Yun Hee Jang, Mario Blanco, William A. Goddard, IIIMSC, 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.

hotoil

coldsea

water

wax oil production pipe wall

Wax: Aggregates

of heavy n-alkanes

at low temperature

pipe blocking

coldsea

water

Comb-like wax inhibitor

Wax inhibitor

(comb-like polymer):

No established

mechanism of action.

coldsea

water

coldsea

water

Wax InhibitionWax InhibitionWax FormationWax Formation

Liquid

Amorphous solid

Ordered crystal

Further growth

Adsorption on pipe

(1) Sequestering mechanismlong 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 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 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 (C18)

good

PAA2 (C18/C1)

good

PAA3 (C22)

poor

PAS2 (C18/C1)

very poor

The same side chain distributionThe same MW

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

n-C7 (liquid) calc.(293 K) expt’l

Density (g/cm3) 0.672 0.002 0.6838 (0.6795 at 298 K)

Hvap (kcal/mol) 8.87 0.3 8.74 0.004

solubility parameter 15.2 MPa1/2 15.1 MPa1/2

n-C32(amorphous) calc. expt’l

Density (g/cm3) 0.816 0.003 0.8124

n-C32(crystal) calc. expt’lHvap (kcal/mol) 52.84 0.8 53.44 0.24 at 298 K

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)

Sequestered wax at 293 K

<PE> = -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

+ AdditiveIncorporation

Crystallization 1

1. Amorphous pure n-C32

2. Amorphous n-C32 with additive

4. Crystalline pure n-C32

3. Crystalline n-C32 with additive

E << 0 CED = 55%

AdditiveSegregation

CED = +80%Less favorable

than above

Crystallization 2

(1 2 3 4) is slower than (1 4). (Crystallization is delayed with additive.)

(E1)

before after

E(incorporation) = EafterEbefore = (E3+E4)(E1+E2) = (E4E2)(E1E3) = Eint(C31)Eint(C7)

PAA1in n-C7

(E2)

puren-C31

(E3)

puren-C7

(E4)

PAA1in n-C31

*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

J/m2 Eint(n-C7)* Eint(n-C31)* Eint(n-C7-to n-C31)

PAA1 -68.5 0.8 -73.7 0.1 -5.2 0.8

PAA2 -68.9 0.3 -74.9 0.2 -6.0 0.4

PAA3 -67.6 0.2 -73.8 0.6 -6.2 0.6

PAS2 -69.8 0.9 -76.4 0.3 -6.6 0.9

-9

-8

-7

-6

-5

-4

-3

0 20 40 60 80 100 120

Relative wax deposit (%)

Ein

co

rpo

rati

on

pe

r a

rea (

J/m

2)

No correlation or reverse correlation to expectation

End-to-end distance average fluctuation*

Pure n-C31 21.6 7.3 2.52

n-C31 with PAA2 22.5 7.3 2.61

n-C31 with PAA3 23.3 6.7 3.04

n-C31 with PAS2 23.2 6.4 2.92

0

5

10

15

20

25

30

35

40

1 3 5 7 9 11 13 15 17 19 21 23 25 27

torsion number (1 to n-4)

Gau

che

po

pu

lati

on

(%

)

pure C31(am)

C31/PAA2

C31/PAS2

pure C31(cr)

0

10

20

30

40

50

60

70

80

1 3 5 7 9 11 13 15 17 19 21 23 25 27

torsion number (1 to n-4)

Sw

itch

fre

qu

ency

(/n

s) pure C31(amor)

C31/PAA1

C31/PAS2

pure C31(cr)

0 10 20 30 40

end-to-end distance (A)

pure c31

c31/PAA2

c31/PAA3c31/PAS2

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 200-600 ps MD

Counted each 1ps

PAS2

PAA3

PAA1

0.5 1.0 1.5 2.0Adsorption energy per area (kcal/mol/A2)

Wa

x d

ep

os

it

Preliminary study:

adsorption of inhibitor on -Fe2O3, 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

-Fe2O3 force field

S. Jiang, et al. J. Phys. Chem. 100, 15760 (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. -Fe2O3)

It looks good so far, but it needs more work.

Larry Smarr (U. Illinois) for supercomputer allocation at NCSA

Yanhua Zhou for Fe2O3 structure and force field