Biomass catalysis in conventional refineries

7 Biomass catalysis in conventional refineries J. A. MELERO, A. GARCÍA and J. IGLESIAS, Universidad Rey Juan Carlos, Spain

Abstract: The feasibility of standard refinery units for the production of biofuels from different biomass-derived feedstocks is highlighted. Special attention is focused on the catalytic cracking and hydrotreating of triglyceriderich biomass feedstock, probably the most suitable for co-processing in conventional refinery conversion units. Opportunities of other highly oxygenated feedstocks such as pyrolysis oils and sugars are also discussed. Conversion of aqueous sugar streams into conventional liquid fuels by coupling of aqueous-phase reforming (APR) with catalytic systems typical of standard petroleum refineries is evaluated. The chemistry, catalysis and challenges involved in the production of biofuels by the co-feeding of biomass feedstock with petroleum streams are reviewed. Key words: triglycerides, sugars, bio-oils, catalytic cracking, hydrotreatment, fuels.

7.1

Introduction

The greatest challenges for the twenty-first century are the reduction of global warming and the provision of energy for transport, heating and electricity. Nowadays, fossil fuels are the main sources for fuels and energy, but the limited deposits of these fossil resources coupled with environmental risks have prompted mankind to look for sustainable resources as alternative to meet the increasing energy demand. Biomass is one of the few resources that has the potential to meet the challenges of sustainable and green energy and hence its use is expected to grow in the foreseeable future. Indeed, several governments have implemented mandatory legislation to increase gross domestic energy from renewable resources, especially biomass. For instance, the US Department of Energy set ambitious goals to derive 20% of transportation fuel from biomass by 2030. On the other hand, the European Union has set a mandatory target of 20% for renewable energy’s share of energy consumption by 2020, as well as a mandatory minimum target of 10% of renewable energy sources in transport. These ambitious targets have contributed to the intensified interest in the development of technology and processes for biomass valorization. Fortunately, the worldwide production capabilities for renewable and sustainable biomass production are enormous. For instance, in the US over 370 million and 1 billion dry tons of biomass are obtained from forest and agricultural lands, respectively. Similarly, large biomass production capacity is available in Europe, which could produce 190 million tons of oil equivalent (Mtoe) of biomass by 2010 with an expected increase up to 199 © Woodhead Publishing Limited, 2011

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300 Mtoe by 2030. To help reach the ambitious goals set by the United States and European Union, a similar system to that used in petroleum refining, called ‘biorefinery’, has been proposed in the future. The chemical and energy integration of biomass transformations in biorefineries is still in its early beginnings and, in the short-to-medium term, biorefinery development will likely incorporate existing petroleum refinery infrastructure to circumvent high capital costs in processing biomass feedstocks. Hence, one promising alternative for the production of biofuels is the co-processing of biomass (cellulosic biomass and triglyceride-based biomass) in conventional oil refineries (Huber and Corma, 2007). This alternative involves the co-feeding of biomass-derived feedstocks with typical petroleum streams in conventional refining units. This strategy has significant advantages as compared with conventional processes for biofuels production. Petroleum refineries are already built and hence the use of existing infrastructure for the production of biofuels would require little capital investment. Furthermore, the production process does not require either secondary reagents, or yields by-products that should have a market share. Finally, a wide range of biofuels produced by means of the same procedures as conventional fuels could be obtained, not only gasoline and diesel, but also LPG, jet fuel, kerosene and fuel oil. Indeed, different oil companies are already investigating this possibility and developing some industrial processes. Two options are suitable for converting biomass-derived feedstocks into biofuels in a petroleum refinery: catalytic cracking and hydrotreating. Cracking reactions in a petrol refinery can be carried out in the presence of a catalyst (FCC unit) and in its absence (thermal units). Thermal units are not considered to be of interest for the production of biofuels since the resulting organic liquid product contains a high oxygenated compound content independent of biomass feedstock, and reduces its interest as transport fuel. In contrast, catalytic cracking is faster and more selective than thermal cracking, which allows working under milder reaction conditions and hence minimizing yield of non-desirable products such as gases, coke and heavy fractions, while maximizing the production of liquid fractions suitable for use as fuel for transport. Hydrogen-based processes are typically more expensive than cracking because these require hydrogen, and hydrogen consumption is even higher when biomass feedstock is processed because of its higher oxygen content in comparison with conventional fuel sources. On the contrary, hydrotreating processes display a higher selectivity towards liquid fraction, minimizing gas and coke production. Figure 7.1 briefly outlines the possibilities for the production of biofuels in a standard refinery unit. The overall challenge with biomass conversion is how to efficiently remove oxygen from the biomass feedstock and produce a molecule that has a high energy density and good combustion properties as current petroleum-derived fuels (reaction 7.1). Catalytic cracking and hydrotreating are very effective at removing

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7.1 Routes to produce biofuels in conventional refineries.

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oxygen from biomass-derived feedstocks. However, the oxygen is not always removed by the optimal pathway, and often undesired products such as coke or acids are formed during the process. CxHyOz → a Cx-b-d-e Hy-2c Oz-2b-c-d + b CO2 + c H2O + d CO + e C [Reaction 7.1] In this sense, Chen et al. (1986) have defined the effective hydrogen index (H/Ceff) as the amount of hydrogen in the fuel which is available for energy production, where H, C, O, N and S correspond to the moles of hydrogen, carbon, oxygen, nitrogen and sulphur, respectively, that are present in the feed (Equation 7.1). [Equation 7.1] As seen in Fig. 7.1, this index for highly oxygenated biomass feedstocks is clearly lower than 1, which means that this feedstock is mainly formed by hydrogendeficient molecules. This index for a mixture of hydrocarbons ranges from 2 (liquid alkanes) to 1 (for benzene). In contrast, triglyceride-based biomass (nonedible vegetable oils and animal fats as well as waste cooking oil) shows a hydrogen index of circa. 1.5 which is quite similar to that of a hydrocarbon mixture. These different values induce distinct chemistry involved in catalytic process which results in different product distribution. In this chapter, the possibilities for converting biomass-derived feedstocks in FCC and hydrotreating refinery units are reviewed. Also discussed are the novel and innovative catalytic procedures for converting sugars-based compounds into conventional liquid fuels (Huber and Dumesic, 2006). This technology involves the coupling of aqueous-phase reforming (APR) with catalytic systems typical of standard petroleum refineries to yield non-oxygenated hydrocarbons.

7.2

Biomass feedstock: availability and diversity

The main biomass feedstocks suitable to be fed into standard refineries can be grouped into two wide categories: carbohydrates and triglycerides. The total current biomass production on the planet is estimated at around 170 billion tonnes and consists of roughly 75% carbohydrates (sugars), 20% lignin and 5% of other substances in minor amounts such as oils, fats, proteins, terpenes, alkaloids, terpenoids and waxes. Carbohydrates (from sucrose, starch, cellulose and hemicellulose) are molecules formed of carbon, hydrogen and oxygen and these are, by far, the most common biomass components found in plant feedstocks. 6-carbon, single-molecule ‘monosaccharide’ sugars (C6H12O6) include glucose, galactose and mannose, while the most common 5-carbon sugars (C5H10O5) are xylose and arabinose. Sucrose is a disaccharide of fructose and glucose which is easily hydrolysable and

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is present in sugar cane and sugar beet. Starch (C6H10O5)n is a polysaccharide composed of glucose molecules, being the most widespread starch crops wheat and corn. Lignocellulosic biomass has three major components: cellulose (40– 50%), hemicellulose (25–35%) and lignin (15–20%). Cellulose is also a polysaccharide of glucose. The distinction from starch is given by the configuration of the bonds formed across the oxygen molecule that joins two hexose units. Starch can be readily hydrolyzed by enzymes or acid attack to the single sugar monomers, while cellulose is much more difficult to hydrolyzed and set free individual glucose monomers. Hemicellulose is a relatively amorphous component that is easier to break down with chemicals and/or heat than cellulose; it contains a mix of C6 and C5 sugars. Lignin is essentially the glue that provides the overall rigidity to the structure of plants and trees and is made of phenolic polymers. Unlike cellulose and hemicellulose are polysaccharides, which can be hydrolyzed into sugars; lignin cannot be used for this purpose. Likewise, lignocellulosic feedstocks can be pyrolyzed in a thermochemical process in absence of oxygen and temperatures in the range of 300–600 °C to yield bio-oil, charcoal and gases. Nevertheless, bio-oils need to be catalytically upgraded in order to enhance their properties as liquid fuels (reduction of oxygen content and acidity). Large amounts of cellulosic biomass can be produced via dedicated crops like perennial herbaceous plant species, or short rotation woody crops. Other sources of lignocellulosic biomass are wastes and residues, like straw from agriculture, wood waste from the pulp and paper industry, and forestry residues. Therefore there is a very wide range of sugar sources that can be found readily available from the utilization of biomass. Many of these can be generated from biomass resources that cannot be utilized as food and can be harvested on land not suitable for crop generation. Triglyceride molecules are the main component of vegetable oils and animal fats. These molecules consist of glycerine coupled in ester form to saturated and unsaturated fatty acids (their chain length ranges between C8 and C20, but 16, 18 and 20 carbons are the most common). The sources of oils and fats are a variety of vegetable and animal raw materials. Soybean, palm, rapeseed and sunflower oil are the most important in terms of worldwide production. Vegetable oils are nowadays mostly used for production of biodiesel by reacting with an alcohol, usually methanol. Like sugar and starch crops, oilseed crops are characterized by low yield and high use of inputs. In the future, non-edible crops, which require lower inputs and are suited to marginal land, may become the most widespread oil crops for biofuels production, especially in dry and semi-arid regions. Other sources of vegetable oils for biofuel conversion can be found in waste streams of food industry, where waste edible oil is mainly generated from commercial services and food processing plants such as restaurants, fast food chains and households.

