Use of Dual Flow Trays

1. Purpose

This procedure explains why Inflection Point Engineering avoids the use of dualflow trays.

2. History

Prior to 1980, Inflection Point Engineering occasionally used dualflow trays in Cat Cracker Fractionator Services, HF Strippers, Isostrippers and EB-Styrene Services. Reports from that era indicated that there was no demonstrable advantage in any of these cases and that poor tray efficiency was observed. In the case of the El Paso Styrene Unit () the dualflow trays were replaced with valve trays and the results improved and the efficiency jumped form 40% to 70%.

Inflection Point Engineering normally specifies dualflow trays for the Water Stripper in a Styrene Reactor Section. This service is extremely fouling. The downcomers of cross flow trays in this service have been reported to foul too quickly to be acceptable.

3. Fractionation Research Inc. (F.R.I.) Work

Considerable data on dualflow trays has been collected at F.R.I. in a four foot column and an eight foot diameter column. A correlation for rating dualflow trays has been developed and is available in the F.R.I. handbook and in F.R.I. computer programs. The 1995 F.R.I. Annual Report has a brief review of this data.

4. Concerns

The F.R.I. data and Inflection Point Engineering experience are in agreement that dualflow trays are prone to low efficiency due to maldistribution of the vapor and liquid on the trays. Distribution and hence efficiency are best near the flood point where there is a substantial head of liquid on the tray. A dualflow tray that is well designed, installed and operating just below its flood point can have efficiencies similar to a conventional valve or sieve tray. However, as the vapor rate is lowered from that point the efficiency drops rapidly. Dualflow trays can also have maldistribution easily generated by poor tray levelness or poor vapor or liquid initial distribution. The larger the tower diameter the harder it will be to obtain good vapor and liquid distribution.

5. Advantages

The three main advantages are as follows.

• High capacity

• Less fouling tendency

• Relatively easy to clean when fouled

As the high capacity frequently comes at the expense of efficiency this is of dubious value. The tolerance to fouling and the easy of cleaning are the areas where dualflow tray use is potentially justified.

Use of dualflow trays requires approval by the appropriate Technology Specialist.