Hydroprocessing of Biomass-Derived Oils and ... - ACS Publications

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Hydroprocessing of Biomass-Derived Oils and Their Blends with Petroleum Feedstocks: A Review Mustafa Al-Sabawi* and Jinwen Chen CanmetENERGY, Natural Resources Canada, Devon, Alberta T9G 1A8, Canada ABSTRACT: Concerns over the declining availability of light conventional crude oils coupled with increasing energy demands and growing environmental concerns have sparked a global interest in the use of renewable oils as potential feedstocks for biofuel production. Over the past 2 decades, a considerable number of research studies in the area of renewable oil processing has been conducted around the world. The present review summarizes recent progress in processing biomass-derived oils, such as pyrolysis bio-oils, edible/inedible vegetable oils, and animal fats, and co-processing these oils with petroleum feedstocks using conventional hydroprocessing technologies, such as hydrotreating and hydrocracking. The main focus of this review is to provide an understanding of the effects of biomass feedstocks on process operation, catalyst performance and deactivation, feedstock conversion, and product yield and quality.

1. INTRODUCTION It is widely acknowledged that the carbon footprint and greenhouse gas (GHG) emissions related to heavy crude oil and bitumen production, upgrading, and refining to produce clean transportation fuels are greater than those related to conventional crudes.1 To reduce the impact of human activities on the environment, practitioners are currently exploring the production of fuels from renewable energy resources, mainly biomass-derived oils, edible and inedible vegetable oils, and animal fats. Such fuels could reduce the carbon footprint, provided that they are produced in a sustainable way (on the basis of life cycle assessment). GHG emissions from the production and processing of renewable oils to produce biofuels are significantly less than those from the production and processing of fossil resources.2 In many developed countries, it is required that biofuels replace from 6 to 10% of petroleum fuels in the near future.3,4 Therefore, co-processing of petroleum feedstocks with biomassderived feedstocks while meeting government regulations has a great potential to reduce the carbon footprint (or GHG Emissions) in the whole processing chain of producing clean transportation fuels from petroleum. Co-processing of petroleum with feedstocks from renewable resources using existing refining catalysts, processes, and technologies also offers other advantages from both technological and economical points of view. Using the existing refining infrastructure and configuration, little additional capital investment is required. However, technical challenges of maintaining or even enhancing process efficiency, retaining catalyst activity and stability, and improving product quality still remain. Over the past 2 decades, there have been a number of studies on catalytic processing of renewable oils, either processed in pure form or co-processed with petroleum-based feedstocks. Reviews by Furimsky5 and Elliott6 have addressed the hydrotreatment of oxygenated model compounds and, to a limited capacity, the hydroprocessing of bio-oils, respectively. Other types of reviews addressing the production of biofuels from renewable resources are available in the literature.7,8 For instance, the review by Balat7 focused on the production of Published 2012 by the American Chemical Society

bioethanol using biochemical techniques, while the review by Nigam and Singh8 summarized information on the generation of biofuels from various sources, including algae, grains, sugars, wood chips, and lignocellulosic biomass via transesterification, gasification, and acid/enzyme hydrolysis methodologies. The objective of this work is to review and evaluate the effects of renewable oil processing on existing hydroprocessing technology, operation, catalyst, and product quality and quantity. This survey focuses on catalytic hydroprocessing of pure renewable oils, including biomass-pyrolysis-derived bio-oil, edible/inedible vegetable oils, and animal fats, as well as coprocessing these oils with petroleum-derived feedstocks. Hydroprocessing technology is known to provide better quality products than other conversion processes, such as transesterification and solvent extraction.9 This review is organized according to the type of biofeedstocks that have been studied in the literature. Under each biofeedstock, the work related to its processing (in its pure form) and its co-processing with conventional petroleum feedstocks is discussed.

2. BIOMASS-DERIVED FEEDSTOCKS 2.1. Pyrolysis Oils (Bio-oils). Biomass pyrolysis oil, also known as bio-oil, is typically a dark brown liquid but can be black or green in color depending upon the chemical composition and the amount of microcarbon content.10 Biooil has the potential to be co-processed with petroleum-derived feedstocks to produce biofuels. However, their chemical and physical characteristics present challenges that need to be overcome. Bio-oils are highly viscous, corrosive, and relatively unstable. They also exhibit poor heating values.11−15 In addition to possessing poor fuel properties,16,17 bio-oils also contain significant amounts of oxygenated compounds. These oxygenated compounds exist as acids, aldehydes, ketones, alcohols, esters, ethers, glycols, and phenols.18 A pyrolysis bioReceived: April 16, 2012 Revised: June 20, 2012 Published: June 21, 2012 5373

dx.doi.org/10.1021/ef3006405 | Energy Fuels 2012, 26, 5373−5399

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Table 1. Studies on the Hydroprocessing of Pyrolysis Bio-oils feedstock HDO pyrolysis oil flash pyrolysis oil fast pyrolysis oil fast pyrolysis oil fast pyrolysis oil fast pyrolysis oil pyrolysis oil pyrolysis oil pyrolysis oil pyrolysis oil pyrolysis oil pyrolysis oil SRGO + pyrolysis oil (30 wt %)

