Review Cite This: Energy Fuels 2019, 33, 5918−5932
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Method Selection for Biojet and Biogasoline Fuel Production from Castor Oil: A Review Masego Molefe, Diakanua Nkazi,* and Hembe Elie Mukaya
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School of Chemical and Metallurgical Engineering, University of the Witwatersrand, 1 Jan Smuts Avenue, Private Bag X3, Braamfontein 2050, Johannesburg, South Africa ABSTRACT: Research has intensified toward the production of biofuels due to increased economic uncertainty and environmental issues associated with petroleum fuel production. A fifth of the global energy demand is derived from transportation fuels such as diesel, jet fuel, and gasoline. Most of the research in literature focuses on the production of biodiesel to supplement petroleum-based diesel. This review evaluates and compares three methods: (1) hydroprocessing, (2) pyrolysis (catalytic cracking), and (3) transesterification to determine the ideal, and simultaneous, biogasoline and biojet fuel production technique from castor oil, a nonedible vegetable oil. The methods are compared on the ability to produce biofuels using in spark-ignition engine or/and aviation. Edible oils have been thoroughly investigated as a biofuel feedstock, which competes with food sources, hence the requirement to switch the focus to nonedible oils. From extensive research, it is clear that transesterification is not adequate on its own. Hydrocracking is the ideal solution as it can simultaneously produce high-quality biojet fuel and biogasoline using one catalyst.
1. INTRODUCTION Petroleum fuel production is plagued by the depletion of nonrenewable sources, economic uncertainty, and environmental issues associated with petroleum fuels. Currently, 88% of energy is derived from fossil fuels such as coal (29%), natural gas (24%), and oil (35%).1 Of all the fuels produced, transportation fuels such as diesel, jet fuel, and gasoline are the most lucrative as they consume one-fifth of the total oil produced.2 Jet fuel is an aviation fuel used in aircraft engines, and it contains mostly olefins. The straight-run kerosene fraction of crude usually corresponds to jet fuel with the main components of linear and branched alkanes and cycloalkanes with a typical carbon chain-length of C12−C16.3 Gasoline, on the other hand, is used for vehicles with spark ignition engines. It is a transportation fuel with branched hydrocarbons in the range of C5−C11, while diesel, a set of monoalkyl esters4 used in diesel engines invented by Rudolf Diesel, lies in the C16−C22 range. The aforementioned fuels can be blended with biofuels in aviation, gasoline, and diesel engines, respectively. Each blend has a term or acronym based on the blending ratios. For example, petrodiesel blends with 20% biodiesel are termed B20.5 The production of biofuels is part of a possible long-term solution to the issues faced when using petroleum to produce liquid fuels. The production of biojet and biogasoline fuels from castor oil can lead to very high returns once commercialized. Selecting the ideal castor oil for the biogasoline and biojet fuel production technique that will be economically feasible, efficient, and reproducible is quite a daunting task when so many different catalysts and technologies have been tested under various conditions. There has not been a solid conclusion in the world of research regarding the best method and triglyceride-based biomass feedstock due to the contextualization required for these bodies of work. This review will evaluate and compare three methods, which are (1) hydroprocessing, (2) pyrolysis (catalytic cracking), and (3) © 2019 American Chemical Society
transesterification. The three methods are considered to be far superior to other technologies when it comes to the production of biodiesel, gasoline, and jet fuel using edible and nonedible vegetable oil feedstock. While the production of biodiesel is not the focal point of this review, the conversion of biodiesel to a biogasoline and biojet fuel will be explored. The type of feedstock that is evaluated is the triglyceride-based biomass, castor oil, as inadequate research has been done on it despite its many beneficial qualities.
