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Enhanced oil recovery and residues utilization of oil sludge through nitric acid pretreatment process
Journal of Environmental Chemical Engineering 7 (2019) 103089
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Enhanced oil recovery and residues utilization of oil sludge through nitric acid pretreatment process Kaichao ZuoJiaoa, Zheng Zhanga, Panjie Lia, Caiyun Jiangb, Jiaxun Liua, Yuping Wanga,
T
⁎
a
School of Chemistry and Material Science, Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control, Nanjing Normal University, Nanjing, 210046, China b Department of Engineering and Technology, Jiangsu Institute of Commerce, Nanjing, 211168, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oil sludge Nitric acid Oil recovery rate Pyrolysis Adsorption
A cost-effective approach to realize both oil recovery and residues utilization of oil sludge (OS) was proposed. It was an oxidation/acidification-assisted pretreatment process, where nitric acid (HNO3) was employed before the procedure of pyrolysis. Effects of dosage and concentration of HNO3 on oil recovery rate from OS were studied. Under the optimal conditions (dosage:5 mL·(g−1 dried OS); concentration: 5 mol·L−1), method of this work demonstrated two advantages: higher oil recovery rate of 27.53% and lower residue pyrolysis temperature of 550 °C were obtained, respectively. Detailed mechanism of such results was investigated via characterizations and analyses, among which saturates, aromatics, resins and asphaltenes (SARA) contents and metals contents were associated with the phenomena. Both oxidation and acidification caused the change of component in OS, giving the two conclusions: more saturates produced and less metals contents left. Meantime, the adsorption behavior of pyrolysis residues was evaluated via methylene blue (MB) adsorption tests. The highest MB adsorption value reached to 89.25 mg·g-1 and Langmuir isotherm model was well fitted the equilibrium data. Overall, the oxidation/acidification-assisted pretreatment of OS through HNO3 can be suggested as an economic and effective option.
1. Introduction With the rapid development of the oil industry in recently years, massive crude oil has been consumed worldwide [1]. Consequently, a large amount of oil sludge (OS) is inevitably produced during the exploitation, transportation and refining processes. It is estimated that over 60 million tons of OS is produced every year around the world [2], while more than 5 million tons of OS from storage tank need to be disposed annually in China [3]. Usually, OS is a complex mixture of water, oil and solid particles, and the oil phase primarily consists of some heavy petroleum hydrocarbons (PHCs) (e.g., long-chain alkanes, aromatics and asphaltenes) [4]. In addition, many hazardous and toxic substances like chemical additives, radioactive material and heavy metals are presented in OS, which may cause high potential risks for both human health and environment [5,6]. Because of the complex components and the high potential risks, OS has been listed as hazardous solid waste [7]. Hence, it is urgent to find a suitable solution to dispose it reasonably. Traditional methods for OS treatment are landfill and incineration, which are not only expensive, but also easy to cause waste of resources
⁎
and secondary environment pollution [5]. In order to reduce the disposal cost and recover oil from OS, the conversion of OS into valuable materials has been attracted extensive attention, and many methods such as solvent extraction [8,9], chemical oxidation [10,11], supercritical water [12,13], and ultrasound [14] have been researched for the treatment of OS. Among many of methods for OS treatment, pyrolysis has received the attention of scholars because of the characters of thorough disposal, less pollution and high recovery rate. Previous researches focused on the pyrolytic carbonization of OS for the preparation of carbonaceous adsorbents with chemical activation by adding ZnCl2 [15,16], H3PO4 [17,18], KOH [19,20] and so on during the process of pyrolysis. The obtained porous carbonaceous adsorbents are usually applied into wastewater treatment to remove the heavy metals [15,21] and dyes [22,23]. However, considering the difference of property and source of OS, the process of pyrolysis is commonly conducted at higher temperature, resulting in higher operational cost and much external energy required [24]. Furthermore, the chemical activation process usually needs massive chemical reagents, which are expensive and need to be removed in an additional washing stage [25,26]. Therefore, in order to decrease the pyrolysis temperature to
Corresponding author. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.jece.2019.103089 Received 3 December 2018; Received in revised form 24 March 2019; Accepted 12 April 2019 Available online 14 April 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103089
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Methylene blue (MB) was purchased from Tianjin Chemical Reagent Research Institute.
save energy and reduce the consumption of chemical reagents to cut cost, finding an economic and effective way of OS disposal should be concerned. HNO3, as a cheap inorganic acid with strong oxidation and acidity, has been widely used in many fields [27–29], but few about the pretreatment of OS to the best of our knowledge. As a consequence, the main objective of this study was to dispose the OS through HNO3 oxidation/acidification-assisted process to increase oil recovery rate and decrease the pyrolysis temperature of residues to prepare adsorbents. Firstly, the effects of dosage and concentration of HNO3 on oil recovery rate from OS were investigated. Then the pyrolysis of OS residues treated by HNO3 (named as HOS) was carried out directly to prepare residue adsorbents. The structure and surface characteristics of pyrolysis residues were characterized by FTIR, SEM and N2 adsorption-desorption isotherms and the adsorption capacity was test by removing MB solution. For a comprehensive comparison, a blank experiment for OS without the treatment of HNO3 (named as OOS) was also carried out.
2.2. Experimental method 2.2.1. Pretreatment of OS 10 g of dried OOS sample was added in a conical flack. A known amount of HNO3 (i.e., 2, 3, 5 or 10 mL·(g−1 dried OS)) with different concentrations (i.e., 1, 3, 5, 10 or 16 mol·L−1) was dosed. Then, the mixture was stirred in magnetic stirrer for 120 min. Finally, the pretreated OOS samples were filtered to obtain HOS samples and dried in an oven for further using. 2.2.2. Recovery of oil Determination method for oil content and oil recovery from OS in this work was based on the previous literature [30]. 5.0 g filtered and dried HOS sample was wrapped with filter paper and placed into Soxhlet extractor, then PE was used as solvent to recover the oil from HOS at 60 °C for 6 h until the solvent in extraction tube was colorless. The HOS residues after extraction were dried and weighted. In order to compare the oil recovery rate under different conditions, a blank experiment of oil recovery from OOS was also carried out. The recovery rate of oil from OS was defined as the mass ratio of liquid product to dried OS and calculated by Eq. (1):
2. Materials and methods 2.1. Materials The OOS sample used in this study was sampled from an oil field, Shandong Province, China. The obtained OOS sample was crushed and dried in an oven at 105 °C for 24 h, then stored in an airtight container before experiments. Ash content of the OOS sample was determined using ASTM method D 482-87. The metals concentration contained in OOS and HOS was measured according to the ASTM method D 5198-92. The saturates, aromatics, resins and asphaltenes (SARA) content of the oil components was obtained according to the ASTM method D 2007-02. The elemental composition including carbon, nitrogen, sulfur and hydrogen in two OS samples was measured by an elemental analyzer (Elementar Vario EL III). Table 1 shows the basic properties of the OOS and HOS. All chemicals used in this experiment were analytical reagent and without any further purification. Nitric acid (HNO3), petroleum ether (PE, 30–60 °C), sodium hydroxide (NaOH) and hydrochloric acid (HCl) used here were purchased from Sinopharm Chemical Reagent Co., Ltd.
