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CO2 and H2S binary sorption on polyamine modified fumed silica
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CO2 and H2S binary sorption on polyamine modified fumed silica
6
Boonyawan Yoosuk a,⇑, Tippayarat Wongsanga b, Pattarapan Prasassarakich b,⇑
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a Renewable Energy Laboratory, National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency, 114 Thailand Science Park, Thanon Phahonyothin, Tambon Khlong Nueng, Amphoe Khlong Luang, Pathum Thani 12120, Thailand b Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
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h i g h l i g h t s Low cost fumed silica modified by PEI800 was developed with the ability to remove both CO2 and H2S.
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The amine amount and amine interaction with H2S or CO2 play crucial role on adsorption performance.
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H2S sorption was determined by thermodynamics, high temperature increased CO2 sorption capacity.
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The fSi-PEI800-40 favors the CO2 sorption and the sorbed CO2 strongly inhibits the sorption of H2S. The sequential sorption of CO2 followed by H2S in double stage system reduced this effect.
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a r t i c l e 2 3 2 5 23 24 25 26 27 28 29 30 31 32 33 34
i n f o
Article history: Received 17 June 2015 Received in revised form 11 November 2015 Accepted 25 November 2015 Available online xxxx Keywords: CO2 removal H2S removal Sorption Amine Fumed silica
a b s t r a c t Low cost fumed silica modified by amine addition with the ability to remove both carbon dioxide (CO2) and hydrogen sulfide (H2S) was developed. The effect of amine type (polyethyleneimine with a molecular weight of 800 (PEI800), aminoethylethanolamine, N-(3-trimethoxysilypropyl) diethylenetriamine, diethylenetriamine, triethylenetetramine and tetraethylenepentamine), loading level and sorption temperature were investigated. The selected sorbent was then evaluated in a single- and double-stage systems. The polymeric amine PEI800 gave the best compromise between the highest CO2 and H2S sorption capacities with minimal amine leaching, and was optimal at a 40% loading level onto fumed silica, in terms of giving the highest breakthrough and saturation capacity. The amount of available amine groups in the sorbent and the interaction between amine groups and CO2 or H2S were the two major factors that affected the sorption capacity and amine efficiency. The sorption of H2S was predominately determined by thermodynamics, while a high temperature helped the diffusion of CO2 molecules from the surface into the bulk of PEI800. The sorbent favored CO2 sorption and the sorbed CO2 hindered the sorption of H2S. However, the sequential sorption of CO2 followed by H2S in a double-stage system was found to solve this problem. Ó 2015 Published by Elsevier Ltd.
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1. Introduction
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The removal of carbon dioxide (CO2) and hydrogen sulfide (H2S) is very important in gas upgrading processes because these gases reduce the heating value and are very corrosive [1,2]. Therefore, efficient methods for their removal are urgently needed. Additionally, carbon capture and storage is a good possibility for reducing the release of CO2 into the atmosphere [3,4]. In the conventional process for gas upgrading, the removal of acid gases is commonly performed by liquid-phase chemical scrubbing with amines. However, this technology suffers from having a high energy
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⇑ Corresponding authors. Tel.: +66 2 218 7517; fax: +66 2 255 5831. E-mail addresses: [email protected] (B. Yoosuk), [email protected] (P. Prasassarakich).
consumption, low absorption/desorption rate, solvent loss and material corrosion. The adsorptive separation of gases, wherein the acid gases, CO2 and H2S interact more strongly with the internal surfaces of the porous material (sorbent), has a high potential for most gas separation applications. This adsorption technology is recognized as an energy efficient technology for acid gas removal provided that a stable material with a high sorption capacity and selectivity toward the acid gases is available. A considerable amount of attention has focused on developing a solid sorbent with a high surface area and sorption capacities for CO2 and H2S that is hence suitable for their removal. The sorption of CO2 has been studied using various supports, such as silica nanoparticles [5], fumed silica (fSi) [6] and mesoporous molecular sieves (MCM-41, MCM-48, SBA-15) [7,8], plus those modified with various amine types, such as polyethylenimine (PEI) [5–8] and
http://dx.doi.org/10.1016/j.fuel.2015.11.080 0016-2361/Ó 2015 Published by Elsevier Ltd.
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triamine (TRI) [8]. Silica nanoparticles modified by PEI are an inexpensive, easy-to-prepare and regenerable adsorbent that have the practical ability to capture CO2 [5]. Fumed silica (fSi) impregnated with PEI (fSi-PEI) was also found to be a superior sorbent for the capture of CO2 directly from air [6]. For CO2 capture, the highly sorbent triamine-grafted pore-expanded mesoporous silica (TRI-PEMCM-41) was not affected by the moisture content in terms of the working sorption capacity developed, and the temperature swing regeneration mode was found to be suitable for desorption above 120 °C [8]. Fossil fuel combustion, which results in the emission of acidic gases, such as CO2, sulfur dioxide (SO2) and nitrogen oxides (NOx), is an important environmental issue. For the selective adsorption of SO2 and NOx in the presence of CO2, amineimpregnated and amine-grafted silica have been found to be effective adsorbents with a good stability [9,10]. Recently, tertiary amine containing poly(propyleneimine) second (G2) and third (G3) generation dendrimers as well as PEI on a nanoporous support [11] and N,N-dimethylpropylamine-grafted-PE-MCM-41 [12] were developed for the selective removal of SO2. Their adsorption capacity was enhanced dramatically in the presence of water vapor in the gas mixture and could be completely regenerated thermally [12]. Additionally, for H2S removal from a model gas mixture using various PEI-loaded mesoporous molecular sieves, including MCM-41, MCM-48 and SBA-15, the PEI-loaded SBA-15 sorbent had the highest saturation capacity, and could be regenerated under mild conditions [13]. A PEI-loaded hierarchical porous silica monolith was developed as a high stability, recyclable H2S sorbent at low temperatures and had a large H2S breakthrough capacity of 1.27 mmol of H2S/g sorbent at 22 °C and this could easily be regenerated at 75 °C [7]. Using the nanoporous composite sorbent PEI/ SBA-15 that had a high sorption capacity, the presence of moisture was found to have a promoting effect on the removal of H2S from gas streams [14]. Recently, high porosity and adsorbent silica xerogels modified by the addition of PEI were developed for the low temperature removal of H2S. Of these, the optimal adsorbent (fSiPEI800-50) was easily regenerated at a mild temperature (optimal of 50 °C) for at least ten successive adsorption–desorption cycles without any detectable loss of adsorption performance or regeneration ability [15]. However, only a few research reports have focused on the development of sorbents with the ability to selectively remove both CO2 and H2S. Early work reported that PEI-loaded MCM-41 or SBA-15 [16] and TRI-PE-MCM-41 [17,18] could adsorb CO2 and H2S from a model gas and subsequently be regenerated. However, these mesoporous silica compounds are presently not commercial available in large quantities and so have to first be synthesized. In this work a low cost fSi was modified by the addition of amine groups. The single and simultaneous sorption of CO2 and H2S over the obtained modified fSi sorbents were investigated in a single and a double fixed-bed system. The influence of the amine type and amine loading level were elucidated and the competitive sorption of CO2 and H2S is discussed.
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2. Experimental
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2.1. Materials
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The fumed silica (fSi) from Concrete Composite Co. Ltd. was selected as the sorbent support, while PEI with a molecular weight (MW) of 800 (PEI800), aminoethylethanolamine (AEEA), TRI, triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) were purchased from Sigma Aldrich and were all used as received.
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2.2. Sorbent preparation
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Modification of the sorbents by the addition of amine groups was performed by wet impregnation with the selected amine (AEEA, PEI800, TEPA, TETA or TRI) using an adaptation of the method of Kamarudin and Alias [19]. Briefly, the fSi powder was dispersed in 150 mL of methanol while the required amount of the respective amine was dissolved in 50 mL of methanol with stirring for about 15 min. The amine solution was added to the dispersed fumed silica solution with continuous stirring and then the methanol (solvent) was evaporated completely from the resultant slurry at 60 °C overnight. The dried sorbents were ground and then vacuum dried at 40 °C overnight, before being used for the sorption. The amine modified fSi sorbents were denoted as fSi-XY, where X indicates the amine precursor (PEI800, AEEA, TRI, TETA or TEPA) and Y is the nominal wt% of amine precursor in the sorbent.
