Article pubs.acs.org/IECR
Aging of Water from Steam-Assisted Gravity Drainage (SAGD) Operations Due to Air Exposure and Effects on Ceramic Membrane Filtration Anthony Y. Ku,*,† Claire S. Henderson,† Matthew A. Petersen,† David J. Pernitsky,‡ and Annie Q. Sun‡ †
GE Global Research, 1 Research Circle, Niskayuna, New York 12309, United States Suncor Energy Inc., 150−6 Ave. SW, Calgary, Canada, T2P 3E3
‡
S Supporting Information *
ABSTRACT: The performance of first generation steam assisted gravity drainage (SAGD) plants for bitumen recovery has improved through operational experience, but there remain ample opportunities for the introduction of technologies that can further improve energy efficiency and plant reliability. Laboratory testing and validation is an important initial step in technology development. A major factor that determines the applicability and validity of testing results is the integrity of process water samples used in lab and field studies for this purpose. The results presented in this paper demonstrate aging of SAGD process water and its direct implication on membrane performance testing. Aging in samples collected after primary bitumen/water separation occurred mainly through reactions of dissolved organic species with air. This resulted in a gradual change in appearance, accompanied by a significantly higher tendency to foul membranes in dead-end filtration tests. The root cause for this change was proposed to be the reaction of phenolic species with oxygen, leading to more compressible and tightly packed filter cakes on the membrane surface. This effect was mitigated by minimizing air exposure during sample collection and handling. These results establish that preventing oxygen exposure to the sample is critical for maintaining sample integrity during a test program. Although this study focuses on filtration, aging effects can also lead to misleading results in laboratory testing of other water treatment processes and must be carefully considered during technology evaluation and development.
■
INTRODUCTION The development of water treatment technologies for the oil sands industry is motivated by both environmental stewardship and economic factors. Large quantities of water are processed in both surface mining (through tailing ponds) and in situ processes. Surface mining processes use hot alkaline water washing to separate the bitumen (oil) from the sand. Run-off water is collected in tailings ponds, which have been the focus of intense regulatory scrutiny.1 In situ recovery processes such as steam-assisted gravity drainage (SAGD) use injected steam to mobilize bitumen.2 In contrast to surface mining operations, SAGD processes recycle water from the produced fluid mixture. Water and bitumen are separated at a centralized processing facility, and the water is treated to remove oil and salts. Steam is generated from the treated water and injected into the reservoir to recover additional oil. Makeup water is added to compensate for losses to the reservoir, as well as blowdown from the treatment processes. Effective solutions for water reuse in in situ processes can reduce the need for makeup water, while also reducing waste disposal costs. The water treatment train in a conventional SAGD design involves several sequential stepsoil removal, softening, and blowdown/concentrate management. Examples of technologies used in first generation SAGD water treatment trains include chemical additives for emulsion breaking, skim tanks, flotation cells, oil removal filters, warm lime softeners, ion exchange columns, thermal evaporators, and crystallizers.3 The performance of first generation plants has improved through operational experience, but there remain ample opportunities for the © 2012 American Chemical Society
development of technologies that can further improve energy efficiency and plant reliability. As the industry matures, additional progress in the areas of process optimization and environmental impact will require increasing insight into the chemistry of the water and its interaction with various unit operations. Laboratory testing and validation is an important initial step in technology development. Experiments performed in controlled lab environments can establish the merits and limitations of new approaches and provide guidance in the design of field pilot tests. These tests can be performed using water samples collected onsite and shipped to a lab, or with simulated water prepared from an analysis of the components. In both cases, it is critical to take steps to ensure the behavior observed in the lab is representative of what will occur in the field. One complication regarding the use of samples shipped from the field is the potential for aging to alter the physicochemical properties leading to spurious results in off-site testing. This is a significant challenge in the handling of crude oils where depressurization and microbial activity are known to alter sample chemistry.4,5 Nonrepresentative lab results can lead to poor design of piloting studies, resulting in lost time and wasted Received: Revised: Accepted: Published: 7170
March 1, 2012 May 4, 2012 May 9, 2012 May 9, 2012 dx.doi.org/10.1021/ie3005513 | Ind. Eng. Chem. Res. 2012, 51, 7170−7176
Industrial & Engineering Chemistry Research
Article
than the tens of hours associated with the SAGD process. Third, tailings pond water is at ambient temperature, whereas SAGD water is maintained at high temperatures to minimize the thermal input required to raise steam. Finally, the nature and quantity of production chemicals may differ, reflecting differences in the nature of mining and in situ recovery processes. As with tailings pond water, advanced analytical tools are becoming available to better understand the water chemistry and its evolution in SAGD processes.12 This paper examines aging in SAGD process water and how it affects lab testing for a potential water treatment technology. A series of experiments was performed to characterize the effects of atmospheric exposure on water physicochemical properties and link the changes observed to ceramic membrane testing results. Fouling behavior in dead-end ceramic membrane tests was further evaluated in order to better understanding the fundamental changes that occurred to the water sample as it was aged. Advanced analytical techniques were leveraged in order to isolate the specific chemical species responsible for the increased fouling tendency. The results of this research are intended to better inform sample handling and technology development practices in the oilfield produced water industry.
