The influence of axial dispersion on carbon dioxide absorption tower performance

Abstract

Carbon dioxide was absorbed from mixtures with nitrogen by countercurrent contact with water in an experimental packed tower. Radial and axial gas concentration profiles were determined from measurements made within the packing. Substantial gas phase channeling was observed. Characterizing the gos flow regime by both piston flow and axial diffusion models yielded mass transfer data and computed axial gas concentration profiles. Differences between the mass transfer results for the two models allowed the influence of axial dispersion to be assessed. Comparison of the piston flow and axially dispersed profiles with the experimental profiles enabled conclusions to be drawn regarding the applicability of the axial diffusion model and the accuracy of available dispersion parameter values.The axial diffusion model appears to be a satisfactory representation of the process. The dispersion coefficients used were found to be too high, which emphasizes shortcomings in the transient response experiments yielding dispersion coefficients. The influence of dispersion on the performance was found to be only moderately adverse. The effect increases with increasing liquid rate, decreasing gas rate, and decreasing packing height. It is improbable that the effect is large enough to account for the difference between industrial scale performance and that predicted from available mass transfer correlations.The spread of fluid element residence times prevailing in an absorption tower invalidates the piston flow assumption inherent in existing design techniques. The resulting longitudinal dispersion will, in some measure, have an adverse influence on the performanoe. The magnitude of this effect has not as yet been determined, however. For carbon dioxide absorbers, in particular, it has been speculated that the wide discrepancy between the actual industrial scale performance and that predicted from standard mass transfer correlations (19,28,31) is due to axial dispersion of the gas phase. In two-phase operation of this nature, substantial gas phase mixing will be induced by the countercurrent liquid flow, particularly at the high water rates necessary to achieve satisfactory absorption of relatively insoluble gases such as carbon dioxide. Published gas-side dispersion coefficients for twophase operation are scarce, but those available (11, 12) would indicate the effect of dispersion on the performance to be significant, especially if projected into the range of industrial liquid rates. The values of dispersion coefficients such as these, obtained from transient response experiments, have not been verified by application to steady state systems of practical interest. Consequently, their accuracy is subject to some question, and they cannot be employed to make definitive predictions.The study described in this paper presents data to indicate the degree to which axial dispersion will affect absorption tower performance. By using published dispersion coefficients, the investigation serves also as a test of their accuracy. The evidence upon...

Frequency Response of the Back-flow Cell Model of Mass-Transfer Processes: Packed-Column Characteristics for Tracer-Gas Absorption

“…9 and 10 indicate that the assumption of constant (k,,a) with respect to water flow rate is a fairly good approximation. Some increase in No, for Carbon Dioxide Runs Nos.…”

“…The form of B in Eq. (9) ensures division of the gains by the amplitude of the particular input wave that is acting. The solution of the quidiagonal matrix system of equations for each assumed frequency o is efficiently carried out by the Gaussian elimination procedure with the recursive relationships given by Conte and Dames (6).…”

Analysis of simultaneous heat and mass transfer in the stripping of methanol from aqueous solutions

“…Gas phase and liquid phase compositions and temperatures were constant across the diameter of the column, at a given packing height (21,22,25). Packing discontinuities at points of liquid redistribution, which occurred every 0.61 m of packed height, did not introduce significant errors.…”

Design of packed, adiabatic absorbers: physical absorption of acid gases in methanol

“…Experimental verification of this method for the case of a single transferring solute was provided by Raal and Khurana (1973) for air-ammonia-water and by Stephenson (1978) for the fractional stripping of krypton-85 with a volatile solvent, C2ClzF2. Stephenson generalized the Treybal approach to include axial dispersion, which is considered important at high liquid flow rates (Brittan and Woodburn, 1966). Feintuch and Treybal (1978) developed a computer algorithm capable of handling multicomponent systems based on the earlier work by Treybal (1969) for singlesolute systems.…”