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2.3. Sorbent characterization
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The functional groups on the surface of each modified sorbent were examined by Fourier transform infrared spectroscopy (FTIR) and KBr was used as the background. The degree of impregnation was determined as the increased nitrogen content by ultimate analysis (LEGO Truespec 628). The nitrogen (N2) adsorption–desorption isotherm was measured on a Quantachrome Autosorb-1 instrument. Pore-size distribution of the samples was determined from the isotherms by the Barrett–Joyner–Hallenda (BJH) method, and the surface area of the sorbents was characterized by the Brunauer–Emmett–Teller (BET) method. The dispersion of amine groups on the surface of the fSi was characterized by scanning electron microscopy (SEM) using a Hitachi model S-3400N microscope.
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2.4. Measurement of CO2 and H2S sorption
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For the CO2 and H2S single component sorption, the sorption was performed in a single stage fixed-bed flow system, coupled with an on-line CO2 and H2S gas chromatography (GC) analyzer (model 490-GC). Approximately 1.0 g of the sorbent was packed into the glass sorption tube (outer and inner diameter of 9.0 and 8.0 mm, respectively). Prior to sorption, the sorbent was heated to 100 °C in N2 (99.999%) at a flow rate of 50 mL/min for 30 min. For evaluation of the CO2 sorption alone, the model gas contained 10% (v/v) CO2 in N2, while for the H2S sorption alone the model gas contained 1% (v/v) H2S in N2. The model gas was introduced into the sorbent bed from the top. When the outlet concentration reached the initial feed concentration, i.e. the sorbent was saturated by CO2 or H2S, the gas line was switched to ultra high purity N2 at a flow rate of 100 mL/min, and the sorbent bed temperature was increased to the desired temperature to perform the desorption. For the simultaneous CO2 and H2S sorption, both single- and double-stage sorption systems were evaluated. For the two-stage system, a two-column system was set up as schematically shown in Fig. 1. About 1.0 g of the sorbent was packed in each column. Prior to sorption, the sorbent was pretreated by heating to 100 °C in N2 (99.999%) at a flow rate of 50 mL/min for 30 min. For sorption, the model of natural gas contained 20% (v/v) CO2, 0.36% (v/v) H2S and 74.8% (v/v) methane (CH4) balanced with N2, and was introduced into the sorbent bed from the top. The sorption in the first and second column was conducted at the previously determined optimal temperatures (80 °C and 30 °C, respectively), using a gas flow rate of 5 mL/min under atmospheric pressure. The CO2 or H2S breakthrough (Cap(BT)) and saturation (Cap(S)) capacities of the sorbents were calculated by integration of the areas above the breakthrough curves, and from the CO2 or H2S con-
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Please cite this article in press as: Yoosuk B et al. CO2 and H2S binary sorption on polyamine modified fumed silica. Fuel (2015), http://dx.doi.org/10.1016/j. fuel.2015.11.080
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Table 1 Surface area, pore volume and ultimate analysis of the fumed silica (fSi) and the PEI800 modified fSi sorbents with different PEI800 loading levels. Sample
Surface area (m2/g)
Pore volume (cm3/g)
fSi fSi-PEI800-20 fSi-PEI800-30 fSi-PEI800-40 fSi-PEI800-50 fSi-PEI800-60
202.8 91.3 81.2 43.5 20.3 4.2
1.860 1.831 1.510 0.820 0.368 0.082
Ultimate analysis (wt%) C
H
N
– 10.5 150 19.8 25.3 29.9
– 2.7 3.5 4.6 6.0 7.1
– 6.3 9.0 11.8 15.5 18.5
Fig. 1. Schematic diagram of the double-stage system used for the sorption of both CO2 and H2S from a mixed gas.
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where BT is the breakthrough time (min), FR is the flow rate (mL/ min), Vmol is the molar volume (24.4 mL/mmol at standard conditions), W is the weight of sorbent (g), Cin is the CO2 or H2S concentration in the initial gas, C0 and Ct are the inlet and outlet concentration of CO2 or H2S, t is the sorption time (min) and N is the molar amount of amine groups (mmol).
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2.5. Stability of adsorbent
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For evaluation of the stability of the adsorbent, the adsorbent capacity was determined after each of 5 successive adsorption– desorption cycles. For sorption in double-stage sorption system, the model gas containing 20% (v/v) CO2, 0.36% (v/v) H2S and 74.8% (v/v) methane (CH4) balanced with N2, was introduced into the sorbent bed. The sorption in the first and second column was conducted at the temperatures (80 °C and 30 °C, respectively), using a gas flow rate of 5 mL/min under atmospheric pressure. For regeneration in double-stage sorption system, when the outlet CO2 and H2S concentration reached the initial inlet concentration (i.e. became saturated), the adsorbent was heated to 90 °C with a N2 flow rate of 100 mL until the eluting H2S and CO2 concentration reached undetectable levels.
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3. Results and discussion
because the loading of PEI800 at 70 wt% resulted in the formation of a yellow waxy solid in the sorbent preparation step. Thus, a 70 wt% of PEI800 is in excess for this fSi and unsuitable as a solid sorbent. In agreement is that, given that the pore volume of the fSi was 1.86 cm3/g with a density of 1.05 g/cm3, the theoretical maximum loading level of 1 g of fSi with PEI800 is 66 wt%. From the ultimate analyses of sorbents, the results confirmed the effectiveness of the impregnation method (Table 1), where the nitrogen content increased proportionally with increasing PEI800 loading levels. The maximum nitrogen content (fSi-PEI800-60) was 18.5%, while the fSi-PEI800-20 sorbent had the lowest nitrogen content (6.3%). That the impregnation greatly increased the nitrogen content of the samples is due to the contribution of the immobilized amines [20], where increased amine loading levels led to an increased nitrogen proportion. With respect to the surface morphology of the amine modified sorbents, as determined by SEM analysis (Fig. 2), the fSi-PEI800 sorbents showed the agglomeration of the primary particles to form larger particles. Comparing the SEM image of the fSi-PEI800-40 sorbent with that for the fSi, it appeared that the PEI800 was uniformly distributed and well dispersed on the fSi surface. The fSi-PEI800-60 showed a similar morphology to that of fSi-PEI800-40 but with a thicker PEI800 layer and larger aggregates. This is similar to that reported previously for PEI modified MCM-48 [21]. Representative FT-IR spectra of the fSi and the PEI800-modified fSi sorbents at a 30, 50 and 60 wt% PEI800 loading level are shown in Fig. 3. For the unmodified fSi, the peaks at 1111 and 812 cm1 are attributed to the Si–O–Si asymmetric and symmetric stretching vibrations, respectively [22]. In the spectrum of the amine modified fSi, the bands at 1567 and 1479 cm1 were attributed to the asymmetric and symmetric bending of the primary amines (NH2), respectively, while that emerging at 1645 cm1 was assigned to the bending of the secondary amines (N(R)H) [23]. With increasing amine loading levels, the peaks for the primary and secondary amines increased accordingly. The bands at 2845 and 2960 cm1 represent the CH2 stretching of PEI800 chains, while those bands at 3380 cm1 could be attributed to the amine N–H stretching vibrations [24].
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3.1. Sorbent characterization
3.2. CO2 and H2S sorption
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The physical properties of these sorbents are presented in Table 1. Although the fSi was comprised of non-porous particles, it exhibited a high surface area (202.8 m2/g) and pore volume (1.86 cm3/g) due to having very fine particles that agglomerated to form the pore-like structures. When the fSi was modified with increasing loading levels of PEI800, the resultant sorbents showed a significant and dose-dependent decrease in the BET surface area (from 2.2- to 48.2-fold at 20 –60 wt% PEI800) and pore volume (from 1.01- to 22.7-fold at 20 –60 wt% PEI800). This is probably because the inter-particle pores had been coated by PEI800. Note that the data for the fSi-PEI800-70 preparation is not available
From computational modeling studies of amine based sorbents, the main reactions between the amine and CO2 or H2S are presented in Eqs. (4)–(7) [25]:
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202 203 204 205 206
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centration in the inlet gas, flow rate, breakthrough and saturation times, and mass of the sorbent using Eqs. (1) and (2), respectively. The amine efficiency (Eff. Amine) was calculated by the ratio of moles of adsorbed CO2 or H2S vs. the number of moles of nitrogen within each sorbent using Eq. (3). 6
in
209
CapðBTÞ ¼
210 212
BT FR C 10 V
Rt CapðSÞ ¼
0
mol
ð1Þ
W in
6
ðC 0 C t Þdt FR C 10 V
mol
W
ð2Þ
213 215 216 217 218 219 220
224 225 226 227 228 229 230 231 232 233 234
239 240 241 242 243 244 245 246 247 248
Eff: Amine ¼
CapðBTÞ or CapðSÞ N
ð3Þ
249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287
290 291
ð4Þ
292 294
CO2 þ 2R2 NH $ R2 NCOO þ R2 NHþ2 ;
ð5Þ
295 297
H2 S þ 2RNH2 $ RNH3 HS;
ð6Þ
298 300
H2 S þ 2R2 NH $ ðR2 NHÞ2 S:
ð7Þ
301 303
CO2 þ 2RNH2 $ R2 NCOO þ
NHþ4 ;
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Fig. 2. Representative SEM images (5000 magnification) of the (A) fSi, (B) fSi-PEI800-20, (C) fSi-PEI800-40 and (D) fSi-PEI800-60.
