As catalytic hydrotreating of liquid biomass has given promising results, the industrial world has given enough confidence to apply it in pilot and industrial scale. The NesteOil Corporation has developed the NExBTL technology for converting vegetable oil (primarily palm oil) into a renewable diesel also known as "green" diesel (Figure 9). Based on this tech­nology the first catalytic hydrotreatment of vegetable oils unit was constructed in Finland in 2007, within the existing Poorvo refinery of NesteOil, with a capacity of 170 kton/hr. The pri­mary feedstock is palm oil, while it can also process rapeseed oil and even waste cooking oil. The same company has constructed a second unit within the same refinery while it has also planned to construct two new units, one in Singapore and one in Rotterdam, with the capacity of 800 kton/yr each.

Oils / Fats

Acids

Bases

Water

H2

The catalytic hydrotreatment technology of 100% waste cooking oil for biodiesel production was developed in the Centre for Research and Technology Hellas (CERTH) in Thessaloniki, Greece [21—24] and later demonstrated via the BIOFUELS-2G project [59], which was co­
funded by the European Program LIFE". In this project WCO was collected from associated restaurants and the produced 2nd generation bio-diesel, to be called "white diesel" was em­ployed. For the demonstration of the new technology, 2 tons of "white diesel’ were pro­duced via catalytic hydrotreatment of WCO based on the large-scale pilot units available in CERTH. The production process simplified diagram is given in Figure 10. The new fuel will be applied to a garbage truck in a 50-50 mixture with conventional diesel in August 2012, aiming to promote the new technology as it exhibits overall yields exceeding 92% v/v.

In the USA the Dynamic Fuels company [60] has constructed in Baton Rouge a catalytic hy­drotreating unit dedicated to oils and animal fats with 285 Mlit capacity. The unit employs the Syntroleum technology based on Fischer-Tropsch for the production of synthetic 2nd gen­eration Biodiesel while it also produces bio-naphtha and bio-LPG. The Bio-Synfining tech­nology of Syntroleum converts the triglycerides of fats and oils into n — and iso-paraffins via catalytic hydrogenation, thermal cracking and isomerization as it is applied in the Fischer — Tropsch wax upgrading to renewable diesel (R-2) and renewable jet (R-8) fuel.

Figure 10. Pilot-scale demo application of WCO catalytic hydrotreatment during the BIOFUELS-2G project [59]

Due to the previously mentioned advantages and benefits, we applied microwave irradia­tion technique to the synthesis of ETBE from EtOH and TBA. Microwave-assisted experi­ments were performed using various microwave apparatuses working at 2.45 GHz frequency, with a power programmable from 0 to 1000W.

1.2. Batch experiments under atmospheric conditions

At first, experiments were carried out in batch mode under atmospheric pressure using the apparatus, shown as an actual image in Figure 5.

12 ІЗ 14 19 20 21 26 27 28

Figure 5. Microwave apparatus for batch synthesis of ETBE under atmospheric conditions (Shikoku Instrumentation Co., Ltd.)

In a typical run, about 0.25mol of EtOH and TBA, and 20 g of catalyst were placed in a reac­tor vessel, and heated using a microwave apparatus described above. Amberlyst 15, an ion exchange resin in H+ form was used as catalyst, unless otherwise specified. GC-FID appara­tus equipped with a CP-Sil 8CB-MS (60mx0.25mm, df=0.25) column for component separa­tion was used for the analyses of the products. Isopropanol was used as an internal standard.

Figure 6 shows typical experimental results obtained under atmospheric conditions. Experi­ments at atmospheric pressures using a batch reactor showed that the yield hardly increased above the 20% level. Similar results were obtained by other researchers [20,21], and the low­
er yield was likely due to the selective dehydration of TBA to IB, a highly volatile com­pound that easily escaped from the reaction zone. The maximum attained temperature was 80 oC, corresponding to the boiling point of the mixture.

