Improved Extraction Procedures for Coal Products Based on the

The enhanced speed of Soxtec also makes it possible to carry out sequential extraction of coal-derived products. Although this work was devoted to coa...
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Energy & Fuels 1996, 10, 1005-1011

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Improved Extraction Procedures for Coal Products Based on the Soxtec Apparatus Luis Membrado Giner,* Jesu´s Vela Rodrigo, Ana Cristina Ferrando Navarro, and Vicente Luis Cebolla Burillo Departamento de Procesos Quı´micos, Instituto de Carboquı´mica (CSIC), Poeta Luciano Gracia 5, 50015 Zaragoza, Spain Received January 26, 1996. Revised Manuscript Received April 16, 1996X

Soxtec, a medium-cost extraction apparatus, was tested against classical Soxhlet extraction applied to coal and coal-derived products. An optimization study of Soxtec operating conditions for our samples led to a reduction of 90% in the total extraction time needed by Soxhlet, with a comparable repeatability of results and similar extraction yields. As Soxtec and Soxhlet share a common set of general operating conditions, product composition is very similar, and most of the differences can be explained in terms of the increased efficiency of Soxtec, probably due to a better mass transfer. A multistep extraction procedure was also studied. Keeping most of the advantages of single-step Soxtec extraction, it also solves some of its problems (mainly the possibility of saturation) and provides some kinetic data to assess the completeness of the extraction. The enhanced speed of Soxtec also makes it possible to carry out sequential extraction of coal-derived products. Although this work was devoted to coal-derived products, these procedures should be applicable to other kinds of samples as well.

Introduction Analytical extraction is a basic laboratory technique. In coal conversion processes (e.g., coal hydroliquefaction and pyrolysis), extraction has been used to quantify process yields and/or as a preparative prefractionation step leading to coal-derived products suitable for further characterization using analytical or spectroscopical techniques.1-3 Direct extraction of coal has also been carried out for fundamental studies on coal structure and coal science.4-7 Likewise, extraction with organic solvents, standardized in some particular cases, is also usual in the coal tar pitch industry.8,9 Single-solvent extraction processes, as well as more or less sophisticated procedures of sequential multisolvent extraction, have been in development for a long time. However, problems such as saturation, or the development of criteria to assess the end of extraction, have received little attention. Likewise, repeatability of methods is rarely reported in the literature. With regard to extraction techniques, reflux extraction was first used, but hot filtration of extract was a serious limitation when volatile organic solvents were used. Soxhlet solved this problem by using thimbles to Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Pullen, J. R. Solvent Extraction of Coal; IEA Coal Research: London, 1981; Report ICTIS/TR16, (2) Mima, M. J.; Schultz, H.; McKinstry, W. E. In Analytical Methods for Coal and Coal Products; Karr, C., Ed.; Academic Press: New York, 1978; Vol. 1, Chapter 19. (3) Tischer, R. E.; Utz, B. R. A Standard Batch Screening Test for Coal Liquefaction Catalysis. A Preliminary Report; U.S. Department of Energy: Washington, DC, 1983; DOE/PETC/TR.83/2. (4) Dryden, I. G. C. Fuel 1951, 30, 145-158. (5) Marzec, A.; Juzwa, M.; Betlej, K.; Sobkowiak, M. Fuel Process. Technol. 1979, 2, 35-44. (6) Larsen, J. W.; Mohammadi, M.; Yiginsu, I.; Kovac, J. Geochim. Cosmochim. Acta 1984, 48, 135-141. (7) Nishioka, M.; Larsen, J. W. Energy Fuels 1988, 2, 351-355. (8) Guille´n, M. D.; Blanco, J.; Canga, J. S.; Blanco, C. G. Energy Fuels 1991, 5, 188-192. (9) ISO Standard 6791-1981. X

