Article pubs.acs.org/IECR
Characteristics and Kinetics of Coked Catalyst Regeneration via Steam Gasification in a Micro Fluidized Bed Yuming Zhang,*,†,‡ Meiqin Yao,‡ Guogang Sun,*,† Shiqiu Gao,‡ and Guangwen Xu‡ †
State Key Laboratory of Heavy Oil Processing, China University of PetroleumBeijing, Beijing 102249, China State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
‡
ABSTRACT: Petroleum residue cracking combined coke gasification (RCCG) process was proposed to regenerate the catalyst via coke-steam gasification for syngas production, thus to solve the problem of excessive heat generated via coke combustion in the industrial fluid catalytic cracking (FCC) process. A commercial FCC catalyst and a bifunctional (BF) catalyst were used as the candidates for the RCCG process, and the BF catalyst was specially designed with both catalytic effects of oil cracking and coke gasification. The regeneration characteristics and kinetics of FCC and BF catalysts were studied using a micro fluidized bed. The results showed that high-quality syngas was produced when regenerating the catalysts via steam gasification and the sum of H2 and CO in the produced gas was over 80 vol % under electrically heated condition.The gasification rate first increased with increasing carbon conversion and then slowly decreased. In comparison with FCC catalyst, the regeneration time of BF catalyst was shortened by more than 30% via addition of alkaline metal oxides and adjustment of its pore size. Homogenous model (HM) and shrinking core model (SCM) were used to calculate the regeneration kinetic parameters of coked catalysts. It was found that the activation energies from these two models were close to each other, while HM had a better fitting relevance for the data than SCM. The activation energy of BF catalyst regeneration was about 115 kJ·mol−1, lower than that of FCC catalyst (150 kJ·mol−1), demonstrating that BF catalyst was easier to regenerate via coke gasification and also justified its bifunctional characteristics. The activation energy of coke gasification on BF catalyst could be further decreased to 45 kJ·mol−1 when introducing 3% oxygen as the gasification reagent. was first cracked into light fractions via contacting with catalyst particles and then conveyed into a distillation tower to get cracking gas (dry gas and liquefied petroleum gas, LPG) and liquid oil. Coke deposited on the catalyst was removed via gasification for syngas instead of combustion in the regenerator and meanwhile for catalyst regeneration. The regenerated hot catalyst was circulated back into the reactor, providing the endothermic heat needed for heavy oil cracking. Moreover, the syngas from coke gasification could be further re-formed into hydrogen and served as the source to refine the cracked oil into clean fuels, which was similar to a flexi-coking process. It is proved by the industrial flexi-coking process that the hydrogen generated via coke gasification is twice the amount needed for hydrogenating its extracted liquids.5 The fundamentals of the RCCG process, such as operation parameters, different feedstocks, and catalysts, have been studied in our previous publications.6,7 The catalyst has a key role to fulfill in the RCCG process. First, the catalyst should have moderate activity for heavy oil cracking to obtain high liquid yield. Second, the catalytic stability should remain stable in the thermal and hydrothermal treating process for catalyst regeneration with steam gasification. A commercial FCC and a self-designed bifunctional (BF) catalyst have been tested for the RCCG process, and their
1. INTRODUCTION Fluid catalytic cracking (FCC) is a refining process to convert heavy feedstocks (i.e., vacuum gas oil, atmospheric residue, and vacuum residue) into light, high-value products. The FCC process will continue to play a key role in refinery in terms of the increasing demand of transportation fuels, such as gasoline and diesel.1 Heavier feedstocks with carbon residue higher than 2 wt % have been used in the industrial FCC process due to the decreasing quality of petroleum, to be known as the resid fluidized catalytic cracking (RFCC) process.2,3 As a result, more coke will be generated and deposited on the catalyst when processing degraded heavy oil with higher carbon residue. The traditional method for FCC catalyst regeneration is to remove the coke via combustion in the regenerator and it will generate excessive heat in the system which should be removed by an external heat exchanger. Hydrogen is usually of great demand for hydrogenation of the cracked oil into clean vehicle fuels, and especially of great shortage in the case of treating heavy oil due to its low H/C ratio.4 Besides, the RFCC process also has strict requirements on the feed quality, e.g., high H/C ratio and low metal content to avoid excessive catalyst consumption and high coke yield. Therefore, only the feeds with relatively low contents of metals, sulfur, and carbon could be the candidates for RFCC process, which are limited and expensive in the refineries. The residue cracking combined coke gasification (RCCG) process was thus proposed to realize the value-added utilization of heavy oil and also to produce syngas via coke gasification instead of combustion, as shown in Figure 1. Petroleum residue © 2014 American Chemical Society
Received: Revised: Accepted: Published: 6316
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Figure 1. Schematic diagram of the residue cracking combined coke gasification (RCCG) process.
