Research Note pubs.acs.org/IECR
Aqueous Dissolved Oil Fraction Removed with Pulsed Corona Discharge Iakov Kornev,† Sergei Preis,*,‡ Elena Gryaznova,† Filipp Saprykin,† Petr Khryapov,† Mikhail Khaskelberg,† and Nikolay Yavorovskiy† †
Institute of High Technology Physics, Tomsk Polytechnic University, 2A Lenin Ave., Tomsk 634050, Russia Department of Chemical Technology, Lappeenranta University of Technology, 34 Skinnarilankatu, Lappeenranta 53850, Finland
‡
ABSTRACT: Costly advanced water treatment methods are necessary to meet the standards for the disposal of oil products polluted waters to commercial fishing water bodies. The objective of the present research is establishing the efficiency of gasphase pulsed corona discharge applied to oil products removal dependent on treatment parameters. The results showed the ability to cost-efficiently purify the produced water to the standard requirements; the maximum efficiency treatment parameters were determined. The mass transfer of oil products was determined to limit the overall process rate. The treatment method efficiency was confirmed in pilot-scale experiments.
■
INTRODUCTION Water pollution with oil components presents probably the most serious environmental problem for its massive character and strict limits for disposal.1 Oil constituents may be dissolved at concentrations of a few tens of milligrams per liter. The highest aqueous solubility is shown by compounds with polar bonds, such as phenolics, low-molecular aromatic benzene-like hydrocarbons, and organic acids.2 Conventional methods of water treatment, although effective against dispersed oil, are unable to remove dissolved components. Membrane filtration and adsorption demonstrate the ability to handle the soluble oil at, however, often unaffordable costs, insufficient flow capacity and susceptibility to fouling.3 The disposal standards show wide variations internationally. Cha et al.4 report Oslo-Paris Commission’s regulation for the produced water discharge limiting the dispersed oil to 30 mg L−1, the United States Environmental Protection Agency (USEPA) and the other commissions set limits for the offshore produced water at similar level achievable by mechanical treatment methods.5 However, the standards for disposal in Russian Federation establish the limit of 50 μg L−1 for the water bodies of the commercial fishing category, covering practically all rivers and lakes.6 Having soluble oil fractions at a few mg L−1 level after, for example, air stripping,7 macroporous polymer extraction,8 cascade membrane filtration,9 or extraction with gas condensate,10 reaching the standard with conventional methods appears to be impossible and requires feasible measures. Application of advanced oxidation processes (AOPs) presents an option for dissolved oil removal,11,12 although the number of publications and full-scale applications remain small.3 The combination of ozonation with sand filtration improves the dissolved oil removal, noticeably reducing chemical oxygen demand (COD).4,13 However, regardless vivid interest toward AOPs, ozone is not massively used in dissolved oil abatement due to its costly synthesis and application.14 Application of pulsed electric discharges producing hydroxyl radicals at the gas−liquid interface presents an energy-efficient alternative in water treatment.15−17 © 2014 American Chemical Society
Earlier, the authors proposed application of pulsed dielectric barrier discharge18 and pulsed corona discharge (PCD) to the treatment of aqueous solutions dispersed in droplets, jets, and films,19 at the surface of which the action of short-living oxidants (O, OH) takes place.20 Such process organization results in surpassing other AOPs in energy efficiency by minimum the factor of 2.17,19 The experimental study was undertaken for the abatement of dissolved fraction of crude oil using gas-phase PCD applied to the model solutions, as well as the specimens of wastewaters of oil distilleries.