Experiments under atmospheric conditions were extended to a continuous flow system us­ing the apparatus shown in Figure 7. A Masterflex digital pump was used to deliver the re­actants into the glass reactor filled with about 50g Amberlyst 15 catalyst. The temperature was set at 70 oC. Flowrates were varied to study the effect of residence times. The residence time of the reactants inside the reactor was calculated based on the reactor void space vol­ume and the flowrates. Products were collected continuously after certain time has elapsed, and until the system reached equilibrium.

Results in Figure 8 show that the yield increases with increasing residence time, getting maximum yield of about 30% at a microwave duty of 13%. Increasing the duty to 20% did not have any significant effect on the yield. The maximum yield of 30% obtained using this method was in agreement with our previous studies on reactive distillation [14] and the re­sults obtained by other researchers [20]. This low yield of ETBE was likely due to the selec­tive dehydration of TBA to IB, a highly volatile compound that easily escaped from the reaction zone. If IB could be allowed to further react with EtOH to produce ETBE, better yield could be obtained. Thus performing the experiments in a sealed reactor vessel was thought to be effective in overcoming this limitation under atmospheric conditions.

It comes from existing food crops like rapeseed (aka Canola), sunflower, corn, and others, af­ter it has been used for other purposes, i. e food preparation ("waste vegetable oil", or WVO), or even in first use form ("straight vegetable oil", or SVO). Not susceptible to micro­bial degradation, high availability, re-used material. It is used in the creation of biodiesel fuel for automobiles, home heating, and experimentally as a pure fuel itself. At present, WVO or SVO is not recognized as a mainstream fuel for automobiles. Also, WVO and SVO are susceptible to low temperatures, making them unusable in colder climates.

Except the chemical ways, butanol can also be obtained from biological ways with the re­newable resources by the microorganism through fermentation. The Clostridia genus is very common for butanol synthesis under anaerobic conditions, and the fermentation products are often the mixture of butanol, acetone and ethanol. A few kinds of Clostridium can utilize cellulose and hemicellulose with the ability of cellulolytic activities (Mitchell et al., 1997; Be­rezina et al. 2009).

Compared with the chemical ways for butanol production, biological ways has the dis­tinct advantages. For example, it can utilize the renewable resources such as wheat
straw, corn core, switch grass, etc. Furthermore, biological process has high product se­lectivity, high security, less by-products. Furthermore, the fermentation condition of buta­nol production is milder than that of chemical ways and the products are easier to separate. The process of biobutanol production with Lignocellulosic feedstocks is as fol­lowing (Fig. 1):

Figure 1. Butanol production process from lignocellulosic feedstocks

For the first step, biomass containing lignocellulosics should be pretreated before they were used as the substrate for the fermentation, except for a few high cellulase activity strains (Ezeji and Blaschek, 2008). The pretreatment methods are different according to the different types of biomass used. There often use dilute sulfuric acid pretreatment, alkaline peroxide pretreatment, steam explosion pretreatment, hydrothermal pretreatment, organic acid pre­treatment etc. Some inhibitors such as acetic acid, furfural, 5- hydroxymethyl furfural, phe­nols etc. that need to be further detoxified. The ordinary detoxification methods are using activated charcoal (Wang et al., 2011), overliming (Sun and Liu, 2012; Park et al., 2010), elec­trodialysis (Qureshi et al., 2008c), membrane extraction (Grzenia et al., 2012) to remove the inhibitors. This step is determined by different feed stock and different pretreatment meth­ods. After the fermentation, the desired product is recovered and purified in the down­stream process. Biological ways has been set up for many years while it was inhibited for industrial application for economic reasons. So, as an alternative fuel, biomass feedstock for biobutanol production must be widely available at low cost (Kent, 2009). Therefore, by using agricultural wastes for butanol production such as straw, leaves, grass, spoiled grain and fruits etc are much more profitable from an economic point of view. Recently, other sources
such as algae culture (Potts et al., 2012; Ellis et al., 2012) also is studied as one substrate for butanol production.

