Separation and Hydroprocessing of HZSM-5 Catalytic Olive Mill Waste

Article pubs.acs.org/EF

Separation and Hydroprocessing of HZSM‑5 Catalytic Olive Mill Waste Sludge Bio-oil Bingji Ma*,† and Foster Agblevor‡ †

College of Agronomy, Henan Agricultural University, Zhengzhou 450002, China USTAR Biofuel Center, Department of Biological Engineering, Utah State University, 620 Grand Avenue, Logan, Utah 84322-4105, United States

‡

ABSTRACT: The catalytic effects of HZSM-5 during olive sludge pyrolysis on the product yields and chemical composition of the bio-oil produced were investigated. Three solvents of increasing polarity (cyclohexane < methylene chloride < acetone) were used to sequentially fractionate the HZSM-5 catalytic bio-oil at 500 °C into four fractions (cyclohexane fraction, methylene chloride fraction A, methylene chloride fraction B, and acetone fraction). The yields for the four fractions cyclohexane, methylene chloride A, methylene chloride B, and acetone were 26.8%, 11.6%, 20.0%, and 39.7%, respectively. The HZSM-5 catalytic bio-oil and noncatalytic pyrolysis bio-oil were characterized by Fourier transform infrared (FT-IR) spectrometry and 1H and 13C nuclear magnetic resonance (NMR) spectrometry, and their organic elemental compositions and calorific values were also determined. Furthermore, major compounds in both bio-oils were identified by gas chromatography−mass spectrometry (GC−MS). Compared with the olive sludge pyrolysis bio-oil obtained without a catalyst, the HZSM-5 catalytic bio-oil had a lower viscosity and a higher calorific value. Chemical characterization revealed that hexadecanenitrile was the major component in the HZSM-5 catalytic bio-oil (23.8%), whereas oleic acid, ethyl oleate, and hexadecanoic acid were the dominant compounds in the noncatalytic bio-oil. Furthermore, the pyrolysis temperature strongly affected the yield of hexadecanenitrile, and most of the hexadecanenitrile could be obtained in a high purity by polarity-based separation plus silica gel column chromatography. Moreover, a series of experiments was conducted at varying temperatures, pressures, catalyst contents, and residence times to study the effects of these parameters on the hydrodeoxygenation reaction in a microreactor. As a result, HZSM-5 catalytic OMWS pyrolysis oil was successfully converted to HDO-upgraded oil. The experimental results indicated that the best performance conditions were 400 °C, 550 psi, 15% catalyst content, and 45-min reaction time for nickel catalytic hydroprocessing of biocrude leading to free-flowing oil.

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INTRODUCTION

Among various processes for the thermochemical conversion of biomass, pyrolysis is considered to be a promising technology for liquid fuel production.7,8 The pyrolysis of olive residues (cuttings and kernels) in a captive sample reactor at atmospheric pressure under helium at a heating rate of 200 °C/s from 300 to 600 °C was conducted by Zabaniotou et al.9 The results showed that, as the final temperature was increased, the percentages of liquid and gaseous products increased and the oil products reached a maximum value of 30% of the dry biomass at about 450−550 °C. Pyrolysis of olive bagasse biomass with selected catalysts in a fixed-bed reactor was carried out by Iĺ knur and Sevgi.10 The maximum bio-oil yields obtained from the pyrolysis olive bagasse were 37.07% and 36.67% upon the use of activated alumina and sodium feldspar, respectively, as catalysts. Presently, bio-oil obtained by fast pyrolysis of biomass is a transportable intermediate that is a potential fossil-fuel substitute. However, compared to fossil fuels, fast-pyrolysis bio-oil have several disadvantages, such as a high oxygen content, low heating value, high viscosity, and viscosity increase over time when heated.11 Therefore, bio-oil derived from biomass must be upgraded by increasing its H/C ratio and reducing its O/C ratio before it can be used as a transportation fuel. Hydro-

