Environ. Sci. Technol. 2002, 36, 3025-3029
Removal of Sulfur Fumes by Metal Sulfide Sorbents JAE BIN CHUNG,† ZHIDONG ZIANG,‡ AND J O N G S H I K C H U N G * ,†,‡ Department of Chemical Engineering and School of Environmental Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea
Removal of sulfur by a transition metal is studied at temperatures of 300-350 °C. Among various metal sulfides tested, only metal sulfides of iron, cobalt, and nickel can remove sulfur fumes as they are transformed into disulfides in the presence of sulfur vapor. The disulfide form can be regenerated into FeS, Co9S8, and Ni3S2, respectively, using hydrogen gas at 350-400 °C. These two reactions of deep sulfidation with sulfur and reduction with hydrogen can be utilized for the removal of sulfur fumes in a process stream and an emission gas.
Introduction Elemental sulfur vapor that is produced in the process industries is generally recovered as liquid sulfur using wash towers or vertical tube condensers (1, 2). When the condenser is not operated properly, however, sulfur “fog” and “mist” could be formed. This condition arises when the heat transfer rate is higher than the mass flow rate. The droplets of sulfur fog are sufficiently small that they follow streamlines such as a gas and, therefore, are not captured effectively by normal coalescing or impingement devices such as condensers or filters. For these reasons, the emission of sulfur compounds (H2S, SO2, and sulfur) in tail gas of the Claus plant, for example, could reach 0.5-1.5% of which more than 30% is contributed from the sulfur that passes through the collecting devices. It is therefore necessary to develop a more efficient way of capturing a low concentration of the elemental sulfur that cannot be captured using conventional devices such as condensers and filters. For the removal of H2S gas, a great deal of research has been carried out using metal oxides or activated carbon. Several metal oxides were used for the removal of hydrogen sulfide from coal-derived fuel gas at high temperatures (500800 °C) (3-11), and used adsorbents can be regenerated using oxygen gas (12-13). Activated carbon was studied for the adsorptive removal of H2S gas at room temperature (14-15). So far there has been no investigation for the removal of sulfur fumes in the gas stream. In this paper we suggest a method for removing a small concentration of sulfur vapor in the gas stream at temperatures 350-400 °C using monosulfides of some transition metal. Investigation was mainly focused on searching metal sulfides effective for the removal of sulfur and finding how to regenerate used absorbents.
Experimental Section Absorbents Preparation. Metal sulfides were prepared by sulfidation of corresponding metal oxides. The sulfidation * Corresponding author phone: +82-562-279-2267; fax: +82-562279-2699; e-mail:
[email protected]. † Department of Chemical Engineering. ‡ School of Environmental Engineering. 10.1021/es011337j CCC: $22.00 Published on Web 05/14/2002
2002 American Chemical Society
reaction was carried out using a packed-bed reactor that was made of 1/2 in. stainless steel tubing. One gram of metal oxide powder packed in the reactor was exposed in a flow of H2S/H2 mixture (H2/H2S ) 4) at 400 °C for 2 h. X-ray diffraction analysis confirmed the formation of metal sulfides. They were then crushed to 100 mesh for later use. Absorption Test and Analysis. Figure 1 shows an apparatus used for the removal experiment of sulfur. The system consists of three packed-bed reactors and one sulfur trap. The first reactor (the reactor I in the figure), made of 1/2 in. stainless steel tubing, was used for generating sulfur vapor from H2S feed. A known concentration of H2S gas in helium was selectively oxidized to elemental sulfur (with the sulfur yield higher than 90%) using a stoichiometric amount of oxygen over chromium oxide catalysts at 300 °C (16).
