Keywords

Lipid hydroprocessing; hydrotreating; hydrocracking; biofuels; aviation biofuels; biojet; renewable fuels

5.1 Introduction

Lipids are long-chain organic compounds predominantly consisting of carbon, hydrogen, and oxygen linkages, classified mainly as glycerides (triglycerides, diglycerides, monoglycerides), free fatty acids, and their derivatives, including phospholipids. These are naturally occurring, and as these compounds are produced by living organisms through their own metabolism, are one of the most abundant usable sources of renewable carbon.

Biofuels can be produced by processing lipids by various routes, including transesterification [1–4], hydrothermal [4,5] reactions, or hydroprocessing [1–4]. All routes have their own limitations and advantages, and in time, with technological developments, the most economical and sustainable route to drop-in biofuel production by lipid processing will prevail [2,3,6–13].

All over the globe, researchers have hydroprocessed various sources of lipids such as soybean oil [14–16], sunflower oil [17–20], palm oil [21,22], rapeseed oil [23–25], castor oil [26,27], tall oil [28], jatropha oil [8,11,29–37], pomace oil [38], and fresh and waste cooking oil [9,39–42] either directly or by co-processing [8,14,18,19,22,25,35,43,44] with gas oil [7–9,11,13,14,18–20,24,25,28–30,39–41,45]. To understand the reaction mechanisms and intermediate compounds for lipid hydroprocessing, researchers have studied model compounds such as tristerian (glycerol tristearate C₁₈) [46], triolean (glucerol trioleate C₁₈) [46], tricaprylin (octanoic acid triglyceride C₈) [47], caprylic acid (octanoic acid C₈) [47], stearic acid [48,49], and oleic acid [50,51] – as well as actual feedstocks such as Jatropha – over different catalysts in hydrogen and inert atmospheres [31–33,45,47–50,52]. Fixed-bed as well as batch reactors and semi-batch reactors have been explored for performing these reactions. Both mono-functional [28,30,47] and bi-functional [8,14,29,31,32] catalysts have been utilized, depending on target reactions, such as hydrotreatment or hydrocracking reactions. Hydrotreatment targets only removal of hetroelements in hydrocarbons such as ‘O, N, S’ and require a catalyst with hydrogenation functionality such as Pd, Pt, Pt-Re, or sulfided NiW, NiMo, CoMo supported over a nonacidic support like γ-Al₂O₃ or activated carbon [11,13,20,28,30,46]. Hydrocracking reactions target production of a wide range of hydrocarbon distillates and require bi-functional catalysts with hydrogenation functionality along with acidic functionality incorporated onto supports such as zeolites, silica-alumina, silico-aluminophosphates, titanosilicates, etc. [8,14,29]. These bi-functional catalysts perform hydrogenation/dehydrogenation, cracking, isomerization, cyclization, and aromatization reactions during lipid hydroprocessing.

Lipids are mostly comprised of triglyceride molecules, which are bulky in size, and these molecules need to access the active sites on the catalyst surface through its porous structure. Moreover, lipids on hydrotreatment produce water, which highlights other important criteria for catalyst selection (ie, hydrothermal stability). The catalyst’s activity, stability, life, and regenerability are the four most important criteria for catalyst preparation and selection. Researchers have modified the conventional catalyst properties and have also developed new nonconventional catalysts for hydroprocessing reactions. To achieve the desired properties, variations in the composition of catalyst supports have been studied as have changes in active metal composition, metal loading, use of different kinds of active metals, and additives, etc. [53–55]. Apart from conventional hydroprocessing catalyst supports, many materials such as clays [56], carbon [11,13,20,28,30,46,57,58], oxides [59–62] like SiO₂, MgO, ZrO₂, TiO₂, and mixed oxide derivatives, such as TiO₂–ZrO₂, TiO₂–Al₂O₃, SiO₂–TiO₂ [57,63–69], zeolites like Na–Y, USY, HY, SAPO [27,34,70–72], mesoporous materials like MCM-41 [73–77], HMS [78], SBA-15 [79,80], and nano-hydroxyapatite [37] have been researched as support materials. Mesoporous aluminosilicate MCM-41 with a combination of large surface area, uniform pore size distribution, and mild acidity and high stability have been reported to have increased hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrocracking performance over conventional alumina- and zeolite-based catalysts [78]. TiO₂–ZrO₂-, TiO₂–Al₂O₃-, and SiO₂–TiO₂-supported catalysts have all shown synergetic effects with increased activity and stability. A unique characteristic of mesoporous materials is the three-dimensional structure with increased surface areas and porosities combined with higher thermal stability. As such, they have been studied for hydroprocessing reactions and for lipid hydroprocessing [23,27,29,30,32,37].

Kinetics and pathways for various reactions occurring during lipid hydroprocessing have been studied, as well as the extent of their effects on the yields of various biofuel components. Major reactions occurring during lipid hydroprocessing are hydrocracking and hydroisomerization reactions, with simultaneous depropanation (C₃H₈ elimination) and hydrodeoxygenation (H₂O elimination), decarboxylation (CO₂ elimination), and decarbonylation (CO elimination) reactions [8,14,19,29,31–33,45,52,80]. The above-mentioned primary reactions are also accompanied by unavoidable gas phase and side reactions, such as water gas shift, methanation, oligomerization, cyclization, and aromatization, which have also been studied. Conditions for minimization of these reactions have been reported in the literature [31–33,45,51,52].

This chapter describes academic and technological advances for the processing of lipids obtained from various sources via hydroprocessing routes to produce biofuels suitable for aviation. The effects of different catalytic systems, operating parameters, reaction pathways, and kinetics involved in the hydroprocessing of lipids are also described. A path forward for the commercial realization of a future aviation biofuel industry via hydroprocessing of lipids is described at the end of the chapter.

5.2 Effect of Catalysts on Lipid Hydroprocessing

5.2.2 Lipid Hydroprocessing Over Various Catalytic Systems

In the literature, researchers have reported lipid hydroprocessing over various catalytic systems, and have classified the liquid products either based on the carbon number distribution (<C₈ as naphtha, C₉–C₁₅ as kerosene, >C₁₅ as diesel) or fractional cut ranges obtained by simulated distillation (IBP-135°C as naphtha, 135–260°C as kerosene, 260-FBP as diesel). Lipids from various sources have been processed either directly or co-processed after mixing with gas oil or other hydrocarbon streams from the refinery. The following section describes more about lipid co-processing and direct processing over various catalytic systems and under different operating conditions.

