Ind. Eng. Chem. Res. 2007, 46, 2661-2664
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Elemental Sulfur Recovery through H2 Regeneration of a SO2-Adsorbed CuO/Al2O3 Youhua Zhao,†,‡ Zhenyu Liu,*,† Zhehua Jia,†,‡ and Xinyan Xing†,‡ State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, People’s Republic of China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China
CuO/Al2O3 sorbent-catalyst has been studied extensively for SO2 removal from flue gases of stationary sources. In these studies the SO2-adsorbed CuO/Al2O3 is treated by reducing reagents such as H2 to yield concentrated SO2, which is further converted in a separate catalytic reactor to valuable products, such as elemental sulfur. This work reports a novel technique, which combines the SO2 removal, H2 regeneration of the sorbent-catalyst, and elemental sulfur recovery into one reactor using a CuO/Al2O3 sorbent-catalyst simply by recycling the effluent gases back to the reactor during H2 regeneration. This combined process can be operated isothermally at 400 °C with 83% yield to elemental sulfur. The catalyst for the elemental sulfur formation is likely to be CuS formed in the H2 regeneration. Most of the Cu species regenerated from the H2 regeneration is Cu0, which converts to CuO and CuSO4 during the SO2 removal stage. Introduction As an effective sorbent-catalyst for SO2 removal and/or simultaneously SO2 and NOx removal from flue gas, CuO/Al2O3 has been studied since 1970s.1 The research focuses have been mainly on characterization of CuO/Al2O3, the mechanism of CuO/Al2O3 sulfation,2,3 and regeneration of the sulfated CuO/ Al2O3 by reducing reagents such as H2, CO, NH3, or hydrocarbons.3-8 The overall SO2 removal process using CuO/ Al2O3 consists of three stages: reaction of CuO with SO2 to form CuSO4 (SO2 removal, termed sulfation), conversion of CuSO4 to CuO and SO2 by a reducing reagent (regeneration of CuO/Al2O3), and treatment or conversion of the released SO2 to products of market value (sulfur recovery).9 The regenerated CuO/Al2O3 regains its activity and is used in the subsequent SO2 removal stage. In the Shell process, the sulfation and the regeneration of CuO/Al2O3 are carried out at 400 °C,1 and the concentrated SO2 released from the regeneration in H2 is reacted with H2S in a catalytic Claus reactor to produce elemental sulfur. In other studies, the released SO2 is converted to sulfuric acid or sulfate using also reactors different from that for SO2 removal. Apparently, all these sulfur recovery methods require additional costs for reactors and catalysts. If it is possible to combine the sulfur recovery stage with the other two stages in one reactor, the overall cost of the process can be reduced. This combination seems possible for elemental sulfur formation because all the three stages in SO2 removal can be carried out at the same temperature, 400 °C, for example, and some of the sulfated CuO/Al2O3 transforms into CuS/Al2O3 in H2 regeneration,6,7 which is likely to be a catalyst for Claus reaction.10 This work reports a recent study on this subject, where CuO/Al2O3 is used for SO2 removal and elemental sulfur recovery in a single fixed bed reactor system. Effect of regeneration temperature on elemental sulfur yield and the activity and stability of CuO/Al2O3 during continued SO2 removal-elemental sulfur recovery cycles are mainly discussed. * To whom correspondence should be addressed. Fax: +86 351 4050320. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
Figure 1. Experimental apparatus.