Table 2 CO2 and H2S sorption capacity of the fumed silica (fSi) and the different amine (20 wt %) modified fSi sorbents. Sorbent
fSi-TRI-20 fSi-AEEA-20 fSi-TETA-20 fSi-TEPA-20 fSi-PEI800-20
CO2 sorption capacity (mmol/gsorb)
H2S sorption capacity (mmol/gsorb)
Cap(BT)
Cap(S)
Cap(BT)
Cap(S)
0.27 0.79 0.93 0.87 0.47
0.62 1.15 1.26 1.43 0.87
0.15 0.44 0.63 0.52 0.59
0.17 0.48 0.66 0.55 0.63
Leachinga of amine
No Some Some Some Minimal
a Some = amine separated from the fSi as droplets, Minimal = amine separated from the fSi as vapor on column surface.
Fig. 3. FT-IR spectra of the fSi and PEI800 modified fSi sorbents (fSi-PEI800-x).
304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319
Stoichiometrically, this limits the sorption capacity to 1 mol of CO2 or H2S for every 2 mol of amine groups. The interactions between the amine group and CO2 or H2S have two potential types of sorption sites. For CO2 sorption, the C atom in the CO2 molecule can interact with a N atom in the amine group in the first type of sorption site, whereas the C atom in the CO2 molecule simultaneously interacts with two N atoms, one from each of two amine groups, in the second type of sorption site [16]. For H2S sorption, the two H atoms in the H2S molecule interact with a N atom in the amine group in the first type of sorption site, whereas each of the two H atoms in the H2S molecule interact with a N atom from one of two amine groups in the second type [16]. 3.2.1. Effect of amine type and loading amount The effect of the amine types used to modify the fSi (20 wt% of PEI800, AEEA, TRI, TETA and TEPA) on the CO2 sorption at 80 °C and on the H2S sorption at 30 °C, in terms of the Cap(BT) and Cap(S), are
summarized in Table 2. The fSi-TRI-20 absorbent showed no amine leaching but had the lowest sorption capacity for both CO2 and H2S and was only marginally higher than that for the unmodified fSi (not shown). The fSi-AEEA-20 absorbent showed a 1.85- to 2.82-fold higher sorption performance for both CO2 and H2S than that for the fSi-TRI-20 sorbent, but some amine leaching was detected. The fSi-TETA-20 sorbent showed the highest CO2 and H2S sorption capacity followed closely by that from the fSi-TEPA-20 sorbent. For the linear polyamines, the performance depended on the polymer chain length and the ratio of primary/secondary amine groups, since primary amines absorb CO2 and H2S faster than secondary amines. Although TEPA and TETA both have two primary amine groups, TETA only has two secondary amines compared to three for TEPA. Although, these two sorbents had high sorption capacities, the volatility of the amines led to a loss of amine loading. Thus, the branched amine polymer PEI800 was evaluated and found to offer a compromise in that although its CO2 adsorption capacity was lower than that for TEPA and TETA, it had a higher performance for H2S sorption and a low level of leaching. Indeed, the minimal level of amine leaching observed was probably due to the loss of lower molecular weight impurities in the PEI800. Regardless, PEI800 was selected as the more optimal amine for modification of the fSi sorbent.
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The effect of the PEI800 loading level (20–60 wt%) on the fSi upon the resultant CO2 sorption capacity of the obtained fSiPEI800 sorbent was evaluated at 80 °C in terms of the breakthrough curves (Fig. 4) and the Cap(BT) and Cap(S) (Table 3). The unmodified fSi had very low Cap(BT) and Cap(S). The low sorption capacity was due to the weak physisorption of CO2 over the fSi. When 20 wt% PEI800 was added, the sorption capacity markedly improved. The breakthrough times increased from 0 to 21.0 min and the Cap(S) increased 1.55-fold from 0.56 to 0.87 mmol/g. This was due to the acid–base reaction between CO2 and the amine (NH2, NH ) sites. As the PEI800 content was increased further up to a 40 wt% loading the breakthrough time and Cap(S) increased, reaching a maximum at 40 wt% PEI800 of 90 min and 2.57 mmol/g, respectively. This implied that increasing the number of amine sites resulted in an improved CO2 sorption capability because the amine was well dispersed on the fSi. However, further increasing the PEI800 loading level above 40 wt% caused a decreased breakthrough time, Cap(BT) and Cap (S) falling down to a 1.67- to 1.27-fold lower level at 60 wt% PEI800 than at 40 wt%. This was probably because at a 40 wt% PEI800 loading level the polymeric amine was better dispersed on the surface of the support, allowing an easier access to amino groups for the incoming gases. At higher PEI800 loading levels, a larger part of the amino groups in PEI800 might not be as accessible due to their poorer dispersion on the support surface and agglomeration of the coated particles. The internal pores of the sorbent were then blocked by the excessive loading of PEI800 [26]. Indeed, the SEM analysis revealed that the fSi-PEI800-60 had a thick PEI800 layer and large aggregates (Fig. 2). The amine efficiency was calculated by comparison of the obtained sorption with the theoretical value for CO2 sorption in order to evaluate the utilization of amine groups in the sorbents. All sorbents had a lower amine efficiency than the theoretical value of 0.5 mol of CO2/mol of NHx [16], as summarized in Table 3. Thus, a significant level of the amine sites was unavailable or inhibited for CO2 binding. This may have resulted from the NHx moieties interacting with the surface silanol groups of the fSi and the inability of the attached spatially distributed, site-isolated amine groups to react with CO2, as well as by the blocking of the pore entrances [27]. The amine efficiency of all sorbents was also affected by the amine loading level, being enhanced with increasing amine loading levels from 20 wt% to a maximum at a 40 wt% loading level and then decreased with higher PEI800 loading levels above 40 wt%, such that the amine efficiency of fSi-PEI800-60 was even lower than that of the fSi-PEI800-20 sorbent. The low amine proximity
Fig. 4. Breakthrough curves of CO2 sorption over fSi-PEI800 sorbents with different amine loading levels.
Table 3 CO2 sorption performance of the fumed silica (fSi) and the PEI800 modified fSi sorbents with different PEI800 loading amounts (20–60 wt%). Sorbent
Breakthrough time (min)
Cap(BT) (mmol/g-sorb)
Cap(S) (mmol/g-sorb)
Amine efficiency (BT)
(S)
fSi fSi-PEI800-20 fSi-PEI800-30 fSi-PEI800-40 fSi-PEI800-50 fSi-PEI800-60
0 21.0 40.0 90.0 69.0 54.0
0 0.47 0.89 2.01 1.54 1.21
0.56 0.87 1.43 2.57 2.14 2.04
– 0.10 0.13 0.22 0.13 0.09
– 0.19 0.21 0.28 0.19 0.15
was another possible reason for the low amine efficiency in the sorbents with a low PEI800 loading level. Increasing the PEI800 loading level up to 40 wt% could dilute this interface effect, and consequently the proportion of utilized amine groups increased greatly with the amine loading level. For higher PEI800 loading levels than 40 wt%, the thickness of the amine film covering the surface increased and led to an increased difficulty in the diffusion of CO2 into the internal amine groups and so decreased the amine capturing efficiency. Accordingly, a 40 wt% PEI800 loading level (fSi-PEI800-40) was selected for all subsequent studies.
