CO2 and H2S Removal from CH4-Rich Streams by Adsorption on

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CO2 and H2S removal from CH4-rich streams by Adsorption on Activated Carbons Modified with K2CO3, NaOH or Fe2O3 Melina C. Castrillon, Karine Oliveira Moura, Caiua Araujo Alves, Moises Bastos-Neto, Diana C. S. Azevedo, Jorg Hofmann, Jens Möllmer, Wolf-Dietrich Einicke, and Roger Glaser Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01667 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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CO2 and H2S removal from CH4-rich streams by Adsorption on Activated Carbons Modified with K2CO3, NaOH or Fe2O3 Melina C. Castrillon †, Karine O. Moura †, Caiuã A. Alves †, Moises Bastos-Neto †, Diana C. S. Azevedo †* Jörg Hofmann ‡,Jens Möllmer ‡, Wolf-Dietrich Einicke § and Roger Gläser ‡,§ †Grupo de Pesquisa em Separações por Adsorção (GPSA), Departamento de Engenharia Química, Universidade Federal do Ceará, Campus do Pici, Bl. 709, 60455-760 Fortaleza, CE, Brazil. Fax: +55 85 33669610, Tel: +55 85 33669611, E-mail: [email protected] ‡Institut für Nichtklassische Chemie e.V. (INC), Permoserstraße 15, 04318 Leipzig, Germany. Fax: +49(0)341 235-2701, Tel: +49(0)341 235-2405, E-mail: [email protected] § Institute of Chemical Technology, Universität Leipzig, Linnéstraße 3, 04103 Leipzig, Germany Fax: +49 341 97-36349, Tel: +49 341 97-36300, E-mail: [email protected]

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KEYWORDS. Biogas; H2S; upgrading; CO2; adsorption; activated carbon.

ABSTRACT. In this study, the adsorption of CO2 and H2S has been investigated on commercial activated carbon Desorex K43 impregnated with K2CO3, NaOH or Fe2O3 in order to assess their potential for "upgrading" and desulfurization of biogas or contaminated natural gas. Different chemical (Fourier transformed infrared spectra (FTIR), X-ray fluorescence (XRF) and pH measurements) and textural characterization techniques (N2 adsorption/desorption isotherms) were used to study the material surface and confirm the presence of K, Na and Fe. Gravimetric experiments of single and binary gas sorption isotherms were used to evaluate CO2 uptake and selectivity with respect to CH4. Breakthrough curves under dry and humid conditions were performed to assess the adsorption of H2S. The materials studied showed high adsorption capacities for both gases: in the range of 0.85 to 4.58 mmol g-1 for H2S and from 1.61 to 1.88 mmol g-1 CO2, under dry conditions and 1 bar. Furthermore, the selectivity of the activated carbons for CO2 in relation to CH4 was in the range of 1.2 – 2.4, Desorex K43-BG being the material with higher adsorption capacity for gases under study. The data obtained by the adsorption experiments were correlated with the textural characteristics and the chemical properties of the materials, which allowed to identify how promising an adsorbent is for the removal of acidic gases from biogas to obtain biomethane. The best compromise between H2S adsorption and CO2/CH4 selectivity was found for the sample containing Na (Desorex K43-Na), which benefited from both a basic surface chemistry and pore size distribution restricted to the micropore range.


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1 Introduction Biogas is a renewable fuel, which is extensively produced nowadays in small facilities by anaerobic biological digestion.1 It mainly contains methane (CH4) and carbon dioxide (CO2) in a proportion ranging from 55 to 70% and 45 to 30 %, respectively. In minor quantities, ammonia (NH3), hydrogen sulfide (H2S) and hydrocarbons may also be found.2 Biogas can be directly used as combustion gas after H2S removal in order to avoid problems of corrosion in pipelines and toxic emissions to the atmosphere.3 Additionally, CO2content may be reduced to bring its calorific power close to that of natural gas, at which condition it may be named as biomethane. To convert biogas to biomethane, two steps are usually required: a cleaning process to remove trace components (H2S is a typical example) and a refining process (upgrading) to adjust calorific power, which consists in the removal (or significant reduction in concentration) of CO2. Such processes usually aim to meet the standards for use as vehicular fuel or for injection into the natural gas grid.4 Desulfurization may be carried out by in-situ reduction of H2S inside the digester vessel upon addition of metal ions (e.g. iron chloride); by using metal oxides (e.g., iron oxide and zinc oxide), by oxidation with air and H2S adsorption on activated carbons.5 The use of activated carbons impregnated with alkali compounds for H2S adsorption has been reported in the literature.6-8 It has been demonstrated that impregnation with basic compounds such as NaOH and K2CO3 leads to significant improvements on sorption capacity towards H2S as a result of a mechanism of physical adsorption followed by chemical reaction. Following the same concept, impregnation with Fe2O3 is also reported to be a promising inorganic compound for H2S removal.9 Possible reaction pathways occurring with H2S on the surface of alkaline materials are shown as follows.


