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High-Pressure H2S Conversion and SO2 Production over γ-Al2O3/FeχS(OH) in Liquid Sulfur Michael A. Shields* and Peter D. Clark Alberta Sulphur Research Ltd., Department of Chemistry, UniVersity of Calgary, #6 - 3535 Research Road N.W. Calgary, Alberta, Canada T2L 2K8
A stainless steel autoclave was used to investigate high-pressure catalytic H2S conversion to sulfur in a liquidsulfur medium. Using the fundamental chemistry developed in earlier low-pressure conversion studies, the objective of this work was to investigate the use of high pressures for increasing overall H2S conversion to elemental sulfur. Using 1.55% H2S (balance N2), the H2S scavenging activity of fresh FeO(OH) was found to increase with increasing operating pressures. Reaction of 1.55% H2S with 0.80% O2 (balance N2) over FeχS(OH) resulted in 87% H2S conversion to elemental sulfur at a reactor operating pressure of 6984 kPa (1000 psig). Incorporating dual-catalyst reaction conditions (γ-Al2O3/FeχS(OH)), 99.6% H2S conversion to elemental sulfur was attained using a feed gas containing 1.33% H2S/1.16% O2 (balance N2) and a reactor operating pressure of 1813 kPa (250 psig). It is suggested that SO2 is generated by the iron catalyst and that H2S conversion occurs by the reaction of H2S with SO2, primarily over γ-Al2O3. A concurrent SO2 kinetics investigation demonstrated very high SO2 production activity over FeχS(OH) as compared to γ-Al2O3. Finally, the dual-catalyst regime proved capable of maintaining high H2S conversions to elemental sulfur under higher H2S-content feed-gas conditions. Overall, higher operating pressures proved to be effective for increasing the total H2S conversion to elemental sulfur. 1. Introduction H2S removal from hydrocarbon gases is a critical step in bringing such feedstocks to the market. With H2S being toxic, unpleasantly odorous, and corrosive, capture and conversion of the H2S is very important. Construction of a Claus sulfur recovery system is justified for large-tonnage sulfur recovery applications, but, for low-content H2S gases, a simpler system is needed. Examples of low-tonnage technology include H2S scavenging and adsorption technologies1-5 as well as gas-phase H2S catalysis (partial oxidation)6-8 and aqueous redox chemistry. Although most of these methods generate very high H2S removal from the gas streams, they have the disadvantage of high capital and operating expenditures. Most notably, scavenging and adsorption methods require regeneration, or waste disposal and reloading of fresh material. Catalytic partial oxidation of H2S to elemental sulfur has the potential to eliminate such steps because continuous processing of the feed gas is possible. For this reason, investigations into new and improved low-tonnage sulfur recovery catalytic processes have received considerable attention in recent years. Catalyzed H2S direct oxidation is a field that has been extensively studied at both sub-dew-point and above dew-point sulfur temperatures using a variety of different catalytic materials, and either O2 or SO2 (eqs 1, 2).
H2S + 1/2O2 f 1/8S8 + H2O
(1)
2H2S + SO2 h 3/8S8 + 2H2O
(2)
The catalysts include a variety of coprecipitated mixed-metal oxides9-14 (many being alumina, silica, vanadia, or titania metal oxides doped with other promoter transition metals and rare earth metals), various activated carbons,15,16 and other unsup* To whom correspondence should be addressed. E-mail: mshields@ ucalgary.ca.
ported catalytic materials. On a laboratory scale, many of these catalysts have proven to be very effective toward H2S conversion using either O2 or SO2 in the feed gas. Not only have high conversions been obtained but mechanistic insights and deactivation effects in many cases have been quite well understood. Unfortunately, very few of these sophisticated catalyst formulations are yet to find their way into industrial practice. γ-Al2O3 is the most common catalyst used industrially for promoting H2S/SO2 conversion to elemental sulfur in Claus plants. As such, the nature of the γ-Al2O3 catalytic surface has been studied with respect to surface acidity/basicity,17 H2S/SO2 adsorption studies, kinetic measurements, catalytic mechanism investigations, and surface deactivation studies (i.e., sulfation).18-20 Physical investigations include macro- and microporosity studies, and effect of pore volume on H2S conversion at sulfur subdew-point temperatures.21 Other work has examined unsupported iron oxide and iron sulfide for the conversion of H2S to elemental sulfur using O2. Although many supported iron oxides (e.g., iron on Al2O3 or SiO2),22-24 mixed-metal iron oxides,25,26 and aqueous iron-based redox systems27 have proven to be effective for promoting H2S conversion to elemental sulfur, unsupported iron oxide has not been used industrially. Fe2O3 is known to react with H2S to produce Fe2S3, as well as FeS2, FeS, and Fe3S4, depending on the temperature maintained during the reaction.28,29 Mechanistically, the direct sulfidation of bulk R-Fe2O3 to iron sulfides has been shown to occur without preceding reduction steps to Fe3O4 or FeO.30 The sulfidation reaction first proceeded through an O-S exchange reaction rather than through reduction of the iron species to Fe3O4 or FeO. Temperature-programmed sulfidation data did not exclude the fact that the eventual reduction of the iron species from Fe3+ to Fe2+ occurred via iron (oxy)sulfides and not only R-Fe2O3. Alumina-supported iron and iron-molybdenum oxide catalysts have also been investigated for sulfidation susceptibility.31 Contrary to the complete sulfidation of unsupported R-Fe2O3, alumina-supported iron oxide was shown to exhibit only partial sulfidation of the iron phase
10.1021/ie071560a CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008
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on the catalyst. For this catalytic material, the relative amount of iron species that could be sulfided was shown to increase with the iron loading. This observation was a direct result of reduced iron diffusivity into alumina under higher iron loadings, thus resulting in a higher concentration of surface iron atoms. This reduced diffusivity of iron into the γ-Al2O3 support was also noted in the presence of molybdenum, resulting in the reduced sulfidibility of the iron-molybdenum oxide catalyst. Compared to unsupported R-Fe2O3, alumina-supported iron and iron-molybdenum oxide catalysts showed a higher resistivity to core sulfidation. This characteristic of alumina-supported iron catalysts for H2S conversion has made these materials very attractive for further scientific and industrial investigations. On the other hand, the facile sulfidation of unsupported iron oxide demonstrates why this material has traditionally been thought of as being catalytically ineffective for prolonged H2S conversion. Although iron sulfides have been used in some catalytic processes, none of these investigations have pertained to H2S conversion. Liquid elemental sulfur has long been viewed as being a roadblock for continuous H2S catalysis under atmospheric conditions because of mass transfer limitations caused by the formation of liquid sulfur in the catalyst pores.32 The initial twophase system (gas-solid) becomes a three-phase system (gasliquid-solid) with the product liquid sulfur eventually limiting the mass transfer of the gas-phase reactants to the active sites on the solid surface. At this point, the elemental sulfur must be evaporated from the pores of the