Characterization of Boiler Blowdown Water from Steam-Assisted

Aug 2, 2012 - These studies can provide insight regarding management options for SAGD disposal water. Standard analytical methods for wastewater ...
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Characterization of Boiler Blowdown Water from Steam-Assisted Gravity Drainage and Silica−Organic Coprecipitation during Acidification and Ultrafiltration Abhijit Maiti,† Mohtada Sadrezadeh,† Subhayan Guha Thakurta,† David J. Pernitsky,‡ and Subir Bhattacharjee*,† †

Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta T6G 2G8, Canada Suncor Energy, Incorporated, Post Office Box 2844, 150 6th Avenue Southwest, Calgary, Alberta T2P 3E3, Canada

‡

S Supporting Information *

ABSTRACT: In thermally enhanced oil recovery operations, particularly in steam-assisted gravity drainage (SAGD), boiler blowdown (BBD) containing high concentrations of dissolved organic matter (DOM), dissolved silica, and total dissolved solids (TDS) is generated. To develop efficient tools for managing this blowdown, a detailed understanding of its chemistry is required. In this study, BBD was evaporated to yield ∼66% condensate and ∼33% concentrate blowdown (CBD). Detailed characterization of the BBD and CBD water was conducted. The effect of acidification was also studied. The acidification coprecipitates the silica and DOM, with over 90% of the silica and over 40% of the DOM precipitating at pH 4. Ultrafiltration treatment was also examined, and a major fraction of the silica and DOM in the CBD was found to foul a 100 kDa ultrafiltration membrane in the pH range of 7.5 to 9. The analysis revealed that the dominant fouling mechanism was cake filtration, indicating the formation of a silica−DOM precipitate layer on the membrane surface. These studies can provide insight regarding management options for SAGD disposal water.

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silicates.2 Hence, conventional industrial water treatment processes direct considerable attention toward removal of these metal ions in order to avoid silicate scale formation. Dissolved organic matter (DOM) can also react with silica. Interactions of DOM with silica and silicates have been studied in the context of drinking water treatment and membrane fouling.3 The main characteristics of conventional oil field produced water (OFPW), SAGD produced water (SAGDPW), oil sands mining process-affected water (OSPW), and SAGD based boiler blowdown (BBD) water are illustrated in Table 1. There is a lack of scientific literature examining the characteristics of SAGD BBD. The characteristics of BBD in Table 1 are from the present study. It is evident that these petroleum produced waters are all characterized by high levels of silica, dissolved solids, and organic matter. Detailed analysis of the properties of the inorganic and organic matter in SAGD boiler blowdown water is lacking. Dissolved organic matter (DOM) is a major constituent in the BBD, and it has been linked to equipment fouling due to the organic and silica coprecipitation.10 However, there has been no systematic study of how these entities interact with each other and lead to aggregation, precipitation, scaling, etc. Such a knowledge base could shed considerable light into the management options for large volumes of these wastewaters, namely, how they foul process equipment, how they respond to chemical treatment, and how to mitigate potential deleterious

INTRODUCTION Steam-assisted gravity drainage (SAGD) is a widely used process for in situ bitumen extraction from oilsands.1 In this process, medium pressure wet steam is injected into the production wells to thermally reduce the viscosity of the bitumen and facilitate its extraction. The steam condensate and the bitumen are extracted as a mixture of bitumen, clay, and water. Typically, 90% of the produced water (PW) is recycled as boiler feedwater. Conventional treatment of produced water includes gravity skim tanks and induced static flotation (ISF) to separate oil and water and warm lime softening (WLS) to remove silica and hardness, after filters for suspended solids removal and weak-acid cationic exchange (WAC) for removal of calcium (Ca2+) and magnesium (Mg2+) ions. The treated water is then used as feed in the steam generators. The boilers used in the SAGD operation are known as once through steam generators (OTSGs). The OTSGs are capable of handling a feed with high total dissolved solids (TDS) content. The use of a high TDS feed also results in a higher blowdown volume in the OTSGs in comparison to standard boilers. The conventionally treated produced water, when converted to steam in an OTSG, generates a boiler blowdown (BBD) which is about 20% of the volume of the boiler feedwater. In current industrial practice, a portion of the BBD is recycled back to the WLS process and the rest is earmarked for disposal. Silica is a concern in industrial process waters as it is often deposited on the walls of process equipment, such as boilers, membranes, filters, evaporators, and heat exchangers as solid fouling layers that are extremely difficult to remove. The presence of minerals, such as magnesium, calcium, aluminum, iron, etc., can exacerbate silica fouling through the formation of © 2012 American Chemical Society

