Mass transfer coefficients and effective area of packing
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The effective mass transfer area (a [subscript e]), liquid film mass transfer coefficient (k [subscript L]), and gas film mass transfer coefficient (k [subscript G]) of eleven structured packings and three random packings were measured consistently in a 0.428 m packed column. Absorption of CO₂ with 0.1 gmol/L NaOH with 3.05 m packing was used to measure a [subscript e], while air stripping of toluene from water with 1.83 m packing was used to measure k [subscript L], and absorption of SO₂ with 0.1 gmol/L NaOH with 0.51 m packing was used to measure k [subscript G]. The experiments were conducted with liquid load changing from 2.5 to 75 m³/(m²*h) and gas flow rate from 0.6 to 2.3 m/s. Packings with surface area from 125 to 500 m²/m³ and corrugation angle from 45 to 70 degree were tested to explore the effect of packing geometries on mass transfer. The effective area increases with packing surface area and liquid flow rate, and is independent of gas velocity. The packing corrugation angle has an insignificant effect on mass transfer area. The ratio of effective area to surface area decreases as surface area increases due to the limit of packing wettability. A correlation has been developed to predict the mass transfer area with an average deviation of 11%. [Mathematical equation]. The liquid film mass transfer coefficient is only a function of liquid velocity with a power of 0.74, while the gas film mass transfer coefficient is only a function of gas velocity with a power of 0.58. Both k [subscript L] and k [subscript G] increase with packing surface area, and decrease with corrugation angle. A new concept, Mixing Point Density, was introduced to account for effect of the packing geometry on k[subscript L] and k [subscript G]. Mixing Point Density represents the frequency at which liquid film is refreshed and gas is mixed. The mixing point density can be calculated by either packing characteristic length or by surface area and corrugation angle: [mathematical equation]. The dimensionless k [subscript L] and k [subscript G] models can then be developed based on the effects of liquid/gas velocity, mixing point density, packing surface area: [mathematical equation] [mathematical equation]. Mi is the dimensionless form of Mixing Point Density (M), which is M divided by a [subscript P]³. Because Mi is only a function of corrugation angle (θ), it is a convenient transformation to represent the effect of θ on mass transfer parameters. An economic analysis of the absorber was conducted for a 250 MW coal-fired power plant. The optimum operating condition is between 50 to 80 % of flooding, and the optimum design is to use packing with 200 to 250 m²/m³ surface area and high corrugation angle (60 to 70 degree). The minimum total cost ranges from $4.04 to $5.83 per tonne CO₂ removed with 8 m PZ.