Evaluation of Wax Inhibitor Performance through Various

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Evaluation of Wax Inhibitor Performance through Various Techniques Jeramie J Adams, Frederic Tort, John F. Schabron, Jenny L. Loveridge, Joseph F. Rovani, and Khalid M Baig Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02693 • Publication Date (Web): 20 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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Evaluation of Wax Inhibitor Performance through Various Techniques Jeramie J. Adams†, Frederic Tort‡, John F. Schabron†, Jenny L. Loveridge†, Joe Rovani, and Khalid Baig †Western Research Institute, 3474 North 3rd Street, Laramie, WY 82082 ‡TOTAL Additifs et Carburants Speciaux, 3 place du Bassin, 69700 Givors, France Abstract. Various techniques were used to compare the effectiveness of a commercially available wax inhibitor (WIA) to a newly developed wax inhibitor (WIEP) using a highly waxy Wyoming crude oil— which causes plugging within wellbores and pipelines. The two additives were compared using centrifuge experiments, cold finger tests, and the precipitation and redissolution Waxphaltene Determinator (WD) method. Centrifuge tube experiments, and cold finger tests, showed that the newly developed WIEP additive was significantly more effective at reducing the amount of ambient temperature wax crystallites in the crude oil, as well as reducing the amount of wax deposited on a cold finger. WD analysis were performed on model compounds to differentiate between shorter and longer n-paraffins. Whole crude oils, ambient temperature waxes centrifuged from the oils, and waxes from cold finger deposits were also analyzed by the WD. Taken together with high temperature gas chromatography, the WD profile of whole crude oils readily distinguishes shorter n-parrafins from the more problematic longer n-paraffins that are prone to crystallization at ambient temperature. The WD analysis profile showed a consistent decrease in wax with WIA concentration to give a linear correlation, however a less consistent change was observed with the WIEP additive. By applying the WD analysis to the additives themselves it was elucidated that the WIEP additive contained components that were highly polar and/or more associated. This observation suggests that components in the WIEP additive may self-precipitate to a greater degree than becoming incorporated with the waxes during the WD separation. This effect caused the WIEP to appear as though it is not as effective as the WIA additive in the WD analysis. Keywords: wax inhibitors, wax classification, Waxphaltene Determinator, waxy crude oils, inhibition efficiency, cold finger, wax deposition, paraffin crystallization, high temperature gas chromatography, nparaffins, wax dispersants

Introduction Paraffininc molecules are ubiquitous to petroleum, and constitute a valuable fraction of material owning to a variety of uses and markets depending upon its carbon number, degree of branching and cyclic content. Given the right conditions, and as the carbon length becomes greater (> C25) and more linear, highly favorable aligning of long paraffin molecules can occur though van der Waals interactions to generate a waxy crystalline nucleus.1 The temperature for the onset of nucleation occurs around the wax appearance temperature (WAT). This nucleation provides a site for additional interactions between solvated, or dispersed, longer paraffinic species, which propagates the growth of the wax crystallite nucleus to form larger paraffin crystals. This growth can lead to precipitation of wax crystallites, and/or additional interactions can take place between the waxy crystallites to form gelled three-dimensional networks.2-3 Precipitated crystallites are often characterized as being macrocrystalline or microcrystalline wax. Macrocrystalline wax (sometimes referred to as paraffinic wax) is enriched in nalkanes of low molecular weights (C16-C40), whereas microcrystalline wax is rich in iso- and cycloalkanes.4-5 In macrocrystalline waxes, the rather uniform geometry of the n-alkane molecules allows Page 1 of 37 ACS Paragon Plus Environment

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them to pack more densely and along a preferred axis to produce highly ordered needles or platelets. Conversely, the more irregular structure of iso- and cyclo-alkanes causes much less uniform packing of these molecules resulting in smaller amorphous crystallites.6 Studies on model systems, designed by doping different waxes into various crude oils or alkane solvents, have shown that wax precipitation is a complex process due to interplays between the various petroleum fractions and the types of waxes. Lighter alkanes can have a co-solubilizing effect on macrocrystaline waxes,7 while resins, and in particular asphaltenes, can have a more dispersing effect depending upon their state of aggregation and solublization. Dispersion of waxes occurs via long aliphatic side chains at the peripheries of asphaltene aromatic cores that interact with paraffinic species to disrupt and terminate further propagation of waxy crystallite growth towards the direction of the aromatic core. 8-13 The effect of asphaltene dispersion is naturally greater for less aromatic asphaltenes due to the availability of longer aliphatic side chains,14 and lower aggregation states of the asphaltene molecules which is likely a function of greater steric crowding due to larger aliphatic side chains. In general, polymeric wax inhibitors, or crystal modifiers, contain long aliphatic sidechains of optimal lengths to sufficiently interact with similar crude oil paraffinic molecules or aliphatic tails of waxy crystallites. These inhibitors usually also contain polar domains that prevent the propagation of crystal growth past the polar domain. Wax inhibitors interact with paraffin waxes through nucleation, adsorption, or co-crystallization to modify wax crystal growth and/or wax-wax interactions or through enhancing the solubility of the paraffin waxes. The inhibitor can also act to increase steric or entropic repulsions, promote interactions with asphaltenes or impart dispersive behavior by incorporating surface-active groups.6, 15 An exception to the commonly used polymeric non-polar/polar structural paradigm, are crystalline-amorphous copolymers which consist of exclusively nonpolar crystalline and amorphous groups. When in solution the crystalline domains lend themselves to self-assembly to form crystalline cores, or plates, surrounded by stabilizing amorphous side chains. These protruding side chains interact favorably with waxy paraffins to localize crystallization. In this way, much smaller wax crystals are formed because the crystalline cores, or plates of the copolymers, act as highly efficient nucleation sites for paraffin molecules thus preventing them from undergoing normal wax nucleation and growth in solution.16 In practice, wax inhibitors are often selected for a particular crude oil based upon empirical approaches, and a significant amount of work is still needed to understand all of the complex interplays between crude oil component if robust predictive models are to be developed that can best match additive formulation with a particular crude. Significant steps have been made in this direction through studies focused on understanding the interplays between different types of waxes and the various crude oil components and how these interactions influence the morphology and structure of wax crystal formation. Recently a physico-chemical approach was applied to determine which crude oil properties are most significant for predicting the performance of several different wax inhibitors.17 This was done by doping different non-waxy crude oils with refined paraffin waxes and treating them with various additives. By using a chemomechanical approach, it was determined that wax inhibitor efficiency was generally lower for heavier crudes and likewise for crudes with higher boiling points, WAT, pour point, aromatics content and with higher amounts of >C25 n-paraffins. Conversely, inhibitor performance improved for crude oils with higher amounts of asphaltenes,18 resins, total acid number, and with lower amounts of >C25 n-paraffins.17 Several others have also observed that wax inhibitor efficiency significantly decreases as the amount of the >C25 n-paraffins increase19-21 or in some cases as the nPage 2 of 37 ACS Paragon Plus Environment

