Novel Bitumen Froth Cleaning Device and Rag Layer Characterization

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Energy & Fuels 2007, 21, 3462–3468

Novel Bitumen Froth Cleaning Device and Rag Layer Characterization Guoxing Gu, Liyan Zhang, Zhenghe Xu,* and Jacob Masliyah Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G6 ReceiVed June 30, 2007. ReVised Manuscript ReceiVed August 13, 2007

A primary bitumen froth product, from a bitumen extraction process, contains approximately 30% water, 10% solids, and 60% bitumen. In bitumen froth treatment vessels, “rag layer” between the top organic phase and the bottom aqueous phase builds up from time to time. The presence of a thick layer of rag renders a two-phase separation into a three-phase separation, causing a significant reduction in oil/water/solids separation efficiency. A novel setup was built in our laboratory, which allows for a two-step water washing of naphthadiluted bitumen froth (NDBF) and buildup of rag layer to be collected efficiently for further investigation of rag formation mechanisms. In the first step of bitumen froth cleaning, 1–5 mm diameter NDBF drops were introduced into an aqueous phase to allow the NDBF drops to rise to the top as the organic phase. In the second step, the top organic phase was then washed using the bottom aqueous phase by circulating the top organic phase back to the aqueous phase. The bitumen froth cleaning at 80 °C was tested for two different naphtha-to-bitumen mass ratios (N/B) of 0.7 and 7. In both cases, after the first washing, the water and solid contents were reduced by more than 91% and 87% of their original levels, respectively. In each of the two cases at N/B mass ratios of 0.7 and 7, the rag layer that formed in the device was collected and fractionated into five fractions, ranging from dry rags, chloroform solubles and insolubles, “asphaltenes”, and deasphalted organics. The emulsification potencies of these five fractions were tested using the simulated model system of heptane/toluene/water. The asphaltenes were characterized using elemental analysis, thermal gravimetric analysis (TGA), and Fourier transform infrared (FTIR) spectroscopy. The results showed that chloroform solubles and insolubles were in a mass ratio of about 1:1. Emulsion tests showed that the chloroform solubles and asphaltenes exhibited the highest emulsion stabilization potency. Elemental analysis and TGA showed that asphaltenes isolated from rags had twice more heteroatoms (N, S, and O) and contained more volatiles and residues than the asphaltenes obtained from normal bulk bitumen. FTIR characterization confirmed the results of elemental analysis.

1. Introduction The bitumen product from a gravity separation vessel employed in a bitumen separation process is called “bitumen froth”. A typical bitumen froth product from a commercial production of Athabasca bitumen contains approximately 30% water, 10% solids, and 60% bitumen. To meet downstream process specifications, the water and solids in the bitumen froth product have to be removed. For example, the bitumen product is required to contain less than 0.5 volume percent total basic sediment and water (BS&W). Centrifugation, inclined plate settler (IPS), hydrocyclone, and asphaltene precipitation thickening are the four major methods used for Athabasca bitumen froth treatment. Athabasca bitumen typically has a density of about 1010 kg/m3 and a viscosity of the order of 100 Pa · s at 25 °C.1 Currently, there are only two commercial bitumen froth treatment processes in Alberta:2 the naphthenic froth treatment (NFT) process (used by Syncrude Canada Ltd. and Suncor Energy Inc.) and the paraffinic froth treatment (PFT) process (used by Albian Sands Energy). The NFT process uses naphtha as a diluent at * To whom correspondence should be addressed. E-mail: [email protected]. (1) Tipman, R. N.; Shaw , R. C. Recent advances in the treatment of oil sands froth. Proceedings of Oil Sands Our Petroleum Future Conference, Edmonton, Alta., Canada, April 4–7, 1993. (2) Romanova, U. G. H.; Yarranton, H. W.; Schramm, L. L.; Shelfantook, W. E Can. J. Chem. Eng. 2004, 82, 710–721.

