Although hydrocracking yields an appealing spectrum for the production of diesel, it is not an attractive option for gasoline. The relatively low extent of branching achieved in hydrocracking yields a product in the gasoline range with a low octane number. In addition, hydrocracking is considered an expensive process due to the high pressure operation and high hydrogen consumption. The fluid catalytic cracking (FCC) process has been investigated as an interesting option for the cracking of FT waxes aimed at the production of FT gasoline (Dupain et al., 2005, 2006; Lappas, 2007; Lappas and Vasalos, 2006; Lappas et al., 2007; Triantafyllidis et al., 2007).

The FCC process is the most important refinery process mainly for the production of gasoline from heavy petroleum fractions such as atmospheric and vacuum gas oil (VGO). In the FCC unit, the long hydrocarbons are cracked in the 480-540°C temperature range over zeolite catalysts to smaller n — and i-paraffins, n — and i-olefins and aromatics. Conventional FCC feedstocks are relatively aromatic, with a high sulphur and nitrogen content, in contrast to FT waxes that are highly paraffinic with extra low aromatics content (< 1 wt.%) and virtually zero sulphur (< 5 ppm) (see Table 19.3). Both the development, therefore, of new catalyst formulations and optimization of the overall process parameters are very critical to optimize the yield and quality of FCC products from FT waxes.

Lappas et al. (2007) compared the crackability of conventional VGO feed and FT wax provided by CHOREN over a typical refinery FCC E-cat. As can be seen in Fig. 19.10, the FT wax is much more crackable than VGO due to the highly paraffinic molecules of wax compared to VGO that contains a significant amount of aromatics. In fact, the cracking rate of the wax molecules was calculated as about 4.2 times faster than that of the VGO molecules. Moreover, coke formation was much less compared to VGO, again due to the paraffinic nature of the feed and the absence of aromatic compounds or coke precursors even at high conversion levels. Very high conversions, over 80 wt.%, can be achieved with conventional

FCC catalysts at very low catalyst/oil ratios and low temperatures. In Table 19.4, a comparison between the two feeds regarding the product distribution at 70 wt.% conversion is given. The table shows that gasoline (C5, 221°C) yield is about the same with both feeds. Gasoline from VGO has, as expected, a higher octane number; however, the research octane number (RON) of the wax gasoline is still acceptable. The RON of the wax gasoline was almost constant and independent of the conversion exactly due to the low aromaticity of this gasoline (Lappas et al., 2004). Dupain et al. (2006) also observed that the cracking of wax to gasoline is a primary reaction with a gasoline selectivity that is independent of conversion level or temperature. Despite the lower R ON number, gasoline from the cracking of FT waxes in an FCC unit is very promising due to the low content of aromatics in the product and the extremely low sulphur and nitrogen concentrations, leading to the production of a very clean gasoline. Moreover, it was found that the diesel range LCO product produced from the catalytic cracking of FT waxes is better than the respective produced from the cracking of conventional FCC feedstocks. The degree of branching in the diesel product is

lower than that of the gasoline, improving marginally the cetane number but acting very beneficially for the diesel cloud point and pour point, in addition to the very low sulphur and nitrogen content (Dupain et al, 2006).

The addition of ZSM-5 additive to a conventional E-cat was found to enhance the cracking rate of FT waxes, enhancing the cracking of gasoline range olefins to gas range olefins and especially propene and butene (Dupain et al., 2006). This was attributed to the diffusions of the initially formed smaller olefins in the ZSM-5 pores. The olefins are not able to leave the ZSM-5 pores rapidly enough, and they are thus easily activated and overcracked to gas range olefins (Dupain et al., 2006). Use of pure ZSM-5 resulted in an octane-enhancing effect of the produced gasoline due to the enhanced formation of olefins and aromatics. Triantafyllidis et al. (2007) investigated the potential utilization of various microporous (zeolites H-Y and H-ZSM-5) and mesoporous [amorphous silica — alumina (ASA) and Al-MCM-41] aluminosilicates as catalysts or active matrices in the cracking of FT waxes towards the production of liquid fuels. Focus was placed on the effect of porous and acidic characteristics of the materials on products yields and properties. According to the authors, the type of catalyst plays a significant role in the product selectivities. The percentage conversion of wax, the product yields [gasoline and liquefied petroleum gas (LPG)] and the RON of the produced gasoline are shown in Fig. 19.11 for different investigated microporous and mesoporous catalysts. The behaviour is typical for the two zeolitic catalysts when used in FCC of petroleum fractions, where H-Y zeolite is being utilized as the main active cracking component of the catalyst and ZSM-5 is being used as an additive in small amounts, leading to lower gasoline and higher LPG yields, and usually to higher RON. Similar trends are observed in Fig. 19.11

for the cracking of FT waxes. One of the main reaction pathways that ZSM-5 catalyzes with higher rates than H-Y is the cracking of paraffins, thus making it very active in the conversion of waxy feedstocks in agreement with the results of Dupain et al. (2006). The 3%-crystalline H-ZSM-5 sample, not diluted with ASA, showed high conversion activity (79 wt.%), very close to that of the diluted catalyst of the crystalline H-ZSM-5. It can thus be suggested that the acid sites present in this sample are much more active for the conversion of wax compared to those of Al-MCM-41 and ASA, although the very low crystallinity H-ZSM-5 sample consists mainly of X-ray diffraction (XRD) amorphous aluminosilicate phase. Figure 19.12 shows the yields (wt.% on feed) of various gasoline components. The data in Fig. 19.12 can also be used for a qualitative comparison of catalytic performance with regard to selectivity towards specific gasoline components, especially in the case of H-Y and H-ZSM-5-based catalysts, which showed a similar percentage conversion of wax (Fig. 19.11). The H-Y-st. catalyst presented a significant selectivity towards the production of branched paraffins (22 wt.% on feed) compared to much lower yields with the rest of the catalysts (3.5-4 wt.%). The increased formation of branched paraffins in gasoline is considered as a major target towards the production of environmentally friendly fuels in accordance with the EU regulations. Olefins were also higher with the H-Y-st. catalyst (15 wt.% on feed) compared to the rest of the catalysts (~12 wt.%), while naphthenes were 1-2 wt.% for all the catalysts. As far as aromatics are concerned, the H-ZSM-5 catalyst led to higher yields compared to the rest of the catalysts. The high RON values of gasoline with the H-ZSM-5 catalyst (~92, see Fig. 19.11) were mainly attributed to the high aromatics content, while in the case of H-Y-st. catalyst, the high RON (~87) was mainly attributed to the relatively high C5-C7 olefins and iso-alkanes yields. The 3%-

H-ZSM-5 sample showed similar trends with the fully crystalline H-ZSM-5 with regard to the yields of gasoline components, except for the case of aromatics, which are significantly lower with the former sample. Interestingly, the RON of the gasoline produced from the 3%-crystalline H-ZSM-5 sample remained considerably high (84). The yield of aromatics with the Al-MCM-41 sample was very low, but they cannot be compared with those of the rest of the catalysts due to the relatively low percentage conversion of wax with the mesoporous catalytic material.

In general, research has shown that the cracking of highly paraffinic FT waxes under FCC conditions can yield an interesting spectrum of renewable fuels, both in the gasoline and diesel range, by adapting the process parameters and catalyst formulations. Optimization of catalyst’s acidic and porosity properties as well as of process parameters is necessary in order to visualize a potential commercialization of the FCC-based upgrading of FT waxes.