Countercurrent columns, mass-transfer coefficients

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. 

Water is to be cooled in a small packed column from 330 to 285 K by means of air flowing countercurrently. The rate of flow of liquid is 1400 cm3/m2 s and the flow rate of the air, which enters at a temperature of 295 K and a humidity of 60%, is 3.0 m3/m2 s. Calculate the required height of tower if the whole of the resistance to heat and mass transfer may be considered as being in the gas phase and the product of the mass transfer coefficient and the transfer surface per unit volume of column is 2 s-1. 

Experimental gas-solid mass-transfer data have been obtained for naphthalene in CO2 to develop correlations for mass-transfer coefficients [Lim, Holder, and Shah, Am. Chem. Soc. Symp. Ser, 406, 379 (1989)]. The mass-transfer coefficient increases dramatically near the critical point, goes through a maximum, and then decreases gradually. The strong natural convection at SCF conditions leads to higher mass-transfer rates than in liquid solvents. A comprehensive mass-transfer model has been developed for SCF extraction from an aqueous phase to CO2 in countercurrent columns [Seibert and Moosberg, Sep. Sci. Techrwl, 23, 2049 (1988) Brunner, op. cit.]. 

The influence of pressure on the mass transfer in a countercurrent packed column has been scarcely investigated to date. The only systematic experimental work has been made by the Research Group of the INSA Lyon (F) with Professor M. Otterbein el al. These authors [8, 9] studied the influence of the total pressure (up to 15 bar) on the gas-liquid interfacial area, a, and on the volumetric mass-transfer coefficient in the liquid phase, kia, in a countercurrent packed column. The method of gas-liquid absorption with chemical reaction was applied with different chemical systems. The results showed the increase of the interfacial area with increasing pressure, at constant gas-and liquid velocities. The same trend was observed for the variation of the volumetric liquid mass-transfer coefficient. The effect of pressure on kia was probably due to the influence of pressure on the interfacial area, a. In fact, by observing the ratio, kia/a, it can be seen that the liquid-side mass-transfer coefficient, kL, is independent of pressure. 

In a countercurrent packed column, n-butanol flows down at the rate of 0.25 kg/m2 s and is cooled from 330 to 295 K. Air at 290 K, initially free of n-butanol vapour, is passed up the column at the rate of 0.7 m3/m2 s. Calculate the required height of tower and the condition of the exit air. Data Mass transfer coefficient per unit volume, hDa = 0.1 s 1. Psychrometric ratio, (h/hDpAs) = 2.34. Heat transfer coefficients, hL = 3hG. Latent heat of vaporisation of n-butanol, A = 590 kJ/kg. Specific heat capacity of liquid n-butanol, Cl = 2.5 kJ/kg K. Humid heat of gas , s = 1.05 kJ/kg K. 

In extraction columns, it is possible to find droplet swarms where the local velocities near the droplet surface are higher, this being due to the lower free area available for the countercurrent flowing continuous phase. Wake and Marangoni influences make the prediction of a physical mass transfer coefficients difficult. With reactive extraction the influence of interfacial kinetics on overall mass transfer is generally not negligible. In any case, a combination of reactive kinetics with any eddy mass transfer model is recommended, whereas the latter could rely on correlations derived for specific column geometries. 

First, we measured thermodynamic and mass transfer data of the multicomponent system olive 0U/CO2 (3,4). The phase equilibria was modulated by correlating the partition coefficients (Kj = y /x ) of each component present in the mixture as a function of the mole fraction of the FFA fraction in the liquid phase (3). Mass transfer studies were performed in a lab-scale countercurrent packed column. The experimental measured mass transfer coefficients were... 

Therefore a much higher mass transfer coefficient can be expected for SFE. A comparison of countercurrent extraction columns using supercritical fluids with those using liquid solvents shows, however, that SFE-columns do not reach an efficiency as high as could be expected because of the solvent properties (table 2). 

Fig. 13. Interfacial areas (a) and true liquid-side mass-transfer coefficients (b) in countercurrent packed columns. (See tabulation on p. 71.)...

The mass transfer coefficient depends on the flow condition of gas and liquid phases, the interface area is influenced by the geometry of the column internals and local velocity of the two phases. The largest driving force for the mass transfer is the concentration difference when the two phases are uniformly distributed over the entire flow area. This is achieved when a countercurrent flow pattern of the two phases without remixing is reached in a theoretical plate. 

The sulfur dioxide content of a gas stream is lowered by 90% by scrubbing with water in a countercurrent packed column. The column is 1.25 m in diameter and 5 m in height. The gas rate is 500 kmol/h, and the inlet sulfur dioxide concentration is 2% mole. The column was designed such that the sulfur dioxide concentration in the bottoms does not exceed 0.04% mole. Determine the required water rate, the number of gas-phase transfer units, the height of the gas-phase transfer unit, and the mass transfer coefficient KyU. The equilibrium relationship for sulfur dioxide is given as Y = 38X. 

