Evaluation of Oil Sludge Ash as a Solid Heat Carrier in the Pyrolysis

Article pubs.acs.org/EF

Evaluation of Oil Sludge Ash as a Solid Heat Carrier in the Pyrolysis Process of Oil Sludge for Oil Production Shuo Cheng,†,‡ Fumitake Takahashi,‡ Ningbo Gao,§ Kunio Yoshikawa,*,‡ and Aimin Li*,† †

School of Environmental Science and Technology, Dalian University of Technology, Dalian, Liaoning 116024, China Department of Environmental Science and Technology, Tokyo Institute of Technology, Suzukakedai Campus G5-8, Yokohama, Kanagawa 226-8503, Japan § School of Energy and Power Engineering, Xi’an Jiaotong University, No.28, Xianning West Road, Xi’an, Shaanxi 710049, P. R. China ‡

ABSTRACT: A pyrolysis study of oil sludge with and without oil sludge ash and quartz sand as solid heat carriers was conducted using a laboratory-scale reactor and thermogravimetric analyzer. The effects of the pyrolysis temperature and solid heat carrier on the product distribution and oil quality were investigated. The results of the oil sludge ash addition case were compared to those of the quartz sand case to evaluate the possibility of using oil sludge ash as a solid heat carrier in the oil sludge pyrolysis process. The chemical characterization of the oil products was performed by FT-IR (Fourier transform infrared spectroscopy) and NMR analyses. Finally, the possible catalytic effect of oil sludge ash and the sulfur transformation pathway were proposed. The results of the experiment demonstrate that the presence of oil sludge ash can increase the oil yield and reduce the optimal reaction temperature from 500 to 450 °C. Oil sludge ash can reduce the coke yield and the carbon residue of the oil product and increase the light oil/heavy oil ratio of the oil product to a greater degree than quartz sand. The presence of pyrrhotite in the oil sludge ash was inferred to be the reason why oil sludge ash is able to reduce the coke yield and carbon residue of the oil product. The results of this study indicate that oil sludge ash is a potential alternative to quartz sand as a solid heat carrier in the pyrolysis process of oil sludge for oil production.

1. INTRODUCTION As one of the most significant solid wastes generated in the petroleum industry,1 oil sludge has been recognized as a hazardous waste in many countries due to its severe environmental and health threats.2−6 Oil sludge from the upstream oil industry (i.e., oilfield sludge) is a viscous and complex mixture of petroleum, water and, mineral admixtures that consist of surface soil, drilling mud residues, and fine suspended particles removed from crude oil. It has been common practice to dispose of oil sludge by incineration or stabilization. However, both of these methods can cause secondary pollution, and neither of them can recover the petroleum component from the oil sludge. Many previous researches7−10 have demonstrated that pyrolysis is an ideal alternative to handle the oil sludge, as it can recover the useful oil product from the oil sludge. Pyrolysis with a solid heat carrier, also called a directly solidheated retort, has been widely used in the coal liquefaction and oil shale retorting processes due to its high oil yield and high thermal efficiency. Aimin Li and Zhengzhao Ma7 applied quartz sand as a solid heat carrier during the pyrolysis process of oil sludge and observed a maximum oil production recovery rate at 550 °C, with sludge/quartz sand = 1:2. Their work demonstrates that the pyrolysis process with a solid heat carrier could represent an economically competitive alternative for the disposal of oil sludge. However, the disposal of the residue and spent solid heat carrier is a problem that must be solved before this process is used commercially. Figure 1A and B show a photograph and scanning electron microscopy (SEM) micrograph of the oil sludge residue, respectively. As shown in Figure 1A, the oil sludge residue from the pyrolysis process is a © 2016 American Chemical Society

Figure 1. Photograph and SEM micrograph of oil sludge residue.

dark, char-like solid with metallic luster. The coke deposits on the surface of the mineral matrix in a flocculent form (as shown in Figure 1B). It consists of carbon and a polar material that is flammable and carcinogenic. These residues cannot be discharged directly. The major solution for handling such residue is to combine the pyrolysis process with fluidized bed technologies. A fluidized bed boiler is able to remove the coke Received: March 18, 2016 Revised: May 25, 2016 Published: May 31, 2016 5970

DOI: 10.1021/acs.energyfuels.6b00648 Energy Fuels 2016, 30, 5970−5979

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Energy & Fuels from the residue and convert it into heat energy. Moreover, the high-temperature ash can be reused as a solid heat carrier. This combined method has been widely investigated in oil shale processing.11 However, few studies have investigated the use of this combined method in the oil sludge case. Therefore, one of the objectives of this study is to verify the possibility of using oil sludge ash as a solid heat carrier in the oil sludge pyrolysis process. The results were compared with the quartz sand addition case. Table 1 lists the main chemical components of the oil sludge ash and quartz sand used in this study. Oil sludge ash contains