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7.3

Catalytic cracking of biomass feedstock

The fluid catalytic cracking (FCC) unit is the most widely used process for the conversion of crude oil into gasoline and other hydrocarbons because of its flexibility to variability in the feedstock and product demands. The reactions occurring in the FCC process include cracking reactions (cracking of alkanes, alkenes, naphthene and alkyl aromatics to lighter products), hydrogen transfer, isomerization and coking reactions (Corma et al., 1994). FCC catalysts usually contain mixtures of Y-zeolite within a silica-alumina matrix, a binder, a clay and some additives. Renewable feedstocks suitable to be fed into FCC units include highly oxygenated biomass such as bio-oils, glycerol, lignin and sugars, as well as triglycerides with low oxygen content.

7.3.1 Catalytic cracking of highly oxygenated feedstock (bio-oils, lignin, glycerol and sugars) Bio-oils are produced by pyrolysis processes where the biomass feedstock is heated in the absence of air, forming a gaseous product, which then condenses. The properties of bio-oils depend on the specific feedstock and conditions of the production process, such as temperature, period of heating, ambient conditions and the presence of oxygen, water and other gases. In fact, bio-oils are a complex mixture of water (15–30 wt%) and different oxygen-containing structures (35–50 wt%), such as hydroxyaldehydes, hydroxyketones, sugars, carboxylic acids and phenols. Due to their oxygen-rich composition, they present low heating value, immiscibility with hydrocarbon fuels, chemical instability, poor volatility, high viscosity, corrosiveness, etc. (Adjaye et al., 1992). Therefore, bio-oils must be upgraded or blended to be used as a replacement for diesel and gasoline fuels. Bio-oils can be upgraded using zeolite catalysts to reduce oxygen content and improve thermal stability (Bridgwater, 1994). Reaction conditions used for these processes are temperatures from 350–500 °C, atmospheric pressure and gas hourly space velocities around 2. The products from this reaction include hydrocarbons (aromatic, aliphatic), water soluble organics, water, oil soluble organics, gases (CO2, CO, light alkanes) and coke. During this process a high number of reactions occur, including dehydration, cracking, polymerization, deoxygenation and aromatization. However, poor hydrocarbon yields and high yields of coke generally occur under reaction conditions, limiting the usefulness of zeolite upgrading. Adjaye and Bakhshi (1995a) studied the FCC upgrading reactions of a number of bio-oil model compounds in a fixed-bed reactor over HZSM-5 catalyst. Also, the reactions of a synthetically prepared volatile feed and bio-oil volatiles over HZSM-5 catalyst were investigated. The objective was to understand and identify the reactions steps involved in the conversion of bio-oil. Based on the results, it is

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7.2 Proposed reaction pathway for the conversion of bio-oil in the presence of HZSM–5 (adapted from Adjaye and Bakhshi, 1995a).

suggested that the bio-oil is initially separated into volatile and non-volatile fractions as soon as the reaction temperature is reached (Fig. 7.2). The non-volatile fraction (which accounts for 37 wt% of the fresh bio-oil) is cracked to yield mainly volatile compounds and a residue by means of polymerization reactions. In addition, some of the non-volatile components also react to form coke. The resultant volatile fraction reacts through processes such as deoxygenation, secondary cracking, oligomerization, olefin formation, hydrogen transfer, cyclation, disproportionation, alkylation and isomerization reactions to form the hydrocarbon-rich product. Polymerization and condensation reactions of some of the oxygenated aromatic compounds and hydrocarbons result in more residue and coke formation. Also, deoxygenation occurs through dehydration to produce water in the aqueous fraction and through decarbonylation and decarboxylation to produce carbon oxides. The hydrocarbon gases are formed from cracking reactions of volatile fraction. Bakhshi and co-workers tested different types of catalysts (HZSM-5, H-Y, mordenite, silicalite and silica alumina) for the upgrading of bio-oil produced from wood (Adjaye and Bakhshi, 1995b, 1995c; Katikaneni et al., 1995a; Sharma and Bakhshi, 1993). Acidic zeolite catalysts revealed to be more effective in the conversion of bio-oil to hydrocarbons. The HZSM-5 catalyst produced the highest amount (34 wt% of feed) of organic liquid products among the tested catalysts, while providing the lowest coke formation. Silica-alumina catalyst was the most effective for minimizing the char formation and H-Y catalyst was superior in minimizing tar formation as well as maximizing the production of

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7.3 Proposed reaction pathway for the conversion of bio-oil (adapted from Adjaye and Bakhshi, 1995c).

aliphatic hydrocarbon. Reaction pathways were proposed for the conversion of bio-oil (Fig. 7.3). It was postulated that bio-oil conversion proceeded as a result of a thermal step followed by a thermocatalytic one. The thermal step promoted separation of biooil into light organic and heavy organic compounds as well as induced the polymerization of bio-oil to char. The thermocatalytic step produced coke, tar, gas, water and the desired organic distillate fraction. Deoxygenation, cracking, cyclation, aromatization, isomerization and polymerization were the main thermocatalytic reactions. In line with this reaction pathway, Bakhshi and co-workers have also developed a two reactor process, where only thermal reactions occur in the first empty reactor, and catalytic reactions take place in the second reactor that contains the catalyst (Srinivas et al., 2000). The advantage of the double reactor system lays on the enhancement of the catalyst life by reducing the amount of coke deposited on the catalyst. The transformation of model bio-oil compounds, including aldehydes, ketones, acids, alcohols, phenols and mixtures over HZSM-5 catalysts, has been largely studied by Gayubo et al. (2004a, 2004b, 2005). They observed that the studied families presented great differences in their reactivity and each one showed a particular reaction pathway. The temperature also influenced the product distribution. Nevertheless, most of the biomass-derived molecules produced large amounts of coke when passed over acidic zeolite catalysts. As a consequence, the same group has carried out studies focused on bio-oil stabilization treatments to minimize coke deposition on the catalyst and to attenuate deactivation. They have demonstrated that co-feeding methanol (around 70 wt%) minimizes coke deposition within and outside the catalyst particles, thereby increasing the viability of crude bio-oil upgrading (Gayubo et al., 2009). Furthermore, the deposition of coke might also be controlled in a specific step of thermal treatment prior to the catalytic reactor, minimizing deposition on the catalyst and thereby attenuating deactivation (Gayubo et al., 2010).

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A possible alternative for the utilization of bio-oils in refineries is their co-processing with the typical FCC feedstock. Graça et al. (2009) have studied the catalytic cracking of mixtures of model bio-oil compounds and gasoil. Model bio-oil compounds representing some of the major oxygenated groups, such as acetic acid, hydroxyacetone and phenol, were added to a standard FCC unit feed, in order to reproduce the bio-oils co-feeding. The mixtures were tested under fluid catalytic cracking (FCC) conditions in a laboratory-scale unit using an industrial FCC equilibrium catalyst (E-CAT) and a mixture of E-CAT and ZSM-5 additive. As a general trend, acetic acid, phenol or hydroxyacetone when mixed with a conventional gasoil increased the overall conversion, reduced the coke yield and increased fuel gas, LPG and gasoline production. On the other hand, the ZSM-5 additive effect was attenuated in presence of the bio-oil compounds, suggesting a preferential interaction of such compounds with the ZSM-5. In the case of phenol and gasoil mixtures, important coke yields were observed. Based on the obtained results, the authors concluded that up to 10 wt% of the studied oxygenated compounds could be processed without major problems in a FCC unit, with the exception of phenol, which might be critical due to the benzene content specification in the gasoline. At the present time, lignin is a by-product of little to no commercial value in the pulp and paper industry, except for its use as a low-grade fuel in Kraft pulping, the dominant pulping process. However, it would be highly desirable to produce value-added products from lignin by breaking its structure to yield aromatic gasoline and propane. Thring et al. (2000) studied zeolite upgrading of lignin with HZSM-5 zeolite as catalyst in a fixed bed reactor operating at atmospheric pressure, over a temperature range of 500–650 °C, and weight hourly space velocities of 2.5–7.5 h−1. The liquid product fraction, which consisted of mostly aromatic hydrocarbons (mainly benzene, toluene and xylene, with toluene dominating), was maximized at 500 °C and a space velocity of 5 h−1. On the other hand, the gas product consisted of olefins, light hydrocarbon gases, CO and CO2 and it was produced at the highest yield at 650 °C and a space velocity of 5 h−1. Among the light hydrocarbon gases produced from the lignin, ethylene and propylene were the olefins produced in the highest quantities. Coke and char formation was particularly high at the low reaction temperatures employed in this work but decreased rather drastically with increasing temperature. For instance, at a space velocity of 5 h−1, 50 wt% of the lignin was converted to coke and char when a reaction temperature of 500 °C was used, compared to only 21 wt% at 650 °C. Small FCC pilot tests were run to determine the crackability of pyrolysis oil and pyrolytic lignin blended with VGO, acting this latter as a hydrogen donor (Marinangeli et al., 2006). Compared to VGO, the pyrolysis oil and pyrolytic lignin tend to form high coke levels. For the blends of VGO with pyrolysis oil or pyrolytic lignin, the acid bio-oils appeared to increase the crackability of the VGO and shift VGO yields toward increased light ends and lower LCO and clarified

