Drip points, number

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. 

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. 

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. 

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. 

Suggested Number of Drip Points/Ft for Random Packings... 

In order to determine Dq, each distributor drip point is represented by a circle. The center of the circle is located where the liquid from each drip point strikes the top of the bed. The area of each circle is proportional to the liquid flow, and the sum of all circle areas equals the tower cross-sectional area. If the liquid is evenly divided among all drip points, the area of each circle equals the tower cross-sectional area divided by the number of drip points. Terms A, B, and C in Eq. (3.1) are then evaluated as follows (Fig. 3.3a). 

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. 

Excessive liquid pressure drop through the distributor should be avoided, because this may restrict the number of drip points. The liquid line to the distributor should contain a control valve or a restriction orifice to let down any excessive pressure. A more satisfactory alternative is to use the high-performance perforated-pipe distributor variation which lets out liquid by gravity instead of pressure. 

Within the above limits and the turndown requirements, there is often an incentive to minimize the maximum liquid depth. The lower the maximum liquid depth, the greater is the number of drip points that can be incorporated, the smaller is the vertical space consumed by the distributor, and the lesser is the... 

The quality of distribution provided by notched-trough distributors is generally somewhat inferior to that achievable by orifice-type distributors. With notched-trough distributors, it is generally difficult to incorporate more than three to four drip points per square foot of column cross-sectional area (111, 438). It may also be difficult to space these drip points evenly. If it is practical to provide a sufficient number of drip points per unit of column area, and to space them evenly, this distributor can provide a distribution as good as an orifice-type distributor. 

In all but very clean noncorrosive services, the actual number of drip points per unit of bed area is usually dictated by the liquid flow rate and plugging tendencies, because these set the total perforation area and perforation diameter (74, 111, 386, 436). Experiences demonstrating the effect of the munber of drip points on liquid distribution have been reported (40, 305, 341). Some discussion is also available in Zanetti s survey (436). 

To improve unit performance, the existing reactor internals were replaced by high performance vortex t) e mixing chambers to optimize liquid-liquid mixing and VLT distribution trays to increase the number of drip points and minimize the dead flow zones adjacent to the reactor wall (Figure 6). The VLT distribution trays also offered low sensitivity to tray levelness and improved stability over a wide operating range. 

Drip points On a packed bed, this refers to the number of points where... 

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