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Ind. Eng. Chem. Res. 1997, 36, 4726-4733
Oxidation of Methyl Mercaptan over an Activated Carbon in a Fixed-Bed Reactor A. K. Dalai* Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5C9
E. L. Tollefson and A. Yang Department of Chemical and Petroleum Engineering, The University of Calgary, Calgary, Alberta, Canada T2N 1N4
E. Sasaoka Faculty of Engineering, Okayama University, Okayama 700, Japan
A gas containing approximately 1000 ppm of methyl mercaptan (CH3SH) was used to test an oxidative reaction system for the purification of gas. Experiments were performed for 3.0 h periods in a fixed-bed reactor containing 0.25-5.0 g of Hydrodarco activated carbon in the temperature and pressure ranges 323-448 K and 122-364 kPa, respectively. The gas hourly space velocity was varied from 938 to 4000 h-1, with the O2/CH3SH ratio varying from 1.1 to 1.33 times the stoichiometric ratio. Dimethyl disulfide was the main product, while CO2 was produced in small amounts. At temperatures above 373 K, 99.99% conversion of the mercaptan was achieved. It was established that higher conversion of CH3SH could be achieved while keeping CO2 production to a minimum by using an O2/CH3SH ratio in the feed gas close to 1.10 times the stoichiometric ratio. Catalyst deactivation occurred due to deposition of dimethyl disulfide on the catalyst. A kinetic study of this process was performed, and a rate equation for the conversion of CH3SH to (CH3)2S2 and H2O was obtained. Since catalyst deactivation occurred by fouling due to deposition of (CH3)2S2 on the catalyst, the initial rates were considered to be global rates without deactivation effects. According to the Langmuir-Hinshelwood model, the overall rate equation was derived on the basis of the mechanism where the rate-determining step is a surface reaction. The rate data obtained using granular activated carbon were collected well with the rate equation. Introduction The natural sources of sulfur emitted to the atmosphere are primarily the reduced sulfur compounds such as dimethyl sulfide, dimethyl disulfide, hydrogen sulfide, and methyl mercaptan, rather than SO2, the predominant industrial emission (Anastasi et al., 1992). Activated carbon has been used in various ways for the removal of H2S and odorous organic sulfur compounds (such as the ones mentioned above) from air and other gas streams. Physical adsorption of such sulfur compounds on activated carbon has been reported by Turk et al. (1989) and Koe and Tan (1990). However, physical adsorption is reversible, with the consequence that H2S or CH3SH may be displaced by the subsequent adsorption of other vapors. Catalytic oxidation of H2S and other sulfur compounds such as SO2 using an activated carbon catalyst has been investigated in the past by Ghosh and Tollefson (1986a,b), Tollefson and Chowdhury (1988), Chowdhury and Tollefson (1990), Richter (1990), Choi et al. (1991), Dalai et al. (1992), Anastasi et al. (1992), and Dalai et al. (1993). Methyl mercaptan (CH3SH) is used at low concentrations in gas as an odorant or stenching agent so that the gas can be readily detected for corrective action if a leak in a natural gas line develops or if a pilot light should cease burning and some gas then escapes into the surrounding area. In some situations it may be necessary to remove this odorant so that a catalytic * Corresponding author. S0888-5885(97)00123-1 CCC: $14.00
reaction system is not poisoned by the sulfur present in the mercaptan. The kinetics and mechanism of the reactions of the CH2SH radicals formed from reaction of CH3SH with O2 have been studied by Anastasi et al. (1992) at atmospheric pressure and 298 K using a pulse radiolysis/kinetic absorption apparatus. A rate constant equal to 8.5 × 10-12 cm3 molecule-1 s-1 was obtained for this reaction. Adsorption spectra due to product species indicated that the reaction with O2 proceeds via addition. Seakins and Leone (1992) conducted a laser flash photolysis/time-resolved FTIR emission study in the reaction of the CH3 radical and O2 and found that CO is a primary product in this reaction. The purpose of the present work has been to identify all the reaction products and to provide a method and kinetic data for the selective oxidation and removal of CH3SH as well as some of the products by sorption on the surface of the catalyst employed. The product identification study was conducted at Okayama University, whereas the data for the kinetic study were generated at The University of Calgary, using granular Hydrodarco activated carbon catalyst at both places. Experimental Section Materials. The activated carbon employed was granular Hydrodarco prepared by ICI America Inc. (Wilmington, DE). Its properties are listed in Table 1. Although this catalyst has lower surface area and iodine number values compared with some commercial activated carbons, it has higher pore volume values. © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4727 Table 1. Properties of Hydrodarco Activated Carbon Catalyst initial surface area, m2/g iodine number moisture content, % particle density, g/cm3 mean pore radius, nm total pore volume, cm3/g pore volume in the range 5-7500 nm, cm3/g particle size, mm
487 550 7.9-8.81 1.3-1.5a 2.9a 1.0a 0.441 2.38 × 0.841
a From manufacturer’s literature: wetted in water.