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slurry oil (CSO), which is an economically attractive outcome. Nevertheless, the high coke levels obtained with both blends (7 and 9%, respectively) would be unacceptable for most FCC units. Glycerol is produced from biomass through fermentation of sugars and mainly by transesterification of vegetable oils during biodiesel production. The glycerol market is currently undergoing radical changes, driven by very large supplies of glycerol arising from biodiesel production. Glycerol is currently too expensive to be used as a fuel; however, as biodiesel production increases, the price of glycerol will decrease. Corma et al. (2007) studied the catalytic cracking of aqueous glycerol and its mixture with VGO in a microactivity test (MAT) reactor at 500–700 °C with six different catalysts, including a fresh FCC catalyst, an equilibrium FCC catalyst with metal impurities, a mesoporous Al2O3, an USY zeolite, a ZSM-5-based FCC additive and an inert silicon carbide. Products from this reaction include olefins (ethylene, propylene and butanes), aromatics, light paraffins (methane, ethane, propane), CO, CO2, H2 and coke. ZSM-5 produced lower coke levels (less than 20% molar carbon yield) and higher levels of aromatic and olefins, whereas other catalysts gave rise to high yields of coke (30–50%) and lower levels of aromatics and olefins in the catalytic cracking of glycerol. When glycerol is fed together with VGO, interactions between the hydrocarbon components and the glycerol reaction intermediates occur, resulting in final selectivities better than those calculated by considering a simple additive effect. These experiments showed that mixtures of VGO with biomass-derived feedstock can help to transfer hydrogen from the VGO to the biomass molecules. One option for further improving the olefin and aromatic yields for co-feeding of glycerol and petroleum-derived feedstocks into a FCC reactor might involve adding ZSM-5 to the FCC catalyst, because ZSM-5 produced more olefins and lower coke amounts than FCC catalyst. Chen (1976) discussed the conversion of carbohydrate materials to petroleumtype hydrocarbons. The process comprised microbial conversion of agricultural carbohydrate materials to alcohols followed by direct conversion of the oxygenated microbial reaction product to a hydrocarbon product comprising a substantial highly aromatic fraction. This latter conversion was carried out in the presence of a ZSM-5 zeolite at about 260–540 °C. Latter, Chen and co-workers (Chen et al., 1986; Chen and Koening, 1990) passed concentrated sugars, including glucose, xylose, starch and sucrose over ZSM-5 at temperatures from 300 °C to 650 °C and observed hydrocarbon, CO, CO2, coke and water as products. The addition of methanol to the feed decreased the amount of coke and increased the hydrocarbon products. The hydrocarbon products consisted of gaseous alkanes (methane, ethane, propane), liquid alkenes and alkanes (butane, pentene, hexane) and aromatics (benzene, toluene, C8–C10 aromatics). One of the problems of this reaction is that when methanol is not used, 40–65% of the carbon is converted into coke. Importantly, this pioneering works showed that sugars can be converted to hydrocarbons by dehydration, decarbonylation and decarboxylation reactions.

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The overall reaction may be written as follows (Reaction 7.2): [Reaction 7.2] Summarizing, catalytic cracking of highly oxygenated feedstocks yields high production of coke which is unacceptable for conventional FCC units (typical coke yields ranging from 1–2 wt%) and hence previous upgrading steps are necessary. Likewise, it has rarely achieved a complete removal of oxygen in the hydrocarbon phase.

7.3.2 Catalytic cracking of triglyceride-based feedstock Catalytic cracking of vegetable oils (e.g. palm, canola, soybean) using acid catalysts such as zeolites (HZSM-5, H-Y and H-mordenite) (Bhatia et al., 1998; Idem et al., 1997; Katikaneni et al., 1995b, 1995c, 1996; Leng et al., 1999; Milne et al., 1990; Ooi et al., 2005; Prasad and Bakhshi, 1985; Prasad et al., 1986a, 1986b; Twaiq et al., 1999), Al- containing mesostructured materials (Al-MCM-41 and Al-SBA-15) (Bhatia et al., 2009; Demirbas, 2009; Idem et al., 1997; Ooi et al., 2004, 2005, 2007; Twaiq et al., 2003a, 2003b) and amorphous materials (alumino-silicates, pillared clays and alumina) (Boocock et al. 1992; Katikaneni et al., 1995b, 1995c; Idem et al., 1997; Vonghia et al., 1995) have been broadly reported in literature to yield mixtures of hydrocarbons suitable as transport fuels. The reaction is usually performed at moderate to high temperatures (300–500 °C) using different oil to catalyst ratios depending on the oil and the catalyst. General reaction pathway of the catalytic cracking of a triglyceride molecule over an acid catalyst is depicted in Fig. 7.4 (Melero et al., 2010a). Once the triglyceride molecule has been primarily decomposed to heavy oxygenated hydrocarbons such as fatty acids, ketones, aldehydes and esters, their reactions to reach other products start by means of the breaking of the C–O and C–C bonds by β-scission reactions. The breaking of the C–O and C–C bonds follows two competitive routes: (1): decarboxylation (CO2) and decarbonylation (CO) reactions followed by C–C bond cleavage of the resulting hydrocarbon radicals or (2): C–C bond cleavage within the hydrocarbon section of the oxygenated hydrocarbon molecule followed by decarboxylation and decarbonylation of the resulting short chain molecule (Idem et al., 1996). The occurrence of these different reaction routes depends on the presence of double bonds in the initial oxygenated hydrocarbon. Whereas C–C bond breaking in α and β position is favoured in presence of unsaturated hydrocarbons molecules, decarboxylation and decarbonylation reactions take place before C–C bond cleavage for saturated oxygenated hydrocarbons since the less endothermic bonding in a saturated hydrocarbon chain is the one associated to the β position of the carbonyl group (Osmont et al., 2007). These secondary cracking reactions finally yield CO, CO2 and water as main oxygenated compounds and a mixture

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7.4 General reaction mechanism for the catalytic cracking of triglyceride molecules over acid catalysts (Melero et al., 2010a).

of hydrocarbons produced by different reactions such as β-scission, hydrogen transfer, isomerization, cyclation or aromatization, some of them being possible because of the presence of an acid catalyst in the reaction system. Furthermore, coke is also formed by means of polymerization reactions (Maher and Bressler, 2007). Products usually obtained by means of the catalytic cracking of vegetable oils and animal fats are depicted in Fig. 7.5 (Melero et al., 2010a). They are usually grouped in an ‘organic liquid product’ (OLP; gasoline, kerosene and diesel fractions), gaseous products (hydrocarbons C1–C5, CO, CO2) water and coke. The oxygen initially present in the feedstock is removed as water (which is easily isolated), CO and CO2. Therefore, there is not a remarkable presence of oxygenated hydrocarbons in the final organic cracking products. The catalyst properties (e.g. crystalline nature, shape selective effect), the reaction conditions (temperature, pressure, space velocity, presence of steam, type of reactor . . .) and the nature of feedstock dramatically influence the conversion and yield towards the different reaction products. Generally, the presence of zeolites increases the yield towards the OLP fraction whereas amorphous catalysts predominantly produce higher amounts of gases (Katikaneni et al., 1995c; Idem et al., 1997; Pioch et al., 1993). Co-feeding steam

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7.5 Simplified scheme of products coming from the catalytic cracking of triglyceride molecules over an acid catalyst (Melero et al., 2010a).

during the reaction process helps to increase both the olefinic compounds formation as well as the durability of the catalyst. This fact takes place because the presence of steam diminishes the coke formation and thus the catalyst deactivation (Katikaneni et al., 1995b). The use of a fluidized bed instead of a fixed bed generally reduces the selectivity towards the OLP fraction due to the shorter contact time which diminishes the possibility of forming liquid hydrocarbons from the olefins C2–C5 oligomerization reactions (Katikaneni et al., 1997). From all the different studies, an OLP with a high concentration of aromatics have been obtained (over 50%) as well as high triglyceride conversion (>80%). Furthermore, the almost null presence of oxygenated hydrocarbons in the final cracking products is confirmed by studies (Katikaneni et al., 1995c, 1997; Leng et al., 1999; Twaiq et al., 2003a). The authors have shown that although the initial decomposition of triglyceride molecule is mainly a thermal process, in the subsequent secondary cracking reactions (hydrogen transfer, isomerization, oligomerization, β-scission, aromatization) the acid catalyst has a crucial role (Twaiq et al., 2003a). Several research centers, universities and companies have been working for years on the co-processing of renewable raw materials in FCC refining units. In the performed studies, the technical feasibility has been shown of co-processing of vegetable oils (palm, rapeseed, soybean or sunflower oils), waste cooking oil and animal fats and vacuum gasoil under FCC conditions (Buchsbaum et al., 2004; Bormann and Tilgner, 1994; Bormann et al., 1993; Carlos de Medeiros et al., 1985; Pinho et al., 2007; Melero et al., 2010b). The operating conditions recorded, as well as the final products obtained after the catalytic cracking reactions, are perfectly compatible with the conditions and products usually

