Regenerable CuO-Based Adsorbents for Low Temperature

Mar 25, 2015 - A series of CuO-based adsorbents for deep removal of H2S at low temperature was prepared by a coprecipitation method. It was found that...
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Regenerable CuO-Based Adsorbents for Low Temperature Desulfurization Application Dai Liu, Shaoyun Chen, Xiaoyao Fei, Chunjie Huang, and Yongchun Zhang* State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China S Supporting Information *

ABSTRACT: A series of CuO-based adsorbents for deep removal of H2S at low temperature was prepared by a coprecipitation method. It was found that CuO-based adsorbents are able to remove H2S from a CO2 stream to less than 0.1 ppm at 40 °C. Among them, Fe−Cu−Al−O adsorbent exhibited the highest breakthrough capacity of 113.9 mg g−1, which is more than 6 times that of pure CuO. The breakthrough capacity was also dependent on the adsorption temperature, space velocity, balance gas, and calcination temperature. The proper adsorption temperature should be lower than 100 °C in the presence of CO2, and a higher space velocity and calcination temperature could decrease the breakthrough capacity significantly. In addition, the CuO-based adsorbents had a regeneration rate of 43−90% in air at a relative low temperature from 100 to 200 °C with a stable breakthrough capacity after four adsorption−regeneration cycles.

1. INTRODUCTION Carbon dioxide (CO2) has been regarded as the main contributor to global warming. CO2 capture, utilization, and sequestration (CCUS) is now accepted as the most effective technology to solve the problem. However, the CO2 captured from coal- or natural-gas-fired power plants and other industrial processes has to be deep purified prior to utilization or pipeline transportation, for it always contains some impurities such as hydrogen sulfide (H2S). H2S is a highly toxic and malodorous gas. In many cases, the sulfur content needs to be reduced to less than parts per million (ppm) levels, for even a trace amount of H2S can cause catalyst poisoning and pipeline corrosion. Adsorption method as an efficient sulfur removal method is required for these applications. Many metal oxide based adsorbents have been studied for the removal of H2S from a series of industrial gas streams at high temperatures (>300 °C).1−6 However, there is a growing requirement to remove H2S at low temperatures ( Zn/Mn ≥ Zn/ Ti/Zr, Zn/Co, Zn/Al > ZnO.12 Jiang et al. concluded that the Cu-rich adsorbents were more suitable than the Zn-rich adsorbents for H2S adsorption at low temperatures.21 The low-temperature desulfurization ability of CuO-based sorbents is much better than that of ZnO-based adsorbents. In spite of the promising properties of CuO-based adsorbents in H2S adsorption, little information is known about their low temperature desulfurization ability, and the high regeneration temperature of CuO-based adsorbents is not applicable in industry.22,23 In this study, a series of CuO-based adsorbents were prepared and tested for the first time toward low-temperature adsorption of H2S from a CO2 stream. The sulfidation and regeneration properties of the adsorbents with/without various promoters were investigated as well as the operation parameters including the effects of adsorption temperature, space velocity, the presence of CO2 in the feed gas, and the calcination temperature. Received: January 13, 2015 Revised: March 23, 2015 Accepted: March 25, 2015

A

DOI: 10.1021/acs.iecr.5b00180 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