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Hydrogen sulfide is a diprotic acid that might react with a hydroxide (NaOH) according to reactions (1) or (1a):7 H2S + NaOH → NaHS+H2O H2S + 2NaOH → Na2S+2H2O

(1) (1a)

For surfaces impregnated with K2CO3, reactions (2) and (2a) may occur in the presence of H2S.7, 10

H2S+K2CO3→KHS+KHCO3

(2)

H2S+K2CO3→K2S+H2CO3

(2a)

The reactions that may occur with H2S on the surface of materials impregnated with iron oxide are:9 Fe2O3 + 3H2S → FeS + FeS2 + 3H2O

(3)

Fe2O3 + 3H2S → Fe2S3 + 3H2O

(3a)

2Fe2S3 + 3O2→ 2 Fe2O3 + 6S

(3b)

Several separation technologies are available for biogas upgrading.4, 11 Membrane separation,12-14 water scrubbing15-16 or PSA (Pressure Swing Adsorption)2, 17 are common examples. PSA processes are based on preferential adsorption and/or diffusion of a given gas (e.g., CO2) from a mixture on a porous adsorbent at high pressure followed by recovery of such gas at low pressure. Silica gel, alumina, zeolites and activated carbons are the most commonly employed adsorbents in this kind of devices.4, 18-20In case of biogas, the most important materials used are carbon molecular sieves allowing for the separation of CO2 by VPSA.21 The binding mechanism is essentially based on physisorption and adsorption-desorption steps must be fully reversible.


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Activated carbon is the adsorbent generally used for the removal of H2S at low concentrations, having great advantages such as its higher surface area and its behavior as surface catalyzed oxidation forming elementary sulfur and sulfate.22-24 Other materials, such as molecular sieves,2526

are used for their thermal and chemical stability. Modification of the material with iron oxides,

like Fe2O3, has been formerly applied due to its notable capacity to reduce H2S loaded streams in the range of 1 ppm to 1000 ppm. Other materials recommended for biogas desulfurization are iron sponges, whose main constituent is hydrated iron oxide (Fe2O3) and wood chips.4 This work aims at the evaluation of three commercial activated carbons obtained from bituminous coal and coconut shell with either alkaline metals or Fe incorporation targeted on cleaning and upgrading of biogas. For this purpose we have investigated H2S adsorption under dry and humid conditions as well as CO2 adsorption depending on CH4 concentrations. Initially, from the breakthrough curves of H2S at 303 K, the retention capacity of samples was estimated. Then, the isotherms of pure gases and binary CO2/CH4 mixtures at different compositions at 298 K were measured. Finally, correlations between the adsorption capacity, CO2/CH4 selectivity and the textural and surface chemistry characteristics of these samples were presented and discussed.

2 Experimental 2.1 Materials Three commercial activated carbons produced by Donau Carbon GmbH (Germany), named as Desorex K43-Na (impregnated with NaOH), Desorex K43-Fe (impregnated with Fe2O3),