Recovery rate =
OOS
HOS
Proximate analysis (wt.%) Moisture a Volatile matters Fixed carbon Ash
6.45 32.61 1.04 59.90
10.23 45.18 1.12 43.47
Ultimate analysis (wt.%) C H N S O
16.55 1.70 0.20 0.51 /
11.17 0.79 0.73 0.14 /
Metal elemental analysis (×10−4 %, by ICP) Mg 3.950 Al 4.419 Fe 5.751 Ca 7.270 K 2.183 Na 2.028 Zn 0.063
0.231 0.140 0.003 1.390 0.084 0.040 0.006
SARA in oil components (%) Saturates Aromatics Resins Asphaltenes a obtained before drying
46.60 34.34 14.18 4.88
(1)
where mb and ma are the dry weight of oil sludge before and after the recovery of oil (g), respectively. The obtained liquid oil samples from HOS and OOS were characterized by using gas chromatography coupled with the mass spectroscopy (GC–MS, Agilent 19091S-433, USA). The column used in the GC was an Agilent HP-5MS Phenyl Methyl Siloxane (30 m × 0.25 mm × 0.25 μm) column. Helium was used as the carrier gas at a nominal flow rate of 1 mL·min−1. The column was held at 40 °C for 1 min and then heated to 300 °C at 10 °C·min−1. The final temperature was held constant for 5 min. The ion source and transfer lines were maintained at 230 °C and 300 °C, respectively and the MS source was set at 70 eV. Mass spectra were recorded from m/z 50 to 240, and the chromatographic peaks were identified by the NIST library. In order to estimate the relative content of compounds in the oil samples, a semiquantitative method was employed using the means of the percentage of area under the curve for each peak of the GC analysis.
Table 1 Basic properties of OOS and HOS. Properties
mb − ma × 100% mb
2.2.3. Residues pyrolysis After the process of oil recovery, the pyrolysis of residues was carried out in a horizontal quartz tube fixed-bed reactor, which was placed in an electrical furnace equipped with temperature-programmed device and thermocouple. During the test, 5.0 g two residues held in a ceramic crucible were loaded in to reactor under N2 atmosphere with the heating rate of 10 °C·min−1. The final pyrolysis temperatures were set at 550 °C for HOS (named as HOS-550) and 700 °C for OOS (named as OOS-700) with a residence time of 120 min at set temperatures, respectively. Then, the two samples were cooled down to room temperature and washed with distilled water until the pH reach to 7. The washed samples were dried in an oven at 70 °C for 12 h. Finally, the dried samples were ground into uniform particles for further characterization and adsorption experiments. 2.2.4. Characterization of samples The thermal behavior of OS samples (HOS and OOS) were examined by thermal analyzer system (TGA, Diamond TG/DTA/DSC, USA) with a 10 °C·min−1 heating rate from 20 °C to 800 °C under N2 flow. The morphologies of the residue samples (HOS-550 and OOS-700) were assessed by Scanning electron microscopy (SEM, Zeiss Ultra Plus Instrument) and Transmission electron microscopy (TEM, H7650, JPN).
60.17 25.22 11.94 2.67
2
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improving oil recovery rate, but excessive dosage results in waste of regent and higher handle cost. Hence, the optimal dosage of HNO3 is selected as 5 mL·(g−1 dried OS). Effect of concentration is then discussed in Fig. 1b, in which dosage of HNO3 is 5 mL·(g−1 dried OS) as previously selected. Obviously, the oil recovery rate of OS after the treatment of HNO3 is better than that without the treatment of HNO3. Compared to the situation of OOS, increasing concentration of HNO3 from 1 to 5 mol·L−1 dramatically increases the oil recovery rate. However, when the concentration is enlarged from 5 to 16 mol·L−1, the recovery rate slightly increases. Meantime, considering lower cost and less reagent, 5 mol·L−1 is selected as the optical concentration of HNO3.
Fourier transform infrared spectroscopy (FTIR, NEXUS670, USA) of samples under different conditions were obtained with over the range of from 400 to 4000 cm−1. The specific surface area and pore structure of residues samples were determined via nitrogen adsorption-desorption at 77 K by gas-sorption analyzer (Micromeritics, ASAP2050, USA). 2.2.5. MB adsorption test The adsorption experiments were conducted in batch mode. Standard solutions (0.1, 0.2, 1.0, 2.0, and 5.0 mg·L−1) for the concentration-absorbance curve were prepared by dissolving MB in deionized water. The concentration of the solutions was determined by a UV–vis spectrophotometer (TU-1900) at 667 nm. The MB concentration can be calculated according to absorbance. In order to evaluate the adsorption capacity of obtained pyrolysis residues, 0.1 g of the pyrolysis residues and a certain amount of standard MB solution (1.5 g·L−1) were mixed in a conical flask. The mixture was then shaken for 30 min at 200 rpm. The absorption capacity of residues was calculated according to the concentration of the solution after adsorption. In adsorption isotherm studies, solutions with different initial MB concentrations (10, 25, 50, 75, and 100 mg·L−1) were used with an equilibrium time of 24 h. In each set, a mixture of 0.1 g HOS-550 and 100 mL of the MB solution was added to a flask and then shaken at 200 rpm. The samples were separated by centrifugation and the MB concentrations were analyzed. A similar procedure was followed containing the same MB concentration range with OOS-700 to be used as a comparison. Each experiment was repeated under identical conditions. The adsorption capacity qe (mg·g−1) of MB were calculated by Eq. (2):
qe = ⎛ ⎝
C0 − Ce ⎞×V M ⎠
3.1.2. SARA analysis Under the optimal conditions, the SARA content of the oil components obtained from OOS and HOS is studied and listed in Table 1. Apparently, more saturates are contained in HOS after the treatment of HNO3, while other three contents (aromatics, resins and asphaltenes) decrease relatively. Such results indicate that HNO3 plays a key role in changing SARA content of the oil component, three contents especially aromatics (expressed as Ar(CH2)2R) are oxidized ([O]) to produce more saturates (expressed as RH). The related reaction equations during this process can be summarized as Eqs. (3) and (4):
Ar (CH2 )2 R
RCOOH
(2)
[O]
−co2
ArCOOH + RCOOH
RH
(3)
(4)
3.1.3. GC–MS analysis Further characterization of oil components obtained from OOS and HOS is analyzed using GC–MS and shown in Fig. 2. Two obtained oil samples are similar in components, and typical peaks of saturated hydrocarbons can be observed in both two profiles, which is consistent with the results reported in previous literature [31]. As the retention time prolong, the carbon numbers of saturated hydrocarbons gradually increase and mainly distribute in the range from C16 to C25. Moreover, the intensity of corresponding peaks is getting stronger for oil samples from HOS than that from OOS, implying that the relative concentration of saturated hydrocarbons is getting higher. The major products obtained from OOS and HOS are mixture of alkanes. Previous study has shown that the presence of aliphatic compounds in OS is very important for the recovery as a fuel source [32]. Besides, it is reported that the hydrocarbons distributed in the range of C9–C25 are good petrochemical industry feedstock [8]. The main compounds identified by GC–MS and their relative contents are listed in Table 2. Results above reveal that more saturated hydrocarbons are produced after the treatment of HNO3, which is in accordance with the results of SARA.
