J. A. MELERO, A. GARCIA and M. CLAVERO, Rey Juan Carlos University, Spain

Abstract: This chapter highlights the feasibility of fluid catalytic cracking (FCC) units for the production of biofuels from different biomass feedstocks. Special attention will be focused on catalytic cracking of triglycerides, which are probably the most suitable feedstocks for their processing in FCC units since they possess density, viscosity and hydrogen/carbon ratio quite similar to those found in vacuum or hydrotreated gasoil usually fed to this refinery conversion unit. Likewise, we will comment on the influence of physicochemical properties of the different biomass feedstocks on the overall refinery facilities upstream FCC unit.

Key words: fluid catalytic cracking, triglycerides, thermal cracking, bio-oils.

15.1 Introduction

One promising alternative for the production of biofuels is the processing of biomass (cellulosic biomass and triglyceride-based biomass) in conventional oil refineries (Huber and Corma, 2007; Lappas et al, 2009). This alternative involves the co-feeding of biomass-derived feedstocks with typical petroleum feedstocks in conventional refining units. This strategy has significant advantages as compared with conventional processes of biofuels production. Petroleum refineries are already built, and hence, the use of existing infrastructure for the production of biofuels would require little capital investment (Huber and Corma, 2007; Holmgren et al.,

2007) . Moreover, a wide range of biofuels might be obtained, not only in the range of gasoline and diesel but also in the range of kerosene or fuel oil. The European Commission has set a goal that by 2020, 10% of transportation fuels in the European Union (EU) will be from renewable sources. Co-feeding biomass-derived molecules into a petroleum refinery could rapidly decrease our dependence on petroleum feedstocks and allow reaching the target of a more sustainable transport.

Several options are available for converting biomass-derived feedstocks into biofuels in a petroleum refinery: (1) Thermal (visbreaking and cocker units) and catalytic [fluid catalytic cracking (FCC) unit] cracking, (2) hydrotreating and (3) hydrocracking. Hydrogen-based processes are typically more expensive than cracking because they require hydrogen, and this consumption is even higher when biomass feedstocks are processed. Likewise, there are other drawbacks that limit the co-processing of biomass in hydrogen-based units, such as the poisoning of catalysts by water coming from hydrodeoxygenation reactions and the low quality of the resulting hydrogenated product to be used as diesel (mainly bad cold properties). Both issues require additional conditioning steps, and hence modification of refinery unit. Cracking reactions in a petrol refinery can be carried out in presence of catalyst (FCC unit) and in its absence (thermal units). Thermal units are not considered of interest for the production of biofuels since the resulting organic liquid product (OLP) contains a high content of oxygenated compounds independently of biomass feedstocks, and this reduces its interest as fuel transport. In contrast, catalytic cracking is faster and more selective than thermal cracking that allows working under milder reaction conditions, and hence minimizing yield towards gases, coke and heavy fractions and maximizing the production of liquid fraction suitable for use as transport fuel. Moreover, the presence of the catalyst shows a great ability to remove the oxygen-containing compounds and convert them into CO, CO2, H2O and a mixture of free oxygen hydrocarbons, although the extent of the oxygen removal is strongly dependent on the features of the initial feedstocks, as will be discussed in this chapter. A simplified reaction pathway for cracking reaction is outlined in Eq. 15.1.

CxHyOz — a Cx-b-d-e Hy-2C Oz-2b-c-d + b CO2 + c H2O + d CO + e C [15.1]

FCC is the most widely used process for the conversion of crude oil into gasoline and other hydrocarbons because of its flexibility to changing the feedstocks and product demands. The FCC process consists of three main steps: reaction process, separation of the products and regeneration of the spent catalysts. In the first step, a hot particulate catalyst is contacted with hydrocarbon feedstocks in a riser reactor to crack it, thereby producing cracked products and spent coked catalyst. After the cracking reaction takes place, the catalyst is largely deactivated by coke. Thus, at the end of the riser reactor, the spent catalyst is separated from the hydrocarbon products, stripped and sent to a fluidized bed regenerator to burn the coke and reactivate the catalyst. The hot catalyst is then recycled to the riser reactor for additional cracking and products are separated in a distillation column. A variety of process configurations and catalysts have been developed for the FCC process. FCC catalysts usually contain mixtures of a Y zeolite within a silica-alumina matrix, a binder, clay and some additives. Using FCC units for biomass conversion does not require any modification in the catalyst or the process itself. Moreover, the co-processing of renewable feedstocks in the FCC unit might involve some other process benefits such as an increase in the coke production, which could help to maintain the thermal balance between the reactor and the regenerator in the FCC unit; higher olefin production in the gas fraction, which favours the application of these compounds to produce polymers, alkylates and tertiary ethers; an increase in the amount of gasoline and in its octane number due to enhancement of aromatization reactions and olefins production and a decrease in the heavy fractions with a low commercial value obtained usually in the FCC unit.

Renewable feedstocks suitable to be fed in FCC units include highly oxygenated biomass such as bio-oils, glycerol, lignin and sugars, as well as triglycerides with

low oxygen content. Figure 15.1 schematizes the different routes to produce biofuels by means of catalytic cracking. The main challenge of this catalytic process is the removal of oxygen from biomass and enriching the hydrogen content of the final hydrocarbon product in order to improve their fuel properties. Chen et al. (1986) have defined the effective hydrogen index, (H/Ceff), where H, C, O, N and S correspond to the moles of hydrogen, carbon, oxygen, nitrogen and sulphur, respectively, which are present in the feed (Eq. 15.2).

(H/C)eff = H — 2°C 3N ~ 2S [15.2]

As seen in Fig. 15.1, this index for highly oxygenated feedstocks is clearly lower than 1, which means that these feedstocks are mainly formed by hydrogen — deficient 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 hydrogen index of ca. 1.5, which is quite similar to that of a mixture of hydrocarbons. These different values induce distinct chemistry involved in cracking process which will result in different product distribution. Likewise, other physical properties such as viscosity can affect dramatically the catalytic performance in the FCC unit.

Nevertheless, for the co-processing of renewable materials in a refinery, it is also necessary to take into account other important issues upstream FCC unit. 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 biomass feedstocks and their mixtures 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 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 FCC reactor system, must also be taken into consideration.