a naphtha-to-bitumen mass ratio of 0.6–0.75, while the PFT process uses paraffinic solvent of C5-C6 mixtures as a diluent at alkane-to-bitumen mass ratio of 2.1–2.5. In both processes, the added solvent facilitates bitumen/water/solids separation by reducing the viscosity of the oil phase and, at the same time, increasing the density difference between the oil and water phases. The NFT process is able to recover more than 98% of the bitumen but needs IPS and/or centrifugal equipment, such as a centrifuge and hydrocyclone, to facilitate bitumen/water/ solids separation at the cost of a product containing 1.0–2.5% residual water and 0.3–0.8% solids. The residual water is in the form of emulsified water droplets, which carry chloride to downstream operations, such as up-grader and refineries. The presence of chlorine causes corrosion problems for the downstream process units. In the case of the PFT process, the residual water and solids can be reduced to less than 0.1 wt % of the bitumen with about 4–10% reduction in bitumen recovery due to rejection of asphaltenes in the form of precipitates. Recently, Cymerman et al. reported3 a two-stage gravity settler for bitumen froth treatment, using naphtha as a diluent at a naphtha-to-bitumen mass ratio (N/B) of 0.5–0.8 for the first stage and 4–10 for the second stage. The naphtha-diluted bitumen froth (NDBF), introduced as a stream to the settler (3) Cymerman, G.; Dougan, P.; Tran, T.; Lorenz, J.; Mayr, C. U.S. Patent US20030029775, Feb 13, 2003.

10.1021/ef7003688 CCC: $37.00  2007 American Chemical Society Published on Web 10/11/2007

NDBF Cleaning DeVice and Rag Layer Characterization

through the bottom aqueous phase, rises to the top and forms the organic phase. The high N/B ratio causes some asphaltenes to precipitate. The precipitates accumulate between the organic and aqueous phases, forming a distinct middle phase.4,5 The formed middle phase is normally called the “rag layer”. Both water droplets and solid particles have to migrate from the top organic phase through the thick and viscous middle rag layer in order to settle down to the bottom of the aqueous phase. In the presence of such a thick and viscous middle rag layer, settling of water droplets and solid particles is inhibited. Most likely, the thick layer of rag renders a two-phase separation into a three-phase separation, causing a significant reduction in oil/ water/solids separation efficiencies. In refineries6,7as well as in in situ crude oil productions,8 the rag layer issue is also a serious concern in most desalter operations and slop oil treatments. It is therefore desirable to explore another approach for bitumen froth cleaning that minimizes cost, hydrocarbon loss, and heat consumption. To achieve these goals, a modified naphthenic froth treatment process has been proposed and tested in our laboratory as reported here. Instead of using multiplestage settlers and introducing the NDBF as a stream to the settler through the bottom aqueous phase, the NDBF was introduced as NDBF drops via a distributor that is located at the bottom aqueous phase of a settler, with the formed top organic phase being continuously circulated back to the same bottom aqueous phase. The distributor has an array of small orifices that allow the feed to break up into small NDBF drops. The aqueous phase washes the rising NDBF drops and facilitates water/solids/ bitumen separation. As anticipated, a significant separation performance was achieved with such a washing scheme. Unfortunately, the formation of rag was intensified, as the skin materials left behind the dispersed water droplets migrating into the bulk aqueous phase. It appears that the higher the separation efficiency is, the faster the rag builds up between the top organic phase and the bottom aqueous phase. The presence of a rag layer can be identified by spinning Trycock samples taken at different elevations of a separation vessel. At present, there is no well-accepted definition of a rag layer, as the spinning conditions (e.g., revolution centrifugal force (RCF), diluent-to-sample volume ratio, temperature, etc.) are quite different from study to study, or from laboratory to laboratory. To mitigate the problem of rag layer buildup, it is extremely important to understand the rag composition and characteristics. However, except for several patents3–8 that are aimed at improving bitumen froth treatment, only Kotlyar et al.9 conducted a detailed study on the rag layer issues. However, the main focus of their study was on bitumen-associated solids that contribute potentially to coke formation and cause fouling in packed bed hydrotreaters. In their study, they first fractionated four rag layer samples into toluene soluble (termed as rag layer bitumen) and insoluble (termed as bitumen-associated solids) fractions; they then subfractionated the toluene soluble fractions into n-heptane soluble and insoluble fractions. The n-heptane insoluble is normally referred to as asphaltenes. They reported (4) Tipman, R.; Long, Y.-C. U.S. Patent US5876592, March 2, 1999. (5) Tipman, R.; Long, Y.-C.; Shelfantook, W. E. U.S. Patent US6214213, April 10, 2001. (6) Shallice, C.; Young, L. R. PCT Int. Appl. WO9416033, July 21, 1994. (7) Ohsol, E. O.; Pinkerton, J. W.; Gillespie, T. E. U.S. Patent US5882506, March 16, 1999. (8) Smith, J. K.; Lopez, T. H.; Means, C. M. U.S. Patent US20050018176, January 27, 2005. (9) Kotlyar, L. S.; Sparks, B. D.; Woods, J. R.; Chung, K. H. Energy Fuels 1999, 13 (2), 346–350.