−1
where C0 is the initial concentration of MB (mg·L ); Ce is the concentration of MB after adsorption (mg·L−1); V is the volume of MB solution (mL); M is the mass of adsorbents used (g). 3. Results and discussion 3.1. Recovery and characterization of oil 3.1.1. Recovery of oil Fig. 1a and b shows the effects of dosage and concentration of HNO3 on oil recovery rate from OS, respectively. Effect of dosage with 5 mol·L−1 HNO3 on oil recovery rate is firstly studied and shown in Fig. 1a. Under the experimental conditions, pretreatment using HNO3 is demonstrated applicable when the dosage exceeds 2 mL·(g−1 dried OS) and the oil recovery rate increase significantly with the increasing dosage of HNO3 from 2 to 5 mL·(g−1 dried OS). After that, the oil recovery rate reaches a plateau with further increase of dosage. The result indicates that suitable increasing dosage of HNO3 is conducive to
Fig. 1. Effects of (a) dosage and (b) concentration of HNO3 on oil recovery rate. 3
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Fig. 2. GC–MS chromatogram of oil recovered from HOS and OOS. Fig. 4. FTIR spectra of OS samples under different conditions.
Table 2 Main compounds identified by GC–MS and their relative contents. No.
Retention
Compound
Formula
Time 1 2 3 4 5 6 7 8 9 10
11.46 11.52 12.03 12.51 12.99 13.44 13.91 14.34 14.87 15.48
Hexadecane Heptadecane Octadecane Heptadecane,2,6-dimethyl Eicosane Heneicosane Docosane Tricosane Tetracosane Heptadecane, 9-octyl
C16H34 C17 H36 C18 H38 C19 H40 C20 H42 C21 H44 C22 H46 C23 H48 C24 H50 C25 H52
Meanwhile, partial ash content (mostly metal contents) contained in OOS is also removed by HNO3 (see Table 1), resulting in the TG curve of HOS final residual quality is lower than that of OOS. Moreover, the final residues pyrolysis temperature drops from 700 °C to 550 °C. Hence, in our work, 700 °C and 550 °C are selected as the appropriate temperatures for OOS and HOS residues pyrolysis, respectively. Fig. 3b shows the thermal degradation curves of two oil samples. Obviously, there is no significant difference between the two oil samples in thermal degradation behavior, while the DTG of HOS are higher than that of OOS, which further reveals that the obtained oil samples are similar in compounds but different in contents.
Peak area (%) OOS
HOS
17.43 100 38.11 54.52 47.83 58.72 62.67 71.86 64.22 71.08
31.64 100 43.46 58.56 49.12 58.36 66.72 81.27 69.54 84.26
3.2.2. FTIR analysis Fig. 4 shows the FTIR spectra of OS samples under different treatment conditions. A broad band between 3300 cm−1 to 3500 cm−1 can be observed in all four conditions, which is assigned to the presence of free and associated hydroxyl groups (OeH stretching bands). For OOS, the peaks at 2852 cm−1 and 2923 cm−1 are attributed to the carbonyl (stretching vibration of C–H bond). The broad band between 1400 cm−1 to 1470 cm−1 and the peaks at 724 cm−1 and 876 cm−1 are the characteristic peaks of CO32− (CaCO3) [34]. After the treatment of HNO3 (HOS), the characteristic peaks of CaCO3 is vanished, whereas the broad band between 1000 cm−1 to 1100 cm−1 and characteristic peaks at 464 cm−1 (bending vibration) and 795 cm−1 (stretching vibration) of Si-O are observed [35]. Besides, the peak intensity of carbonyl group is increased, the peaks of methyl (bending vibration of C–H bond) at 1381 cm−1 and 1465 cm−1, and the peak at 1641 cm−1 (stretching vibration of C]C bond) are produced, respectively. This provides further evidence that more carboxylic acid compounds and alkanes are generated after the treatment of HNO3. However, when the OS samples are treated by pyrolysis, almost all characteristic peaks of
3.2. Characterization of OS and pyrolysis residues 3.2.1. TG analysis Thermogravimetric analysis (TGA) is performed for both solid and oil samples of OOS and HOS. Fig. 3a shows the thermal degradation curves of OOS and HOS samples. The thermal behavior of the OOS shows three main steps. The first loss in weight about 3% below 200 °C is assigned to the evaporation of moisture from samples during heating. The second weight loss about 10% occurs between 200 °C to 600 °C.A mass loss about 27% is observed between 600 °C to 700 °C, and becomes constant after 700 °C,which can be attributed to the decomposition of heavy oil components in OS [33]. While the HOS demonstrates the different thermal behavior, the mass loss about 50% is obtained from 200 °C to 550 °C.This probably because the strong oxidizing property of HNO3 to organics, heavy oil components (aromatics, resins and asphaltenes) in OS are decomposed to generate more lowmolecular saturated hydrocarbons with lower boiling point.
Fig. 3. TG curves for (a) solids and (b) oil samples of HOS and OOS. 4
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Fig. 5. SEM and TEM images of (a and c) OOS-700 and (b and d) HOS-550.
Fig. 6. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of HOS and OOS. Table 3 Specific surface area and pore volume results of HOS-550 and OOS-700. Samples
SBET(m2·g−1)
Smic(m2·g−1)
Smes(m2·g−1)
Vt(cm−3·g−1)
Vmic(cm−3·g−1)
Vmes(cm−3·g−1)
Dp(nm)
HOS-550 OOS-700
68.29 3.13
27.44 3.03
40.85 0.10
0.093 0.016
0.016 0.002
0.077 0.014
5.45 21.1
SBET: BET surface area, Smic: micropore surface area, Smec: mesopore surface area, Vt: total pore volume, Vmic: micropore volume, Vmec: mesopore volume, Dp: average diameter.
