Energy Fuels 2009, 23, 4515–4521 Published on Web 08/14/2009
: DOI:10.1021/ef900358q
Determination of Asphaltenes in Crude Oil and Petroleum Products by the on Column Precipitation Method Estrella Rogel,* Cesar Ovalles,* and Michael E. Moir Chevron Energy Technology Co, 100 Chevron Way, Richmond, California 94802
John F. Schabron Western Research Institute, 365 North ninth Street, Laramie, Wyoming 82072
Energy Fuels 2009.23:4515-4521. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/30/18. For personal use only.
Received April 22, 2009. Revised Manuscript Received July 21, 2009
An improved analytical method for the determination of asphaltene content in crude oils and petroleum products was developed. This method uses an on-column precipitation and redissolution technique coupled with an evaporative light scattering detector (ELSD). Among the improvements is a quantification of the asphaltene content using a calibration procedure, and the redissolution agent is replaced with a blend with a better solvent power. The method requires a small amount of sample, it takes only 20 min to perform and can quantify asphaltene contents as low as 120 ppm with a relatively low error. It can be automated, and it also has the potential to be used on line. It has better repeatability and reproducibility than the traditional gravimetric methods, which are the actual standards. This work also includes results from preparative experiments that support the reliability of the described methodology. All of these characteristics make this on-column method a good candidate for the monitoring of processes in the oil industry.
inert column packing. Later, a similar technique (sequential extraction fractionation (SEF)) was applied to separate petroleum residua into different solubility fractions.4 These early attempts of the use of the technique pointed out that the packing material and the detection technique were two key aspects in the development of a more practical and faster methodology. In particular, it was important to use a packing material able to retain the asphaltenes based on a filtration process and not by adsorption. Problems related to packing material and detection techniques were successfully addressed by Schabron and Rovani2 in their recent development. Polytetrafluoroethylene-packed (PTFE) was used as inert packing material and an evaporative light scattering detector (ELSD) was incorporated into the technique. Another noticeable improvement is the use of high performance liquid chromatography (HPLC) equipment to speed up the analysis time, even though the technique does not involve a chromatographic separation. This technique has been applied to SHARP asphalts and pyrolized residues and has shown high potential for the study of the polar constituents in petroleum materials. In the present work, an improved method to quantify asphaltene content in samples with a wide range of origins and asphaltene content was developed. It is based on the technique developed by Schabron and Rovani2 and includes several important modifications that make it more robust and suitable to be applied to a large range of petroleum materials. The improved technique shows better repeatability than the gravimetric methodologies and incorporates a calibration procedure for the determination of asphaltene contents. A significant number of samples from different sources were
Introduction Asphaltenes are very well known for their tendency to precipitate during production and refining operations, causing significant losses to the oil industry every year. The potential amount of asphaltenes that can precipitate is an important parameter to consider in the design and monitoring of different processes in the oil industry. The current gravimetric methods (i.e., ASTM D6560/IP 143)1 used to determine asphaltenes take several hours to perform and use large amounts of solvents. Another disadvantage of these methods is that they have relatively large errors probably as a result of the extensive manipulation of the samples. For instance, the repeatability and reproducibility of standard test ASTM 6560 are 10% and 20%, respectively. These values apply to samples with asphaltene contents between 0.5 to 30.0%. It is also reported that the error values are significantly higher for asphaltene contents lower than 0.5%. For these reasons, gravimetric methods are impractical as process control tools. Recently, Schabron and Rovani2 developed a new automated separation technique that allows the quantification of asphaltenes using on-column precipitation and redissolution of asphaltenes. On-column separation of asphaltenes was first reported by Boduszynski et al.3 Heptane, toluene, and pyridine were used to extract coal liquids deposited onto an *To whom correspondence should be addressed. Telephone: (510)-2421725. E-mail:
[email protected] (E.R.),
[email protected] (C.O.). (1) Standard Test method for Determination of Asphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products ASTM 6560; ASTM International USA, 2005. (2) Schabron, J. F.; Rovani, J. F. Fuel 2008, 87, 165. (3) Boduszynski, M. M.; Hurtubise, R. J.; Silver, H. F. Anal. Chem. 1982, 54, 372. r 2009 American Chemical Society
(4) Boduszynski, M. M. Energy Fuels 1987, 1, 2.
