Energy Fuels 2010, 24, 2227–2232 Published on Web 11/20/2009
: DOI:10.1021/ef9007906
Microwave-Assisted Procedure for Salinity Evaluation of Heavy Crude Oil Emulsions† )
Diogo P. de Moraes,‡ Fabiane G. Antes,‡ Juliana S. F. Pereira,‡ Maria de Fatima P. dos Santos,§ Regina C. L. Guimar~ aes,§ Juliano S. Barin,‡ Marcia F. Mesko, Jose N. G. Paniz,‡ and Erico M. M. Flores*,‡ Departamento de Quı´mica, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil, §Centro de Pesquisas e Desenvolvimento Leopoldo Am erico Miguez de Mello, TPAP-CENPES/PETROBRAS, 21941-945, Rio de Janeiro, RJ, Brazil, and Departamento de Quı´mica Analı´tica e Inorg^ anica, Universidade Federal de Pelotas, 96010-900, Pelotas, RS, Brazil )
‡
Received July 25, 2009. Revised Manuscript Received October 29, 2009
A procedure for salt extraction of heavy crude oil emulsions (API lower than 14°) was performed using microwave radiation. Emulsion was transferred to high-pressure quartz vessels with further water addition. Microwave radiation was applied for 60 min, and after separation of phases, chloride determination was performed in the water phase by ion chromatography (IC). The sampling procedure, heating time (10-40 min), microwave power (500-800 W), and the number of extractions (1-5) were evaluated. The salt remaining in the oil phase was determined, as chloride, by IC after sample digestion by a microwave-induced combustion technique, and the results confirmed the high efficiency of the proposed microwave-assisted salt extraction procedure. Using the proposed procedure, the salt extraction efficiency from heavy crude oil emulsion was higher than 95%. For a comparison of the results, Cl determination was also performed by inductively coupled plasma-optical emission spectrometry and potentiometric titration. The procedure allowed the salinity determination in less time (1 h) in comparison to the recommended ASTM D 6470 method (more than 4 h), and the use of toxic reagents or demulsifiers was avoided. Up to eight samples could be processed simultaneously, making the proposed procedure suitable for routine analysis in a laboratory scale.
performed in the aqueous extract by potentiometric titration, and results are expressed as NaCl content. Despite the spread use of this method, its application is normally restricted to crude oils with American Petroleum Institute (API) higher than 19°. For crude oils with API lower than 19°, this method presents some drawbacks, such as a long time for sample preparation (more than 4 h) and use of toxic solvents. Heavy crude oil yields more heavy compounds, such as aromatic structures, and a high concentration of asphaltenes and resins. These compounds are responsible for emulsion stability, making it difficult to apply conventional methods for salt extraction.5-7 Nowadays, the depletion of light crude oil reserves has been accompanied by the increase of heavy crude oil production.8 Therefore, new technologies have been investigated to promote the demulsification of heavy crude oil emulsions and, consequently, to remove the salt from the oil phase. Microwave radiation has been successfully used in many fields of chemistry, including organic synthesis,9 sample digestion,10-12 and drying processes.13 Taking into account the
1. Introduction Crude oil is generally produced as a water-in-oil emulsion with high stability because of the presence of natural surfactants. Emulsified water contains high concentrations of salts, mainly sodium chloride, that can be responsible for corrosion during crude oil processing in refineries. In addition, the presence of salts could change the quality of final products as, e.g., petroleum coke and residues of petroleum distillation.1,2 In a laboratory scale, a high content of salt may cause interference in some analysis performed for crude oil characterization, as observed for total acid number determination.3 Generally, the salt concentration should be reduced to values lower than 500 μg g-1 to avoid interferences in crude oil analysis. Salt determination in crude oil is currently performed in a laboratory scale according to the American Society for Testing and Materials (ASTM) D 6470 method.4 This method recommends the salt extraction using xylene at 65 °C followed by extraction with alcohol, acetone, and water in an electrically heated extraction apparatus. Chlorine determination is
(5) Ahmaruzzaman, M.; Sharma, D. K. Energy Fuels 2007, 21, 891– 897. (6) Bhatia, S.; Sharma, D. K. Pet. Sci. Technol. 2006, 24, 1125–1159. (7) Le on, V.; Kumar, M. Biotechnol. Bioprocess Eng. 2005, 10, 471– 481. (8) Rana, M. S.; Samano, V.; Ancheyta, J.; Diaz, J. A. I. Fuel 2007, 86, 1216–1231. (9) Cravotto, G.; Cintas, P. Chem.;Eur. J. 2007, 13, 1903–1909. (10) Luque-Garcı´ a, J. L.; Luque de Castro, M. D. Trends Anal. Chem. 2003, 22, 90–98. (11) Smith, F. E.; Arsenault, E. A. Talanta 1996, 43, 1207–1268. (12) Flores, E. M. M.; Saidelles, A. P. F.; Barin, J. S.; Mortari, S. R.; Martins, A. F. J. Anal. At. Spectrom. 2001, 16, 1419–1423. (13) Maichin, B.; Kettisch, P.; Knapp, G. Fresenius’ J. Anal. Chem. 2000, 366, 26–29.