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parrafins become >C34.22-23 The most effective inhibitors should seek to target these heavier n-paraffins which are most problematic because they become concentrated in waxy deposits.18, 24 It has also been reported that higher concentration of iso- and cylo-paraffins can have a synergistic effect to increase paraffin dispersant effectiveness.20 Formation of waxy crystallites complicates transportation of crude oil because the particles increase the viscosity of the oil and can adhere to surfaces to reduce flow leading to clogging and ultimately declining production. Problems can occur within reservoir wellheads as the crude oil cools to subsurface temperatures, during transportation through pipelines, during transportation by ships and within the infrastructure of production facilites.25-26 Additional difficulties can occur during production, storage and processing leading to large economic and environmental costs worldwide. Mechanical pigging operations are effective to remove blockages and to restore flow in fully or partially blocked pipelines, however it can be costly which is exacerbated by shutdowns and reductions in production. It is generally preferred to take a more preventative approach to mitigate wax fouling. Due to high cost and increased greenhouse emission, maintaining the temperature of the transported crude above its WAT is often unrealistic. A cheaper alternative, widely adopted by the industry, is to add low dosages of polymeric wax inhibitors. Wax inhibitor packages can work by several different mechanisms to solubilize paraffinic materials, impede gel formation, alter the interface and morphology of waxy crystallites, disperse asphaltenes and condition transportation surfaces. Several effective types of inhibitors are commercially available—and as more becomes understood about the interactions between the polymers, the waxy crystallites and the other crude oil components—more effective inhibitors are being designed. Two recent reviews are available which outline many important aspects between different inhibitors and their mechanisms of activity by understanding relationships between wax type, crude oil composition, and inhibitor properties and chemistry.6, 15 An HPLC based separation, termed the Waxphaltene Determinator (WD), has been developed to rapidly provide quantification of wax types in crude oils. The separation injects milligram quantities of petroleum materials into a PTFE packed column which is mixed with methyl ethyl ketone (MEK) at -24°C to precipitate waxy enriched material (>C20) and asphaltenes. Successive redissolution of the precipitated material with heptane at -24°C dissolves a portion of lower molecular weight n-parraffins (C24-C40) and iso-parafins (C40-C60+), referred to as Waxy A. Further redissolution at 60°C with heptane melts and extracts higher molecular weight n-paraffins (C30 to C60) and iso-paraffins (C50C60+), referred to as Waxy B. The WD separation clearly showed that waxy pipeline deposits are highly enriched in the Waxy B fraction which is highly concentrated in longer n-paraffins relative to the parent crude oil.27-28 In this current study two different wax inhibitors, one commercially available and one newly developed, were studied for their effectiveness at treating two different problematic waxy crude oils using centrifuge methods, cold finger experiments, WD analysis, and HTGC.

Experimental Materials. Waxy Wyoming Elliott crude oil was provided by a Wyoming producer, Alvehiem crude oil was provided by a cosponsor of the WRI Heavy Oil Research Consortium, Minnelusa and Dakota crude oils were provided by the University of Wyoming, and the Waxy African crude oil was provided by TOTAL ACS. The two additives used in this study are PAO 103 from Baker Hughes (WIA), which is used to treat paraffinic oils in the field which was also provided by the Wyoming producer, and the newly developed Page 3 of 37 ACS Paragon Plus Environment

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wax inhibitor WIEP 1503 (WIEP) which was provided by TOTAL ACS. Model compounds and solvents (HPLC grade) were purchased from Sigma-Aldrich or other commercial sources. Compositional details of the additives are given in Table 1. Table 1. Compositional details for WIEP and WIA wax inhibitors.

Waxphaltene Determinator (WD) Separation. The automated on-column asphaltene precipitation and re-dissolution experiments were conducted using a Waters 717plus autosampler, a Waters 60F pump with a model 600 controller, a Waters 2489 ultraviolet/visible absorbance detector, and a Waters 2424 evaporative light scattering detector (ELSD). Solutions of the crude oils, model compounds, and collected waxes were prepared as 10% (wt/vol) in chlorobenzene. The solutions were filtered through 0.45 micron syringe filters into autosampler vials. Portions of 20 µL (2 mg or less) were injected for the analytical scale WD separation. The waxes and asphaltenes were precipitated on a ground polytetrafluoroethylene (PTFE) packed column with methyl ethyl ketone at -24 °C. The precipitated materials were re-dissolved using different temperature and solvent combinations. All separation profiles were electronically blank subtracted prior to peak integration.27-28 The Waxphaltene Determinator (WD) divides the heavier (about > 430 °C) waxy components in petroleum into two wax fractions: Waxy A and Waxy B. An example of a WD separation profile is provided in Figure 1. The relative ELSD peak areas from the WD separations were used as an estimate of the weight percent of the various fractions. It should be noted that previous work showed that n-paraffins < C24 are not readily detected in the ELSD because they are too volatile.

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Figure 1. Waxphaltene Determinator separation for West Texas vacuum residuum. Isolation of Wax Enriched Saturates Fractions. Open-column chromatographic separations to isolate the saturates fraction—along with the waxes that are normally insoluble solids at ambient temperature— were performed using a 10 mm id x 200 mm long jacketed glass column that was heated to 60 °C. The elevated column temperature ensures that waxes would be melted and soluble so that they eluted with the saturates fraction. The column was slurry packed in heptane with 20 g of Aldrich grade 62, 60-200 mesh silica gel that had been activated overnight at 120 °C. Weighed samples of 200 µL of the whole oil in heptane were loaded on the top of the column (1 wt. % loading). The wax enriched saturates were eluted with n-heptane and the solvent was removed under gentle rotary evaporation followed by a stream of nitrogen at ambient temperature overnight prior to weighing. High Temperature Gas Chromatography. High temperature GC was performed on the wax enriched saturates fractions that were obtained from the high temperature silica separation. Sample solutions were prepared using toluene, and 1 µL volumes were injected for GC analysis using an Agilent 6890 GC equipped with a flame ionization detector (FID) and ChemStation software was used to analyze the data. The GC column was 15 meter long with and ID of 0.25 mm coated with 0.10 micron film thickness DB-1HT stationary phase manufactured by Agilent J&W. The oven temperature was 60 °C for 0.5 minutes with an initial 15 °C/min temperature ramp to an intermediate temperature of 285 °C, with a second temperature ramp of 10 °C/min to a final temperature of 395 °C for 13.5 minutes. A split injection at a ratio of 10:1 was used, and the glass inlet liner installed in the injection port was an inversion-cup design used at a temperature of 390 °C to provide sufficient residence time to volatilize higher molecular weight range hydrocarbons. The FID was operated at a temperature of 400 °C using flow rates of 400 mL/min air and 40 mL/min hydrogen. In this configuration the separation was capable of eluting and resolving Page 5 of 37 ACS Paragon Plus Environment

