Preliminary results of extraction experiments in an oil shale

Org. Geochem. Vol. 18, No. 3, pp. 313-316, 1992

0146-6380/92 $5.00 + 0.00 Copyright © 1992 Pergamon Press Ltd

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Preliminary results of extraction experiments in an oil shale C. G. BLANCO,J. G. PRADO, M. D. GUILL~N and A. G. BORREGO Instituto Nacional del Carb6n (INCAR), CISC Ap. Co. 73, 33080 Oviedo, Spain Abstract--Soxhlet versus ultrasonic extraction efficiency on the immature Puertollano oil shale has been tested. The yields from the two methods are compared. The extracts were compared by gas chromatography and geochemical parameters such as CPI and pristine/phytane ratio calculated. The ultrasonic extraction procedure is an effective, quick and easy method which does not change the biomarker ratios of the bitumen. Minor efficiencydifferences can be observed among the solvents allowing a choice on the basis of handling properties. Carbon disulfide and chloroform are easiest to eliminate from the extract. Key words---extraction, ultrasonic, soxhlet, oil shale, biomarkers

drofuran and dichloromethane) commercially available from Merck and Fluka, with a purity higher than 99% were used. Solvents were redistilled to increase their purity and tested to prove that they did not contain high molecular weight contaminants. Conditions and experimental procedure were always maintained in order to obtain valid information about the effectiveness of the solvents in extracting the oil shale. Extractions were performed by two methods.

INTRODUCTION

Organic geochemical studies of oil shales usually require organic solvent extraction, followed by characterization techniques such as gas chromatography, nuclear magnetic resonance and gas chromatography-mass spectrometry. Study of oil shale bitumen biological markers is used to deduce its origin and degree of maturation. Several techniques have been proposed for extraction with organic solvents of bitumen from rocks. The most widely used method is soxhlet extraction (Hazai et al., 1988; Kruge et al., 1990), but it has some disadvantages such as long extraction time (Tahiri et al., 1988), work temperature (boiling point of the solvent) or its impracticality for small amounts of sample (Monin et al., 1978). Recently, other extraction procedures have been reported: supercritical extraction (Funazukuri et al., 1988), flash thermal desorption (Crisp et al., 1986; Piittman et al., 1988; Li and Johns, 1990), flow-blending (Radke et al., 1978) and ultrasonic extraction (Farrington et al., 1988; Korth et al., 1988). In all these techniques, the extraction could result in chemical degradation which modifies the structure of the original organic matter. In this context, some studies have been carried out trying to relate the amount and composition of the bitumen from oil shale with the used extractive methods and the organic solvent properties (Saxby, 1976; Monin et al., 1978; Makitra et al., 1984). In this paper, extraction yields from the Puertollano oil shale by soxhlet and ultrasonic methods with different solvents are reported. The influence of the solvents on the geochemical parameters is described. These results are the preliminary data from a larger study that is being carried out on Puertollano oil shale.

(a) Ultrasonic extraction A weighed amount of sample (10 g), crushed to < 2 0 0 # m , was inserted into a glass flask with a measured amount of solvent. The sample/solvent ratio used was 10 g/200 ml. The closed glass flask was immersed in an ultrasonic bath (Selecta ultrasounds, frequency 50 kHz, power 400 W) and kept at 25°C for 2 h. This long period of time was used in order to ensure a thorough extraction. The solution was then filtered under vacuum using a PTFE filter with a pore size of 0.45/am. The residue was washed until the filtrate became totally clear, and dried to a constant weight. The filtrate was isolated from the solvent in a rotary apparatus at reduced pressure. Special care was taken in order to prevent the loss of volatile components. For this reason, extracts were not dried to constant weights and fractionation was avoided. (b) Soxhlet extraction 10 g of sample, crushed to less than 200 #m, were extracted with 200 ml of solvent. The extraction was performed by recycling the solvent, for at least 48 h, until the solvent was clear. The extracted fraction was then filtered as for the sonic extract. The extraction yield (Y), can be considered as a measure of the extractive ability of each solvent, or the solubility of the bitumen in each solvent. Y was calculated by Y = [ ( P - I)/P]*IO0, where P is the weight of the oil shale, and I is the weight of the insoluble material.

EXPERIMENTAL

Eight solvents (carbon disulfide, n-pentane, pyridine, 1,4-dioxane, benzene, chloroform, tetrahyOG IS/3--G

313

314

C . G . BLAr~CO et al.

