Novel Magnetic Demulsifier for Water Removal from Diluted Bitumen

Article pubs.acs.org/EF

Novel Magnetic Demulsifier for Water Removal from Diluted Bitumen Emulsion Junxia Peng, Qingxia Liu, Zhenghe Xu,* and Jacob Masliyah Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada ABSTRACT: The production of conventional crude oil and bitumen often faces the challenges in removing residual water from stable water-in-oil emulsions. The chemical demulsifier is commonly employed to enhance water removal because of its high efficiency and simplicity in operation. In this study, a novel magnetic demulsifier with a surface-active ethyl cellulose (EC) grafted on magnetic nanoparticle surfaces, called M-EC, was investigated for water removal from water-in-diluted bitumen emulsions. The M-EC was demonstrated to be interfacially active and magnetically responsive. The interfacial activity of EC on the surface of novel M-EC nanoparticles allowed them to be effectively attached to otherwise stable emulsified water droplets in diluted bitumen emulsions. The M-EC tagged water droplets were readily removed by an external magnetic field. When a simple magnetic separation was combined with tagging of emulsified water droplets by M-EC nanoparticles, our experimental results showed a more than 90% removal of the original water from the diluted bitumen. Such a combination led to a separation time about 10 times faster than corresponding demulsification by chemical EC. The external magnetic field was found to enhance the coalescence of magnetically tagged water droplets in emulsion, producing a much smaller volume of sludge and hence leading to a minimal hydrocarbon loss to waste aqueous phase. The chemical bonding of interfacially active EC on the surface of magnetic nanoparticles and the magnetic property of M-EC allowed the spent M-EC nanoparticles to be readily recovered by magnetic separation and regenerated by solvent washing. The regenerated M-EC was found to retain its interfacial activity and be effective in breaking the diluted bitumen emulsions after reuse for 10 cycles. Application of M-EC nanoparticles to an industrial bitumen froth showed a minimal water removal of greater than 80%, demonstrating their promising applications to industry demulsification. The current study demonstrated that magnetic demulsification with tailor-designed magnetic demulsifiers represents a new direction of removing emulsified water from heavy oil and diluted bitumen emulsions.

1. INTRODUCTION Most crude oil is produced initially in the form of water-in-oil (W/O) or oil-in-water (O/W) emulsions in oilfields.1−3 These emulsions are highly stable because of the presence of interfacial films formed at the oil/water interface from the interfacially active materials, such as asphaltenes, resins, naphthenic acids, and solid particles (clay and waxes), either naturally present or intentionally added to enhance oil production.4−7 These emulsions are highly undesirable in the petroleum industry because of their sticky nature, causing difficulties in oil transportation and catalyst poisoning in downstream refinery operations.8 Removal of emulsified water by reliable and highly efficient demulsification techniques is therefore of great importance for the petroleum industry.9 Chemical demulsification is widely used for the removal of water from W/O emulsions.10−12 The most commonly used demulsifiers in chemical demulsification are amphiphilic compounds containing both hydrophilic and hydrophobic moieties in the molecular structure, which make them adsorb at the oil/water interface. To break the stable oil/water interface around emulsified water droplets, for example, chemical demulsifiers are in general required to be more surface-active than the emulsion stabilizers,13−15 so that they are capable of occupying the oil/water interface. The occupancy of the oil/water interface by the demulsifiers alters the interfacial properties, such as interfacial tension, mechanical strength, elasticity, and thickness of interfacial films, resulting in flocculation and/or enhanced coalescence of water droplets.16,17 In most cases, the commercial demulsifiers are a non-ionic type of surfactants. The main components of this type of demulsifiers are polyethylene oxide © 2011 American Chemical Society

