Aggregates in Paraffinic Froth Treatment: Settling Properties and

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Fossil Fuels

Aggregates in Paraffinic Froth Treatment: Settling Properties and Structure Dominik Kosior, Edwina Ngo, and Yuming Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01656 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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Aggregates in Paraffinic Froth Treatment: Settling Properties and Structure Dominik Kosior*§‡, Edwina Ngo§, Yuming Xu§ § Natural Resources Canada, CanmetENERGY, 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada ‡ Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Krakow, Poland KEYWORDS bitumen froth, paraffinic froth treatment, settling rate, aggregate structure, Hildebrand solubility parameter

ABSTRACT

The settling rate of aggregates formed during paraffinic froth treatment depends strongly on process conditions. This work examined the influences of process temperature, solvent-tobitumen ratio (S/B), and type of solvent used on the settling rate. Experiments were performed at temperatures ranging from 30 to 90 °C and various S/Bs, using isopentane, n-pentane, and n-hexane as paraffinic solvents. Based on studies of the settling rate, two factors can be emphasized. First, process temperature has the greatest influence on the settling rate of

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aggregates. Second, for a given solvent and temperature, increasing S/B results in an increase of the settling rate. Study of the aggregates formed during paraffinic froth treatment allowed for better understanding of the phenomena affecting the settling rate. Besides developing an aggregate sampling method based on solubility parameters, experimental procedures for determining structural parameters were successfully applied.

1. Introduction Bitumen from the oil sand deposits of northern Alberta is recovered by applying one of two principal methods: surface mining operations1,2 or in situ production (e.g. thermal methods such as steam-assisted gravity drainage, SAGD).2,3 Method selection depends on the deposit characteristic. Surface mining is used for oil sands at depths less than about 80 m, although the main criterion for mining is the stripping ratio defined as the ratio of the volume of overburden that must be excavated to recover a unit volume of oil sand ore. The stripping ratio of oil sand mining operations is usually in the range of 0.4–1.4.2,3 Although in situ production of bitumen continues to expand, mining still has advantages, including high bitumen recovery rates, typically in the range of 95%. Recovery of the bitumen from mineable oil sands is a multi-stage process that includes mining operations,1 warm water extraction,4,5 and froth treatment.6–8 The entire pathway of oil sand treatment is designed to deliver bitumen product of desired properties. The bitumen froth stream from the extraction process typically contains 60 wt % bitumen, 30 wt % water, and 10 wt % solids.8,9 Mineral solids include kaolinite and illite, mainly, with minor amounts of other minerals like montmorillonite and chlorite.9–12 Besides typical mineral solids there are small amounts of heavy minerals such as pyrite, zircon, and rutile.10,13 To meet downstream processing


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requirements and pipeline transportation specifications, processing called froth treatment is needed to remove water and solids. The two main froth treatment technologies, identified by the type of solvent used in froth treatment, are: naphthenic froth treatment (NFT) and paraffinic froth treatment (PFT).2,7 Each method has advantages and weaknesses. Naphtha is a good solvent for all of the bitumen components, while paraffinic solvents precipitate asphaltenes when mixed with bitumen.9,14,15 However, the diluted bitumen from the naphtha-based process contains 3.5– 5.5 wt % water and 0.5–1.2 wt % fine solids, well above acceptable standards for pipeline transport.2,7,16 On the other hand, PFT product contains less than 0.1 wt % water and less than 0.1 wt % mineral solids.2,7,16 The choice of the froth treatment method by plant operators depends on the desired product quality.2,6,9 In PFT, bitumen froth is diluted with a paraffinic solvent at solvent-to-bitumen ratio (S/B) significantly higher than that required for the immediate onset of asphaltene precipitation.9,16 As a result, aggregates are formed by water droplets, dispersed mineral solids, and precipitated asphaltenes.16–18 The settling rate of aggregates in solvent-diluted bitumen is one of the most important process parameters dictating the size of the settlers and other separation equipment needed for bitumen froth treatment. It has been established that the settling rate of aggregates depends on their structure, e.g., compactness, and size.18–21 Therefore, manipulating aggregate structure provides the possibility of enhancing the settling rate. It is known that parameters including S/B,15,16 type of paraffinic solvent,15,18,22 temperature,18,22 and mixing intensity20 can significantly affect the settling rate. One of the main difficulties in experimental studies of bitumen froth treatment is rooted in the fact that the process is run at elevated temperatures and pressures. The use of glass settlers in studies of the diluted froth settling process has proved very beneficial.17,20,22 However, multiple