The development of the design equation for a countercurrent packed tower absorber or stripper begins with a differential mass balance of component A in the gas phase, in a manner similar to that of Example 2.12. However, this time we do not restrict the analysis to dilute solutions or to constant molar velocity. If only component A is transferred (4 G = VA[ = 1.0), considering the fact that the gas-phase molar velocity will change along the column, and that F-type mass-transfer coefficients are required for concentrated solutions, the mass balance is... 

Hqq is related to the individual coefficients by Eq. (16-27a). Unfortunately, it is dangerous to use Eqs. (16-37) and (16-38) to determine the values for Hl and Hq for co-current columns because the correlations are based on data in countercurrent columns at lower gas rates than those used in co-current columns. Reiss (1967) reviews co-current contactor data and notes that the mass transfer coefficients can be considerably higher than in countercurrent systems. Gianetto et al. (1973) operated with a 15-fold velocity increase and observed a 40-fold increase in Icl when liquid-phase resistance controlled. They recommended co-current operation for absorption with chemical reaction. Harmen and Perona (1972) did an economic conparison of co-current and countercurrent columns. For the absorption of CO2 in carbonate solutions where the reaction is slow they concluded that countercurrent operation is more economical. For CO2 absorption in monoethanolamine (MEA), where the reaction is fast, they concluded that countercurrent is better at low liquid fluxes whereas co-current was preferable at high liquid fluxes. 

Gas- and liquid-side mass transfer coefficients in packed absotptkm columns exhibit comfdex dependencies on the gas and liquid rates and the column packing, transfer area will bis a function of the hydrodynamics and packing. Correlations on experimental data, are usually devdoped in terms of the height of a transfisr and lk uid heights of transfer units are tinned 1 die relationships. with countercurrent flows In addition, the interfigial for such situations, based unit (HTU) concqit Gas... 

With known overall mass transfer coefficients kd and /tg, specific volumetric interface area Og and axial dispersion coefficient, the solution gives the actual concentration profile of the key component in the column. In [6.26], methods to measure the longitudinal mixing in countercurrent extraction columns are described and approaches to calculate the Bodenstein number and the axial dispersion coefficient for common extractor designs are given. 

An estimation of the mass transfer coefficients (Kq, Xl), the mass transfer area (fly), and the volume fractions of gas and liquid (ec, el) can be carried out with the correlation equations, which have been developed on the basis of hydrodynamical theories and dimension analysis. The constants incorporated into the equations have subsequently been determined on the basis of experimental data for a number of model systems (such as air-water, oxygen-water, etc.). The dependability of these correlation equations can thus be very different. Usually, the quality of the estimations falls somewhere around 10-30% of the actual values. The correlations presented in the literature should therefore be utilized with great caution, and the validity limitations should be carefully analyzed. However, these correlations are very useful, for example, when performing feasibility studies or planning one s own experimental measurements. A thorough summary of various correlation equations for gas-Kquid reactors is presented by Myllykangas [ 1 ]. Here we will only treat two common gas-Kquid reactors, namely, bubble columns and packed columns, operating in a countercurrent mode. 

A 20 Ibmol/hr effluent stream that is mostly air contains 0.02 mole fraction acetone. Prior to releasing it to the atmosphere the acetone content must be lowered to 100 ppm mole as a maximum. It is proposed to accomplish this by countercurrent water scrubbing in a 3-ft-diameter packed column. The scrubbing water is treated and recycled at the rate of 600 Ibmol/hr with a residual acetone content of 10 ppm mole. Calculate the required packing height if the mass transfer coefficient KyU = 1.75 Ibmol/fE-hr. The equilibrium acetone mole fraction is given by V = 1.67X. 

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. 

Adsorption is very different from absorption, distillation, and extraction. These three processes, detailed in the five previous chapters, typically involve two fluids flowing steadily in opposite directions. In absorption, a gas mixture flows upward through a packed column while an absorbing liquid trickles down. In distillation, a liquid mixture is split into a more volatile liquid distillate and a less volatile bottoms stream. In extraction, two liquid streams move countercurrently to yield an extract and a raffinate. To be sure, in some cases, the contacting may involve near-equilibrium states, and in other cases it may be described with nonequilibrium ideas like mass transfer coefficients. Still, all three units operations involve two fluids at steady state. 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Section 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the mass transfer capacity coefficient term, now used in the model equations, must however be a realistic value corresponding to the hydrodynamic conditions, actually existing within the column and, of course, will be substantially less than that leading to an equilibrium condition. 

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