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Chapter 2
The Importance of Mass Balances: Case Studies of Evaluation of Asphaltene Dispersants and Antifoulants Cesar Ovalles,* Estrella Rogel, Harris Morazan, and Michael E. Moir Petroleum and Material Characterization, Products and Analytical Unit, Chevron Energy Technology Company, Richmond, California 94801, United States *E-mail:
[email protected].
The enormous complexity associated with the composition of petroleum feedstocks and the nature of their reaction products make material balances almost mandatory for the understating of the chemistry, and eventually successful outcome of the experimentation. Unfortunately, there is considerable lack of mass-balance data available in the literature which prevents us from learning when something is not correct or erroneous, draw the appropriate conclusion for a given experiment, and to understand the chemistry involved in the system. During his illustrious career, Dr. Mietek Boduszynski devoted considerable time and effort to analyzing and understanding the mass balances during his lab testing and experimentation. In this work, several case studies are presented for the use of the on-column filtration/redissolution method at elevated temperatures, in which material balances are shown to be an essential element. Nonylphenol formaldehyde resins, phosphopropoxylated asphaltenes, and commercial products were evaluated as dispersants and antifoulants for asphaltene fractions (C7-insolubles), whole virgin crude oils and hydroprocessed samples. The results showed that mass balances were crucial to assess the validity of the asphaltene determination method, the reliability of the data, and the relative effectiveness of the asphaltene-dispersant and antifoulant additives.
© 2018 American Chemical Society Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Introduction Since our early days in science, we learned the importance of applying the conservation of mass to the analysis of physical systems. Without a doubt, mass balance (also called a material balance) is a fundamental tool in the study of science and engineering. So, why should we dedicate time and effort to discuss its importance? There are several reasons to do so. In industry, mass balances are widely employed in engineering, and environmental analyses (1, 2). For example, mass balance theory is used to design chemical reactors, to analyze alternative processes to produce chemicals, as well as to model pollution dispersion and other processes, just to name a few. In petroleum engineering, material balances are used to define the different oil-producing mechanisms and relate the reservoir fluid and rock properties to the subsequent fluid-retrieval processes (3). In other disciplines, mass balances are used to carry out the population, enthalpy, and entropy balances, among many other commercially and academically significant applications (1, 2). In the chemistry lab, we commonly use our lab notebook to record the weights of the materials and carry out mass balances for our experiments. In petroleum chemistry, the complexity associated with the feed composition and their reaction products makes material balances almost mandatory for the understanding of the chemistry involved, and eventually for the favorable outcome of the experimentation. Despite its significance, these data are rarely published in internal reports and articles in peer-reviewed journals, or presented at seminars and technical conferences. Furthermore, many colleagues and fellow scientists consider carrying out mass balances as “part of the job,” and some consider these types of calculations obvious or “not-relevant.” In other words, we, chemists take it for granted. Many times, we have been surprised how these simple calculations are overlooked. Thus, there is considerable lack of mass-balance data published in the literature. Sometimes, this situation prevents us from learning when something is not right or erroneous, draw the correct conclusion for a given experiment, and to understand the chemistry involved in the system under study. Also, mass balances allow us to assess the validity of the work and give an answer to the fundamental question, is this good data? As the old Russian proverb says “trust, but verify” (Доверяй, но проверяй) (made famous by Ronald Reagan). This moment is where Dr. Mietek Boduszynski’s influence takes a role. We have always carried out mass balances, and it is of paramount importance while working with petroleum-derived products. Since his beginning as a scientist, Dr. Boduszynski has devoted considerable time and effort to analyzing and understanding the mass balances within any R&D project he had been involved. Following his teaching, we spent many hours evaluating distillable and product yields, and we did not move to the next phase until we were fully satisfied with the material balances. The significant and successful use of this work-flow methodology is what motivated us to write this chapter. In this work, several case studies are presented for the evaluation of asphaltene dispersants and antifoulants using the on-column filtration/redissolution method at elevated temperatures. Different additives were considered for asphaltene 26 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
fractions (C7-insolubles), whole virgin crude oils, and hydroprocessed samples. The primary objective of this chapter is to show how mass balances were crucial to assess the validity of the new asphaltene determination method, the reliability of the data, and the relative effectiveness of the asphaltene-dispersant and antifoulant additives.
