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Method selection for bio-jet and bio-gasoline fuels production from castor oil: A Review Diakanua Nkazi, Hembe Elie Mukaya, and Masego Molefe Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00384 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019
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Method selection for bio-jet and bio-gasoline fuels production from castor oil: A Review Masego Molefe, Diakanua Nkazi*, Hembe Mukaya School of Chemical and Metallurgical Engineering and 2 School of microbiology, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, 2050, Private Bag X3, Johannesburg, South Africa *Corresponding
author: E-mail:
[email protected], Tel: 011 717 7509
Abstract Research has intensified towards 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, bio-gasoline and bio-jet fuel production technique from castor oil, a non-edible vegetable oil. The methods are compared on the ability to produce biofuels to be used in spark-ignition engine or/and aviation. Edible oils have been thoroughly investigated as biofuel feedstock, which competes with food sources, hence the requirement to switch focus to non-edible oils. From the 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 bio-jet fuel and bio-gasoline using one catalyst. Key words: Castor oil, biofuel, hydrocracking, catalytic cracking, bio-gasoline, bio-jet.
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Contents Abstract.....................................................................................................................................................i 1. Introduction..........................................................................................................................................1 2. Renewable resources and associated challenges .................................................................................2 2.1 Algae..............................................................................................................................................3 2.2 Waste .............................................................................................................................................4 2.3 Vegetable oils ................................................................................................................................5 3. Castor oil..............................................................................................................................................8 3.1 Extraction......................................................................................................................................9 3.1.1 Traditional extraction..............................................................................................................9 3.1.2 Solvent extraction .................................................................................................................10 3.1.3 Cold pressing ........................................................................................................................12 3.2 Refining .......................................................................................................................................14 3.3 Properties .....................................................................................................................................14 3.4 Castor oil derived biofuels for air transport and gasoline blending.............................................15 3.4.1 Biodiesel from castor oil.......................................................................................................15 3.4.2 Bio-gasoline from castor oil .................................................................................................16 3.4.3 Bio-jet fuel from castor oil....................................................................................................17 4. Production routes ...............................................................................................................................19 4.1 Transesterification (Alcholysis)...................................................................................................19 4.1.1 Homogeneous-catalysed transesterification .........................................................................21 4.1.2 Heterogeneous-catalysed transesterification.........................................................................22 4.1.3 Enzyme-catalysed transesterification ...................................................................................23 4.1.4 Non-catalysed transesterification..........................................................................................23 4.2 Pyrolysis.......................................................................................................................................24 4.2.1 Thermal cracking ..................................................................................................................24 4.2.2 Catalytic cracking .................................................................................................................27 4.2.3 Fluid catalytic cracking.........................................................................................................29 4.3 Hydroprocessing ..........................................................................................................................30 4.3.1 Hydrotreating ........................................................................................................................30 4.3.2 Hydrocracking ......................................................................................................................35 5. Biofuel industry collaborations in sub-Saharan Africa......................................................................38 6. Future prospects of castor oil.............................................................................................................39 7. Conclusion .........................................................................................................................................40 8. References..........................................................................................................................................41
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1. Introduction Petroleum fuel production is plagued by the depletion of non-renewable 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 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 esters [4] 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 bio-jet and bio-gasoline fuels from castor oil can lead to very high returns once commercialized. Selecting the ideal castor oil to bio-gasoline and bio-jet 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) transesterification. The three Page | 1 ACS Paragon Plus Environment
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methods are said to be far superior to other technologies when it comes to the production of biodiesel, gasoline and jet fuel using edible and non-edible vegetable oil feedstock. While the production of biodiesel is not the focal point of this review, the conversion of biodiesel to a biogasoline and bio-jet 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 sugar-derived biomass (or edible biomass), and (3) triglyceride-based biomass, which is generally vegetable oil and is the focus of this study [6]. Figure 1 (b) 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 non-edible 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 bio-gasoline production [7]. Figure 1 (a) details the various biofuel production pathways available and currently researched.