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hydrocarbons from approximately dodecane (n-C12) through heptacontane (n-C70). Modification of the conditions can be made to separate waxes of higher carbon number, however this was not deemed necessary for the samples evaluated in the current work. Prior to analysis the toluene solutions were warmed on a hotplate to facilitate complete dissolution of the waxes in the wax enriched saturates fraction, and no insoluble material was evident before GC analyses were performed. The samples were subsequently reanalyzed after the addition of an internal standards mixture consisting of n-C18, n-C24, n-C36, and n-C44. The amounts of internal standard compounds spiked into 1.0 mL of sample solutions were 0.20, 0.20, 0.19, and 0.05 mg, respectively. Peak areas of the internal standards were used to calculate an average FID response factor, which was used to determine the masses of the GC wax peaks in the samples. Differential Scanning Calorimetry (DSC). The DSC profiles were obtained using a TA Instruments DSC Q2000 instrument. Portions of about 10 mg of accurately weighed sample was analyzed taking precautions to avoid significant loss of volatiles and prevent splattering during the initial heat ramp. To obtain the wax appearance temperatures (WAT) of the whole oils, the samples were initially heated to 70 °C at a rate of 10 °C /min, held constant for 5 minutes, and then cooled to -90 °C at a rate of 2 °C/min. The sampling interval was 0.69 seconds per point. The amount of wax was estimated by using an assumed wax heat of fusion of 180 J/g and the measured heat of fusion observed from the cooling profile.29 Thermogravimetric Analysis (TGA). TGA weight loss profiles were generated using a TA Instruments Q5000 TGA (TA Instruments, New Castle, Delaware). Oil samples were place in platinum TGA pans and heated at 20 °C per minute in nitrogen to 600 °C. The sweep gas was switched to air, and samples were held at 600 °C for 20 minutes. Centrifuge Experiments. Material was placed into 100- or 15-mL graduated centrifuge tubes and centrifuged using a Damon IES CU-5000 centrifuge with a 4-position, 40 cm (16 inch) diameter rotor. Centrifugation was conducted at 2,500 to 3,000 rpm at various time intervals. No attempt was made to account for slight changes in viscosity of the oils that could have occurred due to the addition of the liquid additives, or due to the reduction in waxy crystallites by the additives which would decrease the viscosity. Cold Finger Experiments. A 300 mL Parr Instrument Company reactor fitted with a magnetic stirring shaft and a cooling coil in the shape of a “U” was used to collect wax deposits. The u-tube was provided by the manufacture and consists of ¼” outer diameter stainless tubing that extends 5.5” below the bottom of the head of the reactor. The u-tube was connected to a heater chiller bath and a thermocouple was inserted inside the u-tube to monitor the temperature. The fluid in the u-tube was maintained at about 10 °C below the WAT of the crude while the temperature of the crude oil was maintained at about 10 °C above the WAT using a heating mantle and PID temperature controller. 180 mL of crude oil was placed inside the autoclave and the temperature was equilibrated for 1 hour with stirring at around 200 rpm prior to turning on the chiller bath. The time of the wax deposition experiment was 6 hours once the chiller bath was turned on. The autoclave reactor head was removed and placed on a stand to allow excess crude oil to drip from the u-tube that was being used as the cold finger. The wax was recovered from the cold finger by increasing the temperature of the u-tube to 75 °C and melting the wax into a preweighed vial. For experiments using the Elliott crude oil, the temperature of the oil was maintained at 41 ± 2 °C and the temperature of the u-tube was set at 20.0 ± 0.1 °C, while experiments using the African crude oil were Page 6 of 37 ACS Paragon Plus Environment

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conducted using an oil temperature of 51 ± 2 °C and the temperature of the u-tube maintained at 30.0 ± 0.1 °C.

Results and Discussion WD Separation of Model Compounds: Previous work had showed how model compound distributed between the various WD fractions.27-28 In this work the shift from shorter n-paraffins from Waxy A to longer n-alkanes for Waxy B was confirmed by performing WD analysis on Polywax® 500 and 655 mixtures. Polywax 500 is a mixture of synthetic polyethylene from about C20-C76 and Polywax 655 contains higher MW waxes ranging from about C20-C100. The WD separation showed that the Polywax 655 contained 5% more Waxy B material and 5% less Waxy A material than Polywax 500, as shown in Figure 2.

Figure 2. WD results for Polywax 500 and Polywax 655. For these samples, there are no MEK maltenes present. These results are consistent with high temperature gas chromatography (HTGC) profiles which showed that Polywax 655 contained significantly more longer n-alkanes than Polywax 500, as shown in Figure 3. It should be noted that for the WD analysis, the difference between Waxy A and Waxy B was anticipated to be larger between the two Polywax samples. However, all of the interplays between the n-paraffins are not well understood and not all of the Polywax samples could be dissolved in chlorobenzene at ambient temperature. This means that some larger n-alkanes were not dissolved and therefor not analyzed in the WD as the solution is filtered before injection. This could be prevented in the future by heating the samples prior to filtration and using an autosampler injector on the WD that could be maintained at an elevated temperature.


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220

Polywax 500 with Internal Standards

200

nC36

FID Response, pA

180 160 140

nC24

120 nC18

100

nC44

80 60 40 20 0

0

5

10

15

20

25

Time, minutes 140

Polywax 655 with Internal Standards

120

FID Response, pA

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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nC36

100

nC18

nC24

80

nC44

60 40 20 0

0

5

10

15

20

25

Time, minutes

Figure 3. GC profile for Polywax®500 (top) and 655 (bottom), with internal standards. Investigation of Ambient Temperature Insoluble Waxes from Waxy Crude Oils. Three waxy crude oils were studied to compare their n-paraffin distributions in the whole saturates fraction collected by column chromatography at 60°C and to their respective ambient temperature insoluble waxes separated by centrifugation. These fractions were analyzed by HTGC and by the WD separation to better understand how the n-paraffin distribution is manifested in the WD profile. It is important to understand how insoluble ambient-temperature crude oil waxes partition between Waxy A and Waxy B to show which fraction should be targeted by paraffin dispersant additives. Waxy enriched crude oils consisting of Dakota waxy crude oil, Mennelusa crude oil and Alvheim crude oil were used for this study. For the centrifuge separation, 100 mL of the oil was centrifuged until the waxes gathered as a plug at the bottom of the centrifuge tube (usually above the water layer). The centrifuged oil was decanted away from the waxy Page 8 of 37 ACS Paragon Plus Environment

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plug and the residual oil was removed using a lint free wipe. Heptane was added to the waxy plug and the suspension was filtered through at 0.45 micron filter paper to remove any inorganic solids. The Minnelusa and Dakota solids were rinsed with ambient temperature heptane until no more waxy material appeared on the paper. The waxy material from the Alvheim crude were less soluble in ambient temperature heptane requiring them to be rinsed with 60-70 °C heptane to dissolve most of the waxes from the filter paper. The filtrates were collected and the heptane was removed by rotary evaporation. For the chromatographic separation, the wax containing saturates fractions were isolated from the other more polar crude oil components using silica gel and heptane in a column heated to 60 °C. The wax containing saturates fractions collected from silica gel at 60 °C and the centrifuged waxes were analyzed by HTGC.30 In these chromatograms, the broad curved areas below the more well defined GC peaks were not integrated since it was difficult to establish a reliable baseline. The GC profile of the waxcontaining saturates fraction from the Minnelusa crude and the profile for the material spiked with nparaffin standards are provided in Figure 4, and similar profiles for the centrifuged waxes are given in Figure 5. Similarly, Figure 6 shows the GC profiles for the Dakota saturates and Figure 7 shows the GC profiles of the centrifuged Dakota wax, and Figure 8 shows the GC profiles for the Alvheim saturates and Figure 9 shows the profiles for the centrifuged Alvheim wax.