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ascribed to the FCC unit. However, there is a strong effect of the feedstock composition upon the cracking products distribution. The main differences in the processability of the typical petrol feedstock and its mixtures with vegetable oils and animal fats are the production of oxygenated compounds derived by the presence of oxygen molecules in the initial triglycerides of the renewable raw materials. In a typical refinery FCC unit, these oxygen molecules end up mainly as carbon-derived gases (CO and CO2) and, above all, water. The usual conditions of the FCC unit are severe enough to decompose the heavy oxygenated hydrocarbons by means of decarboxylation, decarbonylation or dehydration reactions to yield a mixture of different hydrocarbons. Beyond the production of non-value oxygenated compounds (which are easy to separate from the valuable ones), it is possible to find several differences between the cracking of a hydrocarbon and a vegetable oil in FCC conditions. Triglyceride-based biomass, which lacks of aromatic and refractory compounds, enhances the gas production (a fact even more accentuated in case of the most saturated renewable feedstock) and always reduces the yield towards liquid fractions, especially liquid cycle oil (LCO) and mainly towards decanted oil (DO) (Melero et al., 2010b) (Table 7.1). These results are associated with the higher crackability of vegetable oils and animal fats in comparison with the petrol feedstock. Hence, the gasoline content in the organic liquid product is always enhanced as the percentage of vegetable oil is increased in the initial feedstock (Bormann et al., 1993; Carlos de Medeiros et al., 1985). For example, Bormann et al. (1993) indicate that the percentage of gasoline in the liquid products rises from a 60.3% to 61.1%, when using rapeseed oil instead of vacuum gasoil in cracking experiments. Similar Table 7.1 Products yields from catalytic cracking of feedstocks with different palm oil content. Reaction temperature of 565 °C and a catalyst-to-oil ratio of 4 g catalyst/g oil. Palm oil/VGO (wt%) Product yields (wt%)

0/100

Dry gas

30/70

100/0

2.7

3.8

4.5

LPG

18.8

21.6

24.2

GLN (C5–221 °C)

47.2

42.8

38.2

LCO (221–360 °C)

19.6

16.6

9.9

DO (>360 °C)

8.9

6.0

2.1

Coke

2.8

4.2

5.4

Carboxylic gases (CO + CO2)

1.6

5.3

Water

3.5

10.3

Source: Melero et al., 2010b. LPG: liquid petroleum gases, GLN: gasoline, LCO: liquid cycle oil, DO: decanted oil, VGO: vacuum gas oil.

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results have been obtained by Carlos de Medeiros et al. (1985), whose yield to gasoline in OLP increased 8.6 points when cracking soybean oil instead of the typical vacuum gasoil. This better crackability of triglyceride-based biomass is also clearly confirmed by Melero et al. (2010b) in their gasoline distribution, where the medium (MN; 90–140 °C) and heavy (HN; 140–221 °C) naphtha yields are gradually reduced with the presence of vegetable oil in the feedstock (Table 7.2). In the processing of triglyceride-based materials, decanted oil will be produced by means of polymerization reactions, in contrast with the petrol feedstock, whose decanted oil may be considered as the non-converted fraction. Another important difference in the co-processing of vegetable oils in an FCC unit is their higher yields towards aromatic compounds (Table 7.2). These aromatic compounds are mainly monoaromatic chores, which would end in the gasoline (GLN) fraction (raising its octane number) and diaromatic chores. The yield towards polyaromatic compounds is reduced by the presence of a renewable feedstock, since it does not contain these refractory molecules in its initial Table 7.2 Olefinity of LPG, naphtha distribution in GLN and aromatic content and distribution in the liquid effluent obtained by the catalytic cracking of feedstock with different contents in palm oil (reaction temperature of 565 °C and catalysts-to-oil ratio of 4 gcatalyst goil−1) Palm oil/VGO (wt%) 0/100

30/70

100/0

C3=/C3 TOTAL

0.83

0.80

0.80

n–C4=/C4 TOTAL

0.46

0.45

0.46

i–C4=/C4 TOTAL

0.15

0.13

0.12

LN (C5–90 °C)

40.36

43.71

51.88

MN (90–140 °C)

21.36

18.97

16.65

HN (140–221 °C)

38.28

37.32

31.47

Olefinity of LPG

Naphtha distribution in GLN (wt%)

Aromatic content (wt%) Monoaromatics

26.98

29.41

36.64

Diaromatics

18.61

20.16

22.54

Polyaromatics

16.72

16.06

11.73

Total

62.31

65.63

70.91

Source: Melero et al., 2010b LPG: liquid petroleum gases, GLN: gasoline, LN: light naphtha, MN: medium naphtha, HN: heavy napthha, VGO: vacuum gas oil

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composition, and hence they cannot remain as non-converted molecules in the final products. The high aromaticity of the liquid fraction when cracking renewable raw materials is associated with the removal of hydrogen from the triglyceride molecules to form water (as the main oxygenated compound), increasing olefins formation, which tends to end as aromatic compounds (Dupain et al., 2007). In the case of vegetable oils cracking products, high aromatic contents have been described by several authors. For instance, Adjaye et al. (1995) achieved a liquid with more than 95 wt% of aromatics content after their cracking experiments with rapeseed oil and ZSM-5. Obviously, in the same way that excessive hydrogen elimination from hydrocarbons would produce a higher yield of aromatic compounds, if the removal of hydrogen continues, an increase in the coke production will be produced (highly favoured in the case of the renewable raw materials because of water formation) (Dupain et al., 2007; Melero et al., 2010b). Therefore, coke production is enhanced with the increase of triglyceridebased biomass in the feedstock (Melero et al., 2010b; Ramakrishan, 2004; Buschsbaum et al., 2004; Carlos de Medeiros et al., 1985) as clearly stated in Table 7.1. Recently, a US Department of Energy funded collaboration between UOP, the National Renewable Energy Laboratory and the Pacific Northwest National Laboratory assessed the economics of biofuels integration in petroleum refineries (Holmgren et al., 2007; Marinangeli et al., 2006). Different options were identified for processing vegetable oils and greases in refineries. The use of FCC technology was evaluated for the catalytic cracking of vegetable oils and greases to produce gasoline and liquid cycle oil (LCO). Interestingly, a pretreatment unit to remove catalytic poisons such as alkali metals, and other problematic components such as water and solids, was considered as a crucial step (Fig. 7.6). The pretreated feed can then be introduced as a co-feed with vacuum gas oil (VGO) to yield gasoline and other products. A modified catalytic cracking process was also proposed to yield high-value products such as ethylene and propylene (severe conditions to maximize olefin production).

7.6 Processing approach for catalytic cracking of vegetable oil (Holmgren et al., 2007; Marinangeli et al., 2006).

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Hydrotreating of biomass feedstock

The objective of hydrotreating in a petroleum refinery is to remove sulphur (hydrodesulphurization, HDS), nitrogen (hydrodenitrogenation, HDN), metals (hydrodemetalation, HDM) and oxygen (hydrodeoxygenation, HDO) from the heavy gas oil feedstock. Hydrogen is added with the heavy gas oil feed. Typical catalysts used for hydrotreating include sulphided Co-Mo and Ni-Mo as active phases, and typical reaction conditions consist of temperatures ranging 300– 450°C, pressures of 35–170 bar H2 and liquid hourly space velocities (LHSVs) of 0.2–10 h−1.

7.4.1 Upgrading of bio-oils and co-processing in fluid catalytic cracking (FCC) units The options for directly using bio-oils in refineries are affected by the high acid number, high water content, high oxygen content and high metal content, particularly potassium and calcium. Metals can be removed with guard beds or ion exchange. The low thermal stability, high water content and very high oxygen content of bio-oils make difficult its blending with common refinery intermediate streams such as VGO. Moreover, other serious problem for bio-oils processing comes from the high acid number, which causes corrosion in standard refinery units. The industry standard for refinery vessels is that the total acid number of the blend must be less than 1.5 mg KOH/g. Bio-oils can probably be processed using 317 stainless steel cladding, which is not standard in refinery units. Therefore biooils would require pre-processing in a 317 stainless steel system to reduce their acid number before their processing in typical refinery units (Marinangeli et al., 2006). Since the FCC is the biggest unit and the heart of most refineries, much more development work would be required to minimize risk to the refinery before such an approach was viable. As an alternative to blending, co-processing bio-oil with petrol feedstock in an FCC unit might be possible if a separate feed system were used to inject the bio-oil. Hence, the direct feeding of bio-oils into a standard refinery does not appear to be a straightforward task. Hydrodeoxygenation can be used to reduce bio-oils acidity and oxygen content and to convert bio-oils into a more stable fuel with a higher energy density that has the potential to be blended with petroleum-derived feedstock. Hydrodeoxygenation of bio-oils is performed at moderate temperatures (300–600 °C) with high H2 pressure in the presence of heterogeneous catalysts. Most hydrodeoxygenation of bio-oils has focused on sulphided Co-Mo- and Ni-Mobased catalysts, which are used for hydrotreating industrial feedstocks. When sulphided Co-Mo and Ni-Mo catalysts are used, sulphur must be added to the biooil, otherwise catalyst deactivation will occur. Non-sulphided catalysts, including Pt/SiO2-Al2O3 (Sheu et al., 1988), vanadium nitride (Ramanathan and Oyama, 1995) and Ru have also been used for hydrodeoxygenation.

