Achieve Good Packed Tower Efficiency

Many plants rely on direct-contact mass transfer operations such as distillation, absorption and stripping. Indeed, overall process performance frequently rests heavily upon the efficiency of the mass transfer, which, in turn, depends upon tower design.

Design of a direct-contact mass transfer column must consider two major components. One is the number of theoretical stages (TS) or transfer units required for the separation. (In this article, TS refers to both.) The second is the mechanical selection and design of the tower internals (packing or trays). These two are intimately tied together through the question: What depth of packing or how many trays will provide the number of TS required? The answer depends upon the efficiency of the contacting device and that’s influenced by its mechanical design.

So, we’ll explore the effect of liquid distributor design on the performance of towers using random or structured packing. The number of TS required is calculated with a given liquid-to-vapor (L/V) ratio. Any deviation from this ratio caused by poor (uneven) liquid distribution negatively impacts the separation. This lack of separation often is misinterpreted as poor packing efficiency, that is, a higher-than-expected height equivalent to a theoretical plate (HETP). However, the packing’s mass transfer capability didn’t change — poor liquid distributor design caused a change in L/V ratio and thus the loss in separation efficiency.

Figure 1 -- Impact of L/V ratio: Change in ratio can lead to an equilibrium pinch in which no further separation can occur.

The deviation from the desired L/V ratio in some areas of the tower may result in equilibrium pinching — a condition in which the packing won’t appear to be working because further separation can’t occur no matter what depth of packing is available (Figure 1). Over its efficient operating range, the packing should provide a relatively constant level of vapor/liquid contact and therefore nearly identical interfacial area for mass transfer. So, when you suffer loss of packing efficiency indicated by the lack of separation, focus on a change in L/V ratio due to liquid, or less commonly vapor, maldistribution.

Liquid Distribution
All liquid distributors use an orifice to meter the liquid onto the top of a packed bed. Liquid from the orifice may enter a pipe to avoid being entrained by rising vapor or may impinge upon a spreader plate to improve distribution. The driving force across the orifice is provided either by a static head (from a pool or a standpipe pool of liquid) in a gravity liquid distributor or by pressurized liquid (normally from a pump). Figures 2–4 show various types of liquid distributors.

Figure 2 -- Trough arm distributor: Water pours out of bottom orifices of Sulzer VKG on test stand at FRI.

Uniform liquid distribution is key to obtaining expected performance from a packed bed. The distributor needs to uniformly allocate the liquid for all anticipated flow rates, with an adequate number of pour points for the size of the packing and sufficient open area for vapor passage. Both liquid orifice point-to-point flow uniformity and uniformity of the orifice drip-point pattern across the superficial tower area are crucial to obtaining good packing performance.

Liquid maldistribution affects packing performance in two ways. The local variations, orifice to orifice, in terms of the L/V ratios may cause the compositional pinch shown in Figure 1. And liquid maldistribution over a large section of the tower cross-sectional area may prompt the liquid to flow unevenly through the packing, concentrating at the wall.

Figure 3 -- Another trough arm distributor: This Raschig DTS trough arm distributor features side orifices with discharge tubes.

When a liquid distributor is operated at rates below its designed turndown ratio, it won’t provide uniform flow point-to-point and across the tower cross-sectional area because of low liquid head above the orifices. As the flow uniformity diminishes the separation deteriorates.