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Syngas Dehydration with Ionic Liquids Chengna Dai, Liang Wu, Gangqiang Yu, and Zhigang Lei Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03379 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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Industrial & Engineering Chemistry Research

Syngas Dehydration with Ionic Liquids

Chengna Dai, Liang Wu, Gangqiang Yu, and Zhigang Lei* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing 100029, China

ABSTRACT: This work deals with the feasibility of ionic liquids (ILs) as an alternative drying agent to the conventional solvent (methanol) in the application of syngas dehydration. The screening of ILs by COSMO-RS model shows that [EMIM][Tf2N] is the suitable absorbent for syngas dehydration among various possible ILs. The gas-liquid equilibrium (GLE) experiments for the CO (H2) - [EMIM][Tf2N] systems were carried out. The experimental data were compared with the predicted results by UNIFAC-Lei model. The syngas drying experiments with [EMIM][Tf2N] showed the excellent gas dehydration performance of ILs. On this basis, the conceptual process of syngas dehydration with [EMIM][Tf2N] was simulated and optimized with the rigorous equilibrium stage (EQ) model, where the UNIFAC-Lei model parameters were embedded. It was found that the gas dehydration process with IL exhibits unique advantages in energy consumption, equipment cost, and dehydration performance when compared to the conventional benchmark methanol process. This work confirmed that ILs could be considered as promising alternatives to conventional solvents for syngas drying.

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1. INTRODUCTION It is well-known that syngas is the main product of coal gasification and used as the intermediate for production of various fuels such as synthetic natural gas and synthetic petroleum fuel.1-3 It is mainly composed of carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2). During the production of syngas, gas dehydration and other treatment processes to separate CO2 and VOCs for subsequent production and store are required; otherwise, vapor water contained in syngas would corrode the equipment and block the pipeline by generating hydrates.4,5 At present, methanol is the most used absorbent in the conventional syngas purification process (e.g., Rectisol process).6-8 However, there are still many problems existing in the current process: (1) methanol regeneration requires high energy consumption; (2) the low selectivity of gas/H2O in methanol leads to the wastage of CO and H2, which is adverse to recycle the main components of syngas; (3) the volatility and toxicity of methanol will reduce the quality of syngas product; and (4) a high investment in heat exchangers in methanol dehydration process is needed.6,9 Due to the problems aforementioned, a simple and effective method of syngas dehydration is specially needed. Therefore, this work decides to explore the alternative method to solve the problems existing in the current Rectisol process. Ionic liquids (ILs) have attracted much more attention in recent years due to their excellent and unique physicochemical properties, such as liquid state around room temperature,

negligible

vapor

pressures,

specific

solvent

abilities,

and

high

thermostability.10-16 Most of ILs are highly hygroscopic and exhibit high solubility of H2O,17 and could be promising alternative drying agents to replace methanol used in gas dehydration

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process. Heym et al.18 used the IL [EMIM][EtSO4] as alternative to TEG (triethylene glycol) for nature gas dehydration. But they mainly focused on the capability of nature gas dehydration with ILs, and the thermal stability of [EMIM][EtSO4] is unsatisfactory. Recently, ILs were proposed for CO2 gas drying, exhibiting unique advantages when compared to conventional benchmark solvent triethylene glycol in saving amounts of absorbent and energy consumption.19 Moreover, some researchers have demonstrated that ILs could be using as absorbents to separate impurity and thus purify some gases.20-23 A suitable IL is crucial for ensuring the feasibility and economy of syngas dehydration process. In this work, the COSMO-RS (conductor-like screening model for real solvents) model developed by Klamt24 was first applied to screen ILs for the H2O/CO and H2O/H2 separation. Before the continuous syngas dehydration experiment and process simulation, the gas-liquid equilibrium (GLE) for IL/CO and IL/H2 systems with and without water has to be measured. Moreover, the popular UNIFAC-Lei model was used to predict the GLE. Compared with the COSMO-RS model, which often provides qualitative description of thermodynamic behavior, the UNIFAC-Lei model can provide much better predictions.25,26 Thus, the solubility data of H2 and CO in IL and IL + water were obtained and predicted by the UNIFAC-Lei model. The objective of this work is to retrofit and optimize the conventional syngas dehydration process with ILs as absorbent. The content of this paper is arranged as follows: (1) the COSMO-RS model was first used to screen the most potential IL with high selectivity and capacity to absorb water vapor in syngas; (2) experiments were conducted to measure the GLE data of CO (or H2) in the pure IL and IL + water systems; (3) the UNIFAC-Lei model

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was established on the basis of experimental GLE data, and the binary group parameters were obtained and embedded into the rigorous equilibrium stage (EQ) model RadFrac using the commercial simulation software Aspen Plus; and (4) the syngas dehydration process using IL as absorbent was simulated and optimized, and the results were compared with the conventional syngas dehydration process with methanol as absorbent. The cations and anions involved in this work are listed in Table S1 in Supporting Information. 2. EXPERIMENTAL SECTION 2.1. Materials. The actual syngas consisting of CO, H2, CO2, CH4, N2 and water vapor came from Sinopec Beijing Yanshan Company, and the mole fractions of components in the syngas is given in Table 1. The IL [EMIM][Tf2N] (>99% in mass fration) was purchased from Shanghai ChengJie Chemical Company. Before the experiment, the IL was purified to remove the traces of water and volatile impurities by vacuum rotary evaporator at 413.15 K for 1 h. After the treatment, the water content in [EMIM][Tf2N] was less than 180 ppm. 2.2. Apparatus and Procedure. The solubility data of CO (or H2) in pure [EMIM][Tf2N] and in the mixture of [EMIM][Tf2N] + water were measured in a high-temperature and high-pressure equilibrium view cell by the drainage gas-collecting method. The apparatus and procedure were presented in our previous works.27-29 In principle, both packing column and tray column can be used for implementing the unit operation regarding syngas dehydration with ILs, but the former has such advantages as high throughput, low pressure drop and small liquid holding in actual operation.30 Thus, packing column was adopted in the subsequent study. The continuous syngas dehydration apparatus, consisting of one gas dehydration column (stainless steel cylinder, height of 1 m

