Dewatering of Poor-Quality Bitumen Froth: Induction Time and Mixing

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Dewatering of Poor-Quality Bitumen Froth: Induction Time and Mixing Effects Colin Saraka, Runzhi Xu, Marcio B Machado, Sujit Bhattacharya, Samson Ng, and Suzanne M. Kresta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01613 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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Dewatering of Poor-Quality Bitumen Froth: Induction Time and Mixing Effects Colin Saraka1, Runzhi Xu1, Marcio B Machado1, Sujit Bhattacharya2, Samson Ng2, Suzanne Kresta1 1 – Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada 2 – Research and Development Centre, Edmonton, AB, Canada

Abstract The impact of mixing conditions on the removal of water and solids from high-water, poorquality bitumen froth was explored. Naphtha diluent and a demulsifier were added to improve removal of water and solids from bitumen froth. The mixing and subsequent settling of this system were carried out in the confined-impeller stirred tank (CIST), a lab-scale mixing test vessel with well-characterized, relatively-uniform mixing conditions. A protocol for finding the proper demulsifier dosage at which to study mixing effects was applied successfully. High mixing energy J and the pre-dilution of demulsifier (characterized by its injection concentration IC) improved dewatering and solids removal performance, agreeing with earlier studies in diluted bitumen and bitumen froth of higher quality (Laplante et al, 2015, Arora, 2016). An unexpected finding was that dewatering was significantly delayed in poor-quality froth: it was not detectable until up to 45 minutes in some cases. This induction time was replicated and was clearly impacted by changes in the mixing conditions.

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1. Introduction Bitumen froth treatment is the second stage of separation of water and solids during the bitumen extraction and froth treatment process. Bitumen froth is a mixture of hydrocarbons, water and solids. A typical bitumen froth produced under regular processing conditions has a composition of 60% bitumen, 30% water, and 10% solids. The bitumen froth used for this work is from a poorer-processing ore and had a composition of 50% bitumen, 37% water, and 13% solids. Two types of froth treatment are used in industry.1, 2 Parafinic froth treatment is used in some processes: in this method, an aliphatic diluent is added to the froth to drive asphaltenes out of solution to aid settling. The older process and the subject of this work is naphthenic froth treatment. Naphtha is added as a diluent to the bitumen froth. It improves the removal of water and solids by decreasing the viscosity of the fluid and increasing the density difference between the hydrocarbon and dispersed phases.2 Stable emulsions of water in crude oil are a common cause of process issues. Saline water emulsified in bitumen products can lead to corrosion issues while solids persisting in bitumen products can cause erosion, catalyst fouling and increased production of tailings and lost product. The stability of the mixture is caused in part by interfacial films, or skins, of heavier-oil fractions, particularly asphaltenes. 3, 4 However, since asphaltene is a solubility class – asphaltenes are toluene-soluble but heptane-insoluble – this category includes many molecules, not all of which contribute to emulsion stability, and many of which may destabilize the emulsion.5 Fine solids and other species also contribute to film stability and thus to emulsion stability. 3, 6, 7 The properties of the films which sometimes cover water droplets vary by the source of the bitumen (or other crude oil product), the extraction conditions,8 the nature of the diluent,9 the amount of diluent,9, 10 the amount of demulsifier,11 and the amount and type of solids.12 The size of the emulsified water also depends on many of these factors and is itself an important 2 ACS Paragon Plus Environment

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factor in determining the ease of dewatering. Water droplets above 60 microns will generally separate quickly, while those below 10 microns will remain emulsified indefinitely without further processing; intermediate water droplets require time or centrifugation to separate.13 Water that separates quickly is called “free water” while the difficult-to-separate, small water droplets are referred to as emulsified water. Demulsifier is added to the mixture to enhance separation.14-17 Demulsifier works by two major mechanisms.3 The first mechanism is flocculation, wherein the dispersed phases collect into groups or flocs. The second mechanism is coalescence, wherein small droplets combine to form larger droplets. In practice, most industrial demulsifiers are a mixture of chemicals, promoting multiple mechanisms of demulsification.3 Any demulsifier used to promote coalescence must be able to disrupt or change the properties of the skins which prevent droplets from coalescing. Flocculating aids are often amphiphilic polymers like ethylene oxide/propylene oxide copolymer which report to the interface between phases and bridge water droplets together.3 Demulsifier added to bitumen froth systems is subject to several concurrent phenomena:3 first, it must be dispersed into the continuous phase. Then it must report to the interface between the water and oil phases. In this capacity it competes with the surface-active species. Next, droplets come together through local variations in the flow field, leading to coalescence or flocculation events.18 Flocs and droplets can also break up through contact with turbulent eddies. Mixing is thus a key component of the successful addition and dispersion of demulsifier and the contact, coalescence and breakup of droplets and flocs. To replicate the results of large-scale chemical processes in the turbulent regime, bench scale experiments must also be run under fully turbulent conditions. Bench-scale single-impeller stirred tanks are turbulent in the impeller region but quickly fall off to transitional flow in other regions of the tank: while the impeller region is fully turbulent at a Reynolds number of approximately 20 000, Reynolds numbers of over 300 000 are needed to achieve fully turbulent flow all the way to the top of the tank.19 The geometry used in this work is a confined-impeller stirred tank (CIST).20, 21 The CIST is a labscale vessel; it holds approximately 1 litre of fluid. The tank height is three times its diameter to 3 ACS Paragon Plus Environment