catalytic stability was justified by the recycling tests.8 The detailed regeneration characteristics and kinetics of coked catalyst via steam gasification were investigated in the present study. The previous studies were mainly involved in regenerating coke-containing catalyst via air combustion,9−11 and few publications for FCC catalyst regeneration via coke gasification are found. However, the characteristics and kinetics of coke-steam gasification for carbonaceous materials have been widely studied,12−14 such as coal, petroleum coke, and oil shale, mainly using thermogravimetric (TG) analysis. TG measurement for the reaction kinetics is based on the weight loss data of the samples at different atmospheres under a specified heating program. The samples are confined in a fixed crucible during the reaction process, which leads to great gas diffusion inhibition. As a result, TG analysis could hardly reflect the real regeneration process for FCC catalyst in the fluidized bed regenerator. In the present study, the regeneration characteristics and kinetics of coked catalyst were conducted in a micro fluidized bed coupled with an online mass spectrometer (MS), thus to enable rapid heating of reactant particles and quick measurement of the gaseous products.15 The bed materials (i.e., silica sand) are turbulently fluidized in the reactor, and the pulverized samples are uniformly distributed by the gas and bed materials. Besides, gas products could be quickly entrained by the upward fluidizing gas. All the factors can enhance the mass transfer and decrease the diffusion resistance in a fluidized bed compared to a TG analyzer. The micro fluidized bed analyzer has been used to study different gas−solid reactions (i.e., biomass pyrolysis, CaCO3 decomposition, and graphite combustion, etc.)16,17 and justified its advantage of isothermal differential reaction characteristics. The regeneration characteristics of FCC and BF catalysts were measured isothermally by simulating the reaction conditions in the fluidized bed regenerator of the industrial FCC process. The corresponding reaction kinetic parameters were obtained using different reaction models. By comparing the regeneration characteristics and kinetics of these two catalysts, we could have an insight into the catalyst preparation and provide some reactor design data for the RCCG process.
commercial FCC catalyst and a self-designed BF catalyst. The FCC catalyst was the equilibrium catalyst from a commercial FCC unit, mainly composed of an inert matrix (kaolin), an active matrix (alumina), a binder (silica or silica−alumina), and a Y-zeolite (USY or rare earth exchanged HY). The RCCG process requires the catalyst not only to have moderate activity for heavy oil cracking but also to be with some catalytic effects for coke gasification, thus to reduce the time for catalyst regeneration. The catalyst with catalytic effects of both residue cracking and coke gasification was developed for the RCCG process and named as the bifunctional catalyst. On the basis of knowledge of the FCC catalyst, we first adjusted the cracking acidity of BF catalyst by changing the Si/Al ratio of the matrix and the amount of zeolite and then adding some alkaline metal oxides and improving its pore structure for quick coke gasification. Our previous publication8 illustrated the detailed procedures of BF catalyst preparation. Table 1 shows the main compositions and properties of FCC and BF catalysts. Table 1. Properties and Composition of FCC and BF Catalysts items
FCC
BF
−3
824.3 bulk density (kg·m ) Sauter mean diameter (μm) 62 surface area (m2·g−1) 235.2 pore volume (cm3·g−1) 0.13 average pore diameter (Å) 48.7 XRF Analysis of Catalysts (wt %)
532.5 58 191.8 0.77 129.4
components
Al2O3
SiO2
Na2O
Re2O3
FCC BF
54.15 26.79
37.71 67.38
0.25 0.56
5.37 3.02
The apparatus and operation process for the preparation of coke-containing catalyst were given in our previous publications.6−8 In brief, heavy oil (i.e., vacuum residue) was converted into cracking gas, liquid, and coke in a fluidized bed reactor via contacting with the catalyst particles. Coke deposited on the catalyst was cooled in nitrogen atmosphere and then collected as the gasification samples. The coke content on FCC and BF catalysts was 2.75 and 3.38 wt %, respectively. 2.2. Apparatus and Operation. The schematic diagram of the micro fluidized bed reactor was shown in Figure 2, mainly consisting of the gas-supply and steam-generation units, the
2. MATERIALS AND METHODS 2.1. Sample Preparation. The samples for the experiments were two kinds of coke-containing catalysts, that is, a 6317
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Figure 2. Schematic diagram of the micro fluidized bed analyzer.