■
MATERIALS AND METHODS Experimental Device and Materials. The device is composed of the PCD reactor, the high-voltage (HV) pulse generator, and the tank containing 25 L of circulating solution (Figure 1). The pulse generator consists of a thyristor power switch circuit, followed by a pulse step-up transformer and HV magnetic compression stages, and a pulse compression block.21 The current and voltage waveforms (Figure 2) were registered with the Agilent 54622D oscilloscope using a low-inductance resistive current monitor (Pearson 2878) and an HV divider (Tektronix P6015). The energy released in the electrode systems was calculated by integrating the current and voltage oscillograms. The amplitude of the discharge pulses in voltage was 21 kV and the current was 240 A, for a pulse front-rising time of 60 ns with the repetition frequency of 50−900 pulses per second (pps). The energy delivered to the reactor was 0.34 J per pulse. The energy consumption efficiency of the pulse generator was 67%. The power dissipated in the discharge at maximum pulse repetition frequency (900 pps) was ∼300 W of delivered power; reduced frequencies resulted in proportional powers. Received: Revised: Accepted: Published: 7263
November 4, 2013 March 30, 2014 April 13, 2014 April 14, 2014 dx.doi.org/10.1021/ie403730q | Ind. Eng. Chem. Res. 2014, 53, 7263−7267
Industrial & Engineering Chemistry Research
Research Note
hexane, were further analyzed for opalescence intensity.22 Chemical oxygen demand was measured using the photometric dichromate method described in Standard Methods for the Examination of Water and Wastewater.23 Turbidity was also determined photometrically. Total organic carbon (TOC) analysis was carried out using Vario TOC Cube TOC-analyzer (Elementar, Germany). Ozonation Experiments. The energy efficiency of electric discharge treatment is most often compared with conventional ozonation for similarity in chemistry. For this reason, ozonation experiments were also organized with the model solutions. Ozonation was realized by means of a laboratory ozone generator with a capacity of 0.4 g h−1. Air containing ozone (in a concentration of 6 mg L−1) was delivered with a flow rate of 1.2 L min−1 to the glass cylinder reactor containing 1 L of the solution and dispersed in water by means of sintered glass. The ozone-containing gas bubbles were passed through the 400-mm water layer. Gaseous ozone was determined iodometrically by bubbling the known volume of ozone-containing gas from the ozone generator through the Drexel trap containing acidified 2% potassium iodide solution. Free iodine was titrated using a 0.1 N sodium thiosulfate solution in the presence of starch. All chemicals were taken as supplied by Kriochrome (Russia).
Figure 1. Experimental setup outline.
The reactor utilizes a wire-plate corona geometry: horizontal electrode wires are disposed between vertical grounded-plate electrodes in a set of five parallel sections. The electrodes geometry parameters determining the pulse characteristics were chosen for the maximum pulse energy as HV electrodes made of stainless steel wire 0.5 mm in diameter, positioned at 20 mm from the vertical grounded-plate electrodes with the distance of 30 mm between the HV electrodes. The total length of the HV electrodes was 30 m in 0.2-m sections. Thus, the volume of the discharge zone of the reactor was 40 L. Water is fed to the top of the reactor, where it is dispersed through a perforated plate 200 mm × 200 mm in size, with holes 2.0 mm in diameter producing jets, droplets, and films. Water showers between electrodes to the zone of gas-phase PCD formation, where the treatment with oxidants takes place. The solution returns to the tank for recirculation. The flow rate was varied from 6 L min−1 to 30 L min−1, which corresponds to the gas−liquid contact surface area from 8 m−1 to 37 m−1.19 The model solutions were made by mixing 400 mL of crude oil with 35 L of tap water for 10 min with subsequent settling for 7 days. Such solutions contained from 2 mg L−1 to 7 mg L−1 of oil products extractable with hexane. The 25-L batches were treated in recirculation mode for 42 to 60 min with sampling increment from 3 min to 18 min. The solutions were aerated for 10 min before the start of PCD oxidation. The experiments were tripled, with the deviations of the observed results from each other not exceeding 5%. Analytical Methods. The analysis of oil products content extractable with hexane was carried out using a “Fluorat-023M” fluorimeter (Russia): 100 mL of the water sample containing oil products, vigorously shaken with 10 mL of
■
RESULTS AND DISCUSSION Aeration. Aeration of the samples was carried out in the reactor by circulation at certain air ventilation without the electric discharge. The result showed the decline in oil products content (Figure 3) with the fastest degradation within the first
Figure 3. Dependence of oil products concentration on treatment time with aeration and PCD: (△) aeration, (□) PCD treatment at 200 pps, (○) PCD treatment at 900 pps; liquid flow rate = 12 L min−1 (contact surface 15.6 m−1); air flow rate = 0 and 25 L min−1 in PCD and aeration experiments, respectively (the size of experimental point symbols refer to the standard deviations).