conditions T (°C), P (MPa), and WHSV (h−1)

catalyst T T T T T T T

= = = = = = =

reference

400, P = 13.8, LHSV = 0.4 250−450, P = 13.8 310−375, P = 17.2, LHSV = 0.18−1.12 350, P = 20, batch 200, P = 4, batch 360−390, P = 1.5−3, batch 350−400, P = 5.3−10.4, WHSV = 0.5−3.0

commercial hydrocracking catalyst Co/Al2O3, Ni/Al2O3 Pd/C Ru/C Pt/ZSM-5, Pt/Al2O3 CoMo/Al2O3 Pt/Al2O3/SiO3, sulfided CoMo/γ-Al2O3, Ni−W/γ-Al2O3, Ni−Mo/γAl2O3 sulfided NiMo Ru/C NiMo/γ-Al2O3

T = 200, P = 10 T = 220−310, P = 19, batch T = 100, P = 3, batch

Ru/C NiMo/Al2O3 sulfided CoMo catalyst

T = 230−340, P = 29, batch T = 360, P = 17.2, batch T = 380, LHSV = 2

151 152 151 34 153 154 12 153 77 31 and 32 61 155 77

2.2. Vegetable Oils. The worldwide consumption of petroleum and production of vegetable oil are approximately 4.02 billion and 0.12 billion tons per year, respectively.9 This gap between consumption and production means that, while vegetable oils may be a potential renewable source for fuel production, they will have to be used in small concentrations within co-processed feedstocks or combined with other biomass-derived sources, such as pyrolysis oils, waste oils, and animal fats. Nevertheless, it is still important to understand their properties and behavior during standard refinery hydroprocessing. Many types of vegetable oils have been investigated as renewable biomass-derived feedstocks in the production of biofuels, depending upon the geographical location of the source of the oil. Table 2 presents the various types of vegetable

oil typically contains more than 400 different organic compounds derived primarily from the depolymerization and fragmentation of the three main lignocellulose components, cellulose, hemicellulose, and lignin.19 Hence, bio-oils cannot be used directly in stationary combustion engines and require further upgrading.20 Upgrading of bio-oil produced from biomass for various applications can be accomplished by catalytic hydroprocessing.21−24 Catalytic hydroprocessing has several advantages over other processes, such as fermentation, pyrolysis, and transesterification. For fermentation, feedstocks must be pretreated by saccharification and hydrolysis. Fermentation also requires excessive time. Besides being costly and having high energy consumption, transesterification can only be used to produce biodiesel and not biogasoline. Using pyrolysis, the product quality is dependent upon the feedstock type used. For example, the use of a highly cellulosic feed produces a liquid fraction that contains acids, aldehydes, ketones, alcohols, and phenolic compounds.25,26 Therefore, catalytic hydroprocessing is one of the most viable options for downstream upgrading of bio-oil.19 This review summarizes the work and results of published studies on hydroprocessing of pyrolysis bio-oil. The hydroprocessing of pyrolysis bio-oils is a very important step because upgrading pyrolysis oils often leads to the formation of significant amounts of tars, chars, and coke and to irreversible catalyst deactivation due to the high oxygen content in the feed. 27−29 The elimination of oxygen compounds by hydrotreating results in hydrodeoxygenation (HDO) material that is cleaner and easier to co-process with conventional petroleum-derived vacuum gas oil (VGO) in downstream processes.20 The current state of the art in HDO is based on hydrotreatment of bio-oils over heterogeneous CoMo- and NiMo-based catalysts with high-pressure hydrogen at high temperatures.30 However, most of the published hydrotreating research work on biomass pyrolytic oils involves the use of batch systems rather than continuous systems.30−36 Studies involving the hydroprocessing of bio-oils are summarized in Table 1. It can be observed that there have been several studies on the hydroprocessing of pure pyrolysis bio-oils but very limited work has been conducted on the coprocessing of these bio-oils with conventional petroleum feedstocks.

Table 2. Types of Vegetable Oils Used in Hydroprocessing Studies37,45,50,51 vegetable oil

main source

edibility

canola coconut cottonseed palm rapeseed soybean sunflower

Canada Philippines Greece/Turkey southeast Asia China/Europe U.S.A. Europe

yes yes no yes yes yes yes

oils that have been studied in the context of hydroprocessing and the locations where they are mainly produced. Moreover, these potential renewable feedstocks can be classified on the basis of whether they are suitable for human consumption (edible) or not (inedible). There are also other less common vegetable oils that could be potential feedstocks for biodiesel production, including jatropha, karanja, linseed, rubber seed, mahua, neem, and castor oils. A complete list of edible and inedible vegetable oils as well as different types of animal fats that can be used as biofuel feedstocks can be found in ref 37. It is important to note that high costs are associated with biodiesel production from edible oils because of their high market price. Choosing inedible oils reduces this cost, since 60−80% of the total cost of biodiesel production is attributed to the feedstock itself.39,40 Obviously, the cost of a refined food5374