On Column Filtration/Redissolution Method at Elevated Temperatures The analysis of asphaltenes using the on-column filtration/redissolution method at high temperature (35-195°C) was described elsewhere (4, 5). This method gives the total amount of asphaltene present and allows carrying out mass balances using maltenes and asphaltenes as “reference materials (6)”. These materials were previously separated using ASTM method D-6560 (7). Asphaltenes and feed characterizations can be found elsewhere (4–6). In a typical analysis, the feedstock is dissolved in toluene (0.1% mass/vol) and injected (4 µL) into a previously heated stainless-steel column packed with Poly(tetrafluoroethylene) using n-heptane mobile phase. Figure 1 shows a typical “LC trace” with the temperature and solvent changes throughout the analysis. It is important to mention that the output is called “LC trace” because resembles a liquid chromatograph trace but in the on-column filtration/redissolution method no chromatographic separation takes place. As seen in Figure 1, maltenes (n-heptane solubles) elute from the column as the first peak while precipitated asphaltenes remain inside the column at this temperature. Then, the oven is cooled down to 35°C and when it reaches that temperature the mobile phase is switched to 10% methanol/90% dichloromethane, to redissolve the asphaltenes retained in the column. Once they are eluted, the temperature is increased to 110°C, and finally, the solvent is switched back to 100% n-heptane. As can be seen, the analysis time is approximately 60 min. The quantification of the LC traces was carried out using Mexican VR maltenes and asphaltenes preparatively separated from a Mexican vacuum residue (VR) (7). In a typical preparation, the samples were dissolved in toluene (0.1% m/v for maltenes and 0.01% m/v for asphaltenes), and several volumes of these solutions (0.5, 1, 2, 4, 8 µL) were injected following the procedure described before. As reported previously, maltene and asphaltene response factors were determined by plotting log(areas) versus log(mass) as shown in Figure 2. Linear regressions showed excellent correlations with R2 values in the 0.99 range. The effect of the temperature on the asphaltene determination of Venezuelan VR can be seen in Figure 3. As shown, four different LC traces are presented at four different temperatures (35, 70, 140, and 195°C). At 35°C and 70°C, broad maltene signals are spanning up to ~5 min, and the asphaltene peaks are detected at ~15 min. When the temperature increases to 140°C, the C7-solubles signal is sharper than before, and at the same time, the asphaltene peak slightly decreases in intensity. As the temperature increases to 195°C, a very sharp maltene peak is observed as well as the smallest n-C7-insolubles signal. 27 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 1. Typical LC trace with the temperature and solvent changes throughout the on-column filtration/redissolution method (CAD = Corona Aerosol Detector and MeOH = Methanol).
Mass balances as a function of temperatures for the on-column filtration/ redissolution method for Venezuelan VR are shown in Table 1 in the 35-195°C temperature range. These results are the average of at least two determinations. In general, reasonable mass balances were obtained in the 93-100% range which indicates the validity of the data. Similar closures in the mass balances were reported for asphaltenes and whole VRs from different origins (Mexican and Venezuelan) as well as for visbroken and hydroprocessed samples (4). As seen, the amount of asphaltenes decrease with temperature from 13.29 μg at 35C to 6.93 μg at 195°C (48% reduction). Similar reductions in the percentages of asphaltene precipitation have been reported by various research groups in different parts of the world (8–12). These results give us confidence in the on-column filtration/redissolution method and permit its application to the evaluation of asphaltene dispersants and antifoulants. 28 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 2. Calibration curves for the Mexican VR maltenes and asphaltenes using the on-column filtration/redissolution method.
29 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 3. LC traces showing the effect of temperature on the determination of asphaltene concentration by on-column filtration/redissolution method for a Venezuelan VR.