fluidized bed reactor, the pulse-feeding system, and the product purification and analysis parts. The mixture of argon and steam was used as the fluidizing gas. Also, steam served as the gasification reagent during the reaction process. Argon was the purging gas during the interval of each experiment and used as the calibrating gas during the gasification reaction. Partial oxygen would be introduced into the system as the gasification reagent together with steam due to the exothermic effect of carbon−oxygen reactions. The micro fluidized bed reactor was made of a quartz tube and had an inner diameter of 20 mm and a total length of about 160 mm. The reactor was divided into three sections by two gas distributors, that is, a preheating part filled with inert Al2O3 balls, a reaction area with bed materials (inert silica sand with particle diameters of 100−150 μm), and a purification part for diminishing fine particles. Silica sand was acid-washed, filtered, and calcined to remove the impurities before used as bed materials. High-purity argon, oxygen, and deionized water in the experiments were all commercially available. For the regeneration tests of coked catalysts, 4 g of silica sand was put in the reactor as bed material and about 150 mg of coke-coated catalyst was set in the pulse-feeding container, and then the gas lines were connected , the reaction system was purged, and the furnace temperature was set for gasification. When the preset temperature was reached and the bed materials were uniformly fluidized, the sample was instantly injected into the middle of the high-temperature silica sand by the electromagnetic valve in several milliseconds. The gasification products were quickly stripped out of the reactor by the upward fluidizing gas, then filtered, condensed, dried, and detected by the online MS. The composition of the produced gas was monitored until no more gas components were
detected (i.e., the response lines of MS become stable). Gas composition and concentration were determined using the internal standard method with the calibrating gas. Data collection and processing were accomplished by the selfdeveloped software, and eventually the reaction kinetic parameters were obtained. Each experiment was repeated three times to ensure the relative error less than 3%, and the average values of three experiments were used to calculate the kinetic data. 2.3. Analysis and Characterization. The carbon content of the spent catalyst was measured with a coke analyzer (CS344, LECO). The catalyst sample was burnt in pure oxygen, and its generated gas was analyzed with a gas chromatograph (GC) for the content of CO2, and in turn the carbon content was calculated . The compositions of FCC and BF catalysts were determined using X-ray fluorescence (XRF) spectrometry (AXIOS), and their particle size distributions were determined with the laser particle size analyzer (Malvern Mastersizer 2000). An automatic Brunauer−Emmett−Teller (BET) analyzer (Autosorb-1, Quantachrome) was used to measure the specific surface area and pore structure of the catalyst via N2 adsorption at 77 K and then outgassed in vacuum at 300 °C for 24 h. For a visual comparison, a scanning electron microscope (JSM-6700F JEOL) working with an accelerating voltage of 20 kV and a scanning diameter of 20 mm was used to characterize the surface morphology of the catalysts. MS (PROLINE AMETEK) was adopted to monitor the real-time gas variation during the gasification process. 2.4. Data Processing. The coke on the catalyst was mainly converted into H2, CO, CO2, and CH4 in the gasification reaction, and their corresponding concentrations could be calibrated according to the response values of the MS. So the 6318
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⎛ E ⎞ ⎟ k = A exp⎜ − ⎝ RT ⎠
carbon conversion value of coke gasification was defined by calculating the carbon-containing gas species (i.e., CO, CO2, and CH4) in the syngas, as shown in eq 1. t=t
X=
∫t = 0
t = tg
∫t = 0
( (
FAr × (C CO+ CCH4 + CCO2) 22.4CAr FAr × (C CO+ CCH4 + CCO2) 22.4CAr
) × 100% × 12) dt
E (9) RT where A, E, R, and T are the preexponential factor, activation energy, gas constant, and temperature (K), respectively. The value of ln k is linear with 1/T at a fixed temperature, allowing the determination of E from the slope of the correlation line and also A. ln k = ln(A) −
× 12 dt
(1)
where X (%) is the conversion ratio, t (min) and tg (min) represent the instant and the end of the reaction time, respectively. FAr (mL·min−1) is the flow rate of argon. CAr, CCO, CCH4, and CCO2 (vol %) stand for the volume fractions of Ar, CO, CH4, and CO2, respectively. Gasification rate R (min−1) is defined as the differential of conversion X to gasification time t, R=