Received: May 20, 2012 Revised: July 27, 2012 Published: August 2, 2012 5604

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TDS were measured using standard methods as described in the Supporting Information. We measured both the phenolphthalein alkalinity and the total alkalinity using standard methods.12 Analysis of Dissolved Organic Matter (DOM). The dissolved organic carbon (DOC) was measured by catalytically (Pt)-aided combustion at 680 °C using a TOC analyzer (Shimadzu, Model: TOC-V) after filtering through a 0.45 μm membrane filter (Cellulose acetate, Millipore, USA). The detection range of the TOC analyzer was 3−25 000 mg/L. The water samples were further examined using FTIR (Thermo Nicolet, Nexus 670 FTIR, USA) equipped with an attenuated total reflectance (ATR) accessory. In ATR-FTIR analysis, 600 scans for each sample, were collected at a resolution of 4 cm−1. First, a background scan was performed after pouring 2 mL of deionized water over the surface of the ATR crystal. Following this, the BBD, distillate, and concentrate samples were scanned. Water has very strong IR absorbance in the region of 3000−3600 cm−1. To avoid this interference, scan results in the region of 600−3000 cm−1 were analyzed in this study to determine the organic and silica signatures of these samples. The chemical oxygen demand (COD) of water samples was measured by ISO method 6060.13 Absorbance was measured at 254 nm using a UV−visible spectrophotometer (Varian, Model: Cary 50) with distilled water as the blank. These measurements were used to calculate the SUVA254 (specific UV absorbance) values. Particle Size Measurement of Solids by Electroacoustic Particle Sizer. The turbidity of samples was measured using a turbidity meter (Hach, 2100AN Turbidimeter). The TSS of BBD was found to be very low (∼65 mg/L) from conventional TSS measurements using glass fiber filter. An electroacoustic particle sizer (Dispersion Technologies, DT-1200) was also used to measure the suspended solid size distribution in the water samples. Potentiometric Titration of Raw BBD and Concentrate. An automated variable temperature potentiometric titration system (PCM QC-Titrate, Mandel Scientific) was used to titrate BBD and CBD using standardized 0.12 (M) HCl solution. The potentiometric titration of BBD and concentrate was performed at temperatures of 295 and 348 K. The titration was stopped at different pH values for both BBD and concentrate, and the resultant solutions were collected in test tubes. Any difference in the nature of precipitation and supernatant at different pH was noted. Methanol Extraction of Precipitated Organics from Blowdown Water. A 20 mL portion of the concentrated blowdown water was acidified to pH 2, following which the precipitated solids were separated from the supernatant by decantation and dried at 65−70 °C in an oven. The organic fraction from the precipitate was extracted using 15 mL methanol in a test tube. The methanol extract was separated and dried in a rotary evaporator under vacuum. Also, 10 mL DI water was added to the dried organics. The pH of the aqueous solution was noted to be 5.5. The precipitation behavior of the methanol extracted organic matter was studied. First, pH of the solution was increased stepwise to 10.5 using 0.1 (M) NaOH solution. Following this, 0.1 (M) HCl solution was added to decrease pH stepwise down to a value of 2. The dissolution behavior of precipitated organics at different pH was noted. Detailed preparation method of an aqueous solution of methanol extracted organics from acid precipitated product of CBD is presented in Figure SI-1 in the Supporting Information. Ultrafiltration of Acidified CBD. Ultrafiltration was conducted using 100 kDa polyethersulfone and regenerated cellulose membranes (Spectrum Laboratories) in a dead-end ultrafiltration cell (Millipore, USA) under a transmembrane pressure of 1.36 bar (20 psi). The CBD was acidified to get the end-point pH between 8.0 and 9.0. Then, these acidified solutions were left standing at room temperature for 1 h before the filtration experiment was performed. In another set of experiments, the pH of the CBD samples were adjusted to the same pH range of 9 to 8, following which, the samples were heat-treated at 348−350 K for 1 h and then allowed to cool down to room temperature for another 1 h. Following this, the samples were filtered through 100 kDa polyethersulfone membrane at room temperature.

