Pentafluoroethane Dehydration with Ionic Liquids - Industrial

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Pentafluoroethane Dehydration with Ionic Liquids Gangqiang Yu, Yifan Jiang, and Zhigang Lei Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02790 • Publication Date (Web): 26 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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Industrial & Engineering Chemistry Research

Pentafluoroethane Dehydration with Ionic Liquids

Gangqiang Yu, Yifan Jiang, and Zhigang Lei* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Energy Environmental Catalysis, Beijing University of Chemical Technology, Box 266, Beijing, 100029, China

ABSTRACT: Ionic liquids (ILs) were proposed as promising separating agents for the hydrofluorocarbons (HFCs) pentafluoroethane (R125) dehydration. Taking into account the H2O solubility and selectivity of H2O to R125 in a variety of IL candidates, we determined the hydrophilic [EMIM][BF4] as a satisfactory absorbent. Besides, some theoretical insights into separation mechanism at the molecular level were explored by means of the combination of COSMO–RS model and quantum chemistry calculation. The gas–liquid equilibrium (GLE) data for the binary R125–[EMIM][BF4] and ternary R125–H2O–[EMIM][BF4] systems were measured, and the experimental data were compared with the predicted results by COSMO–RS model. The R125 dehydration experiment with IL was carried out. Furthermore, the continuous processes for R125 dehydration with [EMIM][BF4] and the conventional benchmark solvent tetraethylene glycol (TeEG) as absorbents were simulated and optimized with the rigorous equilibrium (EQ) stage model, in which COSMO–SAC model was selected. It was confirmed that the R125 gas dehydration with IL presented in this work is a typical process intensification technology.

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1. INTRODUCTION In the past few decades, hydrochlorofluorocarbons (HCFCs) or chlorofluorocarbons (CFCs) as refrigerants,1 blowing agents,2 heat transfer fluids,3 and aerosols have been widely applied to industrial fields and daily life.4 However, they have been recently found to be significantly harmful to the ozone layer. As a result, fluorine–substituted hydrocarbons containing less or no chlorine substituent are receiving extensive attention. In many industrial applications, hydrofluorocarbons (HFCs) compounds consisting of only carbon, fluorine, and hydrogen elements have been widely used as leading and prior substitutes to replace the conventional HCFCs and CFCs. Compared to HCFCs or CFCs, the HFCs are more favorable for the environment because of their zero ozone depletion potential, non–flammability, and non–toxicity.5,6 Recently, pentafluoroethane (R125) as one of the relatively new and representative HFCs members is considered as the excellent fire extinguishing agent and refrigerant.7,8 Especially, R125 is generally used in clean agent fire suppression systems and special situations, where water is no longer able to meet the demand. Besides, R125 has zero ozone depletion potential so that it is often used as more suitable, economic, environmental, and alternative refrigerant to replace other conventional refrigerants in daily life and industry. On the other hand, R125 is also extremely safe and simple in storage process by placing it in a slightly pressurized closed container, and this agent is electrically non–conductive and non–corrosive. Unfortunately, when preparing R125 product, there is usually water vapor, and the presence of water vapor will suppress the quality of product gas. Moreover, the water vapor contained in the hydrogen fluoride as a raw material for obtaining R125 product leads 2


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to the formation of hydrofluoric acid, which is extremely corrosive, resulting in the very severe erosion and corrosion of pipelines and equipment as well as threatening human life safety. Thus, it is necessary and urgent to take away water vapor with specific content from R125 gas mixture. At present, the main methods used in industrial gas dehydration include the absorption with liquid solvents,9 adsorption on porous materials,10 membrane separation,11,12 and condensation. Among others, the solvent absorption method due to its simple operation and process is regarded as an industrially preferred technology. For gas dehydration, it is very crucial about how to select a suitable absorbent to achieve an energy saving, emission reduction, environmental protection, economical, and efficient separation process. For this purpose, the conventional organic solvents polyethylene glycols (PEGs),13,14 with relative high boiling points are often used to dehydrate the industrial gases. Unfortunately, there are still many disadvantages for the use of PEGs, e.g., unavoidable volatile solvent loss, entrainment, degradation, equipment corrosion, and high regeneration energy consumption, greatly declining the green degree of absorption process.15 Therefore, it is significant to seek for a more suitable, effective, economic, and alternative liquid solvent to substitute the conventional volatile organic absorbent for industrial gas dehydration. Currently, ionic liquids (ILs) regarded as very promising alternatives to traditional liquid solvents have received considerable attention in the field of gas separation, because of excellent physicochemical properties, e.g., low melting points, excellent thermodynamic stability, tunable chemical structures, and negligible volatility.16,17 In particular, the nonvolatility makes ILs avoid the solvent loss and solvent entrainment in gas product. Thus, 3



