Chemistry of Asphaltenes - American Chemical Society

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8 Size Characterization of Petroleum Asphaltenes and Maltenes G. HALL and S. P. HERRON

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Mobil Research and Development Corporation, Paulsboro, NJ 08066

The molecular size distributions and the size-distribution profiles for the nickel-, vanadium-, and sulfur-containing molecules in the asphaltenes and maltenes from six petroleum residua were determined using analytical and preparative scale gel permeation chromatography (GPC). The size distribution data were useful in understanding several aspects of residuum processing. A comparison of the molecular size distributions to the pore-size distribution of a small-pore desulfurization catalyst showed the importance of the catalyst pore size in efficient residuum desulfurization. In addition, differences between size distributions of the sulfur- and metal-containing molecules for the residua examined helped to explain reported variations in demetallation and desulfurization selectivities. Finally, the GPC technique also was used to monitor effects of both thermal and catalytic processing on the asphaltene size distributions.

R

ecently, petroleum residua have been studied extensively (J, 2) because of the increasing importance of heavier fuels. B o t h the asphaltene (pentane-insoluble) a n d maltene (pentane-soluble) components of residua are of interest, a n d since their properties overlap, a complete study of p e t r o l e u m residua must consider both asphaltenes a n d maltenes. O n e area that has received considerable attention has been the size characterization of asphaltenes a n d maltenes (3, 4, 5). Size distribution data are useful both i n understanding the f u n d a m e n t a l chemistry of asphaltenes a n d maltenes a n d i n observing the effects of various processes o n residua sizes. In this study, six petroleum residua were characterized b y a c o m b i n a t i o n of preparative- a n d analytical-scale gel permeation chromatography ( G P C ) . E a c h r e s i d u u m was separated asphaltene

initially b y pentane deasphalting into a n

and maltene pair, both of w h i c h were separated further b y 0065-2393/81 /0195-0137$05.00/0 © 1981 American Chemical Society

Bunger and Li; Chemistry of Asphaltenes Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

138

CHEMISTRY OF ASPHALTENES

preparative G P C o n Styragel. T h e apparent molecular size distributions were obtained o n a n analytical G P C system

using μ-Styragel

columns.

These

distributions, w i t h the sulfur, n i c k e l , a n d v a n a d i u m measurements f o r each cut, were used to obtain size distribution profiles for the sulfur-, n i c k e l - , a n d v a n a d i u m - c o n t a i n i n g molecules. T h e molecular a n d elemental size d i s t r i b u ­ t i o n data were c o m p a r e d w i t h the pore size distribution data of a small-pore d e s u l f u r i z a t i o n catalyst to illustrate the importance of catalyst pore size f o r efficient desulfurization a n d demetallation. In a d d i t i o n , the effects of both t h e r m a l a n d catalytic processing o n asphaltene size distributions were m o n i ­ t o r e d using these data.

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Experimental Samples.

T a b l e I lists the six residua studied a n d their sulfur, n i c k e l ,

v a n a d i u m , a n d weight percent vacuum

asphaltenes data. T h e A r a b i a n L i g h t is a

(1000 + ° F ) r e s i d u u m ,

while

the other

five

are

atmospheric

(650 + ° F ) residua. T h e samples were a n a l y z e d as received f r o m the refinery distillation tower. Sample Preparation.

T h e residua samples were separated into asphal­

tenes a n d maltenes b y deasphalting the resid w i t h a 25:1 (v/v) amount of n-pentane.

A f t e r stirring, the m i x t u r e was a l l o w e d to sit overnight,

then

filtered through a 0.45-μ porous glass filter. T h e asphaltenes were washed w i t h several portions of pentane a n d d r i e d under v a c u u m at 9 0 ° C . Pentane was evaporated f r o m the filtrate to y i e l d the maltenes. Preparative G P C .