Figure 1. Schematic diagram of the apparatus used for CH3SH oxidation experiments. Table 2. Experimental Conditions A. At The University of Calgary reactor tube diameter ) 12.5 mm reactor length ) 250 mm sample mass of activated carbon ) 0.25-5.0 g gas flow rate ) 200-750 mL/min (STP) (WHSV ) 938-3516 h-1) fraction of methyl mercaptan ) 0.10% volume (in reactant gas) fraction of oxygen ) 0.11-0.13% volume (in reactant gas) temperature ) 343-473 K pressure ) 122-364 kPa B. At Okayama University reactor tube diameter ) 12.5 mm sample mass of activated carbon ) 1.226 g gas flow rate ) 200 mL/min (WHSV ) 4000 h-1) fraction of methyl mercaptan ) 0.1% volume (in reactant gas) fraction of oxygen ) 16 × volume of mercaptan in the feed gas temperature ) 417-473 K pressure ) 1 atm
Gases used in these experiments included nitrogen with a purity above 99.9% and compressed air (medical grade purity 99.9%), both having been obtained from Medigas, Calgary, Alberta, Canada. The feed gas containing 0.1% methyl mercaptan in N2 was from Matheson Gas Products (Whitby, Ontario, Canada). Apparatus. A diagram of the apparatus used in this research at The University of Calgary is shown in Figure 1. It consists of a 12.5 mm i.d., 250 mm long, stainless steel tube surrounded by an electrically-heated aluminum block to provide uniform heating. The granular Hydrodarco activated carbon catalyst was mixed with Ottawa sand (2.38 × 0.841 mm) to keep the total content in the reactor constant at 12.8 cm3 and was supported on a screen placed 20 mm from the bottom of the reactor. The experimental condition used for the oxidation experiments are given in Table 2. Accurate measurements of the flows of gas containing CH3SH and of air were made using mass flowmeters. A steel knockout vessel loaded with glass wool was constructed and incorporated into the system so that
the hot gases leaving the reactor would be cooled and the sulfur, if produced, would be removed. Water vapor produced by the oxidation of CH3SH was removed by calcium chloride traps located downstream from a pressure-reducing valve used to maintain the pressure of the reaction system. Experimental Procedure. The experiments were initiated at The University of Calgary and were partly carried out at Okayama University using Hydrodarco activated carbon catalysts. The data obtained at Calgary were produced over a wide range of process conditions and were used for the kinetic studies. Experiments Done at The University of Calgary. Mass flowmeters (Models 8102-1412FM and 8102-1413FM), obtained from Matheson Gas Products, were calibrated with air and feed gas at the reaction pressure to be investigated using the pressure-reducing valve in the reaction system and passing the gas flow discharged through the valve into a bubble-type buret, through which the gas flow was determined. Before raising the reactor temperature and pressure, the reaction system was flushed with feed gas. When the reaction temperature was reached and the desired pressure set, a sufficient flow of air was established through the mass flowmeter to provide the desired amount of oxygen to oxidize CH3SH in the feed gas stream. Temperatures ranging from 343-473 K were controlled by means of variable transformers supplying power to the electric heaters wound around the reactor. The reaction temperature recorded was that at the bottom of the bed since the gases flowed downward. It was controlled within (1 °C. The temperature measured along the axis of the catalyst bed was found to be within (2 °C of the reaction temperature during the reaction. Part of this variation resulted from not preheating the incoming feed gas sufficiently. Experiments Performed at Okayama University. The experimental reaction system consisted