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and diameter of 0.03 m, filling with the random dixon stainless steel packings with diameter of 4 mm and height of 4 mm), one constant-flux pump (Beijing Satellite Instruments 2PB00C), one buffer tank (stainless steel cylinder), two gas moisture meters (RHD-601, Wuhan Ruihen Company), and one trace moisture analyzer (KLS701, Zibo Kulun Company) to measure the water content in IL, is shown in Figure 1. The syngas flowed into the buffer tank through the pressure and flow controllers, and then entered the bottom of gas dehydration column (the water content was measured beforehand by a gas moisture meter). Meanwhile, the IL [EMIM][Tf2N] regenerated by vacuum rotary evaporator was pumped into the top of gas dehydration column. The water-loaded syngas contacted with [EMIM][Tf2N] countercurrent along the gas dehydration column. The dried syngas flowed out from the top, and the water content was measured online by gas moisture meter. The water-loaded IL [EMIM][Tf2N] was collected in the IL reservoir. Before the water-loaded IL recycled to the syngas dehydration column, it should be regenerated by the vacuum rotary evaporator (type RE-20L). 3. THERMODYNAMIC MODEL 3.1. Screening of ILs by COSMO-RS Model. The COSMO-RS model provides an effective method to predict the thermodynamic properties such as activity coefficients and Henry’s constant for the molecules or ions. It is based on quantum calculations of individual species,31-34 and mainly used as a prior tool when reliable experimental data are not available. COSMOthermX (version C30_1301) is a software package used to predict the thermodynamic properties in liquids, which is an implementation of the COSMO-RS theory. In the COSMO-RS model, an IL is treated as equal molecular mixture of cation and anion. In

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this work, the Henry’s coefficients of gases and water as well as the selectivity of gas/water in ILs with different combinations of cations and anions were predicted using COSMOthermX software. Furthermore, the hydrolytic property and thermostability of cations and anions were analyzed. 3.2. UNIFAC-Lei Model for ILs. The UNIFAC-Lei model for ILs was developed and continually extended by us based on the original UNIFAC model proposed by Fredenslund et al.35 This model has been widely applied by many researchers for the systems containg ILs, and thus was adopted in this work as well. In the model, the skeleton of cation and anion in IL was treated as a whole functional group.36-38 Similar as the conventional UNIFAC model, the activity coefficient γi is calculated by lnγ i = lnγ iC + lnγ iR

(1)

where lnγ iC represents the combinatorial contribution to activity coefficient, and lnγ iR represents the residual contribution. The lnγ iC is caused by the difference of molecular size and shape of different groups, and finally decided by volume parameter Rk and surface parameter Qk, which could be obtained by the COSMO-RS model.39,40 The lnγ iR is the function of group binary interaction parameters αmn and αnm, which could be obtained by correlating the phase equilibrium experimental data. The solubility of CO (1) (or H2) in IL (2) at low pressures can be calculated by

x1 =

y1 P1φ1 (T , P, y1 ) γ 1 P1s

(2)

where P and T are system pressure and temperature, respectively; y1 and x1 are the mole fractions of CO or H2 in gas and liquid phases, respectively; and y1 is approximately equal to 1. φ1 (T , P, y1 ) is the fugacity coefficient of CO or H2 in gas phase as calculated by 6


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Peng-Robinson (PR) equation of state at a certain temperature and pressure. P1s is the saturated vapor pressure of CO or H2, as calculated by Antoine equation.41 The activity coefficient γ 1 of CO or H2 in liquid phase was calculated by the UNIFAC-Lei model, and thus x1 could be deduced. The following objective function (OF) minimized was used to obtain the absent group interaction parameters involved in the systems investigated:  1 OF = min   N

N

∑ 1

xi ,cal − xi ,exp   xi ,exp 

(3)

where xi,exp is the experimental solubility data of syngas component i in the liquid phase obtained from literature, and xi,cal is the solubility data of syngas component i in liquid phase calculated by the UNIFAC-Lei model as aforementioned. N is the number of data points. All the group interaction parameters αmn and αnm obtained in this work are listed in Table 2, while other parameters involved come from reference.42

4. RESULTS AND DISCUSSION 4.1. IL Screening for Syngas Dehydration. The ILs could be designed by combining various cations and anions. To choose the suitable IL with high hygroscopicity and high selectivities of CO to H2O (SCO/H2O) and H2 to H2O (SH2/H2O), the COSMO-RS model was used to screen ILs in terms of the selectivities of the key components (CO and H2) to H2O. The selectivity Si/H2O (i represents CO or H2) based on Henry’s constant (Hi) is defined as

S i / H2O =

Hi H H2O

(4)

In this work, 15 cations and 19 anions were chosen to screen the suitable IL. The predicted selectivity data of CO/H2O and H2/H2O are shown in Figure 2, as calculated by the