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Figure 1. Proposed naphthenic froth treatment process.

that the asphaltene content in the rag layer bitumen was in a range of 21.8–51.8 wt %. The difficulty encountered in rag layer research is representative sampling of rag. In a commercial oil/ water/solids separation vessel, rag builds up gradually and breaks down locally once the weight of the rag exceeds its buoyancy. It is difficult for plant engineers to identify the presence and location of the rag. In a laboratory environment, it is extremely difficult to mimic the formation of rag layer in a continuous operation setup, as pumps are normally required, which may break the rag. With the setup constructed in our laboratory, rags formed under different operating conditions were fractionated and characterized. In this paper, we will describe the novel experimental setup for froth washing and rag layer formation. Froth washing in the NFT process was demonstrated to be an improved viable alternative to the existing NFT process. 2. Experimental Section 2.1. Materials. Raw bitumen froth (30.2% water, 9.7% solids, and 60.1% bitumen by weight) and bulk bitumen as well as partially hydrotreated naphtha were obtained from Syncrude Canada Ltd. Municipal water (City of Edmonton tap water) was used for the tests of rag formations. Toluene (99.8%) and chloroform (HPLC grade) were all purchased from Fisher Scientific. Deionized water produced from a Millipore system was used for emulsification potency tests. To avoid aging effects, the naphtha-diluted bitumen froth (NDBF) was prepared right before each initial and follow-up experiment. Two mass ratios of naphtha to bitumen, N/B ) 0.7 and 7, were used. 2.2. Naphthenic Froth Treatment Process. A schematic of the proposed NFT process is shown in Figure 1. It consists of a 4 L separation vessel with a water jacket, a pump, and a 1 L buffer vessel. The feed was introduced via a distributor located at the bottom of the separation vessel. The distributor has an array of 1 mm diameter orifices for NDBF drop generation. The water jacket is connected to a circulating water bath to control the operating temperature. The operation can be divided into the following two steps: (1) introduction of the NDBF to the separation vessel and (2) circulation of the top organic phase back to the bottom aqueous phase of the separation vessel. The NFT process was tested at two N/B mass ratios, 0.7 and 7. In step 1, half of the separation vessel was initially filled with water; fresh NDBF was fed and distributed into the aqueous phase as 1–5 mm diameter drops, which rose to the top of the aqueous phase to form the organic phase. This step of water washing was stopped when the separation vessel was full and the top continuous organic phase overflowed into the buffer vessel. The organic phase was sampled at the sample point, as shown in Figure 1 for analysis of water and solid contents. The results of water and solid analysis

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Figure 2. Procedure used for determination of the solids in oil samples.

