Improved Chromatographic Technique for Crude Oil Maltenes

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Improved Chromatographic Technique for Crude Oil Maltenes Fractionation Sara Rezaee, Rocio Doherty, Mohammad Tavakkoli, and Francisco M. Vargas Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03328 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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Improved Chromatographic Technique for Crude Oil Maltenes Fractionation Sara Rezaee, Rocio Doherty, Mohammad Tavakkoli, Francisco M. Vargas* Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005, USA

ABSTRACT One common method for describing crude oils according to their composition is by grouping their chemical compounds into four categories: saturates, aromatics, resins, and asphaltenes (SARA). SARA analysis is based on the different degrees of polarizability and polarity of each group1. Most of the available chromatography techniques used for SARA analysis suffer from limitations, such as using large amounts of solvent or sample and yielding irreproducible results. The advantages of previously used methods are combined into a single system for maltenes (contain saturate, aromatic and resin) analysis in order to obtain less loss of volatile components and more reliable results, as well as to use less crude oil sample and solvent. In our proposed method, the system is closed, and the possibility of evaporation is much lower than in an open column. The testing time for maltene analysis reduced from 7 to 2 days, and the amount of solvent and sample decreased by 90% of that which is used in the ASTM D2007 method.

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INTRODUCTION Compositional analysis of crude oil is not feasible due to the hundreds of different types of hydrocarbon that exist. One of the most commonly used methods for characterizing crude oil is SARA analysis, in which crude oil is fractionated into saturates, aromatics, resins, and asphaltenes (SARA) based on their solubility and polarity. The majority of the techniques available for this analysis are based on chromatography.2–4 The oldest method for maltene (deasphalted oil) analysis is the ASTM D2007, which involves a system of two columns of clay and silica gel. Additionally, high-pressure liquid chromatography (HPLC) is used for maltene quantification.5,6 Another commonly used method for SARA analysis is based on thin-layer chromatography (TLC) analysis with flame ionization detection (TLC-FID).7 The HPLC method is among the fastest of the available techniques for SARA analysis. HPLC results are also more reproducible since there are no losses or evaporation in this technique. The HPLC analysis proposed by Suatoni and Swab8 uses silica or alumina columns to fractionate maltene fraction,1 while the improved HPLC method is based on the different affinities of the saturates, resins, and aromatics to an amino-bonded silica column.6 One of the disadvantages of this method is its cost, which is the highest among the different methods that are analyzed. In addition, there is no general HPLC method for all hydrocarbons, and it is necessary to have a specific calibration curve for each type of sample.9 In the HPLC method, the concentration of each maltene fraction is calculated using a calibration curve. Thus, the HPLC method is based on the known concentration of compounds, and optimized flow rates and retention times. However, because the HPLC column properties change over time and usage, the validity of the method, including the calibration curve needs to be verified regularly to make sure that method is still valid after several runs.10,11


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In addition to the different challenges that each of these methods present,3 the reproducibility of the results obtained using different SARA methods is questionable. Bisht et al. Showed that the average differences in the SARA analysis result determined by TLC-FID and ASTM methods could be as high as 9 wt. %.7 By using ASTM D2007 the reproducibility is 4 wt. % for saturate, 3.3 wt. % for aromatic and 1.8 wt. % for resin fractions (Resin content higher than 5%).12 In addition, different researchers showed that sometimes the results are not even repeatable with the same procedure.13,14 SARA analysis has been the subject of a great number of studies because of repeatability and reproducibility issues.5–7,15 Different methods have been proposed such as automated asphaltene determinator coupled with maltene separation16, optical measurement of SARA fractions14, maltene fractionation by flash chromatography17 and improved HPLC techniques.18,19 The new methods are based on the Iatroscan (TLC-FID) method or gravimetric separation followed by liquid chromatography systems. There are some limitations related to using an Iatroscan method such as losses of the light component, which is a special concern when analyzing light and medium gravity oils and challenges to differentiate between the resin and asphaltene fractions.14,16 Although automated techniques for measuring asphaltene content are fast, the kinetics of asphaltene precipitation and aggregation may pose an additional challenge, which in turn might affect the accuracy of its quantification. As it is mentioned earlier HPLC techniques are reliable and fast for SARA analysis but it needs an experienced operator to develop the separation methods and for trouble shooting issues such as the difference in retention times, drifting the retention times, sources of peak tailing, etc.10,11


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In this work, a new methodology for maltene analysis is proposed that aims to overcome some of the limitations of previous methods. The proposed method is based on a chromatography technique by using less sample and solvent in comparison to ASTM D2007. The time of the process is shorter in comparison to other chromatographic systems, and the purity of separated fractions is verified by FTIR analysis. This method is easy to operate, and it does not require a skilled operator to analyze and interpret the results.