this will provide a newly valuable and efficient mean for R125 dehydration. In this work, we like to go a further step to systematically study the R125 dehydration with ILs as absorbents. Therefore, the objectives of this article are composed of the following key points: (1) taking into account the influence of molecular structures of ILs on separation performance (i.e., the selectivity of H2O to R125 as well as the H2O and R125 solubility in a variety of IL candidates), so as to screen out the appropriate IL as separating agent for R125 dehydration; (2) providing some theoretical insights into separation mechanism at the molecular level by means of the combination of COSMO–RS model and quantum chemistry calculation; (3) measuring the gas–liquid equilibrium (GLE) data for the binary system of IL + R125 and ternary system of IL + R125 + H2O, and further validating the predictive applicability of COSMO–RS model; and (4) carrying out the process simulation and optimization of R125 dehydration with ILs as absorbents by using the rigorous equilibrium (EQ) stage model in the case of the COSMO–SAC model being embedded. The simulation results between IL and the benchmark solvent tetraethylene glycol (TeEG) with a high boiling point as absorbents were compared at the industrial scale. In this work, the details on cations and anions (i.e., abbreviations, full names, and structures) are given in Supporting Information Table S1. 2. EXPERIMENTAL SECTION 2.1. Materials. R125, H2O, and [EMIM][BF4] were used in this work. The details on the purities and supplier of untreated and treated experimental materials are given in Supporting Information Table S2. Before experiments, the original IL derived from the supplier was pretreated to eliminate volatile impurities and moisture by a vacuum drying oven (type DZF–6020, Shanghai Yiheng Scientific Instrument Co. Ltd.) at 413.2 K for 4h. The water 4


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content of mass fraction for [EMIM][BF4] after dried is less than 200 ppm as determined by Karl Fischer titration (model KLS 701). It should be noted that the untreated materials purities are provided by the supplier. 2.2. Apparatus and Procedure. 2.2.1. Gas–Liquid Equilibrium (GLE) Experiment. In this work, a high–pressure and high–temperature view–cell apparatus was used to measure the experimental GLE data for binary system of R125 + [EMIM][BF4] and ternary system of R125 + [EMIM][BF4] + H2O by means of the drainage gas–collecting method. Notably, the accuracies of experimental temperature and pressure were detected by an electric heating elements with the accuracy of 0.1 K and a pressure gauge with the accuracy of 0.001 MPa, respectively. More details on experimental apparatus diagram and operating procedure are given in our previous work.18 2.2.2. R125 Dehydration Experiment. The flow diagram of continuous gas dehydration apparatus can be found in Supporting Information Figure S1, where an absorption column with a height of 100 cm and an internal diameter of 3 cm was randomly filled with a large amount of the θ–shaped packings with 4 × 4 × 0.6 mm. Firstly, the dried IL at a given volume flowrate was flowed into the top of absorption column by a liquid pump (Type 2PB3020, Beijing Satellite Manufacturing Factory). Then, the R125 feeding gas with the saturated water content entered into the bottom. The R125 product gas flowed out from the top of absorption column, and the water content was detected by the gas moisture meter (type RHD–601). After absorption, the vacuum drying oven (type DZF–6020) was applied to regenerate the water–loaded IL for recycling. 3. COSMO–RS MODEL 5



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As an efficiently priori thermodynamic model irrespective of experimental data, the COSMO–RS model has received wide attention for predicting some important thermodynamic properties (e.g., phase equilibrium, vapor pressure, gas solubility, and activity coefficient) of the binary and ternary mixture systems.19–23 The GLE for the binary R125 + [EMIM][BF4] or ternary R125 + [EMIM][BF4] + H2O systems is given by

y i Piφ i ( T , P , y i ) = x i γ i Pi Sφ is ( T , Pi s )

(1)

where xi and yi denote the mole fractions of R125 in liquid and gas phases, respectively; γ1 is the activity coefficient of R125 obtained by COSMO–RS model; P is the total pressure of the s

system; P1 refers to the saturated vapor pressure of pure R125 at the system temperature as calculated by the Antoine equation;24 φ i ( T , P , y 1 ) represents fugacity coefficient of R125 in gas phase calculated by the Peng–Robinson (PR) equation; and φis (T , Pi s ) stands for the fugacity coefficient of R125 at P1s . It should be noted that the gas phase is taken on as pure R125 (i.e., yi = 1) resulting from the ILs’ nonvolatility. In this work, the COSMOthermX (version C30_1301) software was employed to screen out the suitable IL for R125 dehydration, analyzing the σ–profiles along with σ–potentials of all of molecules presented in this study, and predicting the activity coefficients of R125–IL system. In the activity coefficient calculation, the entire IL molecule was divided into two independent parts of a cation and an anion. As a result, the R125–IL binary mixture system was treated as a virtual ternary system composed of R125, cation, and anion. A transform relationship of activity coefficients of solute i between the real binary and hypothetical ternary mixtures is given as follows: 6