Figure 2. Voltage and current oscillograms of the pulse. 7264
dx.doi.org/10.1021/ie403730q | Ind. Eng. Chem. Res. 2014, 53, 7263−7267
Industrial & Engineering Chemistry Research
Research Note
minutes of aeration. Nevertheless, aeration in the reactor’s chamber may not remove more than 30%−50% of the starting oil products’ concentration within reasonable time (Figure 3), confirming inability to achieve the polluted water disposal standard. Dependence of Oxidation Efficiency on Pulse Repetition Frequency. Pulsed corona discharge (PCD) treatment results in substantial acceleration of oil products abatement and achievement of lower residual concentrations, compared to aeration (Figure 3), ranging, after 42 min of PCD treatment, from ∼0.03 mg L−1 at 900 pps to 0.1 mg L−1 at 200 pps. The difference at various frequencies is expectable for different concentrations of oxidants in the reactor: more-frequent pulses generate more of short-living oxidants and ozone. The equilibrium concentration of gaseous ozone in the reactor was 1.5−2 mg L−1 at 200 pps, and 5.5−6.0 mg L−1 at 900 pps. Figure 3 shows that treatment at different pulse repetition frequencies gives similar results at, however, different energy consumptions. The dependence of energy efficiency, which is defined as the amount of oil products removed relative to the unit of energy introduced to the reactor (g kW−1 h−1), on the pulse repetition frequency is shown in Figure 4. The efficiency
Figure 5. Degradation of oil products versus treatment time for discontinuous PCD treatment: (△) aeration, (○) 20 s of PCD at 50 pps once per 7 min, (◇) 20 s of PCD at 900 pps once per 7 min, (□) continuous treatment with PCD at 50 pps, liquid flow rate of 12 L min−1 (i.e., contact surface 15.6 m−1) (the size of experimental point symbols refer to the standard deviations).
patterns. The discontinuous PCD shows, expectedly, a rate inferior to that of continuous treatment, since, under the circumstances, the solution is treated with residual ozone only, whereas continuous PCD involves also short-living oxidants. The better effect of the discontinuous treatment with PCD at 900 pps, as compared to discontinuous 50 pps application, is explained by the higher residual ozone concentration at higher frequency, although this effect is accompanied by substantially bigger energy consumption; at 900 pps the discharge energy discontinuously delivered to the reactor was at the same order of magnitude as the one in continuous PCD application at 50 pps. The energy efficiency values for oil products oxidation at 50% removal are 25.8 g kW−1 h−1 for discontinuous treatment at 900 pps and 42.5 g kW−1 h−1 for continuous treatment at 50 pps. The efficiency of discrete PCD at 50 pps for the same purification extent reached as much as 445 g kW−1 h−1, although showing a lower oxidation rate. The discontinuous treatment or the treatment at low frequencies, nevertheless, shows the way of substantial economy achievable in reactions utilizing ozone as an oxidant. Mass Transfer of Oil Products. The dependence of the oil products’ oxidation efficiency on the pulse repetition frequency contrasts with the observation made for refractory humic substances (HS), the oxidation efficiency of which was independent of the frequency and was determined only by the delivered energy.19 Energy efficiency independent of pulse repetition frequency was explained by the negligible role ozone played in HS oxidation. Obviously, ozone plays a significant role in the oxidation of oil products, but the difference between efficiencies (Figure 4) may not be easily explained by the action of ozone only, since the oxidation rate at various frequencies practically remains equal (Figure 3). This points to the additional factors that are affecting oxidation. In absorption with chemical reaction, the mass transfer rate is usually the parameter restraining the overall process rate at higher reaction rates.24 Since the role of ozone appeared to be significant, the reaction rate for oil products oxidation may be high enough, making the mass transfer the restraining stage of the heterogeneous oxidation. The oxidation of oil products at various frequencies behaves, indicating the concentration of oxidants present in excess at frequencies of 100−900 pps (Figure 4). This suggests surface oxidation of oil products limited by their mass transfer from the bulk liquid to the surface: smaller concentration of short-living oxidants and
Figure 4. Dependence of the oxidation efficiency of the oil products (E) on the pulse repetition frequency. [Conditions: liquid flow rate, 12 L min−1 (contact surface 15.6 m−1); oxidation extent, 50%.]
