Orifice/riser distributor

The orifice-riser distributor is designed to lay the hquid carefully onto the bed, with a minimum of contact with gas during the process. It can be designed to provide a large number of liquid streams, with the limit of sufficient liquid head to provide uniform liquid flow through the orifices. The gas risers must oe designed to accommodate the expected variations in flow rate, often with a minimum of pressure drop. For veiy distribution-sensitive packings, it is necessaiy to include pour points in the vicinity of the column wall (to within 25 mm). 

The orifice/riser distributor provides numerous uniformly distributed pour points. The vapor risers must be designed for variations in flow rate, often with a minimum of pressure drop. Typically, risers occupy 20% of the column cross section if the percentage is much lower, there is danger of liquid backing up into the risers. More exact allocation of riser area can be obtained from the simple relationship... 

Distributors and collectors of liquid provide uniform wetting of the packings area and the withdrawal of phases from the packing. The best-known distributor types are orifice-riser, perforated-pipe, spray-nozzles, and through distributors. 

For single- or multiple-bed columns, a distributor is needed for each bed. A center-fed packed distillation column, for example, requires at least two beds. In some cases, the cost of the distrib-utor(s) can approach that of the packing. One can select from several different types of liquid distributors, including trough, orifice/riser, perforated pipe, and spray nozzle. These are illustrated in Figure 12.55 and described below. 

The drip tube/riser distributor is a variation of the orifice/riser device, with tubes added to the orifices to ensure close contact with the packing (not shown in Figure 12.55). When it is necessary that all tubes discharge evenly, a double tubesheet affair can be used with liquid under pressure pressure drop through the orifices and tubes must be estimated carefully for this arrangement. 

Types of liquid distributors are shown in Fig. 5.8-5. The orifice/riser device can be developed in a variety of forms, and the risers ate often of a rectangular cross section. The orifices must be distributed property over the cross section and must offer enough resistance to liquid flow that an adequate head of liquid can be maintained over them. This can lead to difficulties small holes can be eluily plugged, and oui-of-levelness of the distributor can lead to dry sections, especially whan liquid rare is considerably below the design value. 

FIGURE 5.8-5 Types of liquid distributors (a) orifice/riser (6) perforated pipe (c) trough and (d) spray nozzle. 

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.)...

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. 

When a high rate of vapor flow is required, the orifice pan and the weir riser distributors are best avoided. 

Also known as riser tube distributors (RTD) [131], these consist of a flat tray with orifices uniformly spaced to allow liquid to flow onto the packing below. This tray has vapor risers, uniformly spaced, but not interfering with... 

For distributors of any design, including the PAN, it is important to filter the feed or reflux liquid entering tlie distributor to reduce the possibilities of plugging of tlic orifice holes. Otherwise, the landom plugging will cause non-uniform distribution onto the packing below. It is important to avoid leakage around the risers because this can destroy the liquid distiibution pattern [131]. 

Packed-tower efficiency and turndown are strongly dependent on the quality of initial liquid distribution. Uneven distribution may cause local variations in the liquid/gas ratio, localized pinch conditions, and reduced vapor-liquid contact. Figure 14 shows two common liquid distributor types, the ladder type (shown as the top distributor) and the orifice type (shown as the redistributor). The ladder type is a horizontal header of pipes, which are perforated on the underside. The orifice type is a flat perforated plate equipped with round or rectangular risers for gas passage. Other common types of distributors are a header equipped with spray nozzles (spray distributor) and a header of horizontal channels, with V notches cut in the vertical walls of the channels (notched-trough distributor). 

Redistributor design is similar to gravity distributor design. The orifice type is most popular (Fig. 14). A notched-through type requires a liquid collection device above it to feed the liquid onto the distributor. Often, the gas risers are equipped with caps to prevent liquid from dropping through the gas spaces. 

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

When the inlet pipe directly feeds the distributor, the incoming liquid should be fed into the center of the distributor in order to ensure uniform head over all the orifices (111, 305). The feed pipe should be located about 2 to 8 in above the top edge of the distributor pan (305). It is important to ensure the feed flows into the distributor and does not enter the vapor risers. In one case (150a) poor column separation resulted from distributor feed being directly introduced into an open riser. 

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. 

The column did not achieve a relatively sim de sqxaation. Sq>aration was poor because of a pest modification of drilling 4-in boles in the distributor floOT. This caused Ikiuid to bypass the distribution orifices. Problem was solved by equqrping holes with risers. 

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