Sorbent Preparation The γ-Al2O3 support (30-60 mesh, BET surface area of 185 m2‚g-1) was impregnated with an aqueous Cu(NO3)2‚3H2O solution and then calcined at 500 °C for 8 h in a muffle furnace after drying at 50 °C for 8 h and at 110 °C for 5 h. The final sample contains about 8 wt % CuO. Experimental Procedure Figure 1 shows the flow chart of the overall process. First, the SO2 removal stage is carried out at 400 °C by feeding a simulated flue gas, containing 2200 ppm SO2, 6% O2, and balance N2, to the fixed-bed reactor of 15 mm in diameter with 4 g of CuO/Al2O3 (the dotted lines in Figure 1). The feeding velocity of simulated flue gas is 400 mL/min, corresponding to a space velocity of 6000 h-1. When the SO2 removal rate decreases from the initial 100% to 80%, the feed flue gas is switched to pure N2 to purge the reactor for 30 min. The reactor is then fed with H2, at a flow rate of 4 mL/min, and the tail gas from the reactor, at a flow rate of 400 mL/min by a mini-gas recycling pump, to start the regeneration and the elemental sulfur recovery stages (the solid lines in Figure 1). The elemental sulfur formed is collected in the cold trap. The compositions of the inlet and the outlet of the reactor (during the SO2 removal stage) and the recycled gas composition (during the H2 regeneration and elemental sulfur recovery stages)
10.1021/ie0610041 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/13/2007
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Figure 2. SO2 breakthrough curves of different CuO/Al2O3 samples in the sulfation process.
Figure 3. Effect of regeneration temperature on elemental sulfur collection (A) and on SO2 removal capacity (B).
are all determined on-line by a Blazers QMG 422 quadrupolar mass spectrometer. The H2 feeding rate and cumulative H2 fed are controlled and measured by a mass flow controller. To prevent elemental sulfur condensation prior to the cold trap, the tubing between the reactor and the cold trap is maintained at 250 °C with a heating tape while the cold trap is maintained at room temperature. The regeneration and elemental sulfur recovery stages are terminated when the amount of elemental sulfur collected in the cold trap does not increase further. The elemental sulfur is then dried and weighed. The yield of elemental sulfur is defined as
YT (%) ) mT/MT
Figure 4. MS profile of gaseous sulfur species with increasing H2/Cu.
where YT is the sulfur yield at the regeneration temperature of T, mT is the mass of elemental sulfur collected at the regeneration temperature of T, and MT is the mass of regenerable sulfur adsorbed on CuO/Al2O3 which equals the amount of sulfur (elemental sulfur) adsorbed in the subsequent SO2 removal stage. The amount of CuS is determined by the difference in watersoluble Cu (measured by inductively coupled plasma-atomic emission spectrometry, Atomscan 16, TJA, U.S.A.) in the sorbent-catalyst before and after the conversion of CuS to CuSO4. Sulfur content of the sorbent-catalyst is measured by elemental analysis (KZDL-3B, China). The elemental valence is determined by X-ray photoelectron spectroscopy (XPS) measurement, which is carried out on a PHI-5300 ESCA system using Al KR radiation (1486.6 eV) at a residual pressure of 10-10 Torr. Sulfation Process of CuO/Al2O3 Figure 2 shows effluent SO2 concentration during sulfation of fresh and regenerated CuO/Al2O3 at 400 °C. The breakthrough time is about 135 min for the fresh sample and 80 min for the regenerated sample. Literature shows that CuO is the main component for SO2 removal (1) while Al2O3 adsorbs SO2 mainly in the first operation (2) because most of them cannot be regenerated in the conditions used. This agrees with the shortened breakthrough time for the regenerated sample.