2. EXPERIMENTAL SECTION 2.1. Adsorbents Preparation. The pure and metal-doped CuO adsorbents were prepared by a coprecipitation method using Cu(NO3)2, Al(NO3)3, Fe(NO3)3, Co(NO3)2, Ni(NO3)2, and Ce(NO3)3 as precursors of Cu, Al, Fe, Co, Ni, and Ce. The metal salts in desired proportions were dissolved in deionized water and the mixed solution (1.25 M) was placed in a water bath which was heated to 70 °C, and then 20% excess Na2CO3 solution (1.25 M) was added rapidly to the preheated solution under vigorous stirring. The precipitate was aged for 60 min in the mother liquor before it was filtered and washed with deionized water. Finally the obtained solid was dried at 110 °C for 12 h and calcined at 350 °C for 4 h. The finely powdered samples were pressed into disks, crushed, and sieved to 40−60 mesh particles before adsorption tests. The metal-doped CuO samples were denoted as Cu−Al−O and M−Cu−Al−O (M = Fe, Co, Ni, Ce; Cu/Al = 3:1, M/(Cu + Al) = 1:9). 2.2. Adsorbent Characterization. The powder X-ray diffraction (XRD) patterns were collected by a D/max-2400 with Cu Kα radiation (λ = 1.5418, 40 kV, 100 mA). The samples were scanned from 2θ of 10−80° with a step length of 0.02. N2 physisorption measurements were obtained using a Quantachrome AUTOSORB-1-MP at 77 K. Before any measurements were taken, the samples were outgassed at 300 °C for 3 h under vacuum. The specific surface area was calculated using the Brunauer−Emmett−Teller (BET) method. The X-ray photoelectron spectroscopy (XPS) characterization was performed on an ESCALAB MK II X-ray photoelectron spectrometer. The sample binding energies (BEs) were calibrated with the contaminant C 1s line (284.6 eV). 2.3. H2S Adsorption Test. The H2S adsorption test of the adsorbents was conducted on a fixed-bed reactor. The quartz adsorption tube (4 mm i.d.) was placed in an electric furnace whose temperature was monitored by a K-type thermocouple and controlled by a temperature-programmed unit. In a typical adsorption process, when the adsorbent was heated to the required temperature, a gas mixture containing 1000 ppm H2S/ CO2 was introduced from the top at a flow rate of 100 mL min−1. The H2S concentration in the effluent gas was detected by gas chromatography (FULI) with a flame photometric detector (FPD). The breakthrough concentration of H2S was defined as 0.1 ppm. The regeneration of the used adsorbents was performed in the same reactor, and the introduced gas was switched to air at a flow rate of 40 mL/min and heated to the required temperature at a rate of 5 °C/min. The breakthrough capacities of the adsorbents were calculated according to the following equation: Cap(BT) =

Figure 1. XRD patterns of adsorbents used in the work. (a) CuO; (b) Cu−Al−O; (c) Fe−Cu−Al−O; (d) Co−Cu−Al−O; (e) Ni−Cu−Al− O; (f) Ce−Cu−Al−O.

two characteristic peaks at 2θ = 35.5 and 38.7°. The intensity of the two peaks significantly decreased after the addition of Al, Fe, Co, Ni, and Ce oxides, and the diffraction width became broader. However, no diffraction peak for the additives was found in the patterns. The Al phase must well-dispersed in the mixed oxides presented in its amorphous state. The doped Fe, Co, Ni, and Ce phases may also improve the dispersion of CuO or replace Cu2+ in the structure of CuO as reported by Skrzypski et al.13 Among them, the Ce doping should have the best dispersion effect since most of the XRD diffraction peaks for CuO crystal disappeared. 3.2. Desulfurization Evaluation of the Adsorbents. The desulfurization tests for the pure CuO, Cu−Al−O, and M−Cu−Al−O series adsorbents were evaluated at 40 °C, under a flow rate of 100 mL min−1. The breakthrough curves of the adsorbents are shown in Figure 2, and the breakthrough capacities along with the BET surface areas are listed in Table 1. It can be seen that the breakthrough capacity of Cu−Al−O adsorbent (86 mg g−1) is much better than that of the pure CuO (17.7 mg g−1). The addition of Al has increased the BET

BT FR C H2S (34 × 10−6) VmolW

(1)

where Cap(BT) is the breakthrough capacity (mg g−1); BT is the breakthrough time (min), the time when the outlet H2S concentration reaches 0.1 ppm; FR is the flow rate (mL min−1); CH2S is the H2S concentration in the feed gas, generally 1000 ppm; Vmol is the molar volume (24.4 L mol−1, under standard conditions); and W is the sample weight (g).

