Elsevier

Chemical Engineering Science

Volume 153, 22 October 2016, Pages 146-154
Chemical Engineering Science

Analogy of absorption and distillation processes. Wetted-wall column study

https://doi.org/10.1016/j.ces.2016.07.021Get rights and content

Highlights

  • Wetted wall column for absorption as well as distillation experiments.

  • Profile method for evaluation of distillation mass-transfer coefficients.

  • Different dependence of absorption/ distillation kG on the gas velocity.

  • Quality of the diffusivity data/correlations essential for rate-based design.

Abstract

This article is the second one from a pair focusing on the mass transfer fundaments and the analogy between the processes of absorption and distillation. The wetted wall column, a device with known interfacial area, was built for this purpose enabling evaluation of the mass-transfer coefficients instead of the volumetric mass-transfer coefficients usually provided by experiments in the packed columns. The analogy of the processes has been assessed by comparison of the mass-transfer characteristics (HETP, individual mass-transfer coefficients kL and kG) evaluated on the basis of the absorption experiments with those found under the distillation conditions. In the prior paper the results of the absorption experiments performed under distillation-like conditions have been published in the form of dimensionless correlations ShL=0.33ReL0.12ScL0.47andShG=0.012ReLG0.90ScG0.61.

In this paper we present results of the distillation experiments in the form of the HETP data and mass-transfer coefficients evaluated by the profile method. The experimental data are compared with those predicted on the basis of the absorption experiments results. The experimental HETP data acquired on the methanol-n-propanol and ethanol-n-propanol systems agree within 10–20% with the data predicted. The distillation mass-transfer coefficients evaluated by the profile method were correlated in the dimensionless form ShL=0.25ReL0.12ScL0.47andShG=2.3103ReLG1.07ScG0.61. The distillation kL values have been found to be 24% lower and the kG values 13% lower than the ones predicted according to absorption correlations. The gas-phase mass-transfer coefficient, kG, exhibits higher dependence on the vapor velocity than it was found in standard absorption measurements. Similar difference was found also for packed columns in previous works. Apparently, there is at least one unaccounted phenomenon which affects differently the mass-transfer performance of absorption and distillation packed columns. Poor results of the present diffusion models have been noticed in this work preventing deeper study of these differences. Improvement of these models is found to be inevitable for the further research in this area as well as for the reliable rate-based modeling of the industrial absorption and distillation columns.

Introduction

Distillation and absorption in the packed column can be treated as the analogical processes of the mass transfer between the gas/vapor and the liquid flowing on the packing. The assumed analogy implies that the phenomena occurring in both the processes can be described with the same set of models dealing with the hydrodynamic of the phases, phase equilibria and the mass-transfer phenomena. In principle, it should be possible to use the packing mass-transfer characteristic (kLa, kGa and a) measured by standardized experiments under the absorption conditions to predict the packing mass-transfer efficiency under the distillation ones. Despite all the effort already given to this topic the successful recalculation of the mass-transfer parameters from absorption to distillation conditions has never been mastered. Such situation lies the question whether the failures are caused only by the seriously different process conditions (absorption experiments are usually based on cold air/water system while the distillation is performed with hot organic systems) or whether there is a principal difference between those processes, i.e. the processes are not analogical. We re-open this question in this paper, as the rate-based modeling of the absorption and distillation columns relies on the validity of the analogy.

There are too many variables and uncertainties in testing the recalculation from absorption to distillation conditions if the ordinary distillation on the packing is considered. Our attempt to bring some new insight in the situation is based on the following ideas:

  • 1)

    Utilizing direct method of determination of the mass-transfer coefficients under the distillation conditions – the profile method – which enables their direct comparison with results of absorption experiments.

  • 2)

    Bringing the conditions of the absorption and distillation experiments as close together as possible. In an ideal case the phase hydrodynamics and their physical properties should be the same.

  • 3)

    Minimizing the number of phenomena and quantities which affect the mass-transfer process by utilizing the experimental apparatus with known interfacial area – the wetted wall column – and by its utilization for both the absorption as well as the distillation experiments.

This paper is second of a pair. In the prior paper (Haidl et al., 2016a), the results of absorption experiments have been published in the form of the dimensionless correlations for the gas and liquid phase Sherwood numbers:ShL=0.33ReL0.12ScL0.47ShG=0.012ReLG0.90ScG0.61.

The absorption experiments were performed under several conditions using different liquids, gases, their temperatures and flow-rates covering the range of distillation Reynolds and Schmidt numbers so we expect that they can be used for the reliable calculation of the ShL and ShG (and HETP) values also for the distillation conditions. If so, the results would support rationality of the

  • 1)

    Contemporary way of the rate based modeling of both processes

  • 2)

    Description of the mass-transfer coefficients dependence on the process conditions.

The disagreement of the calculated and measured mass-transfer data should alarm us, that

  • 1)

    The number of process affecting phenomena is incomplete

  • 2)

    or/and models of the individual phenomena are insufficient

  • 3)

    or/and the description of the mass-transfer coefficients dependence on the process conditions is poor.

In this paper we bring the separation efficiency of the wetted wall column under distillation conditions in form of the HETP and also as the correlations for gas and liquid Sherwood numbers evaluated by the profile method and their comparison with the values predicted using (1), (2).

Section snippets

Distillation in wetted wall column

Although the wetted wall column was intensively used for the mass-transfer research in the last century, only a few of papers are dealing with its mass-transfer characteristics under distillation conditions. Several attempts to evaluate the contribution of the liquid-side resistance to the overall mass-transfer resistance, i.e. the relative resistance of the liquid phase, RL,rel, have been made.

Johnstone and Pigford (1942) measured the height of transfer unit (HTU) for distillation of five

Wetted wall column

A glass column of ID 25 mm (geometrical area aG=4/d=160 m2/m3) and length of 2.1 m was used. The column was equipped with seven pairs of holes to enable placing liquid and gas sampling devices along the column. Each pair consisted of two conical shaped holes positioned on the opposite sides of the column and was rotated by 90 degrees with respect to the previous one. The vertical distance between the sampling points was 30 cm. The first sampling point was placed 15 cm below the upper column edge.

Results

The liquid and vapor molar fraction profiles along the column were measured by atmospheric distillation of methanol-n-propanol and ethanol-n-propanol systems under total reflux conditions. Altogether 24 M fraction profiles on methanol-n-propanol (MeOH-PrOH) system and 13 profiles on ethanol-n-propanol (EtOH-PrOH) system were measured in range of reflux flow rate from 1 to 3 g/s.

For each experimental profile the overall HETPexp value has been calculated according to Eq. (6). The molar fraction of

Conclusions

This article brings a mass-transfer data (HETP and kL, kG) obtained under distillation conditions in the wetted-wall column distilling methanol–n-propanol mixture under atmospheric pressure and the total reflux. The same type of the column was earlier used for the measurements of the mass-transfer coefficients under absorption conditions with the Reynolds and Schmidt numbers of the phases set to cover their whole range achieved during distillation experiments. The results of the absorption

Acknowledgment

Authors are grateful to Raschig GmbH for financial support of the project and to the Czech Science Foundation for supporting the project through Grant 13-01251S.

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