calculation was made for a 50% decline in the concentration of oil products. One can see that the reduction in pulse repetition frequency from 900 pps to 100 pps results in an almost-linear improvement in oxidation efficiency, from 1.8 g kW−1 h−1 to 12.5 g kW−1 h−1. The PCD treatment of oil products solutions resulted in a moderate reduction of COD, from the initial range of 25−30 mg O2 L−1 by only 6−8 mg O2 L−1 over 42 min of treatment, indicating a low degree of mineralization and possibly surface character of the oil products oxidation with short-living oxidants. Analogous data were obtained for TOC reduction, supporting moderate mineralization of the organic substances: the TOC reduction observed in the PCD oxidation experiments did not exceed 1.5−2.5 mg C L−1 from 7−9 mg C L−1 of starting TOC concentration. Discontinuous PCD Treatment. For clarification of the roles of ozone and short-living oxidants, the experiments with discontinuous PCD treatment were carried out. This consists of the short-time (∼20 s) application of PCD with pauses for a few minutes, utilizing the oxidative potential of residual ozone. The reactor was not ventilated. The concentration of ozone appeared to decrease rather slowly, less than 2-fold during 7 min between PCD applications. Figure 5 shows the effect of treatment under conditions of various PCD application 7265
dx.doi.org/10.1021/ie403730q | Ind. Eng. Chem. Res. 2014, 53, 7263−7267
Industrial & Engineering Chemistry Research
Research Note
249 Wh m−3; the number of cycles varied from 1 to 7 with the energy consumption varying from 0.25 kWh m−3 (30 min) to 1.74 kWh m−3 (210 min). The pulse repetition frequency was 500 pps, giving the pulsed power delivered to the reactor as large as 134 W. Finally, the PCD-treated wastewater was filtered through a sand filter. The result of wastewater treatment is shown in Table 1. One can see the PCD treatment ability to reduce the oil products
ozone at smaller frequency is sufficient for oxidation, making the increased frequency useless accelerating oxidation to a very moderate extent at the consumed power increased proportionally to the frequency. To prove this statement, the series of experiments was carried out, in which oxidation efficiency was followed at a variable liquid flow rate (i.e., gas−liquid contact surface). Earlier, the proportional growth of the contact surface with increased liquid flow rate was established for the PCD reactor within the experimental limits similar to the present research.19 The experiments with oil products solutions were carried out at the contact surface of 8, 23, and 37 m−1 at the pulse repetition frequency of 900 pps, showing the oxidation efficiencies of 0.94, 1.82, and 2.20 g kW−1 h−1, respectively, at 50% oxidation. Comparison of PCD with Ozonation. Assessment comparison of the PCD energy efficiency with the closest competitor, ozonation, was carried out in experiments on oil products oxidation with ozone in the reactor described above (see the Ozonation Experiments section). The energy consumption for ozone synthesis was taken as 20 kWh kg−1 O3 as for oxygen-fed ozone generators.25 The energy efficiency in ozonation taken for 50% oil products removal comprised 4 g kW−1 h−1, exceeding the one for PCD at 900 pps. Nevertheless, the PCD oxidation efficiency is increased with decreasing pulse repetition frequency, and at 400 pps, PCD surpasses ozonation (see Figure 4). The comparison is made in a preliminary approach manner giving only a rough estimation of energy efficiencies: the main energy consumption in conventional ozonation is contained in maintenance of dissolved ozone concentration on account of constant flow of ozone-containing gas. The absence of such drawback presents a substantial advantage of PCD: the treatment takes place without gas transport. Industrial Wastewater Treatment. The PCD method was applied to oil products removal from wastewaters of an oil distillery. The wastewater was separated from the oil sheen in an oil gravitational separator and contained 1.5−2.0 mg L−1 of oil products at high COD (140−200 mg O2 L−1) and turbidity (26−62 FTU). Settling of the wastewater specimens for 24 h resulted in negligibly decreased turbidity, indicating the presence of colloidal substances. The gravitational separator effluent was further clarified by coagulation with subsequent filtration. Aluminum oxychloride was used as a coagulant with the dosage of 60 mg L−1 (20 mg L−1 as Al2O3). Water was clarified after coagulation, using a quartz sand filter (Figure 6). The filtrate entered the PCD reactor through ejector 1 and was recycled using ejector 2 for energy delivery variation. The duration of a cycle at the flow rate of 9 L min−1 and contact surface of 11.7 m−1 was 30 min at the energy consumption of
Table 1. Results of Treatment of the Wastewater of Oil Distillery treatment stage
turbidity (FTU)
oil products (mg L−1)
inflow wastewater coagulation PCD treatment (1.74 kWh m−3) filtration
107.7 5.7 3.8 2.0
4.14 1.07 0.09 0.04
concentration by a factor of ∼10, from 1.07 to 0.09 mg L−1. Also, turbidity decreased from 5.7 to 3.8 FTU, which may be explained by the destabilizing action of oxidants toward colloids by formation of hydrophilic moieties at the surface of colloid particles.4,13,26 Further reduction of both turbidity and oil products is observed in filtration (Table 1). The method showed the ability of PCD method to reduce the content of dissolved oil products in industrial wastewater to the sanitary requirement level, confirming observations made for model solutions.