S0887-0624(96)00018-7 CCC: $12.00

hold the sample. Soxhlet is, by far, the most popular extraction system, as it combines two important advantages: the low cost of the apparatus and its unattended operation. However, it also has several major disadvantages: it usually requires large amounts of solvents and sample, long extraction times (24-48 h), and an additional step to remove the excess solvent, usually using vacuum evaporation. All of these steps require a great deal of time. Moreover, the increased handling of the sample can also cause additional experimental errors. Several alternative extraction techniques have been developed to solve, or at least mitigate, problems found with Soxhlet. Sonication10,11 requires much less time and a lower solvent to sample ratio, despite being more labor intensive (filtration and rotavapor). Although some methods have been developed for coal tar pitches, loss of volatile compounds in oils derived from a highly bituminous coal has been observed.12 Sonication produces good dispersion of samples, but uncontrolled reactions due to cavitation phenomena should not be neglected. In any case, some U.S. Environmental Protection Agency (EPA) organic compound extraction methods (including polycyclic aromatic and related compounds, PAC) in environmental samples are based on both Soxhlet and sonication. Microwave extraction,13 another alternative, shares most of the advantages and disadvantages of sonication. The use of supercritical fluid extraction (SFE) has been growing for a certain time.14 The possibility of modifying different additional factors in the extraction procedure with regard to the (10) Bockrath, B. C.; Schweighardt, F. K. Fuel 1978, 57, 4-8. (11) Delpuech, J. J.; Nicole, D.; Cagniant, D.; Cleon, Ph.; Fouche`res, M. C.; Dumay, D.; Aune, J. P.; Genard, A. Fuel Process. Technol. 1986, 12, 205-241. (12) Cebolla, V. L. Ph.D. Thesis, University of Zaragoza, Spain, 1987. (13) Lo´pez-Avila, V.; Young, R.; Kim, R.; Beckert W. F. J. Chromatogr. Sci. 1995, 33, 481-484. (14) Hawthorne, S. B. Anal. Chem. 1990, 62, 633A-642A.

© 1996 American Chemical Society

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above-mentioned techniques, such as fluid density and/ or the use of modifiers, allows for a potential control of the selectivity of the extraction with respect to different compounds.15 Furthermore, it is a clean technique, which does not require the use of organic solvents.16 However, when a special selectivity is not required, it is still to be demonstrated whether the obtained yields and extraction times justify the cost of the equipment and its technical complexity. This same observation can be applied to accelerated solvent extraction (ASE), a new technique proposed by Dionex. It is based on the extraction of the sample using pressurized organic solvent under an inert atmosphere. Most of these techniques also have the same problem: the experimental conditions needed are different from those required for Soxhlet. This can make difficult the direct comparison of experimental results with those found in the literature. Another alternative method, also recommended by the U.S. EPA, was recently proposed for PAC extraction.17 It is based on a commercially available Soxtec extraction system, which can be described as an improved reflux extraction apparatus. Extraction is carried out in two stages: first, the sample, contained in a thimble, is immersed into the boiling solvent and extracted and, second, the thimble is raised and the sample washed with clean solvent from the reflux condenser. The procedure finishes by removing most of the extracting solvent in the same apparatus, an operation that is easily performed by turning a knob and continuing the reflux evaporation for a little longer. Extraction times have been reported to be considerably shortened, classical Soxhlet advantages are kept, and the excess solvent removal is no longer needed.18 Moreover, experimental errors are partially overcome, and the equipment is not overly expensive. As far as the authors know and despite being a common technique in other fields19 (fat analysis in food), Soxtec has never been used to extract coal and/or coal products. For this reason, the first aim of this work was to evaluate the viability of Soxtec and to develop suitable extraction methods for different coal and coalderived products. Side-by-side comparison with Soxhlet was performed in relation to extraction repeatability, extraction yields, and chemical nature of obtained extracts. Some experiments were included to provide an explanation for the increased efficiency of Soxtec. The studied samples were a lignite, a coal tar pitch, and a hydroliquefaction product. They represent the coalderived products most commonly submitted to extraction. Likewise, they cover the different ranges of extraction yields when using Soxhlet, which could be useful for statistical comparisons. The improvement of the direct application of Soxtec to coal samples was also studied. A multistep procedure and a sequential extraction method were also tested. Both are interesting possibilities permitted by the speed of Soxtec. The multistep extraction procedure, in particular, solves some problems of single-step Soxtec extraction (mainly the possibility of saturation) and allows for easy criteria (15) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1993, 65, 339-344. (16) Miller, D. J.; Hawthorne, S. B.; MacNelly, M. E. P. Anal. Chem. 1993, 65, 1038-1042. (17) Lopez-Avila, V.; Bauer, K.; Milanes, J.; Beckert, W. F. J. AOAC Int. 1993, 76, 864-880. (18) Brumley, W. C. J. Chromatogr. Sci. 1995, 33, 670-685. (19) Foster, M. L.; Gonzales, S. E. J. AOAC Int. 1992, 75, 288-292.