Chromatographic analyses of whole extracts were performed with a Hewlett-Packard 5880A Series II gas chromatograph, equipped with a flame ionization detector (FID). Separation was carried out on a fused-silica capillary column coated with SE-54 stationary phase (McReynolds Polarity = 337). The length and internal diameter of the column were 20 m and 0.22 mm, respectively. The carrier gas was hydrogen with a flow rate of 1.5 ml/min, and the inlet splitting ratio was 1:!00. The detector and injector temperatures were 350 and 300°C respectively. The temperature program was from 50 to 280:C at 4°C/min. The alkane peaks, including those of pristane (2,6,10,14-tetramethylpentadecane) and phytane (2,6,10,14-tetramethylhexadecane) were identified by reference to a standard. Their relative distribution was determined from a response factor calculated from standard mixtures. SAMPLE LOCATION

The Puertollano coal and oil shale field, in the province of Ciudad Real (185 km south of Madrid), is a small outlier of lacustrine, Upper Stephanian B strata, in an area of about 12 by 4 km. The geological structure is very simple, and consists of a shallow anticline flanked by two minor synclines with a general E - W strike and dips of less than 10"C. The Carboniferous succession unconformably overlies Lower Palaeozoic quartzites and cleaved mudstones, and is itself unconformably overlain by Miocene, lacustrine marls. The general succession is mainly lacustrine (sandstones, silts and mudstones) with three well-defined oil shale bands and a few coal seams at the top of the succession. Throughout the section, tufts are present. Its stratigraphic and palaeontologic characteristics have been widely described in previous works (Wallis, 1983; Wagner,

1985). The samples were collected from the B oil shale band (3 m in thickness) which was mined underground until 1966. This is an immature (vitrinite reflectance =0.40%) lamositic oil shale with some contributions from the fresh water algae Botryococcus, cutinite and sporinite.

RESULTS AND DISCUSSION

In Table 1 the means and standard deviations of extraction yields for both methods are shown. Considerable differences in the extractive ability of oil shales by different organic solvents have been reported by Makitra et al. (1984). In this study, no difference is evident in the effectiveness of the solvents used in the soxhlet extraction. The ultrasonic method produced small differences in the extraction yields. Carbon disulfide, tetrahydrofuran and chloroform produced the highest yields and 1,4-dioxane and benzene the lowest. The standard deviation of ultrasonic yields is lower than that of the soxhlet yields. No relation could be established between physicochemical solvent properties (e.g. dipole moment, refraction index, solubility parameters, etc.) and yields in both methods. Carbon disulfide and chloroform have similar yields in both extraction methods; also the standard deviation is lower. Thus, these two solvents are, with their low boiling point, the best for oil shale extraction. To make a detailed comparison of both methods, all extracts were analyzed by gas chromatography. The chromatograms of the whole extracts are similar and the alkane distribution is the same in all cases, regardless of the solvent used for their extraction. Figure 1 shows a representative chromatogram of the Puertollano oil shale, extracted by carbon disulfide ultrasonically. The n-alkanes ranging from Cl, to C32 (labelled according to their carbon number) are the

Table I. Solvent extraction yields from Puertollano oil shale N~

b.p. +

Solvent

disulfide

Ultrasound

Soxhlet

oC

Y(wt%)

Y(wt%)

s

1

carbon

46.2

2.13

0.02

2.29

0.ii

2 3

n-pentane pyridine

36.2 115.3

1.77 1.64

0.i0 0.08

2.36

0.18

4 5 6 7 8

1,4-dioxane benzene chloroform tetrahydrofuran dichloromethane

101.5 80.0 61.2 66.0 39.8

1.45 1.60 2.15 2.25 1.81

0.04 0.15 0.03 0.ii 0.09

2.34

0.09

2.36

0.18

+ B o i l i n g p o i n t s f r o m h a n d b o o k of C h e m i s t r y a n d P h y s i c s , 6 6 t h Edt., W e a s t , R. C. Ed., C h e m i c a l R u b b e r co. B o c a R a t o n , FL, 1985 Y=[(P-I)/P]*I00. weight

(g);

Where

s= s t a n d a r d

P= s a m p l e deviation

weight

(i0 g)

and

I=

insoluble

Extraction experiments on Puertollano oil shale P,

315

21 20

1 9

23

27

22

25

17 + 16

2 ;

15

29 2e

14 31 30

13 =by 12

i

32

-

,

-

*

,

10

.