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γ i bin =

γ i tern xi tern xi bin

=

γ i tern 2 − xi bin

(2)

where xi bin and xi tern stand for the mole fractions of R125 in the real binary system of R125 + IL and in the hypothetical ternary system of R125 + cation + anion , respectively; and

γ i bin and γ i tern denote the corresponding activity coefficients in the binary and hypothetical ternary systems, respectively. Similarly, for the ternary system of R125 + IL + H2O, the solution was considered as a hypothetical quaternary system. More details on calculating activity coefficients using the COSMOthermX software are given in the software tutorials. 4. QUANTUM CHEMISTRY CALCULATION In this work, the optimization of geometry structures of R125, H2O, [EMIM]+, [BF4]–, and [EMIM][BF4] was performed by Gaussian 09 package25, in which the B3LYP/6–31+G* level26,27 along with DFT–D3 dispersion correction function28 was selected. In addition, the basis set superposition error (BSSE)29 correction was employed to calculate the interaction energies between molecules concerned in this work. Moreover, the selected minimum–energy structures were at local minima with no imaginary vibrational frequency. 5. RESULTS AND DISCUSSION 5.1. Screening the suitable ILs for R125 Dehydration. The solubility of solutes (R125 and H2O), which can be reflected by the magnitude of Henry’s constants in ILs, and the selectivity of R125 to H2O are two important physical quantities to determine whether the separation process is effective or not. It is necessary to choose an appropriate IL for R125 dehydration before conducting the experiment. Thus, the COSMO–RS model using the COSMOthermX software aforementioned was applied to screen out the suitable absorbent from 238 IL candidates consisting of 14 cations and 17 anions in the present study by 7



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calculating the Henry’s constants and the selectivity represented by

SR125/H2 O =

H R125 H H2 O

(3)

where SR125/H2 O denotes the selectivity of R125 to H2O, and H R125 and H H2 O refer to the Henry’s constants of R125 and H2O in ILs, respectively. It is clear that the trends of H2O solubility and selectivity of SR125/H2 O are similar (see Figures 1a–b). That is, the high H2O solubility corresponds to the high selectivity for the same IL. Moreover, it was found that H2O solubility and selectivity are mainly affected by the anion. Meanwhile, the R125 solubility in ILs is mainly affected by the anions, although a small portion of cations may also affect the solubility significantly (see Figure 1c). It should be mentioned that the anion with fluorination is beneficial to improving the solubility of H2O, but adverse to enhancing the solubility of R125. Although the selectivity for [EMIM][BF4] isn’t the highest among various kinds of ILs, it is enough to meet the needs of R125 dehydration due to the high SR125/H2O selectivity of more than 200 in [EMIM][BF4] (Figure 1a). Moreover, the relatively cheap price makes [EMIM][BF4] own a good prospect for industrialization. Thus, the hydrophilicity IL [EMIM][BF4] was selected as an appropriate absorbent for capturing water from gas mixture. 5.2. Analysis of the σ–Profiles and σ–Potential. It is known that each different molecule corresponds to distinct σ–profile and σ–potential characteristic curves, which can afford theoretical insights into the molecular interaction between solvents and solutes by means of the COSMO–RS model.30,31 The σ–profile represents the molecular surface charge density (σ) distribution,32 while the σ–potential means the affinity between molecular surfaces. The σ–profiles for all the molecules concerned in this work are illustrated in Figure 2a. One 8


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extremely strong peak in the σ–profile of R125 locates in the non–polar region (–0.0082 < σ < 0.0082 e/Å2), indicating the non–polar characteristics of R125, while the other weak peak at σ = –0.0135 in the hydrogen bond (HB) donor region (σ < –0.0082 e/Å2) results from hydrogen atom in R125 molecule, indicating that it has a weak HB donor ability. One peak in the σ–profile of [EMIM]+ appears around σ = –0.0084 e/Å2, indicating a weak HB donor ability, whereas one strong peak lies in the non–polar region, indicating a strong non–polar capacity. Only does one very strong peak in the σ–profile of [BF4]– locates at σ = 0.0116 e/Å2 within the scope of HB acceptor region (σ > 0.0082 e/Å2) attributed to the strongly electronegative fluorine atoms, indicating that the anion [BF4]– has a relatively strong affinity with other HB donors. Evidently, a very wide range is observed in the H2O σ–profile curve, in which one strong peak appears at σ = –0.016 e/Å2 treated as a HB donor, whereas the other locates at σ = 0.0181 e/Å2 as a HB acceptor. Thus, the strong HB can be formed between H2O and the anion [BF4]– (i.e., the so–called anion effect). The similar conclusions also hold for the σ–potentials as shown in Figure 2b, in which the σ–potential of R125 shows a parabolic shape with σ = 0 as the vertex in the non–polar region. In addition, for the IL [EMIM][BF4] and R125, almost all of the σ–potential values in the non–polar region are negative, and an intersection appears at σ = 0.0032, indicating the intermolecular non–HB interaction between [EMIM][BF4] and R125 in the non–polar region, which results from the interaction between [EMIM]+ and R125. In addition, the negative σ–potential values of [EMIM][BF4] and R125 locate in the HB donor and HB acceptor

