Size Exclusion Chromatography - ACS Publications - American

17 Analysis of Petroleum Crude and Distillates by Gel Permeation Chromatography C. V. PHILIP and RAYFORD G. ANTHONY

Downloaded by CORNELL UNIV on May 12, 2017 | http://pubs.acs.org Publication Date: March 30, 1984 | doi: 10.1021/bk-1984-0245.ch017

Kinetics, Catalysis and Reaction Engineering Laboratory, Department of Chemical Engineering, Texas A&M University, College Station, TX 77843 The currently available high efficiency columns with 5 micron size polystyrene/divinylbenzene copolymer packing, have extended the capability of size exclusion chromatography for the separation of smaller molecular size species in addition to the large polymeric species. Petroleum crude and its refinery products are composed of both larger and smaller molecular components (asphaltenes and distillates). Gel permeation chromatography (GPC) using a 100A PL Gel column and tetrahydrofuran (THF) separates petroleum crude or the refinery product into fractions containing different chemical species such as nonvolatiles (asphaltenes), long chain alkanes and aromatics. GPCs of petroleum crude as well as its d i s t i l l a t i o n cuts are used to illustrate the use of GPC for the analysis of petroleum crude and its refinery products. C u r r e n t l y most r e f i n e r i e s a r e c a p a b l e of p r o c e s s i n g d i f f e r e n t p e t r o l e u m c r u d e s and c a n i n c r e a s e t h e y i e l d o f t h e s e l e c t e d p r o d u c t s on demand. The c o m p o s i t i o n o f t h e c r u d e s v a r i e s d e p e n d i n g on f a c t o r s s u c h as g e o g r a p h i c a l o r i g i n , t h e w e l l l o c a t i o n and d e p t h . C e r t a i n ASTM s p e c i f i c a t i o n s such as A P I g r a v i t y , v i s c o s i t y , d i s t i l l a t i o n temperatures, and f l a s h p o i n t are g e n e r a l l y u s e d f o r t h e e v a l u a t i o n s o f crude as w e l l as i t s r e f i n e r y p r o d u c t s . A number o f s t u d i e s have r e p o r t e d c e r t a i n p h y s i c a l and c h e m i c a l p r o p e r t i e s of r e f i n e r y products (1-5). I t a p p e a r s t h a t t h e ASTM s p e c i f i c a t i o n s , some o f them a r e a few decades o l d , a r e b o t h t i m e consuming t o o b t a i n as w e l l as n o t adequate enough t o guide the crude through the r e f i n e r y process to o b t a i n o p t i m u m p r o d u c t i o n o f d e s i r e d d i s t i l l a t i o n c u t s . Thus, other a n a l y t i c a l t o o l s t o c h a r a c t e r i z e both the crude and i t s r e f i n e r y products a r e needed. 0097-6156/84/ 0245-O257S06.00/ 0 © 1984 American Chemical Society

Provder; Size Exclusion Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


258

SIZE EXCLUSION CHROMATOGRAPHY

G e l p e r m e a t i o n c h r o m a t o g r a p h y (GPC) has been e x t e n s i v e l y used f o r molecular s i z e d e t e r m i n a t i o n s of l a r g e molecular weight species such as polymers (6), c o a l l i q u i d s (7-14), and petroleum asphaltenes (15.16,17), GPC data on a number of compounds such as s t r a i g h t - c h a i n alkanes, amines, a l c o h o l s , m u l t i - r i n g aromat i c s , e t c . (]_) show t h a t t h e r e t e n t i o n v o l u m e i s m a i n l y a f u n c t i o n of t h e l e n g t h of t h e m o l e c u l e r a t h e r t h a n m o l e c u l a r volume, molecular weight or any other molecular s i z e parameter* The steady increase i n the r e t e n t i o n volumes of l a r g e s t r a i g h t c h a i n alkanes suggests t h a t they e x i s t i n the s o l u t i o n i n a s t r e t c h e d s t a t e r a t h e r than i n a c o i l e d s t a t e * I t i s appro p r i a t e t o say t h a t GPC s e p a r a t i o n s a r e m o s t l y on t h e b a s i s of l i n e a r m o l e c u l a r s i z e r a t h e r t h a n any o t h e r m o l e c u l a r s i z e parameter. Longer molecules e l u t e f a s t e r than s h o r t e r molecules because l o n g e r m o l e c u l e s a r e l e s s l i k e l y t o d i f f u s e i n t o t h e l i q u i d t r a p p e d i n s i d e t h e p o r e s . The r e t e n t i o n volume V i n a GPC column i s g i v e n by the equation: T