5
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Fig. 7. (a) MB adsorption valves and (b) adsorption isotherm models of HOS and OOS.
samples. Usually, the specific surface area of carbon materials prepared from OS after activation can reach hundreds or thousands m2·g−1 [19,20]. The specific surface area of obtained HOS-550 is 68.29 m2·g−1. This is because massive oil has been recovered, resulting in a reduction of carbon source in residue, which is negative to the preparation of carbon materials with larger specific surface area. However, HOS-550 exhibits a relatively higher surface area than that of OOS-700 (3.13 m2·g−1), which reveals that pretreatment process can greatly increase the surface area of materials. Similarly, Jarrah produced carbon materials from residue fuel oil by chemical treatment with mixed acids and obtained specific surface area of 50.7 m2·g−1 with adsorption ability of 85.6 mg·g−1 for MB [36]. A comparison of the results for HOS550 and OOS-700 suggests that the HNO3 used has a significant influence on the structure and performance of pyrolysis residues.
Table 4 Kinetic constants for MB adsorption on HOS-550 and OOS-700. Isotherms
Langmuir
Samples
HOS-550 OOS-700
Freundlich HOS-550 OOS-700
Parameters qm (mg·g−1)
KL (L·mg−1)
R2
42.23 18.87 n 7.79 5.97
0.25 0.031 KF (L·mg−1) 2.68 1.38
0.9351 0.8348 R2 0.8790 0.8417
organics disappear, except the C]C bond for HOS-550 and a few of C–H bonds (stretching vibration) for OOS-700. 3.2.3. SEM and TEM analysis Surface morphology of OOS-700 and HOS-550 is characterized by SEM and TEM analysis and shown in Fig. 5. The SEM photograph of OOS-700 presents a layered stacking structure and no obvious pores can be observed in OOS-700 (Fig. 5a). This may be due to during the process of pyrolysis, the high ash content in OOS (Table 1) covers the surface, which is unfavorable to the formation of pores or adsorption. While the rough surface and irregular pore structures can be observed in HOS-550 (Fig. 5b). It indicates that after the treatment of HNO3, most metal contents in OS are removed first and the produced lowmolecular hydrocarbons are carbonized during the process of pyrolysis and positive to form pores. This loosen structure and surface pores are favorable to molecular diffusion and provide sufficient free spaces to molecules, which is helpful for the potential adsorption of dyes in aqueous solution. Fig. 5c and d shows the TEM images of the two samples. Same to the results of SEM, OOS-700 exhibits a layered stacking structure (Fig. 5c), while some uniform white dots that represent pore structures can be observed in HOS-550 (Fig. 5d). The pore size pore size is mainly in the range of 10–20 nm, which can be classified as mesopore.
3.3. Adsorption experiments 3.3.1. MB adsorption valve MB adsorption experiment is carried out to evaluate the adsorption capacity of obtained pyrolysis residues. The MB adsorption values for both HOS-550 and OOS-700 are shown in Fig. 7a. Obviously, the adsorption capacity of HOS-550 (89.25 mg·g−1) is better than that of OOS-700 (23.79 mg·g−1).
3.3.2. Adsorption isotherms The Langmuir isotherm model [37] expressed by Eq. (5) is a widely used adsorption isotherm model while the Freundlich isotherm model [38] shown as Eq. (6) is considered to be an empirical equation that is commonly applied for the correlation of experimental results.
qe =
qm KL Ce 1 + KL Ce
(5)
1
qe = KF Ce n
3.2.4. Nitrogen adsorption-desorption analysis Fig. 6a and b shows the nitrogen adsorption-desorption isotherms of OOS-700 and HOS-550 and the corresponding pore size distributions. The isotherms of these two samples exhibit the similar adsorptiondesorption trend, the curves occur an abrupt increase in nitrogen uptake in relatively higher-pressure ranges with a distinct hysteresis loop (Type H4) which can be classified as a mixture of type I and type IV isotherms and represent a mixed structure of micropore and mesopore according to the International Union of Pure and Applied Chemistry (IUPAC) classification [20] (Fig. 6a). The pore size distributions are shown in Fig. 6b. Obviously, the pore size distributions of samples are mainly mesoporous, which further demonstrating their relatively high mesopore content. Table 3 shows the BET surface area, total volume, micropore and mesopore surface areas, and average pore diameter of
(6)
where KL is the Langmuir isotherm parameter (L·mg−1), qm is the monolayer adsorption capacity (mg·g−1), KF is the Freundlich equation constant and n is a dimensionless heterogeneity factor. Two isotherm models are fitted to the equilibrium data for adsorption of various initial concentrations (10-100 mg L−1) of MB solution on HOS-550 and OOS-700 and exhibited in Fig. 7b. The parameters of the isotherms are listed in Table 4. The R2 values of the Langmuir model are higher than those of the Freundlich model, suggesting the former is more suitable to explain the adsorption behavior. This also suggest that the adsorption of MB is mainly monolayer adsorption. Meanwhile, HOS-550 exhibits a larger adsorption capacity of 40.27 mg·g−1 than 15.56 mg g−1 for OOS-700. 6
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4. Conclusions [12]
A cost-effective method to realize both the oil recovery and the residues utilization was proposed through oxidation/acidification-assisted pretreatment process by introducing HNO3 to dispose OS before pyrolysis process. The optimal operation parameters were: HNO3 dosage of 5 mL·(g−1 dried OOS) and HNO3 concentration of 5 mol·L−1. Under the optimum treatment conditions, the oil recovery rate increased from 4.41% to 27.53% and more saturates were produced according to the result of SARA. At the same time, a lower pyrolysis temperature of 550 °C for HOS than 700 °C for OOS was obtained based on the facts that heavy oil components (mainly aromatics) in OS were oxidized and most metal contents contained in OS was removed by HNO3. In MB adsorption tests, HOS-550 showed a better adsorption capacity of 40.27 mg·g−1 than OOS-700 of 15.56 mg·g−1. Langmuir isotherm model was well fitted to describe the equilibrium data. As the existence of large amounts of OS is severe problem nowadays, the current work provides an alternative approach to pretreat OS through HNO3 to increase oil recovery rate and the pyrolysis residues are applied to wastewater treatment.
[13] [14] [15]
[16]
[17]
[18]
[19]
[20]
Acknowledgments
[21]
This study was supported by the National Natural Science Foundation of China (Grant 51578295), the National Natural Science Foundation of Jiangsu Province (BK20161479), the Educational Commission of Jiangsu Province (16KJB150043), and Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse (Nanjing University of Science and Technology), Qing Lan Project of Jiangsu Province. Additionally, we also appreciate Foundation of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials and a project funded by the priority academic program development of Jiangsu Higher Education Institutions.
