Simultaneous Removal of COS and H 2 S at Low Temperatures over Nanoparticle α-feooh Based Catalysts

Transcription

1 Journal of Natural Gas Chemistry 12(2003)37 42 Simultaneous Removal of COS and H 2 S at Low Temperatures over Nanoparticle α-feooh Based Catalysts Zhihua Gao, Chunhu Li, Kechang Xie State Key Lab of C1 Chemistry and Technology, Institute of Coal Chemical Engineering, Taiyuan University of Technology, Taiyuan , China [Manuscript received July 12, 2002; revised August 16, 2002] Abstract: Catalysts using α-feooh nanoparticles as the active ingredient were tested by a microreactorchromatography assessing apparatus at atmospheric pressure between 25 and 60 with a gas hourly space velocity of 10,000 h 1, while the removal performance of H 2S with catalysts was investigated using the thermal gravimetric method. The results show that the catalysts are highly active for COS hydrolysis at low temperatures ( 60 ) and high gas hourly space velocity, and the highest activity can reach 100%. The catalyst is particularly stable for 12 h, and no deactivation is observed. Nanoparticle α-feooh prepared using hydrated iron sulfate shows higher COS hydrolysis activity, and the optimum calcination temperature for the catalyst is 260. In addition, the catalysts can remove COS and H 2S simultaneously, and 60 is favorable for the removal of H 2S. The compensation effect exists in nanoparticle-based catalysts. Key words: nanoparticle α-feooh, COS hydrolysis, removal of H 2S, compensation effect 1. Introduction A nanoparticle is a small solid particle whose size is less than 100 nm. Because of their volume, surface and quantum effects[1], nanoparticles exhibit special performance in catalysis[2]. Nanoparticles are being used as new catalytic materials and correlative study is the developing trend and main direction of the catalytic field. Over the last two decades, it has become increasingly apparent that emissions of sulphur compounds, including carbonyl sulfide (COS), into the atmosphere have been unacceptably high. From the viewpoint of environmental protection, the removal of sulphur is absolutely necessary. Moreover, sulphur is normally regarded as a significant poison for heterogeneous catalysts (e.g. the Cu/ZnO/Al 2 O 3 methanol synthesis catalyst and the Fe ammonia synthesis catalyst[3]). Well-established procedures have been used for many years to remove sulphur compounds by hydrodesulphurisation that converts the sulphur containing compounds to hydrogen sulphide (H 2 S)[4]. Since it is difficult to remove COS completely using the conventional desulfurization process, an alternative technology has to be used that is based on the formation of hydrogen sulphide by hydrolysis: COS + H 2 O = CO 2 + H 2 S (1) Most studies show that alumina, or alumina supported transition metal oxides, are effective catalysts for the hydrolysis reaction. However, apart from a few studies[5,6] most of these studies concentrate on reaction conditions using temperatures in excess of 200 [7 10]. H 2 S can then be removed by adsorption; namely, two processes were required to remove COS. Recently, there has been intense interest in the use of mild reaction conditions for catalytic reactions of environmental value. Miura et al. [5] reported that COS can be removed at around 50 by a single Corresponding author.

