Grid trays

V-GRID tray Tapered slot M Mod Med Med Med Fixed open, high... 

If a sieve, dual-flow, or grid-tray column is used, the only way to operate the column in a stable manner at the low initial flow rates is to blank offpart of the trays. This increases the vapor velocity through the mixing section, and assmes good contact and an efficient separation. These blanks can be removed at the time of the expansion. 

Hgure 7.5 Cartridge fixed-valve (V-Grid) trays, 23-in ED assembly. (fieprinted courtesy of Nutter Engineering)... 

Iron sulfide pwrticles Ammonium salts Acid coke in alkylation service Check intermediate trays Use V-grid trays Water-soluble deposits... 

Grid tray Similar to a sieve tray. A perforated distillation tray deck, but with no movable valve caps. Represents modern distillation tray design. 

The use of movable valve caps in any service where deposits can accumulate on the tray decks will cause the caps to stick to the tray deck. It s best to avoid this potential problem. Use of grid trays with fixed cap assemblies is preferred for most services. 

For perforated tray decks (sieve or grid trays), I calculate the pressure drop of the vapor flowing through the holes (inches). The pressure drop I want is the weight of liquid on the trays that I calculated above in step c. The idea is to keep the tray from leaking through the tray deck perforations. 

K for sieve trays is 0.3 K for grid trays is 0.6. Only an idiot would use movable valve trays. The valve caps stick to the deck and do not greatly retard tray deck leakage. 

For fouling services, I use Koch-Glitsch pro-valve grid trays with a Vi-inch lift. 

If capacity and vapor flow variability (i.e., turndown) is not an issue, I use sieve trays with 3/8-inch holes (clean service) or Vi-inch holes (dirty services). Sieve trays have about 10 percent less capacity than grid trays. But anyone can make them in a day (i.e., new tray panels) and the price is minimal. 

Use grid trays They are stronger than sieve or valve trays. Again, 1 use 10 gauge rather than the standard 14-gauge deck thickness. 

For such a tower, tray decks should be sieve or grid trays, and never valve cap trays. I will typically use a grid tray with a 1 -inch lift. Also, using 10-gauge rather than 14-gauge trays (which is the industry standard) will double the tray resistance to collapse, due to the relief valves lifting at the bottom of the tower. 

Do not use valve or flutter cap trays. The valve caps stick to the tray deck and promote high-pressure drop and flooding. Use a grid tray with a one-half-inch cap lift. Pro-valves are a good option. 

Fixed V-Grid Figure 8-70 Nutter Out performs Sieve Tray High/High Medium Good... 

Figure 8-70. Nutter V-Grid fixed valve for trays. Used by permission, Nutter Engineering, Harsco Corp., Bui. N-2.

The bottom section of the main column provides a heat transfer zone. Shed decks, disk/doughnut trays, and grid packing are among some of the contacting devices used to promote vapor/liquid contact. The overhead reactor vapor is desuperheated and cooled by a pumparound stream. The cooled pumparound also serves as a scrubbing medium to wash down catalyst fines entrained in the vapors. Pool quench can be used to maintain the fractionator bottoms temperature below coking temperature, usually at about 700°F (370°C). 

As an alternative to trays, especially at low volumetric liquid-to-vapor ratios, packing can be used to promote vapor-liquid contact. One approach is to dump specially shaped pieces of metal, glass, or ceramic material into the column, wherein they are supported on a grid. An example of dumped or random packing is shown in Fig. 7. 

Fjj is the cost factor for tray type (valve, grid, bubble cap, sieve) ... 

A variation of the normal sieve tray is the Turbogrid tray developed by the Shell Development Company. The Turbogrid tray uses slots instead of circular holes and operates as a counterflow plate unit with no downcomers. A typical tray consists of a flat grid of parallel slots. The slots can be stamped perforations in a flat metal sheet or the open spaces between parallel bars. 

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