resources. In addition, false negatives can result in the disqualification of potentially viable candidate technologies. Water can also be simulated by artificially mixing the constituent components found in the field. While this can be simpler logistically, the different processing history of the simulated sample can sometimes result in behavior that is not representative of what can be expected to occur in the field. Practical experience has shown this to be a significant concern in the development of emulsion breaker chemical flocculants and other production chemicals.6,7 Aging of produced water samples is a complex process, involving a variety of possible mechanisms operating at multiple time scales. Short-term effects can be triggered by the pressure changes or cooling of the sample, leading to loss of volatile components or solubility-driven precipitation of some components. Alternately, on longer time scales, effects such as microbial activity result in the selective loss of biodegradable compounds. The exact nature of the aging often varies with the nature of the source water and can manifest as changes in the sample composition, physicochemical properties, and responses to separation processes. The issue of sample aging in oil sands operations was first explored in the 1980s in the context of bitumen recovery from mined material. Syncrude reported that the longer samples were stored, the more caustic addition was required to achieve a target level of bitumen recovery in a hot water flotation process.8,9 Their study attributed this to lower levels of natural carboxylate surfactant formation from the alkaline treatment and noted the effect seemed to apply across a wide range of oil sand grades. They deduced that the root cause was the formation of polyvalent metal ions from mineral oxidation, leading the nonproductive consumption of caustic and altered free surfactant equilibria. Other mechanisms such as dehydration of the sample, direct oxidation of carboxylate groups by exposure to air, and loss of volatile components were also considered, but the experimental data indicated that they played only a minor role in aging. This study noted that aging effects could be monitored by tracking the amount of sulfate in the sample, reversed through the introduction of chelating agents during processing, or arrested by excluding air from the mining process. Density differences can also play a role in aging. Higher density components such as sand and clay particles settle, whereas lighter components such as oil droplets rise. Aging in the context of gravity-based effects can alter the distribution of particles, resulting in a distribution increasingly skewed in favor of smaller particles. Second-order effects involving other mechanisms can cause aggregation, which leads to greater settling rates. For example, microbial activity in tailings ponds can promote settling through the secretion of extracellular polymeric substances, which serve to flocculate the clay particles.10 There is also considerable evidence from the broader petroleum industry that gravity-driven effects can skew laboratory results for emulsion breaking experiments.11 These effects may play in a role in evaluating technologies that are based on size separation or gravity separation processes. Aging effects in water from in situ processes are less welldocumented. Water sampled from SAGD water process trains differ from tailings pond water in several important ways. First, tailings ponds are open to air, whereas the oxygen content in SAGD process water is controlled to prevent corrosion problems in the boiler. Second, the residence times in tailings ponds are on the order of years, which is significantly longer
■
MATERIALS AND METHODS Water Sample Collection. Samples were collected from a SAGD plant operating in Northern Alberta, Canada. Water samples were collected at a point in the process following freephase bitumen/water separation at the outlet of a skim tank. The water had not been processed through a deoiling plant, therefore colloidal oil droplets were assumed to be contained within the sample matrix. The water was collected directly from process sample taps in 4 L glass bottles or 55 gallon epoxy-lined drums without the use of a sample cooler. The sample containers were filled completely and immediately sealed. The samples were shipped by ground freight to the GE site in Niskayuna, NY. Water Handling and Storage. Upon receipt, the drums were opened under a nitrogen blanket and samples were transferred to clean glass bottles for storage. The headspace in the bottles was purged with nitrogen, and the bottles were completely filled and then sealed. Long-Term Aging. Long-term aging of the water was performed under two atmospheric conditions. SAGD process water was transferred from a storage drum into 10 L clear glass containers and the headspace was filled with air or nitrogen. Samples were allowed to age under quiescent conditions for 22 weeks. The headspace of each bottle was