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The reactions involved are quite similar to those occurring in the hydrotreating of petroleum fractions. The catalytic hydrodeoxygenation has been reviewed by Furimsky (2000) and the following order of reactivity of oxygen-containing groups has been put forward: alcohols > ketones > alkyl ethers > carboxylic acids = m- and p-alkyl-substituted phenols = naphthol > phenol > diaryl ethers = o-alkyl-substituted phenols = alkylfurans > benzofurans > dibenzofurans. In view of available biomass feedstocks, the reactivities of phenols, acids and esters are the most relevant. Phenols, which may account for up to 25 wt% of liquids obtained by pyrolysis of lignocellulosic materials, are refractory oxygenates. The overall mechanism for the hydrodeoxygenation of the o-substituted phenols shown in Fig. 7.7 includes two main hydrodeoxygenation reactions, direct hydrodeoxygenation and hydrodeoxygenation via hydrogenated phenol, occurring in parallel (Furimsky et al., 1986). In the latter case, H2O elimination may result in the formation of the intermediate methylcyclohexene species, which are hydrogenated rapidly. The formation of cyclohexene, alkylcyclohexenes and methylcyclopentanes is also shown in Fig. 7.7, although these are only minor products. Guaicacol (GUA) and substituted GUAs have attracted much attention because of their relatively high content in bio-oils and low stability. The hydrotreatment of guaicacol in the presence of CoMo and NiMo catalysts was studied in a batch reactor by Laurent and Delmon (1994a, 1994b). They proposed the mechanism shown in Fig. 7.8, which considers the hydrogenolysis of the

7.7 Mechanism of hydrodeoxygenation of 2–methylphenols (adapted from Furimsky et al., 1986).

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7.8 Mechanism of hydrodeoxygenation of guaiacol (adapted from Laurent and Delmon, 1994a).

methoxy group of guaiacol to catechol and methane as the first stage, followed by the elimination of one OH group from catechol in the second stage to produce phenol. Coke was formed from both guaicacol and the primary product catechol. Elliott and co-workers developed a two-step hydrotreating process for the upgrading of bio-oils using sulphided Co-Mo/Al2O3 or sulphided Ni-Mo/Al2O3 catalysts (Elliot et al., 1988; Elliot and Neuenschwander, 1996). The first step involves a low-temperature (270 °C, 136 bar H2) catalytic treatment that hydrogenates the thermally unstable bio-oil compounds, which would otherwise undergo thermal decomposition to form coke and plug the reactor. The second step involves catalytic hydrogenation at higher temperature (400 °C, 136 bar H2). During this process, 20–30% of the carbon in the bio-oil is converted into gasphase carbon, decreasing the overall yield. Catalyst stability and formation of gums in the lines were identified as points of major uncertainty of the process, and future work is needed to develop improved hydrotreating catalysts. Bio-oil was hydrotreated at high pressures (138–172 bar) and low space velocities (0.1–0.2 LHSV) by Marinangeli et al. (2006). At these high pressures and low space velocities, hydrodeoxygenation predominates. Large quantities of hydrogen are required to generate water during hydrodeoxygenation because of the high level of oxygen (46%) in bio-oil. The resulting hydrotreated oil was then cracked in an FCC or hydrocracker to produce gasoline. This approach is unlikely to be commercially viable because of the high hydrogen requirement and the high capital cost of the hydrotreatment step. The challenges of feeding hydrodeoxygenated pyrolysis oil in standard refineries have been also studied by de Miguel Mercader et al. (2010). Different HDO reaction end temperatures (230–340 °C) were evaluated in a 5 L autoclave, keeping the other process conditions constant (290 bar, 5 wt% Ru/C catalyst), in order to find the required oil product properties necessary for successful FCC co-processing (miscibility with FCC feed and good yield structure: little gas/coke production and good boiling range liquid yields). After hydrodeoxygenation, the upgraded pyrolysis oil underwent phase separation resulting in an aqueous phase, some gases (mainly CO2 and CH4) and an oil phase. Although the oil and the aqueous phase yields remained approximately constant when the HDO reaction

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temperature was increased, a net transfer of organic components (probably hydrodeoxygenated sugars) from the aqueous phase to the oil phase was observed, increasing the carbon recovery in the oil product (up to 70 wt% of the carbon in pyrolysis oil). The upgraded oils were subsequently tested in a lab-scale catalytic cracking unit (MAT reactor), assessing the suitability of HDO oils to be used as FCC feed. In spite of the relatively high oxygen content (from 17–28 wt%, dry basis) and the different properties of the HDO oils, they all could be successfully dissolved in and co-processed (20 wt%) with a long residue, yielding near normal FCC gasoline (44–46 wt%) and light cycle oil (23–25 wt%) products without an excessive increase of undesired coke and dry gas formation, as compared to the base feed. Samolada et al. (1998) reported a two-step process of thermal hydrotreatment and catalytic cracking of biomass flash pyrolysis liquids (BFPLs). Thermal hydrotreatment of BFPLs can be effectively operated producing liquid products which can be upgraded in a refinery. The heavy liquid product of this process (HBFPL), mixed with light cycle oil (LCO) (15/85 wt%/wt%), was considered as a potential FCC feedstock. Commercially available cracking catalysts were found to have an acceptable performance. The obtained bio-gasoline quality is comparable with that of the VGO cracking but with low yields ∼20 wt%. The upgrading of BFPL to transportation fuels by using a hydrotreating technology was also studied in the Chemical Process Engineering Research Institute (CPERI) in collaboration with Veba Oil (Lappas et al., 2009). With the thermal hydrogenation of bio-oil a deoxygenation conversion of 85 wt% was achieved, producing hydrotreated oil with oxygen content of about 6.5 wt%. Due to the low-oxygen content of the thermally hydrotreated bio-oil, it can be separated by distillation in a light and a heavy fraction. The light fraction comprised components mainly in the gasoline and diesel range, and thus it could be directly blended with the corresponding petroleum fractions. The heavy fraction has similar characteristics to conventional VGO. This heavy fraction could be used as co-feed with vacuum gasoils (VGO). The bio-oil co-processing technology proposed by Lappas et al. (2009) is shown in Fig. 7.9. The co-processing of VGO with the heavy fraction showed that the presence of the bio-oil fraction favours the gasoline and diesel production but increases the coke yield. However, depending on the concentration of biomass liquids, it was shown that this option is technically viable for FCC units running with good quality feedstocks, that is, the FCC unit with excess coke burning capacity. Fogassy et al. (2010) have recently evaluated the impact of adding 20 wt% HDO-oil to a conventional FCC feedstock. The VGO and bio-oil mixtures were co-fed into a fixed-bed reactor simulating FCC conditions using an equilibrated industrial FCC catalyst. Co-processing of 20 wt% HDO-oil with VGO gave comparable yields for the gasoline fraction to that of the pure VGO cracking. However, during co-processing, oxygen removal from HDO-oil oxygenated components consumes hydrogen coming from the hydrocarbon feedstock

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7.9 Bio-oil co-processing technology investigated by Lappas et al. (2009).

7.10 Reaction pathways for the catalytic cracking of HDO-oil oxygenates (adapted from Fogassy et al., 2010).

(Fig. 7.10). As a result the final product composition is poor in hydrogen and contains more coke, aromatics and olefins. Moreover, the phenolic fraction was not completely converted.