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Desorex K43-BG (impregnated with K2CO3)27 were studied in this work. CH4 and CO2 were supplied by White Martins Praxair (purity: 99.99 %). H2S was supplied by Linde diluted in N2 at a concentration of 1000 ppm. 2.2 Adsorbent Characterization Fourier transform infrared (FTIR) spectra were obtained using a Vertex 70V FT-IR spectrophotometer. Sample and KBr were ground together at a mass ratio of 5:95 in an agate mortar. FTIR spectra were evaluated in the wavenumber range between 400 and 4000 cm-1, with 128 scans at 4 cm-1 resolution. The chemical composition of the samples was analyzed by x-ray fluorescence analysis (XRF) using an ARL ADVANT`XP+ (Thermo, USA) under helium atmosphere in oxide form. Additionally, pH measurements were carried out according to literature,28 by using a Digimed DM-22 pH-meter. Briefly, 0.40 g activated carbon sample were added to 20 mL distilled water, which were kept under mechanical agitation for about 16 h. pH of the solution was then measured. The experiment was carried out and recorded at 298 K. Textural properties of activated carbon samples were estimated from N2 adsorption/desorption isotherms at 77 K using an Autosorb-iQ3 (Quantachrome Instruments, USA). The samples were pretreated at 423 K under vacuum (10-6 bar) during 6 hours. Specific surface area of all materials was calculated using Brunauer-Emmett-Teller (BET) equation;29 micropore volume was estimated by Dubinin–Radushkevich (DR) model and the total pore volume was evaluated according to the Gurvich rule.30-31 The pore size distribution (PSD) of each sample was obtained using a Density Functional Theory (DFT) kernel for slit-shaped pores.32-33 Microporosity represents the ratio between micropore volume and total pore volume.


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2.3 Determination of Adsorption Capacity 2.3.1 H2S Adsorption H2S breakthrough curves were obtained using a fixed bed setup coupled with a DRÄGER Polytron 7000 electrochemical detector using N2 as carrier gas, as illustrated in Figure 1. Approximately 10 g adsorbent were packed into 10-cm-long stainless steel columns with 1.6 cm internal diameter. They were flushed with N2 at 303 K for 292 h. Experimental feed conditions of breakthrough experiments were H2S in N2 (1000 ppm) at 0 and 70 % relative humidity (RH), flow rate of 200 mL min-1 and atmospheric pressure. The humidity was attained by using a gas saturator (bubbling system) with N2 flow.

Figure 1. Fixed bed unit for determination of H2S breakthrough curves.


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The adsorption capacities were estimated by integrating the area above the breakthrough curve at the breakpoint (t = tb) and at saturation (t =

), using equations (1):

Eq. (1)

where q is the adsorption capacity of the fixed bed at breakthrough or saturation, C0 is the feed concentration and C is the concentration at the outlet of the column as a function of time. ε is the void fraction of fixed bed, Q is the feed flow rate, VL is the column volume and ML is the mass of adsorbent packed in the bed. The breakpoint time was considered as the time elapsed until the H2S concentration at bed outlet reached 5 % of the feed concentration.

2.3.2 CO2 and CH4 Equilibrium Adsorption Isotherms The monocomponent and binary adsorption isotherms for CO2 and CH4 were measured using a magnetic suspension balance by Rubotherm GmbH (Bochum, Germany). Approximately 1.0 g adsorbent was used for the CH4 and CO2 adsorption experiments. The carbon samples were previously outgassed at 393 K, for about 12 h under vacuum (10-3 bar). The isotherms were measured in the pressure range of 0.1 to 10 bar at 298 K. The specific volume of the samples was previously determined by using He, which is practically not adsorbed at the pressure and temperature ranges under study.34 Details regarding data handling might be found elsewhere.34-35 The SIPS (Langmuir-Freundlich) model,36-37 equation (2), was used to fit single component experimental data (CO2 and CH4) of adsorbed amount qi as a function of gas pressure.


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Eq. (2) The fitting parameter qmax,I is the saturation capacity of component I (mmol g-1); bi is related to the adsorbent-adsorbate affinity (bar-1); and ni accounts for the surface heterogeneity. Pi is the partial pressure of component I (bar) and the adsorbed concentration of the component I in equilibrium, qi, is given in mmol g-1. The extended SIPS model,36-37 equation (3), was applied to evaluate binary equilibrium by using the parameters calculated from the fitting procedure for each component independently.