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Enhanced oil recovery and residues utilization of oil sludge through nitric acid pretreatment process Kaichao ZuoJiaoa, Zheng Zhanga, Panjie Lia, Caiyun Jiangb, Jiaxun Liua, Yuping Wanga,
T
⁎
a
School of Chemistry and Material Science, Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control, Nanjing Normal University, Nanjing, 210046, China b Department of Engineering and Technology, Jiangsu Institute of Commerce, Nanjing, 211168, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oil sludge Nitric acid Oil recovery rate Pyrolysis Adsorption
A cost-effective approach to realize both oil recovery and residues utilization of oil sludge (OS) was proposed. It was an oxidation/acidification-assisted pretreatment process, where nitric acid (HNO3) was employed before the procedure of pyrolysis. Effects of dosage and concentration of HNO3 on oil recovery rate from OS were studied. Under the optimal conditions (dosage:5 mL·(g−1 dried OS); concentration: 5 mol·L−1), method of this work demonstrated two advantages: higher oil recovery rate of 27.53% and lower residue pyrolysis temperature of 550 °C were obtained, respectively. Detailed mechanism of such results was investigated via characterizations and analyses, among which saturates, aromatics, resins and asphaltenes (SARA) contents and metals contents were associated with the phenomena. Both oxidation and acidification caused the change of component in OS, giving the two conclusions: more saturates produced and less metals contents left. Meantime, the adsorption behavior of pyrolysis residues was evaluated via methylene blue (MB) adsorption tests. The highest MB adsorption value reached to 89.25 mg·g-1 and Langmuir isotherm model was well fitted the equilibrium data. Overall, the oxidation/acidification-assisted pretreatment of OS through HNO3 can be suggested as an economic and effective option.
1. Introduction With the rapid development of the oil industry in recently years, massive crude oil has been consumed worldwide [1]. Consequently, a large amount of oil sludge (OS) is inevitably produced during the exploitation, transportation and refining processes. It is estimated that over 60 million tons of OS is produced every year around the world [2], while more than 5 million tons of OS from storage tank need to be disposed annually in China [3]. Usually, OS is a complex mixture of water, oil and solid particles, and the oil phase primarily consists of some heavy petroleum hydrocarbons (PHCs) (e.g., long-chain alkanes, aromatics and asphaltenes) [4]. In addition, many hazardous and toxic substances like chemical additives, radioactive material and heavy metals are presented in OS, which may cause high potential risks for both human health and environment [5,6]. Because of the complex components and the high potential risks, OS has been listed as hazardous solid waste [7]. Hence, it is urgent to find a suitable solution to dispose it reasonably. Traditional methods for OS treatment are landfill and incineration, which are not only expensive, but also easy to cause waste of resources
⁎
and secondary environment pollution [5]. In order to reduce the disposal cost and recover oil from OS, the conversion of OS into valuable materials has been attracted extensive attention, and many methods such as solvent extraction [8,9], chemical oxidation [10,11], supercritical water [12,13], and ultrasound [14] have been researched for the treatment of OS. Among many of methods for OS treatment, pyrolysis has received the attention of scholars because of the characters of thorough disposal, less pollution and high recovery rate. Previous researches focused on the pyrolytic carbonization of OS for the preparation of carbonaceous adsorbents with chemical activation by adding ZnCl2 [15,16], H3PO4 [17,18], KOH [19,20] and so on during the process of pyrolysis. The obtained porous carbonaceous adsorbents are usually applied into wastewater treatment to remove the heavy metals [15,21] and dyes [22,23]. However, considering the difference of property and source of OS, the process of pyrolysis is commonly conducted at higher temperature, resulting in higher operational cost and much external energy required [24]. Furthermore, the chemical activation process usually needs massive chemical reagents, which are expensive and need to be removed in an additional washing stage [25,26]. Therefore, in order to decrease the pyrolysis temperature to
Corresponding author. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.jece.2019.103089 Received 3 December 2018; Received in revised form 24 March 2019; Accepted 12 April 2019 Available online 14 April 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
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Methylene blue (MB) was purchased from Tianjin Chemical Reagent Research Institute.
save energy and reduce the consumption of chemical reagents to cut cost, finding an economic and effective way of OS disposal should be concerned. HNO3, as a cheap inorganic acid with strong oxidation and acidity, has been widely used in many fields [27–29], but few about the pretreatment of OS to the best of our knowledge. As a consequence, the main objective of this study was to dispose the OS through HNO3 oxidation/acidification-assisted process to increase oil recovery rate and decrease the pyrolysis temperature of residues to prepare adsorbents. Firstly, the effects of dosage and concentration of HNO3 on oil recovery rate from OS were investigated. Then the pyrolysis of OS residues treated by HNO3 (named as HOS) was carried out directly to prepare residue adsorbents. The structure and surface characteristics of pyrolysis residues were characterized by FTIR, SEM and N2 adsorption-desorption isotherms and the adsorption capacity was test by removing MB solution. For a comprehensive comparison, a blank experiment for OS without the treatment of HNO3 (named as OOS) was also carried out.
2.2. Experimental method 2.2.1. Pretreatment of OS 10 g of dried OOS sample was added in a conical flack. A known amount of HNO3 (i.e., 2, 3, 5 or 10 mL·(g−1 dried OS)) with different concentrations (i.e., 1, 3, 5, 10 or 16 mol·L−1) was dosed. Then, the mixture was stirred in magnetic stirrer for 120 min. Finally, the pretreated OOS samples were filtered to obtain HOS samples and dried in an oven for further using. 2.2.2. Recovery of oil Determination method for oil content and oil recovery from OS in this work was based on the previous literature [30]. 5.0 g filtered and dried HOS sample was wrapped with filter paper and placed into Soxhlet extractor, then PE was used as solvent to recover the oil from HOS at 60 °C for 6 h until the solvent in extraction tube was colorless. The HOS residues after extraction were dried and weighted. In order to compare the oil recovery rate under different conditions, a blank experiment of oil recovery from OOS was also carried out. The recovery rate of oil from OS was defined as the mass ratio of liquid product to dried OS and calculated by Eq. (1):
2. Materials and methods 2.1. Materials The OOS sample used in this study was sampled from an oil field, Shandong Province, China. The obtained OOS sample was crushed and dried in an oven at 105 °C for 24 h, then stored in an airtight container before experiments. Ash content of the OOS sample was determined using ASTM method D 482-87. The metals concentration contained in OOS and HOS was measured according to the ASTM method D 5198-92. The saturates, aromatics, resins and asphaltenes (SARA) content of the oil components was obtained according to the ASTM method D 2007-02. The elemental composition including carbon, nitrogen, sulfur and hydrogen in two OS samples was measured by an elemental analyzer (Elementar Vario EL III). Table 1 shows the basic properties of the OOS and HOS. All chemicals used in this experiment were analytical reagent and without any further purification. Nitric acid (HNO3), petroleum ether (PE, 30–60 °C), sodium hydroxide (NaOH) and hydrochloric acid (HCl) used here were purchased from Sinopharm Chemical Reagent Co., Ltd.
