The chemical nature of conversion products depends both on the structure or type of the zeolite used and the reaction temperatures, because restructuring occurs at the inner surface, which acts as a reaction vessel at the molecular scale. Specific reactions depend on the diameters of pores, the resident time of molecules within the pores or channels and voids of the microporous zeolite, and the temperature. The penetration of lipids into a zeolite is depicted in Fig. 8.5. The scheme is based on [22].

O

II C H

‘ C—— C17H35

HC O C C17H35

O

H2C ^-C^

O C17H35

O

Diffusion

Figure 8.5 Scheme of restructuring triglycerides with shape — selective H-ZSM-5 to aromatic hydrocarbons.

To demonstrate this influence of catalysts and reaction temperature on yields and products, Table 8.4 considers a shape-selective zeolite type H-ZSM-5, commercially available as Pentasil, PZ-2/50H, and Y — zeolite (DAY-Wessalith). The physical characteristics of oils formed from the conversion of animal fat (rendering plant) are depicted [56]. Yields are between 30% and 70%, depending on the type of zeolite and tem­perature. Net calorific values are in the range of 40 MJ/kg compared to

TABLE 8.4 Yields and Physical Characteristics of Hydrocarbons from Catalytic Conversion of Animal Fat Using Zeolite Types H-ZSM-5 (Pentasil, PZ-2/50H) and DAY-Wessalith at Different Temperatures Parameter H-ZSM-5 PZ-2/50H, T = 550°C H-ZSM-5 PZ-2/50H, T = 400°C DAY-Wessalith, T = 400°C Yield, % 31.48 56.74 72.9 NCV, MJ/kg 40.1 40 41.3 Density, g/mL 0.83 0.85 0.81 Viscosity, mm2/s 0.92 1.01 2.29 C, % 88.6 84.5 83.4 H, % 10.7 12.5 13.5 N, % <0.14 <0.14 <0.14 S, % <0.34 <0.34 <0.34

35 MJ/kg of animal fat. All reaction products show relatively low vis­cosity and densities.

Products at T = 400°C. Again, the chemical nature of products formed from animal fat was analyzed by spectroscopic methods (see Fig. 8.6). The IR spectrum reveals the hydrocarbon nature of products. The strong C-H stretching vibrations (frequencies) at 2900 cm-1 is characteristic for alkanes. Functional groups are widely missing. The comparison to diesel from a commercial gas filling station (imprinted spectrum) shows a sim­ilar pattern [37].

Proton resonance spectroscopy depicts the chemical environment of pro­tons in the product formed from the conversion of animal fat. Figure 8.7 shows the dominance of aliphatic protons at chemical shifts of 0.9-2.25 ppm. Aromatic protons absorb at 6.5-8 ppm. The inspection of the ratio of the integral of absorptions reveals 5% aromatics for catalysis at T = 450oC. This is also reflected in the 13C-NMR spectrogram (see Fig. 8.8). However, with increasing temperature in the catalytic bed, the content aromatic alkylbenzenes increase.

Using 13C-NMR spectroscopy in-depth mode (see Fig. 8.9), negative signals at 30-20 ppm are characteristic for CH2-groups. The intensity indicates the presence of long-chain hydrocarbons. Peaks between 140 and 120 ppm denote carbon atoms of aromatic systems. The low inten­sity reflects the low content. Obviously, catalytic cracking over a Y — zeolite widely preserves hydrocarbon moiety in vegetable oil.

Figure 8.6 IR spectrum of hydrocarbons derived from animal fat at 4000C (Y-zeolite catalyst, DAY-Wessalith).

9	4 ppm	1

Minutes Figure 8.10 GC pattern of Y-zeolite conversion product of animal fat at reaction tem­perature T = 400°C. GC-14AShimadzu, column: FS-Supreme-5/H53, 30 m; temperature program: 50°C (5 min); 15°C/min to 320°C (10 min); FID detector at 320°C.