Eq. (3) Despite the non-ideality of the gas mixture, driven by existing interactions between the molecules as well as the polarizabilities of both CO2 and CH4 (29.1 and 25.9 10-25 cm3, respectively) and the quadrupole moment of CO2 (−13.7 x 10-40 cm2),38 the system was assumed to be ideal for modelling purposes. The selectivity of each sample for CO2 towards CH4 was calculated according to Equation 4, considering the mole fractions in the gas and adsorbed phase.39

Eq. (4) where x and y are the molar fractions in the adsorbed and gas phases, respectively, which are obtained from the predictions with the extended SIPS model (Eq.3).


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3 Results and Discussions 3.1 Characterization 3.1.1 Surface Chemistry To evaluate the surface groups, the activated carbon samples were examined by FTIR. Figure 2 shows the spectra of the studied samples, which are quite similar to each other. In all spectra, the OH stretching vibration band at 3430 cm-1 is observed and is consistent with the presence of surface hydroxyl groups on the carbon surface.40 The shoulder in 1636 cm-1 corresponds to a series of overlapping bands, accounting for the stretching vibrations of carboxylic anhydrides groups and the presence of carbonyl groups(-C=O) in quinone and pyridine structures, which are highly conjugated with the polyaromatic matrix.41 The shoulder at around 1570 cm-1 is assigned to stretching vibrations of aromatic rings, conjugated carbonyl groups, and carboxylate groups.41 The band at 1383 cm-1 refers to the presence of alkyl groups (C-H). Bands can also be seen between 1236 cm-1and 935 cm-1, which are typical of -C-O stretching, and could be related to the presence of alcohol and phenols, confirming the presence of OH groups on the materials:42-43 at 1236 cm-1, a more pronounced band for Desorex K43-BG and Desorex K43-Na samples; and at 1126 cm-1, for the carbons impregnated with iron and potassium. Lastly, the band at 935 cm-1, only present in AC Desorex K43-BG, suggests the presence of carboxylic acids functional group.


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Figure 2. FTIR spectra of the samples.

X-ray fluorescence (XRF) analysis offers the possibility to directly quantify the amount of metal species present on the surface of activated carbons and to confirm the presence of impregnated species: Fe, Na, and K. The results are shown in Table 1. Significant amounts of Al and Fe can be observed for all samples. As expected, Desorex K43-BG, impregnated with K2CO3, has a high content of potassium (K), sample Desorex K43-Na, impregnated with NaOH, a high content of sodium (Na), and sample Desorex K43-Fe (Fe2O3 impregnated) has a high content of iron (Fe).


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Table 1. Chemical composition of impregnated activated carbons samples obtained by X-Ray Fluorescence (XRF). Samples Elements

Desorex

Desorex

Desorex

(mass

K43-BG

K43-Fe

K43-Na

%) Si

2.80

3.36

3.34

Na

-

-

4.74

K

8.89

-

-

Al

2.42

2.82

3.00

Fe

2.51

5.04

3.86

Ca

2.47

2.42

2.81

S

0.59

0.71

1.01

Mg

0.36

0.52

0.47

Sodium and potassium are known to promote a high H2S retention by chemisorption, which is potentially favored by the presence of moisture.44 Calcium and magnesium are also reported to improve the adsorption of hydrogen sulfide.10 Furthermore, the occurrence of transition metals, such as iron, allows for the oxidation of H2S into elemental sulfur and sulfur dioxide.45 The probable reactions of H2S with Na, K and Fe species were presented and discussed in the introduction. Therefore, the samples under study in this work are expected to be promising H2S sorbents. The pH-value of the activated carbon samples was determined to understand the interaction between the surface of materials and the gases during the adsorption process. The resulting pHvalues of water with the samples submerged for 16 h are presented in Table 2. Higher pH-values


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for the samples Desorex K43-BG and Desorex K43-Na may be attributed to the presence of alkali metals, as observed in Table 1. A basic surface is expected to favor H2S capture.10, 28, 44 On the other hand, Desorex K43-Fe sample presented a neutral pH-value, mainly due to the absence of alkali metals and the relatively low content of Ca and Mg, as detected in the x-ray fluorescence analysis (Table 1). Table 2. pH-values for the studied activated carbons obtained at 298 K. Material

pH

Desorex K43-BG

9.75

Desorex K43-Fe

7.20

Desorex K43-Na

10.19

3.1.2 Textural Characterization Nitrogen isotherms at 77 K for the studied samples are presented in Figure 3 and textural characteristics obtained from them are summarized in Table 3.All isotherms showed a reversible type Ia behavior,31, 46 which is typical for materials with narrow size distribution of micropores. All samples have supermicropores (below 0.7 nm), which is signaled by the smooth increase at relative pressures up to 0.4 and confirmed in the PSD (Figure 4). The ACs showed specific surface areas in the range of 815-1005 m2g-1 (Table 3), with a high microporosity, which are promising features for gas adsorption.


