Apr-2010

Tray hydraulic operating regimes and selectivity

Tray hydraulics is discussed in terms of froth and spray operating regimes, and possible impacts on selectivity are pointed out by reference to performance data

Ralph Weiland, Nathan Hatcher and Jaime Nava
Optimized Gas Treating

Article Summary

Most gas treating contactors are highly liquid loaded, with good designs operating at vapour loads approaching the jet-flood limit. An optimal design will also have downcomers sized to approach the choke flood limit at the design point, as this allows for the maximum tray active area, hence the smallest column diameter for a given throughput.

At high weir liquid loads, trays operate predominantly in the froth regime, where the gas dispersion rises through continuous liquid, interfacial areas are large and the liquid is highly agitated. This results in high absorption efficiency for both CO2 and H2S. Low weir loads correspond to the spray regime, where the liquid is dispersed as large droplets that are projected through a continuous gas phase as a spray. The droplets behave like nearly stagnant, rigid spheres and have extremely small liquid-side mass transfer coefficients. Consequently, CO2 absorption is severely retarded because its rate is controlled by the liquid-side resistance to mass transfer. This leads to a concomitant improvement in H2S removal.

A number of cases have come to light in which seemingly impossibly low H2S leak rates have been realised and much higher than normal CO2 rejection rates observed from trayed columns. In every case, liquid rates were low and the weirs were long enough to have weir loads of 10 gpm/ft or less, putting the trays well into the spray regime. Simulations could be matched to plant performance data only by using greatly reduced liquid-film coefficients for mass transfer (stagnant liquid), and by increasing the gas-side coefficient as expected for turbulent gas flow past drops.
The spray regime is quite effective in achieving ultra-low H2S leaks and greatly improved CO2 rejection rates and, if conditions cause an existing plant to operate there, selectivity will probably greatly exceed what might be expected. Therefore, under certain circumstances, consideration might be given to trying to operate in this region.

Usually, contactors containing trays are initially designed to operate at 70–80% of jet and downcomer flooding rates using very conventional valve-type trays with few or no hydraulic performance enhancers, such as tapered downcomers and swept-back weirs. These are safe designs because they use older, broadly established technology and leave plenty of room for easy capacity improvement should they fall short in the field and to accommodate the inevitable future increases in plant throughput. Most contactors in gas treating applications have relatively high liquid-to-gas (L/G) ratios, usually quite a bit higher than in distillation, because the large amounts of acid gases to be removed require large solvent flows for absorption. A well-designed tray has a turndown ratio of at least 4:1; in other words, it can operate satisfactorily from a hydraulic standpoint down to about one-quarter or less of design rates. Below three- or four-to-one turndown, liquid starts to weep through the trays to an extent that mass transfer performance begins to suffer.

At the L/G ratios common in gas treating, trays normally operate in the froth regime. In this flow regime, the biphase on the tray has the gas dispersed in a more or less continuous liquid, and the mixture flows across the tray as an intensely agitated froth.

Sometimes a raw gas contains either very small amounts of H2S and CO2, or perhaps a small concentration of H2S and a larger fraction CO2, but the H2S must still be removed to meet a parts-per-million specification. In such cases, only modest solvent flow is needed to treat the gas with MDEA so the L/G ratio in the contactor is low. As the liquid flow is reduced, the froth gives way to a spray because there is insufficient liquid flow to maintain a reasonable volume of liquid on the tray. The still large vapour flow shatters the liquid, converts it to droplets and ejects it into the downcomer, not as a stream of continuous liquid flowing over a weir, but as a swarm of drops (a spray) thrown over the weir and into the downcomer. Interfacial areas, and gas- and liquid-side mass transfer coefficients can be radically different in the froth and spray regimes, with the consequent likelihood that selectivity may be affected significantly.

Tray hydraulics
Maximum tray hydraulic capacity can be limited by any one of three types of flooding mechanism: jet or entrainment flood, downcomer backup flood and downcomer choke flood. The ability of a tray to operate at the lower end of the loading range is limited by weeping and also by entrainment that occurs even in the spray regime of operation as the liquid flow on the tray is decreased while maintaining a high vapour flow rate.

The other low liquid rate limitation is vapour bypassing up downcomers as lowered liquid depths unseal the downcomers. In a perfectly balanced design, jet flood and choke flood occur simultaneously and the column’s diameter is sized to be just adequate for the proposed loads; certainly pressure drop will not be high enough nor downcomer clearance tight enough to induce backup flood problems, nor will vapour bypass up downcomers.

Entrainment flood
Figure 1 shows a typical tray operating diagram illustrating stable limits. The portion of the upper operating line labelled spray entrainment is in the froth regime and corresponds to the upper limit of vapour flow where the tray begins to flood, the so-called jet flood line. This should not be interpreted as a finely drawn, sharp line because the definition of jet flood varies from vendor to vendor. Some vendors call 85% jet flood the vapour velocity at which 10% of the liquid is entrained from the tray and flows with the vapour to the tray above. Others call it the maximum useful capacity. Whether the definition is 80 or 90%, jet flood corresponding to 10 or 20% entrainment is arbitrary and the pinpointing of maximum useful capacity is certainly subjective. However, near the jet flood limit, the entrainment rate is exponentially sensitive to small changes in vapour rate and only a small increase in vapour flow will cause the column to flood. This is not a condition at which one would like to operate because the column approaches instability and becomes harder to control.

Vapour load is best described by the C-factor, usually based on the superficial vapour velocity, uS, through the empty column but sometimes based on the gas velocity through the tray active area or even through the area of the actual openings (valves and so on). The superficial velocity definition is:

Cs = us               ρv
                √  ρL – ρv