Membrado Giner et al.

Figure 1. Scheme of Soxtec apparatus. Table 1. Analytical Data for UL and CTP UL (daf) %C %H %S %N %O moisture ash volatiles fixed carbon DCM THF a

Elemental Analysis 69.3 5.9 3.0 0.9

CTP 93.0 5.1 0.6 0.9 0.9a

Proximate Analysis 7.9 38.1 23.8 30.3 Extraction Yield (Soxhlet, 24 h) 1.9 10.2

84.2 100.0

Oxygen determined directly.

to be applied to determine the end of the extraction process. The procedures developed in this work could be applied to the extraction of products other than those derived from coal. Experimental Section Chemicals. The studied samples were a lignite from the Utrillas basin (Teruel, Spain) (UL) and a hightemperature coal tar pitch (CTP); their main characteristics are shown in Table 1. A hydroliquefied UL (HUL) (solventless reaction, 30 min at 400 °C, H2 at 1200 psi) was also studied. Analytical grade dichloromethane (DCM) and tetrahydrofuran (THF) from Scharlau (Barcelona, Spain) were used as extraction solvents. Soxtec Extraction. A Tecator Soxtec Model HT-2 extractor was used in this work. Figure 1 shows a scheme of its operation. Temperature settings for the heating unit were 90 and 120 °C for DCM and THF, respectively, and all extraction experiments were carried out using aluminum cups (50 mm i.d. and 80 mm height) and 75 mL of extracting solvent. UL and CTP (1-4 g) were usually placed in a 26 × 50 mm Schleicher & Schuell paper extraction thimble and capped with glass wool. Sequential extractions were performed using 33 × 60 mm thimbles. Extracts were dried in a vacuum oven at 50 °C and 15-30 mbar of pressure overnight. Dried samples were stabilized for about 2 h and weighed on a Sartorius analytical balance. Internal