flushed with the appropriate gas for approximately 10−15 min once a week. Accelerated Aging Experiments. Gas was bubbled through water samples in an effort to accelerate the effects of atmospheric exposure. In one set of experiments, air was bubbled through the water using a pipet connected to a plastic tube at a rate of approximately 420 mL/min. Aliquots were collected at intervals of 30 min, 2, 4, 8, and 24 h. The dissolved oxygen (DO), pH, specific conductivity, oxidation reduction potential (ORP), and total organic carbon (TOC) were measured immediately upon collection. A control experiment was performed in which nitrogen was bubbled through the water sample, instead of air. Water samples subjected to bubbling were analyzed or used for filtration experiments immediately after collection. 7171
dx.doi.org/10.1021/ie3005513 | Ind. Eng. Chem. Res. 2012, 51, 7170−7176
Industrial & Engineering Chemistry Research
Article
Filtration Experiments. Ceramic membrane filtration experiments were performed using ceramic anodic aluminum oxide (AAO) membrane discs (Anodisc 25 0.1 μm, Whatman). A stirred ultrafiltration cell (Model 8010, Amicon) was used to perform dead-end filtration tests. In a typical experiment, the membrane was mounted in the test cell and the fluid reservoir filled with deionized water. The cell was pressurized to approximately 40 kPa (6 psi) using compressed nitrogen. Permeate was collected in a bottle on a balance, and the cumulative mass was recorded as a function of time to determine the clean water flux of the membrane. The clean water flux for the membranes used in this study ranged from about 1200 to 1600 L/(m2 h) (LMH). After this initial measurement, the test cell fluid reservoir was refilled with SAGD process water and the measurement process repeated. The test cell was refilled with water whenever the fluid level dropped below about one-third of the initial full volume. Care was taken not to disturb the fouling cake layer on the membrane surface during refilling. The test was continued until the flux dropped below about a quarter of the initial value. After the experiment, the filters were recovered and allowed to dry in air overnight. The solids loading of the water was computed from the mass difference of the filter before and after testing, and the total filtered volume. Dynamic light scattering (DLS) measurements of the permeate were used to confirm the successful removal of particles larger than the pore size. Three separate sets of filtration experiments were performed for the study. The first two sets were conducted using clean ceramic membranes with unaged water (shipped in completely filled containers), aged water, aerated water (accelerated aging), and nitrogen-bubbled water. A third set was conducted to better elucidate the critical fouling processes. Specifically, whether components of the water adsorb to the inner surface of the membrane pores and lead to more rapid flux declines. These tests were conducted with membranes that were presoaked in unaged and aged process water. The membrane was retrieved at the end of the desired soak time and immediately loaded into the ultrafiltration cell. Testing was performed using unaged process water without allowing the membrane to dry. Analytical Techniques. Specific conductivity measurements were performed using an Oakton CON 5 Acorn Series conductivity meter with 1 mS/cm solution as standard. Total organic carbon (TOC) measurements were performed using two different methods based on their local availability. TOC was measured shortly after sample collection using the persulfate-UV method.13 TOC was measured at the GE site using a Shimadzu TOC-5050 TOC analyzer, which is based on the high temperature catalytic combustion method. The pH was measured using an Orion pH probe calibrated using pH 4, 7, and 10 standard solutions. Dissolved oxygen measurements were performed with an Ocean Optics optical DO probe factory calibrated from 0 to 10 ppm over 10−30 °C. Oxidation−reduction potential (ORP) measurements were performed with an Orion ORP meter with platinum electrode, calibrated with Zobell’s solution.14 DLS measurements were performed on the filtrate using a Brookhaven Instruments ZetaPALS instrument operated in DLS mode. Dissolved organic components were analyzed using a twodimensional gas chromatography with time-of-flight mass spectrometry (2DGC-MS) technique. The details of the sample preparation, techniques, instrumentation, and data interpretation are presented elsewhere.12 Samples were
acidified using hydrochloric acid (ACS Plus grade, Fisher) and extracted using methylene chloride (ACS grade, Fisher). The extracts were analyzed on the 2DGC-MS. Scanning electron microscopy (SEM) images were collected using a Zeiss Supra 55VP system scanning electron microscope, equipped with a ThermoFisher Scientific energy dispersive Xray (EDX) spectrometry system. Platinum was sputtered onto the samples prior to imaging to prevent excessive charging. Images were collected at 5 kV.
