Mass transfer coefficients, film column

The thesis of Steward indicates that the overall liquid film and mass transfer coefficients were functions of the gas flow rate and the column pressure and are independent of the liquid flow rate and inlet air temperature. The gas film heat transfer coefficient was found to be a function only of the air flow rate. 

In this process, the two streams flow countercurrently through the column and undergo a continuous change in composition. At any location are in dynamic rather than thermodynamic equilibium. Such processes are frequently carried out in packed columns, in which the liquid (or one of the two liquids in the case of a liquid-liquid extraction process) wets die surface of the packing, thus increasing the interfacial area available for mass transfer and, in addition, promoting high film mass transfer coefficients within each phase. 

In early work on wetted-wall columns, Morris and Jackson,2I) represented the experimental data for the mass transfer coefficient for the gas film hD in a form similar to equation 12.25, though with slightly different indices, to give ... 

Relative Kga valid for all systems controlled by mass transfer coefficient (Kg) and wetted area (a) per unit volume of column. Some variation should be expected when liquid reaction rate is controlling (not liquid diffusion rate). In these cases liquid hold-up becomes more important. In general a packing having high liquid hold-up which is clearly greater than that in the falling film has poor capacity. 

Wetted-wall columns have been used for many years for determining mass-transfer coefficients on the assumption that the interfacial area across which mass transfer occurs can be obtained accurately from the dimensions of the column and a knowledge of the film thickness. It is therefore of considerable practical interest to determine whether the interfacial waves lead to an appreciable increase in the interfacial area of the film, which would introduce a grave uncertainty into such methods of determining mass-transfer coefficients. 

The design of packed column reactors is very similar to the design of packed columns without reaction (Volume 2, Chapter 12). Usually plug flow is assumed for both gas and liquid phases. Because packed columns are used for fast chemical reactions, often the gas-side mass transfer resistance is significant and needs to be taken into account. The calculation starts on the liquid side of the gas-liquid interface where the chemical reaction rate constant is compounded with the liquid side mass transfer coefficient to give a reaction-enhanced liquid-film mass transfer... 

To implement these simulation approaches, the value of the liquid film mass transfer coefficient Kf is required, which for nonporous and porous HPLC particles, can be calculated from literature correlations derived for bath357,400,408 or column models.407,408 For the case with porous particles, the apparent pore liquid mass transfer coefficient Kp can be expressed as an effective pore diffusivity over an average effective diffusion path length, such that... 

Doan, H.D. Fayed, M.E. Entrance effect and gas-film mass-transfer coefficient in a large-diameter packed column. Ind. Eng. Chem. Res. 2000, 39 (4), 1039-1047. 

Au-Yeung, P.H. Ponter, A.B. Estimation of liquid film mass transfer coefficients for randomly packed absorption columns. Can. J. Chem. Eng. 1983, 61, 418-493. 

Ponter, A. B. and Au-Yeung, P. H., Estimating Liquid Film Mass Transfer Coefficients in Randomly Packed Columns, in Handbook of Heat and Mass Transfer, Cheremisinoff, N. P. (Ed.), Gulf Publishing Corp., Houston, TX, Chap. 20, Vol. II, pp. 903-952, 1986. 

First, the overall mass transfer coefRcient k a of the microreactor was estimated to be 3-8 s [43]. For intensified gas liquid contactors, kj a can reach 3 s while bubble columns and agitated tanks do not exceed 0.2 s Reducing the flow rate and, accordingly, the liquid film thickness is a means of further increasing kj a, which is limited, however, by liquid dry-out at very thin films. Despite such large mass transfer coefficients, gas-liquid microreactors such as the falling film device may still operate between mass transfer and kinetic control regimes, as fundamental simulation studies on the carbon dioxide absorption have demonstrated [44]. Distinct concentration profiles in the liquid, and even gas, phase are predicted. 

The parameter 8f is related to the film mass transfer coefficient, which is a function of velocity. For a packed column, the film mass transfer coefficient can be calculated from the following correlation. 

Therefore, in this work a more physically consistent way is used by which a direct account of process kinetics is realised. This approach to the description of a column stage is known as the rate-based approach and implies that actual rates of multicomponent mass transport, heat transport and chemical reactions are considered immediately in the equations governing the stage phenomena. Mass transfer at the vapour-liquid interface is described via the well known two-film model. Multicomponent diffusion in the fdms is covered by the Maxwell-Stefan equations (Hirschfelder et al., 1964). In the rate-based approach, the influence of the process hydrodynamics is taken into account by applying correlations for mass transfer coefficients, specific contact area, liquid hold-up and pressure drop. Chemical reactions are accounted for in the bulk phases and, if relevant, in the film regions as well. 

Cooney, D. O. 1991. Determining External Film Mass Transfer Coefficients for Adsorption Columns. AIChE Journal 37 (8) 1270-1274. 

Aiming separation of liquid mixtures with supercritical fluids, the governing mass transfer resistance normally exists in the liquid phase. Furthermore, the solvent-to-liquid ratios are high. Thus, the favored mode of operation is to run the supercritical solvent as the continuous phase. The liquid phase forms out thin films, rivulets, and droplets. The application of usual mass transfer equation Sh =f(Re, Sc) as given in Table 2.6, enables the calculation of mass transfer coefficients. However, in high-pressure countercurrent columns, one has to consider mutual mass transport. 

The airflow rate coming in is 1900 ft /min. The scrubbing solution flow rate is 0.5 m /min. The gas-phase mass-transfer coefficient for NH3 is kg = 0.35 kgmol/m -hr-atm the corresponding liquid-phase mass-transfer coefficient kg = 0.5 cm/ hr. Henry s law constant is given as 0.0133 m -atm/ kgmol. Assume Dac = Dee in the liquid film. Determine the total gas-liquid interfacial area required to remove 90% of NH3 in the column. 

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