Improved Extraction for Coal Products

air flux of the Soxtec, to help thimble drying, was not used. All extraction yields are calculated on a dry ash free (daf) basis. Soxhlet Extraction. Ten grams of UL, capped with glass wool, was placed in a 33 × 100 mm Whatman paper extraction thimble and extracted with 400 mL of THF or DCM, with extraction times ranging from 1 to 48 h. The unloading speed of the Soxhlet apparatus was around 4 cycles per hour in all cases. Excess solvent was removed in a vacuum rotary evaporator. Extracts were dried, stabilized, and weighed as previously described. All extraction yields are calculated on a daf basis. Temperature Measurements. Temperature measurements were made using a data acquisition system consisting of two thermocouples (Thermocoax, type K, 0.5 mm diameter, -200 to 1000 °C range; and Thermocoax, type S, 1 mm diameter, 0-1700 °C range), a Fluke Hydra 2620 multichannel data acquisition unit, and a HP-95 hand-held computer to receive and store the data. A serial RS-232-C connection was used to send the data from the data acquisition unit to the computer. Thin Layer Chromatography Flame Ionization Detection (TLC-FID) Experiments. An Iatroscan Mark 5 (Iatron Labs Inc., Tokyo) apparatus was used. Samples were developed using n-hexane, toluene, and dichloromethane/methanol (95/5 in volume). Eluents were of analytical grade, from Scharlau (Barcelona, Spain). Flame ionization detector gas flows were of 160 mL‚min-1 for H2 and 2100 mL‚min-1 for air. Samples were scanned at 30 s‚scan-1. Acquisition and treatment of data were carried out using a propietary data acquisition card and BOREAL (Grenoble, France) software. Raw chromatograms were exported to a computer spreadsheet in ASCII form for further treatment. LabCalc (Bomem), which allows for baseline corrections, was also used. A complete description of other operating details can be found in the literature cited herein.20 Results and Discussion One of the differences between Soxhlet and Soxtec is that Soxtec extraction is a two-step process (Figure 1). The initial reflux extraction (boiling) is followed by a separate washing step (rinsing) using fresh solvent from the reflux condenser. It is not possible to make such a distinction in a Soxhlet apparatus. This should be taken into account when one is planning the study of the main aspects of this extraction technique, mainly repeatability, kinetics, and efficiency. As CTP is completely soluble in THF, we only considered its behavior against DCM. UL was studied against DCM and THF. Optimization of the Single-Step Soxtec Extraction Process. It is not possible to disregard the presence of extraction phenomena during the rinsing step. We must therefore consider the effect of different rinsing times for minimum boiling times on the extraction process. Results of extraction percentage of UL, corresponding to a boiling time of 1 min with different rinsing times (see Figure 1), can be found in Table 2. In general, the efficiency of rinsing increases with time, but there is no further significant improvement for times greater than 30 min. From this point on, extraction percentages are very similar, with average values of 2.5 for DCM and 9.9 for THF. Even for a set of experiments corresponding to different rinsing times, the lack of influence of rinsing times greater than 30 min is

Energy & Fuels, Vol. 10, No. 4, 1996 1007 Table 2. Influence of Rinsing Time on Soxtec Extraction Yield (Single-Step, 1 min of Boiling)

rinsing time (min) 0 15 30 60 90 ava SDa

% UL extract DCM THF 0 0.9 2.5 2.4 2.7 2.5 0.2

1.1 7.4 9.8 10.1 9.7 9.9 0.2

% CTP extract DCM 21.2 76.6 78.4 89.0 86.1 84.5 5.5

a Average and standard deviation of values corresponding to 30, 60, and 90 min of rinsing.

Table 3. Influence of Boiling Time on Soxtec Extraction Yield (Single-Step, 30 min of Rinsing) boiling time (min) 1 5 10 15 30 60 120 1440 ava SDa

% UL extract DCM THF 2.5 1.9 1.9 2.0 2.0 2.2 2.1 3.0 2.1 0.1

9.8 11.0 10.0 10.7 10.0 11.0 11.7 14.0 10.9 0.9

% CTP extract DCM 78.4 73.3 79.5 78.0 84.9 85.6 86.0 93.9 85.5 0.6

a Average and standard deviation of values corresponding to 30, 60, and 120 min of boiling.

reflected in low standard deviation (SD) values. Standard deviations were 0.2 in both cases. Results of extraction percentage versus rinsing time in the case of CTP showed a clear variation in the 3090 min range, as can be expected for a more soluble product. For boiling times longer than 1 min, as expected in the routine Soxtec operation, 30 min of rinsing should be enough to provide a correct washing of thimbles for any sample. After selecting 30 min as the routine rinsing time, we studied the influence of the boiling time. Results of extraction percentage of UL and CTP corresponding to different boiling times can be found in Table 3. We can see in this table that the boiling time does not seem to have an important role for UL in the range of 1-120 min. CTP instead shows a higher variation, but in any case extraction yields corresponding to the 30-60 min range for both samples are similar to those obtained for a 24 h Soxhlet extraction (UL-DCM, 1.9%; UL-THF, 10.2%; CTP-DCM, 84.2%). Boiling times longer than 120 min lead to higher extraction yields. There is no significant change in the extraction yields for the 30120 min range [average and SD (In parentheses) were 2.1% (0.1) for UL-DCM; 10.9% (0.9) for UL-THF; and 85.5% (0.6) for CTP-DCM, respectively], and therefore a 30 min boiling time seems the more sensible value of a routine boiling step for any coal or coal-derived sample. Comparison of Extraction Techniques. To compare Soxtec and Soxhlet extraction techniques, we selected UL as the sample to study, mainly because it leads to smaller extraction yields and is therefore less likely to show adequate repeatability. We also chose 1 min of boiling time for Soxtec extraction on the same basis. Table 4 shows a direct comparison between a 24 h Soxhlet extraction and a single-step 1 min of boiling plus 30 min of rinsing Soxtec extraction. Soxtec extraction provides comparable extraction yields and repeatability (RSD%) under these experimental conditions.