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Figure 3. N2 adsorption isotherms at 77 K. Filled symbols and empty symbols show the adsorption and desorption respectively.

All samples showed a high degree of microporosity (73-84%) possibly due to the use chemical agents, which are excellent activating agents to develop microporosity. This finding is supported by the pore size distribution (Figure 4) and has been previously reported.47 The slight difference observed in the textural properties of the impregnated materials occurs due the chemical characteristics of the activation agents used. Impregnation with K2CO3 and NaOH is reported to produce materials with higher surface area and micropore volume.48-49 On the other hand, impregnation with Fe2O3 tends to decrease significantly surface area and micropore volume, with losses reaching nearly 30 % in comparison with the original matrix,50 which might be due to pore obstruction by the chemical agent.


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Table 3. Textural characterization of the samples. SBET

Micropore volume

Total pore volume

Microporosity

(m2g-1)

(cm3g-1)a

(cm3g-1)b

(%)c

Desorex K43- BG

1005

0.36

0.50

73

Desorex K43-Fe

952

0.36

0.43

84

Desorex K43-Na

815

0.30

0.38

79

Material

a

Dubinin–Radushkevich (DR) model 31; b Gurvich Rule 31; c Micropore volume / Total pore volume.


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Figure 4. Pore size distributions obtained by DFT.

3.2 Adsorption 3.2.1 H2S Adsorption The breakthrough curves of H2S on all three materials subjected to dry feed conditions are shown in Figure 4. The results for Desorex K43-Fe (Figure 5) show zero concentration at the bed outlet up to 150 min. On the other hand, the breakpoint for Desorex K43-BG (Figure 5) and Desorex K43-Na (Figure 5) only happened at 2500 and 2900 minutes, respectively. Following these breakpoint values an increase in bed oulet concentration begins until bed saturation (oulet concentration / feed concentration = 1). The calculated adsorbed amount at both conditions, breakpoint and saturation, for each material are shown in Table 4. Desorex K43-BG and Desorex K43-Na showed the highest uptakes, as compared to the very low value obtained for Desorex K43-Fe. These results are in good agreement with the basic character (Table 2). The sample impregnated with Fe presented the


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lowest uptake, which is attributed to its neutral surface. On the other hand, the other two samples clearly show a larger uptake and a much more pronounced dispersion of the breakthrough curve, which could be caused by either hindered pore diffusion or a slow reaction rate.

Figure 5. Experimental breakthrough curves at 303 K.

H2S is a weak diprotic acid with first and second dissociation constants of 7.2 and 13.9, respectively. Thus an enhanced adsorption capacity on basic surfaces is to be expected, because the pH-values obtained for activated carbon are within the range of these dissociation constants. Samples Desorex K43-BG and Desorex K43-Na showed close pH-values, both in the basic range, where as a nearly neutral character was observed for Desorex K43-Fe. The good correlation between H2S adsorption capacities and surface pH-values suggest that the binding mechanism of H2S sorption is likely to be an acid-base reaction.51-52


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Table 4. Adsorbed amounts of H2S (mmol g-1) for the evaluated materials at dry and wet (RH = 70 %) conditions. H2S uptake (mmol g-1)

H2S uptake (mmol g-1)

RH = 0 % (dry)

RH = 70 %

Material at breakpoint

at saturation

at breakpoint

(5 %)

at saturation

(5 %)