catalyst by raising the temperature of the reactor, restoring the original catalyst surface. Although liquid sulfur introduces mass transfer considerations within the reactor, dispersal of a catalyst in liquid sulfur provides some advantages for a low-tonnage sulfur recovery process. First, liquid sulfur provides a large thermal mass capable of absorbing the heat of the reaction associated with H2S conversion. Second, H2S is soluble in liquid sulfur (existing in equilibrium as dissolved H2S and H2Sχ via a reaction with the elemental sulfur), allowing for an increased uptake of H2S. Lastly, product sulfur can be collected by allowing the catalyst to settle to the bottom of an undisturbed sulfur bed. Although sulfur quality would need to be assessed, such a system would prevent the high-temperature, regeneration step currently needed to remove the sulfur from the pores of the catalyst in gas-phase processes. Studies on the catalytic conversion of H2S and SO2 in liquid sulfur under atmospheric conditions and elevated pressure were first carried out by Kerr and Jagodzinski.33,34 Industry convention at that time (as well as today) highlighted the fact that poisoning of the catalyst pores with liquid sulfur was a major concern, reducing H2S conversion and preventing continual operation. However, high equilibrium yields of elemental sulfur at lower temperatures and higher pressures make sub-dew-point operation attractive. With this consideration, Kerr and Jagodzinski predicted that elevated pressures would be enough to maintain high H2S/SO2 conversions at sulfur sub-dew-point temperatures. Using a slurry reactor assembly (160 °C), a lean process gas mixture (2.5% H2S/1.2% SO2) was fed through an aluminacontaining bed of liquid sulfur. H2S conversion, although moderate at low pressure, was virtually complete at higher pressures. This was the first documented case where a Claus reactor operating at sub-dew-point temperatures could maintain high H2S conversions in the presence of liquid sulfur. From this work, the design was patented as the Richards sulfur recovery process (the RSRP process).34 To our knowledge,
however, there has been no follow-up work, through either further scientific studies or commercial application. These studies were followed in the mid-1990s with work by Jacobs Nederland B.V., who performed a series of H2S conversion experiments over various commercial Claus catalysts using a stirred-tank reactor containing liquid sulfur.35 The objective of the study was to investigate a liquid-sulfur stage that could be placed downstream of their selective oxidation Superclaus unit to improve overall H2S conversion to elemental sulfur. Preliminary experiments were performed in a batch autoclave, measuring catalytic H2S conversion in liquid sulfur via a reaction with either SO2 or O2. Control experiments demonstrated the requirement of a catalyst to attain any degree of conversion. Adding various catalysts to the reactor resulted in substantial H2S conversions. Further work using a continuous flow of reactant gases through a Robinson-Mahoney stirredtank reactor showed high catalytic conversions of H2S and SO2, sometimes in excess of the conversions that could be achieved in commercial Claus plants. These laboratory results prompted Jacobs Nederland B.V. to construct a pilot plant to investigate the industrial feasibility of the process (although these studies were limited to ambient pressure). Conversions in excess of 80% were achieved but only at gas residence times >40 s, which were judged to be too long for industrial application. The project did, however, highlight the advantage of using liquid sulfur for a catalytic H2S conversion process. Recent work in our laboratories has focused on the use of unsupported iron oxide (FeO(OH)) in liquid sulfur for the catalytic direct oxidation of H2S to elemental sulfur.36 The fresh FeO(OH) used in our study was shown to be an excellent H2S scavenger in liquid sulfur but had only a low activity for H2S conversion. As a result of the H2S scavenging activity, the steady-state catalytic material was partially sulfided and was characterized as FeχS(OH). Interestingly, the FeχS(OH) displayed a strong propensity to be able to catalyze the oxidation of the liquid sulfur to SO2 (eq 3). 1
FexS(OH)
/8S8 + O2 98 SO2
(3)
Such an observation is interesting in that no information exists regarding low-temperature SO2 production via the catalytic oxidation of liquid sulfur. Instead, other work has demonstrated liquid-sulfur inertness under H2S conversion conditions using a different catalyst surface and composition.37 The formation of in situ SO2 is useful in that it allows the development of new sulfur-recovery processes. In terms of H2S conversion to elemental sulfur, high conversions were obtained using a dualcatalyst scheme incorporating 1 wt % γ-Al2O3/ 5 wt % FeχS(OH) into the reactor.36 Although some direct oxidation activity was observed (eq 4), bulk H2S conversion to elemental sulfur was achieved through the γ-Al2O3 using the in situ generated SO2 (by the FeχS(OH)) to convert the residual H2S to elemental sulfur via the Claus reaction (eq 5). FexS(OH)/Al2O3
H2S + 1/2O2 98 1/8S8 + H2O
(4)
With the Claus mechanism being thermodynamically favored
at lower temperatures and higher pressures, 97% H2S conversion to elemental sulfur was obtained. Considering the conclusions and mechanistic insights generated from our preliminary work,
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future work incorporating the dual-catalyst H2S conversion concept in liquid sulfur was warranted. Increasing reactor operating pressures would be predicted to have a positive effect on the overall H2S conversion to elemental sulfur if the overall rate was diffusion controlled. In a threephase heterogeneous catalytic system (gas-liquid-solid), reactant gases must dissolve in the liquid medium before adsorbing to the solid catalytic surface in order to react. Increasing the concentration of the reactants within the liquid phase would increase the adsorption probability on the surface, thus increasing catalytic activity. Increasing reaction operating pressure is a way to increase reactant concentration in the liquid phase, which, in turn, Henry’s Law states that the partial pressure (pi) of a reactant gas is directly proportional to the concentration (ci) of that reactant dissolved in a given solvent (eq 6).38
pi ) kci
(6)
(where Henry’s constant (k) depends on solvent and temperature). A thermodynamic equilibrium between the gas phase and the liquid phase, however, is a necessity for the law to be valid. Although steady-state reaction kinetics exists for each data set, establishment of a thermodynamic equilibrium remains uncertain. Trending toward that equilibrium, however, would predict that increasing total gas pressure within the reactor would increase the concentration of the reactants in the liquid. Incorporating the prediction from Henry’s Law with Fick’s First Law highlights how catalytic activity improves with pressure.39 Mass transfer in the liquid-sulfur reactor refers specifically to the transport of H2S and O2 from the feed gas, through the liquid sulfur, to the solid catalyst surface. The driving force is diffusion from the high concentration (partial pressure) H2S/O2 in the feed gas to the lower concentration in the liquid film on the catalyst surface. Fick’s First Law defines the flux (J) of the reactant transfer in the liquid through the change in reactant concentration per change in distance (dθ/dχ) (eq 7).