The chemical composition of petroleum-based fuels: gasoline, diesel, and jet fuel, includes linear, branched, and cyclic alkanes, aromatics, and chemical additives [28]. Isoprenoid-based biofuels have the structural diversity to mimic these petroleum compounds, with up to 50,000 known isoprenoid structures including branched and cyclic hydrocarbons with varying degrees of unsaturation [29, 30]. Isoprenoids reported to be potential fuel candidates include: the hemiterpene (C5) isoprene; monoterpenes (C10): terpinene, pinene, limonene, and sabinene; the sesquiterpene (C15) farnesene, and their associated alcohols: isopentenol, terpineol, geraniol, and farnesol [12, 31]. Two metabolic pathways are capable of producing the isoprenoid building blocks isopentenyl pyrophosphate (IPP) and dimethylallyl diphos­phate (DMAPP): the mevalonate (MVA) pathway [32] and the methylerythritol phosphate (MEP) pathway, also known as the 1-deoxy-D-xylulose-5-phosphate (DXP) pathway and the non-mevalonate pathway (Figure 3) [33]. In general, the MVA pathway is found in eukaryotes and archaea while the MEP pathway is utilized by prokaryotes. In agreement with the proposed evolutionary origin of plants, they contain both isoprenoid pathways with the MEP pathway localized in the plastid and the MVA pathway in the cytosol [34]. The MVA and MEP pathways differ with respect to their requirement for carbon, energy, and reducing equiva­lents; this is illustrated by the net balances for IPP biosynthesis from glyceraldehyde-3- phosphate (GAP):

MVA:3 GAP + 3 ADP + 4 NAD(P)+ + 2 P ® IPP + 4 CO2+ 3 ATP + 4 NAD(P)H (1)

MEP:2 GAP + ADP + CTP + P ® IPP + CO2 + ATP + CMP + PP, (2)

Based on these balances, IPP production via the MEP pathway is more efficient at carbon utilization, as only 2 GAPs are required and 1 CO2 is emitted, compared to 3 GAPs and 4 CO2 for the MVA pathway. On the other hand, IPP production via the MVA pathway is more energy efficient overall, resulting in ATP generation and yielding a net gain in reducing equivalents (NAD(P)H). These carbon, energy, and reducing equivalent requirements should be considered when designing a metabolic engineering strategy for isoprenoid biosynthesis.

The MVA pathway interfaces with the primary metabolism at the acetyl-CoA node (Figure 3), and it can be divided into two parts: the top, which involves 3 enzymatic steps to convert acetyl-CoA to MVA, and the 3 enzymatic conversions of the bottom portion to produce IPP from MVA. One novel metabolic engineering strategy compared the efficiencies of the top and bottom portions of the MVA pathway in E. coli using heterologously expressed pathways from 5 different eukaryotic sources. The most efficient top and bottom portions were combined to maximize the yield of isoprenoid building blocks [35]. Accumulation of an intermediate metabolite, 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA), is a known bottleneck in the top MVA pathway, and HMG-CoA was also shown to inhibit cell growth in E. coli [36]. Thus, overexpression of the HMG-CoA reductase (HMGCR) increased MEV production and synthesis of subsequent FPP-derivatives in both E. coli and S. cerevisiae [36—38]. Whole pathway expression and elimination of the HMGCR bottleneck have proven to be successful techniques for enhancing the metabolic throughput of the MVA pathway.

