Phase-Change Reversible Absorption of Hydrogen Sulfide by the

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Phase-change Reversible Absorption of Hydrogen Sulfide by the Superbase 1,5-Diazabicyclo[4.3.0]non-5-ene in Organic Solvents Zhiyong Xu, Wenbo Zhao, Xuhao Xie, Yanhong Li, and Yuan Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05052 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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Industrial & Engineering Chemistry Research

Phase-change Reversible Absorption of Hydrogen Sulfide by the Superbase 1,5-Diazabicyclo[4.3.0]non-5-ene in Organic Solvents Zhiyong Xu, Wenbo Zhao*, Xuhao Xie, Yanhong Li, Yuan Chen Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming,650500, China E-mail: [email protected] Abstract: The phase-change absorption have shown a promising application prospect for acid gas capture because only the gas-rich phase needs to be transported to the stripper for recovery, which could drastically reduce the energy consumption of the regeneration process. In this study, the liquid-liquid phase-change behavior of a new recyclable ternary system, composed of 1,5-Diazabicyclo[4.3.0]non-5-ene (DBN)/hexadecane/hexanol, was evaluated for capturing H2S. The absorption adduct was the [DBNH]+[SH]- salt and the cause of the phase change was attributed to the polarity difference between the upper and lower phases. Furthermore, considering that only the lower phase needed to be heated for regeneration, the gravimetric absorption capacity was calculated to be 0.205 g H2S/g lower phase at 1 bar and 293.15 K. To the best of our knowledge, this is the largest gravimetric absorption capacity for H2S capture obtained to date. Additionally, when the absorption reached equilibrium, DBN, hexanol and H2S were concentrated in the lower phase, while hexadecane mostly remained in the upper phase. The DBN/hexadecane/hexanol system showed that the benign desulfurization efficiency was nearly 100% under the condition that the molar ratio of H2S gas to DBN is not more than 0.6 mol/mol. The cyclic absorption experiments showed that the H2S gas could be easily released by bubbling N2 at 80 °C, which indicated the low energy requirement for the regeneration of the absorbent. Keywords: liquid-liquid phase change, DBN, hexadecane, cyclic absorption 1. Introduction

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Natural gas, which consists mainly of methane (CH4), is one of the cleanest, greenest, economic sources of energy. However, almost over 30% natural gas fields in the world contain hydrogen sulfide (H2S), a highly toxic and corrosive gas.1,

2

On the other hand, H2S is an

important sulfur resource, which can be used to produce sulfuric acid and other sulfur-containing compounds.3,

4

Therefore, removing H2S from natural gas prior to its utilization has important

significance for improving the economic and environmental benefits of natural gas. In the past few years, a number of methods have been proposed for chemical absorption of H2S such as aqueous solutions of alkanolamines. Monoethanolamine (MEA), diethanolamine (DEA), 2-amino-2methyl-l-propanol (AMP) and especially N-methyldiethanolamine (MDEA) are the most widely used alkanolamines for H2S removal in the natural gas industry due to their reversible chemical reactivity to H2S and low cost.5-9 Although the use of alkanolamines in desulfurization has a long history and major strides have been made in H2S removal, there are still inherent drawbacks, including the high energy consumption for H2S desorption, loss of alkanolamine absorbent, transfer of water into the gas stream and especially the limited absorption capacity. As H2S is a very weak acid gas (PKa1=7)10 with weak nucleophilicity, its chemical reactivity is different from that of other acid gases. Accordingly, other organic amines such as amidines and guanidines with a strong basicity and highly proton affinities have been used for the capture

of

H2S.

For

example,

1,8-diazabicyclo[5.4.0]undec-7-ene

(DBU)

and

1,1,3,3-tetramethylguanidine (TMG), as organic superbases with pKa values 12.1 and 13.6, respectively11, 12, can activate H2S via hydrogen transfer processes, they have been intensively investigated. Very recently, Khokarale et al.13 used proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FTIR) spectroscopy to analyze the reaction mechanism of DBU


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with H2S, and confirmed that the H2S molecule react with DBU to form a solid [DBU-H2S] adduct through a protonation process. However, the formation of the solid adduct reduces the absorption efficiency and makes its industrial transportation difficult. Subsequently, a number of researchers proposed the “green” solvent ionic liquids (ILs) for H2S absorption due to their low vapor pressure, high thermal stability and tunable structure.14, 15 Considerable research efforts have been devoted to the use of ILs for H2S capture, early studies determined the solubility of H2S in a wide range of ILs. The H2S solubility in [bmim][PF6] ILs at temperatures from 298.15 to 403.15 K and pressures up to 9.6 MPa was first determined by Jou and Mather.16 Jalili and co-workers reported the solubility of H2S in a variety of imidazolium-based ILs over a range of temperatures and pressures.17-21 Normally, the partial pressure of H2S in natural gas is very low (about 0.05 bar), these normal ILs hardly absorb H2S at low partial pressure due to the weak interaction. Later, Huang et al. studied protic ILs, dual Lewis-base functionalized ILs and tertiary-amine functionalized ILs for H2S absorption.22-25 Although these studies have made important contributions to improve the absorption capacity, their level is still low. More recently, Huang et al.26 determined the H2S solubility in phenolic ILs using different cations, such as [P4444]+, [hmim]+, [DBUH]+ and [TMGH]+. The highest absorption capacity of the [TMGH][PhO] ILs among the four ILs is attributed to the strong basicity of TMG. Unfortunately, ILs often have high viscosity or form a high viscosity product after absorption of H2S, which would be a major barrier for its industrial application.27 Therefore, development of cost-effective methods with higher sorption capacities and superior regeneration ability for H2S capture is highly required. In the field of acid gas capture, the concept of phase-change absorption was put forward in



recent years and has attracted much attention. In general, phase-change absorption solutions consist of two parts, one is the absorbent, which is used to capture acid gas, the other is the solvent, which is used to adjust the viscosity or volatility of the system. A homogeneous solution absorbent will be split into two immiscible phases after the solution absorbed acid gas, one gas-rich and the other gas-lean. Only the gas-rich phase needs to be transported to the stripper for recovery and the gas-lean phase can be reused directly. As a result, the energy consumption of the regeneration process was drastically reduced.28,