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Biofuel Production Processes
A
Catalytic Cracking and hydroprocessing of vegetable oils
Gasoline, Kerosene, Diesel, Aromatics, Olefins
Pyrolysis of Biomass
Bio-oil
Fermentation of sugar obtained from cellulosic biomass
Bio-alcohol
Biodiesel
Biofuel Feedstock
B Oil-based
Edible oil
Ecofining of vegetable oils
Transesterification of vegetable oils
Oil from algae
Waste oil
Biomass
Nonedible oil
Waste materials
Aquatic Biomass
Energy Crops
Forest Products
Figure 1: An overview of the biofuel (A) production route and the (B) feedstock (adapted from Ong and Bhatia [6]) 2.1 Algae
The use of algae as a renewable source has been researched for more than 5 decades [8]. Algae are favoured for their associated high lipid content, high rate of carbon dioxide absorption, low land use, and a 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 Page | 3 ACS Paragon Plus Environment
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production of bio-jet 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, bio-refinery construction, and biofuel certification and marketing) [9]. 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 bio-jet fuel depends on the type of waste [9]. 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 tyres, and agricultural residues using FTS is still quite expensive [9]. 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 bio-aviation and bio-gasoline 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 [10]. 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. Pre-treatment of WCO is also required for bio-gasoline and bio-jet fuels production through hydrocracking [11].
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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 [12]. Vegetable oils are triglycerides moieties, which contain fatty acid chains connected to a glycerol backbone via the carboxylic group [13]. 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) non-edible oil (castor, tall, and jatropha). The current technologies used to produce biofuel are solely dependent on food crops; however, non-edible or waste oils are the most suitable renewable feedstock, which can be upgraded into useful diesel range transportation fuels via different modern technologies [14]. Non-edible 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 [13]. 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 [15], particularly when it comes to the production of biodiesel. Different oil types have been investigated, and the oils that have been studied in the context of hydroprocessing did not include castor oil (Table 1). In fact, castor oil has been classed as the 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 Page | 5 ACS Paragon Plus Environment
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palm oil is said to be easier to process due to a lower content of linoleic and linolenic acids of approximately 12 wt% [15]. Table 2 presents the fatty acid composition of some edible and non-edible oils. Table 1: Types of vegetable oils used in hydroprocessing studies [15] Vegetable oil Canola
Main source Canada
Edibility Yes
Coconut
Philippines
Yes
Cottonseed
Greece/Turkey
No
Palm
Southeast Asia
Yes
Rapeseed
China/Europe
Yes
Soybean
U.S.A
Yes
Sunflower
Europe
Yes
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Table 2: Fatty acid composition (wt%) for different edible and non-edible oils Oleic 18:1 C18H34O2
Linolenic 18:3 C18H30O2
Ricinoleic 18:3 C18H34O3
Arachid ic 20:0 C20H40 O2
Ref
52 - 56.2
4.3 - 6
0
0
[6], [12]
17-17.7
72.9 - 74
0
0
0
[6], [12]
0.9 - 1.5
60.7-64.1
21.2-22.3
8.2 - 11.8
0
0
[6], [12]
40 - 45
5-6
36 - 40.5
9 - 11
0.2 - 0.3
0.3 - 0.4
0
[7], [6], [12]
Jatropha oil
16
6
43
34
0.8
0
0
[7]
Rubber seed oil
10
8
24
39
16
0
0
[7]
Castor oil
1 - 1.1
1 - 3.1
3 - 4.9
1.3 - 5
0
88 - 89.6
0
[7], [12]
Karanja oil
11
7
51
16
2
0
0
[7]
Polanga oil
12
13
34
38
0.3
0
0
[7]
Vegetable oil
Palmitic 16:0 C16H32O2
Stearic 18:0 C18H36O2
Soybean oil
13.9 - 14
2.1 - 4
23.2 - 24
Sunflower oil
6 - 6.4
2.9 - 3
Rapeseed oil
3.5 - 4.8
Palm oil
Linoleic 18:2 C18H32O2
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The choice of vegetable oil depends particularly on its availability, cost, and climate in each country [7]. This could explain why most of the non-edible vegetable oils have not been extensively researched for technologies such as hydroprocessing. Furthermore, the current challenge with the use of non-edible 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 x 106 tons of non-edible oils such as karanja (Pongamia Pinnata), Neem (Azadirachta indica), Palash (Butea monosperma), kusum (Schelchera Trijuga), jatropha, linseed, and castor [16]. Castor oil has trace amounts of linoleic and linolenic acid, sufficiently lower than the content in palm oil (Table 2), which 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 [17]. The oil yield from castor is about 53 wt%, compared to 20 wt% obtained from palm and soybean oils [17]. 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.
