Natural Gas Odorants Desulfurization

This study investigates the use of activated carbon for the odorant removal from natural gas to be used for synthesis gas production for fuel cells. The odorants used in this study were dimethyl sulfide, tetrahydrothiophene, methyl mercaptan, and t-butyl mercaptan. It was concluded that only physical adsorption is contributing towards removal of sulfur species at lower temperatures, whereas the partial oxidation of organic sulfur and subsequent physical adsorption of low volatile oxidation product are the mechanisms for odorant removal at elevated temperatures. Pore size distribution of the activated carbon and vapor pressure of the sulfur species to be removed determine the removal capacity of activated carbon due to capillary condensation. Activated carbon modified with KOH to increase the adsorption of sulfur species as well as acid treated and KMnO4 modified activated carbon to enhance the oxidation process were also the part of this study. The effect of GHSV and temperature on the sulfur species removal capacity of activated carbon was also studied. Introduction Natural gas as it occurs naturally has little or no perceptible odor. In order to make natural gas detectable without the use of instrumentation, the gas is odorized with one or more of a series of sulfur compounds. Odorants in common use include various mercaptans (methyl, ethyl, propyl, isopropyl, t-butyl), organic sulfides (dimethyl, methyl ethyl), thiophane (tetrahydrothiophene), and blends of these. However, the type of sulfur compounds used as odorants in natural gas depends on its geographical source. Table 1 provides the odorant composition used in US pipeline natural gas [1]. However, odorants may have detrimental effects on the reforming catalyst as well as on the fuel cell anode performance if natural gas is used as a reforming fuel for fuel cell applications. Thus, sulfur removal from natural gas is essential to ensure the long life of the reforming catalyst. Table 1. Composition of odorants used in the US pipeline natural gas [1] Natural gas odorant blend Market share % Composition breakdown Mercaptans 40-55 100% mercaptans Mercaptan/alkyl sulfide 40-55 sulfide content is typically 20-50% Thiophane/mercaptan 5 thiophane content is typically 30-50% Thiophane 1 100% thiophane Desulfurization of natural gas depends on the type of sulfur compounds as well as the sulfur content. Adsorption processes using adsorbents such as activated carbon and zeolites are widely used to remove a wide range of sulfur compounds from gaseous emissions [2-4]. For continuous sulfur compounds removal, two similar beds are designed in parallel so that one is being used for adsorption while the other one is being regenerated. In the literature, a number of studies have also been reported on the application of H2S catalytic partial oxidation technology to the desulfurization of sour natural gas [5-9]. H2S(g) + 1⁄2 O2(g) => 1/n Sn + H2O(g) (1) Thermodynamically, Reaction (1) has the potential to remove H2S to the parts-perbillion level below 250 °C. Ghosh et al. [5] were the first to examine the applicability of Reaction (1) using an activated catalyst to sweeten natural gas. At National Energy Technology Laboratory, Gardner et al. [9] successfully demonstrated this technology in removing H2S from a Texaco O2-blown coal-derived synthesis gas. The scope of this work was to determine the performance of the activated carbon or the modified activated carbon for the natural gas odorants removal. Physical adsorption method as well as selective oxidation of sulfur species was explored for odorant removal. Experimental The composition of the simulated natural gas used in this study is provided in Table 2. Two additional gas mixtures containing only sulfur species balanced by N2 were also used in this study. The first gas mixture (Mix-A) contained 100 ppm of dimethyl sulfide (DMS), 100 ppm of tetrahydrothiophene (THT), and balance N2, whereas the second gas mixture (Mix-B) had 100 ppm of methylmercaptan (MM), 100 ppm of t-butylmercaptan (TBM), and balance N2. We decided to use higher concentrations of odorants (100 ppm) instead of normal concentrations (~10 ppm) present in natural gas so the effects are readily and accurately observable analytically. Table 2. Composition of natural gas used in this study A fixed bed reactor system