Orifice distributors are usually of the pan type or of the tunnel type. The former type is best suited for small-diameter columns (<4 ft), while the latter is used in larger-diameter columns (>4 ft).
An orifice pan distributor consists of a pan equipped with circular or rectangular risers for vapor flow and perforations in the pan floor for liquid flow. The pan may rest on a support ring; alternatively, it may be supported on lugs in a manner that provides an annular space for vapor rise between the distributor and the column wall.
Orifice tunnel distributors consist of parallel troughs with perforations for liquid flow in the trough floors. Vapor rises in the space between the troughs. The troughs are often interconnected by cross channels that equalize liquid levels in different troughs. Level-equalizing channels are most important in columns greater than 10 ft in diameter.
Orifice distributors can incorporate a large number of drip points and therefore have the potential for providing better liquid distribution than most other distributor types. This better liquid distribution is not always achieved, the main restricting factors being difficulty in irrigating areas beneath vapor passages and supports, a high sensitivity to uneven spread of drip points, and a high sensitivity to plugging and construction irregularities.
Orifice distributors are capable of handling high liquid loads, with standard orifice pan distributors and orifice tunnel distributors delivering up to 30 and 50 to 70 gpm per square foot of bed, respectively. The open area for vapor flow is relatively low in orifice distributors.
Orifice distributors are also generally larger, more expensive, consume more vertical space, and are more difficult to support than most other distributors. Tunnel orifice distributors provide greater open areas for vapor flow, are easier to support, and are more suitable for large-diameter columns than orifice pan distributors.
Guidelines for selection, design, and operation of orifice distributors are listed below:
- It is frequently difficult to incorporate sufficient open area for vapor rise while maintaining proper irrigation of areas beneath vapor risers. Inadequate irrigation under vapor risers may cause large-scale maldistribution, which can be detrimental to column efficiency. To avoid this problem, it is best to provide a large number of small risers (this can be expensive). The risers can be rectangular or round. One recommends a riser diameter of 4 to 6 in when round risers are used, while another prefers 2 to 4 in. Alternative solutions are installing short tubes which direct liquid to poorly irrigated areas, or drilling holes in the sidewalls of orifice troughs or using criss-cross risers. Such solutions must be carefully engineered and properly tested, and prediction of their performance may be difficult.
- To incorporate sufficient open area with orifice pan distributors, a pan of smaller diameter than the column is sometimes specified, with the annular space between the pan and the wall utilized for vapor rise. This practice may leave a significant fraction of column area un-irrigated. For instance, a 4-in-wide annulus in a 4-ft column will leave 30 percent of the bed cross-sectional area un-irrigated. If it becomes impractical or too expensive to provide sufficient open area without forming poorly irrigated regions, it is best to consider an alternative distributor type.
- Wide support beams or support rings may generate poorly irrigated areas on top of the packing, and therefore large-scale maldistribution. The distributor supports should be carefully reviewed to ensure proper irrigation underneath, especially in the wall region. Sometimes, short tubes can be used to direct liquid to un-irrigated areas under distributor supports, but these need to be carefully designed and tested.
- Supporting an orifice distributor directly on the packing is not recommended, because it may be misaligned during column upsets. In addition, this practice does not permit adequate vapor disengagement from the bed and may cause maldistribution and premature flooding.
- Orifice distributors are favored in foaming services, because liquid drip points are separated from the vapor risers.
- Liquid aeration in pans or troughs can be troublesome in high-pressure, high-liquid-rate, and foaming services. As the liquid enters the distributor liquid pool from above, vapor bubbles are dragged into the liquid (a "waterfall pool" effect). In low-pressure services, these bubbles readily disengage from the liquid and aeration is seldom troublesome. On the other hand, vapor disengagement is slow in foaming and high-pressure services, while in high-liquid-rate services, residence time for vapor disengagement may be too small. Frothing will then occur, causing uneven perforation flow and excessive liquid height in the distributor. The problem is often overcome by using a closed pan or trough, or by other techniques.
- Method for sizing orifice distributors is
Q = 5.46 x Kn x d^2 x h^(0.5)
Q = total liquid flow rate, gpm
n = number of orifices
d = orifice diameter, inches
h = liquid head loss across orifices, inches
K = orifice discharge coefficient
For punched holes, it has been recommended to use K = 0.707 but values as low as 0.62 to 0.63 are sometimes used. Note that h is equal to the liquid height in the pan or troughs minus the vapor head loss (expressed as inches of liquid) in the distributor risers.
Jets leaving the orifices may be unstable; they may move laterally or even break into spray. This can be visualized by experimenting with a household tap. However, jet stability is also dependent on surface irregularities at the orifices, and these are difficult to predict.
To improve jet instability, some high-performance distributors have each perforation equipped with a short, flow-straightening tube. This alleviates spray formation and minimizes any lateral movement of jets. It is recommended using these whenever spray formation is predicted.