v

t

•= V i + K V

P

(1)

where: * the column i n t e r s t i t i a l volume; V * the t o t a l pore v o l u m e ; and Κ the p a r t i t i o n c o e f f i c i e n t , the r a t i o o f t h e a c c e s s i b l e pore v o l u m e t o t h e t o t a 1 pore volume. A l l s o l u t e s e l u t e between and V^ + V . For most g e l columns the value of the r a t i o of t o V i s i n t h e o r d e r of 1 t o 1.3. Consequently the t o t a l number of peaks t h a t can be r e s o l v e d by GPC i s l i m i t e d compared t o other modes o f l i q u i d chromatography. Some mole c u l e s are too l a r g e f o r pores o f the column and they are e l u t e d without s e p a r a t i o n a t ( t o t a l s i z e e x c l u s i o n ) and some other molecules are too s m a l l and they e l u t e at + V ( t o t a l perme a t i o n ) . By s e l e c t i n g t h e c o r r e c t pore s i z e f o r t h e m o l e c u l a r s i z a d i s t r i b u t i o n of s p e c i e s i n t h e s a m p l e , the r e s o l u t i o n of peaks can be i n c r e a s e d . Compared t o o t h e r modes o f l i q u i d c h r o m a t o g r a p h y r e l a t i v e l y l a r g e r s a m p l e s can be s e p a r a t e d , without s i g n i f i c a n t l o s s i n r e s o l u t i o n . The new columns packed w i t h 5 m i c r o n p a r t i c l e s have i n c r e a s e d t h e t h e o r e t i c a l p l a t e counts (manufacturers c l a i m s 40,000 plates/meter) s i g n i f i c a n t l y and hence a n a l y s i s c a n be a c c o m p l i s h e d i n 10-25 m i n u t e s depending on column s i z e and f l o w r a t e s . β

L i n e a r M o l e c u l a r S i z e s f r o m V a l e n c e - B o n d S t r u c t u r e s . I n the absence of any i n t e r a c t i o n s between s o l u t e and s o l v e n t such as hydrogen bonding between s o l u t e and s o l v e n t molecules r e s u l t i n g i n a l a r g e r m o l e c u l a r s i z e , and any i n t e r a c t i o n s between s o l u t e and g e l p a r t i c l e s such a s a d s o r p t i o n w h i c h i s t h e b a s i s f o r l i q u i d chromatography, the molecular l e n g t h can be obtained from the valence bond s t r u c t u r e s . Figure 1 i l l u s t r a t e s the f a c t t h a t r i g i d molecules such as aromatics are expected t o have s m a l l e r l i n e a r m o l e c u l a r s i z e s , and c o n s e q u e n t l y , l a r g e r r e t e n t i o n


17.