Desorex K43-BG

2.58

8.6

----

Desorex K43-Fe

0.82

1.2

----

Desorex K43-Na

4.58

8.2

15.26

30.9

Regarding the adsorbed concentrations at saturation, samples Desorex K43-BG and Desorex K43-Na presented similar capacities: 8.6 mmol g-1 and 8.2 mmol g-1, respectively. BG sample showed the best H2S adsorption capacity at saturation, which is due to a combination of surface basicity together with high surface area and total pore volume. However, Na sample outdid it in terms of adsorption capacity at the breakpoint. The uptake at the breakthpoint is actually more important from a practical point of view, since columns are expected to stop the adsorption cycle at this point in order to be fully regenerated or replaced. The mechanisms of H2S adsorption-oxidation in wet conditions are complex, but are likely to be influenced by two types of active sites: i) the carbon surface itself, and ii) metal oxides or carbonates located on the carbon surface.9 Furthermore,oxygen groups present on the surface of the material, observed in the infrared spectra (Figure 2), may promote H2S gas chemisorption. H2S may also interact with the hydroxyl and carbonyl groups by hydrogen bonding, due to the strong electrostatic interactions present in the system.52 As investigated by other authors, a high


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density of oxygenatedgroups on the surface of the material may lead to the oxidation of H2S and this contributes to the strong retention of this gas.44 The sample with more pronounced basic character (Desorex K43-Na) and the highest H2S uptake at breakpoint was chosen for the measurements with a relative humidity (RH) of 70 %, under the same feed conditions of temperature and H2S concentration. This measurement was carried out in order to evaluate the impact of humidity on the adsorption capacity of the sample. Breakthrough curves at 0 (dry) and 70 % RH (wet) are contrasted in the Figure 6 and show quite different shapes. Not only breakpoint and saturation capacities are enhanced in the presence of moisture, but also the curve with humid feed is much more dispersed, suggesting that a new binding mechanism might be occurring besides the acid-base reaction observed under dry conditions. The removal of H2S at low temperatures (293-473 K) occurs due to reactions with hydrated metal oxide, producing sulphides.9 In addition to that, transition metal ions such as iron, present on the surface of each sample, catalyses the oxidation of H2S in the presence of moisture due to their redox reaction potential.53 A behavior similar to that presented in Figure 6 has been reported by Xiao et. al (2008),23 also using a fixed bed packed with activated carbon impregnated with Na2CO3. A much more delayedand dispersed breakthrough curve was observed in the presence of humidityas compared to that under dry conditions. The authors conducted several tests whereby relative humidity was varied between 0 and 80 % (at 303 K and initial concentration of 200 ppm) to study the effects on H2S adsorption. They observed thatincreasing relative humidity allowed for longer breakthrough times, thus improving the retention of H2S.


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Figure 6. Experimental breakthrough curves for Desorex K43-Na at 303 K under dry and humid conditions.

3.2 CO2 Adsorption Pure component isotherms of CO2 and CH4 are shown in Figure 7 and 8, respectively. As expected and for all samples, CO2 is preferentially adsorbed as compared to CH4. This follows a general trend in any activated carbon since CO2 critical temperature (304.45 K) is significantly higher than that of CH4 (190.55 K), which makes CO2 behave more like a condensable vapor and not as a supercritical gas as occurs with CH4.54 Furthermore, the presence of polar functional groups on the carbon surface, such as the -OH groups detected by FTIR, would lead to the preferential adsorption of CO2 rather than CH4.55-56 Additionally, CO2 is also an acid gas, which will tend to be bound to basic adsorption sites in a similar way as H2S. The measured adsorbed amounts in the studied samples are in agreement with values reported in the literature, as shown in Table 5.


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Figure 7. Adsorption isotherms of CO2 at 298 K. Symbols are experimental data and lines represent the SIPS model.

Figure 8. Adsorption isotherms of CH4 at 298 K. Symbols are experimental data and lines represent the SIPS model.


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The adsorption capacity for CO2 is similar for all samples, indicating that the type of metal impregnated does not affect significantly the adsorption of CO2 and CH4 on the materials. Because the pore size distribution and textural properties are not significantly different for the three samples, it is likely that CO2 and CH4 are essentially adsorbed by physisorption, unlike H2S. Furthermore, a few experimental points from Figures 7 and 8 are obtained in a desorption step (stepwise pressure decreases) to check the occurrence of hysteresis. For all cases, CO2 adsorption and desorption curves overlap, which also indicates physisorption as the prevailing adsorption mechanism as compared to the formation of covalent bonds (chemisorption).