These spectroscopic findings are confirmed by gas chromatogra­phy (GC) [56]. Pyrolyzates (see Fig. 8.10) and commercial diesel (see Fig. 8.11) have a similar GC pattern. However, crude conversion products contain more volatile hydrocarbons.

GC separation on an OV101 capillary [column: 20 m X 0.3 mm, split 1:25; temperature program: 25°C (2 min), 4°C/min to 320°C] reveals double peaks in more detail (see Fig. 8.12). The first peak is for the alkene with a double bond of a given C number. The second peak is for the alkane having the same C number.

cfi
О >

You may use these hydrocarbons as a base for biofuels. However, there are markets for certain fractions of this hydrocarbon mixture. For example, the C-12 to C-18 fraction is a raw material widely used for bulk com­modities. As mineral oil prices increase, it is becoming more financially viable to produce chemical feedstock for commodities and specialities from wastes. Wastes are an energy and carbon source of the future.

Products at T = 550°C. For a given H-ZSM-5-zeolite, the nature of con­version products of lipids (animal fat) shifts to more aromatic com­pounds as the temperature increases. This is demonstrated by different NMR findings [56] for animal fat as a substrate at a reaction temperature of T = 550°C (see Figs. 8.13 through 8.15). Especially, DEPT-135 13C-NMR pattern of oil from catalytic conversion of animal fat at 550°C shows the dominance of aromatic protons and a very low amount of CH2 groups. Chromatographic separation revealed alkylbenzenes (especially 1,3,5-trimethybenzene) as main products [38].

Figure 8.13 1H-NMR spectrogram of hydrocarbons from animal fat at T = 550°C with the commercial catalyst H-ZSM-5 (Pentasil, PZ-2/50H).

Figure 8.14 13C-NMR spectrogram of hydrocarbons from animal fat at T = 550°C with the commercial catalyst H-ZSM-5 (Pentasil, PZ-2/50H).

Heating oil and a conversion product from animal fat have been used in a commercial burner (Buderus, Germany). Both oils resulted in emis­sions within legal limits (see Table 8.5).

A straightforward approach to apply vegetable oil in the most-talked — about biomass-to-liquid-fuel scheme is to use it as a co-substrate in mineral oil refineries. Advantages are low investments for peripheral facilities such as loading and storage and use of an existing infrastruc­ture for distribution and marketing. The processing of rapeseed oil as a feed component in a hydrocracker was described in 1990 [39]. The results are summarized in Table 8.6.

It is worth mentioning that rapeseed oil is converted in the hydrotreat­ment step to paraffins. The oxygen content of the vegetable oil causes an increased consumption of hydrogen to form water. Changes in quality

140 120 100 80 60 40 20 ppm Figure 8.15 DEPT-135 13C-NMR spectrogram of hydrocarbons from animal fat at T = 550°C with the commercial catalyst H-ZSM-5 (Pentasil, PZ-2/50H).

TABLE 8.5 Comparison of Combustion Parameters: Heating Oil versus Oil Derived from Y-Catalytic Conversion of Animal Fat (AF) at T = 400°C Parameter Heating oil Heating oil & oil from AF 1:1 Oil derived from AF Limiting value NCV, MJ/kg 42.0 41.5 41.3 >42* Kinetic viscosity, mm2/s 3.25 2.74 2.51 <6.0* C, % 86.5 85.7 83.4 — H, %, 14.0 13.8 13.5 — N, % <0.14 (d/l) <0.14 (d/l) <0.14 (d/l) — S, % <0.34 (d/l) <0.34 (d/l) <0.34 (d/l) <0.20* NOX, mg/m3 162 186 233 250f SO2, mg/m3 87 26 0 350f Smoke pot no. 0.0 0.4 0.4 1* *DIN 51 603 +TA Luft *1. BImSchV d/l: detection limit

occur in the middle distillate. A lower density and a higher cetane number are a quality-enhancing advantage. A drawback is the susceptibility to freezing point of the fuel. This kind of cold flow behavior would make its use in winter impossible unless special additives are supplemented [40].