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has higher cloud points, pour points, and percentage of carbon residue than conventional diesel.45,46 Moreover, biodiesel has a lower heating value (and, as a result, higher fuel consumption) and inferior low-temperature operability.47,48 In addition to being renewable, diesels derived from vegetable oil also have several advantages over petroleum diesel. Biodiesel has lower sulfur and aromatic contents than conventional diesel. Moreover, biodiesel offers lower engine noise, emits less smoke, and provides higher thermal efficiency.46 It is also found that exhaust emissions, including carbon monoxide and particulate matters, can be reduced remarkably through the use of biodiesel in engines. However, NOx emissions are dramatically increased with biofuels.41,49 Several studies published in the literature have been dedicated to investigate the hydroprocessing of different types of pure vegetable oils, including rapeseed (RSO), sunflower (SFO), palm (PO), soybean (SBO), cottonseed (CSO), and waste cooking oil (WCO). Other work has focused on the hydroprocessing of blends of petroleum feedstocks with these vegetable oils. These studies are summarized in Tables 4 and 5, in which research work performed using animal fat is also included. RSO and SFO are two of the most extensively investigated vegetable oils as potential sources of biofuel. Canola oil, which is a form of RSO having a low amount of erucic acid in its oil and low levels of glucosinolates in its meal, has also been studied. Other types of vegetable oils that have been investigated to some degree include PO, SBO, and CSO. Although the use of WCO has a number of advantages from an environmental perspective, catalytic cracking of WCO has also been explored minimally in published studies. 2.3. Oxygenated Model Compounds. Extensive work has been dedicated to the study of hydroprocessing of model compounds. Table 6 presents a summary of these studies. The focus of model compound work is usually to understand the fundamentals of the reaction chemistry of an individual or a group of compounds to identify reaction mechanisms and to develop kinetic models to predict the behavior of actual complex biomass-derived feedstocks. However, such work provides only partial information on the reaction pathways and ignores the interaction between different types of oxygenated compounds and hydrocarbons during processing.52

quality vegetable oil is higher than that of an unrefined raw vegetable oil because of the number of steps involved in refining the oil, as shown in Figure 1. Even when using inedible

Figure 1. General process of rapeseed refining to produce food-quality rapeseed oil. NRO, neat rapeseed oil (fresh oil after pressing/ extraction from seeds); PRRO, primary refined rapeseed oil (oil after degumming); and RRO, refined rapeseed oil (food-quality oil).38

oils, however, higher costs are associated with producing biodiesel than with producing petroleum diesel.41 The use of inedible over edible vegetable oils is always preferred because of the food-versus-fuel dilemma. From a technical perspective, co-processing of vegetable oils in existing refinery reactors and equipment raises several challenges that must be addressed for optimal operation. Biodiesels derived from vegetable oils, with the exception of coconut oil, have much higher viscosity than conventional diesel fuel at both ambient and elevated temperatures,42 as presented in Table 3, which would necessitate modifications to

3. HYDROPROCESSING Hydroprocessing of oils, whether petroleum- or biomassderived, entails hydrotreating and/or hydrocracking technologies. The primary objective of conventional hydrotreating is to remove impurities present in petroleum feedstocks, such as sulfur and nitrogen, via the addition of hydrogen. In the case of biomass-derived feedstocks, hydrotreating would also be used to remove the high content of oxygen impurities found in such feedstocks. Hydrocracking, on the other hand, is a process that combines catalytic cracking and hydrogenation, wherein hydrocarbon feedstocks are cracked in the presence of hydrogen to produce lighter fuel products. Hydrocracking typically employs different types of catalysts than hydrotreating, as well as more severe operating conditions (higher temperatures and pressures). This section of the review highlights the results of research work conducted over the past 2 decades on the hydroprocessing (hydrotreating and hydrocracking) of renewable oils and their blends with petroleum feedstocks. The biomassderived oils investigated include pyrolysis bio-oils, vegetable

Table 3. Properties of Diesel and Biodiesels from Various Vegetable Oils42 property density at 40 °C (g/mL) kinematic viscosity at 40 °C (cSt) cloud point (K)

no. 2 diesel

canola

used canola

soybean

coconut

0.8278

0.8649

0.8663

0.8674

0.8554

2.52

4.47

4.47

4.02

2.61

273.15

272.15

275.15

270.15

existing engines.9 This high viscosity, coupled with low cetane numbers, could potentially cause several problems in diesel engines, such as engine choking, seizing of fuel injectors, gum formation, and piston sticking during long-term use.43 Another limitation of these renewable fuels is their high acidity and resulting propensity to cause corrosion.44 Biodiesel derived from vegetable oils is also much more reactive to oxygen and 5375


catalyst

Vegetable Oils Mo2N/γ-Al2O3 NiMo/γ-Al2O3 sulfided CoMo/meso-silicates NiMo/γ-Al2O3 NiMo/Al2O3 NiMo/Al2O3 NiMo/Al2O3 NiMo/Al2O3, Mo/Al2O3, Ni/Al2O3 sulfided NiMo/γ-Al2O3 sulfided CoMo/γ-Al2O3 NiMo/γ-Al2O3 NiMo/γ-Al2O3 NiMo/γ-Al2O3, Pd/γ-Al2O3 CoMoS/γ-Al2O3, Ni/SiO2−Al2O3Pt/γ-Al2O3, Ru/Al2O3 NiMo/Al2O3/F Pt/HZSM-22/Al2O3Pt/SAPO-11 CoMo/Al2O3 Pd/SAPO-31 commercial hydrocracking catalyst commercial hydrotreating catalyst NiMo/B2O3−Al2O3 commercial hydrocracking catalyst

feedstock

canola oil canola oil olive-oil-derived byproduct PO RSO RSO RSO RSO RSO RSO (multiple grades) SBO SBO SBO SFO SFO SFO SFO SFO WCO WCO WCO