3. RESULTS AND DISCUSSION 3.1. XRD Analyses. Figure 1 shows the XRD patterns of the CuO-based adsorbents. As we can see, pure CuO displays

Figure 2. Breakthrough curves of H2S adsorption on CuO-based adsorbents. Conditions: T, 40 °C; W, 0.3 g; flow rate, 100 mL min−1; inlet gas mixture, 1000 ppm H2S in CO2. B

DOI: 10.1021/acs.iecr.5b00180 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Fe−Cu−Al−O adsorbent, the surface Fe content is similar to the bulk, and the Cu/Al ratio is higher than it is in the Cu−Al− O adsorbent. It can be deduced that, with the addition of Fe, the content of the surface active component increased, consequently improving the desulfurization ability. The XPS spectra of Fe−Cu−Al−O adsorbent for Fe 2p and Cu 2p are shown in Figure 3. The Fe 2p3/2 appeared around 711.3 eV, and Fe 2p1/2 at 724.5 eV corresponds mainly to typical values of Fe2O3.24,25 The Cu 2p3/2 peak appeared around 933.6 eV, and two satellite peaks about 943 and 963 eV indicated that the copper species are mainly composed of Cu2+.26,27 The XPS results for the Cu species are in agreement with the XRD results which indicated the presence of CuO crystal phases. 3.4. Effect of Adsorption Temperature. The adsorption temperature has a major impact on the desulfurization performance.28 The Fe−Cu−Al−O adsorbent was used to investigate the temperature effect in the range 40−130 °C, and the breakthrough capacities are shown in Figure 4. It was found

Table 1. Breakthrough Capacities and BET Surface Areas of CuO-Based Adsorbentsa adsorbent

Cap(BT) (mg g−1)

Q(H2S/TM)b

BET surf. area (m2 g−1)

CuO Cu−Al−O Fe−Cu−Al−O Co−Cu−Al−O Ni−Cu−Al−O Ce−Cu−Al−O

17.7 86.0 113.9 73.4 55.7 91.1

0.14 0.82 1.06 0.68 0.52 0.85

16.4 72.3 96.7 87.4 86.0 82.1

a Reaction conditions: T, 40 °C; W, 0.3 g; flow rate, 100 mL min−1; inlet gas mixture, 1000 ppm H2S in CO2. bQ(H2S/TM), molar ratio of adsorbed H2S to transition metal on the CuO-based adsorbents.

surface area of CuO from 16.4 to 72.3 m2 g−1 and simultaneously improved its breakthrough capacity about 5 times. The BET surface areas of all the M−Cu−Al−O adsorbents have been further increased when compared to the Cu−Al−O adsorbent. The iron doped adsorbent (Fe−Cu− Al−O) showed the largest BET surface area of 96.7 m2 g−1 and the highest breakthrough capacity of 113.9 mg g−1. The breakthrough capacity of cerium doped adsorbents (Ce−Cu− Al−O) was not significantly improved. The cobalt and nickel doped adsorbents uptake less H2S than the nondoped Cu−Al− O adsorbent. These results also supported the view that the BET surface area is a key parameter in achieving acceptable H2S uptake, but the interaction of H2S with the CuO-based adsorbents is not confined only to the factor of surface area.10 3.3. XPS Analyses. XPS measurements for Cu−Al−O and Fe−Cu−Al−O adsorbents were performed to study the chemical composition of the solid surface and the oxidation state of its metal elements. Table 2 shows the surface atom Table 2. Surface Atom % Composition of CuO-Based Adsorbents Derived from XPS adsorbent

Fe

Cu

Al

Cu−Al−O Fe−Cu−Al−O

− 9.4

52.7 58.6

47.3 32

Figure 4. Breakthrough capacities of H2S on Fe−Cu−Al−O adsorbent at different desulfurization temperatures. Conditions: W, 0.3 g; flow rate, 100 mL min−1; inlet gas mixture, 1000 ppm H2S in CO2.