Olive oil is one of the most important agricultural products of Mediterranean countries and plays an important role in the economy. The Mediterranean region and Middle East produce about 99% of the total world olive fruit. In 2007, the world production of olives was 17.4 Mt, and the amount export approached 3 Mt.1 It is possible to recover 15−22 kg of olive oil and 35−45 kg of olive residue from 100 kg of olive fruits.2 Currently, three main processes are employed to produce olive oil: (1) the traditional batch pressing system, (2) the continuous three-phase processes, and (3) continuous two-phase processes.3 Each process generates a large amount of olive mill wastewater that mainly contains particulate matter from olive pulp, sugars, pectins, tannins, mucilages, organic acids, and polyphenols.4 Olive mill wastewater is becoming one of the major environmental pollutants in the Mediterranean area because it pollutes surface water when spilled. One way to solve this problem is to apply olive mill wastewater directly to croplands to utilize its mineral and organic contents as fertilizer.5 However, land application also has some disadvantages because of the high salt content of olive mill wastewater.6 Over the past several years, ponds and lagoons have been built for the disposal of olive mill wastewater, from which the water evaporates during the dry spring−summer period. The residual sludge is recovered in the fall, before a new olive processing season starts. © 2016 American Chemical Society

Received: August 31, 2016 Revised: November 1, 2016 Published: November 17, 2016 10524

DOI: 10.1021/acs.energyfuels.6b02194 Energy Fuels 2016, 30, 10524−10533

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Energy & Fuels

Figure 1. Polarity-based separation of HZSM-5 catalytic bio-oil from OMWS. impeller mixing speed. Hydrogen was supplied from a reservoir tank through a pressure regulator. The pH value of the bio-oil was measured using a Mettler-Toledo SevenEasy S20 pH meter (Mettler-Toledo Group, Schwerzenbach, Switzerland). The viscosity and density of OMWS hydrodeoxygenation oils were measured at 40 °C on an SVM 3000 Stabinger viscometer (Anton-Paar Inc., Ashland, VA). About 5 mL of sample was used for the analysis. The higher heating values (HHVs) of olive sludge bio-oils were measured on a C 2000 basic IKA calorimeter (IKA Works, Inc., Wilmington, NC). About 500 mg of sample was used for the HHV analysis. All samples were analyzed in duplicate. The carbon, hydrogen, nitrogen, and sulfur contents of OMWS biooils were determined using a FLASH 2000 Series CHNS/O analyzer (Thermo Fisher Scientific Inc., Waltham, MA). Oxygen was determined by difference (100 wt % − CHNS − ash). About 3 mg of sample was loaded into the reactor for analysis. The samples were analyzed in triplicate. The Fourier transform infrared (FT-IR) spectra of OMWS bio-oils were measured using an Avatar 360 FT-IR spectrometer (Thermo Fisher Scientific Inc., Waltham, MA). About 50 mg of each sample was subjected to FT-IR analysis, and the spectra were obtained by scanning the range from 650 to 4000 cm−1. 1H and 13C nuclear magnetic resonance (NMR) spectra of OMWS bio-oils were acquired on a 300 MHz JEOL nuclear magnetic resonance instrument (JEOL Ltd., Tokyo, Japan). For gas chromatography−mass spectrometry (GC−MS) analysis, an HP 5890-SERIES II gas chromatograph coupled to a mass spectrometer fitted with a mass-selective detector (HP 5972) was employed to identify the compounds in the bio-oils. The mass spectrometer was used in electron-impact mode at 70 eV with helium as the carrier gas. Production of Bio-oils. Fast-pyrolysis experiments were carried out using a bench-scale fluidized-bed reactor located in the USTAR Bioenergy Center, Utah State University, Logan, UT. About 100 g of silica sand or HZSM-5 was used as the fluidizing medium. When silica sand was used as the fluidizing medium, a total flow of nitrogen gas at 18.3 L/min was used to fluidize the bed and entrain the feedstock into the reactor. For HZSM-5, a total nitrogen flow of 8 L/min was used. Using a screw feeder, the feedstock was conveyed from the hopper to an entrainment zone where nitrogen gas was used (5 L/min for both catalytic and noncatalytic experiments) to entrain the feed through a jacketed air-cooled feeder tube into the fluidized bed. After pyrolysis, the