H2S + 1/2O2 f S + H2O
(1)
The second reactor (the reactor II in the figure), made of Pyrex glass tubing (OD 10 mm), was used for the sulfur removal experiments. The last reactor (the reactor III), made of 1/2 in. stainless steel tubing, was used to reconvert the sulfur vapor that was emitted from the reactor II into H2S, by using hydrogen gas. Through the blank test, it is confirmed that there is no detectable influence from the reaction between sulfur fumes/H2S and a stainless steal reactor.
S + H2 f H2S
(2)
We confirmed that, using CoMo/γ-Al2O3 and an excess amount of hydrogen (more than 3 times of the stoichiometric amount), sulfur vapor underwent a compete hydrogenation into H2S at temperatures above 300 °C (17). All the lines from the reactor I up to the reactor III were heated to 230-240 °C using heating tapes and heating ovens. The “trap IV” was equipped for the cross checks of the experimental results and filtered the solid sulfur in the gas flow is necessary. The trap IV cooled the outlet gas temperature to room temperature and filtered the solidified sulfur. By weighing the weight changes of the trap IV before and after the reaction, the amount of the accumulated sulfur which was absorbed was measured directly. The sulfur removal test was carried out by the following sequence. First, the feed containing H2S and O2 (H2S/O2 ) 2) in helium was passed through the reactor I and sulfur trap IV in order to measure the H2S remaining after the reaction 1 (This is termed as “Feed A”.). It represents the amount of H2S that is generated after the partial oxidation reaction in reactor I. Second, the “Feed A” was then bypassed the reactor II but passed through the reactor III to reconvert the sulfur fumes into H2S (“Feed B”). So, the “Feed B” is equal to the entire sulfur compound in the inlet flow of the reactor II, including the sulfur fumes. The value is expressed by the H2S concentration. Finally, the “Feed B” was passed additionally through the reactor II in order to carry out the sulfur removal test (“Feed C”). The “Feed C” is the concentration of the outlet sulfur of reaction II. If we let the flow rate of the feed entering through the reactor I be Q1, and the total flow rate (including that of hydrogen) entering through the reactor III be Q2, the percent removal of sulfur in the reactor II can be calculated as follows
% removal of sulfur ) {[H2S in Feed B] - [H2S in Feed C]} × Q2/Q1 {[H2S in Feed B]x Q2/Q1- [H2S in Feed A]}
× 100
where the bracket [] represents the concentration of H2S in the feed. VOL. 36, NO. 13, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic diagram of experimental apparatus for generating sulfur vapor and sulfur removal test and analysis.
TABLE 1. Phase Transformation of Metal Oxides after Presulfiding and Sulfur Absorption metal oxide
stable phasea
presulfiding (400 °C)b
sulfur absorption (300 °C)c
V 2 O5 Cr2O3 MoO3 WO3 MnO2 Fe2O3 CoO NiO Cu2O ZnO
Cr2S3 MoS2 WS2 MnS FeSx Co9S8 NiSx -
d Cr2O3 MoS2/MoO2 WS2/WO2 MnS FeS Co9S8 Ni3S2 CuS ZnS
d Cr2O3 MoS2/MoO2 WS2/WO2 MnS FeS2 CoS2 NiS2 CuS ZnS
a Thermodynamic stable phase at 400 °C. b Presulfiding condition: 20% H2S/H2 for 2 h. c 4500 ppm sulfur, in N2, 100 cm3/min, 1 g sorbent, after 6 h of reaction. d Not detected by XRD.
Characterization of Absorbents. X-ray diffraction patterns of sorbents were obtained using an X-ray analyzer (Mac Science Co., M18XHF). Ni-filtered Cu KR radiation (λ ) 1.5415 Å) was utilized using an X-ray gun operated at 40 kV and 200 mA. Diffraction patterns were obtained at a scan rate of 4°/ min. The identification of the compounds was accomplished by comparison of a measured spectrum with that in JCPDS files. The specific surface area was measured using a BET apparatus (Accusorb 2100E, Micrometrics). The thermal desorption patterns of sulfur from the used adsorbents were monitored by thermal gravimetric analysis (TGA) (PerkinElmer, TGS-2). The flow rate was fixed at 50 mL/min, and the temperature was increased at a rate of 5 °C/min.
