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On the production of hydrotreated jatropha oil using Co-Mo and Ni-Mo catalysts and its blending with petroleum diesel Shailesh J. Patil, and Prakash D. Vaidya Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03305 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017
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Energy & Fuels
On the production of hydrotreated jatropha oil using Co-Mo and Ni-Mo catalysts and its blending with petroleum diesel
Shailesh J. Patil, Prakash D. Vaidya*
Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai-400019, India
*Corresponding author (Tel.: +91-22-3361 2014; Fax: +91-22-3361 1020; Email:
[email protected])
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Abstract Jatropha oil is a prospective non-edible resource for green diesel manufacturing. In this work, diesel-range hydrocarbons (mostly C15-C18 n-paraffins) were produced from the hydrotreatment of jatropha oil over traditional CoMo/Al2O3 and NiMo/Al2O3 catalysts in a fixed-bed reactor. The reaction variables were varied so: temperature 563-653 K, pressure 1.5-3 MPa, H2/oil ratio 200-800 v/v and weight hourly space velocity 1-4 1/h. Oil conversion was maximized (Co-Mo 97%, Ni-Mo 88.6%) at T=653 K and P=3 MPa. Hydrocarbon yield at these conditions was 62.6% (Co-Mo) and 63% (Ni-Mo). These findings were juxtaposed with our latest results on the hydrotreatment of the non-edible karanja oil. From the first-order plots of conversion of triglycerides in jatropha oil, rate constants and energy of activation were found. To improve the cold flow properties of the hydrotreated jatropha oil without isomerization, it was blended with usual diesel in varying proportions. As the concentration of usual diesel in such mixtures increased, the viscosity, cetane number and pour point decreased. Employing tailored blends of hydrotreated vegetable oil and petroleum diesel thus appeared preferential. Finally, the performance of Co-Mo and Ni-Mo catalysts prepared by wet impregnation was tested, but the activity of the commercial catalysts was superior.
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Energy & Fuels 3
1.
Introduction So far, the production of green diesel from hydrotreatment of vegetable oils has been
extensively studied. In this second generation biofuel, also renowned as hydrotreated vegetable oil (HVO), straight-chain paraffins with 15 to 18 carbon atoms are prevalent. It can be made by reacting oil with hydrogen (H2) at high temperature and high pressure over traditional CoMo/Al2O3 and NiMo/Al2O3 catalysts. After the triglycerides are hydrolyzed to produce fatty acids and glycerol, hydrogenolysis follows. Fatty acids are converted into n-paraffins whereas glycerol is transformed into lighter products such as propane. As soon as n-paraffins in HVO are isomerized, they act like the liquid hydrocarbons of usual diesel and such post-treated HVO acquires better cold flow properties. HVO has several benefits, e.g., integration of sustainable liquids in refineries, high process adaptability, low production cost and small ecological trail. Besides, it goes well together with usual diesel and its properties are superior to those of biodiesel. In this work, non-edible jatropha oil (Jatropha curcas) was chosen as feed for producing HVO. Jatropha is crucial feedstock for producing biofuels in India. It easily grows with little or no care in many parts of tropical countries like India. It is robust, perpetual, drought-resistant and adaptable to infertile soil.1 It has low gestation period (2-3 years) and high potential yield (seeds 3-5, oil 1-1.5 tons/hectare).2 Typical hydrotreating catalysts (sulfided Co-Mo and Ni-Mo on high surface area Al2O3), whose high efficacy for hydroprocessing of bio feeds is renowned,3 were selected for this study. In a fixed-bed reactor, we compared the performance of commercial CoMo and Ni-Mo catalysts over wide ranges of temperature (563-653 K), pressure (1.5-3 MPa), H2/oil ratio (200-800 v/v) and weight hourly space velocity (WHSV, 1-4 1/h). Also, the hydrotreatment processing of two non-edible oils - jatropha and karanja - was contrasted.
