Energy Fuels 2010, 24, 5903–5906 Published on Web 10/27/2010
: DOI:10.1021/ef1010989
Capturing H2S(g) by In Situ-Prepared Ultradispersed Metal Oxide Particles in an Oilsand-Packed Bed Column Nashaat N. Nassar and Pedro Pereira-Almao* Department of Chemical and Petroleum Engineering, Alberta Ingenuity Centre for In Situ Energy, University of Calgary, Calgary, Alberta, Canada Received August 16, 2010. Revised Manuscript Received October 5, 2010
The current oil recovery and upgrading processes contribute directly to air pollution problems. H2S(g) is considered one of the major gaseous pollutants in oil recovery and processing. The aim of this study is to investigate the feasibility of methods aimed at the in situ capture of H2S(g) and its conversion into an environmentally neutral final product. In this work, we tested the sorption of H2S(g) into different in situprepared colloidal metal oxides in an oilsand matrix under recovery conditions, namely, ZnO, CuO, NiO, and Al2O3. In addition, the effect of metal oxide concentration and reaction temperature on H2S(g) reactivity was evaluated. Furthermore, commercially available ZnO nanoparticles were tested for comparison. Except for Al2O3, all the considered metal oxides reacted stoichiometrically with H2S(g) at the selected temperature and pressure. An increase in the metal oxide concentration favored the removal of H2S(g). The in situ-prepared ZnO ultradispersed particles were found to be more reactive than the commercial nanoparticles, as a result of their dispersion ability and intrinsic reactivity.
on the other hand, is to capture H2S(g) in situ, subsurface. Our previous work4,5,7,8 aimed at investigating how ultradispersed particles can reduce the environmental footprint of heavy oil recovery and upgrading. To better mimic oilsand processing conditions, in this study an oilsand matrix has been impregnated with ultradispersed metal oxide particles and employed for H2S(g) capture under SAGD conditions for the first time. In addition, the effect of the type of metal oxide, concentration, and reaction temperature was investigated. Furthermore, H2S(g) sorption employing commercial ZnO nanoparticles dispersed in the oilsand matrix has been tested for comparison. ZnO is a widely used sorbent for the removal of H2S(g) in refineries because it has a high sorption rate and capacity.9,10 In addition, the spent ZnO sorbent is environmentally sound. Moreover, ZnO is considered a stable and cost-effective sorbent compared to other metal oxides.10
1. Introduction As world demand for light sweet crude oil is growing, the demand for recovering and upgrading unconventional oils to meet current and future energy needs is growing. The oilsand deposits of northern Alberta represent one of the reliable sources.1 There is a growing interest in developing the huge resource associated with this unconventional oil. The most common recovery concept employs steam-assisted gravity drainage (SAGD).2,3 Heavy oil recovery from oilsands, with current processes, has proven to be challenging from an environmental standpoint. Large amounts of wastewater and air emissions are produced during the recovery and upgrading processes. A common gaseous pollutant produced is hydrogen sulfide [H2S(g)], which is colorless, odorous, highly toxic, corrosive for pipeline, and poisonous for catalysts.4,5 H2S(g) forms primarily from the chemical reaction of heavy oil with water (steam).6 A number of techniques have been employed for the removal of H2S(g) from commercial processes. However, the majority of these techniques deal only with H2S(g) on the surface, once it has been released. Our aim,
2. Experimental Section 2.1. Materials. A mixture of vacuum gas oil (VGO) and vacuum residue (VR) belonging to bitumen from Athabasca, Alberta, was used as a source of heavy oil. The packing material was standard AGSCO Sand (100-140-mesh particle size, AGSCO Corp., Hasbrouck Heights, NJ) and was used for preparing oilsand samples after it was mixed with heavy oil. The following metal precursors were used to prepare the corresponding colloidal metal oxide/hydroxide particles: Zn(NO3)2 3 6H2O (99%, Fisher Scientific, Toronto, ON), Cu(NO3)2 3 2.5H2O (>98.6%, Sigma-Aldrich, Toronto, ON), Ni(NO3)2 3 6H2O (>99.99%, Alfa Aesar, Toronto, ON), and Al(NO3)3 3 6H2O (>98%, SigmaAldrich). Commercial ZnO nanoparticles with diameter of 98%, Sigma-Aldrich) were used for comparison.