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Figure 4. GC profile for Minnelusa saturates spiked with reference n-paraffins.

Figure 5. GC profile for Minnelusa centrifuged waxes spiked with reference n-paraffins.


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Figure 6. GC profile for Dakota saturates spiked with reference n-paraffins.

Figure 7. GC profile for Dakota centrifuged waxes spiked with reference n-paraffins.


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Figure 8. GC profile for Alvheim saturates spiked with reference n-paraffins.

Figure 9. GC profile for Alvheim centrifuged waxes spiked with reference n-paraffins.


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Saturates from the three oils showed very similar n-paraffin distribution profiles, with significant nparaffin contents ranging from about nC12 to nC44. Although the majority of the peaks appear to be nparaffins, there is evidence of branched alkanes or related alkane homologous series in these saturate fractions, especially in the region between C13-C20. The nature of the material below/within the curved baseline is unknown without additional analyses. The weight percent of the saturates eluted from the 60 °C silica column with heptane, and the weight percent of C13-C44 n-paraffin hydrocarbons integrated as HTGC peaks relative to n-paraffin internal calibration standards are listed in Table 2. Table 2. n-paraffin content of saturates separated at 60°C on silica by HTGC. Oil Minnelusa Dakota Alvheim

Weight Percent Saturates Eluted at 60 °C 51.7 60.7 58.4

Weight Percent n-paraffins from HTGC 60.0 56.5 85.0

Similarly, centrifuged waxes were also high in n-paraffin content, however they contained overall more n-paraffins appearing as a bimodal distribution with a significantly higher proportion of > C28 n-alkanes. Smaller paraffins within this bimodal distribution may be due to their co-crystallization within the waxy crystallites, or due to entrainment of crude oil components within the void volume between waxy crystallites during the centrifuging and isolation procedure. No attempts were made to purify the centrifuged wax fraction. The approximate volume percent of the centrifuged ambient temperature insoluble wax, obtained from 100 mL oil, and the weight percent of C13-C44 n-paraffin hydrocarbons integrated from the HTGC peaks relative to n-paraffin internal calibration standards are provided in Table 3. The three samples may be ranked for total insoluble ambient temperature wax content increasing from Minnelusa < Dakota < Alvheim and the order of the n-alkane content in the waxes by HTGC are: Dakota < Minnelusa < Alvheim. Table 3. n-paraffin content of centrifuged waxes by HTGC. Oil Minnelusa Dakota Alvheim

Volume Percent Ambient Centrifuged Waxes 0.4 1.6 3.2

Weight Percent n-paraffins from HTGC 87.0 78.3 92.4

These crude oils and their centrifuged waxes were analyzed by the WD to see if there was a correlation between the centrifuged waxes and the Waxy B material, since it has been reported that the Waxy B material is enriched in pipeline deposits.27 Results from the WD analysis of the crude oils and the centrifuged waxes are provided in Table 4. The WD data represent only the > ~400 °C portion of the material injected which corresponds to waxes of about > C24. The results show that when comparing Waxy A (soluble in heptane at -24 °C) and Waxy B material (insoluble in heptane at -24 °C but soluble in heptane at 60 °C), the Waxy B material is more abundant in the centrifuged waxes relative to the whole oils. Since the HTGC work shows that the centrifuged waxes are enriched in >C28 n-paraffins, and considering that the WD Waxy B material is also enriched in the centrifuged waxes, then the WD Waxy B must also be likewise enriched in larger n-paraffins—or associated n-paraffin structures—of about C28 or larger. These results show that the WD is a convenient way to differentiate between crude oils that have Page 13 of 37 ACS Paragon Plus Environment

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more >C25 n-paraffins which are known to be difficult to treat with wax inhibitors.17 The data also shows that the asphaltenes from the Minnelusa crude oil do not precipitate with, or are concentrated in, the centrifuged waxes. Figure 10 shows a comparison of the WD fractions for the three crude oils. Table 4. n-paraffin content of centrifuged waxes by HTGC. Maltenes

Sample Dakota crude Dakota Cent. Wax Minnelusa crude Minnelusa Cent. Wax Alvheim crude Alvheim Cent. Wax

MEK (-24 °C) 74.29 28.41 81.53 84.15 88.31 71.30

Waxes (heptane-Sol.) A (-24 °C) 14.00 8.12 4.37 1.06 7.57 6.65

B (60 °C) 8.05 62.69 1.42 14.00 2.51 19.90

Asphaltenes Tol-Sol. 1.25 0.37 9.87 0.62 0.64 0.42

CH2Cl2-Sol. 2.42 0.41 2.81 0.17 0.97 1.73

100 90 80 70

ELSD Area Percent

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 50 40 30 20 10 0 Dakota crude

Dakota Cent. Wax MEK Maltenes

Minnelusa Minnelusa crude Cent. Wax Waxy A Waxy B Tol-Sol

Alvheim Alvheim crude Cent. Wax CH2Cl2-Sol

Figure 10. WD analysis of Dakota, Minnelusa, and Alvheim crude oils and centrifuged ambient temperature insoluble waxes from the respective crude oils. DSC was used to evaluate the wax appearance temperature (WAT)27 and total wax content for the whole oils and the centrifuged oils which had some ambient temperature insoluble waxes removed. An example DSC cooling profile for the Dakota whole oil is provided in Figure 11. Exact quantification may be slightly Page 14 of 37 ACS Paragon Plus Environment

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different from sample to sample for the same oil since it may be difficult to obtain a representative sample for the whole oils, especially if they contain some dispersed solid waxes and emulsified droplets of water. Quantification of the wax content using the area is estimated using a heat of fusion of 180 J/g for macrocrystalline wax, and using the heating curves to minimize artifacts introduced by the “cold crystallization” exotherm during heating.27

Figure 11. DSC cooling profile for Dakota crude oil. 21.87 °C is the onset temperature which is the same as the WAT, and 13.17 J/g is the area above the red line which correlates to the amount of the crystallizable fraction. Estimated DSC weight percent wax contents and WAT temperatures are provided in Table 5 for Alvheim, Minnelusa, and Dakota whole oils and the respective centrifuged oils. As expected, when some of the ambient-insoluble waxes are removed by centrifugation the DSC data showed a decrease in the WAT and a decrease in the amount of wax. For these oils, the WAT values are near or above ambient temperature, which is consistent with the presence of suspended insoluble waxes in the whole oils.


