Screening of Pure ILs and DESs for CO 2 Separation, N 2 O Separation, and H 2 S Separation Processes

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Introduction
Every year, large amounts of gases are emitted into the atmosphere, with CO 2 being the primary greenhouse gas [1, 2], N 2 O having a potential impact on global warming that is 310 times greater than that of CO 2 [3], and H 2 S being one of the highest sulfur-containing compounds.Excess emissions of CO 2 and N 2 O result in global warming and climate change, while H 2 S emissions result in acid rain.Terefore, it is necessary to identify efective measures to mitigate the emission of CO 2 , N 2 O, and H 2 S from diferent sources.
Carbon capture and storage (CCS) can be used to reduce CO 2 emissions.Meanwhile, biomass syngas from biomass gasifcation is used in transportation as biofuels.CO 2 separation is required for CCS and biomass syngas purifcation.Te current CO 2 separation technologies have several shortcomings, such as high energy consumption [4], corrosion degradation, and/or large-scale operations, which afect removal efciency.
Te primary sources of anthropogenic N 2 O emissions are nylon production, nitric acid production, and vehicle exhaust emissions.Many N 2 O emission reduction technologies have been developed, such as thermal decomposition and catalytic decomposition.However, they may be constrained by the large energy consumption and increasing CO 2 emissions.
Tere are several technologies for H 2 S separation, such as absorption, oxidation, and adsorption [5].Absorption technology is extensively used for H 2 S separation, whereas aqueous alkanolamine solutions are currently employed in industrial natural gas treatment and sweetening plants [6].However, the application of H 2 S absorption technology is impacted, similar to CO 2 separation, by intensive energy consumption and/or large-scale operations.
Terefore, new gas separation processes for CO 2 , N 2 O, and H 2 S must be developed to curb climate change and protect the environment.Ionic liquids exhibit remarkable properties for gas separation, such as high solubility for CO 2 , N 2 O, and H 2 S.Although ILs have been used in studies related to gas separation [7][8][9][10], the large-scale commercial usage of ILs is limited by their high toxicity, poor biodegradability, and high cost.Deep eutectic solvents have low toxicity, biodegradability, and low cost and are proposed as promising CO 2 absorbents [11][12][13][14].
In our previous work, Gibbs free energy change (∆G) was employed to evaluate the performance of solvents [15,16] for separating CO 2 from biogas and NH 3 from synthetic ammonia purge gas [17].However, the efective absorbents for various gas streams with diferent conditions were not determined.In this study, thermodynamic analysis was utilized for gas separation using pure ILs and DESs.For CO 2 streams, efuent gases, kiln gas, and biomass syngas were chosen.For the H 2 S stream, high-sulfur natural gas was chosen.For the N 2 O stream, adipic acid of-gas was chosen.For various gas streams, several absorbents were screened as potential gas absorbents.Furthermore, the relationship between the performances, the properties, and the critical properties of gas streams was investigated.All of these are employed in the development of new gas separation technologies.

Gas Separation and Thermodynamic Model
2.1.Gas Streams.In this work, efuent gases were chosen as CO 2 streams due to their large emission amounts and low CO 2 concentration, whereas kiln gas was chosen due to its high CO 2 concentration.Biomass syngas is chosen as the CO 2 stream for CO 2 /CO/H 2 .Te high-sulfur natural gas is used as the H 2 S stream for H 2 S/CH 4 , and the adipic acid ofgas is used as the N 2 O stream for N 2 O/N 2 .Table 1 lists the typical conditions of diferent gas streams.

Gas Separation Process.
Te gas separation process using liquid absorbents is shown in Figure 1.Te absorbent quantity (m abs , g abs•g X −1 ) can be calculated using the following equation: where M abs and M x are the molecular weights of the absorbent and X gas, respectively, and g•mol −1 .x a and x s are the molar ratios of the X gas in the absorption tower and desorber, respectively.Te energy consumption required in the gas separation process can be calculated as shown in where n x denotes the molecular weight of the X gas in moles.H x denotes the Henry constant of X gas in the absorbents in bar units.P a and P 1 denote the pressure of the absorption tower and the initial pressure, respectively, in bar units.C p, abs represents the isobaric heat capacities of the absorbents in J•mol −1 •K −1 .T a and T s denote the temperatures of the absorption tower and desorber, respectively.