5.2.2.1 Co-Processing Lipids With Refinery Streams

Lipids have been co-processed with refinery streams of mainly gas oil (both light and heavy) so that the process of production can be carried out in the existing refinery infrastructure and additional capital costs avoided. Researchers have co-processed jatropha [8], soybean [14,43], sunflower [18,19], waste cooking oil [44], and rapeseed oil [25] under hydrotreating conditions and over a nonacidic sulfided metal-based catalyst system, Ni-Mo/Al₂O₃. Very few studies have been done on the co-processing of lipids with gas oil (heavy and vacuum) over hydrocracking catalysts [18,43]. Table 5.2a details lipid co-processing (10–30% lipid in gas oil) at 350°C and 370°C temperatures, 50 bar H₂ pressure, and 1–5 h⁻¹ space velocity over hydrotreating catalyst. Table 5.2b describes lipid co-processing over hydrocracking catalysts. The main aim of these hydrotreating studies was to evaluate the extent of conversion and selectivity for competing deoxygenation and desulfurization reactions; whereas over hydrocracking catalysts, the objective was to maximize the yield of middle distillates of aviation kerosene.

Table 5.2

Yield of Different Products Obtained by Co-processing of Lipids From Various Sources With Refinery Gas Oil Over (a) Hydrotreating, and (b) Hydrocracking Catalysts

a. Ni-Mo/Al2O3 Catalyst at 50 bar Pressure
Feed (Space Velocity)	350°C (10–15% Lipid)					370°C (20–30% Lipid)
	Jatropha (2 h−1) [8]	Soybeana (4 h−1) [34,43]	Sunflower (5.2 h−1) [19]	Rapeseed (2 h−1) [25]	Cooking (1 h−1) [44]	Soybeana (2 h−1) [14,43]	Sunflower (5.2 h−1) [19]	Rapeseeda (2 h−1) [25]	Cooking (1 h−1) [44]
Naphtha (IBP-150)	0	0	0.09	0	0	0	2	0	1
Kerosene (150–250)	9	5	0.05	32.4	5	4.5	1	34.4	8
Diesel (250-FBP)	91	95	99.86	67.6	95	95.5	97	65.6	91
HDS %	96	89	34		92%		48		96%
C17/C18		1.6	0.49			0.75	0.9
C15/C16		1.15	0.5			1	0.75
CO			0.08				0.66
CO2			0.025				0.07
Density, g/cc	0.839	0.846		0.835	0.8508			0.831	0.84

b. Hydrocracking Catalyst at 1000–1500 Nl/L H2 per Feed Ratio
Feed [Reference]	Soybeanb [43]	Sunflowerc [18]	Soybeanb [43]
% Lipid	25	30	40
Naphtha (IBP-150)	0	10	0
Kerosene (150–250)	35	26	30
Diesel (250-FBP)	65	64	70
HDS%	84
C17/C18	1.6		1.58
C15/C16	1.5		1.48
Density, g/cc		0.769	0.818

^a380°C.

^b370°C, 2 h⁻¹.

^c350°C, 1.5 h⁻¹.

Oxygen present in the lipids is similar to heteroatoms (such as sulfur) in fractionated products from crude oil sources. These heteroatoms need to be removed before further processing or utilization of products in any downstream processes. Deoxygenation reactions such as hydrodeoxygenation (H₂O), decarboxylation (CO₂), and decarbonylation (CO) reactions compete with hydrodesulfurization (H₂S) reactions for active sites. It was anticipated that a negative effect of these renewable feedstocks on the catalyst and extent of desulfurization reaction would be observed. However, Kumar et al. [8] demonstrated that co-processing lipid sources (jatropha oil) increased the percentage hydrodesulfurization (%HDS) achieved compared to that achieved when processing gas-oil separately. Bezergianni et al. [44] and Huber et al. [19] also demonstrated that on increasing the percentage of lipid (cooking and sunflower oil, respectively) in gas oil, the %HDS achieved was increased (Table 5.2). High %HDS was observed for jatropha (96%) and soybean (89%) oils, whereas only 34% was observed in the case of sunflower oil, which may be attributed to operation at low severity – that is, at high space velocities (5.2 h⁻¹) as compared to other co-processed lipids (Table 5.2).

Renewable diesel range (250-FBP) products were mainly produced during co-processing reactions because cracking reactions were kept to a minimum – only hydrotreatment reactions were targeted (Table 5.2a). High throughputs with operation at 4 and 5 h⁻¹ space velocity for co-processing of soybean [14,43] and sunflower [6] oils have been reported, respectively. In comparison, waste cooking [44], rapeseed [25], and jatropha [1] oils have been co-processed at lower space velocities of 1–2 h⁻¹. An increase in the kerosene range (150–250°C) products was observed for these lipid sources at lower space velocity (1–2 h⁻¹) due to milder cracking reactions. Rapeseed oil instead showed 32% yield of kerosene range fractions, which were mainly due to co-processing with a light gas oil mixture that contained 28% kerosene range in the feed itself (Table 5.2a). Overall <10% kerosene range products could be obtained at both 350°C and 370°C. Upon increasing the percent of lipid fraction in gas oil (Table 5.2), there was a marginal increase in cracked products yield (naphtha and kerosene), which may be attributed to the increased acidity of the reaction media due to the increased formation of acidic intermediates at high lipid concentrations and higher temperatures.

Unlike Ni-Mo/Al₂O₃ (low-acidity) catalysts, hydroprocessing of lipids from soybean and sunflower sources over hydrocracking catalyst gave high yields of desirable kerosene range hydrocarbons (25–35% yield) at 370°C and 2 h⁻¹ space velocity (Table 5.2b). As a consequence, lower diesel yield was obtained due to cracking of these diesel range hydrocarbons into lower-range distillates.

The ratios of decarboxylation and decarbonylation (carbon rejection) reactions to hydrodeoxygenation (hydrogen addition) reactions (C₁₇/C₁₈ and C₁₅/C₁₆) have also been reported (Table 5.2), which indicates an increase in carbon rejection reactions at higher temperatures (370°C) as compared to lower temperatures (350°C) for sunflower oil at 5 h⁻¹ [19]. A large increase in the concentration of carbon monoxide (eight times) at high temperatures (370°C) due to decarbonylation during co-processing supports this conclusion. Increased carbon rejection reactions were observed for hydrocracking catalysts (higher C₁₇/C₁₈ and C₁₅/C₁₆ ratios) as compared to hydrotreating (Table 5.2a and b).