3. Castor oil Castor oil is colourless or pale yellowish and is used in multiple industries to manufacture chemicals such as greases and lubricants, surface coatings, soaps, pharmaceuticals, and so on [17]. Castor oil dissolves easily in alcohol, ether, glacial acetic acid, chloroform, carbon sulphide, and benzene [18]. Castor oil is produced from castor beans, which are mostly produced in Western India [19]. 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
[18]. Castor bean (Ricinus communis L.) is a
Dicotyledonous Albuminous seed and has been used for years as an industrial oilseed crop
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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 conditions [20] [21]. Ideally, the soil pH should be around 6.0 with moderate levels of fertility. Saline conditions are not favourable 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 [21]. Barnesa, et al., [22] investigated the efficiency of previously researched methods of reducing the toxicity of the by-product 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 minutes. 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 by-product cake. 3.1 Extraction The castor beans (see Figure 4) 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 5) and the oil is either chemically extracted or mechanically expelled [23]. The oil extracted using mechanical pressing has a light colour 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 2 [24]. This method is described as inefficient and time-consuming by Page | 9 ACS Paragon Plus Environment
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Oluwole, et al. (2012) [24] as it only yields approximately 20% of 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.
Boiling: Seeds are boiled in water at 96 °C for 10 minute after cleaning
Drying: The boiled seeds are sun dried on a tray to reduce moisture
Grinding : Seeds are ground into a paste
Cooking: the paste is heated in water until oil oozes out
Scooping & drying: The oil is scooped and dried by heating for moisture reduction
Product: The dried castor oil remains after water evaporati on
Figure 2: Traditional extraction method [24]
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 [21]. Hexane is suitable for vegetable oil extraction due to the efficiency and ease of extraction it provides [25]. Akaranta and Anusiem [25] 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 Page | 10 ACS Paragon Plus Environment
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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 alkyd resins. Akpan, et al. [23] 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 and the weight of oil was determined to be 51.55 g. The following formula was used [26]:
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑜𝑖𝑙 (𝑔)
𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑜𝑖𝑙 𝑒𝑥𝑡𝑟𝑎𝑐𝑡𝑒𝑑 (%) = 𝑊𝑒𝑖𝑔ℎ𝑡 (𝑔)𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 (𝑐𝑎𝑠𝑡𝑜𝑟 𝑐𝑎𝑘𝑒) × 100
(1)
A modified version of Equation (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 𝑊2 = 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑎𝑓𝑡𝑒𝑟 𝑑𝑟𝑦𝑖𝑛𝑔
𝑷𝒆𝒓𝒄𝒆𝒏𝒕𝒂𝒈𝒆 𝒎𝒐𝒊𝒔𝒕𝒖𝒓𝒆 𝒄𝒐𝒏𝒕𝒆𝒏𝒕 (%) =
𝑾𝟏 ― 𝑾𝟐 𝑾𝟏
× 𝟏𝟎𝟎
(2)
Table 3: Determination of percentage oil extracted [23] Parameter Weight of empty flask (M1)
Value (g) 108.6
Weight of thimble (W1)
3.13
Weight of sample + thimble (W2)
33.13
Weight of sample (W1 – W2)
30
Weight of empty flask + oil (M2)
160.15
Weight of oil (M2 – M1)
51.55
Second weight of sample
35.1
Third weight of sample
40.2
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Figure 3: Castor beans
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Figure 4: Castor bean kernels 3.1.3
Cold pressing Apart from solvent extraction, castor oil can also be extracted using a cold press machine. The cold press method has many advantages for vegetable oil extraction such as continuous oil extraction with minimal labour, low capital costs, and high oil recovery under low temperature with very little impact on the oil quality [27].