Table 1. Mass Balances for the Determination of Asphaltene Content by on-Column Filtration/Redissolution Method for a Venezuelan VRa Temp (°C)b
Injec. Amt (μg)c
Maltene Content (μg)d
Asphaltene Content (μ)e
Mass Balancef
% Red. Asph.g
35
40.01
25.63
13.29
97%
-
70
39.91
26.79
12.38
98%
-7%
140
39.68
29.68
10.08
100%
-24%
195
39.67
30.30
6.93
93%
-48%
Average of at least two determinations. b Temperature for asphaltene precipitation. c Amount of sample injected in toluene. d Calibrated using Mexican VR maltenes. e Calibrated using Mexican VR asphaltenes. f Mass balances with respect to the amount of sample injected including the absorbed material (4). g Percentage of reduction vs. that obtained at 35°C a
30 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Evaluation of Asphaltene Dispersants Asphaltene precipitation continues to be a significant issue throughout the petroleum industry. Chemical additives have been widely employed to reduce and control asphaltene precipitation. In upstream operations, asphaltene dispersants are injected downhole and elsewhere in the production system (13–15). During oil producing processes, asphaltene deposition problems originate due to a decrease in the asphaltene solubility. As asphaltenes become less soluble, floccules are formed under operating conditions that can deposit, block pathways, valves and storage tanks. Chemical additives (inhibitors or dispersants) can interact with the asphaltene molecules to increase their solubility in the media and inhibit their precipitation (16–18). In this work, the distinction between asphaltene inhibitors and dispersants are out-of-scope and are used interchangeably. Use of Nonylphenol Formaldehyde Resins (NPFR) Among the additives reported in the literature, alkylphenol and alkylphenolaldehyde resin oligomers are the classes of polymeric asphaltene inhibitors that have been most investigated and are commonly used in the oil industry (18–22). Hydrogen bonding and π-stacking are believed to be the principal mechanisms of interaction between the inhibitor and the asphaltenes (23). The syntheses of the resins were carried out in acidic catalysis (H2SO4), using a reaction mixture of formaldehyde/para-nonylphenol with different molar ratios in freshly distilled cyclohexane (18). Nonylphenol formaldehyde resins (NPFR) have the general structure as shown below (equation 1), where ‘n’ ranges from 4 (average molecular weight, MW ≈ 900 Daltons, hence NPFR-900), 7 (MW ≈ 1600 Daltons, NPFR-1600) to 20 (MW ≈ 4800 Daltons, NPFR-4800) (18).
The characterization of the NPFR by size exclusion chromatography (SEC) showed a polydispersity of ~1.2 for all the three resins synthesized, i.e., NPFR-900, NPFR-1600, and NPFR-4800 (18). A Mexican VR was used to study the behavior of nonylphenol formaldehyde resins as asphaltene-dispersant additives. Figure 4 shows the on-column filtration/redissolution asphaltene analysis in the absence (continuous trace) and the presence (discontinuous trace) of 200 ppm of a nonylphenol formaldehyde 31 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
resin (NPFR-1600) at 195°C (4). As mentioned before, the maltene peak is observed at 2-3 min whereas the asphaltene signal at ~15min. Using the methodology described previously, the maltene and asphaltene contents were calculated (see Table 2). For the run without additive, 10.25 μg (~ 30 wt.% of asphaltenes with respect to the injected sample) was found in the Mexican VR with a mass balance of 95%. In the presence of 200 ppm of a NPFR-1600, a reduction in the asphaltene signal and the concomitant increase in the maltene peak was observed (Figure 4, discontinuous trace). In this case, the asphaltene content is 6.12 μg (~18 wt.%) with a mass balance of ~102%. These results indicate that the NPFR-1600 additive interacts with the asphaltene molecules making them more soluble in heptane than in the case without the additive.
Figure 4. LC traces showing the effect of nonylphenol formaldehyde resin (MW = 1600 g/mol) on the asphaltene determination by on-column filtration/redissolution method for a Mexican VR at 195°C.
Furthermore, the effect of different concentrations of NPFR-1600 additive (from 200 to 1600 ppm) on the reduction of asphaltene content for Mexican VR was studied at 195°C. The results of the mass balances are shown in Table 2. As seen, the asphaltene content decrease whereas the maltene content increase with the NPFR-1600 concentration. Mass balances are in the 95%-102 range indicating the validity of the data. As before, mass balances were crucial to follow the changes in the maltene and asphaltene signals during the evaluation of nonylphenol formaldehyde resins as asphaltene dispersants.
32 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Table 2. Mass Balances of the Effect of Dosage of Nonylphenol Formaldehyde Resin (MW = 1600 g/mol) on the Asphaltene Content for a Mexican VR by on-Column Filtration/Redissolution Method at 195°Ca Additive Conc.b
Injected Amount (μg)c
Maltene Content (μ)d
Asphaltene Content (μg)e
Mass Balancef
Wt. % Asph.g
0
35.55
23.50
10.25
95%
28.8%
200
34.51
28.94
6.12
102%
17.7%
800
34.56
28.82
4.50
96%
13.0%
1600
34.93
30.38
4.04
99%
11.6%
Average of at least two determinations (4). b Additive concentration in ppm = mg/L with respect to the total solution. c Amount of sample injected in toluene. d Calibrated using Mexican VR maltenes. e Calibrated using Mexican VR asphaltenes. f Mass balances with respect to the amount injected solution including the absorbed material (4). g Weight percent of asphaltenes with respect to the amount of sample injected a
To quantify these observations, the reduction of the asphaltene content (as calculated by equation 2) was used to measure the relative efficiency of the dispersants. In general, the more negative this value is, the more efficient the additive is for keeping the asphaltenes dissolved in the hydrocarbon containing feedstock at any given temperature.