were considered as the performance of the first-step froth washing. During the first-step washing, the majority of the solids and emulsified water droplets migrated out of the NDBF drops and the solids settled to the cone-shaped bottom of the separation vessel. The accumulated solids were regularly released from the bottom outlet of the vessel. The feed flow rate was controlled at 15 mL/ min. A period of 2 h was used to introduce the fresh NDBF to the separation vessel. In other words, 2 h was required to wash the fresh NDBF once. In step 2, the fresh NDBF flow was stopped, and the top organic phase was then continuously washed using the bottom aqueous phase by circulating the top organic phase back to the aqueous phase. A 2 h continuous water washing was equivalent to one washing cycle. Six washing cycles were conducted in step 2. An oil sample was taken at the end of each washing cycle at the top sampling point (Figure 1) for the analysis of water and solid content. Water content was obtained by Karl Fischer titration after toluene dilution; and solid content was determined using the procedure shown in Figure 2. As shown in Figure 2, the oil sample was diluted with toluene and centrifuged at 20000g for 30 min. The sediment was then dried in a vacuum oven at 110 °C for 1 h to remove the majority of water and bitumen. To obtain toluene insoluble solids and minimize the adsorption of polar components (such as asphaltenes) on the solids, the dry solids were washed with additional toluene several (N) times until the supernatant became colorless. The purpose of drying the solids before further toluene cleaning was to remove any asphaltenes that were associated with the residue water in the solids, as it has been observed that water-associated asphaltenes are toluene insoluble.10 After removal of water, the resultant asphaltenes became toluene soluble and were removed by toluene cleaning. After drying, the remaining solids were recorded as the solids in the oil sample. 2.3. Rag Formation. While feeding or circulating, solids in the fresh NDBF and in the circulating organic phase settled to the bottom of the separation vessel and were released intermittently. The frequency of solids removal was higher during feeding fresh NDBF than circulating the top organic phase, as more than 85% of the solids in the fresh NDBF settled after one cycle of NDBF washing. During circulating of the top organic phase, a middle rag layer formed gradually between the top organic phase and the bottom aqueous phase. Two quite different rag layers were obtained for the two NDBFs of N/B ) 0.7 and 7. For N/B ) 0.7, the rag layer was visually observed to be loose, deposited partly to the bottom of the separation vessel by itself and almost completely when the rag layer was slightly agitated. However, in the case of N/B ) 7, the rag layer was a viscous and dense phase, behaving as a gel, and agitation did not make it sink. In order to collect rag layers, two different procedures were used. In the case of N/B ) 0.7, all deposits were first released after fresh NDBF feeding of step 1. After continuous circulating of the top organic phase through the bottom aqueous phase for seven cycles (each cycle was equivalent to a run of 2 h), the rag layer was agitated to allow it to sink and be collected with the deposited solids. The collected materials as such are referred to as raw rag. In the (10) Solovyev, A.; Zhang L. Y.; Xu, Z.; Masliyah, J. H. Langmuir films of bitumen at oil/water interface. Energy Fuels, in press.

Figure 3. Procedure of raw rag fractionation.

case of N/B ) 7, the rag layer was stable between the organic and aqueous phases, whereby the rag layer did not sink after agitation. To obtain the rag here, some municipal water was pumped into the aqueous phase, causing the top organic phase to overflow until the aqueous phase penetrated the rag layer and started to overflow, leaving the dense rag layer in the aqueous phase. Finally, the rag layer was collected after releasing the aqueous phase from the bottom of the vessel and was also referred to as raw rag. 2.4. Rag Fractionation. To obtain dry rag, chloroform soluble and insoluble, and n-heptane soluble and insoluble fractions, raw rag is fractionated using the procedures shown in Figure 3. Dry Rag. Dry rag was obtained after removal of both water and naphtha from the raw rag. After mixing the raw rag with toluene, water was first removed from the mixture using a Dean–Stark distillation system operated near toluene’s boiling point. The majority of the added toluene and entrained naphtha was removed at a temperature higher than toluene’s boiling point. After transferring the remaining mixture from the Dean–Stark vessel to a Teflon tube, the residue toluene and naphtha were then removed by blowing air into the mouth of the tube at room temperature. Chloroform Soluble and Insoluble. The dry rag was fractionated into chloroform soluble and insoluble fractions. After ultrasonically dispersing a mixture of the dry rag and chloroform at a dry-ragto-chloroform mass ratio of 1:20 for half an hour, the mixture was centrifuged. The supernatant was collected, and the solid deposit was dried at 80 °C in the vacuum oven. The solids were then cleaned repeatedly using chloroform and ultrasonic dispersion–centrifugation cleaning cycles until the supernatant was colorless. The purpose of vacuum oven drying was to remove residual bound water from the deposits, as water-bound asphaltenes are insoluble in chloroform. Complete removal of residue water from the waterbonded asphaltenes would turn the asphaltenes into chloroform soluble. The final deposit was dried using the air blowing method, and it is referred to as chloroform insoluble. The chloroform soluble fraction was obtained after drying the combined supernatants that resulted from chloroform cleaning in a similar way as drying the chloroform insoluble. Asphaltenes and Deasphalted Organics. The chloroform soluble fraction was further subfractionated into n-heptane soluble and insoluble in a way similar to that of fractionating the dry rag into chloroform soluble and insoluble. The dried deposit is referred to as “asphaltenes”, and the dried supernatant is deasphalted organics. The aforementioned rag fractionation procedure was performed for the two raw rags obtained at N/B ) 0.7 and 7, respectively. 2.5. Characterization of Rag Fractions. The emulsification potencies of the five fractions (dry rag, chloroform soluble and insoluble, “asphaltenes, and deasphalted organic) obtained from NDBF (N/B ) 7) were tested using a heptol (50 vol % heptane + 50 vol % toluene)/deionized water system at a heptol-to-water volume ratio of 1:1 and room temperature. With the help of a sonicator, 0.15 g of each fraction was dispersed in 75 mL of heptol. The solution or suspension was then mixed with 75 mL of deionized water using a shaker for 1 h. Photographs of the sample bottles were taken after the mixtures were left for overnight gravity settling.


