Molecular Simulation of NH3/Ionic Liquid Mixtures for Absorption Heat

Pump CyclesAbhishek KABRA, Tim M. BECKER, Meng WANG*, Carlos A. INFANTE FERREIRA, Thijs J.H. VLUGT

Process and Energy Department, TU Delft

Content

● Introduction● The Absorption Heat Pump Cycle● Property predictions● Results and discussions● Conclusions

May 15 -18, 2017 2HPC 2017

Content

● Introduction● The Absorption Heat Pump Cycle● Property predictions● Results and discussions● Conclusions

May 15 -18, 2017 3HPC 2017

21%

24%3% 18%

24%

10%

Industry Heating Buildings HeatingOther Heat ElectricityTransport Non-Energy Use ● highly efficient space heating & cooling

● utilization of low-grade energy: waste heat, concentrated solar heat

BackgroundWorld Total Final Energy Consumption (IEA)

Absorption heat pump (AHP)

4

Low temp. heat from surroundings

Air Earth Water

High temp. driving heat

AHP

1 J

1 J 2 J

Mid temp. heat application

May 15 -18, 2017 HPC 2017

Motivation

5

Working pairs

Crystallization (H2O/Libr)

Additional process and equipment (NH3/H2O)

Ionic liquid (IL)

• High boiling point• high chemical and thermal stability• Good affinity with refrigerants• Adjustable design of anion and

cationStre

ngth

s

May 15 -18, 2017 HPC 2017

Roles of Molecular Simulation

● To screen the optimum ionic liquid for absorption cycles?

● To determine the performance of the cycle..

May 15 -18, 2017 HPC 2017 6

Content

● Introduction● The Absorption Heat Pump (AHP) Cycle● Property predictions● Results and discussions● Conclusions

7May 15 -18, 2017 HPC 2017

AHP with IL

8

Single-effect AHP

• In steady state.• The solution is at equilibrium leaving GEN, 5 K subcooling leaving ABS,• The refrigerant in outlets of CON or EVA is saturated.• 5 K for the pinch temp. difference in SHX.• Heat losses, pressure losses and pumping work are neglected.A

ssum

ptio

ns

𝐶𝑂𝑃 =𝑄&'(𝑄)*+

𝑓 =𝑚.̇𝑚0̇

=1 − 𝑤4𝑤5 − 𝑤4

CON

EVA2

3

5

6

4 7

Heating Water

Hot Water (Driving Heat)

ABS

SHX

GEN

1

8

Heating Water

Chilled Water

9

10

May 15 -18, 2017 HPC 2017

Content

● Introduction● The Absorption Heat Pump (AHP) Cycle● Property predictions● Results and discussions● Conclusions

9May 15 -18, 2017 HPC 2017

Necessary properties

● Solubility (VLE) --à● Heat capacity● Heat of absorption

• Coefficient of performance• Circulation ratio

• Test cases: [emim][TF2N] & [emim][SCN] with NH3

P-T-x

enthalpies

May 15 -18, 2017 HPC 2017 10

Solubility (VLE)

● Solubility (VLE) à● Monte Carlo(MC) simulations in the osmotic ensemble are

performed to compute the solubility of NH3 in IL

May 15 -18, 2017 HPC 2017 11

Heat capacities of ILs

Cp (ILs) = Cpig + Cpres

↑ ↑• QM calculations • MC in NPT ensemble

May 15 -18, 2017 HPC 2017 12

Enthalpy of absorption

↑ ↑ ↑• MC in osmotic ensemble • NIST • MC in NPT ensemble

])1([ ILNHNHNHsolabs 333 hxhxhh -+-=D

May 15 -18, 2017 HPC 2017 13

resres res

Content

● Introduction● The Absorption Heat Pump (AHP) Cycle● Property predictions● Results and discussions● Conclusions

14May 15 -18, 2017 HPC 2017

Solubilities

(a) (b)

Computed NH3 solubilities (blue/green/cyan/magenta) in (a) [emim][TF2N] and(b) [emim][SCN], compared to solubilities calculated with the experimentaldata (open) and correlated (black), and simulation results of Shi and Maginn(2009) (red) at 308.15 K, 347.15 K, 373.15 K, and 393.15 K.

May 15 -18, 2017 HPC 2017 15

Heat capacities

Comparison between computed total heat capacities (blue), computational results ofMaginn et al. (2014), and experimental measurements of Ge et al. (2008) (green),Paulechka et al. (2007) (cyan), Ferreira et al. (2012) (magenta), Navarro et al. (2013)(purple), and Ficke et al. (2010) (orange) for (a) [emim][TF2N] and (b) [emim][SCN].

(a) [emim][TF2N] (b) [emim][SCN]

May 15 -18, 2017 HPC 2017 16

Enthalpies of Absorption

● Enthalpies of absorption at different cycle conditions

May 15 -18, 2017 HPC 2017 17

Performance prediction

• Prediction is in principle possible, but not precise

May 15 -18, 2017 HPC 2017 18

Content

● Introduction● The Absorption Heat Pump Cycle● Property predictions● Results and discussions● Conclusions

19May 15 -18, 2017 HPC 2017

Conclusion & Outlook

● It is possible to use MS to screen and assess NH3/IL in AHP, but better force fields are required!

● Might be still useful in the complete absence of experimental data.

● Has potential to extend the experimental range and to predict mixture properties.

May 15 -18, 2017 HPC 2017 20

Question?Meng WANG*, Carlos A. INFANTE FERREIRA

M.Wang-2@tudelft.nlProcess and Energy Department,

Delft University of Technology

Backup slides

22

● Simulation details» TraPPe force field for NH3

» ILs force fields are taken from Liu et al. (Ind. Eng. Chem. Res., 51, 7242-7254, 2012.) The anion

and the alkyl part of the cation are considered flexible while the ring of the cation is considered rigid.

» Lennard-Jones interactions are truncated and shifted at 12Å.

» Electrostatic interactions are considered via the Ewald summation technique with a relative precision

of 1074.

» MC simulations are performed using RASPA.

» The solubility is computed by conducting MC simulations in the osmotic ensemble.

May 15 -18, 2017 HPC 2017

Backup slides

23

● Simulation details» Pure component enthalpies are calculated in the NPT ensemble.

» The fluctuation formula in the NPT ensemble is applied to compute the residual part of the heat

capacity.

» The ideal part of the heat capacity is calculated via QM.

» QM calculations of the isolated ions are performed using Gaussian.

» The B3LYP functional with a 6-31+G(2df,p) basis set is used.

» A scaling factor of 0.965 is applied to the vibrational frequencies.

May 15 -18, 2017 HPC 2017

Backup slides

24

● Enthalpies of solutions» Saturated solution (T, P, wNH3)

– for IL

May 15 -18, 2017

( ) ( ) ( ) ( )3 3 3 3, , = , ,NH NH NH IL IL mix NHh T P w w h T w h T h T P w+ +D( ) ( )

00 0

T ILIL pTh T h T C dT= + ò

HPC 2017

Δhabs