J ) -D(dθ/dχ)
(7)
(where the diffusion coefficient (D) depends on temperature, viscosity and particle size). In a steady-state diffusion condition (assuming a perfectly mixed, stirred-tank reactor), the flux into the liquid at the gasliquid interface equals the flux out of the liquid at the liquidsolid interface. Assuming that minimal external diffusion resistance within the liquid sulfur exists in a stirred-tank reactor, increasing the flux (rate of mass transfer) can be achieved by increasing the concentration difference of the reactants between the gas phase and the solid catalytic surface. Higher pressures in the gas headspace would therefore increase the flux through the liquid sulfur, thus increasing catalytic activity. In this work, high-pressure H2S conversion was investigated using a stainless steel stirred-tank reactor with the aim of obtaining higher H2S conversions to elemental sulfur than those previously achieved under low operating pressures. Highpressure H2S scavenging was investigated using fresh FeO(OH), with subsequent steady-state catalytic activity of the FeχS(OH) determined at reactor pressures up to 6984 kPa (1000 psig) when exposed to a stoichiometric 2:1 H2S/O2 (balance N2) feed gas. From these data, the effect of pressure was highlighted through a comparison of uncatalyzed and catalyzed high-pressure H2S conversion. To improve overall H2S conversion to elemental sulfur, a dual-catalyst regime (1 wt % γ-Al2O3/ 5 wt % FeχS(OH)) was tested using feed gases containing H2S/O2/balance
N2 and H2S/SO2/balance N2. Feed-gas variability was then investigated by measuring catalytic H2S conversion to elemental sulfur using higher concentration feed gases. Finally, SO2 production kinetics from liquid-sulfur oxidation were quantified, comparing the activity of FeχS(OH) to the activity of γ-Al2O3. Overall, the primary objective of this work was to determine a minimum reactor operating pressure at which H2S conversion to elemental sulfur exceeds 99% under dual-catalyst experimental conditions. 2. Experimental Section 2.1. Safety Precautions. H2S and SO2 are odorous and extremely toxic. As a result, all of the reaction vessels and gas delivery lines were pressure tested well above any operating pressures outlined in this article. Main gas cylinders were stored in a separate gas storage room, equipped with hard-wired gas detectors linked to automatic shut-off valves. In addition, the complete reactor vessels were also housed in workspace enclosures fitted with H2S detectors wired to an automatic shutdown system to prevent H2S concentrations exceeding 10 ppm in the workspace. 2.2. Reactor Systems, High-Pressure Autoclave. Highpressure H2S conversion data were obtained using a stainless steel stirred-tank reactor. The autoclave used in the study consisted of two individual parts, the upper steel cap and the lower steel body of the autoclave (maximum volume capacity ) 0.30 L). Securing the cap to the autoclave body was achieved by bolting the two parts together using a torque wrench. A steel O-ring was inserted between the lid and the body prior to tightening the bolts, which acted to safely seal the contents of the reactor (gases, liquids, and solids) from the atmosphere. The reactor was housed in an autoclave stand, which was bolted securely to the floor of the experimental workspace. Once inside the fitted housing, the autoclave was bolted at the bottom to the platform and encased by a ceramic furnace, which was used to maintain temperature control. Stirring was achieved in the system using a magnetically driven internal stirrer. Attached to the internal magnet was a hollow-shaft gassing turbine with a four-bladed impeller. In addition to the stirrer, a three-bladed baffling system was inserted within the reactor to inhibit vortex formation and aid in mass transfer, gas recirculation, and mixing. Pressure was monitored upstream, within, and downstream of the reactor. Inlet feed gases entered the vessel through the inlet dip tube (submerged within the liquid-sulfur bed), with gases exiting the reactor via the outlet port on the cap of the autoclave. Outlet gases were scrubbed (using KOH) to remove any residual H2S from the gas stream. The outlet gas lines (as well as the top of the autoclave) were traced with rope heaters and maintained at a temperature of 140 °C to avoid any deposition and solidification of sulfur vapor exiting the reactor. The catalyst-containing sulfur bed was prepared following the structural assembly of the high-pressure autoclave. For all of the FeO(OH)-only experiments, the reaction mixture consisted of 250 g liquid elemental sulfur with 5 wt % FeO(OH) (12.5 g). Following FeO(OH) sulfidation, the catalyst content for the dual-catalyst experiments was 1 wt % γ-Al2O3 (2.5 g)/5 wt % FeχS(OH) (12.5 g original FeO(OH)). All of the reactor slurry mixtures were prepared at 135 °C by melting 250 g of solid sulfur and then subsequently adding the appropriate catalytic materials. The temperature of the reactor was maintained using an electric ceramic heater furnace controlled through a CAL Control 9400 temperature controller (Cole Parmer). A thermocouple immersed in the liquid-sulfur bed was used to monitor the temperature inside the reactor and maintain precise temperature control.