Table 2. Selected Properties and Composition of the Tested Oil Sludge Sample oil sludge proximate analysis (wt% wet basis) moisture volatile fixed carbon ash oil content (wt%)a ultimate analysis (wt%) C H N S Ob HHV(MJ/kg)

Table 1. Chemical Components of Oil Sludge Ash and Quartz Sand compound

oil sludge ash (wt%)

quartz sand (wt%)

SiO2 Al2O3 FexOy CaO K2O MgO Na2O TiO2 SO3 Cl

51.6 17.3 9.2 8.32 3.44 1.6 1.22 1.13 5.48 0.21

99.8 a

5.18 20.02 3.55 71.25 25.79 14.6 2.06 0.08 0.56 2.36 7.86

By a Soxhlet extractor. bBy difference.

constituents from 100%. A bomb calorimeter was employed to measure the heating value of the samples. Table 3 shows the boiling point distribution, carbon residue, and ultimate analysis of the oil extracted by the Soxhlet method from the

Table 3. Properties and Composition of the Extracted Oil 0.12

extracted oil boiling point distribution of the extracted oil (wt%) (ASTM D2887) light and heavy naphtha 70−204 °C light gas oil 204−343 °C heavy gas oil and vacuum residue 343 °C carbon residue (wt%) ultimate analysis (wt%) C H N S Oa

more than 40 wt% metal oxide and approximately 5.5 wt% SO3. The presence of these components unavoidably affects the properties of the oil product. Many previous studies have proven the catalytic effect of the metal oxides (i.e., Al2O3, Fe2O3, and CaO) and the oil sludge ash itself during the pyrolysis of oil sludge.12−16 Our past work13 demonstrated that oil sludge ash can not only increase the oil yield but also decrease the carbon residue of the oil product. These improvements were briefly attributed to the effects of Fe and S in the oil sludge ash. In this study, we provide evidence to support this inference. The other aim of this study is to provide evidence for the previous inference regarding the catalytic effect of oil sludge ash during the pyrolysis of oil sludge. The pyrolysis of oil sludge with oil sludge ash was carried out at different temperatures using a laboratory-scale reactor and thermogravimetric (TG) analyzer. The experiments were also conducted with quartz sand and without a solid heat carrier under the same conditions for comparison. The effects of the thermal conductivity and surface area of the solid heat carrier on the oil yield were investigated. The effect of the oil sludge ash on the quality of the oil product was discussed based on the results of the boiling point distribution, carbon residue, ultimate analysis, NMR, FT-IR (Fourier transform infrared spectroscopy), and XRD (X-ray powder diffraction) analysis.

a

15.92 21.1 62.98 9.3 80.34 10.57 0.85 2.49 5.75

By difference.

oil sludge. The carbon residue of the extracted oil (9.3 wt%) was slightly lower than the vacuum residue from the same oilfield.17 The oil sludge ash was prepared by burning the oil sludge residue from the pyrolysis process in a muffle furnace at 600 °C for 3 h. The ash was crushed and sieved to ≤100 mesh and activated at 550 °C for 1 h before each experimental run. The quartz sand was supplied by SIGMA-Aldrich Co., and it was pretreated in the same manner as the oil sludge ash. 2.2. Experimental Apparatus and Procedures. The pyrolysis experiments were performed in a laboratory-scale reactor and a TG analyzer. The laboratory-scale reactor system consists of a stainless steel stirred tank pyrolyzer that is inserted in an electrical furnace, a condensing system, and a gas product collector, as shown in Figure 2. The inner diameter and height of the pyrolyzer are 80 mm and 375 mm, respectively. The reaction temperature was adjusted by a temperature controller coupled with a K-type thermocouple that was axially inserted into the oil sludge sample. Nitrogen gas with a flow rate of 100 mL/min was passed through the reactor before each experimental run for 1 h to remove the air in the system. The solid heat carrier (300 g) placed in the pyrolyzer was heated at a rate of 20 °C/min from the ambient temperature to the desired temperature (450, 500, 550, and 600 °C). Then, the oil sludge (200 g) was fed to the reactor and held for 60 min with the stirring blade rotating at 5 rpm. The pyrolysis gas was fed into the condensing

