Sieve decks

Refinery FCCunit Propylene-propane splhter A small amoimt of KOH solution fram an upstream dryer was carried over into the column. Once in the column, KOH precipitated, and pltigged sieve decks equqn>sd with yia-in holes. Deposits were removed by acid wash. Small parforatians plug... 

As part of the revamp project, a KOH dryer was installed upstream of the splitter. A small amount of dissolved KOH was carried out of the drier vessel by the splitter feed. Once in the tower, ail of the KOH in solution dropped out as a dry powder and plugged the sieve decks. 

The feed to this tower is usually located close to the top tray. Both the feed tray and the three or four trays located below this point are subject to plugging, fouling, and corrosion. Eyidence of a plugged feed tray is several percent alkylate in the isobulane recycle stream. Replacement of the usual /2-in. sieve hole trays with a "Nutter V-Grid" type tray or a Vi-in. hole sieve deck, has diminished plugging in this service. Monel tray decks and especially Monel downcomers are recommended. 

What had happened The propylene-propane splitter had formerly run for many years without difficulty. After the unit was revamped, the sieve decks had plugged. 

Vapor bubbles up through the sieve holes, or valve caps, on the tray deck, where the vapor comes into intimate contact with the liquid. More precisely, the fluid on the tray is a froth or foam—that is, a mix-... 

When vapor flows through a tray deck, the vapor velocity increases as the vapor flows through the small openings provided by the valve caps, or sieve holes. The energy to increase the vapor velocity comes from the pressure of the flowing vapor. A common example of this is the pressure drop we measure across an orifice plate. If we have a pipeline velocity of 2 ft/s and an orifice plate hole velocity of 40 ft/s, then the energy needed to accelerate the vapor as it flows through the orifice plate comes from the pressure drop of the vapor itself. 

This concept is the basis for tray design for perforated tray decks. While various valve tray vendors maintain that this rule does not hold for their equipment, it is the author s industrial experience that valve trays leak just as badly as do sieve trays, at low vapor hole velocities. To summarize ... 

From the designer s point of view, the top tray of the stripper must have a several times greater number of sieve holes or valve caps on its tray deck than the bottom tray. If, however, all the trays in the stripper are identical, then either the bottom tray will leak (see Chap. 2), or the top tray will flood. Either way, stripping efficiency will suffer. 

F igure 14-36 illustrates the pressure drop of a typical moving valve tray as a function of gas velocity. At low velocities, all valves are closed. Gas rises through the crevices between the valves and the tray deck, with increasing pressure drop as the gas velocity rises. Once point A, the closed balance point (CBP), is reached, some valves begin to open. Upon further increase in gas velocity, more valves open until point B, the open balance point (OBP), is reached. Between points A and B, gas flow area increases with gas velocity, keeping pressure drop constant. Further increases in gas v ocity increase pressure drop similar to that in a sieve tray. 

Not all trays are fouling-resistant. Floats on moving valve trays tend to stick to deposits on the tray deck. Fouling-resistant trays have large sieve holes or large fixed valves, and these should be used when plugging and fouling are the primary considerations. 

Fig. 3.1. Also note that this nonconventional design has the downcomer outlet area as additional active tray area. This additional active area is the tray deck area under the downcomer having valves, bubble caps, or sieve holes that allow the gas to pass through under the liquid downcomer area of the next tray up. ICPD tray programs dealing with the design and rating of sieve, bubble cap, and valve-type trays allow this active area input. This is an option shown in Table 3.1, which is offered in the three tray design/rating computer programs given in this book.

Several equations are applied to calculate sieve tray hole area. Normally sieve tray hole individual diameters are Vie to % in. As for bubble caps, sieve tray hole pitch is the measure from center to center between sieve tray holes. The pitch may be square or triangular, each angle of the triangle being 60°. Section areas are the tray deck area of one pitch markoff area, noted as SECTAREA. For square pitch, one complete hole area is measured for triangular pitch, one-half hole area is measured, noted as SECTHA. The active area divided by SECTAREA equals the number of pitch sections on the tray deck, noted as SECTNO. Equations (3.98) through (3.103) will help clarify these statements. 

For triangular-type sieve tray pitch, use Eq. (3.98) to calculate a single triangular tray deck area, noted as SECTAREA. Each leg of the triangle is an equal pitch measure. One triangle area is equal to SECTAREA in ft2. 

Please note that HOLHA is the total hole area in ft2 on a single tray deck. It is used in Eq. (3.91) to calculate sieve tray jet flood and will be used to calculate sieve tray pressure drop as well. 

THDIA = ratio of sieve tray deck thickness to a single sieve tray hole diameter... 

The height in inches of clear liquid over the sieve tray deck top surface HHDS may next be calculated. Having calculated HHOW in Eq. (3.115), and HHG in Eq. (3.118), HHDS may therefore be calculated in Eq. (3.119). 

A set of sieves with rectangular apertures was used in combination with a set of sieves with square apertures to separate according to shape. The separation was superior to that obtained using a vibrating deck [117],... 

The mass-transfer devices may be sieves (holes), fixed valves, moveable valves, or bubble caps. Fig. 2 shows a selection of mass-transfer devices. The purpose of the device is intimate mixing of the vapor and liquid on the tray deck. An ideal device has high capacity, high flexibility, low leakage, low pressure drop, and low cost. 

Distillation trays are so simple... Sieve tray decks are, after all, hardly more than sheets of metal with a few holes punched in them. This of course is part of the fascination—that the behavior of something so simple can be so difficult to predict with regard to its hydrodynamic and mass transfer performance. 

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