regions, respectively, indicating that [EMIM][BF4] and R125 have a certain affinity with those compounds treated as HB acceptors and donors, respectively. For the σ–potential of 9



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H2O, all of the values are negative in both HB acceptor and HB donor regions except for those in the non–polar region, indicating that H2O can be used as both HB donor and HB acceptor. Moreover, H2O can produce the very strong HB interactions with other molecules as HB donor or HB acceptor due to the very negative σ–potential values of H2O. 5.3. Analysis of the Interaction Energies. The most stable three–dimensional space configuration selected from a variety of possible structural combinations was used to calculate the intermolecular interaction energies by Gaussian 09 software.25 The interaction energies directly reflect the binding strength between molecules, and can be written as ∆EA-B (kJ ⋅ mol −1 ) = 2625.5 {∆EA-B (au) − [ ∆EA (au) − ∆EB (au)]}

(4)

where EA, EB, and EA–B represent the energy of single A, the energy of single B, and the energy of A together with B, respectively, and the symbol “au” means the atom unit. The interaction energies between R125 (H2O) and [EMIM]+ or [BF4]– or the whole IL molecule were calculated by this equation. The calculated results are summarized in Table 1. The most stable spatial structures and intermolecular interactions are shown in Figure 3. Evidently, the interaction energy of H2O–[BF4]– (–51.12 kJ·mol–1) is much stronger than that of H2O–[EMIM]+ (–3.53 kJ·mol–1), but the interaction energy of R125–[BF4]– (–50.46 kJ·mol–1) is somewhat greater than that of R125–[EMIM]+ (–23.56 kJ·mol–1). Moreover, it is noted that the

interaction energy

of

[EMIM][BF4]–H2O

system is stronger

than that of

[EMIM][BF4]–R125. This is in accordance with the conclusion as obtained by COSMO–RS model that H2O solubility in ILs is much larger than R125 solubility, leading to the large selectivity of H2O/R125 as the separation basis of R125 dehydration with ILs (see Figure 1). On the other hand, it is interesting to find that the interaction energy of H2O–[EMIM][BF4] is 10


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significantly close to that of H2O–[BF4]–, bringing about the so–called “anion effect”. However, the interaction energy between R125 and the IL is much closer to the average value of those of R125–[EMIM]+ and R125–[BF4]–. As a whole, the R125 solubility in ILs is mutually affected by both cations and anions, while the H2O solubility in ILs is primarily determined by anions.

5.4. Recognition of the Weak Interaction of Solute–IL Systems. Weak interactions (i.e., non–covalent interactions) including van der Waals (vdW) and HB interactions are present in both small molecules and big molecular complexes. The method of independent gradient model (IGM) analysis based on the atomic density methodology, coupled to a given local descriptor, can give the intuitive 3D picture for a crowd of weak interaction types as proposed by Lefebvre et al.33 Herein, Multiwfn34 and VMD35 were used to carry out the IGM calculation and analysis, in which the high quality data grid was used to obtain the gradient isosurfaces for providing the high–resolution visual images of weak interaction. In this work, we will identify the type of weak interaction of H2O (R125)–cation, H2O (R125)–anion, and H2O (R125)–IL by the IGM analysis. As shown in Figure 4a, one large light green isosurface lies between the fluorine atom in [BF4]– and the hydrogen on the imidazolium ring in [EMIM]+, indicating that the weak HB interaction is formed in this position. Two small green isosurfaces appear in the space between R125 and [EMIM]+ (see Figure 4b), resulting in the vdW dispersion attraction formed between the negatively charged fluorine atoms in R125 molecule and the positively charged hydrogen on [EMIM]+. In the R125 + [BF4]– system, the blue–green isosurface between the fluorine atoms in [BF4]– and the hydrogen atoms in R125 represents the 11



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relatively weak HB formed (see Figure 4c). For the H2O + [EMIM]+ system (see Figure 4d), a relatively small blue–green isosurface appears at the overlapping portion between the electron–rich oxygen in H2O and the electron–deficient hydrogen in the middle of two nitrogens in the imidazole ring, where the strong HB is observed. There is one extremely large dark blue ellipsoid isosurface in the space composed of two fluorine atoms in [BF4]– and two hydrogen atoms in H2O molecule. In this case, the extremely strong HB interaction is formed (see Figure 4e). It is clear that the vdW attraction interaction and HB interaction can be observed for the R125 (H2O)–IL systems as shown in Figures 4f and 4g. Anyway, the conclusions on molecular microstructural analysis obtained by quantum chemistry calculation are consistent with those coming from the theoretical analysis by COSMO–RS model aforementioned.

