Fixed-Bed Hydrogenation at Supercritical Conditions To Form Fatty Alcohols:  The Dramatic Effects Caused by Phase Transitions in the Reactor

Abstract

Natural fatty alcohols are one of the major oleochemicals and can be produced by catalytic hydrogenation of fatty acid methyl esters. In the commercial multiphase process, the hydrogen availability to the solid catalyst limits the reaction rate. By adding propane to the reaction mixture, we can utilize the unique properties of supercritical fluids, properties between those of gases and liquids. Using propane, a substantially homogeneous supercritical phase is created, whereby hydrogen has complete access to the solid catalyst. At high substrate concentrations, a rapid fall of the reaction rate was observed, and the benefits of the propane addition were completely lost. This fall depends on a split of the supercritical reaction mixture into two phases (a substrate-rich and a hydrogen-rich phase). If this phase split occurred using small catalyst particles (e32 µm), the pressure drop over the catalyst bed increased sharply because the formed liquid droplets blocked the void space in the porous catalyst bed. These two phenomena were used to deduce the product and substrate solubility in the reaction mixture. The product showed the most unfavorable solubility which increased with higher pressure. Under our process conditions (150 bar, 280 °C, and 11 mol % hydrogen), a single phase was observed up to 2 mol % (i.e., 15% by mass) product. Besides the minimum pressure in the catalyst bed, substrate transport limitation could be shown to be an important factor in process optimization. Therefore, egg-shell catalysts or fine catalyst particles (100-300 µm) should preferably be used in the continuous supercritical reactors.

Research on Hydrogenation of FAME to Fatty Alcohols at Supercritical Conditions

“…A comparison between the novel hydrogenation technology under supercritical conditions in this work and the conventional process of liquid-phase hydrogenolysis of fatty ester shows that the ratio of hydrogen to FAME in reaction system is reduced by 90-98%, and the temperature and pressure of reaction is decreased significantly also in this work. It can be concluded from the reaction conditions that the pressure of this novel process here is lower by about 5.0 MPa than that for the process reported by other researcher [4] .…”

“…The study on hydrogenation of FAME to FA using propane and carbon dioxide as solvents under supercritical conditions has been reported previously. But the reaction is carried out above 15.0 MPa [2][3][4][5] , and such high operation pressure will seriously increase the capital and operating costs. It is thus clear that a low-pressure hydrogenation process for FAME conversion to FA under supercritical conditions can lead to reduction in the capital and operating costs of the process while the advantages of supercritical reactions are maintained, which can also lead to great social and economic benefits.…”

Sunflower oil hydrogenation on Pd/C in SC propane in a continuous recycle reactor

“…In our experiments, noncondensing conditions were monitored by watching the increase in pressure drop across the reactor in the same way as reported by Van den Hark and Härröd (2001). We observed that one-phase equilibrium conditions are theoretically met but the system requires a finite or longer contact time to reach fluid uniformity.…”

Fatty Acid Deoxygenation in Supercritical Hexane over Catalysts Synthesized Hydrothermally for Biodiesel Production

“…The use of supercritical fluids in deoxygenation can allow the operating temperature and process duration to be decreased. Moreover, compounds such as alcohols and hydrocarbons in the supercritical state can serve as hydrogen atom donors . Thus, no molecular hydrogen is required for the process.…”