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.
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.