CuO + SO2 + 1/2O2 f CuSO4
(1)
Al2O3 + 3SO2 + 3/2O2 f Al2(SO4)3
(2)
It is important to note that significant SO2 release is observed at the early stage of sulfation of regenerated CuO/Al2O3,
suggesting the presence of reductive sulfur species, possibly CuS or Cu2S,4 in the regenerated samples. Effect of Regeneration Temperature Figure 3 shows elemental sulfur yields at various regeneration temperatures after sulfation at 400 °C (Figure 3A) and the subsequent SO2 removal capacity after the regeneration (Figure 3B). No elemental sulfur is collected at the regeneration temperature of 320 °C, although the catalyst shows some SO2 adsorption capacity after the regeneration, indicating mild regeneration activity and minimal YT at this temperature. The amount of elemental sulfur collected increases with increasing regeneration temperature when the regeneration temperature is above 350 °C. A steady maximum amount of elemental sulfur of 17 mg/g-sorb is obtained when the regeneration temperature is above 400 °C. The SO2 adsorption capacity follows the same trend and reaches a steady maximum of 39 mg SO2/g-sorb. These results indicate that 400 °C and higher is favorable for both CuSO4 conversion and elemental sulfur recovery. Distribution of Sulfur Species During the H2 Regeneration Clearly the H2 regeneration involves many reactions with sulfur-containing species either as reactants or as products. Figure 4 shows MS signals of gaseous sulfur species, SO2 and H2S, observed in the regeneration at 400 °C. The H2/Cu is the ratio of cumulative H2 fed to the reactor to the amount of Cu in the catalyst, both in millimoles. The residence time (t0) of the system is about 5 min, which is calculated through dividing the reactor system volume by the velocity of recycling gas, corresponding to a superficial H2/Cu of 0.2. The feed of H2 to the sulfated sorbent-catalyst results in a rapid release of SO2
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Figure 6. Distribution of sulfur species after H2 regeneration at various temperatures. Figure 5. XPS spectra of regenerated CuO/Al2O3.
after a short delay due possibly to the small amount of H2 fed in the short time period and to adsorption of the SO2 released from the upper section by the bottom section in the integrated reactor because the CuO/Al2O3 under regeneration is not saturated with SO2. Possible reactions suggested in the literature include
CuSO4 + 2H2 f Cu + SO2 + 2H2O
(3)
CuSO4 + 4H2 f CuS + 4H2O
(4)
Al2(SO4)3 + 3H2 f Al2O3 + 3SO2 + 3H2O
(5)
At a H2/Cu of 1.3, the SO2 concentration starts to decrease to a stable level. At a H2/Cu of 4.2, a further decrease in SO2 concentration is observed which is accompanied by deposition of elemental sulfur in the cold trap. When the SO2 concentration approaches the baseline at a H2/Cu of about 5.2, a sharp rise of H2S concentration is observed, and the deposition of elemental sulfur becomes minimal. It should be pointed out that the volume of the system undergoes various changes during the regeneration and elemental sulfur recovery process, increases initially up to a H2/Cu ratio of about 4.2, decreases between H2/Cu ratios of 4.2 to 5.2, and then increases thereafter, as evidenced by the expansion of the balloon. This may affect the changes in concentrations of the gaseous sulfur species. These phenomena suggest that the sulfur forms in the system undergo a series of changes in the regeneration: solid sulfates only at a H2/Cu of 0; solid sulfur (sulfates and possibly sulfides) and gaseous SO2 at H2/Cu of 0.5-4.2; solid sulfides, elemental sulfur, and gaseous SO2 at H2/Cu of 4.2-5.2; solid sulfides, elemental sulfur, and gaseous H2S at H2/Cu of 5.2 and higher. This indicates that the sulfates cannot be fully converted to elemental sulfur; the process may follow Clause mechanism (eq 6); and H2S is formed from reaction of SO2 with H2 (eq 7). Al2O3
SO2 + 2H2S 98 3S + 2H2O MS
SO2 + 3H2 98 H2S + 2H2O
(6) (7)
These indications are supported by the XPS analysis of the regenerated sorbent-catalyst. In the S 2p spectra (Figure 5A), the peaks at 162.5 and 168.5 eV indicate the presence of S2and S6+ species, respectively. The satellite of S2- observed at