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glass walls limit visibility, making it difficult to detect and track interfaces, particularly for samples of fast-settling diluted froth. Hence, other methods of tracking the settling rate of aggregates based on ultrasound techniques22 or near-infrared spectrophotometry have been developed.23 Although the settling rate of the aggregates can be determined accurately by visual or ultrasound methods,22 it is difficult to investigate and describe their size and structure using these methods. Samples of diluted bitumen froth collected during typical PFT processing are completely opaque suspensions of aggregates in diluted bitumen. To view the structure of the aggregates, an alternative sample dilution procedure needs to be applied.20 There is no doubt that a precipitation of the asphaltenes is a very important parameter for the PFT efficiency. Thus, understanding asphaltene behavior and predicting their stability are of great industrial interest. The physical nature and the process of precipitations of “primary” asphaltene particles have been investigated intensively for the decades.14,19,24–32 Proposed models for aggregation and precipitation present only limited success in accounting for the experimental facts, especially PFT. The size and structure of aggregates formed during PFT have a crucial influence on their settling rates in diluted bitumen. Systematic studies of these parameters and their relationships with process conditions are therefore of great importance for better understanding and optimization of PFT. This paper presents results of a study of the size and structure of aggregates formed under various PFT conditions. 2. Experimental 2.1. Materials


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A 20-L pail of bitumen froth collected from a commercial oil sand mining operation in Alberta was used as received for the project. The froth composition was determined by Dean-Stark extraction.33 It contained 57.3 wt % bitumen, 32.0 wt % water, and 10.7 wt % solids. Asphaltene content in bitumen, measured using the n-pentane precipitation method according to the modified ASTM D450 procedure, was 17.1 wt%. Reagent-grade isopentane (iC5, Fisher Chemicals, 95%), n-pentane (nC5, OmniSolv, 98%), n-hexane (nC6, Fisher Chemicals, 95%), n-heptane (nC7, Fisher Chemicals, 99%), and toluene (Fisher Chemicals, 99%) were used as received. 2.2. Methods 2.2.1. Settling Tests A detailed description of the experimental setup and the procedure for determining the settling rate are given in our previous paper.22 Briefly, settling tests were carried out in a 630-mL stainless steel autoclave cell (Parr Instruments Co.) with inner diameter of 63 mm and height of 200 mm. The autoclave cell was equipped with four baffles 6 mm wide by 135 mm high, two coupled pitched-blade impellers 35 mm in diameter, a glass window (25 × 155 mm) to allow visual observation, and an ultrasound probe (Imasonic SAS Company). Table 1. Compositions of solvent-bitumen froth mixtures used in settling tests S/B* Solvent (g) Froth (g) 1.4 151.4 188.6 1.6 162.6 177.4 1.8 172.6 167.4 2.0 181.6 158.4 2.2 189.6 150.4 2.4 196.9 143.1 *S/B – solvent-to-bitumen ratio;


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The autoclave vessel was loaded with bitumen froth and solvent at known feed S/B, such that the total mass of mixture was 340 g (see Table 1), and pressurized to 0.83 MPa (120 psi) using nitrogen supplied from an external tank. Then, the content in the autoclave was mixed at 600 rpm and heated over a period of 15 min to the desired temperature (30–90 °C). Once the desired temperature was reached, stirring was continued for another 30 min at 1200 rpm and constant temperature. When mixing was stopped, the settling process inside the vessel was monitored using a digital camera (Nikon D800E) and/or ultrasound velocity profiler (Met-Flow UVP-DUO). Once the settling of aggregates was finished (30–60 min), 150 mL of diluted bitumen (supernatant) sample was transferred by pressure difference into a sampling container. The experiment was run at least two times for each condition investigated. The settling rate was determined by tracking either the upper or lower interface. A detailed description of both methods was presented in a previous paper.22 In short, the upper interface method is based on fitting a linear function to the initial part of the upper interface settling curve; the slope of the fitted line is taken as the settling rate. If neither visual nor sonic tracking of the upper interface is possible due to a high settling velocity or lack of a sharp boundary, then the consolidation point obtained as a local maximum on the lower interface settling curve is used to calculate the settling rate (Us) using

US =

( HC − H0 )

(1)

tC

where H0 is the total height of the bitumen froth-solvent mixture in the autoclave (treated as the upper interface level at t = 0); and Hc and tc are the height and the time at the consolidation point, respectively. Both parameters were determined by visual observation of the lower interface.


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2.2.2. Supernatant Analysis Upon completion of a settling test, the supernatant (diluted bitumen) was collected for further analysis. The density of the diluted bitumen was measured at 25 °C using an Anton Paar density meter (DMA 4500). The solvent in the diluted bitumen sample was removed using a rotary evaporator (Rotavapor) at 120 °C and 2.5 × 10-3 MPa (25 mbar), giving the S/B of the diluted bitumen and generating solvent-free bitumen. Asphaltene content in the bitumen was analyzed using the CANMET asphaltene analyzer (NIR spectroscopy).34 2.2.3. Aggregate Collection Selected runs were repeated in order to collect a sufficient number of aggregates from the hindered settling zone for further analysis. When the desired experimental conditions were reached and stirring was stopped, about 25 mL of diluted froth was collected into a sampling container immersed in a water-ice bath for rapid cooling of the sample. Once the sample was cooled, the container was opened and the sample was immediately transferred into a jar with previously prepared diluent (see below) to prevent further aggregation. Extra portions of diluent were added until a partially transparent mixture was obtained (see Figure 1).

Figure 1. Diluted aggregates in transparent solvent.