deoxygenation (HDO) is an upgrading process that converts oxygenated molecules into lighter hydrocarbons through the catalytic addition of hydrogen. HDO of pyrolysis oil has been extensively studied and demonstrated to reduce the oxygen content of bio-oil and to produce a liquid hydrocarbon mixture without the negative properties of raw bio-oil.12,13 Several research articles have been published dealing with the pyrolysis and chemical composition of olive residues, such as olive stone and olive bagasse.14,15 It is conceivable to apply pyrolysis technology to convert olive mill waste sludge (OMWS) to biofuel, which will simultaneously solve the waste disposal problem and produce green energy. Recently, Hamza conducted the catalytic pyrolysis of OMWS with different catalysts and found that OMWS was a good feedstock for fast pyrolysis.16 Moreover, extraction with solvents of increasing polarity and separation by silica gel column chromatography are good methods for analyzing the chemical compositions of biooils.17,18 This article deals with the fast pyrolysis of OMWS catalyzed by HZSM-5 at 500 °C and reports the chemical characterization of OMWS bio-oil. In this article, we report a method for isolating the valuable compound hexadecanenitrile from HZSM-5 catalytic olive sludge bio-oil by polarity-based separation combined with silica gel column chromatography. Furthermore, an objective of this research was also to upgrade HZSM-5 catalytic OMWS oil at 500 °C by the HDO reaction so as to improve its stability and heating value and lower the viscosity of the bio-oil.

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EXPERIMENTAL SECTION

Materials and Methods. OMWS was collected from a Tunisian disposal pond containing 1−2-year-old materials and olive stone and seeds (Agareb 3030, Sfax, Tunisia). A catalyst containing nickel on silica−alumina (∼65 wt % Ni powder) was purchased from SigmaAldrich. Hydrodeoxygenation reaction experiments were carried out in a 300 mL Parr Series 4560 microreactor (Parr Instrument Company, Moline, IL). A Parr 4848 controller was used to monitor the vessel pressure. The controller also monitored the internal temperature and 10525


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H and 13C NMR Analysis. Approximately 100 mg of each sample was dissolved in 0.6 mL of deuterated chloroform (CDCl3) or dimethyl sulfoxide (DMSO-d6). Chemical shifts were referenced to tetramethylsilane (TMS) for both 1H and 13C NMR spectra. The observed frequency for the 13C nucleus was 75 MHz. The pulse width was 4.9 μs, the acquisition time was 1.88 s, and the recycle delay was 0.5 s. The spectra were obtained with 4000 scans. The observed frequency for the 1 H nucleus was 300 MHz. The pulse width was 6.7 μs, the acquisition time was 3.91 s, and the recycle delay was 1 s. The spectra were obtained with 16 scans. GC−MS Analysis of Bio-oils. A portion of the OMWS bio-oil was dissolved in CH2Cl2 and analyzed by GC−MS. One microliter of liquid sample was injected into the GC injector maintained at 250 °C. The components of the sample were separated on the GC column (HP-5, 30m length, 25-μm internal diameter) and analyzed with the mass detector. The mass was scanned over a mass range from m/z 50 to 500 at a rate of 1.42 s. The GC oven temperature was programmed to increase from 40 to 285 °C at a rate of 10 °C/min and was maintained at 285 °C for 25 min. A delay time of 3 min was applied to eliminate the peak of solvent ions. The G1701BA version B 01.00 GC/MS Control ChemStation software was used to find the fingerprint of each compound in the chromatogram and compare it with 20 already existing similar fingerprints.

