Absorption refrigeration cycles based on ionic liquids: Refrigerant/absorbent selection by thermodynamic and process analysis

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Figure 1. Flowsheet of a single effect absorption refrigeration cycle.

Figure 2. σ-Profile (A) and σ-potential (B) of the refrigerants used in this study.

-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

µ (kcal/mol Å2)

σ (e/Å²)

WATER AMMONIA METHANOL TFE HFC-134a HFC-125 HFO-1234(ze) Pentane

-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03

0 5 10 15 20 25 30

p (σ)

σ (e/Å²)

(B)

(A)

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Figure 3. σ-Profile of cations (A) and anions (B), σ-potential of cations (C) and anions (D) of a representative group of ILs.

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-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03

0 5 10 15 20 25 30

p (σ)

σ (e/Å²)

[EtOHmim]⁺ [1b3mpy]⁺ [P2224]⁺ [emim]⁺ [omim]⁺

-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03

0 5 10 15 20 25 30

p (σ)

σ (e/Å²)

[BF₄]^- [Cl]^- [MeCOO]^- [MeSO

4]^- [NTf2]^-

-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03

-3 -2 -1 0 1

µ (kcal/mol Å2 )

σ (e/Å²)

[BF₄]^- [Cl]^- [MeCOO]^- [MeSO₄]^- [NTf2]^-

-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03

-1.0 -0.5 0.0 0.5 1.0 1.5

µ (kcal/mol Å2)

σ (e/Å²)

[EtOHmim]⁺ [1b3mpy]⁺ [P2224]⁺ [emim]⁺ [omim]⁺

(B) (A)

(C)

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Figure 4. Screening of Henry’s law constants of water (A), ammonia (B), R-134a (C) and n- pentane (D) refrigerants at 298K in 900 ILs calculated by COSMO-RS using [C+A] molecular model.

(D) (B)

(C) (A)

[Cl] [MeCOO] [EtCOO] [Br] [(Me)2PO4] [(Et)2PO4] [MeSO3] [gly] [TMPPh] [(Bu)2PO4] [lac] [I] [NO3] [MeSO4] [EtSO4] [TFA] [BuSO4] [OcSO4] [SCN] [DCN] [NfO] [TfO] [BF4] [TCM] [BETI] [NTf2] [B(CN)4] [FEP] [PF6] [AlCl4]

[N4444]

[P4444]

[N8884]

[P2224]

[P666,14][1b3mpy][dcbmim][N444H][mmpyr][P6661][ompyr][empyr][omim][S444][eeim]

[hxmim][bmim]

[(EtO)emim][OcOHmim][1b3NH2py][EtF3mim][1b3Clpy][mmim][emim][mpy][bpy]

[EtOHmim][choline][mim]

[N11H(EtOH)]

1.0

2.5

1.0

0.5

Anion

Cation

Solute: R-134a

0.25

[FEP] [BETI] [TMPPh] [AlCl4] [NTf2] [B(CN)4] [NfO] [OcSO4] [TCM] [(Bu)2PO4] [PF6] [TfO] [BuSO4] [TFA] [DCN] [(Et)2PO4] [EtCOO] [lac] [SCN] [BF4] [EtSO4] [(Me)2PO4] [MeCOO] [gly] [MeSO4] [NO3] [I] [MeSO3] [Br] [Cl]

[P666,14][dcbmim][N444H][N8884][P6661][P4444][N4444][P2224][ompyr][omim][S444]

[hxmim]

[OcOHmim][1b3mpy][1b3Clpy][bmim][bpy]

[(EtO)emim][1b3NH2py][empyr][emim][eeim]

[mmpyr]

[EtF3mim][mmim]

[EtOHmim][choline][mim][mpy]

[N11H(EtOH)]

10.0

5.0 2.5

1.5 1.0

Anion

Cation

Solute: n-pentane

0.5 [EtCOO] [MeCOO] [TMPPh] [gly] [lac] [(Et)2PO4] [(Me)2PO4] [(Bu)2PO4] [Cl] [MeSO3] [TFA] [Br] [NO3] [DCN] [SCN] [EtSO4] [MeSO4] [BuSO4] [OcSO4] [I] [TfO] [TCM] [NfO] [BF4] [B(CN)4] [NTf2] [BETI] [PF6] [AlCl4] [FEP]

[P2224]

[N4444]

[P4444]

[N8884][empyr]

[P6661]

[P666,14][1b3mpy][mmpyr][ompyr][mmim][S444][emim][eeim]

[(EtO)emim][OcOHmim][1b3NH2py][EtOHmim][EtF3mim][1b3Clpy][dcbmim][N444H][choline][hxmim][bmim][omim][mpy][mim][bpy]

[N11H(EtOH)]

0.05 0.03

0.01 0.005

0.0025

Anion

Cation

Solute: Water

0.0012

[AlCl4] [FEP] [PF6] [BETI] [NTf2] [B(CN)4] [NfO] [TfO] [TCM] [BF4] [OcSO4] [BuSO4] [EtSO4] [MeSO4] [SCN] [DCN] [I] [NO3] [Br] [TFA] [(Bu)2PO4] [MeSO3] [lac] [(Et)2PO4] [(Me)2PO4] [gly] [Cl] [TMPPh] [EtCOO] [MeCOO]

[N11H(EtOH)][EtOHmim][choline][N444H]

[OcOHmim][mim]

[1b3NH2py][EtF3mim][1b3Clpy][dcbmim][hxmim][mmim][omim][bmim][emim][eeim]

[(EtO)emim][P666,14][1b3mpy][mmpyr][P2224][P6661][N4444][P4444][N8884][ompyr][empyr][S444][mpy][bpy]

0.8 1.0

0.5

0.25

0.1

Anion

Cation

Solute: NH₃

0.05

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Figure 5. Excess enthalpy (h^E) and Gibbs free energy (g^E) of selected refrigerant-IL mixtures in terms of intermolecular interaction contributions.

