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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Liquid irrigation along the column wall also is important. Too large a distance between the column wall and the outer distributor pour points (e.g., when a distributor is set on top of a tray ring) cuts the efficiency of the packing. Too many pour points near the column wall may lead to over-irrigation there, resulting in poor separation efficiency because of pinching in some areas of the bed that are under-irrigated or short-circuiting liquid down the tower wall past the packing and the rising vapor [2 ,3].
The distance between the bottom of the liquid distributor and the top of the packing, as well as the distributor open area, must be chosen to avoid liquid entrainment due to high vapor velocity. Typical practice is to put the distributor or redistributor six inches above the bed. The closer the tower is operated near the maximum capacity of the packing, the more care a designer must take to ensure that the liquid streams leaving the distributor don’t break up into droplets and get carried up the vapor risers.

Figure 7 -- Pall rings: FRI data indicate the impact of pour point density on the efficiency of 1-in. rings.

A typical distributor has about 33% open area, so the velocity of the vapor entering the riser area is about three times that of the vapor leaving the top of the packed bed. Some specially designed distributors offer open areas as high as 45%. Liquid that goes up risers and then falls on the distributor can prompt premature distributor flooding by increasing the distributor’s liquid load. Liquid entrainment between packed beds will cause liquid back-mixing and decrease separation in the bed above. If there’s no bed above, the entrainment will be carried out of the tower, lowering the purity of the overhead product.

For optimum packing performance it’s crucial to avoid large-scale or zonal maldistribution [2]. A large-scale maldistribution pattern will more severely lower efficiency than a small-scale random liquid maldistribution. Figure 8 shows the significant impact of blanking off a central area and a chord of the distributor. The height of packing required to even out concentration differences can vary substantially depending upon the severity of maldistribution and the size of the packing. Maldistribution affects large packings (> 2.5 in.) less than small ones. Slight random variations in flow through the recommended number of pour points uniformly distributed over the tower cross-section should minimally impact performance. However, flow variations that are localized or zonal in nature could undermine performance [3].

Figure 8 -- Zonal maldistribution: Blanking off portions of a distributor to create maldistribution lowers packing efficiency as shown by FRI data.

Other Considerations
The liquid distributor often is the limiting factor in turndown unless it’s specifically designed for a high turndown, for example using multi-level liquid orifices. The equation for flow through an orifice (Eq. 2 below) governs the design of the distributor. This means that flow is proportional to the square root of liquid head; so, for example, a turndown of 4:1 would translate into an increase of 1,600% in liquid elevation above a hole. It’s clear that turndown requirements demand careful consideration to avoid tall and expensive liquid distribution devices.
Because separation efficiency tends to deteriorate with increase in packed bed depth, liquid should be redistributed approximately every 30 ft to 35 ft or every 20 TS per bed, whichever is lower. (Some studies indicate that a greater number of TS are possible without the efficiency decrease [4].) When it’s necessary to satisfy structural strength requirements due to bed weight or to introduce feed or withdraw side streams, a liquid redistributor should be used. Redistributors must be placed between beds — and designed and installed as carefully as distributors at the top of the bed. The main difference between a reflux or primary distributor and a redistributor is that the redistributor is between packed beds and therefore has liquid falling on it from the bed above; so it requires vapor riser covers to prevent the falling liquid from entering the risers and being entrained up the tower.

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