5.5. GLE for R125–ILand R125–IL–H2O Systems. In this work, the experimental GLE data for the binary system of R125 + [EMIM][BF4] and ternary system of R125 + [EMIM][BF4] + H2O at different temperatures were measured to identify the effect of temperature, pressure, and H2O on R125 solubility, and compared with the predicted results by COSMO–RS model. Meanwhile, the detailed experimental data and predicted results can be found in Supporting Information Table S3. As shown in Figure 5, the predicted results agree well with experimental data, with the average relative deviations (ARDs) 17.95% and 17.28% for R125 solubility in pure [EMIM][BF4] and in the mixed system of [EMIM][BF4] + H2O, respectively, indicating that the

COSMO–RS

model

enables

to

quantitatively

predict

the

GLE

of

R125–[EMIM][BF4]–H2O mixture system. In addition, it is clear that R125 solubility in 12


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either pure [EMIM][BF4] or [EMIM][BF4] + H2O mixture increases with the decrease of temperature. As pressure increases, R125 solubility increases significantly. It can be seen from Figure 6 that when a small amount of water is present in [EMIM][BF4], R125 solubility decreases significantly. This is favorable for obtaining more amount of R125 product gas due to the decreasing R125 solubility in the IL [EMIM][BF4] in the presence of water in dehydration process.

5.6. Experimental Results and Process Simulation of R125 Dehydration by the EQ Stage Model. The R125 dehydration experiment with [EMIM][BF4] was performed under ambient conditions (25 ℃ and 1 atm). The feed gas stream consisted of R125 gas with saturated water content of 23000 ppm (mole fraction) entered into the absorption column at 500 mL·min–1 volume flow rate under the normal condition. After absorption, the water–loaded IL was regenerated using the vacuum drying oven. Furthermore, R125 dehydration at laboratory scale with [EMIM][BF4] as absorbent was simulated under experimental conditions by the EQ stage model in Aspen Plus (version 8.1). It should be noted that the COSMO–SAC model42 was selected as the property method, into which the σ–profiles of IL were input. On the other hand, [EMIM][BF4] was added as a new component to Aspen database, while other compounds (e.g., H2O and R125) were as conventional substances derived from the built–in Aspen database. In the dehydration process, the effect of two key factors (i.e., the IL volume flow rate VIL and IL initial water content (mass fraction) wH O ) on the purity of R125 product gas 2

represented by the mole faction y1 was studied. The relationship between the VIL with different wH O (320, 698, and 1256 ppm) and gas product purity is shown Figure 7a. It is 2

13



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observed that y1 first increases rapidly, and then reaches a steady state with the increase of VIL. Moreover, y1 will decrease to around 147 ppm when VIL increases to 15 mL·min–1. As a whole, the three curves at different wH O exhibit the same trend. 2

On the other hand, the relationship between wH O and y1 at a fixed value of VIL = 15 2

mL·min–1 is presented as Figure 7b. It can be seen that as wH O increases gradually, y1 2

exhibits a linear increase. This indicates that a low wH O is favorable for improving the 2

quality of R125 gas product. This work demonstrates that the simulated results by EQ stage model conform to experimental data, thus confirming the applicability of EQ stage model with the COSMO–SAC property method for process simulation of R125 dehydration with [EMIM][BF4] as absorbent.

5.7. Process Simulation and Optimization of Continuous R125 Dehydration at Industrial Scale. In this work, we further simulate and compare the continuous R125 dehydration processes by the EQ stage model at industrial scale with the IL [EMIM][BF4] and the conventional solvent TEG as absorbents. For comparison, the R125 gas with the mole fraction water content of 23000 ppm kept at a constant mass flow rate of 1000 kg·h–1 was treated as the simulated feed gas stream at industrial scale, the separation target being the y1 less than 390 ppm. For the flowsheet with IL as absorbent, this process is mainly composed of one gas absorption column, several heat exchangers, and one flash drum used for regenerating absorbent (see Supporting Information Figure S2a). The flowsheet of TeEG process is a little different from the flowsheet of IL process in that a distillation column rather than a flash drum is used for TeEG regeneration (see Supporting Information Figure S2b). The simulated feed gas and IL stream contact countercurrent in absorption column. To 14