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Page 16 of 37

Table 5. DSC data for the original whole crude oils and the crude oils that had some of the ambient temperature insoluble waxes removed by centrifugation. A heat of fusion of 180 J/g was used to estimate the weight percent of wax. The two WAT values for the centrifuged Minnelusa crude oil is due to the appearance to two separate wax crystallization events. WAT °C

Wax Exotherm J/g

Approximate

34.0

3.02

1.3

21.9, 28.3

2.2, 3.1

1.1, 0.73

Dakota Whole Oil

22.9

15.99

6.9

Dakota Centrifuged Oil

20.7

14.95

5.6

Alvheim Whole Oil

26.2

8.07

4.4

Alvheim Centrifuged Oil

20.7

7.30

3.2

Sample Minnelusa Whole Oil Minnelusa Centrifuged Oil

Wax wt. %

Wax Dispersion Additive Evaluation in a High Wax Content Wyoming Crude. A waxy Wyoming Elliott crude oil, which undergoes plugging at the wellbore and in pipelines, was used to evaluate two different wax dispersant additives: a commercially available wax inhibitor (WIA, currently used to treat this reservoir in the field) and a newly developed wax inhibitor (WIEP). A secondary aspect of this study was to determine if the WD analyses could be used to show the effectiveness of the wax inhibitors and to differentiate between which crude oils might be more difficult to treat. If the WD could achieve these goals, it would provide another useful tool in the arsenal of additive formulators and producers alike. The Elliott crude oil was initially treated with 1,000 ppm of WIA, which is the prescribed dosage level provided by the supplier. The WD profiles for the untreated and treated Elliott crude oils are shown in Figure 12. The WD profile shows that the untreated Elliott crude oil had very high wax content containing about 14 wt% of both Waxy A and Waxy B while the additive resulted in lowering the Waxy B fraction to about 3 wt%. The decrease in the Waxy B material also coincided with centrifuging experiments, which showed a significant decrease in the amount of the wax plug in the treated sample, which is shown in Figure 13. It should also be noted that Waxy A also decreases with additive treatment, but as was demonstrated, the Waxy B fraction is enriched in the centrifuged ambient temperature waxes and this fraction correlates with the volume of the waxy plug after centrifuging the crude oil treated by WIA.


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Energy & Fuels

Waxy A

Waxy B

Waxy A Waxy B

Figure 12. Waxphaltene Determinator of Elliott crude oil (top) and the crude with 1,000 ppm WIA (bottom).


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Page 18 of 37

Figure 13. Waxy Wyoming crude oil heated to 70 °C, cooled to ambient temperature, and centrifuged (left). The same crude treated with 1000 ppm of WIA paraffin dispersant additive and treated in the identical manner to the control without WIA (right). These results show that Waxy B material is comprised of associated wax crystallites, and that this particular additive works by co-crystallizing with the waxes to make them more soluble resulting in a reduction in the Waxy B fraction while increasing the more soluble fractions. Evidence for associated wax crystallites in Waxy B is related to the fact that this fraction is eluted by melting at 60 °C while still using heptane as the eluent. The increased temperature serves to disrupt the weak van der Waals interactions that are responsible for causing waxes to associate and crystalize, so that it can melt and dissolve. This experiment shows that the WD can be used with this particular additive to determine the effectiveness of the paraffin dispersant and it may be used to prescribe the proper dose for this additive in a given crude oil. An additional additive concentration dependent experiment was performed using WIA with a different batch of the Elliott crude oil. The Elliott crude oil was treated with 500 ppm, 1000 ppm, 5,000 ppm, 10,000 ppm, 20,000 ppm, and 30,000 ppm of the commercial additive. Higher levels of the additive were not evaluated to determine if all of the crystallized waxes could eventually be dispersed. The treated samples were conditioned at 70 °C, allowed to cool to ambient and then left undisturbed for several days. Prior to cooling, an aliquot was taken and analyzed by the WD. After sitting at ambient conditions, the samples were centrifuged two times at 2,500 rpm for 30 min to form a wax plug at the bottom of the centrifuge tube. The volume of the wax plug and the WD ELSD area percent of Waxy A and Waxy B are given in Table 6. When plotting the amount of Waxy B relative to the amount of WIA used to treat the oil, as shown in Figure 14, the additive had a very high initial efficacy for reducing the amount of the Waxy B material, but Page 18 of 37 ACS Paragon Plus Environment

Page 19 of 37

at higher concentrations it starts to decrease asymptotically after about 5000 ppm. From the WD data, it is also clear that the additive also decreased the amount of the more soluble Waxy A fraction.

Table 6. WD results and the volume of the wax plug after centrifuging for Elliott crude oil (10 mL) treated with different levels of the commercial paraffin dispersant. Sample

MEK Maltenes MEK-Soluble, -24°C

Elliott Control 500 ppm 1000 ppm 5000 ppm 10000 ppm 20000 ppm 30000 ppm

76.4 80.5 81.1 82.9 85.1 88.2 89.6

Waxy A Waxy B Heptane-Soluble, -24°C Heptane-Soluble, 60°C

17.3 15.7 15.7 14.7 13.0 10.5 9.4

6.3 3.9 3.2 2.3 1.9 1.3 1.0

Centrifuged Wax Plug (mL) 4.2 3.2 2.5 2.4 1.6 1.0 0.7

5 4.5

Waxy B, ELSD Area Percent

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

Energy & Fuels

4 3.5 3 2.5 2 1.5 1 0.5 0 0

5000

10000

15000 20000 ppm of WIA

25000

30000

35000

Figure 14. The amount of Waxy B from the WD analysis as a function of the concentration of WIA in Elliott crude oil. Interestingly, there was a good correlation between the amount of the waxy plug isolated by centrifuging and the WD Waxy B peak, as shown in Figure 15. This result shows that for certain crude oil/additive combinations the WD can be successfully used to gauge the effectiveness of the additives. Page 19 of 37 ACS Paragon Plus Environment

Energy & Fuels

7

Waxy B, ELSD Area Percent

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

Page 20 of 37

6 5 4 3 y = 1.426x - 0.3308 R² = 0.9318

2 1 0 0

1

2 3 Wax Centrifuged (mL wax plug)

4

5

Figure 15. Relationship between the amounts of Waxy B obtained from the WD analysis and the volume of the ambient temperature waxy plug after centrifugation.

Over this concentration range, it is clear that that ambient-temperature insoluble waxes still remain in the oil after treatment with high levels of WIA. Figures 14 and 15 show that this particular paraffin dispersant has a limit for the kinds of waxes it can effectively inhibit from crystallizing within this oil at concentrations above 10,000 ppm. For the sample treated at 10,000 ppm, the oil was decanted away and the wax plug and the waxes were dispersed in heptane. The slurry was filtered onto a 0.45 micron filter paper to collect the heptane-insoluble waxes and rinsed sparingly with additional heptane to give a colorless powder. The wax was analyzed by TGA and the derivative of the weight loss curve showed a relatively narrow maximum around 420 °C, indicating that most of material had a boiling point around this range. The actual boiling point of this material may be slightly different because TGA values above 400 °C become complicated by pyrolytic bond breaking. The derivative of the weight lost curve is shown in Figure 16. DSC gives a clearer picture showing that it was a relatively narrow fraction of wax with a melting point around 82 °C (Figure 17). The DSC results show that this wax consists of alkanes in the region around n-C40. This is consistent with other’s work that showed that wax inhibitors are less effective for waxes that have carbon number greater than about C25.13, 17, 19-20, 22-23 This suggests that WIA may not contain long enough aliphatic chains to adequately interact with the larger n-paraffins to co-crystallize with them.6, 23


Page 21 of 37

1.4 1.2

Deriv. weight (%/°C)

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

Energy & Fuels

1 0.8 0.6 0.4 0.2 0 0

100

200

300 400 Temperature (°C)

500

600

700

Figure 16. Derivative of the weight percent for the weight loss curve from the TGA of centrifuged waxes which were not affected by the commercial paraffin dispersant WIA.