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2.3. Reactor Systems, Glass Autoclave. SO2 production kinetic experiments were performed using a Buchi stirred laboratory autoclave system. The reactor was a cylindrical glass vessel with a hemispherical bottom and an external heating jacket for controlling internal reactor temperatures. A stainless steel collar around the upper neck of the autoclave was used to bolt the reactor to the upper steel face-plate of the tripod assembly. Bolted tightly, Teflon gaskets between the glass and the stainless steel securely sealed the inside of the vessel. The tripod structure surrounding the glass autoclave was encased by clear protective shields that allow the reactor contents to be observed. Stirring was achieved using a Buchi magnetic drive system. Attached to the internal magnet of the drive, a hollowshaft gassing turbine was used to mix the gases with the contents of the reactor. A single baffle was used within the autoclave to improve mixing of the reactor contents. In addition to the stirring motor, the steel platform provided fittings for the inlet-feedgas dip tube, a thermocouple for digital temperature readouts from the sulfur bed, an emergency relief valve, and the outlet valve from the reactor. The catalyst-containing sulfur bed was prepared following the structural assembly of the glass autoclave. For all of the kinetic experiments, the reaction mixture consisted of 800 g of liquid elemental sulfur with a 1 wt % catalyst content (either FeO(OH) prior to sulfidation (FeχS(OH)) or γ-Al2O3). All of the reactor slurry mixtures were prepared at 135 °C by melting 800 g of solid sulfur to liquid and then subsequently adding the appropriate catalytic material. The temperature of the reactor was controlled using a Fisher Scientific Isotemp heating circulator by flowing heated silicon oil through the reactor jacket. A thermocouple was then used to independently measure the temperature directly within the liquid-sulfur bed. 2.4. Catalyst Preparation. The iron oxide used in the study was prepared using a wet precipitation method. Fe2(SO4)3‚nH2O (25 g) (Sigma Aldrich) was dissolved in 1.5 L of deionized water and was heated on a hot plate to 70 °C under continuous stirring. Using a peristaltic pump, a 12.5% NH4OH solution was added to the Fe2(SO4)3 solution at a flow rate of 0.6 mL/min. The pH of the solution was monitored continuously as the iron oxide precipitate was formed. After 5-6 h, the precipitation reaction was complete (solution pH 8-9), and solution pH was adjusted to 7.1 using 2 M H2SO4. The iron oxide precipitate was then vacuum-filtered and washed with 1.5 L of deionized water (70 °C). The wet precipitate was collected in a glass dish and was placed in a vacuum oven to dry at 100 °C for 16 h. The overall process was repeated to accumulate the amount needed for the experiments. Prior to being added to the reactor, the fresh iron oxide was shaken through a 45 µm sieve to ensure size uniformity of the particles. The γ-Al2O3 used in the dual catalyst study was a commercial-grade DD-431 catalyst from Alcoa Industrial Chemicals (now BASF). The catalyst (3/16 in. spherical pellets) was crushed and shaken through a 45 µm sieve to ensure particle uniformity with the iron oxide already contained within the liquid sulfur. 2.5. Experimental Procedure, High-Pressure Autoclave. Reactant gases were delivered to the high-pressure stirred-tank reactor by calibrated mass-flow controllers (Advanced Specialty Gases). Feedstock gases were premixed and purchased from Praxair (2% H2S/balance N2, 5% O2/balance N2, 2.5% SO2/ balance N2, 10% H2S/balance N2 and compressed medical air). Downstream of the mass-flow controllers, reactant gases were mixed and fed through the inlet dip tube of the reactor (which was submerged in the bed of liquid sulfur). The stirring rate for the gassing turbine was held at 2000 rpm, the temperature
was maintained at 135 °C during each run, and the reactor pressure was varied between 89 and 6984 kPa absolute (0 and 1000 psig) under each steady-state analysis condition. To initiate the target pressure for each experimental run, bypass valves were installed in the upstream plumbing of the system. When open, these valves direct the gas flow to bypass the mass-flow controllers and go directly into the reactor from the high-pressure gas regulator on the main gas cylinder. With the bypass valves open, the regulator was dialed up to produce the target pressure for each experimental run. With the entire system at the desired pressure, the bypass valves were closed and the mass-flow controllers were set to deliver the appropriate gas flow with the appropriate back-pressure behind the controller. For all of the experiments in this study, the system was operated under continuous gas flow with a total gas-flow rate of 80 mL/min. With respect to the high-pressure autoclave, this corresponded to a gas residence time of 105 s for each experiment. Total flow rates were verified (at each experimental reactor pressure) downstream of the pressure choke valve using a rotameter on the outlet gas stream. Because the aim of the study was to evaluate conversion of H2S in low H2S-content gas streams, the stoichiometric (2:1 H2S/O2) feed-gas ratio was 1.60% H2S/ 0.80% O2 (balance N2), although other inlet gas mixtures were also used. Examples of these include a non-stoichiometric lowlevel H2S/O2 feed gas, a H2S/SO2 (balance N2) feed gas, and a higher H2S/O2-content feed gas. 2.6. Experimental Procedure, Glass Autoclave. Reactant gases for the SO2 production kinetic experiments were delivered into the glass autoclave using calibrated mass-flow controllers (Advanced Specialty Gases). Feedstock gases were premixed and purchased from Praxair (5% O2/balance N2 and 100% N2). Downstream of the mass-flow controllers, reactant gases were mixed and fed through the inlet dip tube of the reactor (which was submerged in the bed of liquid sulfur). The stirring rate for the gassing turbine was held at 1000 rpm, the temperature was maintained at 135 °C during each run, and the reactor pressure was held constant at 296 kPa absolute (30 psig) under each steady-state analysis condition. For all of the experiments in the glass autoclave, the system was operated under continuous gas flow (1.30% O2/balance N2) with a total gas-flow rate of 220 mL/min (corresponding to a gas residence time of 133 s). Total flow rates were verified (at 296 kPa reactor pressure) downstream of the pressure choke valve using a rotameter on the outlet gas stream. 2.7. Data Analysis. For both the high-pressure stirred-tank reactor and the glass autoclave, the gas composition for both the inlet and outlet gas streams were determined using a Varian 4900 gas chromatograph equipped with dual columns and dual thermal conductivity detectors. A molecular sieve column was used to monitor O2 consumption within the reactor, whereas a PoraPLOT U capillary column was used to monitor H2S consumption and SO2 production. Data sets were collected by sampling the inlet gas upstream of the reactor and the outlet gas downstream of the reactor. For steady-state results, the data are reported as the average of triplicate analyses upon reaching steady-state conditions (typically 4-8 h for each experimental condition). 3. Results and Discussion 3.1. High-Pressure H2S Scavenging Using Fresh FeO(OH) in Liquid Sulfur. The scavenging ability of FeO(OH) was investigated for H2S removal from a H2S/N2-containing feed gas. As previously shown, freshly prepared FeO(OH) has a strong ability to scavenge H2S. Using a 1.60% H2S (balance
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Figure 1. H2S scavenging using fresh FeO(OH) in liquid sulfur as a function of pressure (feed gas ) 1.55% H2S/balance N2).