that when the adsorption temperature rose from 40 to 70 °C, the breakthrough capacity significantly increased, from 113.9 to 207.5 mg g−1, but a slight increase in breakthrough capacity was

percent composition of the adsorbent obtained from the XPS results. For the Cu−Al−O adsorbent, the surface Cu/Al ratio is about 1.1, which is much lower than that in the bulk. For the

Figure 3. XPS spectra of (A) Fe 2p and (B) Cu 2p obtained on Fe−Cu−Al−O adsorbent. C

DOI: 10.1021/acs.iecr.5b00180 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research observed as the temperature increased from 70 to 130 °C. Moreover, trace amount of COS was detected before the breakthrough point of H2S when the adsorption was conducted at 130 °C. The breakthrough of COS occurs about 50 min earlier than that of H2S. The same phenomenon was also found by Yang et al., who thought that the COS can be formed via the reaction of sulfides with CO2.29 The reaction can be described as follows:30 MS + CO2 → MO + COS

(2)

where “M” represents metal. The formation of COS must be avoided during the desulfurization process; thus it can be concluded that the Fe−Cu−Al−O adsorbent should be more preferable to be used below 100 °C in the presence of CO2. 3.5. Effect of Space Velocity. Figure 5 shows the effect of space velocity on H2S breakthrough capacity. The flow rates at Figure 6. Effect of different balance gases on H2S adsorption over CuO-based adsorbents. Conditions: T, 40 °C; W, 0.3 g; flow rate, 100 mL min−1; inlet gas mixtures, 1000 ppm H2S in CO2 and 1000 ppm H2S in N2.

Table 3. Breakthrough Capacities of CuO-Based Adsorbents in Different Balance Gasesa Cap(BT) in balance gas (mg g−1) adsorbent

CO2

N2

Cap(BT)(N2)/Cap(BT)(CO2)

CuO Cu−Al−O Fe−Cu−Al−O

17.7 86.0 113.9

25.3 106.3 125.3

1.43 1.24 1.10

Conditions: T, 40 °C; W, 0.3 g; flow rate, 100 mL min−1; inlet gas mixture, 1000 ppm H2S in CO2 and 1000 ppm H2S in N2.

a

breakthrough capacity increased by 43% in H2S/N2, while the breakthrough capacities of Cu−Al−O and Fe−Cu−Al−O increased by 24 and 10%, respectively. This phenomenon allows us to suppose that the negative effect of CO2 may be reduced with the addition of Al and Fe for their dilution effect due to the reaction between CO2 and CuO. 3.7. Effect of Calcination Temperature. Figure 7 and Table 4 show the effect of calcination temperature on breakthrough capacities of the CuO-based adsorbents. As shown in Figure 7, with the calcination temperature increased

Figure 5. Effect of flow rate on breakthrough capacity of Fe−Cu−Al− O adsorbent. Conditions: T, 40 °C; W, 0.3 g; inlet gas mixture, 1000 ppm H2S in CO2.

40, 70, and 100 mL min−1 correspond to weight hourly space velocities (WHSVs) of 8, 14, and 20 L h−1 g−1, and the calculated breakthrough capacities were 156.6, 130.8, and 113.9 mg g−1, respectively. The breakthrough capacity at the WHSV of 8 L g−1 h−1 was about 40% higher than that at the WHSV of 20 L h−1g−1. The trend indicated that high space velocity had a negative effect on the breakthrough capacity. It seems that the gas−solid contact also plays an important role in the H2S adsorption on the Fe−Cu−Al−O adsorbent: a lower space velocity would extend the contact time between H2S and the adsorbent, which can be applied to the ultradeep removal of H2S and increase the breakthrough capacity.4,28 3.6. Effect of CO2 on H2S Adsorption. Typically, CO2 has a strong effect on the low temperature desulfurization performance.28 In this section, the effects of CO2 were investigated for the CuO-based adsorbents using both CO2 and N2 balance with 1000 ppm H2S feed gas mixtures for comparison. The H2S uptake results of these adsorbents are shown in Figure 6 and Table 3. The breakthrough capacities, using either H2S/CO2 or H2S/N2, exhibited the same order of CuO < Cu−Al−O < Fe−Cu−Al−O. However, higher breakthrough capacities of all the adsorbents can be obtained when H2S/N2 is used. In other words, CO2 can compete with H2S adsorption. However, the inhibition degree of CO2 to the adsorbents was not the same. For the CuO adsorbent, its