was used to conduct this series of experiments, which was operated continuously at temperatures up to 350 °C. A summary of reaction conditions is Component Composition Component Composition Component Composition THT 96 ppm N2 3.003 mole% n-propane 2.998 mole% DMS 99 ppm O2 0.200 mole% n-butane 1.253 mole% H2S 0.68 ppm Methane 81.052 mole% n-pentane 0.499 mole% CO2 3.001 mole% Ethane 7.994 mole% given in Table 3. Exit gases including H2S, COS, CS2, SO2, DMS, THT, thiophene, MM, and TBM were analyzed using flame photometric detector (FPD) in different GCs. The catalyst utilized in this study was F600 activated carbon obtained from Calgon Carbon Corporation. Physical properties of sulfur species of interest are listed in Table 4. Table 3. Reaction conditions Condition Range Condition Range Reactor temperature (°C) 25-350 GHSV (hr) 625-2500 Reactor pressure (psig) 10-50 Air flow rate 0-10 sccm Natural gas flow rate 250 sccm Table 4. Physical properties of some sulfur species of interest Sulfur species Boiling point (°C) Vapor pressure (bars) @ 25 °C Vapor pressure (bars) @ 135 °C Dimethyl sulfide 38 0.64 Thiophane 120 0.025 Thiophene 84 0.105 Dimethyl sulfoxide 189 0.0008 0.21 Methyl Mercaptan 8 Dimethyl disulfide 110 0.038 t-Butyl Mercaptan 63 0.55 Di-t-butyl disulfide 200 Results and Discussion Effect of temperature Fig 1 show the effect of temperature on the concentration of sulfur species in the reactor outlet at gas hour space velocity (GHSV) of 2500 hr. At room temperature, a complete removal of THT from natural gas was observed over the time (2 hr) the experiment conducted. THT concentration in the reactor effluent slightly increased with an increase in temperature before further decreased to zero at elevated temperatures. At higher temperatures, dehydrogenation of THT to thiophene takes place and consequently the concentration of thiophene increased with increasing temperature. THT removal capacity of activated carbon decreased considerably with increasing temperature. While DMS removal from natural gas over activated carbon behaved completely different than the THT removal. DMS removal was 89% after 1 hr of run and 0% after 2 hr run at room Fig 1. Effect of temperature on sulfur removal at GHSV of 2500 hr 1 (Inlet: DMS = THT = 100 ppm) 0 20 40 60 80 100 120 20 70 120 170 220 270 320 370 Reaction Temperature (C) R ea ct or o ut le t C on c (p pm ) DMS (1 hr) THT (1 hr) Thiophene (1 hr) DMS (2 hr) THT (2 hr) Thiophene (2 hr) temperature. No DMS removal was observed at 135 °C, but concentration of DMS steadily decreased and, hence, DMS removal increased with increasing temperature. Two lone pair electrons around the sulfide of DMS show a high nucleophilicity, hence, they are susceptible to attack by an electrophile. Oxidation products of DMS will be dimethyl sulfoxide (DMSO) or dimethyl sulfone, having been oxidized by the attack of one or two oxygen atoms respectively. The oxidation of sulfoxide to sulfone is relatively very slow and occurs comparatively higher temperatures because the electron-releasing ability of the sulfoxide sulfur is reduced considerably compared to sulfide sulfur. However, the decomposition of sulfones at elevated temperatures is expected which results in the formation of SO2. The overall reaction occurs according to the equation: At higher temperatures (200-300 °C), the oxidation reaction as well as the adsorption phenomenon is responsible for DMS removal. Oxidation products of DMS (DMSO and dimethyl sulfone) have a considerably low volatility and can have retention through capillary action on micropores of activated carbon. At 350 °C, a significant amount (19 ppm) of final decomposition product SO2 was observed in the reactor effluent. It is also interesting to note that the DMS removal rate after 2 hr of operation was the same as 1 hr operation except at room temperature. Similarly, the oxidation of THT yields tetramethylene sulfone (or sulfolane): Dehydrogenation reaction of THT to thiophene dominates the oxidation reaction of THT. Sulfur’s extra pair of electrons in thiophene molecule is involved in the π cloud as in an aromatic structure. These electrons are not readily available for nucleophilic reactions as of THT. Aromaticity provides high stability to thiophene molecule and, thus, it tends to undergo reactions in which the stabilized aromatic ring is retained. S .. ..