2. EXPERIMENTAL SECTION 2.1. Materials. The oil sludge used in this study comes from Shengli Oil Field in China. Impurities, such as stones and plant leaves, were removed by hand. The sample was crushed and sieved to a particle size of 5−10 mm and dried in a recycle ventilation drier at 30 °C for 24 h. Before the experiments, the sample was stored in an airtight container at 4 °C. The results of the proximate analysis, ultimate analysis, oil content, and higher heating value (HHV) of the oil sludge sample are given in Table 2. The ultimate analysis was performed using a CHNS-932 (VTF900, LECO.co) analyzer. The oxygen was estimated by subtracting the C, H, N, S, and ash 5971


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Energy & Fuels Table 4. Equations and Major Structural Parameters equation

fA =

C T/HT − (Hα + Hβ + Hγ )/2HT C T / HT

Figure 2. Diagram of the laboratory-scale stirred tank reactor system. system, in which the condensable (liquid product) and noncondensable (gas product) parts were separated and collected in a liquid tank and gas bag, respectively. The liquid products were centrifuged, and the oil products were separated from the water product by a separating funnel. The gas products were dried by passing through a silica gel drier, and the composition of the gas was analyzed off-line by gas chromatography (GC) with a thermal conductivity detector and flame ionization detector. The gas product yields were calculated by the yield percentage difference. The coke product yields were determined as the difference between the weight of the pyrolysis residue and the sum of the weight of the matrix part (matrix = oil sludge−oil content) and the solid heat carrier. The gas yields were calculated by weight differences. The pyrolysis experiment was also carried out in a SHIMADZU 60 TG analyzer under a nitrogen flow rate of 100 mL/min flow rate for purging. A 20 mg sample was heated at a rate of 10 °C/min from 40 to 120 °C in an α-Al2O3 crucible and maintained at 120 °C for 20 min. Then, the temperature was increased to the final temperature (300, 400, 450, 500, and 600 °C) and maintained for 30 min. The mixture ratio of the oil sludge and solid carrier was 1:1. All experiments were run in duplicate. The averages of at least two runs are reported in this study. 2.3. Analysis. 2.3.1. 1H and 13C NMR Analyses. 1H and 13C NMR analyses were conducted on a Bruker-Biospin Avance III NMR spectrometer. The obtained NMR spectra were used to determine the average structural parameters of the oil product by bringing the integrated areas of the chemical shifts into the equation list in Table 4 according to the concept of the Brown-Ladner method.18−23 The spectral regions were captured in accordance with the ranges of 1H NMR-Hα (2.0−4.0 ppm); Hβ (1.0−2.0 ppm); Hγ (0.5−1.0 ppm); HA (9.0−6.0 ppm), 13C NMR-C in carbonyl (170−210 ppm); and aromatic C linked to −OH or -OR (150−170 ppm). 2.3.2. Fourier Transform Infrared Spectrometry (FT-IR) Analysis. The FT-IR analysis of the oil product was carried out on a JEOL JIRSPX200 FT-IR spectrometer. The oil samples were prepared as a cast film between two KBr windows, and the range of wave numbers was set from 400 to 4000. 2.3.3. X-ray Diffraction (XRD) Analysis. XRD analysis was used to determine the crystalline phase and mineral composition of the oil sludge ash. The ash sample was scanned as random powder mounts from 5 to 40° (2θ) using Cu Kα radiation with a scan step size of 0.04. Mineral identification was carried out with the High Score Plus software.

HAU /CA =

HA + Hα /2 CA

R T = C T + 1 − HT/2 − CA /2

Iso index = Hγ /Hβ

parameter fA: ratio of aromatic carbon Hα: hydrogens at α−CH, CH2, CH3 (monoaromatic, polyaromatic) Hβ: hydrogens at other positions Hγ: hydrogens at the terminal CH3 HT: number of total hydrogens CT: number of total carbons CA: number of total aromatic carbons, CA = CT × fA RA: number of aromatic rings, RA = (CA − 2)/4 HAU/CA: condensation index of the ring systema HA: hydrogens on the aromatic ring RT: number of total rings RN: number of naphthenic rings, RN = RT − RA Iso-index: iso-paraffin index of the oil product

a

Higher HAU/CA value is associated with a lower condensation degree of the aromatic ring system.

