Molecular Modeling of Wax Inhibition

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Yun Hee Jang, Mario Blanco and William A. Goddard, III*

Materials and Process Simulation Center, Beckman Institute (139-74)
California Institute of Technology, Pasadena, California 91125

Augustin J. Colussi, Michael R. Hoffmann

Department of Chemistry and Chemical Engineering
California Institute of Technology, Pasadena, California 91125

Yongchun Tang, Bob Carlson, Heuy-jyh Chen and Jefferson Creek

Chevron Petroleum Technology Company
1300 Beach Boulevard, La Habra, California 90631


A serious problem in oil production, particularly from deep wells, is deposition of wax on the pipe walls. This occurs when minor, high melting point components [high molecular-weight alkanes (paraffins)] in the crude oil contact the cold pipe walls. This wax deposit causes flow problems in oil production and transportation. Several comb-like polymers having long alkyl sidechains are known to decrease the rate of wax formation, but none are sufficiently effective. Progress in developing more effective inhibitors has been impeded by the lack of an established mechanism connecting the molecular structure to inhibitor efficiency. We have used Molecular Dynamics techniques to investigate the mechanism.

Initially we considered two plausible molecular-level mechanisms:
(a) the sequestering mechanism: long alkane chains in oil selectively partition toward the inhibitors making them less available to nucleate a wax crystal,
(b) the incorporation-perturbation mechanism: inhibitors partition from the oil into amorphous wax deposits ("soft wax") slowing down the crystallization of soft wax to form "hard wax".
We used the MD to predict whether these mechanisms could account for the observed differences in wax inhibition for various poly alkyl acrolates (PAA) and poly alkyl styrenes. We concluded that neither mechanism correlates with observed performance.

This leads us to consider two additional mechanisms:
(c) pipeline inhibitor adsorption mechanism: adsorption of inhibitors on the pipe wall provides an irregular surface that interferes with adsorption of wax to form crystals, and
(d) wax crystal inhibitor adsorption mechanism: adsorption of inhibitors on initial wax nuclei or growing wax crystals inhibits further wax growth.
Ongoing studies provide some support for mechanism (c).

Supported by Chevron Petroleum Technology Company and by NSF.

We thank to Larry Smarr (U. Illinois) for Supercomputer Allocations at NCSA.


Scheme 1. Incorporation-Perturbation mechanism.

PAA1(C18)

PAA2(C18/C1)

PAA3 (C22)

PAS2(C18/C1)

good

good

poor

none

Figure 1. Model inhibitors (poly(alkylacrylate) and poly(p-alkylstyrene)) and their efficiencies.

PAA1 in n-C7 (solvent)

PAA1 in n-C15

PAA1 in n-C31(wax)

Figure 2. Snapshot of conformations of an inhibitor in oil and in amorphous wax (periodic box ~ 30 Angstrom).

Inhibitors

Delta(E/A) (J/m2)

Relative wax deposit (%)

PAA1

-5.2 +/- 0.8

20

PAA2

-6.0 +/- 0.4

24

PAA3

-6.2 +/- 0.6

80

PAS2

-6.6 +/- 0.9

100

Table 1. Energy change during incorporation of inhibitors from n-C7 into n-C31, which shows similar results (within uncertainty of simulation) for different inhibitors with different efficiencies.