dX dt
3. RESULTS AND DISCUSSION 3.1. Regeneration Characteristics of FCC Catalyst. In the preexperiments, the effect of external diffusion on the gasification reaction was determined at different gas flow rates. The results showed that the external diffusion effect of gasification could be negligible in the temperature range of 800−950 °C when the flow rate of argon and steam was 200 mL/min and 0.3 g/min, respectively. Figure 3 shows the gas
(2)
To quantitatively compare the reaction activity of coke between FCC and BF catalysts, the gasification reactivity index Rs (min−1) was proposed by Takarada et al.,18 which was defined as
Rs =
0.5 t0.5
(3)
where t0.5 (min) is the time needed for the conversion ratio up to 50%. There is hardly any reaction function model reported specifically on the catalyst regeneration via coke-steam gasification. However, the reaction kinetics of coal char12,19 and petroleum coke13,20 gasification has been fully studied. The typical reaction models13,19,20 for coke gasification are mainly involved in the shrinking core model and homogeneous model, respectively. Shrinking core model (SCM)21,22 assumes that the gasification reaction occurs only on the surface of spherical reactant particles, and the unreacted core would shrink gradually in the reaction process. The reaction order of SCM is 2/3. When the chemical reaction is the control step, SCM could be expressed as dX = k(1 − X )2/3 dt
Figure 3. Gas concentration versus time for steam regeneration of coked FCC catalyst at 900 °C.
concentration varied with time in the regeneration reaction of coked FCC catalyst at 900 °C. The main gas components for the coke-steam gasification were H2, CO, CO2, and CH4, and their corresponding concentrations first increased and then slowly decreased to zero at the end of the experiments. However, the time needed for different gas species reaching the peak differed from each other, and their release sequence could be proximately summarized as follows: CH4 < COH2 < CO2. The completion time of coke gasification in the micro fluidized bed was about 4−5 min according to the MS curve. Literature study shows that the properties of coke deposited on the catalyst are different from the condensed coke of coal char or petroleum coke.25 The coke component on the catalyst usually has the H/C ratio of 0.3−1.0. Cerqueira et al.26 indicates that there are four main types of coke identified in catalytic cracking of residue oil, that is, reaction or catalytic coke, dehydrogenation or metal coke, Conradson carbon coke, and soft coke (i.e., incomplete stripping and entrained hydrocarbons). Methane is not easy to be produced by the carbon-steam gasification at low temperature ( 0.95) than that of SCM for both FCC and BF catalysts. SCM suggests that the particle size of reactants uniformly decreases during the reaction process, while HM assumes that the particle size is constant as the reaction progresses. As mentioned above, coke distributed as a thin layer on the catalyst. The size of catalyst particles hardly changed even removing all of the coke during the gasification reaction, which was more close to the assumption of HM. Zhang et al.34 investigated the reaction kinetics of steam gasification for chars from heavy oil residue using TG and found that SCM had better fitting relevance than HM. During the kinetic study of petroleum coke gasification in the micro fluidized bed, we also observed that the R2 value of SCM was higher than that of HM, which was different from the conclusion in the current study. The fine char particles pyrolyzed from coal, biomass, or heavy oil (i.e., petroleum coke) usually have similar properties (i.e., composition) in the outer surface and inside cores and will react with gasification reagent on the surface and then shrink uniformly as the reaction proceeds. Apparently, the reaction process of coal char or petroleum coke is more close to the assumption of SCM, thus resulting in a higher R2 value of SCM than that of HM. This justified from another perspective that the characteristics of coke gasification could be distinguished from each other in the different reaction systems, also giving us deeper insight into the mechanisms of coke gasification. For a given reaction model, the reaction rate constant in the regeneration of BF catalyst was higher than that of FCC catalyst, especially at lower reaction temperature. For example, the k value of FCC catalyst regeneration at 800 °C using HM was 0.0706 min−1, while the corresponding k value greatly increased to 0.1196 min−1 for BF catalyst. It was obvious that coke gasification on the BF catalyst was indeed accelerated via
Figure 6. Gasification reactivity index Rs for steam regeneration of coked FCC and BF catalysts.
