Efficient and Quick Method for Saturates, Aromatics, Resins, and

Publication Date (Web): April 29, 2013 ... ionization detector (TLC–FID) is a fast and efficient technique for group-type analysis of hydrocarbon re...
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Efficient and Quick Method for Saturates, Aromatics, Resins, and Asphaltenes Analysis of Whole Crude Oil by Thin-Layer Chromatography−Flame Ionization Detector Harender Bisht,† Manasa Reddy,‡ Manthan Malvanker,† Rahul C. Patil,† Ajay Gupta,† Bibeka Hazarika,† and Asit K. Das*,† †

Reliance Industries Limited, Village Motikhavdi, Jamnagar 361 140, Gujarat, India Nirma University, Serkhej, Gandhinagar Highway, Ahmedabad 382 481, Gujarat, India

‡

ABSTRACT: Saturates, aromatics, resins, and asphaltenes (SARA) analysis by thin-layer chromatography−flame ionization detector (TLC−FID) is a fast and efficient technique for group-type analysis of hydrocarbon residues. The present paper attempts to provide a new method for measurement of SARA fractions of whole crude oils using TLC−FID while addressing the issues related to FID calibration and loss of lighter hydrocarbon components during analysis. The experimental results obtained by the new method are compared to the traditional methods. method,25,26 and (iii) thin-layer chromatography−flame ionization detector (TLC−FID) method.15,27,28 Combination of ASTM D1319, ASTM D6560, and ASTM D2007 is a very useful preparative method which is used for the separation and collection of a large amount of SARA fractions. ASTM D1319 uses a fluorescent indicator for quantification of saturates, olefins, and aromatics in a lighter fraction of crude oil. Whereas ASTM D6560 is used to precipitate and gravimetrically quantify the asphaltene fraction from the heavier fraction, the remaining deasphalted sample is further fractionated by column chromatography, as described in ASTM D2007. These methods are time-consuming and require a large amount of sample and solvents. Proper monitoring of the column eluent is needed to prevent cross-contamination of fractions. We have observed significant cross-contamination particularly for aromatic and resin fractions during ASTM D2007 analysis. Similar to the preparative method described above, the HPLC method also requires extensive sample preparation. Asphaltenic components are required to be removed before injection of the crude oil sample into the HPLC column. Only saturate and aromatic fractions are eluted and quantified by HPLC.25 The resin fraction is back-flushed and quantified gravimetrically. The HPLC method gives additional information about the distribution of the aromatic fraction into mono-, di-, and tri-aromatic compounds. TLC−FID is the quickest method for SARA analysis. Multiple samples can be analyzed simultaneously without extensive sample preparation and asphaltene separation. The sample and solvent required for TLC−FID are very nominal. However, there are two main problems in this method: (i) FID response is not uniform for all components even within individual fractions; therefore, careful calibration of each fraction must be performed with most representative pure

1. INTRODUCTION Crude oil is a complex mixture of thousands of compounds having similar elemental composition.1 Separation, identification, and quantification of each individual compound present in crude oil is very difficult, time-consuming, and expensive. It also requires state-of-the-art analytical equipment, such as Fourier transform ion cyclotron resonance−mass spectrometry (FTICR−MS),2 two-dimensional gas chromatography−mass spectrometry (2DGC−MS),3 all-glass heated inlet system/gas chromatography−magnetic sector−mass spectrometry (AGHIS/GC−magnetic sector−MS),4 and highly skilled human resources. Furthermore, to obtain meaningful conclusions from this analysis, the individual compounds are again grouped into different hydrocarbon classes.5 Therefore, such analysis is mostly confined to academic interest and not used in commercial day to day applications. On the other hand, elemental analysis, even though gives good information about the impurities, does not provide good insight into crude oil composition.6 The elemental composition of crude oil does not change much among them, even though they may have vastly different chemical compositions from a refiner’s point of view. Because of these problems, hydrocarbon-group-type analysis is mostly used for quick characterization of chemical composition of crude oils and their residues. Several analytical methods have been developed for hydrocarbon-group-type analysis of the lighter hydrocarbon fraction, such as gasoline,7−9 kerosene,10 and gas oils.11−14 However, for hydrocarbon-grouptype analysis of crude oils and its heavier residues, saturates, aromatics, resins, and asphaltenes (SARA) analysis is most widely used.15−17 Several useful conclusions, such as compatibility for crude oil blending,18 asphalting stability and fouling tendency,19−21 coking propensity,22 and product stability,23,24 can be predicted by SARA analysis of crude oils. Various methods have been developed for SARA analysis of crude oils. The three most widely used methods are (i) combination of ASTM D1319, ASTM D6560, and ASTM D2007, (ii) high-pressure liquid chromatography (HPLC) © 2013 American Chemical Society