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accommodate more impellers (5-6) and to facilitate the study of settling. Compared to a bench scale stirred tank, the CIST provides a more uniform energy dissipation field, allowing for fully turbulent flow at lower impeller speeds and Reynolds numbers.20 Komrakova, et al. 21 found that most of the vessel is in active circulation: only 5.45% is inactive, compared to 33% in a stirred tank.22 Several studies have used the CIST to study the separation of water from bitumen froth23 and diluted bitumen.24, 25 These studies have consistently shown that although the amount of chemical additive is an important variable, good mixing conditions allow for equally successful separation with substantially less additive. Predilution of the demulsifier is a key strategy used to reduce or eliminate loss of demulsifier effectiveness due to mixing limitations. Despite the importance of mixing, bulk concentration is often the only variable adjusted to improve dewatering. Simply adding more demulsifier is not an optimal solution because increasing the bulk concentration is expensive. Previous studies have found that adjusting mixing variables allows successful separation at a lower bulk concentration for two different commercial demulsifiers and several samples of diluted bitumen.24, 25 Dewatering can also be limited by overdosing: the concentration at which more demulsifier leads to worse separation. Overdosing can be overcome in diluted bitumen by improving mixing.26 This suggests that overdosing is a mesomixing problem due to high local concentration of additive in the feed plume. The mixing variables which can be manipulated to improve performance include injection concentration, mixing time, and the mixing power. Lower injection concentration has been found to effect better separation in diluted bitumen and bitumen froth.23-26 The injection concentration is lowered by diluting the additive chemical in a carrier fluid. The improvement in separation performance is due to lower local concentration of additive: if the demulsifier is added into the system at high concentration and in a quantity that cannot be quickly dispersed by the local mixing conditions, then there will be regions in which high concentrations of demulsifier persist. In these regions of high concentration, alternative mechanisms appear to be engaged: perhaps the demulsifier selfaggregates, or more demulsifier attaches irreversibly to fewer water droplets, lessening the 4 ACS Paragon Plus Environment

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amount available to the rest of system. Whatever the mechanism, it is clear from these studies that high local concentration is a detriment to demulsifier performance which can be overcome by lowering the injection concentration. Increases in mixing time and in the energy dissipated in the impeller swept volume are also associated with better settling in diluted bitumen.24-26 The mixing time and mixing power can be multiplied together to obtain the cumulative mixing energy J which was shown to be associated with better settling in diluted bitumen and average-quality bitumen froth – a froth which produces an average-quality dilbit after processing.23, 24 Cumulative mixing energy has been shown to work as a scaling variable where geometric similarity is not possible or practical, contingent on maintaining fully turbulent flow in the test vessel.27, 28 This work seeks to extend the knowledge of mixing effects into poorer-processing froth by studying the water contend as it changes over the full height of the vessel during settling. the effects of demulsifier bulk concentration, demulsifier injection concentration and mixing energy were assessed. This paper focuses on the separation of water and solids over time as determined by Karl Fisher titration and Dean Stark extraction. It is a subset of the work presented and discussed in a related thesis, 29 in which the data presented here is used in combination with in-situ drop measurements and micrographs in order to study the mechanisms of separation.

2. Experimental The objective of this study is to examine the effects of mixing variables on dewatering of a poor-quality, high-water-content bitumen froth. Three variables are of interest in this study: demulsifier bulk concentration, demulsifier injection concentration and mixing energy. The bulk concentration (BC, ppm) is the amount of active ingredient added to the system and it is measured in ppm on a weight basis (per weight of bitumen froth + naphtha). Two mixing variables are used following earlier work in average quality froth23 and in diluted bitumen.24, 25 The first mixing variable is the injection concentration (IC, wt%). This is a measure of the pre5 ACS Paragon Plus Environment

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dilution of the chemical additive. Injection concentration is treated here as a mixing variable because it accelerates dispersion of the active ingredient and limits the maximum local concentration of chemical additive in the feed plume. The other mixing variable of interest is the cumulative, or total mixing energy (J, J/kg),27 which is the product of the mixing time and the power per mass, or the rate of energy dissipation per unit mass, sometimes also called the mixing intensity. Both time and the local rate of energy dissipation help to disperse the chemical additive: the power per mass is an instantaneous condition which may lead to an equilibrium value, but the cumulative energy gives insight into the dynamic portion of the operating curve. Two campaigns were conducted with a similar experimental procedure. The apparatus and general procedure are presented in Section 2 with additional details and results provided in the sections pertaining to each campaign, detailed in Sections 3 and 4. The appropriate bulk concentration for this froth was determined in the campaign described by Section 3. The effects of injection concentration and mixing energy on dewatering and solids removal are studied in the experimental campaign detailed in Section 4. The demulsifier was provided at 35 wt% active ingredient and diluted in xylenes to the desired level (12 or 21 wt%). Cumulative mixing energies of 425 J/kg and 22 778 J/kg were chosen for the low and high levels, respectively. These values replicate levels used by other authors.24, 25 The test vessel used for studying mixing effects is the confined impeller stirred tank, or CIST, which is shown in Figure 1. It is a jacketed cylindrical mixing vessel with a volume of approximately 1L, a tank diameter of 75 mm and a liquid height of 225 mm. Five Rushton impellers (D=T/2) or six Intermig (D=2T/3) impellers provide active circulation throughout the vessel and a more uniform distribution of turbulence than in a stirred tank (Komrakova et al., 2017), while the height of the tank (H=3T) allows for the analysis of settling trends. There are four sampling ports built into the side of the vessel, each with a septum that can be punctured repeatedly without leaking. The geometry of the tank for two different impeller configurations is specified in Table 1. The locations of the side-sampling ports are specified in Table 2. The top surface of the top impeller is a distance S from the liquid surface, the bottom surface of the