.

.

.

,

20

+

-

•

i

30

.

.

.

.

,

40

.

.

.

.

50

,

60

Time (rain)

Fig. 1. Gas chromatogram of ultrasonic extract with carbon disulfide. Alkane chains are numbered. Pr = pristane, Phy = phytane. main compounds in all extracts studied. Moreover, pristane was found to be the most abundant component. The big difference between the boiling points of the solvents used, ranging from 36.2°C (n-pentane) to 155.3°C (pyridine), and those of the smaller nalkanes considered in this study ( n - C , = 196°C and n-Cl2 = 216°C), ensures that n-Ct2-n-C20 hydrocar-

bons were not released in the solvent evaporation process. Tables 2 and 3 show calculated geochemical parameters. Each value represents at least four data points. CPI* and CPI** data were obtained from Bray and Evans (1961) formula and calculated for C2o-C3oand Ci2-C32 n-alkanes, respectively.

Table 2. Geochemical data from ultrasonic extraction Pr Solvent t -Phy

Pr

Phy

s

s n-C17

s

CPI*

CPI**

n-C18

1 2 3

2 • 54 2.59 2.75

0 • 15 0.17 0.21

1 •47 1.45 1.53

0.01 0.01 0.03

0.62 0.61 0.58

0 • 03 0.03 0.03

1.24 1.23 1.28

0.00 0.04 0 01

1.26 1.22 1.28

0.02 0.04 0.09

4 5 6 7 8

2.55 2.59 2.61 2.46 2.55

0.31 0.01 0.13 0.01 0.11

1.56 1.44 1.52 1.53 1.54

0.01 0.01 0.01 0.00 0.01

0.67 0.60 0.60 0.65 0.65

0.06 0.01 0.03 0.00 0.02

1.21 1.24 1.30 1.24 1.17

0 0 0 0 0

1.20 1.27 1.26 1.27 1.21

0.03 0.02 0.01 0.01 0.02

04 04 09 01 01

P r = P r i s t a n e , P h y = P h y t a n e , s= s t a n d a r d d e v i a t i o n t S o l v e n t n u m b e r s a r e t h e same as in Table 1 1 [nO21+nC23+nC25+ nC2?+nC~; cPI*=

-

~

2 LnCzo+nCz2÷nc24+nc~÷nc28

nC21+nC23+nCz$+nCzT+nc~ 1 +

-

-

nczz+ncz4+nc26+nc28+n~o j

2 Lnc12+nC14+nC16+nC18+nC20+nC22+nC24+nC26+nC28+nC30

nC14+nC16+ncls+nC20+nC22+nC2&+nC26+nC28+nC30+nC32j

C. G. BLANCO et al.

316

Table 3. Geochemical data from soxhlet extraction Pr

Pr

Solvent

Phy

s

s

Phy

s

n-Cl7

CPI*

s

CPI**

s

n-Cl8

1

2.60

0.06

1.47

0.03

0.63

0.03

1.22

0.06

1.21

0.04

3

3.15

0.14

1.59

0.05

0.53

0.01

1.27

0.05

1.25

0.04

6

2.52

0.14

1.49

0.01

0.63

0.04

1.19

0.04

1.14

0.01

8

2.58

0.20

1.51

0.04

0.63

0.03

1.23

0.04

1.20

0.07

symbols

are

the

same

as

in

Table

2

All data are quite similar with the exception of the p r i s t a n e / p h y t a n e ratio resulting from pyridine soxhlet extraction and the CPI values from c h l o r o f o r m soxhlet extraction. The geochemical parameters least influenced by experimental errors are phytane/n-C~s, CPI* a n d CPI**. F r o m results shown in Tables 2 a n d 3, it is evident that all the solvents used are able to extract all the saturated hydrocarbons.

CONCLUSIONS

The ultrasonic extraction m e t h o d is an excellent alternative to soxhlet extraction in b i t u m e n studies, due to the short experimental time, low t e m p e r a t u r e and lack o f t r a n s f o r m a t i o n o f the organic m a t t e r in the rock. C a r b o n disulfide or C h l o r o f o r m are the best solvents. Acknowledgements--A. G. Borrego thanks the Direccion General de Investigaci6n Cientifica y T6cnica (Ministerio de Educaci6n y Ciencia) for a predoctoral fellowship. Doctors L. R. Snowdon and H. Widens are gratefully acknowledged for their valuable comments. REFERENCES

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