Trays vapor-liquid channeling

Uneven vapor flow bubbling-up through the tray deck will promote vapor-liquid channeling. This sort of channeling accounts for many trays that fail to fractionate up to expectations. To understand the cause of this channeling, we will have to quantify total tray pressure drop. 

Figure 2.3 Out-of-level tray causing vapor-liquid channeling.

Meanwhile, all the liquid flow must drain through that portion of the tray that is lower. The net result, as can be seen in Fig. 7.2, is dry packing in one portion of the tower, and overrefluxing in the adjacent portion. This is called vapor-liquid channeling, and this is the root cause of poor fractionation efficiency in any tower. 

In the Coastal Refinery in Aruba, we used sieve trays with one-half-inch holes, which seemed to work fine. Avoid packed towers. They are subject to vapor-liquid channeling and poor fractionation efficiency due to sloppy installation, fouling, or poor liquid feed distribution. When calculating the required hole area for the trays, don t forget that the vapor loads and hence the required tray hole area will substantially diminish as the vapor flows up the column from the reboiler to the feed tray. This will normally require a reduction in the tray deck hole area in proportion to the reduced vapor flow rate. 

Vapor-Liquid Loads and Reflux Ratio Vapor and liquid loads, as well as the reflux ratio, have a small effect on tray efficiency (Fig. 14-43) as long as no capacity or hydraulic limits (flood, weep, channeling, etc.) are violated. 

Another important consideration in tower design is tray downcomers size. At high ratios of liquid flow to vapor flow a proportionally greater area on the tray must be allotted to the downcomer channel opening. Downcomers are designed from basic hydraulic calculations. If the downcomer is inadequately sized and becomes filled with liquid, liquid level will build on the tray above. This unstable situation will propagate its way up to the tower and result in a flooded tower condition. Excessive entrainment can also lead to this same condition and, in fact, is usually the cause of flooding. 

Large fractional hole area, long flow path relative to tray spacing and high liquid flow rate are the key factors leading to the formation or intensification of vapor cross-flow channeling on sieve and valve trays. 

At hi liquid rates (>6 gpm/in of outlet weir), high ratio (>2.5) of flow-path length to tray spacing, and a high fractional hole area (> 11 percent), cross flow of vapor in opposite direction to the liquid can build up froth near tray inlet and center. The froth buildup raises the liquid head in the tray inlet and center. This channels more vapor to the tray outlet region, thus accelerating the cross flow. The inlet froth keeps rising until it reaches the tray above. 

Tray columns (also called plate columns) are particularly well suited for large installations clean, noncorrosive, nonfoaming liquids and low-to-medium liquid flow rate applications. They also are preferred when internal cooling is required in the coluitui. With appropriate tray d ign, cooling coils can be installed on individual trays, or alternatively, liquid can be removed from the column at one tray, cooled, and returned to another tray. Tray columns also are advantageous for separations that require a large number of transfer units because they are not subject to channeling of vapor and liquid streams winch can cause problems in tall packed columns. 

Parallel (Streuber system) or channels mounted crosswise to the (TltoRMANN system) liquid-flow direction are used. Annular channels are also used, in which case the flow on the tray is radial. The type and design of the head slits define the direction of the vapor flow and, therefore, an acceleration or deceleration of the liquid (TRormann tray). 

These surface temperatures correlate with the temperature of the vapor exiting from the chimney tray, which in turn is a function of the reflux rate to a particular quadrant of the packed bed. (This assumes that the vapor flow to the bottom of the bed is well-distributed. Vapor distribution is much more easily achieved than liquid distribution. However, vapor channelling will also result in diminished fractionation efficiency.)... 

---