Table 1. Characteristics of Different Conventional and Unconventional Oil Field Produced Water and SteamAssisted Gravity Drainage Boiler Blowdown (SAGD BBD) Water characteristics

OFPW5,6

pH conductivity (μS/cm) TSS (mg/L) TDS (mg/L) TOC (mg/L) silica (mg/L as silica) Ca2+ (mg/L) Mg2+ (mg/L)

7.4−8.5 1400−5000

a

SAGDPW4

OSPW7−9

SAGD BBDa

7.1 1540

8.9 670−3500

10.3 18 000

97 2500 44−50

65 17 000 2400 90

20−50 19−25

4.2 0.7

700−2000 68−140 7−14

232 46

2−11 0.01−1.3

2.5 0.05−0.15

Data from this study.

effects of their disposal arising from, for instance, their clogging of injection wells during deep well disposal. A relatively simple option of reducing the volume of disposal water in SAGD is to use evaporation of the BBD to obtain a condensate (distillate), which is boiler feed quality water, and concentrated blowdown (CBD) water, which can be disposed. Deep-well injection of this concentrate has several operational limitations, for instance, clogging of wells due to organic−silica coprecipitation, precipitation of products from interaction between the BBD and underground mineral formations or formation water, the possibility of regulatory restrictions, and cost of construction for new deep-well injection facilities.11 The present study primarily focuses on characterization of the inorganic and organic constituents of SAGD BBD and the CBD. We concentrate BBD water by evaporation followed by analysis of the condensate and the concentrate. The characterization study is aimed toward understanding the interactions between the organic matter and the silica in the presence of high amounts of salt, particularly during processes like acidification and membrane filtration. These studies provide considerable insight into the fouling and precipitation behavior of the BBD and CBD water. These studies are used to identify the mechanism of DOM precipitation upon acidification, as well as the nature of silica−organic fouling on ultrafiltration membranes.

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MATERIALS AND METHODS

Water Samples. BBD was received in sealed containers from a SAGD-based in situ bitumen extraction surface treatment plant located in the Athabasca oil sands region of Alberta, Canada. The plant uses the diluted bitumen recovery process. The samples were collected hot and kept in a nitrogen blanket until they were opened for sample analysis. Distillation of BBD. Distillation of BBD was performed at atmospheric pressure in a laboratory distillation setup at 110−115 °C. Two-thirds of the initial volume of BBD was distilled and collected. The remaining one-third concentrate in the reboiler was stored in a capped bottle. BBD, condensate (distillate), and concentrated blowdown (CBD) samples were used for physicochemical characterization. Analysis of Inorganics and Ions. The monovalent and divalent metal ion concentrations in the BBD, distillate, and concentrate samples were measured using a quadrupole-inductively coupled plasma mass spectrophotometer (Perkin-Elmer, Elan 6000 Quadrupole ICPMS, USA) after filtering through a 0.45 μm membrane filter (Cellulose acetate, Millipore, USA) and diluting based on the instrumental detection limit for individual metal ions. The pH, conductivity, and 5605

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RESULTS Characteristics of the Water Samples. The pH, conductivity, alkalinity, TSS, TDS, TOC, COD, and major ion concentrations in BBD, CBD, and condensate are presented in Table 2. The concentrations of other minor

The COD and TOC analysis of the BBD and CBD indicate high concentrations of organic matter (Table 2). The UV254 absorption and the corresponding SUVA254 values indicate presence of high levels of aromaticity in the organic matter.16 Figure 1 compares the ATR-FTIR spectra of BBD, CBD, and

