H2S–CO2 Separation Using Room Temperature Ionic ...

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H2S–CO2 Separation Using Room Temperature Ionic Liquid [BMIM][Br] a

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a

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Haris Handy , Andriyanto Santoso , Andreas Widodo , Jelliarko Palgunadi , Tatang Hernas a

Soerawidjaja & Antonius Indarto

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Department of Chemical Engineering, Institut Teknologi Bandung, Labtek X, Kampus ITB, Bandung, Indonesia Accepted author version posted online: 20 May 2014.Published online: 25 Aug 2014.

To cite this article: Haris Handy, Andriyanto Santoso, Andreas Widodo, Jelliarko Palgunadi, Tatang Hernas Soerawidjaja & Antonius Indarto (2014) H2S–CO2 Separation Using Room Temperature Ionic Liquid [BMIM][Br], Separation Science and Technology, 49:13, 2079-2084, DOI: 10.1080/01496395.2014.908919 To link to this article: http://dx.doi.org/10.1080/01496395.2014.908919

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Separation Science and Technology, 49: 2079–2084, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 0149-6395 print / 1520-5754 online DOI: 10.1080/01496395.2014.908919

H2 S–CO2 Separation Using Room Temperature Ionic Liquid [BMIM][Br] Haris Handy, Andriyanto Santoso, Andreas Widodo, Jelliarko Palgunadi, Tatang Hernas Soerawidjaja, and Antonius Indarto

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Department of Chemical Engineering, Institut Teknologi Bandung, Labtek X, Kampus ITB, Bandung, Indonesia

Solubility and selective absorption of hydrogen sulfide (H2 S) over carbon dioxide (CO2 ) in a room temperature ionic liquid, 1-butyl-3-methylimidazolium bromide ([BMIM][Br]) has been evaluated under ambient temperature and pressure. [BMIM][Br] demonstrated its potential as a solvent for selective removal of H2 S from CO2 /H2 S mixture. Our investigation indicated that H2 S solubility in [BMIM][Br] is comparable to or better than that in commercially available MDEA-based solvents. Meanwhile, CO2 solubility in [BMIM][Br] is lower than that in the same amine resulting in H2 S/CO2 absorption selectivity of within 3.5 to 3.75. The solubility behavior is relatively maintained after 4 times absorption-desorption cycles. A computational molecular study suggested that intramolecular hydrogen bonding interaction between anion Br and hydrogen atom of H2 S could stabilize the complex and resulted lower complexation energy than CO2 interaction with [BMIM][Br]. Based on the experiment results, a separation process employing [BMIM][Br] is proposed to control the CO2 /H2 S ratio existing in a natural gas feed. Keywords carbon dioxide (CO2 ); hydrogen separation; solubility; selectivity

sulfide

(H2 S);

INTRODUCTION Fossil fuels are still expected to become the major energy sources worldwide within the next 50 years or more (1-2). Particularly, demands on natural gas for fuels and for hydrocarbon-based material manufactures have notably increased (2). In the oil and gas productions, along with hydrocarbons, numerous impurities including CO2 and H2 S are co-produced in various concentrations depending on the underground soil characteristics and locations. In the past, oil or gas wells containing significant amount of acid impurities were considered unprofitable. However, with depleting number of

Received 25 August 2013; accepted 24 March 2014. Address correspondence to Antonius Indarto, Department of Chemical Engineering, Institut Teknologi Bandung, Labtek X, Kampus ITB, Jalan Ganesha 10, Bandung 40132, Indonesia. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsst.

