Experimental study on the role of water and kinetic enhancement in the one-step synthesis of methyl isobutyl ketone via catalytic distillation

Abstract

Two pilot scale experiments investigating the one-step synthesis of methyl isobutyl ketone (MIBK) from acetone and hydrogen were conducted via catalytic distillation (CD) utilizing a Pd/Nb2O5/SiO2 catalyst at a reaction temperature of 160 °C and system pressure of about 1.4 MPa. Moisture analysis of liquid samples extracted about the major axis of the CD column revealed that when the reactor was initially operated at 100% reflux as suggested in the patent of Lawson and Nkosi, water had accumulated in the top half of the reactor as evidenced by water concentrations of 9.4 and 21.0 wt% in the reactive and rectification sections respectively, resulting in the suppression of the DAA dehydration reaction and consequently a relatively low MIBK productivity. However, operation of the CD column at 83–97% reflux enabled the rapid in situ separation of water from the reactive section via the overhead distillate stream, which directly resulted in increases in the MIBK productivity and the hydrogen uptake efficiency by factors of about 20. This result demonstrates a unique advantage of CD technology, whereby a substantial kinetic enhancement may be realized from the selective in situ separation of a kinetically relevant inhibiting substrate from the reactant mixture. MIBK in excess of 50 wt% was observed in the reboiler product during this mode of operation, which is significant since state of the art processes for MIBK production typically yield less than 30 wt% MIBK in the reactor effluent. The reboiler product was found to be essentially moisture free, which is also significant since MIBK and water are known to produce azeotropic mixtures resulting in difficult and costly downstream separations. An experiment was conducted to study the effects of the hydrogen and reflux flow rates on the MIBK yield. The CD process for MIBK synthesis was found to be controlled by the rate of external mass transfer of hydrogen. The results indicate that the catalyst wetting efficiency affects the catalytic hydrogenation as evidenced by an increase in mesityl oxide conversion with decreasing reflux flow rate. This suggests the catalyst wetting efficiency is an important design criterion for this process.