Teory.
In thermodynamics, ∆G was used as the evaluation criteria to determine if the isothermal reversible process was spontaneous.As shown in Figure 2, the separation process is nonspontaneous, and the Gibbs free energy change for System 1 is above zero (∆G 1 > 0).Six reversible processes were designed, and the summation is represented as ∆G 2 for Surrounding 2. System 3 is composed of System 1 and Surrounding 2.Moreover, the addition of energy and absorbents causes ∆G 2 to be negative.In Figure 2, ∆G comp represents the isothermal reversible compression process, ∆G abs represents the reversible absorption process of X absorption, ∆G T represents the reversible adiabatic expansion and reversible temperature increasing process of the solution, ∆G des represents the reversible X gas desorption process, ∆G exp represents the isothermal reversible expansion of the other gas, and ∆G T ' represents the reversible adiabatic expansion and the reversible isothermal compression of the X gas.Te optimal operating conditions are achieved when ∆G 3 is equal to 0, and the performance of diferent absorbents can be evaluated.Terefore, using the thermodynamic analysis as proposed in our previous work, the number of absorbents and the energy consumption are combined by Gibbs free energy change [12,13].
∆G for diferent systems and processes can be calculated using the equations given in equations ( 3)- (10).
where n is the number of moles in the gas stream, and y i represents the molar fraction of component i in the stream.

International Journal of Chemical Engineering
Table 1: Te typical condition of gas streams.

Condition
Biomass syngas [18] Efuent gases [19] Kiln gas [20,21] High-sulfur natural gas [22] Adipic acid of-gas [  International Journal of Chemical Engineering 3 where G g i (T, P) and G g i (T a , P a ) are the G values of the gaseous component i at (T, P) and (T a , P a ), respectively, and they are calculated using the formula G � H − TS.Te NIST standard reference database provides the values of H and S for the gas components.
where H x denotes Henry's constant and K x denotes the chemical reaction constant of X gas in absorbents.
where Δ f H j , 298.15 K represents the standard enthalpy change in the formation of j in the absorbents at 298.15 K; S j (298.15K) represents the standard molar entropy of j at 298.15 K; and C p,j represents the isobaric heat capacity of j in the absorbents.
where y x is the mole fraction of X gas in the gas stream, G g i (T a, y x P a ) is the G value of the gaseous component i at (T a , y x P a ).
where G g i (T s , P s ) is the G value of the gaseous component i at (T s , P s ).

The Properties of Absorbents
Te properties of liquid absorbents, including Henry's law constants of diferent gases in ILs/DESs, reaction equilibrium constants, the density of ILs/DESs at 298.15 K, and the heat capacity of the ILs/DESs, have been collected and listed in Table 2.In conventional ILs/DESs, the uncertainty in Henry's law constants of CO 2 , H 2 S, and N 2 O was estimated to be ±8 bar, ±0.4 bar, and ±0.07 bar, respectively.Henry's law constants and reaction equilibrium constants were correlated for some physical ILs and the chemical ILs/DESs as per the CO 2 solubility data described in our previous work [11].Te uncertainties in the densities and the heat capacities of the conventional ILs/DESs were ±63 J•mol −1 •K −1 and ±13 g•cm −3 , respectively, based on the experimental data presented in Table 2. Te names and molar weights of absorbents are listed in Appendix A (Supplementary data available here).

Results and Discussion
Te desorption temperatures of absorbents in this study are identical to those in our previous work [12], which are 299.15-323.15K for physical absorbents, 299.15-345.15K, and 299.15-353.15K for chemical DES and ILs, respectively.Te absorption pressure is iterated until ∆G 3 � 0, and the optimal conditions, absorbent amount, and energy consumption are obtained.As illustrated in Figure 3, the absorption pressures of diferent CO 2 absorbents in physical absorbents are greater than those of chemical ILs/DES for CO 2 streams.For the same IL [Bmim][BF 4 ], the absorption pressures for diferent gas streams are as follows: efuent gases (y CO 2 � 0.12) > kiln gas y CO 2 � 0.25) > adipic acid of-gas (y N 2 O � 0.45) > biomass syngas (y CO 2 � 0.3) > biogas (y CO 2 � 0.4) > high-sulfur natural gas (y H 2 S � 0.32) > synthetic ammonia purge gas (y NH 3 � 0.45).Te key factors infuencing the absorption pressure are gas concentration and gas solubility in absorbents.Te absorption pressures increase with increasing gas concentrations in the gas stream and decreasing gas solubilities.Meanwhile, the absorption pressure shows an increasing trend as gas solubility decreases.Due to the low solubility of N 2 O in [Bmim][BF 4 ], adipic acid of-gas exhibits a higher absorption pressure than biomass syngas.However, due to the high concentration of H 2 S, high-sulfur natural gas shows a lower absorption pressure than biogas.5 and 6 show the absorbent amounts.As the desorption temperature rises, the amount of absorbents decreases, especially at low desorption temperatures.