T h e preparative G P C w o r k was p e r f o r m e d o n the

e x p e r i m e n t a l setup shown i n F i g u r e 1. F o u r 1-in. i . d . glass columns were p a c k e d w i t h Styragel (Waters Associates) w i t h 1 ft 1 0 Â porosity, 2 ft 500 Â 4

porosity, a n d 1 ft 100 Â porosity. T h e Styragel porosités were chosen to give g o o d resolution f o r the entire range of molecular sizes f o u n d i n residua. T w o separate c o l u m n systems were used—one for maltenes, the other for asphaltenes. T e t r a h y d r o f u r a n ( B u r d i c k a n d Jackson " d i s t i l l e d i n glass") a n d p y r i d i n e ( B a k e r Instra-Analyzed) were the m o b i l e phases. A l l of the maltene

sample

was eluted b y tetrahydrofuran; however, p y r i d i n e was r e q u i r e d to remove a

T a b l e I. P e t r o l e u m R e s i d u a A n a l y z e d Description Aramco Arabian Light vacuum Kuwait Lagomedio Prudhoe Bay Wilmington

S(%) 2.77 4.17 4.24 1.80 1.61 1.96

Ni

(ppm) 5 17 13 18 17 93

V

(ppm) 22 80 50 204 49 67

Weight Percent Asphaltenes 3.6 13.2 6.7 5.0 4.8 7.8

Bunger and Li; Chemistry of Asphaltenes Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

8.

HALL AND HERRON

139

Petroleum Asphaltenes and Maltenes

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Figure I. Experimental setup for preparative GPC work

p o r t i o n of the asphaltene sample (5-10%) not eluted b y tetrahydrofuran. A M i l t o n - R o y reciprocating p u m p w i t h stainless steel 316 a n d Teflon surfaces was used f o r tetrahydrofuran. Since p y r i d i n e leaches n i c k e l f r o m stainless steel, a F l u i d m e t e r i n g positive displacement piston p u m p w i t h Teflon a n d c e r a m i c surfaces was used for p y r i d i n e . A 1-ft χ %-in. o.d. A l 0 2

3

(Alcoa F20)

c o l u m n was inserted between the T H F p u m p a n d the G P C c o l u m n to remove a n y peroxides f r o m the tetrahydrofuran. A

100-mg sample

was loaded into a 4 - m L glass sample

loop a n d

t e t r a h y d r o f u r a n was a d d e d to solubilize the sample before the solution was flushed onto the c o l u m n . T h e flow rate was 10 m L / m i n . F o u r to eight passes w e r e m a d e for each sample to obtain sufficient material f o r a l l subsequent analyses. T h e c o l u m n eluate was m o n i t o r e d w i t h a n Isco d u a l b e a m

UV-vis

detector operating at 310 n m a n d 546 n m . T e n - m i l l i l i t e r fractions

were

collected w i t h a n Isco fraction collector triggered b y a siphon counter, a n d after c o m p l e t i o n of a r u n , adjacent 1 0 - m L fractions were c o m b i n e d into five cuts.

Bunger and Li; Chemistry of Asphaltenes Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

CHEMISTRY OF ASPHALTENES

140

T h e fractions were evaporated to near dryness w i t h a roto-evaporator, t h e n transferred to a small, tared beaker for further evaporation u n d e r N o n a 2

hot plate. F i n a l evaporation was accomplished i n a v a c u u m oven at 9 0 ° C . ELEMENTAL ANALYSES.

S u l f u r measurements for preparative cuts a n d

r a w a n d control samples were made o n 15-50-mg samples using A S T M M e t h o d D1552. T h e precision was ± 10% relative for maltenes a n d ± 5% relative for asphaltenes as d e t e r m i n e d b y m u l t i p l e measurements o n several asphaltenes a n d maltenes. F l a m e atomic absorption determinations for both preparative cuts a n d r a w a n d control samples of n i c k e l a n d v a n a d i u m were m a d e o n 10-150-mg samples. T h e precision was ± 10% relative, as d e t e r m i n e d b y replicate

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measurements o n a series of samples, except f o r very l o w metal level samples ( < 1 0 p p m ) , where the precision was ± 20%. PREPARATIVE G P C RESULTS.

T h e sulfur, n i c k e l , v a n a d i u m , a n d weight

d a t a f o r the p r e p runs are g i v e n i n T a b l e II. These r a w data were subsequently corrected f o r solvent residues. A l t h o u g h the highest grade solvents were used, residue weights a n d metal contents must be considered because of the large amounts of solvents evaporated to dryness a n d the s m a l l sample sizes i n v o l v e d . F o r a l l but the p y r i d i n e - c o n t a i n i n g cut f o r the asphaltenes, the corrections w e r e negligible ( CO LO LO

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