■
RESULTS AND DISCUSSION Changes in Appearance. The appearance of SAGD process water sampled downstream of a skim tank had a slightly turbid, yellow appearance at the collection tap. Samples transferred to an on-site lab and stored in covered beakers, which allowed for exposure of the fluid to the atmosphere, gradually darkened in color within two days. Samples stored in containers that were completely filled and immediately sealed retained the yellow appearance for several days. Once these samples were transferred to beakers and exposed to air, the color darkened suggesting the color change was due to air exposure rather than cooling. Water collected in epoxy-lined drums that were completely filled and immediately sealed remained yellow in color, even after several weeks’ transit time to the off-site laboratory. Samples transferred to open beakers and stored for two days at the off-site laboratory darkened in a manner consistent with the behavior observed on-site. Samples stored for several months under an air headspace continued to darken, turning dark brown after a few weeks. Control samples stored under a nitrogen headspace did not change color for several months. Figure 1 shows the appearance of samples examined at the onsite lab, after shipment to an off-site lab, and after extended aging under controlled air and nitrogen headspaces. The presence of air appears to be a factor that is critical for darkening.
Figure 1. Appearance of water under various storage, handling, and aging conditions. On-site lab conditions include the following: (a) immediately after collection, (b) after aging for two days in a sealed bottle with no headspace, and (c) after aging for two days in an unsealed bottle. Samples were shipped to an offsite location in sealed containers. Images of the water were collected: (d) immediately after receipt, (e) after aging for two days in a sealed bottle, (f) after aging for two days in an unsealed bottle, (g) after aging for 22 weeks under a periodically purged nitrogen headspace, and (h) after aging for 22 weeks under a periodically purged air headspace. 7172
dx.doi.org/10.1021/ie3005513 | Ind. Eng. Chem. Res. 2012, 51, 7170−7176
Industrial & Engineering Chemistry Research
Article
Changes in Water Properties. Beyond the appearance change, sample physicochemical property transformation over time, and the impact of those transformations on behavior in filtration tests, was further evaluated. Table 1 lists water
stored with an air headspace for 22 weeks. In all three cases, the water permeated at an average initial flux of around 1400−1600 L/(m2 h) (LMH). Dynamic light scattering measurements on the filtrate confirmed the removal of all colloidal material larger than the pore size of the membrane. The flux declined over time because of the formation of a fouling layer on the surface of the membrane and was more aggressive in aged water. SEM imaging of the membranes showed that the aged water also produced more compact filter cakes on the membrane surface, as seen in the samples aged for 22 weeks. No particles were observed in the pores of membranes tested with water aged up to 90 h. There was some fine particle entrainment observed in membranes tested with water aged for 22 weeks. EDX analysis of the particles showed considerable silicon and oxygen, suggesting that colloidal silica particles and aggregates form in water exposed to air over several weeks. The causal relationship with air was further tested by performing a series of “accelerated” aging experiments. Figure 3 shows the visual appearance and flux decline curves for SAGD process water treated with bubbled air for times ranging from 30 min to 24 h. In each picture, the sealed bottle on the left shows the unaged water and the bottle on the right shows an equivalent sample after bubbling. Bubbling with air accelerated both the color