■
CONCLUSIONS Pulsed corona discharge (PCD) application appeared to be an energy-efficient method of oxidation of dissolved oil products from a few mg L−1 to the sanitary standard (0.05 mg L−1). The PCD method surpasses conventional ozonation in energy efficiency and offers reliable and simple alternative. The mass transfer of oil products was established to be the overall process restraining stage, which gives the possibility of cost-effective choice of treatment parameters. Low-intensity PCD brings benefits in energy efficiency of treatment. The results obtained in experiments with model solutions were successfully confirmed in the treatment of industrial wastewater: the PCD treatment combined with filtration allowed reduction of turbidity and oil products content to the sanitary standards established for polluted waters disposal to the commercial fishery water bodies.
■
AUTHOR INFORMATION
Corresponding Author
*Tel.: +358 401475630. E-mail: sergei.preis@lut.fi. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Hashmat, A. Water Pollution and Its Recent Challenges; Daya Publishing House: Delhi, India, 2012. (2) Lee, K.; Neff, J. N. Produced Water; Springer Science+Business Media LLC: London, 2011. (3) Fakhru’l-Razi, A.; Pendashteh, A.; Abdullah, L. C.; Biak, D. R. A.; Madaeni, S. S.; Abidin, Z. Z. Review of Technologies for Oil and Gas Produced Water Treatment. J. Hazard. Mater. 2009, 170, 530 (DOI: 10.1016/j.jhazmat.2009.05.044). (4) Cha, Z.; Lin, C. F.; Cheng, C. J.; Hong, P. K. Removal of Oil and Oil Sheen from Produced Water by Pressure-Assisted Ozonation and
Figure 6. Outline of the wastewater treatment at the oil distillery. 7266
dx.doi.org/10.1021/ie403730q | Ind. Eng. Chem. Res. 2014, 53, 7263−7267
Industrial & Engineering Chemistry Research
Research Note
Sand Filtration. Chemosphere 2010, 78, 583 (DOI: 10.1016/j.chemosphere.2009.10.051). (5) Veil, J. A.; Puder, M. G.; Elcock, D. Government Develops Innovative, Interactive Drilling-Waste MIS. Oil Gas J. 2004, 102, 31. (6) Decree of Federal Fishing Agency of Russian Federation No. 20, Jan. 18, 2010 (in Russ.). (7) Fang, C. S.; Lin, J. H. Air Stripping for Treatment of Produced Water. J. Petrol. Technol. 1988, 40, 619. (8) Meijer, D. T.; Madin, C. Removal of Dissolved and Dispersed Hydrocarbons from Oil and Gas Produced Water with MPPE Technology to Reduce Toxicity and Allow Water Reuse. APPEA J. 2010, 1 (50th Anniverary Issue). (9) Peng, H.; Tremblay, A. Y. The Selective Removal of Oil from Wastewaters While Minimizing Concentrate Production Using a Membrane Cascade. Desalination 2008, 229, 318 (DOI: 10.1016/ j.desal.2007.10.018). (10) Torvik, H., Bergersen, L.; Paulsen, C. One Year of Operational Experience with C-Tour at Statfjord C. Presented at 3rd NEL Produced Water Workshop, Aberdeen, Scotland, 2005. (11) Kulik, N.; Trapido, M.; Veressinina, Y.; Munter, R. Treatment of Surfactant Stabilized Oil-in-Water Emulsions by Means of Chemical Oxidation and Coagulation. Environ. Technol. 