Figure 4. Water washing performance at N/B ) 0.7.

Similar tests were conducted for the fractions obtained from NDBF at N/B ) 0.7. Several analytical techniques including elemental analysis (EA1108, Carlo Erba Instrument), thermal gravimetric analysis (TGA) (STA 409, Netzsch, Germany), and Fourier transform infrared (FTIR) spectroscopy (FTS 6000, Biorad, USA) were used to characterize the two “asphaltene” fractions obtained from the two NDBFs of N/B ) 0.7 and 7. As a reference, the characterizations were also performed on asphaltene fractions obtained from the bulk bitumen. Elemental analysis was also carried out for the two deasphalted organics. FTIR spectra were obtained using KBr powder as the background. The samples were mixed with dry KBr at a concentration of 1.5 wt % by hand grinding. The spectra were obtained at a spectral resolution of 1 cm-1 over 128 co-scans. TGA of the three asphaltene fractions was conducted at a temperature scan rate of 5 °C/min under nitrogen purge (60 mL/min). The raw rags were also analyzed using an optical microscope. A small amount of the raw rag was first placed on a glass slide under the microscope. A drop of naphtha was placed on the raw rag to dilute the rag so as to obtain a clear image.

3. Results and Discussion 3.1. Formation of Rag Layer. During the water washing of NDBF, a middle rag layer formed and built up gradually between the top organic phase and the bottom aqueous phase for both cases of N/B ) 0.7 and 7. However, there was a clear distinction between the two cases with respect to the properties of the rag layer. In the case of N/B ) 0.7, the rag layer was a loose mixture and settled partly to the bottom of the separation vessel without mechanical agitation and almost completely when the rag layer was slightly agitated mechanically. However, in the case of N/B ) 7, the rag layer was a viscous and dense mixture, behaving like a gel, and mechanical agitation did not make it sink. This finding has a potential implication in a commercial naphthenic bitumen froth treatment where the N/B ratio is 0.7. The issue of rag layer formation and buildup could be mitigated using a mechanical method when the N/B ratio is low. 3.2. Performance of the Proposed Naphthenic Froth Treatment Process. The performance of the proposed NFT process was evaluated on the basis of the water and solid contents in the organic phase as a function of washing cycle. At the end of each cycle, an oil sample was taken at the oil sampling point shown in Figure 1 and was analyzed. The results are shown in Figure 4 for the case of N/B ) 0.7 and Figure 5 for the case of N/B ) 7. The first washing was referred to the case where the fresh NDBF was directly introduced into the separation vessel and formed the top organic phase. For both cases, as shown in Figures 4 and 5, the first washing of the