Results and Discussion Sulfur Removal via Additional Sulfidizaton. Various metal sulfides as potential absorbent of sulfur were prepared by exposing corresponding metal oxides at a presulfiding condition using a feed of H2S/H2 (H2S/H2 ) 1/4) at 400 °C 3026
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FIGURE 2. Breakthrough curves in the sulfur removal tests with sulfides of Fe, Co, and Ni at 350 °C: 4500 ppm sulfur in N2; 100 cm3/min; 1 g sorbent loading. for 2 h (18, 19). The results of X-ray diffraction analyses for the sulfided metals are shown in the third column in Table 1. After the presulfiding, most of the metal oxides were transformed into metal sulfide, the thermodynamically stable phase (shown in the second column in Table 1). Molybdenum and tungsten were sulfided partially remaining an oxide phase, and chromium was not sulfided at all. We failed to detect any phase of vanadium by X-ray diffraction analysis probably because vanadium became amorphous after the sulfidation. The last column in the table shows the results of the sulfur removal experiments that were carried out by passing a feed containing 4500 ppm sulfur over the prepared metal sulfides at 300 °C for 6 h. Only three metal sulfides, FeS, Co9S8, and Ni3S2, had an additional pick up of sulfur and were transformed into a sulfur rich form of metal disulfide,
FIGURE 3. (a) Thermal gravimetric analyses of various metal sulfides: (a) Co9S8, flowing N2; (b) CoS2, flowing N2; (c) FeS2, flowing N2; (d) NiS2, flowing N2; (e) CoS2, flowing air; total gas flow rate is fixed at 45 cm3/min. Temperature was increased at a rate of 5 °C/min. (b) Thermal gravimetric analyses of various metal sulfides: (a) CoS2, flowing H2; (b) NiS2, flowing H2; (c) FeS2, flowing H2; 10% H2 (He balanced) gas flow rate is fixed at 45 cm3/min. Temperature was increased at a rate of 5 °C/min. indicating that they can be used as potential absorbent of sulfur.
FeS(s) + S(g) f FeS2(s)
(3)
Co9S8(s) + 10S(g) f 9CoS2(s)
(4)
Ni3S2(s) + 4S(g) f 3NiS2(s)
(5)
For these three absorbents, additional experiments of the sulfur absorption were carried out in the packed-bed reactor by measuring the sulfur concentration at the reactor outlet as a function of the processing time. The results in Figure 2 establish that sulfur concentration in a process gas can be reduced significantly to a low level via the deep sulfidation reaction over the sulfur-deficient metal sulfides of the three metals (FeS, Co9S8, Ni3S2) at 350 °C. A typical pattern of the breakthrough curve is observed during the absorption (the
FIGURE 4. (a) Regeneration of NiS2 to Ni3S2 using 50 vol % H2 at (a) 350 °C and 400 °C: H2 15 cm3/min; total 30 cm3/min; 1 g sorbent. (b) Regeneration of FeS2 to FeS using 50 vol % H2 at (a) 350 °C and 400 °C: H2 15 cm3/min; total 30 cm3/min; 1 g sorbent. (c) Regeneration of CoS2 to Co9S8 using 50 vol % H2 at (a) 350 °C and 400 °C: H2 15 cm3/min; total 30 cm3/min; 1 g sorbent. deep sulfidation reaction) test. With the three absorbents, over 90-95% of the sulfur removal can be achieved during the period of the steady state. The amount of sulfur that is taken up by the absorbent can be calculated by integrating the area of the removed sulfur as a function of time. After 7 h the amount of sulfur taken up by the absorbent reaches 59.9%, 55.6%, and 61.7% of the saturation value (the maximum theoretical amount of sulfur that can be taken in VOL. 36, NO. 13, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. X-ray diffraction analyses of nickel sulfides after the sulfur absorption and regeneration step.