(PEO) groups (as the hydrophilic part) and polypropylene oxide (PPO) groups (as the hydrophobic part).18−22 Other non-ionic surfactants, such as fatty esters, alkyl phenol ethers, fatty amides, and ethoxylated phenols, just to name a few, are also used as demulsifiers or co-demulsifiers to break down crude oil emulsions.23,24 Despite significant advances in developing effective chemical demusifiers, there are still urgent needs to design more eco-friendly and economically competitive demulsifiers for environmental benefits. Recently, environmentally friendly demulsifiers, such as a special type of bacteria, CO2, and ionic liquids, were reported.25−27 In our recent studies, ethyl cellulose as a biodegradable polymer was discovered to be an effective demulsifier for breaking waterin-diluted bitumen emulsions, achieving a water removal efficiency of greater than 90%.28−30 Introduction of magnetic property into functional materials has recently attracted a wide range of interests, mainly because of its responses to an external magnetic field for quick and easy isolation from complex multiphase systems by magnetic separation.31,32 The demonstrated biological and environmental applications include magnetic imaging, ferrofluids, magnetic soft gels, and novel sorbents.33−36 The implantation of ferrofluid into Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: September 19, 2011 Revised: December 21, 2011 Published: December 22, 2011 2705

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size of water droplets in water-in-diluted bitumen emulsions after settling on a hand magnet. The procedure of the demulsification test is shown in Figure 1. In each test, 10 g of freshly prepared emulsion and

the external or internal phase of emulsions has shown the potential for demulsification of complex emulsion systems.37,38 Ferrofluid is a colloidal suspension consisting of single-domain magnetic nanoparticles in a carrier liquid, such as water or an organic solvent. To inhibit their agglomeration, each magnetic nanoparticle is coated with a surfactant when manufacturing the ferrofluids. Under an external magnetic field, ferrofluids become strongly magnetized,39−41 leading to their wide range of applications in biological and medical fields42−44 and industrial practices.45−47 Magnetic emulsion of narrow drop size distribution,48 for example, was obtained by emulsifying an organic ferrofluid of γ-Fe2O3 nanoparticles coated with oleic acid in aqueous media. A new multifunctional and reversible O/W emulsion49 with magnetic stimuli responses was also prepared by the ferrofluid consisting of oleophilic magnetic nanoparticles as the internal phase. Dubertret et al.50 reported a simple method to produce magnetic and fluorescent emulsions by emulsification in water of an oil containing oleic-acid-stabilized iron oxide particles and tri-n-octylphosphine-stabilized quantum dots. Interestingly, Oder et al.37 implanted ferrofluid into the internal phase of an emulsion and employed magnetic fields to induce coalescence of the dispersed emulsion phase, thus breaking the emulsion. The process is called magnetostatic coalescence. This method is further applied to remove emulsified water from the real crude oil. In this case, a large amount of demulsifiers is needed for an effective separation of water from the oil phase.38 We recently prepared ethyl-cellulose-grafted Fe3O4 nanoparticles (M-EC) consisting of interfacially active ethyl cellulose on the surface of chemically modified, magnetic Fe3O4 nanoparticles.51 The M-EC nanoparticles were found to be interfacially active. For an asphaltene-stabilized water-in-toluene emulsion, the tagging of stable water droplets in the emulsion by M-EC enhanced coalescence and led to rapid separation of the water droplets from emulsion by an external magnetic field. As a part of our ongoing research, M-EC is applied to remove emulsified water from heavy-naphtha-diluted bitumen. As far as we know, this is the first report on a magnetically recyclable nanocomposite as a demulsifier for breaking water-in-diluted bitumen emulsions. In this case, an external magnetic field is used to accelerate the separation of emulsified water droplets tagged using M-EC nanoparticles from diluted bitumen emulsions by magnetic separation. The most recent findings are presented here.