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2.2.4. Diluent Preparation The samples of diluted bitumen froth collected during typical PFT processing were completely opaque suspensions of aggregates in diluted bitumen. For further analysis of the aggregates, a sample dilution procedure was applied. To preserve the aggregate structure, diluent was prepared as a mixture of aromatic and paraffinic solvents (toluene combined with suitable proportions of n-heptane or n-hexane) using the Hildebrand solubility parameter.35–37 The data (S/B and asphaltene content in diluted bitumen) necessary to prepare diluent of the solubility parameter equals to the solubility parameter of diluted bitumen were obtained from analysis of the supernatant (see 2.2.2. Supernatant Analysis). The solubility parameter concept proved to be useful for predicting bitumen and heavy oil solubility behaviors.24,25,38–41 According to the concept of solubility parameter of a liquid (δ), the solubility parameter of diluted bitumen (δdilbit) can be calculated as

δ dilbit = ∑φiδ i

(2)

where ϕi is a volume fraction of the diluted bitumen component (solvent, maltenes, asphaltenes) and δi is its solubility parameter. Table 2. Compounds properties at T = 25 °C Compound iC5 nC5 nC6 nC7 Toluene Maltene Asphaltene

δ (MPa0.5) 13.80 14.30 14.80 15.20 18.20 19.10 21.00

ρ (kg/m3) 621 615 655 680 862 962 1250

η (Pa·s) ×10-4 2.29 2.27 2.96 3.88 5.60 – –


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Solubility parameters of diluted bitumen samples were calculated based on data obtained from the supernatant composition analysis and solubility parameters available in the literature.24,36,42,43 The main weakness of this approach is a lack of data on the Hildebrand solubility parameters at higher temperatures for some components of diluted bitumen (especially maltenes and asphaltenes). It is known that the Hildebrand solubility parameters of organic solvents decrease with increasing temperature36 and that the solubility parameters of maltenes and asphaltenes should change by the same magnitude as for organic solvents. The solubility parameter (δ), density (ρ), and dynamic viscosity (η) of each component at T = 25 °C, found in the literature, are presented in Table 2. 24,36,42,43 Table 3. Volume fractions of solvents calculated for preparation of diluent with δdiluent = δdilbit PFT conditions Solvent nC5 nC5 nC5 nC5 iC5 nC6

S/B (feed)

T (°C)

Diluted bitumen S/B (product)

Asphalthenes* (wt %)

Diluent δdilbit (MPa0.5)

1.6 30 1.82 ±0.03 10.2 ±0.1 15.57 1.6 50 1.77 ±0.04 11.4 ±0.3 15.60 1.6 70 1.75 ±0.02 11.9 ±0.1 15.61 1.6 90 1.73 ±0.01 11.6 ±0.3 15.63 1.6 30 1.92 ±0.01 6.25 ±0.1 15.13 2.4 30 2.71 ±0.01 10.0 ±0.3 15.68 *asphaltene content in bitumen product; **data for φnC6

φtoluene

φnC7

ρ (kg/m3)

0.12 0.13 0.14 0.14 0.10 0.16

0.88 0.87 0.86 0.86 0.90** 0.84

703 704 705 706 675 709

η (Pa·s) ×10-4 4.09 4.10 4.11 4.12 3.18 4.14

Table 3 gives the compositions of prepared and used diluents having the same values of solubility parameter as the diluted bitumen. As seen, the value of δdilbit increases marginally with temperature, which appears to contradict with the previous statement about changes of the Hildebrand solubility parameters with temperature. The reason for this discrepancy is related to three factors: (1) despite the change in process temperature, δdilbit and δdiluent were calculated using literature values of the solubility parameter for T = 25 °C; (2) asphalthene content in diluted bitumen increased slightly with temperature, resulting in marginally higher solubility


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parameters of diluted bitumen; (3) S/B of the product diluted bitumen decreased with temperature, resulting in a slight decrease in the volume fraction of the hydrocarbon solvent – the compound with the lowest δ. 2.2.5. Single-aggregate Experiment The settling velocities of individual aggregates were measured at ambient conditions using an experimental setup (Figure 2) consisting of (i) a square glass column (130 mm in height and 40 × 40 mm in cross section), (ii) fibre-optic illumination, (iii) a light diffuser, and (iv) a Nikon D800E digital camera equipped with a macro lens (AF-S Micro Nikkor 105 mm) and additional macro rings. Pictures of settling aggregates were taken about 20 mm above the bottom of the column, well after the terminal (constant) velocity had been reached. The column was filled with the same diluent as used during sample collection and covered by an aluminum foil cap to prevent solvent evaporation. The aggregates were transferred to the column through a small hole in the middle of the aluminum foil cap using plastic pipettes. Video recordings were processed and analyzed using VirtualDub and imageJ software (GNU General Public License).

Figure 2. Experimental setup for single-aggregate experiments.