FIGURE 6. Repeated absorption with freshly prepared and regenerated nickel sulfide absorbent. the case that sulfur-deficient metal sulfide be converted to metal disulfide) for iron, nickel, and cobalt, respectively. So far there has been no report on the removal of sulfur fumes over solid sorbents at such low temperatures of 300350 °C. Couples of papers deal with the sulfidation of metal alloys in the presence of sulfur. But their purpose was intended to reduce the sulfidation for solving corrosion problems of metal alloy at high temperatures above 700 °C (20-22). To find a possibility of regenerating the saturated metal disulfide back to the sulfur-deficient form, thermal gravimetric analyses were performed for several forms of sulfided metals as shown in Figure 3. Generally metal monosulfide (the case (a) in the figure) is very stable in flowing N2 up to 800 °C. Metal disulfides (the cases (b)-(d)) are less stable than the monosulfide form and begin to lose sulfur in flowing N2 at temperatures of 450-670 °C. When the metal disulfide, CoS2, FeS2, and NiS2, is transformed to Co9S8, FeS, and Ni3S4, the theoretical value of the weight loss is 26, 27, and 26 wt %, respectively. CoS2 and NiS2 were not regenerated fully in 3028
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flowing N2. FeS2 almost reached the calculated value. While CoS2 and NiS2 are so thermally stable, the FeS2 is unstable and the absorbed sulfur could be emitted at the absorption reaction temperature. In flowing air, cobalt disulfide begins to lose sulfur at temperature of 420 °C and then is subsequently transformed into the stable form of metal sulfate. In other aspects, the effect of oxygen on the sulfidation reaction is also shown by the curve (e) in Figure 3. The oxidation and sulfidation reaction of FeS2 and NiS2 occurred competitively below 400 °C. Below 400 °C, the sulfidation reaction of the cobalt sulfide is superior to the oxidation reaction. Figure 3(b) shows the TGA results for hydrogen regeneration. The diluted 10% hydrogen gas was used as the reduction gas. The metal disulfide started to react below 300 °C. At 400 °C, most of the absorbed sulfur was converted to H2S, and the metal disulfides were transformed to their sulfur deficient form. For the low-temperature regeneration of the absorbents, the reductive regeneration by H2 was concluded as the suitable regeneration process. Figure 4(a)-(c) shows a typical pattern of the sulfur desorption from metal disulfide (actually reduction of the disulfide phase to the monosulfide) in a flow of hydrogen gas, convincing to us that the disulfide phases of the three metals can be regenerated to the sulfur-deficient form.
MS2 + H2 f MS2-x + H2S (x ) 1, 1.11, 1.33 for Fe, Co, Ni) (6) X-ray diffraction analysis in Figure 5 shows that, after regeneration with hydrogen gas at 400 °C, the disulfide form is completely transformed into the sulfur-deficient sulfide form. The same results were obtained for all Co and Fe. Figure 6 shows repeated absorption tests of sulfur with fresh and regenerated metal monosulfide. There is no difference in the absorption pattern and capacity between a freshly prepared and a regenerated one. The reductive regeneration roughened the surface of the sorbents and increased the surface to 12.4 m2/g. This increase could effect the sulfidation reactivity. However, regenerated sorbent still has nonporous bulk structure, and the reaction rate is
controlled by the sulfur ion diffusion rate. The overall reaction rate is not strongly affected by the reductive regeneration. Therefore, these three metal monosulfides can be utilized for the removal of elemental sulfur in an emission gas that does not contain oxygen or hydrogen. There are many process industries where a high concentration of H2S (>5%) is removed by the Claus reaction or partial oxidation reaction. The sulfur that is produced by the above H2S removal processes is usually collected using physical trapping devices (coalescence condenser). There has been no method to clean up a low concentration (