higher temperature. The Rs value at 950 °C of both catalysts increased about 7−10 times compared with that at 800 °C. Moreover, at the same reaction temperature, the coke on BF catalyst had higher gasification reactivity than that of FCC catalyst, and consequently BF catalyst was easier to regenerate than FCC catalyst. Coke distributed as a thin layer on the external surface of the catalyst. As a result, the morphological characteristics of coke were greatly determined by the textural properties of the catalyst. From the analysis of the catalysts (Table 1), we can see that BF catalyst had higher pore volume (0.77 cm3·g−1 vs 0.13 cm3·g−1) and also larger average pore diameter (129.4 Å vs 48.7 Å) than FCC catalyst. As a result, the gasification reagent (i.e., steam and oxygen) had more chances to contact with the surface coke and diffuse into the internal pores to remove the carbon due to the developed pore structures of BF catalyst. The SEM images of the coke-coated catalysts in Figure 7 further visualized that BF catalyst had more superficial pores on the surface, while FCC catalyst had a more condensed carbon layer. The deposited coke on the BF catalyst seemed loose and thus was prone to being removed during the coke-steam reaction. In addition, some alkaline metal oxides were added during the preparation of BF catalyst, thus leading to higher contents of Na2O than that of FCC catalyst (0.56 wt % vs 0.11 wt % in Table 1). And other alkaline metals, i.e., K, Ca, and Mg, etc., were also presented in BF catalyst (their contents are very low and not listed in the table), which had pronounced catalytic effects to accelerate coke gasification.30−33
Figure 7. Micrographs of coke-coated FCC (a) and BF (b) catalysts (magnification, ×5000). 6321
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Figure 8. Two kinds of gasification kinetic models for coked FCC (a, b) and BF (c, d) catalysts: (a, c) shrinking core model (SCM); (b, d) homogeneous model (HM).
Table 2. Reaction Rate Constant k and R2 for Regeneration of Coked FCC and BF Catalysts with SCM and HM FCC catalyst
BF catalyst
SCM −1
HM 2
temp (°C)
k (min )
R
800 850 900 950
0.0487 0.1072 0.2212 0.3333
0.9858 0.9384 0.9137 0.9015
−1
SCM −1
2
k (min )
R
0.0706 0.1579 0.3268 0.4966
0.9939 0.9796 0.9639 0.9561
optimizing its preparation, which was consistent with the shorter time needed for BF catalyst regeneration in Figure 5. The distinction of the k value calculated from SCM and HM mainly resulted from the different model assumptions and also the reaction equation. The kinetic parameters were obtained from the correlation of lnk and 1/T according to the Arrhenius equation (eq 9), i.e., for the activation energy and preexponential factor. Figure 9 demonstrates that the reaction data are subjected to a good linear fitting of ln k with 1/T for both FCC and BF catalysts using HM, meaning that coke gasification in the examined conditions is mainly controlled by the chemical reaction. The calculated values of E and A were listed in Table 3 for the regeneration of FCC and BF catalysts, and the activation energy from SCM and HM had good repeatability, that is, about 150 kJ·mol−1 for FCC catalyst and 115 kJ·mol−1 for BF catalyst. The activation energy for coke gasification on BF catalyst decreased by 35 kJ·mol−1 in comparison with that of FCC catalyst, verifying that BF catalyst was easier to regenerate than FCC catalyst via steam gasification. 3.3.2. Steam with Oxygen As Gasification Reagent. Together with steam 3% oxygen was used as the gasification
HM 2
k (min )
R
0.0824 0.1781 0.2806 0.3707
0.9841 0.9809 0.9380 0.9049
−1
k (min )
R2
0.1196 0.2589 0.4129 0.5477
0.9976 0.9975 0.9836 0.9960
Figure 9. Arrhenius equation plot for the kinetic parameters of FCC and BF catalysts regeneration using homogeneous model.
reagent to further promote the coke gasification rate on BF catalyst. Besides, the exothermic coke−oxygen reaction could provide some heat for the carbon steam gasification during the catalyst regeneration, which was necessary for the autothermal operation in the industrial application. Figure 10 presents the carbon conversion ratio varied with reaction time at different 6322
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Table 3. Kinetic Parameters of Regeneration for FCC and BF Catalysts with SCM and HM E (kJ·mol−1) reaction model case I case II case III
a
A (min−1)
SCM
HM
SCM
HM
149.41 114.66 45.45
151.37 116.15 45.19
4.31 × 105 1.88 × 105 44.25
4.31 × 105 3.20 × 105 98.69
a
Case I, Coked FCC catalyst regenerated with pure steam; case II, coked BF catalyst regenerated with pure steam; case III, coked BF catalyst regenerated with steam and 3% O2.