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Table 4. Comparison of Extraction Techniques on UL % UL extract Soxhleta single-step

Soxtecb

av (n ) 6) RSD% av (n ) 6) RSD%

DCM

THF

1.9 4.7 2.6 4.9

10.2 6.0 10.5 5.2

aSoxhlet: 24 h, 400 mL of solvent and 10 g. UL daf. bSoxtec: 1 min of boiling time, 30 min of rinsing time, 75 mL of solvent, and 0.5 g of UL daf.

Figure 4. TLC-FID chromatograms of UL Soxtec extract with DCM and THF (A and C) and UL Soxhlet extract with DCM and THF (B and D). Elution sequence (n-hexane, 38 min.; toluene, 20 min, and DCM/MeOH (95:5 v/v), 5 min.). Peak 1, eluted with n-hexane; peak 2, eluted with toluene; peak 3, eluted with DCM/MeOH (95:5 v/v), and peak 4, uneluted.

Figure 2. Soxtec and Soxhlet extraction yields from UL versus time.

Figure 3. Temperature profiles for Soxtec and Soxhlet operation.

Parallels in behavior in both systems and the greater speed of Soxtec can easily be observed in Figure 2. A common assumption suggested to explain the greater speed in reflux extraction when compared with Soxhlet is based on the presence of overheating effects. Even if it is possible to theoretically disregard the importance of such effects, as solvent should be in both cases very near its boiling temperature and the Soxtec apparatus includes a precise temperature control, some experiments were carried out to confirm this point. We therefore measured the temperature in the heating element and in the extraction solvent, both in Soxtec and Soxhlet, using THF (Figure 3). These temperatures were 102 and 65.8 °C, respectively, for Soxtec (with temperature set at 120 °C in the silicone bath), and 226 and 65.2 °C, respectively, for Soxhlet. The experimental results confirm that there is no foundation for the previously cited assumption and even point out the fact the Soxhlet is more prone to overheating of the final extract than Soxtec. It seems more reasonable to explain the increased speed of Soxtec in terms of differences in mass transfer

from the sample to the solvent. The convective flow associated with solvent heating can favor the diffusion of the extract from the inside of the thimble during the boiling step, and the rinsing step is also more efficient as there is a neat flow of clean solvent passing through the thimble. Mass transfer is performed in Soxhlet mainly by spontaneous diffusion through the thimble walls, and only when the extractor empties is there a neat flow of solvent through the thimble. It is not surprising that a rinsing step of 30 min in Soxtec with a boiling time of 1 min leads to an extraction yield very similar to that from a Soxhlet extraction lasting 24 h (see Table 2). Soxtec and Soxhlet should provide extracts of similar composition, as the extraction conditions are almost the same. Figure 4 shows TLC-FID chromatograms for DCM and THF extracts of UL using Soxtec and Soxhlet. THF is a better extraction solvent, mainly because it has a greater capability to extract polar products. Therefore, TLC-FID areas of peaks that correspond to the more polar compounds should be larger for THF extracts. This is the case for uneluted and DCM/MeOH eluted peaks, of which THF extracts mainly consist. DCM extracts are more sensitive to the differences caused by the extraction technique. Figure 4 also shows a similar composition for DCM extracts. As can be seen in Figure 4A,B, their TLC-FID chromatograms mainly differ in the quantitative distribution of the more polar compounds. In any case, the total amount of compounds corresponding to peaks 3 and 4 is very similar for both techniques (83.0 for Soxtec vs 82.5 for Soxhlet). The small differences in composition could be explained in terms of the slightly higher yield provided by Soxtec extraction, as it should mainly consist of polar compounds. Other Extraction Procedures: Multistep Soxtec Extraction. The extraction speed of Soxtec is a good base to plan new extraction procedures. The aim of these is to solve some problems of Soxtec extraction itself, at the same time trying to avoid some difficulties found in the traditional extraction methods. One of the problems found in Soxhlet extraction is that there is no easy way of knowing if the extraction has been completed. Even for unknown samples that could show a different behavior, extraction is carried out for a constant amount of time in the confidence that it will be enough to extract all of the soluble compounds. The slow speed of Soxhlet extraction prevents the easy