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bottom impeller is a distance C from the bottom of the vessel, and the other three impellers are evenly spaced on the impeller shaft. Full details of the geometry and design of the CIST are provided by Komrakova, et al. 21. The bitumen froth was collected from a pilot run in Syncrude Research and was used as supplied. The froth had relatively high water, solids, and fines content, which have been shown to produce relatively poor-quality dilbit product9: this froth is termed a poor-quality froth in this work. All of the samples were taken from the plant within a short period of time to ensure nearly constant composition and ore body properties. Table 3 shows the range of composition of the samples as well as the fluid properties. The nominal composition of the froth is 50% bitumen, 37% water, and 13% solids. The froth was blended with naphtha and the estimated properties of this mixture are shown as well. The experiment is described in four steps, summarized visually in Figure 2. Pre-Mixing The purpose of premixing is to re-suspend and re-disperse any solids and water that may have settled during storage, and to provide the same initial dispersion conditions for each experiment. The sample can containing bitumen froth is similar to a paint can. It has an internal diameter of 0.1 m. After heating the froth to 70°C in an ethylene glycol bath, the can is fitted with a lid with 2 baffles and the contents are mixed with a down-pumping pitched-blade turbine impeller (D = T/2) at 1000 RPM for 15 minutes. By the end of premixing, the bitumen froth reaches approximately 80°C and the water content is the same at all sampling heights. While heating and pre-mixing the bitumen froth, the naphtha diluent is also heated to 80°C in the same ethylene glycol bath so both fluids are already at the required temperature to run the experiments. The mass of naphtha added is equal to 0.7 of the mass of bitumen in the froth based on the nominal composition of 0.5 weight fraction of bitumen in froth. Thus, the mass of naphtha added is equal to 0.35 of the mass of the bitumen froth. After pre-mixing, a sample of approximately 1 mL of bitumen froth is collected for later KarlFisher (KF) and microscope analysis. The required volume of naphtha is poured into the CIST. The bitumen froth is then added until the liquid level reaches the fill height in the CIST. The lid 7 ACS Paragon Plus Environment

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(which is built in two parts) is installed and the impellers turned on to the naphtha blending speed. Naphtha Blending Naphtha is a mixture of light liquid hydrocarbons and it is considerably less viscous and dense than froth bitumen.1 The bitumen froth and naphtha diluent are blended for 2 min. For all experiments, the speed of the impellers is adjusted to attain a nominal energy dissipation rate of 39 W/kg in the impeller swept volume. Note that this calculation assumes 100% of the energy is dissipated in the impeller region. The mixing conditions were estimated based on a kinematic viscosity of 6.1 × 10-6 m2/s for the bitumen froth and naphtha mixture. The mixing conditions are shown in Table 4. After 2 min of mixing, the impellers are stopped, and a 1-mL sample is collected from the topmost side-sampling port, Z1, for later microscopic and KF analysis. Demulsifier Dispersion After the naphtha and froth are blended, the next step is demulsifier dispersion. In this step the demulsifier is added and mixed through the vessel. The BC, IC, and J settings are different for each campaign and this is discussed in further detail in Sections 3.1 and 4.1. The impellers are turned back on at the new speed and the demulsifier is added with a syringe pump just above the top impeller. Figure 3 shows the geometry at the top of the vessel when Rushton impellers are in use. The feed tube and FBRM probe are 30 mm below the liquid surface. The first impeller is 38 mm below liquid surface when using Rushton impellers and 50 mm below the liquid surface when using Intermig impellers. In both cases the submergence is equal to the impeller diameter. The feed tube is plastic tubing with an internal diameter of 3.2 mm and an outer diameter of 6 mm. The FBRM has an outer diameter of 9.5 mm. Insufficient data were collected from focused-beam reflectance measurement (FBRM) to draw conclusions about the system at this time. The injection rate of demulsifier is 125 mL/hr at low J and 635 mL/hr at high J. These injection rates are determined by the local mixing conditions using a formula derived by Chong, et al. 24 in order to avoid a plume of high concentration resulting from inadequate mixing at the feed 8 ACS Paragon Plus Environment

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point. The impellers are run for the rest of the mixing time to blend the additive throughout the vessel. 30 seconds before the end of the mixing time, a 1-mL sample is collected from the top side-port for microscope and KF analysis. After the specified mixing time, the impellers are stopped and the settling begins. Settling The sampling during settling differs significantly between the two experimental campaigns and is discussed in detail in Sections 3.1 and 4.1.

