**3. Supercritical water process for the production of green fuels from oils/ fats/lipids**

The catalytic hydrothermolysis (CH) is another process which produces regular hydrocarbon transportation fuels from oils or fats. The supercritical water performs the hydrolysis of vegetable oils to produce biocrude oil. This biocrude oil consists of a wide range of compounds such as straight chain, branched chain and cyclic hydrocarbons (alkanes, alkenes and aromatics etc.,) and their distribution varies depending upon the processing conditions and feed stock. An increase in temperature towards critical point the causes oils/fats become miscible with water around 300-330o C [16].

**Figure 3.** Catalytic hydrothermolysis process

The major influencing factors on the yields of biodiesel are type of alcohol, reaction tempera‐ ture, oil to alcohol ratio, reaction time and pressure. The critical temperatures increase with increase in chain length or molecular weight of the alcohol. At the same temperature, the acidity of longer chain alcohols tends to decrease resulting in slower reactivity or slightly more reaction time than the short chain alcohols. However the cold flow properties of the biodiesel produced with long chain alcohols are better than the biodiesel produced with short chain alcohols [12]. All these factors influence the selection of alcohol, as it affects both the economics and energetics of the process. The yield of biodiesel increases with the increase in reaction temperature above the critical conditions of the alcohols. Beyond the optimum temperature, the yield may start decreasing due to degradation of fatty acids at higher temperatures. Usually this also depends on the fatty acid profiles; as poly unsaturated fatty acids (PUFAs) are thermally unstable at higher temperatures. Feed stocks having more PUFAs may give higher yields at slightly lower temperatures than the feed stocks having less PUFAs [13]. The usual

C, which also depending on the other

Yield: 84%

Yield: 86%

Yield: 89%

Reaction time:45 min.

Reaction time:45 min.

Reaction time:10 min.

optimum reaction temperature ranges between 290-350o

Yield: 91%

Yield: 99%

Yield: 94%

**Table 3.** Variation in the yields of biodiesel with different alcohols

Reaction time:24 min.

Reaction time:15 min.

Reaction time:4 min.

**Feed stock FAME FAEE FABE**

Yield: 85%

Yield: 94%

Yield: 94%

As mentioned earlier the long chain alcohols need higher reaction temperatures to get higher yields of biodiesel than the short chain alcohols. The molar ratios of oil to alcohol vary for different feed stocks with different alcohols. This usually ranges between 1:40-45 at optimum reaction temperature. A lower amount of alcohol negatively affects the yields as the reverse transesterification reaction tries to go backwards. On the other hand more alcohol also reduces the yields by changing the critical point of the mixture to higher temperatures, where the optimum temperature of the reaction is not sufficient to perform the forward reaction. This also imposes another economic barrier as this extra alcohol requires more energy to heat, and will need to be recycled after separation process [14]. Supercritical alcohol processing is very

Reaction time:28 min.

Reaction time:45 min.

Reaction time:10 min.

reaction parameters.

168 Biofuels - Status and Perspective

Oil to alcohol molar ratio:

Oil to alcohol molar ratio:

Oil to alcohol molar ratio:

C

C

C

Camelina oil [15] Reaction temperature: 310o

Rapeseed oil [12] temperature: 300o

Rapeseed oil [12] temperature: 350o

1:40

1:42

1:42

During this process, triglycerides undergo hydrolysis reaction to form free fatty acids and glycerol. Compared to thermal cracking and pyrolysis, the formation of gaseous products will be reduced to the minimum in CH process. At higher temperatures, decarboxylation and cyclization reactions of fatty acids and glycerol produce alkanes, alkenes, carbon dioxide and water. The aqueous phase contains small amounts of smaller hydrocarbons and glycerol. Use of external catalyst such as KOH and NaOH enhances the production of alkanes. Use of metal oxide catalysts favors the production of alkenes. [17]. The biocrude oil produced during this process must be hydrotreated or reformed to meet commercial fuel standards. The upgrading process is discussed later in the chapter. Catalytic hydrothermolysis is performed to produce jet fuel and diesel range hydrocarbons. Oil and water are processed at 9:1 (vol. ratio) and between 450-475o C. The resulting biocrude is then upgraded with commercial nickel catalyst to produce jet fuel and diesel range hydrocarbons [18]. The schematic of the CH process is shown in Figure 3. The hydrogen utilization during the CH process can be reduced compared to direct catalytic cracking of oils. The fatty acid profile of feed stock, water to oil ratio, rate of heating and reaction pressure determines the final product properties.