Figure 1. Schematic illustration of the process for the demulsification and recycling tests using M-EC as demulsifiers. 0.15 g of M-EC nanoparticles dispersed in naphtha were thoroughly mixed in a 30 mL polypropylene vial by shaking the mixture in a mechanical shaker for 1 h. The resultant mixture was then transferred to a 10 mL polypropylene vial, which was then capped and sealed with Teflon tape. After settling on the hand magnet in a water bath of 80 °C or at room temperature for 1 h, the water content at 1.5 cm depth from the top surface of the original emulsions (the total depth of 7 cm) was immediately measured with a Karl Fischer titrator at room temperature. Each sample was repeated for 3 times. The water content reported was the average of three repeats. The blank tests were performed for diluted bitumen emulsions without demulsifier addition as a control. The demulsification performance was measured by the percentage of water removed (defined as demulsification efficiency, DE, in %) from the emulsion sample, which is calculated from the measured water content by

DE (%) = (Ct 0 − Ct )/Ct 0 × 100 where Ct0 is the original water content before separation and Ct represents the water content after separation. The greater the DE, the better the demulsifier performs. Micrographs of emulsions at the bottom of the demulsification test tube after settling the emulsion under the hand magnet in the water bath of 80 °C or at room temperature for 1 h were obtained using a Carl Zeiss Axioskop 40 Pol optical microscope equipped with a digital video camera, which was connected to a computer. The emulsion sample without further dilution was placed on a glass slide and covered by a thin glass slide (25 × 25 × 0.2 mm). The image was taken under halogen light. A lower water content and larger water droplet size indicate a higher demulsification efficiency. 2.4. Recycle Test. To test recyclability of the spent magnetic sorbent, the M-EC nanoparticles after the demulsification test were collected using a hand magnet and washed using chloroform (CHCl3) and acetonitrile for 3 times. The M-EC nanoparticles cleaned as such were dried in a vacuum oven at room temperature. The resulting MEC nanoparticles were used for the subsequent demulsification test, which is considered as a 1 time recycle. In this study, 10 time recycles were performed for evaluating the stability of EC anchored on the surface of magnetic nanoparticles.

2. EXPERIMENTAL SECTION 2.1. Materials. Vacuum distillation feed bitumen was provided by Syncrude Canada, Ltd. Heavy naphtha was supplied by Champion Technologies, Inc. The plant recycle process water provided by Syncrude Canada, Ltd. was used to prepare water-in-diluted bitumen emulsions. The process water of pH 8.9 contains 25 ppm Mg2+, 41 ppm Ca2+, 79 ppm SO42−, 527 ppm Na+, 22 ppm K+, 407 ppm Cl−, and 793 ppm HCO3−. 2.2. Preparation of Process Water-in-Diluted Bitumen Emulsions. The emulsions were prepared with plant process water and naphtha-diluted bitumen. When the diluted bitumen was prepared, the naphtha/bitumen mass ratio was fixed at 0.65 as typically encountered in industry practices. After the naphtha/bitumen mixture was shaken in a mechanical shaker for 4 h at 200 cycles/min, 42 g of diluted bitumen was mixed with 2.2 g of process water using a homogenizer (PowerGen homogenizer, 125 W) operated at 30 000 rpm for 3 min. The resulting emulsion contains 5 wt % water and is referred to as diluted bitumen emulsions. The emulsions obtained as such are very stable and extremely complex with drop sizes typically smaller than 5 μm.4−6,28 2.3. Demulsification Test. The ability of M-EC nanoparticles as a demulsifier was determined by measuring the water content and the

3. RESULTS AND DISCUSSION 3.1. Demulsification of M-EC Nanoparticles. In a previous study, we reported the preparation of M-EC by grafting ethyl 2706

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cellulose on the surface of amino-functionalized Fe3 O 4 nanoparticles (M-SiO2−NH2), which were obtained by silanation of silica-coated Fe3O4 nanoparticles (M-SiO2) using 3-aminopropyltriethoxysilane (3-APTES).51 The demulsification efficiency of magnetic nanoparticles with various surface properties was investigated in this study. Figure 2 shows the

Figure 3. Water removal efficiency from diluted bitumen emulsions as a function of M-EC dosage after settling on a hand magnet for 1 h at 80 °C.