potential reservoirs and with increasing oil and gas demands nowadays, those kinds of reservoirs are also seen as valuable sources of energy. Wet scrubbing process employing liquid absorbents, such as Benfield solution and aqueous amine-based system have been widely applied in natural gas surface production facilities to separate acid components from hydrocarbons prior to transportation or further processing (3). When H2 S is present in significant level, it must be removed and recovered rather than being released into the atmosphere. The commonest method used for the recovery has been the Clauss process (4). For the process to work efficiently, the acid gases entering the process must have a high H2 S/CO2 , i.e., H2 S concentration is not less than 55%. Hence, when the acid gas contains a high concentration of CO2 , a highly selective acid gas enrichment unit for increasing the H2 S concentration of the acid gas is required. With regard to acid gas enrichment process the selectivity of industrially amine based solvent is not high enough when H2 S concentration in the acid gas is too low, i.e., less than approximately 3%. Additions of other amines, such as monoethanol amine (MEA) and diethanol amine (DEA) to enhance the selectivity have been also reported but the results are not satisfied due to solvent degradation (5). Even with industrially hindered amine technology, especially design for acid gas enrichment, the selectivity is not yet satisfying. Therefore, it is very challenging to find alternative solvents for the acid gas enrichment process. Currently, room temperature ionic liquids (RTILs), salts melting below 100◦ C have been seen as potentially attractive and novel solvents for chemical separations including for gases separation due to their unique physicochemical properties resulted from the variations or the modifications of the cations and the anions, such as temperature stability, negligible vapor pressure, tunable hydrophylicity/hydrophobicity, and miscibility with hydrocarbons (6). An ionic liquid, even a simple one, such as dialkylimidazolium halide is a complex structure carrying several sites whose sites are capable to specifically respond and interact with a guest molecule. Differences in the solubility capacity of those gases in the RTILs could arise from

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the different nature of interactions between the gas and RTIL molecules (7). RTILs have been the subject of investigations for the separation of CO2 or SO2 generated from fuels combustion processes (8–11). A recent study also clearly shows that weakly acidic acetylene can effectively be separated from etylene using RTILs having basic anions (12, 13). Little attention has been paid to the potential application of RTIL for H2 S capture. Interaction between H2 S, an acidic substance contained in a natural gas stream, and RTIL is expected to be specific and relatively strong. This specific interaction increases the gas solubility giving an opportunity for easy separation of H2 S from less soluble molecules, such as hydrocarbons and CO2 . In general, gas solubility in RTIL is influenced by two factors: (1) acidbase interaction between solvent and solute and (2) free-volume hosting-solute mechanism (14). Room temperature [BMIM][Br] is particularly of interest of study due to the ease of preparation, the stability, and the basicity nature of the anion (15). To the authors’ knowledge, no exploration/research report of [BMIM][Br] ability to separate acid gases, i.e., H2 S and CO2 , was found. For benchmarking, the solubility/selectivity of H2 S and CO2 in commercial liquid absorbents, such as MDEA and activated MDEA were also evaluated. In addition, ab initio analysis of H2 S-[BMIM][Br] was also carried out to give a better understanding on the absorption/complexation mechanism and interaction energy. EXPERIMENTAL RTILs Synthesis RTIL precursors, 1-methylimidazole and 1-bromo butane, were purchased from Sigma Aldrich Co. and were used as received. 1-Butyl-3-methylimidazolium bromide [BMIM][Br] was prepared according to the procedure in literature (12, 13) with modifications. In a typical experiment, 1-methylimidazole is reacted with a slightly excess moles of 1-bromo butane at 60–80◦ C under a reflux condition. Crude ionic liquid was distilled to remove the starting materials and then washed with etyl acetate several times to obtain pure [BMIM][Br]. Purities of the RTILs were found to be higher than 97% as deduced from the proton NMR analysis (500 MHz Agilent NMR spectrometer). Solubility Test CO2 (purity of 99.8%) and H2 S (1.062% − N2 balanced) were purchased from Linde Indonesia and from MESA International/GasChem, USA, respectively. In a typical experiment, 10 g of [BMIM][Br] was charged into a 15 mL Pyrex® glass placed on a weight balance (Precisa XT 220A, Gravimetrics AG, Switzerland, precision 10−4 ). CO2 and/or H2 S stream was continuously bubbled into the liquid absorbent under an ambient condition (26◦ C and 700 mmHg). An online weight measurement technique was used to capture the change of solvent weight due to gas solution. The solvent was considered saturated with the gaseous solute indicated by the absence of a weight fluctuation.