2007, 28, 1345 (DOI: 10.1080/09593332808618896). (12) MacAdam, J.; Ozgencil, H.; Autin, O.; Pidou, M.; Temple, C.; Parsons, S.; Jefferson, B. Incorporating Biodegradation and Advanced Oxidation Processes in the Treatment of Spent Metalworking Fluids. Environ. Technol. 2012, 33, 2741 (DOI: 10.1080/ 09593330.2012.678389). (13) Hong, A.; Nakra, S.; Kao, J.; Hayes, D. Pressure-Assisted Ozonation of PCB and PAH Contaminated Sediments. Chemosphere 2008, 72, 1757 (DOI: 10.1016/j.chemosphere.2008.04.061). (14) Krichevskaya, M.; Klauson, D.; Portjanskaja, E.; Preis, S. The Cost Evaluation of Advanced Oxidation Processes in Laboratory and Pilot-Scale Experiments. Ozone: Sci. Eng. 2011, 33, 211 (DOI: 10.1080/01919512.2011.554141). (15) Hoeben, W.; van Veldhuizen, E.; Rutgers, W.; Kroesen, G. Gas Phase Corona Discharges for Oxidation of Phenol in an Aqueous Solution. J. Phys. D: Appl. Phys. 1999, 32, 133 (DOI: 10.1088/00223727/32/24/103). (16) Pokryvailo, A.; Wolf, M.; Yankelevich, Y.; Wald, S.; Grabowski, L.; van Veldhuizen, E.; Rutgers, W.; Reiser, M.; Glocker, B.; Eckhardt, T.; Kempenaers, P.; Welleman, A. High-Power Pulsed Corona for Treatment of Pollutants in Heterogeneous Media. IEEE Trans. Plasma Sci. 2006, 34, 1731 (DOI: 10.1109/TPS.2006.881281). (17) Preis, S.; Panorel, I. C.; Kornev, I.; Hatakka, H.; Kallas, J. Pulsed Corona Discharge: The Role of Ozone and Hydroxyl Radical in Aqueous Pollutants Oxidation. Wat. Sci. Technol. 2013, 68, 1536 (DOI: 10.2166/wst.2013.399). (18) Boev, S. G.; Muratov, V. M.; Polyakov, N. P.; Yavorovskiy, N. A. Reactor and Method for Water Purification, Patent Application of Russian Federation No. 2136600 RF MKI6 C02F 1/46, 7/00, Dec. 16, 1997. (19) Panorel, I. C.; Kornev, I.; Hatakka, H.; Preis, S. Pulsed Corona Discharge for Degradation of Aqueous Humic Substances. Wat. Sci. Technol. 2011, 11, 238 (DOI: 10.2166/ws.2011.045). (20) Kornev, I.; Yavorovsky, N.; Preis, S.; Khaskelberg, M.; Isaev, U.; Chen, B.-N. Generation of Active Oxidant Species by Pulsed Dielectric Barrier Discharge in Water−Air Mixtures. Ozone: Sci. Eng. 2006, 28, 207 (DOI: 10.1080/01919510600704957). (21) Polyakov, N. P. Nanosecond Ozonators. Instrum. Exp. Technol. 2004, 47, 685 (DOI: 10.1023/B:INET.0000043881.88641.e6). (22) Method of mass concentration measurement for oil products in samples of natural, potable and waste waters using fluorimeter “Fluorat02”. Federal Nature Protection Regulatory Document PNDF 14.1:2:4.128-98, Federal Service of Monitoring of Natural Resources of Russian Federation, Moscow, 1998; 25 pp. (in Russ.). (23) Clesceri, L. S.; Greenberg, A. E.; Eaton, A. D. Standard Methods for the Examination of Water and Wastewater, 19th Edition; American Public Health Association: Washington, DC, 1995.
(24) Danckwerts, P. V. Gas−Liquid Reactions; McGraw−Hill Book Co.: New York, 1970. (25) Pietsch, G. J.; Gibalov, V. I. Dielectric Barrier Discharges and Ozone Synthesis. Pure Appl. Chem. 1998, 70, 1169. (26) Cheng, L.; Bi, X.; Ni, Y. Oilfield Produced Water Treatment by Ozone-Enhanced Flocculation. Energy Educ. Sci. Technol. Part A 2012, 29, 91.
7267
dx.doi.org/10.1021/ie403730q | Ind. Eng. Chem. Res. 2014, 53, 7263−7267