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The settling velocity of an individual aggregate Ua is given by

Ua =

yi +1 − yi ∆t

(3)

where yi, yi+1 are the coordinates of consecutive positions of the aggregate, and ∆t is the interval of photo acquisition. The aggregate equivalent diameter (deq) is calculated using

d eq = 2

Aa

(4)

π

where Aa is a cross-sectional area of the settling aggregate determined by image analysis. 2.2.6. Light Microscopy Photos of individual aggregates from the samples collected during PFT were taken in the glass cell using a light microscope (Zeiss Axio Observer.D1). The Nikon D800E digital camera was coupled to the microscope by means of a suitable adapter and used for image acquisition. To increase the depth of field of the final image, a series of micrographs (20−40) were taken of each aggregate, with the focal point set at evenly spaced intervals into the depth of field. Then, the images in the series were transferred to a computer and stitched together using Adobe Photoshop CS5 software. At least 20 individual aggregates were observed for each PFT condition studied. 3. Results and discussion Settling tests of diluted bitumen froth were performed using three commonly used paraffinic solvents: isopentane, n-pentane, and n-hexane. The main focus was placed on processes carried out for S/B set to 1.6 (the most common value used for PFT in the oil sand industry), but other S/B values were studied as well.


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The settling rates determined for all studied solvents and for the selected solvent-to-bitumen ratios as a function of the temperature are presented in Figure 3. These results confirm that the process temperature of PFT has a profound impact on the settling velocity of the aggregates formed during PFT for all studied S/Bs. It is clear that increase of the process temperature lead to increases of the settling rate. Unfortunately, the reason for such evident dependence of the settling rate on the process temperature has not yet been explained unequivocally.

iC5 ; S/B=1.4 iC5 ; S/B=1.6 nC5 ; S/B=1.6 nC5 ; S/B=1.8 nC6 ; S/B=1.6 nC6 ; S/B=2.4

1000

Settling rate [mm/min]

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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100

10

1 0

20

40

60

80

100

Temperature [oC]

Figure 3. Settling rates of bitumen froth diluted with various solvents at various S/B values as a function of temperature. There is no doubt that process conditions influences product composition. As can be seen in Fig. 4A, asphaltene content in bitumen product (after solvent removal) increases with increasing process temperature, reaching a maximum around 70 °C, and starts to decrease slightly above that temperature, similarly to data presented in the literature.16,22,38,40 In case of data for S/B


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presented in Fig. 4B the analysis is more complex. At first, it is seen that S/B of the product is always higher than intended (feed) S/B. This phenomenon can be related to two main reasons: (i) precipitation of the asphaltenes from the bitumen and (ii) incomplete maltenes extraction. The problem with exact determination of the non-extracted bitumen arises from the porous structure of the underflow, which besides of the non-extracted bitumen also contains trapped diluted bitumen. Next, data on Fig. 4B shows that S/B decreases with increasing process temperature. Observed phenomenon can be associated with (i) changes in asphaltenes solubility with temperature, (ii) better maltenes extraction in higher temperature6 or (iii) loss of the solvent caused by evaporation during the experiments at the higher temperatures.

20

3.0

A

B

2.5

S/B (product)

15

Asphaltene [wt%]

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

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10


5

0 0

20

40

60

80

100

2.0


1.5

1.0 0

Temperature [oC]

20

40

60

Temperature [oC]

80

100

Figure 4. (A) Asphaltene content in bitumen product and (B) product S/B of diluted bitumen as a function of temperature. The dashed line in 4A represents the initial asphaltene content in bitumen before PFT. As seen in Figure 4, the change in the asphaltene content in the bitumen product and the decrease in the S/B of diluted bitumen product are rather small compared to changes in the


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settling rate. Therefore, elucidation of the observed settling rate differences should be sought elsewhere. The most likely factors are changes in physical properties, i.e. density and viscosity, of the continuous phase (diluted bitumen) and changes in aggregate structure and size. Study of the latter possibility is described later in this paper.

100

Settling rate [mm/min]

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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10

iC5; T= 30oC nC5; T= 30oC nC6; T= 30oC

1 1.2

1.4

1.6

1.8

2.0

2.2

2.4

S/B

Figure 5. Settling rates of bitumen froth diluted with various solvents at T = 30 °C as a function

of S/B. Figure 3 also shows that the settling velocity is affected by the process S/B and even more by the type of solvent used. Influences of both factors are well illustrated in Figure 5, where settling rates for all three solvents used are presented as a function of S/B. Figure 5 shows a clear correlation: the higher the S/B, the higher the settling velocity of the aggregates. This phenomenon can be explained using the Richardson–Zaki approximation for the hindered settling rate (Uh):44


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U h = U o ( 1 − φa )

n

(5)

where, ϕa is the volume fraction of the aggregates, n is a coefficient that can vary from 4 to 22, and Uo is the free settling rate of the aggregates. For an impermeable aggregate with porosity ξ moving with low Reynolds number, Uo can be approximated by the improved Stokes equation:18

Uo =

d a2 ( 1 − ξ ) ( ρ p − ρ L ) g

(6)

18η L

where g is the gravitational constant, da is the diameter of the aggregate, ξ is the porosity of aggregate, ηL is the viscosity of the continuous phase, ρp is the density of the primary particles that make up the aggregate, and ρL is the density of the continuous phase. The effect of permeability of the aggregates on settling rate can be neglected, in accordance with the literature, 16,18,45,46

for aggregate porosity lower than 0.80. Based on information obtained from visual

analysis of micrographs of aggregates collected during PFT (Fig. 8) it is unlikely that porosity of studied aggregates was above 0.80. In regard to Figure 5 and eqs 5 and 6, increase of S/B results in reduction of the volume fraction of the aggregates in the diluted bitumen system, which leads to higher settling rate. Moreover, higher S/B (in the studied range) results in greater precipitation of asphaltenes (see Fig. 6), which may reduce the viscosity of the continuous phase (diluted bitumen) and change the aggregate structure. It is known that the density and viscosity of the oil phase change when different solvents are used. However, it has already been shown using theoretical predictions of settling that density and viscosity variations of the oil phase due to the solvent change do not adequately explain the experimental settling rate differences.16 Therefore, other factors, such as structural differences of the aggregates, must be dominant in here.