†
Presented at the 10th International Conference on Petroleum Phase Behavior and Fouling. *To whom correspondence should be addressed. Fax: þ55-55-32209445. E-mail:
[email protected]. (1) Speight, J. G. Handbook of Petroleum Product Analysis; John Wiley and Sons: New York, 2002; p 454. (2) Jayaraman, A.; Saxena, R. C. Corros. Prev. Control 1995, 42, 123– 131. (3) Santos, M. F. P.; Guimar~aes, R. C. L.; Gomes, L. M. B.; Camacho, C. F. B.; Trindade, F. F. Technical Report, Petrobras, Rio de Janeiro, 2006. (4) American Society for Testing and Materials (ASTM). ASTM D 6470-99. Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, 2004. r 2009 American Chemical Society
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: DOI:10.1021/ef9007906
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fast energy transfer to irradiated medium, microwaves could be used to perform the demulsification of heavy crude oil emulsions (and, consequently, reducing interferences in further analysis) in a faster way than conventional methods. The microwave demulsification process allows for the destabilization of emulsions, first, by increasing the temperature (it causes a reduction of the continuous phase viscosity and breaks the outer film of drops allowing for the coalescence) and, second, by rearranging the electrical charge distribution of water molecules while rotating them and moving ions around the drops. These two combined effects could result in emulsion breaking without the addition of any chemical agent.14 There are only few works related to the demulsification of crude oils using microwave radiation, and practically all of them are devoted for synthetic emulsion.15-19 However, despite its relevance, the application of microwave radiation for salt extraction in real samples (especially for heavy crude oils) was not found in the literature. A demulsification procedure, using microwave radiation, was first proposed in a patent by Wolf15 for water-in-oil and oil-in-water emulsions. It was observed that separation efficiency increases for emulsions with higher water content. However, no data were given for salt extraction from the oil phase, and the efficiency of the procedure was not described. Xu et al.16,17 performed a series of studies in a laboratory scale for salt removal of crude oil emulsions using a demulsifier and electric field. After separation of phases, the salt content was analyzed in the oil phase by a microcoulometric detector. It was observed that the use of auxiliary reagents (80 mg L-1 ammonium nitrate) and 50 mg L-1 of demulsifier added to the emulsion before the demulsification procedure increased the efficiency of the desalting procedure in the range of 58-75%.16 The use of the flow system to simulate the process used in oil refineries to desalt crude oil emulsion was also proposed. Emulsion was mixed with water and a demulsifier and demulsification was carried out inside two reactors with electrical field application for 30 min. After optimization, the emulsion was heated at 110-130 °C, using 20 mg L-1 of demulsifier and 5% of water. The efficiency of salt removal was in the range of 80-90%.17 Recently, synthetic crude oil emulsions (API higher than 19°) were used to evaluate the efficiency of the demulsification procedure using microwave radiation.18 In this work, crude oil emulsions containing originally 0.45-2.1% of water were used and only water was added (30-50%) to prepare the synthetic emulsion. Microwave radiation was applied for 15 min, and the temperature was limited to 90 °C. However, only two samples could be simultaneously treated. In another work, microwave radiation was applied for synthetic light crude oil emulsions (API > 19°) containing different amounts of water (25-45%), salt (0-30 g L-1), and pH (7-12).19 The highest efficiency of demulsification was achieved using an emulsion containing 45% water, pH 7, and without previous salt addition. In this procedure, microwave radiation was applied for 15 min at 130 °C. It is important to point out that