MATERIALS AND METHODS Samples The experimental procedure was conducted on four different crude oils from the Middle East and the Gulf of Mexico. The properties of these crude oils at ambient condition are presented in Table 1. The crude oils were first centrifuged to remove any sediment, sand particles, or water present in the system prior to asphaltene separation. Density and viscosity were measured using an Anton Paar DMA 4500 digital vibrating U-tube densitometer and a Cannon-Fenske glass capillary viscometer, respectively. Table 1. Crude oil sample properties. Crude Oil

Density (g.ml-1; 20 °C)

Viscosity (cP; 20 °C)

A

0.884

56.0

C

0.885

22.0

S6

0.834

4.8

S9

0.836

6.0


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The materials needed for the experiment were clay, 500 to 250 μm (30 to 60 mesh), silica gel (>200 sieve size), n-pentane, n-hexane, n-heptane (purity: 99%), dichloromethane (DCM) or toluene (purity: 99.5%) and acetone (purity: 99.9%). Table 2 presents the amounts of clay and silica gel needed for both the ASTM D2007 and improved chromatographic technique. Table 2. Materials needed for the ASTM D2007 method and the improved chromatographic technique. Material Attapulgus Clay Silica gel

Manufacturer/

Particle size

ASTM D2007

Reference No.

(µm)

(g)

Improved chromatographic technique (g)

250-500

200

8

90-710

200

8

Forcoven Products Inc. Catalog No. NC9027284

Clay and silica gel were used to fill in two separate PTFE columns. Figure 1 shows the details of the chromatography columns. The inlet and outlet of each column were connected using 5 pieces 1

1

including-straight connector (Nylon tube fitting, 2″ OD), black O-rings (Buna N, ″ OD), Teflon 2 1

rod, ferrule, and nut (Teflon, 8″) to provide sealing protection against leakages. Since the liquid 1

flow may wash the silica or clay, a 0.2 micron filter paper (PTFE, 2″) was placed in the straight connector to keep them inside the column.


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Straight connector O-ring Ferrule Nut Teflon rod Ferrule Nut b a

Figure 1 Column details: a. PTFE column, b. Detail of Nylon straight connector.

Procedure SARA analysis using the ASTM D2007 method SARA analysis based on ASTM D200712 was performed to obtain reference data with which to compare the results obtained using the new method. Additionally, this experiment provided further understanding of the limitations of using a chromatography method and aided us in developing ideas for improving the new chromatography method used for SARA analysis. After asphaltene separation, the filtered solution went through the maltene (saturates, aromatics, and resins) separation. Maltenes were fractionated based on liquid chromatography by the ASTM D2007 procedure,12 while asphaltene is quantified based on ASTM D6560.20 In this study, toluene, as the solvent of resin and aromatic fractions, was replaced by DCM due to its faster evaporation time.


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The boiling point of DCM is 40 ᵒC, while the boiling point of the lightest aromatic fraction is 80 ᵒC, so there was minimal loss of aromatic fractions during the process of evaporation. Also, DCM has higher solubility parameter 20.2 MPa1/2 in comparison to toluene (18.3 MPa1/2) which makes it a better solvent for washing resin fraction with the solubility parameter reported in the range 21.1-21.8 MPa1/2. 21

Maltene analysis using proposed methodology The heart of the new system, as depicted in Figure 2, consists of two chromatographic columns, one containing silica gel and one containing clay (Attapulgus). The additional equipment required to achieve the separation consists of an HPLC pump and a distillation system, a round-bottom flask, a distillation column, a thermometer, and a condenser. A set of two- and three-way valves to control the direction of the flow is also required. The process was carried out in two steps: adsorption and desorption. During the first step, saturates, aromatics and resins were separated based on their affinity to be adsorbed by the clay or the silica gel column. During the second step, these components were desorbed by washing the columns with the appropriate solvent then being recovered. Both steps are further described below. Adsorption Step: During this step, 1A, 2A, 3A, 4A and 5A valves (Figure 2) were open, while the rest remained closed throughout the adsorption process. Therefore, the flow of the solution followed the brown path indicated in Figure 2. 1 g of maltene which is the filtrate of the asphaltene separation process, diluted with 200 ml of precipitant (n-pentane or n-heptane) was placed into a solution container. The solution was pumped at 1 ml.min-1 through valve 1A and fed into the clay column in which only resins were


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adsorbed. Thereafter, the solution passed through valves 2A and 3A and was fed into the silica column, which adsorbed the aromatic components. The solution continued on its path, passing through the 4A and 5A valves and collecting in the effluent container. After the entire solution passed through the column, the effluent containing the saturate fraction was collected. To separate saturates fraction, n-pentane or n-heptane (the precipitants of asphaltene) was evaporated from the solution at approximately 36.1 °C or 98 °C respectively, and the saturates were dried and weighed. The purity of saturate was checked using FTIR to confirm that there was no aromatic or resin present in the saturate fractions. The amount of saturate is quantified by subtracting the amount of oil from the amount of aromatic, resin and asphaltene. The difference between the measured and calculated saturate fractions was less than 3 Wt. %. Therefore, it is accurate enough and much more time efficient to calculate the saturate as a difference between the oil and the sum of aromatic, resin and asphaltene fractions, assuming a 100 % recovery.


