Distribution liquid

Example 4.1 Oscillatory Variation of Void Fraction Near the Walls of Packed Beds 

It is well known that the wall in a packed bed affects the packing density resulting in void fraction variations in the radial direction. Mueller (1992) developed the following equation to predict the radial void fraction distribution in a cylindrical tower packed with equal-sized spheres  

J0 = Bessel function of the first kind of order zero r = radial distance measured from the wall Consider a cylindrical vessel with a diameter of 305 mm packed with solid spheres with a diameter of 20 mm. Plot the radial void fraction fluctuations near the walls for this packed bed. 

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E] Z = packed height, m of each section with its own liquid distribution. The original work is reported in English units. Cornell et al. (Ref. 81) review early literature. Improved fit of Cornell s ( ) values given by Belles and Fair (Refs. 74 and 75) and in Fig. 5-29. 

Whenever data are available for a given system under similar conditions of temperature, pressure, and composition, equilibrium distribution coefficients (iC = y/x) provide a much more rehable tool for predicting vapor-liquid distributions. A detailed discussion of equilibrium iC vahies is presented in Sec. 13. 

When straight or serrated segmental weirs are used in a column of circiilar cross secdion, a correction may be needed for the distorted pattern of flow at the ends of the weirs, depending on liquid flow rate. The correction factor F from Fig. 14-33 is used direcdly in Eq. (14-112) or Eq. (14-119). Even when circular downcomers are utilized, they are often fed by the overflow from a segmental weir. When the weir crest over a straight segmental weir is less than 6 mm V in), it is desirable to use a serrated (notched) weir to provide good liquid distribution. Inasmuch as fabrication standards permit the tray to be 3 mm Vh in) out of level, weir crests less than 6 mm V in) can result in maldistribution of hquid flow. 

Liquid distribution Good only at high liquid rate Good Good Good... 

Bed limiters commonly are used with metal or plastic tower packings. The primary function of these devices is to prevent expansion of the packed bed, as well as to maintain the bed top surface level. In large diameter columns, the packed bed will not fluidize over the entire surface. Vapor surges fluidize random spots on the top of the bed so that after return to normal operation the bed top surface is quite irregular. Thus the liquid distribution can be effected by such an occurrence. 

Packed bed heights typically vary from 20 ft to 30ft. Many times the location of manholes to provide access to the redistributor will determine the packed depth. Whenever more than 15 theoretical stages are required in one packed bed, good liquid distribution is critical. ... 

Lower pressure drop (less than half) than Raschig rings, also lower HTU (in some systems also lower than Bert saddles), higher flooding limit. Good liquid distribution, high capacity. Considerable side thmst on column wall. Available in metal, plastic and ceramic. 

Although, for most moderators, the surface of a stationary phase in LC can be considered stable at moderator concentrations above about 5%v/v, the results from the same experiments as those carried out by Purnell and his group could still be considered invalid and, at best, would not lead to unambiguous conclusions. Katz et al. [9] avoided this problem by examining liquid/liquid distribution systems using water as one phase and a series of immiscible solvent mixtures as the other and by measuring absolute distribution coefficients as opposed to retention volumes. 

In the range of operating temperatures and compositions, the equilibrium relations are monotonic functions of temperature of the MSA. This is typically true. For instance, normally in gas absorption Henry s coefficient monotonically decreases as the temperature of the MSA is lowered while for stripping the gas-liquid distribution coefficient monotonically increases as the temperature of the stripping agent is increased. 

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... 

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. 

Liquid Distribution Feed, Side Streams, Reflux... 

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. 

Liquid distribution probablv phtys the most important part in the efficient o]veration of a packed tower. good packing from the process viewpoint can be rednced in effectiv eness bv poor liquid distribution across the to ) of its upper surface or the packing sections below mv feed inlet(s) oi rellitx inlets. 

Figure 9-8J. MTS-109 Multipan two-stage liquid distributor for optimum liquid distribution for uniform flow for random and structured packings for low to moderate liquid rates, less than 5 gpm/ft. Also used for redistributor. Used by permission of Nutter Engineering, Harsco Corp., Bull. TI-1, under license from The Dow Chemical Co., protected by U.S. Patents No. 4,472,325 4,808,350 5,013,491.

Figure 9-8L. Full cone spray nozzle as used for liquid distribution. Used by permission of Spraying Systems, Co., Cat. 55.

Turndovn is usually limited to 0.5 1, and liquid distribution can be poor if the sprays are not carefully laid out and the system flow tested for uniformity. Another problem is misting of the liquid from the sprays and the resulting entrainment out of the tower or up to overhead mist eliminators. 

Ov Crall, the careful design of a distributor for liquid in the top of a packed tower, and for the redistribution of liquid flowing dow n multi-section packing in the tower, is essential to good consistent tower performance. However, the liquid flow is not alone, the uniformity of vapor distribution is likewise essential, because non-uniform vapor distribution can cause non-uniform liquid downflow. Then, there is the selection of the packing itself and its characteristics and requirements/sensitivity to the uniform distribution of the liquid and vapor. As earlier emphasized, the level of the distributor tray or trough can be critical to the consistent uniform liquid distribution. 

Liquid Distribution Patterns in Packed Colunms (Data in 6-in., 12-in., and 24-in. Dia. Towers)... 

Percent Liquid Distribution in Inner % Tower Area... 

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]. 

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