change and rate of flux decline during filtration. The rate of flux decline increased with duration of bubbling. The flux decline during filtration results from membrane fouling, and the different rates of fouling, can be understood in the context of two mechanismscake formation and pore blocking. Cake formation involves deposition of a layer of packed solids on the membrane surface. The flux decreases due to the additional hydraulic resistance presented by the filter cake. Pore blocking occurs either through the entrainment of small particles within the pores of the membrane or through the adsorption of species onto the surface of the pores. These mechanisms can occur independently or simultaneously depending on the nature of the water being filtered. Cake formation is clearly an active fouling mechanism in SAGD process water. Separate tests were performed to determine if either pore blocking mechanism was also important. Particle entrainment was evaluated by sectioning the membranes after filtration and examining the internal
Table 1. Summary of Water Quality Measurements at Different Phases of Aging onsite (after cooling)
offsite (as received)
offsite (aged 16 wks)
mS/cm mg/L
0.9 220a
0.9 440b
0.9c 290b
S.U.d mg/L
7.5 2.5
7.1 2.8
6.9 6.1
mV
Not sampled
−240
160c
property
units
ionic conductivity total organic carbon (TOC) pH dissolved oxygen (DO) oxidation− reduction potential (ORP)
a Measured using the persulfate-UV method. bMeasured using high temperature catalytic combustion method. cMeasured after 15 weeks of aging. dStandard units.
properties from each of the sample lifetime phases discussed in this paper. No meaningful differences were observed in the ionic conductivity of the water. The difference in TOC values measured on-site and off-site can be explained in part by the different analytical methods.13 However, the drop in TOC during aging appears to be a real effect. The pH decreased slightly over time, likely due to re-equilibration with atmospheric CO2. For similar reasons, the dissolved oxygen increased. The dissolved oxygen level in a sample stored in air for several months approached the saturation limit of about 7− 10 mg/L. The oxidation−reduction potential increased in a manner consistent with the increasing dissolved oxygen levels in the water samples. Effect of Aging on Filtration. Aging of the water showed dramatic effects on its filterability. Figure 2 shows flux decline curves for dead-end filtration experiments performed using ceramic microfiltration (100 nm pore size) membranes. Data are presented for the following: (i) water tested immediately after transfer from sealed conditions and filtered in ambient atmosphere, (ii) water allowed to stand for 90 h, and (iii) water
Figure 2. Flux decline curves for SAGD process water filtered through 100 nm ceramic membranes. Curves are shown for unaged water (dark blue diamonds), water allowed to stand for 90 h (red triangles), and water stored for 22 weeks under a periodically refreshed air headspace (black circles). Optical and scanning electron microscope images of the filter cake on the surface of the membrane are also shown for the three test conditions. The scale bar on the SEM images is 1 μm. 7173
dx.doi.org/10.1021/ie3005513 | Ind. Eng. Chem. Res. 2012, 51, 7170−7176
Industrial & Engineering Chemistry Research
Article
Figure 3. Effects of bubbling with air on water appearance and filtration characteristics. (a) Flux decline curves are shown for filtration of SAGD process water that had been treated with bubbling air for 30 min (dark red triangles), 2 (purple circles), 4 (green squares), 8 (red diamonds), and 24 h (blue triangles). A control experiment using unaged water is also shown (dark blue diamonds). (b) The appearance of the aged water is shown after different bubbling times. The bottle on the left of each pair contains unaged water.