3. Chemical Dosage Determination The effects of mixing variables and bulk concentration have been studied before in diluted bitumen24, 25 and average-quality bitumen froth,23 but never in a poor-quality froth. Before studying mixing effects in this froth, the first step was to determine a bulk concentration (BC) of additive high enough to provide good separation, low enough that mixing effects would be visible, and low enough to avoid the overdosing range. The sole method of analysis in this first stage of the work was Karl Fischer water titration on samples taken from the top sampling port, 52 mm below the liquid surface.

3.1 Experimental Determination of the optimal bulk concentration began with measuring dewatering under conditions of good mixing (high J and low IC) with varying BC. After identifying a likely BC, the experiment was repeated under poor mixing conditions (low J and high IC) to check for a change in separation. These experiments were also conducted with no demulsifier under both mixing conditions to provide a baseline for comparison. Demulsifier Dispersion Experiments in this campaign were done with either good mixing or poor mixing as given in Table 5. The bulk concentrations tested were 0, 25, 73, 100, 120, 149, 171, 193, 196, and 400 ppm.


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Settling Settling begins as soon as the specified mixing time for demulsifier dispersion is complete. When the impellers stop a timer is started and samples are collected from the top sampling port using an autopipette attached to a metal sampling needle. For the first experiment in the campaign, samples were taken after 3, 5, 7, 10, 30, and 60 minutes of settling based on the procedure developed for average-quality froth.23 After the first few experiments the sample times were modified to every 5 minutes from 5-30 minutes, then after 45, 60, 90, and 120 minutes. Settling was much slower than expected in this poorerprocessing froth. The 1-mL samples are analyzed with Karl Fischer titration. The sample is diluted with a 3:1 mixture of high-purity toluene and isopropanol before analysis.

3.2 Results of Dosage Campaign Although the settling time was extended for some experiments, a water content of less than 1% at 60 minutes was still used as the criterion for successful separation. Figure 4 shows the water content after 60 minutes of settling as the bulk concentration (BC) was varied under good mixing conditions. The final water content after 60 minutes of settling decreased linearly as the bulk concentration was increased under good mixing conditions. The only exceptions to this downward trend were an experiment done with no demulsifier, and an experiment done with a very high dosage of demulsifier to test for overdosing. A bulk concentration of 200 ppm under good mixing conditions gave separation to less than 1% water after 60 min of settling. At the same concentration with poor mixing conditions, the water content at 60 min was 4.5%. This met the objectives identified for the first stage of study, so a bulk concentration (BC=200 ppm) was selected for the rest of the study. In Figure 5 the water content vs. time is plotted for one experiment with poor quality froth, along with an experiment on average-quality froth for comparison. In a previous study with average quality froth23, the water content dropped quickly in the first ten minutes of settling, then slowly in the period from 10 – 60 minutes. The settling proceeded differently in this high10 ACS Paragon Plus Environment

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water-content, poor quality froth: in many experiments, the water content dropped slowly or not at all until 20 – 45 minutes had passed. The water content then dropped quickly during a fast settling period followed by a period of slow settling. All experiments with demulsifier under good mixing conditions followed this pattern. The period during which there is little or no dewatering has been termed the induction time. Figure 6 shows four runs: two that were done with no demulsifier at good and poor mixing conditions, and two that were done at 196/193 ppm with good and poor mixing. The runs with no demulsifier did not exhibit an induction time, so we conclude that the induction time is caused by the demulsifier. The run with demulsifier but poor mixing also did not exhibit an induction time because the demulsifier was not successfully dispersed. Because induction time is connected to demulsifier addition and dispersion, the induction time was monitored more closely in the second campaign while varying mixing effects. With this protocol we successfully identified an appropriate bulk concentration level (200 ppm) at which to study mixing effects. The bulk concentration of additive used in industry is often many times lower than that recommended based on lab studies. This substantial difference between the lab and industrial scale operation merits further study. The induction time is also observed during the next campaign while studying the effects of mixing variables.

4. Analyzing Effects of Mixing After selecting a suitable bulk concentration at which to study mixing effects in this high-watercontent, poor-quality froth the second part of this work set out to test the mixing variables J and IC in a full factorial design with two replicates. Experiments in this campaign used several analytical methods: Dean-Stark extraction to determine oil, water and solids content at the end of the run, and sampling at all 4 heights at various times during the run followed by Karl Fisher water content titration and microscopy. The induction time discovered in the first campaign will be observed and the results will be interpreted considering this interesting phenomenon. Although an FBRM probe was used successfully, the results are not discussed here. Microscope 11 ACS Paragon Plus Environment

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data was also collected but is not discussed in detail: a more comprehensive discussion can be found in the related thesis29.