Figure 5 shows the effect of different concentrations of NPFR-1600 (in ppm or mg/L) on the percentage of reduction of asphaltene content for a Mexican VR at 195°C. As seen, the asphaltene-dispersant activity of the nonylphenol formaldehyde resins (MW = 1600 g/mol) increase with the concentration. Percentages of reductions of the asphaltene content are in the 40-60% range from 200 ppm to 1600 ppm. This phenomenon has been reported by other authors (24–26), and it is attributed to interactions between the dispersant and the active sites of the asphaltene aggregates so that the additive becomes part of the crown surrounding the asphaltene polyaromatic core (26). Nonylphenol formaldehyde resins were also evaluated as asphaltene dispersants for a hydroprocessed product (15.8°API, H/C molar ratio of 1.51, wt. % S = 1.18%) at 195°C. Figure 6 shows the LC traces of the asphaltene 33 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
region (from 18 to 25 min only) of the effect nonylphenol formaldehyde resins of different molecular weights (900, 1600, and 4800 g/mol) on the asphaltene content of a hydroprocessed product. In general, in the presence of the NPFR, reductions of the asphaltene peaks were observed (light, dashed, and dotted traces) in comparison with the run with no additive (darker trace).
Figure 5. Percent of reduction of asphaltene content (equation 2) vs. the concentration of nonylphenol formaldehyde resin (Mol. Weight = 1600 g/mol) for a Mexican VR at 195°C. Error bars = ±5%.
In Figure 6, the LC-traces show different shapes for all three asphaltene dispersant additives. Clearly, the NPFR-900 (light trace) has a larger area than the other two with a long tail finishing around 18 min. NPFR-1600 (dashed curve) has a smaller first peak but also a large tail, whereas NPFR-4800 (dotted curve) has a sharp peak and almost no tail. The results of mass balances (Table 3) show that the values are in the 95-104% range indicating the validity of the data. Furthermore, a careful inspection of the Teflon column showed a brownish precipitated on the top part. A similar observation has been reported before (4–6). Thus, the different shapes of the LC traces for the asphaltene dispersants were attributed to chromatographic effect due to compounds irreversible absorbed in the columns. However, the amounts of these materials are small and were not considered (4, 5, 18). 34 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 6. LC traces at 195°C of the asphaltene region (from 11 to 18 min) showing the effect nonylphenol formaldehyde resins (NPFR of MW = 900, 1600, and 4800 g/mol at 100 ppm) on the asphaltene content of a hydroprocessed product.
Figure 7 shows the percentages of reduction of asphaltene content (calculated using equation 2) versus the molecular weight (MW) of nonylphenol formaldehyde resin (MW = 900, 1600, and 4800 g/mol) for a hydroprocessed product at 195°C. Consistent with the visual observation of the LC traces (Figure 6), NPFR-900 is the least active dispersant (% reduction of asphaltene content = ~40%) whereas as NPFR-1600 and 4800 showed similar activity (~60%). As illustrated in this section, the evaluation of relative activity of nonylphenol formaldehyde resin as asphaltene dispersants was relatively easy to carry out using the on-column filtration/redissolution method at elevated temperature. The use of mass balances to corroborate the validity of the data was of paramount importance during the initial screening of additives and to determine the relative effectiveness of the resins.