Figure 2. Steady-state H2S/O2 conversion over FeχS(OH) in liquid sulfur as a function of pressure (feed gas ) 1.55% H2S/0.80% O2/balance N2).
N2) feed gas at 296 kPa reactor pressure in a glass autoclave, fresh FeO(OH) was able to remove ∼95% of the H2S from the feed in the absence of any inlet-feed O2. Following the high H2S conversions in the scavenging phase, the FeO(OH) became spent, and conversion of H2S from the feed gas dropped significantly. At the end of the experiment, an iron oxide/sulfide material (FeχS(OH)) was produced (eq 8).
FeO(OH) + H2S f FexS(OH) + H2O
(8)
Under high-pressure conditions in this study, the scavenging experiment was first designed to confirm previous observations. In addition, the conversion of H2S using fresh FeO(OH) within the scavenging phase was investigated as a function of pressure. Using a 1.55% H2S (balance N2) feed gas, the scavenging ability of fresh FeO(OH) was shown to be improved at higher pressures (Figure 1). Under atmospheric conditions, H2S removal was determined to be 73%. Applying a moderate pressure (434 kPa), H2S conversion jumped to 95%, increasing then to a maximum of 99.7% conversion at 6984 kPa operating pressure. Following the completion of the scavenging reaction, H2S conversion once again began to trend toward 0% (data not shown). As was expected, the fresh FeO(OH) again reacted with the H2S within the bed of liquid sulfur, leaving an iron oxide/sulfide (FeχS(OH)) as the reaction product.36 Clearly, high reaction pressures increased the reaction kinetics for H2S scavenging using FeO(OH) by increasing the dissolved H2S concentration in the liquid sulfur. The relationship between the pressure in the gas phase and the H2S concentration in the liquid sulfur would predict that higher reactant gas pressures would lead to an increased concentration in the liquid phase. With a higher H2S content in the sulfur, H2S scavenging using FeO(OH) was nearly 100%. 3.2. High-Pressure Catalytic H2S/O2 Conversion and SO2 Production over FeχS(OH) in Liquid Sulfur. With steadystate FeχS(OH) formed within the reactor following the scavenging experiment, catalytic H2S conversion in the presence of O2 was investigated as a function of increasing reactor operating pressures. Using 5 wt % FeχS(OH) at low pressure, previous experiments showed that conversion was achieved through a multistep mechanism where SO2 was generated in situ within the reactor (through the oxidation of liquid sulfur) with subsequent H2S conversion achieved via both direct oxidation with O2 and Claus conversion with the in situ generated SO2. With SO2 acting strictly as an intermediate, the overall reaction consumed H2S and O2 catalytically over FeχS(OH) to form S8 and H2O (eq 9). FexS(OH)
H2S + 1/2O2 98 1/8S8 + H2O
(9)
Figure 3. Steady-state H2S/O2 consumption and SO2 production over FeχS(OH) in liquid sulfur as a function of pressure (feed gas ) 1.55% H2S/ 0.80% O2/balance N2).
Using a stoichiometric 1.55% H2S/0.80% O2 (balance N2) feed gas, H2S, and O2 conversions were measured under atmospheric conditions and increasing operating pressures of up to 6984 kPa (Figure 2). Under these feed conditions, conversion at atmospheric pressure was very low (16% H2S conversion and 24% O2 conversion). Operating the system at 1813 kPa, gas conversion for both H2S and O2 jumped considerably to 75 and 81%, respectively. At 3537 and 6984 kPa operating pressures, H2S conversion increased to 82 and 87%, respectively. Noteworthy, however, was the observation that at higher operating pressures (3537 and 6984 kPa), O2 conversion began to deviate from the H2S conversion within the reactor, measuring virtually 100%. Knowing that FeχS(OH) catalyzes both the oxidation of liquid sulfur to SO2, as well as the conversion of H2S, analysis of the data in terms of gas consumption and gas production was performed (Figure 3). As H2S and O2 conversion increased from atmospheric conditions to 1813 kPa, unconsumed SO2 exiting the reactor increased from 0.10 to 0.20 mol %. Increasing the operating pressure of the reactor further, however, drove the SO2 concentration in the outlet gas stream back down to 0.10 mol % (3537 kPa) and then to trace levels (6984 kPa). The complete consumption of the in situ generated SO2 at 6984 kPa with simultaneous 100% consumption of the O2 in the feed gas indicated that the feed was running lean with respect to the amount of O2 necessary in the feed gas for complete consumption of the H2S at 6984 kPa. Mechanistically, this was either through O2 consumption for SO2 production or O2 consumption in H2S direct oxidation. Although the extent of each elementary step in the overall reaction cannot be totally elucidated, this result highlighted that a lower H2S/O2 ratio than 2:1 H2S/O2 was required for improving total H2S conversion to elemental sulfur. Multiple non-stoichiometric feed-gas ratios (increasing
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Figure 4. Uncatalyzed H2S conversion in liquid sulfur and catalyzed H2S conversion over FeχS(OH) in liquid sulfur as a function of pressure (feed gas ) 1.55% H2S/0.80% O2/balance N2).