7.4.2 Hydrotreating of triglyceride-based feedstock: green diesel Biodiesel is usually prepared by the transformation of vegetable oils and fats through triglyceride transesterification towards fatty acid methyl esters (FAME). This well-established conversion method allows upgrading of these renewable energy resources to get diesel-type fuels, showing high energy content with low

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amounts of oxygen (Huber et al., 2007). However, the final products achieved through this methodology display several properties that do not match the specifications for automotive fuels – for instance high density and boiling point and poor stability against oxidation (Tang et al., 2008), leading to several technical problems, like filters between the fuel tank and engine becoming blocked (Donnis et al., 2009). Besides, the production of FAME leads to the formation of glycerol as a by-product which needs to be marketed – a difficult task because of the abundance of this chemical – to make the process profitable. These and other problems do not make attractive the transesterification of triglyceride oils to conventional petroleum refineries. As an alternative, the transformation of triglycerides in a refinery hydrotreating unit to form lineal alkanes allows production of diesel-like fuels. Though hydrotreating, because of the needing for hydrogen, is much more expensive than other refinery alternatives, such as catalytic cracking, the products achieved through this pathway – green diesel – are pure hydrocarbons indistinguishable from their petroleum counterparts. In fact, green diesel displays a high cetane number, showing good potential to meet current specifications for petroleum-derived diesel-like fuels. In this sense, several studies reveal a meaningful decrease of hydrocarbons, carbon monoxide and nitrogen oxides in diesel engine emissions when using green and conventional diesel mixtures instead of petroleum-derived diesel as fuels (Soveran et al., 1992; Stumborg et al., 1996; Aatola et al., 2008). Apart from these beneficial features, the green diesel option to process triglyceride-oils displays an important advantage over the transesterification production process in hydrotreating units that are actually present in refineries, where high vacuum gasoil (VGO) is already treated with hydrogen. Chemical compatibility allows processing of these feedstocks together with crude-derived fractions in the current infrastructure (Holmgren et al., 2007), so that there is no need to build new plants (Huber and Corma, 2007). From a chemical point of view, the total hydrogenation of triglycerides leads to n-alkanes and propane as the main reaction products and water, CO and CO2 as by-products. This process involves several reactions that can be lumped into two reaction pathways (Donnis et al., 2009; Semejkal et al., 2009): hydrodeoxygenation (HDO) and hydrodecarboxylation (HDC). The former involves the formation of long n-alkanes showing the same number of carbon atoms than the original fatty acid alkyl chain as well as propane, coming from the hydrodeoxygenation of glycerine. On the contrary, hydrodecarboxylation involves the loss of a carbon atom – the atom economy suffers some decrease – coming from the carboxylic group at the fatty acid chain. In this way, the main n-alkanes obtained as products display one less carbon atom than the original fatty acid alkyl chain, this carbon being lost as carbon monoxide or dioxide. An overview of these reaction pathways is depicted in Fig. 7.11. Considering the hydrodeoxygenation mechanism, the mass balance indicates the need for 12 moles of hydrogen per mole of triglyceride plus an additional mole of H2 per double bond present in the fatty acid alkyl chains. This is the faster

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7.11 Proposed scheme for triglyceride hydrotreating main reaction mechanisms. Major products have background tint.

transformation, and thus occurs in the first term, when hydrotreating triglycerides (Huber and Corma, 2007). Thus, for instance, the total hydrogenation of rapeseed oil (4 double bonds per mole) by HDO needs of 16 moles of hydrogen per mole of triglyceride, leading to a mixture of water (six moles), propane (one mole) and a mixture of n-C18 and n-C22 (three moles) – rapeseed oil fatty acid profile is mainly composed by oleic, linoleic and linolenic (C18 ∼35 wt%) acids and erucic acid (C22 ∼ 40 wt%). On the other hand, if considering the hydrodecarboxylation mechanism, only 3 moles of hydrogen are needed to process a mole of triglyceride plus the additional H2 to reduce each double bond (7 moles of H2 per mole of rapeseed oil). This comparison suggests hydrodecarboxylation should be favoured over the hydrodeoxygenation to reduce hydrogen consumption, a critical issue in the profitability of hydrotreating units, but apart from the main reactions proposed in Fig. 7.11, several other chemical gas-phase transformations have to be considered in the HDC mechanism. The mere presence of CO2 in the reaction system leads to the existence of methanation, or at least the partial reduction of the same, as well as the water gas shift reaction (Snåre et al., 2009), though to a minor extent (Fig. 7.12). Thus, it is important to consider these reactions when evaluating the process economy since they lead to the consumption/production of hydrogen.

7.12 Secondary reactions taking place during triglycerides hydrotreating.

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Thus, when considering the methanation of carbon dioxide – or its transformation into carbon monoxide and subsequent methanation – together with the hydrodecarboxylation of starting triglycerides, twelve additional hydrogen moles should be added to the three already considered from the HDC transformation, plus the hydrogen consumed in reducing hydrocarbon bonds. Hydrodecarboxylating rapeseed oil would need 19 moles of hydrogen per mole of triglyceride, leading to n-C17 and n-C21 as main products (three moles), water (six moles), propane (one mole) and methane (three moles). If considering the hydrogen consumption, it seems that the hydrodeoxygenation pathway is more attractive than hydrodecarboxylation, but such a low difference between H2 consumption by both mechanisms and the similarities observed on the final hydrocarbon products do not allow determination of which is the best option, being dependent on the process and on the catalyst used in the hydrotreating unit. The most important challenge when using triglyceride-containing vegetable oils in conventional refinery hydrotreating units seems to be the development of an adequate catalytic technology for treating these new biomass feedstock, but the design of the catalyst depends on the desired reaction pathway. As previously stated, the hydrotreating of triglycerides can follow two reaction pathways, hydrodeoxygenation and hydrodecarboxylation, and both alternatives have been a matter of intensive research looking for suitable catalytic technology for driving these reactions. However, in both cases, the reported research has followed different paths (Krár et al., 2010): current hydrodesulphuration technology in petroleum industry has been investigated for the oxygen removal of triglycerides, because hydrodesulphuration and hydrodeoxygenation are, a priori, rather similar reactions. On the contrary, triglyceride decarboxylation has been investigated through the development and use of a different catalytic technology: supported noble metals (Snåre et al., 2006). The hydrodecarboxylation route in hydrocarbons production from free fatty acids has been investigated through the use of different conventional hydrogenation catalysts, employing a huge variety of active metal species supported on few catalytic supports. Reaction experiments indicate that supported metal carbonbased catalysts display a much higher selectivity towards hydrodecarboxylated products than analogue catalysts based on different supports (Snåre et al., 2006). With regards to the active metal species, palladium and platinum display a much better catalytic performance than other metals like ruthenium, rhodium or iridium. However, in terms of the efficient use of the hydrogen, which is the key factor in hydrotreating units, palladium is superior to platinum because the former mainly drives decarboxylation reactions, whereas platinum produces hydrodecarbonylation – which is the removal of the carboxylic group of free fatty acids leading to carbon monoxide and water, a transformation which involves a higher hydrogen consumption. Nevertheless, the preparation conditions used for these Pd/C materials exert a dramatic influence on their catalytic behaviour, the particle size of the final supported metal species being one of the most important variables determining the catalytic activity (Simakova et al., 2009).

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The use of these catalysts for treating different feedstocks has also been reported. Thus, free fatty acids, alkyl esters and triglycerides have been assayed, leading in every case to the same major products, which are the hydrocarbons – both saturated and unsaturated – formed by an alkyl or alkenyl chain with one carbon atom less than the original fatty acid chain (Kubicˇková et al., 2005). However, the hydrodecarboxylation reaction rate is rather low in comparison with double-bond hydrogenation rate. Besides, hydrotreating free fatty acids seems to be rather effective and fast, being more difficult than the hydrodecarboxylation of alkyl esters (Snåre et al., 2008) and even more complicated triglycerides. These differences seem to be caused by a different reaction mechanism. Thus, whereas the deoxygenation of free fatty acids follows a hydrodecarboxylating pathway, in the case of fatty acid alkyl esters it mostly proceeds via hydrodecarbonylation (Mäki-Arvela et al., 2007), though both types of reactions coexists in both cases (Snåre et al., 2009), and the dominant one can be tuned depending on the catalyst and the reaction conditions; for example, increasing hydrogen concentration enhances the decarboxylating activity. Bearing in mind that hydrodesulphuration and hydrodeoxygenation involve analogous reactions, triglyceride oxygen removal by hydrodeoxygenation seems to be a rather easy task to implement on conventional refinery hydrotreating units used for hydrodesulphuration of petroleum-derived streams. Hydrodesulphurization (HDS) is a widespread and mature technology conventionally used in refineries. HDS usually coexists with HDO and hydrodenitrification (HDN) for the removal of sulphur, oxygen and nitrogen, respectively. The most important industrial catalysts used for HDS conventionally involve alumina-supported molybdenum and tungsten sulphides as main catalytic species, usually promoted with cobalt and/or nickel (Kubicˇka, 2008). Nevertheless, due to the higher catalytic activity, Co-Mo and Ni-Mo are the most widespread catalysts in hydrotreating units (Babich and Moulijn, 2003). These catalysts are employed because of their high resistance against sulphur poisoning, in contrast with noble metals which display a much higher hydrogenating activity but a lower resistance against deactivation with sulphur. Despite the chemical analogy between hydrodeoxygenation and hydrodesulphuration, important different behaviours are found for the HDS catalysts when hydrotreating triglyceride-containing feedstocks in comparison with petroleum-derived streams. These differences are associated with the feedstock nature and the final form of the heteroatoms to be removed (oxygen and sulphur), even though the reaction pathways and mechanisms are rather similar in HDS and HDO. Thus, hydrodesulphurization leads to the formation of sulphidric acid (H2S) whereas hydrodeoxygenation leads to the formation of water (H2O), both of them interacting with the surface of sulphided catalysts (Ferrari et al., 2001). It is well known that sulphidric acid promotes the acid catalytic activity of these catalysts (Laurent and Delmon, 1994b), enhancing the reaction rate of the acid-driven transformations. On the contrary, water seems to exert a negative

