Vapor-liquid redistributor

Each tray in a tower is inherently a vapor-liquid redistributor. The outlet weir, or more exactly the crest height of the liquid overflowing the weir forces the liquid to flow evenly across the tray. Even if the weir... 

Packed-tower efficiency and turndown are strongly dependent on the quality of initial liquid distribution. Uneven distribution may cause local variations in the liquid/gas ratio, localized pinch conditions, and reduced vapor-liquid contact. Figure 14 shows two common liquid distributor types, the ladder type (shown as the top distributor) and the orifice type (shown as the redistributor). The ladder type is a horizontal header of pipes, which are perforated on the underside. The orifice type is a flat perforated plate equipped with round or rectangular risers for gas passage. Other common types of distributors are a header equipped with spray nozzles (spray distributor) and a header of horizontal channels, with V notches cut in the vertical walls of the channels (notched-trough distributor). 

Liquid collectors provide for withdrawal of products, mixing of new feeds with the internal liquid traffic, and remixing to avoid composition gradients after a certain height of packed bed has been traversed. Liquid collectors are often designed as one item to be integral with vapor distributors and liquid redistributors. 

The vapor-liquid separator t5 pe (Fig. 3.96) features an internal knockout drum directly above an orifice redistributor. It consumes more vertical space than the baffle t3q)e, but it has the potential of providing better liquid distribution to the bed below. This device is also most suitable for small columns (386). 

Davies et al demonstrated that when two liquid phases are formed in a distillation column, foaming occurs on those plates with a liquid composition near the two-phase transition [37]. This phenomenon may require a reduction in the normal design vapor flow rate to control the foaming action. Liquid redistributors, as well as the feed distributor, require special designs to maintain uniform liquid-phase composition when operating with two liquid phases in the column. 

One of the functions of a liquid redistributor is to remix the liquid phase so as to bring the entire liquid flow onto the next lower bed at a more uniform composition. To perform this function, the liquid redistributor must intercept all the liquid that is flowing down the column, including any liquid on the column walls. If a new feed is to be introduced, a feed sparger or parting box must be used to predistribute this feed onto the redistributor. The redistributor must maintain the uniform vapor distribution that should have been established at the column bottom. To perform these functions, the redistributor must have a large flow area available that is transverse to the gas risers, because only very low gradient... 

Orifice redistributors (Fig. 3.8a) These are identical to orifice distributors, either the pan or the trough type, except that hats, caps, or strips are usually installed above the risers to prevent liquid from the packed hed above from entering the vapor risers, and also to promote lateral mixing of vapor. 

Wall wipers (or "rosette ) redistributors (Fig. 3.86) This is a collection ring equipped with short projections extending toward the tower center. Liquid removed from the wall is deflected into the projections ("fingers ), which transport it to a desired location in the bed. Wall wipers effectively remove liquid from the wall, but they are only partially effective for counteracting composition pinches. Their ability to counteract composition pinches diminishes as column diameter increases. Therefore, they are only suitable for small columns [< 2 to 3 ft in diameter (74, 305)], where deflection of liquid and vapor by packing particles is sufficient to counteract pinching effects, and where wall flow formation is the main problem. 

Liquid flow must be prevented through the vapor passages. Risers are usually fitted with covers (Fig. 3.8a). Alternatively, a combination support plate-redistributor can sometimes prevent liquid downflow through vapor risers without using riser covers. 

Guidelines 2, 5 to 8, and 10 in Sec. 4.10 are also applicable for redistributor riser covers. Exceptions to these are (1) flat hats are acceptable for redistributors, because a small amount of liquid dropping through the vapor risers can often be tolerated and (2) when vapor distribution is not a major problem (e.g., column diameters smaller than 4 to 6 ft and a large number of vapor risers), uncovered vapor risers in a combination support plate-redistributor may be satisfactory. The top of such uncovered risers should not be more than 4 in below the bottom of the support plate above (305). 

An intermediate liquid feed is almost always introduced into a redistributor. Introducing a vapor-containing feed via a distributor... 

When a sparger pipe enters an intermediate feed, vapor jets must not impinge on redistributor liquid surface or other packed-column internals. Liquid surface agitation in redistributors, or mechanical damage to internals, can cause maldistribution, which can be detrimental to column efficiency. 

Chimney trays (Fig. 4.10a, 6) are used for withdrawing intermediate liquid streams from the column in a packed tower, they are also used as liquid collectors or vapor distributors. Alternative devices used for liquid withdrawal are downcomer trapouts in tray columns, chevron collectors, and some redistributors in packed columns. Compared to these alternative devices, chimney trays have the following advantages ... 

In packed columns, chimney trays are the most effective liquid collection devices for high liquid flow rate services, for interreboilers, and for once-through reboilers. They are also sometimes used for collecting liquid from an upper bed for redistribution (when the redistributor is not self-collecting), and as vapor distributors. 

Redistributor A plate that takes vapor or liquid flowing through a tower and, by means of orifices, forces the fluid to flow more evenly through a packed bed. 

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