Tray columns flow regimes

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. 

It is worth emphasizing that Eqs. (13-61) to (13-68) hold regardless of the models used to calculate the interphase transport rates and EJ. With a mechanistic model of sufficient complexity it is possible, at least in principle, to account for mass transfer from bubbles in the froth on a tray as well as to entrained droplets in a spray, as well as transport between the phases flowing over and through the elements of packing in a packed column. However, a completely comprehensive model for estimating mass-transfer rates in all the possible flow regimes does not exist at present, and simpler approaches are used. 

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. 

In the development of the above series of equations, Zuiderweg has used the work of many prior investigators and relied heavily on the data recently released by Fractionation Research, Inc. (iHki), as repotted by Sakata and Yanagi on the performance of two types of commercial perforated tray. Zuiderweg also presents correlations that define the flow regime transitions, pressure drop, entrainment, column capacity, and other operating panameiers of sieve trays. 

Effect of column diameter (at constant UV and percent of flood). As column diameter increases, both the liquid and vapor flow rates increase as the square of the diameter. The area for vapor flow also increases as the square of the diameter, so the vapor load remains unaffected. On the other hand, the area available for liquid flow only increases in proportion to the diameter. Therefore, the liquid rate per unit of weir length increases, the increase being proportional to the column diameter. The operating point on Fig. 6.29 will therefore shift horizontally to the right, toward the emulsion regime. Increasing the number of liquid passes on the tray reverses the above action, and shifts the operating point back to the left. 

These regimes appear to be an approximate function of the dimensionless flow parameter, introduced earlier in connection with tray and packed column hydraulics ... 

In the froth regime, an increase in vapor flow reduces tray froth density. Froth height above the weir rises, and some of the tray liquid inventory spills over the weir into the downcomers. The expelled liquid ends up in the bottom of the column, and bottom level initially rises (Fig. 16.5). This is opposed to the expected response, and is termed inverse response. 

The incidence of inverse response depends on column internals and vapor rates. It has been suggested (362) that inverse response may occur with valve trays at all vapor rates, while sieve trays may give inverse response at low rates, direct response at high rates, and effectively dead time at intermediate rates. It is unknown whether columns operated in the spray regime experience inverse response in principle they should not, since tray liquid holdup is unaffected by vapor flow rates within this regime (204, 244). 

Solid or fluid particles (bubbles or drops) suspended in another fluid are moving in a large-scale flow. Such a flow can be present in fluidized beds with gas or liquid as continuous phase, for instance dryers, adsorbers, or crystallizers. Fluid particles are moving in bubble or drop columns or in froth (spray or bubble regime) present on coliunn trays. 

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