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COSMO-RS model at 298.15 K. It can be seen that the selectivities of CO/H2O and H2/H2O gas pairs exhibit the same trend, and decrease with the increase of the number of carbon atoms in alkyl side chain of cations. Moreover, given the anion, the selectivity is in the order of [MMIM]+ > [EPY]+ > [EMIM]+ > [C3MIM]+ > [BMIM]+ > [HEMIM]+ > [HMIM]+ > [P4,4,4,4]+. Since more reliable thermodynamic properties for imidazolium-based ILs are available, [EMIM]+ was chosen in this work as the most suitable potential cation for further study. On the other hand, the selectivity in ILs is mainly affected by anions, and the selectivity follows [Cl]- > [CH3SO4]- > [SCN]- > [BF4]- > [Tf2N]- > [PF6]- > [SbF6]-. When choosing the potential anion, the thermostability and viscosity of ILs should also be taken into account at the same time. The comparison of thermal decomposition temperature and viscosity among various ILs with the cation [EMIM]+ is given in Table 3.43,44 It is evident that the decomposition temperature increases in the order of [Ac]- < [Cl]− < [BF4]− < [PF6]− < [Tf2N]−, and [EMIM][Tf2N] can keep stable up to 455 ℃. Under the same conditions such as 25 ℃ and 1 atm, [Tf2N]− has also relatively lower viscosity. In the syngas dehydration process, lower viscosity is beneficial to IL transportation and energy saving. When screening the suitable IL, the solubility and selectivity, combined with the thermostability and negatively hydrolytic property, should be considered together. As a result, [EMIM][Tf2N] is selected as the absorbent for syngas dehydration. In this case, the selectivities of both CO to H2O and H2 to H2O in [EMIM][Tf2N] are as high as 10,000, which is enough to separate the trace water from syngas. Although the solubility and selectivity in [EMIM][Tf2N] can meet the demand of syngas dehydration, the effect of corrosion on engineering materials when using IL should be further

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studied due to the surface reaction of IL in contact with engineering materials. The corrosion experiment was conducted, and the results are given in Table S2 in Supporting Information. Two kinds of metal plates were divided into four groups, and dipped into pure [EMIM][Tf2N] and the mixture of 98 wt% [EMIM][Tf2N] + 2 wt% H2O, respectively, in 150 ℃ for 48 hr. In any case, there are no significant changes observed in weight and surface appearance of the metal plates, indicating that the engineering materials may show a good corrosion resistance to the IL [EMIM][Tf2N] even in the presence of a little water.

4.2. Solubility of CO (H2) in [EMIM][Tf2N] and in the Mixture of [EMIM][Tf2N] + Water. The solubility data of CO (H2) in [EMIM][Tf2N] and in the mixture of [EMIM][Tf2N] + water (98 wt% [EMIM][Tf2N] + 2 wt% H2O) were measured at 313.15, 333.15 and 353.15 K, and the pressure ranging from 1 to 7 MPa. The experiment data are shown in Figures 3 and 4, along with the predicted results by UNIFAC-Lei model. The detailed values, as well as the detailed uncertainties, are listed in Tables S3 and S4 in Supporting Information. It can be seen that CO solubility in both [EMIM][Tf2N] and the mixture of [EMIM][Tf2N] + water increases with the decrease of temperature, whereas H2 solubility shows a reverse trend, i.e., the so-called “inverse” temperature effect. The predicted values of CO and H2 solubility by UNIFAC-Lei model agree well with the experimental data over a wide temperature and pressure range. The average relative deviation (ARD) of CO solubility between experimental data and predicted values by UNIFAC-Lei model is 10.21%, while the ARD of H2 solubility is 16.15%.

4.3. Continuous Syngas Dehydration Experiments with [EMIM][Tf2N]. The syngas dehydration experiments containing a certain amount of H2O with [EMIM][Tf2N] as

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absorbent were conducted at temperature of 25 ℃ and atmospheric pressure. The water-loaded [EMIM][Tf2N] was regenerated by a vacuum rotary evaporator at 140 ℃ and 0.05 atm for 1 hour. After that, the water content in [EMIM][Tf2N] is around 180 ppm (mass fraction) as measured by trace moisture analyzer. The volume flow rate of syngas is kept at 500 mL·min-1 at normal temperature and pressure (NTP), and the volume flow rate (NTP) of [EMIM][Tf2N] ranges from 1 to 30 mL·min-1. In this work, two syngas streams with different water contents (2021 ppm and 6752 ppm, mole fraction) were tested. As shown in Figure 5, the water content of syngas decreases with the increase of IL volume flow rate. When the volume flow rate of IL increases to 15 mL·min-1 (NTP), the water content of syngas tends to be stable for these two syngas streams. After the dehydration with IL, the water content in these two syngas streams decreases to below 400 ppm, which satisfies the industrial requirement.

4.4. Process Simulation of Syngas Dehydration with [EMIM][Tf2N] and Methanol as Absorbents. The syngas dehydration process with [EMIM][Tf2N] and methanol as absorbents were simulated with the EQ stage model using RadFrac module of Aspen Plus software (version 7.2). In the simulation, the syngas components and methanol were used as conventional compounds from the Aspen Plus database, and the IL was defined as a new component by the “User Defined” function. The critical properties of IL were estimated by group contribution methods.45-47 The UNIFAC-Lei model was used to calculate the thermophysical and equilibrium properties of syngas and solvent. The thermophysical properties used in the energy consumption calculation are given in Table S5 in Supporting Information. To validate the reliability of EQ stage model, the comparison between