2. RENEWABLE RESOURCES AND ASSOCIATED CHALLENGES Biofuels generally refer to liquid or gaseous fuels that are produced from renewable sources, and they can be produced using several methods depending on the feedstock and the desired products.6 The current trend in biofuel technology is to use biomass-derived feedstock, which is classified into three categories: (1) cellulosic biomass, (2) starch- and sugarderived biomass (or edible biomass), and (3) triglyceridebased biomass, which is generally vegetable oil and is the focus of this study.6 Figure 1 illustrates the feedstock available to use for biofuel production. Emphasis is particularly placed on using vegetable oils due to their advantageous ease of conversion from triglycerides to liquid transportation fuels compared to other types of biomass since they are high-energy liquids that contain less oxygen.6,7 Triglyceride-based biomass feedstock can be converted into biofuel using various methods which produce different products. Transesterification, or the more current technique ecofining, produces mainly biodiesel, while catalytic cracking is used to produce biofuel made up of typical gasoline, kerosene, and diesel fractions from edible and Received: February 5, 2019 Revised: April 26, 2019 Published: May 7, 2019 5918
DOI: 10.1021/acs.energyfuels.9b00384 Energy Fuels 2019, 33, 5918−5932
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Figure 1. Overview of the biofuel production route and the feedstock (adapted from Ong et al. and Karatzos et al.6,8).
nonedible oils.6 Catalytic cracking of vegetable oil in the presence of a suitable catalyst is said to be one of the most efficient methods for biogasoline production.7 Figure 1 details the various biofuel production pathways available and currently researched. 2.1. Algae. The use of algae as a renewable source has been researched for more than 5 decades.9 Algae are favored for their associated high lipid content, high rate of carbon dioxide absorption, low land use, and high growth rate. In addition, algae do not need land or water to thrive and thus do not compete with food sources. The main reason why the use of algae has not been commercialized in spite of the extensive research is the difficulty in selecting and producing useful algae, and the lipid extraction from algae has not been perfected. The production of biojet fuel from algae still requires a lot of research work for upstream, midstream, and downstream operations (i.e., algal species production and selection, process design and optimization, biorefinery construction, and biofuel certification and marketing).10 2.2. Waste. Waste materials are of low cost and easily available. The use of waste materials simultaneously solves the waste management issues, which currently plague the world. The production route of biojet fuel depends on the type of waste.10 With waste biomass, for example, gasification can be carried out to produce synthesis gas, which will be processed using the Fischer−Tropsch synthesis (FTS) to produce biofuels. The method of upgrading biomass waste such as waste wood, waste tires, and agricultural residues using FTS is still quite expensive.10 The process moves from converting solids to gas and gas to liquids. Using waste cooking oil (WCO), which is already in the liquid form required for bioaviation and biogasoline fuels, is less costly. WCO can be collected from businesses (restaurants) and households in the form of a recycle bin, an implementation that will certainly require public awareness campaigns.11 This method of feedstock collection for biofuel production is not
sustainable as varying cooking oils are used throughout South Africa alone, let alone the other sub-Saharan African countries. One other drawback of using WCO is the several impurities it contains such as free fatty acids (FFA) and water that must be treated before transesterification. Pretreatment of WCO is also required for biogasoline and biojet fuel production through hydrocracking.12 2.3. Vegetable Oils. Vegetable oils are extracted or pressed to obtain crude oil, which usually contains free fatty acids (FFA), water, sterols, phospholipids, odorants, and other impurities.13 Vegetable oils are triglyceride moieties, which contain fatty acid chains connected to a glycerol backbone via the carboxylic group.14 Triglyceride-based agricultural fats and oils can be divided into four types:7 (1) crude vegetable oil (palm, rapeseed, and soybean), (2) used vegetable oil (waste cooking oil), (3) animal fats (lard, tallow), and (4) nonedible oil (castor, tall, and jatropha). The current technologies used to produce biofuel are solely dependent on food crops; however, nonedible or waste oils are the most suitable renewable feedstock, which can be upgraded into useful diesel range transportation fuels via different modern technologies.15 Nonedible vegetable oils are ideal, as they do not compete with food crops. However, these vegetable oils cannot be used directly as liquid fuels as they are quite unstable and highly viscous, and they form carbon deposits in parts of automobile engines.14 Vegetable oils are the most common feedstock for biofuel production due to their high-energy density, liquid nature, and availability as a renewable source.7 Various technologies may be used to produce biofuels, which include solvent extraction, transesterification, catalytic cracking, and hydroprocessing. The hydroprocessing technique has proved to provide better quality products than other conversion processes such as transesterification and solvent extraction,16 particularly when it comes to the production of biodiesel. Different oil types have been investigated, and the 5919
DOI: 10.1021/acs.energyfuels.9b00384 Energy Fuels 2019, 33, 5918−5932
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suggests that it is easier to process than palm oil. Furthermore, castor has a much shorter growing period than jatropha and pongamia, and farmers are more experienced and aware of castor’s cultivation.23 The oil yield from castor is about 53 wt %, compared to 20 wt % obtained from palm and soybean oils.23 Its high ricinoleic acid content makes it unique, and the usefulness of castor oil as a biofuel feedstock should be looked into more than it is currently.