O2 content relative to H2S content) were tested using 5 wt % FeχS(OH) in liquid sulfur. Unfortunately, none of these feedgas streams provided any further improvement to the 87% total H2S conversion to elemental sulfur that was achieved using the stoichiometric feed ratio (data not shown). Higher H2S conversions were obtained, but, only with SO2 breakthrough in the outlet gas stream of the reactor, leaving total sulfur conversion of the H2S to elemental sulfur equal to or less than the previously obtained 87%. 3.3. Effect of Pressure on Uncatalyzed H2S Direct Oxidation and Claus Conversion. An important consideration for high-pressure H2S catalysis in liquid sulfur was determining the effect of higher operating pressures in uncatalyzed reactions. Within the stirred-tank reactor, higher operating pressures would be predicted to increase reaction kinetics by increasing reactant concentrations within the liquid-sulfur phase. Therefore, it was important to determine the effect of pressure alone for promoting gas conversion in an uncatalyzed liquid-sulfur bed. Using the same stoichiometric feed gas as in the 5 wt % FeχS(OH) catalyst experiment (1.55% H2S/0.80% O2/balance N2), uncatalyzed H2S conversions at pressures of up to 6984 kPa were determined (Figure 4). Over an operating pressure range of 89-6984 kPa, uncatalyzed H2S conversion increased linearly. Low H2S conversions (5%) were obtained under atmospheric conditions, which increased to 15% at 1813 kPa, 29% at 3537 kPa, and finally to 49% at 6984 kPa. In addition, it appeared that reactor operating pressures contributed an increasingly higher percentage to the overall catalytic H2S conversion from 1813 to 6984 kPa. Reactor operating pressure became increasingly more significant for overall H2S conversion with higher operating pressures within the reactor compared to catalyzed conversion under identical experimental operating conditions. As predicted, uncatalyzed, high-pressure H2S direct oxidation (using O2) to elemental sulfur was a contributing mechanism for overall H2S removal occurring during the catalytic conversion experiments. Uncatalyzed Claus conversion was subsequently investigated as a function of pressure using a H2S/SO2/balance N2 feed gas. Being an equilibrium-controlled reaction, it was predicted that pressure would drive overall conversion of the H2S/SO2 further toward S8 and H2O. Using a stoichiometric feed gas containing 1.39% H2S/0.72% SO2/balance N2, high-pressure H2S/SO2 conversion was investigated up to reactor operating pressures of 6984 kPa (Figure 5). Under atmospheric conditions, both H2S and SO2 conversions measured only 13%. Gas conversions increased substantially at 1813 kPa, however, with H2S conversion being measured at 74% and SO2 conversion at 69%. Further increases in operating pressures within the reactor pushed conversions even higher.
Figure 5. Uncatalyzed H2S/SO2 conversion in liquid sulfur as a function of pressure (feed gas ) 1.39% H2S/0.72% SO2/balance N2).
Figure 6. Dual-catalyst H2S/O2 conversion over 1 wt % γ-Al2O3/5 wt % FeχS(OH) in liquid sulfur as a function of pressure (feed gas ) 1.33% H2S/ 1.16% O2/balance N2).
At 3537 kPa, gas conversions were approximately 87%, whereas at 6984 kPa, conversions were measured at 89% for both H2S and SO2. In addition, these data sets were in agreement with previous work that showed that the largest increase in H2S conversion under increasing operating pressures occurred when only a moderate pressure was applied on the system. Putting these data into context with the aforementioned uncatalyzed direct oxidation data (Figure 4), uncatalyzed H2S/SO2 conversion at high pressures would also be a significant contributing mechanism to any catalytic H2S/SO2 conversion under identical experimental conditions. 3.4. High-pressure H2S Conversion over FeχS(OH)/γAl2O3 in Liquid Sulfur. Guided by both the uncatalyzed conversion and FeχS(OH)-only catalytic conversion data sets, high-pressure H2S conversion was investigated over FeχS(OH)/ γ-Al2O3 in liquid sulfur. With a fundamental understanding of the catalytic chemistry in the system developed throughout previous work and reiterated in the first half of this article, the objective was now to incorporate a dual-catalyst system to promote higher-level H2S conversion. Using the high-pressure autoclave, a dual-catalyst system was prepared by adding 1 wt % γ-Al2O3 into the previously used 5 wt % FeχS(OH) catalyst/ liquid-sulfur mixture. Following an investigation into an optimized H2S/O2/balance N2 feed-gas ratio, a 1.33% H2S/1.16% O2/balance N2 feed gas demonstrated high-pressure H2S conversions to elemental sulfur in excess of 99% (Figure 6). Under these feed-gas conditions, H2S and O2 conversions were low under atmospheric conditions (56 and 40%, respectively), but, upon increasing the autoclave operating pressure to 434 kPa, H2S and O2 conversions were measured at 98 and 71%, respectively. This result was again consistent with previous data in that a nominal increase in pressure was again shown to have the most dramatic improvement to overall H2S conversion. Increasing the reactor pressure beyond 434 kPa resulted in 99.6% H2S conversion at 1813 kPa operating pressure. More
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Figure 7. Dual-catalyst H2S/O2 consumption and SO2 production over 1 wt % γ-Al2O3/5 wt % FeχS(OH) in liquid sulfur as a function of pressure (feed gas ) 1.33% H2S/1.16% O2/balance N2).
Figure 8. Dual-catalyst H2S/SO2 conversion over 1 wt % γ-Al2O3/5 wt % FeχS(OH) in liquid sulfur as a function of pressure (feed gas ) 1.39% H2S/ 0.72% SO2/balance N2).
importantly, this high-level of H2S conversion from the inletfeed gas resulted in conversion to elemental sulfur. This conclusion was demonstrated through analysis of the same data set in terms of gas consumption (H2S and O2) and gas production (SO2) (Figure 7). Specifically, SO2 was present in the outlet gas stream under atmospheric conditions (0.10 mol %) but decreased as operating pressures were increased within the reactor, with no SO2 detected at operating pressures >1123 kPa. Correlating this observation with the H2S conversions obtained at each operating pressure, it was concluded that not only does H2S conversion increase dramatically with pressure, however, so does H2S conversion to elemental sulfur (i.e., high H2S conversion with complete consumption of any in situ generated SO2 within the reactor). Under these dual-catalyst experimental conditions, high operating pressures (1813 kPa) proved to be advantageous for promoting higher H2S conversions to elemental sulfur compared to any lower-pressure experiments using the dual-catalyst regime or any results obtained using a FeχS(OH)-only catalyst bed. 3.5. High-Pressure Claus Conversion over FeχS(OH)/γAl2O3 in Liquid Sulfur. High-pressure H2S/SO2 conversion over 1 wt % γ-Al2O3/5 wt % FeχS(OH) was investigated using a feed gas containing H2S/SO2/balance N2 to support the prediction that Claus conversion using the in situ generated SO2 was a major mechanism responsible for bulk H2S conversion in the dual-catalyst regime. Using a feed gas containing 1.39% H2S/0.72% SO2/balance N2, data was collected at operating pressures up to 1813 kPa (Figure 8). Results under atmospheric conditions showed H2S conversion and SO2 conversion at 71 and 77%, respectively. Upon increasing the operating pressure to 434 kPa, H2S conversion increased to 91% and SO2 conversion jumped to 96%. Increasing the operating pressure
Figure 9. Rate constants (k) for SO2 production over γ-Al2O3 and FeχS(OH) in liquid sulfur (feed gas ) 1.30% O2/balance N2).
