(48b) Effect of Tray Hydraulic Operating Regime on Selectivity for H2S in Gas Treating

Most gas contactors are highly liquid loaded, with good designs operating at vapor loads approaching the jet-flood limit. A good design also will have downcomers sized to approach the choke flood limit at the design point because this allows for the maximum tray active area, hence the greatest throughput for a given column diameter. At high liquid loads, operation is generally in the froth regime which is characterized by a highly agitated liquid caused by the passage of large bubbles, jets and large-scale bursts of gas. This results in reasonable absorption rates for both CO2 and H2S. At very low weir loads, on the other hand, the liquid is dispersed in the form of droplets in a spray. The droplets are typically about 1-mm diameter, so small they act as nearly rigid spheres and therefore have extremely small liquid-side mass transfer coefficients. Consequently, CO2 absorption is retarded because its rate is controlled by the liquid-side resistance to mass transfer. At atmospheric pressure the droplets move through the gas at Reynolds numbers of about 500, so the gas flow around the droplets is fully turbulent and one might expect rather large gas-side mass transfer coefficients. Several cases have come to light in which impossibly-low H2S leak rates have been realized and much higher than normal CO2 rejection rates observed from trayed columns. In every case, liquid rates were low and the weirs long enough to have weir loads of 10 gpm/ft or less. In one case the weir load was only 2 gpm/ft. H2S leaks of less than 1 ppmv in the treated gas were routinely achieved by the plant when 20 ppmv was to be expected based on the mass transfer characteristics of froths. Simulations could be forced into agreed with plant performance data when liquid-film coefficients for mass transfer were reduced by a factor of ten, and gas-side coefficients were increased by a factor of ten. The fact that the values of the correction factors were the same for all cases lends credence to the conclusion that it is the distinctly different hydraulics in the two operating regimes that is responsible for the greatly superior treating performance. Evidently, although the spray regime may be suboptimal from a column capacity standpoint, it can be used quite effectively to achieve ultra low H2S leaks and greatly improved CO2 rejection rates, a finding that is consistent with the technology embodied in US Patent 4,278,621 issued to Sigmund and Butwell and assigned to Union Carbide Corporation for a tray on which the liquid flow is laminar. Columns cannot always be forced to operate in the spray regime, however, because it's not always possible to provide sufficient total weir length for a given liquid flow, but if weirs can be made long enough, greatly improved separation performance can be achieved (concomitantly with reduced maximum throughput). © 2009 by Optimized Gas Treating, Inc.