Figure 5. Water washing performance at N/B ) 7. Table 1. Mass Percent of Various Fractions Obtained at Two Naphtha-to-Bitumen Ratios (N/B), 0.7 and 7 (Where the Mass Percent Is the Percent of Its Parent Fraction; e.g., at N/B ) 7, Raw Rag Is 6.9% of Raw Froth and “Asphaltenes” Are 55.8% of Chloroform Soluble Organics)

proposed NFT process reduced the water and solids content by more than 91 and 87% of their original levels, respectively. Continuous circulation (step 2) of the top organic phase back to the aqueous phase further reduced the residual water and solids content. The six cycles of water washing provided an option to reduce water and solids below 1%. With increasing washing cycle or time, both water and solids levels in the organic phase were lower in the case of N/B ) 7 than that in the case of N/B ) 0.7, suggesting that circulating the top organic phase back to the aqueous phase was more efficient at higher dilution ratio than at lower dilution ratio. The effect of naphtha dilution on the separation performance will be discussed in detail in the next section. 3.3. Composition of Rag Layer. The mass percents of the rag layers for the two naphtha-to-bitumen ratios are shown in Table 1. In the case of N/B ) 7, the raw rag was 6.9% of the raw froth. The raw rag consisted of 43.2% water, 29.4% naphtha, and 27.4% dry rag. The density of the raw rag was measured (density bottle method) to be 0.992 g/cm3, slightly less than the density of water (0.998 g/cm3) at 22 °C (room temperature), making the rag stay in the middle between the light organic and heavy aqueous phases. It was speculated that the rag layer could have water-in-oil emulsion droplets. However, the microscopic image shown in Figure 6 does not support such a speculation. It is a common and easy way to observe water-inbitumen droplets after solvent dilution under a microscope. Figure 6 shows the absence of spherical water droplets even if the rag was diluted using naphtha or toluene over a wide dilution range on a slide under the microscope, indicating that the rag layer is in the form of a complex dispersion state. The rag layer structure appeared like silt sludge with entrained oils. After water and naphtha removal, the dry rag contained 54.7 and 45.3% chloroform soluble and insoluble, respectively. In the chloroform soluble fraction, there were 55.8% “asphaltenes” and 44.2%


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Figure 6. Microscopic image of rag (N/B ) 7) after dilution by naphtha.

Figure 7. Results of emulsification potency tests for all fractions obtained at N/B ) 7 (oil, 50% heptane + 50% toluene; water, municipal water; fraction additives, 0.2 wt % water; oil-to-water volume ratio, 1:1). Table 2.Comparison of Elemental Analysis Results for Different Fractions Obtained from Rag Layers and Normal Asphaltenes Obtained from Bulk Bitumen description

(C + H) %

(N + S + O) %

H/C

Naphtha-to-Bitumen Mass Ratio ) 7 “asphaltenes” 76.0 24.1 deasphalted organics 94.6 5.4

1.11 1.59

Naphtha-to-Bitumen Weight Ratio ) 0.7 “asphaltenes” 77.4 22.6 deasphalted organics 94.6 5.4 normal asphaltenes 88.1 11.9

1.08 1.54 1.18

deasphalted organics. The high “asphaltene” content in the rag layer could be an indication that asphaltene precipitation occurred during the water washing and the precipitated asphaltenes accumulated in the rag layer. In the case of N/B ) 0.7, the composition in terms of raw rag, water, naphtha, and dry rag is not shown in Table 1, as the rag layer collected from the vessel bottom was the accumulated materials from continuous circulating of the organic phase through the aqueous phase. In this case, it was difficult to know how much rag material was discarded. The dry rag had 50.6 and 49.4% chloroform soluble and insoluble, respectively. In the chloroform soluble, there were 23.7% “asphaltenes” and 76.3% deasphalted organics. In both cases, the content of chloroform insoluble solids was very high, around 50% in their dry rags. The “asphaltenes” obtained from respective chloroform soluble organics are 23.7% (for N/B ) 0.7) and 55.8% (for N/B ) 7). This result is consistent with the results reported by Kotlyar et al.9 where the asphaltenes in the rag layer bitumen were in the range 21.8–51.8%. In the two cases, rag “asphaltene” contents are

Figure 8. TGA thermograms under a nitrogen purge rate of 60 mL/ min for three different asphaltenes at a heating rate of 20 °C/min.