liquid (organics/water) and solid (char/coke) product yields were determined by weighing the reactor, hot-gas filter, water-chilled condensers, and electrostatic precipitator before and after each experiment. Each pyrolysis experiment lasted 2 h, and all experiments were replicated. Separation Procedure of HZSM-5 Catalytic Bio-oil. A 3.95-g sample of HZSM-5-catalyzed olive sludge bio-oil was fractionated by silica gel column chromatography. The glass column was 45 cm in length and 5 cm in diameter loaded with 80 g of silica gel (200−300 mesh). The sample was first eluted with 400 mL of cyclohexane and then eluted with 400 mL of methylene chloride (CH2Cl2). Finally, the sample was eluted with 400 mL of acetone. The CH2Cl2 elution was divided into two parts because of the different colors of two fractions. The first 200 mL CH2Cl2 elution (fraction A) was light in color, whereas the second 200 mL elution (fraction B) was a little darker in color. The solvents were evaporated under a low vacuum on a rotary evaporator to obtain the cyclohexane fraction (0.92 g), CH2Cl2 fraction A (0.46 g), CH2Cl2 fraction B (0.79 g), and acetone fraction (1.57 g). A schematic of the separation process is shown in Figure 1. CH2Cl2 fraction B was subjected to silica gel column chromatography for further purification. About 40 g of silica gel (200−300 mesh) was loaded into the 45 cm × 5 cm glass column, and the sample was eluted with 250 mL of petroleum ethyl/ ethyl acetate mixture (95:5, v/v). The solvent was evaporated under a low vacuum on a rotary evaporator to give 0.48 g of hexadecanenitrile (60.7% of CH2Cl2 fraction B). The column was then eluted with 150 mL of acetone to give about 0.25 g of residue after evaporation of the solvent. Hydrodeoxygenation Reaction. The microreactor vessel was charged with 10 g of HZSM-5 catalytic OMWS pyrolysis oil at 500 °C and either 1 or 1.5 g of catalyst and then flushed with hydrogen three times to displace the air. After the vessel had been charged with either 500 or 550 psi hydrogen, the oil/catalyst slurry was lightly stirred at 250 rpm with a furnace heating the vessel to an internal temperature of either 350 or 400 °C. Once the operating temperature had been reached, the reaction mixture was held at that temperature for either 30 or 45 min. Upon completion of the reaction, the reactor was cooled using cooling coils inside the reactor. The final gas pressure and temperature were recorded, and the gas was trapped for GC analysis. The GC analysis results showed the content of the remaining H 2 , and the H2 consumption was calculated according to the ideal gas law. All hydrodeoxygenation reactions were carried out in duplicate. The reactor contents were recovered and separated as upgraded oil, catalyst, and water. Methanol/toluene (1:1, v/v) was used to recover the remaining oil on the catalyst. Both parts of the oil were combined to determine the yield of final upgraded OMWS oil. The HDO reaction conditions of temperature, pressure, catalyst content, and residence time are listed in Table 1.

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RESULTS AND DISCUSSION Yields of the Fractions from HZSM-5 Catalytic OMWS Oil. The HZSM-5 catalytic pyrolysis OMWS oil at 500 °C was readily fractionated using the polarity-based solvent fractionation method. The yields of various fractionation products shown in Figure 1 did not show any predominant fraction. The yields of the cyclohexane-soluble fraction (26.8%) and the CHCl2-soluble fraction (CH2Cl2 fractions A + B, 31.6%) were close, but the acetone-soluble fraction was slightly higher (39.7%). The fraction of the HZSM-5 catalytic OMWS bio-oil soluble in low-polarity solvents (cyclohexane and CHCl2) was 58.4%, whereas the more polar fraction was 39.7%. The chemical compositions in the cyclohexane and CHCl2 fractions had a low polarity, indicating a rich content of long-chain aliphatic compounds, benzene compounds, and ester compounds. The total amount of the cyclohexane and CHCl2 fractions was over 50% in the crude bio-oil, which also resulted in a low viscosity of the overall oil. Furthermore, the total mass balance was 98.1%, which showed that the separation was highly efficient. Physicochemical Properties of Bio-oils. The elemental analysis of the olive sludge bio-oil catalyzed by HZSM-5 at 500 °C showed high contents of carbon (74.21%) and hydrogen (9.21%), whereas the oxygen content was relatively low (14.09%). The noncatalytic olive sludge bio-oil had lower carbon (66.40%) and higher hydrogen (9.39%) contents. The noncatalytic bio-oil showed a higher content of oxygen (20.81%). In addition, sulfur was not detected in either bio-oil. The HZSM-5 catalytic bio-oil had a higher calorific value (41.07 MJ/kg) than the bio-oil at 500 °C without a catalyst (37.83 MJ/kg) (Table 2). The data in Table 2 also show that the HZSM-5 catalytic bio-oil was less viscous than the noncatalytic bio-oil, although the two had similar densities. The four fractions from HZSM-5 catalytic bio-oil obtained by polarity-based separation showed different

Table 1. HZSM-5 Catalytic OMWS Oil Samples for the HDO Reaction sample

temperature (°C)

pressure (psi)

catalyst content (%)

time (min)

1 2 3 4 5 6 7

350 350 400 400 400 350 400

500 550 500 550 550 500 550

10 10 10 10 15 15 15

45 45 45 45 45 45 30

Table 2. Characteristics of OMWS Bio-oils elemental content (%) sample

C

H

N

O

S

HHV (MJ/kg)

viscosity at 40 °C (mPa·s)

density (kg/L)

OMWS bio-oil catalyzed by HZMS-5 OMWS noncatalytic bio-oil

74.21 66.40

9.21 9.39

2.49 3.45

14.09 20.81

0 0

41.07 37.83

21.08 65.58

0.914 0.922

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Figure 2. FT-IR spectra of OMWS bio-oils.