Recovery rate =
OOS
HOS
Proximate analysis (wt.%) Moisture a Volatile matters Fixed carbon Ash
6.45 32.61 1.04 59.90
10.23 45.18 1.12 43.47
Ultimate analysis (wt.%) C H N S O
16.55 1.70 0.20 0.51 /
11.17 0.79 0.73 0.14 /
Metal elemental analysis (×10−4 %, by ICP) Mg 3.950 Al 4.419 Fe 5.751 Ca 7.270 K 2.183 Na 2.028 Zn 0.063
0.231 0.140 0.003 1.390 0.084 0.040 0.006
SARA in oil components (%) Saturates Aromatics Resins Asphaltenes a obtained before drying
46.60 34.34 14.18 4.88
(1)
where mb and ma are the dry weight of oil sludge before and after the recovery of oil (g), respectively. The obtained liquid oil samples from HOS and OOS were characterized by using gas chromatography coupled with the mass spectroscopy (GC–MS, Agilent 19091S-433, USA). The column used in the GC was an Agilent HP-5MS Phenyl Methyl Siloxane (30 m × 0.25 mm × 0.25 μm) column. Helium was used as the carrier gas at a nominal flow rate of 1 mL·min−1. The column was held at 40 °C for 1 min and then heated to 300 °C at 10 °C·min−1. The final temperature was held constant for 5 min. The ion source and transfer lines were maintained at 230 °C and 300 °C, respectively and the MS source was set at 70 eV. Mass spectra were recorded from m/z 50 to 240, and the chromatographic peaks were identified by the NIST library. In order to estimate the relative content of compounds in the oil samples, a semiquantitative method was employed using the means of the percentage of area under the curve for each peak of the GC analysis.
Table 1 Basic properties of OOS and HOS. Properties
mb − ma × 100% mb
2.2.3. Residues pyrolysis After the process of oil recovery, the pyrolysis of residues was carried out in a horizontal quartz tube fixed-bed reactor, which was placed in an electrical furnace equipped with temperature-programmed device and thermocouple. During the test, 5.0 g two residues held in a ceramic crucible were loaded in to reactor under N2 atmosphere with the heating rate of 10 °C·min−1. The final pyrolysis temperatures were set at 550 °C for HOS (named as HOS-550) and 700 °C for OOS (named as OOS-700) with a residence time of 120 min at set temperatures, respectively. Then, the two samples were cooled down to room temperature and washed with distilled water until the pH reach to 7. The washed samples were dried in an oven at 70 °C for 12 h. Finally, the dried samples were ground into uniform particles for further characterization and adsorption experiments. 2.2.4. Characterization of samples The thermal behavior of OS samples (HOS and OOS) were examined by thermal analyzer system (TGA, Diamond TG/DTA/DSC, USA) with a 10 °C·min−1 heating rate from 20 °C to 800 °C under N2 flow. The morphologies of the residue samples (HOS-550 and OOS-700) were assessed by Scanning electron microscopy (SEM, Zeiss Ultra Plus Instrument) and Transmission electron microscopy (TEM, H7650, JPN).
60.17 25.22 11.94 2.67
2
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improving oil recovery rate, but excessive dosage results in waste of regent and higher handle cost. Hence, the optimal dosage of HNO3 is selected as 5 mL·(g−1 dried OS). Effect of concentration is then discussed in Fig. 1b, in which dosage of HNO3 is 5 mL·(g−1 dried OS) as previously selected. Obviously, the oil recovery rate of OS after the treatment of HNO3 is better than that without the treatment of HNO3. Compared to the situation of OOS, increasing concentration of HNO3 from 1 to 5 mol·L−1 dramatically increases the oil recovery rate. However, when the concentration is enlarged from 5 to 16 mol·L−1, the recovery rate slightly increases. Meantime, considering lower cost and less reagent, 5 mol·L−1 is selected as the optical concentration of HNO3.
Fourier transform infrared spectroscopy (FTIR, NEXUS670, USA) of samples under different conditions were obtained with over the range of from 400 to 4000 cm−1. The specific surface area and pore structure of residues samples were determined via nitrogen adsorption-desorption at 77 K by gas-sorption analyzer (Micromeritics, ASAP2050, USA). 2.2.5. MB adsorption test The adsorption experiments were conducted in batch mode. Standard solutions (0.1, 0.2, 1.0, 2.0, and 5.0 mg·L−1) for the concentration-absorbance curve were prepared by dissolving MB in deionized water. The concentration of the solutions was determined by a UV–vis spectrophotometer (TU-1900) at 667 nm. The MB concentration can be calculated according to absorbance. In order to evaluate the adsorption capacity of obtained pyrolysis residues, 0.1 g of the pyrolysis residues and a certain amount of standard MB solution (1.5 g·L−1) were mixed in a conical flask. The mixture was then shaken for 30 min at 200 rpm. The absorption capacity of residues was calculated according to the concentration of the solution after adsorption. In adsorption isotherm studies, solutions with different initial MB concentrations (10, 25, 50, 75, and 100 mg·L−1) were used with an equilibrium time of 24 h. In each set, a mixture of 0.1 g HOS-550 and 100 mL of the MB solution was added to a flask and then shaken at 200 rpm. The samples were separated by centrifugation and the MB concentrations were analyzed. A similar procedure was followed containing the same MB concentration range with OOS-700 to be used as a comparison. Each experiment was repeated under identical conditions. The adsorption capacity qe (mg·g−1) of MB were calculated by Eq. (2):
qe = ⎛ ⎝
C0 − Ce ⎞×V M ⎠
3.1.2. SARA analysis Under the optimal conditions, the SARA content of the oil components obtained from OOS and HOS is studied and listed in Table 1. Apparently, more saturates are contained in HOS after the treatment of HNO3, while other three contents (aromatics, resins and asphaltenes) decrease relatively. Such results indicate that HNO3 plays a key role in changing SARA content of the oil component, three contents especially aromatics (expressed as Ar(CH2)2R) are oxidized ([O]) to produce more saturates (expressed as RH). The related reaction equations during this process can be summarized as Eqs. (3) and (4):
Ar (CH2 )2 R
RCOOH
(2)
[O]
−co2
ArCOOH + RCOOH
RH
(3)
(4)
3.1.3. GC–MS analysis Further characterization of oil components obtained from OOS and HOS is analyzed using GC–MS and shown in Fig. 2. Two obtained oil samples are similar in components, and typical peaks of saturated hydrocarbons can be observed in both two profiles, which is consistent with the results reported in previous literature [31]. As the retention time prolong, the carbon numbers of saturated hydrocarbons gradually increase and mainly distribute in the range from C16 to C25. Moreover, the intensity of corresponding peaks is getting stronger for oil samples from HOS than that from OOS, implying that the relative concentration of saturated hydrocarbons is getting higher. The major products obtained from OOS and HOS are mixture of alkanes. Previous study has shown that the presence of aliphatic compounds in OS is very important for the recovery as a fuel source [32]. Besides, it is reported that the hydrocarbons distributed in the range of C9–C25 are good petrochemical industry feedstock [8]. The main compounds identified by GC–MS and their relative contents are listed in Table 2. Results above reveal that more saturated hydrocarbons are produced after the treatment of HNO3, which is in accordance with the results of SARA.