PHILIP AND ANTHONY

GPC of Petroleum Crude and Distillates

259


v o l u m e s t h a n s t r a i g h t - c h a i n hydrocarbons of s i m i l a r molecular weight. / ' E f f e c t i v e " L i n e a r M o l e c u l a r S i z e i n S o l u t i o n s . When THF i s used as the mobile l i q u i d phase, c e r t a i n species can form hydro gen bonds w i t h THF e f f e c t i v e l y p r o d u c i n g a c o m p l e x m o l e c u l e w h i c h e x h i b i t s a g r e a t e r l i n e a r m o l e c u l a r s i z e ( F i g u r e 2) and lower r e t e n t i o n volume. When non-polar s o l v e n t s such as toluene are used, the molecular s i z e i s e s s e n t i a l l y unaffected. Phenol f o r m s h y d r o g e n bonds w i t h THF ( F i g u r e 3) r e s u l t i n g i n a 1:1 complex and an increase i n e f f e c t i v e l i n e a r molecular s i z e . GPC i s w i d e l y used f o r the s i z e s e p a r a t i o n as w e l l as f o r the mole c u l a r weight d i s t r i b u t i o n of t y p i c a l polymers. Since molecular l e n g t h i s t h e m a i n b a s i s f o r the GPC s e p a r a t i o n and t h e f a c t t h a t t h e s o l u t e s i z e c a n i n c r e a s e i n c e r t a i n s o l v e n t s , GPC achieves c l a s s s e p a r a t i o n of species which n o r m a l l y have s i m i l a r m o l e c u l a r s i z e s , i n some c o m p l e x m i x t u r e s (8.14). Use o f GPC for separation of coal l i q u i d s into f r a c t i o n s enriched with d i s t i n c t c l a s s of chemical species such as a r o m a t i c s , phenols, asphaltenes mixed w i t h alkanes, i s discussed elsewhere (14). Experimental Samples o f crude o i l and r e f i n e r y p r o d u c t s used i n t h i s s t u d y were obtained from c o m m e r i c i a l as w e l l as from l o c a l sources. The GPC separations were performed on a Waters A s s o c i a t e s Model ALC/GPC 202 l i q u i d chromatograph equipped w i t h a r e f r a c t o m e t e r (Model R401). A Valco v a l v e i n j e c t o r was used t o load about 50 m i c r o l i t e r samples i n t o the column. A 5 micron s i z e 100A PL g e l column (7.5 mm ID, 600 mm long) was used i n t h i s study. Reagent grade t e t r a h y d r o f u r a n (THF) w h i c h was r e f l u x e d and d i s t i l l e d w i t h sodium w i r e i n a n i t r o g e n atmosphere, was used as t h e GPC c a r r i e r s o l v e n t . Flow r a t e was 1 ml per minute. THF was s t o r e d under d r y n i t r o g e n , and a l l s e p a r a t i o n s were c o n d u c t e d i n a n i t r o g e n atmosphere t o prevent the f o r m a t i o n o f peroxides. S t r a i g h t c h a i n alkanes from A p p l i e d Science, aromatics from F i s h e r S c i e n t i f i c Company and p o l y s t y r e n e standards from Waters A s s o c i a t e s were used without p u r i f i c a t i o n f o r the l i n e a r mole c u l a r s i z e c a l i b r a t i o n of the GPC. Since the s o l u b i l i t y of the l a r g e r a l k a n e s i n THF i s v e r y l o w , a p p p r o x i m a t e l y 0.2 -1 mg o f each standard was d i s s o l v e d i n 50 m i c r o l i t e r s o f the THF f o r the molecular s i z e c a l i b r a t i o n s . The samples of crude and d i s t i l l a t e f o r GPC a n a l y s i s were p r e p a r e d by d i s s o l v i n g t h e sample i n d r y a d d i t i v e - f r e e THF t o o b t a i n a 2 5 % s o l u t i o n and t h e s o l u t i o n was f i l t e r e d t h r o u g h micro-pore f i l t e r s ( M i l l i p o r e , 0.5 micrometer s i z e ) . A s o l u t i o n c o n t a i n i n g both the c a l i b r a t i o n standard and the sample was used to determine the molecular s i z e d i s t r i b u t i o n of the sample.



'ebb

=

nCe -

===>

nC

3

n-HEXANE

- PROPANE

C χ •» Κ D


7

==>

=>

- n-HEXANE

nC

c

n-HEXANE

F i g u r e 1· L i n e a r molecules s i z e i n s t r a i g h t - c h a i n alkane carbon u n i t s w i t h comparable aromatic s t r u c t u r e s ( F u e l , 1982) ( 1 4 ) .

n-HEPTANE Figure 2. Phenol-THF Complex (Fuel 1982) (14).


17.