Figure 7. Effect of calcination temperature on breakthrough capacities of adsorbents. Conditions: T, 40 °C; W, 0.3 g; flow rate, 100 mL min−1; inlet gas mixture, 1000 ppm H2S in CO2. D

DOI: 10.1021/acs.iecr.5b00180 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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sulfide (CuS) can be oxidized easily at relatively low temperature (eq 3), but CuS tends to decompose into more stable “Cu+” sulfides when the temperature is above 220 °C (eq 4).

Table 4. Breakthrough Capacities and BET Surface Areas of Adsorbents at Different Calcination Temperaturesa adsorbent

Cap(BT) (mg g−1)

Q(H2S/TM)

BET surf. area (m2 g−1)

CuO-350b CuO-450 CuO-550 Cu−Al−O-350 Cu−Al−O-450 Cu−Al−O-550 Fe−Cu−Al−O-350 Fe−Cu−Al−O-450 Fe−Cu−Al−O-550

17.7 12.7 5.2 86.0 73.4 58.2 119.3 48.1 17.7

0.14 0.10 0.04 0.82 0.70 0.55 1.06 0.43 0.13

16.4 9.5 5.3 72.3 58.9 49.5 96.7 74.8 41.4

2CuS + 3O2 → 2CuO + 2SO2

(3)

2CuS → Cu 2S + S

(4)

It can be concluded that low temperature is more suitable for the regeneration of CuO. To prevent the adsorbent sintering and reduce the energy consumption of the regeneration process, the regeneration temperature should be controlled as low as possible. Therefore, 100 °C was chosen for the regeneration of the CuO-based adsorbents. Four adsorption−regeneration cycles over pure CuO, Cu− Al−O, and Fe−Cu−Al−O were performed in this section to examine the stability of the CuO-based adsorbents. All the desulfurization and regeneration tests were carried out at 100 °C, and their breakthrough capacities are shown in Figure 9. As we can see in Figure 9, the regeneration rate of CuO maintained at about 75% of its initial value after four cycles of adsorption−regeneration. For the Cu−Al−O adsorbent, the breakthrough capacity is different for each cycle and the capacity in the first cycle is as high as 90% of its initial value. However, the desulfurization activity decreased with the increase of adsorption−regeneration cycles, and maintained at about 50% of its initial value finally. The Fe−Cu−Al−O adsorbent showed a regeneration performance similar to that of the Cu−Al−O adsorbent. The degradation of the breakthrough capacity was observed during the first three adsorption−regeneration cycles, and reached a stable breakthrough capacity at about 43% of initial value in the fourth cycle. In multiple adsorption−regeneration cycles, the loss of activity of H2S adsorbents is well-known.32−35 The cumulative breakthrough capacities of the CuO, Cu−Al−O, and Fe−Cu−Al−O adsorbents after four cycles are 57.8, 306.1, and 342.7 mg g−1, respectively. This demonstrated that the CuO-based adsorbents are promising adsorbents for the removal of H2S from a CO2 stream at low temperatures.