Figure 6. Comparison of experimental VLE data to calculated values using COSMO- based/Aspen HYSYS methodology for (A) H2O-IL and (B) methanol-IL pairs.

-4 -3 -2 -1 0 1

E x c es s ener gy ( k c al /m ol )

hÊ (vDW) hÊ (HB) hÊ (MF) gÊ

TFE+[mmim][Cl] AMMONIA+

[(EtOH)mi m][BF4]

HFC-1

34a+[P222 4][Cl] WATER+

[mmim][Cl] HFC-1

25+[mmim][Cl] METHANOL+

[mmim][Cl] HFO-123

4ze+[mmim][Cl] Pentane+[P66

6,14][FEP]

(A) (B)

0 20 40 60 80 100

Calculated vapor pressure (kPa)

Experimental vapor pressure (kPa)

R²=0.982 Slope:0.997

0 20 40 60 80 100 120 140 160 180 200

Calculated vapor pressure (kPa)

Experimental vapor pressure (kPa)

R²=0.974 Slope:0.981

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Figure 7. Comparison of COP and f ratio calculated using COSMO-based/Aspen HYSYS methodology with the data calculated by other authors. Data collected in in Table S4 of Supplementary Material.

Figure 8. COP calculated for each refrigerant/IL pair versus the saturation composition (mass fraction) in the absorber, calculated for the base case operating conditions of absorption refrigeration cycle. Filled symbols correspond to proposed refrigerant-IL pairs in this work.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

C OP

Refrigerant mass fraction in absorber

(A) (B)

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Figure 9. COP calculated for each refrigerant/IL pair versus the mass cooling capacity (MCC) calculated for the case base operating conditions of absorption refrigeration cycle. Filled symbols correspond to selected refrigerant-IL pairs in this work.

0 500 1000 1500 2000 2500

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

C OP

Mass Cooling Capacity (kJ/kg)

WATER AMMONIA METHANOL TFE HFC-134a HFC-125 HFO-1234(ze) Pentane COP_MAX

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Figure 10. Refrigerant+IL mass flow in the pump feed vs f ratio for each refrigerant/IL pair calculated for the case base operating conditions of absorption refrigeration cycle. Filled symbols correspond to selected refrigerant-IL pairs in this work.

0 20 40 60 80 100

0 100 200 300 400 500

Tot al m as s f low pum ped per ev apor at or pow er ( k g/ K W

E

)

f ratio

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Figure 11. Refrigerant+IL mass flow in the pump feed vs f ratio for each refrigerant/IL pair compared to the MCC of the refrigerant, calculated for the case base operating conditions of absorption refrigeration cycle. Filled symbols correspond to selected refrigerant-IL pairs in this work

0 500 1000 1500 2000 2500

0 20 40 60 80 100

Tot al m as s f low pum ped per ev apor at or pow er ( k g/ k W

E

)

Mass Cooling Capacity (kJ/kg)

Minimal mass flow

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Figure 12. Results of the main efficiency parameters of absorption refrigeration cycle for each pair refrigerant-IL selected attending highest absorption capacity. Traditional pairs as H2O/LiBr [16] and NH3/H2O [22] are included to compare.

WATER+[mmi m][Cl]

WATER+LiBr MET

HAN

OL+[mmi m][Cl]

TFE+[mmi m][Cl]

AMMO

NIA+WATER

AMMO

NIA+[(EtOH)mim][BF4]

HFO-1234ze+[mmi m][Cl]

HFC-134a+

[P2224]

[Cl]

Pent ane+

[P666, 14][FEP]

HFC-125+[mmim][Cl]

0 10 20 30 40 50 60 70 80 200 225

Total mass flow pumped per evaporator power (kg/kW E)

Absorbent mass flow Refrigerant mass flow COP

0.0 0.2 0.4 0.6 0.8 1.0

COP

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Figure 13. Calculated COP vs generator temperature for the selected refrigerant-IL systems.

Figure 14. f ratio (A) and required total mass flow rate in the absorber (B) vs generator temperature

60 75 90 105 120 135 150

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

HFC-134a HFC-125 HFO-1234(ze) Pentane

C OP

Generator temperature (ºC)

WATER AMMONIA METHANOL TFE

60 75 90 105 120 135 150

0 50 100 150 200

Total nass flow pumped per evaporator power (kg/kWE)

Generator temperature (ºC)

60 75 90 105 120 135 150

0 5 10 15 20

f ratio

Generator temperature (ºC)

(A) (B)

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Figure 15. Calculated COP (A) and absorber pressure vs evaporator temperature (B).

Figure 16. Calculated f ratio (A) and total mass flow (B) in the absorber vs evaporator temperature.

(B) (A)

-20 -15 -10 -5 0 5 10 15

0.0 0.2 0.4 0.6 0.8 1.0

COP

Evaporator temperature (ºC)

-20 -15 -10 -5 0 5 10 15

1 10 100 1000

Absorber pressure (kPa)

-20 -15 -10 -5 0 5 10 15

0 10 20 30 40 50

f ratio

-20 -15 -10 -5 0 5 10 15

0 200 400 600 800

Total mass flow pumped per evaporator power (kg/kWE)