Heteroatoms are atoms other than carbon (C) and hydrogen (H) and are often encountered into bio — and fossil — based feedstocks. They include sulfur (S), nitrogen (N) and in the case of bio-based feedstocks oxygen (O). In particular oxygen removal is of outmost importance as the presence of oxygen reduces oxidation stability (due to carboxylic and carbonylic double bonds), increases acidity and corrosivity (due to the presence of water) and even reduces the heating value of the final biofuels. The main deoxygenation reactions that take place include deoxygenation, decarbonylation and decarboxylation presented in Schemes 7, 8 and 9 re­spectively [7]. The main products of deoxygenation reactions include n-paraffins, while H2O, CO2 and CO are also produced, but can be removed with the excess hydrogen within the flash drums of the product separation section. It should be noted however that these particular reactions give the paraffinic nature of the produced biofuels, and for this reason the hydrotreated products are often referred to as paraffinic fuels (e. g. paraffinic jet, paraf­finic diesel etc)

R-CH2COOH + 3 H2 ———— ► R-CH2CH3 + 2 H2O

Scheme 7.

R-CH2COOH + H2 ———— ► R-CH3 + CO + H2O

Scheme 8.

R-CH2COOH + H2 ———— ► R-CH3 + CO2

Scheme 9.

The other heteroatoms, i. e. S and N are removed according to the classic heteroatom remov­al mechanisms of the fossil fuels in the form of gaseous H2S and NH3 respectively.

1.1.3. Isomerization

The straight chain paraffinic molecules resulting from the aforementioned reactions, even though they offer increased cetane number, heating value and oxidation stability in the bio­fuels which contain them, they also degrade their cold flow properties. In order to improve the cold flow properties, isomerization reactions are also required, which normally take place during a second step/reactor as they require a different catalyst. Some examples of iso­merization reactions are given in Schemes 10 and 11.

R-CH2-CH2-CH3 ———- ► R-CH-CH3

CH3

Scheme 10.

Scheme 11.