The effect of the initial water content on water removal efficiency at 1.5 wt % M-EC addition ratio was investigated. The results in Figure 4 show a marginal separation of water by

Figure 2. Dewatering efficiency from diluted bitumen emulsions with various types of magnetic nanoparticles at 1.5 wt % or 130 ppm EC after settling on a hand magnet for 1 h at 80 °C.

dewatering performance of different magnetic nanoparticles at 1.5 wt % addition ratio in the naphtha-diluted bitumen emulsion of 5 wt % initial water content. Without the addition of magnetic nanoparticles (assigned as a blank), the water content of the emulsion reduced only marginally from 5 wt % at the beginning to 4.3 wt % after settling at 80 °C for 1 h, as anticipated. With the addition of bare Fe3O4, Fe3O4−SiO2, or Fe3O4−SiO2−NH2 nanoparticles at 1.5 wt % in emulsions, only a slight reduction in the water content was observed. The final water content after treatment was 4.2 wt % for bare Fe3O4, 3.3 wt % for Fe3O4−SiO2, and 3.9 wt % for Fe3O4−SiO2−NH2, suggesting that these particles are not effective in removing water from diluted bitumen emulsions. In contrast, the water content in the emulsion decreased from 4.3 to 0.3 wt % with the addition of 1.5 wt % M-EC, representing a 93% demulsification efficiency. M-EC added at 1.5 wt % of emulsions showed a slightly better performance in water removal efficiency than 130 ppm EC demulsifier, possibly because of the enhanced coalescence and settling of magnetically tagged water droplets by an external magnetic field. These results indicated that M-EC nanoparticles are an effective demulsifier for the water-in-diluted bitumen emulsions and the polymer EC on the surface of M-EC played a crucial role in removing water from water-in-diluted bitumen emulsions. Figure 3 shows water removal efficiency measured by the water content at 1.5 cm depth from the top of the emulsion surface as a function of M-EC dosage. Without M-EC addition, water removal efficiency by natural gravity force was about 5.9%, suggesting an extremely stable emulsion. The water removal efficiency increased to 74.4% at 0.5 wt % M-EC addition and 93.4% at 1.5 wt % M-EC addition. A further increase in M-EC dosage showed a marginal further improvement in water removal efficiency. On the basis of these results, the concentration of M-EC in the following demulsification tests was kept at 1.5 wt % emulsions, which is equivalent to 750 ppm EC by considering 5 wt % EC in M-EC.51 Considering that only a part of the EC grafted on M-EC occupies the interface, a slightly higher EC dosage than chemical EC to accomplish the optimal water removal is not unexpected.

Figure 4. Effect of the initial water content on water removal efficiency from diluted bitumen emulsions with 1.5 wt % M-EC addition after settling on a hand magnet for 1 h at 80 °C.

gravity separation without demulsifier addition, as illustrated by the difference between the split line and triangle symbols. With the addition of 1.5 wt % M-EC demulsifier, a significant reduction in the water content to less than 0.7 wt % was observed up to an initial water content of 25 wt %. It is interesting to note that the higher the water content, the easier the water droplets coalesce and the higher the water removal by M-EC assisted demulsification, as shown by the increased difference in the water content between the blank sample and M-EC-demulsifiered sample. The results further confirm that M-EC is an effective demulsifier for high water content emulsions. In the discussion above, the water removal efficiency by M-EC assisted demulsification was evaluated by monitoring the water content at a fixed depth of 1.5 cm from the top of the emulsion. It is actually more interesting to examine the distribution of the water content at various depths for a given separation time after the addition of M-EC. The results in Figure 5 show a negligible difference in the water content along the test tube for the blank sample, all at around 4.7 wt %. This observation indicates a very stable W/O emulsion without noticeable water separation under 2707