The density of products obtained after co-processing of various lipid sources decreases only a little on increasing temperatures to 370°C, and on increasing % lipid fraction in feed (Table 5.2), which is due to formation of lighter cracked products. The density of products obtained upon co-processing sunflower oil over hydrocracking catalytic systems was comparatively lower due to increased cracking reactions (naphtha formation), even at lower temperature (350°C). On average, the density of products obtained over hydrocracking catalytic systems was lower compared to products formed over hydrotreating catalytic systems (Table 5.2)

5.2.2.2 Direct Processing of Lipid Sources

Lipids have been processed directly under both hydrotreating and hydrocracking conditions in the presence of noble metal and sulfided base metal catalyst systems supported over acidic and nonacidic supports. Tables 5.3 and 5.4 describe the processing of various lipid sources at different temperatures over hydrotreating and hydrocracking catalytic systems, respectively.

Table 5.3

Influence of Temperature Variation on Product Distribution During Hydrotreatment of Lipids From Various Sources

Lipid Source	Castor [26]		Palm [21]			Sunflower [19]			Soybean [14,15]		Jatropha Oil [31]			Cooking [35,36]
Catalyst System	5% Pd/C		NiMo/Al2O3			NiMo/Al2O3			NiMo/Al2O3		CoMo/Al2O3			Commercial HDT Catalyst
Pressure, bar (Space Velocity, h−1)	25		50 (1)			50 (5.2)			50 (2)	92	80 (4)			81(1)
Temperature	300	340	300	330	420	300	350	420	380	400	320	340	360	330	350	398
Naphtha	0	0				5.4	6.3	12	0	2.5	1	1.5	3.35	0	0	5
Kerosene	0	0	0.8	1	12.8	0.4	1	5	3	7.5	2.5	3	7.13	5	5	15
Diesel	40	100	95	93.7	41.6	93.2	91.8	76	97	90	85	88	85	95	95	80
C15/C16		0a	0.2	0.2	0.2	1	1	0.8		1.9				0.8	1.2	1.4
C17/C18	28a	9.7	0.2	0.2	0.2	0.4	0.7	0.8		2.5				1.1	1.3	1.8
m (kerosene)	0	0				0.2	0.05	0.3		0.5				0.1	0.1	0.6
Conversion	100	100	100	100	100	100	100	100		100	92.2	95.1	98.5	92.5	92	96.5

^aMethyl Stearate.

Table 5.4

Product Distribution for Hydroprocessing of Lipids from Various Sources Over Different Hydrocracking Catalyst Systems at 1–1.5 h⁻¹

a. Sulfided
Lipid Source [Reference]	Soybean [16]		Algal [29]	Jatropha [8]		Jatropha [34]								Cooking [9]
Pressure, Bar	40		50	50		70								130
Catalyst System	NiMo/HY		NiMo/Meso-ZSM-5	NiW/SiO2-Al2O3	CoMo/Al2O3	NiMo/MSP-1		NiMo/MSP-2		NiW/MSP-1		NiW/MSP-2		Commercial HC catalyst
Temperature, °C	390	410	410	360	360	400	450	400	450	400	450	400	450	350	370	390
Naphtha						17.3	37	5.6	30.5	14	40	11	28.5	0	1.8	1.2
Kerosene	49.1	55	78.5	19.7 (<C15)	40.2 (<C15)	8	28.5	8	34.5	3.5	24.5	4	37.5	5.3	8.2	19.9
Diesel	18.2	5		80.8	49.2	74.7	34.5	86.4	35	82.5	35.5	85	34	66.7	65	61.0
C15/C16				1.3	0.8	0.5		0.3
C17/C18				2.2	0.7	0.7		0.7
i/n			2.5	1.1	0.3	0.8	2.3	2.5	3.1	1.5	3.5	0.8	3.8
Conversion	98	100	98	100	100	100	100	100	100	100	100	100	100	72	75	82

b. Nonsulfided
Lipid Source	Jatropha [30]				Castor [27]
Pressure, Bar	65 (H2/N2: 85/15)+				30
Catalyst System	Pt/H-ZSM-5	Pt/USY	Pt/CNT	Pt/H-ZSM-5*	25% Ni/SAPO-11	25% Ni/H-Beta	25% Ni/ZSM-5	25% Ni/USY	25%Ni/USY-APTES-MCM-41
Si/Al	23	6.3		23			38
Temperature	270				300
Naphtha	8.0	0.0	0.0	18.2	1.2	97.7	96.8	79.4	13.8
Kerosene	3.8	1.4	13.3	6.0	3.2	1.5	2	19.2	80.3
Diesel	88.4	98.6	86.7	75.8	95.4	0	0	0.8	5.4
C15/C16	0.0	0.8	40.9	1.9
C17/C18	0.4	0.5	0.7	0.1
i/n					0.1	0	0	5.3	4.4
Conversion		31.2	13.6	14.2	99	99	99	99	99

+cat:jatropha:H₂O ratio(1:1:9; *1:10:9). CNT, carbon nanotubes; HY, Y zeolite (H-form); Meso-ZSM-5, hierarchical mesoporous H-ZSM-5 zeolite; MSP, hierarchical mesoporous SAPO-11(MSP-1 (Si/Al: 0.4) & MSP-2 (Si/Al: 0.27)

5.2.2.2.1 Hydrotreating of Lipids

Castor, palm, and sunflower oils were processed by Meller et al. [26], Sirifa et al. [21], and Huber et al. [19] at 300°C (Table 5.3). There was a negligible amount of kerosene range products observed, with predominantly renewable diesel product. Palladium supported over carbon was used as a catalyst for processing castor oil [26] at a low pressure of 25 bar, while palm and sunflower oils were processed using sulfided Ni-Mo supported over alumina as the catalyst at 50 bar pressure. Complete conversion of triglycerides could be observed for all three feedstocks under these conditions. Although complete conversions could be observed at 25 bar pressure and 300°C in the case of castor oil over Pd/C catalyst, only 40% yield of diesel range compounds was observed, unlike the NiMo(S)-alumina catalyst, where a >90% diesel yield was obtained. Sirfa et al. [26] noted the formation of many oxygenated intermediate compounds along with a 28% yield of methyl stearate. Similar oxygenated intermediates were also observed at reduced severity by Kubicka et al. [23] at lower temperatures, by Anand et al. [31] at reduced pressures, and by Huber et al. [3,19] at higher space velocities.

