Weir Distributors

Weir distributors are usually of the weir riser type (Fig. 3.4g) or the notched-trough type (Fig. 3.4/). The former type is commonly used in small-diameter columns ( 4 ft), while the latter is used in larger-diameter columns ( 3 ft), but can also be used in smaller columns. 

A weir riser distributor consists of a pan equipped with cylindrical risers with a V-notch cut in each riser. The V-notch allows liquid to descend countercurrently to the rising vapor. A major disadvantage which renders the weir riser distributor unpopular is the interdependence of the maximum vapor and maximum liquid flow rates. At a tower F-factor (superficial vapor velocity by square root of vapor den- 

In order to reduce the vapor-liquid interaction, standard weir riser pans are usually smaller than the column diameter and are supported on lugs, leaving an annular space for vapor rise between the distributor and the tower wall. This, however, creates an unirrigated region near the column wall, which may cause large-scale maldistribution. Other performance characteristics of this distributor are similar to those of the notched-trough distributor (below). 

Notched-trough distributors consist of parallel troughs with V-notches cut in their sides for liquid flow. Vapor rises through the space between the troughs. 

The quality of distribution provided by notched-trough distributors is generally somewhat inferior to that achievable by orifice-type distributors. With notched-trough distributors, it is generally difficult to incorporate more than three to four drip points per square foot of column cross-sectional area (111, 438). It may also be difficult to space these drip points evenly. If it is practical to provide a sufficient number of drip points per unit of column area, and to space them evenly, this distributor can provide a distribution as good as an orifice-type distributor. 

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Most weir distributors are insensitive to plugging and corrosion. They can handle large volumes of solids as well as liquids near their freezing point. 

Weir distributors are prone to out-of-levelness more than any other distributor because the flow rate through a triangular notch is proportional to the liquid head raised to the power of 2.5 (flow rate through an orifice is only proportional to the liquid head raised to the 0.5 power). With weir distributors, an out-of-levelness of 1 or 2 in is sufficient to cause severe maldistribution (237, 436). One case was reported (436) in which a distributor in a 40-ft tower was installed to a level tolerance of Vie in to avoid this problem. Adjustable leveling screws are often provided and should always be specified with this distributor to enable an in situ level adjustment. The problem is most severe at low liquid rates and tumed-down conditions. 

Generally, redistributors for large-diameter (> 3-ft) columns are of the orifice or weir type. The orifice type is more popular because it does not require the addition of a liquid collector, which consumes vertical space and increases column cost and complexity. Other pros and cons of orifice and weir redistributors, as well as application guidelines for each type of redistributor, are identical to those described earlier for orifice and weir distributors. The general dos and don ts for distributors and for liquid inlets into distributors also extend to redistributors. Additional guidelines unique for selection, design, and operation of redistributors are presented below. 

The feed slurry is introduced into the lower portion of the bowl through a small orifice. Immediately downstream of the orifice is a distributor and a baffle assembly which distribute and accelerate the feed to circumferential speed. The centrate discharges from the top end of the bowl by overflowing a ring weir. Solids that have sedimented against the bowl wall are removed manually from the centrifuge when the buildup of solids inside the bowl is sufficient to affect the centrate clarity. 

Figure 7. Weir trough liquid distributor. (Courtesy of Norton Chemical Process Products Corporation.)...

Adsorbers, distillation colunuis, and packed lowers are more complicated vessels and as a result, the potential exists for more serious hazards. These vessels are subject to tlie same potential haz. uds discussed previously in relation to leaks, corrosion, and stress. However, llicse separation columns contain a wide variety of internals or separation devices. Adsorbers or strippers usually contain packing, packing supports, liquid distributors, hold-down plates, and weirs. Depending on tlie physical and chemical properties of the fluids being passed tlirough tlie tower, potential liazards may result if incompatible materials are used for llie internals. Reactivity with llie metals used may cause undesirable reactions, which may lead to elevated temperatures and pressures and, ullinialely, to vessel rupture. Distillation columns may contain internals such as sieve trays, bubble caps, and valve plates, wliicli are also in conlacl with tlie... 

The number of irrigation or drip-points or entrance points per square foot of flat surface of the tower should be uniform for orifice, weir-type gravity, or pressure distributors, and need not exceed 10 points/ft [82]. This imiformity must not be disturbed by support rings for supporting the distributor itself. The distribution must include the area adjacent to the wall, and the design must not force more liquid at the wall where it contacts the packing. Uniformity of points of distribution to the packing surface is extremely important. The volume flow per point must be carefully calculated. 

Figure 9-8D. Weir-flow distributor. Used by permission of U.S. Stoneware Co., Bull. TA-40 (now, Norton Chemical Process Products Corp.).

Figure 3.50 Liquid distributors with single parting box and weir troughs.

In industrial fluidized bed reactors, the bed height is commonly fixed by an overflow weir. Thus, as the gas velocity, U, increases from Umf, the apparent bed density decreases. An important design principle for the gas distributor is to ensure its sufficient pressure drop for a uniform gas distribution, i.e., without gas channeling, and stable bed operation. Specifically, the total pressure drop across both the distributor and the bed should be in an increasing trend with an increase in the gas velocity. Suppose that the pressure drop across a perforated distributor, Apdistnbuior, with a total orifice area of Ao can be expressed by... 

Liquid inlets. Liquid enters the top tray via a hole in the column shell, often discharging against a vertical baffle or weir, or via a short, down-bending pipe (Fig. 17), or via a distributor. Restriction, excessive liquid velocities, and interference with tray action must be avoided, as these may lead to excessive entrainment, premature flooding, and even structural damage. Disperser units (e.g., perforations, values) must be absent in the liquid entrance area (Fig. 17) or excessive weeping may result. 

For These Two,can Substitute Weir-Type, Distributor Prym Support... 

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