Use of Phosphopropoxylated Asphaltenes During residue processing, the formation of sediments and coke deposits have been correlated to the asphaltene content of the feed (27, 28). It has been suggested that sediments are formed because asphaltenes and other highly refractive materials precipitate at hydroprocessing conditions. In particular, Storm et al. observed a large decrease in the amount of sediments formed during hydroprocessing of heavy ends when the heptane insoluble asphaltenes were removed from the feed (29). 35 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Table 3. Mass Balances of the Effect Nonylphenol Formaldehyde Resins (NPFR) on the Asphaltene Content for a Hydroprocessed Product by on-Column Filtration/Redissolution Method at 195°Ca
Additiveb
Inject. Amount (μg)c
Maltene Content (μg)d
Asphaltene Content (μ)e
Mass Balancef
Wt. % Asph.g
No Additive
40.04
36.44
1.69
95%
4.2%
NPFR-900h
39.96
40.61
1.02
104%
2.6%
NPFR-1600i
40.10
39.90
0.60
101%
1.5%
NPFR-4800j
39.97
38.62
0.57
98%
1.4%
Average of at least two determinations (4). b Additive concentration of 100 ppm (mg/L) with respect to the total solution. c Amount of sample injected in toluene d Calibrated using Mexican VR maltenes. e Calibrated using Mexican VR asphaltenes. f Mass balances with respect to the amount injected including the absorbed material (4). g Weight percent of asphaltenes with respect to the amount of sample injected. h Molecular weight = 900 g/mol. i Molecular weight = 1600 g/mol. j Molecular weight = 4800 g/mol. a
Figure 7. Percentages of reduction of asphaltene content (equation 2) vs. the molecular weight (MW) of nonylphenol formaldehyde resin (MW = 900, 1600, and 4800 g/mol at 100 ppm) for a hydroprocessed product at 195°C. Error bars = ±5%. 36 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
In addition, the same group of authors reported the use of phosphopropoxylated asphaltenes to prevent asphaltene precipitation during catalytic hydrocracking (30). The authors found increases of 9-10% of 1000°F+ residue conversion, hydrodesulfurization and vanadium removal along with ten-fold reduction in the amount of sediment in the reaction with additives versus the additive-free counterpart (30). With these ideas in mind, in this section, the synthesis, characterization, and evaluation of phosphopropoxylated asphaltenes are discussed. The importance of the use of mass balances is also illustrated. The objective is to gain fundamental knowledge on the chemistry and technology associated with these materials as asphaltene dispersant additives and their potential applications throughout the petroleum value chain. The synthesis, characterization, and evaluation of phosphopropoxylated asphaltenes are discussed elsewhere (31). In general, the additives were prepared by functionalizing the asphaltenes from a Mexican VR with PCl3 and then reacting the product with poly (propylene oxide-1000) diol (PPO-1000). Elemental analysis, FTIR, and 31P-NMR characterization results of PPO-1000 are consistent with the structure for the phosphopropoxylated asphaltenes depicted in equation 3:
The use of phosphopropoxylated asphaltenes as asphaltene-dispersant additives was evaluated using gravimetrically separated asphaltenes (ASTM Test Method D-6560) (7). Figure 8 shows the LC traces of the effect of PPO-1000 substitution on the asphaltene determination by on-column filtration/redissolution method. In effect, the addition of 200 ppm of PPO-1000 to a solution of 0.1 wt.% of Mexican VR asphaltenes (discontinuous trace) led a decrease of the asphaltene signal (~26 min) in comparison with the no-additive run (continuous trace). At the same time, the maltene signal at ~ 2 min increase after the addition of the PPO-1000 in comparison with the run without the additive. A careful look at Figure 8 showed that both asphaltene traces (with or without additive) have slightly different shapes and that a shoulder can be seen at the end of the runs (~35-38 min). As reported in Table 4, different concentrations of the phosphopropoxylated asphaltenes were evaluated (50-200 ppm). Mass balances were in the 87-98% range, which in turn, gives confidence in the results obtained. 37 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
As before, the shape of the asphaltene signals was attributed to chromatographic effects of the Teflon column whereas the appearance of the shoulder was due to compounds adsorbed on the columns which eluted when the column is being washed (4, 5, 18). Figure 9 shows the percent of reduction of asphaltene content (equation 2) versus the concentration of PPO-1000 for Mexican VR asphaltenes at 35°C. As seen, as the concentration of the additives increased, the percentages of reduction asphaltene becomes more negative reaching a minimum at -20% at 200 ppm.
Figure 8. LC traces showing the effect of 200 ppm of PPO-1000 on the asphaltene determination by on-column filtration/redissolution method for a Mexican VR.
Volumetric measurements, fluorescence spectroscopy, and solubility determinations were used to study the aggregation behavior of phosphopropoxylated asphaltenes. The results indicate that the mechanism of asphaltene inhibition seems to be related to the ability of these molecules to disrupt the π-π interactions between the polyaromatic moieties of the asphaltenes, inhibiting asphaltene stacking (18, 31). 38 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Table 4. Mass Balances of the Effect of Dosage of Phosphopropoxylated Asphaltenes PPO-1000 on the Asphaltene Content for a Mexican VR Asphaltenes by on-Column Filtration/Redissolution Method at 35°Ca Additive Conc.b
Injected Amount (μg)c
Maltene Content (μg)d
Asphaltene Content (μg)e
Mass Balancef
0
4.21