The flow of CO2 and/or H2 S was regulated at a maximum flow of 100 ± 5 mL/min using a Mass Flow Controller (MFC) Bronkhorst M122132871 (Bronkhorst High-tech BV., The Netherlands) equipped with a Bronkhorst Readout unit (E5700 series). One outlet of the Pyrex® glass was connected to a balloon to maintain the atmospheric pressure during the absorption process. The absorption process was conducted at 700 mmHg. The range of weight fluctuations has been adjusted to ensure the absorption process is within the correct operation window. The temperature of the room (26◦ C) during the experiment was regulated using a domestic air conditioner. The selective absorption of a gas is calculated as follows: Selectivity of H2 S/CO2 =

mol H2 S removed or absorbed mol CO2 removed or absorbed

(1)

The solubilities and selectivities of H2 S and CO2 in nmethyldiethanolamine (MDEA-1) and in commercial mixture of MDEA + activator (MDEA-2) under the same condition have also been evaluated. A solubility experiment was carried out twice for each solvent using a fresh sample. Recycle test was conducted after each solvent was regenerated by heating at > 80◦ C. Computational Simulation Molecular calculation was performed adopting the gas phase model approach to understand the absorption (complexation) energy. A representative method, yet not time consuming, has to be selected to describe the molecular interactions. Density functional theory (DFT) with dispersion-correction module has been used to calculate some models of weak dispersion forces (physical interaction). This method has been successfully implemented for calculations of binding energies of 156 non-covalent biological complexes taken from S22 and JSCH-2005 database combinations of Hobza and co-workers (16) as well as other works (17, 18). All calculations were done using the Gaussian09.d01 program (19), employing the density functional theory (DFT) functionals, such as Becke 3-parameter Lee Yang Parr (B3LYP) and dispersion-corrected functional wB97XD and M062X. For small interaction models, one of the most precise ab initio methods called CCSD, with aug-cc-pVDZ basis, was used. All gas + solvent molecule model geometries were calculated with a complete optimization without any constrains at basis set of 6-311G(2d,p). To compensate the presence of basis set superposition error (BSSE), a counterpoise correction procedure of Boys and Bernardi (20) was also included in the calculation of the potential energy. The binding energy between the solvent and absorbed molecule was calculated according to the following expression: E = Ecomplex − (Esolvent + Egas )

(2)

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H2 S–CO2 SEPARATION USING ROOM TEMPERATURE IONIC LIQUID

RESULTS AND DISCUSSION Solubility-Selectivity of CO2 and H2 S The CO2 absorption trend in various solvents is shown in Fig. 1. The solubility of CO2 in various solvents was found to follow the following order: MDEA-2 > MDEA-1 > [BMIM][Br] > Water ≈ Ethanol. The solubility of CO2 in water reached 0.548 mol CO2 /kg-mol H2 O which is in agreement with 0.515 mol CO2 /kg-mol H2 O from a previous experiment by Duan and Sun (21). The interaction between CO2 and water is dominated by weak O. . .H interaction that plays an important role in the solvation process (22). In the absence of water, specific interaction between nitrogen atom in the amino group of tertiary amine MDEA and carbon atom in CO2 is suggested to be the important key factor of CO2 dissolution (23). Meanwhile, in the presence of water, bicarbonate species is dominantly produced (23). CO2 solubility in [BMIM][Br] was much higher than that in water but still

