Hydrodynamics of a cocurrent downflow of gas and foaming liquid through the packed bed. Part II. Liquid holdup and gas pressure drop

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Abstract

In the present study the results of experiments have been presented whose aim was to determine the values of liquid holdup as well as gas pressure drop through the packing for systems foaming under the pulse flow regime. On the basis of 245 experimental points for the pulse flow regime the verification of the models describing the hydrodynamics of the system has been performed. Attention was focused on the models of Benkrid et al. (Chem. Eng. Sci. 52 (1997) 4021), Pina et al. (AIChE J. 47 (2001) 19) and Fourar et al. (Chem. Eng. Sci. 56 (2001) 5987). It has been concluded that none of the models analysed describes the hydrodynamics of the foaming systems with enough accuracy. Next, based on our own data-base the verification has been carried out of parameters of Benkrid et al. (Chem. Eng. Sci. 52 (1997) 4021) (‘drift flux’ model for εL and boundary layer model for ΔP/H) and Pina et al. (AIChE J. 47 (2001) 19) models as well as the estimation of the values of F*α-functions of Fourar et al. (Chem. Eng. Sci. 56 (2001) 5987) model. Using as the criterion the accuracy of estimation of the values of (ΔP/H) the best results have been obtained by applying Fourar et al. (Chem. Eng. Sci. 56 (2001) 5987) model (for the Ergun constants determined experimentally). The introduction of the estimated F*α-functions into the equations of the model (Eqs. 11 and 12) makes it possible to estimate the liquid holdup with the average absolute relative error not exceeding 9.8% and the pressure drop with an error less than 26%.

Introduction

The subject of the research in the present study is the hydrodynamics of the reactor in which the liquid and gas phases downflow cocurrently along the bed of catalyst particles.

Reactors of this type, called ‘trickle bed reactors’, form a very important group of apparatus used in quite a few branches of chemical industry, mainly in the processes of treating various fractions of crude oil with hydrogen.

In the present, part II of the study the results of experiments have been presented and analysed, in which the values of pressure drop and liquid holdup were determined. The experiments involved a specific group of systems studied i.e. systems which foamed in the high interaction regime.

The aim of the study was to create an experimental database which can be used to verify models describing the hydrodynamics of the system.

In the study much attention has been paid to model the pulsing flow since, as it was mentioned before, the systems investigated foam only in the high interaction regime.

A model which allows to calculate the values of both hydrodynamic parameters closest to the experimental data will be chosen as a result of the carried out verification.

Section snippets

Brief literature survey

Liquid holdup and gas pressure drop in the bed are the two key hydrodynamic parameters whose knowledge is necessary while designing and scaling up of the reactor. They are inseparable and appear together in the momentum balance equations of both fluids. The amount of the liquid held up in the packing is identified with the thickness of the film covering the packing particles. The increase in the thickness of this film limits the void volume of the bed, changes the space available for the gas

Experimental set-up and measurement technique

The diagram of the experimental installation has been presented in Part I of the study [15]. Its main part was a column 57-mm in diameter, filled with a 1.35-m high layer of glass spheres (0.003 m in diameter). Argon, nitrogen and helium were used as the gas phase, while the liquid phase formed water solutions of alcohols (methanol, ethanol and isopropanol) whose concentrations were chosen in such a way as to obtain foaming of the solutions in the high interaction regime. Physicochemical

The results of the experiments

The range of operating parameters used in the experiments has been presented on the flow map (Fig. 4).

Each of the systems tested foamed to a lower or higher degree in the pulse flow regime; however having exceeded the boundary of the regime change from pulsing to trickle flow foaming disappeared. This phenomenon occurred slightly earlier in weakly foaming systems. The appearance of foam in the system is demonstrated in diagram (Fig. 5), in which the measured values of liquid holdup and gas

Modelling of the pulse flow

The data-base, obtained as a result of experiments, comprising 245 points for the pulsing flow has been used to verify the models describing the hydrodynamics of the gas and liquid cocurrent flow through the bed.

The verification carried out will enable to point out the model which gives the values of εL and ΔP/H closest to those determined in the experiments.

Pulse flow regime appears in the reactor at relatively high flow rate of the liquid, thus the assumption that the packing particles are

Conclusions

The results of measurements of liquid holdup and gas pressure drop in the packing for the systems investigated which foamed in the pulse flow regime have been presented in this study. It has been concluded that such systems result in values of higher pressure drop and lower liquid holdup than non-foaming systems at the same velocities of both phases.

The influence of gas density on the parameters measured has been analysed. It has been found that at the same superficial velocity of both phases

Acknowledgements

This study was carried out as part of the research project sponsored by the Polish Committee for Scientific Research (Project No. PBZ/KBN/14/T09/99).

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