Table 2 summarizes the computed cake formation rate constants for experiments performed with naturally aged and
porosity for entrained particles. No material was found inside membranes tested with water subjected to air bubbling. The possibility of pore blocking due to adsorption was tested by soaking membranes before use. No differences were observed in the flux decline curves for the following: (i) a fresh, unsoaked membrane; (ii) a membrane soaked for 24 h in unaged produced water; (iii) a membrane soaked for 24 h in water aged under a quiescent, periodically refreshed air headspace for 22 weeks; and (iv) a membrane soaked for 24 h in water wherein air had been bubbled for 24 h. This suggests that adsorption effects do not become relevant for at least 24 h and that the flux decline trends data observed in Figures 2 and 3 are dominated by the cake mechanism. The data from these experiments is included as Supporting Information. The flux decline curves can be analyzed quantitatively using a cake filtration model. This idealized model applies in situations where the nominal size of the filtered material is larger than the pore size membrane, leading to the accumulation of material in a cake on the surface of the membrane. The overall hydraulic resistance increases with the thickness of the cake layer, and the flux declines as follows:15
1 1 = 2 + Kct 2 J J0
Table 2. Rate Constants for Cake Filtration and Other Test Parameters sample
rate constant Kc (1/(m2 s3)) −11
unaged 2.5 × 10 90 h 9.4 × 10−11 22 weeks 1.3 × 10−7 accelerated aging experiments 30 min 5.1 × 10−10 2h 2.3 × 10−10 4h 4.0 × 10−10 8h 3.1 × 10−9 24 h 6.8 × 10−9
correlation factor, r2
dissolved O2 (mg/L)
0.99 0.96 0.94
2.8 8.2 6.1
0.95 1.00 1.00 0.97 0.96
6.4 7.8 8.3 8.6 8.8
water treated with air bubbling. The rate constant increases with longer or more aggressive exposure to air. Samples treated with bubbling air for just 30 min had DO levels comparable to samples stored under quiescent conditions for 90 h. It is not clear why lower oxygen levels were observed in the sample aged for 22 weeks. The fouling rate constant was higher by a factor of 5. This trend in the flux decline rate constants and dissolved oxygen levels is consistent with a mechanism involving chemical reaction between oxygen and dissolved organic species. The filterable solids loading was computed from the
(1)
where J is the flux, J0 is the initial flux, Kc is an effective rate constant for cake formation, and t is the elapsed time. This model assumes the filter cake is homogeneous and also ignores potential compression effects. 7174
dx.doi.org/10.1021/ie3005513 | Ind. Eng. Chem. Res. 2012, 51, 7170−7176
Industrial & Engineering Chemistry Research
Article
Figure 4. TIC plots from 2DGC-MS analysis of unaged (left) and aged (right) samples. The peaks identified as phenol and alkylated phenol derivatives are noted in both the plots.
mass of the filter cake, divided by the volume filtered. In general, the filter cake solids loading ranged from about 50 mg/ L for the unaged samples to about 60 mg/L for samples with significant aging. The magnitude of this increase was too low to fully explain the change in fouling rates. This suggests that aging changes the structure of the filter cake, in addition to increasing the amount of material deposited on the surface for a given volume of filtered fluid. Raw values for the filterable solids loading and cross-sectional SEM images of the filter cake from the natrually aged and accelerated aging experiments can be found in the Supporting Information. Composition Changes and Potential Aging Mechanism. Analysis of the organic composition of the water using 2DGC-MS can provide insight into the processes that the sample may undergo during aging. The data presented in Figure 4 compare total ion chromatograms (TIC) from the 2DGC-MS analysis of process water before (unaged) and after (aged) the accelerated aging experiment. Several species were absent in the aged sample that were present in the unaged sample. These species were identified by comparing the mass spectra for individual peaks to the NIST Mass Spectral Library. A primary aging mechanism from oxygen exposure is likely related to phenolic compounds present in the water samples. The presence of a relatively substantial amount of phenol and alkyl phenols are shown in Figure 4 before and after the accelerated aging experiment was conducted. These compounds have been shown to be susceptible to oxidation via reaction with dissolved oxygen in the presence of transition metal catalysts or at elevated temperatures.16−18 Products of phenol oxidation may undergo further polymerization reactions that eventually lead to the formation of brown