The MEP pathway requires two primary metabolites as precursors: GAP and pyruvate (PYR) (Figure 3). Compared to the 6 enzymatic steps of the MVA pathway, the MEP pathway is comprised of 7 steps. Metabolic engineering strategies for the MEP pathway have primarily focused on the first two enzymatic steps. Overexpression of 1-deoxy-D-xylulose-5-phosphate synthase (dxs), catalyzing the conversion of GAP and PYR to 1-deoxy-D-xylulose-5-phosphate (DXP), resulted in 6-10-fold increases in the final isoprenoid product [39, 40]. Targeting the next enzymatic step through overexpression of DXP reductoisomerase (dxr) was shown to have little effect on isoprenoid production using the native gene; however, expression of dxs and dxr from Bacillus subtilis improved isoprenoid production 2.3-fold in E. coli [41]. The final step of the MEP pathway was also shown to be rate-limiting, as heterologous expression of IPP isomerases (IPPI) enhanced isoprenoid production in E. coli [42]. Based on its rate-limiting steps, the MEP pathway is a prime candidate for a push-pull metabolic engineering strategy, whereby overexpression of the first step ‘pushes’ carbon flux into the MEP pathway and overexpression of the final step ‘pulls’ the metabolic flux towards the end product. This strategy yielded nearly 2-fold improvements in isoprenoid production in E. coli [43, 44]. Lastly, overexpression of the entire MEP pathway can increase isoprenoid biosynthesis. In fact, Leonard and colleagues demonstrated that 5 additional copies of the MEP pathway genes yielded the highest production, while further increasing the gene copy number to 10 produced lower titers [45].

While targeted gene overexpression may alleviate pathway bottlenecks, the pathway is still subject to native regulatory mechanisms which may limit isoprenoid biosynthesis from either the MVA or MEP pathways. A highly successful strategy for overcoming regulatory limitations is overexpression of the non-native isoprenoid pathway. Expression of the MVA pathway from Saccharomyces cerevisiae in E. coli has enabled higher levels of isoprenoid synthesis compared to engineering the native MEP pathway as the sole isoprenoid pathway [46—50]. The success of this strategy has made it a favorite among metabolic engineers seeking to improve isopre — noid biosynthesis. Farmer and Liao presented a clever approach for regulating the carbon flux into an engineered MEP pathway in E. coli [51]. In this work, a native regulatory circuit was used to control the carbon flux into and through the MEP pathway by regulating expression of two key enzymes: phosphoenolpyruvate synthase (PPS) and isopentenyl diphosphate isomerase (IPPI). Under excess carbon flux, expression of pps and idi was activated using the regulatory circuit, redirecting carbon flux into and through the MEP pathway, yet when the carbon flux was growth limiting, expression of these genes was reduced. This strategy allows for high isoprenoid production without negatively impacting cell growth. As evidence, the regulated pathway improved isoprenoid titers by 50%, while simply placing pps and ippi under control of strong tac promoters resulted in growth inhibition [51]. Native regulatory mecha­nisms are often obstacles limiting isoprenoid biosynthesis, yet they can also be exploited to optimize the flux balance to support both cell growth and isoprenoid production.

Additional targets for improving isoprenoid-based fuel production include precursor supply, cofactor supply, and optimization of the downstream fuel synthesis pathway. Acetyl-CoA is

the precursor for isoprenoid production via the MVA pathway. Overexpression of acetalde­hyde dehydrogenase (ALDH) and acetyl-CoA synthetase (ACS), both of which produce acetyl — CoA, increased the acetyl-CoA supply and subsequently isoprenoid biosynthesis in S. cerevisiae [52]. On the other hand, the MEP pathway requires two precursors from the glycolysis pathway: PYR and GAP. The supply of these metabolites is complicated by the fact that PYR is derived from GAP, and consequently, the PYR/GAP balance is an important metabolic engineering target. The supply of GAP was shown to be limiting in E. coli, as modifying the conversion between PEP and PYR to redistribute the flux toward GAP synthesis increased isoprenoid production [53]. In addition to the carbon precursors, co-factors in the form of energy (ATP, CTP) and reducing equivalents (NADPH) are also required for isoprenoid synthesis. Co-factor supply is often overlooked in strategies for isoprenoid production, yet by improving the availability of NADPH in S. cerevisiae, isoprenoid synthesis through the MVA pathway increased by 85% [54]. This result emphasizes the importance of co-factor availability. Despite optimizing production of the isoprenoid building blocks, the downstream efficiency of assembling the final fuel product may still limit the overall yield. Successful strategies for improving downstream efficiency include overexpression of GPP and FPP synthases [47], overexpression and codon optimization of hemiterpene, monoterpene, and sesquiterpene synthases [41, 47, 48], fusion proteins to localize FPP synthesis and its conversion to sesqui­terpene [47], and downregulation of competing products like squalene [37, 48]. The optimized production of isoprenoid-based fuels requires strategies to address limitations throughout the metabolic pathway, from precursor and co-factor supply to end product synthesis.