oils that have been studied in the context of hydroprocessing did not include castor oil (Table 1). In fact, castor oil has been Table 1. Types of Vegetable Oils Used in Hydroprocessing Studies vegetable oil
edibility
canola coconut cottonseed palm rapeseed soybean sunflower used cooking oil Cerbera manghas rubber seed candlenut corn (maize oil) crambe linseed peanut sesame
yes yes no yes yes yes yes no no no yes yes no yes yes yes
refs 16, 16 16, 16 16, 16, 16, 20 21 21 21 18, 18, 18, 18, 18,
17 18, 19
3. CASTOR OIL Castor oil is colorless or pale yellowish and is used in multiple industries to manufacture chemicals, such as greases and lubricants, surface coatings, soaps, pharmaceuticals, and so on.23 Castor oil dissolves easily in alcohol, ether, glacial acetic acid, chloroform, carbon sulfide, and benzene.24 Castor oil is produced from castor beans, which are mostly produced in Western India.25 Castor is a member of the Euphorbiaceae family; this plant originates in Africa but is found in both wild and cultivated states in all the tropical and subtropical countries of the world.24 Castor bean (Ricinus communis L.) is a Dicotyledonous Albuminous seed and has been used for years as an industrial oilseed crop because of its high seed oil content of approximately 46−55 wt %, its unique fatty acid composition and lubricity, and its ability to grow under varying weather conditions26.27 Ideally, the soil pH should be around 6.0 with moderate levels of fertility. Saline conditions are not favorable for castor production. Castor oil is a naturally occurring and environmentally friendly resource with a good shelf life. Castor oil is obtained from castor seeds, which are poisonous to humans and animals due to the presence of ricin, ricinine, and other allergens, which are toxic.27 Barnesa et al.28 investigated the efficiency of previously researched methods of reducing the toxicity of the byproduct castor meal through boiling and autoclaving. It was discovered that the ricin present can be completely denatured when the whole castor seeds are boiled in water for approximately just over 10 min. Milled seeds will require autoclaving to denature the cytotoxin as boiling is not adequate. Solvent extraction using hexane can break up the oil from the toxin; however, it leaves the poison intact in the byproduct cake. 3.1. Extraction. The castor beans (see Figure 2) need to be dried for a short interval prior to storage and/or threshing and processing as they are harvested before they are fully dried to avoid shattering. In order to process the seeds, the seeds need to be dehusked (Figure 3) and the oil is either chemically extracted or mechanically expelled.29 The oil extracted using
18, 19 18, 19 18, 19
19 19 19 19 19
classified as a less common vegetable oil that could be a potential feedstock for biodiesel production along with jatropha, karanja, linseed, rubber seed, mahua, and neem oils. Rapeseed oil and sunflower oil have been extensively investigated as potential sources of biofuel, while palm oil is said to be easier to process due to a lower content of linoleic and linolenic acids of approximately 12 wt %.16 Table 2 presents the fatty acid composition of some edible and nonedible oils. The choice of vegetable oil depends particularly on its availability, cost, and climate in each country.7 This could explain why most of the nonedible vegetable oils have not been extensively researched for technologies such as hydroprocessing. Furthermore, the current challenge with the use of nonedible oil sources is the high cost of harvesting/extracting the oils as well as the comparatively undeveloped technology in comparison with food-based biofuel production.6 India, being an agriculture-based country, produces approximately 6.7 × 106 tons of nonedible oils such as karanja (Pongamia Pinnata), Neem (Azadirachta indica), Palash (Butea monosperma), kusum (Schelchera Trijuga), jatropha, linseed, and castor.22 Castor oil has trace amounts of linoleic and linolenic acid, sufficiently lower than the content in palm oil (Table 2), which