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The performance of Co-Mo and Ni-Mo catalysts for hydrotreating vegetable oils of sunflower4, rapeseed5-7 and palm8 and model compounds such as fatty acids9 and ester10 is promising. Besides, Mo and Ni/Co are also effective for the jatropha application.11-18 In these works, the influence of catalyst modifiers (P, Si, Ce, La, W and Ti) and feed composition on the process and product was emphasized. For example, Gong et al.11 reported that sulfided NiMoP/Al2O3 is promising for the hydrotreatment process. Liu et al.12,13 found that hydrotreated jatropha oil obtained by using Ni-Mo/SiO2-Al2O3 catalyst was akin to normal diesel. Liu et al.14,15 reported superior performance of non-sulfided NiMo catalysts doped with Ce and La. Kumar et al.16 developed Ni-W, Ni-Mo and Co-Mo for producing green diesel from jatropha oilgas oil mixtures. Using Ni-Mo and Co-Mo catalysts, Sharma et al.17 compared the performance of traditional Al2O3 support with mesoporous titanosilicate. Recently, Satyarthi et al.18 investigated co-processing of jatropha oil with diesel fraction using Co-Mo and Ni-Mo catalysts. Although there are many reported works on jatropha oil, knowledge on hydrotreating kinetics is scarce. Here, we used first-order plots of triglyceride conversion to determine the reaction rate constant. Notwithstanding their high cetane number in the 70-100 range,11 high-melting n-paraffins in HVO are unsuitable for diesel engines, due to their higher viscosity than usual diesel and poor cold-flow properties.7 Thus, isomerization to the low-melting i-paraffins is crucial. To avoid the energy-demanding isomerization of the hydrotreated jatropha oil, we mixed it with usual diesel in varying proportions and the viscosity, cetane number and pour point of such mixtures were measured. Cold-flow properties and viscosity of such blends were superior to those of the HVO product. To the best of our knowledge, there exists no systematic study on the properties of HVO-diesel blends made from jatropha oil.
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Energy & Fuels 5
Past works on jatropha oil hydrotreating led to key insights into improved catalysts and reaction conditions. In this work, we provided new details on diesel blends, substitute feeds and reaction kinetics. These features are not quite reported hitherto and we wish to fill this gap. Additionally, we synthesized Co-Mo and Ni-Mo catalysts by successive wet impregnation method and assessed their performance for HVO production from jatropha oil.
2.
Experimental
2.1. Materials Jatropha oil was purchased from SVM Agro Processor, Nagpur, India. H2 (purity 99.9%) and nitrogen (purity 99.9%) cylinders were purchased from Rakhangi Gas Suppliers, Mumbai. To identify the reaction products in the gas phase, mixtures of H2, methane, ethane, propane and carbon oxides were purchased from Chemtron Science Laboratory Pvt. Ltd., Mumbai. CoMo/γAl2O3 (CoO=4%, MoO3=14%) and NiMo/γ-Al2O3 catalysts (NiO=3.6%, MoO3=14.8% MoO3) were purchased from Strem Chemicals, New Buryport MA (USA) and Sud-Chemie India Pvt. Ltd., New Delhi, correspondingly. For lab-made catalysts, precursor salts of Mo, Co and Ni were purchased from Merck Ltd. BET surface area (245 and 255 m2/g) of the unused CoMo and NiMo catalysts were determined by nitrogen adsorption-desorption measurements (accuracy ±1 m2/g) using Micromeritics ASAP2010 instrument. The respective pore volumes were 0.5 and 0.6 cm3/g. Reagents for degumming of jatropha oil and preparation and analysis of methyl esters were purchased from S. D. Fine Chemicals Pvt. Ltd., Mumbai. Standards for fatty acids, methyl esters and liquid hydrocarbons in diesel were purchased from Aldrich. Usual diesel was obtained from a local petrol filling station in Mumbai.
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2.2. Oil characterization The oil was pretreated by water degumming, acid degumming and TOP degumming processes according to procedures described in a previous work.19 Key oil properties such as saponification value, iodine value, acid value, density and free fatty acid content were measured (see Table 1). Using Thermo Finnegan Analyzer (Flash EA1112 series, Italy), the elemental composition
(N=1.72%,
C=77.03%,
H=9.8%
and
O=10.54%)
was
found.