Table 5. Comparison of CO2 adsorption with different adsorbents. CO2 Capacity

BET

Temperature

(m2 g-1)

(K)

AC Norit R1

3000

293

2.23

Dreisbach et al. (1999)57

WV1050

1615

293

1.69

Rios et al. (2013)58

AC A35/4

-

293

2.0

Heuchel et al. (1999)59

Desorex K43-BG

1005

298

1.88

This Work

Desorex K43-Fe

952

298

1.61

This Work

Desorex K43-Na

815

298

1.67

This Work

Material

at 1 bar

Reference

(mmol g-1)

The presence of oxygen functional groups, as carboxyls and hydroxyls, tends to increase CO2 sorption on microporous materials.60 Furthermore, Furmaniak et al. (2013)61 showed that the presence of carbonyl groups leads to an enhancement in the CO2/CH4 selectivity.


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The SIPS model presented satisfactory correspondence with the experimental data, especially for methane, which is relatively less adsorbed. In the case of CO2, discrepancies between model and experiment become more relevant with increasing pressure (> 5 bar) due to the increasing significance of the molecular interactions disregarded by the model. The calculated isotherm parameters are summarized in Table 6. The parameter related to the adsorbent-adsorbate affinity, b, is higher for CO2 to CH4 in all cases, indicating consistency of the fitting. These results were used to predict binary gas adsorption, as shown in Figure 9, for two different CO2/CH4 compositions. The increase of the amount adsorbed with increasing CO2 mole fraction is consistent with single-component equilibrium data, since CO2 capacity is larger than that of CH4. Table 6. Model parameters for pure gas adsorption isotherms. Material

Gas

qm (mmol g-1)

b (bar-1)

n

CO2

7.330

0.316

0.90

CH4

4.731

0.197

0.60

CO2

7.273

0.294

0.99

CH4

4.368

0.200

0.67

CO2

7.724

0.300

0.88

CH4

2.965

0.260

0.70

Desorex K43-BG

Desorex K43-Fe

Desorex K43-Na

As observed in Figure 9, the SIPS model was accurate enough in the situation studied, even though the CO2-CH4 mixtures are not actually ideal, given the interactions between molecules, which could influence the adsorption behavior of the binary system. A plausible reason for that is the relatively low selectivity (discussed later) for CO2 of all samples lessening those effects.


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Figure 9. Binary adsorption isotherm for CO2-CH4 mixtures. Symbols represent experimental data and lines represent predictions with SIPS model.


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3.2.1 Selectivity As observed in Figure 10 the decrease in selectivity with increasing CO2 concentration in the mixtures is consistent with literature.62 The increase of selectivity with increasing pressure is due to the shape of both CO2 and CH4 isotherms. The capacity for CO2 grows steeper than that of CH4. Moreover, this behavior might also be attributed to a cooperative (energetic) adsorption effect.63

Figure 10. Selectivity for CO2 over CH4.


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The higher selectivity presented in Desorex K43-Na material is attributed to its low CH4 adsorption capacity, since CO2 capacity is similar for all materials. In comparison with data reported in the literature, the selectivities for CO2 at 1 bar of the samples studied in this work are lower (Table 7).

Table 7. Comparison of selectivity for CO2/CH4 binary gas mixture with different adsorbents for 1 bar. Material

T (K)

Y (CO2)

Selectivity

Reference

AC (AX21)

293

0.5

8.0

Kluson and Scaife (2002)64

WV150

293

0.69

8.7

Rios et al. (2013)58

Activated carbon beads

303

0.50

3.6

Wu et al. (2015)38

Honeycomb monolith

299

0.50

2.0

Ribeiro et al. (2008)65

Desorex K43-BG

298

0.30

1.4

0.45

1.2

0.30

1.3

Desorex K43-Fe

Desorex K43-Na

298

This work

This work 0.45

1.2

0.30

2.4

298

This work 0.45

2.1

Despite the relative lower values of selectivity and adsorption capacity for CO2, the greatest advantage of using impregnated carbons such as the ones studied in the present work is the


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possibility of an effective simultaneous removal of H2S during cyclic CO2/CH4 separation (e.g., PSA plants) for as long as H2S does not break through the columns. This may take several months for a column in industrial scale. For such an application, the combined presence of Na and Fe on a microporous activated carbon seem to lead to a very promising adsorbent.