CH2 and CH3 bending, respectively. In addition, the small and sharp band at 1515 cm−1 in Figure 2A is due to secondary aromatic amines or CC stretching in pyridine rings.19 The strong and broad band at 1239 cm−1 in Figure 2A indicates the presence of aryl ethers and vinyl ethers. In addition, the small band at 966 cm−1 is due to ring vibrations and C−H deformations of pyridine.20 For the bio-oil produced from HZSM-5 catalytic pyrolysis (Figure 2B), the strong bands at 2924 and 2853 cm−1 in the spectrum can be assigned to aliphatic CH3 and CH2 groups, respectively. The signal at 2246 cm−1 can be assigned to nitrile

physical properties. The cyclohexane fraction, CH2Cl2 fraction A, and CH2Cl2 fraction B had very low viscosities, whereas the acetone fraction was highly viscous (viscosity data not given here because of an inadequate quantity of samples). FT-IR Spectra. The FT-IR spectra of olive sludge bio-oils are shown in Figure 2. For the noncatalytic bio-oil (Figure 2A), the strong bands at 2922 and 2852 cm−1 in the spectrum are due to aliphatic CH3 and CH2 groups, respectively, indicating a relatively high content of long-chain aliphatic compounds. The broad signal at 1709 cm−1 corresponds to the CO stretch of COOH. The bands of 1456 and 1377 cm−1 are associated with 10527


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Figure 3. NMR spectra of OMWS noncatalytic bio-oil at 500 °C.

compounds. The band at 1594 cm−1 can be assigned to the N−H deformation of nitrogen compounds. The broad signal at 1704 cm−1 indicates the presence of COOH. The bands of 1456 and 1377 cm−1 are associated with CH2 and CH3 bending, respectively. The broad band at 1266 cm−1 in Figure 2B indicates the presence of aryl ethers and vinyl ethers. The band at 1025 cm−1 corresponds to the C−O stretching of carbohydrates. In addition, the peak at 751 cm−1 can be assigned to the β-ring of pyridines. 1 H and 13C NMR Spectra. The 1H NMR spectrum of OMWS noncatalytic bio-oil (Figure 3A) showed the prominent signals of aliphatic CH3 and CH2 protons between 0.8 and 2.4 ppm and olefinic protons between 5.3 and 5.4 ppm. The chemical shift observed at 7.2 ppm was assigned to the CDCl3 solvent. The 13C NMR spectrum of the bio-oil in Figure 3B exhibits signals between 14.3 and 32.1 ppm due to paraffinic carbons; at 129.9−130.4 ppm due to olefinic carbons; and at 178.6 ppm due to C in COOH, which indicates that the oil

contained unsaturated fatty acids. The resonance at 77.4 ppm was assigned to the solvent of CDCl3. The 1H NMR spectrum of the bio-oil catalyzed by HZSM-5 (Figure 4A) showed the major aliphatic protons (0.8−2.7 ppm) of CH3 and CH2 bonded to C. The chemical shifts observed between 7.0 and 7.3 ppm were most likely due to aromatic protons in benzenoid structures and/or in aromatic Ncontaining compounds. The 13C NMR spectrum of this bio-oil (Figure 4B, CDCl3 as the solvent) exhibited terminal CH3 at 14.7 ppm, long-chain CH2 between 17.2 and 32.5 ppm, and signals of aromatic and/or N-containing compounds between 115.8 and 158.3 ppm. The four fractions from HZSM-5 catalytic bio-oil obtained by polarity-based separation were also subjected to NMR analysis. The cyclohexane fraction showed strong aliphatic signals attached to the benzene ring. For the acetone fraction, only very weak carbon signals appeared when CDCl3 was used as the NMR solvent. However, relatively strong signals appeared when the NMR solvent was changed to DMSO-d6. The resonance at 10528


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Figure 4. NMR spectra of OMWS bio-oil at 500 °C catalyzed by HZMS-5.