insoluble precipitates.16,19 Samples and precipitates with similar characteristics were observed in the aged samples generated in this study. Other species that were absent in the aged sample were alkylated benzenes and thiophenes (Supporting Information Figure S2). Accumulation of the phenolic precipitates may have led to the loss of the structurally similar aromatic and thiophene species through a coprecipitation or similar mechanism.20 Metals were not expected to be present in SAGD process water at as high of concentrations as were used in the previously referenced studies. For example the dissolved iron concentration in SAGD process water from a location in the upstream bitumen water separation section of a plant has been reported to be approximately 0.01 μM.6 The concentration of soluble and suspended iron was not tracked in the study. The relative concentration of phenol and methylphenols did not
appear to decrease during aging based on the relative 2DGCMS TIC responses for those species. However, the relatively high concentrations of organics coupled with a constant oxygen feed may have led to the formation of brown insoluble precipitate through a similar mechanism as described above. Volatilization loss of these compounds during bubbling did not appear to be substantial since less volatile compounds (i.e., those that eluted at earlier first-dimension retention times) were still present in the sample. If volatilization did have a substantial impact the organic compositional changes would likely be correlated to species volatility. In the case of this study, differences in composition between the samples were observed over a relatively broad species volatility range as indicated by the peak positions spread across the first dimension residence time. The filtration behavior can be interpreted in terms of these physicochemical changes. The formation of polymeric species can directly increase the amount of material that is removed by filtration. Indirect means are also possible; the reaction products may also flocculate suspended material making it more susceptible to removal by filtration. Moreover, polymeric reaction products may also increase the viscoelastic character of the filter cake, leading to increased compressibility. More detailed analysis of the mechanical properties of the filter cake is needed to conclusively determine if this mechanism fully accounts for the observed behavior. Implications for Other Processes and Mitigation Options. Although the focus in this study has been on the effects of aging on membrane filtration processes, the potential underlying mechanism also provides insight into how aging could impact other water treatment technologies. First, the evolution of organic species in the sample water could impact the efficacy of chemical treatments. Although the aged solution did not show altered adsorption properties on alumina membranes, there were meaningful differences in the packing of the filter cake. This suggests altered material affinity that could lead to overly optimistic results in the evaluation of some absorbent technologies. Second, aging reaction products appear to flocculate and alter the particle size distribution. Again, this was observed indirectly in the packing of the filter cakes and the differences in their hydraulic permeability. Skewed particle size distributions that favor larger particles could also result in overly positive results in processes based on gravity separation and size separation such as flotation or sedimentation. The aging effect can be slowed by minimizing exposure to air during sample collection and handling. Storage under nitrogen 7175
dx.doi.org/10.1021/ie3005513 | Ind. Eng. Chem. Res. 2012, 51, 7170−7176
Industrial & Engineering Chemistry Research
Article
(4) Joshi, N. B.; Mullins, O. C.; Jamaluddin, A.; Creek, J.; McFadden, J. Asphaltene precipitation from live crude oil. Energy Fuels 2001, 15, 979−986. (5) Erstad, K.; Hvidsten, I. V.; Askvik, K. M.; Barth, T. Changes in crude oil composition during laboratory biodegradation: Acids and oilwater, oil-hydrate interfacial properties. Energy Fuels 2009, 23, 4068− 4076. (6) Jennings, D. W.; Shaikh, A. Heat exchanger deposition in an inverted steam-assisted gravity drainage operation. Part 1. Inorganic and organic analysis of deposit samples. Energy Fuels 2007, 21, 176− 184. (7) Wang, S.; Axcell, E.; Bosch, R.; Little, V. Effects of chemical application on antifouling in steam-assisted gravity drainage operations. Energy Fuels 2005, 19, 1425−1429. (8) Schramm, L. L.; Smith, R. G. Some observations on the aging phenomenon in the hot water processing of Athabasca oil sands. Part 1. The nature of the phenomenon. AOSTRA J. Res. 1987, 