29

Jessop et al.30 reported the ternary system of

decane, amidine/(guanidine) and alcohol for highly efficient reversible capture of CO2, which achieved liquid-liquid phase-change due to the polarity variation of the components upon CO2 absorption. Zhang et al.31 introduced a process for CO2 capture using the phase-change absorbent MEA/1-propanol/H2O. They found that the cyclic capacity and initial absorption rates were higher than those of 30 wt% MEA solution. Heldebrant and co-workers32 developed a reversible phase-change system with N,N-dibutylundecanolamine (DBUA) as absorbent and hexane as solvent. The absorption product is precipitated from hexane upon SO2 absorption and is recycled by thermally stripping under vacuum at a temperature of 70 °C. Meanwhile, our research group also investigated the liquid-solid phase-change behavior of diethylenetriamine in nonaqueous systems for CO2 absorption and imidazole in organic solvents for SO2 absorption.33, 34 Considering that H2S is similar to CO2 and SO2 as an acidic gas, the study of phase-change absorption behavior of H2S will be of value in reducing energy consumption. However, the H2S phase-change capture has rarely been reported compared to phase-change absorption of CO2 and SO2. Only Heldebrant35 reported that the solution of dimethylethanolamine (DMEA) and hexane could achieve liquid-liquid phase-change for H2S absorption. It is important to highlight that H2S


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can be rapidly removed from the alkanolamines absorbent at ambient temperature. However, the alkalinity of DMEA is not strong enough that its absorption capacity for H2S is very low. In a word, it can be predicted that by dissolving the organic amine in a suitable solvent, it is possible to cause a liquid-liquid or liquid-solid phase-change after the absorption of H2S. 1,5-Diazabicyclo[4.3.0]non-5-ene (DBN) has the highest gravimetric capacity for CO2 capture among several amidine bases according to the literature.36,

37

Herein, we first report an

efficient liquid-liquid phase-change capture system for H2S with the organic superbase DBN as absorbent and high boiling point, low viscosity organics as solvent to enhance the mass transfer of the absorption process. First, the structure of the absorption product is determined by 1H NMR and FTIR spectroscopy analysis and a possible phase-change mechanism is proposed based on the ultraviolet-visible (UV-vis) spectroscopy results. In addition, the effect of the concentration, temperature and partial pressure on the absorption capacity are also investigated. In addition, the distributions of the composition of the two phases is also systematically determined. The desulfurization performance of the DBN absorbent was measured. Furthermore, the cyclic absorption/desorption properties of H2S are also investigated. Overall, it was found that this novel absorption system is very effective for the capture of H2S, which can be released easily by bubbling N2 at 80 °C. In other words, the absorbent could be regenerated easily via flash vaporization process. 2. Experimental Section 2.1 Materials All chemicals used in the present work were purchased from commercially available sources and

used

as

received

without

further

purification.


The

analytical

reagents


1,5-Diazabicyclo[4.3.0]non-5-ene (DBN, 98.0%), Propylene carbonate (PC, 99.0%), Hexanol (99.0%), Hexadecane (98.0%), N-methylpyrrolidone (NMP, >99.0%), Nile Red (95.0%) and deuterated dimethyl sulfoxide (DMSO-d6, 99.9%+0.03% TMS) were purchased from the Aladdin Reagent Company (Shanghai, China). Analytically pure Butanol (99.0%) was obtained from the Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China), H2S (99.9%) was obtained from Chengdu Hongjin Chemical Co., Ltd. (Chengdu, China), N2 (99.99%) was purchased from Kunming Hongfa Chemical Co., Ltd. (Kunming, China). 2.2 Characterization FTIR spectra were recorded on a Bruker TENSOR 27 FTIR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) in the region 4,000-400 cm−1. After the H2S-rich phase was dissolved in DMSO-d6, 1H NMR spectra were recorded on a Bruker AVANCE III HD NMR spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) at a frequency of 600 MHz. Viscosity was measured on an Anton Paar Lovis falling ball viscometer (Anton Paar GmbH, Graz, Austria) at 20 °C and atmospheric pressure. In addition, The UV–vis spectra were recorded from 350 to 800 nm using a UV-vis spectrophotometer. The components of the two phases at different H2S loadings were determined by gas chromatography (GC) performed on a SHIMADZU GC-2010 Pro Gas Chromatograph system (Shimadzu Corp., Kyoto, Japan) equipped with a SE-30 capillary column (50 m × 0.25 mm × 0.5 μm) and a flame ionization detector (FID) detector. The internal standard was butanol, with the following operating conditions: Inlet temperature 330 °C, detector temperature 330 °C, the capillary column temperature program was at the initial temperature of 120 °C and held for 6 min, then ramped up at 30 °C/min to 150 °C and held for 2 min, and finally ramped up at 50 °C/min to 250 °C, and held for 4 min. The peak area of each test component was


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recorded in the gas chromatograph, and the content of each component of the sample to be tested with the peak area data was calculated. The components of H2S gas at the two phases was calculated by material balance. 2.3 Absorption-Desorption Experiments of H2S The H2S absorption experiment was conducted in a gas-washing bottle at ambient temperature (20 °C) and atmospheric pressure. In a typical process, 1 g of absorbent and 3 g of solvent (i.e. NMP, PC, ethanol, DEGDME, Hexadecane or Hexanol) were placed into the gas-washing bottle, and then pure H2S gas was bubbled into the gas-washing bottle at a flow rate of 10 sccm/min. The weight change upon H2S absorption was monitored using an electronic balance with an accuracy of ±0.1 mg. After the phase-change occurred, the two-phase liquid was separated by decantation or pipetting after centrifugation. The H2S absorption at different partial pressure was controlled by tuning the flow rates of H2S and N2. Desorption experiments were performed using the same equipment used for absorption with continued magnetic stirring. Pure N2 at atmospheric pressure was used as sweep gas and bubbled through the solution at 80 °C at a flow rate of 30 sccm/min. The desorption was assumed to be complete when the mass of the whole gas-washing bottle remained unchanged. All absorption and desorption experiments were performed in fume hood and the excess H2S in the tail gas was absorbed by sodium hydroxide solution for the experimental operation safety and environmental protection. 2.4 Desulfurization Efficiency Experiment A batch mode gas-liquid absorption apparatus was used to determine the H2S desulfurization efficiency. The specific method of operation was reference to Zhuang et al. research work.38 The desulfurization efficiency and the molar ratio of gas to liquid absorbent were calculated by using



the pressure differential and the ideal gas law. 3. Results and Discussion 3.1 Selection of the Absorbent and Solvent To determine the optimal solvent for H2S absorption, different combinations of absorbents and organic solvents were evaluated, as shown in Table 1. The data revealed that a solid product was obtained after H2S was introduced into the pure DBN solution. Khokarale et al. also reported similar phenomenon after bubbling of H2S into the DBU solution.13 It can be predicted that the reaction product of H2S with superbases should be solid. There is no phase-change phenomenon with hexanol, NMP, or PC as solvent when the mass fraction of DBN was 25%. A possible explanation for this phenomenon is that the polarity of the solid product, obtained through the reaction of DBN with H2S, is similar to that of the hexanol, NMP, or PC solvent, resulting in the dissolution of the product in these solvents. Heldebrant et al.35 reported that the H2S absorption product in DMEA was immiscible with hexadecane. It is worth noting that hexadecane has a high boiling point (287 °C), which could effectively reduce the volatilization of the absorption system. Accordingly, in this study we used hexadecane as solvent, and the liquid-solid phase-change phenomenon was observed after H2S gas was bubbled into the solution. Unfortunately, the formation of solid products hindered mass transfer, which is disadvantageous for industrial applications. In order to circumvent this problem, we tried adding another organic solvent to obtain a liquid-liquid phase change. Among the tried organic solvent, namely hexanol, PC and NMP, only hexanol was miscible with the mixed solution of DBN and hexadecane. It should be noted that the liquid-liquid phase-change phenomenon appeared when hexadecane and hexanol were used as mixed solvent to capture H2S. Phase splitting phenomenon was observed once the