Amount of Absorbent. Figures
Te physical [Bmim][DCA] exhibits the lowest amount with values of 44.22 Due to diferences in CO 2 solubility, the amounts of CO 2 absorbents follow the order of chemical absorbents <physical absorbents.Generally, the solubility of CO 2 in chemical absorbents is greater than that in physical absorbents.For diferent CO 2 streams with the same absorbents, the quantity has the following order: high-sulfur natural gas (y H 2 S � 0.32) > synthetic ammonia purge gas (y NH 3 � 0.45) > efuent gases (y CO 2 � 0.12) > kiln gas (y CO 2 � 0.25) > biomass syngas (y H 2 S � 0.3) > biogas (y CO 2 � 0.4) > adipic acid of-gas (y H 2 S � 0.45).Te major reason for the signifcant amount of absorbents is the high gas solubility in highsulfur natural gas and synthetic ammonia purge gas.7 and 8 Due to the high heat of chemical absorbents, the energy consumption of chemical absorbents is larger than that of physical absorbents.Te CO 2 concentrations have an impact on the energy consumption for various CO 2 streams.Te energy consumption sequence is as follows: efuent gases (y CO 2 � 0.12) > synthetic ammonia purge gas (y NH 3 � 0.45) > kiln gas (y CO 2 � 0.25) > biomass syngas (y CO 2 � 0.3) > high-sulfur natural gas (y H 2 S � 0.32) > biogas (y CO 2 � 0.4) > adipic acid of-gas (y N 2 O � 0.45).Te energy consumption for separating CO 2 from efuent gases (CO 2 /N 2 ) is the largest due to the low CO 2 molar ratio, and the energy consumption for separating N 2 O from adipic acid of-gas is the lowest due to the highest N 2 O solubility and a high amount of absorbents.

Screening Absorbents.
Te screening criteria for absorbents include both absorbent amounts (m abs ) and energy consumption (Q tot ).Te physical [Bmim][DCA] was screened between 299.15-323.15K with low amounts of the absorbent (<190 g•gCO 2 −1 for efuent gases, <150 g•gCO 2 −1 for kiln gas, <140 g•gCO 2 −1 for biomass syngas) and low energy consumption (<2 GJ•tonCO 2 −1 for efuent gases, <1 GJ•tonCO 2 −1 for kiln gas, <0.9 GJ•tonCO 2 −1 for biomass syngas).Te chemical [Eeim][Ac] was screened with a lower amount of ILs (<43 g•gCO 2 −1 for efuent gases at 303.15-345.15K, <142 g•gCO 2 −1 for kiln gas at 299.15-345.15K, <68 g•gCO 2 −1 for biomass syngas at 300.15-345.15K).A lower amount of ILs (<293 g•gH 2 S −1 ) was used to screen [Omim][PF 6 ] for high-sulfur natural gas at 299.15-323.15K.A lower amounts of ILs (<108 g•gN 2 O −1 ) was used to screen [P 66614 ][FAP] for adipic acid of-gas at 299.15-323.15K. Te uncertainties of Henry's law constant, heat capacity, and density were taken into consideration while estimating the uncertainties in the iterated results.Te uncertainties in the absorption pressure, the required amount of ILs, and the energy expended for CO 2 separation were estimated to be ±3.97 bar, ±2.47 g•gCO 2 −1 , and ±0.09GJ•tonCO 2 −1 , respectively.Te uncertainties in the absorption pressure, the required amount of ILs, and the energy expended for H 2 S separation were estimated to be ±0.41 bar, ±0.37 g•gH 2 S −1 , and ±0.003 GJ•tonH 2 S −1 , respectively.Te uncertainties in the absorption pressure, the required amount of ILs, and the energy used for N 2 O separation were estimated to be ±0.12bar, ±0.17 ChCl/urea (1:2) ChCl/EG (1:2) ChCl/Gly (1:2) (c)               3.2 1.8     Based on the comparisons of the performances of physical absorbents for CO 2 separation shown in Figures 9  and 10, both energy consumption and the amount required for DEPG are lower than those of [Bmim][DCA] when P a is 12.5 bar and 25 bar, respectively.Tis suggests that the performance of the screened ILs is not superior to that of commercial physical absorbents for DEPG.Based on the comparison of the performances of chemical absorbents for CO 2 streams in Figure 11, [Eeim][Ac] displays a larger absorbent amount and lower energy consumption than those of 30 wt% MDEA and 30 wt% MEA.Assuming that the cost of [Eeim][Ac] is high, the screening of chemical ILs for CO 2 separation must be further researched.However, the volatility of [Eeim][Ac] can be ignored.
From the comparison of the performances of physical absorbents for the H 2 S stream in Figure 12, [Omim][BF 4 ] exhibited higher energy consumption and a lower amount of absorbents.Te comparison of [Omim][BF 4 ] and DEPG shows that the performance of physical IL is not superior to that of DEPG.Te screened ILs have the advantage of low energy consumption and nonvolatility, although they have a higher amount than chemical CO 2 absorbents (30 wt% MDEA and 30 wt% MEA).Te physical H 2 S absorbent consumes less energy than water.
Te cost of conventional ILs is signifcantly higher than that of commercial CO 2 absorbents when operational costs and solvent costs are considered.For instance, the estimated costs of water, DEPG, MEA, and MDEA were estimated to be 0.3     International Journal of Chemical Engineering