*To whom correspondence should be addressed. Telephone: (403) 220-4799. Fax: (403) 220-4852. E-mails:
[email protected] (P. PereiraAlmao), nassar@ucalgary (N. Nassar). (1) Governement of Alberta. Environmental management of Alberta’s oilsands. http://environment.gov.ab.ca/info/library/8042.pdf (accessed September 1, 2010). (2) NationalEnergyBoard Canada’s Oil Sands Opportunities and Challenges to 2015: An Update, The Publications Office, National Energy Board, Calgary, AB, 2006. (3) Butler, R. M. Thermal Recovery of Oil and Bitumen; Prentice-Hall: Englewood, NJ, 1991. (4) Husein, M. M.; Patruyo, L.; Pereira-Almao, P.; Nassar, N. N. J. Colloid Interface Sci. 2010, 342, 253. (5) Nassar, N. N.; Husein, M. M.; Pereira-Almao, P. Fuel Process. Technol. 2010, 91, 169. (6) Hyne, J. B.; Greidanus, J. W.; Tyrer, J. D.; Verona, D.; Clark, P. D.; Clarke, R. A.; Koo, J. H. F. In 2nd International Conference on Heavy Crude and Tar Sands, Caracas, Venezuela, 1982; Vol. I, Chapter 9. r 2010 American Chemical Society
(7) Patruyo, L. M.Sc. Thesis, University of Calgary, Calgary, AB, 2008. (8) Nassar, N. N.; Husein, M. M. In The IASTED International Conference on Environmental Management and Engineering (EME 2009), Banff, AB, 2009, p 135. (9) Wang, X.; Jia, J.; Zhao, L.; Sun, T. Appl. Surf. Sci. 2008, 254, 5445. (10) Sun, J.; Modi, S.; Liu, K.; Lesieur, R. Energy Fuels 2007, 21, 1863.
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Energy Fuels 2010, 24, 5903–5906
: DOI:10.1021/ef1010989
Nassar and Pereira-Almao
Figure 1. Schematic diagram of the experimental setup.
the column and was fed, at atmospheric pressure and 25 °C, to a gas chromatograph equipped with a FPD detector (SRI 8610C, SRI Instruments, Torrance, CA). 2.4. H2S(g) Sorption Experimental Procedure. A 12 g sample of oilsand containing the specified concentration of colloidal metal oxide/hydroxide particles was added to the packed bed column. A leak test was performed by pressurizing the reactor with pure He up to 210 psi. A 1% change in pressure per hour was adopted as the maximum allowable pressure decrease during the leak test. A 210 psi pressure was maintained after the termination of the leak test. He(g) was used for purging the system until O2(g) disappeared completely, as confirmed by the gas chromatograph. Following purging, the column was heated to the desired temperature. When the reactor working pressure and temperature were attained, the actual sorption experiments start by switching of the inlet gas from pure He(g) to a H2S(g)-containing feed. At this point, the zero reaction time for the sorption experiment was considered. The reaction was conducted until saturation; that is, no significant change in H2S(g) inlet and outlet concentrations appeared. Once the breakthrough experiment was complete, the reactor column was cooled to room temperature and a step change was made in the feed composition back to pure He(g), which continues until the column is completely purged. After that, the oilsand sample was discharged from the reactor column, and the column and assembly were washed with toluene. At this stage, the apparatus was ready for another cycle of experiments. A control experiment, with oilsands without the metal oxide particles in the reaction column, was conducted under similar conditions. This was performed to measure the amount of H 2 S(g) reacted or adsorbed with the oilsand matrix. This control experiment was used as a background sorption to be subtracted from the real sorption experiment involving metal oxide particles. 2.5. Data Analysis. The data gathered in the experiments were used to determine the H2S(g) sorption by metal oxide particles present in the oilsand-packed bed. The results of H2S(g) sorption were plotted as the fraction of the H2S(g) concentration in the effluent gas from the oilsand-packed bed column [CH2S(g),e] over that in the feed stream [CH2S(g),i], as per eq 1.