Figure 3. DTG curves for the oil sludge pyrolysis at the final temperatures of 300, 400, 450, 500, 550, and 600 °C.

regions: 163−186 °C, 186−387 °C (300 °C in the 300 °C final temperature case), and 387−547 °C (450 °C in the 450 °C final temperature case, 500 °C in the 500 °C final temperature case). The weight loss in the first region, whose peak appears at 180 °C, is 0.26−0.3 wt%. When the samples were held at 120 °C for 20 min, the free moisture was evaporated before the temperature was increased to 163 °C (the starting temperature of the first region). Moreover, it was reported that the most likely initiation of the free radical chain reaction during pyrolysis is through the cracking of a CS or SS bond24,25 because the CH bond energy is at least 100 kJ/mol higher than that of SS and CS bonds.26,27 Therefore, the weight loss during the first region is mainly due to the decomposition of sulfides (e.g., sulfur alcohols, sulfur ethers, and disulfide groups) in the oil sludge. The evaporation of physically

3. RESULTS AND DISCUSSION 3.1. Effect of Pyrolysis Temperature. Pyrolysis experiments were carried out on 20 mg oil sludge samples in the TG analyzer at final temperatures of 300, 400, 450, 500, and 600 °C. Figure 3 shows the DTG curves of this series of experiments, which can be divided into three characteristic 5972


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second is obtained at 450 °C. When the temperature exceeded 500 °C, the oil yield decreased with further increases in the temperature. The gas yields increase with the temperature, but the coke yields initially decrease with the temperature down to a minimum at 550 °C and then increase slightly at 600 °C. These results are in accordance with the results of the TG experiments. 3.2. Effect of Solid Heat Carrier. 3.2.1. Pyrolysis with Oil Sludge Ash. The pyrolysis experiments of oil sludge with oil sludge ash were conducted at 450, 500, 550, and 600 °C, and the results of the product distribution are given in Figure 5.

bounded water also contributed to this weight loss. The second region of the pyrolysis reaction is observed in the temperature range from 186 to 387 °C, and the weight loss (5.8−9.9 wt%) is mainly related to the volatilization and decomposition of light hydrocarbons and the remainder of the physically bounded water. The peak of the second region in the 300 °C case is not fully developed due to its low final temperature. In the third region (387−547 °C), more high boiling point materials were cracked and volatilized. The weight loss of the oil sludge in this region is 300 °C-1.0 wt%, 400 °C-4.1 wt%, 450 °C-9.3 wt%, 500 °C-10.2 wt% and 600 °C-8.93 wt%. As shown in Figure 3, the peak of the third region did not appear in the 300 °C case, and it is not developed completely in the 400 °C case, possibly because their final temperature is not sufficiently high for the complete decomposition and volatilization of the petroleum hydrocarbons in oil sludge. The third region developed completely when the final temperature was increased to 450 °C. However, the order of the weight loss in this region is 500 °C > 450 °C > 600 °C, which means that the weight loss does not depend on the final temperature in this region. The order of the amounts of total weight loss is 500 °C > 450 °C > 600 °C > 400 °C > 300 °C. Therefore, 500 °C appears to be the optimal final temperature for removing the petroleum from the oil sludge by TG. Some sharp peaks were observed in the DTG curve results (e.g., peaks a and b in Figure 3) as a result of the cage effect of the solid particles in the oil sludge. When the petroleum compositions were heated and vaporized during the pyrolysis process, the solid particles could momentarily trap the vaporized matter. The volume of organic vapor continually increased until it broke through the bounds of the particles and escaped to the outside. Thus, the suddenly weight loss during the pyrolysis process can be observed in the form of sharp peaks in the DTG curves. The TG experiments indicate that the temperature of the oil sludge pyrolysis without a heat carrier should be no less than 450 °C. To investigate the effect of the pyrolysis temperature on the product distribution with depth, pyrolysis experiments were conducted with a 200 g oil sludge sample in a stirred tank reactor at 450, 500, 550, and 600 °C. The results of the gas, oil and coke product distributions are presented in Figure 4. The maximum oil yield (11.45 wt%) is obtained at 500 °C, and the

Figure 5. Product distribution of oil sludge pyrolysis with oil sludge ash at 450, 500, 550, and 600 °C.

With the addition of oil sludge ash, the oil yields were enhanced in all of the temperature conditions compared with the causes without a heat carrier presented above. The maximum was increased to 13.55 wt%, and the optimal temperature was reduced from 500 to 450 °C. This means that addition of oil sludge ash as a solid heat carrier can improve the oil yield and reduce the optimal reaction temperature. The oil yield and coke yield decreased and the gas yield increased with increases in the reaction temperature, as higher temperatures cause more volatile matter to be removed from the oil sludge. Those volatilized matters were further broken into lighter or gaseous hydrocarbons.7 Compared with the cases without a heat carrier, the coke yield was decreased from 5.68 to 6.33 wt% to 3.63−4.43 wt%. In the oil sludge ash cases, more of the heavy fraction, such as asphaltene, which has a tendency to condense to coke during the pyrolysis process, was converted to the lighter fraction and dissolved into the oil product instead of condensing into coke. The gas yield differs only slightly between the cases without a heat carrier (4.01− 5.71 wt%) and with oil sludge ash (4.68−6.03 wt%). 3.2.2. Comparing Oil Sludge Ash with Quartz Sand. The product distribution of oil sludge pyrolysis with oil sludge ash at 450 °C is shown in Figure 6 for comparison with two cases of different grain sizes of quartz sand. The results show that the oil yields of the quartz sand addition cases are slightly higher than that of the oil sludge ash case. Both of the quartz sand cases have a higher coke yield but lower gas yield than the oil sludge ash case, indicating that the quartz sand can better reduce the production of gas product than the oil sludge ash but did not perform as well in terms of coke inhibition.