Upon increasing the temperature to 340°C, methyl stearate disappeared and an increase in the yield of diesel range molecules was observed in the case of castor oil (Table 5.3). On the contrary, there was only a slight reduction in diesel yield and an increase in kerosene and naphtha products yield upon increasing the temperature to 350°C. On further increasing the temperature to 420°C, a drastic decrease in diesel yield was observed; the highest decrease was observed in the case of palm oil (Table 5.3). Increase in cracking and isomerization reactions were observed at higher temperatures, which led to increased kerosene and naphtha yields. A greater reduction in diesel yield was observed in the case of palm oil, not only because of operation at reduced space velocity (palm: 1 h⁻¹; sunflower: 5.2 h⁻¹), but also due to its more saturated composition. Soybean oil was also processed over NiMo/Al₂O₃ catalyst at 50 and 90 bar pressures, and on increasing temperature from 380°C to 400°C, a decrease in diesel yield and an increase in kerosene and naphtha yield were also reported (Table 5.3).

Jatropha [31] and cooking [40,41] oils were processed at a high pressure of 80 bar over CoMo/Al₂O₃ and commercial hydrotreating catalytic systems. At lower temperatures – that is, 320°C (jatropha oil) and 330°C (cooking oil) – only 92% conversion was observed with lower diesel yield (85%) in the case of CoMo/Al₂O₃ with jatropha oil (Table 5.3), as compared to that over commercial hydrotreating catalyst with cooking oil. Lighter cracked products – that is, naphtha and kerosene (3.5% yield) – could be observed even at 4 h⁻¹ space velocity in the case of CoMo/Al₂O₃ systems, whereas only 5% kerosene was observed at 330°C and 1 h⁻¹ space velocity (high severity) in the case of cooking oil (Table 5.3). Diesel yield increased (88%) upon increasing the temperature to 340°C for jatropha oil due to an increase in conversion, whereas it remained almost constant in the case of cooking oil (Table 5.3). On further increasing temperatures to 360°C (jatropha) and 398°C (cooking oil) at 80 bar pressure, there was a decrease in diesel yield and an increase in cracked products yield, and increased formation of kerosene range hydrocarbons (Table 5.3).

Anand et al. [31] demonstrated formation of acidic intermediates upon processing lipid feedstocks over nonacidic Co-Mo/Al₂O₃ catalyst, which justified the increase in cracking reactions under less severe hydrotreating conditions. An increase in decarbonation selectivity (CO, CO₂ elimination) over hydrodeoxygenation selectivity (H₂O elimination) was observed for all feeds at higher temperatures (Table 5.3), except in the case of palm oil, wherein low decarbonation selectivity (low C₁₅/C₁₆ and C₁₇/C₁₈ ratio) was observed. This indicated that heavier lipids such as castor, jatropha, soybean, sunflower, and cooking oils favoured the carbon rejection mechanism at higher temperatures, whereas hydrodeoxygenation for the removal of oxygen was a preferred mechanism in the case of lighter lipid sources such as palm oil (Table 5.3). These facts indicate that the consumption of hydrogen in the case of palm oil would be greater compared with other lipid sources towards deoxygenation reactions.

In summary, oxygenated intermediates were produced at reduced reaction severity (low hydrogen partial pressures, low reaction temperatures, and reduced residence time, that is, high space velocity). Upon increasing the reaction severity, these intermediates converted to deoxygenated products and increased temperatures led to cracking and isomerization reactions. The CoMo/Al₂O₃ catalytic system induced acidity in the reaction media due to formation of acidic intermediates and led to increased cracking reactions and formation of kerosene range molecules even at reduced severity. Lighter lipid sources such as palm oil favoured hydrodeoxygenation reactions over carbon-rejection reactions, whereas heavier lipid sources deoxygenated primarily by carbon-rejection mechanisms at higher temperatures. Generally, hydrotreating cannot directly produce desirable yields of aviation kerosene. But it is possible to convert the diesel – selectively produced from hydrotreating vegetable oil in a first reactor – into aviation kerosene by mild hydrocracking in a second reactor (a two-step process).

5.2.2.2.2 Hydrocracking of Lipids

Hydrocracking of lipids has been studied in the literature over a wide range of catalytic systems, such as conventional sulfided catalysts and their combinations with various hierarchical mesoporous supports, or the nonconventional nonsulfided catalytic systems such as Ni and Pt metal-based catalysts used in reduced conditions over different zeolites, silicoaluminophosphates, and their combinations. Conventional catalysts were evaluated between 40–130 bar (hydrogen pressures) and at temperatures ranging between 360°C and 450°C, whereas nonconventional catalytic systems were evaluated at lower pressures 30–55 bar (hydrogen pressures) and temperatures between 270°C and 300°C.

5.2.2.2.3 Conventional Sulfided Catalysts

Lipids from soybean, algal, jatropha, and waste cooking oils have been processed over Ni-Mo, Ni-W, Co-Mo-based catalytic systems where Mo and W are the active metals providing the required hydrogenation/dehydrogenation ability to the catalyst along with Ni or Co as the promoters, all loaded over an acidic support (Table 5.4a). Nearly complete conversion was observed for all lipid feedstocks except waste cooking oil lipids, with a gradual increase in conversion from 72% to 82% upon increasing temperature observed at 130 bar (Table 5.4a). The conversion calculations as defined by Bezergianni et al. [9] consider boiling fractions above 360°C as unconverted feedstock. Considering the reaction products boiling above 360°C as unconverted lipid from cooking oil, reduced conversions over the commercial hydrocracking catalytic system are inferred.

Cheng et al. [16], Kumar et al. [8], and Verma et al. [29,34] instead studied the conversion of lipids of different sources, and actual concentrations of unconverted feed (measured with chromatography) have been used for conversion calculations. High renewable diesel yield of around 60–80% was obtained at temperatures below 400°C and pressures between 50 and 130 bar (Table 5.4a) for jatropha and cooking oil lipids over Ni-Mo- and Ni-W-based catalysts [8,16,29,34]. Upon further increase of the temperature, the yield of the diesel fraction decreased and more cracked products (ie, kerosene and naphtha) were observed. Comparatively, over a Co-Mo/Al₂O₃ catalyst, low diesel yields were observed (Table 5.4a), along with a 40% yield of cracked (<C₁₅) products. Similar observations of increased cracked product yields over a Co-Mo/Al₂O₃ catalytic system were observed by Anand et al. [31,33]. Increased cracking observed over a Co-Mo catalyst supported on low acidity Al₂O₃ indicated the formation of acidic intermediates in the reaction media that were catalysing the cracking reactions. Moreover, for a Co-Mo catalyst, CO bond breaking was preferred – that is, hydrodeoxygenation was preferred over decarbonation mechanisms (0.8 (C₁₅/C₁₆); 0.7 (C₁₇/C₁₈)) – whereas Ni-W/SiO₂-Al₂O₃ catalytic systems preferred decarbonation reactions over hydrodeoxygenation reactions (1.3 (C₁₅/C₁₆); 2.2 (C₁₇/C₁₈)) due to the acidic SiO₂-Al₂O₃ support, which catalyses CC bond cracking and leads to the formation of CO and CO₂ [8].