4.1 Experimental This campaign used a replicated full-factorial design to test the effects of mixing energy and injection concentration. The bulk concentration was fixed at 200 ppm. The mixing variable levels and factorial codes are given in Table 6. The overall experimental procedure was described in detail in Section 2. This section gives details specific to this campaign.Demulsifier Dispersion In this step the mixing energy and injection concentration are varied while the bulk concentration is held constant. The mixing energy and injection concentration are adjusted to the conditions required by the factorial experimental design shown in Table 6. Details of mixing for the high J and low J conditions are given in Table 7. The injection concentration is varied simply by diluting the chemical additive to either 12 wt% or 21 wt% in xylenes before injection. Xylenes were used as provided by Fisher Scientific at Certified ACS grade. Settling Settling begins as soon as the mixing time for demulsifier dispersion has finished and the impellers stop. A timer is started to monitor the settling time. The first two experiments (coded FA-1 and FB-1) settled for 60 min. After observing that steady state was not reached after 60 min, settling was allowed to continue to 120 minutes for FC-1, FD-1, and the full set of replicates. FA-1 and FB-1 could not be repeated to obtain the last two timepoints because additional bitumen froth samples were not available. Throughout the settling time, 1 mL samples are collected from the four sampling ports using an autopipette attached to a metal sampling needle which pierces the septum in the sampling port. The sampling times are every 5 minutes from 5-30 minutes, then at minutes 45, 60, 90, 120. These samples are all collected at the same angular position and radially halfway between the axis and the wall of the vessel. 12 ACS Paragon Plus Environment

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At the end of settling, large samples are collected with 100 mL syringes and tubing at relative heights of 0.1, 0.5, and 0.9 below the liquid surface. These samples were sent for Dean Stark extraction, which gives the relative compositions of oil, water and solids. These three points were chosen to give a picture of the final, settled profile of water and solids in the tank. Because the sample size is much larger than the small samples taken from the side ports, the water content provided by this method is less sensitive to small local variations in composition. The 1-mL samples are analyzed with microscopy and Karl Fischer titration. A few droplets are collected with a 3 mL plastic syringe and deposited on a slide for microscope observation, with the rest diluted with a 3:1 mixture of high-purity toluene and isopropanol to be used for Karl Fisher titration.

4.2 Results of Mixing Campaign 4.2.1 Dean Stark Water and Solids Content Dean Stark extraction allows determination of the composition of oil, water, and solids content. Figure 7 shows the results for the top 100 mL of the vessel after settling time. There is very good correlation between the water and solids content at this sampling point. It is clear from this data that good mixing conditions are associated with low water content at the top of the vessel. The experiments with two good mixing conditions have the lowest water and solids content, and the experiments with the worst mixing conditions have the highest water and solids content. The experiments with one good mixing condition have intermediate water and solids content. The Dean Stark extraction results show that high mixing energy and low injection concentration lead to good dewatering and solids removal. Multivariate analysis of variance (MANOVA) revealed that both injection concentration and mixing energy have a statistically significant effect (p < 0.05) on the water and solids content at the top height after settling. High mixing energy and low injection concentration are associated with low water content and low solids content at the top of the vessel. This matches previous findings in both diluted bitumen24, 25 and average-quality bitumen froth.23


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Samples were taken for Dean Stark extraction at 50% and 90% of the vessel height as well, but no statistically significant effects were found, nor any other discernable pattern in the results. Regardless of mixing conditions, all experiments except one outlier had approximately 42% water content at the bottom of the vessel. The injection concentration and mixing energy did not have a detectable effect on the water content at the very bottom of the vessel after settling. The importance of mixing energy and injection concentration are clear from performing Dean Stark extraction on the top section of the vessel: both mixing variables have a strong effect on both the final water content and final solids content. High mixing energy and low injection concentration both led to successful dewatering and solids removal. 4.2.2 Water Content Time Trends The water content was measured at four heights over the course of the experiment using Karl Fischer titration. These time trends will be analyzed to determine the efficacy of the demulsifier, as well as any effect the mixing conditions may have on the induction time identified in the previous campaign. Product Layer Figure 8 shows the water settling at all heights. The trends of water content at Z1 (52 mm below the liquid surface) in Figure 8a show induction times where little to no dewatering occurs. The induction time is clearly present in the experiments at low injection concentration (FA and FD), but is less pronounced in the experiments at high injection concentration (FB, FC). The induction time pattern is least pronounced in FC experiments, which are those done at the worst mixing conditions: low mixing energy and high injection concentration. This settling pattern looks like that of an experiment with no demulsifier, as seen in Figure 6: steady reduction in water content with a high final value of 4%. Having both low energy and high injection concentration leads to poor demulsifier performance. The experiments with high mixing energy and low injection concentration (FD) – the best set of mixing conditions – exhibit the longest induction time with the sharpest drop in water content. 14 ACS Paragon Plus Environment

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These same experiments have the lowest water content after 60 minutes of settling — approximately 1.5% — despite having the longest induction time. After 120 minutes, all experiments but FC (poor mixing conditions) settled to less than 1% water at the top height. The difference in final water concentration between the good and poor mixing conditions is more than 3 wt%. Figure 8b shows the water content at height Z2 over time. The water content does not drop until very late in the settling period. The experiments with low injection concentration again have the longest induction times – the induction time at Z2 is 90 minutes for experiments FA and FD. Bottom Layer The water profile at height Z3, 140 mm below the liquid surface, is shown in Figure 8c. No statistically significant correlations between the water content at Z3 and either J or IC were found. In this part of the vessel, the water content varies between 25 – 32% water and remains high for the duration of settling. In average-quality froth, Z3 consistently showed water content less than 5% after 60 minutes of settling.23 In low quality froth, the water-rich layer extends up to Z3 regardless of the mixing conditions. Figure 8d shows the water content over time at the bottom sampling point, 184 mm below the liquid surface and 40 mm above the bottom of the vessel. Note that the data from Karl Fischer titrations only go to 60 minutes. The final data point from Dean Stark extraction is used to supplement understanding. The water content increases over the course of settling, as expected. The water content at the bottom converges on approximately 42 wt% water after 120 minutes of settling. The water content is limited in part because there is no water-continuous layer. In microscope images like Figure 9, the second-lowest layer (Z3) appears to consist of tightly packed, nonspherical water droplets with skins of material preventing coalescence. No micrographs were taken at the bottom height. During sampling and cleaning the bottom layer was observed to be completely opaque and brown in colour and have an oily and gritty consistency.


