The catalytic hydrotreatment of liquid biomass converts the contained triglycerides/lipids into hydrocarbons at high temperatures and pressures over catalytic material under excess hydrogen atmosphere. The catalytic hydrotreatment of liquid biomass process is quite simi­lar to the typical process applied to petroleum streams, as shown in Figure 3. A typical cata­lytic hydrotreatment unit consists of four basic sections: a) feed preparation, b) reaction, c) product separation and d) fractionation.

In the feed preparation section the liquid biomass feedstock is mixed with the high pressure hydrogen (mainly from gas recycle with some additional fresh make-up hydrogen) and is preheated before it enters the reactor section. The reactor section consists normally of two hydrotreating reactors, a first guard mild hydrotreating reactor and a second one where the main hydrotreating reactions take place. Each reactor contains two or more catalytic beds in order to maintain constant temperature profile throughout the reactor length. Within the re­actor section all associated reactions take place, which will be presented in more detail at a later paragraph.

The reactor product then enters the separator section where, after it is cooled down, it enters the high pressure separator (HPS) flash drum in which the largest portion of the gas and liq­uid product molecules are separated. The gas product of the HPS includes the excess hydro­gen that has not reacted within the reactor section as well as the side products of the reactions including CO, CO2 H2S, NH3 and H2O. The liquid product of the HPS is lead to a second flash drum, the low pressure separator (LPS), for removing any residual gas con­tained in the liquid product, and subsequently is fed to a fractionator section. The fractiona­tor section provides the final product separation into the different boiling point fractions that yield the desired products including off-gas, naphtha, kerosene and diesel. The heaviest molecules return from the bottom of the fractionator into the reactor section as a liquid recy­cle stream.

In order to improve the overall efficiency, a liquid recycle stream is also incorporated, which in essence consists of the heavy molecules that were not converted. The gas product from the HPS and LPS, after being treated to remove the excess NH3, H2S, CO and CO2, is com­pressed and fed back to the reactor section as a gas recycle stream in order to maintain a high pressure hydrogen atmosphere within the reactor section.

Gas

-►Naphtha

-►Kerosene

-►Diesel

Figure 3. A typical process diagram of catalytic hydrotreatment of liquid biomass

1.1. Reaction mechanisms

Several types of reactions take place during catalytic hydrotreatment of liquid biomass, based on the type of biomass processed, operating conditions and catalyst employed. The types of reactions that liquid biomass undergoes during catalytic hydroprocessing include: a) cracking, b) saturation, c) heteroatom removal and d) isomerization, which are described in more detail in the following section.

1.1.1. Cracking

As the molecules included in the various types of liquid biomass can be relatively large and complicated, cracking reactions are desired to convert them into molecules of the size and boiling point range of conventional fuels, mainly gasoline, kerosene and diesel. A character­istic reaction that occurs during catalytic hydrotreating of oils / fats is the cracking of trigly­cerides into its consisting fatty acids (carboxylic acids) and propane as shown in Scheme 1 [5][6]. This reaction is critical as it converts the initial large triglycerides molecules of boiling point over 600°C into mid-distillate range molecules (naphtha, kerosene and diesel).

Other cracking reactions may take place however such as those described in Schemes 2 and 3, depending on the type of molecules present in the feedstock. For example Scheme 2 is a cracking reaction which may occur during catalytic hydrotreatment of pyrolysis oil which includes polyaromatic and aromatic compounds. Alternatively Scheme 3 may follow deoxy­genation of carboxylic acids on the produced long chain paraffinic molecules, leading to smaller chain paraffins, during the upgrading of Fischer-Tropsch wax.

O

ch2-o-c-r

I

Ch-o-co-r + 2h2 —► 3r-ch2cooh + ch3-ch2-ch3

CH2-O-C-R

O

Triglyceride carboxylic acid propane

Scheme 1.

Scheme 2.

R — R’ + H2

Scheme 3.

1.1.2. Saturation

Saturation reactions are strongly associated with catalytic hydrotreating as the introduction of excess hydrogen allows the breakage of double C-C bonds and their conversion to single
bonds, as shown in the following reactions. In particular the saturation of unsaturated car­boxylic acids into saturated ones depicted in Scheme 4, is a key reaction occurring in lipid feedstocks. Furthermore other saturation reactions lead to the formation of naphthenes by converting unsaturated cyclic compounds and aromatic compounds as in Scheme 5 and 6, which are likely to occur during upgrading of pyrolysis oils.

RCH = CH-COOH + H2 —► rch2ch2cooh

Scheme 4.

Scheme 5.

Scheme 6.

As a result of this reaction the produced saturated molecules are less active and less prone to polymerization and oxidation reactions, mitigating the sediment formation and corrosion phenomena appearing in engines.