39.5 ppm can be assigned to the solvent of DMSO-d6. The 13C NMR resonances between 13.9 and 33.8 ppm are due to paraffinic carbons, those at 115.1−157.5 ppm can be attributed to the presence of aromatic compounds or N-heterocyclics, and that at 174.6 ppm is due to C in COOH. It is noteworthy that CH2Cl2 fraction A seemed to be a pure compound with little impurity (Figure 5). A detailed NMR analysis showed that CH2Cl2 fraction A was hexadecanenitrile.21 1H NMR (300 MHz, CDCl3) δ ppm: 0.85 (t, J = 7.2 Hz, H-16), 1.25 (m, overlap, H4−H-14), 1.42 (m, H-15), 1.63 (m, H-3), 2.30 (t, J = 6.2 Hz, H2). 13C NMR (75 MHz, CDCl3) δ ppm: 14.3 (C-16), 17.2 (C15), 23.8 (C-3), 25.5−29.8 (C-4−C-14), 32.7 (C-2), 119.9 (C1). In addition, GC−MS showed the mass spectrum of hexadecanenitrile m/z: 236 [M+ − 1], 222, 208, 194, 180, 166, 152, 138, 124, 110, 97, 82, 69, 55, 41, 29, 14. CH2Cl2 fraction B was also dominated by the signals of hexadecanenitrile, as shown by the 13C NMR spectrum. Additionally, CH2Cl2 fraction B exhibited resonances of phenols, which have higher polarity than the aromatic components in the

cyclohexane fraction. To remove the impurities from CH2Cl2 fraction B, CH2Cl2 fraction B was subjected to silica gel column chromatography for further purification. As a result, the 13C NMR spectrum of the residue showed weak signals of hexadecanenitrile, which means CH2Cl2 fraction B was mostly hexadecanenitrile. 13C NMR spectra of the cyclohexane fraction, CH2Cl2 fraction B, and acetone fraction are shown in Figure 6. Major Compounds of Bio-oils Identified by GC−MS. The noncatalytic bio-oil and HZSM-5 catalytic bio-oil were both analyzed by GC−MS. More than 200 peaks were identified in the HZSM-5-catalyzed oil. However, about 50 peaks were detected in the bio-oil without a catalyst, which suggests that HZSM-5 cracked the OMWS chemicals effectively. Briefly, the noncatalytic oil was dominated by oleic acid (28.54% of the total area), ethyl oleate (9.72% of the total area), and hexadecanoic acid (7.28% of the total area), whereas HZSM-5-catalyzed oil showed the prominence of hexadecanenitrile (20.25% of the total area). However, the chromatogram also contained several unknown compounds. The major peaks accounting for more 10529


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Figure 5. NMR spectra of CH2Cl2 fraction A.

components. Oleic acid, ethyl oleate, and hexadecanoic acid were the dominant compounds in the noncatalytic bio-oil, whereas the dominant compound in the HZSM-5 catalytic bio-oil was hexadecanenitrile. To have a clear understanding of the effect of the fast-pyrolysis temperature on the hexadecanenitrile yield, the HZSM-5 catalytic OMWS bio-oil was also produced at 400 and 450 °C and then separated by the same methods. The cyclohexane, CH2Cl2, and acetone fractions and hexadecanenitrile in these two oils were obtained by silica gel column chromatography, and their yields were quantified (Table 5). As a result, the yields of the

than 1% of the total peak area of two bio-oils are summarized as Tables 3 and 4. It was reported that olive stone and seeds are the attractive sources of bioactive and valuable compounds.22 The seed oil is richer in individual sterols, mainly β-sitosterol, which has significant effects on the absorption of cholesterol and bile acid.23 The olive stone and seeds are also richer in total polyunsaturated fatty acids (PUFAs). In our experiments, high yields of fast-pyrolysis bio-oils were obtained from OMWS that had a high calorific value and low viscosity. Furthermore, the chemical analysis of these two bio-oils showed the different major 10530


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Figure 6. 13C NMR spectra of the cyclohexane fraction, CH2Cl2 fraction B, and acetone fraction.