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Figure 10 compares the water content at the end of settling to the induction time, which was defined as the time point at which there was a drop in water content of 20% or more. The outlier (FA-2, 60 min) was not included in the regression. The final water content after settling shows a small but consistent negative correlation with the induction time. There appears to be a trade-off between settling time and the final dewatering performance in poor-quality froth.

5. Conclusions The low-quality bitumen froth examined in this work was prone to a phenomenon which was termed induction time. This is a period at the beginning of settling in which the water content appears to hold steady for more than 20 minutes before declining rapidly. The induction time can extend as long as 45 minutes. This contrasts with average-quality froth which settles quickly within the first 10 minutes in lab studies.23 This induction time appears to be linked to the addition of demulsifier in poor-quality froth, since experiments with no demulsifier did not exhibit an induction time. Experiments with demulsifier but very poor mixing conditions also did not exhibit an induction time, suggesting that the demulsifier was not being dispersed effectively. The presence of an induction time may confound results in settling studies of bitumen froth treatment and lead to difficulties in processing. As in other studies of mixing in naphthenic froth treatment, mixing energy and injection concentration are important variables for the dewatering and solids-removal performance. High mixing energy and low injection concentration lead to low water content at the top of the settling vessel after 60 minutes of settling. After 120 minutes of settling, even one of these conditions was sufficient to obtain low water content. As in previous studies, solids content at the top height correlated with water content after settling.


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6. Acknowledgements The authors would like to thank NSERC (CRDPJ 485317) and Syncrude Canada Ltd. (SCL 485317) for financial support. We would also like to thank Eva Liu for her collaboration with the experiments.

Nomenclature BC CIST CLD dp dpipe D FA FB FBRM FC FD H IC J MANOVA N Np OWS P Re S tmix t VIMP V X z Z β ε εmax μ ν ρ

bulk concentration (demulsifier dosage, ppm) confined-impeller stirred tank chord-length distribution particle/droplet diameter (m) feed pipe diameter (m) impeller diameter (m) factorial run A, both variables low. Number denotes replicate. factorial run B, both variables high focused-beam reflectance measurement factorial run C, low J high IC factorial run D, high J low IC liquid height (m) injection concentration (wt%) energy input (tmix × ε, J/kg) multivariate analysis of variance rotational speed (rpm) power number of impeller (dimensionless) oil, water, and solids as detected by Dean Stark extraction power delivered by impeller(s) (W) Reynolds number (dimensionless) impeller submergence (m) mixing time (min) settling time (min) impeller-swept volume (m3) tank volume (m3) coded variable: +1 for high level, -1 for low level height coordinate, as measured from liquid surface down (mm) sampling height code. Z1 is top, Z4 is bottom. regression coefficient energy dissipation (W/kg) maximum energy dissipation (W/kg) dynamic viscosity (Pa∙s) kinematic viscosity (m2/s) density (kg/m3) 17 ACS Paragon Plus Environment