Table 2. Characteristics of BBD, CBD, and Condensate (Distillate) parameters pH conductivity (μS/cm) total alkalinity (mg/L CaCO3) phenolphthalein alkalinity (mg/L CaCO3) turbidity (NTU) TSS (mg/L) TDS (gravimetric, mg/L) TOC (mg/L) COD (mg/L) UV absorbance (254 nm) SUVA254 color silica (as Si, mg/L) sodium (mg/L) chloride (mg/L) potassium (mg/L) magnesium (mg/L) calcium (mg/L) iron (total, mg/L) boron (mg/L) aluminum (mg/L) arsenic (mg/L)

condensate (distillate)

BBD

CBD

10.3 18090 2700 1300

10.5 55100 5574 2900

7.4 40 6 0

0.6 65 17200 2482 4400 0.654a 4.21 black 95 5199 6715 203 0.68 4.25 2.06 187 0.43 0.34

0.23 190 53715 7776 12000 0.832b 4.28 black 246 16609 21440 624 0.442 2.32 0.68 69 1.34 1.01

0 0 46 28 10.6, silica exists as dissolved silicate and is negatively charged.29−33 For pH < 9.5, condensation of silicate ions occurs and negatively charged small colloids of silica appear in the solution.33 In the presence of higher ionic strength of 0.2−0.3 M, and upon acidification, the range of the electrostatic double layer repulsion between negatively charged silica colloids is considerably reduced and aggregation (polymerization) occurs.31,34 Numerous studies are available in literature about silica precipitation, discussing the effects of acid, ionic strength, pH, concentration of silica, the presence of surfactants, and temperature.29−31,35 Gorrepati et al.31 observed that higher concentration of chloride increases the rate of polymerization of silica and flocculation among the small silica particles. The extent of silica aggregation has been reported to be highest at pH 4−5.33 Roy et al.36 investigated the interaction of both alkaline brine-like and dilute acidic wastes with three injection related lithologies (sandstone, dolomite, and siltstone) at 325 K and 10.8 MPa. They observed that mixing the alkaline waste with solid phases yielded several reaction products like Mg(OH)2, CaCO3, and possibly a type of sodium metasilicate. The abovementioned inorganic reactions of silica, including polyconden-

→ 3(HO)−Si−O−Ca−O−Si−(OH)3 → Ca (silicate) Si(OH)4 + Al(OH)3 → 3(HO)−Si−O−Al−(OH)2 → 3(HO)−Si−Al−(OH)−O−Si(OH)3 → Al (silicate)

SiO2 + (n + 1)H 2O + 2Na + = Na 2SiO3 ·nH 2O + 2H+

From the above, it may be concluded that formation of solid phases could lead to a decrease in pH of the liquid. The decrease in pH further leads to an increase in precipitation of silica present in BBD. Numerous studies have examined organic adsorption onto silica surfaces.37−45 Abramson et al.41 found natural coprecipitation of organic matter and silica due to strong chemical interaction between the siliceous and organic components into natural diatom frustules. Thus, removal of a fraction DOM due to interaction with precipitated silica is possible in progressively acidic environments. Silica polycondensation (gelation) starts at alkaline pH range 10.5−8.0 and the polymerization rate is highest below pH 5.5.33 The precipitation of polymerized products occurs due to charge neutralization by added H+ ions, and the rate of precipitation increases with increase in ionic strength of the solution, which decreases the electrostatic repulsion among the negatively charged silica species.46,47 The long chain polymerized silica species interact with organic matter present in the water via hydrogen bonding and van der Waals attraction.43 Thus, it is highly possible that hydrogen bonding could occur between the hydrogen of Si−OH and oxygen and nitrogen of different functional groups in the DOM of BBD/CBD water. The acidification induced precipitation of the aqueous solution of methanol extracted organic fraction of the CBD shown in Figure 6 indicates that a major fraction of DOM 5610

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precipitated independently of the presence of Si and inorganic matter. However, the presence of silica and high levels of TDS considerably increase the amount of organic matter precipitation. Ultrafiltration with PES and RCA membranes showed a decrease in permeate flux with decreasing solution pH. This is due to the increase in silica−organic coprecipitation with decrease in solution pH. A major fraction of silica and DOM from the CBD was found to be rejected through a cake fouling mechanism in the pH range of 9−7.5. The hydrophobic PES membrane was found to be more severely fouled by silica and organic matter compared to the hydrophilic RCA membrane under identical experimental conditions. We hope this work will provide a comprehensive understanding of the chemistry of SAGD boiler blowdown water and the silica and organic matter coprecipitation in BBD upon acidification and will shed light on the membrane fouling mechanism of SAGD DOM in presence of silica and high salt concentrations during ultrafiltration.

