15.2.1 Catalytic cracking of bio-oils

Bio-oil is a chemically complex mixture of more than 300 oxygenated compounds, the main constituents being acids, aldehydes, ketones, alcohols, glycols, esters, ethers, phenols and phenol derivatives, as well as carbohydrates and a large proportion of lignin-derived oligomers. Liquefaction and pyrolysis are the two major technologies to produce bio-oils. Their properties 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. The possible utilization of bio-oil is, however, limited because of some negative attributes such as low pH, low heating value, high oxygen content and high viscosity. Bio-oil component can be converted into more stable fuels using zeolite catalysts (Bridgwater, 1994). Reaction conditions used for the above process are temperatures from 350°C to 500°C, atmospheric pressure and gas hourly space velocities of around 2 h-1. 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.

Bakhshi and co-workers studied zeolite upgrading of wood-derived fast — pyrolysis bio-oils and observed that between 30 and 40 wt.% of the bio-oil formed coke or char (Adjaye et al., 1996; Katikaneni et al., 1995a; Sharma and Bakhshi, 1993). The ZSM-5 catalyst produced the highest amount (34 wt.% of feed) of OLPs of any catalyst tested. The products in the organic liquid were mostly aromatic for ZSM-5 and aliphatic for SiO2-Al2O3. Gaseous products included CO2, CO, light alkanes and light olefins. However, bio-oils are thermally unstable and thermal cracking reactions occur during zeolite upgrading that leads to a high coke formation. Bakhshi and co-workers also developed a two-reactor process, where only thermal reactions occur in the first empty reactor and catalytic reactions occur in the second reactor that contains the catalyst (Srinivas et al.,
2000). The advantage of the two-reactor system is that it improves catalyst life by reducing the amount of coke deposited on the catalyst.

The transformation of model bio-oil compounds, including alcohols, phenols, aldehydes, ketones, acids and mixtures, has been studied over HZSM-5 catalysts (Fig. 15.2) (Gayubo et al., 2004a, 2004b, 2005). Alcohols were converted into the corresponding olefins at temperatures around 200°C; then, the olefins obtained were transformed into higher olefins (either butenes or C5+ olefins) above 250°C. At temperatures higher than 350°C, the olefins are transformed into C4+ paraffins and a small proportion of aromatics. Phenol has a low reactivity on HZSM-5 and only produces small amounts of propylene and butanes. 2-methoxyphenol also has a low reactivity to hydrocarbons and thermally decomposes generating coke (Gayubo et al, 2004a). Acetaldehyde had a low reactivity on ZSM-5 catalysts, and it also underwent thermal decomposition leading to coking problems. Acetone, which is less reactive than alcohols, converts into C5+ olefins at temperatures above 350°C. These olefins are then converted into C5+ paraffins, aromatics and light alkenes. Acetic acid is first converted to acetone, and that then reacts as above. Products from zeolite upgrading of acetic acid and acetone give considerably more coke than products from alcohol feedstocks (Gayubo et al., 2004b). Therefore, the majority of biomass-derived molecules produce large amounts of coke when passed over acidic zeolite catalysts. Gayubo et al. have recently studied the catalytic transformation of the aqueous fraction of crude bio­oil obtained via the flash pyrolysis of sawdust (from Pinus insignis) at 450°C over

-> Light alkenes: C4~, C3_, C2“

HZSM-5 zeolite (Gayubo et al., 2009, 2010). Previously, the bio-oil has been subjected to stabilization treatments to minimize coke deposition on the catalyst and to attenuate deactivation. 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).

The options for utilizing bio-oils in refineries are affected by its 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. Removal of metals is required before processing because these materials will typically poison catalysts. The low thermal stability, high water content and very high oxygen content make it difficult to blend the bio-oil with common refinery intermediate streams such as vacuum gasoil (VGO). The most serious problem for bio-oil processing is its high acid number that 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-oil can probably be processed using 317 stainless steel cladding, which is not standard in refinery units. Therefore, bio-oils would require pre-processing in a 317 stainless steel system to reduce the acid number before processing in typical refinery units (Holmgren et al., 2007). Since the FCC is the biggest unit and the heart of most refineries, much more development work would be required to minimize refinery risk before such an approach was viable. As an alternative to blending, co-processing bio-oil with petrol feedstocks in an FCC unit might be possible if a separate feed system was used to inject the bio-oil. Hence, the direct feeding of bio-oils into standard refinery does not appear a straightforward task.

Among various upgrading processes, hydrodeoxygenation is a promising alternative to reduce the acidity and oxygen content. Bio-oil was hydrotreated at high pressures (2000-2500 psi) and low space velocities (0.1-0.2 LHSV) by Holmgren et al. (2007). 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. 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 that 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 of approximately 20 wt.%. The co-processing of gasoil with a thermally hydrotreated bio-oil has also been investigated by Lappas et al. (2009). The results showed that the presence of the bio-oil 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.