NaOH (5.0 N, Alfa Aesar) was used as the precipitating agent. All chemicals were used without further purification. 2.2. Preparation of the Ultradispersed Sorbent in an Oilsand Matrix. In situ preparation of colloidal metal oxide/hydroxide particles followed our previous techniques.11-13 Briefly, an aqueous solution of metal salt was exposed to a heavy oil matrix followed by addition of a precipitating agent at a later stage. The composition of the heavy oil matrix was maintained at 80 wt % VGO and 20 wt % VR, and 10 vol % water or aqueous precursor solutions were used. The heavy oil-containing colloidal metal oxide/hydroxide particles were mixed with sand at a ratio of 1:10 (grams per gram). After that, the oilsand-containing metal oxide particles was transferred to a packed bed column and left for 1 h to ensure compaction and equilibration. Care was taken to release any air remaining in the system while the oilsand was passed to the column. To further investigate the ability of oilsand matrices to disperse ex situ-prepared particles for the sorption of H2S(g). Commercially available zinc oxide nanoparticles was tested for the sorption of H2S(g). The particles were dispersed in the oilsand matrix by mixing the oilsand with the ZnO powder. 2.3. Experimental Setup. The experimental setup, which consists mainly of a packed bed column reactor (The Swagelok Co., Solon, OH), gas handling and metering equipment, and a gas chromatograph, is presented schematically in Figure 1. The packed bed column reactor was a stainless steel tube with an inside diameter of 0.35 in and a length of 8.5 in. It was fitted with a perforated brass gas distributor and equipped with a pressure gauge. The column was heated with a heating tape, and the temperature in the bed was measured with a thermocouple. The column was insulated by fiber glass casing. Small glass wool plugs were pushed to the bottom and top of the column to prevent drainage of the oilsand through the column ends. The column is capable of handling a pressure of up to 3000 psi and a temperature of up to 537 °C. The feed streams contained 200 ppm H2S and 1000 ppm N2 in He (balance). The flow rate of the gas stream was controlled at the outlet by a flow controller (a primary gas flow standard, range of 1-6000 cm3/min, A. P. Buck, Inc., Orlando, FL). The exit gas was taken from the top of (11) Nassar, N. N.; Husein, M. M. Fuel Process. Technol. 2010, 91, 164. (12) Nassar, N. N.; Husein, M. M. J. Colloid Interface Sci. 2007, 316, 442. (13) Nassar, N. N.; Husein, M. M. Langmuir 2007, 23, 13093.
f ¼ 5904
CH2 SðgÞ , e CH2 SðgÞ , i
ð1Þ
Energy Fuels 2010, 24, 5903–5906
: DOI:10.1021/ef1010989
Nassar and Pereira-Almao
Figure 2. f vs time for oilsand matrices containing 0.23 M in situprepared ultradispersed ZnO particles at different temperatures, at 210 psi with a feed flow rate of 110 cm3/min.
Figure 3. H2S(g) breakthrough curves presented as f vs time in the presence and absence of ultradispersed ZnO particles in an oilsand matrix, at 200 °C and 210 psi with a feed flow rate of 110 cm3/min.
3. Results and Discussion 3.1. Sorption Mechanism. The sorption of H2S(g) is a two-stage process.5 In step 1, the H2S(g) molecule diffuses through the sand/oil mixture until it reaches the surface of the colloidal metal oxide/hydroxide particle where it is adsorbed and dissociated into two active species, HS- and Hþ. In step 2, sorption takes place, whereby HS- and Hþ react readily with the bulk of the metal oxide/hydroxide particles to form metal sulfide and water, respectively. The reaction stoichiometries are shown in eqs R1 and R2 for divalent and trivalent metals, respectively MOðsÞ þ H2 SðgÞ f MSðsÞ þ H2 OðgÞ
ðR1Þ
M2 O3ðsÞ þ 3H2 SðgÞ f M2 S3ðsÞ þ 3H2 OðgÞ
ðR2Þ Figure 4. f vs time for commercial ZnO nanoparticles dispersed in an oilsand matrix. f values for the control sample and the in situprepared ultradispersed ZnO particles are provided for comparison. Experiments conducted at 200 °C and 210 psi with a feed flow rate of 110 cm3/min.