After
transesterification of jatropha oil with methanol and sodium hydroxide,20 the fatty acid composition was found using gas chromatography (GC) analysis of the methyl esters (BPX-70 column, 50 m × 0.22 mm × 0.25 µm). The results are represented in Table 2. Jatropha oil comprises free fatty acids with even number of carbon atoms (16: palmitic and palmitoleic acids; 18: stearic, oleic and linoleic acids); the detailed composition was so: 14.58% C16 and 85.42% C18. The saturated fatty acid content (palmitic and stearic) was 18.72% whereas the unsaturated acid content (palmitoleic, oleic and linoleic) was 81.28%. The iodine value was high (100 g I2/100 g oil) due to high degree of unsaturation.
2.3. Experimental setup and procedure Trials were made in an apparatus provided by Chemito Technologies Pvt. Ltd., Mumbai. The entire set-up is shown in Fig. 1. It consisted of a SS316 fixed-bed reactor (length=49 cm, diameter=2.54 cm), two electric furnaces, two pre-heaters (PH1 and PH2), two mass flow controllers (MFC) for N2 and H2, high pressure pump, back-pressure regulator (BPR), condenser and gas-liquid separator. To control the process parameters, PID controller and Proficy HMI/SCADA-iFIX software was used. Catalysts were diluted with quartz powder. This mixture was sandwiched between ceramic wool and placed inside the reactor. The remaining portion of
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Energy & Fuels 7
the reactor was filled with glass beads. By this way, complete wetting of the catalyst was ensured; besides, channeling of the liquid feed was avoided. During each trial, the catalyst was pre-treated at 573 K for 2 h under flowing H2 (340 mL/min). Afterwards, it was activated by insitu sulfidation (T=633 K, P=3 MPa, t=3 h) by adding dimethyl disulfide (3 wt. %) to the feed. Thus, the oxidation state of the catalysts was sustained. Jatropha oil was pumped and pre-heated to 373 K using PH1. Then, it was mixed with H2 and heated to 498 K using PH2. This mixture was sent to the catalytic reactor. Subsequently, the hydrotreatment processing was performed for 3 h in the 563-653 K range. On the whole, heating to the desired temperature was achieved within 45 min. The reaction products were condensed and separated in the gas-liquid separator. The gaseous product was separately collected. After water separation, the liquid product was filtered and distilled.
2.4. Product analysis Both gaseous and liquid products were analyzed by gas chromatography technique (GC8610, Thermo Fisher). To analyze the product gas, thermal conductivity detector was used along with HAYESEP-DB packed GC column. Besides unreacted H2, CO, CO2, CH4 and C3H8 were present. For analysis of the liquid product, flame ionization detector (FID) and BPX-5 capillary column (30 m × 0.25 mm × 0.25 µm) were used. 1-Methylnaphthalene was used as internal standard. GC data for the liquid product and usual diesel are shown in Figs. 2a and 2b. The products n-C16H34 and n-C18H38 were formed via hydrodeoxygenation, whereas the other products n-C15H32 and n-C17H36 were formed via decarboxylation and decarbonylation. Cracking reactions favoured at high temperature resulted in the formation of C8 to C14 hydrocarbons. Product constituents were identified using standard samples of the individual paraffins. When
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gas chromatography-mass spectroscopy (GCMS Clarus 500, Perkin Elmer) was used together with a capillary column (Flite-1, 30 m × 0.25 mm × 0.25 µm), the spectrum showed molecular ion peaks at 121, 226, 240 and 254 m/z ratio thereby confirming the presence of C15-C18 hydrocarbons. No paraffins were formed in the absence of the catalyst.
2.5. Reaction metrics Conversion of jatropha oil was found using the relation % Conversion =
. . .
× 100
(1)
Saponification values of the feed and product were determined using AOCS Tl 1a-64 method. Product yield was calculated according the relation % Yield =
! " #$ %& ×
100
(2)
2.6. Blending of hydrotreated jatropha oil with usual diesel Since hydrotreated jatropha oil and usual diesel contain similar hydrocarbons (see Figs. 2a and 2b), it is possible to use this hydrotreated product as a diesel additive. HVO, usual diesel and their mixture have similar calorific heating value.21 In this work, distilled liquid product was blended with usual diesel in varying proportions (20-80, 40-60, 50-50, 60-40 and 80-20%). Cetane number and pour point of HVO product, diesel and the blends were measured. Using Nelson’s method,22 diesel index was determined from the aniline point and API gravity. Knowing the diesel index, the cetane number was found. Earlier, McCoy23 reported cetane number for mixtures of biodiesel with ethanol and usual diesel. Dynamic viscosity measurement of HVO, usual diesel and blends was performed using Anton-Paar Rheometer (MCR 301, Germany) in the 300-373 K range.
