The proposed reaction pathway for catalytic hydrotreatment of pyrolysis oil (Figure 10) implies that the rate of the hydrogenation route should be much higher than the rate of the repolymerisation route to obtain good quality upgraded pyrolysis oil (low molecular weight, low viscosity, low coking tendency). An obvious solution is the development of highly active hydrogenation catalysts. These studies will be reported in the next paragraph of this paper. However, a smart selection of process conditions and reactor configurations may also be considered, particularly to enhance the rate of the hydrogenation/hydrodeoxygenation pathway compared to the repolymerisation pathway. In this respect, it is highly relevant to gain some qualitative insights in the factors that determine the rate of the individual pathways (hydrogenation versus repolymerisation).

A schematic plot is presented in Figure 11, where an envisaged reaction rate (arbitrary values, in mole reactant/ min) is presented versus the actual reaction temperature. The lines drawn are taken in case (i) gas-to-liquid mass transfer determines the overall reaction rate (ii) the catalytic hydrotreatment reactions dominate, and (iii) polymerisation reactions prevail. Figure 11 is derived on basis of simplified kinetics for the glucose hydrogenation — polymerisation reactions, but a detailed outline of all assumptions made is beyond the scope of the presentation here. For this reason the exact values on the x — and y-axes are omitted. The following relations are taken into account to derive Figure 11:

• The conversion rate due to the hydroprocessing reactions RH (mol/m3r. s) can be simplified as a product of the intrinsic kinetic rate expression kR and the surface area

available per reactor volume. Being a catalytic reaction, the influence of temperature can be rather high.

• The overall gas-to-solid mass transfer rate of hydrogen depends on reactor geometry and operating variables. In case of stirred tank reactors (including batch-wise operated autoclaves), the actual stirring rate is important, while in packed bed the catalyst particle wetness is relevant. In both, the concentration of hydrogen (thus hydrogen pressure) is important, together with catalyst particle size, and, to a limited extent, temperature.

• The rate of polymerisation, Rp, will depend largely on the temperature, and, being a reaction with order in reactant(s) > 1 (and probably up to 2 or 3), on the concentration of the reactant.

Temperature (K)

A number of options may be envisaged to promote the hydrogenation pathway:

• Increase the hydroprocessing reaction rate, for instance by a higher catalyst intake or by an increase in the effective hydrogen concentration in the liquid (pressure, application of a solvent with a high hydrogen solubility).

• Reducing the polymerisation reaction, a. o. by performing the initial stabilisation step at a low temperature (< 100°C) and reduction of the concentration of the reactants (a. o. by dilution).

• Increase the overall gas-to-liquid mass transfer rate in case the reaction is performed in the gas-liquid mass transfer limited regime. This may be possible by increasing the mass transfer surface area in the reactor, higher mass transfer coefficient and / or increasing the concentration difference between the gas and the liquid.