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achieve the specific separation target, the operating conditions and design parameter should be optimized. For the R125 dehydration with IL, the influence of some operating and design parameters on separation performance (i.e, y1) was comprehensively investigated by sensitivity analysis. As shown in Supporting Information Figure S3a, y1 starts to decrease sharply as the IL mass flow rate mIL increases, and then tends to stabilize. In addition, increasing the number of theoretical stages Na will help improve the purity of R125 gas product, but when Na is over 5, this effect is no longer significant. Moreover, it is observed that decreasing the absorption column temperature (T1) is favorable for enhancing the y1 (see Supporting Information Figure S3b). On the other hand, increasing temperature (T2) as well as decreasing pressure (P2) for conducting the water–rich IL regeneration in flash drum can result in the good R125 dehydration performance (see Supporting Information Figures S3c–d). Thus, the optimized specifications are given as follows: mIL = 50 kg·h–1, T1 = 20 ℃, Na = 5, T2 = 120 ℃, and P2 = 0.02 atm. As a result, a good dehydration performance is achieved so that y1 is reduced to 376 ppm and the R125 recovery ratio reaches 99.65%. For the R125 dehydration with TeEG, the influence of design parameters (the numbers of theoretical stages of absorption and desorption columns, denoted with Na and Ndes, respectively) and operating conditions (reboiler temperature of desorption column Tboil and TeEG mass flow rate mTeEG) on separation performance was also studied. In the simulation, the operating temperature and pressure of absorption column were set to be T1 = 20 ℃ and P1 = 1 atm as in the IL process. Besides, y1 was specified as less than 390 ppm. It is noting that the reboiler temperature of desorption column for TeEG regeneration must be less than 15



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235 ℃, because a high temperature leads to the decomposition of TeEG. In this case, to achieve the specific separation target, the optimal conditions are: Na = 5, mTeEG = 400 kg·h–1, Tboil = 235 ℃, and Ndes = 6 (see Supporting Information Figure S4). However, y1 in the TeEG process (384 ppm) is slightly higher than that in the IL process (376 ppm), while the recovery ratio of R125 is 99.35% which is also less than that in the IL process. The operating conditions and design parameters for R125 dehydration with different absorbents (i.e., [EMIM][BF4] and TeEG) can be found in Supporting Information Table S4, while the simulation results are summarized in Supporting Information Table S5. Furthermore, it is necessary to make a comparison of energy consumption between [EMIM][BF4] and TeEG processes. Given the same separation target, the total heating and cooling duties in the IL process decrease by 74.76% and 85.96%, respectively, when compared to those in the TeEG process. In addition, the exergy consumption was calculated using the method as proposed by Kutas36

Eexergy = Q (1 −

T0 ) T1

(5)

where Q denotes the energy supplied externally, and T0 and T1 represent the ambient and system temperatures, respectively. As a result, the IL process (0.88 kW) can be reduced up to 84.84%, when compared to the TeEG process (5.83 kW). On the other hand, although a vacuum pump attached to flash drum is used for regenerating IL, the extra electric energy consumption is only 0.4 kW. Anyway, the IL [EMIM][BF4] as absorbent significantly intensifies the R125 dehydration process when compared to the TeEG as absorbent, with such advantages as no solvent loss, no entrainment in gas product, lower energy consumption, and lower equipment investment. 16


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6. CONCLUSIONS This is the first work to systematically study the special gas R125 dehydration with ILs as promising separating agents. The hydrophilic IL [EMIM][BF4] was used as the suitable absorbent to capture water vapor from R125 feeding gas mixture. The COSMO–RS model combined with the quantum chemistry calculation revealed the separation mechanism at the microscopic molecular level. It was found that the anion [BF4]– dictates for R125 dehydration (i.e., the so–called “anion effect”) due to the strong hydrogen bond interaction between H2O and [BF4]–. Moreover, solubility measurement experiments of R125 in pure [EMIM][BF4] and the mixed solvent of [EMIM][BF4] + H2O were conducted. The result exhibits that when the very low water contents are present, R125 solubility in IL is reduced apparently under the identical condition. However, this is helpful for improving the R125 dehydration performance with [EMIM][BF4] due to the absorption loss of R125 gas product decreasing in the presence of water. On the other hand, the experimental data are in good agreement with the predicted results by COSMO–RS model, thus confirming the applicability of COSMO–RS model for the phase equilibrium of mixed system of R125–[EMIM][BF4] and R125–[EMIM][BF4]–H2O. Furthermore, the R125 dehydration experiment with [EMIM][BF4] was systematically performed, exhibiting a good separation performance of ILs in gas dehydration. On this basis, the conceptual design of continuous R125 dehydration processes with [EMIM][BF4] and the conventional benchmark solvent TeEG as absorbents were performed with the rigorous EQ stage model using the COSMO–SAC model as property method. It was proven that the common ILs are effective and efficient for R125 dehydration. 17



■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the website. Detailed specifications of materials used in this work, experimental GLE data, the calculation on uncertainties along with the predicted results by the COSMO–RS model, and the optimized design parameters and operating conditions for continuous R125 dehydration process with different absorbents (i.e., IL and TeEG) at industrial scale can be found in the online version. (.xls file).

