Putting the focus on tray efficiency

A common misconception widely repeated by academia and textbooks is the notion that a tray is a platform carrying liquid smoothly from the inlet downcomer on one side, to the outlet downcomer on the other. In this idealised arrangement, the vapour or gas is evenly sparged into the liquid through perforations in the tray. The narrative continues with the gas rising through the liquid as a swarm of well-defined bubbles which eventually pass through an interface at the top of the biphase. Figure 1 is a sketch of this idealisation. The reality, however, is a different story. The biphase shows extremely turbulent, violent interaction between the vapour and liquid to the extent that it often makes it hard to discern gas from liquid, and even to identify the upper surface of the biphase to the extent that photographing the biphase provides only very blurred, poorly-defined images. Idealisation and reality bear little resemblance to one another. However, hydraulic calculations (pressure drop, liquid depth on the tray, effective density of the biphase) can be quite reliably and usefully done on the idealised basis.

The hydraulic behaviour of trays is not their only important characteristic. Alt-hough hydraulics determines the ultimate vapour and liquid-handling capacity of trays (i.e., plant throughput), the separation one achieves is determined by the tray’s mass transfer characteristics, commonly referred to as tray efficiency, and theoretical stages in what are (unjustifiably) called, state-of-the-art models. In the following, it is posited that except for designs with quite poor vapour-liquid contact in the first place (for example dual-flow and disc-and-doughnut trays), most trays have efficiencies ranging from 92 – 108% depending on the tray design and tray type. This applies to standard hydrocarbon test systems, e.g., C6-C7 and iC4-nC4 mixtures.

Traditionally, tray development has focused primarily on pressure drop and column capacity, and somewhat less on efficiency. With hydrocarbons, efficiencies tend to lie in a relatively narrow range. In reactive systems (common in absorption, especially in gas treating) and in many chemical separations (especially those with highly nonideal phase equilibrium thermodynamics), however, the separation is controlled primarily by mass transfer rate limitations. Mass transfer rates depend on diffusion coefficients of the transferring species, on the interfacial areas for mass (and heat) transfer. In systems of chemicals, these species often interact with each other so that diffusion rates become collaborative even to the extent that a species can diffuse against its own concentration gradient.

This article offers a written and pictorial description of the kind of motion observed on most crossflow trays operating at normal vapour and liquid rates. It describes the effect of pressure on the appearance of the biphase, how the kind of deck perforation visually affects the flows and influences pressure drop, hydraulic capacity, and how hydraulics affects mass transfer.

Chaotic violence

At the design point in a well-designed column, most of the space between trays is filled with the vapour-liquid mixture, also called the biphase. At low L/V ratios and very low pressure (vacuum), liquid tends to be dispersed and vapour continuous (Figure 2), turning the biphase into a concentrated spray of droplets (spray regime). Entrainment occurs when liquid droplets are carried by the vapour onto the tray deck above by being carried by the vapour through the holes in the deck. At high L/V ratios and at high pressure, the biphase tends to appear as a churning mass of geysers, violently interacting to the extent that frequently some of the geysers will pass through the tray above as entrainment – under such conditions, entrainment may not be the most descriptive term. At intermediate L/V ratios and moderate pressure, the biphase has the appearance of an ill-defined mass of vapour and liquid where neither phase is clearly dominant or even clearly identifiable. Often even the place of transition from biphase to vapour is not clearly defined.

Except under vacuum and at quite low vapour flows, the vapour is considered to be dispersed and the liquid continuous, but the vapour rarely adopts the idealised form of bubble warms. Further, it consists of violently interacting jets and large gas and liquid volumes being torn apart by energetically intense interactions. Comparing the biphase to the activity in a washing machine does not adequately describe the turbulence of the interaction if only because of the comparatively much higher vapour volume, which a washing machine can neither produce nor sustain. To drive the phases towards a state of compositional equilibrium, however, intense interaction between phases is exactly what is wanted.

In gas treating operations, typified by carbon dioxide (CO₂) removal from the gas feeding an LNG facility, L/V ratios tend to be high and the tray will be operating close to downcomer choke flood. A downcomer is choke flooded when the total froth flow is unable to get into the downcomer mouth. It is backup flooded when all the liquid is unable to get out of the downcomer bottom, even with the downcomer full of liquid.

Sieves, valves, and other treatments

Vapour flow through sieve holes is vertically directed and no attempt is made through mechanical construction to force or even encourage interaction between adjacent jets except insofar as the sieve holes are close together. With interaction, mixing would be intensified, and better mixing implies better phase contact and better efficiency of mass transfer. Figure 3 is a graphical representation of the jets emanating from sieve holes coalescing into larger jets in which the vapour content largely bypasses the liquid and most of the biphase. Contacting is poor quality, entrainment occurs early, and tray efficiencies tend to be at the low end of the range.

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