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3.0

20

A

B

o

iC5; T = 30 C nC5; T = 30oC nC6; T = 30oC

2.5

S/B (product)

15

Asphaltene [wt%]

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

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10

2.0

1.5

5

iC5; T = 30oC o nC5; T = 30 C nC6; T = 30oC

0 1.2

1.4

1.6

1.8

2.0

2.2

2.4

1.0 1.0

1.5

2.0

2.5

3.0

S/B

S/B

Figure 6. (A) Asphaltene content in bitumen product and (B) product S/B of diluted bitumen as

a function of feed S/B. The dashed line in 6A represents the initial asphaltene content in bitumen before PFT. Besides the physical properties of the continuous phase, the PFT conditions (temperature, type of solvent, S/B) also influence the size and structure of aggregates, significantly affecting the rate of settling. Therefore, studies of aggregate parameters (diameter, density, etc.) and their relationships with process conditions are of a great importance for better understanding of PFT. Data from single-aggregate experiments and from a microscopy study for selected PFT conditions are presented and analyzed below. As mentioned, the S/B for bitumen froth diluted with iC5 and nC5 was set to 1.6 because this value is commonly used for PFT in the oil sand industry. n-Hexane was tested at S/B = 2.4 to maintain asphaltene rejection similar to that for settling tests performed using nC5 at S/B = 1.6.


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6000

5000

Settling velocity [mm/min]

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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4000

3000

2000

1000

T = 30oC T = 90oC

0 0

100

200

300

400

Diameter [µ µm]

500

600

Figure 7. Settling velocities of individual aggregates collected during PFT (nC5, S/B = 1.6) as a

function of aggregate diameter. Solid lines show theoretical results calculated from eq 7. Figure 7 shows settling velocities of individual aggregates collected during PFT as a function of their diameter. The points represent experimental results and the solid lines are curves fitted to those points. The choice of the theoretical curve used for further calculation is strictly related to flow conditions in the studied system. According to results obtained in single-aggregate experiments, the Reynolds numbers of individual settling aggregates fall roughly in a range from 0.1 to 20, i.e. transitional regime. In this work, the curve was estimated according to the relation, which covers both laminar and transition flow regimes, given by Nguyen et al.:47

   d a2 ( ρ a − ρ L ) g  1 Ua =   Ar 18η L 0.749 −0.755 1 + (1 + 0.079 Ar )  96 

(7)


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where Ua is an aggregate velocity, da is an aggregate diameter, ρa and ρL are densities of aggregates and liquid (diluent), respectively, ηL is a liquid viscosity (diluent), and Ar is the Archimedes number defined as

Ar =

d a3 ( ρ a − ρ L ) ρ L g

(8)

ηL2

The aggregate density is the only unknown parameter in eq 7. Thus, fitting of the theoretical curve to the experimental data using the least-squares method allows calculation of ρa. Please note that eq 7 does not take into account aggregate porosity, and the calculated densities of aggregates thus include diluted bitumen entrapped in aggregates structure. Hence, referring to eq 6, it can be written that:

( ρa − ρ L ) = ( 1 − ξ ) ( ρ p − ρL )

(9)

Table 4. Density of aggregates calculated from eq 7 PFT conditions Solvent

S/B

T (°C)

No. of data points

nC5 nC5 nC5 nC5 iC5 nC6

1.6 1.6 1.6 1.6 1.6 2.4

30 50 70 90 30 30

414 478 391 351 520 665

ρa (kg/m3) 1391 1492 1604 1809 1481 1360

Pearson correlation coefficient 0.82 0.87 0.88 0.91 0.89 0.89

Table 4 presents densities of aggregates collected during PFT determined using single-aggregate experiments and eq 7 for selected runs. The data show a clear correlation: the higher the temperature of the process, the higher the density of aggregates. The observed changes in aggregate densities can be result of two effects: (i) lowering of aggregate porosity and/or (ii) partial water removal from the aggregate structures.


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Figure 8 presents micrographs of typical aggregates collected during selected PFT. As the microscopy technique is a time-consuming method, the determination of the statistically small sample size (ca. 20 individuals) as compared to the aggregate population inside the autoclave, was analyzed by visual observation. Despite the mentioned limitation, it was possible to obtain some useful qualitative information about the influence of the PFT conditions on aggregate structure.

Figure 8. Micrographs of aggregates collected during PFT.