4515
pubs.acs.org/EF
Energy Fuels 2009, 23, 4515–4521
: DOI:10.1021/ef900358q
Rogel et al.
Figure 2. Typical output for the asphaltene separation.
material of the column was commercially available polytetrafluoroethylene (PTFE), which was sieved to 40-60 mesh. The concentration of the solution used in the measurement varies according to the estimated amount of the asphaltenes: for crude oils, residues, or processed samples with asphaltene contents larger than 1%, a solution of 0.1 g in 10 mL is prepared; for materials with less than 1%, a solution of 5 g in 10 mL is recommended, even though in some cases when the asphaltene content is very low (on the order of 100-200 ppm), the material can be injected directly into the column without further dilution. For solid materials, such as deposits with large asphaltene contents, (>50%) a solution of 0.01 g in 10 mL is used. Asphaltene Determination by the Gravimetric Method. The asphaltene content was determined using a modification of the ASTM D6560 test.1 In this modified version, A 1/20 sample/ n-heptane ratio is used, and the blend is filtered at 80 o C. The precipitated material is washed using hot heptane prior to drying and weighing. The last traces of precipitate are removed from the digestion beaker using chloroform and then recovered. For Crude Oil AA, this procedure was repeated several times to obtain several grams of asphaltenes in order to compare properties with asphaltenes extracted using the on-column method. Preparative On-Column Separation. The preparative separation of the on-column method was carried out using Crude Oil AA. One gram of this crude oil was dissolved in a 50 mL of dichloromethane. One hundred microliters of this solution was injected into a 10 cc column filled with 40-60 Mesh PTFE using n-heptane as precipitant solvent. After 6 min, the solvent was switched to 10% methanol/90% dichloromethane. Both fractions were collected using an Agilent Technologies 1200 Series preparative HPLC. After one hundred injections (0.2 g) and removal of the solvents, 0.162 g of maltenes (heptane solubles) and 0.029 g of asphaltenes (heptane insolubles) were collected. With respect to the original solution, the total recovery was 95.5%, with 81.1% maltenes and 14.7% asphaltenes. Asphaltene Accelerated Solvent Extraction. For the asphaltene separation using the accelerated solvent extractor (ASE), a Dionex 300 apparatus was used. Two grams of Crude Oil AA was dissolved in the minimum amount of dichloromethane. This solution was stirred with 45 g of PTFE (40-60 Mesh) at room temperature for about an hour. The solvent was removed by heating at 60 °C under nitrogen overnight. The PTFE supported crude oil was placed into a 100 mL stainless steel cell and extracted with heptane at room temperature with 60 min of soaking time. After flushing the cell with nitrogen and removing the solvent, the maltene fraction (heptane solubles) was collected (1.66 g or 83% wt.). The asphaltene containing cell was then extracted three times with 10% methanol/90% dichloromethane at room temperature for 10 min. After removing the solvents, 0.28 g of extracted asphaltene (14%) were isolated. The overall mass balance was 97%.