■ 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 Key R&D Program of China (No. 2018YFB0604902) and the National Natural Science Foundation of China under Grant (No. 21476009).

18


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21



Table Captions Table 1. Interaction Energies of A (R125 or H2O) Combined with B ([EMIM]+, [BF4]–, or [EMIM][BF4])

22


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Table 1. Interaction Energies of A (R125 or H2O) Combined with B ([EMIM]+, [BF4]–, or [EMIM][BF4]) ∆EA–B (kJ·mol–1)

A

B

R125

[BF4]–

–50.46 +

R125

[EMIM]

R125

[EMIM][BF4]

H2O

[BF4]

–23.56 –34.78

–

–51.12 +

H2O

[EMIM]

H2O

[EMIM][BF4]

–3.53 –49.62

23



Figure Captions Figure 1. The common logarithm of the selectivity of R125 to H2O (SR125/H2O) (a) and the Henry’s law constants (MPa) of H2O (lgHH2O) (b) and R125 (lgHR125) (c) in 238 ILs predicted by the COSMO–RS model at 298.15 K.

Figure 2. σ–Profiles of R125, H2O, [EMIM]+, and [BF4] (a), along with σ–potentials of R125, H2O, and [EMIM][BF4] (b) at 298.15 K.

Figure 3. Energy minimum geometric structures of [EMIM][BF4] (a), R125 + [EMIM]+ (b), R125 + [BF4]– (c), H2O + [EMIM]+ (d), H2O + [BF4]– (e), H2O + [EMIM][BF4] (f), and R125 + [EMIM][BF4] (g) as optimized by Gaussian 09 package.

Figure 4. IGM isosurface (s = 0.01) of intermolecular interactions. The isosurfaces are denoted with filling color according to the color bar at bottom with color coding in the ED (electron density) range of –0.05 < sign(λ2)ρ < +0.05 a.u. Blue indicates the strong attractive interaction, and green indicates the vdW dispersion attraction.

Figure 5. R125 solubility in pure [EMIM][BF4] (a) and in the mixed solution of [EMIM][BF4] + H2O (b) under different temperatures. ▲, 353.15 K; ●, 333.15 K ■, 313.15 K. Scattered points, experimental data; solid lines, predicted results by the COSMO–RS model.

24


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Figure 6. Comparison of R125 solubility between pure [EMIM][BF4] (●) and the mixed solution [EMIM][BF4] + H2O (■) at 313.15 K (a), 333.15 K (b), and 353.15 K (c). Scattered points, experimental data; solid lines, predicted results by the COSMO–RS model.

Figure 7. Influence of the VIL on the y1 under different wH O (a) and influence of the wH O 2

2

on the y1 at VIL = 15 mL·min–1 (b). Scattered points, experimental data; solid lines, calculated results by the EQ stage model.

25



(a)

P666,14

3-4 2-3 1-2 0-1

P4444 BMPYR

HMIM N4111 BPY

102

EPY EMIM BMIM C3MIM MMIM HEMIM

bFAP

Tf2N eFAP

PF6 SbF6

TCB

CF3SO3 BF4

CH3SO4

NO3

DCA SCN

TFA TOS

CH3SO3

10

Ac DEPO4

Cation

OMIM BMPY

103

Anion

(b)

P666,14 P4444 BMPYR OMIM BMPY HMIM N4111

10-2

BPY BMIM C3MIM EPY EMIM

10-3

MMIM HEMIM

DEPO4 Ac

TOS TFA CH3SO3

DCA NO3

CF3SO3 SCN CH3SO4

TCB BF4

SbF6 PF6 Tf2N

10-4

Anion

26


-1-0 -2--1 -3--2 -4--3 -5--4

Cation

10-1

bFAP eFAP

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 37

Page 27 of 37

(c)

P666,14

1-2 0-1 -1-0 -2--1

P4444 OMIM

10-1

BMPY HMIM N4111 EMPY BPY

100

Cation

BMPYR

BMIM

10

C3MIM EMIM

eFAP

HEMIM

bFAP

PF6

SbF6

TCB

BF4

Tf2N

SCN

CF3SO3

CH3SO4

NO3

DCA

TFA

TOS

CH3SO3

Ac

MMIM DEPO4

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


Anion

Figure 1. The common logarithm of the selectivity of R125 to H2O (SR125/H2O) (a) and the Henry’s law constants (MPa) of H2O (lgHH2O) (b) and R125 (lgHR125) (c) in 238 ILs predicted by the COSMO–RS model at 298.15 K.