163.7 eV indicates the existence of at least two kinds of sulfide (CuS/Cu2S or Al2S3). Since the elemental analysis shows absence of CuSO4 in the regenerated sorbent-catalyst, the S6+ peak may result only from Al2(SO4)3. In the Cu 2p spectra (Figure 5B), the two peaks centered at 932.5 and 952.5 eV suggest the presence of Cu0 (Cu 2p3/2 at 932.5 eV and Cu 2p1/2 at 952.5 eV), and the satellites with binding energies of 933.3 and 953.3 eV indicate the existence of Cu2+.7 These data indicate that Cu0 is the main species and CuS the minor species on the sorbent-catalyst after the H2 regeneration at 400 °C. Apparently, the results discussed so far indicate that the sulfur species in the sulfated CuO/Al2O3 transform into three sulfurcontaining compounds during the H2 regeneration at 400 °C: (i) Al2(SO4)3 and CuS in the catalyst, (ii) gaseous SO2 and H2S in the system, and (iii) elemental sulfur collected in the cold trap. Figure 6 shows distribution of these sulfur forms at various temperatures. The elemental sulfur increases from 0.28 mmol/ g-sorb to 0.50 mmol/g-sorb, and the gaseous sulfur decreases from 0.23 mmol/g-sorb to 0.08 mmol/g-sorb when the regeneration temperature increases from 350 °C to 400 °C. No CuSO4 exists after the regeneration at regeneration temperatures above 400 °C. The residual sulfur in the catalyst after the regeneration consists of about 0.4 mmol/g-sorb Al2(SO4)3 at all the regeneration temperatures and 0.29 mmol/g-sorb CuS at a regeneration temperature of 400 °C. The constant values for Al2(SO4)3 suggests its formation in the SO2 removal stage and its resistance to H2 regeneration. Thermodynamic calculations shows that ∆rG values for reactions 3-5 are negative in the range of 300-500 °C, and reactions 3 and 4 are exothermic with ∆rH of -8.9 and -244.2 kJ‚mol-1, respectively; reaction 5 is endothermic with ∆rH of 169.3 kJ‚mol-1. Continued Operation Figure 7 shows elemental sulfur yield and SO2 removal capacity during continued sulfation-regeneration cycles at 400 °C. After showing a high initial SO2 removal capacity (1.25 mmol SO2/g-sorb), the consecutive seven cycles maintain a constant SO2 removal capacity of 0.6 mmol SO2/g-sorb and an elemental sulfur yield of 83%. The decreased SO2 removal capacity can be attributed to the formation of Al2SO4, which is not regenerable under the conditions used, while the 83% elemental sulfur yield can be attributed to the formation of H2S, which stays in the gas phase after the regeneration. Obviously CuO/Al2O3 has good stabilities in the sulfation, H2 regeneration, and elemental sulfur recovery.
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83% and a SO2 removal capacity of 0.6 mmol SO2/g-sorb. Some of the Cu in the catalyst is converted into CuS in the regeneration, which may be responsible for catalyzing the reaction for elemental sulfur recovery. Acknowledgment The authors express their grateful appreciation to financial support from the Natural Science Foundation of China (90210034). Literature Cited
Figure 7. Sulfur yield and subsequent SO2 removal capacity in continued cycles at 400 °C.
The results presented so far show that recycle of the tail gas back to the reactor is effective for production of elemental sulfur; CuO/Al2O3 is the sorbent for flue gas SO2 removal and the catalyst, after been in situ transformed, for elemental sulfur recovery; the overall process is superior to the traditional configuration because a separate catalytic converter is not needed. It is worth further noting that the whole process can be carried out in two parallel fixed-bed reactors with periodical switching between the SO2 removal mode and the regeneration/ S-recovery mode, or in two serial moving bed reactors, one for SO2 removal and one for regeneration/S-recovery. Conclusions Elemental sulfur recovery can be integrated with regeneration of SO2 adsorbed CuO/Al2O3 into a single fixed bed reactor when CuO/Al2O3 and H2 regeneration are used and the effluent gases from the regeneration operation is recycled back to the same reactor. Temperatures greater than 350 °C are necessary for regeneration of the sulfated (SO2-adsorbed) sorbent-catalyst and recovery of elemental sulfur. To a CuO/Al2O3 with 8% CuO, the maximum sulfur yield can be obtained at regeneration temperatures of 400 °C and higher, with a sulfur yield of about
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ReceiVed for reView July 31, 2006 ReVised manuscript receiVed December 29, 2006 Accepted February 9, 2007 IE0610041