Received: February 6, 2013 Revised: April 27, 2013 Published: April 29, 2013 3006

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Figure 1. Calibration graph for TLC−FID.

fractions.27,29−31 (ii) Only the high boiling crude oil fractions can be analyzed by this technique. Lighter fractions of the sample become evaporated during spotting, elution, drying, and analysis.27 In this paper, we have tried to address the above two problems so that TLC−FID can be used for accurate and quick analysis of the whole crude oil.

difference of FID response within similar SARA factions of different crude oils, the standards for TLC−FID calibration were prepared from the crude oil residue of a blend of more than 30 crude oils. ASTM D6560 and ASTM D2007 methods were used to separate individual SARA fractions. When the separated SARA fractions were analyzed by TLC−FID, it was observed that aromatic and resin fractions had some cross-contamination; therefore, they were further purified by column chromatography using alumina as an adsorbent and heptane, heptane + toluene, and toluene as eluting solvents. After purification, all SARA standards gave a single peak in TLC−FID analysis showing good purity. Each of these purified standards contained a wide variety of components because of the large number of crude oils used for their preparation. Therefore, they are more representative standards for calibration of the TLC−FID instrument. At least five solutions of different concentrations were prepared for each SARA faction, and the response factor was calculated after plotting the area of the FID peak with the concentration. The response factors for each SARA fractions were calculated by plotting the FID peak area versus the concentration, as shown in Figure 1. We have obtained good calibration data showing R2 values higher than 0.98. The order of the FID response factor is saturates < aromatics < resins < asphaltenes. 2.3.2. TLC−FID Experiments. The residue and whole crude oil samples were dissolved in dichloromethane to make 10−12 and 15− 20 mg/mL solutions, respectively. In the case of whole crude oils, a slightly higher concentration was taken considering the evaporation loss during analysis. An Iatroscan MK6s instrument was used in this study. The hydrogen gas flow rate was maintained at 160 mL/min, and the air flow rate was maintained at 2 L/min. A 1 μL solution was spotted on silica-coated quartz Chromarods. All 10 Chromarods were spotted with a single concentration of each SARA standard to ensure accuracy and repeatability of data. Before actual sample analysis, two blank runs were performed to remove any contaminant on the Chromarods and to ensure a straight baseline. Once spotting of the sample was completed, the Chromarods were kept in n-heptane for 30 min to elute the saturate components. After n-heptane elution, the Chromarods were air-dried for 5 min and kept in toluene for 12 min to elute aromatic components. The Chromarods were again air-dried for 5 min and kept in a mixture of dichloromethane and methanol (95:5) for 3 min to elute the resin components. Asphaltene components do not elute and remain at the bottom of the Chromarods. After drying for another 10 min, the Chromarods were placed in the Iatroscan for FID analysis.

