General guidelines for packed tower liquid distributor design, selection, construction, and operation are presented.
- A liquid distributor (or redistributor) should be used in any location in a packed column where an external liquid stream is introduced.
- It is best to have the packing manufacturer specify and supply the distributor. The user should critically examine and carefully troubleshoot the manufacturer's recommendation and design.
- In order for manufacturers to specify or design a distributor correctly, they must be provided with concise information on the service; its plugging, corrosive, erosive, and foaming tendencies; and of any requirements which may affect distributor selection or design.
- Drilling (or punching) holes or cutting V-notches appears a simple task, and there may be a temptation to "fabricate your own distributor" in the workshop. This practice is dangerous and may lead to disasters, because fabrication irregularities may lead to severe maldistribution and loss of performance in the tower. It is recommended to specify that all perforations (or notches) be
punched (or cut) with the smooth edge of the hole facing the liquid, and that the rough edge is ground smooth free of burrs, but with the smooth edge facing the liquid. There have been cases where in a single distributor some troughs had the rough edge facing the liquid while others had the smooth edge facing the liquid, leading to uneven irrigation.
- Distributor performance should always be water-tested prior to startup with emphasis on critical services and large-diameter (> 8 ft) towers. This test can be performed in situ or at the manufacturer's shop. If not performed in situ, the piping supplying liquid to the distributor should be closely duplicated at the test rig. If maldistribution is apparent, it is best to seek the manufacturer's advice. One experience has been reported where a water test led to the solution of an absorber separation problem which resulted from maldistribution.
- The irrigation pattern at the top of the bed should be closely examined to identify areas of large-scale maldistribution. This should be carried out first on paper at the design stage, and then checked in the water test.
- Another useful paper check is to divide the tower cross-sectional area into three or four concentric radial zones of equal areas. The amount of liquid entering each area should ideally be equal. This check is useful for highlighting under-irrigation of the wall zone. Special attention should be paid to areas directly underneath un-perforated troughs, vapor passages, support beams, support rings, and near the column wall. The manufacturer's advice should be sought if any maldistribution is detected.
- To counteract the tendency of liquid flow toward the wall, a large percentage (> 10 percent) of the total liquid should not enter at the tower wall or within 5 to 10 percent of the tower diameter from the wall (257). At the same time, it is important to ensure that some liquid gets to the wall. One rule of thumb recommends maintaining a set of radially distributed pour points within one packing diameter of the wall. Experience has been reported where the presence of a 3-in-wide orifice pan distributor support ring in a 4-ft column caused a major drop in column efficiency because the area under the support ring was un-irrigated.
- A minimum of four drip points per square foot of bed cross section has been recommended in the industry. Some publications advocate a minimum number of 6 to 10 drip points per square foot of bed or even more. Industry experience suggests that gains from using more than 10 drip points per square foot are marginal, if any, and that packed-bed efficiency can normally be maintained with 5 drip points per square foot. There is, however, some nonconclusive evidence which may dispute this statement. A large number of drip points is often advocated for short packed beds, and for low liquid flow rate applications. 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. Experiences demonstrating the effect of the number of drip points on liquid distribution have been reported.
- The drip points should be evenly spread. Experienced cases demonstrate where a distributor provided more liquid per unit area to some central regions than to some peripheral regions, resulting in maldistribution. Such zonal maldistribution is detrimental to column efficiency.
- The distributor should be located at least 6 to 12 in above the packing to permit vapor disengagement from the bed before passing through the distributor. Greater distances (at least 18 to 24 in) above the bed are recommended for spray distributors. Experience recommends positioning high-performance distributors 4 to 6 in above the bed; some may prefer 6 to 8 in. Some may have concerned that a larger space above the bed may prompt liquid streams issuing from drip points to drift away from their striking target at the top of the packing, thereby leading to uneven irrigation. This drift is less of a problem when jets leaving the orifices are stable. Some may also argues that the close spacing of vapor risers in high-performance distributors alleviates the vapor disengagement problem, and therefore a larger space above the bed serves little purpose. Therefore, it prefers the larger (6 to 8 in) height between the distributor and the top of the bed as a better safeguard against entrainment, frothing, and splashing.
- The plugging potential of a service should not be underestimated. In a lesson learned, 1 lb of solids was sufficient to plug 80 percent of the perforations of a ladder pipe distributor in a 13.5-ft column. Another lesson learned where a small quantity of solids was sufficient to plug an orifice pan distributor. In a third lesson learned, fungus growth caused plugging and consequent over-
flow of an orifice distributor in a water scrubber.