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separation time. The results in Figure 7 show a significant decrease in the water content from original 5 wt % to about 1 wt % within 2 min by applying a hand magnet to the emulsion with 1.5 wt % MEC addition, representing a more than 80% water removal efficiency. In contrast, it takes at least 30 min to achieve the same level of water removal from the emulsion with the same amount of M-EC addition but without applying the hand magnet (data not

Figure 5. Water content at different depths of diluted bitumen emulsions treated with 1.5 wt % M-EC or 130 ppm EC after settling on a hand magnet for 1 h at 80 °C.

gravity separation. In contrast, the addition of 1.5 wt % M-EC with a hand magnet assisted settling reduced the water content of emulsions within the top 6 cm to less than 0.4 wt %, below which the water content increased sharply to above 20 wt %. The results clearly show that more than 90% of water in the original emulsion settled under magnetic forces within 14% bottom volume, i.e., within 1 cm from the bottom of the tube. In comparison to a water content of 18.5 wt % at the depth of 6 cm for the case with EC addition, the addition of M-EC clearly shows a better separation of water into a small volume, leading to a much smaller volume of sludge and less oil loss because of the enhanced separation of M-EC tagged water droplets by magnetic forces. Micrographs of naphtha-diluted bitumen emulsions with and without M-EC addition at different depths of the settling tubes on a hand magnet are shown in Figure 6. Without M-EC

Figure 7. Water removal efficiency from diluted bitumen emulsions with 1.5 wt % M-EC addition as a function of the settling time at room temperature without (GS) and with a hand magnet (MS).

shown in Figure 7). Such a contrast suggests not only effective tagging of emulsified water droplets by M-EC because of its interfacial activity but also effective separation of magnetically tagged water droplets by an external magnetic field. In comparison, the addition of chemical EC led to a negligible removal of water from the emulsion within the first 60 min under the same conditions, i.e., settling at room temperature. All of these results clearly confirm that the magnetic force can assist M-EC for the rapid removal of water from emulsions, even at room temperatures. 3.2. Recycling Tests. As discussed above, the magnetic property of M-EC improved both the demulsification efficiency and kinetics with the aid of magnetic forces. Unlike the conventional once-through chemical demulsifier, a unique feature of M-EC is its ability of being recycled and reused after demulsification, reducing its chemical cost in practical applications in the petroleum industry. The potential of recycling the magnetic demulsifier was studied by conducting a demulsification test using the same testing procedures mentioned above. The experimental procedure of M-EC recycle tests is shown schematically in Figure 1. After each use, the top diluted bitumen phase was separated by decanting while holding the M-EC tagged water droplets at the bottom of the test vial with a hand magnet. The collected (spent) M-EC nanoparticles were washed for 3 times with chloroform to remove residues of diluted bitumen emulsions and used for the subsequent demulsification test. The recycle tests were repeated for 10 cycles. The dewatering efficiency in each cycle is shown in Figure 8. In the first 5 cycles, M-EC nanoparticles showed nearly constant dewatering efficiency of greater than 90% water removal from water-in-diluted bitumen emulsions. The water removal efficiency decreased slightly to below 90% in the last 5 cycles. These results suggest that M-EC nanoparticles have a high demulsification efficiency and exhibit excellent stability. After 10 cycles, M-EC remains fully active without the loss of its interfacial activity. Our study demonstrates that M-EC is a promising magnetic demuslifier for industrial applications. 3.3. Industry Bitumen Froth Cleaning. With the success of water removal from water-in-diluted bitumen emulsions, we

Figure 6. Micrographs of diluted bitumen emulsions at different depths without (a1, a2, and a3) and with 1.5 wt % M-EC (b1, b2, and b3) addition after settling on a hand magnet for 1 h at 80 °C. All micrographs have the same magnification.

