Although different options have been proposed for the post-treatment and upgrading of the FT waxes (Dancuart et al., 2003; de Klerk, 2007; Dupain et al., 2005), it is generally accepted that hydrocracking is the most effective route to maximize the middle distillate yield and it is currently the applied option. Given the small number of commercial FT plants, little technology has been developed specifically for the refining of the FT wax products. In most commercial sites, standard crude oil refining approaches have been used without taking into account the specific characteristics of the FT wax product compared to conventional refinery streams, such as extra low aromatics content (< 1 wt.%) and virtually zero sulphur (< 5 ppm) (see Table 19.3).

Conventional hydrocracking takes places over a bifunctional catalyst with acid sites to provide isomerization/cracking function and metal sites to provide hydrogenation-dehydrogenation function. Platinum, palladium or bimetallic systems (i. e. NiMo, NiW and CoMo in the sulfided form), supported on oxidic supports (e. g. silica-aluminas and zeolites), are the most commonly used catalysts, operating at high pressures, typically over 10 MPa, and temperatures above 350°C.

In recent years, considerable research is ongoing to investigate the effect of the operating conditions, both experimentally (Calemma et al., 2005, 2010; Rossetti et al, 2009) and computationally (Fernandes and Teles, 2007; Pellegrini et al., 2004), and the catalytic material on the yield and quality of the FT wax hydrocracking products. Concerning the operating conditions, it was found that wax hydrocracking requires milder pressure and temperature, as the paraffinic nature of the wax implies higher availability of hydrogen in the unit (little hydrogen consuming aromatics) and thus suppressed coke formation (de Klerk, 2008). FT wax hydrocracking to middle distillates is favoured at pressures ranging from 3 to 5 MPa and temperatures between 250°C and 300°C (Calemma et al., 2010) and yields a product containing light paraffins up to C24, as presented in a product sample chromatograph obtained from FT wax hydrocracking experiments performed in Chemical Process Engineering Research Institute (CPERI) (Fig. 19.7). At these conditions, middle distillate yield (C10-C22) reaches up to 80-85 wt.% at intermediate conversion levels (~60 wt.%) (Calemma et al, 2010). At higher conversions, a small reduction in the middle distillate yield can be observed, indicating an increase of consecutive hydrocracking reactions leading to lighter products. Still, the consecutive reactions are limited, allowing the reaction to be carried out at high conversions without lowering significantly the middle distillate selectivity (Calemma et al., 2010).

Extensive work has also been conducted by our group as part of the EU-funded IP RENEW project that explored technology routes for the production of BTL fuels (Lappas et al, 2004). More specifically, the operating conditions (temperature, pressure, H2/oil ratio) were investigated in experiments with different commercial hydrocracking catalysts in a specially designed hydroprocessing pilot plant unit. Main conclusions were that with all catalysts, hydrocracking temperature appears to play the most important role and influences significantly the product yields, as shown in Fig. 19.8. It was shown that the yields of naphtha and kerosene in the product increase as the temperature increases and so does the conversion.

19.8 Effect of temperature on product yields in the hydrocracking of BTL-FT wax.

However, the diesel yield is maximized at a certain temperature and then decreases as a result of higher conversions achieved at higher temperatures (RENEW, 2008). Moreover, it was shown that the yield of gasoline and diesel in the product decreases as the H2/oil ratio decreases and so does the conversion. The diesel selectivity is also slightly decreased as a result of the decreasing yield and

conversion. Studies by Calemma et al. (2010) showed additionally that the composition of FT diesel, specifically the ratio of iso — and n-paraffins, is also influenced by the operating parameters.

The nature of the catalyst also affects significantly the product quality and yield. Experiments performed in CPERI with three different commercial hydrocracking catalysts showed measurable differences in diesel selectivity at isoconversion as a function of the catalytic material (Fig. 19.9) (RENEW, 2008). Catalysts loaded with a noble metal (particularly Pt) were reported to show better performances in terms of selectivity for hydroisomerization and products distribution in comparison with non-noble metals-based catalyst (Archibald et al., 1960; Gibson et al, 1960). Calemma et al. (2001) reported high diesel selectivities obtained over a Pt/SiO2-Al2O3 catalyst during hydroprocessing of FT waxes and attributed the observed results to the mild Bronsted acidity, high surface area and pore size distribution of the support. Zhang et al. (2001) also showed that Pt performs better than Ni and Pd supported on tungstated zirconia for the hydroisomerization of the model compound n-hexadecane. The use of hybrid catalysts based on Pt/WO3/ZrO2 with addition of sulphated zirconia, tungstated zirconia or mordenite zeolites was studied by Zhou et al. (2003). According to the authors, hybrid catalysts based on Pt/WO3/ZrO2 provide a promising way to obtain higher activity and selectivity for transportation fuels from FT products. Given the high cost of noble metals, hydroprocessing of FT waxes has also been

.9 Product selectivity at isoconversion for different catalytic materials in the hydrocracking of BTL-FT wax.

studied over nickel catalysts (de Haan et al., 2007). de Haan et al. (2007) demonstrated the benefit of using non-sulfided nickel catalysts. In conventional hydroprocessing units, catalysts are sulphated to avoid poisoning by the sulphur species in crude oil. However, in the case of the sulphur-free FT waxes, use of a sulfided catalyst implies the continuous addition of sulphur-containing compounds to avoid catalyst deactivation (de Klerk, 2008). Other advantages of developing a non-sulfided catalyst for the hydrocracking of FT waxes are a simplified, less costly and environmentally friendly process (no H2S in the tail gas) (de Haan et al., 2007). Nickel supported on a commercial silicated alumina yielded results that compare favourably with those of a commercial sulfided NiMo catalyst, with diesel selectivities of 73-77% at a conversion of approximately 52% (de Haan et al., 2007).