2 38 Zhihua Gao et al./ Journal of Natural Gas Chemistry Vol. 12 No process, i.e. utilizing a special form of iron oxides prepared by partially dehydrating α-feooh. This method presented the possibility of simultaneously removing COS and H 2 S at low temperatures. However, the activity of COS hydrolysis over the abovementioned catalysts was lower than that of γ-al 2 O 3, and the catalyst rapidly deactivated. In this paper, nanoparticle α-feooh was prepared by means of homogenous precipitation and used as the active ingredient in order to increase the COS hydrolysis activity and overcome the catalytic deactivation. 2. Experimental 2.1. Catalyst preparation Nanoparticle α-feooh was prepared using homogeneous precipitation. Two kinds of Fe 3+ salt were used for preparation. The first was hydrated iron sulfate (the corresponding catalysts were named s1 s6 according to different preparation conditions), and the second was hydrated iron nitrate (the corresponding catalysts were named n1 n3 according to different preparation conditions). Nanoparticle α-feooh was obtained from hydrated iron sulfate or hydrated iron nitrate, which was hydrolyzed by a freshly prepared urea solution. The solution was heated to 363 K in a flask equipped with a stirrer. The ph value was continuously measured by a combined electrode immersed in the solution. When the change in ph value was less than ±0.2, the reaction was ended. After the reaction, the precipitate was fully washed and dried. The average particle diameter was less than 100 nm[11]. The resultant powder was mixed with CaO and water glass, pressed to form wafers, crushed and sieved into mesh granules and finally calcined at the given temperature for two hours Catalyst evaluation The activity evaluation of COS hydrolysis over the catalyst was carried out in a continuous fixedbed flow micro-reactor made of stainless steel with a catalyst bed diameter of 5 mm. A thermocouple was located approximately in the center of the catalyst bed. The reactor and reaction conditions were evaluated to ensure that mass transfer limitations were not observed in any of the experiments reported. In the absence of catalyst, no conversion was observed. A small amount of catalyst (w= g) was placed in the above-mentioned reactor. After completely purging the air from the reactor with nitrogen at room temperature, the reactor was heated to the reaction temperature ( ) and fed with COS (C(COS)=150 mg/cm 3 ) at atmospheric pressure and a GHSV of 10,000 h 1. H 2 O was supplied by a syringe pump, and the H 2 O/COS ratio was larger than 200 in this study. Steady-state conditions were usually established after a preliminary period, as indicated by the constant COS conversion. The COS preparation method is described in reference 6. The H 2 S removal efficiency was investigated by a thermobalance (Tianjin University, TG-1). The catalyst (17 19 mg) was placed in the reactor and treated with nitrogen in a temperature-programmed manner until the weight of the catalyst and the reaction temperature (25 80 ) remained constant. After treatment, the catalyst was exposed to H 2 S (C(H 2 S)=4.15 g/m 3 ), and the reaction was ended when the catalyst weight no longer increased. The adsorptive capacity of H 2 S over the catalyst was calculated according to the TG curve. The exit gases from the reactor were analyzed by gas chromatograph (Shimadzu GC-9A), which was equipped with a 3.0 m 3 mm (i.d.) stainless steel column of porapak Q and a flame photometric detector (FPD) to analyze COS and H 2 S. COS conversion was calculated as follows: X = (C 0 C)/C 0 100% (2) Where X: Conversion of COS; C 0 : Inlet concentration of COS; C: Outlet concentration of COS. 3. Results and discussion 3.1. Effect of the Fe 3+ salt type on COS conversion Preliminary experiments were carried out to determine the stability of the catalyst. It was found that the catalyst becomes particularly stable for 12 h with no deactivation observed after an initial period of about 1 h. Representative data for the effect of time on stream on COS conversion are shown in Figure 1. The conversion percentages of COS hydrolysis over s1 s3 catalysts are shown in Figure 2. The activity of COS hydrolysis over s1 s3 catalysts is high at low temperatures (less than 60 ), and even at

