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.

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

Wax Inhibition Wax Formation 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

Hydrocarbons and long alkyl sidechains United atom model (SKS) (Siepmann, Karaborni and Smit, Nature, 365, 330 (1993)) Stretching from AMBER with r 0 =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 (r 0,  0 from OPLS) 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)

PAA1 (C 18 ) good PAA2 (C 18 /C 1 ) good PAA3 (C 22 ) poor PAS2 (C 18 /C 1 ) very poor The same side chain distribution The same MW

n-heptane (n-C 7 ) (m.p.183 K; b.p. 372 K) n-C 31 or n-C 32 (amorphous; m.p.~340 K) n-dotriacontane (n-C 32 ) (crystalline) Calc. Average from 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

MD simulations started at various positions of n-C 32 w.r.t. PAA1 in n-C 7 bath Unsequestered wax at 293 K = -741  5* kcal/mol ( ps) Sequestered wax at 293 K = -739  12* kcal/mol (100~200 ps) long alkanes in oil selectively partition toward the inhibitors making them less available to nucleate a wax crystal *Error estimated by the standard deviation between four 25-ps block average No energy gain after sequestering Close contact

 E << 0  CED =  17%  CED =  318% Very favorable + Additive Incorporation Crystallization 1 1. Amorphous pure n-C Amorphous n-C 32 with additive 4. Crystalline pure n-C Crystalline n-C 32 with additive  E << 0  CED = 55%  Additive Segregation  CED = +80% Less favorable than above Crystallization 2 (1  2  3  4) is slower than (1  4). (Crystallization is delayed with additive.)

(E 1 ) beforeafter E(incorporation) = E after  E before = (E 3 +E 4 )  (E 1 +E 2 ) = (E 4  E 2 )  (E 1  E 3 ) = E int (C 31 )  E int (C 7 ) PAA1 in n-C 7 (E 2 ) pure n-C 31 (E 3 ) pure n-C 7 (E 4 ) PAA1 in n-C 31

*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

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

Preliminary study: adsorption of inhibitor on  -Fe 2 O 3, a model of pipewall based on the difference in efficiency between hydrophilic PAA and hydrophobic PAS based on the efficiency increase when inhibitor is added initially From 40~120 ps MD at solid(fix)-vacuum interface  -Fe 2 O 3 force field S. Jiang, et al. J. Phys. Chem. 100, (1996)

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.  -Fe 2 O 3 ) It looks good so far, but it needs more work. Larry Smarr (U. Illinois) for supercomputer allocation at NCSA Yanhua Zhou for  Fe 2 O 3 structure and force field