−1
where C0 is the initial concentration of MB (mg·L ); Ce is the concentration of MB after adsorption (mg·L−1); V is the volume of MB solution (mL); M is the mass of adsorbents used (g). 3. Results and discussion 3.1. Recovery and characterization of oil 3.1.1. Recovery of oil Fig. 1a and b shows the effects of dosage and concentration of HNO3 on oil recovery rate from OS, respectively. Effect of dosage with 5 mol·L−1 HNO3 on oil recovery rate is firstly studied and shown in Fig. 1a. Under the experimental conditions, pretreatment using HNO3 is demonstrated applicable when the dosage exceeds 2 mL·(g−1 dried OS) and the oil recovery rate increase significantly with the increasing dosage of HNO3 from 2 to 5 mL·(g−1 dried OS). After that, the oil recovery rate reaches a plateau with further increase of dosage. The result indicates that suitable increasing dosage of HNO3 is conducive to
Fig. 1. Effects of (a) dosage and (b) concentration of HNO3 on oil recovery rate. 3
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Fig. 2. GC–MS chromatogram of oil recovered from HOS and OOS. Fig. 4. FTIR spectra of OS samples under different conditions.
Table 2 Main compounds identified by GC–MS and their relative contents. No.
Retention
Compound
Formula
Time 1 2 3 4 5 6 7 8 9 10
11.46 11.52 12.03 12.51 12.99 13.44 13.91 14.34 14.87 15.48
Hexadecane Heptadecane Octadecane Heptadecane,2,6-dimethyl Eicosane Heneicosane Docosane Tricosane Tetracosane Heptadecane, 9-octyl
C16H34 C17 H36 C18 H38 C19 H40 C20 H42 C21 H44 C22 H46 C23 H48 C24 H50 C25 H52
Meanwhile, partial ash content (mostly metal contents) contained in OOS is also removed by HNO3 (see Table 1), resulting in the TG curve of HOS final residual quality is lower than that of OOS. Moreover, the final residues pyrolysis temperature drops from 700 °C to 550 °C. Hence, in our work, 700 °C and 550 °C are selected as the appropriate temperatures for OOS and HOS residues pyrolysis, respectively. Fig. 3b shows the thermal degradation curves of two oil samples. Obviously, there is no significant difference between the two oil samples in thermal degradation behavior, while the DTG of HOS are higher than that of OOS, which further reveals that the obtained oil samples are similar in compounds but different in contents.
Peak area (%) OOS
HOS
17.43 100 38.11 54.52 47.83 58.72 62.67 71.86 64.22 71.08
31.64 100 43.46 58.56 49.12 58.36 66.72 81.27 69.54 84.26
3.2.2. FTIR analysis Fig. 4 shows the FTIR spectra of OS samples under different treatment conditions. A broad band between 3300 cm−1 to 3500 cm−1 can be observed in all four conditions, which is assigned to the presence of free and associated hydroxyl groups (OeH stretching bands). For OOS, the peaks at 2852 cm−1 and 2923 cm−1 are attributed to the carbonyl (stretching vibration of C–H bond). The broad band between 1400 cm−1 to 1470 cm−1 and the peaks at 724 cm−1 and 876 cm−1 are the characteristic peaks of CO32− (CaCO3) [34]. After the treatment of HNO3 (HOS), the characteristic peaks of CaCO3 is vanished, whereas the broad band between 1000 cm−1 to 1100 cm−1 and characteristic peaks at 464 cm−1 (bending vibration) and 795 cm−1 (stretching vibration) of Si-O are observed [35]. Besides, the peak intensity of carbonyl group is increased, the peaks of methyl (bending vibration of C–H bond) at 1381 cm−1 and 1465 cm−1, and the peak at 1641 cm−1 (stretching vibration of C]C bond) are produced, respectively. This provides further evidence that more carboxylic acid compounds and alkanes are generated after the treatment of HNO3. However, when the OS samples are treated by pyrolysis, almost all characteristic peaks of
3.2. Characterization of OS and pyrolysis residues 3.2.1. TG analysis Thermogravimetric analysis (TGA) is performed for both solid and oil samples of OOS and HOS. Fig. 3a shows the thermal degradation curves of OOS and HOS samples. The thermal behavior of the OOS shows three main steps. The first loss in weight about 3% below 200 °C is assigned to the evaporation of moisture from samples during heating. The second weight loss about 10% occurs between 200 °C to 600 °C.A mass loss about 27% is observed between 600 °C to 700 °C, and becomes constant after 700 °C,which can be attributed to the decomposition of heavy oil components in OS [33]. While the HOS demonstrates the different thermal behavior, the mass loss about 50% is obtained from 200 °C to 550 °C.This probably because the strong oxidizing property of HNO3 to organics, heavy oil components (aromatics, resins and asphaltenes) in OS are decomposed to generate more lowmolecular saturated hydrocarbons with lower boiling point.
Fig. 3. TG curves for (a) solids and (b) oil samples of HOS and OOS. 4
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Fig. 5. SEM and TEM images of (a and c) OOS-700 and (b and d) HOS-550.
Fig. 6. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of HOS and OOS. Table 3 Specific surface area and pore volume results of HOS-550 and OOS-700. Samples
SBET(m2·g−1)
Smic(m2·g−1)
Smes(m2·g−1)
Vt(cm−3·g−1)
Vmic(cm−3·g−1)
Vmes(cm−3·g−1)
Dp(nm)
HOS-550 OOS-700
68.29 3.13
27.44 3.03
40.85 0.10
0.093 0.016
0.016 0.002
0.077 0.014
5.45 21.1
SBET: BET surface area, Smic: micropore surface area, Smec: mesopore surface area, Vt: total pore volume, Vmic: micropore volume, Vmec: mesopore volume, Dp: average diameter.