the best conditions were achieved using a synthetic emulsion with high water content and pH 7. Despite relatively good results, the characteristics of used synthetic emulsions are not normally found in real samples. In the present work, a microwave-assisted extraction (MAE) procedure using closed vessels was applied for the first time to salinity determination of heavy crude oil (API < 19°), avoiding the use of demulsifiers and toxic reagents. After separation of phases, chloride determination was performed in the water phase by ion chromatography (IC) and inductively coupled plasma-optical emission spectrometry (ICP-OES). In addition, Cl determination was also performed in the oil phase after salt extraction to check the efficiency of the proposed procedure. The follow operational parameters were investigated: sampling, sample mass, water volume, microwave heating program, and number of extraction steps. Results obtained with the proposed procedure were compared to those obtained by the ASTM D 6470 method. 2. Experimental Section 2.1. Instrumentation. A microwave sample preparation system (Multiwave 3000, Anton Paar, Graz, Austria) equipped with eight high-pressure quartz vessels (80 mL) was used for demulsification of heavy crude oil. Temperature and pressure were controlled during the heating program. This equipment was also used for microwave-induced combustion (MIC) to digest heavy crude oil samples obtained after the MAE procedure.21 Chlorine determination was carried out using an ion chromatographic system (Metrohm, Herisau, Switzerland) equipped with a pump (IC liquid handling unit), compact autosampler (model 813), and conductivity detector (model 819). Determinations were performed using a Metrosep A Supp 5 column (150 4 mm inner diameter) with polyvinyl alcohol and quaternary ammonium groups (particle size of 5 μm) and a guard column (Metrosep A Supp 4/5 Guard) with the same packing material and particle size of the analytical column. The mobile phase used was a mixed solution of 3.2 mmol L-1 Na2CO3 and 1.0 mmol L-1 NaHCO3, in a flow rate of 0.7 mL min-1. These conditions were established according to previous works.20,21 A suppressor column (model 833 Suppressor Unit, Metrohm) was used to reduce the conductivity of the mobile phase. The system was also equipped with a dialysis cell, with a cellulose acetate membrane of 0.25 μm and a sample loop of 100 μL. For a comparison of the results, Cl determination was also performed by ICP-OES using a model Spectro Ciros CCD simultaneous spectrometer with axial view configuration (Spectro Analytical Instruments, Kleve, Germany). A crossflow nebulizer coupled to a Scott double pass type nebulization chamber was used throughout. Plasma-operating conditions used in this work were a radio-frequency power generator of 1700 W, and argon flow rates of plasma, auxiliary, and nebulizer were 14.0, 1.00, and 1.00 L min-1, respectively. The selected wavelength for Cl was 134.724 nm, and argon of 99.996% purity (White Martins, Praxair, S~ ao Paulo, Brazil) was used throughout. An automatic titration system (model 6.0502.120, Metrohm) equipped with an Ag/AgCl electrode was used for Cl determination by potentiometric titration. Salt extraction from heavy crude oil emulsions was also performed by the ASTM D 6470 method for further comparison
(14) Chan, C.; Chen, Y. C. Sep. Sci. Technol. 2002, 37, 3407–3420. (15) Wolf, N. O. U.S. Patent 4,582,629, April 15, 1986. (16) Xu, X.; Yang, J.; Gao, J. Pet. Sci. Technol. 2006, 24, 673–688. (17) Xu, X.; Yang, J.; Jiang, Y.; Gao, J. Pet. Sci. Technol. 2006, 24, 1307–1321. (18) Fortuny, M.; Silva, E. B.; Filho, A. C.; Melo, R. L. F. V.; Nele, M.; Coutinho, R. C. C.; Santos, A. F. Fuel 2008, 87, 1241–1248. (19) Fortuny, M.; Oliveira, C. B. Z.; Melo, R. L. F. V.; Nele, M.; Coutinho, R. C. C.; Santos, A. F. Energy Fuels 2007, 21, 1358–1364.
(20) Pereira, J. S. F.; Mello, P. A.; Moraes, D. P.; Duarte, F. A.; Dressler, V. L.; Knapp, G.; Flores, E. M. M. Spectrochim. Acta Part B 2009, 64, 554–558. (21) Pereira, J. S. F.; Diehl, L. O.; Duarte, F. A.; Santos, M. F. P.; Guimar~aes, R. C. L.; Dressler, V. L.; Flores, E. M. M. J. Chromatogr. A 2008, 1213, 249–252.