higher than the asphaltene content of 17% in typical bitumen, indicating that “asphaltenes” concentrated in the rag layer during water washing. It is well known that asphaltenes are a key stabilizer of water-in-oil emulsions. Our recent study11 on interfacial materials isolated from the surface of emulsified heavy water droplets showed that the interfacial film is a monolayer at a high bitumen concentration (N/B ) 0.7) and a multilayer at a low bitumen concentration (N/B ) 19.2). At both low and high bitumen concentrations, asphaltenes are predominant in the interfacial film. The “asphaltenes” accumulated within the rag layer can be considered as the interfacial skin film left in the rag layer of water droplets after coalescence. A much higher “asphaltene” content in the rag layer formed at N/B ) 7 can be accounted for by the multilayer “asphaltene” film left behind. The multilayer film would form soon after naphtha dilution of the raw bitumen froth due to “asphaltene” precipitation. “Asphaltene” precipitation was significantly enhanced at a higher naphtha dilution. On the basis of the data shown in Table 1, in the case of N/B ) 7, the “asphaltenes” in the rag layer were estimated to be 5.7% of the total asphaltenes in bitumen, suggesting that 5.7% of the total asphaltenes were transferred and stayed in the rag, mostly in the form of precipitates. From the separation point of view, higher dilution should be avoided due to asphaltene precipitation and accumulation, which would result in rag layer buildup and hence poor separation of water droplets and solids from diluted bitumen. 3.4. Emulsification Potency. The emulsification potency of the five fractions obtained at N/B ) 7 was tested using a heptol/ deionized water system. The results in Figure 7 show that only chloroform soluble organics and their subfraction of “asphaltenes” formed stable gel-like emulsions, as anticipated from their high asphaltene content (55.8%). However, it was surprising that the dry rag was unable to stabilize an emulsion, although the dry rag contained 30.5% ()54.7% × 55.8%) “asphaltenes”. General knowledge leads us to believe that the dry rag with such a high “asphaltene” content should be able to stabilize a water-in-oil emulsion. In order to understand this observation, the solids present in the rag need to be taken into consideration. Consequently, chloroform insoluble solids and soluble organics were combined to prepare a mixture of the same composition (11) Gu, G.; Zhang, L.; Wu, X.; Xu, Z.; Masliyah, J. Isolation and Characterization of Interfacial Materials in Bitumen Emulsions. Energy Fuels, in press.



Figure 9. FTIR spectra of different asphaltenes.

as the dry rag. The mixture was found to have a similar emulsification potency as compared to that of the dry rag. To further understand this unexpected behavior, the chloroform insoluble solids were separated into hydrophilic and hydrophobic subfractions. The solids were first dispersed in n-heptane using a sonicator for 1 h and then mixed with water at a heptane-towater volume ratio of 1:1 on a shaker for 1 h. The hydrophilic solids were collected from the aqueous phase and hydrophobic solids from the organic phase. The two types of solids were individually mixed with chloroform soluble organics at the same solid-to-organics mass ratio as that present in the dry rag. The results of similar bottle tests for the two new mixtures showed that the mixture containing hydrophilic solids had a significantly lower potency in forming a water-in-oil emulsion than the mixture containing hydrophobic solids. These results would indicate that the hydrophilic solids in the dry rag made it incapable of stabilizing a water-in-oil emulsion. Similar results were obtained for NDBF at N/B ) 0.7. 3.5. Elemental Analysis. Carbon and hydrogen analysis (C and H) was conducted for both “asphaltenes” and deasphalted organics obtained at N/B ) 0.7 and N/B ) 7. For comparison, similar analysis was applied to asphaltenes obtained from bulk bitumen. The mass percents of (C + H) and (N + S + O) are listed in Table 2. The mass percents of heteroatoms (N, S, and O) were calculated to be equal to 100 – mass percent of (C + H). The H/C molar ratio is also listed in Table 2. It can be observed from Table 2 that, for both “asphaltenes” and deasphalted organics, there is a marginal difference in the mass percents of (C + H) and (N + S + O), indicating that no significant compositional difference exists between the fractions obtained at N/B ) 0.7 and N/B ) 7. However, the mass percent of heteroatoms in the “asphaltene” fraction obtained from the rags is about twice that of the asphaltenes obtained from the bulk bitumen, indicating that the “asphaltenes” from the rags are high in their polarity or surface active potencies that would favor the molecules to position at the interface. The H/C molar ratios of the rag “asphaltenes” are slightly lower than the normal asphaltenes, showing slightly higher aromaticities. 3.6. TGA Thermogram. The TGA thermograms of the three asphaltenes in Figure 8 show that, compared with the normal asphaltenes, the rag “asphaltenes” contain species of lower boiling points, especially at less than 200 °C. These lower