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experimental data and simulated results is also shown in Figure 5. It is seen that both agree very well, indicating the validation of the mathematical model established in this work. On this basis, the syngas dehydration processes with IL and methanol as absorbents were simulated and compared in the following cogently interesting aspects: dehydration performance, equipment investment, and energy consumption. It is noted that the conventional syngas purifying process (i.e., Rectisol process) includes the removal of CO2, sulfur compounds and water in series. However, this work only focuses on the removal of water from syngas; thus, the units to remove CO2 and SO2 in the process are not considered here. The simplified flow sheets of syngas dehydration processes with IL and methanol are shown in Figure 6, mainly consisting of an absorption column, a buffer tank, and a solvent recovery unit (flash drum for IL recovery and desorption column for methanol recovery). In the simulation, the initial water content in the feed syngas with the flowrate of 3000 kg·h-1 is at saturated state at the specified temperature and pressure, and the required water content in syngas product was less than 50 ppm. The operating conditions and process simulation results are given in Tables 4 and 5, and the detailed results for each stream are given in Table S6 in Supporting Information. It can be seen that the water content in syngas product is 40 ppm for IL process and 50 ppm for methanol process. In this case, the total heat duties on reboilers and condensers in IL process can decrease by 87.43% and 82.20%, respectively, when compared to methanol process. Although the operation pressure of IL flash drum is 0.05 atm, the extra electric energy consumption for vacuum pump is only 0.10 kW, which can be ignored. The detailed calculation can be found in our previous work.48 Besides, the initial cost of IL absorbent is higher than that of methanol (about 80-fold difference in price);

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however, the operating expenses due to its running losses and energy consumption are much lower. Moreover, the equipment cost of flash drum used in IL process is normally much lower than that of desorption column used in methanol process. Thus, the dehydration process with IL is more efficient in saving energy consumption and equipment investment than with methanol. In addition, the high syngas recovery ratio and the negligible vapor pressure of IL would not introduce impurity into the syngas, indicating that IL is a promising absorbent in syngas dehydration process. Furthermore, the operating and design parameters in the gas dehydration process with IL are optimized. The sensitivity analysis was made to give a comprehensive understanding on the effect of design parameter (the number of theoretical stages of absorption column, Na) and

& IL , temperature Tf, and operating parameters (absorption temperature Ta, IL mass flow rate m pressure Pf of flash drum) on dehydration performance (water content in syngas product,

yproduct). It is found that the water content in syngas product is almost independent on the number of theoretical stages in absorption column as Na is over 4 (see Figure 7(a)). Thus, the number of theoretical stages was determined as 4. As absorption temperature increases, the water content in syngas product increases gradually (see Figure 7(b)). To ensure the water content in syngas product less than 50 ppm and reduce energy consumption, the optimal Ta was chosen as 25 ℃. Besides, it can be seen from Figure 7(c) that with the increase of IL flow rate, the water content in syngas product decreases rapidly, and then tends to be smooth. Considering the energy consumption and dehydration performance together, the mass flow

& IL was chosen as 600 kg·h-1. The temperature Tf and pressure Pf of flash drum rate of IL m are determined to minimize the reboiler energy consumption and improve the purity of

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recycled IL. The Tf and Pf were set as 140 ℃ and 0.05 atm, respectively. Therefore, the

& IL = 600 kg·hr-1, Tf = 140 ℃, and Pf = 0.05 optimized specifications are: Ta = 25 ℃, Na = 4, m atm.

5. CONCLUSIONS In this work, the feasibility of ILs as the alternative drying agent to methanol in syngas dehydration process was evaluated based on the integration of thermodynamic calculation, experiments and process simulation. The Henry’s constants of CO, H2 and H2O in ILs were calculated by COSMO-RS model to obtain the selectivities of CO to H2O ( S H2O/CO ) and H2 to H2O ( S H2O/H2 ) in 285 types of ILs involving 15 cations and 19 anions. When considering the thermostability, selectivity and hydrolytic property together, [EMIM][Tf2N] was chosen as the suitable absorbent. The solubility of the key components (CO and H2) in pure [EMIM][Tf2N] and in [EMIM][Tf2N] + H2O was measured. The predicted results by UNIFAC-Lei model showed a good agreement with the experimental data, indicating that the popular UNIFAC-Lei model could be applied to the subsequent process simulation. The syngas dehydration experiments with [EMIM][Tf2N] were carried out for two syngas streams with different water contents. This new technology enables the water content in syngas product to less than 400 ppm. Noting that the viscosity of the selected IL [EMIM][Tf2N] is only 34 mPa·s at normal temperature and pressure, and thus the viscosity problem is not serious in the application of syngas dehydration. Based on the thermodynamic studies, the conceptual syngas dehydration process with IL was simulated and optimized. It was proven that the new technology presented in this work is simple, effective and efficient, showing a remarkable separation performance, i.e., high

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dehydration performance, low energy consumption, and low equipment investment, when compared to the conventional benchmark methanol process.

■ ASSOCIATED CONTENT The Supporting Information is available free of charge on the website. The abbreviations, names, and chemical structures of cations and anions used throughout the paper, corrosion experiment, the detailed H2 and CO solubility data in pure IL and in the mixture of IL and H2O, the predicted values by the UNIFAC-Lei model, and the detailed process simulation results for each stream.

■ AUTHOR INFORMATION Corresponding Author *Tel.: +86-1064433695. E-mail: [email protected].

ORCID Zhigang Lei: 0000-0001-7838-7207

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China under Grants (Nos. 21476009, 21406007, and U1462104), and the Higher Education and High-quality and World-class Universities (PY201608).