One of the first studies related to glycerol hydrogenolysis was developed by Montassier et al. [40] in the late 1980s. They suggested that over Ru/C catalyst glycerol is first dehydrogen­ated to glyceraldehyde on the metal sites. Next, a dehydroxylation reaction takes place by a nucleophilic reaction of glyceraldehyde with water or with adsorbed — OH species. Finally, hydrogenation of the intermediate yields 1,2-PDO (Figure 9). The main controversial point of this mechanism is the initial dehydrogenation step, which is thermodynamically unfa­vored due to the high hydrogen pressures used [41]. Therefore, in order to shift the equili­brium, glyceraldehyde dehydration should be faster than glycerol dehydrogenation. Otherwise glyceraldehyde would be hydrogenated back to glycerol on the metal sites. Sev­eral authors observed that the addition of a base notably increased glycerol conversion, and this was related to the fact that bases enhance glyceraldehyde dehydration [42—44]. It is inter­esting to point out that when glycerol hydrogenolysis is carried out under alkaline condi­tions, marginal 1,3-PDO selectivities are measured.

Apart from 1,2-PDO, other products stemming from C-C bond cleavage were also report­ed when glycerol hydrogenolysis is conducted under alkaline conditions; mainly, ethyl­ene glycol (EG), methanol and methane. It is suggested that glyceraldehyde can either undergo dehydration or retro-aldolization reactions. The so formed intermediates are hy­drogenated in the last step to yield the products of C-C bond cleavage. Because both the glyceraldehyde dehydration and glyceraldehyde retro-aldol reaction are catalyzed by OH-, the addition of a base increases the glycerol reaction rate but does not improve the selec­tivity to 1,2-PDO [45].

Installation of bioreactors and wastes to use

At the second stage were used the same wastes of first stage, but additionally were employed wastes of tomato, onion, garlic and husk of cape gooseberry (Table 10). The wastes were triturated and mixed with water during three minutes, the relation of wastes: water was 1:2,5 like at first stage. The quantity of wastes employed in each treatment was similar to quantity
used at first stage. The volume of work in each bioreactor was 70% and 30% was dedicated to storage the gas generated.

The highest values of chemical oxygen demand (COD) were obtained in the treatment 2 during the repetition 1 and the treatments 1 and 3 of the repetition 3 respectively, namely in these cases there were more quantity of food available to the microorganisms.

Behavior of pH during the pretreatment and operation of bioreactors

Due to type of wastes employed in the repetition 2, was necessary to add muriatic acid into all bioreactors to achieve the pH of acidification, but there were not response (pH be­tween 3,5 and 4,5). However, during the repetitions 1 and 3, in all treatments was used wastes of orange and lemon, this allowed to apply the pretreatment of acidification, after­wards was added agricultural lime and was reached a pH between 5 and 6, values ade­quate to generate biohydrogen.

The gas generated in all treatments was compound of hydrogen, carbon dioxide, nitrogen and oxygen. The greater value of methane was 3,7% and the less was 0%, in many times the methane was not detected (ND), this mean that the pretreatment to reduce the methanogenic bacteria was satisfactory. The percentage of hydrogen in gas was between 5 and 18,08; this was the highest value in the research and was obtained in the treatment 3 during the repetition 3 when the wastes used were Lettuce and cabbage leaves, orange, lemon and papaya. In the repetition 2 in all treatments, there were not generation of hydrogen (NG). The oxygen content in some repetitions show maybe that some air entered to bioreactor when the samples were taken.