addition, fine water droplets of 2−3 μm in diameter are welldispersed in the entire emulsion from a1 to a3. In contrast, large water droplets of 20−50 μm in diameter are observed at the bottom of the emulsion at b3 with M-EC addition, leaving only a few fine water droplets dispersed in the top (b1) and middle (b2) emulsions. Clearly, M-EC significantly promoted coalescence of emulsified water droplets in water-in-diluted bitumen emulsions. The effects of magnetic property on the dewatering efficiency were also investigated by examining the water content with (MS) and without (GS) applying an external magnetic field to the naphtha-diluted bitumen emulsion at room temperature. The water content of emulsions was measured as a function of the 2708

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Figure 8. Recycling and reuse of the M-EC for demulsification of diluted bitumen emulsions after settling on a hand magnet for 1 h at 80 °C.

Figure 10. Water content at different depths of naphtha-diluted industrial bitumen froth treated with 1.5 wt % M-EC after 5 min settling on a hand magnet at room temperature.

extended our study to cleaning of an industrial bitumen froth as encountered in commercial oil sands operations. The M-EC nanoparticles were applied to the bitumen froth produced by Syncrude commercial plants. The composition of this industrial bitumen froth is shown in Table 1. Figure 9 shows the water

demonstrate that M-EC nanoparticles are able to effectively remove the emulsified water for the industrial bitumen froth.

4. CONCLUSION With the addition of 1.5 wt % M-EC and magnetic separation, the water content (2.5−25 wt %) in naphtha-diluted bitumen can be reduced by more than 93% of its initial value with a much reduced volume of aqueous sludge (tailings). The high demulsification efficiency is attributed to the chemically anchored EC on the surface of magnetic nanoparticles. The same level of water removal with M-EC addition requires at least 30 min under the same conditions but without the external magnetic field. In contrast, EC within the first 60 min at room temperature did not show any noticeable dewatering for heavy-naphtha-diluted bitumen emulsions. Furthermore, the M-EC is shown to be chemically stable without any loss of dewatering efficiency after recycling 10 times. When the M-EC was applied to an industrial bitumen froth, more than 80% of the water in the original bitumen froth was removed in 2 min at room temperature by applying an external magnetic field, indicating the promising application of M-EC in the petroleum industry. This is the first example of a surface-active polymer grafted on magnetic nanoparticles as an effective demulsifier. Because of its high efficiency and rapid kinetics in physical separation with a minimal sludge volume, the technique represents a new direction of removing emulsified water from the diluted bitumen or heavy oil.

Table 1. Composition of Industrial Bitumen Froth (wt %) Industrial bitumen froth

bitumen

solids

water

81.55

5.35

13.10

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Figure 9. Effect of M-EC dosage on water removal efficiency from naphtha-diluted industrial bitumen froth after settling on a hand magnet at room temperature.

AUTHOR INFORMATION

Corresponding Author

*Telephone: 1-780-492-7667. Fax: 1-780-492-2881. E-mail: [email protected].

content of the industrial bitumen froth after demulsification by magnetic separation with different amounts of M-EC addition. Similar to the results obtained with water-in-diluted bitumen emulsions, the water content in diluted industrial bitumen froth can be reduced to below 0.5% in 5 min at room temperature when 1.5 wt % M-EC is added to naphtha-diluted industrial bitumen froth. As shown in Figure 10, no noticeable difference in water distribution is observed along the test tube of the naphthadiluted bitumen froth without demulsifier addition, as anticipated. In contrast, the water content remains below 0.5 wt % from the top to 5 cm depth of naphtha-diluted industrial bitumen emulsions with 1.5 wt % M-EC addition, while the water content at the bottom of the emulsion increases sharply to about 25 wt % at a depth of 6 cm. All of these results clearly

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ACKNOWLEDGMENTS The financial support for this work from the Natural Sciences and Engineering Research Council of Canada under the Industrial Research Chair Program in Oil Sands Engineering is gratefully acknowledged. The authors thank Syncrude Canada, Ltd. for bitumen samples and process water and Champion Technologies, Inc. for providing the heavy naphtha.

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