where M stands for metal.5 The reaction ceases either when all metal oxide is reacted with H2S(g) or when the formed metal sulfide forms a layer that acts as a mass transfer barrier, which in turn limits the diffusion process. This most likely occurred for ex situ application where larger metal oxide particles or aggregates would form. 3.2. Effect of Temperature. To study the effect of temperature on the sorption efficiency, experimental breakthrough data were recorded at different temperatures, namely, 25, 100, and 200 °C. Figure 2 shows the H2S(g) breakthrough curves obtained at different temperatures. The experimental conditions were 210 psi, a feed flow rate of 110 cm3/min, and 0.23 M ZnO. As expected, as the temperature increased from 25 to 200 °C, the breakthrough curve shifts to the right, suggesting an increase in the sorption efficiency of ZnO ultradispersed particles for H2S(g). This result is promising as the considered experimental conditions (i.e., 200 °C and 210 psi) are commonly used for in situ heavy oil recovery. Thus, these conditions were adopted for the subsequent experiments. Similar observations have been reported by Carnes and Klabunde for the sorption of H2S(g) into nanocrystalline zinc oxide.14 It is worth noting that, in our previous study, the sorption efficiency of iron-based ultradispersed particles decreased as the temperature increased.5 This was attributed to the phase change in the crystal structure of iron oxide as well as particle aggregation.4,5 3.3. Effect of the Concentration of Ultradispersed ZnO Particles. Figure 3 shows the H2S(g) breakthrough curves obtained at different concentrations of ZnO in an oilsand
Figure 5. f vs time for different types of in situ-prepared metal oxide ultradispersed particles in an oilsand matrix at 0.23 M, at 200 °C and 210 psi with a feed flow rate of 110 cm3/min.
matrix. The experimental conditions were 200 °C, 210 psi, and a feed flow rate of 110 cm3/min. As expected, the breakthrough time increased as the concentration of ZnO increased. The breakpoints for the ZnO concentrations of 0.23 and 0.40 M were 258 and 438 min, respectively. Calculations of the number of moles of H2S(g) reacted until the breakpoints indicate that a stoichiometric amount of H2S(g) was readily reacted as per reaction R1. This suggests that
(14) Carnes, C. L.; Klabunde, K. J. Chem. Mater. 2002, 14, 1806.
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Energy Fuels 2010, 24, 5903–5906
: DOI:10.1021/ef1010989
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Table 1. Calculated Number of Moles of H2S(g) Reacted up to the Breakpoint, Theoretically and Experimentally at 200 °C and 210 psi type of oxide
concn (mol/L)a
breakpoint (min)
calculated H2S(g) (mmol)b
H2S(g) sorbed at the breakpoint (mmol)c
unsorbed H2S(g) (mmol)
control ZnO NiO CuO Al2O3
0 0.23 0.23 0.23 0.23
12 258 265 264 250
0.225 0.227 0.226 0.682
0.222 0.227 0.226 0.224
0.003 0.0 0.0 0.458
c
a Calculated on the basis of liquid volume (i.e., heavy oil and water). b Calculated on the basis of the stoichiometric ratio as per reactions R1 and R2. Experimental value.