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References 1. Tipman, R., Froth Treatment. In Handbook on Theory and Practice of Bitumen Recovery from Athabasca Oil Sands. Volume II: Industrial Practice, Masliyah, J.; Xu, Z.; Dabros, M., Eds. Kingsley Knowledge Publishing: Canada, 2013. 2. Shelfantook, W. E., A Perspective on the Selection of Froth Treatment Processes. The Canadian Journal of Chemical Engineering 2004, 82, (4), 704-709. 3. Masliyah, J. H.; Czarnecki, J.; Xu, Z., Handbook of Theory and Practice of Bitumen Recovery from Athabasca Oil Sands. Volume I: Theoretical Basis. Kingsley: Canada, 2011. 4. Eley, D. D.; Hey, M. J.; Symonds, J. D., Emulsions of water in asphaltene-containing oils 1. Droplet size distribution and emulsification rates. Colloids and Surfaces 1988, 32, 87-101. 5. Kilpatrick, P. K., Water-in-Crude Oil Emulsion Stabilization: Review and Unanswered Questions. Energy & Fuels 2012, 26, (7), 4017-4026. 6. Gray, M.; Xu, Z. H.; Masliyah, J., Physics in the oil sands of Alberta. Physics Today 2009, 62, (3), 31-35. 7. Sullivan, A. P.; Kilpatrick, P. K., The Effects of Inorganic Solid Particles on Water and Crude Oil Emulsion Stability. Industrial & Engineering Chemistry Research 2002, 41, (14), 33893404. 8. Romanova, U. G.; Valinasab, M.; Stasiuk, E. N.; Yarranton, H. W.; Schramm, L. L.; Shelfantook, W. E., The Effect of Oil Sands Bitumen Extraction Conditions on Froth Treatment Performance. Journal of Canadian Petroleum Technology 2006, 45, (9), 36-45. 9. Romanova, U. G.; Yarranton, H. W.; Schramm, L. L.; Shelfantook, W. E., Investigation of Oil Sands Froth Treatment. The Canadian Journal of Chemical Engineering 2004, 82, (4), 710721. 10. Yang, X. L.; Czarnecki, J., The effect of naphtha to bitumen ratio on properties of water in diluted bitumen emulsions. Colloids and Surfaces a-Physicochemical and Engineering Aspects 2002, 211, (2-3), 213-222. 11. Pensini, E.; Harbottle, D.; Yang, F.; Tchoukov, P.; Li, Z.; Kailey, I.; Behles, J.; Masliyah, J.; Xu, Z., Demulsification Mechanism of Asphaltene-Stabilized Water-in-Oil Emulsions by a Polymeric Ethylene Oxide–Propylene Oxide Demulsifier. Energy & Fuels 2014, 28, (11), 67606771. 12. Chen, Q.; Stricek, I.; Gray, M. R.; Liu, Q., Influence of hydrophobicity distribution of particle mixtures on emulsion stabilization. J Colloid Interface Sci 2016, 491, 179-189. 13. Rao, F.; Liu, Q., Froth Treatment in Athabasca Oil Sands Bitumen Recovery Process: A Review. Energy & Fuels 2013, 27, (12), 7199-7207. 14. Kailey, I., Key Performance Indicators Reveal the Impact of Demulsifier Characteristics on Oil Sands Froth Treatment. Energy & Fuels 2017, 31, (3), 2636-2642. 15. Shehzad, F.; Hussein, I. A.; Kamal, M. S.; Ahmad, W.; Sultan, A. S.; Nasser, M. S., Polymeric Surfactants and Emerging Alternatives used in the Demulsification of Produced Water: A Review. Polymer Reviews 2018, 58, (1), 63-101. 16. Salager, J.-L., Fundamental basis for the action of a chemical dehydrant. Influence of the physical and chemical formulation on the stability of an emulsion. International Chemical Engineering 1990, 30, (1), 103-116. 18 ACS Paragon Plus Environment

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17. Delgado-Linares, J. G.; Alvarado, J. G.; Vejar, F.; Bullon, J.; Forgiarini, A. M.; Salager, J. L., Breaking of Water-in-Crude Oil Emulsions. 7. Demulsifier Performance at Optimum Formulation for Various Extended Surfactant Structures. Energy & Fuels 2016, 30, (9), 7065-7073. 18. Leng, D. E.; Calabrese, R., Immiscible Liquid-liquid systems. In Handbook of Industrial Mixing: Science and Practice, Paul, E.; Atiemo-Obeng, V. A.; Kresta, S. M., Eds. Wiley: 2004. 19. Machado, M. B.; Bittorf, K. J.; Roussinova, V. T.; Kresta, S. M., Transition from turbulent to transitional flow in the top half of a stirred tank. Chemical Engineering Science 2013, 98, 218230. 20. Machado, M. B.; Kresta, S. M., The confined impeller stirred tank (CIST): A bench scale testing device for specification of local mixing conditions required in large scale vessels. Chemical Engineering Research & Design 2013, 91, (11), 2209-2224. 21. Komrakova, A. E.; Liu, Z.; Machado, M. B.; Kresta, S. M., Development of a zone flow model for the confined impeller stirred tank (CIST) based on mean velocity and turbulence measurements. Chemical Engineering Research & Design 2017, 125, 511-522. 22. Bittorf, K. J.; Kresta, S. M., Active volume of mean circulation for stirred tanks agitated with axial impellers. Chemical Engineering Science 2000, 55, (7), 1325-1335. 23. Arora, N. Mechanisms of Aggregation and Separation of Water and Solids from Bitumen Froth using Cluster Size Distribution. University of Alberta, Edmonton, AB, Canada, 2016. 24. Chong, J. Y.; Machado, M. B.; Arora, N.; Bhattacharya, S.; Ng, S.; Kresta, S. M., Demulsifier Performance in Diluted Bitumen Dewatering: Effects of Mixing and Demulsifier Dosage. Energy & Fuels 2016, 30, (11), 9962-9974. 25. Laplante, P.; Machado, M. B.; Bhattacharya, S.; Ng, S.; Kresta, S. M., Demulsifier performance in froth treatment: Untangling the effects of mixing, bulk concentration and injection concentration using a standardized mixing test cell (CIST). Fuel Processing Technology 2015, 138, 361-367. 26. Chong, J. Y.; Machado, M. B.; Bhattacharya, S.; Ng, S.; Kresta, S. M., Reduce Overdosing Effects in Chemical Demulsifier Applications by Increasing Mixing Energy and Decreasing Injection Concentration. Energy & Fuels 2016, 30, (6), 5183-5189. 27. Machado, M. B.; Kresta, S. M., When Mixing Matters: Choose Impellers Based on Process Requirements. Chemical Engineering Progress 2015, 111, (7), 27-33. 28. Liné, A., Energy consumption to achieve macromixing revisited. Chemical Engineering Research and Design 2016, 108, 81-87. 29. Saraka, C. Mixing and Settling Characterization in Low-Quality Bitumen Froth Treatment. University of Alberta, Edmonton, Canada, 2017.