- If the service contains solids, or the liquid is close to its freezing point, a weir-type distributor is the best choice. If it is still desired to use a perforated-pipe, spray, or orifice distributor, a filter should be installed upstream to remove particles that can block the perforations or spray nozzles. Successful applications of this technique have been reported. The opening of the filter elements should be considerably (preferably at least 10 times) smaller than the distributor perforations. The filter arrangement should include a spare filter in parallel and no bypass to ensure that one filter element is always on stream. Good filter maintenance and cleaning are essential; automatically
cleaning filters are sometimes used. The filters should be installed in an accessible location as close to the column as possible. Typical good locations are close to the foot of the vertical rise of liquid feed or reflux, or just upstream of the flashing control valve for flashing feeds. The line downstream of the filter should be adequately flushed or blown to shake free and remove loose rust particles prior to the startup. One lesson learned where the metallurgy of this downstream line was upgraded from carbon to stainless steel to avoid the rust particles. Another lesson learned was reported where spray nozzles plugged due to reliance on filters that were too distant from the column with carbon steel piping downstream of the filters. Orifice distributors with bottom perforations should be avoided in plugging services, even when filters are installed; there have been cases where small solid particles passed through the filter, agglomerated in the pan or troughs, and blocked perforations. In pressure distributors, particularly the spray type, a filter is often (but not always) sufficient.
- Perforation diameters smaller than 1/4-in should be avoided in order to prevent plugging; 1/2-in perforations are preferred. If the service is perfectly clean and noncorrosive, some designers advocate using holes as small as 1/8-in. Corrosion, erosion, and plugging also tend to change perforation diameter (and therefore perforation flow) to a greater extent when perforation diameter is small. On the other hand, the larger the perforation diameter, the lower the number of drip points that can be incorporated in the distributor. Multiple incidents have been experienced in which small distributor perforations were plugged by scale and dirt. In one reported incident, enlarging the perforation diameter and installing an upstream filter with 1/64-in openings eliminated a plugging problem in a ladder pipe distributor.
- In slightly corrosive services, it may pay to use a stainless steel distributor even when carbon steel is satisfactory as the packing material. Successful applications of this practice have been reported. Alternatively, a distributor which is insensitive to corrosion, such as the notched-trough type, can be used.
- When a high liquid flow rate is required, notched-trough, orifice-type, or spray-type distributors are the best selections.
- The vapor risers or channels offer resistance to vapor flow. If vapor pressure drop across the risers becomes equal to the liquid head above the distributor, the distributor will flood. It is therefore important to allow sufficient open area for vapor flow. This open area must be distributed evenly and in a manner that prevents formation of poorly irrigated regions directly beneath the vapor passages.
- When a high rate of vapor flow is required, the orifice pan and the weir riser distributors are best avoided.
- The area directly beneath wide troughs with no bottom perforations (e.g., in notched-trough distributors) should be closely examined to ensure absence of un-irrigated regions.
- Column turndown is commonly set by the turndown of the liquid distributor. Distributor turndown, therefore, is a most important consideration.
- For good turndown, weir-type or some orifice-type distributors are the best selections. Alternatively, the turndown of perforated-pipe, spray, and some orifice-trough distributors can be enhanced by using a dual liquid distributor arrangement. This arrangement consists of two distributors, mounted one above the other. The upper distributor is designed for a higher range of liquid flow rates than the lower distributor. At low liquid flow rates, only the lower distributor is operated; at medium liquid flow rates, only the upper distributor is operated; and at high liquid flow rates, both distributors are operated.
- Distributor levelness affects the quality of distribution, especially under turned-down conditions, when liquid head is low. Careful design and inspection are required to ensure that the distributors are level. Inspection with level gauges is strongly recommended for weir-type distributors. Weir-type distributors should be specified with leveling screws to enable in situ level adjustment.
- Leakage of liquid from the distributor or flanges on the pipes leading to the distributor may cause maldistribution. This is most severe in low liquid flow rate applications.
- Distributor pans and troughs should be deep enough to avoid liquid overflow. The overflowing liquid is likely to give poor irrigation, which will create maldistribution in the bed below. It may also interfere with vapor rise and cause entrainment and premature flooding. Overflowing is most troublesome when liquid flow is high and when the liquid tends to foam or become aerated (e.g., high-pressure services).