Figure 1. Scheme of asphaltene separation using the on-column method.
successfully analyzed following this procedure and the results compared with the gravimetric technique. This work also includes preparative experiments using three methodologies: preparative on-column separation, gravimetric method, and automatic solvent extraction. These studies support the reliability of the improved method to quantify asphaltenes and help us understand the changes in yields and asphaltene properties by different extraction conditions. Experimental Section Materials. A total of 66 petroleum samples were tested including crude oils, vacuum and atmospheric residues, visbroken residues, LC-Fining products, Coker heavy oils, and other hydro- and thermal- treated materials. A Venezuelan crude oil was used as reference material (Crude Oil AA) with the following characteristics: 9.5°API, 1.52 molar ratio H/C, 5.53% of sulfur, and 13.6% of asphaltenes (according to the gravimetric method). Asphaltene Determination by the On-Column Method. Figure 1 shows a general scheme of the method. A solution of the sample in dichloromethane or toluene (if the sample is highly paraffinic) is injected into a column packed with an inert material using n-heptane as the mobile phase. This solvent induces the precipitation of asphaltenes and, as a consequence, their retention in the column. Even though the method was mainly developed for n-heptane, all other short length paraffins (pentane, hexane, octane, etc.) can be used as well. Experiments with pentane have produced similar results. The first eluted fraction from the column with the heptane is the maltenes. After all this fraction has eluted, the mobile phase is switched to a blend dichloromethane/methanol 90/10 v/v. This blend redissolves the asphaltenes. Both maltenes and asphaltenes can be quantified using the ELSD. Figure 2 shows a typical output for the separation. This first peak corresponds to maltenes and the second one to asphaltenes. This method uses HPLC equipment even though no chromatographic separation occurs. The HPLC system consisted of a HP Series 1100 chromatograph and an Alltech ELSD 2000 detector. In this work, the drift tube temperature in the detector was maintained at 75 °C, which is sufficient to permit complete evaporation of the three solvents used: n-heptane, dichloromethane, and methanol. The flow rates used were kept constant during all the experiments. The volumetric flow of the mobile phase was 4.0 mL/min and 3.5 L/min of nitrogen was used for the nebulizing gas of the ELSD. Two different sizes of stainless steel columns were used: 10 mm i.d. 250 mm and 10 mm i.d. 100 mm. No differences were observed as a result of using different sizes. The packing
Results and Discussion Selection of the Redissolution Agent. At the beginning, the experiments were carried out using in dichloromethane as 4516
Energy Fuels 2009, 23, 4515–4521
: DOI:10.1021/ef900358q
Rogel et al.
Figure 3. Comparison of elution patterns using heptane and the following redissolution solvents: (A) dichloromethane only, (B) dichloromethane initially followed by 10% methanol/90% dichloromethane, and (C) 10% methanol/90% dichloromethane only.
Figure 4. On-column determination of asphaltenes using a transparent column.
When the same series of experiments were run without the filter, the formation of a darker ring in the top of the column was detected and also a color gradient from darker in the top of the column to clear in the bottom. This phenomenon may be an indication that a filtration process is occurring where most of the asphaltene particles are retained at the top part of the column. In Figure 4, the formation of the dark area at the top column can be seen confirming what was previously pointed out. Therefore, it can be concluded that the PTFE column acts as a filtration medium and that the asphaltenes are retained in it. The use of a 40-60 mesh-size PTFE is needed to provide a reasonable area for retention but incurs only a small pressure drop. Calibration Procedure. One of the key aspects of the improved technique is the use of ELSD. Theses detectors nebulize the eluent from a chromatography column with a gas, vaporize the droplets formed, and pass a stream containing nonvolatile particles through a light-scattering photometer. The light scattered by the nonvolatile particles is collected and it is a measure of the concentration of the solute in the column effluent. It is assumed that the measured peak area (A) is related to the sample mass (M) by:7