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H2S absorption capacity was higher than 0.1 mol/mol. The upper phase was colorless, and the lower phase was light green in color. In addition, the volume of the lower phase increased, while the volume of the upper phase decreased with the increase of the total loading of H2S. The photograph of specific experimental phenomenon was depicted in Fig. 1. It can be considered that the absorption product was mainly in the lower phase. As it is well established, viscosity has an important effect on the absorption performance of a liquid absorbent. A highly viscous absorbent not only has a slow gas-liquid diffusion rate but also involves a high energy consumption in materials transport. Therefore, the viscosity of the lower phase was measured for the blend of 25 wt% DBN + 45 wt% hexanol + 30 wt% hexadecane absorption system. As shown in Fig. 2, the viscosity increased with the increase of total loading of H2S, and remains unchanged when the molar absorption capacity reached about 1mol/mol. The highest viscosity of the lower phase was 20.86 mPa·s and the viscosity of the solution before absorption was 4.49 mPa·s at 293.15 K. To the best of our knowledge, that value is lower than the reported viscosity of most ILs with or without H2S absorption. For instance, at 298.15 K, the viscosity

of

N,N,N’,N’-tetramethyl-1,6-hexanediamine

bis(trifluoromethylsulfonyl)imide

([TMHDA][Tf2N]) and triethylbutylammonium nicotinate ([N2224][NIA]) reached up to 1,602 mPa·s and 3,307 mPa·s, respectively.24, 25 Regarding the viscosity, compared with most ILs, the DBN/hexadecane/hexanol system are considered to be more applicable for the capture of H2S in the industry. 3.2 Characterization of the H2S Absorption Product The H2S absorption product was obtained over the course of a few minutes after the H2S was bubbled into the solution. To investigate the composition of the absorption product, 1H NMR



analysis was performed on the free DBN, DBN and hexanol mixture solution as well as the absorption product. The data shown in Fig. 3 clearly reveal that the chemical shift of DBN did not change after adding hexanol to the DBN, suggesting that DBN and hexanol will not react with each other. Notably, all these proton chemical shifts demonstrated a downfield shift for the absorption product. Indeed, it is clear that the protons chemical shifts significantly changed from 3.11 to 3.33, 1.63 to 1.86, 2.24 to 2.75 and 3.19 to 3.53 ppm for the H2, H3, H7 and H9 atoms, respectively. A possible explanation for this phenomenon is that the electron density around the H2, H3, H7 and H9 atoms were greatly reduced after the N5 protonation. In addition, A broad peak observed at 4.34 ppm was related to the protonated DBNH+.13 In the previous reports, complexes of DBN with CF3COOH in a 1:1 mole ratio show that the position of the protonation reaction was at N5 atoms.39 These results underscored an important point that the DBN molecules were able to equimolarly interact with H2S gas and formed the [DBNH]+[SH]- salt adduct. In order to gain a deeper insight into the formation of the absorption product, FTIR spectroscopy analysis was performed. The FTIR spectra of the free DBN, the mixture of DBN and hexanol as well as the absorption product are shown in Fig. 4. The sharp but weak peak at 2,570 cm-1 was assigned to S-H stretching vibration, and the broad peaks at 3000-3600 cm-1 were assigned to N-H stretching vibration together with the O-H stretching vibration.35, 40, 41 In addition, a new broad vibrational band appeared at 2,094 cm-1, which could be assigned to ammonium cations.42 Accordingly, consistent with the NMR analysis, the FTIR analysis provides definitive evidence to confirm the formation of the [DBNH]+[SH]- salt, which is composed of a protonated DBN molecule and a hydrogen sulfide anion [SH]-. According to the above analyses, the reaction mechanism between the H2S gas and DBN molecules is proposed as shown in Fig. 5. Based on


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this reaction mechanism, we can arrive the conclusion that 1 mol of DBN can absorb 1 mol of H2S by the chemical reaction. 3.3 Phase-change Mechanism The phase-change mechanism of the DBU superbase in decane and hexanol for the capture of CO2 was first reported by Jessop et al.30 The reason was attributed to the polarity change of the absorbent before and after absorption. Zhang et al.43 reported a absorbents composed of MEA/propanol/H2O formed liquid-liquid two phases after CO2 absorption.

The phenomenon

was attribute to the salting-out ability of the formed [MEAH]+[HCO3]- salts. In order to understand the phase-change mechanism for the capture of H2S, the UV-vis spectra were recorded using Nile Red as a solvatochromic probe to determine the polarity of the solvent. As shown in Fig. 6, the maximum absorption wavelength of Nile Red in the blend of 20 wt% DBN + 60 wt% hexadecane + 20 wt% hexanol appeared at 526 nm, but it appeared at 544 nm in the lower phase. The maximum absorption wavelength was shifted about 18 nm after the absorption of H2S. Based on the pertinent literature, the polarity of the 20 wt% DBN + 60 wt% hexadecane + 20 wt% hexanol solution was comparable to that of benzene (525.4 nm) before the absorption, but it was comparable to that of 1-octanol (544.0 nm) after the absorption.44 These findings indicated the reason for the polar lower phase separated from the nonpolar solvent hexadecane. 3.4 Effect of the Concentration on the Absorption Capacity In order to determine the effect of the concentration of the components on the absorption capacity, the absorption performance of the solutions with different mass fraction of DBN, hexadecane and hexanol was evaluated. In a specific operation process, the total mass of these systems is constant. The concentration of one components did not change, while the



concentrations of the other two components were adjusted as required. We first investigated the effect of the mass fraction of hexadecane and hexanol on the absorption performance. The mass fraction of DBN was fixed at 25 wt% and the mass fraction of hexadecane and hexanol were charged as follows (30, 40, and 50 wt%) and (45, 35, and 25 wt%), respectively. As shown in Fig. 7, the molar and gravimetric absorption capacity did not change. For example, the molar absorption capacity was 1.438, 1.450 and 1.445 (mol H2S/mol DBN) and the gravimetric absorption capacity for each was 0.099 (g H2S/g solution), when the mass fraction of hexanol was 45, 35 and 25 wt%, respectively. In comparison, the absorption rate was slightly lower with the increase of the mass fraction of hexanol. The reason may be that the viscosity of hexadecane (3.474 mPa·s, 20 °C)