PHILIP AND

R e s u l t s and

ANTHONY

GPC

of Petroleum Crude and Distillates

261

Discussions


The s e p a r a t i o n of c o a l l i q u i d s by g e l permeation chromatography u s i n g 100A S t y r a g e l columns and s o l v e n t s such as THF and toluene has been r e p o r t e d e l s e w h e r e (7.8.9.13.14). C o a l l i q u i d s and petroleum crude are s i m i l a r i n t h e i r p h y s i c a l appearance as w e l l as the complexity i n composition. The major d i f f e r e n c e between the two i s t h a t petroleum crude does not c o n t a i n oxygenated com pounds, such as a l k y l a t e d phenols, i n s u b s t a n t i a l q u a n t i t y . In a d d i t i o n , the average l i n e a r molecular s i z e of petroleum d e r i v e d a s p h a l t e n e s (15.16) i s much l a r g e r t h a n t h a t of c o a l d e r i v e d a s p h a l t e n e s (9). L i n e a r Molecular S i z e ; The Best A v a i l a b l e Basis f o r the GPC Se p a r a t i o n . The e l u t i o n p a t t e r n of the GPC u s i n g 5 micron 100A PL g e l column i s i l l u s t r a t e d i n Figure 4 where the GPC s e p a r a t i o n o f a s t a n d a r d m i x t u r e c o n t a i n i n g s t r a i g h t c h a i n a l k a n e s and aromatics i s shown. The polystyrene standard (mol. wt. 2350 and c h a i n l e n g t h 57A) gave a b r o a d peak a t 11 ml r e t e n t i o n v o l u m e . The peak p o s i t i o n i s marked i n the f i g u r e r a t h e r than u s i n g the polystyrene standard i n the mixture i n order to save the nC^HoQ peak from the enveloping e f f e c t of the broad p o l y s t y r e n e peak. The r e n t e n t i o n v o l u m e of s e v e r a l a l i p h a t i c and a r o m a t i c com pounds i n THF and t o l u e n e have been r e p o r t e d (7.). I t i s c l e a r t h a t a r o m a t i c compounds, as e x p e c t e d f r o m t h e i r v a l e n c e bond s t r u c t u r e s , have s m a l l e r l i n e a r molecular s i z e s compared t o nalkanes of s i m i l a r molecular weight. I t i s expected t h a t most of the condensed r i n g aromatics such as naphthlene, anthracene and even b i g ones l i k e coronene (seven f u s e d r i n g s w i t h m o l e c u l a r weight of 300.4) are s m a l l e r than n-hexane (14) and hence have r e t e n t i o n volumes l a r g e r than that of n-hexane. The E f f e c t of "Aromatic" Gel on the S i z e Separation of Aromatic Species. C e r t a i n aromatics such as anthracene, benzopyrene and coronene produce GPC s e p a r a t i o n p a t t e r n s which d e v i a t e from what i s expected from t h e i r molecular lengths. A l l aromatic species have a s l i g h t l y shorter e f f e c t i v e molecular length compared to t h e i r valence bond s t r u c t u r e s . Although anthracene i s about the s i z e of n-hexane, i t has a r e t e n t i o n volume c l o s e to that of nbutane. Benzopyrene ( f i v e fused r i n g s ) has a r e t e n t i o n volume e q u i v a l e n t to propane. The r e t e n t i o n volume of coronene (Figure 1) shows t h a t i t s e f f e c t i v e s i z e i s s l i g h t l y s m a l l e r than t h a t of propane. T h i s type of anomalous b e h a v i o r i s e x p e c t e d f o r a l i m i t e d number of compounds due to t h e i r s t r u c t u r e s a s s o c i a t e d w i t h e x t r e m e a r o m a t i c i t y . The GPC columns a r e packed w i t h s w e l l e d polymer p a r t i c l e s of c o n t r o l l e d pore s i z e formed by the c o - p o l y m e r i z a t i o n of s t y r e n e and d i v i n y l b e n z e n e . Every other c a r b o n atom on t h e p o l y m e r c h a i n has a p h e n y l group f r e e l y hanging. The species w i t h aromatic s t r u c t u r e s can i n t e r a c t w i t h the phenyl groups of the polymer chains of the g e l . The i n t e r -



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Size Exclusion Chromatography - ACS Publications - American

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