Supercritical hydrogen

Figure 5.3 Schematic of a continuous supercritical hydrogenation reactor...

The physico-chemical effect of high pressure, especially in the supercritical state, to enhance the solubility and phase conditions of the components involved. Supercritical hydrogenation, or enzymatic syntheses are offer new steps with high pressure. Supercritical water oxidation at high pressure represents an efficient method for the decontamination of wastes. 

Supercritical hydrogenation is just one example of continuous reactions which can be carried out in SCCO2 solution. Other reactions which have been carried out successfully include Friedel-Crafts alkylation of aromatics by alcohols [64], the dehydration of alcohols to form ethers [65] (using acid catalysts), and the hydroformylation of alkenes [52] (using rhodium catalysts immobilized on Si02). In each of these reactions, it is possible to obtain a selectivity which is at least as good, and often better, than with conventional solvents. However, the precise role of the scCC>2 in these reactions is not as obvious as in supercritical hydrogenation. 

In comparison to the industrial batch process at low pressure and with mass transfer controlled, continuous supercritical hydrogenation has the advantage of accelerating the reaction rate and product formation very significantly. Small-scale laboratory experiments initially indicated an increase in product formation of approximately 500-fold. 

Considerable scientific argument has arisen around the question of whether supercritical hydrogenation reactions proceed faster and more efficiently in either a single or multiple phases indeed conflicting reports have been published [44— 47]. Much of this debate has revolved around the LHSV of a reaction, but this parameter only addresses part of the issue from an industrial perspective. Other important factors which have to be taken into account include catalyst lifetime, overall conversion, and product selectivity, as well as the solvent compression costs need. The situation has been at least partly resolved by a key paper by Nunes da Ponte and co-workers [10]. They have shown that biphasic reactions can sometimes be faster than monophasic ones, because the concentration of subshate (as opposed to H2) is lower under monophasic conditions. 

J. J. Purewal, et al. (2009). Pore size distribution and supercritical hydrogen adsorption in activated carbon fibers. Nanotechnology, 20, 204012 (6pp). 

Yamaguchi, T. (1998) Structure of Subcritical and Supercritical Hydrogen-Bonded Liquids and Solutions, Journal of Molecular Liquids 78, 43-50... 

WO 97/38955 Poliakoff, Martyn Swan, Thomas M. Tacke, Thomas Hitzler, Martin G. Ross, Stephen K. Wieland, Stefan Supercritical Hydrogenation... 

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