Distribution Tray Design

Most liquid distribution devices are proprietary designs developed by process licensors and internals fabricators. Most of the known designs fall into one of four categories. 

The fourth type of liquid distribution device is a bubble cap tray, originally designed for application in fractionation towers. This device employs a number of bubble caps laid out on a regular pitched pattern on a horizontal tray. The bubble cap distributor works on a vapor-assist principle that offers a relatively stable operation compared to a chimney device. The bubble cap is a cap centered concentrically over a standpipe. The sides of the cap are slotted for vapor flow. Liquid flows under the cap and is aspirated by the vapor, flowing upward in the aimular area between the cap and the standpipe, and then down through the standpipe. 

The performance parameter, Sensitivity %, is a measurement of the difference in the liquid delivered by adjacent distributors at different elevations. This parameter is described by the following formula  

Fi = Liquid flow through low distributor Fh = Liquid flow through high distributor Fave = Average flow through all distributors 

When Haldor Topsoe began to investigate the design of a high performance distribution tray, several parameters were recognized as critical to achieve the desired performance efficiency. These are as follows  

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High performance reactor mixing chamber and distribution tray designs have been developed by some licensors which can often be incorporated into existing reactors to improve their performance. 

These contribute to the uniform distribution of liquid as it enters the tray from the downcomer. There are about as many tray designs without weirs as with them. The downcomer without inlet weir tends to maintain uniform liquid distribution itself. The tray design with recessed seal pan ensures against vapor backflow into the downcomer, but this is seldom necessary. It is not recommended for fluids that are dirty or tend to foul surfaces. The inlet weir is objectionable for the same reason. 

The entrance of a liqmd-flashing vapor mixture into the distillation column feed location requires a specially designed distribution tray to separate the vapors from the liquid, w hich must drop onto the packing bed for that section in a uniform pattern and rate. 

Improving liquid flow patterns. A number of special tray designs have been developed to improve liquid velocity distribution on large-diameter trays. Their main applications are vacuum distillation. In pressure distillation, liquid flows are usually high and multipass trays are used, so that stagnant zones are seldom a problem. Some means of improving the liquid flow patterns are... 

Column hardware choice can have a significant influence on the conversion and selectivity such aspects can be properly described only by the NEQ cell model, or by a still more sophisticated model based on computational fluid mechanics (such models have yet to be developed). It is insufficiently realized in the literature that, say, for tray RD columns, the tray design can be deliberately chosen to improve conversion and selectivity. Even less appreciated is the fact that the design methodology for RD tray columns is fundamentally different from that of conventional trays. Liquid residence time and residence time distributions are more important in RD. The froth regime is to be preferred to the spray regime for RD applications this is opposite to the design wisdom normally adopted for conventional distillation. For relatively fast reactions, it is essential to properly model intra-particle dif... 

Figure 13.18 Distributor tray design parameters, (a) Effect of tray spacing on liquid distribution, (b) comparison of tray spacing, and (c) discharge pattern of several tray designs.

Figure 12.12(a) shows two arms of a cable tray designed to be smooth and fiat that have taken a wavy shape, making it completely useless. Using a two-dimensional simulation program one can analyze why this problem occurred. Figure 12.13(a) clearly shows a non-uniform velocity profile indicating a poor distribution of material. 

Ov Crall, the careful design of a distributor for liquid in the top of a packed tower, and for the redistribution of liquid flowing dow n multi-section packing in the tower, is essential to good consistent tower performance. However, the liquid flow is not alone, the uniformity of vapor distribution is likewise essential, because non-uniform vapor distribution can cause non-uniform liquid downflow. Then, there is the selection of the packing itself and its characteristics and requirements/sensitivity to the uniform distribution of the liquid and vapor. As earlier emphasized, the level of the distributor tray or trough can be critical to the consistent uniform liquid distribution. 

Kunesh [126] presents tm overview of the basis for selecting rsuidom packing for a column application. In first deciding between a trayed tower or a packed one, a comparative performance design and its mechanical interpretation should be completed, considering pressure drop, capacity limitations, performance efficiencies (HETP), material/heat balances for each alternate. For one example relating to differences in liquid distribution performance, see Reference 126. 

Bubbles, in fluidized beds, 11 805-806 Bubble size control, 11 805 in fluidized beds, 11 819, 821 Bubble size distribution, 12 14 in foams, 12 11 Bubble tear-offs, 20 229 Bubble tray absorbers, 1 27, 29 design, 1 83-86 Bubble-tube reactor, 25 194 Bubble tube viscometer, 21 739 Bubble two-phase theory of fluidization, 11 805-806... 

At high liquid flowrates, the liquid gradient on the tray can become excessive and lead to poor vapour distribution across the plate. This problem may be overcome by the shortening of the liquid flow-path as in the case of the double-pass and cascade trays. The whole design process is discussed in Volume 6. 

A properly designed valve or sieve tray will act as a vapor redistributor. Thus, poor initial vapor distribution will only lessen the efficiency of the bottom tray. But if a packed-bed vapor distributor does not work properly, vapor channeling will be promoted through the entire bed. 

A convenient approximation is that the distributions of nonkeys require the minimum number of trays as given by Eq. (13.116). Designating the nonkey by subscript nk, that equation becomes... 

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