mixed froth regime

Increasing superficial gas velocity

Distillation: Reactive distillation;

low weirs, low liquid holdup; high weirs, high liquid holdup;

liquid RTD not important liquid RTD very important

Fig. 7.3 Choice of hydrodynamic flow regimes on reactive and non-reactive trays

A tray column can be operated in the spray, mixed froth, or bubbly flow regimes (Fig. 7.3). Conventional (i.e., non-reactive) distillation columns usually operate at high superficial vapor velocities in the spray or froth regime. This is because of the desire to increase throughput in the column and to increase the vapor-liquid interfacial area. There is no need to aim for maximum liquid hold-up in conventional distillation. The situation with respect to RD is quite different. The reaction takes place in the liquid phase and in order to allow more 'room' for chemical reaction we should aim for high liquid hold-ups and liquid-phase residence times. Therefore the preferred regime of operation is the bubbly flow regime, which is achieved by operating at much lower superficial vapor velocities. The choice of higher weirs ensures higher liquid hold-up on the trays. Bubble cap trays are appropriate choices for RD columns, as these have greater liquid holding capacity than sieve trays. Furthermore, there is no danger of weeping of the liquid phase from bubble cap trays. Reverse-flow trays with additional sumps can be used to increase the liquid residence time; a specially designed tray for maximizing liquid hold-up is used in the Eastman process for methyl acetate manufacture [4].

Though the hydrodynamics of distillation tray columns is well researched and documented [5, 6], the available information is usually not sufficient for RD tray design. This is because liquid-phase residence time distribution (RTD) is not usually relevant for conventional distillation but is of crucial importance in RD tray design. Baur et al. [7] have studied the influence of tray hardware design on the conversion, and selectivity, of hydration of ethylene oxide (EO) to ethylene glycol (EG); one of their simulation results (using a rigorous NEQ stage model) is shown in Fig. 7.4.

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