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Energy & Fuels 9
2.7. Catalyst characterization Both Co-Mo and Ni-Mo catalysts were characterized by X-ray diffraction (XRD) technique using Rigaku Miniflex D500 diffractometer and monochromic Cu-Kα radiation. Catalyst crystallinity and metal dispersion on Al2O3 support were examined. Samples were scanned from 10º to 90º with the step 2θ=0.01. In Fig. 3, XRD patterns for fresh and spent catalysts showed peaks at 2θ=37.3º, 46º and 66.5º which are characteristic of alumina.24 No significant peak was observed for CoO and NiO, thereby suggesting complete dispersion of CoO and NiO throughout the surface of alumina. These results were in good agreement with a previous work.7 XRD patterns for spent catalysts showed peaks at 33° which were not seen in those for the fresh catalysts. These peaks were due to MoS2, which is formed by sulfidation of molybdenum.25 Other peak patterns for the fresh and spent catalysts were identical. Thus, we concluded that no major changes in the catalyst surface occurred during reaction. Surface morphology of the catalysts was investigated by Scanning Electron Microscopy (SEM) technique using JEOL-JSM 6653 LA instrument. SEM images for fresh CoMo and NiMo catalysts exhibited irregular shapes (see Figs. 4a and 4b).
2.8. Catalyst synthesis We synthesized CoMo/Al2O3 and NiMo/Al2O3 catalysts using successive wet impregnation technique. Commercial grade γ-Al2O3 powder (Degussa) was screened (20-80 mesh), calcined at 773 K, and impregnated with aqueous solution of (NH4)6Mo7O24·4H2O. After overnight drying at 373 K, the resulting Mo/Al2O3 was calcined at 773 K for 5 h. Cobalt nitrate hexahydrate was then deposited on the calcined Mo/Al2O3 to yield Co-Mo catalyst (nickel nitrate hexahydrate was
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used for the Ni-Mo catalyst). Again, the catalysts were dried overnight at 373 K and calcined at 773 K for 5 h. These Co-Mo and Ni-Mo catalysts exhibited 214 and 231 m2/g BET surface area and 0.4 and 0.5 cm3/g pore volume. Their XRD patterns are shown in Figs. 5a and 5b. Apart from the usual peaks for alumina, a small peak appeared at 23°; this showed the presence of MoO3 over both catalysts. The small peak at 21° for the Co-Mo catalyst corresponds to crystalline CoMoO4, whereas the peak at 26° for the Ni-Mo catalyst is attributed to NiMoO4.
3.
Results and Discussion
3.1. Effect of temperature In the hydrotreatment process, catalyst performance chiefly depends on the reaction temperature. The influence of temperature on the hydrotreatment process was studied in the 563653 K range. Other process variables were fixed: P=3 MPa, WHSV=2 1/h and H2/oil=600 (v/v). The results are shown in Figs. 6a and 6b. As temperature increased, C-C and C-O bond cleavage was easier so that oil conversion was higher. As evident from Figs. 6a and 6b, maximum conversion was seen at 653 K (Co-Mo 97, Ni-Mo 88.6%). Similar effect of temperature on the yield of C15-C18 hydrocarbons was observed. Thus, % yield rose from 24.9 (T=563 K) to 62.6 (T=653 K) over Co-Mo and 21.6 (T=563 K) to 63 (T=653 K) over Ni-Mo catalyst. At high temperature, the acidic sites of the alumina support facilitate the formation of carbeniun intermediates of C15-C18 hydrocarbons. These intermediates undergo cracking reactions and transformation into smaller hydrocarbons (>C15).7 With the rise in temperature, the product thus becomes richer in smaller hydrocarbons (
