Tray sizing

The rigorous column models allow the design engineer to carry out tray sizing and hydraulics calculations for the basic types of distillation trays and for some types of random and structured packing. Different commercial simulators use different tray sizing correlations, but they all follow a method similar to that described in Chapter 11. 

Continuing the problem defined in Example 4.3, use a rigorous simulation to carry out tray sizing and estimate the required column diameter. 

To size the trays in UniSim Design, we must activate the tray sizing utility (from the Tools menu via Tools/Utilities/Tray Sizing). When sieve trays are selected with the default spacing of 609.6 mm (2 ft) and the other default parameters shown in Figure 4.25, then the results in Figure 4.26 are obtained. The column diameter is found to be 4.42 m. 

Tray size (cm) 60x75x4 65x 100x2.2 65x100x2 60x70x3.8 60x70x2.5 ... 

Hettich IR-Dancer sample tray size 200 x 200mm, min. 4 mtp 2 models available (chamber diameter 300 or 360 mm), custom vial holders possible... 

The column cross-sectional area, and hence the tray diameter, is sized such as to prevent flooding. Tray sizing is initially based on preventing entrainment flooding, but is subsequently checked for downcomer flooding and other considerations. Above a certain vapor velocity, the vapor flood velocity, liquid droplets are entrained with the vapor, causing flooding. The tray should be sized such that the actual vapor velocity is below the vapor flood velocity. 

The column will be rated with the nearest larger standard tray size ... 

The trays thereby formed are subsequently filled, lidded and cut out into specific tray sizes and configurations. 

Latest innovations include a double forming operation which reduces the tray size to 20% (Advanced Forming Technology (AFT) process). These tend to use Teflon stretching dies. 

Dynamic simulation of distillation column needs also tray sizing. We considered sieve trays with 0.4 m diameter of and 0.05 m static liquid height. 

When the number of passes is restricted, either by the short liquid path length (3 above) or by fear of maldistribution (4 and 5 above), liquid loads up to 20 gpm per inch of weir can and have been used (138). Care-fiil sizing of trays and downcomers is required with such high liquid loads, since tray sizing correlations often require extrapolation beyond their recommended limits at such loads. Short tray spacinp (< 18 to 24 in) should also be avoided in such applications. 

---