45

is smaller than that of hexanol (5.412 mPa·s, 20 °C)46, which could

facilitate the mass transfer between gas and liquid so that the absorption rate is promoted. Therefore, the amount of hexadecane should be increased as much as possible to improve the absorption rate. However, once the mass fraction of hexadecane was increased to 60 wt% (i.e. hexanol was decreased to 15 wt%), the liquid-liquid-solid three phases were observed upon absorption saturation. The possible reason was that the amount of hexanol was too small to dissolve all of the products. Thus, the concentration of hexanol should be maintained within a suitable range. In order to study the effect of the mass fraction of DBN and hexadecane on the absorption performance, the amount of hexanol was kept at 25 wt% and the mass fraction of DBN and hexadecane were changed as follows (15, 20, 25, and 30 wt%) and (60, 55, 50, and 45 wt%), respectively. As shown in Fig. 8, the molar absorption capacity was basically unchanged. For example, it was 1.474, 1.514, 1.445 and 1.469 mol H2S/mol DBN at wtDBN = 15, 20, 25 and 30%,


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respectively. Noteworthy, the gravimetric absorption capacity of H2S was increased with the increase of the mass fraction of DBN. For example, the gravimetric absorption capacity of H2S was 0.060, 0.084, 0.099 and 0.121 g H2S/g solution at wtDBN = 15, 20, 25 and 30%, respectively. These results indicated that hexadecane barely absorbed H2S and the absorption ability of the system only resulted from DBN. Moreover, if the mass fraction of DBN was further increased to 35 wt% (i.e. hexadecane decreased to 15 wt%), the liquid-liquid-solid three phases were also observed. This is similar to the aforementioned phenomenon. To investigate the effect of the mass fraction of DBN and hexanol on the absorption capacity for H2S, the mass fraction of hexadecane was kept at 60 wt% and the mass fraction of DBN and hexanol were charged as follows (10, 15, and 20 wt%) and (30, 25, and 20 wt%), respectively. The results presented in Fig. 9 reveal that with the increase of the mass fraction of hexanol, the molar absorption capacity was basically unchanged. For instance, it was 1.479, 1.474, and 1.498 mol of H2S/mol of DBN at WtDBN = 10, 15, and 20%, respectively. The gravimetric absorption capacity of H2S was 0.040, 0.060 and 0.083 g H2S/g solution at wt

DBN

= 10, 15, and 20%,

respectively. These results indicated that hexanol also barely absorbed H2S. It also indicated that the excess capacity higher than 1 is due to the physical absorption of the DBN. 3.5 Effect of Temperature and H2S Partial Pressure on the Absorption Capacity It is well known that temperature can affect the solubility of gases in liquid solutions. In order to investigate the influence of temperature on the absorption performance of the system, the absorption performance was evaluated in the temperature range from 293.15 to 333.15 K. The results shown in Fig. 10 reveal that when the temperature rose from 293.15 to 333.15 K, the H2S molar absorption capacity decreased from 1.498 to 1.266 mol/mol. Thus, low temperature is



favorable for this system to capture H2S. Most natural gas fields commonly contain H2S gas at a concentration ranging from 1-5%, and some have even higher concentration. For example, the H2S content in natural gas from the Sichuan Puguang Gas Field ranges from 13-18%. Thus, we further investigated the effect of the partial pressure on the H2S absorption capacity using the 20 wt% DBN + 60 wt% hexadecane + 20 wt% hexanol system as absorbent. Partial pressure only affects the absorption capacity, so the phase splitting phenomenon also occurred at the concentration of 3-20% H2S. As shown in Fig. 11, the H2S absorption capacity decreased with the decrease of the H2S partial pressure. For example, it was 1.08 mol/mol at 0.2 bar, 1.05 mol/mol at 0.1 bar, 0.99 mol/mol at 0.05 bar, and 0.96 mol/mol at 0.03 bar. This result indicated that the absorption capacity kept 1mol/mol even at very low partial pressure. Apparently, we can arrive at the conclusion that chemically absorbed H2S tends to be stable, and the physically absorbed H2S could be released at low partial pressure. 3.6 Comparison of the H2S Absorption Capacities of Different Absorbents The H2S absorption capacity of the DBN/hexadecane/hexanol system was compared with that of other absorbents reported in the literature, as shown in Table 2. The comparison clearly reveals that the molar absorption capacity of H2S in the DBN/hexadecane/hexanol system is higher than that of other alkanolamines solutions or ILs absorbents under the similar conditions. To the best of our knowledge, the largest molar absorption capacity of H2S in liquid absorbents is [TDMAPAH][Ac] ILs, which could reach 1.92 mol/mol at 313.2 K and 1 bar47. In our opinion, the molar absorption capacity of DBN absorbent is second only to [TDMAPAH][Ac] ILs under similar conditions. At the higher temperature of 333.15 K and the pressure of 1 bar, 1 mol DBN absorbent could absorb 1.266 mol H2S, which is more than 1.5 times the absorption capacity of


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traditional 30% MEA solution and [TMGH][PhO] ILs. Such a high molar absorption capacity in this system may originate from the strong basicity of the DBN, as well as from the high proton affinity of DBN to H2S. Therefore, the DBN/hexadecane/hexanol systems is considered to be a promising candidate for natural gas desulfurization. 3.7 Phase Splitting and Component Distribution In order to better understand the effect of the total loading of H2S on the distribution of DBN, hexadecane, hexanol and H2S in two phases, the samples with different H2S loadings, were measured by GC after the two liquid phases were clearly separated. The data presented in Fig. 12 clearly shows that the weight ratio of hexadecane gradually increased from about 67 to up to 97 wt% in the upper phase with the increase of the H2S total loading, while that of hexanol and DBN gradually decreased from about 18 down to 1.4 wt% and from 15 down to 0 wt%, respectively. In comparison, in the lower phase the weight ratio of hexanol and DBN gradually increased from about 34 to 40 wt% and from 32 to 39 wt%, respectively, with the introduction of H2S, but the weight ratio of hexadecane gradually decreased from about 17 down to 2 wt%. As for the distribution of H2S, there was only a trace of H2S in the upper phase and it barely changed with the increase of the H2S total loading. Meanwhile, the weight ratio of H2S gradually increased from about 15 to 20 wt% in the lower phase. Thus, overall, hexadecane was concentrated in the upper phase but hexanol, H2S and DBN remained mostly in the lower phase when the absorption reached saturation. In another word, the formed [DBNH]+[SH]- salt forced hexadecane out from the hexanol solution because of salting-out effect. 3.8 Desulfurization Efficiency of the DBN system The effects of the molar ratio of H2S gas to the DBN on the H2S desulfurization efficiency