3 Journal of Natural Gas Chemistry Vol. 12 No , the conversion of COS can reach 100%. Generally, the activity of n1 n3 catalysts is lower than that of s1 s3. The conversion of COS is about 90% at 140 over n1 n3 catalysts. In our previous study of XRD[11], α-feooh was the main component of s1 s3, but the XRD patterns of n1 n3 displayed two kinds of peaks (α-feooh and α-fe 2 O 3 ). Moreover, FT-IR[11] spectra of s1 s3 had intense peaks at and cm 1, which clearly proved the existence of α-feooh, while those of n1 n3 were very weak. COS hydrolysis is an acid-base catalytic reaction, and the basic capacity of α-feooh is stronger than that of α-fe 2 O 3. Therefore, α-feooh has a higher COS hydrolysis activity than α-fe 2 O 3. Thus, in the following study, all α-feooh nanoparticles were prepared from hydrated iron sulfate Effect of calcination temperature on COS conversion The s3 catalyst was calcined at 110, 260 and 350 for 2 hours. The effect of calcination temperature on COS hydrolysis is shown in Figure 3. The activity of COS hydrolysis over s3 catalyst is the highest when calcination temperature is 260 because α- FeOOH consists of iron-centered oxygen octahedrally joined by the sharing of edges into two-dimensionally infinite layers, with the successive layers held together by hydrogen bonds[5]. When heated to , α- FeOOH loses physically adsorbed water. Upon heat treatment of α-feooh above 200, the hydrogen bonds broke and the structural hydrogen was removed as H 2 O, leaving the reactive surface and uniform interstices[12]. Upon further heat treatment, the interstices collapsed and finally α-fe 2 O 3 was formed. The removal efficiency of COS on α-fe 2 O 3 is less than on α-feooh due to the elimination of chemisorbed water, the decrease of reactive surface and the failure of interstices. Moreover, it is possible to adjust the surface properties and the microstructure by changing the severity of heat treatment. Figure 1. COS conversion as a function of time Reaction conditions: s4 catalyst, temperature 25, GHSV=10,000 h 1, C(COS)=150 mg/cm 3 Figure 2. The conversion of COS hydrolysis over different catalysts Reaction conditions: GHSV=10,000 h 1, C(COS)=150 mg/cm 3 Figure 3. The effect of calcination temperature on COS hydrolysis Reaction conditions: s3 catalyst; reaction temperature: ( )25, ( )30, ( )35, ( )40 ; the other conditions are the same as those in Figure 2

4 40 Zhihua Gao et al./ Journal of Natural Gas Chemistry Vol. 12 No Effect of temperature on COS conversion The conversion of COS hydrolysis over the six catalysts at different temperatures (at ambient pressure and a gas hourly space velocity of 10,000 h 1 ) are shown in Figure 4. The conversion of COS hydrolysis over the six catalysts increases with the increase in temperature, and it is above 95% at 60 over the six catalysts. In our previous works it was tested that COS hydrolysis is a first-order reaction. According to the first-order reaction equation (Equation 3) and the Arrhenius equation (Equation 4): ln(1 X) = 3, 600k/V sp (3) k = Aexp( E/RT ) (4) Where k: Reaction rate constant; V sp : gas hourly space velocity; A: Pre-exponential factor; E: Activation energy. The reaction rate constants for different temperatures are listed in Table 1. The linear plot results of COS hydrolysis are also shown in Table 1. As shown in Table 1, the pre-exponential factor increases with the increase in activation energy. Figure 5 shows the linear relationship between the activation energy and the logarithm of the pre-exponential factor and that the compensation effect exists among the six catalysts. There have been many attempts to explain this compensation effect in recent years. The explanation by Tan[10] suggested that there is an exponential energy distribution of the catalyst surface, and thus the higher activity is not related to the lower activation energy. When comparing the compensation effect equation (lna = be + c) and the Arrhenius relation (lna = E/RT +lnk), we find that they define an isokinetic temperature Ts by equation Ts=1/(R b), where the k s are identical for all values of E. At Ts the relative reaction rates within a group of catalysts undergo an inversion. The activation energy values can be used as a criterion to judge the reaction activity or not depending on the isokinetic temperature Ts due to the existence of the compensation effect. When operating temperature is above Ts, greater activation energy means higher activity. However, while operating temperature is below Ts, lower activation energy means higher activity. The isokinetic temperature of COS hydrolysis over s1 s6 catalysts is Figure 4. The conversion of COS hydrolysis over the six different catalysts at different temperatures Reaction conditions are the same as those in Figure 2 Figure 5. The linear relationship between activation energy and logarithm of preexponential actor Reaction conditions are the same as those in Figure 2 Table 1. Reaction rate constant and the results of a linear plot of COS hydrolysis Catalyst No. k (s 1 ) E(kJ/mol) lna(s 1 ) s s s s s s