Figure 5: Cold press extraction machine (Wikipedia) Yusuf, et al. (2015) [28] 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 Page | 12 ACS Paragon Plus Environment
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physiochemical properties of the cold pressed castor oil. They obtained an oil yield of 39.43% calculated using Equation 3:
𝑷𝒆𝒓𝒄𝒆𝒏𝒕𝒂𝒈𝒆 𝒐𝒊𝒍 𝒚𝒊𝒆𝒍𝒅 (%) =
𝒚𝟏 ― 𝒚𝟐 𝒚𝟏
(3)
× 𝟏𝟎𝟎
Where y1 and y2 are the weights of castor beans before and after oil extraction. Equation 3 is very similar to Equation 2, where the “Weight of oil” is given by y1 - y2. Both Equations 1 and 3 have some errors associated with it: Equation 1 overestimates the amount of oil extracted due to inefficient solvent recovery, and Equation 3 can either underestimate due to inefficient drying and cooling or overestimate from loss of some castor cake during the drying, cooling and weighing stage. Both methods should be tested and averaged to increase efficiency when reporting the oil yields.
Table 4: Physiochemical properties of wild castor seed oil (Yusuf, et al., 2015) Property pH
Value 6.16
ASTM standard -
Specific gravity, 30 °C
0.959
0.957 - 0.961
Refractive index, 30 °C
1.472
1.476 - 1.478
Relative viscosity, 30 °C
1.86
-
Acid value, mg KOH/g oil
2.07
2 (max)
Hydroxyl value, mg KOH/g oil
163.64
160 - 168
Saponification value, mg
175.31
176 - 184
84.18
83 - 88
Peroxide value, ml/g oil
38.00
-
Cold press oil yield, %
39.43
-
KOH/g oil Iodine value (Hanus), g I2/100 g oil
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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 coloured matter through bleaching, and (4) deodorization by steam treatment at high temperature and low pressure [21]. Table 5 shows the difference between crude and refined castor oil. Some researchers do not include refining as they mainly focus on the characterisation of unprocessed oil. Unlike solvent extracted oil, cold pressed castor oil is purer and does not require refining beyond filtration [28]. Table 5: Physiochemical properties of crude and refined solvent extracted castor oil [23] Crude castor oil 6.11
Property pH
Refined castor oil 6.34
Specific gravity
0.9587
0.9587
Refractive index, 28 °C
1.4686
1.4674
Viscosity, St at 28 °C
9.42477
6.4842
1.148
0.869
185.83
181.55
Iodine value, g I2/100 g oil
87.72
84.8
Colour
Amber
Amber
Acid value, mg KOH/g oil Saponification value, mg KOH/g oil
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 [21]. Table 6 illustrates the typical chemical composition of castor oil and Table 7 contains the physical properties of castor oil. Page | 14 ACS Paragon Plus Environment
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Energy & Fuels
Table 6: Chemical composition of castor oil [17] Component Ricinoleic Acid Linoleic Acid Oleic Acid Stearic Acid Palmitic Acid Dihydroxystearic Acid Linolenic Acid Eicosanoic Acid
Percentage (%) 89.5 4.20 3.00 1.00 1.00 0.7 0.3 0.3
Table 7: Physical properties of castor oil [19] Physical properties Viscosity (centistokes) Density (g/mL) Thermal conductivity (W/m.°C) Specific heat (kJ/kg/K) Flash point (°C) Pour point (°C) Melting point (°C) Refractive index
Values 889.3 0.959 4.727 0.089 145 2.7 -2 to -5 1.480
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. (2015). Hajlari et al. (2019) [29] investigated recently the use of castor oil as source for biodiesel production and its impact on the diesel engine performance. The authors found interresting 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 [29]. A comparative summary of biodiesel derived from castor and commercial aviation fuel physiochemical properties is summarized in Table 8. The ASTM D1655 and DEF STAN 91-91 are aviation fuel standards in the USA and UK respectively, with DEF STAN being more stringent on the restrictions. The castor oil derived biodiesel does not meet most of the aviation fuel specifications as it is characterized by a low heat of combustion value, high density and acid number, which negatively affects the engine performance. The high density and low energy contained in the Page | 15 ACS Paragon Plus Environment
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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 [30], [31].