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de Moraes et al. Table 2. Full Factorial Experimental Design To Evaluate Samplinga
Table 1. Characteristics of the Heavy Crude Oil Emulsions Used in Microwave-Assisted Extractiona sample
A
B
C
API (deg) water (%, v/v) salt (μg g-1) asphaltenes (%, m/m)
11.1 11.3 110 11.1
14.0 10.2 6600 1.0
14.2 8.4 17000 6.5
a
Results obtained after sample dehydration.
of the results to the proposed procedure. In this case, an extraction apparatus was used at recommended conditions.4 A mechanical stirrer RZR 1 (Heidolph, Schwaback, Germany) and an oven with air circulation (model 400/2ND, Nova Etica, Brazil) were used for homogenizing and heating the heavy crude oil emulsions. 2.2. Chemicals and Preparation of Samples. All reagents were of analytical grade. Deionized water was further purified using a Milli-Q system (18.2 MΩ cm, Millipore, Billerica, MA) and used in the desalting procedure and to prepare the Cl standards. High-purity concentrated nitric acid, obtained by sub-boiling distillation of reagent grade (Merck, Darmstadt, Germany), was used to clean the vessels after the desalting procedure. A stock solution of chloride (1000 mg L-1) was prepared by dissolving solid sodium chloride (Merck) in water. Standard solutions used for IC determinations were prepared by sequential dilution of this solution. Acetone, ethanol, and toluene (Vetec, Rio de Janeiro, Brazil) were used for the salt extraction step according to the ASTM D 6470 method. A solution of 0.1 mol L-1 silver nitrate was prepared by dissolving appropriate amounts of this salt (Merck) in water. Natural emulsified heavy crude oil samples, identified as A, B, and C, were used. These samples have a API and salt content in the range of 11.1-14.2° and 110-17 000 μg g-1, respectively. The characteristics of the samples investigated are shown in Table 1. Better conditions for sampling were selected after the use of a full factorial experimental design, and results were evaluated using the software The Unscrambler (version 9.7, CAMO AS, Woodbridge, NJ). Heavy crude oil “A” was selected for initial development and optimization of the proposed procedure. Chlorine determination by IC in heavy crude oil emulsion was performed according to previous work.21 2.3. MAE Procedure. After sampling, samples were weighted (10 g) into the quartz vessels and 20 mL of water was added. Five glass spheres (5 mm diameter) were also added to each vessel to avoid sample projection during microwave heating. After the vessels were closed and capped, the rotor was placed inside the microwave cavity and the heating program was started. In the end of the heating program water and oil phases were transferred separately to polypropylene vessels. The aqueous phase was diluted with water to 50 mL for further analysis by IC, ICP-OES, and potentiometric titration. After each run, vessels were cleaned using 6 mL of concentrated HNO3 for 10 min at 1400 W in the microwave oven as recommended by the manufacturer.22 Efficiency of salt removal of the proposed procedure was also evaluated by Cl determination in heavy crude oil (desalted samples) after the use of the microwave-assisted extraction procedure. Heavy crude oil samples were digested by MIC, and Cl determination was carried out by IC as previously described.21 In this work, all of the tests were performed in triplicate and results were expressed as mean and the respective standard deviation.
run
X1
X2
X3
1 2 3 4 5 6 7 8 9 10 11 12
þ þ þ þ þ þ
þ þ þ þ þ þ
0 0 0 0 þ þ þ þ
a X1, heating temperature (80 and 100 °C); X2, stirring time (20 and 30 min); and X3, sampling position (top, middle, and bottom of the vessel).