Mesoporous silicoaluminophospate SAPO-11 (MSP)-based catalytic systems supported with Ni-Mo and Ni-W active metals showed similar yield patterns with increased aviation kerosene and naphtha yield upon increasing temperature from 400°C to 450°C (Table 5.4a). A corresponding decrease in diesel yield was also observed. There was drastic improvement in isomer-to-normal hydrocarbon (i/n) ratios upon increasing the temperature, indicating increased isomerization selectivities over SAPO-11-based catalytic systems at elevated temperatures (Table 5.4a). A maximum aviation kerosene yield of 37.5% was observed for jatropha oil lipids over Ni-W/MSP systems at 450°C temperature and 70 bar pressure, whereas for soybean oil lipids, 49% yield of aviation kerosene was observed over NiMo/HY zeolite catalyst at reduced temperatures and pressures (390°C and 40 bar). Increasing temperature to 410°C, the yield of aviation kerosene further increased over NiMo/HY catalyst (Table 5.4a). Similar observations were made for algal oil lipids, with a maximum aviation kerosene yield of 78% (at 410°C) over hierarchical mesoporous zeolite (ZSM-5)-supported NiMo catalyst [29], which indicated increased lipid cracking ability of these zeolites over other catalytic systems.

Fig. 5.1 shows the product yield distribution for hydrocracking of various vegetable oils over different catalysts. For jatropha oil hydrocracking over mesoporous silica-alumina, hierarchical mesoporous H-ZSM-5 and titanosilicate (MTS) supports loaded with sulfided NiMo, NiW, and CoMo active metals gave the highest yield of naphtha, followed by that of aviation kerosene and diesel, with aviation kerosene ranging between 30–35% at 400–420°C temperatures and 80 bar pressure. However, over CoMo/MTS systems at a reduced temperature of 360°C, cracking of jatropha oil was low, with 80% renewable diesel as the major product, followed by aviation kerosene (~20%) and a negligible yield of naphtha (Fig. 5.1). Increase in acidic intermediates formed [8,31,33], in addition to higher reaction severity, at higher temperature, led to cracking and isomerization reactions, even over low-acidity supports like MTS, as was also observed for CoMo-alumina (Table 5.3) [31]. NiMo/HZSM-5(HSAC), which has hierarchical porous, high surface area, crystalline HZSM-5 zeolite with framework-confined mesoporosity as support, furnished the highest yield of aviation kerosene (~60%) with negligible diesel, from jatropha oil. The same catalyst gave the highest reported yield of aviation kerosene (~80%) when algal oil was used as the feed (Fig. 5.1) [7].

5.2.2.2.4 Nonconventional Nonsulfided Catalysts

Jatropha and castor oil lipids were processed over reduced Pt and Ni catalysts on various supports (Table 5.4b). At low temperatures (270°C) conversions were low (15–30%), even at a high hydrogen partial pressure of 55 bar [28], but nearly complete (99%) conversions were achieved for castor oil at slightly higher temperature (300°C) and a low pressure of 30 bar, indicating that slight changes in reaction temperature have a strong influence on the conversion of lipids [27]. There was increased formation of cracked naphtha range products observed by Murata et al. [30] over a Pt/H-ZSM-5 catalytic system for jatropha feeds, whereas at similar conversion levels Pt/carbon nanotubes gave increased formation of kerosene range molecules (Table 5.4a). This may be attributed to the lower acidity of the carbon nanotube support for Pt as compared to that for H-ZSM-5. Ultrastable Y (USY) zeolites gave two times the conversion (31%) compared to other Pt-loaded catalytic systems (Table 5.4b), indicating Pt/USY as a better catalyst at lower temperature (270°C) for increased diesel production (98.6%). Upon increasing temperature to 300°C, 25% Ni/USY showed complete conversions of castor oil, and a naphtha yield of 79.4% and kerosene yield of 19.2% with negligible diesel yield (Table 5.4b). A similar yield pattern was also observed for H-Beta and USY catalytic systems with 25% Ni loading, and increased naphtha yield was reported. These observations indicate that such highly acidic catalytic systems with 25% Ni loading promoted cracking and isomerization reactions even at 300°C and a low pressure of 30 bar, whereas conventional sulfided Ni-Mo-, Co-Mo-, and Ni-W-based catalyst systems promoted these reactions at higher temperatures, >360°C (Table 5.4a). However, a major limitation for commercial success of the Ni/Zeolite-based catalysts is that they deactivate much more rapidly than conventional sulfided catalysts supported on moderately acidic silica-alumina supports.

Liu et al. [27] also demonstrated for castor oil lipids that a catalytic system with moderate acidity increased the desired aviation kerosene yield to as high as 80% (Table 5.4b) with increased i/n ratios. The yield of aviation range kerosene increased nearly six times using a combination of USY, APTES ((3-aminopropyl)-tri-ethoxysilane), and MCM-41 (Fig. 5.1). Addition of MCM-41 to USY zeolite blocked the strong acid sites, which resulted in greater yields of bio-aviation fuel range compounds and reduced naphtha range compounds (Table 5.4b) [27] (Fig. 5.1). Addition of APTES increased the mesoporosity of the system, which in turn also improved the diffusion (mass transfer) of bulkier triglyceride lipid molecules into the pores of the active sites. Moderate increase in APTES content (5 to 7.5) increased the aviation kerosene yield, but on further increase of the APTES content (7.5 to 10) the yield of cracked products (ie, naphtha and kerosene range molecules) started decreasing. This decrease in cracked products yield was attributed to blockage of active sites/zeolitic pores caused due to excessive use of APTES [27], in addition to negligible moderate acidity shown by high APTES content systems. Liu et al. [27] demonstrated that upon increasing the Si/Al ratio, for 25% Ni/ZSM-5 catalyst, the Lewis acidity decreased, which in turn decreased the yield of cracked lighter distillate products (Fig. 5.1), with gradual increase in middle and heavier distillates. Reduced secondary cracking reactions due to the reduction of strong acidity led to an increase in aviation kerosene yields upon increasing the Si/Al ratio.