5 Journal of Natural Gas Chemistry Vol. 12 No Removal Efficiency of H 2 S In the above experiments, no H 2 S was detected in the tail gas for any of the catalysts examined. This clearly proves that catalysts can simultaneously remove H 2 S. In order to confirm this fact, the removal of H 2 S by the catalysts was performed by a thermobalance in the temperature range from The adsorptive capacity of H 2 S over the s3 catalyst at different temperatures is shown in Figure 6. A desorptive capacity of H 2 S quickly increases in the initial stage of reaction, but the curves tend to stabilize in the final period. Moreover, the H 2 S removal ability of the catalysts at 60 is higher than those at other temperatures, and the optimum temperature for H 2 S removal should therefore be 60. Figure 7 shows the curves of different catalysts for H 2 S removal at 60 and that there are no obvious differences in the adsorptive capacities of H 2 S among the s3 s5 catalysts. In our future research, we plan to examine whether there are competing and independent active sites for COS and H 2 S. In summary, nanoparticle α-feooh based catalysts have a double role. Namely, one acts as a catalyst for COS hydrolysis, and the other acts as an adsorbent to remove H 2 S. The above work presents a new way to remove both COS and H 2 S at low temperatures in only a single process. Figure 7. The adsorptive capacity of H 2 S over different catalysts Reaction conditions: s3 s5 catalysts, C(H 2 S)=4.15 g/m 3, temperature 60, GHSV=10,000 h 1 4. Conclusions Figure 6. The adsorptive capacity of H 2 S over the s3 catalyst at different temperatures Reaction conditions: s3 catalyst, C(H 2 S)=4.15 g/m 3, GHSV=10,000 h 1 (1) Nanoparticle α-feooh based catalysts are particularly stable for 12 h, no deactivation is observed. (2) The type of Fe 3+ salt has a significant effect on COS conversion. Nanoparticle α-feooh catalysts prepared by hydrated iron sulfate show higher values for COS hydrolysis activity. (3) The calcination temperature plays an important role in COS hydrolysis. The optimum calcination temperature is 260. (4) The catalyst is highly active for COS hydrolysis at low temperatures ( 60 ) under high space velocity, and the highest activity can reach 100%. (5) The compensation effect exists in the catalysts. (6) The temperature of 60 is favorable for the removal of H 2 S. Nanoparticle α-feooh based catalysts can remove COS and H 2 S simultaneously.

6 42 Zhihua Gao et al./ Journal of Natural Gas Chemistry Vol. 12 No References [1] Gesser H D, Goswami P C. Chem Rev, 1989, 89 (4): 765 [2] Kagawa M, Honda F, Onodera H et al. Mat Res Bull, 1983, 18(9): 1081 [3] Twigg M V. Catalyst Handbook. Frome: Wolfe Publishers, 1989 [4] Peter Williams B, Nicols C, John W et al. Catal Today, 1999, 49: 99 [5] Kouichi M, Kazuhiro M, Tomohiko I et al. Ind Eng Chem Res, 1992, 31(1): 415 [6] Tan Shishao, Li Chunhu, Liang Shengzhao et al. Catal Lett, 1991, 8: 155 [7] Vincont A Y, Dalla Lana I G, Can J. Chem Eng, 1978, 56: 751 [8] George Z M. J Catal, 1974, 32: 261 [9] Namba S, Shiba T. Kogyo Kagaku Zasshi, 1968, 71: 93 [10] Fiedorow R, Leaute R, Dalla Lana I G. J Catal, 1984, 85: 339 [11] Gao Zhihua, Li Chunhu. Eng Chem Metall (Huagong Yejin, China), 2000, 21(4): 341 [12] Naono H, Fujiwara R J. Colloid Interface Sci, 1980, 26: 406