Figure 4. Product distribution of oil sludge pyrolysis without a heat carrier at 450, 500, 550, and 600 °C. 5973


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The gas compositions of oil sludge pyrolysis with oil sludge ash, quartz sand (20−40 mesh), and without a solid heat carrier at 450 °C are shown in Figure 8. The gas composition includes

Figure 6. Product distribution of oil sludge pyrolysis with different solid heat carriers at 450 °C.

The formation of coke is generally attributed to a condensation reaction that follows the cracking of the volatile fragment of the heavy fraction of the feedstock.28−30 The conversion of the condensation reaction, which reflects the coke-forming tendency of the reaction process, is determined as follows: conversion (%) = (heavy gas oil and vacuum residue (343 °C) of oil product (wt%) + coke yield (wt%))/(heavy gas oil and vacuum residue (343 °C) of extracted oil (wt%)) × 100%. The calculation results of the oil sludge ash, quartz sand (20−40 mesh), and cases without a solid heat carrier (the noaddition case) are presented in Figure 7. The results show that

Figure 8. Gas compositions of oil sludge pyrolysis with oil sludge ash, quartz sand (20−40 mesh), and without a heat carrier at 450 °C.

H2, CH4, CxHy, CO, and CO2. The addition of a solid heat carrier increased the H2 from 26.62 wt% (no-addition case) to 31.62 wt% (quartz sand case) and 34.05 wt% (oil sludge ash case). However, the CH4 and CxHy were reduced in the presence of quartz sand and oil sludge ash. The oil sludge ash has a stronger effect than that of quartz sand, possibly due to the inhibiting effect of the solid heat carrier on the secondary reaction of the volatiles. There is a slight increase in CO2 when oil sludge ash was present, which may be associated with the promotion of oil sludge ash on carboxylic acid decomposition. 3.2.3. Effect of the Physical Properties of the Solid Heat Carrier. We used two types of quartz sand with different grain sizes to investigate the effect of the surface area of the solid heat carrier on the pyrolysis results. The BET surface areas of the oil sludge ash, quartz sand (20−40 mesh), and quartz sand (≤100 mesh) are 5.02 m2/g, 4.04 m2/g, and 16.17 m2/g, respectively. In this experiment, the surface area has only a slight effect on the product distribution even though the surface area of the quartz sand was increased from 4.04 to 16.17 m2/g. This finding corresponds well the report of Watkinson.31 The oil sludge was fed into the reactor with a particle size of 5−10 mm. In the case without a heat carrier, the oil sludge received heat through heat conduction and thermal radiation from the reactor wall. In the presence of the solid heat carrier, the oil sludge was wrapped by heat carrier particles as soon as it was injected into the reactor. Therefore, the heat transfer between the oil sludge and solid heat carrier is generally through heat conduction. Figure 9 presents the relationship between the oil yield and heat transfer coefficient of the heat carriers in this study. The heat transfer coefficients of oil sludge ash (0.81 W/m·K) and quartz sand (1.03 W/m·K) are higher than those of the gas medium (0.25 W/m·K).7,32 As shown, there is an approximately linear relationship between the oil yield and heat transfer coefficient. We can speculate that the oil yield of the oil sludge pyrolysis generally depends on the heat transfer coefficient of the heat carrier. 3.2.4. Effect of Oil Sludge ash on the Oil Product Quality. The quality of the oil products is determined by the carbon

Figure 7. Conversion of condensation reaction of oil sludge pyrolysis with and without heat carrier at different temperatures.

the addition of oil sludge ash is able to significantly reduce the conversion of the condensation reaction, especially at high temperature. However, the conversion of the quartz sand case is nearly identical to that of the case without a heat carrier. This indicates that quartz sand cannot prevent the condensation reaction as well as oil sludge ash, which may explain why the coke yield of the quartz sand cases (as shown in Figure 6) is higher than that of the oil sludge ash case. 5974