■ REFERENCES (1) Bharadwaj, S.S.; Schmidt, L.D. Catalytic partial oxidation of natural gas to syngas. Fuel Process. Technol. 1995, 42, 109-127. (2) Rostrup-Nielsen, J.R. New aspects of syngas production and use. Catal. Today 2000, 63, 159-164. (3) Subramani, V.; Gangwal, S.K. A review of recent literature to search for an efficient catalytic process for the conversion of syngas to ethanol. Energ. Fuels 2008, 22, 814-839. (4) Kohl, A.L.; Nielsen, R. Gas Purification. Gulf Professional Publishing, 1997. 14


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(5) Ueberhorst, S. Energieträger Erdgas: Exploration, Produktion, Versorgung. Verlag Moderne Industrie, 1999. (6) Chen, W.H.; Chen, S.M.; Hung, C.I. Carbon dioxide capture by single droplet using Selexol, Rectisol and water as absorbents: A theoretical approach. Appl. Energ. 2013, 111, 731-741. (7) Hufton, J.; Golden, T.; Quinn, R.; Kloosteran, J.; Wright, A.; Schaffer, C.; Fogash, K. Advanced hydrogen and CO2 capture technology for sour syngas. Energy Procedia. 2011, 4, 1082-1089. (8) Sun, L.; Smith, R. Rectisol wash process simulation and analysis. J. Cleaner Prod. 2013, 39, 321-328. (9) Weiss, H. Rectisol wash for purification of partial oxidation gases. Gas Sep. Purif. 1988, 2, 171-176. (10) Francesco, F.D.; Calisi, N.; Creatini, M.; Melai, B.; Salvo, P.; Chiappe, C. Water sorption by anhydrous ionic liquids. Green Chem. 2011, 13, 1712-1717. (11) Marsh, K.N.; Boxall, J.A.; Lichtenthaler, R. Room temperature ionic liquids and their mixtures—A review. Fluid Phase Equilib. 2004, 219, 93-98. (12) Martins, V.L.; Nicolau, B.G.; Urahata, S.M.; Ribeiro, M.C.; Torresi, R.M. Influence of the water content on the structure and physicochemical properties of an ionic liquid and its Li+ mixture. J. Phys. Chem. B 2013, 117, 8782-8792. (13)

Orchillés,

A.V.;

Miguel,

P.J.;

Vercher,

E.;

Martínez-Andreu,

A.

Using

1-ethyl-3-methylimidazolium trifluoromethanesulfonate as an entrainer for the extractive distillation of ethanol + water mixtures. J .Chem. Eng. Data 2009, 55, 1669−1674. (14) Pereiro, A.B.; Araújo, J.M.M.; Esperança, J.M.S.S.; Marrucho, I.M.; Rebelo, L.P.N. Ionic liquids in separations of azeotropic systems–A review. J. Chem. Thermodyn. 2012, 46, 2-28. (15) Ramenskaya, L.M.; Grishina, E.P.; Pimenova, A.M.; Gruzdv, M.S. The influence of water on the physicochemical characteristics of 1-butyl-3-methylimidazolium bromide ionic liquid. J. Phys. Chem. A. 2008, 82, 1098-1103. (16) Tsanas, C.; Tzani, A.; Papadopoulos, A.; Detsi, A.; Voutsas, E. Ionic liquids as entrainers for the separation of the ethanol/water system. Fluid Phase Equilib. 2014, 379, 148-156. (17) Heym, F.; Haber, J.; Korth, W.; Etzold, B.J.; Jess, A. Vapor Pressure of Water in Mixtures with Hydrophilic Ionic Liquids–A Contribution to the Design of Processes for Drying of Gases by Absorption in Ionic Liquids. Chem. Eng. Technol. 2010, 33, 1625-1634. (18) Zhou, T.; Chen, L.; Ye, Y.; Chen, L.; Qi, Z.; Freund, H.; Sundmacher, K. An overview of mutual 15



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solubility of ionic liquids and water predicted by COSMO-RS model. Ind. Eng. Chem. Res. 2012, 51, 6256-6264. (19) Han, J.; Dai, C.; Lei, Z.; Chen, B. Gas drying with ionic liquids. AIChE J. 2017, DOI 10.1002/aic.15926. (20) Anthony, J.L.; Maginn, E.J.; Brennecke, J.F. Solubilities and thermodynamic properties of gases in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate. J. Phys. Chem. B. 2002, 106, 7315-7320. (21) Song, Z.; Zhou, T.; Zhang, J.; Chen, L.; Qi, Z. Screening of ionic liquids for solvent-sensitive extraction - with deep desulfurization as an example. Chem. Eng. Sci. 2015, 129, 69-77. (22) Zhou, T.; Wang, Z.; Ye, Y.; Chen, L.; Xu, J.; Qi, Z. Deep separation of benzene from cyclohexane by liquid extraction using ionic liquids as solvent. Ind. Eng. Chem. Res. 2012, 51, 5559-5564. (23) Zhou, T.; Wang, Z.; Chen, L.; Ye, Y.; Qi, Z. Freund, H.; Sundmacher, K. Evaluation of the ionic liquids 1-alkyl-3-methylimidazolium hexafluorophosphate as a solvent for the extraction of benzene from cyclohexane: Liquid-liquid equilibria. J. Chem. Thermodyn.. 2012, 48, 145-149. (24) Klamt, A. COSMO-RS: From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design; Elsevier, 2005. (25) Manan, N.A.; Hardacre, C.; Jacquemin, J.; Rooney, D.W.; Youngs, T.G. Evaluation of gas solubility prediction in ionic liquids using COSMOthermX. J. Chem. Eng. Data 2009, 54, 2005-2022. (26) Lei, Z.; Dai, C.; Chen, B. Gas solubility in ionic liquids. Chem. Rev. 2014, 114, 1289–1326. (27) Lei, Z.; Yuan, J.; Zhu, J. Solubility of CO2 in propanone, 1-ethyl-3-methylimidazolium tetrafluoroborate, and their mixtures. J. Chem. Eng. Data. 2010, 55, 4190-4194. (28) Lei, Z.; Han, J.; Zhang, B.; Zhu, J.; Chen, B. Solubility of CO2 in binary mixtures of room-temperature ionic liquids at high pressures. J. Chem. Eng. Data 2012, 57, 2153-2159. (29) Lei, Z.; Qi, X.; Zhu, J.; Chen, B. Solubility of CO2 in acetone, 1-butyl-3-methylimidazolium tetrafluoroborate, and their mixtures. J. Chem. Eng. Data 2012, 57, 3458-3466. (30) Li, H.; Wu, Y.; Li, X.; Gao, X. State‐of‐the‐art in the investigation and applications of advanced distillation technologies in China. Chem. Eng. Technol. 2016, 5, 815-833. (31) Eckert, F.; Klamt, A. Fast solvent screening via quantum chemistry: COSMO-RS approach. AIChE J. 2002, 48, 369-385. (32) Klamt, A. Conductor-like screening model for real solvents: A new approach to the quantitative 16


