(6e) Liquid Continuous Distillation

The bubble column is a simple arrangement of a vessel containing a gas sparger and little else. When operated on a continuous basis, liquid is fed to the column at some selected elevation, and gas is passed up through the vessel in the form of bubble swarms. The device has been used extensively for gas-liquid reactions. Variations include the use of suspended solid catalysts and slurry feeds, in both cases taking on a more complex character because of the third phase. To deal with heat effects, cooling surfaces can be installed within the vessel. There have been some cases where trays have been inserted to mitigate the strong trend toward complete mixing of liquid in the contactor. Some example process applications are fermentations, hydrogenations and oxidations. Experience with bubble columns has been limited to very low gas superficial velocities (< 0.03 m/s), with equivalent fractional gas holdup values of 0.1-0.3, and with unidirectional mass transfer. A large literature exists on bubble column operating characteristics operated under non-distillation conditions at low loadings. In this paper we go far beyond the usual bubble column operating ranges, and report data on liquid-continuous operation up to the capacity limitations of the usual vapor-continuous distillations using trays and packings. The work has been done in the distillation system pilot plant of the Separations Research Program (SRP), with a column diameter of 0.51 m and use of a conventional overhead condenser and a kettle reboiler. Comparisons are made with conventional trays and structured packings for pressure drop, gas holdup, and mass transfer efficiency.