further up to 1813 kPa showed complete consumption of the SO2 in the feed gas with H2S conversions in excess of 90%. Therefore, the dual-catalyst regime did prove to be reasonably effective at promoting high H2S/SO2 conversion from a H2S/ SO2-containing feed gas. A control experiment was performed using a 1 wt % γ-Al2O3 sulfur bed in the absence of any ironbased catalytic material (data not shown). Under these conditions, H2S/SO2 conversion virtually mirrored the results obtained using the dual-catalyst system (Figure 8). This result would be predicted because γ-Al2O3 is well-known for its activity to promote the Claus reaction. As a stand-alone catalyst or incorporated in the dual catalyst system, γ-Al2O3 proved effective for promoting high-pressure Claus conversion in the stirred-tank liquid-sulfur reactor. 3.6. SO2 Production Kinetics over FeχS(OH) and γ-Al2O3 in Liquid Sulfur. Using the glass autoclave stirred-tank reactor, SO2 production kinetics were investigated over FeχS(OH) and γ-Al2O3. Experiments were performed separately for each catalyst using 1 wt % in liquid sulfur. For each catalytic material, a 1.30% O2/balance N2 feed gas was passed through the reactor, the SO2 production was measured, and the rate constant was determined. Previously shown was the fact that the production of SO2 via liquid-sulfur oxidation was first-order with respect to the O2 content in the feed gas, in that doubling the inlet O2 concentration in the feed gas led to a doubling of the SO2 production in the outlet gas stream. Using that data, a rate equation was established (eq 10).
rate (O2 consumption/SO2 production) ) k[O2] (10) Using this rate equation, overall rate constants for the FeχS(OH) and γ-Al2O3 catalysts were experimentally determined. Using the total gas-flow rate (mL/s) and the volume of liquid sulfur at 135 °C (mL), the total-gas residence time in the liquidsulfur bed was calculated (133 s). Under continuously flowing conditions, gas consumption and gas production (mol %) are a function of the gas residence time within the reactor. The rate of reaction (mol %/s) was calculated by dividing SO2 production (O2 consumption) (mol %) by the gas residence time in the liquid sulfur (s). Subsequently, dividing the rate of reaction by the known inlet O2 concentration, the rate constants (k) for SO2 production (O2 consumption) over FeχS(OH) and γ-Al2O3 were determined (s-1) (Figure 9). Under the specified experimental conditions, γ-Al2O3 showed mild activity for the production of SO2 from liquid sulfur with a rate constant of 1.5 × 10-4 s-1. This value, however, was relatively small when compared to the rate constant for SO2 production over FeχS(OH). For FeχS(OH), the rate constant was nearly 15 times higher (2.2 × 10-3 s-1), illustrating that FeχS-
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Figure 10. Specific rate constants (k′) for SO2 production over γ-Al2O3 and FeχS(OH) in liquid sulfur (feed gas ) 1.30% O2/balance N2).
(OH) was a higher-activity SO2 producer compared to γ-Al2O3, so much so that specific activity rate constants (k′) were calculated to account for the surface-area differences between both materials. These were calculated by dividing the rate constant by the surface area of 1 g of steady-state catalytic material. Previous work had detailed the steady-state surface areas for both FeχS(OH) and γ-Al2O3 (15.9 and 253 m2/g respectively).36 The specific rate constant for γ-Al2O3 was calculated to be 7.4 × 10-8 g s-1(m2)-1, whereas the specific rate constant for SO2 production over FeχS(OH) was 1.7 × 10-5 g s-1(m2)-1 (Figure 10). Relatively, FeχS(OH) had a specific rate constant now 230 times higher than γ-Al2O3 compared to the 15 times higher calculated without taking into account the active surface area under steady-state operating conditions. This result highlights the fact that the bulk of the in situ SO2 production in the dual-catalyst regime (γ-Al2O3/FeχS(OH)) occurs via oxidation of elemental sulfur at the FeχS(OH) surface. 3.7. High-pressure H2S Conversion over γ-Al2O3/FeχS(OH) in Liquid Sulfur under Higher H2S Feed Gas Conditions. An experimental program was initiated to investigate dualcatalyst H2S conversion in liquid sulfur using gas streams with higher H2S concentrations. Using a dual-catalyst composition, H2S/O2 conversion data were acquired again as a function of operating pressures of up to 1813 kPa in the high-pressure stirred-tank reactor. The data set, however, was first initiated using a catalyst mixture of 1 wt % γ-Al2O3/1 wt % FeχS(OH). Using a high-pressure H2S/O2 feed gas, a 1 wt % γ-Al2O3/1 wt % FeχS(OH) catalyst mixture proved to be inadequate for high H2S conversions to elemental sulfur (data not shown). After increasing the catalyst content to 2.5 wt % γ-Al2O3/2.5 wt % FeχS(OH), however, conversions improved substantially. Using a feed gas containing 7.20% H2S/4.66% O2/balance N2, dualcatalyst promoted H2S/O2 conversion was measured as a function of reactor operating pressures (Figure 11). Under atmospheric conditions (89 kPa), 59% H2S conversion and 18% O2 conversion were measured. Increasing the autoclave operating pressure to 434 kPa, both H2S and O2 conversion rose to 98 and 62%, respectively. At 1123 kPa, H2S and O2 conversions increased to 99 and 72%, respectively, until finally at 1813 kPa, H2S conversion was 99.3% with an O2 conversion rate of 72.5%. Such results clearly demonstrated that the dual-catalyst system was capable of removing H2S from a gas stream that was 5 times higher in H2S content than in those previously investigated. Not only did total H2S conversion improve under increasing operating pressures but also total H2S conversion to elemental sulfur improved as shown by the decreasing SO2 concentrations in the outlet gas stream (Figure 12). Although a new, optimized gas-flow ratio (H2S/O2/balance N2) was required to compensate
Figure 11. Dual-catalyst H2S/O2 conversion over 2.5 wt % γ-Al2O3/2.5 wt % FeχS(OH) in liquid sulfur as a function of pressure (feed gas ) 7.20% H2S/4.66% O2/balance N2).
