas, which is subsequently stripped from solution, and correlates with the observed disappearance from the chromatograms.
In another test, very low reformate:fuel ratios on a 10 kWe scale were tested for efficacy in view of the methodology described. In these tests, the ratio of reformate gas to liquid fuel feed in HDS reactor (in reaction step 1 for conversion of organic sulfur) was between about 1 standard liter of gas per cubic centimeter (cm3) of the liquid hydrocarbon and about 0.02 standard liters of gas per cm3 of the liquid hydrocarbon. Feed rates were: 7 (slpm): 60 (ccm) and 0.33 (slpm): 15 (ccm), respectively. The HDS reactor was followed immediately by a ZnO bed which facilitated the conversion of elemental sulfur to H2S and the reaction of H2S to form ZnS. Resulting sulfur levels in the liquid fuels for these tests were <0.1 ppm. Even lower gas:liquid ratios can be expected by optimization of the system parameters.
While preferred embodiments of the present invention have been shown and described, it will be apparent to those of ordinary skill in the art that many changes and modifications may be made without departing from the invention in its true scope and broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the spirit and scope of the invention.
This invention was made with United States Government support under Contract No.: W56HZV-06-C-0343 with the U.S. Army Tank Automotive Research, Development and Engineering Center (TARDEC) awarded to Battelle Memorial Institute. The Government has certain rights in the invention.