Hexadecanenitrile is an important intermediate in the chemical industry for the synthesis of surfactant.24 Hexadecanenitrile is also an important additive widely used in lubricants. Moreover, the addition of 0.01−2% hexadecanenitrile or similar aliphatic nitriles to gasoline can prevent the deposition of carbon in the cylinders of an engine.25 In contrast, fatty acids (oleic acid and hexadecanoic acid) are the main compounds obtained during the fast pyrolysis of olive sludge without the addition of catalyst. Thus, HZSM-5 is a useful catalyst that can effectively convert OMWS into hexadecanenitrile. However, the specific

various fractions were found as a function of temperature. At 400 °C, the oil was dominated by the polar acetone fraction, but this fraction was converted into the lower-polarity fraction with increasing temperature. Hence, HZSM-5 can effectively crack the polar compositions in the acetone fraction at higher temperatures. The results also showed that the yield of hexadecanenitrile in the oil at 500 °C was similar to that in the oil at 450 °C, but the yield of hexadecanenitrile in the oil at 400 °C was much lower (11.7%). These results obviously indicate that the pyrolysis temperature significantly affects the yield of hexadecanenitrile. 10531


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the HDO reaction in a microreactor. The yields of HDOupgraded oil under the various conditions are listed in Table 6. Generally, higher yields of upgraded product were obtained at lower temperature, pressure, and catalyst content and shorter residence time, which is comparable to the results reported in the literature.26 The highest yield of 88.8% was achieved at 350 °C, 500 psi, 10% catalyst content, and 45 min. However, 75.9% of the biocrude was converted to the upgraded product at 400 °C, 550 psi, 15% catalyst content, and 45 min. The number of moles of H2 consumed in the HDO reaction are also listed in Table 6. As a whole, H2 consumption exhibited a trend opposite to that of the yield of upgraded oil. The highest H2 consumption was 25.25 mol per kilogram of OMWS pyrolysis biocrude based on the experiments using sample 5, whereas the HDO reaction using sample 1 consumed the lowest amount of hydrogen at 22.88 mol/kg. The upgraded oils were completely free-flowing at room temperature and lighter in color than the original biocrude. When kept in a refrigerator at 0 °C, sample 5 was still a freeflowing liquid, whereas some solid precipitates formed in samples 1−4, 6, and 7. This phenomenon indicates that higher temperatures, pressures, and catalyst contents effectively cracked the high-molecular-weight compounds in the feedstock. As a result, sample 5 was almost colorless, but the other samples still had a little color. Physical Property of HDO Oils. Table 6 reports the physical properties of the HDO-upgraded oils. The pH values of the HDO products ranged from 7.96 to 8.45 and were significantly greater than that of the biocrude oil (6.65). Table 6 also presents the viscosities and densities of the biocrude and its HDOupgraded products. The viscosities of the OMWS upgraded oils ranged from 1.60 to 3.03, significantly lower than that of the biocrude, which indicates the good flow properties of the upgraded oils. There was a slight improvement in the densities of all upgraded products. Compared to the feedstock, the densities of the HDO oils were improved to 0.786−0.835 g/cm3. For the measured HHV results listed in Table 6, a maximum HHV of 45.72 MJ/kg was found for the oil upgraded at 400 °C, 550 psi, 15% catalyst content, and 45-min residence time, whereas the raw bio-oil had a low HHV of 41.07 MJ/kg. The FT-IR spectrum of the OMWS HDO-upgraded oil from sample 5 is shown in Figure 7. Compared to that of the feedstock, the FT-IR spectrum of the HDO oil is very flat, and the broad signals at 1704, 1584, and 1268−817 cm−1 disappeared almost completely, indicting the good performance of the nickel catalyst in the catalytic hydroprocessing of biocrude.