The maximum production of biohydrogen per day obtained at the first; stage was 15 liters, however at the second stage the maximum production was 38 liters, this mean that the production was duplicated during the second stage. In the repetition 3, in the treatment 3 were generated 32 liters of biohydrogen, meanwhile in the treatment 1 were generated 15 liters. The wastes employed sn both treatments were Lettuce and cabbage leaves, orange, lemon and tropical fruits as mango, guava and papaya, the initial pH was lesser to 4,5 during 7 and 8 days, and Che cHof bioreactorsoperation was between5and5,5.

The greater accumulated production was reached in the treatment 2 and the repetition 1, followed by the treatment 3 in the repetition 3. In both cases were used vegetal wastes and tropical fruits in same proportions in addition, initially the wastes were subjected to acid conditions with a pH less to 4,0 during 8 days and a pH for operation of bioreactor between 5 and 5,5. Under those conditions the hydrogen content into gas ranged between 7,5 and 10,5%. The total production of hydrogen in the treatment 2 and the repetition 1 was 317,8 liters in 22 days, with 14,44 liters of H2/day (twice the result from the first stage), and maximum yield of 159 liters of H2/m3 of bioreactor. Other outstanding result was reached when were employed the same wastes, at beginning were applied acid conditions during 7 days under a pH less to 4,5; and then was used a pH for the operation of bioreactor between 5 and 5,5. In this case the percentage of hydrogen into gas ranged between 5 and 18,08% (the last value was the maxi­mum content reached in the research). The production of hydrogen was 231,1 liters of H2 in 22 days with 10,5 liters of H2/day (duplicated the value reached from the first stage) and a maximum yield of 115,6 liters of H2/m3 of bioreactor.

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Figure 17. Productionaccumulatedofhydrogen

A wide scale of monosaccharide or disaccharide or other sugar-based oligomeric or poly­meric substrates can be used as starting material in ABE fermentations. Compere and Griffith [20] studied the effect of substrate types on the yield and distribution of valua­ble ABE products with different strains of Clostridia. Different concentrations (2.5-10%) of arabinose, ribose, xylose, xylane, fructose, glucose, cellobiose, lactose, sucrose, dextrin and cellulose were tested and the amount and the distribution of acetone, ethanol, buta­nol and the H2/CO2 ratio strongly depended on the type of strain, the type of sugar and their concentrations as well. Ounine et al. [21] studied the fermentation of glucose, arabi­nose and xylose with C. Acetobutylicum and found conversion into solvents in 32, 29, and 28%, respectively. Growth yields were similar on the all sugars, but glucose or arabi­nose was consumed in preference to xylose and with faster growth. It means that not on­ly corn, cereals, molasses, but wastes of dairy products [22,23] or agricultural byproducts as corn stalk, corncob, cellulose wastes and other raw materials can also be utilized [24,25]. By using wastes, however, it is an important key factor that the contaminants can prevent the ABE fermentation. For example, when alkaline peroxide pretreated wheat straw was hydrolyzed using cellulolytic and xylanolytic enzymes, and the hydro — lyzate was used to produce butanol using Clostridium beijerinckii P260, the culture pro­duced less than 2.59 g L-1 ABE solvents, but after removal of the formed inhibitor salts with electrodialysis, 22.17 g/L of ABE solvents were formed. This was higher value than the ABE solvent concentration (21.37 g/L) given from glucose. A comparison of use of different substrates (corn fiber, wheat straw) and different pretreatment techniques (di­lute sulfuric acid, alkaline peroxide) suggested that generation of inhibitors was substrate and pretreatment specific [26]. Selection of raw material for ABE process cannot be inde­pendent from the selection of the bacteria strain. For example, cassava, due to its high starch content and low cost, is a promising candidate substrate for large-scale ABE fer­mentation processes. However, the solvent yield from the fermentation of cassava reach­es only 60% of that achieved by fermenting corn. Addition of ammonium acetate (CH^COONHJ to the cassava medium significantly promotes solvent production with a high butanol ratio C. Acetobutylicum mutant (EA 2018). When cassava medium was sup­plemented with 30 mM ammonium acetate, the acetone, butanol and total solvent prodn. reached 5.0, 13.0 and 19.4 g/l, respectively, after 48 h of fermentation which level of sol­vent production is comparable to that obtained from corn medium. Both ammonium (NH4+) and acetate (CH3COO-) were required for increased solvent synthesis [27]