was determined by a batch mode gas-liquid absorption apparatus at 298.15 K. As shown in Fig. 13, the DBN/hexadecane/hexanol system has excellent desulfurization efficiency and the highest desulfurization efficiency was higher than 99%. When the molar ratio of H2S gas to the DBN was less than 0.6 mol/mol, the desulfurization efficiency could reach 99%. If the molar ratio exceeded 0.6 mol/mol, the desulfurization efficiency started to decrease. But, it still remained 95% when the molar ratio of H2S gas to the DBN was about 1mol/mol. If the molar ratio exceeded 1.2 mol/mol, the H2S desulfurization efficiency was lower than 90%. 3.9 Recyclability and stability of the DBN system The recyclability of the solution is intensely important for phase-change absorption. Thus, the absorption recycling properties for the blend of 20 wt% DBN + 60 wt% hexadecane + 20 wt% hexanol solvent were investigated in this study. The five consecutive cycles of H2S absorption and desorption processes are depicted in Fig. 14. It was obviously that the solution exhibited benign reusability at 80 °C, as there was no significant decrease in the maximum absorption capacity during the five consecutive cycles of the absorption process and the H2S can be easily stripped off within about 40 min by bubbling N2. At 70 °C, the solution was substantially non-volatile, but the H2S desorption was incomplete. We improved the desorption temperature to 100 °C. However, the loss of solution became obvious, which resulted in the cyclic absorption capacity decrease. Thus, the optimal desorption temperature was 80 °C. To evaluate the stability of DBN absorbent, the 1H NMR spectra of the lower phase after the first and fifth desorption were compared with that of pure DBN and hexanol solution. As can be seen in Fig. 15, no distinct change occurred in the chemical shifts of absorbent, indicates that the absorbent can be regenerated by heating. In industrial applications of natural gas desulfurization, the gravimetric absorption capacity of H2S gas is more of a concern. Considering that the solvent of hexadecane remained mostly in


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the upper phase and the absorption production could be regenerated via thermal desorption, the cyclic gravimetric absorption capacity was calculated to be 0.205 g H2S/g lower phase at 1 bar and 20 °C by ignoring the existence of hexadecane. Compared with the other alkanolamines solution and ILs absorbents, the DBN/hexadecane/hexanol system exhibits much higher gravimetric absorption capacity. For example, it is about 2.33 times larger than that of the traditional 35 wt/% MDEA solution at 313.15 K and 1 bar (see Table 2). Notably, the molar absorption capacity of the [TDMAPAH][Ac] ILs could reach 1.92 mol H2S/mol ILs, which is the highest molar absorption capacity of H2S so far. However, its gravimetric absorption capacity was only 0.098 g H2S/g ILs at 1 bar and 313.2 K, which was about half of that of DBN/hexadecane/hexanol absorption system.

Conclusions In the present study, an absorption system composed of DBN/hexadecane/hexanol was developed for H2S capture. To the best of our knowledge, the liquid-liquid phase change capture of H2S was observed for the first time. The structure of the absorption product was the [DBNH]+[SH]- salt and the phase-change mechanism was attributed to the polarity difference of the upper and lower phases. The distribution analysis revealed that DBN, hexanol and H2S were concentrated in the lower phase, while hexadecane stayed in the upper phase. The recycling performance of this DBN/hexadecane/hexanol system was relatively stable after traditional heat treatment at 80 °C. Compared with other H2S liquid absorbents, the phase-change solution exhibited low viscosity and high H2S desulfurization efficiency as well as high gravimetric absorption capacity by ignoring the upper phase. The results illustrated that this solution have potential application in natural gas sweetening.



Acknowledgements The authors acknowledge the financial support from National Natural Science Foundation of China (Grant No. 21666011, 21306071), and Analysis and Measure Foundation of Kunming University of Science and Technology (2017M20162207028, 2018T20090102).


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References 1.

Skrtic, L.; Group, R., Hydrogen Sulfide, Oil and Gas, and People's Health. Energy 2006.

2.

Habeeb, O. A.; Kanthasamy, R.; Ali, G. A. M.; Sethupathi, S.; Yunus, R. B. M., Hydrogen sulfide

emission sources, regulations, and removal techniques: a review. Reviews in Chemical Engineering 2017. 3.

Flannigan, K. L.; Wallace, J. L., Hydrogen Sulfide: Its Production, Release and Functions. Springer

Vienna: 2013; p 109-125. 4.

Huang, K.; Feng, X.; Zhang, X. M.; Wu, Y. T.; Hu, X., Ionic Liquids-Mediated Claus Reaction: Highly

Efficient Capture and Conversion of Hydrogen Sulfide. Green Chemistry 2016, 18, (7), 1859-1863. 5.

Li, M. H.; Shen, K. P., Solubility of hydrogen sulfide in aqueous mixtures of monoethanolamine

with N-methyldiethanolamine. J.chem.eng.data 1993, 38, (1), 105-108. 6.

Lawson, J. D.; Garst, A. W., Gas sweetening data: equilibrium solubility of hydrogen sulfide and

carbon dioxide in aqueous monoethanolamine and aqueous diethanolamine solutions. Journal of Chemical & Engineering Data 1976, 21, (1), 20-30. 7.

Fang, Y. J.; Carroll, J. J.; Mather, A. E.; Otto, F. D., Solubility of mixtures of hydrogen sulfide and

carbon dioxide in aqueous N-methyldiethanolamine solutions. Ind.eng.chem.process Des.dev 1982, 21, (4), 539-544. 8.

Jou, F. Y.; Carroll, J. J.; Mather, A. E.; Otto, F. D., The solubility of carbon dioxide and hydrogen

sulfide in a 35 wt% aqueous solution of methyldiethanolamine. Canadian Journal of Chemical Engineering 1993, 71, (2), 264-268. 9.

Roberts, B. E.; Mather, A. E., SOLUBILITY OF CO2 AND H2S IN A HINDERED AMINE SOLUTION.