Figure 10. Carbon conversion X versus time and the change of gas composition for regeneration of BF catalyst using steam and 3% oxygen.
Figure 11. Kinetic profiles of coke gasification on BF catalyst using steam and 3% oxygen (a) shrinking core model (SCM); (b) homogeneous model (HM).
temperatures and the change of gas composition for the BF catalyst regeneration using a mixture of steam and oxygen. When introducing 3% oxygen, the reaction time was greatly decreased at all examined temperatures compared to that using pure steam, especially shortening the regeneration time at lower gasification temperature. For example, only 10 min was needed for completing coke gasification on BF catalyst at 800 °C using the mixture gas as the gasification reagent, while the corresponding time was up to 30 min when using pure steam as the gasification reagent (see Figure 5). When adding 3% O2 into steam as gasification reagent, H2 concentration obviously decreased due to its partial combustion. Correspondingly, the concentration of CO2 was almost doubled because of the carbon oxygen combustion. However, the amounts of H2 and CO were still high in the syngas under electrically heated conditions. Figure 11 converts the data of Figure 10 into the correlation of reaction model and reaction time to define k according to shrinking core model (Figure 11a) and homogeneous model (Figure 11b). The data are subjected to a linear fitting for all of the temperatures, verified by the fact that R2 was over 0.90 for SCM and even reached 0.97 for HM. The kinetic parameters of E and A were derived according to the Arrhenius equation (eq 10), and the results were listed in Table 3 (case III). The E values calculated from SCM and HM were close to each other, that is, about 45 kJ·mol−1 for the regeneration of BF catalyst using a mixture of oxygen and steam. This activation energy was obviously lower than the above-required activation energy (115 kJ·mol−1, case II) using pure steam as gasification reagent, demonstrating that adding oxygen as reagent could greatly promote coke gasification and consequently decrease the time for catalyst regeneration.
4. CONCLUSION A commercial FCC catalyst and a bifunctional (BF) catalyst were used as catalyst candidates for petroleum residue cracking combined coke gasification (RCCG) process. Their regeneration characteristics and kinetics were investigated in a micro fluidized bed analyzer. The BF catalyst with both catalytic effects of oil cracking and coke gasification was specially designed for the RCCG process. Coke gasification with steam on FCC and BF catalysts could produce high-quality syngas, with the contents of (H2 + CO) up to 80 vol % in the syngas, and simultaneously for catalyst regeneration. The coke gasification rate on the catalyst was enhanced at high reaction temperature, and the R value first increased to a peak and then slowly decreased with an increase of the carbon conversion ratio. The regeneration time of BF catalyst was shortened by more than 30% in comparison with that of FCC catalyst at all tested temperatures, mainly due to the more developed pore structures and its high contents of alkaline metal oxides. Shrinking core model (SCM) and homogeneous model (HM) were used to describe the coke gasification reaction kinetics on FCC and BF catalysts. It was found that HM had better fitting relevance for the reaction data than that of SCM, and E values obtained from these two models were close to each other. The activation energy of coke gasification on BF catalyst was about 115 kJ·mol−1, lower than that of FCC catalyst (150 kJ·mol−1), indicating that BF catalyst was easier to regenerate via gasification and also justified its bifunctional characteristics. Introducing 3% oxygen as gasification reagent could greatly decrease the activation energy of coke gasification on BF catalyst to about 45 kJ·mol−1 and thus could further accelerate coke gasification and reduce the time for catalyst regeneration. 6323
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AUTHOR INFORMATION
Corresponding Authors
*(Y.Z.) Tel.: +86-10-89734820. E-mail: yumingzhang@gmail. com. *(G.S.) Tel.: +86-10-89734820. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The study was conducted with the research programs financed by the Science Foundation of China University of Petroleum, Beijing (Grant No. 2462013YJRC021), the National Basic Research Program of China (973 Program, No. 2012CB224801), and the National Nature Science Foundation of China (Grant 21376250). Acknowledgements are also extended to Mr. Deping Yu for his help on the experiments.
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dx.doi.org/10.1021/ie4043328 | Ind. Eng. Chem. Res. 2014, 53, 6316−6324