the solvent to remove asphaltenes from the column as was done in the original Schabron and Rovani2 method. However, we soon realized that the dichloromethane alone is unable to remove all the asphaltenes from the column. First, the stabilization of the column took a long time to achieve repeatable values, and after a short period of use, the values started to show a high deviation from the average. Also, we noticed that washing the column with dichloromethane/ methanol produced a large elution peak consistent with a high amount of material retained on the column. Figure 3 shows a comparison of the method using dichloromethane and the blend dichloromethane/methanol. Also shown is how when the solvent is switched from dichloromethane to the blend, a third peak eluted from the column corresponding to retained material. The use of methanol for decreasing the influence of hydrocarbon adsorption mechanisms from polymeric liquid chromatographic phases has been addressed in the past.5,6 Repeatability problems disappeared once dichloromethane was replaced with the dichloromethane/methanol blend. Precipitation and the Redissolution Process in a Transparent Column. In order to understand how and where the precipitation and redissolution of the asphaltenes takes place, a series of experiments was carried out using a transparent column. The dimensions of the column were 5 mm i.d. 100 mm, and it was designed to withstand pressures up to 90 bar. As in the conventional HPLC column, polystyrene 25 μm frits are used before and after the column. PTFE tubing was used to connect this transparent column to the pump and detector. The same conditions as those used in the on-column method were used, and the analysis of Crude Oil AA samples gave results similar to those obtained using the standard stainless steel column. Once the sample was injected into the heptane containing column, we found that a significant portion of the precipitated asphaltenes was retained in the frit indicating that either they are adsorbed by this material (polyethylene) or the size of the precipitated asphaltene particle is too large to pass through the 25 μm filter. Also, these results indicate that the precipitation occurs before the sample enters the column.
A ¼ C 3 MB
ð1Þ
where C and B are coefficients depending on droplet size, concentration, solvent, nature of solute, and some other variables including the design of the detector.8-10 However, even though ELSD is considered a mass detector, there are serious doubts about its universal response.7 In fact, response depends on density and refractive index of the samples. Even though it has been found that the response of the ELSD to asphaltenes from different sources is basically the same,11 it is highly recommended to have a reliable calibration procedure in place in view of the complex dependence of the response on different factors. The quantification of asphaltenes in the original technique using ELSD is based on a response factor correction. (7) Mengerink, Y.; De Man, H. C. J.; Van der Wal, Sj. J. Chromatogr. 1991, 552, 593. (8) Van der Meeren, P.; Vanderdeelen, J.; Baert, L. Anal. Chem. 1992, 64, 1056. (9) Righezza, M.; Guiochon, G. J. Liq. Chromatogr. 1988, 11, 1967. (10) Luzio, G. A. J. Liq. Chromatogr. Relat. Technol. 2006, 29, 185. (11) Rogel, E.; Ovalles, C.; Moir, M. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2008, 53, 400.
(5) Snyder, R. L. Anal. Chem. 1969, 41, 1223. (6) Carbognani, L. Pet. Sci, Technol. 2003, 21, 1685.
4517
Energy Fuels 2009, 23, 4515–4521
: DOI:10.1021/ef900358q
Rogel et al.
Figure 6. Calibration curves using gravimetrically obtained asphaltenes from virgin materials as calibration standards.
Figure 5. ELSD area as a function of the mass obtained by the gravimetric technique.