Soybean oil was hydroprocessed over Pt-, Pd-, and Ni-based catalysts supported on carbon at 300°C (Fig. 5.1) [55]. Negligible secondary cracking responsible for formation of naphtha range product, and mild primary cracking, led to 15–20% kerosene range middle distillates for the Pt- and Pd-based catalysts. On the contrary, lipid hydroprocessing over 20% Ni supported over carbon yielded mostly kerosene range products (50%) (Fig. 5.1) under similar operating conditions. Studies by both Morgan et al. [46] and Liu et al. [47] over lipids from soybean and castor oils showed Ni-supported catalytic systems suitable for the production of aviation kerosene range hydrocarbons. Carbon-supported catalyst systems are inherently nonacidic in nature and do not catalyse cracking reactions. The higher yield of middle distillate kerosene range products (Fig. 5.1) observed over Ni-based catalyst systems indicated formation of acidic intermediates which promoted cracking of lipids at lower temperature (300°C).

Model triglyceride compounds, both saturated (tristearin, C₅₇H₁₁₀O₆) and unsaturated (triolein, C₅₇H₁₀₄O₆), were also hydroprocessed over Ni-, Pt-, and Pd-based systems supported on carbon [46]. Similar results with increased cracking (kerosene 46–47%) were observed over Ni-based catalyst as compared to Pt- and Pd-based systems (kerosene, 27–34%), along with increased conversion of 81–85% for Ni/C as compared to 22–47% in the case of Pt and Pd systems (Fig. 5.1). Product patterns observed over both saturated and unsaturated lipid molecules were similar over all the catalytic systems, with an exception of increased formation of internal alkenes in the case of triolein feedstocks, which were probably the result of reduced hydrogenation functionalities at 300°C [46].

Ni-based catalytic systems show a promising future for increased production of kerosene range products, but further experimentation and analysis is necessary for determining the catalyst life and properties of these products vis-a-vis specifications for aviation kerosene. Apart from these catalytic systems, various newer conventional and nonconventional catalytic systems are being developed for the production of kerosene range hydrocarbons, but their utilization is subject to the products meeting the ASTM 7566 or Jet A-1 test specifications for aviation kerosene. Ni-Mo and Ni-W catalytic systems supported over acidic mesoporous supports such as SiO₂-Al₂O₃ or ZSM-5 have shown desirable catalytic performance, with properties of kerosene range products meeting the ASTM 7566 specifications. These acidic mesoporous supports, supported with active metals with strong hydrogenation/dehydrogenation ability at 400–450°C temperatures and high hydrogen pressures (80–150 bar), yield the desired isomerization and deoxygenation of lipids to produce aviation range hydrocarbons. Continuous pilot-scale runs and catalytic evaluations have resulted in better understanding of these systems and catalytic life cycles for scale-up and commercial-scale productions. Reduction in reaction temperatures and pressures, with increased productivity of desired aviation kerosene fuel without compromising the quality of the fuel would result in reduced operating costs and increased profit margins, leading to rapid commercialization of these processes.

5.2.2.3 Isomerization Selectivity

The products from vegetable oil hydroprocessing can be used as liquid transportation fuels only if they meet required specifications, such as those defined by ASTM. Isomer-to-normal hydrocarbon ratio is a very important criteria for meeting the desired properties of all liquid transportation fuels. High i/n ratio is necessary for increasing the octane number of gasoline, and for reducing the freeze point of aviation kerosene or pour point of renewable diesel.

Isomerization reactions can occur during lipid hydroprocessing at the acidic sites on the catalyst surface. Generally, alumina support-based catalysts have low acidity and hence do not favour isomerization reactions. On the other hand, acidic supports such as silica-alumina and zeolites provide the required acidity for these reactions. Fig. 5.2 shows i/n hydrocarbon ratios for kerosene range hydrocarbons obtained by the hydroprocessing of various lipids over different acidic and nonacidic catalytic systems. Alumina-based catalytic systems showed almost negligible i/n ratio for algal- and jatropha-based hydroprocessed products at 370°C and 400°C, indicating a negligible effect of temperature on i/n ratio over these NiMo/Al₂O₃-based catalytic systems. Verma et al. [29] hydroprocessed algal and jatropha oil over various hierarchical structured and porous catalytic systems – H-ZSM-5-, SAPO-11-, and SiO₂-Al₂O₃-supported with both sulfided and noble metal-based hydrogenation/dehydrogenation functionalities. They demonstrated that crystalline HZSM-5 (low surface area crystalline (LSAC)) promoted more isomerization reactions as compared to semi-crystalline HZSM-5 (high surface area semi crystalline (HSASC)). Medium-strength acid sites (Brönsted acidity) were responsible for desirable isomerization reactions, whereas strong acidic sites (Lewis acidity) promoted undesired cracking reactions. Liu et al. [27] processed castor oil lipids over 25% Ni/ZSM-5 catalytic systems with varying Si/Al ratios at 300°C and 30 bar, wherein similar results were reported, in other words, increasing isomerization reactions with increasing Si/Al ratio, which was ascribed to lower Lewis acidity (with increasing Si/Al ratio) because of the decrease in extra-framework Al content. Acidic site distribution results shown by both Verma et al. [29] and Liu et al. [27] indicate a decrease in Lewis acidity (strong acid sites) upon increasing the Si/Al ratio. A decrease in the yield of lighter cracked products with increasing Si/Al ratio [27] also supported the fact that cracking was favoured over Lewis acidic sites and isomerization reactions over Brönsted acid sites. Novel mixed catalytic systems (USY, MCM-41, APTES) were also studied by Liu et al. [27] for hydroprocessing of castor oil, and i/n hydrocarbon ratios of 5–6 were obtained, which are comparable to the values obtained over most of the other catalysts, such as NiMo/HZSM-5 (Fig. 5.2).

Over NiMo catalytic systems similar i/n (2.2) were observed for algal oil lipids as well as jatropha oil lipids at 410°C for HZSM-5 (HSASC) supports. Upon increasing the temperature from 380°C to 400°C over HZSM-5 (LSAC) supports, the i/n hydrocarbon ratio increased three times, whereas such a drastic increase was not observed for any other catalytic systems, indicating NiMo/H-ZSM-5 (LSAC) systems were better-suited for isomerization reactions at higher temperatures. Higher isomerization selectivity (i/n=4.5) was reported for Ni-W/H-ZSM-5 (HSASC) as compared to Ni-Mo/H-ZSM-5 (HSASC) (i/n=2.5), indicating the role of the stronger hydrogenation/dehydrogenation ability of ‘W’ over ‘Mo’ supported catalysts for isomerization reactions. However, a similar observation was not reported by Verma et al. [34] in the case of hierarchical mesoporous SAPO-11-supported NiMo and NiW catalytic systems. A slight decrease in the i/n ratio for NiW (i/n=3.8) was reported as compared to that for NiMo (i/n=4.8) catalytic systems (450°C, 60 bar). These differences could be due to the differences in the acidities of the two catalysts. On further increasing the hydrogenation/dehydrogenation ability of the catalytic systems by using noble metal-supported catalysts (0.75% Pt/SAPO-11), a similar i/n ratio (i/n=4) could be obtained at slightly reduced temperatures and pressures (430°C, 50 bar). Even upon further reducing the temperature to 330°C and the pressure to 20 bar, the isomerization selectivity (i/n=4) was maintained, indicating that isomerization reactions were favoured over stronger hydrogenation/dehydrogenation functionalities. Different i/n ratios were obtained for algal and jatropha lipids, indicating that isomerization selectivities are also dependent on feed composition.