Heteroatoms are atoms other than carbon (C) and hydrogen (H) and are often encountered into bio — and fossil — based feedstocks. They include sulfur (S), nitrogen (N) and in the case of bio-based feedstocks oxygen (O). In particular oxygen removal is of outmost importance as the presence of oxygen reduces oxidation stability (due to carboxylic and carbonylic double bonds), increases acidity and corrosivity (due to the presence of water) and even reduces the heating value of the final biofuels. The main deoxygenation reactions that take place include deoxygenation, decarbonylation and decarboxylation presented in Schemes 7, 8 and 9 re­spectively [7]. The main products of deoxygenation reactions include n-paraffins, while H2O, CO2 and CO are also produced, but can be removed with the excess hydrogen within the flash drums of the product separation section. It should be noted however that these particular reactions give the paraffinic nature of the produced biofuels, and for this reason the hydrotreated products are often referred to as paraffinic fuels (e. g. paraffinic jet, paraf­finic diesel etc)

R-CH2COOH + 3 H2 ———— ► R-CH2CH3 + 2 H2O

Scheme 7.

R-CH2COOH + H2 ———— ► R-CH3 + CO + H2O

Scheme 8.

R-CH2COOH + H2 ———— ► R-CH3 + CO2

Scheme 9.

The other heteroatoms, i. e. S and N are removed according to the classic heteroatom remov­al mechanisms of the fossil fuels in the form of gaseous H2S and NH3 respectively.

1.1.3. Isomerization

The straight chain paraffinic molecules resulting from the aforementioned reactions, even though they offer increased cetane number, heating value and oxidation stability in the bio­fuels which contain them, they also degrade their cold flow properties. In order to improve the cold flow properties, isomerization reactions are also required, which normally take place during a second step/reactor as they require a different catalyst. Some examples of iso­merization reactions are given in Schemes 10 and 11.

R-CH2-CH2-CH3 ———- ► R-CH-CH3

CH3

Scheme 10.

Scheme 11.

Gasoline oxygenates such as methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME) and tert-amyl ethyl ether (TAEE) are produced commercially by the reaction of olefins (IB and isoamylenes) or C4-C7 hydrocarbons with MeOH or EtOH, in the presence of homogeneous catalysts (e. g. H2SO4) or a heterogeneous catalysts (e. g. ion — exchange resin) as shown in Figure 1, in case of ETBE.

Figure 1. Reaction scheme for ETBE synthesis using IB as a reactant

The differences of blending characteristics of the ether oxygenates are summarized in Table 1.

Brockwell et al. [3] discussed the process schemes for the production of these ethers. The schemes generally consist of primary reactor, distillation column and additional column to purify the products (e. g. extractor). Due to the upsurge of the demand for gasoline oxygen­ates, the MTBE process having a world production capacity of 20 million tons per year in 1994 [4] is the most established one. With legislations banning MTBE, it is assumed that the currently existing processes for the production of MTBE can be converted for ETBE produc­tion. Similarly, the processes can be integrated for MTBE and TAME production, since these ethers are both produced from MeOH [3].

Some of the patented processes for the production of ether oxygenates are reported in "Re­fining 1996" [5]. The main features of each process are summarized here.