2. EXPERIMENTAL SECTION In total, eight crude oils were analyzed for SARA quantification by three different methods. 2.1. ASTM Methods. In ASTM methods, the lighter components of crude oil were distilled as per ASTM D86. The saturate, olefin, and aromatic components in the distillate fraction were quantified as per ASTM D1319 (fluorescent indicator adsorption) method. Olefins present in the lighter fraction were added to saturates while reporting the SARA analysis. The heavier residue fraction left after D86 distillation was deasphalted as per ASTM D6560 method. Briefly, the crude oil residue was refluxed with 30 times volume of n-heptane. The mixture was cooled, and the precipitated asphaltenes were filtered. The solid asphaltene were washed with a copious amount of n-heptane before drying and quantified gravimetrically. The solvent from the deasphalted filtrate was evaporated in a rotary evaporator. The dried deasphalted residue was weighed and loaded on an adsorbent column as per ASTM D2007 method and eluted with different solvents to collect the saturate, aromatic, and resin fractions. The results of all three methods were combined to obtain complete SARA analysis of the whole crude oil. 2.2. ASTM and TLC−FID Methods. TLC−FID is suitable only for heavy hydrocarbons, because lighter hydrocarbons become evaporated from the TLC−FID rods during elution, drying, and analysis. To use TLC−FID for SARA analysis of the crude oil residue, the crude oil was distilled as per ASTM D86 method to generate lighter and residue fractions. Subsequently, the saturate, olefin, and aromatic components in the lighter fraction were quantified as per ASTM D1319 method, and the SARA analysis of the residue fraction was determined by the TLC−FID method. The amount of SARA fractions of the whole crude oil was calculated after combining the results of the two methods, i.e., ASTM D1319 and TLC−FID. 2.3. New TLC−FID Method. In the third method, the whole crude oil was analyzed only by TLC−FID without any fractionation or deasphalting. 2.3.1. Calibration of TLC−FID. The sensitivity of the FID with respect to the hydrocarbon structure and presence of heteroatoms has been extensively studied.27,29−31 Components present in individual SARA factions of different crude oil may give different FID response because of changes in their structures and heteroatoms. It is very cumbersome to calibrate TLC−FID by SARA standards extracted from individual crude oil before each analysis. To average out the

3. RESULTS AND DISCUSSION Complete SARA analysis of whole crude oil (lighter fraction as well as residue) by ASTM methods requires the use of four different methods, namely, ASTM D86, ASTM D1319, ASTM D6560, and ASTM D2007. ASTM D86 is required to 3007

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Table 1. Crude Oil Propertiesa PINA analysis (wt %) 105 °C

a

TBP distillation data

150 °C

165 °C

105 °C

crude oil

density at 15 °C (g/cm )

API gravity (deg)

P

A

P

A

P

A

CR1 CR2 CR3 CR4 CR5 CR6 CR7 CR8

0.8908 0.8656 0.9117 0.8924 0.8612 0.8298 0.9508 0.9361

27.3 31.9 23.6 27.0 32.7 38.9 17.3 19.6

97.7 96.8 98.6 91.2 97.6 96.1 0.0 95.2

2.3 3.2 1.4 8.8 2.5 3.9 0.0 4.8

89.4 85.2 89.7 84.8 87.7 83.5 0.0 84.3

10.6 4.8 10.4 15.2 12.3 16.5 0.0 15.7

87.5 79.5 81.7 84.6 80.6 74.6 0.0 78.7

12.5 19.5 18.3 15.4 19.4 25.4 0.0 21.3

3

150 °C

165 °C

yield (wt %) 7.4 9.1 5.7 7.1 7.2 10.4 0.3 4.6

11.9 14.9 9.9 12.9 14.8 19.9 0.5 8.4

13.8 16.7 11.5 14.6 17.3 23.7 0.6 10.0

P, paraffin; A, aromatics.

Figure 2. Comparison of SARA results by three different methods.

fractionate crude oil into lighter and heavier fractions. ASTM D1319 (fluorescent indicator adsorption) method is employed for the determination of saturates and aromatics in the lighter fraction by column chromatography. Resins and asphaltenes are not present in the lighter fraction. ASTM D6560 is used for gravimetric estimation of asphaltenes by the precipitation method. ASTM D2007 is used to estimate saturates, aromatics, and resins by column chromatography of the deasphalted residue. TLC−FID has reduced the sample requirement and time of SARA analysis by substituting ASTM D6560 and ASTM D2007 methods. However, it is not used for whole crude oil because there is significant loss of lighter fractions during analysis. Therefore, fractionation of crude oil by D86 method is

still required to generate lighter and heavier fractions. For saturate and aromatic estimation of the lighter fraction, ASTM D1319 is used, whereas the SARA analysis of the heavier fraction can be performed by TLC−FID. Eight crude oils from various sources having different physicochemical properties were used for this study. The details are shown in Table 1. These crude oils are very different in terms of API gravity and density. All of these eight crude oils were characterized for SARA analysis by the three methods described above. The SARA results of the first two methods, i.e., combination of ASTM D1319, D6560, and D2007 and combination of ASTM D1319 and TLC−FID, are in good agreement, except for a few cases, as shown in Figure 2. However, the SARA results of the 3008