Figure 17. DSC profiles of centrifuged waxes which were not affected by the commercial paraffin dispersant WIA. Page 21 of 37 ACS Paragon Plus Environment

Energy & Fuels

A portion of the wax which was unaffected by WIA was also placed in heptane and heated to 60 °C in an oven which caused the wax to become completely dissolved. This result indicates that this fraction would report primarily to the WD Waxy B fraction. This fraction was analyzed by the WD which showed that it was in fact significantly more enriched in Waxy B material. However, it also did show some Waxy A and a larger than expected amount of MEK soluble material. This may be due to the process of dissolving the wax in ambient temperature chlorobenzene that causes some of the lighter n-paraffins to become preferentially dissolved since not all of the sample was soluble in ambient temperature chlorobenzene. As mentioned earlier, this could be improved by using an auto sampler that kept the chlorobenzene samples at an elevated temperature to solubilize all of the waxes during sample injection. Figure 18 shows the WD profiles for the Elliott crude oil, the centrifuged ambient temperature-insoluble waxes, and the waxes which were unaffected by WIA.

80 70 60

ELSD Area Percent

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

Page 22 of 37

50 40 30 20 10 0 Elliott Crude Elliott Cent. Wax Elliott WIA Insol. MEK Maltenes Waxy A Waxy B Tol-Sol CH2Cl2-Sol

Figure 18. WD results for Elliott crude oil, ambient temperature centrifuged waxes, and waxes which remained insoluble after treatment with WIA.

The waxy Elliott material that was not affected by WIA was also analyzed by HTGC. The GC profile is shown in Figure 19, which show a relatively narrow distribution of n-paraffins ranging from C36 to C50 with a mean of C42. These paraffins are consistent with the DSC results of this material and what is expected for WD Waxy B material. Page 22 of 37 ACS Paragon Plus Environment

Page 23 of 37

120

Elliott WIA Insoluble Material

100

FID Response, pA

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

Energy & Fuels

80

nC18

nC24 nC36

60

nC44

40

20

0

5

10

15

20

25

Time, minutes

Figure 19. GC profile for centrifuged waxes from Elliott crude oil after being treated with WIA at 10,000 ppm, with internal standards. A second wax inhibitor, WIEP, was tested for its effectiveness in the Elliott crude oil and to provide a comparison to WIA. The supplier of the WIEP additive provided the amount of active material in the inhibitor and the amount of active material in WIA was estimated by removing the solvent under vacuum and obtaining a weight of the residue. The WIEP additive had 38 wt% active material and the WIA had approximately 13 wt% active material. Therefore, the Elliott crude oil was treated at 347 and 3470 ppm of WIEP, which is the same relative weight of active material in WIA at 1000 ppm and 10,000 ppm. Fresh treated samples were prepared for this study and conditioned at 70 °C for 30 min and an aliquot was sampled for WD analyses. 10 mL of the solution were placed into 15 mL graduated centrifuge tubes which were allowed to cool overnight prior to centrifuging twice at 2,500 rpm for 30 min. The tubes were inverted to obtain the volume of the wax plug. From these tests it was clear that the WIEP additive had a much greater effect on reducing the amount of ambient temperature insoluble wax that could be centrifuged. For the WIA additive, there was a small decrease in the amount of centrifuged wax when going from 1000 to 10,000 ppm: 2.5 and 2.0 mL, respectively. However, the WIEP additive continued to significantly reduce the amount of centrifuged wax when going from 347 to 3470 ppm: 1.3 and 0.7 mL, respectively. Although the WIEP additive performed better at reducing the amount of ambient temperature centrifuged wax, the WD results did not reflect this since it did not show as large of an effect at reducing the Waxy B peak. The WD profile and the amounts of the centrifuged wax plug for the untreated Elliott crude oil and treated oils, and the WD profile of waxy deposit collected from a cold finger experiment (see later) are shown in Table 7.


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Page 24 of 37

Table 7. WD results and the volume of ambient temperature wax plug after centrifuging untreated Elliott crude oil (control) and the same sample of Elliott treated with two different levels of WIA and WIEP additives. The last row shows the WD analysis of wax deposited from untreated Elliott crude oil on a cold finger at 21 °C with the oil stirred at 40 °C. Sample

MEK Maltenes Soluble MEK -24°C

Waxy A Soluble Heptane -24°C

Waxy B Soluble Heptane 60°C

Centrifuged Wax Plug (mL)

Elliott Control Elliott, 1000 ppm WIA Elliott, 10,000 ppm WIA Elliott, 347ppm WIEP Elliott, 3470 ppm WIEP Elliott Wax from Cold Finger

76.4 78.0 83.3 79.2 78.5 67.1

17.3 18.2 15.1 15.4 17.7 16.5

6.3 3.9 1.6 5.5 3.9 16.2

4.3 2.5 2.0 1.3 0.7 -

From the results in Tables 6 and 7, it is obvious that as the additive concentration increases it becomes more effective at decreasing the amount of centrifuged wax and the amount of Waxy B. However, the WD results show that the WIEP additive does not decrease Waxy B as much as the WIA additive, even though it significantly outperforms WIA in the centrifuge experiments. It should be noted that the centrifuge experiments are in agreement with cold finger experiments described later. Some possible explanations for the discrepancy between the centrifuge experiments and WD results for the oil treated with WIEP are: (1) the WIEP additive could be highly insoluble in MEK causing it to self-precipitate independently of the waxes causing it to appear as if it has little effect during the WD separation, or (2) The WD separation could preferentially remove or strip a major active component in the additive preventing it from interacting with the waxes. Of these, option (1) is the most plausible based upon WD analysis of the WIA and WIEP additives. WD results in Figure 20 show that WIEP contains mostly Waxy B material and a significant amount of polar toluene-soluble material. This suggests that the majority of the WIEP additive would rapidly precipitate at the same time as the n-paraffins in MEK at -24 °C causing competition between self-crystallization (or association) of additive components and co-crystallization of additive components with waxes. Several repeated WD analysis of the Elliott crude oil treated, at different concentrations with the WIEP additive, showed only a small effect on reducing the amount of Waxy B material despite reducing the amount of centrifuged waxes by as much as 94% by volume. It is well known that there is a complex interplay between the self-crystallization/solvation state of the polymeric portion of additives and how it will interact with waxy crystallites to exhibit the proper effect to disperse the crystallites by either nucleation, adsorption, co-crystallization and solubilization.6, 15 Regarding (2), this is unlikely as the terpolymer in WIEP consists of highly polar maleic anhydride groups causing it to be mostly insoluble in -24 °C MEK.31


Page 25 of 37

80 70 60

ELSD Area Percent

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

Energy & Fuels

50 40 30 20 10 0 WIA MEK Maltenes Waxy A

Waxy B

WIEP Tol-Sol CH2Cl2-Sol

Figure 20. WD analysis of WIA and WIEP.