neither mass transfer nor reaction kinetics would hinder the reactivity between H2S(g) and the ZnO particles, within the residence time of the H2S(g) in the column. Similar observations have been reported in our previous study about the sorption of H2S(g) into ultradispersed colloidal FeOOH particles formed in situ in water-in-oil microemulsion system.4,5 3.4. Commercial ZnO Particles as Sorbents. To validate the effectiveness of the in situ-prepared ZnO particles for H2S(g) sorption, the sorption of H2S(g) was compared to that of commercially available ZnO nanoparticles. The experiments were conducted at 200 °C and 210 psi with a feed flow rate of 110 cm3/min. The concentration of ZnO was fixed at 0.23 M. The samples were compared on a per mole basis. Figure 4 shows the H2S(g) breakthrough curves for the control sample, a sample containing commercial ZnO nanoparticles, and a sample containing in situ-prepared ZnO particles. It is evident from Figure 4 that the in situ-prepared ZnO particles were much more efficient in H2S(g) sorption than the commercial nanoparticles. This can be attributed to the good dispersion with the in situ-prepared particles. Similar observations were reported in our previous studies with water-in-oil microemulsions containing in situ-prepared iron oxide particles and ex situ commercial nanoparticles.4,5 The in situ-prepared nanoparticles outperformed the commercial ones, because it was found that particle aggregation and poor dispersion can occur during ex situ application. This impacts the surface area, which in turn hinders the sorption efficiency.4,5,15 These observations were also reported by Carnes and Klabunde,14 who tested the reactivities of metal oxide nanoparticles toward H2S(g) at high temperatures. The authors reported that the in house-prepared nanoparticles are much more efficient than the commercially available oxides in H2S(g) scrubbing. The authors showed that the reaction molar ratios [moles of H2S(g) per mole of oxide] were 1:2.4 and 1:32 for in house-prepared and commercial zinc oxide nanoparticles, respectively.14 This again supports the fact that in situ-prepared sorbents not only are more efficient in H2S(g) scrubbing but also are cost-effective for reducing the environmental footprint of oilsand industry as they can be prepared in situ, thus minimizing the processing cost for pumping, transportation, instrumentation, etc.
3.5. Other Ultradispersed Metal Oxide Sorbents. Sorptions of H2S(g) into selected metal oxides such as NiO, CuO, and Al2O3 that are commonly used in heavy oil upgrading were tested at 200 °C and 210 psi with a feed flow rate of 110 cm3/min. The individual concentration of metal oxide in the oilsand sample was fixed at 0.23 mol/L, per volume of heavy oil. Figure 5 shows the breakthrough curves of sorption of H2S(g) into the selected metal oxides. The figure shows that all the selected metal oxide (except Al2O3) succeeded in the sorption of the H2S(g) under the considered experimental conditions. Calculations of the number of moles of H2S(g) reacted up to the breakpoint indicated that H2S(g) was stoichiometrically sorbed by NiO, CuO, and ZnO in situprepared particles as per reaction R1. However, apparently Al2O3 was not very reactive toward H2S(g). This is not surprising as the reaction of H2S(g) with Al2O3 is thermodynamically unfavorable.14 Table 1 shows the results obtained. Nonetheless, Al2O3 was still capable of removing an appreciable amount of H2S(g) because of its accessible surface area. Similar observations have been reported by Carnes and Klabunde, who found that ZnO nanoparticles react nearly to a stoichiometric portion with H2S(g), while Al2O3 nanoparticles exhibited little reactivity.14 4. Conclusions This study has shown that in situ-prepared ultradispersed metal oxide particles in an oilsand matrix are efficient scrubbers for H2S(g) under oilsand recovery conditions. It was shown that a stoichiometric amount of H2S(g) was readily reacted with in situ-prepared metal oxide particles at 200 °C and 210 psi. Increasing the reaction temperature from 25 to 200 °C favored the removal of H2S(g) stoichiometrically. Removal of H2S(g) was enhanced by increasing the metal oxide concentration in the oilsand matrix. Furthermore, the in situ-prepared metal oxide particles were more efficient for H2S(g) removal than the commercially available nanoparticles because of their dispersion abilities, high surface areas, and intrinsic reactivities. Acknowledgment. We acknowledge the funding from the Alberta Ingenuity Centre for In Situ Energy (AICISE). Thanks are due to Mr. Luis Alberto Pineda for helping in the experimental setup and Mrs. Maha AbuHafeetha Nassar for drawing the experimental setup.
(15) Husein, M. M.; Nassar, N. N. Curr. Nanosci. 2008, 4, 370.
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