Table 4. Studies on the Hydroprocessing of Pure Vegetable Oils

T T T T T T T T T T T T T T T T T T T T T = = = = = = = = = = = = = = = = = = = = =

400, P = 8.3, WHSV = 0.9, H2/oil = 810 L/L 300−400, P = 1.8−8.5, batch 250, P = 3, WHSV = 2.7 350, P = 4−9, WHSV = 2, H2/oil = 20 mol/mol 260−280, P = 3.5, WHSV = 1−4, H2/oil = 50 mol/mol 260−340, P = 7, WHSV = 1.0 310−360, P = 7−15, WHSV = 1.0, H2/oil = 920 N m3/m3 260−280, P = 3.5, WHSV = 0.25−4.0 260, P = 3.5, WHSV = 1−4, H2/feed = 50 mol/mol 310, P = 3.5, WHSV = 2, H2/feed = 100 mol/mol 350−400, P = 1−20, batch 360, P = 14, batch 400, P = 9.2, batch 280−380, P = 2−8, LHSV = 0.75−3, H2/oil = 400−600 N m3/m3 280−380, P = 3−8, WHSV = 1−4, H2/oil = 250−400 N m3/m3 380, P = 4−6, WHSV = 1.0, H2/oil = 500−600 N m3/m3 310−360, P = 2, WHSV = 0.9−1.6, H2/oil = 1000 N m3/m3 360−420, P = 18 330−398, P = 8.3, WHSV = 1.0, H2/oil = 4000 scfb 300−350, P = 7, batch 350−390, P = 13.8, WHSV = 1.5, H2/oil = 6000 scfb

conditions T (°C), P (MPa), and WHSV (h−1)

103 91 156 102 157 89 92 83 157 38 85 88 158 4 101 and 112 98 99 159 128 and 129 130 131

reference

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Table 5. Studies on the Co-hydroprocessing of Vegetable Oils and Animal Fat with Petroleum Feedstocks feedstock biomass component CSO PO PO

petroleum component HDS diesel gas oil straight run diesel vacuum distillate atmospheric gas oil (AGO) light gas oil (LGO) LGO straight run diesel straight run diesel HGO HT-VGO, SRGO HVO SRGO VGO

RSO RSO RSO RSO RSO SBO SFO SFO SFO SFO SFO

percent biomass in the feed mixture

conditions T (°C), P (MPa), HSV (h−1)

catalyst

reference

10 wt % 2.5−10 wt % 10 wt %

Vegetable Oils CoMo/Al2O3 T = 305−345, P = 3, WHSV = 5−25 CoMo/γ-Al2O3 T = 310−350, P = 3.3, WHSV = 0.7−1.4, commercial hydrotreating T = 300−330

5 wt %

commercial NiMo

T = 400−420, P = 18, LHSV = 1.0, H2/oil = 1000 scfb

113

6.5 vol %

T = 360−380, P = 3.5−5.5, LHSV = 1.0, H2/feed = 1000 N m3/m3

104

15, 25 vol %

NiW/NaY NiW/TiO2NiW/ ZrO2NiMo/TiO2NiMo/ Al2O3 sulfided NiMo/γ-Al2O3

T = 350, P = 4.5, LHSV = 1.5, H2/feed = 250−500 N L/L

94

10−20 wt % 10 wt %

NiMo/Al2O3 commercial hydrotreating

T = 320−380, P = 3−5, LHSV = 2.0, H2/feed = 500 N m3/m3 T = 300−330

108 84

10−80 wt %

commercial hydrotreating

T = 300−330

84

5−100 wt % 10, 30 vol %

NiMo/Al2O3 commercial catalysts

T = 300−380, P = 6−8, LHSV = 1.0, H2/feed = 600 N m3/m3 T = 350, P = 13.8, LHSV = 1.5

100 30

5−50 wt % 20 wt % 10, 30 vol %

sulfided NiMo/Al2O3 NiMo/Al2O3 BEA/Al2O3 commercial hydrocracking catalysts sulfided NiW/SiO2−Al2O3 and NiMo/Al2O3

T = 300−450, P = 5, LHSV = 4.97, H2/feed = 1600 mL/mL T = 320−350, P = 3−6, WHSV = 1−4, H2/feed = 500 v/v T = 350, P = 6.8−13.8, LHSV = 1.5, H2/oil = 6000 scfb