Cold Finger Tests. To better simulate expected field performance, cold finger tests were performed on the Elliott crude oil treated with the WIA and WIEP additives. A cold finger apparatus was constructed from a Parr reactor that had a u-tube attached which was fitted to a chiller bath as shown in Figure 21. The temperature of the oil was maintained at 41 ± 2 °C and the temperature of the u-tube was set at 20.0 ± 0.1 °C, which is approximately 10 °C above and below the WAT of the oil (WAT ~32 °C measured by DSC), respectively. The untreated Elliott crude oil control, the Elliott crude oil treated with 1000 and 10,000 ppm WIA, and the Elliott crude oil treated with 347 and 3470 ppm WIEP were subjected to at least two different cold finger tests. Table 8 gives the summary of the amount of wax collected for each of the cold finger experiments.


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Page 26 of 37

Thermal Well U-tube Stirrer Figure 21. Cold finger apparatus showing the u-tube coated with wax from a control experiment using Elliott crude oil.


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Energy & Fuels

Table 8. Amount of wax collected during cold finger experiments for untreated Elliott crude oil and the oil treated with WIA and WIEP at two different concentrations. The oil temperature was maintained at about 41 °C, the cold finger u-tube was maintained at 20 °C and the oil was contacted with the cold finger for 6 hrs with mechanical stirring. The number of experimental runs was dictated by the availability of the samples. Elliott Crude Oil Control 1,000 ppm WIA 10,000 ppm WIA 347 ppm WIEP 3,470 ppm WIEP

1 (g) 0.928 1.026 0.931 0.721 0.172

Experiment Run 2 (g) 3 (g) 0.904 0.881 0.807 0.931 0.823 0.737 0.734 0.28 0.206

4 (g) 0.889 -

Data from the cold finger experiments showed that the WIEP additive was more effective at reducing the amount of wax accumulated at 20 °C, which is consistent with results from the centrifuge tube experiments. Treated samples with WIA had very little effect on the amount of wax deposited on the cold finger, regardless of the concentration. This is not surprising since WIA treated oils still contained a significant amount of ambient temperature wax, as observed in the centrifugation experiments, which could easily collect on the cold finger. For the oil treated with 374 ppm WIEP, there was a slight reduction in the amount of wax collected on the cold finger, which is consistent with the centrifuge tube experiments which showed that the treated sample has a significantly lower amount of wax than the 10,000 ppm WIA treated sample. At the higher concentration of WIEP, 3470 ppm, there was a very significant reduction in the amount of wax collected on the cold finger, which is also consistent with the fact that this level of treatment produced the smallest amount of wax that could be centrifuged. Figure 22 shows a comparison of the wax deposit on the cold finger for the untreated Elliott crude oil and the Elliott crude oil treated with 3470 ppm of WIEP.


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Page 28 of 37

Figure 22. The left picture shows the wax coating the cold finger u-tube from the untreated Elliott oil and the picture on the right shows the u-tube with very little wax for the Elliott oil treated with 3470 ppm of WIEP. Both experiments were conducted under identical conditions. The waxy deposit from a cold finger experiment, carried out with the untreated Elliott crude oil, was analyzed by the WD and by HTGC. The WD data is presented above in Table 6 and the GC profile is shown in Figure 23. As expected, the GC profile shows a bimodal distribution of waxes with an enrichment of larger n-paraffin waxes centered around C42, which is consistent with waxes that were collected from the Elliott crude oil treated with 10,000 ppm WIA as shown in Figure 19. Likewise, by the WD analysis there was a significant increase in the Waxy B fraction, 6 % Waxy B in the Elliott crude oil and 16% Waxy B in the deposit.


Page 29 of 37

120

Elliott Wax Deposited on Cold Finger nC18

100

FID Response, pA

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

Energy & Fuels

nC24 nC36

80

60 nC44

40

20

0

5

10

15

20

25

Time, minutes

Figure 23. GC profile of Elliott wax deposited on cold finger, with internal standards. For comparison, HTGC data were collected for the untreated Elliott crude oil and the wax centrifuged from the Elliot crude oil, and an overlay of the GC profiles are shown in Figure 24. Unexpectedly, the GC profile of the centrifuged wax from the Elliott crude oil did not exhibit a significant bimodal distribution showing an increase in higher order n-paraffins, even after recentrifuging the wax. After recentrifuging there was only a slight overall increase in all the n-paraffins relative to the whole crude oil (Figure 24). This result is interesting because the centrifuged wax is a semisolid and clearly different in physical appearance from the whole crude oil. This may be due to strong co-crystallization n-paraffins C25.

120

Centrifuged Wax from Elliott Treated with 347 ppm WIEP with Internal Standards

100

FID Response, pA

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

Energy & Fuels

nC18 nC24

80

nC36

60

40 nC44

20

0

5

10

15

20

25

Time, minutes

Figure 26. GC profile for wax centrifuged from Elliott crude oil after treatment with 347 ppm of WIEP, with internal standards. Evaluation of WIA and WIEP on a Highly Waxy African Crude Oil. The additives WIA and WIEP were used to treat a highly waxy African crude oil that had more than twice as much waxes than the Elliott crude oil as judged by DSC, as well as having a WAT that was about 10 °C higher. The DSC cooling heat flow profiles for the Elliott and African crude oils are shown in Figure 27. The African crude oil contains two distinct cooling events, a smaller one with at WAT at about 40 °C and a significantly larger one at about 33 °C. This large amount of higher temperature crystalizing wax indicates that this crude oil would likely have greater amount of larger n-paraffins than the Elliott crude oil. This was confirmed by HTGC, as shown in Figure 28.


Energy & Fuels

0.75 Elliott 0.7

African

Heat Flow (W/g)

0.65 0.6

WAT 33°C, 40 °C 22% Wax

0.55 0.5

WAT 32°C, 10% Wax

0.45 0.4 -60

-40

-20

0 20 Temperature (°C)

40

60

Figure 27. DSC heat flow cooling profile for the Elliott and African crude oils. 120

Waxy African Control

100

FID Response, pA

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

Page 32 of 37

nC18

Elliott Control nC24

80 nC36

60

40 nC44

20

0

5

10

15

20

25

Time, minutes

Figure 28. Overlay of the GC profiles for untreated waxy Elliott and African crude oils, with internal standards. Since the African crude oil was a gelled solid at ambient temperature this made it difficult to centrifuge the crystallized waxy phase from the liquid phase and determine reliable volumes of the centrifuged waxy plug. Therefore, the African crude oil was warmed to 30 °C and centrifuged to produce a wax plug which was then analyzed by HTGC. This material was similar to other centrifuged waxes in that it was significantly enriched in n-paraffins > C25. Figure 29 shows an overlay of the GC profiles for the untreated waxy African crude oil and the centrifuged wax. Page 32 of 37 ACS Paragon Plus Environment

