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Molecular Modeling of Wax Inhibition

Molecular Modeling of Wax Inhibition. Yun Hee Jang , Mario Blanco, William A. Goddard, III MSC, Beckman Institute, Caltech Augustin J. Colussi, Michael R. Hoffmann Department of Chemistry and Chemical Engineering, Caltech Yongchun Tang, Bob Carlson, Huey-jyh Chen, Jefferson Creek

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Molecular Modeling of Wax Inhibition

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  1. Molecular Modeling of Wax Inhibition Yun Hee Jang, Mario Blanco, William A. Goddard, III MSC, Beckman Institute, Caltech Augustin J. Colussi, Michael R. Hoffmann Department of Chemistry and Chemical Engineering, Caltech Yongchun Tang, Bob Carlson, Huey-jyh Chen, Jefferson Creek Chevron Petroleum Technology Co.

  2. Wax problem Wax: Aggregates of heavy n-alkanes at low temperature  pipe blocking cold sea water cold sea water hot oil wax oil production pipe wall Wax inhibitor (comb-like polymer): No established mechanism of action. cold sea water cold sea water Comb-like wax inhibitor

  3. Proposed Mechanisms Wax Formation Wax Inhibition Liquid  Amorphous solid  Ordered crystal  Further growth  Adsorption on pipe (1) Sequestering mechanism long alkanes in oil selectively partition toward the inhibitors making them less available to nucleate a wax crystal (2) Incorporation-perturbation mechanism inhibitors partition from the oil into amorphous wax ("soft wax") slowing down the crystallization of soft wax to form "hard wax” (3) Wax crystal adsorption mechanism adsorption of inhibitors on initial wax nuclei or growing wax crystals inhibits further wax growth (4) Pipeline adsorption mechanism adsorption of inhibitors on the pipe wall provides an irregular surface that interferes with adsorption of wax to form crystals Objective of this work:Establish mechanism by investigating each of them

  4. Force field used in MD simulation Hydrocarbons and long alkyl sidechains United atom model (SKS) (Siepmann, Karaborni and Smit, Nature, 365, 330 (1993)) Stretching from AMBER with r0=1.54 Å from SKS Acrylate backbones (around -COO-) VdW: OPLS (Briggs, Nguyen and Jorgensen, J. Phys. Chem. 95, 315 (1991)) Charge: HF/6-31G** calculation Torsion: fitted to HF/6-31G** torsion energy curve for model systems Stretching/bending/inversion: AMBER (r0,0 fromOPLS) Styrene backbones (around phenyl ring) DREIDING (Mayo, Olafson and Goddard, J. Phys. Chem.94, 8897 (1990)) Torsion: checked to reproduce ab initio torsion potential for model system (G. Gao)

  5. Model inhibitors PAA1 (C18) good PAA2 (C18/C1) good PAA3 (C22) poor PAS2 (C18/C1) very poor The same MW The same side chain distribution

  6. Model oil / wax n-heptane (n-C7) (m.p.183 K; b.p. 372 K) n-C31 or n-C32 (amorphous; m.p.~340 K) n-dotriacontane(n-C32) (crystalline) • Calc. • Average from 200-600 ps • NPT dynamics • error from std. dev. of • block averages • Expt’l • J. Chem. Eng. Data • 9, 231 (1994) • CRC handbook of • chemistry and physics

  7. Why n-C31/32 as model wax?

  8. Sequestering long alkanes in oil selectively partition toward the inhibitors making them less available to nucleate a wax crystal MD simulations started at various positions of n-C32 w.r.t. PAA1 in n-C7 bath Unsequestered wax at 293 K <PE> = -741  5* kcal/mol (100-200 ps) *Error estimated by the standard deviation between four 25-ps block average Sequestered wax at 293 K <PE> = -739  12* kcal/mol (100~200 ps) No energy gain after sequestering Close contact

  9. Incorporation-Perturbation Crystallization 1 DCED = +318% Very favorable 4. Crystalline pure n-C32 1. Amorphous pure n-C32 + Additive Incorporation - Additive Segregation DE << 0 DCED = -17% DE << 0 DCED = 55% Crystallization 2 DCED = +80% Less favorable than above 2. Amorphous n-C32 with additive 3. Crystalline n-C32 with additive (1  2  3  4) is slower than (1  4). (Crystallization is delayed with additive.)

  10. Incorporation before after PAA1 in n-C7 pure n-C7 (E1) (E3) pure n-C31 PAA1 in n-C31 (E2) (E4) E(incorporation) = Eafter-Ebefore = (E3+E4)-(E1+E2) = (E4-E2)-(E1-E3) = Eint(C31)-Eint(C7)

  11. Incorporation energetics *Interaction energy between inhibitor with oil/wax *averaged over 200~600 ps of MD simulations *normalized by average contact area *error estimated from duplicate runs for each system No correlation or reverse correlation to expectation

  12. Wax conformation change? Counted each 1ps *average over 55 n-C31’s of standard deviation of end-to-end distance along time 200-600 ps MD Incorporated inhibitors disturb conformation relaxation of wax for crystallization? No

  13. Adsorption on hydrophilic pipewall? based on the difference in efficiency between hydrophilic PAA and hydrophobic PAS based on the efficiency increase when inhibitor is added initially Preliminary study: adsorption of inhibitor on a-Fe2O3, a model of pipewall From 40~120 ps MD at solid(fix)-vacuum interface a-Fe2O3 force field S. Jiang, et al. J. Phys. Chem.100, 15760 (1996)

  14. Summary Sequestering mechanism? No. No energy difference between sequestered and unsequestered state There is no preference for wax molecules to be sequestered by inhibitor. Incorporation-Perturbation mechanism? No. It cannot explain the difference in efficiency between PAA and PAS. Adsorption of inhibitor on hydrophilic surface (e.g. a-Fe2O3) It looks good so far, but it needs more work. Acknowledgement Larry Smarr (U. Illinois) for supercomputer allocation at NCSA Yanhua Zhou for a-Fe2O3 structure and force field

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