Table 2. Fatty Acid Composition (wt %) for Different Edible and Nonedible Oils vegetable oil soybean oil sunflower oil rapeseed oil palm oil jatropha oil rubber seed oil castor oil karanja oil polanga oil
palmitic 16:0 C16H32O2
stearic 18:0 C18H36O2
oleic 18:1 C18H34O2
linoleic 18:2 C18H32O2
linolenic 18:3 C18H30O2
ricinoleic 18:3 C18H34O3
arachidic 20:0 C20H40O2
ref
13.9−14 6−6.4
2.1−4 2.9−3
23.2−24 17−17.7
52−56.2 72.9−74
4.3−6 0
0 0
0 0
6, 13 6, 13
3.5−4.8 40−45 16 10
0.9−1.5 5−6 6 8
60.7−64.1 36−40.5 43 24
21.2−22.3 9−11 34 39
8.2−11.8 0.2−0.3 0.8 16
0 0.3−0.4 0 0
0 0 0 0
6, 13 6, 7, 13 7 7
1−1.1 11 12
1−3.1 7 13
3−4.9 51 34
1.3−5 16 38
0 2 0.3
88−89.6 0 0
0 0 0
7, 13 7 7
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extraction due to the efficiency and ease of extraction it provides.32 Akaranta and Anusiem32 compared a bioresource solvent (known as feint, which is a liquid industrial waste obtained from the distillery) with the commercial hexane. The results obtained indicated that higher temperatures and longer extraction times produce the highest castor oil yields. At the same temperature and extraction time, feint proves to yield better results. The oil extracted using feint can be converted from a nondrying to a drying oil through dehydration and can subsequently be used to produce air-drying alkyl resins. Akpan et al.29 determined the percentage of castor oil extracted using the information collected and tabulated in Table 3. The total weight of the sample tested was 155.30 g,
Figure 2. Castor beans.
Table 3. Determination of Percentage Oil Extracted29
Figure 3. Castor bean kernels.
parameter
value (g)
weight of empty flask (M1) weight of thimble (W1) weight of sample + thimble (W2) weight of sample (W1 − W2) weight of empty flask + oil (M2) weight of oil (M2 − M1) second weight of sample third weight of sample
108.6 3.13 33.13 30 160.15 51.55 35.1 40.2
and the weight of oil was determined to be 51.55 g. The following formula was used:33
mechanical pressing has a light color and low free fatty acids (FFAs), and it is about 45% of the oil present in the seeds. The remainder of the oil can only be recovered through solvent extraction. 3.1.1. Traditional Extraction. The traditional method of oil extraction is currently still practiced in Nigeria and India as detailed in Figure 4.30 This method is described as inefficient and time-consuming by Oluwole et al.30 as it only yields approximately 20% of the castor oil. The excessive boiling is also quite energy intensive, making it an expensive process if it were to be commercialized. The boiling also requires water, a scarce resource which needs to be conserved, particularly in South Africa. 3.1.2. Solvent Extraction. The crushed seeds are the feedstock for the solvent extraction method, wherein a Soxhlet or commercial extractor is used in conjunction with a suitable solvent. Solvents which may be used are hexane, heptane, and petroleum ethers.27 Hexane is suitable for vegetable oil
percentage oil extracted (%) =
weight of oil (g) × 100 weight (g) of sample (castor cake)
(1)
A modified version of eq 1 is also used to calculate the percentage moisture content, and the “weight of moisture” is given by W1 − W2, where W1 = original weight of sample before drying and W2 = weight of sample after drying. W − W2 × 100 percentage moisture content (%) = 1 W1 (2) 3.1.3. Cold Pressing. Apart from solvent extraction, castor oil can also be extracted using a cold press machine (see Figure 5). The cold press method has many advantages for vegetable oil extraction such as continuous oil extraction with minimal labor, low capital costs, and high oil recovery under low temperature with very little impact on the oil quality.34
Figure 4. Traditional extraction method.30,31 5921