Conclusions
In this study, green absorbents were analyzed based on ∆G to screen the absorbents for CO 2 separation from efuent gases, kiln gas, and biomass syngas; H 2 S separation from high-sulfur natural gas (H 2 S/CH 4 ); and N 2 O separation from adipic acid of-gas (N 2 O/N 2 ).[Bmim][DCA] and [Eeim][Ac] were screened for CO 2 separation using m abs and Q tot as criteria.[Omim][PF 6 ] and [P 66614 ][FAP] were screened for H 2 S separation and N 2 O separation, respectively.According to comparisons between the screened absorbents and the DEPG, the physical IL has a higher energy consumption and higher m abs than the DEPG.When compared with 30 wt% MDEA and 30 wt% MEA, the chemical IL has a lower Q tot and is involatile.Lower energy consumption and a higher amount of H 2 S separation were observed when screened [Omim][BF 4 ] was compared to H 2 O.
Due to the diferences in gas concentration in gas streams and gas solubility in absorbents, the amount of absorbents has the following order: high-sulfur natural gas > synthetic ammonia purge gas > efuent gases > kiln gas > biomass syngas > biogas > adipic acid of-gas.Te order of energy consumption is as follows: efuent gases > synthetic ammonia purge gas > kiln gas > biomass syngas > high-sulfur natural gas > biogas > adipic acid of-gas.

Figure 2 :
Figure 2: Te thermodynamic analysis of the coupling process of gas separation with absorbents.

Figure 4 :
Figure 4: Te absorption pressures of (a) 10 physical absorbents for high-sulfur natural gas and (b) 8 physical absorbents for adipic acid of-gas.

Figure 6 :
Figure 6: Te absorbent amount of (a) 10 physical absorbents for high-sulfur natural gas and (b) 8 physical absorbents for adipic acid of-gas.

Figure 8 :
Figure 8: Te energy consumption of (a) 10 physical absorbents for high-sulfur natural gas and (b) 8 physical absorbents for adipic acid ofgas.

Figure 9 :
Figure 9: Te comparison of the performances of physical absorbents with P a at 12.5 bar for CO 2 streams.

Figure 10 :
Figure 10: Te comparison of the performances of physical absorbents with P a at 25 bar for CO 2 streams.

Figure 12 :Figure 11 :
Figure 12: Te comparison of the performances of physical absorbents for the H 2 S stream.

Table 2 :
Henry's constant of X gas (H x ) in physical absorbents, the density at 298.15 K (ρ 298.15 ), and the isobaric heat capacity (C p ) of CO 2 absorbents.P a within 299.15-323.15K with values of 4.40-22.25 bar among the eight physical ILs that were analyzed for H 2 S/CH 4 separation. lowest -11.15 g•gCO 2 ] exhibited the lowest amount with values of 104.59-6.71g•gN 2 O −1 and 4.72-14.74g•gH 2 S −1 for adipic acid of-gas and high-sulfur natural gas, respectively, within the range of 300.15-323.15K.
Table3lists the screened absorbents and their values of T s , P a , m abs , and Q tot .Omim][PF 6 ] is compared with water.Te fndings of P a , m abs , and Q tot of commercial absorbents for gas separation from efuent gases, kiln gas and biomass syngas, biogas, high-sulfur natural gas, and adipic acid ofgas are displayed in Table4with the set desorption temperature.Te results of m abs and Q tot for[Bmim][DCA] and DEPG are displayed in Figures9 and 10.Figure11displays the predicted results of m abs and Q tot for chemical[Eeim][Ac] at 338.15 K, 30 wt% MEA at 393.15 K, and 30 wt% MDEA at 393.15 K.

Table 3 :
Te optimal condition (T s , P a ), the absorbents amounts (m abs ), and the energy consumption (Q tot ) of the screened absorbents.Furthermore, with technological advancement, the cost of ILs will drop, and the cost diference between conventional ILs and commercial absorbents will reduce.