considerably lower compared to MDEA-1 or MDEA-2. The absorption of CO2 by MDEA is comparable to the data from Zhang and Chen under a similar experimental condition (24). Interaction between CO2 and [BMIM][Br], is influenced by both anion and cation as one O atom of CO2 is closed to H of butyl group and the second O atom of CO2 is near to anion atom, Br (see Fig. 2). This proposed that the interaction is a result from the balance between free-volume cavity mechanism and acid-base interaction (14). Recycle tests up to four cycles indicated that the CO2 absorption capacities for [BMIM][Br], MDEA-1, and MDEA-2 were relatively similar. This means that these solvents, especially [BMIM][Br], could be regenerated similar to MDEA-1 and MDEA-2 without losing the performance. In the case of H2 S gas, the solubility of H2 S in [BMIM][Br] is higher compared to the solubility H2 S in MDEA or in other commercial amine solvents. The H2 S and CO2 solubilities in three different solvents, tabulated in Table 1, clearly indicate that [BMIM][Br] absorbs H2 S more selective than MDEA or commercial amine giving higher H2 S/CO2 selectivity ratio. Different from CO2 absorption, interaction between [BMIM][Br] and H2 S is dominated by interaction between the anion atom, Br, and S atom of H2 S.

0.20 CO2 Absorption (mol/mol)

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where Esolvent and Egas are the monomer energies (kcal/mol) of the solvent and CO2 , respectively, in relaxed mode. Ecomplex is the surface energy (kcal/mol) of the complex obtained after the BSSE correction.

[MMIM]-Br Ethanol 96% Water MDEA-1 MDEA-2

0.15

0.10

0.05

0.00 0

50

100

150

200 250 Time (minute)

300

350

400

FIG. 1. CO2 solubility with contact time in various solvents at ambient temperature and pressure (25 ◦ C and 1.0 atm).

Note: data point is an average value from four experiment cycles.

FIG. 2. Optimized geometry of (a) CO2 and (b) H2 S with [BMIM][Br].

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TABLE 1 CO2 and H2 S solubilities and selectivities at ambient temperature and pressure Solvent

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MDEA-1 MDEA-2 [BMIM][Br]

H2 S (mol/mol)

CO2 (mol/mol)

Selectivity H2 S/CO2

0.0183 0.0206 0.0313

0.0481 0.1392 0.0090

0.38 0.15 3.48

Computational Simulation Thermodynamic factors and geometries of molecular interaction between CO2 or H2 S and [BMIM][Br] were analyzed using the ab initio molecular calculation method. Figure 2 shows the optimized geometries of CO2 or H2 S when interacting with [BMIM][Br] at B3LYP/6-311G(2d,p) level of theory. Table 2 supports the above experimental result showing the absorption energy of CO2 into [BMIM][Br] is lower than CO2 into water. Solubility comparison simulation of CO2 in MDEA solvent was not conducted here as the mechanism was slightly different as the mechanism consists a set of reaction including shuttle mechanism. Gupta et al. have calculated that the total absorption energy is c.a. −10 kcal/mol, depending on the temperature and concentration (23). This suggests that absorption of CO2 using MDEA will be the highest among water and [BMIM][Br]. As the absorption energy of CO2 into [BMIM][Br] is smaller than MDEA, it supposes that the absorption and desorption process will be much easier than MDEA. It requires only 80◦ C to recover the pure [BMIM][Br] with almost similar performance after 4 cycle runs. Unfortunately, no comparable data of H2 S solubility in MDEA was found. The interaction between MDEA and H2 S is classified as chemical interaction and recovering the solvent could be more difficult. By using a solvent such as [BMIM][Br] that forms physical interaction with H2 S (with E = −8 to −5 kcal/mol, see Table 2), the solvent will be easier to be recycled or purified. A comparable result between simulated Gibbs’ energy difference of CO2 and H2 S absorption and