From micrographs analysis it is possible to distinguish three main components of aggregates. The brightest parts of the aggregates, often having spherical shape, are the water droplets attached to the aggregates. Their yellowish color is probably related to the fine solids dispersed inside of the water droplets. The shining spots, yellow and orange, with irregular shapes are the


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mineral particles. Finally, black and dark green areas indicate the presence of organic phase, most probably precipitated asphaltenes. It is clearly seen that increasing the process temperature has a significant effect on the aggregate structure. Aggregates produced at higher temperature have more compact (less porous) structure with reduced water content. As seen in Figure 8, the type of solvent also influences aggregate structure. Aggregates produced by nC5 and nC6 at T = 30 °C contain mixtures of all components in the whole structure, i.e. water, solids, and asphaltenes, while those produced by iC5 have a main core built of solids and asphaltenes with small water droplets attached to it. 60 iC5 ; S/B=1.4 iC5 ; S/B=1.6 nC5 ; S/B=1.6 nC5 ; S/B=1.8 nC6 ; S/B=1.6 nC6 ; S/B=2.4

50

U/F Height [mm]

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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40

30

20 0

20

40

60

o

80

100

Temperature [ C]

Figure 9. Height of PFT underflow after 60 min of settling as a function of temperature.

The aggregates structure, especially information about diluted bitumen trapped underflow (U/F) after settling is finished can be reflected in final volume or height of underflow. Figure 9 presents U/F height for all studied conditions as a function of temperature. The trend is clear: the


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higher the process temperature the lower the final underflow height. The observed phenomenon might be a result of the changes in the aggregate structure. Higher temperature results in formation of less-fluffy, more-compacted aggregates. The very interesting aspect seen is that the observed changes do not follow trends related to the asphaltene precipitation (Figure 4A).

0.4

U/F Height / Froth Mass [cm/g]

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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0.3

0.2

0.1 iC5; T = 30oC nC5; T = 30oC nC6; T = 30oC

0.0 1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

S/B

Figure 10. Normalized height of PFT underflow after 30 min of settling as a function of S/B.

The analysis of the U/F heights as a function of S/B imposes some difficulties related to the test procedure. As the autoclave vessel was always loaded with the same total mass of mixture, i.e. 340 g, higher S/B resulted in reduced bitumen froth mass (see Table 1). Consequently, less underflow was produced during the experiment. To compare the relative influences of S/B on the structure of the compacted sediment, the underflow height needs to be normalized for the bitumen froth mass. Figure 10 presents the normalized height of the compacted sediment as a function of S/B for iC5, nC5, and nC6. Small changes in the normalized U/F height indicate


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rather negligible impact of S/B on aggregate structure. Therefore, the increase of hindered settling velocity with increase of S/B (presented in Figure 5) is the result of a decrease in the volume fraction of aggregates and the physical properties of the continuous phase, as discussed above in reference to eq 5. Table 5. Average diameter of aggregates from single-aggregate experiment PFT conditions S/B Solvent T (°C) ratio nC5 1.6 30 nC5 1.6 50 nC5 1.6 70 nC5 1.6 90 iC5 1.6 30 nC6 2.4 30

Average diameter (µm)

135 ± 57 157 ± 69 142 ± 48 145 ± 63 158 ± 76 137 ± 79

The single-aggregate experiment delivers information, not only about aggregate densities, but also about their sizes, i.e. average diameter and size distribution. Note that these results should be treated with caution as the aggregates with diameters smaller than 50 µm were excluded from the data analysis due to limitations related to camera resolution (ca. 120 pix/mm). Table 5 presents average diameters of aggregates collected during PFT for selected conditions for the aggregates fraction larger than 50 µm. These results show that the process temperature as well as the type of solvent have rather minor influences on the average diameter of the aggregates – measured changes of aggregate size are small. Figure 11 shows the size distributions of aggregates collected during PFT (nC5, S/B = 1.6) for two different temperatures, 30 and 90 ºC. The number of aggregates used to prepare Fig. 11 is equal to the number of data points obtained in single-aggregate experiment (see Table 4). Again, the process temperature has a rather small effect on the average diameters and size distributions


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of aggregates during PFT. Therefore, the observed significant changes of hindered settling rate with process temperature are not due to variations in the aggregate size, but rather to their structure.

25 T = 30oC T = 90oC

20

Frequency [%]

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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15

10

5

0 0

100

200

300

400

Diameter [µ µm]

500

600

Figure 11. Size distribution of aggregates collected during PFT (nC5, S/B = 1.6).

It should be underlined again that exclusion of aggregates smaller than 50 µm from this analysis may have had a profound impact on the overall observation of the settling process. Exclusion of the smaller aggregates probably does not interfere with calculations of aggregate densities, as these are estimated using a theoretical function fitted to the experimentally obtained points over the wide range of aggregate sizes. However, average diameters and size distributions of aggregates should be treated with caution as they can be altered by exclusion of small aggregates (< 50 µm) from data analysis.