Page 33 of 37

120

African Centrifuged Wax African Control

100

FID Response, pA

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

Energy & Fuels

nC18 nC24

80 nC36

60

40 nC44

20

0

5

10

15

20

25

Time, minutes

Figure 29. Overlay of the GC profiles for the untreated waxy African crude oil and wax centrifuged from the oil, with internal standards. The African waxy crude oil was treated with the newly developed WIEP additive between 200, 400, and 1000 ppm and the treated samples were subjected to cold finger tests. For these tests the crude oil temperature wax maintained at 51 °C and a cold finger temperature was held constant at 30 °C. These parameters are analogous to what was used for the Elliott cold finger tests so that the oil temperature was maintained at 10 °C above the WAT and the cold finger temperature was 10 °C below the WAT. There was no observable effect in the ability of the additives to reduce the amount of wax deposited during cold finger experiments, under these dosage levels and conditions. It is not surprising that the WIEP additive had little effect on this crude oil since it contained significantly more n-paraffins than the Elliott oil. This is consistent with literature reports that show that wax inhibitors become increasingly less effective for crudes with higher WAT, higher overall wax content, and higher >C25 n-paraffin content.18-24 WD analysis of the African crude oil also showed that it contained significantly more total wax (Waxy A + Waxy B) than the Elliott crude oil, and more than twice the amount of longer n-paraffin Waxy B material. These results are consistent with the DSC data. WD analysis of the wax deposited on the cold finger from the untreated crude oil also showed a preferential enrichment in Waxy B material, which is also consistent with the Elliott results. An increase of Waxy B material in the deposits during the cold finger experiments is consistent with what occurs with pipeline deposits.27 Table 9 shows the WD results for the untreated African Crude oil and the wax collected on the cold finger.


Energy & Fuels

Table 9. WD results for the untreated African crude oil and the wax from the untreated African crude deposited on the cold finger. Maltenes MEK (-24 °C)

Sample African Crude Oil African Wax from Cold Finger

Waxes (heptane-Sol.) A (-24 °C) B (60 °C)

47.3 45.8

30.6 24.3

21.8 29.5

Asphaltenes Tol-Sol. CH2Cl2-Sol. 0.2 0.2

0.2 0.2

The cold finger wax deposit from the untreated African crude oil was analyzed by HTGC. Similar to the deposit collected when using the untreated Ellliott crude oil, there was a significant enrichment in the distribution of higher order n-paraffins that were centered around n-C47, as shown in Figure 30. These results suggest that the African crude oil may need to be treated with significantly more additive, or more likely with a different additive package that contains a higher dispersing potential and a polymer with significantly longer alkyl side chains to better interact with the longer n-parrafins. Treating these high wax content crudes remains a challenge to the industry. African Control African Centrifuged Wax African Crude from Cold Finger

120 nC18

100

FID Response, pA

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

Page 34 of 37

nC24 nC36

80

60 nC44

40

20

0

5

10

15

20

25

Time, minutes

Figure 30. Overlay of the GC profiles for an untreated waxy African crude oil, wax centrifuged from the oil, and wax deposited during cold finger experiments for the same crude oil.

Conclusion An automated HPLC based separation, known as the Waxphaltene Determinator (WD), easily discriminates between shorter n-paraffins, as found in the Waxy A fraction, and longer n-paraffins prone to crystallization at ambient temperature, as found in the Waxy B fraction. A waxy Wyoming Elliott crude oil, that has deposition problems at wellbores and in pipelines, was treated with a commercially available wax inhibitor (WIA) at various dosage levels. Changes in the WD separation profile were very effective at quantifying the change in the amount of ambient temperature oil-insoluble wax that could be obtained by centrifugation. The amount of centrifuged wax decreased with increasing amounts of the Page 34 of 37 ACS Paragon Plus Environment

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additive and correlated linearly with a decrease in the WD Waxy B fraction. However, for the same oil treated with the newly developed additive package (WIEP), the WD separation profile did not show changes in the fractions relative to the additive performance as observed by centrifugation and cold finger experiments. This was attributed to the solubility of the WIEP components which were significantly more polar, and/or self-associated, than those in the WIA additive. This likely causes the additive, or certain components in the additive, to become precipitated independently from the waxes during the separation. This gives rise to a situation where the additive components self-precipitated with other additive components to a greater extent than becoming incorporated within the n-paraffins crystalline structures. This is an important limitation for the WD separation, when it comes to evaluating the effectiveness of various additive packages, which needs to be investigated further. This limitation can easily be recognized by prior screening of the pure additives by the WD separation. Additives with components that are mostly soluble in the MEK and heptane at -24 °C would be ideal candidates for further evaluation to determine crude oil wax treatment efficacy by this method. Centrifuge experiments showed that the WIEP additive was more effective than WIA at reducing ambient temperature waxes in the Elliot crude oil, and cold finger experiments confirmed that the additive significantly reduced wax deposition at the tested conditions. The WIA and WIEP additives were also used to treat a waxy African crude oil with approximately twice as much wax. Treatment of up to 1000 ppm of the additives showed no reduction in the amount of wax collected during cold finger experiments. Differential scanning calorimetry, WD, and high temperature gas chromatography (HTGC) analyses showed that the African crude oil contained significantly more overall wax and more longer >C25 n-paraffins than the Elliott crude oil, which is likely why the additives showed no effect at reducing the amount of crystallized wax in this oil. HTGC effectively showed that ambient temperature insoluble waxes are more enriched in >C25 waxes, and the WD analyses showed that these waxes were more enriched in the Waxy B fraction. The WD separation was also shown to be a useful tool to identify crude oils that may be difficult to treat with additives based upon the total amount of waxy and especially by the amount of longer n-paraffin enriched Waxy B present in the sample. Funding. Funding for this project was sponsored in part by the WRI Heavy Oil Research Consortium funded by ExxonMobil, TOTAL, HTRI, Petrobras, and Shell. Additional funding, the African crude oil and the WIEP additives were provided by TOTAL ACS. Acknowledgement. The authors would like to acknowledge the local Wyoming producer who provided us with samples of the Elliott crude oil and WIA additive. References. 1. Paso, K. G.; Fogler, H. S., Influence of n-Paraffin Composition on the Aging of Wax-Oil Gel Deposits. Thermodynamics, AIChE Journal 2003, 49, 3241-3252. 2. Machado, A. L. d. C.; Lucas, E. F., Poly(ethylene-co-vinyl acetate) (EVA) Copolymers as Modifiers of Oil Wax Crystallization. Pet. Sci. Technol. 1999, 17, 1029-1041. 3. Vignati, E.; Piazza, R.; Visintin, R. R. G.; Lapasin, R.; D'Antona, P.; Lockhart, T. P., Wax Crystallization and Aggregation in a Model Crude Oil. J. Phys. Condens. Matter 2005, 17, S3651-S3660. 4. Musser, B. J.; Kilpatrick, P. K., Molecular Characterization of Wax Isolated from a Variety of Crude Oils. Energy Fuels 1998, 12, 715-725. Page 35 of 37 ACS Paragon Plus Environment

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