precipitates on its own. This behavior suggests the protonation of organic acid salts present in CBD/BBD to free acid forms, which become insoluble in acidic media and precipitate. Similar organic precipitation from oil sands produced water during acidification was observed by Jennings and Shaikh.10 Numerous studies have examined organic acids in petroleum and Canadian bitumen.48−52 Thus, it is likely that DOM removal through precipitation at acidic pH can be attributed to both the protonation of organic acid salt and hydrogen bond interaction between organics and precipitated silica. Modes of Membrane Fouling by Silica−Organic Matter Precipitation upon Mild Acidification. When the ionic strength of the feed solution is low, the trend of permeation flux with pH can be explained by considering the electrostatic double layer interactions between the solutes and membranes.53−55 If the pH of the solution is between the isoelectric point (IEP) of the solute and the membrane, there will be attractive electrostatic solute−membrane interaction, leading to a more intense fouling and, consequently, decreased permeate flux. Otherwise, electrostatic repulsion hinders fouling, thereby causing a less severe permeate flux decline.56−58 In the present work, however, ionic strength of the feed (CBD) is very high (>0.5 M), which diminishes the effect of electrostatic repulsion.47 Decreasing pH causes precipitation of silica particles and coprecipitation of organic compounds, which are adsorbed on the surface of the silica nanoparticles. Deposition of silica particles and organic compounds on the membrane surface leads to the formation of a cake layer with its resistance to permeate flow increasing with decreasing pH. It is of interest to note that, while silica and organic matter precipitation is not visible during acidification above pH 7, such precipitation is quite significant at a pH of 9 during membrane filtration, as observed in our flux decline studies. This may be attributed to the selective permeation of the low molecular weight components through the membrane, which influences the local (membrane surface) concentrations of the different species in the concentration polarization layer. This, in turn, affects the reaction kinetics and equilibrium constants of polycondensation of silica, formation of silicates, as well as silica−organic matter association that eventually deposit on the membrane as a cake upon mild acidification. It is worth noting that the fouling is more intense for PES membrane at pH 9 compared to pH 8. The DOM and chloride rejection are also higher at pH 9 (refer to Table 3). We hypothesize that the silica nanoaggregates that are formed at pH 9 deposit more readily on the hydrophobic PES membrane forming a cake with lower permeability for DOM and chloride. However, at pH 8, silica aggregates into larger particles forming a cake with higher permeability for DOM and chloride.

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ASSOCIATED CONTENT

S Supporting Information *

Standard analytical methods for wastewater characterization, preparation method for methanol extracted dissolved organics, minor inorganic ion concentration, ATR-FTIR scan of acid precipitate and supernatant of BBD, and characteristics of UF membranes used for filtration experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work through the NSERC Industrial Research Chair in Water Quality Management for Oil Sands Extraction is gratefully acknowledged. We are also grateful to Conoco Phillips Canada for the gracious donation of boiler blowdown water samples. We thank Dr. Ramesh Sharma for insightful discussions. The opinions expressed and the conclusions drawn from the study are those of the authors and should not be considered as views or endorsements of the funding organizations or water sample donors for this research.

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REFERENCES

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CONCLUDING REMARKS On the basis of FTIR analysis, evaporation and concentration does not change the chemical signature of the dissolved organic matter and silica in the CBD compared to BBD. Concentration did result in the precipitation of a portion of the dissolved Si, Ca, Mg, and Fe. Acidification of BBD and CBD results in the precipitation of DOM and Si. A visible precipitate forms below pH 7, with almost complete precipitation below pH 4. Acid titration end-points were found at pH 7.1−7.3 and 3.2. The characterization results of the CBD reveal that coprecipitation of silica and dissolved organics is highly specific to solution pH. The results of extraction experiments show that organic acids 5611

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