Spray distributors are pipe headers with spray nozzles fitted on the underside of the pipes. They are most popular in heat transfer and scrubbing services and are infrequently used in fractionation. Services where spray distributors are common include refinery crude towers, FCC main fractionators, and refinery vacuum towers.
Spray distributors are also used in very small columns (where a single spray nozzle adequately covers the entire cross-sectional area), and in applications where a large vapor-handling capacity is most important. Some designer engineers recommend avoiding spray distributors in small diameter columns.
The quality of distribution provided by spray distributors may be inferior to any of the others because the spray cones create areas of uneven irrigation, the spray cones are often non-homogeneous, and because a significant amount of liquid is directed toward the wall. Factors such as spray angle, height of the spray nozzles above the bed, nozzle construction, and nozzle pattern set the quality of distribution. The effect of these factors on the quality of distribution is poorly understood, and good nonproprietary ground rules for spray distributor design are unavailable. In short packed beds, the sprays themselves may significantly contribute to mass and heat transfer. This can partly account for the favorable experiences with spray distributors in refinery vacuum towers, which typically have short beds.
Other performance characteristics of spray distributors are generally similar to perforated-pipe distributors. Like perforated-pipe distributors, spray distributors are of simple construction, are easy to support, and are inexpensive. Compared to perforated-pipe distributors, spray distributors offer an even larger open area, a greater liquid flow rate, easier replacement of corroded or eroded sections, and more extensive irrigation. On the downside, spray distributors require higher pumping horsepower than other distributors, and an overhead mist eliminator is mandatory in order to control entrainment. Compared to perforated-pipe distributors, spray distributors also consume much more vertical space.
Guidelines for selection, design, and operation of spray distributors are listed below:
- Typically, spray distributors use wide-angle (120°) sprays, and are located 18 to 36 inches above the bed, providing irrigation area of the order of 5 to 10 ft2 per spray nozzle. Typical pressure drops are 5 to 30 psi.
- Some overlap of the spray footprints at the top of the packing is beneficial. An overlap of about 100 percent (double coverage) has been recommended for uniform irrigation.
- The sprays should be as homogeneous as possible. Liquid distribution is sensitive to even a slight spray non-homogeneity. The sprays should be inspected for homogeneity during distributor water tests. If desired, a sample of the nozzles to be specified can be obtained ahead of time, hooked to a water pipe, and tested for homogeneity before the nozzles are specified.
- Areas of either overspraying or underspraying near the column wall may severely degrade liquid distribution and should be avoided.
- Spray distributors are not suitable for vapor-containing streams.
- Like perforated-pipe distributors, spray distributors are sensitive to plugging, corrosion, and erosion. The sensitivity to plugging is somewhat lower with spray distributors, while the sensitivity to corrosion and erosion is somewhat higher compared to perforated-pipe distributors because of the higher liquid velocity. "Nonplugging" spray nozzles are available, but their effectiveness is uncertain. If a spray distributor is used with solid-containing streams, adequate filtration is mandatory. There were cases where not only the spray nozzles but also the header plugged.
- Spray distributors should never be embedded in the packing, because this is likely to cause premature flooding. For good distribution, a spray distributor containing more than a single spray nozzle should be placed at least 18 in, and preferably more than 24 in, above the bed.
- Like perforated-pipe distributors, the spray distributor turndown ratio is about 2:1. Excessive liquid flow rates may create a mist problem, while low liquid flow rates reduce the cone diameter and may create poorly irrigated areas. For this reason, oversized spray nozzles must be avoided. As with perforated pipe distributors, a dual liquid distributor can be used to enhance turndown.
* Reproduced from Distillation Operation by Henry Kister