5
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Fig. 7. (a) MB adsorption valves and (b) adsorption isotherm models of HOS and OOS.
samples. Usually, the specific surface area of carbon materials prepared from OS after activation can reach hundreds or thousands m2·g−1 [19,20]. The specific surface area of obtained HOS-550 is 68.29 m2·g−1. This is because massive oil has been recovered, resulting in a reduction of carbon source in residue, which is negative to the preparation of carbon materials with larger specific surface area. However, HOS-550 exhibits a relatively higher surface area than that of OOS-700 (3.13 m2·g−1), which reveals that pretreatment process can greatly increase the surface area of materials. Similarly, Jarrah produced carbon materials from residue fuel oil by chemical treatment with mixed acids and obtained specific surface area of 50.7 m2·g−1 with adsorption ability of 85.6 mg·g−1 for MB [36]. A comparison of the results for HOS550 and OOS-700 suggests that the HNO3 used has a significant influence on the structure and performance of pyrolysis residues.
Table 4 Kinetic constants for MB adsorption on HOS-550 and OOS-700. Isotherms
Langmuir
Samples
HOS-550 OOS-700
Freundlich HOS-550 OOS-700
Parameters qm (mg·g−1)
KL (L·mg−1)
R2
42.23 18.87 n 7.79 5.97
0.25 0.031 KF (L·mg−1) 2.68 1.38
0.9351 0.8348 R2 0.8790 0.8417
organics disappear, except the C]C bond for HOS-550 and a few of C–H bonds (stretching vibration) for OOS-700. 3.2.3. SEM and TEM analysis Surface morphology of OOS-700 and HOS-550 is characterized by SEM and TEM analysis and shown in Fig. 5. The SEM photograph of OOS-700 presents a layered stacking structure and no obvious pores can be observed in OOS-700 (Fig. 5a). This may be due to during the process of pyrolysis, the high ash content in OOS (Table 1) covers the surface, which is unfavorable to the formation of pores or adsorption. While the rough surface and irregular pore structures can be observed in HOS-550 (Fig. 5b). It indicates that after the treatment of HNO3, most metal contents in OS are removed first and the produced lowmolecular hydrocarbons are carbonized during the process of pyrolysis and positive to form pores. This loosen structure and surface pores are favorable to molecular diffusion and provide sufficient free spaces to molecules, which is helpful for the potential adsorption of dyes in aqueous solution. Fig. 5c and d shows the TEM images of the two samples. Same to the results of SEM, OOS-700 exhibits a layered stacking structure (Fig. 5c), while some uniform white dots that represent pore structures can be observed in HOS-550 (Fig. 5d). The pore size pore size is mainly in the range of 10–20 nm, which can be classified as mesopore.
3.3. Adsorption experiments 3.3.1. MB adsorption valve MB adsorption experiment is carried out to evaluate the adsorption capacity of obtained pyrolysis residues. The MB adsorption values for both HOS-550 and OOS-700 are shown in Fig. 7a. Obviously, the adsorption capacity of HOS-550 (89.25 mg·g−1) is better than that of OOS-700 (23.79 mg·g−1).
3.3.2. Adsorption isotherms The Langmuir isotherm model [37] expressed by Eq. (5) is a widely used adsorption isotherm model while the Freundlich isotherm model [38] shown as Eq. (6) is considered to be an empirical equation that is commonly applied for the correlation of experimental results.
qe =
qm KL Ce 1 + KL Ce
(5)
1
qe = KF Ce n
3.2.4. Nitrogen adsorption-desorption analysis Fig. 6a and b shows the nitrogen adsorption-desorption isotherms of OOS-700 and HOS-550 and the corresponding pore size distributions. The isotherms of these two samples exhibit the similar adsorptiondesorption trend, the curves occur an abrupt increase in nitrogen uptake in relatively higher-pressure ranges with a distinct hysteresis loop (Type H4) which can be classified as a mixture of type I and type IV isotherms and represent a mixed structure of micropore and mesopore according to the International Union of Pure and Applied Chemistry (IUPAC) classification [20] (Fig. 6a). The pore size distributions are shown in Fig. 6b. Obviously, the pore size distributions of samples are mainly mesoporous, which further demonstrating their relatively high mesopore content. Table 3 shows the BET surface area, total volume, micropore and mesopore surface areas, and average pore diameter of
(6)
where KL is the Langmuir isotherm parameter (L·mg−1), qm is the monolayer adsorption capacity (mg·g−1), KF is the Freundlich equation constant and n is a dimensionless heterogeneity factor. Two isotherm models are fitted to the equilibrium data for adsorption of various initial concentrations (10-100 mg L−1) of MB solution on HOS-550 and OOS-700 and exhibited in Fig. 7b. The parameters of the isotherms are listed in Table 4. The R2 values of the Langmuir model are higher than those of the Freundlich model, suggesting the former is more suitable to explain the adsorption behavior. This also suggest that the adsorption of MB is mainly monolayer adsorption. Meanwhile, HOS-550 exhibits a larger adsorption capacity of 40.27 mg·g−1 than 15.56 mg g−1 for OOS-700. 6
Journal of Environmental Chemical Engineering 7 (2019) 103089
K. ZuoJiao, et al.
4. Conclusions [12]
A cost-effective method to realize both the oil recovery and the residues utilization was proposed through oxidation/acidification-assisted pretreatment process by introducing HNO3 to dispose OS before pyrolysis process. The optimal operation parameters were: HNO3 dosage of 5 mL·(g−1 dried OOS) and HNO3 concentration of 5 mol·L−1. Under the optimum treatment conditions, the oil recovery rate increased from 4.41% to 27.53% and more saturates were produced according to the result of SARA. At the same time, a lower pyrolysis temperature of 550 °C for HOS than 700 °C for OOS was obtained based on the facts that heavy oil components (mainly aromatics) in OS were oxidized and most metal contents contained in OS was removed by HNO3. In MB adsorption tests, HOS-550 showed a better adsorption capacity of 40.27 mg·g−1 than OOS-700 of 15.56 mg·g−1. Langmuir isotherm model was well fitted to describe the equilibrium data. As the existence of large amounts of OS is severe problem nowadays, the current work provides an alternative approach to pretreat OS through HNO3 to increase oil recovery rate and the pyrolysis residues are applied to wastewater treatment.
[13] [14] [15]
[16]
[17]
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Acknowledgments
[21]
This study was supported by the National Natural Science Foundation of China (Grant 51578295), the National Natural Science Foundation of Jiangsu Province (BK20161479), the Educational Commission of Jiangsu Province (16KJB150043), and Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse (Nanjing University of Science and Technology), Qing Lan Project of Jiangsu Province. Additionally, we also appreciate Foundation of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials and a project funded by the priority academic program development of Jiangsu Higher Education Institutions.
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