of a glass reactor which was operated at atmospheric pressure. The experimental conditions used at Okayama University are given in Table 2. The catalyst (activated carbon as mentioned above) was supported by a glass wire mesh. The temperature just under the mesh was measured and controlled as the reaction temperature. The sulfur species in inlet and outlet gases from the reactor were analyzed on-line using a GC (Yanako G3810) equipped with a flame photometric detector. A 3 m long glass column with the packing PEG-5000 supported by TPA 60/80 was used in this GC. The oven and the detector temperatures were set at 110 and 200 °C, respectively. Analytical Methods Employed at The University of Calgary. An SR1 8610 gas chromatograph (Wennick Scientific Corp., Ottawa, Canada) and a Packard Bell 286 computer loaded with the “peaksample” software were used to determine the sulfur compounds, namely CH3SH, SO2, (CH3)2S, (CH3)2S2, and CS2, in the product gas. A 0.32 mm i.d. and 30 m long fused silica capillary was used as the chromatographic column. The GC was equipped with a flame photometric detector (FPD) in which a flow of hydrogen produced a hydrogen-rich flame in which sulfur compounds were burnt to release photons of discrete frequencies which passed through a 393 nm filter. Primary and secondary hydrogen flows of 30 mL/min, air flow of 100 mL/min, and helium flow of 50 mL/min were maintained in the GC. During analysis there was a temperature ramp from 27 to 100 °C at a rate of 15 °C/min with a FPD voltage of 500 mV. A Varian-Aerograph (Hansenway, Palo Alto, CA) Series
4728 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 2. Activity and stability of activated carbon catalyst at 417 K, 88 kPa, 4000 h-1, and an O2/CH3SH ratio of 8.0 times the stoichiometry that is required for the oxidation reaction.
Figure 3. Activity and stability of activated carbon at 448 K, 88 kPa, 4000 h-1, and an O2/CH3SH ratio of 8.0 times the stoichiometry that is required for the reaction.
1700 gas chromatograph and a HP 3396 Series II integrator (Hewlett Packard, Avondal, PA) were used to determine non-hydrocarbon components (N2, CO2) in the product gas with a Poropak Q column. Some of the experiments were repeated and were reproducible with these analytical techniques within (1.5%. Results and Discussion The experiments conducted at The University of Calgary were more elaborate and were obtained using a wide range of process conditions. These results were used in developing the kinetic equation for the oxidation process. The study conducted at Okayama University was for identification of various sulfur compounds and other products obtained in this process. Studies Conducted at Okayama University. The experiments were conducted at atmospheric pressure and at 417-473 K using 1.23 g of activated carbon. The concentrations of CH3SH and O2 in the feed gas were 0.1 and 0.4%, respectively. The conversion of CH3SH was virtually 100% at these temperatures. (CH3)2S2 and H2O were the main reaction products. CH3SH appeared in the product stream when the supply of air (for oxidation) was cut off (see Figure 2). These results indicate that CH3SH was oxidized with oxygen and mostly to (CH3)2S2. Trace amounts of CS2 and (CH3)2S were produced at 473 K. CO2 was produced in small quantities and increased with reaction temperature as well as with the supply of oxygen to the reaction. It was established that higher conversion of CH3SH could be achieved while keeping CO2 production to a minimum by using an O2/CH3SH ratio in the feed gas close to 1.10 times the stoichiometric ratio. A catalyst deactivation study at 417 K indicated a decrease in CH3SH conversion from 99.99 to 98.7% over a 6 h period (see Figure 3). At lower temperatures (
