Guidelines for introducing liquid into parting boxes and distributor pan.
In most small columns (<3 to 4 ft in diameter), the inlet pipe directly feeds the distributor pan (or trough). In larger columns, liquid is fed from the inlet pipe into one or more parting boxes, which then feed liquid to the distributor pan (or troughs) via perforations or notches. Incorrect feeding of liquid into the parting boxes or into the distributor can generate uneven liquid surface in the distributor, causing mal-distribution in the packed bed below. Below are recommended guidelines for introducing liquid into parting boxes and distributor pans (or troughs).
- Feed velocities leaving the feed pipe or feed sparger should not exceed 10 ft/s and preferably be less than 4 to 5 ft/s. High velocities may disturb the liquid surface or cause excessive aeration in the distributor or parting box.
- Parting boxes should be sparger-fed. The feed points must fall between the troughs and not over the perforations
or weirs feeding liquid from the parting box to the troughs. An even better practice is to feed parting boxes from a feeder box.
- Sparger pipes feeding parting boxes should be oriented parallel to and centered directly above the centerline of each parting box. This prevents the entering liquid from missing the parting box.
- Sparger pipes feeding parting boxes should be designed according to the criteria in Section 4 (except for criteria 10 and 12). Achieving uniform distribution out of the sparger perforations is not critical because the parting box equalizes liquid level. The sparger must be designed so that it does not induce wave formation or excessive hydraulic gradient in the parting box. Special attention is needed to item 2 in Section 4, since the horizontal velocity component can form waves or push liquid against the narrow wall of the parting box. This, in turn, can cause poor distribution into the troughs, or induce liquid overflow at the narrow wall. In one acid tower, this overflowing liquid was entrained by rising vapor, leading to an acid emission problem. Use of flow-straightening tubes can avoid this problem.
- When the inlet pipe directly feeds the distributor, the incoming liquid should be fed into the center of the distributor in order to ensure uniform head over all the orifices. The feed pipe should be located about 2 to 8 in above the top edge of the distributor pan. It is important to ensure the feed flows into the distributor and does not enter the vapor risers. In one lesson learned, poor column separation resulted from distributor feed being directly introduced into an open riser. It is best to "ell" the pipe down and continue it vertically downward for a short distance to prevent feed from entering the vapor risers. The distributor should be inspected for absence of risers directly beneath the pipe outlet.
- The liquid fall height from the inlet pipe or sparger to the parting box or distributor should be minimized to prevent excessive splashing, frothing, or liquid aeration, and uneven liquid surface. A feeder box should minimize these problems.
- The depth of liquid in the parting box is of major importance. If too shallow, surface disturbances in the parting box may cause mal-distribution among the troughs; if too high, they may cause liquid overflow, which in turn may induce mal-distribution in the packed bed below.
- Waves on the liquid surface in the parting box must be avoided. The potential for wave formation can best be assessed in a water test. If waves are observed, either the feed sparger should be modified or baffles should be added to the parting box.
- Parting boxes may contain perforations in their bottom, V-notches or rectangular notches or perforations in their sides, or both. Bottom perforations are generally preferred because they generate the least disturbance to the liquid surface in the distributor troughs. The disturbances can be further mitigated by equipping perforations with tubes submerged in the trough liquid. Liquid entering the troughs via bottom perforations possesses no horizontal momentum, thus reducing the tendency to form an uneven crest and surface waves. Bottom perforations also avoid the possibility of liquid missing the troughs and are less sensitive than V-notches to out-of-levelness. Since the number of parting box perforations is generally much smaller than distributor drip points, larger perforation diameters are used. These larger perforations are far less sensitive to plugging and corrosion than the smaller perforations used in orifice distributors.
- Whenever possible, internal flanges should be avoided or minimized in the feed pipes. If one or more such flanges are necessary, they should be carefully installed and inspected to prevent leakage. Significant leakage of internal flanges in trough distributors is likely to generate large-scale mal-distribution.
Read further for additional general guidelines for packed tower liquid distributor design, selection, construction, and operation.