Mass-transfer effects in the biphasic hydroformylation of propylene

The hydroformylation of propylene to butyraldehyde is an important industrial process. In this reaction system, carbon monoxide, hydrogen, and propylene are converted to n-butyraldehyde in an aqueous phase containing a water-soluble rhodium catalyst. This reaction system consists therefore of three different phases: the aqueous catalyst phase, the organic butyraldehyde phase, and the gas phase. A modeling study indicated that an optimum value of the production rate is reached at a certain power input. It was shown that mass transfer plays an important role in this reaction system and that accurate knowledge of the mass-transfer parameters in the gas-liquid-liquid system is necessary to predict and to optimize the production rate. Mass-transfer experiments with CO, H-2, and propylene in water and in solutions consisting of water with different amounts of n-butyraldehyde were carried out in a stirred autoclave. The results indicated that the addition of small amounts of butyraldehyde caused a relatively small increase increase in k(L)a, which was probably caused by a larger increase of the gas-liquid interfacial area, a, accompanied by a decrease in the mass-transfer coefficient, k(L). Near the point of maximum solubility of butyraldehyde, a strong (factor of 3) increase in k(L)a was observed. The most logical explanation for this is an increase in k(L), because measurement of the interfacial area did not show such an increase. This increase in k(L), i.e., the disappearance of the initial decrease, might be due to the disappearance of the rigid layer of butyraldehyde molecules at the gas-liquid interface by formation and spreading of a butyraldehyde layer around the bubble.