Table 3. Major Peaks of Noncatalytic Bio-oil (>1%) Identified by GC−MS no.

retention time (min)

percentage (%)

1 2 3 4 5 6 7 8 9

14.149 16.379 16.450 18.916 19.624 19.849 20.746 20.793 20.934

1.29 1.60 1.27 1.98 7.28 2.75 1.88 3.33 4.62

10 11 12 13 14

21.477 21.583 21.808 22.351 22.469

28.54 9.72 5.85 2.22 2.39

chemical name pentadecane 8-heptadecene 1-heptadecene heptadecane hexadecanoic acid hexadecanoic acid, ethyl ester unknown 2-heptadecanone 9-octadecenoic acid (Z), methyl ester oleic acid ethyl oleate 2-nonadecanone hexadecanamide unknown

Table 4. Major Peaks of HZSM-5-Catalyzed Bio-oil (>1%) Identified by GC−MS no.

retention time (min)

percentage (%)

chemical name

1 2 3 4 5 6 7 8 9 10 11 12

3.987 9.545 16.225 16.378 16.449 16.650 18.809 19.093 19.435 21.075 21.383 22.104

1.10 1.00 1.08 1.36 1.67 1.05 2.31 20.25 1.00 3.08 2.01 1.24

undecane 4-ethylphenol 1-methylnonylbenzene unknown decylbenzene 1-heptadecene dodecylbenzene hexadecanenitrile unknown octadecanenitrile nonadecanenitrile hexadecanamide

Table 5. Yields of HZSM-5 Catalytic Bio-oil Fractions and Hexadecanenitrile at Different Pyrolysis Temperatures yields (%) fraction

400 °C oil

450 °C oil

500 °C oil

cyclohexane CHCl2 (fraction A + fraction B) acetone hexadecanenitrile

27.6 19.9 46.4 11.7

30.2 30.6 30.9 21.2

31.1 33.8 27.9 23.8

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mechanisms of the different thermochemical conversions are still unclear. Effect of HDO Reaction. A series of experiments was conducted at varying temperatures, pressures, catalyst contents, and residence times to study the effects of these parameters on

CONCLUSIONS This study focused on the fast pyrolysis of olive mill waste sludge to obtain bio-oils. HZSM-5 catalyst was investigated in terms of the effects of the catalyst on the product yield and chemical

Table 6. HDO-Upgraded Oil Yields, H2 Consumption, and Physical Properties sample

HDO oil yield (%)

H2 consumption (mol/kg)

HHV (MJ/kg)

pH

viscosity at 40 °C (mPa·s)

density (g/cm3)

1 2 3 4 5 6 7

88.8 86.0 78.5 77.1 75.9 82.0 79.9

22.88 24.70 24.09 24.16 25.25 24.79 24.52

43.13 44.65 44.62 44.45 45.72 45.61 45.07

8.30 8.25 8.45 7.96 8.20 8.40 8.44

3.03 2.98 2.03 1.96 1.60 1.86 2.07

0.835 0.829 0.821 0.817 0.804 0.803 0.786

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Energy & Fuels

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composition of the bio-oil product. In addition, polarity-based separation with column chromatography was employed to separate the HZSM-5 catalytic bio-oil, yielding four fractions. The results showed that the HZSM-5 catalytic bio-oil had a low viscosity and a high calorific value close to that of light petroleum products. Chemical characterization indicated that hexadecanenitrile was the major component in the HZSM-5 catalytic oil at 500 °C, whereas oleic acid, ethyl oleate, and hexadecanoic acid were the dominant compounds in the bio-oil obtained without a catalyst. Furthermore, the pyrolysis temperature strongly affected the yield of hexadecanenitrile, and most of the hexadecanenitrile could be obtained in high purity by the polarity-based separation process combined with silica gel column chromatography. Moreover, HZSM-5 catalytic OMWS pyrolysis oil was successfully converted at 500 °C to its HDO-upgraded oil. The experimental results indicated a good performance for the nickel catalytic hydroprocessing of biocrude leading to the free-flowing oils. The yields of upgraded oil were higher at lower temperature, pressure, catalyst content, and residence time, but the oil obtained under these conditions exhibited higher O and N contents and a lower HHV. The results also indicated that temperature, pressure, catalyst content, and residence time are all important parameters in removing oxygen from oxygenated compounds in OMWS biocrude.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Utah Science Technology and Research (USTAR) Program is acknowledged for funding support. B.M. thanks the Chinese Visiting Scholars Program (201509160001).

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REFERENCES

(1) Food and agriculture data. Food and Agriculture Organization of the United Nations, 2009. http://faostat.fao.org/. (2) Pütün, A. E.; Uzun, B. B.; Apaydin, E.; Pütün, E. Fuel Process. Technol. 2005, 87, 25−32. 10533


Separation and Hydroprocessing of HZSM-5 Catalytic Olive Mill Waste

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