Chemical Engineering Communications 1988, 64, (1), 105-111. 10. Serjeant; E., P., Ionisation constants of organic acids in aqueous solution. Pergamon Press: 1979; p 811-829. 11. Salmi, H.; Allonas, X.; Ley, C.; Defoin, A.; Ak, A., Quaternary ammonium salts of phenylglyoxylic acid as photobase generators for thiol-promoted epoxide photopolymerization. Polymer Chemistry 2014, 5, (22), 6577-6583. 12. Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A., Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: unification of different basicity scales. Journal of Organic Chemistry 2005, 70, (3), 1019-28. 13. Khokarale, S. G.; Mikkola, J. P., Hydrogen sulfide gas capture by organic superbase 1,8-diazabicyclo-[5.4.0]-undec-7-ene through salt formation: salt synthesis, characterization and application for CO2 capture. Rsc Advances 2018, 8. 14. Blanchard, L. A.; Dan, H.; Beckman, E. J.; Brennecke, J. F., Green processing using ionic liquids and CO2. Nature 1999, 399, (6731), 28-29. 15. Zhang, Z.; Xie, Y.; Li, W.; Hu, S.; Song, J.; Jiang, T.; Han, B., Hydrogenation of carbon dioxide is promoted by a task-specific ionic liquid. Angew Chem Int Ed Engl 2010, 120, (6), 1143-1145. 16. Jou, F. Y.; Mather, A. E., Solubility of Hydrogen Sulfide in [bmim][PF 6 ]. International Journal of Thermophysics 2007, 28, (2), 490. 17. Jalili, A. H.; Rahmatirostami, M.; Ghotbi, C.; Hosseinijenab, M.; Ahmadi, A. N., Solubility of H2S in Ionic Liquids [bmim][PF6], [bmim][BF4], and [bmim][Tf2N]. Journal of Chemical Thermodynamics 2009, 41, (9), 1052-1055.



18. Rahmati-Rostami, M.; Ghotbi, C.; Hosseini-Jenab, M.; Ahmadi, A. N.; Jalili, A. H., Solubility of HS in ionic liquids [hmim][PF], [hmim][BF], and [hmim][TfN]. Journal of Chemical Thermodynamics 2009, 41, (9), 1052-1055. 19. Sakhaeinia, H.; Taghikhani, V.; Jalili, A. H.; Mehdizadeh, A.; Safekordi, A. A., Solubility of H 2 S in 1-(2-hydroxyethyl)-3-methylimidazolium ionic liquids with different anions. Journal of Chemical Thermodynamics 2010, 298, (2), 303-309. 20. Jalili, A. H.; Shokouhi, M.; Maurer, G.; Hosseini-Jenab, M., Solubility of CO 2 and H 2 S in the ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate. Journal of Chemical Thermodynamics 2013, 67, (6), 55-62. 21. Sakhaeinia, H.; Jalili, A. H.; Taghikhani, V.; Safekordi, A. A., Solubility of H2S in Ionic Liquids 1-Ethyl-3-methylimidazolium Hexafluorophosphate ([emim][PF6]) and 1-Ethyl-3-methylimidazolium Bis(trifluoromethyl)sulfonylimide ([emim][Tf2N]). Journal of Chemical & Engineering Data 2010, 55, 5839-5845. 22. Huang, K.; Zhang, J. Y.; Hu, X. B.; Wu, Y. T., Absorption of H2S and CO2 in Aqueous Solutions of Tertiary-Amine Functionalized Protic Ionic Liquids. Energy & Fuels 2017, 31, (12). 23. Huang, K.; Zhang, X. M.; Xu, Y.; Wu, Y. T.; Hu, X. B.; Xu, Y., Protic ionic liquids for the selective absorption of H2S from CO2: Thermodynamic analysis. Aiche Journal 2014, 60, (12), 4232-4240. 24. Huang, K.; Cai, D. N.; Chen, Y. L.; Wu, Y. T.; Hu, X. B.; Zhang, Z. B., Dual Lewis Base Functionalization of Ionic Liquids for Highly Efficient and Selective Capture of H 2 S. Chempluschem 2014, 79, (2), 241-249. 25. Huang, K.; Zhang, X. M.; Hu, X. B.; Wu, Y. T., Hydrophobic protic ionic liquids tethered with tertiary amine group for highly efficient and selective absorption of H2S from CO2. Aiche Journal 2016, 62, (12), 4480-4490. 26. Huang, K.; Zhang, X. M.; Zhou, L. S.; Duan-Jian-Tao; Fan, J. P., Highly Efficient and Selective Absorption of H 2 S in Phenolic Ionic Liquids: A Cooperative Result of Anionic Strong Basicity and Cationic Hydrogen-Bond Donation. Chemical Engineering Science 2017, 173, 253–263. 27. Chiappe, C.; Pomelli, C. S., Hydrogen Sulfide and Ionic Liquids: Absorption, Separation, and Oxidation. Topics in Current Chemistry 2017, 375, (3), 52. 28. Liang, Z.; Rongwong, W.; Liu, H.; Fu, K.; Gao, H.; Cao, F.; Zhang, R.; Sema, T.; Henni, A.; Sumon, K., Recent progress and new developments in post-combustion carbon-capture technology with amine based solvents. International Journal of Greenhouse Gas Control 2015, 40, 26-54. 29. Zhuang, Q.; Clements, B.; Dai, J.; Carrigan, L., Ten years of research on phase separation absorbents for carbon capture: Achievements and next steps. International Journal of Greenhouse Gas Control 2016, 52, 449-460. 30. †, D. J. H.; †, P. G. J., ,; ‡, C. A. T.; And, C. A. E., ‡; Liotta‡, C. L., The Reaction of 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) with Carbon Dioxide. Journal of Organic Chemistry 2005, 70, (13), 5335-5338. 31. Zhang, W.; Jin, X.; Tu, W.; Ma, Q.; Mao, M.; Cui, C., A Novel CO2 Phase Change Absorbent: MEA/1-Propanol/H2O. Energy & Fuels 2017, 31, (4). 32. Heldebrant, D. J.; Koech, P. K.; Yonker, C. R., A reversible zwitterionic SO2-binding organic liquid. Energy & Environmental Science 2010, 3, (1), 111-113. 33. Zhang, Z.; Zhao, W.; Nong, J.; Feng, D.; Li, Y.; Chen, Y.; Chen, J., Liquid–Solid Phase‐Change Behavior of Diethylenetriamine in Nonaqueous Systems for Carbon Dioxide Absorption. Energy Technology 2017, 5, (3), 461-468.