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carrier. There was no significant difference between the quartz sand and cases without a heat carrier in the carbon residue at the temperatures of 450 and 500 °C. However, when the temperature reached 600 °C, the carbon residue of the case without a heat carrier increased rapidly from 2.61 wt% to 4.4 wt %. This index increases with increasing temperature in the oil sludge ash cases but is consistently lower than those of the cases without a heat carrier. The results indicate that presence of oil sludge ash can not only enhance the oil yield during the pyrolysis process but also reduce the carbon residue and increases the light oil/heavy oil ratio to a greater extent than the quartz sand. Table 5 shows the H/C ratio, heteroatom (S, N, O)/C ratio, and heating value of the oil product from oil sludge pyrolysis with and without oil sludge ash at 450 °C. The H/C ratio of the case with oil sludge ash (1.65) is higher than that of the case without a heat carrier (1.53), which means that the degree of saturation of the oil sludge ash case oil product is higher than that of the case without a heat carrier. In Figure 8, the weight percentage of H2 gas increased due to the addition of oil sludge ash. This result seems to be contradictory to the H/C ratio tendency. However, according to the calculation results for the actual H weight (actual weight of H element = weight percentage of H2, CH4, or CxHy × weight percentage of H element in H2, CH4, or CxHy × weight of gas product) in the gas products, the gas product for the no addition case contained 3.49 g of hydrogen, which is higher than that of the oil sludge ash addition case (3.01 g). The results indicate that more H elements were carried away from the condensable product in the no addition case. Therefore, the results agreed with the H/ C ratio of the oil product. The higher heating value of the oil sludge ash case also supports this conclusion. The lower heteroatom/C ratio of the oil sludge ash case shows that the addition of oil sludge ash is able to decrease the amount of heteroatoms in the oil product. These results correspond with the lower carbon residue of the oil sludge ash case that is explained above, as heteroatoms mainly concentrate in the

Figure 9. Relationship between the oil yield and heat transfer coefficient of the heat carrier.

residue index, H/C ratio, heteroatom (S, N, O)/C ratio, heating value, and light oil/heavy oil ratio, which is expressed as light oil/heavy oil (w/w) = light and heavy naphtha (70−240 °C)/(light gas oil (204−343 °C) + heavy gas oil and vacuum residue (343 °C)), boiling point distribution was measured according to ASTM D2887). The carbon residues (showing the tendency of coke formation) and light oil/heavy oil ratios of the oil product from oil sludge pyrolysis with and without a heat carrier at different temperature conditions are shown in Figure 10. The light oil/heavy oil ratios of the cases with oil sludge ash and quartz sand added are generally lower than those of the cases without a heat carrier due to the larger middle and heavy fractions in the oil product. As noted above, these increases mainly came from the cracking of the asphaltene or precursor coke. The carbon residues of the oil sludge ash cases are notably lower than those of the quartz sand and cases without a heat

Figure 10. Carbon residue and light oil/heavy oil ratio of the oil product from oil sludge pyrolysis with and without a heat carrier at different temperatures. 5975


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Energy & Fuels Table 5. Quality Indexes of Oil Products H/C ratio (mol)

S/C (mol) × 10−2

N/C (mol) × 10−2

O/C (mol) × 10‑2

heating value (MJ/kg)