97 160 119

T = 340−380, P = 5, LHSV = 2−4, H2/feed = 1500 mL/mL

137

T = 380, P = 5.5, LHSV = 1.0, H2/feed = 1000 N m3/m3

104

waste soy oil (WSO)

gas oil

25−100 wt %

tallow

AGO

6.5 vol %

120 126 84

Animal Fat NiMo/Al2O3

Table 6. Studies on the Hydroprocessing of Oxygenated Model Compounds Typically Found in Biomass-Derived Feedstocks feedstock (model compounds) acetic acid esters, acid fatty acids guaiacol guaiacol, cresol, furan ketones, guaiacol methyl esters methyl hexadecanoate methyl laurate mixture of aldehydes, ketones, acids, alcohols oleic acid oleic acid, tripalmitin phenol SRGO + guaiacol (0.5 wt %)

conditions T (°C), P (MPa), WHSV (h−1)

catalyst NiMo/γ-Al2O3 Pd/C Pd/C Rh/ZrO2, Pd/ZrO2, Pt/ZrO2 Pt/ZSM-5, Pt/Al2O3 CoMo/Al2O3 sulfided NiMo/γ-Al2O3, sulfided NiMo/γAl2O3 Ni/HZSM-5 NiMo/γ-Al2O3 Ru-based Shvo catalyst

T T T T T T T

Mo2N/γ-Al2O3, VN/γ-Al2O3, WN/γ-Al2O3 Pt/γ-Al2O3 NiMo/Al2O3, CoMo/Al2O3 sulfided CoMo/Al2O3

reference

200, P = 3, batch 300−360, P = 1.7−4, batch 300−360, P = 1.5−2.7, batch 100, 300, P = 8, batch 200, P = 4, LHSV = 2−6 280, 300, P = 7, batch 250−300, P = 1.5

31 86 87 62 153 71 117

T = 160−240, P = 2−4, batch T = 300−400, P = 1.8−8.5, batch P = 0.5−1.0, batch

161 91 65

T = 380−410, P = 7.15, WHSV = 0.45, H2/feed = 810 L/L T = 320, P = 2, batch T = 250, P = 1.5 T = 280−360, P = 4, LHSV = 1−2

103

= = = = = = =

162 67 78

involved hydrogenation at a higher temperature (400 °C and 13.8 MPa). Samolada et al.55 also reported similar yields of hydrotreated bio-oils (∼40 wt %) during bench-scale experiments using commercial NiMo and CoMo catalysts, while Grange et al.56 specified that almost 50 wt % yields of hydrocarbons should be expected during high-temperature deep hydrorefining of pyrolysis oils. It should be noted that, while the refined oil yield increases at higher hydroprocessing temperatures, the rate at which the yield increases tends to diminish at temperatures above 350 °C.57,58 In addition, at low temperatures, the influences of other operating parameters,

oils, WCOs, animal fats, and mixtures of oxygenated model compounds. 3.1. Pure Pyrolysis Bio-oils. 3.1.1. Yield of Refined Biooils. It has been shown that catalytic hydrotreating of raw pyrolysis bio-oil can result in 30−50% yields of refined bio-oil, depending upon operating conditions. Such yields were attained by Elliott and co-workers,53,54 who developed a twostage hydrotreating process for upgrading thermally unstable bio-oils derived from pyrolysis using CoMo/Al2O3 catalysts. The first stage in their process involved low-temperature operation (270 °C and 13.8 MPa), while the second stage 5377


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such as space velocity and hydrogen flow, on hydroprocessing results are minor.6 Similar to the effect of the temperature, it has been reported that increasing the hydrogen pressure enhances the yield of refined bio-oil, especially at lower feed flow rates. Rocha et al.59 observed a slight reduction in refined oil yield when they increased the hydrogen pressure from 2.5 to 5 MPa during twostage cellulose hydropyrolysis−HDO experiments, but they also observed an increase of 15 wt % in conversion when the pressure was increased from 5 to 10 MPa. Almost an equal split between the liquid oil product and gas product was achieved.59 At higher hydrogen gas velocities, however, the trend was slightly different, with the maximum refined oil product yield being achieved at 5 MPa. Hence, the effect of the hydrogen pressure on the oil yield was most pronounced at lower gas velocities. 3.1.2. Deoxygenation. The HDO of bio-oils involves reactions that eliminate the oxygen from the oils, mainly in the form of water.5 Two types of HDO of pyrolysis bio-oils have been investigated in the literature: thermal HDO and catalytic HDO. The former is able to achieve 78−85% oxygen removal, whereas the latter is capable of achieving 88−99.9% based on the work by Samolada et al.55 The properties of flash pyrolysis bio-oil and HDO bio-oil are presented in Table 7.