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influence on the catalytic activity of HDS catalysts, but this is only because of the presence of H2S, since H2S and H2O compete for their coordination to metal sites (Laurent and Delmon, 1994b). In case of hydrotreating sulphur-free feedstocks, like triglyceride containing feedstocks, H2S is absent and thus water is free to interact with hydrogenating sites, leading to the inhibition of the hydrogenation reactions (S¸enol et al., 2005a). The work from S¸enol et al. (2005b) described the hydrogenation of model aliphatic methyl esters in presence of alumina-supported sulphided CoMo and NiMo catalysts as a sequence of three reaction pathways: the formation of alcohols which evolve towards hydrocarbons by dehydration, the deesterification between the alcohol and the carboxylic acid functionality and the hydrogenation of the carboxylic acid towards hydrocarbon, either passing through the alcohol or not. Thus, dehydration, hydrolysis and hydrogenation reactions are present at the same time when hydrotreating these triglyceride-containing feedstocks, the two first reactions being promoted by acid catalysis whereas the last one is driven by the hydrogenating activity. In fact, some decarboxylating activity (S¸enol et al., 2005b) has also been found to be present, which could not be avoided, at least under the employed reactions conditions. This last reaction is also driven by the acid sites of these catalysts, whose presence is associated to sulphydryl acid groups (Ferrari et al., 2001). A proof of this behaviour is the low hydrodecarboxylating activity of the same catalysts when used as non-sulphided metal oxides (S¸enol et al., 2005b). One possibility to avoid this deactivation is to drive these HDO reactions in the presence of sulphidric acid, but the enhancement of acid activity leads to a meaningful increase of the extent of hydrodecarboxylation reactions (Laurent and Delmon, 1994b; Ferrari et al., 2001). On the contrary, hydrogenolysis and hydrogenation reactions are consequences of the presence of sulphur vacancies associated with molybdenum atoms (Leliveld et al., 1998). When treating more realistic feedstocks like sunflower (Krár et al., 2010) or waste cooking oils (Bezergianni et al., 2010a, 2010b), the observed reaction pathways were in essence the same as those previously described for model compounds, suggesting that the performance of conventional hydrodesulphuration catalysts was almost the same. However, several interesting results are found when treating triglyceride feedstocks. Increasing the reaction temperature leads to a much higher extent in the removal of oxygen, though this enhancement on triglyceride conversion is accompanied by a much higher rate in hydrodecarboxylation, isomerization and even hydrocracking reactions, which are more important when using NiMo/γ-Al2O3 catalysts (Gusmão et al., 1989; da Rocha Filho et al., 1993; Šimácˇek et al., 2009). Thus, though the triglycerides conversion is higher insofar as the operation temperature increases, the yield towards green diesel decreases – a proof of this fact is the decreasing of the cetane number in the final product or the increasing of the bromine index when operating at high temperature (∼400 °C). In this way, it seems preferable to be operating at

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lower temperatures and recirculating the heavy fraction and the residue to maximize the yield towards biodiesel. Some modifications of conventional hydrodesulphuration catalysts have been reported, looking for better catalytic activity of the same in triglyceride HDO treatments. Most of these improvements lay on the modification of the catalytic support, mainly tackled through the enhancement of the support surface area (Kubicˇka et al., 2009) or the assay of different supports like silica or silica alumina mixed oxides (Liu et al., 2009; Kubicˇka et al., 2010). Nevertheless, though the performance of these modified HDS catalysts is good there are still scarce examples of the adaptation of the same for hydrotreating triglyceride-containing feedstocks, and much more effort needs to be made. As an alternative, the use of conventional HDS catalyst for treating mixed blends composed by triglyceridecontaining oils and petroleum-derived oils is now becoming of more and more interest to petroleum refineries. This strategy is readily applicable in conventional refineries without the need to implement large modifications to existing hydrotreating units. The coprocessing of vegetable oils with petroleum-derived streams has been tackled through the use of conventional hydrodesulphuration catalysts instead of supported noble metals because of the sulphur resistance of the former. Thus, Huber et al. (2007) reported the use of sunflower oil together with heavy vacuum oil (HVO) as feedstock to be treated in a hydrotreating unit in presence of a conventional sulphided NiMo/γ-Al2O3 catalyst. Interestingly, this treatment option produced, under certain conditions, much better results than treating both feedstocks separately. Thus, treating both feeds together led to a higher amount of straight alkyl chains in the range C15 to C18 than if treating pure sunflower oil – the given reason for this improvement is that dilution of free fatty acids (FFA) inhibits polymerization and hydrocracking reactions (Lappas et al., 2009). Besides, since hydrodesulphuration is a much slower reaction than alkane production from the vegetable oils, the use of feedstocks mixtures does not affect the rate of desulphuration. Similar findings to this pioneer work have also been found using CoMo/ γ-Al2O3 catalyst in the hydrotreatment of cottonseed oil (Sebos et al., 2009). In addition, quality enhancement of several properties of the final products were also found, like a cetane index (Bezergianni et al., 2009) which showed higher values compared to that achieved when treating only petroleum fractions. The direct application of the existing infrastructure in petroleum refineries for treating petroleum streams–vegetable oils mixtures makes possible extensive industrial application in the near future. Much important industrial experience has been gained in the use of vegetable oils and animal fats in existing industrial hydrotreating units, and much research effort has been carried out by oil refining companies. Most of these developments consist of modified gasoil hydrotreating processes to which a blend of gasoil and triglyceride-containing mixtures is fed (Mayeur et al., 2009; O’Connor et al.,

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7.13 Flow diagram and reaction steps of the UOP/ENI Ecofining process.

2008). However, Neste Oil OYJ, a Finnish oil refining company, has taken a step forward and has licensed a new process, NExBTL, for the production of green diesel from pure vegetable oils and fats (Snåre et al., 2009; Markkanen et al., 2010). The first two production plants (170 000 tons per year) for this process were built in Porvoo refinery. In addition, two new plants, one in Rotterdam and another in Singapore, are now under construction (800 000 tons per year). A similar process is that licensed by UOP/ENI, the Ecofining process (Kalnes et al., 2007; Baldiraghi et al., 2009), which also involves the same two reaction stages as the NExBTL process (Fig. 7.13). Thus, the two distinct stages comprise the hydrodeoxygenation step into which triglyceride-based biomass and hydrogen are fed. Obviously, as previously stated, not only HDO reactions occur during this step, but HDC and subsequent methanation also takes place in this first reaction step. The second stage involves the hydroisomerization of the deoxygenated product to improve its cold properties. Light ends can be used to produce hydrogen, which is then recycled to the reaction stages in order to increase the profitability of the processes. In essence, both processes, NExBTL and Ecofining, are rather similar and the final products coming from the process are the same: light naphtha, propane and green diesel. This last is the major component, comprising more than 80 wt% of the final products. Final properties of this green diesel are rather similar to those achieved through the production of Fischer–Tropsch liquid fuel from syngas. Table 7.3 lists the properties calculated for diesel-like fuels, including green diesel and its blending with conventional diesel fuel. These fuels are featured by a high cetane number, which involves very good engine efficiency, low oxygen content,

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Table 7.3 Properties for different diesel-like fuels obtained through different techniques Diesel fuel1

Property

Biodiesel2

Green diesel3 Blend4

Cetane number

53

50

70–90

Oxygen content (wt%)

0

11

0

Sulphur content (wt ppm)

<10

<10

<1

Distillation (°C)

180–360

340–355

265–320

57.8 0 4.7 249–341

Lower heating value (MJ kg−1)

35.7

38

44

36.5

Cloud point (°C)

−5

−5

−10–20

−4.1

Stability

Baseline

Low

Baseline

Baseline

Specific gravity (kg m−3)

835

883

780

827

1 4

Aatola et al., 2008; 2 Huber and Corma, 2007; 3 Kalnes et al., 2007; Mayeur et al., 2009.

which makes green diesel more similar to petroleum-derived fuels than conventional biodiesel, and very low sulphur content. The boiling range of green diesel is rather similar to that of conventional diesel fuel, preventing vaporization problems in the combustion chamber. Besides, green diesel displays very high stability in contrast with biodiesel. Finally, the low specific gravity of this fuel makes possible the upgrading of low-value and high-density refinery streams. With regards to the economic and environmental impact of these processes in conventional petroleum refineries, the lifecycle assessment studies of both the Ecofining (Kalnes et al., 2007, 2009) and the NExBTL (Gärtner et al., 2006) processes indicates that these processes are competitive with biodiesel when operating moderately sized units. However, the profitability of the processes depends on the price differential between crude petroleum and renewable plant oils. From an environmental point of view, the production processes for green diesel are clearly superior to conventional diesel fuel production, both in terms of energy consumption and greenhouse gas emissions, though, in these cases, differences with conventional diesel fuels can become negligible depending on the starting triglyceride-containing oil or even the farming practices employed.