1.53 1.65

0.82 0.76

0.60 0.50

1.31 0.68

43.27 44.42

450 °Cno add 450 °Coil sludge ash

Table 6. Average Structural Parameters of the Oil Product by 1H and 13C NMR

fA

RT

RA

RN

HAU/CA

iso index

C in COOH/COOR

CA link to OH/OR

450 °Cno addition 450 °Coil sludge ash extracted oil

0.29 0.27 0.26

7.15 3.88 5.30

5.70 3.77 3.63

1.45 0.11 1.67

0.75 1.24 1.02

0.32 0.55 0.28

0.32 0.03 0.63

0.09 0.04 0.30

asphaltene fraction, which is the precursor of the carbon residue. In the view of oil recovery, oil sludge ash is a potential alternative to quartz sand. 3.3. NMR Analysis Results. The average structural parameters (fA, RT, RA, RN, HAU/CA), and iso-paraffin index of the oil products were calculated according to the results of the 1H NMR spectra. The definitions of fA, RT, RA, RN, and HAU/CA are listed in Table 4. The iso-paraffin index presents the degree of isomerization of the oil product. The amount of carbon in the COOH/COOR groups and the aromatic carbons linked to OH/OR were obtained from the 13C NMR spectra. The calculation results of the oil product from pyrolysis with and without oil sludge ash at 450 °C are provided in Table 6. The result of the extracted oil is provided for comparison. After the pyrolysis process, the aromatic carbon ratio (fA) increased regardless of whether the oil sludge ash was added due to the aromatization during the pyrolysis process. However, the increase in the case without a heat carrier (from 0.26 to 0.29) is greater than that in the case in which oil sludge ash was added (from 0.26 to 0.27), which means that the absence of oil sludge ash led to a more rapid trend of aromatization in the oil sludge pyrolysis. The case without a heat carrier has the highest total ring number (RT) but the lowest condensation index (HAU/CA), indicating that the ring systems in the oil product of the case without a carrier have a more condensed ring structure than the cases with oil sludge ash and extracted oil. In contrast, the case with oil sludge ash has the lowest total ring number and highest condensation index (HAU/CA), which indicates that it contains a less condensed ring structure. The reduction of the C in  COOH/COOR from 0.63 (extracted oil) to 0.32 (no heat carrier) is mainly due to the thermal decomposition of carboxylic acid. This value further decreased to 0.03 in the case with oil sludge ash, which means that the presence of oil sludge ash can improve the decarboxylation. 3.4. FT-IR Analysis Results. The IR of the oil products of the cases with oil sludge ash (PO) and extracted oil (EO) was broken into two wavenumber ranges, 400−1300 cm−1 and 1300−3200 cm−1, which are shown in Figures 11 and 12, respectively. In Figure 11, the presence of a SiO or AlO group at 465 cm−1 in the EO is due to the mineral particles remaining in the extracted oil. This peak disappeared in the PO case, as the mineral particles were removed from the oil product in the pyrolysis process. The peaks distributed in area 1 show the chain length distribution of the oil sample. The peak at 728 cm−1 demonstrates the existence of a CH2 group in C(CH2)nC, where n ≥ 4. The peaks for n = 3−1 occur from 744 to 781 cm−1.33−35 The n ≥ 4 peak was weakened after the pyrolysis with oil sludge ash. In contrast, the peaks for n =

Figure 11. FTIR spectrum of the oil product (from 400 to 1300 cm−1).

Figure 12. FTIR spectrum of the oil product (from 1300 to 3200 cm−1).

3−1 appeared in the IR of PO. These changes may be due to the scission of the long-chain alkanes into shorter ones during the pyrolysis process. The coexistence of peaks at 908 and 991 cm−1 in PO can be assigned to a terminal olefin (α olefin), which is one of the characteristic products of long n-alkane thermal cracking.36 Areas 2 and 3 in Figure 12 present the aliphatic CH2 and CH3 deformation vibration zone and the aliphatic CH zone of the oil sample IR, respectively. Peak (a) in area 2 (1376 5976


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Energy & Fuels cm−1: the symmetric deformation vibration δ CH3) indicates the presence of branched hydrocarbons.15 This peak became stronger in the PO case due to the increasing amount of branched hydrocarbons in the oil product. This is confirmed by the iso-paraffin index results in Table 6, where the iso-paraffin index of PO (0.55) is higher than that of EO (0.38). The small peak at 1642 cm−1 in the PO case indicates the production of CC aromatic and olefinic skeletal vibrations during the pyrolysis process. Area 4 of Figure 12 is assigned to the zone of CO stretching absorption in carboxyl or carbonyl groups. Because the peak positions in this area are difficult to separate from each other, we took the second derivative of the absorbance from 1660 to 1830 cm−1, and the results are given in Figure 13. The

Figure 14. XRD spectrum of oil sludge ash.

produce Fe2(SO4)3, which has been shown in past work to effectively improve the oil quality during the oil sludge pyrolysis.12 Therefore, the presence of metal oxides in the oil sludge ash is inferred to promote the isomerization of the oil product as an acid catalyst. Pyrrhotite (Fex−1S) was also observed in the XRD spectrum of oil sludge ash, which is considered to be the active ingredient of coke inhibitor.43 The presence of pyrrhotite in the oil sludge ash may be the reason that the oil sludge ash can reduce the coke yield and carbon residue of the oil product in the pyrolysis process. Figure 15 presents the DTG curves of the oil sludge pyrolysis with and without oil sludge ash at the final temperature of 450

Figure 13. Second derivative of the absorbance of the oil product from 1660 to 1830 cm−1.