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calculation of solvation phenomena. J. Phys. Chem. 1995, 99, 2224-2235. (33) Klamt, A.; Jonas, V.; Bürger, T.; Lohrenz, J.C. Refinement and parametrization of COSMO-RS. J. Phys. Chem. A 1998, 102, 5074-5085. (34) Klamt, A.; Eckert, F.; Arlt, W. COSMO-RS: An alternative to simulation for calculating thermodynamic properties of liquid mixtures. Annu. Rev. Chem. Biomol. Eng. 2010, 1, 101-122. (35) Fredenslund, A.; Jones, R.L.; Pransnitz, J.M. Group contribution estimation of activity coefficients in nonideal liquid mixture. AIChE J. 1975, 27, 1086-1099. (36) Lei, Z.; Zhang, J.; Li, Q.; Chen, B. UNIFAC model for ionic liquids. Ind. Eng. Chem. Res. 2009, 48, 2697-2704. (37) Lei, Z.; Dai, C.; Liu, X.; Xiao, L.; Chen, B. Extension of the UNIFAC model for ionic liquids. Ind. Eng. Chem. Res. 2012, 51, 12135-12144. (38) Lei, Z.; Dai, C.; Wang, W.; Chen, B. UNIFAC model for ionic liquid-CO2 systems. AIChE J. 2014, 60, 716-729. (39) Banerjee, T.; Singh, M.K.; Sahoo, R.K.; Khanna, A. Volume, surface and UNIQUAC interaction parameters for imidazolium based ionic liquids via polarizable continuum model. Fluid Phase Equilib. 2005, 234, 64-76. (40) Santiago, R.S.; Santos, G.R.; Aznar, M. UNIQUAC correlation of liquid–liquid equilibrium in systems involving ionic liquids: The DFT–PCM approach. Part II. Fluid Phase Equilib. 2010, 293, 66-72. (41) Stull, D.R. Vapor pressure of pure substances. Organic and inorganic compounds. Ind. Eng. Chem. Res. 1947, 39, 517-540. (42) Lei, Z.; Dai, C.; Yang, Q.; Zhu, J.; Chen, B. UNIFAC model for ionic liquid-CO (H2) systems: an experimental and modeling study on gas solubility. AIChE J. 2014, 60, 4222-4231. (43) Ngo, H.L.; Compte, K.; Hargens, L.; McEwen, A.B. Thermal properties of imidazolium ionic liquids. Thermochim. Acta. 2000, 357, 97-102. (44) Zhou, Z.B.; Matsumoto, H.; Tatsumi, K. Low-melting, low-viscous, hydrophobic ionic liquids: 1-alkyl (alkyl ether)–3-methylimidazolium perfluoroalkyl trifluoroborate. Chem. Eur. J. 2004, 10, 6581-6591. (45) Gardas, R.L.; Coutinho, J.A.P. A group contribution method for viscosity estimation of ionic liquids. Fluid Phase Equilib. 2008, 266, 195-201. (46) Gardas, R.L.; Coutinho, J.A.P. Group contribution methods for the prediction of thermophysical 17



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and transport properties of ionic liquids. AIChE J. 2009, 55, 1274-1290. (47) Valderrama, J.O.; Zarricueta, K. A simple and generalized model for predicting the density of ionic liquids. Fluid Phase Equilib. 2009, 275, 145-151. (48) Yu, G.; Dai, C.; Wu, L.; Lei, Z. Natural gas dehydration with ionic liquids. Energy Fuels 2017, 31, 1429-1439.

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Table Captions Table 1. The Syngas Composition Investigated in This Work

Table 2. The New Obtained Group Binary Interaction Parameters (αmn and αnm) in the UNIFAC-Lei Model

Table 3. Thermal Decomposition Temperature and Viscosity of ILs with the Cation [EMIM]+

Table 4. Optimized Specifications and Operating Conditions for Syngas Dehydration Processes

Table 5. Comparison of the Process Simulation Results Between IL and Methanol

19



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Page 20 of 36

Table 1. The Syngas Composition Investigated in This Work component

CO

H2

CO2

CH4

N2

H2O

mol%

4.1 - 6.6

30.8 - 34.2

40.0 - 43.1

16.4 - 17.2

1.9 - 2.6

0.2 - 0.7

20


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Table 2. The New Obtained Group Binary Interaction Parameters (αmn and αnm) in the UNIFAC-Lei Model Main groups

αmn

αnm

H2O

1265.10

1358.21

H2O

809.99

740.66

m

n

CO H2

21



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Page 22 of 36

Table 3. Thermal Decomposition Temperature and Viscosity of ILs with the Cation [EMIM]+ Decomposition temperature

Viscosity /mPa·s

ILs /℃ [EMIM][Cl]

285

-

[EMIM][PF6]

375

-

[EMIM][BF4]

412

41

[EMIM][Tf2N]

455

34

[EMIM][Ac]

221

-

[EMIM][SCN]

-

35

[EMIM][CH3SO4]

-

82

[EMIM][SbF6]

-

67

22


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Table 4. Optimized Specifications and Operating Conditions for Syngas Dehydration Processes Absorbents