4 Conclusions Impregnated activated carbons were studied as adsorbents for CO2 and H2S. Results show that the presence of basic alkali metal salts on the adsorbent surface improves the retention of H2S by improving surface nature, but it did affect CO2 capacity under the studied conditions. Considering the similarity of the textural properties of the studied samples and the corresponding CO2 isotherms, one may infer that physical adsorption is the main mechanism occurring in the adsorption of this gas. Regarding the sorption of H2S, the surface basicity, rendered by the presence of alkali metals, coupled with favorable textural properties, seems to lead to a combined binding mechanism of physical adsorption and chemical reaction. Additionally, the presence of humidity in the system increases the adsorption of H2S, confirming the important role of water in the H2S retention. The best compromise between H2S adsorption and CO2/CH4 selectivity was found for the sample containing Na and Fe (Desorex K43-Na), which benefited from both a basic surface chemistry and pore size distribution restricted to the micropore range. In comparison with samples developed for CO2 capture, adsorption capacity and selectivity of the studied impregnated materials are not the highest. Nevertheless, if one should consider a hybrid (single step) process


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for removing both CO2 and H2S (desulfurization and upgrading) the use of ACs impregnated with alkali metals, such as Na, might be an interesting and possibly economically favorable alternative for the treatment of Biogas, which is very commonly rich in acidic gases.


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ACKNOWLEDGMENT The authors acknowledge financial support from CNPq (ConselhoNacional de DesenvolvimentoCientífico e Tecnológico), CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and DAAD (DeutscherAkademischerAustauschdienst).


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NOMENCLATURE Symbols C

concentration in the gas phase, mol L-1

q

adsorption capacity, mmol g-1

C0

feed concentration, mol L-1

Q

is the feed flow rate, mL min-1

VL

column volume, cm3

ML

mass of adsorbent packed in the bed, g

qmax,I

saturation capacity of component I, mmol g-1

bi

parameter related to the adsorbent-adsorbate affinity, bar-1

ni

accounts for the surface heterogeneity

Pi

partial pressure of component I, bar

qi

concentrationof the component I in equilibrium, mmol g-1.

SBET

Surface Area, m2g-1


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FIGURE CAPTIONS Figure 1. Fixed bed unit for determination of H2S breakthrough curves. Figure 2. FTIR spectra of the samples. Figure 3. N2 adsorption isotherms at 77 K. Filled symbols and empty symbols show the adsorption and desorption respectively. Figure 4. Pore size distribution obtained by DFT. Figure 5. Experimental breakthrough curves at 303 K. Figure 6. Experimental breakthrough curves for Desorex K43-Na at 303 K under dry and humid conditions. Figure 7. Adsorption isotherms of CO2 at 298 K. Symbols are experimental data and lines represent the SIPS model. Figure 8. Adsorption isotherms of CH4 at 298 K. Symbols are experimental data and lines represent the SIPS model. Figure 9. Binary adsorption isotherm for CO2-CH4 mixtures. Symbols represent experimental data and lines represent predictions with SIPS model. Figure 10. Selectivity for CO2 over CH4.


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TABLES Table 1. Chemical composition of impregnated activated carbons samples obtained by X-Ray Fluorescence (XRF). Table 2. pH-values for the studied activated carbons obtained at 298 K. Table 3. Textural characterization of the samples. Table 4. Adsorbed amounts of H2S (mmol g-1) for the evaluated materials at dry and wet (RH = 70%) conditions. Table 5. Comparison of CO2 adsorption with different adsorbents. Table 6. Model parameters for pure gas adsorption isotherms. Table 7. Comparison of selectivity for CO2/CH4 binary gas mixture with different adsorbents for 1 bar.


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AUTHOR INFORMATION Corresponding Author Departamento de EngenhariaQuímica, Universidade Federal do Ceará, Campus do Pici, Bl. 709, 60455-760 Fortaleza, CE, Brazil. Fax: +55 85 33669610, Tel: +55 85 33669611, E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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CO2 and H2S Removal from CH4-Rich Streams by Adsorption on

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