5.3 Kinetics, Reaction Mechanisms, and Pathways

The reaction mechanism and the pathway followed for lipid conversion into various hydrocarbons mainly depend on the type of catalyst system chosen for the conversion. Lipids from various sources mostly contain triglycerides, along with some diglycerides, monoglycerides, and free fatty acids. These molecules contain double bonds (Table 5.1) along with heteroatoms (mainly oxygen), which need to be removed prior to use.

Over a hydrotreating catalytic system, such as CoMo or NiMo supported over low acidity Al₂O₃, the unsaturated lipid molecules are primarily saturated and deoxygenated to yield a corresponding saturated hydrocarbon [21,23,45]. During deoxygenation, the glycerol linkage of the triglyceride molecule is broken to produce a propane molecule along with three corresponding carboxylic acid molecules (Scheme 5.1a) [7]. In the case of a multifunctional catalyst with cracking/isomerization functionality and hydrogenation/dehydrogenation ability, cracking and isomerization reactions are observed [7,31,32]. In addition, cyclization and aromatization reactions may also be observed if conjugated double bonds are present [5]. Gas phase compositions of products obtained over hydrotreating and hydrocracking catalysts for palm and jatropha oil (Table 5.5) indicate that 30–60% of the gaseous products is propane, which is predominantly produced from cleavage and hydrogenation of the triglyceride molecule (CO bond).

The oxygen present in these complex molecules can be removed either by deoxygenation reactions (ie, hydrodeoxygenation, H₂O) or by decarbonation reactions (CO and CO₂). Following the deproponation reaction, the intermediate acid molecule produced undergoes a deoxygenation reaction to yield hydrocarbon products. The depropanation and deoxygenation reactions can either occur simultaneously or consecutively – that is, depropanation to produce acid followed by deoxygenation of acidic molecules [23]. The route by which the lipid molecules or the acid intermediate molecules are deoxygenated (ie, the deoxygenation pathways) also depends on the processing conditions chosen and catalyst functionality (acidity/hydrogenation ability). Strong hydrogenation catalytic systems over nonacidic supports – that is, hydrotreating catalysts such as sulfided CoMo, NiMo supported over Al₂O₃ – favour hydrogenation reactions leading to the breakage of CO bonds. These bonds are present in the acid linkages and upon reaction form hydrocarbons as the major product with H₂O as the main by-product, along with small quantities of CO and CO₂; whereas hydrocracking catalysts with acidic supports such as silica-alumina, silicoaluminophospates, zeolites, etc., favour CC bond breaking. This leads to comparatively more CO₂ and CO formation than H₂O. Accordingly, increased molar yields of CO and CO₂ are observed over hydrocracking catalysts compared to hydrotreating catalysts (Table 5.5).

The produced deoxygenated hydrocarbons undergo further cracking and isomerization reactions to yield lower boiling point range hydrocarbons. Anand et al. [31] and Sharma et al. [32] studied various lumped kinetic models for hydroconversion of jatropha oil lipids over CoMo catalyst systems supported over Al₂O₃ and MTS. They considered all the possible reactions and the reaction products formed during the hydroconversion of lipids. Triglycerides in lipid feed were considered as a single entity and the liquid hydrocarbon products produced after hydroprocessing were lumped according to their corresponding carbon ranges – that is, gasoline (C₅–C₈), kerosene (C₉–C₁₄), gasoline+kerosene (<C₁₅), diesel (C₁₅–C₁₈), and oligomerized product (>C₁₈). Rate equations were framed for the models and reaction parameters were estimated for different reaction pathways based on the experimental results (Table 5.6). Titanosilicate-based catalyst systems showed reduced rates for triglyceride conversion at 320°C as compared to alumina-supported catalyst systems. The marginally higher acidity of MTS as compared to the alumina-supported catalyst [32], led to direct cracking of the lipid molecules into naphtha and kerosene (Scheme 5.1b), whereas in the case of the CoMo/Al₂O₃ system at 320°C, the triglycerides were initially deoxygenated to yield diesel range products, which were further cracked into lower range naphtha and kerosene range hydrocarbons (Scheme 5.1c). The conversion pathways for lipids over these catalyst systems were different, due to alternative metal support interactions and the different physicochemical properties of these systems. The MTS system catalysed an alternative pathway for the formation of lower range hydrocarbons directly from the triglyceride at lower temperatures. The rate of formation of primary cracked products (ie, gasoline and kerosene range compounds over MTS catalyst) were lower when compared to the Al₂O₃ system (Table 5.6). There was also increased conversion and corresponding diesel rate of formation in the case of Al₂O₃ catalyst systems as compared to MTS systems, indicating CoMo/Al₂O₃ systems were better-suited for the formation of diesel range compounds at lower temperature (320°C).

Upon further increase of the temperature to 360°C in the case of the CoMo/Al₂O₃ catalyst, a shift in the reaction pathway was observed from only deoxygenation reactions to direct cracking into naphtha and kerosene range products, along with internal conversion of diesel range products into cracked kerosene range products (Scheme 5.1d). There was an increased (four to six times) rate of formation of naphtha, kerosene along with a four-unit increase in the rate of formation for diesel range hydrocarbons upon increasing temperature (360°C) as compared to those obtained at lower temperature, 320°C (Table 5.6). The substantial cracking and change in the reaction pathway observed upon increasing temperature (320–360°C) was attributed to acidic intermediates produced over the CoMo/Al₂O₃ catalyst system, which promoted cracking reactions at 360°C. Reaction pathways were affected by the acidic intermediates produced during the hydroconversion of lipids over different catalysts. These intermediates promoted cracking reactions and changed the type of reactions that occurred. Over MTS these promoted mild cracking reactions along with oligomerization reactions at lower temperatures of 300°C and 320°C.