- The flow area of the vapor risers is set by the allowable pressure drop. Too large an area may promote poor irrigation; too small an area leads to excessive pressure drop. The pressure drop must be low enough to satisfy system criteria and to avoid excessive liquid backup in the pan or troughs. Excessive liquid backup will overflow into the vapor risers, leading to maldistribution and possibly premature flooding. It has been recommended to make riser area 15 to 45 percent of tower cross-sectional area. Typically, riser pressure drop is 0.25 in of liquid. Methods for calculating riser pressure drops are detailed elsewhere.
- Liquid depth in the distributor pan or troughs dictates the riser height. The operable liquid depth ranges from 1/2 in at minimum liquid flow rates to 1 in below the top of the riser at maximum flow rates. An additional margin of 1/2 to 1 in or greater at each end of the range is frequently justified.
Excessive liquid depth may spill liquid into the vapor risers, causing maldistribution and possibly premature flooding. Liquid spillage is promoted when the liquid is aerated, agitated, or when plugging occurs. It is recommended that the normal liquid head be set at 50 to 70 percent of the riser height. Another recommendation is to setting the liquid
level such that no spillage occurs when 10 to 15 percent of the orifices are plugged. There is a lesson learned with one column that achieved poor efficiency because liquid spilled into the vapor risers. The spill was caused by undersizing of the total orifice area.
Insufficient liquid depth is likely to cause maldistribution and may allow some perforations to dry, permitting vapor flow through them. The lower the liquid depth, the greater the sensitivity of the irrigation pattern to distributor out-of-levelness, fabrication irregularities, perforation corrosion, and liquid surface agitation. In addition, the fall height of liquid from the feed pipe to the distributor increases as liquid depth in the distributor declines, thus increasing aeration and liquid surface agitation.
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 distributor cost and support requirements. Riser height is usually about 6 inch for standard orifice distributors and 8 to 12 inch for high-performance variations.
- Turndown ratios of orifice pan distributors are relatively high, with ratios of up to 4:1 achievable with standard designs. The turndown ratio is lower with tunnel orifice distributors, with standard designs achieving ratios of about 2.5:1. Higher ratios can be achieved by using taller pans or troughs and taller gas risers.
- Orifice distributors should be avoided in services where plugging may occur, such as when solids are present or
when liquid is close to its freezing point. In one reported case, plugging of an orifice pan distributor (with large orifices) in a fouling service caused liquid mal-distribution and poor separation efficiency. Compared to perforated-pipe distributors, orifice distributors have lower liquid velocities, larger liquid residence time, and open pans (or troughs) into which solids can be carried over during upsets and which can overflow when perforations are plugged. All these factors render orifice distributors more sensitive to plugging than even perforated-pipe distributors. If it is still desired to use an orifice distributor with a solids-containing stream, adequate filtration is mandatory, but may be insufficient to avoid plugging. If deposits adhering to the column top head may drop into the distributor troughs (or pan), a closed-trough design or trough covers should be considered.
A technique sometimes used to minimize plugging and its effect on distribution is to provide perforations or V-notches in the side of the troughs; alternatively, perforations or V-notches near the top of the troughs can be provided to ensure an even overflow.
The technique of using side openings can be troublesome, because the liquid jets follow a complex trajectory motion. As a result, the points at which the jets hit the top of the packing become a complex function of the liquid head in the troughs and of the vertical distance from the openings to the top of the packing. This may generate maldistribution. To overcome this problem, deflecting baffles are often installed in front of the orifices, and these deflect the liquid downward to a desired location. These baffles have the added advantage of breaking the liquid jets and converting them into liquid sheets (a "men's urinal wall" principle). With structured packings, liquid sheets are advantageous; they are introduced perpendicular to the crimp openings, thus ensuring even irrigation to all flow channels. However, the effectiveness of the baffles in converting the point sources into liquid sheets maybe low, even nonexistent. A lesson learned of one distributor water test where the baffles were effective in deflecting the liquid directly downward to the desired locations and in giving even irrigation, but completely ineffective in converting the point sources into liquid sheets.
Another technique to minimize plugging is to equip each orifice with a short drip tube which rises vertically above the bottom of the pan, so that solids settling at the bottom of the pan do not enter the orifices.
The above techniques must be carefully engineered and adequately tested to ensure even irrigation.
- Orifice distributors are best avoided in corrosive services, because some orifices may expand more than others.
- The method of drilling the perforations is important. Fabrication irregularities on the top surface of the pan or troughs may increase the flow resistance of some perforations compared to others. On the other hand, fabrication irregularities on the bottom surface may induce liquid flow along the bottom face of the pan or trough, and therefore,
- Orifice distributors are sensitive to out-of-levelness and to liquid surface agitation, particularly when liquid depth is either low or close to the point of overflowing the risers. Both out-of-levelness and liquid surface agitation cause uneven liquid depths, and therefore an uneven irrigation pattern to the bed below. At high liquid rates, both may cause uneven and premature liquid overflow into vapor risers. Levelness tolerances of 1/8 and 1/4 inch have been recommended for orifice distributors in towers 1.5 to 8 ft and 8 to 20 ft inch diameter, respectively.
* Reproduced from Distillation Operation by Henry Kister