Figure 2. Reactivity scale of oxygenated groups under hydrotreatment conditions.6

temperatures.61 Similar results were achieved during model compound studies involving guaiacol and phenol.62,63 The temperature−HDO relation was also observed by Samolada et al.,55 who conducted high-severity deoxygenation of 99.9% at 343 °C versus only 88% at 327 °C. Elliott et al.53 were able to produce a refined oil product containing only 1 wt % oxygen after the 400 °C second-stage hydrotreatment process described earlier. While close to 90% HDO conversion can take place at low hydrotreating severities/temperatures, it is only at high deoxygenation levels (>95%) that the liquid product meets the specifications for standard crude oils in terms of the carbon−hydrogen ratio, density, and oxygen content.55 Sheu et al.12 reported a moderate increase in oxygen removal between 350 and 375 °C, with this rate increasing drastically between 375 and 400 °C during the hydrotreatment of pyrolytic oil produced from pine sawdust and bark in a trickle bed reactor system. Oxygen removal from bio-oil feeds can also be affected by the hydrogen pressure and the type of catalyst used. Rocha et al.59 were able to remove an additional 10−20% more oxygen when they increased the hydrogen pressure from 2.5 to 10 MPa. With regard to the type of catalysts, Soltes et al.64 showed that Pt catalysts are very active in removing oxygen (27−45%) from pyrolysis bio-oil compared to conventional NiMo and CoMo catalysts (15−39%). It has also been shown that the sulfided form of the CoMo-based catalyst was much more active than the oxide form, while the opposite is true for Nibased catalysts, with the oxide form being more active than the sulfided forms.6 Although Ni-based catalysts exhibit activities similar to those of sulfided CoMo-based catalysts, the former provide much higher gas yields and consume more hydrogen in the process.6 The chemistry and reaction mechanisms of hydroprocessing of pyrolysis oils containing various oxygenated compounds have been discussed by a number of researchers.19,65,66 Table 8, taken from Xu et al.,31 presents a simplified list of bio-oiloxygenated compounds and their corresponding products after hydrotreatment. Included in the reactions that take place during the hydrotreatment of pyrolysis bio-oils is esterification. During the hydrotreatment of bio-oils derived from fast pyrolysis of pine sawdust over the MoNi/γ-Al2O3 catalyst under mild conditions, Xu et al.31 observed a higher water content in the

Table 7. Comparison of Properties of Flash Pyrolysis Bio-oil and Hydrodeoxygenated Bio-oil60 property density at 15 °C (g/mL) viscosity (cP) moisture (wt %) higher heating value (MJ/ kg) elemental analysis C (wt %) H (wt %) O (wt %) S (wt %) H/C ratio (dry)

flash pyrolysis biooil

hydrodeoxygenated biooil

24.8 59 (at 40 °C) 24.8 22.6

0.80−0.93 1.0−4.6 (at 23 °C) 20 28 103 9 11 95 0.54

>128 28−30 >20 29 105 9 11 95 0.65

125 25−27 >20 25 103 10 11 66 0.06

120 23−25 >20 23 103 10 11 42 0.05

95

339

330

344

323

turn, less susceptible to degradation than food-quality RSO that had been extensively treated and had some of its natural antioxidants destroyed during processing.38 It has been reported in the literature that there are substantial differences in the fuel properties of vegetable oils and their derived biodiesels.37 This is expected because triglycerides are esters of glycerine with different fatty acids and the proportions of the various acids vary from oil to oil. Even when biofuels are obtained from different vegetable oils at similar reaction conditions, there are evident variations in their properties. While biofuel derived from PO hydrotreating, for instance, had a similar density to that of biofuel produced from soybean and canola oils, it clearly had the highest cloud point (∼25 °C) and the lowest pour point (∼18 °C). Biodiesel derived from SBO, on the other hand, had the lowest cloud point (21.1 °C).82 Palm-oil-derived biofuel also had the lowest viscosity among the biofuels82 but was still slightly higher than that of conventional diesel fuel.109 It was observed that the bromine index of the palm OLP was higher than that of the product derived from RSO but significantly lower than the product derived from SFO, even when all three vegetable oils were processed at similar conditions. Mikulec et al.104 conducted hydroprocessing tests of PO, RSO, and SFO at 340−350 °C, 4.5 MPa, space velocity of 0.8 h−1, and H2/feed ratio of 500 N m3/m3 and reported bromine numbers of 54, 71, and 43 mgBr2/ 100 g for the corresponding liquid products. 3.3.4. Deoxygenation. Studies have shown that both reaction pathways, HDO and HDC, are responsible for elimination of oxygen (deoxygenation) and the formation of hydrocarbon compounds, as discussed earlier, with the rate of each reaction dependent upon reaction conditions and the type of catalyst used.38,94 With regard to the effect of reaction conditions on oxygen removal, it was observed that operating at 5384


