(666a) Homogeneous Liquid Phase Epoxidation of Light Olefins: Mass Transfer Investigations

Propylene oxide and ethylene oxide are bulk intermediate chemicals for a broad spectrum of consumer products such as glycols, polyols, and glycol ethers. The industrially practiced processes for the manufacture of these epoxides are highly energy intensive and plagued with selectivity issues. Recently, we demonstrated an environmentally benign alternative for the selective synthesis of the epoxides [1,2]. The proposed concept involves the transport of the olefin (either propylene or ethylene) from the gas phase into the aqueous phase containing the dissolved catalyst methyltrioxorhenium and oxidant hydrogen peroxide to form their corresponding epoxides. The oxidation of olefins is highly selective towards the desired product and produces water, a harmless byproduct. In this process, the transport of the olefin substrate from the gas phase to the liquid phase is the rate-limiting step. To better understand this step, mass transfer studies were carried out in a 50 ml stirred vessel (a ReactIR). Propylene uptake experiments were performed at ambient temperature and constant propylene pressure, achieved by replenishing the propylene dissolving into the liquid phase from an external reservoir. The transient pressure profiles in the external reservoir at varying agitation speeds are therefore a direct measure of the rate of propylene dissolution. A model was developed to estimate the gas/liquid mass transfer coefficient. The rate of propylene dissolution increased with stirring speed with no change beyond 1000 rpm. The amount of propylene transferred into the liquid phase at saturation is consistent with published data and that predicted by the thermodynamic models. The measured mass transfer coefficients at various stirring speeds are consistent with those predicted from correlations. Kinetic studies performed under conditions of negligible gas/liquid mass transfer limitations will also be presented and discussed.

References:

1. Lee, H-J.; Shi, T-P.; Busch, D. H.; Subramaniam, B. A greener, pressure intensified propylene epoxidation process with facile product separation. Chem. Eng. Science, 2007, 62, 7282-7289.

2. Lee, H-J.; Ghanta, M.; Busch, D. H.; Subramaniam, B. Towards a CO2 -free ethylene oxide process: Homogeneous ethylene oxide in gas-expanded liquids. Web published. doi: 10.1016/j.ces.2009.02.008