7.5

Production of conventional liquid fuels from sugars

One of the main drawbacks of several technologies described above for the synthesis of biofuels is the need for large quantities of hydrogen, which involves high operation costs that have to be compensated for through higher prices for the obtained fuel products, making these renewable feedstock-derived fuels less competitive in comparison with those obtained from petroleum. One alternative

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for the required hydrogen is the catalytic reforming of oxygenated hydrocarbons coming from renewable biomass sources, such as ethanol (Cruz et al., 2008), glycerol (Wen et al., 2008), sugars (Davda and Dumesic, 2004) or even waste paper and wood (Valenzuela et al., 2006), in liquid water under moderate temperature conditions, called aqueous-phase reforming (APR). This procedure allows obtaining large quantities of hydrogen in liquid phase, in which the water– gas shift reaction is favourable, leading to the production of H2 with very low amounts of carbon monoxide in a single step reaction – in contrast to conventional processes used for the production of hydrogen from conventional processes using non-renewable sources of hydrocarbons. The starting point of aqueous phase reforming is the availability of oxygenated hydrocarbons, which can be obtained from a huge variety of biomass feedstocks. These are transformed into several oxygenated compounds such as polysaccharides, sugars, furans and lower molecular weight compounds (Rinaldi and Schüth, 2009; Rinaldi et al., 2010) which can be processed through APR to obtain hydrogen and light oxygenated hydrocarbons. The reactions taking place in the hydrogen production by APR involves cleavage of C–C, C–H and O–H bonds. These bonds are easily cleaved on the surface of several catalysts, mainly based on metals with hydrogenating activity, where carbon monoxide is formed. Subsequent transformation of CO into CO2 through the water–gas shift reaction leads to the formation of hydrogen, as the main hydrogen production pathway. Depending on the starting oxygenated hydrocarbon, the reaction mechanism is more or less complicated, but in any case, these can be summarized as C–C and C–O bonds cleavage reactions, dehydration, hydrogenation and dehydrogenation reactions, as it is shown in Fig. 7.14. In this way, the transformation of oxygenated hydrocarbons by APR allows obtaining multiple possible products, ranging from hydrogen to alkanes. Bearing in mind the complicated and multiple reaction pathways taking place during APR of oxygenated hydrocarbons, if hydrogen production is desired, discerning requirements are needed for the catalyst to be used in APR transformation. First, the catalyst must promote C–C bond cleavage and CO removal from the surface of the same. The first reaction is needed because it leads to the formation of hydrogen and carbon monoxide. The second is needed because carbon monoxide depresses the activity of the catalysts, thus CO must be removed from the surface of the catalyst to continue with the cleavage of more C–C, C–H and O–H bonds. On the contrary, the catalyst should present very low activity in C–O cleavage and CO/CO2 methanation reactions, because both of them lead to hydrogen consumption. Most of the catalysts used in these transformations involve one or more metal species showing hydrogenating activity, such as Pd, Pt, Ni, Ru, Rh or Ir (Huber and Dumesic, 2006), or their mixtures, all of them supported on different solids, like alumina, silica or carbon, as the most employed supports (Shabaker and Dumesic, 2004; Tanksale et al., 2007).

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7.14 Reaction pathways involved in APR of oxygenated compounds (adapted from Davda et al., 2005).

Though hydrogen production through biomass-derived APR is an attractive option, the alkane synthesis pathways have attracted much attention in the last years, as it allows production of biofuels in a single step from sugar-based renewable feedstock (Chheda et al., 2007; Huber et al., 2004). A proof of this interest is the process licensed by Virent Energy Systems, known as the BioForming process (Dumesic and Roman-Leshkov, 2009), whose main steps are depicted in Fig. 7.15. This process takes advantage of APR as the core of the procedure, generating intermediate oxygenated hydrocarbons achieved through the treatment of biomass-derived products such as sugars. These sugars, and their derivatives, are treated by hydrogenolysis and hydrogenation to form light oxygenated alkanes which are fed to the APR step, in which the hydrogen required for the previous steps is produced. During the APR stage, several products are produced, as well as hydrogen, and though depending on the nature of the starting compound, these can be classified as alcohols, acids, ketones and aldehydes. Some fraction of these derive into hydrogen and carbon dioxide during the APR, whereas the remaining fraction are subsequently treated through different pathways (condensation, hydrodeoxygenation, dehydration, oligomerization . . .) to form larger alkanes which can be used as fuels (diesel, gasolines or kerosene). Since the reaction routes to achieve the alkane fraction as the main products are different to those required for maximizing the hydrogen production, a different catalyst is required for the APR stage in the BioForming process. Thus, not only hydrogenation, C–C and C–O bonds cleavage are required, but also dehydration

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© Woodhead Publishing Limited, 2011

7.15 Virent BioForming process flow diagram.

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reactions are needed, so that the employed catalysts should present a bifunctional activity. On the one hand, the metal species are required to promote the already described reactions for H2 production; on the other hand, acid activity is needed to drive the dehydrogenation reactions. As an illustrative example, Fig. 7.16 depicts the reaction routes occurring in the transformation of glucose to different alkanes, involving APR, dehydration, hydrogenation and condensation transformations. Thus, Virent’s BioForming process combines the APR procedure for hydrogen production with several hydrogenation, dehydration and base-catalyzed condensation reactions to prepare saturated hydrocarbons, suitable for the formulation of liquid fuels, and starting from renewable biomass derived carbohydrates such as sugars. Preliminary economic analysis suggests that converting sugars to conventional liquid fuels using this technology can economically compete with petroleum fuels at crude oil prices greater than US $60/bbl.

7.6

Future trends

Co-feeding biomass-derived molecules into a petroleum refinery could rapidly decrease our dependence on petroleum feedstocks and it can be considered an interesting short-term step towards fully chemical- and energy-integrated longterm biorefineries. Potential biomass feedstocks to be fed to standard refineries include mainly cellulosic biomass and triglyceride-based feedstock. Vegetable oils and fats are the easiest feedstock to convert into liquid fuels because of their high energy density and low oxygen content. Gasoline and diesel fuel can be produced from catalytic cracking and hydrotreating of triglycerides molecules. Co-processing of triglyceride-based biomass in FCC units improves gases and gasoline yields, and decreases heavy fractions. Thus, co-processing vegetable oils and animal fats in the FCC unit can be considered as an alternative for the production of bio-gasoline and bio-olefins C2–C4, as main products. Diesel fuel produced from hydrotreating of vegetable oils has been reported to have better fuel properties than biodiesel. Cellulose-based biomass is more difficult to convert into a biofuel because it is a solid with a low energy density. The first step for utilization of cellulosic biomass in a petroleum refinery is to overcome the recalcitrant nature of this material and convert it into a liquid product, which is done by fast pyrolysis to produce biooils, or by hydrolysis routes to produce aqueous sugars and solid lignin. Catalytic cracking of bio-oils, sugars, and lignin produces olefins and aromatics from biomass-derived feedstocks. Unfortunately, large amounts of coke are produced under typical FCC conditions and hence the improvement of reaction conditions must be addressed in the future. Likewise, the resultant hydrocarbon mixture usually contains a relevant presence of oxygenated compounds which limit its use

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© Woodhead Publishing Limited, 2011

7.16 Reaction pathways for the conversion of glucose into liquid alkanes (adapted from Huber and Dumesic, 2006; Huber et al., 2004).

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as transport fuel. Hydrotreating of bio-oils seems to be a required upgrading step to incorporate this feedstock in a refinery. Most of the biomass conversion processes in refineries need a large amount of hydrogen in order to remove oxygen and yield high-energy density fuels. It is likely that in the future this hydrogen could be produced by using renewable energy sources such as the sun, wind or biomass. Hopefully, the transformation of carbohydrates towards hydrogen using APR processes might be a good alternative to supply renewable hydrogen in conventional refineries. Moreover, the use of waste biomass from agriculture/forest residues (lignocellulosic biomass) to urban triglyceride-based residues (waste cooking oils, non-edible animal fats and even lipid residues of wastewater) is a priority to produce clean biofuels. Although, as seen in this chapter, biomass valorization can use successful commercial technology developed for petroleum refining and petrochemicals, we have to take into account that petroleum-derived feedstocks are chemically different to biomass-derived feedstocks. It is likely that as heterogeneous catalysis has made it possible to efficiently convert our petroleum-derived resources to fuels; it will also allow us to convert our renewable biomass resources to fuels. However, we honestly think that intensified efforts should involve the development of new heterogeneous catalytic materials specifically designed for this renewable feedstock rather than simply applying the ‘old’ catalyst technology, developed for petroleum refining, to new substrates. With this new feedstock come also new catalytic opportunities. Finally, for the co-processing of renewable materials in a refinery, it is necessary to take also into account other important issues upstream conversion units. The stability of refining streams in the storage, pre-heating or separation devices of a refinery is well known, as well as the compatibility with the materials of the different systems. However, this behaviour is still unknown for biomassfeedstocks and their mixture with petrol feedstocks. Stability problems during their storage might occur as a consequence of low thermal and oxidative stability of renewable raw materials as well as corrosion problems that might arise from the presence of free fatty acids. Likewise, stability and corrosion of these mixtures under higher temperature similar to that found in feed lines and heat exchangers prior to the conversion reactor system must be also taken in consideration. Hence, depending of features of the feedstock, some pre-treatment might be required before feeding to the conversion unit.

7.7

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