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Adsorption step

Water out

Condenser

Distillation column

Water in

Heating mantle

B

B 2 A

B A 4

5

A

Silica

Clay

Recovered Saturates

A

A

1

3

B

B

HPLC pump 2-way valve

Maltene fraction diluted by precipitant

3-way valve

* Not drawn to scale

Figure 2. Improved chromatographic technique; Adsorption step22 (From Doherty et al. CRC Press, 15-73, 2018). Desorption Step: After completing the adsorption process, the columns were washed separately with a solvent to recover resins and aromatics fractions. 200 ml of a mixture of DCM (50 vol. %) and acetone (50 vol. %) were used to wash the clay column to desorb the resins fraction. In this step, as depicted in Figure 3a (solid line), valves 1A, 2B, and 5B were open, and the rest remained closed. The silica gel column was washed with a 200 ml of a mixture of DCM (62 vol. %) and hexane (38 vol. %) to desorb the aromatic fraction. The amount of solvent (200 ml) and pump flow rate (1 ml.min-1) during desorption step was optimized in such a way that a certain level of liquid is always present in the solvent container. As Figure 3b (solid line) indicates, for washing, silica gel column valves


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1B, 3B, 4B, and 5B were open, and the remaining valves were closed. When the solvent passed through the columns, it dissolved resins and aromatics from the clay and the silica gel media, respectively. The solution continued to the distillation system. b. Desorption step Washing silica gel column Aromatic separation

a. Desorption step Washing clay column Resin separation

Water out

Water out

Condenser

Condenser

Distillation column

Water in

Recovered resin

Distillation column

Water in B 2

B

A

Heating mantle 5

Heating mantle

B

A

Recovered aromatic B 4

5

Clay

Silica

1

A

1

3

B

B

B

HPLC pump

HPLC pump

DCM and Acetone

Silica

Clay

2-way valve

2-way valve

3-way valve

3-way valve


DCM and Hexane


Figure 3. Improved chromatographic technique; Desorption step, a) Washing clay column, b) Washing silica gel column22 (From Doherty et al., CRC Press, 15-73, 2018). Distillation Process: The solution of solvent, DCM (50%) + acetone (50%), and resins leaving the clay column was fed into the distillation column from the top and was collected in a round-bottom flask. The temperature of the heating mantle was fixed in such a manner that the solvent evaporated without burning the resins. The temperature of the distillation column was monitored using a thermometer


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at the top of the column to ensure that the vapor was pure solvent. The pure solvent was collected in a solvent container after passing through the condenser. For the aromatic fractions, the process of distillation was exactly the same, with the exception that the solvent used for washing the silica was DCM (62 vol. %) and hexane (38 vol. %). Once the solvents leaving the clay and silica gel columns became clear, desorption process was stopped, and both resins and aromatics were dried in an oven and subsequently weighed.

RESULTS AND DISCUSSION To determine the optimum pump flow rate and maximum adsorption efficiency, three different pump flow rates (1, 3, and 5 ml.min-1) were tested. The purity of the effluent was checked using FTIR to ensure that there is no aromatic or resin in the saturate fraction. Figure 4 presents the IR results of the saturate fraction while the pump flow rates were 1, 3, and 5 ml.min-1. The peak at 1600 cm-1 corresponds to the aromatic C=C bonds, and the shoulder at 3050 cm-1 corresponds to aromatic C—H stretching. The peaks in the wavelength range of 28003000 cm-1 are related to CH2 and CH3 in aliphatic chains. The peaks in the range 1350-1500 cm-1 are related to the CH3 functional groups.23 Based on these results, it can be concluded that the flow rates of 3 and 5 ml.min-1 were not acceptable, as the IR spectra, Figure 4b, and c, indicate some peaks related to the aromatic fraction at 1600 and 3050 cm-1. Therefore, 1 ml.min-1 was chosen as an appropriate flow rate during the adsorption step in order to prevent impurities in the effluent.