Improved Extraction for Coal Products

Energy & Fuels, Vol. 10, No. 4, 1996 1009 Table 5. Comparison of Single-Step and Multistep Soxtec Extraction Procedures (30 min of Total Boiling Time, 30 min of Rinsing Time) Soxtec extraction

% UL extract (THF)

% CTP extract (DCM)

single step multistep A (3 steps) multistep B (6 steps) multistep C (12 steps) av SD

10.0 10.6 10.4 11.2 10.7 0.4

84.9 88.1 92.7 95.6 90.3 4.8

Table 6. Saturation Effects on CTP-DCM in Multi- and Single-Step Soxtec Extractions CTP mass (g)

Figure 5. Operation scheme for single and multistep Soxtec extractions.

acquisition of any kind of kinetic data. On the other hand, the limited amount of solvent involved in Soxtec extraction can lead to saturation problems21 when we deal with very soluble or relatively large amounts of sample. A multistep Soxtec extraction may be a good compromise, solving both difficulties and at the same time keeping most of the advantages of Soxtec related to the total amount of time needed to complete the process and the reduced sample handling. Figure 5 shows a scheme of such a procedure. Multistep Soxtec extraction provides three values: the total yield of the extraction and the amount of extract corresponding both to the last extraction and to the rinsing steps. These additional values should provide an easy way of characterizing the end of the extraction process, as they make it possible to estimate a maximum for the amount of unrecovered extract. Therefore, repeatability, possible extract saturation, and termination criteria were studied for the multistep Soxtec extraction procedure. Three different sequences of multistep extraction were considered, each with a total boiling time of 30 min. Sequence A consists of 3 boiling steps of 10 min each, sequence B uses 6 boiling steps of 5 min each, and sequence C is made up of 12 boiling steps of 2.5 min each. Each sequence also includes 30 min of rinsing after the boiling steps. A second extraction of the sample was also carried out in some cases using 10 min of boiling and 30 min of rinsing. From here on this appears as sequence D. Two sets of sample and solvent were tested, each showing different behavior when extracted: UL with THF and CTP with DCM. 1. Repeatability. Multistep procedures could be considered an interesting alternative if they do not lead to large loss of repeatability. UL extracted with THF, using sequence B, was selected for this test. The moderate yield of this extraction, and the number of steps (six), should make it suitable to detect differences in RSD% related to the multistep procedure. Total extraction yield was 10.8% with a 9.7 RSD% for six experiments with separate weighing of the extract from each step. As can be seen in Table 4, these values compare acceptably with those corresponding to the ULTHF single-step Soxtec extraction (10.5% and 5.2 RSD%). There was no significant improvement of RSD% when the step extracts were accumulated in the way established in the routine multistep procedure (Figure 5). RSD% should be better for sequences with a smaller

0.5 1 1.5 2 4 av SD

mass/volume ratio (g/L)

single-stepa extract %

multistepb extract %

6.7 13.3 20.0 26.7 53.3

84.9 90.1 87.9 71.5 69.9 80.9 9.5

88.1 87.2 87.6 88.8 85.6 87.5 1.2

a 30 min boiling and 30 min rinsing times. b Sequence A (3 steps with 10 min of boiling each and 30 min of rinsing).