Figure 12. Dual-catalyst H2S/O2 consumption and SO2 production over 2.5 wt % FeχS(OH)/ 2.5 wt % γ-Al2O3 in liquid sulfur as a function of pressure (feed gas ) 7.20% H2S/4.66% O2/balance N2).
for the adjusted catalyst-content ratio in the experiment, the dualcatalyst mixture proved effective in maintaining high H2S conversions to sulfur over a wider H2S/O2 feed-gas concentration range than previously observed. 4. Conclusions The results presented throughout this article support the conclusion that a liquid-sulfur stirred-tank reactor containing γ-Al2O3 and FeχS(OH) could be industrially applicable as a lowtonnage sulfur recovery unit for handling sour-gas streams. As hypothesized, operation of the system at significantly higher pressures was shown to be extremely advantageous for total H2S conversion to elemental sulfur. Fundamental mechanisms previously elucidated remained consistent with the data acquired at higher pressures. Fresh FeO(OH) was shown to have a higher H2S scavenging activity at higher reactor operating pressures. Under steady-state catalytic conditions using 5 wt % FeχS(OH), 90% H2S conversion to elemental sulfur was obtained, but only at reactor operating pressures of 6984 kPa. Incorporating the dual-catalyst regime (1 wt % γ-Al2O3/5 wt % FeχS(OH)) into the high-pressure stirred-tank reactor, 99.6% H2S conversion to elemental sulfur was achieved at a reactor operating pressure of only 1813 kPa. The dual-catalyst regime, therefore, produced higher H2S conversions at significantly lower operating pressures than results obtained using only 5 wt % FeχS(OH). SO2 kinetic experiments quantified the observation that FeχS(OH) is more active for SO2 production from liquid-sulfur oxidation compared to γ-Al2O3. Finally, high H2S conversions (99.3% at 1813 kPa) were achieved using a higher H2S-content feed gas, demonstrating feed-gas versatility under dual-catalyst reaction conditions. Overall, the objective of achieving 99+% H2S conversion to elemental sulfur was achieved using a high-pressure, dual-
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catalyst regime in the liquid-sulfur stirred-tank reactor. Proving the concept of high-pressure H2S conversion, the utility of the concept might have a niche industrially. Lowering the gas residence time within the reactor is an area where improvements would add value to the project. However, our objective of modeling our experiments to resemble a stand-alone lowtonnage recovery process (e.g., sulfur recovery from a lowcontent H2S gas stream, sulfur pit off-gas processing) would allow much more flexibility in gas residence times, the reason being the volume of H2S required for conversion to elemental sulfur would be relatively low compared to larger-scale sulfur recovery operations. Previous work investigating liquid-sulfur reactors was performed with the objective of using it for tailgas processing downstream of a Claus plant.35 The requirement for low gas residence times was emphasized in that case because the liquid-sulfur reactor had to catalytically convert H2S to elemental sulfur at a minimum rate to keep up to the large H2S volume throughput in the Claus plant. High conversions (>80%) could not be maintained below a gas residence time of 40 s, preventing the liquid-sulfur reactor from being incorporated in the Claus tail-gas conversion. Although lowering the total-gas residence time of 105 s in this article would be an improvement, it would not necessarily be a requirement depending on the type of gas stream requiring H2S removal. Modeling the residence time to fit a specific process, the system has the potential to be a stand-alone reactor for pressurized, low-tonnage H2S conversion. Traditionally, reactor pressurization for H2S conversion systems has been avoided. However, high-pressure H2S conversion might prove useful for a sour-gas processing application where the liquid-sulfur reactor is set up to handle a pressurized sour sample prior to depressurization. Also, modest compression of a low-pressure feed gas might prove advantageous technically and economically because high H2S conversions to elemental sulfur can be achieved. For this reason, future research will continue to be oriented toward maintaining high conversions at minimal reactor operating pressures. Regardless of industrial applicability at this point, the high-pressure H2S conversion data supported the chemistry developed to date on the project. Future work will focus on new reactor designs and variable feed-gas modeling (incorporating low-level hydrocarbons and H2O). Acknowledgment We would like to thank Alberta Sulfur Research Ltd. and its membership for its financial support of the project. Literature Cited (1) Schaack, J. P.; Chan, F. Hydrogen Sulfide Scavenging. Oil Gas J. 1989, 87, 45. (2) Fox, I. Highly Reactive Iron Oxide Agents and Apparatus for Hydrogen Sulfide Scavenging. U.S. Patent, 4,246,244, 1982. (3) Teixeira da Silva, F. Hydrogen Sulphide Scavenging by Porous Magnetite. Trans. Inst. Min. Metall. 2005, 114, C245. (4) Jamal, A.; Leppin, D.; Fisher, K. S. Design of Direct Injection H2S Scavenging Systems. LRGCC Conference Proceedings, 2005. (5) Nagl, G. J. Removing H2S from Gas Streams. Chem. Eng. 2001, 108, 97. (6) Van Nisselrooy, P. F. M. T.; Lagas, J. A. SUPERCLAUS Reduces Sulfur Dioxide Emission by the Use of a New Selective Oxidation Catalyst. Catal. Today 1993, 16, 263. (7) Cameron, L. C. Aquitaine Improves Sulfreen Process for More Hydrogen Sulfide Recovery. Oil Gas J. 1974, 72, 110. (8) Lagas, J. A.; Borsboom, J.; Berben, P. H. Selective-Oxidation Catalyst Improves Claus Process. Oil Gas J. 1988, 86, 68. (9) Davydov, A. A.; Marshneva, V. I.; Shepotko, M. L. Metal Oxide in Hydrogen Sulfide Oxidation by Oxygen and Sulfur Dioxide. I. The Comparison Study of the Catalytic Activity. Mechanism of the Interactions between H2S and SO2 on Some Oxides. Appl. Catal., A 2003, 244, 93.
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ReceiVed for reView November 16, 2007 ReVised manuscript receiVed January 28, 2008 Accepted February 3, 2008 IE071560A