27



30

(a) 25

R125 H2O

p (σ)

20

+

[EMIM] [BF4]

15 10 5 0 -0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

σ (e/Å2)

0.6

(b)

R125 H2O

0.4

µ(σ) [kcal/(mol·Å2)]

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

[EMIM][BF4]

0.2 0.0 -0.2 -0.4 -0.6 -0.8 -0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

σ (e/Å2)

Figure 2. σ–Profiles of R125, H2O, [EMIM]+, and [BF4] (a), along with σ–potentials of R125, H2O, and [EMIM][BF4] (b) at 298.15 K.

28


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2.237 Å 2.233 Å

(b)

(a)

2.073 Å

2.235 Å 2.049 Å (d)

(c)

2.042 Å

2.042 Å 2.188 Å (e)

(f)

29


1.758 Å


2.189 Å

2.025 Å

(g)

C

H

O

N

B

F

Figure 3. Energy minimum geometric structures of [EMIM][BF4] (a), R125 + [EMIM]+ (b), R125 + [BF4]– (c), H2O + [EMIM]+ (d), H2O + [BF4]– (e), H2O + [EMIM][BF4] (f), and R125 + [EMIM][BF4] (g) as optimized by Gaussian 09 package.

30


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(b) R125 + [EMIM]+

(a) [EMIM][BF4]

(d) H2O + [EMIM]+

(c) R125 + [BF4]-

(e) H2O + [BF4]-

(f) H2O + [EMIM][BF4]

31



(g) R125 + [EMIM][BF4]

-0.05

0.05 H-bond

Dispersion

Figure 4. IGM isosurface (s = 0.01) of intermolecular interactions. The isosurfaces are denoted with filling color according to the color bar at bottom with color coding in the ED (electron density) range of –0.05 < sign(λ2)ρ < +0.05 a.u. Blue indicates the strong attractive interaction, and green indicates the vdW dispersion attraction.

32


Page 32 of 37

Page 33 of 37

0.08 0.07

(a)

0.06 0.05

x1

0.04 0.03 0.02 0.01 0.00

0

1

2

3

4

5

6

4

5

7

8

9

P/bar

0.05

(b) 0.04

0.03

x1

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


0.02

0.01

0.00 0

1

2

3

6

7

8

P/bar

Figure 5. R125 solubility in pure [EMIM][BF4] (a) and in the mixed solution of [EMIM][BF4] + H2O (b) under different temperatures. ▲, 353.15 K; ●, 333.15 K ■, 313.15 K. Scattered points, experimental data; solid lines, predicted results by the COSMO–RS model.

33



0.09

(a)

0.08 0.07 0.06

x1

0.05 0.04 0.03 0.02 0.01 0.00

0

1

2

3

4

5

5

6

6

7

P/bar

0.07

(b)

0.06 0.05 0.04

x1

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 34 of 37

0.03 0.02 0.01 0.00 0

1

2

3

4

P/ bar

34


7

8

Page 35 of 37

0.06

(c)

0.05 0.04

x1

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


0.03 0.02 0.01 0.00

0

1

2

3

4

5

6

7

8

9

P/ bar

Figure 6. Comparison of R125 solubility between pure [EMIM][BF4] (●) and the mixed solution [EMIM][BF4] + H2O (■) at 313.15 K (a), 333.15 K (b), and 353.15 K (c). Scattered points, experimental data; solid lines, predicted results by the COSMO–RS model.

35



2400

(a)

320 ppm 698 ppm 1256 ppm

y1 (ppm)

2000 1600 1200 800 400 0

0

5

10

15

20

25

30

-1

VIL (mL·min )

600

(b)

500

y1 (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

Page 36 of 37

400 300 200 100 0

500

1000

1500

wH O 2

2000

2500

3000

(ppm)

Figure 7. Influence of the VIL on the y1 under different wH O (a) and influence of the wH O 2

2

on the y1 at VIL = 15 mL·min–1 (b). Scattered points, experimental data; solid lines, calculated results by the EQ stage model.

36


Page 37 of 37

Table of Content (TOC) Graphic R125

IL

H-bonding

R125 + H2O + IL

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


R125 + H2O

H2O + [BF4]-

H2O + [EMIM]+

IL + H2O

37


Pentafluoroethane Dehydration with Ionic Liquids - Industrial

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