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3.2.2. Effect of temperature on the CO2 and H2S single-column sorption The Cap(BT) of CO2 or H2S sorption at various temperatures on the fSi-PEI800-40 sorbent is illustrated in Fig. 5, where the Cap(BT) for CO2 sorption increased from 1.13 to 2.08 mmol/g as the temperature increased from 30 to 80 °C. For H2S sorption, the opposite trend was observed and the Cap(BT) decreased as the temperature increased. Wang et al. [28] proposed a more detailed mechanism for CO2 sorption on PEI-impregnated mesoporous solid supports, suggesting that gas sorption includes two kinetic rates; (i) a rapid sorption over the surface layers of PEI, and (ii) the diffusion and sorption inside the bulk multilayers of PEI. The first step is controlled by the isothermal equilibrium of CO2 sorption, while the second step is controlled by the rate of CO2 diffusion. According to this, since the sorption of CO2 is an exothermic reaction, the sorption capacity would be expected to decrease with an increasing temperature. However, in this study, the sorption capacity increased with increasing temperature, presumably because the higher temperature facilitated the diffusion of CO2 molecules from the surface into the bulk of PEI by overcoming the kinetic barrier [16,29]. This then resulted in the higher sorption capacity at the higher temperature,
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Fig. 5. Breakthrough capacity of CO2 or H2S sorption on the fSi-PEI800-40 sorbent at various temperatures.
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even though the increased temperature does not thermodynamically favor the sorption of CO2 on the surface and in the bulk of PEI. In addition, this effect was due to the faster reaction kinetics between CO2 and the amino groups at higher temperatures. In the case of H2S sorption, that the lower temperature favored an increase in the H2S equilibrium sorption capacity indicated that the sorption of H2S over the fSi-PEI800-40 sorbent was predominately determined by thermodynamics rather than by the kinetics of H2S diffusion. Chen et al. [7] explained this observed trend as being due to the weak thermodynamic stability of the bonds between the amine groups and H2S. The weak acid–base interaction between H2S and the amine groups is not high enough to allow the amine groups on the surface to bind and retain H2S molecules at higher temperatures, but it is high enough to adsorb H2S molecules on the surface at room temperature. On the other hand, the lower kinetic barrier for the diffusion of the adsorbed H2S in the PEI800 bulk, as estimated by a computational chemistry approach [13], would facilitate the transfer of the adsorbed H2S on the surface into the PEI800 bulk inside the pores [13]. The computational study of Ma et al. [16] revealed that the estimated kinetic barrier for diffusion of the sorbed CO2 from the surface into the bulk of PEI was threefold higher than that for the diffusion of the sorbed H2S, indicating that the diffusion of the sorbed CO2 from the exposed surface into the bulk of PEI is much more difficult than that for the sorbed H2S. Thus, a higher sorption temperature is required for CO2. However, if the temperature is higher than the critical point, it will promote desorption. Moreover, it is seen that the CO2 sorption capacity is comparatively less sensitive to the temperature than H2S, at least within this evaluated temperature range. 3.2.3. Simultaneous sorption of CO2 and H2S In this work, the mixed gas (20% (v/v) CO2, 0.36% (v/v) H2S and 74.8% (v/v) CH4 balanced with N2) was used to be the model of natural gas with which the polyamine modified fumed silica sorbent would be implemented. To investigate the competitive sorption of CO2 and H2S, the adsorption onto the fSi-PEI800-40 sorbent of the mixed CO2 and H2S gas was performed at 30 °C and 80 °C in a single-stage process. The breakthrough curves of CO2 and H2S sorption in a single column (Fig. 6(A)) confirmed the temperature dependence of the sorption capacity for each gas. However, considering the breakthrough curves at 30 °C, the breakthrough time of CO2 was much higher than that of H2S. This is different from the data indicated in Fig. 5, showing that at 30 °C the sorption capacity of H2S was slightly higher than that of CO2, and so implied that CO2 strongly inhibits the H2S sorption under this condition. This phenomenon has also been observed with the PEI/MCM-41 sorbent [16]. The higher heat of adsorption of CO2 favors its sorption on the fSi-PEI800-40 sorbent and then the sorbed CO2 hinders further H2S sorption. The breakthrough curves of CO2 and H2S sorption in the doublestage system (Fig. 6(B)) clearly revealed that the effect of CO2 on the sorption of H2S is decreased in this double-stage system. The Cap(BT) of H2S was 0.045 mmol/g, which was lower than the expected value, presumably because some CO2 escaped from the first column into the second column where it was adsorbed and inhibited H2S sorption. Another phenomenon that might happen is the partial replacement of sorbed H2S molecules by CO2 molecules since CO2 has a higher adsorptive affinity than H2S. Thus, fSi-PEI800-40 sorbent was found to have the good sorption capacity for both CO2 and H2S sorption which show the best capacity of each gas on this sorbent at different temperature (80 °C for CO2 and 30 °C for H2S). Additionally, the presence of CO2 strongly inhibited the H2S sorption in the simultaneous CO2 and H2S sorption. The results show that the sequential sorption of CO2 followed by H2S in a double stage system was found to solve
Fig. 6. Breakthrough curves of the simultaneous CO2 and H2S sorption on the fSiPEI800-40 sorbent in a (A) single-stage or a (B) double-stage system.
this problem and successfully remove both CO2 and H2S from a mixed gas and so this polyamine modified fumed silica as low cost sorbent has a high potential for application in the acid gas treatment process.
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3.3. Stability of the fSi-PEI800-40 adsorbent
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The regeneration performance of the fSi-PEI800-40 adsorbent was evaluated, as this is one of the more important properties of an adsorbent for any practical application. In order to examine the stability of the fSi-PEI800-40 adsorbent in the adsorption and regeneration processes, 5 sequential adsorption–desorption cycles were conducted, with the adsorption performed at 80 °C (1st column) and 30 °C (2nd column) and a model gas (20% CO2, 0.36% H2S and 74.8% CH4) flow rate of 5 mL/min, and the regeneration performed at 90 °C and a N2 (99.999%) flow rate of 10 mL/min. Both were performed at atmospheric pressure. The breakthrough CO2 and H2S sorption capacities did not significantly change on 1st, 2nd and 3rd cycles, remaining within the narrow ranges of 3.36–3.494 and 0.053–0.063 mmol/g-sorb for CO2, and H2S, respectively (Fig. 6). However, the breakthrough CO2 and H2S sorption capacities slightly decreased on 4th and 5th cycles (Cap(BT) = 2.822–2.957 and Cap(BT) = 0.051 mmol/g-sorb for CO2 and H2S, respectively) due to accumulation of very small amount of residual CO2 and H2S in adsorbent and this possibly required longer regeneration time in 4th and 5th cycles for the case of high CO2 and H2S concentration feed. However, these results showed that the adsorption performance of fSi-PEI800-40 was stable. Therefore, this adsorbent could be regenerated under mild conditions for at least 5 cycles without loss of CO2 and H2S adsorption activity and so may be suitable for CO2 and H2S removal (see Fig. 7).
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Fig. 7. H2S and CO2 sorption capacity of the fSi-PEI800-40 over five successive adsorption–desorption cycles.
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4. Conclusions
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A series of amine modified fSi sorbents were prepared by impregnation. The resultant sorbents could remove CO2 and H2S under mild conditions. Among the five different evaluated amines, PEI800 exhibited a medium and high sorption capacity for CO2 and H2S, respectively, but also had a low amine leaching level and so was selected as the optimal amine. The amount of available amine groups in the sorbent and the interaction between amine groups and CO2 or H2S are the two major factors that affect the sorption capacity and amine efficiency. The fSi-PEI800-40 sorbent showed the highest CO2 sorption capacity (2.57 mmol/g-sorb). At higher amine loading levels, the thickness of the amine film covering the surface increased and led to an increased difficulty in the diffusion of CO2 into the internal amine groups. Higher temperatures facilitated the diffusion of CO2 from the surface into the bulk of the PEI800 by overcoming the kinetic barrier, while a lower temperature favored an increased H2S sorption capacity. The presence of CO2 strongly inhibited the H2S sorption in the simultaneous CO2 and H2S sorption. This effect can be reduced by using a doublestage sorption system with CO2 sorption first followed by H2S removal.
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Acknowledgments
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The authors gratefully acknowledge the funding support from the Center for Petroleum Petrochemicals and Advanced Materials, Chulalongkorn University and National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (P-12-01184 project). We would like to thank Dr. Robert Butcher for proof reading the article and suggestions.
