Column distillation flow regime

Stirred tanks are modeled assuming that both phases are well mixed. Tray columns are usually modeled as well mixed on each tray so that the overall column is modeled as a series of two-phase, stirred tanks. (Distillation trays with tray efficiencies greater than 100% have some progressive flow within a tray.) When reaction is confined to a single, well-mixed phase, the flow regime for the other phase makes little difference but when the reacting phase approximates piston flow, the flow regime in the other phase must be considered. The important cases are where both phases approximate piston flow, either countercurrent or cocurrent. 

Baffle columns and shower tray columns, shown in Fig. 2.27, are characterized by relatively low liquid dispersion and very low pressure drops. The major application of this type of flow regime is in cooling towers, where the water flows across wooden slats and very large volumes of gas are handled. Here economics dictate that fans rather than compressors be used. Some gas absorption and vacuum distillation columns employ baffle or shower trays. 

Heat transfer drops off> rate than expected and Ap increases faster than expected fouling because of oversized kettle reboiler on distillation column or change in pH or flow regime laminar when design was turbulent or higher level of contamination in fluids or crud carry over from upstream equipment (e.g. silica from catalyst in upstream reactor) or compensation for oversize by reduced coolant flow-rate. 

The overall efficiency lumps together everything that happens in the columns. What variables would we expect to affect column efficiency The hydrodynamic flow properties such as viscosity and gas flow rate would affect the flow regime, which affects efficiency. The mass transfer rate, which is affected by the diffusivity, will in turn affect efficiency. Overall efficiency is usually smaller as the separation becomes easier increases). The column size can also have an effect. Correlations for determining the overall efficiency will be discussed in Chapter 10. For now, we will consider that the overall efficiency is determined from operating experience with similar distillation columns. 

In high-pressure (>200-psi) distillation, vapor velocity is low and liquid flow rate is relatively high, which coincides with an operating point in the emulsion regime. If the column operates at low liquid... 

---