Table 3. Results Obtained for Chloride Determination in Heavy Crude Oil Emulsion by IC after MIC Decomposition Using Full Factorial Design To Evaluate Sampling Conditions (n = 3) chloride (μg g-1) sampling position top middle bottom
20 min, 80 °C
30 min, 80 °C
20 min, 100 °C
30 min, 100 °C
68.9 ( 4.2 64.2 ( 3.2 65.0 ( 4.1
62.9 ( 2.8 67.6 ( 6.3 66.3 ( 4.1
66.8 ( 4.4 66.9 ( 4.4 69.5 ( 3.6
63.7 ( 4.7 68.2 ( 5.7 68.7 ( 4.5
conditions of heavy crude oil samples. In general, chloride is associated with microdrops of water, and if their distribution is not homogeneous, an erroneous data evaluation could occur. Optimization of experimental parameters was carried out using two-level (stirring time of 20 and 30 min and temperature of 80 and 100 °C) and three-level (sampling of the top, middle, and bottom of the vessel) full factorial designs (Table 2). Results of factorial design were processed by analysis of variance (ANOVA) with a confidence level of 95%. Sample aliquots of emulsified heavy crude oil were digested by microwave-induced combustion for further Cl determination by IC. Results are shown in Table 3. A statistical difference was not observed for the Cl concentration in heavy crude oil emulsions using different stirring times, heating temperatures, and positions of sampling. Therefore, taking into account the better handling of the sample, samples were arbitrarily heated at 100 °C for 20 min and sampling was performed in the top of the vessel for subsequent tests. 3.2. Optimization of the MAE Procedure of Heavy Crude Oil Emulsions. Initially, the operational parameters of the proposed procedure were evaluated using the crude oil sample “A” (API of 11.1°, water and salt contents of 11.3% and 110 μg g-1, respectively). After optimization, the procedure was applied for other heavy crude oil samples. For this study, the sample mass was arbitrarily set at 10 g and water volumes of 6, 10, 20, and 30 mL were investigated. The chlorine determination was performed in the water phase by IC, and the obtained results were expressed as NaCl content (micrograms of salt per gram of heavy crude oil emulsion), as shown in Figure 1. As expected, the salt extraction efficiency increased when a higher volume of water was used. This behavior was also observed by other authors that reported that the increase of the water volume increased salt transfer to added water, resulting in higher extraction efficiency.17 This fact could be
3. Results and Discussion 3.1. Evaluation of Heavy Crude Oil Emulsion Sampling. A preliminary study was performed to check the best sampling (22) Microwave Sample Preparation System, software version v1.27Synt, Anton Paar GmbH, Graz, Austria, 2003.
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Figure 3. Influence of the microwave power on the microwaveassisted salt extraction procedure, with 10 g of emulsified heavy crude oil sample A (n = 3).
Figure 1. Influence of the water volume on the microwave-assisted salt extraction procedure, with 10 g of emulsified heavy crude oil sample A (n = 3).
Table 4. Microwave Heating Program Used in the Desalting Procedure for Heavy Crude Oil Emulsions parameter power (W) ramp time (min) heating time (min) cooling time (min) maximum temperature (°C) maximum pressure (bar)
800 30 30 20 260 70
consequently, for salt extraction. In the present work, microwave radiation power was evaluated from 500 to 800 W. The correspondent results are shown in Figure 3. As can be observed, a higher salt extraction was obtained when a microwave power of 800 W was applied. In this condition, the heat transfer to the heavy crude oil emulsion was improved because of the faster temperature increase of added water. A higher microwave power was not tested because of safety aspects. The operational conditions after the optimization of the MAE procedure are shown in Table 4. To evaluate the performance of microwave radiation in comparison to conventional heating for breaking heavy crude oil emulsions, an experiment was also carried out using microwave heating in an open system (model Ethos 1 Microwave Oven, Milestone, Italy). Heavy crude oil emulsion A (10 g) was mixed with 20 mL of water and heated using a microwave oven for 1 h at 120 °C (800 W, atmospheric pressure). The same procedure was repeated using conventional heating using a conventional oven. After the cooling time and phase separation (20 min), the water phase was removed for salt determination by potentiometric titration. Each procedure was repeated 3 times. The results (mean and standard deviation) obtained after microwave heating and conventional heating were 28.0 ( 2.5 and 5.8 ( 1.1 μg of NaCl per gram of heavy crude oil emulsion, respectively. Salt extraction for conventional heating was about 80% less effective than extraction using microwave heating. However, as it could be expected, for both procedures, salt extraction efficiency was lower than that using microwave heating in closed vessels as in the proposed method (60.1 ( 2.8 μg g-1 of NaCl after one microwave extraction step). Therefore, it demonstrates that, despite the general lower separation efficiency using conventional or atmospheric pressure microwave heating, the salt extraction using microwaves is more efficient than using conventional heating. It is also important to point out that phase separation was more efficient when microwave heating was used, giving a much clearer sample solution for analysis by potentiometric titration.
