Drip points

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. 

These distributors are fabricated of pipe lengths tied to a central distribution header (usually) %vith orifice holes drilled in the bottom of the various pipe laterals off the header. This style of distributor can be fed by pressure or gravity for clean fluids. The gravity feed is considered better for critical distillation application when uniformity of the flow of the drip points (or flow points) through out the cross-section of the tower is extremely important, and is excellent for low flow requirements such as below 10 gpm/ft2 [131]. 

For low purity hydrocarbon fraction, the number of drip or separate flow points shoitid be 6-10 drip points/ft of tower cross-section surface area. 

Pressure drop data for several styles and arrangements of drip point grid tile are given in Figures 9-25A-9-25E. These are not included in the general GPDC correlations for random packings. 

Figure 9-25A. Pressure drop in inches of water per foot of height, drip point tile, shape 6295 with crossed flue arrangement. Used by permission of General Refractories Co.

Drip-point grids (continuous flue) Style 6146 0.0154 0.23 3,500-30,000... 

Molstad, M. C., J. F. McKinney and R G. Abbey, Performance of Drip-Point Grid Tower Packings, Trans. Amer. Inst, of Chem. Engr. Vol. 39, No. 5, 605 (1943). 

Molstad, M. C., McKinney, J. F. and Abbey, R. G. Trans. Am. Inst. Chem. Eng. 39 (1943) 605. Performance of drip-point grid tower packings, III. Gas-film mass transfer coefficients additional liquid-film mass transfer coefficients. 

Large drops wander through the bed to the downstream edge, grow there by accretion of other drops, and break off by a drip-point formation method. 

In the ideal case, the number of openings (pour or drip points) of the distributor should be equal to the physical density of the bed channels (Stanek, 1994). In practice, in beds of high diameter, this number should be between 35 and 251 openings/m2 or alternatively, at least one opening for every 10-12 particles that occupy the cross-section of the bed (Klemas and Bonilla, 1995). 

One way out of this problem, is to increase APL, the pressure drop of the liquid flowing through the orifice holes. This could be done, by increasing the orifice hole liquid velocity. We could drill fewer orifice holes. Unfortunately, this would decrease the number of drip points per square foot of tower area (6 to 10 is a good target). This would reduce vapor-liquid contacting efficiency. Or, we could have smaller orifice holes. But too small a hole would probably plug with corrosion products. 

Figure 14-67c shows results of tests in which flows from individual distributor drip points were varied in a gaussian pattern (maxi-mum/mean = 2). When the pattern was randomly assigned, there was no efficiency loss. When the variations above the mean were assigned to a "high zone, and those below the mean to a "low zone, HETP rose by about 20 percent. With structured packing, both random and zonal maldistribution caused about the same loss of efficiency at the same degree of maldistribution. 

Figure 9.4 shows little wall flow near the top of the bed. This is because the liquid distributor drip points stop short of the wall (the unirrigated ring at the top of the bed was 1 in wide in Hoek s measurements). With increasing depth below the top of the bed, liquid... 

As column diameter is increased, it becomes more difficult to maintain the number of distributor drip points per unit area (3,40). AIbo, the frection of unirrigated area under the top distributor (near the column wall) may vary. These changes can leed to enhancement of maldistribution in the prototype. It was recommended (170) to use the same number of drip points per unit area and to ensure that liquid is distributed to the column wall both in the prototype and in the pilot column. 

In the pilot column distributor, use the same number of drip points per unit area as in the prototype and ensure that liquid is distributed to the wall in the same manner as in the prototype. 

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