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the error is much higher if the sample has been prepared after asphaltene separation. The difference of SARA values obtained by the new TLC− FID method and traditional methods is more pronounced, as shown in Table 3. The average differences in the values of SARA determined by the new TLC−FID and ASTM methods are 10.8, 11.8, 4.0, and 1.5 wt %, respectively. Whereas the average differences in the SARA values determined by the new TLC−FID and ASTM + TLC−FID methods are 11.9, 10.3, 1.6, and 1.6 wt %, respectively The discrepancy in the SARA results obtained by the new TLC−FID method and ASTM methods can be explained if the evaporation losses during TLC−FID analysis of whole crude oil are taken into account. Asphaltenes and resins are highly polar and high boiling fractions of crude oil; therefore, their evaporation can be ruled out during the analysis. The remaining two fractions that can become evaporated during analysis are saturates and aromatics. The distributions of saturates and aromatics in the lighter fractions of eight crude oils are shown in Table 1. The lighter fractions of most of the crude oils are predominantly composed of saturates, with very little aromatic components. Therefore, even if all of the evaporation losses are assigned to the saturate fraction for the sake of simplicity and ease of calculation, the error thus introduced will be within the permissible limits. The selection of the boiling range of various lighter fractions was performed on the basis of the typical true boiling point (TBP) distillation used to quantify the yields of various fractions in crude oils. Generally, TBP distillation is performed to obtain the following 11 fractions: (1) 15 °C, (2) 15−105 °C, (3) 105−150 °C, (4) 150−165 °C, (5) 165−227 °C, (6) 227− 270 °C, (7) 270−370 °C, (8) 370−390 °C, (9) 390−410 °C, (10) 410−565 °C, and (11) 565+ °C. To ascertain the boiling range of crude components that are prone to evaporation during TLC−FID analysis, we have normalized the SARA results based on the yield of three TBP factions, i.e., initial boiling point (IBP)−105 °C, IBP−150 °C and IBP−165 °C. The results of these normalizations are compared to the other two methods. 3.1. Case 1: IBP−105 °C Fractions Are Evaporating during TLC−FID Analysis. If we analyze the distribution of saturates and aromatics in the IBP−105 °C fraction of eight crude oils, as shown in Table 1, the saturate content is >90%. Furthermore, all eight crude oils analyzed in this study do not have more than 10 wt % of this fraction. Therefore, the normalization of TLC−FID results of whole crude oil was

third method, i.e., whole crude oil by TLC−FID, show significantly lower saturates for most of the crude oils in comparison to the two methods. The values of the other three fractions, especially the aromatic fractions, are higher for most of the crude oils in the TLC−FID method. All four panels are drawn on the same scale to show the relative distribution and the extent of deviation of the SARA fractions by different methods. As seen from Figure 2, a typical pattern of SARA distribution was observed for all eight crude oils analyzed in this study. The saturate content is the highest, and the asphaltene content is the lowest. It is also observed that the deviation in results of TLC−FID and ASTM methods is more pronounced for saturate and aromatic fractions. In some of the crude oils, there is a significant difference in SARA results of the standard methods also. Table 2 shows the difference in SARA (wt %) determined by ASTM and ASTM + TLC−FID methods. Table 2. Difference in SARA Values in Traditional Methods difference in SARA (wt %) of ASTM versus TLC−FID + ASTM methods crude oil CR1 CR2 CR3 CR4 CR5 CR6 CR7 CR8 difference (wt %)

minimum maximum average

S

A

R

A

9.1 2.3 1.9 6.7 3.6 1.1 4.1 4.6 1.1 9.1 4.2

5.4 0.8 7.7 3.0 9.1 2.0 1.1 0.4 0.4 9.1 3.7

5.3 2.5 4.7 4.9 5.7 2.7 3.3 2.3 2.3 5.7 3.9

1.6 1.1 1.4 1.1 0.2 0.4 0.3 2.7 0.2 2.7 1.1

As described in Table 2, the SARA values of eight crude oils obtained by the traditional methods are also varying significantly in some cases. The average differences in the weight percent of SARA determined by the two traditional methods are 4.2, 3.7, 3.9, and 1.1 wt %, respectively. In some cases, the difference is as high as 9 wt %. The repeatability (same operator) limits of saturates and aromatics in ASTM D2007 are 2.1 and 2.3 wt %, respectively. Whereas the reproducibility (different operator) limits for saturates and aromatics are much higher at 4.0 and 3.3 wt %, respectively. Moreover, the method (ASTM D2007) clearly mentioned that