Figure 2. Influence of the heating time on the microwave-assisted salt extraction procedure, with 10 g of emulsified heavy crude oil sample A (n = 3).
explained by the fast heating of free water added to the system.23 Free water added to emulsion preferentially absorbs the microwave radiation, and the vapor formed passed by the crude oil phase, destabilizating and heating the emulsion, improving the coalescence rate.14,23 It is important to point out that using closed vessels the water vapor condensates inside the vessel walls, resulting in a refluxing process that contributes to salt extraction. No statistical difference was observed between the results obtained using 20 and 30 mL of water (t test, 95% confidence level), and for further studies, 20 mL of water was used. The heating time was also investigated in the range of 10-40 min, as shown in Figure 2. In this case, a ramp time of 30 min was applied to avoid sample projection. It was possible to observe that chloride extraction was improved with the increase of the heating time. However, for heating times of 30 and 40 min, no significant difference (t test, 95% confidence level) was observed in salt extraction and 30 min was chosen for subsequent tests. These results are in agreement with those found by Tan et al.24 They compared the heating time and effect of the demulsifier concentration using conventional and microwave heating to break water-in-oil and oil-in-water crude oil emulsions and reported that microwave heating was more efficient than conventional heating. Using microwave heating, a lower demulsifier concentration and shorter heating and sedimentation time were needed to achieve the same demulsification efficiency. The main advantage of using microwaves is that less time is required for emulsion breaking and, (23) Fang, S.; Chang, B. K. L.; Lai, P. M. C.; Klaila, W. J. Chem. Eng. Commun. 1988, 73, 227–239. (24) Tan, W.; Xiao-Gang, Y.; Tan, X. Sep. Sci. Technol. 2007, 42, 1367–1377.
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3.3. Evaluation of an Additional Extraction Step. After optimization of the proposed procedure, it was observed that the results obtained for the salt concentration were lower in comparison to those obtained after sample dehydration by distillation (Table 1). Therefore, taking into account the high viscosity of samples, additional extraction steps were studied to improve the efficiency of salt extraction to the water phase. After the end of the microwave heating program, water was removed, an additional volume (20 mL) was added to heavy crude oil, and microwave heating was applied again. The same procedure was repeated up to five sequential steps. The results obtained after each extraction step for sample “A” are shown in Figure 4. An increase of extracted salt to the water phase for additional extraction steps was observed. For sample “A”, it was necessary to use three subsequent steps to ensure quantitative removal of the salt from heavy crude oil emulsion. After three extractions, the total extracted salt was 106 ( 7 μg g-1. Using four and five extractions, the chloride content was below the limit of quantification (LOQ, 10σ) of the IC technique (5 μg g-1), and for subsequent studies, three extractions steps were used for sample “A”. The same study was performed for other samples of heavy crude oil emulsions, and the results obtained for chloride determination by IC, ICP-OES, and potentiometric titration after the use of microwave radiation are shown in Table 5. Results obtained by the proposed procedure were compared to those obtained by the ASTM D 6470 method. It was observed that, for each sample, a different number of extraction steps was necessary. For sample A, at least three extractions should be used to ensure quantitative extraction of salt from the heavy crude oil emulsions. However, for samples B and C, two and one extractions were, respectively, enough to ensure quantitative salt extraction. The agreement of the results obtained by IC, ICP-OES, and potentiometric titration was better than 95%. In view of different characteristics between heavy crude oil emulsions,
it is possible to propose three extractions as a general procedure for microwave-assisted salt evaluation. 