experimental was found to be close to each other (last row of Table 2). Figure 2 clearly shows that H2 S is positioned closer to the cation of the RTIL than CO2 . The H-bond of H2 S-cation (2.38 Å) is much shorter than that of CO2 -cation (3.18 Å). This implies that H2 S-[BMIM][Br] interaction is stronger than CO2 -[BMIM][Br] interaction. In addition, the theoretical calculation of Gibbs (G) differences between CO2 and H2 S interaction supports the experimental value shown in last row of Table 2. Taking an example of data at 313 K for CO2 or H2 S − [BMIM][Br], the experimental Gibbs difference (GCO2 GH2S ) between CO2 and H2 S is 0.76 kcal/mol (comparable to 1.1−1.7 kcal/mol by theoretical calculation). In other words, H2 S-[BMIM][Br] complex is thermodynamically more stable than CO2 -[BMIM][Br] complex. A complete assessment of the experimental and theoretical comparison study is undergoing. Proposed Configuration Process Experimental results demonstrated that [BMIM][Br] is a potential alternative for a selective H2 S separation from CO2 . Furthermore, thermodynamic analysis indicates that H2 S interacts specifically yet physically with ionic liquid suggesting easy regeneration of the solvent for a cycle absorption process. Based on these findings, the common Acid Gas Enrichment (AGE) configuration as shown in Fig. 3 could be applied without any modification. The configuration allows for H2 S enrichment for a feasible Clauss process, In general, the oxidation reaction in the Clauss process requires sufficiently high level of H2 S concentration in the feed (∼ 40–100%) (4). However, the advantage of using [BMIM][Br] as the solvent will reduce the consumption energy in the regeneration unit. The configuration for acidic gases removal consists of two absorption-desorption units. The first scrubber-stripper columns (the larger one) employs aqueous amine for the absorption of all acid impurities mainly CO2 and H2 S from a raw feed gas to obtain a sweet gas. The second or smaller unit is utilized to separate and enrich H2 S heading for the Clauss process. After entering the second scrubber, the acid gas stream

TABLE 2 Binding energy calculation of some DFT functional calculations Complexation energy (kcal/mol) Solvent

B3LYP

M062X

wB97XD

CCSD

Literature

ECO2 −H2 O ECO2 −MDEA ECO2 −[BMIM][Br] EH2 S−[BMIM][Br] GCO2 −H2 S

−1.52

−2.43

−2.44

−2.16

−2.67 −5.38 1.1

−4.33 −8.4 1.5

−4.47 −7.73 1.71

−2.31 −102 0.763

Note: 1 reference (21); 2 estimation temperature dependence of reference [22]; 3 estimation from [BMIM][PF6] due to the similarity of the H2 S/CO2 selectivity.

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H2 S–CO2 SEPARATION USING ROOM TEMPERATURE IONIC LIQUID Sweet Gas CO2 to injection or atmosphere

H2S Absorber Column

Main Absorber Column

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Raw Gas

H2S to injection or Claus Process

Ionic Liquid Regeneration Column

Main Regeneration Column

Acid Gas Enrichment unit

FIG. 3. Absorption-desorption unit for the separation and enrichment of H2 S from CO2 employing [BMIM][Br].

from the first process is treated with [BMIM][Br] to capture H2 S. CO2 rich gas leaves the top of scrubber column while the H2 S-rich [BMIM][Br] is pumped out to the stripper column for H2 S collection and solvent regeneration. Desorption of H2 S and regeneration of ionic liquid take place in the H2 S stripper column under an atmospheric pressure. Common MDEA process for AGE is usually able to increase H2 S concentration ∼4 times higher than its concentration in the feed. As the absorption selectivity ratio of H2 S over CO2 using [BMIM][Br] is greater than MDEA in this experiment, it is expected that [BMIM][Br] performs better for this process. CONCLUSIONS In summary, we have demonstrated that TSIL [BMIM][Br] can be used for CO2 and H2 S gas absorption-separation process. Preliminary H2 S to CO2 absorption tests showed that the CO2 to H2 S selectivity uptake of [BMIM][Br] may reach 3.48 under room temperature and pressure. This value is higher than the absorption using commercial amine solvents. Moreover, lower absorption energy of CO2 absorption compared to commercial amines suggesting less energy requirement for the stripping process. FUNDING The authors acknowledge a generous funding from the 2013 Alumni Progam of Korea Institute of Science and Technology (KIST), Seoul, Republic of Korea. REFERENCES 1. Royal Dutch Shell (2008) Shell energy scenario to 2050. The Hague, Netherland. 2. International Energy Agency (2012) World Energy Outlook 2012. Paris, France. 3. UOP (2009) Overview of UOP gas processing technologies and applications. Illinois, US.

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