peak at 1708 cm−1 for CO in the COOH group33,37,38 disappeared in the IR of PO due to the thermal decomposition of carboxylic acid. The peak for CO in ketone or aldehyde (1716−1722 cm−1) and CH in aldehyde39 (2729 cm−1 in Figure 12) become stronger in the PO case. These trends suggest that a dehydrogenation reaction of alcohols with ketone7 and aldehyde formation may occur during the pyrolysis process. The decreasing of CA linked to OH or OR (Table 6) in the PO case also support this inference. Considering the high metal oxide content in oil sludge ash, the reaction of carboxylic acids with metal oxides in oil sludge ash is another possible source of ketones in PO.40−42 3.5. Possible Catalytic Effect of Oil Sludge Ash. The isomerization of the oil product, which is determined by the iso-paraffin index, is the determinative feature of the cracking over acid catalyst that follows the carbonium ion reaction. Table 6 shows that the iso-paraffin index of the case with oil sludge ash added (0.55) is higher than that of the case without a heat carrier (0.32). This result indicates that the isomerization of the oil product is caused by not only thermal cracking but also oil sludge ash addition. The oil sludge ash used in this study contains over 40 wt% of metal oxide. Figure 14 shows the XRD spectrum of the oil sludge ash. γ-Al2O3, hematite (Fe2O3), and lime (CaO), whose catalytic effects have been proven by previous studies, were detected. According to the chemical composition of oil sludge ash listed in Table 1, there is approximately 5.5 wt% SO3 in oil sludge ash. The SO3 is able to react with H2O and hematite to

Figure 15. DTG curves for the oil sludge pyrolysis with and without oil sludge ash at the final temperature of 450 °C.

°C. The heating rate is 20 °C/min, and the mixture ratio of oil sludge and oil sludge ash is 1:1. As noted above, the weight loss at approximately 180 °C is mainly due to the decomposition of sulfides and the evaporation of physically bounded water. During this decomposition process, H2S would be released.27 However, the weight loss peak disappeared when oil sludge ash was added. This result indicates that the H2S and H2O might be absorbed by the oil sludge ash. After pyrolysis and burning, this portion of the sulfur might remain in the oil sludge ash in the 5977


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form of SO3 or pyrrhotite. A possible pathway of the sulfur transformation is shown in Figure 16.

REFERENCES

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Figure 16. Pathway of sulfur transformation.

In this manner, the organic sulfur in the oil sludge feedstock can be converted into inorganic sulfur and work as a coke inhibitor (e.g., pyrrhotite) or catalyst (e.g., Fe2(SO4)3) during the pyrolysis of oil sludge with oil sludge ash. Further research is needed to verify this pathway.

4. CONCLUSIONS

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Article

• The optimal temperature of oil sludge pyrolysis without a solid heat carrier for oil recovery is 500 °C. • The presence of oil sludge ash can improve the oil yield and reduce the optimal reaction temperature to 450 °C. • The coke yield was decreased by the addition of oil sludge ash because the heavy fraction, which has a tendency to condense to coke during the pyrolysis, was converted to a lighter fraction and dissolve into the oil product instead of condensing to coke. • The coke yield of the oil sludge ash case is lower than that of the quartz sand case, indicating that oil sludge ash can inhibit the condensation reaction during the pyrolysis to a greater extent than quartz sand. • The presence of oil sludge ash can reduce the carbon residue and increase the light oil/heavy oil ratio to a greater extent than quartz sand. • Oil sludge ash is able to decrease the amount of heteroatoms in the oil product. • The NMR analysis of the oil product shows that the addition of oil sludge ash can reduce the aromatic carbon ratio and total ring number of the oil product and decrease the degree of condensation of the ring system in the oil product molecule. • The result indicates that the isomerization of the oil product was caused by not only thermal cracking but also the presence of oil sludge ash. • The presence of pyrrhotite in the oil sludge ash may be the reason why oil sludge ash can reduce the coke yield and carbon residue of the oil product in the pyrolysis process. • Oil sludge ash is a potential alternative to quartz sand as a solid heat carrier in the pyrolysis process of oil sludge for oil recovery.

AUTHOR INFORMATION

Corresponding Authors

*[email protected]. *[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express our sincere gratitude for the facilities and support provided by Dalian University of Technology and to Shengli Oilfield, China for supplying oil sludge. 5978


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Energy & Fuels (39) Ernö, P.; Philippe, B.; Martin, B. Structure Determination of Organic Compounds, Inc.; Springer-Verlag: Berlin Heidelberg, 2009; pp 310−316. (40) Leung, A.; Boocock, D. G. B.; Konar, S. K. Energy Fuels 1995, 9, 913−920. (41) March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structures, 2nd ed.; McGraw-Hill: New York, 1977; pp 448. (42) Wade, L. G., Jr Organic Chemistry; Pentice-Hall: Englewood Cliffs, NJ, 1987; pp 898−989. (43) Herrick, D. E.; Tierney, J. W.; Wender, I.; Huffman, G. P.; Huggins, F. E. Energy Fuels 1990, 4, 231−236.

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Evaluation of Oil Sludge Ash as a Solid Heat Carrier in the Pyrolysis

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