Table 3. Difference in SARA Values in Traditional Methods versus the New TLC−FID Method difference in ASTM versus new TLC−FID crude oil CR1 CR2 CR3 CR4 CR5 CR6 CR7 CR8 difference (wt %)

minimum maximum average

difference in ASTM + TLC−FID versus new TLC−FID

S

A

R

A

S

A

R

A

12.6 5.8 13.9 7.8 18.8 15.4 2.8 9.2 2.8 18.8 10.8

14.4 5.3 13.2 1.2 21.3 17.8 6.8 14.0 1.2 21.3 11.8

4.3 2.2 3.8 7.3 2.7 2.0 3.7 6.0 2.0 7.3 4.0

2.5 2.7 4.2 0.7 0.2 0.3 0.2 1.2 0.2 4.2 1.5

21.7 8.0 12.0 1.0 15.2 16.5 6.9 13.8 1.0 21.7 11.9

19.8 6.1 5.5 1.8 12.2 15.8 7.9 13.6 1.8 19.8 10.3

1.1 0.3 0.9 2.4 3.0 0.7 0.4 3.7 0.3 3.7 1.6

0.9 1.6 5.6 0.4 0.0 0.1 0.5 3.9 0.0 5.6 1.6

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Figure 3. Comparison of SARA results after normalization (105 °C cut).

Table 4. Difference in SARA Values in Traditional Methods versus New TLC−FID ASTM versus new TLC−FID

case 1 (IBP−105 °C)

case 2 (IBP−150 °C)

case 3 (IBP−165 °C)

ASTM + TLC−FID versus new TLC−FID

difference (wt %)

S

A

R

A

S

A

R

A

minimum maximum average minimum maximum average minimum maximum average

1.4 14.7 8.2 0.9 14.1 6.6 1.6 14.9 6.1

2.0 17.8 9.7 0.1 14.6 8.2 0.3 14.6 5.1

2.0 7.3 4.0 2.0 7.3 4.0 2.9 9.9 5.9

0.3 1.7 0.8 0.3 1.9 0.8 0.1 3.5 1.2

3.7 17.3 9.3 1.3 14.8 7.5 0.6 13.9 6.8

0.8 16.4 7.8 0.7 14.6 6.3 0.1 11.6 5.8

0.3 3.7 1.6 0.3 3.7 1.6 0.2 6.2 2.3

0.2 2.8 1.0 0.2 2.6 1.0 0.1 4.9 1.4

performed on the assumption that the IBP−105 °C fraction is becoming evaporated during analysis. To further simplify the analysis, the total amount of this fraction was added to saturates because the total aromatic content of this cut is about 1 wt % of the whole crude oil. Therefore, this normalization will introduce a maximum error of 1 wt % in saturates and aromatics, which is much lower than the limit reported in ASTM methods. The normalized results are shown in Figure 3. As evident from Figure 3, the normalized results of TLC− FID analysis of whole crude oil are closer to the other two methods. However, there is still a significant difference particularly in saturate and aromatic fractions, as shown in Table 4 (case 1). The average differences in the normalized values (case 1) of SARA fractions determined by the new TLC−FID and ASTM

methods have reduced to 8.2, 9.7, 4.0, and 0.8 wt %, respectively. Whereas the average differences in the normalized SARA values determined by the new TLC−FID and ASTM + TLC−FID methods have reduced to 9.3, 7.8, 1.6, and 1.0 wt %, respectively. 3.2. Case 2: IBP−150 °C Fractions Are Evaporating during TLC−FID Analysis. To further reduce the difference between TLC−FID and ASTM methods, the SARA values of the new TLC−FID method were normalized with the assumption that up to 150 °C fraction is evaporating during analysis. The saturate content in the 150 °C fraction is >83 wt % for all crude oils in this study, as shown in Table 1. The maximum amount of 150 °C cut is