Contents Columns

Streams

[EMIM][Tf2N] Absorption column Temperature (℃) Pressure (MPa) Total stages Flash drum /Desorption column Temperature (℃) Pressure (atm) Total stages Mole reflux ratio Distillate rate (kg·h-1) Feed stream Temperature (℃) Pressure (atm) Component flowrate (kg·h-1) CO H2 CO2 CH4 N2 H2O Total flowrate (kg·h-1) Absorbent stream Temperature (℃) Pressure (atm) Component flowrate (kg·h-1) CO H2 CO2 CH4 N2 H2O [EMIM]+[Tf2N]methanol Total flowrate (kg·h-1)

methanol

25 2.5 4

25 2.5 4

140 0.05 -

62 1 10 0.05 159

25 2.5

25 2.5

240 82 2255 338 82 3 3000

240 82 2255 338 82 3 3000

25 2.5

25 2.5

0.04 0.14 599.82 600

0.16 0.01 2.82 187.00 190

23



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Page 24 of 36

Table 5. Comparison of the Process Simulation Results Between IL and Methanol Absorbents

Contents Streams

Product stream

Heat dutya

Flash drum Desorption column

methanol

28

22

Temperature (℃) -1

Recovery of main syngas component

[EMIM][Tf2N] Mass flowrate (kg·h ) Water content of the syngas product (mole fraction) Composition CO H2 CH4 Reboiler (kW)

2949 40 ppm

2998 50 ppm

99.58% 99.99% 99.30% 6.91

99.58% 99.99% 99.29% -

Condenser (kW)

2.36

-

Reboiler (kW)

-

55.00

Condenser (kW)

-

49.44

6.91

55.00

Cool Stream Inlet Temperature (℃)

25

25

Cool Stream Outlet temperature (℃)

118

40

Heat duty (kW)

22.87

2.36

Inlet Temperature (℃)

45

45

Outlet temperature (℃)

25

25

Heat duty (kW)

4.56

2.64

Total heating duty (kW) Heat exchanger

Cooler1

Cooler2

Inlet Temperature (℃)

140

-

Temperature of outlet (℃)

25

-

Heat duty (kW)

2.35

-

9.27

52.08

Total cooling duty (kW) a

The total heating duties for gas dehydration processes with IL and methanol are the energies required for

the flash drum and the reboiler of desorption column, respectively. The total cooling duties for gas dehydration processes with IL and methanol are the energies required for cooler 1, cooler 2, and the condenser of desorption column.

24


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Figure Captions Figure 1. The experimental flowchart of syngas dehydration with IL. 1, syngas feed; 2, pressure regulating valve; 3, volume flow controller; 4, water storage tank; 5, gas moisture meter; 6, gas drying column; 7, IL reservoir; 8, constant-flux pump; 9, IL store tank.

Figure 2. Common logarithms of the selectivities of H2 to H2O (a) and CO to H2O (b) in 285 ILs calculated by the COSMO-RS model at 298.15 K.

Figure 3. Solubility of H2 (x1) in pure [EMIM][Tf2N] and in the mixture of [EMIM][Tf2N] + H2O at 313.15 K (a), 333.15 K (b), and 353.15 K (c). Solid lines, predicted results by the UNIFAC-Lei model; scattered points, experimental data. ●, [EMIM][Tf2N]; ▼, 98 wt% [EMIM][Tf2N] + 2 wt% H2O.

Figure 4. Solubility of CO (x1) in pure [EMIM][Tf2N] and in the mixture of [EMIM][Tf2N] + H2O at 313.15 K (a), 333.15 K (b), and 353.15 K (c). Solid lines, predicted results by the UNIFAC-Lei model; scattered points, experimental data. ●, [EMIM][Tf2N]; ▼, 98 wt% [EMIM][Tf2N] + 2 wt% H2O.

Figure 5. Effect of the volume flow rate VIL of the entry IL on the water content (mole fraction) yproduct in gas product. Solid lines, calculated by the EQ stage model; scattered points, experimental data. ●, Water content in entry syngas is 2021 ppm; ○, Water content in entry syngas is 6752 ppm.

Figure 6. Continuous syngas dehydration processes with the IL [EMIM][Tf2N] (a) and methanol (b) as absorbents.

25



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

Figure 7. The effect of the number of theoretical stages in absorption column (Na) (a), the temperature of absorption column (Ta) (b), the IL mass flow rate ( m& IL ) (c), the temperature of flash drum (Tf) ) (d), and the pressure of flash drum (Pf) (e) on the water content (mole fraction) in syngas product (yproduct). & IL = 600 kg·h-1, Tf = 140 ℃, Pf = 0.05 atm; (b) Na = 4, Pa = 0.1 MPa, (a) Ta = 20 ℃, Pa = 0.1 MPa, m m& IL = 600 kg·h-1, Tf = 140 ℃, Pf = 0.05 atm; (c) Na = 4, Ta = 25 ℃, Pa = 0.1 MPa, Tf = 140 ℃, Pf = 0.05

& IL = 600 kg·h-1, Pf = 0.05 atm; (e) Na = 4, Ta = 25 ℃, Pa = atm; (d) Na = 4, Ta = 25 ℃, Pa = 0.1 MPa, m & IL = 600 kg·h-1, Tf = 140 ℃. 0.1 MPa, m

26


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5

5

6

3

1

8

2 9

4

7

Figure 1. The experimental flowchart of syngas dehydration with IL. 1, syngas feed; 2, pressure regulating valve; 3, volume flow controller; 4, water storage tank; 5, gas moisture meter; 6, gas drying column; 7, IL reservoir; 8, constant-flux pump; 9, IL store tank.

