The Use of Supercritical Fluids to Reduce the Number of Phases in Catalytic Hydrogenation: The Reaction of Fatty Acid Methyl Esters to Fatty Alcohols
Doktorsavhandling, 2000

Supercritical fluids have unique properties - between those of liquids and gases - making them attractive reaction media. In this study, propane was used to change a heterogeneous catalyst from being substrate-covered in gas-liquid hydrogenation reactions into hydrogen-covered in our experiments. For the investigated process this resulted in an increase in the reaction rate by 2 orders of magnitude and high yields. The standard high-pressure (i.e. 200-300 bar) alcohol process consists of a multi-phase system: liquid (substrate and product), gas (hydrogen) and solid (catalyst). Hydrogen availability for the catalyst is the rate-determining step in this process, and therefore only a fraction of the catalytic potential is used. Dissolving hydrogen and substrate/product into one supercritical phase, consisting of mainly propane, eliminates the gas-liquid interface and allows a surplus of hydrogen at the catalyst. In the lab-scale fixed-bed reactor (i.e. 0.05-5 ml), the activity of commercial copper catalysts has been studied as a function of the temperature, hydrogen and substrate concentration in the reaction mixture. The influence of mass transport and catalyst deactivation on the process was also investigated. At 280 ºC, a system pressure of 150 bar and about 80 mol% propane, reaction times of some seconds were needed to reach high product yields. The reaction rates were comparable to those in gas-phase hydrogenation, and product space velocities of 100 h-1 were reached. In some cases, transport limitation of substrate, not hydrogen, occurred. To improve the mass transport small catalyst particles, preferably between 100-300 µm, were used. At high substrate concentrations (> 2 mol%, i.e. 15 wt. %.), a rapid fall in the reaction rate and an increase in the pressure drop were observed. This was interpreted as a phase split and shows the necessity of having the reaction mixture in a single phase. From these observations phase-equilibria data were deduced. It was possible to control overhydrogenation of the fatty alcohols to alkanes. With high hydrogen concentrations (20 mol%), catalyst deactivation was reduced and reached values similar to those in industrial processes.

fatty alcohols

single phase

deactivation

mass transport

phase reduction

supercritical fluids

hydrogenation

propane

kinetics

catalysis

Författare

Sander van den Hark

Institutionen för livsmedelsvetenskap

Ämneskategorier

Kemi

ISBN

91-7197-968-9

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 1652