number of steps and higher extraction yields. Some data in the following sections confirm this assumption. 2. Saturation. Table 5 shows results for single-step and multistep Soxtec extractions for UL with THF and CTP with DCM. Total extraction yields and RSD% compare well in any case, both when single-step extraction is considered against the multistep procedures and also when comparison is made between different multistep procedures. Standard deviation values are 0.4 for UL and 4.8 for CTP. As there were no noticeable saturation effects for these samples, the amount of CTP was increased while the amount of solvent (DCM) was maintained. Table 6 shows the results for these experiments. Saturation effects did not appear for CTP amounts of sample below 1.5 g when a single-step Soxtec extraction procedure was used. As was expected, when more than 1.5 g was used, saturation effects appeared and multistep procedures were clearly better, with good RSD% (1.35%) for sequence A, even for different amounts of sample. As CTP gives a higher extraction yield, it was reasonable to expect better RSD% than those found for UL and THF. Data in Tables 5 and 6 confirm the possibility of saturation effects in single-step Soxtec extraction when the ratio of sample amount to extraction volume is high. The data also show the absence of such effects for the multistep procedure. Saturation was also confirmed by carrying out a second conventional extraction of thimbles holding 2 and 4 g of CTP. An additional 15% of extract was obtained in both cases, and this was the difference in extraction yield found for these amounts of sample when single-step and multistep extraction procedures were compared. 3. Ending Criteria for the Extraction Process. As extraction processes usually follow asymptotic kinetics, the linear extrapolation for the total extraction time of any of the steps included in a multistep procedure leads to an estimate of the extraction limit. This value should be progressively lower and, with a high enough number of steps, equal to the extraction yield of a product for a given amount of time. Figure 6 illustrates this behavior for sequence C applied to CTP and UL (parts A and B, respectively). A complete extraction should show a horizontal asymptote, and the comparison of the linear extrapola-

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Figure 7. TLC-FID chromatograms of multistep sequential Soxtec extraction: (A) n-hexane fraction; (B) toluene fraction; (C) DCM fraction; and (D) THF fraction. Elution sequence: n-hexane, 38 min; toluene, 3 min; and DCM/MeOH (95:5 v/v), 30 s.

Figure 6. (A, top) Extraction yields from CTP versus time: multistep Soxtec [sequences A (2), B (9), and C (b)] with second single-step extraction (D) and extraction finalization criteria to sequence C. (B, bottom) Extraction yields from UL versus time: multistep Soxtec [sequences A (2), B (9), and C (b)] with second single-step extraction (D) and extraction finalization criteria for sequence C. Table 7. Extraction Completion Percentage (ECP) to CTP, UL, and Hydroliquefied UL [Values Extrapolated to 60 min (Boiling plus Rinsing)] sequence

sample (solvent)

calcd yield % from step 2

5

8

11

real last yield % ECR

C (12 steps) CTP (DCM) 638.7 192.5 124.3 107.5 99.5 UL (THF) 74.3 24.9 15.3 12.2 12.1 39.3 A (3 steps) HULa (THF) 46 a

95.6 11.2 38.6

0.96 0.93 0.98

Hydroliquefied Utrillas lignite.

tion of the yield corresponding to the last step for the total extraction time (boiling plus rinsing) against the experimental yield of the extraction can be used as a criterion to determine the completeness of the extraction. As we need to compare extraction processes leading to very different yields, we should develop some kind of relative value for this purpose. We have used a simple ratio of real extraction yield versus the extrapolated value and named it “extraction completion ratio (ECR)” (Table 7). It is calculated by the formula ECR ) real extraction yield (%)/extrapolated extraction yield (%), where extrapolated extraction yield (%) ) accumulated extraction yield for steps 1 to n - 1 (%) + [total extraction time (minutes) × (last step yield (%)/ step time (minutes))]. The higher the number of steps, the smaller the difference should be. In our case, the extrapolated value to 60 min (boiling plus rinsing) for sequence C using CTP and DCM was 99.52%, while for the real extraction it was 95.63%. ECR