Table 1. Repeatability Standard Test Deviations of the On-Column Method
This correction is calculated by comparing the average response for weighed asphaltenes to the average response for weighed maltenes. According to Schabron and Rovani,2 the ratio of the two factors is 1.4, which is the square of the ratio of the density of the polar asphaltenes to the density of the maltenes. For heavy samples, this ratio seems to be constant, and this procedure works well. However, when a more diverse group of samples comprising a wide range of asphaltene contents were used to test the method, it was found that the responses for maltenes did not correlate very well with the maltene content. Figure 5 shows a correlation between the recorded peak area and the relative content of asphaltenes and maltenes measured by the gravimetric method. As can be seen, asphaltene areas show a good correlation with asphaltene content. For maltenes, the results indicate a larger dispersion of the data probably due to two factors:
asphaltene content 100 ppm-10000 ppm 1%-5% 5%-10% 10%-20% 20%-30% more than 30%
7-28 0.0013-0.27 0.0416-0.22 0.0220-0.64 0.0268-0.83 0.1092-0.63
a vacuum residue. Test 3 and test 1 were run more than two months apart, also indicating that no degradation in the column occurred during that time due to use. As can be seen, the results show a relatively good agreement between the different calibration curves. This can be translated in differences not larger than 5% when the different calibration curves are used to determine the asphaltene content of real samples. Finally, new calibration curves are required when other nonsolvents, such as (pentane, hexane, octane, etc.), are used as precipitant agents. Evaluation of the Method. The standard deviation of this method was determined for 66 samples including crude oils and petroleum products from different sources. Also, some samples prepared by dilution of virgin and processed materials were used (mostly to test very low concentrations). The maximum and minimum repeatability standard test deviations found are shown in Table 1 depending on the asphaltene content. These values are considerably lower than that reported for the gravimetric method.1 The repeatability of a reference sample (Crude Oil AA) was determined using several months of data. It was found that in this case, the standard deviation was 1.4 (approximately 5% error) with ca. 60% of the test values within þ σ and 100% of the test values within þ 2σ. A plot of the results is shown in Figure 7. Other important aspects to evaluate the performance of any analytical technique are the detection and quantification limits. According to previous studies using ELSDs, the detection limit can be determined by:7 2xN 1=x M load ¼ M ð3Þ H
(1) Maltenes are more diverse than asphaltenes, which means that density and refractive index vary widely among them, and as a result, the signal changes. (2) Maltenes can be evaporated during the detection process; as for some of the sample, the distillation data showed up to ca.10% of volatile material below 230 °C. It is clear from Figure 5 that the asphaltene/maltene response ratio changes as a function of the concentration when a diverse group of samples is tested. On the basis of these results, a new quantification method is proposed on the basis of a calibration procedure using a reference material. The calibration procedure was developed based on eq 1. An asphaltene solution prepared using asphaltenes extracted from virgin samples is used to this end. The recommended concentration for this solution is 0.0100 g in 10 mL of dichloromethane. The area A is determined for different injection volumes in the range from 1 to 14 μL. Then, the areas of the peaks are related to the masses M according to: log M ¼ D log A þ E
repeatability standard deviations
ð2Þ
where Mload is the detection limit, M is the injected mass of analyte, N/H is the ratio of the noise to the peak high, and x is the slope of the calibration curve. The detection limit for asphaltenes using the Alltech ELSD 2000 detector was found to be 0.40 μg. Under the conditions of the test, this mass value corresponds with an
where D and E are the calibration constants. Regression analysis was used to calculate the calibration constants. Figure 6 shows a set of different calibration curves run for the same column. In this case, two different heptane asphaltenes were injected: AA from a crude oil and AB from 4518
Energy Fuels 2009, 23, 4515–4521
: DOI:10.1021/ef900358q
Rogel et al.
Figure 7. Repeatability of the reference sample (Crude Oil AA) for the on-column method. Figure 10. Comparison between the on-column method and the gravimetric method for processed samples with high asphaltene content (>1% wt.).
Figure 8. Comparison between the on-column method and the gravimetric method for virgin samples with high asphaltene content. Figure 11. On-column separation of gravimetric asphaltenes from a processed sample (visbroken residue) showing a large maltene area.
Figure 9. Comparison between the on-column method and the gravimetric method for virgin samples with low asphaltene content.
asphaltene content of 40 ppm in crude oils and petroleum samples. The quantification limit is considered to be 3 times the detection limit; therefore, this value for asphaltenes using the Alltech ELSD 2000 detector was found to be 1.20 μg. Under the conditions of the test, this mass value corresponds with an asphaltene content of 120 ppm in crude oils and petroleum samples. Correlation with Gravimetric Methods. Since heptane insolubles or asphaltenes are defined by the method of extraction, it is impossible to determine the bias in asphaltene determinations. However, it is possible to compare results from different techniques. To this end, several samples were tested using the new technique and the ASTM D6560
Figure 12. Comparison between the on-column method and the gravimetric method for processed samples with low asphaltene content (