3, 195−214. (9) Schramm, L. L.; Smith, R. G. Some observations on the aging phenomenon in the hot water processing of Athabasca oil sands. Part 2. The mechanism of aging. AOSTRA J. Res. 1987, 3, 215−224. (10) Bordenave, S.; Kostenko, V.; Dutkoski, M.; Grigoryan, Al.; Martinuzzi, R. J.; Voordouw, G. Relation between the activity of anaerobic microbial populations in oil sands tailings ponds and the sedimentation of tailings. Chemosphere 2010, 81, 663−668. (11) Poindexter, M. K.; Chuai, S.; Marble, R. A.; Marsh, S. C. Solid content dominates emulsion stability predictions. Energy Fuels 2005, 19, 1346−1352. (12) Petersen, M. A.; Grade, H. Analysis of steam assisted gravity drainage produced water using two-dimensional gas chromatography with time-of-flight mass spectrometry. Ind. Eng. Chem. Res. 2011, 50, 12217−12224. (13) Method 5320 Total Organic Carbon In Standard Methods for the Examination of Water and Wastewater, 21st ed.; APHA: Washington DC, 2005. (14) Method 2580 Oxidation-Reduction Potential (ORP). Standard Methods for the Examination of Water & Wastewater, 21st ed.; APHA: Washington DC, 2005. (15) Hermia, J. Constant pressure blocking filtration laws − application to power law non-Newtonian fluids. Trans. Inst. Chem. Eng. 1982, 60, 183−187. (16) Sadana, A.; Katzer, J. R. Catalytic oxidation of phenol in aqueous solutions over copper oxide. Ind. Eng. Chem. Fund. 1974, 13, 127−134. (17) Colarieti, M. L.; Toscano, G.; Greco, G., Jr Soil-catalyzed polymerization of phenolics in polluted waters. Water Res. 2002, 36, 3015−3022. (18) Devlin, H. R.; Harris, I. J. Mechanism of the oxidation of aqueous phenol with dissolved oxygen. Ind. Eng. Chem. Fund. 1984, 23, 387−392. (19) Pierpoint, W. S. o-Quinones Formed in Plant Extracts and their Reactions with Amino Acids and Peptides. Biochem. J. 1969, 112, 609− 616. (20) Wilberg, K. Q.; Nunes, D. G.; Rubio, J. Removal of phenol by enzymatic oxidation and flotation. Braz. J. Chem. Eng. 2000, 17, 907− 913.
was effective in arresting the chemical reactions that lead to accelerated fouling. In practice, this suggests that rapid filling of containers, leaving no headspace, followed by immediate sealing, and the use of nitrogen blanketing during handling should be strongly considered in studies that involve the shipping and handling of SAGD process water samples for offsite testing.
■
CONCLUSION Aging in SAGD process water collected after a skim tank occurs primarily through reactions of dissolved aromatic species with air. This results in a gradual change in appearance and is accompanied by a significantly higher tendency for fouling in membrane filtration tests. The fouling results in an increased resistance in the filter cakes formed by filtered material, and may be due to changes in structure rather than differences in absolute solids loading. While this paper focused on filtration, aging effects can also lead to misleading results in laboratory testing of other water treatment processes and must be carefully considered during technology evaluation. Aging effects can be mitigated by minimizing air exposure during sample collection and handling.
■
ASSOCIATED CONTENT
S Supporting Information *
Data on the morphology of the filter cake layers from the fouling experiments and effects of adsoprtion effects from presoaking of the membranes before filtration is available. This information is available free of charge via the Internet at http:// pubs.acs.org/.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +1-518-387-4628. Fax: +1-518-387-7563. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors gratefully acknowledge Brent Scott, Jennifer Sergent, and Pragnesh Parekh from the Suncor Central Laboratory, and Scott Bennett and Calvin Gee from Suncor In situ Technical Services for their assistance with the collection and handling of water samples at the production site. The authors also acknowledge Lauraine Denault and Hans Grade from GE Global Research for their help in sample analysis. Finally, we thank Jerome Ybema from Suncor, and David Polizzotti and James Rawson from GE for their insightful comments and helpful discussions on this manuscript.
■
REFERENCES
(1) Scott, A. C.; Mackinnon, M. D.; Fedorak, P. M. Naphthenic acids in Athabasca oil sands tailings water are less biodegradable than commercial naphthenic acids. Environ. Sci. Technol. 2005, 39, 8388− 8394. (2) Butler, R. M. Steam-assisted gravity drainage: Concept, development, performance and future. J. Can. Pet. Tech. 1994, 33, 44−50. (3) Heins, W. F. Is a paradigm shift in produced water treatment technology occurring at SAGD facilities? J. Can. Pet. Tech. 2010, 49, 10−15. 7176
dx.doi.org/10.1021/ie3005513 | Ind. Eng. Chem. Res. 2012, 51, 7170−7176