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel 4 5 3
CO2 and H2S binary sorption on polyamine modified fumed silica
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Boonyawan Yoosuk a,⇑, Tippayarat Wongsanga b, Pattarapan Prasassarakich b,⇑
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a Renewable Energy Laboratory, National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency, 114 Thailand Science Park, Thanon Phahonyothin, Tambon Khlong Nueng, Amphoe Khlong Luang, Pathum Thani 12120, Thailand b Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
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h i g h l i g h t s Low cost fumed silica modified by PEI800 was developed with the ability to remove both CO2 and H2S.
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The amine amount and amine interaction with H2S or CO2 play crucial role on adsorption performance.
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H2S sorption was determined by thermodynamics, high temperature increased CO2 sorption capacity.
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The fSi-PEI800-40 favors the CO2 sorption and the sorbed CO2 strongly inhibits the sorption of H2S. The sequential sorption of CO2 followed by H2S in double stage system reduced this effect.
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a r t i c l e 2 3 2 5 23 24 25 26 27 28 29 30 31 32 33 34
i n f o
Article history: Received 17 June 2015 Received in revised form 11 November 2015 Accepted 25 November 2015 Available online xxxx Keywords: CO2 removal H2S removal Sorption Amine Fumed silica
a b s t r a c t Low cost fumed silica modified by amine addition with the ability to remove both carbon dioxide (CO2) and hydrogen sulfide (H2S) was developed. The effect of amine type (polyethyleneimine with a molecular weight of 800 (PEI800), aminoethylethanolamine, N-(3-trimethoxysilypropyl) diethylenetriamine, diethylenetriamine, triethylenetetramine and tetraethylenepentamine), loading level and sorption temperature were investigated. The selected sorbent was then evaluated in a single- and double-stage systems. The polymeric amine PEI800 gave the best compromise between the highest CO2 and H2S sorption capacities with minimal amine leaching, and was optimal at a 40% loading level onto fumed silica, in terms of giving the highest breakthrough and saturation capacity. The amount of available amine groups in the sorbent and the interaction between amine groups and CO2 or H2S were the two major factors that affected the sorption capacity and amine efficiency. The sorption of H2S was predominately determined by thermodynamics, while a high temperature helped the diffusion of CO2 molecules from the surface into the bulk of PEI800. The sorbent favored CO2 sorption and the sorbed CO2 hindered the sorption of H2S. However, the sequential sorption of CO2 followed by H2S in a double-stage system was found to solve this problem. Ó 2015 Published by Elsevier Ltd.
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1. Introduction
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The removal of carbon dioxide (CO2) and hydrogen sulfide (H2S) is very important in gas upgrading processes because these gases reduce the heating value and are very corrosive [1,2]. Therefore, efficient methods for their removal are urgently needed. Additionally, carbon capture and storage is a good possibility for reducing the release of CO2 into the atmosphere [3,4]. In the conventional process for gas upgrading, the removal of acid gases is commonly performed by liquid-phase chemical scrubbing with amines. However, this technology suffers from having a high energy
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⇑ Corresponding authors. Tel.: +66 2 218 7517; fax: +66 2 255 5831. E-mail addresses: [email protected] (B. Yoosuk), [email protected] (P. Prasassarakich).
consumption, low absorption/desorption rate, solvent loss and material corrosion. The adsorptive separation of gases, wherein the acid gases, CO2 and H2S interact more strongly with the internal surfaces of the porous material (sorbent), has a high potential for most gas separation applications. This adsorption technology is recognized as an energy efficient technology for acid gas removal provided that a stable material with a high sorption capacity and selectivity toward the acid gases is available. A considerable amount of attention has focused on developing a solid sorbent with a high surface area and sorption capacities for CO2 and H2S that is hence suitable for their removal. The sorption of CO2 has been studied using various supports, such as silica nanoparticles [5], fumed silica (fSi) [6] and mesoporous molecular sieves (MCM-41, MCM-48, SBA-15) [7,8], plus those modified with various amine types, such as polyethylenimine (PEI) [5–8] and
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triamine (TRI) [8]. Silica nanoparticles modified by PEI are an inexpensive, easy-to-prepare and regenerable adsorbent that have the practical ability to capture CO2 [5]. Fumed silica (fSi) impregnated with PEI (fSi-PEI) was also found to be a superior sorbent for the capture of CO2 directly from air [6]. For CO2 capture, the highly sorbent triamine-grafted pore-expanded mesoporous silica (TRI-PEMCM-41) was not affected by the moisture content in terms of the working sorption capacity developed, and the temperature swing regeneration mode was found to be suitable for desorption above 120 °C [8]. Fossil fuel combustion, which results in the emission of acidic gases, such as CO2, sulfur dioxide (SO2) and nitrogen oxides (NOx), is an important environmental issue. For the selective adsorption of SO2 and NOx in the presence of CO2, amineimpregnated and amine-grafted silica have been found to be effective adsorbents with a good stability [9,10]. Recently, tertiary amine containing poly(propyleneimine) second (G2) and third (G3) generation dendrimers as well as PEI on a nanoporous support [11] and N,N-dimethylpropylamine-grafted-PE-MCM-41 [12] were developed for the selective removal of SO2. Their adsorption capacity was enhanced dramatically in the presence of water vapor in the gas mixture and could be completely regenerated thermally [12]. Additionally, for H2S removal from a model gas mixture using various PEI-loaded mesoporous molecular sieves, including MCM-41, MCM-48 and SBA-15, the PEI-loaded SBA-15 sorbent had the highest saturation capacity, and could be regenerated under mild conditions [13]. A PEI-loaded hierarchical porous silica monolith was developed as a high stability, recyclable H2S sorbent at low temperatures and had a large H2S breakthrough capacity of 1.27 mmol of H2S/g sorbent at 22 °C and this could easily be regenerated at 75 °C [7]. Using the nanoporous composite sorbent PEI/ SBA-15 that had a high sorption capacity, the presence of moisture was found to have a promoting effect on the removal of H2S from gas streams [14]. Recently, high porosity and adsorbent silica xerogels modified by the addition of PEI were developed for the low temperature removal of H2S. Of these, the optimal adsorbent (fSiPEI800-50) was easily regenerated at a mild temperature (optimal of 50 °C) for at least ten successive adsorption–desorption cycles without any detectable loss of adsorption performance or regeneration ability [15]. However, only a few research reports have focused on the development of sorbents with the ability to selectively remove both CO2 and H2S. Early work reported that PEI-loaded MCM-41 or SBA-15 [16] and TRI-PE-MCM-41 [17,18] could adsorb CO2 and H2S from a model gas and subsequently be regenerated. However, these mesoporous silica compounds are presently not commercial available in large quantities and so have to first be synthesized. In this work a low cost fSi was modified by the addition of amine groups. The single and simultaneous sorption of CO2 and H2S over the obtained modified fSi sorbents were investigated in a single and a double fixed-bed system. The influence of the amine type and amine loading level were elucidated and the competitive sorption of CO2 and H2S is discussed.
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2. Experimental
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2.1. Materials
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The fumed silica (fSi) from Concrete Composite Co. Ltd. was selected as the sorbent support, while PEI with a molecular weight (MW) of 800 (PEI800), aminoethylethanolamine (AEEA), TRI, triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) were purchased from Sigma Aldrich and were all used as received.
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2.2. Sorbent preparation
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Modification of the sorbents by the addition of amine groups was performed by wet impregnation with the selected amine (AEEA, PEI800, TEPA, TETA or TRI) using an adaptation of the method of Kamarudin and Alias [19]. Briefly, the fSi powder was dispersed in 150 mL of methanol while the required amount of the respective amine was dissolved in 50 mL of methanol with stirring for about 15 min. The amine solution was added to the dispersed fumed silica solution with continuous stirring and then the methanol (solvent) was evaporated completely from the resultant slurry at 60 °C overnight. The dried sorbents were ground and then vacuum dried at 40 °C overnight, before being used for the sorption. The amine modified fSi sorbents were denoted as fSi-XY, where X indicates the amine precursor (PEI800, AEEA, TRI, TETA or TEPA) and Y is the nominal wt% of amine precursor in the sorbent.
