Types of Liquid Distributors

Any liquid distributor gives some maldistribution, because for practical reasons, liquid can only be divided into a limited number of streams. From these point sources the liquid spreads. The main considerations in selecting a distributor for a given service are compatibility with the service and avoiding large-scale maldistribution. 

Most of the common tjq)es of commercial liquid distributors are shown in Fig. 3.4 and compared in Table 3.1. Table 3.1 was compiled using information available in Refs. 74, 111, 142, 212, 224, 237, 257, 305, 319, 386, and 438, together with the author s experience. 

Several modern designs, often referred to as high-performance dis- 

The common gravity distributors are the weir type and the orifice type. Both types can handle large liquid flow rates. The weir tjq)e is generally one of the least troublesome distributors and has an excellent turndown, but it can usually provide only a limited number of drip points and is extremely sensitive to levelness and liquid surface agitation. The orifice type may suffer from corrosion and plugging, but it can be designed with a large number of drip points to provide superior liquid distribution. 

Each of the common distributor types is discussed in detail below. 

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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. 

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.)...

This procedure is only concerned with liquid or two phase flow, and does not consider gas/vapor type distribution devices. There are several types of liquid distributors for packed columns that are also not part of this... 

Another type of investigations showing the maldistribution of the liquid phase is the determination of the holdup distribution using die gamma tomography method [141]. The eiqieriments are carried out with metal Pall ring 24.4 mm in a 600 mm diameter column. Ihree types of liquid distributors are used. The first two are presented schematically in Fig. 5. Tire thud (point source inlet) feeds the liquid as a single jet in the centre of the column cross-section. 

Westerterp et al. (W5) measured interfacial areas in mechanically agitated gas-liquid contactors. The existence of two regions was demonstrated At agitation rates below a certain minimum value, interfacial areas are unaffected by agitation and depend only on nominal gas velocity and the type of gas distributor, whereas at higher agitation rates, the interfacial areas are... 

It is difficult to compare the performance of various spray towers since the type of spray distributor used influences the results. Data from Hixson and Scott 33 and others show that KGa varies as G70-8, and is also affected by the liquid rate. More reliable data with spray columns might be expected if the liquid were introduced in the form of individual drops through a single jet into a tube full of gas. Unfortunately the drops tend to alter in size and shape and it is not possible to get the true interfacial area very accurately. This has been investigated by Whitman et a/. 34 , who found that kG for the absorption of ammonia in water was about 0.035 kmol/s m2 (N/m2), compared with 0.00025 for the absorption of carbon dioxide in water. 

Imafuku et al.46 measured the gas holdup in a batch (i.e., no liquid flow) three-phase fluidized-bed column. They found that the presence of solids caused significant coalescence of bubbles. They correlated the gas holdup with the slip velocity between the gas and liquid. They found that the gas holdup does not depend upon the type of gas distributor or the shape of the bottom of the column when solid particles are completely suspended. Kato et al.53 found that the gas holdup in an air-water-glass sphere system was somewhat less than that of the air-water system and that the larger solid particles showed a somewhat smaller... 

Future work should concentrate on large diameter (both short and tall) bubble columns. The applicability of the existing correlation should be examined for these columns for a variety of organic liquids and electrolytes and different types of gas distributors. Very little is known about the bubble dynamics and the gas holdup in a bubble column containing highly viscous polymeric solution and water soluble polymer solutions. This area should be explored. 

FIGURE 5.8-6 Effect of liquid distributor type on packing efficiency (Ref. 15) for I In- Pall rings, 24 psia, cyclcbexane-n-heplane. 

Very important for all types of packings is a uniform liquid distribution at the top of the bed and a limitation of bed height to 6 or 8 m. Beneath each bed, the liquid has to be collected, mixed, and redistributed. These measures intend to suppress the so-called maldistribution of liquid because it strongly affects mass transfer rates. The design of liquid distributors, liquid collectors, support grids, etc., should provide a large open area not to hinder the countercurrent flow of gas and liquid. 

Figure 8-5 shows the configuration at the top of the tower. The reflux was distributed by an orifice plate chimney type, pan liquid distributor. Vapor flowed up through the chimneys reflux was supposed to be evenly distributed by the /2-in. holes drilled in the pan across the top of the packed bed. To determine if the reflux was truly being equally distributed across the structured bed, I proceeded as follows ... 

The fourth type of liquid distribution device is a bubble cap tray, originally designed for application in fractionation towers. This device employs a number of bubble caps laid out on a regular pitched pattern on a horizontal tray. The bubble cap distributor works on a vapor-assist principle that offers a relatively stable operation compared to a chimney device. The bubble cap is a cap centered concentrically over a standpipe. The sides of the cap are slotted for vapor flow. Liquid flows under the cap and is aspirated by the vapor, flowing upward in the aimular area between the cap and the standpipe, and then down through the standpipe. 

End Effects Analysis of the mass-transfer efficiency of a packed cohimn should take into account that transfer which takes place outside the bed, i.e., at the ends of the packed sections. Inlet gas may veiy weU contact exit liquid below the bottom support plate, and exit gas can contact liquid from some types of distributors (e.g., spray nozzles). The bottom of the cohimn is the more hkely place for transfer, and SU-vey and KeUer [Chem. Eng. Prog., 62(1), 68 (1966)] found that the... 

The term mist generally refers to liquid droplets from submicron size to about 10 /xm. If the diameter exceeds 10 /xm, the aerosol is usually referred to as a spray or simply as droplets. Mists tend to be spherical because of their surface tension and are usually formed by nucleation and the condensation of vapors (6). Larger droplets are formed by bursting of bubbles, by entrainment from surfaces, by spray nozzles, or by splash-type liquid distributors. The large droplets tend to be elongated relative to their direchon of mohon because of the action of drag forces on the drops. 

Liquid distribution in a packed bed is a function of the internal vapoi/liquid traffic, the type of packing employed, and the quality of the liquid distributors mounted above the packed bed. Vapor distribution is controlled by the internal vapor/liquid traffic, by the type of packing employed, and by the quality of the vapor distributors located below the packed beds. 

WTien using plate-type distributors, an out-of-level condition can cause serious channeling of liquid down one part of the column and gas up the other. Prov ision should be... 

The type of distribution to select depends on the sensitivity of the tower performance to the liquid distribution as discussed earlier. Norton s [83] data indicate that the sensitivity of tower performance to liquid distribution quality depends only on the number of theoretical stages in each bed of packing achierable at its System Base HETP [83]. Tower beds of high efficiency packing are more sensitive to liquid distribution quality than shorter beds of medium efficiency packing [83]. It is important to extend the uniformity of the distributor all the way to within one packing particle diameter of the tower wall [85]. 

Structured packing requires specially designed distributors recommended by the respective manufacturers to ensure the same important uniform liquid distribution across any bed of this type of packing. 

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