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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Bunching the orifice together to make room for a gas riser or a tray ring can cause micro maldistribution. The methods for laying out the liquid orifice pattern differ slightly for random packing [1] and structured packing [9] because random packing spreads liquid in the shape of an inverted cone, 360° spreading, while structured packing spreads liquid more in one direction than the other and depends upon the rotation between adjacent layers to get 360° liquid spreading.

Orifice density. The number of orifices per unit area affects packing efficiency. With too few, liquid is essentially maldistributed on a micro scale because orifices are so far apart that a large portion of packing remains un-wetted, resulting in poor interfacial contact area, improper L/V ratios and vapor bypassing. However, using a few large orifices does somewhat decrease the cost of a distributor and the problem of fouling. On the other hand, using many small orifices will boost separation efficiency but also the cost of the distributor and the chance of plugging. Generally it’s best to keep the density between 4 and 10 points/ft2 [2]. The use of spreader plates on the orifice discharge allows a lower orifice count while maintaining good distribution [9].

Depth of the liquid pool above the orifice. In a gravity head distributor this is the sum of the two components: hOA = hO + hPD (1) where hOA is the total liquid head in the liquid distributor, in.; hO is the orifice head, in.; and hPD is the distributor head loss on the vapor side, in. It is good practice to add one to two inches to hOA when determining the physical height of the vapor risers to prevent liquid from entering the vapor risers at maximum flow rate.

The flow through the orifice is defined [8] as: QO = 19.636 Kd2hO< sup> .5>(2) where QO is single orifice flow, gpm; d is orifice diameter, in.; hO is head at the orifice without hPD, ft of liquid; and K is the discharge coefficient.

The head required for flow through n orifices in a distributor can be calculated by rearranging Eq. 2 and changing hO to inches: hO = 0.0311 QT2/K2d4n2 (3)

Rearranging Eq. 3 gives the number of orifices required for a specified head: n = 0.1764 QT/Kd2hO0.5 (4)
where QT is total liquid flow to distributor, gpm.

It can be seen from Eq. 2 that the flow, if all else is held constant, is a square root function of liquid head above the orifice. Therefore, at low heads (< 1 in.) the flow is very sensitive to small changes in liquid depth. A ±¼-in. variation in head changes flow +11.8% to -13.4%. Thus, an out-of-level distributor with a 1-in. deep pool having a ¼-in. wave action will have 25.2% flow variation. If the pool depth is increased to 2 in. the same ¼-in. head variation will be reduced to +6.1% to -6.5% for a maximum variation of 12.6%. Thus, a minimum liquid pool depth of 2 in. is in line with the recommendation in Ref. 1 of an orifice-to-orifice variation of ±5%.

 

Figure 9 -- Point versus zonal maldistribution:
The nature of liquid maldistribution, whether random or random zonal, changes the impact on packing efficiency.

 


For random packing, Fractionation Research, Inc. (FRI) found that a flow variation of ±12.5% randomly spread across the tower cross-sectional area didn’t degrade the efficiency of 1-in. metal Pall rings. However, if the flow variation isn’t randomly spread across the tower but isolated in zones where all low-flowing orifices are in one area and high-flowing orifices in another area, the apparent packing efficiency declined by 20% [2] (Figures 8 and 9). For structured packing the results were similar but the apparent packing efficiency declined for both the distributed random flow variation and the zonal maldistribution distribution [2].

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