in this case was 0.96. A second extraction of the sample (sequence D) led to a total extraction yield of 96.32%; as expected, this was smaller than the extrapolated value, and the ECR increased to 0.97. In the case of UL, the extrapolated value was 12.05% and the real yield was 11.18%, with an ECR equal to 0.93. Sequence D led to a total yield of 11.9%, increasing the ECR to 0.99. The higher increase of ECR in the case of UL reflects the fact that UL extraction does not show a horizontal asymptote in the conditions used in our experiments. This can be explained in terms of the different nature of CTP and UL as extraction products.22 This is not the only possible ending criterium. While to obtain ECR one more weighing is needed (amount of extract from rinsing in our case), slope values can be calculated using only two weight values (total and last step extract). We could, for example, use the slope of the last extraction step or some particular combination of ECR and slope. This kind of data could be less selfexplanatory, but they might provide a more precise estimate of the achievement of a horizontal step. Nevertheless, slope-based criteria could be less useful against samples that do not show a clear horizontal asymptote. The criteria to be used for a given sample can be left to the analyst to reflect more precisely the particular needs of each case. Other Modified Procedures. The flexibility and speed of Soxtec also make it possible to carry out the sequential extraction of a sample using different solvents. Figure 7 shows results from a sequential extrac(20) Vela, J.; Cebolla, V. L.; Membrado, L.; Andre´s, J. M. J. Chromatogr. Sci. 1995, 33, 417-425. (21) Valca´rcel, M.; Luque de Castro, M. D.; Tena, M. J. Extraccio´ n con Fluidos Supercrı´ticos en el Proceso Analı´tico; Editorial Reverte´: Barcelona, Spain, 1993. (22) Hawthorne, S. B.; Galy, A. B.; Smith, V. O.; Miller; D. J. Anal. Chem. 1995, 67, 2723-2732.

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liquefied UL versus the original UL makes it possible to use a shorter sequence with good results Conclusions

Figure 8. Extraction yields from UL liquefaction versus time in a multistep soxtec (sequence A) with second single-step extraction (D) and extraction finalization criteria.

tion of UL using four solvents with 1 min of boiling time and 60 min of rinsing time for each of them. The composition of the fractions was monitored with TLCFID. The yield for each fraction was 0.4% for n-hexane, 0.5% for toluene, 0.3% for DCM/MeOH, and 8.0% for THF. When all of these values are added, the total 9.2% is very similar to the yield for the direct extraction of UL using THF. Application. The multistep procedure of Soxtec extraction was applied to several samples from the hydroliquefaction of UL. Figure 8 shows the behavior of hydroliquefied UL using sequence A and THF as solvent. It can be seen that the extrapolated value for extraction yield was 39.3%, while the real yield was 38.6%. ECR was 0.98. Sequence D led to a yield of 38.8, increasing the ECR to 0.99. The higher solubility of

The Soxtec extraction technique was studied against the Soxhlet extraction for coal and coal-derived products. Soxtec seems to be a very interesting alternative given that it provides similar or better results, in terms of extraction yield and repeatability, and reduces the time needed for carrying out extractions by up to 90%. The speed and flexibility of Soxtec make it possible to carry out multistep procedures that, while requiring around 15% of the time needed to perform a Soxhlet extraction, allow for a simple characterization of kinetic aspects of the extraction process, especially an ending criterium, and avoid the possibility of saturation of the extract. There is a moderate loss of repeatability in this case for samples of low solubility. The nature and amount of the particular sample to be extracted, and the degree of confidence desired in the results from the extraction process, are the points that the analyst should consider when determining the extraction procedure. Soxtec can also be used for sequential extraction of coal-related products and should provide products very similar in nature to those derived from Soxhlet extraction. Most of these results should be applicable to other kinds of samples as well. Acknowledgment. We are grateful to the ECSC (European Coal and Steel Community, Project 7220-EC/ 765) and the Spanish DGICYT (Project PB93-0100) for their financial support. EF960018O