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Table 5. Physiochemical Properties of Crude and Refined Solvent Extracted Castor Oil
Yusuf et al.35 extracted castor oil from wild Ricinus communis seeds found in Nigeria using mechanical cold pressing at a temperature below 45 °C; Table 4 details the physiochemical Table 4. Physiochemical Properties of Wild Castor Seed Oil35 property
value 6.16 0.959 1.472 1.86 2.07 163.64 175.31 84.18 38.00 39.43
crude castor oil29,36
refined castor oil29
pH specific gravity refractive index, 28 °C viscosity, St at 28 °C acid value, mg KOH/g oil saponification value, mg KOH/g oil iodine value, g I2/100 g oil color
6.11 0.9587−0.9589 1.4686−1.4722 9.42477 1.1−1.148 159.26−185.83 83.7−87.72 amber
6.34 0.9587 1.4674 6.4842 0.869 181.55 84.8 amber
extracted oil, cold pressed castor oil is purer and does not require refining beyond filtration.35 3.3. Properties. Castor oil properties vary based on the extraction method used. For example, cold-pressed castor oil has lower acid and iodine values and a slightly higher saponification value than solvent extracted oil.27 Table 6 illustrates the typical chemical composition of castor oil, and Table 7 contains the physical properties of castor oil.
Figure 5. Cold press extraction machine used in the authors’ laboratory.
pH specific gravity, 30 °C refractive index, 30 °C relative viscosity, 30 °C acid value, mg KOH/g oil hydroxyl value, mg KOH/g oil saponification value, mg KOH/g oil iodine value (Hanus), g I2/100 g oil peroxide value, mL/g oil cold press oil yield, %
property
Table 6. Chemical Composition of Castor Oil23
ASTM standard 0.957−0.961 1.476−1.478 2 (max) 160−168 176−184 83−88
component
percentage (%)
ricinoleic acid linoleic acid oleic acid stearic acid palmitic acid dihydroxystearic acid linolenic acid eicosanoic acid
89.5 4.20 3.00 1.00 1.00 0.7 0.3 0.3
Table 7. Physical Properties of Castor Oil physical properties
properties of the cold pressed castor oil. They obtained an oil yield of 39.43% calculated using eq 3: y − y2 percentage oil yield (%) = 1 × 100 y1 (3)
2
−1
viscosity (mm s ) density (g/mL) thermal conductivity (W/m °C) specific heat (kJ/kg K) flash point (°C) pour point (°C) melting point (°C) refractive index
where y1 and y2 are the weights of castor beans before and after oil extraction. Equation 3 is very similar to eq 2, where the “weight of oil” is given by y1 − y2. Both eqs 1 and 3 have some errors associated with them: eq 1 overestimates the amount of oil extracted due to inefficient solvent recovery, and eq 3 can either underestimate due to inefficient drying and cooling or overestimate from loss of some castor cake during the drying, cooling, and weighing stages. Both methods should be tested and averaged to increase efficiency when reporting the oil yields. 3.2. Refining. The main aim of refining vegetable oils is to remove impurities and undesired substances such as FFAs and colloidal matter. Refining makes the oil more suitable for storage and involves four steps: (1) removal of solid and colloidal matter through settling and filtration, (2) neutralization of FFAs using alkali such as NaOH, (3) removal of colored matter through bleaching, and (4) deodorization by steam treatment at high temperature and low pressure.27 Table 5 shows the difference between crude and refined castor oil. Some researchers do not include refining as they mainly focus on the characterization of unprocessed oil. Unlike solvent
values
ref
6.27−8.84 0.957−0.961 4.727 0.089 145 2.7 −2 to −5 1.472−1.480
25, 37 25, 35 25 25 25 25 25 25, 35