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Review

decarbonylation reactions are independent of the type of vegetable feedstock used. As for other operating parameters, it was reported that lower H2/oil volume ratios (200 N m3/m3) also promoted HDC reactions over HDO reactions.98 Increasing the hourly space velocity from 0.9 to 1.6 h−1 led to reductions in C17/C18 and C15/C16 ratios in the liquid product. Thus, it can be deduced that HDC is predominant at lower space velocities.99 The relative rates of HDC and decarbonylation reactions were also compared in a study by Huber et al.,97 who quantified the amounts of CO and CO2 generated during SFO hydroprocessing. The yields of these gases were almost the same and increased by approximately the same magnitude with an increasing temperature. This suggests that the rates of HDC and decarbonylation reactions are similar and that these reactions may occur via similar mechanisms, as illustrated in Figure 5. It is important to note that HDC and decarbonylation reactions were dominant compared to direct hydrogenation of fatty acids, especially as the temperature increased.99 It should be pointed out that hydroprocessing at higher temperatures could lead to permanent deoxygenation of vegetable oils. Kubička and Kaluža83 did not detect any traces of triglycerides during RSO hydrotreating at temperatures above 270 °C because of their complete conversion. Therefore, it can be concluded that deoxygenation reactions of triglycerides are irreversible at such temperatures. Hydrotreating at even higher temperatures can lead to substantial deoxygenation. Monnier and co-workers103 reported 90% and nearly 100% deoxygenation of canola oil using a Mo2N/Al2O3 catalyst and a commercial sulfided NiMo catalyst in tests conducted at 400 °C and 8.35 MPa H2 pressure, respectively. Similar results were achieved by Kikhtyanin and co-workers,99 who, using a Pd/SAPO-31 catalyst, were able to achieve 86 wt % saturated hydrocarbons in the OLP (with the remainder being oxygenated compounds) during SFO hydroprocessing at 310 °C and 0.9 h−1. In fact, the physical appearance of the organic phase obtained at this temperature was different; it was described to be cloudy or have the heterogeneous nature of a suspension of fine white crystals with a melting temperature above 50 °C. On the other hand, when the temperature was increased to 340 °C, no traces of oxygenates were detected in the OLP, which was clear and homogeneous. It is important to note, however, that reducing the residence time of SFO in the reactor (by increasing the space velocity to 1.6 h−1) at 340 °C yielded an OLP containing about 6 wt % oxygenated compounds. Hence, reducing the process severity (temperature and residence time) leads to incomplete oxygen removal during vegetable oil hydroprocessing. As for the effect of the pressure on oxygenated compounds, Guzman et al.102 observed various oxygenated species in the product stream from PO hydroprocessing at lower pressures (2.5 MPa). These species included octadecenes, octadecanol, nhexadecanoic acid, octadecanal, hexadecyl hexadecanoate, and octadecyl hexadecanoate. The presence of these intermediate compounds indicates that complete conversion/deoxygenation of PO was not attained at this low pressure. When the pressure was elevated to 9 MPa, however, no traces of these compounds were apparent. The type of catalyst plays an important role in vegetable oil deoxygenation. Higher oxygen removal rates were obtained using bimetallic catalysts versus monometallic catalysts. This was confirmed by the RSO hydrotreating work by Kubička and Kaluža,83 who achieved deoxygenation rates using NiMo/Al2O3

higher hydrogen pressures and lower reaction temperatures favored the HDO pathway over the HDC pathway,104 as presented in Figure 4. This might lead to higher H 2 consumption because HDC reactions do not consume H2. While operating under conditions that favor HDC reactions reduces H2 consumption, CO2 produced via HDC may participate in subsequent reactions that consume H2, such as methanization. Such reactions would not only require more H2 but also lead to a reduced diesel yield, making HDC less attractive than HDO.94 Note that because HDO is promoted at lower temperatures and higher pressures, the formation of noctadecane is more favorable than that of n-heptadecane under these conditions.92 A number of studies have established that increasing the reaction temperature or decreasing the pressure during vegetable oil hydrotreating promotes HDC reactions over HDO reactions. With regard to the effect of the temperature, Kikhtyanin et al.99 observed that raising the operating temperature from 320 to 360 °C during SFO hydroprocessing caused an increase in the C17/C18 ratio in the liquid product from a range of 1−4 to a range of 8−10 (Figure 6). This

Figure 6. Dependence of the C17/C18 ratio of products obtained from hydroprocessing of SFO upon the temperature.99 Weight hourly space velocity (WHSV) = 0.9 h−1 (blue ●), 1.2 h−1 (pink ■), and 1.6 h−1 (orange ▲).

indicates that HDC reaction rates were enhanced significantly over HDO reactions. Others have suggested that, while HDC reactions were favored at higher temperatures, the amounts of C17 and C18 compounds present in the product pool were roughly the same at 450 °C.97 Kikhtyanin et al.99 did report that the amount of water formed during SFO hydroprocessing was fairly consistent, ranging between 3 and 4 wt % over the range of temperatures used (310−360 °C). This indicates that, while water-generating HDO reactions are not as favored as HDC reactions at higher temperatures, as previously discussed, HDO reactions do not slow at higher temperatures; however, HDC reactions are greatly expedited. With regard to pressure, research work by Guzman et al.102 on hydroprocessing of PO at 350 °C and 1.5−3 MPa using a NiMo/γ-alumina catalyst showed that the ratios of C17/C18 and C15/C16 in the generated liquid product decreased with the reaction pressure. This indicates that decarboxylation and decarbonylation reactions are not favored at higher pressures. Similar results have already been reported with other types of vegetable oils,98,104 which confirmed that HDO, HDC, and 5385