boiling point species in the rag “asphaltenes” appear to be n-heptane insoluble. Clearly, the molecular composition of the rag “asphaltenes” is different from the normal asphaltenes. Another distinct feature of the rag “asphaltenes” is their higher coke content than that of the normal asphaltenes when the coking temperature was above 550 °C, as evidenced by the higher leveling off plateaus. The rag “asphaltenes” from NDBF at N/B ) 0.7 has the highest value of plateau. This observation would indicate that the rag “asphaltenes” are mixtures of lower molecular weight species (that were vaporized or decomposed at a lower temperature than 500 °C) and higher molecular weight spices (that were not vaporized or decomposed at a higher temperature than 500 °C) than normal asphaltenes. It is evident that asphaltenes of higher and lower molecular weights selectively accumulate at the oil/water interface, and hence accumulate in the rag layer. 3.7. FTIR Spectra. FTIR spectra of the three asphaltene samples normalized at 2928 cm-1 are shown in Figure 9. Clearly, the absorbencies of the two rag “asphaltenes” are higher than the normal asphaltenes throughout the whole spectrum range, indicating a higher heteroatom content in the rag “asphaltenes” than in the normal asphaltenes. This result is consistent with elemental analysis results. The observed bands at 2928, 2856, 1460, and 1380 cm-1 for all three asphaltene samples are attributed to stretching and bending vibrations of aliphatic CH2 and CH3 groups. The broad band at 3200–3400 cm-1 is characteristic of functional groups with nitrogen (R–NH2, Ar–NH2, R–NHR, and Ar–NH-R, where Ar represents aromatic ring carbon and R the aliphatic chain carbon). The bands at 3621–3698 cm-1 (free OH) and the bands at 920, 1024, 1039, and 1111 cm-1 (Si–O) are characteristic of kaolinite clays,11 indicating a small amount of kaolinite clay present in the rag “asphaltenes” but not in the normal asphaltenes. In other words, some very fine “asphaltene”-bound solids were suspended in chloroform and were not removed completely. The most informative bands in Figure 9 are those at about 1700 and 1600 cm-1, which are free of clay interference. The former is attributed to CdO stretching and the latter to the aromatic ring. On the basis of the FTIR spectra of Athabasca bitumen and its components reported in the literature,12 the band (12) Wu, X. Energy Fuels 2003, 17 (1), 179–190.


of the aromatic rings appears always at the exact wavenumber of 1600 cm-1. The spectral feature around 1700 cm-1 was attributed to the presence of carboxylates. 4. Conclusions The performance of naphthenic froth treatment (NFT) was shown to be improved by water washing of the diluted bitumen froth. With only one-step washing (step 1), the water and solids contents were reduced by more than 91 and 87% of their original levels, respectively. Continuous circulation of the top organic phase back to the aqueous phase further reduced the residual water and solids content. The novel setup allowed us to generate and collect rag layer materials efficiently. The two “asphaltenes” isolated from the rag layer material at different naphtha-to-bitumen mass ratios of 0.7 and 7 were of similar elemental composition. However, the content of heteroatoms (N, S, and O) was twice that in normal asphaltenes. The higher content of heteroatoms in the

Gu et al

rag “asphaltenes” than in the normal asphaltenes was confirmed from FTIR spectra, as evidenced by higher absorption intensity over the whole wavelength range. The results of compositional analysis showed that chloroform solubles and insolubles in the rag layer material were in a ratio of about 1:1. TGA results suggested that the rag “asphaltenes” were a mixture of species with lower and higher boiling points than the normal asphaltenes. In other words, the “asphaltenes” obtained from rag layer contained more volatiles and coking residues than the normal asphaltenes obtained from the bulk bitumen. The results of emulsification potency tests for the five fractions—dry rag, chloroform soluble and insoluble, “asphaltenes”, and deasphalted organics—revealed that the chloroform soluble and asphaltenes exhibited the highest emulsion stabilization potency. Acknowledgment. We thank the NSERC Industrial Research Chair in Oil Sands Engineering (held by JHM) for financial support. EF7003688

Novel Bitumen Froth Cleaning Device and Rag Layer Characterization

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