Perforated plates without downcomers

Perforated Plate Without Downcomers (Thud-Flaw, from FRI)... 

Perforated plates without downcomers have only recently been included in commercial equipment. The data for rating the performance is not adequately covered in the literature, since the present developments in industrial equipment have not been released. The information included here is based only on available data and experience, yet it may serve as a basis for rating, because the basic nature of the contact is quite analogous to the sieve tray. The limits of performance are not well defined therefore the methods oudined cannot be considered firm. However, they are adequate for many applications and as the basis for further study. 

Hole size is as important in perforated plates without downcomers as far the sieve tray. Published data limits a full analysis of the relationships however, the smaller holes, Ys-in., Me-in., 4-in. appear to give slightly higher efficiencies for the same tray spacing [47]. Unfortunately the data [69] for the larger %-in. holes was not evaluated for efficiencies. Experience has indicated efficiencies equal to or only slightly, 10-15%, less for M-in. holes w hen compared to Me-in. holes for some systems. Holes as small as Mfrin., %2-in. and Me-in. were considered unsatisfactory for high surface tension materials such as water [47]. 

A tower separates a weak ammonia solution. Design trays using perforated plates without downcomers for the following conditions as determined from the column performance calculations. 

There is essentially no published work on specific tests with these trays as relates to entrainment, etc. However, the very close similarity between a perforated plate without downcomers and one with downcomers is sufficient to justify using some data for one in the design of the second. 

On a perforated plate the liquid side mass transfer coefficient kLa and gas side mass transfer coefficient k( a, based on the column volume, vary linearly with the dispersion height. The true liquid- and gas-side mass transfer coefficients and first increase with the dispersion height and then go through a maximum and decrease slightly (123). Sharma and Gupta (124) attribute this to different behavior of the density of dispersion and the average bubble size with increase in gas flowrate, which leads to a phase inversion point. These authors correlate their experimental data for 10 cm i.d. perforated plates without downcomers by the following expressions... 

This is the case with diameter determination. The relation of Equation 8-250 for the perforated tray or sieve tray with downcomers can be used for the plate without downcomers. Generally, the liquid level and foam-froth height will be higher on this tray, hence the ralue of h., clear liquid on the tray, may range from 1-in. to 6-in. depending on the service. 

The perforated plate, punched plate, or Dual-Flow plate are terms used to refer to a tray operating without downcomers, with rapor and liquid passing countercurrent through perforations in the tray. The Dual-Flow term has been coined by Fractionation Research, Inc., and its design know-how is restricted to contributing members, and cannot be presented in this book. 

Perforated plates that fit into bubble columns like sieve trays are still used in cocurrent bubble columns (11). Usually the resultant free area of the openings is very small, i.e. 1 - 5 % of the total plate area. These plates may totaly suppress backmixing by forming a gas buffer beyond each plate which prevents liquid backflow. Such plates cannot be used in countercurrent flow in as much as the required pressure drop through the holes prevents the liquid downflow. Several possibilities have been suggested for the utilization of perforated plates in countercurrent bubble columns, i.e. special kinds of downcomers with or without control valves (12) (13) or additional pulsing... 

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