3.4. Comparison of the Proposed MAE Procedure with the ASTM Method. A comparison of ASTM and the proposed microwave-assisted extraction procedure is shown in Table 6. For the ASTM D 6470 method, the sample was homogenized with a mixer for 15 min (3000 rpm) and an aliquot (about 40 g) was dissolved in 40 mL of xylene at 65 °C in a water bath. Further, this aliquot was extracted with alcohol, acetone, and water in an electrically heated apparatus, and the mixture was boiling at a vigorous rate. After phases settled, the water phase is filtered and analyzed by potentiometric titration.4 Therefore, the procedure is long; toxically reagents should be used; and if the sample contains sulfur in high concentrations, it is necessary to apply an additional step to remove the compounds. Consequently, a low sample throughput was observed. In comparison to the ASTM D 6470 method, the proposed microwave-assisted procedure used only water for extraction and it could be performed in 1 h. Even if additional extraction steps are necessary to ensure the complete salt extraction using the proposed procedure, the time for extraction is lower in comparison to the ASTM method. Moreover, the analysis of different heavy crude oil samples having a similar water content by the proposed procedure presented good agreement with the ASTM method. Using the microwaveassisted procedure, up to eight samples can be processed simultaneously in one extraction step (up to 8 samples/4 h, considering three extraction steps), which allows for a convenient sample throughput. One of the main aspects is related to the aqueous extract compatibility to different determination techniques, such as IC, ICP-OES, and potentiometric titration. Consequently, the proposed procedure can be used by different laboratories according to their Table 6. Comparison between the Proposed Microwave-Assisted Salt Extraction Procedure and Recommended ASTM Method for Salinity Evaluation procedure parameter sample mass (g) reagents determination technique
40 70 mL of xylene 25 mL of alcohol 15 mL of acetone potentiometric titration
sample throughput 1 sample/4 h reflux step not available pressure and not available temperature control
Figure 4. Influence of additional extraction steps for sample A, with 10 g of emulsified heavy crude oil sample (n = 3).
a
microwave-assisted procedure
ASTM D 6470
10 20 mL of water IC ICP-OES potentiometric titration up to 8 samples/3 ha available available
Considering three extractions for each sample.
Table 5. Salt Concentration in Water of Extraction from Heavy Crude Oil Emulsions after the Desalting Step Using the Microwave-Assisted Procedure and the ASTM Method (n = 3)a NaCl (μg g-1) sample A B C a
number of extractions
MAEb IC
MAEb ICP-OES
MAEb titration
ASTM D 6470
3 2 1
106 ( 7 6500 ( 368 16000 ( 1100
107 ( 9 6511 ( 382 15943 ( 1231
108 ( 9 6532 ( 423 16544 ( 1261
110 ( 10 6600 ( 620 17000 ( 1400
Results obtained by IC, ICP-OES, and titration (microwave-assisted salt extraction and ASTM method). b Microwave-assisted salt extraction.
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instrumentation available for salinity evaluation. In addition, the pressure and temperature could be controlled during the procedure, which is an important aspect for safety reasons. The efficiency of salt extraction of the proposed procedure was higher than 95% for all investigated samples. Therefore, salt extraction from heavy crude oil using microwave radiation was considered suitable for analytical purposes. 3.5. Chlorine Determination in the Oil Phase after Microwave-Assisted Extraction. After demulsification, the oil phase was collected and decomposed by the microwave-induced combustion technique for further chloride determination by IC. This study was performed to check the concentration of residual salt in desalted oil. The results obtained for samples A, B, and C were 5.39 ( 0.38, 146 ( 6, and 16.6 ( 1.6 μg g-1 of NaCl, respectively. These values correspond to 95.1, 97.8, and 99.9% of efficiency of desalinization of heavy crude oil emulsions. It is important to point out that it was possible to remove the major part of the salt content (higher than 95%) of heavy crude oil emulsions to values lower than 500 μg g-1, avoiding interferences in the characterization step. The proposed procedure was suitable to remove the salt content from heavy crude oil emulsions with very low API and stability, using only water under microwave radiation.
4. Conclusions The proposed procedure for the demulsification process in a closed vessel using microwave radiation presented efficiency higher than 95% for different samples of heavy crude oil emulsions. Results obtained using microwave radiation were in agreement with those using the ASTM D 6470 method, showing the applicability of the proposed procedure. Moreover, the proposed procedure could be carried out in less time (1 h) when compared to the ASTM method (more than 4 h). In addition, the desalting process could be performed using only water, avoiding the use of toxic reagents or demulsifiers, which is an important aspect related to the green chemistry recommendations. The use of the microwave-assisted procedure is relatively easy to be performed, and up to eight samples could be simultaneously processed. The water phase obtained after the desalting procedure was suitable for Cl determination by different techniques, such as IC, ICP-OES, and potentiometric titration. Acknowledgment. The authors are grateful to CNPq and CAPES for supporting this study and also to CENPES/PETROBRAS for support and donation of samples.
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