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4. Conclusions Examination of the influence of process temperature and solvent-to-bitumen ratio on the settling rate shows that both factors caused noticeable increases of the settling velocity of aggregates, irrespective of the solvent used. A temperature change from 30 to 90 ºC resulted in an increase of the settling rate by an order of magnitude. Solvent-to-bitumen ratio also affected the settling rate of aggregates, but its influence was not as significant as that of temperature. The settling rate dependence upon the solvent used had the following order: isopentane > n-pentane > n-hexane. Process temperature showed a notable impact on aggregate structure: higher temperature caused formation of more compacted aggregates and enhanced the release of water droplets as a separate phase, and resulted in increased aggregate densities. Without doubt, the observed changes in aggregate structure with process temperature had a crucial influence on the settling rate. The fact that the height of compacted sediment decreased with increase of process temperature suggested that less solvent-diluted bitumen was entrapped in the aggregate structures. Surprisingly, both process temperature and solvent-to-bitumen ratio did not noticeably affect the average diameter of aggregates (which was in the range of 130–160 µm) nor the particle size distribution. AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] Present Addresses


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§ Natural Resources Canada, CanmetENERGY, 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada ‡ Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Krakow, Poland Notes

The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank dr. Kim Kasperski for her insightful comments and suggestions. The authors acknowledge the late distinguished scientist Dr. Tadek Dabros, who made significant contributions to oil sand research and development. REFERENCES (1)

Demorest, T.; Read, P.; Jonah, K.; Payne, F.; Kosik, W.; Beers, R. In Handbook on Theory and Practice of Bitumen Recovery from Athabasca Oil Sands Volume II: Industrial Practice; Czarnecki, J., Masliyah, J., Xu, Z., Dabros, M., Eds.; Kingsley Knowledge Publishing: Canada, 2013; pp 41–70. (2) Gray, M. R. Upgrading Oilsands Bitumen and Heavy Oil; University of Alberta Press: Edmonton, Canada, 2015. (3) Schramm, L. L.; Isaacs, E. In Handbook on Theory and Practice of Bitumen Recovery from Athabasca Oil Sands Volume II: Industrial Practice; Czarnecki, J., Masliyah, J., Xu, Z., Dabros, M., Eds.; Kingsley Knowledge Publishing: Canada, 2013; pp 603–636. (4) Clark, K. A. Trans Can Inst Min Met. 1944, 47, 257–274. (5) Masliyah, J.; Zhou, Z. J.; Xu, Z.; Czarnecki, J.; Hamza, H. Can. J. Chem. Eng. 2004, 82 (4), 628–654. (6) Shelfantook, W. E. Can. J. Chem. Eng. 2004, 82, 704–709. (7) Tipman, R. N. In Handbook on Theory and Practice of Bitumen Recovery from Athabasca Oil Sands Volume II: Industrial Practice; Czarnecki, J., Masliyah, J., Xu, Z., Dabros, M., Eds.; Kingsley Knowledge Publishing: Canada, 2013; pp 211–254. (8) Rao, F.; Liu, Q. Energy Fuels 2013, 27, 7199–7207. (9) Masliyah, J. H.; Czarnecki, J.; Xu, Z. Handbook on Theory and Practice of Bitumen Recovery from Athabasca Oil Sands Volume I: Theoretical Basis; Kingsley Knowledge Publishing: Canada, 2011. (10) Sparks, B. D.; Kotlyar, L. S.; O’Carroll, J. B.; Chung, K. H. J. Pet. Sci. Eng. 2003, 39 (3), 417–430.


