Orifice Distributors

Orifice distributors are usually of the pan type (Fig. 3.4d) or of the tunnel type (Fig. 3.4e). The former t3rpe is best suited for small-diameter columns ( 4 ft), while the latter is used in larger-diameter columns ( 4 ft). High-performance variations of the Fig. 3.4e distributor are shown in Fig. 3.5a, c and 3.6a, d. 

An orifice pan distributor consists of a pan equipped with circular or rectangular risers for vapor flow and perforations in the pan floor for liquid flow. The pan may rest on a support ring alternatively, it may be supported on lugs in a manner that provides an annular space for vapor rise between the distributor and the column wall. 

Orifice tunnel distributors consist of parallel troughs with perforations for liquid flow in the trough floors. Vapor rises in the space between the troughs. The troughs are often interconnected by cross channels that equalize liquid levels in different troughs (Fig. 3.6d). Level-equalizing channels are most important in coliunns greater than 10 ft in diameter (111, 349). 

In some high-performance designs (e.g.. Fig. 3.5c), side orifices and deflection baffles are used (75a, 224a) item 9 below has further details. 

Orifice distributors are capable of handling high liquid loads, with 

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High Viscosity and Surface Tension Bravo (Paper presented at the AIChE Spring National Meeting, Houston, Tex., 1995) studied a system that had 425-cP viscosity, 350 mN/m surface tension, and a high foaming tendency. He found that efficiencies were liquid-phase-controlled and could be estimated from theoretical HTU models. Capacity was less than predicted by conventional methods which do not account for the high viscosity. Design equations for orifice distributors extended well to the system once the orifice coefficient was calculated as a function of the low Reynolds number and the surface tension head was taken into account. 

For a typical design 2 ft (0.61m) of extra height is required for each redistributor when using pan-orifice distributors and 4 ft (1.22 m) of extra height is required for each redistributor when using trough-orifice distributors. 

Pressurized liquid distributors use pressure drop across an orifice to deliver a smooth liquid distribution to a packed bed. Two main types exist, pipe-orifice distributors and spray distributors. Of the two, spray distributors are the most common. In specifying either type, the main criteria to be set are the pressure drop available for use and the minimum orifice size. These... 

In pipe-orifice distributors, each orifice delivers a liquid stream to the packed bed. Pipe orifice distributors have the advantage that they are relatively compact and occupy a small vertical space in the tower. However, they tend to be very expensive. A separate orifice is needed for each drip point onto the packing. Typical designs call for between 5 and 15 drip points per square foot (50 and 150 drip points per square meter). The minimum orifice size is set by the fouling tendency of the system. Liquid velocities leaving the orifice should be kept to under lOft/sec (3m/sec). This prevents excessive liquid splashing on the top of the packed bed. 

Pan-orifice distributors depend on liquid height on a pan to provide sufficient head for the liquid to fiow through holes in the pan to the packed bed below. Pan-orifice distributors also include vapor chimneys for vapor to rise past the tray. 

Figure 3.4 Common types of liquid distributors, (a) Ladder pipe distributor (6) perforated ring distributor (c) spray distributor id) orifice pan distributor (e) tunnel orifice distributor (f) notched-trough distributor (g) weir-riser distributor. (Parts a and c to f reprinted courtesy of Norton Company part b, reprinted courtesy of Koch Engineering Company, Inc. part g, reprinted courtesy ofGlitsch, Inc.)...

Orifice distributors are also generally larger, more expensive, consume more vertical space, and are more difficult to support than most other distributors. Tunnel orifice distributors provide greater open areas for vapor flow, are easier to support, and are more suitable for large-diameter columns than orifice pan distributors. 

Guidelines for selection, design, and operation of orifice distributors are listed below. 

Supporting an orifice distributor directly on the packing is not recommended, because it may be misaligned during column up-... 

Orifice distributors are favored in foaming services, because liquid drip points are separated from the vapor risers (111, 349). 

Methods for sizing orifice distributors were described by Chen (74) Fadel (111), and Kaiser (183a). Some highlights are described below. A most useful relationship is (74, 150a)... 

Orifice distributors are best avoided in corrosive services, because some orifices may expand more than others. 

If the service contains solids, or the liquid is close to its freezing point, a weir-type distributor is the best choice. If it is still desired to use a perforated-pipe, spray, or orifice distributor, a filter should be installed upstream to remove particles that can block the perforations or spray nozzles (237,305). Successful applications of this technique have been reported (237,346). 

Orifice distributors with bottom perforations should be avoided in plugging services, even when filters are installed there have been cases where small solid particles passed through the filter, agglomerated in the pan or troughs, and blocked perforations. In pressure distributors, particularly the spray type, a filter is often [but not always (297)] sufficient. 

Orifice redistributors (Fig. 3.8a) These are identical to orifice distributors, either the pan or the trough type, except that hats, caps, or strips are usually installed above the risers to prevent liquid from the packed hed above from entering the vapor risers, and also to promote lateral mixing of vapor. 

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