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1 ml/min Transmittance, %

3 ml/min

Transmittance, %

a 3900

3100 2300 1500 Wavenumber, , cm-1

3050

1600

b

700

3900

3100 2300 1500 Wavenumber, cm-1

700


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3050

1600

c 3900

3100 2300 1500 Wavenumber, cm-1

700

Figure 4. FTIR results of saturate fraction when the pump flow rate is: a) 1 ml.min-1, b) 3 ml.min-1 and c) 5 ml.min-1. Figure 5 depicts the FTIR spectra of aromatic fractions by testing three pump flow rates (1, 3 and 5 ml.min-1). In Figure 5b and c, the peak at 1700 cm-1 belonged to the carboxylic functional group,24 which indicates the presence of some resin fractions in the silica gel column. Therefore, it can be concluded that 3 and 5 ml.min-1 are not appropriate flow rates for the adsorption process. At 1 ml.min-1, the FTIR spectra indicate that there is no more carboxylic acid in the silica gel column, and the resin fraction was adsorbed completely by the clay column.


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a 3900

3100 2300 1500 Wavenumber, cm-1

700

1700

b 3900

3100 2300 1500 -1 Wavenumber, cm

700

Transmittance, %

5 ml/min

1700

c 3900

3100

2300 1500 -1 Wavenumber, cm

700

Figure 5. FTIR spectra of aromatic fraction when the pump flowrate is a) 1 ml.min-1, b) 3 ml.min-1, and c) 5 ml.min-1. Four crude oils (S9, S6, C, and A) were fractionated using an improved chromatographic technique in order to conduct the maltene analysis. Asphaltene content was determined based on ASTM D6560,20 using n-pentane as the precipitant. The results are provided in Figure 6. The maltene analysis was obtained after 2 days, and the maximum standard deviation for this method was approximately 4 Wt. %. 80

60

Wt. %

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S9

S6

C

A

40

20 0 Saturate

Aromatic

Resin

Asphaltene (C5+)

Figure 6. SARA analysis of four crude oils.


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Maltene analysis was also performed using the ASTM D2007 method.16 Figure 7 presents the SARA fractionation based on both improved chromatographic technique and ASTM D2007 methods. The reported result is the average of four measurements. The percent relative standard deviations in the values of SARA determined by ASTM D2007 methods are 6, 7, 3, and 0.1 Wt. %, respectively, whereas the percent relative standard deviations in the SARA values determined by an improved chromatographic technique are 4, 3.5, 2, 0.1 Wt. %. The process of ASTM D2007 method was time-consuming; it took approximately 10 days to complete the SARA analysis which includes the separation of asphaltene and maltene fractionation, while the improved chromatographic technique took only 5 days. Furthermore, the ASTM D2007 method required large amounts (500 ml pentane, 2000 ml DCM, and hexane for the S6 or S9 analysis) of solvent, while, in the proposed method, the efficiency of separation was increased by solvent recirculation. Pure solvents washed the adsorbed resins and aromatics in each cycle. Additionally, the solvent can be recovered at the end of the process, thus decreasing the costs associated with using large amounts of solvent. Also, it reduces the risk of exposure to the chemical to laboratory personnel. Data from several sources have identified that the ASTM D2007 method has some limitations, such as being time-consuming, using a large amount of solvent and sample, and generating larger standard deviation in comparison to the other methods.1,3,14,15 These limitations are consistent with the findings of this study.


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S6 crude oil

S9 crude oil Improved Chromatographic Technique (This study) ASTM-D2007

80 60 40

100

Improved Chromatographic Technique (This study) ASTM-D2007

80

Wt. %

100

Wt. %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20

20

0

0 Saturate

Aromatic

Resin

Asphaltene (C5+)

Saturate

Aromatic

Resin

Asphaltene (C5+)

Figure 7. Comparison of SARA analysis based on the improved chromatographic technique and ASTM D2007. CONCLUSIONS In this study, a new method for maltene analysis is proposed that boasts several unique advantages. In our new method, approximately 90% less solvent is required than when using the ASTM D2007 method, and it is fully recovered. The time required for maltene analysis, once the asphaltene is removed, is reduced to 2 days from the 7 days required with the ASTM D2007 column. The collected data are found to be more reproducible since the volatile components are not separated and are measured as a part of saturates and aromatics. Another advantage of our method is that it relies upon low-cost, readily available equipment and easy to perform and analyze the data. Furthermore, in TLC-FID, it is impossible to collect the extracted fractions due to the small amounts of the sample used. Although in HPLC method it is possible to collect the separated maltene fractions, it is needed to run the test several times to get a significant amount of each fraction, depending on the intended application. However, by using the new proposed analysis appropriate amount of maltene fractions for further analysis can be separated.


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AUTHOR INFORMATION Corresponding Author *Telephone: 713-348-2384. Email: [email protected]

ACKNOWLEDGMENTS The authors gratefully acknowledge the generous financial support of the members of the Rice University Consortium on Petroleum Thermodynamics and Flow Assurance.

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Improved Chromatographic Technique for Crude Oil Maltenes

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