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Tables and Figures

Table 1. Vessel geometry for two different impeller configurations. Intermigs are used for low mixing intensity, and Rushtons are used for high mixing intensity. Impeller

Intermig

Rushton

T (m) number of impellers H D C S

0.0762 6 3T 2T/3 D/3 D

0.0762 5 3T T/2 D/3 D

Table 2. Sampling port locations using the liquid surface as reference plane. Total liquid height is 225 mm. The angular location of the ports is at the mid-baffle plane. Sampling Point Height below Surface (mm)

Z1 Z2 Z3 Z4

52 96 140 184


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Table 3. Range of compositions of bitumen froth and fluid properties of bitumen froth and froth/naphtha blend. Properties of bitumen froth were provided by Syncrude Research. Properties of blend estimated from dilbit measured at 80°C (Chong et al., 2016a). Fluid mixture

Properties

Bitumen Froth

composition Bitumen (wt%)

48.8 - 50.3

Water (wt%)

36.0 - 39.2

Solids (wt%)

11.4 - 13.6

viscosity (mPa∙s) at 60°C

5900

viscosity (mPa∙s) at 80°C

1100

density (kg/m3)

1140

Bitumen Froth +

naphtha to bitumen ratio (wt basis)

0.7

Naphtha Blend

estimated viscosity (mPa∙s)

7.1

estimated density (kg/m3)

860

Table 4. Mixing conditions during naphtha blending. The impeller speed is set to give the same power per mass in the impeller swept volume for all runs. Impeller

Intermig

Rushton

N (rpm) Re single-impeller Np power per mass (W/kg) impeller swept volume (L)

1060 7240 0.63 6.55 0.168

600 2367 4.2 1.67 0.0431

power per mass in impeller swept volume (W/kg)

38.8

38.6

mixing time (min) mixing energy (J/kg)

2 4652

2 4633


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Table 5. Summary of mixing conditions used to select bulk concentration. Mixing Condition

IC (wt %) J (J/kg) BC (ppm)

good mixing (RT)

12

22778 varied

poor mixing (Intermig) 21

425

Table 6. Variable levels for 2-level full factorial design in two factors: mixing energy and injection concentration. Two replicates of each of the four experiments were completed in randomized order. Factorial Factor Level (X) -1

1

J (J/kg)

425

22778

IC (wt%)

12

21

BC (ppm)

200 (fixed)

Table 7. Mixing conditions during demulsifier dispersion. Rushton impellers are used for high J, while Intermigs are used for low J. Impellers

Intermig

Rushton

Run codes

FA, FC

FB, FC

N (rpm)

400

600

Re

2732

2367

single-impeller Np

1.5

4.2

power per mass (W/kg)

0.84

1.67

power per mass in swept volume (W/kg)

4.96

38.61

mixing time (min)

2

10

J (J/kg)

425

22778


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Figure 1. Jacketed CIST showing 4 side-sampling ports, Rushton impellers and shaft, baffle assembly, feed injection port, and FBRM probe.


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Figure 2. Visual summary of experimental apparatus and procedure. Step 1 takes place in a glycol heating bath. Steps 2-4 all take place in the mixing test cell, which allows for well-controlled mixing and for sampling during settling.

Figure 3. Geometry of upper portion of mixing vessel, showing the relative positions of the feed tube, FBRM probe, and top impeller (using Rushton impellers).


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Figure 4. Final water content as bulk concentration is varied. A water content of 1% (dashed line) or less is considered successful.

Figure 5. Illustration of dewatering process with 3 stages significant induction time in low quality froth. Average-quality froth is shown for comparison (Arora, 2016).


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Figure 6. Comparison of good and poor mixing conditions with no demulsifier (0 ppm) and with a target of 200 ppm. The experiment with demulsifier and good mixing provides the lowest final water content. The other three experiments also do not exhibit an induction time.


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Figure 7. Dean Stark results for water and solids content in the top sample (h/H = 0.1) after settling (120 min except where noted) for all experiments. Good mixing conditions are denoted (+ -); bad mixing as (- +).


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Figure 8. Water content vs. settling time at (a) Z1, 52 mm below liquid surface (b) Z2, 96 mm below liquid surface (c) Z3, 130 mm below liquid surface and 84 mm above bottom (d) Z4, 184 mm below liquid surface and 40 mm above bottom.


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Figure 9. Micrograph of sample taken after 60 minutes of settling at second-lowest height Z3, 84 mm above the bottom of the vessel. Micrographs were not taken at the lowest sampling height Z4. The morphology of water droplets at this height, as well as observations in the lab and composition information obtained from Dean Stark extraction, indicate the bottom of the settling vessel is characterized by a tightly packed configuration of water droplets that are protected from coalescence by skins of material.


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Figure 10. Induction time compared to water content after 60 and 120 min of settling. After settling, water content in the product layer correlates with induction time.


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Dewatering of Poor-Quality Bitumen Froth: Induction Time and Mixing

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