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(11) Liu, J.; Xu, Z.; Masliyah, J. AIChE J. 2004, 50 (8), 1917–1927. (12) Kaminsky, H. A. W.; Etsell, T. H.; Ivey, D. G.; Omotoso, O. Can. J. Chem. Eng. 2009, 87 (1), 85–93. (13) Kaminsky, H. A. W.; Etsell, T. H.; Ivey, D. G.; Omotoso, O. Miner. Eng. 2008, 21 (4), 264–271. (14) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149–152. (15) Xu, Y. Energy Fuels 2018, 32 (3), 2801–2810. (16) Long, Y.; Dabros, T.; Hamza, H. In Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O., Sheu, E., Hammami, A., Marshall, A., Eds.; Springer New York, 2007; pp 511–547. (17) Long, Y.; Dabros, T.; Hamza, H. Fuel 2002, 81, 1945–1952. (18) Long, Y.; Dabros, T.; Hamza, H. Fuel 2004, 83, 823–832. (19) Rahmani, N. H. G.; Dabros, T.; Masliyah, J. H. Energy Fuels 2005, 19, 1099–1108. (20) Zawala, J.; Dabros, T.; Hamza, H. Energy Fuels 2012, 26, 5775–5781. (21) Casas, Y. Settling Rates of Asphaltenes and Solids from Diluted Bitumen. MSc, University of Calgary: Calgary, 2017. (22) Kosior, D.; Ngo, E.; Dabros, T. Energy Fuels 2016, 30 (10), 8192–8199. (23) Long, Y.; Dabros, T. Energy Fuels 2005, 19, 1542–1547. (24) Mannistu, K. D.; Yarranton, H. W.; Masliyah, J. H. Energy Fuels 1997, 11 (3), 615–622. (25) Andersen, S. I.; Speight, J. G. J. Pet. Sci. Eng. 1999, 22 (1–3), 53–66. (26) Rahmani, N. H. G.; Dabros, T.; Masliyah, J. H. Ind. Eng. Chem. Res. 2005, 44 (1), 75–84. (27) Maqbool, T.; Balgoa, A. T.; Fogler, H. S. Energy Fuels 2009, 23, 3681–3686. (28) Mullins, O. C. Energy Fuels 2010, 24 (4), 2179–2207. (29) Mullins, O. C.; Sabbah, H.; Eyssautier, J.; Pomerantz, A. E.; Barré, L.; Andrews, A. B.; Ruiz-Morales, Y.; Mostowfi, F.; McFarlane, R.; Goual, L.; Lepkowicz, R.; Cooper, T.; Orbulescu, J.; Leblanc, R. M.; Edwards, J.; Zare, R. N. Energy Fuels 2012, 26 (7), 3986– 4003. (30) Haji-Akbari, N.; Masirisuk, P.; Hoepfner, M. P.; Fogler, H. S. Energy Fuels 2013, 27, 2497–2505. (31) Ghosh, A. K.; Chaudhuri, P.; Kumar, B.; Panja, S. S. Fuel 2016, 185, 541–554. (32) Vilas Bôas Fávero, C.; Maqbool, T.; Hoepfner, M.; Haji-Akbari, N.; Fogler, H. S. Adv. Colloid Interface Sci. 2017, 244, 267–280. (33) Dean, E. W.; Stark, D. D. J. Ind. Eng. Chem. 1920, 12, 486–490. (34) Long, Y.; Dabros, T.; Hamza, H. Can. J. Chem. Eng. 2004, 82, 776–781. (35) Hildebrand, J. H.; Scott, R. L. The Solubility of Non-Electrolytes, 3rd Ed.; Reinhold: New York, 1950. (36) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters, 2nd Ed.; CRC Press: Boca Raton, FL, 1991. (37) Hansen, C. M. Hansen Solubility Parameters: A User’s Handbook, Second Edition; CRC Press, 2007. (38) Andersen, S. I.; Stenby, E. Fuel Sci. Technol. Int. 1996, 14, 261–287. (39) Wiehe, I. A.; Yarranton, H. W.; Akbarzadeh, K.; Rahimi, P. M.; Teclemariam, A. Energy Fuels 2005, 19 (4), 1261–1267. (40) Wiehe, I. A. Process Chemistry of Petroleum Macromolecules; Chemical Industries; CRC Press: Boca Raton, FL, 2008. (41) Wiehe, I. A. Energy Fuels 2012, 26 (7), 4004–4016.


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(42) Aray, Y.; Hernández-Bravo, R.; Parra, J. G.; Rodríguez, J.; Coll, D. S. J. Phys. Chem. A 2011, 115 (42), 11495–11507. (43) Lide, D. R. CRC Handbook of Chemistry and Physics, 88th Edition; CRC Press, 2007. (44) Richardson, J. F.; Zaki, W. N. Trans. Inst. Chem. Eng. 1954, 32, 35–53. (45) Tang, P.; Raper, J. A. Powder Technol. 2002, 123 (2), 114–125. (46) Camenen, B.; Pham van Bang, D. Cont. Shelf Res. 2011, 31 (10), S106–S116. (47) Nguyen, A. V.; Stechemesser, H.; Zobel, G.; Schulze, H. J. Int. J. Miner. Process. 1997, 50, 53–61.


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Figure 1. Diluted aggregates in transparent solvent. 126x210mm (72 x 72 DPI)


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Figure 2. Experimental setup for single-aggregate experiments. 143x115mm (300 x 300 DPI)


Energy & Fuels 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

Figure 3. Settling rates of bitumen froth diluted with various solvents at various S/B values as a function of temperature. 155x150mm (96 x 96 DPI)


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Figure 4. (A) Asphaltene content in bitumen product and (B) product S/B of diluted bitumen as a function of temperature. The dashed line in 4A represents the initial asphaltene content in bitumen before PFT. 227x112mm (96 x 96 DPI)


Energy & Fuels 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

Figure 5. Settling rates of bitumen froth diluted with various solvents at T = 30 °C as a function of S/B. 150x149mm (96 x 96 DPI)


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Figure 6. (A) Asphaltene content in bitumen product and (B) product S/B of diluted bitumen as a function of feed S/B. The dashed line in 6A represents the initial asphaltene content in bitumen before PFT. 226x111mm (96 x 96 DPI)


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Figure 7. Settling velocities of individual aggregates collected during PFT (nC5, S/B = 1.6) as a function of aggregate diameter. Solid lines show theoretical results calculated from eq 7.


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Figure 8. Micrographs of aggregates collected during PFT. 47x31mm (300 x 300 DPI)


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Figure 9. Height of PFT underflow after 60 min of settling as a function of temperature. 153x151mm (96 x 96 DPI)


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Figure 10. Normalized height of PFT underflow after 30 min of settling as a function of S/B. 153x151mm (96 x 96 DPI)


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Figure 11. Size distribution of aggregates collected during PFT (nC5, S/B = 1.6). 151x152mm (96 x 96 DPI)


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Aggregates in Paraffinic Froth Treatment: Settling Properties and

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