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2.3. Sorbent characterization
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The functional groups on the surface of each modified sorbent were examined by Fourier transform infrared spectroscopy (FTIR) and KBr was used as the background. The degree of impregnation was determined as the increased nitrogen content by ultimate analysis (LEGO Truespec 628). The nitrogen (N2) adsorption–desorption isotherm was measured on a Quantachrome Autosorb-1 instrument. Pore-size distribution of the samples was determined from the isotherms by the Barrett–Joyner–Hallenda (BJH) method, and the surface area of the sorbents was characterized by the Brunauer–Emmett–Teller (BET) method. The dispersion of amine groups on the surface of the fSi was characterized by scanning electron microscopy (SEM) using a Hitachi model S-3400N microscope.
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2.4. Measurement of CO2 and H2S sorption
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For the CO2 and H2S single component sorption, the sorption was performed in a single stage fixed-bed flow system, coupled with an on-line CO2 and H2S gas chromatography (GC) analyzer (model 490-GC). Approximately 1.0 g of the sorbent was packed into the glass sorption tube (outer and inner diameter of 9.0 and 8.0 mm, respectively). Prior to sorption, the sorbent was heated to 100 °C in N2 (99.999%) at a flow rate of 50 mL/min for 30 min. For evaluation of the CO2 sorption alone, the model gas contained 10% (v/v) CO2 in N2, while for the H2S sorption alone the model gas contained 1% (v/v) H2S in N2. The model gas was introduced into the sorbent bed from the top. When the outlet concentration reached the initial feed concentration, i.e. the sorbent was saturated by CO2 or H2S, the gas line was switched to ultra high purity N2 at a flow rate of 100 mL/min, and the sorbent bed temperature was increased to the desired temperature to perform the desorption. For the simultaneous CO2 and H2S sorption, both single- and double-stage sorption systems were evaluated. For the two-stage system, a two-column system was set up as schematically shown in Fig. 1. About 1.0 g of the sorbent was packed in each column. Prior to sorption, the sorbent was pretreated by heating to 100 °C in N2 (99.999%) at a flow rate of 50 mL/min for 30 min. For sorption, the model of natural gas contained 20% (v/v) CO2, 0.36% (v/v) H2S and 74.8% (v/v) methane (CH4) balanced with N2, and was introduced into the sorbent bed from the top. The sorption in the first and second column was conducted at the previously determined optimal temperatures (80 °C and 30 °C, respectively), using a gas flow rate of 5 mL/min under atmospheric pressure. The CO2 or H2S breakthrough (Cap(BT)) and saturation (Cap(S)) capacities of the sorbents were calculated by integration of the areas above the breakthrough curves, and from the CO2 or H2S con-
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Table 1 Surface area, pore volume and ultimate analysis of the fumed silica (fSi) and the PEI800 modified fSi sorbents with different PEI800 loading levels. Sample
Surface area (m2/g)
Pore volume (cm3/g)
fSi fSi-PEI800-20 fSi-PEI800-30 fSi-PEI800-40 fSi-PEI800-50 fSi-PEI800-60
202.8 91.3 81.2 43.5 20.3 4.2
1.860 1.831 1.510 0.820 0.368 0.082
Ultimate analysis (wt%) C
H
N
– 10.5 150 19.8 25.3 29.9
– 2.7 3.5 4.6 6.0 7.1
– 6.3 9.0 11.8 15.5 18.5
Fig. 1. Schematic diagram of the double-stage system used for the sorption of both CO2 and H2S from a mixed gas.
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where BT is the breakthrough time (min), FR is the flow rate (mL/ min), Vmol is the molar volume (24.4 mL/mmol at standard conditions), W is the weight of sorbent (g), Cin is the CO2 or H2S concentration in the initial gas, C0 and Ct are the inlet and outlet concentration of CO2 or H2S, t is the sorption time (min) and N is the molar amount of amine groups (mmol).
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2.5. Stability of adsorbent
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For evaluation of the stability of the adsorbent, the adsorbent capacity was determined after each of 5 successive adsorption– desorption cycles. For sorption in double-stage sorption system, the model gas containing 20% (v/v) CO2, 0.36% (v/v) H2S and 74.8% (v/v) methane (CH4) balanced with N2, was introduced into the sorbent bed. The sorption in the first and second column was conducted at the temperatures (80 °C and 30 °C, respectively), using a gas flow rate of 5 mL/min under atmospheric pressure. For regeneration in double-stage sorption system, when the outlet CO2 and H2S concentration reached the initial inlet concentration (i.e. became saturated), the adsorbent was heated to 90 °C with a N2 flow rate of 100 mL until the eluting H2S and CO2 concentration reached undetectable levels.
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3. Results and discussion
because the loading of PEI800 at 70 wt% resulted in the formation of a yellow waxy solid in the sorbent preparation step. Thus, a 70 wt% of PEI800 is in excess for this fSi and unsuitable as a solid sorbent. In agreement is that, given that the pore volume of the fSi was 1.86 cm3/g with a density of 1.05 g/cm3, the theoretical maximum loading level of 1 g of fSi with PEI800 is 66 wt%. From the ultimate analyses of sorbents, the results confirmed the effectiveness of the impregnation method (Table 1), where the nitrogen content increased proportionally with increasing PEI800 loading levels. The maximum nitrogen content (fSi-PEI800-60) was 18.5%, while the fSi-PEI800-20 sorbent had the lowest nitrogen content (6.3%). That the impregnation greatly increased the nitrogen content of the samples is due to the contribution of the immobilized amines [20], where increased amine loading levels led to an increased nitrogen proportion. With respect to the surface morphology of the amine modified sorbents, as determined by SEM analysis (Fig. 2), the fSi-PEI800 sorbents showed the agglomeration of the primary particles to form larger particles. Comparing the SEM image of the fSi-PEI800-40 sorbent with that for the fSi, it appeared that the PEI800 was uniformly distributed and well dispersed on the fSi surface. The fSi-PEI800-60 showed a similar morphology to that of fSi-PEI800-40 but with a thicker PEI800 layer and larger aggregates. This is similar to that reported previously for PEI modified MCM-48 [21]. Representative FT-IR spectra of the fSi and the PEI800-modified fSi sorbents at a 30, 50 and 60 wt% PEI800 loading level are shown in Fig. 3. For the unmodified fSi, the peaks at 1111 and 812 cm1 are attributed to the Si–O–Si asymmetric and symmetric stretching vibrations, respectively [22]. In the spectrum of the amine modified fSi, the bands at 1567 and 1479 cm1 were attributed to the asymmetric and symmetric bending of the primary amines (NH2), respectively, while that emerging at 1645 cm1 was assigned to the bending of the secondary amines (N(R)H) [23]. With increasing amine loading levels, the peaks for the primary and secondary amines increased accordingly. The bands at 2845 and 2960 cm1 represent the CH2 stretching of PEI800 chains, while those bands at 3380 cm1 could be attributed to the amine N–H stretching vibrations [24].
237
3.1. Sorbent characterization
3.2. CO2 and H2S sorption
288
238
The physical properties of these sorbents are presented in Table 1. Although the fSi was comprised of non-porous particles, it exhibited a high surface area (202.8 m2/g) and pore volume (1.86 cm3/g) due to having very fine particles that agglomerated to form the pore-like structures. When the fSi was modified with increasing loading levels of PEI800, the resultant sorbents showed a significant and dose-dependent decrease in the BET surface area (from 2.2- to 48.2-fold at 20 –60 wt% PEI800) and pore volume (from 1.01- to 22.7-fold at 20 –60 wt% PEI800). This is probably because the inter-particle pores had been coated by PEI800. Note that the data for the fSi-PEI800-70 preparation is not available
From computational modeling studies of amine based sorbents, the main reactions between the amine and CO2 or H2S are presented in Eqs. (4)–(7) [25]:
289
202 203 204 205 206
207
centration in the inlet gas, flow rate, breakthrough and saturation times, and mass of the sorbent using Eqs. (1) and (2), respectively. The amine efficiency (Eff. Amine) was calculated by the ratio of moles of adsorbed CO2 or H2S vs. the number of moles of nitrogen within each sorbent using Eq. (3). 6
in
209
CapðBTÞ ¼
210 212
BT FR C 10 V
Rt CapðSÞ ¼
0
mol
ð1Þ
W in
6
ðC 0 C t Þdt FR C 10 V
mol
W
ð2Þ
213 215 216 217 218 219 220
224 225 226 227 228 229 230 231 232 233 234
239 240 241 242 243 244 245 246 247 248
Eff: Amine ¼
CapðBTÞ or CapðSÞ N
ð3Þ
249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287
290 291
ð4Þ
292 294
CO2 þ 2R2 NH $ R2 NCOO þ R2 NHþ2 ;
ð5Þ
295 297
H2 S þ 2RNH2 $ RNH3 HS;
ð6Þ
298 300
H2 S þ 2R2 NH $ ðR2 NHÞ2 S:
ð7Þ
301 303
CO2 þ 2RNH2 $ R2 NCOO þ
NHþ4 ;
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Fig. 2. Representative SEM images (5000 magnification) of the (A) fSi, (B) fSi-PEI800-20, (C) fSi-PEI800-40 and (D) fSi-PEI800-60.