Table 8: Biodiesel international standards and castor oil derived biodiesel physiochemical properties Specification Density at 15 °C,
ASTM D1655 [32] 0.775-0.840
DEF STAN 91-91 [32] 0.775-0.840
Biodiesel from castor oil (B100) 0.924 - 0.9268
g/cm3 Viscosity at -20 °C,
Ref [30], [18], [29]
Max 8.0
Max 8.0
-
0.100
0.015
0.220 – 1.87
[30], [33]
Min 38
Min 38
165 – 186.5
[33]
Min 42.8
Min 42.8
35.86 - 37.9
[30], [12],
mm2/s Acid no., mg KOH/g Flash point, °C Heat of combustion, Mj/kg Freezing point, °C Sulfur, wt%
[18], [29] Max -40
Max -47
-
0.3
0.3
0.01
[12]
3.4.2 Bio-gasoline 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 [34]. Furthermore, the use of bioethanol in South Africa is not encouraged as the main sources for bioethanol production are prohibited [35]. Butanol isomers have a higher energy content and are less aggressive towards 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 towards materials and therefore requires processing [36]. Alcohols can be converted to ethers as gasoline components. The dehydration of ethanol and methanol forms
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ETBE and DME (dimethylether), respectively, which can be used as additives to gasoline fuel [36]. Bio-gasoline 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. (2009) [37] worked on producing bio-gasoline 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. (2012) [38] also reported the results of bio-gasoline from palm oil-derived biodiesel in terms of conversion and gasoline fraction yields. Liu, et al. (2015) [39] produced bio-gasoline from castor oil as a by-product using hydrocracking in attempts to produce bio-jet fuel. They also did not test the compatibility of the fuel with the current spark ignition engines. The work done on the production of bio-gasoline from castor oil is almost non-existent and should be explored. Table 9: GC-MS analysis of palm oil derived bio-gasoline [37]
1
Hydrocarbon molecule C8
Composition (wt.%) 5.13
2
C9
37.97
3
C10
4.60
4
C13
5.06
5
C15
37.26
6
C17
7.54
7
C19
2.44
No.
3.4.3 Bio-jet fuel from castor oil Not a lot of research has been conducted for bio-aviation fuel from castor oil. Liu, et al. (2015) [39] first reported on conversion of castor oil to bio-jet fuel through hydrocracking [40]. The authors claim that the reported bio-jet fuel has been compared to Jet fuel No. 3 and Page | 17 ACS Paragon Plus Environment
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met the basic jet-fuel mixing requirements (as summarized in Tables 10 and 11). The authors did not detail the method of biofuel characterization and the results meet nearly all the criteria of the ASTM D7566 standard; hence, the results may not be reliable and researchers should take precaution prior to taking the reported results at face value. Zhou, et al. (2016) [40] worked on the conversion of castor oil into “jet fuel” through selective dimerization and hydrodeoxygenation. The authors reported that selectivity towards jet fuel went up as high as 90% and the fuel properties are summarized in Table 10 [40]. Table 10: Properties of jet fuel range products and standard of jet fuel Standard of jet fuel-No.3 (from ASTM D7566)
HDO production from castor oil [39]
38
46
46
41
Viscosity (mm2 S-1)