Page 20 of 29

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


34. Wang, Y.; Zhao, W.; Chai, M.; Li, G.; Jia, Q.; Chen, Y., Phase-Change Absorption of SO2 by Imidazole in Organic Solvents and Conversion of the Absorption Product in the Presence of Water and Oxygen. Energy & Fuels 2017. 35. Koech, P. K.; Rainbolt, J. E.; Bearden, M. D.; Zheng, F.; Heldebrant, D. J., Chemically selective gas sweetening without thermal-swing regeneration. Energy & Environmental Science 2011, 4, (4), 1385-1390. 36. Bungo Ochiai; Kensuke Yokota; Asuka Fujii; Daisuke Nagai, ‡ And; Takeshi Endo, Reversible Trap−Release of CO2 by Polymers Bearing DBU and DBN Moieties. Macromolecules 2008, 41, (4), 1229-1236. 37. Pereira, F. S.; Agostini, D. L. D. S.; Santo, R. D. D. E.; Deazevedo, E. R.; Bonagamba, T. J.; Job, A. E.; González, E. R. P., A comparative solid state 13C NMR and thermal study of CO2 capture by amidines PMDBD and DBN. Green Chemistry 2011, 13, (8), 2146-2153. 38. Zhuang, Q.; Clements, B., CO 2 capture by biphasic absorbent—absorption performance and VLE characteristics. Energy 2018, 147. 39. Wiench, J. W.; Stefaniak, L.; Grech, E.; Bednarek, E., Two amidine derivatives studied by 1H, 13C, 14N, 15N NMR and GIAO-CHF calculations. Journal of the Chemical Society Perkin Transactions 1999, 4, (4), 885-890. 40. Koga, K.; Takami, A.; Koda, S., IR spectra of H 2S isolated in free-standing crystals of Kr and Xe. Chemical Physics Letters 1998, 293, (3), 180-184. 41. Miller, D. D.; Chuang, S. S. C., The Effect of Electron-Donating Groups and Hydrogen Bonding on H2S Capture over Polyethylene Glycol/Amine Sites. Journal of Physical Chemistry C 2016, 120, (2). 42. Silverstein; Robert, M.; Silverstein, R. M.; Webster, F. X.; Webster; Francis, X., Spectrometric identification of organic compounds. John Wiley: 1991; p 3316. 43. Yang, F.; Jin, X.; Fang, J.; Tu, W.; Yang, Y.; Cui, C.; Zhang, W., Development of CO2 phase change absorbents by means of cosolvent effect. Green Chemistry 2018, 20, (10). 44. Deye, J. F.; Berger, T. A.; Anderson, A. G., Nile Red as a solvatochromic dye for measuring solvent strength in normal liquids and mixtures of normal liquids with supercritical and near critical fluids [Erratum to document cited in CA112(18):171383n]. Analytical Chemistry 1990, 62, (6), 615-622. 45. Schmidt, R.; Griesbaum, K.; Behr, A.; Biedenkapp, D.; Voges, H. W.; Garbe, D.; Paetz, C.; Collin, G.; Mayer, D.; Höke, H., Hydrocarbons. Wiley‐VCH Verlag GmbH & Co. KGaA: 2014; p 57–58. 46. Trenzado, J. L.; Matos, J. S.; González, E.; Esther Romano, A.; Caro, M. N., Study on Properties Derived from Densities and Viscosities for the Ternary Systems (Methyl Pentanoate or Methyl Heptanoate) + Octane + 1-Hexanol and Their Binary Subsystems at Various Temperatures. Journal of Chemical & Engineering Data 2003, 48, (4). 47. Zheng, W.; Wu, D.; Feng, X.; Hu, J.; Zhang, F.; Wu, Y. T.; Hu, X. B., Low viscous Protic ionic liquids functionalized with multiple Lewis Base for highly efficient capture of H 2 S. Journal of Molecular Liquids 2018. 48. Rebolledo-Libreros, M. E.; Trejo, A., Gas solubility of H 2 S in aqueous solutions of N -methyldiethanolamine and diethanolamine with 2-amino-2-methyl-1-propanol at 313, 343, and 393K in the range 2.5–1036kPa. Fluid Phase Equilibria 2004, 224, (1), 83-88. 49. Jou, F. Y.; And, F. D. O.; Mather, A. E., Solubility of H2S, CO2, and Their Mixtures in an Aqueous Solution of 2-Piperidineethanol and Sulfolane. Journal of Chemical & Engineering Data 1998, 43, (3), 409-412.



50. Mazloumi, S. H.; Haghtalab, A.; Jalili, A. H.; Shokouhi, M., Solubility of H2S in Aqueous Diisopropanolamine + Piperazine Solutions: New Experimental Data and Modeling with the Electrolyte Cubic Square-Well Equation of State. Journal of Chemical & Engineering Data 2012, 57, 2625-2631. 51. Huang, K.; Cai, D. N.; Chen, Y. L.; Wu, Y. T.; Hu, X. B.; Zhang, Z. B., Thermodynamic validation of 1 ‐alkyl‐3‐methylimidazolium carboxylates as task‐specific ionic liquids for H2S absorption. Aiche Journal 2013, 59, (6), 2227–2235. 52. Wang, B.; Zhang, K.; Ren, S.; Hou, Y.; Wu, W., Efficient capture of low partial pressure H2S by tetraethyl ammonium amino acid ionic liquids with absorption-promoted solvents. Rsc Advances 2016, 6, (103).


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Table 1. Liquid-liquid phase-change absorption of H2S by absorbent in organic solvent (25 wt%) absorbent

DBN

solvent —

Solid

Hexanol

No change

NMP

No change

PC

No change

Hexadecane

Liquid-solid

Hexadecane + NMP

Immiscible

Hexadecane + PC

Immiscible

Hexadecane+ Hexanol

Liquid-liquid

Table 2. Comparison of H2S absorption capacities of different organic amine absorbent Absorbents

Concentration a

T/K

PH2S b

Mol Loading c

Mass Loading d

ref.