Table 2 CO2 and H2S sorption capacity of the fumed silica (fSi) and the different amine (20 wt %) modified fSi sorbents. Sorbent
fSi-TRI-20 fSi-AEEA-20 fSi-TETA-20 fSi-TEPA-20 fSi-PEI800-20
CO2 sorption capacity (mmol/gsorb)
H2S sorption capacity (mmol/gsorb)
Cap(BT)
Cap(S)
Cap(BT)
Cap(S)
0.27 0.79 0.93 0.87 0.47
0.62 1.15 1.26 1.43 0.87
0.15 0.44 0.63 0.52 0.59
0.17 0.48 0.66 0.55 0.63
Leachinga of amine
No Some Some Some Minimal
a Some = amine separated from the fSi as droplets, Minimal = amine separated from the fSi as vapor on column surface.
Fig. 3. FT-IR spectra of the fSi and PEI800 modified fSi sorbents (fSi-PEI800-x).
304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319
Stoichiometrically, this limits the sorption capacity to 1 mol of CO2 or H2S for every 2 mol of amine groups. The interactions between the amine group and CO2 or H2S have two potential types of sorption sites. For CO2 sorption, the C atom in the CO2 molecule can interact with a N atom in the amine group in the first type of sorption site, whereas the C atom in the CO2 molecule simultaneously interacts with two N atoms, one from each of two amine groups, in the second type of sorption site [16]. For H2S sorption, the two H atoms in the H2S molecule interact with a N atom in the amine group in the first type of sorption site, whereas each of the two H atoms in the H2S molecule interact with a N atom from one of two amine groups in the second type [16]. 3.2.1. Effect of amine type and loading amount The effect of the amine types used to modify the fSi (20 wt% of PEI800, AEEA, TRI, TETA and TEPA) on the CO2 sorption at 80 °C and on the H2S sorption at 30 °C, in terms of the Cap(BT) and Cap(S), are
summarized in Table 2. The fSi-TRI-20 absorbent showed no amine leaching but had the lowest sorption capacity for both CO2 and H2S and was only marginally higher than that for the unmodified fSi (not shown). The fSi-AEEA-20 absorbent showed a 1.85- to 2.82-fold higher sorption performance for both CO2 and H2S than that for the fSi-TRI-20 sorbent, but some amine leaching was detected. The fSi-TETA-20 sorbent showed the highest CO2 and H2S sorption capacity followed closely by that from the fSi-TEPA-20 sorbent. For the linear polyamines, the performance depended on the polymer chain length and the ratio of primary/secondary amine groups, since primary amines absorb CO2 and H2S faster than secondary amines. Although TEPA and TETA both have two primary amine groups, TETA only has two secondary amines compared to three for TEPA. Although, these two sorbents had high sorption capacities, the volatility of the amines led to a loss of amine loading. Thus, the branched amine polymer PEI800 was evaluated and found to offer a compromise in that although its CO2 adsorption capacity was lower than that for TEPA and TETA, it had a higher performance for H2S sorption and a low level of leaching. Indeed, the minimal level of amine leaching observed was probably due to the loss of lower molecular weight impurities in the PEI800. Regardless, PEI800 was selected as the more optimal amine for modification of the fSi sorbent.
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The effect of the PEI800 loading level (20–60 wt%) on the fSi upon the resultant CO2 sorption capacity of the obtained fSiPEI800 sorbent was evaluated at 80 °C in terms of the breakthrough curves (Fig. 4) and the Cap(BT) and Cap(S) (Table 3). The unmodified fSi had very low Cap(BT) and Cap(S). The low sorption capacity was due to the weak physisorption of CO2 over the fSi. When 20 wt% PEI800 was added, the sorption capacity markedly improved. The breakthrough times increased from 0 to 21.0 min and the Cap(S) increased 1.55-fold from 0.56 to 0.87 mmol/g. This was due to the acid–base reaction between CO2 and the amine (NH2, NH ) sites. As the PEI800 content was increased further up to a 40 wt% loading the breakthrough time and Cap(S) increased, reaching a maximum at 40 wt% PEI800 of 90 min and 2.57 mmol/g, respectively. This implied that increasing the number of amine sites resulted in an improved CO2 sorption capability because the amine was well dispersed on the fSi. However, further increasing the PEI800 loading level above 40 wt% caused a decreased breakthrough time, Cap(BT) and Cap (S) falling down to a 1.67- to 1.27-fold lower level at 60 wt% PEI800 than at 40 wt%. This was probably because at a 40 wt% PEI800 loading level the polymeric amine was better dispersed on the surface of the support, allowing an easier access to amino groups for the incoming gases. At higher PEI800 loading levels, a larger part of the amino groups in PEI800 might not be as accessible due to their poorer dispersion on the support surface and agglomeration of the coated particles. The internal pores of the sorbent were then blocked by the excessive loading of PEI800 [26]. Indeed, the SEM analysis revealed that the fSi-PEI800-60 had a thick PEI800 layer and large aggregates (Fig. 2). The amine efficiency was calculated by comparison of the obtained sorption with the theoretical value for CO2 sorption in order to evaluate the utilization of amine groups in the sorbents. All sorbents had a lower amine efficiency than the theoretical value of 0.5 mol of CO2/mol of NHx [16], as summarized in Table 3. Thus, a significant level of the amine sites was unavailable or inhibited for CO2 binding. This may have resulted from the NHx moieties interacting with the surface silanol groups of the fSi and the inability of the attached spatially distributed, site-isolated amine groups to react with CO2, as well as by the blocking of the pore entrances [27]. The amine efficiency of all sorbents was also affected by the amine loading level, being enhanced with increasing amine loading levels from 20 wt% to a maximum at a 40 wt% loading level and then decreased with higher PEI800 loading levels above 40 wt%, such that the amine efficiency of fSi-PEI800-60 was even lower than that of the fSi-PEI800-20 sorbent. The low amine proximity
Fig. 4. Breakthrough curves of CO2 sorption over fSi-PEI800 sorbents with different amine loading levels.
Table 3 CO2 sorption performance of the fumed silica (fSi) and the PEI800 modified fSi sorbents with different PEI800 loading amounts (20–60 wt%). Sorbent
Breakthrough time (min)
Cap(BT) (mmol/g-sorb)
Cap(S) (mmol/g-sorb)
Amine efficiency (BT)
(S)
fSi fSi-PEI800-20 fSi-PEI800-30 fSi-PEI800-40 fSi-PEI800-50 fSi-PEI800-60
0 21.0 40.0 90.0 69.0 54.0
0 0.47 0.89 2.01 1.54 1.21
0.56 0.87 1.43 2.57 2.14 2.04
– 0.10 0.13 0.22 0.13 0.09
– 0.19 0.21 0.28 0.19 0.15
was another possible reason for the low amine efficiency in the sorbents with a low PEI800 loading level. Increasing the PEI800 loading level up to 40 wt% could dilute this interface effect, and consequently the proportion of utilized amine groups increased greatly with the amine loading level. For higher PEI800 loading levels than 40 wt%, the thickness of the amine film covering the surface increased and led to an increased difficulty in the diffusion of CO2 into the internal amine groups and so decreased the amine capturing efficiency. Accordingly, a 40 wt% PEI800 loading level (fSi-PEI800-40) was selected for all subsequent studies.