27



P6,6,6,14 P4,4,4,4 OMIM HMIM BMPY BMPYR N4,1,1,1 EMPY BPY BMIM C3MIM EMIM EPY MMIM HEMIM

102 103

104 105

5-6 4-5 3-4 2-3 1-2

Ac DEPO4 Cl CH3SO3 TFA NO3 TOS DCA CH3SO4 SCN CF3SO3 TCB BF4 Tf2N PF6 SbF6 eFAP pFAP bFAP

106

6-7

(a)

2

10 103 104

105

P6,6,6,14 P4,4,4,4 OMIM BMPYR HMIM BMPY N4,1,1,1 EMPY BPY BMIM C3MIM EPY MMIM EMIM HEMIM

6-7 5-6 4-5 3-4 2-3 1-2

Ac DEPO4 Cl CH3SO3 TFA NO3 TOS DCA CH3SO4 SCN CF3SO3 BF4 Tf2N TCB PF6 SbF6 eFAP pFAP bFAP

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 28 of 36

(b)

Figure 2. Common logarithms of the selectivities of H2 to H2O (a) and CO to H2O (b) in 285 ILs calculated by the COSMO-RS model at 298.15 K.

28


Page 29 of 36

7 6

(a)

5

P/MPa

4 3 2 1 0 0.000

0.005

0.010

0.015

0.020

0.025

x1 7 6

(b)

P/MPa

5 4 3 2 1 0 0.000

0.005

0.010

0.015

0.020

0.025

0.030

x1 7 6

(c)

5

P/MPa

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


4 3 2 1 0 0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

x1

Figure 3. Solubility of H2 (x1) in pure [EMIM][Tf2N] and in the mixture of [EMIM][Tf2N] + H2O at 313.15 K (a), 333.15 K (b), and 353.15 K (c). Solid lines, predicted results by the UNIFAC-Lei model; scattered points, experimental data. ●, [EMIM][Tf2N]; ▼, 98 wt% [EMIM][Tf2N] + 2 wt% H2O. 29



5

(a)

P/MPa

4 3 2 1 0 0.00

6

0.01

0.02

x1

0.03

0.04

0.05

(b)

P/MPa

5 4 3 2 1 0 0.00

6

0.01

0.02

0.01

0.02

x1

0.03

0.04

0.05

0.03

0.04

0.05

(c)

5

P/MPa

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

4 3 2 1 0 0.00

x1

Figure 4. Solubility of CO (x1) in pure [EMIM][Tf2N] and in the mixture of [EMIM][Tf2N] + H2O at 313.15 K (a), 333.15 K (b), and 353.15 K (c). Solid lines, predicted results by the UNIFAC-Lei model; scattered points, experimental data. ●, [EMIM][Tf2N]; ▼, 98 wt% [EMIM][Tf2N] + 2 wt% H2O. 30


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550 500 450 yproduct (ppm)

DFADFAF344

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


400 350 300 250 200 150

0

5

10

15

20

25

30

VIL (mL·min-1)

Figure 5. Effect of the volume flow rate VIL of the entry IL on the water content (mole fraction) yproduct in gas product. Solid lines, calculated by the EQ stage model; scattered points, experimental data. ●, Water content in entry syngas is 2021 ppm; ○, Water content in entry syngas is 6752 ppm.

31



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Page 32 of 36

Water Product gas

IL

Cooler2 Cooler1 Offgas

Absorption column

Flash drum

Feed

Heat exchanger Buffer tank Pump (a)

Product gas Methanol

Desorption column

Absorption column

Offgas

Cooler1

Feed

Heat exchanger

Water

Buffer tank

(b)

Pump

Figure 6. Continuous syngas dehydration processes with the IL [EMIM][Tf2N] (a) and methanol (b) as absorbents.

32


Page 33 of 36

160

(a)

140

yproduct (ppm)

120 100 80 60 40 20 1

2

3

4

5

6

7

8

9

40

45

Na

200 180

(b)

160

yproduct (ppm)

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


140 120 100 80 60 40 20 0

0

5

10

15

20

25

30

35

Ta (℃)

33



80

(c)

yproduct (ppm)

70 60 50 40 30 20

400

500

600

700 •

800

900

1000

-1

mIL (kg·h )

70

(d)

60

yproduct (ppm)

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

50

40

30 110

120

130

140

150

160

Tf (℃)

34


170

Page 34 of 36

Page 35 of 36

140

(e) 120 100

yproduct (ppm)

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


80 60 40 20 0.00

0.05

0.10

0.15

0.20

0.25

Pf (atm)

Figure 7. The effect of the number of theoretical stages in absorption column (Na) (a), the temperature of absorption column (Ta) (b), the IL mass flow rate ( m& IL ) (c), the temperature of flash drum (Tf) ) (d), and the pressure of flash drum (Pf) (e) on the water content (mole fraction) in syngas product (yproduct). & IL = 600 kg·h-1, Tf = 140 ℃, Pf = 0.05 atm; (b) Na = 4, Pa = 0.1 MPa, (a) Ta = 20 ℃, Pa = 0.1 MPa, m

m& IL = 600 kg·h-1, Tf = 140 ℃, Pf = 0.05 atm; (c) Na = 4, Ta = 25 ℃, Pa = 0.1 MPa, Tf = 140 ℃, Pf = 0.05 & IL = 600 kg·h-1, Pf = 0.05 atm; (e) Na = 4, Ta = 25 ℃, Pa = atm; (d) Na = 4, Ta = 25 ℃, Pa = 0.1 MPa, m & IL = 600 kg·h-1, Tf = 140 ℃. 0.1 MPa, m

35



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Page 36 of 36

Table of Content (TOC) Graphic

Water Product gas

IL

Cooler2 Cooler1 Absorption column

Offgas Flash drum

Feed

Heat exchanger Buffer tank Pump

36


Syngas Dehydration with Ionic Liquids - ACS Publications - American

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