DBN DBN MEA DEA MDEA MDEA AMP MIPA e PEDA f PZ [emim][Tf2N] [emim][Ac] [DMEAH][Ac] [DMEAH][For] [BDMAEE][AcO] [TMGH][PhO] [TMGH][PhO] [N2222][Gly] [TDMAPAH][Ac]

20 20 30 25 11.8 35 18.9 30 45 21.5 100 100 100 100 50 100 100 50 50

293.15 333.15 333.15 311.15 298.15 313.15 313.15 313.15 313.15 313.15 303.15 293.15 303.15 303.15 298.15 333.15 298.15 313.15 313.15

0.03 - 1 1 1.21 1.01 0.08 - 1.82 1.03 0.13-1.76 1.01 1.11 1.02 1 0.1 - 1 1 1 1 1 0.1 - 1 0.05 1

0.96 - 1.50 1.27 0.74 0.86 0.66 - 1.18 0.87 0.83-1.02 0.16 1.06 1.23 0.07 0.22 - 0.59 0.20 0.11 1.00 0.78 0.69 - 0.97 0.94 1.92

0.127 - 0.205 0.169 0.124 0.070 0.022 - 0.040 0.088 0.060 - 0.127 0.026 0.126 0.105 0.006 0.045 - 0.117 0.050 0.031 0.078 0.127 0.112 - 0.158 0.080 0.098

this work this work Li et al.5 Lawson et al.6 Jou et al.7 Mather et al.8 Roberts9 Trejo et al.48 Jou et al.49 Jalili et al.50 Sakhaeinia et al.19 Huang et al.51 Huang et al.23 Huang et al.23 Huang et al.22 Huang et al.26 Huang et al.26 Wu et al.52 Wu et al.47

a mass

fraction.

b bar. c mol

H2S/mol amine. H2S/g lower phase. e 1-amino-2-propanol. f 2-Piperidineethanol. dg



A1

A2

A3

A4

A5

A6

A1

A1

A2

A1

A1

A1

Serial number

A1

A2

A3

A4

A5

A6

Total H2S loading (mol/mol)

0

0.15

0.34

0.46

0.87

1.27

Fig. 1 Effect of total H2S loading on phase separation. 22 20 18

Viscosity/mPa•s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

16 14 12 10 8 6 4 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Total H2S loading(mol/mol)

Fig. 2 Viscosity of the lower phase with different absorption capacity of H2S.

Fig. 3 1H NMR spectra of (a) Free DBN, (b) DBN and Hexanol and (c) the product [DBNH]+[SH]- salt using external DMSO-d6 (2.50 ppm) as a reference.


Page 25 of 29

a

transmittance

b

c 2570

3500

3000

2094

2500

2000

1500

1000

-1

Wavenumber(cm )

Fig. 4 FTIR spectra of (a) Free DBN, (b) DBN and Hexanol and (c) the product [DBNH]+[SH]salt. N

N

+ H2S N

N H SH

Fig. 5 Reaction mechanism of DBN with H2S. 1.0 0.9

Befor absorption After absorption

0.8 0.7

544

Abs

0.6

526

0.5 0.4 0.3 0.2 0.1 0.0 400

500

600

700

800

Wavelength(nm)

Fig. 6 UV-vis spectra of Nile Red in DBN before and after absorption of H2S. 1.6

0.10

1.4 1.2

Total H2S loading(g/g)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.8 25% DBN+45% Hexanol 25% DBN+35% Hexanol 25% DBN+25% Hexanol

0.6 0.4 0.2 0.0

0

10

20

30

40

50

60

70

80

0.08 0.06 25% DBN+45% Hexanol 25% DBN+35% Hexanol 25% DBN+25% Hexanol

0.04 0.02 0.00

0

10

20

Time (min)

30

40

50

60

70

80

Time (min)

(a) (b) Fig. 7 The effect of hexadecane/hexanol ratio on the absorption capacity. (a) molar absorption capacity, (b) gravimetric absorption capacity.



1.6

0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00

1.2



1.4

1.0 0.8 15% DBN+25% Hexanol 20% DBN+25% Hexanol 25% DBN+25% Hexanol 30% DBN+25% Hexanol

0.6 0.4 0.2 0

10

20

30

40

50

60

70

80

90

15% DBN+25% Hexanol 20% DBN+25% Hexanol 25% DBN+25% Hexanol 30% DBN+25% Hexanol 0

10

20

30

Time (min)

40

50

60

70

80

90

Time (min)

(a) (b) Fig. 8 The effect of DBN/hexadecane ratio on the absorption capacity. (a) molar absorption capacity, (b) gravimetric absorption capacity. 1.6

0.09

1.4

0.08

1.2

0.07



1.0 0.8 0.6

10% DBN+60% Hexadecane 15% DBN+60% Hexadecane 20% DBN+60% Hexadecane

0.4

0.06 0.05 0.04 0.03

10% DBN+60% Hexadecane 15% DBN+60% Hexadecane 20% DBN+60% Hexadecane

0.02

0.2

0.01

-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Time (min)

Time (min)

(a) (b) Fig. 9 The effect of DBN/hexanol ratio on the absorption capacity. (a) molar absorption capacity, (b) gravimetric absorption capacity. 1.6


1.4 1.2 1.0 293.15K 303.15K 318.15K 333.15K

0.8 0.6 0.4 0.2 0

10

20

30

40

50

60

70

80

Time (min)

Fig. 10 H2S absorption loading at different temperatures. 1.2 1.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

0.8 0.6 3% 5% 10 % 20%

0.4 0.2 0.0 0

10

20

30

40

50

60

Time/min


70

Page 27 of 29

Fig. 11 H2S absorption loading under different partial pressures. 1.0

0.40

0.8

0.35

0.6 0.4

Hexanol DBN Hexadecane H2S

0.30

Hexanol DBN Hexadecane H2S

Weight Ratio(%)

Weight Ratio(%)

0.2 0.0

0.25 0.20 0.15 0.10 0.05

-0.2 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.00 0.0

1.6

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Total H2S Loading(mol H2S/mol DBN)

Total H2S Loading(mol H2S/mol DBN)

(a) (b) Fig. 12 The individual weight ratio of all the compontents in the two phase, (a) Upper phase, (b) Lower phase.

Desulfurization efficiency/%

100 80 60 40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Molar ratio of gas to liquid(mol/mol)

Fig. 13 The desulfurization efficiency of DBN/hexadecane/hexanol system. 1.6

70C 80C 100C

1.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2

0

100

200

300

400

500

Time/Min

Fig. 14 Five times absorpation-desorpation cycle experiments at different temperature (absorption condition: 1 atm with a H2S flow rate of 30 sccm; desorption conditions: 70 °C in oil bath, 80 °C in oil bath and 90 °C in oil bath with a N2 flow rate of 30 sccm).



c

b

a 3.5

3.0

2.5

2.0

1.5

1.0

0.5

f1(ppm)

Fig. 15 1H NMR spectra of (a) pure DBN and Hexanol, (b) the first desorption of lower phase liquid and (c) the fifth desorption of lower phase liquid using external DMSO-d6 (2.50 ppm) as a reference.


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Graphical Abstract

Highlights: An approach to develop H2S phase-change absorbent was proposed for the first time. Viscosities of the DBN/hexadecane/hexanol solution are lower than 21 mPa·s with or without H2S absorption. The absorption system show highest gravimetric absorption capacity for H2S capture among alkanolamines solution and ILs. New recyclable absorption system for natural gas desulfurization in industrial was open up.


Phase-Change Reversible Absorption of Hydrogen Sulfide by the

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