Conditions: T, 40 °C; W, 0.3 g; flow rate, 100 mL min−1; inlet gas mixture, 1000 ppm H2S in CO2. bAdsorbent was calcinated at 350 °C for 4 h. a

from 350 to 550 °C, the breakthrough capacities of all these adsorbents decreased and those of the CuO and Fe−Cu−Al−O adsorbents decreased more obviously than that of the Cu−Al− O adsorbent. The breakthrough capacity of Fe−Cu−Al−O adsorbent became less than that of the Cu−Al−O adsorbent when it was calcined at 450 and 550 °C. The BET surface areas of all the adsorbents decreased with the increase of calcination temperature. After being calcined at 550 °C for 4 h, the surface area of Cu−Al−O adsorbent (49.5 m2 g−1) is higher than that of Fe−Cu−Al−O (41.4 m2 g−1). The results clearly indicate that alumina calcined at high temperature is more stable, which can reduce the sintering of CuO, but the iron oxide seems easier to sinter at high temperatures.31 Therefore, the suitable calcination temperature of the CuO-based adsorbents in our experiments is 350 °C. 3.8. Regeneration Performance. Figure 8 shows the H2S breakthrough capacities of CuO at various regeneration

4. CONCLUSIONS A series of CuO-based adsorbents were prepared for the selective removal of H2S from a CO2 stream at low temperature. Several conclusions from the above discussion can be drawn as follows: 1. The CuO-based adsorbents are promising regenerable adsorbents which could effectively remove H2S from a CO2 stream to less than 0.1 ppm at low temperatures. The Fe−Cu− Al−O adsorbent exhibited the highest breakthrough capacity of 113.9 mg g−1 at 40 °C. In the temperature range of 40−130 °C, the breakthrough capacity of Fe−Cu−Al−O adsorbent increased with the adsorption temperature, but the recommended adsorption temperature should be below 100 °C in the presence of CO2 because the new impurity COS would probably be generated above 100 °C. A higher breakthrough capacity can be obtained at lower space velocities. 2. The CO2 will inhibit the adsorption of H2S on the CuObased adsorbents, but the inhibitory effect on CuO can be weakened with the addition of Al and Fe. 3. Low calcination temperature is preferable for the CuObased adsorbents. With the increase of calcination temperature, the adsorption capacity and BET surface area of the CuO-based adsorbents both declined.

Figure 8. Effect of regeneration temperature on breakthrough capacity of CuO. Reaction conditions: T, 40 °C; W, 0.3 g; flow rate, 100 mL min−1; inlet gas mixture, 1000 ppm H2S in CO2. Regeneration conditions: flow rate, 40 mL min−1; 6 h at different regeneration temperatures in air.

temperatures from 100 to 500 °C. The regenerated CuO exhibited a breakthrough capacity up to 75% of its initial value at low regeneration temperatures (≤200 °C). The breakthrough capacity decreased with the increase of regeneration temperature above 200 °C. It can be explained that copper E

DOI: 10.1021/acs.iecr.5b00180 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 9. Breakthrough capacities of multiple regeneration cycle tests of (A) CuO, (B) Cu−Al−O, and (C) Fe−Cu−Al−O adsorbents. Reaction conditions: T, 40 °C; W, 0.3 g; flow rate, 100 mL min−1; inlet gas mixture, 1000 ppm H2S in CO2. Regeneration conditions: T, 100 °C; flow rate, 40 mL min−1; 6 h in air.

4. A relatively low temperature of 100 °C was applied to regenerate the CuO-based adsorbents in air, and these adsorbents could reach a stable regeneration rate after four cycles of adsorption−regeneration. The CuO-based adsorbents are promising regenerable adsorbents for low temperature removal of H2S from a CO2 stream, though the adsorbents cannot be fully regenerated. The reason for the decrease of the regeneration rate is under investigation by the authors.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of the CuO-based adsorbents are provided (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 411 84986322. Fax: +86 411 84986322. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Tamhankar, S. S.; Bagajewicz, M.; Gavalas, G. R.; Sharma, P. K.; Flytzani-Stephanopoulos, M. Mixed-Oxide Sorbents for High-TemperF

DOI: 10.1021/acs.iecr.5b00180 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.5b00180 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX