Distribution liquid, tray inlet

At low liquid rates ( 1 gpm/in of weir), the tray liquid flow profile strongly depends on the liquid distribution at the tray inlet (e.g., inlet weir) but is practically independent even of msyor disturbances at the outlet weir (163). The importance of Liquid distribution at tray inlet was also noted by others (159,160,164,165). 

It is difficult to state whether the tray inlet or outlet geometry has a greater influence on liquid distribution between the passes. Some simulator tests (384) imply that at low liquid loads (1 to 2 gpm per inch of outlet weir), tray inlet geometry would be far more important. It is uncertain whether this con-... 

A recessed seal pan distributes liquid to the tray with an upward vertical motion rather than a horizontal motion containing a vertical downflow component. This results in better aeration at the inlet edge, increases both capacity and efficiency, and avoids precipitant weeping. Using a recessed seal pan can also be exploited to reduce the tray s inlet calming zone. 

Distribution. With tray towers, liquid distribution to the reboiler is uneven. With wide-boiling mixtures, this may prompt composition pinches. Further, light components can be depleted near the inlet or the top of the bundle, causing temperature pinches. In most cases, however, distribution is not a major problem because the bundle is small and internal reboiler AT is large. 

For most trays, liquid flows across an active area of the tray and then into a downcomer to the next tray below, etc. Inlet and/or outlet weirs control the liquid distribution across the tray. Vapor flows up the tower and passes through the tray active area, bubbling up through (and thus contacting) the liquid flowing across the tray. The vapor distribution... 

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 number of valve caps that can be fitted on a tray is at best an estimate unless a detailed tray layout is prepared. However, a standard has evolved for low- and moderate-pressure operations a 3 x 2 y2 in pattern that is the tightest arrangement available, accommodating about 14caps/ft2 (150 caps/m2). The active area does not take into account liquid-distribution areas at the inlet and outlet, nor edge losses due to support rings, nor unavailable space over tray-support beams. In smaller columns, it is possible that as much as 25 percent of the active tray area may not be available for functioning valves. For this column, which operates at low pressure, select the standard 3 x 2 /2 in pitch. 

For sloped downcomers, the critical liquid velocity is at the bottom, insofar as final disengagement of vapor is concerned. The total volume of the filled portion or lire downcomer can be used in estimating residence lime, For downcomers with bottom recesses, where the liquid must make an extra mm before entering the tray, the pressure lora under the downcomer may he estimated as twice that calculated from Eq. (5.7-30). This rule of thumb applies also to the case where an inlet weir is used io distribute the liquid after it has flowed under die downcomer baffle. 

Inlet weirs (Fig. 6.21a) and recessed seal pans (Fig. 6.216) are primarily used for achieving a downcomer seal in cases where a potential sealing problem exists and clearance under the downcomer is limited by one of the design criteria previously cited (Sec. 6.19). These devices provide a positive seal on the tray imder all conditions and ensure that the second and third sealing criteria (Sec. 6.18) are always satisfied. Sometimes it is argued that these devices improve liquid distribution to the tray, but this function is usually performed satisfactorily by the downcomer outlet (48, 172, 257, 404) and can rarely justify using either device. One exception is when the downcomer is circular... 

Inlet Impingement Device The purpose of the inlet impingement device is to diffuse the velocity profile of the fluids entering the reactor inlet nozzle and create an even pattern of liquid dispersion across the top liquid distribution tray. 

Weirs The depth of liquid on the tray required for gas contacting is maintained by an overflow (outlet) weir, which may or may not be a continuation of the downspout plate. Straight weirs are most common multiple V-notch weirs maintain a liquid depth which is less sensitive to variations in liquid flow rate and consequently also from departure of the tray from levelness circular weirs, which are extensions of circular pipes used as downspouts, are not recommended. Inlet weirs (Fig. 6.10) may result in a hydraulic jump of the liquid [67] and are not generally recommended. In order to ensure reasonably uniform distribution of liquid flow on a single-pass tray, a weir length of from 60 to 80 percent of the tower diameter is used. Table 6.1 lists the percentage of the tower cross section taken up by downspouts formed from such weir plates. 

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