Achieve Good Packed Tower Efficiency

Proper design of liquid distributors plays a key role in getting the most from packings.

By Frank Rukovena and Tony Cai, Fractionation Research, Inc.

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where A is the cross-sectional area and WP is the wetted perimeter of the riser.
The friction factor, f, is computed using the Colebrook equation for turbulent pipe flow:
(10) where ε is the relative roughness of the pipe, which is about 0.0018 in. for stainless steel. If the risers are noncircular pipes, the hydraulic diameter, Dh, needs to be used. The vapor Reynolds number, Re, is based on Dh.

The vapor velocity, Vv, in the riser was approximated using weighted vapor rates and riser opening areas. The weighted vapor rate was calculated by multiplying total vapor rate by the ratio of the riser opening area to total opening area of the column cross-section at the distributor level.

The pressure drops calculated from Eq. 5 were converted to head losses via: hPD = Δp/ρLg (11)

Effects of horizontal liquid velocity on orifice flow. The “still liquid pool” above the orifice should be sufficiently deep to prevent orifice flow instability over the expected distributor operating range, as discussed above. The pool should have large enough volume to keep the horizontal velocity low so as not to significantly vary the liquid head above orifices due to the Bernoulli effect. High horizontal liquid velocity in a distributor trough or arm reduces flow through the orifices by lowering the pressure above the orifice in high velocity areas and increasing the head above orifices in areas where the liquid slows down or stops flowing. The head change caused by the horizontal velocity change of the liquid in a trough can be calculated [1] by:
hv = 0.187 Vh2 (12) where hv is the velocity head, in.; and Vh is the horizontal liquid velocity, ft/s.
It’s best to keep the horizontal velocity under 1.25 ft/s at the minimum head condition. The effect of the horizontal velocity also depends on the total liquid head above the orifice, with less impact on distributors with high liquid head.

Feed pipes. The importance of feed pipe design for distributors or redistributors often is underestimated. It’s critical that liquid leaving the feed pipe doesn’t upset the hydraulics in the distributor. It shouldn’t enter the vapor risers or be entrained by the vapor exiting the vapor riser. This can be accomplished if the liquid is discharged below the top of the vapor risers.

The liquid leaving the feed pipe shouldn’t impinge on the orifices of the distributor or parting box. Otherwise it’ll generate high horizontal velocities and waves across the orifices that will cause flow variations out of orifices. Kister [7] recommends keeping the velocity under 10 ft/s, preferably less than 5 ft/s. As tower diameter increases the design of the feed pipe becomes more complicated, especially if it goes directly into the distributor and not a parting box. Ref. 5 gives criteria for designing perforated pipe distributors and suggests a pressure drop across the orifice of ten times the velocity head of the liquid in the feed pipe for a ±5% flow variation among orifices. This is better than what’s required in most applications but is a good design starting point.

Obtain optimum performance
The design of liquid distributors and redistributors plays an important role in the performance of packed towers. To realize the packing’s full mass transfer-efficiency it’s imperative that liquid entering a packed bed is evenly distributed, so it wets the entire packing surface as soon as possible, and is flowing uniformly across the tower area at the same L/V ratio as used in the TS calculation. To achieve this, the designer must pay close attention to the hydraulics of liquid distributors and redistributors. Distribution of vapor entering the bottom of a packed bed whether it’s coming from a vapor inlet or the bed below, although not the focus of this article, deserves equal consideration.

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