Upon co-processing the lipids from soy and jatropha oils a marginal increase in the rate of the HDS reaction was observed upon increasing the lipid concentration in gas oil (Table 5.6). A decrease in activation energy required for HDS reactions over NiMo/Al₂O₃ catalytic systems also supports the above conclusion. Reduction in the activation energy upon increasing the lipid concentration may be attributed to different feed/catalyst interactions that occur in the presence of lipids (which produces acidic intermediates). Activation energy on the order of 40 kJ/mole for the conversion of triglycerides were observed in the case of the MTS systems [32], compared to only 26 kJ/mole in the case of the Al₂O₃ system [31]. This indicated that different conversion pathways were followed over these catalytic systems at lower temperatures (<320°C). Very high activation energy for hydrodeoxygenation reactions was observed (130–150 kJ/mole) while co-processing lipids over NiMo/Al₂O₃ catalyst systems as compared to direct processing of lipids, indicating direct processing of lipids is advantageous due to the absence of competing HDS reactions.

A very high activation energy of around 127 kJ/mole was required for the conversion of jatropha oil lipids into aviation fuels over the CoMo/Al₂O₃ catalyst system. A catalyst system with lower activation energy for the formation of aviation kerosene would therefore be beneficial in terms of increasing yields as well as reducing the cost of production of these fuels.

5.4 Path Forward and Challenges

Aviation kerosene is a global commodity, and due to various global drivers, the aviation sector is at the brink of needing to develop alternative fuels. Many organizations such as DARPA, IATA, NATO, and the European Union (EU) have already taken enormous interest in the development of an alternative aviation fuel, funding projects that will bring green jet fuel to commercial flights and to strategic applications. It is expected that by 2050 biofuels will reduce aviation emissions by 80% [83,84]. United Airlines signed an agreement with AltAir to buy 15 million gallons of Bio-jet fuel over a period of three years starting in 2014. This fuel is for use in flights departing from their Los Angeles hub. AltAir Fuels will retrofit idle portions of a Los Angeles petroleum refinery to produce 30 million gallons of advanced biofuels and chemicals per year from nonedible natural oils and agricultural wastes using a two-step process. This renewable jet fuel is expected to achieve at least a 50% reduction in greenhouse gas emissions on a lifecycle basis [84].

In light of the above (both fuel demand and environmental issues), it can be said that aviation fuel from renewable sources has great potential and could provide a clear alternative solution to fossil fuels. For the commercial production of hydrocarbons from vegetable oil (lipid), major challenges with respect to hydroprocessing technology are as follows: (1) high hydrogen requirement, (2) high exothermicity of the reaction, (3) continuous feed supply for large production, and (4) feed pretreatment issues.

High hydrogen requirement: A hydrocracking unit is typically the largest hydrogen consumer in the refinery, and hydroprocessing of vegetable oil requires much more (nearly two to three times) hydrogen compared to a typical hydrocracker. In addition to typical hydrocracking reactions, other reactions like hydrodeoxygenation and saturation of a high percentage of unsaturated components in some vegetable oils like jatropha are the main reason for such high hydrogen consumption. The hydrogen requirement may also increase as unsaturated hydrocarbons form due to unwanted cracking reactions becoming saturated and requiring extra hydrogen, which adds additional cost to the process. For commercial viability, purification and recycling of the unused hydrogen is necessary to make the process economical. Even though simple water washing or amine washing is generally done to recycle unused hydrogen, it cannot remove certain impurities, specifically CO, which may adversely affect catalyst performance. New technologies like membrane purification give better hydrogen purity compared to conventional systems. Impurities such as CO, CO₂, and methane can be removed using such membrane systems to obtain the desired (99.9%) purity of hydrogen in the recycle stream.

High exothermicity of the reaction: The conversion of renewable feedstocks such as plant, animal, and algal oil triglycerides and free fatty acids into aviation fuel and other hydrocarbons is highly exothermic. Therefore, quench hydrogen requirements, reactor bed temperature controls, and high deactivation rates for the catalyst are major concerns for commercialization of the processes. The high exothermicity of these reactions also leads to excess cracking and coke formation reactions in the catalyst pores, further leading to high pressure drops, low catalyst life, and hence, a less economical process.

Continuous feed supply for large production: To make the process competitive, it is necessary to have a continuous supply of feedstock to be converted into fuel, while taking care to avoid food-versus-fuel issues and deforestation. Besides waste cooking oils, cultivation of oil-seed bearing trees (afforestation) could be one of the ways to increase the feed supply in an environment-friendly way. Government support in terms of policy making and funding is a must for advancing renewable aviation fuel technology.

Feed pretreatment issue: Most of the vegetable oil contains significant quantities of impurities (such as metals, Na, K, Ca, etc.) and large amount of phosphorus, which need to be completely removed prior to conversion into aviation fuel so as to maintain catalyst life. To increase catalyst life it is important to pretreat the feed as it contains metals that may cause catalyst poisoning. The metals in vegetable oil are generally present along with phospholipids. To remove these metals and phospholipids, a ‘water degumming’ process and guard bed catalysts are required.

Overcoming the above challenges can make the renewable aviation fuel from the hydroprocessing of lipids competitive with fossil-based aviation fuel. Fuel coming from renewable sources has marginally higher calorific value as well as low sulfur and aromatics content, which results in lower emissions compared to fossil fuels.

5.5 Conclusion

It is clear that there is a great need for alternative sources of aviation fuel. Due to stringent specifications for aviation fuels, higher investment in terms of engine/aircraft modifications, and complications in terms of fuel certification strategy, etc., alternative aviation fuels that are drop-in by nature are the most promising solutions to meeting increased fuel and environmental demands.

Hydroprocessing routes to the production of alternative aviation fuels have become a well-established technology, though economically not yet cost-competitive due to the higher cost of animal- and plant-derived triglycerides/lipids. Among different catalysts reported and discussed in this chapter, the sulfided mesoporous catalysts with moderate acidity and higher surface area are the best. Since these catalysts are similar to currently used hydrocracking catalysts, they are easier to retrofit in the current refinery infrastructure for large-scale production, even under co-processing conditions. As discussed above, if challenges like regular feedstock availability, control of exothermicity of the reaction by using modern strategies (liquid quench, heat integration), pretreatment of feedstock to increase the catalyst life, and recycle gas purification are met, and government support to farmers/suppliers to streamline these alternative feedstocks are provided, it is expected that aviation fuel could be produced from animal- and plant-based oils (lipids) competitively.