Molecular Dynamics Investigation of Triethylene Glycol in Hydrated LTA Zeolite

Transcription

1 U N I V E R S I T E T E T I B E R G E N Institute of Physics and Technology Molecular Dynamics Investigation of Triethylene Glycol in Hydrated LTA Zeolite Emphasis on Evaluation of Potential Models Bjørnar Jensen a, Tatiana Kuznetsova a, Bjørn Kvamme a, Even Solbraa b, Andrea Carolina Machado Miguens b a Department of Physics and Technology, University of Bergen, Allégaten 55, 5007 Bergen, Norway b STATOIL, Arkitekt Ebbels vei 10, 7053 Ranheim, Norway

2 Agenda Background Force Fields Bulk Zeolite Interfacial Systems Conclusions

3 Background: the application Institute of Physics and Technology Water accompanies natural gas from reservoirs Produced gas must be dehydrated to meet product specifications Water drop-out can lead to possible clathrate formation Currently carried out by means of molecular sieves, utilizing LTA zeolites Reservoir Molecular Sieves General Processing Pipeline Customer

4 Operational challenges related to glycol and molsieve dehydration processes Molecular sieves are designed to have lifetime of 5 years, but experience show that they have to be replaced every 2-3 year Frequent replacement of adsorbents is extremely costly due to interrupted operations Analysis of used adsorbents indicate high amounts of liquid and thermally degraded triethylene glycol, as well as traces of other production chemicals

5 Background: the problems Institute of Physics and Technology Adsorbent not performing according to specifications Adsorbent poisoning Lower efficiencies Increased downtime and costs We lack of knowledge of adsorbate/adsorbent interactions

6 Zeolite LTA-4Å Unit cell: a = b = c = Å α = β = γ = 90 Chemical Formula: Na [Al 12 Si 12 O 48 ] 8 Legend: Blue: Sodium Lime: Silicon Red: Oxygen White: Aluminum

7 Atomistic-level simulations: the force fields Csányi et al [1] LJ 12-6 Moderate charges Faux et al [2] Hybrid Significant charges Jackson and Catlow [3] Buckingham Significant charges ALL forcefield developed and so far used for bulk zeolites Water-Water: SPC/E [4] Water-Zeolite: Leherte et al [5] Methane: OPLS-UA [6] 1. Csányi et al., J. Phys. Chem. C, 2009, 113, Faux et al., J. Phys. Chem. B, 1997, 101, R. A. Jackson & C. R. A. Catlow, Mol. Simul., 1988, 1:4, Berendsen et al., J. Phys. Chem., 1987, 91, Leherte et al., J. Mol. Catal. 1989, 54, Jorgensen et al., J. Am. Chem. Soc., 1984, 106, 22,

8 Counter-Ions in zeolite: the RDFs Institute of Physics and Technology Sodium versus oxygen T = 298 K, t = 0.5 fs

9 Counter-Ions in zeolite: the heat maps Blue heat-map: Least counter-ion movement Red heat-map: Most counter-ion movement Green heat-map: Medium counter-ion movement (x,y)-coordinates of sodium plotted every 0.5ps for 1000 frames

10 Institute of Physics and Technology Water in zeolite: the heat maps Blue heat map: vacant channels - reduced water movement Red heat map: occupied channels - increased water movement Green heat map: medium water movement (x,y)-coordinates of water plotted every 0.5ps for 1000 frames

11 Water Diffusion in LTA-4Å: mean square displacement T = 298 K, t = 0.5 fs Frames 0.5 ps

12 Fitting to Einstein s diffusion equation: Faux et al.

13 Fitting to Einstein s diffusion equation: J&C SM1

14 Fitting to Einstein s diffusion equation: J&C SM2

15 Summary Force Fields The original Faux et al. used too long time steps Improved results obtained with shorter time steps J&C SM2 has erratic MSD, larger diffusivity coefficient and std. dev. Faux et al. and J&C SM1 show similar performance. Faux et al. preferred due to availability of LJ interactions and less extreme charges

16 Initial flooding of LTA Zeolite with methane at 503 K Institute of Physics and Technology Legend: Brown: Potassium Cyan: Methane Windows blocked by fixed potassium counter-ions Lime: Silicon Red: Oxygen White: Aluminum Potassium released and moved away 150 frames immediately upon release of counter-ions. methane enters the zeolite Potassium (brown) drawn every 10 fs for 150 frames, 1.5 ps.

17 Methane flooding in the absence of outer counter-ions Legend: Brown: Potassium Cyan: Methane Lime: Silicon Methane rapidly enters when counter-ions moves into bulk zeolite Increased counter-ion concentration blocks inner cavities Methane unable to penetrate further than outermost cavities Red: Oxygen White: Aluminum

18 Methane at 503 K Without Counter-Ions Multiple frames of methane plotted every 50 fs for 250 ps Higher loading in outer cavities Methane jumps from one cavity to another Greater tendency to diffuse between outermost cavities

19 Triethylene Glycol Force field: OPLS-AA, Jorgensen et al, J. Am. Chem. Soc., 1996, 118, Lorentz-Berthelot mixing rules for cross-interactions Initial system configuration. Fixed TEG and zeolite. Methane, water and potassium allowed to move.

20 Interfacial Zeolite: improved electrostatic potential needed TEG in cavity after initially being ejected Immense electrostatic interactions stretches bonds in polar molecules beyond all reasonable expectations Imbalanced electrostatic potential in interface Calls for more moderate charges to describe zeolite in interfacial systems

21 Conclusions and suggestions for further work Faux et al. performs well as a bulk force field Currently not feasible modeling interfacial systems with these force fields Interfacial systems need tailored force fields Electrostatic potential must be improved Estimate new partial charges from ab initio Investigate the possibility of a pure LJ all atom force field for zeolite