388
3.2.2. Effect of temperature on the CO2 and H2S single-column sorption The Cap(BT) of CO2 or H2S sorption at various temperatures on the fSi-PEI800-40 sorbent is illustrated in Fig. 5, where the Cap(BT) for CO2 sorption increased from 1.13 to 2.08 mmol/g as the temperature increased from 30 to 80 °C. For H2S sorption, the opposite trend was observed and the Cap(BT) decreased as the temperature increased. Wang et al. [28] proposed a more detailed mechanism for CO2 sorption on PEI-impregnated mesoporous solid supports, suggesting that gas sorption includes two kinetic rates; (i) a rapid sorption over the surface layers of PEI, and (ii) the diffusion and sorption inside the bulk multilayers of PEI. The first step is controlled by the isothermal equilibrium of CO2 sorption, while the second step is controlled by the rate of CO2 diffusion. According to this, since the sorption of CO2 is an exothermic reaction, the sorption capacity would be expected to decrease with an increasing temperature. However, in this study, the sorption capacity increased with increasing temperature, presumably because the higher temperature facilitated the diffusion of CO2 molecules from the surface into the bulk of PEI by overcoming the kinetic barrier [16,29]. This then resulted in the higher sorption capacity at the higher temperature,
399
Fig. 5. Breakthrough capacity of CO2 or H2S sorption on the fSi-PEI800-40 sorbent at various temperatures.
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even though the increased temperature does not thermodynamically favor the sorption of CO2 on the surface and in the bulk of PEI. In addition, this effect was due to the faster reaction kinetics between CO2 and the amino groups at higher temperatures. In the case of H2S sorption, that the lower temperature favored an increase in the H2S equilibrium sorption capacity indicated that the sorption of H2S over the fSi-PEI800-40 sorbent was predominately determined by thermodynamics rather than by the kinetics of H2S diffusion. Chen et al. [7] explained this observed trend as being due to the weak thermodynamic stability of the bonds between the amine groups and H2S. The weak acid–base interaction between H2S and the amine groups is not high enough to allow the amine groups on the surface to bind and retain H2S molecules at higher temperatures, but it is high enough to adsorb H2S molecules on the surface at room temperature. On the other hand, the lower kinetic barrier for the diffusion of the adsorbed H2S in the PEI800 bulk, as estimated by a computational chemistry approach [13], would facilitate the transfer of the adsorbed H2S on the surface into the PEI800 bulk inside the pores [13]. The computational study of Ma et al. [16] revealed that the estimated kinetic barrier for diffusion of the sorbed CO2 from the surface into the bulk of PEI was threefold higher than that for the diffusion of the sorbed H2S, indicating that the diffusion of the sorbed CO2 from the exposed surface into the bulk of PEI is much more difficult than that for the sorbed H2S. Thus, a higher sorption temperature is required for CO2. However, if the temperature is higher than the critical point, it will promote desorption. Moreover, it is seen that the CO2 sorption capacity is comparatively less sensitive to the temperature than H2S, at least within this evaluated temperature range. 3.2.3. Simultaneous sorption of CO2 and H2S In this work, the mixed gas (20% (v/v) CO2, 0.36% (v/v) H2S and 74.8% (v/v) CH4 balanced with N2) was used to be the model of natural gas with which the polyamine modified fumed silica sorbent would be implemented. To investigate the competitive sorption of CO2 and H2S, the adsorption onto the fSi-PEI800-40 sorbent of the mixed CO2 and H2S gas was performed at 30 °C and 80 °C in a single-stage process. The breakthrough curves of CO2 and H2S sorption in a single column (Fig. 6(A)) confirmed the temperature dependence of the sorption capacity for each gas. However, considering the breakthrough curves at 30 °C, the breakthrough time of CO2 was much higher than that of H2S. This is different from the data indicated in Fig. 5, showing that at 30 °C the sorption capacity of H2S was slightly higher than that of CO2, and so implied that CO2 strongly inhibits the H2S sorption under this condition. This phenomenon has also been observed with the PEI/MCM-41 sorbent [16]. The higher heat of adsorption of CO2 favors its sorption on the fSi-PEI800-40 sorbent and then the sorbed CO2 hinders further H2S sorption. The breakthrough curves of CO2 and H2S sorption in the doublestage system (Fig. 6(B)) clearly revealed that the effect of CO2 on the sorption of H2S is decreased in this double-stage system. The Cap(BT) of H2S was 0.045 mmol/g, which was lower than the expected value, presumably because some CO2 escaped from the first column into the second column where it was adsorbed and inhibited H2S sorption. Another phenomenon that might happen is the partial replacement of sorbed H2S molecules by CO2 molecules since CO2 has a higher adsorptive affinity than H2S. Thus, fSi-PEI800-40 sorbent was found to have the good sorption capacity for both CO2 and H2S sorption which show the best capacity of each gas on this sorbent at different temperature (80 °C for CO2 and 30 °C for H2S). Additionally, the presence of CO2 strongly inhibited the H2S sorption in the simultaneous CO2 and H2S sorption. The results show that the sequential sorption of CO2 followed by H2S in a double stage system was found to solve
Fig. 6. Breakthrough curves of the simultaneous CO2 and H2S sorption on the fSiPEI800-40 sorbent in a (A) single-stage or a (B) double-stage system.
this problem and successfully remove both CO2 and H2S from a mixed gas and so this polyamine modified fumed silica as low cost sorbent has a high potential for application in the acid gas treatment process.
485
3.3. Stability of the fSi-PEI800-40 adsorbent
489
The regeneration performance of the fSi-PEI800-40 adsorbent was evaluated, as this is one of the more important properties of an adsorbent for any practical application. In order to examine the stability of the fSi-PEI800-40 adsorbent in the adsorption and regeneration processes, 5 sequential adsorption–desorption cycles were conducted, with the adsorption performed at 80 °C (1st column) and 30 °C (2nd column) and a model gas (20% CO2, 0.36% H2S and 74.8% CH4) flow rate of 5 mL/min, and the regeneration performed at 90 °C and a N2 (99.999%) flow rate of 10 mL/min. Both were performed at atmospheric pressure. The breakthrough CO2 and H2S sorption capacities did not significantly change on 1st, 2nd and 3rd cycles, remaining within the narrow ranges of 3.36–3.494 and 0.053–0.063 mmol/g-sorb for CO2, and H2S, respectively (Fig. 6). However, the breakthrough CO2 and H2S sorption capacities slightly decreased on 4th and 5th cycles (Cap(BT) = 2.822–2.957 and Cap(BT) = 0.051 mmol/g-sorb for CO2 and H2S, respectively) due to accumulation of very small amount of residual CO2 and H2S in adsorbent and this possibly required longer regeneration time in 4th and 5th cycles for the case of high CO2 and H2S concentration feed. However, these results showed that the adsorption performance of fSi-PEI800-40 was stable. Therefore, this adsorbent could be regenerated under mild conditions for at least 5 cycles without loss of CO2 and H2S adsorption activity and so may be suitable for CO2 and H2S removal (see Fig. 7).
490
Please cite this article in press as: Yoosuk B et al. CO2 and H2S binary sorption on polyamine modified fumed silica. Fuel (2015), http://dx.doi.org/10.1016/j. fuel.2015.11.080
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Fig. 7. H2S and CO2 sorption capacity of the fSi-PEI800-40 over five successive adsorption–desorption cycles.
514
4. Conclusions
515
534
A series of amine modified fSi sorbents were prepared by impregnation. The resultant sorbents could remove CO2 and H2S under mild conditions. Among the five different evaluated amines, PEI800 exhibited a medium and high sorption capacity for CO2 and H2S, respectively, but also had a low amine leaching level and so was selected as the optimal amine. The amount of available amine groups in the sorbent and the interaction between amine groups and CO2 or H2S are the two major factors that affect the sorption capacity and amine efficiency. The fSi-PEI800-40 sorbent showed the highest CO2 sorption capacity (2.57 mmol/g-sorb). At higher amine loading levels, the thickness of the amine film covering the surface increased and led to an increased difficulty in the diffusion of CO2 into the internal amine groups. Higher temperatures facilitated the diffusion of CO2 from the surface into the bulk of the PEI800 by overcoming the kinetic barrier, while a lower temperature favored an increased H2S sorption capacity. The presence of CO2 strongly inhibited the H2S sorption in the simultaneous CO2 and H2S sorption. This effect can be reduced by using a doublestage sorption system with CO2 sorption first followed by H2S removal.
535
Acknowledgments
536
541
The authors gratefully acknowledge the funding support from the Center for Petroleum Petrochemicals and Advanced Materials, Chulalongkorn University and National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (P-12-01184 project). We would like to thank Dr. Robert Butcher for proof reading the article and suggestions.
542
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Please cite this article in press as: Yoosuk B et al. CO2 and H2S binary sorption on polyamine modified fumed silica. Fuel (2015), http://dx.doi.org/10.1016/j. fuel.2015.11.080