Energy & Fuels

Review

catalysts. The low gas yield in this case might be attributed to the lower severity of hydroprocessing (i.e., low operating temperatures). Moreover, canola oil hydrotreating tests performed at 400 °C and 8.35 MPa using Mo2N/Al2O3 provided middle distillate yields of only 38−48 wt %, whereas 80 wt % of middle distillates was achievable using a commercial sulfided Mo catalyst.103 As mentioned earlier, hydrogenation, HDO, and HDC reactions proceed at different rates depending upon reaction conditions and the type of catalyst used. Changes in the respective reaction rates can be measured by changes in the product distribution. The proportions of products, such as noctadecane/n-heptadecane, or the amounts of generated CO2 and H2O can be used to assess the rates of HDO and HDC, respectively. The effects of reaction conditions and catalyst type on these reactions have already been discussed in this review. The discussion in this section on product distribution will focus mainly on the compositions of each of the major products obtained during vegetable oil hydroprocessing. The gaseous product of RSO hydroprocessing consists mainly of light hydrocarbons (methane and propane) and carbon oxides (CO2 and CO),92 which is in agreement with previous deoxygenation studies.81,89,94,95,111 Other researchers also reported ammonia and hydrogen sulfide gases,99 as well as C4−C5 hydrocarbon products.82 The formation of CO and methane occurs via the reduction of the primarily formed CO2, which leads to higher consumption of H2, whereas the formation of propane originates from the glycerol part of triglycerides. It was reported that the concentration of methane formed via methanization increased with the operating temperature and pressure, decreased with the space velocity, but did not change significantly with the alteration of the H2/oil volume ratio.98 H2 consumption followed similar trends. Despite these variations, however, the concentration of methane in the gaseous product did not usually exceed 6% at any of the reaction conditions, as observed by Krár et al.98 The amount of propane in the gaseous product was also dependent upon the temperature. Kikhtyanin et al.99 generated a range of light hydrocarbons when hydroprocessing SFO in the gaseous product (C4−C8) at 330 °C, with propane making up 66% of this fraction. Increasing the reaction temperature to 350 °C caused the propane content to drop to less than 20%, while the amount of C4−C8 hydrocarbons, formed mainly from the cracking of long-chain alkanes, increased to 52% of the total hydrocarbon gas product. In fact, a study by Hancsók et al.112 showed that the gas product could contain as much as 40% isobutane during partial hydrocracking of SFO. With regard to carbon oxides, Kubička and Kaluža83 reported that the highest concentrations of these gases were obtained using the Ni/Al2O3 catalyst when compared to the Mo/Al2O3 or NiMo/Al2O3 catalyst, indicating that the Ni/Al2O3 catalyst used in the study had the least hydrogenation activity. Furthermore, Ni promotes HDC reactions over HDO reactions, which mainly take place with Mo catalysts. As for other gaseous species, Šimácě k et al.92 obtained a significant amount of propane (up to 2.5 vol %) relative to the other gaseous species (0.05−3.0 vol % combined) in their RSO hydroprocessing study using bimetallic catalysts (NiMo/ Al2O3), which have been reported to be highly active in converting triglycerides.83 Dependent upon the overall conversion and rate of deoxygenation, the OLP fraction could consist of C4−C20 hydrocarbons, alkyl esters, unconverted or evolved carboxylic

that were 3.5 and 19 times higher than those using monometallic Mo/Al2O3 and Ni/Al2O3, respectively. The contact time at a given reaction temperature had to be doubled and quadrupled for Mo and Ni catalysts, respectively, to achieve similar deoxygenation rates obtained with NiMo/Al2O3. These results are in qualitative agreement with previous HDS work.110 In these studies, the enhanced activity of bimetallic catalysts, such as NiMo/Al2O3, was attributed to the presence of Ni, which functions as an activity promoter of Mo catalysts in the deoxygenation of triglycerides. The optimal ratio of Ni/Mo is 0.3. Kubička and Kaluža83 also detected larger differences in the rates of triglyceride conversion and oxygen removal among Ni/ Al2O3, NiMo/Al2O3, or Mo/Al2O3, suggesting that different reaction pathways exist with different catalysts. It was found that deoxygenation using Ni/Al2O3 occurred primarily via HDC of the fatty acids that were formed from triglycerides, whereas Mo/Al2O3 promoted the hydrogenation steps of HDO. The variation in reaction pathways was attributed to the different electronic properties of the Ni and Mo sulfided phases. Similar selectivities to HDC products using a sulfided monometallic Ni catalyst were reported with Pt/Al2O3111 and Pd/C catalysts86,87 at reaction temperatures between 260 and 325 °C. As for bimetallic catalysts containing the MoS2 phase, studies showed that they promoted both HDO and HDC reactions.89,94,95 As previously discussed, the grade of vegetable oil used can influence the hydrotreating process, especially when it comes to oxygen removal. Kubička and Horácě k38 evaluated different grades of RSO feedstocks during hydrotreating experiments under the same experimental conditions (310 °C, 3.5 MPa H2, H2/oil ratio of 100 mol/mol, and liquid space velocity of 2 h−1) using a commercial CoMo/γ-Al2O3 catalyst. The different grades of RSO included neat, degummed, refined food-quality, and waste RSO. They found that, at early run times (

Hydroprocessing of Biomass-Derived Oils and ... - ACS Publications

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