3.4. Castor Oil Derived Biofuels for Air Transport and Gasoline Blending. 3.4.1. Biodiesel from Castor Oil. Significant research has been performed for castor oil-derived biodiesel, and the results support that biodiesel is not as efficient as aviation fuel as was stated by Hari et al.10 Hajlari et al.38 recently investigated the use of castor oil as a source for biodiesel production and its impact on the diesel engine performance. The authors found interesting results on the use of castor biodiesel produced in the compression-ignition engine. The authors consider the chemical formula of the produced biodiesel (castor oil ethyl ester) as C18H33O2, which is not in the carbon range of aviation fuel.38 A comparative summary of physiochemical properties of biodiesel derived from castor and commercial aviation fuel is summarized in Table 8. The ASTM D1655 and DEF STAN 91-91 are aviation fuel standards in the U.S. and U.K., respectively, with DEF STAN being more stringent on the restrictions. The castor oil 5922
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Energy & Fuels Table 8. Biodiesel International Standards and Castor Oil Derived Biodiesel Physiochemical Properties specification
ASTM D165541
DEF STAN 91-9141
biodiesel from castor oil (B100)
0.775−0.840 max 8.0 0.100 min 38 min 42.8 max −40 0.3
0.775−0.840 max 8.0 0.015 min 38 min 42.8 max −47 0.3
0.924−0.9268
24, 38, 39
0.220−1.87 165−186.5 35.86−37.9
39, 42 42 13, 24, 38, 39
0.01
13
density at 15 °C, g/cm viscosity at −20 °C, mm2/s acid no., mg KOH/g flash point, °C heat of combustion, MJ/kg freezing point, °C sulfur, wt % 3
castor oil to biojet fuel through hydrocracking. The authors claim that the reported biojet fuel has been compared to Jet Fuel No. 3 and met the basic jet-fuel mixing requirements (as summarized in Tables 10 and 11).37 The authors did not detail
derived biodiesel does not meet most of the aviation fuel specifications as it is characterized by a low heat of combustion value and high density and acid number, which negatively affect the engine performance. The high density and low energy contained in the biodiesel (low heating value) makes it more difficult for the aviation engine to start, and the high acid number indicates that the biodiesel will be too corrosive for the engine.39,40 3.4.2. Biogasoline from Castor Oil. Biomass-derived alcohols such as bioethanol and methanol derived fuel are often used for gasoline blending. Ethanol has the advantage of a higher octane number and lower sulfur content than petroleum gasoline; however, the energy potential is significantly lower and it cannot be used as the sole fuel for spark ignition engines.43 Furthermore, the use of bioethanol in South Africa is not encouraged as the main sources for bioethanol production are prohibited.44 Butanol isomers have a higher energy content and are less aggressive toward materials but have lower octane numbers and higher production costs than ethanol. Methanol, on the other hand, cannot be used in large amounts because it is extremely aggressive toward materials and therefore requires processing.45 Alcohols can be converted to ethers as gasoline components. The dehydration of ethanol and methanol forms ETBE and DME (dimethyl ether), respectively, which can be used as additives to gasoline fuel.45 Biogasoline from vegetable oils has been explored and is said to be fully compatible with petroleum gasoline but is to be tested in spark ignition engines by researchers. Nasikin et al.46 worked on producing biogasoline from palm oil through simultaneous cracking and hydrogenation using a NiMo/ zeolite catalyst. The reported results were only in terms of the composition of the hydrocarbons present and are detailed in Table 9. Usomboon et al.47 also reported the results of
Table 10. Properties of Jet Fuel Range Products and Standard of Jet Fuel
property freezing point (°C) density (kg m−3) flash point (°C) viscosity (mm2 S1−) sulfur content (%) olefins content (%) aromatics content boiling point (°C)
temperature (° C)
carbon range
composition (%)
ref
palm waste cooking waste cooking
300 180 250
C8−C12 C6−C12 C6−C12
47.70 35.70 59.50
46, 48 48, 49, 50 48, 49, 50
standard of Jet Fuel No. 3 (from ASTM D7566)
HDO production from castor oil: Jet Fuel No. 3 (1:1)37
HDO production from castor oil37
biojet fuel51
38
46
46
41
